On the formation of solid state crystallized intrinsic polycrystalline germanium thin films
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On the formation of solid state crystallized intrinsic polycrystalline germanium thin films Zhiguo Meng, Zhonghe Jin, and Gururaj A. Bhat Department of Electrical and Electronic Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Paul Chu Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong
Hoi S. Kwok and Man Wong Department of Electrical and Electronic Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong (Received 19 September 1996; accepted 21 April 1997)
A two-step heat treatment process has been employed to crystallize low pressure deposited thin films of amorphous germanium. Large grain p-type polycrystalline germanium with a Hall effect hole mobility of greater than 300 cm2yVs has been obtained. Films with near intrinsic conductivity, necessary for the construction of practical enhancement-mode insulated-gate thin film transistors, were obtained by introducing phosphorus as a compensating dopant. High Hall effect electron mobility of 245 cm2yVs has been measured on the resulting n-type polycrystalline germanium thin films. I. INTRODUCTION Polycrystalline thin film transistors (TFT’s) have been used in a number of important applications. These include three-dimensional stacked transistors for static random-access memories, pixel, and drive transistors for liquid crystal display (LCD) panels, integrated sensors, etc. In many such applications, low processing temperature is required.1 In the case of LCD’s, the constraint on the processing temperature is quite stringent since the inexpensive glass substrates on which the transistors are formed typically cannot withstand temperature much higher than 550 ±C. Currently, silicon (Si) based amorphous or polycrystalline TFT’s are being used, the former typically in the pixel arrays2 of LCD’s. The higher electron and hole mobilities of the latter make it possible to realize also complementary metal-oxide-semiconductor (CMOS) control and driver circuits directly on the inexpensive glass substrates. The main drawback of the conventional technique of forming the transistor active layers of Si-based polycrystalline TFT’s by the pyrolysis of silane (SiH4 ) gas in a low pressure chemical vapor deposition (LPCVD) system is the high temperature required, typically around 600 ±C. For reduced-temperature processing, polycrystalline alloys of silicon and germanium (Si1–x Gex ) have been extensively studied and reported.3,4 An added advantage of using Si1–x Gex thin films is their higher carrier mobilities than those found in pure Si thin films. Recently, the use of polycrystalline Ge (polygermanium) thin film as a semiconducting material with the potential to enable even lower processing temperature and provide even higher carrier mobilities has been investigated.5 2548
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 10, Oct 1997
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The deposition and the charac
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