R-sapphire Substrates Research Articles

Evolution of the structural and microstructural characteristics of SrSn1 − xTixO3 thin films under the influence of the composition, the substrate and the deposition method

SrSn1−xTixO3 thin films were grown on R-sapphire and (100) LaAlO3 single crystal substrates by two different routes: chemical solution deposition (CSD) and pulsed laser deposition (PLD). Structural and microstructural characteristics of the films were determined by X-ray diffraction (θ–2θ, ω- and φ-scans) and field emission scanning electron microscopy. Pure perovskite phase was obtained for all of the compositions, whatever the method of deposition and the substrate nature. On R-sapphire, a randomly oriented growth (polycrystalline) was observed for all of the compositions deposited by CSD while (h00) preferential orientation was attained when deposition was done by PLD, in particular for SrTiO3 composition. The phi-scan performed on this sample revealed that the (100) oriented grains present an in-plane ordering (epitaxial growth) with respect to the substrate with an alignment of the [011] direction of the film along the 121¯ direction of the substrate, explained on the basis of misfit considerations and interface arrangements. All of the films grown on (100) LaAlO3 exhibited an epitaxial growth with an in-plane relationship ⟨010⟩film // ⟨010⟩substrate. As for the thin film microstructure, porosity, homogeneity, shape and size of the grains were strongly influenced by Ti content in the SrSn1−xTixO3 solid solution, and also by the nature of the substrate and by the deposition method. Moreover, the influence of the composition and thin film growth on the photoluminescence of SST films were also evaluated.

MOCVD growth of CdO very thin films: Problems and ways of solution

In this paper the growth of CdO by the MOCVD technique at atmospheric pressure has been studied in order to achieve very thin films of this material on r-sapphire substrates. The growth evolution of these films was discussed and the existence of a threshold thickness, below which island-shaped structures appear, was demonstrated. Some alternatives to reduce this threshold thickness have been proposed in the frame of the analysis of the crystal growth process. The morphology and structural properties of the films were analyzed by means of SEM and HRXRD. High-quality flat CdO samples were achieved with thicknesses up to 20nm, which is five times thinner than the values previously reported in the literature.

Synthesis and thermal stability of Pt3Si, Pt2Si, and PtSi films grown by e-beam co-evaporation

Platinum silicide thin films formed via solid–solid reaction of Pt–Si interfaces play an important role as interconnects in microelectronics. This “self-aligned” silicide formation, however, is diffusion-limited and only Pt2Si and PtSi phases form. Also, their usefulness in electronic applications at high temperatures is limited due to agglomeration effects. Here, e-beam co-evaporation is presented as a means of fabricating well-defined films of Pt3Si as well as the Pt2Si and PtSi phases familiar to conventional, self-aligned silicide technology. Pt–Si films with a range of compositions (silicon atomic fractions of XSi = 0.00–0.77) were grown on r-sapphire substrates by co-deposition of independently controlled Pt and Si evaporant fluxes at 400 °C. Phase, morphology, and electronic properties were analyzed upon deposition and after vacuum annealing for 48 h at 1000 °C. As-deposited films with XSi < 0.70 are polycrystalline and at higher Si concentrations are fully amorphous. Valence spectra from Pt, Pt2Si, and PtSi films confirm previous reports and a valence spectrum is presented for the stoichiometric Pt3Si phase; these data provide experimental support for recently proposed revisions to the standard transition metal–silicon bonding model. As-deposited films, nominally 200 nm thick, are electrically conductive in the range 0.1–4.0 × 106 S/m and remain so after vacuum annealing for 48 h at 1000 °C. The phase, morphological, and electrical stabilities are attributed to the finely grained as-deposited morphologies of co-evaporated Pt silicide films, which hinder agglomeration at 1000 °C for much greater times than do morphologies of traditional, self-aligned silicides grown via solid-state reaction.

Comparison of PtSi Films Grown by Solid-State Reaction and by E-Beam Co-Evaporation: Thermal Stability in Air at 1000 °C

ABSTRACTTwo different methods were used to synthesize 200 nm thick single-phase, orthorhombic-PtSi films: (i) e-beam co-evaporation (EBC) of Pt and Si onto r-sapphire substrates and (ii) solid-state reaction (SSR) of sputtered Pt films on Si (100) wafers. Morphology, electrical conductivity, and crystalline structure were characterized for as-grown films and for films annealed in air at 1000 °C via scanning electron microscopy (SEM), 4-pt conductivity measurements, andin situX-ray diffraction (XRD). As-grown EBC films exhibit columnar grain morphology and slight (101) crystalline texture, while SSR films exhibit granular morphology with many voids and a strong (002) texture. Above 600 °C, EBC PtSi films rapidly oxidize to form crystalline Pt3Si and amorphous SiO2phases. Around 1000 °C, the Pt3Si phase melts and c-Pt grains nucleate. After air annealing for 6 h at 1000 °C, room-temperature XRD shows that the oxidized EBC films consist of Pt3Si and Pt phases within a SiO2matrix and become electrically insulating. SSR films initially form with a (002) o-PtSi orientation and above 900 °C they recrystallize to preferred (101) texture and exhibit an unchanged electrical conductivity and a stable film morphology during 48 h of air annealing at 1000 °C. Separate oxidation mechanisms are proposed for the two film types.

Morphological and Microstructural Evolution and Related Impurity Incorporation in Non-Polar a-Plane GaN Grown on r-Sapphire Substrates **Supported by the National Natural Science Foundation of China under Grant No 61204006, and the Fundamental Research Funds for the Central Universities under Grant No K50511250002.

Effects of the growth temperature on morphological and microstructural evolution of a-plane GaN films grown on r-plane sapphires by metal organic chemical vapor deposition are investigated by atomic force microscopy and secondary ion mass spectroscopy (SIMS). Surface morphology, structural quality and related impurity incorporation are very sensitive to the growth temperature. A significant difference of yellow luminescence is observed and attributed to the incorporation of carbon into GaN films, which is confirmed by SIMS analysis. Our results show that the sample with triangular-pit morphology has significantly higher concentrations of oxygen than the other sample with pentagon-like pit morphology, which is induced by the existence of an N-face in triangular pits.

Surface Acoustic Wave Humidity Sensors Based on ZnO Piezoelectric Films Sputtered on R-Sapphire Substrates**Supported by the National Natural Science Foundation of China under Grant Nos 11174142, 11304160 and 11404147, the National Basic Research Program of China under Grant No 2012CB921504, the PAPD Project, the Natural Science Foundation of Jiangsu Higher Education Institutions of China under Grant No 13KJB140008, and the

ZnO films on R-sapphire substrates are prepared and characterized by x-ray diffraction and scanning electron microscopy, which indicate that the thin films are well crystallized with texture. Love wave and Rayleigh wave are used for fabrications of humidity sensors, which are excited in and [0001] directions of the ZnO piezoelectric films, respectively. The experimental results show that both kinds of sensors have good humidity response and repeatability, and the performances of the Love wave sensors are better than those of the Rayleigh wave sensors at room temperature. Moreover, the theoretical calculations of the mass sensitivity of the sensors are also carried out and the calculated results are in good agreement with the experimental measurements.

Study of defect management in the growth of semipolar (11-22) GaN on patterned sapphire

This work describes, using mainly transmission electron microscopy as an investigation tool, the nature and behaviour of the crystalline defects which are present in (11-22) semipolar GaN films grown epitaxially on patterned r-sapphire substrates using a 3 step growth process. The microstructure at these different growth stages is described. The independent 3D-crystallites nucleated on the substrate surface contain threading dislocations resulting from the epitaxy on c-sapphire facets and basal stacking faults (BSFs), mainly in the −c-wings. These defects are concentrated in a few hundred nanometre wide stripe-like regions emerging on the top facet of the islands. By a careful choice of the growth conditions, these defective regions may be overgrown by defect-free material, blocking their propagation towards the coalesced surface. However, when the 3D crystals coalesce, new dislocations together with very few BSFs are created at the coalescence boundaries. These coalescence defects propagate to the surface of the films in (0001) planes. In summary, the control of the nucleation and propagation of the crystalline defects allows obtaining large area semipolar films with very low defect densities: 7 × 107 cm−2 for TDs and 70 cm−1 for BSFs.

Room temperature ferromagnetism in epitaxial Cr2O3 thin films grown on r-sapphire

We report on the epitaxial growth and magnetic properties of Cr2O3 thin films grown on r-sapphire substrate using pulsed laser deposition. The X-ray diffraction (XRD) (2θ and Φ) and TEM characterization confirm that the films are grown epitaxially. The r-plane (011¯2) of Cr2O3 grows on r-plane of sapphire. The epitaxial relations can be written as [011¯2] Cr2O3 ‖ [011¯2] Al2O3 (out-of-plane) and [1¯1¯20] Cr2O3 ‖ [1¯1¯20] Al2O3 (in-plane). The as-deposited films showed ferromagnetic behavior up to 400 K but ferromagnetism almost vanishes with oxygen annealing. The Raman spectroscopy data together with strain measurements using high resolution XRD indicate that ferromagnetism in r-Cr2O3 thin films is due to the strain caused by defects, such as oxygen vacancies.

Humidity responses of Love wave sensors based on [formula omitted] ZnO/R-sapphire bilayer structures

Love wave humidity sensors are fabricated by sputtering (1 1 2¯ 0) ZnO thin films on R-sapphire substrates and lithographing interdigital transducers on the surfaces of ZnO films. The response characters of the sensors to humidity are investigated experimentally and theoretically. The experimental data show nonlinear function relations between oscillation frequency shifts of the sensors and relative humidity variations. Meanwhile, the phenomena can be explained by combining Brunauer–Emmett–Teller model for physical adsorption isotherms and surface acoustic wave propagation theory in inhomogeneous media. The experimental and theoretical results are in good agreement with each other.

Molecular beam epitaxy growth of SnO2 using a tin chemical precursor

The authors report on the development of a molecular beam epitaxy approach for atomic layer controlled growth of phase-pure, single-crystalline epitaxial SnO2 films with scalable growth rates using a highly volatile precursor (tetraethyltin) for tin and rf-oxygen plasma for oxygen. Smooth, epitaxial SnO2 (101) films on r-sapphire (101¯2) substrates were grown as a function of tin precursor flux and substrate temperatures between 300 and 900 °C. Three distinct growth regimes were identified where SnO2 films grew in a reaction-, flux-, and desorption-limited mode, respectively, with increasing substrate temperature. In particular, with increasing tin flux, the growth rates were found to increase and then saturate indicating any excess tin precursor desorbs above a critical beam equivalent pressure of tin precursor. Important implications of growth kinetic behaviors on the self-regulating stoichiometric growth of perovskite stannates are discussed.

The S concentration dependence of lattice parameters and optical band gap of a-plane ZnOS grown epitaxially on r-plane sapphire

High quality non-polar a-plane ZnO1−xSx films were deposited epitaxially on r-sapphire substrates by pulsed laser ablating a ZnS ceramic target in O2 atmosphere. The in-plane orientation relationship between ZnO1−xSx films and sapphire substrates was revealed by X-ray diffraction φ-scans as ZnO1−xSx[1¯100] || sapphire [1¯1¯20] and ZnO1−xSx [0001] || sapphire [1¯101]. Both lattice constants a and c expand with increasing S content in the ZnO1−xSx alloys. Large lattice misfit along either ZnO1−xSx [0001] or ZnO1−xSx[1¯100] indicated a domain matching rather than lattice matching epitaxy mode for a-plane ZnO1−xSx on r-plane sapphire. Optical measurements show high transparency of the films in visible range. The band gap of ZnO1−xSx becomes narrower with S fraction increasing up to 43%, which behaves similar to the band gap of c-plane ZnO1−xSx films grown on c-plane sapphire.

Growth and characterization of self-assembled Cd1−xMgxO (0 ≤ x ≤ 1) nanoparticles on r-sapphire substrates

In this work, the growth of isolated Cd1−xMgxO nanoparticles on r-sapphire substrates is extended to the entire range of Mg content (0 ≤ x ≤ 1) using the spray pyrolysis method. The sizes of the Cd1−xMgxO nanoparticles were in the ranges 4–6 nm and 15–30 nm (with a nanoparticle density of 1010 cm−2). The composition of the nanoparticles was determined using transmission electron microscopy energy dispersive X-ray analysis (TEM-EDAX), while the compound formation was confirmed using X-ray photoelectron spectroscopy. A systematic decrease in the a lattice parameter of Cd1−xMgxO on increasing the Mg content substantiated the successful incorporation of Mg2+ ions into the cubic CdO lattice. Single-phase cubic Cd1−xMgxO nanoparticles were obtained in the range of nominal Mg content 0% to 30% (0 ≤ x ≤ 0.26), whereas a Mg content of about 40% (nominal) forced a phase separation where Cd1−xMgxO and Mg1−xCdxO co-exist (the Mg nominal content was 40% to 90%), providing experimental evidence of the phase separation that exists in the MgO–CdO system. This phase separation was detected using selected area electron diffraction and X-ray diffraction measurements. Cross-sectional transmission electron microscopy analysis shows the growth of a single layer of isolated and crystalline Cd1−xMgxO nanoparticles over the r-sapphire substrate.

Evaluation of superconducting pickup coils with high Q for 700 MHz NMR

In this study, we propose a helmholtz-type coil using a high-temperature superconductor (HTS) for 700 MHz NMR pickup coils with a high quality factor (Q). We analyzed the coil microwave resonant properties of the helmholtz-type one turn coil made using an HTS (DyBa2Cu3O7−δ: DyBCO coil) and the same type of coil made from normal metal (Cu coil) through electromagnetic simulations. The Q of the DyBCO coil at 77 K and the Cu coil at 77 K were 3295 and 482 in the simulations. The one turn coils were fabricated using 200-μm-thick DyBCO thin film deposited on an r-sapphire substrate and 3^m-thick Cu thin film on an r-sapphire substrate. The Q of the DyBCO coil was 2920 at 77 K, and that of the Cu coil was 423 at 77 K. These experimental results indicate that the DyBCO coils are more advantageous than the Cu coils for high-sensitivity NMR measurements.

Preparation and modification of VO2 thin film on R-sapphire substrate by rapid thermal process

The VO2 thin film with high performance of metal–insulator transition (MIT) is prepared on R-sapphire substrate for the first time by magnetron sputtering with rapid thermal process (RTP). The electrical characteristic and THz transmittance of MIT in VO2 film are studied by four-point probe method and THz time domain spectrum (THz-TDS). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and search engine marketing (SEM) are employed to analyze the crystalline structure, valence state, surface morphology of the film. Results indicate that the properties of VO2 film which is oxidized from the metal vanadium film in oxygen atmosphere are improved with a follow-up RTP modification in nitrogen atmosphere. The crystallization and components of VO2 film are improved and the film becomes compact and uniform. A better phase transition performance is shown that the resistance changes nearly 3 orders of magnitude with a 2-°C hysteresis width and the THz transmittances are reduced by 64% and 60% in thermal and optical excitation respectively.

One-step growth of isolated CdO nanoparticles on r-sapphire substrates by using the spray pyrolysis methodology

In spite of the remarkable properties of CdO, there are only a few reports on CdO nanostructures, especially on isolated NPs. In this paper, we analyze the growth of isolated CdO nanoparticles (NPs) on 0.5° miscut r-sapphire substrates by using the spray pyrolysis methodology in its classical configuration. A systematic study has been performed to optimize the growth parameters such as precursor concentration, growth time, spray rate and growth temperature to obtain CdO NPs (size: 6–10 nm to 35–100 nm depending on the growth conditions). The study shows the control over the size and density of the CdO nanoparticles that can be achieved by adjusting the growth parameters. The CdO nanoparticles were assembled in a parallel chain on the steps of the substrate surface (density: 2.1 ± 0.1 × 1010 NPs per cm2), which provided nucleation sites for the CdO adatoms, in addition to the free surface bonds on the substrate. A surface treatment of the r-sapphire substrate prior to the growth process provided additional nucleation sites for CdO nanoparticles resulting in an increase of the density (3.3 ± 0.2 × 1010 NPs per cm2). High-resolution transmission electron microscopy, X-ray diffraction and selected area electron diffraction analysis display the polycrystalline nature of the CdO samples, whereas X-ray photoelectron spectroscopy study confirmed the compound formation. The study shows the versatility of the spray pyrolysis technique to grow isolated nanoparticles of CdO on substrates in a single step, without any template assistance and/or post annealing treatment.

Correlation between structure and semiconductor-to-metal transition characteristics of VO2/TiO2/sapphire thin film heterostructures

This study focuses on the role of strain and thin film epitaxy on the semiconductor-to-metal transition (SMT) characteristics of single crystalline VO2 thin films. The VO2/TiO2 heterostructures of controlled orientations were epitaxially grown on m-cut, r-cut and c-cut sapphire substrates. Detailed structural investigations were performed using high-resolution X-ray diffraction (2θ–θ and φ scans) and high-resolution transmission electron microscopy techniques to correlate SMT properties with microstructural characteristics. Monoclinic (M1) VO2 thin films with (100), (001) and (2¯01) out-of-plane orientations were grown on TiO2(101)/r-sapphire, TiO2(100)/c-sapphire and TiO2(001)/m-sapphire platforms, respectively. The in-plane alignments across the interfaces were established to be [010](100)VO2||[010](101)TiO2, [100](001)VO2||[001](100)TiO2 and [010](2¯01)VO2‖[010](001)TiO2 for r-sapphire, c-sapphire and m-sapphire substrates, respectively. We were able to tune the SMT temperature of VO2 epilayers from ∼313K to 354K (bulk Tc≈340K). The SMT characteristics were interpreted based upon the residual strain in the VO2 lattice, particularly along the c-axis of tetragonal VO2. This research introduces the VO2-based single crystalline heterostructures as a potential candidate for a wide range of applications where different transition temperatures are required.

Strain Distributions in Non-Polar a-Plane InxGa1−xN Epitaxial Layers on r-Plane Sapphire Extracted from X-Ray Diffraction

By using x-ray diffraction analysis, we investigate the major structural parameters such as strain state and crystal quality of non-polar a-plane InxGa1−xN thin films grown on r-sapphire substrates by metalorganic chemical vapour deposition. The results of the inplane grazing incidence diffraction technique are analyzed and compared with a complementary out-of-plane high resolution x-ray diffraction technique. When the indium composition is low, the a-plane InxGa1−xN layer is tensile strain in the growth direction (a-axis) and compressive strain in the two in-plane directions (m-axis and c-axis). The strain status becomes contrary when the indium composition is high. The stress in the m-axis direction σyy is larger than that in the c-axis direction σzz. Furthermore, strain in the two in-plane directions decrease and the crystal quality becomes better with the growing of the InxGa1−xN film.

Characteristics of surface acoustic waves excited by (1120) ZnO films deposited on R-sapphire substrates.

(1120)-textured ZnO films are deposited on Rsapphire substrate by RF magnetron sputtering and the effects of deposition conditions on ZnO films are investigated. The chemical compositions of the ZnO films are characterized by X-ray photoelectron spectroscopy and the results indicate that the ratio of latticed oxygen to zinc increases with increasing of oxygen concentration in the sputtering gas, which demonstrates the improvement of crystal structures in ZnO films. To investigate the characteristics of surface acoustic waves excited by the (1120)-textured ZnO films, SAW delay lines based on layered structures of (1120) ZnO film/R-sapphire substrate are fabricated, in which Rayleigh and Love modes are excited along the (0001)- and (1100)-directions of the ZnO films, respectively. The phase velocities and electromechanical coupling factors of both wave modes are characterized as functions of the film-thickness-to-wavelength ratio. The acoustic properties of the layered structures are calculated using the transfer matrix method. The experimental and theoretical results are in good agreement with each other.

Growth and characterization of polar and nonpolar ZnO film grown on sapphire substrates by using atomic layer deposition

We investigated the electrical and the optical properties of polar and nonpolar ZnO films grown on sapphire substrates with different crystallographic planes. High resolution X-ray results revealed that polar c-plane (0001), nonpolar m-plane (10-10) and a-plane (11-20) ZnO thin films were grown on c-plane, m- and r-sapphire substrates by atomic layer deposition, respectively. Compared with the c-plane ZnO film, nonpolar m-plane and a-plane ZnO films showed smaller surface roughness and anisotropic surface structures. Regardless of ZnO crystal planes, room temperature photoluminescence spectra represented two emissions which consisted of the near bandedge (~380nm) and the deep level emission (~500nm). The a-plane ZnO films represented better optical and electrical properties than c-plane ZnO, while m-plane ZnO films exhibited poorer optical and electrical properties than c-plane ZnO.

Effects of lateral overgrowth on residual strain and In incorporation in a-plane InGaN/GaN quantum wells on r-sapphire substrates

We investigated the effects of the patterned lateral over-growth on the residual strain in GaN templates and In incorporation in a-plane InGaN/GaN quantum wells grown on a r-sapphire substrate, by utilizing micro-photoluminescence and Raman scattering spectroscopy. Strong enhancement of emission intensity is observed from the wing area. We report a reduction in the residual strain and different In incorporation in the wing area. The InGaN quantum wells on the merged area have higher In composition with smaller residual strain of the GaN layers underneath.