Quantum-well Samples Research Articles

Temperature Dependent Optical Properties of Ultrathin InAs Quantum Well

Temperature-dependent photoluminescence (TDPL) and time-resolved photoluminescence (TRPL) of ultrathin InAs quantum wells (QWs) in GaAs matrix have been investigated to understand the optical properties of carriers. Samples containing different thicknesses of InAs (0.5, 0.75, 1, 1.2, 1.4 monolayers) have been used for this study. The PL peak position of InAs with temperature does not follow the Varshni model at low temperatures. The activation energy (EA) of these QWs has been calculated from TDPL. As expected, the thinnest QW sample (0.5 monolayer) results in the smallest EA of 23 meV, whereas the thickest QW sample (1.4 monolayer) results in the highest EA of 79 meV. Carrier lifetime has been calculated from TRPL measurement for varying temperatures. At 10 K, the carrier lifetime increased almost linearly from 250 to 800 ps with the InAs QW thickness. Thicker InAs QW results in a longer carrier lifetime, which has been explained by the carrier escape model. Higher temperatures resulted in a decrease in carrier lifetime, which suggests carrier escape is dominating the temporal decay behavior.

The Influence Mechanism of Quantum Well Growth and Annealing Temperature on In Migration and Stress Modulation Behavior.

This study explores the effects of growth temperature of InGaN/GaN quantum well (QW) layers on indium migration, structural quality, and luminescence properties. It is found that within a specific range, the growth temperature can control the efficiency of In incorporation into QWs and strain energy accumulated in the QW structure, modulating the luminescence efficiency. Temperature-dependent photoluminescence (TDPL) measurements revealed a more pronounced localized state effect in QW samples grown at higher temperatures. Moreover, a too high annealing temperature will enhance indium migration, leading to an increased density of non-radiative recombination centers and a more pronounced quantum-confined Stark effect (QCSE), thereby reducing luminescence intensity. These findings highlight the critical role of thermal management in optimizing the performance of InGaN/GaN MQWs in LEDs and other photoelectronic devices.

Effect of varying threading dislocation densities on the optical properties of InGaN/GaN quantum wells with intentionally created V-shaped pits

The effect of varying threading dislocation densities on the internal quantum efficiencies (IQEs) of InGaN quantum wells (QWs), with and without intentionally created “V-pits,” is reported here. InGaN QW samples grown on GaN-on-sapphire templates with threading dislocation densities of <1 × 108 and <1 × 109 cm−2 are compared, with and without GaN/InGaN superlattice (SL) layers incorporated to intentionally open up the threading dislocation cores and form large-size “V-pits.” The formation of “V-pits” is confirmed by cross-sectional transmission electron microscopy to initiate from threading dislocations in the SL layers. The densities of the pits are confirmed by plan-view SEM to agree with the substrate threading dislocation densities. The experimental room temperature IQEs of the “V-pit” QW samples are enhanced to 15% ± 1% compared to 6% ± 2% for conventional QW samples. Both conventional and “V-pit” samples show insensitivity to the magnitude of the dislocation densities with respect to IQE performance, while the “V-pit” samples show shifts in the peak emission wavelengths compared to the conventional samples, attributed to strain modulation. This study provides additional understanding of the causes of the observed insensitivity of the IQEs to different threading dislocation densities.

Excitons in (Al,Ga)N quantum dots and quantum wells grown on (0001)-oriented AlN templates: Emission diagrams and valence band mixings

The luminescence efficiency of AlxGa1−xN quantum dots (QDs) and quantum wells (QWs), buried in AlN cladding layers and emitting in the ultraviolet range between 234 and 310 nm, has been investigated. The growth and optical properties have been done using similar aluminum composition (varying from 0.4 to 0.75) for both QDs and QWs. In order to compare as much as possible the optical properties, the QWs were fabricated with a growth time tuned such that the QW width is similar to the average height of the QDs. The photoluminescence (PL) showed emission ranging from 4 to 5.4 eV, putting into evidence differences in terms of full width at half maximum, PL intensity, and asymmetry of the line shape between QDs and QWs. The results show shorter wavelengths and a slightly narrower PL linewidth for QWs. To determine the light emission dependence with the electric field direction in the crystal, the evolutions of the emission diagrams for all samples were recorded along two orthogonal directions, namely, the “in-plane” (growth) and the “on-side” directions, from which the light emission was collected. For the whole QDs and QWs samples' series, the shapes of the emission diagram indicate emission in both in-plane and on-side directions, as evidenced by intra-valence band mixings caused by strain effects combined with the anisotropic Coulomb interactions that are particularly contributing to the polarization at wavelengths below 260 nm. Furthermore, the degree of polarization, determined for each sample, showed good agreement with results from the literature.

Surface plasmon coupling effects on the behaviors of radiative and non-radiative recombination in an InGaN/GaN quantum well

Although surface plasmon (SP) coupling has been widely used for enhancing the emission efficiency of an InGaN/GaN quantum well (QW) structure, the interplay of the carrier transport behavior in the QW with SP coupling, which is a crucial mechanism controlling the SP-coupling induced QW emission enhancement, is still an issue not well explored yet. To understand the effects of SP coupling on the radiative and non-radiative recombination behaviors of carriers in a QW structure, the temperature-dependent time-resolved photoluminescence spectroscopies of two QW samples of different indium contents with surface Ag nanoparticles are studied. A two-single-exponential model is used for calibrating their radiative and non-radiative decay times. The SP coupling process, which transfers carrier energy from a QW into the SP resonance mode for effective radiation and increases the effective radiative recombination rate, produces energy-dependent carrier depletion and, hence, disturbs the quasi-equilibrium condition of carrier distribution. In this situation, a strong carrier transport process occurs targeting a new quasi-equilibrium condition that enhances non-radiative recombination and, hence, reduces the benefit of using the SP coupling technique. To alleviate this problem of SP-coupling induced energy loss, a weak energy-dependent or broad-spectrum SP coupling process is recommended.

Optical dielectric function and electronic band structure of dilute GaAsBi-GaAs multiquantum wells by using variable angle spectroscopic ellipsometry

ObjectivesThis letter reports about the electronic band structure of dilute GaAsBi-GaAs quantum wells (QWs) experimentally studied by variable angle spectroscopic ellipsometry (VASE) and theoretically by nextnanomat software by solving symmetric double quantum well. MethodologyThe GaAsBi-GaAs quantum wells (QWs) samples were grown by molecular beam epitaxy (MBE). X-Ray diffraction (XRD) measurements were performed to confirm the incorporation of Bismuth in GaAs. Raman measurements in all samples were performed to investigate the lattice vibration properties. Spectroscopic ellipsometry was used first time on bismide quantum wells to estimate the optical functions and critical point transition energies of the innovative GaAsBi quantum wells. A parameterized semiconductor model was used to demonstrate the dielectric function of alloys as a combination of Gaussian oscillators focused on critical points in the band structure. Then from this model, energy values of those critical points are extracted. ResultsThe bandgap was measured for QWs samples for a range of bismuth concentration (1 %, 2 % and 3 %) and bulk sample with Bismuth concentration 0 % and 5 %. PL measurements of bulk and quantum wells samples showed reduction in bandgap with the increase in Bismuth concentration. This reduction in bandgap for QW samples was also confirmed experimentally by VASE and theoretically by using nextnanomat software. ConclusionThis research work gives important electronic and optical parameters for the Bismide quantum wells to design optoelectronic devices using these innovative alloys.

Inhibited exciton spontaneous emission in InGaAs/GaAs quantum well by the phase-related scattering field of gold nanoparticles

We have obtained long lifetime exciton emission in an InGaAs/GaAs quantum well (QW) when the QW film is transferred onto a silicon substrate covered by Au nanoparticles. It is found that the exciton lifetime increases from 0.301 ± 0.003 ns for the as-grown QW sample to 88 ± 6 ns for a QW film with Au nanoparticles, i.e., the spontaneous decay rate is inhibited with a factor of 1/300. The experimental condition of observing long lifetime excitons is very sensitive to the separation distance z between the QW and Au nanoparticles because phase-related scattering field mainly occurs at kz ∼1, where k is the wave vector. A classical dipole oscillator dynamic equation driven by the scattering field of Au nanoparticles can be employed to elucidate the experimental results.

Role of Intra‐Band Relaxation of Holes and Tunneling of Electrons in Carrier Relaxation in AlGaAs/GaAs Quantum Well

The role of intra‐band hole relaxation and tunneling of electrons in the carrier relaxation in Al0.7Ga0.3As/GaAs quantum well (QW) is studied by performing pump‐probe transient reflectivity measurements with a femtosecond laser at room temperature. The ultrafast optical response of the QW sample is measured with an aim of understanding various ultrafast processes occurring on the time scale of ∼100 fs to 10 ps. Carriers are injected into the QW layer via a resonant photoexcitation of electron–hole pairs enabling a sharp rise of the signal. Carrier relaxation inside QW is dominated by non‐radiative processes like phonon‐mediated intra‐band relaxation of holes and tunneling of electrons out of the QW plane and their subsequent capture by the midgap states in the AlGaAs barrier. Subsequent to the initial thermalization within 100 fs, carrier relaxation is mainly driven by the tunneling of electrons and trapping by the midgap states over a time scale of 3–8 ps. In between, photoexcited holes are seen to relax from light to heavy hole states of QW within 1–2 ps, leading to a delayed rise of transient reflectivity signal. Systematic efforts are made to isolate various contributions which showed phonon re‐absorption leading to saturation effect in intraband relaxation.

A quantitative analysis of electronic transport in n- and p-type modulation-doped GaAsBi/AlGaAs quantum well structures

Electronic transport properties of as-grown and thermally annealed n- and p-type modulation-doped GaAsBi/AlGaAs quantum well (QW) structures were investigated. Hall mobility of as-grown, n- and p-type modulation doped QW structures are found from raw experimental data as ∼1414 and 95 cm2 Vs−1 at room temperature. A comparison between reported two-dimensional (2D) electron density determined from the analyses of Shubnikov de Haas oscillations and the 2D Hall electron density indicates a presence of parallel conduction in barrier layer (AlGaAs) and QW layer (GaAsBi) in n-type samples, therefore a parallel channel conduction theory is used to separate the electron mobility in the QW and the barrier layers in n-type modulation doped GaAsBi/AlGaAs QW structure. The extracted electron mobility of the as-grown n-type GaAsBi/AlGaAs QW sample is determined as ∼5975 cm2 Vs−1 at 4.2 K, which is closer to the electron mobility in GaAs. It is found that thermal annealing at lower temperature than growth temperature increases electron mobility of 2D electron gas, while annealing at higher temperature than growth temperature decreases electron mobility. The temperature dependence of the extracted electron mobility using parallel conduction approximation is analytically calculated by considering possible scattering mechanisms. Analysis of temperature-dependent electron mobility shows that the dominant scattering mechanisms are interface roughness (IFR), acoustic, and alloy-potential scatterings at low and intermediate temperature range, and IFR and optical phonon scattering at the high-temperature range in n-type modulation doped GaAsBi/AlGaAs QW structures.

The strain, energy band and photoluminescence of GaAs0.92Sb0·08/Al0.3Ga0.7As multiple quantum wells grown on GaAs substrate

GaAsSb based materials have become the promising system for infrared semiconductor lasers and detectors. In this article, the strain, energy band structures and photoluminescence (PL) of GaAs0.92Sb0·08/Al0.3Ga0.7As strained quantum wells (QWs) grown with molecular beam epitaxy are systematically analyzed both theoretically and experimentally. The theoretical results are derived by Kane's model and k⋅p method, and the optical properties of a high-quality GaAs0.92Sb0·08/Al0.3Ga0.7As strained QWs sample are thoroughly investigated by excitation- and temperature-dependent PL measurements. The theoretical results show the strain has significant influence on the band structure of QWs. In experimental part, it is found that the light-hole exciton emission coexists with the heavy-hole exciton line in the temperature range of 50 K–300 K. However, the emission of localized excitons, which is caused by the nonuniformity of component in the GaAsSb well layer, takes over the light-hole exciton emission at lower temperatures (<50 K).

Electronic transport in n-type modulation-doped AlGaAs/GaAsBi quantum well structures: influence of Bi and thermal annealing on electron effective mass and electron mobility

We investigate electronic transport properties of as-grown and annealed n-type modulation-doped Al0.15Ga0.85As/GaAs1−xBix (x = 0 and 0.04) quantum well (QW) structures using magnetotransport measurements in the temperature range 4.2 K and 60 K and at magnetic fields up to 18 T. Thermal annealing process was applied at two different temperatures, 700 °C and 350 °C during 60 s and 180 s, respectively. We find that electron effective mass and 2D electron density in as-grown Bi-containing sample are slightly lower than that in Bi-free one. Furthermore, quantum electron mobility and quantum scattering time are observed to be decreased in Bi-containing samples. The annealing process at 700 °C causes a slight increase in electron effective mass and 2D electron density. A negligible decrease in electron effective mass and an increase in 2D electron density are determined following annealing at 350 °C. The observed change in electron effective mass following thermal annealing process is attributed to changing 2D electron density in the samples. No improvement on quantum electron mobility and quantum scattering time are observed following thermal annealing at both process temperatures. We determine that one electron subband (e1) for as-grown and annealed (at 700 °C for 60 s) Bi-containing QWs and two electron subbands (e1 and e2) for the annealed (at 350 °C for 180 s) GaAsBi QW sample and the Bi-free QW sample contribute to electronic transport. Our results reveal that there is no significant direct effect of Bi on effective electron mass, but an indirect effect, in which Bi can provoke changes in 2D electron density and hence causes not to observe actual band-edge electron mass but a deviation from its band-edge value. Therefore, it can be concluded that dispersion curve of conduction band does not change as an effect of Bi incorporation in GaAs.

Characterization of III-V dilute nitride based multi-quantum well p-i-n diodes for next generation opto-electrical conversion devices

This work investigates the temperature dependent photoluminescence (PL) and current-voltage (I-V) characterization of a GaInNAs/GaAs p-i-n diode with multiple quantum wells (MQWs). Two different diode samples based on 10- and 20-undoped quantum wells (QWs) were considered for this work. The QWs of Ga0.952In0.048N0.016As0.984 p-i-n diode were 10 nm thick and separated by 10 nm GaAs barriers. They were grown above GaAs substrates using a molecular beam epitaxy (MBE) system. The temperature dependence of PL measurement showed good agreement with the effective bandgap of GaAs and GaInNAs QWs materials. The 10 QWs and 20 QWs samples show PL peaks at λ = 1076 nm and λ = 1086 nm with 300 K temperature, respectively, compared to calculated value of λ = 1087 nm using band-anti crossing (BAC) model. Furthermore, peak PL energy (eV) as a function of temperature, T (K) indicated blue and red shifts as the temperature dropped from 300 K to 100 K and 100 K to 50 K, respectively. With further decrease in temperature below 50 K, pronounced S-shaped pattern is exhibited as reported for dilute nitride material systems. Meanwhile, I-V characteristics of the p-i-n diodes show that the dark current density at -6.0 V reversed bias is relatively low with 472.8 nA/cm2 and -22.6 nA/cm2 for 10 and 20 QWs devices, respectively. Under illumination at peak wavelength, the 20 QWs produce twice to that of 10 QWs in current density. Thus, a GaInNAs MQWs diode is suitable for photodetector operating near 1.0 µm wavelength and solar cells with high efficiency photovoltaics

Study of direct bandgap type-I GeSn/GeSn double quantum well with improved carrier confinement

The GeSn-based quantum wells (QWs) have been investigated recently for the development of efficient GeSn emitters. Although our previous study indicated that the direct bandgap well with type-I band alignment was achieved, the demonstrated QW still has insufficient carrier confinement. In this work, we report the systematic study of light emission from the Ge0.91Sn0.09/Ge0.85Sn0.15/Ge0.91Sn0.09 double QW structure. Two double QW samples, with the thicknesses of Ge0.85Sn0.15 well of 6 and 19 nm, were investigated. Band structure calculations revealed that both samples feature type-I band alignment. Compared with our previous study, by increasing the Sn composition in GeSn barrier and well, the QW layer featured increased energy separation between the indirect and direct bandgaps towards a better direct gap semiconductor. Moreover, the thicker well sample exhibited improved carrier confinement compared to the thinner well sample due to lowered first quantized energy level in the Γ valley. To identify the optical transition characteristics, photoluminescence (PL) study using three pump lasers with different penetration depths and photon energies was performed. The PL spectra confirmed the direct bandgap well feature and the improved carrier confinement, as significantly enhanced QW emission from the thicker well sample was observed.

Transmission electron microscopy of Ga(Sb, Bi)/GaSb quantum wells with varying Bi content and quantum well thickness

We analyze the microstructure of a series of Ga(Sb, Bi)/GaSb quantum wells (QW) with varying Bi content and QW thicknesses using transmission electron microscopy. The structures are grown on GaSb(001) substrates by low-temperature molecular beam epitaxy. Although Ga(Sb, Bi) is regarded as a highly-mismatched alloy, the material is of remarkable quality and no extended defects or nanoclusters are detected in the pseudomorphic layers. Regardless their different Bi content (ranging from 4% to 11%) all QW samples seem to be homogeneous in composition and no composition fluctuations are detected. The QW/barriers are nevertheless affected by thickness fluctuations, where the thickness slightly varies from QW to QW but also laterally. Despite the samples exhibit well-defined QW/barrier interfaces, we note that the Ga(Sb, Bi)-on-GaSb interface is rougher than the GaSb-on-Ga(Sb, Bi) one. The morphological smoothing effect at the GaSb-on-Ga(Sb, Bi) interface is attributed to the well-known surfactant behavior of Bi in connection with Bi surface segregation, as evidenced from experimental Bi distribution profiles.

Optical properties of n- and p-type modulation doped GaAsBi/AlGaAs quantum well structures

In this work, optical properties of n- and p-type modulation doped GaAsBi/AlGaAs single quantum well (QW) heterostructures are investigated via temperature- and excitation power-dependent photoluminescence (PL) and integrated photoluminescence (IPL) studies and the results are compared to the n- and p-type GaAs/AlGaAs QW structures to determine the influence of Bi and doping type on the optical properties. The results of the temperature dependent PL peak energy reveal that, as an effect of doping type, the temperature dependence of the PL peak energy exhibit different characteristic for n- and p-type samples. The temperature dependence of the PL peak energy reveals S-shaped trend for the n-type GaAsBi/AlGaAs QW sample. On the other hand, the characteristic follows Varshni law for the p-type GaAsBi/AlGaAs QW sample. The observed S-shaped behaviour for the n-type sample is explained by considering the contribution of the Bi-induced states above valence band (VB) to the PL. As for p-type sample, the localised states-related contribution to the PL signal is drastically diminished, resulting in an almost S-shape free temperature dependence of the optical transition energy, which can be explained by the compensation of acceptor-like states. The observed PL spectra of n- and p-type samples successfully reconstructed by two Gaussian peaks, which are assigned to optical transitions due to recombination of free and localised excitons. The localised exciton-related peak is observed to be very weak in p-type sample compared to that in n-type one. The allowed transitions in GaAsBi QW is calculated by self-consistently solving the Schrödinger-Poisson equation to identify the origin of the observed transitions. A comparison of the PL results of the Bi-containing samples with the results of the Bi-free ones is exhibited approximately 80 meV/Bi% decrease in the fundamental optical transition. Using the excitation-dependent IPL measurements, it is found that at low temperatures and under low excitations, recombination process is under the effect of Shockley-Read-Hall (SRH) non-radiative process. When the excitation power density increases, radiative recombination becomes dominant. At higher-temperatures and in the high-intensity regime, we observed Auger effect in recombination process for n- and p-type GaAsBi samples, but the effect of Auger loss is observed to be much less in the p-type GaAsBi sample due to enhanced spin orbit split-off energy as a result of incorporation of bismuth. Furthermore, when we compare the result for Bi-free and Bi-containing p-type samples, again, Auger recombination is found to be less effective for the Bi-containing sample.

Light-emitting-diode Lambertian light sources as low-radiant-flux standards applicable to quantitative luminescence-intensity imaging.

Planar-type Lambertian light-emitting diodes (LEDs) with a circular aperture of several tens of μm to a few mm in diameter were developed for use as radiant-flux standard light sources, which have been in strong demand for applications such as quantitative or absolute intensity measurements of weak luminescence from solid-state materials and devices. Via pulse-width modulation, time-averaged emission intensity of the LED devices was controlled linearly to cover a wide dynamic range of about nine orders of magnitude, from 10 μW down to 10 fW. The developed planar LED devices were applied as the radiant-flux standards to quantitative measurements and analyses of photoluminescence (PL) intensity and PL quantum efficiency of a GaAs quantum-well sample. The results demonstrated the utility and applicability of the LED standards in quantitative luminescence-intensity measurements in Lambertian-type low radiant-flux level sources.

Estimation of electron spin polarization from circularly polarized photoluminescence in strained quantum wells

A general method based on the valence band mixing is proposed to establish a correlation between electron spin polarization and the degree of circular polarization of luminescence in a strained quantum well (QW). Key issues that are critical for establishing such a correlation are highlighted by invoking 4 band k.p calculations in a strained QW system. The proposed method takes care of the complex valence band mixing in strained QWs by simultaneously analyzing the heavy hole (e-hh) and light hole (e-lh) related luminescence transitions along the quantization direction. It is shown that the ratio of the degree of circular polarizations measured for the e-hh and e-lh features is a sensitive probe to estimate the values of band mixing. The method is reasonably supported by carefully conducted experiments on a tensile strained GaAsP/AlGaAs QW sample in the temperature range of 10 to 150 K. The proposed method shall be helpful for understanding the spin injection/transfer across the interfaces of QW, especially for the cases where heavy hole-light hole coupling is expected to be strong, and both the features are experimentally observed. It might lead to new directions in the field of semiconductor spin-optoelectronics in QWs.

The effect of an InP cap layer on the photoluminescence of an InxGa1–xAs1–yPy/InzAl1– zAs quantum well heterostructure

The effect of an InP cap on the photoluminescence (PL) spectrum of an InGaAsP/InAlAs quantum well (QW) is investigated using excitation power and temperature dependent PL. An as-grown sample with the InP cap layer shows an inverted interface created between InP and InAlAs that has a transition energy very close to the transition energy of the QW; consequently, there is an overlap between them. On the other hand, the QW sample with the cap layer etched away does not have a feature due to the inverted interface; even at very low power, the only observed feature is due to the QW transition.

Emission characteristics of high-gain GaN-based Vertical-Cavity Surface-Emitting Lasers

GaN-based vertical-cavity surface-emitting lasers (VCSELs) with high optical gain and short cavity lifetime are favorable for the generation of ultra-short pulses in the blue and green regions. In our previous works, 6 and 2 picosecond short-pulses have been generated from gain-switched InGaN VCSELs with 3- and 10-period InGaN/GaN quantum wells (QWs) in the active layers by using an up-conversion measurement system. To further increase the gain of the VCSEL for the generation of even shorter pulses, 20-period InGaN/GaN QWs samples were fabricated. The emission characteristics of these high-gain VCSELs were investigated and analyzed under the optical pumping at room temperature.

Spectral dynamical behavior in two-section, quantum well, mode-locked laser at 1.064 μm**Project supported by the National Basic Research Program of China (Grant Nos.~2013CB933304 and 2012CB932701), the National Natural Science Foundation of China (Grant Nos. 61274125 and 61435012), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB01010200).

In this study, two-section mode-locked semiconductor lasers with different numbers of quantum wells and different types of waveguide structures are made. Their ultrashort pulse features are presented. The spectral dynamical behaviors in these lasers are studied in detail. In the simulation part, a two-band compressive-strained quantum well (QW) model is used to study thermally induced band-edge detuning in the amplifier and saturable absorber (SA). A sudden blue shift in laser spectrum is expected by calculating the peak of the net gain. In the experiment part, the sudden blue shift in the emission spectrum is observed in triple QW samples under certain operating conditions but remains absent in single QW samples. Experimental results reveal that blue shift phenomenon is connected with the difference between currents in two sections. Additionally, a threshold current ratio for blue-shift is also demonstrated.