We have investigated the growth condition suitable for homoepitaxial diamond growth of phosphorus (P)-doped films on vicinal (001) substrates with a misorientation angle of 5 degrees using the high-power-density microwave-plasma (MWP) chemical-vapor-deposition (CVD). The P-doped layers were grown with H-2-diluted (1%) CH4 gas containing P(CH3)(3) with P/C ratio of 0.99% at various substrate temperatures ranging from 960 to 1210 degrees C by using a conventional MWPCVD system with a quartz-tube chamber, It is found from exciton-related cathodoluminescence spectra taken at approximate to 80 K and secondary ion mass spectrometry profiles that reasonably-high-quality P-doped layers with containing a substantial amount of substitutional P donors were grown on vicinal (001) substrates only at substrate temperatures of approximate to 1160 degrees C although the sample surface completely lost the original flatness with a roughness of approximate to 0.3 nm. This indicates that the suitable process window should be rather narrow for the P-doped diamond homoepitaxial growth on the vicinal (001) substrate using the present MWPCVD with 1% CH4 source gas and that the crystalline steps should play an important role on the growth process appropriate for the P atom incorporation to substitutional sites. (C) 2015 Elsevier B.V. All rights reserved.