Enhanced Minority Carrier Lifetimes in GaAs/AlGaAs Core−Shell Nanowires through Shell Growth Optimization N. Jiang,* ,† Q. Gao, † P. Parkinson, ‡ J. Wong-Leung, †,§ S. Mokkapati, † S. Breuer, † H. H. Tan, † C. L. Zheng, ∥ J. Etheridge, ∥,⊥ and C. Jagadish † † Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia ‡ Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3 PU, United Kingdom § Centre for Advanced Microscopy, The Australian National University, Canberra, ACT 0200, Australia ∥ Monash Centre for Electron Microscopy, Monash University, Victoria 3800, Australia ⊥ Department of Materials Engineering, Monash University, Victoria 3800, Australia * S Supporting Information ABSTRACT: The effects of AlGaAs shell thickness and growth time on the minority carrier lifetime in the GaAs core of GaAs/AlGaAs core−shell nanowires grown by metal−organic chemical vapor deposition are investigated. The carrier lifetime increases with increasing AlGaAs shell thickness up to a certain value as a result of reducing tunneling probability of carriers through the AlGaAs shell, beyond which the carrier lifetime reduces due to the diffusion of Ga−Al and/or impurities across the GaAs/AlGaAs heterointerface. Interdiffusion at the heterointerface is observed directly using high-angle annular dark field scanning transmission electron microscopy. We achieve room temperature minority carrier lifetimes of 1.9 ns by optimizing the shell growth with the intention of reducing the effect of interdiffusion. KEYWORDS: GaAs/AlGaAs, nanowires, minority carrier lifetimes, MOCVD, HAADF-STEM S emiconductor nanowires are investigated as key compo- nents for future optoelectronic devices. 1−8 With their one- dimensional geometry and the ability to form radial or axial heterostructures, nanowires and their heterostructures show significant advantages over their planar counterparts and thus a potential for creating high performance devices, such as solar cells, 2,4,6,7,9,10 photodetectors, 11−13 light-emitting diodes, 14,15 transistors, 8 and lasers. 8,16 III−V semiconductors, with their direct band gap and high electron mobility, are excellent materials for advanced optoelectronic devices. Among the III− V semiconductor nanowires, GaAs/(Al,Ga)As core−shell nanowires are prime candidates for investigation, 17,18 since they are lattice-matched materials and planar GaAs/AlGaAs heterostructures have been extensively studied and used in the electronics and optoelectronics industries. 14,15,19 To match the internal quantum efficiency achieved by state- of-the-art GaAs-based planar optoelectronic devices, a high- quality GaAs/AlGaAs heterointerface is required in GaAs/ AlGaAs core−shell nanowires to reduce surface recombination velocity (SRV) and obtain long room temperature minority carrier lifetimes (τ mc ). 19−21 Due to the high surface-to-volume ratio in nanowires, the electronic properties such as carrier lifetimes and mobility are extremely sensitive to the surface and interface states. Research on planar structures has shown that an almost “surface-effect-free” GaAs epilayer with a SRV of only ∼50 cm/s and microsecond τ mc can be achieved by replacing the free GaAs surface with a GaAs/AlGaAs heterointerface. 22,23 However, the longest τ mc in GaAs/AlGaAs core−shell nano- wires are still in the nanosecond regime. 24−26 Both the growth techniques 25 and the nanowire crystal structure 27−29 are reported to influence carrier lifetimes in GaAs/AlGaAs core− shell nanowires. The effect of the AlGaAs shell, on the other hand, has not yet been extensively studied. However, orders of magnitude difference in τ mc have been observed between nanowires differing only in their AlGaAs shell growth temperature in our previous study. 24 Thus, to be able to produce high-quality GaAs nanowires suitable for device applications, it is important to understand the effect of shell growth parameters on the structural and optoelectronic properties of the nanowires. To obtain a high-quality AlGaAs layer, a high process temperature is very important, particularly for growth utilizing metal−organic chemical vapor deposition (MOCVD). 20,24 Compared with GaAs grown under similar conditions, AlGaAs typically leads to higher O impurity incorporation arising from the trimethylaluminium precursor. 30 Furthermore, the stronger Al−C (65 kcal/mol) bond energy in comparison to Ga−C (57.5 kcal/mol) bond energy also results in a higher level of background C incorporation. 31,32 Compositional uniformity is a further issue because of the difference in diffusion length Received: June 27, 2013 Revised: September 14, 2013 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A dx.doi.org/10.1021/nl4023385 | Nano Lett. XXXX, XXX, XXX−XXX