Interfacial scattering of hot electrons in ultrathin Au/Co films

Abstract

We have used room-temperature, ballistic electron emission microscopy (BEEM) to measure hot-electron transport through ultrathin Au/Co multilayer structures deposited onto Si. The samples consist of Au/Co/Si or (Au/Co)n/Au/Si diodes, sputter deposited at 175 or 300 K, where n is the number of repeat layers. The thin-film Co attenuation length, λCo, is extracted from the BEEM spectra as a function of Co thickness, in single Co layer samples. Similarly, the interface attenuation number, or the number of Co/Au interfaces required for a 1/e attenuation, is determined from the multi-interface samples. BEEM barrier heights of Au/Co/Si decrease with increasing Co thickness (for thicknesses <1 nm), as the film becomes continuous and develops a Schottky barrier for Co or CoSi2(<0.7 eV). For these diodes, λCo, increases from 0.3 to 0.5 nm, each with an estimated uncertainty of 0.1 nm, when the deposition temperature is decreased from 300 to 175K. This result is associated with decreased silicide formation at the lower deposition temperature. When Co is isolated from the Si with a 2 nm Au layer, the barrier height is stable (0.82 eV), and λCo, increases further to 0.8±0.1 nm. The λCo values are independent of electron energy over the range measurable (1–1.8 eV). The interface attenuation number was 1.8 and 25 interfaces for 0.6 and 1.2 nm total Co thickness, respectively (in structures with a constant total Au thickness of 8 nm). Thus, significant interface scattering is observed in the thinner sample and negligible scattering when the Co thickness was doubled. Further BEEM measurements on similar superlattices are being carried out to understand these results.