Predominant Si Doping through Au Catalyst Particles in the Vapor

Feb 3, 2011 - Doping of Self-Catalyzed Nanowires under the Influence of Droplets. Yunyan Zhang ... Koji Muraki. Japanese Journal of Applied Physics 20...
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Predominant Si Doping through Au Catalyst Particles in the Vapor-Liquid-Solid Mode over the Shell Layer via the Vapor-Phase Epitaxy Mode of InAs Nanowires Guoqiang Zhang,* Kouta Tateno, Satoru Suzuki, Hideki Gotoh, and Tetsuomi Sogawa NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan ABSTRACT: InAs semiconductor nanowires are expected to have applications in high-mobility nanoelectronics. Understanding the dopant distribution will be critical for the fabrication of high-performance devices based on nanowires. We study the n-type doping of InAs nanowires, using Si2H6 as a doping precursor, and clarify the predominant Si doping through Au catalyst particles. Using a series of segments in a single nanowire with a tapered shape and corresponding nanowire-channel field effect transistor characteristics, we show that the dopant atom incorporates predominantly via the Au-catalyzed vapor-liquidsolid mode, which accompanies the shell growth in the vapor-phase epitaxy mode. We determine the electrically active doping concentrations via the Au-catalyzed vapor-liquid-solid and vapor-phase epitaxy modes in the InAs nanowire to be 1.37  1018 and 1.57  1017 cm-3 under a In/Si source flow ratio of 600. This work developed a new method for the characterization of dopant distribution in semiconductor nanowires and the result provides more opportunities for the formation of modulation-doped core-shell nanowires and novel nanostructures.

1. INTRODUCTION Semiconductor nanowires (NWs) have a wide range of potential applications, ranging from electronics1,2 and photonics3 to biochemistry and medicine.4 They are expected to play an important role as functional device elements in nanoscale electronic devices. The electrical properties of semiconductor NWs depend strongly on their crystalline structure and composition. It is therefore important to measure their homogeneities in crystalline structure and composition at the level of single wires and to correlate these with NW dimensions and optical properties.5 In addition, to modulate the electrical properties of the semiconductor NWs, dopants must be introduced into NWs with controlled concentration. The doping technique mainly has two aspects: in situ or ex situ doping into NWs and measurement of dopant concentration and distribution in NWs. In situ doping can be achieved by introducing dopant sources during NW growth.6 Ex situ doping can be achieved by annealing NWs in a dopant ambient or by ion injection.7 However, measuring the carrier (dopant) concentration and distribution in NWs remains a challenge because the widely used Hall measurement (secondary ion mass spectroscopy, SIMS) is extremely difficult to perform on a one-dimensional object with a tiny diameter. Very recently, an atom-probe method has been developed to measure the dopant distribution in semiconductor NWs and Perea et al.8 used this technique to characterize the phosphors dopant distribution in Ge NWs. Scanning photocurrent microscopy9 and capacitance-voltage methods10 have also been used to measure the carrier distribution in NWs. The carrier concentration and mobility in semiconductor NWs can also be obtained by analyzing the transfer characteristics of NW-channel field effect r 2011 American Chemical Society

transistors (FETs).11 Raman spectroscopy has been used to characterize dopant distribution in Si12,13 and SiGe NWs.14 Up to now, research on the measurement of dopant distribution in NWs has focused on elemental semiconductor NWs, mainly Si and Ge NWs.7-14 Though III-V compound-semiconductor NWs have many applications in optical and highmobility devices, there have been few reports about the characterization of the dopant distribution in III-V NWs.15,16 As a high-mobility material, InAs is a good candidate for high-speed nanoelectronics.17-21 Recently, Tomioka et al.22 were able to grow vertical InAs NWs on Si substrate. This opens the way to the integration of high-mobility InAs into Si-based technology and provides more opportunities for InAs NW-based nanoelectronics. Consequently, it is necessary to study the dopant distribution and understand the doping behavior in individual InAs NWs for desired electrical properties. The particle-seeded method via the vapor-liquid-solid (VLS)23 mode has been widely used for NW growth.1-4 The NW usually consists of two parts grown via different modes: the core in the VLS mode and the shell in the vapor-phase epitaxy (VPE) mode on NW side surfaces.24 Owing to the different growth modes, dopant atoms have different ways of incorporating into the crystalline lattice, which may result in an inhomogeneous distribution of dopant concentration and different conduction types between the two parts in a single NW. Colombo et al.25 recently reported that Si dopant atoms in GaAs Received: July 13, 2010 Revised: November 26, 2010 Published: February 3, 2011 2923

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The Journal of Physical Chemistry C NWs show different conduction types in the regions grown by the VLS and VPE modes and that a p-n junction can therefore be naturally formed by one dopant. The difference in the concentration and conduction type of dopants in the VLS and VPE regions in NWs provides new opportunities for novel devices and new tools for building modulation-doped NW-based core-shell nanostructures.26 However, we still have very little understanding about the dopant distribution induced by the different growth mechanisms. Most research has focused on the synthesis and electrical characterization of InAs NWs;17-21 there has not been any study about the dopant distribution and carrier concentration in NWs along the axial and radial directions. In the present work, we studied the distribution of Si dopant concentration in individual InAs NWs grown by the Au-catalyzed VLS method and found that the Si dopant incorporates predominantly via the Au-catalyzed VLS mode, which accompanies the shell growth in the VPE mode. This work allows the formation of some modulation-doped core-shell NWs in a natural way and provides more opportunities for the growth of some novel nanostructures.

2. EXPERIMENTAL METHODS Commercially available 40-nm Au colloidal particles were dispersed on InAs (001) substrates by the spin-coating method to conduct NW growth. The InAs NWs were grown in a lowpressure (76 Torr) MOVPE system.24,27,28 The flow rates of trimethylindium (TMIn) and AsH3 were 9.6 and 200 μmol/min, respectively. Si2H6 was the Si dopant source, and the flow rate was 1.6  10-2 μmol/min for Si-doped InAs NWs. The substrate temperature was 460 °C. The growth time was 5 min. The NWs were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for morphological and structural characterizations, respectively. To determine whether the Si atoms incorporate predominantly through the NW side surface in the VPE mode or through the Au catalyst in the VLS mode, we designed a series of experiments. We prepare Si-doped InAs NWs with a tapered shape and the tapered NW consists of core and shell parts grown in the VLS and VPE modes, respectively. Normally, the VPE region increases from the tip of the NW. Assuming there are constant dopant concentrations in VLS and VPE parts, the dopant distribution along the NW axial direction should exhibit an increasing or decreasing tendency, depending on the incorporation efficiency of dopant atoms in the core VLS and shell VPE parts. About the measurement method of the dopant distribution, we used the technique that field-effect carrier concentration and mobility can be estimated by analyzing the transfer characteristics of NW-channel FET.17-21 At room temperature, the carrier concentration is roughly equal to the dopant concentration. Figure 1 shows a schematic diagram of a tapered NW-channel FET device. To quantitatively assess the doping along the NW growth direction, we fabricated NW-channel FET devices with multiple parallel metallic contacts spanning the NW, as shown in Figure 1. Each pair of adjacent contacts along with the back gate forms a NW-FET whose characteristics depend on the dopant concentration of the segment. Data about the carrier concentration of each segment can be obtained from their transfer characteristics. By establishing a growth model of doped NWs, we can obtain an equation describing the dopant distribution along the NW growth axis. Using the equation, we can fit the experimental data and then

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Figure 1. Schematic diagram of a tapered NW-channel FET device with multiple electrodes. The NW consists of VLS and VPE parts and there is an increasing ratio of VPE growth from the tip of the NW. If it is assumed that there are constant dopant concentrations in VLS and VPE growth, the dopant distribution along the axial direction exhibits an increasing or decreasing tendency, depending on the incorporation efficiency of dopant atoms in VLS and VPE parts. The carrier concentration of each segment can be estimated from its transfer characteristics. At room temperature, the carrier concentration is roughly equal to the dopant concentration. By this method, we can study the dopant profiling along the NW axis and clarify the difference in the doping efficiency between VLS and VPE parts.

estimate the dopant concentration by extrapolating the equation of the fitting curve. By this technique, we can study the dopant profiling along the NW axis and clarify the difference in doping efficiency in the core VLS and shell VPE parts.

3. RESULTS AND DISCUSSION First, we need the doped InAs NWs with both core VLS and shell VPE parts. By optimizing the growth conditions, we obtained tapered Si-doped InAs NWs with pronounced VPE growth. Figure 2a-c shows cross-sectional SEM images of Sidoped InAs NWs grown on an InAs (001) substrate. The diameters near the root and tip of the NW are 160 and 40 nm, respectively. A significant difference in diameter between the root and tip indicates pronounced growth in the VPE mode on the NW sidewalls. Figure 2d is a schematic diagram of a tapered NW with the core VLS and radial VPE parts. Figure 3a shows a TEM image of an InAs NW dispersed on a grid, and panel b shows an enlarged image of the area in the white rectangle. Figure 3c shows a HRTEM image of the NW. Lattice fringes can be clearly seen in the image. The diffraction pattern indicates that the NW has a wurtzite crystal structure and the growth direction is c-orientation. Note also that there are no stacking faults in the NW. The stacking fault may have some effect on dopant distribution along the axial and radial directions. The singlecrystalline feature of the NW ensures the homogeneous incorporation efficiency of dopant atoms for each growth mode. To probe the electrical properties and doping characteristics of the InAs NWs, we fabricated InAs NW-channel FET devices with metallic contacts. NWs suspended in ethanol were deposited on a degeneratively doped Si wafer covered with 500-nm SiO2 dielectric film. Contact electrodes (Ni/Au 20/180 nm) were defined by e-beam lithography and metal evaporation. Before being loaded into the chamber for metallic deposition, the NWs were chemically immersed in a solution of (NH4)2Sx for 20 s to remove the surface oxides and passivate the surface by forming a sulfide termination. Devices were annealed in H2 gas at 300 °C for 30 s by rapid thermal processing (RTP). All electrical measurements were performed in a probe station at room 2924

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Figure 2. Cross-sectional SEM images of InAs NWs epitaxially grown on a InAs (001) substrate. The InAs NWs were grown along the two [111] directions and are therefore inclined to the (001) substrate. The regions near the tip and root of a NW, as indicated by two dashed rectangles in panel a, are shown in panels b and c, respectively. The diameters near the root and tip of the NW are 160 and 40 nm, respectively. The difference in diameter between the root and tip is large, indicating pronounced growth by the VPE mode on the NW sidewalls. (d) Schematic diagram of the NW structure with both Auseeded VLS and shell VPE regions.

temperature. Figure 4 shows a SEM image of a Si-doped InAs NW-channel FET with multiple Ni/Au metallic electrodes on a single InAs NW. The device characteristics used here are (i) the drain current (Id) versus the applied drain-source bias (Vds) at fixed values of the gate source bias (Vgs) (output characteristics) and (ii) Id versus Vgs at fixed values of Vds (transfer characteristics). Because NW FET characteristics typically exhibit gate hysteresis, for consistency we always measure the transfer characteristics by sweeping the back gate bias from -4 to 4 V. The contact performance of these metallic electrodes is examined by comparing two- and four-terminal measurements. Figure 5a shows the two- and four-terminal measurement results for a segment of the InAs NW indicated by the white arrow. There is a very small difference between them. The contact resistance was estimated to be 600 ( 100 Ω, which is very small compared with that of the NW. When it is taken into account that the contact area is extremely small, the specific contact resistivity is fairly low, ∼2.0  10-7 Ω 3 cm2. Such low contact resistivity ensures that the effect on electrical measurements induced by the contact resistance is negligible. Figure 5b shows a typical Id-Vgs curve of an InAs NW-channel FET under drain voltages ranging

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Figure 3. Structure of InAs NWs characterized by TEM. (a) A single InAs NW dispersed on a TEM grid. (b) Enlarged image of the area in the white square in panel a. (c) High-resolution TEM image of the area in the gray square in panel b. Lattice fringes can be clearly seen in panel c. (Inset) Corresponding diffraction pattern. The pattern indicates the crystal structure is wurtzite and the growth direction is c-orientation. The NW is purely wurtzite without any planar defects including stacking faults.

Figure 4. SEM image of a NW-FET with multiple Ni/Au (20/180 nm) metallic electrodes on a single InAs NW. The substrate is SiO2 (thickness 500 nm)/ Si.

from 20 to 100 mV with a step of 20 mV. Gate dependence can be clearly seen and the NW-channel FET works in the n-channel depletion mode. Figure 6 shows the transfer characteristics of each segment of the NW shown in Figure 4. Each segment shows an n-type 2925

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radius for our device. Consequently, the carrier density and mobility at Vg = 0 V can be estimated by the following two equations: N¼

2εε0 Vth nð2h=rÞer 2

μ ¼ ln ð2h=rÞ=ð2πεε0 Vsd Þ

Figure 5. (a) Two- and four-terminal measurements of a segment of the InAs NW indicated by the white arrow. The contact resistance is very small compared with that of the NW. (b) Typical Id-Vgs characteristics of a InAs NW-channel FET under different drain voltages (Vds). Gate dependence is observed.

feature from the Id-Vg characteristics under a series of drain voltages. The transconductance at the zero gate voltage shows an increasing tendency from the tip of the NW. Assuming that transport in the NW is diffusive at room temperature; the field effect mobility of carriers can be estimated from the transconductance of the FET. The carrier density (N) in the NW can be calculated as18 N ¼ Cg ðVg -Vth Þ=eL ð1Þ where Cg is the gate capacitance, Vg is the gate voltage, Vth is the threshold voltage, e is the electron charge, and L is the NW length. Since the NW conductance G = Neμ/L = Cg(Vg - Vth)μ/L2, the mobility of carriers, μ, can be estimated from the transconductance ð2Þ dG=dVg ¼ μCg =L2 When the NW diameter is much smaller than the gate oxide thickness, Cg can be approximated as ð3Þ Cg ¼ 2πεε0 L=ln ð2h=rÞ where ε = 3.9 is the dielectric constant of the SiO2, ε0 is the permittivity of free space, h = 500 nm is the oxide layer thickness, L = 2.62 μm is the channel length of each segment, and r is the NW

ð4Þ dId dVg

ð5Þ

where the threshold gate voltages of each segment can be extracted from the Id-Vg characteristics and dId/dVg is the transconductance at the zero gate voltage (see Figure 6). The values of average radius of four segments from the tip to the root of the NW are 32.2, 45.4, 58.5, and 71.6 nm, respectively. Using eqs 4 and 5, we extract the carrier (i.e., electron) density and mobility of the InAs NW segment in Figure 6. Figure 7 plots the electron density (N) as a function of distance from the NW tip. The mobility varies in a small range [(1.26-1.43)  103 cm2 3 s-1 3 V-1] and exhibits a small decreasing tendency along the NW growth direction. The carrier concentration exhibits an increasing tendency along the NW growth direction. This is in good agreement with the small decreasing tendency of the mobility since carrier scattering by dopant atoms is enhanced with increasing dopant concentration. Compared with recent electron mobility values reported for other n-channel InAs NW FETs, our values are lower. Bryllert et al.19 reported an electron mobility of 3000 cm2/(V 3 s) in vertical InAs NW transistors with a cylindrical gate structure, and Dayeh et al.20 reported a value of 2740 cm2/(V 3 s) for back-gated InAs NW FETs (The mobility value was calculated by the same method used in this work for comparison with our data.) The electron mobility values are approximately twice as high as ours. Notably, the InAs NWs in those reports were also grown by the MOVPE system, but the NWs are nominally undoped. The reduction of the electron mobility in our Si-doped InAs NWs is due to the scattering of ionized Si dopant atoms in the Si-doped InAs NWs. On the other hand, the InAs segments exhibit a relatively large negative threshold voltage of 6-10 V, which also indicates a relatively high level of ionized dopants in the NWs. Note that the electron mobility values in Figure 7 are much lower than that of the bulk. Presumably, the mobility in NWs is significantly degraded by the surface scattering because surface states are usually formed due to the existence of dangling bonds on the NW surface. Jiang et al.21 reported an electron mobility as high as 11 500 cm2/(V 3 s) at room temperature in InAs/InP core-shell NWs, indicating the NW surface should be responsible for the low mobility and a capping layer is very effective to improve the mobility. The mobility may also be decreased due to the wurtzite crystal structure of the Si-doped InAs NWs because the wurtzite structure has larger effective electron mass than that of zincblende structure.29,30 To describe the distribution of carrier concentration along the NW axial direction shown in Figure 7, we established a simple growth and doping model. Regarding the NW growth and doping, we have three assumptions: (i) Very few Au atoms into NWs are incorporated into NWs so the shrinkage of the core VLS part induced by the consumption of Au atoms can be neglected. (ii) The diffusion length of the dopant source molecule (Si2H6) along the NW sidewalls is long enough to ensure a constant doping efficiency in both VLS and VPE parts with growth. (iii) The diameter of the NW has a linear variation along the axial direction and the diameter of each segment can be 2926

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Figure 6. Output and transfer characteristics, Id-Vg at various Vsd values and g-Vg at Vsd = 0.1 V, of each segment of the InAs NW shown in the SEM images (white arrows).

About assumption ii, up to now, there have been no data about the diffusion length of Si2H6 along the InAs NW side face. Many researchers believe that the adatoms and (or) molecules on the NW side face normally have a fairly high diffusion length because the NW side provide a very smooth surface.31,32 On the other hand, the Au-catalyzed VLS growth may be accompanied by a Au-rich monolayer on the NW side surface,33 which can also enhance the migration of adatoms and (or) molecules on the NW side face. Therefore, we think the assumption is reasonable. Figure 8a is a schematic diagram of dopant incorporation pathways and the NW growth model. In the model, there are homogeneous doping concentrations in the VLS and VPE parts and the core VLS part has a uniform diameter. On the basis of the geometric shape of the tapered NW measured by SEM, one can obtain the following equation of the carrier concentration (N): Figure 7. Plot of N and as a function of distance from the NW tip. (Inset) Schematic diagram of Si-doped InAs NW. Diameter fluctuation in each NW segment results in the error bars for N. Undoped NWs usually show a mobility of 3  103 cm2/(V 3 s).17,18 In these Si-doped InAs NWs, electron mobility is in the range of (1.29-1.53)  103 cm2/(V 3 s). The reduction of the electron mobility is caused by scattering of the ionized Si dopant atoms. The increasing tendency of N along the growth direction is in good agreement with the small decrease in electron mobility.

estimated. According to the top-view images taken by SEM for the NWs grown on (111) substrates, the InAs NWs typically exhibit a hexagonal cross section. Our TEM characterization indicates that the InAs NWs are wurtzite-structured. So the InAs NWs consist of six identical side faces.

NðxÞ ¼

ðNVLS -NVPE Þ þ NVPE ð1 þ 0:21xÞ2

ð6Þ

where x is the distance from the NW tip (in micrometers) and NVLS and NVPE are carrier concentrations in the VLS core part and radial VPE part, respectively. The factor of 0.21 is decided by the shape parameters of the Si-doped InAs NW; namely, the length and the diameters at the tip and root of the NW. We fit the experimental data in Figure 7 using eq 3, and the fitting curve of the distribution of carrier concentration is shown in Figure 8b. It can be seen that the experimental data is well fitted by the model. The equation of the fitting curve is 12:12 þ 1:57 ð7Þ NðxÞ ¼ ð1 þ 0:21xÞ2 2927

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Figure 9. Schematic diagram of Si doping pathways via the VLS and VPE modes. The purple circles represent Si source molecules and Si atoms in the InAs lattice. For the doping pathway in the VPE mode, the molecules adsorb on the NW sidewalls, migrate, and decompose, and finally dopant atoms incorporate into the InAs crystal lattice. Desorption always occurs throughout the process. The steps are indicated by the dotted arrows. Si incorporation via Au particles comprises several steps: absorption and dissolvation of source molecules into the Au particle, transport of Si to the liquid-solid interface, and incorporation into the InAs crystal lattice, as indicated by the solid arrows.

Figure 8. (a) Schematic diagram of dopant incorporation pathways and the NW doping model. NVLS and NVPE are carrier concentrations in the VLS core part and radial VPE part, respectively. The diameters near the tip and root of the NW are 38 and 180 nm, respectively. The NW length is 18 μm. (b) Fitting of N as a function of distance from the NW tip based on the model shown in panel a. (0) Experimental data; (---) fitting curve. The fitting result shows that NVLS and NVPE are 1.37  1018 and 1.57  1017 cm-3, respectively. (Inset) Schematic diagram of a tapered NW with VLS and VPE parts.

where the unit of N(x) is 1017 cm-3. By extrapolating the x in eq 7 to 0 and infinite distance from the NW tip, one can get the carrier concentration values in the VLS and VPE parts. The values of NVLS and NVPE are estimated to be 1.37  1018 and 1.57  1017 cm-3, respectively. Since the measurements were all performed at room temperature, the dopants are all ionized and the carrier concentration is approximately equal to the dopant concentration. Namely, the Si dopant concentrations in the VLS and VPE parts are 1.37  1018 and 1.57  1017 cm-3, respectively.

This result indicates a significant difference of dopant concentration between the core VLS and shell VPE parts. It also clarifies that Si atoms are more easily incorporated via the Au-seeded VLS mode than via the VPE mode on NW sidewalls. Consequently the carrier concentration in the core part is higher. The doping density in NWs, including VLS and CVD grown regions, is affected by the local distribution of doping sources, which depends on the NW density and homogeneity. If the NWs are randomly distributed on a substrate, a doping-density variation is probably caused. If NWs are regularly grown on a substrate with a defined pattern, like a hexagonal pattern, the distribution of doping sources become homogeneous and the doping-density variation in NWs should therefore become small. The InAs NWs studied in this work are not regularly aligned on the substrate, and we therefore found that not all NWs show same carrierconcentration values as the NW shown in Figure 4. But for individual NWs, the core region grown the VLS mode always shows a higher carrier concentration than the region grown by the VPE mode, indicating that the trend of the predominant doping through catalyst particles does not change because the local distribution of sources near each NW is homogeneous. To obtain a full understanding of the difference in the doping behavior of the VLS and VPE parts, we discuss the difference in the doping process during NW growth. Figure 9 is a schematic diagram of the Si doping pathways via the VLS and VPE modes. For the doping pathway in the VPE mode, the molecules are adsorbed on the NW sidewalls. Some of them desorb from the sidewalls; others migrate along the sidewalls, decompose, and are incorporated finally into the crystalline lattice. For the VPE mode, the desorption occurs very easily and the decomposition is also not complete.34 For the doping pathway via the VLS mode, the molecules are adsorbed on the surface of the Au particle and then decompose and dissolve into it. For the VLS mode, the desorption of the molecules from the Au particle hardly occurs and the dissolving process proceeds very quickly because the Au 2928

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The Journal of Physical Chemistry C particle during growth is in a liquidus state. The absorption and decomposition percentage of source molecules via the VLS mode are nearly 100%. The subsequent growth of Si-doped InAs NWs from the Au particle is indeed liquid-phase epitaxy, which proceeds under a near-equilibrium condition. As a result, the doping efficiency via Au particles strongly depends on the concentration of dopants in Au and the solubility of Si dopants in solidus InAs crystals. The segregation coefficient of Si in melt growth of InAs is close to 1,35 indicating that the Si dopant has relatively high equilibrium concentration in solid. The concentration of Si dopants in Au becomes, therefore, the key factor that decides the doping efficiency. At temperatures as low as 363 °C, Au and Si can form a eutectic phase with the atomic ratio of ∼18.5%. At the growth temperature of 460 °C of our Si-doped InAs NWs, the Si composition in liquidus Au is in the range of 17%-22% from the binary phase diagram.36 Even the solubility in nanoparticles is higher than that in bulk state due to the Gibbs-Thomson effect.24 Such high solubility of Si in Au catalyst ensures that Si source molecules absorbed on Au particles can be fully incorporated into the InAs NWs. Owing to the liquidus state of Au particles, the Si source molecules absorbed on the surface of Au particles are very difficult to desorb from the liquidus particles. Consequently, the incorporation of Si via the Au catalyst particles is quite easy compared with that via the VPE mode on the NW sidewalls. Noting that the mole ratio of TMIn to Si2H6 sources is 600, if the Si atoms ideally have a high incorporation efficiency into the InAs NW, as shown above, the ideal Si impurity concentration in the InAs NW grown by the VLS mode should be 2.2  1021 cm-3. However, the doping level via the Au-catalyzed VLS mode in the InAs NW is calculated to be 1.37  1018 cm-3, which is far lower than the ideal value. Probably two reasons should be responsible for the result: (i) Part of the Si atoms incorporated into the InAs NW are electrically inactive. The phenomenon often occurs in III-V compound semiconductors.37 (ii) The segregation coefficient of Si in Au solution growth of InAs seems to be fairly low compared with that of Si in the melt growth of InAs because the Si solubility in liquidus Au is in the range of 17%-22% in the atomic composition. Therefore, the Si concentration in InAs NWs is probably significantly lower than that in the Au particle. Notably, some dopant atoms show higher doping efficiency in the shell VPE part in Si and Ge NWs. Tutuc et al.11 reported that phosphorus (P) incorporates predominately via VPE growth on Ge NW sidewalls. Perea et al.8 confirmed by the atom-probe method that P has a higher concentration in the shell layer grown in the VPE mode. Kawashima et al.12 reported the growth of high-boron (B)-concentration layers by VPE on the sidewalls of Si NWs. In the present work, Si incorporates predominantly into InAs NWs via the Au catalyst particle. The doping distributions of P (B) in Ge (Si) NWs and Si in InAs NWs are noticeably different in the core VLS and shell VPE parts. This is mainly because of the extremely low solubility of B and P in Au. The solubility of B and P in Au is extremely low at temperatures lower than 900 °C.36 Therefore, the number of B (P) atoms doped into Si (Ge) NWs via Au catalysts is very limited. On the other hand, compared with the growth via Au catalysts, the B (P) atoms can be easily incorporated into Si (Ge) via the VPE mode on NW sidewalls. Active levels of B incorporation exceeding 1  1020 cm-3 have been achieved by in situ doping in Si epilayers by use of B2H6 and SiH4 sources.38 As a result, the B (P) exhibits higher doping concentrations in the shell layer of Si (Ge) grown by the VPE mode.

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4. CONCLUSIONS In conclusion, we studied the n-type doping of InAs NWs with Si2H6 as a doping precursor. Using different segments in a single NW along the growth direction and corresponding FET characteristics, we showed that Si dopant atoms incorporate predominantly via the Au-catalyzed VLS mode, which accompanies shell growth in the VPE mode. We determined the doping levels via the Au-catalyzed VLS and VPE modes in the InAs NW to be 1.37  1018 and 1.57  1017 cm-3, respectively. This work developed a new method for the characterization of dopant distribution in semiconductor NWs, and the result provides more opportunities for the formation of modulation-doped core-shell NWs and novel nanostructures. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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