Nearly degenerate ground state of phosphorus donor in diamond

We investigate phosphorus (P) donors in P-doped diamond epitaxial films by means of electron paramagnetic resonance spectroscopy (EPR). In a diamond thin film on IIa diamond substrate, we could observe an EPR signal of the NIMS1 center, which has been assigned to a neutral P donor with a ${D}_{2d}$ symmetry. In contrast, we could not observe this center in another free-standing diamond film with the same P concentration $([\mathrm{P}]\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3})$. This striking contrast can be reasonably accounted for in a model where the P donor in diamond has a nearly degenerate ground state due to ${t}_{2}$ and $e$, in contrast to P donors in group-IV semiconductors like Si and Ge, where ${a}_{1}$ singlet ground state is present. We observed a strong uniaxial strain only in the former diamond thin film, which was confirmed by Raman microscopy and preferential orientations of NIMS1 EPR centers. This strain splits the nearly degenerate ground state of the P donor, resulting in the observation of the NIMS1 center. We point out that the electronic system of the substitutional P donor in diamond is similar to that of interstitial Li donor in Si.