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 Cu2+ 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 Cu2+/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.
This paper concerns optimization studies of the growth of cadmium telluride, an important II-VI compound semiconductor, using electrochemical atomic layer epitaxy (EC-ALE). The importance of the potentials used to deposit atomic layers of Cd and Te, as well as the potential used to strip excess Te, were investigated. These potentials were used in a cycle, an EC-ALE cycle, to form deposits one atomic layer at a time, using a sequence of surface limited reactions. The optimal potentials for the CdTe EC-ALE cycle included Cd deposition at ) 0.65 V, Te deposition at ) 0.35 V and bulk Te stripping at ) 0.70 V. The deposits obtained were stoichiometric, with a Te/ Cd atomic ratio of 1.01 from electron probe microanalysis (EPMA). Electrochemical quartz crystal microbalance (EQCM) studies of the optimal condition indicated that about a third of the deposited Cd was oxidatively stripped at the potential used to deposit Te. Glancing angle X-ray diffraction studies showed a (111) preferred orientation for the deposit, while room temperature near infrared absorption measurements indicated a direct band gap of 1.5 eV.
This paper describes various studies undertaken to devise a deposition cycle for the formation of normalSb2normalTe3 , 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 normalSb2normalTe3 EC-ALD cycle, on a Au substrate, consisted of −0.22V for Sb deposition, and −0.35V for Te. Bulk Te stripping at was performed at −0.70V . The conditions using a TiN substrates included Sb deposition at −0.20V , Te deposition, and bulk stripping at −0.35 and −0.70V , 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. normalSb2normalTe3 was shown to grow successfully on patterned TiN substrates with 200nm vias.
The composition gradient and properties of magnetic 5-100 nm thin CoNiFe films electrodeposited on Cu or Pt substrate were studied. It was found that the average elemental composition of CoNiFe, obtained by ICP analysis, changes during electrodeposition. The extent of anomalous co-deposition achieved at deposit thickness <100 nm was found to be several times larger than in thicker CoNiFe films. The partial current densities for all three metals (Co, Ni, Fe) increase during the time of electrodeposition and gives rise to stable value when the thickness reaches about 100 nm. The partial current density for hydrogen evolution decreases and becomes stable at the thickness >100 nm. The observations related to the experimental results could be explained through a modified Bockris-Drazic-Despic reduction mechanism. The time-dependent dynamics of roughening surface exhibits two characteristic regions, i.e. the first with fast roughening at the thickness <100 nm and the second with slow roughening at the thickness >100 nm. The stress evolution show typical compressive-tensile-compressive behavior in the thickness range 5-100 nm. The crystal structure of 20 nm CoNiFe films is mixed fcc + bcc crystallites with the larger grain size close to the substrate while thick films show fcc-rich structure. The mechanism of coercivity in CoNiFe films is governed by magnetoelastic anisotropy and follows Neel's thickness dependence relation, i.e. H c = c t −n . The "volcano" type of H c -t curve obtained at 5-50 nm was explained taking into account: change of composition, stress, crystal structure and roughness.
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