<RESEARCH PUBLICATIONS> I. SrTiO3 Thin Film Oxygen Sensor (2006-2011)
Stoichiometric SrTiO3 or BaTiO3, which have a d0 configuration, can be treated as band insulators with a band gap of about 3.2 eV. They have been used as materials for ceramic capacitors. In electron-doped SrTiO3 or BaTiO3, doped electrons enter the bottom of the empty Ti 3d band. They show apparent semiconductive carrier transport properties at room temperature and above; however, the properties are not the same as the one of conventional semiconductors, such as Si or Ge. In SrTiO3 and BaTiO3, the electron effective mass (m*) is higher than me=9.1*10-31 kg [m*=hbar^2/(4ta0^2)=hbar^2/(2Ja0^2), where hbar is the Planck's constant, t is the transfer integral, J is the overlap integral, a0 is the lattice constant]. For example, the effective mass in 1.4*1020 cm-3 (0.85 at.%) electron-doped SrTiO3 was estimated to be m*=1.2 me for light electrons and to be m*=7.0 me for heavy electrons by Chang et al. [Y. J. Chang, A. Bostwick, Y. S. Kim, K. Horn, E. Rotenberg, Phys. Rev. B 81 (2010) 235109.]. Chang et al. observed that the Fermi surface of SrTiO3 consists of three degenerate ellipsoids (dxy, dyz, and dzx) above 105 K, and that dxy band has lower minimum energy than the doubly degenerate dyz and dzx bands below 105 K. In n-type semiconductive (electron-doped) SrTiO3 or BaTiO3, electronic transport properties can be affected by the polarization field resulting from ionic displacements, such as the displacement of Ti4+ and O2- in a TiO6 octahedron, namely, the electrical conduction in electron-doped SrTiO3 or BaTiO3 is controlled by polarons [K. A. Muller, J. Supercond. 12 (1999) 3; E. V. Bursian, Y. G. Girshberg, E. N. Starov, Phys. Status Solidi B 46 (1971) 529.]. At SrTiO3 surfaces, adsorbed electronegative molecules such as oxygen (O2) induce local distortions of TiO6 unit cells. As a result, carrier-electrons become frequently trapped near the oxygen-adsorbed surfaces of SrTiO3. Such a phenomenon can be used for monitoring trace amounts of oxygen at room temperature. Note that the mobility perpendicular to the "c axis" is relatively high and the one parallel to it is relatively low; however, the mobility in a "deformed" lattice (e.g. BaTiO3) is lower than that in a "non-deformed" lattice (e.g., bulk SrTiO3). For references, the splitting of the 3d-t2g band at the gamma point in BaTiO3 increases from 400 to 330 K and to be about 30 meV [F. M. Michel-Calendini, R. N. Blumenthal, J. Am. Ceram. Soc. 54 (1971) 515, 577.]. Particularly in nanometer-scale SrTiO3 thin films, the surface adsorbates such as O2 significantly affect the electrical conductivity of the films. At room temperature, the electrical conductivity of nanometer-scale SrTiO3 thin films can be explained by Holstein's nonadiabatic small polaron theory. The electrical conductivity and the polaron size of the films depend on oxygen concentration, which is reasonable by assuming the modulation of polaron size due to oxygen adsorption.
[10] T. Hara, K. Shinozaki, Effect of Oxygen Adsorption on Polaron Conduction in Nanometer-Scale Nb5+-, Fe3+-, and Cr3+-doped SrTiO3 thin films, Jpn. J. Appl. Phys. 50 (2011) 065807. [9] T. Hara, T. Ishiguro, K. Shinozaki, Ultraviolet-Light-Induced Desorption of Oxygen from SrTiO3 Surfaces, Jpn. J. Appl. Phys. 50 (2011) 041502. [8] T. Hara, T. Ishiguro, K. Shinozaki, Aging Effect on Oxygen-Sensitive Electrical Resistances of SrTiO3 Thin Film, Jpn. J. Appl. Phys. 50 (2011) 061501. [7] T. Hara, T. Ishiguro, K. Shinozaki, Annealing Effects on Sensitivity of Atomic-Layer-Deposited (ALD) SrTiO3-Based Oxygen Sensors", Jpn. J. Appl. Phys. 49 (2010) 09MA15. [6] T. Hara, T. Ishiguro, K. Shinozaki, Oxygen-concentration-dependent electrical resistances of SrTiO3-based thin films, Jpn. J. Appl. Phys. 49 (2010) 041104. [5] T. Hara, T. Ishiguro, SrTiO3-based sensors for in situ monitoring of trace levels of oxygen, J. Ceram. Soc. Jpn. 118 (2010) 300. [4] T. Hara, T. Ishiguro, SrTiO3-based microfabricated oxygen sensors, Jpn. J. Appl. Phys. 48 (2009) 09KA17. *Note that the sensors were fabricated using MEMS technologies. [3] T. Hara, T. Ishiguro, N. Wakiya, K. Shinozaki, Oxygen sensitivity of perovskite-type dielectric thin films, Mater. Sci. Eng. B 161 (2009) 142. [2] T. Hara, T. Ishiguro, Oxygen sensitivity of SrTiO3 thin film prepared using atomic layer deposition, Sens. Actuators B 136 (2009) 489. [1] T. Hara, T. Ishiguro, N. Wakiya, K. Shinozaki, Oxygen sensing properties of SrTiO3 thin films, Jpn. J. Appl. Phys. 47 (2008) 7486.
* PLD-SrTiO3-based sensors were prepared at the Shinozaki-Sakurai laboratory of Tokyo Institute of Technology. ALD-SrTiO3-based sensors were fabricated by a MEMS foundary. I think that the devices will be mass-produced at MEMS foundaries.
II. (Ba,Sr)TiO3 Thin Film Capacitor (2003-2005)
[6] T. Hara, Electronic structures near surfaces of perovskite type oxides, Mater. Chem. Phys. 91 (2005) 243. [5] T. Hara, Defect-related leakage behavior and degradation mechanisms of (Ba,Sr)TiO3 films, Integr. Ferroelectr. 70 (2005) 79. [4] T. Hara, Electron-detrapping from localized states in the band gap of (Ba,Sr)TiO3, Solid State Commun. 132 (2004) 109. [3] T. Hara, Electrical characteristics of (Ba,Sr)TiO3 films accounted by partially depleted model, Microelectron. Eng. 75 (2004) 316. [2] T. Hara, Intentionally inserted oxygen depleted (Ba0.5Sr0.5)TiO3 layers as a model of DC-electrical degradation, IEEE Trans. Device Mater. Reliab. 4 (2004) 670. [1] T. Hara, Leakage behavior of DC electrically degraded (Ba,Sr)TiO3 thin films, IEEE Trans. Device Mater. Reliab. 4 (2004) 268.
DIscussion -> http://www.geocities.jp/ferroelectricmaterials/discussiontfceng.html (ex.) Q1: Can a depletion layer be narrowed when a reverse-bias voltage is applied? It appears to be unexplainable. A1: The depletion layer can become narrow when a reverse-bias voltage is applied because of the ionization of deep-level donors in defective (Ba,Sr)TiO3 and the decrease in the dielectric constant caused by the bias voltage applied to the depletion layer of (Ba,Sr)TiO3. The depletion layer becomes narrow when a reverse-bias voltage is applied because of the decrease in the dielectric constant caused by the bias voltage applied to the depletion layer [T. Hara, "Electrical characteristics of (Ba,Sr)TiO3 films accounted by partially depleted model", Microelectron. Eng. 75 (2004) 316]. I think that no explanation is necessary for this phenomenon. First, I will explain what is meant by pure semiconductors without deep-level donors. Conduction electrons are discharged from an n-type semiconductor toward the electrode on the forward-bias side. However, on the reverse-bias side, hardly any electrons are supplied from Pt to the n-type semiconductor because of the Schottky barrier. Thus, when only shallow-level donors exist in the depletion layer, the depletion layer is widened (deep depletion) by the ionization of these shallow-level donors to flow electrons. Next, I will discuss impure semiconductors with deep-level donors. Let us consider the relaxation current when a small reverse-bias voltage is applied to a Pt/n-type (Ba,Sr)TiO3 Schottky contact. There are deep-level donors in a (Ba,Sr)TiO3 film with oxygen vacancies. The deep-level donors are ionized as a result of the lowering of quasi-Fermi level EImref in accordance with the equation n=NCexp{-(EC-EImref)/kBT}, because the conduction electron density n temporarily decreases on the reverse bias side. Therefore, it is considered that the density of the ionized donors increases and the depletion layer becomes narrow. In the above equation, NC is the effective density of states of the conduction band and EC is the bottom energy of the conduction band. Note that the depletion layer of some conventional semiconductors with a low purity becomes narrow owing to the ionization of the deep-level donors [C. W. Jen, C. L. Lee, Solid-State Electron. 24 (1981) 949; E. H. Rhoderick, Rev. Phys. Technol. 1 (1970) 81]. When the density of the donors to be ionized is high, the barrier height slightly decreases owing to the positive charges of the donors and the image charges generated in the electrodes. According to Maeda [K. Maeda, J. Vac. Sci. Technol. B 19 (2001) 268], the Schottky barrier of Au/Si slightly decreases from 0.80 to 0.72 eV when the donor density increases from 1.5*1014 to 8.0*1016 cm-3. Here, the Fermi level of (Ba,Sr)TiO3 is extended toward the interface between (Ba,Sr)TiO3 and Pt so that the Fermi level intersects the depletion layer and reaches the Fermi level of Pt under zero bias voltage. When a reverse-bias voltage is applied, the barrier height remains the same when viewed from the Pt side (in fact, the height slightly decreases, which is disregarded for simplicity), and the quasi-Fermi level of (Ba,Sr)TiO3 decreases. It can be easily imagined that the ionization of deeper-level donors is required as it approaches the electrode interface. It has been reported that Cr3+, which serves as an acceptor in the Sr(Ti,Cr)O3 used in resistance random access memories (RRAMs), is oxidized to Cr4+ on both the anode and cathode sides [M. Janousch et al., Adv. Mater. 19 (2007) 2232], which indicates that deep depletion does not occur in such a system.
<WORKING EXPERIENCES> - Taiyo Yuden Co., Ltd. (http://www.ty-top.com/), 2003/10-2012/03 Research and Development on SrTiO3-Based Thin Film Oxygen Sensor (Type I, simply structured Electrode/SrTiO3/Single Crystal Substrate; Type II, fabricated using MEMS processes) and (Ba,Sr)TiO3-Based Thin Film Capacitor - Kyocera Crop. (http://global.kyocera.com/), 1998/12-2003/10 Research and Development on (Ba,Sr)TiO3-Based Thin Film Capacitor, Organic Semiconductor Devices, and Lithium Battery - Sony Energytech Co., Ltd., 1997/12-1998/12 Research and Development on Lithium Battery - Izumomurata Mfg Co., Ltd. (http://www.murata.co.jp/izumomurata/), 1993/04-1997/12 Research and Development on Ceramic Capacitor
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