Identification and control of defects in nitrogen-doped ZnO

Abstract

ZnO is regarded as a wide band gap semiconductor with high-efficient luminescent properties due to the stable existence of excitons at room-temperature. However, the prominent opto-electronic properties have not been achieved in real application because the defects existing in the material are quite complex in terms of form and property. Also, the p-type doping of ZnO is well-known to be extremely hard. As a result, more and more recent works have shown that the properties of the ZnO material are of high-possibility to be determined by the defects in ZnO. For instance, in p-type doped ZnO, free holes could be a result of complex defects rather than the anticipated dopants. Therefore, it is quite important to investigate the forms, properties, and control of various defects in ZnO. In this article, we will discuss the properties and control of various intrinsic and extrinsic defects in ZnO by reviewing the recent progress on related studies. Firstly, the formation, form, and control technique of the zinc interstitial related donors will be discussed. It is shown that the zinc interstitial related defect is an important compensating source to holes in nitrogen-doped ZnO material. The defects could be identified and tracked quantitively by a combination of a batch of characterization methods, such as electron paramagnetic resonance, X-ray photoelectron spectroscopy, and Raman backscattering spectroscopy. The defects could be controlled by appropriately setting the condition of growth and/or post thermal treatment. Secondly, the main origin of possible acceptors in nitrogen-doped ZnO material has been pointed out. Intrinsic vacancies in combination with the extrinsic dopants, nitrogen, could be possible acceptors with shallow activation energy. Meanwhile, zinc vacancy small clusters could also be a candidate responsible for the shallow states of acceptors in ZnO. Moreover, novel complex acceptors as proposed by first-principles theoretical calculations have been experimentally introduced and realized. Under O-rich condition during nitrogen doping of ZnO, nitrogen molecule (N2) or nitrogen-hydrogen (NH x ) complexes could be incorporated on the zinc site, having shallow activation energy to contribute holes. As a result, the origin of p-typeness realized in nitrogen-doped ZnO could be partially understood. Combining the photoluminescence characterization, the corresponding relation between the defects and the optical emission lines has been established. Such a “fingerprint” could be utilized to assist identify defects in ZnO. Thirdly, we will discuss the isovalent-acceptor (IA) co-doping scheme, like tellurium-nitrogen or sulfur-nitrogen co-doping in ZnO. Such a co-doping technique could be a feasible route for obtaining high-efficient p-type ZnO material by suppressing compensation and enhancing activation rate of free holes. Zinc interstitials related donor-like complexes could be well suppressed while the co-doped isovalent element is beneficial to stabilizing nitrogen, restoring strain mismatch, and elevating the valence band maximum (VBM). The elevated VBM can further facilitate the ionization of acceptors, making the p-typeness much prominent. At last, using the IA co-doping technique, we will show the realization of ZnO homojunction light-emitting diode made by IA-co-doped ZnO nano-rod on high-quality ZnO thin films at room temperature and other applications of ZnO material to devices, like high-performance non-volatile memory and broadband photo-detector. The current problem, perspective and outlook of the defects and p-type doping in ZnO have been briefly discussed at the end of the article.