New Insights into Q-Carbon: Novel Interlayer in Gan-Diamond Integration for Improved Heat Dissipation

The recent surge in high-power electronic device research has led to the development of various wide bandgap materials. GaN is such a material with immense potential due to its many impressive characteristics, such as high electron mobility and high breakdown voltage, leading to smaller device footprints with better power densities and high efficiency. However, heat dissipation has become a significant concern in high-power semiconductor devices made of GaN, which stems from the combination of low thermal conductivity and increasing power density. This study aims to resolve this issue by incorporating a Q-carbon (quenched form of carbon) interlayer to form continuous diamond films, which have the highest thermal conductivity of any material. Q-carbon is a novel form of carbon that has been rapidly cooled after melting, resulting in its unique properties and potential for use in electronic applications and device fabrication. The growth of large-area Q-carbon has been restricted to low thermally conductive substrates due to the non-equilibrium nature of the synthesis process and a narrow range of parameters. This study addresses the abovementioned limitations by conducting a comprehensive parameter optimization of diamond-like carbon (DLC), the fundamental structure necessary for synthesizing Q-carbon. Initially, the deposition of DLC films onto sapphire substrates is accomplished through the utilization of graphitic carbon targets via pulsed laser deposition (PLD), and subsequent pulsed laser annealing (PLA) is employed to generate Q-carbon, with a comprehensive regulation of laser, deposition, and annealing parameters. The DLC films deposited in their as-formed state exhibit an sp³ content ranging from 15% to 82%, which is achieved by optimizing the laser energy density, frequency, and deposition temperature during the process of PLD. SLIM simulation is employed to determine the optimal energy level of pulsed laser annealing (PLA) necessary for the synthesis of extensive Q-carbon. The experimental outcomes are then replicated by subjecting diverse diamond-like carbon (DLC) films to PLA with varying energy densities, spanning from 0.3 J/cm² to 1.2 J/cm², to obtain Q-carbon. The present study demonstrates that the synthesis of large-area Q carbon can be achieved by adjusting the parameters of PLD and PLA. This research broadens the scope of feasible thermally conductive substrates and provides fundamental parameters for future investigations on Q-carbon. Subsequently, we focus on GaN as a relatively higher thermally conductive substrate (conductivity ~200 Wm^-1K^-1) compared to the other substrates in the existing literature to employ the knowledge of large-area Q-carbon growth on various DLC films. This study incorporates a thin Q-carbon interlayer by utilizing low sp³ DLC film as the precursor and attempts to provide a solution to address the challenges associated with the integration of diamond and GaN for necessary thermal management capability. Incorporating a low thermally conductive DLC film and appropriate annealing parameters assist in achieving a Q-carbon interlayer on GaN. The utilization of Q-carbon on GaN serves as a nucleation substrate for producing high-quality HFCVD diamond with uniformity while protecting the GaN layer from deterioration and resolving the thermal mismatch between GaN and diamond. The distinctive combination of features possesses the capability to address the predicament of heat management in GaN devices, thereby advancing the attainment of GaN's theoretical potential in the domains of wireless/5-g communications, high-power devices, and other intricate microelectronic systems.