GaN homoepitaxial layers

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

Figure 14.13 Surface AFM irDages of (a) (1122) GaN substrate and GaN homoepitaxial layers grown at (b) 1050 and (c) 975 °C where the scale bar is drawn in (a). Peak-to-valley distances are (a) 13.9 nm, (b) 9.9 nm, and (c) 1.0 nm. (d) is a surface Normarski microscope image of the GaN homoepitaxial layer grown at 975 °C.

Figure 14.16 In-plane PL polarization property of a (1122) GaN homoepitaxial layer (open circles) and an InGaN/GaN MQW (closed squares) at 10 K. 0 = 0° corresponds to the [1100] direction, whereas 6 = 90° corresponds to the [1123] direction.

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 ... 

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... 

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. 

The crystallinity (such as defect density) and/or properties of GaN strongly depend on the growth conditions. HVPE GaN has a threading dislocation density of about 10 -10 cm, although GaN crystal quality has been improved in recent years and is expected to be improved, further. HVPE growth (Maruska Tietjen, 1969) is a very popular method of fabricating both crystalline substrates and homoepitaxial layers because of the high... 

Optical properties of GaN plates with a semipolar (1122) surface sliced from thick HVPE layers as well as of homoepitaxial layers grown by MOCVD on such semipolar substrates have also been reported [106,107]. As in the case of the nonpolar bars, low-temperature PL spectra exhibit narrow DBE lines and well-resolved Xa and Xb EEs. However, the polarized PL spectra measured in backscattering geometry show some peculiarities, the most prominent of... 

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. 

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]). 

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. 

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