*2.2. Growth of Amorphous High-κ Oxides on GaN-based Materials*

The surfaces of GaN-based materials (GaN, AlGaN, InGaN, etc.) are typically characterized by the presence of large concentrations of defects (e.g., nitrogen vacancies, structural/morphological imperfections, residual contaminations, etc.) that can result in large leakage current and low performance and device reliability. Kerr et al. [67] demonstrated by density functional theory simulations that the defect sites, such as Ga dangling bonds and Ga-Al metal bonds, are responsible for the formation of states in the band gap. These interfacial trap states could be removed by annealing procedures before or after gate dielectric deposition. Moreover, especially from the perspective of high-κ gate oxide deposition, the removal of contaminations is crucial for increasing the density of precursor nucleation sites. Hence, pre-deposition surface treatments are needed to improve high-κ oxide quality. Systematic studies [68–76] have reported on the effect of several pre-treatments, and the principal cleaning/activation methods have been based on the use of wet chemical solutions [68–72,77,78] or plasma/gas actions [73–76]. Generally, the piranha (H2O2:H2SO4) solution is used for the cleaning of carbon contaminations, but some oxidation of the nitride surface can occur [70,71]. On the other hand, chloride acid (HCl) solution is efficient for the removal of metallic contaminations (eventually present from device processing) or residual oxygen on the surface. However, chlorine itself could be a residual contamination of the system [70]. Finally, hydrofluoric acid (HF) treatment is effective for the elimination of unwanted native oxide formation but is not efficient for carbon contamination [70,71]. Brennan et al. [71] compared the nucleation efficiency of the Al precursor with/without the cleaning of the surface by sequential use of acetone, methanol, isopropanol, and HF 2% solution. It was clear, from the results of an XPS study after each ALD cycle, that the decrease in the Ga-O concentration induced by the HF etch resulted in a stronger interaction between the Al precursor and the Ga surface. Nepal et al. [69] compared the effects of three different chemical solutions (i.e., piranha, diluted HF, and diluted HCl), finding that: (i) the single HCl pre-treatment provides 10–30 nm-sized particles, indicating a three-dimensional nucleation; (ii) the HF-based treatments produced an improvement in the electrical behaviour; (iii) the best dielectric properties, in terms of smaller hysteresis and lower density-trap state values, were obtained on the piranha-treated surface. Finally, Schilirò et al. [72] showed a comparison among several chemical solution combinations (i.e., piranha, HCl/ HF, and piranha/HF). In particular, it was shown that, although the Al2O<sup>3</sup> thin films treated with each solution possessed identical structural properties, adherent, uniform, and amorphous, there were some intrinsic differences depending on the adopted surface pre-cleaning. In fact, under a TEM electron beam, the films deposited after piranha treatment showed the formation of polycrystalline grains, while epitaxial layers were formed for samples deposited after HF based treatments. This was an indication that in the case of HF-based treatments, the deposition process occurred on a very clean AlGaN surface, which could act as seed layer for the formation of epitaxial films. Moreover, investigation of the initial growth stages by AFM demonstrated that the smallest three-dimensional grain nucleation resulted in deposition on HF-HCl-treated surfaces, which could ensure a cleaner surface in order to allow ideal layer-by-layer ALD growth.

It could be concluded that the pre-deposition treatments of GaN-based surfaces with HF cleaning provided Al2O<sup>3</sup> films with the best dielectric properties [69,71,72].

An alternative route to cleaning by chemical solution is represented by "in situ" cleaning process based on H2/N<sup>2</sup> (forming gas) or NH<sup>3</sup> plasma actions [68]. The impact of N<sup>2</sup> and forming gas on the growth and interfacial characteristics of Al2O<sup>3</sup> on AlGaN/GaN heterostructures was explored by Qin et al. [73], who demonstrated by XPS investigation that C contamination was effectively reduced by both N<sup>2</sup> and forming gas plasma. The latter also decreased the number of Ga-O bonds, improving the Al2O<sup>3</sup> nucleation. In regard to plasma action effects before high-κ deposition, the same group contributed with a large

variety of studies [73,75,76]. In particular, the effects of O2, N2, and forming gas plasma annealing were evaluated, comparing the electrical behaviour in terms of interface state density with the results obtained by XPS analyses. The formation of oxynitride bonds (Ga-O-N) increased the number of interface defects and that among all the studied treatments, the forming gas action was the most efficient.

In this context, it has to be emphasized that the semiconductor surface preparation and the deposition conditions may induce different insulting behaviours after the first film growth stages. As an example, Schilirò et al. [79] recently reported different behaviour in the early growth stages of Al2O<sup>3</sup> thin films deposited on AlGaN/GaN heterostructures by thermal or plasma-enhanced ALD. In particular, they provided evidence that the PE-ALD process occurred under ideal layer-by-layer growth because of the efficiency of the O2-plasma agent, which acted directly on the Al precursor. On the other hand, the T-ALD approach resulted in a nucleation process of the Al2O<sup>3</sup> film similar to the island-growth model and a higher susceptibility to charge trapping [79].

Summarizing, surface preparation prior to high-κ oxide deposition is a crucial issue, including in the case of GaN-based materials, and can be carried out by many procedures. The aim is the cleaning of C residues, which are detrimental for the oxides' nucleation, and the elimination of Ga-N-O bonds, which are the main centres of interfacial electronic defects. These two issues are generally addressed by non-oxidizing plasma action or by HF treatments.
