The three important classes of IR semiconducting materials are III-V alloys, II-VI alloys, and IV-VI alloys. The relevant elements of the periodic table are shown in Fig. 5.1. The elemental Group IV semiconductors have a zinc blende lattice structure and are characterized by a purely covalent bond; the direct bandgap at the Γ point of the Brillouin zone covers the range from insulating (diamond) through semiconducting (Si, Ge) to semi-metallic (Sn). However, the fundamental absorption edge in these Group IV semiconductors is indirect and is not determined by the direct gap at Γ ¼ 0. The III-V alloy semiconductors are also zinc blende in nature, but their IR absorption edges are direct and controlled by the direct bandgap at Γ ¼ 0. Their bonds are largely covalent but also contain a degree of ionic bonding, which is thought to play a role in the observation of the somewhat wider bandgaps found in these materials relative to their Group IV counterparts.1 The ionic aspect of the bond decreases the electronic wave functions between atoms, resulting in a larger amplitude in the variation of the periodic potential and a larger bandgap. II-VI alloys represent an extrapolation of the III-V alloy theme, with an even larger degree of ionic bonding and considerably larger bandgaps than the corresponding Group IVs. Their IR bandgaps are also direct at Γ ¼ 0 and possess the somewhat unique quality of extending down to 0 eV by the incorporation of semi-metallic binary alloys. The group IV-VI semiconductors have a rock salt crystal lattice and possess a largely ionic bond. They differ in that their direct bandgap lies at the edge of the Brillouin zone, at the L point, with virtually symmetrical conduction and valence bands.
It is of interest to examine the properties of these three semiconductor classes with a view to determining their potential impact on future-generation IRFPAs. The author is fully conversant with the vagaries of II-VI alloys, and these materials will be considered first, followed by III-Vs and then IV-VIs. These properties will be discussed primarily in a qualitative manner, so as to easily follow the simple physics of the semiconductor.
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