Control of Grain Size and Texture of Poly-Si with Atomic Hydrogen Under In Situ Ellipsometric Observation
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K. NAKAMURA, T. AKASAKA, D. He and I. SHIMIZU, Tokyo Institute of Technology, The Graduate School, Nagatsuta, Midoriku, Yokohama, Japan ABSTRACT A systematic study was performed to fabricate poly-Si thin films on glass substrates. Amorphous tissues were efficiently removed by forming a textured structure with the aid of atomic hydrogen. According to the real-time observation by spectroscopic ellipsometry(SE), the grain growth was enhanced by permeation of atomic hydrogen into the sub-surface. Hall measurements were made over a rather wide temperature range: 100 K--350 K. A rather high Hall mobility of 20 cm 2 /Vs or more was obtained at room temperature due to the reduction of amorphous tissues remaining at grain boundaries. INTRODUCTION
Poly-Si thin films are one of the systems of primary interest as materials for "Giant Microelectronics" fabricated on glass substrates. [1] Extensive attempts were made to deposit poly-Si thin films on glass by various methods such as laser annealing [2] and low temperature growth from the solid phase.[3] In spite of enthusiastic efforts toward this aim, so far, there were no techniques satisfying completely all the requirements for application to practical devices. As far as productivities are concerned, the plasma-enhanced PECVD has some attractive features for making a-Si:H on glass. Although microcrystalline Si thin films with grain size of 10-20 nm were made by the PE-CVD in an analogous manner to the fabrication of a-Si:H, so far the mobilities of carriers were insufficient for application to devices.[4] In recent years, poly-Si thin films with grains of 100-300 nm dia. were successfully grown on glass from fluorinated precursors made by PE-CVD.[5],[6] In addition, a field effect mobility of around 40 cmn/Vs was obtained for the films fabricated from the fluorinated precursors.[5] In this study, we investigated systematically the chemical reactions that promote grain growth by the aid of atomic hydrogen under in situ observation with spectroscopic ellipsometry. In addition, Hall effect measurements using cells in the Hall-bar geometry with 6 terminals 871
Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
were made to deepen our understanding of carrier-transport in poly-Si thin films. EXPERIMENTAL Poly-Si thin films were made by PE-CVD using two reactors, the conventional RF-glow of silane with assistance of atomic hydrogen and the remote-type plasma using microwave.[5] The real time observations of the growing surface were performed with a spectroscopic ellipsometer (Jobin-Yvon UVISEL). XRD, Raman, FTIR and TEM measurements were made to evaluate structures. Measurements of Hall effect were made over a rather wide temperature range (100 K to 350 K) for the n-type films using the Hallbar geometry with 6 Al-terminals made by photolithographic technology. RESULTS AND DISCUSSION Selection of precursors In PE-CVD, the selection of the chemical species resulting from plasma-induced decomposition is one of the important factors for the aim of controlling the
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