Quantum-well Structures Research Articles

Strain engineering of transverse electric and transverse magnetic mode of material gain in GeSn/SiGeSn quantum wells

8-band k · p Hamiltonian together with envelope function approximation and planewave expansion method are applied to calculate the electronic band structure and material gain for Ge1−wSnw/SiyGe1−x−ySnx/Ge1−wSnw quantum wells (QWs) grown on virtual Ge1-zSnz substrates integrated with Si platform. It is clearly shown how both the emission wavelength in this material system can be controlled by the content of virtual substrate and the polarization of emitted light can be controlled via the built-in strain. In order to systematically demonstrate these possibilities, the transverse electric (TE) and transverse magnetic (TM) modes of material gain, and hence the polarization degree, are calculated for Ge1−wSnw/SiyGe1−x−ySnx/Ge1−wSnw (QWs) with the strain varying from tensile (ε = +1.5%) to compressive (ε = −0.9%). It has been predicted that the polarization can be changed from 100% TE to 80% TM. In addition, it has been shown that SiyGe1−x−ySnx barriers, lattice matched to the virtual Ge1-zSnz substrate (condition: y = 3.66(x-z)), may ensure a respectable quantum confinement for electrons and holes in this system. With such material features Ge1−wSnw/SiyGe1−x−ySnx/Ge1−wSnw QW structure unified with Ge1-zSnz/Si platform may be considered as a very prospective one for light polarization engineering.

Zero-Dimensional Lead-Free Hybrid Perovskite-like Material with a Quantum-Well Structure

Low-dimensional perovskites are an emerging class of materials with high stability and excellent optoelectronic properties. Herein, we introduce a novel, lead-free, zero-dimensional perovskite-like material, (1,3-propanediammonium)2Bi2I10·2H2O, for optoelectronic applications. This material exhibited good moisture and thermal stability under ambient conditions. Single-crystal X-ray diffraction analysis revealed a quantum-well structure having the inorganic Bi2I104– clusters periodically arranged in the crystallographic “c” axis separated by a distance of 5.36 A, sandwiched by independent layers of organic cations. The density functional theory calculations showed that the oxygen in water molecules has a significant contribution to the band edges of the material. The photodetector device fabricated using this material showed an efficient charge separation at low voltage (1 V) due to the good electronic conduction between the Bi2I104– dimer units.

Deep-UV emission at 260 nm from MBE-grown AlGaN/AlN quantum-well structures

Deep ultraviolet (UV) emission at 260 nm has been obtained from high Al content AlGaN/AlN multi-quantum well (MQW) structures by plasma-assisted molecular beam epitaxy (PA-MBE) on AlN template on sapphire substrates. The effect of III/V ratio and growth temperature on structural and optical properties of AlGaN was studied. It was observed that Al content in AlGaN material increased with increasing Al flux ratio or growth temperature. By altering growth conditions, AlGaN with high Al content of 47–80% was obtained. Further studies indicate that AlGaN layers have excellent surface morphology and optical properties grown in slightly metal-rich condition. As for AlGaN/AlN MQW, the emission wavelength was tuned by controlling the thickness of AlGaN quantum well, and deep ultraviolet emission below up to 260 nm was obtained from AlGaN (2.4 nm)/AlN(6.9 nm) MQW at room temperature.

High-Power 810-nm Passively Mode-Locked Laser Diode With Al-Free Active Region

We report on a mode locked semiconductor laser for high-power pulse generation in the 810-nm waveband. This is based on a novel GaAsP/GaInP single quantum-well structure, with Aluminum-free active region. Average output powers higher than 40 mW are reported in narrow ridge two-section devices. A 1.65-mm-long laser with 70 μm saturable absorber yields a stable mode locked regime at a 23-GHz repetition rate as evidenced by a record low RF linewidth of 75 kHz over a wide range of gain currents. Other laser dynamics are also reported at lower frequencies.

Exciton spectroscopy of optical reflection from wide quantum wells

Optical spectroscopy of resonant reflection has been used for both experimental and theoretical studies of the exciton-light interaction in wide, high-quality quantum-well structures with the widths exceeding the exciton Bohr radius by an order of magnitude or more. The light mixes the low-lying confined exciton states which are captured by the generalized model developed by M. M. Voronov et al. [Phys. Solid State 49, 1792 (2007)]. We demonstrate that the high-energy confined states in the wide QWs can still be described by the standard model in which the amplitude reflection coefficient from the QW is a sum of individual size-quantized exciton resonances. The excitonic parameters extracted from fitting the experimental spectrum to the standard model agree with those obtained by the numerical solution of the two-particle Schr\odinger equation in a rectangular quantum well. The measured and microscopically calculated spectra are compared with those found with the widely used model of the center-of-mass exciton quantization and the polaritonic model. The comparison shows that the two approximate models considerably underestimate the interaction of confined excitons with light because they ignore the strong modification of the exciton wave function near the QW interfaces.

Theoretical study of optical properties of non-polar BAlGaN/AlN quantum wells lattice-matched to AlN

Light emission characteristics of non-polar ultraviolet (UV) BxAlyGa1−x−yN/AlN quantum well (QW) structures lattice-matched to AlN substrate were theoretically investigated using the multiband effective-mass theory. The peak intensity of the non-polar BAlGaN/AlN QW structure lattice-matched to AlN is found to be much larger than that of the c-plane BAlGaN/BAlGaN QW structure lattice-matched to AlN. Also, the lattice-matched BAlGaN/AlN QW structure shows a large polarization ratio and its value changes from 0.7 to 0.9 in an investigated range of carrier density. We expect that non-polar BAlGaN/BAlGaN QW structures lattice-matched to AlN could be used as a UV light source with a higher crystal quality.

Extraordinary magnetic field effects mediated by spin-pair interaction and electron mobility in thermally activated delayed fluorescence-based OLEDs with quantum-well structure

We fabricated quantum-well organic light-emitting diodes (QW-OLEDs) based on thermally activated delayed fluorescence (TADF) and measured their magnetic field effects curves over various magnetic field ranges.

Strain-optoelectronic coupling properties of externally deformed nanoribbons with embedded quantum well

Strain-engineering provides a critical mechanism for tuning the band structure of quantum wells (QWs). However, due to the inherent rigidity of the semiconductor materials, the strain induced by lattice-mismatch is restricted to be invariable, uniform, thus cannot be subject to direct mechanical loading processes. Therefore, the flexible nanoribbons (NRs) in a variety of configurations offer entirely new research perspectives in modulation of the optoelectronic characteristic of QWs by curvature-induced inhomogeneous strain. In this paper, we propose a strategy to fabricate the QW layer embedded in uniaxially wavy NRs. To thoroughly figure out the internal strain-optoelectronic coupling mechanism, we establish a simple and accurate calculation model for externally deformed QW NRs using the eight-band k·p perturbation method for the first time. Luttinger-Kohn-Pikus-Bir Hamiltonian (LKPBH) for strained semiconductor is adopted and solved by finite-difference method (FDM). Theoretical calculations reveal inclined band edges, which result in the quantum-confined stark effect (QCSE) in the bended QW NRs. Continuously and periodically varied band gap of the wavy QW is revealed with a blue-shift from peak to valley of the sample NR, which agrees well with the μ-photoluminescence measurements. Further adjustments of the external configuration are explored to study the impact of the larger deformed structure on the curvature and band gap of the wavy QW within the confinement of the fracture limit.

Resonant Tunneling Diode by Means of Compound Armchair Boron/Nitride and Graphene Nanoribbons

The band gap of armchair graphene nanoribbons (AGNRs) can be modulated by replacing the carbon atoms with boron/nitride (BN) atoms to produce the compound nanoribbons, while the width of ribbons remains constant. By introducing BN doping atoms in the proper positions along the ribbon length, a double-barrier quantum-well structure is constructed. Consequently, negative differential resistance properties can be obtained by a combination of armchair BN nanoribbons (ABNNRs) and AGNRs as the compound ABNxGyNRs, in which x and y denote the number of BN and C atoms in the ribbon width, respectively. The proposed resonant tunneling diode (RTD), called an armchair BN graphene nanoribbon resonant tunneling diode (ABNGNR-RTD), is investigated in three different platforms including W, S, and H shapes. The numerical tight-binding model along with non-equilibrium Green’s function formalism is taken into account to study the electronic properties of the proposed RTD. The performance of the ABNGNR-RTD is examined in terms of device characteristics such as peak-to-valley ratio (PVR) and power dissipation. Based on the presented results, the performance of H-shaped devices is better than those of the other two cases in terms of PVR and power dissipation. In addition, the electronic properties of ABNGNR-RTDs can be modified by varying the relative width of ABNNRs with respect to AGNRs.

Strong suppression of In desorption from InGaN QW by improved technology of upper InGaN/GaN QW interface

The aim of this work is to elucidate how different growth mode and composition of barriers can influence the QW properties and their PL and to find optimal QW capping process, to suppress the In desorption from QWs and to maintain the QW PL efficiency. It concentrates on the technology procedure for growth of upper quantum well (QW) interfaces in InGaN/GaN QW structure when different temperature for QW and barrier epitaxy is used. We have found that optimal photoluminescence (PL) results were achieved, when the growth after QW formation was not interrupted, but immediately continued during the temperature ramp by the growth of (In)GaN capping layer with small introduction of In precursor into the reactor. Optimal barrier between QW with respect to PL results was found to be pure GaN. We have shown according to SIMS and HRTEM results that by this technological procedure the InGaN desorption was considerably suppressed and three times higher In concentration and two times thicker QWs were achieved for the same QW growth parameters without deterioration of PL intensity in comparison to sample with usually used thin GaN low temperature capping protection. Additionally, for samples covered by the QW capping layer during the temperature ramp the defect band is almost completely missing, thus we can conclude that this defect band is connected with quality of the upper QW interface.

Luminescence of HgCdTe– and InAsSb–based quantum–well heterostructures

Photoluminescence of single quantum-well (SQW) HgCdTe-based structures and electroluminescence of multi quantum-well (MQW) InAsSb-based structures were studied. SQW HgCdTe-based heterostructures were grown by molecular beam epitaxy, while MQW-based InAsSb heterostructures were fabricated by metal-organic chemical-vapour deposition. The specifics of the electronic structure of the materials used is considered in relation to the possibility of observation of optical transitions between the quantization levels of electrons and first and/or second heavy and light hole levels.

Experimental Study of Spontaneous Emission in the Bragg Multiple Quantum Wells Structure of InAs Monolayers Embedded in a GaAs Matrix

Time-resolved photoluminescence of a Bragg structure of InAs-monolayer quantum wells in GaAs matrix was experimentally studied with. Comparison of luminescence patterns from the side and from the surface of a sample showed that Bragg-type ordering of quantum wells leads to a substantial alteration of the photoluminescence spectra including appearance of additional radiative modes. The sample side spectrum contains a single line corresponding to a ground state of an exciton. The surface spectrum at high excitation levels a new radiation line appears whose frequency and propagation angle correspond to the Bragg condition for quantum wells. A numerical calculation of the modal Purcell factor explains why the radiative emission amplification occurs only at a set of specific angles and frequencies, as opposed to the whole range that satisfies the Bragg condition.

On the Application of Strain-Compensating GaAsP Layers for the Growth of InGaAs/GaAs Quantum-Well Laser Heterostructures Emitting at Wavelengths above 1100 nm on Artificial Ge/Si Substrates

Stressed InGaAs/GaAs quantum-well laser structures are grown by gas-phase epitaxy from organometallic compounds on GaAs substrates and artificial Ge/Si substrates based on Si(001) with an epitaxial metamorphic Ge layer. To suppress the relaxation of elastic stresses during the growth of InGaAs quantum wells with a high fraction of In, strain-compensating GaAsP layers are applied. The structural and radiative properties of the samples grown on substrates of various types are compared. Stimulated radiation at wavelengths up to 1.24 μm at 300 K is obtained for structures grown on GaAs substrates and at wavelengths up to 1.1 μm at 77 K, for structures grown on Ge/Si substrates.

General way to obtain multiple defect modes in multiple photonic quantum-well structures.

We introduce a method to obtain and describe the multiple defect modes in the multiple photonic quantum-well structure [(AB)mC]n(AB)m. It is found that the relationship between structural parameters and defect modes can be obtained by analytically solving the dispersion relationship for the photonic crystal with its periodic unit [(AB)mC], and a laconic and unifying expression can be used to describe the energy levels of both the interface modes and defect modes. The internal mechanism to determine the related parameters is revealed. We believe that these findings can be used to provide direct guidance for practical application.

P-type wurtzite GaN/AlGaN quantum well structures for normal incidence inter-subband photodetectors at 1.55 µm

The inter-subband (ISB) absorption of wurtzite (WZ) p-type GaN/AlGaN quantum well (QW) structures was theoretically investigated as a function of Al content in the barrier by using the multiband effective mass theory. The GaN/AlGaN QW structure with x = 0.7 shows the peak wavelength of the TE-polarized absorption spectrum at 1.55 µm. The peak intensity of the TE-polarized absorption spectrum is much higher than that of the TM-polarized absorption spectrum. We expect that a p-type WZ GaN/AlGaN heterostructure with a relatively low Al content (x = 0.7) will be attractive for a normal incidence photodetector application for fiber-optic communications because the free hole concentration of about 1 × 1017 cm−3 is expected to be possible on the experimental side.

Study of direct bandgap type-I GeSn/GeSn double quantum well with improved carrier confinement

The GeSn-based quantum wells (QWs) have been investigated recently for the development of efficient GeSn emitters. Although our previous study indicated that the direct bandgap well with type-I band alignment was achieved, the demonstrated QW still has insufficient carrier confinement. In this work, we report the systematic study of light emission from the Ge0.91Sn0.09/Ge0.85Sn0.15/Ge0.91Sn0.09 double QW structure. Two double QW samples, with the thicknesses of Ge0.85Sn0.15 well of 6 and 19 nm, were investigated. Band structure calculations revealed that both samples feature type-I band alignment. Compared with our previous study, by increasing the Sn composition in GeSn barrier and well, the QW layer featured increased energy separation between the indirect and direct bandgaps towards a better direct gap semiconductor. Moreover, the thicker well sample exhibited improved carrier confinement compared to the thinner well sample due to lowered first quantized energy level in the Γ valley. To identify the optical transition characteristics, photoluminescence (PL) study using three pump lasers with different penetration depths and photon energies was performed. The PL spectra confirmed the direct bandgap well feature and the improved carrier confinement, as significantly enhanced QW emission from the thicker well sample was observed.

Two-dimensional type-II Dirac fermions in a LaAlO3/LaNiO3/LaAlO3 quantum well

The type-II Dirac fermions that are characterized by a tilted Dirac cone and anisotropic magneto-transport properties have been recently proposed theoretically and confirmed experimentally. Here, we predict the emergence of two-dimensional type-II Dirac fermions in LaAlO3/LaNiO3/LaAlO3 quantum-well structures. Using first-principles calculations and model analysis, we show that the Dirac points are formed at the crossing between the dx2-y2 and dz2 bands protected by the mirror symmetry. The energy position of the Dirac points can be tuned to appear at the Fermi energy by changing the quantum-well width. For the quantum-well structure with a two-unit cell thick LaNiO3 layer, we predict the coexistence of the type-II Dirac points and the Dirac nodal line. The results are analyzed and interpreted using a tight-binding model and symmetry arguments. Our findings offer a practical way to realize the 2D type-II Dirac fermions in oxide heterostructures.

Mg x Zn1–x O/ZnO Quantum Well Photodetectors Fabricated by Radio-Frequency Magnetron Sputtering

Mg x Zn 1-x O/ZnO quantum well (QW) photodetectors were fabricated by a radio-frequency magnetron sputtering system. The QW structure largely reduced the leakage current, from 2.6 × 10 -7 to 1.0 × 10 -9 A, by a factor of 260. The reduced leakage current enhanced the photo-to-dark current ratio from 1.35 × 10 3 to 4.25 × 10 4 , a 32-fold increase for the photodetectors with the QW structure. The 340 nm/560 nm rejection ratio was increased by a magnitude of 5.6 compared to that of photodetectors without QW structure because the electron-hole pairs generated by low-energy photons were confined in the Mg x Zn 1-x O/ZnO QW.

High hole mobility in strained In0.25Ga0.75Sb quantum well with high quality Al0.95Ga0.05Sb buffer layer

We have demonstrated high hole mobility in strained In0.25Ga0.75Sb quantum well (QW) structure with a high quality Al0.95Ga0.05Sb buffer layer for future single channel complementary metal-oxide-semiconductor circuits. The Al0.95Ga0.05Sb buffer layer is important to achieve low substrate leakage and guarantee good channel material quality and high hole mobility. We grew buffer layers with various Sb effective flux conditions using molecular beam epitaxy to obtain high crystal quality and proper electrical properties. We systematically evaluated the relationship between the crystal quality and electrical properties using X-ray diffraction, atomic force microscope, Raman, and the Hall effect measurement system. Then, on this optimized buffer layer, we grew the In0.2Al0.8Sb/In0.25Ga0.75Sb/linear-graded Al0.8Ga0.2Sb QW structure to obtain high hole mobility with compressive strain. Moreover, the compressive strain and hole mobility were measured by Raman and Hall effect measurement system. The results show a compressive strain value of 1.1% in In0.25Ga0.75Sb QW channel, which is very close to the theoretical value of 1.1% from lattice mismatch, exhibiting the highest hole mobility of 1170 cm2/V s among reported mobility in In0.25Ga0.75Sb QW. Furthermore, it was able to be fabricated as p-type Fin-FET and shown the excellent electrical characteristics with low Smin and high gm.

Design of InGaN-ZnSnN2 quantum wells for high-efficiency amber light emitting diodes

InGaN-ZnSnN2 based quantum wells (QWs) structure is proposed and studied as an active region for high efficiency amber (λ ∼ 600 nm) light emitting diodes (LEDs), which remains a great challenge in pure InGaN based LEDs. In the proposed InGaN-ZnSnN2 QW heterostructure, the thin ZnSnN2 layer serves as a confinement layer for the hole wavefunction utilizing the large band offset at the InGaN-ZnSnN2 interface in the valence band. The barrier layer is composed of GaN or AlGaN/GaN in which the thin AlGaN layer is used for a better confinement of the electron wavefunction in the conduction band. Utilizing the properties of band offsets between ZnSnN2 and InGaN, the design of InGaN-ZnSnN2 QW allows us to use much lower In-content (∼10%) to reach peak emission wavelength at 600 nm, which is unachievable in conventional InGaN QW LEDs. Furthermore, the electron-hole wavefunction overlap (Γe-h) for the InGaN-ZnSnN2 QW design is significantly increased to 60% vs. 8% from that of the conventional InGaN QW emitting at the same wavelength. The tremendous enhancement in electron-hole wavefunction overlap results in ∼225× increase in the spontaneous emission radiative recombination rate of the proposed QW as compared to that of the conventional one using much higher In-content. The InGaN-ZnSnN2 QW structure design provides a promising route to achieve high efficiency amber LEDs.