*3.3. Modified Al2O<sup>3</sup> Gate Dielectrics*

Besides the use of pure Al2O<sup>3</sup> films, other approaches involving the use of Al2O3-based bilayer gate stack dielectrics, interface engineering techniques or Al2O3-based compound materials have been investigated to combine the properties of Al2O<sup>3</sup> with the favorable properties of other dielectric materials.

Kambayashi et al. [170] applied a SiO2/Al2O<sup>3</sup> gate stack (layers indicated from top to bottom) in hybrid GaN MIS-FETs, thus demonstrating a high-performance device with a channel mobility of 192 cm2/Vs. Using a SiO2/Al2O<sup>3</sup> gate stack, Guo and del Alamo [171,172] studied the origin of PBTI and negative bias temperature instability (NBTI) in hybrid GaN MIS-FETs. It was shown that for a composite SiO2/Al2O<sup>3</sup> gate oxide, the resulting Vth shifts are due to electron trapping or detrapping in pre-existing oxide traps and the generation of oxide traps near the oxide/GaN interface. Van Hove et al. [173] applied an ALD-Al2O3/in situ MOCVD-Si3N<sup>4</sup> gate bilayer stack in AlGaN/GaN MIS-HEMTs to achieve excellent electrical device characteristics with lower gate leakage currents, more stable threshold voltages and reduced current collapse when compared to Al2O3-gated MIS-HEMTs. Capriotti et al. [174] investigated the fixed interface charges between the AlGaN and the Al2O3/in situ SiN gate stack of AlGaN/GaN MIS-HEMTs. Colon and Shi [90] fabricated AlGaN/GaN MIS-HEMTs with low gate leakage currents using an ALD-HfO2/Al2O<sup>3</sup> bilayer stack as well as an ALD-HfAlO<sup>x</sup> ternary compound as gate dielectrics to achieve a higher dielectric constant than Al2O<sup>3</sup> and a higher conduction band offset, thermal stability and crystallization temperature than HfO2. However, both the HfO2/Al2O<sup>3</sup> and HfAlOx-gated MIS-HEMTs still showed low transconductance, high interface state density and pronounced current collapse. The energy band alignment of MOCVD-HfAlO to GaN was investigated by Liu et al. [175,176], reporting a conduction band offset of 2.2 eV and minimum values of interface trap density in the range of 1–3 <sup>×</sup> <sup>10</sup><sup>11</sup> cm−<sup>2</sup> eV−<sup>1</sup> at the HfAlO/GaN interface. Hatano et al. [177] demonstrated reduced gate leakage and the improved operation and thermal stability of AlGaN/GaN MIS-HEMTs using a ZrO2/Al2O<sup>3</sup> gate stack dielectric.

Other approaches based on the use of Al2O3-based composite materials have also been reported. Partida-Manzanera et al. [178] investigated the potential of a ternary phase of Ta2O<sup>5</sup> and Al2O<sup>3</sup> as gate dielectrics to achieve higher permittivity than Al2O3, and hence enhance the transconductance of AlGaN/GaN MIS-HEMTs. Although a higher transconductance and reduced gate leakage current were achieved, the C–V curves did not feature the characteristic step at the forward bias in the spill-over regime, indicating a high density of trap states at the dielectric/AlGaN interface. Kikuta et al. [179] applied Al2O3/SiO<sup>2</sup> nanolaminate films deposited by ALD on GaN to obtain a gate dielectric material with a larger conduction band offset to GaN and a higher crystallization temperature than pure Al2O<sup>3</sup> films in order to reduce gate leakage currents. The composition of Al and Si in the oxide and the resulting oxide properties of the permittivity, breakdown field and leakage currents could be controlled and tuned by the numbers of ALD cycles. Compared to pure Al2O<sup>3</sup> films, a higher breakdown field and better reliability were obtained for the SiO<sup>2</sup> composition, from 0.21 to 0.69. Similarly, Mitrovic et al. [180] suggested that Al2O3/TiO<sup>2</sup> nanolaminates can also be favorable as gate dielectrics, and they very recently investigated the band alignment to the GaN and the permittivity of the Al2O<sup>3</sup> layers doped with Ti, corresponding to TixAl1−xOy. Although the permittivity of TixAl1−xO<sup>y</sup> increased significantly with the increasing Ti content, a small conduction band offset for all compositions was obtained. However, Le et al. [181,182] reported excellent characteristics with good insulating properties for MIS-HEMTs using AlTiO deposited by ALD as a gate dielectric.

Current research has also focused on the "doping" by fluorine ions (F−) of Al2O<sup>3</sup> gate dielectric films in order to control the threshold voltage of MIS-HEMTs towards normally off operation [183,184]. The latter can be obtained by implanting F− ions into the AlGaN

barrier prior to the dielectric ALD. After the ALD-Al2O<sup>3</sup> deposition, the incorporated F − ions can act as a source of negative fixed charges, compensating the intrinsic positive charges in the dielectric and shifting the Vth of the device in positive bias direction. It is worth mentioning that a previous physical approach based on the fluorine incorporation via plasma etching under the gate to shift the device threshold voltage was demonstrated by Cai et al. [185]. Using an ALD-Al2O<sup>3</sup> gate dielectric combined with a fluorine-based plasma treatment, Chu et al. [186] demonstrated normally off Al2O3-gated MIS-HEMTs with a breakdown voltage of 1200 V.

An interesting process was used by Liu et al. [79] and Yang et al. [187], who improved the performance and the Vth stability of the Al2O3-gated hybrid MIS-FETs by inserting a monocrystalline AlN interfacial layer via plasma-enhanced atomic layer deposition (PEALD) at the Al2O3/GaN interface to block oxygen from the GaN surface and prevent the formation of oxygen-related interface traps. Al2O3/AlN/GaN structures showed a small frequency dispersion in the C–V curves and a Dit in the range of 1011–10<sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> , determined using the conventional conductance method. Similarly, Yang et al. [188] and Chen et al. [189] used an in situ low-damage plasma treatment based on NH<sup>3</sup> and N<sup>2</sup> prior to the ALD-Al2O<sup>3</sup> deposition to effectively remove the native oxide while forming an ultrathin monocrystal-like nitridation interlayer (NIL) at the Al2O3/GaN interface. The N<sup>2</sup> plasma treatment was also demonstrated to compensate for VN-related defects at the surface. After a PDA was carried out at 500 ◦C in O<sup>2</sup> ambient, the Al2O3/NIL-gated MIS structures showed a lower interface trap density in the range of 1–6 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> eV−<sup>1</sup> , resulting in AlGaN/GaN MIS-HEMTs with improved performance [189].

Finally, a very promising approach proposed by Asahara et al. [190] consists in using a sputtered AlON film as a gate dielectric, obtained by introducing nitrogen into Al2O3. An atomically abrupt high quality AlON/AlGaN interface with extremely low <sup>D</sup>it values ranging from 1.2 to 1.4 <sup>×</sup> <sup>10</sup><sup>11</sup> cm−<sup>2</sup> eV−<sup>1</sup> and improved bulk properties were achieved, resulting in excellent C–V characteristics with negligible frequency dispersions and a markedly suppressed gate leakage current. Similar results were obtained by Wang et al. [191], who deposited AlON films by inserting thin AlN alternating layers into Al2O3. As shown in Figure 7, Ueda et al. [192] very recently applied AlON films deposited by ALD combined to a PDA in O<sup>2</sup> for shifting the Vth so to realize the normally off operation in the recessed-gate AlGaN/GaN MIS-HEMTs, with a negligible hysteresis in the transfer characteristics, a reduced off-state leakage current, a breakdown voltage of 730 V, an on-state resistance of 270 mΩ for a 10 A drain current rating and impressive switching performance, indicating the great potential of AlON as gate dielectric technology. <sup>Ω</sup>

**Figure 7.** Transfer and output characteristics of recessed-gate AlGaN/GaN MIS-HEMTs using AlON as gate dielectric and subjected to PDA in O<sup>2</sup> atmosphere, reported by Ueda et al. [192]. The positive shift of Vth obtained by O<sup>2</sup> annealing for the AlON-gated transistor is shown in (**a**), while (**b**) reports the transfer curves without hysteresis obtained after applying a maximum gate voltage up to 10 V. The output characteristics of AlON-gated MIS-HEMTs in the on-state and off-state are shown in (**c**,**d**), respectively.

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