Characterization of GaN epitaxial films grown on SiN x and TiN x porous network templates

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1 Characterization of GaN epitaxial films grown on SiN x and TiN x porous network templates J. Xie a,, Y. Fu a, Ü. Özgür a, Y. T. Moon a, F.Yun a, and H. Morkoç a, H. O. Everitt b,a. Sagar c, R. M. Feenstra c, C. K. Inoki d and T. S. Kuan d, L. Zhou e and D. J. Smith e a Dept. of Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia b Dept. of Physics, Duke University, Durham, North Carolina c Dept. of Physics, Carnegie Mellon University, Pittsburgh, PA d Dept. of Physics, SUNY at Albany, Albany, New York e Center for Solid State Science, Arizona State University, Tempe, Arizona ABSTRACT We report on the structural, electrical, and optical characterization of GaN epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on SiN x and TiN x porous templates in order to reduce the density of extended defects. Observations by transmission electron microscopy (TEM) indicate an order of magnitude reduction in the dislocation density in GaN layers grown on TiN x and SiN x networks (down to ~10 8 cm -2 ) compared with the control GaN layers. Both SiN x and TiN x porous network structures are found to be effective in blocking the threading dislocation from penetrating into the upper layer. Supporting these findings are the results from X-Ray diffraction and low-temperature photoluminescence (PL) measurements. The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably for the layers grown using SiN x and TiN x layers, which generally suggests the reduction of edge and mixed threading dislocations. In general, further improvement is observed with the addition of a second SiN x layer. The room temperature decay times obtained from biexponential fits to time-resolved photoluminescence (TRPL) data are increased with the inclusion of SiN x and TiN x layers. TRPL results suggest that primarily point-defect and impurity-related nonradiative centers are responsible for reducing the lifetime. The carrier lifetime of 1.86 ns measured for a TiN x network sample is slightly longer than that for a 200 µm-thick high quality freestanding GaN. Results on samples grown by a new technique called crack-assisted lateral overgrowth, which combines in situ deposition of SiN x mask and conventional lateral overgrowth, are also reported. Keywords: GaN, MOCVD, SiN x, TiN x, Porous Network, Dislocation, SEM, TEM, XRD, TPRL. 1. INTRODUCTION In past decades, III-V Nitrides devices based on conventional heteroepitaxy growth techniques, such as metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), have succeeded in high-power ultraviolet to visible light-emitting diodes, high-performance ultraviolet detectors, and field effect transistors. 1,2 However, the advances are hindered by the high density of threading dislocations (TDs) as well as associated point defects which limited the device performance, lifetime, and reliability. Although many groups have tried to develop the techniques of single crystal growth of GaN, the success is limited by the size, cost, and techniques themselves. 3-6 Currently, the most popular substrates used for III-V nitrides are still sapphire and SiC. Typical dislocation densities for GaN grown on sapphire are in the cm -2 range using AlN or GaN buffer layers. Although SiC has a much smaller lattice mismatch to GaN, the dislocation densities of GaN grown on SiC are comparable to that of GaN grown on sapphire due to the large thermal mismatch. 7 Moreover, serious cracks formed in thick GaN layers grown on SiC substrates by MOCVD are undesired for device applications. 8 A promising growth technique, hydride vapor-phase epitaxy (HVPE), can offer free-standing GaN templates with mid-10 6 cm -2 dislocation densities. As high as 160,000 cm -2 /Vs lowtemperature mobility was reported for AlGaN/GaN grown on thick Zn-compensated HVPE GaN templates. 10 On the other hand, growth techniques based on epitaxial lateral overgrowth 11,12 (ELOG) concept reduced dislocation densities significantly. Dislocation densities as low as 10 5 cm -2 were achieved in the wing areas. Wafer scale templates are also xiej@vcu.edu, Also with the U.S. Army Research Office, Research Triangle Park, Durham, NC 27709

2 possible for device applications. Pendeo-epitaxy concept was proposed to produce a continuous low dislocation density layers. 13 One disadvantage of ELOG is that it requires ex situ SiN x or SiO x deposition and multi-step photolithography which increases processing cost. So other in situ techniques based on SiN x or TiN x porous networks are very promising for reducing TDs without any photolithography or even any ex situ processing. Maskless and one-step processing is also desirable to eliminate contamination. The basic concepts of in situ network are similar to ELOG. Instead of well-oriented strip masks, micro-scale self-formed SiN x or TiN x network acts as a lateral overgrowth mask. When using SiN x network, growth starts from nano-openings in the porous network, without any etching process or annealing, while for growth on TiN x network, in situ annealing or etching process is required for forming lateral overgrowth openings. Recently, in HVPE growth, TiN x networks were used for separation of thick GaN films from sapphire by cooling down. 21 Similar to SiN x porous network, TiN x porous network would be very promising in MOCVD growth for reduction of dislocation densities. In this study, we report on the structural, electrical, and optical characterization of GaN epitaxial layers grown by MOCVD on SiN x and TiN x porous templates. A new growth technique named crack-assisted lateral overgrowth, which combines in situ deposition of SiN x mask and conventional lateral overgrowth, is also reported. 2. EXPERIMENT All GaN epilayers in this study were grown in a low-pressure vertical cold-wall MOCVD system, using (0001) sapphire or n-type (0001) 6H-SiC as a substrate, and Hydrogen as carrier gas. Trimethylgallium (TMGa), trimethylaluminum (TMAl), trimethylindium (TMIn), silane (SiH 4 ) and ammonia (NH 3 ) were used as sources of Ga, Al, In, Si and N, respectively. The effects of SiN x network study were investigated on three types of templates: (1) 3µm GaN epilayer on sapphire with low temperature (LT)-GaN buffer layer, (2)100 nm LT-GaN buffer layer on SiC, and (3)100 nm high temperature (HT)- AlN buffer. Amorphous porous SiN x layers were deposited on these buffer layers at 1020 C, 200 Torr for 5 min by flowing 50 sccm of 100 ppm silane that was diluted by hydrogen. This was followed by the overgrowth of GaN at 1030 C with a V/III ratio of Two samples were grown on sapphire substrates, one without SiN x network (reference sample) and one with SiN x. For each of the two types of templates on SiC substrates three samples were grown. The first sample was grown with a single SiN x layer inserted after the 100 nm buffer layer, while the second sample had two SiN x layers: first one deposited after the 100 nm buffer layer, and the second one after 2 µm overgrown GaN. A third sample which had no SiN x network was used as a reference sample. The total thicknesses of the samples were up to 8 µm. The TiN x porous networks were applied on two types of GaN templates: (1) 0.7 µm GaN on sapphire with LT-GaN buffer layer, and (2) 1 µm GaN on SiC with HT-AlN buffer layer. After the deposition of 10, 20, or 50 nm-thick Ti thin films ex situ by e-beam evaporation the templates were immediately transferred into the MOCVD growth chamber. Then, TiN x porous networks were formed by in situ annealing under H 2 +NH 3 flow. Finally, GaN layers were grown with various thicknesses to achieve coalesced surface. Crack-assisted lateral overgrowth was performed on AlGaN/GaN heterostructures with sapphire substrates. Growth of 300 nm Al 0.35 Ga 0.65 N on 2 µm GaN generated cracks due to lattice mismatch. In situ deposited SiN x served as a lateral overgrowth mask and cracks served as windows. GaN seed layer networks were formed through cracks. Finally, after the deposition of a 12 µm-thick GaN layer with a V/III ratio of 4000, flat surface was achieved. Further details of the growth can be found in reference 22. Surface morphology evolutions of all samples described in this study were investigated using a JEOL JSM 6060 scanning electron microscopy (SEM). A high-resolution X-ray diffraction (XRD) system (X Pert-MPD TM, Philips) was used to examine the crystalline quality. Hall (7504 HMS TM, Lake Shore Cryotronics) and Capacitance-Voltage (CV) measurements were performed to determine room temperature mobility, temperature-dependent mobility, and doping profile. Atomic force microscope (AFM) (MultiMode TM, Digital Instruments) was used to investigate the surface atomic roughness of GaN. TEM was employed to investigate the effectiveness of various growth techniques applied here on reducing TDs. To study the radiative efficiency of GaN overgrown layers, time-resolved photoluminescence (TRPL) was employed at room temperature using a ~45 ps resolution Hamamatsu streak camera.

3 3. RESULTS AND DISCUSSION 3.1 Growth with SiN x porous network SiN x network was formed in situ by depositing a few nanometers thick amorphous SiN x on GaN buffer layers. The coverage of SiN x porous network is the key parameter for TD reduction and is very sensitive to silane concentration, flow rate, and deposition time. The growth conditions of SiN x porous network layers were optimized using scanning Auger microscopy mapping (SAM). 17 Full coverage with SiN x was observed to result in polycrystalline GaN growth On sapphire substrates Figure 1 shows cross-sectional TEM images of a ~3µm-thick GaN overgrown on a ~3 µm GaN/sapphire template using a SiN x porous network layer. It can be seen that SiN x porous network significantly affects the structure of the dislocations in the overgrown layer. Some pits can be seen from SEM images. Those pits have V-shape in cross section with stable {1101} facets. Towards {1101} facets of those pits are bent dislocations (marked as b ). Dislocation bending was mostly completed in the initial island growth stage, from which one can conclude that the island density (or pore density, or coverage of SiN x ) affects the amount of dislocation bending. 23 Bending of dislocations enhances the possibility of interaction between them (for annihilation) that would then reduce the overall number of TDs. XRD results for this sample showed 5.3 and 5.8 arc-min full width at half maximum (FWHM) values for the (0002) and (1012) rocking curve peaks, respectively, compared to 5.0 and 6.7 arc-min for the control sample without any SiN x network layer. m Figure 1: Cross-sectional TEM images of a ~3 µm thick GaN film overgrown on a ~3 µm thick GaN template, with a SiN x network layer (marked by a white arrow). Dislocations bend ( b ) towards the walls (growth facets) of the pit. The SiN x network layer acts as a mask ( m ) blocking the propagation of dislocations. Some dislocations pass through the interlayer and extend straight into the overgrown film ( s ) On SiC substrates Figure 2 shows a representative cross-sectional TEM image of the samples with SiN x. In this particular sample grown on LT-GaN buffer, two SiN x networks were used for dislocation reduction. Here, the dislocation density is reduced dramatically by the first SiNx network. The dislocation density reduction by the second SiN x network, however, is not as significant as the first one. The cross-sectional TEM image (not shown here) of the control sample without the SiN x layer also shows significant dislocation reduction at the LT-GaN/GaN overlayer interface. To evaluate the effects of SiN x layers we thus have to rely on the comparison of dislocation densities observed at the top GaN overlayer. For more accurate numbers for dislocation density, plan-view TEM images need to be evaluated; however, because of the high crack density in the overgrown layer, we were not able to prepare specimens for plan-view TEM. Nevertheless, crosssectional TEM observations indicate that the total dislocation density can be reduced by the SiN x network and further reduced by the second SiN network to cm -2 on LT-GaN buffer.

4 Figure 2: Cross-sectional TEM micrograph of a GaN overlayer grown with the insertion of two SiN x nanoporous network layers, on a LT-GaN buffer layer by MOCVD using a 6H-SiC substrate. Table 1 summarizes the results on samples grown with ~100 nm LT-GaN buffer using SiC substrates. XRD rocking curve measurements were carried out for both symmetric (0002) and asymmetric (1012) reflections and were found to be consistent with the TEM data. The X-ray rocking curve FWHM values show consistent improvement for (1012) reflection after insertion of one and two SiN x porous network layers. The (1012) FWHM value of 26.0 arc min for the control sample was reduced drastically to 15.6 arc min when one SiNx network was used, and to 12.0 arc min when second SiN x network was inserted. The FWHMs of the (0002) reflection, however, remained roughly the same for the control (10.7 arc min), single SiN x network (10.9 arc min), and two SiN x network (9.6 arc min) samples. It is known that in wurtzite GaN crystals, the symmetric (0002) reflection is affected by pure screw dislocations with Burgers vector b=[0001], while the asymmetric (1012) diffraction is more sensitive to edge-type (b=1/3[1120]) and mixed-type dislocations. Accordingly, the x-ray-diffraction data shown in Table 1 suggest that the edge dislocation density has been effectively reduced by the use of a single-layer SiN x, and was further filtered by a second layer of SiN x. Table 1: Summary of the XRD FWHM data, TEM dislocation density, and crack density data for GaN overlayers grown with one and two SiNx networks using low-temperature GaN and high-temperature AlN buffer layers on 6H-SiN substrates. GaN Crack XRD FWHM TEM dislocation density thickness Density (arc min) (µm) (mm -1 (cm -2 ) ) control (total) LT-GaN One SiN x layer (total) buffer HT-AlN buffer Two SiN x layers (total) Control (s) (e) One SiN x layer (s) (e) Two SiN x layers (s) (e) Table 1 also summarizes the set of samples grown on ~100nm HT AlN buffer using SiC substrates. X-ray FWHM of the (0002) reflection shows no significant difference between control sample and SiN x samples. However, the effect of SiN x network on FWHM of the (1012) reflection peak is significant. With a single SiN x network layer, (1012) reflection FWHM is reduced to 4.9 arc min compared to 8.5 arc min value for the control sample. With the addition of a second SiN x network layer, no significant change was observed in the FWHM value of the (1012) reflection. Plan view TEM images of this set of samples are shown in Figure 3. Screw type and edge type dislocation densities are obtained as cm -2, cm -2 for the control sample, cm -2 and cm -2 for the single SiN x network layer sample, and cm -2 and cm -2 for the two SiN x network layers sample, respectively. For evaluation of the crystalline quality, XRD is not sufficient because X-ray probes the entire depth of the samples. Therefore, cross sectional and plan-view TEM images are more accurate for determining the effect of SiN x networks. Based on the analysis of the XRD and TEM data in Table 1, it is reasonable to conclude that most of the blocked dislocations are edge type (or mixed type), not screw type. Electron diffraction patterns for the GaN layers overgrown

5 on SiNx networks further reveal that they are free of any c-axis tilt typically found over the wing regions of samples grown by the ELOG process. 24 Figure 3: Plan-view TEM micrographs of GaN overlayers grown on HT-GaN nucleation layers, (a) without SiN x (control sample), (b) with one SiN x network layer, and (c) with two SiN x network layers, by MOCVD on 6H-SiC substrates. e stands for edge-type dislocations and s screw-type dislocations. Table 2 summarizes the room-temperature TRPL decay constants, amplitude ratios and 10K PL linewidths for the GaN samples grown with one and two SiN x network layers. The TRPL decays for all the samples were well characterized by a biexponential decay function A1 exp( t / τ1) + A2 exp( t / τ 2 ). 17,25 From Table 2, it can be seen that independent of the buffer layer used, the decay constants and the amplitude ratio A 2 /A 1 exhibit a trend of increase with the inclusion of SiN x network layers. Similar trend is confirmed for the 10K photoluminescence (PL) linewidth of the donor-bound exciton (D 0 X) emission. For growth on LT-GaN buffer, one SiN x network layer didn t affect the D 0 X linewidth; however, significant decrease was achieved with two SiN x network layers. For growth on HT-AlN buffer, D 0 X linewidth decreased mainly after the inclusion of the first SiN x network, but stayed unchanged when the second SiN x network layer was inserted. TABLE 2: TRPL decay constants and amplitude ratios (at 200 µj/cm 2 excitation density) for GaN samples grown with one and two SiN x network layers on both LT-GaN and HT-AlN buffer layers. LT-GaN buffer layer HT-AlN buffer layer control One SiN x layer Two SiN x layers control One SiN x layer Two SiN x layers t 1 (ns) t 2 (ns) A 2 /A K PL D 0 X (mev) In order to investigate the effect of Si diffusion from the SiN x layer on the electron concentration profile of the overgrown GaN epilayer, one additional sample was grown on a 100 nm HT AlN buffer using SiC substrate. To achieve the best surface morphology measured by AFM, chamber pressure was changed from 76 Torr to 200 Torr after 2 µm GaN growth on the SiN x porous network. The total thickness of the overgrown layer was ~3 µm. Room temperature CV characteristics were measured at 1MHz on 300 µm-diameter GaN/Au Schottky diodes. Figure 4 shows the carrier concentration profile vs. depletion depth for this sample. The inset of Figure 4 shows the C - V profile down to 35 V. The carrier concentration of this particular sample can be analyzed in three regions. Part I shows an average carrier concentration of 3x10 16 cm -3 that corresponds to the top 1 µm-thick GaN layer grown at 200 Torr. Part II shows a very low carrier concentration of 6x10 14 cm -3. In part III, carrier concentration gradually increases to cm -3 up to the depletion depth we can achieve. The low carrier concentration of part II can be attributed to an increase of carbon incorporation efficiency due to the relatively low growth pressure of 76 torr. 26,27 This can be understood by considering that the hydrogen etching effect for the removal of the deep acceptor from the dissociation of Ga(CH 3 ) 3 source molecules on the growing GaN surface will decrease as the chamber pressure decreases by reducing the effective number of hydrogen molecules staying in the chamber while there is no noticeable change in the Ga adsorption rate on the growing surface (or the growth rate of GaN) in the employed pressure range. It is noteworthy to point out that the

6 carrier concentration in part III increased toward the porous SiN x layer. The increase of carrier concentration can be attributed to the apparent Si diffusion up to over 0.7 µm from the interface between the overgrown GaN and the SiN x layer. This implies that the interface side of GaN overgrown on the porous SiN x network will be heavily silicon doped, resulting in a formation of highly degenerate impurity interlayer. Figure 4: Electron concentration profile calculated from the C-V profile for 3 µm GaN grown on SiN x porous network using HT-AlN buffer on a SiC substrate. In addition to dislocation reduction, SiN x network can also be used to reduce the density of cracks generated by the thermal mismatch between SiC substrates and GaN epilayers. Figure 5 shows typical optical images for the control sample on LT-GaN buffer, the one SiN x network layer sample on LT-GaN buffer and the two SiN x network layer sample on HT-AlN buffer. The highest crack density was observed for the control sample on LT-GaN buffer (8.0 mm - 1 ). By inserting one SiN x network layer, a crack density of 3.9 mm -1 could be obtained in the overgrown layer. The lowest crack density was achieved for the two SiN x network layer sample grown on HT-AlN buffer (0.7 mm -1 ). Crack densities obtained for all samples are summarized in Table 1. Figure 5: Optical images showing crack lines on the MOCVD-grown GaN surface for (a) control sample on LT-GaN buffer (b) sample with one SiN x network layer on LT-GaN buffer and (c) sample with two SiN x network layers on HT-AlN buffer. The horizontal scale bar indicates 1 mm. 3.2 Growth with TiN x porous network After e-beam deposition, the thin Ti film was continuous and mirror like. After NH 3 :H 2 mix gas annealing, TiN x networks were formed. The morphology of TiN x networks was very sensitive to the annealing temperature, gas ratio (NH 3 :H 2 ), annealing time, and Ti thickness. After annealing in pure NH 3 (7 liter/min flow rate) for 30 min, sample surface remained flat with a few small pores. Based on the XRD analysis, this nitrided Ti was composed of (111)- oriented TiN x, suggesting the formation of a continuous TiN x film on top of the GaN template. High-resolution TEM indicated that this TiN x layer was polycrystalline. In samples annealed in pure hydrogen for the same duration, we

7 observed peeling off of most parts of Ti films from GaN, which may be due to enhanced decomposition of GaN below Ti without the presence of N supplied by ammonia. Provided a proper annealing time was used, porous TiN x networks could be obtained with NH 3 :H 2 ratios ranging from 1:1 to 1:3. We propose that formation of the porous TiN x takes place over three stages. 20,28 First, the Ti film is turned into TiN x by nitridation, as confirmed by EDS analysis and high resolution TEM observations. Second, the continuous TiN x layer gets etched by hydrogen or decomposed at high annealing temperature. The etching or decomposition is likely to be initiated from boundaries of TiN x polycrystalline because defective regions are more likely to be attacked than others. This is consistent with the appearance of enclosed chains of small holes on the sample surface after several minutes of annealing. With continued annealing, these holes became larger, accompanied by the formation of new openings inside the grain boundaries. Finally, GaN partially decomposes in these openings and the porous TiN x networks are formed. Figure 6: SEM images of TiN network formed after NH3:H2 (1:3) (a) 10 min and (b) 20 min annealing in NH 3 :H 2 (1:3) for 10 nm Ti deposited on 1µm GaN grown on a SiC substrate. Figure 6(a) shows a typical surface morphology of TiN x porous network formed after 10 min NH 3 :H 2 (1:3) annealing. Ti thickness for this particular sample was 10nm and was deposited on 1µm GaN grown on a SiC substrate with HT- AlN buffer. It can be seen that high density porous network has a distribution in opening sizes, but most openings have a size below 1 µm. By increasing annealing time only, the density and size of openings increased as shown in Figure 6(b). Another set of samples with 20 nm Ti deposited on 0.7µm GaN using sapphire substrates exhibited similar TiN x porous network evolutions by changing annealing time and annealing gases ratio. But to get similar network morphology much longer annealing time was needed. As a result, some GaN openings decomposed completely down to substrates. 20 Detailed sample structures and annealing parameters are summarized in Table 3. We also increased the Ti thickness to 50nm, but most areas of the Ti layer were highly resistive to annealing independent of how much we increased the annealing time. Although some openings were observed, they had too low density to serve as overgrowth windows. Table 3: Summary of Ti thickness, annealing conditions and XRD data for GaN overgrown on 1µm GaN on SiC with HT-AlN buffer and 0.7µm GaN on sapphire with LT GaN buffer. template Annealing GaN XRD FWHM TEM TDs Ti overgrowth (arc min) (cm -2 ) (nm) Time NH (min) 3 : H thickness 2 (µm) edge screw : µm GaN on SiC with HT-AlN buffer : : µm GaN on sapphire with LT GaN buffer : For GaN growth at high temperatures on continuous TiN x layers obtained by annealing in pure ammonia, only sparse hexagonal GaN islands were observed. This result suggests poor wetting of the GaN nucleation layer on TiN x, similar to GaN growth on SiO x or SiN x. 29 For the porous TiN x network, GaN growth nucleates selectively at the pores by first

8 filling the voids inside the openings, followed by formation of small isolated GaN islands on the surface. With further growth, adjacent GaN islands expand and coalesce to form mesas. These mesas extend both vertically and laterally on TiN x, leading eventually to full coalescence. It should be noted that average distance between GaN islands is 2~3 µm, while sub-micron distances were observed between many tiny openings in Figure 6. This suggests that although high density of tiny openings formed on the TiN x network, they could not act as effective nucleation sites. 1µm 1µm Figure 7: Cross sectional TEM images with (a) g=[0002] (b) g=[1120] for GaN overgrown on TiN x porous network formed by 10 min annealing of 10 nm Ti on 1µm GaN using a SiC substrate. Representative cross sectional TEM images of GaN overgrown on TiN porous network are shown in Figure 7 for different g-vectors. This sample is with TiN x network formed by 10 min annealing of 10 nm Ti on 1µm GaN using a SiC substrate. Figure 7(a) and 7(b) were taken at almost exactly the same area. In GaN templates below the TiN x network, a much higher TD density is observed in Figure 7(b) than that in Figure 7(a), suggesting that the predominant TDs have edge-type component (b =1/3[1120]), consistent with data published earlier. 20 Most TDs emanating from the GaN template are blocked from penetrating to the upper layer by the TiN x and the TDs above the GaN template are mainly of mixed type (~70%). This indicates that the number of TDs blocked by the TiN x network is especially significant for edge-type dislocations. Although a small fraction of TDs pass through the network openings and enter the upper layer, many of them bend nearly 90 during the lateral overgrowth process and extend laterally, finally annihilating each other by forming half-loops. It is noted that this dislocation interaction mechanism is observed to occur more effectively in Figure 7(b) within the first 0.2µm above the TiN x. There are only a few TDs that extend further away vertically, and occasionally reach the top surface. Discontinuous planar voids with sub-micron size are observed above the TiN x, due to ELOG and poor wetting of GaN on TiN x. These voids result in the elimination of the interface between TiN x and the laterally overgrown GaN, which may help to release the interfacial stress and enhance the quality of coalesced fronts, which is consistent with previous observations. 30 For the sample grown on TiN x network with longer annealing time (20 min), coverage of TiN x network was decreased as shown in Figure 6(b). But the dislocation reduction is similar as observed from cross sectional TEM images. TDs were again effectively blocked or bent by the TiN x network. However, due to the larger openings, more dislocations continuously extend to the overgrown layer through openings. This suggests that optimized coverage (in terms of annealing time for particular Ti thickness) is needed to improve the quality of the overgrown layer. We also investigated the effects of 20 nm thick Ti. To get suitable TiN x network for growth, 30 min and 60 min annealing times were used. Although the templates were changed to be 0.7 µm GaN on sapphire substrates, the dislocation density reduction analyzed via TEM study was similar to what was observed for growth on 1µm GaN on SiC with HT-AlN buffer. Dislocation densities for different types of samples are also summarized in Table 3. It s obvious for both types of templates that TiN x porous network reduces the edge type dislocations by an order of magnitude, screw type and mix type dislocations by half. This is similar to what was observed in SiN x porous network samples discussed in Sec Increasing Ti thickness is likely to reduce dislocation more efficiently. Thicker Ti results in less density of openings which means less nucleation through openings, and finally less dislocation density in the overgrown layer. However, thicker Ti will introduce difficulty in surface coalescence. The thickness required to achieve a flat surface for the overgrown GaN layer was found to be 3 µm for 10 nm Ti layers, but 13 µm for 20 nm Ti layers. This huge difference may be attributed to the low lateral overgrowth rate when hexagonal islands are formed through openings. With smaller

9 density of openings, more time is needed for coalescence. FWHM values of rocking curve peaks that show a trend consistent with TEWM observations are all listed in Table 3. One additional sample (not listed in Table 3) was grown for TRPL characterization. The Ti thickness used was 10 nm. The biexponential decay times were resolved as (t 1,t 2 ) =(0.47 ns, 1.86 ns) which are significantly longer than those of the control sample [(t 1,t 2 ) =(0.13 ns, 0.30 ns)], and are comparable to those of the freestanding HVPE GaN template [(t 1,t 2 ) =(0.34 ns, 1.73 ns)]. 31 Longer carrier lifetimes indicate that the radiative efficiency of GaN can be greatly improved by using the TiNx network. The improvement in decay times for the samples with SiN x and TiN x templates compared to the control GaN samples reflects the fact that TDs act as nonradiative recombination channels. However, no direct correlation is observed between the XRD linewidths and the decay times within the SiN x and TiN x network samples. The observed sample-tosample variation in the decay times may be ascribed to point defect-like nonradiative centers that cannot be detected by the TEM. Here, it should also be noted that the TD density in the samples with TiN x layers (10 18 cm -3 ) is two orders of magnitude larger than that in the HVPE-grown freestanding GaN sample (10 6 cm -3 ). These results indicate that threading dislocations do not limit the emission efficiency at room temperature as much as other types of nonradiative recombination centers like point defects and impurities. 3.3 Growth with crack assisted lateral overgrowth Cracks generated by lattice mismatch between AlGaN and GaN can serve as epitaxy overgrowth windows similar to the patterned strip windows in conventional ELOG. During deposition of 300 nm Al 0.35 Ga 0.65 N on a 2 µm-thick GaN template using c plane sapphire substrates, cracks were generated as networks along [1120] directions. In situ deposited SiN x served as the overgrowth mask. Unlike SiN x network discussed in Sec. 3.1, SiN x used here was thick enough to avoid formation of pores. Different growth stages are displayed in Figure 8. At early stages of the growth, after 10 min, wire-like GaN seed lines formed along the cracks with {1101} facets as displayed in Figure 8(a). Then, these seed lines grew laterally and vertically [Figure 8(b)], and finally coalesced [Figure 8(c))]. A cross sectional SEM of seed layers is also displayed in Figure 8(d) and {1101} facets are marked. This growth method is similar to the conventional ELOG. However, there is no need for any photolithography process since the SiN x mask is deposited in situ. The only drawback of this growth method is that the cracks are along the [1120] direction, while in conventional ELOG strip patterns are along the [1100] direction. Strips along [1120] have a much less lateral growth rate than those along [1100], and have {1101} facets in a large growth window and are therefore difficult to coalesce. In order to enhance the lateral growth rate of the {1100} facets, different growth conditions were reported in literature: Ga-rich ambient with lower ammonia partial pressure, 32 Mg doping, 33 relatively high temperature and low pressure, 34 or ammonia flow modulation. 35 However, no conclusive evidence has been found and further study is needed to get reproducible conditions.

10 50µm 10µm {1101} 5µm 2µm Figure 8: SEM images for different stages of lateral overgrowth through cracks: (a) after 10 min, (b) after 2 hours (c) after 6 hours, and (d) cross section of wire-like seed layers with {1101} facets. Figure 9: Cross sectional TEM image of the 12 µm-thick overgrown GaN by crack assisted lateral overgrowth under two-beam diffraction conditions with g=[1120]. A void formed by cracks and dislocation bending are indicated by the arrows. A typical cross sectional TEM image of GaN overgrown using crack assisted lateral overgrowth is shown in Figure 9. Voids (indicated by arrow) are observed due to cracks into the GaN template. In some areas, much bigger voids can been observed [Figure 8(d)] because of decomposition of GaN starting from cracks. In the overgrown layer, dislocations are effectively bent by lateral overgrowth. XRD rocking curve measurements revealed that c-axis tilt is apparent in

11 these samples, as in the conventional ELOG. Another important point is that due to low lateral growth rate, high crack density might be desirable for a quick coalescence. Fortunately, crack density can be controlled by increasing Al composition or AlGaN thickness. The biexponential TRPL decay times for samples with different growth times on a high density crack template are summarized in Table 4. A high crack density was obtained by increasing the Al composition in the AlGaN while keeping its thickness the same. Decay times increase significantly with increasing growth time while the coverage of lateral overgrown layer increased. This suggests the overgrown layer has a much less density of point-defect and impurity-related non-radiative centers than the GaN template. Excellent optical quality of the overgrown GaN was also confirmed by PL measurements. Room temperature PL spectrum of the overgrown GaN epilayer prepared by crack-assisted lateral overgrowth, shows a strong band edge luminescence at ev. In 10 K PL spectrum, the donor-bound exciton (D 0 X), free exciton A (FX A ), and free exciton B (FX B ) emission lines are clearly observed at 3.482, 3.488, and ev, respectively, for a 12 µm thick fully coalescence layer. When compared to other two growth methods discussed above, crack assisted overgrowth results in decay times [(t 1,t 2 ) =(0.200 ns, ns)], longer than those for the SiN x network samples, but shorter than those for the TiN x network samples. Table 4: TRPL decay constants of overgrown GaN layers with different overgrowth time on high crack density template. GaN template Growth time (hours) t 1 (ns) t 2 (ns) SUMMARY We have studied three different growth methods for dislocation reduction. TEM studies indicate an order of magnitude reduction in the dislocation density in GaN layers grown on TiN x and SiN x networks. Both SiN x and TiN x porous network structures were found to be effective in blocking the threading dislocation from penetrating into the overgrown layer, especially for edge type dislocations. FWHM values of XRD rocking curve peaks were consistent with TEM observations. Crack assisted lateral overgrowth, which is similar to the conventional ELOG, was also compared to growth with TiN x and SiN x networks. Reduction of dislocation density by crack assisted lateral overgrowth was confirmed by TEM. The room temperature TRPL decay times obtained for samples grown by three different methods were compared. TRPL results suggest that primarily point-defect and impurity-related non-radiative centers are responsible for reducing the lifetime. The carrier lifetime of 1.86 ns measured for a TiN x network sample is slightly longer than that for a 200 µm-thick high quality freestanding GaN. 5. ACKNOWLEDGEMENTS The work at VCU, CMU and SUNY Albany is funded by ONR as part of a DURINT program under Grant No. N , monitored by Dr. Colin Wood, and the work at ASU was partially supported by ONR-grant N The Duke portion of this work was funded in part by U.S. Army Research Office Grant No.W011NF- 04-D-0001/DI #0002. REFERENCES 1. H. Morkoç, Nitride Semiconductors and Devices 2 nd edition (Springer, Berlin 2005). 2. J. W. Orton and C.T. Foxon, Rep. Prog. Phys (1998). 3. S. Porowski, J. Crystal Growth 189/ (1998). 4. M. Aoki, H. Yaman, M. Shimada, T. Sekiguchi, T. Hanada, T. Yao, S. Sarayama, and F. J. DiSalvo, J. of Crystal Growth 218, 7 (2000). 5. M. Bockowski, Cryst. Res. Technol. 36, 771 (2001).

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