Metal-Free Germanium-Induced Crystallization of Amorphous-Si on Glass
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Metal-Free Germanium-Induced Crystallization of Amorphous-Si on Glass A. Akhavan, L. Rezaee, J. Derakhshandeh, S. Mohajerzadeh* and A. Goodarzi Thin Film Lab, Dep. of Elect. & Comp. Eng., University of Tehran, Tehran, Iran Email: [email protected], [email protected]
ABSTRACT A metal-free germanium-induced crystallization technique is reported to realize poly-silicon films on glass at temperatures as low as 480o C. The reduction in crystallization temperature is achieved by using ultra-violet illumination during thermal treatment of the amorphous silicon layer. The annealed samples have been studied using SEM and XRD analyses. Also the electrical conductivity of samples with various annealing conditions has been examined. Conductivity of the samples annealed in the presence of ultra-violet exposure shows more than three orders of magnitude improvement compared to that of amorphous films. Samples treated without UV exposure do not show much enhancement. Size of poly-silicon grains is 0.3µm, as revealed using SEM study.
INTRODUCTION Polycrystalline silicon has drawn attention of researchers for its prospective application in realization of thin film transistors. Despite the usefulness of such material for large area electronics, its use has been limited to special glasses, mainly due to a high processing temperature needed for crystallization. Several techniques have been introduced to reduce the annealing temperature during crystal growth. The most important of which are excimer laser annealing (ELA) and solid phase crystallization (SPC). While ELA is a low temperature process, it requires expensive facility [1]. On the other hand SPC is energy consuming and requires temperatures above 550o C[2]. Metal induced crystallization (or MIC for short) has been successful in reducing the crystallization temperature to values around 400o C[3]. Many researchers have attempted various types of MIC to lower the treatment temperature and realize thin film transistors on glass [4]. Metal contamination, however, is a main concern in all of these approaches. The use of germanium as a clean seed of crystallization has been attempted by Subramanian et. al.[5]. A lateral growth of the order of 0.5ìm has been achieved at temperatures above 500o C and small geometry TFT’s have been realized. Insignificant lateral growth as well as a high annealing temperature, seem to be the main obstacles in using this approach for many applications [6]. In this paper we use ultra-violet illumination during thermal treatment to reduce the crystallization temperature to values as low as 480°C. Various samples have been realized for this study and the electrical conductivity of the annealed samples have been examined to study the crystallization of the silicon films. Also SEM and XRD were used to further examine the physical characteristics of the films. Both techniques seem to corroborate the results of our conductivity measurements.
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