Abstract
This paper reviews the recent progress in acceptor doping of ZnO that has been achieved with a focus toward the optimum strategy. There are three main approaches for generating p-type ZnO: substitutional group IA elements on a zinc site, codoping of donors and acceptors, and substitution of group VA elements on an oxygen site. The relevant issues are whether there is sufficient incorporation of the appropriate dopant impurity species, does it reside on the appropriate lattice site, and lastly whether the acceptor ionization energy is sufficiently small to enable significant p-type conduction at room temperature. The potential of nitrogen doping and formation of the appropriate acceptor complexes is highlighted although theoretical calculations predict that nitrogen on an oxygen site is a deep acceptor. We show that an understanding of the growth and annealing steps to achieve the relevant acceptor defect complexes is crucial to meet requirements.
Highlights
Zinc oxide has been investigated for many years, its potential for photonic and electronic applications has led to significant resurgence in interest during the last decade
Its use as a widely diverse functional material is enhanced by the fact that it can be grown in bulk, thin film, and nanostructures, examples of the latter being nanowires, nanobelts, and other morphologies that are dependent upon growth conditions
Many of the potential device applications, require both donor and acceptor doping, and growth of reproducible and stable p-type ZnO has been difficult to achieve. This doping asymmetry problem in which n-type doping is achieved while p-type is quite problematic is well known [3], and widespread development of ZnO-based devices has been inhibited
Summary
Zinc oxide has been investigated for many years, its potential for photonic and electronic applications has led to significant resurgence in interest during the last decade. Advantages that ZnO has in comparison to GaN are availability of a native substrate, relative ease of wet chemical etching for device fabrication, its large exciton binding energy of approximately 60 meV [10] and biexciton binding energies on the order of the 25 meV thermal energy at room temperature [11] These latter characteristics make it attractive for low threshold and large differential quantum efficiency photonic devices in the UV and blue portion of the electromagnetic spectrum. We will first address the origin of background donors, which is highly dependent upon whether the material is grown in a Znrich or an O-rich environment, the role of H as a donor in ZnO, and the issue of the acceptor ionization energy of substitutional N on the O sublattice We follow this with a summary of recent reports on the p-type behavior for the various potential doping schemes and conclude with our outlook
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