Surface Charging of n-type Gd 2 O 3 and HfO 2 thin Films
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1111-D07-16
Surface charging of n-type Gd2O3 and HfO2 thin films David Wooten,1 Ya. B. Losovyj,2,3 J. Petrosky,1 J. McClory,1 Jinke Tang,4 Wendong Wang,4 and P.A. Dowben3 1
Air Force Institute of Technology, 2950 Hobson Way, Wright Patterson Air Force Base, OH 45433-7765, U.S.A. 2
Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Highway, Baton Rouge, LA 70806, U.S.A. 3
Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, P.O. Box 880111, Lincoln, NE 68588-0111, U.S.A. 4
Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, U.S.A.
ABSTRACT Gd2O3 and HfO2 films exhibit temperature dependent photovoltaic charging. The dielectric properties of the surface and concomitant photovoltaic surface charging increases following oxygen exposure under ultrahigh vacuum conditions suggesting that the surface conductivity plays a role in the surface photovoltage charging. INTRODUCTION Surface charging is common in the photoemission of insulators leading to the use of a compensating charge mechanism (typically electron flood guns) in photoemission spectroscopy. Surface photovoltage is a means for providing insight into the surface charge carriers as well. Since the initial recognition and use of surface photovoltage (SPV) charging of semiconductors, characterization methods have tended to follow the method implemented by Brattain and Bardeen in 1953 [1] and the technique has been subsequently much exploited [2]. Surface photovoltage effects that occur with temperature, substrate doping, and metal deposition [3-5] have provided a means by which phonon based recombination and Schottky barrier heights can be measured using ultraviolet photoemission spectroscopy. While photoemission induced surface photovoltage on semiconductors is a well established phenomenon [2], there are surprisingly few surface photovoltage studies that characterize the surface charging. We have utilized surface photovoltage here to investigate the surface charging of two notable 4f oxide dielectrics. Although considered a high K dielectric, one can fabricate a heterojunction diode of n-type HfO2 with silicon by introducing donor states in a HfO2 layer though oxygen vacancies introduced during the pulsed laser deposition process [6,7]. This formation of donor states is also accomplished with Gd2O3 [8]. The surfaces of these oxides, however, probably contain more defects than the material interior, and the lower coordination of the surface can lead to small amounts of lattice relaxation at the surface. There is no a priori reason for the surface conductivity to resemble the bulk conductivity.
EXPERIMENT HfO2 films were deposited on single crystal silicon (100) p-type (and n-type) substrates using pulsed laser deposition (PLD) at a growth rate of about 0.15 Å/s. The HfO2 target was prepared by standard ceramic techniques using HfO2 powders as described elsewhere [7-9]. The Gd2O3 was grown on Si(100) using a pure Gd2O3 targe
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