We demonstrate the growth by molecular beam epitaxy of a metal/semiconductor composite consisting of epitaxial semimetallic ErSb particles in a GaSb matrix. The ErSb nucleates in an island growth mode leading to the spontaneous formation of nanometer-sized particles. These particles are found to preferentially grow along a [011] direction on a (100) GaSb surface. The particles can be overgrown with GaSb to form an epitaxial superlattice consisting of ErSb particles between GaSb spacer layers. The size of the ErSb particles increases monotonically with the deposition. The carrier concentrations in the superlattices are found to be dependent on both the size and density of the ErSb particles. Smaller particles and closer layer spacings reduce the hole concentration in the film. (C) 2004 American Institute of Physics.

Direct epitaxial integration of III-V optoelectronic devices on Si offers a substantial manufacturing cost and scalability advantage over heterogeneous integration via wafer bonding. The challenge in utilizing direct epitaxy of III-Vs on Si is that epitaxial growth introduces high densities of crystalline defects that limit device performance and lifetime. As an optical gain medium, quantum dots exhibit a unique tolerance to crystalline defects due to their three-dimensional quantum confined structure.

Quantum dot lasers epitaxially grown on Si are showing promise for achieving low-cost, scalable integration with silicon photonics. Their atom-like, inhomogeneously broadened, discrete density of states yields unique gain properties that show promise for improved performance and new functionalities relative to their quantum well counterparts (even on native substrates). By reducing the dislocation density in III-V/Si material and improving quantum dot size homogeneity, several world record results have been achieved for epitaxial laser performance on silicon.

A subset of the results achieved include continuous-wave threshold currents below 1 mA in micro-scale ring laser cavities, single-facet output powers of 175 mW at 20 °C, continuous wave lasing up to 105°C, near zero linewidth enhancement factor, isolator-free stability at optical feedback levels of up to 90%, and record long device lifetimes on silicon of more than 100 years at 35°C based on extrapolated 8,000-hour aging studies, and >100,000 h lifetimes at 60°C from extrapolated 4,000-hour aging studies. These results show potential to revolutionize integrated photonics through economic advantages and performance capabilities not achievable in quantum well lasers.

An investigation of the optical transmission properties of semimetallic ErAs films grown by molecular beam epitaxy reveals a maximum in transmission around 1.55 mu m. The semitransparent window extends from similar to 1.5 to 2.3 mu m. These films were found to have resistivities less than 7x10(-5) Omega cm and permit similar to 85% transmission for a 150 nm film and similar to 97% transmission for a 15 nm film with respect to a GaAs substrate at 1.55 mu m. These results suggest that ErAs may be a useful material for applications requiring transparent contacts from 1.5 to 2.3 mu m. Polycrystalline films of ErAs were grown on sapphire substrates to investigate optical properties of ErAs in the visible region. (c) 2006 American Institute of Physics.

The low-frequency power spectrum has been measured for ErAs:InAlGaAs diodes and shows at least a ten-times reduction of 1/f noise compared to traditional Al Schottky diodes on the same semiconductor material. These junctions are grown by molecular beam epitaxy, preventing oxidation and other contamination at the junction. The major noise source for these devices is attributed to the sidewalls and not the junction itself. Low-frequency oscillations have also been observed and associated with a deep trap level estimated to be similar to 200 meV below the conduction band edge by the comparison of diodes with different InAlGaAs compositions and confirmed by deep level transient spectroscopy. This deep level could be associated to erbium incorporation in the depletion region.

We demonstrate subpicosecond photocarrier lifetimes at 1.55 mum in GaSb/ErSb nanoparticle superlattices grown by molecular beam epitaxy. Pump-probe measurements were made with a 1.55 mum mode-locked laser in transmission geometry to determine the photocarrier lifetime. The lifetime is found to be dependent on the size of the ErSb particles, amount of ErSb, and the distance between layers of particles. Through manipulation of these three parameters the photocarrier lifetime can be tuned down to less than 300 fs, the temporal limit of our experiment. (C) 2004 American Institute of Physics.

We demonstrate a molecular beam epitaxy growth technique to create intentionally textured interfaces between semimetallic ErAs and GaAs that result in interface orientations that are different from the substrate. To grow the textured interfaces, ErAs is deposited on (100) GaAs and islands form in an island growth mode. Enough GaAs is then deposited to fill the space between islands with semiconductor. More ErAs is deposited on the ErAs/GaAs surface and the new ErAs islands are thought to nucleate on the exposed top surface of the partially covered islands. The process is repeated to "stack" the islands. By quadratically increasing the amount of ErAs deposited in each repetition, the islands form into cones/pyramids that coalesce into a complete film with an intentionally roughened interface. Compared to a smooth interface between ErAs and GaAs, the Schottky barrier height between textured ErAs and n-GaAs can be reduced from similar to 0.81 to similar to 0.52 eV, and the Schottky barrier height between ErAs and p-GaAs can be increased from similar to 0.38 to similar to 0.63 eV. (c) 2006 American Vacuum Society.

High-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy was used to determine the atomic structure of interfaces between epitaxial ErAs layers with the cubic rock salt structure and In0.53Ga0.47As and GaAs, respectively. All layers were grown by molecular beam epitaxy. We show that the interfacial atomic arrangement corresponds to the so-called chain model, in which the zinc blende semiconductor is terminated with a Ga layer. Image analysis was used to quantify the expansion between the first ErAs plane and the terminating Ga plane. In the HAADF images, a high intensity transfer from the heavy Er columns into the background was observed in the ErAs layer, whereas the background in In0.53Ga0.47As was of much lower intensity.

In this paper, we explore electron filtering as a technique to increase the Seebeck coefficient and the thermoelectric power factor of heterostructured materials over that of the bulk. We present a theoretical model in which the Seebeck coefficient and the power factor can be increased in an In0.53Ga0.47As-based composite material. Experimental measurements of the cross-plane Seebeck coefficient are presented and confirm the importance of the electron filtering technique to decouple the electrical conductivity and Seebeck coefficient to increase the thermoelectric power factor.

In contrast to traditional metal-semiconductor (i.e., Schottky) junctions, molecular beam epitaxy (MBE)-grown ErAs:InAlGaAs heterojunctions have recently been shown to provide highly "engineerable" electrical rectification characteristics through the tuning of the Schottky barrier height, while maintaining the very low specific capacitance. This letter reports an approximate 10x improvement in the low-frequency noise performance by using MBE-grown ErAs as the Schottky contact instead of evaporated aluminum. The low-frequency noise power spectrum of ErAs devices has been observed to have a 1/f(1.0) frequency dependence. Constant-current bias-dependent measurements have shown a 1.8 power law dependence of the noise spectral density on dc current. (c) 2006 American Institute of Physics.