Structure-dependent vibrational lifetimes of hydrogen in silicon

The lifetimes of the Si-H vibrational stretch modes of the H(*)(2) ( 2062 cm(-1)) and HV.VH((110)) ( 2072.5 cm(-1)) defects in crystalline Si are measured directly by transient bleaching spectroscopy from 10 K to room temperature. The interstitial-type defect H(*)(2) has a lifetime of 4.2 ps at 10 K, whereas the lifetime of the vacancy-type complex HV.VH((110)) is 2 orders of magnitude longer, 295 ps. The temperature dependence of the lifetime of H(*)(2) is governed by TA phonons, while HV.VH((110)) is governed by LA phonons. This behavior is attributed to the distinctly different local structure of these defects and the accompanying local vibrational modes.     

First-principles calculations of vibrational lifetimes and decay channels: hydrogen-related modes in Si

The vibrational lifetimes and decay channels of local vibrational modes are calculated from first principles at various temperatures. Our method can be used to predict the temperature dependence of the lifetime of any normal mode in any crystal. We focus here on the stretch modes of H2*, H+(BC), and VH x HV in Si. The frequencies are almost identical, but the lifetimes vary from 4 to 295 ps. The calculations correctly predict the lifetimes for T > 50 K and illustrate the critical importance of pseudolocal modes in the decay processes of high-frequency local vibrational modes.     

Anomalous LO phonon lifetime in AlAs

The temperature dependence of the frequencies and linewidths of the Raman-active longitudinal optical (LO) phonons in GaAs and AlAs have been measured. The low-temperature lifetime of the LO phonon in AlAs is found to be 9.7 ps, very close to the corresponding GaAs value of 9.5 ps. This contradicts early theoretical predictions. The agreement between theory and experiment can be restored when the accidental degeneracy between the AlAs LO phonon frequency and a feature in the two-phonon density of states is taken into account.     

Dumbbell rattling in thermoelectric zinc antimony

Inelastic neutron scattering measurements on thermoelectric Zn4Sb3 reveal a dominant soft local phonon mode at 5.3(1) meV. The form factor of this local mode is characteristic for dumbbells vibrating preferably along the dumbbell axis and can be related to a vibration of Sb dimers along the c axis. The Lorentzian width of the mode corresponds to short phonon lifetimes of 0.39(2) ps and yields an estimate of the thermal conductivity that agrees quantitatively with recent steady state measurements. Heat capacity measurements are consistent with an Einstein mode model describing the local Sb-dimer rattling mode with an Einstein temperature of 62(1) K. Our study suggests that soft localized phonon modes in crystalline solids are not restricted to cagelike structures and are likely to be a universal feature of good thermoelectric materials.     

Volume-dependence of phonon frequencies and mode Grüneisen parameters for Si and Ge

The volume-dependence of phonon frequencies for Si and Ge is studied using higher-order perturbation and local Heine-Abarenkov model pseudopotential. The obtained mode Grüneisen parameters at atmospheric pressure are in good agreement with the experimental data. The mode Grüneisen parameters obtained for the Raman, TO, LO and LA modes increase slightly with decreasing volume. The other TA mode Grüneisen parameters decrease strongly as function of the volume strain, and give important contribution to the anharmonic properties of Si and Ge. The pressure dependence of phonon frequencies for Raman and some modes at the zone boundaries are estimated over the whole region of the compressed volume before the pressure induced covalent-metallic phase transition.     

Investigation on thermal conductivity of graphene/Si heterostructure with different defect ratios and sizes

The thermal conductivity (TC) of graphene/Si heterostructures with different defect ratios and sizes was investigated using the molecular dynamics method. As the defect ratio of heterostructure increased, the TC decreased first sharply and then slowly under a high temperature stage. The TC of heterostructure also showed a significant size effect. This phenomenon was explained by phonon dispersion and flip competition. The phonon density of states for the graphene heterostructure with different defect ratios and sizes was obtained to understand the thermal transport mechanism. Analysis showed that with the increase in the defect ratio and when the flexural modes of the heterostructure became weak, the longitudinal and transverse modes gradually dominated the phonon transport. This phenomenon can be explained that the Si atom vibration was harder in the vertical plane than that of graphene. The vibration mode hindered the heat carrier of graphene and affected heat transport to the heterostructure.     

Anharmonic phonon lifetimes in carbon nanotubes: evidence for a one-dimensional phonon decay bottleneck

High-resolution Raman spectroscopy is applied to suspended single-walled carbon nanotubes (SWNTs) to elucidate the puzzling differences in the lifetime of the radial breathing mode (RBM) obtained from different experimental techniques. Whereas recent tunneling experiments suggest a room temperature RBM lifetime as long as 10 ns, previous Raman experiments yield lifetimes shorter than 2 ps. The lifetimes obtained in this study are longer than 5 ps-a significant step in the direction of the tunneling results. We argue that the remaining discrepancy is due to the existence of phonon decay bottlenecks caused by the one-dimensional nature of nanotubes. Numerical simulations of the RBM decay show that it is possible to reconcile the short lifetimes measured spectroscopically with the long lifetimes obtained in tunneling experiments.     

Surface and interface optical phonon modes and electron-phonon interaction in a multi-shell spherical system

Within the framework of the dielectric continuum model, interface optical(IO) and surface optical(SO) phonon modes and the Fr?hlich electron-IO (SO) phonon interaction Hamiltonian in a multi-shell spherical system were derived and studied. Numerical calculation on CdS/HgS/H₂O and CdS/HgS/CdS/H₂O spherical systems have been performed. Results reveal that there are two IO modes and one SO mode for the CdS/HgS/H₂O system, one SO mode and four IO modes whose frequencies approach the IO phonon frequencies of the single CdS/HgS heterostructure with the increasing of the quantum number l for CdS/HgS/CdS/H₂O. It also showed that smaller l and SO phonon compared with IO phonon, have more significant contribution to the electron-IO (SO) phonon interaction. Received 16 October 2001 and Received in final form 23 January 2002 Published online 25 June 2002     

Vibrational lifetime of bond-center hydrogen in crystalline silicon

The lifetime of the stretch mode of bond-center hydrogen in crystalline silicon is measured to be T1 = 7.8+/-0.2 ps with time-resolved, transient bleaching spectroscopy. The low-temperature spectral width of the absorption line due to the stretch mode converges towards its natural width for decreasing hydrogen concentration C(H), and nearly coincides with the natural width for C(H) approximately 1 ppm. The lifetimes of the Si-H stretch modes of selected hydrogen-related defects are estimated from their spectral widths and shown to range from 1.6 to more than 37 ps.     

Polaron relaxation dynamics in InAs/GaAs self-assembled quantum dots

Polaron decay in n-type InAs quantum dots has been investigated using energy dependent, mid-infrared pump–probe spectroscopy. By studying samples with differing ground state to first excited state energy separations the relaxation time has been measured between 40 and 60 meV. The low-temperature decay time increases with increasing detuning between the pump energy and the optical phonon energy and is maximum (55 ps) at 56 meV. From the experimentally determined decay times we are able to extract a low-temperature optical phonon lifetime of 13 ps for InAs QDs. We find that the polaron decay time decreases by a factor of 2 at room temperature due to the reduction of the optical phonon lifetime.     

The intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes

Estimates of the intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes have been presented from the application of Fermi’s golden rule formula based upon an elastic continuum model for cubic anharmonicity. In particular, results have been presented for the lowest non-zero mode in both nanostructures, and also the breathing mode in the nanotube. Except for the ultrathin nanowire, the lifetime increases with size and decreases with an increase in temperature. Typically, these modes have a lifetime of the order of nanoseconds, almost a thousand times larger than the lifetimes of optical phonon modes in the corresponding bulk materials. Also, at room temperature the lifetime of the lowest non-zero mode is nearly an order of magnitude larger in the (20,20) nanotube than in the nanowire of similar thickness (width 2.2 nm).     

Isotope dependence of the lifetime of the vibration of oxygen in silicon

By simply changing the isotopes of the Si atoms that neighbor an oxygen Oi atom in crystalline silicon, the measured decay rate tau of the asymmetric-stretch vibration (nu3=1136 cm-1) of oxygen (Oi) in silicon changes by a factor of approximately 2.5. These data establish that nu3 decays by creating one nu1 symmetric-stretch, local-vibrational mode of the Si-Oi-Si structure. If the residual energy (nu3-nu1) is less than the maximum frequency num of the host lattice, as for 28Si-16O-28Si in natural silicon, then it is emitted as one lattice mode, and tau depends on the density of one-phonon states at nu3-nu1. If (nu3-nu1)>num, as for 16O in single-isotope 30Si silicon, two lattice modes are created in addition to nu1, increasing tau. Prediction of tau for a particular defect clearly requires a detailed knowledge of that defect.     

Coherent optical phonon oscillations in bulk GaN excited by far below the band gap photons

We report on generation of coherent optical phonon oscillations in 150 microm thick bulk GaN. With photon energy far below the band gap, the generation mechanisms of coherent phonon modes of A1(LO), high- and low-frequency E2 are revealed to be the impulsive stimulated Raman scattering. We find that one among the two degenerate E2 modes is selectively detected with a proper choice of probe polarization. Dephasing times range from 1.5 to 70 ps for different modes, and phonon-three-photon absorbed carrier interactions are compared between the A1(LO) and the E2 mode.     

Sound-based analogue of cavity quantum electrodynamics in silicon

A quantum mechanical superposition of a long-lived, localized phonon and a matter excitation is described. We identify a realization in strained silicon: a low-lying donor transition (P or Li) driven solely by acoustic phonons at wavelengths where high-Q phonon cavities can be built. This phonon-matter resonance is shown to enter the strongly coupled regime where the "vacuum" Rabi frequency exceeds the spontaneous phonon emission into noncavity modes, phonon leakage from the cavity, and phonon anharmonicity and scattering. We introduce a micropillar distributed Bragg reflector Si/Ge cavity, where Q?10(5)-10(6) and mode volumes V?25λ(3) are reachable. These results indicate that single or many-body devices based on these systems are experimentally realizable.     

N型4H-SiC低温拉曼光谱特性

利用拉曼散射技术对N型4H-SiC单晶材料进行了30～300 K温度范围的光谱测量。实验结果表明,随着温度的升高,N型4H-SiC单晶材料的拉曼峰峰位向低波数方向移动,峰宽逐渐增宽。分析认为,晶格振动随着温度的升高而随之加剧,其振动恢复力会逐渐减小,使振动频率降低;原子相对运动会随温度的升高而加剧,使得原子之间及晶胞之间的相互作用减弱,致使声学模和光学模皆出现红移现象。随着温度的升高,峰宽逐渐增宽。这是由于随着温度的升高声子数逐渐增加,增加的声子进一步增加了散射概率,从而降低了声子的平均寿命,而声子的平均寿命与峰宽成反比,因此随着温度的升高峰宽逐渐增宽。声子模强度随温度升高呈现不同规律,E₂(LA),E₂(TA),E₁(TA)和A₁(LA)声子模随着温度升高强度单调增加,而E₂(TO),E₁(TO)和A₁(LO)声子模强度出现了先增后减的明显变化,在138 K强度出现极大值。分析认为造成原因是由于当温度高于138 K时,高能量的声子分裂成多个具有更低能量的声子所致。     

Raman scattering as a probe of the tensile strain distribution in GaAs grown on Si(111) by molecular beam epitaxy

Epitaxial layers of GaAs grown on Si substrates, where the layer thickness greatly exceeds any critical thickness based on mismatch in lattice constant alone, have been shown to be under tensile strain for temperature at or below 300 K. This "thermal" strain arises from the difference in thermal expansion coefficients between GaAs and Si. We have performed Raman experiments on GaAs layers grown on both Si (001) and Si (111) substrates. We have observed a shift in the optical modes towards lower frequencies which is indicative of tensile strain in the GaAs layers, this is greater in the (111) growth direction than in the (001) one. In order to investigate the strain distribution as a function of distance from the GaAs/Si interface we have measured Raman spectra after successive removing of the epitaxial layer by chemical etching. We have found out that the strain decreases with increasing distance from the interface. We have developed the theory of Cerdeira et al. (1) to determine quantitatively the strain present in the heteroepitaxial layers. We have used, for the first time, polarization selection rules to separate the various components of the optical phonon modes. According to the theory we have observed that the doubly degenerate TO phonon line exhibits both a splitting and shift with strain, while only a shift is observed for the LO phonon line. In conformity with Cerdeira we have remarked that the strain dependence of the LO phonon is equal to that of the TO phonon mode observed in crossed polarization configuration.     

Structural transitions in silicon induced by a femtosecond laser pulse: The role of an electron-hole plasma and phonon-phonon anharmonicity

It is shown by the methods of time-resolved self-reflection and linear reflection that irradiation of a silicon target by a 100-fs laser pulse induces successive structural transitions of the target material to new crystal and liquid metal phases, which can occur during the laser pulse or 0.1–10³ ps after the pulse termination, depending on the excitation conditions. The thresholds of these structural transitions are determined, and “’soft” phonon modes involved in them are identified, which represent “hot” short-wavelength LA modes. The dynamics of the structural transitions in silicon in the time interval from 0.1 to 10³ ps is described using the model of instability of phonon modes caused by an electron-hole plasma and intra-and intermode phonon-phonon anharmonic interactions     

Coupled ultrafast lattice and polarization dynamics in ferroelectric nanolayers

We report the first analysis of the polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer system by femtosecond x-ray diffraction. Two Bragg reflections provide information on the coupled dynamics of the two relevant phonon modes for ferroelectricity in perovskites, the tetragonal distortion and the soft mode. Optical excitation of the SrRuO(3) metal layers generates giant stress (>1 GPa) compressing the PbZr(0.2)Ti(0.8)O(3) layers by up to 2%. The resulting change of tetragonality reaches a maximum after 1.3 ps. As a result, the ferroelectric polarization P is reduced by up to 100% with a slight delay that is due to the anharmonic coupling of the two modes.     

Vibrational properties and phase transitions in II-VI materials: lattice dynamics, ab initio studies and inelastic neutron scattering measurements

Inelastic neutron scattering measurements were carried out to determine the phonon density of states of ZnSe and interpreted with lattice dynamical computations (ab initio as well as a potential model). Calculations are also reported for other II-VI compounds, ZnTe and ZnS. Vibrational (phonon spectra and Grüneisen parameters), and thermal (negative thermal expansion and non-Debye specific heat) properties have been calculated and found to be in good agreement with available experimental data. This model has been further employed to study the pressure-induced solid-solid phase transitions exhibited by these compounds and the results have been compared with experimental data. Total energy calculations for zincblende and SC16 phases of ZnSe were carried out employing the pseudopotential approach under the local density approximation (LDA) as well as the generalized gradient approximation (GGA). The density functional perturbation theory is applied to study the vibrational properties of the zincblende and SC16 phases of ZnSe. An investigation of the pressure dependence of the phonon frequencies shows that the existence of the (experimentally undetected) SC16 phase as a thermodynamically stable high pressure phase is impeded due to dynamical instabilities. A detailed investigation of the polarization of phonons of different energies for the various phases of these compounds indicates that in the case of the zincblende phase the low energy modes are librational, while in the rocksalt phase the low energy modes are bending modes. Further, in ZnTe the low energy bending modes display a larger amplitude of bending than that in ZnSe and ZnS.     

Vibrational lifetimes and frequency-gap law of hydrogen bending modes in semiconductors

Vibrational lifetimes of hydrogen and deuterium related bending modes in semiconductors are measured by transient bleaching spectroscopy and high-resolution infrared absorption spectroscopy. We find that the vibrational lifetimes follow a universal frequency-gap law; i.e., the decay time increases exponentially with increasing decay order, with values ranging from 1 ps for a one-phonon process to 265 ps for a four-phonon process. The temperature dependence of the lifetime shows that the bending mode decays by lowest-order multiphonon process. Our results provide new insights into vibrational decay and the giant isotope effect of hydrogen in semiconductor systems.