Polycrystalline Germanium and Silicon-Germanium Alloys on Plastic for Realization of Thin-Film Transistors
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Polycrystalline Germanium and Silicon-Germanium Alloys on Plastic for Realization of Thin-Film Transistors B. Hekmatshoar, D. Shahrjerdi, S. Mohajerzadeh, A. Khakifirooz1, M. Robertson2 and E. Asl Soliemani Department of Electrical and Computer Engineering, Thin Film laboratory, University of Tehran, Tehran, Iran, +98-21 801 1235, e-mail: [email protected] 1 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Boston, Massachusetts 2 Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada ABSTRACT Device-quality polycrystalline Ge layers have been grown on flexible poly-ethylene terephthalate (PET) substrates by means of stress-assisted Cu-induced crystallization at temperatures as low as 130˚C and employed for fabrication of depletion-mode poly-Ge thin-film transistors (TFTs). These TFTs show an ON/OFF ratio of 104 and an effective hole mobility of 110 cm2/Vs. The stress-assisted crystallization technique has been extended to crystallize SiGe alloys at low temperatures for possible fabrication of poly-SiGe TFTs on plastic. As a result, poly-Ge seeded poly-crystalline SiGe layers with 40% Si content are grown at a low annealing temperature of 180˚C in the presence of 0.05% equivalent compressive strain. 1. INTRODUCTION Thin film transistors (TFTs) are widely employed as drivers in display technology. Integration of display drivers reduces cost, weight and size of the displays. Also, avoiding complexity in interconnection to external drivers increases reliability. High performance transistors increase the efficiency of integration for display drivers which reduces weight and size of the displays, as well as the chance of avoiding complexity of their interconnections to external drivers [1]. Amorphous Si and Ge TFTs are rather poor in field effect mobility, so polycrystalline semiconductors are preferred for enhanced performance. Conventional solid phase crystallization techniques need high temperatures, which considerably increases the cost of crystallization. Furthermore, flexible substrates which are proper candidates for portable electronic applications need much lower processing temperatures [2]. Various methods have been proposed for reducing the crystallization temperature including the metal-induced crystallization (MIC) and metal-induced lateral crystallization (MILC). Ni-induced lateral crystallization of Si [3,4], Al-induced [5,6] and Cu-induced [7] crystallization of Ge have been widely investigated. MIC with Al reduces the crystallization temperature of a-Ge to values as low as 100°C, but the high percentage of the metal contamination involved severely degrades the semiconductor properties, on the other hand Cu-induced crystallization of germanium requires a high temperatures of about 400°C which is still too high for flexible plastic substrates. We have recently developed a stress-assisted Cu-induced crystallization technique which reduces the crystallization temperature of a-Ge to as low as 130ºC [8]. In this technique, apply
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