Fabrication of a p-NiO/n-Si Heterojunction Diode by UV Oxidation of Ni Deposited on n-Si
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Fabrication of a p-NiO/n-Si Heterojunction Diode by UV Oxidation of Ni Deposited on n-Si Dongyuan Zhang, Kazuo Uchida and Shinji Nozaki Graduate School of Informatics and Engineering The University of Electro-Communications 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan ABSTRACT The metallic nickel (Ni) deposited on an n-Si substrate with resistivity of 4 – 6 ͼcm was oxidized by the ultra-violet (UV) oxidation technique to form a p-NiO/n-Si heterojunction diode. The rectifying current-voltage (I-V) characteristic confirmed formation of a pn junction. The capacitance-voltage (C-V) characteristic further identified an abrupt p+n junction between NiO and n-Si. The photocurrent increased with the increased wavelength of laser under illumination of the diode. The voltage-dependent photocurrent suggests that the carriers generated in the depletion region of Si was effectively collected but not outside the depletion region. A low diffusion length of holes was attributed to Ni diffusion in Si caused by the substrate heating during the UV oxidation. INTRODUCTION Nickel oxide NiO is a wide-band-gap semiconductor with a band gap of 3.8 eV. Its resistive switching phenomenon was extensively studied for a potential memory application [1]. We, however, paid a great attention to the p-type conductivity of NiO and have been motivated to form a heterojuntion pn junction of NiO with another n-type semiconductor. Although the method mostly commonly used to deposit metal oxide semiconductors such as ZnO and NiO is sputtering, the plasma damage often deteriorates the quality of metal oxide semiconductors. We have developed the UV oxidation technique to form NiO by oxidation of metallic nickel. In order to form semiconducting NiO, the UV oxidation parameters such as temperature and oxidation time were optimized using the Raman scattering measurement of the NiO films [2-3]. The UV oxidation of metal can be applied to form various oxide semiconductors including ZnO. As a preliminary study of heterojunction formation with oxide semiconductors, the p-NiO/n-Si heterojunction diode was made by UV oxidation of the metallic Ni on an n-Si substrate and characterized. EXPERIMENT Metallic nickel was deposited on n-Si substrate with resistivity of 4 – 6 ͼcm by electronbeam (e-beam) vacuum evaporation. Before the deposition, the n-Si substrate was ultrasonically cleaned in acetone, ethanol and deionized water each for 5 minutes subsequently. The pressure of the e-beam evaporation chamber was about 5 – 7×10-6 torr during evaporation. The thickness of the deposited Ni film was controlled with a crystal monitor and confirmed with a dektak profilometer. The 50 nm-thick Ni film was formed by e-beam evaporation and then UV-oxidized
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in oxygen atmosphere at 350 oC under UV illumination using a metal halide lamp. The oxidation time of 1 hour was long enough to completely oxidize Ni, and the thickness of NiO measured using a dektak profilometer and ellipsometer was about 100 nm. Platinum was deposited to form a NiO ohmic contact with a diameter of
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