Synthesis and properties of p -type nitrogen-doped ZnO thin films by pulsed laser ablation of a Zn-rich Zn 3 N 2 target
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Y. Che, Z.D. Hu, and B. Liu Functional Materials Group, IMRA America, Inc., Ann Arbor, Michigan 48105 (Received 14 March 2007; accepted 8 May 2007)
Epitaxial ZnO thin films doped uniformly with nitrogen at 1020 atoms/cm3 were fabricated by pulsed laser ablation of a Zn-rich Zn3N2 target. The films grown at 300 °C and annealed at 600 °C in O2 showed p-type conductivity. Two acceptor levels at 105 and 224 meV were determined by temperature-dependent Hall and photoluminescence measurements of the p-type samples. Transmission electron microscopy studies revealed that the p-type ZnO films consist of 10–20 nm columnar grains with a high density of defects and grain boundaries that may facilitate the annihilation of native donors and the activation of acceptors during postdeposition annealing.
I. INTRODUCTION
There has been an increasing interest in ZnO for applications in optoelectronics such as solid-state lighting and ultraviolet (UV) light detection. However, because its valence band maximum lies well below the p-type pinning energy, ZnO is asymmetric and resistant to shallow acceptor doping.1 Despite recent reports of ZnObased light-emitting devices,2 the fabrication of robust p-type ZnO is still limited by various issues such as a low dopant solubility due to the intrinsic band structure of ZnO,3 self- and doping-induced compensation,4 and passivation by hydrogen.5 Theoretically, the substitution of oxygen by nitrogen gives the shallowest acceptor among group V elements, with an activation energy of 400 meV.6 Therefore, a high nitrogen concentration is required to produce hole concentrations on the order of 1017 to 1018 cm−3. Yet, there is a marked trade-off between nitrogen solubility and structural quality since ZnO films of good epitaxial quality require high growth temperatures, while the nitrogen solubility generally decreases with the growth temperature. Plasma-assisted growth, which relies on the dissociation of nitrogen gases such as N2, N2O, or NO, has been in some cases successful in fabricating p-type ZnO.2,7
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0294 J. Mater. Res., Vol. 22, No. 8, Aug 2007
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However, the complex kinetics of reactions involving nitrogen and its radicals during growth results in the incorporation of undesired nitrogen-related donors such as (N2)O or NO–(N2)O.8,9 Electron paramagnetic resonance studies have also shown that molecular nitrogen (N2−) can act as an acceptor in ZnO crystals,10 illustrating the difficulty of controlling how nitrogen incorporates the ZnO film. In this paper, we report the fabrication of reproducible and stable p-type ZnO thin films by pulsed laser ablation of a Zn-rich Zn3N2 target with which the nitrogen solubility issue can be overcome. Because of the characteristic high kinetic energies of the plume ions, nitrogendoped epitaxial ZnO films can be grown at low temperatures. We found that the concentration of nitrogen was as high as 1020 atoms/cm3 in t
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