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Surface chemical structure and doping characteristics of boron-doped Si nanowires fabricated by plasma doping
Accepted Manuscript Title: Surface chemical structure and doping characteristics of boron-doped Si nanowires fabricated by plasma doping Authors: Seung-Hoon Oh, Jin-Won Ma, Jung Min Bae, Yu-seon Kang, Jae-Pyung Ahn, Hang-Kyu Kang, Jimin Chae, Dongchan Suh, Woobin Song, Sunjung Kim, Mann-Ho Cho PII: DOI: Reference:
S0169-4332(17)31317-X http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.015 APSUSC 35950
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-6-2016 12-1-2017 2-5-2017
Please cite this article as: Seung-Hoon Oh, Jin-Won Ma, Jung Min Bae, Yu-seon Kang, Jae-Pyung Ahn, Hang-Kyu Kang, Jimin Chae, Dongchan Suh, Woobin Song, Sunjung Kim, Mann-Ho Cho, Surface chemical structure and doping characteristics of boron-doped Si nanowires fabricated by plasma doping, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dopant activations of plasma-doped (PD-) Si NWs are investigated with RTA time Chemical states and Fano factor of PD-Si NWs are changed with the dopant activation Both electrical and optical conductivity of PD-Si NWs increase greatly and coherently Carrier generation by PD process is more effective at 1D Si NW than 2D Si substrate
1
Surface chemical structure and doping characteristics of boron-doped Si nanowires fabricated by plasma doping
Seung-Hoon Oh 1, Jin-Won Ma 1,3, Jung Min Bae 1, Yu-seon Kang 1,3, Jae-Pyung Ahn 2, Hang-Kyu Kang 1, Jimin Chae 1 Dongchan Suh 3, Woobin Song 3, Sunjung Kim 3 and Mann-Ho Cho 1,* 1
Institute of Physics and Applied Physics, Yonsei University, Seoul 03722, Republic of Korea
2 3
Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
Process Development Team, Semiconductor R&D Center, SAMSUNG, Hwaseong-si 18448, Republic of Korea
Abstract We investigated the conduction characteristics of plasma-doped Si nanowires (NWs) after various rapid thermal annealing (RTA) times. The plasma doping (PD) process developed a highly-deposited B layer at the NW surface. RTA process controls electrical conductivity by mediating the dopant diffusion from the surface layer. The surface chemical and substitutional states of the B plasma-doped Si NWs were analyzed by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. To elucidate the detailed structure of the NWs, we analyzed the change in the optical phonon mode caused by the incorporated B atoms.
For this purpose, we examined Fano resonance by the
investigation of the asymmetry, line-width, and phonon wavenumber in Raman spectra. The changes in symmetry level of the Raman peak, phonon lifetime, and internal strain were closely related to the number of electrically activated borons, which was drastically increased with RTA time. The change in electrical and optical characterizations related to the doping characteristics of the NWs was investigated using a 4-point probe and terahertz time-domain spectroscopy (THz–TDS). The resistivity of the NWs was 3000 times lower after the annealing process compared to that before the annealing process, which is well consistent with the optical conductivity data. The data provide the potential utility of PD in conformal doping for three-dimensional nanodevices.
Key world: Plasma doping, Si NW, conductivity change
Correspondence and requests for materials should be addressed to M.-H. Cho ([email protected]) 2
Introduction The semiconductor industry has employed bulk silicon channels in metal-oxide-semiconductor field-effect transistors for several decades [1]. However, this industry has recently faced demands for high-performance, easily integrated, scaled-down devices [2]. In searching for new channels with higher mobility, the industry has explored various materials and nanoscale structures [3]. New channel materials, such as graphene and MoS2, and new structures, such as fin field-effect transistors (FinFETs) and all-around gate field-effect transistors [4-6], have been extensively researched. The especially high performance of FinFETs is attributed to the movement of many carriers on the three-dimensional (3D) interface between the gate oxide and channel when the gate voltage is applied [7]. Due to its 3D structure, the FinFET-based channel can be formed by a conformal doping process. In general, the performance of a FinFET depends on the dopant distribution in the near-surface region of the sidewalls. Therefore, a specific doping process is required to uniformly dope the conformal structure. At present, the electrical properties of semiconductors are usually enhanced by ion implantation (IIP) using accelerated ions. As an ex-situ doping system, IIP can control the depth profile of the dopants by tuning the ion energy [8]. However, IIP process contains some disadvantages to nanomaterials because the high kinetic energy of the accelerated ions would damage their surfaces [9]. Moreover, the non-isotropic beam direction of the doping method is unsuitable for conformal structures. To avoid surface damage, new doping methods have been attempted in FinFET technology.
Plasma
doping (PD) is expected to solve the problem of IIP because PD is an ex-situ conformal doping process that avoids surface damage in 3D structures and nanomaterials [10-11]. The number of active carriers on the channel surface depends on the density of PD. In intensive PD process, the dopant is easily activated by rapid thermal annealing (RTA) process because the dopant is somewhat already activated under the high plasma power. Many researchers have analyzed the doping characteristics of nanomaterials doped by PD; particularly, a FinFET is a model structure for this purpose. However, most of the FinFET research has highlighted the advantages of conformal doping, compared to IIP [12-13]. The surface chemical structure and structural properties of nanomaterials subjected to PD 3
have been rarely reported. The surface and interface characteristics of nanodevices can be closely related to the electrical properties because most nanodevices have a high surface-to-volume ratio [14]. Therefore, by controlling the surface charge carriers on nanostructures through PD, we can realize a blueprint for high-performance nanodevices. In this study, we activated plasma-doped Si nanowires (NWs) by RTA and investigated their electrical characteristics through their chemical states and structural changes on the surface as functions of RTA time. Under the RTA process, dopants located to the outer surface region was diffused the inner surface and simultaneously activated through the substitutional process. PD significantly improved the electrical properties of the nanomaterials, which is dependent on RTA times. The changes in the conductivity of the NWs were closely related to the chemical state of dopants and electron–phonon interactions.
Material and Methods Si NWs were synthesized by the vapor–liquid–solid (VLS) method using an ultrahigh vacuum chemical vapor deposition (UHV–CVD) system [15]. The catalyst was a 2-nm-thick Au film deposited on a clean Si (111) substrate in a metal chamber by thermal evaporation at a growth pressure of 1 × 10-6 torr. The sample was transferred in situ to the growth chamber and annealed at 650°C under 1 × 10-8 torr for 5 min. After the annealing, Au–Si alloy droplets were formed on the Si substrate. The Si NWs were then grown for 1 h by using a mixture of SiH4 (precursor gas) and H2 (carrier gas). The SiH4:H2 composition ratio was 10:200 and a fixed total pressure of 0.05 torr was maintained by a feedback system using a throttle valve and a baratron gauge [16-17]. Next, the oxidized surface of the NWs grown in UHV–CVD was etched using 1% hydrofluoric (HF) solution. The PD into Si NWs was performed with BF3 (dopant gas). The conditions for stable doping were optimized as 200 W plasma power, 4000 V accelerating voltage, 1 × 10-4 torr processing pressure, and 3 min. s doping time. The doping-induced changes in morphology and crystalline structure were investigated using a transmission electron microscope (TEM; JEM-ARM200F, JEOL). To electrically 4
activate the boron dopants and diffuse them uniformly into the inner surface, we subjected the plasmadoped Si NWs to RTA in a N2 ambience at 1000°C (the best temperature for boron activation) [18]. To investigate the dependence of the electrical properties on active dopant concentration, we varied the RTA time as 1, 3, and 5 min. The chemical bonding state of the NWs was evaluated by high-resolution x-ray photoelectron spectroscopy (HRXPS) using monochromatic Al Kα (1486.6 eV) with a pass energy of 23.5 eV under a base pressure of 5 × 10-8 torr. The binding energies were calibrated by corelevel spectra using the C 1s spectrum (284.5 eV). To deconvolute the XPS core-level spectra, we removed the background by a Shirley-type subtraction. The vibrational mode changes induced by the dopant were analyzed by Raman spectroscopy and explained by Fano resonance. The Raman spectra were obtained by a Horiba Jobin–Yvon LabRarm Armis spectrometer. The excitation source was a 514.5-nm line of an Ar ion laser. The Raman-scattered light signal was collected by a backscattering geometry using a ×50 microscope objective lens. The spot diameter of the Raman excitation beam was approximately 1 μm. The device for the 4-point probe measurement was fabricated using a nanomanipulator (MM3A, Kleindeck) installed in a scanning electron microscope (Quanta 3D, FEI). The electrical conductivity of the NWs was calculated from the resistance data acquired by the 4-point probe, correcting for the geometric factors (diameter and effective length). The experimental setup of the THz–TDS is shown in Supporting Information S1. A 1-kHz pulse train of 2.5 mJ, comprising 800nm pulses each with a pulse duration of 150 fs full-width at half-maximum (FWHM), was generated by a Ti:sapphire regenerative amplifier system . Terahertz pulses were generated by optical rectification of the fundamental pulse in a (10 × 10 × 1) mm3 <110> ZnTe crystal. The transmitted terahertz radiation was detected through an electro-optic sampling method in another <110> ZnTe nonlinear crystal with dimensions of (10 × 10 × 3) mm3. The signal was collected by a lock-in amplifier (Stanford Research System, SR830) phase-locked to an optical chopper. To extract the frequency-dependent transmission characteristics, the THz–TDS requires two transmitting signals: a signal Esample(t) with the sample and a signal Eref(t) without the sample. The transmission amplitude and phase in the frequency domain was computed by fast Fourier transform (FFT). The amplitude A(ω) of the ratio of the two spectra, Esample(ω)/Eref (ω), was calculated and analyzed to obtain the 5
frequency-dependent transmission amplitude.
Result and Discussion Figure 1 displays scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR–TEM) images of the as-grown Si NWs on the Si substrate, which shows the vertical growth of the Si NWs on the substrate. The NWs were uniformly grown to approximate lengths of 4 μm with the growth direction of [111], as shown in Fig. 1(a) and (b). We checked the diameters of the grown NWs (80–120 nm) and classified them into size groups. As shown in the histogram, most of the Si NWs were ~100 nm in diameter. Moreover, all Si NWs exhibited a singlecrystalline structure without any defect in the HR–TEM images. In particular, the HR–TEM image of the plasma-doped Si NWs (Figure 1(c)) revealed a 5.8-nm-thick amorphous layer on the NW surface. Felch et al. [11] and Takamizawa et al. [13] also reported an amorphous doping layer on the 3D surface of a plasma-doped FinFET. deposited on the surface of 3D structure.
Using high density of Plasma, the dopant layer can be The thickness of the amorphous doping layer clearly
decreased after annealing process, as shown in Figure 1(d). To confirm the diffusion of the dopant into the NWs after annealing process, we investigated the dopant profile by secondary ion mass spectroscopy (SIMS). As SIMS profiling in the depth direction on the surface is not possible for the vertically grown NWs, we instead profiled the dopant distribution in the two-dimensional (2D) structure of the Si substrate. As shown in Supporting Information S2, the plasma-doped boron in the intrinsic Si wafer was predominantly distributed over the surface. However, the dopant was extensively diffused into the substrate after the annealing process and then the diffusion was almost saturated at annealing times of over 3 min. Thus, the reduction of surface layer thickness is caused by the boron diffusion. The detailed chemical structure of the BF3-doped Si NWs during the RTA process was revealed by XPS measurements. The change with RTA-time in the Si sub-oxides (Si+1, Si+2, Si+3, Si+4), interstitial Si, interfacial state (Si-B-O), and activation state (Si-B) was analyzed by detailed curve-fitting. Figures 2(a) and (b) show the Si 2p and B 1s XPS core-level spectra of the plasma6
doped Si NWs at different RTA times using BF3 as the source material, respectively. The Si 2p corelevel spectra exhibit the Si–Si bonding peak at 98.9 eV and Si+1 after cleaning process using 1% HF solution for 2 min (Figure 2(a)). However, the surface states was significantly changed after the plasma doping process. In particular, various oxidation and interstitial Si states were detected in the plasma-doped Si NWs. The interstitial Si state, indicating Si–Si bond breakage, reflected surface damage to the Si NWs by the accelerated boron ions during the PD process (Figure 2(a)). These surface defects generated by the surface damage easily bond with oxygen to stabilize the surface energy [19]. Thus, the multiple oxidation states of Si+1, Si+2, Si+3, and Si+4 detected at 100–102 eV resulted from chemical reactions between external oxygen and the Si elements at the damaged surfaces of the PD Si NWs. However, the doped boron does not induce any Si–B bonding state between 102.1 eV and 100.5 eV, but B–B homopoloar bonds were clearly observed at 188 eV. These bonding states indicate that the doped B did not react with Si; in other words, none of the Si atoms were substituted with B. Based on the escape depth of the photoelectrons in the XPS measurements, we infer that B was predominantly located on the Si NW surface during the plasma process, indicating that the observed thin film in the TEM images was a deposited B layer. The presence of a B2O3 peak in the B 1s core-level spectra indicates that boron and oxygen chemically reacted during the PD process. In a quasi-binary system, ternary (B–Si–O) bonding states such as borosilicate can also be formed [20]. This state was drastically increased after the RTA process, stabilizing the interface. Moreover, substitutional Si–B bonding state is induced on the NW surface after the annealing process. After the annealing process, time-dependent changes in peak intensity were clearly observed. In particular, as the annealing time increased, the intensity of the borosilicate and B–Si peaks gradually increased, while the B–B peak was attenuated. These peak changes are consistent with the surface layer changes in the TEM image. As the high-temperature (1000°C) RTA process activates the B diffusion into NWs [18], we conclude that B was substituted into the Si lattice, generating holes on the NW surfaces. During high-temperature annealing, the substitutional exchange of Si with the diffusing B 7
dopants changes the bonding structure of the Si matrix. To evaluate the resulting change in the bonding characteristics, we measured the bonding vibration mode in the micro-Raman spectrum. The PD-induced change in the Si optical phonon mode can be deduced from the Fano resonance in the Raman peaks. Fano resonance is generated when the continuum state couples with discrete states, which affect the optical phonon line shape of the continuum state [21]. Typically, the amplitude of the continuum state varies slowly with energy, whereas the resonant scattering intensity changes quickly. These resonance characteristics cause asymmetries in the Raman peak profiles (see the main peak near 520 cm-1 in Figure 3(a)). The coupled state suggests a close relationship between the quantity of electrically activated dopants and the degree of asymmetry in the line shape of the continuum state. In the main peak near 520 cm-1, the enhanced asymmetry reflects the increased number of activated dopants, which is consistent with the B changes in the XPS data. The doping characteristics can be derived from the Fano equation, which accounts for the asymmetry in the optical measurements and is given by (𝑞+𝜀)2
𝐼(𝜔) = 𝐼0 (1+𝜀2 ), where ω is the wavenumber, I0 is the pre-factor, q is the asymmetry parameter, and ε = (ω - ωp)/Γ, where ωp and Γ denote the phonon wavenumber and the linewidth parameter, respectively. The Raman data were successfully fitted by the Fano equation, as shown by the representative example in Figure 3(b). The optical phonon mode of the as-grown Si NWs has a symmetric line shape (asymmetry factor = 0) because there is no coupling between Si and B. However, the Si optical phonon mode of plasma-doped Si NWs exhibits a slightly higher asymmetric line shape at low wavenumbers than at high wavenumbers. The opposite effect appears in the line shape of the annealed plasma-doped Si NWs. As the asymmetric change is sensitive to the surface condition, it is attributed to the formation of surface oxides. Comparing the surface region changes on as-grown Si NWs and plasma-doped Si NWs, we infer that the surface oxide defects altered the localized surface carriers on the latter, affecting the shape of the optical phonon peak. However, the asymmetric line shape of the plasma-doped Si NWs was dramatically changed by the RTA process, which activates the B in the Si 8
matrix. The doped B is activated through the generation of substitutional B–Si bonding during the high-temperature RTA process [18]. Since the Si–B bonding gradually is increased with annealing time, as shown in XPS data, the asymmetry parameter is also increased with RTA time, peaking at 5 min (Figure 3(a)). Table 1 lists the phonon wavenumbers calculated by the Fano equation; the calculations are consistent with the observed Raman peak shift. Finally, the number of B atoms in the Si NWs affecting the Si states is increased with RTA time, resulting inthe extensive increase in the continuum state (Si) coupled with the discrete state (B). The increased B activation also manifests in the wavenumber range 610–640 cm-1. Natural boron exists as two stable isotopes, 10B and 11B. During the RTA process, some of the Si lattice sites are substituted with
10
B or
11
B, generating local
vibrational modes at 618 cm-1 and 640 cm-1, respectively. The intensity ratio of 10B to 11B is nearly 1:4, which is consistent with the natural proportions of the two isotopes. The increased local vibrational peaks of the B isotopes confirm that the active dopant concentration is increased with the RTA time. For a detailed investigation of the doping characteristics, we analyzed the FWHM of the Raman peaks, which is inversely proportional to the phonon lifetime [22]. The RTA process dramatically increases the FWHM of the optical phonon peak of the plasma-doped Si NWs, as shown in Table 3. The phonon lifetime, which the phonon decays back to the valence band, was a rapidly decreasing function of RTA time. The photo-excited active carriers coexist with phonons in a virtual state rather than the ground state. In the virtual state, phonons couple with carriers to induce carrier–phonon scattering [23]. This phenomenon drastically reduces the relaxation time of the phonons in the ground state. According to the lifetime–FWHM relationship, the change in doping concentration of the plasma-doped Si NWs clearly reflects the change in FWHM in the Fano equation. The internal strain induced by the doping substitution can also characterize the doping characteristics. This strain may be caused the interstitial sites observed on the plasma-doped Si NWs. According to Horn et al. [24], boron dopants contract the distance between Si lattices in a concentration-dependent manner because Si–B bonds are shorter than Si–Si bonds. Therefore, as the boron substitution increases with RTA time, the tensile strain should increase. The tensile strain 9
changes the phonon mode, causing a peak shift, as shown in Figure 3(d). With increasing RTA time, the phonon wavenumber of the plasma-doped Si NWs decreases by 3.0 cm-1 from that of the as-grown Si NWs. This result well supports the generation of tensile strain in the NWs. To confirm the change in electrical conductivity caused by the PD and annealing processes, we measured the current–voltage characteristics at different RTA times using a 4-point probe. To ensure reliable data, we selected typical Si NWs with approximate lengths and diameters of 4 µm and 100 nm, respectively. The current–voltage characteristic is strongly linear, indicating ohmic contact (Figure 4(c)). The resistivity of the plasma-doped Si NWs (2.13 × 10−1 Ω∙cm) is not significantly decreased compared to that of the as-grown Si NW (2.82 × 10−1 Ω∙cm). However, the conductivity is decreased after the annealing process, reflecting the generation of substitutional B in Si as shown in the increase in the chemical bonding of Si-B. As also observed in the XPS, this bonding was absent in the plasma-doped Si NWs. The highly dense plasma and ~keV energies damage the NW surfaces, deforming the Si sites and forming surface sub-oxides (Si+1, Si+2, Si+3), as shown in the XPS data. The surface damage caused by plasma doping induces carrier scattering during the electrical conduction process. Consequently, resistivity is slightly higher in plasma-doped Si NWs than in as-grown Si NWs. The resistivity of the plasma-doped Si NWs significantly and abruptly decreases after the RTA process, reaching 8.02 × 10−5 Ω∙cm (12,467 Ω−1∙cm−1 of conductivity) after annealing for 5 min. As shown in the Raman data of the annealed Si NWs, the substitution of Si with boron dopants considerably affects three parameters in the Fano equation. The changing shape of the Raman peak related to the formation of Si–B bonding can also be confirmed by resistivity data. Therefore, the changing resistivity of the NWs with increasing annealing time can be attributed to the high surface density of B after annealing at 1,000°C [25]. As the resistivity is inversely proportional to the conductivity, the electrical conductivity of the plasma-doped Si substrate increases from 0.12 to 16.62 Ω−1∙cm−1 as the annealing time increases to 5 min in the supporting information of S2, which is not comparable with that of the plasma-doped Si NW. The change in plasma doped NW, which far exceeded those in the plasmadoped Si substrate, is related to local dopant concentrations and the conformal doping characteristics 10
of PD. As shown in the Supporting information of S3 for the SIMS depth profile, the saturated diffusion length of B from the surface to bulk Si exceeded 300 nm after 3 min of annealing. In the Si NWs, the plasma-doped B was confined within the NW diameter (~100 nm); consequently, the dopant concentration was significantly increased in the NW system than in the Si substrate. The measured electrical conductivities and active dopant concentrations obtained from the I–V curves are presented in Table 2. To investigate the relationship of the active dopant concentration with electrical conductivity in the Si substrate, we measured the Hall value of the plasma-doped Si substrate (see S2 in Supporting Information). The active dopant concentration in the plasma-doped Si NWs annealed for 5 min, derived from the 4-point probe data, was 1.71 × 1021 atoms/cm3, which is over 150 times that in the plasma-doped Si substrate. This result suggests that the conformal doping characteristics of PD are significantly effective in nanosized 3D structures. The optical characteristics of the plasma-doped Si NWs were investigated by THz–TDS. The THz pulse was perpendicularly incident on the substrate with vertically aligned NWs. Figure 5(a) presents the raw THz–TDS data of plasma-doped Si NWs at various RTA annealing times. The delay times in the experimental data were converted to frequency units through FFT (Figure 5(b)). Optical conductivity is inversely proportional to transmittance. The transmittances of the plasma-doped Si NWs and as-grown Si NWs are very similar, indicating that the optical conductivities of both materials were nearly identical. On the other hand, the transmittance is significantly reduced in RTAannealed plasma-doped Si NWs. The transmittance change with increasing annealing time is consistent with the changing electrical conductivity (Table 2), which can be explained by the increased carrier number at longer annealing times. Regarding the the NWs on the substrate as a thin film, we calculated the optical conductivity by the thin-film equation using the experimental data [26],:
σ𝑜𝑝𝑡𝑖𝑐𝑎𝑙 =
𝑡 (1+𝑡𝑁𝑊 )( 𝑁𝑊 −1) 𝑡𝑠𝑢𝑏
𝑍0 𝑑
,
(3)
where tNW and tSub denote the transmittances of the NW and substrate, respectively. Z0 is the 11
refractive index of Si and d is the film thickness. Here, we substituted d with the NW length and corrected the result by a filling factor, as shown in Supporting Information S4. The optical conductivities of the as-grown Si NWs and plasma-doped Si NWs were approximately zero (Figure 5(d)). However, after annealing for 5 min, the conductivity of the plasma-doped Si NWs is increased by 3000 times. The THz–TDS data reflecting the changes in optical conductivity induced by electrically activated carriers is also consistent with the electrical measurement results.
Conclusion We demonstrated the utility of the PD process in concentrating the dopants in Si NWs. The surface chemical bonding and electrical conduction measurements reveal a clear positive relationship between the active dopant level of plasma-doped Si NWs and RTA time. Meanwhile, the XPS data provide the changes in the interstitial Si sites, surface oxides, and Si–B bonding, which is also closely related to the shape and shift of the Raman peaks. The bonding characteristics is altered as B was substitutionally exchanged with Si.
Specifically, the optical phonon peak of Si is largely broadened
by activated borons at the Si substitutional sites. Through the calculation of the asymmetry parameter (the Fano factor) by the Fano equation, we can extract the change in dopant concentration. The generation of many carriers at longer annealing times greatly increases the electrical and optical conductivity of the plasma-doped Si NWs, compared to the plasma-doped Si substrate. Therefore, The doping method especially benefits the conformal structure of the NWs, and is a suitable doping process for 3D nanostructures.
12
Acknowledgements The authors are grateful for the valuable help in the experiments performed using the fs-THz spectroscopy beamlines at the Pohang Light Source (PLS). Funding : This work was supported by an Industry-Academy joint research program between Samsung Electronics-Yonsei University and the Korea Research Institute of Standards and Science (KRISS) under the Metrology Research Centre Project.
13
FIGURE CAPTIONS
Figure 1. (a) SEM data of vertical Si NWs fabricated by VLS growth in UHV–CVD. Each NW is capped by a Au catalyst (droplet). The average Si NW diameter was approximately 100 nm. (b) TEM image of as-grown Si NW with a [111] growth direction. The substrate grew in the same direction. (c) After the doping process, a 5.9-nm-thick PD layer was uniformly formed on the NW surface. (d) The PD layer gradually thinned as B diffused into the inner surface.
Figure 2. XPS core-level spectra of plasma-doped Si NWs in the regions of (a) Si 2p and (b) B 1s. The formation of Si–B and borosilicate on the NW surface increased with RTA time. Panel (a) shows that the interstitial Si and sub-oxides were drastically changed after the RTA process. In (b), elemental B reduced with increasing RTA time as it diffused from the outer to inner surface of the NWs.
Figure 3. (a) Raman spectra of plasma-doped Si NWs at various RTA times (1, 3, and 5 min). The Si optical phonon peak is largely broadened after RTA process. (b) Local vibration of B isotopes in Si lattice. (c) Raman peak fitted by the Fano equation. (d) Peak shift of the Si optical phonon peak is related to the internal strain induced by B substitution.
Figure 4. (a) Experimental setup for the 4-point probe of plasma-doped Si NW. (b) SEM image of PD Si NW measured by the 4-point probe. Metal pattern was fabricated using a Pt ion source. A voltage ranging from -1 V to 1 V was applied to the first and forth probes. The second and third probes measured the voltage drop when a 100-mA current was uniformly applied to the NWs. (c) and (d) I–V curve and resistivity data of the plasma-doped Si NWs acquired using the 4-point probe method. The changes in resistivity reflect the large increase in activated carrier number after the RTA process.
Figure 5. (a) THz–TDS experimental data of plasma-doped Si NWs. Transmittance signal was decreased after the RTA process. (b) The time delays were converted to frequency units using FFT. (c) Transmittance data of plasma-doped Si NWs divided by that of Si substrate to reveal the optical information on the Si NW. (d) Optical conductivity data of plasma-doped Si NWs. 14
TABLES Table1. Fano-equation fitting parameters to the observed Si optical phonon peak of plasma-doped Si NWs. The fitting parameters are drastically changed by the RTA process.
Sample
Asymmetry parameter
Line width (cm-1 )
Peak position (cm-1)
Si NWs
0
3.2
520.8
PD SiNWs
0
4.1
520.2
PD SiNWs (RTA 1min)
6.6
5.5
518.8
PD SiNWs (RTA 3min)
7.3
7.1
517.9
PD SiNWs (RTA 5min)
7.7
7.9
517.2
Table 2. Electrical properties of plasma-doped Si NWs. To avoid geometrical effects, the diameter and effective length of the NWs were set to 100 nm and 3 μm, respectively.
Sample
Current at 1V (μA)
Resistivity ( 10-5Ω∙cm)
Current density at 1V (μA /m2)
Active dopants concentration (1016atoms/cm3)
Si NW
1.23
21322
39.2
9.11
PD Si NW
0.93
28249
29.35
6.36
419
62.5
13300
20800
1640
15.9
52200
85100
3260
8.02
104000
171000
PD Si NW (RTA3min) PD Si NW (RTA3min) PD Si NW (RTA5min)
15
Table 3. Optical conductivity data of plasma-doped Si NWs.
Sample
Optical conductivity
Si NWs
4.32
PD Si NW
3.12
(Ω-1∙cm-1)
PD Si NW
8749
(RTA 1min) PD Si NW
11010
(RTA 3min) PD Si NW
13200
(RTA 5min)
16
References [1] A. K. Buin, A. Verma, Svizhenko A., M. P. Anatram, Significant enhancement of hole mobility in [110] silicon nanowires compared to electrons and bulk silicon, Nano Lett. 2008, 8 (2) 760-765 [2] J. Xiang, W. Lu, Y. J. Hu, Y. Wu, H. Yan, C. M. Lieber, Ge/Si nanowire heterostructures as highperformance field-effect transistors, Nature 2006, 441, 489-493. [3] L. Liao, Y. C. Lin, M. Q. Bao, R. Cheng, J. W. Bai, Y. A. Liu, Y. Q. Qu, K. L. Wang, Y. Huang, X. F. Duan, High-speed graphene transistors with a self-aligned nanowire gate, Nature 2010, 467(16) 305-308. [4] S. Kim, A. Konar, W. S. Hwang, J. H. Lee, J. Lee, J. Yang, C. Jung, H. Kim, J. B. Yoo, J. Y. Choi, Y. W. Jin, S. Y. Lee, D. Jena, W. Choi, K. Kim, High-mobility and low-power thin-film transistors based on multilayer MoS2 crystals, Nature Comm. 2012, 3, 1011. [5] D Sacchetto, M. H. Ben-Jamaa, G DeMicheli, Y usuf Leblebici, Fabrication and characterization of vertically stacked gate-all-around Si nanowire FET arrays, Proceedings of the European Solid State Device Research Conference 2009, 245-248. [6] A. Konar, J. Mather, K. Nayak, M. Bajaj, R. K. Pandey, S. Dhara, K. V. R. M. Murali, M. M. Deshmukh, Carrier Transport in High Mobility InAs Nanowire Junctionless Transistors, Nano Lett. 2015, 15(2), 1684–1690. [7] J. W. Han, D. I. Moon, J. S. Oh, Y. K. Choi, M. Meyyappan, Vacuum gate dielectric gate-allaround nanowire for hot carrier injection and bias temperature instability free transistor, App. Phys. Lett. 2014, 104(25), 253506. [8] N. Abdelmalek, F. Djeffal, M. Meguellati, T. Bendib, Numerical Analysis of Nanoscale Junctionless MOSFET Including Effects of Hot-Carrier Induced Interface Charges, Advanced Materials Research 2014, 856, 137-141. [9] H. Tellez, A. Aguadero, J. Druce, M. Burriel, S. Fearn, T. Ishihara, D. S. McPhail, J. A. Kilner, New perspectives in the surface analysis of energy materials by combined time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high sensitivity low-energy ion scattering (HS-LEIS), J. Anal. At. Spectrom 2014, 29, 1361-1370. [9] L. Porte, C. H. Devilleneuve, M. Phaner, Scanning Tunneling Microscopy Observation of Local Damages Induced on Graphite Surface by Ion-Implantation, J. Vac. Sci. Technol. B 1991, 9(2), 106417
1067. [10] M. Takase, K. Yamashita, A. Hori, B. Mizuno, Shallow source/drain extensions for pMOSFETs with high activation and low process damage fabricated by plasma doping, Electron Devices Meeting, 1997. IEDM '97. Technical Digest., International 1997, 475-478. [11] S. Felch, C. Hobbs, J. Barnett, H. Etienne, J. Duchaine, M. Rodgers, S. Bennett, F. Torregrosa, Y. Spiegel, L. Roux, Plasma doping of silicon fin structures, The 11th International Workshop on Junction Technology 2011, 22-25. [12] J. W. Lee, Y. Sasaki, M. J. Cho, M. Togo, G. Boccardi, R. Ritzenthaler, G. Eneman, T. Chiarella, S. Brus, N. Horiguchi, G. Groeseneken, A. Thean, Plasma doping and reduced crystalline damage for conformally doped fin field effect transistors, Appl. Phys. Lett. 2013, 102, 223508. [13] H. Takamizawa, Y. Shimizu, Y. Nozawa, T. Toyama, H. Morita, Y. Yabuuchi, M. Ogura, Y. Nagai, Dopant characterization in self-regulatory plasma doped fin field-effect transistors by atom probe tomography,
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[14] E. C. Garnett, Y. C. Tseng, D. R. Khanal, J. Q. Wu, J. Bokor, P. D. Yang, Dopant profiling and surface analysis of silicon nanowires using capacitance-voltage measurements, Nat. Nanotechnol. 2009, 4(5), 311-314. [15] J. W. Ma, W. J. Lee, J. M. Bae, K. S. Jeong, S. H. Oh, J. H. Kim, S. H. Kim, J. H. Seo, J. P. Ahn, H. Kim, M. H. Cho, Carrier Mobility Enhancement of Tensile Strained Si and SiGe Nanowires via Surface Defect Engineering, Nano Lett. 2015, 15(11), 7204. [16] W. J. Lee, J. Ma, J. Bae, M. H. Cho, J. P. Ahn, Effects of hydrogen on Au migration and the growth kinetics of Si nanowires, CrystEngComm. 2011, 13(2), 690-696. [17] W. J. Lee, J. W. Ma, J. M. Bae, M. H. Cho, J. P. Ahn, Generation of planar defects caused by the surface diffusion of Au atoms on SiNWs, Mater. Res. Bull. 2012, 47(10), 2739-2743. [18] A. E. Michel, W. Rausch, P. A. Ronsheim, R. H. Kastl, Rapid Annealing and the Anomalous Diffusion of Ion-Implanted Boron into Silicon,
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[22] D. H. Song, F. Wang, G. Dukovic, M. Zheng, E. D. Semke, L. E. Brus, T. F. Heinz, Direct measurement of the lifetime of optical phonons in single-walled carbon nanotubes, Phys. Rev. Lett. 2008, 100, 225503. [23] X. W. Fu, Z. M. Liao, R. Liu, J. Xu, D. P. Yu, Size-Dependent Correlations between Strain and Phonon Frequency in Individual ZnO Nanowires, ACS Nano 2013, 7(10), 8891–8898. [24] F. H. Horn, Densitometric and Electrical Investigation of Boron in Silicon, Phys. Rev. 1955, 97, 1521. [25] K. K. Lew, L. Pan, T. E. Bogart, S. M. Dilts, E. C. Dickey, J. M. Redwing, Y. F. Wang, M. Cabassi, T. S. Mayer, S. W. Novak, Structural and electrical properties of trimethylboron-doped silicon nanowires, Appl. Phys. Lett. 2004, 85(15), 3101. [26] R. A. KAindl, M. A. Carnahan, J. Orenstein, D. S. Chemla, H. M. Christen, H. Y. Zhai, M. Paranthaman, D. H. Lowndes, Far-infrared optical conductivity gap in superconducting MgB2 films, Phys. Rev. Lett. 2002, 88, 027003.
19
(a)
(b)
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S. H. Oh, et. al., Figure
(b)
Si 2p
B 1s
PD Si NW RTA 5min
PD Si NW RTA 5min
PD Si NW RTA 3min
PD Si NW RTA 3min
Intensity (arb. units)
Intensity (arb. units)
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PD Si NW Interstitial Si Sub-oxide +1
SiO2
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Si NW Si-Si +1
Si 106
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S. H. Oh, et. al., Figure
PD SiNWs RTA5min PD SiNWs RTA3min PD SiNWs RTA1min PD SiNWs SiNWs
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S. H. Oh, et. al., Figure
S0169-4332(17)31317-X http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.015 APSUSC 35950
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-6-2016 12-1-2017 2-5-2017
Please cite this article as: Seung-Hoon Oh, Jin-Won Ma, Jung Min Bae, Yu-seon Kang, Jae-Pyung Ahn, Hang-Kyu Kang, Jimin Chae, Dongchan Suh, Woobin Song, Sunjung Kim, Mann-Ho Cho, Surface chemical structure and doping characteristics of boron-doped Si nanowires fabricated by plasma doping, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dopant activations of plasma-doped (PD-) Si NWs are investigated with RTA time Chemical states and Fano factor of PD-Si NWs are changed with the dopant activation Both electrical and optical conductivity of PD-Si NWs increase greatly and coherently Carrier generation by PD process is more effective at 1D Si NW than 2D Si substrate
1
Surface chemical structure and doping characteristics of boron-doped Si nanowires fabricated by plasma doping
Seung-Hoon Oh 1, Jin-Won Ma 1,3, Jung Min Bae 1, Yu-seon Kang 1,3, Jae-Pyung Ahn 2, Hang-Kyu Kang 1, Jimin Chae 1 Dongchan Suh 3, Woobin Song 3, Sunjung Kim 3 and Mann-Ho Cho 1,* 1
Institute of Physics and Applied Physics, Yonsei University, Seoul 03722, Republic of Korea
2 3
Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
Process Development Team, Semiconductor R&D Center, SAMSUNG, Hwaseong-si 18448, Republic of Korea
Abstract We investigated the conduction characteristics of plasma-doped Si nanowires (NWs) after various rapid thermal annealing (RTA) times. The plasma doping (PD) process developed a highly-deposited B layer at the NW surface. RTA process controls electrical conductivity by mediating the dopant diffusion from the surface layer. The surface chemical and substitutional states of the B plasma-doped Si NWs were analyzed by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. To elucidate the detailed structure of the NWs, we analyzed the change in the optical phonon mode caused by the incorporated B atoms.
For this purpose, we examined Fano resonance by the
investigation of the asymmetry, line-width, and phonon wavenumber in Raman spectra. The changes in symmetry level of the Raman peak, phonon lifetime, and internal strain were closely related to the number of electrically activated borons, which was drastically increased with RTA time. The change in electrical and optical characterizations related to the doping characteristics of the NWs was investigated using a 4-point probe and terahertz time-domain spectroscopy (THz–TDS). The resistivity of the NWs was 3000 times lower after the annealing process compared to that before the annealing process, which is well consistent with the optical conductivity data. The data provide the potential utility of PD in conformal doping for three-dimensional nanodevices.
Key world: Plasma doping, Si NW, conductivity change
Correspondence and requests for materials should be addressed to M.-H. Cho ([email protected]) 2
Introduction The semiconductor industry has employed bulk silicon channels in metal-oxide-semiconductor field-effect transistors for several decades [1]. However, this industry has recently faced demands for high-performance, easily integrated, scaled-down devices [2]. In searching for new channels with higher mobility, the industry has explored various materials and nanoscale structures [3]. New channel materials, such as graphene and MoS2, and new structures, such as fin field-effect transistors (FinFETs) and all-around gate field-effect transistors [4-6], have been extensively researched. The especially high performance of FinFETs is attributed to the movement of many carriers on the three-dimensional (3D) interface between the gate oxide and channel when the gate voltage is applied [7]. Due to its 3D structure, the FinFET-based channel can be formed by a conformal doping process. In general, the performance of a FinFET depends on the dopant distribution in the near-surface region of the sidewalls. Therefore, a specific doping process is required to uniformly dope the conformal structure. At present, the electrical properties of semiconductors are usually enhanced by ion implantation (IIP) using accelerated ions. As an ex-situ doping system, IIP can control the depth profile of the dopants by tuning the ion energy [8]. However, IIP process contains some disadvantages to nanomaterials because the high kinetic energy of the accelerated ions would damage their surfaces [9]. Moreover, the non-isotropic beam direction of the doping method is unsuitable for conformal structures. To avoid surface damage, new doping methods have been attempted in FinFET technology.
Plasma
doping (PD) is expected to solve the problem of IIP because PD is an ex-situ conformal doping process that avoids surface damage in 3D structures and nanomaterials [10-11]. The number of active carriers on the channel surface depends on the density of PD. In intensive PD process, the dopant is easily activated by rapid thermal annealing (RTA) process because the dopant is somewhat already activated under the high plasma power. Many researchers have analyzed the doping characteristics of nanomaterials doped by PD; particularly, a FinFET is a model structure for this purpose. However, most of the FinFET research has highlighted the advantages of conformal doping, compared to IIP [12-13]. The surface chemical structure and structural properties of nanomaterials subjected to PD 3
have been rarely reported. The surface and interface characteristics of nanodevices can be closely related to the electrical properties because most nanodevices have a high surface-to-volume ratio [14]. Therefore, by controlling the surface charge carriers on nanostructures through PD, we can realize a blueprint for high-performance nanodevices. In this study, we activated plasma-doped Si nanowires (NWs) by RTA and investigated their electrical characteristics through their chemical states and structural changes on the surface as functions of RTA time. Under the RTA process, dopants located to the outer surface region was diffused the inner surface and simultaneously activated through the substitutional process. PD significantly improved the electrical properties of the nanomaterials, which is dependent on RTA times. The changes in the conductivity of the NWs were closely related to the chemical state of dopants and electron–phonon interactions.
Material and Methods Si NWs were synthesized by the vapor–liquid–solid (VLS) method using an ultrahigh vacuum chemical vapor deposition (UHV–CVD) system [15]. The catalyst was a 2-nm-thick Au film deposited on a clean Si (111) substrate in a metal chamber by thermal evaporation at a growth pressure of 1 × 10-6 torr. The sample was transferred in situ to the growth chamber and annealed at 650°C under 1 × 10-8 torr for 5 min. After the annealing, Au–Si alloy droplets were formed on the Si substrate. The Si NWs were then grown for 1 h by using a mixture of SiH4 (precursor gas) and H2 (carrier gas). The SiH4:H2 composition ratio was 10:200 and a fixed total pressure of 0.05 torr was maintained by a feedback system using a throttle valve and a baratron gauge [16-17]. Next, the oxidized surface of the NWs grown in UHV–CVD was etched using 1% hydrofluoric (HF) solution. The PD into Si NWs was performed with BF3 (dopant gas). The conditions for stable doping were optimized as 200 W plasma power, 4000 V accelerating voltage, 1 × 10-4 torr processing pressure, and 3 min. s doping time. The doping-induced changes in morphology and crystalline structure were investigated using a transmission electron microscope (TEM; JEM-ARM200F, JEOL). To electrically 4
activate the boron dopants and diffuse them uniformly into the inner surface, we subjected the plasmadoped Si NWs to RTA in a N2 ambience at 1000°C (the best temperature for boron activation) [18]. To investigate the dependence of the electrical properties on active dopant concentration, we varied the RTA time as 1, 3, and 5 min. The chemical bonding state of the NWs was evaluated by high-resolution x-ray photoelectron spectroscopy (HRXPS) using monochromatic Al Kα (1486.6 eV) with a pass energy of 23.5 eV under a base pressure of 5 × 10-8 torr. The binding energies were calibrated by corelevel spectra using the C 1s spectrum (284.5 eV). To deconvolute the XPS core-level spectra, we removed the background by a Shirley-type subtraction. The vibrational mode changes induced by the dopant were analyzed by Raman spectroscopy and explained by Fano resonance. The Raman spectra were obtained by a Horiba Jobin–Yvon LabRarm Armis spectrometer. The excitation source was a 514.5-nm line of an Ar ion laser. The Raman-scattered light signal was collected by a backscattering geometry using a ×50 microscope objective lens. The spot diameter of the Raman excitation beam was approximately 1 μm. The device for the 4-point probe measurement was fabricated using a nanomanipulator (MM3A, Kleindeck) installed in a scanning electron microscope (Quanta 3D, FEI). The electrical conductivity of the NWs was calculated from the resistance data acquired by the 4-point probe, correcting for the geometric factors (diameter and effective length). The experimental setup of the THz–TDS is shown in Supporting Information S1. A 1-kHz pulse train of 2.5 mJ, comprising 800nm pulses each with a pulse duration of 150 fs full-width at half-maximum (FWHM), was generated by a Ti:sapphire regenerative amplifier system . Terahertz pulses were generated by optical rectification of the fundamental pulse in a (10 × 10 × 1) mm3 <110> ZnTe crystal. The transmitted terahertz radiation was detected through an electro-optic sampling method in another <110> ZnTe nonlinear crystal with dimensions of (10 × 10 × 3) mm3. The signal was collected by a lock-in amplifier (Stanford Research System, SR830) phase-locked to an optical chopper. To extract the frequency-dependent transmission characteristics, the THz–TDS requires two transmitting signals: a signal Esample(t) with the sample and a signal Eref(t) without the sample. The transmission amplitude and phase in the frequency domain was computed by fast Fourier transform (FFT). The amplitude A(ω) of the ratio of the two spectra, Esample(ω)/Eref (ω), was calculated and analyzed to obtain the 5
frequency-dependent transmission amplitude.
Result and Discussion Figure 1 displays scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR–TEM) images of the as-grown Si NWs on the Si substrate, which shows the vertical growth of the Si NWs on the substrate. The NWs were uniformly grown to approximate lengths of 4 μm with the growth direction of [111], as shown in Fig. 1(a) and (b). We checked the diameters of the grown NWs (80–120 nm) and classified them into size groups. As shown in the histogram, most of the Si NWs were ~100 nm in diameter. Moreover, all Si NWs exhibited a singlecrystalline structure without any defect in the HR–TEM images. In particular, the HR–TEM image of the plasma-doped Si NWs (Figure 1(c)) revealed a 5.8-nm-thick amorphous layer on the NW surface. Felch et al. [11] and Takamizawa et al. [13] also reported an amorphous doping layer on the 3D surface of a plasma-doped FinFET. deposited on the surface of 3D structure.
Using high density of Plasma, the dopant layer can be The thickness of the amorphous doping layer clearly
decreased after annealing process, as shown in Figure 1(d). To confirm the diffusion of the dopant into the NWs after annealing process, we investigated the dopant profile by secondary ion mass spectroscopy (SIMS). As SIMS profiling in the depth direction on the surface is not possible for the vertically grown NWs, we instead profiled the dopant distribution in the two-dimensional (2D) structure of the Si substrate. As shown in Supporting Information S2, the plasma-doped boron in the intrinsic Si wafer was predominantly distributed over the surface. However, the dopant was extensively diffused into the substrate after the annealing process and then the diffusion was almost saturated at annealing times of over 3 min. Thus, the reduction of surface layer thickness is caused by the boron diffusion. The detailed chemical structure of the BF3-doped Si NWs during the RTA process was revealed by XPS measurements. The change with RTA-time in the Si sub-oxides (Si+1, Si+2, Si+3, Si+4), interstitial Si, interfacial state (Si-B-O), and activation state (Si-B) was analyzed by detailed curve-fitting. Figures 2(a) and (b) show the Si 2p and B 1s XPS core-level spectra of the plasma6
doped Si NWs at different RTA times using BF3 as the source material, respectively. The Si 2p corelevel spectra exhibit the Si–Si bonding peak at 98.9 eV and Si+1 after cleaning process using 1% HF solution for 2 min (Figure 2(a)). However, the surface states was significantly changed after the plasma doping process. In particular, various oxidation and interstitial Si states were detected in the plasma-doped Si NWs. The interstitial Si state, indicating Si–Si bond breakage, reflected surface damage to the Si NWs by the accelerated boron ions during the PD process (Figure 2(a)). These surface defects generated by the surface damage easily bond with oxygen to stabilize the surface energy [19]. Thus, the multiple oxidation states of Si+1, Si+2, Si+3, and Si+4 detected at 100–102 eV resulted from chemical reactions between external oxygen and the Si elements at the damaged surfaces of the PD Si NWs. However, the doped boron does not induce any Si–B bonding state between 102.1 eV and 100.5 eV, but B–B homopoloar bonds were clearly observed at 188 eV. These bonding states indicate that the doped B did not react with Si; in other words, none of the Si atoms were substituted with B. Based on the escape depth of the photoelectrons in the XPS measurements, we infer that B was predominantly located on the Si NW surface during the plasma process, indicating that the observed thin film in the TEM images was a deposited B layer. The presence of a B2O3 peak in the B 1s core-level spectra indicates that boron and oxygen chemically reacted during the PD process. In a quasi-binary system, ternary (B–Si–O) bonding states such as borosilicate can also be formed [20]. This state was drastically increased after the RTA process, stabilizing the interface. Moreover, substitutional Si–B bonding state is induced on the NW surface after the annealing process. After the annealing process, time-dependent changes in peak intensity were clearly observed. In particular, as the annealing time increased, the intensity of the borosilicate and B–Si peaks gradually increased, while the B–B peak was attenuated. These peak changes are consistent with the surface layer changes in the TEM image. As the high-temperature (1000°C) RTA process activates the B diffusion into NWs [18], we conclude that B was substituted into the Si lattice, generating holes on the NW surfaces. During high-temperature annealing, the substitutional exchange of Si with the diffusing B 7
dopants changes the bonding structure of the Si matrix. To evaluate the resulting change in the bonding characteristics, we measured the bonding vibration mode in the micro-Raman spectrum. The PD-induced change in the Si optical phonon mode can be deduced from the Fano resonance in the Raman peaks. Fano resonance is generated when the continuum state couples with discrete states, which affect the optical phonon line shape of the continuum state [21]. Typically, the amplitude of the continuum state varies slowly with energy, whereas the resonant scattering intensity changes quickly. These resonance characteristics cause asymmetries in the Raman peak profiles (see the main peak near 520 cm-1 in Figure 3(a)). The coupled state suggests a close relationship between the quantity of electrically activated dopants and the degree of asymmetry in the line shape of the continuum state. In the main peak near 520 cm-1, the enhanced asymmetry reflects the increased number of activated dopants, which is consistent with the B changes in the XPS data. The doping characteristics can be derived from the Fano equation, which accounts for the asymmetry in the optical measurements and is given by (𝑞+𝜀)2
𝐼(𝜔) = 𝐼0 (1+𝜀2 ), where ω is the wavenumber, I0 is the pre-factor, q is the asymmetry parameter, and ε = (ω - ωp)/Γ, where ωp and Γ denote the phonon wavenumber and the linewidth parameter, respectively. The Raman data were successfully fitted by the Fano equation, as shown by the representative example in Figure 3(b). The optical phonon mode of the as-grown Si NWs has a symmetric line shape (asymmetry factor = 0) because there is no coupling between Si and B. However, the Si optical phonon mode of plasma-doped Si NWs exhibits a slightly higher asymmetric line shape at low wavenumbers than at high wavenumbers. The opposite effect appears in the line shape of the annealed plasma-doped Si NWs. As the asymmetric change is sensitive to the surface condition, it is attributed to the formation of surface oxides. Comparing the surface region changes on as-grown Si NWs and plasma-doped Si NWs, we infer that the surface oxide defects altered the localized surface carriers on the latter, affecting the shape of the optical phonon peak. However, the asymmetric line shape of the plasma-doped Si NWs was dramatically changed by the RTA process, which activates the B in the Si 8
matrix. The doped B is activated through the generation of substitutional B–Si bonding during the high-temperature RTA process [18]. Since the Si–B bonding gradually is increased with annealing time, as shown in XPS data, the asymmetry parameter is also increased with RTA time, peaking at 5 min (Figure 3(a)). Table 1 lists the phonon wavenumbers calculated by the Fano equation; the calculations are consistent with the observed Raman peak shift. Finally, the number of B atoms in the Si NWs affecting the Si states is increased with RTA time, resulting inthe extensive increase in the continuum state (Si) coupled with the discrete state (B). The increased B activation also manifests in the wavenumber range 610–640 cm-1. Natural boron exists as two stable isotopes, 10B and 11B. During the RTA process, some of the Si lattice sites are substituted with
10
B or
11
B, generating local
vibrational modes at 618 cm-1 and 640 cm-1, respectively. The intensity ratio of 10B to 11B is nearly 1:4, which is consistent with the natural proportions of the two isotopes. The increased local vibrational peaks of the B isotopes confirm that the active dopant concentration is increased with the RTA time. For a detailed investigation of the doping characteristics, we analyzed the FWHM of the Raman peaks, which is inversely proportional to the phonon lifetime [22]. The RTA process dramatically increases the FWHM of the optical phonon peak of the plasma-doped Si NWs, as shown in Table 3. The phonon lifetime, which the phonon decays back to the valence band, was a rapidly decreasing function of RTA time. The photo-excited active carriers coexist with phonons in a virtual state rather than the ground state. In the virtual state, phonons couple with carriers to induce carrier–phonon scattering [23]. This phenomenon drastically reduces the relaxation time of the phonons in the ground state. According to the lifetime–FWHM relationship, the change in doping concentration of the plasma-doped Si NWs clearly reflects the change in FWHM in the Fano equation. The internal strain induced by the doping substitution can also characterize the doping characteristics. This strain may be caused the interstitial sites observed on the plasma-doped Si NWs. According to Horn et al. [24], boron dopants contract the distance between Si lattices in a concentration-dependent manner because Si–B bonds are shorter than Si–Si bonds. Therefore, as the boron substitution increases with RTA time, the tensile strain should increase. The tensile strain 9
changes the phonon mode, causing a peak shift, as shown in Figure 3(d). With increasing RTA time, the phonon wavenumber of the plasma-doped Si NWs decreases by 3.0 cm-1 from that of the as-grown Si NWs. This result well supports the generation of tensile strain in the NWs. To confirm the change in electrical conductivity caused by the PD and annealing processes, we measured the current–voltage characteristics at different RTA times using a 4-point probe. To ensure reliable data, we selected typical Si NWs with approximate lengths and diameters of 4 µm and 100 nm, respectively. The current–voltage characteristic is strongly linear, indicating ohmic contact (Figure 4(c)). The resistivity of the plasma-doped Si NWs (2.13 × 10−1 Ω∙cm) is not significantly decreased compared to that of the as-grown Si NW (2.82 × 10−1 Ω∙cm). However, the conductivity is decreased after the annealing process, reflecting the generation of substitutional B in Si as shown in the increase in the chemical bonding of Si-B. As also observed in the XPS, this bonding was absent in the plasma-doped Si NWs. The highly dense plasma and ~keV energies damage the NW surfaces, deforming the Si sites and forming surface sub-oxides (Si+1, Si+2, Si+3), as shown in the XPS data. The surface damage caused by plasma doping induces carrier scattering during the electrical conduction process. Consequently, resistivity is slightly higher in plasma-doped Si NWs than in as-grown Si NWs. The resistivity of the plasma-doped Si NWs significantly and abruptly decreases after the RTA process, reaching 8.02 × 10−5 Ω∙cm (12,467 Ω−1∙cm−1 of conductivity) after annealing for 5 min. As shown in the Raman data of the annealed Si NWs, the substitution of Si with boron dopants considerably affects three parameters in the Fano equation. The changing shape of the Raman peak related to the formation of Si–B bonding can also be confirmed by resistivity data. Therefore, the changing resistivity of the NWs with increasing annealing time can be attributed to the high surface density of B after annealing at 1,000°C [25]. As the resistivity is inversely proportional to the conductivity, the electrical conductivity of the plasma-doped Si substrate increases from 0.12 to 16.62 Ω−1∙cm−1 as the annealing time increases to 5 min in the supporting information of S2, which is not comparable with that of the plasma-doped Si NW. The change in plasma doped NW, which far exceeded those in the plasmadoped Si substrate, is related to local dopant concentrations and the conformal doping characteristics 10
of PD. As shown in the Supporting information of S3 for the SIMS depth profile, the saturated diffusion length of B from the surface to bulk Si exceeded 300 nm after 3 min of annealing. In the Si NWs, the plasma-doped B was confined within the NW diameter (~100 nm); consequently, the dopant concentration was significantly increased in the NW system than in the Si substrate. The measured electrical conductivities and active dopant concentrations obtained from the I–V curves are presented in Table 2. To investigate the relationship of the active dopant concentration with electrical conductivity in the Si substrate, we measured the Hall value of the plasma-doped Si substrate (see S2 in Supporting Information). The active dopant concentration in the plasma-doped Si NWs annealed for 5 min, derived from the 4-point probe data, was 1.71 × 1021 atoms/cm3, which is over 150 times that in the plasma-doped Si substrate. This result suggests that the conformal doping characteristics of PD are significantly effective in nanosized 3D structures. The optical characteristics of the plasma-doped Si NWs were investigated by THz–TDS. The THz pulse was perpendicularly incident on the substrate with vertically aligned NWs. Figure 5(a) presents the raw THz–TDS data of plasma-doped Si NWs at various RTA annealing times. The delay times in the experimental data were converted to frequency units through FFT (Figure 5(b)). Optical conductivity is inversely proportional to transmittance. The transmittances of the plasma-doped Si NWs and as-grown Si NWs are very similar, indicating that the optical conductivities of both materials were nearly identical. On the other hand, the transmittance is significantly reduced in RTAannealed plasma-doped Si NWs. The transmittance change with increasing annealing time is consistent with the changing electrical conductivity (Table 2), which can be explained by the increased carrier number at longer annealing times. Regarding the the NWs on the substrate as a thin film, we calculated the optical conductivity by the thin-film equation using the experimental data [26],:
σ𝑜𝑝𝑡𝑖𝑐𝑎𝑙 =
𝑡 (1+𝑡𝑁𝑊 )( 𝑁𝑊 −1) 𝑡𝑠𝑢𝑏
𝑍0 𝑑
,
(3)
where tNW and tSub denote the transmittances of the NW and substrate, respectively. Z0 is the 11
refractive index of Si and d is the film thickness. Here, we substituted d with the NW length and corrected the result by a filling factor, as shown in Supporting Information S4. The optical conductivities of the as-grown Si NWs and plasma-doped Si NWs were approximately zero (Figure 5(d)). However, after annealing for 5 min, the conductivity of the plasma-doped Si NWs is increased by 3000 times. The THz–TDS data reflecting the changes in optical conductivity induced by electrically activated carriers is also consistent with the electrical measurement results.
Conclusion We demonstrated the utility of the PD process in concentrating the dopants in Si NWs. The surface chemical bonding and electrical conduction measurements reveal a clear positive relationship between the active dopant level of plasma-doped Si NWs and RTA time. Meanwhile, the XPS data provide the changes in the interstitial Si sites, surface oxides, and Si–B bonding, which is also closely related to the shape and shift of the Raman peaks. The bonding characteristics is altered as B was substitutionally exchanged with Si.
Specifically, the optical phonon peak of Si is largely broadened
by activated borons at the Si substitutional sites. Through the calculation of the asymmetry parameter (the Fano factor) by the Fano equation, we can extract the change in dopant concentration. The generation of many carriers at longer annealing times greatly increases the electrical and optical conductivity of the plasma-doped Si NWs, compared to the plasma-doped Si substrate. Therefore, The doping method especially benefits the conformal structure of the NWs, and is a suitable doping process for 3D nanostructures.
12
Acknowledgements The authors are grateful for the valuable help in the experiments performed using the fs-THz spectroscopy beamlines at the Pohang Light Source (PLS). Funding : This work was supported by an Industry-Academy joint research program between Samsung Electronics-Yonsei University and the Korea Research Institute of Standards and Science (KRISS) under the Metrology Research Centre Project.
13
FIGURE CAPTIONS
Figure 1. (a) SEM data of vertical Si NWs fabricated by VLS growth in UHV–CVD. Each NW is capped by a Au catalyst (droplet). The average Si NW diameter was approximately 100 nm. (b) TEM image of as-grown Si NW with a [111] growth direction. The substrate grew in the same direction. (c) After the doping process, a 5.9-nm-thick PD layer was uniformly formed on the NW surface. (d) The PD layer gradually thinned as B diffused into the inner surface.
Figure 2. XPS core-level spectra of plasma-doped Si NWs in the regions of (a) Si 2p and (b) B 1s. The formation of Si–B and borosilicate on the NW surface increased with RTA time. Panel (a) shows that the interstitial Si and sub-oxides were drastically changed after the RTA process. In (b), elemental B reduced with increasing RTA time as it diffused from the outer to inner surface of the NWs.
Figure 3. (a) Raman spectra of plasma-doped Si NWs at various RTA times (1, 3, and 5 min). The Si optical phonon peak is largely broadened after RTA process. (b) Local vibration of B isotopes in Si lattice. (c) Raman peak fitted by the Fano equation. (d) Peak shift of the Si optical phonon peak is related to the internal strain induced by B substitution.
Figure 4. (a) Experimental setup for the 4-point probe of plasma-doped Si NW. (b) SEM image of PD Si NW measured by the 4-point probe. Metal pattern was fabricated using a Pt ion source. A voltage ranging from -1 V to 1 V was applied to the first and forth probes. The second and third probes measured the voltage drop when a 100-mA current was uniformly applied to the NWs. (c) and (d) I–V curve and resistivity data of the plasma-doped Si NWs acquired using the 4-point probe method. The changes in resistivity reflect the large increase in activated carrier number after the RTA process.
Figure 5. (a) THz–TDS experimental data of plasma-doped Si NWs. Transmittance signal was decreased after the RTA process. (b) The time delays were converted to frequency units using FFT. (c) Transmittance data of plasma-doped Si NWs divided by that of Si substrate to reveal the optical information on the Si NW. (d) Optical conductivity data of plasma-doped Si NWs. 14
TABLES Table1. Fano-equation fitting parameters to the observed Si optical phonon peak of plasma-doped Si NWs. The fitting parameters are drastically changed by the RTA process.
Sample
Asymmetry parameter
Line width (cm-1 )
Peak position (cm-1)
Si NWs
0
3.2
520.8
PD SiNWs
0
4.1
520.2
PD SiNWs (RTA 1min)
6.6
5.5
518.8
PD SiNWs (RTA 3min)
7.3
7.1
517.9
PD SiNWs (RTA 5min)
7.7
7.9
517.2
Table 2. Electrical properties of plasma-doped Si NWs. To avoid geometrical effects, the diameter and effective length of the NWs were set to 100 nm and 3 μm, respectively.
Sample
Current at 1V (μA)
Resistivity ( 10-5Ω∙cm)
Current density at 1V (μA /m2)
Active dopants concentration (1016atoms/cm3)
Si NW
1.23
21322
39.2
9.11
PD Si NW
0.93
28249
29.35
6.36
419
62.5
13300
20800
1640
15.9
52200
85100
3260
8.02
104000
171000
PD Si NW (RTA3min) PD Si NW (RTA3min) PD Si NW (RTA5min)
15
Table 3. Optical conductivity data of plasma-doped Si NWs.
Sample
Optical conductivity
Si NWs
4.32
PD Si NW
3.12
(Ω-1∙cm-1)
PD Si NW
8749
(RTA 1min) PD Si NW
11010
(RTA 3min) PD Si NW
13200
(RTA 5min)
16
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19
(a)
(b)
(c)
(d)
S. H. Oh, et. al., Figure
(b)
Si 2p
B 1s
PD Si NW RTA 5min
PD Si NW RTA 5min
PD Si NW RTA 3min
PD Si NW RTA 3min
Intensity (arb. units)
Intensity (arb. units)
(a)
PD Si NW RTA 1min Borosilicate Si-B
PD Si NW Interstitial Si Sub-oxide +1
SiO2
+2
+3
(Si , Si , Si )
PD Si NW RTA 1min Borosilicate
B-Si
PD Si NW B-B B-O Si NW
Si NW Si-Si +1
Si 106
104
102
100
98
Binding Energy (eV)
96
194 192 190 188 186 184 182
Binding Energy (eV)
S. H. Oh, et. al., Figure
PD SiNWs RTA5min PD SiNWs RTA3min PD SiNWs RTA1min PD SiNWs SiNWs
a)
0.8
0.6
Peak broadening
0.4
Local vibrational peak of B
0.2
0.10
PD Si NWs RTA5min PD Si NWs RTA3min PD Si NWs RTA1min PD Si NWs Si NWs
b) 0.08
Intensity(arb.unit)
Intensity(arb.unit)
1.0
0.0
11
B
0.06
10
0.04
B
0.02
0.00 600
480 500 520 540 560 580 600 620 640 660
610
-1
Raman shift(cm )
0.6
FWHM 0.4
0.2
520
540
560
Raman shift(cm-1)
660
PD Si NWs RTA5min PD Si NWs RTA3min PD Si NWs RTA1min PD Si NWs Si NWs
d) Red shift
500
650
Raman shift(cm )
0.8
0.0 480
640 -1
Experiment data Fitting curve
c)
630
Intensity(arb.unit)
Intensity(arb.unit)
1.0
620
580
480
500
520
540
560
580
600
620
-1
Raman shift(cm )
S. H. Oh, et. al., Figure
(a)
(b)
(1)
(2)
(4)
0.003
30000
(c)
28000 26000
0.002 0.001
Resistivity
Current(A)
24000
0.000
-0.001
(d)
(3)
PD Si NW RTA5min PD Si NW RTA3min PD Si NW RTA1min PD Si NW Si NW
22000 20000 80 60
PD Si NW RTA5min -0.002 PD Si NW RTA3min PD Si NW RTA1min PD Si NW -0.003 Si NW -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
40 20 0
Voltage(V)
S. H. Oh, et. al., Figure 4
PD Si NWs RTA5min PD Si NWs RTA3min PD Si NWs RTA1min PD Si NWs Si NWs
60
FFT Ampulitude(arb.unit)
a)
Transmittance
3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0
50 45 40 35 30 25 20 15 10 5
-4000-3000-2000-1000 0
0 0.0
1000 2000 3000 4000
0.4
Delay time(ps)
TNWs+Substrate/TSubstrate
1.2
c)
PD Si NWs RTA5min PD Si NWs RTA3min PD Si NWs RTA1min PD Si NWs Si NWs
1.0 0.8
0.8
1.2
1.6
2.0
2.4
2.8
Frequancy(THz) 20000
Optical conductivity
1.4
PD Si NWs RTA5min PD Si NWs RTA3min PD Si NWs RTA1min PD Si NWs Si NWs
b)
55
15000
d)
PD Si NWs RTA5min PD Si NWs RTA3min PD Si NWs RTA1min PD Si NWs Si NWs
10000
0.6 0.4
5000
0.2 0
0.0 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Frequancy(THz)
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Frequancy(THz)
S. H. Oh, et. al., Figure