Optical Characteristics of Shallow and Deep Level Defects in Epitaxial ZnO

Zinc oxide and associated II-VI oxide semiconductor alloys (CdO, MgO, BeO) possess bandgap energies that span the visible and ultraviolet spectral regions, and are of major interest for semiconductor light emitting devices at these wavelengths. The achievement of high quality ZnO substrates and experiments that demonstrate a close lattice match for CdZnO and MgZnO suggest that these materials may provide heterostructures with low defect density, leading to high device efficiency and reliability. In order to achieve these devices, an understanding of defects in the material is important to obtain low background carrier concentrations necessary for intentional doping, and high radiative efficiency. In this article, some notable optical characteristics of defects observed in ZnO materials grown by molecular beam epitaxy are described. Similar to other wide bandgap materials, one of the key challenges for ZnO emitters is the achievement of highly conductive p-type ZnO, requiring a stable shallow acceptor. A conventional approach for achieving p-type ZnO is a Group V substitutional impurity for oxygen. Nitrogen is the best match with respect to atomic radius, and provides the shallowest acceptor level. We have observed p-type ZnO using nitrogen doping (p~10cm). Photoluminescence spectra of nitrogen doped ZnO show an acceptor related transition (Fig. 1) 0.1 1