Number Of Quantum Wells Research Articles

High performance thin quantum barrier InGaN/GaN solar cells on sapphire and bulk (0001) GaN substrates

We demonstrate high performance InGaN/GaN multiple quantum well solar cells with thin quantum barriers and spectral response extending to 460 nm. Devices grown on bulk (0001) GaN substrates with up to 50 quantum wells (QWs) outperform those grown simultaneously on sapphire due to the lower threading dislocation density. Increasing the number of QWs eventually leads to performance degradation of devices grown on both substrates. Solar cells are demonstrated with peak external quantum efficiencies up to 60%, open circuit voltages up to 2.28 V, fill factors up to 80%, and conversion efficiencies up to 2.4% under 1 sun AM0 equivalent illumination.

Single junction a-Si:H solar cell with a-Si:H/nc-Si:H/a-Si:H quantum wells

A single junction hydrogenated amorphous silicon (a-Si:H) solar cell with incorporating hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H (nc-Si:H/a-Si:H) multiple quantum wells (MQW) in the intrinsic region is proposed. Its theoretical realization has been made on the basis of existing quantum well solar cell models to determine the influence of the incorporated MQW over the conversion efficiency of a-Si:H solar cell. The atomic energy states and density of states of nc-Si:H/a-Si:H quantum wells (QW) are simulated at various QW widths. Based on the simulated results the absorption coefficient of nc-Si:H/a-Si:H QW is estimated and these values along with the evaluated atomic energy states of QW are used to optimize the QW width for the designed quantum well solar cell. The optimum well width is found to be 10nm, where the absorption coefficient increases in three steps starting from 1.0×104cm−1 to 3.0×104cm−1 for photon energy of 1.2eV to 1.4eV, respectively, and thereafter remained constant for higher energies. The estimated absorption coefficient of 10nm QW is involved in the calculations of photocurrent density to study the effect of number of incorporated QW on short circuit current density (JSC), open circuit voltage (VOC) and efficiency (η) of a-Si:H solar cell. The JSC, VOC, and η are presented for up to 100 QW incorporated a-Si:H solar cell. The results show that the conversion efficiency of a-Si:H solar cell is directly related to the number of incorporated QW, which is mainly due to the growing short circuit current density with increasing number of QW. The current–voltage plot for 100-QW a-Si:H solar cell with intrinsic region of 5.5μm showed a peak power of 13.3mW/cm2.

Numerical study of the suppressed efficiency droop in blue InGaN LEDs with polarization-matched configuration

In blue InGaN light-emitting diodes (LEDs), the intuitive approaches to suppress Auger recombination by reducing carrier density, e.g., increasing the number of quantum wells (QWs) and thickening the width of wells, suffer from nonuniform carrier distribution and more severe spatial separation of electron and hole wave functions. To resolve this issue, LED structures with thick InGaN wells and polarization-matched AlGaInN barriers are proposed theoretically. Furthermore, the number of QWs is reduced for the purpose of mitigating the additional compressive strain in AlGaInN barriers. Simulation results reveal that, in the proposed structures, the quantum-confined Stark effect in strained wells is nearly eliminated through the utilization of polarization-matched barriers, which efficiently promotes internal quantum efficiency. Furthermore, the phenomenon of efficiency droop is also markedly improved because of the uniformly distributed or dispersed carriers, and accordingly the suppressed Auger recombination.

Performance Optimization of Multiple Quantum Well Transistor Laser

A comprehensive physical model that emphasizes carrier tunneling between quantum wells (QWs) in the base of transistor lasers (TLs) is developed. This model relies on a set of multilevel coupled rate equations solved by computationally efficient numerical methods for simulating both steady state and transient TL operations. Our approach also features the explicit dependence of the structure design on device parameters such as optical confinement factor and carrier density-dependent gain. It also accounts for operation behaviors such as bandwidth roll-off and critical base width not yet addressed in the literature. Simulation results show significant enhancement in optical bandwidth as well as threshold current reduction when multiple QWs are incorporated within the base region. It predicts the dominance of tunneling transport of carriers for barriers thinner than 13.5 nm in a system with 7-nm QWs. However, the optimum QW number depends on the structure design as well as TL biasing conditions. For this purpose, we define a performance parameter as a TL figure of merit that can be maximized by optimizing both the base and the cavity designs.

MOVPE growth for photodiodes in 2.5 µm region with InGaAs/GaAsSb type‐II quantum wells

AbstractLow dark current photodiodes (PDs) in the short wavelength infrared (SWIR) region over 1.7 µm are expected to be used for many applications. InGaAs/GaAsSb type‐II quantum well (QW) structures are considered to be attractive material systems for realizing low dark current PDs, owing to lattice‐matching to InP substrates. In this report, we describe the successful operation of pin‐PDs with InGaAs/GaAsSb QWs with InP capping layers grown by metal‐organic vapor phase epitaxy (MOVPE). Distinct X‐ray diffraction satellite peaks and emissions from type‐II transition were observed. Electrical and optical characteristics of pin‐PDs, such as dependence of responsivity on the number of QWs, were investigated. Dark current was 9.0 µA/cm2 at 233 K and an external quantum efficiency (EQE) of 48% was obtained. These results indicate that it is possible to reduce the dark current of InGaAs/GaAsSb type‐II PDs by using the InP capping layers grown by MOVPE. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlations between the morphology and emission properties of trench defects in InGaN/GaN quantum wells

Atomic force microscopy (AFM) and scanning electron microscopy (SEM) with cathodoluminescence (CL) were performed on exactly the same defects in a blue-emitting InGaN/GaN multiple quantum well (QW) sample enabling the direct correlation of the morphology of an individual defect with its emission properties. The defects in question are observed in AFM and SEM as a trench partially or fully enclosing a region of the QW having altered emission properties. Their sub-surface structure has previously been shown to consist of a basal plane stacking fault (BSF) in the plane of the QW stack, and a stacking mismatch boundary (SMB) which opens up into a trench at the sample surface. In CL, the material enclosed by the trench may emit more or less intensely than the surrounding material, but always exhibits a redshift relative to the surrounding material. A strong correlation exists between the width of the trench and both the redshift and the intensity ratio, with the widest trenches surrounding regions which exhibit the brightest and most redshifted emission. Based on studies of the evolution of the trench width with the number of QWs from four additional MQW samples, we conclude that in order for a trench defect to emit intense, strongly redshifted light, the BSF must be formed in the early stages of the growth of the QW stack. The data suggest that the SMB may act as a non-radiative recombination center.

Efficiency of GaInAs/GaAs quantum-well lasers upon inhomogeneous excitation of quantum wells

A model for calculating the power characteristics of laser structures taking into account inhomogeneous excitation of quantum wells (QWs), recombination in the barrier regions, and nonlinear gain effects is developed. It is shown that, with increasing number of QWs, the output power of the structures at first considerably increases and then slightly decreases. In a wide range of injection currents, the optimal number of QWs is . The inhomogeneity of QW excitation increases with increasing injection current and decreases the laser power compared to homogeneous excitation.

A quantum-well superlattice solar cell for enhanced current output and minimized drop in open-circuit voltage under sunlight concentration

Insertion of quantum wells (QWs) extends the absorption edge to a longer wavelength than the value of a p-i-n cell without the QWs, which is preferable for the improved current matching of a InGaP/GaAs/Ge multijunction cell. The QWs, however, reduce the open-circuit voltage (Voc) and degrade the fill factor; the latter is significant for a large number of QWs that are mandatory for sufficient light absorption. As a structure to minimize these drawbacks, a QW superlattice, a strain-balanced In0.13Ga0.86As (4.7 nm)/GaAs0.57P0.43 (3.1 nm) stack, was implemented by metalorganic vapour-phase epitaxy. It brought about an enhancement in short-circuit current density (3.0 mA cm−2) with a minimal drop in Voc(0.03 V) compared with a p-i-n cell without the superlattice. The collection efficiency of photocarriers from the wells to an external circuit was evaluated: the efficiency was above 0.95 for the superlattice, while it was below 0.8 at a large forward bias for a conventional QW cell with thicker barriers. With the fast electron–hole separation in the superlattice owing to tunnelling transport, the superlattice cell exhibited a steeper increase in Voc as a function of the sunlight concentration ratio than the conventional QW cell: at the concentration ratio of 50, the value of Voc for the superlattice cell was almost equivalent to the value of the GaAs p-i-n cell without QWs. As a possible mechanism behind such an enhancement in Voc, photocurrent generation by two-step photon absorption was observed, using the electron ground state of the superlattice as an intermediate state.

Structural Amelioration of 920 Nm Optically Pumped Semiconductor Vertical External-Cavity Surface Emitting Laser (OPS-VECSEL)

920 nm optically pumped semiconductor vertical external-cavity surface emitting laser (OPS-VECSEL) has an important application in laser display. We constructed and optimized a 920 nm OPS-VECSEL with active region of In0.09Ga0.91As quantum well (QW) system pumped by 808 nm laser diode module. By the finite element method, self-consistent solutions of the semiconductor electronic and optical equations are realized to calculate the characteristics parameters of OPS-VECSEL. The performances of device especially the mode, the threshold and the optical-optical translation efficiency were analyzed by dealing with different number of QWs (1, 2 and 3) in one period, QW depth, barrier width, the component and dimension of the non-absorption layer. We chose an improved structure of them. On this basis, we ameliorated the number of QW periods and the simulation showed that in order to obtain high performance device, the choice of the number of QW periods must be cautious.

Universal behavior of photoluminescence in GaN-based quantum wells under hydrostatic pressure governed by built-in electric field

Correlation between the photoluminescence (PL) energy at ambient pressure and the pressure coefficient of photoluminescence is studied in quantum wells (QWs) based on nitride alloys, such as InGaN/GaN, GaN/AlGaN, and GaN/InAlN, grown along the polar direction [0001] of the wurtzite structure. Analyzing previously published and new experimental data, we have found that for InGaN/GaN QWs independent of In content (in the range between 6% and 25%) and also QW number and QW width, a linear relationship between these two parameters occurs. The presented experimental results are in agreement with numerical calculations carried out in the framework of the k→⋅p→ method with excitonic effects, provided that nonlinear piezoelectricity and nonlinear elasticity are taken into account. The performed analytical analysis indicates that the slope of the linear relationship between the pressure coefficient of photoluminescence and the photoluminescence energy at ambient pressure is determined by the logarithmic derivative of the built-in electric field with respect to pressure. Then, we show that the pressure coefficient of photoluminescence depends linearly on the photoluminescence energy at ambient pressure also in GaN/AlGaN and GaN/InAlN QWs. In GaN/AlGaN QWs, the slope of this dependence slightly decreases with Al content in the barriers. For GaN/InAlN QWs, we predict an unusual dependence of this slope on In content, which is associated with the vanishing built-in electric field in structures with 30% of In. For all studied nitride systems, a reasonable agreement between the experimental and theoretical results is achieved when the effects of nonlinear piezoelectricity and nonlinear elasticity are taken into account.

Effect of the number of quantum wells in the active region on the linearity of the light-current characteristic of a semiconductor laser

The light-current characteristic of a semiconductor laser with multiple quantum wells (QWs) is calculated, with the delayed capture of charge carriers from the waveguide region into the wells taken into account. It is shown that increasing the number of QWs is a more effective way to improve the power characteristics of a laser, compared with an increase in the velocity of carrier capture into each of the wells. For example, using two QWs as the active region leads to a substantial increase in the internal quantum efficiency of stimulated emission and to a significantly better linearity of the light-current characteristic of the laser, compared with a single-well structure. At the same time, using three or more QWs only slightly improves the power characteristics of the laser, compared with the double-well structure. Thus, a double-well structure is the optimal as regards high output power and simplicity of growth.

An improved carrier rate model to evaluate internal quantum efficiency and analyze efficiency droop origin of InGaN based light-emitting diodes

A carrier rate model taking carrier delocalization into account is presented to analyze current dependent internal quantum efficiency of InGaN based light-emitting diodes (LEDs). By fitting normalized experimental internal quantum efficiency-current curves, both injection efficiency and radiative recombination efficiency depending on current can be obtained. Based on the fitting results from two LED samples with and without the InGaN interlayer beneath the active regions of 5 InGaN quantum wells (QWs), carrier delocalization and carrier leakage are believed to lead to the efficiency droop effect under considerable and even larger injection, respectively. By investigating two LED samples with 8 and 10 QWs, it is found that the 8-QWs LED has the highest radiative recombination efficiency over 80% and the 10-QWs one has the highest injection efficiency over 50% under 120 A/cm2. This means that increasing QW number is an effective method to suppress droop effect.

Optimal number of quantum wells for blue InGaN/GaN light-emitting diodes

Blue InGaN/GaN light-emitting diodes with different numbers of quantum wells (QWs) are numerically investigated. With increase of QW number, the output power and forward voltage are both increased consistently. In terms of power conversion efficiency, the optimal number of QWs increases with the increase of injection currents and is twelve at experimental current of 42 A/cm2. Simulation results show that the bias voltage in QW plane is reduced with increase of QW number. This effect improves the current spreading and uniformity of carrier distribution in QW plane which results in the reduction of electron leakage and enhancement of emission efficiency.

量子阱数量变化对InGaN/AlGaN LED的影响

The optical properties of the InGaN/AlGaN light-emitting diodes(LEDs) with varied quantum well(QW) numbers are studied numerically.The simulated results show that the single quantum-well(SQW) structure has the best optical performance with small current less than 100 mA.However,the 9-QWs structure obtains higher output power and IQE than other structures when the current is larger than 700 mA,and the efficiency droop is dropped markedly from 14.10% to 5.15% at 700 mA comparing with that of the SQW structure.Therefore,9-QWs is the optimized structure for high-power LEDs.It can be explained as follows:(1) The 9-QWs structure acquires higher capacity of carrier confinement due to the smaller band bending effect and the effective hole potential barrier heights formed by the electron block layer(EBL).(2) When the QW number is less than 9,the lower capacity of carrier confinement and lower radiative recombination rate in the active region reduce the performance of the device,though the turn-on voltage is lower.(3) When the QW number is more than 9,the structure shows higher peak internal quantum efficiency(IQE) and less serious efficiency droop compared with 9-QW structure.However,the smaller IQE and larger band bending effect make it worse for a high power structure.

Investigation of spectral regulation in dual- wavelength light-emitting diodes by using the selective p-doped barriers

The electrical and the optical characteristics of dual-wavelength light-emitting diode (LED) with the newly designed selective p-doped barriers are investigated numerically. The simulation results show that the selective p-doped barriers can improve the distribution equilibria of electron and hole concentrations in each quantum well (QW). The radiative recombination rate of QW is enhanced remarkably when specific number of p-doped barriers is adopted, and the electron leakage current is suppressed obviously with this new design. Therefore, the internal quantum efficiency is improved and the trend of efficiency drooping with the increase of current injection is also alleviated. Moreover, the curve peaks of the spectrum become quite uniform when the specific number of vertically-stacked QWs is adopted, and the spectral regulation of the dual-wavelength LED is more effective.

The characterization of electroplex generated from the interface between 2-(4-trifluoromethyl-2-hydroxyphenyl)benzothiazole] zinc and N,N′-diphenyl-N,N′- bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine

The electroplex between (2-(4-trifluoromethyl-2-hydroxyphenyl)benzothiazole) zinc [Zn(4-TfmBTZ)2] as an electron-acceptor and N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) as an electron-donor was characterized by bilayer, blend, and multilayer quantum-well (MQW) device, respectively. The blend composition and quantum-well number are effective parameters for tuning electroluminescence color. White light with high color purity and color rendering index (CRI) was observed from these devices based on Zn(4-TfmBTZ)2/NPB. Moreover, the blend and MQW devices all exhibit high operation stability, hence excellent color stability. For the device with 5 mol% NPB in blend layer, its Commission International Del’Eclairage (CIE) coordinate region is x=0.28–0.31, y=0.33–0.35 and CRI is 83.3–91.2 at 5–9 V. For MQW structure device with NPB of 60 nm thickness, its CIE coordinate region is x=0.29–0.32, y=0.31–0.34 and CRI=87.9–92.5 at 10–15 V. Such high color stability and purity and CRI, being close to ideal white light, are of current important for white OLED.

Strong exciton-photon coupling in a monolithic ZnO/(Zn,Mg)O multiple quantum well microcavity

We report on strong exciton-photon coupling in an epitaxially grown (Zn,Mg)O-based λ-microcavity (MC) containing four 3.5 nm wide ZnO quantum wells (QWs) as active layers. At 5 K, the observed Rabi splitting in absorption is 26 meV, while the inhomogeneous linewidth of A and B excitons in similar QWs without a MC is about 10 meV. The strong coupling regime (SCR) is lost between 150 K and 200 K due to additional homogeneous broadening. From transfer matrix calculations, we deduce that increasing the number of QWs per cavity length can extend the SCR up to room temperature.

Modelling of a diode laser with a resonant grating of quantum wells and an external mirror

A three-dimensional numerical model of a diode laser with a resonant grating of quantum wells (QWs) and an external mirror is developed and used to calculate diode laser pulses that are long compared to the time of reaching a stationary regime and are short enough to neglect heating of the medium. The consistent solutions of the Helmholtz field equation and the system of diffusion equations for inversion in each QW are found. A source of charge carriers can be both an electron beam and a pump laser beam. The calculations yielded the longitudinal and radial profiles of the generatedfield, as well as its wavelength and power. The effective threshold pump current is determined. In the created iteration algorithm, the calculation time linearly increases with the number of QWs, which allows one to find the characteristics of lasers with a large number of QWs. The output powers and beam divergence angles of a cylindrical laser are calculated for different cavity lengths and pump spot radii. After calculating the fundamental mode characteristics, high-order modes were additionally calculated on the background of the frozen carrier distributions in the QW grating. It is shown that all the competing modes remain below the excitation threshold for the pump powers used in the experiment. The calculated and experimental data for the case of pumping by a nanosecond electron beam are qualitatively compared.

Modeling of vertical external cavity semiconductor laser with MQW resonant structure

Numerical model is developed for modeling pulsed operation of a vertical external cavity semiconductor laser (VECSEL) with a resonant gain structure. Properties of optical modes for compound resonator formed by Bragg reflector, chip boundary and external spherical mirror were studied. For above threshold operation carrier density in each of quantum wells (QW) obeys non-linear diffusion equation with a source supplied by electron-hole pairs generated in barrier layers by pump radiation or fast electron beam. A new iteration procedure for round-trip operator evaluation was developed, which provides linear growth of computation time with the number of QWs. Results of numerical simulations are reported for a VECSEL comprising 25 QW in resonant configuration and the output spherical mirror with curvature radius 3 cm. Variation of distance to the external mirror is found to result in notable changes in power and optical quality of the output beam. The decisive role of gain and index non-linearity in these changes is identified. A range of values of distance to external mirror is found where iterative procedure does not converge. In another range, the resulting solution depends on the initial conditions for the iteration procedure.

High quantum efficiency InGaN/GaN multiple quantum well solar cells with spectral response extending out to 520 nm

We demonstrate high quantum efficiency InGaN/GaN multiple quantum well (QW) solar cells with spectral response extending out to 520 nm. Increasing the number of QWs in the active region did not reduce the carrier collection efficiency for devices with 10, 20, and 30 QWs. Solar cells with 30 QWs and an intentionally roughened p-GaN surface exhibited a peak external quantum efficiency (EQE) of 70.9% at 390 nm, an EQE of 39.0% at 450 nm, an open circuit voltage of 1.93 V, and a short circuit current density of 2.53 mA/cm2 under 1.2 suns AM1.5G equivalent illumination.