Temperature Dependent Hall Measurements on PLD Thin Films
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0957-K03-02
Temperature Dependent Hall Measurements on PLD Thin Films Holger von Wenckstern, Matthias Brandt, Gregor Zimmermann, Jörg Lenzner, Holger Hochmuth, Michael Lorenz, and Marius Grundmann Halbleiterphysik, Universität Leipzig, Leipzig, 04103, Germany
ABSTRACT The electrical properties of epitaxial ZnO thin films grown on sapphire substrates by pulsed laser deposition were investigated by temperature dependent Hall measurents. The thin films investigated were grown at different oxygen partial pressures ranging from 10-2 mbar to 1 mbar. The formation of a degenerate layer, determining the low temperature Hall data, depends on the oxygen partial pressure applied during growth. Further, the formation of such a layer can be correlated to the grain size of the samples. The thermal activation energy of dominant donors decreases in tendency with increasing oxygen partial pressure p(O2); it is about 100 meV for p(O2) ≤ 3 × 10-2 mbar and about 30 meV for p(O2) ≥ 0.1 mar. The concentration of donors and compensating acceptors increases with increasing p(O2).
INTRODUCTION Electrical properties of ZnO are investigated for about half a century. In the last years studies on ZnO single crystals and ZnO thin films are conducted by more and more research groups due to reports of p-type ZnO. The availability of high conducting p-type ZnO will allow optoelectronic devices based on ZnO; which has outstanding material properties as, e.g., the high exciton binding energy of about 60 meV (allowing excitonic recombination at room temperature and above). Temperature dependent Hall measurements have been reported for state of the art ntype ZnO single crystals [1,2,3]. The influence of electron accumulation layers at the sample surface on Hall measurements was discussed [2,3], a two-layer model was successfully applied to correct experimental data[3]. The species of dominant donors and with that the thermal activation energy Ed depends on the growth method. The thermal activation energy of donors in ZnO crystals grown by seeded chemical vapor deposition is Ed ~ 37 meV and Ed ~ 67 meV [4], respectively, in crystals grown by pressurized melt growth it is Ed ~ 26 meV [5], and in crystals grown by the hydrothermal method a value of Ed ~ 340 meV was reported [1]. Epitaxial ZnO thin films grown by molecular beam epitaxy (MBE) on sapphire substrates exhibit a highly conducting layer at the sapphire/ZnO interface, which can be prevented growing a ZnO:N buffer layer [6]. The thermal activation energy of such crystals is about 110 meV. Exposure to a remote hydrogen plasma increases the conductivity of epitaxial thin films grown by MBE on sapphire substrates, such crystals also have a highly conducting layer independent of the exposure to the remote H2-plasma [7]. We report the electrical properties deduced from temperature dependent Hall measurements of epitaxial thin films grown by pulsed-laser deposition on sapphire substrates.
EXPERIMENT ZnO thin films were grown by pulsed-laser deposition (PLD) on 10 × 10 mm2 a-plane sapphire substrate
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