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1. Magnetic properties and spin reorientation of hexagonal HoFeO3 thin films

   https://doi.org/https://doi.org/10.1016/j.tsf.2021.138596  

   R.C. Rai, C. Horvatits (April 2021, Thin solid films)

   null (Ed.)

   We present structural, magnetic, and optical properties of hexagonal HoFeO3/Al2O3 thin films deposited by Magnetron Sputtering. The x-ray diffraction patterns of HoFeO3 thin films show the c-planes of a hexagonal structure. The magnetization data display an antiferromagnetic transition temperature, TN∼120 ± 5 K and the magnetization-field hysteresis loops were measured below 100 K, confirming a weak ferromagnetism arising from a spin canting of the Fe3+ moments. The magnetization data also show an anomaly around ∼40 K due to a spin-reorientation transition caused by the Ho3+- Fe3+ interactions. We observed comparable magnetization along the ab plane and c axis although the spin canting of Fe3+ sites has a preferential component along the c axis, suggesting that the Ho3+- Fe3+ interactions dominate in the low temperature magnetic structures of hexagonal-HoFeO3. The observed electronic excitations at ∼2.29, 2.87, 3.82, 4.79, and 6.53 eV have been assigned to the Fe3+ d to d on-site as well as O 2p to Fe 3d, Ho 6s, and 5d charge-transfer excitations, respectively. The room temperature energy band gap of the hexagonal-HoFeO3 thin film was measured to be ∼1.99 ± 0.04 eV. 

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2. Effect of post-deposition annealing on crystal structure of RF magnetron sputtered germanium dioxide thin films

   https://doi.org/10.1116/6.0003960  

   Abed, Ahmad Matar; Peterson, Rebecca L (October 2024, Journal of Vacuum Science & Technology A)

   In this work, we demonstrate the growth and phase stabilization of ultrawide bandgap polycrystalline rutile germanium dioxide (GeO2) thin films. GeO2 thin films were deposited using RF magnetron sputtering on r-plane sapphire (Al2O3) substrates. As-deposited films were x-ray amorphous. Postdeposition annealing was performed at temperatures between 650 and 950 °C in an oxygen or nitrogen ambient. Annealing at temperatures from 750 to 950 °C resulted in mixed-phase polycrystalline films containing tetragonal (rutile) GeO2, hexagonal (α-quartz) GeO2, and/or cubic (diamond) germanium (Ge). When nitrogen was used as the anneal ambient, mixed GeO2 phases were observed. In contrast, annealing in oxygen promoted stabilization of the r-GeO2 phase. Grazing angle x-ray diffraction showed a preferred orientation of (220) r-GeO2 for all crystallized films. The combination of O2 annealing and O2 flux during growth resulted in r-GeO2 films with highly preferential alignment. Using electron microscopy, we observed an interfacial layer of hexagonal-oriented GeO2 with epitaxial alignment to the (11¯02) Al2O3 substrate, which may help stabilize the top polycrystalline r-GeO2 film. 

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3. Annealing Induced Structural Phase Change of Hexagonal‐LuFeO3 Thin Films

   R. C. Rai, D. Mckenna (April 2018, Ceramic Transactions Series)

   This chapter presents structural, optical, and magnetic properties of multiferroic LuFeO3 thin films, deposited on single crystal sapphire and YSZ substrates by an RF magnetron sputtering system. Growth temperature and annealing are found to be critical to stabilize hexagonal LuFeO3 thin films. Radio‐Frequency (RF) Magnetron Sputtering is relatively cost effective and one of the most commonly used methods for the deposition of oxides. An RF Magnetron Sputtering offers flexibility in terms of controlling the growth conditions, maintaining the stoichiometry, and a higher deposition rate. When the lattice strain is released due to annealing, the thin film can form bigger granular structures, as observed in the AFM image, by the nucleation process. The inset shows an example of the energy band edge fitting with the direct energy band gap model. 

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4. MOCVD growth and band offsets of κ-phase Ga ₂ O ₃ on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) YSZ substrates

   https://doi.org/10.1116/6.0002106  

   Bhuiyan, A F; Feng, Zixuan; Huang, Hsien-Lien; Meng, Lingyu; Hwang, Jinwoo; Zhao, Hongping (December 2022, Journal of Vacuum Science & Technology A)

   Epitaxial growth of κ-phase Ga 2 O 3 thin films is investigated on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) oriented yttria stabilized zirconia (YSZ) substrates via metalorganic chemical vapor deposition. The structural and surface morphological properties are investigated by comprehensive material characterization. Phase pure κ-Ga 2 O 3 films are successfully grown on GaN-, AlN-on-sapphire, and YSZ substrates through a systematical tuning of growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, whereas the growth on c-sapphire substrates leads to a mixture of β- and κ-polymorphs of Ga 2 O 3 under the investigated growth conditions. The influence of the crystalline structure, surface morphology, and roughness of κ-Ga 2 O 3 films grown on different substrates are investigated as a function of precursor flow rate. High-resolution scanning transmission electron microscopy imaging of κ-Ga 2 O 3 films reveals abrupt interfaces between the epitaxial film and the sapphire, GaN, and YSZ substrates. The growth of single crystal orthorhombic κ-Ga 2 O 3 films is confirmed by analyzing the scanning transmission electron microscopy nanodiffraction pattern. The chemical composition, surface stoichiometry, and bandgap energies of κ-Ga 2 O 3 thin films grown on different substrates are studied by high-resolution x-ray photoelectron spectroscopy (XPS) measurements. The type-II (staggered) band alignments at three interfaces between κ-Ga 2 O 3 and c-sapphire, AlN, and YSZ substrates are determined by XPS, with an exception of κ-Ga 2 O 3 /GaN interface, which shows type-I (straddling) band alignment. 

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5. Signature of structural distortion in optical spectra of YFe2O4 thin film

   https://doi.org/10.1063/1.4942753  

   Rai, R_C; Hinz, J.; Petronilo, G_X_A; Sun, F.; Zeng, H.; Nakarmi, M_L; Niraula, P_R (February 2016, AIP Advances)

   We report structural, optical, and electro-optical properties of polycrystalline YFe2O4 thin films, deposited on (0001) sapphire substrates using the electron-beam deposition technique. The optical spectra of a 120 nm YFe2O4 show Fe d to d on-site and O 2p to Fe 3d, Y 4d, and Y 5s charge-transfer electronic excitations. Anomalies in the temperature dependence data of the charge-transfer excitations and the splitting of the 4.46 eV charge-transfer peak strongly suggest a structural distortion at 180 ± 10 K. Evidence of such a structural distortion is also manifested in the surface resistance versus temperature data. In addition, the YFe2O4 thin film at low temperatures shows strong electro-optical properties, as high as 9% in the energy range of 1 - 2.5 eV, for applied electric fields up to 500 V.cm−1. 

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