Thin Films, Transparent Conductive Materials

Scientists have begun using self-replicating rapid prototyper (RepRap) 3-D printers to manufacture open source digital designs of scientific equipment. This approach is refined here to develop a novel instrument capable of performing automated large-area four-point probe measurements. The designs for conversion of a RepRap 3-D printer to a 2-D open source four-point probe (OS4PP) measurement device are detailed for the mechanical and electrical systems. Free and open source software and firmware are developed to operate the tool. The OS4PP was validated against a wide range of discrete resistors and indium tin oxide (ITO) samples of different thicknesses both pre-and post-annealing. The OS4PP was then compared to two commercial proprietary systems. Results of resistors from 10 to 1 MΩ show errors of less than 1% for the OS4PP. The 3-D mapping of sheet resistance of ITO samples successfully demonstrated the automated capability to measure non-uniformities in large-area samples. The results indicate that all measured values are within the same order of magnitude when compared to two proprietary measurement systems. In conclusion, the OS4PP system, which costs less than 70% of manual proprietary systems, is comparable electrically while offering automated 100 micron positional accuracy for measuring sheet resistance over larger areas.

A new method of preparing highly conductive ultra-thin indium tin oxide for plasmonic-enhanced thin film solar photovoltaic devices. Abstract Recent numerical modeling of plasmonic metallic nanostructures have shown great potential as a method of light management in thin-film nanodisc-patterned hydrogenated amorphous silicon (a-Si:H) solar photovoltaic (PV) cells. A significant design challenge for such plasmonic-enhanced PV devices is the requirement for ultra-thin transparent conducting oxides (TCOs) with high transmittance (low loss) and low enough resistivity to be used as device top contacts/electrodes. Most work on TCOs is on relatively thick layers and the few reported cases of thin TCO showed a marked decrease in conductivity. Recent work on ultra-thin TCOs of aluminum-doped zinc oxide, indium-doped tin oxide and zinc oxide revealed an unavoidable trade-off between transmittance and resistivity when fabricated with conventional growth methods. Ultra-thin films showed a tendency to be either amorphous and continuous or form as isolated islands. This results in poor electrical properties, which cannot be improved with annealing as the delicate thin films nucleate to form grain clusters. In order to overcome this challenge, this study investigates a novel method of producing ultra-thin (<40 nm) high quality TCOs. First, ~80nm ITO films are sputtered in various argon-oxygen atmospheres and annealed to increase conductivity. The most promising materials were then reduced in thickness with a controlled low-cost room-temperature cyclic wet chemical etching process to reach the desired thickness. The degradation in the electrical conductivity was tracked as a function of thickness. The sheet resistance of 36nm thin films was observed to be of the same order compared to the much thicker commercial ITO films currently used as transparent electrodes in PV and other opto-electronic devices. Experimental optical properties of the shaved films were then used in an optimized model of nano-disc plasmonic a-Si:H solar cells. Simulations indicate that optical enhancement greater than 21% are possible in the 300 – 730 nm wavelength range, when compared to the reference cell. Using the novel chemical shaving method described here, high-quality ultra-thin ITO films capable of improving the efficiency of thin film a-Si:H solar cells have been demonstrated. The methods employed in the optimization process are well established and economically viable, which provide the technical potential for commercialization of plasmonic based solar cells. A new method of preparing highly conductive ultra-thin indium tin oxide for plasmonic-enhanced thin film solar photovoltaic devices.

This study reports on the properties of nitrogen doped titanium dioxide (TiO 2) thin films considering the application as transparent conducting oxide (TCO). Sets of thin films were prepared by sputtering a titanium target under oxygen atmosphere on a quartz substrate at 400 or 500°C. Films were then doped at the same temperature by 150 eV nitrogen ions. The films were prepared in Anatase phase which was maintained after doping. Up to 30at% nitrogen concentration was obtained at the surface, as determined by in situ x-ray photoelectron spectroscopy (XPS). Such high nitrogen concentration at the surface lead to nitrogen diffusion into the bulk which reached about 25 nm. Hall measurements indicate that average carrier density reached over 10 19 cm-3 with mobility in the range of 0.1 to 1 cm 2 V-1 s-1. Resistivity about 3.10-1 cm could be obtained with 85% light transmission at 550 nm. These results indicate that low energy implantation is an effective technique for TiO 2 doping that allows an accurate control of the doping process independently from the TiO 2 preparation. Moreover, this doping route seems promising to attain high doping levels without significantly affecting the film structure. Such approach could be relevant for preparation of N:TiO 2 transparent conduction electrodes (TCE).

There is an enormous scope for graphene and its analogues in electromagnetic interference (EMI) applications, especially as microwave absorbers. Recent studies have shown that graphene analogues exhibit excellent electromagnetic shielding through uniquely designed 3D macroporous foams or as ultrathin papers and thus, are inevitable towards developing hybrid architectures. Herein, we provide a comprehensive evaluation of the key structural and mechanistic attributes of both graphene-based foams and papers towards EM shielding applications. Furthermore, we emphasize on the crucial aspect of architecture and alignment of graphene in both foam and paper towards enhancing EMI shielding for emerging applications.

The opportunity for substantial efficiency enhancements of thin film hydrogenated amorphous silicon (a-Si:H) solar photovoltaic (PV) cells using plasmonic absorbers requires ultra-thin transparent conducting oxide top electrodes with low resistivity and high transmittances in the visible range of the electromagnetic spectrum. Fabricating ultra-thin indium tin oxide (ITO) films (sub-50 nm) using conventional methods has presented a number of challenges; however, a novel method involving chemical shaving of thicker (greater than 80 nm) RF sputter deposited high-quality ITO films has been demonstrated. This study investigates the effect of oxygen concentration on the etch rates of RF sputter deposited ITO films to provide a detailed understanding of the interaction of all critical experimental parameters to help create even thinner layers to allow for more finely tune plasmonic resonances. ITO films were deposited on silicon substrates with a 98-nm, thermally grown oxide using RF magnetron sputtering with oxygen concentrations of 0, 0.4 and 1.0 sccm and annealed at 300 °C air ambient. Then the films were etched using a combination of water and hydrochloric and nitric acids for 1, 3, 5 and 8 min at room temperature. In-between each etching process cycle, the films were characterized by X-ray diffraction, atomic force microscopy, Raman Spectroscopy, 4-point probe (electrical conductivity), and variable angle spectroscopic ellipsometry. All the films were polycrystalline in nature and highly oriented along the (222) reflection. Ultra-thin ITO films with record low resistivity values (as low as 5.83 × 10−4 Ω·cm) were obtained and high optical transparency is exhibited in the 300–1000 nm wavelength region for all the ITO films. The etch rate, preferred crystal lattice growth plane, d-spacing and lattice distortion were also observed to be highly dependent on the nature of growth environment for RF sputter deposited ITO films. The structural, electrical, and optical properties of the ITO films are discussed with respect to the oxygen ambient nature and etching time in detail to provide guidance for plasmonic enhanced a-Si:H solar PV cell fabrication.

Surgical lenses in laparoscopes and arthroscopes “fog” during surgery. Fogging increases by up to 40% the duration of surgery, the rate of infections, and the instances of scarring due to tissue exposure to air from repeated withdrawal of the
scopes for wiping and cleaning of both fog, and bodily fluids, such as blood and synovial fluids. Thin film growth models derived from classical nucleation and growth theory, reveal that thin films condense on surfaces via 3 possible mechanisms:
2-dimensional (Frank-VanDerMerwe), 3-dimensional (Volmer-Weber), or mixed (Stranski-Krastanov). Modeling nucleation on surfaces shows that 2-D layer-by-layer condensation maintains transparency while 3-D condensation forms droplets that
refract at gas/fluid interfaces leading to opacity or “fogging”. This ProteinKnox™ model for lenses made from bio-compatible polymers and silica led us to a nano-scale molecular mesh applied as a bio-identical emulsion. ProteinKnox™ [1-5] meets a
100% success rate in eliminating fogging for up to 240 minutes over 300 experiments. Twenty surgical trials in the OR yield a success rate of 90 %, with loss of vision due to the presence of blood or blood proteins, not fogging. We studied the common blood protein, heparin, which prevents coagulation, with the ProteinKnox™ model. Heparin behaves like H2O on hydrophobic surfaces. It does not prevent fogging nor interferes with 2-D condensation. Next, we investigated fibrinogen as an agonist agent because it causes coagulation. Fibrinogen applied to various surfaces in emulsions prepared in accordance with the ProteinKnox™ model can prevent not only fogging but blood wetting. We show that proteins combined with the proper emulsion avoid whole blood accumulation on lenses, and validates the ProteinKnox™ model.

Using ion beam modification, films composed of synthesized
‘‘interphases’’ of ordered silica on OH-passivated (1 X 1) Si(100) underwent surface electro-chemical changes quantified by surface free energy via Sessile drop method and contact angle analysis using Young’s equation and Van Oss theory. IBMM caused the surface free energies initially ranging from 26.0 mJ/m2 to 57.3 mJ/m2 to converge to 43.1–45.4 mJ/m2 for various passivated and as-received wafer samples alike. Although TMAFM also identified topographic changes, these changes did not correlate to the change of surface free energies. Ion beam modification of the ordered silica film on Si(100) surface is analyzed using 3.045 MeV 16O(a, a)16O nuclear resonance scattering (NRS) in conjunction with channeling in (1 1 1) direction, which demonstrated the convergence of the partially ordered oxygen to amorphous at about 55 microCoulombs/mm2 He++ flux. Additionally, Si surface peak channeling in (1 0 0) and (1 1 1) directions also experienced an uptrend in areal density as incident ion flux increased, while the rotating random Si signal height remains stable, showing a disruption in the surface order during IBMM.

The distinction of opto-electrical properties in case of aluminium, gold and silver random and ordered nano-holed layers was demonstrated. It was found that transmittance drops due to the shortening of plasmon polaritons propagating length within the Anderson localization effect, while the sheet resistance increases in regard of the decrement of the volume of metal connections. Gold and silver nano-holed layers possess the transmittance of more than 80% and the sheet resistance of 20 Ohm/sq, regardless of the arrangement of nano-holes. Aluminium nano-holed layers demonstrate comparable parameters only with the ordered patterns.