Theoretical and experimental studies are presented to understand the initial stages of growth of InGaAs on GaAs. Thermodynamic considerations show that, as strain increases, the free-energy minimum surface of the epilayer is not atomically flat, but three-dimensional in form. Since by altering growth conditions the strained epilayer can be grown near equilibrium or far from equilibrium, the effect of strain on growth modes can be studied. In situ reflection high-energy electron diffraction studies are carried out to study the growth modes and surface lattice spacing before the onset of dislocations. The surface lattice constant does not change abruptly from that of the substrate to that of the epilayer at the critical thickness, but changes monotonically. These observations are consistent with the simple thermodynamic considerations presented.

For the last two decades, polymer solar cells (PSCs) have been a cynosure of the photovoltaic community, as evidenced by the growing number of patent applications and scientific publications. Efforts to achieve high power conversion efficiency in PSC, propelled by advances in device architecture, material combination, and nanomorphology control, evolved into poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-Butyric-Acid-Methyl Ester (PCBM) bulk heterojunction PSCs, which had been the best seller in PSC research for a decade. Subsequently, PSC research was redirected towards the synthesis of low bandgap materials and optimization of tandem cells, which led to a power conversion efficiency of ∼13%. Even though this efficiency may not be sufficient enough to compete with that of inorganic solar cells, unique properties of PSCs, such as mass roll-to-roll production capability, as well as flexibility and lightness, suggest their niche market opportunities. In this review, an overview of developments in PSCs is presented during the last three decades encompassing pre- and post-P3HT:PCBM era. Emphasis is given on evolution in device architecture, coupled with material selection for pre-P3HT:PCBM era, and synthesis of low-bandgap materials, coupled with a tandem structure for post-P3HT:PCBM era. Last but not least, efforts toward the longer operational lifetime of PSCs by encapsulation are reviewed.

Resonant interband tunneling diodes on silicon substrates are demonstrated using a ${\mathrm{Si/Si}}_{\mathrm{0.5}}{\mathrm{Ge}}_{\mathrm{0.5}}\mathrm{/Si}$ heterostructure grown by low temperature molecular beam epitaxy which utilized both a central intrinsic spacer and δ-doped injectors. A low substrate temperature of 370 °C was used during growth to ensure a high level of dopant incorporation. A B δ-doping spike lowered the barrier for holes to populate the quantum well at the valence band discontinuity, and an Sb δ-doping reduces the doping requirement of the n-type bulk Si by producing a deep $n^{+}$ well. Samples studied from the as-grown wafers showed no evidence of negative differential resistance (NDR). The effect of postgrowth rapid thermal annealing temperature was studied on tunnel diode properties. Samples which underwent heat treatment at 700 and 800 °C for 1 min, in contrast, exhibited NDR behavior. The peak-to-valley current ratio (PVCR) and peak current density of the tunnel diodes were found to depend strongly on δ-doping placement and on the annealing conditions. PVCRs ranging up to 1.54 were measured at a peak current density of 3.2 kA/cm².

Hydrogen silsesquioxane (HSQ) is a high-resolution negative-tone inorganic resist for electron beam lithography. Investigations on the smoothness of the surfaces of thin films (less than 100 nm thick) have been conducted for nanolithography applications. It is demonstrated that films at thicknesses down to 25 nm have very low rms roughness and are defect free. Using 50 kV electron beam lithography, we demonstrate the achievement of isolated 6-nm-wide lines and 27 nm period gratings in 30 nm HSQ films on silicon substrates. These results are superior to those achieved with poly(methylmethacrylate) resist and demonstrates the versatility of HSQ for nanolithography.

AlN/GaN resonant tunneling diodes grown on low dislocation density semi-insulating bulk GaN substrates via plasma-assisted molecular-beam epitaxy are reported. The devices were fabricated using a six mask level, fully isolated process. Stable room temperature negative differential resistance (NDR) was observed across the entire sample. The NDR exhibited no hysteresis, background light sensitivity, or degradation of any kind after more than 1000 continuous up-and-down voltage sweeps. The sample exhibited a ∼90% yield of operational devices which routinely displayed an average peak current density of 2.7 kA/cm² and a peak-to-valley current ratio of ≈1.15 across different sizes.