Superconductivity in heavily B-doped diamond layers deposited on highly oriented diamond films

We deposited a [100]-oriented B-doped diamond layer by three methods to clarify the effects of film morphology on the transition from metallic to superconducting diamond. Heavily B-doped [100]-oriented diamond layers were deposited on [first method] undoped polycrystalline diamond films with [111] faces, [second method] highly oriented undoped diamond (HOD) thin films with a pyramidal surface, and [third method] thick undoped HOD films with a pyramidal surface. We confirmed that the B-doped layer in the third method was oriented in the [100] direction by scanning electron microscopy (SEM). The highest transition temperatures were Tc(onset) = 5.0 K and Tc(zero) = 3.1 K for the B-doped layer deposited on a thick HOD film with a pyramidal surface under a zero magnetic field. By contrast, Tc(onset) was 4.1 K for a heavily B-doped diamond layer deposited on a thin HOD film with a pyramidal surface, and was 3.9 K for a heavily B-doped diamond layer deposited on an undoped polycrystalline diamond film. These differences in Tc for our samples are affected by disorder and effective hole-carrier doping in each sample. Using the third method, we successfully deposited a high-quality B-doped [100] layer in three steps: (first step) depositing a [100] HOD film on a Si [100] substrate, (second step) depositing an HOD film with a pyramidal surface, and (third step) depositing a [100]-oriented B-doped layer. The change in the electronic states due to the B-doping of diamond films and the film morphology were investigated by x-ray photoelectron spectroscopy (XPS) measurements and band calculations.

► B-doped [100]-oriented diamond layer is deposited on undoped polycrystalline diamond films with [111] faces.
► B-doped diamond layer which is deposited on thick highly oriented undoped diamond film is highly [100] oriented.
► XPS C1s and valence band (C2s, sp2, sp3) spectra in B-doped diamond films reveal B-doping dependence.
► B-doping level is estimated to be about 2-5 × 1021 cm−3 from the phase diagram of Tc vs boron concentration.
► B-doped [100]-oriented diamond film on thick highly oriented undoped diamond thin film has a Tc equal to the highest value.