9.2.5 Compound Semiconductors

Compound semiconductors are inorganic compounds containing two or more elements with the electrical properties of semiconductors. In principle, all types of semiconductor devices can be made from compound semiconductors but, in practice, silicon dominates the market for electronic devices. Compound semiconductors fill specialized niches, and are particularly important for LEDs, lasers, and optoelectronic devices.

The most important compound semiconductors for laser applications are the III V compounds, which contain equal amounts of elements from group IIIa and group Va of the periodic table. The important elements are listed below:

The simplest of these materials are "binary" compounds containing two elements, such as gallium arsenide (GaAs), indium phosphide (InP), and gallium nitride (GaN). Each of these compounds has its own set of characteristics, including energy levels, band gap, and atomic spacing in the crystalline lattice.

Other elements can be added to the compound as long as they maintain the balance of equal numbers of atoms from group III and group V, which have different valence. This is desirable because it allows adjusting properties of the semiconductor, particularly the size of the band gap. For example, replacing some gallium in gallium arsenide with aluminum increases the band-gap energy in gallium aluminum arsenide (GaAlAs). Such compounds containing three elements are called ternary and are written in the form Ga_{1 x}Al_xAs, where x is a number between 0 and 1. This format indicates what we said above, that the number of gallium atoms plus the number of aluminum atoms must equal the number of arsenic atoms.

Adding a fourth element to make a quaternary compound gives more flexibility and control over material properties. An example is indium gallium arsenide phosphide (InGaAsP), which is written In_{1 x}Ga_xAs_{1 y}P_y, where both x and y are numbers between 0 and 1. In this case, the total number of indium and gallium atoms must equal the number of arsenic and phosphorus atoms. It is also possible to have three elements in one group and only one in the second, such as indium gallium aluminum phosphide (InGaAlP), written as In_{1 x y}Ga_xAl_yP, where both x and y are numbers between 0 and 1 which together add to less than 1. In this case, the total number of indium, gallium, and aluminum atoms must equal the number of phosphorous atoms to form a semiconductor crystal.

Ternary and quaternary compounds are harder to fabricate into good crystals, but they offer an ability to control the band gap and lattice constant, which is invaluable in fabricating devices such as diode lasers. Practical concerns impose some limits on material characteristics. It is difficult to grow ternary and quaternary compounds in bulk, so they normally are deposited on substrate wafers made of binary compounds, mostly GaAs and InP. Successful deposition requires careful matching of the lattice spacing of the deposited compound with the substrate, limiting the blends that are easy to fabricate. This affects the composition and wavelengths of semiconductor lasers, as described later in this chapter.

Light emission has been demonstrated from two other families of compound semiconductors. One is silicon carbide (SiC), composed of two group IV elements with four valence electrons. The other family is called II VI compounds because they contain elements with two and six valence electrons. They come mostly, but not entirely, from columns IIB and VI of the periodic table. The most important elements in these compounds are:

The II VI compounds fall into two broad groups. Compounds of zinc and cadmium with group VI elements have large band gaps and can emit visible light. Compounds of lead and tin with group VI elements have small band gaps and emit in the infrared.