Venkatram Venkatasamy

Copper Nanofilm Formation by Electrochemical ALD

This paper describes the formation of Cu nanofilms using atomic layer deposition (ALD) via surface-limited redox replacement, also referred to as monolayer-restricted galvanic displacement. An automated flow-cell electrodeposition system was employed to make Cu nanofilms using 100, 200, and 500 ALD cycles. The cycle was composed of a sequence of steps: Pb underpotential deposition (UPD), rinsing with blank, introduction of Cu 2+ at open circuit, and exchange of the Pb atoms for Cu, rinsing with a blank. The open-circuit potential was used to follow the replacement, exchange, of Pb for Cu, which shifted from that used to deposit Pb UPD (-0.44 V) up to the equilibrium potential for Cu 21 /Cu or -0.013 V upon a complete exchange. The resulting Cu films appeared homogeneous from inspection, optical microscopy, and scanning electron microscopy. Electron probe microanalysis showed no Pb in deposits formed using -0.44 V for Pb UPD. However, for deposits formed with Pb deposition at potentials more negative than -0.44 V, Pb was evident in the deposit. A prominent Cu(111) peak was displayed in the X-ray diffraction pattern for the Cu nanofilms. Morphology studies of the Cu films were performed using ex situ scanning tunneling microscopy and attested to the layer-by-layer growth of the Cu film. The 250 nm flat terraces suggested that a surface may have become smoother during growth rather than roughened as normally experienced during the electrodeposition or growth of thin films in general. A decrease in coulometry for Pb UPD during the first 30 cycles could also be interpreted as a decrease in surface roughness, or surface repair during ALD.

Formation of HgSe Thin Films Using Electrochemical Atomic Layer Epitaxy

The growth of HgSe using electrochemical atomic-layer epitaxy (EC-ALE) is reported. EC-ALE is the electrochemical analog of ALE, where electrochemical surface-limited reactions referred to as underpotential deposits, generally result in the formation of an atomic layer of an element, under controlled potential. HgSe thin films were formed on gold substrates using two reactant solutions: a solution of Hg 2 + complexed with ethylenediaminetetraacetic acid and a HSeO - 3 ion solution. X-ray diffraction analysis showed a zinc blende structure for the deposits, with a strong (111) preferred texture, and an average grain size of 425Δ. Electron probe microscope analysis showed near-stoichiometric deposits. Fourier transform infrared spectroscopy reflection absorption measurements suggest two bandgaps: 0.42 and 0.88 eV.

ALD Approach toward Electrodeposition of Sb2Te3 for Phase-Change Memory Applications

This paper describes various studies undertaken to devise a deposition cycle for the formation of Sb 2 Te 3 , a phase-change memory material, by electrochemical atomic layer deposition (EC-ALD). The importance of deposition potentials to the formation of deposits of Sb and Te, were investigated. The resulting potentials were then used in an EC-ALD cycle to form deposits one atomic layer at a time, using a sequence of surface limited reactions. The optimal deposition potentials for the Sb 2 Te 3 EC-ALD cycle, on a Au substrate, consisted of -0.22 V for Sb deposition, and -0.35 V for Te. Bulk Te stripping at was performed at -0.70 V. The conditions using a TiN substrates included Sb deposition at -0.20 V, Te deposition, and bulk stripping at -0.35 and -0.70 V, respectively. The deposits obtained were stoichiometric, with a Te/Sb atomic ratio of 1.5 from electron probe microanalysis. Glancing angle X-ray diffraction studies showed a (015) preferred orientation for the deposit. Sb 2 Te 3 was shown to grow successfully on patterned TiN substrates with 200 nm vias.

Studies of HG(1-x)CDxTe (MCT) formation by electrochemical atomic layer deposition (ALD), and investigations into bandgap engineering

Films of Hg (1-x) Cd x Te (MCT) were grown using electrochemical atomic layer deposition, the electrochemical analog of atomic layer epitaxy, and atomic layer deposition (ALD). The present study describes the growth of MCT via electrochemical ALD, using an automated electrochemical flow cell deposition system. The system allows potential control and solution exchange as desired. Deposits were characterized using X-ray diffraction, electron probe microanalysis, and reflection absorption Fourier transform infrared spectroscopy. The as-deposited films showed strong (111) preferred orientation. No postdeposition annealing was required. Changes in deposit composition showed the expected trend in bandgaps: the more Hg the lower the bandgap, but with some significant deviations. Deposit composition was controlled using a superlattice deposition program. Hg 0.5 Cd 0.5 Te and Hg 0.8 Cd 0.2 Te deposits resulted in bandgaps of 0.70 and 0.36 eV, respectively. Electrochemical quartz crystal microbalance studies, using an automated flow cell, indicated that some deposited Cd was stripping at potentials used to deposit Hg. In addition, redox replacement of Cd for Hg was evident, a function of the greater stability of Hg than Cd.