We report on the design and demonstration of 𝛽𝛽-(AlGa)2O3/Ga2O3 modulation doped heterostructures to achieve high sheet charge density. The use of a thin spacer layer between the Si delta-doping and heterojunction interface was investigated in 𝛽𝛽 -(AlGa)2O3/Ga2O3 modulation doped structures. We find that that this strategy enables higher 2DEG sheet charge density up to 6.1x10 12 cm -2 with mobility of 147 cm 2 /Vs. The presence of a degenerate 2DEG channel was confirmed by the measurement of low temperature effective mobility of 378 cm 2 /V-s and a lack of carrier freeze out from low temperature capacitance voltage measurements. The electron density of 6.1x10 12 cm -2 is the highest reported sheet charge density obtained without parallel conduction channels in an (AlGa)2O3/ Ga2O3 heterostructure system. With a high theoretical breakdown field strength of 8 MV/cm 1,2 , 𝛽𝛽-Ga2O3 has the potential to be useful in several high frequency 3,4 and power switching applications 5,6 . The high break down field enables shrinking the overall device footprint which results in improved frequency performance for power switching devices and increased output power density for RF power amplifiers. Besides the superior breakdown field strength, the availability of native 𝛽𝛽-Ga2O3 substrates 7-9 enables high quality epitaxial growth using techniques such as molecular beam epitaxy 10,11 , metal organic chemical vapor deposition 12,13 , halide vapor phase epitaxy 14,15 and pulsed laser deposition 16,17 .For lateral power devices it is essential for the channel to be placed close to the gate. Firstly, for enhancement mode devices (which are preferred for power electronics), a lower gate-to-channel spacing leads to higher gate-to-channel capacitance, and therefore enables higher sheet charge density for the same gate voltage swing. In addition, a scaled channel allows for better control of gate-drain electric field and lateral scaling of gate length. The former is important in achieving high average breakdown field strength while the latter is crucial in improving the frequency of operation in RF power amplifiers and reducing on resistance in power switching devices.Lateral devices like 𝛽𝛽-Ga2O3 MESFETs for high frequency application have been demonstrated with high on/off ratio and breakdown voltage but the performance of these devices is mainly limited by the low mobility (50-90 cm 2 /V-s) 3,4 . Introducing

A new record-high room temperature electron Hall mobility (µ RT = 194 cm 2 /V•s at n ~ 8×10 15 cm -3 ) for β-Ga 2 O 3 is demonstrated in the unintentionally doped thin film grown on (010) semi-insulating substrate via metalorganic chemical vapor deposition (MOCVD). A peak electron mobility of ~9500 cm 2 /V•s is achieved at 45 K. Further investigation on the transport properties indicate the existence of sheet charges near the epi-layer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epi-layer and the interface, originated from both surface contamination as well as growth environment. Pre-growth hydrofluoric acid cleaning of the substrate lead to an obvious decrease of Si impurity both at interface and in epi-layer. In addition, the effect of MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β-Ga 2 O 3 film with very low bulk charge concentration, even a reduced sheet charge density can play an important role in the charge transport properties.

We report on the origin of high Si flux observed during the use of Si as a doping source in plasma assisted MBE growth of -Ga2O3. We show on the basis of secondary ion mass spectroscopy (SIMS) analysis that Si flux is not limited by the vapor pressure of Si but by the formation of volatile SiO. The low sublimation energy of SiO leads to weak dependence of the SiO flux of Si cell temperature and a strong dependence on the background oxygen pressure. Extended exposure to activated oxygen results in reduction of SiO flux due to the formation of SiO2 on the Si surface. The work reported provides key understanding for incorporating Si into future oxide-based semiconductor heterostructure and device MBE growth.The high breakdown voltage [1] and availability of bulk substrates grown from melt [2-4] makes -Ga2O3 promising for various applications, including power switches [5,6], high frequency amplifiers [7,8] and high temperature electronics [9,10].High quality epitaxial growth with low defect density and a wide range of controllable n-type doping [2][3][4]11] are critical enablers for these applications. Variety of growth techniques like molecular beam epitaxy [12,13], metal organic chemical vapor deposition [14], halide vapor phase epitaxy [15] and low pressure chemical vapor deposition [16] have been utilized to realize high quality epitaxial layers of -Ga2O3. Since the conduction band of -Ga2O3 is largely made up of Ga s oribitals, group II elements like Si (30 meV), Ge (30 meV) and Sn (60 meV) provide shallow donor levels [9, 17, 18,] and a detailed understanding of the doping process of each one of them is critical in realizing the full potential of -Ga2O3 based devices.Si is the preferred n-type shallow donor in many III-V materials like GaN (20 meV) and GaAs (6 meV) due to its low activation energy and compatibility with epitaxial growth processes. In the case of molecular beam epitaxy, elemental high-purity silicon is typically used, with the effusion cell maintained at high temperatures (typically 1000 °C -1300 °C) required for sublimation of the solid Si. This typically provides excellent control of doping density up to 10 20 cm -3 [19,20].Plasma and ozone-based molecular beam epitaxy growth have been used to obtain Si-doped n-type -Ga2O3 with excellent transport properties [9,21]. However, the cell temperatures necessary to achieve Si doping have been found to be significantly lower [9,22,23] than that used typically for other non-oxide material systems. Since Si has a high sticking coefficient, this

Wide and ultra-wide band gap semiconductors can provide excellent performance due to their high energy band gap, which leads to breakdown electric fields that are more than an order of magnitude higher than conventional silicon electronics. In materials where p-type doping is not available, achieving this high breakdown field in a vertical diode or transistor is very challenging.We propose and demonstrate the use of dielectric heterojunctions that use extreme permittivity materials to achieve high breakdown field in a unipolar device. We demonstrate the integration of a high permittivity material BaTiO3 with n-type β-Ga2O3 to enable 5.7 MV/cm average electric field and 7 MV/cm peak electric field at the device edge, while maintaining forward conduction with relatively low on-resistance and voltage loss. The proposed dielectric heterojunction could enable new design strategies to achieve theoretical device performance limits in wide and ultrawide band gap semiconductors where bipolar doping is challenging.

Auger analysis of oxidized GaAs surfaces, heat treated in vacuo, has been used to establish an accurate value for the oxide desorption temperature Tox. Major differences are found in the value of Tox for the surface oxides produced by thermal and ozone oxidation: 582±1 °C and 638±1°C, respectively. These temperature differences are also confirmed by reflection high-energy electron diffraction observations of the thermal cleaning of GaAs substrates prior to epitaxial growth in a molecular beam epitaxy system. It is suggested that the measured temperatures can be used in establishing appropriate growth conditions for ‘‘indium-free’’ GaAs substrates during molecular beam epitaxial growth.