Advances in n-Type Chemical Vapor Deposition Diamond Growth: Morphology and Dopant Control

ConspectusDiamond, a wide bandgap semiconductor, has captivated researchers for decades due to its exceptional properties. While p-type doping has dominated the field, the advent of n-type diamond, doped by nitrogen or phosphorus, has unlocked novel prospects for diverse applications. Nonetheless, the chemical vapor deposition (CVD) of n-type diamond faces substantial hurdles, particularly concerning crystalline quality and dopant concentration control. In this Account, we summarize our progress in developing high quality CVD n-type diamond films. Our research initiates with nitrogen introduction into the CH4/H2 CVD plasma for depositing polycrystalline diamond films. The addition of 4% N2 gas induces the formation of ultra-nanosized diamond grains through CN species, but further increases in nitrogen content result in grain agglomeration into larger sizes. Fixing 3% of N2 in the CVD plasma, we explore the influence of methane concentration on N-doped nanocrystalline diamond (NCD) films. At a low methane concentration of 1%, faceted diamond grains are formed, while increasing methane to 15% yields nanoneedles encased in nanographitic phases, featuring a low resistivity of 90 Ω·cm. We further investigate P-doped polycrystalline diamond films, where preliminary examinations of P-doped NCD reveal well-defined grain structures but also morphological imperfections and twin boundaries, with a phosphorus incorporation of ≈1019 cm–3. Our investigations also cover P-doped (110)-textured polycrystalline CVD diamond films, finding that the phosphorus concentration varies with grain misorientation and that higher phosphine concentrations lead to a more uniform distribution. Additionally, we note that an increase in the [P]/[C] ratio in the CVD plasma of P-doped diamond growths leads to the transformation of NCD to ultra-NCD, reducing residual stress, and affecting film quality. In a complementary investigation, we explore the codoping of NCD films with nitrogen and phosphorus, observing a transition from micron-sized faceted diamond grains to nanosized grains with increasing nitrogen content at a fixed amount of phosphorus concentration in the CVD plasma.Exploring diamond's potential as a semiconductor, our research group investigated the captivating properties of P-doped single crystal diamond films, given a shallower donor energy level of 0.6 eV compared to nitrogen's deep donor level at 1.7 eV. Our findings indicate optically active defects with various electronic levels, using a doping range from 1016 to 1019 cm–3 in (111)-oriented P-doped diamond epilayers. However, challenges like formation of defects, persist for this orientation. In contrast, (100)-oriented diamond films are renowned for the p-type conductivity and high crystalline quality, though achieving n-type conductivity remains a challenge. Our research highlights the critical role of varying methane concentration during CVD in influencing both crystalline quality and phosphorus concentration. Elevated methane concentrations are found to induce surface degradation, affecting film quality and doping level. Surprisingly, (110)-oriented P-doped single crystal diamond growth demonstrates promising results with a 33 μm/h deposition rate using only 1% methane concentration. Furthermore, the off-angle from the (110) orientation can potentially impact film quality, indicated by cathodoluminescence spectroscopy, offering exciting prospects for future research.The insights provided in this Account will illuminate the CVD growth of n-type diamond films, contributing to the advancement of diamond-based devices.