Structural and optical properties of MgxAl1-xHy gradient thin films:: a combinatorial approach

The structural, optical and dc electrical properties of MgxAl1-x (0.2 <= x <= 0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the MgxAl1-x layer undergoes a metal-to-semiconductor transition and MgH2 is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH2 and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction H-1(N-15,alpha gamma)C-12. Besides MgH2, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x=0.33). This suggests that the film is partially transformed into Mg(AlH4)(2) at around this composition.