Carriers In Quantum Wells Research Articles

Effect of polarization state on optical properties of blue-violet InGaN light-emitting diodes

The optical properties of blue-violet InGaN light-emitting diodes under normal and reversed polarizations are numerically studied. The best light-emitting performance under normal and reversed polarization is obtained in a single quantum-well structure and double quantum-well structure, respectively. The key factors responsible for these phenomena are presumably the carrier concentration distribution and the amount of carriers in quantum wells. The turn-on voltage of light-emitting diodes under reversed polarization is lower than that of light-emitting diodes under normal polarization, due mainly to lower potential heights for electrons and holes in the active region.

Role of carrier reservoirs on the slow phase recovery of quantum dot semiconductor optical amplifiers

The gain and phase recovery dynamics of quantum-dot (QD) semiconductor optical amplifiers are calculated, including all the optical transitions involved in successive carrier recovery processes. The carrier recovery dynamics of inhomogeneously broadened QDs is simulated by solving 1088 coupled rate equations. The respective contributions of QD states and quantum-well carrier reservoirs to the gain and phase changes are identified both temporally and spectrally. We show that the slow phase recovery component of the QD ground state is induced by the slow carrier dynamics of the carrier reservoir due to a slowly varying line shape function of the refractive index change.

Effect of pump reflections in vertical external cavity surface-emitting lasers

A numerical analysis of the influence of pump reflections on the carrier generation rate and the uniformity of carrier population in the quantum wells (QWs) of vertical-cavity surface-emitting lasers is presented. We have applied an approach allowing us to study the carrier distribution over the absorbing layers and the QW layers of the structure which is quite general and for an arbitrary function of the local carrier generation rate. When analysing the dual-wavelength VECSEL (Leinonen T et al 2007 Opt. Express 15 13451–6), we have found that application of carrier transport blocking layers can result in a highly uniform QW carrier population. The pump reflections are shown to have a great impact on the carrier distribution in the device under study.

Carrier capture in InGaAsSb∕InAs∕InGaSb type-II laser heterostructures

Experimental studies of the electron and hole concentration dynamics in the barrier of GaSb-based type-II quantum-well (QW) heterostructures were performed. Capture of electrons and holes was studied separately in specially designed and grown laser heterostructures with QWs only for electrons or only for holes. The difference between electron and hole relaxation rates is explained by corresponding QW carrier confinement energies.

Chirp in a transistor laser: Franz-Keldysh reduction of the linewidth enhancement

A quantum well (QW) transistor laser with the capability of high current density, a collector current ensuring minimal operational change (beyond laser threshold) in base QW carrier population, a favorable long narrow emitter laser geometry, and built-in base-collector Franz-Keldysh absorption (with IE+IB+IC=0), is measured for chirp-related behavior. A relatively low transistor laser linewidth enhancement factor αe∼0.7 is obtained despite the resolution limitations of the measurement apparatus.

Localized states in the active region of blue LEDs related to a system of extended defects

Blue light-emitting diodes (LEDs) based on InGaN/GaN quantum wells (QWs) with different characters of the system of extended defects (SEDs) threading through the active region have been studied using the current-voltage (I–U), capacitance-voltage (C–V), and deep-level transient spectroscopy (DLTS) measurements in the dark and under illumination with white light in a temperature range from 100 to 450 K. The DLTS curves exhibit broad E1 and E2 peaks with amplitudes dependent on the illumination. This behavior can be explained assuming the presence of localized states related to SEDs in the active region of the LED. The LEDs with more developed SEDs are characterized by a greater concentration of donor-type traps, which leads to an increase in the density of free charge carriers in QWs, which screen the electron-hole interaction. This circumstance can be among the factors responsible for a severalfold decrease in the quantum efficiency of such LEDs.

Time-Resolved Studies of Excitonic Dynamics in a Wide II-VI Quantum Well by a Femtosecond Pump-Probe Reflectivity

The time-resolved reflection was studied on wide quantum wells (QW) based on semimagnetic semiconductors. The QW made of Cd1-xMnxTe were embedded between (Cd,Mg)Te barriers. The use of 160 A wide QW allowed observation of up to 7 excitonic lines associated to excited states of carriers confined in the QW. Several samples with concentration of Mn ions in QWs ranging from 0 to 2.5% were used. The giant Zeeman splitting was primarily used to assign the observed lines to heavy and light hole excitons. The time-resolved reflection experiments were performed in a pump-probe set-up with a tunable Ti:Al2O3 femtosecond pulsed laser as a light source. The delay between pump and probe pulses was precisely controlled by a delay line. The same laser was a source for both the pump and the probe pulses. The probe is an spectrally broad (>40nm) laser pulse taken directly from the laser. The pump is a spectrally narrow line selected from the dispersed laser spectrum. Using the pump pulses narrowed spectrally to about 1nm it was possible to excite resonantly selected states in a quantum well. The independent circular polarizers on the pump and probe beams allowed to study the evolution of populations of states with different spin orientations. The experimental set-up was similar to those used in Refs. [1] and [2] to study ultrafast processes in narrow QW, i.e. the interplay between excitons, trions and carriers in QWs. In the present experiments we extended such studies on the excited states in wide QW. In particular we observed a significant quenching of the absorption of the excited states by the high population of excitons in the fundamental excitonic subband. It was also possible to study the dynamics of the relaxation from the excited levels to the ground states.

Self-consistent modeling of escape and capture of carriers in quantum wells

A self-consistent model for escape and capture of carriers in quantum wells is described. The model accounts for carrier dynamics in the quantum wells by calculating the rates of escape and capture of photo-excited carriers in the wells, and including these rates in a self-consistent solution of the Poisson, Schrödinger, and drift–diffusion equations. The results obtained are in good agreement with those experimentally measured or theoretically calculated by others. Furthermore, the escape and capture rates calculated are in compliance with the requirements of detailed balance theory.

A near-room-temperature all-optical polarization switch based on the excitation of spin-polarized “virtual” carriers in quantum wells

Near-room-temperature operation of an all-optical polarization switch based on the virtual excitation of spin-polarized carriers in semiconductor quantum wells is demonstrated. The device is shown to exhibit a pulse-width-limited switching time, a contrast ratio of >18dB, an optical bandwidth of ∼3THz, and an energy throughput of >0.1% using a thin (40 wells) GaAs∕AlGaAs sample. The results of differential transmission measurements are used to identify the dominant switching mechanisms and to monitor the spin and temporal dynamics of the carriers excited during the operation of the switch.

Probing carriers in two-dimensional systems with high spatial resolution by scanning spreading resistance microscopy

In this work, cross-sectional scanning spreading resistance microscopy (SSRM) is used to profile carriers in quantum wells (QWs). The investigated structures consist of InGaAs wells of different widths sandwiched between Si-doped InP barriers. It is demonstrated that SSRM is indeed capable of detecting electrons in the quantum wells with high lateral resolution and that the SSRM signal shows a systematic trend for the different wells. Clear dips in the resistance signal are observed at the quantum wells and imply accumulated electron densities higher than in the surrounding barriers. Carrier density in the QW is found by using the calibration curve obtained from the resistance measurements on reference layers sample. It is also shown that only at certain appropriate tip-sample bias conditions the depletion regions in the barriers adjacent to the wells are resolved. Finally, we demonstrate that under very low forward biases the full width at half maximum of the observed resistance dips in SSRM data is nearly equal to the geometric QW widths.

Significance of dimensionality and dynamical screening on hot carrier relaxation in bulk GaAs and quantum wells

We have found theoretically that the reduced dimensionality from bulk GaAs to quantum wells has a strong effect on hot carrier relaxations at highly excited carrier densities. The distinct density of state and the dynamical screening cause hot carriers in quantum wells to relax significantly slower than in the bulk when the carrier density is above a critical value of 2×10 18 cm −3. With the random phase approximation, the dynamical screening in quantum wells appears to be much stronger than that in the bulk and as important as the hot phonon effect at high carrier densities. We also found that the dependence of the average energy-loss rates on the well width in quantum wells becomes more appreciable when Al compositions are high.

Modeling of multiple-quantum-well solar cells including capture, escape, and recombination of photoexcited carriers in quantum wells

A self-consistent numerical Poisson-Schrodinger-drift-diffusion solver is described for simulation of multiple-quantum-well (MQW) Al/sub x/Ga/sub 1-x/As-GaAs solar cells. The rates of escape, capture, and recombination of photoexcited carriers in quantum wells embedded in the intrinsic region of a p-i-n device are self-consistently incorporated in the model. The performance of the device for various quantum-well configurations is investigated and the device characteristics are related to the dynamics of capture, escape, absorption, and recombination of carriers in the quantum wells. Our results show that the incorporation of MQWs in the intrinsic region of a p-i-n solar cell can improve the conversion efficiency of non-optimal devices, if the device is designed based on careful consideration of the behavior of the photoexcited carriers in the quantum wells. Specifically, we found out that an Al/sub 0.1/Ga/sub 0.9/As-GaAs cell with multiple quantum wells of 150 /spl Aring/ is more efficient than an identical single bandgap Al/sub 0.1/Ga/sub 0.9/As cell with no quantum wells, but less efficient than a single bandgap GaAs cell without such quantum wells.

Electron heating by a strong longitudinal electric field in quantum wells

The electron heating by a strong longitudinal electric field and the energy losses due to the scattering of nonequilibrium electrons by polar optical phonons in rectangular GaAs/AlGaAs quantum wells are studied. A simple model is suggested to calculate the rate of energy losses due to the scattering of electrons by nonequilibrium optical phonons. Some of the experimental results on the heating of charge carriers in quantum wells are discussed, and it is shown that taking nonequilibrium optical phonons into account significantly improves the agreement between the theoretical and experimental data.

Spin-Photocurrent in p-SiGe Quantum Wells Under Terahertz Laser Irradiation

A detailed study of the circular photogalvanic effect (CPGE) in SiGe structures is presented. It is shown that the CPGE becomes possible because of the built-in asymmetry of quantum wells (QWs) in compositionally stepped samples and in asymmetrically doped structures. The photocurrent arises due to optical spin orientation of free carriers in QWs with spin splitting in k-space. It is shown that the effect can be applied to probe the macroscopic in-plane symmetry of low dimensional structures and allowing to conclude on Rashba or Dresselhaus terms in the Hamiltonian.

Numerically stable and flexible method for solutions of the Schrodinger equation with self-interaction of carriers in quantum wells

A numerically stable method to calculate the quantum states of carriers based on the variational principle is proposed. It is especially effective for the carriers confined in the quantum wells under the influence of self-interaction of the carriers. In this treatment, a wave function is defined as a set of scalar numbers based on the finite-difference approach. An action defined as the expectation value of a Hamiltonian becomes a multivariate function of the wave function. Application of numerical multidimensional minimization procedures to the action can achieve stable convergence even under the conditions where the conventional self-consistent approach to Schrodinger and Poisson equations fails to give solutions. Application to the calculations of ground states in modulation-doped single quantum wells is demonstrated, and quantitative comparison to the conventional method is also presented. This method has implications not only for numerical procedures, but also for the numerical realization of the variational principle, a fundamental concept in physics.

Removal of spin degeneracy inp-SiGe quantum wells demonstrated by spin photocurrents

Spin photocurrents requiring a system lacking inversion symmetry, become possible in SiGe based quantum well (QW) structures due to their built-in asymmetry. We report on circular and linear photogalvanic effects induced by infrared radiation in (001)- and (113)-oriented $p\ensuremath{-}{\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ QW structures and analyze the observations in view of the possible symmetry of these structures. The circular photogalvanic effect arises due to optical spin orientation of free carriers in QW's with spin-split subbands. It results in a directed motion of free carriers in the plane of the QW. We discuss possible microscopic mechanisms that could remove the spin degeneracy of the electronic subband states.

Photogalvanic effects in quantum wells

Circular and linear photogalvanic effects induced by far-infrared radiation have been investigated in both n-type and p-type quantum wells (QWs) of various point symmetry groups. The circular photogalvanic effect arises due to optical spin orientation of free carriers in QWs which results in a directed motion of free carriers in the plane of a QW perpendicular to the direction of light propagation. Due to selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right to left.

Circular Photogalvanic Effect in SiGe Semiconductor Quantum Wells

ABSTRACTThe photogalvanic effects, which require a system lacking inversion symmetry, become possible in SiGe based quantum well (QW) structures due to their built-in asymmetry. We report on observations of the circular and linear photogalvanic effects induced by infrared radiation in (001)-and (113)-orientedp–Si/Si1–xGex QW structures and analyse these observations in view of the possible symmetry of these structures. The circular photogalvanic effect arises due to optical spin orientation of free carriers in QWs with band splitting in k-space which results in a directed motion of free carriers in the plane of the QW. We discuss possible mechanisms that give rise to spin-splitting of the electronic subband states for different symmetries.

Quantum control of charge carriers in quantum wells

We present theoretical studies of the quantum control of charge carriers in a direct-current-biased (DC-biased) asymmetric double quantum well (ADQW). The objective in this work is to find the laser pulse that drives an electronic wave packet to maximum overlap with a target distribution in a chosen well at a chosen time. We use a genetic algorithm to determine the optimal parameters of the excitation pulses. The robustness of the control is analyzed with respect to the magnitude of the DC field, and two types of defects in the ADQW potential profile, a point defect and a notch. In all cases studied, the genetic algorithm is able to reoptimize the laser field to achieve the desired objective. Finally, we consider the effects of coupling the ADQW to a harmonic bath. We find that a coupling large enough to distort the effective potential significantly does not eliminate the control. Our results indicate that control in the ADQW is quite robust, which should make it a favorable system for further experimental study. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 393–402, 2000

Quantum control of charge carriers in quantum wells

We present theoretical studies of the quantum control of charge carriers in a direct-current-biased (DC-biased) asymmetric double quantum well (ADQW). The objective in this work is to find the laser pulse that drives an electronic wave packet to maximum overlap with a target distribution in a chosen well at a chosen time. We use a genetic algorithm to determine the optimal parameters of the excitation pulses. The robustness of the control is analyzed with respect to the magnitude of the DC field, and two types of defects in the ADQW potential profile, a point defect and a notch. In all cases studied, the genetic algorithm is able to reoptimize the laser field to achieve the desired objective. Finally, we consider the effects of coupling the ADQW to a harmonic bath. We find that a coupling large enough to distort the effective potential significantly does not eliminate the control. Our results indicate that control in the ADQW is quite robust, which should make it a favorable system for further experimental study. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 393–402, 2000