Improvement In Electrical Properties of Polycrystalline Silicon Films by The H 2 O Vapor Annealing Method
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IMPROVEMENT IN ELECTRICAL PROPERTIES OF POLYCRYSTALLINE SILICON FILMS BY THE H2O VAPOR ANNEALING METHOD
6QUJK[WMK5COGUJKOC-CVUWOK#UCFC;QUJK[WMK6UWPQFCCPF;QUJK[CUW-CPGMQ Tokyo University of Agriculture and Technology, Faculty of Technology, Tokyo 184-8588, JAPAN #$564#%6
Improvement of electrical properties by heat treatment with high-pressure H2O vapor was discussed for 7.4x1017 cm-3 phosphorus-doped pulsed laser crystallized silicon films. The analysis of the electrical conductivity resulted in that 1.3x106-Pa-H2O vapor annealing at 270oC for 3 h reduced the density of defect states from 5.5 x1018 cm-3 (as crystallized) to 5.0x1017 cm-3 for tail states and from 1.0x1018 cm-3 (as crystallized) to 3.0x1017 cm-3 for deep level defect states. The potential barrier height at grain boundaries decreased from 0.34 eV (as crystallized) to 0.05 eV by the heat treatment. The combination of oxygen plasma with high-pressure H2O vapor annealing effectively reduced the densities of defect tail sates as well as deep level defect states. It achieved a high performance of thin film transistors with a threshold voltage of 1.3 V and an effective mobility of 160 cm2/Vs.
INTRODUCTION Reduction of defects in silicon films at low temperatures is very important for fabrication of electronic devices, such as, thin film transistors (TFTs) and solar cells [1-3]. We have reported a simple heat treatment with high-pressure H2O vapor for this purpose [4]. Reduction of the threshold voltage and increase of the effective mobility of polycrystalline silicon thin film transistors (poly-Si TFTs) have been observed by heat treatment with highpressure H2O vapor [5]. Improvements of electrical properties of SiO2 films, SiO2/Si interfaces and poly-Si films have been achieved by the present method [6-9]. In this paper, we discuss reduction of defects of laser crystallized silicon films by heat treatment with high-pressure H2O vapor. Change in the electrical conductivity caused by reduction of the density of localized defect states is presented. Numerical analysis of the electrical conductivity using a statistical thermodynamical program is conducted in order to estimate change in the density of the defect states located at deep energy level as well as tail states. Improvement of characteristics of poly-Si TFTs by heat treatment with high-pressure H2O vapor is also demonstrated. The distribution of defect states in the channel region is also analyzed using the numerical calculation program of transfer characteristics. EXPERIMENTAL 7.4x1017 cm-3-phosphorus-doped 50-nm-thick amorphous silicon films were formed on glass substrates using low-pressure chemical vapor deposition (LPCVD) and ion implantation methods. Polycrystalline silicon films were fabricated through rapid meltregrowth induced by 30 ns pulsed XeCl excimer laser heating in vacuum. Laser energy density was increased from 160 mJ/cm2 (crystallization threshold) to 400 mJ/cm2 in 20 mJ/cm2 steps. Samples were then placed in a pressure-proof stainless-steel chamber with a
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