Formation of Hydrogen Related Defects and Nano-Voids in Plasma Hydrogenated ZnO

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Formation of Hydrogen Related Defects and Nano-Voids in Plasma Hydrogenated ZnO Reinhart Job Department of Mathematics and Computerscience, University of Hagen, Universitätsstr. 27, Hagen, D-58084, Germany ABSTRACT Using µ-Raman spectroscopy (µRS) and cathodoluminescence (CL) analyses, the impact of hydrogen plasma treatments on sintered zinc oxide (ZnO) samples was studied. 1 hour H-plasma treatments (150 W, 13.56 MHz) were applied at substrate temperatures between 250 °C and 500 °C. µRS and CL analyses show that plasma hydrogenation causes significant defects in ZnO samples; i) non-specified defect species are established with a maximal density upon H-plasma exposure at 350 °C substrate temperature, and ii) the formation of oxygen vacancies (VO) can be traced. Moreover, µRS reveals vibration modes of H2 molecules trapped in nano-voids. The experimental results indicate that those nano-voids are created by a coalescence of the VO defects. INTRODUCTION The wide band gap semiconductor ZnO (∼3.3 eV at 300 K) provides a wide range of promising electronic, optoelectronic and even mechanical applications. It has a large exciton binding energy (∼60 meV) making the fabrication of low-threshold excitonic laser diodes possible [1, 2]. Furthermore, sintered ZnO is an established raw material for varistor devices, whose performance highly depend on defects and structural disorder [3]. Recently, the interaction of hydrogen with ZnO became a significant research focus, since hydrogen has a strong impact on the defect structure and is also suspected to generate a shallow donor state in ZnO (e.g. [1, 2]). To investigate the impact of hydrogen on ZnO, sintered ZnO samples were treated by H-plasma at various applied substrate temperatures up to 500 °C. The samples were analyzed by µRS and CL. It is to point out that this paper is an extension to a report presented at the last MRS fall meeting [4]. EXPERIMENTAL ZnO pellets were prepared from high purity ZnO powder (Merck). To stiffen the pellets, ZnO powder was mixed with an acrylic resin (Elvacite®) and acetone and homogenized by stirring. During stirring acetone evaporates. The homogenized mixture was mortared, pressed to pellets (∅: ∼2 cm, thickness: ∼3 mm) and then cut into small pieces (ca. 5 × 5 × 3 mm3), which finally were sintered in a vacuum furnace at ∼10-5 mbar. The heating profile had a maximum of 900 °C, which was attained within 1 hour applying a linear heating ramp. After annealing at 900 °C for 1.5 hours the furnace was switched off, and the temperature was reduced down to below 150 °C within about 1.5 hours. Then the samples were taken out of the furnace. During heating Elvasite® completely evaporates from the samples as was proven by µRS. Hydrogenation was applied for 1 hour in a plasma setup (13.56 MHz, 150 W, bias voltage: ∼500 V, 5.4⋅10-3 mbar) at various substrate temperatures between 250 °C and 500 °C [4]. The Hplasma was stabilized by an admixture of argon (H2 flux: 50 sccm, Ar flux: 20 sccm).


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H-plasma: 1 h, 500 °C