(45c) Effect of Deposition Method on Valence Band Offsets of SiO2 and Al2O3 on (Al0.14Ga0.86)2O3

The β-(Al_xGa_1-x)₂O₃/Ga₂O₃ heterostructure is attracting attention for field-effect transistors (HFETs) that have shown impressive and improving performance. The formation of a two-dimensional electron gas enhances the transport properties, with advantages in device performance. These heterostructures extend the range of options for Ga₂O₃ based electronics, which is a welcome addition to potential ultra-wide bandgap semiconductors with power figure-of-merits well beyond SiC and GaN. The absence of p-type doping for Ga₂O₃ and its alloys means only majority carrier devices are possible and the availability of (Al_xGa_1-x)₂O₃/Ga₂O₃ heterostructures for scaled field effect transistors is important if radiofrequency applications are to be addressed by Ga₂O₃. The options for gate dielectric are limited in the case of (Al_xGa_1-x)₂O₃ (referred to here in short as AGO), which exhibits a bandgap of around 5 eV and must utilize dielectric material with a bandgap of 7 eV of higher to have adequate offsets in both the conduction and valence bands. Additionally, the gate dielectric must also be able to be deposited without disrupting the oxide surface. The two most common gate dielectrics in general for Ga₂O₃ and specifically for β-(Al_xGa_1-x)₂O₃/Ga₂O₃ have been Al₂O₃ and SiO₂. Both of these dielectrics are thermodynamically stable on the AGO. Recent studies have shown the surface of β-Ga₂O₃ is easily degraded by exposure to plasma environments involving energetic ion bombardment. An advantage of sputtering is that it is broadly available and can produce pure oxides, but sputtered oxides can have plasma-induced damage.

In this paper we show that there are differences of up to 1 eV in band alignments for the common gate dielectrics SiO₂ and Al₂O₃ on single crystal (Al_0.14Ga_0.86)₂O₃, depending on whether they are deposited by sputtering or Atomic Layer Deposition. In the case of Al₂O₃, this changed the band alignment from nested (type I) to staggered gap (type II). The valence band offset at eachheterointerface was measured using X-Ray Photoelectron Spectroscopy and was determined to be -0.85 eV for sputtered Al₂O₃ and 0.23 eV for ALD Al₂O₃ on β-(Al_0.14Ga_0.86)₂O₃, while for SiO₂ it was 0.6 eV for sputtered and 1.6 eV for ALD. These results are consistent with recent results showing that the surface of Ga₂O₃ and related alloys are susceptible to severe changes during exposure to energetic ion environments.