Large In-plane Compressive Strain Research Articles

Deposition-Temperature-Mediated Selective Phase Transition Mechanism of VO2 Films

A clear experimental explanation of the contribution of Mott and Peierls transitions to the insulator–metal transition (IMT) characteristics in vanadium dioxide (VO2) is still lacking. Examining the crystal and electronic structures of epitaxial VO2 films grown at various deposition temperatures, a Mott or a Peierls transition was observed. The VO2 film deposited at 500 °C showed suppressed Peierls transition characteristics because of the large in-plane compressive strain in the insulating phase. The VO2 films deposited at 600 and 650 °C had a higher IMT temperature because of the relaxation of both the in-plane and out-of-plane strain, and there were abundant V4+ states. Therefore, it was related to a collaborative Mott–Peierls transition. Finally, the VO2 film deposited at 720 °C showed a suppressed Mott transition because of the abundance of V3+ states in the insulating phase. Furthermore, an analysis of the electronic structure of the insulating and metallic phases using in situ X-ray photoelectron spectroscopy and X-ray absorption spectroscopy provide a complete band diagram to support the above explanation of the deposition-temperature-dependent IMT characteristics.

Phase transformation in two-dimensional covalent organic frameworks under compressive loading.

As a new class of two-dimensional (2D) materials, 2D covalent organic frameworks (COFs) are proven to possess remarkable electronic and magnetic properties. However, their mechanical behaviours remain almost unexplored. In this work, taking the recently synthesised dimethylmethylene-bridged triphenylamine (DTPA) sheet as an example, we investigate the mechanical behaviours of 2D COFs based on molecular dynamics simulations together with density functional theory calculations. A novel phase transformation is observed in DTPA sheets when a relatively large in-plane compressive strain is applied to them. Specifically, the crystal structures of the transformed phases are topographically different when the compressive loading is applied in different directions. The compression-induced phase transformation in DTPA sheets is attributed to the buckling of their kagome lattice structures and is found to have significant impacts on their material properties. After the phase transformation, Young's modulus, band gap and thermal conductivity of DTPA sheets are greatly reduced and become strongly anisotropic. Moreover, a large in-plane negative Poisson's ratio is found in the transformed phases of DTPA sheets. It is expected that the results of the compression-induced phase transformation and its influence on the material properties observed in the present DTPA sheets can be further extended to other 2D COFs, since most 2D COFs are found to possess a similar kagome lattice structure.

The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.

Effect of nitrogen upon structural and magnetic properties of FePt in FePt/AlN multilayer structures

This paper investigates the effect of the addition of nitrogen in FePt layers for ultrathin FePt/AlN multilayer structures. X-ray diffraction results reveal that a compressive stress relaxation occurs after annealing owing to the release of interstitial nitrogen atoms in the FePt layers. The introduction of nitrogen also induces a large in-plane compressive strain during grain growth not seen in FePt deposited without nitrogen. This strain is considered to decrease the driving force for (111) grain growth and FePt ordering.

Influence of compressive strain on oxygen distribution in La0.7Ba0.3MnO3 thin films

The effects of annealing ambient on the structural and transport properties of epitaxial La0.7Ba0.3MnO3 films suffering large compressive strain on LaAlO3 substrates (LBMO/LAO) have been investigated. Independent of oxygen or argon ambient, a high temperature (900 °C) post-annealing process leads to a c-axis contraction, which can be attributed to the oxygen transfer from the in-plane to the out-of-plane direction. Meanwhile, the transport properties are similar with each other for both the Ar-annealed and O2-annealed films. The comparison of the ambient annealing effects between the highly strained LBMO/LAO films and negligibly strained LBMO films on SrTiO3 substrates indicates that a large in-plane compressive strain favors decreasing oxygen deficiency in the films, especially in the ab-plane.

Antiferrodistortive Structural Phase Transition in Compressively-Strained Epitaxial SrTiO3 Film Grown on (La, Sr)(Al, Ta)O3 Substrate

(001)-epitaxial SrTiO3 films were grown on (La, Sr)(Al, Ta)O3 substrates for investigating the impact of biaxial strain on the antiferrodistortive (AFD) structural phase transition in SrTiO3. The films were fully constrained by the substrates, which resulted in the large in-plane compressive strain of -0.9%. The AFD transition temperature estimated using temperature controlled x-ray diffraction and transmission electron microscope was over 150K higher than the theoretical prediction, but rather supported the experimental results reported on SrTiO3 films on LaAlO3 substrates.

Influence of Epitaxial Growth Orientation on Residual Strain and Dielectric Properties of (Ba0.3Sr0.7)TiO3 Films Grown on In-Plane Compressive Substrates

Orientation dependence of the strain in epitaxial (Ba0.3Sr0.7)TiO3 films and the dielectric properties have been investigated using in-plane compressive substrates. (100)- and (111)-epitaxial films were grown on (100)- and (111)-cSrRuO3/SrTiO3 substrates by rf magnetron sputtering. The residual strain was found to be remarkably different in those films. The (100)-epitaxial films are fully constrained by the substrate, resulting in the large in-plane compressive strain. On the other hand, the (111)-epitaxial films are almost fully relaxed. As results, for the (100)-epitaxial films, the apparent Curie-Weiss temperature was much higher than that of unstrained bulks. This trend was also confirmed for the films on SrRuO3/(La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates.

Effect of large compressive strain on low field electrical transport in La0.88Sr0.12MnO3 thin films

We have investigated the effect of large in-plane compressive strain on the electrical transport in La0.88Sr0.12MnO3 in thin films. For achieving large compressive strain, films have been deposited on single crystal LaAlO3 (LAO, a = 3.798 Å) substrate from a polycrystalline bulk target having average in-plane lattice parameter aav = (ab + bb)/2 = 3.925 Å. The compressive strain was further relaxed by varying the film thickness in the range ∼6–75 nm. In the film having least thickness (∼6 nm) large increase (c = 3.929 Å) in the out-of-plane lattice parameter is observed which gradually decreases towards the bulk value (cbulk = 3.87 Å) for ∼75 nm thick film. This shows that the film having the least thickness is under large compressive strain, which partially relaxes with increasing film thickness. The TIM of the bulk target ∼145 K goes up to ∼235 K for the ∼6 nm thin film and even for partially strain relaxed ∼75 nm thick film TIM is as high as ∼200 K. This enhancement in TIM is explained in terms of suppression of Jahn–Teller distortion of the MnO6 octahedra by the large in-plane compressive strain. We observe a large enhancement in the low field magnetoresistance (MR) just below TIM in the films having partial strain relaxation. Thick films of 6 and 20 nm have MR ∼14% at 3 kOe that almost doubles in 35 nm film to ∼27%. Similar enhancement is also obtained in the case of the temperature coefficient of resistivity. The near doubling of low field MR is explained in terms of delocalization of weakly localized carriers around TIM by small magnetic fields.

A high strain two-stack two-color quantum well infrared photodetector

A high strain two-stack, two-color, InGaAs/AlGaAs and AlGaAs/GaAs quantum well infrared photodetector for midwavelength infrared (MWIR) and long wavelength infrared (LWIR) detection has been demonstrated. Each stack is designed to have detection in one of the two atmospheric windows, 3–5 μm and 8–12 μm, respectively. The MWIR stack has employed 35% of indium in the InGaAs well, which not only achieved peak wavelength at 4.3 μm, but also obtained very high peak responsivity of Rp=0.65 A/W, using 45° light coupling. Normal incidence without grating coupling also has high responsivity with 40%–50% in the MWIR stack and 35%–45% in the LWIR stack, respectively, compared with the 45° incidence. Despite the large in-plane compressive strain induced by the high indium concentration, the device is highly uniform and has very low dark current in the MWIR stack. The background limited temperature is 125 K for the MWIR stack with a cutoff wavelength λc=4.6 μm, and is 70 K for the LWIR stack with λc=10 μm.