Investigations of the co-doping of boron and lithium into CVD diamond thin films

Lithium has been incorporated into heavily boron-doped single-crystal (SCD), microcrystalline (MCD) and nanocrystalline diamond (NCD) films at concentrations up to ~2 × 10²⁰ cm^-3 using Li₃N as a solid-state Li source for in-diffusion and diborane as the B source. The quality, morphology, electrical resistance and concentration of B and Li dopants present in a range of B+Li co-doped SCD, MCD and NCD films have been studied. Analysis of the SIMS depth profiles for Li enabled the diffusion constants, D, to be measured (in units of cm² s^-1) as: 2.5 × 10^-15, 1.3 × 10^-14 and 7.0 × 10^-14 for SCD, MCD and NCD, respectively, at 1100 K. The value for D for SCD agrees closely with that in the literature, while the much larger values for the polycrystalline films provide direct evidence that Li can diffuse rapidly along or through diamond grain boundaries at elevated temperatures. If prolonged diffusion allows the Li to reach the Si substrate, the Si acts as a sink for Li absorbing large quantities and reducing its concentration in the diamond film.