Microwave Plasma CVD of Silicon Nanocrystalline and Amorphous Silicon as a Function of Deposition Conditions
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Microwave Plasma CVD of Silicon Nanocrystalline and Amorphous Silicon as a Function of Deposition Conditions J-H Jeung, Hak-Gue Lee, Lihong Teng and W.A. Anderson, SUNY at Buffalo, Electrical Engineering, 208 Bonner Hall, Buffalo, NY, 14260. Abstract Using ECR-CVD (electron cyclotron resonance-chemical vapor deposition) , we can make amorphous-silicon (a-Si) and nanocrystalline (nc-Si) thin films. We are looking forward to improve the photo/dark conductivity ratio ( σ p / σ d ) by measuring the photo and dark currentvoltage (I-V). In the ECR deposition, there are several factors which we can control and adjust for improved results, such as the amounts of silane and argon, the vacuum, and the temperature of the substrate. These become the critical factors for ECR deposition in order to make better films. Input gases consist of Ar, 2%SiH4 in He and H2. In the process, SiH4 is decomposed into SiHx. A residual gas analyzer (RGA) gives composition in the plasma. Because Ar possibly etches the substrate and Si is to be deposited, the best RGA signal is obtained with low Ar content. This work serves to correlate process conditions, RGA signals and electrical data. The best RGA signal occurs for 5 mTorr Ar, 60 mTorr SiH4:He, and power of 600 W. Best value of dark conductivity ( σ d ) was 1.53 x 10-9 S/cm and1.58x 10-5 S/cm for photo conductivity ( σ p ). High value of σ p and low value of σ d indicate material with fewer defects. Adding extra H2 improves the photo-conductivity ( σ p ). Applications of these films are heterojunction solar cells and thin film transistors. The heterojunction solar cell had a structure of metal grid/ 500oA of aSi:H/p-Si wafer/Ohmic contact. These cells gave an open circuit voltage (Voc) = 0.51 (V) and short circuit current density (Jsc) = 5.5 mA/cm2 under 50mW/cm2 tungsten halogen lamp. Thin film transistors using nc-Si, with gate length/width (L/W) =450/65 gave field effect mobility of 18 cm2/V-s, and Ion/Ioff of 1.25x105. Introduction ECR plasma deposition can control the properties of the plasma so that it can be applied to deposit nanocrystalline silicon (nc-Si) thin films or amorphous silicon (a-Si) for thin film transistors (TFT’s) and solar cells [1]. With ECR deposition, ion bombardment and etching during growth can be controlled. Also, low pressures can minimize the radical-radical reactions. Other benefits of rf- generated plasma processing includes high fraction of ionization and dissociation, high electron kinetic temperature, and no need for an electrode inside the chamber which can reduce the contamination of samples [2]. In-situ mass spectroscopy [3] is used in the analysis of the plasma during the deposition. In the ECR process, we need to induce a good argon plasma and also reduce the defects from argon ions. Therefore, before beginning the deposition, it is helpful to check the RGA (residual gas analyzer) signals and try to obtain a good signal from the RGA. By using excess hydrogen, we can reduce dangling bonds that are present in the a-Si. This produces a-Si:H. It can improv
