Kinetics of interfacial microstructural variation across insulator-thermoelectric semiconductor interface and its effects on thermoelectric properties of magnesium silicide thin films

Abstract

Thermoelectric semiconductor magnesium silicide epitaxial films were fabricated on insulating (001) alumina (Al2O3) and (111) magnesium oxide (MgO) substrates using low temperature (220 °C) RF magnetic sputtering followed by high temperature (500 °C) annealing. Cross sectional studies of interfaces by bright field transmission electron microscopy reveals the formation of an interfacial region over a length scale of ∼30 to 120 nm for films deposited on MgO substrate and magnesia buffered Al2O3 substrates. A more detailed analysis using scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) studies clearly showed that Mg2Si film grown on magnesia buffer and MgO substrate indicate the formation of O and Si rich interfacial layer over a range of length scales (30–120 nm), while there is no such formation for the film deposited on Al2O3 substrate. The electrical conductivity of the film deposited on alumina substrate shows ∼60% higher value than the film on magnesia substrate. Further, the Seebeck coefficient measurement showed almost similar magnitude, at the same time of opposite polarity between film on alumina substrate (positive) and the film on magnesia substrate (negative). Changes in the carrier concentration arising due to the formation of an interfacial layer can be primary influence for the observed anomaly between two films. The theoretical modeling based on standard space charge kinetics across the multi-layer structures comprising alumina substrate-magnesia buffer-Mg2Si thin film, clearly indicates the possibility of oxygen migration from magnesia buffer layer when annealing temperature is increased above ∼750 °C.