Optically biased photoreflectance spectroscopy of a GaAs epitaxial layer and an AlGaAs/GaAs quantum well

We propose optically biased photoreflectance (OBPR) spectroscopy, which is performed by continuous illumination of a secondary monochromatic light on a sample with conventional photoreflectance (PR), as a useful tool to investigate the internal electric fields dependence of the PR signals associated with band to band and quantum level transitions. Line shape of the PR signal has a strong dependence on the internal electric field. In OBPR, if a secondary incident light is absorbed, the internal electric field is suppressed by the photo-generated electron–hole pairs. On the other hand, if the secondary light is not absorbed, the internal electric field is not affected. Through the OBPR investigation of a GaAs epitaxial layer and an AlGaAs/GaAs quantum well, we are able to obtain an absorption like spectrum by performing a wavelength scan of a secondary monochromatic light. The results of OBPR measurements at each PR peak position show the contribution of the electric fields modification by the photo-generated carriers in each layers to the PR signals that are related to band gap and quantum level transitions.     

Photoreflectance spectroscopy of semiconductor structures at hydrostatic pressure: A comparison of GaInAs/GaAs and GaInNAs/GaAs single quantum wells

The pressure dependence of optical transitions in Ga_0.64In_0.36As/GaAs and Ga_0.64In_0.36N_0.01As_0.99/GaAs single quantum well (SQW) structures were studied in photoreflectance (PR) spectroscopy. In order to apply high hydrostatic pressure, up to ∼11 kbar, the liquid-filled clamp-pressure cell with a sapphire window for optical access has been adopted in the PR set-up with the so called ‘bright configuration’. It has been found that the linear hydrostatic pressure coefficient for the ground state transition are equal to 8.6 and 7.3 meV/kbar for the GaInAs/GaAs and GaInNAs/GaAs SQWs, respectively. This result shows that the incorporation of only 1% of N atoms into GaInAs/GaAs leads to ∼15% decrease in the pressure coefficient. In addition, a non-linearity in the pressure dependence of the ground state transition has been resolved for the GaInNAs/GaAs SQW.     

Impurity photoconductivity in strained <Emphasis Type="Italic">p</Emphasis>-InGaAs/GaAsP heterostructures

The impurity-photoconductivity spectrum is observed for strained quantum wells of the p-InGaAs/GaAs solid solution at T= 4.2 K. In addition to the broad photoconductivity band attributed to the transitions from the acceptor ground state to the continuum of the first size-quantization subband, the spectrum exhibits a peak due to the transitions from the ground state to the excited localized acceptor state, a band corresponding to the transitions to the resonance states associated with the second heavy-hole size-quantization subband, and a narrow photoconductivity peak (Fano resonance) in the spectral range corresponding to the optical-phonon energy.     

Investigation of various optical transitions in GaAs/Al0.3Ga0.7As double quantum ring grown by droplet epitaxy

This work examines the optical transitions of a GaAs double quantum ring (DQR) embedded in Al_0.3Ga_0.7As matrix by photoreflectance spectroscopy (PR). The GaAs DQR was grown by droplet epitaxy (DE). The optical properties of the DQR were investigated by excitation‐intensity and temperature‐dependent PR. The various optical transitions were observed in PR spectra, whereas the photoluminescence (PL) spectrum shows only the DQR and GaAs band emissions. The various optical transitions were identified for the GaAs near‐band‐edge transition, surface confined state (SCS), DQR confined state, wetting layer (WL), spin–orbital split (E_GaAs + Δ_o), and AlGaAs band transition. PR spectroscopy can identify various optical transitions that are invisible in PL. The PR results show that the GaAs/AlGaAs DQR has complex electronic structures due to the various interfaces resulting from DE.     

Polarization dependence of two-photon transitions in quantum wells

Summary We give explicitly the polarization dependence of two-photon subband-subband transitions in semiconductor quantum wells. We consider transitions from heavy-hole subbands as well as from light-hole subbands. We study the polarization dependence in the case of absorption of one photon having an energy of the order of the band gap and one having an energy of the order of the subband separation. We show that the absorption structure depends on the polarization of the low-energy photon. We also give, in the case of equal photons with in-plane linear polarizations, the dependence of the transition rate on the angle between the polarizations.     

Photoluminescence due to exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We have investigated photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well at 10 K under intense excitation conditions. It has been found that a PL band due to exciton–exciton scattering, the so-called P emission, is observed in addition to the biexciton PL under an excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical phonon. On the other hand, the P band could never be observed at an excitation energy much higher than the exciton energy, where a band-filling phenomenon appears in the PL spectrum. Furthermore, we confirmed the existence of optical gain leading to stimulated emission in the energy region of the P band using a variable-stripe-length method.     

Photoluminescence properties of exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We report on the photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well under high-density-excitation conditions at excitation energies near the fundamental exciton energies. The biexciton-PL band is dominant in a relatively low-excitation-power region. The PL originating from exciton–exciton scattering, the so-called P emission, suddenly appears with an increase in excitation power. The excitation-energy dependence of the intensity of the P-PL band indicates that the excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical (LO) phonon is the most efficient for the P PL. This suggests that the LO-phonon scattering plays an important role in the relaxation process of excitons leading to the P PL. The appearance of the P-PL band remarkably suppresses the intensity of the biexciton-PL band; namely, the exciton–exciton scattering process prevents the formation of biexcitons. Furthermore, we have confirmed the existence of optical gain due to the exciton–exciton scattering process with use of a variable-stripe-length method.     

Orientational dependence of hole effective masses in quantum-well laser structures

Valence energy subbands and hole effective masses for quantum-well structures have been calculated, using GaInAs–InGaAsP and GaAs–AlGaAs material systems as an example. A Luttinger–Kohn 4 ×  4 hamiltonian with heavy-hole and light-hole band mixing was used in the calculations. Systematic numerical results have been presented for a range of growth directions, material parameters and quantum well widths.     

用光伏谱方法研究InGaAs/GaAs应变量子阱的性质

采用低温光伏谱方法，研究了应变In_0.18Ga_0.82/GaAs单量子阱结构中各子能级之间的光跃迁，并与理论计算的结果进行比较，对光伏谱的谱峰跃迁能量随温度变化的分析，表明量子阱中的应变与温度基本无关．研究了光伏谱的谱峰半高宽度随温度的变化关系．讨论了声子关联、混晶组分起伏及生长界面不平整对光伏谱谱峰宽度的影响 关键词：     

Optical transitions in AlGaAs/GaAs quantum wires on GaAs(6 3 1) substrates studied by photoreflectance spectroscopy

We present the synthesis and characterization of a system of self-assembling GaAs quantum wires (QWRs) embedded in Al_x Ga_1−x As barriers grown by molecular beam epitaxy on GaAs(6 3 1)-oriented substrates. We studied the optical transitions in the QWRs as a function of temperature (T) by photoreflectance (PR) spectroscopy. The energy transitions were extracted from the PR spectra employing the third-derivative functional form, and they were compared with the transitions theoretically calculated from both, a model of QWRs with cylindrical geometry and a model of a conventional square quantum well. The results show a good agreement between experimental and theoretical data in the case of the QWR model, and from this comparison we were able to identify up to 12 different transitions in the PR spectra and to study their behavior dependent on temperature.     

Photoreflectance spectroscopy of a Ga0.62In0.38N0.026As0.954Sb0.02/GaAs single quantum well tailored at 1.5 μm

Optical properties of a Ga_0.62In_0.38As_0.954N_0.026Sb_0.02/GaAs single quantum well (SQW) tailored at ∼1.5 μm have been investigated by photoreflectance (PR) spectroscopy. The identification of the optical transitions was carried out in accordance with theoretical calculations, which were performed within the framework of the usual envelope function approximation. Using this method, four confined states for both electrons and heavy holes have been found and the optical transitions between them have been determined. The obtained result corresponds to a conduction band offset ratio close to 80%. In addition, the effect of ex situ annealing has been investigated. Lineshape analysis of the PR transitions shows that one of the phenomena responsible for the blueshift of QW transitions is the change in the nitrogen nearest-neighbour environment from Ga-rich to In-rich environments.     

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.     

GaAs/AIAs superlattices under pressure: steady-state and subpicosecond time-resolved measurements

Abstract

Low-temperature photoluminescence measurements under hydrostatic pressure were performed on [100]-, [311]-and [111]-grown GaAs/AlAs superlattices. The indirect optical transitions for all three growth directions were identified by their characteristic pressure dependences as originating from the X point of the AlAs conduction band. Subpicosecond-time-resolved measurements on a GaAs/AIAs superlattice show a decrease of electron transfer times from the GaAs layer into the A1As layer with pressure from 400 fs to 5Ofs and an intensity dependence of the pressure-induced crossover from type I to type II.     

Electron and hole levels of InAs quantum dots in a GaAs matrix

Photoluminescence, capacitance-voltage and transmission electron microscopy studies have been carried out on structures containing a sheet of a self-assembled InAs quantum dots formed in GaAs matrices after the deposition of a 1.7 ML of InAs at 480°C. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the capacitance-voltage technique. From analysis of photoluminescence and capacitance-voltage measurements it follows that the quantum dots have electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 meV and 170 meV above the top of the GaAs valence band.     

Threshold and probability of impact ionization by electrons in narrow-gap p-type semiconductors with highly degenerate holes

The process of electron-initiated impact ionization of states in the heavy-hole band is investigated theoretically for a p-type narrow-gap semiconductor with energy bands governed by the Kane dispersion law, subject to the condition that the Fermi level of the holes lies in the valence band. The dependence of the minimum electron energy for ionization of a state at the Fermi level in the valence band on the heavy-hole Fermi momentum is determined. The probability of impact ionization for electrons with near-threshold energies is calculated for the case in which the heavy-hole Fermi momentum exceeds the hole threshold momentum for the given ionization process. Relations between the temperatures of holes and electrons with energies of the order of the threshold value are found, thereby establishing the validity domain of the final results. Fiz. Tverd. Tela (St. Petersburg) 39, 275–279 (February 1997)     

Photoreflectance study of InAs quantum dots on GaAs(n 1 1) substrates

InAs self-assembling quantum dots (SAQDs) were grown on GaAs(n 1 1) substrates (n=2,3,4,5) by molecular beam epitaxy. Their structural and optical properties were studied by reflection high-energy electron diffraction, atomic force microscopy (AFM) and photoreflectance spectroscopy (PR). The PR spectra from 0.7 to 1.3 eV presented transitions associated to the SAQDs. The energy transitions were obtained by fitting the PR spectra employing the third derivative line-shape model. For n=2,4,5, two functions were required to fit the spectra. For n=3 only one function was required, in agreement with the more uniform SAQDs size distribution observed by AFM on GaAs(3 1 1)A. Franz–Keldysh oscillations (FKO) were observed in the PR spectra at energies higher than the GaAs band gap. From the FKO analysis we obtained the GaAs built-in internal electric field strength (F_int) at the InAs/GaAs(n 1 1) heterointerface. From F_int we made an estimation of the GaAs strain at the heterointerface.     

Photoluminescence and photoreflectance studies on δ-Doped In0.15Ga0.85As/GaAs quantum wells

Experimental and theoretical studies on δ-doped In_0.15Ga_0.85As/GaAs quantum wells are reported. Photoreflectance (PR) and Photoluminescence (PL) spectra are measured and compared with results of band structure and PL line shape calculations. The dominating structure seen in the PL spectra is related to the δ-doping well. Its line shape is well described by k_∥-non-conserving radiative transitions.     

Magnetoreflectance and luminescense study of beryllium acceptors in selectively doped GaAs/AlxGa1−xAs multiple quantum wells

We present a magneto-optical study of p-type GaAs/Al_xGa_1−xAs quantum wells doped with Be acceptors over the central one-third of the GaAs layers. Using magneto-reflectance and magneto-luminescence spectroscopies, we have investigated (a) interband Landau transitions and (b) transitions from the conduction band Landau levels to the Be acceptors. Binding energies of the acceptors were determined and the dependence of the impurity ground state energy on magnetic field has been studied.     

Optical properties of GaInNAs/GaAs prepared by molecular beam epitaxy

Optical properties of GaInNAs are studied with emphasis on photoluminescence (PL) and photoreflectance (PR) using GaInNAs single quantum wells (GaInNAs-SQW) and thick GaInNAs layers lattice-matched to GaAs (LM-GaInNAs) grown by solid-source molecular-beam epitaxy. First, a blue-shift of band-gap energy as a result of a rapid thermal annealing (RTA) has been confirmed by both PL and PR. Temperature-dependent PL and PR spectra were measured. At low temperature, GaInNAs-SQWs exhibited a PL due to the localized exciton (LE), and the localization was strongly reduced by RTA. A very high-quality GaInNAs-SQW exhibited very intense PL with small half-width (10–20 meV for 8–300 K) without LE emission. In LM-GaInNAs, low-temperature PL exhibited both the LE emission and a deep PL band. Temperature-dependent PL showed that LM-GaInNAs contains large number of non-radiative centers. PR spectra of LM-GaInNAs were composed of more than two transitions, and their relative intensity depended on temperature and annealing.     

Effects of hydrostatic pressure on intrawell and interwell excitons in a strained GaAs/GaAlAs double quantum well system

Binding energies of intrawell and interwell excitons are investigated in a GaAs/GaAlAs double quantum well system in the presence of hydrostatic pressure applied in the z-direction. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of pressure. The pressure dependent photoionization cross section for a charged exciton placed at the center of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross section on photon energy is carried out for the charged excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The results show that the charged exciton binding energy, interband emission energy and the photoionization cross section depend strongly on the well width and the hydrostatic pressure. Our results are compared with the other existing literature available.