Letter pubs.acs.org/NanoLett
Metamorphic GaAs/GaAsBi Heterostructured Nanowires Fumitaro Ishikawa,*,† Yoshihiko Akamatsu,† Kentaro Watanabe,‡ Fumihiko Uesugi,§ Shunsuke Asahina,∥ Uwe Jahn,⊥ and Satoshi Shimomura† †
Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Transmission Electron Microscopy Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ∥ SM Business Unit, JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan ⊥ Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany ‡
S Supporting Information *
ABSTRACT: GaAs/GaAsBi coaxial multishell nanowires were grown by molecular beam epitaxy. Introducing Bi results in a characteristic nanowire surface morphology with strong roughening. Elemental mappings clearly show the formation of the GaAsBi shell with inhomogeneous Bi distributions within the layer surrounded by the outermost GaAs, having a strong structural disorder at the wire surface. The nanowire exhibits a predominantly ZB structure from the bottom to the middle part. The polytipic WZ structure creates denser twin defects in the upper part than in the bottom and middle parts of the nanowire. We observe room temperature cathodoluminescence from the GaAsBi nanowires with a broad spectral line shape between 1.1 and 1.5 eV, accompanied by multiple peaks. A distinct energy peak at 1.24 eV agrees well with the energy of the reduced GaAsBi alloy band gap by the introduction of 2% Bi. The existence of localized states energetically and spatially dispersed throughout the NW are indicated from the low temperature cathodoluminescence spectra and images, resulting in the observed luminescence spectra characterized by large line widths at low temperatures as well as by the appearance of multiple peaks at high temperatures and for high excitation powers. KEYWORDS: Semiconductor nanowires, GaAsBi, GaAs, roughening, luminescence
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optic telecommunications. Optical devices based on GaAs, such as laser optical amplifiers working in the near-infrared regime, incur severe intrinsic losses related to nonradiative Auger recombination. To circumvent this problem, employing dilute bismide alloys has recently attracted attention in thin-film technology. Dilute bismide GaAsBi has a number of interesting properties, including a large reduction in the band gap of up to 88 meV/Bi % with enormous concomitant spin−orbital splitting.16−19 These characteristics of such alloys are particularly well suited to the fabrication of infrared emitters and photodetectors.18 Tunability of the band gap and spin− orbit splitting energy near the valence band enables the suppression of nonradiative Auger-related recombinations as well as control of the spin−orbit splitting.18,20−22 Indeed, the large lattice mismatch between GaAs and GaAsBi as well as their miscibility gap produces defects in GaAsBi alloys, which limit the impact of this material system.23,24 Therefore, NW development should aim to reduce the formation of these defects. Dislocation penetration is filtered at a nanowire side-
he study of semiconductor nanowires (NWs) has attracted considerable interest because of their potential application in nanostructured electronic and optoelectronic devices. The introduction of group III−V compound semiconductors into NWs provides dynamic control over the electronic band structure of these systems.1,2 With the sophisticated nanoscale growth techniques recently developed, NWs can now be fabricated into compound alloys with desired constituent compositions. Multilayered heterostructures with radial core/shell or axial heterostructures can also be obtained, allowing control of the material properties and endowment with desired functions. These heterostructures can serve as charge carriers, light emitters, and waveguides, making NWs attractive building blocks in nanoelectronics and nanophotonics.3−7 Moreover, advanced epitaxial techniques used to fabricate NWs can overcome the large mismatches between lattice constants and thermal coefficients of group III−V semiconductors and Si, allowing the possibility of integration of NW-based optoelectronic devices with the mature microelectronic technology based on Si.8,9 To date, optical output from group III−V NWs has been demonstrated within the ultraviolet to near-infrared spectral range.10−15 Much effort has been made obtain emission with longer wavelengths, which has great significance in photovoltaics, medical diagnosis, and fiber© XXXX American Chemical Society
Received: June 11, 2015 Revised: October 1, 2015
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DOI: 10.1021/acs.nanolett.5b02316 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. (a−d) SE images of the 45°-tilted GaAs/GaAsBi nanowires; (b) enlarged image of the rectangle area of (a); (c) and (d) focused images of the top and bottom of a single nanowire for the areas delimited by the orange boxes in (b); (e) SE images of the 45°-tilted reference GaAs nanowire sample grown under identical conditions with the GaAs/GaAsBi nanowire, except for the interrupted Bi flux; (f) enlarged image of an area from the same sample.
facet during growth.25,26 Furthermore, the enlarged critical thickness of the nanowire heterostructures compared with that of thin films realizes coherent growth of the wide lattice mismatched materials, where the strain is shared by the overlayer and the underlayer.27 Consequently, monolithic integration of NWs with dilute bismide heterostructures on Si could produce promising materials with high crystal quality, suitable for optoelectronic applications. Until now, the growth characteristics and the resulting properties of high-mismatch Bicontaining GaAs nanowires were not known. There are many open questions concerning the Bi introduction potential, its impact on the NWs growth characteristics, the properties of the GaAsBi nanowire, and their difference to those of thin films. Hence, here we report the synthesis of dilute bismide GaAsBi/ GaAs heterostructured NWs on Si(111) substrates and an investigation of their properties. Materials and Methods. The sample was grown by molecular beam epitaxy on phosphorus-doped n-type Si(111) substrates (information on the growth mechanism of core− shell nanowires can be found in the Supporting Information). The sample was heated to 580 °C under an As4 beam at an equivalent pressure of 1 × 10−5 mbar. Growth of the GaAs nanowire core started upon opening the Ga shutter. The formation of the GaAs nanowire core was initiated by constituent Ga-induced vapor−liquid−solid growth. The Ga
supply was set to match a planar growth rate of 1.0 ML/s on GaAs(001). The GaAs core was grown for 15 min at the requisite temperature to initiate longitudinal wire growth. Next, the growth was interrupted for 10 min, and the substrate temperature was reduced to 550 °C to induce crystallization of the Ga catalyst on the tip of the NWs to GaAs. After crystallization of the catalyst, lateral growth became dominant, leading to core−shell-type nanowires with precisely controlled shell diameters.28−30 We then supplied a Ga flux for 15 min in order to form a GaAs shell about 50 nm thick.29 Next, we interrupted the growth once more, lowering the substrate temperature to 350 °C, at which the subsequent growth of GaAsBi could proceed.17,18,23,24 We then supplied appropriate Ga and Bi fluxes to form GaAsBi and subsequently GaAs; these layers were grown for 15 min each. The beam equivalent pressure of Bi was adjusted to 6 × 10−8 mbar. The nanowires were then expected to form GaAs cores surrounded by a GaAs/ GaAsBi/GaAs multishell, whereby each of the layers was grown for 15 min. For comparison, we grew a GaAs nanowire without Bi flux irradiation; otherwise, the growth conditions were identical to those for GaAsBi heterostructured NWs. A thinfilm sample on the GaAs(001) substrate that was simultaneously grown with the NWs sample on the Si(111) substrate by placing it side-by-side on the substrate mounting block was examined by X-ray diffraction analysis, which revealed that the B
DOI: 10.1021/acs.nanolett.5b02316 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. BF-TEM image of a single GaAs/GaAsBi nanowire (leftmost). Right series: ADF-STEM and EDS elemental mapping images for the areas delimited by the squares in the BF-TEM or ADF-STEM images. The black arrows label the twin defects. The red arrows in the Bi EDS-mapping images indicate large Bi concentrations.
thin GaAsBi film of 250 nm contained about 5% Bi (Supporting Information). For comparison of the structural characterization, reference GaAs NWs were grown with the identical growth conditions of the GaAs/GaAsBi NWs, except the interrupted Bi flux. The structural characteristics of the NWs were investigated by field-emission scanning electron microscopy (FE-SEM) using a JSM-7800F Prime instrument (JEOL, Japan) as well as by transmission electron microscopy (TEM). Cross-sectional analysis with constituent elemental mapping was conducted using a scanning transmission electron microscope (STEM, JEM-ARM200F, JEOL, Japan) operating at 200 kV, equipped with spherical aberration correctors for both TEM and STEM (CEOS GmbH, German) and with a dual silicon drift-detector (SDD) energy-dispersive X-ray spectroscope (EDS). The obtained EDS data were analyzed using the analytical package NORAN System 7 (Thermo Fisher Scientific, USA) to quantify the concentration of each investigated element from the intensity maps. For the TEM measurements, finely sliced samples of thickness
