Silver catalyzed ultrathin silicon nanowires grown by low-temperature ...

JOURNAL OF APPLIED PHYSICS 107, 096105 共2010兲

Silver catalyzed ultrathin silicon nanowires grown by low-temperature chemical-vapor-deposition Joerg V. Wittemann,a兲 Wolfram Münchgesang, Stephan Senz, and Volker Schmidt Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany

共Received 26 February 2010; accepted 19 March 2010; published online 7 May 2010兲 In this work we demonstrate the synthesis of monocrystalline silicon nanowire using silver particles as catalysts at temperatures of less than 500 ° C by means of ultrahigh vacuum chemical vapor deposition. The nanowires were grown epitaxially on silicon substrates and had diameters of about 10 nm. We furthermore show that the silver remnants can be wet chemically removed with potassium ferricyanide and sodium thiosulfate. © 2010 American Institute of Physics. 关doi:10.1063/1.3393601兴

II. EXPERIMENTAL PROCEDURES

Low p-doped silicon substrates were cleaned by standard two-step Radio Corporation of America 共RCA兲 cleaning.18 Subsequently the wafers were dipped in diluted hydrofluoric acid to obtain silicon oxide free and hydrogen terminated silicon surfaces. The substrates were then immediately transferred to the noncommercial ultrahigh vacuum 共UHV兲 system having a base pressure better than 5 ⫻ 10−10 mbar. Less than a monolayer silver was evaporated 共Alfa-Aesar; purity: 99.9%兲 onto the heated silicon substrate 共300 ° C兲. Elevating the substrate temperature increases the surface mobility of silver atoms, so that instead of a silver film, silver nanopar-

A 0.54 eV D 0.26 eV

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: ⫹49345-5582 760.

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Ag

1000

400 a兲

CB

A 0.29 eV

α

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D 0.29 eV

L+α 0.93%

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The standard technique for silicon nanowire 共SiNW兲 growth is chemical vapor deposition 共CVD兲 of silicon precursors using gold as catalyst via the vapor-liquid-solid 共VLS兲 mechanism.1–6 Liquid metal-semiconductor alloy droplets form at the surface above the eutectic temperature. Silicon precursor molecules, e.g., monosilane 共SiH4兲, predominantly crack at the droplet surface and silicon gets incorporated into the droplets. At the liquid-solid interface between the eventually supersaturated droplet and the substrate a wire forms due to silicon precipitation. The nanowire synthesis using gold is very robust; gold, however, exhibits unfavorable properties to electronic industry. Therefore, a lot of effort was put into identifying substitute catalysts.3,7–9 Surprisingly little attention was paid to silver as alternative for SiNW synthesis although silver exhibits—compared to gold—distinct advantages regarding chemical treatment and electronic properties. Surfacial silver can be removed effectively 共shown below兲, which is particularly important as metal contaminations are problematic for semiconductor processing. Gold creates two impurity energy levels within the silicon band gap—one acceptor level 0.54 eV below the conduction band 共CB兲; one donor level 0.29 eV above the valence band 共VB兲,10 schematically depicted in Fig. 1共a兲. Deep level energy traps in silicon strongly influence the minority carrier lifetime. In a simplified model, energy levels become more detrimental the closer they are to the band gap middle.10 Therefore, the two recombination centers created by silver—a donor level 0.26 eV above the VB and an acceptor level 0.29 eV below the CB are much less effective.11 Naturally the carrier lifetime is inversely proportional to the trap density, i.e., the Ag concentration, in silicon. In order to minimize silver contamination one should decrease the growth temperature as both the diffusivity and the equilibrium solubility of silver in silicon increase with temperature.12 In Fig. 1共b兲 the binary phase diagram of silver and silicon is depicted.13 In accordance with this, growth of silicon whiskers with large diameters above the eutectic temperature of 836 ° C via the VLS mechanism was shown.1,2,4,14 Also synthesis of thick amorphous silicon whis-

kers well below the eutectic temperature was demonstrated by Tatsumi et al.15 In this respect, the Ag–Si phase diagram 关see Fig. 1共b兲兴 is similar to the binary phase diagrams of aluminum-silicon or gold-germanium;16 and for aluminum catalyzed SiNWs8 as well as for gold catalyzed germanium nanowires17 growth below the eutectic temperature via a supersaturated solid-solution was shown. In the following we present the fabrication of ultrathin SiNWs grown below 500 ° C with silver particles. First, the preferred growth direction of silver catalyzed wires is analyzed. Afterwards, the wet chemical removal of the surfacial catalyst is examined via x-ray photoelectron spectroscopy 共XPS兲 and high resolution transmission electron microscopy 共HRTEM兲. In addition the crystallinity of the SiNWs and their epitaxial alignment with the substrate is studied.

Temperature [oC]

I. INTRODUCTION

VB

L 836 oC

α + Si

0

Atomic Percent Silicon

10

FIG. 1. 共a兲 Deep levels energies of Ag and Au in Si band gap. 共b兲 Schematic of Ag rich side of the Ag–Si binary phase diagram.

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© 2010 American Institute of Physics

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< 211 11>

0.2

(a) (b)

(c) 0.1

100 nm 5

10

15

0

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FIG. 2. 共Color online兲共a兲 Top view SEM of Ag catalyzed SiNW on 共100兲 substrate 共grids indicate growth directions兲. 共b兲 Histogram of preferred growth directions vs diameter. 共c兲 Ag catalyzed SiNWs on 共110兲 substrate.

ticles readily form. By this approach, an additional high temperature annealing step to break up the metal film can be avoided. The wafers with the silver particles were transferred into the cold-wall UHV-CVD process chamber, wherein nanowire synthesis was carried out by a conventional thermal CVD process. The silicon wafer temperature was checked with a pyrometer 共LumaSense Technologies; spectral response: 900 nm兲. The error of the given substrate temperature is—conservatively estimated—less than ⫾ 30 K. High resolution XPS scans of the Ag 3d and Si 2p lines were performed after silver particle deposition, after SiNW growth, and after silver removal 共non–monochromatic Al source; step size: 0.1 eV; exposure time: 5 s per measurement point; pass energy: 50 eV兲. To eliminate surface charging effects, spectra were calibrated to the Si 2p line 共99.0 eV for p-type Si19兲. Position and intensity of XPS peaks were determined by fitting a Voigt function cum Shirley background.20,21 For silver removal, first the wafer with the silver catalyzed SiNWs was dipped in diluted hydrofluoric acid 共1%兲 for one minute, in order to get rid of potential silica layers covering the catalyst surface. Afterwards, the wafer was placed for 5 min in a mixture of 2.1 g potassium ferricyanide and 4.91 g sodium thiosulfate dissolved in 200 ml purified water. Subsequently the sample was rinsed for 1 min in purified water and transferred into the UHV system for XPS analysis. III. RESULTS AND DISCUSSION

Figure 2共a兲 shows a top view scanning electron microscopy 共SEM兲 micrograph of SiNWs grown on a silicon 共100兲 wafer at 490 ° C for 10 min at a monosilane partial pressure of 0.2 Torr. SEM micrographs were taken with a JEOL JSM 6701F microscope. The mean diameter of the SiNW in this experiment was 10.2 nm with a standard deviation of 2 nm. The growth directions of the nanowires can be deduced by relating the observed growth directions in the top view electron micrograph 关Fig. 2共a兲兴 to the stereographic projections of a 共100兲 surface. Transparent grids and colored arrows sketch the projections of the associated directions in Fig. 2共a兲. As can be seen therein, one third of the possible 具211典 orientations coincide with the 具111典 directions. These contributions are disentangled by assuming that all 具211典 directions are equally probable, the 具211典 and 具111典 counts are

corrected accordingly. The directions of roughly 300 wires were evaluated. For about 15% of SiNWs distinct growth directions could not be assigned due to their undefined geometry, which is indicative for a high defect density among these wires. Figure 2共b兲 shows a number density histogram of the different nanowire directions as a function of their diameter. The histogram indicates that the 具110典 directions are slightly favored at diameters of about 10 nm. A preferential 具110典 growth direction is often observed considering SiNWs in this diameter range.6,22 In order to attain a higher yield of perpendicular SiNWs, growth was carried out on a Si 共110兲 substrate 关see Fig. 2共c兲兴 as described before, using 0.1 Torr of silane at 500 ° C for 20 min. The 45° tilted view SEM micrograph in Fig. 2共c兲 suggests synthesis of a fair percentage of perpendicular NWs. The applied temperatures never exceeded 500 ° C, which is more than 300 K below the eutectic temperature. The binary phase diagram for bulk materials does not apply for nanosized objects; the calculated lowering of the silversilicon eutectic temperature was 45 K for particles with a diameter of 10 nm.23 This indicates that the catalyst particle was solid throughout the process and the growth scheme should therefore be vapor-solid-solid instead of VLS. In this regime the silicon equilibrium concentration is less than 1% as depicted in Fig. 1共b兲. This is about one order of magnitude smaller than the concentration in Au–Si catalyst droplets. A low silicon concentration is essential for growth of axial heterostructure nanowires with abrupt interfaces—particularly for tunneling field effect transistors—since the transition between two sections is sharper when the solubility in the catalyst is lower.24 As mentioned above, a major obstacle for implementing bottom up SiNWs in electronic devices is the ubiquitous metal contamination of the catalyst. In particular, the complete removal of gold in a one step wet etching process25 has turned out to be challenging. Yet grown SiNWs need to be free of catalyst remnants26,27 in order to use them as building blocks in devices. In contrast to gold, residual silver can be removed from the surface and the SiNW more easily due to a lower chemical stability of silver. One well-known cleaning recipe is based on a solution containing potassium ferricyanide and sodium thiosulfate. In order to verify the effectivity of the removal procedure, we performed UHV XPS measurements. XPS spectra were taken 共a兲 after depositing silver nanoparticles onto a 共100兲 p-type silicon wafer, 共b兲 after SiNW growth, and 共c兲 after wet chemical Ag removal. The high resolution XPS spectra corresponding to the three different processing steps are shown in the lower half of Figs. 3共a兲–3共c兲 along with respective SEM micrographs above. The peaks for silicon 共Si 2p兲 and for silver 共Ag 3d5/2 at 368.3 eV and Ag 3d3/2 at 374 eV兲 are investigated in the respective binding energy intervals.19 The XPS spectra of the Ag nanoparticles on the Si wafer 关see Fig. 3共a兲兴 as well as of the grown Si nanowires 关see Fig. 3共b兲兴 clearly indicate the presence of both Si and Ag. The ratio between the XPS intensities of Si 2p and Ag 3d5/2 increases from 1.2 共after silver deposition兲 to 2.0 共after the CVD process兲, see Fig. 3共b兲. This rise in intensity reflects the relative increase in silicon surface area due to the growth of the SiNWs. The


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50 nm Arbitrary Units

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IV. CONCLUSION

Ag 3d

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FIG. 3. 共Color online兲共a兲 Top: top view SEM micrographs after 共a兲 Ag deposition, 共b兲 SiNW growth. 共c兲 Ag removal. Bottom: corresponding high resolution XPS spectra of silver 共3d5/2, 3d3/2兲 and silicon 共2p兲.

SEM micrograph in Fig. 3共b兲 furthermore depicts the presence of silver catalyst particles on the SiNWs tips. For the wet-chemical removal of silver 共for details see experimental section兲, the sample was taken out of the UHV environment. The final XPS scan in Fig. 3共c兲 proofs the successful silver removal. In addition to the crystalline silicon 2p peak at 99 eV, also a peak at 102.2 eV—indicative for fractional oxidation of silicon—can be seen in Fig. 3共c兲. This oxidation originates from the silver removal in aqueous solution. In addition to XPS measurements, also HRTEM cross sectional micrographs were taken with a JEOL JEM 4010 microscope to prove the efficiency of the silver removal pro¯ 1兴 oriented SiNW 共zone axis is cess. Figure 4共a兲 shows a 关21 关011兴兲 homoepitaxially grown at 480 ° C for 5 min at 0.1 Torr monosilane partial pressure on a 共100兲 silicon substrate. After growth the silver catalyst was removed as conducted above. In contrast to gold removal studies,25 no residual silver could be observed in any TEM micrograph. The HRTEM micrograph in Fig. 4共b兲 displays a silver catalyzed 关110兴 oriented SiNW grown at 490 ° C on a 共110兲 Si surface for 30 min. Note that in this case disilane with a partial pressure of 3 ⫻ 10−3 Torr was used as precursor. One can also see in Fig. 4共b兲 that the Ag catalyst detached from the nanowire tip, presumably during TEM sample preparation. The fast Fourier transform 共FFT兲 of the wire and parts of the substrate 关see Fig. 4共c兲兴 suggests that the wire grew epitaxially and as a single crystal, though, about half of the nanowires from the (220) Ag cattal alys ystt ys

(113)

[211]

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(b)

(111) (002)

(c) (d)

[110] (111)

[110]

[112]

[110] [100] [011]

[110] [011]

10 nm

same process show defects, such as stacking faults and/or ¯ 10兴 zone axis; twins. In Fig. 4共d兲 a HRTEM micrograph 共关1 共110兲 Si substrate兲 shows a straight SiNW with 关112兴 growth ¯ 兲 twin boundary direction exhibiting as the only defect a 共111 extending along the whole growth axis. This a well known defect phenomenon28 for 具211典 SiNWs.

[001]

10 nm

FIG. 4. 共Color online兲共a兲 Cross-sectional HRTEM of SiNW after silver removal. 共b兲 HRTEM from 关110兴 oriented SiNW on 共110兲 Si substrate; Ag catalyst detached from tip. 共c兲 FFT of entire SiNW and substrate from 共b兲. ¯ 兲 twin boundary 共d兲 HRTEM micrograph of 关112兴 oriented SiNW with 共111 plane. Same scale bar in 共b兲 and 共d兲.

In summary we have introduced ultrathin silver catalyzed SiNWs as promising candidate for application in electronic industry. The preferred growth orientations of the SiNWs were identified as 具110典. Wires were found to be epitaxial though some exhibited planar defects. Successful post-growth Ag catalyst removal was verified using XPS. ACKNOWLEDGMENTS

This work was funded by the joint MPG and Fraunhofer–Gesellschaft nanoSTRESS project. The authors thank S. Hopfe for TEM sample preparation. Above all we would like to express our deep gratitude to Professor U. Gösele 共deceased 8 Nov. 2009兲 for his abundant support and encouragement. R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 共1964兲. R. S. Wagner, W. C. Ellis, S. M. Arnold, and K. A. Jackson, J. Appl. Phys. 35, 2993 共1964兲. 3 V. Schmidt, S. Senz, and U. Gösele, Z. Metallkd. 96, 427 共2005兲. 4 R. S. Wagner and W. C. Ellis, Trans. Metall. Soc. AIME 233, 1053 共1965兲. 5 E. I. Givargizov, J. Cryst. Growth 31, 20 共1975兲. 6 V. Schmidt, S. Senz, and U. Gösele, Nano Lett. 5, 931 共2005兲. 7 V. A. Nebol’sin, A. A. Shchetinin, A. A. Dolgachev, and V. V. Korneeva, Inorg. Mater. 41, 1256 共2005兲. 8 Y. W. Wang, V. Schmidt, S. Senz, and U. Gösele, Nat. Nanotechnol. 1, 186 共2006兲. 9 J. Arbiol, B. Kalache, P. Recca i Cabarrocas, J. R. Morante, and A. Fontcuberta i Morral, Nanotechnology 18, 305606 共2007兲. 10 S. M. Sze, Physics of Semiconductor Devices, 2nd ed. 共Wiley, New York, 1981兲. 11 J. M. Fairfield and B. V. Gokhale, Solid-State Electron. 8, 685 共1965兲. 12 K. Graff, Metal Impurities in Silicon-Device Fabrication, 2nd ed. 共Springer, Berlin, 2000兲. 13 L. Weber, Metall. Mater. Trans. A 33, 1145 共2002兲. 14 G. A. Bootsma and H. J. Gassen, J. Cryst. Growth 10, 223 共1971兲. 15 Y. Tatsumi, M. Shigi, and M. Hirata, J. Phys. Soc. Jpn. 45, 703 共1978兲. 16 T. B. Massalski, Binary Alloy Phase Diagrams, 2nd ed. 共ASM International, Materials Park, OH, 1990兲, Vol. 1. 17 S. Kodambaka, J. Tersoff, M. C. Reuter, and F. M. Ross, Science 316, 729 共2007兲. 18 W. Kern, J. Electrochem. Soc. 137, 1887 共1990兲. 19 J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook Of X-Ray Photoelectron Spectroscopy 共Physical Electronics, Eden Prairie, Minn., 1995兲. 20 R. Hesse, P. Streubel, and R. Szargan, Surf. Interface Anal. 39, 381 共2007兲. 21 D. Briggs and M. P. Seah, Practical Surface Analysis, 2nd ed. 共Wiley, Chichester, 1990兲. 22 Y. Wu, Y. Cui, L. Huynh, C. J. Barrelet, D. C. Bell, and C. M. Lieber, Nano Lett. 4, 433 共2004兲. 23 V. Schmidt, J. V. Wittemann, and U. Gösele, Chem. Rev. 110, 361 共2010兲. 24 M. T. Björk, J. Knoch, H. Schmid, H. Riel, and W. Riess, Appl. Phys. Lett. 92, 193504 共2008兲. 25 S. H. Christiansen, J. W. Chou, M. Becker, V. Sivakov, K. Ehrhold, A. Berger, W. C. Chou, D. S. Chuu, and U. Gösele, Nanotechnology 20, 165301 共2009兲. 26 J. Bauer, F. Fleischer, O. Breitenstein, L. Schubert, P. Werner, U. Gösele, and M. Zacharias, Appl. Phys. Lett. 90, 012105 共2007兲. 27 S. Hoffmann, J. Bauer, C. Ronning, T. Stelzner, J. Michler, C. Ballif, V. Sivakov, and S. H. Christiansen, Nano Lett. 9, 1341 共2009兲. 28 A. H. Carim, K. K. Lew, and J. M. Redwing, Adv. Mater. 13, 1489 共2001兲. 1 2
