In our recent study Xu et al (2002 Chem. Phys. Lett. 364 57-63), a phase transformation from the hexagonal to the tetragonal structure in the annealed ZnO films on silicon was studied by atomic force microscopy. Cathodoluminescence (CL) and glancing-angle x-ray diffraction analysis of the ZnO films indicated that such a transformation is due to the generation of a tetragonal zinc silicate. In order to identify the formation mechanism of the zinc silicate and the bottom broadening of the UV band, a depth profile secondary ion mass spectroscopy experiment was carried out. The results show that vast atomic diffusion between the ZnO film and the silicon substrate occurred due to the annealing temperature. Such interdiffusion can create not only a mixed crystal of ZnO and Zn 2 SiO 4 , but also an amorphous silicon dioxide (a-SiO 2) in a deep range from the surface to the interface of the ZnO/Si system. The a-SiO 2 is most probably the source of the 453 nm blue band hidden in the tail of the 390 nm UV band, since the blue band agrees with the CL spectra of the amorphous quartz glass and the thermally oxidized silicon. ZnO film has been widely studied for a variety of applications in piezoelectric acoustic wave devices [1, 2], varistors [3, 4], optical waveguides [5], substrates or buffer layers for the growth of GaN [6, 7], or as a material for light-emitting diodes [8]. In addition, ZnO deposited on silicate glass has been widely used as a transparent conducting oxide buffer in the construction of semiconductor film solar cells [9]. A ZnO/Si heterojunction was also investigated as a candidate for a mono-junction solar cell [10]. Under such conditions, it is necessary to
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