Growth and Characterization of MBE Parametric Variations Upon Polarization Independent Strained Films

This project was designed to comprehensively investigate the effects of molecular beam epitaxy (MBE) growth variations upon tensile-strained films. Using two dimensional (2-D) structures, such as quantum wells of variable configurations, we worked to observe the electro-optical response of polarization independence. The latter had been studied for compressively strained systems. However, the proof of principle for the feasibility of tensile-strained single and double quantum wells, particularly in the GaAs as opposed to the InP material system, had been much less investigated by comparison. The significance of this project, from a Basic Energy Sciences perspective, has been in its contribution to the understanding of how MBE parameters and sample design effect the physics of strained materials which themselves have been tailored to react with polarization insensitivity. Fundamental issues of materials structure control, growth parameter modifications, and characterization methods were investigated. Both single and double quantum well systems were used with both symmetric and asymmetric well widths for the latter. In this work, we essentially investigated both the growth of and potential applications for quantum based structures that use tensile strain to induce polarization independence

Engineering the I-V characteristics of an asymmetric double barrier device with variable period GaAs/AlAs superlattice injectors

This study represents a first-time examination of an asymmetric double barrier structure using variable period superlattice emitters. The layer thickness and period of the superlattices were systematically varied in order to define a unique emitter state in each structure grown. In each case, the emitter state was determined theoretically by calculating the miniband formation in the constituting superlattice. The relationship between the emitter state defined by the superlattice structure parameters and the current-voltage behavior of the diode at 77 K has been investigated here. © 1997 Elsevier Science S.A

Electrophoretic deposition of carbon nanotubes on 3-amino-propyl-triethoxysilane (Aptes) surface functionalized silicon substrates

Fabrication of uniform thin coatings of multi-walled carbon nanotubes (MWCNTs) by electrophoretic deposition (EPD) on semiconductor (silicon) substrates with 3-aminopropyl-triethoxysilane (APTES) surface functionalization has been studied extensively in this report. The gradual deposition and eventual CNT film formation on the organosilane treated silicon substrates is greatly assisted by the Coulombic force of attraction existing between the positively charged –NH2 surface groups of APTES and the acid treated, –COOH− functionalized negatively charged nanotubes migrating towards the anode (silicon) surface. The superior CNT coatings produced by EPD in comparison to the dip/immersion coating and the influence of isopropyl (IPA) based EPD suspension in the fabricated film quality have also been revealed in this study. The effect of varying APTES concentration (5%–100%) on the Raman spectroscopy and thickness of the deposited CNT film has been discussed in details, as well. The organosilane assisted deposition approach has eliminated the need of metal deposition in the EPD technique and has established an economical, fast and entirely room temperature solution-based fabrication strategy of CNT thin films for a wide range of next generation electronic applications

Characterization of AOT capped cadmium sulfide quantum dots using fluorescence spectroscopy

Colloidal cadmium sulfide quantum dots (Q-CdS) capped with dioctyl sulfosuccinate (AOT) were synthesized and characterized. Fluorescence spectroscopy is used to verify quantized dot formation, size, and emission wavelength. Optical characterization of this form is necessary for fundamental investigation of hybrid electronic materials and their potential use in next-generation devices. © 2010 Wiley Periodicals, Inc

Fluorescence spectroscopy characterization of cadmium sulfide quantum dots on metal, insulator, and semiconductor substrates

Colliodal cadmium sulfide quantum dots (Q-CdS), ranging in size from 1 to 5 nm and capped with dioctyl sulfosuccinate (AOT), were synthesized and deposited upon substrates of gold (Au), mica, and gallium arsenide (GaAs). Fluorescence spectroscopy is used to identify the range and maxima of excitation and emission spectra of each structure. A red shift in emission spectra from the Au to mica to GaAs substrate based samples is observed for the smallest-sized quantum dots. The midrange and largest-sized dots display the longest emission wavelengths when physisorbed to the Au substrates with a shift to shorter and then longer emission wavelengths for the mica and GaAs-based samples, respectively. The display of emission spectra throughout the visible range for the Q-CdS deposited on all three types of substrates bodes well for future optical device applications using hybrid (organic-inorganic) electronic materials. © 2011 Wiley Periodicals, Inc

Resonant tunneling with superlattice emitters

In this study we examine the effect of superlattice (SL) structures as electron injectors to asymmetric double barrier resonant tunneling diodes (ADBRTDs). The experiment consisted of growing devices with five periods of GaAs/AlAs SLs prior to the ADBRT structure in the growth direction. The periods of SL used in our characterization were 50 angstrom/ 50 angstrom, 30 angstrom/ 30 angstrom and 15 angstrom/ 15 angstrom. Observations of the effect of the SL period on the first resonance level in forward bias and reverse bias were made. Phonon assisted tunneling was also observed

Surface enhanced Raman spectroscopy on silver-nanoparticle-coated carbon-nanotube networks fabricated by electrophoretic deposition

In this study, the efficiency of silver nanoparticle (AgNP) decorated carbon nanotube (CNT) based porous substrates has been investigated for surface-enhanced Raman spectroscopy (SERS) applications. The fabrication of uniform thin coatings of carbon nanotubes is accomplished by Electrophoretic Deposition (EPD) on organosilane functionalized silicon substrates. The deposition process exemplifies a fast, reproducible and single-step room temperature coating strategy to fabricate horizontally aligned porous CNT network. Surfactant stabilized AgNPs were deposited on the CNT networks by immersion coating. The acquired Raman spectra of Rhodamine6G (R6G) analyte examined on the fabricated Ag-CNT-Si substrates exhibited enhanced signal intensity values when compared to SERS-active planar AgNP-Si substrates. An overall enhancement factor of ~;109 was achieved for the tested analyte which enables pushing the limit of detection to 1 × 10-12 M (1 pM). The enhancement can be attributed to the large surface area offered by the AgNP-CNT porous network, which is expected to increase the number of effective hot spots for the SERS effect. © 2014 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht

Structural and optical characterization of InAs nanostructures grown on high-index InP substrates

The structural and optical properties of InAs layers grown on high-index InP surfaces by molecular beam epitaxy are investigated in order to understand the self-organization of quantum dots and quantum wires on novel index surfaces. Four different InP substrate orientations have been examined, namely, (1 1 1)B, (3 1 1)A, and (3 1 1)B and (1 0 0). A rich variety of InAs nanostructures is formed on the surfaces. Quantum wire-like morphology is observed on the (1 0 0) surface, and evident island formation is found on (1 1 1)A and (3 1 1)B by atomic force microscopy. The photoluminescence spectra of InP (1 1 1)A and (3 1 1)B samples show typical QD features with PL peaks in the wavelength range 1.3-1.55 μm with comparable efficiency. These results suggest that the high-index substrates are promising candidates for production of high-quality self-organized QD materials for device applications

Growth mode and strain relaxation of InAs on InP (111)A grown by molecular beam epitaxy

Growth mode and strain relaxation of molecular-beam-epitaxy grown InAs/InAlAs/InP (111)A system have been investigated using reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and photoluminescence measurements. In direct contrast to the well-studied InAs/GaAs system, our experimental results show that the InAs grown on InAlAs/InP (111)A follows the Stranski-Krastanov mode. Both self-organized InAs quantum dots and relaxed InAs islands are formed depending on the InAs coverage. Intense luminescence signals from both the InAs quantum dots and wetting layer are observed. The luminescence efficiency of (111)A samples is comparable to that of (001) samples, suggesting the feasibility of fabricating quantum dot optoelectronic devices on InP (111)A surfaces. © 1999 American Institute of Physics