Interaction of Boron and Phosphorus Impurities in Silicon Nanowires

Article pubs.acs.org/JPCC

Interaction of Boron and Phosphorus Impurities in Silicon Nanowires during Low-Temperature Ozone Oxidation Naoki Fukata,*,† Jun Kaminaga,†,‡ Ryo Takiguchi,‡ Riccardo Rurali,*,§ Mrinal Dutta,† and Kouichi Murakami‡ †

International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan ‡ Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan § Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de Bellaterra, 08193 Bellaterra, Barcelona, Spain ABSTRACT: In doped Si nanowires (SiNWs) boron (B) atoms segregate to the surface oxide layers during thermal oxidation, while phosphorus (P) atoms preferentially pile up in Si crystalline regions close to the Si/SiO2 interface. Here we report on micro-Raman scattering and electron spin resonance (ESR) measurements showing that B atoms can be stabilized at the crystalline Si core region in codped SiNWs with average diameters of 20−30 nm because of the strong interaction between B and P atoms during thermal oxidation below 800 °C. Theoretical calculation clearly demonstrated the effect of B−P pairing, which can stabilize the B atoms in the Si side. In the B−P pairing configuration, dopant passivationbeyond simple compensationoccurs, making the impurities electrically inactive.

1. INTRODUCTION Functionalization of semiconducting silicon nanowires (SiNWs) by impurity doping is one of the most important processes for their application as next-generation field effect transistors, sensors, and solar cells,1−6 and controlling dopant concentration, distribution, and electrical activation is essential for optimizing the performances of these applications. Many studies of the growth and characterization of SiNWs have been reported. 1−18 The states of dopant atoms have been characterized by various methods such as scanning tunneling spectroscopy (STS), near-edge X-ray absorption fine structure (NEXAFS), atom probe tomographic analysis, Raman scattering, electron spin resonance (ESR), and various electrical measurements.10−18 For example, STS measurements have provided current−voltage curves for SiNWs, which clearly showed an enhancement of the electrical conductivity by boron (B) doping.10 NEXAFS measurements showed that phosphorus (P) atoms are found to be inside the core of the SiNWs as well as at the interface between silicon oxide and core silicon, but not on the surface of the as-prepared nanowire.11 Atom probe measurements clarified the axial and radial distribution of dopant atoms in SiNWs,8 and the radial active dopants distribution within a single n-type SiNWs has been also investigated by Kelvin probe microscopy (KPFM) measurements.12,13 Raman and ESR measurements showed that boron (B) and P atoms are doped in the crystalline Si core of SiNWs, where they are electrically active.14−16 The carrier relaxation and diffusion in photoexcited single SiNW have been examined by ultrafast optical microscopy, although this is not the result of intentional impurity doping.17 © 2013 American Chemical Society

In addition to the studies of the chemical bonding, electronic states and electrical activities of dopant atoms in SiNWs, it is important to understand the behavior of dopant atoms during thermal oxidation, one of the important processes in the fabrication of gate dielectric films of ultralarge-scale integrated circuit (ULSI). In particular, the low-temperature formation of high quality gate dielectric film consisting of silicon dioxide (SiO2) has become important to reduce the thermal budget during the fabrication of ULSI devices. Thermal oxidation using ozone gas is one of the possible solutions to achieve a good quality of SiO2 layer at low temperature. Recently, the behaviors of dopant atoms in SiNWs during thermal oxidation were investigated in singly doped and codoped SiNWs with B and P atoms.19 The results of Raman scattering and ESR measurements convincingly showed that B atoms segregate to surface oxide layers during thermal oxidation, while P atoms preferentially pile up in the Si crystalline regions nearby the interface,19 as a direct result of the increased stabilty of B atoms in SiO2 rather than in Si, while conversely P atoms are more stable in Si than in SiO2. The same tendency was later confirmed by theoretical calculations.20 However, the effect of low-temperature oxidation using ozone gas and the interaction between B and P atoms have never been investigated in SiNWs. Understanding the interaction of impurity atoms is becoming increasingly important, since SiNWs have been extensively considered for the next-generation high-efficiency solar cell materials.21,22 Here, pn junctions are radially formed in each Received: July 8, 2013 Revised: September 9, 2013 Published: September 10, 2013 20300

dx.doi.org/10.1021/jp406713p | J. Phys. Chem. C 2013, 117, 20300−20307

The Journal of Physical Chemistry C

Article

Figure 1. (a) Schematic illustration of the ozone oxidation system. Photographs of (b) the UV lamp and (c) the sample stage with shade to suppress the direct UV irradiation are also shown.

Figure 2. High-resolutional images of SiNWs (a) before oxidation and after (b) ozone oxidation at 600 °C and (c) normal oxidation at 900 °C. The oxidation times are 180 min. (d) A schematic illustration of thermal oxidation of SiNWs. (e) Dependence of the thickness of the surface oxide layer on oxidation temperature.

2. EXPERIMENTAL AND THEORETICAL METHOD

nanowire by the controlled diffusion of impurities or by CVD growth. Therefore, in order to control the pn junction formation in SiNWs with small diameters by impurity diffusion, it is important to investigate the interaction between B and P atoms. In this paper, we report on the interaction between B and P atoms during thermal oxidation. We prepared SiNWs codoped with B and P and characterized the impurity interaction during thermal oxidation by Raman scattering and ESR measurements. The average diameter of as-grown nanowires is 20−30 nm. Here, the crystalline core diameter is about 15−20 nm and the thickness of oxide layer is 5 nm. In order to gain additional insight on the atomic scale mechanisms underlying such interaction, we performed first-principles electronic structure calculations of isolated P and B impurities as well as B−P aggregates. These results indicate that the B−P pairing can stabilize B atoms in Si crystal side in SiNWs, hindering their segregation to the oxide.

SiNWs were grown by catalytic laser ablation. Laser ablation was done at 1200 °C in flowing Ar gas at 50 sccm using a Si target with Ni as the metal catalyst and B and P as the dopant atoms. We used a frequency-doubled Nd:YAG laser (wavelength 532 nm). The laser power was at about 180 mJ per pulse with a pulse duration of 7 ns at 10 Hz. We used two different types of targets: Si89Ni1B10 and Si95(Ni2P)5. SiNWs codoped with both B and P were synthesized by ablating two targets at the same time. To investigate the effect of oxidation of SiNWs, specimens were thermally oxidized at 500−900 °C in O2 gas at a pressure of 200 Torr after the synthesis of the SiNWs. We also performed thermal oxidation using ultraviolet (UV) assisted ozone oxidation at 500, 600, and 800 °C. The schematic view of the setup is shown in Figure 1a. An UV lamp with a wavelength of 172 nm and power density of 17 mW/cm2 was used (Figure 1b). In order to avoid the direct UV irradiation and the direct bombardment of energetic ionized oxygen species, the specimens were put under the shade (Figure 1c). 20301



Article

Figure 3. (a) Raman spectra observed for B-doped SiNWs before and after normal oxidation at 900 °C. The inset is the magnification of the B local vibrational peaks. (b) ESR signal observed for P-doped SiNWs before and after normal oxidation at 900 °C. Dependence of (c) the peak intensity of 11 B local vibrational peak and (d) the ESR signal intensity of conduction electrons on thermal oxidation time.

diameter of the wires, as also pictorially illustrated in Figure 2d. This is clearly seen in Figure 2b where the ozone oxidation effectively increases the thickness of surface oxide layer, resulting in the decrease in the Si crystalline core region. It is noteworthy that thermal oxidation occurred at the low temperature of 600 °C. We also performed thermal oxidation at 500−900 °C in O2 gas atmosphere, which, hereafter, we refer to as normal oxidation. No significant changes were observed after normal oxidation below 600 °C, while the SiNWs were thermally oxidized at higher temperature than 600 °C as shown in Figure 2c. The enhanced oxidation rate by ozone oxidation at low temperature is due to the release of the first-excited state of oxygen atoms O (3P) from O3 by a thermal decomposition reaction at a Si surface.28,29 The dependences of the thickness of surface oxide layer on oxidation temperature are summarized in Figure 2e. As it can be seen, the thickness of oxide layer does not significantly depend on the oxidation temperature for the ozone oxidation. We considered different scenarios to explain our observations. First, we considered the effect of the decrease in the mean free pass of the first-excited state of oxygen atoms O (3P) at higher temperatures. However, the temperature in the atmosphere does not significantly change if the substrate temperature is increased. Hence, this effect must be ruled out. Second, we considered the rate limiting effect of O3 cleavage during the oxidation. According to a previous study of ozone oxidation in bulk Si, the thickness of the oxide layer increased with the ozone oxidation temperature.28 Therefore, it is also difficult to explain our results with the rate limiting effect of O3 cleavage. Finally, we considered the self-limiting oxidation

Micro-Raman scattering measurements were performed at RT with a 100× objective and a 532 nm excitation light. The excitation power was set to about 0.01−0.02 mW to prevent local heating.23 ESR measurements were carried out at 4.2 K using an X-band ESR spectrometer with a magnetic field modulation of 100 kHz to investigate the state of P donors and defects in the SiNWs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the structure of the SiNWs. The first-principles electronic structure calculations were performed within density-functional theory (DFT) as implemented in the SIESTA code.24 We used a double-ζ polarized basis set to expand the one-electron wave function and accounted for the core electrons via norm-conserving pseudopotentials. The exchange-correlation energy was calculated within the generalized gradient apporximation (GGA).25 We modeled the Si/SiO2 interface using the α-quartz crystalline structure of the oxide in slab geometry (see Figure 5a). Previous works showed that the conclusions are not qualitatively modified by considering explictely the amporphous nature of the oxide or the nanowire geometry.20,26,27 All the structures discussed were relaxed until the forces on the atoms were lower than 0.04 Å/eV.

3. RESULTS AND DISCUSSION 3.1. Structual Change by Thermal Oxidation. Representative TEM images of SiNWs before and after ozone oxidation at 600 °C are displayed in Figure 2a,b and nicely show how the oxidation time was used to control the effective 20302



Article

9.5, respectively. The electrically active B concentration can be deduced to be on the order of 1019 cm−3 using these fitted values. The B local vibrational peak and the Fano broadening in the Si optical phonon peak significantly decreased with increasing annealing time by the normal oxidation at 900 °C. This happens because of the segregation of B atoms into the surface oxide layer during thermal oxidation.19 The normal oxidation at high temperature of 900 °C introduces Si selfinterstitials into crystalline Si core of SiNWs.31−37 These two effects, high temperature annealing and introduction of defects, enhance the segregation of B atoms into the surface oxide layer. Figure 3c shows the comparison between the normal oxidation and the ozone oxidation at 600 °C in addition to the data obtained for the normal oxidation at 900 °C. The intensity of B local vibrational peak and the Fano broadening did not show the significant decrease by the normal oxidation at 600 °C, while a gradual decrease was observed for the ozone oxidation. This is due to the enhanced oxidation by using ozone. The results of Figure 3c are in good agreement with those of TEM measurements for the sample after the normal and the ozone oxidation at 600 °C. The same analysis was carried out for P-doped SiNWs as shown in Figure 3b,d. The g-value of 1.998 corresponds to that of the conduction electrons in Si, clearly indicating that P atoms were doped in the substitutional sites of the crystalline Si core of SiNWs during the growth and are electrically activated in these sites.15,16 The electrical active P donor concentration can be roughly estimated to be about 1019 cm−3 by the analysis of the line width.15,38,39 This value is of the same order of the electrical active B concentration in singly B-doped SiNWs. The other ESR signal at 2.006 is attributed to interfacial defects between the Si core and the surface oxide of SiNWs, the socalled Pb centers.15,16 The intensity of conduction electron signal was not affected by the normal oxidation at 600 °C, while it was gradually decreased by the ozone oxidation at the same temperature. This is also due to the enhanced oxidation with ozone. The reduction of the conduction electron signal is due to the deactivation of P donors caused by the segregation of P atoms. Two possibilities are considered for the deactivation of P donors. The first is due to the formation of P donor pairs, as also reported by theoretical calculations.20,40 The second is due to the segregation from the Si core region to the surface oxide layer during thermal oxidation. A new signal with a g-value of 2.002 appeared. This ESR signal is probably due to defects in the surface oxide of SiNWs, the EX centers.41,42 The reduction of the ESR signal of conduction electrons is slower than that of the intensity of B local vibrational peak by the normal oxidation at 900 °C, showing the segregation of P atoms is much slower than that of B atoms. This means that P atoms tend to accumulate in the Si region around the interface of SiNWs, whereas B atoms preferentially segregate in SiO2.19 For B, this tendency is also in good agreement with the recent result of theoretical calculations.20 On the other hand, the annealing behaviors of B and P atoms by the ozone oxidation at 600 °C did not show a big difference. As already described, the B segregation can be enhanced by the reaction with Si selfinterstitials compared with P segregation. The Si selfinterstitials produced by oxidation processes can easily diffuse into Si crystalline region and effectively interact with B atoms in deeper region due to the high temperature. The ozone oxidation increases the thickness of oxide layers even at the low temperature of 600 °C. However, the reaction between Si self-interstitials and B atoms is expected to be low at 600 °C.

Figure 4. Dependences of the ESR signal intensity of conduction electrons and the intensity of B local vibrational peaks as a function of layer-by-layer HF etching time. The intensities after etching were normalized by those before etching. Furthermore, the intensity of B local vibrational peak was normalized by the intensity of the Si optical phonon peak and the volume of SiNWs. The inset illustrates the etching process.

effect. The promotion of oxidation induces compressive stress in SiNWs, and the formation of such a highly stressed oxide increases the activation energy of oxidant diffusivity, showing a resultant decrease in oxidation rate due to the self-limiting oxidation effect.30 Ozone oxidation forms thick oxide layer on the surface of SiNWs at lower temperature. Hence, the selflimiting oxidation easily occurred during the low-temperature ozone oxidation, resulting in the almost flat curve of Figure 2e (red symbols), and we thus conclude that it is the most likely cause of our observations. 3.2. Segregation Behaviors of B and P in SiNWs. In order to investigate the segregation behaviors of B and P atoms in SiNWs, we performed Raman scattering and ESR measurements. Raman measurements clarify the bonding structures and the electrical activity of B atoms through the observation of B local vibrational peaks and the Fano broadening in the Si optical phonon peak, while ESR measurements clarify the status of P atoms through the observation of conduction electron signals. Figure 3a,c shows the results of Raman measurements for B-doped SiNWs. The intense peak at around 519−520 cm−1 is due to the Si optical phonon peak, and the peaks observed at 618 cm−1 and 640 cm−1 have been assigned to the local vibrational peak of 11B and 10B in SiNWs.14,15 The peak intensity of B local vibrational peak was normalized by that of Si optical phonon peak, and therefore it corresponds to the B concentration in SiNWs. The Si optical phonon peak showed an asymmetric broadening toward higher wavenumbers. This is due to the Fano effect by high concentration B doping in SiNWs.14,15 The two important parameters, q (the asymmetry parameter) and Γ (the line width parameter), obtained by fitting the Fano equation, were estimated to be about 14.0 and 20303



Article

Figure 5. (a) Dependence of the peak intensity of 11B local vibrational peak on ozone oxidation time. The ozone oxidations were performed for codoped SiNWs at 600 and 800 °C. The data for B-doped SiNWs oxidized at 600 °C are also shown in the dotted line. Dependence of (b) the ESR signal intensity and (c) the width of conduction electrons for codoped SiNWs on ozone oxidation time.

Therefore, the decrease of B local vibrational peak mainly corresponds to the distribution of B atoms in SiNWs. On the other hand, the P segregation is not enhanced by the Si selfinterstitials. Hence, no big differences were observed in the segregation behaviors between B and P at 600 °C. 3.3. Radial Dopant Profiles of B and P in SiNWs. The initial dopant profiles are important to discuss the segregation and interaction behaviors of dopant atoms in SiNWs during thermal oxidation. The distribution of P atoms in SiNWs has been reported previously.19 We also performed the same layerby-layer etching experiments using HF solution and Raman measurements to investigate the distribution of B atoms in SiNWs. The result is shown in Figure 4 in addition to the data of P atoms. The distribution of B atoms also showed the almost same tendency as that of P atoms. The B local vibrational mode for the as-grown nanowires is of the same strength for the singly and codoped situations. This suggests that the B concentration is almost the same for the singly and codoped SiNWs. As for the codoped SiNWs, it is impossible to investigate the distribution of P atoms from our layer-by-layer etching experiments since the conduction electron signal completely disappeared by the compensation of B atoms. However, the distributions of P and B atoms are expected to be largely overlapping because (i) the distribution of P and B

atoms are almost the same in singly P and B doped SiNWs shown in Figure 4, (ii) P and B compensate each other, and (iii) the average diameter is only 20 nm; thus, it is difficult to imagine that P and B have significantly different profiles in the as-grown wire. 3.4. Interaction between B and P Atoms in Codoped SiNWs. In order to investigate the interaction between B and P atoms in SiNWs, the ozone oxidation was performed for SiNWs codoped with B and P atoms. We also applied same techniques of Raman and ESR measurements. The results are summarized in Figure 5. The peak intensity of the B local vibrational peak did not decrease at 600 °C, but rather slightly increased as shown in Figure 5a,b,d. We performed several experiments to assess the reproducibility of these results, and no significant deviations were observed. As already reported and explained, the total number of B atoms is not significantly changed by codoping,43 and the distribution of B and P atoms is expected to be very similar in the codoped SiNWs. The intensity of B local vibrational peak was normalized by that of the Si optical phonon peak. If the ratio of B to Si increases by the reduction of Si atoms due to thermal oxidation, the B concentration increases in the SiNWs. On the basis of these points, we concluded that this slight increase means that the concentration of B atoms increased. Hence, the result of Figure 20304



Article

atoms to surface oxide layer which becomes again allowed by the lack of interaction between B and P, resulting in the reactivation of P donors in the codoped SiNWs. Additionally, the line width of ESR signal of conduction electrons increased with increasing oxidation time (Figure 5f). This is due to the increase in the electrical active P concentration in codoped SiNWs and also means the reactivation of P donors in codoped SiNWs.15,38,39 3.5. Theoretical Modeling of B−P Pairing in Codoped SiNWs. We performed first-principles electronic structure calculations to validate the picture that emerges from the experimental observations, i.e., the interaction between B and P atoms during the ozone oxidation at 600 °C. First we performed theoretical calculations for Si singly doped with B and P atoms, respectively (Figure 6). In agreement with recent theoretical calculations,20,26,27 we found that B atoms are more stable in the SiO2 side by constructing a split interstitial configuration at a Si site. On the Si side of the interface, on the other hand, B most stable configuration is a substitutional− interstitial complex, where a B impurity substitutes a host lattice site and interacts with the displaced Si atom that occupies a neighboring tetrahedral site. In bulk Si this is a very stable defect, and kick-outobserved for instance in SiC44does not occur. However, if one of these configurations happens to be close to the interface, B segregation is favored, the Si interstitial takes back its own lattice site, the B atoms move to an interstitial position in the SiO2, and 0.7 eV per diffusing B atom is gained45 (see Figure 6c). Next, we calculated the formation energy of a P substitutional in Si as a function of the distance from the interface finding that, in agreement with the experiments, P piles up at the interface (see Figure 6b). The segregation reaction of B is inhibited when a P at the interface, on the Si side, is present. As shown in Figure 5d, now the B atom has to overcome a barrier and increase its energy of approximately 0.6 eV to move across the interface and reach the SiO2. We have estimated the aggregation energy of the B−P complex on the Si side of the interface (left-hand side of Figure 6d) to be 1.49 eV. Other B−P aggregates considered consistently gave values in the 1−1.5 eV range. An important consequence of this mechanism is that B and P are going to be electrically inactive, in agreement with our experimental observations reported in Figure 5a,c,d,e. It is not a simple compensation, but rather dopant passivation due to the formation of a B−P chemical bond, as previously reported for SiNWs46 and SiC.47 These results are in good agreement with the experimental observation that during low-temperature ozone oxidation B atoms stay in the Si core (Figure 5a,b,d) and deactivate the P donors (Figure 5c,e), providing a strong evidence for B−P pairing in the codoped SiNWs. The formation of the B−P pairing is considered to effectively occur in NW structures because of the inhomogeneous distribution of dopant atoms in SiNWs. The distribution of dopant atoms in SiNWs becomes radially inhomogeneous due to the two reasons: vapor−solid (VS) deposition and the stress relaxation. If the VS deposition occurred during the growth of SiNWs, dopant concentrations become higher near the surface of SiNWs.8 Theoretical calculation also suggested that dopant atoms tend to locate at near surface region since the region allows a larger relaxation of the lattice.48 In these cases, dopant atoms in SiNWs easily form pair or clustering structures at near surface region and show different dopant dynamics during thermal oxidation compared to bulk Si.

Figure 6. (a) Computational atomic model of the Si/SiO2 interface. (b) Formation energies of P substitutionals as a function of their distance from the interface. Filled symbols correspond to the case where an O vacancy, a common point defect in SiO2, is present close to the interface. Segregation of a B impurity without (c) and with (d) a substitutional P nearby. The diffusion reaction (BSi−Sii)Si ⇒ Biox, where a B atom moves from a substitutional (BSi)−interstitial (Sii) complex in the Si side to become an interstitial on the oxide side (Bi) is favored by 0.7 eV (c), but has to overcome a barrier in presence of a P substitutional, leading to an increase of the total energy of 0.6 eV (d). Yellow, red, green, and blue spheres denote Si, O, B, and P atoms, respectively. Orange spheres denote Si self-interstitials.

5 shows that B atoms in codoped SiNWs tend to pile up in the Si region around the interface of SiNWs as well as P atoms in SiNWs during the ozone oxidation at 600 °C. Interestingly, the segregation behavior of B atoms in codoped SiNWs is quite different from SiNWs doped with only B (Figure 3a,c). This result indicates a non-negligible interaction between B and P atoms in codoped SiNWs, namely the formation of the B−P pairing. The peak shift of B local vibrational peak was not observed by the pairing of B and P by Raman measurements since the shift should be very small considering the slight difference of reduced mass of Si and P. The formation of the B−P pairing and the stability are corroborated by our theoretical calculations described below. The intensity of the B local vibrational peak decreased again at 800 °C with increasing oxidation time, and the tendency becomes close to the result observed for the ozone oxidation of B-doped SiNWs at 600 °C, showing that the interaction between B and P vanishes at 800 °C. The dependence of the conduction electron signal on the ozone oxidation time also corroborates the interaction between B and P at 600 °C (Figure 5c,e). The ESR signal of conduction electron was not observed by ozone oxidation at 600 °C, which indicates compensation by B acceptors, confirming that B atoms do not segregate to surface oxide layer but tend to pile up at Si side with P atoms. On the other hand, the ESR signal of conduction electrons was observed by ozone oxidation at 800 °C. The ESR signal intensity increased with increasing oxidation time and showed the anticorrelation with the intensity of the B local vibrational peak shown in Figure 5e. This is due to the segregation of B 20305



Article

(9) Bierman, M. J.; Lau, Y. K. A.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Dislocation-Driven Nanowire Growth and Eshelby Twist. Science 2008, 320, 1060−1063. (10) Ma, D. D. D.; Lee, C. S.; Lee, S. T. Scanning Tunneling Microscopic Study of Boron-Doped Silicon Nanowires. Appl. Phys. Lett. 2001, 79, 2468−2470. (11) Tang, Y. H.; Sham, T. K.; Jurgensen, A.; Hu, Y. F.; Lee, C. S.; Lee, S. T. Phosphorus-Doped Silicon Nanowires Studied by near Edge X-ray Absorption Fine Structure Spectroscopy. Appl. Phys. Lett. 2002, 80, 3709−3711. (12) Koren, E.; Berkovitch, N.; Rosenwaks, Y. Measurement of Active Dopant Distribution and Diffusion in Individual Silicon Nanowires. Nano Lett. 2010, 10, 1163−1167. (13) Koren, E.; Hyun, J. K.; Givan, U.; Hemesath, E. R.; Lauhon, L. J.; Rosenwaks, Y. Obtaining Uniform Dopant Distributions in VLSGrown Si Nanowires. Nano Lett. 2011, 11, 183−187. (14) Fukata, N.; Chen, J.; Sekiguchi, T.; Okada, N.; Murakami, K.; Tsurui, T.; Ito, S. Doping and Hydrogen Passivation of B in Silicon Nanowires Synthesized by Laser Ablation. Appl. Phys. Lett. 2006, 89, 203109. (15) Fukata, N. Impurity Doping in Silicon Nanowires. Adv. Mater. 2009, 21, 2829−2832. (16) Fukata, N.; Chen, J.; Sekiguchi, T.; Matsushita, S.; Oshima, T.; Uchida, N.; Murakami, K.; Tsurui, T.; Ito, S. Phosphorus Doping and Hydrogen Passivation of Donors and Defects in Silicon Nanowires Synthesized by Laser Ablation. Appl. Phys. Lett. 2007, 90, 153117. (17) Seo, M. A.; Yoo, J.; Dayeh, S. A.; Picraux, S. T.; Taylor, A. J.; Prasankumar, R. P. Mapping Carrier Diffusion in Single Silicon Core− Shell Nanowires with Ultrafast Optical Microscopy. Nano Lett. 2012, 12, 6334−6338. (18) Wang, Y.; Lew, K. K.; Ho, T. T.; Pan, L.; Novak, S. W.; Dickey, E. C.; Redwing, J. M.; Mayer, T. S. Use of Phosphine as an n-Type Dopant Source for Vapor−Liquid−Solid Growth of Silicon Nanowires. Nano Lett. 2005, 5, 2139−2143. (19) Fukata, N.; Ishida, S.; Yokono, S.; Takiguchi, R.; Chen, J.; Sekiguchi, T.; Murakami, K. Segregation Behaviors and Radial Distribution of Dopant Atoms in Silicon Nanowires. Nano Lett. 2011, 11, 651−656. (20) Kim, S.; Park, J.-S.; Chang, K. J. Stability and Segregation of B and P Dopants in Si/SiO2 Core−Shell Nanowires. Nano Lett. 2012, 12, 5068−5073. (21) Allon, I. H.; Peidong, Y. Semiconductor Nanowires for Energy Conversion. Chem. Rev. 2010, 110, 527−546. (22) Peng, K. Q.; Lee, S. T. Silicon Nanowires for Photovoltaic Solar Energy Conversion. Adv. Mater. 2011, 23, 198−215. (23) Piscanec, S.; Cantoro, M.; Ferrari, A. C.; Zapien, J. A.; Lifshitz, Y.; Lee, S. T.; Hofmann, S.; Robertson, J. Raman Spectroscopy of Silicon Nanowires. Phys. Rev. B 2003, 68, 241312(R). (24) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The SIESTA Method for ab Initio Order-N Materials Simulation. J. Phys.: Condens. Matter 2002, 14, 2745−2779. (25) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (26) Oh, Y. J.; Hwang, J.-H.; Noh, H.-K.; Bang, J.; Ryu, B.; Chang, K. J. Ab Initio Study of Boron Segregation and Deactivation at Si/SiO2 Interface. Microelectron. Eng. 2012, 89, 120−123. (27) Oh, Y. J.; Noh, H.-K.; Chang, K. J. First-Principles Study of the Segregation of Boron Dopants near the Interface between Crystalline Si and Amorphous SiO2. Physica B 2012, 407, 2989−2992. (28) Chao, S. C.; Pitchai, R.; Lee, Y. H. Enhancement in Thermal Oxidation of Silicon by Ozone. J. Electrochem. Soc. 1989, 136, 2751− 2752. (29) Nishiguchi, T.; Saitoh, S.; Kameda, N.; Morikawa, Y.; Kekura, M.; Nonaka, H.; Ichimura, S. Rapid Oxidation of Silicon Using UVLight Irradiation in Low-Pressure, Highly Concentrated Ozone Gas below 300 °C. Jpn. J. Appl. Phys. 2007, 46, 2835−2839. (30) Fukata, N.; Oshima, T.; Okada, N.; Kizuka, T.; Tsurui, T.; Ito, S.; Murakami, K. Phonon Confinement and Self-Limiting Oxidation

This kind of dopant passivation will become a crucial problem for the next-generation solar cell using SiNWs since pn junctions are expected to be radially formed in SiNWs and B and P atoms closely distribute in core−shell regions, respectively.

4. CONCLUSIONS In conclusion, the segregation behaviors of B and P atoms and the interaction between them were investigated by both experiments and theoretical calculations. For singly B-doped SiNWs, B atoms easily segregate to the SiO2 side. However, the B impurities can stay at the Si side by pairing with P atoms in codoped SiNWs during thermal oxidation carried out at less than 800 °C. These observations were corroborated by firstprinciples calculations which demonstrated that the B−P interaction stabilizes the B atom in the Si side, inhibiting its diffusion to the oxide. This phenomenon is not a simple compensation, but rather dopant passivation due to the formation of a B−P chemical bond. In SiNWs with smaller diameters, the B−P pairing seems to occur with a higher probability and show different dopant dynamics during thermal oxidation compared to bulk Si.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (N.F.). *E-mail [email protected] (R.R.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partly supported by the Innovation Research Project on Nanoelectronics Materials and Structures and the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan. This study was also supported by the Japan Science and Technology Agency (JST). Funding under Contract FIS2012-37549-C05-05 and computational resources at the Centro de Supercomputación de Galicia (CESGA) are greatly acknowledged.

■

REFERENCES

(1) Li, Y.; Qian, F.; Lieber, C. M. Nanowire Electronic and Optoelectronic Devices. Mater. Today 2006, 9, 18−27. (2) Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; Gösele, U.; Samuelson, L. Nanowire-Based One-Dimensional Electronics. Mater. Today 2006, 9, 28−35. (3) Pauzauskie, P. J.; Yang, P. Nanowire Photonics. Mater. Today 2006, 9, 36−45. (4) Fan, H. J.; Werner, P.; Zacharias, M. Semiconductor Nanowires: From Self-Organization to Patterned Growth. Small 2006, 2, 700− 717. (5) Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312, 242−246. (6) Rurali, R. Colloquium: Structural, Electronic, and Transport Properties of Silicon Nanowires. Rev. Mod. Phys. 2010, 82, 427−449. (7) Chockla, A. M.; Korgel, B. A. Seeded Germanium Nanowire Synthesis in Solution. J. Mater. Chem. 2009, 19, 996−1001. (8) Schlitz, R. A.; Perea, D. E.; Lensch-Falk, J. L.; Hemesath, E. R.; Lauhon, L. J. Correlating Dopant Distributions and Electrical Properties of Boron-Doped Silicon Nanowires. Appl. Phys. 2009, 95, 162101. 20306



Article

Effect of Silicon Nanowires Synthesized by Laser Ablation. J. Appl. Phys. 2006, 100, 024311. (31) Masetti, G.; Solmi, S.; Soncini, G. Oxidation-Rate Dependence of Phosphorus Diffusivity in Silicon. Philos. Mag. 1976, 33, 613−621. (32) Hu, S. M. Formation of Stacking Faults and Enhanced Diffusion in the Oxidation of Silicon. J. Appl. Phys. 1974, 45, 1567−1573. (33) Matsumoto, S.; Ishikawa, Y.; Niimi, T. Oxidation Enhanced and Concentration Dependent Diffusions of Dopants in Silicon. J. Appl. Phys. 1983, 54, 5049−5054. (34) Gossmann, H. J.; Rafferty, C. S.; Luftman, H. S.; Unterwald, F. C.; Boone, T.; Poate, J. M. Oxidation Enhanced Diffusion in Si BDoping Superlattices and Si Self-Interstitial Diffusivities. Appl. Phys. Lett. 1993, 63, 639−641. (35) Stolk, P. A.; Gossmann, H. J.; Eaglesham, D. J.; Poate, J. M. Implantation and Transient Boron Diffusion: the Role of the Silicon Self-Interstitial. Nucl. Instrum. Methods Phys. Res. 1995, B 96, 187−195. (36) Jain, S. C.; Schoenmaker, W.; Lindsay, R.; Stolk, P. A.; Decouter, S.; Willander, M.; Maes, H. E. Transient Enhanced Diffusion of Boron in Si. J. Appl. Phys. 2002, 91, 8919−8941. (37) Kageshima, H.; Uematsu, M.; Akagi, K.; Tsuneyuki, S.; Akiyama, T.; Shiraishi, K. Theoretical Study of Excess Si Emitted from Si-Oxide/ Si Interfaces. Jpn. J. Appl. Phys. 2004, 43, 8223−8226. (38) Maekawa, S.; Kinoshita, N. Electron Spin Resonance in Phosphorus Doped Silicon at Low Temperatures. J. Phys. Soc. Jpn. 1965, 20, 1447−1457. (39) Quirt, J. D.; Marko, J. R. Absolute Spin Susceptibilities and Other ESR Parameters of Heavily Doped n-Type Silicon. I. Metallic Samples. Phys. Rev. 1972, B5, 1716−1728. (40) Kim, Y. S.; Chang, K. J. Segregation of Nearest-Neighbor Donor-Pair Defects to Si/SiO2 Interfaces. Appl. Phys. Lett. 2005, 87, 041903-1−041903-3. (41) Baumer, A.; Stutzmann, M.; Brandt, M. S.; Au, F. C. K.; Lee, S. T. Paramagnetic Defects of Silicon Nanowires. Appl. Phys. Lett. 2004, 85, 943−945. (42) Wang, R. P. Defects in Silicon Nanowires. Appl. Phys. Lett. 2006, 88, 142104. (43) Fukata, N.; Mitome, B. Y.; Seoka, M.; Matsushita, S.; Murakami, K.; Chen, J.; Sekiguchi, T. Codoping of Boron and Phosphorus in Silicon Nanowires Synthesized by Laser Ablation. Appl. Phys. Lett. 2008, 93, 203106. (44) Rurali, R.; Godignon, P.; Rebollo, J.; Ordejón, P.; Hernández, E. Theoretical Evidence for the Kick-out Mechanism for B Diffusion in SiC. Appl. Phys. Lett. 2002, 81, 2989−2991. (45) We notice that according to our calculations segregation is favored even for neutral defects, while Oh et al.26 observe it only for charged defects. We attribute this difference to the large number of stable configuration for a B interstitial in the flexible network of crystalline α-quartz; thus, we might be comparing different B defects. (46) Moon, C.-Y.; Lee, W.-J.; Chang, K. J. Formation of Dopant-Pair Defects and Doping Efficiency in B- and P-Doped Silicon Nanowires. Nano Lett. 2008, 8, 3086−3091. (47) Rurali, R.; Godignon, P.; Rebollo, J.; Hernández, E.; Ordejón, P. First-Principles Study of n-Type Dopants and Their Clustering in SiC. Appl. Phys. Lett. 2003, 82, 4298−4300. (48) Peelaers, H.; Partoens, B.; Peeters, F. M. Formation and Segregation Energies of B and P Doped and BP Codoped Silicon Nanowires. Nano Lett. 2006, 6, 2781−2784.

20307


Interaction of Boron and Phosphorus Impurities in Silicon Nanowires

Recommend Documents