Thermally induced structural change of a-Ge:H/a-GeNx multilayer structures

Abstract

Multilayer structures of a-Ge:H/a-GeNx and a-Ge/a-GeNx were prepared by a reactive-sputtering technique and their structural stability was studied through thermal anneals. High-resolution transmission-electron-microscope analyses show that crystallization takes place only in a-Ge:H(a-Ge) layers without disturbing atomically smooth and uniform a-Ge:H(a-Ge)/ a-GeNx interfaces. The crystallization temperature of a-Ge:H(a-Ge) layers increases with either decreasing thickness of those layers or increasing thickness of a-GeNx layers. The rise in crystallization temperature is most remarkable when the layer thickness becomes smaller than a couple of hundred angstroms. On the basis of the experimental results, the structural stability of multilayer films is discussed in the light of classical nucleation theory using the free-energy change of the system. It is demonstrated that the crystallization temperature is strongly affected both by layer thickness and by the nature of a heterointerface which phenomenologically explains well the experimental observations.