Benito Alén

Optical investigation of type II GaSb∕GaAs self-assembled quantum dots

We have studied the emission and absorption properties of type II GaSb∕GaAs quantum dots embedded in a p-i-n photodiode. The excitation power evolution provides clear signatures of the spatially separated confinement of electrons and holes in these nanostructures. We have estimated the confinement potential for the holes to be ∼500meV, leading to an intense room temperature emission assisted by recapture processes from the wetting layer. Photocurrent measurements show strong absorption in the wetting layer and in the quantum dots at room temperature which are important for photodetection applications based in this system.

Synthesis and luminescence properties of electrodeposited ZnO films

Zinc oxide (ZnO) films have been grown on gold (111) by electrodeposition using two different OH− sources, nitrate and peroxide, in order to obtain a comparative study between them. The morphology, structural and optical characterization of the films were investigated depending on the solution used (nitrate and peroxide) and the applied potential. Scanning electron microscopy pictures show different morphologies in each case. X-ray diffraction confirms that the films are pure ZnO oriented along the (0002) direction. ZnO films have been studied by photoluminescence to identify the emission of defects in the visible range. A consistent model that explains the emissions for the different electrodeposited ZnO films is proposed. We have associated the green and yellow emissions to a transition from the donor OH− to the acceptor zinc vacancies (VZn−) and to interstitial oxygen (Oi0), respectively. The orange-red emission is probably due to transitions from the conducting band to Oi− and OZn0 defects and the inf...

Column-by-column compositional mapping by Z-contrast imaging.

A phenomenological method is developed to determine the composition of materials, with atomic column resolution, by analysis of integrated intensities of aberration-corrected Z-contrast scanning transmission electron microscopy images. The method is exemplified for InAs(x)P(1-x) alloys using epitaxial thin films with calibrated compositions as standards. Using this approach we have determined the composition of the two-dimensional wetting layer formed between self-assembled InAs quantum wires on InP(001) substrates.

Optical transitions and excitonic recombination in InAs/InP self-assembled quantum wires

InAs self-assembled quantum wire structures have been grown on InP substrates and studied by means of photoluminescence and polarized-light absorption measurements. According to our calculations, the observed optical transitions in each sample are consistent with wires of different heights, namely from 6 to 13 monolayers. The nonradiative mechanism limiting the emission intensity at room temperature is related to thermal escape of carriers out of the wires.

Carrier recombination effects in strain compensated quantum dot stacks embedded in solar cells

In this work we report the stacking of 50 InAs/GaAs quantum dot layers with a GaAs spacer thickness of 18 nm using GaP monolayers for strain compensation. We find a good structural and optical quality of the fabricated samples including a planar growth front across the whole structure, a reduction in the quantum dot size inhomogeneity, and an enhanced thermal stability of the emission. The optimized quantum dot stack has been embedded in a solar cell structure and we discuss the benefits and disadvantages of this approach for high efficiency photovoltaic applications.

Single Photon Emission from Site-Controlled InAs Quantum Dots Grown on GaAs(001) Patterned Substrates

We present a fabrication method to produce site-controlled and regularly spaced InAs/GaAs quantum dots for applications in quantum optical information devices. The high selectivity of our epitaxial regrowth procedure can be used to allocate the quantum dots only in positions predefined by ex-situ local oxidation atomic force nanolithography. The quantum dots obtained following this fabrication process present a high optical quality which we have evaluated by microphotoluminescence and photon correlation experiments.

Single quantum dot emission at telecom wavelengths from metamorphic InAs/InGaAs nanostructures grown on GaAs substrates

We report on the growth by molecular beam epitaxy and the study by atomic force microscopy and photoluminescence of low density metamorphic InAs/InGaAs quantum dots. subcritical InAs coverages allow to obtain 108 cm−2 dot density and metamorphic InxGa1−xAs (x=0.15,0.30) confining layers result in emission wavelengths at 1.3 μm. We discuss optimal growth parameters and demonstrate single quantum dot emission up to 1350 nm at low temperatures, by distinguishing the main exciton complexes in these nanostructures. Reported results indicate that metamorphic quantum dots could be valuable candidates as single photon sources for long wavelength telecom windows.

Formation and optical characterization of single InAs quantum dots grown on GaAs nanoholes

We present a study of the structural and optical properties of InAs quantum dots formed in a low density template of nanoholes fabricated by droplet epitaxy on GaAs (001). The growth conditions used here promote the formation of isolated quantum dots only inside the templated nanoholes. Due to the good optical quality and low density of these nanostructures, their ensemble and individual emission properties could be investigated and related to the particular growth method employed and the quantum dot morphology.

Scanning x-ray excited optical luminescence microscopy in GaN

In this work, an imaging tool to investigate optical inhomogeneities with site and chemical sensitivities has been integrated in a hard x-ray microprobe. Freestanding GaN and epitaxially grown GaN:Mn on α-Al2O3 are used to exploit the unprecedented scanning x-ray excited luminescence technique. Optical images of the radiative recombination channels are reported for several impurities and defect centers in sapphire and GaN compounds. Within the experimental accuracy, a visible nonuniformity characterizes the Mn centers in good correlation with former x-ray fluorescence map. Expanding the microprobe versatility, x-ray absorption spectroscopy in both photon collection modes (x-ray excited luminescence and x-ray fluorescence) is finally presented from a freestanding GaN layer.

Enhancement of the room temperature luminescence of InAs quantum dots by GaSb capping

The authors have studied the use of antimony for the optimization of the InAs∕GaAs(001) self-assembled quantum dot (QD) luminescence characteristics in the 1.3μm spectral region. The best results have been obtained by capping InAs QDs with 2 ML of GaSb grown on top of a 3 ML GaAs barrier separating the InAs and the GaSb layers. This results in an order of magnitude enhancement of the room temperature luminescence intensity at 1.3μm emission wavelength.