Epitaxy compounds

T. L. Wade, Th. A. Sorenson, J. L. Stick-ney. Epitaxial compound electrodeposition in Interfacial Electrochemistry. Theory, Experiment and Applications (Ed. A. Wi ckowski), Marcel Dekker, New York-Basel, 1999, p. 757. 

LPE is one of the earliest techniques used to produce epitaxial compound semiconductors. It also is one of the simplest and most reliable. In this technique, elemental or compound reservoirs are held at their melting temperatures. The temperature must be maintained very accurately. The deposition substrate then is passed over the melt and a film is allowed to solidify from solution. LPE systems are generally quartz tube furnaces with graphite boats for the melts, and slider assemblies with a load lock. The... 

III-V compound semiconductors with precisely controlled compositions and gaps can be prepared from several material systems. Representative III-V compounds are shown in tire 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 tire binary compounds arises from tlieir availability as tire substrate material needed for epitaxial growtli of device stmctures. 

Figure C2.16.2 shows tire gap-lattice constant plots for tire III-V nitrides. These compounds can have eitlier tire WTirtzite or zincblende stmctures, witli tire wurtzite polytype having tire most interesting device applications. The large gaps of tliese materials make tliem particularly useful in tire preparation of LEDs and diode lasers emitting in tire blue part of tire visible spectmm. Unlike tire smaller-gap III-V compounds illustrated in figure C2.16.3 single crystals of tire nitride binaries of AIN, GaN and InN can be prepared only in very small sizes, too small for epitaxial growtli of device stmctures. Substrate materials such as sapphire and SiC are used instead.

Germane is used primarily to produce high purity germanium metal or epitaxial deposits of germanium on substrates for electronics by thermal decomposition at about 350°C (see Germaniumand germanium compounds). 

The main advantages that compound semiconductor electronic devices hold over their siUcon counterparts He in the properties of electron transport, excellent heterojunction capabiUties, and semi-insulating substrates, which can help minimise parasitic capacitances that can negatively impact device performance. The abiUty to integrate materials with different band gaps and electronic properties by epitaxy has made it possible to develop advanced devices in compound semiconductors. The hole transport in compound semiconductors is poorer and more similar to siUcon. Eor this reason the majority of products and research has been in n-ty e or electron-based devices. 

Although a great number of compound semiconductor devices make use of epitaxy to form the cote vertical stmcture of the device, ion implantation (qv) is a powerful tool in creating both horizontal and vertical modifications to a device. Ion implantation can be used to dope a semiconductor either fi- or / -type by using appropriate species. Implantation can also be used to render a region semi-insulating or to initiate multilayer intermixing. 

Arsenic from the decomposition of high purity arsine gas may be used to produce epitaxial layers of III—V compounds, such as Tn As, GaAs, AlAs, etc, and as an n-ty e dopant in the production of germanium and silicon semiconductor devices. A group of low melting glasses based on the use of high purity arsenic (24—27) were developed for semiconductor and infrared appHcations. 

The crystallographic requirement for tire formation of G-P zones is that the material within the zones shall have an epitaxial relationship with the maUix, and tlrus the eventual precipitate should have a similar unit cell size in one direction as tha maUix. In dre Al-Cu system, the f.c.c. structure of aluminium has a lattice parameter of 0.4014 nm, and the tetragonal CuAl2 compound has lattice parameters a — 0.4872 and b — 0.6063 nm respectively. 

Recent texts have assembled impressive information about the production, characterisation and properties of semiconductor devices, including integrated circuits, using not only silicon but also the various compound semiconductors such as GaAs which there is no room to detail here. The reader is referred to excellent treatments by Bachmann (1995), Jackson (1996) and particularly by Mahajan and Sree Harsha (1999). In particular, the considerable complexities of epitaxial growth techniques - a major parepisteme in modern materials science - are set out in Chapter 6 of Bachmann s book and in Chapter 6 of that by Mahajan and Sree Harsha. 

Generally, epitaxial films have superior properties and, whenever possible, epitaxial growth should be promoted. The epitaxial CVD of silicon and III-V and E-VI compounds is now a major process in the semiconductor industry and is expected to play an increasingly important part in improving the performance of semiconductor and optoelectronic designs (see Chs. 13-15). 

Silicon Epitaxy. Silicon epitaxial films have superior properties. The applications are, however, limited by the high temperature of deposition, which is generally above 1000°C. These reactions use chlorinated compounds of silicon (tetrachloride, trichlorosilane, or dichlorosilane) as precursors as follows ... 

Most of these compounds were originally prepared by liquid-phase epitaxy. That process is now largely replaced by MOCVD, particularly in the case of gallium arsenide, gallium arsenic phosphide, and gallium aluminum phosphide. 

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