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

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.