Textured growth and electrical characterization of zinc sulfide on back-end-of-the-line compatible substrates

Scaling of transistors has enabled continuous improvement in the performance of logic devices, especially with contributions from materials engineering. However, there is a need to surpass the horizontal limitations in chip manufacturing and incorporate the vertical or third dimension. To enable monolithic three-dimensional integration of high-performance logic, one needs to solve the fundamental challenge of low temperature (<450 °C) synthesis of high mobility n-type and p-type semiconductor thin films that can be utilized for the fabrication of back-end-of-line (BEOL) compatible transistors. Metal oxides, especially indium oxides alloyed with gallium and tungsten, are promising n-type semiconductor channel materials; however there is a lack of p-type channel materials that can meet the stringent synthesis conditions of BEOL manufacturing. Zinc sulfide (ZnS), a transparent wide bandgap semiconductor, has shown room temperature p-type conductivity when doped with copper and nitrogen, and crystallizes below 400 °C. Here, we report growth of crystalline thin films of ZnS by pulsed laser deposition on a variety of amorphous and polycrystalline surfaces, including silicon nitride (SiNx), thermal silicon dioxide (SiO2), yttrium oxide (Y2O3), hafnium dioxide (HfO2), sapphire (Al2O3), platinum (Pt), and titanium nitride (TiN). X-ray diffraction scans show out-of-the-plane texturing of ZnS on all surfaces. In-plane crystalline quality is investigated using grazing incidence wide-angle x-ray scattering measurements. Surface and interface quality is measured using x-ray reflectivity and atomic force microscopy measurements. Electrical characterization of the ZnS films is done by J-V measurements of ZnS on platinum and metal-oxide-semiconductor capacitor (MOSCAP) measurements of ZnS on SiO2 on heavily doped silicon. The J-V measurements indicate low leakage current on the order of 10−5 A/cm−2 with an electric field of 0.40 MV cm−1, and the MOSCAP characteristics show bilayer capacitor behavior, which points to ZnS being highly intrinsic with very low unintentional, electrically active point defects. Further work on doping ZnS with copper or other p-type candidate dopants is needed to demonstrate ZnS as a dopable wide bandgap semiconductor for channels compatible with BEOL manufacturing. This work showcases the capability of the novel thin film growth technique of a wide bandgap sulfide semiconductor under BEOL compatible conditions with potential for technological applications in transistor manufacturing.