Homoepitaxial GaN

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. 

As a consequence E2 varies with substrate material. Taking homoepitaxial GaN on bulk GaN as a reference, variations as large as the values in TABLE 5 can be found [30]. 

Comparative studies of diffusion of Zn into heteroepitaxial GaN layers on sapphire and bulk pressure grown crystals showed that dislocations play a very important role in the diffusion process in heteroepitaxial GaN layers [29], The penetration of Zn into bulk crystal is a few orders of magnitude slower than into the heteroepitaxial layer. A similar effect of reduced diffusion due to the reduced density of dislocations can be expected for homoepitaxial GaN layers. Consequently this can improve the quality of GaN p-n junctions. 

FIGURE 3 Temperature dependent photoluminescence of a homoepitaxial GaN layer (0.4 p.m) grown by RMBE. At 4 K photoluminescence linewidths of the bound excitons are as narrow as 0.5 meV (after [31]). 

S. Porowski, Bulk and homoepitaxial GaN-growth and characterization, J. Cryst. Growth 189-190 (1998) 153-158. 

R. J. Molmr, Hydride Vapor Phase Epitaxial Growth of TTI-V Nitrides T. D. Moustakas, Growth of III-V Nitrides by Molecular Beam Epitaxy Z. Liliental-Weber, Defects in Bulk GaN and Homoepitaxial Layers C G. Van tie Walk and N. M. Johnson, Hydrogen in III-V Nitrides... 

GaN, relaxed homoepitaxial layers, Mg-doped bulk (low free-electron concentration) 3.1885 0.0003 5.1850 0.0001 [6]... 

II) slightly strained homoepitaxial layer of a small free electron concentration (about 1017 cm 3) on highly conductive GaN substrates ... 

Bulk plate shaped GaN crystals do not have threading dislocations along the c-axis which would end at the (0001) surfaces. This is very different in comparison with GaN layer crystals grown on any substrate. It is also important with respect to application of these plates as substrates for homoepitaxial growth, since threading dislocations in a substrate propagate into the epitaxial layers. 

Based on TEM studies of the defects in bulk GaN it appears that this material is suitable as a substrate for epitaxial growth [30]. In addition, optimisation of gas flow and purity, and also cleaning of the bulk surfaces are necessary to obtain high crystal quality in the homoepitaxial layers... 

GaN crystals of both low and high electric conductivity can be grown under high pressure of nitrogen. Routinely grown GaN single crystals in a form of hexagonal platelets with surface area 60 - 100 cm2 and dislocation densities lower than 105 cm 2 can be used as substrates for homoepitaxy. 

Bulk growth of GaN and AIN has been achieved by a sublimation method and a sublimation sandwich method. Bulk GaN and AIN bulk crystals were proved to have high crystallinity. It will improve the quality of nitride-based optoelectronic devices, if these bulk crystals are used as substrates for homoepitaxial growth. The size of the bulk GaN, however, is not large enough at this moment, and enlargement of bulk GaN may be necessary. 

The first homoepitaxial growth on high-pressure-grown single crystals of GaN was reported by Pakula et al [5] in 1996. Since then, a number of authors have performed similar experiments using as-grown GaN crystals as substrates [6-10], Despite many important scientific discoveries on homoepitaxial layers, the authors faced problems related to the lack of a proper surface preparation, which was the main reason that the layers were inhomogeneous (for example, variations of the half-width and the position of the photoluminescence peaks were observed). The recent development of the surface preparation made it possible to overcome this problem and to study the phenomena described below. 

FIGURE 1 X-ray rocking curve for the GaN homoepitaxial layer on the highly conductive substrate. 

State-of-the-art, RMBE grown GaN/sapphire exhibits photoluminescence at 2 K which is dominated by a free A exciton (FEA), with visible transitions of excited FEAs and two excited FEBs, and linewidths are as narrow as 3 meV [47], Using GaN substrates for homoepitaxial RMBE growth, two donor bound excitons with an energy spacing of 0.9 meV have been resolved for the first time in low temperature photoluminescence (see FIGURE 3) [48], Additionally, the free C exciton becomes visible at about 80 K. 

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