NiO is a promising alternative to p-GaN as a hole injection layer for normally-off lateral transistors or low on-resistance vertical heterojunction rectifiers. The valence band offsets of sputtered NiO on c-plane, vertical geometry homoepitaxial GaN structures were measured by x-ray photoelectron spectroscopy as a function of annealing temperatures to 600 °C. This allowed determination of the band alignment from the measured bandgap of NiO. This alignment was type II, staggered gap for both as-deposited and annealed samples. For as-deposited heterojunction, ΔEV = 2.89 eV and ΔEC = −2.39 eV, while for all the annealed samples, ΔEVvalues were in the range of 3.2–3.4 eV and ΔECvalues were in the range of −(2.87–3.05) eV. The bandgap of NiO was reduced from 3.90 eV as-deposited to 3.72 eV after 600 °C annealing, which accounts for much of the absolute change in ΔEV − ΔEC. At least some of the spread in reported band offsets for the NiO/GaN system may arise from differences in their thermal history.more » « less
- Award ID(s):
- NSF-PAR ID:
- Publisher / Repository:
- American Vacuum Society
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology A
- Page Range / eLocation ID:
- Article No. 053401
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
There is increasing interest in the alpha polytype of Ga2O3 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-Ga2O3 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 Ga2O3. 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-Ga2O3 heterojunction, ΔEV = −2.8 and ΔEC = 1.6 eV, while after 600 °C annealing the corresponding values are ΔEV = −4.4 and ΔEC = 3.02 eV. These values are 1−2 eV larger than for the NiO/β-Ga2O3 heterojunction.
The characteristics of sputtered NiO for use in pn heterojunctions with Ga2O3 were investigated as a function of sputtering parameters and postdeposition annealing temperature. The oxygen/ nickel and Ni2O3/NiO ratios, as well as the bandgap and resistivity, increased as a function of O2/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 O2/Ar ratio increasing from 1/30 to 1/3. By sharp contrast, the bandgap and Ni2O3/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-Ga2O3. The breakdown voltage of NiO/β-Ga2O3 heterojunction rectifiers was also a strong function of the O2/Ar flow ratio during deposition, with values of 1350 V for 1/3 and 830 V for 1/30.
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