Correlation between morphology and transport properties of Au/Co/Au/Si wedge ultra-thin film

Abstract

We report the thermoelectric power (TEP) and electrical resistivity characterization supported by structural and surface morphological properties on a wedge shaped ultra-thin sample of Au/Co/Au/Si (100) with Co have varying thickness from 0.5 nm to 4 nm across the sample. The sample was prepared by Ion Beam Sputtering method, with an ultrathin layer of Co sandwiched between two layers of Au (3 nm thicknesses each). The observed TEP results showed that the system is semiconductor in nature and the concentration of the charge carrier is inversely proportional to the Seebeck coefficient (S) and also concentration of charge carrier modifies with the film thickness leading to a change in thermo power, which is found to be higher at lower thicknesses of Co sandwiched layer. The type of charge carrier was also determined to be electrons in this system. The observed results are in good agreement with the resistivity measurement carried out as a function of film thickness indicating a decrease in resistivity as a function of film thickness due to enhancement in charge carrier mobility. The thermal activation energy was seen to increase from ∼29.5 meV to ∼280.3 meV as the film thickness increased from the minimum to the highest. Corresponding Atomic Force Microscopy (AFM) measurement revealed island type grain growth with the introduction of a significant amount of roughness, while Magnetic Force Microscopy (MFM) experiments exhibit randomly oriented magnetic domain structures. The overall results suggest that lowest thickness film shows semiconducting nature, while variable range hopping phenomena due to some localized energy levels near the valence and conduction band is observed in the highest thickness film. This result is also well supported by XRD measurement, which revealed crystalline nature of the films. Such low cost, easy to fabricate materials may find applications in Peltier cooling or energy harvesting applications without the harmful carbon dioxide emission. For this, advance experiments will be done in future.