Damage-free lift-off of epitaxial HgCdTe thin films for future curved infrared imaging array applications

Epitaxial thin films can offer unprecedented definition of an electrocatalyst surface, defining the crystallographic orientation, interaction with a current collector, and providing an atomically flat surface for quantitative spectroscopy yielding mechanistic insights. This review introduces experts in thin films to the important parameters of electrochemical setups, techniques, and methods for quantifying electrocatalytic activity and comparing it across the literature. Examples are drawn from the literature measuring oxygen electrocatalysis on epitaxial oxide thin films. We share best practices in the robust measurement of intrinsic activity of thin films, including methods of electrical contact and assessment of charge transport in situ. We conclude by highlighting recent insights from epitaxial films in understanding oxygen electrocatalysis.

ABSTRACTWe report the direct measurement of elastic strain effect on the electrical and magnetic properties of single domain epitaxial SrRuO3 thin films, using a lift-off technique. The as-grown films on vicinal (001) SrTiO3 substrates are subjected to elastic biaxial compressive strain within the plane and tensile strain normal to the plane. In contrast, the lift-off films prepared by chemical etching of SrTiO3 substrates, are completely strain free with bulk like lattice. Our measurements indicate that the elastic strain can significantly affect the electrical and magnetic properties of epitaxial ferromagnetic SrRuO3 thin films. For the strained films, the Curie temperature (Tc) was suppressed to 150K and the saturation magnetic moment (Ms) was decreased to 1.15μB/Ru atom as compared to a Tc of 160K and Ms of 1.45μB/Ru atom for the strain free films. These property changes are attributed to the structural distortion due to the elastic strain in the as-grown epitaxial thin films. Our results provide direct evidence of the crucial role of lattice strain in determining the properties of the perovskite epitaxial thin films.

Although there is considerable interest in BiFeO3 owing to its versatile physical properties, which make it suitable for a wide range of applications, its high leakage current is a significant limitation. Among various methods for reducing the leakage current, substitution with transition-metal or rare-earth elements is widely recognized as the most effective approach. Herein, to enable in-depth studies of the physical properties of BiFeO3, high-quality epitaxial BiFeO3 thin films with a low leakage current must be formed. However, owing to the difficulty of controlling the element doping when pulsed laser deposition is used for epitaxial thin-film growth, studies on substitutional doping based on epitaxial BiFeO3 thin films have not been systematically carried out. In this regard, we establish an innovative approach for overcoming the high leakage current of BiFeO3 by fabricating artificially engineered superlattice-based epitaxial BiFeO3 thin films in which there is a significant reduction of the leakage current. The control of the element doping in epitaxial BiFeO3 thin films is easily regulated precisely at the atomic-scale level. The results of this study strongly suggest that superlattice-based epitaxial BiFeO3 thin films can be a cornerstone for exploring the reliable fundamental physical properties of substitutional doping in epitaxial BiFeO3 thin films.

By lifting an epitaxial thin film off its growth substrate, we directly and quantitatively demonstrate how elastic strain can alter the magnetic and electrical properties of single-domain epitaxial SrRuO3 thin films (1000 Å thick) on vicinal (001) SrTiO3 substrates. Free-standing films were then obtained by selective chemical etching of the SrTiO3. X-ray diffraction analysis shows that the free-standing films are strain free, whereas the original as-grown films on SrTiO3 substrates are strained due to the lattice mismatch at the growth interface. Relaxation of the lattice strain resulted in a 10 K increase in the Curie temperature to 160 K, and a 20% increase in the saturation magnetic moment to 1.45 μB/Ru atom. Both values for the free-standing films are the same as that of the bulk single crystals. Our results provide direct evidence of the crucial role of the strain effect in determining the properties of the technologically important perovskite epitaxial thin films.

The bulk photovoltaic effect (BPE) is a nonlinear light-matter interaction in which light is converted into electrical current. Unlike the junction based PV mechanism, which relies on an interface to induce band bending between two differently doped regions or materials, or the Dember effect involves uses non-uniform incident light intensity to generate asymmetrical populations of holes and electrons, the BPE occurs in materials which lack inversion symmetry or break time-reversal symmetry. The process in which light is converted into an electrical current by the BPE involves aspects of crystal structure, electronic structure, and carrier transport. The BPE is influenced by material synthesis not unlike the materials science and engineering paradigm of structure, properties, performance, and processing. This broad perspective is important to consider, as modern research in the BPE, e.g., light detection under a broad range of wavelengths, exploring the mechanisms of the BPE, and improving the power conversion efficiency, reveals other influences that can alter the observed response. To distinguish and elucidate the contribution from the BPE, BaTiO₃ epitaxial thin films grown by pulsed laser deposition were used a model system. The main goals of this thesis are (1) investigate the roles of biaxial strain and crystallographic orientation on the BPE response and (2) to probe the influence of electrode materials and their resistivity on BPE photocurrent in epitaxial thin films. These results deepen practical understanding of the BPE, which may help to improve the BPE response in future ferroelectric photovoltaic thin films.

Polarized neutron grazing incidence diffraction on magnetic epitaxial thin films

We have investigated the intrinsic thermoelectric properties of epitaxial β-FeSi2 thin films and the impact of phosphorus (P) doping. Epitaxial β-FeSi2 thin films with single phase were grown on Si(111) substrates by two different techniques in an ultrahigh-vacuum molecular beam epitaxy (MBE) system: solid-phase epitaxy (SPE), where iron silicide films formed by codeposition of Fe and Si at room temperature were recrystallized by annealing at 530°C to form epitaxial β-FeSi2 thin films on Si(111) substrates, and MBE of β-FeSi2 thin films on epitaxial β-FeSi2 templates formed on Si(111) by reactive deposition epitaxy (RDE) at 530°C (RDE + MBE). Epitaxial SPE thin films based on codeposition had a flatter surface and more abrupt β-FeSi2/Si(111) interface than epitaxial RDE + MBE thin films. We investigated the intrinsic thermoelectric properties of the epitaxial β-FeSi2 thin films on Si(111), revealing lower thermal conductivity and higher electrical conductivity compared with bulk β-FeSi2. We also investigated the impact of doping on the Seebeck coefficient of bulk and thin-film β-FeSi2. A route to enhance the thermoelectric performance of β-FeSi2 is proposed, based on (1) fabrication of thin-film structures for high electrical conductivity and low thermal conductivity, and (2) proper choice of doping for high Seebeck coefficient.

Critical current properties in YBCO coated IBAD tapes

To achieve the epitaxial thin films deposition, it is necessary to use properly oriented substrates, with or without buffer layers, matching the lattice parameters of the epitaxial thin film we want to grow. In this work, the deposition of epitaxial Bi2SiO5(200) and BiFeO3(001) thin films on Si(001) substrates by pulsed electron deposition (PED) technique is reported without special substrate preparation. The deposition of epitaxial BSO(200) and T-BFO(001) directly onto Si(001) substrates during a single target deposition process is relevant and presents enormous potential to reduce costs and improve practicality, interface quality and BFO integration efficiency with Si(001) substrates.

This paper reports on an abnormally high Curie temperature ( $T_{c}$ ) of a Pb(Mg1/3, Nb2/3)O3-PbTiO3 (PMN-PT) epitaxial thin film on Si prepared by sputter deposition with fast cooling. This deposition method was previously applied to Pb(Mn, Nb)O3-Pb(Zr, Ti)O3, and a c-axis-oriented epitaxial film with high $T_{c}$ was obtained. Using the same method, a crack-free 2- $\mu \text{m}$ -thick PMN-PT thin film was epitaxially grown on a Si substrate covered with buffer layers. The piezoelectricity, $\vert \text{e}_{31,f}\vert $ , was as large as 18~19 C/m2 under an electric field ranging from 25 to 75 kV/cm. The temperature characteristics of the dielectric constant and crystalline structure were significantly different from those of a bulk single crystal of PMN-PT, and suggested $T_{c}$ higher than 500 °C. The enhanced $T_{c}$ was possibly caused by thermally induced compressive strain received from the Si substrate. This approach can be an effective method for breaking the well-known tradeoff relationship between piezoelectricity and $T_{c}$ of a piezoelectric thin film. 0.

We present a full scale nonlinear thermodynamic model based on a Landau-Ginzburg-Devonshire formalism and the theory of dense polydomain structures in a multiparameter space to predict the phase stability of (001) oriented ${\text{PbTiO}}_{3}$ epitaxial thin films as a function of film thickness and epitaxial strain. The developed methodology, which accounts for electrostatic boundary conditions as well as the formation of misfit dislocations and polydomain structures, produces a thickness-strain phase stability diagram where it finds that the rotational phases (the so-called $r$ and $ac$ phases) in epitaxial ${\text{PbTiO}}_{3}$ are possible only in a very small window. We find that for experimentally used thickness or strains (or both) that often fall outside this window, the film is in either single phase tetragonal ($c$ phase) or in a $c/a/c/a$ polydomain state; this explains why rotational polar domains are rarely observed in epitaxial ferroelectric thin films.

A theoretical discussion on the internal quantum efficiencies of the epitaxial single crystal GaSb thin film cells with different p–n junctions

Hydrogen in thin epitaxial metal films and superlattices: structure, magnetism, and transport

The development of perovskite oxide epitaxial films and their freestanding membranes for the next-generation integrated and miniaturized electronics has attracted a lot of attention due to their promising multiferroic properties and low dimensionality. Strontium titanate, (SrTiO3, STO), as one typical perovskite incipient ferroelectric materials, has been reported to encounter significant challenges in inducing stable ferroelectricity and magnetism at room temperature, especially in the case of freestanding STO membranes. Here we deliberately introduced Ti deficiency/O-vacancy into the targeted STO thin films via defect-strain engineering to prepare the non-stoichiometric epitaxial thin films with large Sr/Ti ratios. The prepared non-stoichiometric STO thin films (Sr/Ti = 0.6/0.4) exhibited stable tetragonal phase with coexistent relaxor ferroelectricity and magnetism, which was thought to be associated with the presence of polar nanoregions. Importantly, owning to the defect-strain engineering, the freestanding non-stoichiometric STO membranes well preserved the initial compressive strain state when releasing from the rigid substrate. The freestanding STO membranes thus exhibited a polar tetragonal phase with significant ferroelectricity and magnetism. This study offers an effective approach for achieving ferroelectricity and magnetism in STO materials, both for epitaxial thin films and freestanding membranes, which gives their great potential in multifunctional semiconductor applications.

High-entropy oxides are emerging materials with unique functionalities. However, while various bulk-type high-entropy oxides have been developed, limited research has been pursued for high-entropy oxide thin films. In this paper, we report the synthesis of LiCr1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6O2 epitaxial thin films with a layered rocksalt structure via the pulsed laser deposition technique. We obtained a single-phase epitaxial thin film by tuning the laser fluence and substrate temperature. In addition, we fabricated a thin-film-type solid-state battery with the epitaxial thin film as a positive electrode. The battery exhibited stable operation. The results of this study provide a platform for synthesizing high-entropy oxide epitaxial thin films with the layered rocksalt structure.