Publications

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The goal of this project is to search for Majorana fermions (a new quantum particle) in a topological superconductor (a new quantum matter achieved in a topological insulator proximitized by an s-wave superconductor). Majorana fermions (MFs) are electron-like particles that are their own anti-particles. MFs are shown to obey non-Abelian statistics and, thus, can be harnessed to make a fault-resistant topological quantum computer. With the arrival of topological insulators, novel schemes to create MFs have been proposed in hybrid systems by combining a topological insulator with a conventional superconductor. In this LDRD project, we will follow the theoretical proposals to search for MFs in one-dimensional (1D) topological superconductors. 1D topological superconductor will be created inside of a quantum point contact (with the metal pinch-off gates made of conventional s-wave superconductors such as niobium) in a two-dimensional topological insulator (such as inverted type-II InAs/GaSb heterostructure).

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We utilize the unique dispersion properties of leaky plasmon polaritons in epsilon-near-zero (ENZ) thin films to demonstrate thermal radiation control. Owing to its highly flat dispersion above the light line, a thermally excited leaky wave at the ENZ frequency out-couples into free space without any scattering structures, resulting in a narrowband, wide-angle, p-polarized thermal emission spectrum. We demonstrate this idea by measuring angle- and polarization-resolved thermal emission spectra from a single layer of unpatterned, doped semiconductors with deep-subwavelength film thickness (d / λ 0 ∼ 6 × 10 - 3, where d is the film thickness and λ 0 is the free space wavelength). We show that this semiconductor ENZ film effectively works as a leaky wave thermal radiation antenna, which generates far-field radiation from a thermally excited mode. The use of semiconductors makes the radiation frequency highly tunable by controlling doping densities and also facilitates device integration with other components. Therefore, this leaky plasmon polariton emission from semiconductor ENZ films provides an avenue for on-chip control of thermal radiation.

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Wafer-level step-stress experiments on high voltage Npn InGaP/GaAs HBTs are presented. A methodology utilizing brief, monotonically increasing stresses and periodic, interrupted parametric characterization is presented. The method and various examples of step-stressed HBTs illustrate the value of the technique for screening the reliability of HBT wafers. Degradation modes observed in these InGaP/GaAs HBTs closely correspond to a subset of those in other, longer types of reliability experiments and can be relevant in a reliability screen. A statistical sampling of HBT wafers reveals a consistently realized critical destructive limit over a very narrow power range, which indicates that thermal stress is the main cause of degradation. When stepped just shy of the destructive limit, electrical characteristics are capable of revealing gradual degradation. The end state of stressing typically involves shorting of both the base-emitter and base-collector junctions. Interrupted characterization revealed cases where baseemitter shorts preceded base-collector shorts and other cases where base-collector shorts occurred first. Examples of degradation include reductions in reverse breakdown voltage, increases in the offset voltage, and drops in current gain. These wafer-level stepstress techniques show promise for reducing the large time lag between wafer fabrication and useful reliability screening in HBTs.

Wafer-level step-stress experiments on high voltage Npn InGaP/GaAs HBTs are presented. A methodology utilizing brief, monotonically increasing stresses and periodic, interrupted parametric characterization is presented. The method and various examples of step-stressed HBTs illustrate the value of the technique for screening the reliability of HBT wafers. Degradation modes observed in these InGaP/GaAs HBTs closely correspond to a subset of those in other, longer types of reliability experiments and can be relevant in a reliability screen. A statistical sampling of HBT wafers reveals a consistently realized critical destructive limit over a very narrow power range, which indicates that thermal stress is the main cause of degradation. When stepped just shy of the destructive limit, electrical characteristics are capable of revealing gradual degradation. The end state of stressing typically involves shorting of both the base-emitter and base-collector junctions. Interrupted characterization revealed cases where baseemitter shorts preceded base-collector shorts and other cases where base-collector shorts occurred first. Examples of degradation include reductions in reverse breakdown voltage, increases in the offset voltage, and drops in current gain. These wafer-level stepstress techniques show promise for reducing the large time lag between wafer fabrication and useful reliability screening in HBTs.

Quantum-engineered multiple stage photovoltaic (PV) devices are explored based on InAs/GaSb/AlSb interband cascade (IC) structures. These ICPV devices employ multiple discrete absorbers that are connected in series by widebandgap unipolar barriers using type-II heterostructure interfaces for facilitating carrier transport between cascade stages similar to IC lasers. The discrete architecture is beneficial for improving the collection efficiency and for spectral splitting by utilizing absorbers with different bandgaps. As such, the photo-voltages from each individual cascade stage in an ICPV device add together, creating a high overall open-circuit voltage, similar to conventional multi-junction tandem solar cells. Furthermore, photo-generated carriers can be collected with nearly 100% efficiency in each stage. This is because the carriers travel over only a single cascade stage, designed to be shorter than a typical diffusion length. The approach is of significant importance for operation at high temperatures where the diffusion length is reduced. Here, we will present our recent progress in the study of ICPV devices, which includes the demonstration of ICPV devices at room temperature and above with narrow bandgaps (e.g. 0.23 eV) and high open-circuit voltages. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

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