Hot Phonon Effects Research Articles

Hot-electron energy-loss rate via PO-phonon scattering in CdZnSe/ZnSe quantum wire

The hot-electron energy-loss rate conditioned by confined and interface polar-optical phonons for a Cd 0.35Zn 0.65Se quantum wire embedded in ZnSe medium is investigated theoretically. It is shown that the inclusion of the polar-optical phonon confinement effects is crucial for accurate calculation of the energy-loss rate in quantum wire. Taking into account the nonequilibrium phonon populations, the hot-electron energy-loss rate is derived by a model, which includes the lowest subband occupation and the phonon confinement effects. The contribution of intersubband transitions to electron energy-loss rate as well as the relaxation time of the electron temperature with and without hot-phonon bottleneck effect is estimated.

Feature Article: Hot phonons in GaN channels for HEMTs

In the Feature Article [1] a fluctuation technique is used to obtain the hot electron energy relaxation time (blue symbols in the cover picture) and the hot phonon lifetime (red) for a biased two-dimensional channel in an AlGaN/AlN/GaN heterostructure as a function of the supplied electric power. The close proximity of red and blue symbols illustrates that the relaxation is mainly determined by the phonon conversion into other vibrational modes being able to drain the Joule heat out of the channel. The red inset shows the momenta of hot phonons launched by hot electrons. The green line presents the electron energy relaxation time obtained by Monte Carlo simulation for lightly doped GaN under the assumption that hot phonons are absent. A strong effect of hot phonons on the electron energy relaxation is evident. The author Arvydas Matulionis is Professor of Physics and head of the Fluctuation Research Laboratory of the Vilnius Semiconductor Physics Institute. He investigates hot electron fluctuations and other high electric field effects in semiconductor heterostructures for microwave electronics. The Editor's Choice “Anomalous charge relaxation in channels of pentacene-based organic field-effect transistors: a charge transient spectroscopy study” by I. Thurzo et al. [2] is also part of the present issue.

Influence of longitudinal electric field on the hot-phonon effect in quantum wells

Using the Huang-Zhu model [K. Huang and B.-F. Zhu, Phys. Rev. B 38, 13377 (1988)] for the optical phonons and associated carrier-phonon interactions in semiconductor superlattices, the effects of longitudinal electric field on the energy-loss rates (ELRs) of hot carriers as well as on the hot-phonon effect (HPE) in GaAs∕AlAs quantum wells (QWs) are studied systematically. Contributions of various bulklike and interface phonons to the hot-carrier relaxation are compared in detail, and comprehensively analyzed in relation to the intrasubband and intersubband scatterings for quantum cascade lasers. Due to the broken parity of the electron (hole) states in the electric field, the bulklike modes with antisymmetric potentials are allowed in the intrasubband relaxation processes, as well as the modes with symmetric potentials. As the interface phonon scattering is strong only in narrow wells, in which the electric field affects the electron (hole) states little, the ELRs of hot carriers through the interface phonon scattering are not sensitive to the electric field. The HPE on the hot-carrier relaxation process in the medium and wide wells is reduced by the electric field. The influence of the electric field on the hot-phonon effect in quantum cascade lasers is negligible. When the HPE is ignored, the ELRs of hot electrons in wide QWs are decreased noticeably by the electric field, but slightly increased by the field when considering the HPE. In contrast with the electrons, the ELRs of hot holes in wide wells are increased by the field, irrespective of the HPE.

Hot-Electron Transport and Noise in GaN Two-Dimensional Channels for HEMTs

Accumulation of non-equilibrium longitudinal optical (LO) phonons (termed hot phonons) is considered as a possible cause for limitation of frequency of operation of GaN-based high-electron-mobility transistors (HEMTs). The experimental data on noise temperature of hot electrons at a microwave frequency as a function of supplied electric power is used to extract information on hot phonons: the hot-phonon lifetime, the equivalent hot-phonon temperature, the effective occupancy of hot-phonon states involved into electron-LO-phonon interaction. The possible ways for controlling the hot-phonon effect on electron drift velocity through variation of electron density, channel composition, and hot-phonon lifetime are discussed. The expected dependence of hot-electron drift velocity on hot-phonon lifetime is confirmed experimentally. A self-consistent explanation of different frequency behaviour of InP-based and GaN-based HEMTs is obtained from a comparative study of hot-phonon effects.

A deterministic study of hot phonon effects in a 2D electron gas channel formed at an AlGaN/GaN heterointerface

We investigate the hot phonon effects on the transport properties of a two-dimensional electron gas formed in an AlGaN/GaN heterostructure. For this purpose, we use a deterministic numerical scheme to solve the coupled system of Boltzmann transport equations. The envelope wave functions of the confined carriers are self-consistently calculated from the Schrodinger-Poisson system. The simulation results show that the electron drift velocity is reduced by hot phonons for moderate and high electric fields, but become enhanced for low fields. This interesting behavior is elucidated by virtue of the energy balance equation using an analytic electron temperature model. We find good agreement to experimental data when hot phonon and degeneracy effects are taken into account.

Monte Carlo simulation of hot phonon effects in resonant-phonon-assisted terahertz quantum-cascade lasers

We study the influence of nonequilibrium optical phonons on the electron transport in resonant-phonon-assisted terahertz (THz) quantum-cascade lasers (QCLs). The hot phonon effect is included in the Monte Carlo simulation by introducing a time constant accounting for the decay of polar optical phonons into other phonon modes. We find that nonequilibrium polar optical phonons may modify the electron-phonon interaction, affect the electron distribution in different subbands, and consequently change the device current. We conclude that the hot phonon effect should be taken into account in the study of the transport properties of resonant-phonon-assisted THz QCLs.

Carrier transport and confinement in polarization-induced three-dimensional electron slabs: Importance of alloy scattering in AlGaN

By exploiting the difference in spontaneous and piezoelectric polarization between GaN and compositionally graded layers of strained AlGaN, we demonstrate three-dimensional electron slabs of tunable widths (30–100 nm) and densities (1–5×1018cm3). Removal of ionized impurity scattering results in relatively high mobilities limited by alloy scattering at low temperatures, and by a combination of alloy and polar optical phonon scattering at room temperature. Owing to the tunable electron-electron interactions in such slabs, they offer an ideal system to probe high-field transport physics in the III-V nitride semiconductors in general, and the hot-phonon effect in particular.

Optical study of hot electron transport in GaN: Signatures of the hot-phonon effect

The hot-phonon lifetime in GaN is measured by temperature- and electric field-dependent photoluminescence studies of a n-type channel. The rate of increase of electron temperature with the external electric field provides a signature of nonquilibrium hot-phonon accumulation. Hot-electron temperatures are measured directly as a function of applied electric fields, and by comparing theoretical models for electron energy-loss into acoustic and optical phonons, a hot-phonon lifetime of τph=3 to 4 ps is extracted.

Study of the Temperature Dependence of E2 and A1(LO) Modes in ZnO

ABSTRACTRaman scattering measurements were carried out on a bulk, single crystal of wurtzite ZnO over a temperature range from 80 to 760 K and the temperature-dependent shift and broadening of the E2high and A1(LO) modes was analyzed. The E2high mode exhibits a visibly asymmetric line shape that can be related to the interaction with the continuum of acoustic two-phonon density of states. A Fermi resonance model was used to describe the E2high temperature dependence. On the other hand, the anharmonic shift and broadening of the A1(LO) mode are adequately accounted for by a decay model with a dominating Ridley channel involving TO and LA modes. Phonon lifetimes of ∼0.9 and 0.5 ps are found for the E2high and A1(LO) modes, respectively, which corroborates that anharmonic decay involves in both cases a three-phonon process. The A1(LO)lifetime is one order of magnitude lower than that of GaN, which suggests that hot phonon effects should be expected to play a less relevant role in carrier relaxation in ZnO as compared with GaN.

A deterministic solver for the transport of the AlGaN/GaN 2D electron gas including hot-phonon and degeneracy effects

The transport of the two-dimensional electron gas formed at an AlGaN/GaN heterostructure in the presence of strain polarization fields is investigated. For this purpose, we develop a deterministic multigroup model to the Boltzmann transport equations. The envelope wave functions for the confined electrons are calculated using a self-consistent Poisson–Schrödinger solver. The electron gas degeneracy and hot phonons are included in our transport equations. Numerical results are given for the dependence of macroscopic quantities on the electric field strength and on time and for the electron and phonon distribution functions. We compare our results to those of Monte Carlo simulations and with experiments.

Ultrafast carrier dynamics in ZnO nanorods

Free exciton and above-band-gap free carrier dynamics in ZnO nanorods have been investigated at room temperature with a femtosecond transient transmission measurement. Following the photoexcitation of above-band-gap free carriers, an extremely fast external thermalization time on the order of 200 fs can be observed. Under high excitation, hot phonon effects were found to delay the carrier cooling process. While the photoexcitation energy was tuned to match the free exciton transition, stable exciton formation can be uncovered while no evident exciton ionization process can be found unless the photoexcited exciton density exceeded the Mott density.

Phonon lifetimes and phonon decay in InN

We report on the Raman analysis of A1(LO) (longitudinal optical) and E2 phonon lifetimes in InN and their temperature dependence from 80 to 700 K. Our experimental results show that among the various possible decay channels, the A1(LO) phonon decays asymmetrically into a high energy and a low energy phonon, whereas the E2 phonon predominantly decays into three phonons. Possible decay channels of the A1(LO) phonon may involve combinations of transverse optical and acoustic phonons. Phonon lifetimes of 1.3 and 4 ps were measured at 80 K for the A1(LO) and the E2 phonons, respectively. This rather long A1(LO) phonon lifetime suggests that hot phonon effects will play a role in InN for carrier relaxation.

Resonant Raman spectroscopy on InN

AbstractThe novel use of resonant Raman spectroscopy to elucidate the band gap of InN is illustrated in a study of MBE‐grown films. This technique can distinguish between electronic transitions related to the InN from transitions related to defects and impurities that are so typical for current InN material. Using excitation energies from 1.49 eV (830 nm) to 2.54 eV (488 nm), we identify a critical point in the InN band structure within ≈200–300 meV below 1.5 eV. The origin of this critical point, whether band gap or higher critical point, is discussed. Furthermore, Raman results are presented on the temperature dependence of the InN phonons. Analysis of the data provides information on phonon lifetimes and decay mechanisms, important to assess whether hot phonon effects need to be considered in future InN devices. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier Recombination, Relaxation, and Transport Dynamics in InN

AbstractKnowledge of the carrier recombination, relaxation, and transport processes in InN materials is essential for determining the applicability of this material system in photonic and electronic applications. In this article, we provide a review of time-resolved spectroscopy experimental techniques and our recent results using these techniques to measure transient processes in InN. Specifically, subpicosecond differential transmission experiments were used to determine the carrier recombination lifetime and the carrier thermalization time of InN. In those experiments, we observed a fast initial hot carrier cooling followed by a slower recombination process. At short times after pulsed excitation, modeling of the observed relaxation suggests that the dominant energy relaxation process is longitudinal optical phonon scattering modified by a strong hot phonon effect at room temperature. An inverse proportionality between the carrier lifetime and the free electron concentration was found. This suggests that donor-like defects or impurities may stimulate the formation of non-radiative recombination centers. Furthermore, we report the measurements of in-plane carrier transport and hole mobility of an InN epilayer by time-resolved transient grating spectroscopy using subpicosecond pulses at 800 nm and ∼1900 nm for grating writing and probing, respectively. The ambipolar diffusion coefficient Da = 2.0 cm2/s and hole mobility µh = 39 cm2/Vs at 300 K near the InN surface were determined by monitoring the transient grating kinetics at various grating periods.

The effect of hot phonons on the drift velocity in GaN/AlGaN two dimensional electron gas

We present the effect of hot-phonon production on the high-field drift velocity in the steady state in GaN/AlGaN heterostructures . The results are compared with a model taking into account the effect of hot phonon production on the high-field transport of electrons. Experimental results show that the drift velocity saturates at around v d ≈3.8×10 5 cm / s at an electric field of F≈8.9×10 2 V / cm at 3.8 K . The theoretical calculations show that the enhancement of the momentum relaxation rate due to the production of non-drifting hot phonons reduces the drift velocity at high field. The reduction in the drift velocity increases with increasing population of non-drifting non-equilibrium phonons (LO phonons).

The effect of hot phonons on the drift velocity in GaN/AlGaN two dimensional electron gas

We present the effect of hot-phonon production on the high-field drift velocity in the steady state in GaN/AlGaN heterostructures. The results are compared with a model taking into account the effect of hot phonon production on the high-field transport of electrons. Experimental results show that the drift velocity saturates at around v d ≈3.8×10 5 cm/ s at an electric field of F≈8.9×10 2 V/ cm at 3.8 K . The theoretical calculations show that the enhancement of the momentum relaxation rate due to the production of non-drifting hot phonons reduces the drift velocity at high field. The reduction in the drift velocity increases with increasing population of non-drifting non-equilibrium phonons (LO phonons).

Monte Carlo simulation of hot-phonon effects in a biased AlGaN/GaN channel

Ensemble Monte Carlo simulation of two-dimensional electron transport is performed for a biased undoped AlGaN/GaN channel. The effect of hot phonons on electron drift velocity and energy dissipation is investigated, the model is found correct within a factor of ∼2 when LO-phonon conversion lifetime is assumed to be 0.35 ps.

Microwave noise in AlGaN∕GaN channels

Recent investigation of microwave noise of nominally undoped AlGaN/GaN channels is reviewed. The noise is agitated in a two-dimensional electron gas by an electric field applied in the plane of electron confinement. The experimental results are compared with those of Monte Carlo simulation and with simple semi-empirical formulas. The importance of hot-phonon effects is emphasised.

Numerical analysis of the relaxation of photoexcited carriers and the hot-phonon effect in GaN

The ultrafast relaxation kinetics of the photo-injected plasma in polar semiconductors has been analyzed with certain details resorting to a first-principles quantum theory based on a nonequilibrium ensemble formalism (the non-equilibrium statistical operator method). Particular attention was given to the so-called hot-phonon effect that accompanies the rapid relaxation processes in the photo-injected plasma. This is done by studying the process of generation and decay of the nonequilibrium longitudinal optical (LO) phonon population per mode. Numerical calculations are presented for the GaN, particular attention being paid to the influence of the carriers effective mass and the time relaxation of the LO-phonons population in the process.

Time-resolved spectroscopy of recombination and relaxation dynamics in InN

Subpicosecond resolution differential transmission measurements of an InN epilayer have been employed to probe the carrier recombination dynamics and hot carrier relaxation processes in these materials at room temperature. We observed a fast initial hot carrier cooling followed by a slower recombination process with characteristic decay times of 300–400 ps. At short times after pulsed excitation, modeling of the observed relaxation suggests that the dominant energy relaxation process is longitudinal optical phonon scattering modified by a strong hot phonon effect. At longer times, a redshift of the peak energy in the differential transmission spectra was observed. This redshift is consistent with a reduction of the bandfilling effect that occurs as the photoexcited carriers recombine.