J.L. Casas Espinola

Thermal activation of excitons in asymmetric InAs dots-in-a-well InxGa1−xAs∕GaAs structures

Photoluminescence, its temperature dependence, and photoluminescence excitation spectra of InAs quantum dots embedded in asymmetric InxGa1−xAs∕GaAs quantum wells [dots in a well (DWELL)] have been investigated as a function of the indium content x (x=0.10–0.25) in the capping InxGa1−xAs layer. The asymmetric DWELL structures were created with the aim to investigate the influence of different barrier values at the quantum dot (QD)/quantum well interface on the photoluminescence thermal quenching process. The set of rate equations for the two stage model for the capture and thermal escape of excitons in QDs are solved to analyze the nature of thermal activation energies for the QD photoluminescence quenching process. The two stage model for exciton thermal activation was confirmed experimentally in the investigated QD structures as well. The localization of nonradiative defects in InAs∕InGaAs DWELL structures is discussed on the base of comparison of theoretical and numerically calculated (fitting) results.

Thermal ionisation of ground and multiply excited states in InAs quantum dots embedded into InGaAs/GaAs MQW

The photoluminescence (PL) spectra of highly uniform self-assembled InAs quantum dots (QDs) embedded in In 0.15 Ga 0.85 As multi-quantum-well (MQW) heterostructures have been investigated at variable temperatures. This paper presents the PL bands, connected with ground (GS) and multi-excited states (ES) in QDs. Not equidistant optical transitions have been revealed. Spectral peak shifts and PL intensity variations in the temperature range 12-220 K for all PL bands are analyzed. The activation energy of the temperature quenching processes for GS and 4 ES optical transitions in InAs QDs are measured. The mechanism of these processes and the positions of the energy levels in QDs are discussed as well.

Photoluminescence variation in dot-in-a-well structures with different InAs quantum dot densities

Photoluminescence (PL) and its temperature dependence have been investigated in InAs quantum dots (QDs) embedded in In0.15Ga0.85As∕GaAs quantum wells (QWs). The QD density varied from 1.1×1011 down to 1.3×1010cm−2 with an increase in QD growth temperature. Three stages have been revealed in the thermal decay of the PL intensity in InAs QDs. A variety of activation energies of PL thermal decay are discussed. Numerical simulations of experimental PL thermal decay curves give possibility to analyze the area of localization of nonradiative defects in InGaAs∕GaAs QW structures with different InAs QD densities.

Carrier dynamics in InAs quantum dots embedded in InGaAs/GaAs multi quantum well structures

Ground and multi excited state photoluminescence, as well as its temperature dependence, in InAs quantum dots embedded in symmetric InxGa1-xAs/GaAs (x = 0.15) quantum wells (DWELL) have been investigated. The solution of the set of rate equations for exciton dynamics (relaxation into QWs or QDs and thermal escape) solved by us earlier is used for analysis the variety of thermal activation energies of photoluminescence thermal quenching for ground and multi excited states of InAs QDs. The obtained solutions were used at the discussion of the variety of activation energies of PL thermal quenching in InAs QDs. It is revealed three different regimes of thermally activated quenching of the QD PL intensity. These three regimes were attributed to thermal escape of excitons: i) from the high energy excited states of InAs QDs into the WL with follows exciton re-localization; ii) from the InxGa1-xAs QWs into the GaAs barrier and iii) from the WL into the GaAs barrier with their subsequent nonradiative recombination in GaAs barrier.

Emission and Structure Varieties in ZnO:Ag Nanorods Obtained by Ultrasonic Spray Pyrolysis

Scanning electronic microscopy (SEM), X ray diffraction (XRD) and photoluminescence (PL) have been applied to the study of the structural and optical properties of ZnO nanocrystals prepared by the ultrasonic spray pyrolysis (USP) at different temperatures. The variation of temperatures and times at the growth of ZnO films permits modifying the ZnO phase from the amorphous to crystalline, to change the size of ZnO nanocrystals (NCs), as well as to vary their photoluminescence spectra. The study has revealed three types of PL bands in ZnO NCs: defect related emission, the near-band-edge (NBE) PL, related to the LO phonon replica of free exciton (FE) recombination, and FE second-order diffraction peaks. The PL bands related to the LO phonon replica of FE in PL spectra measured at room temperature testify on the high quality of ZnO films prepared by the USP technology.

Impact of capping layer type on emission of InAs quantum dots embedded in InGaAs/InxAlyGazAs/GaAs quantum wells

The optical and structural properties of In0.15Ga0.85As/InxAlyGazAs/GaAs quantum wells with embedded InAs quantum dots (QDs) were investigated by the photoluminescence (PL), its temperature dependence, X-ray diffraction (XRD), and high resolution (HR-XRD) methods in dependence on the composition of capping InxAlyGazAs layers. Three types of capping layers (Al0.3Ga0.7As, Al0.10Ga0.75In0.15As, and Al0.40Ga0.45In0.15As) have been used and their impact on PL parameters has been compared. Temperature dependences of PL peak positions in QDs have been analyzed in the range of 10–500 K and to compare with the temperature shrinkage of band gap in the bulk InAs crystal. This permits to investigate the QD material composition and the efficiency of Ga(Al)/In inter diffusion in dependence on the type of InxAlyGazAs capping layers. XRD and HR-XRD used to control the composition of quantum well layers. It is shown that QD material composition is closer to InAs in the structure with the Al0.40Ga0.45In0.15As capping layer...