Single Quantum Wells Research Articles

Luminescence of HgCdTe– and InAsSb–based quantum–well heterostructures

Photoluminescence of single quantum-well (SQW) HgCdTe-based structures and electroluminescence of multi quantum-well (MQW) InAsSb-based structures were studied. SQW HgCdTe-based heterostructures were grown by molecular beam epitaxy, while MQW-based InAsSb heterostructures were fabricated by metal-organic chemical-vapour deposition. The specifics of the electronic structure of the materials used is considered in relation to the possibility of observation of optical transitions between the quantization levels of electrons and first and/or second heavy and light hole levels.

Time of carrier escape and recombination coefficients in InGaN quantum-well active regions of blue, cyan, and green light-emitting diodes

Addressing the recently observed unexpected low-current saturation of differential carrier lifetime (DLT) in the active region of InGaN-based light-emitting diodes (LEDs), a novel procedure is suggested for the evaluation of both escape time of carriers from the InGaN quantum wells and their recombination coefficients. The procedure uses coupled measurements of external quantum efficiency and DLT as a function of LED continuous-wave operating current. The procedure has been applied to three LEDs with InGaN multiple quantum well (QW) active regions emitting blue, cyan, and green light. Laser diodes operating at different wavelengths were employed for small-signal resonant pumping of the LED active regions in order to understand a possible dependence of the escape time and recombination coefficients on the excitation wavelength. The revealed wavelength dependence of the Shockley-Read-Hall recombination coefficient in the green LED is attributed to lateral non-uniformity of the InGaN QW with high Indium content. The radiative and Auger recombination coefficients evaluated by the above procedures in all three LEDs turned out to be close to those reported earlier for the LEDs with single QW active regions. The obtained recombination coefficients are used for further performance comparison of the studied devices. The suggested procedure is helpful for the investigation of carrier escape and the optimization of III-nitride devices, such as photo-detectors, solar cells, and quantum-cascade lasers.

High-mobility indirect excitons in wide single quantum well

Indirect excitons (IXs) are bound pairs of electrons and holes confined in spatially separated layers. We present wide single quantum well (WSQW) heterostructures with high IX mobility, spectrally narrow IX emission, voltage-controllable IX energy, and long and voltage-controllable IX lifetime. This set of properties shows that WSQW heterostructures provide an advanced platform both for studying basic properties of IXs in low-disorder environments and for the development of high-mobility excitonic devices.

752-MHz Modulation Bandwidth of High-Speed Blue Micro Light-Emitting Diodes

In this paper, the optical frequency response of gallium nitride-based blue micro-LEDs with different quantum well numbers in the active region is presented. Single quantum well (SQW) blue micro-LEDs have about 1.6 times higher modulation bandwidth ( $f_{-3\,\text {dB}}$ ) by comparison with triple quantum well blue micro-LEDs. The larger bandwidth enhancement of the SQW micro-LEDs can be attributed to more carriers stored in a quantum well leading to higher recombination rate and lower recombination lifetime. The high $f_{-3\,\text {dB}}$ up to 752 MHz is demonstrated in the SQW micro-LED with a light-emitting area of $10\,\,\mu \text {m} \times 10\,\,\mu \text{m}$ . It suggests that micro-LEDs designed with fewer quantum well numbers may have the potential for the application of visible light communication (VLC) technology.

Modeling of tunneling current density of GeC based double barrier multiple quantum well resonant tunneling diode

The double barrier quantum well (DBQW) resonant tunneling diode (RTD) structure made of SiGeSn/GeC/SiGeSn alloys grown on Ge substrate is analyzed. The tensile strained Ge1−zCz on Si1−x−yGexSny heterostructure provides a direct band gap type I configuration. The transmission coefficient and tunneling current density have been calculated considering single and multiple quantum wells. A comparative study of tunnelling current of the proposed structure is done with the existing RTD structure based on GeSn/SiGeSn DBH. A higher value of the current density for the proposed structure has been obtained.

Polarization-Sensitive Fourier-Transform Spectroscopy of HgTe/CdHgTe Quantum Wells in the Far Infrared Range in a Magnetic Field

Spectra of magnetoabsorption and Faraday rotation in HgTe/CdHgTe heterostructures with single and double quantum wells in high magnetic fields up to 11 T have been studied by the Fourier-transform spectroscopy method. The study of Faraday rotation spectra makes it possible to determine the sign of resonance circular polarization of transitions between Landau levels of carriers, which allows identifying observed intraband and interband transitions in the far and middle infrared ranges.

Exciton localization and structural disorder of GaAs1−xBix/GaAs quantum wells grown by molecular beam epitaxy on (311)B GaAs substrates

In this work, we have investigated the structural and optical properties of GaAs(1−x)Bix/GaAs single quantum wells (QWs) grown by molecular beam epitaxy on GaAs (311)B substrates using x-ray diffraction, atomic force microscopy, Fourier-transform Raman (FT-Raman) and photoluminescence spectroscopy techniques. The FT-Raman results revealed a decrease of the relative intensity ratio of transverse and longitudinal optical modes with the increase of Bi concentration, which indicates a reduction of the structural disorder with increasing Bi incorporation. In addition, the PL results show an enhancement of the optical efficiency of the structures as the Bi concentration is increased due to important effects of exciton localization related to Bi defects, nonradiative centers and alloy disorder. These results provide evidence that Bi is incorporated effectively into the QW region. Finally, the temperature dependence of the PL spectra has evidenced two distinct types of defects related to the Bi incorporation, namely Bi clusters and pairs, and alloy disorder and potential fluctuation.

Optimization of optical characteristics of In0.29Ga0.71As0.99N0.01/GaAs straddled nano-heterostructure

Optical gain and Modal gain with Lasing wavelength for In 0.29 Ga 0.71 As 0.99 N 0.01 /GaAs Straddled Heterostructures. Designing of a nanoscale Quantum Well (QW) heterostructure with a well thickness of ∼60 Å is critical for many applications and remains a challenge. This paper has a detailed study directed towards designing of In 0.29 Ga 0.71 As 0.99 N 0.01 /GaAs straddled nanoscale-heterostructure having a single QW of thickness ∼60 Å and optimization of optical and lasing characteristics such as optical and mode gain, differential gain, gain compression, anti-guiding factor, transparency wavelength, relaxation oscillation frequency (ROF), optical power and their mutual variation behavior. The outcomes of the simulation study imply that for the carrier concentration of ∼2 × 10 18 cm −3 the optical gain of the nano-heterostructure is of 2100 cm −1 at the wavelength is of 1.30 μm. Though the obtained gain is almost half of the gain of InGaAlAs/InP heterostructure, but from the wavelength point of view the InGaAsN/GaAs nano-heterostructure is also more desirable because the 1.30 μm wavelength is attractive due to negligible dispersion in the silica based optical fiber. Hence, the InGaAsN/GaAs nano-heterostructure can be very valuable in optical fiber based communication systems.

Useful studies on the optical properties of annealed and non annealed InGaN/GaN single QW LEDs, with different dopings in the barrier

In this paper we have studied comprehensively annealed and non annealed single quantum well (QW) light emitting diode (LED) structures with different doping concentrations in the barriers. Computations have been carried out through the self-consistent solutions of Schrödinger and Poison equations. Wide changes in the transition energies (TE) and the transition probabilities (TP) are observed due to the doping variation. On annealing, with the increase of diffusion length, the results show that depending on the doping concentration, the TE increases monotonically as well as it goes through red-blue shifts. It is also observed that, for lower doping, TP increases and/or decreases with the diffusion length. The variation of TP also depends on the operating current. After a critical doping in the barriers, on annealing, the optical performances of QW LEDs deteriorate. Through change in doping and annealing, the optical performance of the InGaN/GaN QW LEDs may be improved. This information should be of importance to the optoelectronic designer.

Trade-off between bandwidth and efficiency in semipolar (202¯1¯) InGaN/GaN single- and multiple-quantum-well light-emitting diodes

InGaN/GaN light-emitting diodes (LEDs) with large modulation bandwidths are desirable for visible-light communication. Along with modulation speed, the consideration of the internal quantum efficiency (IQE) under operating conditions is also important. Here, we report the modulation characteristics of semipolar (202¯1¯) InGaN/GaN (LEDs) with single-quantum well (SQW) and multiple-quantum-well (MQW) active regions grown on free-standing semipolar GaN substrates with peak internal quantum efficiencies (IQEs) of 0.93 and 0.73, respectively. The MQW LEDs exhibit on average about 40–80% higher modulation bandwidth, reaching 1.5 GHz at 13 kA/cm2, but about 27% lower peak IQE than the SQW LEDs. We extract the differential carrier lifetimes (DLTs), RC parasitics, and carrier escape lifetimes and discuss their role in the bandwidth and IQE characteristics. A coulomb-enhanced capture process is shown to rapidly reduce the DLT of the MQW LED at high current densities. Auger recombination is also shown to play little role in increasing the speed of the LEDs. Finally, we investigate the trade-offs between the bandwidth and efficiency and introduce the bandwidth-IQE product as a potential figure of merit for optimizing speed and efficiency in InGaN/GaN LEDs.

Stacking of ZnSe/ZnCdSe Multi-Quantum Wells on GaAs (100) by Epitaxial Lift-Off

Here we present stacking of GaAs/ZnSe/ZnCdSe single-quantum well (QW) structures using epitaxial lift-off (ELO). Molecular beam epitaxy (MBE)-grown II–VI QW structure was lifted using our standard ELO technique. The QW structures were transferred onto glass plates and then subsequent layers stacked on top of each other to form a triple-QW structure. This was compared to an MBE-grown multiple-QW (MQW) structure of similar design. Low-temperature (77 K) photoluminescence (PL) spectroscopy was used to compare the two structures and showed no obvious degradation of the ELO stacked layer. It was observed that by stacking the single QW layer on itself we could increase the PL emission intensity beyond that of the grown MQW structure while maintaining narrow line width.

Calculation of Energy States of Excitons in Square Quantum Wells

The ground and excited energy states of excitons in single square GaAs-based quantum wells are found by the numerical solution of the three-dimensional Schrodinger equation. This equation is obtained within the envelope-function formalism from the exciton energy operator using the spherical approximation of the Luttinger Hamiltonian. Precise results for the exciton states are achieved by the finite-difference method. The radiative decay rates of the calculated states are also determined.

Large-scale atomistic simulations demonstrate dominant alloy disorder effects in GaBixAs1−x/GaAs multiple quantum wells

Bismide semiconductor materials and heterostructures are considered a promising candidate for the design and implementation of photonic, thermoelectric, photovoltaic, and spintronic devices. This work presents a detailed theoretical study of the electronic and optical properties of strongly-coupled GaBi$_x$As$_{1-x}$/GaAs multiple quantum well (MQW) structures. Based on a systematic set of large-scale atomistic tight-binding calculations, our results reveal that the impact of atomic-scale fluctuations in alloy composition is stronger than the inter-well coupling effect, and plays an important role in the electronic and optical properties of MQW structures. Independent of QW geometry parameters, alloy disorder leads to a strong confinement of charge carriers, a large broadening of the hole energies, and a red shift in the ground-state transition wavelength. Polarisation-resolved optical transition strengths exhibit a striking effect of disorder, where the inhomogeneous broadening could exceed an order of magnitude for MQWs, in comparison to a factor of about three for single quantum wells. The strong influence of alloy disorder effects persists when small variations in the size and composition of MQWs typically expected in a realistic experimental environment are considered. The presented results highlight the limited scope of continuum methods and emphasise on the need for large-scale atomistic approaches to design devices with tailored functionalities based on the novel properties of bismide materials.

Enhancement of Photoconductivity by Carrier Screening Effect in n-GaSb/InAs/p-GaSb Heterostructure with Single Deep Quantum Well

n-GaSb/n-InAs/p-GaSb heterostructure with a single InAs QW was grown for the first time by MOVPE. Photocurrent spectra were obtained at reverse bias in the range from 0 to 0.8 V. It was shown that the photocurrent increases nonlinearly. The maximum of differential photoconductivity is archived at low applied voltage up to 0.2 V. This effect was explained by electrostatic screening of electrons localized in QW.

Lattice-matched double dip-shaped BAlGaN/AlN quantum well structures for ultraviolet light emission devices

Ultraviolet light emission characteristics of lattice-matched BxAlyGa1−x−y N/AlN quantum well (QW) structures with double AlGaN delta layers were investigated theoretically. In contrast to conventional single dip-shaped QW structure where the reduction effect of the spatial separation between electron and hole wave functions is negligible, proposed double dip-shaped QW shows significant enhancement of the ultraviolet light emission intensity from a BAlGaN/AlN QW structure due to the reduced spatial separation between electron and hole wave functions. The emission peak of the double dip-shaped QW structure is expected to be about three times larger than that of the conventional rectangular AlGaN/AlN QW structure.

Investigation of regime switching from mode locking to Q-switching in a 2 µm InGaSb/AlGaAsSb quantum well laser.

A two-section InGaSb/AlGaAsSb single quantum well (SQW) laser emitting at 2 μm is presented. By varying the absorber bias voltage with a fixed gain current at 130 mA, passive mode locking at ~18.40 GHz, Q-switched mode locking, and passive Q-switching are observed in this laser. In the Q-switched mode locking regimes, the Q-switched RF signal and mode locked RF signal coexist, and the Q-switched lasing and mode-locked lasing happen at different wavelengths. This is the first observation of these three pulsed working regimes in a GaSb-based diode laser. An analysis of the regime switching mechanism is given based on the interplay between the gain saturation and the saturable absorption.

Characteristics of OMVPE grown GaAsBi QW lasers and impact of post-growth thermal annealing

Laser diodes employing a strain-compensated GaAs1−xBix/GaAs1−yPy single quantum well (SQW) active region were grown by organometallic vapor phase epitaxy (OMVPE). High resolution x-ray diffraction, room temperature photoluminescence, and real-time optical reflectance measurements during the OMVPE growth were used to find the optimum process window for the growth of the active region material. Systematic post-growth in situ thermal anneals of various lengths were carried out in order to investigate the impacts of thermal annealing on the laser device performance characteristics. While the lowest threshold current density was achieved after the thermal annealing for 30 min at 630 °C, a gradual decrease in the external differential quantum efficiency was observed as the annealing time increases. It was observed that the temperature sensitivities of the threshold current density increase while those of lasing wavelength and slope efficiency remain nearly constant with increasing annealing time. Z-contrast scanning transmission electron microscopic) analysis revealed inhomogeneous Bi distribution within the QW active region.

The charge confinement effect of quantum-well Alq[formula omitted]-based OLEDs by dual-pulsed transient electroluminescence

In this paper, the dual-pulsed transient electroluminescence (EL) characteristics in organic light-emitting diodes (OLEDs) with NPB/Alq3 multiple quantum-well (MQW) structures are investigated in detail. The threshold duty-ratio ( δth) is found in the NPB/Alq3 MQW devices, whereas the bilayer NPB/Alq3 device does not appear. Then the δth in single and double wells are 0.68 and 0.35, respectively. In the single QW device, the second pulsed intensity will be greater than the first pulsed strength after reaching the δth, while the double QW device is the opposite. This is due to the charge diffusion effect in the Alq3 well layer near the cathode of MQW devices. As the well numbers increase, it is different from the fall time remaining almost constant, the transient delay time will increase linearly, which is totally caused by the recombination position of excitons.

On the optical polarization properties of semipolar (202¯1) and (202¯1¯) InGaN/GaN quantum wells

In the framework of k·p-theory, semipolar (202¯1) and (202¯1¯) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, (202¯1) QWs consistently exhibit a lower degree of linear polarization (DLP) than (202¯1¯) QWs. To understand this peculiarity, we investigate the optical properties of (202¯1) and (202¯1¯) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k·p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with calculations. We observe a strong correlation between DLPs and spectral widths, which indicates that inhomogeneous broadening affects the optical polarization properties. Considering indium content fluctuations, QW thickness fluctuations, and the localization length of charge carriers, we argue that different broadenings apply to each subband and introduce a formalism using effective masses to account for inhomogeneous broadening in the calculation of the DLP. We conclude that the different DLP of (202¯1) and (202¯1¯) QWs might be related to different effective broadenings of their valence subbands induced by the rougher upper QW interface in (202¯1), by the larger sensitivity of holes to this upper interface due to the polarization field in (202¯1), and/or by the different degrees of localization of holes.

Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: A first-principles perspective to recent synthesis

Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed.