Growth of GaN on an AIXTRON Close Coupled Showerhead System

Transcription

1 Growth of GaN on an AIXTRON Close Coupled Showerhead System

2 Theory MOVPE growth

3 Alternative names for MOVPE MOCVD: Metal Organic Chemical Vapor Deposition MOVPE: Metal Organic Vapor Phase Epitaxy OMVPE: Organo Metallic Vapor Phase Epitaxy Often all three expressions are used interchangeably

4 Basic Principle of the MOVPE Process A gas mixture containing the precursors needed for growth, and if necessary for doping, is passed over a heated substrate. The precursor molecules pyrolyze leaving the atoms, e.g., Ga and N atoms on the substrate surface. The atoms bond to the substrate surface and a new crystalline layer is grown, in this case GaN

5 Principle of LP-MOVPE H 2 gas blending reactor scrubbing system H 2, N 2 P=100 Torr Ga(CH 3 ) 3 + NH 3 GaN + 3CH 4 vacuum pump TMGa, NH 3 throttle valve High purity, precise mixing sapphire substrate, T ~ 1040 C Safety Production and research reactors filter unit Crystal quality, uniformity, reproducibility

6 Group III MO Precursors Bubblers TMGa - (CH 3 ) 3 Ga H2 in H2 plus TMGa out Concentration is function of vapour pressure and bubbler pressure: F F MO MO MO H PH ( PBubbler P 2 2 MO) F MO P F H 2 P P P Bubbler Note that the vapour pressure is related to the temperature of the bubbler. MO P MO Therefore the molar flow of MO is controlled by the bubbler pressure, the bubbler temperature and the flow of H2 through the bubbler

7 Growth rate (mm/min) MOVPE growth regimes (I) kinetically limited regime growth is limited e. g. by desorption kinetics of CH T ( C) III II I Slope = E V growth exp E RT (II) mass transport limited (III) high temperature fall-off - various causes are possible: Evaporation of deposited film Surface species desorption Parasitic processes adduct formation, upstream wall deposition (110) (100) /T (1/K) Schematic for MOVPE of GaAs; after: D. H. Reep, S. K. Ghandhi, J. Electrochem. Soc. 130 (1983)

8 MOVPE growth regime for GaN Growth rate dependence in transport limited regime: Reaction kinetics are the fastest processes - gas phase transport to growing surface becomes bottleneck. Get weak dependence of growth rate on temperature due to the following : V growth T 0.7 Growth rate T -1 V growth D Y MO p D p -1 D T 1.7 V growth = growth rate = gas density D = diffusion coeff. Y MO = MO species mass fraction p = pressure T = temperature Pressure dependence due to reaction kinetics Linear dependence of growth rate on G III partial pressure

9 MOVPE growth regime for GaN For GaN growth, standard growth occurs at ~ 1040 C at the overlap of regimes II and III Mostly mass transport controlled, with a significant etching/desorption rate Growth rate = Mass transport growth rate Etch rate The etch rate is controlled by temperature, pressure and atmosphere H2 causes much more etching than NH3 or N2 In addition, with higher pressure, higher temperature and growth of Al containing alloys, there are more parasitic reactions, which can also reduce the growth rate. The exception to these growth conditions is the growth of the low temperature nucleation layer, which occurs at 530 C this is in the kinetic regime I

10 Theory Close-Coupled Showerhead Reactor

11 Precursor flow Complete separation of ammonia and metal-organic precursors until they reach the chamber Combination occurs 11mm from the surface of the wafers Susceptor and wafers at >1000 C, showerhead surface ~150 C (showerhead water temp = 50 C) Ideal for GaN and AlGaN growth where pre-reactions are most significant especially at higher pressures and temperatures

12 pressure drop p [mbar] pressure drop p [mbar] Close Coupled Showerhead Reactor : global pressure management Pressure drop between plenum and process chamber significantly higher than across the process chamber. This results in excellent uniformity CCS 19x2" - Pressure drop along the close space process chamber 6.0x10-3 Regime: T SH = 80 C, T Susc = 1060 C, H 2 : NH 3 = 1:1 5.0x x10-3 CCS 19x2" - Pressure drop along the group-iii tubes 1.2 Regime: T SH = 80 C, H 2 : NH 3 = 1:1 1.0 Q total = 70 slm x Q total = 50 slm 2.0x10-3 Q total = 50 slm x10-3 Q total = 30 slm 0.2 Q total = 30 slm process pressure [Torr] total flow rate Q total [slm] showerhead p lateral process chamber susceptor surface p tubes

13 vertical velocity (m/s) 3D Jetting from showerhead injectors mm dist. from gas inlet 1 mm dist. from gas inlet 2 mm dist. from gas inlet 3 mm dist. from gas inlet 5 mm dist. from gas inlet NH 3 plenum MO plenum length coordinate (m) Extreme case NH3 plenum 90% NH3, MO plenum 100% H2 Complete jet dissipation at distance of ~ z = 5 mm (carrier 1 NH 3 ) ~ z = 2 mm (carrier 2 - MOs)

14 The Close-Coupled Showerhead Reactor Axial Component Independent of Radius Uniform vertical gas supply from close-coupling a showerhead No side effect and no fast rotation Vertical component and boundary layer constant with radius Excellent uniformity under wide parameter window Growth independent of wafer position and showerhead size All reactor sizes use the same parameter space Process is directly scaleable between reactor sizes

15 Close Coupled Showerhead Reactor 19x2 : Flow dynamics at gas inlet tubes (2) Showerhead design study for the CCS 19 x 2" Distribution of precursor TMGa (mass fraction in logarithmic scale) Distribution of decomposition by-product MMGa (mass fraction) Q(H 2 ) = 20 slm, Q(NH 3 ) = 20 slm, p = 100 Torr, T = 1030 C Uniform boundary layer of growth limiting group-iii species. Decomposition complete by middle of process chamber

16 Close Coupled Showerhead Reactor : Chemical boundary layer theory Why is the chemical boundary layer so important? determines the growth rate explains parameter dependencies in CCS reactor How is the chemical boundary layer defined? distance from concentration maximum to growing surface Showerhead gas inlet Susceptor surface Computed MMGa species distribution T = 1030 C, p = 100 Torr, =100 rpm Concentration maximum MMGa : thickness of chemical boundary layer attributed to decomposition by-product MMGa

17 Close Coupled Showerhead Reactor : Chemical boundary layer theory (2) How does the growth rate depend on the chemical boundary layer thickness? The thinner the boundary layer, the higher the growth rate Growth rate versus boundary layer thickness for mass transport limited growth: V growth D c max MO c surface MO Showerhead gas inlet Susceptor surface Computed MMGa species distribution V growth = growth rate c max MO = maximal concentration of MO species in the gas phase c surface MO = concentration of MO species on the substrate surface D = diffusion coefficient of MO species

18 Close Coupled Showerhead Reactor : Chemical boundary layer theory (3) How to control the chemical boundary layer thickness in a CCS by process parameters? mainly by carrier gas flow rate and composition in a CCS rotation speed pressure does not depend on (NB: growth rates may depend on pressure due to gas phase parasitic reactions) Rules of thumb: square root of flow rate Q 1/2 (Q is total flow) (at constant precursor concentration) square root of (gas density) -1-1/2 ; e. g. (N 2 ) 14 * (H 2 )

19 Theoretical Conclusions The growth rate of GaN is controlled principally by mass transport to the substrate It is possible to calculate the change in growth rate to a first approximation with basic diffusion and boundary layer equations The showerhead reactor is an intrinsically uniform reactor - in practice and in simulation The showerhead design avoids jetting or turbulence, and minimises pre-reactions

20 Growth in practice Typical structures

21 Typical LED structure on sapphire Active region contacts pgan InGaN QWs ngan ugan Sapphire This is a very simplified type of LED structure there are many improvements which have occurred to improve the design: Flip chip removal of chip from sapphire onto heat sink ITO contacts for improved emission Many other tricks for improving light extraction and therefore external quantum efficiency

22 Growth of LED structure on sapphire High T ugan ngan layer 5x Quantum Wells pgan Low T GaN nucleation layer

23 Growth of LED structure on sapphire Low temperature nucleation layer ~ 30nm at 530 C Poor quality, poly-crystalline layer

24 Growth of LED structure on sapphire Annealing reduces the nucleation layer to islands of GaN

25 Growth of LED structure on sapphire Annealing reduces the nucleation layer to islands of GaN And then to only a few islands

26 Growth of LED structure on sapphire Annealing reduces the nucleation layer to islands of GaN And then to only a few islands before growth starts again, and the islands grow

27 Growth of LED structure on sapphire Annealing reduces the nucleation layer to islands of GaN And then to only a few islands before growth starts again, and the islands grow.and coalesce to a flat film And then the device layers are grown on top

28 Growth of InGaN quantum wells Standard growth 2 temperature wells and barriers: Well growth: 740 C needed due to instability of indium in InGaN Carrier gas N2: hydrogen etches the indium away, resulting in GaN rather than InGaN Slow growth for better quality Typically around 25Å thickness Can decompose at higher temperatures, losing indium, and so shortening the emission wavelength Lower temperature for green LEDs ~700 C need more indium incorporation for the longer wavelength Roughly 1.5nm wavelength change per C change in temperature

29 Wavelength/nm Growth of InGaN quantum wells There are 4 ways to change the emission wavlength of the MQW: Well thickness: thinner shorter wavelength Due to quantum effects TMIn in gas phase: less shorter wavelength Due to lower indium content in QW This is a linear relationship Growth rate of well: slower shorter wavelength Faster growth incorporates more indium As a consequence, higher TMGa flow (counter-intuitively) results in longer wavelength Growth temperature: higher shorter wavelength InGaN is unstable at growth temperatures, and so higher temperatures make it harder to incorporate so much indium, resulting in shorter wavelength. Therefore there are many conditions to achieve the same wavelength of emission, so it is difficult to optimise the MQW Indium concentration in gas phase

30 Growth of InGaN quantum wells Barrier growth: 860 C for higher quality growth of GaN barriers Fast temperature ramping between wells and barriers to reduce decomposition of wells Very slow growth during the ramping to lock in the indium in the InGaN wells Typically Å thickness First 3 barriers are lightly silicon doped (around 7E17) to improve conductivity, and therefore to reduce forward voltage (Vf) Last two barriers undoped in order to keep separation between the n- and p-doped layers

31 Growth of pgan pgan is difficult to characterise, due to low mobility and carrier concentration Has to be grown at <950 C to avoid decomposing the quantum wells sometimes as low as 800 C for green LEDs. Also, for the same (kinetic) reasons, the growth should not be too slow Need very high p-doping in the final ~5nm to allow for good contacting of the device (around 4% Mg/Ga ratio), due to the large band gap of GaN Increasingly, companies are experimenting with rough pgan finish, to improve the light extraction from the device. Now in production in Taiwan

32 Growth in practice Defect reduction

33 Main defects Threading dislocations these form at the interface with the sapphire and are maintained as growth progresses these can affect device performance Pure screw type rare ~1% Pure edge type common, but easier to reduce Mixed type combination of screw and edge harder to remove Misfit or in-plane dislocations remove strain in the layers, and do not affect most device properties Crystalline imperfections most commonly in GaN, these are V N nitrogen vacancies these can affect device properties

34 Threading dislocation reduction These are formed due to the large lattice mismatch between sapphire and GaN. Typically at the interface, density >10 10 cm -2 Reduced by roughening the layer, and then coalescing it a process which partially simulates ELOG

35 Threading dislocation reduction Initially there are many dislocations in the islands

36 Threading dislocation reduction Initially there are many dislocations in the islands As the island grows laterally, some of these dislocations bend into the plane

37 Threading dislocation reduction Initially there are many dislocations in the islands As the island grows laterally, some of these dislocations bend into the plane But as the surface flattens, the density stays constant

38 Threading dislocation reduction Initially there are many dislocations in the islands As the island grows laterally, some of these dislocations bend into the plane But as the surface flattens, the density stays constant

39 Threading dislocation reduction Longer coalescence results in larger islands, and more dislocation reduction this can be achieved by Higher pressure growth (reduces growth rate due to parasitic effects) Lower V/III ratio can result in more N-vacancies, due to less NH3 Lower temperature can result in more N-vacancies due to less NH3 breakdown, and lower mobility of atoms on the surface However, longer coalescence needs more time, and thicker growth length of coalescence depends on the needs of the device Another important step is to ensure the nucleation layer and wafer preparation (nitridation, baking) is optimised this improves the interface, and therefore also improves the subsequent layers

40 Growth in practice Growth rates

41 Factors affecting growth rate According to theory, the growth rate is affected by the boundary layer thickness, diffusion, and partial pressure of MOs these are changed by: Carrier gas density (and therefore NH3 in carrier) Showerhead susceptor gap Total flow rate Molar flow of MOs (bubbler pressure/temp and MFC setting) These affect growth rate, as predicted, but there are also other effects due to gas phase pre-reactions and etching of the GaN surface

42 Factors affecting growth rate Etching of the GaN surface Negative growth rate Increased etching with: H2 carrier gas (reduced under N2) Reduced NH3 flow (NH3 stabilises the GaN) Increased temperature Gas phase pre-reactions (Parasitic reactions) Reactions between TMGa and NH3 in the gas phase form small particles and reduce growth rate Increased pre-reactions with Increased temperature Increased NH3 flow Increased pressure TMAl and TEGa are more severely affected by pre-reactions than TMGa due to higher reactivity of the precursors

43 Growth in practice Special case doping

44 Sheet Resistance (Ohm/sq.) for 2 micron layer Si doped ngan - very straightforward 600 c.c. [Si] sol [Si] sol α SiH 4 / TMGa [Si/Ga] sol [Si/Ga] gas SiH 4 /TMGa 14ppm c.c. 1e18cm -3 SiH 4 /TMGa 100ppm c.c. 7e18cm -3 LED Mob = f(c.c.) SiH 4 /TMGa Rs Works well up to 1e19cm e18 cm [SiH4 eq./ TMGa] gas (ppm)

45 Resistivity, ( cm) Growth results/ Mg doping-[graph3] Mg doped pgan not so simple c.c. [Mg] sol c.c. = [Mg] sol x activation ratio c.c. [Mg] sol / 100 << [Mg] sol Poor activation (passivation) [Mg] sol Cp 2 Mg / TMGa? [Mg] sol = f (P tot, Q tot, T ) Poor incorporation example: c.c. = 1e18 cm -3? Needs [Mg] sol 1e20 cm -3! Defects, poor quality, low mobility Cp 2 Mg / TMGa > 1% = ppm!! Effect of growth atmosphere on GaN:Mg resistivity T = 1000 o C, P = 100 Torr d = 1 m, R g = m/h V / III = Grp N 2 III: = 730%H2 / 20 slm + 70%N2 Grp N III: 2 = 0100%H2 / 20 slm(no N2) Data from Cambridge University 6x2 system, Jan. 02 From Hall data on thick layers [Mg]/([Mg] + [Ga]) gas (%) Cp 2 Mg / TMGa : defects : Rs More constraints for quality:1) limit p-gan growth temperature and growth time 2) increasing pressure reduces incorporation

46 c (Ohm/cm 2 ) Rs (kohm) Analysis of Mg:GaN using TLM data C 950C 900C trend to high leakage current For Cp 2 Mg / TMGa > 1%, Rs increases Confirms the trend observed at Cambridge University for lower ratios 600 Good LEDs No conclusion on temperature effect as different growth rates were used low brightness For p-gan grown at 100 Torr, 44slm Iv good for Cp 2 Mg / TMGa = 1.8 ~ 2% Cp 2 Mg/TMG (%) 0.30 For metallization process used here contact resistance best for 3.5 ~ 4% C 950C 900C But should depend on chip process In addition, Rc decreased for lower growth rate of the p+ surface layer probable ITOsputter issue TLM data occasionally helped to detect problems from ITO sputtering Cp 2 Mg/TMG (%)

47 Improving pgan layers Better quality pgan with: Higher total flow (increases precursor usage) Higher pressure (more parasitic reactions, need increased Cp 2 Mg flow) Higher NH3 flow (more parasitic reactions, need more precursors) Lower growth rate (takes more time, more likely to affect MQW) Higher showerhead temperature (reasons not clear) pgan has to be optimised for the device lowest resistivity does not result in the best LEDs the pgan also has to provide holes for recombination pgan also balanced against stability of MQW if pgan is grown too slowly at a high temperature, the MQW will degrade

48 Growth in practice Tricks for improvement of LEDs

49 Trick layers for LED improvement Basic LED structure on sapphire is: ugan coalescence layer ngan MQW undoped pgan Contact layer However, there are additional improvements which can be made to this simple structure, and which some companies have found useful. Warning: These layers are not well understood, and so they may not be easy to optimise, or to combine in the same recipe. They may also need different optimisation if the LED structure is to be grown on a different substrate. Key terms: V f : forward voltage normally that needed for 20mA I R : reverse current leakage current through the device, i.e. the amount of current which does not cause EL

50 Trick layers for LED improvement 1. Doped barriers in MQW reduce V f, but may also decrease brightness. Do not dope the final 2 barriers 2. palgan this layer is inserted directly above the MQW. This is usually considered to act as a more resisitive layer which helps spread the current from the pgan contact, and so give a better pattern of current density 3. Thick InGaN layer (or InGaN-GaN superlattice inserted below the MQW Intended to affect the strain in the QWs The InGaN layer puts the QWs under less compression at room temperature 4. nalgan inserted about 1µm below the MQW Reduces the dislocation density But can result in more compressive strain in the MQW

51 Other Tricks for LED improvement Roughening the pgan Increases the light extraction efficiency if the roughening is correct But, makes the contacts to the device harder to make and harder to control Popular in Taiwan Patterned sapphire wafers Triangular patterns on the sapphire substrates If designed properly, reduce the dislocation density of the LED structure Also increase the light extraction Home made in each company Popular in Korea

52 Growth in practice Other devices

53 Alternative growth schemes on sapphire Possible to grow using an AlN nucleation layer rather than GaN This is less well understood very different properties from GaN nucleation layer the high bond strength of AlN means that it will not etch or anneal at 1040 C Harder to clean from the reactor Often thin layer ~ 5nm at ~800 C, followed by island growth and fast coalescence of GaN Has given good results particularly in Japan. Also now popular in Taiwan due to patent issues Alternative sapphire substrates: Patterned sapphire: substrate is etched so that it has pyramidal islands around 2μm tall. GaN is then grown on top of these used to approximate ELOG to produce low dislocation density Semi polar and non-polar GaN: a-plane, m-plane GaN is grown on various substrates should reduce piezo-electric effects in devices. But, difficult to get high quality GaN in these orientations

54 HEMT structure Needs: Resistive GaN Al 0.25 Ga 0.75 N layer ~25nm thick Ideally needs heat conductive substrate for devices Most commonly grown on SiC and Si Problems: Hard to characterise without full processing Semi-insulating silicon is easily doped in GaN reactor (by Al, Ga) AlGaN GaN AlGaN AlN Silicon AlGaN GaN AlN SiC

55 Growth of GaN on silicon

56 Can grow on 2, 4 and 6 silicon wafers

57 Further devices on silicon Many devices are possible with GaN on silicon DBR structures LED structures HEMTs Growth on 4, 6, 8 and even 12 substrates [8 (111) silicon is rare, and 12 (111) is not yet available] Problems: Large lattice mismatch gives poor quality Large thermal expansion mismatch can lead to cracking and bowing unless special growth structures are used Non-transparency makes high efficiency LEDs difficult Can t grow GaN directly on silicon due to reactions between TMG and silicon Al and Ga in the reactor can easily dope the semi-insulating silicon at growth temperatures of 1100 C

58 Basics of growing GaN on silicon There are many different schemes for growing GaN on silicon, but typically these include: AlN buffer layer between silicon and GaN Dislocation reduction layers Strain management layers these ensure that the GaN remains crack free, and also that the wafers are flat when cooled to room temperature. GaN AlGaN AlN AlGaN used to reduce dislocations and to manage the strain in the GaN layer Silicon

59 Strain management layers In order to prevent cracking or bowing when the wafer is returned to room temperature: Need to grow the GaN in strong compression This can be done by growing the GaN on relaxed layers with a smaller lattice size, e.g. AlGaN and AlN This results in a bowed wafer during GaN growth at 1040 C Therefore temperature must be radially controlled for large wafers during GaN growth. Critical for LED growth This is possible for large wafers in a CCS reactor, using the 3-zone heater This is applicable for: 4 on 3x2 6 on 7x2 8 on 19x2 12 on Crius

60 Typical buffer and strain control layers: The standard buffer and nucleation layer is high temperature AlN 1100 C, low V/III ratio (400), low growth rate (0.5um/hr) This is sometimes preceded by a low temperature AlN layer to protect the silicon from being doped by the Ga in the reactor The strain control layers are normally AlGaN 500nm Al 0.5 Ga 0.5 N, 1050 C, low V/III ratio followed by 500nm Al 0.2 Ga 0.8 N, 1050 C OR Graded AlN to GaN over 1µm In addition to these AlGaN layers, there may be other layers to reduce the dislocation density of the layers: AlGaN/GaN superlattices can help reduce the dislocation density Low temperature AlN interlayers are used to control strain and reduce dislocation density Then 1um GaN, followed by HEMT structure, or other active layers, such as MQW

61 Necessary ingredients in growth on silicon Need high quality AlN, to ensure the GaN layer grows in compression because: Dislocations allow relaxation in the GaN High quality at the initial layer continues through the later layers Need high compression GaN to: Avoid cracking Result in a flat wafer when cooled to room temperature, or when cooled to MQW temperature whichever is more important for the structure. It is not possible to have the wafer flat at 2 stages of the process Allow growth of uncracked device structures on top

62 Thickness Uniformity Thickness uniformity of GaN on a 6 silicon wafer grown on CCS 7x2 reactor - 1.7%: (uniformity was not optimised before this run)

63 Additional difficulties with growth on large silicon wafers Heating the wafers must be very carefully controlled: If the centre of the wafer becomes even 1 C hotter than the outside, it becomes extremely bowed, and this results in >30 C temperature drop to the outside If there is a large temperature difference between the centre and the outside of the wafer, plastic deformation (i.e. slip lines) occurs Therefore it is important to use some method for observing the wafers for either curvature or temperature changes. AIXTRON is developing a diode array which can measure temperature in-situ. Seen here in 7x2 configuration

64 Growth in practice Managing the reactor

65 Temperature balancing of the reactor There are three separate heater zones in the reactor these need to be balanced for uniform growth. Need to balance the power in heater Zones A, B and C so that the measured temperature in probes 1, 2 and 3 is the same Probe 1 Probe 2 Probe 3 Balancing must be done on a clean susceptor with no wafers on it so that it approximates a black body These settings will change slightly as the reactor becomes coated the showerhead becomes less reflective so zones A and B will need more power. This will stabilise after several runs Zone A Heater Zone B Zone C

66 Average temperature ( C) Stability of the reactor Showerhead needs to be coated for temperature distribution to be stabilized. Average surface temperature may change by up to 20 C during the first 2 hours of growth/coating Experiment carried out in a 7x2 system after SH vacuum-cleaning using two recessed optical probes. P = 100 Torr, R = 2.5 m/hr Calibrations performed on 19x2 CCS show temperature change of ~15 C from the situation after clean + bake to the situation after one n-gan run time (s) The effect is mostly due to the change in showerhead surface from reflective, after cleaning, to semi-reflective, after Bake, and then to absorbing, after first run (dummy coating run). Effect may still go on after coating run at slower rate: < 0.1% / run

67 wavelength (nm) Stability of the reactor 0.1% per run temperature change can cause a drift of 1nm per run. Example: Series of blue LED runs performed in a 7x2 reactor. SH was cleaned every 12 runs SH was coated for 2 hours at 100 Torr after cleaning. Wavelength changes by 0.75 nm/run. Effect may depend on process SH clean + coat SH clean + coat Stable production would require a correction for well temp of 0.5 C/run run number In addition, use of layers such as AlGaN and AlN may cause a change in the emissivity of susceptors, or change the reflectance of the showerhead, resulting in a change in surface temperature on wafer, despite the same setpoint in the recipe. Therefore the condition of the reactor may not be identical when sets of runs take place several weeks apart

68 Stability of the reactor Typical examples (3x2 FT) Coated showerhead Clean showerhead AlGaN coated susceptor Clean susceptor

69 Stability of the reactor Solution: Baking the reactor after each run (around 1.5 hour complete process) keeps the susceptor clean and stable The thick coating on the reactor is relatively stable, both in absolute temperature and in temperature across the susceptor - any changes must be taken into account when analysing results After many runs (around 30), the showerhead may begin to deposit particles on the susceptor at this point it is necessary to brush and vacuum the showerhead, before starting the process again In addition, the runs should be grouped together into batches with similar materials use, if possible Susceptors should be designated for separate tasks, e.g. one for AlN buffer layers, one for use only with GaN, one for the temperature calibration, etc. It may also be necessary to coat new susceptors. For example a susceptor for use with AlN will go white with use, and so a new susceptor will need to have a thick AlN coating before it will be stable Typical procedure: Clean showerhead coating run bake run (experimental run bake run) ~30 And repeat Gives good experimental reproducibility

70 Growth in practice Uniformity

71 Uniformity Thickness and MQW wavelength uniformity in CCS reactor are primarily governed by temperature there are 2 factors: Uniformity across the susceptor this is controlled by the 3 zone heater as discussed in the previous section Wafer bowing this can be controlled to some extent by growth conditions, and structure grown This becomes more important for large wafer sizes For large wafers, the change in temperature due to the bowing of the wafer can be compensated for by the 3 zone heater, for example on a 4 wafer in a 3x2 reactor, or a 6 wafer on a 7x2 reactor

72 Wafer Bowing effects on thickness uniformity Due to the underside of the wafer being hotter than the top, wafers tend to become dished as they are heated up This results in a colder edge to the wafer, due to better thermal contact with the susceptor at the centre, and worse at the edge Which results in faster growth at the edge (growth rate decreases as temperature increases, as discussed earlier), and therefore a thicker film A typical growth pattern for a susceptor with a uniform temperature distribution is shown right This may be partially compensated for by growing the structure in compression, which forces the wafer to bow in the opposite direction cooler hotter cooler

73 Wafer Bowing effects on MQW wavelength uniformity Depending on the thickness of GaN before the MQW, and the strain state it is grown in, the wafer will have a different bow when it gets cooled to MQW temperature Top is shown a wafer which is still concave at MQW temp the uniformity may be improved with a thicker GaN layer Bottom is shown a wafer which has moved to convex curvature at MQW temp the uniformity may be improved by a thinner GaN layer

74 Growth in practice Summary

75 Growth conditions and effects Quality Growth rate Coalescence Increased V/III ratio (Increased NH3 flow) Increased Temperature Increased Pressure Increased Growth rate Increased Reduced Faster Increased Reduced Faster Increased Reduced Slower Decreased - Unchanged

76 Insitu tools Interferometer

77 Interferometer Hardware A laser beam is channelled down a port in the showerhead onto the wafer, and a reflection reaches the detector There is interference between the top surface of the growing film and the surface of the substrate. Resulting in an oscillation in the amplitude of the signal when it is plotted against time during growth This allows calculation of the thickness equivalent to one oscillation: t = λ/(2n) Where λ = 635nm and n = 2.4 (refractive index of GaN) So for one oscillation, t = 133nm for GaN For AlN, t = 150nm And then the growth rate of the film can be found time

78 Interferometer Effects Reduction in the average amplitude of the oscillation implies a general roughening of the film Reduction of the amplitude of the oscillation implies an increase in waviness, i.e. large scale non-flatness on the film (not necessarily a bad thing). Can also be caused by use of on-axis wafers Another reason for a change in the amplitude of the oscillation is the growth of a material of different refractive index, e.g. AlGaN. This would be clearly shown for the growth of a DBR structure

79 On-axis and miscut sapphire wafers On axis is usually c-plane 0.1. This can result in a large variety of surfaces. When close to 0 offset, can result in large islands, see right. Miscut is usually 0.1 to 0.3 : Better reproducibility More terraced surface As seen right, the on-axis wafer (red line) shows significant damping in the oscillations from the interferometer monocrystal 0.25 miscut Miscut On axis

80 Characterisation Photoluminescence (PL) Including white light thickness mapping

81 Principle of PL A UV laser is used to excite a material and produce a peak equivalent to its bandgap. In an LED structure, this will result in a peak at about the same wavelength that the LED would emit, and a GaN peak around 360nm. The intensity of PL emission can also be an indication of the brightness of EL emission (although not always) In a HEMT structure, there will also be the GaN peak, but in addition, if the laser is at a short enough wavelength, e.g. 266nm, there will be an AlGaN peak from which the composition of the AlGaN can be calculated

82 Intensity/a.u. Processing errors Top image shows an LED wafer taken and analysed in standard mode Clearly there are strange effects on the wafer likely to be artefacts. This is corrected by using integrated analysis as shown in the lower image, which gives a much smoother and more believable result. From the graph of one scan clearly there are several peaks Fabry-Pérot fringes Caused by interference between the layer and the substrate. These are artifacts which can legitimately be removed by integrating the spectrum Wavelength/nm

83 Principle of white light thickness measurement A white light is reflected from the surface of the sample, and the intensity vs. wavelength of the reflected signal is recorded. By analysing the spacing between the fringes, it is possible to get a thickness measurement. t 2n Where t is the thickness, n is the refactive index, N is the number of peaks and λ 1 and λ 2 are the wavelengths of the two peaks. 1 N 1 This is then processed into a thickness map by the Accent RPM2000 mapper as shown right

84 Artefacts from white light thickness measurement For thin samples, or rough samples, it may be difficult to get good or accurate results The example on the left suggests a step change from the top of the wafer to the bottom, which is obviously an artefact The basic pattern can be checked against thickness fringes seen under fluorescent lighting. The blue spectrum shows good data, while the red spectrum shows limited poor quality data Thicker films and flatter surfaces produce the best data

85 Characterisation X-ray diffraction (XRD)

86 Principle of XRD XRD can be used to measure several things: Quality of films, measuring ω-scans (called rocking curves) Thickness of thin layers, particularly superlattices and quantum wells, using ω/2θ scans (also sometimes called rocking curves) Alloy composition, using either ω/2θ scans or peak finding to get a- and c-parameters of the material Strain states of various layers, including AlGaN, InGaN and QW layers The basic setup of an XRD machine is shown below:

87 Principle of XRD For XRD analysis, a mosaic model is assumed, where the GaN film is assumed to be composed of a large number of crystalline grains, all slightly misaligned from one another. Tilt (left) and twist (right) are shown in the diagrams below These types of misalignment are measured using ω-scans. An ω-scan involves the x-ray diffractometer being set up with 2θ at the diffraction angle for the appropriate reflection, and then the sample (ω) is moved: On the (002) reflection, this allows analysis of the tilt in the film associated with the screw and mixed type dislocation density On the (102) or (101) reflection, the scan allows analysis of the twist in the film this is associated with edge type dislocation density There are other effects which broaden the ω-scan peaks, but the dominating factor in GaN films is that of misalignment Effectively, we can regard the ω-scan as measuring one lattice spacing at different orientations, and the ω/2θ scan as measuring different lattice spacing at the same orientation.

88 Measured composition/% Al XRD and PL measurement of AlGaN composition The graph below shows a set of identical samples which were analysed for AlGaN composition with both XRD and Accent RPM2000. PL shows a higher AlGaN composition XRD shows a straighter line, with linear response to TMA vs TMG ratio This is due to assuming Vegard s Law for XRD AlGaN composition a linear function, while the PL uses a fitted curve. Generally, it doesn t matter which is used, but it is useful to have a comparison. Also, it should be noted that the PL data is likely to shift with different strain states in the AlGaN layers PL data XRD data Molar ratio of TMA/(TMG + TMA)/%

89 Characterisation Atomic Force Microscopy (AFM)

90 Principle of AFM AFM on GaN is usually done in tapping mode The microscope scans a rapidly oscillating silicon tip across the sample If there are any changes in amplitude of that oscillation then the sample height is moved by a small amount to return the oscillation to normal (all done in fractions of a second) This results in a scan which shows the height to which the sample was moved at each point, which is effectively a topographical map of the sample area. Artefacts: These are relatively easy to get in AFM due to the sensitive nature of the technique Typically damaged tips, or material picked up by the tip give bad results However generally a very powerful tool for giving fast results

91 Example of AFM on GaN This example shows the amplitude image, which effectively shows a derivative of the height with respect to the left-right direction. Significant features: Terraces 2.5Å high equal to ½ unit cell height Large pits mixed or screw type dislocations. These pin the terraces. Small pits purely edge type dislocations So, we can calculate screw/mixed type dislocation density as ~17disl n /9μm 2 = 2x10 8 cm

92 Characterisation Hall measurements

93 Sample preparation There are several designs of sample these show the best and the worst! Good Acceptable Bad Also, care should be taken to avoid the contact going down the side of the sample: Good Bad This could allow conduction between the GaN and sapphire, which can give misleading p-type measurements Contacting with indium is satisfactory so long as the contacts are Ohmic

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