Order and Epitaxy during Electrochemical Layer By Layer Growth of Semiconductor Thin Films

Electrochemical Atomic Layer Epitaxy (ECALE), also known as Electrochemical Atomic Layer Deposition (E-ALD) [1], is a method based on Under Potential Deposition (UPD) which allows the layer by layer growth of compounds on single crystal substrates.In the ECALE method [1] a compound can be deposited on a substrate by an alternate UPD of the elements that form the compound. UPD is a surface- limited phenomenon, so the electrodeposition in UPD conditions is generally limited to one atomic layer. ECALE has been demonstrated to be a very effective method for the production of thin films of semiconductor compounds with high grade of crystallinity [2-3]. UPD processes on single crystals are substrate dependent, thus different structures of the deposited layer should be expected on substrates with different orientation. Substrate orientation dependency was shown in a previous investigation on the deposition of CdS films on Ag(111), Ag(110) and Ag(100) [2]. Aim of the present work was the in-situ characterization of the growth process of CdS thin films on Ag(111). In-situ structural characterization of thin films is generally a rather complex task as it requires high brilliance x-ray sources given the small thickness of the film, the high background of the electrochemical environment and the attenuation due to the presence of the solvent. Nevertheless synchrotron light sources can provide such an high photon flux to perform this kind of experiment. Surface X-ray diffraction was used to identify the crystallographic structure and to investigate the epitaxial order on the surface. In-situ characterization of the growth process was performed on ID03 beamline at ESRF (Grenoble) using an electrochemical flow cell specifically designed for this purpose. Even if it's well established that the sequential deposition of UPD layers of Cd and S on Ag(111) results in an ordered multilayer structure with lattice parameters comparable to the one of the wurtzite CdS structure [3], several aspects of the growth process were not yet fully understood. In-situ experiments allowed to record structural changes of the in-plane order and details of the out-of-plane relaxation of the film that could not be observed before. One of the most interesting fact observed during the growth process was the dependency of the epitaxial order on the potential used for the UPD deposition of Cd. In figure X-ray reflectivity curve measured during CdS growth on Ag(111).

[1] Gregory and J. L. Stickney, J. Electroanal. Chem. 300, 1991, 543

[2] F. Carla', et al., J. Phys. Chem. C, 12, 6132 (2014)

[3] M.L. Foresti, et al. Electrochimica Acta, 51, 25, 5532 (2006)