High electron density β-(Al0.17Ga0.83)2O3/Ga2O3 modulation doping using an ultra-thin (1 nm) spacer layer

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

N-type doping of low-pressure chemical vapor deposition grown β-Ga2O3 thin films using solid-source germanium

“…Devices with power densities reaching as high as 1 GW/cm 2 14 and breakdown voltages up to 8 kV 8 were already realized. In addition to unipolar devices, rapid progress has been made in studying β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures [15][16][17][18] and integration of p-type materials with β-Ga2O3 19 .…”

“…Growth of β-Ga2O3 has already been studied using MBE for a variety of film orientations (010), (-201), ( 001) and (110) and dopants(Si, Sn and Ge) [30][31][32][33][34] . A variety of lateral FETs have been fabricated using MBE-grown β-Ga2O3 films 2,18 . Whereas PLD is primarily utilized for studying heteroepitaxial growth of Ga2O3 polymorphs 35 and achieving heavily doped n + β-Ga2O3 layers 25 .…”

Growth and characterization of metalorganic vapor-phase epitaxy-grown β-(Al x Ga1−x )2O3/β-Ga2O3 heterostructure channels

“…2DEG sheet charge densities between 1x10 12 -5 x10 12 cm -2 and mobilities of 75 -180 cm 2 /Vs have been achieved using MBE technique. Currently the maximum sheet charge density reported in MBE-grown β-(AlxGa1-x)2O3/β-Ga2O3 is less than 5 x 10 12 cm -2 for a single heterostructure without parallel channel in β-(AlxGa1-x)2O3 22 . Furthermore, composition of MBE-grown β-(AlxGa1-x)2O3 films is limited to x < 0.25 because of the limited growth temperature window 23 .…”

“…However, all the current literature is based on MBE-grown β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures [17][18][19][20]22 . 2DEG sheet charge densities between 1x10 12 -5 x10 12 cm -2 and mobilities of 75 -180 cm 2 /Vs have been achieved using MBE technique.…”

“…Reports of charge freeze out at low temperature have been observed in β-(AlxGa1-x)2O3/β-Ga2O3 single and double heterostructures 20,21 and uniformly doped β-(AlxGa1-x)2O3 thin films 29 . High background charge (1.5x10 17 -3x10 18 cm -3 ) was required to model mobility in MBE-grown β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures 19,22 . The above analysis indicates that getting a very high 2DEG charge in β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures is still challenging.…”