Q. Gan

Direct measurement of strain effects on magnetic and electrical properties of epitaxial SrRuO3 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 A 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.

Control of the growth and domain structure of epitaxial SrRuO3 thin films by vicinal (001) SrTiO3 substrates

We report the effect of both miscut angle (α) and miscut direction (β) of vicinal substrates on the epitaxial growth and domain structure of isotropic metallic oxide SrRuO3 thin films. The thin films have been grown on vicinal (001) SrTiO3 substrates with α up to 4.1° and β up to 37° away from the in-plane [010] axis. Single-crystal epitaxial (110)o SrRuO3 thin films were obtained on vicinal SrTiO3 substrates with a large miscut angle (α=1.9°, 2.1°, and 4.1°) and miscut direction close to the [010] axis. Decreasing the substrate miscut angle or aligning the miscut direction close to the [110] axis (β=45°) resulted in an increase of 90° domains in the plane. The films grown on vicinal substrates displayed a significant improvement in crystalline quality and in-plane epitaxial alignment as compared to the films grown on exact (001) SrTiO3 substrates. Atomic force microscopy revealed that the growth mechanism changed from two-dimensional nucleation to step flow growth as the miscut angle increased.

Growth mechanisms of epitaxial metallic oxide SrRuO3 thin films studied by scanning tunneling microscopy

We report the deliberately controlled growth of epitaxial metallic oxide SrRuO3 thin films in three distinctly different growth modes. Scanning tunneling microscopy and x-ray diffraction indicate that the growth mechanism for films on exact (001) SrTiO3 substrates is two-dimensional nucleation, which results in a two domain in-plane structure. As the miscut angle of vicinal (001) SrTiO3 substrates is increased, the growth mechanism changes to step flow which leads to single domain thin films. Films on (001) LaAlO3 substrates have an incoherent three-dimensional island growth due to the large lattice mismatch, resulting in a bulk-like lattice. The vast difference in the growth mechanisms of these films leads to a corresponding difference in their electrical transport and magnetic behavior. Such nanoscale control of growth mechanism, surface morphology, and domain structure can be very important in the fabrication of novel perovskite oxide devices.

Domain structure of epitaxial SrRuO3 thin films on miscut (001) SrTiO3 substrates

The microstructure of epitaxial SrRuO3 thin films grown on vicinal (001) SrTiO3 substrates with miscut angle of 1.9° and miscut direction of 12° away from [100] direction was studied using transmission electron microscopy (TEM). Cross-section as well as plan-view TEM studies revealed that these films are single domain with the in-plane epitaxial orientation relationship of SrRuO3[001]//SrTiO3[010] and SrRuO3[110]//SrTiO3[100]. This result is in contrast to the previous studies of the SrRuO3 thin films grown on exactly (001) SrTiO3, which are composed of two types of [110] domains with nearly the same volume fraction. The occurrence of these different domain structures is attributed to the step-flow growth of the film on the substrate surface due to the miscut.

Strain stabilized metal–insulator transition in epitaxial thin films of metallic oxide CaRuO3

We report the observation of both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO3 deposited under identical conditions. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single-crystal (100) SrTiO3 substrates, metallic films with lattice parameters close to the bulk material grew on (100) LaAlO3 substrates and poor crystalline quality SrTiO3 substrates. It is believed that a strain induced substitution of the small Ru4+ cations by the larger Ca2+ cations occurs, breaking the conduction pathway within the three-dimensional network of the RuO6 octahedra and leading to a metal–insulator transition. This unique phenomenon, which is not observed in bulk material, can be significant in technologically important epitaxial perovskite oxide heterostructures.

Lattice distortion and uniaxial magnetic anisotropy in single domain epitaxial (110) films of SrRuO3

Single domain epitaxial (110) films of SrRuO3 exhibit uniaxial magnetic anisotropy instead of the biaxial anisotropy observed in the bulk material. The magnetic easy axis for the film is along the orthorhombic [010] direction below TC, and it rotates toward the [110] perpendicular direction as temperature decreases. The [100] direction, which is also magnetically “easy” in the bulk, becomes “hard” in the film. X-ray diffraction experiments show that this unique transformation of magnetic anisotropy is related to a distortion from the bulk orthorhombic lattice into a triclinic structure in the epitaxial film, such that the lattice along the [010] direction expands while its [100] counterpart contracts. The distortion appears to arise from rotation and tilt of RuO6 octahedra. The finding indicates that the magnetic anisotropy in epitaxial SrRuO3 films is rooted in the crystalline anisotropy influenced by strong spin–orbit interactions.

Uniform deposition of YBa2Cu3O7 thin films over an 8 inch diameter area by a 90° off‐axis sputtering technique

The uniform deposition of YBa2Cu3O7 (YBCO) thin films over an 8‐in.‐diam. area, using a 3‐in.‐diam. sputtering target and optimized substrate rotation in a single target 90° off‐axis sputtering technique, is reported. Two dimensional maps of the thickness profile of YBCO films deposited on a stationary substrate have been obtained using surface profilometry. These thicknesses were used in a computer simulation to predict which distance of the target from the center of the substrate rotation will produce the maximum area with uniform thickness. The films deposited on substrates mounted on a rotating arm displayed uniform thickness ( 87.5° K) and critical current density (Jc 4.2 K>2×107 A/cm2) over an 8‐in.‐diam area.

Initial Stage Nucleation and Growth of Epitaxial SrRuO3 Thin Films on (0 0 1) SrTiO3 Substrates

We have investigated the initial stage nucleation and growth of epitaxial SrRuO3 thin films grown on both polished (as received) and buffered HF (BHF) etched single crystal (0 0 1) SrTiO3 substrates by 90° off-axis sputtering. Atomic force microscopy indicates a dramatic difference in the initial stage growth of SrRuO3 films on the two substrates. The films on polished substrates nucleate as rectangular islands, which merge together to form a continuous film as the thickness increases. Complete coverage is obtained at film thickness of ∼20 nm. In contrast, the film on BHF etched substrate nucleates as finger-shaped islands at the step sites and continues to grow by adatom diffusion to the step sites. Complete coverage is obtained at a film thickness of ∼10 nm. This difference in the initial stage nucleation is attributed to the difference in surface morphology and termination layer of the two substrates. However, the thicker films on both as received and BHF etched substrates have identical surface morphologies. Such studies on the initial stage nucleation will also help us understanding the growth kinetics and development of surface morphology and interfaces in multilayered perovskite thin film heterostructures and devices.

Effects of miscut of the SrTiO3 substrate on microstructures of the epitaxial SrRuO3 thin films

We have studied the microstructures of epitaxial SrRuO 3 thin films on both exact and miscut (001) SrTiO 3 substrates with miscut angle of 1.9° and miscut direction of 12° away from the in-plane [100] axis by transmission electron microscopy (TEM). Among six possible types of domains, the (110) domains [i.e. the (110) planes are parallel to the (001) planes of SrTiO 3 ] are favored during the growth of the SrRuO 3 film. The SrRuO 3 thin films on exact (001) SrTiO 3 are composed of two types of (110) domains in the plane with nearly the same volume fraction. Each type has an in-plane orientation relationship with respect to the substrate of either SrRuO 3 [110]//SrTiO 3 [100] and SrRuO 3 [001]//SrTiO 3 [010] (the X-type), or SrRuO 3 [110]//SrTiO 3 [010] and SrRuO 3 [001]//SrTiO 3 [100] (the Y-type). However, the SrRuO 3 thin films grown on the miscut (001) SrTiO 3 are composed of mainly X-type domain.

Epitaxial thin film heterostructures of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3

Abstract We have grown stoichiometric pure perovskite phase Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) thin films and PMN-PT/SrRuO3 heterostructures on miscut (100) SrTiO3 substrates by pulsed laser deposition. X-ray diffraction θ–2θ scans show that the PMN-PT films are purely c-axis oriented. The off-axis Φ scans show that the heterostructures grow “cube-on-cube” with an in-plane epitaxial arrangement of PMN-PT[100], [010] // SrRuO3[001] // SrTiO3[100] and PMN-PT[010], [100] // SrRuO3[110] // SrTiO3[010]. The crystalline quality of the films found to be comparable to that of bulk single crystals. The AFM images show that the SrRuO3 and PMN-PT layers have smooth surfaces with root mean square roughness of 9A. These epitaxial heterostructures can be used for the fabrication of piezoelectric devices.