Substrates for epitaxial growth

When both donors and acceptors are present, compensation results, whereby the electrons supplied by the donor are given to the acceptor. Thus, the free carrier concentration can be considerably reduced below that expected from introducing a known donor or acceptor if the opposite type of dopant is unintentional. For example, semi-insulating (SI) InP (used as a substrate for epitaxial growth) can be made by incorporating low levels of Fe3+ as a deep acceptor (reduced to Fe2+) to compensate for unintentional n-type doping in the sample [19]. 

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... 

In earlier days of the GaN research, the r-plane sapphire has often been used as substrate for epitaxial growth mainly because this form of the sapphire was cheaper and most readily available at that time. There was also an expectation... 

Erom a crystallographic point of view, (1120) a-plane SiC is a better substrate for epitaxial growth of a-plane GaN than the r-plane sapphire. First, the layer and substrate have identical crystal orientation and the epitaxial relationships... 

In addition to these direct long-range forces there may also exist effective long-range forces, produced by some medium or substrate. An especially drastic effect is expected for epitaxial growth on a semiconductor. If adsorbate atoms are different from the substrate, the adsorbed layers have a lattice constant different from that of the substrate. In the case of thick adsorbate layers, an instability then appears on the surface of the crystal such that the surface undergoes wavy deformation, which might even lead to... 

Many of the various techniques associated with metal film preparation have recently been reviewed by Klemperer (76). Much of the catalytic work with thick continuous films has used a cylindrical reaction vessel (Fig. 7a). This cylindrical geometry permits a cylindrical sleeve of mica sheet to be inserved and used as the film substrate for epitaxial film growth... 

The single-crystal GaAs wafers are used as substrates for the growth of very thin layers of the same or other III-V compounds having the desired electronic or optical properties. Such crystal growth, in which the substrate determines the crystallinity and orientation of the grown layer, is called epitaxy, and a variety of epitaxial growth techniques are used in III-V display and device production. 

Epitaxial Layers. Epitaxial deposition produces a single crystal layer on a substrate for device fabrication or a layer for multilevel conductive interconnects which may be of much higher quality than the substrate. The epitaxial layer may have a different dopant concentration as a result of introducing the dopant during the epitaxial growth process or may have a different composition than the substrate as in silicon on sapphire. Methods used for epitaxial growth include chemical vapor deposition (CVD), vapor phase epitaxy (VPE), liquid phase epitaxy (LPE), molecular beam epitaxy (MBE) and solid phase epitaxy (SPE). 

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. 

A very close compensation between donors and acceptors is sometimes required to obtain, for instance for epitaxial growth, substrates with a resistivity close to the intrinsic one. In the case of GaAs, this can be realized nearly... 

III-V compound semiconductors with precisely controlled compositions and gaps can be prepared from several material systems. Representative III-V compounds are shown in the gap-lattice constant plots of figure C2.16.3. The points representing binary semiconductors such as GaAs or InP are joined by lines indicating ternary and quaternary alloys. The special nature of the binary compounds arises from their availability as the substrate material needed for epitaxial growth of device structures. 

Figure C2.I6.2 shows the gap-lattiee constant plots for the III-V nitrides. These compoimds can have either the wurtzite or zineblende struetures, with the wurtzite polytype having the most interesting device applications. The large gaps of these materials make them particularly useful in the preparation of LEDs and diode lasers emitting in the blue part of the visible speetrum. Unlike the smaller-gap III-V compoimds illustrated in figure C2.I6.3 single crystals of the nitride binaries of AIN, GaN and InN can be prepared only in very small sizes, too small for epitaxial growth of device structures. Substrate materials such as sapphire and SiC are used instead.

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