J. Y. Choi

Effects of oxygen pressure and Mn-doping on the electrical and dielectric properties of Bi5Nb3O15 thin film grown by pulsed laser deposition

Bi5Nb3O15 (B5N3) thin films grown at temperatures above 500 °C developed a crystalline B5N3 phase that decomposed into a BiNbO4 phase in the film grown at 700 °C, probably due to Bi2O3 evaporation. The leakage current density of the B5N3 film grown at 200 °C under an oxygen pressure (OP) of 100 mTorr was high at approximately 6.3 × 10−7 A cm−2 at 0.2 MV cm−1, with a small breakdown field of 0.24 MV cm−1 due to the presence of intrinsic oxygen vacancies. The electrical properties of the B5N3 films improved with increasing OP during the growth but the dielectric constant (er) decreased. The Mn-doping also improved the electrical properties of the B5N3 films grown under low OP by producing doubly ionized oxygen vacancies, which decreased the number of the intrinsic oxygen vacancies. Furthermore, the Mn-doping increased the er value of the B5N3 film, confirming the effectiveness of such doping in improving both the electrical and the dielectric properties of the B5N3 film. In particular, the 5.0 mol% Mn-doped B5N3 film grown at 200 °C under an OP of 100 mTorr exhibited a low leakage current density of 7.9 × 10−8 A cm−2 at 0.2 MV cm−1 and a large breakdown field of 0.4 MV cm−1 with a high er of 64.

Random Si nanopillars for broadband antireflection in crystalline silicon solar cells

We demonstrate the fabrication of shallow Si nanopillar structures at a submicron scale which provides broadband antireflection for crystalline Si (c-Si) solar cells in the wavelength range of 350 nm–1100 nm. The Si random nanopillars were made by reactive ion etch (RIE) processing with thermally dewetted Sn metals as an etch mask. The diameters and coverages of the Si nanopillars were adjusted in a wide range of the nanoscale to microscale by varying the nominal thickness of the Sn metals and subsequent annealing temperatures. The height of the nanopillars was controlled by the RIE process time. The optimal size of the nanopillars, which are 340 nm in diameter and 150 nm in height, leads to the lowest average reflectance of 3.6%. We showed that the power conversion efficiency of the c-Si solar cells could be enhanced with the incorporation of optimally designed Si random nanopillars from 13.3% to 14.0%. The fabrication scheme of the Si nanostructures we propose in this study would be a cost-effective and promising light trapping technique for efficient c-Si solar cells.

Nuclear magnetic resonance study of distinct proton dynamics in SnCl4·5H2O

H nuclear magnetic resonance was employed to study the proton dynamics in SnCl 4·5H2O, which comprises two coordi- nated and three uncoordinated crystalline water molecules in each unit cell. Two types of proton sites were distinguished dynamically as well as structurally. In particular, only one of them was shown to undergo an apparent order-disorder phase transition. q 2001 Published by Elsevier Science Ltd.