Energy band alignment in MoS₂/HfO₂: Transfer-related artifacts and interfacial effects

Abstract

The knowledge of energy band alignment in heterojunctions with atomically thin transition metal dichalcogenides (TMDs) is critical for their use in advanced electronic and optoelectronic devices. Despite considerable efforts, the measurement of energy band offset across heterojunctions has been challenging, especially for van der Waals bonded stacks. Key obstacles are related to the scarce and often inconsistent information regarding the bandgap of the TMD layer and the offset between the conduction and valence bands, which is usually inferred from different measurement techniques and samples. To overcome this obstacle, we report combined internal photoemission (IPE) and photoconductivity measurements from 3-monolayer (ML) MoS2 films, grown by chemical vapor deposition on sapphire and transferred onto HfO2-covered silicon. We compare the spectral threshold of electron IPE in this heterostructure with IPE data from the Si/HfO2 interface, yielding the value of the electrostatic potential variation. To improve band offset predictions, we examine the applicability of the classical electron affinity rule by deriving characteristic energies. Our results show that electronic properties at 2D TMD/insulator interfaces depend on the interface processing prior to the 2D material transfer, allowing for the modification of band offsets by adjusting the interface. Furthermore, the measured photoconductivity spectra of 3ML MoS2 allow us to evaluate the bandgap of the TMD layer, which, combined with the IPE barriers, establishes the interface band diagram of a heterojunction. The presented IPE-based experimental approach can be extended to other two-dimensional TMDs for determining the corresponding band alignment schemes. It evaluates the impact of processing, such as solvent-based MoS2 transfer, which introduces a dipole and alters band alignment.