Improved temporal resolution in diffusion driven photodetectors

We recently demonstrated a photovoltaic detector based on the Dember voltage and optical interference effects.1 In brief, we created an interference pattern in a semiconductor with two electrodes. The interference pattern is positioned to generate a photocarrier grating with a maximum density at one electrode and a minimum at the other. The Dember voltage associated with this density difference decays at the same rate as does the photocarrier grating (i.e., D a k g 2 ). In effect, this device substitutes the time that it takes for a photocarrier grating to decay for the transit time. In the devices discussed previously, the grating period was limited to 1.4 μm by imaging optics and resulted in response times of a few tens of picoseconds. Here we report measurements on a new generation of these detectors that allow the grating period to be reduced to 0.1 μm and which have greatly reduced electrical parasitics. These devices are made from 500-Å silicon on sapphire wafers contacted to an electrooptic wafer. The second wafer allows us to perform electrooptic sampling and is required because these devices are photovoltaic and consequently cannot be used as a sampling gate.