Raman Characterization of B and Ge Distribution in Individual B

The distribution of Ge composition and active B concentration in individual B-doped Si1−xGex alloy nanowires (SiGeNWs) grown by the vapor liquid sol...
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J. Phys. Chem. C 2009, 113, 5467–5471

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Raman Characterization of B and Ge Distribution in Individual B-Doped Si1-xGex Alloy Nanowires Chiharu Nishimura,† Go Imamura,† Minoru Fujii,*,† Takahiro Kawashima,‡ Tohru Saitoh,‡ and Shinji Hayashi† Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe UniVersity, Rokkodai, Nada, Kobe 657-8501, Japan, and Panasonic Corporation, 3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japan ReceiVed: December 25, 2008; ReVised Manuscript ReceiVed: February 12, 2009

The distribution of Ge composition and active B concentration in individual B-doped Si1-xGex alloy nanowires (SiGeNWs) grown by the vapor liquid solid (VLS) process is studied by micro Raman spectroscopy by shifting the measurement position from the catalyst side to the substrate side. The Raman data reveal that there is strong correlation between the Ge composition and the active B concentration within individual SiGeNWs. We show that B-doped SiGeNWs have a core-shell structure with a low-Ge composition and low-B concentration core grown by the VLS process covered with a high-Ge composition and high-B concentration shell grown by the conformal deposition. Introduction In recent years, semiconductor nanowires (NWs) have attracted increasing interest because of their unique electronic properties and possible device applications as transistors,1,2 chemical sensors,3,4 and photovoltaic cells.5-10 Among various kinds of NWs, group IV NWs such as SiNWs,5,7-11 GeNWs,12-15 and Si1-xGex NWs (SiGeNWs)1,2,16,17 are most intensively studied due to their high compatibility with the highly developed complementary metal-oxide semiconductor (CMOS) technology and possible integration in future electronic devices. Especially, SiGeNWs1,2,16-18 are fascinating materials because of the high carrier mobility, the wide controllability of the electronic structure, and the possibility for the modulation of the band gap within a nanowire. Up to now, SiGeNW-based nand p-type MOS field-effect transistors (MOSFET) have been realized, and their high performance has been demonstrated.1,2 SiGeNWs are usually grown by the vapor liquid solid (VLS) process by using metal nanoparticles as catalysts.1,2,16,17 VLSgrown SiGeNWs usually become conical due to a conformal deposition of a Si1-xGex layer on the sidewall of a VLS-grown core during the growth.17,19,20 Because Ge composition of a shell is not necessarily the same as that of a core, SiGeNWs have core-shell structures.19-21 Recently, Lew et al. reported a systematic approach to synthesize SiGeNWs with controlled composition by the VLS growth and demonstrated the growth of homogeneous SiGeNWs with minimal Ge deposition on the outer surface of the wires.17 However, because of the complex growth process as compared to that of Si or Ge NWs, there still remain unknown factors that determine the morphology of SiGeNWs. For example, the composition of SiGeNWs is reported to be determined not only by the growth conditions but also by the diameter when it is below ∼50 nm.16 The situation becomes much more complicated when dopant gases are supplied during the growth of SiGeNWs for in situ impurity doping because of different pyrolysis temperatures * Corresponding author. E-mail: [email protected]. † Kobe University. ‡ Panasonic Corp.

between Si, Ge, and dopant precursors, and different solublities in catalyst. Therefore, although electric properties have been successfully controlled by doping,1,2 the behavior of doped impurities in SiGeNWs, that is, the radial and axial distributions of impurities, activation rate, etc., has not been fully understood. Furthermore, the effects of dopant gases on the composition and distribution of Ge in SiGeNWs are not well-known. These problems cannot be accessed by conductance measurements, and development of a new characterization technique is required. In a previous work,20 we demonstrated that mapping of Raman spectra along a SiGeNW axis by micro Raman spectroscopy and analysis of the spectral shape provide useful information for the axial and radial distributions of Ge composition in an individual SiGeNW. By applying a similar technique, we also analyzed the distribution of active B concentration along an axis of individual B-doped SiNWs.22 The technique is based on the analysis of Fano-type Raman spectral shape arising from the interaction between phonon Raman scattering and electronic Raman scattering due to intravalence band hole transitions.22 In this work, we apply the same technique to B-doped SiGeNWs. By mapping Raman spectra along the axis of individual B-doped SiGeNWs, distributions of active B concentration as well as that of Ge composition along the axis are obtained. Furthermore, the analysis of Raman spectral shape in each position provides information on the distribution of B and Ge in the radial direction. We show that B-doped SiGeNWs have a core-shell structure with a low-Ge composition and low-B concentration core covered with a high-Ge composition and high-B concentration shell, and that there is strong correlation between the Ge and B concentration. Experimental Section SiGeNWs were synthesized via gold (Au)-catalyzed VLS growth using a cold-wall infrared (IR) lamp-heated chemical vapor deposition (CVD) apparatus. To attach the catalysts to the Si substrates, thermally oxidized Si (100) wafers were first treated with oxygen plasma and dipped into 1 wt % solution of 3,5-diaminopentyltrimethoxysilane. The substrates were then dipped into a solution of Au-colloid (Tanaka Kikinzoku Co.)

10.1021/jp811406y CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

5468 J. Phys. Chem. C, Vol. 113, No. 14, 2009 for 5-30 min. After being dried, the substrates were loaded into the CVD chamber. The flow rates of Si2H6 precursor, GeH4 precursor, and H2 were 200, 200, and 1080 sccm (cubic centimeters per minutes at STP), respectively. The gas pressure and temperature for the CVD growth were 0.3 Torr and 450 °C, respectively. The growth duration was 10 min. For the growth of B-doped SiGeNWs, B2H6 (1.25 sccm) was added. Other growth conditions were the same as those of undoped SiGeNWs, that is, 0.3 Torr, 450 °C, and 10 min. For both undoped and B-doped SiGeNWs, the length was 7-12 µm, and the diameter at the catalyst side was 80-100 nm. The morphology of the SiGeNWs was studied by a transmission electron microscope (TEM) (JEOL JEM-2010) operated at 400 kV. For the TEM observations, the SiGeNWs were separated from the growth substrates by ultrasonication in ethanol, and the solvent containing the SiGeNWs was then dropped onto a microgrid. For Raman measurements of individual SiGeNWs, they were separated from the growth substrates by ultrasonication in ethanol, and the diluted solution was dropped on Au (∼100 nm in thickness)-coated Si(100) substrates. The purpose of Au coating was to avoid a Raman signal from Si substrate. Raman spectra were measured by a confocal microscope (50× objective lens, N.A. ) 0.75) equipped with a single monochromator and a charge-coupled device (CCD). The excitation source was the 632.8 nm light from a He-Ne laser. This wavelength is apart from the resonant condition of phonon Raman scattering of Si and is suitable for the observation of Fano resonance at the Si-Si modes. Rayleigh scattering was removed by a low pass filter (Semrock: LP02633RU-25). Raman signals from the region of 2 µm in diameter on substrates were collected. Although the diameter of SiGeNWs was smaller than the diffraction limit of the resolution, they were observable from scatter of imaging light. From the image, isolated SiGeNWs were selected. The catalyst side and the other side (the substrate side) of NWs could be distinguished by the slightly conical shape of SiGeNWs; that is, the substrate side looks darker than the catalyst side. Raman spectra were recorded along a SiGeNW from the catalyst side to the substrate side by changing the measurement position by 2 µm for each step.

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Figure 1. Typical TEM images of (a) undoped and (b) B-doped SiGeNWs. Distribution of conical angle (θ) estimated from TEM images of (c) undoped SiGeNWs and (d) B-doped SiGeNWs.

Results and Discussion As-Grown Undoped and B-Doped SiGeNWs. Figure 1a and b shows typical TEM images of undoped and B-doped SiGeNWs, respectively. Both NWs are conical. The formation of the conical shape is caused by conformal deposition of a Si1-xGex layer on the sidewall of a VLS-grown Si1-xGex core because of the low pyrolysis temperature of GeH4.11,17,19 In fact, in the present growth condition, without GeH4, that is, when only Si2H6 and H2 are supplied, nonconical NWs are grown. By comparing Figure 1a and b, we notice that the B-doped SiGeNW is more conical than the undoped SiGeNW. To discuss the difference of the shape quantitatively, we estimate the conical angle (θ) from TEM images for more than 100 NWs. Figure 1c and d shows histograms of the conical angles of the undoped and B-doped SiGeNWs, respectively. We can see a clear difference between them. In undoped SiGeNWs, θ is distributed from 0° to 3°, while in B-doped SiGeNWs, θ smaller than 1° cannot be found. The increase in the conical angle by B doping indicates that B2H6 enhances conformal growth of a Si1-xGex layer. A similar effect has been reported for SiNWs and GeNWs.12,23 In heavily B-doped SiNWs, the substrate side was very defective23 because of conformal deposition of defective high-B concentration layers, and high temperature annealing was

Figure 2. High-resolution TEM image of B-doped SiGeNW at the substrate side.

necessary to improve the crystallinity and to activate doped B. To study the crystallinity of B-doped SiGeNWs, we performed high-resolution TEM (HRTEM) observations. Figure 2 shows a typical HRTEM image of the substrate side of a B-doped SiGeNW. The image shows that the NW is defect-free single crystal. A similar image was observed in the whole range of the NW. Figure 3a shows micro Raman spectra of an individual undoped SiGeNW measured at the positions A-F indicated in the illustration. Raman spectra were measured from the catalyst side to the substrate side. Signals from the region of 2 µm in diameter are detected in each measurement. Three major peaks can be seen at 500-520, 400-410, and 280-290 cm-1, which correspond to optical phonons caused by the motions of adjacent Si-Si, Si-Ge, and Ge-Ge pairs, respectively.24 The peak around 420 cm-1 is assigned to the local vibration mode of the Si-Si pairs modulated by the adjacent Ge atoms.25 The Raman

B-Doped Si1-xGex Alloy Nanowires

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Figure 3. Raman spectra of (a,b) undoped and (c,d) B-doped SiGeNWs measured at the positions A-F indicated in the illustration. (b and d) Expansion of the region of the Si-Si mode of (a) and (c), respectively (490-520 cm-1). (e) Expansion of the region between 480 and 580 cm-1 of (c). Solid curves in (e) represent the results of fitting by the Fano resonance formula.

intensity depends on the measurement position; the intensity is smaller at the catalyst side than at the substrate side. This is mainly due to different volumes caused by the conical shape of SiGeNWs and to different compositions discussed later. To make the comparison of the spectral shape easier, the spectra are normalized at the position of the Si-Si mode. The intensity of the Ge-Ge and Si-Ge modes with respect to the Si-Si mode slightly increases by changing the measurement position from A to F. Figure 3c shows micro Raman spectra of an individual B-doped SiGeNW. In contrast to the undoped SiGeNW, the relative intensities of the Ge-Ge and Si-Ge modes increase significantly from the catalyst side to the substrate side. This indicates strong Ge-composition gradient along the growth axis of the B-doped SiGeNW. Figure 4a plots the relative intensity of the Ge-Ge mode as a function of the distance from the catalyst. The “0” and “9” represent the data of the undoped and B-doped SiGeNWs, respectively. We can see a clear difference between them. The relative intensity of the Ge-Ge mode of the B-doped SiGeNW increases from the catalyst side to the substrate side, while that of the undoped SiGeNW is almost constant. This result combined with the measured conical angle in B-doped SiGeNWs suggests that B doping not only accelerates the conformal deposition rate but also increases the Ge composition of the conformally deposited layer. Figure 3b and d shows expansions of the Si-Si mode of Figure 3a and c, respectively. In both cases, the mode splits into two peaks, which implies that the NW consists of two regions with different Ge compositions. In a previous paper,20 we deconvoluted the peak into two Lorentzian functions and obtained the wavenumbers of the peaks as a function of the measurement position. The analysis of the data revealed that

Figure 4. (a) Relative intensities of Si-Si and Ge-Ge Raman modes as a function of distance from the catalyst. “0” and “9” represent the data of undoped and B-doped SiGeNWs, respectively. (b) Coupling strength (1/q) as a function of distance from the catalyst. (c) Coupling strength (1/q) as a function of the intensity ratio of the Ge-Ge and Si-Si Raman modes (IGeGe/ISiSi). Dashed lines are guides to the eyes.

the SiGeNWs have a core-shell structure with a low-Ge composition core grown by the VLS process covered with a high-Ge composition shell grown by the conformal deposition. In the case of Figure 3b, Ge composition of the core estimated from the Raman shift is 0.19 at the catalyst side and 0.23 at the substrate side, while that of the shell is 0.29 and 0.31 at the catalyst and substrate sides, respectively. In B-doped SiGeNWs (Figure 3d), the measurement position dependence of the spectral shape is qualitatively the same as that of the undoped SiGeNW; that is, the intensity of the low-wavenumber peak with respect to the high-wavenumber one increases from the catalyst side to the substrate side. This indicates that the volume of high-Ge composition region increases to that direction and is consistent with the data in Figure 4a. The largest difference between Figure 3b and d is the width of the peaks. In B-doped SiGeNWs, the peaks are much broader, and as a result the splitting is less pronounced. The broadening is mainly due to the Fano resonance caused by B-doping. Note that the Fano resonance not only modifies the spectral shape but also shifts the peak. Therefore, Ge composition of B-doped SiGeNWs cannot be estimated from the Raman peak wavenumbers. The characteristic features of the Fano resonance are an asymmetric Raman spectrum with a long tail toward the highwavenumber side and a dip at the low-wavenumber side. In Figure 3c, we can clearly see these features especially at the substrate side of the SiGeNW. The observation of Fano

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resonance is evidence that B atoms are doped into the substitutional sites of the SiGeNW and electrically active. The degree of the asymmetry depends on the doping level. Therefore, active B concentration in SiGeNWs can be estimated by fitting the Raman spectra by the Fano resonance line shape expressed as

I(ω) ) A

ω - ω0 (q + ε)2 ,ε ) 2 Γ 1+ε

(1)

where q is the asymmetry parameter (1/q is sometimes referred to as the coupling strength), Γ is the line width parameter and is related to the phonon lifetime, and ω0 is the renormalized phonon frequency in the presence of coupled scattering.23,26 Note that modification of Raman spectral shape by the phonon confinement effect is negligible in the present rather thick SiGeNWs, and thus eq 1 is applicable. The influence of the electronic Raman scattering on the phonon Raman scattering appears in q; a larger value of 1/q implies stronger interference and higher concentration of active B atoms. In Figure 3c, because the Si-Si mode splits into two peaks, the spectra should be fitted by the sum of two Fano line shapes. However, the formula is too complicated, and too many fitting parameters result in a large uncertainty in the accuracy of the parameters. Therefore, in this work, we regard the double peak as a single one and fit the whole Si-Si mode by a single Fano resonance line shape. The results of the fitting are shown in Figure 3e. The fitting is not perfect but can represent the characteristic feature of the Fano resonance well, and thus the coupling strength (1/q values) obtained from the fitting can be used as a measure of the doping level. Figure 4b shows the coupling strength as a function of the distance from the catalyst. The coupling strength increases from the catalyst side to the substrate side. This suggests a conformal deposition of a high-B concentration layer. Furthermore, small coupling strength in the catalyst side indicates that doping of B atoms via metal catalyst is very limited. The comparison of Figure 4a to b reveals that the measurement position dependence of B concentration is very similar to that of Ge concentration. To study the correlation between them, we plot the 1/q values as a function of the intensity ratio of the Ge-Ge and Si-Si Raman modes in Figure 4c. We can see strong correlation between them; that is, high-Ge composition region is always more heavily B doped. The correlation strongly suggests that B2H6 enhances conformal deposition of high-Ge and -B concentration layers, which results in a large distribution of Ge and B concentration along the growth axis of SiGeNWs. Effects of Annealing. In a previous work on as-grown B-doped SiNWs,23 we demonstrated that the heavily B-doped substrate side is very defective and not all B atoms are active. On the other hand, as can be seen in Figure 2, in the case of B-doped SiGeNWs, the crystallinity is very good, and clear Fano resonance can be seen in as-grown SiGeNWs. To confirm that almost all B atoms are active in as-grown samples, we annealed B-doped SiGeNWs and compared the Raman spectra to those of as-grown B-doped SiGeNWs. The annealing was performed by a rapid thermal processing (RTP) in nitrogen gas atmosphere at 800 or 950 °C for 1 min. Figure 5a and b shows Raman spectra of the catalyst and the substrate sides, respectively, of as-grown and annealed B-doped SiGeNWs. We can see no distinct difference between Raman spectra of as-grown and annealed NWs in both the catalyst and the substrate sides. In fact, the coupling strength obtained from

Figure 5. Raman spectra of as-grown and annealed B-doped SiGeNWs: (a) catalyst side and (b) substrate side.

the spectra of the substrate side in Figure 5b is almost the same, that is, 0.36, 0.23, and 0.28 for as-grown and 800 and 950 °C annealed samples, respectively. Therefore, in B-doped SiGeNWs, almost all B atoms are activated during the growth. This is probably due to the lower growth temperature of SiGe than that of Si. In fact, in CVD growth of Si1-xGex thin films, the addition of GeH4 is known to enhance the growth rate, that is, to decrease the growth temperature due to complex interactions between source gases and the low pyrolysis character of GeH4 gas.27 Conclusion B-doped SiGeNWs grown by the VLS process were studied by micro Raman spectroscopy and TEM. By analyzing Fano type spectral shape and the intensity ratio of the Ge-Ge and Si-Si modes, distributions of electrically active B concentration and Ge compositions were obtained simultaneously. The Raman results revealed that there is strong correlation between Ge and B concentration within B-doped SiGeNWs; that is, the heavily B-doped region always had high Ge composition. The Raman results combined with TEM observations indicate that B-doped SiGeNWs have a core-shell structure with a low-Ge composition and low-B concentration core grown by the VLS process covered with a high-Ge composition and high-B concentration shell grown by the conformal deposition. The results also suggest that B2H6 enhances the conformal deposition of the shell. The enhanced conformal deposition results in larger distribution of Ge composition and B concentration along the axis and makes SiGeNWs more conical. Acknowledgment. This work is supported in part by a Grantin-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References and Notes (1) Whang, S. J.; Lee, S. J.; Yang, W. F.; Cho, B. J.; Kwong, D. L. Appl. Phys. Lett. 2007, 91, 072105. (2) Kim, C.-J.; Yang, J.-E.; Lee, H.-S.; Jang, M.; Jo, M.-H.; Park, W.H.; Kim, Z. H.; Maeng, S. Appl. Phys. Lett. 2007, 91, 033104. (3) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (4) Hahm, J.; Lieber, C. M. Nano Lett. 2004, 4, 51. (5) Peng, K.; Xu, Y.; Wu, Y.; Yan, Y.; Lee, S.-T.; Zhu, J. Small 2005, 1, 1062. (6) Shim, H.-S.; Na, S.-I.; Nam, S. H.; Ahn, H.-J.; Kim, H. J.; Kim, D.-Y.; Kim, W. B. Appl. Phys. Lett. 2008, 92, 183107. (7) Cheng, Y.; Fang, G.; Li, C.; Yuan, L.; Ai, L.; Chen, B.; Zhao, X.; Chen, Z.; Bai, W.; Zhan, C. J. Appl. Phys. 2007, 102, 083516. (8) Peng, K.; Wang, X.; Lee, S.-T. Appl. Phys. Lett. 2008, 92, 163103.

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