Achieve p -type conduction in N-doped and (Al,N)-codoped ZnO thin films by oxidative annealing zinc nitride precursors
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N-doped and (Al,N)-codoped ZnO films were synthesized by oxidative annealing of (Zn + Zn3N2) films, which were fabricated by reactive magnetron sputtering. Both nand p-type conductions were obtained in these ZnO:N and ZnO:AlN films. Optimal oxidation treatments for achieving p-type ZnO are annealing at 400–600 °C for 10–60 min, depending on the film thickness and morphology. The electric properties were found to be very sensitive to the annealing conditions and film structure. As-deposited (Zn + Zn3N2) films with and without Al addition had carrier concentrations of 1021–1022 cm−3. After conversion to ZnO, the n-type films had a carrier concentrations up to 1019 cm−3, whereas the p-type ZnO:N films had hole concentrations of 1014–1016 cm−3. (Al,N)-codoping increased the hole concentration of p-type film to 1018 cm−3 despite a decrease in Hall mobility. The photoluminescence properties of the p-type ZnO films were also investigated. The synthesis of p-type ZnO:AlN by oxidative annealing is believed to provide an alternative approach to realize p-type conduction in codoped ZnO film by using N2 as the N source.
I. INTRODUCTION
As one of the most important transparent conducting oxides, the wide-band-gap semiconductor ZnO has been considered as a promising material for optoelectronic devices, such as ultraviolet light-emitting diodes and laser diodes. For development of these ZnO-based devices, it is necessary to synthesize both high-quality n- and p-type ZnO films. It is well known that undoped ZnO is naturally n-type because of the existence of intrinsic defects, such as oxygen vacancies, zinc interstitials, etc. To increase the carrier concentration, group III element doping is normally used. It has been found that doping by Al, Ga, etc. can reduce the resistivity of ZnO to a very low 1 level. On the other hand, due to the self-compensating effect in wide-band-gap semiconductors, it is difficult to prepare p-type ZnO films with low resistance. The first demonstration of p-type ZnO was published in 1997.2 Since then, many other reports have been published, although some reported high mobilities or carrier concentrations were thought to be unreasonable.3,4 All of the common
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Address all correspondence to this author. e-mail: [email protected], [email protected] Present address: School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798. DOI: 10.1557/JMR.2007.0364 2668 J. Mater. Res., Vol. 22, No. 10, Oct 2007 http://journals.cambridge.org Downloaded: 04 Apr 2015
group V elements, i.e., N, P, As, and Bi, have been successfully used to make p-type ZnO.1,5–7 Based on the calculations by the first-principles pseudopotential method, the best doping element for p-type ZnO is N.8 According to Look and Claflin,4 more than half of the p-type ZnO samples reported in the literature have been doped with N. N-doped p-type ZnO films have been grown by chemical vapor deposition (CVD),2 molecularbeam epitaxy (MBE),9 spin coating,10 pulsed laser deposition (PLD),11 and sputtering.12 A number
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