Electrical properties of nominally undoped β-Ga2O3 crystals grown by the Czochralski method from an iridium crucible under a carbon dioxide containing atmosphere were studied by temperature dependent conductivity and Hall effect measurements as well as deep level transient spectroscopy. All crystals were n-type with net donor concentrations between 6 × 1016 and 8 × 1017 cm−3. The Hall mobility of electrons was on average 130 cm2/Vs at room temperature and attained a maximum of 500 cm2/Vs at 100 K. The donor ionization energy was dependent on the donor concentration. Extrapolation of this dependence to zero concentration yielded a value of about 36 meV for isolated donors agreeing well with the ionization energy derived from effective-mass theory. Three deep electron traps were found at 0.55, 0.74, and 1.04 eV below the conduction bandedge. The trap at EC – 0.74 eV was detected in all samples with concentrations of 2 – 4 × 1016 cm−3. This concentration is comparable to that of compensating acceptors we have to take into account for an explanation of the temperature dependent electron density. Therefore, under the assumption that the electron trap at EC – 0.74 eV possesses acceptor character, this trap could be the dominating compensating acceptor in our crystals. Besides, a value of ΦB = (1.1 ± 0.1) V was determined for the Schottky barrier height of Ni on the (100) surface of n-type β-Ga2O3.

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The Schottky barrier height of Au deposited on (100) surfaces of n-type β-Ga2O3 single crystals was determined by current-voltage characteristics and high-resolution photoemission spectroscopy resulting in a common effective value of 1.04 ± 0.08 eV. Furthermore, the electron affinity of β-Ga2O3 and the work function of Au were determined to be 4.00 ± 0.05 eV and 5.23 ± 0.05 eV, respectively, yielding a barrier height of 1.23 eV according to the Schottky-Mott rule. The reduction of the Schottky-Mott barrier to the effective value was ascribed to the image-force effect and the action of metal-induced gap states, whereas extrinsic influences could be avoided.

Transparent semiconducting β-Ga 2 O 3 single crystals were grown by the Czochralski method from an iridium crucible under a dynamic protective atmosphere to control partial pressures of volatile species of Ga 2 O 3 . Thermodynamic calculations on different atmospheres containing CO 2 , Ar and O 2 reveal that CO 2 growth atmosphere combined with overpressure significantly decreases evaporation of volatile Ga 2 O 3 species without any harm to iridium crucible. It has been found that CO 2 , besides providing high oxygen concentration at high temperatures, is also acting as a minor reducing agent for Ga 2 O 3 . Different coloration of obtained crystals as well as optical and electrical properties are directly correlated with growth conditions (atmosphere, pressure and temperature gradients), but not with residual impurities. Typical electrical properties of the n-type β-Ga 2 O 3 crystals at room temperature are: ρ = 0.1 -0.3 Ωcm, μ n,Hall = 110 -150 cm 2 V -1 s -1 , n Hall = 2 -6×10 17 cm -3 and E Ionisation = 30 -40 meV. A decrease of transmission in the IR-region is directly correlated with the free carrier concentration and can be effectively modulated by the dynamic growth atmosphere. Electron paramagnetic resonance (EPR) spectra exhibit an isotropic shallow donor level and anisotropic defect level. According to differential thermal analysis (DTA) measurements, there is substantially no mass change of β-Ga 2 O 3 crystals below 1200 °C (i.e. no decomposition) under oxidizing or neutral atmosphere, while the mass gradually decreases with temperature above 1200 °C. High resolution transmission electron microscopy (HRTEM) images at atomic resolution show the presence of vacancies, which can be attributed to Ga or O sites, and interstitials, which can likely be attributed to Ga atoms.

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.

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...

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We present a new approach for scaling-up the growth of β-Ga2O3 single crystals grown from the melt by the Czochralski method, which has also a direct application to other melt-growth techniques involving a noble metal crucible. Experimental and theoretical results point to melt thermodynamics as the crucial factor in increasing the volume of a growing crystal. In particular, the formation of metallic gallium in the liquid phase in large melt volumes causes problems with crystal growth and eutectic or intermetallic phase formation with the noble metal crucible. The larger crystals to be grown the higher oxygen concentration is required. The minimum oxygen concentration ranges from about 8 to 100 vol.% for 2 to 4 inch diameter cylindrical crystals, challenging the use of iridium crucibles in a combination with such high oxygen concentrations. A specific way of oxygen delivery to a growth furnace with the iridium crucible allows to minimize the formation of metallic gallium in the melt and thus obtaining large crystal volumes while decreasing the probability of the eutectic formation.