Electrical and Optical Properties of n-type and p-type ZnO
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Electrical and Optical Properties of n-type and p-type ZnO D.C. Look1,2 and B. Claflin1,2 Semiconductor Research Center, Wright State University, Dayton, OH 45435 2 Materials and Manufacturing Directorate, Air Force Research Laboratory, WrightPatterson AFB, OH 45433 1
ABSTRACT In recent years, ZnO has been proposed for new electronic and optoelectronic devices, such as transparent transistors and UV light-emitting diodes (LEDs). The LED application will require both n-type and p-type ZnO, but the latter is difficult to produce, and progress in this area will require a detailed knowledge of the various impurities and defects that affect the electrical and optical properties. The dominant donors in as-grown ZnO are usually thought to be interstitial H and substitutional AlZn, with activation energies of about 40 and 65 meV, respectively. However, interstitial Zn and its associated complexes may also contribute free electrons. The dominant acceptor, at least in vaporphase-grown material, is the Zn vacancy; however, substitutional NO is also present, although sometimes passivated by H. To produce p-type ZnO, it is necessary to dope with acceptor-type impurities, and some success has been achieved with N, P, As, and Sb. However, only N has been proven to have simple substitutional character (NO), and more complicated acceptor structures, such as AsZn-2VZn, have been proposed for some of the other group V elements. Both homostructural and heterostructural UV LEDs have been fabricated, although not of high luminescent power so far. The main objective of this paper is to review the Hall-effect and photoluminescence results on n-type and p-type ZnO. INTRODUCTION The age of solid-state white lighting is fast approaching, mainly because III-N-based blue and UV light-emitting diodes (LEDs) are now available and can be combined with appropriate phosphors to produce white light [1]. Progress in this area is rapid, and it is projected that costs will be low enough, and efficiencies high enough, to justify replacement of the present lighting infrastructure in the USA by 2025. The power savings due to the increased efficiency will be enormous, approximately $135B per year, by one estimate. Besides LEDs, blue and UV laser diodes (LDs) will also be very important for the CD and DVD data storage industries, and blue LDs are already making their way into the marketplace. The commercial possibilities of UV LEDs and LDs have fostered a new interest in ZnO, which like GaN is also a wide-bandgap semiconductor material, and one which is fundamentally a brighter emitter than GaN. Worldwide research on ZnO has already produced high-quality bulk and epitaxial n-type material, but not reproducible ptype material [2]. Thus, it is imperative to understand the electrical and optical activities of the various background impurities and native defects, as well as the efficacy of the potential acceptor dopants. Many experimental and theoretical techniques have been employed in this quest, and we will discuss several of them, although concentra
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