Wide Quantum Wells Research Articles

Stimulated emission in the 2.8-3.5 μm wavelength range from Peltier cooled HgTe/CdHgTe quantum well heterostructures.

We report stimulated emission in the 2.8-3.5 μm wavelength range from HgTe/CdHgTe quantum well (QW) heterostructures at temperatures available with thermoelectric cooling. The structures were designed to suppress the Auger recombination by implementing narrow (1.5 - 2 nm wide) QWs. We conclude that Peltier cooled operation is feasible in lasers based on such structures, making them of interest for spectroscopy applications in the atmospheric transparency window from 3 to 5 μm.

Effect of Transverse Electric Field on Polariton Reflectance Spectra of Wide Quantum Wells

Excitonic waves propagating in thick GaAs/AlGaAs quantum wells form an interference pattern, which is observed as oscillations in the reflectance spectra. Oscillations in the spectra of wide GaAs quantum wells should reveal a non-trivial behavior as a function of the external uniform electric field as it is found in the theory developed in the present work. The oscillations are suppressed as the electric field increases and then reappear again with an opposite phase at the further rise of the field.

Surface plasmon resonance aided analysis of quantum wells for photonic device applications

The present report aims at analyzing the quantum well (QW) structures using the highly sensitive surface plasmon resonance (SPR) technique in Otto configuration. The optical properties of ZnO and Ni doped ZnO (NiZnO) based QW structures grown using Pulsed laser deposition technique are investigated. An optimal value of QW width (8nm) is deduced from the SPR study for maximum charge carrier confinement which is in coherence with the photoluminescence (PL) studies conducted on the same QW structures. The metal-dielectric interfaces were modelled primarily for the design of QW structures to be analysed experimentally. The mechanism behind the coupling of QWs with SP excitation has been proposed theoretically and experimentally to substantiate the results of PL and SPR studies. The variation in the SPR curves observed as a function of the QW width can be attributed to the interaction of evanescent SP field originating at the Au/QW interface and electroabsorptive behaviour of QWs towards the SP field.

Reduced Efficiency Roll-Off and Enhanced Stability in Perovskite Light-Emitting Diodes with Multiple Quantum Wells.

Mixed-formamidinium (FA) and -cesium (Cs) cations were used to fabricate multiple quantum well (MQW) perovskite light-emitting diodes (PeLEDs). The partial substitution of FA with Cs facilitates the formation of wider quantum wells, which can effectively reduce efficiency roll-off by suppressing Auger recombination. The device has a peak external quantum efficiency (EQE) of 7.8% at a current density of 125 mA cm-2, maintaining an EQE of ∼6.6% at a high current density of 500 mA cm-2. Due to the improved stability of mixed-cation structure, we achieve a PeLED device with a lifetime of ∼31 h under a constant current density of 10 mA cm-2.

5a-F-12 GaAs広い量子井戸における励起子発光の偏光

5a-F-12 GaAs広い量子井戸における励起子発光の偏光

Characterization of individual layers in a bilayer electron system produced in a wide quantum well

Here, we report on a transparent method to characterize individual layers in a double-layer electron system, which forms in a wide quantum well, and to determine their electron densities. The technique relies on the simultaneous measurement of the capacitances between the electron system and gates located on either side of the well. Modifications to the electron wave function due to the population of the second subband and the appearance of an additional electron layer can be detected. The magnetic field dependence of these capacitances is dominated by quantum corrections caused by the occupation of Landau levels in the nearest electron layer. The technique should be equally applicable to other implementations of a double layer electron system.

Synthetic Control over Quantum Well Width Distribution and Carrier Migration in Low-Dimensional Perovskite Photovoltaics.

Metal halide perovskites have achieved photovoltaic efficiencies exceeding 22%, but their widespread use is hindered by their instability in the presence of water and oxygen. To bolster stability, researchers have developed low-dimensional perovskites wherein bulky organic ligands terminate the perovskite lattice, forming quantum wells (QWs) that are protected by the organic layers. In thin films, the width of these QWs exhibits a distribution that results in a spread of bandgaps in the material arising due to varying degrees of quantum confinement across the population. Means to achieve refined control over this QW width distribution, and to examine and understand its influence on photovoltaic performance, are therefore of intense interest. Here we show that moving to the ligand allylammonium enables a narrower distribution of QW widths, creating a flattened energy landscape that leads to ×1.4 and ×1.9 longer diffusion lengths for electrons and holes, respectively. We attribute this to reduced ultrafast shallow hole trapping that originates from the most strongly confined QWs. We observe an increased PCE of 14.4% for allylammonium-based perovskite QW photovoltaics, compared to 11-12% PCEs obtained for analogous devices using phenethylammonium and butylammonium ligands. We then optimize the devices using mixed-cation strategies, achieving 16.5% PCE for allylammonium devices. The devices retain 90% of their initial PCEs after >650 h when stored under ambient atmospheric conditions.

Minimising efficiency roll-off in high-brightness perovskite light-emitting diodes

Efficiency roll-off is a major issue for most types of light-emitting diodes (LEDs), and its origins remain controversial. Here we present investigations of the efficiency roll-off in perovskite LEDs based on two-dimensional layered perovskites. By simultaneously measuring electroluminescence and photoluminescence on a working device, supported by transient photoluminescence decay measurements, we conclude that the efficiency roll-off in perovskite LEDs is mainly due to luminescence quenching which is likely caused by non-radiative Auger recombination. This detrimental effect can be suppressed by increasing the width of quantum wells, which can be easily realized in the layered perovskites by tuning the ratio of large and small organic cations in the precursor solution. This approach leads to the realization of a perovskite LED with a record external quantum efficiency of 12.7%, and the efficiency remains to be high, at approximately 10%, under a high current density of 500 mA cm−2.

Large optical gain from the 2D-transition metal dichalcogenides of MoS2/WSe2 quantum wells

In this work, the optical gain in the new class of 2D-materials is calculated for MoS2/WSe2 single quantum well (QW). For this purpose, numerical solution of the k.p equation at (K+, K−) points of the Brillouin zone is done in order to obtain the energy levels and their corresponding wave functions of electrons and holes in QW. Then, we discussed the band gap dependence on temperature of MoS2 and WSe2 materials. Further, we examined the temperature and the QW width effects on the optical and the maximum of gain. We found that MoS2/WSe2 QW can satisfy the lasing requirements and can give interesting results about the performance of laser emitting devices. The results are compared to the available data reported in the literature.

Effect of transverse electric field on polariton reflectance spectra of wide quantum wells

AbstractExcitonic waves propagating in thick GaAs/AlGaAs quantum wells form an interference pattern, which is observed as oscillations in the reflectance spectra. Oscillations in the spectra of wide GaAs quantum wells should reveal a non-trivial behavior as a function of the external uniform electric field as it is found in the theory developed in the present work. The oscillations are suppressed as the electric field increases and then reappear again with an opposite phase at the further rise of the field.

Electron Effective Mass and g Factor in Wide HgTe Quantum Wells

The magnetic-field (0 T < B < 9 T) dependence of the longitudinal and Hall resistances at fixed temperatures (2 K < T < 50 K) for the HgCdTe/HgTe/HgCdTe system with a HgTe quantum well 20.3 nm in width are measured. The activation analysis of the magnetoresistance curves is used as a tool for identifying the mobility gaps between neighboring Landau levels. The activation-energy values obtained from the temperature dependences of the longitudinal resistance in the plateau regions of the quantum Hall effect with the filling factors ν = 1, 2, 3 make it possible to estimate the effective mass and the g factor of electrons in the system under study. Indications concerning the possibility of large values of the g factor (≅ 80) are obtained.

Manifestation of PT symmetry in the exciton spectra of quantum wells

The reflection and photoluminescence excitation spectra under circular polarization of the incident light are measured in wide quantum wells whose width is much larger than the exciton Bohr radius. It is found that, in the absence of a magnetic field, the relation between the intensities of even and odd absorption peaks is reversed upon reversal of the sign of circular polarization.

Electron mobility of inverted InAs/GaSb quantum well structure

We report on the electron mobility of an inverted and a non-inverted type II InAs/GaSb quantum well (QW) structures. It is found that the inverted QW, which has much thicker well width than the non-inverted one, has a smaller electron mobility than the non-inverted one for a wide range of temperature. The result disagrees with the established understanding that the electron mobility increases with increasing the QW width at very low temperature. The smaller mobility is caused by larger electron effective mass since the bottom of the conduction band of the inverted QW becomes a hole-like band due to the mixing between the conduction band and the valence band.

Influence of well width fluctuations on recombination properties in semipolar InGaN quantum wells studied by time- and spatially-resolved near-field photoluminescence

Scanning near-field photoluminescence spectroscopy has been applied to distinguish the relevance of quantum well (QW) alloy composition and well width fluctuations on emission linewidth and recombination times in semipolar (202¯1) plane InGaN QWs. It has been found that well width fluctuations, compared to variations of InGaN alloy composition, play a negligible role in defining the photoluminescence linewidth. However, the well width strongly affects the recombination times. Prolonged radiative and nonradiative carrier lifetimes in wide QWs have been attributed to electron and hole separation by in-plane electric fields caused by nonplanarity of QW interfaces.

Photoelectric characteristics of quantum dots photodetector and fluorescence detection application

ABSTRACTThe photodetector based on AlAs/GaAs/AlAs heterostructures has a layer of self-assembled InAs quantum dots (QDs) and In0.15Ga0.85As quantum well (QW) imbedded in GaAs wide QW. A peculiar negative differential capacitance characteristic has been clearly observed near 100 K. A simple model is proposed and used to qualitatively describe and demonstrated that the InAs monolayer can effectively absorb photo-excited carriers. A high sensitivity micro-spectrometer based on 64 pixels QDs photodetector array has been designed and used for fluorescence detection.

Donor binding energies in a GaN/ZnGeN2 quantum well

Binding energies of a donor atom within a GaN/ZnGeN2 quantum well structure have been investigated. Hydrogenic type wave functions are assumed and the Schrödinger and Poisson's equations are solved self-consistently. The binding energies of the donor states 1s and 2p± associated with the first subband in the quantum well and the transition energies between them are represented as a function of the quantum well width, the donor position and the external magnetic field. The ground state 1s and the excited states 2p± have maximum binding energies as much as 64 and 11 meV, occurring in quantum wells of widths 10 and 18 Å, respectively. The binding energies are found to be an asymmetric function of the donor position with respect to the quantum well center because of the asymmetry in the band profile introduced by the built-in electric field in the structure. An external magnetic field up to 10 T is included into the calculations and it is seen that the excited states show a small Zeeman splitting, very close to the bulk GaN value, because of the heavy effective mass of electron.

Use of micro-photoluminescence as a contactless measure of the 2D electron density in a GaAs quantum well

We compare micro-photoluminescence (μPL) as a measure of the electron density in a clean, two-dimensional (2D) system confined in a GaAs quantum well (QW) to the standard magneto-transport technique. Our study explores the PL shape evolution across a number of molecular beam epitaxy-grown samples with different QW widths and 2D electron densities and notes its correspondence with the density obtained in magneto-transport measurements on these samples. We also measure the 2D density in a top-gated quantum well sample using both PL and transport and find that the two techniques agree to within a few percent over a wide range of gate voltages. We find that the PL measurements are sensitive to gate-induced 2D density changes on the order of 109 electrons/cm2. The spatial resolution of the PL density measurement in our experiments is 40 μm, which is already substantially better than the millimeter-scale resolution now possible in spatial density mapping using magneto-transport. Our results establish that μPL can be used as a reliable high spatial resolution technique for future contactless measurements of density variations in a 2D electron system.

Surface transport and quantum Hall effect in ambipolar black phosphorus double quantum wells.

Quantum wells (QWs) constitute one of the most important classes of devices in the study of two-dimensional (2D) systems. In a double-layer QW, the additional "which-layer" degree of freedom gives rise to celebrated phenomena, such as Coulomb drag, Hall drag, and exciton condensation. We demonstrate facile formation of wide QWs in few-layer black phosphorus devices that host double layers of charge carriers. In contrast to traditional QWs, each 2D layer is ambipolar and can be tuned into n-doped, p-doped, or intrinsic regimes. Fully spin-polarized quantum Hall states are observed on each layer, with an enhanced Landé g factor that is attributed to exchange interactions. Our work opens the door for using 2D semiconductors as ambipolar single, double, or wide QWs with unusual properties, such as high anisotropy.

Topological approach to quantum Hall effects and its important applications: higher Landau levels, graphene and its bilayer

In this paper the topological approach to quantum Hall effects is carefully described. Commensurability conditions together with proposed generators of a system braid group are employed to establish the fractional quantum Hall effect hierarchies of conventional semiconductors, monolayer and bilayer graphene structures. Obtained filling factors are compared with experimental data and a very good agreement is achieved. Preliminary constructions of ground-state wave functions in the lowest Landau level are put forward. Furthermore, this work explains why pyramids of fillings from higher bands are not counterparts of the well-known composite-fermion hierarchy – it provides with the cause for an intriguing robustness of ν = 7/3 , 8/3 and 5/2 states (also in graphene). The argumentation why paired states can be developed in two-subband systems (wide quantum wells) only when the Fermi energy lies in the first Landau level is specified. Finally, the paper also clarifies how an additional surface in bilayer systems contributes to an observation of the fractional quantum Hall effect near half-filling, ν = 1/2 .

Excitons in tunnel coupled CdTe and (Cd,Mn)Te quantum wells

The photoluminescence (PL) from structures containing Cd0.95Mn0.05Te and CdTe quantum wells (QWs) separated by a narrow (1.94 nm) barrier was studied. The PL lines of comparable intensities from several possible exciton states were observed simultaneously at energy distances substantially exceeding kT. This means that the energy transfer in the studied systems is slower than the radiative recombination of the confined excitons. For the CdTe QW width of about 8.7–9 nm, indirect excitons with the electron and heavy hole chiefly localized in the CdTe and Cd1−xMnxTe QWs, respectively, were detected in the magnetic field. These indirect excitons have PL energy of about 10–20 meV above the PL line of the direct excitons in the CdTe QW. The observation of the PL from the indirect excitons which are not the lowest excitations in the structure is a distinctive feature of the system.Photoluminescence intensity dependence on the energy and the magnetic field.