Raman Vaidyanathan

Electrochemical formation of a III–V compound semiconductor superlattice: InAs/InSb

Abstract We report on the use of electrochemical atomic-layer epitaxy (EC-ALE) to grow thin-films of the III–V compounds InAs, InSb, and an InAs x Sb 1− x superlattice. EC-ALE is a method for forming compound semiconductors with improved control, compared to other electrodeposition methodologies. It involves the use of surface limited reactions to form deposits an atomic layer at a time, in a cycle. The EC-ALE cycle uses underpotential deposition (upd) to form atomic layers of each of the component elements. One cycle ideally produces one monolayer (ML) of the desired compound. Studies to optimize the InAs cycle are reported, specifically the dependence on the In and As deposition potentials. These studies show that the potentials must be adjusted for each of the first 25 or more cycles, as a contact potential between the Au substrate and the growing semiconductor develops. After deposition of this initial ‘buffer layer’, steady state conditions are reached, and the same potentials can be used without change, for the remaining cycles. The formation of InSb has also been investigated, and the EC-ALE growth of InSb deposits is reported for the first time. Due to a 6% lattice mismatch, and a less than fully optimized cycle, the InSb deposits on Au appear composed of 70 nm particles. By combining the InAs and InSb programs, a superlattice was formed with 41 periods, where each period involved ten cycles of InAs followed by ten cycles of InSb. X-ray diffraction (XRD) indicated a period of 5.5 nm, whereas a 7.4 nm period was expected, based on 1 ML/cycle and the (111) interplanar spacing, derived from the lattice constants for InAs and InSb. Given the stoichiometry of the resulting deposit, and the shorter periodicity observed, it appears that 1 ML/cycle of InAs was formed, while only a 1/2 ML/cycle of InSb was obtained. IR absorption measurements indicate that the deposit was red shifted relative to the lower bandgap compound, InSb (0.17 eV), which is consistent with a type II superlattice. If an alloy had been formed, the bandgap should have been a linear function of the bandgaps and relative mole fractions of InAs and InSb, or about 0.31 eV, twice the observed bandgap.

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.