Final Report. Amber Green Emitters Targeting High Temperature Applications AGETHA IST INFORMATION SOCIETIES TECHNOLOGY (IST) PROGRAMME

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1 Final Report Amber Green Emitters Targeting High Temperature Applications AGETHA IST INFORMATION SOCIETIES TECHNOLOGY (IST) PROGRAMME Compiled by: Brian Corbett: NMRC This document is offered for dissemination at the sole discretion and on the sole responsibility of the receiving party. Neither the Community nor the authors can accept any responsibility or liability for the use that might be made of data appearing therein or for any consequences arising therefrom. This document must not be reproduced without this cover page and disclaimer intact, unless written permission to do so has been obtained. All rights reserved. AGETHA Final Report 1 IST

2 Summary... 4 Section 2: Device Modelling and Physics (WP2)... 7 Introduction...7 Key Achievements of Optical Modelling:...7 Brief summary of Optical modelling work:...7 Initial studies...7 Optimisation of PdAg AlGaN/GaN RCLED device...9 Understanding of the importance of QW position...10 Interaction with project partners...12 Electrical Modelling and New Physics...14 Current Spreading...14 Operating Voltage...15 Light-Current-Voltage and Power Efficiency Modelling...17 Section 3: Epitaxial Growth (WP4-7) Key achievements...19 Outline of the epitaxial growth work...19 Basic Studies : Growth optimisation of GaInN/GaN MQW Heterostructures...20 Main characterisation results (MBE Growth at UPM)...20 Main characterisation results (MOCVD Growth at CRHEA) Growth optimisation of GaAlN/GaN Bragg Mirror...26 Main characterisation results (MBE Growth at UPM)...26 Main characterisation results (MOCVD Growth at CRHEA) Growth optimisation of GaInN/GaN MQW LED and RCLED structures...32 Main characterisation results (MOCVD Growth at CRHEA)...32 Main Characterisation results ( MBE growth at UPM)...35 Main characterisation results (MOCVD Growth at CRHEA)...38 References on Growth:...40 Section 4 Device Fabrication (WP8, WP13) Key achievements...41 RC-LED structure...41 Fabrication process...42 ZrN/ZrB2 based contacts (sputtered, IET)...43 Section 5: Device Characteristics (WP1, WP9, WP10) Key achievements...51 Introduction...51 Forward voltage:...52 EL-peak wavelength and FWHM:...53 Switching times/ Small signal modulation bandwidth measurements:...54 Assessment of impact of TSSOP packaging process on device performance...56 Speed measurements...57 Data link measurements...59 Temperature dependence measurements...64 Output power variations with temperature...69 Reliability studies on GaN devices...70 Failure in circular contact LEDs and RCLEDs...71 Detector response:...73 Conclusions...74 References...74 Section 6 Safety issues and environment (WP11) First year...75 Second year...76 Third year...76 AGETHA Final Report 2 IST

3 Appendix A: Agetha Metrics Journal Publications...78 Conference Publications...81 PhD theses...84 PostDoc Personnel...84 Postgraduate Students:...84 Patents :...84 Spin-off companies :...85 Conferences/Workshops organised :...85 AGETHA Final Report 3 IST

4 Summary The AGETHA Consortium has researched the design, growth, fabrication, application and safety issues associated with RCLEDs emitting around nm. These devices are based on InGaN quantum wells grown on sapphire substrates for applications in datacommunications over low cost and consumer friendly step index plastic optical fibre (POF). The project has demonstrated the worlds first InGaN RCLED and the worlds highest speed (250Mb/s) communication link based on InGaN (RC)LEDs. The devices have small signal bandwidths of over 100MHz, have forward voltages less than 3.6V at 20mA and POF coupled powers of 200uW. These components have an advantage over red (650nm) in high speed links between 50 and 100m in industrial, aerospace, automotive and buildings. Increased length will be possible with graded index POF. Lower speed links are also demonstrated in the form of a replacement (and extension) of a laser link between a video camera and a monitor. High volume consumer applications based on an alternative to free space laser or copper links. Additional applications are where the component needs to be placed in a hot environment such as an engine but issues associated with the fibre need to be addressed. Another possible application is the Taxi Aid Camera System (TACS) which has been developed by Airbus to be implemented on the A It comprises external view cameras positioned in the forward edge of the aircraft fin and the forward section of the belly fairing. Fibre optics links the two cameras to an interface unit in the cockpit. A data rate at just under 300Mbps is being used in this first system. It is expected that developments of this system will be installed in the A380 and other future aircraft. It is also to be expected that the required data rate for such a system will increase as more cameras and other sensors are added to the network. Lengths up to 200m may be required for these networks and these networks will have a high temperature requirement on the cables (at least 125 C) due to the external positioning of the cameras. Several differences between GaN and previously successful GaAs based RCLEDs have been identified: The first is that the refractive index contrast of AlGaN distributed Bragg reflectors (DBR) is reduced compared with GaAs structures. The second is that growth of high aluminium composition AlGaN is difficult due to the high growth temperatures and the thermal and lattice mismatch in this material system. The result is that a high contrast (ie AlN/GaN) highly reflective (>60%) epitaxial DBR was not possible. An alternative approach of removing the substrate is very difficult as the optimum effect is obtained with the mirror being as close as possible to the quantum wells thereby reducing the thickness of supporting GaN material. To circumvent this problem we changed the design of the structure from a top surface emitter to a bottom (through substrate) emitter. The highly reflective mirror is now created by the metal contact which serves a dual role as an injecting ohmic contact. The new challenges are then in optimising this metal to GaN interface, the incorporation of this the metal with its attendant phase on reflection into the optical model, the optimisation of the current spreading, developing growth processes to meet the designs, developing the fabrication processes and the package technology for this new design. In AGETHA Final Report 4 IST

5 addition to the RCLED with an output reflector based on an AlGaN/GaN DBR this approach also permits a Resonant LED (RLED) where the enhancement in optical emission is created by the positioning of the quantum wells with respect to the metal mirror. The report is organised as follows. The second section describes the modelling from an optical and electrical point of view. Various cavity geometries were considered with different mirror configurations and cavity lengths. These show that a short (1λ) cavity with high contrast DBRs have the best extraction efficiency (8.5%) into 0.5 NA POF but this cavity is difficult to realise in practice due to the issues discussed above. Nevertheless a 3.4% extraction efficiency can be obtained with a 3 λ, metallic mirror with a 30% AlGaN based DBR. In addition the advantages of the single mirror RLED structure was explored and an efficiency enhancement of 3.5 over an LED is possible. The optimum device will have uniform electrical injection over the full device area. However this is compromised by the doping configuration (doping of the mirror increases strain), doping levels, carrier mobilities and contact resistances. We show that as the p type contact resistance is reduced the current spreading disimproves and the electron mobility dominates. A grid contact design is shown to be the solution to this problem. Further details and the final design rules are given in D20. Section 3 describes the achievements in the epitaxy of InGaN RCLED structures. Major improvements in the quality of all aspects the material have been obtained over the duration of the project resulting in high optical and structural quality GaInN/GaN MQW LED and RCLED structures, emitting at 500nm have been developed using both MOCVD and MBE techniques. The device performances of such LED and RCLED structures have been found in good agreement with the physical properties of these structures. The impact of the strain induced defects on the optical output power and on the reliability of the devices has been demonstrated. LED and RCLED wafers with a low misfit dislocation density, a very low surface morphology roughness as measured by AFM (RMS=10Å, Rmax = 20Å) and a narrow linewidth (FWHM=22nm) of the 500nm emission peak have been successfully grown using a two step process on template GaN buffer layers. Such wafers have led to an improved stability of the devices. High reflectivity DBR mirrors were realised using MBE but difficulties were encountered with MOCVD overgrowth. Section 4 describes the fabrication challenges overcome during the project. Low contact resistivity contacts were realised using PdAg and ZrN/ZrB 2. Essential to achieving these values was the surface preparation techniques which were optimised. An additional requirement on the p contact was the simultaneous high reflectivity. A non-alloyed Pd Ag contact was shown to be effective with a reflectivity of >70%. The phase on reflection is shown to be an important parameter in the optical design and this was experimentally determined for a number of metal configurations. Device degradation was studied and identified as being due to parallel micropipes which shorted due to internal heating to the melting point of GaN. Improvements in the growth technique avoided these problems. AGETHA Final Report 5 IST

6 Section 5 describes the characteristics of the packaged devices. Due to the use of a bottom surface emitting device a new package was introduced. A robust, reliable, compact, board mountable, high speed TSSOP package was configured for the devices. Fully packaged devices were used to demonstrate both high speed (250Mb/s over 50m) signaling. Devices from AGETHA grown material had higher bandwidth that those fabricated with commercial material. The bandwidth depended on the current density (contact area and current level), contact geometry, wavelength (decreasing with increasing wavelength) and quantum well configuration. Section 6 provides an overview of the safety issues associated with the growth of GaN. These issues are associated with incorporation of Al, Ga and In with the pump oil while significant amount of cyanides can be emitted from the mist oil separator. The abatement of ammonia is important due to the large volumes utilised. D21 describes these systems in more detail. During this time GaN devices have become of increasing importance worldwide due primarily to applications in lighting. This consortium has developed European expertise in growth, modelling, technology and packaging for a datacomm application. An international conference (ISBLLED) was organised by the consortium members and a special session was devoted to the Agetha project. An European workshop on Safety and Toxicology relating to GaN epitaxy was organised for growers, lab managers and maintenance personnel. The use of GaN devices also facilitated a number of projects as pre-university and undergraduate level. Appendix A lists the publications, presentations and dissemination associated with AGETHA. AGETHA Final Report 6 IST

7 Section 2: Device Modelling and Physics (WP2) Contributing Partners: TCD, Surrey Introduction The objective of this workpackage was to investigate the performance and determine the optimum designs for RCLED and RCD devices emitting at 510nm and 570nm using optical and electrical computational models. Due to the evolution of the project most effort in this workpackage focussed on understanding the performance and optimising the designs of RCLED devices emitting around 510nm. In order to achieve this goal, a wide range of detailed simulations were initially performed to produce device designs, which were then continuously updated in light of the characterised performance of fabricated devices, developments in growth and fabrication capabilities, and the results of continued modelling work. Detailed reports summarising the results of the device modelling work and containing specific device designs were produced in Milestones M3 (month 6), M8 (month 12), M18 (month 18), and M24 (month 24). In this section the main highlights and key achievements of the device modelling work during the project are outlined, with reference to detailed discussions of issues in previous reports where relevant. Key Achievements of Optical Modelling: 1. An extensive investigation of the light extraction efficiency of different cavity and single mirror structures. A comprehensive range of structures with different types of mirror (metallic, SiO 2 /TiO 2 DBR, GaN/AlGaN DBR), cavity configuration (1λ, 2λ etc) and device type (RCLED or single mirror, substrate or surface emitting) were modelled. 2. A complete optimisation of the epitaxial design of the chosen generic RCLED structure was performed. This structure consisted of a PdAg metal top mirror and an AlGaN/GaN bottom DBR mirror. The optimisation was performed as a function of the Al fraction in the DBR and QW emission linewidth. This enabled growers to clearly identify the improvement in RCLED efficiency achievable by altering these parameters. 3. A key result of this work was the development of an understanding of the importance of correctly positioning the QW relative to the metal mirror. It was shown that the efficiency is more dependent on this parameter than any cavity parameters. 4. The provision of detailed designs accounting for the growth capabilities of individual partners (e.g. higher Al fraction in DBR for MBE growth in UPM). Also the comparison of measured performance from devices with modelled predictions. Brief summary of Optical modelling work: Initial studies Since very little work had previously been published on the optimum design of GaN RCLEDs it was necessary to initially consider a broad range of possible cavity AGETHA Final Report 7 IST

8 configurations in our simulations. Figure 1 shows the range of top and bottom mirror structures and central cavity types considered. While there was a wide range of possible combinations of these cavity components certain structures were rapidly eliminated. A top emitting RCLED or a top emitting single mirror device requires a bottom mirror offering high reflectivity over the entire QW emission spectrum. In order to avoid expensive and difficult lift-off processes, the choice of bottom mirror was limited to an AlGaN/GaN DBR. Limitations on the aluminium fraction in the DBR and the number of DBR pairs by growth considerations limited the reflectivity achievable and hence the performance of any top emitting device (see M3 report). Therefore work focussed on substrate emitting devices, with a high reflectivity top mirror, either metallic or SiO 2 /TiO 2 DBR, and a lower reflectivity bottom AlGaN/GaN DBR through which the light was extracted. Metallic (Al,Ag,Pd) SiO 2 /TiO 2 DBR AlGaN/GaN DBR (for single mirror top emitting device) No mirror Top mirrors considered 3λ cavity 1λ cavity Central cavities considered (MQW active region centrally positioned) AlGaN/GaN DBR No mirror (for single mirror substrate emitting device) Bottom mirrors considered Figure 1. Mirror and cavity components considered in initial array of cavity configurations. A number of different metal mirrors were considered including Al, Ag and Pd, with a composite PdAg structure chosen as the optimum solution providing the required electrical properties while maintaining a high reflectivity of 70%. The high refractive index contrast between alternate layers in the SiO 2 /TiO 2 DBR results in an extremely highly reflective mirror, greater than 99% for only 5 DBR pairs. The higher reflectivity of the SiO 2 /TiO 2 DBR results in a higher extraction efficiency for devices incorporating this top mirror compared to the PdAg metal mirror. However due to the insulating nature of the SiO 2 /TiO 2 DBR a thin metal (AuNi) current spreading layer has to be included below the DBR in order to achieve a uniform current across the device. The high absorption in this current spreading layer severely degrades the performance of the SiO 2 /TiO 2 DBR AGETHA Final Report 8 IST

9 devices as can be seen from the results in table 1. During these simulations the composition of the bottom DBR and the location of the QWs in the cavity were optimised for each top mirror type. Aluminium fractions of 0.3 (limitation of MOCVD growth) and 1.0 (best possible) in the bottom AlGaN/GaN DBR were considered. The conclusion of this work was that while the SiO 2 /TiO 2 top and AlN/GaN bottom mirror combination offers the best extraction efficiency, the alterations to this design required to achieve a uniform current injection (i.e. the AuNi layer) results in a lower efficiency than a device incorporating a PdAg top mirror. This conclusion combined with the fabrication and electrical design considerations resulted in a focussing of effort by the consortium on substrate emitting devices with a PdAg top mirror. Top Mirror Bottom Mirror Extraction Efficiency into NA=0.5 1 λ cavity 3 λ cavity Metallic PdAg Al 0.3 Ga 0.7 N/GaN DBR 4.1% 3.4% SiO 2 /TiO 2 Al 0.3 Ga 0.7 N/GaN DBR 5.1% 4.1% SiO 2 /TiO 2 + AuNi Al 0.3 Ga 0.7 N/GaN DBR 3.3% 2.8% Metallic PdAg AlN/GaN DBR 5.7% 3.8% SiO 2 /TiO 2 AlN/GaN DBR 8.5% 5.1% SiO 2 /TiO 2 + AuNi AlN/GaN DBR 4.1% 3.1% Table 1. Modelled extraction efficiencies of various substrate emitting RCLED structures. Other issues covered in these initial optical simulations were - the tolerance of the designs to variations in the specified epitaxy (see report M8) - the optimum number/position of QWs - optimising the designs for lower emission NAs (see separate report to the commission) Optimisation of PdAg AlGaN/GaN RCLED device Having chosen the optimum generic cavity structure, extensive simulations were performed to optimise the RCLED design for maximum extraction efficiency as functions of Al fraction in the bottom DBR and QW emission linewidth. These simulations were performed for both 1λ and 3λ devices (1λ design offers higher efficiency, but 3λ design enables the processing of intracavity contacts). For each combination of Al fraction in the AlGaN/GaN DBR and the intrinsic InGaN/GaN QW emission linewidth, three design parameters were simultaneously optimised : 1. The thickness of the central cavity. Small adjustments to the thickness of the cavity adjust the angle of the cavity mode resonance for a fixed cavity order (1λ, 3λ/2, 2λ ). The optimum cavity thickness for maximum extraction efficiency is dependent on the NA into which emission is considered. AGETHA Final Report 9 IST

10 2. The number of AlGaN/GaNDBR pairs, which determines the reflectivity of the DBR mirror. Adjusting the reflectivity of the DBR mirror adjusts the cavity mode linewidth, which determines the range of wavelengths/angles over which emission is enhanced. 3. The position of the QWs in the cavity. The magnitude of the E-field of the extracted mode at the location of the QWs determines the coupling of emission to the extracted mode. The QWs are positioned at an antinode of the extracted mode to maximize the extraction efficiency. The complete results of this optimisation procedure were presented and discussed in M24, highlighting the importance of minimising the QW emission linewidth and maximising the Al fraction in the DBR. The results for the case of a 1-λ device optimised for an emission NA of 0.5 are shown in figure 2. Al fraction in bottom DBR Pairs Pairs 11 Pairs 3 Pairs 4 Pairs 5 Pairs 6 Pairs 7 Pairs 8 Pairs 9 Pairs FWHM of QW emission linewidth (nm) Figure 2. Contour plot of maximum extraction efficiency and corresponding number of DBR pairs as functions of Al fraction in DBR and QW emission linewidth for 1λ device into an emission NA of 0.5. The black lines show extraction efficiencies and shading shows the optimum number of DBR pairs. Understanding of the importance of QW position In the course of simulating the optical properties of the PdAg-AlGaN/GaN RCLED it was noted that the reduction in efficiency for incorrectly positioning the QWs within the cavity was greater than for having the incorrect cavity resonance wavelength. Specifically it was crucial to the RCLED extraction efficiency that the QWs were positioned correctly with respect to the metal mirror. The relative importance of these two design parameters is shown in figure 3. AGETHA Final Report 10 IST

11 It was further noted that the efficiency of vices could be improved by moving the QWs (a) to an antinode nearer the metal mirror. Previously (b de ) the QWs had been positioned at the central antinode in the cavity. In light of this re t, the difficulty in growing DBRs, and 0.04 sul 0.04 the limited efficiency gain from having a bottom AlGaN/GaN DBR with only a 0.3 Al fra 0.03 ction in a 3λ case (Al fraction limited by growth, λ needed for intra-cavity contacts), a series of single mirror designs were produced in year 3 of the project. These designs fo 0.02 cussed on optimally positioning the QWs wit 0.02 h respect to the metal mirror. The first set of designs positioned the QWs at the first antinode from the metal mirror ( 20nm metal mirro 0.01 r-qw separation). These devices were fa 0.01 bricated, however the extremely thin p- layer resulted in carrier leakage through the active region and no electro-luminescence was 0.00observed. In order to overcome this proble 0.00 m a second set of designs with the QWs now move d to the second anti-node from the Me Th metal mirror ( 129nm metal mirror-qw tal Mirror-QW separation (nm) ickness Top + Bottom Cavity Spacer layers (nm) separation) was produced. Extraction Efficiency, NA=0.5 Figure 3. Variation in extraction efficiency of 1λ RCLED with (a) QW position in cavity of optimal thickness and (b) central cavity thickness with QW optimally positioned. Extraction Efficiency, NA=0.5 Metal mirror-qw spacing (nm) λ and 1λ RCLED designs ~3λ ~2λ Extraction efficiency NA= ~1λ DBR - QW spacing (nm) No DBR Resonant single mirror designs Figure 4. Contour plot of modeled extraction efficiency into NA=0.5 of metal- Al 0.3 Ga 0.7 N/GaN DBR RCLED as functions of top and bottom cavity spacer layers. The locations of the QWs for the 1λ and 3λ RCLED designs and for two resonant single mirror designs are marked. AGETHA Final Report 11 IST

12 It is important to note that while these devices are only single mirror structures, that the exploitation of constructive interference effects by correctly positioning the QWs produces an efficiency enhancement factor 3.5 compared to a bulk LED (no mirror) device. In comparison a single mirror device with the QWs positioned at a large distance from the metal mirror would only offer an enhancement factor of <2. Indeed the modelled extraction efficiency of the resonant single mirror GaN structure of 3.3% (13%) into an NA of 0.5 (1.0) is similar to the modelled extraction efficiency of the red InGaAlP RCLEDs of 3.5% (14%), without requiring the growth of a DBR. Interaction with project partners While the optical modelling work was performed in TCD there was considerable interaction with project partners to ensure that the designs produced would have the required electrical properties and also would be compatible with the capabilities of the partners responsible for the growth and processing of the devices. The evolving requirements on device design produced by electrical device modelling performed by WP2 partners at the University of Surrey were considered in the optical modelling work. The sometimes-contradictory requirements for optimum electrical and optical device performance were discussed in the WP2 milestone reports. The individual capabilities and preferences of the three growth groups in terms of the Al fraction in the DBR, the QW emission wavelength, and MQW design were accounted for in the optical modelling work. A series of detailed designs were produced for each grower, and the designs were updated as both the growth capabilities and our understanding of optical design issues evolved. Similar interaction with the processing group in the NMRC resulted in a convergence towards the optimum top metal contact and ensured the compatibility of any designs with processing tolerances. The other aspect of the interaction of the optical modelling part of WP2 with other WPs, was in attempting to relate the measured performance of fabricated devices to the modelled performance. As mentioned above and discussed in detail in M24, the position of the QWs with respect to the top metal mirror in either single mirror or RCLED devices is crucial. Since the position of the QWs in the device does not affect the spectral position of the cavity mode in a RCLED device it is difficult to determine any information regarding the QW position from the emission spectrum of these devices. However simulations showed that the shape of the spectrally integrated farfield should vary depending on the metal mirror-qw separation. The measured farfield from a resonant single mirror AGETHA device is compared to the simulated results in figure 5. The results indicate that the QW is near optimally positioned with respect to the metal mirror. It should be noted that a similar result would be obtained if the metal mirror had a very low reflectivity (i.e. a device without any mirror), however separate measurements previously confirmed the reflectivity of the PdAg top mirror layer. Attempts were also made to reproduce the measured angle resolved spectra from various RCLED and single mirror devices. While reasonable agreement was obtained for some devices, considerable variation in the measured results from nominally similar devices off different wafers made comparisons with the model difficult. AGETHA Final Report 12 IST

13 (a) Extraction Efficiency (NA=0.5) Metal mirror-mqw separation (nm) Normalised Farfield Intensity (b) Measured farfield for single mirror device Angle (degrees) Figure 5. (a) Variation in extraction efficiency with separation between metal mirror and QWs in single mirror device. (b) The normalised simulated farfields for metal mirror-qw separations indicated by the coded vertical lines in graph (a). The measured farfield of a fabricated device is also shown. The shape of the measured farfield is a closest match to that for a device with the QWs optimally positioned. AGETHA Final Report 13 IST

14 Electrical Modelling and New Physics During this final period of the project our main concern has been to tie together the twodimensional electrical simulation work using the Atlas program from Silvaco with the analytical model of Guo and Schubert[1] and to try to model the light-current-voltage characteristics of some devices. The main points, considered in mode detail in deliverable D20, are summarised in the following sections. Current Spreading The model of Guo and Schubert predicts, on the basis of certain simplifications, that the current spreading length [the distance over which the current density falls by a fixed amount] is controlled by the thickness and conductivity of the n- and p-gan layers, and the specific contact resistance of the metal contact to the p-gan. In most devices good current spreading is obtained because of the relatively high specific contact resistance of the p-gan contact. For example, from the report for milestone M24 we showed that for a specific contact resistance of 0.01Ωcm 2 and typical material parameters that the current spreading length was 150µm. This current spreading is obtained at the cost of an increased operating voltage. This is a problem because our devices have a low operating voltage specification. Alternatively applications in solid state lighting are compromised because the power dissipated in this series resistance limits the power efficiency of the device. One outstanding question was how good is the Guo and Schubert model compared to a full two-dimensional calculation for mesa devices of a finite size? We found that, in the limit as the mesa width increased the two-dimensional simulations agreed well with the analytical model in the worst case when the specific contact resistance was zero. As the width of the mesa is reduced the spreading length calculated from the 2D simulation increased indicating that the analytical model can be used as a conservative estimate of the spreading length. For mesa contacts where the n-gan contact is located on both sides of the p-gan contact, as in an interdigited structure the mesa width can be twice the spreading length without compromising the uniformity of the current spreading. This explains why, in most commercial devices the light output looks uniformly distributed. However for small mesa areas the operating voltage increases because of the contact resistance. For large devices as improvements to the device doping and contact metalisation are developed current spreading effects will require a different mask design most likely an interdigitated pattern. In this project the devices are required to operate over a wide range of temperatures. Consequently we studied how the current spreading length changed with temperature. We found that for a 1-λ RCLED the spreading length changed from 80µm at 300K to 35µm at 400K. This assumes that the specific contact resistance decreases by a factor of 10 between the two temperatures as a result of the increasing hole carrier density as the temperature is increased. This shows that the electrical properties of the device must be specified in terms of current spreading at the highest temperature of operation. 1 X. Guo and E. F. Schubert, Appl. Phys. Lett. 78, 3337, (2001). AGETHA Final Report 14 IST

15 It is clear that for the specific contact resistances that can currently be achieved there seems to be little advantage to be obtained in an interdigitated pattern on the basis of current spreading. However, as this device parameter decreases this situation should be kept under review. Operating Voltage The results of current-voltage (I-V) measurements as a function of temperature show that the maximum operating voltage across the device will be at the lowest specified operating temperature. We performed calculations showing where the voltage is dropped across the device on the basis of a simple series resistance model for typical material parameters and with a specific contact resistance of 0.01Ωcm 2. The results of Table 1 below are for a device current of 50mA with a circular mesa diameter indicated. The two important things to note here are (i) the large voltage drop across the p-contact, particularly for small devices and (ii) the large voltage drop in the n-gan between the mesa and the n- contact. Device diameter (µm) p- contact Voltage across 1-λ Cavity Structure (V). p- layer n- layer To n- Contact p- contact Voltage across 3-λ Cavity Structure (V). p- layer n- layer To n- contact Table 1: Voltage drops for 1-λ and 3-λ cavity devices for I = 50mA at 300K for three different circular p-gan mesa diameters. It is clear that the voltage is rather a strong function of the n-type resistivity. Even so it does not appear to be possible to achieve an operating voltage of 3 or 3.5V with a circular mesa design. The only alternative is to adopt an interdigitated mesa structure. For the material properties used in Table 1 and with the same device surface area we have tried to reduce the operating voltage. A possible set of mask designs are shown later. The samples designated A have a 25µm wide digit, those designated B have a 50µm wide digit while those designated C have a 100µm wide digit. The calculations assume a central contact of 100x100µm for C devices and a 50x50µm contact for devices A and B. The remaining surface area of the active region is made up of a digit whose width is outlined above. In this way the results given in Table 2 for the different designs are independent, as far as is possible, of the detailed mesa pattern. AGETHA Final Report 15 IST

16 Device area (µm 2 ) Voltage across 1-λ Cavity Structure (V). Voltage across 3-λ Cavity Structure (V). p- contact p- layer n- layer To n- contact p- contact p- layer n- layer To n- contact 17671A B C A B C A B C Table 2: Interdigitated Mesa Designs to investigate n-contact effects The results of Table 2 indicate that for a 3-λ structure an operating voltage of 3.2V should be achievable for the largest emitting area with a 25µm wide digit, increasing to 3.5V for the smaller device with the same digit size. The voltage drop across the p- contact needs to decreased if the smallest device is to operate below 5.4V. For a 1-λ structure an operating voltage of 3.3V should be achievable for the 25µm wide digit. The NMRCs experimental results on samples patterned with an interdigitated structure seem to show little improvement in operating voltage but sample to sample variations may be masking any trend. To summarise then we have found that the operating voltage of a device with a circular mesa cannot be reduced to 3.5V or less. However, using an interdigitated structure with a 25µm wide digit allows a voltage below 3.5V to be achieved for both 3-λ and 1-λ devices. Perhaps it is also worth adding a note of caution. If a non-radiative mechanism is introduced as a result of an increased mesa perimeter some of the advantage of an interdigitated structure will be lost. However, we believe this effect should not be significant if appropriate surface passivation is included as part of the device processing. AGETHA Final Report 16 IST

17 Light-Current-Voltage and Power Efficiency Modelling Finally, on the assumption that uniform current spreading has been achieved we have used a one-dimensional drift-diffusion model to calculate the light-current and currentvoltage characteristics of conventional LED structures although the main conclusions should also be applicable to RCLED devices. First we note that such simulations are not straightforward because of the large bandgap, the large energy separation of the quasi Fermi levels from the band edges and the large band offsets. We were fortunate that Dr A Onischneko, funded under MOTIFES, another (a) EU project [CSG (GRD )], was able to make his code available and to help in some of the calculations reported here. (b) Figure 6: (a) Schematic band structure of InGaN QWs. (b) Model band structure (in red) where radiative recombination only allowed in regions correspondings to QWs. We found that it was quite straightforward to get reasonable agreement with measured lightcurrent characteristics, at least over the range of currents we usually investigate experimentally. However, we found it very difficult to get reasonable agreement with the current-voltage characteristics of an InGaN/GaN device even when the effect of the series resistance of the p- GaN contact was taken into account. In contrast we could obtain reasonable fits to a GaN homojunction device from Madrid. This gave us some confidence that the basic modelling was working correctly but that some aspect of the device design was missing. The most obvious feature not normally included in such calculations directly is the influence of the peizoelectric field introduced in the InGaN as a result of the strain. This is shown schematically in Fig. 6(a) where the left of the figure corresponds to the substrate side of the structure. We developed the approximate model shown in Fig. 6(b). Here the difference in energy between the GaN barriers is reproduced by displacing the valence band edge as shown by the red lines. The radiative emission from the QWs is modelled by radiative emission in the GaN layers in the regions corresponding to the QWs. Clearly the emission wavelength in such a model is incorrect as is the two-dimensional density-of-states. However, we considered here that these effects are incorporated within the radiative recombination rate as a fitting parameter. The results obtained are shown in Fig. 7. AGETHA Final Report 17 IST

18 10 2 Experimental Data Piezoelectric Model A Piezoelectric Model B Current (ma) Voltage (V) Figure 7: Experimental I-V data for CRHEA T9 LED at 300K. Curves represent drift-diffusion modelling (A) with piezoelectric field as shown in Fig. 6, and (B) with reverse direction of the piezoelectric field. Clearly the agreement is rather good for one field direction. The reverse field direction is shown for the same device parameters. Clearly the agreement is poor. We have not been able to get good agreement between theory and experiment with this reversed field direction even allowing wide variations in the fitting parameters. Using these simulations for L-I and I-V we have been able to qualitatively predict some of the features of the power efficiency of LEDs as a function of current and temperature. AGETHA Final Report 18 IST

19 Section 3: Epitaxial Growth (WP4-7) Contributing Partners: CRHEA, UPM and TRT Key achievements High optical and structural quality GaInN/GaN MQW LED, RLED and RCLED structures, emitting at 500nm have been developed. The device performances of such LED and RCLED structures have been found in good agreement with the physical properties of these structures. The impact of the strain induced defects on the optical output power and on the reliability of the devices has been demonstrated. LED and RCLED wafers with a low misfit dislocation density, a very low surface morphology roughness as measured by AFM ( RMS=10Å, Rmax = 20Å) and a narrow linewidth (FWHM=22nm) of the 500nm emission peak have been successfully grown using a two step re-growth process on template GaN buffer layers. Such wafers have led to an improved stability of the devices : Outline of the epitaxial growth work Growth studies in the frame of AGETHA project have been focused in a first step on very basic studies such as GaN, GaInN, GaAlN bulk materials, GaInN/GaN MQW heterostructures and GaAlN/GaN Bragg Mirrors. The following step in the growth optimisation of LED and RCLED structures, absolutely critical for the device performances, which has required a lot of efforts, has been the obtaining of a high quality P+/N junction. A real breakthrough in the optical output power of the devices has been obtained for optimised electrical and structural properties of the P+/N interfaces of the LED or RCLED structures. Finally, reliability studies, carried out during the last year of the project, in straight collaboration with grower and processing partners, have established that strain induced defects could be the key parameter for the device stability. Basic Studies : LED or RCLED structures have been grown by MOCVD at CRHEA and TRT and by MBE at UPM. Their physical properties have been optimised using high resolution X- Ray diffraction(hr-xray), room temperature photoluminescence mapping measurements, Transmission Electron Microscopy (TEM) and Reflectivity measurements. AGETHA Final Report 19 IST

20 1. Growth optimisation of GaInN/GaN MQW Heterostructures Main characterisation results (MBE Growth at UPM) The growth optimisation of GaInN/GaN MQW structures performed by UPM was initiated using sapphire substrates during the first year of the Project, due to the lack of GaN templates. Once the GaN templates were available (via CHREA) work was continued using these substrates. The initial step was the calibration of the In composition using a series of InGaN bulk layers. Two important results were concluded: (i) the quality of the bulk InGaN layers is substantially better (10 times higher PL emission) when using GaN templates, and (ii) the possibility of growing InGaN bulk layers covering an In-composition range between 22 % (482 nm) and 33 % (579 nm) which included the 510 nm targeted wavelength of the Project. The second stage of this work was the growth of InGaN/GaN MQW structures with high In content (up to 32 %) and high quality, emitting at 510 nm. This was possible by adjusting the In/Ga molecular flux ratio and using a low growth temperature ( ºC range). Figure 1 shows the low temperature PL spectra of In 0.28 Ga 0.72 N/GaN MQWs structures with different InGaN well thickness (i.e. 10, 20 and 30 Å) and a fixed GaN barrier (60Å) emitting in the range of 2.4 to 2.8 ev. PL Intensity (a. u.) K 1 mw M576 30Å InGaN x 5 M577 20Å InGaN x nm M578 10Å InGaN x Energy (ev) Figure 1. Low temperature PL of In 0.28 Ga 0.72 N/GaN MQWs structures with different InGaN well thickness (i.e. 10, 20 and 30 Å) and a fixed GaN barrier (60Å). AGETHA Final Report 20 IST

21 The In composition in these structures was determined by HRXRD data using both symmetric and asymmetric reflections, in order to take in account the strain present in the layers. This technique was also very useful to estimate the quality of the barrier/well interfaces by checking the appearance or not of satellite peaks in the XRD profile. Main characterisation results (MOCVD Growth at CRHEA) - Indium concentration in InGaN QWs. At room temperature, the bandgap of hexagonal GaN is 3.42 ev whereas it is 0.7 ev for hexagonal InN [1], so the targeted wavelength of 510 nm (2.43 ev) in the RCLEDs structures of AGETHA, is reached by the use of InGaN/GaN MQWs. In MOCVD, the most natural way to increase the indium concentration in InGaN is to decrease the growth temperature. Fig.2 shows a linear relationship of [In] with Temperature in thick (bulklike) InGaN films. [In] was measured from the lattice parameters a and c, obtained by X-ray reciprocal space mapping around the asymmetrical (-1015) reflection peak of InGaN. Since InGaN is lattice-matched with the underlying GaN substrate, a correction for strain was done to extract the relaxed parameters, and finally the actual In concentration in InGaN. Good agreement was found with the [In] values measured by calibrated Energy Dispersion Spectrometry (EDS). In the case of thick (140 nm) InGaN layers, the photoluminescence (PL) severely degrades for growth temperature below 780 C. However, the growth of QWs (thickness < 5 nm) of good optical quality can be achieved at growth temperatures as low as C. Extrapolation of the curve in Fig.7 to this temperature range yields an In concentration of 18-22%. Below 660 C, the PL efficiency of the QWs is found to drop rapidly. Therefore an In concentration of about 20% appears to be acceptable. 25 In mole fraction (%) C 20% In InGaN (140 nm) GaN Growth Temperature Fig.2 : In mole fraction in InGaN as a function of growth temperature, measured by XRD (black squares) and EDS (open squares). AGETHA Final Report 21 IST

22 It is important to know the intrinsic band gap energy of In x Ga 1-x N with x = 0.2. Fig. 3 displays the bandgap energy of In x Ga 1-x N at room temperature, measured by Photodeflection Spectroscopy (PDS), as a function of x. Bandgap E g (ev) b = 1.6 ev GaN / InGaN (140 nm) E g = x E g (InN) + (1-x) E g (GaN) - b*x (1-x) E G (GaN) = 3.42 ev E G (InN) = 0.7 ev (Ref 1) 300 K In mole fraction (%) Fig.3 : Bandgap (E g ) of In x Ga 1-x N versus x, at 300K. E g was determined by PDS and x was obtained by X-ray diffraction (XRD) According to our experimental data in Fig.3, which give a relationship between the bandgap and the In content in InGaN, the intrinsic bandgap of pseudomorphic (strained) In 0.2 Ga 0.8 N is 2.62 ev (473 nm). As already noted, the optimum temperature window for the QW growth is C. This restriction limits the In concentration to 20%. At first sight, it seems unlikely to reach the targeted emission energy of 2.43 ev (510 nm). However, wurzite structures exhibit spontaneous and piezoelectric polarizations along the [0001] direction (growth direction), giving rise to the so-called Quantum Confined Stark Effect (QCSE) [2]. This effect results in a redshift of the excitonic emission, which strongly increases with increasing well width. Indium composition fluctuations were also often invoked in the literature [3] to account for the redshift of the QW emission. In that case, excitons recombine at potential minima in indium-rich regions. AGETHA Final Report 22 IST

23 All our results of cross- sectional TEM images on green emitting QWs give no clear evidence of such fluctuations. On the other hand, we have remarked that the PL emission of InGaN QWs could be tuned from violet to yellow, simply by varying the width of the QWs. This behaviour is shown in Fig.4. The structures consist of a 2µm thick GaN template, a In 0.2 Ga 0.8 N single QW and a 10 nm GaN cap layer. (arb.units) PL Intensity x20 x5 X3 InGaN SQW PL@300K Growth temp. 680 C Growth time : 5' G1171 4' 30'' G1172 4' x1 3' G1162 G1170 2'30'' x2 G Photon Energy (ev) Fig.4 : PL spectra of InGaN QWs of different thicknesses. The growth rate is approximately 1 nm/min and the deposition time is ranging from 2.5 to 5 min. Examination of the PL in Fig.4 leads to the following remarks : For thin (2.5 nm) QWs (sample G1163), the transition energy is broad and greater than the intrinsic bandgap of 2.62 ev. This indicates that quantum confinement of free carriers in this QW dominates. AGETHA Final Report 23 IST

24 As the width of the QW increases, the PL peak energy decreases until reaching values far below the intrinsic bandgap energy. This behaviour is thought to be the consequence of strong piezoelectric fields in the wells. The decreasing PL intensity and redshift of the PL emission with increasing well width is characteristic of the QCSE. Despite the 20% In concentration in InGaN QWs, it is thus possible to reach emission wavelengths in the green region (510 nm), simply by increasing the QW width. However, the emission intensity drops. In the next paragraph, we summarise the optimised growth parameters used for the QW growth, in our vertical MOCVD reactor. - Optimisation of the growth parameters Our best conditions for the MOCVD growth of the active region of the green AGETHA LEDs are : (i) InGaN QW : Growth temperature within the range C Φ( TMI ) Flux ratio : 3, V Φ( TEG) III V G ~ 1 nm / min, deposition time ~ 4 min (ii) GaN barrier Growth temperature 970 C V 2500 III V G ~ 5 nm / min, deposition time ~ 2 min. Fig.5 shows typical PL spectra of SQWs, using different temperature growth conditions Fig.5 : PL of SQW structures : G1856 : T G = 700 C/ 4 min, G1162 : T G = 680 C/ 4 min, G1870 : T G ramp from 660 to 700 C (3 min) + 1 min at 700 C G1879 : T G ramp from 660 to 700 C (4 min) + 5 sec at 700 C AGETHA Final Report 24 IST

25 Inspection of Fig.5 leads to a few remarks: InGaN grown at 700 C gives a too short PL emission wavelength (450 nm). Decreasing the growth temperature by 20 C (680 C) allows to obtain a green emission, but the PL spectrum is very broad (FWHM = 70 nm). This could be due to increased In composition fluctuations in the QW. Ramping the temperature during the growth of the QW has a beneficial effect on both the PL intensity and the FWHM of the PL peak. A bright PL peak near 510 nm is obtained in sample G1879: InGaN is grown under a temperature ramp starting from 660 C to 700 C during 4 min, using TMIn and TEGa. After a short time lag (5 sec), the TMIn flux is cut off and the temperature is quickly increased up to 970 C in 1 min. The GaN barrier is then grown at this temperature, using TMGa. The reason as to why a temperature ramp enhances the PL emission is still not fully understood, and beyond the scope of this report. It can be invoked that the QW exhibits an indium gradient that in turn induces a bandgap gradient. This would favor a better overlap of the electron and hole wave functions in the QW, thus counterbalancing the QCSE that tends to separate the electron and hole towards each side of the well. AGETHA Final Report 25 IST

26 2. Growth optimisation of GaAlN/GaN Bragg Mirror Main characterisation results (MBE Growth at UPM) The growth of AlGaN/GaN Bragg mirrors posed a series of problems that made of this study a difficult one. The first issue has to do with the low contrast in the refractive indexes between the GaN and AlGaN layers, which forced to have a superlattice (SL) structure with a large number of periods and/or AlGaN layers with a high Al content in order to obtain a mirror with a high reflectivity (> 50 %). Both of these requirements increased the thermal and lattice mismatch between the GaN and AlGaN layers, giving raise to the appearance of cracks on the surface that degradates the growth and operation of the emitting device. A second difficulty laid on the fact that each layer (GaN and AlGaN) is grown at a different optimum temperature. The first growths consisted in 10-periods AlGaN/GaN Bragg structures with a nominal GaN thickness of 52.8 nm. The nominal thickness of the AlGaN layer varied between 54.6 and 56.4 nm depending on the Al composition (from 20 to 40% respectively). The nominal/theoretical thicknesses of each layer were chosen from the results of a simulation program in order to obtain a maximum reflectivity centered at 510 nm. All growths were performed without interruption between the GaN and AlGaN layers but increasing rapidly the growth temperature, about 20ºC, from the GaN layer to the AlGaN one. This method leads to clean and abrupt interfaces as observed by TEM images, as well as surface roughness (rms) of 0.8 nm (for a 3x3 µm 2 scan area) from AFM measurements, that was better than the initial surface roughness of the GaN template. Figure 6 shows the reflectivity spectra of three of these structures with reflectivities between 48 and 53 % at nm, close to the maximum theoretical value attainable (for this design below 60%). AGETHA Final Report 26 IST

27 M 556 M 617 M 640 Reflectivity nm λ (nm) Figure 6: Measured reflectivity spectra of different Bragg structures consisting in 10 periods AlGaN / GaN superlattices. After the 1 st -year review meeting (month 12), it was decided to continue this study using 7 periods AlGaN/GaN SLs instead of 10 periods and a lower Al content in the AlGaN layers, for the 3-λ cavity design. Bragg mirrors with reflectivity above 30% (almost identical to the maximum theoretical value) centered at 512 nm were obtained. Figure 7 shows the reflectivity spectrum of one of these mirrors together with the simulation results and the details of the SL structure. AGETHA Final Report 27 IST

28 Reflectivity (%) Measured. Simulated. DBR characteristics: nm λ = 38 nm Simulation results: * AlGaN layer: % Al = 17 % (from XRD) Thickness = 54.5 nm. * GaN: Thickness = 53.0 nm nm λ (nm) Figure 7: Reflectivity spectra (simulated and measured) of a 7-periods AlGaN/GaN DBR structure to be used in a 3-λ RCLED device. Another important issue regarding the growth of these DBRs structures is the homogeneity of the reflectivity value and centered wavelength throughout the whole wafer piece used for growth. In our particular case, growth is performed on square pieces (1.8 cm side), and a mapping of the reflectivity value and centered wavelength obtained shows that the variation of these parameters is less than 2%, proving MBE as a technique leading to samples with an excellent homogeneity. During the fourth quarter of the second year special effort was dedicated to the design and growth of DBRs to be incorporated in 1-λ cavity structures. For this particular device, the mirrors are Si-doped and consist in a 6-periods AlGaN/GaN with a nominal Al content of 50 %. The reflectivity spectrum of one of these DBRs is shown in Figure 8. A reflectivity value of 50 % at 520 nm is obtained. AGETHA Final Report 28 IST

29 60 50 R = nm Measurement. Simulation. Reflectivity (%) nm λ (nm) Figure 8: Reflectivity spectra (simulated and measured) of a 6-periods Si-doped AlGaN/GaN DBR structure to be used in a 1-λ RCLED design. With these results in mind, we can confirm the viability of the MBE technique to fabricate high quality Bragg mirrors, meeting the specifications required for the RCLED emitting at 510 nm. Main characterisation results (MOCVD Growth at CRHEA) The distributed Bragg reflectors (DBRs) entering into the RCLED structure consist of a series of (AlGaN/GaN) pairs. The optical thickness λ/4 of each layer was controlled during growth by following the oscillations of a reflected laser beam nm). After cooling, the optical thicknesses decrease, mainly due to the diminution of the refractive indexes. The resulting relectivity spectra at room temperature have the typical shape shown in Fig.9, with a peak centered around 520 nm. 0,67 0,56 E (ev) 2,7 2,6 2,5 2,4 2,3 2,2 2,1 G1810 centre 0,45 R (%) 0,33 0,22 0,11 Fig.9 : Reflectance of a 10x (Al 0.3 Ga 0.7 N / GaN) DBR grown on a 1.5 µm thick GaN template 0,00 AGETHA Final 450 Report λ (nm) IST

30 The thickness gradient along the wafer (due to inhomogeneous fluxes in the reactor) tends to shift the spectra towards shorter wavelengths near the wafer edge, thus large surface area having a suitable stopband centered at 510 nm can be obtained. Due to the lattice mismatch between AlGaN and GaN, tensile stress accumulates in the DBR during the deposition of the quarter wave stacks, and cracks are found to occur in the structure, from an aluminium concentration as low as 20% (see Fig.10). Cracks must be avoided since they may severely degrade the performances of the RCLEDs. G1757 Fig.10 : Optical micrograph showing a network of cracks at the surface of a 10 x (Al 0.2 Ga 0.8 N / GaN) DBR. Several attempts were made in order to alleviate the tensile stress accumulated in the DBR, responsible for the crack formation : (i) (ii) (iii) insertion of a thick AlN layer (λ/4n) during the first sequence of the DBR growth. insertion of a thin (5 nm) AlN layer deposited either at low temperature (600 C) or high temperature (1100 C) prior to the DBR growth. Insertion of a quarter wave AlN/GaN superlattice. All these treatments have resulted in the total suppression of cracks. However, AFM and TEM measurements have shown that the dislocation density could increase by a factor of 10, compared to conventional DBRs. The DBRs with AlN interlayers have the typical morphologies shown in Fig.11. Here, the dislocation density reaches /cm 2. AGETHA Final Report 30 IST

31 Mixte/Screw Edge G1689 G1689 Fig.11 : Cross sectional TEM and AFM images of a DBR structure with a λ/4n thick AlN layer. A summary of various DBRs properties is given in table I: DBR InterLayer dislocation dens. (cm -2 ) Crack G x 13% X medium G x 28% X ( ) many G x 15% thick AlN X G x 10% X few G x 14% X medium G x 16% X medium G x 20% X many G1802 7x 20% AlN 700 C X G x 20% AlN 1100 C X G x 30% SL AlN/GaN In order to evaluate the effect of dislocations on the optical properties of the LED structures, a single quantum well was grown on top of two conventional DBRs (G1754 and G1757) and a DBR with a AlN interlayer (G1689). The PL is shown in Fig12. X AGETHA Final Report 31 IST

32 PL Intensity (arb. units) 1 SQW on DBRs 10x (Al x Ga 1-x N / GaN) G1757 x = 0.2 G1689 x = 0.15 AlN (70 nm) Photon Energy (ev) G1754 x = 0.1 GaN (~50 Å) InGaN SQW GaN (~0.7µm) DBR GaN template (~ 2 µm) Fig.12 : Room temperature PL spectra of an InGaN SQW grown on different DBRs. We clearly observe a drop in the PL intensity for the SQW grown on the G1689 DBR, which exhibits the highest dislocation density (~10 10 cm -2 ). 3. Growth optimisation of GaInN/GaN MQW LED and RCLED structures Main characterisation results (MOCVD Growth at CRHEA) P-N junction One of the key issue of the AGETHA contract is to obtain devices operating at a forward voltage V F as low as possible. This implies to decrease the series resistance R S. The main contributions to R S in our LED or RCLED designs come from the lateral current flow along the n-type layer, and from the specific p-type contact resistance. Separate Hall measurements on n-type (Si doped) GaN templates yields n = 3-6 E18 cm -3, µ = cm 2 /V/s. This gives a resisitivity lying between and Ω.cm. The use of multiple finger patterns allows to keep the n-layer resistance low, even when the n- layer is only 100 nm thick (case of 1λ cavities). Contribution from the p-contact resistance remains the principal source of series resistance. This contribution can be reduced by increasing the p doping level. Indeed, the AGETHA Final Report 32 IST

33 specific contact resistance is proportional to exp C, where p is the hole density [4]. p It is thus essential to achieve a high p-doping. Latest results on the G1902 RLED supplied to NMRC give a V F as low as 4 V at 30 ma. This could be obtained by growing a p layer with a Mg gradient (high Mg surface concentration). During the course of the AGETHA project, optimisation of the p doping was done, using a series of samples with different Mg content. The Mg source was the biscyclopentadienyl magnesium (Cp 2 Mg). The growth temperature was 1090 C and the total pressure in the reactor was 30 kpa. The samples underwent a post-growth thermal annealing at 750 C for 15 min under nitrogen in order to activate the dopant. Fig.13 shows the room temperature hole concentration p, measured by Hall effect, and the corresponding global Mg concentration, measured by SIMS, as a function of Cp 2 Mg flux. The TMGa flux was kept constant at 62 µmol/min. 1E20 [Mg] SIMS 1E19 Cp 2 Mg Concentration [cm -3 ] 1E18 1E17 G1584 G1307 G1306 G1303 G1296 G1547 G1297 G1565 G1566 G1563 G1564 [p] Hall Fig.13 : Total Mg concentration measured by SIMS, and hole concentration measured by Hall effect at 300K, as a function of flux ratio (Cp) 2 Mg/TMGa. G1473 G1295 1E16 G Flux ratio (Cp) 2 Mg / TMGa [%] For [Mg] < 3x10 19 cm -3, the free hole density agrees well with the classical model of compensated p-type semiconductor, with a donor density N D = 3.5x10 17 cm -3, and a Mg acceptor level E A = ± ev [5]. N D is actually the residual n-type defect concentration in undoped GaN, and the value of E A was confirmed by temperature dependent Hall measurements on Mg doped samples. A dramatic drop of the free hole concentration occurs at high Mg doping, suggesting that Mg introduces auto-compensating impurities. It is interesting to note that this change in electrical properties is accompanied by the formation of Mg-rich pyramidal inversion domains (PIDs), a few nm wide, easily seen by TEM [6]. Their density is sufficient to account for an auto-compensating mechanism. AGETHA Final Report 33 IST

34 Using proper growth conditions, a maximum hole density of 2 x cm -3 can be achieved. The mobility remains however very low, i.e < 10 cm 2 /V/s and consequently the resistivity is superior to 0.3 Ω.cm. RCLED structures It is worth mentioning that all RCLED structures that used AlN interlayers in the DBR have resulted in degraded performances. The results are summarized in Table II. Sample Structure Run V F at 20 ma (Volt) P opt at 20 ma (µw) Peak λ (nm) G1781 3λ DBR 17 pairs, 10% Al M G1803 3λ DBR 7 pairs, 20% Al LT AlN 5 nm M G1806 3λ DBR 10 pairs, 20% Al HT AlN 5 nm M G1711 3λ DBR 17 pairs, 20% Al HT AlN 70 nm nnnm nm M green G λ DBR 8pairs, 40% Al SL AlN/GaN M46 14 < 1 blue Table II : RCLEDs structures processed and tested at NMRC. Only one RCLED structure (G1781) has given satisfactory results. The cavity was grown on a conventional DBR with a fairly low aluminium content (10%) in order to avoid cracks. Of course, the index of refraction contrast between GaN and Al 0.1 Ga 0.9 N is small, leading to a narrow stopband, and a significant penetration depth of the emitting light into the DBR, making our cavity enhancement weak. The additional benefit from a 10% AlGaN/GaN DBR being minimal, an alternative structure was proposed by TCD, referred to as resonantly enhanced LED (RLED). In this structure, the DBR is lacking, and the QWs are located at an antinode position from the metallic p-side mirror. Such structures were grown (G1881, G1902) and promising results were obtained in the AGETHA consortium AGETHA Final Report 34 IST

35 Main Characterisation results ( MBE growth at UPM) Once the growth of the InGaN/GaN MQW structures on GaN templates had been successfully achieved, the next step was the growth and fabrication of a conventional P/N junction (LED) emitting at 510 nm using this MQW as the active area. The layout of the grown structure, with the nominal thickness of each layer, is presented in Figure 27. A substrate temperature of 660ºC was employed for the growth of the initial thin GaN, the GaN:Si (n-side) and the GaN:Mg (p-side) layers. The whole MQW structure (5 periods) was grown at 550ºC, stopping the growth and decreasing the substrate temperature after the growth of the first GaN barrier. During the growth of the last GaN barrier the substrate temperature was increased again to 660ºC. The best p-type doping results were obtained using a Mg cell temperature of 350ºC. Figure 14 also shows the RT electroluminescence spectra of different MQWs LED structures with different In-content InGaN layers (from 0 to 25 %). 5 x In x Ga 1-x N / GaN GaN:Mg GaN:Si GaN template Sapphire 0.3 µm 20 Å In x Ga 1-x N 50 Å GaN 0.8 µm thin (200nm) GaN layer Normalized EL intensity (a. u.) 1.0 In content: X~0 0.8 X~16 % X~21 % X~25 % 0.6 RT CW@20 ma Energy (ev) Figure 14 : Schematic of the conventional P/N junction (LED) structure and RT electroluminescence spectra for different In composition InGaN QWs. During the second year of the Project, the growth optimisation of the MQW LED structures has been followed by the RCLED growth studies. Two different RCLEDs designs were grown: a 3 λ structure and a 1-λ one The 3-λ RCLED consisted in a DBR structure of 7-periods of Al 0.17 Ga 0.83 N (~54 nm) / GaN (~52.5 nm) and a MQW active zone formed by 5 periods of In 0.25 Ga 0.75 N (~2.2 nm) / GaN (~5 nm). The top mirror of the cavity is formed with a metallisation of 200 nm of metallic Al layer on top of the device. Figure 15 shows the RT electroluminescence of this device together with the reflectivity spectrum, both measured from the back side of the device (i.e. through the sapphire substrate). AGETHA Final Report 35 IST

36 λ dip = 500 nm Reflectivity (a.u.) M ma CW, RT Cavity resonance FWHM 38 nm EL Intensity (a.u.) λ (nm) Figure 15 : Room temperature electroluminescence and reflectivity spectrum of a 3-λ RCLED device, both measured from the back side (i.e. through the sapphire substrate). The 1-λ design included a Si-doped DBR structure of 6 periods of Al 0.44 Ga 0.56 N (~60 nm) / GaN (~57 nm) and a MQW active area of 5 periods of In 0.28 Ga 0.72 N (~2 nm) / GaN (~3.3 nm). Figure 16 shows the RT electroluminescence and reflectivity spectra of such a device, measured from the back side (i.e. through the sapphire substrate). λ dip = 501 nm Reflectivity (a.u.) FWHM 39.5 nm Cavity resonance M784 5 ma CW, RT EL Intensity (a.u.) λ (nm) Figure 16 : Room temperature electroluminescence and reflectivity spectrum of a 1-λ RCLED structure, both measured from the back side (i.e. through the sapphire substrate). AGETHA Final Report 36 IST

37 In summary, during the second year of the Project, MBE growth produced RCLEDs with a EL intensity more than 10 times greater than conventional MBE-grown LEDs from the first year, justifying the use of MBE-grown Bragg mirrors to improve the efficiency of the devices. Although the absolute power figures are still lower than those corresponding to devices grown by MOCVD, the MBE-grown Bragg mirror properties are still of higher quality and meet the specifications required by the RCLED with a lower number of periods in the AlGaN/GaN superlattice. During the last year of the Project, the work has been concentrated on the MBE growth of DBRs structures that were sent to Valbonne (CRHEA) to continue the MOCVD-growth of the rest of the RCLED device on top of it. The attempts of these regrowths have not been very successful. The MBE-grown GaN layer seems to degrade when it is exposed to typical MOCVD growth temperatures (> 950ºC) not allowing the growth of high quality InGaN/GaN quantum wells. New approaches for the initial stage of the regrowth process must be pursued for this task. AGETHA Final Report 37 IST

38 Main characterisation results (MOCVD Growth at CRHEA) GaN on sapphire is currently obtained by MOCVD in a two-dimensional (2D) growth mode. The process starts with the nitridation of Al 2 O 3 under ammonia at high temperature (typically 1050 C). Then a GaN buffer layer is deposited at low temperatures ( C) to promote a uniform dispersion of nuclei that can effectively cover the substrate. Afterwards, the deposition of GaN is stopped, and the temperature is raised. The epitaxial growth is then allowed to proceed, giving rise to a 2D GaN film with threading dislocations densities of about cm -2. In CRHEA, an original growth process, referred to as 3D growth process, has been implemented [7,8], in order to obtain better quality GaN materials. Prior to the deposition of the buffer layer, the nitridated substrate is shortly exposed to silane and ammonia at 1080 C. This treatment fundamentally changes the growth mode during the subsequent growth steps. Fig.17 displays the laser nm) reflectivity recorded during the progress of the growth process. Upon annealing up to 1080 C, the GaN buffer layer changes drastically its microstructure by mass transport recrystallisation into small GaN seeds. The recrystallisation, inducing a 2D-3D transition, increases the diffuse scattering of the laser beam. This appears as a continuous decrease of the reflectivity signal (step 4 in Fig.17). As shown in Fig.18, the shape of the GaN islands strongly depends on the composition of the carrier gas (N 2 + H 2 or pure H 2 ). The growth of GaN starts during the reflectivity drop, allowing the GaN seeds to expand both laterally and vertically. The reflectivity pass through a dip and progressively increases until showing oscillations characteristic of a 2D growth process. The coalescence into a 2D flat film may last between 20 min and 1 hour, depending on the initial buffer morphology, especially the average distance between the GaN islands ) 3D islands formation 1080 C Reflectivity ) Buffer 270 Å C 1) Nitridation 1080 C 2) SiH 4 treatment 5) Coalescence C time (min) Fig.17 : Reflectivity spectra (HeNe laser) recorded during the MOCVD growth of GaN. The coalescence (step 5) occurs more or less quickly following the shape of the initial buffer morphology. Buffer (A) in Fig.18 : green curve, buffer (B) in Fg.18 : blue curve. AGETHA Final Report 38 IST

39 A B 1 µm 1 µm Fig.18 : GaN buffers after thermal recrystallisation at 1080 C under a mixture H 2 + N 2 (A), pure H 2 (B). Table III summarizes the electrical and structural properties of the resulting two GaN epilayers obtained in Fig.17. Sample A Fast coalescence Sample B Slow coalescence n (cm -3 ) µ (cm 2 /V/s) ω (arc.sec) Rocking curve Dislocation density ρ (cm -2 ) AFM mean roughness Scan 60x60µm 2 ~3 x x 10 8 ~ 40 Å ~5 x x 10 8 > 60 Å nanopipes The 3D growth mode is very efficient in reducing the dislocations density in GaN (by a factor of 50). Sample B exhibits half as much dislocations as sample A, but the slow coalescence process induces nanopipes and additional roughness. For these reasons, the fast coalescence process was systematically adopted for the fabrication of the GaN templates entering into the LED and RCLED structures of the AGETHA project. AGETHA Final Report 39 IST

40 References on Growth: [1] V. Yu. Davydov, A.A. Klochikhin, V.V. Emtsev, D.A. Kurdyukov, S.V. Ivanov, V.A. Vekshin, F. Bechstedt, J. Futhmueller, J. Aderhold, J. Graul, A.V. Mudryi, H. Harima, A. Hashimoto, A. Yamamoto, and E.E. Haller Phys. Stat. Sol. (b) 234, N 3, (2002). [2] T. Takeuchi, C. Wetzel, S. Yamaguchi, H. Sakai, H. Amano, I. Akasaki, Y. Kaneko, S. Nakagawa, Y. Yamaoka, and N. Yamada App. Phys. Lett. 73, N 12, 1691 (1998). [3] H.K. Cho, J.Y. Lee, N. Sharma, C. Humphreys, G.M. Yang, C.S. Kim, J.H. Song, and P.W. Yu Appl. Phys. Lett. 79, N 16, 2594 (2001). [4] A. Weimar, A. Lell, G. Bruderl, S. Bader, and V. Harle Phys. Stat. Sol. (a) 183, N 1, 169 (2001). [5] P. Vennéguès, M. Leroux, S. Dalmasso, M. Benaissa, P. de Mierry, P. Lorenzini, B. Damilano, B. Beaumont, J. Massies, and P. Gibart Submitted to Phys. Rev. B (April 2003). [6] M. Leroux, P. Vennéguès, S. Dalmasso, M. Benaissa, E. Feltin, P. de Mierry, B. Beaumont, B. Damilano, N. Grandjean, and P. Gibart Phys. Stat. Sol. (a) 192, N 2, 394 (2002). [7] S. Haffouz, H. Lahreche, P. Vennéguès, P. de Mierry, B. Beaumont, F. Omnès, and P. Gibart. Appl. Phys. Lett. 73, 1278 (1998). [8] P. Vennegues, B. Beaumont, S. Haffouz, M. Vaille, and P. Gibart J. Cryst. Growth 187, 167 (1998). AGETHA Final Report 40 IST

41 Section 4 Device Fabrication (WP8, WP13) Contributing Partners: NMRC, IET. Key achievements A viable fabrication process has been developed for substrate emitting RC-LEDs and R-LEDs. R-LEDs are LEDs, with the active layers placed at resonance with the top mirror (no bottom DBR-mirror). Inter-digitated p-contacts have reduced the forward voltage to 3.4 V at 20 ma, and increased the bandwidth to 90 MHz at 20 ma. Novel p- contacts based on ZrN/ZrB 2 have been developed. These contacts display good thermal stability. The growers have sent approximately 150 wafers/pieces during the duration of the project. All of these have been processed and evaluated for feedback. There has been close co-operation with both designers and growers, as we have evaluated new designs, and have sought to overcome problems as they occurred. As a result of such co-operation we have jointly improved the device lifetime on high-brightness Thales wafer material. Six batches of devices have been packaged at Infineon. RC-LED structure Figure 1 shows the RC-LED structure in cross-section, while Fig.2 shows an array of finished devices in plan view. In this photograph the central p-contact and the surrounding n-contact both appear in black. One device has been probed, and light can be seen coming through the 5 µm un-coated separation between the p-and n-contacts. The main light emission is downwards. Reflecting p- t t p-gan n-contact Micro InGaN n-contact GaN, nid n-gan DBR (AlGaN/GaN) Sapphire light emission Fig. 1. RC-LED structure AGETHA Final Report 41 IST

42 The advantages of this substrate emitting design are: It removes the need for a high-reflectivity DBR, which is very difficult to realise in AlGaN/GaN It lowers the forward voltage, as the top contact does not have to be thin and transparent It avoids light losses due to shadowing by the contact pads It is compatible with flip-chip bonding It results in a very simple fabrication process, and compact devices as the p- contact = mirror = bondpad If a top emitting structure had been chosen, the bottom mirror would have had to be highly reflective. This would mean either a highly reflective DBR, which is very difficult to realise in the AlGaN/GaN material system, or removal of the GaN epitaxial layers from the sapphire substrate. Substrate removal would be difficult to achieve, and is expected to reduce device yields significantly. Moreover, substrate removal would mean very thick cavities, and therefore very weak resonance enhancement of EL-intensity. The main disadvantage of our bottom emitting approach was that it required a new packaging strategy, as Infineon s standard RC-LED package for RC-LEDs emitting at 650 nm is designed for top-emitting devices. Fabrication process The fabrication process consists of the following steps: 1. Wafer cleaning and p-contact deposition 2. Lithography level 1, and p-contact definition by wet etching 3. ICP-etch of GaN, to expose the n-layer 4. Lithography level 2, and n-metal deposition (lift-off process) 5. On-wafer device testing 6. Sapphire substrate thinning and device separation (dicing saw) For the p-contacts, two alternative contact schemes were explored: 1. ZrN/ZrB 2 based contacts (sputtered, IET) 2. PdAg based contacts (electron beam evaporated, NMRC) After the p-contact definition, both sets of wafer pieces went through the same processing sequence for the remainder of the process. The following paragraphs describe the main processing steps in more detail. AGETHA Final Report 42 IST

43 ZrN/ZrB2 based contacts (sputtered, IET) The AGETHA project is targeting high temperature applications of GaN-based amber/green RCLEDs. This puts much more stringent requirements on the quality of ohmic contacts, compared to conventional/rt applications demands. In particular, as the outcome of the project are devices operating at temperatures up to C the issue of thermal stability of p-type contact was crucial. The lack of efficient p-type doping of GaN makes the fabrication of low resistivity p-type ohmic contacts extremely difficult. Due to the high ionisation energy of Mg, the shallowest acceptor dopant, concentration of holes at RT is usually two orders of magnitude lower than that of Mg. On the other hand, the solubility limit of Mg in GaN is ~5x10 19 at.cm -3. Moreover, residual hydrogen accumulates in the superficial layer of p-gan causing free carrier concentration in the subsurface volume to be lower than in the bulk. To solve the p-type contact problem we have developed a metallisation: enabling to drain hydrogen from the semiconductor subcontact region under heat treatment and assuring the thermal stability of final metal/semiconductor system. The new metallisation consists of ZrN/ZrB 2 bilayer. Our choice was based on the following premises: (i) zirconium possesses one of the highest absorptive capabilities for hydrogen of metal hydride systems, (ii) ZrN and ZrB 2 are distinguished for their low resistivities and high melting points. Fabrication of ohmic contacts involved elaboration of surface cleaning, optimisation of metallization deposition and pattering processes, and adjustment of heat treatment procedure. The results of our studies towards the development of low resistivity Zr-based p-type contacts, including contact electrical, structural and optical properties, are presented in the Final Report on WP13: Thermally Stable Ohmic Contacts to Nitridebased RCLEDs. As the thermal integrity of p-gan/zrn/zrb 2 system was a key issue, we have studied the evolution of the contact microstructure under annealing and proved its stability up to C. We have also demonstrated stable electrical and structural properties of Zr-based contacts upon aging at C for 100h, followed by C for 100h aging. Figure 2 shows the microscopic image of p-gan surface with ZrN/ZrB 2 metallisation. SIMS profiles of as-deposited and annealed contact, presented in figures 2b and c, prove that the subcontact 20nm thick p-type layer remained almost intact after ohmic contact fabrication. AGETHA Final Report 43 IST

44 a) b) 10 c) 8 Ga SIMS Signal [ c/s ] 10 7 AEC-864 p-gan(20nm) 10 6 ZrN ZrN/ZrB 2 (50/50nm) as-deposited N ZrB Mg In SIMS Signal [ c/s ] ZrB 2 ZrN Ga AEC-864 p-gan(20nm) ZrN/ZrB 2 (50/50nm) C N In Mg Depth [ µm ] Depth [ µm ] Figure 2. a)microscopic image of LED surface with ZrN/ZrB 2 metallisation, b) SIMS profile of the as-deposited contact, c) SIMS profile of contact annealed at C AGETHA Final Report 44 IST

45 PdAg based contacts (electron beam evaporated, NMRC): Good contacts to p-gan can be obtained with metals that have a high work function, i.e. Pt (5.3 ev), Pd (5.0 ev), Ni (4.9 ev), and Ag (4.7 ev). We experimented with each of these metals and selected palladium as it yielded the best adhesion to the GaN, as well as ohmic contacts as deposited, i.e. without any need for thermal annealing. In order to obtain high optical reflectance, we kept the palladium thickness very low, and overlaid this layer with a thicker layer of silver. Silver is the metal with the highest reflectance at 510 nm, while palladium is semi-transparent at thicknesses of the order of a few nanometers. We tried to make the palladium layer as thin as possible, but noticed that when we went below 3 nm both the contact resistance and the adhesion of the compound PdAg-metallisation to the GaN started to deteriorate. Our standard metallisation scheme became: 3nm Pd/ 50 nm Ag/ 30 nm Ni/ 300 nm Au. The nickel and the gold were added for wire bonding purposes. TEM analysis at Thales showed that this metallisation had an epitaxial structure when deposited on GaN, see Fig. 3. Fig. 3. Pd-Ag based contact metallisation, grown epitaxially onto p-gan

46 Fig. 4 shows the results of circular transmission line measurements (c-tlm), to determine the specific contact resistivity of the Pd-Ag based contacts. The insert shows the c-tlm patterns. If the measurement current is kept low, the measurements can be done directly on LED material. This removes the need for specially grown p- GaN layers for these TLM measurements. The specific contact resistivity is a strong function of the surface p-doping level in the semiconductor. For the measurements in Fig. 6 we used commercial LED wafer material with a p-doping concentration of 4x10 17 cm -3. The fitted line corresponds to a specific contact resistivity of 4x10-3 Ω.cm 2. 25,000 Run M44: Commer cial waf er. 20,000 Rpp, Ohm 15,000 10,000 5, ln(r/r) Fig. 4. c-tlm pattern and c-tlm results for 3nm Pd/ 100nm Ag/ 30 nm Ni contacts to p-gan, as deposited The optical reflectance of the PdAg metallisation is shown in Fig. 5. PdAg 100 Reflectance (%) Wavelength (nm) Fig. 5. Measured reflectance at a GaN/ 3nm Pd/ 100 nm Ag interface

47 Fig. 6 shows how the optical reflectance changes as the palladium thickness increases Reflectivity (%) Pd thickness (nm) Fig 6. Calculated reflectance at a GaN/ Pd/ 100 nm Ag interface, for various Pd thicknesses Determination of phase change upon reflection at a GaN/ Pd/ Ag interface: We determined the the phase change upon reflection at the GaN-to-metal interface as follows: We coated a glass slide with a 1-λ cavity, consisting of a dielectric DBR, a SiO 2 cavity and a Pd/Ag (3/100 nm) top mirror We measured the reflectance spectrum, through the substrate We calculated the same spectrum, from the known layer structure, and then fitted the calculated spectrum to the measured one by allowing the n and k- values of the metal to vary from their nominal values. The n- and thickness values of the dielectric coatings were also allowed to vary Fig. 7a shows the measured spectrum, as well as the calculated spectrum Having determined the n, k, and thickness values by fitting, we then placed in the model a GaN layer into contact with the PdAg bi-layer, and allowed the programme to calculate the phase change upon reflection and the reflectance, both as a function of wavelength. Figures 9b and 9c show the results The phase change upon reflection is an important input into the design of a R-LED or RC-LED. Knowledge of this phase change is used to ensure that the quantum wells are placed at anti-node positions, with respect to the metal mirror in the case of a R- LED, and with respect to the cavity for a RC-LED

48 Reflectance (%) λ cavity-pd3-ag100 nm measured fitted Wavelength (nm) Fig. 7a. Measured and fitted reflectivity spectrum for a 1-λ cavity PdAg Reflectance phase (deg) Wavelength (nm) Fig. 7b. Extracted phase change upon reflection, GaN-Pd3-Ag100 nm interface PdAg 100 Reflectance (%) Wavelength (nm) Fig. 7c. Extracted reflectance, GaN-Pd3-Ag100 nm interface

49 60 50 Rpp, Ohm ln(r/r) Conclusion: We have experimentally established the phase change that occurs at the GaN-Pd(3nm)-Ag(100nm) interface. At 510 nm the value of this phase change is 104 degrees, while the reflectance value at this wavelength is 67%. Wet etching of p-metal/ ICP of GaN: We etched the Pd/ Ag/ Ni/ Au metallisation in aqua regia, at room temperature. The GaN mesa was etched in an ICP etcher with the same resist mask. Lithography level 2, and n-metal deposition (lift-off process): The n-contact metallisation consists of an electron beam evaporated multi-layer: Ti (100 nm)/ Al 50 nm)/ Pt (30 nm)/ Au (300 nm). It is evaporated into windows in a bilayer resist pattern. The unwanted metal is removed together with the resist, by immersion in resist remover. Prior to n-metal deposition the GaN surface is prepared by a short de-oxidation step in buffered oxide etchant (BOE), rinsed in running DI, and then briefly immersed in an NH 4.S x solution. The wafer piece is then rinsed again in running DI, and blown dry with oxygen free nitrogen (OFN). The resulting contacts are ohmic, as deposited. Fig. 10 shows TLM results from such n-contacts. The fitted line corresponds to a specific contact resistivity of 3x 10-5 Ω.cm 2. This is a good value for an n-contact to GaN. Fig. 8. c-tlm results for Ti/Al/Pt/Au contacts to n-gan, as deposited (ρ c = 3x 10-5 Ω.cm 2 ) On-wafer device testing: The following characteristics were measured on-wafer: EL-spectrum EL-intensity into an 0.5 NA fibre I-V curves L-I curves Angle-dependent EL, to check for resonance Feedback has been provided to the growers on all the growth structures that we received. Wafer materials that looked promising were subjected to a quick lifetime screening test. Prior to packaging at Infineon, recent wafer materials were also sent to

50 our partners at TCD for bandwidth measurements. Table 2 shows the characteristics of some wafer pieces. Table 2: Main results from on-wafer characterisation Wafer id. V F at 20 ma (V) EL-peak (nm) EL-intensity into 0.5 NA (µw), (*) 3 db bandwidth (MHz), (*) 1781 (RC-LED) 786 (RC-LED) 1881 (R-LED) 919 (RC-LED) Commercial (LED) N/A Not available (*) at 20 ma N/A Sapphire substrate thinning and device separation (dicing saw): The best wafer pieces were lapped down with diamond grits, polished and diced into individual devices with a dicing saw. Because of the hardness of the sapphire material, the dicing had to be done very slowly, and the depth of the cuts had to be built up gradually with multiple cuts. The devices were sorted by visual inspection, and sent to our partners at Infineon for packaging in the new TSSOP package. Conclusions: 1. A viable fabrication process for RC-LEDs and R-LEDs has been developed 2. Good V F, good switching speed and device lifetime have been demonstrated 3. A very large number of structures (149 off) has been processed and evaluated, and there has been close co-operation with the growers 4. Novel concepts to improve the device lifetime have been developed, and one of these is the subject of a patent application 5. Six batches of LEDs and RC-LEDs have been packaged at Infineon

51 Section 5: Device Characteristics (WP1, WP9, WP10) Contributing Partners: Infineon, BAE Systems, Surrey, TCD, NMRC Key achievements Devices on 1881 wafer material have reached the target wavelength of 510 nm. However, on this material we observe a strong increase in switching speeds as the EL wavelength increases above 480 nm. This limits the attainable bandwidth. Devices on 919 wafer material have reached an EL wavelength of 499 nm, with good EL-intensity, and good bandwidth. However, the improvements in EL intensity and EL wavelength have come with a decrease in device lifetime. The cause of this deterioration seems to be the presence of pinholes in the wafer material, which short circuit the p-n junction. Different attempts have been made to reduce the density of these pinholes. At the time of writing this report, the lifetime results have improved from poor to variable, with some contacts still operational after 800 hours at 30 ma drive current, albeit at much reduced EL intensity. On-wafer bandwidth measurements have been provided by TCD. These measurements included (a) comparison of devices from different wafers (b) comparisons of different contact geometries (c) comparisons with commercial devices bought off the shelf (d) the impact of temperature, and (e) the correlation between bandwidth and operating voltage, wavelength and light output. Introduction In this section we present the device characteristics of the five most relevant RC- LED, R-LED and LED batches, and compare these with the project specifications. Table 1 provides a summary of the key parameters and their specifications. Table1: AGETHA specifications Parameter Requirement Forward Voltage (50mA) 3V Wavelength 510nm Spectral bandwidth 20nm Switching times (10% to 90%) 3ns Coupling into 1mm POF (0.47NA) 50% Output power into 1mm POF (0.47NA) I F = 10mA) 200µW Power temperature coefficient -0.2 %/K Voltage temperature coefficient -3mV/K Wavelength temperature coefficient 0.16nm/K Table 2 provides an overview of the results on the five most relevant device batches, and compares the results with the AGETHA specifications

52 Table 2: Overview of the characterisation results on the five most relevant device batches Batch RCLED-2A RCLED-2B LED-3 R-LED RCLED-4 Wafer External SPEC V F at 50 ma (V) (3.9) Wavelength (nm) FWHM (nm) Switching time (ns) Bandwidth, 20 ma (MHz) Coupling efficiency (%) P OPT,10 ma in POF(µW) T.C. Power (%.K -1 ) T.C. Voltage (mv.k -1 ) T.C. Wavelength (nm.k -1 ) The boxes have been coloured where the results meet the specifications. We notice that both the commercial LED material, and the 1881 R-LED material met the target wavelength of 510 nm, but both these wafers yielded slow switching speeds, making them unsuitable for high speed data communications. The shorter wavelength 1781 batch, and the two other batches yielded switching times close to the specified time of 3 ns. In the following sections, we will discuss the results on each of the keyspecifications. Forward voltage: Figure 1 shows the forward I-V characteristics on the wafer material used for LED-3, comparing three electrode patterns, A, B, and C. Pattern B yields the lowest forward voltage values, mainly due to the fact that the p-contact area is largest on this pattern. We will see later on in this chapter that there is a trade-off between forward voltage and bandwidth, as for maximum bandwidth the current density needs to be maximised. We furthermore observe that the best forward voltages are obtained on the materials with the highest p-doping level. This is because the specific p-contact resistivity is a function of p-doping near the GaN surface. On the commercial LED material shown in Fig. 1, we have p=3.5 x cm -3, and we obtain a forward voltage of V F =3.3 V at 10 ma and 4.3 V at 100 ma. These are respectable values for devices with such a small p-contact area. However, they do not meet the very tight specification on V F

53 Wafer material used for LED Current (ma) "B" "A" "C" Voltage (V) Fig.1. Forward I-V curves on the wafer material used for LED-3. Comparison between the three electrode patterns A, B and C. EL-peak wavelength and FWHM: Figures 2 and 3 show the EL-spectra for 1881, and peaks at 510 nm, with a FWHM of 31 nm, while 919 peaks at 499 nm with a FWHM of 23 nm. Fig. 2. EL-spectrum for

54 Fig. 3. EL-spectrum for devics from wafer 919 Switching times/ Small signal modulation bandwidth measurements: In this measurement a sinusoidal electrical signal of small amplitude superimposed on a d.c. bias is applied to the LED device. The light output from the device is detected by a fast photodiode, and the amplitude of the resulting electrical signal at the frequency of the applied modulation is recorded. As the frequency of the applied modulation approaches the bandwidth of the LED, which is typically limited by the recombination time of the carriers in the active region, the amplitude of the detected signal from the emitted light decreases. The 3dB bandwidth is defined as the frequency at which the detected electrical signal falls by 3dB from the low frequency amplitude. The measurement enables us to determine the response time of the LED under different conditions. By performing the measurement at different d.c. biases the variation in carrier lifetime with carrier density can be determined. The primary focus of the work was to provide feedback as to the maximum transmission rate that could be achieved by these devices at a range of drive currents. As a first simple approximation a data rate (NRZ encoding) a factor of 2 greater than the small signal bandwidth should be achievable. However the actual data rate limitation of the RCLED/LED varies depending on capacitance effects and also the electronics used to drive the device. In our measurements a 2GHz network analyser is used to both generate the high frequency modulation applied to the LED and analyse the amplitude of the electrical signal generated by the avalanche photodiode (APD) used to detect the optical output. The system response of the measurement set-up was measured using a fast laser diode. A large number of devices with different contact geometries, off different wafers and across the same wafer were measured. The bandwidths of the devices were determined as functions of bias current in the 1mA-20mA range and at 10 C, 40 C and 70 C. Nearly all the devices measured showed the expected increase in bandwidth with bias current due to the higher carrier density in the active region

55 The variation in bandwidth with operating temperature was also investigated. The results shown in table 3 show that the change in bandwidth was less than 20% over the 60 C range from 10 C to 70 C. The bandwidth of the devices processed from AGETHA material decreased with increasing temperature while the bandwidth of the devices from the commercial material increased. Device Bandwidth at 20mA (MHz) % change over Wafer Contact 10 C 40 C 70 C 10 C 70 C µm diam % 1881 Interdigitated % Commercial 150µm diam % Commercial Interdigitated % Table 1. Variation in bandwidth of selected devices with temperature. The bandwidth of certain AGETHA LEDs is significantly greater than that of either the commercial LEDs or the LEDs processed from commercial material under AGETHA. It is clearly important to try and get an understanding of the origin of this increased speed so that it can be fully exploited. While any comparison with the commercial devices is confused by the lack of knowledge regarding their structure (they may be operating at very low current densities), the lateral dimensions of the devices processed from commercial material are known. In order to explore any correlation between bandwidth and light output power or emission wavelength these properties are plotted in figure 4 for all devices measured at TCD. 3dB Bandwidth at 20mA (MHz) (a) Agetha devices Agetha processed devices InGaN Commercial devices Light output at 20mA integrated ±45 (µw) (b) 3dB Bandwidth at 20mA (MHz) Agetha devices Agetha processed devices InGaN Commercial devices Peak emission wavelength (nm) Figure 4 (a) Plot of bandwidth as a function of emitted power (the light output of the commercial devices is scaled by 0.17 (= 0.33 (cup reflector) 0.5(epoxy))) (b) Plot of bandwidth as a function of peak emission wavelength

56 It is well known that by increasing non-radiative recombination centres in the active region, the response time of the LED can be reduced due to the fast nature of such processes. If the high bandwidth of the AGETHA devices is due to increased nonradiative effects we would expect a lower light output from the faster LEDs. Figure 4(a) shows that this is partially true with the slower commercial LEDs and LEDs processed from commercial material offering high output powers. However within the AGETHA devices there is no clear trend of increasing speed with decreasing light emission. Therefore without further studies it is impossible to conclude whether it is a non-radiative effect or some other factor such as a reduced active region volume that is responsible for the high bandwidth of the AGETHA devices. Another correlation that has been observed in results from both TCD and Infineon has been a decrease in device speed with increasing wavelength around 500nm. Whether such a correlation is merely coincidental, and what the origin of such a correlation would be is not clear. However the evidence shown in figure 4(b) and seen in other results from Infineon suggests there may be a relationship. Again further results are needed but such a correlation would have serious implications for GaN RCLEDs/LEDs operating in the POF transmission window at 510nm. Assessment of impact of TSSOP packaging process on device performance TCDs ability to perform extensive characterisation measurements for both on-wafer and packaged devices, enabled an assessment of the impact the TSSOP packaging process on the LED device performance. These measurements were performed on devices processed from the commercial wafer material because of the high uniformity of device performance across the wafer. The measured L-I, I-V and bandwidth characteristics of devices before and after packaging were identical to within the expected variation in device performance across the wafer

57 Speed measurements To perform pulse-measurements at the TSSOP package a special printed circuit board was made under consideration of high frequency design rules. In order to get proper HF-termination, a small series resistor was mounted on the board. The eye-diagram shown in 5 shows the performance of the RCLED #17 at 30mA CW current and a 2 volt (peak-to-peak) signal (100Mbps, PRBS). Due to the low sensitivity of the silicon photodiode at 500nm and the low sensitivity of the broadband receiver, the signal is noisy. The eye-diagram shows a switching time of around 4ns, which is sufficient for 100Mbps transmission. Figure 5: Eye-diagram of RCLED #17 (919 wafer) (100Mbps PRBS-signal, 30mA CW current, 2Vp-t-p) Further system demonstrations have been made with RCLED #14, with eye-diagrams being shown in, Fig 6 and Fig 7. As can be seen an open eye is still achieved after 50m of the low (0.3) NA but the low optical power for this device closes the eye after 100m of 0.5NA fibre. The speed response of the device is still sufficient for 100Mbps over 100m but a greater output power is required

58 Figure 6: RCLED #14, 100Mbps transmission (30mA, 1.5V p-t-p) after 60cm of 0.5NA POF Figure 7: 100Mbps transmission (30mA, 1.5 V p-t-p) after 50m of NA0.3 POF

59 Figure 8: 100Mbps transmission (30mA, 1.5 V p-t-p) over 100m standard 0.5 NA POF Data link measurements As well as the eye-diagram measurements, data links were established to perform bit error rate (BER) measurements. For these demonstrations, the TSSOP packaged devices were mounted on a Mindspeed LED drive board driven with a 5.5 volt power supply. The fibre was aligned to the output of the device using X-Y-Z alignment stages. To improve the coupling efficiency into the POF, polishing of the fibre ends was performed. The ESKA Premier POF Fibre was prepared by polishing using Aluminium Oxide papers of 9 micron and 0.3 micron grade on a hard rubber surface. Typical results are shown in Fig 9 though to Fig 12. SMA connectors were prepared by bonding the plastic fibre into the connector and using an SMA polishing puck. Bare fibre ends were also prepared using an SMA connector with the fibre held in the ferrule (but not bonded in). The fibres were inspected using a PriorSpec Inspection microscope. A larger aperture ST polishing puck was used to polish fibre with the outer jacket intact (for alignment at the detector housing). Again hand pressure was used to keep the fibre in place whilst polishing. As can be seen good quality finishes for the fibres were achieved

60 Figure 9: Polishing on 0.3micron film. Al2O3. IPA lube Figure 11: Rough polish on 9micron film, loose in SMA connector body with SMA puck. Hard rubber surface Figure 10: Further polishing on 9 micron film. Al 2 O 3 Figure 12: Further Polishing on 0.3micron film The output from the fibre was detected with a receiver board put together by Infineon. It consists of a preamp and postamp IC with a fast silicon photodiode in an SFH case to allow easy coupling to the POF. BER measurements were made over 1m, 10m, 50m and 100m lengths of standard 0.47NA POF. PRBS data (2 7-1) was used for all of these measurements. After testing the available packaged devices for output power and forward current three were used for the BER tests. These are listed in Table 1 along with device 26 used in the extended temperature range tests. Devic p-contact Peak Output power at Forward voltage at e wavelength 10mA 10mA star nm 80.8 µw 3.96 V (A) star nm 55 µw 3.93 V (A) star (A) nm 1.5 µw 3.22 V 34 Comb (B) m 2.8 µw 2.82 V Table 1: Device parameters of RCLEDs used in BER measurements

61 A summary of the main results of the BER measurements is shown in Table 2. Of the three devices used for these measurements device 24 was first used but failed after results at 1m and 50m of fibre had been taken. Device 34 was then used and results taken at 1m, 10m, 50m and 100m. Finally the higher power device 18 was used and the best BER measurements at 10m and 100m were obtained with this device. However, the power of this device fell by over half before it could be used to test a 50m link. Had this been its first test it is believed that a data rate of 250Mbps with a BER of better than was achievable. Devic Length of Data rate BER Edata Oeye Eeye e fibre 24 1m 250Mbps < m 125Mbps < m 310Mbps < m 250Mbps < m 280Mbps < m 125Mbps < m 150Mbps < m 50Mbps < m 125Mbps m 50Mbps m 250Mbps m 300Mbps m 250Mbps < m 300Mbps < m 310Mbps < m 125Mbps < Table 2: Summary of BER measurements made over 0.47NA POF While these BER measurements were being taken, eye-diagrams were recorded of the optical signal (via a silicon APD) and of the electrical signal out of the receiver. Over 1m of fibre the highest data rate that a good BER was recorded for was 310Mbps. As can be seen, even when the optical eye-diagram is very noisy (Fig 13), the electrical eye (Fig 14) is clean with only jitter effects remaining, while the data stream (Fig 15) appears very clean. Over 100m of fibre a BER of < was obtainable at 125Mbps, the optical eye-diagram for this (Fig 18) is very closed but 61-

62 only a small amount of added jitter is seen in the electrical eye-diagram (Fig 16). The optical eye diagram over 1m at the data rate is also shown for comparison (Fig 19). Figure 13 Capture of part of the data stream from the receiver for device 34, at 310Mbps after 1m of 0.47NA POF (BER < ) Figure 15: Eye-diagram of the optical signal from the APD for device 34, at 310Mbps after 1m of 0.47NA POF (BER < ) Figure 14: Eye-diagram of the electrical signal from the receiver for device 34, at 310Mbps after 1m of 0.47NA POF (BER < ) Figure 16: Eye-diagram of electrical signal after receiver for device 18, over 100m of 0.47NA POF at 125Mbps (BER < ) 62-

63 Figure 17: capture of part of the data stream after receiver for device 18, over 100m of 0.47NA POF at 125Mbps (BER < ) Figure 18: Eye-diagram of optical signal after APD for device 18, over 100m of 0.47NA POF at 125Mbps (BER < ) Figure 19: Optical eye-diagram after the APD for device 18, over 1m of 0.47NA POF at 125Mbps 63-

64 Temperature dependence measurements Standard PMMA POF has a specified operating temperature of -40 C to +85 C. Future high temperature POF should allow operation at higher temperatures and it was desirable to know how the AGETHA GaN devices would operate over an extended temperature range. To allow this measurement device 26, one of the TSSOP packaged devices with no lens, was coupled into a 300µm core diameter step index silica fibre. This was then glued in place using a UV cured adhesive: Norland Optical Adhesive Type 68. With a typical cured refractive index of 1.54, the adhesive has a rated operating range of 60 C to +90 C. It is a clear, colourless liquid photopolymer that will cure when exposed to ultraviolet light, typically in the range 350 to 380nm. Despite the limited high temperature range of this adhesive, it was hoped that it would aid in maintaining alignment at higher temperatures given that no external movement was expected. The test chamber used at BAE SYSTEMS ATC-Filton was a TAS Series 3 environmental cabinet. The facility is capable of combined temperature and humidity cycling and has a test volume of 600mm 3. The chamber performance is summarised in Table 3 and a photograph of the facility is shown in Figure 20. Parameter Temperature range Humidity range Test volume Ramp rates Values 75 C to +180 C 10 to 98% relative humidity (Max +95 o C, Min +10 o C 600 x 600 x 600mm Typically 2.2 deg/minute average (faster on heating, slowest rates cooling below ~ -30 o C) Table 3: TAS environmental chamber specifications Items under test can be positioned on a shelf near the middle of the chamber. Access for optical/electrical leads etc. is gained through side ports via slits cut in the rubber sealing bungs. The chamber had internal sensors for temperature and humidity. For the measurement performed for AGETHA, the chamber was controlled through TAS PCbased control software (via the RS232 interface). The main parameter studied in these tests was temperature. The humidity within the chamber was not specifically controlled and does vary during the heating/cooling cycles. A typical humidity sensor can give ambiguous readings below zero, which suggest humidity is present. However, these readings are not to be interpreted too rigidly because they are almost certainly localised and relate to effects such as frost build-up on the sensor or the imperfect sealing of the chamber volume (e.g. through bung seals). The humidity sensor is only designed to give accurate readings within the 0 to 95 C range. 64

65 Figure 20 TAS Series 3 environmental test cabinet. (Capable of temp range -75 C to +180 C) The wavelength was analysed using a StellarNet EPP2000C radiometer with a SMA fibre optic interface. This has a 600 lines/mm holographic grating and 25micron slit. A 2048 pixel CCD detector to gives an optical resolution of ~1nm. The typical wavelength range of this instrument is 200 to 850nm. It was found that the coupling efficiency into the silica fibre was too small to enable reliable power measurements to be made with the usual power meter, however, it was possible to get a comparative power measurement with the radiometer. This allowed the power variations with temperature to be monitored. While the device was in the environmental chamber, it was operated under DC conditions with a constant supply voltage. As can be seen from Figure 21, during operation the device produced two noticeable falls in output power as well as variations due to the temperature conditions. These were between 80 C and 90 C during the temperature rise and when the device reached the peak temperature of 125 C. These two glitches can be best attributed to ageing effects in the device, although a misalignment in the fibre at the high temperature can not be ruled out. With an ageing effect, this would cause a change in the bias current of the device, which could be expected to have an affect on the wavelength and spectral width of the output as well as the output power. The performance of the device between 120 C and -55 C showed no obvious device ageing effects and is considered to be the most reliable data for determining the thermal coefficients of the device. This is shown in Figure 22 with a linear trend line superimposed on the data. This indicates a thermal coefficient of the peak wavelength of 0.013nm per C. The spectral width of the output was seen to increase with temperature as shown in Figure 23. The full width half maximum being 24.8nm at -55 C and 29.6nm at 125 C. 65

66 Peak wavelength (nm) peak wavelength temperature power Temperature (Centigrade) Figure 21 Variation of peak wavelength and output power with temperature for device 26. Wavelength Temperature Figure 22: Variation of peak wavelength with temperature for device 26 66

67 Spectral width (nm) Temperature (C) Figure 23: Variation of spectral width with temperature for device 26 Figure 24: Optical spectral output at 20 C for device 26 67

68 Figure 25: Optical spectral output at 125 C for device 26 Figure 25: Optical spectral output at -55 C for device 2 68

69 Output power variations with temperature Output power (arbitary units) Temperature (C) Figure 26: Output power variation with temperature for device 26 As shown in Figure 26, the output power reached a peak at -30 C and fell to 75% of this value at 120 C, this indicates a thermal coefficient of %/K. These values also include the fibre coupling changes and device ageing variations that would have occurred during these measurements. These results indicate very promising thermal stability for this device taken from the 919 wafer and packaged in the TSSOP case without a lens. 69

70 Reliability studies on GaN devices While many devices have been grown, some have been more stable under continuous current injection than others, but over time the light output has either become dimmer gradually, or flickered on and off until the device failed. This has happened for a variety of materials and mask designs, as discussed below. The first mask layout had top emitting LEDs with square overall shape as shown in Figure 27. n p Figure 27. Picture of layout of top-emitting square LEDs, with schematic of layer structure As the device was run under forward bias, it could be seen that emission was not uniform across the emitting surface, but occurred at several bright spots. These spots were quite stable. However, there appeared another spot, as indicated by the arrow in Fig 28a, which then formed a track across the surface of the device. This track went to the p-contact pad, and then quickly extended around the perimeter of the pad. At this stage, device output had dropped significantly, and the device failed. Videos of this have been taken, clearly showing the progress of the track, three frames of which are shown in Figure 28. Figure 28. Track progress with device under bias (a) track starting in bottom left hand corner, (b) progressing towards the p-contact, (c) running around the edge of the p- contact. 70

71 Figure 29. Device surface after failure Further analysis of the track showed that the GaN had been severely pitted, indicating that it had actually melted. We think that the track is made by local short-circuits which take so much current that the material melts, and so the current has to go to the nearest point, creating more local heating and melting, thus forming a track across the surface. Failure in circular contact LEDs and RCLEDs. As the above effect may have been partly due to a non-uniform current spreading layer, substrate emitting devices were made. In this case, the whole top surface is covered with a reflective metal that acts both as p-contact and top mirror. As the metal is highly conductive, any further problems must be associated with the GaN material only. However, the devices were still unstable the light output was observed to flicker on and off, or suddenly drop. Occasionally, the light could suddenly increase again to a former level. These effects were captured while attempting to take a light-current-voltage curve, as in Figure 30. Figure 30. LI curve for circular contact RCLED showing unstable light output. A clear correlation is seen between light output and voltage. When the light intensity drops, the voltage also drops, when the light intensity increases the voltage also increases. This suggest a short-circuiting effect where short-circuits are created then fused. The nearfield images of a device before and after failure are shown in Fig. 31. Figure 31. Nearfield image of a substrate emitting circular contact RCLED before and after degradation 71

72 Two noticeable large dark areas are clearly visible at top and bottom, with two lesser dark-spots to the left of the image also more pronounced. If the material is actually melting, the local heating must be very high. The melting temperature of GaN is 2800 K [1], and the Planck spectrum at this temperature has a large component in the near infrared. To investigate this further, a long-wavelength-pass filter was inserted into the imaging system, as shown in Figure 32. In this arrangement, only radiation at wavelengths between 870 nm and 1.9 µm will be detected. As the device normally emits in the blue-green region of the spectrum (450<λ<520 nm), the image will be dark except for any infrared flashes due to melting. Figure 32. Set up for measuring the heat failure of circular contact RCLEDs. Due to the GaAs wafer the green emission is blocked and the only wavelengths recorded on the camera were infra-red, between 870 nm and 1.9 µm. Figure 33. Device has transient hot-spots which flash for less than one second. As can be seen in Figure 33, flashes are indeed observed. These last for just less than one second, as the heat takes time to dissipate from the original burst. Hence we conclude that there is some migration in the material, probably as the metal migrates down a tubular defect common in this material. This at first causes a short-circuit, but this takes so much current it then melts, producing a bright flash in the infra-red due to the heat. After this event, the melted point may go open circuit and the device goes back to its high-emission state, or there may be a residual resistive short-circuit down the side-walls of the resulting hole in the material. This acts as a parallel resistance, and the light output is not recovered. As the device accumulates more of these the output drops to unusable levels. To avoid this problem, effort went into the growth of better material. Different GaN templates were tried, including commercial ones, interrupted growth and swapping of templates between CRHEA and Thales to check for reactor differences. As a result of 72