Carlos Anthony Sanchez

An AlN/Al0.85Ga0.15N high electron mobility transistor

An AlN barrier high electron mobility transistor (HEMT) based on the AlN/Al0.85Ga0.15N heterostructure was grown, fabricated, and electrically characterized, thereby extending the range of Al composition and bandgap for AlGaN channel HEMTs. An etch and regrowth procedure was implemented for source and drain contact formation. A breakdown voltage of 810 V was achieved without a gate insulator or field plate. Excellent gate leakage characteristics enabled a high Ion/Ioff current ratio greater than 107 and an excellent subthreshold slope of 75 mV/decade. A large Schottky barrier height of 1.74 eV contributed to these results. The room temperature voltage-dependent 3-terminal off-state drain current was adequately modeled with Frenkel-Poole emission.

Ultrathin Flexible Crystalline Silicon: Microsystems-Enabled Photovoltaics

We present an approach to create ultrathin (<;20 μm) and highly flexible crystalline silicon sheets on inexpensive substrates. We have demonstrated silicon sheets capable of bending at a radius of curvature as small as 2 mm without damaging the silicon structure. Using microsystem tools, we created a suspended submillimeter honeycomb-segmented silicon structure anchored to the wafer only by small tethers. This structure is created in a standard thickness wafer enabling compatibility with common processing tools. The procedure enables all the high-temperature steps necessary to create a solar cell to be completed while the cells are on the wafer. In the transfer process, the cells attach to an adhesive flexible substrate which, when pulled away from the wafer, breaks the tethers and releases the honeycomb structure. We have previously demonstrated that submillimeter and ultrathin silicon segments can be converted into highly efficient solar cells, achieving efficiencies up to 14.9% at a thickness of 14 μm. With this technology, achieving high efficiency (>;15%) and highly flexible photovoltaic (PV) modules should be possible.

Microfabrication of microsystem-enabled photovoltaic (MEPV) cells

Microsystem-Enabled Photovoltaic (MEPV) cells allow solar PV systems to take advantage of scaling benefits that occur as solar cells are reduced in size. We have developed MEPV cells that are 5 to 20 microns thick and down to 250 microns across. We have developed and demonstrated crystalline silicon (c-Si) cells with solar conversion efficiencies of 14.9%, and gallium arsenide (GaAs) cells with a conversion efficiency of 11.36%. In pursuing this work, we have identified over twenty scaling benefits that reduce PV system cost, improve performance, or allow new functionality. To create these cells, we have combined microfabrication techniques from various microsystem technologies. We have focused our development efforts on creating a process flow that uses standard equipment and standard wafer thicknesses, allows all high-temperature processing to be performed prior to release, and allows the remaining post-release wafer to be reprocessed and reused. The c-Si cell junctions are created using a backside point-contact PV cell process. The GaAs cells have an epitaxially grown junction. Despite the horizontal junction, these cells also are backside contacted. We provide recent developments and details for all steps of the process including junction creation, surface passivation, metallization, and release.

Sensitivity of on-resistance and threshold voltage to buffer-related deep level defects in AlGaN/GaN high electron mobility transistors

The influence of deep levels defects located in highly resistive GaN:C buffers on the on-resistance (RON) and threshold voltage (Vth) 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 RON (~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.

Cost analysis of flat-plate concentrators employing microscale photovoltaic cells for high energy per unit area applications

Microsystems Enabled Photovoltaics (MEPV) is a relatively new field that uses microsystems tools and manufacturing techniques familiar to the semiconductor industry to produce microscale photovoltaic cells. The miniaturization of these PV cells creates new possibilities in system designs that can be used to reduce costs, enhance functionality, improve reliability, or some combination of all three. In this article, we introduce analytical tools and techniques to estimate the costs associated with a hybrid concentrating photovoltaic system that uses multi-junction microscale photovoltaic cells and miniaturized concentrating optics for harnessing direct sunlight, and an active c-Si substrate for collecting diffuse sunlight. The overall model comprises components representing costs and profit margin associated with the PV cells, concentrating optics, balance of systems, installation, and operation. This article concludes with an analysis of the component costs with particular emphasis on the microscale PV cell costs and the associated tradeoffs between cost and performance for the hybrid CPV design.

Effect of low dose γ-irradiation on DC performance of circular AlGaN/GaN high electron mobility transistors

The changes in direct current performance of circular-shaped AlGaN/GaN high electron mobility transistors (HEMTs) after 60Co γ-irradiation doses of 50, 300, 450, or 700 Gy were measured. The main effects on the HEMTs after irradiation were increases of both drain current and electron mobility. Compton electrons induced from the absorption of the γ-rays appear to generate donor type defects. Drain current dispersions of ∼5% were observed during gate lag measurements due to the formation of a virtual gate between the gate and drain resulting from the defects generated during γ-irradiation.

Back-contacted and small form factor GaAs solar cell

We present a newly developed microsystem enabled, back-contacted, shade-free GaAs solar cell. Using microsystem tools, we created sturdy 3 µm thick devices with lateral dimensions of 250 µm, 500 µm, 1 mm, and 2 mm. The fabrication procedure and the results of characterization tests are discussed below. The highest efficiency cell had a lateral size of 500 µm and a conversion efficiency of 10%, open circuit voltage of 0.9 V and a current density of 14.9 mA/cm2 under one-sun illumination.

Ultra-thin single crystal silicon modules capable of 450 W/kg and bending radii <1mm: Fabrication and characterization

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.

Nanopatterning and bandgap grading to reduce defects in CdTe solar cells

We present simulation and experimental results proving the feasibility of a novel concept to increase efficiency of CdTe based solar cells. In order to achieve

Effect of 5 MeV proton radiation on DC performance and reliability of circular-shaped AlGaN/GaN high electron mobility transistors

0.50/W price in CdTe based modules, higher efficiencies need to be attained. The high defect density due to lattice-mismatch between CdS and CdTe reduces lifetime, voltage, and efficiency of the cells. We propose the use of a graded composition structure and a patterned substrate to reduce defects, increase lifetime, and efficiency of the cells. Innovative simulations using high-fidelity molecular dynamics predict that defect-free films are possible if the CdTe film is graded with Zn and is constructed as nano-islands with sizes below 90 nm. Both graded structure and nano-islands reduce the lattice-mismatch stresses. Also, the graded composition creates a back surface field and an enhanced ohmic contact. We have attempted to grow ZnTe and CdTe films on CdS substrates using a template of micro and nano-islands. Selective growths on patterned substrates have shown fewer grain boundaries when the island size decreases below 300 nm. Also, larger grain sizes were obtained using a CdTe/ZnTe stack when compared to a single layer CdTe. The simulation and experimental results demonstrate for the first time the ability to use nanopatterned substrates to enhance uniformity in thin film solar cells.