GaAs-like Longitudinal Optical Research Articles

Strain related relaxation of the GaAs-like Raman mode selection rules in hydrogenated GaAs1−xNx layers

The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAs1−xNx (x = 0.01) layers—with different H doses and similar low-energy irradiation conditions—was investigated by micro-Raman measurements in different scattering geometries and compared with those of epitaxial GaAs and as-grown GaAs1−xNx reference samples. A relaxation of the GaAs selection rules was observed, to be explained mainly on the basis of the biaxial strain affecting the layers. The evolution of the LO phonon frequency with increasing hydrogen dose was found to heavily depend on light polarization, thus suggesting that a linear relation between strain and the frequency of the GaAs-like LO phonon mode should be applied with some caution. Moreover, photoreflectance measurements in fully passivated samples of identical N concentration show that the blueshift of the GaAs-like LO frequency, characteristic of the hydrogenated structures, is dose-dependent and strictly related to the strain induced by the specific type of the dominant N-H complexes. A comparison of photoreflectance results with the finite element method calculations confirms that this dependence on the H dose is due to the gradual replacement of the N-2H complexes responsible for the electronic passivation of N with N-3H complexes, which are well known to induce an additional and sizeable lattice expansion.

Screening effects of photogenerated carriers on terahertz radiation from coherent GaAs-like longitudinal optical phonons in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells

We investigated terahertz radiation from coherent GaAs-like longitudinal optical (LO) phonons in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells for clarifying the screening effects of photogenerated carriers. We observed the intense quasi-monochromatic terahertz wave from the coherent GaAs-like LO phonon, which originates from the initial polarization enhanced by the strong piezoelectric field. The intensity of the coherent GaAs-like LO-phonon band exhibited a saturation behavior as the pump power was increased. We evaluated the saturation behavior in terms of excitation efficiency of the terahertz wave from the coherent GaAs-like LO phonon using the parameter of unit-power intensity. From the pump-power dependence of the unit-power intensity, we conclude that the screening effect of high density photogenerated carriers on the piezoelectric field causes saturation of the terahertz-wave intensity from the coherent GaAs-like LO phonon.

Raman scattering from GaAs/AlGaAs multiple quantum well structures grown by two-step molecular beam epitaxy

We report polarized and spatially resolved Raman scattering from GaAs/Al0.3Ga0.7As multiple quantum well (MQW) structures grown by a two-temperature step molecular beam epitaxy method, in which an initial GaAs buffer layer is deposited at low temperature and a subsequent GaAs layer is deposited at high temperature. Polarized Raman spectra revealed several transverse optical (TO) and longitudinal optical (LO) phonons, i.e., GaAs, GaAs-like, and AlAs-like TO and LO phonons. The Raman peaks related to the AlGaAs barriers were significantly enhanced under resonant excitation near the band gap energy of AlGaAs, In addition, resonant Raman scattering from the side edges of the samples exhibited symmetry-forbidden GaAs-like and AlAs-like LO phonons. To investigate stress and strain evolutions in the GaAs buffer layers, spatially resolved Raman measurements were performed along the growth direction, obtained via the side edges of the samples. Interestingly, Raman maps across the GaAs layers showed that the usually doubly degenerate GaAs TO phonons were split into two distinct phonons. Depending on the two-step growth temperatures, the peak energies of the TO phonons evolved differently along the growth direction. The built-in stress and strain depth-profiles in the GaAs buffer layers were estimated using a biaxial assumption.

Intense monochromatic terahertz electromagnetic waves from coherent GaAs-like longitudinal optical phonons in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells

We demonstrate that, in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells, the terahertz electromagnetic wave from the coherent GaAs-like longitudinal optical (LO) phonon is enhanced by a piezoelectric field originating from a tensile strain in the GaAs layer. The presence of the tensile strain is confirmed using Raman scattering spectroscopy. The Fourier power spectrum of the terahertz waveform shows that the intensity of the terahertz band of the coherent GaAs-like LO phonon increases as the index n approaches 1. The amplitude of the GaAs-like LO phonon is proportional to the piezoelectric field in the strained GaAs layer.

Micro-Raman study on the improvement of luminescence efficiency of GaAsN alloys

We have used in situ micro-Raman spectroscopy to clarify the mechanism of the luminescence efficiency improvement of GaAsN alloys by laser irradiation. Raman peak intensity of GaAs-like longitudinal optical (LO) mode phonon was observed to increase with the laser irradiation time. With increasing laser power density, the Raman intensity of GaAs-like LO mode phonon was found to increase more rapidly and to be saturated in a shorter time. Changes in the Raman intensity can be expressed by the same equation used for the luminescence efficiency improvement. Time constants of changes in the Raman intensity were almost in agreement with those of the luminescence efficiency improvement. This indicates that the increase in Raman intensity of GaAs-like LO phonon is closely related to the luminescence efficiency improvement.

Characterization of terahertz electromagnetic waves from coherent longitudinal optical phonons in GaAs/AlAs multiple quantum wells

The characteristics of terahertz (THz) electromagnetic waves emitted from coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells were investigated. It was found that the amplitude of the THz wave from the coherent LO phonon is increased linearly with increasing number of periods of the quantum well. This result demonstrates that the THz wave originates from the superposition of the oscillatory polarization induced by the coherent GaAs-like LO phonon in each quantum-well layer. The amplitude of the THz wave is proportional to the excitation-power density below about 0.4 μJ∕cm2, and its radiation power reaches about 1 μW at the excitation-power density of 2 μJ∕cm2. Moreover, the amplitude of the THz wave shows a sinusoidal change with the polarization angle of the pump pulse. These results indicate that the coherent LO phonon emitting the THz wave is generated through the combination of the following two mechanisms: impulsive stimulated Raman scattering and instantaneous screening of the surface electric field.

Intense terahertz radiation from longitudinal optical phonons in GaAs∕AlAs multiple quantum wells

We report the generation of intense terahertz (THz) electromagnetic waves from GaAs∕AlAs multiple quantum wells (MQWs) using ultrashort laser pulses. Pronounced THz waves from the coherent GaAs-like longitudinal optical (LO) phonons with a frequency of 8.8 THz were observed in the time region over 6 ps, which indicates the long dephasing time. The power of the THz waves from the coherent GaAs-like LO phonons in the GaAs∕AlAs MQW was enhanced to be about 100 times larger than that of those from coherent LO phonons in an epitaxial-growth GaAs film. We discuss the origin of the enhancement of the THz wave from coherent GaAs-like LO phonon in the MQW.

Characteristics of coupled mode of excitonic quantum beat and coherent longitudinal optical phonon in GaAs/AlAs multiple quantum wells

We have investigated the coupled mode of excitonic quantum beats and coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells with the splitting energy of heavy- and light-hole excitons close to the LO phonon energy. The time-domain signals show the coherent oscillation of the coupled mode between the quantum beat and the LO phonon with a fast dephasing time and the long-lived coherent GaAs-like LO phonon oscillation. The dependence of the coupled-mode frequency on splitting energy exhibits an anticrossing behavior. We demonstrate that the amplitude of the long-lived coherent LO phonon is manipulated by controlling the coupled mode with a double-pulse excitation technique.

Raman scattering from InGaAs/GaAs quantum dot structures grown by atomic layer molecular beam epitaxy

We report Raman scattering studies of optical phonons in InGaAs/GaAs quantum dot (QD) structures grown by atomic layer molecular beam epitaxy to explore formation of QDs and relaxation of strain. The QDs were grown by alternate supply of InAs and GaAs with deposition periods of n=3, 5, and 7. Raman scattering reveals longitudinal optical (LO) and transverse optical phonon responses related to the sample structure. Importantly, a Raman response at ∼237 cm −1 was observed. This Raman response is attributed to the InAs-like LO phonons of InGaAs QDs, indicating clear evidence of the formation of the QDs. Moreover, both the GaAs and the GaAs-like LO phonon energies are shifted downward with increasing n from 3 to 7, suggesting that the strain relaxation occurs and the QDs grow in size.

Enhancement of coherent LO phonons by quantum beats of excitons in GaAs/AlAs multiple quantum wells

We have investigated the dynamical properties of the coherent GaAs-like longitudinal optical (LO) phonon and the quantum beat of excitons in GaAs/AlAs multiple quantum wells with different splitting energies of the heavy-hole and light-hole excitons by using a reflection-type pump–probe technique. It has been found that the coherent GaAs-like LO phonon in the multiple quantum well is remarkably enhanced as the splitting energy approaches to the LO phonon energy of GaAs. The pump-energy dependence of the intensity of the coherent GaAs-like LO phonon is similar to that of the excitonic quantum beat in the multiple quantum wells with the splitting energy nearly equal to the LO phonon energy of GaAs. We conclude that the enhancement of the coherent LO phonon is induced by the coupling of the excitonic quantum beat with the coherent GaAs-like LO phonon through the longitudinal polarization.

Coupling of coherent longitudinal optical phonons to excitonic quantum beats in GaAs/AlAs multiple quantum wells

We have investigated the dynamical properties of the coherent oscillations due to the GaAs-like longitudinal optical (LO) phonon and the quantum beat of excitons in two samples of GaAs/AlAs multiple quantum wells with different splitting energies of the heavy-hole and light-hole excitons. The coherent oscillations were measured with a reflection-type pump-probe technique. In the multiple quantum well with the splitting energy almost equal to the energy of the GaAs-like LO phonon, the intensity of the coherent LO phonon are markedly enhanced in comparison with that of the other sample, and the pump-energy dependence of the intensity of the coherent GaAs-like LO phonon shows similar profile to that of the excitonic quantum beat. Moreover, the intensity of the coherent GaAs-like LO phonon is linearly increased with an increase in pump power, which is contrary to a usual generation mechanism, i.e., the displacive excitation of coherent phonon leading to a square dependence. These facts indicate that the excitonic quantum beat acts as a driving force for the coherent GaAs-like LO phonon in the case that the splitting energy of the heavy-hole and light-hole excitons is almost equal to the LO phonon energy.

Dynamical Properties of Coherent Plasmons Coupled with LO Phonons in an InAs/GaAs Strained Superlattice

We have investigated the coherent plasmon photoexcited in the electron miniband of an InAs/GaAs strained superlattice (SL) using a reflection-type pump-probe technique. Under the condition that the center energy of the pump pulses is tuned to the fundamental transition energy of the SL, the coherent plasmon coupled with the longitudinal optical (LO) phonon of GaAs that is the barrier layer is observed. The time-domain signals of the coherent plasmon coupled with the GaAs-like LO phonon drastically change with varying the generation and detection energies. From the energy dependence of the time-domain signals, we discuss the relaxation process of the carriers photoexcited in the miniband of the SL.

Defect and strain redistribution in InxGa1−xAs/GaAs multiple quantum wells studied by resonant Raman scattering

Resonant Raman scattering by longitudinal optical (LO) phonons in the GaAs barriers of InxGa1−xAs/GaAs multiple quantum well structures has been used to study the onset of strain relaxation as the number of quantum wells is increased. The intensity of scattering from one and two GaAs LO phonons for excitation in resonance with the GaAs E0+Δ0 band gap is found to be highly sensitive to the formation and/or redistribution of defects and the buildup of strain in the barriers, and thus to strain relaxation in the InxGa1−xAs quantum wells. This behavior allows us to detect the onset of strain redistribution, even in samples where the frequencies of both the GaAs LO phonon in the barrier layers and the GaAs-like LO phonon in the InxGa1−xAs quantum wells show no measurable shift. Here, the GaAs LO phonon frequency observed in structures with fully strained InxGa1−xAs wells and unstrained GaAs barriers is used as a reference.

Phonon mode study of near-lattice-matched InxGa1−xAs using micro-Raman spectroscopy

We identify the four allowable phonon modes in InxGa1−xAs on InP:InAs-like transverse optical (TO) (225±2 cm−1), InAs-like longitudinal optical (LO) (233±1 cm−1), GaAs-like TO (255±2 cm−1), and GaAs-like LO (269±1 cm−1), using the selectivity of first-order Raman scattering off the (100) normal surface and the (011) cleaved plane and detailed line-shape analysis employing a sequential simplex optimization procedure. Raman scattering off the (011) cleaved plane was achieved for the first time in thin-film InGaAs using microprobing capabilities (∼1 μm). We also identify another phonon mode R* at 244 cm−1 which is attributed to an alloy disorder mode in these films. For the five identified phonon modes, a linear relationship between the Raman frequencies and composition determined from x-ray diffraction was determined for near-lattice-matched conditions (0.42<1−x<0.52).

Raman scattering study of thermal interdiffusion in InGaAs/InP superlattice structures

Interdiffusion process of InGaAs/InP superlattice structures by thermal annealing of 700–850 °C is studied by Raman spectroscopy. Peak intensities and peak energies of InAs-, GaAs-, and InP-like longitudinal optical (LO) phonon modes change with thermal annealing temperature and time. Depth profiles of the group III and group V atoms are estimated quantitatively by measuring the variations of the peak energies of the LO phonon modes and the ratios of the mode intensities. The energy shift of the GaAs-like LO peak showing a small broadening, the existence of nonenergy shift InP LO phonon peak and the emerging of InP-like LO peak in the lower energy region indicate that the interdiffused superlattice consists of uniform compositional InGaAsP well and InP barrier layers and sharp interfaces. It is found that the resulting InGaAsP quaternary alloy is roughly lattice-matched to InP (<±0.5%). It is also found that the diffusion coefficient in the well region is larger than that in the barrier region, and that the interdiffusion coefficient D0 and activation energy Ea are 8.56×1010 cm2/s and 5.82 eV, respectively. The interdiffusion in this superlattice is determined by the diffusion in the InP region.