Surface Treatments and Modifications of Si Based MIS Photodiodes
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Silicon (Si) based electronics have been widely developed and studied in industries. As an important branch of Si based electronics, photodiode technologies are widely used in photonic detection and the photovoltaic field. Low-cost thin-film Si is especially useful for solar cells. We studied the metal induced growth (MIG) of micro-crystal Si (mc-Si) and use of Schottky diodes to evaluate the mc-Si. This thesis focused on the fabrication of metal-insulator-semiconductor Schottky photodiodes combining with different types of Si, insulator layers, pre and post surface treatments. To grow mc-Si, 500Å thickness palladium (Pd) and 300Å thickness cobalt (Co) were evaporated as the metal catalyst on the silicon substrate covered with silicon dioxide. The 5 um thickness mc-Si films were deposited using two-step DC sputtering at 575°C. The Pd and Co were consumed to form silicide to provide nucleation sites for mc-Si growth at 50 W low power step. Then, mc-Si films were grown at a high deposition rate in the 150 W high power step. SiO 2 grown by thermal oxidation, Al 2 O 3 and HfO 2 achieved by atomic layer deposition were applied to MIG-Si, single crystal n-Si and single crystal p-Si as insulator layers. Au and Cr were used to form a Schottky junction on n-Si and p-Si, respectively. The I-V performance of all three groups of different Si based photodiodes under both dark and illumination conditions were illustrated in the thesis. The fill factor, barrier height, ideality factor and m-factor in log-log plot were calculated. SiO 2 acted as good insulator layer on all three different Si based MIS photodiodes. The photo I-V measurements of devices based on MIG-Si, n-type sc-Si and p-type sc-Si with SiO 2 as an insulator layer provided a Voc of 0.18, 0.2 and 0.44V; a J sc of 6.59×10 -4-7.39×10 -3 and 1.29×10 -2 A/cm 2 ; and a fill factor of 0.4, 0.28 and 0.39, respectively. In addition, Al 2 O 3 and HfO 2which are widely used in capacitors with high dielectric constants, were applied to MIS photodiodes by atomic layer deposition. To achieve high quality Al 2 O 3 or HfO 2 films, pretreatments on the silicon surface and post annealing of insulator film were used. As a result, pretreatment on single crystal n-Si showed a large improvement comparing with the same structure device with SiO 2 as an insulator. Ozone cleaned HF-last n-type sc-Si with 20Å HfO2 gave the best result: a V oc of 0.48 V, a J sc of 1.42×10 -2 A/cm 2and a fill factor of 0.37.