Operando SXRD study of the structure and growth process of Cu2S ultra-thin films

Andrea Giaccherini,Giordano Montegrossi,Serena Cinotti,Alessandro Lavacchi,Massimo Innocenti,Francesco Di Benedetto,Francesco Vizza,Francesco Carlà,Roberto Felici,Annalisa Guerri

doi:10.1038/s41598-017-01717-0

Abstract

Electrochemical Atomic Layer Deposition (E-ALD) technique has demonstrated to be a suitable process for growing compound semiconductors, by alternating the under-potential deposition (UPD) of the metallic element with the UPD of the non-metallic element. The cycle can be repeated several times to build up films with sub-micrometric thickness. We show that it is possible to grow, by E-ALD, Cu2S ultra-thin films on Ag(111) with high structural quality. They show a well ordered layered crystal structure made on alternating pseudohexagonal layers in lower coordination. As reported in literature for minerals in the Cu-S compositional field, these are based on CuS3 triangular groups, with layers occupied by highly mobile Cu ions. This structural model is closely related to the one of the low chalcocite. The domain size of such films is more than 1000 Å in lateral size and extends with a high crystallinity in the vertical growth direction up to more than 10 nm. E-ALD process results in the growth of highly ordered and almost unstrained ultra-thin films. This growth can lead to the design of semiconductors with optimal transport proprieties by an appropriate doping of the intra metallic layer. The present study enables E-ALD as an efficient synthetic route for the growth of semiconducting heterostructures with tailored properties.

Highlights

Present efforts of materials and surfaces science focus on the development of facile manufacturing techniques which either use processes with a low energy consumption or result in growth with high quality crystalline films
We report on the operando Surface X-ray Diffraction (SXRD) structural characterization of Cu-S films during their Electrochemical Atomic Layer Deposition (E-ALD) growth, with the aim of understanding the growth mechanism and of determining the main structural characteristics of the deposited thin film
The electrode surface was cleaned under Ultra High Vacuum (UHV) conditions by performing several cycles of Ar+ sputtering at 1 kV followed by annealing at 900 K for about 1 min

Summary

Introduction

Present efforts of materials and surfaces science focus on the development of facile manufacturing techniques which either use processes with a low energy consumption or result in growth with high quality crystalline films. High crystallinity and low concentration of defect accumulated during the growth is the main focus in order to opportunely control the electronic and transport properties of the materials In this context, our work proposes the use of thin layers of sulphides deposited by an electrodeposition technique that allows the deposition of single atomic layers with an accurate control of the crystal structure. In the case of binary and ternary sulfides including only Cd and/or Zn the obtained phases can be excellently referred to the known structural models[11, 12], allowing a straightforward interpretation of the relative physical and semiconducting properties, the same does not hold true as well in the case of Cu sulfides It is well known, that the Cu-S system presents several phases with different stoichiometry and crystal structure[13] and that the relationships between Cu sulfides and other elements as Zn or Sn evidence only very limited solid solution ranges[14, 15]. This relation has been used in this work to estimate the stoichiometry of the film

Methods

Results

Conclusion