Imaging and spectroscopy of Au nanoclusters in yttria-stabilized zirconia films using ballistic electron/hole emission microscopy

Abstract

Ballistic Electron/Hole Emission Microscopy (BEEM/BHEM) has been applied to imaging and spectroscopy of Au nanoclusters (NCs) embedded into ultrathin (≈6 nm thick) yttria-stabilized zirconia (YSZ) films on Si substrates prepared using magnetron sputtering and annealing. Such structures are promising for resistive switching in non-volatile memory applications. According to the High Resolution Cross-sectional Transmission Electron Microscopy (HR X-TEM) data, the nearly spherical Au NCs of 1.5 to 3.5 nm in diameter were arranged almost in a single sheet between the YSZ layers. The BEEM images demonstrated the spots of increased collector current of 1 to 2.5 nm in size which are related to ballistic electron tunnelling via the Au NCs. The BEEM/BHEM spectra of the NCs demonstrated stepwise features attributed to the quantum confined states in the Au NCs. By using the best fit between the quantum confined energy states determined from the BEEM/BHEM spectra, the estimate of the NC diameter and the ones calculated for a spherical quantum dot yielded 1.7 to 3.3 nm which is consistent with the HR X-TEM data. The results of the present study demonstrate the capabilities of BEEM/BHEM in characterizing ultrathin dielectric films with metal NCs in research and also in industry.