Seeing (infra)Red: InGaAs Conquers Imaging, Sensing, Telecom
- Sunday, 01 January 2006
Indium gallium arsenide, or InGaAs, is an alloy of gallium arsenide and indium arsenide. In a more general sense, it belongs to the InGaAsP quaternary system that consists of alloys of indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP), and gallium phosphide (GaP). As gallium and indium belong to Group III of the Periodic Table, and arsenic and phosphorus belong to Group V, these binary materials and their alloys are all III-V compound semiconductors.
Largely, the electrical and optical properties of a semiconductor depend on its energy bandgap and whether the bandgap is “direct” or “indirect.” The energy bandgaps of the 4 binary members of the InGaAsP quaternary system range from 0.33 eV (InAs) to 2.25 eV (GaP), with InP (1.29 eV) and GaAs (1.43 eV) falling in between. A semiconductor will only detect light with photon energy larger than the bandgap, or, to put it another way, with a wavelength shorter than the cutoff wavelength associated with the bandgap. This “long wavelength cutoff” works out to 3.75 μm for InAs and 0.55 μm for GaP, with InP at 0.96 μm and GaAs at 0.87 μm.
By mixing two or more of the binary compounds, the properties of the resulting ternary and quaternary semi-conductors can be tuned to intermediate values. The challenge is that not only does the energy bandgap depend on the alloy composition, so also does the resulting lattice constant. The lattice constants range from 5.4505 Å (GaP) to 6.0585 Å (InAs), with GaAs at 5.6534 Å and InP at 5.8688 Å. The relationship between the lattice constant and the long wavelength cutoff of the four ternary alloys in the InGaAsP family are shown in Figure 1.
The InAs/GaAs alloy is referred to as InxGa1-xAs, where x is the proportion of InAs and 1-x is the proportion of GaAs. The lattice constants and long wavelength cutoffs of these alloys are depicted as red lines in Figure 1. The challenge is that while it’s possible to make thin films of InxGa1-xAs by a number of techniques, a substrate is required to hold up the thin film. If the thin film and the substrate do not have the same lattice constant, then the properties of the thin film will be severely degraded.
For a variety of reasons, the most convenient substrate for InxGa1-xAs is indium phosphide. High-quality InP substrates are available with diameters as large as 100 mm. InxGa1-xAs with 53 percent InAs is often called “standard InGaAs” without noting the values of “x” or “1-x” because it has the same lattice constant as InP and, therefore, the combination leads to very high-quality thin films.
Standard InGaAs has a long wavelength cutoff of 1.68 μm. The material is sensitive to the wavelengths of light that suffer the least signal dispersion and degredation in glass fiber (1.3 μm and 1.55 μm), it detects “eye-safe” lasers (wavelengths longer than 1.4 μm), and it is the optimum wavelength for detecting the natural glow of the night sky.