Single Quantum Wells Research Articles

A unified method to extract the effective mobility in InGaAs metal-insulator-semiconductor field-effect-transistors using scattering-parameters

In this letter, we propose a unified method to extract the effective mobility (μeff) of In0.52Al0.48As/In0.7Ga0.3As/In0.52Al0.48As single-quantum-well (SQW) metal-insulator-semiconductor field-effect-transistors (MISFETs). The proposed method relies only on the measured high-frequency scattering-parameters (S-parameters) of the MISFETs in the linear regime. Two key metrics of MOS devices, intrinsic output conductance (go_i) and intrinsic gate capacitance (Cg_i), were extracted directly from the measured S-parameters using two-port network parameter theories, allowing us to compute the effective mobility of the MOS devices. Since the method only requires the small-signal S-parameter measurement data, it would be applicable to any kind of FETs and could be fruitful for studying the dependence of the effective mobility on lateral electric field intensity.

Features of Magneto-Intersubband Oscillations in HgTe Quantum Wells

Magneto-intersubband oscillations (MISOs) in single quantum wells of the gapless semiconductor HgTe have been studied experimentally. It has been shown that, in contrast to MISOs in double quantum wells based on broadband semiconductors GaAs and InGaAs and theoretical predictions, magnetic fields at which antinodes of high-frequency oscillations are observed coincide with minima rather than with maxima of MISOs of the magnetoresistance ρxx. It has been assumed that this feature is due to the suppression of the resonance of the probability of transitions between Landau levels of two spectral branches split by the strong spin—orbit coupling.

Analysis of a Direct-Bandgap GeSn-Based MQW Transistor Laser for Mid-Infrared Applications

We propose the design and analysis of a group-IV material-based (Ge0.84Sn0.16/Si0.09Ge0.8Sn0.11) multiple quantum wells (MQW) transistor laser (TL) for mid-infrared applications. The base region incorporates Ge0.84Sn0.16/Si0.09Ge0.8Sn0.11 MQW structures pseudomorphically grown on silicon through GeSn virtual substrate, compatible with CMOS platforms for cost-effective integration of electronic and photonic circuits. With the introduction of a certain amount of tin (α-Sn) content (Sn > 6%) into germanium (Ge), GeSn alloy shows direct bandgap, thereby achieving the population inversion condition. In addition, the use of MQW structures such as the active region enhances gain due to quantum–confinement effects, and better carrier utilization over its bulk counterpart. A comparison is made between the theoretical finding for the proposed Ge0.84Sn0.16 MQW TL and the available experimental and theoretical data for currently employed InGaAs-based single quantum well (SQW) and MQW TLs. Estimated results show that a lower threshold base current of ∼ 2.65 mA and high modulation BW of ∼ 53 GHz can be achieved, which ensures the proposed GeSn-based MQW TL can be a good alternative for III–V-based TL.

GaAs1-xSbx/GaAs single quantum well for long wavelength photonic devices

Carrier dynamics in GaAs1-xSbx/GaAs single quantum well (SQW) is investigated in this report. With Sb incorporation (x = 0.352, 0.405), the photoluminescence (PL) emission peaks exhibit characteristics of GaAs1-xSbx in low temperature and GaAs above 200 K. In power dependent PL, the intensities reveal sublinear power relationship as localized and free excitons are involved at 10 K. The power exponent is in agreement with the degree of localization energy present in SQW and carrier kinetics for various recombination mechanisms are also discussed by deriving respective rate equations.

Electrical properties of n-HgCdTe MIS structures with HgTe single quantum wells

For the first time, the electrical characteristics of the metal–insulator–semiconductor (MIS) structures based on n(p)-Hg1−xCdxTe grown by molecular beam epitaxy including HgTe single quantum well (SQW) with thickness of 6.5 nm were investigated. SQW significantly influences the voltage, frequency, and temperature dependencies of the admittance of the MIS structure. When the SQW thickness is less than the critical thickness, there are numerous sharp maxima in the capacitance–voltage (C–V) curve, and when the thickness of the well is close to the critical thickness, a wide maximum is observed in the C–V characteristics associated with overshoot of minority charge carriers from SQW. A distinction is revealed between the effect of radiation on capacitive maxima caused by the escape of charge carriers from SQW and the recharging of deep levels in the epitaxial film bulk.

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.

InGaN quantum wells with improved photoluminescence properties through strain-controlled modification of the InGaN underlayer

The effect of internal strain on the luminescence properties of an InGaN single quantum well (SQW) was investigated as a function of modification via an underlayer (UL). Single In0.25Ga0.75N QWs (λ = 520 nm) 3 nm thick were grown on various ULs on a sapphire substrate, where the two UL types included (1) a buffer layer onto which an InGaN layer with a very small amount of In was inserted and (2) a buffer layer grown using different carrier gases. The SQWs were then analyzed by temperature-dependent time-resolved photoluminescence, scanning electron microscopy and cathodoluminescence. The experimental results show that the density of non-radiative recombination centers and the level of potential fluctuation in the SQWs decrease with insertion of an InxGa1−xN UL possessing a quite low but sufficient indium content (x = 0.007). The density of non-radiative recombination centers in the SQW on the H2 carrier-grown UL, however, is large.

Effect of exchange electron-electron interaction on conductivity of InGaAs single and double quantum wells

We report on experimental study of quantum conductivity corrections for two-dimensional electron gas in GaAs/InGaAs/GaAs single and double quantum wells in a wide temperature range (1.8–100) K. We perform a comparison of our experimental data for the longitudinal conductivity in zero magnetic field to the theory of interaction-induced corrections to the transport coefficients. In the temperature range from 10 K up to (45–60) K, which covers the ballistic interaction regimes for our samples, rather a good agreement between the theory and our experimental results has been found.

Optical properties of c-Plane InGaN/GaN single quantum wells as a function of total electric field strength

We present low temperature photoluminescence spectra from four InGaN/GaN single quantum well structures where the total electric field across the quantum wells was varied by the manipulation of the surface polarization field, which is of opposite sign to the electrostatic built-in field originating from spontaneous and piezoelectric polarization intrinsic to the material. We find that, overall, the photoluminescence peak emission energy increases and its full width at half maximum decreases with decreasing total internal electric field. Using an atomistic tight-binding model of a quantum well with different total internal electric fields, we find that the calculated mean and standard deviation ground state transition energies follow the same trends with field as our experimentally determined spectral peak energies and widths. Overall, we attribute this behavior to a reduction in the quantum confined Stark effect and a connected reduction in the variation of ground state transition energies with decreasing electric field, respectively.

MBE growth strategy and optimization of GaAsBi quantum well light emitting structure beyond 1.2 μm

GaAs1−xBix/AlGaAs quantum wells (QWs) with varying As/Ga beam equivalent pressure (BEP) ratios were grown by solid source molecular beam epitaxy at a relatively high temperature of 350–400 °C intended for light emitting applications with wavelengths beyond 1.2 μm. Both the Bi content and the photoluminescence (PL) intensity were found to be highly dependent on As2 flux, especially for the case of growing GaAsBi at a relatively high temperature. A graded index separate confinement GaAsBi/AlGaAs single QW with 5.8% Bi exhibited a strong PL emission at 1.22 μm. The growth strategy to incorporate considerable Bi into GaAs at a relatively high temperature through meticulous control of the As/Ga BEP ratio and compensation of Bi flux is demonstrated to be effective in guaranteeing a high Bi content as well as an optimal optical performance of GaAsBi QWs.

InGaN Platelets: Synthesis and Applications toward Green and Red Light-Emitting Diodes.

In this work, we present a method to synthesize arrays of hexagonal InGaN submicrometer platelets with a top c-plane area having an extension of a few hundred nanometers by selective area metal-organic vapor-phase epitaxy. The InGaN platelets were made by in situ annealing of InGaN pyramids, whereby InGaN from the pyramid apex was thermally etched away, leaving a c-plane surface, while the inclined {101̅1} planes of the pyramids were intact. The as-formed c-planes, which are rough with islands of a few tens of nanometers, can be flattened with InGaN regrowth, showing single bilayer steps and high-quality optical properties (full width at half-maximum of photoluminescence at room temperature: 107 meV for In0.09Ga0.91N and 151 meV for In0.18Ga0.82N). Such platelets offer surfaces having relaxed lattice constants, thus enabling shifting the quantum well emission from blue (as when grown on GaN) to green and red. For single InGaN quantum wells grown on the c-plane of such InGaN platelets, a sharp interface between the quantum well and the barriers was observed. The emission energy from the quantum well, grown under the same conditions, was shifted from 2.17 eV on In0.09Ga0.91N platelets to 1.95 eV on In0.18Ga0.82N platelets as a result of a thicker quantum well and a reduced indium pulling effect on In0.18Ga0.82N platelets. On the basis of this method, prototype light-emitting diodes were demonstrated with green emission on In0.09Ga0.91N platelets and red emission on In0.18Ga0.82N platelets.

Efficient carrier transport in GRIN-SCH transistor lasers

We report analytical and theoretical results of simulation of a graded-base, single quantum well (SQW) transistor laser (TL). Using an appropriate carrier transport model, device performances for different confinement structures are studies. Physical parameters including the diffusion constant and optical confinement factor are calculated, and the dependence of the optical response on both current level and structure design (e.g. base doping and cavity length) is investigated. Simulation results show that using graded index layers of AlξGa1 − ξAs (ξ: 0.1 → 0) in the left-hand side of the QW and AlξGa1 − ξAs (ξ: 0.05 → 0) in the right-hand side of the QW (instead of GaAs in the base region) increases the optical output power by a factor of 3, eliminates completely the resonance peak, and most interestingly increases optical bandwidth by ∼37% compared to the conventional (i.e. non-graded base) structure.

Coherent Resonant Tunneling through Double Metallic Quantum Well States.

Study of resonant tunneling through multimetallic quantum well (QW) structure is not only important for the fundamental understanding of quantum transport but also for the great potential to generate advanced functionalities of spintronic devices. However, it remains challenging to engineer such a structure due to the short electron phase coherence length in metallic QW system. Here, we demonstrate the successful fabrication of double-QW structure in a single fully epitaxial magnetic tunnel junction (MTJ) heterostructure, where two Fe QW layers are sandwiched between three MgAlO x tunnel barriers. We show clear evidence of the coherent resonant tunneling through the discrete QW states in the two QWs. The coherent resonant tunneling condition is fulfilled only when the middle barrier between the two QWs is thin enough and available QW states are present simultaneously in both QWs under a certain bias. Compared to the single QW structure, the resonant tunneling in double-QW MTJ produces strong conductivity oscillations with much narrower peak width (about half) owing to the enhanced energy filtering effect. This study presents a comprehensive understanding of the resonant tunneling mechanism in MTJ with multiple QWs, which is essential for future development of new spintronic devices operating in the quantum tunneling regime.

Direct lift-off and the piezo-phototronic study of InGaN/GaN heterostructure membrane

Internal polarizations, caused by the intrinsic material properties and lattice mismatch during material epitaxial growth, exist inherently in III-nitrides and limit significantly their practical applications. Here, InGaN/GaN single quantum well (SQW) structure grown on sapphire substrate was directly exfoliated by electrochemical (EC) etching. The separated InGaN/GaN SQW heterostructure membranes (SQW-HMs) exhibit substantial relaxation of internal strain and polarization compared to the as-grown samples. To further modulate/tune the internal polarizations and corresponding opto-electronic properties of the SQW-HMs, the piezo-phototronic effect was introduced by applying external stress on the freestanding membrane under ultraviolet (UV) light illumination. The obtained photoluminescence (PL) intensity can be effectively modulated in different manners under tensile and compressive straining conditions. Energy diagrams of the SQW-HMs under free and straining conditions were carefully analyzed to illustrate the working mechanisms of the piezo-phototronic modulations. This research provides a new approach to optimize the performances of III-nitride devices and expand their further applications in optical communications and optical integration systems.

Numerical simulation of UV LEDs with GaN and BGaN single quantum well

The objective of this work is to simulate a single quantum well ultraviolet light emitting diode (LED) based on AlGaN/GaN/AlGaN and AlGaN/BGaN/AlGaN, by using TCAD Silvaco simulator. The first structure has a GaN quantum well taken between two layers, of n-AlGaN and p-AlGaN. The second one has a BGaN quantum well instead of GaN. We fix the concentration of the boron in BGaN to only 1% and we vary the thickness of GaN and BGaN quantum well layer from 7 to 20 nm, for the two structures. As results, we obtain respectively for GaN-LED and BGaN-LED, a maximum current of 0.52 and 0.27 mA, a maximum power spectral density of 1.935 and 6.7 W cm−1 eV−1, a maximum spontaneous emission of 3.34 × 1028 and 3.43 × 1028 s−1 cm−3 eV−1, and a maximum Light output power of 0.56 and 0.89 mW.

Growth of Si/Si1-xGex/Si (x < 0.1) Quantum Wells by Modulating the Ge Molecular Flow. II. Study of Properties

Single quantum wells in the Si1-xGex/Si (x < 0.1) system grown in the mode of molecular Ge flow modulation in forming the solid solution layer are studied. The grown heterostructures are investigated by the low-temperature photoluminescence method, and their properties were compared with the properties of similar heterostructures grown without modulation of the Ge molecular flow. It is shown that modulation of the Ge flow has almost no effect on the spectral properties of the quantum wells.

Beats of Quantum Oscillations of the Resistance in Two-Subband Electron Systems in Tilted Magnetic Fields

Electron transport in single GaAs quantum wells of widths from 22 to 46 nm with two populated quantum-confinement subbands ES and EAS is investigated at a temperature of T = 4.2 K in tilted magnetic fields B 72° and α > 85°, respectively. The origin of this unexpected behavior of magnetointersubband oscillations in tilted magnetic fields is discussed.

G‐Factor Anisotropy Inversion in InGaAs 2D Nanostructures

The inversion or sign change of the electron g‐factor anisotropy in thin‐layer semiconductor nanostructures is investigated theoretically and gauged for InGaAs asymmetric single and double quantum wells (QWs). The g‐factor anisotropy in these 2D nanostructures is given by the difference between the longitudinal and transverse components; it is a fine sensor of the confining potential and in InGaAs structures it is determined by the Rashba spin–orbit coupling. In the presence of structure inversion asymmetry (SIA) the g‐factor anisotropy is expected to invert at a critical well width. This effect can be useful technologically and is here analyzed in detail with InGaAs/InP asymmetric multi‐layer structures. The g‐factor anisotropy in these structures is calculated in a fine grid around the inversion point, using 8‐band kp Kane model based envelope function theory for the nanostructure, and perturbation theory for the calculation of the effective g factor. It is shown that the anisotropy inversion can be seen only in asymmetric structures with very thin layers, near the limit of no bound states allowed, and corresponding to the electron being pushed out of the confining region. The inversion point, or critical well width for the g‐factor anisotropy inversion in Insulator/InGaAs/InP QWs is determined to be ≈4 nm. For double or coupled QWs it is found that the inversion can be observed only with very thin tunneling barriers around 1 nm wide.

Three-Dimensional Numerical Study on the Efficiency Droop in InGaN/GaN Light-Emitting Diodes

The efficiency droop characteristics of single quantum well (SQW) InGaN/GaN light-emitting diodes (LEDs) including the phase-space filling (PSF) effect are predicted by a three-dimensional (3-D) numerical simulation. The carrier transport is based on the solution of the 3-D non-linear Poisson and drift-diffusion equations for both holes and electrons. A modified formulation of the Shockley-Reed-Hall (SRH) coefficient is proposed to describe the SRH carrier lifetime behavior, which increases at a low excitation level and decreases at a higher one. The current crowding causes a non-uniform distribution of the carrier concentration in the active layer that leads to the inversion of the local internal quantum efficiency (IQE) under the n-pad region when the injection current density increases from low to high levels. To further understand the correlation of the efficiency droop with the PSF effect, we systematically investigate carrier transport in the SQW InGaN/GaN LEDs and how the different PSF effect coefficients affect the current-voltage curve and IQE. The lumped IQE found in this study agrees well with previous experimental measurements. Moreover, the PSF effect has a strong impact on the IQE behavior including its peak and droop in efficiency.

Growth of Si/Si1_xGex/Si (x &lt; 0.1) Quantum Wells by Modulating Ge Molecular Flow. I. Molecular Beam Epitaxy

Single quantum wells in the system Si1_xGex/Si with x < 0.1 are grown by molecular-beam epitaxy in the mode of periodic Ge molecular flow interruption during the formation of the alloy layer. It allowed an increase in the operating current of the source electron gun, which provided a more stable molecular Ge flow. The grown heterostructures are studied by transmission scanning electron microscopy methods