The Local Electronic structure at Grain Boundaries and Hetero- Interfaces in ZnO Thin Films Grown by Laser Deposition.
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The Local Electronic structure at Grain Boundaries and Hetero- Interfaces in ZnO Thin Films Grown by Laser Deposition. Alexander Kvit, Gerd Duscher*, Chunming Jin and Jagdish Narayan, NSF Center for Advanced Materials and Smart Structures, Department of Materials Science and Engineering, NCSU, Raleigh, NC; * Joint position in Oak-Ridge National Laboratory, TN.
The structure and chemistry of interfaces and grain boundaries are known to influence the optical and electrical properties of wide-band gap semiconductors structures. ZnO/AlN/Si(100) heterostructures grown by laser deposition were studied by conventional and high-resolution transmission electron microscopy (HRTEM). The local electronic structure of ZnO grain boundaries was investigated by high resolution Z-contrast imaging using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy in a scanning mode. Zcontrast imaging and EELS were performed simultaneously enabling direct correlations between interface chemistry and local structure to be made. ZnO grain boundaries are composed of a periodic array of a basic structural unit. On the basis of the electron energy-loss near-edge structure (ELNES) of zinc and oxygen edges associated with the ZnO- grain boundaries, the corresponding electronic spectrum was discussed. It is generally agreed that defects in II-VI compounds play more important and sometimes detrimental role in comparison with Si and Ge. Therefore, identification of extended and point defects are of considerable importance to the understanding of the electrical and optical properties of wide-gap II-VI compounds. For example, many studies of the compensation mechanism in these compounds have been controversial due to the complexity of interaction between impurities and defects. On the other hand, the perturbation introduced by extended defects such as dislocations, in II-VI compounds also have relevance to the performance of II-VI devices. The optical and electrical properties of such defects are important since they can interact with the carrier transport and recombination processes. One of the important questions in the physics of II-VI semiconductors and their applications is the influence of extended defects like dislocations and grain boundaries on the electrical properties of these materials. Do dislocations or GB introduce any levels in the bandgap or, alternatively, do relevant impurities segregate there, thus initiating a compensation process? The pronounced radiative recombination from dislocation regions in II-VI compounds is an exciting but not a unique phenomenon. One of the most popular examples in this regard is the recombination of D- series in Si and SiGe. As for the II-VI compounds, the two sets of bands named Z- and Y- series have traditionally been attributed to extended defects. Unusual optical spectra referred to as Y- and Z- luminescence systems have been observed in various II-VI compounds1. The Huang-Rhys parameter, S, which describes the strength of coupling to LO phonons, is abnormally
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