Rapid Thermal Annealing of Amorphous Silicon Thin Films Grown by Electron Cyclotron Resonance Chemical Vapor Deposition
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Rapid thermal annealing of amorphous silicon thin films grown by electron cyclotron resonance chemical vapor deposition Pei-Yi Lin, Ping-Jung Wu and I-Chen Chen* Institute of Materials Science and Engineering, National Central University, Jhongli 32001, Taiwan
ABSTRACT Hydrogenated amorphous silicon (a-Si:H) thin films were deposited on pre-oxidized Si wafers by electron cyclotron resonance chemical vapor deposition (ECRCVD). The rapid thermal annealing (RTA) treatments were applied to the as-grown samples in nitrogen atmosphere, and the temperature range for the RTA process is from 450 to 950 °C. The crystallization and grain growth behaviors of the annealed films were investigated by Raman spectroscopy and X-ray diffraction (XRD). The onset temperature for the crystallization and grain growth is around 625 ~ 650 °C. The crystalline fraction of annealed a-Si:H films can reach ~ 80% , and a grain size up to 17 nm could be obtained from the RTA treatment at 700 °C. We found that the crystallization continues when the grain growth has stopped. INTRODUCTION Due to its extensive applications on solar cells and thin film transistors, the hydrogenated amorphous silicon (a-Si:H) film has been fabricated by various techniques, such as by glow discharge , plasma enhanced chemical vapor deposition (PECVD), hot wire chemical vapor deposition (HWCVD) and electron cyclotron resonance chemical vapor deposition (ECRCVD) . Among various growth methods of silicon thin films, ECR-CVD is a promising, low temperature process to achieve high deposition rate and expected to be a potential approach for the next generation of Si thin film solar cell production since ECR plasma can generate high electron density, low energy ions and highly active species. Recrystallization of a-Si:H has been studied as an approach for the production of inexpensive and lower defect-density nanocrystalline and polycrystalline thin films. Nanocrystalline Si has better optical and electrical properties than a-Si:H in terms of the device stability under light soaking in solar cells and gate bias stress in thin film transistors. The thermal annealing treatment is a way to enhance the grain growth and crystallinity, and reduce the defects in the as-grown a-Si:H films, thus they could highly improve the performance of Si thin film devices. Three conventional thermal treatments are used to anneal the material: excimer laser annealing (ELA), furnace annealing (FA), rapid thermal annealing (RTA). Among them, RTA process is most desirable because of its simplicity, fast processing and low capital cost. However, there has been little work done on investigation of annealing effects on ECRCVD-grown films. In this study, we investigated the post-growth annealing effects on ECRCVD-grown a-Si:H films by RTA processes. Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for characterization and structure examination.
EXPERIMENT The undoped a-Si:H films were deposited on thermally oxidized Si wafers to a thic