Electronic structures of $\mathrm{B}2p$ and $\mathrm{C}2p$ levels in boron-doped diamond films studied using soft x-ray absorption and emission spectroscopy

X-ray absorption (XAS) and emission (XES) spectroscopy near B K and C K edges have been performed on metallic ($\sim 0.1\mathrm{at.}\%$ B, B-diamond) and semiconducting ($\sim 0.03\mathrm{at.}\%$ B and N, BN-diamond) doped diamond films. Both B K XAS and XES spectra show a metallic partial density of states (PDOS) with the Fermi energy of $185.3\mathrm{eV}$, and there is no apparent boron-concentration dependence in contrast to the different electric property. In C K XAS spectrum of B-diamond, the impurity state ascribed to boron is clearly observed near the Fermi level. The Fermi energy is found to be almost same with the top of the valence band of nondoped diamond: $E_V=283.9\mathrm{eV}$. C K XAS of BN-diamond shows both the B-induced shallow level and N-induced deep and broad levels as the in-gap states, in which the shallow level is in good agreement with the activation energy $(E_a=0.37\mathrm{eV})$ estimated from the temperature dependence of the conductivity; namely, the change in $\mathrm{C}2p$ PDOS of impurity-induced metallization is directly observed. The electric property of this diamond is ascribed mainly to the electronic structure of $\mathrm{C}2p$ near the Fermi level. The observed XES spectra are compared with the discrete variational $\mathrm{X}\alpha$ ($\mathrm{DVX}\alpha$) cluster calculation. The $\mathrm{DVX}\alpha$ result supports the strong hybridization between $\mathrm{B}2p$ and $\mathrm{C}2p$ observed in XAS and XES spectra, and suggests that the small amount of boron $(⩽0.1\mathrm{at.}\%)$ in diamond occupies the substitutional site rather than interstitial site.

Figure 1

B K XAS and XES spectra of B- and BN-diamonds. The incident angle was set to $\theta =20°$. The excitation energy of XES measurement was $200\mathrm{eV}$.

Figure 2

B K XES spectra of B- and BN-diamonds. (a) The incident-angle dependence of B-diamond. The inset shows excitation-energy dependence and shows B K XES spectrum of $h\text{−}\mathrm{B}\mathrm{N}$. (b) The excitation-energy dependence of B-diamond, in which the spectrum with $E_{\mathrm{ex}}$ of $185.5\mathrm{eV}$ being magnified 5 times. The spectrum of BN-diamond with $E_{\mathrm{ex}}$ of $200\mathrm{eV}$ is also shown.

Figure 3

C K XAS spectra of B-, BN-, and nondoped diamonds. (a) Overall features of those spectra. (b) Detailed spectra of the in-gap states. The XAS and XES of nondoped diamond are also shown. The dashed lines indicate the top of the valence band of nondoped diamond and the impurity level expected from the activation energy of BN-diamond, respectively.

Figure 4

C K XES spectra of B-diamond: (a) XES spectra with $E_{\mathrm{ex}}$ of $284\mathrm{eV}$ (near the Fermi energy) and of $300\mathrm{eV}$. C K XAS spectrum of B-diamond is also shown (two arrows indicate two $E_{\mathrm{ex}}$ positions). (b) C(N.N.) XES spectra derived from the subtraction of the elastic peak from observed XES spectra with $E_{\mathrm{ex}}$ $284\mathrm{eV}$ (near the Fermi energy). The inset shows C(N.N.) XES and C K XAS spectra.

Figure 5

(Color online) Comparison of observed B K XAS and XES, and C K XES spectra with $\mathrm{DVX}\alpha$ simulations. (a) Observed XAS and XES spectra, (b) $\mathrm{DVX}\alpha$ results of substitutional case, and (c) $\mathrm{DVX}\alpha$ results of the interstitial case. In the inset pictures of (b) and (c), red and blue balls represent boron and N.N. carbon atoms, respectively.