Quantum-well Heterostructures Research Articles

Strain modulation effect of superlattice interlayer on InGaN/GaN multiple quantum well

The strong piezoelectric field in InGaN/GaN heterostructure quantum wells severely reduces the light emission efficiency of multiple quantum well (MQW) structures. To address this issue, a strain modulation interlayer is commonly used to mitigate the piezoelectric polarization field and improve the luminescence performance of the devices. To investigate the influence and mechanism of strain modulation in the InGaN/GaN superlattice (SL), epitaxial wafers with an n-type InGaN/GaN SL interlayer sample, and their corresponding control samples are prepared. The measured temperature-dependent photoluminescence (PL) spectra of the epitaxial wafers, show that the introduction of an SL interlayer leads to a shorter-wavelength emission and enhancement of internal quantum efficiency. As the temperature increases, a blue shift of the PL peak is observed. However, for the sample with an SL interlayer, the blue shift of the PL peak with temperature increasing is relatively small. Electroluminescence (EL) experiments indicate that the introduction of an SL interlayer significantly increases the integrated intensity of the EL peak and reduces its full width at half maximum. These phenomena collectively indicate that the incorporation of a superlattice interlayer can partly suppress the quantum-confined Stark effect (QCSE) that affects the light emission efficiency. Theoretical calculations show that the introduction of a superlattice strain layer before growing an active multiple quantum well can weaken the polarization-induced built-in electric field in the active quantum well, reduce the tilt of the energy band in the multiple quantum well active region, increase the overlap of electron and hole wave functions, enhance the emission probability, shorten the radiative recombination lifetime, and promote competition between radiative recombination and non-radiative recombination, thereby achieving higher recombination efficiency and improving light emission intensity. This study provides experimental and theoretical evidence that the strain modulation SL interlayer can effectively improve the device performance and offer guidance for optimizing the structural design of devices.

Sub-nanosecond free carrier recombination in an indirectly excited quantum-well heterostructure

Nanometer-thick quantum-well structures are quantum model systems offering a few discrete unoccupied energy states that can be impulsively filled and that relax back to equilibrium predominantly via spontaneous emission of light. Here we report on the response of an indirectly excited quantum-well heterostructure, probed by means of time and frequency resolved photoluminescence spectroscopy. This experiment provides access to the sub-nanosecond evolution of the free electron density, indirectly injected into the quantum wells. In particular, the modeling of the time-dependent photoluminescence spectra unveils the time evolution of the temperature and of the chemical potentials for electrons and holes, from which the sub-nanosecond time-dependent electron density is determined. This information allows to prove that the recombination of excited carriers is mainly radiative and bimolecular at early delays after excitation, while, as the carrier density decreases, a monomolecular and non-radiative recombination channel becomes relevant. Access to the sub-nanosecond chronology of the mechanisms responsible for the relaxation of charge carriers provides a wealth of information for designing novel luminescent devices with engineered spectral and temporal behavior.

Single-Exciton Photoluminescence in a GaN Monolayer inside an AlN Nanocolumn.

GaN/AlN heterostructures with thicknesses of one monolayer (ML) are currently considered to be the most promising material for creating UVC light-emitting devices. A unique functional property of these atomically thin quantum wells (QWs) is their ability to maintain stable excitons, resulting in a particularly high radiation yield at room temperature. However, the intrinsic properties of these excitons are substantially masked by the inhomogeneous broadening caused, in particular, by fluctuations in the QWs' thicknesses. In this work, to reduce this effect, we fabricated cylindrical nanocolumns of 50 to 5000 nm in diameter using GaN/AlN single QW heterostructures grown via molecular beam epitaxy while using photolithography with a combination of wet and reactive ion etching. Photoluminescence measurements in an ultrasmall QW region enclosed in a nanocolumn revealed that narrow lines of individual excitons were localized on potential fluctuations attributed to 2-3-monolayer-high GaN clusters, which appear in QWs with an average thickness of 1 ML. The kinetics of luminescence with increasing temperature is determined via the change in the population of localized exciton states. At low temperatures, spin-forbidden dark excitons with lifetimes of ~40 ns predominate, while at temperatures elevated above 120 K, the overlying bright exciton states with much faster recombination dynamics determine the emission.

Modelling of GaAsSb/InAs type-II QW heterostructure and simulation of its optical gain characteristics under (100), (001) and (110) directional pressure

A multi band (particularly, six band) k. p approach has been adopted for modelling the GaAsSb/InAs/AlSb W-shaped quantum well (QW)-heterostructure (a type-II with broken bandgap band alignment) and simulation of behaviour of optical gain characteristics under uni- and bi-axial pressures. The motivation behind designing such QW heterostructure is its high optical gain in MIR (mid infrared region) with noteworthy tuneable optical characteristics. The tuneable characteristics under uni-axial (100) and (001) pressure and bi-axial (110) pressure have been studied through the simplification of a multiband band k. p Hamiltonian. For all the directional pressures, the pressure range was fixed from 1 GPa to 3 GPa (with the interval of 0.5 GPa). For the (100) directional pressure, a significant increase in the gain (from 3570 to 7800/cm) was noted showing the red shifted wavelength with increasing order of pressure; while (001) directed pressure has caused to reduce the gain (3550–1158/cm) with red shifted wavelength. The (110) directed pressure has also caused to enhance the gain considerably (2550–7597/cm) with change in wavelength towards high wavelength. The study of gain characteristics was performed considering 1.5 × 1012 cm−2 2-D charge carrier density, which proves the tunability of the designed heterostructure in various tuneable optoelectronic devices.

Complex exciton dynamics with elevated temperature in a GaAsSb/GaAs quantum well heterostructure

Exciton dynamics in a GaAsSb/GaAs quantum well (QW) heterostructure were investigated via both steady state and transient photoluminescence. The measurements at 10 K demonstrated the coexistence of localized excitons (LEs) and free excitons (FEs), while a blue-shift resulting from increased excitation intensity indicated their spatially indirect transition (IT) characteristics due to the type-II band alignment. With increasing temperature from 10 K, the LEs and FEs redistribute, with the LEs becoming less intense at relatively higher temperature. With increasing temperature to above 80 K, electrons in GaAs are able to overcome the small band offset to enter inside GaAsSb and recombine with holes; thus, a spatially direct transition (DT) appeared. Hence, we are able to reveal complex carrier recombination dynamics for the GaAsSb/GaAs QW heterostructure, in which the “S” shape behavior is generated not only by the carrier localization but also by the transformation from IT to DT with elevated temperature.

Intraband carrier relaxation in mid-infrared (3–4 μm) HgCdTe based structures: Effect of the carrier heating on the operating temperatures of bulk and quantum-well lasers

In this work, we study stimulated emission (SE) at 3–4 μm wavelengths from optically pumped bulk HgCdTe and HgTe/CdHgTe quantum-well heterostructures. It is proposed that, under intense excitation of such structures, energy relaxation of hot electrons occurs mostly via electron–hole scattering, while the relaxation of hot holes is direct, phonon-mediated. By balancing carrier generation/recombination and heating/cooling processes, we outline heat-induced limits to the operating temperatures of mid-IR HgCdTe lasers. Based on the existing experimental results for the SE around 3.5 μm, we predict that lasing at this wavelength may be achieved in 2.5 μm-pumped optical converters at temperatures as high as Tmax ∼ 270 K.

Evaluation of Effective Mass in InGaAsN/GaAs Quantum Wells Using Transient Spectroscopy.

Transient spectroscopies are sensitive to charge carriers released from trapping centres in semiconducting devices. Even though these spectroscopies are mostly applied to reveal defects causing states that are localised in the energy gap, these methods also sense-charge from quantum wells in heterostructures. However, proper evaluation of material response to external stimuli requires knowledge of material properties such as electron effective mass in complex structures. Here we propose a method for precise evaluation of effective mass in quantum well heterostructures. The infinite well model is successfully applied to the InGaAsN/GaAs quantum well structure and used to evaluate electron effective mass in the conduction and valence bands. The effective mass m/m0 of charges from the conduction band was 0.093 ± 0.006, while the charges from the valence band exhibited an effective mass of 0.122 ± 0.018.

Enhanced quantum efficiency in flexible AlGaN deep-ultraviolet light-emitting diodes by external strain

Flexible electronic devices have great application potential in the field of next-generation consumer electronics. In this paper, we have demonstrated that applying external bending on AlGaN-based flexible deep-ultraviolet light-emitting diodes (DUV LEDs) can modulate the electrical characteristics of the quantum-well heterostructures. The internal quantum efficiency of DUV LEDs can be significantly improved by applying external strain on the device in bend-up mode. In addition, the peak emission of the DUV LEDs can be significantly tuned by bending the device into concave or convex curvatures. This desirable feature allows a single device to be applied in different environments and fields by applying external strain.

Density matrix theory of the charge and current wave formation at fast pulsed photoemission through a double quantum well

Within the framework of the density matrix method, general formulas are obtained that are convenient for describing fast pulsed photoemission that occurs in a time less than or on the order of the times of relaxation processes inside the photocathode. Expressions for the elements of the density matrix are found by solving the kinetic equation that takes into account the alternating electromagnetic field of light pumping and inelastic scattering of electrons. The derived formulas are applied for the numerical-analytical study of a one-dimensional model of wavelike spatiotemporal modulation of a photoelectron pulse of suitable duration during its passage through a double-well quantum-well heterostructure deposited on a volumetric planar photocathode. This modulation is a quantum beat that occurs as a result of excitation and subsequent slow oscillatory decay of the superposition of the doublet of quasistationary states of the heterostructure. It is possible to provide prolongation of generation and even amplification of waves of charge density and current density of photoelectrons when the photocathode is exposed to a periodic sequence of light pulses.

Yellow to green excitonic emission of nearly lattice-matched Zn1−zCdzSe/Zn1−xCdxSe/Zn1−yMgySe (z > x) quantum wells grown on GaAs(0 0 1)

We report the results obtained from the design, epitaxial elaboration, and characterization of nearly lattice matched ZnCdSe quantum wells (QWs) with ZnMgSe barriers, specifically designed with emission in selected wavelengths in the yellow-green spectral range. The quantum wells were deposited on ZnSe/GaAs(0 0 1); ZnSe and ZnMgSe materials were grown by molecular beam epitaxy and ZnCdSe by submonolayer pulsed beam epitaxy in a sequential layer-by-layer mode. The three ZnCdSe layers of the QWs consist of a central region with i monolayers, 3 ≤ i ≤ 7, of Zn1−zCdzSe and at each side are 8 monolayers of Zn1−xCdxSe with Cd contents such that x < z in order to obtain a staggered potential. The barriers are made of Zn1−yMgySe adjusting the composition for lattice matching to the lateral regions of Zn1−xCdxSe, then, only the central quantum well region is lattice mismatched. The low temperature photoluminescence spectra of the QW heterostructures with 3, 4, 6 and 7 MLs central regions of Zn1−zCdzSe (z ∼ 0.77 ± 0.02) surrounded at each side by 8 MLs thick Zn1−xCdxSe layers (x = 0.25) presented excitonic emission at 14 K around 2.337, 2.292, 2.197 and 2.176 eV, respectively, that are in very good agreement with the model calculations. The only modification of the heterostructures was the layer thickness of the central region, indicating the feasibility of these QWs with minimized strain for excitonic emission in the yellow-green spectral region.

Electron tunneling through double-electric barriers on HgTe/CdTe heterostructure interface

We investigate theoretically the carrier transport in a two-dimensional topological insulator of (001) HgTe/CdTe quantum-well heterostructure with inverted band, and find distinct switchable features of the transmission spectra in the topological edge states by designing the double-electric modulation potentials. The transmission spectra exhibit the significant Fabry–Pérot resonances for the double-electric transport system. Furthermore, the transmission properties show rich behaviors when the Fermi energy lies in the different locations in the energy spectrum and the double-electric barrier regions. The opacity and transparency of the double-modulated barrier regions can be controlled by tuning the modulated potentials, Fermi energy and the length of modulated regions. This electrical switching behavior can be realized by tuning the voltages applied on the metal gates. The Fabry–Pérot resonances leads to oscillations in the transmission which can be observed in experimentally. This electric modulated-mechanism provides us a realistic way to switch the transmission in edge states which can be constructed in low-power information processing devices.

Theoretical exploration of the optoelectronic properties of InAsNBi/InAs heterostructures for infrared applications: A multi-band k·p approach

We have investigated numerous electronic and optical aspects of the InAsNBi material, considering a multi-band k·p Hamiltonian. We have scrutinised electronic band dispersion, changes in spin–orbit splitting energy (so), charge carrier effective mass, band offset, and optical gain variations of InAsNBi and Quantum Well (QW) structures lattice-matched to InAs. The band gap of InAs0.9209N0.0291Bi0.05 is observed to be 131 meV under lattice-matched conditions, and the operating wavelength equivalent to this bandgap is 9.46 μm, which corresponds to the atmospheric Long Wavelength Infrared (LWIR) communication window of 8–12 μm. In order to tune the bandgap of a quantum confined heterostructure over a wide range, strain analysis plays a pivotal role. While compressive strain of 0.7% offers a band gap of 96 meV, tensile strain of 1.16% generates a band gap to 178 meV, which is almost equivalent to the room temperature bandgap of InSb. Under the influence of strain, spin-orbit splitting energy (Δso) spans the range from 0.31 eV to 0.45 eV. In addition to this, the inclusion of N and Bi into InAs reduces the effective mass of electrons to 0.01343 m0, which is ∼0.5 times of InAs. We have also computed the band structure of the InAsNBi/InAs QW heterostructure and found that under the influence of tensile strain with N = 2.5% and Bi = 1%, there is a changeover from Type-I to Type-II heterostructure. Indeed, InAsNBi can be a potential material for realizing various optoelectronic applications such as atmospheric LWIR communication, intermediate band solar cells, and night vision cameras.

Realizing crack-free high-aluminum-mole-fraction AlGaN on patterned GaN beyond the critical layer thickness

Wide-bandgap III-nitride heterostructures are required for a variety of device applications. However, this alloy system has a large lattice constant and thermal expansion coefficient mismatch that limits the alloy composition and layer thickness for many heteroepitaxial device structures. Consequently, various methods have been devised to allow the heteroepitaxial growth of AlInGaN heterostructures to accommodate this inherent strain. In this work, we describe a non-planar-growth approach that enables the deposition of crack-free high-Al-mole-fraction AlxGa1−xN on patterned GaN/sapphire templates and bulk GaN substrates with large-area mesas. We have studied the effects of the patterned mesa width, the mesa etch depth, and the gap between the mesas on the heteroepitaxy of AlxGa1−xN superlattices with an average Al molar fraction 0.11 &amp;lt; x¯ &amp;lt; 0.21 and non-planar overgrowth growth thicknesses up to 3.5 μm. Similar to the planar growth approach, increasing the thickness and Al mole fraction of the AlxGa1−xN superlattices leads to surface cracking when exceeding the critical layer thickness. However, limiting the mesa dimension in one direction enables strain mitigation and drastically increases the critical layer thickness. Additionally, larger etch depths of the mesas increase the Al alloy composition and thickness for crack-free AlGaN heteroepitaxy whereas the gap in between the mesas seems to have no crucial influence. We demonstrate that the Al alloy composition and layer thicknesses of such heterostructures can be increased far beyond the critical layer thickness for planar growth and demonstrate the growth of a crack-free full AlxGa1−xN/GaN quantum-well laser heterostructure designed for operation at ∼370 nm.

Electromechanical behaviors in piezotronic quantum wells based on a quantum-corrected phenomenological theory

Piezotronic devices have attracted a great deal of attention due to their potential applications in self-powered tactile sensing, nano-device memory, human-electronic interface, etc. As the size of piezotronic devices shrinks, some interesting quantum effects begin to appear. In this paper, we establish a theory oriented to the engineering application of piezoelectric semiconductors, called quantum-corrected phenomenological (QCP) theory, by coupling the density-gradient theory and the linear piezoelectricity theory through Gauss's law. For numerical verification, we specifically studied the electromechanical behaviors in GaN/AlGaN heterostructure quantum wells (QWs) with both infinite and finite barrier height. The results of electron density, electric potential, and quantum potential are provided, and their dependence on the doping density, the applied stress, and the Al mole fraction is investigated. Some interesting quantum effects are revealed, and their influencing mechanisms are well investigated from a macroscopic perspective. Not only do the conclusions drawn in this paper enrich the fundamental understanding of the piezotronic effect in a QW structure, but also the proposed QCP theory can serve as a valuable tool for future device engineering.

Room‐Temperature Yellow Emission of a High Cd Content (x = 0.70), Highly Strained, Layer‐by‐Layer Grown Zn1−xCdxSe/ZnSe Quantum Well

The results of the growth and characterization of an 8 monolayers (MLs) thick Zn1−xCdxSe/ZnSe quantum well (QW) with quite high Cd content (x = 0.70) are presented. At room temperature (RT), the QW presents yellow excitonic emission at 2.179 eV (569 nm, same color as the yellow line of a Kr ion laser). Despite the large Cd content, the RT photoluminescence spectrum shows a well‐defined, symmetric excitonic peak with relatively narrow full width at half maximum, indicating a good structural quality of the QW that is attributed to the sequential layer‐by‐layer growth mode achieved by the use of the submonolayer pulsed beam epitaxy (SPBE) technique. The ZnSe barriers are grown by molecular beam epitaxy (MBE); the QW heterostructure is deposited on top of a GaAs(001) substrate at 275 °C. Scanning transmission electron microscopy indicates a homogeneous smooth QW layer. The evolution of the excitonic emission energy with increasing temperature in the 19–300 K range shows the well‐known S‐shaped behavior that is interpreted in terms of exciton migration.

Band Structure Engineering Based on InGaN/ZnGeN2 Heterostructure Quantum Wells for Visible Light Emitters

Band structure engineering based on InGaN/ZnGeN2 heterostructure quantum wells (QWs) is proposed to address the long-standing charge separation challenge in visible light emitters using polar InGaN...

EFFECTS OF TEMPERATURE AND A TRANSVERSAL QUANTIZING MAGNETIC FIELD ON THE FORBIDDEN BAND OF A QUANTUM WELL

This article considers the temperature dependence of the band gap in quantum-well heterostructures in the presence of a transverse quantizing magnetic field. An analytical expression is obtained for determining the band gap of a rect angular quantum well at various magnetic fields and temperatures. The proposed formulas well exp lain the experimental results obtained for quantum-well semiconductor structures

(Invited) Isotopically Engineered Silicon Testbeds for Advanced CMOS and Quantum Information

Silicon isotope engineering invokes a new paradigm into materials science for development of advanced CMOS technology and practical quantum computers [1]. Current development of planer-type CMOS devices confronts an immediate issue for downscaling limit, demanding a basal changeover to gate-all-around transistors employing three-dimensional (3D) nanostructures, such as vertical nanowires or stacked nanosheets. Recently, nanoscale metrology of 3D electronic devices is enabled by means of atom probe tomography (APT) having nearly atomic resolution [2]. We extended this APT technique to verify Si self-diffusion in isotopically modulated nanopillars, which allowed spatial visualization of individual isotope atoms crossing the heterointerface [3]. As a result, Si self-diffusivity in gate-oxidized nanopillars having a diameter of ~200 nm was found to remain the same as the one measured for standard planar oxidation. More recently, process tracking technique based on transmission electron microscope (TEM) was established for identical Si nanopillars, which led to unveil a gate-oxidation overview for various conditions [4]. In addition, the silicon isotope engineering provides a crucial solution to eliminate unwanted decoherence for Si-based spin qubits. We adopted the isotope engineering to tailor high-quality CVD-grown 28Si epi-wafers [1], where state-of-the-art Si-MOS nanoelectronic devices were implemented to demonstrate two-qubits operations encoded by single donor electron and nuclear spins [5]. In parallel, the isotopically enriched wafers were employed to demonstrate single and two spin-qubit operations in 28Si-MOS quantum dots (QDs) that were defined by standard CMOS lithography [6]. Meanwhile, we accomplished isotope enrichment of strained-Si/SiGe quantum-well (QW) heterostructures, where the QDs were placed away from gate-oxide interface traps. Thanks to efficient suppression of magnetic nuclear-spin noise, the isotopically enriched 28Si/SiGe QDs prolonged electron-spin dephasing time, consequently recording the highest single-spin gate fidelities of 99.93% in benchmark test [7]. However, it was revealed that inherent charge noise induces temporal fluctuation of qubit frequency. Moreover, the low-energy excited states responsible for Si valley degeneracy should be well-lifted for the higher-fidelity operations, including spin-qubit readout and initialization. Our recent 28Si/SiGe heterostructures exhibit large valley splitting, demonstrating a well-defined QW interface having fewer atomic-scale steps [8]. And also, major impurities of carbon and oxygen can be suppressed below 1016 cm-3. In order to address the remaining source of step generation, we developed direct imaging scheme for crystallographic tilting effects on the basis of the TEM diffraction [9]. It has been identified that the local lattice rotation is preferentially caused in the proximity of the dislocations having threading components. This fact provides experimental signature for the global step generation at the QW interface.This work has been supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118069228, JST-ACCEL Grant Number JPMJAC1301, and the Center for Spintronics Research Network, Keio University. The authors acknowledge fruitful collaborations with Yasuyoshi Nagai, Yasuo Shimizu, Tetsuo Endoh, Seigo Tarucha, Mark Eriksson, Andrew Dzurak and Andrea Morello.[1] K. M. Itoh and H. Watanabe, MRS Communications 4, 143 (2014).[2] D. J. Larson et al., MRS Bulletin 41, 30 (2016).[3] R. Kiga et al., J. Appl. Phys. 127, 045704 (2020).[4] T. Wako et al., in preparation.[5] J.T. Mohonen et al., Nature Nanotechnol. 9, 986 (2014).[6] M. Veldhorst et al., Nature Nanotechnol. 9, 981 (2014); M. Veldhorst et al., Nature 526, 410 (2015).[7] J. Yoneda et al., Nature Nanotechnol. 13, 102 (2018).[8] S. Miyamoto et al., IEEE IEDM – Technical Digest, pp.6.4.1 - 6.4.4 (2018).[9] K. Takeuchi et al., in preparation.

Detuning Axis Pulsed Spectroscopy of Valley-Orbital States in Si / Si - Ge Quantum Dots

Silicon-quantum-dot qubits must contend with low-lying valley excited states that are sensitive functions of the quantum-well heterostructure and disorder; quantifying and maximizing the energies of these states are critical to improving device performance. We describe a spectroscopic method for probing excited states in isolated $\mathrm{Si}$/$\mathrm{Si}$-$\mathrm{Ge}$ double quantum dots using standard baseband pulsing techniques, easing the extraction of energy spectra in multiple-dot devices. We use this method to measure dozens of valley excited-state energies spanning multiple wafers, quantum dots, and orbital states, which are crucial for evaluating the dependence of valley splitting on quantum well width and other epitaxial conditions. Our results suggest that narrower wells can be beneficial for increasing valley splittings, but this effect can be confounded by variations in growth and fabrication conditions. These results underscore the importance of valley-splitting measurements for guiding the development of $\mathrm{Si}$ qubits.

From InGaN pyramids to micro-LEDs characterized by cathodoluminescence

We present a study of the optical properties of various steps in the process of fabricating micro light-emitting diodes (μ-LEDs) based on quantum wells embedded in micron-sized InGaN platelets. In this study, we focus on structures for red emission, but the technology is equally suitable for the less technologically challenging blue and green emitting μ-LEDs. The starting point is growth of an InGaN pyramid with a sub-micron sized hexagonal base. The pyramid is flattened to create a flat top c-facet for the subsequent InGaN quantum well growth. We compare two approaches, reshaping of the pyramid by high-temperature annealing; and by chemical mechanical polishing. Their merits are discussed. The flattened platelets are used as templates to grow low-strain single quantum wells in both heterostructures and full LEDs including n- and p-barriers on either side of the quantum well. The structures are investigated in terms of homogeneity in peak energy position and intensity using hyperspectral cathodoluminescence imaging. We observe that the main contribution to inhomogeneity is the growth of the initial pyramid, that exhibits regular, facet driven variations in the In incorporation.