Chandru Thambidurai

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.

E-ALD of Cu Nanofilms on Ru/Ta Wafers Using Surface Limited Redox Replacement

bCEA, LETI, MINATEC, F38054 Grenoble, France This paper describes the formation of Cu nanofilms on a Ru/Ta-coated wafer using electrochemical atomic layer deposition E-ALD. The initial steps involved cleaning and oxide removal from the Ru/Ta substrate using ultrahigh vacuum electrochemical system. Auger spectroscopy was used to follow the relative amounts of oxygen, Ru, and Cu on the wafer: as-received, after electrochemical treatment, after ion bombardment, and after Cu deposition. An automated flow cell electrodeposition system was employed to grow Cu nanofilms with up to 200 cycles using surface limited redox replacement SLRR reaction. The open-circuit potential was used to follow the exchange of Pb for Cu in the SLRR reaction. Electron probe microanalysis was used to determine the homogeneity of the Cu films. The use of a complexing agent citrate greatly improved the homogeneity. Different concentrations of citrate were investigated, containing 2 or 4 mM citrate. Atomic force microscopy images of the Cu films showed the ingress morphology to be the same as the egress when 4 mM citrate was used. A prominent Cu111 peak was displayed in the X-ray diffraction pattern for 200 cycles of Cu grown on the Ru/Ta-coated wafer.

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.

PtRu nanofilm formation by electrochemical atomic layer deposition (E-ALD).

The high CO tolerance of PtRu electrocatalysis, compared with pure Pt and other Pt-based alloys, makes it interesting as an anode material in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). This report describes the formation of bimetallic PtRu nanofilms using the electrochemical form of atomic layer deposition (E-ALD). Metal nanofilm formation using E-ALD is facilitated by use of surface-limited redox replacement (SLRR), where an atomic layer (AL) of a sacrificial metal is first formed by UPD. The AL is then spontaneously exchanged for a more noble metal at the open-circuit potential (OCP). In the present study, PtRu nanofilms were formed using SLRR for Pt and Ru, and Pb UPD was used to form the sacrificial layers. The PtRu E-ALD cycle consisted of Pb UPD at -0.19 V, followed by replacement using Pt(IV) ions at OCP, rinsing with blank, then Pb UPD at -0.19 V, followed by replacement using Ru(III) ions at OCP. PtRu nanofilm thickness was controlled by the number of times the cycle was repeated. PtRu nanofilms with atomic proportions of 70/30, 82/18, and 50/50 Pt/Ru were formed on Au on glass slides using related E-ALD cycles. The charge for Pb UPD and changes in the OCP during replacement were monitored during the deposition process. The PtRu films were then characterized by CO adsorption and electrooxidation to determine their overpotentials. The 50/50 PtRu nanofilms displayed the lowest CO electrooxidation overpotentials as well as the highest currents, compared with the other alloy compositions, pure Pt, and pure Ru. In addition, CO electrooxidation studies of the terminating AL on the 50/50 PtRu nanostructured alloy were investigated by deposition of one or two SLRR of Pt, Ru, or PtRu on top.