Initial phase formation in Nb/Si multilayers deposited at different temperatures

Abstract

The initial phase formation in Nb/Si multilayers deposited at 25 and 200 °C was studied by high-resolution transmission electron microscopy and x-ray diffraction. When Nb/Si multilayers were deposited at 25 °C, the multilayers with a modulation period L of 4 nm were in the amorphous state, with Nb-rich amorphous silicide layers and Si-rich amorphous silicide layers. The multilayers with a modulation period L of 100 nm also had an amorphous structure, consisting of intermixed layers of amorphous Nb silicide between the amorphous Si (a-Si) and amorphous Nb (a-Nb) layers. The initial amorphization reaction in Nb/Si multilayers is thermodynamically and kinetically favored. When the multilayers were deposited at 200 °C, a crystalline cubic Nb3Si phase with AuCu3 structure was formed in the multilayer samples. The interfacial energy and modified heat of formation are used to explain why the crystalline phase is formed and Nb3Si is the first phase formed during deposition at so low a temperature.