Transport Properties of Polycrystalline SiGe Thin Films Grown on SiO 2
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B9.22.1
Transport Properties of Polycrystalline SiGe Thin Films Grown on SiO2 Minoru Mitsui1, Keisuke Arimoto1, Junji Yamanaka1, Kiyokazu Nakagawa1, Kentarou Sawano2 and Yasuhiro Shiraki3 1 Center for Crystal Science and Technology, University of Yamanashi, Miyamae-Cho, Kofu 400-8511, Japan. 2 The University of Tokyo, Tokyo 113-8656, Japan. 3 Musashi Institute of Technology, Tokyo 158-8557, Japan. ABSTRACT Transport properties of polycrystalline Si1-xGex (x = 0, 30, 50 and 70) thin films on SiO2 were studied by Hall measurements and transport properties of the TFTs fabricated on the films were characterized. Si1-xGex films were p-type in spite of non-doping. Room temperature hole densities of Si1-xGex films increased from 5 x 1013 to 5 x 1016 cm-3 as Ge concentration increased from 30 % to 70 %. The acceptor levels in Si1-xGex were located at 0.43, 0.40 and 0.34 eV for x= 0.3, 0.5 and 0.7 from valence band, respectively. The high leakage current of SiGe-TFTs was observed and drain current could not be turned off even when the high gate voltage was applied. The acceptor density increased with increasing annealing temperature from 700 ºC to 800 ºC. The leakage currents were independent of the annealing temperature and is thought to originate from Ge-related defects in grain boundaries. INTRODUCTION The applications of thin film transistors (TFTs) to flat panel displays are very widespread. Polycrystalline Si (poly-Si) is used as the active layer for the TFTs. Low temperature crystallization of amorphous Si (a-Si) on SiO2 is a key to fabricate advanced TFTs. However the crystallization temperature of a-Si by solid phase crystallization (SPC) is higher than 600 ºC. Therefore, low cost glass substrate cannot be used because of its low softening temperature. Poly-SiGe film can be formed at the lower crystallization temperature than poly-Si film since the energy of Si-Ge bond is lower than that of Si-Si bond. The nucleation and growth mechanisms of SiGe have been investigated by many researchers [1-3]. However, there have been only a few reports on the electronic properties [4-6], and they were only on SiGe films with the Ge concentration less than 30%. In this paper, we have investigated the influence of Ge concentration (30%-70%) on transport properties of SiGe films by Hall measurements and characterized the transport properties of the TFTs fabricated on the SiGe films. The annealing temperature dependence of transport properties of SiGe films was also studied.
B9.22.2
EXPERIMENTAL DETAILS Amorphous-Si1-xGex (x=0, 0.3, 0.5 and 0.7) films with thicknesses of 100nm were deposited by solid-source molecular beam epitaxy (MBE) system on quartz substrates at room temperature. The MBE system consists of an e-gun evaporator for Si deposition and an effusion cell for Ge deposition. Raman spectroscopy and transmission electron microscopy analysis revealed that the as-deposited Si1-xGex films were amorphous and Si1-xGex films annealed above 700 ºC were polycrystalline. For Hall measurements, the Hall bar geometry was fabricate
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