Low-temperature synthesis of diamond films by photoemission-assisted plasma-enhanced chemical vapor deposition.

Photoemission-assisted plasma-enhanced chemical vapor deposition (PA-PECVD), a process in which photoelectrons emitted from a substrate irradiated with ultraviolet light are utilized as a trigger for DC discharge, was investigated in this study; specifically, the DC discharge characteristics of PA-PECVD were examined for an Si substrate deposited in advance through hotfilament chemical vapor deposition with a nitrogen-doped diamond layer of thickness ~1 μm. Using a commercially available Xe excimer lamp (hν= 7.2 eV) to illuminate the diamond surface with and without hydrogen termination, the photocurrents were found to be 3.17×10¹² and 2.11×10¹¹ electrons/cm²/s, respectively. The 15-fold increase in photocurrent was ascribed to negative electron affinity (NEA) caused by hydrogen termination on the diamond surfaces. The DC discharge characteristics revealed that a transition bias voltage from a Townsend-to-glow discharge was considerably decreased because of NEA (from 490 to 373V for H₂ gas and from 330 to 200V for Ar gas), enabling a reduction in electric power consumption needed to synthesize diamond films through PA-PECVD. In fact, the authors have succeeded in growing high-quality diamond films of area 2.0 cm² at 540 ℃ with a discharge power of only 1.8 W, plasma voltage of 156.4 V, and discharge current of 11.7mA under the glow discharge of CH₄/H₂/Ar mixed gases. In addition to having only negligible amounts of graphite and amorphous carbon, the diamond films exhibit a relatively high diamond growth rate of 0.5 μm/h at temperatures as low as 540 ℃, which is attributed to Ar⁺ ions impinging on the diamond surface, and causing the removal of hydrogen atoms from the surface through sputtering. This process leads to enhanced CH_x radical adsorption, because the sample was applied with a negative potential to accelerate photoelectrons in PA-PECVD. [ABSTRACT FROM AUTHOR]