Core-Shell GaAs-AIAs Nanowires Grown by MBEHadas Shtrikman'", Ronit Popovitz-Biro', Palle von Huth', Andrey Kretinin' and Moty Heiblum'

Abstract - The objective of this work was to achieve controlover MBE growth of GaAs core-shell nanowires using the goldassisted VLS method, thereby facilitating formation of either pureWurtzite (WZ) or pure Zinc Blende (ZB) wires. Growth of WZtype wires relies on initial nucleation of a thin (stOnm) GaAs wirewhich, based upon theoretical calculations, is expected to be freeof SF. Preferential nucleation of pure ZB type wires required arespective reduction of the supersaturation, so as to simulate theconditions, which dominate the growth of epitaxy/bulk material.A shell composed of AlAs layer and a capping GaAs layer wasapplied in situ to both types of wires in order to form confinedstructures for optical and electronic applications.

I. BACKGROUND

The growth of nanowires of semiconductor materials israpidly emerging as one of the most promising topics in nano­science and nano-technology [I]. In particular, the synthesis ofnanowire heterstructures is being more widely pursued in thequest to increase the versatility of optical and electronicdevices. Core-shell nanowires, where a narrower band gapsemiconductor nanowire is embedded in a wider band gapsemiconductor material, are of significant importance forproducing devices with enhanced optical output [2] or ballisticelectronic transport [1]. The AlAs/GaAs system is particularlyattractive in view of the low mismatch between the twomaterials and the high intrinsic mobility of electrons within thehigh purity GaAs.

Much of the growth of core-shell III-V nanowires was doneso far using MOCVD [3-5], though some recently publishedwork relate to the MBE growth of such structures, particularlyin the absence of a gold catalyst [6, 7]. As in 2D systems MBEgrowth is significantly advantageous for pursuing growth ofpure high mobility structures.

II. MA1ERIALS AND METHODS

The synthesis of the nanowires was carried out in a Ribersolid source MBE system. A thin (-1 nm) gold layer wasevaporated in situ on the (III)B GaAs substrates immediatelyafter the oxide blow-off, and subsequently heated well above

I Braun Center for Submicron Research, The WeizmannInstitute of Science, Rehovot, Israel.2 Electron Microscopy Unit, The Weizmann Institute ofScience, Rehovot, Israel.*Contacting Author: Hadas Shtrikman is at the Braun Centerfor Submicron Research, The Weizmann Institute of Science,Rehovot 76100, Israel. (phone: 972-8-9342585; fax: 972-8­9344128; e-mail: hadas.shtrikman@weizann.ac.il).

978-1-4244-3544-9/10/$25.00 ©2010 IEEE

Fig. I . SEM images of (left) 45° and (right) top views of pencil shape GaAscore-shell nanowires grown on a (111)8 surface.

the eutectic temperature to form the gold droplets. Growth wascarried out at a nominal bulk growth rate of IML/s, and groupVillI ratio of 200. Substrate temperature was 550 and 600 °Cfor growth of WZ and ZB, respectively, and the shell wasgrown at 600 °C for both. A two minutes growth interruptionwas introduced at both interfaces of the AlAs shell. A GaAscapping layer was grown under the same conditions as theAlAs shell and a Ga flux as during growth of the core.

III. RESULTS

In this work we have exploited our previous results were we haveshown that pure WZ or ZB wires can be grown by MBE usingthewellpursued VLSmethod[8,9].

A. Wurtzite WiresGrowth of the prominently occurring WZ GaAs wires relies

on theoretical calculations which demonstrate that for thinenough (~lOnm) wires the WZ structure is dominant over theZB (known to be favorable in the bulk and epitaxy growth)[10]. Therefore, such thin wires tend to be free of SF. Theaxially growing nanowires extend laterally when growthproceeds beyond the mean migration length of Ga atoms, thusproducing thicker GaAs wires, yet with no SF, which serve asthe core. Growth of the core was followed by coating with anAlAs shell and a GaAs protection layer which prevents theoxidation of the AlAs.

Fig. 1 shows a SEM image of as grown GaAs core-shellnanowires depicting their final pencil shape as well as thehexagonal faceting. The wires having a WZ structure growperpendicular to the substrate in the <0001> direction (Fig. 2a­c). EDS analysis of the as grown wires was used for generalconfirmation of the elemental composition, whereas TEM of aFIB produced cross-section, taken from the center of suchwire, exposed the actual thicknesses of the individual layers.The double diameter distribution of gold droplets yieldedamong the long wires extending to 3-6 microns (whichoriginate from small diameter droplets; 10-30nm), short onesresulting from the large diameter (-IOOnm) droplets.

Fig. 2: (a) TEM ofa single WZ core-shell nanowire ; (b) HRTEM and (c) electron diffraction showing the WZ structure and <000 1> growth direction; (d)TEM of a single ZB core-shell nanowire ; (e) HRTEM and (f) electron diffraction showing the ZB structure and <001> growth direction.

B. Zinc Blende WiresGaAs is known to assume the ZB structure in its bulk form

as well as at the initial nucleation and upon discontinuation ofthe supply of source materials [11]. Therefore, it is speculatedthat reducing the supersaturation during the entire processwould enhance nucleation and growth of ZB rather than WZwires. We proposed to induce a locally reducedsupersaturation by applying a particularly thick gold layerwhich upon heating produces a double size distribution of golddroplets. The thick droplets as well as the surface in betweenthem absorb much of the supplied material, thus locallyreducing the supersaturation in the vicinity of other dropletswhich develop into ZB wires among the WZ ones. Due totheir large diameter the WZ wires are limited to a length of afew hundred nanometers, whereas the ZB ones evidently seenby their tilt angle to the growth surface are a couple of micronslong (see Fig. 3).

Fig. 3. SEM images (45'view) of rectangular GaAs core-shell nanowiresgrown on (III )B surface at high temperature using a thick gold layer.

The ZB wires, which grow along the <001> or the <011>direction, assume either a square or a rectangular cross-section.Therefore, such wires lying on the TEM grid are likely to bealigned with the core-shell interface parallel to the electron

beam, thus enabling imaging of the interfaces without a needof producing a cross-section (Fig. 2d-t). The AlAs shell isclearly seen (lower contrast) having two continuous interfaceswith the GaAs core and capping layer.

ACKNOWLEDGMENT

The authors are indebt to Perla Kacman and her group forthe theoretical calculations.

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