In situ STM study of CdTe ECALE bilayers on gold

Scanning tunneling microscopy (STM) has been used in situ to study the epitaxial growth of CdTe bilayers electrodeposited onto Au single-crystal surfaces. The films were produced by the alternate reductive underpotential deposition of Te and Cd by the electrochemical atomic layer epitaxy (ECALE) technique. The kinetics of electrocrystallization were found to be consistent with an instantaneous 2-D nucleation and growth model. Atomic resolution images of the corresponding first UPD structures of tellurium prior to Cd deposition in a CdSO4 solution have been obtained. Cd/Te bilayer structures formed by Cd UPD on Au(111)-(12x12)Te, Au(110)-c(2x8)Te, and Au(100)-(2x2)Te were Au(111)-(3x3)CdTe, Au(110)-(2x3)CdTe, and Au(100)-c(2x2)CdTe bilayers, respectively. To maintain stoichiometry, bilayer formation was accompanied where necessary (increasing in the order Au(111)-(3x3)CdTe < Au(110)-(2x3)CdTe < Au(100)-c(2x2)CdTe) by a significant rearrangement of the Te structure during bilayer growth. This process was found to take place without three-dimensional nucleation and resulted in the exposure of large Au(100) patches randomly distributed on the terraces, on which Cd was deposited. In striving for structural epitaxy on the gold substrates, structures exhibiting a compression of the bulk lattice planes were observed for cases where both excess and insufficient Te in the UPD layer was present to form the bilayer of ideal density. This observed inhomogeneity in the growth of Au(100)-c(2x2)CdTe was consistent with a Cd-induced structure in the Te stripping peak on Au(100) and a Coulometric excess of adsorbed Cd in the overall layer.