Carriers In Quantum Wells Research Articles

Simulation of quantum states in solid-state ferromagnetic nanostructures

Purpose of the article. Identification and analysis of mathematical expressions for the energy spectrum of charge carriers in nano-sized magnetic films of nickel and Ni-Cu (substrate), as well as NiO-Ni-Cu (substrate) structures.Methods. In this work, based on basic quantum mechanical concepts and taking into account the boundary conditions for coupled quantum wells, expressions for the energy spectrum of electrons are obtained. The ferrimagnetic properties of nickel are taken into account through the effective mass value. The solution of nonlinear equations that determine discrete energy levels was achieved by numerical methods using the Mathcad mathematical package.Results. Transcendental expressions for the energies of free charge carriers in quantum wells of ferromagnetic nickel films are obtained, and the influence of the position of nickel on the copper substrate is shown. It has been shown that the use of a copper substrate leads to an increase in the density of energy levels in the nickel nanofilm. The influence of the oxide layer on single-electron states in nanofilms of nickel and its oxide is considered within the framework of the Anderson model. Based on the numerical solution of the transcendental equations obtained in the work, the influence of the ratio of the thicknesses of a ferromagnetic metal and its oxide on the energy levels of electrons localized in the oxide layer is shown.Conclusions. The formulas presented in the work for the energy spectrum take into account the energy relief of a complex quantum well, the dimensions of the film, surface oxide, and the significant effective mass in the region of the ferromagnetic film. It has been shown that an increase in the effective mass in magnetic heterostructures leads to an increase in the electron density of states. It was found that the density of electronic states in the region of surface nickel oxide is practically independent of the thickness of the nickel film. The results and conclusions of the work can be used for theoretical prediction of the physical properties of magnetic nanostructures, in particular spintronic elements.

Study on the Quantum Confinement of Photo-Generated Carriers in Quantum Wells

According to the classical theory, multi-quantum wells (MQWs) have quantum confinement effect on photo-generated carriers, which limits its application in light-to-electric devices. However, relevant experiments showed that a large proportion of photo-generated carriers can escape from MQWs sandwiched in the p-type layer and n-type layer (PIN structure), but not in the NIN structure. In order to study this phenomenon carefully, we applied positive and negative bias to an NIN structure respectively to simulate the PIN structure. By analyzing the photoluminescence (PL) spectra, we observed a weak escape behavior of photon-generated carriers among QWs in NIN structure. The experiment results indicate that strong electric field could drive carriers to escape from QWs rather than relaxation and recombination, while in NIN structure the inhomogeneous distribution of the electric field intensity reduces the carrier transport efficiency. The further study will give new ideas to design and produce photoelectric devices.

In-Well and Barrier Coupling Pump of Semiconductor Disk Laser

We report an in-well and barrier coupling pump scheme of semiconductor disk laser (SDL). An in-well-absorbed (940 nm) and a barrier-absorbed (808 nm) pump source were used to measure laser output characteristics. The output power of the coupling pump can be significantly improved compared to the in-well pump only scheme under similar thermal load conditions. The power scaling of the coupling pump SDL is caused by the interaction dynamics of the photoexcited carrier between quantum-well and barrier. The results are discussed with a rate equation model that considers the injected into the quantum-well carrier and the barrier carrier in the active region.

Influence of interface states on built-in electric field and diamagnetic-Landau energy shifts in asymmetric modulation-doped InGaAs/GaAs QWs

The impact of interface defect states on the recombination and transport properties of charges in asymmetric modulation-doped InGaAs/GaAs quantum wells (QWs) is investigated. Three sets of high-mobility InGaAs QW structures are systematically designed and grown by the metal-organic vapor phase epitaxy technique to probe the effect of carrier localization on the electro-optical processes. In these structures, a built-in electric field drifts electrons and holes towards the opposite hetero-junctions of the QW, where their capture/recapture processes are assessed by temperature-dependent photoreflectance, photoluminescence, and photoconductivity measurements. The strength of the electric field in the structures is estimated from the Franz Keldysh oscillations observed in the photoreflectance spectra. The effects of the charge carrier localization at the interfaces lead to a reduction of the net electric field at a low temperature. Given this, the magnetic field is used to re-distribute the charge carriers and help in suppressing the effect of interface defect states, which results in a simultaneous increase in luminescence and photoconductivity signals. The in-plane confinement of charge carriers in QW by the applied magnetic field is therefore used to compensate the localization effects caused due to the built-in electric field. Subsequently, it is proposed that under the presence of large interface defect states, a magnetic field-driven diamagnetic-Landau shift can be used to estimate the fundamental parameters of charge carriers from the magneto-photoconductivity spectra instead of magneto-photoluminescence spectra. The present investigation would be beneficial for the development of high mobility optoelectronic and spin photonic devices in the field of nano-technology.

Wide-spectrum and high-homogeneity InGaN tunnel-junction light-emitting diode stacked by two unit light-emitting diodes

A wide-spectrum InGaN tunnel-junction (TJ) light-emitting diode (LED) stacked by two unit LEDs with high homogeneity is investigated numerically. Effort is made to balance the emission intensity from the multiple-quantum well active regions of individual unit LEDs by optimizing the materials of quantum wells (QWs) and quantum barriers (QBs) as well as the thickness of QBs. Upon optimization, the proposed TJ-LED structure is of high radiative recombination efficiency and uniformly distributed carriers in QWs. The physical mechanisms relevant to the desirable output performance are explored. The results of this study may be of great help for the application of cascaded LEDs or micro-LEDs with minimal number of unit LEDs.

Time-Resolved Photoluminescence in Heterostructures with InGaAs:Cr/GaAs Quantum Wells

The results of studies of the time-resolved photoluminescence in semiconductor heterostructures containing two noninteracting InGaAs quantum wells in a GaAs matrix are reported. One of the quantum wells was undoped, and the other was uniformly doped with chromium atoms (InGaAs:Cr). It was shown that the introduction of Cr had a profound effect on the recombination lifetime of charge carriers in quantum wells. The change in the photoluminescence intensity after excitation cannot be described by a monoexponential decay function, which is attributed to a change in the built-in electric field of the surface barrier in the quantum wells because of screening by photoexcited charge carriers.

Фотолюминесценция с временным разрешением в гетероструктурах с квантовыми ямами InGaAs:Cr/GaAs

We present the results of time-resolved photoluminescence measurements carried out for semiconductor heterostructures containing two non-interacting quantum wells in the GaAs matrix: an undoped InGaAs quantum well and a quantum well uniformly doped with chromium atoms (InGaAs : Cr). It has been shown that the introduction of Cr significantly affects the recombination lifetime of carriers in quantum wells. The change in the intensity of photoluminescence, starting from the moment of excitation, is not described by a monoexponential decay function, which is explained by a change in the built-in electric field of the surface barrier in quantum wells due to screening by photoexcited carriers.

Спектры остаточной фотопроводимости в гетероструктурах с квантовыми ямами HgTe/CdHgTe

Spectra of persistent photoconductivity (PPC) in HgTe/CdHgTe quantum well (QW) heterostructures of both n- and p-types have been investigated at T = 4.2 K. PPC is shown to be either positive (increase of carrier concentration in QW) or negative depending on a wavelength of the illumination. As a general trend, PPC maxima in n-type sample correspond to PPC minima in p-type samples and vice versa. It is discovered that in p-type samples the illumination with certain wavelengths results in the freezing out of free carriers in QWs but not in the conversion of the conductivity type. The latter indicates a significant role in the PPC mechanism of the built-in electric field that is switching off at the QW neutralization.

Enhancement of Auger recombination induced by carrier localization in InGaN/GaN quantum wells

The origin of efficiency droop in state-of-the-art quality In GaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two m-plane In GaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar In GaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in In GaN/GaN QWs.

Direct Observation of Carrier Transportation Process in InGaAs/GaAs Multiple Quantum Wells Used for Solar Cells and Photodetectors**Supported by the National Natural Science Foundation of China under Grant Nos 11574362, 61210014, and 11374340, and the Innovative Clean-Energy Research and Application Program of Beijing Municipal Science and Technology Commission under Grant No Z151100003515001.

The resonant excitation is used to generate photo-excited carriers in quantum wells to observe the process of the carriers transportation by comparing the photoluminescence results between quantum wells with and without a p—n junction. It is observed directly in experiment that most of the photo-excited carriers in quantum wells with a p—n junction escape from quantum wells and form photocurrent rather than relax to the ground state of the quantum wells. The photo absorption coefficient of multiple quantum wells is also enhanced by a p—n junction. The results pave a novel way for solar cells and photodetectors making use of low-dimensional structure.

Direct observation of the carrier transport process in InGaN quantum wells with a pn-junction**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574362, 61210014, and 11374340) and the Innovative Clean-energy Research and Application Program of Beijing Municipal Science and Technology Commission, China (Grant No. Z151100003515001).

A new mechanism of light-to-electricity conversion that uses InGaN/GaN QWs with a p–n junction is reported. According to the well established light-to-electricity conversion theory, quantum wells (QWs) cannot be used in solar cells and photodetectors because the photogenerated carriers in QWs usually relax to ground energy levels, owing to quantum confinement, and cannot form a photocurrent. We observe directly that more than 95% of the photoexcited carriers escape from InGaN/GaN QWs to generate a photocurrent, indicating that the thermionic emission and tunneling processes proposed previously cannot explain carriers escaping from QWs. We show that photoexcited carriers can escape directly from the QWs when the device is under working conditions. Our finding challenges the current theory and demonstrates a new prospect for developing highly efficient solar cells and photodetectors.

Influence of localized surface plasmons on carrier dynamics in InGaN/GaN quantum wells covered with Ag nanoparticles for enhanced photoluminescence

An analysis of the complex mechanisms of enhanced photoluminescence (PL) from InGaN/GaN quantum wells (QWs) by surface plasmon (SP) coupling is reported. Silver nanoparticles were deposited on the QWs to generate a wide surface plasmon resonance (SPR) band covering both PL excitation and emission wavelengths. Significant enhancement of the integrated PL intensity, a blue shift of the PL peak wavelength, a suppression of the relative contribution of PL from In-rich quantum dot-like structures, and an increased PL decay time were observed in temperature-dependent and time-resolved PL spectroscopy. In addition to an enhanced radiative recombination rate by resonant coupling the spontaneous emissions to SPs, QW carrier dynamics can be significantly affected by SPR-enhanced, light-induced local fields. The latter occurs because of screening of the quantum-confined Stark effect and the enhanced localization of carriers.

Interband and intraband radiation from the n-InGaAs/GaAs heterostructures with quantum wells under the conditions of injection in high lateral electric fields

The interband and intraband radiation from the n-InGaAs/GaAs heterostructures with the double and triple tunnel coupled and selectively doped quantum wells (QWs), which is appeared under the lateral electric field and in the presence of hole injection from the anode contact, has been investigated. A steep increase of the interband radiation intensity was found at the fields of E≥1.7kV/cm. This effect should be related to the big lifetime of the injected charge carriers (~10−6s) which exceeds by three orders of magnitude the lifetime in the similar bulk direct-band semiconductor. Its reason lies in spatial separation of the injected holes and electrons between coupled wells, firstly, by the built-in transverse electric field between wells and, secondly, due to the real-space transfer of carriers heated by the lateral electric field from the wide well to the narrow δ-doped one. Furthermore, an increase of the carrier concentration due to injection leads to an increase of that transition intensity and, consequently, to an intensity increase of the radiative intersubband transitions of carriers in QWs which results in a steep intensity increase of the far (50–120µm) infrared radiation.

The effect of ferromagnetic Mn-delta-doped layer on the emission properties of GaAsSb/GaAs and InGaAs/GaAsSb/GaAs heterostructures

We have studied the emission characteristics and circularly polarized electroluminescence of light-emitting diodes based on heterostructures with a single (GaAs/GaAsSb/GaAs) or two-layer (GaAs/InGaAs/GaAsSb/GaAs) quantum well (QW) and a Mn-delta-doped layer in the GaAs barrier. The ferromagnetic effect of the delta-layer of Mn on the spin polarization of carriers in QWs based on type-II heterostructures has been observed and studied for the first time. The observed phenomena are described using a model of the exchange interaction of Mn ions in the barrier and holes in the QW.

Hot Carriers in Quantum Wells for Photovoltaic Efficiency Enhancement

In a hot carrier solar cell, the steady-state carrier population is hot relative to the surrounding lattice. This requires an absorber material which restricts carrier-phonon interaction and, therefore, reduces entropic loss during thermalization. The limiting efficiency of these devices approaches 85%: the Carnot limit for a solar energy collector. A spectroscopic analysis of GaAsP/InGaAs quantum well structures shows that carrier cooling in single quantum well samples is dominated by the rate of radiative recombination, leading to unprecedented carrier cooling lifetime (τ = 5.8 ±0.1 ns). This exceptional lifetime arises due to state saturation, frustrating the carrier scattering processes. A steady-state carrier population temperature >100 K above the lattice temperature is measured under illumination equivalent to 10 000 Suns. We calculate the projected efficiency >40% for a device with these characteristics, amounting to a 3% efficiency enhancement over equivalent single-junction devices.

Efficiency Enhancement of Blue InGaN Light-Emitting Diodes With Shallow First Well

Efficiency enhancement of blue InGaN/GaN light-emitting diodes (LEDs) with shallow first well is investigated. Energy band diagrams, carrier concentrations in quantum wells, spontaneous emission spectra, light-current-voltage performance curves, and internal quantum efficiency are studied. Simulation results show that the LED is markedly improved when regular first well is replaced by shallow first well due to reduced polarization field and thereby enhanced injection efficiency of carriers in quantum wells. The proposed structure is beneficial for the blue InGaN LED even at a high level of current injection.

Spin-polarized half-metallic state of a ferromagneticδlayer in a semiconductor host

We discuss a model which is apt to describe the appearance of a spin-polarized half-metallic state around a single δ layer of a magnetic transition metal embedded into a nonmagnetic semiconductor host. We show that ferromagnetism in this system can be attributed to the intrinsic physical properties of the δ layer. The relevant physicaleffectsdescribedbyourmodelarethehybridizationoftheelectronstatesofthemetalandsemiconductor atoms, the charge redistribution around the δ layer, and the electron-electron correlation on a metal atom, which is the driving force of ferromagnetism. We obtain the mean-field phase diagram of the model at zero and finite temperature, both in the case when the chemical potential is fixed, and when the density of particles on the δ layer isfixed.Therelevanceofourresultsinconnectionwithnumericalandexperimentalresultsontheso-calleddigital magnetic heterostructures, in the absence and in the presence of a quantum-well carrier channel, is eventually discussed.

Modeling of injection characteristics of polar and nonpolar III-nitride multiple quantum well structures

Carrier confinement and injection characteristics of polar and nonpolar III-nitride quantum well (QW) light-emitting diode or laser diode structures are compared. We demonstrate that strongly inhomogeneous QW injection in multiple-QW (MQW) active region is one of the possible reasons holding back the advance of nonpolar laser structures. In polar structures, strong interface polarization charges induce the nonuniform carrier distribution among the active QWs so that the extreme p-side QW always dominates the optical emission. On the contrary, in nonpolar MQW structures, the inhomogeneity of QW populations is supported mainly by QW residual charges and the prevailing QW is the one closest to the n-side of the diode. For both polar and nonpolar structures, the QW injection inhomogeneity is strongly affected by the QW carrier confinement and becomes more pronounced in longer wavelength emitters with deeper active QWs. We show that in nonpolar structures indium incorporation into optical waveguide layers improves the uniformity of QW injection. On the contrary, QW injection in polar structures remains inhomogeneous even at high-indium waveguide layer compositions. We show, however, that polarization-matched design of the electron-blocking layer can noticeably improve the injection uniformity in polar MQW structure and enhance the structure internal quantum efficiency.

Numerical Simulation of Temporal and Spectral Variation of Gain and Phase Recovery in Quantum-Dot Semiconductor Optical Amplifiers

We numerically investigate the gain and phase recovery dynamics of quantum-dot (QD) semiconductor optical amplifiers (SOAs) by simultaneous consideration of the temporal and spectral variations. We solve 1088 coupled rate equations to simulate the carrier recovery dynamics of the inhomogeneously broadened QD ensemble, where QD electron and hole states are considered separately. The gain and phase recovery responses induced by an ultrafast pump pulse are calculated by considering different carrier relaxation time constants and spectral line shape functions involved in the successive carrier recovery process. The interband transition is assumed to have a Lorentzian line shape function and the intraband free-carrier absorption is described by the Drude model. We demonstrate that the actual behavior of the gain and phase recovery in QD SOAs can be clearly understood by simultaneously considering the temporal and spectral behavior of the gain and phase recovery responses, which are visualized by means of plots in the time and wavelength domains. We identify how the respective QD state-the ground state, the excited state, and an upper state-and quantum-well carrier reservoirs contribute to the gain and phase recovery of QD SOAs both temporally and spectrally.

Dynamics of photoluminescence and recombination processes in Sb-containing laser nanostructures

The dynamics of interband photoluminescence has been studied at various temperatures and excitation levels in structures with quantum wells based on InGaAsSb alloys and barriers based on AlGaAsSb and AlInGaAsSb alloys. The lifetimes of optically injected charge carriers in quantum wells at various temperatures and levels of optical excitation have been experimentally determined. An increase in the recombination rate in structures with deeper InGaAsSb/AlGaAsSb quantum wells for electrons is attributed to manifestation of resonant Auger recombination. The Auger recombination brings about heating of electrons and holes in lower subbands of dimensional quantization. The temperature of charge carriers in the course of Auger recombination is estimated using the equation for balance of power with accumulation of nonequilibrium optical phonons taken into account. The studied structures were used to fabricate lasers of two types with lasing wavelength of approximately 3 μm; it is shown that the use of a quinary alloy as the material for the barrier leads to an improvement in the characteristics of the lasers.