Al Composition Dependence of Band Offsets for SiO₂ on α-(Al_xGa_1−x)₂O₃

Abstract

Valence band offsets for SiO2 deposited by Atomic Layer Deposition on α-(AlxGa1-x)2O3 alloys with x = 0.26–0.74 were measured by X-ray Photoelectron Spectroscopy. The samples were grown with a continuous composition spread to enable investigations of the band alignment as a function of the alloy composition. From measurement of the core levels in the alloys, the bandgaps were determined to range from 5.8 eV (x = 0.26) to 7 eV (x = 0.74). These are consistent with previous measurements by transmission spectroscopy. The valence band offsets of SiO2 with these alloys of different composition were, respectively, were −1.2 eV for x = 0.26, −0.2 eV for x = 0.42, 0.2 eV for x = 0.58 and 0.4 eV for x = 0.74. All of these band offsets are too low for most device applications. Given the bandgap of the SiO2 was 8.7 eV, this led to conduction band offsets of 4.1 eV (x = 0.26) to 1.3 eV (x = 0.74). The band alignments were of the desired nested configuration for x > 0.5, but at lower Al contents the conduction band offsets were negative, with a staggered band alignment. This shows the challenge of finding appropriate dielectrics for this ultra-wide bandgap semiconductor system.

Band Alignment of Al2O3 on α-(AlxGa1-x)2O3

“…We have recently found that SiO 2 deposited on the α-(Al x Ga 1-x ) 2 O 3 alloys shows band alignments that are staggered type II for x < 0.5 and straddling type I for x > 0.5, with conduction band offsets >1.3 eV across the composition range examined. 72 Even this large gap dielectric will only provide adequate valence band offsets over a limited range of compositions. 72 Note also that we find the bandgap of the ALD Al 2 O 3 is slightly smaller than the α(Al 0.74 Ga 0.26 ) 2 O 3 , which is a common feature when comparing amorphous thin films to polycrystalline films.…”

“…72 Even this large gap dielectric will only provide adequate valence band offsets over a limited range of compositions. 72 Note also that we find the bandgap of the ALD Al 2 O 3 is slightly smaller than the α(Al 0.74 Ga 0.26 ) 2 O 3 , which is a common feature when comparing amorphous thin films to polycrystalline films. [22][23][24][25][26] The optical bandgap of a-Al 2 O 3 single crystal is 8.7-8.8 eV.…”

Determination of anisotropic optical properties of MOCVD grown m-plane α-(Al x Ga1−x )2O3 alloys

“…46,47) So far, some investigations of the optical properties of α-(Al x Ga 1−x ) 2 O 3 have been made by Ito et al 48) and Jinno et al 38) employing optical transmission measurements. Dang et al 35) analyzed a Tauc plot and Uchida et al 49) Chen et al 36) and Xia et al 50) utilized X-ray photoelectron spectroscopy (XPS) to determine bandgap values and/or bowing parameters. But since the corundum crystal structure is anisotropic, a polarization dependent investigation of the material properties is crucial.…”