Early Growth Stages of Aluminum Oxide (Al₂O₃) Insulating Layers by Thermal- and Plasma-Enhanced Atomic Layer Deposition on AlGaN/GaN Heterostructures

In this paper, a comparative study of the early growth stages of aluminum oxide (Al2O3) thin dielectric layers deposited by thermal- (T-ALD) and plasma-enhanced atomic layer deposition (PE-ALD) on AlGaN/GaN heterostructures is presented. In particular, the evolution of the morphological features and the electrical properties of all the deposited Al2O3 layers has been investigated at the nanoscale by conductive-atomic force microscopy and correlated to electrical measurements on metal–insulator–semiconductor capacitors. The different insulating characteristics and their relative evolution upon increasing the film thickness are an indication of the occurrence of two different ALD growth mechanisms. As a matter of the fact, chemical characterization by X-ray photoelectron spectroscopy also corroborated the nanoscale electrical results and provided evidence that the PE-ALD process occurs under an ideal layer-by-layer growth because of the efficiency of the O2-plasma agent which acts directly on the Al precursor. The T-ALD approach, in contrast, shows a nucleation process similar to the island growth model. Moreover, the interface defects have been imaged at the early growth stages, and their evolution has been recorded as a function of deposition approaches. The detailed investigation at the nanoscale provided the quantification of the oxide trap charges and the evaluation of their effects on the final insulating properties upon increasing the film thickness. Since their limitation is a crucial issue for the correct operation of electronic devices, the present investigation demonstrated that PE-ALD is the proper deposition process.