Effects of Dielectric Stoichiometry on the Photoluminescence Properties of Encapsulated WSe2 Monolayers

TL;DR

This study investigates how the stoichiometry of encapsulating oxides affects the photoluminescence of WSe2 monolayers, revealing that stoichiometric oxides preserve optical properties better than sub-stoichiometric ones.

Contribution

It demonstrates that oxide stoichiometry critically influences the optical and electrical properties of encapsulated WSe2 monolayers, providing insights for reliable device integration.

Findings

Sub-stoichiometric oxides induce electrical doping and reduce emission.

Physical deposition of stoichiometric oxides preserves optical properties.

Atomic layer deposition results in cluster-like oxide coatings.

Abstract

Two-dimensional transition-metal-dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact performances. However atomically thin materials are highly sensitive to surrounding dielectric media, which imposes severe limitations to their practical applicability. Hence for their suitable integration into devices, the development of reliable encapsulation procedures that preserve their physical properties are required. Here, the excitonic photoluminescence of WSe2 monolayer flakes is assessed, at room temperature and 10 K, on mechanically exfoliated flakes encapsulated with SiOx and AlxOy layers employing chemical and physical deposition techniques. Conformal flakes coating on untreated - non-functionalized - flakes is successfully demonstrated by all the techniques except for atomic layer deposition, where a cluster-like oxide coating is observed. No significant compositional or strain state changes in the flakes are detected upon encapsulation by any of the techniques. Remarkably, our results evidence that the flakes' optical emission is strongly influenced by the quality of the encapsulating oxide - stoichiometry -. When the encapsulation is carried out with slightly sub-stoichiometric oxides two remarkable phenomena are observed. First, there is a clear electrical doping of the monolayers that is revealed through a dominant trion - charged exciton - room-temperature photoluminescence. Second, a strong decrease of the monolayers optical emission is measured attributed to non-radiative recombination processes and/or carriers transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation.