Abstract
Self-assembled nanowires are posed to be viable alternatives to conventional planar structures, including the nitride epitaxy for optoelectronic, electronic and nano-energy applications. In many cases, current injection and extraction at the nanoscopic scale are essential for marked improvement at the macroscopic scale. In this investigation, we study the mechanism of nanoscale current injection and the origin of improvement of the flow of charged carriers at the group-III nitride semiconductor surface and metal-semiconductor interfaces. Conductive atomic force microscopy (c-AFM) and Kelvin probe force microscopy (KPFM) enable a rapid analysis of the electrical and morphological properties of single and ensemble nanostructures. The surface potential and current injection of AlGaN nanowire-based LEDs are spatially mapped before and after surface treatment with KOH solution. Treated-nanowires showed an improved current spreading and increased current injection by nearly 10×, reduced sub-turn-on voltage (as low as 5 V), and smaller series resistance. The reduced contact potential confirms the lower semiconductor/metal barrier, thus enabling larger carriers flow, and correlates with the 15% increase in injection efficiency in macroscopic LEDs. The improvement leads to the normalization of nanoscale electrical conducting properties of UV AlGaN-based nanowire-LEDs and lays the foundation for the realization of practical nanowire-based device applications.
Highlights
AlGaN-based UV nanowires light emitters have recently attracted attention due to their unique properties [1,2,3] that partly overcome the drawbacks of the more mature planar technology, such as spontaneous and piezoelectric polarization fields [4], high dislocation density [5], low p-doping efficiency [6], and light extraction efficiency [7]
In this work we address the nanoscale current injection limitation of UV AlGaN nanowires by analyzing, for the first time, the origin that prevents the efficient carrier flow and distribution uniformity. Conductive atomic force microscopy (c-AFM) and Kelvin probe force microscopy (KPFM) techniques were used to measure the single nanowires, and to map the current and the contact potential difference distributions of ensembles of nanowires
This study aims to give a deep understanding of the nanometer scale current injection mechanism among the self-assembled AlGaN-based nanowires, and provides insights towards enhanced current spreading, injection efficiency and overall UV device performance demonstration
Summary
AlGaN-based UV nanowires light emitters have recently attracted attention due to their unique properties [1,2,3] that partly overcome the drawbacks of the more mature planar technology, such as spontaneous and piezoelectric polarization fields [4], high dislocation density [5], low p-doping efficiency [6], and light extraction efficiency [7]. GroupIII nitride nanowires can be grown on CMOS-compatible and cost-effective substrate such as Si [8,9] and highly conductive metal substrates [10,11,12,13,14] This allows better electrical and heat dissipation compared to the commercial AlN and sapphire substrates, and improves the overall device performance [15,16]. They offer a promising future for applications in air purification, water disinfection, curing, phototherapy and the generation of high resolution displays [17,18]. The small contact area and the self-assembled growth of nanowires, lead to nonuniformity and low carrier injection which subsequently lowers the injection efficiency
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