Heteroepitaxial growth of tungsten oxide films on silicon(100) using a BaF2 buffer layer

Multidomained heteroepitaxial WO, films were grown on Si(100) substrates using a (111)-oriented BaF2 buffer layer at the WO3-Si interface. The 30-nm-thick BaF2 layer, grown by very low rate molecular-beam epitaxy, consisted of four equivalent crystalline domains oriented about the BaF2[111] axis, which provided templates for heteroepitaxial WO3 film growth. The WO3 films were grown by electron cyclotron resonance oxygen plasma-assisted electron beam evaporation of a WO3 source, and the temperature range was varied between 25 degreesC and 600 degreesC. At an optimal deposition temperature of approximately 450 degreesC, monoclinic-phase WO3 films were produced, which consisted of coexisting (002), (020), and (200) in-plane orientations with respect to the BaF,(111)/Si(100) substrate. During growth, an interfacial barium tungstate (BaWO4) reaction product formed at the WO3-BaF2 interface. The{112} planes of this BaWO4 layer also have a multidomained heteroepitaxial orientation with respect to the BaF2(111) buffer layer. Postdeposition annealing experiments in air for 24 h at 400 degreesC indicated that the heteroepitaxial BaWO4 and WO3 layers remain stable. A thermodynamic argument is used to explain the BaWO4 interfacial reaction during initial growth stages, and kinetically limited diffusion processes through the BaWO4 layer coupled with lattice matching across the WO3-BaWO4 interface are proposed to be responsible for the formation of stable WO3 films at later growth stages.