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H3.3.1

Hydrogen Bonding in ZnO N. H. Nickel and K. Brendel Hahn-Meitner-Institut Berlin Kekuléstr. 5, D-12489 Berlin, Germany.

ABSTRACT Hydrogen effusion measurements reveal that the total hydrogen concentration in ZnO ranges from 5.2×1016 cm-3 for single crystal ZnO to 3×1021 cm-3 for polycrystalline ZnO thin-fims. From the H effusion spectra the hydrogen chemical potential is determined as a function of the H concentration that can be related th the H density-of-states distribution. Single crystal ZnO exhibiting the lowest H concentration reveals six peaks in the H density-of-states located between 0.59 and 1.4 eV below the H transport site. With increasing H concentration the amount of H accommodated with binding energies larger than 1.0 eV increases significantly.

INTRODUCTION Recently, zinc oxide is attracting a lot of interest because of its electrical and optical properties for a variety of applications ranging from UV light emitting diodes and lasers to piezoelectric devices. However, and major drawback is the fact that the zinc oxide generally exhibits n-type conductivity. Over the last decade it has been established that presence of hydrogen in a semiconductor influences its electrical properties. In most semiconductors H acts as a compensating center neutralizing intentionally incorporated dopants. The behavior of hydrogen is different in zinc oxide. Results obtained from first-principles density-functional calculations suggest that hydrogen acts as a shallow donor independent of the position of the Fermi energy. Therefore, it might be the source of the traditionally observed n-type conductivity in ZnO [1]. This theoretical prediction was confirmed by muon spin rotation measurements [2] and nuclear double-resonance spectroscopy [3]. Hall-effect measurements performed on state-ofthe-art single-crystal the zinc oxide revealed to the presence of two donor state [4], one of which was identified as the hydrogen show donor that occurs at a concentration of about 6×1016 cm-3 [3]. In fact, hydrogen effusion measurements performed on undoped state-of-the-art ZnO single crystals revealed a total hydrogen concentration of about 5.2×1016 cm-3 [5]. This value is an excellent agreement with the observed concentration of hydrogen shallow donors. In this paper we present hydrogen effusion data obtained from state-of-the-art single crystal ZnO and sputtered zinc oxide thin films. By analyzing the effusion data of the hydrogen densityof-states distribution (H DOS) can be derived. Single-crystal the zinc oxide exhibits six peaks in the H density of states distribution. The most prominent peak is located at about 0.86 eV be low that H transport states. The total hydrogen concentration in the sputter deposited zinc oxide films varies between 1020 and 3×10 21 cm-3. Moreover, the majority of H atoms are accommodated with binding energy exceeding 1.0 eV.

H3.3.2

EXPERIMENTAL DETAILS The single crystal zinc oxide samples used in this study were c-axis oriented crystals with an “O” face and a thickness of 0.5 mm. The ZnO thin