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- Journal of Vacuum Science & Technology A
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- National Science Foundation
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There is increasing interest in the alpha polytype of Ga 2 O 3 because of its even larger bandgap than the more studied beta polytype, but in common with the latter, there is no viable p-type doping technology. One option is to use p-type oxides to realize heterojunctions and NiO is one of the candidate oxides. The band alignment of sputtered NiO on α-Ga 2 O 3 remains type II, staggered gap for annealing temperatures up to 600 °C, showing that this is a viable approach for hole injection in power electronic devices based on the alpha polytype of Ga 2 O 3 . The magnitude of both the conduction and valence band offsets increases with temperature up to 500 °C, but then is stable to 600 °C. For the as-deposited NiO/α-Ga 2 O 3 heterojunction, ΔE V = −2.8 and ΔE C = 1.6 eV, while after 600 °C annealing the corresponding values are ΔE V = −4.4 and ΔE C = 3.02 eV. These values are 1−2 eV larger than for the NiO/β-Ga 2 O 3 heterojunction.
Abstract The band alignment of sputtered NiO on β -Ga 2 O 3 was measured by x-ray photoelectron spectroscopy for post-deposition annealing temperatures up to 600 °C. The band alignment is type II, staggered gap in all cases, with the magnitude of the conduction and valence band offsets increasing monotonically with annealing temperature. For the as-deposited heterojunction, Δ E V = −0.9 eV and Δ E C = 0.2 eV, while after 600 °C annealing the corresponding values are Δ E V = −3.0 eV and Δ E C = 2.12 eV. The bandgap of the NiO was reduced from 3.90 eV as-deposited to 3.72 eV after 600 °C annealing, which accounts for most of the absolute change in Δ E V −Δ E C . Differences in thermal budget may be at least partially responsible for the large spread in band offsets reported in the literature for this heterojunction. Other reasons could include interfacial disorder and contamination. Differential charging, which could shift peaks by different amounts and could potentially be a large source of error, was not observed in our samples.
The characteristics of sputtered NiO for use in pn heterojunctions with Ga 2 O 3 were investigated as a function of sputtering parameters and postdeposition annealing temperature. The oxygen/ nickel and Ni 2 O 3 /NiO ratios, as well as the bandgap and resistivity, increased as a function of O 2 /Ar gas flow ratio. For example, the bandgap increased from 3.7 to 3.9 eV and the resistivity increased from 0.1 to 2.9 Ω cm for the O 2 /Ar ratio increasing from 1/30 to 1/3. By sharp contrast, the bandgap and Ni 2 O 3 /NiO ratio decreased monotonically with postdeposition annealing temperatures up to 600 °C, but the density of films increased due to a higher fraction of NiO being present. Hydrogen is readily incorporated into NiO during exposure to plasmas, as delineated by secondary ion mass spectrometry measurements on deuterated films. The band alignments of NiO films were type II-staggered gaps with both α- and β-Ga 2 O 3 . The breakdown voltage of NiO/β-Ga 2 O 3 heterojunction rectifiers was also a strong function of the O 2 /Ar flow ratio during deposition, with values of 1350 V for 1/3 and 830 V for 1/30.
Valence band offsets for SiO 2 deposited by Atomic Layer Deposition on α -(Al x Ga 1-x ) 2 O 3 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 SiO 2 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 SiO 2 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 bandmore »
The band alignment of Atomic Layer Deposited SiO2on (InxGa1−x)2O3at varying indium concentrations is reported before and after annealing at 450 °C and 600 °C to simulate potential processing steps during device fabrication and to determine the thermal stability of MOS structures in high-temperature applications. At all indium concentrations studied, the valence band offsets (VBO) showed a nearly constant decrease as a result of 450 °C annealing. The decrease in VBO was −0.35 eV for (In0.25Ga0.75)2O3, −0.45 eV for (In0.42Ga0.58)2O3, −0.40 eV for (In0.60Ga0.40)2O3, and −0.35 eV (In0.74Ga0.26)2O3for 450 °C annealing. After annealing at 600 °C, the band alignment remained stable, with <0.1 eV changes for all structures examined, compared to the offsets after the 450 °C anneal. The band offset shifts after annealing are likely due to changes in bonding at the heterointerface. Even after annealing up to 600 °C, the band alignment remains type I (nested gap) for all indium compositions of (InxGa1−x)2O3studied.