Electron transport in epitaxial metal films studied by ballistic electron emission microscopy and the resisitivity measurements.

摘要

The studies of electron transport in thin metallic films are motivated by a need to understand how electron reflections at metal surfaces and interfaces affect the resistivity of thin interconnect lines in chips. In this Thesis, we have characterized the resistivity of thin Al(111) and Cu(111) films that were grown on CaF₂/Si(111) by molecular beam epitaxy (MBE), and performed ballistic electron emission microscopy (BEEM) investigations of thin Al films. The structure of the films was studied by reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and transmission electron microscopy (TEM).; It was found that nearly single crystal epitaxial Al grows directly on CaF₂, while epitaxial Cu can be grown only on an Al seed layer. The resistivity increase in thin Al films agreed with a model that assumed complete diffuse electron scattering at surfaces and a value of 15.5 ± 1.5 nm for the mean free path of the conduction electrons. For Cu, the resistivity increase in thin films was too large to be explained by diffuse surface scattering, and was attributed to scattering at grain boundaries.; In the course of studying thin film resistivity, we have also developed a technique to measure thickness of nanometer-thick films by AFM, a method to initiate the CaF₂ growth without exposing bare Si surface to vacuum, and a simple formula to estimate influence of thickness inhomogeneity on the effective film resistivity.; The BEEM experiments on Al films deposited on Au/Si, aimed at obtaining more information about the electron transport in Al, resulted in the hot electron attenuation length values between 1.4 nm and 1.8 nm. Such small values were attributed to high defect density in the samples, resulting from alloying between Al and Au.; On the other hand, we have shown theoretically that the BEEM attenuation length may be several times shorter than the resistivity-measured mean free path, even if the main scattering mechanism is due to phonons in both cases. These calculations highlighted different sensitivity of the two techniques to small-angle scattering events, and necessity to consider angular dependence of scattering probability in simulations of electron transport in metals.