Direct Characterization of Buried Interfaces in 2D/3D Heterostructures Enabled by GeO₂ Release Layer

Inconsistent interface control in devices based on two-dimensional materials (2DMs) has limited technological maturation. Astounding variability of 2D/three-dimensional (2D/3D) interface properties has been reported, which has been exacerbated by the lack of direct investigations of buried interfaces commonly found in devices. Herein, we demonstrate a new process that enables the assembly and isolation of device-relevant heterostructures for buried interface characterization. This is achieved by implementing a water-soluble substrate (GeO₂), which enables deposition of many materials onto the 2DM and subsequent heterostructure release by dissolving the GeO₂ substrate. Here, we utilize this novel approach to compare how the chemistry, doping, and strain in monolayer MoS₂ heterostructures fabricated by direct deposition vary from those fabricated by transfer techniques to show how interface properties differ with the heterostructure fabrication method. Direct deposition of thick Ni and Ti films is found to react with the monolayer MoS₂. These interface reactions convert 50% of MoS₂ into intermetallic species, which greatly exceeds the 10% conversion reported previously and 0% observed in transfer-fabricated heterostructures. We also measure notable differences in MoS₂ carrier concentration depending on the heterostructure fabrication method. Direct deposition of thick Au, Ni, and Al₂O₃ films onto MoS₂ increases the hole concentration by >10¹² cm^-2 compared to heterostructures fabricated by transferring MoS₂ onto these materials. Thus, we demonstrate a universal method to fabricate 2D/3D heterostructures and expose buried interfaces for direct characterization.