Graphene diffusion barrier for forming ohmic contact on N-polar n-type GaN for high-power vertical-geometry light-emitting diodes

Abstract

Graphene sheets were employed as a diffusion barrier to form highly reliable ohmic contacts to N-polar n-GaN for high-power vertical-configuration LEDs (VLEDs). Before annealing, both Ti/Al and Ti/graphene/Al contacts exhibit ohmic behavior. After annealing at 250 °C, unlike the Ti/Al contact, the Ti/graphene/Al contacts remain ohmic with a contact resistivity of 7.5 × 10−4 Ω cm2. Secondary ion mass spectroscopy (SIMS) results show that the graphene sheets serve as a barrier to the outdiffusion of Ga atoms into the electrode. On the basis of SIMS and X-ray photoemission spectroscopy results, the ohmic and degradation behaviors of the Ti-based contacts are described and discussed.

Review of Graphene as a Solid State Diffusion Barrier

“…Typical I–V characteristics of Ti/Al samples with and without graphene barriers before and after annealing at 250 °C. Reproducd with permission . Copyright 2014, The Japan Society of Applied Physics.…”

“…Since graphene itself is readily oxidized, it is important to provide oxidation barriers when it is used a transparent conducting layer. The oxidation of graphene‐based transparent contact layers to AlGaN/GaN/AlGaN UV light‐emitting diodes (LEDs) was found to be suppressed by the use of SiN X passivation layers . Although graphene may be an effective transparent conductive contact layer to UV‐LEDs, oxidation of these layers at high operating temperatures has been an issue.…”

“…GaN‐based UV‐LEDs deposited with SiN X by plasma‐enhanced CVD showed minimal degradation of light output intensity and I–V characteristics, because the graphene‐based UV transparent conductive layers were shielded from the oxygen molecules. This is a simple and effective approach for maintaining the advantages of graphene conducting layers as electrodes on UV‐LEDs …”

“…Previous barrier experiments with graphene have concentrated on preventing corrosion on Cu substrates. Table 1 summarizes the literature on the use of graphene diffusion barriers . Chen et al have reported that graphene coatings improve the oxidation resistance of Cu and Cu/Ni alloys in air up to 200 °C and in 30% hydrogen peroxide solutions.…”

“…This is important for mitigating the interdiffusion of contact metals or reaction of the metals with the underlying semiconductor during thermal processing and device operation, especially for Ohmic contacts whose resistance is severely compromised during intermixing or reaction. The incorporation of graphene as a diffusion barrier may also add flexibility when designing process schemes to integrate graphene as a channel material on existing semiconductors …”