ABSTRACT
In this chapter, we discuss the use of graphene for multiple purposes in semiconductor devices, including AlGaN/GaN light-emitting diodes and as a diffusion barrier in contacts to Si. Precautions are needed to avoid degradation of the graphene during semiconductor processing. For example, graphene layers on SiO2/Si substrates were exposed to chemicals or gases commonly used in semiconductor fabrication processes, including solvents (isopropanol, acetone), acids (HCl), bases (ammonium hydroxide), UV ozone, H2O, and O2 plasmas. The recovery of the initial graphene properties after these exposures was monitored by measuring both the layer resistance and Raman 2D peak position as a function of time in air or vacuum. Solvents and UV ozone were found to have the least affect while oxygen plasma exposure caused an increase in resistance of more than 3 orders of magnitude. Recovery is accelerated under vacuum but changes can persist for more than 5 h. Careful design of fabrication schemes involving graphene is necessary to minimize these interactions with common processing chemicals. Optimized UV ozone cleaning of graphene layers on SiO2/Si substrates is shown to improve contact resistance of e-beam evaporated Ti/Au contacts by 3 orders of magnitude (3 × 10−6 Ω cm2) compared to untreated surfaces (4 × 10−3 Ω cm2). Subsequent annealing at 300°C lowers the minimum value achieved to 7 × 10−7 Ω cm2. Ozone exposure beyond an optimum time (6 min in these
experiments) led to a sharp increase in sheet resistance of the graphene, producing degraded contact resistance. The oxidation of graphene-based highly transparent contact layers to AlGaN/GaN/AlGaN ultraviolet (UV) light-emitting diodes (LEDs) was suppressed by the use of SiNX passivation layers. The oxidation is initiated at the unsaturated carbon atoms at the edges of the graphene and reduces the UV light intensity and degrades the current-voltage characteristics. We also demonstrated large-area suspended graphene on GaN nanopillars, which were predened by natural lithography and inductively coupled plasma etching. The thermal properties of the suspended and supported graphene were investigated by varying the underlying surface structures from at-top to sharp-cone morphologies. The heat transfer was effective even when the contact area between the suspended graphene and the supporting substrate was small. The extremely high thermal conductivity of the graphene can improve the thermal management in GaN-based high-power electronic and optoelectronics devices, which is critical for their long-term reliability. Finally, the insertion of chemically vapor-deposited graphene layers between Al metallization and Si substrates and between Au and Ni metal layers on Si is shown to provide a signicant reduction in spiking and intermixing of the metal contacts and reaction with the Si. The graphene prevents reaction between Al and Si up to temperatures of at least 700°C and of Au and Ni up to 600°C.
