Textured growth and electrical characterization of Zinc Sulfide on back-end-of-the-line (BEOL) compatible substrates

Abstract

Scaling of transistors has enabled continuous improvements in logic device performance, especially through materials engineering. However, surpassing horizontal limitations in chip manufacturing requires a vertical, third dimension. Three-dimensional integration of high-performance logic demands solving the challenge of low-temperature (less than 450{\deg}C) synthesis of high-mobility n-type and p-type semiconductor thin films for back-end-of-line (BEOL) compatible transistors. Metal oxides, particularly indium oxides alloyed with gallium and tungsten, are promising n-type channel materials, but suitable p-type materials for BEOL remain scarce. Zinc sulfide (ZnS), a wide band-gap semiconductor, shows room-temperature p-type conductivity when doped with copper and crystallizes below 400{\deg}C. Here, we report growth of crystalline ZnS thin films by pulsed laser deposition on amorphous and polycrystalline surfaces including silicon nitride, thermal silicon dioxide, yttrium oxide, hafnium dioxide, sapphire, platinum, and titanium nitride. X-ray diffraction reveals out-of-plane texturing across all surfaces, while grazing incidence wide-angle X-ray scattering probes in-plane crystalline quality. Surface and interface properties are assessed using X-ray reflectivity and atomic force microscopy. Electrical characterization via J-V measurements (ZnS on Pt) and metal-oxide-semiconductor capacitor (ZnS on silicon dioxide) measurements show low leakage current ($10^{-5} A/cm^2$ at 0.40 MV/cm) and bilayer capacitor behavior, suggesting ZnS is highly intrinsic with minimal electrically active defects. Further work on doping ZnS with copper or other p-type elements is needed to realize ZnS as a dopable wide band-gap semiconductor for BEOL integration. This work demonstrates a novel thin-film growth method for sulfide semiconductors under BEOL-compatible conditions.