Structural investigation and electronic band transitions of nanostructured TiO2 thin films

Titanium dioxide (TiO₂) thin film was deposited on n‐Si (100) substrate by reactive DC magnetron sputtering system at 250 °C temperature. The deposited film was thermally treated for 3 h in the range of 400‐1000 °C by conventional thermal annealing (CTA) in air atmosphere. The effects of the annealing temperature on the structural and morphological properties of the films were investigated by X‐ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD measurements show that the rutile phase is the dominant crystalline phase for the film annealed at 800 °C. According to AFM results, the increased grain sizes indicate that the annealing improves the crystalline quality of the TiO₂ film. In addition, the formation of the interfacial SiO₂ layer between TiO₂ film and Si substrate was evaluated by the transmittance spectra obtained with FTIR spectrometer. The electronic band transitions of as‐deposited and annealed films were also studied by using photoluminescence (PL) spectroscopy at room temperature. The results show that the dislocation density and microstrain in the film were decreased by increasing annealing temperature for both anatase and rutile phases. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pressure Dependence of C–V Curve for Ferroelectric PbTiO3 Thin Films Prepared by RF Multi-target Magnetron Sputtering

Lead titanate thin films were prepared in-situ by RF magnetron sputtering method in the substrate temperature region of 550 °C ˜ 650 °C using Ti and Pb metal targets and oxygen gas. The films deposited at 650 °C for 60 min – when the applied power to the Ti and Pb target and deposition pressure were 200 W, 40 W, and 1.8 × 10⁻² Torr, respectively — had stoichiometric composition ratio and shown X-ray diffraction patterns of tetragonal structure like that of the powder. For the application to the piezoelectric field effect transistor (PI-FET), the substrate of SiO₂ (50 nm)/p-Si(100) was used. Leakage current density of the thin film was 7 × 10⁻⁸ A/cm² at 100 kV/cm, dielectric breakdown voltage was 1.8 MV/cm, and so it showed high insulator characteristics. Capacitance-voltage curve was measured as a function of pressure. The variation width of the capacitance was 6.5 pF at bias voltage of –4 volts, and the value increased linearly according to the pressure up to about 6 kgf/cm², and was saturated at the pressure of 8 kgf/cm².     

Thermally oxidized zirconium nanostructured films grown on Si substrates

Zirconium thin films grown on Si substrates by a planar magnetron sputtering system were thermally oxidized at oxygen ambient within 523‐823 K resulting in zirconium oxide films with various stoichiometries. XRD analysis of the ex situ oxidized films revealed the phases at different oxidation temperatures. To achieve a reasonable fit between the experimental and SIMNRA simulated RBS spectra of the prepared samples; it was required to introduce a SiO₂buffer layer in the simulated target between Si substrate and ZrO₂ film. The presence of this intermediate SiO₂ layer was confirmed by observation of SiO₂ phase in the XRD patterns of all the thermally oxidized samples. Using RBS analysis data, the effect of oxidation temperature on the stoichiometry of zirconium oxide films and thickness of ZrO_xand SiO₂ films were investigated. XRD patterns of thermally oxidized Zr films also revealed that crystallization of zirconium oxide films was initiated at about 673 K and was almost completed at 823 K. Diffusion of oxygen atoms through surface layer was investigated and the effective activation energy for oxygen mass transport was estimated to be 1.75 eV using RBS data and Arrhenius relation. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reactions between sol–gel coatings and some technical glass substrates during consolidation

SiO₂ and Na₂O–SiO₂ coatings have been applied on float glass and other technical glass substrates by a sol–gel dip-coating process. After drying and baking these films at temperatures up to 500 °C and for times up to 1020 min, the in-depth profiles of the different constituents were measured by secondary neutral mass spectrometry (SNMS). Sn, Al, and Si turned out to be immobile, whereas a diffusion coefficient of ≈10⁻¹⁷ cm²/s could be evaluated for Mg at 500 °C for the transport from float glass into the films. Ca diffused a little faster, however, especially for the Na₂O–SiO₂ films a saddle point and finally a peak occurred in the interface region. This interface peak was even stronger for Na, showing quite anomalous profiles. The mechanism of this peak formation is explained mainly as an up-hill diffusion process. According to this model at the interface non-bridging –O⁻ ions are formed, whose electroneutrality has to be maintained by mobile cations like Na⁺ and Ca²⁺, even diffusing against their own concentration gradient. The other glass substrates, two borofloat glasses and an alkali-poor display glass showed similar but less pronounced effects.     

Temperature dependence of growth orientation and surface morphology of Li‐doped ZnO thin films on SiO2/Si substrates

ZnO thin films doped with Li (ZnO:Li) were deposited onto SiO₂/Si (100) substrates by direct‐current sputtering technique in the temperature range from room temperature to 500 °C. The crystalline structure, surface morphology and composition, and optical reflectivity of the deposited films were studied by X‐ray diffraction (XRD), Scanning Electron Microscopy (SEM), X‐ray Photoelectron Spectroscopy (XPS) and optical reflection measurements. Rough surface p‐type ZnO thin film deposition was confirmed. The results indicated that the ZnO:Li films growed at low temperatures show c‐axis orientation, while a‐axis growth direction is preferable at high temperatures. Moreover, the optical reflectivity from the surface of the films matched very well with the obtained results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of zirconium oxide thin films on silicon substrate

The growth and characterization of zirconium oxide (ZrO₂) thin films prepared by thermal oxidation of a deposited Zr metal layer on SiO₂/Si were investigated. Uniform ZrO₂ thin film with smooth surface morphology was obtained. The thermal ZrO₂ films showed a polycrystalline structure. The dielectric constant of the ZrO₂ film has been shown to be 23, and the equivalent oxide thickness (EOT) of the ZrO₂ stacked oxide is in the range of 3.38–5.43 nm. MOS capacitors with ZrO₂ dielectric stack show extremely low leakage current density, less than 10^?6 A/cm² at ?4 V. Consequently, using this method, high-quality ZrO₂ films could be fabricated at oxidation temperature as low as 600 °C.     

Composition of Ge+ and Si+ implanted SiO2/Si layers: Role of oxides in nanocluster formation

X-ray photoelectron spectroscopy (XPS) has been used to examine the atomic content of implanted SiO₂/Si layers. In particular, an XPS analysis permits to identify elemental Ge and Si, as well as GeO₂ precipitations in SiO₂ matrices. The XPS results reveal valuable information not only about the formation mechanism of Ge and Si nanoclusters but also on the annealing kinetics of SiO₂ whose properties are known to be significantly altered during the process of ion implantation and subsequent annealing. The composition of ion beam-modified SiO₂ layers strongly depends on the annealing temperature. With respect to germanium implanted samples a possibility of Ge nanocrystals formation appears at high (above 1000 °C) annealing temperatures. It has been shown that an intermediate step in the Ge oxide formation is necessary for the creation of Ge nanoclusters. Additionally, the presence of a subsurface zone GeO_x (about 100 nm thick) predicted in kinetic three-dimensional lattice simulations has been confirmed. In the case of Si⁺ implanted samples substoichiometric silicon oxide lines in the XPS spectra of a SiO₂ layer for all samples have been observed. No evidence of a line connected to the Si–Si bonding has been observed even at the highest annealing temperatures, at which only stoichiometric SiO₂ has been detected.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Glass formation through hydrolysis of Si(OC2H5)4 with NH4OH and HCl solution

Monolithic silica glass was obtained by heating the gels prepared by hydrolyzing Si(OC₂H₅)₄ with NH₄OH and HCl solution. The effect of the condition of hydrolysis of Si(OC₂H₅)₄ on glass formation was examined by measuring the bulk density, the infrared spectra and the thermal shrinkage of the gel on heating. The gel prepared by hydrolysis with NH₄OH solution consisted of numerous spherical particles, the bulk density being about 0.8. This gel abruptly shrank at about 1050°C, being converted to the pore free material similar to fused silica. The conversion of the gel to glass followed the sintering model in which the viscous flow controlled the sintering process. The viscosity and the activation energy for viscous flow were calculated on the basis of the Frenkel equation. On the other hand, the spherical particles were not observed in the gel prepared by hydrolysis with HCl solution. The bulk density of the gel was about 1.8. This gel was converted to glass at about 700°C, which was lower than the temperature of glass formation for the gel obtained by hydrolysis with NH₄OH solution.     

Infrared spectroscopy of Li-diborate glassy thin films

This work presents a structural investigation of Li-borate thin film electrolytes prepared by rf-sputtering from targets having the nominal Li₂O–2B₂O₃ composition. Thin films of ca. 1 μm were deposited on Si wafers and gold-covered Si substrates under argon, and their infrared spectra were measured in the 30–5000 cm⁻¹ range. The measured spectra of thin films were compared with those calculated on the basis of the infrared properties of the bulk Li-diborate glass and by considering all optical effects in the film/substrate system. The results showed that the thin films have the key structural features of the bulk glass, but exhibit also differences in the short-range order structure and in the Li ion-site interactions. These findings were discussed in terms of cooling rate differences between melt-quenching for bulk glass and sputtering for thin films.     

Periodic Multilayers MoSi2/Si with a Large Number of Periods

Different approaches to decreasing mechanical stresses developed in MoSi₂/Si multilayers with increase of number of periods due to structural reconstruction in layers of amorphous silicon and nanocrystalline MoSi₂ were studied by scattering CuKα X-ray radiation at small and large angles, cross-sectional electron microscopy and micrometry of substrate sag. It was shown that effective relaxation of mechanical stresses in MoSi₂/Si multilayers is achieved by annealing them at ∼320 °C during 1 hour, or by deposition of layers at substrate temperature ∼320 °C, or by increasing sputtering gas pressure up to 7 × 10^—3 Torr in case of argon. Optimal conditions for deposition of MoSi₂/Si multilayers with periods N > 10³ and high reflectivity of X-rays with wavelengths 12.4—20.0 nm are: substrate temperature T_s = 220 °C, argon pressure P_Ar = 3 × 10^—3 Torr, layer deposition rate 1 nm/s.     

High resolution 29Si NMR study of the structure and devitrification of lead-silicate glasses

High speed magic-angle rotation of glass samples in the strong polarizing field of a superconducting magnet yields high-resolution ²⁹Si NMR spectra. Using this technique glasses of various composition, PbO·SiO₂, (PbO)₂·SiO₂, and (PbO)₄·SiO₂ were studied and the influence of thermal treatment followed. Crystallization of PbO·SiO₂ glass has been found to be a complex process leading to a structure identical with that of the mineral alamosite. The ²⁹Si NMR spectrum of crystalline alamosite consists of three lines in agreement with the structure determined by X-ray diffraction.     

Growth and characterization of highly c‐axis textured SrTiO3 thin films directly grown on Si(001) substrates by ion beam sputter deposition

Highly c‐axis textured SrTiO₃ (STO) thin films have been directly grown on Si(001) substrates using ion beam sputter deposition technique without any buffer layer. The substrate temperature was varied, while other parameters were fixed in order to study effect of substrate temperature on morphology and texture evolution of STO films. X‐ray diffraction, pole figure analysis, atomic force microscope, and high‐resolution electron microscopy were used to characterize and confirm quality and texture of the STO films. The experimental results show that optimum substrate temperature to achieve highly c‐axis textured films is at 700 °C. The full width at half maximum (FWHM) of 002_STO was found to be 2° and fraction of (011) orientation was as low as 1%. The surface morphology was Volmer‐Weber growth mode with a small roughness ∼1 nm. The lowest leakage current density (5.8 μA/cm² at 2 V) and the highest dielectric constant (ε_STO ∼ 98) were found for highly c‐axis textured films grown at 700 °C. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Development of novel surfactant-templated nanostructured thin SiO2, and SiO2–TiO2 films

Nanostructured oxidic thin films are of interest due to their applicability in many different fields like filtration technology, catalysis, biomaterials and self purification. In this paper, we present sol–gel derived SiO₂ and SiO₂–TiO₂ thin films with a new fingerprint-like nanostructure. The films were produced by dip-coating glass slides into the sol and a temperature treatment at 500 and 600 °C. The characterization methods were FEGSEM and AFM. The dimensions of the nanostructured surface pattern were calculated from the AFM data with the power spectral density method.     

Effect of laser fluence on c‐axis orientation of LiNbO3 piezoelectric films on nanocrystalline diamond by pulsed laser deposition

Completely c‐axis oriented LiNbO₃ piezoelectric films have been deposited on nanocrystalline diamond (NCD)/Si substrates with SiO₂ buffer layer by pulsed laser deposition. The amorphous SiO₂ buffer layer was formed on NCD/Si substrates by sol‐gel method. The c‐axis orientation and crystallinity of LiNbO₃ films are strongly dependent on the laser fluence, and the laser fluence 3.6 J/cm² is found to be the optimal value for the growth of oriented LiNbO₃ film, which has a smooth surface with composed of a large mount of uniform grains. The average surface roughness of LiNbO₃ films is about 6.7 nm.     

Surface Chemical Bonding States and Ferroelectricity of Pb(Zr0.52Ti0.48)O3 Thin Films

The surface chemical bonding states and the ferroelectric properties of sol‐gel deposited lead zirconate titanate [Pb(Zr_0.52Ti_0.48)O₃, PZT] thin films coated on (111)Pt/Ti/SiO₂/Si substrates were investigated. X‐ray photoelectron spectroscopy (XPS) was used to determine the oxidation state of the surface and the chemical composition as a function of depth in ferroelectric PZT thin layers. Values for the dielectric constant and dissipation factor at 1 kHz for the 300 nm‐thick film were 1214 and 0.014 for the film annealed at 520 °C, and 881 and 0.015 for a film annealed at 670 °C. Measured values for the remanent polarization (P_r) and coercive field (E_c), from polarization‐electric field (P‐E) hysteresis loops biased at 10 V at a frequency of 100 Hz, were 16.7, 14.4 μC/cm² and 60, 41.7 kV/cm for 520 °C and 670 °C. The leakage current density (J) was 72 and 96 nA/cm² at an applied field of 100 kV/cm. It was found that the bonding states of lead and oxygen in the surface regions could be correlated with the ferroelectric properties of the integrated thin layers.     

Vibrational spectra of vapor-deposited binary phosphosilicate glasses

Room temperature infrared transmission spectra in the range 4000-250 cm^?1 of binary phosphosilicate glass (PSG) films deposited by reacting argon- or nitrogen-diluted PH₃SiH₄O₂ mixtures on heated silicon substrates at 300–400° C have been obtained across the whole composition range. In all the as-deposited binary films, an absorption at ≈1300 cm^?1 characteristics of the P=O vibration was found to persist, together with a couple of broad absorptions in the regions 1200-900 cm^?1 and 500 cm^?1. Using a differential infrared technique the broad feature in the higher frequency region has been resolved into two well-defined bands at ≈1100 and 970 cm^?1. A detailed analysis shows that the intensity variation of the differential band at ≈1100 cm^?1 conforms well, at least to 50 mol% P₂O₅, to a simple structural model that yields an analytic distribution of POSi linkages as a function of composition by assuming chemical mixing in the vapor-deposited P₂O₅SiO₂ system. Furthermore, the system may be written as (P=O)₂ O₃SiO₂ in order to emphasize the similarity of its coordination scheme with that of the B₂O₃SiO₂ system studied earlier. The nature of these CVD films has also been elucidated by thermal and water treatments.     

Structure and photoelectrical properties of SiO2/Si/SiO2 single quantum wells prepared under ultrahigh vacuum conditions

SiO₂/Si/SiO₂ single quantum wells (QWs) were prepared under ultrahigh vacuum conditions in order to study their structural, chemical and photoelectrical properties with respect to a possible application in photovoltaic devices. Amorphous silicon (a-Si) layers (thickness <10 nm) were deposited onto quartz glass (SiO₂) substrates and subsequently oxidized with neutral atomic oxygen at moderate temperatures of 600 °C. Under these conditions, the formation of suboxides is mostly suppressed and abrupt Si/SiO₂ interfaces are obtained. Crystallization of a-Si QWs requires temperatures as high as 1000 °C resulting in a nanocrystalline structure with a small amorphous fraction. The spectral dependence of the internal quantum efficiency of photoconductivity correlates well with the nanocrystalline structure and yields mobility lifetime products of <10^?7 cm² V^?1. This rather low value points towards a strong influence of Si/SiO₂ interface states on the carrier mobility and the carrier lifetime in Si QWs. Electronic passivation of interface states by subsequent hydrogen treatment in forming gas enhances the internal quantum efficiency by nearly one order of magnitude.     

Crystallization of bismuth titanate and bismuth silicate grown as thin films by atomic layer deposition

Bismuth silicate and bismuth titanate thin films were deposited by atomic layer deposition (ALD). A novel approach with pulsing of two Bi-precursors was studied to control the Si/Bi atomic ratio in bismuth silicate thin films. The crystallization of compounds formed in the Bi₂O₃–SiO₂ and Bi₂O₃–TiO₂ systems was investigated. Control of the stoichiometry of Bi–Si–O thin films was studied when deposited on Si(1 0 0) and crystallization was studied for films on sapphire and MgO-, ZrO₂- and YSZ-buffered Si(1 0 0). The Bi–Ti–O thin films were deposited on Si(1 0 0) substrate. Both Bi–Si–O and Bi–Ti–O thin films were amorphous after deposition. Highly a-axis oriented Bi₂SiO₅ thin films were obtained when the Bi–Si–O thin films deposited on MgO-buffered Si(1 0 0) were annealed at 800 °C in nitrogen. The full-width half-maximum values for 200 peak were also studied. An excess of bismuth was found to improve the crystallization of Bi–Ti–O thin films and the best crystallinity was observed with Ti/Bi atomic ratio of 0.28 for films annealed at nitrogen at 1000 °C. Roughness of the thin films as well as the concentration depth distribution were also examined.