  The longest spin coherence times for nitrogen-vacancy (NV) centers at room
temperature have been achieved in phosphorus-doped n-type diamond. However,
difficulty controlling impurity incorporation and the utilization of highly
toxic phosphine gas in the chemical vapor deposition (CVD) technique pose
problems for the growth of n-type diamond. In the present study, n-type diamond
samples were synthesized by CVD using tert-butylphosphine, which is much less
toxic than phosphine. The unintentional incorporation of nitrogen was found to
be suppressed by incrementally increasing the gas flow rates of H2 and CH$_4$.
Hall measurements confirmed n-type conduction in three measured samples
prepared under different growth conditions. The highest measured Hall mobility
at room temperature was 422 cm$^2$/(Vs). In the sample with the lowest nitrogen
concentration, the spin coherence time ($T_2$) increased to 1.62 $\pm$ 0.10 ms.
Optically detected magnetic resonance spectra indicated that all of the
measured NV centers were aligned along the [111] direction. This study provides
appropriate CVD conditions for growing phosphorus-doped n-type diamond with
perfectly aligned NV centers exhibiting long spin coherence times, which is
important for the production of quantum diamond devices.
