GOX 2022 Session DI-MoP: Dielectric Interfaces Poster Session

Recently, high quality alloys of β-(Al_xGa_1-x)₂O_3, have been grown demonstrating excellent properties for use in high power, high frequency, and high voltage systems and devices such as wireless communication, satellite electronics, and electrified transportation.A natural step to follow the formation and study of these alloys is the study of their thin film heterostructures and subsequent devices. In order to support this, the heterostructure band offsets must be known well enough to model device performances. These values will vary slightly with crystal direction, i.e., the growth plane, as will the optimized growth conditions and film quality. Theoretical predictions for the monoclinic (β) aluminum oxide/gallium oxide interface predict a type-II interface with a maximum VBO value of 0.33 eV for Al₂O₃.

The presented study focuses on the stable (010) heterointerface. Several films of high quality β-(Al_0.21Ga_0.79)₂O₃ were grown by metal organic chemical vapor deposition on bulk (010) oriented β-Ga₂O₃.The indirect bandgap of the β-(Al_0.21Ga_0.79)₂O₃ was determined through optical transmission to be 4.69 eV with a direct transition of 5.37 eV, while β-Ga₂O₃ was confirmed to have an indirect bandgap of 4.52 eV with a direct transition of 4.94 eV. Theoretical calculations for this interface predict a type-II band alignment with a small VBO of only 0.08 eV and a conduction band offset (CBO) of 0.4 eV for a fully strained, 21% Al film on (010) β-Ga₂O₃.Experimentally, in the presented work, the band offsets for this β-(Al_0.21Ga_0.79)₂O₃/β-Ga₂O₃ (010) heterojunction were then measured using x-ray photoelectron spectroscopy.The resulting band alignment was determined to be of type II with the valence and conduction band edges of the β-(Al_0.21Ga_0.79)₂O₃ being -0.26eV and 0.43 eV, respectively above those of the β-Ga₂O₃ (010). These values can now be used to help better design and predict the performance of β-(Al_xGa_1-x)₂O₃ heterojunction-based devices.

For ultra-WBG semiconductors, the development of dielectrics that can hold a much larger electric field demands high-quality films free of buried charges and traps. For Ga₂O₃, Al₂O₃ has proven to be a promising gate dielectric¹, typically grown using ALD at ~250 °C utilizing trimethylaluminum (TMA) and H₂O as precursors. Recently, MOCVD grown Al₂O₃ on β-Ga₂O₃ was demonstrated [2], using TMA and O₂ precursors but grown at 600 °C, with Ar carrier gas. In this prior work, a lower fixed charge of 2 x 10¹² cm^-2 compared to 3.6 x 10¹² cm^-2 in ALD Al₂O₃ on β-Ga₂O₃ was observed². Higher growth temperatures lead to efficient pyrolysis of metal-organic sources like TMA, resulting in lower unintentional C content. The presence of C leads to donor and acceptor level creation, hence low C content allow lower leakage, higher V_BR, and reduced interface-state densities of MOS devices³. In addition, dielectrics grown within the MOCVD reactor (in-situ), as opposed to ALD chamber (ex-situ), avoid a regrowth interface, lowering interface-state densities⁴. Using triethylaluminum (TEA) instead of TMA, may improve the quality of in-situ Al₂O₃ as the ethyl radical is readily desorbed from the growth surface by the β-hydride elimination of ethene, reducing the C in Al₂O₃⁵.

In this work, the Al₂O₃ dielectric growth using TEA and O₂ precursors in a 7x2” MOCVD reactor with close injection showerhead is reported, grown at 500-900°C on Sn-doped Ga₂O₃ and Si substrates. A constant TEA flow of 4.75 µmol/min was introduced into the reactor using Ar and N₂ carrier gases while the O₂ flow was varied as 100, 400, and 700 sccm, resulting in growth rates (GR) of 0.6, 1.4, and 1.7 nm/min, respectively. Compared to 1.2 nm/min GR reported using TMA precursor in an R&D MOCVD reactor [2], these GR demonstrate the prospect of TEA as a potential alternative to further improve the in-situ Al₂O₃ dielectric quality. The influence of N₂ as the sole carrier gas was also studied by introducing a TEA flow of ~3.4 µmol/min at an O₂ flow of 400 sccm, resulting in a 7x lower thickness variation across a 2” wafer without affecting the GR. The C content in Al₂O₃ resulting from TEA and TMA precursors, the use of alternate O precursors like N₂O, the D_it and V_BR comparison with other in-situ dielectrics like SiO₂ or AlSiO_x will be quantified using CV and IV measurements on MOSCAP structures and reported at the conference.

1. K. D. Chabak et al. APL, vol. 109, no. 21, 2016 2. S. Roy et al. AEM, vol. 7, no. 11, 2021 3. M. Uenuma et al. AIP Adv., vol. 8, no. 10, 2018 4. S. H. Chan et al. APEX, vol. 11, no. 4, 2018 5. A. C. Jones, Chem. Soc. Rev. vol. 26, 1997