DOI: 10.1021/cg100395a
(SiC)x(AlN)1-x Solid-Solution Substrate for High Temperature and High Power Devices
2010, Vol. 10 3508–3514
Narsingh B. Singh,* Brian Wagner, Andre Berghmans, David J. Knuteson, Sean McLaughlin, David Kahler, Darren Thomson, and Matthew King Northrop Grumman Corporation, ES- ATL, 1212 Winterson Road, Linthicum, Maryland 21090 Received March 24, 2010; Revised Manuscript Received June 30, 2010
ABSTRACT: SiC/(SiC)x(AlN)1-x films were deposited on on-axis Si-face 6H-SiC(0001) substrates by the physical vapor transport (PVT) method. The chemistry of SiC/(SiC)x(AlN)1-x is similar to that of many hexagonal materials that have no polytype. In turn, using this material system as a substrate will reduce the defects in epitaxially grown films. We report the growth and characterization results of a (SiC)x(AlN)1-x solid solution substrate suitable for GaN epitaxial growth. (SiC)x(AlN)1-x crystals were grown by physical vapor transport (PVT) in a vertical geometry. We used the solid-solution ranging from 30 to 80% AlN in the mixture. The quality was characterized by X-ray, PL, and SEM. X-ray rocking curves showed that crystals with a fwhm of less than 200 arcsec could be grown. We observed a hexagonal morphology, and we did not observe micropipes. The suitability of this substrate was evaluated by growing a GaN film on this substrate using the MOCVD method. The morphology, nucleation, and grain growth of GaN were studied to understand the source of defects in the film and to understand the overall growth mechanism.
1. Background For several decades researchers have been involved in developing GaN/AlN/InN high power and low noise microwave devices for improved commercial and military systems. In particular, efficient, broad-band power radio frequency (rf) transmitters are needed with high linearity, as well as low noise rugged receivers for Transmit/Receive modules (T/R modules). For this purpose the current state of highly defective nitride film technologies can only be looked upon as a “stop gap” until high quality reliable defect-free structures are obtained. The poor quality of the GaN film is the biggest show stopper in making these practical devices. The poor quality of the film is due to the unavailability of a lattice-matched and chemically matched substrate for GaN. The substrate choices are Si, SiC, GaAs, AlN, GaN, and ZnO. Some of the oxides, such as ZnO, have been considered, but these react with byproducts during the epitaxial growth. Several experiments1-5 have been performed to grow pure and doped AlN large diameter crystals. Edgar1,2 and his co-workers have studied SiC-AlN alloys on the AlN rich side and demonstrated growth of AlN crystals. Because of low pressure and reactivity of Al with crucible materials, it was observed that growth of large crystals always involved impurity incorporation in the AlN bulk materials. Experiments using pure aluminum and nitrogen5,6 resulted in transparent large colorless needles (Figure 1a) with a long aspect ratio. Along with these long millimeter size needles, we observed growth of hexagonal small crystallites (Figure 1b). For the growth of large crystals, we raised the temperature above 2150 C, and the AlN single crystal changed to a yellow color due to impurities. A typical polished material is shown in (Figure 1c). This indicated that very high quality transparent materials of this composition could not be achieved concurrently as large crystals. In other words, the temperature required to achieve a high AlN vapor pressure and grow large *E-mail:
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single crystals also unfortunately leads to incorporation of impurities from the crucible. Because crucible stability at high temperature was always a problem, impurities coming out of crucibles always created contamination in the AlN crystals. Various properties of the potential crucible materials;the vapor pressure and the tendency for these crucible elements to become incorporated into the AlN crystal;were studied by Edgar et al.2,7 The results of detailed studies on contamination from crucibles by Edgar’s group are summarized in ref 2. Typically GaN growth is performed on AlN thin films deposited on SiC wafers. 6H-SiC and 4H-SiC have significant defects;especially micropipes, which have been shown to be due to screw dislocations and stacking faults. Some have attributed this as due to the inherent crystallographic structures of 6H- and 4H-SiC. These structures are the main cause of stacking faults. In the present paper, we report the results of thick films of SiC-AlN solid solutions grown on (0001) 6H-SiC with a wide range of compositions and their characteristics. 2. Experimental Method 2.1. Growth of Material. SiC-AlN solid-solution growth was carried out by the physical vapor transport method in a manner similar to that used for silicon carbide and aluminum nitride described in refs 3-5. All the growth experiments were performed in a vertical transport geometry in which the source material was transported upward into a cooler zone. The growth was performed in an induction furnace in the modified PVT configuration capable of maintaining a temperature up to approximately 2300 C. The coils were 600 in diameter, and graphite liners were used for the insulation. Graphite liners were annealed several days to eliminate outgassing. The aspect ratio of the flow, i.e. transport path and crucible diameter, can be altered by the height and the diameter of the crucible. We used on-axis 6H-SiC substrates for growing (SIC)x(AlN)1-x materials. The typical growth parameters are described in Table 1. r 2010 American Chemical Society
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Figure 1. AlN crystals grown above 2100 C: (a) high purity Al and N2 at low temperature; (b) hexagonal fat needles of AlN; (c) crystal grown by vertical PVT using AlN powder above 2150 C.
Figure 3. Morphology of AlN-SiC solid solution film grown on a 6H-SiC wafer; magnification 30.
the morphology, composition, and crystallinity. A Hitachi FE-SEM equipped with a PGT EDS was used to examine the film surface and composition. The suitability of SiC-AlN as a substrate was evaluated by growing a GaN film on the prepared material. 3. Results and Discussion Figure 2. Schematic of a (SIC)x(AlN)1-x solid-solution growth furnace. Table 1. Typical Growth Parameters starting material ambient gas chamber pressure source temp Ts-Tg growth time substrate
AlN þ SiC powders nitrogen 5-250 Torr 1850-2050 C ∼60-80 K ∼10-20 h 6H-SiC(001) on-axis
Figure 2 shows a schematic diagram of the growth chamber. Since the vapor pressure of AlN is lower than that of SiC at the growth temperature, compositional inhomogeneities could be a problem; however, the powders sinter and mix in a nitrogen atmosphere well before reaching 2000 C. Following this process, the alloy sublimes as a compound, thereby allowing the final film composition to match the original AlN/SiC ratio. 2.2. Characterization. The quality of SiC-AlN was evaluated by X-ray diffraction, SEM, and PL methods. We used a Bede D1 X-ray diffractometer equipped with a microsource and a triple-bounce Ge crystal beam conditioner for Omega curves and Omega-2Theta curves to determine the peak positions. The quality of the film was evaluated by studying
Thick AlN-SiC alloy crystals ranging from 1 to 500 μm thick were grown on on-axis Si-face 6H-SiC (0001) substrates by the sublimation-recondensation method from a mixture of AlN and SiC powders in a N2 atmosphere. Growth was in the vertical geometry with source material in the bottom. This created a destabilizing thermal condition. The color of the crystal film changed from clear to dark green with increasing growth temperature. This indicated some impurity incorporation at higher (>1975 C) temperatures. Figure 3 shows the typical morphology of alloy films grown on 6H-SiC wafers. The large hexagonal growth steps indicate wurtzitic structures. Careful observations and concentration measurements showed these facets were AlN rich. Figure 4a shows the morphology and (b) EDX results, confirming larger AlN concentrations. This was consistently observed in many parts of the film. In a separate growth run when we used higher AlN contents, we observed (Figure 5) smaller faceted hexagons. As we increased the AlN content, we observed that the hexagons have smaller steps and the film became smoother. Figure 5 shows the virgin surface for the 67% AlN and 33% SiC composition. This had some hexagonal growth steps on the top of the smooth film, which indicates that nucleation can occur on the top of the smooth film in the form of hexagons. When we grew in the composition range of 75% AlN and 25% SiC, smooth surfaces were observed. Because of high AlN
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Figure 4. (a) Area indicating hexagonal growth steps. They appear to be rich in AlN, and the hexagonal facet appears to be aluminum-rich. (b) 56% Al and 44% Si.
Figure 5. Several hexagonal facets. AlN rich alloy was observed in the faceted area: 67% AlN and 33% SiC composition. Image dimensions are 5 μm 5 μm.
Figure 6. SiC-AlN films grown by using 75% SiC and 25% AlN source materials; the smooth facet free regions of (a) area A and (b) area B were scanned for composition.
concentration, this composition has a closer lattice match with GaN compared to SiC substrates. When films were grown in the 80% to 75% SiC region, the smoothness (Figure 6) increased. The film grown by using 75% SiC and 25% A1N source materials is shown in the smooth facet free regions of (a) area A and (b) area B. The cross-sectional studies with a focused ion beam (FIB) of several films showed that the smooth regions extended from 5 to 40 μm. Figure 7 shows results with approximately 8.8 and 34.5 μm (SiC)(AlN) film thicknesses.
Figure 7. Smooth areas of the sample show the continuous film that varies in thickness from (a) ∼9 μm (b) to ∼35 μm.
Edgar et al.2,7 reported the crystal structure and compositional changes near the two interfaces of an AlN/(AlN)x(SiC)1-x/ 4H-SiC heterostructure by cross-sectional transmission electron microscopy. A compositional transition layer adjacent to the
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Figure 8. X-ray Omega - 2Theta curves showing the position of film SiC-AlN alloy relative to SiC substrate.
Figure 9. Omega curve for the alloy, which was very much comparable to that for SiC, indicating good crystal quality.
4H-SiC substrate suggested interdiffusion between the substrate and the alloy layer. Above the transition layer, the alloy exhibited the typical 2H-polytype. They had voids at the AlN/ (AlN)x(SiC)1-x interface, due to the decomposition of the (AlN)x(SiC)1-x layer before the AlN layer had completely coalesced. The nominally pure AlN layer contained approximately 8% of Si and C, possibly coming from the decomposition of the alloy layer and/or the substrate during growth of the AlN layer. They observed that both interfaces were abrupt, to less than 50 nm, with low densities of threading dislocations. Dislocations in both the (AlN)x(SiC)1-x and AlN layers were not threading but ran parallel to the (0001) planes. We did not observe threading at the interface. The morphology and interface mixing is very complex for the SiC-AlN system, and several papers have been published8-12 on the AlN-SiC interface reconstructions. Almost all these are based on synthesis of small grains by the reaction of silicon carbide and aluminum nitride. Studies by XRD and STEM-EDX analysis showed that
they are single phase solid solutions. These studies showed dense, equiaxial grain structure of the wurtzite SiC-AlN solid solution. Hilmas and Tien10 suggested that AlN addition helps in stabilizing the 2H-polytype of -SiC, resulting in fine equiaxed 2H-SiC and AlN solid solution grains. AlN solid solution grains inhibit formation of the 6H-SiC grains, since AlN (2H) will not go into solid solution in the SiC (6H) structure, effectively pinning the growth of the 6H-SiC grains. Similar experiments11,12 on SiC/AlN composites showed that controlled interfacial solid solutions affect the interfacial bonding and the effect of AlN on the mechanical properties. Almost all of these suggest that two types of abrupt interfaces have been suggested for SiC-AlN alloys. These are based on Si-N, C-Al and C-N, Si-Al bonds. The expectation is that Si-N and Al-C are more stable. The formation energy of the AlN/SiC interface is positive, and AlN/SiC (111) and (001) junctions with long-bond interface energy have lower density of interface bonds. These abrupt AlN/SiC (111) and (001) interfaces are
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Figure 10. Omega - 2Theta curve for an alloy of the composition SiC 80% and 20% AlN, Δ2θ = 1300. Numbers represent peak positions along the Omega - 2Theta axis.
Figure 11. X-ray reciprocal space scan showing the position of pure SiC and SiC-AlN alloy.
charged and, therefore, thermodynamically unstable. The charge neutrality can be restored in these systems by atomic intermixing of the interface. The simplest arrangements giving rise to neutral interfaces are those with one mixed N/C plane or Al/Si plane. This hypothesis indicates that SiC mixed with AlN will not grow as both 4H-SiC and 6H-SiC. The quality of solid-solution produced in our study was very good. Figure 8 is an X-ray Omega - 2Theta curve showing the position of the SiC-AlN alloy peak relative to the SiC substrate. Figure 9 shows the alloy rocking curve, where the small fwhm indicates good crystallinity. In particular, the scans show that the SiC-AlN film quality is comparable to that of the SiC substrate, given the similar fwhm values. Figure 10 shows an Omega - 2Theta curve for an alloy of the composition 80% SiC and 20%AlN, 2θ = 130000 . The fwhm is very small in this case as well, indicating very good crystal quality. The peaks present in this scan represent reflections from the wurtzite phases stabilized in the alloy (160 and 650 arcsec), along with the SiC substrate (10 arcsec). This is in contrast to
Figure 12. (a) Morphology of initial nucleation on SiC-AlN substrate. (b) Growth morphology of GaN and formation of small angle boundaries.
those at pure AlN reflection, which would be present at approximately 720 arcsec. It is unclear which reflections are represented by the SiC-AlN alloy peaks, considering the significant phase intermixing which takes place in these substrates.
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Figure 13. Device structure for Si/SiC/AlN/GaN: X-ray Omega curve showing the quality of GaN (1 μm).
characterized and had the following characteristics: The X-ray rocking curve for Si/SiC/AlN/GaN was taken, and results are shown in Figure 13. The X-ray Omega curve for a GaN (1 μm) film showed a fwhm of 404 arcsec. Photoluminescence (PL) was taken (Figure 14) to evaluate the characteristics of the film and to evaluate emission due to any residual impurities. The GaN band-edge emission occurs at 363.9 nm or 3.4 eV with a fwhm = 7.8 nm or 72 meV. A large yellow band was identified at 570 nm with an intensity of 0.01 V. The observed values are typical of GaN produced and reported in the literature. 4. Summary Figure 14. GaN band-edge emission at 363.9 nm or 3.4 eV with a fwhm = 7.8 nm or 72 meV.
The Omega - 2Theta scans and reciprocal scan shown in Figure 11 clearly show the peaks of SiC and the alloy. The peak splitting along the Omega Rel axis suggests that the alloy film is tilted 254.9 arcsec = 0.0708 from the SiC substrate surface. Also, we did not observe arrays of micropipes generally observed in 4H- and 6H-SiC materials. Subsequently, we used the SiC-AlN alloy film as a substrate for the growth of a GaN film. This SiC-AlN alloy film/substrate was grown on a Æ0001æ SiC wafer. The lattice parameter for the grown alloy was a = 3.10 A˚, which is much closer to GaN compared to the a = 3.071 A˚ value for SiC. This was an excellent film and suitable for GaN epitaxial growth. The growth of GaN was performed using the Northrop Grumman standard MOCVD process currently used for GaN growth on AlN buffer substrates. The grown film of GaN was studied by evaluating the X-ray rocking curve and morphology by SEM (Figure 12). Nucleation phenomena at alloy substrate were studied by running a short-term experiment. Figure 12 shows the morphology of initial nucleation on a SiC-AlN substrate. We observed the formation of small angle boundaries. As GaN grows, the fwhm is 404.2 arcsec. The GaN peak is located at the expected location for epitaxial (0001) GaN. Also, it is important to note that the orientation was maintained from substrate, to alloy material, and finally to GaN; thus, the GaN is in fact epitaxial. The grown film was
We have studied the feasibility of the growth of the (SiC)x(AlN)1-x alloy, which has the potential for a better lattice matched substrate and buffer for GaN epitaxial growth. Based on the concentration of AlN and SiC in the solid solution, one can tune the lattice parameters of (SiC)x (AlN)1-x alloys. We used several compositions, from 30 to 80% AlN, and the material was grown by the physical vapor transport method using SiC and AlN powders. These SiC- and AlN-based alloys were grown in the temperature range 1700-2050 C by PVT. These alloys have favorable lattice parameters compared to pure SiC and hence are better matched for the growth of GaN for high power devices. The quality was characterized by X-ray, PL, and SEM. X-ray rocking curves showed that crystals with fwhm of less than 200 arcsec could be grown. We observed a hexagonal morphology, and we did not observe micropipes. The suitability of this substrate was evaluated by an experiment performed to grow GaN film on this substrate using the MOCVD method. The nucleation and growth of GaN on this substrate was studied to understand the coarsening and grain growth of GaN. Also, the (SiC)x(AlN)1-x alloy can create a heterostructure for AlN and SiC substrates using the (AlN)x(SiC)1-x alloy for GaN based HEMT and HBT devices. Acknowledgment. The authors thank Dr. Michael Aumer for his help during MOCVD of GaN and Rich Brooks, Ron
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Twist, and James Arnold for their help during microscopic characterization by SEM, EDX, and Auger.
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Singh et al. (5) Singh, N. B.; Jones, E.; Berghmans, A.; Wagner, B. P.; Jelen, E.; McLaughlin, S.; Knuteson, D. J.; King, M.; Fitelson, M.; Kahler, D. J. Cryst. Res. Technol., in press. (6) Wagner, B. P.; Singh, N. B.; Berghmans, A.; Knuteson, D. J.; Kahler, D.; McLaughlin, S.; Hawkins, J.; Golombeck, J. J. Electron. Mater. 2008, 37, 379. (7) Edgar, J. Private Communication, 2008. (8) Li, J.-F.; Watanabe, R. J. Mater. Sci. 1991, 26, 4813. (9) Xu, Y.; Zangvil, A.; Landon, M.; Thevenot, F. J. Am. Ceram. Soc. 1992, 75, 325. (10) Hilmas, G. E.; Tien, T.-Y. J. Mater. Sci. 1999, 34, 5613. (11) Miura, M.; Yogo, T.; Hirano, S.-I. J. Ceram. Soc. Jpn. 1993, 101, 1281. (12) Huang, J.-L.; Jih, J.-M. J. Am. Ceram. Soc. 1996, 79, 1262.
