Growth of highly oriented LiTaO3 thin film on Si with amorphous SiO2 buffer layer by pulsed laser deposition

High-quality single-phase, c-axis textured LiTaO₃ thin films have been deposited on Si(100) substrate with amorphous SiO₂ buffer layer for optic waveguide application by pulsed laser deposition under optimized conditions of 30 Pa oxygen pressure and 650 °C. The amorphous SiO₂ buffer layer with a thickness of 100 nm was coated on the Si(100) by thermal oxidation at 1000 °C. Li-enriched LiTaO₃ ceramic target was used during the deposition. In order to study the influence of oxygen pressure on the orientation, crystallinity and morphology, different oxygen pressures (10 Pa, 20 Pa, 30 Pa and 40 Pa) were used. X-ray diffraction (XRD) results showed that LiTaO₃ thin films exhibited highly c-axis orientation under 30 Pa. It was observed by scanning electron microscopy (SEM) that the as-grown film in the optimal conditions was characterized by a dense and homogeneous surface without cracks, and the average grain size was in the order of 25 nm.     

Control of crystallographic orientation of LiNbO3 films grown by electron-cyclotron resonance plasma sputtering on TiN films

We investigated the orientation of domains in LiNbO₃ (LN) thin films grown by electron-cyclotron resonance plasma sputtering on TiN films with various crystalline states. Deposition at 400 °C on an amorphous TiN produced partially crystallized and apparently c-axis-oriented LN. When TiN crystallized at 460 °C to become polycrystalline grains, the roughened surface randomized the orientation of LN. At 600 °C, the reaction of TiN with oxygen atoms supplied from the plasma created a TiO_x layer. Rapid thermal annealing of amorphous LN films at 460 °C was the best solution for removing these disorientation factors, but annealing of amorphous LN on poly-crystalline TiN yielded no c-axis-oriented domains.     

X-ray reflectivity study of r.f.-sputtered thin SiO2 films

X-ray reflectivity investigations have been performed on thin amorphous SiO₂ films obtained by r.f.-sputtering of fused quartz onto Si single crystals in an argon atmosphere.This is a non-destructive technique to determine thickness and density as well as the interface and surface roughness. For the SiO₂/Si system (small differences in density) the X-ray reflectivity is sufficiently sensitive to provide precise information on the occurence of an intermediate layer we obtained only on well-polished silicon wafers.The thickness of the films is compared with that obtained from ellipsometry data and mechanical scanning measurements (stylus method).     

Enhanced photoluminescence of CaTiO3:Pr phosphor films deposited on SiO2 buffered Si substrates

CaTiO₃:Pr³⁺ films have been prepared by pulsed-laser deposition method on SiO₂-buffered Si substrates, and their microstructure and photoluminescence properties have been compared with those of the films deposited directly on bare Si substrates. The SiO₂ buffer layers were prepared using thermal oxidization and HF-etching. Photoluminescence intensities of CaTiO₃:Pr³⁺ films on the SiO₂-buffered Si substrates are significantly higher (up to 800%) than those of the films on bare Si substrates, which is attributed to the low refractive index and low light absorption of the SiO₂ buffer layer. This study reveals that the presence of the buffer layer is effective in improving the red emission brightness of CaTiO₃:Pr³⁺ films without sacrificing the surface roughness.     

The Influence of the Microstructure and Morphology of CeO 2 Buffer Layer on the Properties of YBCO Films PLD Grown on Ni Tape

YBa₂Cu₃O_7?δ films were deposited on CeO₂-buffered nickel substrates, with different buffer thickness. Full width at half maximum of rocking curve, Δω, of CeO₂ and yttrium barium copper oxide (YBCO), as well as the critical temperature, T _c, of YBCO were shown to be strongly dependent on buffer thickness. They behave similarly but not proportional to the buffer thickness increase. This and the fact that Δω vs. buffer thickness and T _c vs. buffer thickness for YBCO behave similar with RMS roughness vs. thickness of CeO₂ indicates that the surface peculiarity of buffers is responsible for YBCO properties. More precisely, the surface of CeO₂ films prepared by the chemical solution route based on propionic acid is prone to agglomerate (de-wet) and the degree of agglomeration depends in an intricate way on buffer thickness. We showed that nor RMS roughness neither (00 l) texture degree can define alone the surface suitable for c-axis YBCO nucleation. The {111} faceted grains (even in the case of high (00 l) texture) and other defects generated by agglomeration supply a low fraction of (00 l) flat terminations of buffer that affect the nucleation of c-axis-oriented YBCO phase. Moreover, the thermal instability of the surface morphology of CeO₂ buffers (further development of de-wetting process, {111} faceted grains, etc. during superconducting layer deposition) influence the quality of YBCO films.     

The problems of native SiO2 layer removing for epitaxial growth of YSZ film on Si

The preparation processes of epitaxially grown YSZ (Yttrium stabilized ZrO₂) buffer layers on silicon (100) wafers were investigated. The “etching” procedure, at which the thin (∼5 nm) SiO₂ native amorphous layer from the Si surface was reduced to volatile SiO by deposition of a few nm thick Zr layer and subsequent annealing at low pressure, was monitored by mass spectrometer. The subsequent YSZ layer was deposited by evaporation or RF sputtering technique and examined by XRD and TEM observations. The results show that the epitaxy of YSZ layer is strongly influenced by efficiency of amorphous SiO₂ reduction at Si surface.     

Fabrication of free standing LiNbO3 single crystal micro-platelets and their integration to Si-on-insulator platforms

Free standing single crystal LiNbO₃ micro-platelets (mm long and 1 μm thick) have been obtained from a z-cut LiNbO₃ wafer by ion implantation and thermal treatment. X-ray diffraction measurement has proved that they have a single crystal structure. Their surface roughness and film quality have been investigated by atomic force microscope and field emission scanning electron microscope. These micro-platelets are transferable and bondable to other materials which require material properties of LiNbO₃. They have been transferred, positioned and bonded to SiO₂/LiNbO₃, SiO₂/Si, and Si-on-insulator (SOI: Si/SiO₂/Si) by direct bonding method with optimum annealing conditions. For SiO₂/Si and SOI substrates, there were large thermal mismatch between LiNbO₃ and Si. They were, however, bonded at high temperature since these ultra thin micro-platelets were flexible and stretchable. Finally, to realize multifunctional SOI applications, a hybrid structure of LiNbO₃ film and Si waveguide has been demonstrated.     

Effect of SiO2 buffer layer thickness on the properties of ITO/Cu/ITO multilayer films deposited on polyethylene terephthalate substrates

Transparent conductive ITO/Cu/ITO films were deposited on polyethylene terephthalate (PET) substrates with a SiO₂ buffer layer by magnetron sputtering using three cathodes at room temperature. The effect of the SiO₂ buffer layer thickness on the electrical and optical properties of ITO/Cu/ITO films was investigated. The ITO/Cu/ITO film deposited on the 40 nm thick SiO₂ buffer layer exhibits a sheet resistance of 143Ω/sq and transmittance of 65% at 550 nm wavelength. Highly transparent ITO/Cu/ITO films with a transmittance of 80% and a sheet resistance of 98.7Ω/sq have been obtained by applying −60 V substrate bias.     

Nanostructural characterization of YBCO films on metal tape with textured buffer layer fabricated by pulsed-laser deposition

Thick YBa₂Cu₃O_7-x (YBCO) films with high critical current density (J_c) values were deposited by pulsed-laser deposition (PLD) on Hastelloy with a textured CeO₂/Gd₂Zr₂O₇ buffer layer. Both cross-sectional and plan-view TEM specimens of the YBCO films were prepared, and then the nanostructural characterization of the films was performed by transmission electron microscopy (TEM). The YBCO films less than 1 μm thick were predominantly composed of c-axis-oriented grains, however, many a-axis-oriented grains, which grew larger with the increase of the thickness of the YBCO film, were formed beyond about 1 μm from the CeO₂ interface. We found Y₂O₃ and copper oxides between a- and c-axes-oriented grains. In particular, Y₂O₃ grains were formed between the {001} plane of an a-axis-oriented grain and the {100} or {010} plane of a c-axis-oriented grain. The orientation relationships between Y₂O₃ and YBCO are found to be; (001)YBCO//(001)Y₂O₃ and (100)YBCO//(110)Y₂O₃. In addition, we also found gaps between YBCO grains. Since a-axis-oriented grain growth and the formation of Y₂O₃, copper oxides and the gaps are considered to reduce the J_c values of the YBCO film, it is important to determine the optimum process conditions to suppress the nucleation of a-axis-oriented grains, impurity oxides and gaps.     

Enhanced ferroelectric properties of Pb(Zr0.80Ti0.20)O3/PbO thin films prepared by radio frequency magnetron sputtering

The Pb(Zr_0.80Ti_0.20)O₃ (PZT) thin films with and without a PbO buffer layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by radio frequency (rf) magnetron sputtering method. The PbO buffer layer improves the microstructure and electrical properties of the PZT thin films. High phase purity and good microstructure of the PZT thin films with a PbO buffer layer were obtained. The effect of the PbO buffer layer on the ferroelectric properties of the PZT thin films was also investigated. The PZT thin films with a PbO buffer layer possess better ferroelectric properties with higher remnant polarization (P_r = 25.6 μC/cm²), and lower coercive field (E_c = 60.5 kV/cm) than that of the films without a PbO buffer layer (P_r = 9.4 μC/cm², E_c = 101.3 kV/cm). Enhanced ferroelectric properties of the PZT thin films with a PbO buffer layer is attributed to high phase purity and good microstructure.     

Structural and dielectric properties of Ba0.7Sr0.3TiO3 thin films grown on thin Bi layer-coated Pt(111)/Ti/SiO2/Si substrates

In this work, the growth and study of dielectric properties of Ba_0.7Sr_0.3TiO₃ (BST) thin films grown on thin Bi layer coated Pt(111)/Ti/SiO₂/Si substrates, depending on thin Bi layer thickness is reported. The BST thin film (thickness 180 nm) grown on 10-nm-thick Bi layer exhibited more improved structural and dielectric properties than that grown on bare Pt(111)/Ti/SiO₂/Si substrate. The 10-nm-thick Bi layer in optimum configuration was effective for the grain growth of BST phase and suppressed the formation of the oxygen-deficient layer at the interface between the BST thin film and bottom electrode, which resulted in an increase in dielectric constant and a decrease in leakage current density of the Pt/BST thin film/Pt capacitor.     

GISAXS study of Si nanocrystals formation in SiO2 thin films

We present a study on amorphous SiO/SiO₂ superlattice using grazing incidence small-angle X-ray scattering (GISAXS). Amorphous SiO/SiO₂ superlattices were prepared by high vacuum evaporation of 3 nm thin films of SiO and SiO₂ (10 layers each) on Si(100) substrate. After the evaporation, samples were annealed at 1100 °C for 1 h in vacuum, yielding Si nanocrystals formation. Using a Guinier approximation, the shape and the size of the crystals were obtained.     

Electrical and structural properties of LiNbO<Subscript>3</Subscript> films,grown by RF magnetron sputtering

Nanocrystalline films of LiNbO₃ on substrates (001)Si and (001)Si–SiO₂ were synthesized by the method of RF magnetron sputtering. The elemental composition, structure of the LiNbO₃ films, and also—electrical properties of heterostructures (001)Si–LiNbO₃ and (001)Si–SiO₂–LiNbO₃ were studied. The dielectric constant of the LiNbO₃ films calculated from the capacitance at the accumulation region was about 28. The resistivity was 1·10⁹ ohm cm for films on (100)Si and 1.6·10¹¹ ohm cm for films on (001)Si–SiO₂. It has been determined that transmission of the current in the studied structures during direct biases is defined by hopping conduction, and, during reverse biases—by the Poole–Frenkel effect.     

Selective growth of ZnO nanorods on SiO2/Si substrates using a graphene buffer layer

A promising strategy for the selective growth of ZnO nanorods on SiO₂/Si substrates using a graphene buffer layer in a low temperature solution process is described. High densities of ZnO nanorods were grown over a large area and most ZnO nanorods were vertically well-aligned on graphene. Furthermore, selective growth of ZnO nanorods on graphene was realized by applying a simple mechanical treatment, since ZnO nanorods formed on graphene are mechanically stable on an atomic level. These results were confirmed by first principles calculations which showed that the ZnO-graphene binding has a low destabilization energy. In addition, it was found that ZnO nanorods grown on SiO₂/Si with a graphene buffer layer have better optical properties than ZnO nanorods grown on bare SiO₂/Si. The nanostructured ZnO-graphene materials have promising applications in future flexible electronic and optical devices.     

Thin magnetite films on an oxidized silicon surface: Raman spectroscopy study

Polycrystalline films of magnetite (Fe₃O₄) formed by the reactive sputtering of iron in oxygen on Si(001) substrates covered by thin (1.4 nm) or thick (1200 nm) SiO₂ layers have been studied by Raman spectroscopy. It is established that (i) the α-Fe₂O₃ phase is formed due to the laser-induced heating in magnetite films synthesized on thick SiO₂ layers and (ii) the formation of α-Fe₂O₃ phase depends on the thickness of the buffer SiO₂ layer.     

Synthesis of mono and bi-layer of Si nanocrystals embedded in a dielectric matrix by e-beam evaporation of SiO/SiO2 thin films

Si nanocrystals (ncs) were synthesized by e-beam evaporation of SiO/SiO₂ multilayer structures and subsequent annealing at high temperatures. We focused our attention on the possibility to manipulate the ncs dimension by changing the SiO layer thickness. Time of flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy analyses of the annealed samples indicated that the thermal treatment induces a phase separation in the SiO layer. The formation of Si nanocrystals was confirmed by high-resolution transmission electron microscopy, although in the thinnest layers (below 2 nm) the Si clusters are probably amorphous. Preliminary results indicate that a similar approach is suitable also for synthesizing Ge nanocrystals.     

GISAXS study of Si nanoclusters in SiO/SiO2 layers

We present a study on amorphous SiO/SiO₂ superlattice using grazing-incidence small-angle X-ray scattering (GISAXS). Such nanostructured material might be of great interest for photovoltaic conversion and optoelectronics. Amorphous SiO/SiO₂ superlattices were prepared by magnetron sputtering of thin SiO and SiO₂ films (20 layers each) on Si (1 0 0) substrate. Rotation of the Si substrate during evaporation ensures homogeneity of the films over the whole substrate. After the evaporation, the samples were annealed at 1100 °C for 1 h in vacuum. The analysis of the 2-D GISAXS pattern has shown that Si nanocrystals are formed in the remaining SiO₂ films in the annealed samples. From the 2-D GISAXS pattern, their shape, size and inter-particle distance are determined.     

Deposition of ZnO thin films on SiO2/Si substrate with Al2O3 buffer layer by radio frequency magnetron sputtering for high frequency surface acoustic wave devices

ZnO films with c-axis (0002) orientation have been grown on SiO₂/Si substrates with an Al₂O₃ buffer layer by radio frequency magnetron sputtering. Crystalline structures of the films were investigated by X-ray diffraction, atomic force microscopy and scanning electron microscopy. The center frequency of the surface acoustic wave (SAW) device with a 4.8 μm thick Al₂O₃ buffer layer was measured to be about 408 MHz, which was much higher than that (265 MHz) of ZnO/SiO₂/Si structure and approaches that (435 MHz) of ZnO/sapphire. It is a possible way as an alternative for the sapphire substrate for the high frequency SAW device applications, and is also useful to integrate the semiconductor and high frequency SAW devices on the same Si substrate.     

The effect of synthesis temperature on structural and magnetic properties of Fe3O4 films grown on the SiO2/Si(001) surface

Magnetite (Fe₃O₄) films have been deposited on a SiO₂/Si(001) surface at various temperatures by reactive sputtering of iron in oxygen. The influence of the synthesis temperature on the structure and magnetic properties of Fe₃O₄/SiO₂/Si(001) samples have been studied by the methods of reflection high-energy electron diffraction, atomic force microscopy, Raman spectroscopy, and vibrating sample magnetometry. An optimum substrate temperature for the formation of polycrystalline films with the best magnetic characteristics has been determined.     

Effects of annealing in O2 and N2 on the microstructure of metal organic chemical vapor deposition Ta2O5 film and the interfacial SiO2 layer

Ta₂O₅ films were made by metal organic chemical vapor deposition (MOCVD) and annealed at various temperatures under N₂ and O₂ ambients in a conventional furnace and a rapid thermal reactor (RTR). The microstructure and composition of the Ta₂O₅ film and the interfacial SiO₂ layer before and after various annealing treatments were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Auger electron spectroscopy (AES). The as-deposited Ta₂O₅ film has an amorphous structure. The surface topology of the as-deposited Ta₂O₅ film is smooth without any apparent features. Annealing of the as-deposited film results in crystallization to an orthorhombic structure with (1 0 0) preferred orientation, and an increase in surface roughness, with the appearance of grain boundaries under AFM. The crystallization temperature varies in the various annealing treatments. An interfacial SiO₂ layer was found between the as-deposited/annealed Ta₂O₅ films and silicon substrate. The annealing treatments result in an increase in thickness of the SiO₂ layer and roughness changes of the Ta₂O₅/SiO₂/Si interfaces, which are discussed in terms of element diffusion and thermodynamic stability.