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Thermal Delocalization of Excitons in GaAs/ AlGaAs Quantum Well Tube Nanowires Teng Shi, Howard E Jackson, Leigh Morris Smith, Nian Jiang, Hark Hoe Tan, and Chennupati Jagadish Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04864 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016
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Thermal Delocalization of Excitons in GaAs/AlGaAs Quantum Well Tube Nanowires Teng Shi,† Howard E. Jackson,† Leigh M. Smith,† Nian Jiang,° H. Hoe Tan,° and Chennupati Jagadish° †
Department of Physics, University of Cincinnati, Cincinnati, OH 45221-0011
°Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia
Keywords: Nanowires, Heterostructures, Quantum Well, Quantum Dots e-mail address:
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ABSTRACT
We use temperature-dependent photoluminescence (PL), photoluminescence imaging and timeresolved photoluminescence measurements to gain insights into the localization of excitons in single 2 nm GaAs/AlGaAs quantum well tube nanowires. PL spectra reveal the coexistence of localized and delocalized states at low temperatures, with narrow quantum dot-like emission lines on the high energy side of a broad emission band, and delocalized states on the low energy side. We find that the high energy QD-like emissions are metastable, disappearing at higher temperatures with only delocalized states (quantum well tube ground states) surviving. By comparing temperature- and time- dependent PL we develop a theoretical model which provides insights into the confinement potentials and relaxation dynamics which localize the excitons in these quantum well tube nanowires.
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Introduction High quality group III-V semiconductor nanowire heterostructures are excellent candidates as the basis for novel optical devices and optoelectronic applications, such as lasers, photodetectors and field-effect transistors.1–9 In the past few decades, researchers have worked to optimize the growth conditions in order to grow high quality core only and core/shell nanowires.10–13 However, one limitation of these III-V core and core/shell nanowires is the lack of the flexibility of bandgap engineering or the control of quantum wavefunctions. Recently, intensive studies have been conducted on GaAs/AlGaAs radial heterostructured nanowires with quantum confinement. Quantum-confined core-multishell GaAs/AlGaAs nanowires can easily be fabricated through either MOCVD- or MBE-grown GaAs quantum well tubes (QWT) embedded in AlGaAs barriers.14–20 These GaAs/AlGaAs QWT nanowires are found to exhibit both high optical quantum efficiency, as well as confinement energies of excitons as high as 400meV for 1.5 nm QWT width.14,15 Recent spatially-resolved photoluminescence (PL) measurements have shown that excitons can become localized for narrow (< 5 nm) quantum well tubes, resulting in a broad spectrum of ultranarrow emission lines on the high energy side of the luminescence band. High resolution spatially-resolved PL measurements show the coexistence of localized and delocalized states in a 2 nm GaAs/AlGaAs QWT nanowires at low temperatures. Such localized states are seen to persist up to 80 K, but only delocalized excitons are observed at higher temperatures.15
We
present
below
temperature-dependent
photoluminescence
(PL),
photoluminescence imaging and time-resolved photoluminescence measurements which provide, via a phenomenological theoretical model, a new understanding of the complex carrier dynamics in these narrow QWT nanowires.
With this understanding we gain new insights into the
localization potentials caused by the alloy or well-width disorder present in the quantum wells or barriers.
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2nm QWT temperature-dependent PL spectra To gain insight into this process, we use temperature-dependent PL spectroscopy to investigate 2 nm GaAs/AlGaAs quantum well tube (QWT) nanowires. These QWTs are grown on a (111)Boriented GaAs substrate by 50 nm Au-catalyzed MOCVD growth with overall diameters of ~150 nm and total length of ~4 µm. Detailed nanowire growth information can be found elsewhere.15 Cross-sectional HAADF STEM images of a 2 nm QWT in Figure 1(a) has shown a well-defined GaAs QWT (brighter hexagonal band) with AlGaAs barrier layers on both sides. Figure 1(a) also shows clear evidence of 3-fold featured symmetry suggested by Zheng et al.21 Along the three B directions there are bands of higher Al concentration and slightly narrower GaAs quantum well widths, while along the three A directions there are somewhat lower concentration bands of Al rich growth and slightly wider quantum well widths. Figures 1(b) and (c) are the higher magnification images around two hexagonal corners from the top region of Fig. 1(a), which show the distinct change in QWT width along the different facets. Here, 2 nm GaAs/AlGaAs QWT nanowires were mechanically transferred onto the center of a 4 mm hemisphere solid immersion lens (SIL) with a refractive index of n=2. The SIL was mounted to the cold finger of a continuous-flow liquid helium optical cryostat. The orientation of the NW long axis was set to be parallel to the entrance slit of the spectrometer. A pulsed 2.1 eV excitation laser was defocused so that the nanowire was illuminated uniformly with an excitation density of 14 W/cm2. The PL emission from a single nanowire was collected through a 50×/0.5NA microscope objective and then imaged onto the entrance slit of a DILOR triple-stage spectrometer with spectral resolution of ~200 µeV and a spatial resolution of 500 nm. The PL emission was then dispersed by a grating, and finally collected by a 2048×512 pixel liquid nitrogen-cooled CCD.
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Figure 1: (a) low magnification HAADF-STEM Cross-sectional image of a 2 nm GaAs/AlGaAs QWT. The AlGaAs layers are light and dark grey which reflect the aluminum concentration, while the GaAs layers are nearly white. (b) High magnification image of (a) along the 112B direction. (c) High magnification image of (a) along the 112A direction. (d) Photoluminescence spectra of a single 2 nm QWT at temperatures from 10 K to 120 K. All spectra are scaled to the same intensity range. The PL spectra of a single 2 nm GaAs/AlGaAs QWT as a function of temperature are shown in Figure 1(d). PL spectra at different temperatures were taken under the same excitation conditions and normalized to the same intensity. At 10 K, a large number of ultranarrow QD-like emissions are observed on the high energy side (emission energies > 1.85 eV at 10 K). On the low energy side (emission energies < 1.85 eV at 10 K), such sharp lines are not seen. Instead, several broad PL emissions are detected. At higher temperatures, the linewidths of individual peaks become broader and the emission centers shift to lower energies. The quenching of QD-
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like emissions on the high energy side starts at low temperatures and all of the localized states in the 2 nm QWT quench completely by ~100 K. At 120 K, only three major peaks are survived, and these states experience a redshift of ~20 meV as the temperature increases from 10 K to 120 K. Fickenscher et al. reported that in GaAs/AlGaAs QWT nanowires, the ground states for electron and hole states are strongly confined to the hexagonal corners and the confinement energies are extremely sensitive to QW width fluctuations.14 We attribute these broad PL emissions on the low energy side to the ground states originating from the thick corners of the 2 nm QWT along the 112A directions (Fig. 1c). The localized and metastable states likely result from narrower parts of the QWT where alloy or well-width fluctuations cause excitons to become localized. As the temperature increases, these localized excitons on the high energy side appear to escape from the weak confinement potential and recombine through other channels so that finally only the ground states emit, resulting in a substantial narrowing of the PL band. Such PL linewidth narrowing is also seen in self-assembled QD systems due to the carrier transfer between wetting layer or QW and QDs at temperatures under 150K.22–26
PL spatial imaging of 2nm QWT at different temperatures To obtain more detailed information of the localized PL emissions from the 2 nm QWT, spatially- and energy-resolved PL maps from the same nanowire were also obtained as a function of temperature. Nonresonant and defocused excitation of 14 W/cm2 was used to illuminate the entire nanowire. Using a high-index SIL hemisphere with slit-confocal spectroscopy provides distinct advantages with both increased light collection efficiency and an increased spatial resolution down to ~500 nm.15
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Figure 2: Two-dimensional PL maps of a single 2 nm QWT at (a) 20 K, (b) 40 K, (c) 80 K and (d) 120 K under nonresonant excitation, showing that the localized states quench, but the ground states still exist at higher temperatures. Scale bar at different temperatures indicate the quenching of the PL intensities of the overall emission. White dash lines indicate the boundaries of localized and delocalized states at different temperatures. Figure 2 shows the false-colored two-dimensional PL maps of a single 2 nm QWT nanowire at four different temperatures (20K, 40K, 80K and 120K). The vertical axis indicates the axial positions of PL emissions from a 2 nm QWT with a length of 4 µm, and the horizontal axis represents the emission energy. A number of observations can immediately be made. First, the overall emission intensity decreases sharply by 120 K (in fact essentially no PL is observed with these excitation power densities above 140 K.) At 20 K, the 2 nm QWT PL image shows sharp
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PL emission of localized states from specific axial positions along the NW long axis, particularly along the higher energy part of the PL band. The PL intensity of the localized states decreases with increasing temperature and the PL response on the low energy side begins to dominate, resulting in substantial line narrowing of the inhomogeneous PL band from 145 meV to 85 meV. By 120 K, the localized states quench completely leaving delocalized ground states which emit along the entire length of the (see Figure 2(d)).15 To quantify this behavior, we divide the PL emission band into two parts: the localized states along the high energy side (>1.85 eV at 10 K), and the delocalized states along the lower energy side (
