Karin Hinzer

Encoding with bursting, subthreshold oscillations, and noise in mammalian cold receptors

Mammalian cold thermoreceptors encode steady-state temperatures into characteristic temporal patterns of action potentials. We propose a mechanism for the encoding process. It is based on Plants ionic model of slow wave bursting, to which stochastic forcing is added. The model reproduces firing patterns from cat lingual cold receptors as the parameters most likely to underlie the thermosensitivity of these receptors varied over a 25C range. The sequence of firing patterns goes from regular bursting, to simple periodic, to stochastically phase-locked firing or skipping. The skipping at higher temperatures is shown to necessitate an interaction between noise and a subthreshold endogenous oscillation in the receptor. The basic period of all patterns is robust to noise. Further, noise extends the range of encodable stimuli. An increase in firing irregularity with temperature also results from the loss of stability accompanying the approach by the slow dynamics of a reverse Hopf bifurcation. The results are not dependent on the precise details of the Plant model, but are generic features of models where an autonomous slow wave arises through a Hopf bifurcation. The model also addresses the variability of the firing patterns across fibers. An alternate model of slow-wave bursting (Chay and Fan 1993) in which skipping can occur without noise is also analyzed here in the context of cold thermoreception. Our study quantifies the possible origins and relative contribution of deterministic and stochastic dynamics to the coding scheme. Implications of our findings for sensory coding are discussed.

InAs/GaAs(100) self-assembled quantum dots: arsenic pressure and capping effects

We explore growth effects leading to size and compositional limitations in the production of self-assembled quantum dots (QD) emitting at long wavelengths. Molecular beam epitaxy grown QDs are studied as a function of arsenic pressure at a specific InAs coverage, and as a function of InAs coverage for three arsenic pressures. As a function of increasing the arsenic pressure used in QD growth, the photoluminescence (PL) of capped QDs is first redshifted at low arsenic pressures, and then blueshifted at high arsenic pressures. Microscopy of uncapped QDs shows that as the arsenic pressure increases, the QD density increases while the average QD width and height decrease monotonically; these trends are consistent with the shift in PL for the high arsenic pressure samples, but are inconsistent with the shift in PL for the low-pressure samples. This points to a modification of the QDs during capping. We discuss prior reports pertaining to arsenic pressure and capping effects, and in this context describe our observations of the effects of adjusting the arsenic pressure on the formation of QDs and the mechanism by which QDs may be modified during capping.

Electrical Scanning Probe Microscopy: Investigating the Inner Workings of Electronic and Optoelectronic Devices

Semiconductor electronic and optoelectronic devices such as transistors, lasers, modulators, and detectors are critical to the contemporary computing and communications infrastructure. These devices have been optimized for efficiency in power consumption and speed of response. There are gaps in the detailed understanding of the internal operation of these devices. Experimental electrical and optical methods have allowed comprehensive elaboration of input–output characteristics, but do not give spatially resolved information about currents, carriers, and potentials on the nanometer scale relevant to quantum heterostructure device operation. In response, electrical scanning probe techniques have been developed and deployed to observe experimentally, with nanometric spatial resolution, two-dimensional profiles of the electrical resistance, capacitance, potential, and free carrier distribution, within actively driven devices. Experimental configurations for the most prevalent electrical probing techniques based on atomic force microscopy are illustrated with considerations for practical implementation. Interpretation of the measured quantities are presented and calibrated, demonstrating that internal quantities of device operation can be uncovered. Several application areas are examined: spreading resistance and capacitance characterization of free carriers in III-V device structures; acquisition of electric potential and field distributions of semiconductor lasers, nanocrystals, and thin films; scanning voltage analysis on diode lasers—the direct observation of the internal manifestations of current blocking breakdown in a buried heterostructure laser, the effect of current spreading inside actively biased ridge waveguide lasers, anomalously high series resistance encountered in ridge lasers—as well as in CMOS transistors; and free-carrier measurement of working lasers with scanning differential spreading techniques. Applications to emerging fields of nanotechnology and nanoelectronics are suggested.

Optimization of antireflection coating design for multijunction solar cells and concentrator systems

Photovoltaic solar cells are a route towards local, environmentally benign, sustainable and affordable energy solutions. Antireflection coatings are necessary to input a high percentage of available light for photovoltaic conversion, and therefore have been widely exploited for silicon solar cells. Multi-junction III-V semiconductor solar cells have achieved the highest efficiencies of any photovoltaic technology, yielding up to 40% in the laboratory and 37% in commercial devices under varying levels of concentrated light. These devices benefit from a wide absorption spectrum (300- 1800 nm), but this also introduces significant challenges for antireflection coating design. Each sub-cell junction is electrically connected in series, limiting the overall device photocurrent by the lowest current-producing junction. Therefore, antireflection coating optimization must maximize the current from the limiting sub-cells at the expense of the others. Solar concentration, necessary for economical terrestrial deployment of multi-junction solar cells, introduces an angular-dependent irradiance spectrum. Antireflection coatings are optimized for both direct normal incidence in air and angular incidence in an Opel Mk-I concentrator, resulting in as little as 1-2% loss in photocurrent as compared to an ideal zero-reflectance solar cell, showing a similar performance to antireflection coatings on silicon solar cells. A transparent conductive oxide layer has also been considered to replace the metallic-grid front electrode and for inclusion as part of a multi-layer antireflection coating. Optimization of the solar cell, antireflection coating, and concentrator system should be considered simultaneously to enable overall optimal device performance.

Estimating cell temperature in a concentrating photovoltaic system

A temperature calibrated equivalent circuit model of a high efficiency CPV solar cell is used to simulate a measured six-cell module J-V curve to estimate its average operating temperature. The simulation is based on a two diode equivalent circuit model for each subcell of a representative triple junction cell. Module J-V curves in a real CPV system were measured with a test station that performs continuous voltage sweeps allowing cells to reach a well defined thermal equilibrium during measurement. The average electrical power extracted during measurement is then used to determine the cell temperature when they are operating at their maximum power point. It is shown that the cells would operate at 42 ± 2 C° above ambient (32 ± 2°C abs.) given the ambient conditions during the measurement.

The Effects of Absorption and Recombination on Quantum Dot Multijunction Solar Cell Efficiency

The key characteristics of quantum dot (QD)-enhanced multijunction solar cells (MJSC) are explored theoretically by focusing on the generation and recombination rates throughout the QD layers in the middle subcell. The quantum dots are modeled using an effective medium to describe light absorption, confinement, and recombination properties. We report an 8% increase in the short-circuit current density accompanied by a 3% drop in an open-circuit voltage for a QD- enhanced MJSC at 1 sun illumination (1 kW/m2) compared with a control MJSC without QD. The drop in an open-circuit voltage is due in part to the increased recombination rates in the depletion region, decreased carrier lifetimes in the QDs, and the increased recombination rates resulting from carrier escape and capture. Overall, these contribute to an absolute increase in efficiency of over 1% for the studied QD-enhanced MJSC design for a QD density of 125 QD/μm2.

Ultrahigh efficiencies in vertical epitaxial heterostructure architectures

Optical to electrical power converting semiconductor devices were achieved with breakthrough performance by designing a Vertical Epitaxial Heterostructure Architecture. The devices are featuring modeled and measured conversion efficiencies greater than 65%. The ultrahigh conversion efficiencies were obtained by monolithically integrating several thin GaAs photovoltaic junctions tailored with submicron absorption thicknesses and grown in a single crystal by epitaxy. The heterostructures that were engineered with a number N of such ultrathin junctions yielded an optimal external quantum efficiencies approaching 100%/N. The heterostructures are capable of output voltages that are multiple times larger than the corresponding photovoltage of the input light. The individual nanoscale junctions are each generating up to ∼1.2 V of output voltage when illuminated in the infrared. We compare the optoelectronic properties of phototransducers prepared with designs having 5 to 12 junctions and that are exhibiting volt...

Multijunction Solar Cell Designs Using Silicon Bottom Subcell and Porous Silicon Compliant Membrane

A novel approach to the design of multijunction solar cells on silicon substrates for 1-sun applications is described. Models for device simulation, including porous silicon layers, are presented. A silicon bottom subcell is formed by diffusion of dopants into a silicon wafer. The top of the wafer is porosified to create a compliant layer, and a III-V buffer layer is then grown epitaxially, followed by middle and top subcells. Because of the resistivity of the porous material, these designs are best suited to high-efficiency 1-sun applications. Numerical simulations of a multijunction solar cell that incorporates a porous silicon-compliant membrane indicate an efficiency of 30.7% under AM1.5G, 1-sun for low-threading dislocation density, decreasing to 23.7% for a TDD of 107 cm-2.

Five-volt vertically-stacked, single-cell GaAs photonic power converter

The high-efficiency conversion of photonic power into electrical power is of broad-range applicability to many industries due to its electrical isolation from the surrounding environment and immunity to electromagnetic interference which affects the performance and reliability of sensitive electronics. A photonic power converter, or phototransducer, can absorb several watts of infrared laser power transmitted through a multimode fiber and convert this to electrical power for remote use. To convert this power into a useful voltage, we have designed, simulated, and fabricated a photovoltaic phototransducer that generates >5 V using a monolithic, lattice-matched, vertically-stacked, single-cell device that eliminates complex fabrication and assembly steps. Experimental measurements have demonstrated a conversion efficiency of up to 60.1% under illumination of ~11 W/cm2 at a wavelength of 835 nm, while simulations indicate that efficiencies reaching 70% should be realistically achievable using this novel design.

Low noise InAs/InP quantum dot C-band monolithic multiwavelength lasers for WDM-PONs

A 50-GHz-spacing C-band multiwavelength laser (MWL) module based on an InAs/InP quantum-dot (QD) Fabry-Perot cavity has been developed for the first time. We have evaluated its system performance to show our QD-MWL potentials for WDM-PONs.