Thermoelectric Properties of Epitaxial β-FeSi2 Thin Films on Si(111) and Approach for Their Enhancement

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.

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.

A detailed study of the role of oxygen vacancies in determining the effective mass and high temperature (300–1000 K) thermoelectric properties of La-doped epitaxial SrTiO3 thin films is presented. It is observed that at intermediate temperatures, a transition from degenerate to non-degenerate behavior is observed in the Seebeck coefficient, but not electrical conductivity, which is attributed to heterogeneous oxygen non-stoichiometry. Heikes formula is found to be invalid for the films with oxygen vacancies. By fitting the spectroscopic ellipsometry (SE) data, obtained in the range 300–2100 nm, using a Drude-Lorentz dispersion relation with two Lorentz oscillators, the electrical and optical properties of the films are extracted. Using the excellent agreement between the transport properties extracted from SE modeling and direct electrical measurements, we demonstrate that an increase in concentration of oxygen vacancies results in a simultaneous increase of both carrier concentration and electron effective mass, resulting in a higher power factor.

Critical current properties in YBCO coated IBAD tapes

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.

High-quality Mn2-xCrxSb (x = 0.01, 0.04, and 0.1) epitaxial thin films were grown on SrTiO3 (STO) (001) single-crystal substrates using molecular beam epitaxy. Magnetotransport and magnetic measurements reveal that the x = 0.01 sample undergoes a quasi-ferrimagnetic (I) [Q-FIM(I)]-to-ferrimagnetic (II) [FIM(II)] spin reorientation (SR) transition and a giant magnetoresistance (MR) associated first-order ferrimagnetic(II)-to-antiferromagnetic (AFM) phase transition upon cooling, resulting in the AFM ground state with a weak in-plane net moment. Upon increasing the doping level from x = 0.01 to 0.1, both the SR transition and the first-order magnetic transition are suppressed. For x = 0.1, the former transition is suppressed, leaving only the Q-FIM(I)-to-AFM transition within the whole temperature region. TAFM-FIM shows almost similar changes upon the application of either in-plane or out-of-plane magnetic fields. TAFM-FIM values of the x = 0.01 and 0.04 samples are much higher than those of the Mn2-xCrxSb bulk with similar doping levels, which can be understood by the clamping effect from STO substrates. For each thin-film sample, the MR effect is observed near TAFM-FIM and disappears in the high temperature Q-FIM(I) phase and low temperature AFM phase, indicating that MR is related to the spin-dependent electron scattering during the first-order magnetic phase transition. Based on the magnetotransport and magnetic data, a magnetic phase diagram is established for the Mn2-xCrxSb films in the low doping level region.

(001) oriented epitaxial Pt bottom electrode was prepared on Si(001) substrate using four fold buffer layer of ST/LSCO/CeO 2 /YSZ. On the epitaxial Pt bottom electrode, we found that both epitaxial and polycrystalline BST thin film can be prepared by the change of oxygen gas flow procedure during heating up to deposition temperature of BST. Without oxygen gas flow on the heating process of the Pt bottom electrode, BST thin film was changed into polycrystalline, on the other hand, when the Pt bottom electrode was heated in 100 mTorr of oxygen pressure, epitaxial grown BST thin film was realized. This means that the orientation of BST thin film can be controlled by the oxygen flow procedure during heating of the Pt bottom electrode. Dielectric constant and tunability of epitaxial BST thin film changed with the oxygen pressure during heating and deposition of BST thin film. This suggests that oxygen pressure during heating and deposition is the key to control the orientation and properties of BST thin film.

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.

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.

Bismuth vanadate (BiVO4) has emerged as one of the highest performing metal oxide photoelectrodes for solar energy-to-fuels applications [1, 2]. This achievement has been largely attributed to the development of high-quality synthesis techniques. Specifically, epitaxial synthetic routes to producing lattice-matched, near single-crystalline quality BiVO4 with minimal extrinsic defects have been essential to understand the fundamental and PEC properties of BiVO4 that are inaccessible by conventional bulk synthesis techniques. In addition, epitaxial synthesis may also be employed as a strategy to further improve the PEC properties of BiVO4 through, for instance, altering the band structure and enhancing the carrier dynamics by strain engineering. Hence, the ability to produce high-quality single-phase epitaxial BiVO4 films is desirable.To date, molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and pulsed laser deposition (PLD) [3] have been used to fabricate epitaxial BiVO4 thin films, with PLD having the advantage of relatively simple experimental setup and material versatility. However, an oft-noted drawback in conventional PLD using single oxide targets is off-stoichiometry in the deposited films; this is especially so for BiVO4 which requires a Bi-rich compound target to achieve a stoichiometric BiVO4 film [4]. This problem is also expected in general for multinary oxides composed of elements with very different ablation properties. To alleviate the limitations of conventional PLD, we employ for the first time alternate-target layer-by-layer PLD as a more elegant approach to produce high quality epitaxial BiVO4 films. Briefly, constituent oxide targets are alternately ablated by an excimer laser (λ= 248 nm) to build the desired film one unit cell thick at a time. Stoichiometry is controlled by the number of shots corresponding to a specified laser fluence for the constituent oxide target.To demonstrate the method, we deposited epitaxial BiVO4 films onto (001)-oriented yttrium-stabilized zirconia (YSZ) using 4N-pure Bi2O3 and V2O5 targets. Films grown are single phase BiVO4 as shown by x-ray diffraction. Out-of-plane diffraction peaks indexed to BiVO4(00l) suggests epitaxy of the film onto the substrate; reciprocal space maps of the asymmetric BiVO4(208) and YSZ(204) peaks further confirm the BiVO4(001) || YSZ(001) epitaxial relationship (top figure). Rocking curves of the BiVO4 (004) peak (FWHM ~ 0.015-0.041) indicate high crystalline perfection of the BiVO4 film, almost approaching that of the YSZ substrate (FWHM ~ 0.010). Thickness dependent-rocking curve studies reveal that BiVO4 films are strained to the substrate for film thicknesses under 22 nm; above this critical thickness, film relaxation ensues. In turn, the resulting optoelectronic properties of the BiVO4 film is dictated by its relaxation state. The optical band gap narrows with film relaxation as observed with spectroscopic ellipsometry. Moreover, steady-state photoluminescence emission spectroscopy reveals a sub-bandgap state (A 2, bottom figure) associated with strained BiVO4 films, on top of a state consistent with band-to-band recombination (B1 ). A higher energy sub-bandgap state (A 1, bottom figure) develops as the film relaxes. The implications of the relaxation state on the charge carrier dynamics and photoelectrochemical properties of BiVO4 will be discussed. Figure: (top) reciprocal space maps of the asymmetric BiVO4(208) and YSZ(204) peaks; (bottom) photoluminescence emission spectra of strained and relaxed BiVO4 films.

NiFe epitaxial thin films were prepared on MgO(100) and MgO(110) single-crystal substrates heated at 300° C by ultra-high vacuum molecular beam epitaxy. In the early stage of film growth on MgO(100) substrate, formation of NiFe (112¿0) epitaxial thin film with hcp structure is observed by in-situ RHEED. With increasing the film thickness, fcc-NiFe(100) phase appears and the RHEED intensity from fcc-NiFe(100) crystal increases. X-ray diffraction and high-resolution transmission electron microscopy indicate that the resulting film structure is fcc-NiFe(100), suggesting that the metastable hcp-NiFe (112¿0) phase is observable only during film growth process. On the contrary, NiFe(110) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fcc</sub> single-crystal film grows epitaxially on MgO(110) substrate. Atomically sharp boundaries are recognized between NiFe thin films and MgO substrates, where misfit dislocations are periodically introduced in the films at the NiFe/MgO interfaces reducing the lattice mismatches. Out-of-plane and in-plane lattice spacings of these films agree with the values of bulk NiFe crystal with very small errors of less than 0.5%, suggesting that the strains in the NiFe films are very small.

Thin film perovskite-type oxide SrTiO3 has been grown epitaxially on Si(001) substrate by molecular beam epitaxy. Reflection high energy electron diffraction and x-ray diffraction analysis indicate high quality SrTiO3 heteroepitaxy on Si substrate with SrTiO3(001)//Si(001) and SrTiO3[010]//Si[110]. The SrTiO3 surface is atomically as smooth as the starting substrate surface, with a root mean square roughness of 1.2 Å observed by atomic force microscopy. The thickness of the amorphous interfacial layer between SrTiO3 and Si has been engineered to minimize the device short channel effect. An effective oxide thickness &amp;lt;10 Å has been obtained for a 110 Å thick dielectric film. The interface state density between SrTiO3 and Si is 6.4×1010 cm−2 eV−1, and the inversion layer carrier mobilities are 221 and 62 cm2 V−1 s−1 for n- and p-channel metal–oxide–semiconductor devices with 1.2 μm effective channel length, respectively. The gate leakage in these devices is two orders of magnitude smaller than a comparable SiO2 gate dielectric metal–oxide–semiconductor field effect transistors.

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.

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.

A “misfit strain-temperature” phase diagram for BiFeO3 thin film was constructed based on the Landau–Devonshire theory with the mechanical substrate effect. It is found that the polarization instabilities result in the formation of the monoclinic phases in BiFeO3 thin films. The effective substrate lattice parameter has been introduced to calculate the film thickness dependence of the polarization and the dielectric constants. The theoretical results are in agreement with the experimental data for the thickness dependence of ferroelectric properties of the BiFeO3 epitaxial thin films on SrTiO3 and Si substrates.