Single Quantum Wells Research Articles

Electronic transport in strained p-modulation Be-doped Ga0.8In0.2As/GaAs single quantum well

We report on the electronic transport properties of p-modulation Be-doped Ga0.8In0.2As/GaAs single quantum well. The experiments included the spectral photoluminescence between 8 and 300 K, and Hall effect measurements at temperatures between 14 and 300 K. The effect of strain which induces splitting of the valence band as light and heavy hole bands on transport is discussed. The calculated band alignment of the GaInAs sample using model-solid theory including strain effects indicates large conduction band discontinuities and a much smaller valance band discontinuity in GaInAs. The effect of the conduction and valance band discontinuity on the electronic transport properties is also discussed.

InGaN/GaN quantum wells for polariton laser diodes: Role of inhomogeneous broadening

Contrary to the case of III-nitride based visible light-emitting diodes for which the inhomogeneous linewidth broadening characteristic of InGaN-based multiple quantum well (MQW) heterostructures does not appear as a detrimental parameter, such a broadening issue can prevent a microcavity (MC) system entering into the strong light-matter coupling regime (SCR). The impact of excitonic disorder in low indium content (x ∼ 0.1) InxGa1–xN/GaN MQW active regions is therefore investigated for the subsequent realization of polariton laser diodes by considering both simulations and optical characterizations. It allows deriving the requirements for such MQWs in terms of absorption, emission linewidth, and Stokes shift. Systematic absorption-like and photoluminescence (PL) spectroscopy experiments are performed on single and multiple In0.1Ga0.9N/GaN quantum wells (QWs). Micro-PL mappings reveal a low temperature PL linewidth of ∼30 meV, compatible with SCR requirements, for single QWs for which the microscopic origin responsible for this broadening is qualitatively discussed. When stacking several InGaN/GaN QWs, a departure from such a narrow linewidth value and an increase in the Stokes shift are observed. Various possible reasons for this degradation such as inhomogeneous built-in field distribution among the QWs are then identified. An alternative solution for the MC design to achieve the SCR with the InGaN alloy is briefly discussed.

Stacking faults and interface roughening in semipolar (202¯1¯) single InGaN quantum wells for long wavelength emission

The microstructure of InGaN single quantum wells (QWs) grown in semipolar (202¯1¯) orientation on GaN substrates was studied by transmission electron microscopy. Stress relaxation in the lattice mismatch InxGa1−xN layer was realized by forming partial misfit dislocations associated with basal plane stacking faults (BPSFs). For given composition x = 0.24, BPSFs formation was observed when the QW thickness exceeded 4 nm. The high density of partial threading dislocations that bound the BPSFs is detrimental to light-emitting device performance. Interface roughening (faceting) was observed for both upper and lower QW interfaces (more pronounced for upper interface) and was found to increase with the thickness of the QW. BPSFs had a tendency to nucleate at roughened interface valleys.

Applied electric field and hydrostatic pressure effects on quasistationary states in single and coupled GaAs-(Ga,Al)As quantum wells

We have studied the effects of hydrostatic pressure and an uniform electric field on the electron energy levels GaAs-(Ga,Al)As single quantum wells (QWs) and coupled double quantum wells (DQWs) by using the Enderlein's method to solve exactly the Schrodringer equation. Numerical results were obtained using the density of states (DOS) as a function of the applied electric field, hydrostatic pressure, Al concentration, and the geometry as well. We found that the quasistationary ground and excited states energy diminish with and the applied electric field, increase with the confinement potential and the width of central barrier in the DQW. In the latter structure we observed the anti-crossing between the first and second quasistationary energy levels. We found that the applied electric field and the hydrostatic pressure modify the period of Pulsations in QWs.

Coaxial Multishell (In,Ga)As/GaAs Nanowires for Near-Infrared Emission on Si Substrates

Efficient infrared light emitters integrated on the mature Si technology platform could lead to on-chip optical interconnects as deemed necessary for future generations of ultrafast processors as well as to nanoanalytical functionality. Toward this goal, we demonstrate the use of GaAs-based nanowires as building blocks for the emission of light with micrometer wavelength that are monolithically integrated on Si substrates. Free-standing (In,Ga)As/GaAs coaxial multishell nanowires were grown catalyst-free on Si(111) by molecular beam epitaxy. The emission properties of single radial quantum wells were studied by cathodoluminescence spectroscopy and correlated with the growth kinetics. Controlling the surface diffusivity of In adatoms along the NW side-walls, we improved the spatial homogeneity of the chemical composition along the nanowire axis and thus obtained a narrow emission spectrum. Finally, we fabricated a light-emitting diode consisting of approximately 10(5) nanowires contacted in parallel through the Si substrate. Room-temperature electroluminescence at 985 nm was demonstrated, proving the great potential of this technology.

INVESTIGATION OF MATERIAL GAIN OF In0.90Ga0.10As0.59P0.41/InP LASING NANO-HETEROSTRUCTURE

In this paper, we have proposed a step separate confinement heterostructure (SCH) based lasing nano-heterostructure In 0.90 Ga 0.10 As 0.59 P 0.41/ InP consisting of single quantum well (SQW) and investigated material gain theoretically within TE and TM polarization modes. In addition, the quasi Fermi levels in the conduction and valence bands along with other lasing characteristics like anti-guiding factor, refractive index change with carrier density and differential gain have also been investigated and reported. Moreover, the behavior of quasi Fermi levels in respective bands has also been correlated with the material gain. Strain dependent study on material gain and refractive index change has also been reported. Interestingly, strain has been reported to play a very important role in shifting the lasing wavelength of TE mode to TM mode. The results investigated in the work suggest that the proposed unstrained nano-heterostructure is very suitable as a source for optical fiber based communication systems due to its lasing wavelengths achieved at ~1.35 μm within TM mode, while ~1.40 μm within TE mode.

In situ X-ray investigation of changing barrier growth temperatures on InGaN single quantum wells in metal-organic vapor phase epitaxy

The effects of GaN quantum barriers with changing growth temperatures on the interfacial characteristics of GaN/InGaN single quantum well (SQW) grown on GaN templates by metalorganic vapour phase epitaxy were in situ investigated by X-ray crystal truncation rod (CTR) scattering and X-ray reflectivity measurements at growth temperature using a laboratory level X-ray diffractometer. Comparing the curve-fitting results of X-ray CTR scattering spectra obtained at growth temperature with that at room temperature, the InxGa1-xN with indium composition less than 0.11 was stabile of the indium distribution at the interface during the whole growth processes. By using several monolayers thickness GaN capping layer to protect the InGaN well layer within temperature-ramping process, the interfacial structure of the GaN/InGaN SQW was drastically improved on the basis of the curve-fitting results of X-ray CTR scattering spectra, and the narrow full width at half-maximum and strong luminous intensity were observed in room temperature photoluminescence spectra.

Rapid thermal annealing effect on GaAsBi/GaAs single quantum wells grown by molecular beam epitaxy

The effect of rapid thermal annealing on the optical and structural properties of GaAsBi/GaAs quantum wells (QWs) is investigated. The photoluminescence (PL) spectra of the samples are measured at 80 K and room temperature before and after rapid thermal annealing, to ascertain any improvement in the optical quality of the material. The impact of annealing temperature on QW interface quality, layer composition, and thicknesses are studied with x-ray diffraction. For a 60 second annealing time, the low temperature peak PL intensity increases to a maximum of 1.8 times the original intensity at an annealing temperature of 500 °C. Validating this optimum annealing temperature, the room temperature PL peak intensity is seen to increase by 2.2 times. The peak position exhibits a minor blueshift of 15 meV throughout the 450–700 °C temperature range, while annealing at 750 °C produces a blue-shift on the order of 100 meV, indicating out-diffusion of bismuth from the QW. Degradation of the QW interfaces with annealing temperatures above 550 °C is observed. The composition and thickness of the QWs remained constant up to 700 °C. Significant out-diffusion of bismuth and QW thinning are observed at an annealing temperature of 750 °C.

The Characteristics of Exciton in Single and Multiple Quantum Wells

The Characteristics of Exciton in Single and Multiple Quantum Wells

Quasistationary states in single and double GaAs–(Ga,Al)As quantum wells: Applied electric field and hydrostatic pressure effects

We have calculated exactly the energy of electron quasistationary states in GaAs–(Ga,Al)As single and double quantum wells under the action of applied electric field and hydrostatic pressure by using Enderlein׳s method to solve the Schrödinger equation. Numerical results were obtained by means of the density of states as a function of the applied electric field, hydrostatic pressure, Al concentration and the structure geometry as well. We found two regions very well differentiated in energy; for lower values there are quasistationary states and for higher, fast oscillations. The quasistationary ground and excited energy states diminish with the well width and the applied electric field, and increase with the confinement potential and the width of the central barrier in the double quantum well. In the latter structure we observed the anti-crossing between the first and second quasistationary energy levels, phenomena which certainly depend on the central barrier width. Otherwise, in the region of fast oscillations, the period of Franz–Keldysh oscillation type in single quantum well and double quantum well increases with the applied electric field and the number of nodes augments with the well width. Also, we found that the increase of the central barrier height in the double quantum well diminishes the number of nodes, while the applied hydrostatic pressure changes the length of pulsations in both structures.

Atomistic tight-binding study of electronic structure and interband optical transitions in GaBixAs1−x/GaAs quantum wells

Large-supercell tight-binding calculations are presented for GaBixAs1−x/GaAs single quantum wells (QWs) with Bi fractions x of 3.125% and 12.5%. Our results highlight significant distortion of the valence band states due to the alloy disorder. A large full-width-half-maximum (FWHM) is estimated in the ground state interband transition energy (≈33 meV) at 3.125% Bi, consistent with recent photovoltage measurements for similar Bi compositions. Additionally, the alloy disorder effects are predicted to become more pronounced as the QW width is increased. However, they are less strong at the higher Bi composition (12.5%) required for the design of temperature-stable lasers, with a calculated FWHM of ≈23.5 meV at x = 12.5%.

Exchange–correlation and temperature effects on plasmons in strongly correlated two‐dimensional electron systems: Finite‐temperature local‐field‐correction theory combined with angle‐resolved Raman spectroscopy

Using the two types of the finite‐temperature (T) local‐field corrections, namely, the Singwi–Tosi–Land–Sjölander (STLS) and Hubbard (H) approximations, we investigate the exchange–correlation (XC) and T effects on plasmons (PLs) in strongly‐correlated two‐dimensional electron systems with ultralow density. In a density‐parameter range of and in a Fermi‐T normalized T range of 0.5 ≤ T/TF ≤ 8.4, we analyse those PL dispersions, which were observed extensively from single quantum wells by angle‐resolved Raman spectroscopy, but which have not been analysed theoretically so far. Even in our strongly correlated electron systems, the size of the XC holes can be well normalized by the inverse Fermi wave number . Close comparison with the experiment shows that the STLS approximation can be expected to give a quantitative description of the PLs in a smaller wave‐number (q) range of q kF, but that it overestimates the XC effect in a larger q range of q kF. The H approximation is insufficient to explain the remarkable XC effects. The conspicuous T dependence in the PL dispersion can be ascribed largely to the T dependence of the constituent electronic transitions of the PLs through the Fermi–Dirac distribution function.

X‐ray investigations of GaInN single quantum wells grown by atomic layer epitaxy and metalorganic vapor phase epitaxy

AbstractA set of GaN/GaInN single quantum wells (SQWs) were grown on c‐plane GaN/sapphire templates using metal organic vapor phase epitaxy (MOVPE) and atomic layer epitaxy (ALE). The structural information on the interfaces and surfaces was obtained by fitting the experimental X‐ray spectra and was compared between MOVPE and ALE mode. The X‐ray crystal truncation rod (CTR) scattering measurement results showed that the samples grown by ALE had sharper interfaces than those grown by MOVPE. Both the X‐ray reflectivity (XRR) curve‐fitting results and atomic force microscope (AFM) results indicated that the surfaces of the ALE‐grown SQWs were smoother than those of MOVPE‐grown SQWs. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of semipolar {11$ \bar 2 $2} light‐emitting diodes using a hole blocking layer

AbstractIn this study, we fabricated {11$ \bar 2 $2} light‐emitting diodes (LEDs) using a single quantum well (SQW) and hole blocking layer (HBL) with various Al compositions and investigated the effect of the HBL on the characteristics of the LED. The electroluminescence (EL) intensity increased with an increase of the Al composition of the HBL. Furthermore, the EL intensity increased by a factor of six when the Al composition of the HBL increased by 25%. The forward voltages of the LEDs with the HBLwere approximately 3 V regardless of the Al composition of the HBL. The prevention of hole overflow using the HBL is demonstrated by the injection current dependence of the EL intensity and the blueshift by comparing the SQW LEDs with the HBLs with the SQW LEDs without the HBL and a multiple quantum well LED. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface photo-voltage characterization of GaAs/AlGaAs single quantum well laser structures grown by molecular beam epitaxy

We present surface photo-voltage (SPV) measurements on molecular beam epitaxy (MBE) grown single quantum well (SQW) laser structures. Each layer in the hetero-structure has been identified by measurement of the SPV signal after a controlled sequential chemical etching process. These results have been correlated with high resolution x-ray diffraction and photoluminescence (PL) measurements. Quantum confined Stark effect and the carrier screening of electric field have been taken into consideration both theoretically and experimentally to account for the differences observed in SPV and PL results. It is shown that SPV can be used as a very effective tool for evaluation of hetero-structures involving multiple layers.

Atomic-scale nanofacet structure in semipolar and InGaN single quantum wells

Atomic-scale nanofacets were observed in semipolar and InGaN quantum wells (QWs)/GaN quantum barriers interfaces. Transmission electron microscopy studies showed that these nanofacets were mainly composed of , and planes, which led to significant fluctuations in QW thickness. Atom probe tomography studies were carried out to visualize the nanofacet structure. The In composition in the InxGa1−xN alloys followed a binominal distribution despite the formation of the nanofacet structure. One-dimensional (1D) Schrödinger–Poisson drift-diffusion simulation showed that these nanofacets and associated QW thickness fluctuations will lead to a large wavelength shift and a broadened spectral linewidth for semipolar QWs.

Determination of oscillator strength of confined excitons in a semiconductor microcavity

We have achieved a significant experimental Rabi-splitting (3.4 meV) for confined polaritons in a planar semiconductor $\lambda$ microcavity for only a single quantum well (SQW) of GaAs (10 nm) placed at the antinode. The Rabi-splitting phenomena are discussed in detail based on the semiclassical theory, where two coupled harmonic oscillators (excitons and photons) are used to describe the system. In this way, we can obtain the dispersion curve of polaritons, the minimum value for the cavity reflectance and the oscillator strength to reach the strong coupling regime. This approach describes an ensemble of excitons confined in a SQW and includes a dissipation component. The results present a weak coupling regime, where an enhanced spontaneous emission takes place, and a strong coupling regime, where Rabi-splitting in the dispersion curve can be observed. The theoretical results are confronted with experimental data for the reflectance behavior in resonant and off-resonant conditions and present a great accuracy. This allows us to determine the oscillator strength of the confined excitons in the SQW with great precision.

Optical properties of ZnO/(Zn, Mg)O quantum wells

This paper reviews the optical properties of ZnO/(Zn, Mg)O single quantum wells grown by molecular beam epitaxy. Both heteroepitaxial quantum well growth along the polar c-direction and homoepitaxial quantum well growth on the nonpolar M plane cases are considered. The optical properties of these quantum wells are investigated by using reectance, continuous wave photoluminescence, and time-resolved photoluminescence spectroscopies. The quantum-conned Stark eect dominates the properties of the excitons for polar quantum wells. The magnitude of the internal electric eld that is induced by both spontaneous and piezoelectric polarizations is determined by comparing the experimental results with a variational calculation of excitonic energies and lifetimes. For nonpolar quantum wells, the optical spectra reveal strong in-plane optical anisotropies, as predicted by the group theory. Moreover, the radiative recombination of free excitons is dominating the quantum well photoluminescence even at room temperature.

Size and ternary mixed crystal effects on interband absorption in wurtzite ZnO/MgxZn1-xO quantum wells

Adopting a numerical method of solving self-consistently the Schrödinger equation and Poisson equation, the eigenstates and eigenenergies of electrons (holes) in a two-dimensional electron-hole gas are obtained for wurtzite asymmetric ZnO/MgxZn1-xO single quantum wells (QWs). In our computation, a realistic heterostructure potential is used, in which the influences from energy band bending, material doping and the built-in electric field induced by spontaneous and piezoelectric polarizations are taken into account. Furthermore, based on the Fermi's golden rule, the optical absorptions of electronic interband transitions in QWs, and their size and ternary mixed crystal effects are discussed. The results indicate that the increase of the Mg component in MgxZn1-xO enhances the build-in electric field, which forces electrons (holes) to approach to the left (right) barrier. This causes the interband transition absorption peak to decrease exponentially and to be blue-shifted. For different widths of QWs, the calculated results show that absorption peak decreases and transition energy shows a red shift with the increase of well width. The above conclusions are expected to give a theoretical guidance for improving the opto-electronic properties of materials and devices made of heterostructures with suitable optical absorption spectra and wave lengths.

Growth and characterization of Si doped InGaAs/GaAs Single quantum well

The effect of silicon on In GaAsSi/GaAs quantum well structures is investigated using high resolution X-ray diffraction technique (HRXRD) . The growth of quantum well is studied by surface electron microscopy (SEM). The surface morphology is studied using atomic force microscopy (AFM). It is observed from the AFM images that the roughness increases as Si doping increases. The incorporation of silicon also induces alloy non- uniformity in the quantum well, leading to an increased surface roughness. We found that the FWHM of the rocking curves of Si-doped InGaAs increase with increasing Si concentration, indicating the degradation of the crystal quality with increasing doping level.