Reza J Kashtiban

Abstract: InGaN/GaN multilayer quantum dot structures produced by MOCVD techniques on c-plane sapphire were studied by transmission electron microscopy (TEM) and photoluminescence (PL) techniques. Indium fluctuations ranging from 1-4 nm were observed with both energy filtered TEM (EFTEM) and high angle annular dark field (HAADF) scanning TEM. The existence of V-shaped defects with nucleation centres at the termination of threading dislocation were observed in HAADF images. There was also evidence of the formation of large quantum dots at low densities from lattice HRTEM images. This was further confirmed by PL measurements through the observation of a single sharp line at low power with the typical saturation behaviour at higher power excitation.

Abstract: Germanium (Ge) nanocrystals (NCs) embedded in alumina thin films were produced by deposition on silicon (111) substrates using radio-frequency (RF) magnetron sputtering. By changing growth condition and annealing parameters samples with large and small Ge-NCs were produced. The average size of NCs in the sample with larger NCs was estimated to be 7.2, 30.0 and 7.6 nm, and 5.0, 7.0 and 4.8 nm for the sample with smaller NCs, according to X-ray diffraction (XRD), Raman and high resolution transmission electron microscopy (HRTEM) results, respectively. Both, XRD and Raman peak positions of the larger NCs are shifted to higher angles and larger wave numbers in relation to the Ge bulk values, whereas the Raman peak was red-shifted for the smaller NCs indicating phonon confinement. HRTEM shows twinned structures, which is an indication of relaxation. Strain evaluation of the bigger NCs gave values < 0.5 % which is within the estimated error of the evaluation technique.

Abstract: Er-doped SiO2 and Si nano-crystals (NCs) embedded in a SiO2 matrix were produced by ion beam implantation of Si (100) substrates. After annealing Er ions agglomerate in different positions with different compositional properties in samples with and without Si implants. HAADF and EELS show that in the sample with Si implants the Si and Er distribution is identical and within a band of ~110nm width ~75nm below theSiO2 surface whereas in the sample with no excess Si, Er forms on average much larger, amorphous aggregates, presumably an Er-oxide, in the SiO2 matrix with tendency to move towards the surface of the SiO2 layer.

Abstract: We present a study of silicon (Si) and erbium (Er) coimplanted silica (SiO2) in which we observe, by combining high resolution scanning transmission electron microscopy and selective electron energy loss spectroscopy (EELS), a high spatial correlation between silicon nanocrystals (Si-NCs), Er, and oxygen (O) after a single high temperature (1100 °C) anneal. The observation of a spatial overlap of the EELS chemical maps of dark field (DF) images at the Er N4,5, Si L2,3, and O K edges is concomitant with an intense room temperature infrared luminescence around 1534 nm. We suggest that these observations correspond to Er–O complexes within an amorphous silicon (a-Si) shell at the Si-NC/SiO2 interface. The presence of a crystalline phase at the Si-NC center, verified by high resolution electron micrographs and DF diffraction contrast images and the low solubility of Er in crystalline Si (c-Si) would tend to suggest a preferential Er agglomeration toward the Si-NC/SiO2 interface during formation, particularly when high concentrations of both Si and Er are obtained in a narrow region of the SiO2 after coimplantation. The absence of narrow Stark related features in the Er emission spectrum at low temperature and an inhomogeneous broadening with increasing temperature, which are characteristic of Er confined by an amorphous, rather than a crystalline host further support these hypotheses. After comparing the luminescence to that from a SiO2:Er control sample prepared in exactly the same manner but without Si-NCs, we find that, despite the observed spatial correlation, only a small fraction ( ∼ 7%) of the Er are sensitized by the Si-NCs. We ascribe this low fraction to a combination of low sensitizer (Si-NC) density and Auger-type losses arising principally from Er ion-ion interactions.

Abstract: An optical and structural study of InGaN/GaN quantum dots (QDs) is reported. With increasing InGaN deposition time, the dominant emission changes from wetting layer (WL) to QDs, and a strong redshift of the emission occurs. Emission from localized WL states is observed, with a density and nature very different to that due to the QDs. Structural measurements reveal a disordered WL, consistent with the form of the WL photoluminescence excitation spectra. (C) 2009 American Institute of Physics. [doi:10.1063/1.3226645]

Abstract: SiGe quantum dots in a dielectric matrix show numerous interesting properties, which are very relevant for applications in different nano-based devices. Homogeneous sizes and uniform spatial distribution of quantum dots in the host matrix are very important parameters for their potential applications. Here we present a study of structural properties of SiGe quantum dots formed in amorphous silica matrix by magnetron sputtering technique. We investigate deposition conditions leading to the formation of dense and uniformly sized quantum dots, distributed homogeneously in the matrix. A detailed analysis based on grazing-incidence small-angle x-ray scattering revealed the dependences of quantum dot sizes, size distributions, spatial arrangement and concentration of quantum dots in the matrix, as well as the Si:Ge content on the deposition conditions

Abstract: Two kinds of superlattices (i) with and (ii) without growth interrupt (GI) after deposition of 1.77 monolayers (ML) of InAs on GaAs (0 0 1) were grown by solid-source molecular beam epitaxy (MBE) and assessed by transmission electron microscopy (TEM) techniques, double crystal X-ray diffraction (DCXRD) and photoluminescence (PL) measurements in order to gain an understanding of the structural and compositional properties. In case (i) formation of coherent dislocation free self-organized quantum dots (SOQDs) with 2.8-3.2 nm height and 13-16 nm lateral size was observed, whereas in case (ii) no quantum dots had formed. In order to better understand the implication of growth interruption for the formation mechanism, highly localised assessment of the composition of the QD was carried out via atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS). (c) 2008 Elsevier Ltd. All rights reserved.

Abstract: We have investigated the structural properties of Si1-xGex nanocrystals formed in an amorphous SiO2 matrix by magnetron sputtering deposition. The influence of deposition parameters on nanocrystals size, shape, arrangement and internal structure was examined by X-ray diffraction, Raman spectroscopy, grazing incidence small angle X-ray scattering, and high resolution transmission electron microscopy. We found conditions for the formation of spherical Si1-xGex nanocrystals with average sizes between 3 to 13 nm, uniformly distributed in the matrix. In addition we have shown the influence of deposition parameters on average nanocrystal size and Ge content x