Band-Offset Degradation in van der Waals Heterojunctions

Two-dimensional materials-based van der Waals heterojunctions (vdWHs) are promising candidates for tunnel field-effect transistors (TFET) because of their atomically clean and electronically sharp junction interfaces and lattice mismatch-free characteristics. The tunneling behaviors of the TFETs greatly depend on the energy band alignments between different layers. By first-principles analyses, the band offsets in vdWHs are found to be very sensitive to the vdW interaction and temperature, especially for the type-III heterojunctions, which can be switched to the type-II configuration. This band-offset degradation is attributed to the interlayer charge transfer, generating built-in electric fields and potential differences, consistent with the plate condenser model. The charges are mainly transferred by two mechanisms: spontaneous and thermal excitation-assisted electron tunneling. The first one is caused by the electron wave function overlaps and energy difference of the valence band maximum and conduction band minimum, which belong to different materials, whereas the latter one is strongly correlated to the thermal excitations. The band-offset degradation is a general phenomenon in vdWHs, especially for those with a type-III band alignment. Therefore, attention should be paid in TFET investigations, since they are switched on by changing the band alignments to the type-III configuration.