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1. Type II band alignment of NiO/α-Ga ₂ O ₃ for annealing temperatures up to 600 °C

   https://doi.org/10.1116/6.0002257  

   Xia, Xinyi; Li, Jian-Sian; Chiang, Chao-Ching; Yoo, Timothy Jinsoo; Hershkovitz, Eitan; Ren, Fan; Kim, Honggyu; Kim, Jihyun; Jeon, Dae-Woo; Park, Ji-Hyeon; et al (December 2022, Journal of Vacuum Science & Technology A)

   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. 

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2. Impact of process anneals on high-k/β-Ga ₂ O ₃ interfaces and capacitance

   https://doi.org/10.1116/6.0002264  

   Hawkins, Roberta; Wang, Xinglu; Moumen, Naim; Wallace, Robert M.; Young, Chadwin D. (March 2023, Journal of Vacuum Science & Technology A)

   Gallium oxide (β-Ga 2 O 3 ) is becoming a popular material for high power electronic devices due to its wide bandgap and ease of processing. In this work, β-Ga 2 O 3 substrates received various annealing treatments before atomic layer deposition of HfO 2 and subsequent fabrication of metal–oxide–semiconductor (MOS) capacitors. Annealing of β-Ga 2 O 3 with forming gas or nitrogen produced degraded capacitance–voltage (C–V) behavior compared to a β-Ga 2 O 3 control sample with no annealing. A sample annealed with pure oxygen had improved C–V characteristics relative to the control sample, with a higher maximum capacitance and smaller flat-band voltage shift, indicating that oxygen annealing improved the C–V behavior. X-ray photoelectron spectroscopy also suggested a reduction in the oxygen vacancy concentration after O 2 annealing at 450 °C, which supports the improved C–V characteristics and indicates that O 2 annealing of β-Ga 2 O 3 may lead to better MOS device performance. 

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3. In situ MOCVD growth and band offsets of Al ₂ O ₃ dielectric on β-Ga ₂ O ₃ and β-(Al _x Ga _1−x ) ₂ O ₃ thin films

   https://doi.org/10.1063/5.0104433  

   Bhuiyan, A F; Meng, Lingyu; Huang, Hsien-Lien; Hwang, Jinwoo; Zhao, Hongping (October 2022, Journal of Applied Physics)

   The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al 2 O 3 dielectrics on β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films is investigated as a function of crystal orientations and Al compositions of β-(Al x Ga 1−x ) 2 O 3 films. The interface and film qualities of Al 2 O 3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al 2 O 3 dielectrics with abrupt interfaces on (010), (100), and [Formula: see text] oriented β-(Al x Ga 1−x ) 2 O 3 films. The surface stoichiometries of Al 2 O 3 deposited on all orientations of β-(Al x Ga 1−x ) 2 O 3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al 2 O 3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(Al x Ga 1−x ) 2 O 3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the [Formula: see text] orientation. The results from this study on MOCVD growth and band offsets of amorphous Al 2 O 3 deposited on differently oriented β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films will potentially contribute to the design and fabrication of future high-performance β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 based transistors using MOCVD in situ deposited Al 2 O 3 as a gate dielectric. 

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4. NiO/β-(Al _x Ga _1−x ) ₂ O ₃ /Ga ₂ O ₃ heterojunction lateral rectifiers with reverse breakdown voltage >7 kV

   https://doi.org/10.1116/6.0002393  

   Wan, Hsiao-Hsuan; Li, Jian-Sian; Chiang, Chao-Ching; Xia, Xinyi; Ren, Fan; Masten, Hannah N.; Lundh, James Spencer; Spencer, Joseph A.; Alema, Fikadu; Osinsky, Andrei; et al (May 2023, Journal of Vacuum Science & Technology A)

   NiO/β-(Al x Ga 1− x ) 2 O 3 /Ga 2 O 3 heterojunction lateral geometry rectifiers with diameter 50–100  μm exhibited maximum reverse breakdown voltages >7 kV, showing the advantage of increasing the bandgap using the β-(Al x Ga 1− x ) 2 O 3 alloy. This Si-doped alloy layer was grown by metal organic chemical vapor deposition with an Al composition of ∼21%. On-state resistances were in the range of 50–2180 Ω cm 2 , leading to power figures-of-merit up to 0.72 MW cm −2 . The forward turn-on voltage was in the range of 2.3–2.5 V, with maximum on/off ratios >700 when switching from 5 V forward to reverse biases up to −100 V. Transmission line measurements showed the specific contact resistance was 0.12 Ω cm 2 . The breakdown voltage is among the highest reported for any lateral geometry Ga 2 O 3 -based rectifier. 

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5. Si doping in MOCVD grown (010) β-(Al _x Ga _1−x ) ₂ O ₃ thin films

   https://doi.org/10.1063/5.0084062  

   Bhuiyan, A. F.; Feng, Zixuan; Meng, Lingyu; Fiedler, Andreas; Huang, Hsien-Lien; Neal, Adam T.; Steinbrunner, Erich; Mou, Shin; Hwang, Jinwoo; Rajan, Siddharth; et al (April 2022, Journal of Applied Physics)

   In this work, the structural and electrical properties of metalorganic chemical vapor deposited Si-doped β-(Al x Ga 1−x ) 2 O 3 thin films grown on (010) β-Ga 2 O 3 substrates are investigated as a function of Al composition. The room temperature Hall mobility of 101 cm 2 /V s and low temperature peak mobility (T = 65 K) of 1157 cm 2 /V s at carrier concentrations of 6.56 × 10 17 and 2.30 × 10 17  cm −3 are measured from 6% Al composition samples, respectively. The quantitative secondary ion mass spectroscopy (SIMS) characterization reveals a strong dependence of Si and other unintentional impurities, such as C, H, and Cl concentrations in β-(Al x Ga 1−x ) 2 O 3 thin films, with different Al compositions. Higher Al compositions in β-(Al x Ga 1−x ) 2 O 3 result in lower net carrier concentrations due to the reduction of Si incorporation efficiency and the increase of C and H impurity levels that act as compensating acceptors in β-(Al x Ga 1−x ) 2 O 3 films. Lowering the growth chamber pressure reduces Si concentrations in β-(Al x Ga 1−x ) 2 O 3 films due to the increase of Al compositions as evidenced by comprehensive SIMS and Hall characterizations. Due to the increase of lattice mismatch between the epifilm and substrate, higher Al compositions lead to cracking in β-(Al x Ga 1−x ) 2 O 3 films grown on β-Ga 2 O 3 substrates. The (100) cleavage plane is identified as a major cracking plane limiting the growth of high-quality Si-doped (010) β-(Al x Ga 1−x ) 2 O 3 films beyond the critical thicknesses, which leads to highly anisotropic and inhomogeneous behaviors in terms of conductivity. 

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