David H. Tomich

Reduced auger recombination in mid-infrared semiconductor lasers

A quantum-design approach to reduce the Auger losses in λ = 2 μm InGaSb type-I quantum well edge-emitting lasers is reported. Experimentally realized structures show a ∼3 × reduction in the threshold, which results in 4.6 × lower Auger current loss at room temperature. This is equivalent to a carrier lifetime improvement of 5.7 × and represents about a 19-fold reduction in the equivalent “Auger coefficient.”

Band gap formation in graphene by in-situ doping

We report the formation of band gaps in as-grown stacks of epitaxial graphene with opposite doping. Control of in-situ doping during carbon source molecular beam epitaxy growth on SiC was achieved by using different carbon sources. Doping heterostructures were grown by stacking n-type material from a C60 source on p-type material from a graphite filament source. Activation energies for the resistivity and carrier concentration indicated band gaps up to 200 meV. A photoconductivity threshold was observed in the range of the electrical activation energies. Band gap formation is attributed to electric fields induced by spatially separated ionized dopants of opposite charge.

Progress in orientation-patterned GaAs for next-generation nonlinear optical devices

Orientation-patterned GaAs (OPGaAs) shows great promise as a nonlinear optical material for frequency conversion in the 2-5 μm and 8-12 μm regions. We report recent progress in each of the three main areas of OPGaAs development: fabrication of patterned templates using a combination of wafer bonding and MBE techniques; thick-layer HVPE growth; and material and OPO device characterization. This work has led to significant improvements in material quality, specifically reduced optical loss, increased sample thickness, improved patterned domain fidelity, and greater material uniformity. Advances in material quality have in turn enabled demonstration of OPO devices operating in the 3-5 μm spectral region. Optical loss and OPO performance measurements on a series of OPGaAs samples are presented, with the goal of understanding how these properties are influenced by growth conditions, and how OPO performance may be improved. Research continues on understanding loss mechanisms, correlating performance with material properties, transitioning the technology into an industrial process, and extending it to additional materials.

Self-assembly of heterojunction quantum dots

The fabrication of a self-assembled heterojunction quantum dot structure composed of multiple materials is reported. This structure consists of a composite dot formed of an initial core of one material which results from normal self-assembly, followed by the epitaxy of a crown composed of a similarly strained material. Finally the entire dot structure is capped with a barrier material closely lattice matched to the substrate. In this demonstration, self-assembled InAs quantum dots were first formed on a GaAs substrate and subsequently crowned with GaSb. The entire structure was encapsulated with a GaAs cap layer. Atomic force microscopy shows that additional nucleation between the InAs layers has been minimized and cross-sectional transmission electron microscopy shows the formation of the composite structure.

Study of interfaces in GaInSb/InAs quantum wells by high-resolution X-ray diffraction and reciprocal space mapping

The electrical and optical properties of advanced epitaxial structures, such as quantum wells and superlattices are strongly influenced by the quality of their interfaces. The GaInSb/InAs system is particularly important because of its promise for use in infrared detection in the 8-14 μm ranges. In this material system the two compounds do not share a common anion, and different bond types exist at the interface depending upon the growth parameters. We have looked at single quantum-well structures grown with solid source molecular beam epitaxy such that a distinct interface type would be found in each sample. X-ray rocking curves and full dynamical simulations were performed for each quantum well structure. Quantum wells with three types of interfaces were grown and analyzed; random interfaces, Sb-like interfaces, and As-like interfaces formed with a one monolayer group III deposition followed by a five second group V soak. With the exception of the interfaces, all three SQWs have nominally the same structure; 5000 A of GaSb buffer layer grown upon a GaSb substrate, a 150 A quantum well with 35 A Ga 0.75 In 0.25 Sb barriers and a final GaSb cap layer of 50 A thickness. Well-resolved Pendellosung fringes were found in all samples indicating high quality in the epitaxial layers and interfaces. The SQW with the As-like interfaces had the highest degree of quality as evidenced by persistence of the fringes.

Recent VECSEL developments for sensors applications

Vertical external cavity surface emitting lasers (VECSELs) have proven themselves to be a suitable semiconductor answer to many solid-state lasers. Their simplicity makes them a very versatile platform for accessing wavelengths from the UV through the THz through direct and frequency-converted emission. This wavelength flexibility, combined with an optical cavity accommodating additional tuning or nonlinear elements, make the VECSEL a uniquely suited solution to a variety of applications. We will present recent AFRL progress in VECSELs and potential applications for these lasers.

Growth and spectroscopic ellipsometry evaluation of composite layers of ErAs and InAs nanoparticles

Metal nanoparticles coupled to semiconductor quantum dots have been studied recently due to the enhancement in absorption, emission, and nonlinearities expected from these hybrid structures. These properties stem from the ability of the metal to focus light as well as shift the phase, which occurs at the metal–dielectric interface. To date, most quantum dots metal nanoparticle couples are formed by the attachment of a ligand to both particles. The extension of this idea to bulk semiconductor films is being attempted by the formation of a composite structure of ErAs, which forms semimetallic nanoparticles (SMNP) in GaAs, and InAs self-assembled quantum dots (SAQD). In this work, the authors analyze structures composed of periods of InAs SAQDs and ErAs SMNPs and analyze these with spectroscopic ellipsometry in the spectral region 0.7–4.0 eV with 0.02 eV steps. Initially, individual structures composed of InAs SAQD stacks or ErAs SMNP stacks, both capped with layers of GaAs, are analyzed. The authors have al...

Electrical and structural characterization of a single GaSb∕InAs∕GaSb quantum well grown on GaAs using interface misfit dislocations

Interface misfit formation has been used for the growth of high mobility GaSb∕InAs single quantum wells (SQW) formed on GaAs substrates. The SQW structure was topped with 800A GaSb, followed by 100A GaSb:Si (5×108cm−3), 10nm GaSb, 10nm InAs, and finally 250nm GaSb on a GaAs substrate. The structural quality was examined using high resolution x-ray diffraction and transmission electron microscopy. Reciprocal space mapping indicated that the GaSb was completely relaxed. A high resolution x-ray rocking curve showed good agreement between the proposed structure and the simulation, assuming that all layers were relaxed to the GaSb lattice, and clearly showed interference fringing from individual layers. Atomic force microscopy showed the film appeared textured, and that the final growth occurred by step flow growth. The observed peak-to-peak roughness was 7nm over a 100×100μm2 square area. Plane view transmission electron microscopy analysis showed a nearly regular array of Lomer dislocations responsible for t...

Nanofabricated quantum dot array formation through annealing of nano-patterned planar InAs

Quantum dots (QDs) are typically formed using a self-assembly process that results in random placement and size distributions, thus limiting their applicability for many devices. In this work, we report a process which uses nano-patterned planar InAs and subsequent annealing under As stabilized conditions to produce QDs with uniform placement and size distribution. The authors demonstrate the ability to form ordered QD arrays with a density of 3 × 1010 dots/cm2 and QD base widths of <30 nm. The authors achieved photoluminescence from the patterned area at a temperature below 100 K.

Shape changes in patterned planar InAs as a function of thickness and temperature

Quantum dots have the potential to produce devices with enhanced properties. However, many quantum dot devices require the quantum dots to have a precise size and a precise location for optimum operation. So far approaches such as directed assembly and self assembly have failed due to the random effects resulting during nucleation of the quantum dots. InAs grown under metal rich conditions can remain planar as opposed to forming the self assembled quantum dot morphology. Recently we have demonstrated that planar InAs when patterned via tip-based scribing and then annealed under an As pressure typical for self-assembled quantum dot growth reorganizes and assumes a 3D morphology. We have been studying this process as a potential method to precisely locate quantum dots with definable sizes. In this work we report change in the morphology for different thickness of planar InAs for various pattern dimensions and annealing temperatures. We have analyzed the composition of the films after annealing to determine the effect induced in the films from patterning resulting from scribing. Using this approach, arrays of 3D InAs mounds have been formed with mounds having base dimensions of 800, 500, and 350Å. These results demonstrate that the smaller patterns are less stable and coarsening becomes more dominant.