Dependence of the Superconducting Transition Temperature on the Doping Level in Single-Crystalline Diamond Films

Homoepitaxial diamond layers doped with boron in the ${10}^{20}–{10}^{21}{\mathrm{c}\mathrm{m}}^{-3}$ range are shown to be type II superconductors with sharp transitions ($\sim 0.2\mathrm{K}$) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity in those 001-oriented single-crystalline films is about $5–7{10}^{20}{\mathrm{c}\mathrm{m}}^{-3}$. The $H-T$ phase diagram has been obtained from transport and ac-susceptibility measurements down to 300  mK.

Figure 1

SIMS profiles for ion mass $m=23$ obtained using $\mathrm{C}{\mathrm{s}}^{+}$ primary ions on samples 2, 4, and 5. In the case of the thicker samples, sputtering was interrupted before reaching the buffer layer.

Figure 2

Dependence of the superconducting transition temperature $T_c$ on the boron concentration $n_{\mathrm{B}}$: solid squares—this work, open circles—from 1. The different $T_c$ values were obtained from the onset of the diamagnetic signal (see inset for the real (solid symbols) and imaginary (open symbols) parts of the magnetic susceptibility.

Figure 3

(a) Temperature dependence of the electrical resistance at indicated magnetic fields for the film with $n_{\mathrm{B}}=19\times {10}^{20}{\mathrm{a}\mathrm{t}.\mathrm{c}\mathrm{m}}^{-3}$. The dashed lines correspond to the two criteria used for the determination of $H_{c2}$ (see Fig. 4), i.e., $R/R_N=90\%$ and 10% ($R_N$ being the normal state resistance). (b) Temperature dependence of the real part of the magnetic susceptibility at indicated magnetic fields for the film with $n_{\mathrm{B}}=9\times {10}^{20}{\mathrm{a}\mathrm{t}.\mathrm{c}\mathrm{m}}^{-3}$. $H_{c2}$ has been deduced from the onset of the diamagnetic signal.

Figure 4

$H-T$ phase diagram for the films with $n_{\mathrm{B}}=19\times {10}^{20}{\mathrm{a}\mathrm{t}.\mathrm{c}\mathrm{m}}^{-3}$ and $n_{\mathrm{B}}=9\times {10}^{20}{\mathrm{a}\mathrm{t}.\mathrm{c}\mathrm{m}}^{-3}$. The solid circles (diamonds) have being deduced from temperature sweeps of the electrical resistance [see Fig. 3a] for $R/R_N=90\%$ ($R/R_N=10\%$). The gray area is an indication of the width of the transition. The solid squares were defined from the onset of the ac susceptibility [see Fig. 3b]. The solid lines are fits to the data using classical theory [10], for which the only free parameter is $H_{c2}(0)$. The dotted line is a guide to the eye.