Phonons In Ge Research Articles

Thermal resistance from non-equilibrium phonons at Si–Ge interface

As nanostructured devices become prevalent, interfaces often play an important role in thermal transport phenomena. However, interfacial thermal transport remains poorly understood due to complex physics across a wide range of length scales from atomistic to microscale. Past studies on interfacial thermal resistance have focused on interface-phonon scattering at the atomistic scale but overlooked the complex interplay of phonon-interface and phonon-phonon scattering at microscale. Here, we use the Peierls-Boltzmann transport equation to show that the resistance from the phonon-phonon scattering of non-equilibrium phonons near a Si–Ge interface is much larger than that directly caused by the interface scattering. We report that non-equilibrium in phonon distribution leads to significant entropy generation and thermal resistance upon three-phonon scattering by the Boltzmann's H-theorem. The physical origin of non-equilibrium phonons in Ge is explained with the mismatches of phonon dispersion, density-of-states, and group velocity, which serve as general guidance for estimating the non-equilibrium effect on interfacial thermal resistance. Our study bridges a gap between atomistic scale and less studied microscale phenomena, providing comprehensive understanding of overall interfacial thermal transport and the significant role of phonon-phonon scattering.

Ion irradiation-induced effects on optical and structural properties of Ge nanoparticles synthesized by laser ablation

We present here the effects of swift heavy ion irradiation on the optical and structural properties of Germanium (Ge) nanoparticles (NPs) synthesized by using laser ablation of bulk Ge wafer in liquids. The Ge NPs have been irradiated with 120 MeV Ag ions of various fluences ranging from 5 × 1012 to 1 × 1014 ions/cm2. The effect of ion irradiation on optical and structural properties of the Ge NPs has been studied with transmission electron microscopy (TEM), Raman scattering, and photoluminescence (PL) emission measurements. We have observed a decrease in the average size of the Ge NPs upon increasing the fluence of irradiation, amorphization at high fluence, and smaller NPs started to grow in the vicinity of larger sized NPs under ion irradiation. Therefore, the proper selection of fluence can be tuned to modify the existing NPs and synthesize new NPs. The NPs became amorphous at the higher fluence with the core remaining crystalline while the shell transformed into an amorphous phase. From Raman scattering results, the shift observed in peak position and the increase in the broadening in the Ge phonon vibrational peak indicates a reduction in NPs size. The PL emission is also blue-shifted upon increasing the irradiation fluence.

Thermal Transport across a Semiconductor/Semiconductor Interface: A First-Principles-Based Approach

Controlling heat flow across material heterojunctions is important for thermoelectrics and thermal management of electronic devices. A clear physical understanding of what transport mechanism dominates near an interface can help benefit technology. We present a theoretical investigation of phonon transport across a semiconductor/semiconductor interface, specifically Si/Ge, and demonstrate how inelastic scattering and non-equilibrium effects play a key role. We treat phonon transport with the McKelvey-Shockley flux method, which is efficient, captures ballistic and non-equilibrium effects, inelastic scattering, and has shown excellent agreement with the more computationally-demanding Boltzmann equation. The Si and Ge phonon dispersions and 3-phonon scattering rates, serving as input for the transport modeling, are calculated from first-principles. The results show that, while the maximum phonon frequency in Si is nearly double that of Ge, significant heat currents are carried by the high-frequency Si phonons above the Ge cutoff. When approaching the interface, inelastic scattering redistributes energy to the phonon frequencies that can transfer elastically across the Si/Ge junction. We explain how this collective reorganization of phonons is driven by non-equilibrium effects near the interface. We also include a model for the contact resistance of an ideal interface, that depends on both phonon dispersions, which provides a lower limit for a given material combination. These results help provide clear physical insights into what controls phonon transport at semiconductor/semiconductor interfaces.

Optical properties of hybrid Si1−xGex/Si quantum dot/quantum well structures grown on Si by RPCVD

We experimentally investigated the optical properties of structures consisting of a Si1−xGex/Si quantum well and stacked quantum dots on Si substrates. The Ge composition determined for Si1−xGex quantum wells and dots was approximately 20% and 30%, respectively. Three Raman peaks observed at approximately 520, 410, and 295cm−1 correspond to vibration of Si–Si, Si–Ge, and Ge–Ge phonons, and a peak related to Si1−xGex quantum dots was observed at 490cm−1. Photocurrent spectra were dominated by transitions related to the quantum dots and quantum well corresponding to the energy gap and split-off band for Si and energy for Si1−xGex quantum dots and wells.

Characterization of reduced pressure chemical vapor deposited Si0.8Ge0.2/Si multi-layers

We have investigated the Si0.8Ge0.2/Si multi-layer grown directly onto the Si (001) substrates using reduced pressure chemical vapor deposition. The thicknesses of the Si0.8Ge0.2/Si multi-layer were determined using transmission electron microscopy. From the results of energy-dispersive X-ray spectroscopy and X-ray diffraction analyses on the Si0.8Ge0.2/Si multi-layer, Ge composition in the Si1−xGex layers was determined as ∼20% and the value of residual strain ε of the Si0.8Ge0.2 layer is calculated to be 0.012. Three peaks are observed in Raman spectrum, which are located at approximately 514, 404, and 303cm−1, corresponding to the vibration of Si–Si, Si–Ge, and Ge–Ge phonons, respectively. The photoluminescence spectrum originates from the radiative recombinations both from the Si substrate and the Si0.8Ge0.2/Si multi-layer. For the Si0.8Ge0.2/Si multi-layer, the transition peaks related to the quantum well region observed in the photocurrent spectrum were preliminarily assigned to e–hh and e–lh fundamental excitonic transitions.

Optical properties of Si 0.8Ge 0.2/Si multiple quantum wells

The optical property was studied on the Si 0.8Ge 0.2/Si strained multiple quantum well (MQW) structure grown using ultra-high vacuum chemical vapor deposition (UHV-CVD). Three peaks are observed in Raman spectrum, which are located at about 510, 410, and 300 cm −1, corresponding to the vibration of Si–Si, Si–Ge, and Ge–Ge phonons, respectively. The photoluminescence (PL) spectrum originates from the radiative recombinations both from the Si substrate and the Si 0.8Ge 0.2/Si MQW. For Si 0.8Ge 0.2/Si strained MQW, the transition peaks related to the MQW region observed in the photocurrent (PC) spectrum were preliminarily assigned to electron–heavy hole (e–hh) and electron–light hole (e–lh) fundamental excitonic transitions.

Overlayer Surface-Enhanced Raman Spectroscopy for Studying the Electrodeposition and Interfacial Chemistry of Ultrathin Ge on a Nanostructured Support

Ultrathin films of germanium (Ge) have been electrodeposited onto surface-enhanced raman spectroscopy (SERS)-active, polycrystalline gold (Au) nanoparticle film electrodes from aqueous solutions containing dissolved GeO2. An overlayer SERS strategy was employed to use the SERS-activity of the underlying Au electrode to enhance the Raman signatures separately for the Ge phonon mode and vibrational modes of surface groups. Electrochemical and spectroscopic data are presented that demonstrate monolayer-level detection of the electrodeposited material and the preparation of crystalline Ge films exhibiting quantum-confinement effects. Potential-dependent Raman spectra are shown that identify electrodeposition conditions where Ge films can be deposited with either long- or short-range crystalline order. Raman spectra collected with electrodeposited Ge films immersed in solutions containing CN-(aq) did not indicate a significant presence of pinholes that exposed the underlying Au(s) substrate. Raman spectra were also collected that identified a potential-dependence for Ge hydride formation at the interface of these films. Separate spectra were collected for the oxidative dissolution of Ge in solution and the complete dry oxidation of Ge to GeOx in air. These data sets cumulatively represent the first demonstration of the overlayer SERS strategy to follow surface chemical processes at crystalline, nanostructured, Ge materials in situ and in real time.

The contributions of composition and strain to the phonon shift in [formula omitted] alloys

Strained and relaxed Ge 1− x Sn x alloys grown on Si(001) substrates by molecular beam epitaxy have been studied by Raman scattering. The compositional dependence of the Ge–Ge Raman mode has been measured to be Δ ω ( x ) = − 30.3 x cm − 1 for the strained alloys while it is Δ ω ( x ) = − 83.1 x cm − 1 for the relaxed ones, which are in good agreement with theoretical predictions. The results clearly show that, with the increase of Sn concentration, the Ge–Ge phonon frequency decreases much more slowly in the strained alloys than in the relaxed ones.

Microstructural, chemical bonding, stress development and charge storage characteristics of Ge nanocrystals embedded in hafnium oxide

We have grown Ge nanocrystals (NCs) (4.0–9.0 nm in diameter) embedded in high-k HfO2 matrix for applications in floating gate memory devices. X-ray photoelectron spectroscopy has been used to probe the local chemical bonding of Ge NCs. The analysis of Ge–Ge phonon vibration using Raman spectroscopy has shown the formation of compressively stressed Ge NCs in HfO2 matrix. Frequency dependent electrical properties of HfO2/Ge-NCs in HfO2/HfO2 sandwich structures have been studied. An anticlockwise hysteresis in the capacitance–voltage characteristics suggests electron injection and trapping in embedded Ge NCs. The role of interface states and deep traps in the devices has been thoroughly examined and has been shown to be negligible on the overall device performance.

Polarized Raman study of self‐assembled Ge/Si dots under hydrostatic pressure

AbstractWe have studied the correlation between the morphology of Si‐capped Ge islands grown on Si substrates and their vibrational properties as measured by Raman scattering at ambient and under high hydrostatic pressure. For this study we have grown by molecular beam epitaxy a sample that exhibits a size distribution with two distinct island types having different aspect ratios called pyramids and domes. In accordance, Raman spectra show a double‐peak structure at the frequencies of the Ge–Ge vibrational mode (∼300 cm–1) as well as for the Si–Ge mode (around 400 cm–1). Pairwise, different selection rules have been found for these modes depending upon the Raman polarization configurations, suggesting a correlation between both Ge dot sizes and the double peak structure in the Raman spectra. Finally, a different strain status of the dots was observed by studying the pressure coefficient of the Ge–Ge phonon mode, resulting in a value of 3.3 and 3.6 cm–1/GPa for pyramids and domes, respectively. The main conclusion of our work is that the double‐peak structure observed in Raman spectra is indicative of a bimodal dot size distribution in contrast with what is proposed in previous work within the so‐called core‐shell model [M. Y. Valakh et al., Nanotechnol. 16, 1464 (2005)]. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vibrational relaxation of adsorbates at semiconductor surfaces: H on Ge(100)

Infrared pump-sum frequency generation (IR pump-SFG) probe spectroscopyis used to study the vibrational energy relaxation of hydrogen adsorbed ona Ge(100) surface. We found that the symmetric Ge–H stretch mode at1992 cm−1 (υs = 1) for theGe(100)-(2 × 1):H surface has a population relaxation time(T1)of 0.70 ± 0.07 ns at a substrate temperature of 323 K which is of the same order asSi(100)-(2 × 1):H. The population of the stretch mode decays exponentially in the temperature rangetested from 273 to 400 K. The observed decay constants decrease from a value of the orderof one nanosecond to a value of the order of 100 picoseconds, when the substratetemperature is increased from 273 to 400 K. This strong temperature dependence isconsistent with a decay via a multiphonon process, which may be facilitated by the rapidincrease of the density of states associated with the low-frequency TA mode.From the evaluation of the temperature dependence we suggest that the Ge–Hstretch mode decays by simultaneously exciting three bending mode quanta at530 cm−1 and more than two bulk Ge phonons.

Inelastic strain relaxation in the Ge quantum dot array

Raman spectra of Si∕Ge∕Si (100) structures with Ge quantum dots (QDs) obtained by means of the low-temperature (200–300°C) molecular beam epitaxy are investigated. An extremely narrow Ge phonon line of the “anomalously” high intensity and a doublet structure of the Ge–Si mode are observed. These features are explained by the formation of a pure pseudomorphic state of a QD array to a Si substrate. Additional broad Ge phonon lines related to inelastic strain relaxation are found under the variation of growth conditions. The observed strain relaxation is strongly nonuniform for the two well known mechanisms of inelastic strain relaxation.

Growth of Ge islands and nanocrystals using RF magnetron sputtering and theircharacterization

Ge nanocrystals of different sizes and densities have been grown on p-Si(100) substrates byradio-frequency magnetron sputtering. We have also synthesized Ge nanocrystals embedded in aSiO2 matrix, using a suitable target, followed by post-deposition annealing at900 °C ina N2 atmosphere. Atomic force and transmission electron micrographsrevealed the formation of Ge clusters on the Si and within theSiO2 matrix, respectively. Infrared and x-ray photoelectron spectroscopy were used to study thechemical bonding in the deposited films. Raman spectra for Ge embedded in theSiO2 matrix showa peak at 300 cm−1 corresponding to the Ge–Ge phonon mode. A photoluminescence study has been performedat room temperature to detect the light emission from the Ge nanocrystals.

Resonant Raman scattering in GeSi/Si superlattices with GeSi quantum dots

The resonant Raman scattering in GeSi/Si structures with GeSi quantum dots has been analyzed. These structures were formed at various temperatures in the process of molecular-beam epitaxy. It has been shown that Raman scattering spectra recorded near resonances with the E 0 and E 1 electronic transitions exhibit the lines of Ge optical phonons whose frequencies differ significantly from the corresponding values in bulk germanium. In the structures grown at low temperatures (300–400°C), the phonon frequency decreases with increasing excitation energy. This behavior is attributed to Raman scattering, which is sensitive to the size of quantum dots, and shows that quantum dots are inhomogeneous in size. In the structures grown at a higher temperature (500°C), the opposite dependence of the frequency of Ge phonons on excitation energy is observed. This behavior is attributed to the competitive effect of internal mechanical stresses in quantum dots, the localization of optical photons, and the mixing of Ge and Si atoms in structures with a bimodal size distribution of quantum dots.

Structure and photoluminescence of Ge nanoparticles embedded in SiO2 gel glasses fabricated at different temperatures

Abstract Ge nanocrystals embedded in bulk SiO 2 glasses were prepared by combining the sol–gel process and the heat treatment in a H 2 gas atmosphere. The size of Ge nanoparticles decreases with a decrease in the heat treatment temperature in a H 2 gas atmosphere, and thus the crystalline Ge–Ge phonon vibration mode shifts to lower frequencies. The SiO 2 gel glasses doped with Ge nanoparticles prepared at 500 °C showed a strong room temperature photoluminescence (PL) with peaks at 572, 609, 673, and 730 nm under 514.5 nm Ar + laser excitation. When the heating temperature is increased to 600–700 °C, the peak positions of the PL spectra hardly vary. However, the PL intensity decreases tempestuously. The PL arises from the ultra small Ge clusters instead of nanocrystalline Ge with the diamond structure.

Raman scattering studies of Ge/Si islands under hydrostatic pressure

AbstractRaman scattering under hydrostatic pressure is used to investigate the phonon modes of self‐assembled Ge islands grown by solid source molecular beam epitaxy. Our results show that the overlapping spectra of the Ge–Ge phonon and Si 2TA modes occur around 304 cm–1 at ambient pressure can be resolved at relatively low pressure of about 3 kbar. The mode Grüneisen parameter of the Ge–Ge phonon mode in quantum dot is found to be γ = 0.84 ± 0.01. The normalized Raman intensity profiles of Ge–Ge mode, exhibits a resonance enhancement peak at 2.37 eV. The pressure coefficient of this resonating electronic transition is ∼2.7 ± 0.5 meV/kbar, which is significantly smaller than the pressure shift of the E1 transition in bulk Ge. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of hydrostatic pressure on Raman scattering in Ge quantum dot superlattices

Self-organized Ge/Si quantum dots (QD's) in strained Si/Ge short-period superlattices are studied by Raman scattering under hydrostatic pressure excited in-resonance and off-resonance with the confined Ge-like E 1 transition of Ge using 488 and 514.5 nm lines from an argon-ion laser and 632.8 nm line from a He–Ne laser. The Raman spectra of Ge–Ge, Si–Ge and Si-2TA modes of the QD's were obtained as a function of pressure in the range of 1–70 kbar. Our results show that the mode Grüneisen parameter of the Ge–Ge phonon mode in QD's is found to be γ=0.81±0.01, which is smaller than that of the bulk Ge. We observe resonance effects with the confined Ge-like E 1 transition and the pressure coefficient of this resonating electronic transition obtained is ∼5±1 meV kbar −1.

Phonons in self-assembled Ge/Si structures

We present the results of an investigation dealing with fundamental vibrations in periodical Ge/Si structures with small-size Ge quantum dots (QDs) performed using macro- and micro-Raman spectroscopy under resonant and off-resonant conditions. Samples with different number of repetition of Ge and Si layers contain Ge QDs with an average dot base size of 15 nm and a QD height of 2 nm . Periodic oscillations observed in the low-frequency region of the Raman spectra are assigned to folded LA phonons in the Ge QD superlattices. The measured phonon frequencies are in a good agreement with those calculated using the Rytov model. These oscillations are superimposed with a broad continuous emission originating from the whole acoustic dispersion branch due to a breaking up of translational invariance. The Raman spectra of the structure with single Ge QD layer reveal a series of peaks corresponding to LA phonons localized in the Si layer. Using the measured phonon frequencies and corresponding wave vectors the dispersion of the LA phonons in the Si is obtained. The longitudinal-acoustic wave velocity determined from the dispersion is 8365 ms −1 and in excellent agreement with that derived from the Brillouin study. In the optical phonon range, the LO and TO phonons localized in Ge QDs are observed. The position of the LO Ge phonons shifts downwards with increasing excitation energy (from 2.5 to 2.7 eV) indicating the presence of a QD size distribution in Ge dot superlattices. Raman scattering from Ge QDs is size-selectively enhanced by the resonance of the exciting laser energy and the confined excitonic states.

Size-selective Raman scattering inself-assembled Ge/Si quantum dot superlattices

Self-organized Ge quantum dot (QD) superlattices havingproperties of two- and zero-dimensional structures were investigatedby Raman spectroscopy. Longitudinal optical (LO) Ge phonons, longitudinal(L) Ge-Si phonons and folded longitudinal acoustic (LA) phononssuperimposed on a strong continuous emission were studied underresonant conditions. The measured phonon frequencies of folded LAphonons up to 15th order are in good agreement with thosecalculated using the Rytov model applied usually to planarsuperlattices. The low-frequency continuous emission can beexplained in terms of a breakdown of crystal momentum conservationfor resonant Raman processes involving acoustic phonons. Afrequency enlargement of the continuous emission band and a downwardshift of the LO Ge phonons with increasing excitation energy(2.54-2.71 eV) are attributed to electron and phonon sizeconfinement in the small Ge QDs resonantly contributing to thescattering process. The size of the QDs involved in the process isestimated from analysis of the frequency position of their opticalphonons.

Dispersion of the long-wavelength phonons in Ge, Si, GaAs, quartz, and sapphire

We have used the picosecond ultrasonic technique to measure the dispersion of long-wavelength longitudinal acoustic phonons in Ge, Si, GaAs, z-cut quartz, and sapphire. In these experiments, a subpicosecond light pulse was used to generate a strain pulse at one surface of the sample. After propagation through the sample, the shape of this strain pulse was modified because of the phonon dispersion. From this change in shape, the magnitude of the dispersion could be determined. The results are compared with various lattice dynamical models. We also report on measurements of the temperature dependence of the sound velocity and the acoustic attenuation.