(Invited) Beta- Gallium (Aluminum) Oxide Epitaxial Thin Films, Heterostructures and in-Situ Dielectrics for High Performance Electronics
Authors: Krishnamoorthy Sriram , Bhattacharyya Arkka , Peterson Carl , Roy Saurav 
Published: December 2024

In this talk, we will provide an overview of epitaxial growth, doping and channel engineering techniques towards β-Ga2O3 high-performance power and high frequency devices. Metal-organic vapor phase epitaxy (MOVPE) offers several advantages such as high carrier mobility, scalability, and in-situ growth of various heterostructures including Aluminum Gallium Oxide ternary alloys and Al2O3 in-situ dielectrics.

Study of intentional silicon doping is undertaken to understand uniform doping, delta doping and modulation doping in β-Ga2O3 thin films and heterostructures. Low temperature epitaxy is demonstrated with sharp doping profiles and high carrier mobility, enhancing the growth window of high quality β-Ga2O3 thin films. By understanding surface riding of dopants, delta doping is achieved in MOVPE-grown β-Ga2O3 with a sharp doping profile by controlling the substrate temperature. This also enabled the first demonstration of MOVPE-grown pure 2D electron channel in β-Ga2O3 material system without any parallel conduction in the Aluminum Gallium Oxide barrier layer. The absence of parallel conduction is confirmed using low temperature hall measurements. A low-temperature undoped buffer scheme developed resulted in record high electron mobilities in uniformly Si-doped β-Ga2O3 thin films. Material quality is further evidenced by high current β-Ga2O3 fin shape transistors with power figure of merit close to 1 GW/cm2. In-situ growth of Al2O3 dielectrics on β-Ga2O3 epilayers with breakdown field strength as high as 6-10 MV/cm is achieved, showing promise towards realization of β-Ga2O3 MOSFETs with in-situ dielectrics.

Vertical power devices require high purity, low-doped drift layers. To this effect, we report on the growth of both intentionally low-doped (~ 7 x 1015 cm-3 ) and unintentionally doped (UID) β-Ga2O3 homoepitaxial films (~ 2 x 1015 cm-3) using MOCVD with high room temperature mobilities and thicknesses of up to 187-190 cm2/V•s and 4.5- 6.2 μm respectively. By optimizing the VI/III ratio, 6.3 μm UID film was grown with a flat charge density profile (verified by isolated Van der Pauw Hall and vertical C-V measurements) of 2.4 x 1015 cm-3, a high Hall mobility of 190 cm2/V•s, and RMS surface roughness values of 1.3 nm. Vertical Schottky barrier diodes were formed on the 6.3 μm thick drift layer and the J-V measurements showed a high rectification ratio of >109. Finally, breakdown measurements were performed and the parallel plane field at the center of the anode was found to be 2.05 MV/cm (without any field management), which is the highest reported for any MOVPE-grown vertical diode. The mobility values in the intentional and unintentionally doped films are the highest reported at these background concentration levels and suggest very low compensation in the grown films. The growth of these high-mobility and low background concentration thick β-Ga2O3 films is a crucial step forward towards realizing thick epitaxial drift layers for efficient > 10 kV vertical power devices, which can be very attractive for grid-scale electronics.

We acknowledge funding from the Coherent/II–VI Foundation Block Gift Program and Air Force Office of Scientific Research, under Award No. FA9550-21-1-0078 (Program Manager: Dr.Ali Sayir).