p‑Type InN Nanowires S. Zhao, † B. H. Le, † D. P. Liu, ‡ X. D. Liu, ‡ M. G. Kibria, † T. Szkopek, † H. Guo, ‡ and Z. Mi* ,† † Department of Electrical and Computer Engineering, McGill University 3480 University Street, Montreal, Quebec H3A 0E9, Canada ‡ Department of Physics, McGill University, 3600 University Street, Montreal, Quebec, H3A 2T8, Canada * S Supporting Information ABSTRACT: In this Letter, we demonstrate that with the merit of nanowire structure and a self-catalytic growth process p-type InN can be realized for the first time by “direct” magnesium (Mg) doping. The presence of Mg acceptor energy levels in InN is confirmed by photoluminescence experiments, and a direct evidence of p-type conduction is demonstrated unambiguously by studying the transfer characteristics of InN nanowire field effect transistors. Moreover, the near- surface Fermi-level of InN can be tuned from nearly intrinsic to p-type degenerate by controlling Mg dopant incorporation, which is in contrast to the commonly observed electron accumulation on the grown surfaces of Mg-doped InN films. First-principle calculation using the VASP electronic package further shows that the p-type surface formed on Mg-doped InN nanowires is highly stable energetically. KEYWORDS: InN, nanowire, Mg doping, p-type III-nitride semiconductors are critical for a wide range of applications including solid-state lighting, ultraviolet photonics, photovoltaics, high-power electronics, and biosensors; the III- nitrides are regarded as the next Si. 1,2 One grand challenge for III-nitride semiconductors is the realization of p-type InN, which severely limits their device applications. To date, the study of p-type doping into InN is limited to InN thin films 3-7 and a direct evidence for the presence of free holes is still lacking due to the commonly measured surface electron accumulation on the grown surfaces of InN thin films, 8,9 which greatly overwhelms the p-type conduction. Recently, by cleaving an InN thin film the absence of surface electron accumulation was observed on nonpolar planes, 10,11 which sheds light on the possibility to measure p-type conduction directly. However, obtaining nonpolar grown surfaces without surface electron accumulation has not been possible mostly due to the lack of suitable substrates. In this regard, resorting to other low-dimensional structures with their optical and electrical properties being largely determined by nonpolar planes may provide a feasible solution. It has been found that the lateral surfaces of InN nanowires grown directly on Si substrates are typically nonpolar m-planes, 12,13 which largely determine the optical and electrical properties of InN nanowires because these nonpolar planes form the majority of the nanowire surface area. The nanowire approach can therefore offer a promising route to achieving p-type conduction. However, InN nanowires grown by conventional methods, such as the vapor -liquid-solid (VLS) process 14 and spontaneous formation process, 13 typically exhibit tapered surface morphology, and thus poor optical and electrical properties. These nominally nondoped InN nanowires possess very large residual electron density (on the order of 10 18 cm -3 , or higher) and high density of accumulated electrons on the lateral nonpolar surfaces 12,15,16 due to the formation of extensive n-type defects and their preferential incorporation into the near-surface region. 17 Furthermore, recent studies 18 have shown that defect species such as V N and O N are present (e.g., by self-compensation) and stable in p-type materials, further enhancing the residual electron density. These factors and the resulting strong n-type characteristics underscore the difficulty in realizing p-type conductivity in InN nanowires. For similar reasons “direct” p-type doping has not been possible in any of the technologically important narrow-bandgap semi- conducting nanowires, including InN, InAs, and InSb. Recently, we have demonstrated that intrinsic InN nanowires can be achieved by an improved molecular beam epitaxy (MBE) technique 17,19 that involves the use of an in situ deposited In seeding layer to promote the nucleation and formation of InN nanowires. A self-catalytic growth process, together with an ultrahigh vacuum MBE environment, can largely eliminate any undesirable impurity atom incorporation thus further minimize the formation of extensive surface defects. Consequently, the resulting InN nanowires possess extremely low residual electron densities (in the range of 10 13 to 10 15 cm -3 ) 20 and are absent of any surface electron accumulation on the lateral nonpolar surfaces. 17,21,22 This progress has made the realization of p-type doping into InN nanowires possible. In this Letter, we investigate the direct Received: August 17, 2013 Revised: September 12, 2013 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A dx.doi.org/10.1021/nl4030819 | Nano Lett. XXXX, XXX, XXX-XXX