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Register a new userEpitaxial Growth of Perovskite SrBiO$_3$ Film on SrTiO$_3$ by Oxide Molecular Beam Epitaxy
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Hole-doped perovskite bismuthates such as Ba$_{1-x}$K$_x$BiO$_3$ and Sr$_{1-x}$K$_x$BiO$_3$ are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate SrBiO$_3$ on SrO-terminated SrTiO$_3$ (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature co-deposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. SrBiO$_3$ has a pseudocubic lattice constant $sim$4.25 AA, an $sim$8.8% lattice mismatch on SrTiO$_3$ substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature co-deposition, the recrystallization method can produce higher quality 1-6 unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.
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We report the successful growth of tetragonal FeS film with one or two unit-cell (UC) thickness on SrTiO3(001) substrate by molecular beam epitaxy. Large lattice constant mismatch with the substrate leads to high density of defects in single UC FeS, while it has been significantly reduced in double UC thick film due to the lattice relaxation. The scanning tunneling spectra on the surface of FeS thin film reveal the electronic doping effect of single UC FeS from the substrate. In addition, at the Fermi level, the energy gaps of approximate 1.5 meV are observed in films of both thicknesses at 4.6 K and below. The absence of coherence peaks of gap spectra may be related to the preformed Cooper-pairs without phase coherence.
SrMoO$_3$ is a promising material for its excellent electrical conductivity, but growing high-quality thin films remains a challenge. Here we synthesized epitaxial films of SrMoO$_3$ using the molecular beam epitaxy (MBE) technique under a low oxygen-flow rate. Introduction of SrTiO$_3$ buffer layers of 4--8 unit cells between the film and the (001)-oriented SrTiO$_3$ or KTaO$_3$ substrate was crucial to remove impurities and/or roughness of the film surface. The obtained film shows improved electrical conductivities as compared with films obtained by other techniques. The high quality of the SrMoO$_3$ film is also verified by angle-resolved photoemission spectroscopy (ARPES) measurements showing clear Fermi surfaces.
Variations in growth conditions associated with different deposition techniques can greatly affect the phase stability and defect structure of complex oxide heterostructures. We synthesized superlattices of the paramagnetic metal LaNiO3 and the large band gap insulator LaAlO3 by atomic layer-by-layer molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) and compared their crystallinity, microstructure as revealed by high-resolution transmission electron microscopy images and resistivity. The MBE samples show a higher density of stacking faults, but smoother interfaces and generally higher electrical conductivity. Our study identifies the opportunities and challenges of MBE and PLD growth and serves as a general guide for the choice of deposition technique for perovskite oxides.
Epitaxial films of NdFeAsO were grown on GaAs substrates by molecular beam epitaxy (MBE). All elements including oxygen were supplied from solid sources using Knudsen cells. The x-ray diffraction pattern of the film prepared with the optimum growth condition showed no indication of impurity phases. Only (00l) peaks were observed, indicating that NdFeAsO was grown with the c-axis perpendicular to the substrate. The window of optimum growth condition was very narrow, but the NdFeAsO phase was grown with a very good reproducibility. Despite the absence of any appreciable secondary phase, the resistivity showed an increase with decreasing temperature.
Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate DBCO. We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film. It is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CuO2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one corresponding to the superconductivity and the other one to the pseudogap. While the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.
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