Valence band tail states in disordered superlattices embedded in wide parabolic AlGaAs well

Optical properties of intentionally disordered multiple quantum well (QW) system embedded in a wide AlGaAs parabolic well were investigated by photoluminescence (PL) measurements as functions of the laser excitation power and the temperature. The characterization of the carriers localized in the individual wells was allowed due to the artificial disorder that caused spectral separation of the photoluminescence lines emitted by different wells. We observed that the photoluminescence peak intensity from each quantum well shifted to high energy as the excitation power was increased. This blue-shift is associated with the filling of localized states in the valence band tail. We also found that the dependence of the peak intensity on the temperature is very sensitive to the excitation power. The temperature dependence of the photoluminescence peak energy from each QW was well fitted using a model that takes into account the thermal redistribution of the localized carriers. Our results demonstrate that the band tails in the studied structures are caused by alloy potential fluctuations and the band tail states dominate the emission from the peripheral wells. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4730769]

Magnetophotoluminescence study of GaAs/AlGaAs coupled double quantum wells with bimodal heterointerface roughness

This work reports on the results of magnetophotoluminescence (MPL) measurements carried out in a sample containing two Al0.35Ga0.65As/GaAs, coupled double quantum wells (CDQWs), with inter-well barriers of different thicknesses, which have the heterointerfaces characterized by a distribution of bimodal roughness. The MPL measurements were performed at 4 K, with magnetic fields applied parallel to the growth direction, and varying from 0 to 12 T. The diamagnetic shift of the photoluminescence (PL) peaks is more sensitive to changes in the confinement potential, due to monolayer variations in the mini-well thickness, rather than to the exciton localization at the local potential fluctuations. As the magnetic field increases, the relative intensities of the two peaks in each PL band inverts, what is attributed to the reduction in the radiative lifetime of the delocalized excitons, which results in the radiative recombination, before the excitonic migration between the higher and lower energy regions in each CDQW occurs. The dependence of the full width at half maximum (FWHM) on magnetic field shows different behaviors for each PL peak, which are attributed to the different levels and correlation lengths of the potential fluctuations present in the regions associated with each recombination channel. (C) 2011 Elsevier B.V. All rights reserved.

Circularly Polarized Photoluminescence as a Probe of Density of States in GaAs/AlGaAs Quantum Hall Bilayers

Polarized magnetophotoluminescence is employed to study the energies and occupancies of four lowest Landau levels in a couple quantum Hall GaAs/AlGaAs double quantum well. As a result, a magnetic field-induced redistribution of charge over the Landau levels manifesting to the continuous formation of the charge density wave and direct evidence for the symmetric-antisymmetric gap shrinkage at v = 3 are found. The observed interlayer charge exchange causes depolarization of the ferromagnetic ground state.

Theoretical and experimental study of the excitonic binding energy in GaAs/AlGaAs single and coupled double quantum wells

This paper discusses the theoretical and experimental results obtained for the excitonic binding energy (Eb) in a set of single and coupled double quantum wells (SQWs and CDQWs) of GaAs/AlGaAs with different Al concentrations (Al%) and inter-well barrier thicknesses. To obtain the theoretical Eb the method proposed by Mathieu, Lefebvre and Christol (MLC) was used, which is based on the idea of fractional-dimension space, together with the approach proposed by Zhao et al., which extends the MLC method for application in CDQWs. Through magnetophotoluminescence (MPL) measurements performed at 4 K with magnetic fields ranging from 0 T to 12 T, the diamagnetic shift curves were plotted and adjusted using two expressions: one appropriate to fit the curve in the range of low intensity fields and another for the range of high intensity fields, providing the experimental Eb values. The effects of increasing the Al% and the inter-well barrier thickness on Eb are discussed. The Eb reduction when going from the SQW to the CDQW with 5 Å inter-well barrier is clearly observed experimentally for 35% Al concentration and this trend can be noticed even for concentrations as low as 25% and 15%, although the Eb variations in these latter cases are within the error bars. As the Zhao's approach is unable to describe this effect, the wave functions and the probability densities for electrons and holes were calculated, allowing us to explain this effect as being due to a decrease in the spatial superposition of the wave functions caused by the thin inter-well barrier.

Dynamics of photoexcited carriers in the presence of disorder in radial heterostructured GaAs/AlGaAs/GaAs nanowires

Time-resolved photoluminescence was employed to study electron-hole dynamics in radial heterostructured GaAs/AlGaAs/GaAs core/inner shell/outer shell nanowires. It was found that impurity random potential results in a red shift of the recombination time maximum with respect to the photoluminescence peak energy.

Diffusion of Zn into GaAs and AlGaAs from isothermal Liquid-phase epitaxy solutions

In this work we present results of zinc diffusion in GaAs using the liquid phase epitaxy technique from liquid solutions of Ga‐As‐Zn and Ga‐As‐Al‐Zn. Using silicon‐doped n‐GaAs substrates, working at a diffusion temperature of 850 °C, and introducing a dopant concentration ranging 1018–1019 cm−3, the most important findings regarding the diffusion properties are as follows: (a) zinc concentration in the solid depends on the square root of zinc atomic fraction in the liquid; (b) the diffusion is dominated by the interstitial‐substitutional process; (c) the diffusivity D varies as about C3 in the form D=2.9×10−67C3.05; (d) aluminum plays the role of the catalyst of the diffusion process, if it is introduced in the liquid solution, since it is found that D varies as (γAsXlAs)−1; (e) the zinc interstitial is mainly doubly ionized (Zn++i); (f) the zinc diffusion coefficient in Al0.85 Ga0.15 As is about four times greater than in GaAs; (g) by means of all these results, it is possible to control zinc diffusion processes in order to obtain optimized depth junctions and doping levels in semiconductor device fabrication.

Performance analysis of AlGaAs/GaAs tunnel junctions for ultra-high concentration photovoltaics

An n(++)-GaAs/p(++)-AlGaAs tunnel junction with a peak current density of 10 100Acm(-2) is developed. This device is a tunnel junction for multijunction solar cells, grown lattice-matched on standard GaAs or Ge substrates, with the highest peak current density ever reported. The voltage drop for a current density equivalent to the operation of the multijunction solar cell up to 10 000 suns is below 5 mV. Trap-assisted tunnelling is proposed to be behind this performance, which cannot be justified by simple band-to-band tunnelling. The metal-organic vapour-phase epitaxy growth conditions, which are in the limits of the transport-limited regime, and the heavy tellurium doping levels are the proposed origins of the defects enabling trap-assisted tunnelling. The hypothesis of trap-assisted tunnelling is supported by the observed annealing behaviour of the tunnel junctions, which cannot be explained in terms of dopant diffusion or passivation. For the integration of these tunnel junctions into a triple-junction solar cell, AlGaAs barrier layers are introduced to suppress the formation of parasitic junctions, but this is found to significantly degrade the performance of the tunnel junctions. However, the annealed tunnel junctions with barrier layers still exhibit a peak current density higher than 2500Acm(-2) and a voltage drop at 10 000 suns of around 20 mV, which are excellent properties for tunnel junctions and mean they can serve as low-loss interconnections in multijunction solar cells working at ultra-high concentrations.

InAs/AlGaAs quantum dot intermediate band solar cells with enlarged sub-bandgaps

In the last decade several prototypes of intermediate band solar cells (IBSCs) have been manufactured. So far, most of these prototypes have been based on InAs/GaAs quantum dots (QDs) in order to implement the IB material. The key operation principles of the IB theory are two photon sub-bandgap (SBG) photocurrent, and output voltage preservation, and both have been experimentally demonstrated at low temperature. At room temperature (RT), however, thermal escape/relaxation between the conduction band (CB) and the IB prevents voltage preservation. To improve this situation, we have produced and characterized the first reported InAs/AlGaAs QD-based IBSCs. For an Al content of 25% in the host material, we have measured an activation energy of 361 meV for the thermal carrier escape. This energy is about 250 meV higher than the energies found in the literature for InAs/GaAs QD, and almost 140 meV higher than the activation energy obtained in our previous InAs/GaAs QD-IBSC prototypes including a specifically designed QD capping layer. This high value is responsible for the suppression of the SBG quantum efficiency under monochromatic illumination at around 220 K. We suggest that, if the energy split between the CB and the IB is large enough, activation energies as high as to suppress thermal carrier escape at room temperature (RT) can be achieved. In this respect, the InAs/AlGaAs system offers new possibilities to overcome some of the problems encountered in InAs/GaAs and opens the path for QD-IBSC devices capable of achieving high efficiency at RT.

Highly conductive p++-AlGaAs/n ++-GaInP tunnel junctions for ultra-high concentrator solar cells

Tunnel junctions are key for developing multijunction solar cells (MJSC) for ultra-high concentration applications. We have developed a highly conductive, high bandgap p  + + -AlGaAs/n  + + -GaInP tunnel junction with a peak tunneling current density for as-grown and thermal annealed devices of 996 A/cm 2 and 235 A/cm 2, respectively. The J–V characteristics of the tunnel junction after thermal annealing, together with its behavior at MJSCs typical operation temperatures, indicate that this tunnel junction is a suitable candidate for ultra-high concentrator MJSC designs. The benefits of the optical transparency are also assessed for a lattice-matched GaInP/GaInAs/Ge triple junction solar cell, yielding a current density increase in the middle cell of 0.506 mA/cm 2 with respect to previous designs.

AlGaAs/GaAs photovoltaic converters for high power narrowband radiation

AlGaAs/GaAs-based laser power PV converters intended for operation with high-power (up to 100 W/cm(2)) radiation were fabricated by LPE and MOCVD techniques. Monochromatic (lambda = 809 nm) conversion efficiency up to 60% was measured at cells with back surface field and low (x = 0.2) Al concentration 'window'. Modules with a voltage of 4 V and the efficiency of 56% were designed and fabricated.

Highly conductive p++-AlGaAs/n++-GaInP tunnel junctions for operation up to 15,000 suns in concentrator solar cells

In the last few decades there has been great interest in III-V multijunction solar cells (MJSC) for concentrator applications due to their promise to significantly reduce the cost of electricity. Being formed by series connection of several solar cells with different bandgaps, a key role in a MJSC structure is played by the tunnel junctions (TJ) aimed to implement such series connection. Essentially, tunnel junctions (tunnel diodes or Esaki diodes) are thin, heavily doped p-n junctions where quantum tunneling plays a key role as a conduction mechanism. Such devices were discovered by Nobel laureate Leo Esaki at the end of 1950. The key feature of tunnel junctions for their application in MJSC is that, as long as quantum tunneling is the dominant conduction mechanism, they exhibit a linear I-V dependence until the peak tunneling current (Jp) is reached. This initial ohmic region in the I-V curve is ideal for implementing low-loss interconnections between the subcells with different energy bandgaps that constitute a MJSC.

Magnetotransport in a wide parabolic well superimposed with a superlattice

The electron properties of artificially disordered superlattices embedded in a wide AlGaAs parabolic well were investigated in a strong magnetic field. We demonstrated that in the extreme quantum limit the interlayer disorder results in formation of a new correlated phase. A nearly uniform electron distribution over the superlattice wells was found in a weak magnetic field. However, a nonuniform phase with partially localized electrons, representing well-developed fractional quantum Hall effect features, was observed in high magnetic field (at the filling factor v < 1). A distinct magnetic field-induced transition separates these two phases. (C) 2011 American Institute of Physics. [doi:10.1063/1.3576134]

Contrasting LH-HH subband splitting of strained quantum wells grown along [001] and [113] directions

Contrasting responses for the temperature tuning of the electronic structure in semiconductor quantum wells are discussed for heterolayered structures grown along (001) and (113) directions. The temperature affects the strain modulation of the deformation potentials and the effective optical gap is tuned along with the intersub-band splitting in the valence band. A multiband theoretical model accounts for the characterization of the electronic structure, highlighting the main qualitative and quantitative differences between the two systems under study. The microscopic source of strain fields and the detailed mapping of their distribution are provided by a simulation using classical molecular-dynamics technics.

Delocalization-localization transition of plasmons in random (GaAs)(m)(Al(0.3)Ga(0.7)As)(6) superlattices

The transition of plasmons from propagating to localized state was studied in disordered systems formed in GaAs/AlGaAs superlattices by impurities and by artificial random potential. Both the localization length and the linewidth of plasmons were measured by Raman scattering. The vanishing dependence of the plasmon linewidth on the disorder strength was shown to be a manifestation of the strong plasmon localization. The theoretical approach based on representation of the plasmon wave function in a Gaussian form well accounted for by the obtained experimental data.