Mapping of internal photoemission in metal-insulator-metal heterosystems

Internal photoemission (IPE) in heterosystems is determined by light absorption and electron hole (e-h) pair generation in the layers of the systems. Lateral inhomogeneities of the dielectrical properties, layer thickness variations or local defects can for example cause lateral variations in the IPE signal. With photon energies smaller than the band gap of the insulator one can selectively probe e-h pair generation in the back and top metal layers. For the lateral mapping of IPE a Ag-TaO-Ta heterosystem is raster scanned across the focus of a Schwarzschild objective (NA=0.4). The illumination with 400 nm laser radiation results in a focus diameter of 7 μm limiting the spatial resolution of the IPE microscopy. Mapping across the 70 μm wide edge of the top metal film shows transport effects in the top silver electrode as well as the increasing excitation of carriers in the tantalum backelectrode with decrasing top electrode thickness. The variation of the top electrode thickness and comparison to simulated excitation profiles allows the investigation of excited electron transport in the metal-insulator-metal junction. The IPE map shows photocurrent variations correlated and uncorrelated to the simultaneously measured backscattered radiation and thus reflects variations in the local optical excitation in the top electrode and in the local transport through the insulator. The combination of photocurrent spectroscopy with high spatial resolution provides a new tool for the characterization of buried interfaces.