Domain Matching Epitaxy Paradigm Research Articles

Atomic-Scale Insights on Large-Misfit Heterointerfaces in LSMO/MgO/c-Al2O3

Understanding the interfaces in heterostructures at an atomic scale is crucial in enabling the possibility to manipulate underlying functional properties in correlated materials. This work presents a detailed study on the atomic structures of heterogeneous interfaces in La0.7Sr0.3MnO3 (LSMO) film grown epitaxially on c-Al2O3 (0001) with a buffer layer of MgO. Using aberration-corrected scanning transmission electron microscopy, we detected nucleation of periodic misfit dislocations at the interfaces of the large misfit systems of LSMO/MgO and MgO/c-Al2O3 following the domain matching epitaxy paradigm. It was experimentally observed that the dislocations terminate with 4/5 lattice planes at the LSMO/MgO interface and with 12/13 lattice planes at the MgO/c-Al2O3 interface. This is consistent with theoretical predictions. Using the atomic-resolution image data analysis approach to generate atomic bond length maps, we investigated the atomic displacement in the LSMO/MgO and MgO/c-Al2O3 systems. Minimal presence of residual strain was shown at the respective interface due to strain relaxation following misfit dislocation formation. Further, based on electron energy-loss spectroscopy analysis, we confirmed an interfacial interdiffusion within two monolayers at both LSMO/MgO and MgO/c-Al2O3 interfaces. In essence, misfit dislocation configurations of the LSMO/MgO/c-Al2O3 system have been thoroughly investigated to understand atomic-scale insights on atomic structure and interfacial chemistry in these large misfit systems.

Characterization of MgO/TiN bilayer deposited on cube-textured copper using pulsed-laser deposition technique

Here, we demonstrate heteroepitaxial growth of MgO/TiN thin films on flexible metal foil of copper substrates using pulsed laser deposition. X-ray and electron diffraction measurements revealed that the epitaxial MgO/TiN bilayer was oriented along [002] direction. The large mismatch between TiN film and Cu substrate was effectively reduced by the 5/6 and 6/7 variations of domains. Despite the local irregularity of misfit dislocations at TiN/Cu, the domain-matching epitaxy paradigm is a remarkably accurate theory. MgO/TiN bilayer is chemically homogeneous and Mg, Ti, or Cu atoms do not segregate into the low-angle grain boundaries regardless of MgO/TiN films thickness. Thus, it appears that thickness may be reduced up to the value in which TiN fully covers the rough surface of Cu tape. Unfortunately, issues related to the presence of in-plane TiN contraction, probably generated by the difference in thermal expansion coefficient, and the origin of misfit dislocations irregularity at TiN/Cu interface remain unresolved. Our findings can make a substantial contribution to further research on Cu-based coated conductors and help in better understanding the ways that MgO/TiN bilayer are grown on Cu tape.

Catalyst-assisted epitaxial growth of ferromagnetic TiO2/TiN nanowires

We report a novel method of growth for single-crystalline TiO2/TiN nanowires through oxidation of epitaxial TiN nanowires on Si-SiO2 (amorphous) and c-sapphire (crystalline) as practical substrates. We propose that the laser ablated Ti and N diffuse into molten Au to form TiN nanodots where the growth rate of nanowires is directly proportional to the laser ablation flux due to high diffusion in molten Au. The TiN nanowires were grown by Pulsed Laser Deposition method using Au as a catalyst. The TiN nanowires were then oxidized to create TiO2/TiN core-shell nanowires. The growth of TiO2 (rutile) occurs by domain matching epitaxy paradigm in such a way that (002) planes of the TiO2 match with (200) plane of TiN, where the TiO2 thickness can be tuned by adjustment of oxidation time and temperature. This design provides a core-shell structure of TiO2/TiN nanowires integrated with silicon and sapphire substrates. The Rutile TiO2 nanowires show ferromagnetic behavior, while the as-grown TiN exhibits diamagnetic behavior. The SEM, TEM, and EBSD are used to characterize the microstructure and atomic alignments of TiO2 nanowires. The simple method of oxidation combined with tunable magnetic properties provides benefits to many smart applications where the magnetic field can be used as an external stimulation.

The Catalyst-Assisted Epitaxial Growth of Ferromagnetic TiO <sub>2</sub>/TiN Nanowires

We report a novel method of growth for single-crystalline TiO2/TiN nanowires through oxidation of epitaxial TiN nanowires on Si/SiO2 (amorphous) and sapphire (crystalline) as practical substrates. We propose that the laser ablated Ti and N diffuse into molten Au to form TiN nano-dots where the growth rate of nanowires is directly proportional to the laser ablation flux due to high diffusion in molten Au. The TiN nanowires were grown by Pulsed Laser Deposition method using Au as a catalyst. The TiN nanowires were then oxidized to create TiO2/TiN core-shell nanowires. The growth of TiO2 (rutile) occurs by domain matching epitaxy paradigm in such a way that (002) planes of the TiO2 match with (200) plane of TiN, where the TiO2 thickness can be tuned by adjustment of oxidation time and temperature. This design provides a core-shell structure of TiO2/TiN integrated with silicon and sapphire substrates. The Rutile TiO2 nanowires show ferromagnetic behavior, while the as-grown TiN exhibits diamagnetic behavior. The SEM, TEM, and EBSD are used to characterize the microstructure and atomic alignments of TiO2 nanowires. The simple method of oxidation along with tunable magnetic properties provides benefits to many smart applications where the magnetic field can be used as an external stimulation.

High-quality TiN/AlN thin film heterostructures on c-sapphire

We have developed TiN/AlN/c-sapphire epitaxial heterostructures and compared it with TiN/c-sapphire epitaxial heterostructures, needed for GaN-based LEDs and lasers. AlN is used as a buffer layer to provide a high misfit strain and facilitate the 2D growth on sapphire. The large misfit strain between sapphire and AlN makes this substrate a great candidate for GaN-based devices because it guarantees a full relaxation of AlN thin films through domain matching epitaxy paradigm. TiN can also act as an excellent contact and bottom electrode for Ⅲ-Ⅴ nitrides. Also, the introduction of TiN as a buffer layer decreases the critical thickness beyond which dislocations can grow in GaN thin films due to higher misfit strain compared to sapphire, which also improves the quality of potential GaN thin films. The selected-area-electron-diffraction patterns, scanning transmission electron microscopy, and transmission Kikuchi diffractions along with atomic arrangement simulations revealed that films are epitaxial with the following relationships: TiN<101>‖AlN[1¯21¯0]‖sapphire[011¯0] (in-plane), and TiN<111>‖AlN[0001]‖sapphire[0001] (out-of-plane). This is equivalent to a 30° rotation of Al basal plane in AlN with respect to that in sapphire. In TiN/c-sapphire epitaxial platforms, there is a 30° rotation: TiN<101>‖sapphire[011¯0] (in-plane), and TiN<111>‖sapphire[0001] (out-of-plane). It is shown that these heterostructures are fully relaxed in terms of misfit strains and only thermal strain stays as unrelaxed. The domain matching epitaxy paradigm is used to rationalize the epitaxial growth. The details of dislocations nucleation and glide in these heterostructures were studied and the results also discussed to elucidate the mechanism of strain relaxation.

Control of Structural and Electrical Transitions of VO2 Thin Films.

Unstrained and defect-free VO2 single crystals undergo structural (from high-temperature tetragonal to low-temperature monoclinic phase) and electronic phase transitions simultaneously. In thin films, however, in the presence of unrelaxed strains and defects, structural (Peierls) and electronic (Mott) transitions are affected differently, and are separated. In this paper, we have studied the temperature dependence of structural and electrical transitions in epitaxially grown vanadium dioxide films on (0001) sapphire substrates. These results are discussed using a combined kinetics and thermodynamics approach, where the velocity of phase transformation is controlled largely by kinetics, and the formation of intermediate phases is governed by thermodynamic considerations. We have grown (020) VO2 on (0001) sapphire with two (001) and (100) in-plane orientations rotated by 122°. The (100)-oriented crystallites are fully relaxed by the paradigm of domain-matching epitaxy, whereas (001) crystallites are not relaxed and exhibit the formation of a few atomic layers of thin interfacial V2O3. We have studied the structural (Peierls) transition by temperature-dependent in situ X-ray diffraction measurements, and electronic (Mott) transition by electrical resistance measurements. A delay of 3 °C is found between the onset of structural (76 °C) and electrical (73 °C) transitions in the heating cycle. This temporal lag in the transition is attributed to the residual strain existing in the VO2 crystallites. With this study, we suggest that the control of structural and electrical transitions is possible by varying the transition activation barrier for atomic jumps through the strain engineering.

Thin film bi-epitaxy and transition characteristics of TiO2/TiN buffered VO2 on Si(100) substrates

ABSTRACTBi-epitaxial VO2 thin films with [011] out-of-plane orientation were integrated with Si(100) substrates through TiO2/TiN buffer layers. At the first step, TiN is grown epitaxially on Si(100), where a cube-on-cube epitaxy is achieved. Then, TiN was oxidized in-situ ending up having epitaxial r-TiO2. Finally, VO2 was deposited on top of TiO2. The alignment across the interfaces was stablished as VO2(011)║TiO2(110)║TiN(100)║Si(100) and VO2(110) /VO2(010)║TiO2(011)║TiN(112)║Si(112). The inter-planar spacing of VO2(010) and TiO2(011) equal to 2.26 and 2.50 Å, respectively. This results in a 9.78% tensile misfit strain in VO2(010) lattice which relaxes through 9/10 alteration domains with a frequency factor of 0.5, according to the domain matching epitaxy paradigm. Also, the inter-planar spacing of VO2(011) and TiO2(011) equals to 3.19 and 2.50 Å, respectively. This results in a 27.6% compressive misfit strain in VO2(011) lattice which relaxes through 3/4 alteration domains with a frequency factor of 0.57. We studied semiconductor to metal transition characteristics of VO2/TiO2/TiN/Si heterostructures and established a correlation between intrinsic defects and magnetic properties.

Epitaxial integration of TiO2 with Si(100) through a novel approach of oxidation of TiN/Si(100) epitaxial heterostructure

ABSTRACTIn this study, we provide a novel approach to the epitaxial integration of TiO2 with Si(100) and investigate the defect mediated ferromagnetism in TiO2 structure. Epitaxial TiO2 thin films were grown on a TiN/Si(100) epitaxial heterostructure through oxidation of TiN where a single crystalline rutile-TiO2 (r-TiO2) with a [110] out-of-plane orientation was obtained. The epitaxial relationship is determined to be TiO2(1$\bar 1$0)||TiN(100) and TiO2(110)||TiN(110). We rationalized this epitaxy using the domain matching epitaxy paradigm. First TiN is grown epitaxially on Si(100). Subsequently, TiN/Si(100) samples are oxidized to create r-TiO2/TiN/Si(100) epitaxial heterostructures. The details of the mechanism behind the oxidation of single crystalline TiN to TiO2 was investigated using atomic scale high resolution electron microscopy techniques. Defects introduced to the heterostructure during oxidation caused ferromagnetism in TiO2 thin film which is reversible and can be tuned by controlling oxygen partial pressure. The source of magnetization is correlated with the presence of oxygen vacancy leading to introduction of two localized states; hybrid and polaron among neighboring Ti atoms, and titanium vacancy providing four holes to form molecular oxygen. We present structure property correlations and its impact on the next generation solid state devices.

Diamagnetism to ferromagnetism in Sr-substituted epitaxial BaTiO3 thin films

We report on the ferromagnetic-like behavior in otherwise diamagnetic BaTiO3 (BTO) thin films upon doping with non-magnetic element Sr having the composition Ba0.4Sr0.6TiO3 (BST). The epitaxial integration of BST (∼800 nm) thick films on Si (100) substrate was achieved using MgO (40 nm) and TiN (20 nm) as buffer layers to prepare BST/MgO/TiN/Si (100) heterostructure by pulsed laser deposition. The c-axis oriented and cube-on-cube epitaxial BST is formed on Si (100) as evidenced by the in-plane and out-of-plane X-ray diffraction. All the deposited films are relaxed through domain matching epitaxy paradigm as observed from X-ray diffraction pattern and A1TO3 mode (at 521.27 cm−1) of Raman spectra. As-deposited BST thin films reveal ferromagnetic-like properties, which persist up to 400 K. The magnetization decreases two-fold upon oxygen annealing. In contrast, as-deposited un-doped BTO films show diamagnetism. Electron spin resonance measurements reveal no evidence of external magnetic impurities. XRD and X-ray photoelectron spectroscopy spectra show significant changes influenced by Sr doping in BTO. The ferromagnetic-like behavior in BST could be due to the trapped electron donors from oxygen vacancies resulting from Sr-doping.

Structural, magnetic and magnetotransport properties of bi-epitaxial La0.7Sr0.3MnO3 (110) thin films integrated on Si (001)

We report the growth of bi-epitaxial La0.7Sr0.3MnO3 (110) thin films on Si (001) substrate with cubic yttria stabilized zirconia (c-YSZ)/SrTiO3 (STO) buffer layers by pulsed laser deposition. The La0.7Sr0.3MnO3 and STO thin films were grown with a single [110] out-of-plane orientation and with two in-plane domain variants, which is confirmed by XRD and detailed TEM studies. The growth of STO on c-YSZ can be explained by the paradigm of domain matching epitaxy. The epitaxial relationship between STO and c-YSZ can be written as [110] (001) c-YSZ ‖ [1¯11¯] (110) STO (or) [110] (001) c-YSZ ‖ [1¯12¯] (110) STO. The La0.7Sr0.3MnO3 thin films are ferromagnetic with Curie temperature 324 K and showed metal to insulator transition at 285 K. The La0.7Sr0.3MnO3 thin films showed hysteresis loops in magnetoresistance when magnetic field is applied along both in-plane (110) and out-of-plane [110] directions. The highest magnetoresistance obtained in this study is −32% at 50 K and 50 kOe for in-plane configuration, whereas the room-temperature magnetoresistance is −4% at 10 kOe and −17% at 50 kOe. The hysteresis in the magnetoresistance and the controlled domain boundaries in bi-epitaxial La0.7Sr0.3MnO3 films integrated on Si can offer significant advantages over the polycrystalline counterparts.

Epitaxial growth of rutile TiO2 thin films by oxidation of TiN/Si{100} heterostructure

We have integrated epitaxial TiO2 on a TiN/Si(100) platform through oxidation of TiN. The oxidation of TiN(100)/Si(100) results in the formation of an epitaxial rutile-TiO2 (r-TiO2) with a [110] out-of-plane orientation. We have studied in detail the r-TiO2 epitaxy and the epitaxial relationship is determined to be TiO2(11¯0)||TiN(100) and TiO2(110)||TiN(110). We rationalized this epitaxy using the domain matching epitaxy paradigm. Below the r-TiO2 epitaxial layer, we observed cuboids, which are mostly voids. We described the mechanism of oxidation where Ti out diffusion during oxidation leads to collapse of the nitrogen octahedron. This collapse makes neighboring Ti bonds weaker, promoting these Ti atoms to diffuse out next. Thus, cuboids filled with atomic nitrogen are formed, which then form N2 gas. The N2 pressure in these cuboids was estimated to be as high as 359 MPa, assuming all N2 is retained in the cuboids. This pressure can exceed the fracture stress of TiO2 and leads to rupture of thin TiO2 surface, which has been observed under certain conditions.

Ferroelectric and ferromagnetic properties in BaTiO3 thin films on Si (100)

In this paper, we report on the epitaxial integration of room temperature lead-free ferroelectric BaTiO3 thin (∼1050 nm) films on Si (100) substrates by pulsed laser deposition technique through a domain matching epitaxy paradigm. We employed MgO and TiN as buffer layers to create BaTiO3/SrRuO3/MgO/TiN/Si (100) heterostructures. C-axis oriented and cube-on-cube epitaxial BaTiO3 is formed on Si (100) as evidenced by the in-plane and out-of-plane x-ray diffraction, and transmission electron microscopy. X-ray photoemission spectroscopic measurements show that Ti is in 4(+) state. Polarization hysteresis measurements together with Raman spectroscopy and temperature-dependent x-ray diffraction confirm the room temperature ferroelectric nature of BaTiO3. Furthermore, laser irradiation of BaTiO3 thin film is found to induce ferromagnetic-like behavior but affects adversely the ferroelectric characteristics. Laser irradiation induced ferromagnetic properties seem to originate from the creation of oxygen vacancies, whereas the pristine BaTiO3 shows diamagnetic behavior, as expected. This work has opened up the route for the integration of room temperature lead-free ferroelectric functional oxides on a silicon platform.

Effect of substrate temperature on the microstructural properties of titanium nitride nanowires grown by pulsed laser deposition

The current work reports on the growth and microstructural characterization of titanium nitride (TiN) nanowires on single crystal silicon substrates using a pulsed laser deposition method. The physical and microstructural properties of the nanowires were characterized using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The corrosion properties of the TiN nanowires compared to TiN thin film were evaluated using Direct Current potentiodynamic and electrochemical impedance spectroscopy. The nanowires corroded faster than the TiN thin film, because the nanowires have a larger surface area which makes them more reactive in a corrosive environment. It was observed from the FESEM image analyses that as the substrate temperature increases from 600 °C to 800 °C, there was an increase in both diameter (25 nm–50 nm) and length (150 nm–250 nm) of the nanowire growth. There was also an increase in spatial density with an increase of substrate temperature. The TEM results showed that the TiN nanowires grow epitaxially with the silicon substrate via domain matching epitaxy paradigm, despite a large misfit.

Nanosecond laser switching of surface wettability and epitaxial integration of c-axis ZnO thin films with Si(111) substrates

We have achieved integration of polar ZnO[0001] epitaxial thin films with Si(111) substrates where cubic yttria-stabilized zirconia (c-YSZ) was used as a template on a Si(111) substrate. Using XRD (θ–2θ and φ scans) and HRTEM techniques, the epitaxial relationship between the ZnO and the c-YSZ layers was shown to be [0001]ZnO ∥ [111]YSZ and , where the [] direction lies in the (0001) plane, and the [] direction lies in the (111) plane. Similar studies on the c-YSZ/Si interface revealed epitaxy as 〈111〉YSZ ∥ 〈111〉Si and in-plane 〈110〉YSZ ∥ 〈110〉Si. HRTEM micrographs revealed atomically sharp and crystallographically continuous interfaces. The ZnO epilayers were subsequently laser annealed by a single pulse of a nanosecond excimer KrF laser. It was shown that the hydrophobic behavior of the pristine sample became hydrophilic after laser treatment. XPS was employed to study the effect of laser treatment on surface stoichiometry of the ZnO epilayers. The results revealed the formation of oxygen vacancies, which are envisaged to control the observed hydrophilic behavior. Our AFM studies showed surface smoothing due to the coupling of the high energy laser beam with the surface. The importance of integration of c-axis ZnO with Si(111) substrates is emphasized using the paradigm of domain matching epitaxy on the c-YSZ[111] buffer platform along with their out-of-plane orientation, which leads to improvement of the performance of the solid-state devices. The observed ultrafast response and switching in photochemical characteristics provide new opportunities for application of ZnO in smart catalysts, sensors, membranes, DNA self-assembly and multifunctional devices.

Interface Magnetism in Epitaxial BiFeO3-La0.7Sr0.3MnO3 Heterostructures Integrated on Si(100)

We report on the heteroepitaxial growth of ferroelectric (FE)-antiferromagnetic (AFM) BiFeO3 (BFO) on ferromagnetic La0.7Sr0.3MnO3 (LSMO), integrated on Si(100) using pulsed laser deposition via the domain matching epitaxy paradigm. The BFO/LSMO films were epitaxially grown on Si(100) by introducing epitaxial layers of SrTiO3/MgO/TiN. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photo absorption spectroscopy, and atomic force microscopy were employed to fully characterize the samples. Furthermore, we have investigated the magnetic behavior of this five layer heterostructure, in which a d(5) system (Fe(3+)) manifested in FE-AFM BFO is epitaxially conjoined at the interface to a multivalent transition metal ion such as Mn(3+)/Mn(4+) in LSMO. The temperature- and magnetic field-dependent magnetization measurements reveal an unexpected enhancement in magnetic moment and improved magnetic hysteresis squareness originating from the BFO/LSMO interface. We observe a stronger temperature dependence of HEB when the polarity of field cooling is negative as compared to positive field cooling. We believe such an enhancement in magnetic moment and magnetic coupling is likely directly related to an electronic orbital reconstruction at the interface and complex interplay between orbital and spin degrees of freedom, similar to what has previously been reported in the literature. Future work will involve the linearly polarized X-ray absorption measurements to prove this hypothesis. This work represents a starting step toward the realization of magneto-electronic devices integrated with Si(100).

Recent progress in thin film epitaxy across the misfit scale (2011 Acta Gold Medal Paper)

This paper discusses recent progress in thin film epitaxy across the misfit scale through the paradigm of domain matching epitaxy (DME). This epitaxy across the misfit scale is critical for integrating multifunctionality on a chip and creating smart structures for next-generation solid-state devices. There are three sources of strains that are cumulative at the growth temperature, and the relaxation process starts during the growth process. Upon cooling, unrelaxed lattice, thermal and defect strains give rise to net residual strains. In large misfit (ε⩾10%) systems, where lattice misfit strain is predominant, it can be relaxed completely, and then only thermal and defect strains remain upon cooling. In low misfit systems, all three sources contribute to the residual strain upon cooling, as result of incomplete lattice relaxation. The predominant strain relaxation mechanism in thin films is by nucleation of dislocations at the free surface, as the nucleation energy in the bulk is considerably higher. At the free surface, the activation barrier for dislocation nucleation is considerably lower at the steps. Since the step formation energy is lower under a compressive stress compared with tensile stress, it reduces nucleation energy under compressive stress and lowers the critical thickness compared with tensile stresses in thin films. Once the dislocation nucleates, it propagates or glides to the interface to relieve the strain. However, if lattice frictional stress in the film is high, most dislocations may not reach the interface, depending upon the growth temperature and rate. Thus, these two key steps, dislocation nucleation and propagation, play a critical role in the thin film relaxation process. Once the dislocations reach the interface, the atomic structure of the dislocation at the heterointerfaces determines its electronic properties, specifically trapping and recombination characteristics. It is found that the atomic structure of the dislocation is determined by the interplay between strain and chemical free energies. Thus, the dislocations (representing missing or extra planes) play a critical role in the relaxation of thin film heterostructures. This paper focuses on epitaxy across the misfit scale, based upon matching of integral multiples of lattice planes. If the misfit falls between the integral multiples, it is accommodated by the principle of domain variation, where domains alternate to accommodate the misfit. Details of epitaxy from low misfit (∼4%) in Ge/Si) to large misfit (∼22%) in TiN/Si are shown. In III-nitride/sapphire and II-oxide/sapphire systems, this paper deals with polar orientations, where misfit is uniform in the basal plane, and non-polar orientations, where misfit varies over an order of magnitude in the film plane. It is shown that the DME paradigm is key to the integration of thin film heterostructures across the misfit scale and other complex systems such as vanadium oxide and PZT systems on Si(100) substrates for the integration of functionalities on a computer chip. Finally, it is shown that the formation of epitaxial and self-assembled nanodots on Si(100) provides a critical advance, with tremendous implications for information and data storage and related nanomagnetics applications.

Thin film epitaxy and near bulk semiconductor to metal transition in VO2/NiO/YSZ/Si(001) heterostructures

Abstract

Phase Tuning, Thin Film Epitaxy, Interfacial Modeling, and Properties of YSZ-Buffered TiO2 on Si(001) Substrate

We have studied systematically the influence of pulsed laser deposition variables on microstructure and properties of TiO2 epitaxial thin films where integration with Si(100) substrate was achieved by cubic yttria-stabilized-zirconia (c-YSZ) buffer layer. Details of crystallographic and atomic arrangements across the interfaces are discussed in the light of the domain matching epitaxy paradigm. The single crystalline rutile films were obtained at higher substrate temperatures and lower oxygen pressures; whereas, the growth of epitaxial anatase films was promoted by decreasing the temperature and increasing the oxygen pressure. We showed that crystallographic structure of the TiO2 films is determined mainly by the termination structure of the c-YSZ layer and the bonding characteristics across the TiO2/c-YSZ interface. Using 2θ and φ scans of XRD, the epitaxial relationship between Si(100) substrate and the zirconia buffer layer was shown to be cube-on-cube: (001)[100]YSZ∥(001)[100]Si. Furthermore, the epitaxial relationships at the rutile/zirconia and the anatase/zirconia interfaces were determined as (100)[01̅1]rutile∥(001)[010]YSZ and (001)[110]anatase∥(001)[100]YSZ, respectively. The proposed crystallographic arrangements as well as the epitaxial growth were confirmed by high resolution TEM diffraction and imaging. XPS and XRD studies showed less defect content and favorable crystallinity in the films grown at higher temperatures. AFM results revealed that the finest domain size and the smoothest surface were obtained at the intermediate deposition temperatures. Based on the four-point electrical measurements, the heterostructures deposited at 500 °C were more conductive than those grown at 300 and 700 °C. Photocatalytic activity of the films was studied through decomposition of 4-chlorophenol under UV illumination. The maximum photocatalytic reaction rate constants were determined as 0.0124 and 0.0087 min–1 for the anatase and the rutile films grown at 500 °C.

Structure–property correlation in epitaxial (2 0 0) rutile films on sapphire substrates

We have investigated the influence of the deposition variables on photocatalytic properties of epitaxial rutile films. Despite a large lattice misfit of rutile with sapphire substrate, (200) epitaxial layers were grown on (0001)sapphire by domain matching epitaxy paradigm. Using φ-scan XRD and cross section TEM, the epitaxial relationship was determined to be rutile(100)||sapphire(0001), rutile(001)||sapphire(10−10), and rutile(010)||sapphire(1−210). Based on the XRD patterns, increasing the repetition rate introduced tensile stress along the film normal direction, which may arise as a result of trapped defects. Formation of such defects was studied by UV–VIS, PL, and XPS techniques. AFM studies showed that the film roughness increases with the repetition rate. Finally, photocatalytic performance of the layers was investigated through measuring decomposition rate of 4-chlorophenol on the films surface. The films grown at higher frequencies revealed higher photocatalytic efficiency. This behavior was mainly related to formation of point defects which enhance the charge separation.

VO2 thin films: Defect mediation in room temperature ferromagnetic switching characteristics

Vanadium dioxide (VO2) has tremendous potential in multifunctional device applications related to spintronics, switching, and magnetic recording. We have discovered that the room temperature ferromagnetism (RTFM) in undoped vanadium oxide epitaxial films can be switched on and off by altering the cooling ambient conditions which exhibit a sharp electrical transition at 341 K. By correlating the structural and ferromagnetic properties in VO2, we envisage the potential for creation of novel multifunctional solid-state devices. High-quality epitaxial VO2 thin films were grown on c-sapphire (0001) substrates, under different ambient conditions via the domain matching epitaxy paradigm. The observed RTFM has its origin in the valence charge defects with unpaired electrons in V+3 in VO2 thin films, where the concentration of the defects could be varied with oxygen partial pressure. The VO2 films-with a high ferro- to paramagnetic transition (Curie) temperature around 500 K estimated by fitting the magnetization data to the Bloch’s T3/2 law, a saturated magnetization of 18 emu/cm3, and with a finite coercivity of 40 Oe at 300 K-can be useful for integrated smart sensors operable at room temperature.