Microwave Plasma CVD of Silicon Nanocrystalline and Amorphous Silicon as a Function of Deposition Conditions

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 current-voltage (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%SiH 4 in He and H 2. In the process, SiH 4 is decomposed into SiH x . 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 SiH 4 :He, and power of 600 W. Best value of dark conductivity (σ d ) was 1.53 × 10 -9 S/cm and1.58 × 10 -5 S/cm for photo conductivity (σ p and low value of σ d indicate material with fewer defects. Adding extra H 2 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/ 500°A of aSi:H/p-Si wafer/Ohmic contact. These cells gave an open circuit voltage (V oc ) = 0.51 (V) and short circuit current density (J sc ) = 5.5 mA/cm 2 under 50mW/cm 2 tungsten halogen lamp. Thin film transistors using nc-Si, with gate length/width (L/W) =450/65 gave field effect mobility of 18 cm 2 /V-s, and Ion/Ioff of 1.25×10 5 .