Morphology and resistivity of Al thin films grown on Si (1 1 1) by molecular beam epitaxy

Abstract

Thin films of aluminium metal with varying thickness between 10 and 200 nm were grown on (1 1 1) Si substrates at 250 °C under UHV conditions using molecular beam epitaxy (MBE). Grown thin films were characterized by in situ X-ray photoelectron spectroscopy, and ex situ X-ray diffraction, atomic force microscopy and temperature-dependent electrical resisitivity measurements. The results showed that (i) films grow via 3D-island Volmer–Weber growth mechanism, (ii) with increasing film thickness the average grain size increases and the coalescence takes place for thickness >60 nm, and (iii) independent of the thickness, films grow with (1 1 1) orientation. The room-temperature value of resistivity contrary to the predictions of existing theoretical models is found to increase monotonically up to a thickness of 40 nm. This anomalous feature was understood in terms of the film morphology, whereby charge transport takes place via variable range hopping (VRH). For film thickness ⩽50 nm, the resistivity vs. temperature curves exhibited a metal-to-insulator (M–I) transition at low temperature, and analysis of insulating region supported the 2D-VRH mechanism of charge transport. However, for thickness ⩾60 nm the resistivity decreased sharply and the M-I transition disappeared. The bulk value of resistivity (2.59 μΩ cm) was obtained for thickness >200 nm.