High Resolution X-ray Diffraction Analysis of Gallium Nitride/Silicon Carbide Heterostructures H.M. Volz 1, R.J. Matyi 2, and J.M.

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1 Copyright (C) JCPDS-International Centre for Diffraction Data High Resolution X-ray Diffraction Analysis of Gallium Nitride/Silicon Carbide Heterostructures H.M. Volz 1, R.J. Matyi 2, and J.M. Redwing 3 1 Materials Science Program, University of Wisconsin, Madison, WI Department of Materials Science and Engineering, University of Wisconsin, Madison, WI Advanced Technology Materials, Inc., Danbury, CT 681 Abstract We have performed a comprehensive diffraction analysis of GaN and GaN/(Ga,Al)N heterostructures synthesized by metal-organic chemical vapor deposition with and without buffer layers on basal plane (<1> oriented) bulk SiC to better understand the intrinsic structural characteristics of epitaxial GaN and the effect of growth conditions on the structure. In virtually all cases we find that simple double crystal rocking curves exhibit multiple peaks that cannot be interpreted with ease. In contrast, high resolution maps of the diffuse intensity distribution about the substrate and layer reciprocal lattice points have been very useful in articulating the defect structure of these materials. Typically, the SiC 6 reflection exhibits very little broadening parallel to the <6> direction but showed a relatively uniform intensity distribution perpendicular to <6>, suggesting large mosaic spread but little strain. In the case of the epitaxial GaN layers, the extent of the diffuse scatter off the 6 point is a convenient and reliable semi-quantitative measure of the defect structure in these materials. Our results demonstrate the utility of high resolution triple crystal diffraction methods in generating useful relationships between the epitaxial growth conditions and structural characteristics of GaN/SiC materials. Introduction Although silicon-based technologies are being widely utilized for a variety of electronic devices, the band structure of silicon makes it very inefficient for use in photonic, light-emitting applications. Because of its indirect bandgap, photon emission from silicon requires phonon absorption or simultaneous emission, which occur only infrequently. Thus the probability of an electronic transition successfully ejecting a photon is greatly reduced, and other materials must be utilized for applications that necessitate a high number of radiative transitions. Direct bandgap materials filling this requirement include various III-V systems such as nitrides, SiC, and II-VI semiconductors. These materials have a broad range of bandgap energies that can generate photons covering the spectrum from infrared to ultraviolet [1]. There is great interest in the heteroepitaxial growth of GaN and its ternary alloy (Ga,Al)N due to the potential use of these materials in advanced optoelectronic applications. The primary interest in GaN heterostructures arises from their potential importance for optoelectronic applications, more specifically blue-green laser diodes. Since they possess wide, direct bandgaps (approximately 3.5 ev for GaN and 6.2 ev for AlN), one of the many possible applications for these nitrides, combined with existing group III-arsenide based red-emitting devices, would be red-green-blue color displays. In fact, color displays using discrete assembled nitride and arsenide LED s have been demonstrated. Other fields investigating the advantages of blue-green laser devices include optical recording, communication, defense, and other scientific applications. 3

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright (C) JCPDS-International Centre for Diffraction Data Material a (Å) c (Å) GaN AlN SiC Table 1: Lattice parameters of heteroepitaxial and substrate materials As with most electronic devices, the realization of the aforementioned applications will heavily depend upon the ability to predict and control defects in the system. Undoped GaN is usually n- type, with electron mobilities ranging from 2 to 5 cm 2 /V-sec, and carrier concentrations from to /cm 3. Nitrogen vacancies are commonly described as the mechanism for conductivity in GaN [3]. Common methods for growing GaN-based heterostructures include molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Often the growth is performed on substrates such as SiC, which offers a favorable combination of similarity in lattice parameter and crystal symmetry when compared to GaN. Unfortunately, SiC itself often exhibits a relatively high degree of structural imperfection [2] ; this can be reflected in a degradation of the structural quality of epitaxial GaN or (Ga,Al)N grown on SiC. An additional concern when growing these heteroepitaxial structures is the mismatch in lattice parameter between the different films. The larger the discrepancy between the distances separating crystallographic planes of atoms from one material to the next (i.e., the larger the difference in lattice parameter), the greater the strain developed at the interface between those two films. This strain is usually accommodated by the generation of misfit defects, such as dislocations, which have a detrimental effect on electro-optical properties. Because of both its matching lattice parameter and its crystallographic similarities to these nitrides, silicon carbide is a widely used substrate for these systems. All materials used in this study (SiC, AlN, and GaN) are of space group P6 3 mc. Table 1 shows the lattice parameters of these materials. If the growth is performed on SiC (1), then the mismatch parallel to the growth interface (in the a direction) is 3.8% for GaN and 1.2% for AlN. With such a large mismatch, misfit defects will be generated in all films, thereby reducing the electrical and optical performance of these materials. However, using a buffer layer of AlN between the substrate and a GaN film can minimize this effect. In an attempt to observe the nature of the defects in these structures, several characterization techniques have been applied to heteroepitaxial films. Some of the most popular methods involve X-ray techniques. A variety of diffraction methods have been applied to GaN heterostructures [1, 4-6]. For example, double crystal rocking curves (DXRC) are diffraction scans that involve rotation of the sample axis, ω, small angular displacements from a Bragg reflection while leaving the detector stationary. The breadth of the peak, which is often reported as the full angular width at half the maximum intensity (FWHM), is an indicator of the amount of defects in a sample. Reciprocal space maps (RSM) have recently gained popularity for more precisely determining the quality of epitaxial structures. In this high resolution technique, multiple scans are taken from a diffractometer in triple crystal mode (i.e., a diffractometer with a multiple reflection incident beam monochromator and a post-specimen stationary analyzer crystal). Defects introduced to the structure will broaden the observed diffraction peaks in two ways. First, compositional variations

4 Copyright (C) JCPDS-International Centre for Diffraction Data S/λ S o /λ AlN SiC GaN and/or strain in the lattice will locally change the lattice parameter. This will be manifest in variations from the exact Bragg condition in the θ/2θ direction (parallel to the l direction). Secondly, mosaic spread in the sample will create extra intensity away from the Bragg angle when rotating the sample axis, ω (perpendicular to the l direction). The resulting data is ususally plotted as a contour plot with one axis representing a rotation in ω and the other representing movement in θ/2θ. The contours represent equal diffracted intensity regions in reciprocal space. When working with RSMs from AlN/GaN/SiC heterostructures, the c lattice parameters presented above indicate that the d-spacing is largest for the gallium nitride and smallest for the aluminum nitride. This means that the diffracted intensity distribution from gallium nitride 2 should have the smallest reciprocal space coordinates and the aluminum nitride 2 the largest, with an intermediate peak from the silicon carbide 6 (Figure 1). Because of the thickness of the epitaxial layers, this work assumes that are layers are fully relaxed. However, bulk silicon carbide is frequently structurally defective, and many processing variables exist during growth that can potentially affect the final structure of the heteroepitaxial films. Thus, the main purpose of this work is to determine through high resolution X-ray diffraction the factors that influence the quality of epitaxial GaN and (Ga,Al)N grown on SiC substrates. Experimental H AlN, 2 H SiC, 6 H GaN, 2 Figure 1. Theoretical distribution of diffracted intensity in reciprocal space Growth was performed with MOCVD using trimethyl gallium (TMGa), trimethyl aluminum (TMAl), and ammonia (NH 3 ). Growth rates were approximately 2-3 µm/hr, and the nominal growth temperature for AlN was 11 C. Silicon carbide (1) substrates with an approximate 4 miscut were used. In this hexagonal material, the repeat distance is six atomic layers, meaning that the stacking sequence would be of the form ABCACB. This is often referred to as 6H-SiC. 2 X-ray diffraction characterization was performed using a Bede D 3 diffractometer system. This system is equipped with a two crystal, four reflection monochromator and a three reflection analyzer. The X-ray source was Cu Kα radiation from a Rigaku rotating anode generator. The primary focus of the high resolution X-ray diffraction characterization was on double crystal rocking curves and reciprocal space maps from symmetric reflections (l). Although recent work emphasizes the information obtained from asymmetric reflections such as the (114) [7], they were not necessary in this study because of the wealth of information obtained from the reciprocal space maps of symmetric reflections. The effect of several parameters on epitaxial quality was investigated. These included the aluminum content of the heteroepitaxial AlGaN layer, the presence of a graded AlGaN layer between the SiC substrate and GaN layer, and the growth temperature of the AlN.

5 Copyright (C) JCPDS-International Centre for Diffraction Data Results AlN/GaN/SiC heteroepitaxial structure Figure 2 illustrates the general schematic of a typical heteroepitaxial structure in this system. A substrate of (6) 6H silicon carbide was used. On top of the SiC, a.14 µm thick layer of n-type aluminum gallium nitride was grown with a nominal aluminum concentration of 34%. The next epitaxial layer was a 1. µm thick layer of n-type gallium nitride, followed by a cap layer of.1 µm undoped aluminum nitride. The reciprocal space map corresponding to the structure described above is shown in Figure 3. On the axes of the reciprocal space map, q is.1 µm undoped AlN 1. µm n-type GaN.14 µm n-type AlGaN (34% Al) (1) 6H-SiC Figure 2: Nominal layer compositions and thicknesses in an AlN/GaN/SiC heteroepitaxial structure defined as the deviation from the exact Bragg condition, where q z represents deviation parallel to the l direction and q x signifies deviation perpendicular to that l direction. The peak of the SiC 6 was arbitrarily set to the q x = q z = position. The mosaic spread from the silicon carbide is particularly noteworthy. There are misorientation regions present at q x values of approximately -45 and +7 µm -1, which are mimicked in the GaN intensity distribution. However, there is no surface streak visible from this sample (i.e., broadening of peaks in the q z direction), which would indicate termination of a perfect crystal lattice. This confirms the high level of surface defects present in these materials. 1 5 AlN q z (µm)-1 SiC AlGaN -1-1 GaN Figure 3: Reciprocal space map from a typical heteroepitaxial structure, as shown in Figure 2

6 Intensity Intensity Copyright (C) JCPDS-International Centre for Diffraction Data Using Vegard s law, which assumes a linear dependence of lattice parameter on composition, the aluminum concentration can be experimentally determined by calculating the fractional position of the AlGaN peak between the AlN and GaN peaks. With this technique, the aluminum concentration in the AlGaN was found to be 22%, versus the expected 34%. This information was fed back to the crystal growers involved in this project. Once again, this work assumes fully relaxed layers. Figure 4 shows the (6) double crystal rocking curve from the same sample. This DXRC was taken with a wide open detector (no analyzer crystal), such that the diffracted wavevector was integrated over several degrees. The integration of the intensity is particularly deceptive for samples with large mosaic spread, with the result that significantly less information is available from this graph than from the reciprocal space map. For example, intensity from the AlN peak is added to the SiC peak, so that the resulting curve shows broad peaks from the substrate and thick GaN layer, with little to no information about the AlN and AlGaN layers. The effect of a graded AlGaN layer Rocking Angle (arcseconds) The first variable investigated in this work was the presence of a graded, undoped AlGaN layer between the GaN and SiC, instead of an AlGaN layer of given concentration. This structure and its double crystal rocking curve are shown in Figure 5. A (1) SiC was once again used as a substrate material. However, the first epitaxial layer grown on the substrate was a graded aluminum gallium nitride layer, instead of an AlGaN layer of homogeneous concentration. The next layers, though, remain essentially the same: a one micron layer of n-type gallium nitride, followed by a slightly thicker.4 µm layer of undoped aluminum nitride. Similar to the previous section, the double crystal rocking curve (Figure 5) for this material does Figure 4. Double crystal rocking curve of sample shown in Figure 2..4 µm undoped AlN µm n-type GaN Graded, undoped AlGaN (1) 6H-SiC Rocking Angle (arcseconds) Figure 5: Nominal structure and double crystal rocking curve from graded AlGaN sample

7 Copyright (C) JCPDS-International Centre for Diffraction Data not provide as much information as the reciprocal space map, shown above in Figure 6. The graded AlGaN layer is clearly shown in this reciprocal space map by the streak of intensity in the q z direction. From the constancy of the intensity distribution between the AlN and GaN, it appears to be graded quite uniformly. One could argue that a single longitudinal scan would show this grading effect. However, information about mosaic spread, as shown by the distribution of intensity in the q x direction, would not come through in a single longitudinal scan. Mosaic spread is indeed present in the SiC, and is mimicked by both the GaN and AlN layers. This spread is not Gaussian, but rather multi-modal with misorientation regions at q x values of -6 and +3 µm -1. The width in q x of the graded AlGaN layer of roughly 3 µm -1. The effect of aluminum content qz (µm) Figure 6: Reciprocal space map of graded AlGaN sample shown in Figure 4. The second parameter investigated was the effect of the aluminum content of the AlGaN on these heterostructures. Figures 7 and 8 (next page) show the samples studied and their respective reciprocal space maps. Both samples utilized the same (1) SiC substrate as the previous heteroepitaxial structures, followed by a graded, undoped layer of aluminum gallium nitride. The difference between these two and the previous examples lies in the top epitaxial layer. In Figure 7, the uppermost layer is a.8 µm thick layer of n-type aluminum gallium nitride with a nominal aluminum concentration of 34%, whereas the last epitaxial layer grown on the sample illustrated in Figure 8 had a.7 µm thick layer of n-type aluminum gallium nitride, with a significantly lower nominal aluminum concentration of 6%. In both of these samples, there is still mosaic spread in the SiC peak, which is dictating the spread in the heteroepitaxial layers. This is visible both in the q x width of the graded AlGaN layer, as well as in the presence of misorientation regions in the GaN layer at the same q x positions as

8 Copyright (C) JCPDS-International Centre for Diffraction Data µm n-type AlGaN (34% Al) Graded, undoped AlGaN (1) 6H-SiC qz (µm) Figure 7: Structure and reciprocal space map of sample containing n-type AlGaN with 34% Al.7 µm n-type AlGaN (6% Al) Graded, undoped AlGaN qz (µm) (1) 6H-SiC Figure 8: Structure and reciprocal space map of sample containing n-type AlGaN with 6% Al similar regions in the SiC substrate (+5 µm -1 in the higher aluminum content sample and -45 and +3 µm -1 in the lower Al content sample). Comparable misorientation regions in the AlN layer could not be distinguishable from background noise. -1-1

9 Copyright (C) JCPDS-International Centre for Diffraction Data The higher aluminum content of the sample in Figure 7 would decrease the lattice spacing, causing a shift in the intensity distribution to higher q z values. This is observed in that the main AlGaN peak is at a q z value of -11 µm -1 in the lower Al content sample, but at µm -1 in the higher Al content sample. Unlike the previous RSM (Figure 6), the grading of these AlGaN layers is not very uniform. Extra regions of intensity are present, especially at higher aluminum concentrations (i.e., larger q z values, such as +3 µm -1 ). This suggests that higher Al concentrations may be especially sensitive to MOCVD growth conditions. The sample with a higher aluminum content also has a tail of intensity leading toward the GaN peak position, possibly suggesting a variable Al concentration in the top AlGaN layer. The effect of AlN growth temperature.5 µm AlN 1. µm GaN.1 µm AlGaN (3% Al) (1) 6H-SiC Figure 9. Nominal structure for investigation of AlN growth temperature effects Lastly, the effect of the AlN growth temperature on the heteroepitaxial layers was investigated. Three samples were grown with the structure shown in Figure 9 above. The substrate, (6) 6H-SiC, is the same material used in previous samples. In this instance, the next layer grown was a.1 µm thick aluminum gallium nitride with a nominal aluminum concentration of 3%. This layer was followed by growing a 1. µm layer of gallium nitride, and then capped by a very thin (.5 µm) layer of aluminum nitride, whose growth temperature was variable. AlN growth temperatures of 55 C, 9 C, and 11 C were used in the three samples examined. Figure 1 shows the reciprocal space maps at the three growth temperatures. At 55 C, there is no AlN peak visible in the reciprocal space map, meaning that the AlN is either not depositing, or 1 55 C 9 C 11 C 1 1 q z (µm) q z (µm) q z (µm) Figure 1: Effect of AlN growth temperature on AlN/GaN/SiC heterostructures

10 Copyright (C) JCPDS-International Centre for Diffraction Data is amorphous. An intense AlN peak is present at 9 C, and a weaker peak exists at 11 C, implying that higher temperatures alters and perhaps reduces the crystalline perfection in the sample. A poor quality SiC substrate again adversely affected the quality of the heteroepitaxial layers: the mosaic spread present in the substrate at the low temperature is mimicked by the AlGaN and GaN layers. But in the samples used at higher temperatures, mosaic spread is virtually absent in the substrate, and subsequently lacking in the epitaxially grown layers. Conclusions Reciprocal space mapping of heteroepitaxial structures enabled diffracted intensity due to mosaic spread and compositionally graded layers to be clearly seen, providing much more information on the growth of these materials than conventional double crystal rocking curves. The crystal truncation rod, or surface streak, that is commonly observed in reciprocal space from high quality Si and GaAs samples is never seen in the GaN/SiC heterostructures, indicating that the diffraction properties of these samples is primarily kinetic in nature. Another observation from RSMs of these heteroepitaxial structures is that 9 C is the optimum growth temperature for the AlN cap layer. The lack of intensity from AlN at a lower temperature (55 C) indicates an absent or amorphous AlN layer, whereas decreased intensity at a higher growth temperature (11 C) suggests deterioration in the crystallinity of the sample. After analyzing of the parameters investigated, including the Al content of the AlGaN, the presence of a graded AlGaN layer, and the AlN growth temperature, the determining factor in the quality of the heteroepitaxial layers was the quality of the SiC substrate. Mosaic spread in the SiC substrates was a problem, due to the tendency of AlN, GaN, and AlGaN layers to mimick the mosaicity of the layer on which they are grown. References 1. W.J. Choyke and G. Pensl, MRS Bulletin, 22, (1997) 2. C.J. Eiting, P.A. Grudowski, and R.D. Dupuis, JOM, 49, 27 (1997) 3. N.D. Perkins, Doctoral Thesis, University of Wisconsin-Madison, (1997) 4. Saxler, M.A. Capano, W.C. Mitchel, P. Kung, X. Zhang, D. Walker and M. Razeghi, Mat. Res. Soc. Symp., 449, 477 (1997). 5. Lafford, N. Loxley and B.K. Tanner, ibid., p Goorsky, A.Y. Polyakov, M. Skowronski, M. Shin and D.W. Greve, ibid., p B. Heying, X.H. Wu, S. Keller, Y. Li, D. Kapolnek, B.P. Keller, S.P. DenBaars and J.S. Speck, Appl. Phys. Lett., 68, 643 (1996).