Surface Preparation of Single Crystals for ZnO Homoepitaxy
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Surface Preparation of Single Crystals for ZnO Homoepitaxy Christian Neumann1, Stefan Lautenschläger1, Swen Graubner1, Niklas Volbers1, Bruno K. Meyer1, Jürgen Bläsing2, and Alois Krost2 1 I. Physikalisches Institut, Justus-Liebig-Universitaet Giessen, Giessen, 35392, Germany 2 Institut für Experimentalphysik, Otto-von-Guericke-Universität Magdeburg, Magdeburg, 39106, Germany ABSTRACT For the homoepitaxial growth of ZnO it is inevitable to obtain a regular crystalline single crystal surface prior to growth. Commercially available, hydrothermally grown ZnO single crystals show amorphous surfaces due to mechanical cutting and polishing. Here we present the results of a thermal treatment on these ZnO single crystals. After annealing, a regular crystalline oxygen terminated surface can be obtained. Changes in surface roughness, residual defect concentration and electrical properties can be shown. The bulk crystallinity though was not affected. INTRODUCTION ZnO as a wide-band-semiconductor with a band gap of 3.3 eV at room temperature is a promising material for optoelectronic devices. Due to a still not completely understood defect structure, reproducible p-type doping is still the open question in this material system. Thus, it is essential to grow thin films of high crystalline perfection with low residual intrinsic and extrinsic defect concentrations to achieve reliable p-type conductivity. The first step to do so is to have a substrate standard with enables a defined nucleation of the gaseous precursor species on the single crystal surface and thus regular growth of the epitaxial layers. Previous works [1,2] show that a high temperature annealing step is sufficient to prepare these regular crystalline surfaces. Here we show the necessity of modifying the surface morphology of commercially available ZnO single crystals by a thermal annealing step to obtain regular crystalline surface structures. EXPERIMENTAL A series of two-side-polished c-plane oriented ZnO crystal wafers cut from one bulk single crystal by CrysTec were annealed in a high temperature quartz glass tube oven at different temperatures ranging from 650°C to 1150°C for 1h. During the annealing step, a constant oxygen flow was applied over the crystal surfaces to ensure efficient material transport. The surface morphology was investigated locally by atomic force microscopy (Quesant AFM Q250) and integrally by x-ray reflectivity. Also, the crystallinity of the samples was checked by high resolution x-ray diffraction (HRXRD Seifert 3003HR). The impurity incorporation in the films has been determined by secondary ion mass spectroscopy (SIMS) using a RIBER MIQ-256 SIMS system with oxygen primary ions. The electrical properties of the crystals have been examined using a hall measurement system in the van-derPauw geometry using a magnetic field of 1 T.
DISCUSSION ZnO single crystal properties Prior to the annealing step, both surfaces of the polished ZnO crystals show no characteristic crystalline features. For instance, Fig. 1 shows a surface sc
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