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Silicon infrared photodetector using sub-bandgap transitions


This dissertation details the use of silicon as near- infrared photodetectors based on two different optical sub -bandgap transition mechanisms: band to surface state transition and band to sub-bandgap transition. For maximizing these effects and realizing ultra-high resolution imager arrays, a vertical nanowire structure is used. Nanoimprinting Lithography (NIL) is used to practically obtain nano-scaled patterns on a large area. In the case of band to surface state transitions, vertical silicon nanowire arrays have been fabricated through the use of UV NIL and conventional etching technology. They have an ability to detect pico watts level of light per nanowire at 1550nm IR light and a femto watt level at 635nm visible light. Their broad band detection spectrum and high sensitivity arise from the high surface to volume ratio. The high surface to volume ratio induces a built-in field in the radial direction, which confines the carriers in the core of the wire and substantially increases the lifetime of the confined carriers. These effects allow us to obtain a responsivity of 100A/W per nanowire to 1550nm light at 170K. Measurement results of our silicon nanowire arrays also show the limitation for the application of commercial products due to the high dark current at room temperature and the surface state uncertainty. Thus, we propose a core shell structure which frees the device characteristics from surface state effects while enormously enhancing the gain and reducing the dark current. IR detection in core shell nanowires can also occur by sub-bandgap transitions. To validate the core shell structure for the detection of IR wavelengths, the absorption coefficient model by sub-bandgap transition is proposed. To enhance absorption coefficients at IR wavelengths, the Franz-Keldysh effect, the spatial confinement effect, and the impurity state and quasi-2D state transition are considered in our physical model. To validate our proposed model, we also provide the method to measure the absorption coefficient value using a lossy medium model. The measured value is close to the simulation results, with an absorption coefficient of 10/ cm at 1480nm. These model and measurement results are very beneficial in designing high responsivity silicon IR detectors

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