Electron irradiation induced crystallization of amorphous MgAl 2O 4

Abstract

Using transmission electron microscopy (TEM), an electron beam was used to irradiate cross-sectional TEM specimens of an amorphous MgAl 2O 4 thin film on a single crystal spinel substrate. Amorphous MgAl 2O 4 thin films of 160 nm thickness were initially formed by 400 keV Xe ion irradiation at liquid nitrogen temperatures in single crystal spinel substrates. It was observed that electron irradiation at 300 keV induced a phase transformation from an amorphous to a metastable crystalline structure. Crystallization of amorphous MgAl 2O 4 was observed to occur both at the amorphous/crystalline interface by epitaxial growth and in the amorphous region by nucleation and growth. Under large doses of electron irradiation, a polycrystalline metastable phase was formed over the entire amorphous region. In contrast, thermal annealing at 600 °C for 1 h only produced negligible crystallization at the amorphous/crystalline interface. The major driving force for crystallization is the energy difference between the amorphous phase and the metastable crystalline phase. Electron irradiation provides the necessary defects to kinetically drive the transformation.