We deposited hydroxyapatite (HAp) thin films on Si(1 0 0) substrates by means of electron cyclotron resonance (ECR) plasma sputtering from a HAp target and characterized their structural properties by X-ray diffraction (XRD) and Fourier transform infrared absorption spectroscopy. Deposition in the presence of an H2O vapor at room temperature incorporated H2O and OH species in the deposited films. Post-annealing in an O-2 ambient self-organized OH- and PO43- functional groups in HAp crystals. The XRD patterns revealed randomly orientation when the annealing temperature ranged between 700 and 900 degrees C. In contrast, preferentially c-axis-oriented HAp crystals nucleated after prolonged annealing at 550-600 degrees C. The possible scenario for the preferred orientation is that C-plane terminated HAp crystallites were initially created in the near-surface region, and the following crystallization proceeded exclusively on the seed surface. After post annealing in a vacuum or in an Ar gas ambient at 900 degrees C, films were reduced into tricalcium phosphate, increasing photoabsorption in the infrared range. (c) 2013 Elsevier B.V. All rights reserved.