*3.1. Al2O3/(Al)GaN Structures*

The presence of defects acting as traps or fixed charge centers within the Al2O<sup>3</sup> films and at the Al2O3/(Al)GaN interface is of critical importance because of their potential to affect the threshold voltage and the gate leakage currents of the MIS gate structures [51], eventually deteriorating the operational stability and the reliability of the insulated-gate GaN-based devices.

For ALD-Al2O3/(Al)GaN structures, a positive fixed charge arising from donor-type interface states and/or defect levels in the bulk Al2O<sup>3</sup> was often reported [112–115]. In this regard, Esposto et al. [107] and Son et al. [116] pointed out that fixed charges at the Al2O3/GaN interface shifted the flat-band voltage (VFB) in the C–V curves of Al2O3/GaN capacitors. A shift of the VFB towards the negative bias direction in Al2O3/GaN structures was observed by Kaneki et al. [115]. Similar shifts in the C–V characteristics attributed to interface states acting as fixed charges were reported for Al2O3/AlGaN/GaN structures by Mizue et al. [26] and Yatabe et al. [73]. Nishiguchi et al. [38] reproduced the observed negative shift in the C–V curve of Al2O3/AlGaN structures, assuming an effective fixed positive charge of +1.2 <sup>×</sup> <sup>10</sup><sup>13</sup> cm−<sup>2</sup> in the Al2O<sup>3</sup> layer or at the Al2O3/AlGaN interface.

In line with this, annealing treatments have been reported to affect the VFB and Vth of Al2O3/(Al)GaN structures as a result of a change in the defect levels in Al2O<sup>3</sup> films [117,118]. For example, Hashizume et al. [114] reported a VFB recovery of Al2O3/GaN structures after a postmetallization annealing (PMA) in N<sup>2</sup> at 200–400 ◦C, possibly attributed to the reduction of the donor-type interface states and/or the defect levels in the bulk. Similarly, Hung et al. [119] obtained a VFB recovery by PMA in H2/N<sup>2</sup> forming gas at 400–550 ◦C. Zhou et al. [120] showed a permanent positive shift of the Vth in ALD-Al2O3-gated MIS-HEMTs after a postdeposition annealing (PDA) at 600 ◦C in N2, which was also suggested to be caused by a reduction of the deep-level bulk or interface traps. For similar reasons, a recovery of the Vth of MIS-HEMTs towards positive bias values was reported by Nishiguchi et al. [38] when using a reverse-bias anneal at 300 ◦C in air, and by Nakazawa et al. [121] with an anneal process at 750 ◦C in O<sup>2</sup> atmosphere.

The exact nature of the fixed charges in the bulk of the as-deposited Al2O<sup>3</sup> or in the vicinity of the Al2O3/(Al)GaN interfaces is still under debate, with native defects in the oxide layer or dangling bonds at the interface being the major candidates. Choi et al. [122] investigated the impact of native point defects in Al2O<sup>3</sup> by first-principle calculations, revealing that oxygen vacancies introduce charge-state transition levels near the GaN conduction band edge, which can act as border traps close to the Al2O3/n-GaN interface or as source of leakage current through the dielectric. However, other defects such as aluminum vacancies and interstitials have been identified to act as fixedcharge centers [122]. Weber et al. [123] also suggested that aluminum vacancy and oxygen interstitial defects introduce negatively charged centers while the aluminum interstitials act as positively charged centers, affecting carrier scattering in the channel and the threshold voltage of the device. Moreover, Liu et al. [124] studied the energy levels of the oxygen vacancy in Al2O3. Shin et al. [125] and Kim et al. [126] identified oxygen and Al dangling bonds as the origin of the fixed charges in ALD-Al2O3. Huang et al. [127] suggested that these defective dangling bonds, which are also associated to fixed positive charges and acceptor-like border traps, can be suppressed by the substitution of H2O as an oxygen source with O<sup>3</sup> for the ALD deposition of Al2O3. Other groups have also demonstrated the influence of using different ALD precursors and different deposition temperatures on oxide charges, as well as the interface traps of Al2O<sup>3</sup> films [128–132].

Defect states inside Al2O<sup>3</sup> can affect the leakage current of the MIS gate structures through trap-assisted tunneling mechanisms. For Al2O3-gated MIS-HEMTs under forward bias, Liu et al. [133] and Yoshitsugu et al. [131] showed that trap-assisted tunneling (TAT) and Poole–Frenkel emissions (PFE) are dominant at medium electric fields and temperatures above 0 ◦C, whereas Fowler–Nordheim tunneling (FNT) dominates at high electrical fields and temperatures below 0 ◦C. In addition, Yoshitsugu et al. [131] estimated a TATrelated trap energy of about 1.0 eV below the conduction band minimum of Al2O3. Wu et al. [134] instead suggested that TAT is the dominant transport mechanism in high oxide fields, with trap energies of ~1.1–1.2 eV, while PFE was responsible for medium oxide field gate current transport. Recently, Heuken et al. [135] also suggested that the time-dependent dielectric breakdown (TDDB) of ALD-Al2O<sup>3</sup> films occurs with the presence of an initial defect density in the film and is then related to the formation of a percolation path by randomly generated defects in the oxide under stress bias. The time to breakdown was found to be thermally activated, with an activation energy of 1.25 eV, similar to the reported values of the activation energy of TAT in Al2O<sup>3</sup> at a high oxide field [131,134].

While defect states and bulk traps acting as fixed charges mostly affect the absolute value of the threshold voltage, the charging and discharging of bulk traps, especially border traps near the Al2O3/(Al)GaN interface, and interface traps deeply located in the bandgap of the (Al)GaN at the Al2O3/(Al)GaN interface can induce significant dynamic instabilities of the threshold voltage and of the drain current during device operation due to their slow detrapping behavior. A schematic illustration of the band diagram of Al2O3/GaN and Al2O3/AlGaN/GaN structures, including border and interface traps, is shown in Figure 4.

**Figure 4.** Schematic band diagram of the (**a**) Al2O3/GaN structure and (**b**) Al2O3/AlGaN/GaN heterostructure at equilibrium, showing border traps near the Al2O3/(Al)GaN interface and interface traps at the Al2O3/(Al)GaN interface. E<sup>C</sup> and E<sup>V</sup> are the conduction and valence bands of (**a**) GaN and (**b**) AlGaN, respectively. E<sup>F</sup> denotes the Fermi energy.

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For these reasons, many groups have focused their efforts on the characterization and minimization of trap states at the dielectric/(Al)GaN interface of the MIS gate structures. Figure 5 illustrates a summary of the interface trap density (Dit) distributions reported in the literature for Al2O3/(Al)GaN structures. Note that the best results reported in each reference have been illustrated in Figure 5. The Terman method [136] and conductance method [137] are often used to estimate the interface trap state densities of Al2O3/GaN structures. Differently, since for Al2O3/AlGaN/GaN structures the evaluation of interface trap states is more challenging due to the presence of a double interface (Al2O3/AlGaN and AlGaN/GaN) complicating the potential distribution over the structure, more advanced techniques such as conductance dispersion techniques [138–140] and frequency and/or temperature-dependent capacitance voltage measurements [26,73,141] are employed. More detailed overviews on the characterization of the electronic states at the insulator/(Al)GaN interfaces of GaN-based MIS-HEMTs with respect to their applicability and potential limitations are given by Ramanan et al. [142] and Yatabe et al. [49].

− **Figure 5.** Interface density distributions (column) extracted from literature for (I) Al2O3/GaN, (II) Al2O3/GaN/AlxGa1xN/GaN and (III) Al2O3/AlxGa1−xN/GaN structures. The corresponding reference is indicated at the bottom of the graph for each column. The conduction band minimum E<sup>C</sup> of GaN and AlGaN is set at 0 eV as reference. The valence band maximum E<sup>V</sup> of GaN and AlGaN, accordingly to the bandgap values of 3.4 eV and 3.9 eV, respectively, are also illustrated as dashed lines.

The results reported in Figure 5 highlight the presence of high-density interface trap states, especially at energies close to the conduction and valence band edges of (Al)GaN. For Al2O3/GaN interfaces, minimum values of the interface state densities in the range of 1010–10<sup>11</sup> cm−<sup>2</sup> eV−<sup>1</sup> have been reported [114,115,143–146]. In comparison, Al2O3/AlGaN interfaces have shown minimum values of interface state densities that are about one order of magnitude higher [26,27,38,73,145,147–150]. Mizue et al. [26] suggested that this difference can be due to oxygen incorporation into AlGaN or to a higher density of defects in the AlGaN layer. Note also that some groups investigated Al2O3/GaN/AlGaN/GaN structures where a thin GaN layer (~1–3 nm) was present on top of the AlGaN layer, possibly affecting the distribution of the interface trap states [105,150–153]. A very thin GaN cap layer is indeed often included in the AlGaN/GaN epitaxial material, as it also helps to protect the AlGaN surface and to reduce leakage currents. Gregušová et al. [150] obtained an interface trap state density that was two to three times lower for the Al2O3 gated AlGaN/GaN structures with a GaN cap compared to ones without a GaN cap. On the contrary, Tapajna et al. [ ˇ 106] reported almost the same C–V characteristics and interface trap state distributions for Al2O3/(GaN)/AlGaN/GaN structures with and without a GaN

cap layer. For ALD-Al2O3/AlGaN/GaN structures, Mizue et al. [26] estimated the trap states density distribution at the ALD-Al2O3/AlGaN interface for the first time, showing that trap states with densities higher than 1 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> exist at the Al2O3/AlGaN interface. To evaluate the near-midgap electronic states at room temperature (RT), a photoassisted C–V method using photon energies less than the AlGaN bandgap was developed [26,73]. For states close to the valence band of (Al)GaN, Matys et al. [154,155] developed a method based on the measurement and simulations of the photo-capacitance of MIS gate heterostructures. Combining this method with the photoassisted capacitance– voltage technique, the interface state density in the entire band gap at the Al2O3/AlGaN interface was determined, revealing the presence of a large amount of trap states with Dit values higher than 1 <sup>×</sup> <sup>10</sup><sup>13</sup> cm−<sup>2</sup> eV−<sup>1</sup> also near the valence band edge [148].

When using Al2O<sup>3</sup> films on (Al)GaN, particular attention has to be given to the temperature processes applied after the dielectric deposition. Hori et al. [108] showed that the annealing process at 800 ◦C for the ohmic contact formation applied after the ALD-Al2O<sup>3</sup> deposition created a large number of microcrystalline regions in the Al2O<sup>3</sup> layer, causing a pronounced increase of the leakage current of the Al2O3/n-GaN structures. To prevent this effect, an "ohmic-first" approach with a SiN protection layer was applied, which maintained the amorphous phase in the atomic configuration of Al2O3, leading to a sufficient suppression of the leakage current. In addition, protecting the surface with a SiN layer during annealing resulted in the low interface trap densities of less than <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> extracted from the C–V characteristics of the Al2O3/GaN structures.

Other processing steps for the fabrication of GaN devices are also critical and can affect the interface quality and the electrical properties of the Al2O3/(Al)GaN structures. To achieve normally off operation, recessed gates are often employed in MIS-HEMTs or hybrid MIS-FETs. For this reason, the influence of inductively coupled plasma (ICP) etching on the interface properties of Al2O3/(Al)GaN structures has also been investigated. Yatabe et al. [73] estimated the state density distribution at the Al2O3/AlGaN interface of MIS structures subjected to ICP dry etching of the AlGaN surface, using for the first time the combination of the photoassisted C–V method and the modeling of the C–V curves [26,156]. Trap state densities higher than 2 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> were obtained at the Al2O3/AlGaN interface of the ICP-etched structures [73]. Without the ICP etching of AlGaN, a nearmidgap Dit of about 1 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> or less was obtained. Similarly, Kim et al. [144] also investigated the effects of a Cl2-based ICP etching on the interface properties of Al2O3/GaN structures. From the X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses, it was shown that the ICP etching caused a disorder of the chemical bonds at the GaN surface. This resulted in high-density trap states with a density larger than 1 <sup>×</sup> <sup>10</sup><sup>13</sup> cm−<sup>2</sup> eV−<sup>1</sup> near the conduction band edge of the GaN at the Al2O3/GaN interface, which was suggested to include defects related to nitrogen vacancy (VN) levels. A decrease of the interface state density was obtained by applying a PDA process in N<sup>2</sup> at 400 ◦C, which partially recovered the VN-related levels, thus increasing the chemical bond order at the GaN surface. Yatabe et al. [149] also reported that the ICP etching of the AlGaN surface introduced a monolayer-level crystalline roughness, the disorder of the chemical bonds and various types of defect complexes including VN, resulting in high trap state densities of up to 8 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> at the Al2O3/AlGaN interface. Fang et al. [157] also reported that Cl2-based ICP etching enhanced the deep centers at the GaN surface originating from V<sup>N</sup> and other defect complexes.

Other studies have demonstrated the importance of PDA and PMA treatments to minimize the interface trap states at the Al2O3/(Al)GaN interface. From the TEM investigations, Hashizume et al. [114] revealed that PMA in N<sup>2</sup> at 300–400 ◦C led to a uniform distribution of the lattice constant near the interface of the ALD-Al2O3/GaN MIS structures, which resulted in excellent C–V characteristics almost without frequency dispersion and a reduced Dit ranging from 1 to 4 <sup>×</sup> <sup>10</sup><sup>10</sup> cm−<sup>2</sup> eV−<sup>1</sup> at energies near the conduction band edge. Similar values of Dit at the Al2O3/GaN interface after PMA in N<sup>2</sup> at 400 ◦C were also very recently obtained by Ando et al. [158]. Ando et al. [147] also demonstrated that a PMA

in N<sup>2</sup> at 300 ◦C led to a similar reduction of the electronic states at the ALD-Al2O3/AlGaN interface. Kaneki et al. [115] pointed out that annealing under reverse bias at 300 ◦C in air for 3 h is also beneficial to decrease the interface state density of ALD-Al2O3/GaN structures, and it is more effective than PDA in N<sup>2</sup> at 400–700 ◦C, probably due to a relaxation of the dangling bonds and/or the point defects at the GaN surface. Moreover, almost no shift of the VFB with respect to the expected value was observed in the C–V curves due to the reduction of the donor-type interface states and/or defect levels in the bulk Al2O3. Similar effects of the reverse-bias annealing were obtained by Nishiguchi et al. [38] for ALD-Al2O3/AlGaN structures. Winzer et al. [143] reported that PDA in O<sup>2</sup> or forming gas (H2/N2) at 500 ◦C were more efficient for decreasing the traps at the Al2O3/GaN interface than PDA in N<sup>2</sup> at the same temperature. A very low interface trap density of less than <sup>5</sup> <sup>×</sup> <sup>10</sup><sup>11</sup> cm−<sup>2</sup> eV−<sup>1</sup> was achieved for Al2O3/GaN structures treated by forming gas PDA at 500 ◦C. However, it was also reported that forming gas PDA resulted in a detrimental increase of the leakage currents of the Al2O<sup>3</sup> films. Similar results were reported by Long et al. [159], where the effect of trap passivation during the forming gas anneal was correlated to the incorporation of hydrogen at the interface.

Similar to annealing processes, surface treatments are also effective in reducing interface trap states at the Al2O3/(Al)GaN interface. Hori et al. [27,145] demonstrated that an N2O-radical treatment can decrease interface states both at the Al2O3/GaN and Al2O3/AlGaN interfaces. For Al2O3/AlGaN structures, the interface state density was estimated to be 1 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> or less around the midgap and 8 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> near the conduction band edge [27]. Calzolaro et al. [151] recently reported a significant reduction of frequency dispersion of the C–V characteristics of Al2O3/GaN/AlGaN/GaN structures after a remote O<sup>2</sup> plasma-based surface treatment prior to the ALD-Al2O<sup>3</sup> deposition combined with a PMA in N<sup>2</sup> at 350 ◦C. The Dit was estimated to be reduced to a value in the order of 2 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> near the conduction band edge.

Trapping mechanisms at the Al2O3/(Al)GaN interface are especially critical for Al-GaN/GaN MIS-HEMTs under forward gate bias, where electrons can spill over from the 2DEG channel towards the dielectric by overcoming the AlGaN barrier and become trapped at the Al2O3/(Al)GaN interface [34–37]. Similarly, charge trapping in high-density electronic states at the interface has been reported to lead to a significant screening of the gate electric field and the consequent loss of control of the surface potential of the barrier layer, causing the degradation of the current linearity and the saturation of the current at forward bias in AlGaN/GaN MIS-HEMTs [38]. In this regard, the next section focuses on reviewing the recent progress on the performance of Al2O3-gated MIS-HEMTs.
