We have investigated properties of heavily-B-doped diamond (HBD) alms homoepitaxially grown with boron-to-carbon (B/C) mole ratios ranging from 1000 to 5000 ppm in the source gas mainly by using X-Ray diffraction (XRD), cathocloluminescence (CL), and Hall effect measurements. Each HBD layer was deposited on a vicinal (001) substrate of high-pressure/high-temperature synthesized Ib-type diamond with 5 degrees misorientation angle by means of high-power-density microwave-plasma chemical-vapor-deposition method with a source gas composed of 4% CH4 in H-2 and H-2-diluted B(CH3)(3). XRD data indicated that the lattice constant of the B-doped layer slightly decreased for the B/C ratios <= 3000 ppm while slightly increasing for that of 5000 ppm, suggesting that for the latter HBD sample a part of the incorporated B atoms behaved differently from the remaining other B atoms. By contrast the Hall data indicated that all the HBD samples had a degenerate feature only at temperatures well below room temperature (RT), above which a semiconducting feature was evident, and that the density of the degenerate holes steeply increased from 1.3 x 10(9) to 1.2 x 10(21) cm(-3) with increases in the incorporated B density, [B], from 1.2 x 10(20) to 5.9 x 10(20) cm(-3). This drastic change in the hole density strongly suggested the presence of a [BI-dependent impurity band. Their evident near-band-edge CL spectra taken at RT and 85 K demonstrated that radiative transition features in the HBD layers considerably varied for the B/C ratios studied. The CL peaks were consistently assigned by assuming both the presence of an impurity band and a slight bandgap shrinkage. These observed features are discussed in relation to the energy separation between the low mobility impurity band assumed and the valence band in the high quality HBD layer which are not merged in energy. (C) 2015 Elsevier B.V. All rights reserved,