Boron-related infra-red absorption in homoepitaxial diamond films

In this paper, the evolution of the infra-red absorption of high-quality epitaxial diamond films versus boron concentration is reported. Homoepitaxial diamond films were grown by microwave-plasma-assisted chemical vapour deposition on type Ib crystals. From 2 x 10(17) to 8 10(20) cm(-3) of boron were incorporated in the diamond from the gas phase during growth. The main absorption features are (1) a single phonon absorption around 160 meV, (2) a series of three lines at 304, 347 and 363 meV due to excited states of the bound hole, (3) a photoionisation continuum, and (4) a phonon replica of these electronic transitions. The evolution of these absorption with the boron concentration is presented and discussed. A strong increase in the photoionisation cross-section for boron concentrations above 5 x 10(18) cm(-3) is observed. New linear relationships between the boron concentration and the integrated absorption at 347 meV and the photoionisation are proposed. From the evolution of the lines due to transitions to boron-excited states, we deduced the width of the impurity band versus the boron concentration. (C) 1998 Elsevier Science S.A. All rights reserved.