Publications

Abstract not provided.

Abstract not provided.

The influence of deep levels defects located in highly resistive GaN:C buffers on the on-resistance (RON) and threshold voltage (V th) of AlGaN/GaN high electron mobility transistors (HEMTs) power devices was studied by a combined photocapacitance deep level optical spectroscopy (C-DLOS) and photoconductance deep level optical spectroscopy (G-DLOS) methodology as a function of electrical stress. Two carbon-related deep levels at 1.8 and 2.85 eV below the conduction band energy minimum were identified from C-DLOS measurements under the gate electrode. It was found that buffer-related defects under the gate shifted Vth positively by approximately 10%, corresponding to a net areal density of occupied defects of 8 × 1012 cm-2. The effect of on-state drain stress and off-state gate stress on buffer deep level occupancy and RON was also investigated via G-DLOS. It was found that the same carbon-related deep levels observed under the gate were also active in the access region. Off-state gate stress produced significantly more trapping and degradation of R ON (∼140%) compared to on-state drain stress (∼75%). Greater sensitivity of RON to gate stress was explained by a more sharply peaked lateral distribution of occupied deep levels between the gate and drain compared to drain stress. The overall greater sensitivity of RON compared to Vth to buffer defects suggests that electron trapping is significantly greater in the access region compared to under the gate, likely due to the larger electric fields in the latter region. © 2013 IOP Publishing Ltd.

In this paper we propose a stacked multi-junction solar cell design that allows the intimate contact of the individual cells while maintaining low resistive losses. The cell design is presented using an InGaP and GaAs multi-junction cell as an illustrative example. However, the methodologies presented in this paper can be applied to other III-V cell types including InGaAs and InGaAsP cells. The main benefits of the design come from making small cells, on the order of 2×10-3 cm2. Simulations showed that series resistances should be kept to less than 5 ω for devices up to 400 μm in diameter to keep resistance power losses to less than 1%. Low resistance AuBe/Ni/Au ohmic contacts to n-type InGaP are also demonstrated with contact resistivity of 5×10-6 ωcm-2 when annealed at 420°C. © 2013 IEEE.

We present ultra-thin single crystal mini-modules built with specific power of 450 W/kg capable of voltages of >1000 V/cm2. These modules are also ultra-flexible with tight bending radii down to 1 mm. The module is composed of hundreds of back contact microcells with thicknesses of approximately 20 μm and diameters between 500-720 μm. The cells are interconnected to a flexible circuit through solder contacts. We studied the characteristics of several mini-modules through optical inspection, evaluation of quantum efficiency, measurement of current-voltage curves, and temperature dependence. Major efficiency losses are caused by missing cells or non-interconnected cells. Secondarily, damage incurred during separation of 500 μm cells from the substrate caused material detachment. The detachment induced higher recombination and low performance. Modules made with the larger cells (720 μm) performed better due to having no missing cells, no material detachment and optimized AR coatings. The conversion efficiency of the best mini module was 13.75% with a total Voc = 7.9 V. © 2013 IEEE.

We present ultra-thin single crystal mini-modules built with specific power of 450 W/kg capable of voltages of >1000 V/cm2. These modules are also ultra-flexible with tight bending radii down to 1 mm. The module is composed of hundreds of back contact microcells with thicknesses of approximately 20 μm and diameters between 500-720 μm. The cells are interconnected to a flexible circuit through solder contacts. We studied the characteristics of several mini-modules through optical inspection, evaluation of quantum efficiency, measurement of current-voltage curves, and temperature dependence. Major efficiency losses are caused by missing cells or non-interconnected cells. Secondarily, damage incurred during separation of 500 μm cells from the substrate caused material detachment. The detachment induced higher recombination and low performance. Modules made with the larger cells (720 μm) performed better due to having no missing cells, no material detachment and optimized AR coatings. The conversion efficiency of the best mini module was 13.75% with a total Voc = 7.9 V. © 2013 IEEE.

Microsystem technologies have the potential to significantly improve the performance, reduce the cost, and extend the capabilities of solar power systems. These benefits are possible due to a number of significant beneficial scaling effects within solar cells, modules, and systems that are manifested as the size of solar cells decrease to the sub-millimeter range. To exploit these benefits, we are using advanced fabrication techniques to create solar cells from a variety of compound semiconductors and silicon that have lateral dimensions of 250 - 1000 μm and are 1 - 20 μm thick. These fabrication techniques come out of relatively mature microsystem technologies such as integrated circuits (IC) and microelectromechanical systems (MEMS) which provide added supply chain and scale-up benefits compared to even incumbent PV technologies. © The Electrochemical Society.

Abstract not provided.

Surface acoustic wave (SAW) devices are used as sensing elements in the best performing portable chemical detectors. The SAW device, with a selectively absorbing chemical coating, serves as a mass sensor which preferentially responds to various chemical exposures. To obtain the highest performance, a number of criteria must be optimized, including SAW microwave insertion loss, impedance matching, electrode design configuration, RF shielding, chemically absorbent coating area, electronic measurement approach, and microfluidic packaging. A properly optimized system can be sensitive to chemical exposures the parts-per-trillion range. We report on a design optimization approach consisting of multiple comparison experiments made with competing designs. © 2012 IEEE.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Photonic crystals (PC) can fundamentally alter the emission behavior of light sources by suitably modifying the electromagnetic environment around them. Strong modulation of the photonic density of states especially by full threedimensional (3D) bandgap PCs, enables one to completely suppress emission in undesired wavelengths and directions while enhancing desired emission. This property of 3DPC to control spontaneous emission, opens up new regimes of light-matter interaction in particular, energy efficient and high brightness visible lighting. Therefore a 3DPC composed entirely of gallinum nitride (GaN), a key material used in visible light emitting diodes can dramatically impact solid state lighting. The following work demonstrates an all GaN logpile 3DPC with bandgap in the visible fabricated by a template directed epitaxial growth. © 2012 SPIE.

Abstract not provided.

We report on the application of MEMS and other microsystem technologies to photovoltaic (PV) cells, modules, and systems, taking advantage of several, significant benefits that are realized as the size of solar cells decrease to sub-mm length scales. To demonstrate these effects, we have developed both crystalline silicon and III-V PV cells. These cells are from 2 to 20 microns thick and from 250 microns to one millimeter across. We have demonstrated conversion efficiencies of up to 14.9% for a 14 micron thick crystalline silicon PV cell. This work contributes to two broad PV applications: 1) highly flexible PV modules with conversion efficiencies greater than 20%, and 2) commercial/utility scale PV systems using moderate concentration flat plate modules with simple single-axis or coarse dual-axis tracking. Cost models indicate that systems based on these technologies can achieve unsubsidized energy costs of less than $0.10/kWh. © The Electrochemical Society.

Abstract not provided.

Abstract not provided.