Homoepitaxially grown crystals

In Fig. 3a, cubic and octahedral faces are evident, and in Fig. 3b the twinned crystals with pseudo-fivefold symmetry can be clearly seen. This twinned fivefold symmetry is prevalent in CVD diamond thin films and apparently never develops on homoepitaxially grown crystals.Balllike diamond crystals are grown at high supersaturationsf (Fig. 3c). 

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. 

Regarding homoepitaxial growth of cBN on a cBN seed, a clear boundary between the grown crystal and the seed crystal was obtained when LiCaBN2 was used as a solvent (184,199). When LijBNj was used, it appeared difficult to make a clear interface. 

In molecular beam epitaxy (MBE) [317], molecular beams are used to deposit epitaxial layers onto the surface of a heated crystalline substrate (typically at 500-600° C). Epitaxial means that the crystal structure of the grown layer matches the crystal structure of the substrate. This is possible only if the two materials are the same (homoepitaxy) or if the crystalline structure of the two materials is very similar (heteroepitaxy). In MBE, a high purity of the substrates and the ion beams must be ensured. Effusion cells are used as beam sources and fast shutters allow one to quickly disrupt the deposition process and create layers with very sharply defined interfaces. Molecular beam epitaxy is of high technical importance in the production of III-V semiconductor compounds for sophisticated electronic and optoelectronic devices. Overviews are Refs. [318,319],... 

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. 

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. 

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. 

Molecular beam epitaxy. Epitaxial techniques are techniques of arranging atoms in single-crystal fashion on crystalline substrates so that the lattice of the newly grown film duplicates that of the substrate. If the film is of the same material as the substrate, the process is called homoepitaxy, epitaxy, or simply epi. The most important applications here are Si epi on Si substrates and GaAs epi on GaAs substrates. If the deposit is made on a substrate that is chemically different, the process is termed heteroepitaxy. An important application is the deposition of silicon on an insulator (SOI) e.g. with sapphire (AI2O3) as the insulator in the silicon on sapphire (SOS) process. 

SiC has been grown epitaxially by the methods of liquid-phase epitaxy (LPE), chemical vapor deposition (CVD), and molecular beam epitaxy (MBE). Because the epitaxial growth temperature of SiC is very high compared with other semiconductors, the materials suitable for the substrates are restricted to refractory crystals. SiC itself (homoepitaxial growth) and other materials, such as Si and TiC (heteroepitaxial growth), have been used as substrates. 

---