MgO Substrates Research Articles

Growth of highly oriented crystalline gold nanoislands on MgO(001) substrates for surface-enhanced Raman scattering chips by pulsed laser deposition

Highly oriented crystalline gold nanoislands (NIs) are self-assembled on MgO(001) substrates by pulsed laser deposition. Morphologies of the gold NIs are significantly dependent on the substrate temperature during gold deposition. Symmetrical-shaped NIs with Au(111) orientation are existence parallel to the MgO(001) substrate at low temperatures deposition (350 and 550 °C), while square-shaped and hexagonal-shaped gold NIs are co-existence whose crystallinity was Au(001) and Au(111), respectively. Extinction spectra of the gold NIs on MgO(001) substrates show that wide-range of photo-energy is disappeared. For a simple and useful plasmonic device, the produced gold NIs on MgO(001) substrates are applied to the surface-enhanced Raman scattering (SERS) chips. The SERS properties of the chips are evaluated using 4-MBA as a model analyte. From the SERS signal of a 4-MBA ring breathing mode, we obtain a significant enhancement factor over 107, which is approximately 10 times higher than that of a conventional SERS chip.

The anomalous Hall effect controlled by residual epitaxial strain in antiferromagnetic Weyl semimetal Mn3Sn thin films grown by molecular beam epitaxy

The large anomalous Hall effect (AHE) in antiferromagnetic(AFM) Weyl semimetal Mn3Sn attracts intensive attentions in spintronics. Here, we report the structural property of high quality Mn3Sn thin film on insulator substrate MgO(110) by molecular beam epitaxy (MBE), and AHE in control of residual mismatch strain between Mn3Sn film and substrate. We are able to grow strain-free Mn3Sn(10 1¯ 0) films or alternatively strained Mn3Sn(11 2¯ 0) films via a three-step process. The strain-free Mn3Sn film has large anomalous Hall conductivity up to 30 Ω-1cm−1 at room temperature, which is comparable to bulk Mn3Sn. In contrast, AHE is switched off in strained Mn3Sn film due to piezomagnetic effect under a uniaxial compress strain of ∼2.0%. These findings provide a deeper understanding on AFM spintronic applications.

Effect of the growth orientation on the physical properties of Sr2CoNbO6 thin films

We study the effect of the growth orientation on the structural, electronic, and hence transport properties of Sr2CoNbO6 thin films grown on the orthorhombic NGO(100) and cubic MgO(100) substrates. The x-ray diffraction patterns show the growth of the thin film along the a axis resulting in the asymmetric (b≠c) in-plane compressive strain in the case of NGO(100), whereas along the c axis with tensile strain in the case of MgO(100) substrate. The temperature dependent resistivity measurements indicate the lower electronic conductivity for the film grown on the NGO(100) substrate, which is found to be correlated with the higher degree of the oxygen deficiencies and, hence, a larger concentration of the insulating Co2+ in this sample. Further, the x-ray photoemission spectroscopy measurements show that Sr and Nb are present in the 2+ and 4+ valence state, whereas Co exists in the 2+, 3+, and 4+ states, a fraction of which was found to vary with the growth orientation. Moreover, the analysis of leakage current using the sum exponent model indicates the presence of two different relaxation mechanisms in these samples.

Preparation of Pb(Mg1/3Nb2/3)TiO3–PbTiO3 thick films with highly preferred orientation via screen printing

Rhombohedral 0.75Pb(Mg1/3Nb2/3)TiO3–0.25PbTiO3 (PMN-25PT) and tetragonal 0.65Pb(Mg1/3Nb2/3)TiO3–0.35PbTiO3 (PMN-35PT) thick films with a highly preferred orientation were prepared via screen printing on MgO and YSZ ceramic substrates. The use of oriented BaTiO3 thick films as template layers was effective in forming the oriented PMN-25PT and PMN-35PT thick films. The orientation degrees of both thick films were over 0.85. The formation process of the thick films was examined using electron backscatter diffraction. PMN-25PT grains grew on the BaTiO3 template layers and aligned with the BaTiO3 orientation direction. Finally, the PMN-25PT and PMN-35PT thick films prepared on MgO had better electrical properties than the thick films fabricated on YSZ.

Optimization of the superconducting properties of NbTiN thin films by variation of the N2 partial pressure during sputter deposition

We report the effect of nitrogen partial pressure on the growth of niobium-titanium nitride (NbTiN) thin films through reactive sputtering of the NbTi target in the presence of Ar and N2 atmosphere. The N2 partial pressure was varied from 5.8% to 15.15% with respect to the Ar flow available in the chamber. We measured a critical temperature (T C) as high as 15.77 K for an N2 partial pressure of 6.8% for a 50 nm film deposited on an MgO substrate. The epitaxial growth of the NbTiN films is evident from the observed (200) and (400) XRD peaks with respect to the substrate. The superconducting properties were analyzed with respect to the observed values of T C, ΔT C, temperature dependence of the upper critical field (B C2), coherence length (ξ), diffusion coefficient (D), and the thermally activated flux flow behavior following the Arrhenius equation. The T C variation observed for the entire spectrum of N2 partial pressure variation is ∼11%, with T C getting saturated towards the higher end of the N2 content. While the T C variation with respect to N2 content is non-linear, the variation of B C2(0), diffusion coefficient, and zero temperature coherence length ξ(0) show a linear dependence. While the B C2(0) values show an increasing trend, D and ξ(0) values show a decreasing trend with respect to the increase of N2 partial pressure. It is concluded that the optimum window of N2 partial pressure for the reactive sputtering of NbTiN using a target of Nb0.7Ti0.3 for the present experimental conditions, in terms of epitaxial growth and obtaining the highest T C is within the range of 5.8% to 8.51%.

Field-free spin-orbit torque switching assisted by in-plane unconventional spin torque in ultrathin [Pt/Co

Electrical manipulation of magnetization without an external magnetic field is critical for the development of advanced non-volatile magnetic-memory technology that can achieve high memory density and low energy consumption. Several recent studies have revealed efficient out-of-plane spin-orbit torques (SOTs) in a variety of materials for field-free type-z SOT switching. Here, we report on the corresponding type-x configuration, showing significant in-plane unconventional spin polarizations from sputtered ultrathin [Pt/Co]N, which are either highly textured on single crystalline MgO substrates or randomly textured on SiO2 coated Si substrates. The unconventional spin currents generated in the low-dimensional Co films result from the strong orbital magnetic moment, which has been observed by X-ray magnetic circular dichroism (XMCD) measurement. The x-polarized spin torque efficiency reaches up to −0.083 and favors complete field-free switching of CoFeB magnetized along the in-plane charge current direction. Micromagnetic simulations additionally demonstrate its lower switching current than type-y switching, especially in narrow current pulses. Our work provides additional pathways for electrical manipulation of spintronic devices in the pursuit of high-speed, high-density, and low-energy non-volatile memory.

Photoinduced Nonthermal Reduction of the Coercive Field in FePt and FePt0.84Rh0.16 Epitaxial Thin Films in the L10 Phase

The time-resolved magneto-optical Kerr effect in epitaxial thin films of the FePt compound and the FePt0.84Rh0.16 solid solution with the perpendicular magnetic anisotropy on MgO (001) substrates has been studied. The evolution of hysteresis loops at short (100 fs–1 ns) and long (1–20 ms) time scales after the excitation by a femtosecond light pulse has been studied. Long-lived nonthermal reduction of the coercive field has been detected. The coercive field is recovered in several milliseconds. It has been proposed to explain the observed phenomenon by the excitation of high-Q-factor acoustic resonances in the substrate/film system and to the strong magnetoelastic interaction in FePt and FePt0.84Rh0.16 films.

Laser irradiation effects in FeRh thin film

The effect of laser irradiation in the energy range from 20 mW to 200 mW was investigated in 109 nm thick Fe51Rh49 film deposited on an MgO (100) substrate. The initial, A1 structure with fully paramagnetic magnetic ordering was achieved after irradiating the samples with 120 keV Ne+ ions with a fluence of 1 × 1016 ion cm−2, as it was confirmed by conversion-electron Mössbauer spectroscopy. At higher powers physical damage of the layer was observed, while in the lowest power case, magnetic force microscopy revealed a well-defined magnetic structure reflecting the laser irradiation pattern. The presented results have the potential to be employed for laser ablation or allows the fabrication of arbitrary ferromagnetic pattern within a homogeneous paramagnetic FeRh thin films.

Theoretical investigation on mechanical, thermal, and ultrasonic properties of epitaxial nanostructured ZrN layers growth on MgO (001) substrate

Theoretical investigation on mechanical, thermal, and ultrasonic properties of epitaxial nanostructured ZrN layers growth on MgO (001) substrate

Resonant Excitation of the Ferroelectric Soft Mode by a Narrow-Band THz Pulse.

This study investigates the impact of narrow-band terahertz pulses on the ferroelectric order parameter in Ba0.8Sr0.2TiO3 films on various substrates. THz radiation in the range of 1-2 THz with the pulse width of about 0.15 THz was separated from a broadband pulse with the interference technique. The 375 nm thick BST film on a MgO (001) substrate exhibits enhanced THz-induced second harmonic generation when excited by THz pulses with a central frequency of 1.6 THz, due to the resonant excitation of the soft phonon mode. Conversely, the BST film on a Si (001) substrate shows no enhancement, due to its polycrystalline state. The 800 nm thick BST film on a MgO (111) substrate demonstrates the maximum of a second harmonic generation signal when excited by THz pulses at 1.8 THz, which is close to the soft mode frequency for the (111) orientation. Notably, the frequency spectrum of the BST/MgO (111) film reveals peaks at both the fundamental and doubled frequencies, and their intensities depend, respectively, linearly and quadratically on the THz pulse electric field strength.

Co2Fe(Ti0.5Al0.5) epitaxial thin films: Structural and magnetic properties of a Heusler alloy with Z-site transition metal substitution

We investigate the structural, static, and dynamic magnetic properties of epitaxial Heusler Co2Fe(Ti0.5Al0.5) (CFTA) alloy thin films with thickness varying from 6 nm to 80 nm grown by sputter beam epitaxy on cubic MgO(001), MgAl2O4(001), and hexagonal Al2O3(112̄0) substrates. X-ray diffraction measurements indicate epitaxial growth of CFTA thin films with B2 chemical ordering, with cubic [001] and [220] CFTA axes normal to the cubic and hexagonal substrates, respectively. Microstructure analysis of films grown on MgO substrates reveals a uniformly oriented epitaxial crystal with small variations consistent with strain distortions, providing an explanation for the relatively large X-ray rocking curve values found. Meanwhile, films on Al2O3(112̄0) substrates reveal columnar growth with frequent in-plane grain rotations. A pronounced four-fold magnetocrystalline anisotropy is observed in epitaxial thin films grown on cubic substrates. A pronounced uniaxial anisotropy for films grown on Al2O3(112̄0) substrates is observed. A saturation magnetization of ∼5.0μB/f.u. (where f.u. represents formula unit) is obtained at room temperature, slightly smaller compared to the expected value based on the Slater-Pauling rule. Ferromagnetic resonance spectroscopy finds an effective damping parameter and inhomogeneous linewidth broadening comparable to those found in parent compound Co2FeAl, which suggests that Ti substitution can be achieved without negatively affecting the magnetic properties of the system.

Strains in Fe/Cr/Fe trilayers and (Fe/Cr)5/Fe multilayers epitaxied on MgO and MgO/SrTiO3

Fe/Cr/Fe trilayers and multilayers are prepared as model systems designed to furnish simple data comparable with calculation results for diffusion properties in nuclear materials. Their structure (epitaxy, residual strains and dislocations) is characterized in detail. The film structure (strain and stress) is shown to be different on MgO20nm/SrTiO3 and MgO substrates due to the residual strain in the MgO buffer layer on SrTiO3. Superlattices with high crystalline quality are prepared, with Fe and Cr in coherent epitaxy. In-plane residual strain in Fe is +0.45(13)% on MgO substrates and decreases from 1.70(9)% to 0.47(2)% when increasing the thickness of the trilayers on MgO/SrTiO3 substrates. These strains enhance the contrast between Fe and Cr, opening the way to future kinetics studies using x-ray diffraction in this system, which is far more efficient (non-destructive and rapid) than high resolution transmission electron microscopy with electron energy loss spectroscopy or atom probe tomography.

Enhanced magnetic properties and leakage current mechanism in bismuth lanthanum ferrite thin films coupled with impedance and magnetic studies

A highly crystalline and pure phase Bi0.993La0.007FeO3 (BLFO) thin films have been grown using a pulsed laser deposition technique. The structural analysis confirmed the thin film is well crystallized with a rhombohedral crystal structure with an R3c space group. The BLFO phase and composition are also confirmed by transmission electron microscopy and EDAX analysis. The surface chemistry of thin films, analysed by X-ray photon spectroscopy, reveals Fe's presence in Fe3+ and Fe2+ states. At RT, the deposited film exhibited high dielectric permittivity (∼4 × 102) and low dielectric loss (∼10−1). The isothermal magnetization on highly oriented (001)-MgO and LaAlO3 (LAO) substrates is investigated. The response is compared to the randomly oriented polycrystalline films on low-cost substrates (Pt/TiO2/SiO2/Si and n-type Si substrate). The enhanced saturation magnetization of 57 emu/cm3 and 36 emu/cm3 at 10 K and 300 K are observed at 2.5 kOe applied field for the film grown on the LAO substrate. Additionally, the low leakage current density of the film is obtained around ∼1 × 10−7 A/cm2 at a field of 100 kV/cm. Acquiring single-phase BLFO thin films with enhanced electrical and magnetic properties can be relevant in designing multiferroic materials for multiple-state memories, magnetic field sensors, and spintronics applications.

Secondary electron emission of B-doped/undoped double-layer heteroepitaxial single crystal diamond film

A B-doped/undoped double-layer single crystal diamond structure was proposed to enhance secondary electron emission (SEE), and the double-layer single crystal diamond film was heteroepitaxially grown on MgO substrate by microwave plasma chemical vapor deposition. The crystal quality, surface morphology and conductivity of the film were studied. It is found that the film has high crystal quality, the full width at half maximum of X-ray diffraction rocking curve is 0.14°, and the surface roughness is small. For these reasons, the B-doped/undoped double-layer single crystal diamond film has better SEE performance than the B-doped/undoped double-layer polycrystalline diamond film. At the incident electron energy of 2 keV, the SEE yield of B-doped/undoped double-layer single crystal diamond reached 22.4. The physical model calculation results indicate that at a low B doping concentration, less sp2 carbon and smaller surface roughness are beneficial to improve the SEE yield.

Realization of tunable fabrication from Cu nanoparticles film towards Cu2O thin film by pulse laser deposition method

In this paper, the tunable fabrication from Cu nanoparticles (NPs) film to Cu2O thin film on single-crystal MgO (100) substrates via pulse laser deposition (PLD) has been realized by precisely adjusting the oxygen pressure. In-situ X-ray photoelectron spectroscopy confirms that the chemical state of Cu is altered from Cu0 to mixed state of Cu0 and Cu1+, and then to Cu1+. X-ray diffraction results reveal the main crystal structure of the films is changed from Cu (200) to Cu2O (110) with the increase of oxygen pressure. The morphology of three typical samples (Cu NPs film, Cu@Cu2O core-shell NPs films and Cu2O thin film) was record by transmission electron microscopy. According to the analysis, the mechanism of oxygen is to absorb on the surface of MgO (100) substrate and act as an oxidant in the growth of the films. The increase of oxygen pressure is beneficial to the formation of Cu@Cu2O core-shell NPs with larger diameters and the transition from Cu@Cu2O core-shell NPs film to continuous Cu2O (110) thin film. This work provides essential information for the fabrication of Cu2O thin films by PLD with Cu target at low oxygen pressure.

Transparent and highly conductive La-doped strontium stannate thin film growth by PI-MOCVD method on LaAlO3 and MgO substrates of (100), (110), (111) orientations

Wide band gap (4.6 eV) lanthanum doped SrSnO3 semiconductors of high electron mobility have potential applications in transparent oxide semiconductor technologies. However, there is a lack of data on the electrical properties of La:SrSnO3 both in single crystals and in thin films with an orientation other than (001). We show that after applying a modified metalorganic chemical vapor deposition method it is possible to deposit relaxed thick (001), (110) and (111) oriented La:SrSnO3 films on LaAlO3 and MgO substrates. Thin films on LaAlO3 substrates exhibit comparatively high charge carrier mobility values: µ100=71 cm2V−1s−1, µ110=72 cm2V−1s−1 and µ111=67 cm2V−1s−1. In this article, using the results of X-ray diffraction and crystallite orientation distribution modeling, as well as scanning and transmission electron microscopy data, the link between the obtained charge carrier mobility (µ100 ≈ µ110 > µ111) and film's microstructure and morphology is discussed.

Epitaxial growth of NbN thin films for electrodes using atomic layer deposition

The epitaxial growth of NbN thin film was accomplished via atomic layer deposition (ALD) for the first time using NbCl5 and NH3 as the Nb precursor and nitrogen source at a deposition temperature of 450 ℃. The cubic NbN thin film was grown epitaxially on a cubic MgO crystal through the coherent lattice matching between NbN and MgO with a small lattice mismatch (∼2.8%). A high concentration of Cl impurity of 4–5% remained in NbN thin films grown on a SiO2 substrate using ALD. However, the Cl impurity concentration decreased to ∼ 2% in the epitaxially grown NbN thin films, which facilitated the epitaxial growth of NbN thin films on the MgO substrate. The origin was attributed to a residual strain at the NbN/MgO interface, which induced a bond length change in Nb-N-Cl. The bond length change may promote Cl desorption during NbN ALD because an in-plane compressive strain in the NbN film and an in-plane tensile strain in the MgO surface were observed. Finally, the epitaxially grown NbN thin film exhibited a 50% lower resistivity than that grown with a polycrystalline phase based on the enhanced carrier mobility owing to the improved crystallinity of epitaxial NbN thin films.

Phase purity and surface morphology of high-Jc superconducting Bi2Sr2Ca1Cu2O8+δ thin films

Bi2Sr2Ca1Cu2O8+δ (Bi-2212) thin films with thicknesses less than 50 nm (<20 unit cells) are grown by pulsed-laser deposition (PLD) onto (001) LaAlO3 (LAO) single crystal substrates. Phase-pure and smooth c-axis oriented Bi-2212 films with optimal oxygen doping, critical temperature Tc0 up to 86 K, and critical current density Jc(60 K) above 1 MA/cm2 are obtained for samples that are annealed in situ at temperatures below 700 °C. At higher temperature Bi-2212 films on LAO substrates partially decompose to non-superconducting impurity phases, while films on MgO and SrTiO3 substrates are stable. The broadening ΔTc of the metal-to-superconductor resistive phase transition in magnetic fields is much larger for thin films of Bi-2212 as compared to YBa2Cu3O7. The magnetic field-induced suppression of Tc0 is stronger for Bi-2212 films containing impurity phases as compared to the phase-pure Bi-2212 films. The degradation of LAO substrate crystals after several steps of deposition and chemical removal of the Bi-2212 layer is investigated. New, commercially prepared substrates provide Bi-2212 films with smallest surface roughness (3 nm) and strong out-of-plane texture. However, thin films of almost the same quality are obtained on re-used LAO substrates that are mechanically polished after the chemical etching.

High-Index Epitaxial Fe Films Grown on MgO(113).

The epitaxial growth of high-index Fe films on MgO(113) substrates is successfully achieved using direct current (DC) magnetron sputtering, despite the significant lattice constant mismatch between Fe and MgO. X-ray diffraction (XRD) analysis is employed to characterize the crystal structure of Fe films, revealing an Fe(103) out-of-plane orientation. Furthermore, our investigation reveals that the Fe[010] direction is parallel to the MgO[11¯0] direction within the films plane. These findings provide valuable insights into the growth of high-index epitaxial films on substrates with large lattice constant mismatch, thereby contributing to the advancement of research in this field.

Employing high-temperature-grown SrZrO3 buffer to enhance the electron mobility in La:BaSnO3-based heterostructures

We report a synthetic route to achieve high electron mobility at room temperature in epitaxial La:BaSnO3/SrZrO3 heterostructures prepared on several oxide substrates. Room-temperature mobilities of 157, 145, and 143 cm2 V−1 s−1 are achieved for heterostructures grown on DyScO3 (110), MgO (001), and TbScO3 (110) crystalline substrates, respectively. This is realized by first employing pulsed laser deposition to grow at very high temperature the SrZrO3 buffer layer to reduce dislocation density in the active layer, then followed by the epitaxial growth of an overlaying La:BaSnO3 active layer by molecular-beam epitaxy. Structural properties of these heterostructures are investigated, and the extracted upper limit of threading dislocations is well below 1.0×1010 cm−2 for buffered films on DyScO3, MgO, and TbScO3 substrates. The present results provide a promising route toward achieving high mobility in buffered La:BaSnO3 films prepared on most, if not all, oxide substrates with large compressive or tensile lattice mismatches to the film.