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Junction Properties of Metal/SrTiO3 Systems

  • Takashi Shimizu (a1) and Hideyo Okushi (a1)

Electrical properties of Nb-doped SrTiO3 (STO:Nb) Schottky barrier (SB) junctions have been investigated in detail for a comprehensive understanding of metal/oxide interfaces. Using a high-purity ozone surface treatment, rectification ratio over 9th order of magnitude has been successfully obtained, while without the surface treatment, anomalous large reverse bias leak currents were observed in the current-voltage characteristic of the junctions. The X-ray photoelectron spectroscopy (XPS) shows that carbon contamination which adsorbed the STO:Nb surface in air, induces surface states in the band gap of the STO:Nb, which probably originate the large reverse bias leak currents of the metal/STO:Nb junctions. Thus we present importance, of surface treatment for oxides to obtain controllability and reproducibility of the electrical properties of the oxide devices. Photocapacitance spectroscopy has been performed to investigate deep levels due to bulk defects and impurities in the Au/STO:Nb junctions. The photocapacitance spectra clearly indicate existence of the deep levels in the Au/STO:Nb and the concentration of the deep levels were of the order of 1013∼1015 cm−3. These values are too low to affect the Fermi level pinning at the interface if the deep levels exist in the near surface region of the bulk STO:Nb. We have shown some interesting electrical properties, characteristic of the SB junction of the dielectric oxide compared with that of the conventional semiconductor's. The schematic band diagram of the Au/STO:Nb junction with the intrinsic low permittivity layer at the interface has been proposed, which explains all the characteristic electrical properties. Considering the chemical trend of the SB height (SBH) estimated from the J-V results, we have pointed out the importance of the metal reactivity for understanding the formation mechanism of the SBH.

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7 Cleaning condition of the STO:Nb substrate was slightly different from the conventional ozone cleaning that described in the experimental section. In case of the STO:Nb, whose XP spectra are shown in fig. 3 and fig. 4, substrate surface was cleaned at room temperature under 800 Pa 03 with ultraviolet (UV) light. However, results of the XP spectra are almost same as those of the conventional ozone cleaning, which has been already published in: Shimizu, T., Okushi, H., Bull. Electrotech. Lab. 63, 5 (1999).
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