John D. Perkins

Large dielectric constant (ε/ε0>6000) Ba0.4Sr0.6TiO3 thin films for high-performance microwave phase shifters

We deposited epitaxial Ba0.4Sr0.6TiO3 (BST) films via laser ablation on MgO and LaAlO3 (LAO) substrates for tunable microwave devices. Postdeposition anneals (∼1100 °C in O2) improved the morphology and overall dielectric properties of films on both substrates, but shifted the temperature of maximum dielectric constant (Tmax) up for BST/LAO and down for BST/MgO. These substrate-dependent Tmax shifts had opposite effects on the room-temperature dielectric properties. Overall, BST films on MgO had the larger maximum dielectric constant (e/e0⩾6000) and tunability (Δe/e⩾65%), but these maxima occurred at 227 K. 30 GHz phase shifters made from similar films had figures of merit (ratio of maximum phase shift to insertion loss) of ∼45°/dB and phase shifts of ∼400° under 500 V (∼13 V/μm) bias, illustrating their utility for many frequency-agile microwave devices.

Titanium-doped indium oxide: A high-mobility transparent conductor

We report on the effects of titanium doping (0–7at.%) on the optical and electrical properties of In2O3 using combinatorial deposition and analysis techniques. Maximum mobilities are observed at Ti concentrations of 1.5–2.5at.% and are >80cm2∕Vs in sputtered films. The carrier concentration increased with titanium content to a high of 8.0×1020cm−3. Data show that one carrier is generated per added Ti between 1 and 3at.%. Conductivities up to 6260Ω−1cm−1 were observed. These remained very high >5000Ω−1cm−1 across a wide compositional range. The optical transparency is high (>85%) in a wide spectral range from 400nm to at least 1750nm. The work function of titanium-doped indium oxide varies substantially over the studied compositional range.

Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Combinatorial studies of Zn-Al-O and Zn-Sn-O transparent conducting oxide thin films

Abstract In this work, we discuss the development of combinatorial deposition and analysis tools for the investigation of and the optimization of transparent conducting oxides. Library deposition by co-sputtering followed by optical analysis is shown to be a facile way to achieve these goals. Initial work focused on Zn-Al-O libraries with low Al contents as a test case. Subsequent work has focused on the ZnO-SnO2 tie line. Local maxima in the composition dependence of the conductivity were found for Zn/Sn ≈2:1 (Zn2SnO4) and Zn/Sn ≈1:1 (ZnSnO3). For these two representative stoichiometries, constant composition films have also been grown by pulsed laser deposition.

Li ion diffusion measurements in V2O5 and Li(Co1−xAlx)O2 thin-film battery cathodes

Abstract Thin films of V 2 O 5 and LiCoO 2 were deposited by pulsed laser deposition (PLD) and the chemical diffusion coefficients, D were measured by potentiostatic intermittent titration technique (PITT). The PLD-grown V 2 O 5 and LiCoO 2 films are electrochemically similar to bulk powders and thin films produced by other techniques. In crystalline V 2 O 5 , the maximum and minimum D were found to be 1.7×10 −12 cm 2 s −1 and 5.8×10 −15 cm 2 s −1 respectively, with a general trend for D to rise in single-phase regions. In amorphous V 2 O 5 films, D was initially 5×10 −13 cm 2 s −1 and decreased steadily to 1.2×10 −13 cm 2 s −1 at Li 0.4 V 2 O 5 . The decrease in D then became more gradual with a final value of 5.52×10 −14 cm 2 s −1 at Li 1.5 V 2 O 5 . The chemical diffusion coefficient of Li in LiCoO 2 films ranged from 1×10 −12 –4×10 −11 cm 2 s −1 with a pronounced minimum at Li 0.7 CoO 2 . Thin films of LiCo 0.5 Al 0.5 O 2 also deposited by PLD exhibited limited cycling capabilities and an upper bound of D =9×10 −13 cm 2 s −1 .

High-mobility transparent conducting Mo-doped In2O3 thin films by pulsed laser deposition

Highly conductive and transparent Mo-doped indium oxide (IMO) thin films were grown on glass and (100) yttria-stabilized zirconia (YSZ) single-crystal substrates by pulsed laser deposition. The electrical, optical, and structural properties were measured for films grown from 0, 1, 2, and 4 wt % Mo-doped targets. Films grown from the 2 wt % Mo-doped target had the best overall properties. In particular, for biaxially textured 2 wt % Mo IMO films grown on (100) YSZ, the conductivity was ∼3000 S cm−1 with a mobility greater than 95 cm2 V−1 s−1. In the visible, the optical transmittance normalized to the substrate was greater than 90%.

rf magnetron sputter deposition of transparent conducting Nb-doped TiO2 films on SrTiO3

rf magnetron sputtering, an established and scalable large area deposition process, is used to deposit Nb:TiO2 and Ta:TiO2 films onto (100) SrTiO3 substrates at temperatures TS ranging from room temperature to 400°C. Optical, electrical, and structural properties similar to those reported for pulsed laser deposition grown films were obtained. In particular, the most conducting Ti0.85Nb0.15O2 films, grown at TS≈375°C, are epitaxially oriented anatase films with conductivity of 3000Scm−1, carrier concentration of 2.4×1021cm−3, Hall mobility of 7.6cm2V−1s−1, and optical transparency T>80% from 400to900nm. The conductivity is strongly correlated with the intensity of the anatase (004) x-ray diffraction peak.

Controlling single-wall nanotube diameters with variation in laser pulse power

Abstract We demonstrate that laser peak pulse power can be employed to tune carbon single wall nanotube (SWNT) diameters. The production of SWNTs was investigated at room temperature and at 1200°C. The diameters were shifted to smaller sizes in both cases as the pulse power was increased. SWNT size distributions and yields were studied with Raman spectroscopy and transmission electron microscopy. The evolution of the material quality with laser energy parameters offers insight in to SWNT formation mechanisms. These studies should aid in the development of methods for the rational control of SWNT growth.

Structural properties of hot wire a-Si:H films deposited at rates in excess of 100 Å/s

The structure of a-Si:H, deposited at rates in excess of 100 A/s by the hot wire chemical vapor deposition technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). The films examined in this study were chosen to have roughly the same bonded H content CH as probed by infrared spectroscopy. As the film deposition rate Rd is increased from 5 to >140 A/s, we find that the short range order (from Raman), the medium range order (from XRD), and the peak position of the H evolution peak are invariant with respect to deposition rate, and exhibit structure consistent with a state-of-the-art, compact a-Si:H material deposited at low deposition rates. The only exception to this behavior is the SAXS signal, which increases by a factor of ∼100 over that for our best, low H content films deposited at ∼5 A/s. We discuss the invariance of the short and medium range order in terms of growth models available in the literature, and relate changes in th...

Pulsed Laser Deposition and Characterization of Crystalline Lithium Cobalt Dioxide ( LiCoO2 ) Thin Films

The pulsed laser deposition of LiCoO 2 and LiCo 0.5 Al 0.5 O 2 thin films was investigated as a function of deposition conditions. The initial growth process optimization focused on films grown on (200)-textured SnO 2 -coated glass substrates. Film growth was also investigated on ZnO and indium-tin oxide (ITO) coated substrates. For both LiCoO 2 and LiCo 0.5 Al 0.5 O 2 , dense uniaxially textured (003)-oriented films of the layered LiCoO 2 phase were grown on the SnO 2 coated substrates. The grain size increased substantially with increased substrate temperature in the range from 400 to 700°C. For constant current cycling between 3.5 and 4.4 V vs. Li, the best crystalline LiCoO 2 films, grown at T s = 700°C, p O2 = 2000 mTorr, had an initial discharge capacity of ∼118 8 mAh/g (0.43 Li/Co) which decreased ∼0.5% per cycle. For LiCo 0.5 Al 0.5 O 2 , although, as predicted, the films did have a higher cathode potential, the charge capacity was signilicantly lower than that for LiCoO 2 films, Finally, functional LiCoO 2 thin film cathodes were grown on flexible ITO-coated Upilex polymer substrates at T s = 300°C.