Tofail et al. 3 have found that the dipole of HA is the hydroxyl (OH) ion, which lies along the crystallographic c-axis within the tunnel formed by phosphate (PO 4) tetrahedra. In adjacent tunnels, the OH ions could point in a parallel direction or in an anti-parallel one. We believe that the high-temperature calcination crystallized the HA film on silicon and converted most of the OH pairs to a parallel configuration. Thus the HA developed a domain structure with randomly oriented dipoles. Upon cooling, sufficient domains reoriented in polarity so as to result in a net polar structure. This is due to the crystalline character of the silicon substrate and texturing. The photocurrent is caused by the silicon alone.