Raman Characterization of Active B-Concentration Profiles in

May 29, 2009 - Rokkodai, Nada, Kobe 657-8501, Japan, and Panasonic Corporation, 3-1-1 Yagumo-Nakamachi, Moriguchi,. Osaka 570-8501, Japan...
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J. Phys. Chem. C 2009, 113, 10901–10906

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Raman Characterization of Active B-Concentration Profiles in Individual p-Type/Intrinsic and Intrinsic/p-Type Si Nanowires Go Imamura,† Takahiro Kawashima,‡ Minoru Fujii,†,* Chiharu Nishimura,† 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: February 24, 2009; ReVised Manuscript ReceiVed: April 23, 2009

Active B-concentration profiles of modulation doped p-type/intrinsic (p-i) and intrinsic/p-type (i-p) silicon nanowires (SiNWs) grown by a vapor-liquid-solid process were studied by analyzing Fano-type spectra in Raman spectroscopy. The analysis of Raman spectra of as-grown, annealed, and oxidized p-i and i-p SiNWs revealed that B atoms are mainly doped from the surface by conformal deposition of a heavily doped layer on the side wall of a SiNW. The surface doping results in strong gradient in B concentration along the axial and radial directions. Introduction Semiconductor nanowires (NWs) have been attracting considerable research attention in recent years because of their unique physical properties and their possible applications as transistors,1,2 photovoltaic cells,3 and chemical sensors.4,5 Among different kinds of semiconductor NWs, silicon (Si) NWs (SiNWs) have most extensively been studied because of their high compatibility with the highly developed CMOS technology and possible integration in future electronic devices. SiNWs are usually grown by a vapor-liquid-solid (VLS) process.6 The vapor precursors of silicon decompose at a metal catalyst droplet, diffuse through the catalyst, and then are solidified at the catalyst-SiNWs interface. One of the crucial issues for the development of SiNW-based electronic devices is the control of dopants within SiNWs and also the development of a technique to characterize concentration and distribution of active impurities in SiNWs. A number of studies have been conducted on the synthesis and characterization of impurity-doped SiNWs.7-13 Especially, alternatively boron (B) and phosphorus (P) doped SiNWs have been grown for electronic, optoelectronic, and photovoltaic applications.3,14-16 Doped SiNWs are usually characterized by the electrical properties.7,9-11 However, the method is not powerful enough to study the distribution of dopants along radial and axial directions, the activation ratio, the mechanism of doping, etc. Because the crystallinity and microstructures of B- and P-doped SiNWs are reported to depend on the kind of dopant sources and doping levels,8-10 a simple technique to characterize distribution of active impurities in doped SiNWs is highly demanded. Recently, direct three-dimensional (3D) visualization of dopant distribution in SiNWs was demonstrated by using a pulsed-laser atom probe tomography analysis.13 From the 3D image, preferential accumulation of dopants on the surface region of SiNWs is clearly shown. Although this technique is very powerful to characterize dopant distribution in SiNWs, a * To whom correspondence should be addressed. E-mail: fujii@ eedept.kobe-u.ac.jp. † Kobe University. ‡ Panasonic Corporation.

drawback is that the activation ratio and the distribution of active dopants cannot be obtained. Raman spectroscopy is a powerful tool to characterize active p-type impurities in semiconductor NWs.17,18 In heavily B-doped SiNWs, Fano resonance due to the interference between discrete phonon Raman scattering and continuous electric Raman scattering from intravalence band-hole transitions is observed and from the spectral shape concentration of active B atoms can be estimated.19,20 The detection limit of this method is around 1 × 1019 atoms/cm3. In previous work,18 we studied B-doped SiNWs by Raman spectroscopy. We measured Raman spectra of individual B-doped SiNWs at the catalyst side and the other side and demonstrated the gradient of active B concentration along the growth axis of SiNWs. In this work, we perform micro-Raman studies of individual SiNWs with controlled impurity profiles, i.e., p-type/intrinsic (p-i) and intrinsic/p-type (i-p) SiNWs. By mapping Raman spectra along the growth direction, distribution of active B concentration is studied. Furthermore, from the analysis of the spectral shape, distribution of active B concentration in the radial direction is discussed. The effects of annealing in N2 and O2 ambience on doping profiles are also discussed. Experimental Procedure SiNWs were synthesized via gold (Au) catalyzed VLS growth by using an infrared lamp-heated cold-wall chemical vapor deposition (CVD) apparatus.17,18,21,22 First, thermally oxidized Si(100) wafers (6 in. in diameter) were treated with oxygen plasma and dipped into 1 wt % solution of 3,5-diaminopentyltrimethoxysilane to attach the catalysts to the wafers. The wafers were then dipped into a Au-colloidal solution for 5-30 min. After drying, the wafers were loaded into the CVD chamber. The gas pressure and temperature for CVD growth were 0.3 Torr and 450 °C, respectively. For the growth of p-type SiNWs, the flow rate of Si2H6 precursor and H2 were 100 and 1510 sccm (cubic centimeters per minute at STP), respectively, and that of B2H6 (5% in a hydrogen mixture) were 6.25 or 12.5 sccm. The doping modulation in SiNWs was achieved by switching on and off the dopant gas flow during the VLS growth. For the growth of p-i SiNWs, a p-type region was first grown

10.1021/jp901679k CCC: $40.75  2009 American Chemical Society Published on Web 05/29/2009

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with the B2H6 flow rate of 6.25 or 12.5 sccm for 7.5 min, then the supply of B2H6 was stopped and an intrinsic region was continuously grown for another 7.5 min. For the growth of i-p SiNWs, an intrinsic region was first grown for 10 min, then the supply of B2H6 was started to grow a p-type region for 5 min. The diameter and length of SiNWs were 80-100 nm and 12-20 µm, respectively. For Raman measurements of individual SiNWs, they were separated from the growth substrates by ultrasonication in ethanol, and the diluted solution was dropped on a Au (∼100 nm in thickness) coated Si(100) wafer. The purpose of Au coating was to avoid a Raman signal from Si substrate. Raman spectra were measured by a confocal microscope (100× objective lens, N.A. ) 0.9) 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. From the image, isolated SiNWs were selected and the Raman spectra were measured along a SiNW from the catalyst side to the substrate side by changing the measurement position by 1 µm for each step. Signals from the area of 1 µm in diameter were detected. The catalyst and substrate sides of SiNWs could be distinguished by the slightly conical shape of SiNWs; that is, the substrate side looked darker than the catalyst side. For quantitative estimation of active B concentration from Fano spectral shape, Raman spectra of reference samples with known active B concentration were measured at the same condition. The reference samples were prepared by B implantation into Si on quartz (SOQ; 200 nm Si layer) and annealing. The doping levels of the reference samples were estimated from resistivity measurements and simulation. Results and Discussion p-Type/Intrinsic Si Nanowires. Figure 1 shows Raman spectra of as-grown p-i SiNWs measured at different positions. The distance measured from the catalyst is shown in the figure. We performed Raman measurements for more than 10 single SiNWs and found that they can be classified into two groups from the spectral shape. The probability of finding each group is almost the same. Parts a and b of Figure 1 show representative spectra of the two groups. In both cases, the spectral shape changes along the axis of SiNWs. The catalyst side shows a sharp symmetric peak at 520 cm-1 due to optical phonons of Si. The full-width at half-maximum (fwhm) of the peak at the catalyst side is close to that of bulk Si crystal (about 3.6 cm-1 in our measurement setup). On the other hand, it is much broader in the substrate side, where B2H6 was supplied during the growth. Interestingly, the origin of the broadening is different between the two groups. In Figure 1a, the existence of an amorphous component at 480 cm-1 results in the broadening of the spectra. This suggests that defective SiNWs are grown during the supply of B2H6. In our previous work,17 we studied B-doped SiNWs by transmission electron microscopy. The highresolution and dark-field images clearly demonstrated that the catalyst side is a defect-free single crystal, while the substrate side is covered by a defective polycrystalline layer. The defective layer is considered to contribute to the amorphous component. On the other hand, in Figure 1b, a signature of Fano resonance, i.e., a long tail at the high-wavenumber side and a dip at the low-wavenumber side of the peak, can clearly be seen. The observation of the Fano resonance is the direct evidence that B is doped into substitutional sites of the SiNW and is electrically active.19,20 Figure 2a shows Raman spectra of a p-i SiNW annealed at 950 °C for 1 min in a N2 atmosphere. The annealing was

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Figure 1. Typical Raman spectra of as-grown p-i SiNWs measured at different positions. Spectra are measured from the catalyst side to the substrate side by changing the measurement position by 1 µm for each step. The distance measured from the catalyst is shown in the figure. (a) and (b) are obtained from different SiNWs. Spectral shape of the substrate side is different between (a) and (b). The probabilities to find type (a) and type (b) SiNWs are almost the same.

performed prior to removing SiNWs from the growth substrates. The spectra of the catalyst side are symmetric, while very clear Fano resonance can be seen in the substrate side, especially in the region from 13 to 17 µm from the catalyst. Contrary to the as-grown SiNWs, individualities are not observed between measured SiNWs. This implies that nominally p-type region of as-grown p-i SiNWs are always B-doped, although they are not always activated as can be seen in Figure 1a. At present, the origin of the NW-to-NW variation of the activation rate is not clear. A possible explanation is small gradient of the growth temperature (about 20 °C) within a 6 in. wafer. In this work, SiNWs grown on thermally oxidized Si wafers are removed from the substrates by ultrasonication in ethanol to make black ink containing SiNWs. The diluted ink is dropped on Au-coated Si wafers for Raman measurements. Mixing of SiNWs grown in the whole region of a wafer in ethanol may cause the variation of the observed activation rate. Figure 2b shows Raman spectra of a p-i SiNW annealed at 1100 °C for 1 min. In contrast to SiNWs annealed at 950 °C, Fano resonance appears in the whole region. B atoms are thus distributed over the whole region of the SiNW. The analysis of Raman spectral shape provides quantitative information for the active impurity concentration.20 To estimate the distribution of active impurities in SiNWs, we fit the Raman spectra by the Fano resonance line shape expressed as

I(ω) ) A

(q + ε)2 , 1 + ε2

ε)

(ω - ω0) Γ

(1)

p-Type/Intrinsic and Intrinsic/p-Type Si Nanowires

Figure 2. Typical Raman spectra of p-i SiNWs annealed at different temperatures: (a) annealed at 950 °C for 1 min, (b) annealed at 1100 °C for 1 min. Spectra are measured from the catalyst side to the substrate side by changing the measurement position by 1 µm for each step. The distance measured from the catalyst is shown in the figure.

where ω0 is the renormalized phonon frequency in the presence of coupled scattering. q is the asymmetry parameter (1/q is sometimes referred to as the coupling strength), and Γ is the line width parameter and is related to the phonon lifetime. It should be mentioned here that modification of Raman spectral shape by the phonon confinement effect is negligible in the present rather thick SiNWs. The influence of the electric 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. From the analysis of the spectra of reference samples, we estimate that 1/q of 0.1 corresponds to the B concentration of ∼2 × 1019 atoms/cm3 and 1/q of 0.5 corresponds to ∼1 × 1020 atoms/cm3. For annealed SiNWs, the spectra could be fitted almost perfectly by eq 1, while for as-grown SiNWs, the spectral shape was not reproduced by eq 1. One of the reasons for this disagreement is the existence of amorphous components. However, even for samples with no amorphous components (e.g., Figure 1b), the spectral shape is significantly different from the Fano line shape. The spectra in Figure 1b looks like the combination of a Fano line shape and a sharp symmetric peak, suggesting that the SiNW is not homogeneous but consists of an almost intrinsic region and a heavily B-doped region. We thus tried to reproduce the spectra by the sum of the Fano line shape and the Lorentzian function,

I(ω) ) A

Γ22 (q + ε)2 + B , 1 + ε2 (ω - ω2)2 + Γ22

ε)

(ω - ω1) Γ1 (2)

The relative contribution of the Fano-type spectrum and the Lorentzian-type spectrum can be given by the ratio of A and B.

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Figure 3. Expansion of Raman spectra obtained from the catalyst side and substrate side of as-grown and annealed p-i SiNWs: (a) as-grown/ catalyst side (Figure 1b, spectrum at 1 µm from the catalyst), (b) asgrown/substrate side (Figure 1b, spectrum at 13 µm from the catalyst), (c) annealed at 950 °C/catalyst side (Figure 2a, spectrum at 1 µm from the catalyst), and (d) annealed at 950 °C/substrate side (Figure 2a, spectrum at 18 µm from the catalyst). The solid curves are the results of fitting by eq 2. Contributions of the Fano line shape and Lorentzian function are expressed by red and blue dotted curves, respectively.

Figure 3 shows the results of the fitting by eq 2 for as-grown (a and b) and 950 °C annealed (c and d) SiNWs at the catalyst (a and c) and substrate (b and d) sides. Black squares represent experimental spectra, while solid curves are the results of fitting by eq 2. In parts b and d of Figure 3, red and blue dotted curves are the contributions of the first and second terms of eq 2. All the spectra can be well fitted by the formula. In parts a and c of Figure 3, the contribution of the Fano-type spectra to the whole spectra ((A)/(A + B)) is nearly 0, while (A)/(A + B) is almost 1 in Figure 3d. These SiNWs are thus considered to be almost homogeneous. On the other hand, in the substrate side of the as-grown SiNW (Figure 3(b)), (A)/(A + B) is 0.182. The spectrum thus consists of an intrinsic and a heavily B-doped regions. In Figure 4, the coupling strength (1/q) values obtained by the fitting are plotted as a function of the distance from the catalyst. Figure 4a shows the results for annealed p-i SiNWs. The data for as-grown SiNWs are not shown because the data are largely scattered between SiNWs, as we demonstrated in Figure 1. In the case of 950 °C annealing, the coupling strength values of the region from 0 to 12 µm from the catalyst are very small, meaning that active B concentration of this region is below our detection limit (∼1 × 1019 atoms/cm3). It starts to increase at 13 µm and continues to increase to the substrate side. The p-i structure is thus achieved. The reason that B concentration of the p-type region is not flat but exhibits position dependence will be discussed later. The p-i profile is strongly modified by annealing at 1100 °C (Figure 4a). The coupling strength values of the intrinsic region increase significantly and the active B concentration becomes almost flat over the whole region. The same phenomenon was observed for the samples with different B concentration (B2H6 ) 12.5 sccm). The observed complete corruption of the p-i structure by 1100 °C annealing is considered to be induced by efficient diffusion of B atoms from the p-type region to the intrinsic region.17 Probably, B atoms diffuse along the side wall

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Figure 4. Coupling strengths (1/q) as a function of the distance from the catalyst for (a) annealed and (b) oxidized p-i SiNWs.

Figure 6. Typical Raman spectra of as-grown i-p SiNWs. Spectra are measured from the catalyst side to the substrate side by changing the measurement position by 1 µm for each step. The distance measured from the catalyst is shown in the figure. (a) and (b) are obtained from different SiNWs. Spectral shape of the substrate side is different between (a) and (b). The probabilities to find type (a) and type (b) SiNWs are almost the same. Inset in (a) is the expansion of the peak at the substrate side.

Figure 5. Typical Raman spectra of p-i SiNWs oxidized at different temperatures and durations: (a) oxidized at 950 °C for 1 min, (b) oxidized at 1100 °C for 1 min, and (c) oxidized at 1100 °C for 5 min. Spectra are measured from the catalyst side to the substrate side by changing the measurement position by 1 µm for each step. The distance measured from the catalyst is shown in the figure.

of SiNWs. Further studies are necessary to understand the mechanism of efficient B diffusion during the high temperature annealing. We also study the effects of oxidation on the distribution of active B concentration. Figure 5 shows Raman spectra of p-i SiNWs oxidized at different temperatures and durations: (a) 950 °C for 1 min, (b) 1100 °C for 1 min, and (c) 1100 °C for 5

min. The experimental setup for oxidation is the same as that of annealing except for the ambience, i.e, O2 gas instead of N2 gas. Coupling strength values obtained from the fittings by eq 2 are shown in Figure 4b. When oxidized at 950 °C for 1 min, the p-i structure remains, although the maximum coupling strength is smaller than that of SiNWs annealed in N2 ambience at the same temperature (Figure 4a). With increasing the oxidation temperature and duration, the maximum values of the coupling strength become small, and after 5 min oxidation at 1100 °C, Fano resonance almost disappears. During oxidation, SiO2 grows from the side wall of SiNWs toward the core by incorporating B atoms. The observed drastic decrease of the B concentration implies that high B concentration layers exist on the surface of SiNWs. This is consistent with the surface doping model previously reported.13,17,18 In a previous work on B-doped p-type SiNWs,18 we demonstrated that B concentration depends on the distance from the catalysts and suggested that B atoms are mainly doped by conformal deposition of a high B concentration layer on the side wall of a low B concentration core grown by the VLS process.18 This model explains the gradient of B concentration in the p-type region of p-i SiNWs. The anomalous spectral shape of the as-grown SiNWs (Figure 3b) also evidence the surface doping model. The sharp symmetric spectrum comes from a low B concentration core and the Fano-type spectrum from a high B concentration shell. Intrinsic/p-Type Si Nanowires. Figure 6 shows Raman spectra of i-p SiNWs measured at different positions. Similar to p-i SiNWs, the spectral shape of as-grown SiNWs is distributed. Parts a and b of Figure 6 are two representative

p-Type/Intrinsic and Intrinsic/p-Type Si Nanowires

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Figure 8. Coupling strengths (1/q) as a function of the distance from the catalyst for (a) annealed and (b) oxidized i-p SiNWs.

Figure 7. Typical Raman spectra of i-p SiNWs annealed at different temperatures: (a) annealed at 950 °C for 1 min, and (b) annealed at 1100 °C for 1 min. Spectra are measured from the catalyst side to the substrate side by changing the measurement position by 1 µm for each step. The distance measured from the catalyst is shown in the figure.

spectra; amorphous components are relatively large in Figure 6a, while Fano resonance is seen in Figure 6b. It is very important to note that the amorphous component and the Fano resonance appear in the nominally “intrinsic” regions, while the spectra of the p-type region is rather symmetric. Figure 7a shows Raman spectra of an i-p SiNW annealed at 950 °C for 1 min. The spectra of the “intrinsic” region is much broader than that of the p-type region. The spectra of the “intrinsic” region cannot be well fitted by eq 1, and eq 2 is necessary to reproduce them. This means that the nominally “intrinsic” region consists of a low B concentration region and a heavily B-doped region. Figure 7b shows Raman spectra of an i-p SiNW annealed at 1100 °C for 1 min. Clear Fano resonance can be seen in the whole region and the spectra can be fitted by eq 1, indicating that B atoms are distributed over the whole region of the SiNW. Figure 8 shows the coupling strength obtained by fitting the spectra by eq 2 as a function of the distance from the catalyst. In the sample annealed at 950 °C (Figure 8a), the coupling strength increases as the distance from the catalyst increases in the p-type region. This behavior is similar to the p-type region in p-i SiNWs. In p-i SiNWs, the coupling strength was almost 0 in the intrinsic region (Figure 4). On the other hand, in the i-p SiNWs, the coupling strength is very large in the nominally “intrinsic” region. Therefore, the “intrinsic” region is heavily B-doped, although the supply of B2H6 is started at the middle of the growth. This result again supports the surface doping model. Although intrinsic SiNWs are grown at the first stage, a heavily B-doped layer is conformally deposited on the side wall of the intrinsic region during the growth of the p-type region. As a result, the whole region of i-p SiNWs are B-doped. Because the “intrinsic” region consists of an intrinsic SiNW core and a heavily B-doped shell, the spectral shape becomes

Figure 9. Typical Raman spectra of i-p SiNWs oxidized at different temperatures and durations: (a) oxidized at 950 °C for 1 min, (b) oxidized at 1100 °C for 1 min, and (c) oxidized at 1100 °C for 5 min. Spectra are measured from the catalyst side to the substrate side by changing the measurement position by 1 µm for each step. The distance measured from the catalyst is shown in the figure. Insets in (a) and (b) are expansions of the peaks at the substrate side.

the combination of the Fano line shape and the Lorentzian function. In fact, if we closely look at the spectra of the “intrinsic” region in as-grown i-p SiNWs, we can see a splitting of the peak (inset in Figure 6a). The clear splitting of the peak indicates that B concentration does not change gradually to the radial direction but it changes rather abruptly at the core-shell interface.

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Figure 9 shows Raman spectra of i-p SiNWs oxidized at different conditions. The coupling strength are plotted in Figure 8b. Similar to p-i SiNWs, the coupling strength becomes small at higher temperature and longer duration oxidation. It is very interesting to note that the splitting of the peak in the “intrinsic” region remains even after oxidation at 950 and 1100 °C for 1 min (inset in Figure 9), indicating that the core-shell structure holds after oxidation. When oxidized at 1100 °C for 5 min, Fano resonance almost completely disappears, suggesting that the heavily B-doped shell is almost completely oxidized.

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References and Notes

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JP901679K

Conclusion By analyzing Fano-type asymmetric Raman spectra, we investigated the active B-concentration profiles of modulation doped p-type/intrinsic and intrinsic/p-type SiNWs grown by the VLS process. The analysis of Raman spectra of as-grown, annealed, and oxidized p-i and i-p SiNWs revealed that B atoms are mainly doped from the surface by conformal deposition of a heavily doped layer on the side wall of SiNWs. This results in strong inhomogeneity of B concentration along the radial and growth axis. The present results indicate that Raman spectroscopy is a powerful tool for the characterization of active impurities in SiNWs. Finally, it should be mentioned that the surface doping model proved in this work is not necessarily a general phenomenon of the VLS growth of SiNWs. In growth conditions different from the present work, especially when the dopant sources are different and the doping level is much lower, different doping processes may be possible. The different doping process may result in different B distribution. Acknowledgment. This work is supported by a Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.