Daniel Granados

In(Ga)As self-assembled quantum ring formation by molecular beam epitaxy

The effect of growth conditions on the morphological properties of InAs/GaAs(001) quantum dots covered by a thin (<3 nm) GaAs cap has been studied by atomic force microscopy. Each dot turns into an elongated nanostructure at 540 °C upon deposition of the cap in As4 atmosphere, while structures with two humps are obtained when capping at 500 °C. The use of As2 atmosphere instead of As4 at 500 °C leads to the formation of quantum rings. Photoluminescence spectroscopy and polarization photoluminescence (PL) at 15 K show dramatic changes due to the different kinds of confinement. This allows the possibility of tailoring PL emission by controlling the size and shape.

Manipulating exciton fine structure in quantum dots with a lateral electric field

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero.

Atomic-scale structure of self-assembled In(Ga)As quantum rings in GaAs

We present an atomic-scale analysis of the indium distribution of self-assembled In(Ga)As quantum rings (QRs) which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20nm which is consistent with the observed electronic radius of QR structures.

Room temperature emission at 1.6μm from InGaAs quantum dots capped with GaAsSb

Room temperature photoluminescence at 1.6μm is demonstrated from InGaAs quantum dots capped with an 8nm GaAsSb quantum well. Results obtained from various sample structures are compared, including samples capped with GaAs. The observed redshift in GaAsSb capped samples is attributed to a type II band alignment and to a beneficial modification of growth kinetics during capping due to the presence of Sb. The sample structure is discussed on the basis of transmission electron microscopy results.

Laser devices with stacked layers of InGaAs/GaAs quantum rings

Stacked layers of In(Ga)As on GaAs(001) self-assembled quantum rings (QR) for laser application have been studied. Several samples with three stacked QR layers have been grown by molecular beam epitaxy with GaAs spacers from 1.5 to 14?nm. The optical and structural properties have been characterized by photoluminescence spectroscopy and by atomic force microscopy, respectively. For GaAs spacers larger that 6?nm, the stacked QR layers present similar properties to single QR layers. A semiconductor laser structure with three stacked layers of QR separated 10?nm in the active region has been grown. This spacer ensures well-developed rings with optical emission like that of a single layer. Laser diodes have been processed with 1?2?mm cavity lengths. The stimulated emission is multimodal, centred at 930?nm (77?K), with a threshold current density per QR layer of 69?A?cm?2. In this work, it is demonstrated that stacking rings is possible, and that a broad area laser with three QR layers can be fabricated successfully.

Vertical order in stacked layers of self-assembled In(Ga)As quantum rings on GaAs (001)

Stacked layers of self-assembled In(Ga)As quantum rings on GaAs grown by solid source molecular beam epitaxy are studied by ex situ atomic force microscopy (AFM), low temperature photoluminescence (PL) and cross-sectional transmission electron microscopy (XTEM). The influence of the strain field and InAs segregation on the surface morphology, optical properties and vertical ordering of three quantum ring layers is analyzed for GaAs spacers between layers from 1.5 to 14 nm. AFM and PL results show that samples with spacers >6nm have surface morphology and optical properties similar to single layers samples. XTEM results on samples with 3 and 6 nm GaAs spacers show that the rings are preserved after capping with GaAs, and evidence the existence of vertically ordered quantum rings.

Organic Covalent Patterning of Nanostructured Graphene with Selectivity at the Atomic Level.

Organic covalent functionalization of graphene with long-range periodicity is highly desirable-it is anticipated to provide control over its electronic, optical, or magnetic properties-and remarkably challenging. In this work we describe a method for the covalent modification of graphene with strict spatial periodicity at the nanometer scale. The periodic landscape is provided by a single monolayer of graphene grown on Ru(0001) that presents a moiré pattern due to the mismatch between the carbon and ruthenium hexagonal lattices. The moiré contains periodically arranged areas where the graphene-ruthenium interaction is enhanced and shows higher chemical reactivity. This phenomenon is demonstrated by the attachment of cyanomethyl radicals (CH2CN(•)) produced by homolytic breaking of acetonitrile (CH3CN), which is shown to present a nearly complete selectivity (>98%) binding covalently to graphene on specific atomic sites. This method can be extended to other organic nitriles, paving the way for the attachment of functional molecules.

Emission polarization control in semiconductor quantum dots coupled to a photonic crystal microcavity

We study the optical emission of single semiconductor quantum dots weakly coupled to a photonic-crystal micro-cavity. The linearly polarized emission of a selected quantum dot changes continuously its polarization angle, from nearly perpendicular to the cavity mode polarization at large detuning, to parallel at zero detuning, and reversing sign for negative detuning. The linear polarization rotation is qualitatively interpreted in terms of the detuning dependent mixing of the quantum dot and cavity states. The present result is relevant to achieve continuous control of the linear polarization in single photon emitters.

Customized nanostructures MBE growth: from quantum dots to quantum rings

Studies for growing control methods of quantum dots (QD) sizes and dot/cap intermixing for tuning QD optical properties have particular interest nowadays. Morphological shape surface changes of QD due to thin capping overgrowth of GaAs under different growing conditions has been reported by different authors [1, 2] as a powerful technique to obtain different nanostructures. In this work, control over shape and optical emission of molecular beam epitaxy (MBE) thin capping overgrown InAs QD on GaAs[100] have been studied, using ex-situ Atomic Force Microscopy (AFM) and Photoluminescence (PL).

Determination of the energy levels on InAs quantum dots with respect to the GaAs conduction band

The electronic energy levels of InAs quantum dots (QDs) have been measured with respect to the GaAs conduction band edge by employing capacitance–voltage spectroscopy with a method which accurately measures the height of the Schottky barrier. To do so, two samples have been grown by molecular beam epitaxy and processed in parallel with Ohmic and Schottky contacts. One sample has no InAs (neither wetting layer nor QDs) which allows an accurate measurement of the Schottky barrier height when the flat band condition is achieved on the device. The measured Schottky barrier height is 760 meV. The second sample has embedded InAs QDs. The measured s-like states of these QDs are 230 meV below the GaAs conduction band.