LA Phonons Research Articles

Optic and Acoustic Phonons in Strained Hexagonal GaN/AlGaN Multilayer Structures

The effects of the layer thickness and alloy composition on the Raman spectra of hexagonal GaN/Al x Ga 1-x N multilayer structures have been investigated. Our findings show that the A 1 (TO) optic mode can be regarded as propagating through the structure, whereas other optic modes are confined to the GaN or Al x Ga 1-x N layer. We report the first observation of folded acoustic modes in short-period GaN/Al x Ga 1-x N superlattices. The dispersion of the LA phonon branch was determined by using different Raman scattering configurations. The sound velocity for GaN/Al x Ga 1-x N SL with Al content x = 0.28 was found to be 8410 m/s.

Optic and Acoustic Phonons in Strained Hexagonal GaN/AlGaN Multilayer Structures

The effects of the layer thickness and alloy composition on the Raman spectra of hexagonal GaN/Al x Ga 1-x N multilayer structures have been investigated. Our findings show that the A 1 (TO) optic mode can be regarded as propagating through the structure, whereas other optic modes are confined to the GaN or Al x Ga 1-x N layer. We report the first observation of folded acoustic modes in short-period GaN/Al x Ga 1-x N superlattices. The dispersion of the LA phonon branch was determined by using different Raman scattering configurations. The sound velocity for GaN/Al x Ga 1-x N SL with Al content x = 0.28 was found to be 8410 m/s.

Theory of coherent acoustic phonons inInxGa1−xN/GaNmultiple quantum wells

A microscopic theory for the generation and propagation of coherent LA phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump laser and is treated theoretically using the density matrix formalism. We use realistic wurzite bandstructures taking valence-band mixing and strain-induced piezo- electric fields into account. In addition, the many-body Coulomb ineraction is treated in the screened time-dependent Hartree-Fock approximation. We find that under typical experimental conditions, our microscopic theory can be simplified and mapped onto a loaded string problem which can be easily solved.

Acoustic phonon generation from quasi-2D hole gas in quantum wells

The angular and polarization dependences of acoustic phonon emission from a quasi-2D hole gas in quantum wells are theoretically investigated. The contribution of both deformation potential and piezoelectric coupling is considered in the hole–phonon interaction. We have also taken into account the effect of anisotropy of the valence band which gives rise to the hole–phonon interaction due to deformation potential coupling for both LA and TA phonons. Finally, the theory is applied to calculate the rates of acoustic phonon emission in GaAs quantum wells.

Resonant Brillouin Scattering of Exciton-Polaritons in β-ZnP2

The secondary emission spectra of β-ZnP 2 under the resonant excitation in the 1 s exciton region were investigated in detail. Several scattering lines with the Stokes shifts of less than 6 meV were found. By analyzing the experimental results using a model based on the exciton-polariton picture, we attributed the observed scattering lines to the Brillouin lines. The sound velocities of the LA and TA phonons were derived to be v LA =4.8 (±0.2)×10 5 cm/s and v TA =2.8 (±0.2)×10 5 cm/s, respectively. The resonant enhancement of the Brillouin lines was observed around the transverse exciton energy E T , which reflects high transmissivity and a small group velocity of the exciton-polaritons at E T . We also found that the Rayleigh scattering line was resonantly enhanced between the longitudinal exciton energy E L and E T due to the surface roughness, which explains the drastic reduction of the luminescence from the lower branch polaritons in intensity reported earlier. The excitation energy dependence of the Brillouin line width is also discussed.

Carrier relaxation and quantum decoherence of excited states in self-assembled quantum dots

We report systematic measurements of photoluminescence excitation spectra and dephasing times ${(T}_{2}'\mathrm{s})$ on various excited states of hundreds of individual quantum dots (QDs). From the variation of ${T}_{2}'\mathrm{s}$ with the energy separation between excited states and the ground state ${(E}_{\mathrm{rel}}),$ we identified two distinct regions of ${E}_{\mathrm{rel}}$ where LO phonon emission and hole relaxation via LA phonon emission play as dominat dephasing mechanisms. We also found a clear evidence of significantly slow energy relaxation in the ${E}_{\mathrm{rel}}$ range where these phonon emission processes are suppressed due to the reduction of interaction phase space.

Absorption and Emission of Acoustic Phonons by a Two‐Dimensional Electron Gas

We have observed acoustic phonons, absorbed as nanosecond heat pulses directly by the two-dimensional electron gas (2DEG) of a GaAs-Al 0.3 Ga 0.7 As Modulation Doped Field Effect Transistor (MODFET) structure. The effect of phonons was to increase the electron temperature of the 2DEG and also the mobility. From a comparison with the effect of electrical heating, it was possible to determine the absolute level of the phonon power observed. Finally by comparison with data on TA and LA phonon absorption by the 2DEG, it could be concluded that the deformation potential was the dominant electron-phonon coupling mechanism in GaAs.

Frictional drag between nonequilibrium charged gases

The frictional drag force between separated but coupled two-dimensional electron gases of different temperatures is studied using the non-equilibrium Green function method based on the separation of center-of-mass and relative dynamics of electrons. As the mechanisms of producing the frictional force we include the direct Coulomb interaction, the interaction mediated via virtual and real TA and LA phonons, optic phonons, plasmons, and TA and LA phonon-electron collective modes. We found that, when the distance between the two electron gases is large, and at intermediate temperature where plasmons and collective modes play the most important role in the frictional drag, the possibility of having a temperature difference between two subsystems modifies greatly the transresistivity.

Optically driven qubits in artificial molecules

We present novel models of quantum gates based on coupled quantum dots in which a qubit is regarded as the superposition of ground states in each dot. Coherent control on the qubit is performed by both a frequency and a polarization of a monochromatic light pulse illuminated on the quantum dots. We also show that a simple combination of two single qubit gates functions as a controlled NOT gate resulting from an electron-electron interaction. To examine the decoherence of quantum states, we discuss electronic relaxation contributed mainly by LA phonon processes.

Phonon generation and decay in hydrogenated amorphous silicon

We report on investigations of the temporal evolution of nonequilibrium phonon populations in hydrogenated amorphous silicon held at 1.8 K. Two types of a-Si:H were examined, one grown by plasma enhanced chemical vapor deposition (PE) and one by hot-wire assisted chemical vapor deposition (HW). Phonons were created during the relaxation and recombination of optically excited charge carriers, and detected by means of pulsed anti-Stokes Raman spectroscopy. In the same setup, pulsed luminescence experiments were performed, under identical experimental conditions. The temporal shape of the Raman signals turned out to be determined both by the electronic processes responsible for the phonon generation and by the anharmonic decay of the excited phonon population itself. In the PE films we observed a slowly decaying $(\ensuremath{\gg}100$ ns) contribution to the Raman signal, which was not present in the HW layers. We propose a model to explain this slow background as resulting from laser-induced fast nonradiative recombination of mobile with localized charge carriers. The results of the pulsed luminescence experiments support our model. In addition, phonon decay times were observed to be the same in all samples: decay times of $\ensuremath{\sim}70$ ns were obtained for LA- and TO-like vibrations, whereas TA-like vibrations decayed faster $(<10$ ns) than could be resolved with the experimental setup. We propose that the extreme longevity of the LA and TO phonons is related to the microstructure of amorphous silicon.

Effect of pressure on the phonon properties of europium chalcogenides

Lattice vibrational properties of europium chalcogenides have been investigated at high pressure by using a simple lattice dynamical model theory viz. the three-body force rigid ion model (TRIM) which includes long range three-body interaction arising due to charge transfer effects. The dispersion curves for the four Eu-chalcogenides agree reasonably well with the available experimental data. Variation of LO, TO, LA and TA phonons with pressure have also been studied at the symmetry points of the brillouin zone (BZ) for Euchalcogenides for the first time by using a lattice dynamical model theory. We have also calculated the one phonon density of states and compared them with the first order Raman scattering results. The calculation of one phonon density of states for Eu-chalcogenides has also been extended up to the phase transition pressure. We observed a pronounced shift in phonon spectrum as pressure is increased.

Excitonic relaxation involving exciton–acoustic phonon interactions in quantum wells

We investigate exciton–acoustic phonon interaction for both LA and TA phonons obtained from deformation potential and piezoelectric interactions in quantum wells. Exciton–acoustic phonon interaction operator is derived as a function of phonon wave vector q for both LA and TA phonons. The emission rates of LA and TA phonons due to excitonic relaxations are also calculated as a function of phonon wave vector, exciton temperature, exciton density, and well width in GaAs quantum wells. Our results show that LA and TA phonons are equally effective in the excitonic transition involving acoustic phonon emission in GaAs quantum wells.

Triplet-to-singlet conversion of 1s excitons in β-ZnP 2

We observe the luminescence spectrum of the singlet exciton polariton ( E‖c) in β-ZnP 2 at 2 K under excitation into the lowest triplet exciton absorption ( E‖b) by measuring the excitation spectra. A sharp excitation peak is observed at the triplet exciton energy E t for each excitation spectrum, which appears through the anti-Stokes acoustic phonon scattering of the triplet excitons into the singlet exciton polariton states lying above 2 meV. The luminescence spectrum is obtained by plotting the intensities of the excitation peaks against the monitoring energy. The spectrum has similar shape to the singlet polariton luminescence. The excitation efficiency increases steeply again below the low-energy tail of the ordinary singlet luminescence. Since the singlet exciton polariton has the character of light in the low-energy region, the scattered polariton goes out easily from the crystal surface. As the monitoring energy is lowered, the excitation spectra have almost equidistant (∼0.14 meV) satellite peaks on both sides of the main peak at E t. The energy distance corresponds to the LA phonon energy connecting the light branch with the bottleneck in the triplet exciton polariton.

Use of an Optical Microcavity to Probe Exciton Relaxation in Strained V-Groove Quantum Wires

State-of-the-art InGaAs/GaAs V-groove quantum wires were successfully embedded in a wavelength-size planar Bragg microcavity. The cavity induced control of the wire spontaneous emission spectrum and pattern is used to probe the dynamics of hot carrier relaxation in one dimension following pulsed ps optical excitations. Slow relaxation is observed at 60 K and high carrier density, as a result of hot LO-phonon reabsorption and carrier phase space filling. Slower relaxation is observed at 10 K and low carrier density, partly due to the low carrier–LA phonon scattering rate.

Ballistic phonon studies in the lowest Landau level

We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall regime at Landau level filling factors of ν= 1 3 , 2 5 and 1 2 . The technique used can resolve the interaction of the two-dimensional electron system with LA and TA phonons and has been used to measure the temperature variation of the heat capacity of a single layer of electrons at ν= 1 3 . The energy gaps at ν= 1 3 have also been measured and found to be in good agreement with theory. The roles of compressible and incompressible regions in the phonon absorption process are discussed. Angle resolved measurements at ν= 2 5 are also in good agreement with theory.

Excitonic relaxation through phonon interactions in quantum wells

Excitonic processes involving LA, TA, and LO phonons are studied in quantum wells. We have derived expressions for exciton–acoustic phonon interactions for both deformation potential and piezoelectric couplings. We have taken into account the effect of anisotropy of the valence band which gives rise to exciton–phonon interaction due to deformation potential coupling for both LA and TA phonons. The rates of excitonic relaxation are also derived involving all three types of phonons. Finally, the rates of phonon emissions are calculated in GaAs quantum wells.

Decay of the longitudinal optical phonons in wurtzite GaN and Al xGa 1− xN

Decay of the longitudinal optical (LO) phonons in wurtzite GaN and Al x Ga 1− x N ( x=0.1) has been studied by subpicoseond time-resolved Raman spectroscopy. In contrast to the usually believed 2LA decay channel for LO phonons in other semiconductors, our experimental results show that, among the various possible decay channels, the LO phonons in wurtzite GaN and Al x Ga 1− x N ( x=0.1) decay primarily into a large wave vector TO and a large wave vector LA or TA phonons. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves.

Effects of stimulated optical phonon decay on hot carriers in GaAs

We have considered energy transfer from highly nonequilibrium electron gas to crystal lattice via emission of high-frequency longitudinal optical (LO) phonons and their subsequent transformations. At high intensity of electron excitation, the daughter LA phonons resulting from LO-phonon decay accumulate in considerable numbers and cause the LO-phonon decay to be a stimulated process. The stimulated decay effectively decimates the LO phonons and thus prevents their reabsorption by the electrons. These effects being incorporated in description of the electron–phonon kinetics, we have obtained considerable intensification of energy transfer from the electron subsystem. We also qualitatively discuss the effects of stimulation for alternative channels of LO-phonon decay.

New aspects of the b*-LA phonon of CuGeO 3

We performed high resolution measurements of the b*-LA phonon branch of CuGeO 3 by inelastic neutron scattering. From the results it was clarified that the b*-LA broad branch reported earlier is composed of two phonon branches with excitation energies very nearby. The second branch close to this b*-LA mode cannot be assigned in the space group Pbmm and it may indicate that a twisting motion of GeO 4 tetrahedra couples to the b*-LA mode. This coupling may be an indispensable ingredient for the spin-Peierls transition in CuGeO 3.

Stimulated decay of nonselectively pumped optical phonons in GaAs

We have considered the processes determining the steady-state population of longitudinal optical ~LO! phonons being intensively produced in GaAs by electrons optically excited or accelerated in strong external electric field. The emitted LO phonons pass their energy to acoustic phonon systems through anharmonic decay into longitudinal acoustic ~LA! phonons. The latter emerge as almost monoenergetic and concentrated in a narrow layer in the reciprocal space. Had the occupation numbers of k states within this layer grown to unity, the decay process would have become stimulated, effectively desolating the built-in LO states and giving rise to various nonlinear phenomena. We show that this is actually the case by tracing LA phonon kinetics and calculating the number of phonons in each k state available for feedback. The results obtained substantially clarify the physical picture of energy transfer processes mediating heat removal in GaAs. @S0163-1829~99!02131-1#