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 $(\left[\mathrm{P}\right]\sim 1\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3})$. 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.

Figure 1

(a) EPR spectra of P-doped diamond(111) epitaxial films. Only in sample B, was the NIMS1 center of shallow P donor observed, in addition to H1 center (a typical carbon defect in epitaxially grown diamond). (b) Angular dependence of EPR signals in sample B measured at 20 K. Open squares and solid squares correspond to NIMS1 center and H1 center, respectively. Dotted curves are simulated by EPR parameters of NIMS1 shown in Table 2.

Figure 2

Spin densities of NIMS1 center and P concentrations measured by SIMS. Open circle is refereed from Ref. [14]. A solid line expresses one-to-one correlation between NIMS center and P doping.

Figure 3

EPR spectra of sample A measured at 20 and 4.2 K. (a) Only H1 center can be observed at both temperatures, where a 1.4-mT doublet structure indicates characteristic ${}^1\mathrm{H}$ hyperfine interaction of H1 center. (b) Simulation of the NIMS1 EPR signal with spin density of $1\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$ (dotted lines). The NIMS1 center was not detectable at either temperature. Vertical scale is common to both the 20 and 4.2 K signals.

Figure 4

CL spectra of sample A and sample B. “${\mathrm{FE}}_{TO}$” and “${\mathrm{BE}}_{\mathrm{TO}}$” peaks originate from free excitons of diamond and binding excitons of neutral P donors, respectively. Vertical scale is common both to sample A and sample B.

Figure 5

(a) Cross-sectional Raman mapping of diamond Γ-phonon Raman peak on each edge face of sample A and sample B. In sample A, a free-standing P-doped epitaxial film is shown in the range between 2 and 50 μm. In sample B, regions over 50 μm indicate the air region. Raman spectra were measured at every 1-μm step on each edge face of samples. (b) Raman shifts as a function of cross-sectional positions. Only in sample B, was an irregular Raman shift of $+0.1\mathrm{c}{\mathrm{m}}^{-1}$ detected in the P-doped region, which is due to the presence of a strong compressive strain in this region (see details in text). In addition to the stepwise change, a gradual change in the Raman shift was also observed in sample B. Its magnitude depended on measured positions in sample B. Furthermore, base levels for the Raman shift varied from 1330.35 to $1331.25\mathrm{c}{\mathrm{m}}^{-1}$ in sample B. Such behaviors may relate to an inhomogeneity of the IIa substrate of sample B. Regardless of the variations, the stepwise change of $+0.1\mathrm{c}{\mathrm{m}}^{-1}$ was always observed in every line profile of sample B. In sample A, a relatively weak gradual change was observed. Fluctuated Raman intensities in sample A may originates from a surface roughness of its edge face.

Figure 6

Presence of a strong strain in sample B revealed by NIMS1 EPR center. (a) $D_{2d}$-symmetric distorted structure of NIMS1 center (substitutional P donor). A tetragonal $D_{2d}$ symmetry generates three equivalent orientations along the [100], [010], and [001] axes. The epitaxial growth direction along [111] is also indicated. (b) EPR spectrum of NIMS1 in sample B, when B // [110]. In this B direction, [001] orientation is distinguishable from other two orientations. In addition to EPR spectrum, the fitting peaks for experimental signals of NIMS1 center and the simulated peaks of NIMS1 center for the ideal population are shown. (c) Population ratios of NIMS centers with three orientations in an ideal uniform population and in sample B. The nonuniform population in sample B indicates the presence of a strong uniaxial strain in P-doped epitaxial layer on IIa-diamond substrate. (d) Schematic view of sample B and its off direction.

Figure 7

Schematic illustration of $1s$ ground states and excited states of substitutional P donors in Si and diamond, in addition to those of interstitial Li donors in Si. Electronic states $(a_1,e$, and $t_2$, etc.) shown in (a), (b), and (c) are taken from Refs. [34]. Excited energy levels $\left(2p_0,2p\pm ,3p_0\right)$ in (d) and (e) are taken from Ref. [35]. Valley-orbit splitting in $1s$ ground state of P donor are schematically drawn, because they have not yet been determined. Solid arrows represent unpaired electrons occupying each $1s$ ground state. The electronic system of the substitutional P donor in diamond is similar to that of interstitial Li donor in Si.

Figure 8

Temperature dependence of normalized EPR intensity of NIMS1 center. Solid triangles and open squares correspond to EPR measurements on IIa-diamond substrate/P-doped epitaxial film with P doping of $1\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$ (sample B) and of $1\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$ (Ref. [36]), respectively. The solid curve indicates normal temperature dependence of EPR intensity according to Curie's law, which was largely separated from experimental results (expressed approximately by a dashed smooth curve). This unusual dependence of the NIMS1 center suggests a close distribution of ground and excited states of substitutional P donor in diamond.