Vanessa Sih

The disintegration of GaSb/GaAs nanostructures upon capping

Atom probe tomography and cross-sectional scanning tunneling microscopy show that GaSb/GaAs quantum dots disintegrate into ring-like clusters of islands upon capping. Band transition energies calculated using an 8-band k.p model of the capped dots with the observed dimensions are consistent with emission energies observed in photoluminescence data. These results emphasize the need for full three-dimensional characterization to develop an accurate understanding of the structure, and thus the optical properties, of buried quantum dots.

Spin lifetime measurements in GaAsBi thin films

Photoluminescence spectroscopy and Hanle effect measurements are used to investigate carrier spin dephasing and recombination times in the semiconductor alloy GaAsBi as a function of temperature and excitation energy. Hanle effect measurements reveal the product of g-factor and effective spin dephasing time (gTs) ranges from 0.8 ns at 40 K to 0.1 ns at 120 K. The temperature dependence of gTs provides evidence for a thermally activated effect, which is attributed to hole localization at single Bi or Bi cluster sites below 40 K.

Long-Lived Hole Spin/Valley Polarization Probed by Kerr Rotation in Monolayer WSe2.

Time-resolved Kerr rotation and photoluminescence measurements are performed on MOCVD-grown monolayer tungsten diselenide (WSe2). We observe a surprisingly long-lived Kerr rotation signal (∼80 ns) at 10 K, which is attributed to spin/valley polarization of the resident holes. This polarization is robust to transverse magnetic field (up to 0.3 T). Wavelength-dependent measurements reveal that only excitation near the free exciton energy generates this long-lived spin/valley polarization.

Analysis of defect-free GaSb/GaAs(001) quantum dots grown on the Sb-terminated (2 × 8) surface

Multilayer and single layer GaSb/GaAs(001) quantum dot structures were grown on an Sb-terminated (2 × 8) surface reconstruction and compared to those grown on an As-terminated (2 × 4) surface reconstruction. Uncapped quantum dots grown on the (2 × 8) surface were approximately 25% smaller in diameter and had a larger width/height aspect ratio. Quantum dots grown on both surfaces were defect free at the quantum dot/spacer layer interface. The dots did not appear to be fully compact when imaged by transmission electron microscopy, which may be due to dissolution and/or quantum ring formation. The quantum dot photoluminescence peak for dots grown on the (2 × 8) surface was brighter but at the same energy as that of dots grown on the (2 × 4) surface. This was likely the result of a higher areal density of dots on the (2 × 8) surface and a lower tendency for them to intermix during capping, resulting in dots of similar size for both samples after capping. Quantum dots grown on the (2 × 8) surface also displaye...

Surface plasmon resonances of Ga nanoparticle arrays

We have examined the influence of particle and chain diameter on surface plasmon resonance (SPR) energy of 2D and 1D Ga nanoparticle (NP) arrays fabricated using focused-ion-beam irradiation of GaN surfaces. Maxima in the extinction spectra suggest the presence of SPR at visible and near-infrared wavelengths. The SPR energies increase with decreasing NP or chain diameter, due to particle diameter-dependent dipole interactions within the metallic NPs. The SPR quality factors are comparable to those reported from Ag and Au NPs, suggesting Ga NPs as a promising alternative plasmonic material.

Lateral patterning of multilayer InAs/GaAs(001) quantum dot structures by in vacuo focused ion beam

We report on the effects of patterning and layering on multilayer InAs/GaAs(001) quantum dot structures laterally ordered using an in vacuo focused ion beam. The patterned hole size and lateral pattern spacing affected the quantum dot size and the fidelity of the quantum dots with respect to the lateral patterns. 100% pattern fidelity was retained after six layers of dots for a 9.0 ms focused ion beam dwell time and 2.0 µm lateral pattern spacing. Analysis of the change in quantum dot size as a function of pattern spacing provided a means of estimating the maximum average adatom surface diffusion length to be approximately 500 nm, and demonstrated the ability to alter the wetting layer thickness via pattern spacing. Increasing the number of layers from six to 26 resulted in mound formation, which destroyed the pattern fidelity at close pattern spacings and led to a bimodal quantum dot size distribution as measured by atomic force microscopy. The bimodal size distribution also affected the optical properties of the dots, causing a split quantum dot photoluminescence peak where the separation between the split peaks increased with increasing pattern spacing.

Photoluminescence imaging of focused ion beam induced individual quantum dots.

We report on scanning microphotoluminescence measurements that spectrally and spatially resolve emission from individual InAs quantum dots that were induced by focused ion beam patterning. Multilayers of quantum dots were spaced 2 μm apart, with a minimum single dot emission line width of 160 μeV, indicating good optical quality for dots patterned using this technique. Mapping 16 array sites, at least 65% were occupied by optically active dots and the spectral inhomogeneity was within 30 meV.

Photoluminescence of patterned arrays of vertically stacked InAs/GaAs quantum dots

We report on photoluminescence measurements of vertically stacked InAs/GaAs quantum dots grown by molecular beam epitaxy on focused ion beam patterned hole arrays with varying array spacing. Quantum dot emission at 1.24 eV was observed only on patterned regions, demonstrating preferential nucleation of optically active dots at desired locations and below the critical thickness for dot formation at these growth conditions. Photoluminescence measurements as a function of varying focused ion beam irradiated hole spacing showed that the quantum dot emission intensity increased with decreasing array periodicity, consistent with increasing dot density.

Atom probe tomography analysis of different modes of Sb intermixing in GaSb quantum dots and wells

Different modes of intermixing are observed in GaSb/GaAs layers via atom probe tomography. The intermixing length scale for quantum wells of varying thickness is on the order of a monolayer, but three times longer for the wetting layer of a quantum dot structure. The former arises from segregation of Sb and/or surface-induced intermixing via detachment from step edges. The latter is dominated by surface-induced intermixing due to disintegration of the GaSb dots upon capping.

Optimizing nanophotonic cavity designs with the gravitational search algorithm

Designing photonic crystal cavities with high quality factors and low mode volumes is of great importance for maximizing interactions of light and matter in metamaterials. Previous work on photonic crystal cavities has revealed dramatic improvements in performance by fine-tuning the device design. In L3 cavities, slight shifts of the holes on the edge of the cavity have been found to greatly increase quality factors without significantly altering the mode volume. Here we demonstrate utilizing a nature inspired search algorithm to efficiently explore a large parameter space. The results converge upon a new cavity model with a high quality factor to mode volume ratio (Q/V = 798,000 (λ/n)(-3)).