A set of phosphorus doped CVD diamond films was investigated by infrared ab
sorption spectroscopy. The photo-ionisation continuum intensity increases c
onsistently with the phosphorus concentration. Two peaks, at 523 and 562 me
V, were observed for the first time, and are attributed to electronic trans
itions from the phosphorus ground level to the 2P(0) and 2P(+/-) excited st
ates, in good agreement with the effective mass approximation. The optical
ionisation energy deduced is 600 meV (+/-20 meV), consistent with Hall effe
ct measurements. (C) 2000 Elsevier Science Ltd. All rights reserved.