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Contactless Optical Characterization of Carrier Dynamics in Free-Standing InAs-InAlAs Core-Shell Nanowires on Silicon Xinxin Li, Kailing Zhang, Julian Treu, Lukas Stampfer, Gregor Koblmueller, Fatima Toor, and John Prineas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04226 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019
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Nano Letters
Contactless Optical Characterization of Carrier Dynamics in Free-Standing InAs-InAlAs Core-Shell Nanowires on Silicon Xinxin Li 1, 2, Kailing Zhang 1, 2, Julian Treu4, Lukas Stampfer4, Gregor Koblmueller4, Fatima Toor1,2, 3and John P. Prineas 1, 2, 3, *
1Department
of Physics and Astronomy, 2Optical Science and Technology Center, 3Department of Electrical and Computer Engineering, University of Iowa, IA 52242, USA 4Walter
Schottky Institut and Physics Department, Technical University Munich, Garching 85748, Germany
ABSTRACT Contactless time-resolved optical pump-probe and external quantum efficiency measurements were performed in epitaxially grown freestanding wurtzite indium arsenide/indium aluminum arsenide (InAs-InAlAs) coreshell nanowires on Si (111) substrate from 77K to 293K. The first independent investigation of Shockley-Read-Hall, radiative and Auger recombination in InAs-based NWs is presented. Although the Shockley-Read Hall recombination coefficient was found to be at least two orders of magnitude larger than the average experimental values of other reported InAs materials, the Auger recombination coefficient was reported to be ten-fold smaller. The very low Auger and high radiative rates result in an estimated peak internal quantum efficiency of the core-shell nanowires as high as 22% at 77K, making these nanowires of potential interest for high efficiency mid-infrared emitters. A greater than two-fold enhancement in minority carrier lifetime was observed from capping nanowires with a thin InAlAs shell due to passivation of surface defects.
KEY WORDS Pump-probe spectroscopy, minority carrier lifetime, core-shell nanowires, Shockley-Read-Hall, radiative, Auger recombination rate, surface/interface recombination velocity
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Indium Arsenide (InAs) nanowires (NWs) have gained increasing attention due to their numerous applications. The high electron mobility and narrow direct band gap make them a promising material for high-speed electronic devices such as transistors1-2 and infrared (IR) photovoltaic devices, including short-wavelength infrared (SWIR)3 and mid-infrared (MIR) photodetectors4-5. Various growth techniques of InAs NWs have been explored to gain control over dimensions6-8, microstructure, and electronic and optical properties. Non-catalytically grown NWs eliminate unwanted metal seeds such as Au9-10, which can contaminate the NWS and create deep level traps that enhance non-radiative recombination. Other trap states for charge carriers have been identified by the prevalent wurtzite/zincblende crystal-phase intermixing in InAs NWs, where their trapping and deactivation behavior has been characterized11. Moreover, surface passivation and encapsulation of NWs have been found essential to increase the carrier mobility12-13, to prolong carrier lifetime12, 14, to reduce the photoluminescence (PL) linewidth15 and to enhance the PL emission intensity16. The ability to grow NWs with reduced amounts of material compared to planar layers, and on low cost lattice mismatched substrates such as silicon (Si) further add to their technological interest. The minority carrier lifetime of a plethora of III-V semiconductor NWs has been widely investigated at room temperature. Gallium nitride (GaN) NWs were found to have carrier lifetime as long as 2.5ns due to apparently weak influence of surface traps compared to other III-V semiconductors17. Through manipulating and passivating surface states18, the minority carrier lifetime of gallium arsenide/ aluminum gallium arsenide (GaAs/AlGaAs) core-shell NWs was enhanced to 1.9ns, much longer than that of unpassivated GaAs NWs (2.4ps) with comparable diameter19. InAs NWs with similar radial size19, on the other hand, only have carrier lifetime of 290ps. Since there are no reports about the carrier lifetime of InAs-based core-shell NWs, the interplay of carrier lifetime and non-radiative recombination with passivation, and the question of whether these types of NWs are suitable for MIR optoelectronic applications remain open. On the one hand, carrier recombination in NWs is claimed to be mostly limited by surface recombination 20. However, the validity of this statement has to be evaluated since the Auger recombination process becomes profound under high carrier injection in narrow gap materials. Carrier recombination may have contributions from both bulk and surfaces9, 21. Yet the individual contribution of radiative and nonradiative recombination including Shockley-Read-Hall (SRH) and Auger recombination to bulk recombination has not been investigated. A thorough understanding of charge carrier recombination mechanisms in InAs-based NWs is therefore crucial. Without any electrical contacts or fabrication steps, the ultrafast differential transmission and external quantum efficiency (EQE) measurements are non-destructive to the samples of interest and thus enable us to determine carrier dynamics in the free-standing NWs attached to the substrate. In the current study, the aforementioned optical techniques along with the widely used ABC model22-24 were applied for the first time to characterize the recombination rates and branching ratios of three constituent recombination mechanisms, known as SRH, radiative and Auger recombination, in InAs/InAlAs core-shell NWs under varying excess carrier density. InAs/InAlAs interface SRH recombination is found to be the dominant process at low carrier density (
