Junction Properties of Metal/SrTiO 3 Systems
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preliminary results of junction properties of metal/Nb-doped SrTiO3 systems, including extrinsic origin of interface states, characteristic junction properties originating from their dielectric properties and deep levels observed using the photocapacitance method, are presented. EXPERIMENTAL The Schottky barrier (SB) junctions were fabricated on 0.01 wt.% Nb-doped SrTiO 3 (STO:Nb) (001). To prevent effect of grain boundary and/or low crystalinity, Verneuil grown single crystal substrates were used. They were as-polished substrates chemically etched by concentrated HNO 3 acid for 2 minutes or so-called "stepped substrates" chemically etched by buffered HF. Each substrates were annealed in flowing 02 at 1000 0C for 1 hour and at 400 °C for 4 hours to obtain the substrate surface having well-ordered surface steps and atomically flat terraces [1][2]. Before deposition of metals for the SB junction in ultra-high vacuum chambers, cleaning procedure using high-purity ozone was performed in the same chambers, in which substrate surfaces were annealed in the high purity ozone atmosphere (the ozone flux was about 1016 molecules-cm 2 ) at 400-500 0C for 20-40 minutes (we call this procedure as "ozone cleaning"). The pressure of the chamber during the ozone cleaning was 1-3 X 10ioPa. The reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements showed that the surface reconstruction and the surface morphology are almost unchanged after the ozone cleaning [3]. After the ozone cleaning, the substrates were cooled down spontaneously in the ozone atmosphere to 50-100 °C at which metal electrodes were deposited for fabricating metal/STO:Nb junctions. The electrodes for the SB junctions were deposited in situ through the stainless steal mask of holes soon after stopping introduction of the high-purity ozone gas. Electrode metals for the SB junctions were mainly Au, while Cu, Ag and Pt electrodes were also used. Electrodes for the Ohmic contacts were Al and Ti deposited backside of the STO:Nb substrates to form SB diodes. Although guide-ring electrodes to prevent edge effect in electrical measurements were not able to be deposited, in-situ preparation will be advantageous to prevent the STO:Nb surface from mechanical damages compared with preparation methods using photo lithography technique, which requires chemical etching and/or ion etching of the STO:Nb surface. The electrical measurements were performed in a vacuum chamber using tungsten probes. Current density-voltage (J- V) and capacitance-voltage (C-V) characteristics were measured using a picoampere meter and an impedance analyzer, respectively. The measurement frequencies of the C-V measurements were between 100 Hz and 1 MHz. Using a single-grating monochrometer, monochromatic light was irradiated on Au/STO:Nb SB junctions, to study photocurrent and photocapacitance characteristics. The schematic of the measurement system is
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shown in Fig. 1. The photocurrents were measured using the picoampere meter under 0 bias voltage. The pho
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