Brandon Aguirre

Resistive Switching of SnO2 Thin Films on Glass Substrates

Resistive switching of SnO2 thin films deposited by RF magnetron reactive sputtering at room temperature was investigated. Ag/SnO2/Ti structures were fabricated on glass substrates for current-voltage characteristics evaluation. Repeatable unipolar switching was observed using a compliance current of 10 mA and limiting the reset voltage between 0.8 and 1.2 V. Different top contact area were fabricated indicating a filamentary forming mechanism. Furthermore, a retention memory analysis was performed indicating an acceptable device behavior through time. An Ohmic conduction process was found in LRS and HRS. However for HRS, Ohmic conduction was observed only at voltages lower than 0.3 V. At higher voltages, conduction is not explained well by Ohmic, Poole-Frankel, Schottky emission, or space-charge-limited conduction. This indicates that a material structural change occurs at voltages above 0.3 V which is the onset to switching from HRS to LRS.

Fabrication and characterization of a co-planar detector in diamond for low energy single ion implantation

We demonstrate low energy single ion detection using a co-planar detector fabricated on a diamond substrate and characterized by ion beam induced charge collection. Histograms are taken with low fluence ion pulses illustrating quantized ion detection down to a single ion with a signal-to-noise ratio of approximately 10. We anticipate that this detection technique can serve as a basis to optimize the yield of single color centers in diamond. The ability to count ions into a diamond substrate is expected to reduce the uncertainty in the yield of color center formation by removing Poisson statistics from the implantation process.

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

Power maximization in III–V sub-millimeter, radial front contacted cells for thin micro-concentrators

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.

Ion-damage-free planarization or shallow angle sectioning of solar cells for mapping grain orientation and nanoscale photovoltaic properties

Sub millimeter scale solar cells coupled with medium concentration lenses can reduce the balance of system costs of concentrating photovoltaics by creating thin and highly efficient concentrators with relaxed tracking requirements. This paper shows the design, fabrication, simulation, and testing of micro-sized photovoltaics that have unique perimeter front contacts outside the optical collection area. The design of the device considered the need for low resistance current carrying layers while minimizing optical losses. The paper also shows the successful fabrication of InGaAs cells as well as of GaAs cells transferred onto silicon substrates. The simulations and experimental measurements show that small cells of this type 1) suffered from slightly lower voltage levels caused by proportionally larger dark currents 2) peaked their efficiencies at higher concentration levels compared to larger ones 3) performed better overall.

Photovoltaics of ordered CdTe/CdS nanoarrays

Ion beam milling is the most common modern method for preparing specific features for microscopic analysis, even though concomitant ion implantation and amorphization remain persistent challenges, particularly as they often modify materials properties of interest. Atomic force microscopy (AFM), on the other hand, can mechanically mill specific nanoscale regions in plan-view without chemical or high energy ion damage, due to its resolution, directionality, and fine load control. As an example, AFM-nanomilling (AFM-NM) is implemented for top-down planarization of polycrystalline CdTe thin film solar cells, with a resulting decrease in the root mean square (RMS) roughness by an order of magnitude, even better than for a low incidence FIB polished surface. Subsequent AFM-based property maps reveal a substantially stronger contrast, in this case of the short-circuit current or open circuit voltage during light exposure. Electron back scattering diffraction (EBSD) imaging also becomes possible upon AFM-NM, enabling direct correlations between the local materials properties and the polycrystalline microstructure. Smooth shallow-angle cross-sections are demonstrated as well, based on targeted oblique milling. As expected, this reveals a gradual decrease in the average short-circuit current and maximum power as the underlying CdS and electrode layers are approached, but a relatively consistent open-circuit voltage through the diminishing thickness of the CdTe absorber. AFM-based nanomilling is therefore a powerful tool for material characterization, uniquely providing ion-damage free, selective area, planar smoothing or low-angle sectioning of specimens while preserving their functionality. This enables novel, co-located advanced AFM measurements, EBSD analysis, and investigations by related techniques that are otherwise hindered by surface morphology or surface damage.

Comparison of Gain Degradation and Deep Level Transient Spectroscopy in pnp Si Bipolar Junction Transistors Irradiated With Different Ion Species

Cadmium telluride (CdTe) is a good absorber layer for solar cells with a predicted efficiency of 29% however the highest reported efficiency is only ≪17% due to various loss mechanisms. One particular loss is attributed to non-uniformity in grain shape and size by Karpov. Simulation of Karpovs model yields counter-intuitive insight that highlights how empirical data can lead researchers to wrong conclusions. A method to grow ordered arrays of ZnTe/CdTe heterostructures is presented. The method has potential for probing the electronic characteristics of individual photodiodes at the granular level.

Method for electrical-structural correlation in isolated CdTe/CdS islands

We studied the effect of light ion and heavy ion irradiations on pnp Si BJTs. A mismatch in DLTS deep peak amplitude for devices with same final gain but irradiated with different ion species was observed. Also, different ions cause different gain degradation when the DLTS spectra are matched. Pre-dosed ion-irradiated samples show that ion induced ionization does not account for the differences in DLTS peak height but isochronal annealing studies suggest that light ions produce more VP defects than heavy ions to compensate for the lack of clusters that heavy ions produce. The creation of defect clusters by heavy ions is evident by the higher content of E4 and

Nanoscale photovoltaic performance in micro/nanopatterned CdTe-CdS thin film solar cells

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Record breaking solar cells : ZnxCd(1-x)Te graded bandgap nanoarrays.

defects compared to light ions.