GaN on sapphire

The preceding bandgap values are given for low (2 - 10 K) temperature (T). Of primary importance is the temperature dependence of the bandgaps, in particular their value at room temperature (RT). From the previous discussion, different T dependencies are expected, due to the different strain state of the samples studied. For GaN on sapphire, both the residual epitaxial strain and the thermoelastic strain are compressive, whereas for GaN on SiC, the thermoelastic part of the residual strain is tensile. This leads to very different temperature dependencies between low T and RT, as shown in FIGURE 1. Note that results different from those shown on FIGURE 1 have also been reported [8],... 

TABLE 2 lists some determinations of the coefficients of the above equations, principally for homoepitaxial GaN and GaN grown on (0001) sapphire. In spite of the scatter in their values, the corresponding curves are very similar. The A excitonic gap is typically 70 meV lower at RT than at helium temperature for GaN on sapphire. The RT temperature coefficient of the bandgap is -0.45 +0.1 meV/K. Whether the A-B and A-C valence band splittings are nearly constant [23,25] or vary with T [21] is still controversial. 

FIGURE 1 Resistivity versus temperature for MBE, MOCVD and HVPE GaN on sapphire. 

FIGURE 3 Uncorrected (for degenerate interface layer) and corrected Hall concentrations for HVPE GaN on sapphire. The solid line is a theoretical fit. 

Compared to GaN on sapphire substrate, GaN on 6H-SiC tends to show a narrower XRC profile for both symmetrical and (10-10) diffraction as shown in TABLE 1. The narrowest figure was reported on bulk GaN grown by high pressure solution growth [15],... 

An co-mode high resolution XRD pattern of free-standing bulk GaN is shown in FIGURE 7 [8], The FWHMs (full width at half maximum) of 48 arcsec, 32 arcsec, and 38 arcsec for (0002), (101-1) and (101-2) planes, respectively, are more than one order of magnitude narrower than those of the best GaN on sapphire showing the high crystalline symmetry of the bulk crystals. 

FIGURE 4 Typical temperature programme for growth of GaN on sapphire. The sapphire substrate can be nitrided by flowing NH3 during the heat treatment. 

The compositions of the ternary layers were evaluated from the lattice parameters measured using X-ray diffraction (see Datareview A1.2) and from the positions of the photoluminescence peaks. Both methods gave the same results, if the bowing parameters of 3.2 eV and 0.1 eV, for InGaN and AlGaN, respectively, were used (as proposed by Takeuchi et al [25] for strained layers on relaxed GaN on sapphire). 

The regions of coalescence for GaN stripes oriented along [llOO] contained occasional voids which originated from the LE0-GaN/Si02 interfaces and terminated within the LEO-GaN film thickness. A continuous, epitaxial, nearly defect free GaN layer always formed above these voids. Continuous voids or cracks throughout the LEO-GaN films have not been observed, as is commonly reported for selective growth of GaN on sapphire substrates [15],... 

In this section we are concerned with epitaxial deposition. The word Greek taxis can mean an arrangement or a positioning. The Greek preposition epi in this context means upon. Epitaxial, then, means that the deposited layers are arranged on something namely, the substrate or layers already deposited. The particular arrangement is crystalline. The term epitaxial deposition is reserved for crystalline deposition. Epitaxial is further refined to include homoepitaxial and heteroepitaxial. In homoepitaxial deposition, the deposited material is the same as the substrate silicon on silicon and diamond on diamond are examples of homoepitaxial deposition. In heteroepitaxial deposition the deposited material is different from the substrate diamond on silicon or GaN on sapphire. 

D. J. Diaz, T. L. Williamson, I. Adesida, P. W. Bohn, and R. J. Molnar, Morphology evolution and luminescence properties of porous GaN generated via Pt-assisted electroless etching of hydride vapor phase epitaxy GaN on sapphire, J. Appl. Phys. 94, 7526-7534 (2003). 

H. Aida, S.-W. Kim, T. Suzuki, K. Koyama, N. Aota, T. Doi, T. Yamazaki, Surface planarization of GaN-on-sapphire template by chemical mechanical polishing for subsequent GaN homoepitaxy, ECS J. Sohd State Sci. Technol. 3 (2014) P163—P168. 

H Amano, I Akasaki, K Hiramatsu, N Koide. Effects of the buffer layer in metalorganic vapor phase epitaxy of GaN on sapphire substrate. Thin Solid Films 163 415, 1988. 

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