Low Temperature Metal-Free Fabrication of polycrystalline Si and Ge TFTs by PECVD Hydrogenation

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Low Temperature Metal-Free Fabrication of polycrystalline Si and Ge TFTs by PECVD Hydrogenation Pouya Hashemi 1, Jaber Derakhshandeh 1, Bahman Hekmatshoar 1, Shamsoddin Mohajerzadeh 1, Yaser Abdi 1, and Michael D. Robertson 2 1 Thin film laboratory, Department of Electrical and Computer Engineering, University of Tehran, Tehran, Iran 2 Department of Physics, Acadia University, Wolfville, Nova Scotia, B4P 2R6, Canada ABSTRACT Poly-crystalline Si and Ge layers were grown at low temperatures on glass substrates by successive hydrogenation and annealing steps, with no need to any metal incorporation. Hydrogenation is performed in an RF-PECVD apparatus with different powers of hydrogen plasma and the annealing step is carried out in the same system in N2 ambient. This leads to formation of granular Si and Ge structures with average grain size of less than 100nm at temperatures as low as 250ºC and 150ºC, respectively. The effect of hydrogen plasma power at various temperatures on the crystallinity of the layers has been studied by SEM and TEM analyzes. Successive hydrogenation and annealing at respective temperatures of 150ºC and 200ºC for Ge layer and 300ºC for Si layer would result in a device-quality polycrystalline Ge and Si layers which have been employed for fabrication of thin-film transistors. These TFTs show the mobility of 80cm2/Vs and 4cm2/Vs and ON/OFF ratio of more than 103 and 5×104 for Ge and Si, indicating the feasibility of this technique for applications in large-area electronics. INTRODUCTION Poly-crystalline Si and Ge are of the most promising materials for applications in large-area electronics. Carrier mobility is the most prominent parameter determining the electrical behavior of semiconductor devices, which may be considerably improved by crystallization. However, the potential complexity and cost of the crystallization step imposed by high-temperature annealing requirements makes them incompatible with low-temperature flexible substrates aimed for reelto-reel processing or flexible display applications. Therefore, especial attention has been paid to low-temperature crystallization techniques. Several approaches have been proposed for lowering the crystallization temperature of semiconductors, including the metal-induced crystallization (MIC) [1], metal-induced lateral crystallization (MILC) [2] and Excimer laser annealing (ELA) [3]. In the previous works, Al [4] and Cu-induced [5] crystallization of Ge and Pa [6] and Ni-induced [7] crystallization of Si have been studied. We have recently devised a stress-assisted metal-induced crystallization technique that reduces the crystallization temperature of Si [8] and Ge [9-10] to 310°C and 130°C, respectively. In this technique, mechanical compressive stress is externally applied to the flexible plastic substrate during annealing by inward bending of the substrate, leading to formation of high mobility poly crystalline Ge and Si. In this article, we propose a method for non-metal-induced crystallization of Si and Ge, by means of successive hydr