Incorporation of lithium in single crystal diamond: diffusion profiles and optical and electrical properties

Doping of diamond by in-diffusion of lithium in natural type Ia and IIa single crystal diamond plates has been investigated. The lithium incorporation was realized by direct in-diffusion from a Li2O source at temperatures between 400 and 650 °C. Using secondary ion mass spectroscopy (SIMS) depth profiling, high concentrations of lithium, up to a level of 8.0 × 1021 cm−3 and gradually decreasing to 1.0 × 1019 cm−3 at a depth of 0.5 μm below the surface, were measured. From these profiles the diffusion constant of lithium in diamond was determined to be DLi(T) = 2 × 10−10exp (−0.9 eV/kT) cm2s−1. The the activation energy for diffusion of 0.9 ± 0.3 eV is in agreement with the theoretical literature value of 0.85 eV. Cathodoluminescence spectroscopy showed that the blue band-A luminescence, which is characteristic for natural type IIa diamond, is largely quenched by the presence of lithium. A shift of the maximum luminescence intensity towards higher energy values has been found. From photothermal deflection spectroscopy and optical transmission measurements no new optically active defect is observed after lithium in-diffusion. However, these measurements indicated the occurrence of a large concentration of light scattering centres, the size of which is 10 nm or less. This suggests that lithium is clustered in the crystal volume just below the specimen surface and therefore is not electrically active.