Electron beam-induced crystallization of Al₂O₃ gate layer on β-Ga₂O₃ MOS capacitors

Electron irradiation was observed to induce crystallization of amorphous Al₂O₃ films grown by atomic layer deposition on β-Ga₂O₃ substrates. Growth of large, strongly oriented crystalline γ-Al₂O₃ regions was induced using conventional-mode transmission electron microscopy (TEM) and observed to propagate outward from the interface as well as from the previously crystallized Al₂O₃. A few nm of epitaxial Al₂O₃ was already visible at the beginning of the crystallization front propagation. The phenomenon is not explained by electron beam-induced heating, which amounted to less than 1 K at all times. Direct measurement of the beam current permitted quantitative correlation between electron dose rates and crystallization rates. Enlarging the electron beam to reduce current density was found to slow the propagation of the crystallization front. Furthermore, a factor of 4 smaller electron dose was required for a given rate using 100 keV electrons as compared to 200 keV, indicating that crystallization is driven by ionization-induced atomic rearrangement within the gate layer. Lattice spacing between the oxygen sub-lattices of β-Ga₂O₃ and γ-Al₂O₃ are favorable for the nucleation of crystallites at the interface. Multivariate statistical analysis of electron energy loss spectroscopy (EELS) data also showed evidence of diffusion between Al and Ga in the substrates and gate oxides, respectively. These structural transformations at the semiconductor-insulator interface are expected to influence the device electrical behavior and are relevant to the continued refinement of β-Ga₂O₃ device technology.