Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/NanoLett
Producing Atomically Abrupt Axial Heterojunctions in Silicon− Germanium Nanowires by Thermal Oxidation Hsin-Yu Lee, Tzu-Hsien Shen, Chen-Yu Hu, Yun-Yi Tsai, and Cheng-Yen Wen* Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan S Supporting Information *
ABSTRACT: Compositional abruptness of the interfaces is one of the important factors to determine the performance of Group IV semiconductor heterojunction (Si/Ge or Si/SiGe) nanowire devices. However, forming abrupt interfaces in the nanowires using the common vapor−liquid−solid (VLS) method is restricted because large solubility of Si and Ge in the Au eutectic liquid catalyst makes gradual composition change at the heterojunction after switching the gas phase components. According to the VLS growth mechanism, another possible approach to form an abrupt interface is making a change of the semiconductor concentration in the eutectic liquid before precipitation of the second phase. Here we show that the composition in AuSiGe eutectic liquid on SiGe nanowires of low Ge concentration (≤6%) can be altered by thermal oxidation at 700 °C. During the oxidation process, only Si is oxidized on the surface of the eutectic liquid, and the Ge/Si ratio in the eutectic liquid is increased. The subsequently precipitated SiGe step at the liquid/solid interface has a higher Ge concentration (∼20%), and a compositionally abrupt interface is produced in the nanowires. The growth mechanism of the heterojunction includes diffusion of Si and Ge atoms on nanowire surface into the AuSiGe eutectic liquid and step nucleation at the liquid/nanowire interface. KEYWORDS: Silicon−germanium nanowires, axial heterojunction, interfacial abruptness, thermal oxidation, step growth
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face.4,13−15 Lowering the semiconductor solubility in the liquid catalyst is helpful to improve interfacial abruptness. In order to reduce the reservoir effect, several approaches have been proposed, including using solid catalysts that have low Si and Ge solubility via the vapor−solid−solid (VSS) mechanism,8,16 in situ alloying the catalyst to decrease the eutectic solubility,17 and applying Sn catalysts which have low eutectic solubility.18 Although narrow heterojunction interfaces can be produced by these methods, morphological changes, such as kinking or diameter change, of nanowires are sometimes present after the formation of heterojunction. In VLS, a nanowire is formed by repeated step nucleation and step growth at the liquid/nanowire interface.19 When a silicon−germanium (Si−Ge) nanowire is growing, the Ge/Si ratio in the composition of each step is the same as that in the eutectic liquid catalyst. If the Ge/Si ratio in the eutectic liquid is changed abruptly for the formation of a heterojunction, the produced interface should accordingly exhibit an abrupt compositional change. We propose a method to create such a compositional change in the eutectic liquid to form a compositionally abrupt interface in Si−Ge nanowires. The tendency of Si to form an oxide is stronger than that of Ge.20,21
roup IV semiconductor heterojunction nanowires, such as Si/Ge or Si/SiGe alloy, are potentially useful in electronics, bandgap engineering, and thermoelectric applications.1−4 The small diameter of the nanowires is beneficial for avoiding the formation of misfit dislocations, which usually appear in thin film heterojunction structures of materials with a large lattice mismatch.5−8 The heterojunctions that are restricted in thin-film structures can therefore be realized in nanowires. Besides the structural perfection, compositional abruptness at the interface is another important factor in the properties and performance of heterojunction nanowire devices. Group IV nanowires are routinely grown via the vapor− liquid−solid (VLS) mechanism in chemical vapor deposition (CVD),9 pulsed laser ablation,4 and molecular beam epitaxy systems.10 They can be epitaxially formed on substrates with low defect density and well-defined size distribution.11,12 In the VLS mechanism, liquid catalysts, for example, AuSi or AuGe eutectic liquid, absorb semiconductor material from gas precursors and precipitate semiconductor solid at the liquid/ solid interface to form nanowires. Axial heterojunctions in nanowires can be produced simply by switching the gas precursors; however, because of large solubility of Si and Ge in the eutectic liquid (>20%), the compositional transition in the eutectic liquid can not be instant. As a consequence, the produced heterojunctions inevitably have a diffuse inter© XXXX American Chemical Society
Received: August 10, 2017 Revised: November 6, 2017 Published: November 29, 2017 A
DOI: 10.1021/acs.nanolett.7b03420 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Oxidation of nanowires was performed under the normal ambient air in a furnace. The oxidation temperatures used in this study were 700 and 900 °C. After oxidation, the nanowires were quenched to room temperature. This oxidation process was performed soon after the nanowires were prepared, so as to avoid the formation of an interfacial oxide layer, which could interrupt mass transfer between the nanowire and the Au metal at nanowire tips.22 In the control experiments, the as-grown Si−Ge nanowires were etched in KI/I2 solution to remove Au nanoparticles on nanowire tips and sidewalls before thermal oxidation. Figure 1a shows the TEM image of the Si1−xGex (x = 0.06) (denoted by SiGe in the following discussion) nanowire after thermal oxidation in air at 700 °C for 6 h, with the presence of Au eutectic liquid on the tip. This oxidation process results in the formation of a uniform amorphous oxide shell (in a thickness of 50 nm) on nanowire sidewalls and an oxide whisker (in a length of 300 nm) atop the Au nanoparticle. By contrast, if the nanowire surface has been etched to remove Au before oxidation, only a uniform oxide layer is formed surrounding the SiGe nanowire, as shown in Figure 1b. Most importantly, the oxidation process produces a sharp axial heterojunction interface (indicated by an arrow in Figure 1a,c) in the Au/SiGe nanowires. The EDS composition line profiles in Figure 1d show that the concentration of Ge changes abruptly from 6% to 20% across the interface, which is between the original SiGe (Si0.94Ge0.06) nanowire and a Ge-rich section. The Ge concentration in the Ge-rich section increases gradually from 20% to 70% along the axial direction. In the high-angle annular dark-field STEM (HAADF-STEM) image of the heterojunction (Figure 1e), no misfit dislocations are observed at the interface. The intensity in the HAADF-STEM image is sensitively proportionally to Z1.7, where Z is the atomic number,23 so that the intensity in Figure 1e contains the compositional information. The intensity profile of the (111) lattice planes (Figure 1f) indicates that the compositional change at the heterojunction interface is within one atomic plane. Because the oxidation temperature is higher than the eutectic temperatures of Au−Si and Au−Ge (∼360 °C),24,25 a AuSiGe eutectic liquid droplet forms on the nanowire tip during oxidation. When the liquid droplet is solidified at the end of oxidation, another section of silicon−germanium alloy is precipitated between the Ge-rich section and the Au tip, labeled as the final precipitation in Figure 1c. The distribution of chemical elements in the heterojunction nanowire in Figure 1a is shown in Figure 2. Ge appears in the whole nanowire core (Figure 2b). It also appears in the Au area because of solubility of Ge in solid Au.25 By contrast, the distribution of Si and O (Figure 2c and d, respectively) covers the whole nanowire. It is therefore confirmed that the oxide layer on nanowire sidewalls and the oxide whisker are composed of only Si and O. Figure 3a−c illustrate the kinetics of the heterojunction formation in SiGe nanowires at 700 °C. The length of the Gerich section and the thickness of the Si oxide layer both increase with the oxidation time but exhibit a saturation tendency. The driving force for the formation of the Ge-rich section in nanowires is due to the oxidation of Si; therefore, the thickness of the Ge-rich section can be controlled by the oxidation time. Termination of the oxidation process is revealed in the results after high-temperature (900 °C) oxidation (Figure 3d−f). The growth of the Ge-rich section continues until the lateral oxidation ends. The AuSiGe eutectic liquid is tapered in
It is expected that, when the AuSiGe eutectic liquid droplet on a Si−Ge nanowire is exposed to oxygen, only Si oxide will form on the surface of the droplet. Because Si is selectively extracted, the Ge/Si ratio in the eutectic liquid is increased. If Ge atoms are subsequently supplied into the eutectic liquid to make sufficient supersaturation for nucleation, then a step with a higher Ge concentration will be precipitated at the liquid/ nanowire interface. A compositionally abrupt interface can therefore be created. In this report, we demonstrate that the formation of a heterojunction based on the above mechanism is realized by thermally oxidizing Si−Ge (Si1−xGex, x = 0.03 and x = 0.06) nanowires with the AuSiGe eutectic liquid on nanowire tips. An atomically abrupt axial heterojunction interface is formed in the Si−Ge nanowires after oxidation. We describe the structure and chemical element distribution in the heterojunction nanowires based on transmission electron microscopy (TEM) observations. The role of the AuSiGe eutectic liquid in the formation of the heterojunction will be discussed. Our results suggest a model for the growth of a nanowire section of higher Ge concentration on the original Si−Ge nanowire with a compositionally abrupt interface. Finally, the effect of interdiffusion on interfacial abruptness will be described. Silicon−germanium nanowires in this research were grown in a cold-wall ultrahigh vacuum chemical vapor deposition (UHVCVD) system using the VLS method. The base pressure of the system was 10−9 Torr. Si (111) substrates were used for nanowire growth. They were cleaned in acetone, isopropanol, and 10% hydrogen fluoride solution, sequentially, and then immediately loaded into the UHV-CVD chamber. The chamber was equipped with in situ metal evaporators so that the deposited metal on the growth substrate was not exposed to the ambient environment. A 1 nm thick Au film was evaporated onto the substrate. At the growth temperature, the Au film reacted with the Si substrate to form eutectic liquid catalysts for the VLS nanowire growth. The gas precursors were diluted disilane (2% Si2H6 in helium) and diluted germane (2% GeH4 in helium). In this study, Si1−xGex nanowires of two Ge concentrations, x = 0.03 and x = 0.06, were respectively prepared by adjusting the flow ratio between the gas precursors. The growth direction of the nanowires was along the [111] axis of Si. After 2 h of growth, the nanowires had an average diameter of 150 nm and a length of about 1 μm. The nanowires were scraped off the growth substrate and transferred onto an amorphous carbon supporting film on a Cu grid for TEM analysis. For cross-sectional sample preparation, nanowires were embedded in epoxy, followed by mechanical polishing and Ar ion beam thinning. TEM analysis was performed in a JEOL AEM 2010F microscope and a JEOL AEM 2100F microscope equipped with a probe-type corrector for the spherical aberration of the objective lens. Both microscopes were operated at 200 kV. Energy dispersive spectroscopy (EDS) compositional information was obtained in the scanning transmission electron microscopy (STEM) mode. The Si and Ge compositional ratio in the EDS analysis was calibrated by measuring standard Si1−xGex thin films of different Ge/Si ratios. These Si1−xGex thin films were deposited on Si (111) substrates in the UHV-CVD chamber by varying the flow ratio of Si2H6 and GeH4 gases. The composition of the standard Si1−xGex film was determined from the lattice constant, which was measured in an X-ray diffractometer, using the Vegard’s law. B
DOI: 10.1021/acs.nanolett.7b03420 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 2. STEM-EDS composition maps of (a) Au, (b) Ge, (c) Si, and (d) O of the heterojunction nanowire in Figure 1a. The dashed line labels the position of the interface between the original SiGe nanowire and the Ge-rich section. The length of the dashed line also indicates the width of the whole nanowire including the oxide shell.
Figure 1. (a) TEM image of a SiGe (Si0.94Ge0.06) nanowire after oxidized in air at 700 °C for 6 h. The Au catalyst used for the VLS SiGe nanowire growth remained at the nanowire tip during the oxidation process. A segment with higher Ge concentration (the Gerich section) is formed in the nanowire. The arrow indicates the interface between the original SiGe nanowire and the Ge-rich section. (b) TEM image of a SiGe nanowire oxidized at 900 °C for 1 h without Au on nanowire surface. (c) High-angle annular dark-field scanning TEM (HAADF-STEM) image of the heterojunction formed after the same oxidation conditions as panel a. The arrow indicates the heterojunction interface. Another section of Si−Ge alloy is precipitated between the Ge-rich section and the Au tip during the cooling process. (d) STEM energy dispersive spectroscopy (STEMEDS) concentration line profiles of Au, Ge, and Si measured along the nanowire in panel c. The bottom dashed line labels the position of interface between the Ge-rich section and the SiGe nanowire, and the top dashed line labels the interface between Au and the final precipitate. (e) HAADF-STEM lattice image of the heterojunction formed in the SiGe nanowire, which is oxidized at 700 °C for 1.5 h. The viewing direction is along the [112] crystallographic axis. A coherent (111) interface is formed between the Ge-rich section and the SiGe nanowire. (f) Intensity profile of the image labeled in panel e. The two dashed lines label the transition region from SiGe to the Gerich section. The width of the interface is less than 0.36 nm.
Figure 3. (a−c) STEM dark-field images of SiGe nanowires after oxidized at 700 °C for 1.5, 6, and 13 h, respectively. (d−f) STEM darkfield images of SiGe nanowires after oxidized at 900 °C for 15 min, 1 h, and 4 h, respectively. The interface between the Ge-rich section and the original SiGe nanowire is indicated by arrow. The scale is 100 nm. Panels a−d and f are of the same magnification.
coalesce to form Au nanoparticles via the Ostwald ripening mechanism.28 In Figure 3, we find that there is no Au nanoparticle on the Ge-rich section and suggest that the Gerich section is newly formed because of the oxidation process. The Si oxide whisker on nanowire tip has a faster growth rate than the oxide layer on nanowire sidewalls (Figure S1 in Supporting Information). Such an enhanced oxide growth by the Au tips on Si nanowires has been observed,28,29 and the proposed mechanism for it includes (1) thermal etching of the
prolonged oxidation (Figure 3c,e) because the oxidation rate near the liquid tip is faster than that on the side of the liquid.26 Eventually, the eutectic liquid is separated into small droplets (Figure 3f). During the VLS nanowire growth, Au atoms in the liquid catalyst diffuse to nanowire sidewalls and substrate surface.27 In the subsequent thermal oxidation process, the surface Au atoms C
DOI: 10.1021/acs.nanolett.7b03420 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Ge atoms should diffuse into the Si−Ge nanowire, as evidenced in Figure 4c. A similar phenomenon is reported in the oxidation of Si/SiGe superlattice fin structure; the ejected Ge atoms diffuse not only into the SiGe lattice, but also transversely into the Si layers.30 On the basis of the above observations, the mechanism of the heterojunction formation in SiGe nanowires is proposed, as schematically illustrated in Figure 5. When a SiGe nanowire is
Si nanowires by AuSi eutectic liquid and (2) diffusion of Si atoms through the AuSi eutectic liquid to supply oxide formation.22,29 Here, the AuSiGe eutectic liquid also enhances the growth of the silicon oxide whisker, and the Ge/Si ratio in the composition of the eutectic liquid is consequently increased. If the Ge-rich section is precipitated as a result of solidification of the eutectic liquid at ∼360 °C, then its volume should not vary with different oxidation times. Therefore, the precipitation mechanism is excluded for the formation of the Ge-rich section. Figure 4a is the cross-sectional TEM image of an oxidized SiGe nanowire, which was etched to remove surface Au before
Figure 5. Schematic illustration of the formation mechanism of the heterojunction structure in SiGe nanowires by oxidation: (a) AuSiGe eutectic liquid forms when the temperature is higher than the eutectic point of the Au−Ge−Si ternary alloy; (b) because of oxidation, Si and Ge atoms are ejected and diffuse to the eutectic liquid; (c) while a Si oxide whisker is forming above the nanowire tip, the Ge-rich section is precipitated at the interface between the AuSiGe eutectic liquid and nanowire; (d) a final silicon−germanium segment is precipitated from the AuSiGe eutectic liquid after cooling.
Figure 4. (a) Cross-sectional TEM image of a SiGe nanowire oxidized at 700 °C for 6 h after the surface Au has been etched. (b, c) STEMEDS maps of Si and Ge, respectively, of the nanowire core in panel a. (d) Cross-sectional TEM image of a SiGe nanowire oxidized with the presence of surface Au at 700 °C for 6 h. (e, f) STEM-EDS maps of Si and Ge, respectively, of the nanowire core in panel d.
heated to high temperature, eutectic liquid, which contains Au, Si, and Ge, is formed on the nanowire (Figure 5a). A silicon oxide layer is grown on the surface of the nanowire in the following oxidation process (Figure 5b). Ge atoms are then ejected at the SiGe/oxide interface; besides, because of a large volume expansion when silicon transforms to silicon oxide (the density of Si atoms in silicon oxide is lower than that in the Si diamond cubic lattice), some Si atoms are also ejected from the oxide layer. These ejected Ge and Si atoms do not accumulate inside the SiGe nanowire core, but diffuse at the nanowire/ oxide interface. Absorption of these Ge and Si atoms by the AuSiGe eutectic liquid increases the oxidation kinetics; the oxide thickness of the heterojunction nanowire formed after oxidized at 900 °C for 1 h is 62 nm (Figure 3e), but that of the nanowire oxidized without the AuSiGe eutectic liquid by the same conditions is only 50 nm (Figure 1b). The formation of Si oxide whiskers on the nanowire tip consumes Si in the AuSiGe eutectic liquid so that the Ge/Si ratio in the liquid is increased. Meanwhile, the interfacial Ge and Si atoms diffuse to the liquid. Once the total amount of Ge and Si in the eutectic liquid reaches the solubility limit, a Ge-
oxidation. The image shows an amorphous shell surrounding the SiGe core. Since the shell contains only Si and O, the Ge atoms must have been ejected from the shell region during oxidation. Figure 4b and c show that Ge atoms indeed accumulate in the interior of the nanowire core. On the other hand, for the SiGe nanowire that is oxidized in the presence of Au (Figure 4d), there is no accumulation of Ge atoms inside the SiGe nanowire core (Figure 4e,f). Instead, Ge appears at the interface between the core and the oxide layer. These Ge particles are precipitated from the AuSiGe eutectic liquid droplets on nanowire sidewalls (Figure S2 in Supporting Information). Thus, it is suggested that the ejected Ge atoms due to Si oxidation should diffuse at the interface and some of them diffuse to the AuSiGe eutectic liquid on nanowire tips. The AuSiGe eutectic liquid at the tip is a reservoir of the ejected Ge during the oxidation process. Without it, the ejected D
DOI: 10.1021/acs.nanolett.7b03420 Nano Lett. XXXX, XXX, XXX−XXX
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results show that the heterojunction interface between the original SiGe nanowire (6% Ge) and the Ge-rich section (20% Ge) has a sufficient thermal budget for maintaining the abruptness in the postgrowth device fabrication process. In conclusion, atomically abrupt axial heterojunction interfaces are formed in Si−Ge (Si1−xGex, x = 0.03 and 0.06) nanowires after a thermal oxidation process. The Ge concentration changes abruptly between the original Si−Ge nanowire and a Ge-rich section. Such a transition in composition verifies our assumption that the Ge/Si ratio in the AuSiGe eutectic liquid can be increased by oxidation of the Si atoms from the eutectic liquid. Because the Ge/Si ratio in the eutectic liquid has been increased, the initial step growth of the Ge-rich section has a higher Ge concentration than that of the original Si−Ge nanowire. Consequently, an abrupt heterojunction interface is produced. Our results show that the thermal oxidation treatment is a controllable method to produce heterojunctions in Si−Ge nanowires. The formation of heterojunctions in Si−Ge alloy nanowires by this method is due to the miscibility and a relatively large difference of oxidation tendency between Si and Ge.
rich solid is precipitated at the liquid/nanowire interface (Figure 5c). Because a stable and uniform Si oxide shell confines the area of the liquid/nanowire interface, the Ge-rich section and the Si−Ge nanowire have the same thickness and axial direction. The morphological change that sometime appears in the CVD direct heterojunction growth methods does not occur. Finally, a section of silicon−germanium alloy is precipitated in the cooling step (Figure 5d). The Ge-rich section is epitaxially grown on the (111) surface of the original SiGe nanowire, as shown in Figure 1e. The (111) plane of Si is a favorable interface between Si and AuSi eutectic liquid during the VLS Si nanowire growth, and step growth of the Si(111) bilayer repeats on this interface.31 The flat (111) interface in Figure 1e implies that the (111) plane of the original SiGe nanowire also appears in contact with the AuSiGe eutectic liquid during the oxidation process. It is suggested that the formation of the Ge-rich section is also by step growth of the (111) bilayer at the liquid/nanowire interface. Each precipitated (111) bilayer has the same Ge/Si ratio in its composition as that in the eutectic liquid. Because the oxidation of Si has raised the Ge/Si ratio in the liquid before the growth of the Ge-rich section, the first precipitated Ge-rich bilayer has a higher Ge/Si ratio relative to the original SiGe nanowire, resulting in the formation of an atomically abrupt interface. In the continuous growth of the Ge-rich section, the Ge concentration gradually increases (as shown in Figure 1d), implying that the consumption of Si in the eutectic liquid should be faster than the supply of Si; otherwise, the Ge/Si ratio does not increase. Interestingly, the maximum Ge concentration in the Ge-rich section never reaches 100% by our growth conditions; after oxidation for 13 h at 700 °C, Ge concentration reaches a plateau at about 70% (Figure S3 in Supporting Information). In Figure 1d, the Ge concentration in the final precipitate varies from 50% to 40%, rather than the highest Ge concentration in the Ge-rich section. This deficiency of Ge concentration is attributed to the 3% solubility of Ge in Au at the eutectic temperature.25 Ge remains in the solid Au tip, as observed in Figure 2a and b. The heterojunction structure formed in Si1−xGex alloy nanowires with a lower Ge concentration (x = 0.03) is also consistent with the above growth model (Figure S4 in Supporting Information). A sharp heterojunction is formed between the original Si0.97Ge0.03 nanowire and a Ge-rich section, after the Si0.97Ge0.03 nanowire is oxidized at 700 °C for 6 h. The Ge concentration changes abruptly from 3% to 15% in less than 1 nm at the interface. The maximum concentration in the Gerich section is also about 70%. These heterojunction nanowires contain the original Si1−xGex nanowire section and a Ge-rich section. The thermal conductivity of the original Si1−xGex nanowire (e.g., x = 0.06) is higher than that of the Ge-rich section (0.2
