From Smart Electronic Materials: Fundamentals and Applications
5.2 IMPORTANT MATERIAL SYSTEMS
A large number of semiconductor systems are exploited for optoelectronic devices for detection and emission. The choice of the material depends upon a number of issues; some dependent on physics, some on technology, and some on market forces. We will discuss some of the important factors that play a role in the choice of a material for light detector or emitter. We will discuss inorganic semiconductors in this section. Organic semiconductors will be discussed later in this chapter.
Bandstructure
Optical processes are strongest for direct gap semiconductors because of the momentum conservation law. Materials like Si and Ge have weak optical properties. Indirect gap semiconductors can be used for light detection, but they are not suitable for light emission.
During the emission process the energy of the photon is very close to the bandgap energy, E g
| (5.1) |  |
and the wavelength is
| (5.2) |  |
The wavelength ? c is also the cutoff wavelength if the material is used as a detector, i.e., the material is transparent and, therefore, unresponsive to longer wavelengths. The need for a particular optical wavelength thus determines the bandgap needed and the materials used.
In Tables 5.1 and 5.2 we show some of the important materials and their bandgaps. Factors that are crucial in determining the desired wavelength include:
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As shown in Fig. 5.3, demands from optical communication, where 1.55 m and 1.3 m light has the lowest optical fiber loss and lowest dispersion, respectively.
Figure 5.3: Optical attenuation...
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