Letter pubs.acs.org/NanoLett
Nickel/Platinum Dual Silicide Axial Nanowire Heterostructures with Excellent Photosensor Applications Yen-Ting Wu,† Chun-Wei Huang,† Chung-Hua Chiu,† Chia-Fu Chang,† Jui-Yuan Chen,† Ting-Yi Lin,† Yu-Ting Huang,† Kuo-Chang Lu,‡ Ping-Hung Yeh,§ and Wen-Wei Wu*,† †
Department of Materials Science and Engineering, National Chiao Tung University, No. 1001, University Road, East District, Hsinchu City 30010, Taiwan ‡ Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan City 70101, Taiwan § Department of Physics, Tamkang University, No. 151, Yingzhuan Road, Tamsui District, New Taipei City 25137, Taiwan S Supporting Information *
ABSTRACT: Transition metal silicide nanowires (NWs) have attracted increasing attention as they possess advantages of both silicon NWs and transition metals. Over the past years, there have been reported with efforts on one silicide in a single silicon NW. However, the research on multicomponent silicides in a single silicon NW is still rare, leading to limited functionalities. In this work, we successfully fabricated β-Pt2Si/Si/θ-Ni2Si, β-Pt2Si/θ-Ni2Si, and Pt, Ni, and Si ternary phase axial NW heterostructures through solid state reactions at 650 °C. Using in situ transmission electron microscope (in situ TEM), the growth mechanism of silicide NW heterostructures and the diffusion behaviors of transition metals were systematically studied. Spherical aberration corrected scanning transmission electron microscope (Cs-corrected STEM) equipped with energy dispersive spectroscopy (EDS) was used to analyze the phase structure and composition of silicide NW heterostructures. Moreover, electrical and photon sensing properties for the silicide nanowire heterostructures demonstrated promising applications in nano-optoeletronic devices. We found that Ni, Pt, and Si ternary phase nanowire heterostructures have an excellent infrared light sensing property which is absent in bulk Ni2Si or Pt2Si. The above results would benefit the further understanding of heterostructured nano materials. KEYWORDS: Silicide, nanowires, in situ TEM, photosensor, heterostructure, ternary phase
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With the purpose of exploring the potential of Si NWs, solid state reactions had been introduced to fabricate silicide NW heterostructures. This method manufactures silicide NWs through the diffusion of metal atoms into Si NW, followed by reacting with Si atoms and forming silicides. With the aid of better control of atom diffusion length and direction, silicide NW heterostructures can be fabricated in any shape or form. To date, some functional silicide NW heterostructures had been studied, such as NiSi,21−24 PtSi25,26 and Cu3Si27 NW heterostructures for their excellent electronic properties, and made into powerful field-effect transistors (FETs). Despite many reports on one silicide phase in a single NW heterostructure, the study of multicomponent phases in a single NW heterostructure is still rare. In this work, we investigated a unique solid state reaction among a Si NW, Ni and Pt metal contacts to fabricate Pt2Si and
owadays, silicon-based materials with traditional topdown approached such as photolithography and etching processes are still the core of semiconductor industry. However, as devices continue to scale down, these traditional processes will encounter serious physical restrictions.1 Therefore, a bottom-up process through self-assembly method, for the formation of 1D NWs, sheds light on the path to a new field of future electronic devices. Among numerous NWs, metal silicide NWs have been widely studied for their excellent compatibility with Si device processing and special physical properties such as ideal contact resistance (NiSi,2 PtSi3), thermoelectricity (CrSi2,4 MnSi1.75−7), magnetism (MnSi,8 CoSi9−11), and fieldemission (TiSi2,12 Ti5Si313). In addition to binary silicide NWs, ternary silicide NWs, for instance, Fe 1−x Co x Si 14 and Fe1−xMnxSi,15 had been synthesized with several exceptional characteristics as well. Typically, most silicide NWs are synthesized through thermal evaporation based on chemical vapor transport (CVT)16,17 or chemical vapor deposition (CVD)18−20 mechanism, which is low cost and great yield but nondirectional, limiting synthesis of single phase silicide NWs. © XXXX American Chemical Society
Received: October 23, 2015 Revised: January 13, 2016
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DOI: 10.1021/acs.nanolett.5b04309 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Ni2Si dual NW heterostructures. Through an in situ TEM, the whole reaction process can be directly observed and controlled; thus, we could obtain various NW heterostructures, including Pt2Si/Ni2Si with and without a Si gap, and Pt, Ni, and Si ternary NW heterostructures. It has been reported that schottky detectors, such as PtSi/Si junctions, have an excellent IR detection property.28 However, schottky-base detectors need to operate with huge phase interfaces, which is a serious physical limit with devices scale down. Therefore, we investigated electrical and IR photon sensing properties of the NW heterostructures, demonstrating promising potential in future advanced nanodevices. Furthermore, based on this unprecedented method, if we change metal contact, we would obtain different silicide NW heterostructures with other interesting physical properties. Methods. In this work, the crystalline Si NWs with high aspect ratio were grown on Si wafers in a three zone horizontal furnace through a vapor evaporation and deposition process with a vapor−liquid−solid (VLS) mechanism.29,30 The asgrown Si NWs were detached from the Si substrate into ethanol by ultrasonic vibration and dispersed on the Si3N4 membrane samples. 150 nm thick Ni contact electrodes were fabricated by E-beam lithography and E-gun evaporation. Prior to Ni deposition, surface oxide around the contact region was removed by buffered oxide etch (BOE). Subsequently, focused ion beam (FIB) was used to deposit 400 nm thick Pt contact electrodes (Figure S1). Real time observation of solid state reactions was carried out in an in situ TEM (JEOL 2100F combined with Protochips Aduro300 holder) with a video recorder which has a time resolution of 1/30 s. The reaction was investigated at 650 °C and pressure in the sample stage was about 3 × 10−6 Torr. Structure and composition of the silicide NW heterostructures were analyzed by Cs-corrected STEM (JEOL JEM-ARM200F) and energy dispersive spectrometry (EDS). At the last step, FIB was used again to deposit Pt metal wires connecting the outer electrodes and silicide NW heterostructures in order to measure the electrical and photon sensing properties. Semiconductor analyzers (Agilent B1500) and 940 nm wavelength infrared light lamp were used to measure these properties at ambient condition. Results and Discussion. Figure 1a shows the TEM image of a Si NW with Pt and Ni contact pads before annealing. After annealing at 650 °C for a while, dark contrast regions emerged at both edges of the Si NW near two metal pads and EDS mapping shows Ni and Pt signals (Figure 1b and S2). Therefore, we verified Ni and Pt atoms could diffuse into Si NWs through Si/Pt or Si/Ni interfaces and form silicides simultaneously. According to the high resolution TEM (HRTEM) image of the Pt silicide region in Figure 1c, we identified this silicide to be β-Pt2Si with [012] zone axis. This result is different from 2-D thin film system, where Pt2Si appears at around 300 °C and transforms into PtSi after Pt metal source is exhausted at 500 °C.31,32 However, in 1-D NW system, Pt metal contact electrodes are unlimited source as compared with 1-D Si NWs; thus, Pt2Si can exist at over 500 °C. On the other hand, HRTEM image of the Ni silicide region in Figure 1d shows the structure is θ-Ni2Si with [111] zone axis. In previous studies, Ni2Si is known as a competitive diffusion limited phase.33,34 Here, with such a high reaction temperature (650 °C) and small-diameter Si NWs we used (40 nm), Ni atoms did not spend much time on diffusion so that Ni2Si could emerge.
Figure 1. (a) TEM image of an as-fabricated Si NW based solid state reaction device where Pt contact is on the left and Ni contact is on the right. (b) TEM image of the device after annealing at 650 °C. The arrows indicate the reaction interface of Pt and Ni silicides. (c) HRTEM image of β-Pt2Si and the inset showing the corresponding FFT diffraction pattern with [012] zone axis. (d) HRTEM image of θNi2Si and the inset showing the corresponding FFT diffraction pattern with [111] zone axis.
Other than the above Pt2Si/Si/Ni2Si NW heterostructures, if we further prolonged the annealing time until two silicide/Si interfaces met each other, the Pt2Si/Ni2Si NW heterostructure, shown in Figure 2a, could be fabricated. Parts b−d of Figure 2 are EDS mapping corresponding to the dash box region in Figure 2a. On the basis of bright field and dark field STEM images in Figure 2, parts e and f, the Pt2Si/Ni2Si interface could be clearly found due to the large difference in Z-contrast between Pt and Ni atoms. The HRTEM image at the Pt2Si/ Ni2Si interface in Figure 2g demonstrates that Pt2Si and Ni2Si have [100]//[101] and (001)//(101̅) epitaxial relationships with a lattice misfit of 12.9%. However, the epitaxial interface is rough due to the strain caused by the misfit between the lattice spacing. The interaction between the two silicides is of interest; therefore, we let Pt and Ni atoms diffuse to obtain Pt,Ni,Si ternary NW heterostructures, as shown in Figure S3. With further analysis, we provided EDS line profile across the Pt,Ni,Si ternary region (Figure S5), indicating that Pt and Ni atoms were truly dissolved into the silicide matrix.33,35 According to inorganic crystal structure database (ICSD), there is no evidence for Pt, Ni, and Si ternary compounds. Thus, the Pt,Ni,Si ternary region must consist of Pt silicide and Ni silicide. Figure 3a shows a Pt2Si/Ni2Si NW heterostructure with EDS mapping in Figure S4a; HRTEM images in Figure 3, parts b and 3c, indicate that the silicide phases were Pt2Si and Ni2Si, respectively. After further annealing, this NW turned into a ternary heterostructure (Figure S4b). Parts d and e of Figure 3 are HRTEM images in the same positions after annealing, showing the silicide phases were still Pt2Si and Ni2Si, which means there was no phase transformation during the ternary region formation, but doping-like behavior instead. We propose two mechanisms of how foreign metal atoms exist in the silicide matrix. One is substitution mechanism as schematically illustrated in Figure 3f. For Ni2Si silicide matrix, with the fact that Ni2Si and Pt2Si share the same hexagonal structure, foreign Pt atoms could substitute Ni atoms in Ni2Si lattice and form PtxNi2‑xSi ternary phase. For Pt2Si silicide matrix, it formed B
DOI: 10.1021/acs.nanolett.5b04309 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. (a) TEM image of a Pt2Si/Ni2Si NW heterostructure. (b−d) EDS mapping corresponding to the dash box region in part a. (e) Bright field and (f) dark field STEM images of the Pt2Si/Ni2Si interface. (g) HRTEM image of the Pt2Si/Ni2Si interface. The insets show the FFT diffraction patterns of Ni2Si and Pt2Si, respectively.
Figure 3. (a) TEM image of a Pt2Si/Ni2Si NW heterostructure. (b, c) HRTEM images before annealing in the Pt silicide region and Ni silicide region, respectively. (d, e) HRTEM images after annealing in the Pt silicide region and Ni silicide region, respectively. (f) Schematic illustration of the Pt, Ni, and Si ternary NW heterostructure.
NixPt2‑xSi in the same manner. The other is interstitial mechanism, where part of foreign metal atoms squeezed into the silicide matrix and caused the silicide lattice expansion. Parts c and e of Figure 3 show Ni2Si lattice spacing expanded about 10% after annealing. Figure S6a is the top view of Ni2Si crystal from [001] direction and notably, the Ni2Si (001) plane is a parallelogram with equal sides. Figure S6b shows half of Ni2Si (001) plane is a regular triangle. Pt atoms will squeeze into the largest interstitial site located at the center of the regular triangle, as shown in Figure S6c. Red arrows point the lattice spacing, and calculation indicates that the lattice spacing change is about 13%, being consistent with the experimental results. As for Pt2Si, parts b and d of Figure 3 show the lattice spacing expanded less than 5% due to the larger interstitial site in Pt2Si crystal. Moreover, some dark particles appeared in both two silicides regions, as shown in Figures 2a and 3a. The EDS point analysis in Figure S7 shows that the metal content at the particle is much higher than that at the silicide NW; thus, those dark particles are possibly pure metal precipitates since metal contact electrodes were unlimited sources with respect to 1-D Si NWs. During the reaction, metal atoms continuously diffused into NWs and precipitated at grain boundaries or NW surfaces when the metal was supersaturated. The growth kinetics of Pt2Si and Ni2Si was investigated through dynamic observation by in situ TEM (Movie S1). Figure 4a is a plot of Pt2Si and Ni2Si length as a function of
annealing time. For Ni2Si growth rate, it obviously has two steps which were faster at the beginning (85.3 nm/s) and then slower (20.7 nm/s) later. At the first step, Ni atoms diffused into Si and formed Ni2Si, but the growth rate slowed down at the second step since the Ni atoms had to diffuse through the previously formed Ni2Si with a diffusion coefficient smaller than that through Si. In addition, real time growth observation (Movie S2) indicates that Ni2Si at the center grew faster than that at the edge and Ni2Si/Si interfaces; there is a positive curvature in Figure 4b. This is attributed to the fact that the interfacial energy for Ni2Si/SiOx is higher than that for Si/SiOx and Ni2Si prefers to nucleate at the core of NWs due to the lower energy barrier. Ni2Si diffusion and nucleation behaviors are schematically illustrated in Figure 4d. As for Pt2Si growth rate, there is only one step of approximately 51.5 nm/s. The single growth rate suggested that Pt atoms diffused along the Si NW surface, so that the Pt diffusion was independent of the Pt2Si length. Additionally, the real time observation (Movie S3) and negative curvature of Pt2Si/Si interface, shown in Figure 4c, demonstrate that the interfacial energy for Pt2Si/SiOx is likely lower than that for Si/SiOx. As a result, Pt2Si tends to nucleate at the NW surface instead of the center. In the same way, a schematic illustration of Pt2Si diffusion and nucleation C
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Figure 4. (a) Plot of Ni2Si and Pt2Si reaction length as a function of time. (b, c) High-magnification image of the Ni2Si/Si and Pt2Si/Si interfaces, respectively. Schematic illustration of (d) Ni atoms and (e) Pt atoms diffusion mechanism. The red arrow indicates the metal atoms diffusion path.
Figure 5. I−V measurements for (a) 100% and (b) 80% Pt, Ni, and Si ternary NW heterostructure between ±0.5 V bias in the dark and under exposure to 940 nm infrared light. (c) 100% Pt, Ni, and Si ternary NW heterostructure switching behaviors for time-dependent photoresponse with 940 nm infrared light turned on and off repeatedly. (d) Photoresponse and recovery time for a single cycle. (e, f) Schematic illustration of the photon sensing mechanism with the Pt, Ni, and Si NW heterostructure in the dark condition and under exposure of infrared light.
behaviors is shown in Figure 4e. HRTEM images of the Pt2Si/ Si and Ni2Si/Si interfaces are shown in Figure S8. For the above NW heterostructures, some electric and photon sensing properties were also explored. Two-terminal I− V measurements in Figure S9a show that all the NW heterostructures, including Ni2Si/Pt2Si, 50% and 80% Pt,Ni,Si ternary NW heterostructures, behave as typical Ohmic contact. Furthermore, the Ni2Si/Pt2Si NW heterostructure has lowest resistance (about 11.8 kΩ, black curve), which corresponds to the resistivity of 595.3 μΩ·cm. As for ternary NW heterostructures (red and blue curves), the resistivity is higher when the ternary region is larger. This phenomenon is probably attributed to a number of defects caused by larger lattice expansion based on interstitial mechanism in ternary regions; the defects capture free carriers, contributing to higher resistivity. Once we can change the resistivity of the ternary NW heterostructures by exciting those captured carriers through photons or something else, we will be able to produce tiny NW sensors. Therefore, in photon sensing property measurements, we compared the current gain in the dark and exposure to infrared light (940 nm). The results show that Ni2Si/Pt2Si NW heterostructures do not respond to infrared light with a bias of ±0.5 V. In contrast, 100% and 80% Pt,Ni,Si ternary NW heterostructures have interaction with the infrared light, as shown in Figure 5, parts a and b, respectively. In Figure 5a, the photocurrent is 152.61 nA (red curve) with a bias of 0.5 V, which is 5.6 times larger than dark current (28.14 nA, black curve). However, for the 80% Pt,Ni,Si ternary NW heterostructures in Figure 5b, the current gain is too small to be observed since the dark current is too high. Furthermore, Figure 5c exhibits I−t measurement of 100% Pt, Ni, and Si
ternary NW heterostructures with three on/off cycles at a bias of 0.5 V; each cycle is 200 s. We can find alternatively switching behaviors in this test. As for detailed switching time analysis, Figure 5d is the enlargement of the 300 to 400 s in Figure 5c, and the response time and recovery time are 0.2 and 0.1 s, respectively. The mechanism for the photon sensing is described below. Figure 5e is a schematic illustration of the ternary NW heterostructure in dark conditions; the black dots are defects in the ternary region, and red dots symbolize the captured free carriers or unbounding electrons. Under exposure infrared light, as illustrated in Figure 5f, part of the captured conducting carriers or unbounding electrons were excited by infrared light and escaped from the defects, leading to the increasing current. The defect-induced photon sensing mechanism also suggests that no response from the Ni2Si/ Pt2Si NW heterostructures to infrared light is due to lack of Pt,Ni,Si ternary region with numerous defects. Moreover, some of the excited carriers were gripped by other defects during exposure, contributing to the slightly unstable output current at Figure 5c. This also demonstrates the defect-induced photon sensing property. As we compare other oxide photodetectors, it is not a prerequisite for this to create any holes and react with oxygen,36 so that the response time and recovery time are extremely rapid. According to a previous study,28 PtSi/Si Schottky-based detectors require huge phase interfaces to D
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(3) Liu, B.; Wang, Y.; Dilts, S.; Mayer, T. S.; Mohney, S. E. Silicidation of Silicon Nanowires by Platinum. Nano Lett. 2007, 7, 818−824. (4) Zhou, F.; Szczech, J.; Pettes, M. T.; Moore, A. L.; Jin, S.; Shi, L. Determination of Transport Properties in Chromium Disilicide Nanowires via Combined Thermoelectric and Structural Characterizations. Nano Lett. 2007, 7, 1649−1654. (5) Aoyama, I.; Fedorov, M. I.; Zaitsev, V. K.; Solomkin, F. Y.; Eremin, I. S.; Samunin, A. Y.; Mukoujima, M.; Sano, S.; Tsuji, T. Effects of Ge Doping on Micromorphology of MnSi in MnSi∼1.7 and on Their Thermoelectric Transport Properties. Jpn. J. Appl. Phys. 2005, 44, 8562. (6) Shivaprasad, S. M.; Anandan, C.; Azatyan, S. G.; Gavriljuk, Y. L.; Lifshits, V. G. The Formation of MnSi(111) Interface at Room and High Temperatures. Surf. Sci. 1997, 382, 258−265. (7) Pokhrel, A.; Degregorio, Z. P.; Higgins, J. M.; Girard, S. N.; Jin, S. Vapor Phase Conversion Synthesis of Higher Manganese Silicide (MnSi1.75) Nanowire Arrays for Thermoelectric Applications. Chem. Mater. 2013, 25, 632−638. (8) Higgins, J. M.; Ding, R.; DeGrave, J. P.; Jin, S. Signature of Helimagnetic Ordering in Single-Crystal MnSi Nanowires. Nano Lett. 2010, 10, 1605−1610. (9) Seo, K.; Lee, S.; Yoon, H.; In, J.; Varadwaj, K. S. K.; Jo, Y.; Jung, M.-H.; Kim, J.; Kim, B. Composition-Tuned ConSi Nanowires: Location-Selective Simultaneous Growth along Temperature Gradient. ACS Nano 2009, 3, 1145−1150. (10) Williams, H. J.; Wernick, J. H.; Sherwood, R. C.; Wertheim, G. K. Magnetic Properties of the Monosilicides of Some 3d Transition Elements. J. Appl. Phys. 1966, 37, 1256−1256. (11) Tsai, C. I.; Yeh, P. H.; Wang, C. Y.; Wu, H. W.; Chen, U. S.; Lu, M. Y.; Wu, W. W.; Chen, L. J.; Wang, Z. L. Cobalt Silicide Nanostructures: Synthesis, Electron Transport, and Field Emission Properties. Cryst. Growth Des. 2009, 9, 4514−4518. (12) Xiang, B.; Wang, Q. X.; Wang, Z.; Zhang, X. Z.; Liu, L. Q.; Xu, J.; Yu, D. P. Synthesis and Field Emission Properties of TiSi2 Nanowires. Appl. Phys. Lett. 2005, 86, 243103. (13) Lin, H. K.; Tzeng, Y. F.; Wang, C. H.; Tai, N. H.; Lin, I. N.; Lee, C. Y.; Chiu, H. T. Ti5Si3 Nanowire and Its Field Emission Property. Chem. Mater. 2008, 20, 2429−2431. (14) Schmitt, A. L.; Higgins, J. M.; Jin, S. Chemical Synthesis and Magnetotransport of Magnetic Semiconducting Fe1−xCoxSi Alloy Nanowires. Nano Lett. 2008, 8, 810−815. (15) Hung, M. H.; Wang, C. Y.; Tang, J.; Lin, C. C.; Hou, T. C.; Jiang, X.; Wang, K. L.; Chen, L. J. Free-Standing and Single-Crystalline Fe1−xMnxSi Nanowires with Room-Temperature Ferromagnetism and Excellent Magnetic Response. ACS Nano 2012, 6, 4884−4891. (16) Song, Y.; Schmitt, A. L.; Jin, S. Ultralong Single-Crystal Metallic Ni2Si Nanowires with Low Resistivity. Nano Lett. 2007, 7, 965−969. (17) Szczech, J. R.; Schmitt, A. L.; Bierman, M. J.; Jin, S. SingleCrystal Semiconducting Chromium Disilicide Nanowires Synthesized via Chemical Vapor Transport. Chem. Mater. 2007, 19, 3238−3243. (18) Liang, Y. H.; Yu, S. Y.; Hsin, C. L.; Huang, C. W.; Wu, W. W. Growth of Single-Crystalline Cobalt Silicide Nanowires with Excellent Physical Properties. J. Appl. Phys. 2011, 110, 074302. (19) Lee, C. Y.; Lu, M. P.; Liao, K. F.; Lee, W. F.; Huang, C. T.; Chen, S. Y.; Chen, L. J. Free-Standing Single-Crystal NiSi2 Nanowires with Excellent Electrical Transport and Field Emission Properties. J. Phys. Chem. C 2009, 113, 2286−2289. (20) Chiu, W. L.; Chiu, C. H.; Chen, J. Y.; Huang, C. W.; Huang, Y. T.; Lu, K. C.; Hsin, C. L.; Yeh, P. H.; Wu, W. W. Single-crystalline δNi(2)Si Nanowires with Excellent Physical Properties. Nanoscale Res. Lett. 2013, 8, 290−290. (21) Wu, W. W.; Lu, K. C.; Chen, K. N.; Yeh, P. H.; Wang, C. W.; Lin, Y. C.; Huang, Y. Controlled Large Strain of Ni Silicide/Si/Ni Silicide Nanowire Heterostructures and Their Electron Transport Properties. Appl. Phys. Lett. 2010, 97, 203110. (22) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Single-crystal Metallic Nanowires and Metal/Semiconductor Nanowire Heterostructures. Nature 2004, 430, 61−65.
accept as many excited carriers as possible. In contrast, the ternary NW heterostructure detectors do not need any phase interface since the defects are sufficient in the silicide matrix; in other words, phase interfaces are ubiquitous in the whole NW. With above advantages, ternary NW heterostructure detectors may be the most efficient and smallest photon detectors in the future. Conclusions. In this study, we have successfully fabricated controllable Pt2Si/Si/Ni2Si, Pt2Si/Ni2Si, and Pt, Ni, and Si ternary NW heterostructures at 650 °C through solid state reactions. We found that Ni2Si and Pt2Si have [100]//[101] and (001)//(101)̅ epitaxial relationships with large strain. At the Pt, Ni, and Si ternary region, we confirmed this area is composed of PtxNi2‑xSi and NixPt2‑xSi, which formed by substitution mechanism. Additionally, interstitial mechanism occurred in ternary region as well and caused lattice spacing expansion. The in situ observations indicate that Ni2Si and Pt2Si are very different in terms of growth kinetics and formation mechanism. Furthermore, we also conducted electrical and photon sensing properties measurements for all the NW heterostructures. Ni2Si/Pt2Si NW heterostructures have the best resistivity of approximately 595.3 μΩ·cm. Although Pt,Ni,Si ternary NW heterostructures have higher resistivity owing to the defects in ternary region, these defects contribute to the excellent infrared light sensing property, which is absent in Ni2Si/Pt2Si NW heterostructures, bulk Ni2Si or Pt2Si.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04309. Fabrication procedures of the Si NW solid state reaction devices, EDS mapping and line scan for Pt2Si/Si/Ni2Si and Pt,Ni,Si NW heterostructures through solid state reactions, the Ni2Si crystal structure, I−V measurements for NW heterostructures, and full captions for the movies (PDF) Dynamic growth process of a Pt 2 Si/Ni 2 Si NW heterostructure (AVI) Dynamic growth process of Ni2Si (AVI) Dynamic growth process of Pt2Si (AVI)
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AUTHOR INFORMATION
Corresponding Author
*(W.-W.W.) E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support by Ministry of Science and Technology through Grants 103-2221-E-009-222-MY3, 104-2221-E-009-050-MY4, and 102-2221-E-006-077-MY3.
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Nickel/Platinum Dual Silicide Axial Nanowire Heterostructures with Excellent Photosensor Applications Yen-Ting Wu,† Chun-Wei Huang,† Chung-Hua Chiu,† Chia-Fu Chang,† Jui-Yuan Chen,† Ting-Yi Lin,† Yu-Ting Huang,† Kuo-Chang Lu,‡ Ping-Hung Yeh,§ and Wen-Wei Wu*,† †
Department of Materials Science and Engineering, National Chiao Tung University, No. 1001, University Road, East District, Hsinchu City 30010, Taiwan ‡ Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan City 70101, Taiwan § Department of Physics, Tamkang University, No. 151, Yingzhuan Road, Tamsui District, New Taipei City 25137, Taiwan S Supporting Information *
ABSTRACT: Transition metal silicide nanowires (NWs) have attracted increasing attention as they possess advantages of both silicon NWs and transition metals. Over the past years, there have been reported with efforts on one silicide in a single silicon NW. However, the research on multicomponent silicides in a single silicon NW is still rare, leading to limited functionalities. In this work, we successfully fabricated β-Pt2Si/Si/θ-Ni2Si, β-Pt2Si/θ-Ni2Si, and Pt, Ni, and Si ternary phase axial NW heterostructures through solid state reactions at 650 °C. Using in situ transmission electron microscope (in situ TEM), the growth mechanism of silicide NW heterostructures and the diffusion behaviors of transition metals were systematically studied. Spherical aberration corrected scanning transmission electron microscope (Cs-corrected STEM) equipped with energy dispersive spectroscopy (EDS) was used to analyze the phase structure and composition of silicide NW heterostructures. Moreover, electrical and photon sensing properties for the silicide nanowire heterostructures demonstrated promising applications in nano-optoeletronic devices. We found that Ni, Pt, and Si ternary phase nanowire heterostructures have an excellent infrared light sensing property which is absent in bulk Ni2Si or Pt2Si. The above results would benefit the further understanding of heterostructured nano materials. KEYWORDS: Silicide, nanowires, in situ TEM, photosensor, heterostructure, ternary phase
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With the purpose of exploring the potential of Si NWs, solid state reactions had been introduced to fabricate silicide NW heterostructures. This method manufactures silicide NWs through the diffusion of metal atoms into Si NW, followed by reacting with Si atoms and forming silicides. With the aid of better control of atom diffusion length and direction, silicide NW heterostructures can be fabricated in any shape or form. To date, some functional silicide NW heterostructures had been studied, such as NiSi,21−24 PtSi25,26 and Cu3Si27 NW heterostructures for their excellent electronic properties, and made into powerful field-effect transistors (FETs). Despite many reports on one silicide phase in a single NW heterostructure, the study of multicomponent phases in a single NW heterostructure is still rare. In this work, we investigated a unique solid state reaction among a Si NW, Ni and Pt metal contacts to fabricate Pt2Si and
owadays, silicon-based materials with traditional topdown approached such as photolithography and etching processes are still the core of semiconductor industry. However, as devices continue to scale down, these traditional processes will encounter serious physical restrictions.1 Therefore, a bottom-up process through self-assembly method, for the formation of 1D NWs, sheds light on the path to a new field of future electronic devices. Among numerous NWs, metal silicide NWs have been widely studied for their excellent compatibility with Si device processing and special physical properties such as ideal contact resistance (NiSi,2 PtSi3), thermoelectricity (CrSi2,4 MnSi1.75−7), magnetism (MnSi,8 CoSi9−11), and fieldemission (TiSi2,12 Ti5Si313). In addition to binary silicide NWs, ternary silicide NWs, for instance, Fe 1−x Co x Si 14 and Fe1−xMnxSi,15 had been synthesized with several exceptional characteristics as well. Typically, most silicide NWs are synthesized through thermal evaporation based on chemical vapor transport (CVT)16,17 or chemical vapor deposition (CVD)18−20 mechanism, which is low cost and great yield but nondirectional, limiting synthesis of single phase silicide NWs. © XXXX American Chemical Society
Received: October 23, 2015 Revised: January 13, 2016
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Nano Letters Ni2Si dual NW heterostructures. Through an in situ TEM, the whole reaction process can be directly observed and controlled; thus, we could obtain various NW heterostructures, including Pt2Si/Ni2Si with and without a Si gap, and Pt, Ni, and Si ternary NW heterostructures. It has been reported that schottky detectors, such as PtSi/Si junctions, have an excellent IR detection property.28 However, schottky-base detectors need to operate with huge phase interfaces, which is a serious physical limit with devices scale down. Therefore, we investigated electrical and IR photon sensing properties of the NW heterostructures, demonstrating promising potential in future advanced nanodevices. Furthermore, based on this unprecedented method, if we change metal contact, we would obtain different silicide NW heterostructures with other interesting physical properties. Methods. In this work, the crystalline Si NWs with high aspect ratio were grown on Si wafers in a three zone horizontal furnace through a vapor evaporation and deposition process with a vapor−liquid−solid (VLS) mechanism.29,30 The asgrown Si NWs were detached from the Si substrate into ethanol by ultrasonic vibration and dispersed on the Si3N4 membrane samples. 150 nm thick Ni contact electrodes were fabricated by E-beam lithography and E-gun evaporation. Prior to Ni deposition, surface oxide around the contact region was removed by buffered oxide etch (BOE). Subsequently, focused ion beam (FIB) was used to deposit 400 nm thick Pt contact electrodes (Figure S1). Real time observation of solid state reactions was carried out in an in situ TEM (JEOL 2100F combined with Protochips Aduro300 holder) with a video recorder which has a time resolution of 1/30 s. The reaction was investigated at 650 °C and pressure in the sample stage was about 3 × 10−6 Torr. Structure and composition of the silicide NW heterostructures were analyzed by Cs-corrected STEM (JEOL JEM-ARM200F) and energy dispersive spectrometry (EDS). At the last step, FIB was used again to deposit Pt metal wires connecting the outer electrodes and silicide NW heterostructures in order to measure the electrical and photon sensing properties. Semiconductor analyzers (Agilent B1500) and 940 nm wavelength infrared light lamp were used to measure these properties at ambient condition. Results and Discussion. Figure 1a shows the TEM image of a Si NW with Pt and Ni contact pads before annealing. After annealing at 650 °C for a while, dark contrast regions emerged at both edges of the Si NW near two metal pads and EDS mapping shows Ni and Pt signals (Figure 1b and S2). Therefore, we verified Ni and Pt atoms could diffuse into Si NWs through Si/Pt or Si/Ni interfaces and form silicides simultaneously. According to the high resolution TEM (HRTEM) image of the Pt silicide region in Figure 1c, we identified this silicide to be β-Pt2Si with [012] zone axis. This result is different from 2-D thin film system, where Pt2Si appears at around 300 °C and transforms into PtSi after Pt metal source is exhausted at 500 °C.31,32 However, in 1-D NW system, Pt metal contact electrodes are unlimited source as compared with 1-D Si NWs; thus, Pt2Si can exist at over 500 °C. On the other hand, HRTEM image of the Ni silicide region in Figure 1d shows the structure is θ-Ni2Si with [111] zone axis. In previous studies, Ni2Si is known as a competitive diffusion limited phase.33,34 Here, with such a high reaction temperature (650 °C) and small-diameter Si NWs we used (40 nm), Ni atoms did not spend much time on diffusion so that Ni2Si could emerge.
Figure 1. (a) TEM image of an as-fabricated Si NW based solid state reaction device where Pt contact is on the left and Ni contact is on the right. (b) TEM image of the device after annealing at 650 °C. The arrows indicate the reaction interface of Pt and Ni silicides. (c) HRTEM image of β-Pt2Si and the inset showing the corresponding FFT diffraction pattern with [012] zone axis. (d) HRTEM image of θNi2Si and the inset showing the corresponding FFT diffraction pattern with [111] zone axis.
Other than the above Pt2Si/Si/Ni2Si NW heterostructures, if we further prolonged the annealing time until two silicide/Si interfaces met each other, the Pt2Si/Ni2Si NW heterostructure, shown in Figure 2a, could be fabricated. Parts b−d of Figure 2 are EDS mapping corresponding to the dash box region in Figure 2a. On the basis of bright field and dark field STEM images in Figure 2, parts e and f, the Pt2Si/Ni2Si interface could be clearly found due to the large difference in Z-contrast between Pt and Ni atoms. The HRTEM image at the Pt2Si/ Ni2Si interface in Figure 2g demonstrates that Pt2Si and Ni2Si have [100]//[101] and (001)//(101̅) epitaxial relationships with a lattice misfit of 12.9%. However, the epitaxial interface is rough due to the strain caused by the misfit between the lattice spacing. The interaction between the two silicides is of interest; therefore, we let Pt and Ni atoms diffuse to obtain Pt,Ni,Si ternary NW heterostructures, as shown in Figure S3. With further analysis, we provided EDS line profile across the Pt,Ni,Si ternary region (Figure S5), indicating that Pt and Ni atoms were truly dissolved into the silicide matrix.33,35 According to inorganic crystal structure database (ICSD), there is no evidence for Pt, Ni, and Si ternary compounds. Thus, the Pt,Ni,Si ternary region must consist of Pt silicide and Ni silicide. Figure 3a shows a Pt2Si/Ni2Si NW heterostructure with EDS mapping in Figure S4a; HRTEM images in Figure 3, parts b and 3c, indicate that the silicide phases were Pt2Si and Ni2Si, respectively. After further annealing, this NW turned into a ternary heterostructure (Figure S4b). Parts d and e of Figure 3 are HRTEM images in the same positions after annealing, showing the silicide phases were still Pt2Si and Ni2Si, which means there was no phase transformation during the ternary region formation, but doping-like behavior instead. We propose two mechanisms of how foreign metal atoms exist in the silicide matrix. One is substitution mechanism as schematically illustrated in Figure 3f. For Ni2Si silicide matrix, with the fact that Ni2Si and Pt2Si share the same hexagonal structure, foreign Pt atoms could substitute Ni atoms in Ni2Si lattice and form PtxNi2‑xSi ternary phase. For Pt2Si silicide matrix, it formed B
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Figure 2. (a) TEM image of a Pt2Si/Ni2Si NW heterostructure. (b−d) EDS mapping corresponding to the dash box region in part a. (e) Bright field and (f) dark field STEM images of the Pt2Si/Ni2Si interface. (g) HRTEM image of the Pt2Si/Ni2Si interface. The insets show the FFT diffraction patterns of Ni2Si and Pt2Si, respectively.
Figure 3. (a) TEM image of a Pt2Si/Ni2Si NW heterostructure. (b, c) HRTEM images before annealing in the Pt silicide region and Ni silicide region, respectively. (d, e) HRTEM images after annealing in the Pt silicide region and Ni silicide region, respectively. (f) Schematic illustration of the Pt, Ni, and Si ternary NW heterostructure.
NixPt2‑xSi in the same manner. The other is interstitial mechanism, where part of foreign metal atoms squeezed into the silicide matrix and caused the silicide lattice expansion. Parts c and e of Figure 3 show Ni2Si lattice spacing expanded about 10% after annealing. Figure S6a is the top view of Ni2Si crystal from [001] direction and notably, the Ni2Si (001) plane is a parallelogram with equal sides. Figure S6b shows half of Ni2Si (001) plane is a regular triangle. Pt atoms will squeeze into the largest interstitial site located at the center of the regular triangle, as shown in Figure S6c. Red arrows point the lattice spacing, and calculation indicates that the lattice spacing change is about 13%, being consistent with the experimental results. As for Pt2Si, parts b and d of Figure 3 show the lattice spacing expanded less than 5% due to the larger interstitial site in Pt2Si crystal. Moreover, some dark particles appeared in both two silicides regions, as shown in Figures 2a and 3a. The EDS point analysis in Figure S7 shows that the metal content at the particle is much higher than that at the silicide NW; thus, those dark particles are possibly pure metal precipitates since metal contact electrodes were unlimited sources with respect to 1-D Si NWs. During the reaction, metal atoms continuously diffused into NWs and precipitated at grain boundaries or NW surfaces when the metal was supersaturated. The growth kinetics of Pt2Si and Ni2Si was investigated through dynamic observation by in situ TEM (Movie S1). Figure 4a is a plot of Pt2Si and Ni2Si length as a function of
annealing time. For Ni2Si growth rate, it obviously has two steps which were faster at the beginning (85.3 nm/s) and then slower (20.7 nm/s) later. At the first step, Ni atoms diffused into Si and formed Ni2Si, but the growth rate slowed down at the second step since the Ni atoms had to diffuse through the previously formed Ni2Si with a diffusion coefficient smaller than that through Si. In addition, real time growth observation (Movie S2) indicates that Ni2Si at the center grew faster than that at the edge and Ni2Si/Si interfaces; there is a positive curvature in Figure 4b. This is attributed to the fact that the interfacial energy for Ni2Si/SiOx is higher than that for Si/SiOx and Ni2Si prefers to nucleate at the core of NWs due to the lower energy barrier. Ni2Si diffusion and nucleation behaviors are schematically illustrated in Figure 4d. As for Pt2Si growth rate, there is only one step of approximately 51.5 nm/s. The single growth rate suggested that Pt atoms diffused along the Si NW surface, so that the Pt diffusion was independent of the Pt2Si length. Additionally, the real time observation (Movie S3) and negative curvature of Pt2Si/Si interface, shown in Figure 4c, demonstrate that the interfacial energy for Pt2Si/SiOx is likely lower than that for Si/SiOx. As a result, Pt2Si tends to nucleate at the NW surface instead of the center. In the same way, a schematic illustration of Pt2Si diffusion and nucleation C
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Figure 4. (a) Plot of Ni2Si and Pt2Si reaction length as a function of time. (b, c) High-magnification image of the Ni2Si/Si and Pt2Si/Si interfaces, respectively. Schematic illustration of (d) Ni atoms and (e) Pt atoms diffusion mechanism. The red arrow indicates the metal atoms diffusion path.
Figure 5. I−V measurements for (a) 100% and (b) 80% Pt, Ni, and Si ternary NW heterostructure between ±0.5 V bias in the dark and under exposure to 940 nm infrared light. (c) 100% Pt, Ni, and Si ternary NW heterostructure switching behaviors for time-dependent photoresponse with 940 nm infrared light turned on and off repeatedly. (d) Photoresponse and recovery time for a single cycle. (e, f) Schematic illustration of the photon sensing mechanism with the Pt, Ni, and Si NW heterostructure in the dark condition and under exposure of infrared light.
behaviors is shown in Figure 4e. HRTEM images of the Pt2Si/ Si and Ni2Si/Si interfaces are shown in Figure S8. For the above NW heterostructures, some electric and photon sensing properties were also explored. Two-terminal I− V measurements in Figure S9a show that all the NW heterostructures, including Ni2Si/Pt2Si, 50% and 80% Pt,Ni,Si ternary NW heterostructures, behave as typical Ohmic contact. Furthermore, the Ni2Si/Pt2Si NW heterostructure has lowest resistance (about 11.8 kΩ, black curve), which corresponds to the resistivity of 595.3 μΩ·cm. As for ternary NW heterostructures (red and blue curves), the resistivity is higher when the ternary region is larger. This phenomenon is probably attributed to a number of defects caused by larger lattice expansion based on interstitial mechanism in ternary regions; the defects capture free carriers, contributing to higher resistivity. Once we can change the resistivity of the ternary NW heterostructures by exciting those captured carriers through photons or something else, we will be able to produce tiny NW sensors. Therefore, in photon sensing property measurements, we compared the current gain in the dark and exposure to infrared light (940 nm). The results show that Ni2Si/Pt2Si NW heterostructures do not respond to infrared light with a bias of ±0.5 V. In contrast, 100% and 80% Pt,Ni,Si ternary NW heterostructures have interaction with the infrared light, as shown in Figure 5, parts a and b, respectively. In Figure 5a, the photocurrent is 152.61 nA (red curve) with a bias of 0.5 V, which is 5.6 times larger than dark current (28.14 nA, black curve). However, for the 80% Pt,Ni,Si ternary NW heterostructures in Figure 5b, the current gain is too small to be observed since the dark current is too high. Furthermore, Figure 5c exhibits I−t measurement of 100% Pt, Ni, and Si
ternary NW heterostructures with three on/off cycles at a bias of 0.5 V; each cycle is 200 s. We can find alternatively switching behaviors in this test. As for detailed switching time analysis, Figure 5d is the enlargement of the 300 to 400 s in Figure 5c, and the response time and recovery time are 0.2 and 0.1 s, respectively. The mechanism for the photon sensing is described below. Figure 5e is a schematic illustration of the ternary NW heterostructure in dark conditions; the black dots are defects in the ternary region, and red dots symbolize the captured free carriers or unbounding electrons. Under exposure infrared light, as illustrated in Figure 5f, part of the captured conducting carriers or unbounding electrons were excited by infrared light and escaped from the defects, leading to the increasing current. The defect-induced photon sensing mechanism also suggests that no response from the Ni2Si/ Pt2Si NW heterostructures to infrared light is due to lack of Pt,Ni,Si ternary region with numerous defects. Moreover, some of the excited carriers were gripped by other defects during exposure, contributing to the slightly unstable output current at Figure 5c. This also demonstrates the defect-induced photon sensing property. As we compare other oxide photodetectors, it is not a prerequisite for this to create any holes and react with oxygen,36 so that the response time and recovery time are extremely rapid. According to a previous study,28 PtSi/Si Schottky-based detectors require huge phase interfaces to D
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(3) Liu, B.; Wang, Y.; Dilts, S.; Mayer, T. S.; Mohney, S. E. Silicidation of Silicon Nanowires by Platinum. Nano Lett. 2007, 7, 818−824. (4) Zhou, F.; Szczech, J.; Pettes, M. T.; Moore, A. L.; Jin, S.; Shi, L. Determination of Transport Properties in Chromium Disilicide Nanowires via Combined Thermoelectric and Structural Characterizations. Nano Lett. 2007, 7, 1649−1654. (5) Aoyama, I.; Fedorov, M. I.; Zaitsev, V. K.; Solomkin, F. Y.; Eremin, I. S.; Samunin, A. Y.; Mukoujima, M.; Sano, S.; Tsuji, T. Effects of Ge Doping on Micromorphology of MnSi in MnSi∼1.7 and on Their Thermoelectric Transport Properties. Jpn. J. Appl. Phys. 2005, 44, 8562. (6) Shivaprasad, S. M.; Anandan, C.; Azatyan, S. G.; Gavriljuk, Y. L.; Lifshits, V. G. The Formation of MnSi(111) Interface at Room and High Temperatures. Surf. Sci. 1997, 382, 258−265. (7) Pokhrel, A.; Degregorio, Z. P.; Higgins, J. M.; Girard, S. N.; Jin, S. Vapor Phase Conversion Synthesis of Higher Manganese Silicide (MnSi1.75) Nanowire Arrays for Thermoelectric Applications. Chem. Mater. 2013, 25, 632−638. (8) Higgins, J. M.; Ding, R.; DeGrave, J. P.; Jin, S. Signature of Helimagnetic Ordering in Single-Crystal MnSi Nanowires. Nano Lett. 2010, 10, 1605−1610. (9) Seo, K.; Lee, S.; Yoon, H.; In, J.; Varadwaj, K. S. K.; Jo, Y.; Jung, M.-H.; Kim, J.; Kim, B. Composition-Tuned ConSi Nanowires: Location-Selective Simultaneous Growth along Temperature Gradient. ACS Nano 2009, 3, 1145−1150. (10) Williams, H. J.; Wernick, J. H.; Sherwood, R. C.; Wertheim, G. K. Magnetic Properties of the Monosilicides of Some 3d Transition Elements. J. Appl. Phys. 1966, 37, 1256−1256. (11) Tsai, C. I.; Yeh, P. H.; Wang, C. Y.; Wu, H. W.; Chen, U. S.; Lu, M. Y.; Wu, W. W.; Chen, L. J.; Wang, Z. L. Cobalt Silicide Nanostructures: Synthesis, Electron Transport, and Field Emission Properties. Cryst. Growth Des. 2009, 9, 4514−4518. (12) Xiang, B.; Wang, Q. X.; Wang, Z.; Zhang, X. Z.; Liu, L. Q.; Xu, J.; Yu, D. P. Synthesis and Field Emission Properties of TiSi2 Nanowires. Appl. Phys. Lett. 2005, 86, 243103. (13) Lin, H. K.; Tzeng, Y. F.; Wang, C. H.; Tai, N. H.; Lin, I. N.; Lee, C. Y.; Chiu, H. T. Ti5Si3 Nanowire and Its Field Emission Property. Chem. Mater. 2008, 20, 2429−2431. (14) Schmitt, A. L.; Higgins, J. M.; Jin, S. Chemical Synthesis and Magnetotransport of Magnetic Semiconducting Fe1−xCoxSi Alloy Nanowires. Nano Lett. 2008, 8, 810−815. (15) Hung, M. H.; Wang, C. Y.; Tang, J.; Lin, C. C.; Hou, T. C.; Jiang, X.; Wang, K. L.; Chen, L. J. Free-Standing and Single-Crystalline Fe1−xMnxSi Nanowires with Room-Temperature Ferromagnetism and Excellent Magnetic Response. ACS Nano 2012, 6, 4884−4891. (16) Song, Y.; Schmitt, A. L.; Jin, S. Ultralong Single-Crystal Metallic Ni2Si Nanowires with Low Resistivity. Nano Lett. 2007, 7, 965−969. (17) Szczech, J. R.; Schmitt, A. L.; Bierman, M. J.; Jin, S. SingleCrystal Semiconducting Chromium Disilicide Nanowires Synthesized via Chemical Vapor Transport. Chem. Mater. 2007, 19, 3238−3243. (18) Liang, Y. H.; Yu, S. Y.; Hsin, C. L.; Huang, C. W.; Wu, W. W. Growth of Single-Crystalline Cobalt Silicide Nanowires with Excellent Physical Properties. J. Appl. Phys. 2011, 110, 074302. (19) Lee, C. Y.; Lu, M. P.; Liao, K. F.; Lee, W. F.; Huang, C. T.; Chen, S. Y.; Chen, L. J. Free-Standing Single-Crystal NiSi2 Nanowires with Excellent Electrical Transport and Field Emission Properties. J. Phys. Chem. C 2009, 113, 2286−2289. (20) Chiu, W. L.; Chiu, C. H.; Chen, J. Y.; Huang, C. W.; Huang, Y. T.; Lu, K. C.; Hsin, C. L.; Yeh, P. H.; Wu, W. W. Single-crystalline δNi(2)Si Nanowires with Excellent Physical Properties. Nanoscale Res. Lett. 2013, 8, 290−290. (21) Wu, W. W.; Lu, K. C.; Chen, K. N.; Yeh, P. H.; Wang, C. W.; Lin, Y. C.; Huang, Y. Controlled Large Strain of Ni Silicide/Si/Ni Silicide Nanowire Heterostructures and Their Electron Transport Properties. Appl. Phys. Lett. 2010, 97, 203110. (22) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Single-crystal Metallic Nanowires and Metal/Semiconductor Nanowire Heterostructures. Nature 2004, 430, 61−65.
accept as many excited carriers as possible. In contrast, the ternary NW heterostructure detectors do not need any phase interface since the defects are sufficient in the silicide matrix; in other words, phase interfaces are ubiquitous in the whole NW. With above advantages, ternary NW heterostructure detectors may be the most efficient and smallest photon detectors in the future. Conclusions. In this study, we have successfully fabricated controllable Pt2Si/Si/Ni2Si, Pt2Si/Ni2Si, and Pt, Ni, and Si ternary NW heterostructures at 650 °C through solid state reactions. We found that Ni2Si and Pt2Si have [100]//[101] and (001)//(101)̅ epitaxial relationships with large strain. At the Pt, Ni, and Si ternary region, we confirmed this area is composed of PtxNi2‑xSi and NixPt2‑xSi, which formed by substitution mechanism. Additionally, interstitial mechanism occurred in ternary region as well and caused lattice spacing expansion. The in situ observations indicate that Ni2Si and Pt2Si are very different in terms of growth kinetics and formation mechanism. Furthermore, we also conducted electrical and photon sensing properties measurements for all the NW heterostructures. Ni2Si/Pt2Si NW heterostructures have the best resistivity of approximately 595.3 μΩ·cm. Although Pt,Ni,Si ternary NW heterostructures have higher resistivity owing to the defects in ternary region, these defects contribute to the excellent infrared light sensing property, which is absent in Ni2Si/Pt2Si NW heterostructures, bulk Ni2Si or Pt2Si.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04309. Fabrication procedures of the Si NW solid state reaction devices, EDS mapping and line scan for Pt2Si/Si/Ni2Si and Pt,Ni,Si NW heterostructures through solid state reactions, the Ni2Si crystal structure, I−V measurements for NW heterostructures, and full captions for the movies (PDF) Dynamic growth process of a Pt 2 Si/Ni 2 Si NW heterostructure (AVI) Dynamic growth process of Ni2Si (AVI) Dynamic growth process of Pt2Si (AVI)
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AUTHOR INFORMATION
Corresponding Author
*(W.-W.W.) E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support by Ministry of Science and Technology through Grants 103-2221-E-009-222-MY3, 104-2221-E-009-050-MY4, and 102-2221-E-006-077-MY3.
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