Hydrogen-induced surface modifications of ZnO single crystals


Hydrogen-induced surface modifications of ZnO single crystals

Prochazka, I.; Cizek, J.; Anwand, W.; Brauer, G.; Grambole, D.; Schmidt, H.

ZnO is a wide band gap semiconductor with a variety of applications including UV light emitting diodes and lasers, optoelectronic devices, and gas sensors. Due to a progress in crystal growth, high quality single crystals are nowadays available. However, properties which are essential for any of the applications mentioned are significantly influenced by the presence of lattice defects. A detailed characterization of lattice defects in ZnO crystals is, therefore, a key task in order to understand their physical properties. Defect studies of hydrothermally grown ZnO single crystals revealed that hydrogen is the most important impurity in ZnO crystals [1]. It was found that hydrogen is coupled with zinc vacancies in the form of vacancy-hydrogen complexes. Moreover, it has been demonstrated that a huge amount of hydrogen (30 at.-%) can be introduced into ZnO crystals by electrochemical loading [2].

Surface changes in ZnO crystals electrochemically doped with hydrogen were investigated in this work using slow positron implantation spectroscopy (SPIS) combined with atomic force microscopy (AFM) and optical microscopy (OM). It was found that hexagonally shaped pyramids were formed on the surface (Fig. 1). All these pyramids have the same crystallographic orientation as the ZnO matrix. The formation of pyramids can be explained by hydrogen-induced plastic deformation which is realized by slip in the [0001] direction. Such a picture is supported (i) by AFM where terraces of a height comparable with the c-lattice parameter were found at the base of the pyramids, and (ii) by SPIS which revealed a defected subsurface layer, being formed by the hydrogen-induced plastic deformation and exhibiting an enhanced concentration of open-volume defects.

Fig. 1 Optical microscopy image (taken using oblique light) of the surface of a ZnO crystal electrochemically loaded with hydrogen.

References
[1] G. Brauer, et al., Phys. Rev. B 79, 115212 (2009).
[2] J. Čížek, et al., J. Appl. Phys. 103, 053508 (2008).

Keywords: ZnO; vacancy-hydrogen complexes; hydrogen-induced plastic deformation; positron annihilation spectroscopy; atomic force microscopy

  • Lecture (Conference)
    12th International Workshop on Slow Positron Beam Techniques for Solids and Surfaces (SLOPOS-12), 01.-06.08.2010, Magnetic Island/North Queensland, Australia
  • Open Access Logo Journal of Physics: Conference Series 262(2011), 012050
    DOI: 10.1088/1742-6596/262/1/012050
    Cited 1 times in Scopus

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