Wide Quantum Wells Research Articles

High transparency induced superconductivity in field effect two-dimensional electron gases in undoped InAs/AlGaSb surface quantum wells

We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in 24 nm wide indium arsenide surface quantum wells. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor ν = 2 in magnetic fields of up to B = 18 T, at electron densities up to 8 ×1011/cm2. Peak mobility is 11 000 cm2/Vs at 2 ×1012/cm2. Large Rashba spin–orbit coefficients up to 124 meV Å are obtained through weak anti-localization measurements. Proximitized superconductivity is demonstrated in Nb-based superconductor-normal-superconductor (SNS) junctions, yielding 78%–99% interface transparencies from superconducting contacts fabricated ex situ (post-growth), using two commonly used experimental techniques for measuring transparencies. These transparencies are on a par with those reported for epitaxially grown superconductors. These SNS junctions show characteristic voltages IcRn up to 870 μV and critical current densities up to 9.6 μA/μm, among the largest values reported for Nb-InAs SNS devices.

Nuclear spin relaxation mediated by donor-bound and free electrons in wide CdTe quantum wells

Nuclear spin relaxation mediated by donor-bound and free electrons in wide CdTe quantum wells

Optical, magnetic, and transport properties of two-dimensional III-nitride semiconductors (AlN, GaN, and InN) due to acoustic phonon scattering.

In this paper, the magneto-optical transport (MOT) properties of III-nitride Pöschl-Teller quantum well (QW) semiconductors, including AlN, GaN, and InN, resulting from the acoustic phonon interaction are thoroughly investigated and compared by applying the technique of operator projection. In particular, a comparison is made between the Pöschl-Teller QW results and the square QW ones. The findings demonstrate that the MOT properties of III-nitride QW semiconductors resulting from acoustic phonon scattering are strongly influenced by the quantum system (QS) temperature, applied magnetic field, and QW width. When the applied magnetic field and QS temperature increase, the absorbing FWHM in AlN, GaN, and InN increases; on the other hand, it diminishes when the QW's width increases. The absorbing FWHM in GaN is smaller and varies slower compared with AlN; inversely, it is larger and varies faster compared with InN. In other words, the absorbing FWHM in AlN is the largest and the smallest in InN. Compared to the square QW in AlN, GaN, and InN, the absorbing FWHMs in the Pöschl-Teller QW vary more quickly and have greater values. The absorbing FWHMs resulting from the acoustic phonon interaction in III-nitrides are strongly dependent on the nanostructure's geometric shape and parameters. Our findings provide useful information for the development of electronic devices.

High‐Quality Pure‐Phase MA‐Free Formamdinium Dion‐Jacobson 2D Perovskites for Stable Unencapsulated Photovoltaics

AbstractIt is generally difficult to achieve high quality in low‐n‐value (MA‐free) FA‐based Dion‐Jacobson (DJ) 2D perovskites due to their low crystallinity, random well‐width distribution of multiple quantum wells (QWs) with disordered orientation, which has seriously hindered further advances in photovoltaics. Meanwhile, pure‐phase DJ 2D perovskites with uniform QWs width and fixed low‐n values are highly desirable for high efficiency and stability. Herein a novel 4APP(FA)3Pb4I13‐based quasi‐DJ‐2D perovskite is presented first, where 4APP is 4‐aminopiperidinium. The remarkable rigidity of 4APP and its strong hydrogen bonding with the perovskites prolong the perovskite intrinsic stability. the tri‐solvent engineering approach is further established by matching solvent properties to achieve pure‐phase DJ‐2D perovskites with uniform QWs width structure and finely oriented crystal grains, which enhance the power conversion efficiency of unencapsulated solar cells by 48%, representing one of the highest values in MA‐free DJ 2D perovskite with n≤4. Remarkably, the unencapsulated devices retain 95% of their initial PCE after 2000 h of operation under 1‐sun illumination at 40 °C, and 3000 h exposure in the air at 85 °C and 60–90% relative humidity (RH). The simultaneous control of rigidity and phase purity in DJ‐2D perovskites will open up new directions for fundamental research and application exploration.

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.

Bright Emission at Reverse Bias After Trailing Edge of Driving Pulse in Wide InGaN Quantum Wells

In group‐III‐nitride quantum wells (QWs), a strong piezoelectric field is formed. A built‐in potential from the p–n junction is working in the opposite direction depending on an externally applied voltage. Furthermore, the electric field in the QW can be partially screened by a high charge carrier density. Herein, the influence of these effects on recombination and spectrum for over 10 nm wide InGaN QWs is investigated. The very low overlap of electrons and holes would suggest inefficient devices. However, it is shown that thick QWs can be more effective and reach high optical gain. This can be explained by the screening of the electric field, resulting in a high overlap of excited electron and hole states that enable lasing. Herein, a pulsed electrical excitation scheme is used, where carrier injection at forward bias and predominant recombination at zero or reverse bias are separated in time. The interplay between the piezoelectric field and the built‐in potential on carrier recombination in dependence on an external bias voltage is observed. In particular, a strong increase of the radiative recombination rate after the trailing edge of the driving pulse is observed.

Regulation of Quantum Wells Width Distribution in Quasi-2D Perovskite Films for High-Performance Photodetectors.

Dynamic optimization of the quantum-well (QW) width distribution in quasi-2D halide perovskite thin films is an effective approach for tuning the properties of photoelectric devices. Here, that the QWs width distribution in quasi-2D perovskite films can be controlled only by using hydroiodic acid (HI) as an additive is demonstrated. A uniform distribution of the colloidal particle size in the quasi-2D perovskite precursor solution is achieved through the formation of soluble iodoplumbate coordination complexes, PbI3 - from the reaction of HI with PbI2 , resulting in an improved phase purity in the final film. Density functional theory calculations indicate that the ideal n value quasi-2D perovskite reaction pathway through the PbI3 - complex has a lower enthalpy of formation than the random nucleation pathway without the HI additive. Benefiting from this merit, a high-quality quasi-2D perovskite film with optimized phase purity delivered a balanced carrier diffusion length and improved carrier mobility. The resultant photodetectors exhibited a light on/off ratio of 50000, a responsivity of 0.96 A W-1 , and a detectivity of 5.7 × 1012 Jones at 532nm. In addition, the state-of-the-art device maintained more than 80% of its initial photocurrent after 720h of storage at 30% relative humidity.

Theoretical and Experimental Studies on Material Gain for Wide Polar InGaN Quantum Well‐Mechanism Leading to Electric Field Screening and Lasing

AbstractThe width of polar InGaN quantum wells (QWs) is usually limited to a few nanometers to ensure a sufficient overlap between the wave functions of the ground electron and hole state. This paper presents the results of material gain measurements for thin (2.6 nm), wide (10.4 nm), and extremely wide (25 nm) InGaN QWs. The comparison of experimental data with theoretical predictions allows for correlation of the current density with the carrier concentration, which contributes to the positive material gain, and determination of the coefficients in the standard ABC model. It is shown that the mechanism in wide and narrow polar QWs that leads to electric field screening and lasing is different. For wide QWs, the optical transitions between excited states become dominant in the gain spectrum. The reason for such a gain mechanism is the built‐in electric field, which must first be screened. The ground electron and hole states play a very important role in the screening; however, their contribution to the material gain and lasing in wide QWs is very weak.

Modeling of Stacking Faults in 4H-SiC n-Type Epilayer for TCAD Simulation

Current–voltage characteristics of an n-type 4H-silicon carbide (SiC) epilayer containing a stacking fault (SF) were analyzed using a technology computer-aided design (TCAD) simulation. In the simulation, the SF was modeled by a quantum well (QW) formed in the conduction band, which traps electrons and induces a potential barrier. The simulation analysis clarified that the electron conductance in the n-type epilayer containing a SF was dominantly determined by the potential barrier height. Based on this insight and the experimental results obtained from our previous study, the energetic depth and width of the QW in the conduction band were deduced for four types of SFs. The temperature dependence of the experimental current–voltage characteristics of Schottky barrier diodes (SBDs), containing the SF, was effectively reproduced by adopting the deduced QW depth and width, proving the feasibility of the proposed simulation model quantitatively predicting the impacts of SFs on SiC unipolar device performances.

Recombination in Polar InGaN/GaN LED Structures with Wide Quantum Wells

The analysis of the photoluminescence of polar light emitting diode (LED) structures with a 25 nm In0.17Ga0.83N quantum well is reported. The observed emission most likely originates from a set of energetically close excited states (ES), while the ground state decays only extremely slowly on a μs‐to‐ms timescale, that is, long‐living charge accumulates there. However, this population can also be quantified spectroscopically by applying short reverse voltage pulses. The emission from ES is effective and nearly exponential with decay time constants of 1.5 and 8 ns at 6 and 300 K, respectively, and is likely dominated by radiative processes. These findings point to a possible route to improved polar device architectures.

High‐Efficiency Quasi‐2D Perovskite Light‐Emitting Diodes Using a Dual‐Additive Strategy Guided by Preferential Additive‐Precursor Interactions

AbstractQuasi‐2D perovskites are promising emitters in light‐emitting diodes (LEDs) due to their natural quantum well (QW) structure and tunable exciton binding energy. Synergistic regulation of the defect passivation and QW width distribution of quasi‐2D perovskites is crucial to achieve high‐performance perovskite‐based LEDs. Herein, a combination of two additives, i.e., 3‐(diaminomethylidene)‐1,1‐dimethylguanidine (Metformin) and 1,4,7,10,13,16‐hexaoxacyclooctadecane (Crown) having preferential interactions with different quasi‐2D perovskite precursors is proposed to synergistically passivate the defects and regulate the QW width distribution of quasi‐2D perovskites. Metformin additive interacts strongly with lead bromide, mainly passivating the defects. Crown additive has preferential interactions with organic spacer phenethylammonium bromide, which dominates the QW width distribution and induces quasi‐2D perovskites with uniform dimensions. The dual additives of Metformin and Crown interact preferentially with distinct precursors, allowing to regulate the QW width distribution and defect density of quasi‐2D perovskites synergistically. A photoluminescence quantum yield as high as 91.5% is achieved for the quasi‐2D perovskite prepared with the dual‐additive strategy. Moreover, high‐efficiency quasi‐2D perovskite LEDs were successfully fabricated with a maximum external quantum efficiency of 21.3% and a maximum luminance over 30 000 cd m−2. This work provides a preferential interaction‐guided dual‐additive strategy to fabricate high‐efficiency quasi‐2D perovskite LEDs without anti‐solvent treatment.

Factors Affecting Terahertz Emission from InGaN Quantum Wells under Ultrafast Excitation

InGaN quantum wells (QWs) grown on c-plane sapphire substrate experience strain due to the lattice mismatch. The strain generates a strong piezoelectric field in QWs that contributes to THz emission under ultrafast excitation. Physical parameters such as QW width, period number, and Indium concentration can affect the strength of the piezoelectric field and result in THz emission. Experimental parameters such as pump fluence, laser energy, excitation power, pump polarization angle, and incident angle can be tuned to further optimize the THz emission. This review summarizes the effects of physical and experimental parameters of THz emission on InGaN QWs. Comparison and relationship between photoluminescence properties and THz emission in QWs are given, which further explains the origin of THz emission in InGaN QWs.

Evidence for "dark charge" from photoluminescence measurements in wide InGaN quantum wells.

Wide (15-25 nm) InGaN/GaN quantum wells in LED structures were studied by time-resolved photoluminescence (PL) spectroscopy and compared with narrow (2.6 nm) wells in similar LED structures. Using below-barrier pulsed excitation in the microsecond range, we measured increase and decay of PL pulses. These pulses in wide wells at low-intensity excitation show very slow increase and fast decay. Moreover, the shape of the pulses changes when we vary the separation between them. None of these effects occurs for samples with narrow wells. The unusual properties of wide wells are attributed to the presence of "dark charge" i.e., electrons and holes in the ground states. Their wave functions are spatially separated and due to negligible overlap they do not contribute to emission. However, they screen the built-in field in the well very effectively so that excited states appear with significant overlap and give rise to PL. A simple model of recombination kinetics including "dark charge" explains the observations qualitatively.

Spectral properties of a broadband far infrared photodetector with a new design of active region

A quantum theory of spectral parameters and oscillator strengths of quantum transitions in an active region, which contains cascades of wide quantum wells with a complicated potential profile is developed. A new spatial design of the cascade is calculated and proposed with such an asymmetric arrangement of the wells and barriers, in which, without an applied electric bias, the magnitudes of oscillator strengths are considerable and one-way resonant-tunneling transport of electrons is observed. As a result, it becomes possible to ensure a successful functioning of the broadband photodetector in the far IR range.

Terahertz Ratchet Effect in Interdigitated HgTe Structures

The emergence of ratchet effects in two-dimensional materials is strongly correlated with the introduction of asymmetry into the system. In general, dual-grating-gate structures forming lateral asymmetric superlattices provide a suitable platform for studying this phenomenon. Here, we report on the observation of ratchet effects in $\mathrm{Hg}\mathrm{Te}$-based dual-grating-gate structures hosting different band-structure properties. Applying polarized terahertz laser radiation we detect linear and polarization-independent ratchets, as well as a radiation-helicity-driven circular ratchet effect. Studying the ratchet effect in devices made of quantum wells (QWs) of different thickness, we observe that the magnitude of the signal substantially increases with decreasing QW width with a maximum value for devices made of QWs of critical thickness hosting Dirac fermions. Furthermore, sweeping the gate voltage amplitude we observe sign-alternating oscillations for gate voltages corresponding to $p$-type conductivity. The amplitude of the oscillations is more than two orders of magnitude larger than the signal for $n$-type conducting QWs. The oscillations and the signal enhancement is shown to be caused by the complex valence band structure of $\mathrm{Hg}\mathrm{Te}$-based QWs. These peculiar features of the ratchet currents make these materials an ideal platform for the development of terahertz applications.

Confinement factor and carrier recombination of InGaAsP/InP quantum well lasers

Low-dimensional materials have attracted significant attention in developing and enhancing the performance of quantum well lasers due to their extraordinary unique properties. The optical confinement factor is one of the most effective parameters for evaluating the optimal performance of a semiconductor laser diode when used to measure the optical gain and current threshold. The optical confinement factor and the radiative recombination of single quantum wells (SQW) and multi-quantum wells (MQW) for InGaAsP/InP have been theoretically studied using both radiative and Auger coefficients. Quantum well width, barrier width, and number of quantum wells were all looked at to see how these things changed the optical confinement factor and radiative and non-radiative recombination coefficients for multi-quantum well structures. It was found that the optical confinement factor increases with an increase in the number of wells. The largest value of the optical confinement factor was determined when the number of wells was five at any width. The optical confinement coefficient was 0.23, 0.216, and 0.203 for the number of wells (3, 4, and 5) and well width (27, 19.5, and 15) nm, respectively. In addition, the radiative recombination coefficient increases with the width of the quantum well after 5 nm, and it is much bigger than that of its bulk counterparts.

Regulation of Quantum Wells Width Distribution in 2D Perovskite Films for Photovoltaic Application

AbstractSolution‐processed 2D perovskite films generally contain mixed quantum wells (QWs) with multiple well width distribution, which seriously weakens the charge transfer. To achieve regulation of the QW width, strategies to optimize the crystallization dynamics of 2D perovskite films are urgently needed. In this review, systematic summary on QW distribution and guidelines for 2D perovskite phase regulation is provided, aiming to establish a general manual for preparing efficient 2D perovskite solar cells (PSCs). The factors affecting the distribution of multiple‐QWs in 2D perovskite films, including component engineering, additive engineering, process optimization, are first generalized. Then an extensive review of these factors that are widely used to reconstruct 2D perovskite crystallization process is conducted. Leveraging these insights, the effect of QWs distributions on 2D PSCs properties is also summarized. Similarly, considering the crystallization kinetics and device performance, the QWs width control of 2D perovskite films from the aspects of ligand engineering, precursor design, and fabrication optimization, is rationalized. Finally, an outlook on how to realize ordered QWs distribution in perovskite films for efficient 2D PSCs is proposed.

Heavy-hole–light-hole exciton system in GaAs/AlGaAs quantum wells

Light-hole (lh) excitons (Xlhs) in quantum wells (QWs) are hardly studied compared with heavy-hole (hh) excitons (Xhhs), mainly due to the difficulties of their experimental observation. In this paper, a comprehensive study of both types of excitons in high-quality GaAs/AlGaAs QWs of different widths is performed. We focus on the energy positions of exciton resonances and the exciton-light interaction. The effect of mixing lhs and hhs in GaAs-based structures on these exciton characteristics is investigated. The corrections to the exciton energy due to mixing are only a fraction of millielectronvolts for wide QWs, in which the energy levels of Xhhs and Xlhs are close to each other. It is also experimentally found that the oscillator strength of Xlhs is $\ensuremath{\sim}2.5$ times less than that of Xhhs. This value noticeably deviates from the 3:1 oscillator strength ratio known for optical transitions between free electron and hole states. This deviation originates from the different squeezing of the Xlh and Xhh wave functions due to distinct values of the effective masses of the Xlh and Xhh in the heterostructure. Mixing of hh and lh valence bands is not so important.

Polarization-Induced Phase Transitions in Ultra-Thin InGaN-Based Double Quantum Wells.

We investigate the phase transitions and the properties of the topological insulator in InGaN/GaN and InN/InGaN double quantum wells grown along the [0001] direction. We apply a realistic model based on the nonlinear theory of elasticity and piezoelectricity and the eight-band k·p method with relativistic and nonrelativistic linear-wave-vector terms. In this approach, the effective spin–orbit interaction in InN is negative, which represents the worst-case scenario for obtaining the topological insulator in InGaN-based structures. Despite this rigorous assumption, we demonstrate that the topological insulator can occur in InGaN/GaN and InN/InGaN double quantum wells when the widths of individual quantum wells are two and three monolayers (MLs), and three and three MLs. In these structures, when the interwell barrier is sufficiently thin, we can observe the topological phase transition from the normal insulator to the topological insulator via the Weyl semimetal, and the nontopological phase transition from the topological insulator to the nonlocal topological semimetal. We find that in InGaN/GaN double quantum wells, the bulk energy gap in the topological insulator phase is much smaller for the structures with both quantum well widths of 3 MLs than in the case when the quantum well widths are two and three MLs, whereas in InN/InGaN double quantum wells, the opposite is true. In InN/InGaN structures with both quantum wells being three MLs and a two ML interwell barrier, the bulk energy gap for the topological insulator can reach about . We also show that the topological insulator phase rapidly deteriorates with increasing width of the interwell barrier due to a decrease in the bulk energy gap and reduction in the window of In content between the normal insulator and the nonlocal topological semimetal. For InN/InGaN double quantum wells with the width of the interwell barrier above five or six MLs, the topological insulator phase does not appear. In these structures, we find two novel phase transitions, namely the nontopological phase transition from the normal insulator to the nonlocal normal semimetal and the topological phase transition from the nonlocal normal semimetal to the nonlocal topological semimetal via the buried Weyl semimetal. These results can guide future investigations towards achieving a topological insulator in InGaN-based nanostructures.

Linewidths and energy shifts of electron-impurity resonant states in quantum wells with infinite barriers

The linewidths and the energy shifts of the resonant states of the impurity electron in GaAs-based quantum wells (QWs) with infinite barriers are calculated. The two-dimensional Schr\"{o}dinger equation for the charge impurity in the QW is solved by the developed finite-difference method combined with the complex-scaling technique. A dependence of the linewidths and energy shifts on the QW width for the impurity localized in the center of QW is studied. The calculated results extend and improve theoretical estimations of these quantities in the GaAs-based QW by Monozon and Schmelcher [Phys. Rev. B 71, 085302 (2005)]. In particular, in agreement with their studies we obtain that resonant states originating from the second quantum-confinement subband have negligibly small linewidths. In contrast to previous estimations, we show that for the QW widths of the order of the impurity's Bohr radius the linewidths of resonant states associated to the third quantum-confinement subband linearly depend on the thickness of the QW. We show how the previous theoretical predictions can be improved for such QW widths. We also calculate linewidths for the case when the electron impurity is localized away from the center of the QW.