This letter reports the small-signal and large-signal performances at high drain voltage (V DS ) ranging up to 60 V for a 0.5 µm gate length two-dimensional hole gas diamond metal-oxide-semiconductor field-effect transistor with a 100-nm-thick atomic-layer-deposited Al 2 O 3 film on a IIa-type polycrystalline diamond substrate with (110) preferential surfaces. This diamond FET demonstrated a cutoff frequency (f T ) of 31 GHz, indicating that its carrier velocity was reaching 1.0 × 10 7 cm/s for the first time in diamond. In addition, a f T of 24 GHz was obtained at V DS = −60 V, thus giving a f T × V DS product of 1.44 THz•V. This diamond FET is promising for use as a high-frequency transistor under high voltage conditions. Under application of a high voltage, a maximum output power density of 3.8 W/mm (the highest in diamond) with an associated gain and power added efficiency were 11.6 dB and 23.1% was obtained when biased at V DS = −50 V using a load-pull system at 1 GHz.

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As previously reported, postdeposition annealing at 800 °C and higher simultaneously crystallizes atomic-layer-deposited (ALD) Al2O3 films and reduces the current in Al/ALD-Al2O3/(0001) GaN capacitors by two orders of magnitude. This current reduction is caused by the enhancement of conduction band offset from 1.4 to 1.8 eV, as revealed by the space-charge-controlled field emission analysis. Selected area electron diffraction (SAED) patterns demonstrate that the crystallized films consist of twinned (111)-oriented cubic γ-Al2O3 with an epitaxial relation of Al2O3 ⟨01¯1⟩∥ GaN ⟨21¯1¯0⟩. The SAED patterns additionally include spots that are specific to triaxially tripled γ-Al2O3. The aforementioned epitaxy is due to the similarity of hexagonal close-packed sublattices between oxygen on a (111) γ-Al2O3 plane and nitrogen on a (0001) GaN plane. However, the hexagonal close-packed lattice constant of γ-Al2O3 is 12% smaller than that of GaN, necessitating domain matching epitaxy. The thickness of the interfacial transition layer caused by the large misfit is estimated to be thinner than four monolayers of oxygen sublattice, by using the methodology developed here. Based on these results, the effect of Al2O3 crystallinity on the characteristics of Al2O3/GaN capacitors, such as conduction current, dielectric breakdown, interface states, and bias instability, was comprehensively captured. According to x-ray diffraction analyses, Al2O3 films crystallize at 700 °C, which is ∼100 °C lower than the threshold temperature estimated by transmission electron microscope observations. This difference was possibly caused by locally crystallized Al2O3 films, as confirmed by the slightly reduced current. These findings form a basis for improving ALD-Al2O3 films as gate insulator.

Atomic-layer-deposited (ALD) Al2O3 films are the most promising surface passivation and gate insulation layers in non-Si semiconductor devices. Here, we carried out an extensive study on the time-dependent dielectric breakdown characteristics of ALD-Al2O3 films formed on homo-epitaxial GaN substrates using two different oxidants at two different ALD temperatures. The breakdown times were approximated by Weibull distributions with average shape parameters of 8 or larger. These values are reasonably consistent with percolation theory predictions and are sufficiently large to neglect the wear-out lifetime distribution in assessing the long-term reliability of the Al2O3 films. The 63% lifetime of the Al2O3 films increases exponentially with a decreasing field, as observed in thermally grown SiO2 films at low fields. This exponential relationship disproves the correlation between the lifetime and the leakage current. Additionally, the lifetime decreases with measurement temperature with the most remarkable reduction observed in high-temperature (450 °C) O3-grown films. This result agrees with that from a previous study, thereby ruling out high-temperature O3 ALD as a gate insulation process. When compared at 200 °C under an equivalent SiO2 field of 4 MV/cm, which is a design guideline for thermal SiO2 on Si, high-temperature H2O-grown Al2O3 films have the longest lifetimes, uniquely achieving the reliability target of 20 years. However, this target is accomplished by a relatively narrow margin and, therefore, improvements in the lifetime are expected to be made, along with efforts to decrease the density of extrinsic Al2O3 defects, if any, to promote the practical use of ALD Al2O3 films.

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