For high electron mobility transistors (HEMTs) power transistors based on AlGaN/GaN heterojunction, p-GaN gate has been the gate topology commonly used to deplete the two dimensional electron gas (2-DEG) and achieve a normally-OFF behavior. But fully recessed MIS gate GaN power transistors or MOSc-HEMTs have gained interest as normally-OFF HEMTs thanks to the wider voltage swing and reduced gate leakage current when compared to p-GaN gate HEMTs. However the mandatory AlGaN barrier etching to deplete the 2-DEG combined with the nature of the dielectric/GaN interface generates etching-related defects, traps, and roughness. As a consequence, the threshold voltage (V_TH) can be unstable, and the electron mobility is reduced, which presents a challenge for the integration of a fully recessed MIS gate. Recent developments have been studied to solve this challenge. In this paper, we discuss developments in gate recess with low impact etching and atomic layer etching (ALE) alongside surface treatments such as wet cleaning, thermal or plasma treatment, all in the scope of having a surface close to pristine. Finally, different interfacial layers, such as AlN, and alternative dielectrics investigated to optimize the dielectric/GaN interface are presented.

Electrical and material properties of plasma‐enhanced atomic layer deposited (PE‐ALD) AlON on dry‐etched n‐type GaN substrates are investigated for nitrogen concentration ranging from 1.5% to 7.1%. Firstly, an increase in flat‐band voltage (V_FB) and its hysteresis with increasing nitrogen concentration is reported. The increase in V_FB is associated with the nitrogen content whereas the increase in hysteresis to the presence of impurities (hydroxyl groups and carbon‐related impurities). Alongside the nitrogen concentration, the impact of different post‐deposition annealing (PDA) temperatures is studied (400–800 °C). Stable AlON layers and interfaces with etched GaN substrates are reported with slight gallium oxide growth or gallium diffusion towards the dielectric layer. Finally, with increasing PDA temperature, an increase in V_FB and a significant reduction of both V_FB hysteresis and interface state density (D_it) are observed, notably at the measuring temperature of 150 °C. These results present a promising pathway toward more reliable and stable normally‐OFF GaN‐based MOS‐channel high electron mobility transistors (MOSc‐HEMTs).

Homogeneous AlOxNy thin films with a controlled nitrogen composition, up to 7.1%, were grown on GaN substrate using plasma-enhanced atomic layer deposition. This was achieved through repeated cycles consisting of AlN thin layers deposition and subsequent in situ O2 plasma oxidation under optimized conditions. We demonstrate that the nitrogen concentration in AlOxNy films can be finely tuned from 1.5% to 7.1% by a precise control of the AlN thin film thickness from 0.25 to 0.96 nm before each oxidation step. It is shown also that higher AlN thickness, ≥1.96 nm, leads to an incomplete plasma oxidation of AlN films, resulting in the formation of AlOxNy/AlN-like structures. Several structural and physic-chemical characterizations, including focused ion beam-scanning transmission electron microscopy, Auger electron spectroscopy, XPS, and time-of-flight secondary ion mass spectrometry were performed to confirm these findings. These results highlight the importance of this approach for growing homogeneous AlOxNy thin films with controlled nitrogen doping, which can be used as dielectrics in gate stacks for high performance GaN-based transistors.

This scientific paper describes the chemical study of different Al2O3/GaN interfaces found in the new generation of high electron mobility transistors developed for power electronics applications. The stake for these interfaces is the limitation, ideally the absence of an oxidized gallium layer hampering the good electrical behavior of the semiconductor. These structures have been studied through time-of-flight secondary ion mass spectrometry (ToF-SIMS), magnetic SIMS (M-SIMS), and atomic force microscopy (AFM) analyses. Two structures were considered: a bilayer Al2O3 10 nm–GaOx 2 nm on GaN and a Al2O3 10 nm single layer deposited on preliminary etched GaN. The first sample was used as a dedicated reference sample where an actual gallium oxide layer was grown, whereas the second one was a technologically relevant structure. Several experimental conditions were compared for the ToF- and M-SIMS analyses; in particular, three angles of incidence for the primary Cs+ sputter beam (65°, 61°, 45°) were used, leading to diverse depth resolutions and roughnesses, as revealed by surface topography analysis provided by AFM. Among the different incidence angles, it was found that the best experimental conditions were those obtained by ToF-SIMS analysis with an incidence angle of 45°, generating the least roughness.

In this paper, we discuss the combined characterization by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and hard x-ray photoelectron spectroscopy (HAXPES) of the Al2O3/GaN interface for the next generation of MOS-gate GaN-based devices. The final properties of these devices strongly depend on the quality of this critical interface. The results highlight that gallium oxidation at this interface is enhanced when increasing atomic layer deposited (ALD) Al2O3 thicknesses from 3 nm up to 20 nm. Moreover, we highlight how Al2O3 (O3/H2O)/GaN structures reduce the oxidation of gallium compared to Al2O3 (H2O) structures, where O3 and H2O are the oxygen precursors for ALD of Al2O3. In addition, through ToF-SIMS measurements we show the effect in terms of contaminants (hydrogen, carbon, halide) at the Al2O3/GaN interface depending on the oxygen precursor used.