Modulation-doped heterostructures are a key enabler for realizing high mobility and better scaling properties for high performance transistors. We report the realization of a modulation-doped two-dimensional electron gas (2DEG) at the β-(Al0.2Ga0.8)2O3/Ga2O3 heterojunction by silicon delta doping. The formation of a 2DEG was confirmed using capacitance voltage measurements. A modulation-doped 2DEG channel was used to realize a modulation-doped field-effect transistor. The demonstration of modulation doping in the β-(Al0.2Ga0.8)2O3/Ga2O3 material system could enable heterojunction devices for high performance electronics.

No abstract

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.

Threshold voltage instability was observed on β-Ga 2 O 3 transistors using double-pulsed current-voltage and constant drain current deep level transient spectroscopy (DLTS) measurements. A total instability of 0.78 V was attributed to two distinct trap levels, at E C -0.70 and E C -0.77 eV, which need to be mitigated for future applications. The traps are likely located near the gate-drain edge and below the delta-doped layer, which is determined through the DLTS technique and an understanding of the fill and empty biasing conditions. The trap modulation was consistent with a gate leakage-based trap filling mechanism, which was demonstrated. It is likely that Fe is playing a role in the observed dispersion due to the close proximity of the Fe substrate. Index Terms-Gallium oxide, MESFETs, trapping effects, V T instability. I. INTRODUCTION G ALLIUM oxide in its beta phase has received a lot of attention recently for potential applications in high voltage and high frequency devices due to its wide bandgap of ∼4.8 eV and theoretical breakdown field of ∼8 MV/cm, leading to a ∼4× increase in the Baliga figure of merit compared to GaN [1]. β-Ga 2 O 3 also offers the advantage of growing large area bulk substrates from inexpensive melt methods [2]-[5]. Bulk crystalline substrate availability enables homoepitaxy which supports low concentrations of crystallographic defects, such as threading dislocations, compared to growth on lattice-mismatched substrates. There are limited