Person:

Kim, Sang Bok

SnS is a promising earth-abundant material for photovoltaic applications. Heterojuction solar cells were made by vapor deposition of p-type tin(II) sulfide, SnS, and n-type zinc oxysulfide, Zn(O,S), using a device structure of soda-lime glass/Mo/SnS/Zn(O,S)/ZnO/ITO. A record efficiency was achieved for SnS-based thin-film solar cells by varying the oxygen-to-sulfur ratio in Zn(O,S). Increasing the sulfur content in Zn(O,S) raises the conduction band offset between Zn(O,S) and SnS to an optimum slightly positive value. A record SnS/Zn(O,S) solar cell with a S/Zn ratio of 0.37 exhibits short circuit current density ((J_{sc})), open circuit voltage ((V_{oc})), and fill factor (FF) of (19.4 mA/cm^{2}), 0.244 V, and 42.97%, respectively, as well as an NREL-certified total-area power-conversion efficiency of 2.04% and an uncertified active-area efficiency of 2.46%.

Bottom-gate thin film transistors with amorphous zinc tin oxide channels were grown by atomic layer deposition (ALD). The films maintained their amorphous character up to temperatures over 500 (^{\circ})C. The highest field effect mobility was ~13 (cm^2/V^.s) with on-to-off ratios of drain current ~10(^9)-10(^{10}). The lowest subthreshold swing of 0.27 V/decade was observed with thermal oxide as a gate insulator. The channel layers grown at 170 (^{\circ})C showed better transistor properties than those grown at 120 (^{\circ})C. Channels with higher zinc to tin ratio (~3-4) also performed better than ones with lower ratios (~1-3).

Atomic layer deposition (ALD) of tin oxide ((SnO_2)) thin films was achieved using a cyclic amide of Sn(II) (1,3-bis(1,1-dimethylethyl)-4,5-dimethyl-(4R,5R)-1,3,2-diazastannolidin-2-ylidene) as a tin precursor and nitric oxide (NO) as an oxidant gas. Film properties as a function of growth temperature from (130-250^{\circ}C) were studied. Highly conducting (SnO_2) films were obtained at (200-250^{\circ}C) with the growth per cycle of (~1.4 \mathring{A})/cycle, while insulating films were grown at temperatures lower than (200^{\circ}C). Conformal growth of (SnO_2) in holes of aspect-ratios up to ~50 : 1 was successfully demonstrated.

Atomic Layer Deposition (ALD) was used to grow single-crystalline epitaxial layers of high-k dielectric oxides on semiconductors with remarkably few defects or traps at the interfaces. La2O3 on GaAs(111) produced record-breaking transistors with both n-and p-channels, and CMOS circuits entirely in GaAs, including inverters, logic circuits and 5-stage ring oscillators. More conventionally oriented GaAs(100) substrates with etched (111) slopes also produced working transistors. ALD also grew single-crystalline epitaxial La2O3 films on Ge(111), and (Ca, Mg)O films on GaN(0001) substrates with high-quality epitaxial interfaces. These processes can be run in commercial ALD reactors using precursors produced by the Dow Chemical Company.

We demonstrate for the first time that a singlecrystalline epitaxial MgxCa1−xO film can be deposited on gallium nitride (GaN) by atomic layer deposition (ALD). By adjusting the ratio between the amounts of Mg and Ca in the film, a lattice matched MgxCa1−xO/GaN(0001) interface can be achieved with low interfacial defect density. High-resolution X-ray diffraction (XRD) shows that the lattice parameter of this ternary oxide nearly obeys Vegard’s law. An atomically sharp interface from cross-sectional transmission electron microscopy (TEM) confirmed the high quality of the epitaxy.High-temperature capacitance−voltage characterization showed that the film with composition Mg0.25Ca0.75O has the lowest interfacial defect density. With this optimal oxide composition, a Mg0.25Ca0.75O/AlGaN/GaN metal−oxide −semiconductor high-electron-mobility (MOS-HEMT) device was fabricated. An ultrahigh on/off ratio of 1012 and a near ideal SS of 62 mV/dec were achieved with this device.

Storage of photovoltaic and wind electricity in batteries could solve the mismatch problem between the intermittent supply of these renewable resources and variable demand. Flow batteries permit more economical long-duration discharge than solid-electrode batteries by using liquid electrolytes stored outside of the battery. We report an alkaline flow battery based on redox-active organic molecules that are composed entirely of Earth-abundant elements and are nontoxic, nonflammable, and safe for use in residential and commercial environments. The battery operates efficiently with high power density near room temperature. These results demonstrate the stability and performance of redox-active organic molecules in alkaline flow batteries, potentially enabling cost-effective stationary storage of renewable energy.