SrTiO3 Shell Nanowires Grown by Molecular Beam

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GaAs core / SrTiO3 shell nanowires grown by Molecular Beam Epitaxy Xin Guan, Jeanne Becdelievre, Benjamin Meunier, Abdennacer Benali, G. Saint-Girons, Romain Bachelet, Philippe Regreny, Claude Botella, Geneviève Grenet, Nicholas Blanchard, Xavier Jaurand, Mathieu G. Silly, Fausto Sirotti, Nicolas Chauvin, Michel Gendry, and José Penuelas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05182 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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Graphical abstract 229x119mm (96 x 96 DPI)

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SEM image of the GaAs NWs grown on Si(111) substrate (a). TEM images of a single GaAs NW showing its morphology and multi-twinned domains close to the NWs tip (b), lattice resolved TEM image of a GaAs NWs, with its Fourier transform in inset (c). 169x139mm (96 x 96 DPI)


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Illustration of the capping/decapping and of the two-step SrTiO3 growth process (a, c, e, g). RHEED pattern measured along [110] directions: at 350°C after the decapping of the As protective shell (b), after the growth of SrTiO3 buffer during 15 minutes at 350°C (d), after the annealing at 550°C during 15 minutes (f), after the growth of SrTiO3 during 15 minutes at 550°C (h). 150x159mm (96 x 96 DPI)


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Two-dimensional element mapping of Ga, As, Sr, Ti and O, respectively for a single NW (a) and its corresponding Scanning Transmission Electron Microscopy image for a spot size of 1.5 nm) (b) Line-scan performed along the NW diameter (c). 200x109mm (96 x 96 DPI)


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TEM images showing an abrupt (a) and an amorphous (b) GaAs core / SrTiO3 shell NW interface. 239x119mm (96 x 96 DPI)


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XPS core levels of GaAs / SrTiO3 NWs measured at room temperature. 209x219mm (96 x 96 DPI)


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Photoluminescence spectra of decapped GaAs NW array (a), GaAs / SrTiO3 core / shell NW array (b), GaAs / AlGaAs core / shell NW array (c) and GaAs / AlGaAs / SrTiO3 core / shell NW array (d). For sample (b) and (d) the SrTiO3 was grown using the two-step method. The measurements were performed at 300 K. 254x190mm (96 x 96 DPI)


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GaAs core / SrTiO3 shell nanowires grown by Molecular Beam Epitaxy X. Guan1, J. Becdelievre1, B. Meunier1, A. Benali1, G. Saint-Girons1, R. Bachelet1, P. Regreny1, C. Botella1, G. Grenet1, N. P. Blanchard2, X. Jaurand3, M. G. Silly4, F. Sirotti4, N. Chauvin5, M. Gendry1, J. Penuelas1

1

Institut des Nanotechnologies de Lyon - Université de Lyon, UMR 5270 - CNRS, Ecole

Centrale de Lyon, 36 avenue Guy de Collongue, F-69134 Ecully cedex, France 2

Institut Lumière Matière (ILM), UMR5306 Université Lyon 1- CNRS Université de Lyon,

69622 Villeurbanne cedex, France 3

Centre Technologique des Microstructures, Université Claude Bernard Lyon1, 5 rue Raphael

Dubois-Bâtiment Darwin B, F-69622, Villeurbanne Cedex, France 4

Synchrotron SOLEIL (TEMPO beamline), l’Orme des Merisiers, Saint-Aubin, 91192 Gif-

sur-Yvette, France 5

Institut des Nanotechnologies de Lyon - Université de Lyon, UMR 5270 - CNRS, INSA-

Lyon, 7 avenue Jean Capelle, 69621 Villeurbanne, France * To whom correspondence should be addressed. E-mail: [email protected]

KEYWORDS: Core/shell nanowires, hybrid nanowires, functional oxides, semiconductor, interface 1 ACS Paragon Plus Environment

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ABSTRACT: We have studied the growth of a SrTiO3 shell on self-catalyzed GaAs nanowires grown by vapor-liquid-solid assisted molecular beam epitaxy on Si(111) substrates. To control the growth of the SrTiO3 shell, the GaAs nanowires were protected using an arsenic capping/decapping procedure in order to prevent uncontrolled oxidation and/or contamination of the nanowire facets. Reflection high energy electron diffraction, scanning electron microscopy, transmission electron microscopy and x-ray photoelectron spectroscopy were performed to determine the structural, chemical and morphological properties of the heterostructured nanowires. Using adapted oxide growth conditions, it is shown that most of the perovskite structure SrTiO3 shell appears to be oriented with respect to the GaAs lattice. These results are promising for achieving one-dimensional epitaxial semiconductor core / functional oxide shell nanostructures.

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1. Introduction: Semiconductor nanowires (NWs) have been intensively studied over the last few years because of their great potential in microelectronics [1-5], photonics [6-8], life science [9,10], energy harvesting based on NWs solar cells [11, 12] or as nanogenerators [13, 14]. Recently, core/shell NW heterostructures have attracted much attention due to their enhanced or original physical properties coming from carrier confinement [15, 16], high specific surface area or due to the coupling of the physical properties from the core and the shell [17-19]. Although most of the published works on core/shell NWs concern semiconductors of the same family, integration of heterogeneous crystalline materials such as silicon [20-21], silicide [22] or metals [23] on GaAs NWs has been recently achieved opening the way to the fabrication of original devices. Owing to their wide-ranging properties, that are complementary to those of semiconductors, functional oxides with perovskite structure are a very promising shell material. Moreover, the growth of semiconductor core / functional oxide shell NWs could be an alternative means to fabricate one dimensional functional oxide nanostructures as perovskite oxide NWs are difficult to prepare by the Vapor-Liquid-Solid (VLS) method despite recent promising results [24, 25]. However, achieving an epitaxial interface in semiconductor/perovskite oxide heterostructures is challenging due to the rapid oxidation of classical semiconductors such as Si or III-V when exposed to air or O2 [26, 27]. To prevent such phenomenon, Mc Kee et al. [28] developed a strategy to passivate the Si substrate surface with half a monolayer of Sr that allows the growth of SrTiO3 thin films directly on Si (001) by molecular beam epitaxy (MBE). Such interface engineering has attracted considerable work due to the possibility to grow functional oxides such as BaTiO3 [29], Pb(Zrx,Ti1-x)O3 [30] PMN-PT [31] on Si substrates. More recently, SrTiO3 was successfully grown on GaAs substrates [32-38], which is promising for the development of heterostructures coupling light emission/absorption and piezoelectricity or ferroelectricity [39, 3 ACS Paragon Plus Environment

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40]. In a similar way, the fabrication of SrTiO3 layers on the facets of GaAs NWs could open the way to the integration of functional oxides on semiconducting NWs. The aim of this work is to fabricate GaAs core / SrTiO3 shell NWs. Such a core/shell heterostructure has potential applications for the fabrication of field effect transistors, GaAs being the channel and SrTiO3 the high-K dielectric gate. However, in order to fabricate such device, low interface state density is needed [41] which can be achieved by protecting the NW facets before the oxide growth with a reversible As capping [42] and / or by accurately controlling the oxide growth conditions in order to limit surface oxidation or other undesirable interfacial reactions. Moreover, SrTiO3 can be seen as a model system for the growth of perovskite oxides on semiconducting III-V NWs in order to combine piezoelectric or ferroelectric oxides with GaAs NWs. Even if the interest in growing GaAs core / oxide shell NWs is great, to our knowledge, the only work concerning such nano-objects reports a controlled oxidation to produce AlGaOx shell that wraps the GaAs core [43]. This work reports the first attempt to grow a functional oxide directly on III-V NWs. 2. Nanowire growth: GaAs / SrTiO3 NWs were grown on p-doped Si(111) substrates by MBE. Before being introduced in ultrahigh vacuum (UHV), the epi-ready Si substrates were cleaned for 5 minutes in ethanol and acetone in order to remove surface contaminations. Substrates were then outgassed in UHV for a few minutes at 200°C. GaAs NWs were grown in a MBE reactor dedicated to III-V semiconductors equipped with a Ga Knudsen cell and a valved cracked As source. The SrTiO3 shell was grown in a MBE reactor dedicated to the growth of functional oxides and equipped with Sr and Ti effusion cells. O2 injection was controlled in a differentially-pumped pre-chamber connected to the reactor via a butterfly valve. The substrate temperature was controlled by a calibrated thermocouple and checked by a


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pyrometer. During growth, the sample holder was continuously rotated to enhance growth homogeneity. Both MBE reactors are equipped with Reflection High Energy Electron Diffraction (RHEED). Self-catalyzed GaAs NWs were grown on silicon substrate by MBE using the VLS method with Ga droplets as catalyst [45-49]. First, the Si substrate temperature was increased to 530°C and 1 monolayer of Ga was deposited at a deposition rate of 1.4 Å/s on the substrate which was still covered by its 2 nm-thick native oxide. At this temperature, the deposition leads to the formation of Ga liquid droplets on the surface. These droplets locally decompose the native oxide [48]. Then, the substrate temperature was increased up to 610°C to grow the GaAs NWs. The Ga deposition rate was 2.1 Å/s in units of equivalent 2D GaAs growth rate as measured from RHEED oscillations during Ga-limited growth and the As4 beam equivalent pressure was set at 3.3x10-6 Torr which corresponds to a deposition rate of 3 Å/s as measured from RHEED oscillations during As-limited growth [50]. The Ga shutter and the As valve were opened at the same time to initiate NWs growth. After 15 minutes, the Ga shutter was closed and the temperature was decreased down to room temperature with a As4 flux in order to avoid GaAs decomposition. A representative SEM image of the sample is shown in Figure 1(a). The NW average length is 1.0 µm with an average diameter of 45 nm and their density is about 7 NWs/µm². The NWs do not show any tapering phenomenon and the Ga catalyst is no longer visible at the NW tip. GaAs nanocrystals are also visible on the surface in between the NWs. TEM images of GaAs NWs are shown in Figure 1.b-c. The GaAs NW of Figure 1.b is quite representative of the other NWs on the substrate and exhibits four different domains. From the bottom to the top, the first domain is a zinc-blende (ZB) phase with very few structural defects. The second domain, about 30 nm in length, is located at roughly a hundred nm from the NW tip and contains a large number of twining defects along the growth axis and several short wurtzite (WZ) segments. The third domain is made of a WZ region of around 50 nm 5 ACS Paragon Plus Environment

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length. The last domain exhibits a pure ZB structure: it corresponds to the consumption of the Ga catalyst droplet during the cooling down of the sample under As. Such a phase sequence has already been reported [51-53] and is supposed to be a consequence of an evolution of the Ga droplet shape during the NW growth. Figure 1.c shows a lattice-resolved TEM image of a GaAs NW in the pure ZB region (the inset shows its Fourier transform), the NW growth axis is [111] and the zone axis is [1-10]. The lattice parameter is about 5.62 Å, nearly equal to that of bulk GaAs (5.65 Å). The facets are covered by a smooth amorphous layer which corresponds to a GaAs oxide formed during the transfer to the microscope. In order to avoid the formation of such an amorphous surface oxide that would be critical for the growth of the crystalline SrTiO3 shell, an As protective capping layer was grown on the GaAs NWs to prevent them from oxidation during the transfer from the III-V MBE reactor to the oxide MBE reactor prior to the growth of the SrTiO3 shell. Similar reversible capping procedure was recently used to grow Al2O3 shell on the facets of III-V NWs with reduced interface defects [42]. The As capping layer was grown by the deposition of As2 at room temperature for 120 minutes. The diffraction pattern was in accordance with an amorphous As capping layer (further details concerning the As capping and decapping can be found in ref [54]). Figure 2 shows the RHEED pattern measured at different stages of SrTiO3 growth. The first stage (see Figure 2.a) consisted in the transfer of the As-capped GaAs NWs to the oxide MBE reactor and the thermal desorption of the As capping layer in order to obtain clean GaAs NW facets. The sample temperature was slowly increased from room temperature up to 350°C. At room temperature, no diffraction pattern was visible, in agreement with the presence of an amorphous As capping layer. At 350°C a diffraction pattern became visible (see Figure 2.b), which testifies for the As capping layer desorption. The RHEED pattern was that of GaAs grown along the [111] direction. However, several spots (marked by arrows in Figure 2.b) are indicative of the presence of twins, as commonly reported in GaAs NWs [55, 56]. In order to 6 ACS Paragon Plus Environment

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avoid the formation of an amorphous oxide on the GaAs NW facets we used the two-step growth process as proposed by Niu et al. [57] for growing SrTiO3 films on Si substrates. The first step (see Figure 2.c-d) consisted in depositing an amorphous SrTiO3 buffer layer at 350 °C under low O2 partial pressure of 5x10-8 Torr in order to obtain a relatively sharp interface. Annealing at 550°C in UHV for 15 minutes leads to crystallization of this SrTiO3 buffer layer (Figure 2.f). In the second step (see Figure 2.g) further SrTiO3 growth is performed at higher temperature (550°C) and higher O2 partial pressure (1x10-6 Torr) which is possible thanks to the SrTiO3 buffer layer grown at low temperature that acts as a passivation film, which prevents damaging the SrTiO3 / GaAs interface. After SrTiO3 deposition at low temperature, the RHEED pattern does not show any change except a brighter background typical of an amorphous layer (Figure 2.d). However rings appeared during the annealing due to the SrTiO3 crystallization without preferential orientation (Figure 2.f). During the second step of SrTiO3 growth, rings become more intense (Figure 2.h). All the rings observed in Figure 2.h are in agreement with bulk SrTiO3 (see Supporting Information). In order to investigate the impact of the growth temperature on the structural and morphological properties of the NWs, a series of samples was prepared. The one step deposition of 40 nominal monolayers of SrTiO3 under low O2 partial pressure of 5x10-8 Torr was performed for comparison with the two-step method. Our results show that at low growth temperature (350°C) SrTiO3 is mainly amorphous while at high temperature the SrTiO3 is crystalline but strongly dewetted (see Supporting Information). The composition of the NWs was studied by Energy Dispersive Xray Spectroscopy (EDS) mapping as shown in Figure 3.a-b. The results clearly show the presence of the SrTiO3 shell wrapping the GaAs core. Interestingly, the NW tip exhibits a SrTiO3 enrichment when compared to the rest of the NW. This is probably the consequence of the verticality of the NWs and of the geometry of the MBE reactor that induced a preferential growth on the tip when compared to the facets. However radial growth is also evidenced


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without a tapering effect. A slight composition variation is visible along the cross section of Figure 3.c that could be due to shadowing, such inhomogeneity is observed on several NWs (see Supporting Information). 3. Structure, morphology and chemistry at the interface Figure 4 shows TEM images of the GaAs NWs after growth of SrTiO3. The NW surface is relatively rough because SrTiO3 seems to form agglomerated nanocrystals or facetted shell. The GaAs surface is well covered by SrTiO3 even if some NWs exhibit regions free of SrTiO3 probably due to shadowing effects during the oxide growth because of the inclination of the NW axis with respect to the Sr and Ti flux. The total equivalent thickness of SrTiO3 was 10 nm. As the density of NWs is 7 NWs/µm², their diameter 45 nm and their length 1.0 µm, the exposed GaAs surface is equivalent to 98% of the 2D surface. Assuming the growth of a twodimensional SrTiO3 layer on GaAs as reported in Ref [40], the equivalent SrTiO3 shell thickness should be about 5 nm, in accordance with the TEM image. In Figure 4.a, some regions clearly exhibit Moiré fringes due to the overlapping of SrTiO3 and GaAs lattices. SrTiO3 nanocrystals are unambiguously identified thanks to their lattice spacing. Moreover the chemical composition of the nanocrystals measured by EDS is in accordance with SrTiO3 stoichiometry (see Supporting Information). Most of the SrTiO3 layer appears to be oriented with respect to the GaAs lattice. Measurements performed on the spots of the upper Fourier transform in inset of Figure 4.a (2.29 Å and 2.76 Å) are in good agreement with [002] and [110] reflections of SrTiO3. The alignment of SrTiO3 [110] with GaAs [111] results in a high mismatch that induces the formation of interfacial dislocations as shown in the filtered image of the interface (inset of Figure 4.a). The interface is abrupt but not perfectly flat, which could be the consequence of SrTiO3 / GaAs interfacial reaction or microfacetting of the GaAs NW (note that {111} micro-facets have been reported [59]).


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Figure 4.b shows another interface between a SrTiO3 nanocrystal and the GaAs NW core. Although the interface is not abrupt and seems to be partially amorphous, some parts of the SrTiO3 layer are epitaxially grown on the GaAs lattice. The SrTiO3 lattice is clearly identified, the Fourier transform of the lattice shows three series of spots related to {110} planes, the zone axis is [-111]. From this image, we deduced that the SrTiO3 [1-12] is aligned with the NW growth direction which is GaAs [111]. If GaAs NW facets were {1-10} planes as reported in ref [58], SrTiO3 [-111] would be aligned with GaAs [11-2]. Such alignment would have minimized the mismatch because it would be 2.3% along SrTiO3 [1-12] // GaAs[222] and SrTiO3 [-111] // GaAs[11-2] (a model of the interface is shown in the Supporting Information). To summarize, our TEM images show sharp or amorphous interface (Supporting Information). As for a classical two-dimensional growth, the amorphous interfacial layer can be formed after the SrTiO3 growth which is compatible with epitaxial growth under oxygen partial pressure. However some NWs exhibit polycrystalline areas, particularly at the NW tip (Supporting Information). X-ray Photoelectron Spectroscopy (XPS) core level measurements were performed after the SrTiO3 growth at Soleil synchrotron facility (TEMPO beamline) with a photon energy of 750 eV and a resolution of about 70 meV [60]. Figure 5 shows As 3d, Ga 3d, Sr 3d, Ti 2p and O 1s core levels before annealing, the fitting parameters can be found in the Supporting Information. A Shirley background was subtracted and peaks were fitted using Voigt functions. The Ti 2p core level is made of a single spin-orbit doublet (Ti4+) with a spin-orbit splitting of 5.73 eV [61-63]. The Sr 3d core level exhibits a main spin-orbit doublet with Sr 3d5/2 at 133.2 eV and a spin-orbit splitting of 1.74 eV as in bulk SrTiO3 [62-64] and two other spin-orbit doublets at 133.8 eV and 134.5 eV for Sr 3d5/2. These latter peaks have been related to the presence of stacking faults and SrO respectively [63, 64]. The formation of SrO in MBE-grown SrTiO3 arises from Sr segregation at the surface and at the interface during 9 ACS Paragon Plus Environment

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growth. The Ga 3d core level consists in a spin-orbit doublet corresponding to GaAs with Ga 3d5/2 = 18.9 eV and a spin-orbit splitting of 0.4 eV [65], and in an intense component at 20.1 eV corresponding to Ga oxides. Contribution from O 2s and Sr 4p core levels is also reported. The As 3d peak position at 44.8 eV corresponds to As oxide, and not to the As 3d doublet (spin orbit of 0.69 eV), which would appear at around 41 eV between Sr 4s / Ti 3p peaks and As oxide peak. Therefore, an oxidized GaAs has been formed either at the SrTiO3/GaAs interface during SrTiO3 growth or at the GaAs NW surface after growth due to a partially inefficient passivation of GaAs NWs. The core level O 1s also shows three components at 532.15 eV, 530.94 eV and 529.74 eV attributed to SrTiO3, As oxide and Ga oxide. Figure 6.a shows the photoluminescence (PL) spectrum of GaAs NWs after As capping / decapping procedure in UHV. The excitation was provided by a 532 nm continuous wave diode-pumped solid-state laser (50 mW) and the detection by a nitrogen cooled silicon CCD detector coupled to a monochromator. The peak located at 1.43 eV is in agreement with the emission energy of the ZB phase of GaAs NWs at room temperature [66]. Then, the two-step SrTiO3 deposition method was applied to the decapped GaAs NWs. As shown in Figure 6.b, PL of GaAs has been barely detected, while a rather low broad band at about 1.25 - 1.3 eV originates from defects of the parasitic 2D layer (it should be noted that similar spectra were obtained for SrTiO3 shell grown at lower temperature, see Supporting Information). The suppression of PL for this decapped-GaAs / SrTiO3 core / shell NWs could be the consequence of interface defects caused by oxidation during the SrTiO3 growth (despite the moderate growth temperature and oxidation pressure used during the first growth step) or inter-diffusion of the metallic species (Sr, Ti) into the NWs. To avoid the occurrence of poor light emission property of GaAs core, an AlGaAs passivating shell was grown at 515°C around the GaAs core. Growth rates for Al and Ga were 0.17 ML/s and 0.5 ML/s, respectively, while the As beam equivalent pressure (BEP) was identical to that for the growth 10 ACS Paragon Plus Environment

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of GaAs core. The AlGaAs shell was grown for 8 min, and the equivalent shell thickness was about 47 nm. The PL peak of GaAs / AlGaAs NWs (Figure 6.c) is quite similar to that of Figure 6.a, but 5 times more intense, showing the efficiency of the AlGaAs passivation. Finally, GaAs / AlGaAs core / shell NWs were capped with As layer and then transferred into the MBE chamber dedicated to functional oxides. After the As-decapping, SrTiO3 was grown by using the two-step method. An evident strong peak arises at 1.43eV (Figure 6.d), in accordance with result of the sample before SrTiO3 growth (Figure 6.c). From time-resolved PL measurements we extracted a lifetime of 300 ps and 209 ps for GaAs / AlGaAs NWs and GaAs / AlGaAs / SrTiO3 NWs respectively (see Supporting Information). It is well clarified that the AlGaAs passivation can avoid non-radiative recombination centers at the GaAs interface introduced by the fabrication of a SrTiO3 shell. 4. Conclusion and perspectives: The growth of GaAs core / SrTiO3 shell NWs has been achieved by MBE. Using an As capping / decapping method, the As-protected GaAs NWs were transferred in air without oxidation of the GaAs facets which is necessary for the further growth of functional oxides. By means of a two-step method for the oxide growth, we obtained partially oriented SrTiO3 layer covering the GaAs NWs although as shown by XPS analysis and TEM images the interface is not abrupt and probably prevents a perfect epitaxial growth. However, the desorption of the As oxides can be achieved by annealing in UHV at 500°C, and the GaAs / SrTiO3 NWs show a good thermal stability which is promising for the further growth of functional oxides such as ferroelectric BaTiO3. Surface preparation and desorption strategies are in progress in order to obtain an abrupt interface free of amorphous interlayer or Ga oxides. The first strategy consists in the preparation of the GaAs NW facets with Ti, Sr or Zintl-Klemm intermetallic phases [67]. The second strategy consists in growing the functional oxide at temperature high enough for both As oxides and Ga oxides to desorb. In summary, 11 ACS Paragon Plus Environment

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our study provides the first evidence that it is possible to obtain monocrystalline epitaxial shell of functional oxides monolithically integrated on semiconducting NWs. Associated Content Supporting Information. 1. Sample preparation and effect of the growth temperature: 2. Shadowing: 3. GaAs / SrTiO3 interface and SrTiO3 structural quality: 4. X-ray photoelectron spectroscopy analysis and thermal stability: 5. Photoluminescence measurements Acknowledgements: This work has been partly funded by the French Agence Nationale pour la Recherche (ANR), project COSCOF ANR 12JS10 00301. Chinese Scholarship Council (CSC) is acknowledged for its financial support. The authors thank the CLYM and NanoLyon for access to equipment and J. B. Goure for technical assistance.


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Figures:

Figure 1: SEM image of the GaAs NWs grown on Si(111) substrate (a). TEM images of a single GaAs NW showing its morphology and multi-twinned domains close to the NWs tip (b), lattice resolved TEM image of a GaAs NWs, with its Fourier transform in inset (c).


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Figure 2: Illustration of the capping/decapping and of the two-step SrTiO3 growth process (a, c, e, g). RHEED pattern measured along [110] directions: at 350°C after the decapping of the As protective shell (b), after the growth of SrTiO3 buffer during 15 minutes at 350°C (d), after the annealing at 550°C during 15 minutes (f), after the growth of SrTiO3 during 15 minutes at 550°C (h).


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Figure 3: Two-dimensional element mapping of Ga, As, Sr, Ti and O, respectively for a single NW (a) and its corresponding Scanning Transmission Electron Microscopy image for a spot size of 1.5 nm) (b) Line-scan performed along the NW diameter (c).


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Figure 4: TEM images showing an abrupt (a) and an amorphous (b) GaAs core / SrTiO3 shell NW interface.


Nano Letters

Sr4s & Ti3p

Ga3d

O2s & Sr4p

Intensity (arb. units)

Intensity (arb. units)

As3d

48

46

44

42

40

38

36

34

28

26

24

Binding energy (eV)

22

20

18

16

Binding energy (eV)

Sr3d

Ti2p Intensity (arb. units)

Intensity (arb. units) 139

138

137

136

135

134

133

132

131

468

466

464

Binding energy (eV)

462

460

458

Binding energy (eV)

O1s Intensity (arbitrary units)

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534

532

530

528

Binding energy (eV)

Figure 5: XPS core levels of GaAs / SrTiO3 NWs measured at room temperature.


456

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Figure 6: Photoluminescence spectra of decapped GaAs NW array (a), GaAs / SrTiO3 core / shell NW array (b), GaAs / AlGaAs core / shell NW array (c) and GaAs / AlGaAs / SrTiO3 core / shell NW array (d). For sample (b) and (d) the SrTiO3 was grown using the two-step method. The measurements were performed at 300 K.


SrTiO3 Shell Nanowires Grown by Molecular Beam

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