Anharmonic Phonon Decay Research Articles

Anharmonic phonon decay in CdSexTe1−x compound thin films

Resonant Raman scattering experiments were employed to investigate composition dependence of exciton-phonon coupling and anharmonic phonon decay in CdSexTe1−x thin films with x values between 0 and 1. The energy variations of the CdTe- and CdSe-like longitudinal optical phonons (LO) with compositions were found to be well described by the random-element isodisplacement model. Fröhlich interaction was enhanced in CdSexTe1−x films with x values of ∼0.2, and strong additive combination phonons mLO1 + nLO2 have been observed, here, m and n are integers describing Raman scattering orders. The experimental results demonstrate that the anharmonic decay of the additive combination LO1 + LO2 mode was dominated by the cascade relaxation process, namely, the decay can be described by successive decaying of the individual LO1 and LO2 modes. Transverse acoustical phonon (TA), whose phonon density of states varies rapidly with alloy composition, played critical role in the anharmonic decay for the CdSexTe1−x films with different compositions.

Carrier–phonon interaction and anharmonic phonon decay in ZnS thin film studied by resonance Raman scattering

AbstractResonance Raman scattering was carried out on polycrystalline zinc sulfide (ZnS) thin film. The resonance Raman spectra revealed strong carrier–phonon interaction where longitudinal optical phonon (LO) with overtones up to the fourth order and its combination phonons with transverse acoustic (TA) phonons were observed, namely, nLO + mTA (where n and m are integers). The resonance Raman scattering processes were well explained within the framework of the cascade scattering model. The wavenumber of the LO phonon with increased temperature was dominated by anharmonic phonon decay. Detailed experimental data fittings showed that thermal expansion contribution to the nLO phonon overtones differed from the fundamental LO phonon due to the involvement of the scattering of the LO phonons at the Brillion zone edge. The negative value of the mode‐Grüneisen parameter for the TA phonon slowed down the wavenumber shift of the combination nLO + mTA phonon modes with increasing temperature compared to the nLO phonons. The anharmonic phonon decay of combination nLO + mTA phonons were dominated by anharmonic phonon decay of nLO phonons.

Temperature dependence of growth-sector-dependent Raman spectra of boron-doped diamonds synthesized at high-pressure high-temperature

Single crystals of boron-doped diamond (BDD) were synthesized by the temperature gradient method in high-pressure and high-temperature conditions in the Fe–Al–B–C system, and multisectoral diamond plates were extracted. Temperature-dependent (77–600 K) high-resolution Raman spectroscopic studies have been carried out to investigate the behavior of anharmonic phonon decay in the {001}, {113}, and {111} growth sectors of multisectoral diamond plates with different content of boron impurities (⩽80 ppm) and compare with the data for undoped IIa diamond. Micro-Fourier transform infrared spectroscopy was used to estimate the spatial distribution of uncompensated boron impurity [Na-Nd] in BDD plates by analyzing boron-related absorption peaks. The plates were shown to have non-uniform growth-sector-dependent content of uncompensated boron impurity in the range from 1.1 × 1018 to 1.4 × 1019 cm−3. The effects of anharmonic decay (damping) of optical phonons in BDD are studied by modeling the temperature dependence of phonon frequency and linewidth of the diamond’s F2g and boron-induced vibrational modes. The extrapolated zero-temperature optical phonon linewidth and frequency and the anharmonic nature of their linear relationship are determined as a function of the growth sector and boron doping. The predominant mechanisms and parameters of the anharmonic decay of optical phonons are determined, which is of fundamental importance for the thermal conductivity of semiconductor materials. The anharmonic phonon decay remained the predominant process at higher temperatures, irrespective of the doping level.

Thickness-Dependent Terahertz Permittivity of Epitaxially Grown PbTe Thin Films

The exceptional thermoelectric properties of PbTe are believed to be associated with the incipient ferroelectricity of this material, which is caused by strong electron–phonon coupling that connects phononic and electronic dynamics. Here, we have used terahertz time-domain spectroscopy measurements to generate complex permittivity spectra for a set of epitaxially grown PbTe thin films with thicknesses between 100 nm and 500 nm at temperatures from 10 K to 300 K. Using a Drude–Lorentz model, we retrieved the physical parameters of both the phononic and electronic contributions to the THz permittivity. We observed a strong decrease, or softening, of the transverse optical phonon mode frequency with decreasing temperature, determining a thickness-independent negative ferroelectric-transition critical temperature, while we found a thickness-dependent anharmonic phonon decay lifetime. The electronic contribution to the permittivity was larger in thinner films, and both the carrier density and mobility increased with decreasing temperature in all films. Finally, we detected a thickness-dependent longitudinal optical phonon mode frequency, indicating the presence of plasmon–phonon coupling.

Assessment of the anharmonic phonon decay, Fröhlich interactions, and phonon lifetime in Zn0.98Mn0.02S thin films through temperature‐dependent Raman spectroscopy

AbstractThis report accounts for the occurrence of the temperature‐induced variation in the phonon properties and the spatial variation in the Raman spectra for the spray‐deposited Zn0.98Mn0.02S thin films. The study of the temperature dependence of the Raman line position, line width, and the phonon lifetime using the theoretical models that consider the thermal expansion and three‐ and four‐phonon anharmonic effects is the novelty of this work. The lifetime of the LO phonon in Zn0.98Mn0.02S thin film was as low as 0.234 ps, indicating material suitability for optical limiting applications. The intensity ratio I2LO/I1LO ranges within 0.5 at most of the (x, y) positions in the Raman mapping data, indicating the nanocrystalline nature of Zn0.98Mn0.02S.

Existence of Equistable Quasi-Microphases in Degenerate Silicon: Revelation through an Atypical Thermal Response of the Fano–Raman Parameter

Temperature-induced quasi-phase transition in degenerate crystalline silicon has been studied using temperature-dependent Raman spectroscopy. Atypical temperature dependence of Raman spectral width has been observed, which shows a parabolic nature, making it inconsistent with the existing anharmonic phonon decay theory. This has been analyzed by considering the presence of multiple phonon decay pathways, which depend differently under thermal stimuli. The data have been analyzed to explore the possibility of the existence of quasi-microphases and their reversible temperature-induced phase transition. Thermal hysteresis has been investigated to understand the phase stability and relative activation energy. A theoretical model has been developed to explain the parabolic nature of the Raman width vs temperature curve, showing a good agreement between the two. Temperature-dependent coupling parameters for electron–phonon interactions have been calculated, which confirm the existence of quasi-microscopic temperature-induced phases. The comprehensive analysis of the temperature-dependent variation of Raman parameters and developed theoretical model thereafter may be useful in understanding temperature-dependent electrical properties in electronic-grade doped semiconductors.

Off‐State Drain Leakage Current Mechanism in GaN High Electron Mobility Transistors from Thermal Storage Test

The effect of the thermal storage test on GaN high electron mobility transistor (HEMT) is investigated in this study by observing off‐state drain leakage current and on‐state hysteresis and maximum transconductance. The recovery post thermal stress is also observed which points to native defects within the heterostructure as the leakage values after 72 hrs of recovery are measured to be one order higher than the pristine device. On‐state hysteresis recovery and off‐state drain leakage are dominated by similar defects as observed from the characteristic recovery times. Trap analysis through drain current spectroscopy reveals an additional trap level of EC – 0.83 eV from the thermal storage test. The electrical measurements are in close correlation with the Raman active mode leading to reduced lifetimes (≈0.44 ps) at higher temperatures, where anharmonic phonon decay is observed following the reported Balkanski model. This gives an overall picture of the phonon‐induced defect generation from the thermal stress and the recovery from the reduced scattering probability within the material observed after 72 hrs recovery; also observed within the HEMTs from electrical characterization.

Nonlinear Temperature-Dependent Phonon Decay in Heavily Doped Silicon: Predominant Interferon-Mediated Cold Phonon Annihilation.

A nonlinear Fano interaction has been reported here which is manifest in terms of a parabolic temperature-dependent phonon decay process observable in terms of a Raman spectral parameter. Temperature-dependent Raman spectroscopic studies have been carried out on heavily and moderately doped crystalline silicon to investigate the behavior of anharmonic phonon decay in semiconductor systems where Fano interactions are present inherently. Systematic study reveals that in heavily doped systems an interferon-mediated decay route exists for cold phonons present at lower temperatures (<475 K) where Fano coupling is stronger and dominates over the typical multiple-phonon decay process. On the other hand, the anharmonic phonon decay remains the predominant process at higher temperatures irrespective of the doping level. Temperature-dependent phonon self-energy has been calculated using experimentally observed Raman line-shape parameters to validate the fact that the nonlinear decay of phonons through interferon mediation is a thermodynamically favorable process at low temperatures.

Colossal phonon drag enhanced thermopower in lightly doped diamond

Diamond is one of the most studied materials because of its unique combination of remarkable electrical, mechanical, thermal and optical properties. Using a fully self-consistent ab initio theory of coupled electron-phonon transport, we reveal another striking behavior: a huge drag enhancement of the thermopower of lightly doped diamond. Thermopower values of around 100,000 μV K−1 are found at 100 K, significantly exceeding the highest previously measured value in the correlated metal FeSb2, and occurring at much higher temperatures. The enormous thermopower in diamond arises primarily from exceptionally weak anharmonic phonon decay around and below 100 K that facilitates efficient momentum exchange between charge carriers and phonons through electron-phonon interactions. Exceedingly large thermoelectric power factors are also identified. This work gives insights into the physics of the coupled electron-phonon system in solids and advances our understanding of thermoelectric transport in the regime of strong drag.

Anharmonic phonon coupling and decay of optical phonons in polycrystalline CdSe thin films

The lifetime of longitudinal optical (LO) phonon is a key parameter that significantly affects the performance of semiconductor optoelectronic devices. Herein, in situ temperature-dependent resonance Raman scattering experiment has been performed to study the phonon anharmonicity in sputter-deposited CdSe thin films. Strong multiphonon scattering processes involving 1LO + TO, 2LO-TO, 2LO + TO, 2LO+ E2low and TO-E2low are well resolved and allow us to investigate the anharmonic decay of 1LO, 2LO, and 1LO + TO phonon modes. Fundamental 1LO and its overtone 2LO phonon modes asymmetrically decay into a pair of transverse optical (TO) and transverse acoustical (TA) phonon at the ‘A’ point of the Brillouin zone along Γ−A direction. With increasing temperature, enhanced anharmonic phonon coupling and phonon decay led to an almost linear decrease in the wavenumber of both modes and a nearly linear increase in their full-width half maxima. The composite 1LO + TO mode exhibited a ∼ 30% faster decay rate compared to the decay rate of the fundamental 1LO phonon mode.

Temperature dependence of exciton–phonon coupling and phonon anharmonicity in ZnTe thin films

AbstractIn ZnTe, longitudinal optical (LO) phonons are strongly coupled with free exciton via long‐range order Froehlich interactions. In this study, we ex situ and in situ monitored the effect of annealing temperature on exciton–phonon (e‐ph) coupling in vacuum evaporated ZnTe thin films by resonance Raman scattering experiment. The 350°C air‐annealed thin film exhibited strong e‐ph coupling and nLO phonons up to sixth order were observed in the Raman spectrum. Moreover, a series of (n − 1)LO + TO and (n + 1)LO‐TO combination phonon modes appeared at low and high wavenumber shoulders of the nLO phonon modes, respectively. The phenomenon of e‐ph coupling was in situ monitored by temperature‐dependent Raman scattering experiment for 350°C air‐annealed thin film. During the in situ measurements, the energy of free exciton was decreased and the condition of outgoing resonance was achieved with nLO phonon. The observation of strong overtones phonon scattering and combination phonon modes allowed us to study anharmonic phonon decay of the zone center nLO phonons. The decay of the fundamental 1LO and its overtone 2LO phonon was mainly dominated by three‐phonon process. For higher order LO phonons with n ≥ 3, the four‐phonon process contribution was increased in the decay process with increasing order.

Role of defects and grain boundaries in the thermal response of wafer-scale hBN films

With more widespread applications of nanotechnology, heat dissipation in nanoscale devices is becoming a critical issue. We study the thermal response of wafer-scale hexagonal boron nitride (hBN) layers, which find potential applications as ideal substrates in two dimensional devices. Sapphire-supported thin hBN films, 2′′ in size and of different thicknesses, were grown using metalorganic vapour phase epitaxy. These large-scale films exhibit wrinkles defects and grain boundaries over their entire area. The shift of phonon mode with temperature is analysed by considering the cumulative contribution of anharmonic phonon decay along with lattice thermal expansion, defect, and strain modulation. The study demonstrates that during heat treatment the strain evolution plays a dominating role in governing the characteristics of the wrinkled thinner films. Interestingly we find that both defects and strain determine the spectral line-width of these wafer-scale films. To the end, from Raman line-width, the changes in phonon lifetime in delaminated and as-grown films is estimated. The results suggest the possibility of a reduction in thermal transport in these wafer-scale films compared to their bulk counterpart.

Phonon-Phonon Interactions in Strongly Bonded Solids: Selection Rules and Higher-Order Processes

We show that the commonly used lowest-order theory of phonon-phonon interactions frequently fails to accurately describe the anharmonic phonon decay rates and thermal conductivity ($\kappa$), even among strongly bonded crystals. Applying a first principles theory that includes both the lowest-order three-phonon and the higher-order four-phonon processes to seventeen zinc blende semiconductors, we find that the lowest-order theory drastically overestimates the measured $\kappa$ for many of these materials, while inclusion of four-phonon scattering gives significantly improved agreement with measurements. We have identified new selection rules on three-phonon processes that help explain many of these failures in terms of anomalously weak anharmonic phonon decay rates predicted by the lowest-order theory competing with four-phonon processes. We also show that zinc blende compounds containing boron (B), carbon (C) or nitrogen (N) atoms have exceptionally weak four-phonon scattering, much weaker than in compounds that do not contain B, C or N atoms. This new understanding helps explain the ultrahigh $\kappa$ in several technologically important materials like cubic boron arsenide, boron phosphide and silicon carbide. At the same time, it not only makes the possibility of achieving high $\kappa$ in materials without B, C or N atoms unlikely, but it also suggests that it may be necessary to include four-phonon processes in many future studies. Our work gives new insights into the nature of anharmonic processes in solids and demonstrates the broad importance of higher-order phonon-phonon interactions in assessing the thermal properties of materials.

Enhanced anharmonic phonon coupling and decay dominated by low‐energy phonons in CdS nanowires

AbstractLow‐energy optical phonons with energy in the order of that of acoustic phonons are readily populated at room temperature, and therefore, they play a key role in thermal, electronic, and thermo‐electric fundamental processes in a solid material. High‐quality crystalline cadmium sulfide (CdS) nanowires with a diameter of several tens of nanometer and a length up to several micrometers (3–5 μm) demonstrate strong exciton–phonon coupling. This strong coupling interaction allows us to observe low‐energy optical phonons and their anharmonic couplings to high‐energy phonon modes. In this study, second and higher order multiphonons related to the phonon were observed in near resonance Raman scattering condition (excitation wavelength of 532 nm). The clear observation of the multiphonon modes makes it possible to study composite phonon anharmonic decay. The experimental results show that low‐energy phonons (both optical and acoustical) dominate anharmonic phonon decay in CdS.

Isotopic effects on phonon anharmonicity in layered van der Waals crystals: Isotopically pure hexagonal boron nitride

Hexagonal boron nitride (h-BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state 10BN neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-crystal h-BN. Phonon frequency and lifetime are measured in the 80–600-K temperature range for 10B-enriched, 11B-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in 10B-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.

Scattering of carriers by coupled plasmon-phonon modes in bulk polar semiconductors and polar semiconductor heterostructures

We present a general treatment of carrier scattering by coupled phonon-plasmon collective modes in polar semiconductors, taking anharmonic phonon decay into account and self-consistently calculating carrier momentum relaxation rates and carrier mobility in a parabolic band model. We iteratively solve the weak-field Boltzmann equations for carriers and collective modes and obtain their nonequilibrium distribution functions. Both the scattering rates and the anharmonic decay of the coupled modes are expressed through the total dielectric function of the semiconductor, consisting of a damped lattice dielectric function, and a temperature dependent random phase approximation dielectric function for the carrier plasma. We show that the decay of the coupled modes has a significant effect on the contribution to the mobility limited by carrier-coupled mode scattering. We also propose a scalar quantity, the phonon dissipation weight factor, with which this effect can be estimated from an analytic expression. We apply this treatment to dynamically screened electron-longitudinal optical phonon scattering in bulk polar semiconductors, and to dynamically screened remote phonon scattering in polar heterostructures where monolayers of ${\mathrm{MoS}}_{2}$ are sandwiched between various polar dielectrics. We find that a dynamic treatment of the remote phonon scattering yields mobilities up to 75% higher than a static screening approximation does for structures which consist of a monolayer of ${\mathrm{MoS}}_{2}$ between hafnia and silica. Moreover, we show that accounting for the nonzero thickness of the ${\mathrm{MoS}}_{2}$ interface layer has an important effect on the calculated mobility in the same structure.

Evolution of the 128-cm−1 Raman phonon mode with temperature in Ba(Fe1–xCox)2As2 (x = 0.065 and 0.2)**Project supported by the Agence Nationale de la Recherche through Grant PNICTIDES.

We report electronic Raman scattering measurements on Ba(Fe1−xCox)2As2 (x = 0.065 and 0.2) single crystals with Raman shifts from 9 cm−1 up to 600 cm−1 in the symmetry of B1g with respect to 1 Fe unit cell. When the crystals are cooled down, the evident quasielastic peaks of Raman spectra occur only in the crystal with x = 0.065, which is due to the contribution of orbital ordering between xz and yz Fe 3d orbitals, as we reported in another work. Here, we analyze the Eg phonon at 128 cm−1, which has the same function form of its Raman tensors as those of xz and yz Fe 3d orbitals in these two crystals respectively. Unlike their electronic continuums, no anomalies are found in the Eg phonons of these two samples, which simply follows the expressions corresponding to the anharmonic phonon decay into acoustic phonons with the same frequencies and opposite momenta. Our results indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping and there is not any indication of coupling between charge nematicity and Eg phonon mode from our experimental results, which is consistent with the results in our previous work.

Influence of anharmonic phonon decay on self-heating in Si nanowire transistors

Anharmonic phonon-phonon scattering is incorporated into an electro-thermal quantum transport approach based on the nonequilibrium Green's function formalism. Electron-phonon and phonon-phonon interactions are taken into account through scattering self-energies solved in the self-consistent Born approximation. While studying self-heating effects in ultra-scaled Si nanowire transistors, it is found that the phonon decay process softens the artificial accumulation of high energy phonons caused by electron relaxations close to the drain region. This leads to an increase of the device current in the ON-state and a reduction of the effective lattice temperature.

Raman study of the temperature and magnetic-field dependence of the electronic and lattice properties of MnSi

The temperature and magnetic-field dependence of lattice and carrier excitations in MnSi is studied in detail using inelastic light scattering. The pure symmetry components of the electronic response are derived from the polarization-dependent spectra. The $E$ and ${T}_{2}$ responses agree by and large with longitudinal and optical transport data. However, an anomaly is observed right above the magnetic ordering temperature ${T}_{\mathrm{C}}=29$ K that is associated with the fluctuations that drive the transition into the helimagnetic phase first order. The ${T}_{1}$ spectra, reflecting mostly chiral spin excitations, have a temperature dependence similar to that of the $E$ and ${T}_{2}$ symmetries. The response in the fully symmetric ${A}_{1}$ representation has a considerably weaker temperature dependence than that in the other symmetries. All nine Raman active phonon lines can be resolved at low temperature. The positions and linewidths of the strongest four lines in $E$ and ${T}_{2}$ symmetry are analyzed in the temperature range $4&lt;T&lt;310$ K. Above 50 K, the temperature dependence is found to be conventional and given by anharmonic phonon decay and the lattice expansion. Distinct anomalies are observed in the range of the helimagnetic transition and in the ordered phase. Applying a magnetic field of 4 T, well above the critical field, removes all anomalies and restores a conventional behavior highlighting the relationship between the anomalies and magnetism. The anomaly directly above ${T}_{\mathrm{C}}$ in the fluctuation range goes along with an anomaly in the thermal expansion. While the lattice constant changes continuously and has only a kink at ${T}_{\mathrm{C}}$, all optical phonons soften abruptly, suggesting a direct microscopic coupling between spin order and optical phonons rather than a reaction to magnetostriction effects.

A Conjoined Electron and Thermal Transport Study of Thermal Degradation Induced During Normal Operation of Multigate Transistors

A 3-D full-band particle Monte Carlo (MC) simulator, with full electron and phonon dispersion and a 2-D quantum correction is self-consistently coupled to a phonon MC simulator. The coupling entails feeding the phonon data obtained from the 3-D electrical MC to the phonon MC. The phonon MC reciprocates by providing the resulting spatial temperature map, which is used in the electron MC, with temperature-dependent scattering table, in a self-consistent manner. A key feature of our model is its ability to delineate the influence of the various phonon modes on the electronic transport through the application of anharmonic phonon decay and full phonon dispersion. The electrothermal simulator developed is utilized to assess the performance of silicon-on-insulator (SoI) multigate (MG) MOSFET with nanoscale cross sections. This paper shows that the hotspot in inversion mode SoI MG MOSFET with 20-nm gate length permeates into the channel as the cross section is reduced (covering ~ 50% of the channel for the 5 nm × 5 nm cross section). Furthermore, cross-sectional scaling, a key design rule to mitigate short-channel effects, degenerates device performance well beyond the ideal current gain limits of MG MOSFET architecture of double-gate, trigate, and gate-all-around MOSFET. Consequently, at the sub-20-nm scale adding more gate does not necessarily improve performance.