Longitudinal Optical Phonon Lifetime Research Articles

Multiphonon anharmonicity of MgO

The anharmonic lattice dynamics of MgO has been studied at ambient conditions and at high temperatures by infrared spectroscopy and inelastic x-ray scattering measurements combined with density functional perturbation theory calculations. The agreement between the measured phonon energies and widths with ab initio calculated values provides a direct and pertinent test of the validity of advanced theoretical methods. Long observed anharmonic features in the infrared reflectivity find a clear explanation in terms of well-defined multiphonon scattering processes and lattice dynamics peculiarities, also responsible for a significant and sharp reduction of the longitudinal optical phonon lifetime at critical finite wave vectors. Our work highlights the importance of multiphonon scattering processes on the collective dynamics and related material properties.

Elastic scattering by hot electrons and apparent lifetime of longitudinal optical phonons in gallium nitride

Longitudinal optical (LO) phonons in GaN generated in the channel of high electron mobility transistors (HEMT) are shown to undergo nearly elastic scattering via collisions with hot electrons. The net result of these collisions is the diffusion of LO phonons in the Brillouin zone causing reduction of phonon and electron temperatures. This previously unexplored diffusion mechanism explicates how an increase in electron density causes reduction of the apparent lifetime of LO phonons, obtained from the time resolved Raman studies and microwave noise measurements, while the actual decay rate of the LO phonons remains unaffected by the carrier density. Therefore, the saturation velocity in GaN HEMT steadily declines with increased carrier density, in a qualitative agreement with experimental results.

Density-dependent electron transport and precise modeling of GaN high electron mobility transistors

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors (HEMTs). Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 × 107 cm/s at a low sheet charge density of 7.8 × 1011 cm−2. An optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated longitudinal optical (LO) phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

Ultraviolet Raman spectra of few nanometer thick silicon-on-insulator nanofilms: Lifetime reduction of confined phonons

We show that in Si-on-insulator (SOI) nanofilms (NFs), phonon-confinement-induced longitudinal optical (LO) phonon Raman band asymmetric broadening is much weaker than thickness-inversely-proportional symmetric broadening associated with phonon lifetime reduction. We discuss the origin of the effect, determine LO phonon lifetime in SOI NFs and modify Richter-Campbell-Fauchet model describing the confined-phonon Raman band shape by taking the symmetric broadening into account. We also discuss connection of the effect with SOI NF thermal conductivity reduction.

Ultraviolet Raman spectra of single uncoated and SiO2-coated silicon-on-insulator nanowires: Phonon boundary scattering, wave-vector relaxation and stress

We study Raman spectra of single straight Si-on-insulator (SOI) nanowires (NWs) at the 364 nm excitation wavelength. Uncoated SOI NW Raman band downshift and asymmetric broadening appeared to be smaller than those reported for NW ensembles, where these effects are enhanced due to additional wave-vector relaxation associated with NW imperfections. We observe NW-diameter-inversely proportional symmetric Raman band broadening associated with the phonon boundary scattering (PBS). NW longitudinal optical phonon lifetime and mean free path are determined from the PBS band broadening. SiO2-coated NWs display stress transforming from tensile to compressive with a decrease in the NW width.

Effect of hot phonon lifetime on electron velocity in InAlN/AlN/GaN heterostructure field effect transistors on bulk GaN substrates

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructure field effect transistors (HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields present in the HFET channel, and thus, we interpret the minimum intrinsic transit time in terms of the hot phonon decay. At the gate bias associated with the minimum transit time, we determined the average electron velocity for a 1.1 μm gate length device to be 1.75±0.1×107 cm/sec.

Studies of electron–phonon and phonon–phonon interactions in InN using ultrafast Ramanspectroscopy

Subpicosecond time-resolved Raman spectroscopy has been employed to investigateelectron–phonon interactions and phonon dynamics in InN. The electron–longitudinaloptical phonon scattering rate and the decay dynamics of longitudinal optical phonons inInN have been directly measured. Our results indicate that hot-phonon effects can play animportant role in the electron relaxation and transport in InN. The carrier dependence ofthe lifetime of the longitudinal optical phonons has also been measured. The resultssuggest that more theoretical work is needed to account for the dependence ofthe lifetime of longitudinal optical phonons on the photoexcited carrier density.

Observation of femtosecond carrier thermalization time in indium nitride

Ultrafast carrier thermalization in n-type indium nitride (InN) with an electron concentration of 3.8×1018 cm−3 was investigated by femtosecond transient transmission measurements at room temperature with different wavelengths. An extremely fast carrier external thermalization time on the order of 400 fs was observed, which is much faster than all previous reports. This observed femtosecond thermalization time is consistent with a prediction based on a Coulomb screening effect. Through wavelength dependent and power dependent studies, even with a 400 fs thermalization time, we did not observe any evidence of the existence of the hot phonon effect, which agrees with a recent report that a longitudinal optical phonon lifetime could be shorter than 300 fs in specific InN samples.

Effects of high carrier densities on phonon and carrier lifetimes in Si by time-resolved anti-Stokes Raman scattering

The relaxation times T1 of nonequilibrium populations of elementary excitations in Si are studied by time-resolved Raman scattering using a subpicosecond pump-probe method. Incoherent anti-Stokes Raman scattering is used to monitor the decay of the nonequilibrium populations of holes and the generation and decay of zone-center longitudinal optical (LO) phonons. At lower levels of laser excitation, hole and LO phonon T1 lifetimes are less than 0.2ps and greater than 1.6ps, respectively. At higher laser intensities, the lifetime of holes increases and the lifetime of LO phonons decrease toward a common value of T1∼0.4ps.

Stimulated-emission-induced enhancement of the decay rate of longitudinal optical phonons in III–V semiconductors

We explore the feasibility of reducing the lifetime of longitudinal optical phonons in InP by injecting coherent longitudinal acoustic modes of frequency given by the difference between those of the longitudinal and transverse optical phonons. Calculations show that a ten-fold reduction in the lifetime can be attained using a 40 mW acoustic source. The increase in the phonon decay should reduce the scattering rate of hot electrons in fast devices.

Subpicosecond Time-Resolved Raman Studies of Non-Equilibrium Excitations in Wide Bandgap GaN

Non-equilibrium phonon dynamics in wurtzite GaN have been studied by subpicosecond time-resolved Raman spectroscopy. The population relaxation time of longitudinal optical phonons was directly measured to be τ ≅ (5 ± 1) ps at T = 25 K. The experimental results on the temperature dependence of the lifetime of longitudinal optical phonons suggest that the primary decay channels for these phonons are the decay into (i) one transverse optical phonon and one high energy, longitudinal or transverse acoustical phonons; and (ii) one transverse optical phonon and one E2 phonon.

Subpicosecond Time-Resolved Raman Studies of Nonequilibrium Excitations in Wurtzite GaN

ABSTRACTNon-equilibrium electron distributions as well as phonon dynamics in wurtzite GaN have been measured by subpicosecond time-resolved Raman spectroscopy. Our experimental results have demonstrated that for electron densities n ≥ 5 × l017cm−3, the non-equilibrium electron distributions in wurtzite GaN can be very well described by Fermi-Dirac distribution functions with the temperature of electrons substantially higher than that of the lattice. The population relaxation time of longitudinal optical phonons was directly measured to be τ ≅ 5 ± 1 ps at T = 25 K. The experimental results on the temperature dependence of the lifetime of longitudinal optical phonons suggest that the primary decay channels for these phonons are the decay into (1) one transverse optical phonon and one high energy, longitudinal or transverse acoustical phonons; and (2) one transverse optical phonon and one E2 phonon.

Nonlinear gain coefficients in semiconductor lasers: effects of carrier heating

Nonlinear gain coefficients due to the effects of carrier heating are derived from the rate equations of carrier energy transfer in semiconductor lasers. We find that, in the modulation responses of semiconductor lasers, stimulated recombination heating will affect the resonant frequency and damping rate in a same form as the effects of spectral hole burning, while free carrier absorption heating will only affect the damping rate. The effects of injection heating and nonstimulated recombination heating are also discussed. The carrier energy relaxation time is calculated from first principles by considering the interactions between carriers and polar optical phonons, deformation potential optical phonons, deformation potential acoustic phonons, piezoelectric acoustic phonons. At the same time, the hot phonon effects associated with the optical phonons are evaluated because their negligible group velocity and finite decay time. We show that the carrier-polar longitudinal optical phonon interaction is the major channel of carrier energy relaxation processes for both electron and holes. We also point out the importance of the longitudinal optical phonon lifetime in evaluating the carrier energy relaxation time. Neglecting the finite decay time of longitudinal optical phonons will significantly underestimate the carrier energy relaxation time, this not only contradicts the experimental results but also severely underestimates the nonlinear gain coefficients due to carrier heating. The effects of spectral hole burning, stimulated recombination heating, and free carrier absorption heating on limiting the modulation bandwidth in semiconductor lasers are also discussed.

Theoretical calculation of longitudinal-optical-phonon lifetime in GaAs

The anharmonic decay of longitudinal-optical (LO) phonons in zinc-blende semiconductors has been studied. Based on an approach in which the anharmonic crystal potential is estimated using the theory of elasticity, the lifetime of LO phonons via emission of two acoustic phonons is calculated as a function of lattice temperature and phonon wave vector. Application of this model to bulk GaAs shows an excellent agreement with available experimental data. Since the parameters employed in the model can be obtained experimentally, the approach provides a useful tool to investigate LO-phonon lifetimes in semiconductors.