V. D. Dasika

Design and analysis of photonic crystal coupled cavity arrays for quantum simulation

Through experimental study of an array of coupled photonic crystal cavities, we find that the intercavity coupling is significantly larger than the fabrication-induced disorder, a necessary condition for the generation of strongly correlated photons.

Growth and characterization of LuAs films and nanostructures

We report the growth and characterization of nearly lattice-matched LuAs/GaAs heterostructures. Electrical conductivity, optical transmission, and reflectivity measurements of epitaxial LuAs films indicate that LuAs is semimetallic, with a room-temperature resistivity of 90 μΩ cm. Cross-sectional transmission electron microscopy confirms that LuAs nucleates as self-assembled nanoparticles, which can be overgrown with high-quality GaAs. The growth and material properties are very similar to those of the more established ErAs/GaAs system; however, we observe important differences in the magnitude and wavelength of the peak optical transparency, making LuAs superior for certain device applications, particularly for thick epitaxially embedded Ohmic contacts that are transparent in the near-IR telecommunications window around 1.3 μm.

Growth and properties of rare-earth arsenide InGaAs nanocomposites for terahertz generation

We explore the electrical, optical, and structural properties of fast photoconductors of In0.53Ga0.47As containing a number of different rare-earth arsenide nanostructures. The rare-earth species provides a route to tailor the properties of the photoconductive materials. LuAs, GdAs, and LaAs nanostructures were embedded into InGaAs in a superlattice structure and compared to the relatively well-studied ErAs:InGaAs system. LaAs:InGaAs was found to have the highest dark resistivities, while GdAs:InGaAs had the lowest carrier lifetimes and highest carrier mobility at moderate depositions. The quality of the InGaAs overgrowth appears to have the most significant effect on the properties of these candidate fast photoconductors.

Growth and characterization of single crystal rocksalt LaAs using LuAs barrier layers

We demonstrate the growth of high‐quality, single crystal, rocksalt LaAs on III‐V substrates; employing thin well-behaved LuAs barriers layers at the III-V/LaAs interfaces to suppress nucleation of other LaAs phases, interfacial reactions between GaAs and LaAs, and polycrystalline LaAs growth. This method enables growth of single crystal epitaxial rocksalt LaAs with enhanced structural and electrical properties. Temperature-dependent resistivity and optical reflectivity measurements suggest that epitaxial LaAs is semimetallic, consistent with bandstructure calculations in literature. LaAs exhibits distinct electrical and optical properties, as compared with previously reported rare-earth arsenide materials, with a room-temperature resistivity of ∼459 μΩ-cm and an optical transmission window >50% between ∼3-5 μm.

Correlating structure, strain, and morphology of self-assembled InAs quantum dots on GaAs

We report on the use of a direct x-ray phase retrieval method, coherent Bragg rod analysis, to characterize self-assembled InAs quantum dots (QDs) grown epitaxially on GaAs substrates. Electron density maps obtained close to the x-ray absorption edges of the constituent elements are compared to deconvolute composition and atomic spacing information. Our measurements show no evidence of a wetting layer and reveal bowing of the atomic layers throughout the QD, extending from the QD-substrate interface. This leads to a half-layer stacking shift which may act to partially decouple the QDs electronically from the substrate.

Surfactant-assisted growth and properties of rare-earth arsenide InGaAs nanocomposites for terahertz generation

We explore the effects of surfactant-mediated epitaxy on the structural, electrical, and optical properties of fast metal-semiconductor superlattice photoconductors. Specifically, application of a bismuth flux during growth was found to significantly improve the properties of superlattices of LuAs nanoparticles embedded in In0.53Ga0.47As. These improvements are attributed to the enhanced structural quality of the overgrown InGaAs over the LuAs nanoparticles. The use of bismuth enabled a 30% increase in the number of monolayers of LuAs that could be deposited before the InGaAs overgrowth degraded. Dark resistivity increased by up to ∼15× while carrier mobility remained over 2300 cm2/V-s and carrier lifetimes were reduced by >2× at comparable levels of LuAs deposition. These findings demonstrate that surfactant-mediated epitaxy is a promising approach to enhance the properties of ultrafast photoconductors for terahert generation.

Increased InAs quantum dot size and density using bismuth as a surfactant

We have investigated the growth of self-assembled InAs quantum dots using bismuth as a surfactant to control the dot size and density. We find that the bismuth surfactant increases the quantum dot density, size, and uniformity, enabling the extension of the emission wavelength with increasing InAs deposition without a concomitant reduction in dot density. We show that these effects are due to bismuth acting as a reactive surfactant to kinetically suppress the surface adatom mobility. This mechanism for controlling quantum dot density and size has the potential to extend the operating wavelength and enhance the performance of various optoelectronic devices.

Minimized open-circuit voltage reduction in GaAs/InGaAs quantum well solar cells with bandgap-engineered graded quantum well depths

The use of InGaAs quantum wells with composition graded across the intrinsic region to increase open-circuit voltage in p-i-n GaAs/InGaAs quantum well solar cells is demonstrated and analyzed. By engineering the band-edge energy profile to reduce photo-generated carrier concentration in the quantum wells at high forward bias, simultaneous increases in both open-circuit voltage and short-circuit current density are achieved, compared to those for a structure with the same average In concentration, but constant rather than graded quantum well composition across the intrinsic region. This approach is combined with light trapping to further increase short-circuit current density.

Conductivity and structure of ErAs nanoparticles embedded in GaAs pn junctions analyzed via conductive atomic force microscopy

We have used conductive atomic force microscopy to investigate the influence of growth temperature on local current flow in GaAs pn junctions with embedded ErAs nanoparticles grown by molecular beam epitaxy. Three sets of samples, one with 1 ML ErAs deposited at different growth temperatures and two grown at 530 °C and 575 °C with varying ErAs depositions, were characterized. Statistical analysis of local current images suggests that the structures grown at 575 °C have about 3 times thicker ErAs nanoparticles than structures grown at 530 °C, resulting in degradation of conductivity due to reduced ErAs coverage. These findings explain previous studies of macroscopic tunnel junctions.

Growth rate and surfactant-assisted enhancements of rare-earth arsenide InGaAs nanocomposites for terahertz generation

We report the effects of the growth rate on the properties of iii-v nanocomposites containing rare-earth-monopnictide nanoparticles. In particular, the beneficial effects of surfactant-assisted growth of LuAs:In0.53Ga0.47As nanocomposites were found to be most profound at reduced LuAs growth rates. Substantial enhancement in the electrical and optical properties that are beneficial for ultrafast photoconductors was observed and is attributed to the higher structural quality of the InGaAs matrix in this new growth regime. The combined enhancements enabled a >50% increase in the amount of LuAs that could be grown without degrading the quality of the InGaAs overgrowth. Dark resistivity increased by ∼25× while maintaining carrier mobilities over 3000 cm2/V s; carrier lifetimes were reduced by >2×, even at high depositions of LuAs. The combined growth rate and surfactant enhancements offer a previously unexplored regime to enable high-performance fast photoconductors that may be integrated with telecom compone...