Single Quantum Well Structures Research Articles

Enhancement of electron transport mobility in GaAs/InGaAs asymmetrically doped narrow quantum well pHEMT structure

We study the effect of asymmetric doping concentrations on the electron mobility μ in GaAs/InGaAs-based single quantum well (SQW) as well as double quantum well (DQW) pseudomorphic high electron mobility transistor (pHEMT) structures. Unequal doping in the substrate and surface barriers (n d1 and n d2 ) causes asymmetric distributions of subband wave functions, ψ 0 and ψ 1, which influence the subband scattering rate matrix elements (SSRME), thereby affecting the subband mobility μ n . For narrow well widths (w w ), in SQW structures, mostly a single subband is occupied. We show that an increase in n d2 , keeping n d1 fixed, enhances μ nonlinearly. The interface roughness (ir-) scattering mostly dominates μ in thin wells (w w < 70 Å), while generally, μ is determined by ionized impurity (ii-) scattering and to some extent by alloy disorder (ad-) scattering. The influence of ir-scattering enhances, while ad-scattering diminishes, by reducing n d2 . For DQW, a double subband is occupied. In a symmetric DQW structure at resonance, n d1 = n d2 , ψ 0 and ψ 1 equally extend into both the wells. For a minor variation, say n d1 > n d2 , ψ 0 mostly lies in one well while ψ 1 is in the other well. In the case of n d1 < n d2 , the distribution reverts. The substantial changes in ψ 0 and ψ 1 influence the intra- and inter-SSRME differently through intersubband effects, leading to nonlinear μ n as a function of n d2 . Taking n d1 + n d2 = 3 × 1018 cm−3, we show that for w w1 = w w2 = 80 Å, a shallow dip in μ occurs at n d1 = n d2 = 1.5 × 1018 cm−3. Whereas, for w w1 = 60 Å and w w2 = 100 Å, the dip in μ occurs near the corresponding resonance, n d1 = 2.3 × 1018 cm−3 and n d2 = 0.7 × 1018 cm−3. Our results of nonlinear μ can be utilized for performance analysis of pHEMT.

Investigations on the high performance of InGaN red micro-LEDs with single quantum well for visible light communication applications

In this study, we have demonstrated the potential of InGaN-based red micro-LEDs with single quantum well (SQW) structure for visible light communication applications. Our findings indicate the SQW sample has a better crystal quality, with high-purity emission, a narrower full width at half maximum, and higher internal quantum efficiency, compared to InGaN red micro-LED with a double quantum wells (DQWs) structure. The InGaN red micro-LED with SQW structure exhibits a higher maximum external quantum efficiency of 5.95% and experiences less blueshift as the current density increases when compared to the DQWs device. Furthermore, the SQW device has a superior modulation bandwidth of 424 MHz with a data transmission rate of 800 Mbit/s at an injection current density of 2000 A/cm2. These results demonstrate that InGaN-based SQW red micro-LEDs hold great promise for realizing full-color micro-display and visible light communication applications.

Photoluminescence properties of GaAsBi single quantum wells with 10% of Bi

A set of single quantum well (SQW) samples of GaAs1-xBix with x ~ 0.1 and p-doped GaAs barriers grown by molecular beam epitaxy was investigated by the temperature-dependent photoluminescence (PL) spectroscopy. Those GaAsBi SQW structures showed a high crystalline quality, a smooth surface and sharp interfaces between the layers and exhibited a high PL intensity and a lower than 100 meV PL linewidth of QW structures. Temperature dependence of the optical transition energy was S-shape-free for all investigated structures and it was weaker than that of GaAs. An analysis of the carrier recombination mechanism was also carried out indicating that the radiative recombination is dominant even at room temperature. Moreover, numerical calculations revealed that a higher Be doping concentration leads to an increased overlap of the electron and heavy hole wave functions and determines a higher PL intensity.

Comprehensive optimization of electronic and optical properties of polar III-nitride laser

A detailed investigation is performed to obtain optimized optical performance of polar III-nitride single quantum well (SQW) semiconductor lasers by band engineering through symmetric and asymmetric tuning of device parameters. Electronic, optical, and threshold characteristics are calculated to analyze the effects of band engineering on device performance. Here, the InN-In0.25Ga0.75 N SQW structure is chosen as a case study. This work shows a significant improvement in optical properties from higher wavefunction overlap integral obtained from band modification due to the tuning which also provides higher momentum matrix elements, spontaneous emission rate, and optical gain, as well. The maximum TE-polarized optical gain for the symmetric structure is found as around 6291 cm−1 for 6 A well width and 6 A barrier width at 710 nm, while for asymmetric laser structure, a maximum TE-polarized optical gain of around 6225 cm−1 at around 800 nm is observed for 6 A well width with asymmetric barrier width of 6 A and 8 A. The optimization is carried out over 500–1600 nm wavelength range to obtain the optimum structure for maximum optical performance at different wavelengths. A time-efficient genetic algorithm-based optimization is also performed that provides the same optimization results in reduced time.

Investigation of optical polarization characteristics for an AlGaN-based quantum well structure

The anisotropic optical properties of an AlGaN-based quantum well (QW) structure are investigated by using the effective-mass theory. Firstly, the crystal-field bowing parameter (bcr = −0.087) and the slope (bs = 8) of the crystal-field splitting energy (∆cr) — in-plane strain (εxx) curve are obtained by comparing the calculated degree of polarization (DOP) of the AlGaN alloy with the experimental values. Then, the DOP of Al0.35Ga0.65N/AlxGa1-xN single QW structures with various Al-content (x = 0.5, 0.45, 0.55) in the barrier layer is calculated by employing the obtained parameters. The DOP is enhanced from −43.15% to −4.47% with increasing Al-content at J = 100 A cm−2. By increasing the injection current density or the residual strain in the active region, the DOP can be increased/decreased by modulating the Al component of the barrier layer due to the different polarization characteristics of the transition levels. In addition, the AlGaN-based QW structure with lower Al-content in the barrier layer shows the larger spontaneous emission rate.

Effects of 1-MeV Electron Irradiation on the Photoluminescence of GaInNAs|GaAs Single Quantum Well Structure

Minimizing the impact of radiation-induced degradation in dilute nitride based optoelectronic devices is crucial in its applications. The effects of 1-MeV electron irradiation (of 1 × 1014–1 × 1016 e/cm2 range) on undoped GaInNAs|GaAs single quantum-well (QW) structure has been studied by low-temperature photoluminescence (PL). PL spectra of GaInNAs|GaAs QW are measured before and after electron irradiation. The results show a slight enhancement of the PL intensity in relatively low electron fluence, and then subsequent deterioration of PL with the increase of cumulative electron fluences. The enhancement in PL intensity at low electron doses is explained by recombination-enhanced defect reaction model, and the degradation at high electron doses is explained by irradiation-induced defects in the lattice.

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.

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.

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.

Lattice-matched double dip-shaped BAlGaN/AlN quantum well structures for ultraviolet light emission devices

Ultraviolet light emission characteristics of lattice-matched BxAlyGa1−x−y N/AlN quantum well (QW) structures with double AlGaN delta layers were investigated theoretically. In contrast to conventional single dip-shaped QW structure where the reduction effect of the spatial separation between electron and hole wave functions is negligible, proposed double dip-shaped QW shows significant enhancement of the ultraviolet light emission intensity from a BAlGaN/AlN QW structure due to the reduced spatial separation between electron and hole wave functions. The emission peak of the double dip-shaped QW structure is expected to be about three times larger than that of the conventional rectangular AlGaN/AlN QW structure.

Internal quantum efficiency and carrier dynamics in semipolar (2021) InGaN/GaN light-emitting diodes.

The internal quantum efficiencies (IQE) and carrier lifetimes of semipolar (202¯1¯) InGaN/GaN LEDs with different active regions are measured using temperature-dependent, carrier-density-dependent, and time-resolved photoluminescence. Three active regions are investigated: one 12-nm-thick single quantum well (SQW), two 6-nm-thick QWs, and three 4-nm-thick QWs. The IQE is highest for the 12-nm-thick SQW and decreases as the well width decreases. The radiative lifetimes are similar for all structures, while the nonradiative lifetimes decrease as the well width decreases. The superior IQE and longer nonradiative lifetime of the SQW structure suggests using thick SQW active regions for high brightness semipolar (202¯1¯) LEDs.

Defect engineering in AlGaN-based UV optoelectronic heterostructures grown on c-Al2O3 by plasma-assisted molecular beam epitaxy

ABSTRACTAlGaN-based SQW heterostructures grown by plasma-assisted molecular beam epitaxy on c-Al2O3 substrates have been studied with high resolution transmission electron microscopy (HR TEM), photoluminescence spectroscopy and x-ray diffraction. The high-temperature (780°C) synthesis of the AlN buffer layer nucleated on c-Al2O3 by a migration enhanced epitaxy and including several ultra-thin GaN interlayers grown under moderate N-rich conditions was shown to be the optimum approach for lowering the threading dislocations density down to 108-109 cm-2. HR TEM study has confirmed the fine structure of single quantum wells (SQW) formed by a sub-monolayer digital alloying technique and revealed different kinds of compositional inhomogeneities in the AlxGa1-xN barrier layers of the heterostructures, including the formation of Al-rich barriers induced by the temperature-modulated epitaxy and the spontaneous compositional disordering along the growth axis for x=0.6-0.7. The influence of these phenomena on the parameters of the mid-UV stimulated emission observed in the SQW structures has been studied as well.

Shallow-terrace-like interface in dilute-bismuth GaSb/AlGaSb single quantum wells evidenced by photoluminescence

Photoluminescence (PL) measurements are performed on one GaSb/AlGaSb single-quantum-well (SQW) sample and two dilute-bismuth (Bi) GaSb/AlGaSb SQW samples grown at 360 and 380 °C, at low temperatures and under magnetic fields. Bimodal PL features are identified in the dilute-Bi samples, and to be accompanied by abnormal PL blueshift in the sample grown at 360 °C. The bimodal PL features are found to be from similar origins of band-to-band transition by magneto-PL evolution. Analysis indicates that the phenomenon can be well interpreted by the joint effect of interfacial large-lateral-scale islands and Al/Ga interdiffusion due to Bi incorporation. The interdiffusion introduces about 1-monolayer shrinkage to the effective quantum-well thickness, which is similar to the interfacial islands height, and the both together result in an unusual shallow-terrace-like interface between GaSbBi and AlGaSb. A phenomenological model is established, the Bi content of isoelectronic incorporation and the exciton reduced effective mass are estimated for the GaSbBi sample grown at 380 °C, and a value of about 21 meV/% is suggested for the bandgap bowing rate of GaSbBi. An effective routine is suggested for determining the Bi content and the depth of the shallow-terraces at interface in dilute-Bi SQW structures.

A nonlinear gain model for multiple quantum well transistor lasers

The gain spectra of single quantum well (SQW) heterojunction bipolar transistor laser (HBTL) is calculated with the gain coefficients and transparency carrier density as a function of the device structure factors by taking into account intraband relaxation. Inter-well coupling effects in a multiple quantum well (MQW) structure are considered as a correction factor in our approach. We introduce a six-order polynomial expression for a nonlinear carrier-dependent gain of MQW-HBTL considering ground, first excited and second excited states. We show that our gain model is notably more accurate than usual logarithmic equations, particularly in a large signal regime. Using computationally efficient numerical methods with a comprehensive rate-equation-based model, we obtain good agreement with experimental results for small signal, large signal and switching analysis of the SQW and MQW structures. We discuss the influence of different structure factors on the switching behaviour as well as on the optical bandwidth of the HBTL. Specifically, turn-on times (τon) of 160 and 540 ps are obtained for SQW and 4 QW structures, while τon considerably decreases below 100 ps for 6 QWs due to competing effects between tunneling and thermionic emission followed by carrier diffusion over the barriers. Finally, τon can be minimized independently of the QW number when a barrier width of ≈7 nm is used in the HBTL structure.

In situ X-ray measurements of MOVPE growth of InxGa1−xN single quantum wells

GaN/InxGa1−xN/GaN single quantum wells (SQWs) have been grown on c-plane GaN/sapphire substrates using MOVPE system. PL (photoluminescence) and AFM (atomic force microscope) measurements demonstrate good quality of after-growth thermal-annealed SQWs. In situ XRD (X-ray diffraction), XRR (X-ray reflectivity), and X-ray CTR (crystal truncation rod) scattering measurements were successfully conducted on the SQWs under the NH3+N2 ambient at 1103K. The analysis results of the XRR and the X-ray CTR spectra at 1103K and at 300K on the same sample matched well. It demonstrated that In0.09Ga0.91N SQW structure with several ML (monolayer) InGaN thicknesses was successfully investigated using the XRR and CTR scattering measurements at 1103K.

Electronic and optical properties of ZnO/(Mg,Zn)O quantum wells with and without a distinct quantum-confined Stark effect

The luminescence properties of polar ZnO/(Mg,Zn)O quantum wells (QWs) are determined, besides confinement effects, by a redshift caused by the Stokes shift and the quantum-confined Stark effect (QCSE). We present a comprehensive study of photoluminescence and optical transmission measurements to separate these two effects. Single QW structures have been prepared by pulsed laser deposition on a-plane sapphire exhibiting excitons with and without a distinct QCSE. The QCSE leads to a redshift of the QW luminescence maximum beneath the free exciton energy in ZnO as well as a change of the dynamics from a single exponential decay function to a non-exponential one, well described by a stretched exponential decay function. The internal electric field was evaluated to 0.66 MV/cm. In the absence of an internal electric field, the Stokes shift was determined to drop from 64 meV down to 24 meV with increasing well width. Additionally, with increasing temperature, the QCSE vanishes as the internal electric field is screened by free charge carriers, leading to a drop of the redshift above 150 K to a value comparable to the Stokes shift determined for the QWs without a distinct QCSE. The radiative decay time increases linearly at low temperatures for both types of QWs, indicating free exciton emission as the major recombination channel.

Photoluminescence Enhancement in GaN/InGaN Multi-Quantum Well Structures as a Function of Quantum Well Numbers: Coupling Behaviors of Localized Surface Plasmon

Photoluminescence (PL) efficiency enhancement produced by deposition of Ag/SiO2 core/shell nanoparticles (NPs) on the GaN/InGaN multi-quantum well structure was studied as a function of quantum well (QW) number in the structure. For back-surface laser excitation the strongest PL enhancement by 1.5 times was observed for single QW structure, the weakest for the 5QW structure. The result is ascribed to Ag/SiO2 NPs localized surface plasmons coupling to the MQW region and the dependence of the coupling efficiency on the distance to the NP layer. Simple modeling suggests that the coupling efficiency starts to decrease for distances higher than about 50 nm.

The Pulling Power of Pairs

Physics The functionality and characteristics of many semiconductor devices depend crucially on the movement of electron-hole pairs in response to an applied electric field. In such ambipolar devices, it is typically the heavier holes that dominate the transport properties. As such, the packets of charged pairs move in the direction of the holes. Some experiments, however, exhibit behavior opposite to that expected motion and have been interpreted in terms of a correlation field, or friction, between the electrons and holes. Yang et al. use a transient grating spectroscopy technique to probe the electron and hole pair packets as they drift and diffuse in a single quantum-well structure. An electron-hole density grating is imprinted in the quantum well by two interfering laser beams. Monitoring the diffraction of a probe beam through the grating as it evolves and decays provides a detailed picture of the electrons' and holes' dynamics. With a model, the authors are then able to determine the degree of friction between them. The technique should provide a route to better understanding the operation of such ambipolar devices and, from the polarization dependence, the operation of spintronic devices based on the spin properties of the carriers. Phys. Rev. Lett. 106 , 247401 (2011).

High Efficiency Red Phosphorescent Organic Light Emitting Diodes with Single Quantum Well Structure

We report highly efficient red phosphorescent organic light-emitting diodes with a triplet p-i-n single quantum well (QW) structure. This p-i-n single QW structure is realized using p-doped and n-doped wide band-gap hole and electron injection and transporting layers with narrow band-gap host and dopant materials. The maximum current and power efficiencies of 11.1 cd/A and 14.4 lm/W, respectively, are demonstrated by a good carrier confinement effect of QW. Very low driving voltage characteristics of 3.7 ∼ 3.9 V at 1000 cd/m2 are realized by almost no injection barrier with high conductivity configuration in our p-i-n structure.

Effect of normal and reversed polarizations on optical characteristics of ultraviolet-violet InGaN laser diodes

The optical characteristics of ultraviolet-violet InGaN laser diodes with different numbers of quantum wells under normal and reversed polarizations are numerically investigated. For the laser structures under normal polarization, the lowest threshold current is obtained when the number of quantum wells is two in the spectral range of 380–408 nm. For the laser structures under reversed polarization, the single quantum-well laser structure possesses the lowest threshold current. The simulation results suggest that the physical origin for these phenomena is caused by the sufficiently suppressed electron and hole leakage currents when the laser diode is under reversed polarization.