Silicon metal-semiconductor-metal photodetectors: Ion-implanted high-speed near-infrared photodiodes and position-sensitive photodetectors

Silicon metal-semiconductor-metal (MSM) photodetectors were studied for two applications: high-speed photodetection at near-infrared wavelengths (700-1000 nm) and high-resolution position detection. In the high-speed photodetection application, the long absorption length ({dollar}sim{dollar}5-20 {dollar}mu{dollar}m) inherent in silicon at these wavelengths result in an unacceptable diffusion tail in the impulse response of the photodetectors. Introducing a damaged layer {dollar}sim{dollar}1 {dollar}mu{dollar}m below the Si surface by a simple ion-implantation step reduced the diffusion tail while maintaining the higher mobility crystalline region above for carrier transport and making the Ni Schottky contacts. Two types of implant damage were studied: divacancies (from F-ion implantation) and nanovoids (from He-ion implantation followed by a high-temperature anneal). For both types of implantation damage, the bandwidth of the device at 980 nm increased from {dollar}<{dollar}1 GHz to more than 3 GHz for the nanovoid detectors and 9 GHz for the F-implanted detectors, with only a modest loss in quantum efficiency in the nanovoid detectors. While an increase in the absorption coefficient was measured for both types of implant damage, this did not translate into a higher signal in the actual devices because of the large number of recombination centers present in the implanted material. The ultimate effect of implantation was reduction in the diffusion tail (increase in speed) rather than both a higher signal and faster speed.; In the second application, the photocurrent from crystalline Si MSM photodiodes was monitored as a function of the laser beam position inside the photodetector's active area. One-dimensional (1-D) and two-dimensional (2-D) sensors were fabricated and position sensitivity was tested at different laser wavelengths, powers and laser spot diameters. A position resolution of 4 nm was achieved, limited by mechanical vibrations. The position resolution predicted from the electronic noise limit is 25 pm/{dollar}surd{dollar}Hz. A model for the position-sensitive signal incorporating surface recombination, diffusion and laser spot diameter was developed using a Green function technique.