Graphene as Transparent Conductive Film for GaN-Based Vertical Cavity Surface Emitting Lasers

Abstract

There is a need for lasers in the 370nm - 470nm spectrum (near ultraviolet - blue) for applications such as high storage, high resolution printers and biomedical applications. GaN-based vertical cavity surface emitting laser (VCSEL) is one interesting option due to the wide bandgap of the GaN and the 2D array capability and cost-effective production of VCSELs. Because of the high resistivity of p-GaN, these devices require the use of a transparent conductive film (TCF) to effectively laterally spread the current across the active region. The actual material used so far is ITO wish has problems related to the deposition technique and the constantly increasing price of the material. Graphene, a one atom layer thick (0.35 nm) graphite derivative 2D material, stands as a very good option due to its outstanding properties (high mobilities, high transmittance, etc.). In order to demonstrate the feasibility of replacing ITO with a graphene TCF the production, transfer, sheet resistance and contact resistivity to p-GaN have to be investigated and further developed. In this work a low-pressure cold-wall CVD reactor was used for reproducible production of large domains graphene films on 99.995% Cu foils of 50 µm. Different transfer techniques were assessed and reproducible graphene transfer was achieved by improving a recently developed method. The method is based on the separation of the graphene from the Cu foil by H2 bubbles formation on the surface of the Cu foil that was used as a cathode in an electrolytic cell with aqueous 0.25M NaOH solution. Different methods to improve the sheet resistance of the graphene were evaluated (artificially deposited dual layer films, doping with FeCl3, S1813 and PVA). Devices were created to characterize the mobility, sheet resistance, carrier concentration, maximum current densities and contact resistivity to p-GaN. A GaN-based LED using a graphene TCF was fabricated and showed efficient current spreading by the graphene based TCF. The results show that graphene is an interesting option for TCF however, more effort is needed to further improve the contact resistivity.