Enhanced interlayer electron transfer by surface treatments in mixed-dimensional van der Waals semiconductor heterostructures

TL;DR

This study demonstrates that surface treatments of III-V substrates significantly influence excitonic behavior and electron transfer in WS₂ monolayers, enabling control over carrier density and exciton localization in heterostructures.

Contribution

It reveals how native oxide removal and surface passivation of substrates modulate excitonic properties and interlayer electron transfer in 2D material heterostructures.

Findings

Native oxide removal enhances electron transfer from WS₂ to substrate.

Surface passivation suppresses localized exciton emission.

Surface engineering controls carrier density and exciton delocalization.

Abstract

We investigate the excitonic species in WS$_{2}$ monolayers transferred onto III-V semiconductor substrates with different surface treatments. When the III-V substrates were covered with amorphous native oxides, negatively charged excitons dominate the spectral weight in low-temperature near-resonance photoluminescence (PL) measurements. However, when the native oxides of the III-V substrates were reduced, neutral excitons begin to dominate the spectral weight, indicating a reduction in the electron density in the WS$_{2}$ monolayers. The removal of the native oxides enhanced the electron transfer from the WS$_{2}$ monolayer to the III-V substrate. In addition, an additional shoulder-like PL feature appeared $\sim$50 meV below the emission of neutral excitons, which can be attributed to the emission of localized excitons. When the III-V substrate surface was passivated by sulfur after the reduction of the native oxides, neutral excitons still dominated the spectral weight. However, the low energy PL shoulder disappeared again, suggesting the effective delocalization of excitons through the substrate surface passivation. Surface engineering of the semiconductor substrates for two-dimensional (2D) materials can provide a novel approach to control the carrier density of the 2D materials, to implement deterministic carrier localization or delocalization for the 2D materials, and to facilitate the interlayer transfer of charge, spin, and valley currents. These findings open the avenue for novel device concepts and phenomena in mixed-dimensional semiconductor heterostructures.