Article pubs.acs.org/crystal
Integration of GaN Crystals on Micropatterned Si(0 0 1) Substrates by Plasma-Assisted Molecular Beam Epitaxy Fabio Isa,*,†,‡ Caroline Chèze,§ Marcin Siekacz,∥,⊥ Christian Hauswald,§ Jonas Laḧ nemann,§,@ Sergio Fernández-Garrido,§ Thomas Kreiliger,† Manfred Ramsteiner,§ Yadira Arroyo Rojas Dasilva,# Oliver Brandt,§ Giovanni Isella,‡ Rolf Erni,# Raffaella Calarco,§ Henning Riechert,§ and Leo Miglio∇ †
Laboratory for Solid State Physics, ETH Zurich, Otto-Stern-Weg 1, CH-8093 Zurich, Switzerland L-NESS and Department of Physics, Politecnico di Milano, Via Anzani 42, I-22100 Como, Italy § Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany ∥ Institute of High Pressure Physics, Polish Academy of Sciences, ul. Sokolowska 29/37, 01-142 Warszawa, Poland ⊥ TopGaN sp. z.o.o., al. Prymasa Tysiąclecia 98, 01-424 Warszawa, Poland # Electron Microscopy Center EMPA, Swiss Federal Laboratories for Materials Science and Technology, Uberlandstrasse 129, CH-8600 Dübendorf, Switzerland ∇ L-NESS and Department of Materials Science, Università di Milano-Bicocca, Via Cozzi 55, I-20125 Milano, Italy ‡
S Supporting Information *
ABSTRACT: We present an innovative approach to integrate arrays of isolated, strainfree GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donorbound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that a reduction of threading dislocations is achieved in their top part.
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INTRODUCTION GaN and related III-nitride compounds have recently received considerable attention because of the fabrication of highly efficient and bright visible- and UV light-emitting1−3 laser diodes made out of these materials.4,5 The integration of GaNbased optoelectronic devices on Si(0 0 1) substrates would benefit from the well-established, reliable, and cheap Si CMOS platform. Nevertheless, this integration process requires solutions to the problems related to the heteroepitaxial growth, namely, different crystal symmetry and large mismatch between lattice and thermal expansion coefficients. As already widely reported in the literature, the epitaxial growth of wurtzite GaN on planar6−11 and patterned12,13 Si(1 1 1) substrates is favored by the 3-fold surface symmetry, compared to the 4-fold surface symmetry of Si(0 0 1).14 Additionally, the use of deeply patterned substrates with faceted features at the micrometersize scale would be beneficial for reducing the density of dislocations,15,16 as well as for improving the light extraction efficiency17−19 and directionality20,21 in GaN-based lightemitting diodes. These issues call for the analysis of GaN © 2015 American Chemical Society
nucleation and growth on high-index Si surfaces, e.g., {1 1 3} and {15 3 23}, which is a challenging task not widely investigated so far.22,23 In this work, we demonstrate by plasmaassisted molecular beam epitaxy (PAMBE) an innovative approach to integrating regular arrays of strain-free GaN crystals on micrometer-sized faceted Si pillars deeply patterned in Si(0 0 1) substrates. The GaN crystals are eventually bounded by inclined {0 0 0 1} facets with optical properties comparable to those of similar structures deposited on planar Si(1 1 1) substrates.
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EXPERIMENTAL SECTION
The Si substrates consist of 3−7 Ω cm n-type 4 in. Si(0 0 1) wafers. The substrates are deeply patterned by the Bosch24 process, leading to the formation of arrays of square, 8 μm tall Si pillars, with a 2 μm long side and separated by 3, 4, and 5 μm wide trenches.25 After RCA Received: May 27, 2015 Revised: September 1, 2015 Published: September 14, 2015 4886
DOI: 10.1021/acs.cgd.5b00727 Cryst. Growth Des. 2015, 15, 4886−4892
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cleaning26 and oxide removal in 5% diluted HF, the patterned Si(0 0 1) substrates are loaded into the low-energy plasma-enhanced chemical vapor deposition (LEPECVD)27 reactor for epitaxial deposition of 2.5 μm of Si at 750 °C at a growth rate of 0.27 nm/s. Subsequently, they are diced in 1 cm × 1 cm chips (containing patterns with different pillar spacing) and cleaned by an oxygen plasma. Prior the substrates being loaded into the plasma-assisted molecular beam epitaxy (PAMBE) reactor, they are cleaned again using the same procedure described above. The PAMBE reactor (P600 MBE by DCA instruments) is equipped by radiofrequency N2 plasma sources (SVT Associates and Addon) for active N, and standard solid-source effusion cells for Ga and Al. The angle of the cells with respect to the substrate surface normal is 44°. The Ga coverage is monitored in situ during growth by reflection highenergy electron diffraction (RHEED) and laser reflectivity.28 In the MBE growth chamber, the Si substrates are heated to ∼875 °C to remove any residual SiOx from the surface. Subsequently, they are cooled to 580 °C for the growth of the AlN buffer layer. This buffer layer is deposited in two steps. First, 17 MLs of Al are deposited at 580 °C. Then the substrate temperature is ramped up to 850 °C, with a ramp rate of 40 °C/min, and the sample is nitridated for 180 s with an active N flux of 1.7 × 1014 atoms cm−2 s−1. Second, at 850 °C, the AlN buffer layer is grown with a thickness ranging from 50 to 200 nm with N and Al fluxes of 1.7 × 1014 and 3.4 × 1014 atoms cm−2 s−1, respectively. This procedure leads to a conformal (thickness variation within 20%) AlN buffer (see Figure S1) with a root-mean-square (rms) surface roughness of 5 nm on the Si pillar top that prevents the formation of amorphous silicon nitride and Ga etching of the Si surface.29 GaN growth is performed under intermediate metal-rich conditions30 at 610 °C. The N and Ga fluxes are 1.7 × 1014 and 4.6 × 1014 atoms cm−2 s−1, respectively. The continuous presence of approximately two Ga MLs30,31 on the sample surface during growth is controlled by RHEED and laser reflectivity.28 The nominal thickness tGaN of GaN crystals is varied between 410 nm and 4.5 μm. The morphological analysis of GaN crystals is performed with a scanning electron microscope (SEM) and an atomic force microscope (AFM) using Zeiss Ultra 55 field emission and Veeco Innova microscopes, respectively. The lateral growth of GaN on Si micropillars is studied by crosssectional SEM images after cutting through the middle of the crystal along the ⟨1 1 0⟩ Si direction using a Zeiss NVision 40 focused ion beam (FIB). The GaN crystal orientation on different Si facets is analyzed by electron backscatter diffraction (EBSD) using an EDAX DigiView IV EBSD detector attached to the scanning electron microscope. The analysis of threading dislocations is conducted using a Philips CM 30 transmission electron microscope (TEM) at 300 kV, and the sample is prepared by mechanical polishing, dimple grinding, and ion milling. The GaN polarity is assessed using a convergent beam electron diffraction technique and the simulation software JEMS. Confocal micro-Raman spectroscopy is performed at room temperature in backscattering geometry using a 473 nm laser focused to a spot size of 1 μm. The optical properties of GaN/AlN/Si crystals are investigated at room temperature (RT) and 10 K by means of both microphotoluminescence (micro-PL) and cathodoluminescence (CL). The micro-PL measurements are performed in confocal geometry using a 325 nm He−Cd laser, with a maximal power of 30 mW, focused on the sample surface by a 40× objective, giving a spot size of 1.5 μm. The micro-PL map with an area of 31 μm × 31 μm is attained by moving the sample in the ⟨1 1 0⟩ Si directions in 1 μm steps. The CL measurements are performed using a Gatan MonoCL 3 system attached to the SEM with an incident electron energy of 5 keV.
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Figure 1. (a−c) Top and (d) perspective view SEM images of (a) a 2 μm large Si pillar (5 μm wide trench) after the deposition of 2.5 μm of Si by LEPECVD. (b−d) GaN crystals deposited on top of Si pillars shown in panel a. In panel b, the GaN thickness tGaN = 3.2 μm, while it is 4.5 μm in panels c and d. In panel a, the different Si facets are delimited by blue dashed lines and labeled by x:{1 1 1}, y:{15 3 23}, and z:{1 1 3}. In panel d, the red arrows indicate the downwardoriented crystal areas.
exhibit 4 × {1 1 3} facets in the apex of the crystals that are surrounded by 4 × {1 1 1} facets and, close to the corners, 8fold symmetric {15 3 23} crystal facets. In Figure 1a, the {1 1 1}, {15 3 23}, and {1 1 3}, facets are indicated by the letters x, y, and z, respectively, and they cover a surface of 13.4 ± 0.2, 29.8 ± 0.2, and 0.50 ± 0.05 μm2, respectively. More information about the morphology of the pristine Si(0 0 1) pillars can be found in ref 25. Figure 1b−d shows top and perspective view scanning electron microscope (SEM) images of 3.2 and 4.5 μm GaN deposition by PAMBE on a dense array of Si pillars, each one faceted as shown in Figure 1a. Nucleation occurs everywhere on the Si profile, but (see Figure 1d) the GaN deposited on the Si(0 0 1) surface between Si pillars, on pillar vertical sidewalls, and on downward-oriented areas (indicated by red arrows) exhibits a rough morphology. In contrast, GaN deposited on the {1 1 1}, {1 1 3}, and {15 3 23} Si facets consists of compact layers with smooth surfaces (see Figure 2c and related
Figure 2. (a) EBSD analysis of the early stages of epitaxial growth: GaN 410 nm/AlN 100 nm deposited on different Si crystal facets x:{1 1 1}, z:{1 1 3}, and y:{15 3 23}. The color map indicates the crystal orientation perpendicular to its surface. (b) Cross-sectional SEM image of a [GaN 40 nm/(Al,Ga)N 10 nm] × 5 superlattice on AlN/ Si{1 1 1}. (c) AFM image of the GaN surface after deposition of tGaN = 3.2 μm on the x:{1 1 1} Si facets. (d) AFM profile along the ⟨1 0 1̅ 0⟩ direction extracted from the area marked by the dashed line in panel c.
RESULTS AND DISCUSSION
The epitaxial growth of 2.5 μm of Si by low-energy plasmaenhanced chemical vapor deposition (LEPECVD)27 on Si pillars with a flat (0 0 1) top surface leads to the formation of isolated and multifaceted crystals, as shown in Figure 1a. They 4887
DOI: 10.1021/acs.cgd.5b00727 Cryst. Growth Des. 2015, 15, 4886−4892
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Figure 3. SEM cross-sectional images of GaN/AlN crystals (a) with different GaN thicknesses (tGaN) of 0.41, 3.2, and 4.5 μm on Si pillars spaced by 5 μm and (b) with a thickness tGaN of 4.5 μm deposited on Si pillars with 3, 4, and 5 μm gap widths. The crystals are FIB cut along the ⟨1 1 0⟩ Si direction through their middle. The GaN crystal profile (labeled by d) is measured as a function of the vertical position (indicated by Z) and averaged over three different crystals from the same pattern (error in d is ±50 nm). Z is set to be 0 at the intersection between the {15 3 23} and downward-oriented Si facets. The labels hp and ht indicate the distance between GaN and AlN surfaces on the {111} Si facets and on the crystal apex, respectively.
Table 1. Growth and Structural Parameters of GaN Crystals Deposited on Patterned Si(0 0 1) Substratesa GaN nominal thickness tGaN (μm)
Si pillar gap (μm)
4.5 4.5 4.5 3.2 0.41 0.41
5 4 3 5 5 4
GaN crystal gap (μm) 0.71 0.32 0.24 1.95 2.66 1.57
± ± ± ± ± ±
0.06 0.06 0.06 0.05 0.05 0.05
hp (μm)
ht (μm)
± ± ± ± ± ±
3.4 ± 0.1 3.34 ± 0.09 3.4 ± 0.1 3.30 ± 0.06 0.40 ± 0.01 0.40 ± 0.01
2.33 2.33 2.33 2.26 0.28 0.28
0.05 0.09 0.09 0.09 0.01 0.02
hp/ht 0.69 0.70 0.69 0.68 0.70 0.70
± ± ± ± ± ±
0.03 0.05 0.05 0.04 0.04 0.07
hp/tGaN 0.52 0.52 0.52 0.71 0.68 0.68
± ± ± ± ± ±
0.08 0.09 0.09 0.09 0.04 0.06
The Si pillar gap and GaN crystal gap refer to the distances along the ⟨1 1 0⟩ direction between Si pillars before any epitaxial deposition and after GaN growth, respectively. Parameters hp and ht, as defined in Figure 2a, indicate the distance between GaN and AlN surfaces on the Si {1 1 1} facets and on the crystal apex, respectively. a
despite the initial 5 nm RMS surface roughness of the buffer, a smooth growth front is recovered after about 100 nm deposition. The low surface roughness and complete coalescence of GaN deposited on Si {1 1 1} facets is confirmed by the AFM image shown in Figure 2c, which has a rms surface roughness of 0.7 nm on an area of 700 nm × 700 nm. The AFM line scan along the ⟨1 0 1̅ 0⟩ direction (displayed in Figure 2d) reveals that the edges of GaN layers on Si {1 1 1} facets are highly stepped, consisting of {0 0 0 1} terraces with an ∼2.1 nm (8 MLs) step height. The nucleation of GaN microcrystals on different Si facets is investigated by electron backscatter diffraction (EBSD). The results are presented in Figure 2a. Here, the analysis of the crystal orientation shows that in the central part of the large x: {1 1 1} and y:{15 3 23} Si facets, GaN microcrystals have {0 0 0 1} planes parallel to the Si heterointerface. Convergent beam electron diffraction analysis indicates that GaN layers deposited on Si{1 1 1} facets are N-polar. In contrast, at the edges between adjacent facets and on the small z:{1 1 3} surfaces, the crystal orientation is random. Consequently, a high density of
discussion). The rough morphology of GaN layers in the former regions is mostly related to a different surface symmetry of cubic Si and wurtzite III-N that leads to the formation of a microcrystal with different orientations14 and high surface roughness.32,33 The rms surface roughness of GaN grown on Si(0 0 1), measured by an atomic force microscope (AFM) over an area of 5 μm × 5 μm, is 15 nm. Additionally, shadowing of the impinging atom fluxes due to the presence of closely spaced micrometer-tall GaN/Si crystals reduces the amount of deposited material, which in turn inhibits the coalescence of GaN microcrystals. The morphology of GaN crystals grown on Si pillars strongly depends on the thickness (tGaN) of the deposited material. Indeed, for a thickness tGaN of 410 nm (see Figure 2a), the morphology is characterized by uncoalesced microcrystals anywhere on the Si profile. With an increase in the amount of deposited GaN, they start to coalesce34 at tGaN = 1.0 μm (not shown here), and finally, for tGaN > 3 μm (see Figure 1d), they form compact and smooth layers on the Si {1 1 1} facets. The cross-sectional SEM image of Figure 2b displays a (GaN 40 nm/(Al,Ga)N 10 nm) × 5 superlattice deposited on a 50 nm thick AlN buffer layer on a Si {1 1 1} facet. It reveals that 4888
DOI: 10.1021/acs.cgd.5b00727 Cryst. Growth Des. 2015, 15, 4886−4892
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impinging atom fluxes affects only the lower part of the GaN crystals. Indeed, the accessible solid angle for the atomic Ga and N fluxes to impinge on the crystal surface increases monotonically with Z. The impact of the GaN morphology on the distribution of threading dislocations is elucidated by the TEM analysis of Figure 4. Here, three GaN crystal grains are found to be
stacking faults (SFs) is expected at the coalescence boundaries between adjacent facets. The growth profile and the morphology of GaN crystals are analyzed by first cutting them through the middle along the ⟨1 1 0⟩ Si direction by means of a focused ion beam (FIB), and then by measuring in cross-sectional SEM images the lateral dimension d as a function of vertical position Z. Figure 3a shows GaN crystal width d as a function of vertical position Z, for different GaN thicknesses on the same pattern with a 5 μm gap between 2 μm wide Si pillars. The distances between the GaN and AlN surfaces on the {1 1 1} Si facets and on the crystal apex are labeled by hp and ht, respectively, and the values are summarized in Table 1. Initially, GaN microcrystals resulting from a tGaN of 0.41 μm (see Figure 2a) are conformal to the Si pillars, and the dependence of d(Z) closely follows the shape profile of the underlying faceted Si crystal. With an increase in the amount of deposited material to 3.2 and 4.5 μm (see Figure 3a), the GaN crystals expand laterally, reaching a maximal value (trend indicated by the black arrow) of d = 5.6 μm at Z = 1.0 μm and d = 6.3 μm at Z = 1.8 μm, respectively. For GaN crystals with a thickness tGaN of 3.2 μm, in the Z range of 1.0−2.0 μm, the lateral dimension d stays constant, leading to the formation of vertical surfaces. The formation of vertical surfaces is ascribed to the progressively reduced distance between neighboring crystals with the increase in the amount of deposited material (see GaN crystal gap data reported in Table 1). Therefore, the flux of impinging atoms on each GaN crystal lateral surface is progressively limited by the shadowing of other surrounding structures that are getting closer with the increase in the amount of deposited material.35−37 This shadowing effect between adjacent GaN crystals is more evident in Figure 3b for a smaller Si pillar gap width. Finally, for larger values of Z, GaN crystals exhibit a growth front that is parallel to the {1 1 1} Si facets, terminating with a small flat top area, located at the intersection among the four inclined facets, which extends for ∼0.7 ± 0.2 μm2 (see also Figure 1b). Table 1 shows that for all GaN crystals, regardless of their thickness or Si pillar gap width, the ratio hp/ht stays constant within the experimental error at a value of ∼0.7. This result implies that the top part of GaN crystals is constituted by crystal facets parallel to the Si {1 1 1} surface. GaN crystals with a thickness tGaN of 4.5 μm have an hp/tGaN ratio lower by ∼25% than that for tGaN values of 0.41 and 3.2 μm. This effect could be tentatively attributed to the high stability of the crystal facets, which therefore inhibits the further evolution of morphology.38 The period of the Si pillar pattern influences the final morphology of GaN crystals, too. Figure 3b shows that with a decrease in the distance between Si pillars from 5 to 4 μm and to 3 μm, the initial lateral expansion of GaN crystals is inhibited, favoring their growth only in the vertical direction. The predominant factor responsible for the vertical growth is shadowing of the impinging atom fluxes by adjacent growing crystals and the reduced material diffusion length compared to the micrometer size of the features.35−37,25 Indeed, the Z range in which d is constant progressively decreases with the increase in the Si pillar gap. Interestingly, the value of ht for crystals with a tGaN of 4.5 μm is not affected by a different distance between Si pillars, implying that a larger volume of material is deposited on Si pillars with a wider gap width. Therefore, the shadowing of
Figure 4. Compilation of bright-field TEM images around the [4 2̅ 2̅ 3] zone axis of GaN (tGaN = 4.5 μm) grown on a Si pillar separated by a 3 μm gap. Three GaN crystal grains are nucleated on the different Si facets and can be distinguished by a different color contrast. The sample is not cut exactly through the middle of the Si pillar.
nucleated on the different Si facets. Because of the inclination of the Si facets and the vertical growth of GaN, threading dislocations running along the c-direction are mostly confined to the lower part of the crystal, resulting in a reduced density in the upper region. The spontaneous photon emission of GaN crystals deposited on patterned Si(0 0 1) substrates is investigated by means of cryogenic (10 K) and room-temperature (RT) microphotoluminescence (micro-PL) and cathodoluminescence (CL). The optical analysis is performed on GaN samples with a deposited thickness of >3 μm, guaranteeing smooth and coalesced GaN layers on Si {1 1 1} facets (see Figure 2c) on Si pillars separated by 5 μm wide gaps. Panels a and b of Figure 5 present the micro-PL spectrum of GaN deposited on Si {1 1 1} facets at RT and 10 K, respectively. Remarkably, at RT (Figure 5a), the micro-PL data reveal no detectable photon emission in the yellow spectral range, marked by a black dashed circle, between 2.1 and 2.25 eV. The absence of PL emission in the yellow spectral range indicates a reduced density of point defects, especially Ga vacancies and their complexes acting as deep acceptors.39,40 At a higher photon energy, the free A exciton transition (XA) peak dominates the spectrum, being located at 3.415 ± 0.001 eV. The micro-PL spectrum recorded at 10 K for a GaN crystal on Si {1 1 1} facets exhibits broad spectral features with a low integrated intensity in the photon energy range of 3.1−3.4 eV, and a high-intensity peak related to the neutral donor-bound A exciton (D0,XA) recombination at higher energy. 4889
DOI: 10.1021/acs.cgd.5b00727 Cryst. Growth Des. 2015, 15, 4886−4892
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Figure 5. Micro-PL spectra (a) at 300 K and (b) at 10 K of GaN on the Si {1 1 1} facet (see Figure 2c). In panel a, the dashed black circle shows the yellow spectral range between 2.1 and 2.25 eV where no PL signal distinguishable from the noise level is detected.
Figure 6. (a) Micro-PL intensity map at 10 K around the (D0,XA) emission superimposed with a top view SEM image of GaN crystals (tGaN = 3.2 μm; Si pillar spacing of 5 μm) taken in an analogous sample area. (b) Micro-PL spectra at 10 K of GaN (tGaN = 3.2 μm) deposited on Si {1 1 1} facets (blue curve) and on the Si(0 0 1) unpatterned area (black curve).
to those of the material deposited on the Si pillars. Indeed, Figure 6b compares the micro-PL spectra of GaN on a Si {1 1 1} facet (blue curve) and Si(0 0 1) surface (black curve). As described before, the exciton PL transition of GaN deposited on the Si {1 1 1} facet is located at 3.471 eV with a fwhm of 10 meV. Differently, in the case of GaN/Si(0 0 1), the PL exciton transition is broader (fwhm = 30 ± 1 meV) and red-shifted by 6 ± 1 meV. The red-shift of the PL exciton emission is ascribed to residual biaxial tensile strain due to different thermal expansion coefficients of Si, AlN, and GaN.51,52 The GaN biaxial tensile strain, responsible for the red-shift in the unpatterned Si(0 0 1) area, is calculated53,54 to be (5 ± 1) × 10−4. Cracks are observed (see Figure S2 of the Supporting Information) in the unpatterned GaN/Si(0 0 1) region, while they are absent for the material deposited on Si pillars. The residual tensile strain in the GaN/Si(0 0 1) unpatterned area is also measured by confocal micro-Raman microscopy. The GaN E2 (high) phonon transition of material deposited on Si pillars is observed at the same Raman shift of a strain-free GaN crystal (566.4 cm−1). Differently, it is red-shifted by 1.1 ± 0.2 cm−1 for GaN grown on the unpatterned Si(0 0 1) area (see Figure S3 of the Supporting Information). This finding indicates55,53,54 a residual biaxial tensile strain of (4 ± 1) × 10−4, in good agreement with the value derived from the micro-PL measurements. The room-temperature CL measurements shown in Figure 7 confirm the good crystal quality of GaN deposited on Si{1 1 1} facets. The material deposited on these facets exhibits an
The broad defect-related PL spectral features are attributed to carriers recombining radiatively on crystal defects, particularly on E, I1, and I2 stacking faults, approximately at energies of 3.30, 3.35, and 3.40 eV, respectively. The broadening of the PL spectral features and the energy of the peaks are related to the distribution of stacking fault thickness and their interaction with other defects.41−43 The (D0,XA) transition is centered at 3.471 ± 0.001 eV and exhibits a full width at half-maximum (fwhm) of 10 ± 1 meV. The energy of the exciton transition indicates that GaN on Si pillars is free of biaxial tensile strain.7,44 The fwhm of the (D0,XA) transition for GaN deposited with our approach on a patterned Si(0 0 1) substrate is comparable to that (range of 10−20 meV) of GaN layers grown by MBE45−47 or metal− organic CVD48 on Si(1 1 1) substrates and by far better than that of GaN layers deposited on (0 0 1) oriented and stress compliant silicon-on-insulator substrates.49 A GaN exciton transition with a fwhm of
