Plasma induced surface modification of sapphire and its influence on graphene grown by PECVD

TL;DR

This study investigates how plasma-induced surface modifications of sapphire substrates affect the quality and defect structure of graphene grown via PECVD, aiming to improve graphene synthesis on dielectric surfaces.

Contribution

It demonstrates how controlling sapphire surface terminations influences graphene defect types and crystallinity, advancing catalyst-free graphene growth techniques.

Findings

Hydrophilic (Al-terminated) surfaces reduce graphene defects.

Different sapphire terminations lead to vacancy or boundary defects in graphene.

Optimizing surface termination improves graphene crystallinity.

Abstract

The catalyst-free synthesis of graphene on dielectrics prevents the damage induced by the transfer process. Although challenging, to master this synthesis would boost the integration of graphene on consumer electronics since defects hinder its optoelectronic properties. In this work, the influence of the different surface terminations of c-plane sapphire substrates on the synthesis of graphene via plasma-enhanced chemical vapour deposition (PECVD) is studied. The different terminations of the sapphire surface are controlled by a plasma etching process. A design of experiments (DoE) procedure was carried out to evaluate the major effects governing the etching process of four different parameters: i.e. discharge power, time, pressure and gas employed. In the characterization of the substrate, two sapphire surface terminations were identified and characterized by means of contact angle measurements, being a hydrophilic (hydrophobic) surface the fingerprint of an Al- (OH-) terminated surface, respectively. The defects within the synthesized graphene were analysed by Raman spectroscopy. Notably, we found that the ID/IG ratio decreases for graphene grown on OH-terminated surfaces. Furthermore, two different regimes related to the nature of graphene defects were identified and depending on the sapphire terminated surface are bound either to vacancy or boundary like defects. Finally, studying the density of defects and the crystallite area, as well as their relationship with the sapphire surface termination paves the way for increasing the crystallinity of the synthesized graphene.