Electrical characterization of defect states in local conductivity domains in ZnO:N,As layers
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P- and n-type conductivity domains in dual-doped ZnO:As+N layers grown by metal organic vapor phase epitaxy on GaN–sapphire templates were electrically microcharacterized by scanning capacitance microscopy (SCM) and scanning surface potential microscopy (SSPM) techniques with respect to their defect states. The p-type domains were found to be dominated by two acceptors with thermal activation energies of about 80 and 270 meV, as observed by transient SCM scans at different temperatures. Optically excited SSPM scans revealed defect-to-band transitions at 400, 459, and 505 nm omnipresent in both domain types as well as a shallower transition at 377 nm exclusively in the p-type regions. According to the similar energy levels, the optical transitions at 377 and 400 nm are assigned to acceptor states, whereby the 80-meV acceptor is probably responsible for the conversion from n- to p-type regions in the domains.
I. INTRODUCTION
Because of some unique material properties (e.g., a very high exciton binding energy of ∼60 meV in combination with a large direct band gap of about 3.37 eV at room temperature), ZnO has an extremely high application potential for use in optoelectronics systems in the ultraviolet/blue/green spectral range. However, before the commercial realization of ZnO-based light emitters can start, the question of successful p-type doping of ZnO has to be solved. In the last few years, a couple of different acceptordoping approaches have been tested, from the exploitation of intrinsic Zn vacancy acceptors1 via conventional monodoping with group-I elements (Li, Na, and K) on Zn sites2 or group-V elements (N, P, As) on O sites,3,4 to codoping and cluster doping with N and Ga simultaneously.5,6 However, despite the variety of approaches and some promising reports on successful acceptor doping, the problems in achieving a reproducible, controllable, and enduringly stable p-type ZnO remain. In a recent work,7 we studied the effect of conventional monodoping with nitrogen and arsenic as well as simultaneous doping with both acceptor species (hereaf-
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Address all correspondence to this author. e-mail: [email protected] This paper was selected as the Outstanding Meeting Paper for the 2006 Materials Research Society Fall Meeting Symposium K Proceedings, Vol. 957. DOI: 10.1557/JMR.2007.0238 J. Mater. Res., Vol. 22, No. 7, Jul 2007
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ter called “dual-doping”) by scanning capacitance microscopy (SCM). The main result of our work was that these doping approaches result in the formation of local n- and p-type conductivity domains induced by structural inhomogeneities instead of uniformly doped p-type ZnO layers. Moreover, in this study we observed for some dual-doped ZnO:As+N samples dominant p-type regions and good surface morphology only locally disturbed by minor defect- and grain-related n-type parts. Similar domain formation from GaN is already known8 and has been attributed to selective dopant incorporation. In the present article, we
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