The underpotential deposition and surface characterization of metal and chalcogenide atomic layers used in electrochemical atomic layer epitaxy (EC-ALE): cadmium, tellurium, and sulfur
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The formation of high quality compound semiconductors is facilitated by controlling growth at the atomic level. Electrochemical atomic layer epitaxy (EC-ALE) has been developed to electrodeposit compound semiconductor thin films one atomic layer at a time, by the sequential underpotential deposition (UPD) of each element from a separate solution, in a cycle. Two important parameters for controlling growth during EC-ALE are the electrode potential and solution composition, which influence the deposit structure and stoichiometry. Studies of the first monolayer of a compound and its constituent elements are the most important; they form the interface between the substrate and deposit, which must accommodate any lattice mismatch, and can greatly influence the substrate morphology of the deposit as it grows. Surface sensitive techniques were used to study the structure and composition of cadmium, tellurium, and sulfur atomic layers, as well as cadmium telluride monolayers on Au(111): Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). CdTe formation using a two-step EC-ALE process resulted in the formation of (v7Xv7)R19.1° and (3X3) structures with 1:1 and 2:1 stoichiometry, respectively. Both structures were formed using either cadmium or tellurium as a first layer on Au(111). Using a three-step procedure to form cadmium telluride produced the highest quality monolayer, relative to the two-step methods. Ordered cadmium atomic layers were electrodeposited at underpotentials on Au(111), where coadsorbed anions were structure determining, and served to stabilize the layers from spontaneous oxidation during removal from solution. These layers can be used to form Cd-containing compound semiconductors such as CdTe, CdSe, or CdS using EC-ALE. Ordered sulfur atomic layers were formed on Au(111) from alkaline solutions of sulfide, thiosulfate, and thiourea. A range of solution compositions and electrode potentials were found where the same 1/3 monolayer (v3Xv3)R30° sulfur layer can form, which can be used in an EC-ALE cycle to form compound semiconductors such as ZnS, CdS, and CuInS2.