Invited Speakers

Much excitement has surrounded the accelerating development of β-Ga2O3 for electronics due to its ultrawide band gap, high breakdown voltage, compatibility with many dopants, and comparative ease of producing large substrates via melt-growth techniques. Our research has focused on growth and characterization of Czochralski (CZ) and vertical gradient freeze (VGF) single crystals of β-Ga2O3 with various dopants, including donors (Zr, Hf), acceptors (Mg, Zn, Ni, Cu, Mn), and alloying elements (Al, Sc, In). We find in general that doping in CZ and VGF materials can be different and sometimes non-uniform due to the interaction with crucible material (Ir), selective evaporation, and thermal profile. 

Recent developments, such as sub-oxide MBE, and several new approaches to controllable n-type doping have continued to advance the state-of-the-art for MBE growth of gallium oxide. These works have also shown the important role sub-oxide species play in the molecular beam epitaxy (MBE) growth processes for this material, especially in terms of growth rate and dopant incorporation. Building on this understanding, I will present our team’s experimental effort exploring the relationship between compensating acceptor formation in MBE grown β-Ga2O3 and the Ga2O desorption reaction, as controlled by the metallic gallium flux in our plasma enhanced MBE growth technique.

Recently, many prototype vertical power devices using thick homoepitaxial layers (~10 μm) grown on β-Ga2O3 substrates by halide vapor phase epitaxy (HVPE) have been reported. As is well known, HVPE can grow high-purity β-Ga2O3 layers at high speeds of about 10 μm/h [1]. On the other hand, growth of β-Ga2O3 by metalorganic vapor phase epitaxy (MOVPE) has not been readily explored due to concerns about the violent reaction of metalorganics (MOs) of Ga with O2, and the resulting carbon (C) and hydrogen (H) contamination of the grown layers. However, MOVPE of β-Ga2O3 has been attracting significant attention in recent years due to the possibility of diverting equipment technology developed for the mass production of GaAs- and GaN-based devices. 

Ga2O3 RF transistors have been reported, and a milestone current cut-off frequency fT of 30 GHz was achieved in β-(AlxGa1-x)2/Ga2O3 modulation-doped FET (MODFET) and a maximum oscillation frequency fmax of 48 GHz was realized by employing a 60 nm thin channel [1, 2]. Despite these promises, the output current density, frequency performance, and output powers are far from expected due to its low carrier density, low channel mobility, severe short channels effects, as well as poor thermal conductivity [1]. Very recently, we have demonstrated shallow implantation to produce a two dimensional electron gas (2DEG) like channel in bulk Ga2O3 and deliver a high fT/fmax of 29/35 GHz in MOSFET with deep sub-micron gate lengths [3]. 

Deep acceptor doping is a standard process used in beta-phase gallium oxide (β-Ga2O3) electronic devices for compensating background electron concentrations to create highly-resistive or semi-insulating component layers needed to enable high voltage and RF devices. Iron (Fe), magnesium (Mg) and nitrogen (N) are known acceptors with Fe being most common. However, with its primary energy level at EC-0.8 eV that is associated with the FeGa defect, device biasing and operating temperature can modulate its charge state depending on device design, which can cause operational instabilities in β-Ga2O3 transistors, and for high voltage devices can lead to increased leakage current.

The unique material properties of Gallium Oxide make it promising for a range of future applications, but innovative materials and device engineering are needed to translate these ultimate material limits to real technology. This presentation will discuss our recent work on epitaxy, heterostructure design, and electrostatics to achieve high-performance B-Ga2O3 lateral and vertical electronic devices and photodetectors. We will first discuss recent advances in materials growth and device design for lateral structures which enabled promising transistor demonstrations in the Gallium Oxide material system. Inserting an extreme-permittivity dielectric between the metal and semiconductor is an elegant way to prevent premature tunneling. 

Our modern society's increasing dependence on various electrical and electronic systems makes it necessary to develop highly efficient switching technologies. It is estimated that around 90% of electrical energy will flow through power electronic systems by 2050. Even if Ga2O3-based systems were only 1% more efficient, up to 600 TWh per year could be saved worldwide by 2050 (this corresponds to Germany's current total demand). Therefore our research focus on this innovative material as the next high performance material for high power devices.

In this presentation, we highlight recent progress and challenges in development of β-Ga2O3 materials growth and processing with a focus on high voltage Schottky diodes and vertical transistors. We also place an emphasis on transitioning the materials growth, doping, and processing from the (010) orientation to the scalable (by EFG bulk growth) (001) orientation.