Fabrication of Q-Carbon Nanostructures and Subsequent Formation of High-Quality Diamond on β-Ga₂O₃

The ultrawide bandgap (UWBG) oxides and nitrides, particularly beta-gallium oxide (β-Ga₂O₃), are promising material systems for next-generation power devices in commercial and military applications. For most electronic and RF applications, UWBG semiconductor-based devices, including β-Ga₂O₃, can operate at much higher voltages, frequencies, and temperatures than commercially available options, including silicon carbide and gallium nitride. However, the remarkably low anisotropic thermal conductivity of β-Ga₂O₃ (11-27 Wm^-1 K^-1) is a significant bottleneck for high-power device applications as it affects the device performance and results in self-heating effects (SHE), which severely limits output power density and maximum current flow of the circuit. To address this issue, the incorporation of a diamond layer that can operate as a heat spreader in Ga₂O₃-based devices has been investigated due to its extraordinarily high thermal conductivity (~ 2000 W/m/k). The deposition of high-quality diamond coatings onto non-diamond substrates, such as β-Ga₂O₃ films, is a complex task due to several inherent nucleation, growth, stress, and adhesion-related issues. The absence of carbon solubility in β-Ga₂O₃, a large mismatch in surface energies with a significant difference in thermal expansion coefficient between β-Ga₂O₃ and diamond, and unfavorable decomposition phenomena during CVD make it difficult to achieve uniform diamond layers with suitable adhesion and nucleation density. This study introduces a novel method for depositing diamond on β-Ga₂O₃ films using a diamond-like carbon (DLC) through pulsed laser deposition (PLD) and successive pulsed laser annealing (PLA) to form a quenched carbon (Q-carbon) layer, demonstrating improved precision and control in the synthesis of diamond coatings for optoelectronic and electronic devices. In this investigation, we have created strong Q-carbon/α-carbon and Q-carbon/nanodiamond heterostructures on β-Ga₂O₃ films using laser annealing of amorphous carbon films with nanosecond laser pulses above the melt threshold. The energy density required for the PLA is calculated by modeling the laser-solid melt interaction, and a maximum melt regrowth velocity of 12.5 m/s is achieved at a laser energy density of 0.4 J/cm². Raman studies and X-ray photoelectron spectroscopy (XPS) demonstrate the presence of a high <i>sp³ </i>content of ~ 81% in the Q-carbon region. Nanodiamonds formed by the PLA process exhibit a distinct Raman peak at 1320 cm^-1, with a red shift of approximately 12 cm^-1 due to the phonon confinement effect. This research reveals a considerable increase in the nucleation density during diamond deposition by hot filament chemical vapor deposition (HFCVD). A very high seeding density with high-quality delamination-free diamond film growth is possible on the Q-carbon region of the film due to the presence of densely packed diamond tetrahedra in Q-carbon which act as nucleation sites for diamond growth. The nucleation density, diamond quality, and stress values were compared for diamonds on Q-carbon coated β-Ga₂O₃ and uncoated β-Ga₂O₃ films. The incorporation of a Q-carbon intermediate layer significantly reduces the compressive stress in diamond films on β-Ga₂O₃ to 0.37 GPa, compared to 1.23 GPa for films deposited on uncoated substrates. The phase purity of the diamond can be measured by its FWHM of signature Raman peak, with a manually calculated value that matches the direct value obtained from the deconvoluted signal. The values obtained were obtained to be 11 cm^-1 and 16.66 cm^-1 for Q-carbon coated β-Ga₂O₃ and uncoated β-Ga₂O₃, respectively. Thus, this study addresses the critical challenges, i.e., poor adhesion and large thermal mismatch between diamonds and β-Ga₂O₃films, which has tremendous implications in realizing efficient β-Ga₂O₃-based high power devices.