Electrical Properties and Defect States of Laser Crystallized Polycrystalline Silicon Films
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ELECTRICAL PROPERTIES AND DEFECT STATES OF LASER CRYSTALLIZED POLYCRYSTALLINE SILICON FILMS Tadashi Watanabe, Nobuyuki Andoh, Toshiyuki Sameshima Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan ABSTRACT In this paper, changes in electrical properties of laser crystallized silicon films doped with 8.5×1017-cm-3-phosphorus atoms as a function of laser shot number are investigated. The samples are treated with plasma hydrogenation for 30 sec at 130 Pa at 250 °C and additional H2O vapor heat treatment at 260 °C for 3 hours with 1.3 MPa. The electrical conductivity at room temperature become about 10-6~10-5 S/cm as laser shot number increases from 1 to 100. After hydrogenation and additional H2O vapor heat treatment, electrical conductivity remarkably increases to 100~101 S/cm. At laser irradiation of 20 or 50 shots after both treatments, the density of defect at deep level states and tail states are determined 1.15×1017 cm-3 and 5.7×1017 cm-3 using an analysis program. Potential barrier height at grain boundary is 0.048 eV. The effective carrier density and carrier mobility are markedly increased up to1017 cm-3 and 209 cm2/Vs by hydrogenation and additional H2O vapor heat treatment.
INTRODUCTION Polycrystalline silicon films are applied to thin film transistors (TFTs) and solar cells[1-5]. There is an advantage of using low melting point substrate because laser crystallized silicon films are formed by rapid melt and solidification. The laser crystallized silicon films have fine grains and many defects at their grain boundaries [6-9]. The hydrogen plasma treatment has been used for reduction of defect density in polycrystalline silicon films and to improve the electrical properties at low temperature [10-12]. H2O vapor heat treatment is also considered as a reduction method for defect density [13-15]. In this paper, changes in electrical properties of laser crystallized silicon films as a function of laser shot number are investigated using hydrogenation and H2O vapor heat treatment. We developed an analysis program using the finite element method (FEM) combined with statistical thermo dynamical conditions with defect states localized at grain boundary for estimation of the defect states density in the band gap of silicon films [16-18].
EXPERIMENTAL DETAILS 50nm amorphous silicon films are formed on quartz substrates by low-pressure chemical vapor deposition (LPCVD) at 425°C. 8.5 ×1017-cm-3-phosphorus atoms are doped through SiO2 film covered on the silicon films by ion implantation with energy of 80 keV. After elimination of top SiO2 films, aluminum electrodes with thickness of 100 nm are patterned on the samples. The amorphous silicon films are then annealed by 28 ns-pulsed XeCl excimer laser irradiation at 250 °C at vacuum level of 3×10-6 Torr. Multiple steps laser irradiation is carried out. The laser energy density is increased from 150 to 350 mJ/cm2 in step of 25 mJ/cm2. The six different laser crystallized silicon films are fabricated by laser irradiation of 1,5,10
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