Plasma enhanced atomic layer deposition of Al2O3 gate dielectric thin films on AlGaN/GaN substrates: The role of surface predeposition treatments

Abstract

Al2O3 thin films were deposited by plasma enhanced atomic layer deposition (PEALD) from trimethylaluminum precursor and oxygen plasma at 250 °C on AlGaN/GaN heterostructures. Before deposition, the sample surfaces were treated with the following solutions: (A) H2O2:H2SO4 (piranha), (B) fluoride acid (HF) + HCl, and (C) piranha + HF for 10 min each. Transmission electron microscopy analysis revealed that, independently from the surface preparation, all the films are adherent and uniform with thicknesses of about 27–28 nm. However, a different structural evolution has been observed under electron beam effect. In particular, while all the as-deposited films were found to be amorphous, the formation of polycrystalline grains was observed on the sample deposited after the A treatment. On the other hand, oriented layers were formed on the samples deposited after B and C treatments. This result is an indication that in the case of HF-based treatments, the PEALD occurred on a very clean AlGaN surface, which can act as seed layer for the formation of epitaxial films when some energy is provided. Atomic force microscopy (AFM) showed smoother Al2O3 films when grown after HF based treatments. Moreover, further AFM investigation of the initial growth stages (3 nm thick Al2O3 layers) demonstrated that the smallest three-dimensional grain nucleation resulted in deposition on C-treated surfaces. Finally, the predeposition treatments of AlGaN/GaN surfaces with the HF cleaning provided Al2O3 films having the most promising dielectric properties.