X-ray structural study of amorphous Mo-Ge films

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Abstract
Atomic-scale structural changes with composition in sputtered amorphous Mo-Ge films with compositions from pure Ge to 70 at. % Mo have been studied with radial distribution function (RDF) and differential anomalous scattering techniques, extended x-ray absorption fine structure (EXAFS), and small-angle x-ray scattering. The complementary structural information from large-angle scattering and EXAFS was investigated. The combined techniques indicate that specific chemical ordering between Mo and Ge, much like that in the Ge-rich intermetallic compounds, plays a predominant role in determining the local structures in the Ge-rich amorphous material and continues to play an important role in the Mo-rich material. Three structural regions with composition are distinguished. Structural region I, the semiconductor-metal transition region, extends from a-Ge to about 23 at. % Mo. A Mo-modified amorphous structure with distinct local order like that in the Ge-rich compounds coexists with tetrahedral a-Ge on a very fine size scale and rapidly modifies the remaining tetrahedral a-Ge with continued addition of Mo. All indications of tetrahedral a-Ge disappear at about 23 at. % Mo. Region II extends from about 23 to roughly 50 at. % Mo. RDF’s and EXAFS show little change with composition in the average structure across this region, which is characterized by strong ordering of Ge about Mo at short distances, long Mo-Mo first-nearest-neighbor distances, and a lack of preferred Ge-Ge distances. The collapse of the long Mo-Mo first-nearest-neighbor distances delimits regions II and III and occurs as the material becomes Mo-rich. At 65 at. % Mo, an RDF and a Mo differential distribution function (DDF) look much like RDF’s of typical melt-quenched metal-metalloid glasses. Ge EXAFS and DDF’s change little between regions II and III, suggesting that the Mo-Ge interaction persists as the material becomes Mo-rich.