Buried silicide mirror improves efficiency of resonant-cavity detector
Silicon-based photodetectors would be attractive for long-wavelength focal-plane arrays (FPAs) except for their low quantum efficiency. Readout circuits on FPAs are generally silicon-based, and integrating them with silicon detectors would avoid such problems as thermal mismatch that result from hybridized approaches. Researchers at the Defence Research Agency (Malvern, England) have demonstrated that a p-type silicon germanium-silicon (SiGe-Si) resonant-cavity device with quantum wells above a high-reflectance tungsten silicide layer can enhance photoresponse by a factor of eight. The current best performance comes from hybridized n-type gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) quantum-well infrared photodetectors. But the SiGe-Si device, using normally incident radiation at the resonance wavelength, creates a standing optical wave in the cavity that increases quantum efficiency to a value comparable to the GaAs/AlGaAs device. The researchers say that making the device so the quantum wells are only in regions of high electric field could enhance the photo response by as much as 16 times. The device operates in the 8-12-µm band. Varying device composition and the width of the quantum wells allows adjustment of the peak spectral responsivity to a desired wavelength. See www.dera.gov.uk/dera.htm.