**3. Al2O<sup>3</sup> for Insulated-Gate GaN Devices**

Table 1 reports the physical parameters of the energy bandgap (EG), conduction band offset (∆EC) and valence band offset (∆EV) obtained experimentally from amorphous Al2O<sup>3</sup> films deposited on GaN and AlGaN by various deposition methods. Note that the bandgap of the amorphous Al2O<sup>3</sup> ranges between 6.7 eV and 7.6 eV depending on the method of the oxide film growth, and it is lower than the value for the crystalline bulk α-Al2O<sup>3</sup> (8.8 eV–9 eV) considered in the theoretical calculations (Figure 3). In fact, it is well known that the E<sup>G</sup> of Al2O<sup>3</sup> compounds strongly depends on its crystallographic phase [96,97]. Momida et al. investigated the structure of amorphous Al2O<sup>3</sup> by first-principles calculations, concluding that the reduction of the bandgap of amorphous Al2O<sup>3</sup> compared to crystalline Al2O<sup>3</sup> could be related to the changes in the density of the Al2O<sup>3</sup> compounds and the average coordination number of Al atoms [98]. Toyoda et al. showed that annealing at temperatures of 800 ◦C led to phase transformations of the Al2O<sup>3</sup> films from amorphous to crystalline, which correlated to a significant increase in the energy bandgap and the modification of the conduction band discontinuity [99]. Afanas'ev et al. pointed out that for Al2O<sup>3</sup> films treated at temperatures above 800 ◦C, the widening of the Al2O<sup>3</sup> bandgap with the phase transformation from amorphous to crystalline mostly occurred at the valence band side [96,97]. Differently, Yang et al. revealed that the annealing processes at a lower temperature of 650 ◦C can affect the band bending of GaN but has almost no effect on the Al2O3/GaN band offset [100]. The decrease of the bandgap of amorphous Al2O<sup>3</sup> has also been associated with defect-induced states located in the bandgap [101]. This could explain the large discrepancy between the theoretical (Figure 3) and experimental (Table 1) values of ∆EV. In fact, since in the case of Al2O<sup>3</sup> the valence band maximum states are associated with the O 2p states, and the conduction band minimum states are associated with the Al 3s, 3p states [102], the rehybridization between Al 3s, 3p and O 2p modifies the charge transfer between Al and O and consequently decreases the bandgap, thus increasing the valence band maximum [51]. In contrast to ∆EV, the experimental values of ∆E<sup>C</sup> obtained for the Al2O3/(Al)GaN system are consistent with the theoretical predictions and make Al2O<sup>3</sup> a suitable dielectric for insulated-gate GaN-based transistors.

In addition to the physical properties of the bandgap of Al2O<sup>3</sup> and the band offsets in the Al2O3/(Al)GaN system, high-quality dielectric layers in terms of defects and bulk traps and an Al2O3/(Al)GaN interface with a low interface trap density are required to deliver a high performance and highly efficient MIS gate structure, as discussed above. It is important to mention that these properties strongly depend on the deposition technique and temperature, the crystalline structure of the film and the surface and annealing treatments [51]. Among the techniques explored for the deposition of Al2O<sup>3</sup> films, such as sputtering [103], the oxidation of a thin Al layer [53] and MOCVD [104–106], the ALD technique is widely used. The main advantages of the ALD method are the low deposition temperature (<350 ◦C), the excellent film thickness control as well as the high uniformity and conformality, which have enabled the deposition of high-quality Al2O<sup>3</sup> films and Al2O3/(Al)GaN interfaces compared to other methods. Nevertheless, despite substantial progress in the ALD technology, large amounts of defects in the as-deposited Al2O<sup>3</sup> bulk material and interface traps at the Al2O3/(Al)GaN interface are still present and still hinder the success of the insulated-gate GaN devices [1].

**Table 1.** Energy bandgap (EG), conduction band offset (∆EC) and valence band offset (∆EV) measured for Al2O<sup>3</sup> films on GaN and AlGaN. The deposition method is reported in the second column, where ALD = atomic layer deposition; PEALD = plasma-enhanced atomic layer deposition; CVD = chemical vapor deposition; MBD = molecular beam epitaxy; ECR = electron cyclotron resonance. In addition, the measurement method is noted in column 3, where C–V = capacitance–voltage measurements; F–N = Fowler–Nordheim characteristics; IPE = internal photoemission; XPS = X-ray photoelectron spectroscopy; UPS = ultraviolet photoelectron spectroscopy; XAS = X-ray absorption spectroscopy.

