Single-crystal Sapphire Substrates Research Articles

Substrate temperature and oxygen pressure dependence of pulsed laser-deposited Sr ferrite films

The effect of substrate temperature and oxygen pressure on the microstructure, magnetic and magneto-optical properties of Sr ferrite (SrM) films grown on (001) single-crystal sapphire substrates by pulsed laser deposition has been investigated. Polycrystalline SrM films with perpendicular magnetic anisotropy could be prepared under a wide range of oxygen pressures and relatively high temperatures, sufficient to crystallise the material. However, an almost exclusive c-axis orientation normal to the film plane could be attained only at a narrow operational window centered at 0.1 mbar and 840°C. The magneto-optical properties of the films were comparable to those of the bulk barium hexaferrite single-crystal material. In addition, results obtained by atomic force microscopy provide convincing evidence that the growth of Sr ferrite on sapphire takes place by a spiral growth mechanism.

Residual Stress Effects on the Fracture of Tantalum Nitride Films

ABSTRACTIn this study, continuous microscratch testing was used to determine the effects of compressive residual stresses on the fracture of thin tantalum nitride films. The films were sputter deposited at room temperature onto single crystal sapphire substrates to a thickness of 600 nm. Some films were left in the as-deposited condition while the remaining films were vacuum annealed at 300°C. The only discernible change in structure was a surface rearrangement of atoms into parallel arrays of striations on the vacuum annealed samples revealing a high compressive residual stress in the films. These stresses had a strong effect on fracture as shown by the as-deposited films which buckled readily during scratch tests forming uniform-width and circular blisters while the vacuum annealed films exhibited a much lower susceptibility to fracture. Application of mechanics-based models for film buckling defined the levels of compressive residual stress and their effect on interfacial fracture.

Ion beam induced epitaxial recrystallization of alumina thin films deposited on sapphire

We have investigated ion beam induced epitaxial recrystallization (IBIER) of amorphous alumina thin films deposited on c-axis single crystal sapphire substrates. The alumina thin films, deposited by electron-beam evaporation, were about 66 and 90 nm thick, and doped with Fe cations to cation concentrations of 0 and 2 at.%, respectively. The as-deposited samples were subsequently irradiated at temperatures of 400, 500, and 600°C with either 180 keV oxygen ions or 360 keV argon ions. The process of IBIER was characterised with Rutherford backscattering spectrometry combined with ion channeling techniques. A kinetics study showed that the process of IBIER in alumina is characterised by an effective activation energy of 0.23 ± 0.05 eV. This value is one order of magnitude lower than that (5.0 eV) determined from the thermal annealing studies for the phase transformation of gamma-to-alpha. Our preliminary study also revealed that the presence of Fe cations in the films inhibits the IBIER process.

Superconductivity and electrical properties in single-crystalline ultrathin Nb films grown by molecular-beam epitaxy.

High-quality superconducting ultrathin Nb films were grown on sapphire single-crystal substrates by molecular-beam epitaxy, of which thicknesses ranged from 1.2 to 9.0 nm. They were found to be single crystalline by reflection high-energy electron diffraction. Size effects appeared remarkably in the normal conduction range. Superconducting transition temperatures (${\mathit{T}}_{\mathit{c}}$'s) of these films were higher than in the past reports for the same thicknesses. The thickness (d) dependence of ${\mathit{T}}_{\mathit{c}}$ was ln[${\mathit{T}}_{\mathit{c}}$(d)]\ensuremath{\propto}-1/d, which could be explained in terms of the proximity effect. A theory of localization effects in two-dimensional superconductors by Maekawa and Fukuyama, on the other hand, could not be applied to the present experiments, since the prepared ultrathin films had better crystallinity than those discussed in the theory.

Formation of iron or chromium doped epitaxial sapphire thin films on sapphire substrates

This work summarizes results of a simple procedure to incorporate dopants into the near surface region of single-crystal sapphire. We demonstrate the formation of iron-doped and chromium-doped sapphire thin films by solid-phase epitaxial growth. Amorphous alumina films of about 200–350 nm thickness were deposited onto single-crystal sapphire substrates. Fe or Cr ions were introduced into the films during deposition. A post-deposition thermal process was performed in oxidizing ambients at 800–1400 °C to induce epitaxial growth and to incorporate dopants. The epitaxial relationship of the grown film with the substrate was confirmed by both ion channeling and cross-sectional transmission electron microscopy. The growth kinetics were determined by time-resolved reflectivity measurements for different dopant concentrations. Ion channeling angular scans revealed that the Fe or Cr ions are incorporated onto octahedral sites (Al3+ sites) in the corundum structure as expected in equilibrium. External optical transmittance measurements exhibited absorption in the near ultraviolet range associated with the Fe3+ state. The substitution of Cr for Al3+ was also confirmed by the observation of R1 and R2 luminescence lines characteristic of ruby. The doping procedure has potential applications in the fabrication of thin film planar optical waveguides and thin film stress sensors.

Microstructure development of sol-gel derived epitaxial LiNbO3 thin films

Film growth and microstructural evolution were investigated for sol-gel derived LiNbO3 thin films deposited on lattice-matched single-crystal substrates. Epitaxial LiNbO3 films of about 100 nm nominal thickness were prepared by spin coating a solution of the lithium niobium ethoxide on sapphire (0001) substrates and annealing at 400 °C or 700 °C in a humidified oxygen atmosphere. These films exhibited an epitaxial relationship with the substrate of the type LiNbO3 (0001) || α-Al2O3 (0001) and LiNbO3 [100] || α-Al2O3 [100] as determined by x-ray pole figure analysis. Transmission electron microscopy indicated the epitaxial films annealed at 400 °C consisted of slightly misoriented ∼5 nm subgrains and of numerous ∼10 nm enclosed pores. The microstructure and orientation development of these films was consistent with a heteroepitaxial nucleation and growth mechanism, in which epitaxial nuclei form at the substrate surface and grow upward into an amorphous and porous intermediate film: Epitaxial films annealed at 700 °C contained larger 150-200 nm subgrains and pinholes. Misorientations between adjacent subgrains appeared to be significantly smaller in films annealed at 700 °C than those in films annealed at 400 °C. Hydrolysis of the alkoxide precursor solution prior to spin coating promoted the development of polycrystalline films on single-crystal sapphire substrates. Infrared spectra and thermal analysis indicated that, independent of the degree of the solution hydrolysis, nucleation of LiNbO3 was immediately preceded by decomposition of an amorphous carbonate intermediate phase.

An X-ray diffraction study of epitaxial lithium tantalate films deposited on (0001) sapphire wafers using r.f. diode sputtering

Epitaxial thin films of lithium tantalate have been r.f. diode sputtered onto (0001) single-crystal sapphire substrates. X-ray diffraction results show that, as the Li/(Li + Ta) ratio decreased below 0.5, the LiTaO 3 (LTO type) c-axis lattice constant increased. An additional lithium tantalate phase is observed to coexist with the LTO-type phase at a Li/(Li + Ta) ratio of around 0.4. When the Li/(Li + Ta) ratio becomes close to 0.25, this additional phase is the only phase observed in the sputter-deposited thin film. Tilt-angle X-ray diffraction results confirmed that this additional phase is ilmenite-type (IL type) Li( 1 − x) TaO 3.

Properties and Fracture Resistance of Thin Polycrystalline Aluminum Films on Sapphire Substrates

AbstractWe employed nanoindentation, continuous microscratch testing, and high resolution TEM to determine the effect of structure on the properties and resistance to fracture of thin polycrystalline aluminum films deposited onto single crystal sapphire substrates at 25°C and 250°C. These films had a nominal thickness of 90 nm and a grain size of 160 nm. The elastic and plastic properties were similar for both films. The elastic moduli superimposed, increasing from bulk aluminum values at the surface to sapphire values at the interface. Hardness values also superimposed, but were constant through the film thickness at a value between aluminum and sapphire. In contrast, susceptibility to fracture varied markedly between the films with the 25°C film exhibiting abrupt failure along the film-substrate interface while the 250'C film gave no indication of fracture in the film, along the interface, or in the substrate under the conditions tested.

Magneto-optical properties of Sr-ferrite films produced by pulsed laser ablation

The effect of substrate temperature and oxygen pressure on the microstructure and magneto-optical (m-o) properties of Sr-ferrite films grown on [001] single crystal sapphire substrates by pulsed laser deposition has been investigated. The results demonstrated that smooth c-axis oriented films, could only be prepared at a narrow operational window of the useful oxygen pressure and at high substrate temperatures. The best films were deposited at a substrate temperature of 840/spl deg/C, and in an oxygen atmosphere of about 0.1 mbar. These were of good optical quality, hard to scratch, with c-axis oriented perpendicular to the film plane and exhibited bulk m-o properties.

Optical Switching of Coherent VO2 Precipitates Embedded in Sapphire

AbstractIn this work, we report the formation of a new type of active or “smart” surface that is produced by ion implantation and thermal processing. By co-implanting vanadium and oxygen into a single-crystal sapphire substrate and annealing the system under appropriate conditions, it was possible to form buried precipitates of vanadium dioxide that were crystallographically oriented with respect to the host AI2O3 lattice. The implanted VO2 precipitate system undergoes a structural phase transition that is accompanied by large variations in the optical transmission which are comparable to those observed for thin films of VO2 deposited on sapphire. Co-implantation with oxygen was found to be necessary to ensure good optical switching behavior.

The Microstructure and Properties of Layered Oxide Thin Films Fabricated by MOCVD

ABSTRACTFor the first time, layered oxide thin films of SrBi2Ta2O9 (SBT) with very good ferroelectric properties have been prepared by direct-liquid-injection MOCVD technique. The SBT films were deposited onto both Pt/Ti/SiO2/Si wafers and single-crystal sapphire substrates to measure their phase formation, microstructure and ferroelectric properties. Crystalline SBT phase had been observed at temperatures as low as 500°C. With increasing deposition temperature above 500°C, the grain size of SBT thin films was increased from 0.01 μ m to 0.2 μm. The films were found to be dense and homogenous. Typically, SBT thin films of 200 nm thick with grain size about 0. 1μm have 2Pr around 10 μC/cm2 at 5V, Ec 55.7 kV/cm, and dielectric constant around 100. The leakage 9 2 currents were as low as 8 × 10−9 A/cm2 at 150 kV/cm. The films also showed fatigue-free characteristics: no fatigue was observed up to 1.4 × 1010 switching cycles. This development of MOCVD technique for good quality SBT films make their integration into high density nonvolatile memories relatively easy.

Structure and properties of sol-gel processed (Pb,La)TiO3 ferroelectric thin films

Abstract The influence of heat treatment and substrate materials on PLT thin films by sol-gel processing has been researched. Epitaxial growth PLT thin films with perovskite-type structure on sapphire, SrTiO3 and MgO single crystal substrates have been prepared. The epitaxial relations are (100)PLT28/(100)SrTiO3, (111)PLT28//(0001)sapphire and (100)PLT14//(100)MgO. The PLT polycrystal thin films with perovskite-type structure on Si single crystal and quartz glass substrates have been prepared. The remanent polarization Pr and the coercive field Ec of PLT15 ceramic thin films are 7.7 μc/cm2 and 34 kv/cm at 4 KHz respectively. The optical transmittance of PLT28 ceramic thin films within the wavelength range of λ = 500–1000 nm is approximately 80%.

Deposition of diamond films on sapphire: studies of interfacial properties and patterning techniques

Polycrystalline diamond films were grown on single crystal sapphire substrates using hot filament chemical vapour deposition (CVD). Problems with poor adhesion, stress and film cracking became severe for deposited areas greater than about (100 μm) 2. Scanning electron microscopy analysis showed the films to be failing both at the interface and in the diamond layer itself. Transmission electron microscopy cross-sections of the interface showed that the interface was clean and free from non-diamond carbon impurities. Spallation problems in the diamond film could be reduced by introducing a barrier layer of epitaxial silicon grown on the sapphire prior to the diamond CVD step. Patterned silicon-on-sapphire wafers were then used as substrates for CVD of diamond in order to define features of linewidth more than 10 μm in the diamond films. Two methods were used: selective nucleation and lift off.

The growth of K-β″-Al 2O 3 single-crystal films on single-crystal α-Al 2O 3 substrates

The overgrowth of K-β″-Al 2O 3 on single-crystal sapphire substrates has been investigated using carbonate, chloride and β″ (β )-Al 2O 3 vapour sources at temperatures between 950°C and 1250°C. Single-crystal film is obtained on the sapphire substrate using a (K, Li)-β″-Al 2O 3 powder vapour source at 1120-1250°C. The mechanism of growth and the chemical and crystallographic factors involved are discussed.

Annealing treatment of pulsed laser ablated Pb-(Mg, Nb)-Ti-O thin films

Abstract Laser ablated thin films of thickness 1 to 2 pm with a nominal composition 0.9 Pb3(MgNb2)O9-0.1 PbTiO3 were prepared on the r-plane of single crystal sapphire substrates, which were not heated during ablation. As-deposited films were amorphous. The perovskite phase started to crystallize at 350 °C so that the films composed of perovskite with a small amount of pyrochlore after 6 hours annealing at 400 °C. The fraction of pyrochlore increased with rising temperature and attained 100% at 900 °C. This effect was favoured by the evaporation of lead on surface.

Effects of dopants in PZT films

Undoped and lanthanide (La and Nd) doped Pb(ZrxTi1-x)03, i.e., PZT, ferroelectric thin films were prepared by metallorganic decomposition (MOD) and spin-coating. The precursors for making the undoped PZT films were derived from lead acetate, zirconium n-propoxide, and titanium isopropoxide. In addition, lanthanum acetylacetonate and neodymium acetate were introduced into the precursor solution to accomplish the doping of the corresponding elements. Both undoped and doped PZT films were coated onto Pt/Ti/Si02/Si and single-crystal sapphire substrates to various thicknesses and annealed at a range of temperatures and times. The effects of lanthanide dopants in PZT films were studied with regard to microstructure, Curie temperatures, crystal distortion, optical properties, and electrical properties. The results indicate that the addition of La and Nd dopants tends to enhance perovskite phase formation and improve the packing densities and electrical properties of PZT films.

Incorporation of Iron Cations Into Epitaxial Sapphire Thin Films by CO-Evaporation and Subsequent Thermal Annealing

AbstractIron-doped sapphire thin films have been successfully epitaxially grown onto sapphire single crystal substrates by electron beam deposition and subsequent thermal annealing. Amorphous A12O3 thin films, about 280–390 nm thick, cation doped with iron have been deposited on [0001] oriented sapphire substrates. Iron doping with cation concentrations (a ratio of Fe content to total cation content) up to 5 at.% can be incorporated into the octahedral sites of Al-cation sublattice during the epitaxial regrowth process at 1000–1400 C, as determined by Rutherford Backscattering Spectrometry and ion channeling measurements. Cross-sectional Transmission Electron Microscopy shows the presence of two distinct regions in the annealed films. One exhibits the epitaxial relationship with the sapphire substrate and the second region has amorphous type of contrast. External optical transmittance measurements in the ultra violet and visible light range have exhibited the absorption associated with Fe3+. This study has demonstrated a simple method of incorporating dopants into single crystal sapphire, which has potential in the fabrications of thin film planar optical waveguiles.

Magneto-optical properties of CdMnTe films on quartz glass and sapphire substrates in the transparent, visible region

Magneto-optical properties in the visible wavelength region are studied on CdMnTe films grown on quartz glass and sapphire (0001) single crystal substrates by MBE. Good quality films with sharp spectra of X-ray diffraction are obtained on the sapphire substrates polished carefully and pre-heated up to 950/spl deg/C. Weak (111) film-growth is observed for films deposited on the quartz glass substrates. A value of Faraday rotation angle of -0.31 deg/cmG at 700 nm is measured in the Cd/sub 0.59/Mn/sub 0.41/Te film grown on the sapphire substrate. The values for the films on the quartz glass substrates are 1/3 of those on sapphire. It is shown that the specific rotation in CdMnTe depends strongly upon the crystalline quality of films deposited on the substrate. >

Interface Roughness Effects on Adhesion of Ta2N Films.

AbstractIn this study, we employed continuous microscratch testing to determine the effects of interface roughness on adhesion and fracture toughness of thin Ta2N films. These films were sputterdeposited on single crystal sapphire and polycrystal alumina substrates to a thickness of 0.5 μm. Comparison of the results showed that the interfacial fracture energy increased from 0.4 J/m2 for films on the single crystals to 1.5 J/m2 for films on the polycrystals with a corresponding increase in fracture toughness values. These results are consistent with crack deflection and interface roughness models and are useful for understanding adhesion and toughness on the submicron scale. The results also show that the continuous microscratch technique is a viable approach to determining adhesion and toughness of bi-material systems.

Effect of Heat Treatment on Adhesion in the Cr/Al2O3 System

ABSTRACTContinuous microscratch experiments have been carried out to investigate the effect of various heat treatments on adhesion in the Cr/Al2O3 system. After samples were prepared by vapor depositing 0.32 µm thick Cr films on (1102) surfaces of single crystal sapphire substrates, they were annealed at different temperatures at controlled oxygen partial pressures. In the subsequent continuous microscratch tests, a conical diamond indenter with a 1 µm tip radius was driven simultaneously into and across the sample until a load drop occurred. Optical and scanning electron microscopy observations demonstrate that a portion of the Cr film delaminated from the Al2O3 substrate at that point. The interfacial strain energy release rate was calculated from the load at failure combined with the size and geometry of the delaminated region. The work of adhesion was found to decrease dramatically on annealing in an H2-Ar mixture at 250°C.