Effects of Post-Deposition Processing on the Ultimate Grain Size in Metastable Semiconductor Thin Films to be Used in IR Detectors

ABSTRACTThrough post-deposition annealing in a differential scanning calorimeter (DSC), we have manufactured both thin (200 nm) and bulk (8000 nm) single phase films of crystalline Ge1–xSnx, using rf sputtering. The Sn concentrations produced ranged up to 31 at.%, well beyond the solid solubility limit of this system. There was a marked difference, in the asdeposited structure, between thick and thin films produced under the same deposition conditions. Quantitative models for both systems are given in this paper and were deduced frorn DSC measurements in conjunction with electron microscopy. The metastable crystalline state in the thin films formed by nucleation and growth from an amorphous phase; whereas in the thick films, the desired phase was already present in the as-deposited films and only growth of preexisting grains was observed upon post-deposition annealing. When annealed to high temperature, the Sn phase separates from the alloys and we postulate here that it does so by nucleation and growth of β-Sn. With this hypothesis, the Sn separation in the 8000 nm thick films was accurately modeled by a two-mechanism process, however, in the 200 nm thick films, only one phase separation mechanism was necessary to accurately fit the data. Both models were corroborated by the subsequent melting behavior of the phase separated Sn which, though it varied depending on the sample being measured, always exhibited a melting endotherm starting 25–35°C lower than the bulk melting temperature of Sn. Speculation on the reasons for this are presented.