Joanna Mirecki Millunchick

Unified model of droplet epitaxy for compound semiconductor nanostructures: Experiments and theory

We present a unified model of compound semiconductor growth based on kinetic Monte Carlo simulations in tandem with new experimental results that can describe and predict the mechanisms for the formation of various types of nanostructures observed during droplet epitaxy. The crucial features of the model include the explicit and independent representation of atoms with different species and the ability to treat solid and liquid phases independently. Using this model, we examine nanostructural evolution in droplet epitaxy. The model faithfully captures several of the experimentally observed structures, including compact islands and nanorings. Moreover, simulations show the presence of Ga/GaAs core-shell structures that we validate experimentally. A fully analytical model of droplet epitaxy that explains the relationship between growth conditions and the resulting nanostructures is presented, yielding key insight into the mechanisms of droplet epitaxy.

Thermal emission in type-II GaSb/GaAs quantum dots and prospects for intermediate band solar energy conversion

The electronic structure and thermal carrier capture and escape mechanisms are studied for GaSb/GaAs quantum dots with a type-II band alignment using admittance spectroscopy. Clear signatures are observed corresponding to confined quantum dot states with extracted activation energy of 0.337 eV and the thermal capture cross section in the range from 2.10 × 10−16 to 1.19 × 10−13 cm2. The thermal emission rates in the GaSb/GaAs quantum dots are significantly lower than prior reports for type-I systems, where optical emission is predicted to be the dominant process in an intermediate band solar cells under solar concentration.

Filling of hole arrays with InAs quantum dots

Focused ion beams are used to pattern GaAs(001) surfaces with an array of nanometer-deep holes upon which deposition of InAs results in quantum dot formation at the hole location. Experiments show that the size and quantity of quantum dots formed depend on growth parameters, and ion dose, which affects the size and shape of the resulting holes. Quantum dots fabricated in this fashion have a photoluminescence peak at 1.28 eV at 77 K, indicating that the ion irradiation due to patterning does not destroy their optical activity. Kinetic Monte Carlo simulations that include elastic relaxation qualitatively model the growth of dots in nanometer-deep holes, and demonstrate that growth temperature, depth of the holes, and the angle of the hole sidewalls strongly influence the number of quantum dots that form at their perimeter.

Cooperative nucleation leading to ripple formation in InGaAs/GaAs films

In0.25Ga0.75As epilayers were grown on GaAs (001) substrates (1.8% misfit strain) by molecular beam epitaxy to investigate the two-dimensional to three-dimensional transition as a function of thickness (t⩽30 MLs). Tapping-mode atomic force micrographs show the evolution of the morphology as a function of thickness. As the film is deposited, the nucleation of 3D islands followed by cooperative nucleation of pits is observed. As the thickness increases, both islands and pits continue to nucleate and grow until they coalesce, resulting in a fully formed ripple morphology running along the [110]. The ripples also exhibit a secondary alignment roughly along the 〈310〉 which is attributed to the nucleation of islands with {136} faces.

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.

Small signal modulation characteristics of red-emitting (λ = 610 nm) III-nitride nanowire array lasers on (001) silicon

The small signal modulation characteristics of an InGaN/GaN nanowire array edge- emitting laser on (001) silicon are reported. The emission wavelength is 610 nm. Lattice matched InAlN cladding layers were incorporated in the laser heterostructure for better mode confinement. The suitability of the nanowire lasers for use in plastic fiber communication systems with direct modulation is demonstrated through their modulation bandwidth of f-3dB,max = 3.1 GHz, very low values of chirp (0.8 A) and α-parameter, and large differential gain (3.1 × 10−17 cm2).

Mechanisms of ring and island formation in lattice mismatched droplet epitaxy

Lattice mismatched GaSb nanostructures were grown using droplet epitaxy. In this method, liquid Ga droplets are deposited on GaAs substrates and then exposed to a Sb flux at various temperatures. At increasing temperature and droplet volumes, the morphologies changed from two-dimensional islands to nanoholes, three-dimensional islands, rings, and clusters of islands. A theoretical model describes the relationship between the volume of the droplet and the final nanostructure, and is validated by kinetic Monte Carlo simulations. The combined experimental and simulation results demonstrate another process to obtain complex nanostructures, widening the design window for devices.

Work in progress - using screencasts to enhance student learning in a large lecture Material Science and Engineering course

University lecturing is changing as a result of increasing student populations, increasing student diversity, and transformative technologies. One of the newest technological developments is the availability of screencasts, recordings that capture audio narration along with computer screen images. This study documents the strategic use of screencasts in a Material Science and Engineering (MSE) course, and examines their impact on student learning and satisfaction in the large lecture environment. Screencasts posted to the course management site included solutions to homework and quizzes and mini-lectures that explain topics identified by students as unclear. Survey results indicate that the majority of students responding found all of the screencasts helpful regardless of whether they found a concept difficult or not. But other data suggest that the impact on student learning could be even greater, as both faculty and students learn to utilize this new resource. Future course iterations will refine the uses of screencasts and continue analyzing their impact.

Focused ion beam modification of surfaces for directed self-assembly of InAs/GaAs(001) quantum dots

Controlled nucleation of InAs quantum dots has been achieved by Ga + focused ion beam modification of GaAs(100) surfaces. Quantum dots may be induced in irradiated regions despite the fact that the deposited thickness is less than the critical thickness for their formation under typical growth conditions when the ion dose is greater than 10 13 ions cm −2 .W e also fi nd that the dot density increases with increasing ion dose, and reaches saturation for D > 10 14 ions cm −2 . Parameters such as dot height and diameter are unaffected by the dose level. Thus, we show that the increase in dot density is a result of diffusion of adatoms from outside the patterned region. The mechanism for enhanced quantum dot formation is due to the formation of monolayer deep holes created in the substrate by the ion beam, which may be used to form regular arrays of quantum dots.

Mechanisms of nanodot formation under focused ion beam irradiation in compound semiconductors

We have examined the responses of GaAs, InP, InAs, and AlAs to 30 keV focused ion beam (FIB) irradiation and applied a unified model that consistently explains the observed effects. Nanodots were observed to form on GaAs, InP, and InAs under irradiation at normal incidence, while nanodots are not observed on AlAs. The FIB response and nanodot formation behavior of each material is discussed with regard to a few basic material properties and a model for nanodot creation and growth by the action of preferential sputtering and Ostwald ripening. The model predicts the development of a stable average nanodot size with increasing ion dose, with the average nanodot size depending on the excess group III adatom yield, adatom surface diffusion rate, and surface tension. These predictions qualitatively agree with the experimentally observed trends for GaAs and InP. They also agree for the initial nanodot formation on InAs, but this material system exhibits a sudden transition in the nanodot size distribution. The m...