Wide Quantum Wells Research Articles

Quantum-confined Stark effect and mechanisms of its screening in InGaN/GaN light-emitting diodes with a tunnel junction.

Nitride-based light-emitting diodes (LEDs) are well known to suffer from a high built-in electric field in the quantum wells (QWs). In this paper we determined to what extent the electric field is screened by injected current. In our approach we used high pressure to study this evolution. In LEDs with a narrow QW (2.6 nm) we found that even at a high injection current a large portion of built-in field remains. In LEDs with very wide QWs (15 and 25 nm) the electric field is fully screened even at the lowest currents. Furthermore, we examined LEDs with a tunnel junction in two locations - above and below the active region. This allowed us to study the cases of parallel and antiparallel fields in the well and in the barriers.

Investigation of Factors Affecting the Radiation Intensity of Quantum Wells

We study the possibilities of increasing the light-emission intensity of quantum wells. We describe the influence of the quantum-well width and the effective mass of charge carriers on the radiation intensity. Since the formation of negatively charged layers near the quantum well can increase the photon emission, we propose to form the charged layers with the use of ion implantation. The results of the study can be implemented in the field of quantum-size light-emitting structures and sensors.

Electronic mobility limited by optical phonons in symmetric MgxZn1-xO/ZnO quantum wells with mixed phases

In symmetric MgxZn1-xO/ZnO quantum wells (QWs), which are the basic structures of high electronic mobility transistors (HEMTs), the electron states and optical phonon modes are clarified with the dielectric continuum model, uniaxial model, and force balance equation. Then, the electronic mobility affected by optical phonons is obtained around room temperature by a weight model combined with Lei-Ting's force-balance equation, in consideration of mixed phases in MgxZn1-xO (0.37<x < 0.62). Our results show that the potentials of QW's barriers, sharply influenced by the built-in electric field, present a novel fluctuation in mixed phases region with increasing Mg composition x, contrasting to the results only influenced by ternary mixed crystals effect. Meanwhile, compared rocksalt (RS) phase barriers with wurtzite (WZ) ones in QWs, the confinement on both optical interface and confined phonons is stronger in the well and weaker in the barriers. Furthermore, the total electronic mobility in the QWs is mainly influenced by WZ and RS phases, in small (x < 0.37) and large (x > 0.62) Mg composition regions, respectively. In WZ phase, the mobility first reaches a minimum due to the strong polarizations, then rises to a maximum in RS phase. It indicates that the restriction of electronic mobility from different phases should be a primary consideration for the designation of HEMTs. Strong temperature and size dependences of the mobility are also revealed as well. Relatively thicker well width of QWs is more beneficial to increase electronic mobility.

Dimension control of in situ fabricated CsPbClBr2 nanocrystal films toward efficient blue light-emitting diodes

In the field of perovskite light-emitting diodes (PeLEDs), the performance of blue emissive electroluminescence devices lags behind the other counterparts due to the lack of fabrication methodology. Herein, we demonstrate the in situ fabrication of CsPbClBr2 nanocrystal films by using mixed ligands of 2-phenylethanamine bromide (PEABr) and 3,3-diphenylpropylamine bromide (DPPABr). PEABr dominates the formation of quasi-two-dimensional perovskites with small-n domains, while DPPABr induces the formation of large-n domains. Strong blue emission at 470 nm with a photoluminescence quantum yield up to 60% was obtained by mixing the two ligands due to the formation of a narrower quantum-well width distribution. Based on such films, efficient blue PeLEDs with a maximum external quantum efficiency of 8.8% were achieved at 473 nm. Furthermore, we illustrate that the use of dual-ligand with respective tendency of forming small-n and large-n domains is a versatile strategy to achieve narrow quantum-well width distribution for photoluminescence enhancement.

Laser field effect on self-polarization of a donor impurity in a finite [formula omitted] quantum well

The self-polarization and donor binding energy are calculated for a square Ga1−xAlxAs/GaAs quantum well under an laser field. The donor binding energy and self-polarization are obtained as a function of the laser field parameter, donor impurity position and quantum well-width. The calculations are made by means of variational and finite differences methods using the effective-mass approximation. Our results show that the laser field has remarkable effect on the self-polarization and donor binding energy. These results can be useful for studies on the electronic and optical properties of a quantum well under the influence of a laser field.

Modeling of Exciton Exchange Interaction in GaAs/AlGaAs Quantum Wells

In this work, we study the exchange interactions between two excitons in the GaAs/AlGaAs quantum wells of various widths. We numerically solved the Schrodinger equation for an exciton in a quantum well to find the two-exciton wave functions and to calculate the exchange integral. The results suggest that the strongest interactions between excitons occur in the quantum wells of widths of about 40–50 nm, with the exchange energy being of about of 9 μeV for an exciton density of 1/μm2.

Exciton-polariton interference controlled by electric field

Linear in the wave-vector terms of an electron Hamiltonian play an important role in topological insulators and spintronic devices. Here we demonstrate how an external electric field controls the magnitude of a linear-in-K term in the exciton Hamiltonian in wide GaAs quantum wells. The dependence of this term on the applied field in a high quality sample was studied by means of the differential reflection spectroscopy. An excellent agreement between the experimental data and the results of calculations using semi-classical non-local dielectric response model confirms the validity of the method and paves the way for the realisation of excitonic Datta-and-Das transistors. In full analogy with the spin-orbit transistor proposed by Datta and Das [Appl. Phys. Lett. {\bf 56}, 665 (1990)], the switch between positive and negative interference of exciton polaritons propagating forward and backward in a GaAs film is achieved by application of an electric field with non-zero component in the plane of the quantum well layer.

The impurity states in different shaped quantum wells under applied electric field

In this paper, the impurity states in square, parabolic and V-shaped quantum wells (QWs) are calculated by the plane wave method under the theoretical framework of effective mass envelope function approximation. In different shaped QWs, the impurity binding energies in 1s-like state and 2p-like state have the same trend with the increase of QW width. However, the transition energies of impurity states between 1s-like state and 2p-like state in three shaped QWs are different. When the width of QW is fixed, the transition energy in square QW is the minimum and that in V-shaped QW is the maximum. The effect of electric field is larger for the on-center donor impurity. The effect of electric field on the impurity states is the largest in square QW and the smallest in V-shaped QW.

Origin of carrier localization in AlGaN-based quantum well structures and implications for efficiency droop

We investigate carrier localization in Al-rich AlGaN/AlN quantum well (QW) structures. Low temperature time-resolved photoluminescence (PL) experiments reveal a strong variation of the carrier decay times with detection photon energy, suggesting a strong impact of carrier localization, which is found to depend primarily on the QW width. In combination with time-integrated PL measurements and numerical band structure calculations, we are able to provide conclusive evidence that the localization strength in AlGaN-based QW structures is directly coupled to the oscillator strength, providing an explanation for its strong dependence on the QW width. This is further supported by the observation of a strong polarization field dependency of the carrier localization, which excludes excitons and may be explained by the accumulation of electrons close to the QW interface, while holes are independently localized across the QW. We complete our discussion by proposing a model to explain the well-known phenomenon of efficiency droop in accordance with our findings, suggesting delocalization-induced Auger recombination as the responsible loss channel.

Self-polarization of a donor impurity for the first excited state in an Ga1-xAlxAs/GaAs quantum well

In this study, self-polarization (SP) for the first excited state in the finite quantum well is presented by taking into account the hydrostatic pressure and the external electric field. We calculated the SP of the first excited state by using the variational method under the effective mass approximation. These calculations were made for different values of the quantum well-width, hydrostatic pressure, external electric field and impurity positions. Our results indicate that the self-polarization of the first excited state in finite quantum well depends remarkably on the hydrostatic pressure, external electric field strength and impurity position.

Optical Gain Characteristics of BGaAs/GaP Quantum Wells

Light emitters integrated with Si platform are highly desirable for photonic integrated circuits, however, manufacturing them remains difficult. In this work, BGaAs/GaP quantum well (QW) structures are proposed as a promising solution of the challenge. These QWs can be grown on GaP/Si templates, which are intensively developed for recent years. An 8-band k·p model, envelope function approximation, self-consistency in solving of Schrodinger and Poisson equations with parabolic approximations of the indirect valleys and Fermi golden rule are used to calculate and analyze the material optical gain spectra of the QWs. A positive material gain is found for the QWs with 10–35% BAs mole fraction, with zinc-blende BGaAs epilayers grown on the GaP(001) substrates as direct gap semiconductors. It is predicted that such structures emit red light with wavelengths from the range of 730–690 nm. Optimal QWs widths for maximal TE and TM gain polarizations are below the critical thickness of BGaAs grown on the GaP(001). Presented results clearly indicate that BGaAs/GaP QW system is a very promising gain medium for Si-compatible photonic integrated circuits.

Quantum piezotronic devices based on ZnO/CdO quantum well topological insulator

Abstract Developing emergent quantum piezotronic devices based on third-generation semiconductors has become the growing tendency for going beyond conventional device applications. Wurtzite ZnO is currently the leading piezoelectric semiconductor for developing high performance piezotronic nanodevices. In this study, we propose a quantum piezotronic device of ZnO quantum wells (QWs) topological insulator. Stress-induced piezoelectric field can drive the QW to be topological insulator, and the critical stress is highly dependent on the QW width. An effective Hamiltonian is calculated to identify the existence of gapless edge states, and further used to study electronic transport in quantum point contact (QPC) structure. The transport conductance can be controlled at edge states and bulk states by externally applied stress under various QPC widths and Fermi levels. A topological insulator device is proposed for manipulating quantum states among the normal and topological insulator, and robustly against the applied stress. This study provides a microscopic picture to understand piezotronic effect on topological insulator states of ZnO quantum well, and paves the way towards the flexibly integrated applications of topological insulator.

Quantum-well laser diodes for frequency comb spectroscopy.

We demonstrate simple optical frequency combs based on semiconductor quantum well laser diodes. The frequency comb spectrum can be tailored by choice of material properties and quantum-well widths, providing spectral flexibility. We demonstrate the correlation in the phase fluctuations between two devices on the same chip by generating a radio-frequency dual comb spectrum.

Influence of interface modes compared to confined modes on phonon-assisted cyclotron resonance effect in quantum wells

Abstract We study the influence of interface phonon modes on the phonon-assisted cyclotron resonance (PACR) effect in GaAs-AlAs quantum well (QW) structures by calculating the absorption power (AP) based on the operator projection technique in comparison with bulk and confined phonon modes in previous study [N. D. Hien, Superlattices Microstruct. 131 (2019) 86]. The full width at half maximum (FWHM) of the PACR absorption peak is obtained by utilizing the profile technique. On the other hand, the FWHM is described as functions of the QW width and temperature for different kinds of phonon modes (interface, bulk, confined modes) presented to compare. The calculations clearly demonstrate important roles of interface-mode as the QW width is small enough. Also, the result indicates that the interface optical modes also gives a considerable contribution to the electron–phonon scattering as compared with that of bulk and confined optical phonon modes.

Magneto-intersubband oscillations in two-dimensional systems with an energy spectrum split due to spin-orbit interaction

In the present paper we study magneto-intersubband oscillations (MISO) in HgTe/Hg$_{1-x}$Cd$_x$Te single quantum well with "inverted" and "normal" spectra and in conventional In$_{1-x}$Ga$_x$As/In$_{1-y}$Al$_y$As quantum wells with normal band ordering. For all the cases when two branches of the spectrum arise due to spin-orbit splitting, the mutual arrangement of the antinodes of the Shubnikov-de Haas oscillations and the maxima of MISO occurs opposite to that observed in double quantum wells and in wide quantum wells with two subbands occupied and does not agree with the theoretical predictions. A "toy" model is supposed that explain qualitatively this unusual result.

Modelling and optical response of a compressive-strained AlGaN/GaN quantum well laser diode

The effects of the quantum well (QW) width, carrier density, and aluminium (Al) concentration in the barrier layers on the optical characteristics of a gallium nitride (GaN)-based QW laser diode are investigated by means of a careful modelling analysis in a wide range of temperatures. The device’s optical gain is calculated by using two different band energy models. The first is based on the simple band-to-band model that accounts for carrier transitions between the first levels of the conduction band and valence band, whereas the second assumes the perturbation theory (k.p model) for considering the valence intersubband transitions and the relative absorption losses in the QW. The results reveal that the optical gain increases with increasing the n-type doping density as well as the Al molar fraction of the AlxGa1–xN layers, which originate the GaN compressive-strained QW. In particular, a significant optical gain on the order of 5000 cm–1 is calculated for a QW width of 40 Å at room temperature. In addition, the laser threshold current density is of few tens of A/cm2 at low temperatures.

Excitons in Cu2O : From quantum dots to bulk crystals and additional boundary conditions for Rydberg exciton-polaritons

We propose schemes for calculation of optical functions of a semiconductor with Rydberg excitons for a wide interval of dimensions. We have started with a zero-dimensional structure (quantum dot), then going to one-dimensional (quantum wire), two-dimensional (quantum wells and wide quantum wells), and finally three-dimensional bulk crystals; our analytical findings are illustrated numerically, showing an agreement with available experimental data. Calculations including exciton-polaritons are performed; the case of a large number of polariton branches is discussed, and obtained theoretical absorption spectra show good agreement with experimental data.

Strongly localized carriers in Al-rich AlGaN/AlN single quantum wells grown on sapphire substrates

Carrier dynamics in AlGaN-based single quantum well (QW) structures grown on sapphire are studied by means of time-integrated and time-resolved photoluminescence spectroscopy (PL) in a wide temperature range from 5 K to 350 K. The samples cover a broad compositional range, with aluminum contents ranging between 42% and 60% and QW widths between 1.5 nm and 2.5 nm. All samples reveal the characteristic “S”-shape temperature dependence of the PL emission energy as frequently reported in InGaN-based systems, albeit with significantly larger localization strengths of up to 60 meV. It is shown that in the compositional range investigated, carrier localization is determined primarily by the QW width and, in contrast, exhibits a much weaker dependence on aluminum concentration. By the combination of time-integrated and time-resolved PL measurements, the localization of carriers is demonstrated to have a significant impact on the recombination dynamics of AlGaN/AlN QWs grown on sapphire, heavily affecting the internal quantum efficiency and efficiency droop even in standard LED operation conditions.

Optically active energy states of the exciton in quantum wells of various widths

The optically active energy states of the exciton in GaAs/Al0.3Ga0.7As quantum wells of various widths are calculated by the direct solution of the three-dimensional Schrodinger equation. A dependence of energies on the quantum well width as a parameter is obtained for widths up to 200 nm. The results are compared with the model of quantization of the exciton as a whole in wide quantum wells. The radiative decay rates for several lowest exciton states are also found.

Intersubband and intrasubband electron scattering by acoustic-phonons in quantum wells

We study the intersubband and intrasubband electron scattering by acoustic-phonons in quantum wells by comparing the cyclotron-resonance (CR) full-width at half-maximum (FWHM) when without intersubband transitions with that when having intersubband transitions in quantum-well structure due to electrons-acoustic phonons scattering. Using operator projection technique, the expressions for CR magneto-optical absorption powers (MOAPs) are obtained. Using graphical and numerical method, we obtain the dependence of the MOAPs on the photon energy, temperature, quantum-well width, and external magnetic field. We obtain the FWHM as profile of the curve based on the profile technique. The FWHM is calculated for both when without intersubband transitions and when having intersubband transitions. The influence of the temperature, external magnetic field, and well width on the FWHM have been considered and discussed. The results for the CR-peak's FWHM in the case without intersubband transitions are compared with those in the case having inter-subband transitions. In addition, the contribution of acoustic phonons in the range of low- and high-temperatures have also been considered and compared.