Atomically Abrupt Silicon–Germanium Axial Heterostructure

Letter pubs.acs.org/NanoLett

Atomically Abrupt Silicon−Germanium Axial Heterostructure Nanowires Synthesized in a Solvent Vapor Growth System Hugh Geaney,†,# Emma Mullane,†,# Quentin M. Ramasse,‡ and Kevin M. Ryan*,† †

Materials and Surface Science Institute and Department of Chemical and Environmental Sciences, University of Limerick, Ireland SuperSTEM Laboratory, SciTech Daresbury Campus, Daresbury WA4 4AD, United Kingdom

‡

S Supporting Information *

ABSTRACT: The growth of Si/Ge axial heterostructure nanowires in high yield using a versatile wet chemical approach is reported. Heterostructure growth is achieved using the vapor zone of a high boiling point solvent as a reaction medium with an evaporated tin layer as the catalyst. The low solubility of Si and Ge within the Sn catalyst allows the formation of extremely abrupt heterojunctions of the order of just 1−2 atomic planes between the Si and Ge nanowire segments. The compositional abruptness was confirmed using aberration corrected scanning transmission electron microscopy and atomic level electron energy loss spectroscopy. Additional analysis focused on the role of crystallographic defects in determining interfacial abruptness and the preferential incorporation of metal catalyst atoms near twin defects in the nanowires. KEYWORDS: Heterostructure nanowires, silicon, germanium, interfacial abruptness, solvent vapor growth, aberration corrected STEM analysis

B

To circumvent this issue, a variety of approaches have been explored with particular focus on catalyst alloying and solid phase catalysis. A successful method which allowed narrowing of the interfacial region in Si/Ge heterostructure NWs was shown by Ross et al. who exploited the decreased solubility of Si and Ge in a solid Au/Al18 (and more recently Ag/Au24) alloy catalyst to reduce the impact of the reservoir effect using a VSS growth method. However, Au is not an ideal group IV NW catalyst due to it being an electron trap and is also incompatible with complementary metal−oxide−semiconductor (CMOS) processing.25,26 Recently, “Type B” catalyst materials which possess eutectic compositions much lower than 1% Si and Ge have been identified as attractive group IV NW growth catalysts.27,28 In particular, In,29,30 Sn,31 and Bi32 are low melting point materials, facilitating low growth temperatures for pure Si and Ge NWs. This low eutectic solubility for Type B catalysts has also been exploited to tailor the interfacial width in Si/Ge heterostructure NWs using varied Ga percentages in Ga/ Au alloy nanoparticle seeds.19 This process resulted in the formation of reduced mixed Si/Ge interfaces (with a size reduction from approximately 60 to 6 nm) but required quite complex catalyst alloying processes within a CVD growth system. Pure type B catalyst materials have not been investigated in the formation of Si/Ge heterostructure NWs despite the potential for operating in a VLS growth mode while simultaneously allowing for the formation of abrupt heterojunctions due to a decreased reservoir effect.

ottom-up fabricated Si and Ge nanowires (NWs) have attracted significant research interest due to their wide ranging suitability for applications in transistors,1,2 photovoltaic cells,3,4 and Li-ion storage.5,6 Early synthetic efforts focused on developing catalytic processes which were suitable for generating highly anisotropic structures of either Ge or Si.7,8 However, advances in the synthetic control achievable within chemical vapor deposition (CVD) based systems led to the advent of more complex heterostructure NWs, including Si/Ge and III/VI compound semiconductor materials.9−14 These heterostructures have taken the form of axial or radial geometries with growth methods for Si/Ge and Si/SiGe heterostructure NWs focusing primarily on Au mediated vapor−liquid−solid (VLS) methods.15−17 Of utmost importance within the heterostructure NW field is the requirement for abrupt interfaces between the Si and Ge segments as they determine the suitability of these NWs for device architectures such as tunnel and avalanche transistors.18−21 Au catalyzed methods for axial heterostructure NW formation have struggled in producing abrupt heterojunctions due to the “reservoir effect”, where residual Si or Ge atoms remain within the catalyst after vapor source cessation and are incorporated into the NW, resulting in a diffuse boundary between the various sections.22 This effect is due to Au being a “Type A” catalyst whereby the respective eutectic alloys with Ge and Si contain more than 20% of the group IV atoms.23 Hence, when working in a VLS driven growth regime for Type A catalyst materials, this residual Si or Ge within the catalyst will be incorporated into the growing NW, prohibiting the formation of an abrupt heterojunction. This diffuse region has been typically observed as being of the same order as the catalyst diameter, meaning it is more pronounced for NWs with larger diameters.22 © 2013 American Chemical Society

Received: January 12, 2013 Revised: March 5, 2013 Published: March 21, 2013 1675

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We have recently developed a cost-effective high boiling solvent vapor growth (SVG) system which allows the formation of either Si or Ge NW using various catalytic processes.33−36 This route represents an intermediate between standard glassware based solution approaches and CVD methods which couples the high yields and low cost associated with the former and offers the excellent synthetic control and versatility of the latter. Here we present the growth of abrupt axial Ge/Si heterostructure NWs within the vapor phase of a SVG system using a thermally evaporated Sn catalyst layer. The NWs were formed using a one-pot wet chemical based SVG method, and this is the first time that axial heterostructure NWs have been formed outside of CVD and laser ablation approaches. Aberration-corrected STEM analysis coupled with atomic resolved EELS analysis confirms the abruptness of the Si/Ge interface while additional microscopic and spectroscopic analysis was used to identify crystallographic defects and the preferential incorporation of metal dopants. The ability to generate complex heterostructure NWs in a wet chemical system demonstrates the versatility of this growth protocol for the formation of highly functional NWs with excellent control of interfacial abruptness. Results and Discussion. Sn was chosen as catalyst material for heterostructure NW growth as it has recently been highlighted as an effective NW catalyst for both Si and Ge, is abundant, and crucially forms eutectic alloys with less than 1% of either element.31,37 This eutectic composition compares extremely favorably with alternative pure metal catalysts such as Au,38 Al,39 and germanide and silicide (“Type C”) forming metals such as Cu40 and Ni.41 The Sn catalyst islands employed here were formed in situ from a thermally evaporated Sn layer directly on a substrate, simplifying catalyst preparation for NW growth. Heterostructure NW growth was carried out within a costeffective, versatile high boiling point solvent vapor growth (SVG) system (depicted schematically in Figure 1) which is suited to NW growth using various catalytic approaches.33−35,42 Initially, silane was formed in situ by thermally decomposing phenylsilane (PS) along previously reported reaction pathways43 with Si NW growth occurring via a vapor−liquid−solid (VLS) mechanism. Following the Si NW segment growth, the reaction temperature was reduced from 460 to 430 °C, and triphenylgermane was injected. This temperature profile was necessary for two reasons: It was required to quench the Si NW growth (as there was residual silane vapor in the system) while also negating the influence of the increased reactivity of the Ge precursor triphenylgermane (TPG) in comparison to PS which would have led to uncontrolled Ge NW growth. At the reaction temperature of 430 °C, TPG breaks down in situ to form germane,44 and pure Ge segments were grown from the preformed Si NWs. The Sn seed also formed in situ from the annealing of evaporated layer allowing the growth of highly dense Si/Ge NWs (ranging from 20 to 100 nm in diameter and up to 2 μm in length) directly from the underlying growth substrate (Figure 1b). In type B catalysts, the seed diameter does not typically correlate directly with the wire diameter due to the different wetting behavior between the liquid seed and solid wire for these low melting point metals.29,30,45 In our case, there is a clear broadening of the Ge segment in comparison to the Si segment in the heterostructure (Figure 1c). This is consistent with the seed/NW diameter ratios of 2.25:1 for Si and 1.75:1 for Ge observed when silicon and germanium are

Figure 1. (a) Schematic illustrating the heterostructure NW growth method within a simple wet chemical based approach. Sn seeds were formed in situ from an evaporated Sn layer on the substrate. (b) SEM image showing examples of heterostructure NWs grown from the substrate. A number of NWs have been artifically colored to show the different sections. (c) Artificially colored high angle annular dark field image of a single Si/Ge heterostructure NW.

grown separately with tin seeds (Supporting Information, Figure 1). The interfaces of a number of the Si/Ge heterostructure NWs were examined using high-angle annular dark field (HAADF) imaging in a scanning transmission electron microscope (STEM), along with atomic-resolution electron energy loss spectroscopy (EELS) analysis. The HAADF and simultaneously acquired bright field (BF) STEM images taken at the interface of a heterostructure NW with a diameter of ca. 25 nm are shown in Figure 2a and b.The inset in Figure 2a is a lower magnification view of that NW, while an example image for a NW showing the Sn catalyst and Ge and Si sections is shown in Supporting Information, Figure 2. In the conditions used, the contrast of HAADF (or “Z-contrast”) images is approximately proportional to Zn (n = 1.7−2), where Z is the atomic number of the observed material. A high magnification view of the region indicated by the red box in Figure 1a at the Si/Ge interface is shown in Figure 2c. The HAADF intensity clearly suggests a sharp transition over one atomic plane (shown schematically by the dotted line) from the upper Si region to the higher Z Ge region underneath. To assess precisely the interface sharpness, an EELS chemical profile was recorded by moving the electron probe serially across the interface along the line indicated by the red arrow and recording the Ge L2,3 and Si K EELS edges (Figure 2e). The integrated EELS intensities (Figure 2d, bottom) are atomically resolved: their oscillations follow the simultaneously recorded HAADF signal (Figure 2d, top) and correspond to the successive atomic planes across the interface. Strikingly, the transition from pure Si to pure Ge occurs within 1−2 atomic planes. At the position indicated by the dotted line (the same 1676

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position is indicated on Figure 2c and d) both chemical signals are still ∼50% of their maximum values, indicating a plane possibly containing a mix of Si and Ge (this could however be partly attributed to EELS signal delocalization).47 This interfacial abruptness is far sharper than those noted for any Si/Ge heterostructure NWs previously grown via VLS approaches (where a mixed interface is typically of the order of the NW diameter)22 and appears better or is at least similar to the vapor−solid−solid (VSS) grown heterostructure NWs described by Chen et al. whose instrument did not allow as precise an atomic scale characterization as performed here18,24 It was noted in this study that the NWs did not grow exclusively along the ⟨111⟩ growth directions. As a result, the growth does not necessarily proceed through the formation of new (111) layers under the catalyst as previously reported.18 This variation may be due to the nature of the Sn as a type B catalyst whereby the low surface tension of the metal causes a somewhat more unstable growth front. Defects within NWs play an important role in determining their suitability for device applications as they can influence the electrical and mechanical properties of the nanostructures.49 Significant efforts have been made toward analyzing and controlling defect formation within NWs,36,50,51 with a recent report showing defect transfer directly from solid catalyst seeds to the resultant NWs.52 However, the impact of defects on the interfacial abruptness of Si/Ge heterostructure NWs has not been investigated. The NW presented in Figure 3 exhibits twin defects that originate in the Si segment of the NW (as the Si segment is grown first) and continue within the Ge segment after switching to Ge growth. The HAADF image of the interface with two clear twin defects present is shown in Figure 3a (the low magnification BFSTEM image inset shows how the defects propagate throughout the wire). The interfacial region highlighted is presented in Figure 3b and analyzed using EELS in Figure 3c. In contrast to the abrupt interface presented in Figure 2, EELS analysis shows there is a clear region (ca. 4 nm) where a mixed Si/Ge composition is noted. This is further evident in the gradual increase noted in the HAADF intensity profile. It seems likely that these twin defects are acting as a

Figure 2. Interfacial abruptness determination of Si/Ge heterostructure NWs. (a) High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image of Si/Ge interface taken from the heterostructure NW inset. (b) Corresponding bright field STEM image taken from the same heterostructure NW. (c) Unprocessed cut-out from the survey image which corresponds to the area indicated by the red box in a. (d) HAADF intensity profile and EELS line scans taken along the arrow defined in c. The HAADF intensity can be seen to correspond to the abrupt compositional variation presented in the EELS line scan (95% Gelest) was injected into the flask via the septum cap. The Ge segment was allowed to grow for 3 min. The NW growth was quenched by turning off the furnace and allowing the reaction vessel to cool to room temperature before stopping the Ar flow, opening the flask, and retrieving the growth substrate. Analysis. SEM analysis was performed on a Hitachi SU-70 system operating between 3 and 20 kV. The Sn substrates were untreated prior to SEM analysis. All scanning transmission electron microscopy work was carried out on a Nion UltraSTEM100 microscope58 operated at 100 keV primary beam energy. In the conditions used for the experiments, the microscope forms a ∼0.8 Å probe with a convergence semiangle of 31 mrad. The HAADF detector semiangular range was calibrated as 85−195 mrad for Z-contrast imaging. A Gatan Enfina spectrometer was used to acquire electron energy loss spectra. Although the native energy spread of the beam delivered by the cold field emission emitter of the microscope is 0.35 eV, the spectrometer was set up so both Ge L2,3 and Si K edges could be recorded simultaneously, resulting in an energy resolution (estimated by the full width at half-maximum of the

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NOTE ADDED AFTER ASAP PUBLICATION This Letter was published ASAP on March 25, 2013. The Table of Contents graphic has been modified. The correct version was published on March 27, 2013.

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dx.doi.org/10.1021/nl400146u | Nano Lett. 2013, 13, 1675−1680