We present a unified model of compound semiconductor growth based on kinetic Monte Carlo simulations in tandem with 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.

As the size of gallium nitride (GaN) transistors is reduced in order to reach higher operating frequencies, heat dissipation becomes the critical bottleneck in device performance and longevity. Despite the importance of characterizing the physics governing the thermal transport in thin GaN films, the literature is far from conclusive. In this letter, we report measurements of thermal conductivity in a GaN film with thickness ranging from 15–1000 nm grown on 4H-SiC without a transition layer. Additionally, we measure the thermal conductivity in the GaN film when it is 1 μm-thick in the temperature range of 300 < T< 600 K and use a phonon transport model to explain the thermal conductivity in this film.

Silicon carbide is used as a substrate for high-power GaN devices because of its closely matched lattice spacing with GaN and its high thermal conductivity. In these devices, thermal resistance at the GaN–SiC interface is a bottleneck to heat flow, making this property an important factor in device design. In this letter, we report the first measurements of the thermal boundary conductance of epitaxial GaN grown directly on SiC without a transition layer. We find that the thermal boundary conductance increases from approximately 230 MW/m2K at 300 K to 330 MW/m2K at 600 K. Our measured values are in good qualitative agreement with the diffuse mismatch model for thermal boundary conductance and are in good quantitative agreement when we include a correction factor based on the ratio of Debye temperatures of the two materials. We also report the thermal conductivity of the GaN film, the thermal conductivity of 4H-SiC, and the thermal boundary conductance between Ni and GaN.

Terahertz intersubband photodetectors are developed based on GaN/AlGaN quantum wells grown on a free-standing semi-polar ð20 2 1Þ GaN substrate. These quantum wells are nearly free of the polarization-induced internal electric fields that severely complicate the design of nitride intersubband devices on traditional c-plane substrates. As a result, a promising bound-to-quasi-bound THz photodetector design can be implemented. Pronounced photocurrent peaks at the design frequency near 10 THz are measured, covering frequencies that are fundamentally inaccessible to existing arsenide intersubband devices due to reststrahlen absorption. This materials system provides a favorable platform to utilize the intrinsic advantages of nitride semiconductors for THz optoelectronics.

We propose a novel solar cell architecture consisting of multiple fiber-based photovoltaic (PV) cells. Each PV fiber element is designed to maximize the power conversion efficiency within a narrow band of the incident solar spectrum, while reflecting other spectral components through the use of optical microcavity effects and distributed Bragg reflector (DBR) coatings. Combining PV fibers with complementary absorption and reflection characteristics into volume-filling arrays enables spectrally tuned modules having an effective dispersion element intrinsic to the architecture, resulting in high external quantum efficiency over the incident spectrum. While this new reflective tandem architecture is not limited to one particular material system, here we apply the concept to organic PV (OPV) cells that use a metal-organic-metal-dielectric layer structure, and calculate the expected performance of such arrays. Using realistic material properties for organic absorbers, transport layers, metallic electrodes, and DBR coatings, 17% power conversion efficiency can be reached.