Ambrosini, S.

Booth, Tim

Bøggild, Peter

Fragiacomo, Giulio

Rubini, S.

Savenko, Alexey

Wagner, Jakob Birkedal

Self catalyzed GaAs nanowires grown on Si-treated GaAs substrates were studied with a transmission electron microscope before and after annealing at 600◦C. At room temperature the nanowires have a zincblende structure and are locally characterized by a high density of rotational twins and stacking faults. Selected area diffraction patterns and high-resolution transmission electron microscopy images show that nanowires undergo structural modifications upon annealing, suggesting a decrease of defect density following the thermal treatment

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Downloaded from orbit.dtu.dk on: Dec 18, 2017In situ transmission electron microscopy analyses of thermally annealed self catalyzedGaAs nanowires grown by molecular beam epitaxyAmbrosini, S.; Wagner, Jakob Birkedal; Booth, Tim ; Savenko, Alexey; Fragiacomo, Giulio; Bøggild,Peter; Rubini, S.Published in:In situ transmission electron microscopy analyses of thermally annealed self catalyzed GaAs nanowires grownby molecular beam epitaxyPublication date:2011Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Ambrosini, S., Wagner, J. B., Booth, T., Savenko, A., Fragiacomo, G., Bøggild, P., & Rubini, S. (2011). In situtransmission electron microscopy analyses of thermally annealed self catalyzed GaAs nanowires grown bymolecular beam epitaxy. In In situ transmission electron microscopy analyses of thermally annealed selfcatalyzed GaAs nanowires grown by molecular beam epitaxy18th Int. Symp. “Nanostructures: Physics and Technology”Ekaterinburg, Russia, June 20–25, 2011c© 2011 Ioffe InstituteIn situ transmission electron microscopy analyses of thermally an-nealed self catalyzed GaAs nanowires grown by molecular beamepitaxyS. Ambrosini1,2,3, J. B. Wagner4, T. Booth5, A. Savenko4,5,G. Fragiacomo5, P. Boggild5 and S. Rubini1,21 TASC National Laboratory, 34149 Trieste, Italy2 Sincrotrone Trieste S.C.p.A, 34149 Trieste, Italy3 UNITS, Universit degli Studi di Trieste, 34100 Trieste, Italy4 DTU CEN, Center for Electron Nanoscopy, Technical University of Denmark, DK-2800, Kgs. Lyngby, Denmark5 DTU Nanotech, Department of Micro and Nanotechnology, Technical University of Denmark, 2800 Kgs. Lyngby,DenmarkAbstract. Self catalyzed GaAs nanowires grown on Si-treated GaAs substrates were studied with a transmissionelectron microscope before and after annealing at 600◦C. At room temperature the nanowires have a zincblendestructure and are locally characterized by a high density of rotational twins and stacking faults. Selected areadiffraction patterns and high-resolution transmission electron microscopy images show that nanowires undergostructural modifications upon annealing, suggesting a decrease of defect density following the thermal treatment.IntroductionGaAs nanowires (NWs) grown by catalyst assisted meth-ods are typically affected by the presence of structural de-fects. Stacking faults are common, and the two GaAs poly-types, zincblend (ZB) and wurtzite (WZ) are often inter-mixed along the growth axis of a single nanowire. The twoGaAs polytypes of GaAs have indeed a small cohesive en-ergy difference (24 meV per III-V atom pair [1]). The occur-rence of one or the other of the possible stacking has beenrecently predicted on thermodynamic basis in terms of therelative weight of the surface energies of the different phasesinvolved in the NW growth, i.e. solid, liquid and gaseous [2].Recently, wide effort has been devoted to careful tune thegrowth conditions in order to obtain pure ZB or pure WZNWs [3, 4]. Self-catalyesd NWs typically display ZB struc-ture but twins and stacking faults are generally present [5].We give here a preliminary account of a study of the ther-mal stability of self-catalyzed GaAs NWs, especially withrespect to the stability of stacking faults.Experimental detailsNWs have been synthesized by solid source molecular beamepitaxy using a Riber 32P apparatus: (111)B GaAs wafershave been deoxidized and 3-4 monolayers of Si (1 monolayer= 1,36 A˚) have been epitaxially deposited. Air exposureand outgassing in ultra high vacuum completed the sub-strates preparation for self catalyzed NW growth [6]. NWswere grown at a growth temperature of 620◦C with an Asto Ga beam-equivalent pressure ratio of 12. In these growthconditions a two-dimensional growth rate of 1 µm/hour isobserved. A suspension of the as-grown nanowires in ethanolproduced by sonication was then applied to a nickel Quan-tifoil TEM grid and examined in the transmission electronmicroscope (Tecnai G2 operating at 200 keV). The anneal-ing step consisted of 10 minutes at 600◦C inside the TEMchamber using a sample holder with a platinum heating el-ement (Gatan Inc.). High temperature mesurements werepossible only in diffraction mode, while for high resolutionthe absence of thermal drift was mandatory: to acquire highresolution images, then, 30 minutes were waited after theannealing.Results and discussionFig. 1. (a) SEM image of GaAs NWs on (111)B GaAs substratesas grown. (b) HRTEM image of a NW in the ZB region at thebase; (c): filtered FFT of (b); (d): HRTEM image of the sameNW after the annealing step; (e) filtered FFT of (d). The insetsin (b) and (d) show details of the crystal lattice. Notice that theFFTs are aligned with the HRTEM images.In fig. 1 the result of the annealing step on the NWbody where it had pure ZB crystal structure is visible. InFig. 1(a) a scanning electron microscope image of the NWsas grown on the (111)B GaAs substrate. In 1(b) the NWbody with twinned zones of alternating contrast along thegrowth axis. The inset in 1(b) and the fast Fourier transform(FFT) of the image, visible in 1(c), confirms the stackingto be ZB, diffraction spots are indexed according to this12structure. In 1(d) the HRTEM image of the same NW showsa greater uniformity possibly due to a tilt of the crystalafter the annealing step, though the detail of the atomiclattice is still visible in the inset. In 1(e) the FFT of 1(d):spot indexes show the stacking to be ZB. Notice that theFFTs in 1(c) and 1(e) are aligned with the HRTEM image.Fig. 2. Low magnification TEM images (left) and correspondingSAD patterns taken from the black circled zone (right) of a NWbefore (top) and after (bottom) the annealing step. Notice theblack spots on the NW body in the region next to the SADaperture in (c).In fig. 2 the low magnification image of a NW tip is vis-ible, before in 2(a) and after in 2(c) the annealing step. Inboth 2(a) and 2(c), the black circle region shows where theSAD pattern in 2(b) and 2(d), respectively, were taken; no-tice that the diffractograms in 2(b) and 2(d) are 90◦ rotatedwith respect to 2(a) and 2(c). The diffractogram in 2(b)is compatible with a defected region with stacking faultsand/or rotational twins, whereas the diffractogram in 2(d)is much simpler and strongly suggests the absence of multi-ple crystal phases in that area. Although a slight movementor rotation of the NW as result of the annealing step (sameas previous image) could be the reason of the disappearanceof some diffraction spots, distances and positions of the maindiffraction peaks show pure ZB phase in the selected area.It should be pointed out that the slight different selectedarea between 2(a) and 2(c) is a result of the thermal driftat high temperature. Fig 2(c) shows other particulars, ifcompared with 2(a): first, the Ga nanoparticle on top ofthe NW has a different morphology; second, the presence ofgreater contrast spots on the NW body, close to the selectedarea, suggests a partial dewetting of the Ga nanoparticle. Inthis case, Ga atoms have migrated down the NW body andsegregated in smaller Ga clusters; this should be taken intoaccount in considering transport or electronic properties ofannealed NWs. Their position inside the NW body couldn’tthough be resolved by this TEM analysis.With an energy difference of 24 meV per III-V atom pair be-tween ZB and WZ GaAs polytypes and thermal energy em-ployed for these experiments of 75 meV, structural changesin NWs are very likely to be triggered.ConclusionsThese preliminary results show that structural changes takeplace when self catalyzed GaAs NWs undergo thermal an-nealing. In particular, where the stacking was ZB, it re-mains ZB and where the structure was defected it seems tobecome simpler towards pure ZB. The incidental evidenceof Ga diffusion within the NW body moreover confirms theeffectiveness of post growth annealing procedures to affectNW structure. An annealing step in As atmosphere insidethe growth chamber after the growth interruption and otherTEM analyses performed with a lower electron beam energyare upcoming points of this investigation.References[1] C. Y. Yeh et al , Phys. Rev. B46, 10086 (1992).[2] F. Glas et al , Phys. Rev. Lett. 99, 146101 (2007).[3] H. Shtrikman et al , Nano Lett. 9, 215 (2009).[4] H. Shtrikman et al , Nano Lett. 9, 1506 (2009).[5] G. Cirlin et al , Phys. Rev. B 82, 35302 (2010).[6] S. Ambrosini et al , submitted to Nanoresearch (2011).

In situ transmission electron microscopy analyses of thermally annealed self catalyzed GaAs nanowires grown by molecular beam epitaxy

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In situ transmission electron microscopy analyses of thermally annealed self catalyzed GaAs nanowires grown by molecular beam epitaxy

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