A Sn-doped (100) β-Ga2O3 epitaxial layer was grown via metal-organic vapor phase epitaxy (MOVPE) onto a singlecrystal, Mg-doped semi-insulating (100) β-Ga2O3 substrate. Ga2O3-based Metal-Oxide-Semiconductor Field effect Transistors (MOSFETs) with a 2 µm gate length (LG), 3.4 µm source-drain spacing (LSD) and 0.6 µm gate-drain spacing (LGD) were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the off-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

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We studied the growth of Si-and Sn-doped homoepitaxial β-Ga 2 O 3 layers on (010)-oriented substrates by metal organic vapor phase epitaxy (MOVPE). At optimal growth conditions (850 • C, 5 mbar) the layers were smooth with RMS roughness values of ∼600 pm. A microstructural study by transmission electron microscopy (TEM) revealed a very high crystalline perfection of the layers. No dislocations or planar defects were observed within the field of view of TEM. Using Si as dopant, the free electron concentration could be varied in a range between 1 × 10 17 and 8 × 10 19 cm −3 , while with Sn the doping range was restricted to 4 × 10 17 −1 × 10 19 cm −3 . This was explained by a pronounced Sn memory effect in the MOVPE reactor that hampers achieving low carrier densities and by incorporation issues that limit the doping efficiency at high Sn doping levels. The electron mobility for a given doping density increased from ∼50 cm 2 /Vs for n = 8 × 10 19 cm −3 to ∼130 cm 2 /Vs for n = 1 × 10 17 cm −3 independently of the dopant. These values match the best literature data relative to β- Transparent Oxide Semiconductors (TSOs) are an emerging class of materials, which combine high electrical conductivity with transparency down to the deep UV region.1,2 Among TSOs, monoclinic β-Ga 2 O 3 is one of the most interesting compounds thanks to a wide bandgap (∼4.8 eV) that leads to a calculated electric breakdown field strength of 8 MV/cm. 3,4 The most promising application of β-Ga 2 O 3 is in the field of high power electronics, where it is predicted to outperform the leading technology based on SiC and GaN. A key advantage of β-Ga 2 O 3 is that native substrates can be fabricated from bulk single crystals grown from the melt by Floating Zone (FZ), 5 Edge-Defined Film Fed Growth (EFG) 6 and Czochralski (CZ) 7,8 methods. The growth of homoepitaxial β-Ga 2 O 3 has been mainly investigated by molecular beam epitaxy (MBE), 9-11 and only rarely by metal organic vapor phase epitaxy (MOVPE).12-14 For both epitaxial techniques Schottky diodes and field-effect transistors have been realized. 15,16 The structural quality of (100) β-Ga 2 O 3 layers grown by MOVPE has been relatively poor so far, with maximum electron mobilities of ∼40 cm 2 /Vs, 13 contrary to values achieved for layers grown by MBE on (010)-oriented substrates that are comparable to those of bulk material (>100 cm 2 /Vs). 10 A further improvement of the growth of β-Ga 2 O 3 by MOVPE is desirable, since MOVPE is more suitable for large-scale production.Here we report on the MOVPE-growth of Si-and Sn-doped epitaxial β-Ga 2 O 3 layers on (010) β-Ga 2 O 3 substrates. We selected this substrate orientation to investigate whether it promotes the growth of layers with higher crystalline perfection. Moreover, thermal conductivity in β-Ga 2 O 3 is anisotropic with the highest value along the [010] direction (27.0 W/mK at RT) and the lowest one along the [100] direction (10.9 W/mK at RT). 17 Heat dissipation in devices fabricated on (010)-oriented substrates is then predicted to be a...

Sn-doped gallium oxide (Ga2O3) wrap-gate fin-array field-effect transistors (finFETs) were formed by top-down BCl3 plasma etching on a native semi-insulating Mg-doped (100) β-Ga2O3 substrate. The fin channels have a triangular cross-section and are approximately 300 nm wide and 200 nm tall. FinFETs, with 20 nm Al2O3 gate dielectric and ∼2 μm wrap-gate, demonstrate normally-off operation with a threshold voltage between 0 and +1 V during high-voltage operation. The ION/IOFF ratio is greater than 105 and is mainly limited by high on-resistance that can be significantly improved. At VG = 0, a finFET with 21 μm gate-drain spacing achieved a three-terminal breakdown voltage exceeding 600 V without a field-plate.

Epitaxial β‐Ga2O3 layers have been grown on β‐Ga2O3 (100) substrates using metal‐organic vapor phase epitaxy. Trimethylgallium and pure oxygen or water were used as precursors for gallium and oxygen, respectively. By using pure oxygen as oxidant, we obtained nano‐crystals in form of wires or agglomerates although the growth parameters were varied in wide range. With water as an oxidant, smooth homoepitaxial β‐Ga2O3 layers were obtained under suitable conditions. Based on thermodynamical considerations of the gas phase and published ab initio data on the catalytic action of the (100) surface of β‐Ga2O3 we discuss the adsorption and incorporation processes that promote epitaxial layer growth. The structural properties of the β‐Ga2O3 epitaxial layers were characterized by X‐ray diffraction pattern and high resolution transmission electron microscopy. As‐grown layers exhibited sharp peaks that were assigned to the monocline gallium oxide phase and odd reflections that could be assigned to stacking faults and twin boundaries, also confirmed by TEM. Shifts of the layer peak towards smaller 2θ values with respect to the Bragg reflection for the bulk peaks have been observed. After post growth thermal treatment in oxygen‐containing atmosphere the reflections of the layers do shift back to the position of the bulk β‐Ga2O3 peaks, which was attributed to significant reduction of lattice defects in the grown layers after thermal treatment.

The authors have applied positron annihilation spectroscopy to study the vacancy defects in undoped and Si-doped Ga2O3 thin films. The results show that Ga vacancies are formed efficiently during metal-organic vapor phase epitaxy growth of Ga2O3 thin films. Their concentrations are high enough to fully account for the electrical compensation of Si doping. This is in clear contrast to another n-type transparent semiconducting oxide In2O3, where recent results show that n-type conductivity is not limited by cation vacancies but by other intrinsic defects such as Oi.

Heteroepitaxial Ga2O3 was grown on c-plane sapphire by molecular beam epitaxy, pulsed-laser deposition, and metalorganic chemical vapor deposition. Investigation by scanning transmission electron microscopy (STEM) revealed the presence of a three-monolayer-thick pseudomorphically grown layer of trigonal α-Ga2O3 at the interface between the c-plane sapphire substrate and the β-Ga2O3 independent of the growth method. On top of this pseudomorphically grown layer, plastically relaxed monoclinic β-Ga2O3 grew in the form of rotational domains. We rationalize the stable growth of the high-pressure trigonal α-phase of Ga2O3 in terms of the stabilization of the α-Ga2O3 phase by the lattice-mismatch-induced strain.

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