Identifying field-tunable surface resonance states on black phosphorus

TL;DR

This study reveals that surface resonance states on black phosphorus significantly influence its electrostatic response, with tunable surface states affecting tunneling conductance and screening, crucial for nanoscale device design.

Contribution

The paper demonstrates, through scanning tunneling spectroscopy, that surface resonance states dominate electric field screening on black phosphorus and introduces a model explaining their impact.

Findings

Surface resonance states dominate screening near the valence band edge.

Tunneling conductance spectrum shows a tunable dip with electric field.

Bulk band edges remain pinned despite surface state dynamics.

Abstract

Surface resonance states are electronic states localized near the surface while remaining hybridized with bulk bands. These states can strongly modify the electric-field response of semiconductors. Here, we demonstrate using scanning tunneling spectroscopy that on black phosphorus, surface resonance states near the valence-band edge dominate the screening of a strong external electric field. We observe in the tunneling conductance spectrum a pronounced dip with an energy continuously tunable by the local electric field in the tunneling junction. Meanwhile, we also notice that the bulk band edges remain effectively pinned, indicating efficient surface screening and suppression of bulk band bending. We interpret the conductance dip as the consequence of a field-dependent tunneling barrier: as the external electric field drives the surface resonance band into the band gap, the coupling between the surface resonance states and the bulk states is suppressed, leading to a reduced tunneling probability. Our simplified model based on this mechanism reproduces our main experimental findings. Our results highlight surface-localized states as a critical component in the electrostatic response of semiconductors, which must be taken into consideration in the design and operation of nanoscale semiconductor devices.