ZnO/MgZnO Quantum Wells

Exciton binding energies in ZnSSe/MgSSe single quantum wells (SQWs) are calculated to study their exciton properties in detail. The heavy-hole exciton binding energies are larger than the light-hole exciton binding energies in narrow wells because the degree of confinement of the heavy-hole excitons is larger than that of the light-hole excitons in these SQWs. The heavy-hole exciton binding and heavy-hole excitonic transition energies calculated for ZnSSe/MgSSe SQWs are comparable to those measured for ZnSe/MgS quantum wells. [DOI: 10.1380/ejssnt.2011.219]

In this study, we investigated the exciton binding energy and interband transition between the electron and heavy-hole for the single and double quantum wells which have different hyperbolic-type potential functions subject to electric, magnetic, and non-resonant intense laser fields. The results obtained show that the geometric shapes of the structure and the applied external fields are very effective on the electronic and optical properties. In the absence of the external fields, the exciton binding energy is a decreasing function of increasing well sizes except for the strong confinement regime. Therefore, for all applied external fields, the increase in the well widths produces a red-shift at the absorption peak positions. The magnetic field causes an increase in the exciton binding energy and provides a blue-shift of the absorption peak positions corresponding to interband transitions. The effect of the electric field is quite pronounced in the weak confinement regime, it causes localization in opposite directions of the quantum wells of the electron and hole, thereby weakening the Coulomb interaction between them, causing a decrease in exciton binding energy, and a red-shift of the peak positions corresponding to the interband transitions. Generally, an intense laser field causes a decrease in the exciton binding energy and produces a red-shift of the peak positions corresponding to interband transitions.

We have theoretically investigated the 1.55 μm p-type doped GaNAsBi-based double quantum wells (DQWs) using the (16 × 16) BAC model combined with a self-consistent calculation. We have found that the coupling effect becomes more pronounced by reducing barriers width. The optical performance of the structure is enhanced when the DQWs are coupled and doped. Moreover, a p-i-n heterojunction based on GaAs/GaNAsBi/GaAs DQWs designed for infrared photodetection was developed. The optimization of well parameters such as the bismuth composition, the well width, and the doping densities give rise to p-i-n heterojunction emitting at the wavelength 1.55 μm. The computed gain can reach the value 3 104 cm−1. The quantum confined stark effect on the optical properties of studied structures is also discussed.

GaN‐based optoelectronic structures have been successfully applied in the fabrication of blue light emitting diodes (LEDs) and laser diodes [1]. The fabrication of high‐efficient green LEDs remains challenging which is referred to as the “green gap”. In GaN‐based green LEDs, the active region consists of InxGa1‐xN quantum wells (QWs) and GaN barrier layers (BLs) where the emitting wavelength is controlled by theIncontent (x). However,Inincorporation often results in inferior crystal quality due to the miscibility gap and lattice mismatch when theIncontent is increased [2]. Although conventionalc‐plane InxGa1‐xN /GaN QWs have demonstrated the highest crystal quality, the quantum efficiency remains low as a result of the polarization field along thec‐axis which tilts the conduction and valence bands thus reducing the recombination probability (i.e., quantum confined Stark effect, QCSE). Semipolar InxGa1‐xN/GaN structures, on the other hand, offer a good compromise between reduced QCSE and acceptable crystal quality [3].In this study, we investigated the crystal quality of two sets of InxGa1‐xN/GaN structures grown on (11‐22) and (10‐11)‐semipolar planes. The samples were grown on sapphire by using metalorganic vapor phase epitaxy (MOVPE). The weak‐beam dark‐field technique (WBDF) was applied to investigate the distribution of the threading dislocations (TDs). Figure 1 shows the WBDF images of a (11‐22)‐semipolar sample. The GaN growth starts on thec‐plane‐like facets by MOVPE, forming triangular shaped stripes. The stripes subsequently coalesce forming a closed {11‐22}‐semipolar surface. TDs originate from the inclined c‐plane‐like GaN/sapphire interface and propagate inc‐direction and then bend 90° to thea‐directions. Most of the dislocations accumulate around the coalescence area and penetrate the QWs. The bending phenomenon is related to the triangular shape of the original GaN stripes and the SiNxinterlayers deposited during the growth of GaN.By using high‐resolution TEM, we characterized the local structures of the QWs. Figure 2 shows the experimental HRTEM images of a (10‐11)‐semipolar sample acquired from the active region. Structural factors, such as the thickness of the QWs, the interfacial sharpness between the QWs and the BLs and the stacking faults formed with in the active region, can be analyzed with atomic resolution. Stacking faults are observed and confirmed to be the I2intrinsic type. [4]

Quantum-confined stark effect on an exciton in a spherical quantum well of a GaAs cluster embedded in Al xGa 1−xAs

The possibility of pressure‐induced increase of exciton–LO‐phonon coupling in ZnCdSe/ZnSe quantum wells is studied. The ground state binding energies of the heavy hole excitons are calculated using a variational method with consideration of the electron–phonon interaction and the pressure dependence of the parameters. The results show that for quantum wells with intermediate well width, the exciton binding energy and the LO‐phonon energy may coincide in the course of pressure increasing, resulting in the increase of exciton–LO‐phonon coupling. It is also found that among the pressure‐dependent parameters, the influence of the lattice constant is the most important one. The changes of both the effective masses and the dielectric constants have obvious effects on the exciton binding energy, but their influences are counterbalanced.

AlGaN‐based deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) hold great promise for a wide range of applications, including water sterilization, biochemical sensing, and UV curing. However, their efficiency is fundamentally limited by the quantum‐confined Stark effect (QCSE) originating from strong spontaneous and piezoelectric polarization fields in c‐plane quantum wells (QWs). These internal fields induce pronounced band bending and spatial separation of electron and hole wavefunctions. In this work, a multiple quantum well (MQW) design with a linearly graded‐aluminum composition is introduced along the [0001] direction of the QWs. The grading suppresses polarization discontinuities at quantum barrier (QB)/quantum well (QW) interfaces, reducing interface‐bound charges, and simultaneously generates distributed bulk polarization charges that create a reverse internal electric field. This counter‐field mitigates QCSE, resulting in flattened band profiles and improved wavefunction overlap. Technology computer‐aided design (TCAD) simulations using the SILVACO Advanced Technology Large‐Area Simulator (ATLAS) platform show an increase in electron–hole overlap from 92.45% to 98.26%, a 44.16% enhancement in light output power, a 63.6% reduction in internal electric field, and a 0.7 V decrease in forward voltage. The novelty of this work lies in applying in‐well linear compositional grading as a polarization‐field engineering technique for AlGaN QWs, enabling simultaneous enhancement of optical and electrical performance.In this work, a practical, scalable, and simulation‐verified pathway toward high‐efficiency AlGaN‐based DUV LEDs is reported.

ABSTRACTEdge emission characteristics of optically pumped GaN-AlGaN double heterostructures and quantum wells are examined. The samples, which were grown by metalorganic vapor phase epitaxy, are photoexcited with light from a pulsed nitrogen laser. The pump light is focused to a narrow stripe on the sample surface, oriented perpendicular to a cleaved edge, and the edge luminescence is collected and analyzed. We first compare emission characteristics of highly excited GaN-AlGaN double heterostructures grown simultaneously on SiC and sapphire substrates. Polarization resolved spectral properties of edge luminescence from both structures is studied as a function of pump intensity and excitation stripe length. Characteristics indicative of stimulated emission are observed, particularly in the sample grown on SiC. We then present results demonstrating laser emission from a GaN-AlGaN separate-confinement quantum-well heterostructure. At high pump intensities, band edge emission from the quantum well exhibits five narrow (∼1 Å) modes which are evenly spaced by 10Å to within the resolution of the spectrometer. This represents the first demonstration of laser action in a GaN-based quantum-well structure.

The quantum-confined Stark effect (QCSE) is an established optical modulation mechanism, yet top-performing modulators harnessing it rely on costly fabrication processes. Here, we present large modulation amplitudes for solution-processed layered hybrid perovskites and a modulation mechanism related to the orientational polarizability of dipolar cations confined within these self-assembled quantum wells. We report an anomalous (blue-shifting) QCSE for layers that contain methylammonium cations, in contrast with cesium-containing layers that show normal (red-shifting) behavior. We attribute the blue-shifts to an extraordinary diminution in the exciton binding energy that arises from an augmented separation of the electron and hole wavefunctions caused by the orientational response of the dipolar cations. The absorption coefficient changes, realized by either the red- or blue-shifts, are the strongest among solution-processed materials at room temperature and are comparable to those exhibited in the highest-performing epitaxial compound semiconductor heterostructures.

Exciton binding energy in semiconductor quantum wells and in heterostructures

We investigate photo-luminescence (PL) and cathodo-luminescence (CL) spectra of InAsP/InP quantum well (QW) structures under exposure to low energy ions, and more generally to the particles generated in a plasma reactor typically used for etching such materials. The samples contain QWs of constant thickness (7 to 8 nm) and graded As/P concentration such that each QW produces, at low temperature, a PL line at a well-defined and resolved energy. Adjusting the depth of the QWs below the sample surface, the PL/CL of these QWs is thus the signature of the depth-dependent interaction with the ions and particles from the plasma. We have exposed our QW structures to different kinds of plasmas, containing variable mixtures of halogen-based molecules / hydrocarbons / nitrogen and Ar. Previous studies [1], in particular based on a similar approach of using the QW as an internal probe of the ion – material interaction, have shown that atomic ions with energies in the range of 100 to a few 100’s eV penetrate III-V materials (like GaAs or InP) through a channeling process [2], even if their direction of impingement is not favorable for channeling. The angular dispersion is in general sufficient to produce channeling. The ions channel a certain distance into the crystal, and then stop due to de-channeling mechanisms. As a consequence, a quenching of the PL for the QWs in the region where the de-channeling occurred takes place. We extended the analysis looking not only at the intensity of the PL lines, but also at their spectral characteristics upon exposure to the ion fluxes. Fig. 1 illustrates a selection of the effects that we were able to identify and explain [3]. On fig. 1-a, a sample was exposed to the particles flux from a plasma discharge in SiCl4/H2/Ar. Under the conditions of this plasma discharge, etching actually occurs at a rate ≈ 250-300 nm/min., and we had to be careful not to etch the whole QW structure immediately. Fig. 1-a shows that: - the signal for the two QWs at the lower wavelength seems to disappear. These QWs are the ones closest to the sample surface - the emission bands from the other QWs exhibit energy shifts Transmission electron microscopy and secondary ion mass spectrometry (SIMS) show that actually the two QWs closest to the top surface have not been etched. SIMS shows the penetration of Cl-species below the surface, which probably play a role in the quenching of the PL for these QWs The sample on fig. 1-a was undoped. On fig. 1-b, the same experiment is conducted for a sample with the same QW sequence as that of fig. 1-a, but n-type doped such that a large built-in electric field is created across the QWs region. One can see that this changes completely the shape of the PL lines. The reason for this change can be attributed to the quantum confined Stark effect (QCSE), as shown in [4]. Interestingly, as seen on fig. 1-b, the effect of the interaction with the etching ions is significantly more pronounced than for the undoped sample. We have shown that this is caused by a specific channeling interaction involving ions (probably Cl- ions) which strongly screen the QCSE. Using plasmas containing N2 (which effect is to reduce the etch rate), very significant blue shifts of the PL lines are seen (up to more than 7 nm). These were associated with the incorporation of N deep in the sample, occupying interstitial positions, thus creating local compressive lattice deformation that leads to the blue shifts [3]. All these effects will be discussed in the context of a model that we established to describe the modifications of the luminescence properties of the QW structures under the effect of low-energy ion bombardment. We will also describe experiments on samples where we take benefit of the plasma etching to define nanoscale structures and examine the spatial variation of the PL/CL spectra after interaction with the ion beams. [1] D. L. Green, E. L. Hu, P. M. Petroff, V. Liberman, M. Nooney, and R. Martin, J. Vac. Sci. Technol., B 11, 2249 (1993). [2] R. Germann, A. Forchel, M. Bresch, and H. P. Meier, J. Vac. Sci. Technol., B 7, 1475 (1989). [3] J.P. Landesman, J. Jiménez, C. Levallois, F. Pommereau, C. Frigeri, A. Torres, Y. Léger, A. Beck, and A. Rhallabi, J. Vac. Sci. Technol., A 34, 041304-1 (2016). [4] L. Viña, E. E. Mendez, W. I. Wang, L. L. Chang and L. Esaki, J. Phys. C: Solid State 20, 2803 (1987). Figure 1

We report on the properties of nonpolar a-plane (Zn,Mg)O/ZnO quantum wells (QW) grown by molecular beam epitaxy on r plane sapphire and a plane ZnO substrates. For the QWs grown on sapphire, the anisotropy of the lattice parameters of the (Zn,Mg)O barrier gives rise to an unusual in-plane strain state in the ZnO QWs, which induces a strong blue-shift of the excitonic transitions, in addition to the confinement effects. We observe this blue-shift in photoluminescence excitation experiments. The photoluminescence excitation energies of the QWs are satisfactorily simulated when taking into account the variation of the exciton binding energy with the QW width and the residual anisotropic strain. Then we compare the photoluminescence properties of homoepitaxial QWs grown on ZnO bulk substrate and heteroepitaxial QWs grown on sapphire. We show that the reduction of structural defects and the improvement of surface morphology are correlated with a strong enhancement of the photoluminescence properties: reduction of full width at half maximum, strong increase of the luminescence intensities. The comparison convincingly demonstrates the interest of homoepitaxial nonpolar QWs for bright UV emission applications.

Binding energy and diamagnetic susceptibility of a donor in a GaAs/Al0.3Ga0.7As Double Quantum Well (DQW) for various pressures, impurity locations and well width have been computed numerically using variational technique. The non-parabolicity of the conduction band of the well and barrier materials have been included through effective mass. Since it is a function of subband energy it largely affects the binding energy. Effect of Γ-X crossover due to applied pressure is also considered. Stress applied to the DQW will reduce the barrier height and this greatly enhances the localization of the donor. The binding energy and diamagnetic susceptibility of both non-parabolic and parabolic cases have been compared. The influence of band crossover on the behavior of binding energy and diamagnetic susceptibility at the critical pressures has been critically analysed.

The multiple quantum wells (MQW) Mg0.27Zn0.73O/ZnO have been grown by the pulsed laser deposition method with different well widths Lw. The interface roughness of quantum wells was inherited from the bottom one and did not exceed 1 nm. We observed the quantum confinement effect showing up in the blue shift of the exciton peak in the low temperature (8 K) photoluminescence and absorption spectra at well width reduction. The exciton binding energy of the two-dimensional structures Mg0.27Zn0.73O/ZnO was two times higher in comparison with the bulk ZnO. It has been established that Einstein’s characteristic temperature ΘE sharp increase with reduction of well width Lw up to Lw=2.6 nm. It has been revealed that the discontinuity ratio of conduction and valence bands in the heterostructure Mg0.27Zn0.73O/ZnO is 0.65/0.35. We demonstrated the abrupt increase in quantum efficiency at a reduction of the well width that allowed us to observe the optically excited stimulated emission in ZnO quantum wells with the excitation threshold of ∼210 kW/cm2.

Investigation of defect structures in InGaN/GaN multiple quantum wells grown on Si(1 1 1) substrate by metalorganic vapor phase epitaxy