Extension of long wavelength response by modulation doping in extrinsic germanium infrared detectors

We have investigated a new concept for infrared detectors based on multilayer epitaxy and modulation doping. This permits a high doping concentration and lower excitation energy in the photodetecting layer as is necessary for longer wavelength response, without incurring the detrimental effects of increased dark current and noise as would be the case with conventional detector designs. Germanium photodetectors using conventional materials and designs have a long wavelength cutoff in the infrared at 138 μm, which can only be extended through the inconvenient application of mechanical stress or magnetic fields. As a result of this approach which we arrived at from theoretical considerations and subsequently demonstrated experimentally, the long wavelength cutoff for germanium extrinsic detectors was extended beyond 200 μm, as determined by direct infrared optical measurements.