UC Riverside

This dissertation presents results of the experimental studies of graphene thermal properties and discusses possibilities of graphene applications for thermal management of high-power density electronic devices. The results reported here are divided into two parts. In part one, I describe fabrication and testing of graphene and few-layer graphene (FLG) heat spreaders for high-power AlGaN/GaN transistors. The mechanically exfoliated graphene-graphite layers have been transferred on top of AlGaN/GaN field-effect transistors and attached close to the heat generating regions near the metal contacts. The micro-Raman spectroscopy was used for in situ monitoring of the temperature of the hotspots. It was demonstrated that the temperature of the hot spots can be lowered by ~20 °C in transistors operating at ~13 W/mm power. The latter corresponds to an order-of-magnitude increase in the device lifetime. Simulations results supported our conclusions and indicated that graphene-graphite heat spreaders perform even better in AlGaN/GaN transistors on sapphire substrates. The proposed approach for heat removal at micrometer and nanometer scale represents a transformative change in thermal management of electronics.

In part two of this dissertation, I report the results of investigation of thermal conductivity of thin films made of a novel nanostructured graphene material, which consists of graphene nanoribbons encapsulated in single walled carbon nanotubes. The temperature dependent Raman spectrum of this material was measured in order to obtain the temperature coefficients of G+ peak and 2D peaks. Using the Raman optothermal technique, I determine the local temperature rise due to laser heating from the shifts in Raman peak positions. A finite element analysis method was conducted to simulated heat dissipation in the samples and to determine their effective thermal conductivity. The obtained results suggest that this hybrid graphene - carbon nanotube material can be used as fillers in thermal interface materials.