Si-Si Modes Research Articles

Investigation of Phonon Spectral Model for Silicon-Germanium By Inelastic X-Ray Scattering and Raman Spectroscopy

Background and purpose Silicon-germanium (SiGe) alloy thin films are expected to be candidates for next-generation electronic and thermoelectric device materials because they have much lower thermal conductivity than Si and Ge single crystals and high hole-mobility. It is gradually becoming important to understand the physical properties of SiGe in devices. Raman spectroscopy is widely used to evaluate strain, Ge fraction, and temperature from Raman shift. The Raman shifts are Ge fraction dependent, and asymmetric broadening of the Raman spectra occurs at the low energy side. This is because the momentum conservation is relaxed due to the random atom position, and the phonons other than the zone center (Γ point) are excited. Raman scattering of alloy materials can be explained by a spatial correlation model [1] to understand nanoscale characterization. However, Raman spectral fitting of SiGe using a spatial correlation model has not been performed because the phonon dispersion curves of SiGe are unclear. In this study, we investigated the accurate Raman spectral fitting of SiGe using a spatial correlation model based on the SiGe phonon dispersion curve observed by X-ray inelastic scattering (IXS) to understand the local ordering of atoms in SiGe alloy. Experiments The (001)-oriented bulk SiGe samples were prepared by two different growth methods: the Czochralski (Ge fraction: 16%) [2] and the traveling liquidus zone (Ge fraction: 32 and 45%) [3] methods. The crystal orientation and Ge fraction were confirmed by X-ray diffraction measurement.The IXS measurements were performed at the BL35XU beamline of the SPring-8 synchrotron facility [4]. The measurements were performed at room temperature using a reflection geometry. The incident X-ray energy was 17.795 keV, which corresponds to Si (9 9 9) reflection, and the overall energy resolution is 3 meV. The measurements were performed along the Γ-X ([00q]) direction.We obtained the Raman spectra of Si-Si, Si-Ge, and Ge-Ge vibrational modes derived from the bulk SiGe. The focal length and wavenumber resolution of the Raman spectrometer were 2,000 mm and 0.1 cm-1, respectively. The excitation light source was a green laser (wavelength: 532 nm). Results and discussion Figure 1 shows the phonon dispersion curves ([00q]) of the longitudinal optical (LO) mode for Si-Si vibration obtained by IXS. The phonon dispersion curves of bulk Si are shown in Ref. [5]. As the Ge fraction increases, the phonon dispersion curve shifts to the lower energy side and the curvature decreases. Figure 2 shows the results of Raman spectral fitting of the Si-Si mode for bulk SiGe (Ge fraction: 32%) using a spatial correlation model. We used the model for fitting as shown in the inset of Fig. 2, where q, L, and α are momentum, spatial correlation length, and spatial correlation weighting parameter, respectively. ω and Γ0 are Raman shift and natural width, respectively [6]. ω(q) is the phonon dispersion curve. We reproduced the spectrum based on the LO mode phonon dispersion curve of SiGe, and the Roodenko model was applied to α [7]. The fitting results showed that the asymmetric broadening in Raman spectra is reproduced by an application of the spatial correlation model based on the phonon dispersion curves by IXS. In addition, we found that the values of L for Si-Si bonds in bulk SiGe using a spatial correlation model was small as the Ge fraction increases.In conclusion, we investigated the SiGe Raman spectral model based on the SiGe phonon dispersion curve. The proposed model provides an accurate Raman spectral fit and clarifies the nanoscale characterization of the SiGe alloy. Acknowledgements The IXS measurements were performed at SPring-8 with the approval of JASRI (Proposal Nos. 2019A167, 2019B1750, 2020A1463, and 2020A0662). This work was supported by JSPS KAKENHI Grant Number 21K14201, Japan.

Optical Properties of Multilayered Staggered SiGe Nanodots Depending on Si Spacer Growth Temperature

Introduction Uniformly ordered SiGe nanodot (ND) has the potential for optoelectronic device application. By engineering Si surface morphology, heteroepitaxial SiGe layer thickness distribution can be controlled by self-ordering [1]. The self-ordering is an attractive phenomenon for fabricating a vertically ordered stack of SiGe ND fabrication. Yamamoto et al. have successfully fabricated the SiGe staggered NDs by controlling the balance between the surface energy and the residual strain of the Si spacer [2]. We have evaluated the strain state, the optical characteristics, and the band structure of the SiGe staggered and dot-on-dot NDs by Raman and PL (Photoluminescence) spectroscopy, and clarified that the strain state not only in the SiGe NDs but also in the Si spacer affects the emission energy [3]. The surface energy can be controlled by the growth temperature of the Si spacer, and it is expected to modify the dot shape and size. Indeed, the cross-sectional transmission electron microscopy (TEM) observations show that the lower Si spacer growth temperature results in a smaller dot size. However, it has not been sufficiently clarified the effect of the strain induced in NDs and the optical properties of the multilayered staggered NDs realized by the difference in the Si spacer growth temperature. Therefore, we investigated the strain and the luminescence properties of those multilayered staggered SiGe NDs by Raman and PL spectroscopy. Experiment The staggered NDs (designated structure: {Si0.6Ge0.4 NDs /50 nm Si spacer} × 10 cycles) on the Si spacer with the growth temperature of 625, 650, 675, 700, and 725℃ were prepared, which were fabricated on the Si substrate by a reduced pressure chemical vapor deposition. Figure 1 shows the cross-sectional TEM images of the staggered NDs on the Si spacers grown at 625 and 675℃. These samples were evaluated by Raman and low-temperature PL spectroscopy. The Raman spectrometer has a 2,000 mm focal length and a high resolution of approximately 0.1 cm-1. A diode-pumped solid-state (DPSS) laser with a wavelength of 532 nm and a beam diameter of approximately 1 µm as the excitation source was used. The PL measurement setup was equipped with a spectrometer which contains an InGaAs diode array detector, with a measurable wavelength range between 0.9 and 1.7 μm. This spectrometer can control the sample stage temperature between 4 and 300 K with a helium compressor and a heater device. A He-Cd laser with a wavelength of 325 nm and a DPSS laser with a wavelength of 532 nm, here the beam diameters of approximately 50 µm, were utilized as the excitation source. The relatively wide slit of 100 µm enabled obtaining a high PL intensity with the wavelength resolution of approximately 4 nm. Results and Discussion Raman spectrum of the SiGe NDs on the Si spacer grown at the temperature of 625℃ is shown in Fig. 2. The Raman peaks of the Si-Si mode derived from Si spacers and SiGe NDs were observed around 520 cm-1 and 510 cm-1, respectively. Figure 3 shows the Raman shift of Si-Si mode from the SiGe NDs on the Si spacers grown at 625, 650, 675, 700, and 725℃. The large compressive strain seems to be induced in the staggered SiGe NDs compared to the strain-free Raman shift of the single crystalline Si0.6Ge0.4 [4], and the higher growth temperature of the Si spacer leads to stronger compressive strain. Figure 4 shows the PL spectra derived from the SiGe NDs on the Si spacers grown at 625, 650, 675, 700, and 725℃. As shown in Fig. 4, we confirmed the TO-phonon-assist line derived from the Si spacer, transverse-optical (TO)-phonon-assist line, and Non-phonon (NP) line related to SiGe NDs. All staggered NDs show stronger SiGe-derived PL, compared to the TO-phonon-assist line derived from Si spacers. The transition energy of the TO-phonon-assist line derived from SiGe NDs indicates that the higher growth temperature for the Si spacer leads to a wider bandgap. This behavior of the transition energy might be caused by the strain in the SiGe NDs and/or the Si spacers. In conclusion, lower Si spacer growth temperature leads to a smaller size of the SiGe NDs and the red-shift of the luminescence due to the strain state of SiGe NDs and/or Si spacer in the staggered structure.

Evaluation of Thermal Transport Characteristics of Three-Dimensional Self-Ordered Multilayered Silicon-Germanium Nanodots

1. Background and purpose Silicon-germanium (SiGe) alloys have much lower thermal conductivity than Si and Ge single crystals and are expected to be candidates for next-generation thermoelectric device materials of internet of things society. It is important to achieve both high carrier mobility and low thermal conductivity on the basis of the dimensionless figure of merit [1]. There are several studies on the thermoelectric properties of SiGe with low-dimensional structures such as superlattice and nanodots [2,3] to dramatically decrease thermal conductivity. However, the relationship between the nanoscale thermal properties and the phonon scattering mechanism of self-ordered multilayer SiGe nanodots with high uniformity grown by Stranski-Krastanov [4] mode is unclear. In this study, we demonstrated the thermal transport characteristics of the self-ordered multilayer SiGe nanodots. 2. Experiments Si/SiGe superlattice and nanodots samples were fabricated by reduced pressure chemical vapor deposition. We set to 20 cycles of Si0.65Ge0.35 / Si superlattice, the staggered Si0.65Ge0.35 nanodots (alternately stacked) and the dot-on-dot Si0.65Ge0.35 nanodots (vertically stacked) by changing growth temperature and precursors for Si spacer growth [4], as shown in Fig. 1. Thermal properties were evaluated by the Time Domain Thermoreflectance (TDTR) method. The strain states and Ge fraction were evaluated by Raman spectroscopy. The focal length and wavenumber resolution of the Raman spectrometer were 2,000 mm and 0.1 cm-1, respectively. The excitation light source was a green laser (wavelength: 532 nm). We obtained the Raman spectra of Si-Si, Si-Ge, and Ge-Ge vibrational modes derived from the SiGe regions. 3. Results and discussion Figure 2 shows the thermal conductivity for the SiGe region of the superlattice and nanodots (dot-on-dot and staggered structures) obtained by TDTR method. We found that the SiGe nanodots with the dot-on-dot structure have the highest thermal conductivity, while the SiGe nanodots with the staggered structure and SiGe superlattice have almost the same thermal conductivity. We consider that the heat flow easily penetrates from the top (surface) to the bottom (Si substrate interface) in the dot-on-dot structures. On the other hand, it is possible that the staggered SiGe nanodots can play a sufficient role as a phonon scatterer compared to the dot-on-dot structures since the superlattice structure and the staggered SiGe nanodots have almost the same thermal properties. In conclusion, we demonstrated the possibility of controlling phonon scattering by changing the arrangement of the self-ordered multilayer SiGe nanodots. Acknowledgement This work was supported by JSPS KAKENHI Grant Number 21K14201, Japan.

Temperature Dependence of Raman Peak Shift in Single-Crystalline Silicon-Germanium

We demonstrate the relationship between Raman shift ω and temperature T (dω/dT) of silicon-germanium (SiGe) for Si–Si, Si–Ge, and Ge–Ge vibration modes which should be useful in local temperature evaluation of SiGe devices at submicron levels. We investigated the dω/dT of single-crystalline SiGe for Si–Si, Si–Ge, and Ge–Ge vibration modes and its dependence on the Ge fraction using variable-temperature Raman spectroscopy. We clarified that the (dω/dT)s for Si–Si, Si–Ge, and Ge–Ge are fairly constant for all single-crystalline SiGe samples. Therefore, the anharmonic vibration of Si–Si, Si–Ge, and Ge–Ge modes has no Ge-fraction dependence in SiGe. The peak shifts help define the temperature on the submicron-scale surface.

Temperature Dependence of Raman Peak Shift in Single-Crystalline Si-Rich Sige

Background and Objectives Silicon-germanium (SiGe) is used for thermoelectric devices since its thermal conductivity is lower than either pure Si or Ge. Indeed, the material was used in components of the Voyager and New Horizons deep-space craft because of its superior nanoscale heat conductivity. Improving SiGe for better thermoelectric devices requires ever more precise measurements of nanoscale heat conductivity and a comprehensive study on its phonon-scattering mechanism. Especially, a relationship between Raman shift ω and temperature T (dω/dT) is required through the highly precise measurements of its temperature-dependence. Burke et al. have reported the Ge fraction dependence of dω/dT for Si-Si, Si-Ge, and Ge-Ge vibration modes of polycrystalline bulk SiGe samples with high laser power excitation [1]. However, the reported values of dω/dT may be affected by phonon scattering at grain boundaries and the local heating caused by high laser power excitation. Here, we investigated dω/dT of single-crystalline bulk SiGe and its dependence on the Ge fraction by using variable-temperature Raman spectroscopy. Experiments The (001)-oriented single-crystalline Si-rich SiGe bulk samples were prepared by two different growth methods; the Czochralski (Ge fraction: 16%) [2] and traveling liquidus zone (TLZ) methods (Ge fraction: 32 and 45%) [3]. All samples were confirmed to be strain-free by X-ray diffraction (XRD). We also measured Raman spectrum of single-crystalline Si as a reference. For Raman spectroscopy measurements, the wavelength of the excitation source was 532 nm, the focal length of the spectrometer was 2,000 mm, the laser power at the sample surface was kept to less than 1 mW. The sample temperatures were set between 50 and 300°C in 50°C steps, and the Raman signal was measured twice at each temperature (i.e., during heating and cooling). Results and Discussion Raman spectra of Si-Si mode in SiGe (Ge fraction: 16%) are shown in Fig. 1(a). With increasing temperature, the full width at half maximum (FWHM) of the Raman spectra increased and shifted toward the lower side of wavenumber. This wavenumber shift is considered to be caused by the anharmonic vibrations becoming dominant as the temperature increases, and the phonon frequency decrease due to the thermal expansion of the crystal. Figure 1(b) shows the temperature dependences of the Si-Si Raman peak shifts. We experimentally clarified the dω/dT are approximately constant in each bulk SiGe sample, as shown in Fig. 1(b). Figure 1(c) shows the Ge fraction dependence of -(dω/dT) for Si-Si mode. As a result, we found that dω/dT for Si-Si mode is independent of Ge fraction unlike the study of Burke et al. It was reported that the dω/dT depends on the coefficient of linear thermal expansion and anharmonic constants [4]. From the obtained dω/dT values in the investigated range of Ge fraction, we consider that the anharmonic constants of SiGe alloy do not change by Ge fraction. Thus, the Si-Si anharmonic vibration is supposed to be not affected by the Ge fraction in SiGe alloy. In conclusion, we determined dω/dT= -0.023 cm-1/℃ for Si-Si mode from experiments with single-crystalline bulk SiGe samples and low laser power excitation. Acknowledgement This work was supported by Grant-in-Aid for Scientific Research (21K14201).

In-line Raman spectroscopy for gate-all-around nanosheet device manufacturing

In-line Raman spectroscopy for compositional and strain metrology throughout front-end-of-line (FEOL) manufacturing of next-generation gate-all-around nanosheet field-effect transistors is presented. Thin and alternating layers of fully strained pseudomorphic Si(1 − x)Gex and Si were grown epitaxially on a Si substrate and subsequently patterned. Intentional strain variations were introduced by changing the Ge content (x = 0.25, 0.35, 0.50). Polarization-dependent in-line Raman spectroscopy was employed to characterize and quantify the strain evolution of Si and Si(1 − x)Gex nanosheets throughout FEOL processing by focusing on the analysis of Si-Si and Si-Ge optical phonon modes. To evaluate the accuracy of the Raman metrology results, strain reference data were acquired by non-destructive high-resolution x-ray diffraction and from destructive lattice deformation maps using precession electron diffraction. It was found that the germanium-alloy composition as well as Si and Si(1 − x)Gex strain obtained by Raman spectroscopy are in very good agreement with reference metrology and follow trends of previously published simulations.

Evaluation of Phonon Dispersion Relation for Bulk Silicon Germanium by Inelastic X-ray Scattering

1. Introduction Silicon germanium (SiGe) is one of the promising candidates as a next-generation material for thermoelectric devices as an ambient energy harvesting technique because it has low thermal conductivity. For a SiGe thermoelectric device, it is important to reveal the phonon scattering mechanism for the alloy material and the thermal transport in miniaturization to the nanoscale. However, the origin of the low thermal conductivity of the SiGe alloy has not been investigated in a sufficiently rigorous manner yet. In addition, the correlation among the thermal conductivity, the phonon dispersion curve, and the lifetime remains mostly unclear.From the above, we focused on direct phonon properties evaluation by inelastic X-ray scattering (IXS) with synchrotron radiation. IXS is a powerful technique that reveals the phonon dispersion curves in contrast to Raman spectroscopy, which can only give optical phonon modes at the Brillouin zone center (the Γ point). In this study, we evaluated the phonon dispersion curve of bulk SiGe by IXS. 2. Experimental method The single-crystalline SiGe samples for IXS were prepared by two different growth methods: the Czochralski (Cz) [1] and traveling liquidus zone (TLZ) methods [2]. The sample information is summarized in Table 1. Prior to the IXS measurements, the crystal orientation and Ge fraction were confirmed by X-ray diffraction (XRD) and electron backscattering pattern (EBSP) measurements, respectively.The IXS measurements were performed at the BL35XU beamline of the SPring-8 synchrotron facility. The measurements were conducted at room temperature using a reflection geometry. The incident X-ray energy was set to 17.795 keV, which corresponds to Si (9 9 9) reflection, and the overall energy resolution was around 3 meV. The incident X-ray beam size was approximately 60 x 75 μm2. The measurements were conducted along the Γ-X ([00q]) direction. 3. Results and Discussion As an example of phonon dispersion of bulk Si1-x Ge x , we show the IXS results of x = 0.45 in Fig. 1 obtained by peak positions of IXS spectra. As a result, all optical and acoustic phonon modes for bulk SiGe were observed as shown in Fig. 1. In addition, the energy of the optical phonon modes (Si-Si, Si-Ge, and Ge-Ge modes) obtained by IXS at the zone center (Γ point) was good agreement with results of Raman spectroscopy (λ = 532 nm). Both Raman and IXS indicate that, at around Γ point, the phonon energies of Si-Si and Ge-Ge modes tend to be lower than that of pure Si and Ge, respectively. These behaviors can be considered to be caused by random distribution of Si and Ge atoms in the alloy. On the other hand, it was revealed that the acoustic phonon (LA and TA modes) energies are located between those of pure Si [3] and Ge [4]. Based on the above, the origin of the dramatic thermal conductivity reduction of the SiGe alloy, which has been reported in previous study [5], may not be derived from the reduction of group velocity, but the low phonon lifetime reduction. Acknowledgements The IXS measurements were performed at SPring-8 with the approval of JASRI (Proposal Nos. 2016A1496, 2017B1630, 2019A1678, and 2019B1750). The authors thank Dr. Koji Usuda (KIOXIA Corp.) for his great support with analyzing the IXS results throughout the work.

Evaluation of Strain-Shift Coefficients for SiSn by Raman Spectroscopy

1. Introduction Silicon tin (SiSn) is a new promising candidate material for next-generation optical devices since its energy band-gap is theoretically expected to a direct band-gap as Sn fraction increases [1, 2]. The lattice constant difference (strain states) between the substrate and the SiSn thin film directly affects a band-gap, a carrier mobility, and device design. Therefore, it is important to evaluate strain states in SiSn thin film and accurate strain evaluation techniques are indispensable.Raman spectroscopy is one of the powerful strain evaluation techniques because it is a high spatial resolution and nondestructive measurement. However, the strain shift coefficient of SiSn has not been experimentally investigated in contrast to silicon germanium, germanium tin, and highly accurate strain measurement cannot be performed by Raman spectroscopy. In this study, Raman measurement was performed to extract the strain-shift coefficient of SiSn thin films, accurately. 2. Experimental method SiSn thin films (Sn fraction: 0.50, 0.90, 1.8, 2.0, and 6.0%) were grown on (001)-Si substrate by molecular beam epitaxy. Their thickness was set to 50 nm. Sn fraction and in-plane biaxial strain were confirmed by X-ray diffraction (XRD).The Si1-x Sn x thin films were evaluated by Raman spectroscopy. Excitation source of ultraviolet laser (λ = 355 nm) was used and focal length of the Raman spectrometer was 2,000 mm. Wavenumber resolution is approximately 0.1 cm-1. The Raman spectra of the Si1-x Sn x thin films for Si-Si vibration mode were measured to determine the strain-shift coefficient. 3. Results and Discussion Figure 1 shows Raman spectra of Si1-x Sn x with x of 0.06 and bulk Si (Si-Si mode) for comparison. As a result, asymmetrical broadening on the low-wavenumber side with slight peak shift in the Raman spectrum for the SiSn thin film were observed. In the Raman scattering, this broadening is often explained using the so-called spatial correlation model, which reproduces the relaxation of momentum conservation due to random atom positions.Figure 2 shows the relationship between Raman peak shift of SiSn for Si-Si mode (Δω LO) and in-plane strain estimated by reciprocal space mapping using XRD. In this study, D’Costa model (see below equation) was used to obtain strain-free Raman shift for Si-Si mode [3]: ω 0 = 520 – 141.8x. (1)It was revealed that the relationship between Raman peak shift of SiSn for Si-Si mode and in-plane strain has clear linear dependence. From the slope of the straight line which calculated by linear approximation as shown in Fig. 2, the value of the strain-shift coefficient b was found to be -732 cm-1. This value shows good agreement with that of strained Si [4] and SiGe [5]. On the basis of the result, it is suggested that the strain-shift coefficient b is not affected by random atom positions (alloy state), and the value of b may not change by composition and strain state.In conclusion, these insights play an important role in the strain evaluation for the next-generation SiSn device by Raman spectroscopy and in the understanding the relationship between phonon behavior and strain. Acknowledgements This work was supported partly the Japan Society for the Promotion of Science (JSPS) (19K21971 and 17J08240).

Evaluation of Phonon Dispersion Relation for Bulk Silicon Germanium by Inelastic X-ray Scattering

We report on phonon dispersion curves of bulk silicon germanium (Si1-x Ge x ) alloy with the x value of 0.16, 0.32, 0.45, and 0.72 obtained by inelastic x-ray scattering (IXS) with synchrotron radiation. All phonon dispersion relations of optical (Ge-Ge, Si-Ge, Si-Si modes) and acoustic modes were observed along the Γ-X ([00q]) direction obtained by energy positions of IXS spectra. The phonon dispersion curves of bulk Si1-x Ge x showed behaviors similar to those of pure Si and Ge. We also confirmed Ge fraction dependence of acoustic modes and revealed that the acoustic phonon energies are located between those of pure Si and Ge. From the above, it has been suggested that low thermal conductivity of SiGe alloy may contribute to phonon lifetime rather than group velocity.

Evaluation of Thermal Conductivity Characteristics in Polycrystalline Silicon - The Effect of Nanostructure in the Grains

1.Introduction Polycrystalline silicon (poly-Si) is a promising candidate for channel materials such as thin film transistors (TFTs). Moreover, poly-Si is also expected for the next-generation thermoelectric device materials. It has been reported that poly-Si has low thermal conductivity due to their grain boundaries and defects, and so on [1,2]. We have previously confirmed that nanostructures exist in poly-Si grains, and these nanostructures become the phonon scatters [3]. However, the detailed effect of annealing on the nanostructure size has not been sufficiently. In this study, we focused on only the nanostructure size by changing the additional annealing conditions during poly-Si thin film fabrication with keeping the same grain size, and evaluated the thermal conductivity characteristics of the poly-Si thin film. 2.Experiments The 100 nm thick SiO2 films were thermally grown on Si substrates. The 150 nm thick Amorphous Si (a-Si) thin films were deposited at 510 °C by low pressure chemical vapor deposition (LPCVD) using the mono-silane (SiH4) gases under the pressure of 0.4 Torr. The a-Si thin films on SiO2/Si substrate were then annealed at 700 °C 2 hrs (Sample A), 700 °C 2hrs + 900 oC 2hrs (Sample B) and 700 °C 2 hrs +1100 °C 2 hrs (Sample C), respectively. The grain size is determined by the first annealing. Therefore, the grain size of each sample is almost the same at 600 nm.The fabricated samples were measured by UV Raman spectroscopy for the investigation of thermal conductivity characteristics. The excitation source was the UV laser (λ=355 nm), and the focal length of the spectrometer was 2,000 mm. The laser power was controlled by a variable neutral density (ND) filter in the incident light path from 1 to 10 mW in 1 mW steps. Five-point Raman spectra measurements were performed on each sample to evaluate the thermal conductivity characteristics. 3.Results and Discussion Figure 1 shows the laser-power-dependent Raman spectra of Sample A. It was confirmed that the higher laser power resulted in the higher Raman spectral intensity. In addition, it was confirmed the Raman spectrum tends to shift toward lower wavenumber as the laser power increases. It has been reported that the Raman spectrum for Si-Si mode shift toward lower wavenumber due to the thermal effect as temperature increases. Therefore, it is considered that the change in the Raman shift is due to the temperature change caused by local laser heating. Table 1 shows the average nanostructure size of each sample. The nanostructure size is calculated by the full width at half maximum (FWHM) of the UV Raman spectroscopy [4]. Figure 2 shows that the laser-power-dependent Raman peak shift from the poly-Si thin films. It was confirmed that there is a difference in zero power intercept due to phonon confinement effect. It is considered that the Raman spectrum for sample A shifted due to the phonon confinement effect possibly caused by the smaller nanostructures inside the grain. In addition, it was also confirmed that the discrepancy in the Raman shifts between each sample becomes larger as the laser power increases, and it indicates that there is a difference in the temperature change for each sample. The relation between Raman shift and temperature are described by following Eq. (1) [5].dω/dT = -0.024 cm-1/K (1)Figure 3 shows that temperature in poly-Si thin film measured by Si Raman peak shift. It was revealed that Sample B has lower thermal conductivity despite the nanostructure size is larger. It may need to consider not only size but also density of the nanostructure to understand these phenomena. In conclusion, we clearly indicated that not only the average grain size but also the nanostructures inside grains should be carefully considered to control thermal conductivities. REFERENCE Y. Nakamura, Sci. Technol. Adv. Mater. 19, 31 (2018).K. Valalaki, N. Vouroutzis, and A. G. Nassiopoulou, J. Phys. D: Appl. Phys. 49, 315104 (2016).H. Takeuchi, R. Yokogawa, K. Takahashi, K. Komori, T. Morimoto, N. Sawamoto and A. Ogura, 236th ECS Meeting. G04-1210 (2019).H. Yamazaki, M. Koike, M. Saitoh, M. Tomita, R. Yokogawa, N. Sawamoto, M. Tomita, D. Kosemura, and A. Ogura, Sci. Rep. 7,16549 (2017).D. Fan, H. Sigg, R. Spolenak, and Y. Ekinci, Phys. Rev. B 96, 115307 (2017). Figure 1

Evaluation of Strain-Shift Coefficients for SiSn by Raman Spectroscopy

We report on the strain-shift coefficient of silicon tin (SiSn) alloys obtained by ultraviolet (UV) Raman spectroscopy. The values of the Raman peak shift of the Si-Si mode for the SiSn (Sn fraction: 0.5, 0.9, 1.8, 2.2, and 6.0%) thin films were found to increase with Sn fraction and it indicates that compressive strain is induced in the SiSn films. The strain-shift coefficient of the Si-Si mode obtained from linear fits to the measurement data (UV Raman spectroscopy and X-ray diffraction reciprocal space mapping) has a good agreement with the values of the strain-shift coefficients about strained Si and strained silicon germanium, and commercial high-purity Si reported by previous studies. The strain-shift coefficient determined by in this work contribute to realize strain measurements for the next-generation SiSn devices such as near-infrared optical and thermoelectric devices.

Temperature Measurement for Si Nanowire Thermoelectric Generators By Operand Raman Spectroscopy

1. Background and purpose Silicon nanowire (SiNW) is a promising candidate for next-generation thermoelectric materials, because they maintain both low-thermal conductance and high electric conductance simultaneously [1]. It has been reported that thermoelectric power is enhanced by miniaturizing the SiNW [2], and it is very important to measure the temperature difference across the SiNW for examination of thermoelectric characteristics. However, it becomes gradually difficult to measure the temperature with miniaturization of the SiNW. From the above, we focused on the temperature measurement using Raman spectroscopy. In this study, we demonstrated the measurement of temperature difference for the SiNW thermoelectric generator using operand Raman spectroscopy, in order to evaluate of thermoelectric characteristics accurately. Experimental method We fabricated SiNWs using an SOI substrate (SOI layer 55 nm/Buried oxide 145 nm/Si substrate 750 μm) via electron beam lithography and ICP-RIE. Thermal oxidation was performed in dry oxygen ambient at 850 oC for 3h, to form 20 nm thick oxide film on the surface of the SiNWs. The height and width of the SiNW were approximately 35 nm and 100 nm, respectively. The length of SiNWs were assigned to 8, 46, and 90 μm. Phosphorus (P) ions were then implanted at an acceleration energy of 25 keV, with a dose of 1.0 × 1015 ions/cm2 followed by activation annealing at 950 °C for 10 min under vacuum conditions. Next, the SiO2 layer at the pad regions was removed via photolithography and a wet-etching process using buffered hydrofluoric acid. Then, the Si pads and both ends of the SiNWs were nickelized by depositing a Ni film followed by rapid thermal annealing at 410 °C for 20 min. Thus, the both ends of the SiNWs were connected to the NiSi pads. Then, thermally conductive AlN film was deposited onto one of the NiSi pads via RF reactive sputtering to facilitate heat conduction from the heat source, where Al and an Ar/N2 mixture are used as the sputtering target and sputtering gas, respectively. Finally, a forming gas anneal was conducted at 400 °C. The fabricated SiNW-μTEG consists of 8 arrays of 400 legs [3]. Temperature measurement was performed using Raman spectrometer equipped with custom-made microthermostat consisting of a heater and a platinum thermometer on an aluminum nitride ceramic plate. Figure 1 shows schematic of operand Raman measurement. The focal length of the Raman spectrometer was 2,000 mm and the wavenumber resolution was approximately 0.1 cm-1. The excitation source was UV (λ = 355 nm) laser with the penetration depth for bulk Si of approximately 5 nm. From obtained Raman spectra of SiNW regions, temperature was estimated. Results and Discussion Figure 2 shows time transition of Raman shift obtained by operand Raman spectroscopy. The Raman shifts were measured at the edge on the heat source side and the middle of the 46 μm-long SiNWs. As shown in Fig. 2, an apparent change in Raman shift was observed, and the Raman peak tends to shift toward lower wavenumbers over time. It has been reported that the Raman spectrum for Si-Si mode shifts toward lower wavenumbers and broadened by a thermal effect as temperature rises. To obtain thermal conductivity properties in detail, the temperature in SiNW region was estimated using the relation dω/dT = −0.024 cm−1/K [4]. Figure 3 shows time transition temperature obtained by operand Raman spectroscopy. As shown in Fig. 3, we confirmed that the temperature increases over time by operand Raman spectroscopy. Moreover, it was found that the heat transfer tends to be delayed at the SiNW region away from the heat source (L=23 μm). Based on the above, we consider that operand Raman spectroscopy is a novel promising technique that can measure the temperature in a very small region. Acknowledgements This work was supported partly by JST CREST Grant Number JPMJCR15Q7 and JPMJCR19Q5, and the Japan Society for the Promotion of Science (JSPS) through a JSPS Fellows (17J08240).

Composition and strain analysis of Si1-xGex core fiber with Raman spectroscopy

The fabrication and characterization of Si1-xGex core fiber have attracted much attention because of its great application potential in new optoelectronic devices. In this work, by assembling two semi-cylindrical monocrystalline Si and Ge rods into a silica tube, we present a fabrication method to draw Si1-xGex core silica clad fiber with graphite furnace. Raman spectra analysis reveals that in all regions of the core formed the Si1-xGex alloy. The optical microscopic photograph shows that in the core of a diameter of 36 μm distributed the bright and dark regions, where it was further proved by Raman spectroscopy that the bright regions are Ge-rich areas and the dark regions are rich in silicon. By recording the Raman spectra of consecutive regions, it was found that with the increase of Ge content (x<0.5) the peak intensity of Si-Ge mode obviously increases, similar to the intensity of Ge-Ge mode, while the peak intensity of Si-Si mode decreases. Then we made a quantitative analysis of the components and strain by mapping the Raman spectra of the fiber core. The experimental results show that the Ge content mainly distributes between 0.1 and 0.8, concentrating between 0.2 and 0.3, and the strain distribution on the surface is obtained at the same time.

Enhanced Electron Mobility in Nonplanar Tensile Strained Si Epitaxially Grown on SixGe1-x Nanowires.

We report the growth and characterization of epitaxial, coherently strained SixGe1-x-Si core-shell nanowire heterostructure through vapor-liquid-solid growth mechanism for the SixGe1-x core, followed by an in situ ultrahigh-vacuum chemical vapor deposition for the Si shell. Raman spectra acquired from individual nanowire reveal the Si-Si, Si-Ge, and Ge-Ge modes of the SixGe1-x core and the Si-Si mode of the shell. Because of the compressive (tensile) strain induced by lattice mismatch, the core (shell) Raman modes are blue (red) shifted compared to those of unstrained bare SixGe1-x (Si) nanowires, in good agreement with values calculated using continuum elasticity model coupled with lattice dynamic theory. A large tensile strain of up to 2.3% is achieved in the Si shell, which is expected to provide quantum confinement for electrons due to a positive core-to-shell conduction band offset. We demonstrate n-type metal-oxide-semiconductor field-effect transistors using SixGe1-x-Si core-shell nanowires as channel and observe a 40% enhancement of the average electron mobility compared to control devices using Si nanowires due to an increased electron mobility in the tensile-strained Si shell.

Shell morphology and Raman spectra of epitaxial Ge−SixGe1−x and Si−SixGe1−x core-shell nanowires

We investigate the shell morphology and Raman spectra of epitaxial Ge−SixGe1−x and Si−SixGe1−x core−shell nanowire heterostructures grown using a combination of a vapor−liquid−solid (VLS) growth mechanism for the core, followed by in-situ epitaxial shell growth using ultra-high vacuum chemical vapor deposition. Cross-sectional transmission electron microscopy reveals that the VLS growth yields cylindrical Ge, and Si nanowire cores grown along the ⟨111⟩, and ⟨110⟩ or ⟨112⟩ directions, respectively. A hexagonal cross-sectional morphology is observed for Ge-SixGe1-x core-shell nanowires terminated by six {112} facets. Two distinct morphologies are observed for Si-SixGe1-x core-shell nanowires that are either terminated by four {111} and two {100} planes associated with the ⟨110⟩ growth direction or four {113} and two {111} planes associated with the ⟨112⟩ growth direction. We show that the Raman spectra of Si- SixGe1-x are correlated with the shell morphology thanks to epitaxial growth-induced strain, with the core Si-Si mode showing a larger red shift in ⟨112⟩ core-shell nanowires compared to their ⟨110⟩ counterparts. We compare the Si-Si Raman mode value with calculations based on a continuum elasticity model coupled with the lattice dynamic theory.

Spectroscopic ellipsometry and X-ray diffraction studies on Si1-xGex/Si epifilms and superlattices

Comprehensive optical and structural properties are reported on several MBE grown thin Si1-xGex epifilms and Si1-xGex/Si superlattices with low Ge contents by using spectroscopic ellipsometry (SE), high resolution x-ray diffraction (HR-XRD) and Raman scattering (RS). For thin Si1-xGex films, our appraised results of the optical dielectric functions from SE spectra fitted to the parameterized models have revealed discrepancies with the existing data. While E1 and E1+Δ1 critical point energies have shown similarities, their amplitudes uncovered ∼25% larger value for the E1 band-edge, and ∼10% larger value for the E1+Δ1 band-edge. In our samples, the observed vibrational peaks in the RS are classified as unstrained SiSi, GeGe and GeSi modes. These mode assignments in Si1-xGex alloys are evaluated compared and discussed with the available RS data.

Coherently Strained Si-SixGe1-x Core-Shell Nanowire Heterostructures.

Coherently strained Si-SixGe1-x core-shell nanowire heterostructures are expected to possess a positive shell-to-core conduction band offset, allowing for quantum confinement of electrons in the Si core. We report the growth of epitaxial, coherently strained Si-SixGe1-x core-shell heterostructures through the vapor-liquid-solid mechanism for the Si core, followed in situ by the epitaxial SixGe1-x shell growth using ultrahigh vacuum chemical vapor deposition. The Raman spectra of individual nanowires reveal peaks associated with the Si-Si optical phonon mode in the Si core and the Si-Si, Si-Ge, and Ge-Ge vibrational modes of the SixGe1-x shell. The core Si-Si mode displays a clear red-shift compared to unstrained, bare Si nanowires thanks to the lattice mismatch-induced tensile strain, in agreement with calculated values using a finite-element continuum elasticity model combined with lattice dynamic theory. N-type field-effect transistors using Si-SixGe1-x core-shell nanowires as channel are demonstrated.

Effects of alloy disorder and confinement on phonon modes and Raman scattering in SixGe1−x nanocrystals: A microscopic modeling

Confinement and alloy disorder effects on the lattice dynamics and Raman scattering in Si1−xGex nanocrystals (NCs) are investigated numerically employing two different empirical inter-atomic potentials. Relaxed NCs of different compositions (x) were built using the Molecular Dynamics method and applying rigid boundary conditions mimicking the effect of surrounding matrix. The resulting variation of bond lengths with x was checked against Vegard's law and the NC phonon modes were calculated using the same inter-atomic potential. The localization of the principal Raman-active (Si-Si, Si-Ge, and Ge-Ge) modes is investigated by analysing representative eigenvectors and their inverse participation ratio. The dependence of the position and intensity of these modes upon x and NC size is presented and compared to previous calculated results and available experimental data. In particular, it is argued that the composition dependence of the intensity of the Si-Ge and Ge-Ge modes does not follow the fraction of the corresponding nearest-neighbour bonds as it was suggested by some authors. Possible effects of alloy segregation are considered by comparing the calculated properties of random and clustered SixGe1−x NCs. It is found that the Si-Si mode and Ge-Ge mode are enhanced and blue-shifted (by several cm−1for the Si-Si mode), while the intensity of the Si-Ge Raman mode is strongly suppressed by clustering.

Influence of a Thickness and Percentage of Ge in Optical Properties of Thin Films of Si1-xGex Grown by LPCVD

Optical properties of Si1-xGex thin films obtained by LPCVD, with different thickness and percentages of Ge are investigated in this work. Raman spectroscopy was employed for the determination of Ge percentage (x) and strain (ε) present on the films, using equations that involve the frequencies of Si-Si, Si-Ge and Ge-Ge Raman vibrational modes. From these calculations, an increase in the percent of Ge is observed with the rise of time of deposition. The refractive index of each film was determined by Ellisometry, where an abrupt increase of this parameter is detected for films of lower thickness. This result could be due to the quantum confinement effects as a consequence of the thickness of these films, which is comparable with the excitonic Bohr diameter of Ge. The best result was obtained to the film Si0.63Ge0.37 grown at 30s with a refractive index n= 3.141 for λ= 632.9 nm, thickness of 62.7 nm and a density of nanoparticle about 1.0x1010 cm-2 with an average size of ~ 47 nm.

Low-temperature metal-induced crystallization of hydrogenated amorphous Si1-xGex (0.25≤x≤1) thin films with Au solution

Low-temperature (∼ 400 °C) metal-induced crystallization of hydrogenated amorphous silicon–germanium thin films using Au solution has been investigated by X-ray diffraction, Raman spectra, scanning electron microscopy and atomic force microscopy. It was shown that Au solution significantly promotes the crystallization of the films at low temperatures. The effects of annealing temperature, Ge fraction in the films and the concentration of Au solution on the structure and morphology of the films were analyzed. An increase in crystallinity was observed on increasing the annealing temperature and concentration of Au solution. High-frequency shifts of Ge-Ge and Si-Ge modes and a low-frequency shift of the Si-Si mode were found on increasing the Ge fraction. Compared with Au-induced crystallization, the Au solution induced crystallization tends to give large crystal grains.