Electrorefraction in quantum dots

We developed a model to calculate the electrorefraction due to the Quantum Confined Stark Effect in strained cylindrical QDs. We followed a numerical matrix diagonalization method using an expansion in plane-wave states. The correct polarization dependence is obtained by using the 4X4 Luttinger-Kohn Hamiltonian for the valence band. The exciton wavefunction is also calculated using a diagonalization approach. We will present the absorption spectra as a function of applied electric field and dot dimensions and discuss the implications for the electrorefraction

Experimental analysis of the efficiency of heterostructure GaAs-AlGaAs solar cells

Some years ago Multiple Quantum Wells (MQW) solar cells were introduced as an alternative to obtain high efficiencies. Based on the simple Shockley diode model, the short-circuit current could be increased without loss in the open-circuit voltage. Applying the Detailed Balance Theory, including radiative recombination in the i-region, leads to less optimistic predictions of the limiting efficiency of MQW cells. We present an experimental study in order to compare the efficiency of MQW solar cells with heterostructure cells with graded Al compositions and single bandgap solar cells. Compared to the homojunction AlGaAs cell, an increase of the short-circuit current is observed by the incorporation of GaAs in the i-region. However, the open-circuit voltage is reduced by the implementation of GaAs, due to an increase of the non-radiative recombination current. To estimate the maximum possible open-circuit voltage, the radiative recombination current is determined by measuring the light emission as a function of the applied voltage. From this experiment we conclude that the maximum possible open-circuit voltage of all the heterostructure cells is considerably lower than the homogeneous AlGaAs cell and close to the value of the GaAs cell, showing the relation between the open-circuit voltage and the smallest bandgap in the cell. The measured curves can be well predicted by calculations based on the Detailed Balance Theory. We find no principal advantage of MQW cells over cells with graded composition or single bandgap cells

Low field Monte-Carlo calculations in heterojunctions and quantum wells

We present results of low-field Monte-Carlo calculations and compare them with experimental results. The negative absolute mobility of minority electrons in p-type quantum wells, as found in recent experiments, is described quite well

Self-organized ordered quantum dot molecules (Invited)

No abstract

InAs/InP quantum dots emitting in the 1.55 μm wavelength region by inserting ultra-thin GaAs and GaP interlayers

In this letter, we demonstrate electrically pumped continuous-wave lasing at room temperature in microring lasers, which employ a quantum-dot gain medium. Lasing occurs in the important 1.55-??m telecom wavelength range. The 2-??m-wide ring waveguides are made from InGaAsP–InP (100) material suitable for active–passive photonic integrated circuits. Lasing in rings down to 22 ??m in diameter is found, with a threshold current of 12.5 mA. Index Terms—Integrated optics, microring lasers, quantum dots (QDs), semiconductor lasers

Random electric fields and impurity diffusion in δ layers

Impurity diffusion in the d layer during the process of its growth has been considered. Experiments show that the spreading of the impurity profile has a complex dependence on the in-plane impurity concentration. We carried out the numerical simulation of the self-consistent diffusion problem for the impurities moving in their own random electric field and have shown that at some critical impurity concentration in d layer the impurity distribution function perpendicular to the layer acquires a non-Gaussian character

Anisotropic Corbino magnetothermopower in a quantum Hall system

A Corbino-geometry contact configuration combined with a scanning laserspot as a heating source, is used for a thermovoltage mapping in a GaAs-AlGaAs quantum Hall device. For an isotropic system, the Corbino thermopower yields the diagonal component epsilon /sub xx/ of the thermoelectric tensor, which should be zero under the prevailing condition of phonon drag. The experiments reveal that epsilon /sub xx/ is large and anisotropic with respect to the crystallographic directions. The observations yield conclusive evidence that inhomogeneities are the origin for the existence of epsilon /sub xx

Determination of the intra - and inter-subband scattering times in a degenerate dimensional electron gas

Measurements of magnetoresistance oscillations have been performed for a twodimensional electron gas where two subbands are occupied to study processes of inter- and intra-subband scattering. Analysis of the temperature dependence of the oscillations allowed us to estimate independently inter- and intra-subband scattering times in the subbands. It has been found that the mobility in the first subband is mainly determined by the intra-subband scattering events, while that in the second subband is limited by processes of inter-subband scattering

Ordered quantum dot molecules and single quantum dots formed by self-organized anisotropic strain engineering

An ordered lattice of lateral InAs quantum dot (QD) molecules is created by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs superlattice (SL) template on GaAs(311)B by molecular-beam epitaxy, constituting a Turing pattern in solid state. The SL template and InAs QD growth conditions, such as the number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers, are studied in detail for an optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy. The average number of InAs QDs within the molecules is controlled by the thickness of the upper GaAs separation layer on the SL template and the (In,Ga)As growth temperature in the SL. The strain-correlated growth in SL template formation and QD ordering is directly confirmed by high-resolution x-ray diffraction. Ordered arrays of single InAs QDs on the SL template nodes are realized for elevated SL template and InAs QD growth temperatures together with the insertion of a second InAs QD layer. The InAs QD molecules exhibit strong photoluminescence (PL) emission up to room temperature. Temperature-dependent PL measurements exhibit an unusual behavior of the full width at half maximum, indicating carrier redistribution solely within the QD molecules. ©2005 American Institute of Physic