Development of a "near-surface"-photodetector based on anomalous above-band-gap absorption band of SiGe

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Silicon-on-insulator (SOI) substrates have been in the focus of attention these days particularly from the perspective of developing high speed, less power-hungry electronics. The looming silicon-photonics on the other hand is to bring photonics/optoelectronics onto this otherwise purely electronic SOI chip. The problem then is that the conventional architecture of photonics/optoelectronics devices, at it stands, would not allow for a highly integrated circuit design, which can be traced back to the indirect band-gap of silicon that only weakly absorbs light thereby leaving a fairly large footprint.In this study, an attempt is made to create a new class of strongly-absorbing, on-chip quantum photodetector that takes advantage of an anomalous near-surface absorption band centered near 12eV developing in oxidized SiGe due to the relevant unique electronic states as opposed to those pertaining to the bulk. This above-band-gap absorption band was reproducible even in silicon-based systems short of Ge, which shows that it has to do with Si and hence its near-surface defects launched therein. A series of MSM (metal-semiconductor-metal) photodetecotrs prototypes with Schottky links was tested to examine such characteristics as spectral sensitivity in relation to wavelength selective detection, dynamic range of photoresponse, dark current, polarization-insensitiveness, and transient response including bandwidth reaching the subnanosecond domain. Interestingly, the absence of hole burning indicates that the 1.2eV band is not homogeneously broadened. Besides the issues that had been addressed in the original proposal, electroabsorption and electroreflectance were discovered during the course of this study, which holds considerable promise in building electro-optic modulator and tunable detector. The fabrication of an arrayed detector for mulitichannel spectroscopy and transparent fringe detection will remain as a subject of future study.