Thick Quantum Wells Research Articles

Experimental and Theoretical Examination of the Photosensitivity Spectra of Structures with In0.4Ga0.6As Quantum Well-Dots of the Optical Range (900–1050 nm)

Solar cells based on a new type of nanostructure (quantum well-dots) have been studied. These quantum well-dots (QWDs) are In0.4Ga0.6As layers with marked composition and thickness modulations. The energies of the observed optical transitions in QWDs turn out to be close to the energies of transitions involving a light hole and a heavy one in a two-dimensional quantum well of the same nominal thickness and composition. The ground-state peak is characterized by strong TE polarization (>70%), while the shorter-wavelength peak is almost unpolarized (<10%).

Effect of indium accumulation on the growth and properties of ultrathin In(Ga)N/GaN quantum wells

In this work, we present the investigation of InN/GaN multiple-quantum-well (MQW) growth by plasma-assisted molecular beam epitaxy using in-situ reflection high-energy electron diffraction (RHEED) to monitor the growth process. The analysis of the RHEED intensity and pattern transitions identified an indium surface accumulation even with a nominal thickness of InN as small as 0.5 monolayer (ML). This result explicitly shows that, even at low growth temperatures of ~550 °C, not all of the supplied indium is incorporated into the quantum well (QW). Moreover, the residual indium can become incorporated into the GaN matrix on either side of the QW. Both QW thickness and the photoluminescence (PL) emission energy showed a self-regulating behavior. The apparent thickness did not exceed 2 MLs even when the deposited InN thickness is as large as 5 MLs. The PL emission shows a continuous redshift with the deposited InN from ~370 nm for 0.5 ML until it saturates at ~423 nm for >2 ML. Based on the observed growth phenomena, a qualitative growth model was developed to explain the self-limited epitaxial growth of ultrathin In(Ga)N/GaN QWs.

Quantizing nanolaminates as versatile materials for optical interference coatings.

In this paper, the theoretical foundation of quantizing nanolaminates is explained, and the dependence of the optical band gap on quantum-well thickness is demonstrated. The production is investigated by applying molecular dynamics growth simulation and by correlating the results with layers deposited by ion beam sputtering and atomic layer deposition. The properties of manufactured nanolaminates are then compared to the theoretical behavior, and good agreement is found.

Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites

Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm2 s−1, which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.

Экспериментальное и теоретическое исследование спектров фоточувствительности структур с квантовыми ямами-точками In-=SUB=-0.4-=/SUB=-Ga-=SUB=-0.6-=/SUB=-As оптического диапазона 900-1050 nm

Investigation of photovoltaic converters based on a novel type of nanostructures referred to as quantum-well dots (QWD) which represents In0.4Ga0.6As layer with pronounced modulation of composition and thickness was carried out. The energies of the optical transitions in QWD were found to be very close light and heavy hole based transitions in the quantum well of similar composition and thickness. QWD ground state peak was found to be strongly TE polarized (&gt;70%), whereas the short-wavelength peak is nearly unpolarized (&lt;10%).

Effects of quantum well thickness and aluminum content of electron blocking layer on InGaN-based laser diodes

The effects of quantum well (QW) thickness and aluminum content of electron blocking layer (EBL) on device performance of InGaN-based laser diodes (LDs) are numerically investigated with LASTIP. It is found that the device performance of the 3.0-nm-thick QW LD is the best. The threshold current increases and the output power at 120 mA decreases when the QW thickness is too thin or too thick. Actually, the optical and electrical characteristics of InGaN-based LDs demonstrate that the optical confinement factor decreases and optical loss increases when the QW thickness is too thin. The stimulated recombination rate decreases due to the poorer overlap of electron–hole wave functions and the enhanced polarization-induced built-in electric field when the well thickness is too thick. Moreover, the calculation results of LDs with different aluminum compositions of EBL demonstrate that the effectiveness of EBL would be enhanced through increasing aluminum content when the thickness of QWs decreases, because there is a reduction of ground-state energy level and the energy difference between the ground state and the top of the quantum barrier.

Compositional Control in 2D Perovskites with Alternating Cations in the Interlayer Space for Photovoltaics with Efficiency over 18.

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge-transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA)n Pbn I3 n +1 (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low-n and high-n QWs and suppressed nonradiative charge recombination. Therefore, the additive-treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open-circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden-Popper, and Dion-Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

Synthetic Variation and Structural Trends in Layered Two-Dimensional Alkylammonium Lead Halide Perovskites

We report the cooling-induced crystallization of layered two-dimensional (2D) lead halide perovskites with controllable inorganic quantum-well thicknesses (n = 1, 2, 3, and 4), organic-spacer chain...

Sidewall Quantum Wires on GaAs (001) Substrates

We study the structural, optical, and transport properties of sidewall quantum wires on GaAs(001) substrates. The QWRs are grown by molecular beam epitaxy (MBE) on GaAs(001) substrates prepatterned with shallow ridges. They form as a consequence of material accumulation on the sidewalls of the ridges during the overgrowth of a quantum well (QW) on the patterned surface. The QWRs are approximately 200 nm-wide and have emission energies red-shifted by 27meV with respect to the surrounding QW. Spatially resolved spectroscopic photoluminencence studies indicate that the QW thickness reduces around the QWRs, thus creating a 4 meV energy barrier for the transfer of carriers from the QW to the QWR. We show that the QWRs act as efficient channels for the transport of optically excited electrons and holes over tens of {\mu}m by a high-frequency surface acoustic wave (SAW). These results demonstrate the feasibility of efficient ambipolar transport in QWRs with sub-micrometer dimensions, photolithographically defined on GaAs substrates.

Optical properties of III-nitride laser diodes with wide InGaN quantum wells

In this paper we show that, despite a high piezoelectric field, a wide InGaN quantum well (QW) can be more effective as the active region of laser diodes (LDs) than the thin ones usually used. The optical gain in the LDs with a single wide QW is studied. It is shown that the differential gain in a LD with 10.4 nm QW is higher than in a LD with three 2.6 nm thick QWs. The high optical gain in a wide QW is interpreted as originating from transitions through excited states. Additionally, a substantial difference in lasing spectra between LDs with wide and thin QWs is found.

Detection of optically injected spin polarized electrons in the L-valley of AlGaAs through polarization resolved photoluminescence excitation spectroscopy

GaAs/AlGaAs multi quantum well (QW) architecture is employed to study the optical injection of spin polarized electrons in Al0.22Ga0.78As material over the excitation energy range of 1.85–3.4 eV. By analyzing the degree of circular polarization (DCP) of photoluminescence from QW, it is seen that though the magnitude of DCP is affected by the QW thickness, its spectral dependence is mainly governed by the barrier layer. In addition to a usual maximum in DCP near Γ-point, a local maximum at 3.2 eV is also observed. This is identified as a signature of optical spin orientation in the L-valley of AlGaAs. The comparison of DCP spectra with that of a ZrO2 coated structure confirms that its origin is deep rooted in the bulk material and is therefore robust against surface treatments. A two level model for the transfer of spin polarized electrons from L to Γ valley is proposed to estimate the spin relaxation time of electrons which is found to be large in the L-valley of AlGaAs when compared with GaAs. This could be an interesting aspect for the fabrication of AlGaAs based spintronic devices. Further the QW architecture proposed here is useful for studying the injection of spin polarized carriers in indirect bandgap semiconductors.

Strain-free GaN/InAlN chirped short-period superlattice electron-blocking layer for 450 nm InGaN laser diode

In this study, we optimized the lattice-matched GaN/In0.18Al0.82N chirped short-period superlattice (C-SPSL) electron-blocking layer for a laser diode emitting at 450 nm. The effective bandgap of C-SPSL depends upon the quantum well (QW) and quantum barrier (QB) thickness of C-SPSL. In this study the In0.18Al0.82N QB thickness is constant at 0.5112 nm (1 unit layer (UL) = 1 lattice constant thickness) and the GaN QW thickness is varied as 1 UL, 3 UL, and 5 UL. The estimated effective bandgap for 15 periods of 1 UL GaN/1 UL In0.18Al0.82N SPSL is ~4.2 eV, for four periods of 3 UL GaN/1 UL In0.18Al0.82N SPSL it is ~3.93 eV and for two periods of 5 UL GaN/1 UL In0.18Al0.82N SPSL it is 3.62 eV. Wave-function hybridization and the built-in electric field play an important role in the bandgap behavior of C-SPSL. The electron leakage decreased from 2534.6 A cm−2 to ~14 A cm−2 while hole transportation improved from 7.7 kA cm−2 to 10 kA cm−2 at 10 kA cm−2 injection current density. The light output power per facet improved from 146 mW to 255 mW. Slope efficiency increased from 0.548 W A−1 to 0.924 W A−1 with the C-SPSL design.

Electron exchange energy of neutral donors inside a quantum well

We calculated the exchange energy of a pair of donor-bound electrons placed in the middle of an infinite quantum well (QW). In order to obtain this energy for any interdonor distance and for any QW thickness, we have first adapted to a QW the method developed by Gor'kov and Pitaevskii [L. P. Gor'kov and L. P. Pitaevskii, Dokl. Akad. Nauk SSSR 151, 822 (1963)] for a three-dimensional (3D) distribution of donors, and calculated the asymptotic form of the exchange energy. Second we have calculated the exchange energy of a ``helium atom'' in a QW; and third, inspired by the interpolation procedure proposed by Ponomarev et al. [I.V. Ponomarev et al., Phys. Rev. B 60, 5485 (1999)], we have obtained an interpolated expression for any interdonor distance. The obtained exchange energy is written in units of effective hartree, and the distance between the donors, as well as the width of the QW, are expressed in units of effective Bohr radius. We calculated the exchange energy for some commonly studied semiconductor materials, and discussed also the relationship between the exchange energy and the spin relaxation time for a donor concentration close to the insulator-metal transition.

Phosphor-free white emission from InGaN quantum wells grown on in situ formed submicron-scale multifaceted GaN stripes

We demonstrate in situ self-organization of multifaceted GaN stripes on m-sapphire substrates. The GaN stripes aligned along <1¯1¯23> with submicron-scale {112¯2}, {101¯1}, {202¯1}, {112¯4}, and {0001} facets are identified and employed as growth templates of InGaN quantum wells (QWs). Under identical growth conditions the emission spectrum of the InGaN QWs is determined by the GaN stripe shaping and atomic structures of the stripe surface. For instance, the growth rate (thickness) of the InGaN QWs is proportional to the layer spacing and dangling bond density of specific growth face and plays a key role in emission wavelength. Accordingly, using the stripe templates the InGaN QWs on different stripe facets have different emission wavelengths. Broad photoluminescence peak covering ultraviolet, blue, green, yellow, amber, and red emissions is observed for the InGaN QWs grown on various sidewall facets of the stripe templates. The chromaticity diagram indicates a cool-daylight white emission with a correlated color temperature of 5997 K.

Intersubband plasmons induced negative refraction at mid-IR frequency in heterostructured semiconductor metamaterials

In this contribution, we report on negative refraction effect occurring in layered semiconductors. The origin of this effect is attributed to the presence of intersubband plasmon resonance induced by the electronic confinement of the electrons in thick quantum wells. Relying on the effective medium theory, we analyse both the in plane and the out of plane effective optical properties of highly doped ZnO/ZnMgO semiconductor material. We theoretically show and experimentally demonstrate that thicknesses and doping levels of each layer can be carefully chosen to feature strong intersubband transitions, leading to type 1 and type 2 hyperbolic response. Understanding the optical properties of intersubband materials in the frame of hyperbolic materials would shed new light on which physical mechanisms are controlling the radiative decay of intersubband plasmon excitations. This approach could be further utilized to designing efficient mid-IR sources.

Multicolored-light emission from InGaN/GaN multiple quantum wells grown by selective-area epitaxy on patterned Si(100) substrates

GaN-based light-emitting diodes (LEDs) structures with microstripes of InGaN/GaN quantum wells were grown on the Si(111) planes of microscale V-grooved Si(100) substrate by metal–organic vapor-phase epitaxy. This microstriped microfacet structure is composed of two semipolar (1 $$ \bar{1} $$ 01) planes and two polar (0001) planes. The indium (In) composition and thickness of quantum wells grown on semipolar (1 $$ \bar{1} $$ 01) plane differ from polar (0001) plane, causing the variation of local emission wavelength along the planes. Using the cathodoluminescence (CL) techniques, it was found that the emission wavelength is longer at the top of the (0002) planes, whereas it is shorter at the bottom of the (0002) plane and the (1 $$ \bar{1} $$ 01) plane. The continuous change in the properties of the quantum wells results in a uniform multicolored emission band. The results show that unique structures could uniformly activate multicolored light to realize the integrated light emission over the most visible spectrum. In some situations, this arrangement can emit white light. This method can be used to produce integrated white-light sources for monolithically integrated white-light-emitting diodes.

Dependence of indium content in monolayer-thick InGaN quantum wells on growth temperature in InxGa1-xN/In0.02Ga0.98N superlattices

We investigate the In content in single monolayer (ML)-thick InxGa1-xN quantum wells (QWs) as a function of the growth temperature ranging from 650 °C to 480 °C, stacked in a superlattice (SL). The SLs were grown by plasma-assisted molecular beam epitaxy using high N-flux. For the evaluation of the indium concentrations, scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) studies were combined with local lattice parameter measurements obtained from high-resolution transmission electron microscopy (HRTEM) images. The mean In content in the QWs increases from 11% to 23% when the growth temperature decreases from 650 °C to 610 °C. Further decrease in the growth temperature results in a saturation of the mean In content. Our experiments show that a substantial reduction of the growth temperature is not a practical way to obtain pseudomorphically grown InN MLs on GaN(0001). The InGaN QW thickness is limited to 1 ML and is not affected by a change of growth temperature. For two SL structures grown at constant temperatures of 640 °C and 600 °C, increase in the In content in the QWs causes a shift in the peak emission from 382 to 395 nm, as was measured by cathodoluminescence at 7 K. The application of X-ray diffraction studies to analyze the composition of InGaN ML-thick QWs in SLs is discussed.

Transmission electron microscopy of Ga(Sb, Bi)/GaSb quantum wells with varying Bi content and quantum well thickness

We analyze the microstructure of a series of Ga(Sb, Bi)/GaSb quantum wells (QW) with varying Bi content and QW thicknesses using transmission electron microscopy. The structures are grown on GaSb(001) substrates by low-temperature molecular beam epitaxy. Although Ga(Sb, Bi) is regarded as a highly-mismatched alloy, the material is of remarkable quality and no extended defects or nanoclusters are detected in the pseudomorphic layers. Regardless their different Bi content (ranging from 4% to 11%) all QW samples seem to be homogeneous in composition and no composition fluctuations are detected. The QW/barriers are nevertheless affected by thickness fluctuations, where the thickness slightly varies from QW to QW but also laterally. Despite the samples exhibit well-defined QW/barrier interfaces, we note that the Ga(Sb, Bi)-on-GaSb interface is rougher than the GaSb-on-Ga(Sb, Bi) one. The morphological smoothing effect at the GaSb-on-Ga(Sb, Bi) interface is attributed to the well-known surfactant behavior of Bi in connection with Bi surface segregation, as evidenced from experimental Bi distribution profiles.

Theory and design of InxGa1−xAs1−yBiy mid-infrared semiconductor lasers: type-I quantum wells for emission beyond 3 μm on InP substrates

We present a theoretical analysis and optimisation of the properties and performance of mid-infrared semiconductor lasers based on the dilute bismide alloy InxGa1−xAs1−yBiy, grown on conventional (001) InP substrates. The ability to independently vary the epitaxial strain and emission wavelength in this quaternary alloy provides significant scope for band structure engineering. Our calculations demonstrate that structures based on compressively strained InxGa1−xAs1−yBiy quantum wells (QWs) can readily achieve emission wavelengths in the 3–5 μm range, and that these QWs have large type-I band offsets. As such, these structures have the potential to overcome a number of limitations commonly associated with this application-rich but technologically challenging wavelength range. By considering laser structures having (i) fixed QW thickness and variable strain, and (ii) fixed strain and variable QW thickness, we quantify key trends in the properties and performance as functions of the alloy composition, structural properties, and emission wavelength, and on this basis identify routes towards the realisation of optimised devices for practical applications. Our analysis suggests that simple laser structures—incorporating compressively strained InxGa1−xAs1−yBiy QWs and unstrained ternary In0.53Ga0.47As barriers—which are compatible with established epitaxial growth, provide a promising route to realising InP-based mid-infrared diode lasers.

Radiative and non-radiative recombination of thermally activated magneto-excitons probed via quasi-simultaneous photoluminescence and surface-photovoltage spectroscopy

The effect of the magnetic field on radiative and non-radiative mechanisms of charge carriers in GaAs/AlGaAs quantum wells (QWs) is investigated via quasi-simultaneous magneto-photoluminescence (PL) and magneto-surface photo-voltage (SPV) spectroscopy. At low-temperature, the luminescence intensity of ultra-low disordered GaAs/AlGaAs QWs generally increases under strong magnetic perturbation. Even at relatively high-temperature (100 K), the magnetic field driven enhancement of PL intensity is observed for thick QWs. On the other hand, it is found that the PL intensity of narrow QWs gradually decreases under a strong magnetic field at 100 K. The magnetic field driven enhancement (suppression) of radiative recombination efficiency for wide (narrow) QWs is investigated by considering the oscillator strength, thermal effects, and carrier re-distribution in energy states. Also, the charge carriers which escape from narrow QWs or are captured by interface defects are probed via magneto-SPV measurements. Radiative recombination and thermionic emission of charge carriers, investigated by magneto-PL and magneto-SPV spectroscopy, provide a clear guideline of the critical QW width which would be essential for magnetic field driven high-temperature operation of advanced emission based-devices.