Single Quantum Well Structures Research Articles

Optical study of APCVD-grown Si 1− xGe x/Si quantum well structures under different post-growth annealing conditions

Single and double Si 1− x Ge x /Si quantum well (QW) structures, which were grown by atmospheric pressure chemical vapor deposition (APCVD), are characterized by photoluminescence and secondary-ion mass spectrometry. Systematic post-growth annealing treatments are carried out at temperatures between 600 and 1100°C in pure N 2 ambient. The interdiffusion between the Si layer and the Si 1− x Ge x well layers occurs at the annealing temperature around 900°C. From SIMS measurements for single QW structures we have estimated the activation energy which is about 3.9 eV in the temperature range between 950°C and 1100°C. The double QW structures show a similar value. The intensity of the exciton recombination related to carriers confined in the double QW structures decreases with increasing annealing temperatures and becomes strongly suppressed at 750°C. When the annealing temperature is increased further, the intensity of the QW emission recovers in the QW structures.

On the improvement in thermal quenching of luminescence in SiGe/Si structures grown by molecular beam epitaxy

Thermal quenching of photoluminescence (PL) from SiGe/Si quantum well (QW) structures grown by molecular beam epitaxy is shown to be more severe when grown at a lower temperature. The mechanism responsible for the thermal quenching of PL is discussed as being due to thermally activated nonradiative recombination channels, related to defects in both Si barriers and SiGe QW. Nonradiative defects in Si can be rather efficiently deactivated by post-growth treatments such as hydrogenation and thermal annealing, leading to a significant improvement in the thermal quenching behavior of PL from single QW structures. Nonradiative defects in SiGe are found to be thermally stable, on the other hand, evident from the experimentally observed minor role played by post-growth thermal annealing in the thermal quenching of PL from multiple QW structures.

Saturation spectroscopy of electronic states in a magnetic field in InAs/Al xGa 1− xSb single quantum wells

We have carried out saturation spectroscopy of cyclotron resonance in a semiconducting InAs/Al 0.5Ga 0.5Sb single quantum well using the UCSB free electron laser and have extracted an effective Landau level lifetime using an n-level rate equation model. The effective lifetime shows strong oscillations (>an order of magnitude) with frequency. Minima are shifted to higher frequencies than those given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. We have also used this technique to investigate the origins of two lines: the X-line and cyclotron resonance in a “semimetallic” InAs/Al 0.1Ga 0.9Sb single quantum-well structure. Results show that the two lines are of different origin.

Role of localized excitons in the stimulated emission in ultra-thin CdSe/ZnSe/ZnSSe single quantum-well structures

Photo-pumped lasing properties have been investigated in a CdSe/ZnSe/ZnSSE single quantum wells (SQWs) with the well-layer thickness (LW) of 1, 2 and 3 monolayer (ML). At 20 K, the laser threshold for the SQW withLW= 1 ML was the lowest in spite of the smallest active layer thickness. The carrier (exciton) sheet density at the threshold (n)thwas estimated to be as low as 7 × 1010cm−2, which is well below Mott's screening density. Time-resolved photoluminescence has revealed that the localized biexciton band, whose peak energy agrees with the lasing peak, appeared on the low-energy side of the main PL peak at this level of carrier concentrations. Theoretical calculation has also shown that the localized biexciton recombination has to be taken into account for the lasing process. On the contrary, thenthvalues of the SQWs with 2 and 3 ML are 1 order of magnitude larger than that of the SQW with 1 ML. This may be due to the smaller oscillator strength of both localized excitons and localized biexcitons because of the larger inhomogeneous broadening, resulting in an increased carrier density for achieving optical gain sufficient to overcome the reflection losses.

High magnetic field study of pressure effect on the spin–spin coupling among excitons and Mn ions in 2D- and 3D-CdMnTe systems

Pressure effects on the spin exchange interaction in CdTe/Cd 1− x Mn x Te ( x=0.095, 0.24) single quantum well (SQW) structure with different quantum well widths and bulk Cd 1− x Mn x Te ( x=0.15) were investigated through photoluminescence (PL) measurements under magnetic fields up to 40 T. We took into account a linear magnetization term in the determination of spin exchange parameters from Zeeman shifts of PL peaks. Negative pressure dependence of the exchange constant between the exciton spin and the Mn spin was found for the barrier in the CdTe/Cd 1− x Mn x Te ( x=0.24) SQW structure and Cd 1− x Mn x Te ( x=0.15).

Electronic band structures and optical gain spectra of strained wurtzite GaN-Al/sub x/Ga/sub 1-x/N quantum-well lasers

The electronic band structures, density-of-states, and optical gain spectra for wurtzite GaN-Al/sub x/Ga/sub 1-x/N quantum wells are studied theoretically based on the Hamiltonian derived using the k.p method. We investigate the dependence of the optical gain and transparent current density on the well width, barrier height, and strain using a numerical approach with high accuracy. The mole fraction of Al in the barrier material is progressively increased to study the effects of quantum confinement and compressive strain. A higher Al mole fraction in the barrier leads to improvement of the TE optical gain and suppression of the TM optical gain. Furthermore, we demonstrate that a reduction of the well width offers improved modal gain over all radiative current densities. We also predict a transparent current density of 250 A/cm/sup 2/ for the GaN-Al/sub x/Ga/sub 1-x/N single quantum-well (QW) structure. Our results suggest that a suitable combination of thin well width and large barrier height should be selected in improving the TE optical gain in wurtzite GaN-Al/sub x/Ga/sub 1-x/N single QW.

Conduction-band offset in a pseudomorphic GaAs/In0.2Ga0.8As quantum well determined by capacitance–voltage profiling and deep-level transient spectroscopy techniques

The conduction-band offset ΔEC has been determined for a molecular beam epitaxy grown GaAs/In0.2Ga0.8As single quantum-well structure, by measuring the capacitance–voltage (C–V) profiling, taking into account a correction for the interface charge density, and the capacitance transient resulting from thermal emission of carriers from the quantum well, respectively. We found that ΔEC=0.227 eV, corresponding to about 89% ΔEg, from the C–V profiling; and ΔEC=0.229 eV, corresponding to about 89.9% ΔEg, from the deep-level transient spectroscopy (DLTS) technique. The results suggest that the conduction-band discontinuity ΔEC obtained from the C–V profiling is in good agreement with that obtained from the DLTS technique.

Investigation of inhomogeneities in (Al, Ga, In)N heterostructures by STEM and cathodoluminescence

Wurtzite (Al, Ga, In)N heterostructures grown by metal organic vaper phase epitaxy (MOVPE) were studied using quantitative analytical scanning transmission electron microscopy (STEM) and cathodoluminescence (CL). The STEM results can be summarized as follows. The interfaces in e.g. In 0.12Ga 0.88N/GaN single quantum wells (SQWs) appear to be asymmetric. The lower interface in the growth direction is more abrupt than the upper one, where a grading in the In concentration is significant. We found composition fluctuations in the nanometer range within the InGaN QWs, which are supposed to cause the localized exitonic behavior of the observed photoluminescence (PL) emission. In the sample with 17 nm thick InGaN QWs we observed relaxation effects, which are not present in the thin QWs of 2 nm thickness. CL results on both InGaN/GaN and AlGaN/GaN SQW structures show generally inhomogeneous emission intensity in panchromatic CL micrographs on a 1 μm scale, which is related to local variations of the interface quality. CL spectra recorded from defect sites in AlGaN/GaN SQWs are dominated by the so-called ‘yellow-emission’. In the samples containing InGaN layers, the grown-in hexagonal pyramids and mesa-like structures as well as micropipes were commonly observed.

Strong Piezoelectric Effects in Unstrained GaN Quantum Wells

AbstractOptical properties of GaN/AIGaN single quantum well (SQW) and multi quantum well (MQW) structures grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire have been characterised by low temperature photoluminescence. Photoluminescence (PL) peaks corresponding to emission from very narrow (10 Å wide) SQWs are blue shifted with respect to the bulk GaN emissions but reveal strong red shift for wider SQWs (40 and 60 Å wide). Structural properties of SQW and MQW structures have been characterised by X-ray reciprocal lattice mapping in order to determine the strain conditions and composition of ternary A1GaN alloys. The results clearly demonstrate that in all structures under investigation the ternary (barrier) alloys are fully strained to underlying MOCVD-grown GaN/sapphire substrates. Despite of the fact that GaN quantum wells (QW) are unstrained, photoluminescence experiments clearly reveal the presence of extremely strong electric fields in the QW regions, which can be attributed to the interplay of the piezoelectric-type polarisation in the well and barrier layers due to Fermi level alignment.

Thermal Stability Of SiGe/Si Quantum Well Structures Grown By APCVD

AbstractSingle and double Si1-xGexx/Si quantum well (QW) structures, which were grown by atmospheric pressure chemical vapor deposition (APCVD), are characterized by photoluminescence and secondary ion mass spectrometry. Systematic post-growth annealing treatments were carried out at temperatures between 600°C and 1100°C in pure N2 ambient. The interdiffusion between the Si layer and the Si1-xGex, well layers occurs at the annealing temperature around 900°C. The diffusion coefficient is deduced at different temperatures from SIMS measurements for single QW structures. The activation energy is about 3.9 eV in the temperature range between 950°C and 1100°C. The double QW structures show a similar value, but the accurate value is more difficult to obtain because it is more complicated to analyze the SIMS profile of the double QW structures. The intensity of the exciton recombination related to carriers confined in the double QW structures decreases with increasing annealing temperatures and becomes strongly suppressed at 750°C. When the annealing temperature is increased further, the intensity of the QW emission recovers. The results indicate that nonradiative centers were generated at annealing temperature of about 750°C

High Magnetic Field Photoluminescence Study of Pressure Effect on Spin Exchange Interaction in a CdTe/Cd1-xMnxTe Single Quantum Well Structure.

Photoluminescence (PL) measurements in CdTe/Cd1-xMnx Te (x=0. 24) single quantum well (SQW) structure with different quantum well (QW) widths and bulk Cd1-xMnxTe (x=0. 15) under high hydrostatic pressures up to 2. 68 GPa and DC magnetic fields up to 30 T are reported. We analyze the Zeeman shift of the band-edge exciton PL peaks, taking into account a linear magnetization term, and obtain pressure dependence of spin exchange parameters. Negative pressure dependence of the p-d hybridization is found for the barrier of the SQW structure and the bulk sample. A possibility that the hybridization is weakened due to the pressure induced relaxation of the distortion around Te atoms is proposed.

Spatially resolved cathodoluminescence spectra of InGaN quantum wells

Spatially resolved cathodoluminescence (CL) spectrum mapping revealed a strong exciton localization in InGaN single-quantum-wells (SQWs). Transmission electron micrographs exhibited a well-organized SQW structure having abrupt InGaN/GaN heterointerfaces. However, comparison between atomic force microscopy images for GaN-capped and uncapped SQWs indicated areas of InN-rich material, which are about 20 nm in lateral size. The CL images taken at the higher and lower energy side of the spatially integrated CL peak consisted of emissions from complementary real spaces, and the area was smaller than 60 nm in lateral size.

Phonon dispersion and electron–polar optical phonon interaction in coupled quantum-well structures in the modified image–charge ansatz approach

Phonon dispersion and Fröhlich interaction in coupled quantum-well structures are derived by using a modified image–charge ansatz that shows that the mode symmetry in the surface phonon dispersion of single quantum-well (QW) structures is modified by the addition of a thin inner barrier. The electron–phonon scattering rates are evaluated for phonons in the entire coupled QW configuration and compared with the corresponding rates calculated for phonons in the separated-single QW approximation. Our calculations show that the separated-single QW phonon model provides reasonable estimates of the electron–phonon scattering rates except for intersubband processes when the quantized energy separation between the initial and final electron states is in resonance with the polar optical phonon energy.

Metalorganic Vapor Phase Epitaxy Growth of a High-Quality GaN/InGaN Single Quantum Well Structure Using a Misoriented SiC Substrate

A high-quality GaN/InGaN single quantum well (SQW) structure has been successfully grown using a misoriented 6H–SiC substrate, the face of which is tilted from (0001) toward [1120] by 3.5°, by low pressure metalorganic vapor phase epitaxy (MOVPE). A sharp emission, whose full-width at half maximum (FWHM) was 24.3 meV, from the GaN/InGaN SQW structure was observed at 385 nm in the 77 K photoluminescence spectrum. From the transmission electron microscopy (TEM) analysis, the dislocations in the GaN film grown on the misoriented substrate were bent from the c-direction, and the threading dislocations to the InGaN film on the GaN film were decreased. For the InGaN film grown on the misoriented substrate, only the sharp band edge emission, whose FWHM was 92.3 meV, was observed at 385 nm in the PL spectrum at 77 K. The dislocation density, which was estimated from TEM photographs, in the InGaN film on the GaN film grown on the misoriented substrate was about 5×108 cm-2, which was nearly half of that grown on the (0001) substrate.

Simulation of the capacitance–voltage characteristics of a single-quantum-well structure based on the self-consistent solution of the Schrödinger and Poisson equations

The numerical self-consistent solution of the coupled Schrödinger and Poisson equations is used to simulate the C–V characteristic of Schottky barrier heterostructures with a single quantum well (SQW). This model is applied to study n-type SQW structures based on InGaAs/InAlAs. It has been shown from analysis of the C–V characteristics of a SQW structure that it is possible to extract information about the energy position of subband levels and the distribution of electron density in the QW. We have demonstrated that due to the two-dimensional distribution of electron gas in the QW the apparent concentration profile NC–V–W derived from the C–V characteristic fails to describe the free electron density distribution in the QW layer. However, the number of the NC–V–W peaks indicates the quantity of electron subband levels in the QW situated below the Fermi level at zero reverse bias.

Optical properties of quaternary GaInAsSb/AlGaAsSb strained quantum wells

The electronic states in quaternary GaInAsSb/AlGaAsSb strained quantum well (QW) structures grown by molecular beam epitaxy have been investigated both theoretically and experimentally. For strained multiple quantum wells (MQWs), strong luminescence, and well-resolved excitonic absorption peaks are observed even at room temperature, which is indicative of the good quality of our quaternary sample. By fitting the experimental results to the theoretical calculations, we find that the light holes are in well regions (type I MQWs) and the conduction band offset ratio . The critical temperature for the excitonic-polariton - mechanical-exciton transition in this quaternary MQW structure was found to be . The measured intersubband absorption of about is in good agreement with the theoretical calculation. The transition from type I MQWs to type II MQWs for light holes is also predicted theoretically. In the photoluminescence spectra the sharp exciton resonances have been attributed to localized excitons for temperature and to free excitons at higher temperatures up to room temperature. We conclude that the dominant luminescence quenching mechanism in this quaternary system is mainly that of the trapped excitons thermalizing from the localized regions below 100 K, and the thermal carrier activation from the first electron and heavy-hole subbands to the second electron and heavy-hole subbands at higher temperatures. The strength of the exciton - phonon coupling is determined from the linewidth analysis. The inhomogeneous linewidth and homogeneous broadening in both MQW and single-quantum-well (SQW) structures have been discussed. We conclude that the experimental result of stronger exciton - phonon coupling in the quaternary SQW structure will lead to partial ionization of excitons at higher temperatures (above 125 K), in good agreement with the line-shape analysis of the luminescence spectra which clearly shows the presence of band-to-band recombination.

Gain Characteristics of Optical-Gate Switches Using InGaAs/GaAs Strained-Layer Quantum Wells

Gain characteristics in single, double and triple InGaAs/GaAs strained-layer quantum-well structures have been measured in a wide wavelength range, and an optical-gate switch with the 0.9 µ m wavelength band has been examined for use in an integrated device with low driving current operation. Gate switches are classified into two types: small-gain switches and large-gain switches. A switch with 0 dB gain is produced by reducing the active layer volume through the use of a single quantum-well structure with 1.8 mA driving current. A large-gain switch with 15 dB gain under a no-gain-saturation condition is achieved using a large optical confinement structure: a triple quantum-well structure with 10 mA driving current. The driving current dependence of the gain spectrum for the single, double, and triple quantum-well gate switches shows that carrier injection to the second quantized state plays an important role in the saturation of the increase in gain when the driving current is increased.

Growth and optical investigation of quaternary [formula omitted] quantum wells

Infrared photoluminescence and high sensitive absorption measurements were performed on a quaternary GaInAsSb AlGaAsSb strained multiple quantum well (MQW), as well as single quantum well (SQW) structures grown by molecular-beam epitaxy, to investigate its band offsets and subband behavior. Strong luminescence and well-resolved excitonic absorption peaks are observed even at room temperature, which is indicative of the good quality of our quaternary sample. By fitting the experimental results to the theoretical calculations, we find that the light holes are confined in well regions for Ga 0.75In 0.25As 0.04Sb 0.96 Al 0.22Ga 0.78As 0.02Sb 0.98 QWs (type I MQW) with a conduction-band offset ratio of Q c = 0.66 ± 0.01. The transition from type I to type II for light holes is predicted theoretically and demonstrated directly by photoluminescence spectra in the SQW structures.

Observation of excitonic polariton and broadening of room-temperature exciton in strained InGaAs/GaAs quantum wells

Photoluminescence and absorption spectra of strained InGaAs/GaAs single quantum well (SQW) and multiple quantum well (MQW) structures as a function of well width have been investigated in detail. It has been demonstrated that the strength of the exciton-LO phonon coupling is quite stronger in InGaAs/GaAs SQW structures than that of InGaAs/GaAs MQW structures by aid of the temperature-dependent linewidth analysis. The critical temperature for the excitonic polariton-mechanical exciton transition in InGaAs/GaAs quantum well structures is found to be ∼35 K.

Electron–interface-phonon interaction and scattering in asymmetric semiconductor quantum-well structures

The interaction Fr\ohlich-like Hamiltonian between an electron and interface optical phonons in asymmetric single and step quantum-well (QW) structures is obtained and studied. We observe that the behaviors of electron--interface-phonon coupling functions as functions of wave number k have two anomalies. The first is that, in an asymmetric single QW and general step QW, the interaction between an electron and the lowest frequency interface mode is large in the vicinity of the Brillouin-zone center. The second anomaly is that in step QW's the interaction between an electron and the fifth interface mode has a maximum which relates to the inflection point in the dispersion (${\mathrm{\ensuremath{\omega}}}_{5\mathrm{\ensuremath{-}}}$k) plot. The physical origins of these two anomalies have been analyzed. The electron--interface-phonon-scattering rates for intrasubband and intersubband transitions in asymmetric single and step QW structures are also calculated and are given as functions of well width, step width, and step height. It is shown that the electron scattering depends strongly on the potential parameters, and the usual selection rules for these transitions break down in asymmetric heterostructures.