Stephen Michael Cox

Formation of In2Se3 thin films and nanostructures using electrochemical atomic layer epitaxy

Abstract The formation of the III–VI compound In 2 Se 3 , at room temperature by electrochemical atomic layer epitaxy (EC-ALE) is reported here. EC-ALE involves the use of surface limited reactions to form atomic layers of the elements making up a compound (In and Se) in a cycle. In electrodeposition, surface limited reactions are referred to as under potential deposition, and generally result in the formation of an atomic layer of the depositing element. These layers are deposited alternately in a cycle, resulting in the formation of a one monolayer of the compound, In 2 Se 3 . Cyclic voltammograms were used to determine approximate deposition potentials for each element. An automated deposition program was used to form thin films of In 2 Se 3 , with from up to 350 cycles. Electron probe microanalysis was performed to determine the stoichiometry of the thin films. The atomic ratio of Se/In in the thin films was found to be 3/2. X-ray diffraction of 350 cycle films indicated the deposits contained beta phase In 2 Se 3 . Band gaps were determined by FT-IR reflection absorption measurements, and found to be 1.73 eV. The surface morphology was determined by atomic force microscopy (AFM), suggesting that the deposits consist of 100 nm crystallites. Deposits on rougher substrates resulted in still smaller crystallites, and a blue shift in the band gap, possibly due to quantum confinement. Photoelectrochemical measurements suggested a band gap of 1.82 eV. In 2 Se 3 nanostructures were electrodeposited inside the pores (200 nm) of commercial polycarbonate membrane using EC-ALE. AFM images indicated that nanostructures were higher then expected, for 200 cycles of deposition. Studies of the Au vapor-deposited on the membrane showed that it had ingressed into the holes, accounting for most of the extra height. Microprobe data suggested that the total coverage was 1/6th that observed for a thin film, consistent with the observed coverage of nanostructures.