Growth by laser ablation of Ti-based oxide films with different valency states

x La_2/3+yTiO_3-δ perovskite (with δ≤0.5) were deposited by the laser ablation technique from Li_0.33La_0.56TiO₃ targets. Their growth onto MgO substrateswas studied as a function of the oxygen pressure. For films grown in vacuum (10^-6 mbar), a La_0.63TiO_2.5 composition was obtained, meaning that Ti³⁺ alone is present in the films, while Li ions are not incorporated under these conditions. This material shows good electric conductivity (ρ=500 mΩ cm). By contrast, insulating films with a Li_0.1La_0.70TiO₃ composition corresponding to the Ti⁴⁺ species were obtained at high oxygen pressures (>0.05 mbar). For all conditions, textured films were grown with different orientations depending on the temperature and the oxygen pressure. Received: 10 September 1997/Accepted: 24 November 1997     

Room-temperature epitaxial growth of CeO2(001) films on YSZ buffered Si(001) substrates

2 (001) epitaxial thin films deposited on Si(001) with yttria-stabilized zirconia buffers have been obtained for the first time at room temperature by pulsed-laser deposition. The influence of oxygen pressure on the crystal quality of CeO₂ was studied for the films deposited at 100 °C. The rocking curve full width at half maximum of the CeO₂(002) peak for films deposited at room temperature and 100 °C was between 1° and 2°, for oxygen pressures below 3×10^-2 mbar. The best crystal quality was obtained at around 3×10^-3 mbar. Epitaxial growth at room temperature was confirmed by cross-sectional transmission electron microscopy. Scanning electron microscopy and atomic force microscopy revealed very smooth surfaces for oxygen pressure below 3×10^-2 mbar, with rms roughness values around 0.3 nm over 5 μm×5 μm. Received: 25 September 1997/Accepted: 22 April 1998     

Growth and characteristics of reactive pulsed laser deposited molybdenum trioxide thin films

Molybdenum trioxide thin films were prepared by reactive pulsed laser deposition on Corning 7059 glass substrates. The influence of oxygen partial pressure and deposition temperature on the structure, surface morphology and optical properties of these films was studied to understand the growth mechanism of MoO₃ thin films. The films formed at 473 K in an oxygen partial pressure of 100 mTorr exhibited predominantly a (0k0) orientation, corresponding to an orthorhombic layered structure of α-MoO₃. The evaluated optical band gap of the films was 3.24 eV. The crystallite size increased with increase of deposition temperature. The films formed at an oxygen partial pressure of pO₂=100 mTorr and at a deposition temperature greater than 700 K exhibited both (0k0) and (0kl) orientations, representing α-β mixed phases of MoO₃. The films formed at an oxygen partial pressure less than 100 mTorr were found to be sub-stoichiometric with α-β mixed phases. The investigation revealed the growth of polycrystalline and single-phase orthorhombic-layered-structure α-MoO₃ thin films with composition nearly approaching the nominal stoichiometry at moderate substrate temperatures in an oxygen partial pressure of 100 mTorr. Received: 9 April 2001 / Accepted: 6 August 2001 / Published online: 17 October 2001     

Oxygen-segregation-controlled epitaxy of Y2O3 films on Nb(110)

We demonstrate a very simple and reliable method of manufacturing clean, single-crystalline Y₂O₃ films on Nb(110) substrates in situ. The method exploits the oxygen bulk contamination of Nb as a source of clean oxygen. For substrate temperatures above 800 K oxygen segregation to the Nb surface is so efficient, that yttrium becomes oxidized during deposition without any background oxygen pressure required in the ultrahigh vacuum system. The crystallinity and stoichiometry of these films can be tuned by the deposition temperature. For Y deposition at 1300 K the formation of well-ordered (111)-oriented Y₂O₃ films is achieved. Received: 19 April 2000 / Accepted: 20 April 2000 / Published online: 23 August 2000     

Anomalies of attenuation and phase velocity of surface acoustic waves in YBa2Cu3O7‒δ thin films in the vicinity of Tc

We present measurements of the attenuation and phase velocity of surface acoustic waves in thin YBa₂Cu₃O films as a function of temperature, in magnetic fields up to 3.6 T applied parallel to the c-axis of the films. We have observed anomalies in both, the attenuation and the phase velocity in the vicinity of the superconducting critical temperature which do not depend on the magnetic field. Possible origins of these anomalies, observed, to our knowledge, for the first time in YBa₂Cu₃O thin films, are discussed and compared to bulk acoustic wave experiments. We present a kind of feedback technique for surface acoustic waves which improves the sensitivity of this type of measurement. The actual sensitivity limits are mentioned. Received: 7 August 1997 / Revised: 7 November 1997 / Accepted: 17 November 1997     

Conducting properties of In2O3:Sn thin films at low temperatures

Electrical conductivity, Hall effect and magnetoresistance of In₂O₃:Sn thin films deposited on a glass substrates at different temperatures and oxygen pressures, have been investigated in the temperature range 4.2–300 K. The observed temperature dependences of resistivity for films deposited at 230 °C as well as at nominally room temperatures were typical for metallic transport of electrons except temperature dependence of resistivity of the In₂O₃:Sn film deposited in the oxygen deficient atmosphere. The electrical measurements were accompanied by AFM and SEM studies of structural properties, as well as by XPS analysis. It is established that changes of morphology and crystallinity of ITO films modify the low-temperature behavior of resistivity, which still remains typical for metallic transport. This is not the case for the oxygen deficient ITO layer. XPS analysis shows that grown in situ oxygen deficient ITO films have enhanced DOS between the Fermi level and the valence band edge. The extra localized states behave as acceptors leading to a compensation of n-type ITO. That can explain lower n-type conductivity in this material crossing over to a Mott-type hopping at low temperatures. Results for the low temperature measurements of stoichiometric ITO layers indicate that they do not show any trace of metal-to-insulator transition even at 4.2 K. We conclude that, although ITO is considered as a highly doped wide-band gap semiconductor, its low-temperature properties are very different from those of conventional highly doped semiconductors.     

Carbonaceous phases by IR laser-induced decomposition of 3-butyn-2-one

2 laser radiation in the gas phase results in the deposition of carbonaceous films composed of a graphitic and monohydrogenated carbon, in which H is dominantly bonded to sp³ sites. The films incorporate oxygen in the topmost layers. The technique is suitable for the preparation of thin C-based films at low substrate temperatures. Received: 23 July 1997/Accepted: 18 November 1997     

Contributions to photodoping in oxygen-deficient YBa2Cu3Ox films

We report our studies on the superconducting and normal-state properties of metallic thin films ( 52 K) exposed to long-term white-light illumination (photodoping). It was observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. At low temperatures, both the Hall mobility and the Hall number were photoenhanced, whereas, at temperatures slightly below room temperature, the Hall mobility initially showed an abrupt increase followed by a long-term decrease, and the Hall number increased even stronger than at low temperatures. The enhancement of the film's superconducting transition temperature T_c, caused by photodoping, exhibited the same temperature dependence as the enhancement of the Hall number, being largest ( 2.6 K) at high temperatures. From the asynchronous behavior of the Hall quantities, we conclude that both the photoassisted oxygen ordering and charge transfer mechanisms contribute to photodoping. The relative contributions of both mechanisms and, thus, the electronic properties of the photoexcited state are strongly temperature dependent. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after termination of the illumination. The relaxation saturated somewhat below the initial, undoped value, similarly to the decrease of the Hall mobility, observed upon long illumination. These latter findings give evidence for a competition between the oxygen ordering and thermal disordering processes during and after the photoexcitation in the high-temperature range. Received: 13 October 1997 / Accepted: 19 November 1997     

Surface morphology and surface chemical states of epitaxial Pb(Zr0.95Ti0.05)O3 thin films grown on SrTiO3(100) by sputter deposition

0.95 Ti_0.05)O₃ thin films of an orthorhombic perovskite structure were obtained on SrTiO₃(100) substrates by radio frequency sputter deposition. The surface morphology of the films was investigated with atomic force microscopy, scanning electron microscopy, and reflection electron microscopy. It is shown that the film surfaces are rather bumpy. There are undulations of about 400 nm in length in an in-plane direction. The mean roughness perpendicular to the surface is 39.6 nm, for the film thickness of 0.45 μm. The surface roughening was probably caused by island-shaped nucleation and growth during the film growth. It has also been found that some gorges and a number of small pits remain at the film surfaces. The surface chemical states of the films were characterized by using X-ray photoelectron spectroscopy. A Pb enrichment layer and a large amount of adsorbed oxygen have been found at the surfaces of the films. Near the film surface Pb and Zr exist mainly in the forms of, besides Pb(Zr,Ti)0₃, metal Pb, metal Zr, oxygen-chemisorbed Pb, and various lead oxides. In addition, a small amount of lead, whose binding energy of Pb 4f_7/2 is much lower than that of metal Pb, was observed at the film surfaces, but its chemical state is unknown up to now. Received: 2 June 1997/Accepted: 22 September 1997     

Al2O3 thin films by plasma-enhanced chemical vapour deposition using trimethyl-amine alane (TMAA) as the Al precursor

2 O₃ thin films by plasma-enhanced chemical vapour deposition (PECVD) using trimethyl-amine alane (TMAA) as the Al precursor. The thin films were deposited on both Si and quartz silica (SiO₂) substrates. Deposition rates were typically 60 Å min^-1 keeping the TMAA temperature constant at 45 °C. The deposited Al₂O₃ thin films were stoichiometric alumina with low carbon contamination (0.7–1.3 At%). The refractive index ranged from 1.54 to 1.62 depending on the deposition conditions. The deposition rate was studied as a function of both the RF power and the substrate temperature. The structure and the surface of the deposited Al₂O₃ thin films were studied using X-ray diffraction, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Received: 20 May 1997/Accepted: 12 June 1997     

Pulsed laser deposition of ZnO as conductive buffer layer of (001)-LiNbO3 thin films

1 (LO) mode peak at 579 cm^-1, indicating oxygen deficiency in the films. Significant dependence of the electrical property for the films on substrate temperature was shown. The I–V relation of the films exhibited non-ohmic behavior. Whereas the films deposited above 500 °C showed a metal-like property, at a lower substrate temperature semiconducting thin films were achieved. The (001)-oriented LiNbO₃/ZnO heterostructure was successfully prepared on quartz fused and (001) sapphire plates. Received: 6 April 1998/Accepted: 26 May 1998     

Growth and characterization of silicon nitride films on various underlying materials

Characteristics of silicon nitride (SiN_x:H) films, grown by plasma enhanced chemical vapor deposition (PECVD) on various metals such as Ta, IrMn, NiFe, Cu, and CoFe at various temperatures down to 100 °C, were studied using measurements of BHF etch rate, surface roughness and Auger electron spectroscopy (AES). The results were compared with those obtained for SiN_x:H films on Si. The deposition rate of SiN_x:H films increased slightly as deposition temperature decreased, and showed a weak dependence on the underlying materials. The surface of the nitride films deposited on all underlying materials at lower temperatures (below 150 °C) became rougher. In particular, a bubble-like surface was observed on the nitride film deposited on NiFe at 100 °C. At higher deposition temperatures (above 200 °C), SiN_x:H films on all the above metals had small RMS values, except for films on Cu which cracked at 250 °C. BHF (10:1) etch rate increased dramatically for nitride films deposited below 150 °C. For different underlying films, the BHF etch rate was quite different, but exhibited the same trend with decrease in deposition temperature. AES measurements showed that Si and N concentrations in the SiN_x:H films were only slightly different for the various deposition temperatures and underlying materials. AES depth profile of nitride films indicated that both surface O content and the depth of oxygen penetrating into SiN_x:H increased for low temperature-deposited films. However, there was no observed oxygen signal from within the films, even for films deposited at 100 °C, and both Si and N concentrations were uniform throughout the film. Received: 26 October 2001 / Accepted: 2 March 2001 / Published online: 20 June 2001     

二氧化钒薄膜的低温制备及其性能研究

针对VO₂薄膜在微测辐射热计上的应用，采用射频反应溅射法，在室温下制备氧化钒薄膜；研究了氧分压对薄膜沉积速率、电学性质及成分的影响.通过调节氧分压，先获得成分接近VO₂的非晶化薄膜，再在400℃空气中氧化退火，便可制得高电阻温度系数，低电阻率的VO₂薄膜，电阻温度系数约为-4%/℃，薄膜方块电阻为R_□为100—300kΩ；薄膜在室温下沉积，400℃下退火的制备方法与微机电加工(micro electromechanic 关键词： 二氧化钒 电阻温度系数 氧分压 射频反应溅射法     

Thermal desorption spectroscopy of fullerene films containing endohedral Li@C60

60 and produced by low-energy ion bombardment. The desorption properties of the films were studied as a function of the film thickness, ion energy, and ion dose. The results provide strong corroborative evidence for the endohedral nature of the Li-containing species and also clearly indicate the prescence of Li₂@C₆₀ in the films. The binding energy in these films is significantly larger than in pure C₆₀ films and evidence of degradation of the fullerenes is seen at high ion energies and/or large ion doses. Received: 6 August 1997/Accepted: 8 August 1997     

The effect of annealing on the room temperature ferromagnetism in co-sputtered In2O3: C thin films

In this paper, we report investigation of room temperature (RT) ferromagnetism in In₂O₃ (InO) thin films doped with carbon prepared by the co-sputtering method. InO thin films both undoped and C doped with varied thicknesses in the range of 45 to 80 nm were synthesized on Si substrates with varied C concentrations. The carbon concentration was varied from 1.6 to 9.3 at%. The undoped InO films showed no trace of ferromagnetism. Carbon doped films (InO:C) exhibited ferromagnetism at RT, which was of the orders of 10⁻⁵ emu and varied strongly with C concentrations. It is observed that the magnetization reached a maximum value of 5.7 emu/cm³ at 4 at% C. Annealing of the InO:C films in an oxygen environment resulted in a decrease in the magnetization, indicating the crucial role of oxygen vacancies in the films. It is concluded that the oxygen vacancies were important and compete with C substitution for the RT ferromagnetism.     

Structural, electrical, and surface characteristics of La0.5Sr0.5CoO3 thin films prepared by pulsed-laser deposition

A parametric study of the growth of La_0.5Sr_0.5CoO₃ (LSCO) thin films on (100) MgO substrates by pulsed-laser deposition (PLD) is reported. Films are grown under a wide range of substrate temperature (450–800 °C), oxygen pressure (0.1–0.9 mbar), and incident laser fluence (0.8–2.6 J/cm²). The optimum ranges of temperature, oxygen pressure, and laser fluence to produce c-axis oriented films with smooth surface morphology and high metallic conductivity are identified. Films deposited at low temperature (500 °C) and post-annealed in situ at higher temperatures (600–800 °C) are also investigated with respect to their structure, surface morphology, and electrical conductivity. Received: 20 November 1998 / Accepted: 6 July 1999 / Published online: 21 October 1999     

Influence of atmospheric exposure of tris (8-hydroxyquinoline) aluminum (Alq<Subscript>3</Subscript>): a photoluminescence and absorption study

We have investigated the effects of atmospheric exposure on the properties of tris (8-hydroxyquinoline) aluminum (Alq₃) thin films by photoluminescence (PL) and UV-Vis absorption measurements. Alq₃ films were evaporated on glass substrates at different temperatures. The influence of annealing on the environmental stability of the films has also been investigated. It has been found that deposition at higher substrate temperature and annealing of the samples deposited at room temperature yield an improvement in the environmental stability of the films, i.e. less decrease in the PL intensity over time with exposure to atmosphere, as well as increased PL intensity. To investigate further the effects of the air exposure, films deposited at room temperature were stored for four days in air, nitrogen, and oxygen. No decrease in PL intensity has been found for storage in nitrogen, while the decrease for the film stored in oxygen was smaller than that for the film stored in air, indicating that both humidity and oxygen play a role in the PL intensity decrease in Alq₃ thin films. PACS 78.55.Kz; 78.40.Me     

Structural and optical properties of Ge20SbxSe80-x films

20 Sb_xSe_80-x (where 10≤x≤40 at.%) thin films. The optical absorption results indicate that the absorption mechanism is due to non-direct transition. The optical gap of the as-deposited films was found to decrease monotonically with increasing antimony content, a result which was interpreted on the basis of the chemical-bond approach proposed by Bicerno and Ovshinsky. Annealing of the Ge₂₀Sb₄₀Se₄₀ films at temperatures higher than 450 K was found to decrease the optical gap and increase the refractive index of the investigated film. Increasing the amount of crystalline GeSe₂ and Sb₂Se₃ phases while increasing the annealing temperature could be responsible for the continuous decrease of the optical gap of the Ge₂₀Sb₄₀Se₄₀ film. Received: 4 November 1997/Accepted: 16 December 1997     

Influence of oxygen on structural and optoelectronic properties of CdS thin film deposited by magnetron sputtering technique

In this study, CdS thin films with thicknesses of approximately 100 nm were deposited at a substrate temperature of 100 °C by a sputtering technique under two different ambient conditions—pure Ar ambient and Ar/O₂ (99:1) ambient—at deposition power densities of 1.0 and 2.0 W/cm², respectively The films were polycrystalline with a preferential orientation along the (002) crystal plane; however, the films deposited in the Ar/O₂ ambient exhibited reduced crystallinity. Furthermore, the crystallite sizes, micro-strains, and dislocation densities of the films were significantly affected by oxygen diffusion into the films’ crystal structures. The CdS films deposited in the Ar/O₂ ambient demonstrated higher optical transmittance and higher bandgaps. Morphologies observed from scanning electron microscopy images revealed that the grains of the films were also significantly affected by the oxygen present in the deposition ambient. Additionally, photoluminescence analysis revealed that the sulfur vacancies in the CdS films were partially filled by oxygen atoms, causing significant variations in the electrical properties of the films.     

Structural manipulation of ultrathin organic films on metal surfaces: the case of decane thiol/Au(111)

3 (CH₂)₉]SH) have been adsorbed on Au(111) single crystals both via vacuum deposition and from an ethanolic solution. The epitaxial structure of the ultrathin organic films has been identified at room temperature via low-energy electron diffraction to be c(4×2)R30° for the solution grown film and rectangular c(23×) for the vacuum deposited film. These structures correspond to molecules adsorbed on the surface with their carbon chains flat on the surface (vacuum deposited) and nearly perpendicular (solution grown). It is demonstrated that this orientation can be changed reversibly in vacuum via either annealing the films or exposing them to additional gas. Received: 7 February 1997/Accepted: 27 May 1997