High Quality Al₂O₃/(111) (OH)-Terminated Diamond Interface for MOSFETs Fabrication

The efficiency of power devices is critical nowadays for the rise of renewable energy. Wide-bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), have advantages over silicon due to their high breakdown electric field and high-temperature operation. However, p-channel wide-bandgap FETs have poor performance compared with their n-channel counterparts due to low hole mobility, which leads to a high ON resistance and high conduction loss. This has constrained the production of complementary circuits with integrated n-channel and p-channel transistors. Diamond is a wide-bandgap material that has a bulk intrinsic hole mobility very high respectively to SiC and GaN, which makes it promising for use in high-mobility p-channel transistors. Due to the high optical phonon frequency and high acoustic phonon velocity of diamond, the number of phonons that participate in carrier scattering is small. Having all the others wide band gap semiconductors characteristics like high electric field, high breakdown voltage and high thermal conductivity also makes diamond attractive for power applications.<br/>Most diamond power metal–oxide–semiconductor field-effect transistors (MOSFETs) have been fabricated using H-terminated diamond surfaces (and afterwards the surface transfer doping) with their unique properties of two-dimensional hole gas (2DHG). The downside of this concept comes from the close proximity of the negatively charged surface acceptors with the 2DHG, the carrier mobility is limited by surface impurities, and increasing the carrier concentration dramatically decreases the hole mobility. Moreover, the negative charges cause a positive shift in the threshold voltage, leading to normally ON characteristics.<br/>In this work we try to find an alternative by realizing p-type OH-terminated diamond surface, taking advantage of the fact that applying a negative potential to a MOS stack leads to an accumulation regime of holes on the diamond surface. However, this happens only if the Fermi level is not pinned due to interface states. Therefore, a good interface quality between diamond and oxide is needed. We realized a p-type diamond with a boron concentration approximately 10¹⁷ atom/cm³ and a thickness of 0.5 μm on (111) EDP substrate followed by a OH-termination using a wet vapour annealing^[1]. We have then realized some metal–oxide–semiconductor capacitors (MOSCAP) on top of it and studied their behaviour.<br/>The frequency-dependent capacitance-voltage (C–V) characteristics were examined from depletion to accumulation regimes. The latter one was clearly observed with a very small leakage current. Moreover, we reached accumulation regime around 0V which can lead to a realization of a MOSFET with normally OFF characteristics with very high current in ON state, thanks to the 2DHG. With these promising results, we finally fabricate MOSFETs using the same procedure as MOSCAPs.<br/>In the first part of this presentation, the method will be described in details. Then, the C-V characteristics will be reported and deeply analysed. Lastly, we will show and discuss the first results obtained regarding the MOSFETs and comparing them with finite element simulation as well.<br/><b>References</b><br/>[1] Ryo Yoshida et al, “Formation of atomically flat hydroxyl-terminated diamond (111) surfaces via water vapor annealing”, Applied Surface Science, 458 (2018)