NANO LETTERS
Pressure-Modulated Alloy Composition in Si(1-x)Gex Nanowires
2009 Vol. 9, No. 5 1775-1779
Uri Givan and Fernando Patolsky* School of Chemistry, The Raymond and BeVerly Sackler Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel Received December 3, 2008; Revised Manuscript Received March 27, 2009
ABSTRACT Si(1-x)Gex nanowires (NWs) constitute promising building blocks for future electronic and optoelectronic devices due to the enhanced tuneability of their physical properties, achieved mainly by controlling their chemical composition. In this study, the pressure dependence of the chemical composition, growth and tapering rates and crystalline structure of Si(1-x)Gex NWs grown by the CVD-VLS technique was investigated. It is demonstrated for the first time, that the composition of single crystal Si(1-x)Gex NWs can be readily modulated between ca. x ) 0.75 to x ) 0.25, simply by altering the total growth pressure while keeping all other growth parameters fixed. Moreover, this procedure does not cause any undesired structural or morphological side effects. Growth pressure is hence concluded to be the most significant parameter for tailoring Si(1-x)Gex NWs electron and phonon mobility, band gap, and so forth. The observed alloy-composition control phenomena can be explained by the interplay between the pressure-dependent unimolecular decomposition of the individual precursor gases, SiH4 and GeH4, at the given experimental conditions that leads to a direct modulation of the decomposed/activated Si/Ge precursors ratio in the gas feedstock and is finally reflected in the composition of the obtained binary alloy nanowires. In addition, a silicon-germanium cooperative growth mechanism is suggested to account for the observed growth rate pressure dependence and enhanced growth rates.
One-dimensional semiconductor nanowires (NWs) have the potential to play a key role as functional components as well as interconnectors in future electronic and optoelectronic devices. Of all possible NWs chemical compositions, group IV (Si, Ge, SiGe) NWs are gifted with the advantages of compatibility with conventional CMOS processes, as well as of relatively easy and low cost synthetic preparation by the CVD-VLS technique. While silicon and germanium nanowires (SiNWs, GeNWs) were intensively studied during the last decades,1-9 binary alloy Si(1-x)GexNWs were first produced only a few years ago,10-15 and since then attracted rapidly increasing interest due to their variety of unique properties. The silicon-germanium complete miscibility enables a wide range of Si(1-x)Gex NWs chemical composition which can be modulated through growth parameters such as temperature or precursor gases ratio. The Si(1-x)Gex NW’s chemical composition has a dramatic effect over several significant physical properties, for example, electron and phonon mobility, band gap, and lattice parameters. Some of these effects were thoroughly investigated by the thin film research community, leading to new applications in thermoelectricity,16 photodetectors,17 heterojunction bipolar transistors (HBT),18 optic-fiber communication,19 and so forth. Since Si(1-x)Gex NWs performance is expected to exceed that of thin films, they may facilitate the production of new * To whom correspondence should be addressed. E-mail: fernando@post. tau.ac.il. 10.1021/nl803657z CCC: $40.75 Published on Web 04/15/2009
2009 American Chemical Society
devices and may be used as a test case for basic studies in the field of disordered alloy systems.20 The enhanced properties tuneability of Si(1-x)Gex NWs over the single element NWs (SiNW, GeNw, etc.) grown via the bottom-up approach, require careful growth planning and rigorous understanding of the influence of growth parameters on the resulting NW’s characteristics. Other synthetic techniques such as laser-assisted growth (LAG)10 and pulsed laser ablation CVD21 demonstrated NW composition control by changing the growth substrate location in the furnace or through inserting SiGe segments into SiNWs, thus creating a heterostructure superlattice. Using the CVD-VLS synthesis technique, an increased compositional control was achieved by changing the following growth parameters: inlet precursor gases ratio, growth temperature and catalyst diameter.22,23 The fact that the simultaneous change of both temperature and inlet gases ratio led to a relatively wider range composition control,24 points out that the separate effect of each one of the aforementioned growth parameters is quite limited. Though growth temperature has been shown to influence over the resulting alloy NWs composition, it is generally accompanied by undesired tapering (non catalytic radial growth) and crystal structure modulation.22 Also, the catalyst diameter has a minor influence on the alloy composition,23 and excessive GeH4 partial pressure results in tapering as well.25 In this study, we sought after a more applicable and direct growth parameter that will control a wider range of the
resulting Si(1-x)Gex NW’s chemical compositions without leading to any undesired side effects. The total pressure during growth is shown to be the optimal parameter through which this goal can be achieved. We report for the first time the influence of the total growth pressure on Si(1-x)Gex NWs composition, structure, growth rate, and tapering rate, and find it to be superior to other plausible growth parameters such as growth temperature and inlet gases partial pressures for tailoring the Si(1-x)Gex NWs chemical composition and hence their physical properties. All NWs produced in this work were grown by the VLS technique in a home-built computer-controlled hot-wall CVD setup using 20 nm Au nanoparticles (AuNPs) as the catalyst, high purity silane (SiH4), and germane (10% GeH4 in H2 carrier) as precursor gases, and high purity (99.9999%) argon and hydrogen as carrier gases. Si substrates were cut to the required size (ca. 1.2 × 3 cm), cleaned with acetone and isopropyl alcohol (IPA), and dried using a dry N2 stream. In order to improve AuNPs adhesion, the substrates were dipped in a poly-L-lysine solution (0.1 v/w, Ted Pella) for 2 min, washed with deionized distilled water (DDW), and dried in a dry N2 stream. After drying, the substrates were dipped in a 1/100, IPA diluted, AuNPs solution (7 × 1011 particles/ ml, Ted Pella) for two minutes followed by washing and drying in the aforesaid manner. Finally, organic traces were removed by a 200 W oxygen plasma step for 2 min. Substrates were then placed at the center of a horizontal quartz tube in a hot-wall furnace that was vacuumed to 5 × 10-3 Torr for 10 min followed by an additional 10 min purge with Ar and H2 flow of 10 and 20 standard centimeter cube per minute (sccm) respectively. Flow rates were determined using computer-controlled calibrated mass flow controllers (MFC). The growth step for all Si(1-x)Gex NWs was carried out at 350 °C (monitored by a thermocouple) with the following gas inlet flows: 10%GeH4, 1 sccm; SiH4, 5 sccm; and H2, 100 sccm. Growth duration and pressure varied between 20-40 min and 50-1000 Torr, respectively, with the pressure being computer controlled via a throttle valve defining the flow to the vacuum pump. A postgrowth step of Ar and H2 purging, while cooling down the sample, ended the growth process. Time-of-flight secondary ion mass spectroscopy (TOFSIMS) samples were grown on GaAs substrates prepared by the same manner described above with the exception of dipping in a nondiluted AuNPs solution. Our experience indicates that TOF-SIMS measurements performed on a Si substrate resulted in a high Si background signal (ca. 40%) regardless of the NW density. The use of a substrate which contains neither Si nor Ge was thus found to be necessary. NWs morphology was then investigated on the growth substrates employing field emission high resolution scanning electron microscope (FEG-HRSEM, JEOL JSM-6400), or FEG environmental SEM (FEG-ESEM, Quanta 200). Growth rates were calculated from the obtained NW’s length divided by growth time at the relevant pressure. For structural studies using FEG high resolution transmission electron microscopy (FEG-HRTEM, Philips Tecnai F20), the NWs were separated from the substrate by 1776
Figure 1. Calibration plots of growth (red) and tapering (black) rates dependence on NWs growth total pressure. Inset: SEM image of a typical Si(1-x)Gex nanowires growth substrate.
sonication in IPA followed by dispersing the solution onto carbon-coated copper grids. High resolution images and FFT revealed the growth direction and determined the lattice parameters. Compositional analysis was carried out by either energy dispersive X-ray spectroscopy (EDS) in HRTEM on nanowires grown on standard silicon wafers, or by TOFSIMS (PHI 2100 TRIFT II) using as-grown high density Si(1-x)GexNWs on a GaAs substrate and employing gallium metal ion gun (LMIG) with a 600 pA beam and 15 kV beam voltage.26 EDS quantification was done using thin films model with K factors of 1 and 2.083 for Si and Ge K lines, respectively. The pressure influence on Si(1-x)Gex NWs growth was distinguished by systematically increasing the growth step total pressure from 50 to 1000 Torr, while maintaining all other growth parameters constant. In order to suppress radial growth we chose the growth temperature to be 350 °C, inlet gas ratio (GeH4/(GeH4+SiH4)) to be 0.02, and a 100 sccm flow of H2. Growth duration varied from 20 to 40 min and resulted in long NWs (up to 50 µm) which enabled a detailed study of both growth and tapering rates.27 The Au-tip was readily observed by SEM or HRTEM (unless broken during transfer onto TEM grid), suggesting a VLS growth mechanism. The pressure dependence of growth and tapering rates are demonstrated in Figure 1. While the growth rate seems to be linear throughout the entire investigated pressure range, the tapering rate remains constant (within the error limits) with a negligible value of about 0.8 nm per 4 µm (or less than one atomic level per µm) between 50 to 500 Torr, and then increases up to a rather low value of less than 0.9nm/ µm at 1000 Torr. Figure 2 shows low resolution, high resolution, and FFT images of Si(1-x)Gex NWs grown under different total pressures. Over the whole pressure span Si(1-x)Gex NWs exhibit smooth surfaces with a thin (ca. 3nm) layer of amorphous silicon oxide (confirmed by EDX measurements). The Nano Lett., Vol. 9, No. 5, 2009
Figure 2. Low magnification (top series) and high resolution (bottom series) TEM images of ca. 20 nm diameter Si(1-x)Gex NWs grown at 350 °C, inlet precursors gases ratio (GeH4/GeH4+SiH4) of 0.02 and total growth pressures between (A) 50 Torr to (F) 1000 Torr. Scale bars are 0.6 µm for top series and 8.5 nm for bottom series. Insets: (A-top) TEM image of a NW Au-tip suggesting VLS growth mechanism; (C-bottom) FFT analysis of the HRTEM image in panel C exhibits [111] growth direction; (F-top) high-magnification TEM image of the orange circle enclosed area showing a Ge nucleation grain.
percentage of Ge in the outer oxide layer is extremely small and the outer oxide layer was found to be essentially silicon oxide. At pressures higher than 750 Torr some grains (previously reported for Si(1-x)Gex NWs grown at relatively high GeH4 partial pressure28) appeared on the nanowires surface and became more apparent at 1000 Torr. Furthermore, all Si(1x)Gex NWs in this study were single crystal. As previously reported1,12,14,23 most Si(1-x)Gex NWs exhibit [111] growth direction regardless of their chemical composition. The crystalline Si(1-x)Gex NWs diameter showed a slight increase with raising the pressure (and hence the Ge content) from 18 nm at 50 Torr to 24 nm at 1000 Torr. This fact can be explained by the GeNWs tendency to have larger diameter than the AuNPs catalyst,2 thus the diameter increases with higher Ge content. The previous report of nanowire composition dependence upon the catalyst diameter9 could not account for this effect, since the same 20 nm AuNPs were used throughout the whole pressure range. Nevertheless, it must be emphasized that using AuNPs catalyst of different diameters other than 20 nm does not reduce the resulting nanowires composition control ability, but might, according to published reports,9 change the chemical composition range. EDS chemical composition analysis reveals (for both the radial and longitudinal axes) at all growth pressures a compositional uniformity within ca. (3% accuracy. Notably, the disordered alloy is indeed expected to exhibit some diversity when high resolution and small spot sizes are used for the EDS analysis. Figure 3 shows EDS results taken along a distance of 16 µm on a Si(1-x)Gex NW grown at 100 Torr. The quantitative analysis determines a Si content of 67.7% (x ) 0.68) with a deviation of less than ca. (3%. The readily pressure-modulated Si content in Si(1-x)Gex NWs is shown in Figure 4. A tendency shift is observed around 500 Torr when the sharp decrease in Si content milds into a somewhat less dramatic linear-like slope. Qualitative SIMS results (Figure 4 inset) exhibit similar trends but with higher Si content than for the obtained EDS analysis. The discrepancy between EDS and SIMS results can be ascribed Nano Lett., Vol. 9, No. 5, 2009
Figure 3. (A) Low magnification TEM image of a 16 µm Si0.7Ge0.3 NW grown under a total pressure of 100 Torr. The scale bar is 2 µm. (B) Energy-dispersive X-ray spectra corresponding to the area enclosed in the five circles along the NW in (A).
to the qualitative nature of the SIMS results or to the GaAs substrate’s influence on the measured Si/Ge signals. The latter two assumptions are currently under further investigation. The clear dependence of the composition upon growth pressure manifested above is not directly predicted from thermodynamically driven VLS models.28-31 Published literature has given little attention to the chemical nature of the precursor reactions kinetics as an influencing step during the nanowires growth, although it is well known that pressure may have a dramatic influence on gas-phase reaction kinetics, particularly when two chemically different gas precursor species are present during the nanowire growth, each one having its own pressure-dependent behavior under the given experimental conditions, as in our case. Thus, pressure effects on growth kinetics and final chemical composition cannot be neglected. Generally, the NW growth from gas precursors 1777
Figure 4. Growth pressure dependence of Si(1-x)Gex NW’s Si content extracted from energy-dispersive X-ray spectroscopy (EDS). Inset: Growth pressure dependence of Si(1-x)Gex NW’s Si content extracted from qualitative TOF-SIMS experiments.
consists of four main steps: (1) precursor molecule activation or decomposition reaction in the gas phase; (2) adsorption of reactive species onto the nanoparticle catalyst surface; (3) diffusion of precursor atoms, for example, Si or Ge, into the droplet alloy; and (4) supersaturation/crystallization to form a NW. The proposed growth models differ mainly by choosing the rate-limiting kinetic step and usually refer to incorporation and decomposition as a single step, or neglect the gas phase decomposition step altogether. Among the four steps outlined above, the first one will be most dominant in influencing the chemical composition and growth kinetics of nanowires in the presence of gas precursors having different chemical kinetics of decomposition such as silane and germane.32 It is important to note that under different growth conditions such as molecular beam epitaxy (MBE) or cold-wall CVD systems, the NW synthesis is performed at relatively high vacuum (∼10-6 Torr). Under such conditions gas molecules have mean free paths much larger than the growth chamber or only the growth substrate is heated and gas precursors remain at low temperatures during the process, so that gasphase reactions influence over the NW growth can be neglected. In contrast, gas-phase reactions have a profound influence upon both growth kinetics and nanowire composition in our hot-wall CVD-based NW synthesis setup. Though a full discussion of the possible explanations of these findings is beyond the scope of this paper, we chose to inspect the feasibility of attributing the pressure dependence of both the NW composition and the growth rate to the individual precursor gases decomposition kinetics and the known cooperative growth of Si and Ge.33 The unimolecular decomposition kinetics of silane and germane were extensively studied over the last decades due to the rising interest in Si, Ge, and SiGe thin films for the CMOS industry.33-38 Specifically, the pressure dependence was investigated over a wide range of temperatures (800-1500 K) and pressures (10-2-106 Torr) relevant for the CVD growth of thin films.35,36 Though both silane and germane exhibit an increase in the unimolecular decomposition rate 1778
Figure 5. (A) Growth rates of Si(1-x)GexNWs (a) and SiNWs (b) grown under same growth conditions. (B) ESEM images of Ge grains obtained under the same growth conditions at total growth pressures of 100 (left), 500 (middle), and 1000 Torr (right). Scale bars are 0.9 µm.
while raising the pressure from 50 to 1000 Torr, the germane unimolecular decomposition rate at 800 K increases by at least 2 orders of magnitude more then the silane’s rate over the same pressure range.34,35 Furthermore, if extrapolated to lower temperatures, the germane-to-silane pressure-dependent decomposition rate ratio seems to increase. We therefore assume that for a fixed inlet gases ratio, (Siin/Gein ) 50) at each given growth pressure the decomposed Si/Ge ratio (Sidecomposed /Gedecomposed) will decrease, leading to a decrease of the Si/Ge ratio in the AuNP catalyst with pressure, resulting in the same compositional trend observed in the Si(1-x)Gex NWs. The cooperative growth of Si and Ge films (also referred to as nonlinear growth or heterogeneous growth) was observed in SiGe thin films growth and assumed to account for the increase of the observed growth rates with Ge concentration.33,38 In order to evaluate its relevancy to our study, we compared the growth rate of the mutual growth to the growth rate of each of the components grown separately under the same conditions. Figure 5A compares SiNWs and Si(1-x)Gex NW growth rates throughout the entire total growth pressure range. Note that Ge individual growth did not yield any NWs growth under the entire pressure range investigated here. Instead, highly dense nucleation grains were produced, which can be attributed to non-catalytic growth as reported by other groups using similar growth setups and experimental conditions.39 Successful synthesis of germanium nanowires at low-partial pressures of germane (similar to our experiments) was only achieved by using coldwall CVD synthesis set-ups.40 SEM images of the Ge nucleation grains are shown in Figure 5B. Although both Nano Lett., Vol. 9, No. 5, 2009
SiNWs and Si(1-x)GexNWs growth rates exhibit an augmentation with increasing pressure, we conclude from the vast deviation between their values and from the absence of GeNWs growth, that under the above-mentioned experimental conditions cooperative growth has dramatic influence on the growth process rate.33 To conclude, the pressure influence on CVD-VLS grown Si(1-x)Gex NW’s chemical composition, crystalline structure and on both growth and tapering rates was investigated. We found that by simply increasing the pressure from 50 to 1000 Torr, while holding all other growth parameters fixed, the chemical composition of the resulting Si/Ge alloy nanowires could be readily varied from Si-rich to Ge-rich regimes without any undesired crystalline structural changes or tapering. We suggest that, combined with other growth parameters such as temperature and inlet gases ratio, the entire Si(1-x)Gex NWs compositional range can be achieved, leading to the complete utilization of the enhanced tuneability of the physical properties of Si(1-x)Gex NW’s. We also suggest that the germane’s stronger pressure-dependence of decomposition rate in comparison to the silane’s decomposition at the given experimental conditions (Tgrowth 350 °C and total growth pressures between 50-1000 Torr) leads to the observed modulation of the alloy nanowires composition. Higher pressures result in lower Si/Ge ratios in the feedstock and to the observed compositional ratio in the produced Si(1-x)Gex NWs. We also suggest that a cooperative growth mechanism of Ge and Si causes the enhanced growth rate observed through the whole pressure range. Nevertheless, auxiliary studies will be performed to further examine those hypotheses. Acknowledgment. The authors thank Dr. Ela Strauss for TOF-SIMS measurements and Dr. Yossi Lereah for useful discussions and experimental support regarding HRTEM and EDS measurements. This work was in part financially supported by the Legacy Fund-Israel Science Foundation (ISF) and the German-Israel Foundation (GIF).
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