Theda Daniels-Race

HIGH-DENSITY INAS NANOWIRES REALIZED IN SITU ON (100) INP

High-density InAs nanowires embedded in an In0.52Al0.48As matrix are fabricated in situ by molecular beam epitaxy on (100) InP. The average cross section of the nanowires is 4.5 x 10 nm(2). The linear density is as high as 70 wires/mu m. The spatial alignment of the multilayer arrays exhibit strong anticorrelation in the growth direction. Large polarization anisotropic effect is observed in polarized photoluminescence measurements

Effects of the matrix on self-organization of InAs quantum nanostructures grown on InP substrates

We have studied the influence of matrix materials on the self-organization of InAs nanostructures grown on InP substrates by molecular-beam epitaxy. Our results show that InAs quantum dots are formed on InAlGaAs, whereas quantum-wire-like structures are produced on InAlAs and InGaAs. Tuning from vertical anticorrelation in InAs/InAlAs superlattices to vertical correlation in InAs/InGaAs and InAs/InAlGaAs superlattices is observed, which is explained by the size effects in the nanostructure–nanostructure interaction.

Influence of indium composition on the surface morphology of self-organized InxGa1−xAs quantum dots on GaAs substrates

InxGa1-xAs self-organized quantum dots with x=1.0, 0.5, and 0.35 have been grown by molecular beam epitaxy. The areal density, distribution, and shapes have been found to be dependent on x. The dot shape changes from a round shape for x=1.0 to an elliptical shape for x less than or equal to 0.5. The major axis and minor axis of the elliptical InxGa1-xAs dots are along the [(1) over bar 10] and [110] directions, respectively. The ordering phenomenon is also discussed. It is suggested that the dot-dot interaction may play important roles in the self-organization process

Self-organization of wire-like InAs nanostructures on InP

The initial InAs growth on InP(1 0 0) during molecular beam epitaxy has been investigated. The as-grown islands were shaped like nanowires and formed dense arrays over the entire surface in the 3–6 monolayer InAs deposition range. The wires were oriented along the [View the MathML source 1 0] direction. Transmission electron microscopy images confirm that the wires are coherently grown on the substrates. Our results suggest that the coherent wire-shaped island formation may be a possible method to fabricate self-organized InAs nanowires.

Lateral correlation of InAs/AlInAs nanowire superlattices on InP(001)

The appearance of InAs quantum-wire-like morphology on an AlInAs buffer layer grown by molecular-beam epitaxy on nominal InP(001) surfaces is investigated. Lateral composition modulation in the AlInAs buffer layer is suggested to play an important role in the formation of InAs nanowires. For InAs/AlInAs nanowire superlattices, the InAs nanowires are laterally correlated with respect to growth directions. By changing the spacer thickness, no evidence of vertical correlation is observed. The lack of vertical correlation is ascribed to the asymmetrical cross-sectional shape of the nanowires.

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

Measurement of the GaAs/AlAs valence‐band offset from a single quantum well near the Γ‐X crossover

An iterative procedure which utilizes the type‐I and type‐II transitions of a GaAs/AlAs single quantum well is used to determine the GaAs/AlAs valence‐band offset. The iteration is based on the fact that the confinement energy is not sensitive to the change of the barrier potential. It has the advantage that knowledge of the thickness of the quantum well is not necessary to determine the valence‐band offset. Both the type‐I and type‐II transitions can be observed in the low‐temperature photoluminescence spectrum if the GaAs is thin enough so that the lowest‐energy level of the electron in the GaAs well is higher than the X‐conduction‐band minimum of the adjacent AlAs. We illustrate this procedure with a 35 A GaAs/AlAs single quantum well, and the valence‐band offset is found to be 36% of the Γ‐gap difference.

A needle probe to detect surface enhanced Raman scattering (SERS) within solid specimen

A needle probe has been developed to obtain surface enhanced Raman scattering data from within a solid specimen located remotely from the spectrometer. It produces the high signal strength of a single mode optical fiber but with a negligible fiber induced background. The observed Raman signal strength is comparable to that obtained with a microscope objective of the same numerical aperture in a conventional spectrometer arrangement and many times larger than that of probes using two fibers.

Surface enhanced Raman spectroscopic substrate utilizing gold nanoparticles on carbon nanotubes

This paper reports a new low cost technique for fabricating Surface Enhanced Raman Spectroscopy substrates. A Gold (Au) nano-metallic structure for surface enhancement is created by depositing Au nanoparticles on a Multi-wall Carbon Nanotube layer previously deposited on the etched Aluminum foil. A low cost, simple method is used to deposit the nanotubes. Huge enhancements have been observed in both in vitro and in vivo measurements.

Dependence of current-voltage characteristics on Al mole fraction in GaAs/AlxGa1−xAs asymmetric double barrier structures

The effect of barrier Al mole fraction, 0.2⩽x⩽0.8, on tunneling currents has been studied for a set of asymmetric GaAs/AlxGa1−xAs double barrier structures. The barrier widths of each sample were scaled so that barrier transmission coefficients for different samples should be approximately equivalent at the first resonant tunneling peak. Structures were grown by molecular beam epitaxy; by adjusting Ga and Al cell temperatures, the full range of Al mole fractions could be achieved in AlxGa1−xAs barrier layers while maintaining a nearly constant growth rate of about 1 μm/h. Current-voltage measurements are in agreement with theoretical estimates and indicate good sample quality.