pubs.acs.org/Langmuir © 2009 American Chemical Society
Gold Removal from Germanium Nanowires Joshua B. Ratchford,† Irene A. Goldthorpe,‡ Yun Sun,§ Paul C. McIntyre,‡ Piero A. Pianetta,§ and Christopher E. D. Chidsey*,† † ‡
Department of Chemistry, Stanford University, Stanford, California 94305-5080, Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-220, and §Stanford Synchrotron Radiation Lightsource, Stanford University, Stanford, California 94305-0210 Received March 1, 2009. Revised Manuscript Received April 19, 2009
We report the selective removal of gold from the tips of germanium nanowires (GeNWs) grown by chemical vapor deposition on gold nanoparticles (AuNPs). Selective removal was accomplished by aqueous hydrochloric acid solutions containing either potassium triiodide or iodine. Measurement of the residual number of gold atoms on the GeNW samples using inductively coupled plasma-mass spectrometry shows that 99% of the gold was removed. Photoemission spectroscopy shows that the germanium surfaces of these samples were not further oxidized after treatment with these liquid etchants. Auger electron spectroscopy shows that AuNPs that did not yield GeNWs contain germanium and also that the addition of gaseous HCl to GeH4 during GeNW growth increased the selectivity of germanium deposition to the AuNPs.
Introduction The growth of germanium nanowires (GeNWs) from gold nanoparticles (AuNPs) may permit the synthesis of germanium electronic elements in nanoelectronic devices. However, gold has electron energy levels in the band gap of germanium that enhance the equilibration of charge carriers across the band gap, potentially leading to charge leakage through transistors made from this material.1 Unless gold is effectively removed from GeNWs, the use of AuNPs for the synthesis of germanium electronic elements in nanoelectronic devices may not be adopted. However, it is challenging to remove gold, a noble metal, without damaging germanium, which is generally found to be quite reactive. To address this problem, we have investigated methods to selectively remove gold from the tips of GeNWs with liquid etchants. We initially tried to remove the gold using an aqueous solution containing H2O2 and HCl, a mixture that is used to remove metals from the surface of silicon wafers,2,3 or an aqueous solution containing HNO3 and HCl, called “HNO3-HCl” below, an ancient method used to dissolve gold.4 However, these conventional liquid etchants readily dissolved the GeNWs, presumably because of the formation of water-soluble oxidized germanium species. Previously, we reported our discovery5 that an aqueous solution containing I2, KI, and HCl, which is known to contain the I3- anion,6,7 removed all gold visible by transmission electron microscopy and scanning electron microscopy with minimal etching of the GeNWs. We called this solution “triiodide-HCl”.5 Inductively coupled plasma-optical emission *To whom correspondence should be addressed. E-mail:
[email protected]. (1) Bracht, H.; Stolwijk, N. A.; Mehrer, H. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 14465–14477. (2) Kern, W.; Puotinen, D. A. RCA Rev. 1970, 31, 187–206. (3) Kern, W. J. Electrochem. Soc. 1990, 137(6), 1887–1892. (4) CRC Handbook of Chemistry and Physics, 58th ed.; CRC Press: Boca Raton, FL, 1978. (5) Woodruff, J. H.; Ratchford, J. B.; Goldthorpe, I. A.; McIntyre, P. C.; Chidsey, C. E. D. Nano Lett. 2007, 7, 1637–1642. (6) Williams, K. R.; Gupta, K.; Wasilik, M. J. Microelectromech. Syst. 2003, 12, 761–778. (7) Davis, A.; Tran, T. Hydrometallurgy 1991, 26(2), 163–177.
Langmuir 2009, 25(16), 9473–9479
spectroscopy (ICP-OES) measurements of gold in triiodideHCl used to treat a GeNW sample showed that more than 75% of the gold was removed from the sample.5 An aqueous solution containing I2 and KI but without the added HCl removed gold from the GeNW tips but substantially etched the GeNWs.5 In this paper, we report a more accurate quantification of the amount of gold removed from GeNW and related control samples after treatment with liquid etchants using a more sensitive method, inductively coupled plasma-mass spectrometry (ICP-MS). We also report the effects that these liquid etchants have on the surfaces of GeNW samples. We also report the discovery of another liquid etchant containing I2 and HCl, without the added KI, that removes gold from GeNW tips selectively and with minimal etching of GeNWs. We call this solution iodine-HCl. Treatment of GeNW samples with triiodide-HCl or iodine-HCl removed 99% of the gold from GeNW samples, with minimal etching of the GeNWs. The ICP-MS measurements show that treatment of GeNW samples with HNO3-HCl also removed more than 99% of the gold from the samples, but this treatment dissolved the GeNWs. The chemical composition of the sample surfaces was determined using photoemission spectroscopy after treatment with triiodide-HCl or iodine-HCl. We found that HCl suppressed the oxidation of germanium during treatments with triiodide-HCl or iodine-HCl, consistent with the known HCl passivation of germanium.8-10 The elemental composition of selected regions on GeNW samples was determined using Auger electron spectroscopy. We found that those AuNPs that did not yield GeNWs contained germanium. We also confirmed, as others have reported,11 that the addition of HCl to the gas mixture during GeNW growth suppressed the deposition of germanium onto the surface of the silicon substrate. (8) Adhikari, H.; McIntyre, P. C.; Sun, S. Y.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 2005, 87(26), 263109. (9) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 15466–15472. (10) Lu, Z. H. Appl. Phys. Lett. 1996, 68, 520–522. (11) Kamins, T. I.; Briggs, G. A. D.; Williams, R. S. Appl. Phys. Lett. 1998, 73, 1862–1864.
Published on Web 05/06/2009
DOI: 10.1021/la900725b
9473
Article
Ratchford et al.
Experimental Section Water was purified with a Millipore four-bowl purification system. The HNO3-HCl mixture was made by adding 1 volume of 37% by mass aqueous HCl (Fisher, Trace metal grade) to 1 volume of 70% by mass aqueous HNO3 (Fisher, Trace metal grade). The triiodide-HCl solution was made by adding 9 volumes of an aqueous solution that contained 1% by mass of I2 (Merck, Supra pure) and 39% by mass KI (EMD AnalaR) to 1 volume of 37% HCl. No solids were visible in the triiodide-HCl. The iodine-HCl solution was made by adding 115 mg of I2 to 5.00 mL of 37% HCl. Because I2 is not very soluble in aqueous solutions without an added iodide salt, the I2 and HCl mixture was ultrasonicated for 30 min before use. The final solution was light brown and produced a purple vapor, presumably gaseous iodine, from which we infer that the HCl was saturated with I2. To determine the amount of iodine in the iodine-HCl solution, the iodine that did not dissolve was vacuum-filtered from the HCl solution until dry. The mass of the remaining solid I2 was 91 mg, from which we infer that the solution was 0.5% by mass iodine. If a significant amount of the residual I2 sublimed during vacuum filtration and drying, then this value is an upper estimate of the mass fraction of iodine in the solution. An aqueous 1 ppb iridium internal standard was made by diluting a 1000 μg/mL standard solution of iridium in 20% by mass aqueous HCl (Alfa Aesar, Iridium, AAS standard solution, specpure) with 1% by mass aqueous HCl. An aqueous 1 ppb gold standard was made by diluting a 1000 μg/mL standard solution of gold in 20% HCl (Alfa Aesar, Gold, AAS standard solution, specpure) with 1% HCl. The polished surface of a silicon (100) wafer (10-20 Ω cm, p type, boron doped) was coated with a photoresist then diced into 5 mm 5 mm chips. The photoresist was removed from the chips with an acetone rinse. Then, the chips were added to a mixture composed of 1 volume neat H2SO4 (Fisher) to 1 volume 31% by mass aqueous H2O2 (Kanto), which was heated to 90 C for 10 min. The Si chips were rinsed with 1 L of water before they were added to a mixture composed of 1 volume 37% HCl to 1 volume 31% H2O2 to 4 volumes water, which was heated to 90 C for 10 min. The mixture was then decanted from the silicon chips, which were then rinsed with another 1 L of water. Some of these chips, called “Si samples” below, were stored in 7 mL polyethylene vials for subsequent ICP-MS measurements without ever intentionally exposing them to gold. These were used to determine the gold detection limit of our measurement method. The remaining Si chips were immersed into 1% by mass aqueous HF (Kanto) for 5 min to remove silicon oxide formed during the cleaning. AuNPs were deposited by placing a 50 μL drop of a mixture composed of 10 volumes of a commercial citrate-stabilized 40 nm gold colloid solution (BBC International, 9.0 1010 AuNPs/mL) to 1 volume 1% HF on top of the polished surface of a chip.5 The AuNP deposition solution was drained from the surface after 5 min. Some of these chips with AuNPs, called “AuNP samples” below, were stored in 7 mL polyethylene vials for later ICP-MS measurement to determine how well the liquid etchants could remove AuNPs deposited on the surface of silicon with no subsequent GeNW growth. To grow GeNWs, the remaining AuNP samples were heated in a gaseous mixture consisting of 0.6 Torr GeH4, 0.6 Torr HCl, and 28.8 Torr H2, flowing through a lamp-heated, cold-wall CVD chamber at 510 sccm. The temperature was raised to 375 C for 2 min and then held at 300 C for 10 min to minimize germanium deposition on the sidewalls of the GeNWs.12 These chips, called “GeNW samples” below, were stored in 7 mL polyethylene vials. Either a 200 μL volume of the HNO3-HCl liquid etchant13 or a 500 μL volume of the triiodide-HCl or iodine-HCl liquid etchants was added to the samples in the vials. After either 20 s or 10 min, the (12) Jagannathan, H.; Deal, M.; Nishi, Y.; Woodruff, J.; Chidsey, C.; McIntyre, P. C. J. Appl. Phys. 2006, 100, 024318. (13) The HNO3-HCl liquid etchant was added to the vial by adding 100 μL of HNO3 first and then 100 μL HCl.
9474 DOI: 10.1021/la900725b
liquid etchant was removed from the vial. The sample was rinsed with approximately 10 mL of aqueous HCl and immediately dried in isopropanol vapor for 10 s.14 Drying GeNW samples grown on Si(111) chips with isopropanol vapor had previously been found to prevent the collapse of GeNWs and adhesion to the silicon surface that had been observed when rinsed chips were dried in air.5,15 Aqueous solutions for ICP-MS measurement were made from samples by adding 200 μL of HNO3-HCl to the vial. We observed that the mass of a GeNW sample did not measurably change after treatment with HNO3-HCl, but a SEM image of this sample showed that no GeNWs remained. Thus, we assumed that the HNO3-HCl treatment only dissolved the gold and germanium but no significant amount of silicon from the surfaces of the samples. After 10 min, the HNO3-HCl mixture was diluted with 100 μL of water and 700 μL of the iridium internal standard and mixed. The chip was never removed from the vial. Some of the solution was aspirated into the Teflon nebulizer (PFA-μ-Flow 100-4093) of a Finnigan Element ICP-MS for 30 s before measurement of 197Au+ and 193Ir+ mass chromatograms for the next 30 s, consuming in total 0.225 mL of the solution. After each measurement, the nebulizer was rinsed with 1% HNO3 for 30 s to eliminate carry over of gold and iridium to the other measurements. The ICP-MS analysis of the same solution was then repeated, consuming another 0.225 mL of the solution. Because mixtures of HF and HNO3 dissolve silicon, 150 μL of 25% HF was then added to the remaining volume of the solution to dissolve some of the chip to measure any residual gold on the chip.4 After 10 min, this solution was aspirated into the nebulizer as described above and another measurement was made. SEM images of a GeNW sample after the HF addition show that the silicon surface was roughened and pitted. We found that approximately one-third of the mass of the chip was consumed. The instrument’s software reported the average signals for 197Au+ and 193Ir+. The standard deviation of the average signals for 197Au+ and 193Ir+ using four sequential measurements of the 1 ppb gold standard or the 1 ppb iridium internal standard was 3% or less. The number of gold atoms on an AuNP sample was determined by counting the AuNP coverage in SEM images of the sample and multiplying by the number of gold atoms expected in a 40 nm particle and the surface area of the sample. This estimated number, NAu,0, was 3.5 1014 gold atoms. The ratio of the average signal for 197Au+ to the average signal for 193Ir+ in the solution prepared from this reference sample, IAu,0/IIr,0, was 41. This value was used to determine the number of gold atoms, NAu,1, in the other solutions using eq 116 NAu, 1 ¼ NAu, 0
IAu, 1 =IIr, 1 IAu, 0 =IIr, 0
ð1Þ
where IAu,1/IIr,1 is the ratio of the average signal for 197Au+ to the average signal for 193Ir+ in the other solutions. The measurement of IAu,1/IIr,1 was repeated to confirm the reproducibility. Using this measurement method, the detection limit was found to be 7 1010 gold atoms on a 5 mm 5 mm chip, as measured from the control Si samples. The number of additional gold atoms removed by the HF addition, 4NAu,2, was calculated using eq 217 ΔNAu, 2 ¼ NAu, 0
! IAu, 2 =IIr, 2 -IAu, 1 =IIr, 1 ðV1 -Vs Þ IAu, 0 =IIr, 0 V1
ð2Þ
where V1 is the initial volume of the solution, 1.000 mL. Vs is the volume of the solution used in the previous two ICP-MS (14) Samples treated with triiodide-HCl were rinsed with 1 M HCl, while samples treated with iodine-HCl were rinsed with 12 M HCl. (15) GeNW samples used for ICP-MS measurement were not dried in isopropanol vapor. (16) See the Supporting Information for the derivation of eq 1. (17) See the Supporting Information for the derivation of eq 2.
Langmuir 2009, 25(16), 9473–9479
Ratchford et al.
Article
measurements, 2 0.225 mL = 0.450 mL. IAu,2/IIr,2 is the ratio of the average signal for 197Au+ to the average signal for 193Ir+ after HF addition. Photoemission measurements were made at the Stanford Synchrotron Radiation Lightsource (SSRL) to determine the chemical composition of GeNW surfaces after treatment with triiodide-HCl and iodine-HCl. Immediately after growth, the GeNW samples were passed through laboratory air into a desiccator filled with argon gas at ambient pressure. The desiccator was then evacuated with house vacuum for 20 s. The samples were stored in a chamber filled with argon gas at atmospheric pressure after transportation to SSRL. High-resolution Ge 3d spectra were obtained with a photon energy of 80 eV on beamline 8-1 or 200 eV on beamline 10-1. The elemental composition of selected areas on GeNW samples was obtained using a PHI-700 scanning Auger nanoprobe with a 20 kV electron acceleration voltage and a 10 nA electron gun current. Au MNN, Ge LMM, or Si KLL Auger electron spectra were each collected by co-adding 30 1 eV/s scans within their characteristic energy ranges.
Results and Discussion Figure 1 shows plan view SEM images of GeNW samples after growth and after treatment with triiodide-HCl. As others have reported, the GeNWs are directed along the Æ111æ crystal directions of the silicon (100) surface.12 Many AuNPs did not yield GeNWs. Figure 1a shows that the AuNPs that did not yield GeNWs appeared to have moved on the silicon surface, leaving tracks with bright spots at one end. Figure 1b shows that the bright spots at the tips of GeNWs and at the ends of the AuNP tracks disappeared after treatment with triiodide-HCl for 20 s and rinsing with 1 M HCl. In our previous report,5 we observed that the initial 40 nm diameter of as-grown GeNWs was reduced by approximately 8 nm after treatment with triiodide-HCl for 20 s and 1 M HCl rinsing.18 A GeNW sample was treated with triiodide-HCl for 10 min to see if increasing the treatment time would remove more gold from the GeNW sample in ICP-MS measurements; however, the GeNWs dissolved. Therefore, the amount of gold on this GeNW sample was not measured. Figure 2a shows that the bright spots at the tips of the GeNWs disappeared after a GeNW sample was treated with iodine-HCl for 20 s and rinsed with 12 M HCl but that the bright spots at the ends of AuNP tracks did not. Another GeNW sample was treated with iodine-HCl for 10 min to see if the remaining bright spots at the ends of AuNP tracks could be removed without dissolving the GeNWs. Figure 2b shows that indeed increasing the treatment time to 10 min removed the bright spots from the tips of GeNWs and from the ends of the AuNP tracks without dissolving the GeNWs. From this result, we infer that iodine-HCl is less reactive toward germanium than triiodide-HCl, probably because it is less concentrated in I2 and possibly because it contains substantially more aqueous HCl. Interestingly, GeNWs partially dissolved after a GeNW sample was treated with iodine-HCl for 20 s and rinsed with 1 M HCl rather than 12 M HCl.19 We hypothesize that molecular iodine from the iodine-HCl solution adsorbs to the passivated GeNW surface. Upon rinsing, the germanium surface loses sufficient passivation, is oxidized by the adsorbed iodine, and dissolves as germanium chlorides. Figure 3a (18) Cross-section SEM images of the samples in Figures 1b and 2b reveal that the GeNWs adhered to the silicon surface even after the samples were dried in isopropanol vapor, following treatment with triiodide-HCl. Unlike the vertical GeNWs grown from a Si(111) surface in our previous report, these GeNWs start out inclined toward the surface and therefore may be more prone to adhering to the substrate, even though they were dried in isopropanol vapor. (19) See the Supporting Information for a SEM image of the sample treated with iodine-HCl for 20 s and rinsed with 1 M HCl.
Langmuir 2009, 25(16), 9473–9479
Figure 1. Plan view SEM image of GeNW samples on Si(100) substrates (a) after growth showing GeNWs and the AuNPs that did not yield GeNWs tracking on the silicon surface and (b) after 20 s treatment with triiodide-HCl and 1 M HCl rinsing.
shows the surface of a GeNW sample after treatment with HNO3-HCl for 10 min. This treatment completely dissolved the GeNWs. A shorter, 20 s treatment with HNO3-HCl dissolved the visible brown film from another GeNW sample, presumably dissolving most or all of the GeNWs.20 The bright spots in SEM images of AuNP samples disappeared after samples were treated with triiodide-HCl, iodine-HCl, or HNO3-HCl for 20 s or 10 min. Because ICP-MS has a large dynamic range21 and high sensitivity for heavy elements, we used this method to measure the number of gold atoms in aqueous solutions after treatment of the chips with liquid etchants. The third column in Table 1 lists the number of gold atoms calculated from eq 1 in solutions made from samples treated with HNO3-HCl after various prior treatments. The fourth column in Table 1 lists the corresponding (20) The brown coating on GeNW samples remained after treatment with triiodide-HCl or iodine-HCl for 20 s or with iodine-HCl for 10 min, followed in each case by aqueous HCl rinsing. (21) We measured a linear response for gold over 4 orders of magnitude of a serial dilution of 100 ppb gold standard solution.
DOI: 10.1021/la900725b
9475
Article
Ratchford et al.
Figure 3. Plan view SEM image of a GeNW sample after treatment with HNO3-HCl for 10 min.
Figure 2. Plan view SEM images of GeNW samples after (a) 20 s treatment with iodine-HCl and 12 M HCl rinsing and (b) after 10 min treatment with iodine-HCl and 12 M HCl rinse.
fraction of gold removed by these prior treatments. Because the HNO3-HCl treatment also dissolved germanium, we infer that these values are due to gold that may have been embedded underneath germanium as well as any gold remaining on the surface of the sample. Note that prior treatment reduced the number of gold atoms in the ICP-MS solutions. For instance, the ICP-MS solution made from a GeNW sample after a prior HNO3-HCl treatment for 10 min had more than 99.9% fewer atoms than the number of gold atoms in the solution made from a GeNW sample that had no prior treatment.22 Table 1 shows that the GeNW samples that were treated with triiodide-HCl for 20 s, iodine-HCl for 10 min, or HNO3-HCl for 20 s had 99% fewer gold atoms than the GeNW sample that had no prior treatment. This level of gold removal is significantly greater than what we were previously able to measure using ICP-OES.5 (22) For example, we calculated the fraction of gold atoms removed from the GeNW treated with HNO3-HCl for 10 min prior to the ICP-MS sample data in column 3 of Table 1 in the following preparationusingthe corresponding 10 expression: 41000 41000 100% - 41000 100%g99:9% gold atoms removed
9476 DOI: 10.1021/la900725b
The fifth column in Table 1 lists the increase in the number of gold atoms in the solutions, calculated from eq 2, after HF was added to dissolve about a third of the silicon chip. We observed an increase in the number of gold atoms in every solution. This did not result from unintentional gold contamination by the added HF because a control solution containing HNO3, HCl, and HF23 had fewer gold atoms than the observed increases for all solutions made from GeNW or AuNP samples. Also, this did not simply result from mass interference by dissolved silicon because the increase observed for the Si sample was less than all increases observed in solutions made from GeNW or AuNP samples. A simple explanation for the increase in the number of gold atoms after some of the silicon of GeNW samples was dissolved is that gold atoms diffused into the chip when it was heated during GeNW growth. However, the observed increases for GeNW samples are similar to those for most of the control AuNP samples, which were not heated. For example, the number of gold atoms in the solution made from the GeNW sample treated with HNO3-HCl for 10 min increased by 7 1010 gold atoms after some of the silicon was dissolved, whereas the number of gold atoms in the solution made from the AuNP sample treated with HNO3-HCl for 10 min increased by 9 1010 gold atoms after some of the silicon was dissolved. From these data, we conclude that the increases were not due to gold that had diffused into the chip but rather were caused by residual gold somehow adsorbed on the surface of the chip that was displaced into the solution after HF was added to the solution to dissolve some of the silicon. Calculation of the number of gold atoms that could diffuse into a silicon chip under the GeNW growth conditions supports our conclusion of negligible diffusion into the chip. Using the diffusion coefficient and equilibrium concentration of gold in silicon at 375 C extrapolated from literature values obtained at high temperatures, we find that only 6 substitutional gold atoms and 1300 interstitial gold atoms can diffuse into the chip.24-26 (23) A total of 100 μL of 70% HNO3, 100 μL of 37% HCl, 100 μL of 49% HF, and 700 μL of the 1 ppb Ir standard solution. (24) Bullis, W. M. Solid-State Electron. 1966, 9, 143–168. (25) Wilcox, W. R.; Lachapelle, T. J. J. Appl. Phys. 1964, 35, 240–246. (26) See the Supporting Information for these calculations.
Langmuir 2009, 25(16), 9473–9479
Ratchford et al.
Article
Table 1. Gold Atoms Removed by the 10 Min HNO3-HCl Treatment after Various Prior Treatments as Determined by ICP-MS sample GeNW
prior treatment
NAu,1 (1 1010) removed by HNO3-HCla
none HNO3-HClc HNO3-HCld triiodide-HClc iodine-HCld none HNO3-HClc HNO3-HCld triiodide-HClc iodine-HCld none
41000 350 10 540 510 35000 38 15 14 380 7
fraction of gold atoms removed (%)
4NAu,2 (1 1010) removed by adding HFb
130 12 7 27 36 AuNP 430 99.9 7 99.9 9 99.9 30 99 90 Si 2 none 5 a A total of 200 μL of HNO3-HCl solution diluted with 100 μL of water and 700 μL of the 1 ppb iridium internal standard. b Remaining solution (0.550 mL) further diluted with 100 μL of 25% HF. c A 20 s treatment. d A 10 min treatment. 99 99.9 99 99
Figure 4. High-resolution Ge 3d photoemission spectrum obtained at 80 eV photon energy from (a) a GeNW sample immediately after growth and (b) after 20 s treatment with triiodide-HCl and 1 M HCl rinsing.
These numbers are significantly less than the limit of detection for gold with ICP-MS and are consistent with our lack of evidence that gold diffused into the chip during GeNW growth. The chemical passivation of the GeNW surfaces is essential for their manipulation under ambient conditions and will be required for fabrication of high-performance electronic devices made from GeNWs.8,9 To determine how the chemical composition of GeNW surfaces was affected by treatment with triiodide-HCl or iodine-HCl, we obtained high-resolution Ge 3d photoemission spectra from GeNW samples. Figure 4a shows the Ge 3d photoemission spectrum, obtained with 80 eV Langmuir 2009, 25(16), 9473–9479
Figure 5. High-resolution Ge 3d photoemission spectrum at 80 eV photon energy from (a) a GeNW sample after growth, (b) after 20 s treatment with triiodide, without added HCl and without 1 M HCl rinsing, and (c) after immersion in 1 M HCl for 5 min. DOI: 10.1021/la900725b
9477
Article
Figure 6. High-resolution Ge 3d photoemission spectrum obtained at 200 eV photon energy from a GeNW sample (a) after growth and (b) after treatment with iodine-HCl and 12 M HCl rinsing.
photon energy, from a GeNW sample after growth.27 The spectrum shows the expected 3d doublet peak with a shoulder indicative of oxidized forms of germanium. Fitting the spectrum with known chemical shifts for various forms of oxidized germanium shows that the shoulder is composed of Ge1+ and some Ge2+.28 Figure 4b shows that these features remained after the sample was treated with triiodide-HCl and rinsed with 1 M HCl. We infer that the Ge1+ and Ge2+ features are due to the formation of germanium monochloride and germanium dichloride species.8 We wished to determine the effect of the triiodide anion, I3-, in the absence of HCl, on the chemical composition of the germanium surface. Therefore, we obtained the photoemission spectra, shown in Figure 5, from a GeNW sample treated with an aqueous solution of I2 and KI, without added HCl.29 Prior to the treatment, the sample used in this experiment showed a diffuse peak because of germanium in a higher oxidation state (labeled Ge3+ in Figure 5a), indicating more adventitious oxidation than seen in the previous sample (Figure 4a). However, the area of this peak increased dramatically and shifted to a higher binding energy after the sample was treated with the I2 and KI solution for 20 s (labeled Ge3+ and Ge4+ in Figure 5b). A similar peak was also observed from another GeNW sample treated with 10% by mass H2O2 (Kanto) for 3 s, suggesting that a germanium oxide layer may be formed on the GeNW surface in both cases.30 (27) Growth conditions were 1 Torr GeH4 and 29 Torr H2 flowing through the reactor at 300 sccm. (28) Sun, S. Y.; Sun, Y.; Liu, Z.; Lee, D. I.; Peterson, S.; Pianetta, P. Appl. Phys. Lett. 2006, 88, 021903. (29) A total of 1% by mass I2, 39% by mass KI, and 60% by mass water. (30) See the Supporting Information for the photoemission spectrum of the GeNW sample treated with 10% H2O2 for 3 s.
9478 DOI: 10.1021/la900725b
Ratchford et al.
Figure 7. Secondary electron image and corresponding Ge LMM and Au MNN Auger electron signals from nanostructures, indicated with black arrows, on a GeNW sample grown in the presence of HCl. The dashed line shows the Ge LMM Auger electron signal from the silicon surface of a GeNW sample grown in the absence of HCl.
Figure 5c shows that the area of the diffuse peak was dramatically reduced after the sample was treated with 1 M HCl for 5 min.8,9 On the basis of these and previous results,5 we conclude that HCl suppresses the formation and dissolution of oxidized germanium species during treatment with triiodide-HCl. Figure 6 shows the effect of iodine-HCl treatment. Interestingly, the area of the diffuse peak (fitted to a peak assigned to Ge3+ and one assigned to Ge4+) increased after the sample was treated with iodine-HCl and rinsed with 12 M HCl.31 We hypothesize that the high concentration of HCl in iodine-HCl suppressed the dissolution of oxidized germanium species formed by the iodine, thus increasing the area of the diffuse peak in the photoemission signal. We used Auger electron spectroscopy to determine the elemental composition of selected nanostructures on GeNW samples after growth. Arrows within the secondary electron image in Figure 7 indicate points from which Au MNN and Ge LMM Auger electron spectra were obtained.32 Gold and germanium were observed at the tip of a GeNW (arrow 1 in Figure 7) and at the bright spot at the end of an AuNP track (arrow 2 in Figure 7). (31) Because of the Stanford Synchrotron Research Lightsource user schedule, we were unable to observe GeNW samples treated with iodine-HCl on the beamline 8-1 with 80 eV photon energy but instead used beamline 10-1 with 200 eV photon energy. (32) Si KLL Auger electrons were observed from all nanostructures. We attributed this to analyzing the samples in plan view, because the Si KLL Auger electron signals were not observed from a GeNW tip in cross-section orientation.
Langmuir 2009, 25(16), 9473–9479
Ratchford et al.
Article
These results show that germanium is indeed on both AuNPs that yielded GeNWs and those that did not. Germanium was observed from the body of the GeNW and from the body of the AuNP track, but gold was not observed from these structures.33 A small amount of germanium was observed from the silicon surface of this sample (arrow 3 in Figure 7); however, more germanium was observed from the silicon surface of a GeNW sample that was grown in the absence of HCl (dashed line in Figure 7). This result shows that the addition of HCl to the gas mixture during GeNW growth increases the selectivity of germanium deposition to the gold catalysts but does not appear to inhibit the growth of GeNWs.34
Conclusion In conclusion, we have shown that gold can be selectively removed from the tips of GeNWs by treatment with triiodideHCl or iodine-HCl. ICP-MS measurements showed that our optimal treatments of GeNWs with these liquid etchants remove 99% of the gold from the sample. Photoemission spectra collected of GeNW samples after treatment showed that HCl in the liquid etchants suppress the oxidation of the GeNWs and that either triiodide-HCl or iodine-HCl may be well-suited for treating germanium surfaces prior to electronic device fabrication. Auger electron spectra collected from GeNW samples showed that germanium is on both AuNPs that yielded GeNWs and those that did not, and the addition of HCl to the gas mixture during GeNW growth increased the selectivity of germanium deposition to the AuNPs. (33) See the Supporting Information for AES spectra taken from the body of the GeNW and from the body of the AuNP track. (34) Ratchford, J. B.; Goldthorpe, I. A.; McIntyre, P. C.; Chidsey, C. E. D. Appl. Phys. Lett. 2009, 94, 044103.
Langmuir 2009, 25(16), 9473–9479
Acknowledgment. This work was supported by DARPA/ SPAWAR Grant N66001-04-1-8916 and the SRC contract number 2006-VJ-1429 Task 1429.001. The authors thank their colleagues Yoshio Nishi, H. S. Philip Wong, James McVittie, Makoto Koto, Jacob H. Woodruff, Hemant Adhikari, Hemanth Jagannathan, Paul W. Leu, Yuan Zhang, Jason M. Parker, and Shu Hu for the shared use of the CVD system used for GeNW growth. The authors thank Chuck Hitzmann of the Stanford Nanocharacterization Laboratory of Stanford University for assistance with the Auger electron spectroscopy analysis and Rob Franks of the Marine Analytical Laboratory of the University of California;Santa Cruz for assistance with the ICP-MS analysis. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Supporting Information Available: Lower magnification SEM images of GeNW samples after treatment with liquid etchants, a SEM image of a GeNW sample treated with iodine-HCl for 20 s and then rinsed with 1 M HCl, a high-resolution Ge 3d photoemission spectrum obtained from a GeNW sample after treatment with 10% by mass H2O2 for 3 s, secondary electron image with corresponding AES spectra obtained from several selected nanostructures on a GeNW and an AuNP track, derivations of eqs 1 and 2, and calculation of the expected number of gold atoms in a silicon chip heated to 375 C. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la900725b
9479
