MD-Simulations of the Interface Structure of Wurtzite type Thin Films on Sapphire (00.1) and (01.2)Substrates

Wilfried Wunderlich, Tomohisa Kato, Hitoshi Ohsato, Takashi Okuda,
Nagoya Institute of Technology, Dept. Mat. Sci. Eng. Gokiso, Showa-ku, 466-8555 Nagoya, Japan
Tel: +81-90-4119-1290, Fax: +81-52-735-5294 e-mail: wi-wunder@rocketmail.com

Wurtzite thin films, like GaN, AlN, ZnO, ZnS or CdO, upon sapphire substrates are promising new electronic devices. For Alumina and Zinc Oxide good Buckingham potential parameters are available for Molecular Dynamics simulations which were performed using the "Moldy" software program which also includes the long range Coulomb interaction. The crystal structure of the ZnO/Al2O3 interface has been calculated and is compared to a previous model for other Wurtzite materials based on experimental results. In detail, three different models for the atomic structure at the interface are considered as input models, but only composite models avoiding the surfaces show reliable results. Concerning the structure of the (00.1) substrate on atomic level, four input models can be considered, two of each with long-stick- or zigzag-bonding at the interface, and two of each with the polar vector pointing towards or away from the interface. The zigzag-bonding was found to be more stable than the long-stick-bonding. During all calculations one structure occurs most frequently. In this interface structure locally two structural units appear, where both types of bonding, between the Wurtzite-cation to Oxygen and the Wurtzite-anion to Aluminium, are occurring. In particular, one structural unit consists of a Zinc atom at the interface bonded to three Oxygen atoms of the Alumina, with the same coordination number 4 as in the Wurtzite lattice. The other structural unit consists of an Aluminum atom at the interface bonded to four Oxygen of Wurtzite and four of Alumina, with the same coordination number 8 as in the Corundum lattice. Although the interface is coherent, the periodicity length is rather large and due to the occurrence of misfit dislocations in three different directions on the interface plane, the atomic structure in between them is not well-defined. The atomic structure of the misfit dislocation core is about one atomic distance in thickness and height which is less than assumed in the previous model. In the case of (01.2) substrates the surface is rough on the atomic level and therefore the interface structures gets less welldefined.

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