Infrared Lattice Vibrations of Nitrogen-doped ZnO Thin Films
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1035-L07-07
Infrared Lattice Vibrations of Nitrogen-doped ZnO Thin Films Makoto Hirai1, and Ashok Kumar2 1 Nanomaterials & Nanomanufacturing Research Center, University of South Florida, Tampa, FL, 33620-5350 2 Department of Mechanical Engineering, University of South Florida, Tampa, FL, 33620-5350 ABSTRACT Nitrogen (N) is the most promising p-type dopant for zinc oxide (ZnO), and its bonding state must be governed by the substitution of N atoms into anion sites. We have synthesized undoped ZnO and N-doped ZnO thin films by utilizing a pulsed laser deposition (PLD) method. The N-doped ZnO thin film possessed shorter c-axis length than the un-doped ZnO thin film. This fact seems to be owing to that Zn-N bond length is shorter than Zn-O bond length in wurtzite structure. Besides, from the result of Fourier transform infrared (FT-IR) measurement, the absorption peak of the N-doped ZnO thin film emerged at 406 cm-1, and was attributed to transverse optical (TO) phonon of E1 mode. The infrared lattice vibrations of the N-doped ZnO thin films can be induced by the complex factors consisting of not only the decreases in reduced mass and interionic distance, but also the increase in covalency. INTRODUCTION The II-VI semiconductors, most of which crystallize into wurtzite structure with space group C6v4, tend to have large direct band gap. Recently, much attention has been based for the development of ultraviolet and blue light emitting diodes and lasers, as they have a great potential to contribute toward the reduction in energy consumption. However, the development of such optoelectronic devices has been impeded due to the difficulty in p-type doping, which is mainly attributed to self-compensation of shallow acceptors by native donor defects. Zinc oxide (ZnO) is a transparent II-VI semiconductor with a direct band gap of 3.4 eV. Nitrogen (N) is widely considered to be the most promising p-type dopant, producing a shallow acceptor level in ZnO compared to the other V-group elements as phosphorus (P) and arsenic (As).1 Until now, several research groups have already synthesized N-doped ZnO thin films with p-type conductivity.2-4 Additionally, since Pauling’s electronegativity values of Zn, N and O atoms are 1.65, 3.04 and 3.44 respectively,5 the substitution of N atoms for partial O atoms in wurtzite structure is predicted to cause the slight change in bonding state. However, this change seems to affect interionic distance and phonon frequency, whereas the experimental data and overall discussion related to such phenomena cannot be enough. In this investigation, un-doped ZnO and N-doped ZnO thin films have been fabricated by utilizing a pulsed laser deposition (PLD) method, furthermore, the major carrier of the N-doped ZnO thin film was judged from the current-voltage (I-V) curve for homojunction structure. Moreover, examining c-axis length and phonon frequency of the thin films with x-ray and infrared techniques, we have investigated the effect of N doping for bonding state of the ZnO thin films. It is extremely important
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