Growth and Characterization of Poly-SiGe Prepared by Reactive Thermal CVD
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Growth and Characterization of Poly-SiGe Prepared by Reactive Thermal CVD Jianjun Zhang a,b, Kousaku Shimizua and Jun-ichi Hannaa a Imaging Science and Engineering Laboratory, Tokyo Institute of Technology 4259 Nagatsuta Midori-ku Yokohama, 226-8503 Japan b Institute of Photoelectronic Thin Film Device and Technology, Nankai University, Tianjin 300071, China ABSTRACT We have prepared poly-Si1-xGex thin films with different germanium contents (Ge=5%~40%) by reactive thermal CVD. In this study, the Ge content was controlled by varying the source gases GeF4 flow rate at a fixed Si2H6 flow rate. The effects of GeF4 flow rate on growth rate, film crystallinity, and electrical properties were investigated. The films were always polycrystalline when GeF4 was introduced even in a small amount, while only amorphous film deposited without GeF4. With an increase in GeF4 flow rate, Ge content and conductivity of the films increased and its activation energy decreased. When GeF4 flow rate over a certain value, the growth rate decrease and finally no film could be deposited. These behaviors are discussed in relation with a role of GeF4 for the crystal growth at a low temperature. INTRODUCTION Polycrystalline silicon thin-film transistors (poly-Si TFTs) fabricated on glass substrates have attracted increasing attention for switching elements and integrated drivers in liquid crystal displays and organic light emitting displays. Nowadays, the poly-Si films on glass substrates are successfully realized with two different techniques, i.e., the excimer laser annealing [1,2] and metal-induced solid phase crystallization of a-Si:H thin films [3,4]. The direct deposition of high quality poly-Si films at low-temperatures, however, has not been established yet in spite of high potential in their industrial production. Generally, poly-Si films directly deposited by CVD have poor crystallinity at the initial stage of the film growth because immediate nucleation on the substrate does not take place in conventional CVDs. A lot of effort has been made to improve the crystallinity of films so far, especially in plasma-enhanced CVDs, but a thicker film, e.g., more than several hundreds nm is still needed to establish the crystallinity high enough to apply for the TFTs [5,6]. For the essential improvement in the film crystallinity, a technique that enables us to form nuclei directly on the substrate surface and subsequently to grow the nuclei to the grains should be the most important issue in the low-temperature growth of polycrystalline films. We have investigated a new type of thermal chemical vapor deposition technique for low-temperature growth of poly-Si thin films, which we call reactive thermal CVD, and succeeded in the direct nucleation on the substrate and their successive growth to the grains by using Si2H6 and GeF4 as reactive source gasses [7,8]. With this technique, high Si content (95 atm%) poly-Si1-xGex films have been prepared and applied to the TFTs successfully. The TFTs
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fabricated with 200nm thick poly-Si1-xG
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