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

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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.