Showing papers by "C. M. Sotomayor Torres published in 1998"

Problems of the nanoimprinting technique for nanometer scale pattern definition

Abstract: We have tested nanoimprint lithography, a new and promising technique for nanometer-scale pattern definition. Preliminary experiments reveal that, besides severe sticking and adhesion problems, the problem of material transport is one inherent to this technique. There are clear indications that most of the effects found may be understood in terms of material transport. We performed experiments within a well defined pressure and temperature window which ranged from 60 to 100 bar and from 50 to 90 °C above the glass transition temperature of the poly(methylmethacrylate)-like polymer used. As a result, the quality of imprint is evaluated with respect to full area pattern transfer, based on a qualitative scanning electron microscope investigation of the fully imprinted area of 2 cm × 2 cm patterned with features of different size and shape. Optimum conditions for imprint quality are found around 100 bar and 90 °C above Tg for the specific polymer used. Although material transport will limit nanoimprint perfor...

Elastic relaxation of dry-etched Si/SiGe quantum dots

Abstract: Elastic relaxation of the compressive strain due to the lattice mismatch between SiGe and Si has been studied with both x-ray diffraction and Raman scattering in small (30\char21{}100 nm) dry-etched Si/SiGe quantum dots fabricated from high-quality multilayers grown on (001)-oriented Si. The Raman spectroscopic investigations showed that the dot alloy layers have relaxed by approximately 65% from their fully strained value and that a compensating tensile strain has been induced in the Si layers. The relaxation is essentially independent of the dot size and the values derived experimentally compare well with analytical and numerical model calculations.

Shear strains in dry etched GaAs/AlAs wires studied by high resolution x-ray reciprocal space mapping

Abstract: We have fabricated GaAs/AlAs quantum wires and quantum dots by means of molecular beam epitaxy, electron beam lithography, and subsequent reactive ion etching using SiCl4 and O2 The nominal periods are 300 nm and 350 nm for both wire and dot samples High resolution x-ray reciprocal space maps of the 350 nm samples exhibit not only satellites corresponding to a periodicity of 350 nm but also additional satellites corresponding to a period of three times 350 nm, whereas there are no such extra peaks in the maps of the 300 nm samples These secondary satellites are shown to be associated with a discretization effect in electron beam writing Moreover, we found, that the shear strain in the wires has a distinct influence on the intensities of these weak extra satellites Hence, they provide a sensitive means for the assessment of shear strains in elastically relaxed quantum wires

Optical Properties of InP Quantum Wires Grown in Hollow Cylindrical Channels of Chrysotile Asbestos

Abstract: The behaviour of a large ensemble of nanostructures is of fundamental and practical interest since it cannot be simulated with bulk materials [1]. The discreteness of the energy spectrum added to its low density of states and controllable anisotropy, features inherent to each individual nanostructure, make these ensembles very attractive for application in, for example, non-linear optics. However, to meet a practical demand such nanostructures should be assembled in large arrays. The classical example of such ensembles is an array of quantum dots in a glass matrix, but this approach cannot provide neither ordered nor dense arrays and so far has been shown to work well only for II-VI semiconductors [2]. A self-organised strategy allows the growth of dense well-correlated arrays of quantum wires (QWRs) or dots on the crystal surface. Recently this method was extended to sequential growth of multiple dot-containing layers in order to construct three dimensional (3D) ensembles [3]. However, ordering in such arrays poses a significant challenge for five or more layers of quantum dots due to the extremely subtle nature of the interaction inducing the ordering. In contrast to the above techniques the template method based on structural confinement offers an opportunity to prepare very large 3D ensembles of nanoparticles with crystal-like ordering. A template in this case is a crystalline dielectric with structural voids, e.g. a zeolite or an opal. Voids in a porous dielectric lend themselves for infilling with a ≪guest≫ material [4]. The nano-scale size of these voids forces the “guest” material to follow their size and strictly limit the size distribution of nanostructures.

RT exciton waveguiding and lasing in submonolayer CdSe–(Zn, Mg)(S, Se) structures

Abstract: We demonstrate exciton waveguiding and short wavelength lasing (460 nm at 300 K) in quantum dot structures based on submonolayer superlattices. High resolution electron microscopy studies reveal that CdSe submonolayer insertion in a ZnSe matrix forms an array of 2D nanoscale islands. Strong modulation in the optical reflectance spectra is observed in a 20 period submonolayer superlattice structure at energy corresponding to the localized exciton state, and the lasing occurs directly in the spectral region of the exciton-induced enhancement of the refractive index.

RT lasing via nanoscale CdSe islands in a (Zn,Mg)(S,Se) matrix

Abstract: New approach for high performance lasing in semiconductors using an effect of exciton-induced waveguiding is proposed and realized. We demonstrate RT lasing in a structure without external waveguide formed by cladding of an active region with wider bandgap layers. Stacked arrays of nanoscale CdSe islands inserted directly in a wide bandgap matrix are used as an active media in our case. The lasing wavelength is 460 nm (2.69 eV) at 300K while the bandgap of the matrix material is 2.86 eV, providing an ultimate shift toward blue spectral range for the same matrix material.

Optical Properties and Lasing in CdSe-Submonolayers in a (Zn,Mg)(S,Se) Matrix

Abstract: We have proposed and realized a new type of optoelectronic structures we refer to as excitonic waveguides which allow an ultimate shift towards the violet spectral range (460 nm at 300 K for the ZnMgSSe/GaAs materials system). The excitonic waveguiding effect is realized at the low energy side of the strong exciton absorption peak due to localized states, caused by ultrathin narrow-gap insertions. To realize the zero-phonon lasing mechanism resonant to the spectral region of resonantly enhanced refractive index one needs to break the k-selection rule preventing direct radiate exciton recombination having large in-plane k-vector dominating at high excitation densities and observation temperatures. This can be done by localizing excitons at nanoscale CdSe-rich islands formed by ultrathin CdSe insertions.