Deposition parameter dependence of hydridation-dehydridation effects of bilayered magnesium-nickel/palladium thin films

Abstract

In this study, (i) Pd films deposited at various sputtering pressure OPd, and (ii) Mg-Ni/Pd bilayered films deposited at various relative sputtering powers of the two elements, substrate temperatures and thicknesses of Pd overcoats were prepared. Their elemental composition, structure and change of compound film resistivity during hydridation and dehydridation were measured and correlated. A Pd film deposited at a higher sputtering pressure OPd is more porous due to more severe scattering among particles in vacuum during deposition. During hydridation, the film structure experiences substantial volumetric expansion, so that some pores are closed to cause a drop in electrical resistivity p. The structure of a Pd film deposited at a lower OPd has denser structure, so that the change in p during hydridation is dominated by a metal-to-hydride transition, and is increased as a consequence. A method is developed to analyze XPS data for determining the elemental and phase composition of a Mg-Ni/Pd film. It is assumed that such a film is composed of a Mg-O phase, and a metallic phase consisting of Pd and a Mg-Ni phase. The relative fractions of these phases and the elemental composition of each phase are obtained as functions of depth according to the Pd3d5/2, Ni2p3/2, Mg1s and O1s photoelectron spectra, with the interference from Pd3p3/2 photoelectrons to be removed from the O1s spectrum. For Mg-Ni/Pd films, in general (i) the film structure formed at room temperature is highly disordered; (ii) the concentration of Pd is the highest at the top surface and drops with increasing depth; and (iii) a Mg-O layer is formed between the Pd overcoat and the underlying Mg-Ni layer. By using a larger sputtering power ratio PMg : PNi the film deposited has a higher overall Mg content. Interdiffusion between Pd and Mg is more readily to occur. A thicker Mg-O layer with a higher Mg-to-O ratio is formed. In the first hydridation process (12 hrs in 15% H2 in Ar), the film shows a larger fractional change of <p> (an increase of 328 times was observed for a film deposited at PMg: PNi = 5.2). The dynamic range of the change of <p> in subsequent hydridation-dehydridation cycles is also larger. For a film deposited at a smaller PMg : PNi ratio, the overall Mg content is lower. The diffusion range of Pd into the Mg-Ni layer is shorter. The Mg-O layer is thinner and the Mg-to-O ratio is lower. The Mg-to-Ni ratio in the Mg-Ni layer is smaller. It shows a smaller fractional change in <p> at the first initial hydridation process and a smaller dynamic range of the change in <p> in the subsequent hydridation-dehydridation cycles. Crystallization of Mg2Ni occurs at substrate temperature Ts = 350 oC. A film deposited at a higher Ts has a thinner Mg-O layer since oxygen is supposed to be more difficult to diffuse in the crystallized structure. The Mg-to-Ni ratio in the Mg-Ni phase is smaller since Mg is more readily to be re-evaporated during deposition. It also has a lower as-deposited <p>. However, <p> does not have any observable change in an hydrogen-containing environment, because the migration of hydrogen in a crystallized film structure is more difficult than in a defective one. The addition of a Pd overcoat not thinner than 2 nm is important for a film to give switching of <p> by hydridation. However, the addition Pd overcoat as thick as 10 nm cannot eliminate the appearance of a Mg-O layer just under the Pd layer. With a thicker Pd overcoat, the fractional change of <p> in first hydridation and the dynamic range of the change of <p> in subsequent hydridation-dehydridation cycles are smaller, because the Pd layer give rise to more significant electrical shorting effect to reduce the overall apparent <p> value of the compound film.