Schottky barrier formation at metal -quantum well interfaces studied with ballistic electron emission microscopy

A number of possible near- and long-term semiconductor device technologies rely on abrupt metal-semiconductor interfaces with nm-dimensions, and/or with internal nm-scale inhomogeneity. It is therefore very important to be able to probe the electronic properties of these buried interfaces with sub-10 nm resolution and to find out the impact of "small-size" effects on their transport properties.;In our study we used Ballistic Electron Emission Microscopy (BEEM) and finite-element electrostatic modeling to quantify how "small-size" effects modify the energy barrier at metal/semiconductor quantum wells (QWs), formed by making Schottky contacts to cleaved edges of GaAs. Our model semiconductor heterostructure is formed as a sequence of AlGaAs/GaAs/AlGaAs layers and contains a sequence of GaAs quantum wells with thickness between 1 and 15 nm. Thin Au films were deposited on the cleaved side of these heterostructures using photolithography techniques or shadow mask evaporation. The Schottky barrier height (SBH) measurements as a function of QW thickness showed that the SBH value increases as the QW thickness is decreased, by up to ∼140 meV for a 1 nm thick QW. This is mostly due to a large quantum-confinement increase (∼200 meV for a 1nm QW), modified by smaller decreases due to "environmental" electric field effects. Our modeling gave excellent quantitative agreement with measurements for a wide range of QW thickness when both these effects are considered. Our results show that "small-size" effects such as quantum confinement and environmental electric field effects strongly influence the transport properties of nm-small contacts.;In a separate study, the cleaved QW were used as nm-apertures to estimate the resolution of BEEM as a function of metal film thickness. We found that BEEM resolution degrades as the top metal film layer is made thicker, from ∼12 nm for a 4 nm thick Au layer, up to ∼22 nm for a 15 nm thick Au layer. These estimates of BEEM resolution are much larger than several early experimental estimates, but are consistent with theoretical models of the lateral spreading of hot electrons in the metal film that include multiple hot-electron scattering at interfaces and in the bulk of the metal film.;We also proposed a theoretical BEEM model that explicitly considers the hot electron spatial distribution at the metal-SC interface, spatial variations in the metal-SC interface transmission function and the quantum confinement effects inside the QWs. (Abstract shortened by UMI.).