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Preparation of the ZnO substrate surface

Paweł Skupiński ¹, Krzysztof Grasza ^1,2, Andrzej Mycielski ¹, Elżbieta Łusakowska ¹, Halina Sakowska ²

1. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
2. Institute of Electronic Materials Technology (ITME), Wólczyńska 133, Warszawa 01-919, Poland

Abstract

ZnO is a material with a great potential for the applications in UV light emitters, spin functional devices, gas sensors, transparent electronics and surface acoustic wave devices. Large bulk crystals are usually grown by the hydrothermal method. In our Institute we use a less expensive chemical vapor transport (CVT) method to grow ZnO crystals.

Very important factor, which drive the efforts to obtain large single crystals of zinc oxide is the possibility to use the ZnO wafers as substrates for homoepitaxy and epitaxial growth of GaN layers in MBE ammonia-free methods. ZnO is a good candidate because the lattice parameters for both materials are almost the same. The preparation of substrate surface plays a crucial role in epitaxial growth applications.

This work presents a method to obtain ZnO substrates with a very smooth surface. The surface of our samples was investigated using the atomic force microscopy (AFM). We present a series of experimental results and a method based on those results to obtain surfaces with RMS= 0.5 nm. Surface of such RMS was made by short mechanical polishing with two different powders having grain size of 20-30 μm and 5-6 μm and finally by two-phase mechano-chemical polishing in colloidal silica of various pH. The first phase of mechano-chemical polishing was made in aquatic colloid solution of SiO₂ nanoparticles reduced to pH=6 by hydrochloric acid. This part of polishing removes the layer degraded by mechanical polishing. The next step was polishing in colloidal solution reduced by nitric acid to pH=7. In this step we obtained very smooth surface. Short, low temperature annealing was a complementary procedure applied in order to remove the amorphous layer, keeping the roughness low.

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