Phonons In GaAs Research Articles

Assessment of Optical and Phonon Characteristics in MOCVD-Grown (AlxGa1-x)0.5In0.5P/n+-GaAs Epifilms.

Quaternary (AlxGa1-x)yIn1-yP alloys grown on GaAs substrates have recently gained considerable interest in photonics for improving visible light-emitting diodes, laser diodes, and photodetectors. With two degrees of freedom (x, y) and keeping growth on a lattice-matched GaAs substrate, the (AlxGa1-x)0.5In0.5P alloys are used for tuning structural, phonon, and optical characteristics in different energy regions from far-infrared (FIR) → near-infrared (NIR) → ultraviolet (UV). Despite the successful growth of (AlxGa1-x)0.5In0.5P/n+-GaAs epilayers, limited optical, phonon, and structural characteristics exist. Here, we report our results of carefully examined optical and vibrational properties on highly disordered alloys using temperature-dependent photoluminescence (TD-PL), Raman scattering spectroscopy (RSS), and Fourier-transform infrared reflectivity (FTIR). Macroscopic models were meticulously employed to analyze the TD-PL, RSS, and FTIR data of the (Al0.24Ga0.76)0.5In0.5P/n+-GaAs epilayers to comprehend the energy-dependent characteristics. The Raman scattering and FTIR results of phonons helped analyze the reflectivity spectra in the FIR region. Optical constants were carefully integrated in the transfer matrix method for evaluating the reflectivity R(E) and transmission T(E) spectra in the NIR → UV regions, validating the TD-PL measurements of bandgap energies (EgPL).

Phosphides‐Based Terahertz Quantum‐Cascade Laser

Due to their high optical phonon energies GaInP/AlGaInP heterostructures are a promising active medium to solve the problem of creating compact semiconductor sources with an operating frequency range of 5.5–7 THz. In this work, the temperature dependences of gain and absorption at 6.4–6.9 THz have been calculated for a GaInP/AlGaInP‐based quantum‐cascade laser (QCL) with two quantum wells in the cascade and a metal–metal waveguide. The dielectric function and mode losses for a 10 μm thick Au–Au waveguide, based on ternary InGaP and quaternary AlGaInP semiconductors, have been investigated in details. A laser structure that provides a mode gain of over 100 cm−1 with a maximum operating temperature about 100 K is proposed. The results of this study open the way to the development of a QCL for operation in a significant part of the GaAs phonon absorption band region, which is inaccessible for existing QCLs.

Thin active region HgCdTe-based quantum cascade laser with quasi-relativistic dispersion law.

HgCdTe is promising as a material to solve a problem of the development of semiconductor sources with an operational frequency range of 6-10 THz due to the small optical phonon energies and electron effective mass. In this study, we calculate the dependence of the metal-metal waveguide characteristics on the number of cascades for the 3-well design HgCdTe-based quantum cascade laser at 8.3 THz. It is shown that four cascades are sufficient for lasing at a lattice temperature of 80 K due to the large gain in the active medium. The results of this study provide a way to simplify the fabrication of thin active region HgCdTe-based quantum cascade lasers for operation in the range of the GaAs phonon Reststrahlen band inaccessible to existing quantum cascade lasers.

Theory for coherent control of longitudinal optical phonons in GaAs using polarized optical pulses with relative phase locking

A theoretical model for the coherent control of longitudinal optical phonons in GaAs (001) by double-pulse excitation was derived using a simplified band model and the allowed and forbidden Raman scattering. The time evolution of the electron-phonon states was calculated with the density-matrix formalism and second-order perturbation. The amplitude of the longitudinal optical phonons controlled by the two pulses was obtained as a function of the delay between the pulses for several polarization conditions. For parallel-polarized pulses, electronic and phonon interference fringes were predicted, which were independent of the crystal orientation and the ratio between the allowed and forbidden Raman scattering intensities. For orthogonally polarized pulses, only phonon interference fringes were obtained at an angle of $\ensuremath{\pi}/4$ from the [100] direction. When one of the pulses was along the [100] direction, electronic interference fringes were induced by the allowed Raman scattering, although the pump pulses do not interfere themselves.

Polaronic interaction in a single modulation-doped GaAs quantum well with the Feynman-Hellwarth-Iddings-Platzman approximation

Absolute far-infrared magnetotransmission experiments have been performed in magnetic fields up to 33.5 T on a series of single GaAs quantum wells doped with different electron concentrations. The transmission spectra have been simulated with a multilayer dielectric model. The imaginary part of the optical response function, which reveals singular features related to the electron-phonon interactions, has been extracted. In addition to the expected polaronic effects due to the longitudinal-optical phonon of GaAs, additional interactions with interface phonons are observed. The main interaction is analyzed quantitatively with the Feynman-Hellwarth-Iddings-Platzman model, which is shown to predict correctly the concentration of carriers beyond which the Fr\"ohlich interaction is completely screened.

Coherent control of optical phonons in GaAs by relative-phase-locked optical pulses under perpendicularly polarized conditions

Coherent control of the optical phonons in GaAs is realized via paired femtosecond pulses that are relative phase-locked and have perpendicular polarizations. To investigate polarization effects, only phonon interference occurs, which contrasts previous experiments with parallel-polarized pulses, which produced as well electronic interference. The results suggest that for an n-GaAs(001) sample the diagonal components of the Raman tensor determinate in the generation of coherent longitudinal optical phonons.

Influence of piezoelectric phonons on the magneto optical transition linewidth in GaN and GaAs

We compare piezoelectric phonon scattering mechanism with optical phonon one in both Gallium arsenide (GaAs) and Gallium nitride (GaN) materials of a quantum well through the their contribution to the magneto-optical transition linewidths (MOTLWs). Applying the projection-operator and the profile methods to compute the magneto-optical conductivity tensor (MOCT), magneto-optical absorption power (MOAP), and MOTLWs. Numerical calculation results show that the MOTLWs increase as the temperatures and the magnetic fields increase, but decrease as the electron density and the well width increase for both GaN and GaAs materials. In particular, the MOTLWs due to piezoelectric phonons vary sharper and have larger value than they do due to optical phonons in both GaN and GaAs materials, and the MOTLWs of GaN are larger than those of GaAs for both piezoelectric and optical phonon scattering mechanisms. As small enough quantum-well-width, the piezoelectric phonon modes play an important role and they should be considered in studying the magneto optical transition properties in low-dimensional electron systems. • Influence of piezoelectric phonons on the magneto-optical transition linewidths (MOTLWs) in GaN and GaAs is studied. • MOTLWs are affected strongly by temperature, magnetic field, electron density, and well width. • MOTLWs for piezoelectric phonon are compared with those for optical phonon in both GaN and GaAs. • MOTLWs of GaN are compared with those of GaAs for both piezoelectric and optical phonons. • Influence of piezoelectric phonon on the MOTLWs is considerable and cannot be neglected.

High-resolution analogue of time-domain phonon spectroscopy in the transmission electron microscope: UEM Imaging of GaAs Phonon Dynamics

High-resolution analogue of time-domain phonon spectroscopy in the transmission electron microscope: UEM Imaging of GaAs Phonon Dynamics

HgCdTe-based quantum cascade lasers operating in the GaAs phonon Reststrahlen band predicted by the balance equation method.

The lack of radiation sources in the frequency range of 7-10 THz is associated with strong absorption of the THz waves on optical phonons within the GaAs Reststrahlen band. To avoid such absorption, we propose to use HgCdTe as an alternative material for THz quantum cascade lasers thanks to a lower phonon energy than in III-V semiconductors. In this work, HgCdTe-based quantum cascade lasers operating in the GaAs phonon Reststrahlen band with a target frequency of 8.3 THz have been theoretically investigated using the balance equation method. The optimized active region designs, which are based on three and two quantum wells, exhibit the peak gain exceeding 100 cm-1 at 150 K. We have analyzed the temperature dependence of the peak gain and predicted the maximum operating temperatures of 170 K and 225 K for three- and two-well designs, respectively. At temperatures exceeding 120 K, the better temperature performance has been obtained for the two-well design, which is associated with a larger spatial overlap of weakly localized lasing wavefunctions, as well as, a higher population inversion. We believe that the findings of this work can open a pathway towards the development of THz quantum cascade lasers featuring a high level of optical gain due to the low electron effective mass in HgCdTe.

The possibility of difference frequency generation in the GaAs phonon reststrahlen band within dual-chip GaAs-based lasers

We consider the possibility of difference frequency generation in the GaAs phonon reststrahlen band within dual-chip GaAs-based lasers at room temperature. Sufficient generation efficiency is achieved via the resonant increase of GaAs second order nonlinear susceptibility in this spectral range. The outcoupling power conversion efficiency is anticipated to be up to 4 × 10−7 W−1 in the laser design studied.

Phonon thermodynamics and elastic behavior of GaAs at high temperatures and pressures

The phonons of wurtzite and zinc blende GaAs were calculated at simultaneously elevated temperature and pressure, and elastic constants were calculated as functions of pressure. Pressure caused instabilities of shorter-wavelength transverse acoustic modes in both wurtzite and zinc blende GaAs, causing them to fall to zero at 18 and 20 GPa, respectively. The Born stability criteria, which depend on elastic constants and only long wavelength phonons, therefore overestimated the pressure needed to induce instability at 0 K. At elevated temperatures, explicit anharmonicity pushes the onset of instability to higher pressures in both wurtzite and zinc blende GaAs. Phonon linewidth and densities of states data showed that the quasiharmonic approximation failed to account for temperature-induced phonon frequency shifts, and the quasiharmonic approximation became less reliable at elevated pressure. In general, the number of three-phonon processes increased with pressure, thereby increasing the temperature-driven broadening of phonon spectral lineshapes.

Anharmonic coupling, thermal transport and acoustic wave attenuation in cubic semiconductors and bismuth.

In this work we present our recent results of the ab initio calculations of anharmonic coupling in cubic semiconductors and bismuth. Our results allow us to explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy in the subterahertz domain, which shows a plateau between 0.6 and 1 THz at low temperatures. The plateau is explained by the competition between different phonon-phonon scattering processes such as Herring’s mechanism, which dominates at low frequencies, saturates, and disappears at higher frequencies. We found an excellent agreement between measurements performed by some of us, and new ab initio calculations of third-order anharmonic processes. We predict that the same phenomenon should occur in other cubic semiconductors. In the case of bismuth, we discuss the occurrence of the hydrodynamic heat transport regime at low temperatures, in consistency with the experimental observations. Bismuth is one of the rare materials in which second sound has been experimentally observed. Our calculations predict the occurrence of the Poiseuille phonon flow in Bi between 1.5 K and 3.5 K for sample size of 3.86 mm and 9.06 mm, in consistency with the experimental observations. We will also discuss a Gedanken experiment allowing to assess the occurrence of the hydrodynamic regime in any bulk material.

Raman study of InAs/GaAs quantum dot solar cells

We report polarized and resonant Raman study of InAs/GaAs quantum dot solar cell (QDSC) structures. Raman spectra obtained from the top surfaces of the samples suggested that the formation of InAs QDs induced tensile strain in the overgrown GaAs layers. Furthermore, a longitudinal optical phonon-plasmon (LPP) coupled modes were observed in the p-type GaAs layers. The tensile strain was increased with an increase in the QD size. The hole concentrations estimated by fitting the individual LPP coupled modes were in the range of 2.4–3.5 × 1018 cm−3. Resonant Raman spectra obtained from the cleaved sides, where the QDs were located, showed a 225 cm−1 mode in parallel polarization configurations. Based on accurate analysis, this mode was identified as the LA(X) phonon of GaAs.

(Invited) Phonon Heat Transport Processes and Thermal Conductivity of II-Oxide and III-V Superlattices: Wave-like Coherent Transport in CaTiO3/SrTiO3 Superlattices and Ballistic-Diffusive Transport in GaAs/Alas Superlattices

The thermal conductivity of superlattices is strongly reduced as compared to the thermal conductivity of the individual parent materials comprising these multilayered structures due to phonon-scattering and thermal boundary resistances at the superlattice period interfaces. However, given the relatively length scale of the superlattice periods compared to typical phonon wavelengths and mean free paths, the vibrational heat transport in superlattices can give rise to unique phonon scattering processes. For example, partially ballistic and partially diffusive phonon transport in superlattices in which long wavelength phonons effectively do not “see” the interfaces on the order of the superlattice period can occur if superlattice periodicity is short enough. As another example, under certain conditions in which phonon coherence is not destroyed at superlattice interfaces, the emergence of a “minimum” in the superlattice thermal conductivity as a function of interface density can be realized. In this work, we present a series of works on III-V (e.g., GaAs/AlAs) and II-oxide (CaTiO3/SrTiO3) superlattices in which we observe unique phonon thermal transport properties based on measurements of thermal conductivity of these superlattice thin films. First, we will discuss our work on CaTiO3/SrTiO3 superlattices in which we observe coherent phonon transport based on the observation of a minimum in thermal conductivity of superlattice thin films as a function of interface density. When the period thickness decreases to below the phonon coherence length of the phonons in CaTiO3 and SrTiO3, an increase in interface density leads to an increase in thermal conductivity, a phenomenon that indicates a “particle-wave-like” crossover and coherent phonon transport influencing the in phonon thermal conductivity in superlattices. This observation is enabled by the chemically and structurally abrupt nature of the CaTiO3/SrTiO3 interfaces in these superlattices. Next, we will discuss our work on GaAs/AlAs superlattices in which we observe a ballistic-diffusive cross over based on the total thickness of the superlattice. We report on the room temperature thermal conductivity of AlAs-GaAs superlattices (SLs), in which we systematically vary the period thickness and total thickness between 2–24 nm and 20.1–2,160 nm, respectively. The thermal conductivity increases with the SL thickness and plateaus at a thickness around 200 nm, showing a clear transition from a quasiballistic to a diffusive phonon transport regime. These results demonstrate the existence of classical size effects in SLs, even at the highest interface density samples. Our results reveal that the change in thermal conductivity with period thickness is dominated by incoherent (particlelike) phonons in these III-V superlattice films, whose properties are not dictated by changes in the AlAs or GaAs phonon dispersion relations. This work demonstrates the importance of studying both period and sample thickness dependencies of thermal conductivity to understand the relative contributions of coherent and incoherent phonon transport in the thermal conductivity in SLs. “Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices,” Nature Materials, 13, 168–172 (2014).“Interplay between total thickness and period thickness in the phonon thermal conductivity of superlattices from the nanoscale to the microscale: Coherent versus incoherent phonon transport,” Physical Review B, 97, 085306 (2018).

Femtosecond Pump Probe Spectroscopy Implementation for Semiconductors

Two different pump-probe (PP) setups were developed successfully with different femtosecond pulse lasers. Using a PP setup with an ultra-short pulse laser, the excitation of coherent phonons in GaAs was measured for a calibration and an accuracy test of the developed setup. The frequencies of the coherent phonon modes were in good agreement with reported values [1, 2]. The setups for ZnSe and GaAs were transmission and reflection–type, respectively. When using the ultra-short pulse laser, the signal in the PP experiment was measured by a balanced photo diode.In the other PP experimental setup, built to measure the transient transmittance of bulk ZnSe, the light source and detector differed from the previous PP setup. A strong pulse laser was successfully used for the spectrally resolved pump probe experimental setup. A broadband, high-resolution spectrometer (HR4000CG-UV-NIR) was used as the detector.

Terahertz driven amplification of coherent optical phonons in GaAs coupled to metallic dog-bone resonators

Two-dimensional terahertz spectroscopy on an AlAs/GaAs nanostructure covered by field-enhancing dog-bone resonators shows signatures of coherent optical phonon amplification. Amplification is due to stimulated phonon emission by a terahertz-driven electron current.

Breakdown of Herring's processes in cubic semiconductors for subterahertz longitudinal acoustic phonons

In the present work we explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy $\omega$ in the sub-THz domain. These attenuations along the [100] direction show a plateau between 0.6 and 1 GHz at low temperatures. We found an excellent agreement between measurements performed by some of us, and new \textit{ab initio} calculations of third-order anharmonic processes. The formation of the plateau is explained by the competition between different phonon-phonon scattering processes as Herring's mechanism, which dominates at low frequencies, saturates and disappears. The plateau is shown to be determined by the phononic final-state phase-space available at a given temperature. We predict that a change of scattering mechanism should also show up in the attenuation of silicon around 1.2-1.7 THz, and argue that the attenuation plateau is a general feature of cubic semiconductors.

Phonon thermodynamics and elastic behavior of GaN at high temperatures and pressures

The phonons of wurtzite and zinc blende GaAs were calculated at simultaneously elevated temperature and pressure, and elastic constants were calculated as functions of pressure. Pressure caused instabilities of shorter-wavelength transverse acoustic modes in both wurtzite and zinc blende GaAs, causing them to fall to zero at 18 and 20 GPa, respectively. The Born stability criteria, which depend on elastic constants and only long wavelength phonons, therefore overestimated the pressure needed to induce instability at 0 K. At elevated temperatures, explicit anharmonicity pushes the onset of instability to higher pressures in both wurtzite and zinc blende GaAs. Phonon linewidth and densities of states data showed that the quasiharmonic approximation failed to account for temperature-induced phonon frequency shifts, and the quasiharmonic approximation became less reliable at elevated pressure. In general, the number of three-phonon processes increased with pressure, thereby increasing the temperature-driven broadening of phonon spectral lineshapes.

Interplay between total thickness and period thickness in the phonon thermal conductivity of superlattices from the nanoscale to the microscale: Coherent versus incoherent phonon transport

We report on the room temperature thermal conductivity of AlAs-GaAs superlattices (SLs), in which we systematically vary the period thickness and total thickness between $2--24\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and $20.1--2,160\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively. The thermal conductivity increases with the SL thickness and plateaus at a thickness around 200 nm, showing a clear transition from a quasiballistic to a diffusive phonon transport regime. These results demonstrate the existence of classical size effects in SLs, even at the highest interface density samples. We use harmonic atomistic Green's function calculations to capture incoherence in phonon transport by averaging the calculated transmission over several purely coherent simulations of independent SL with different random mixing at the AlAs-GaAs interfaces. These simulations demonstrate the significant contribution of incoherent phonon transport through the decrease in the transmission and conductance in the SLs as the number of interfaces increases. In spite of this conductance decrease, our simulations show a quasilinear increase in thermal conductivity with the superlattice thickness. This suggests that the observation of a quasilinear increase in thermal conductivity can have important contributions from incoherent phonon transport. Furthermore, this seemingly linear slope in thermal conductivity versus SL thickness data may actually be nonlinear when extended to a larger number of periods, which is a signature of incoherent effects. Indeed, this trend for superlattices with interatomic mixing at the interfaces could easily be interpreted as linear when the number of periods is small. Our results reveal that the change in thermal conductivity with period thickness is dominated by incoherent (particlelike) phonons, whose properties are not dictated by changes in the AlAs or GaAs phonon dispersion relations. This work demonstrates the importance of studying both period and sample thickness dependencies of thermal conductivity to understand the relative contributions of coherent and incoherent phonon transport in the thermal conductivity in SLs.

Ab initio calculations of the phonon and thermal properties of a (GaAs)1/(AlAs)1 superlattice and comparing them with the Ga0.5Al0.5As alloy

In this paper the structural, phonon and thermal properties of a (GaAs)1/(AlAs)1 superlattice have been investigated and compared with the phonon and thermal properties of a Ga0.5Al0.5As alloy and the bulks of GaAs and AlAs. The Debye temperature and the specific heats at constant pressure and volume have been calculated in the temperatures range of 0K to 1000K and at the pressure of 0GPa. It has been observed that the phonon density of state diagram and the thermal properties of the (GaAs)1/(AlAs)1 superlattice are similar to that of the Ga0.5Al0.5As alloy, but the Debye temperature and specific heat of the (GaAs)1/(AlAs)1 superlattice are a little higher than those of the Ga0.5Al0.5As alloy, because it has a structure with lower symmetry.