(Invited) Rare Earth Oxides on Wide Band Gap Semiconductors
Authors: Mitrovic Ivona Z. , Finch Harry , Jones Leanne A.H. , Dhanak Vinod R. , Hannah Adrian N. , Valizadeh Reza , Renz Arne Benjamin B. , Shah Vishal Ajit , Gammon Peter Michael , Mawby P. A. 
Published: July 2022

Globally, most recent power electronics converter and device technology has been driven by the unprecedented demand seen within the automotive sector [1]. The latter demands power converters with near 100% energy efficiency, lightweight, compact and reliable. Wide band gap (WBG) semiconductor materials such as GaN and 4H-SiC have emerged as contenders to replace Si in many power electronics applications. Currently, GaN devices based on the high electron mobility transistor (HEMT) architecture are limited commercially to a maximum of 650 V [2,3]. GaN HEMTs have traditionally suffered from a poor thermal conductivity and the “current collapse” phenomenon, limiting their ability to operate within harsh environments. On the other hand, 4H-SiC has a few reliability issues that limit its ubiquitous application in the automotive sector [2]. Currently the GaN based Metal-Insulator-Semiconductor (MIS)-HEMT device is seen to demonstrate superior performance in power electronics applications over the Schottky gate counterpart, due to its inherently lower gate leakage current, together with the ability to provide larger forward gate voltage swing by engineering the threshold voltage between depletion and enhancement mode operation and also an improved gate-drain breakdown voltage. High band gap gate dielectric materials are preferable as they can provide higher tunnelling barriers for electrons and holes, which result in lower gate leakage current. Furthermore, high dielectric constant (high-k) material is also necessary for improved electrostatic control over the channel and improved on-current, which in-turn results in higher transconductance. In terms of SiC based devices, the use of SiO2 proves to be a bottleneck in exploiting full potential of SiC due to the low value of dielectric constant of SiO2 [4]. Since the oxide is subjected to an electric field that is ~2.5x the field in the semiconductor, the breakdown of SiO2 occurs first, and hence causes the breakdown of SiO2/4H-SiC based devices well below the critical electric field of SiC. This shortcoming led to the exploration of high-k dielectrics as gate stacks in 4H-SiC MIS based devices as well as for surface passivation. It is worth noting that SiO2 is the only dielectric commercially used for SiC devices largely due to the unavailability of a reliable high-k dielectric alternative. Most published results focus on use of Al2O3 and HfO2 films [5], however they have yet to become used mainstream in 4H-SiC devices.
In this paper, two rare earth high-k oxides, Y2O3 and Sc2O3, both with dielectric constant >10, have been studied in terms of their suitability as gate dielectrics on GaN and 4H-SiC. The oxide films were prepared by radio frequency (RF) magnetron sputtering (Sc2O3) and ion gun sputtering (Y2O3). The substrates were cleaned with Kr ions with anode current of ~ 50 mA and accelerating voltage of 3 kV for 1-2 hours. Then metallic Y was sputtered using pressure of 1.5 x 10-5 mbar and current of 26 mA, which was followed by exposure to O2 to form Y2O3 films. Sc2O3 films were deposited using Moorfields nano-PVD (physical vapour deposition) equipment with circular Sc2O3 target of 99.99% purity using RF power of 60 W and a gas flow of 0.5 sccm for 3 nm film and 3 sccm for 40 nm film. The films were also deposited simultaneously on Si to be used as reference samples for variable angle spectroscopic ellipsometry (VASE) measurements to ascertain their thickness and optical properties. The X-ray photoelectron spectroscopy (XPS) was used to characterise comprehensively the oxide/semiconductor interface. The capacitance voltage measurements on MIS stacks were used to determine permittivity of deposited oxides. The complete band alignment of oxide/WBG semiconductor as well as comprehensive comparison to state of the art data will be presented and discussed in full paper.

Acknowledgement. The UKIERI IND/CONT/G/17-18/18 and F.No.184-1/2018(IC) project funded by the British Council; UKRI GCRF GIAA award 2018/19 and EP/P510981/1, funded by the EPSRC, UK.

References. [1] http://www.yole.fr/Compound_Semiconductor_Monitor_Q1.aspx. [2] Li et al., Materials 14, 5831 (2021); [3] Gonzalez et al. IEEE Trans. Ind. Electron. 67, 7375 (2020); [4] A. Siddiqui et al., J. Mater. Chem. C 9, 5055 (2021); [5] Bencherif et al., Appl. Phys. A 126, 854 (2020).