Silicon-Based Metal-Semiconductor-Metal Detectors
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Silicon-Based MetalSemiconductor-Metal Detectors
Basic Considerations Figure 1 shows the absorption coefficients for Si, Ge, and several compound semiconductors. The pronounced indirect bandgap of Si leads to the gentle increase of the absorption coefficient with energy, starting from the indirect gap value of 1.1 eV and growing slowly to a = 5 X 103 cm"1 at 2 eV. Note the striking contrast to the compound semiconductors with their direct bandgap, which show a steplike onset of the band-toband absorption as soon as their gap values are exceeded. Germanium actually is an indirect semiconductor with an indirect gap value of 0.76 eV (at 4 K). Since the direct gap of Ge is only slightly larger and amounts to 0.85 eV, its absorption is quickly dominated by direct processes. Therefore pure germanium is well-suited as an absorbing material for a photodetector at 1.3 or 1.5 /am.
Ch. Buchal and M. Loken Introduction Photodetectors must provide fast and efficient conversion of photons to charge carriers. When considering semiconductor light sources, the indirect bandgap of silicon and germanium represents a serious obstacle to radiative electron-hole recombinations. Momentum conservation demands the simultaneous interaction of the electron-hole pair with a momentummatching phonon. As a consequence, radiative recombinations are five orders of magnitude less probable in Si if compared to a direct semiconductor such as GaAs. Although the absorption of a photon and the generation of an electron-hole pair may be considered as the inverse process to emission, photon absorption within indirect semiconductors is a highly probable process if the photon energy is sufficient to bridge the energy gap in a direct process. The resulting electronhole pair is created in an excited state and relaxes sequentially. The ubiquitoussilicon solar cells operate this way. In the visible spectral range, Si photodetectors have demonstrated fast and efficient performance, being readily adapted for optoelectronic applications and being fully compatible to standard-silicon processing schemes. Optical interconnects (optolinks) operating with visible light are easy to install and service because "you see what you do" without infrared (ir) conversion optics. Probably these optolinks are best suited for data transfer within computers or within local computer clusters and between sensors and processors in machines, plants, vehicles, or air- and spacecraft. Silicon-based photodetectors lend themselves directly to these applications. Long-distance optocommunication uses the 1.3- and 1.5-/nm wavelengths. Silicon itself is highly transparent in this regime. To extend the operational range of siliconMRS BULLETIN/APRIL 1998
based devices into this regime, the use of a germanium layer is a very good choice. Another approach takes advantage of the internal photoeffect at the Schottky barrier between silicon and a metal. Silicon photodiodes are very popular detectors for the range from 0.4 to 1 /im. In general they use a p-n junction or a p-i-n structure, both in r
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