T.J. Zhang

Electromagnetic modes and prism-film coupling in anisotropic planar waveguides of epitaxial (101) rutile thin films

We report optical waveguiding in single‐crystal, epitaxial (101) oriented rutile (TiO2) thin films grown on (1120) sapphire (α‐Al2O3) substrates using the metal‐organic chemical vapor deposition technique. The electromagnetic field distributions and propagation constants for asymmetric planar waveguides composed of an anisotropic dielectric media applicable to these films are derived. Modifications to the prism‐film coupling theory for this anisotropic case are also discussed. By application of this model to (101) oriented rutile thin films, we directly obtain values of the ordinary and extraordinary refractive indexes, n0 and ne, of the rutile thin films as well as film thicknesses. We obtain typical values of the refractive indexes (n0=2.5701±0.0005; ne=2.934±0.001) near to those for bulk rutile single crystals indicating the exceptional quality of these films.

Study of defects and interfaces on the atomic scale in epitaxial tio2 thin films on sapphire

Abstract TiO2 thin films grown on (11 0) sapphire (α-Al2O3) at 800°C by the metallo-organic chemical vapour deposition technique have been characterized by transmission electron microscopy. The TiO2 thin films are single-crystal rutile. The epitaxial orientation relationship between the rutile thin films (R) and the sapphire substrates (S) is (101)[010]R‖(110)[0001]s. Growth twins are commonly observed in the films with (101) twin planes and twinning directions. The atomic structure of twin boundaries and TiO2-α-Al2O3 interfaces have been investigated by high-resolution electron microscopy. When the interfaces are viewed in the direction of [010]R-[0001]s, the interfaces appear structurally coherent along the direction of [10]R-[100]s. The small misfit (0.5%) is accommodated at interface steps. In contrast, in the direction of [10]R-[100]s, the interfaces are semicoherent. Growth mechanisms are discussed, based on information about the atomic structure of the interfaces.

Raman scattering and x‐ray diffractometry studies of epitaxial TiO2 and VO2 thin films and multilayers on α‐Al2O3(112̄0)

Epitaxial thin films of TiO2 and VO2 single layers and TiO2/VO2 multilayers were grown on (1120) sapphire (α‐Al2O3) substrates using the metalorganic chemical vapor deposition technique and were characterized using Raman scattering and four‐circle x‐ray diffractometry. X‐ray diffraction results indicate that the films are high quality single crystal material with well defined growth plane and small in‐plane and out‐of‐plane mosaic. Single‐layer films are shown to obey the Raman selection rules of TiO2 and VO2 single crystals. The close adherence to the Raman selection rules indicates the high degree of orientation of the films, both parallel and perpendicular to the growth plane. Selection rule spectra of two and three layer TiO2/VO2 multilayers are dominated by the VO2 layers with only minimal signature of the TiO2 layers. Due to the low band gap of semiconducting vanadium dioxide, we attribute the strong signature of the VO2 layers to resonant enhancement of the VO2 Raman component accompanied with abs...

Heteroepitaxy of TiO2, VO2 films and TiO2/VO2 multilayers grown on sapphire (1120) by metallo-organic chemical vapor deposition

Abstract Single-crystal TiO 2 and VO 2 films in single-layer and multilayer configurations have been successfully grown on sapphire (11 2 0) by the metallo-organic chemical vapor deposition technique. The epitaxial orientation relationships were determined by the X-ray diffraction and transmission electron microscopy techniques. High resolution electron microscopy studies were performed to examine the microstructures of the films at atomic level.

Microstructure of epitaxial VO2 thin films deposited on (1120) sapphire by MOCVD

Epitaxial VO[sub 2] thin films grown on (11[bar 2]0) sapphire ([alpha]--Al[sub 2]O[sub 3]) substrates by MOCVD at 600 [degree]C have been characterized by conventional electron microscopy and high resolution electron microscopy (HREM). Three different epitaxial relationships between the monoclinic VO[sub 2] films and sapphire substrates have been found at the room temperature: (I) (200)[010] monoclinic VO[sub 2]//(11[bar 2]0)[0001] sapphire; (II) (002)[010] monoclinic VO[sub 2]//(11[bar 2]0)[0001] sapphire; and (III) (020)[102] monoclinic VO[sub 2]//(11[bar 2]0)[0001] sapphire. Epitaxial relationships II and III are equivalent to each other when the film possesses tetragonal structure at the deposition temperature: i.e., they can be described as (010)[100] tetragonal VO[sub 2]//(11[bar 2]0)[0001] sapphire and (100)[010] tetragonal VO[sub 2]//(11[bar 2]0)[0001] sapphire. HREM image shows that the initial nucleation of the film was dominated by the first orientation relationship, but the film then grew into the grains of the second and the third (equivalent to each other at the deposition temperature) epitaxial relationships. Successive 90[degree] transformation rotational twins around the [ital a]-axis are commonly observed in the monoclinic films.

Structure and property of heteroepitaxial TiO2/VO2 multilayers

Abstract Various types of TiO2/VO2 multilayer structures have been prepared on sapphire (11 2 0) substrates by a low-pressure metal-organic chemical vapor deposition process. X-ray diffraction and transmission electron microscopy techniques were used to study the crystallinity and epitaxial relationships of the deposited films. High-resolution electron microscopy was used to examine the microstructure of the overlayers and interfaces. Electrical resistivity measurements were performed to investigate the metal-semiconductor phase transition of VO2 layers in multilayer structures.

Microstructure of epitaxial VO[sub 2] thin films deposited on (11[bar 2]0) sapphire by MOCVD

Epitaxial VO[sub 2] thin films grown on (11[bar 2]0) sapphire ([alpha]--Al[sub 2]O[sub 3]) substrates by MOCVD at 600 [degree]C have been characterized by conventional electron microscopy and high resolution electron microscopy (HREM). Three different epitaxial relationships between the monoclinic VO[sub 2] films and sapphire substrates have been found at the room temperature: (I) (200)[010] monoclinic VO[sub 2]//(11[bar 2]0)[0001] sapphire; (II) (002)[010] monoclinic VO[sub 2]//(11[bar 2]0)[0001] sapphire; and (III) (020)[102] monoclinic VO[sub 2]//(11[bar 2]0)[0001] sapphire. Epitaxial relationships II and III are equivalent to each other when the film possesses tetragonal structure at the deposition temperature: i.e., they can be described as (010)[100] tetragonal VO[sub 2]//(11[bar 2]0)[0001] sapphire and (100)[010] tetragonal VO[sub 2]//(11[bar 2]0)[0001] sapphire. HREM image shows that the initial nucleation of the film was dominated by the first orientation relationship, but the film then grew into the grains of the second and the third (equivalent to each other at the deposition temperature) epitaxial relationships. Successive 90[degree] transformation rotational twins around the [ital a]-axis are commonly observed in the monoclinic films.

Structural properties of epitaxial TiO sub 2 films grown on sapphire (11 2 0) by MOCVD

Titanium dioxide thin films were grown on sapphire (11{bar 2}0) substrates in a low-pressure metal-organic chemical vapor deposition system at temperatures ranging from 400 to 800 {degree}C. Raman scattering, x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy techniques were employed to characterize the structural properties of the deposited films. The resultant phases and structures of the deposited films depended on both the growth temperature and the substrate surface properties (surface imperfections, steps, etc.). At the growth temperature of 800 {degree}C, single-crystal rutile films were obtained reproducibly with two possible epitaxial relationships. At lower temperatures (400 to 775 {degree}C), the deposited films can be epitaxial or polycrystalline with highly oriented grains. The similarity between the atomic arrangements of the substrate and the film is discussed in detail to explain the observed epitaxial relationships and abruptness of the interfaces.

Structural properties of epitaxial TiO2 films grown on sapphire (11

Titanium dioxide thin films were grown on sapphire (11{bar 2}0) substrates in a low-pressure metal-organic chemical vapor deposition system at temperatures ranging from 400 to 800 {degree}C. Raman scattering, x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy techniques were employed to characterize the structural properties of the deposited films. The resultant phases and structures of the deposited films depended on both the growth temperature and the substrate surface properties (surface imperfections, steps, etc.). At the growth temperature of 800 {degree}C, single-crystal rutile films were obtained reproducibly with two possible epitaxial relationships. At lower temperatures (400 to 775 {degree}C), the deposited films can be epitaxial or polycrystalline with highly oriented grains. The similarity between the atomic arrangements of the substrate and the film is discussed in detail to explain the observed epitaxial relationships and abruptness of the interfaces.

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Titanium dioxide thin films were grown on sapphire (11{bar 2}0) substrates in a low-pressure metal-organic chemical vapor deposition system at temperatures ranging from 400 to 800 {degree}C. Raman scattering, x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy techniques were employed to characterize the structural properties of the deposited films. The resultant phases and structures of the deposited films depended on both the growth temperature and the substrate surface properties (surface imperfections, steps, etc.). At the growth temperature of 800 {degree}C, single-crystal rutile films were obtained reproducibly with two possible epitaxial relationships. At lower temperatures (400 to 775 {degree}C), the deposited films can be epitaxial or polycrystalline with highly oriented grains. The similarity between the atomic arrangements of the substrate and the film is discussed in detail to explain the observed epitaxial relationships and abruptness of the interfaces.