Optical properties of SrTiO<Subscript>3</Subscript> thin films by pulsed laser deposition

SrTiO₃ thin films were prepared on a fused-quartz substrate by pulsed laser deposition (PLD). Dense and homogeneous films with a thickness of 260 nm were prepared. Optical constants (refractive index n and extinction coefficient k) were determined from the transmittance spectra using the envelope method. The optical band gap energy of the films was found to be 3.58 eV, higher than the 3.22 eV for bulk SrTiO₃, attributable to the film stress exerted by the substrate. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The refractive index and the packing density for the PLD-prepared SrTiO₃ thin films are higher than those for the SrTiO₃ films prepared by physical vapor deposition, sol–gel and RF sputtering. Received: 18 March 2002 / Accepted: 7 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Anisotropic in-plane strain in W-doped (Ba,Sr)TiO<Subscript>3</Subscript> thin films deposited by pulsed-laser deposition on (001)MgO

The structural and the microwave dielectric properties of Ba_xSr_(1-x)TiO₃ films (BST) with x=0.5, 0.6 and 0.7, containing 1 mol % W have been investigated. The films were grown by pulsed-laser deposition on MgO (001) substrates at a temperature of 720 °C in an oxygen pressure from 3 to 500 mTorr. The film structures were determined by X-ray diffraction. The lattice parameters were fitted to a tetragonal distortion of a cubic lattice. The out-of-plane lattice parameter (c) was calculated from the position of the (004) reflection. Using c, the in-plane lattice parameter, a, was calculated from the position of the (024) and (224) reflections. A deviation in the calculated values for a, beyond the systematic error, was found in the in-plane lattice parameter, suggesting an in-plane orthorhombic distortion (a, a’). Films with x=0.7 showed a minimum in-plane distortion due to a better lattice match with the substrate. The ratio of the in-plane and out-of-plane lattice parameters was calculated as a measure of the lattice distortion (a/c and a’/c). The dielectric properties of the films deposited were measured at room temperature at 2 GHz using gap capacitors fabricated on top of the dielectric film. For all Ba/Sr ratios investigated in the W-doped material, the dielectric Q (1/cosδ) was observed to be insensitive to the oxygen deposition pressure. A peak in the change in the dielectric constant, as a function of an applied electric field (0–80 kV/cm), was observed for films deposited in 50 mTorr of oxygen. The largest K-factor, K=(ε(0)-ε(V )/ε(0)×Q(0)), for films deposited from a BST x=0.6 (1 mol % W-doped) target was observed in the film that had a minimum in-plane strain, where a∼a’ and c was greater than a and a’. Received: 4 July 2002 / Accepted: 5 July 2002 / Published online: 17 December 2002 RID="*" ID="*"Permanent address: Nuclear Research Center–Negev, P.O. Box 9001 Beer-Sheva, Israel RID="**" ID="**"Corresponding author. Fax: +1-202/767-5301, E-mail: horwitz@ccsalpha3.nrl.navy.mil     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Effect of substrate temperature and annealing temperature on the structural, electrical and microstructural properties of thin Pt films by rf magnetron sputtering

Influence of both substrate temperature, T_s, and annealing temperature, T_a, on the structural, electrical and microstructural properties of sputtered deposited Pt thin films have been investigated. X-ray diffraction results show that as deposited Pt films (T_s = 300, 400 °C) are preferentially oriented along (1 1 1) direction. A little growth both along (2 0 0) and (3 1 1) directions are also noticed in the as deposited Pt films. After annealing in air (T_a = 500-700 °C), films become strongly oriented along (1 1 1) plane. With annealing temperature, average crystallite size, D, of the Pt films increases and micro-strain, e, and lattice constant, a₀, decreases. Residual strain observed in the as deposited Pt films is found to be compressive in nature while that in the annealed films is tensile. This change in the strain from compressive to tensile upon annealing is explained in the light of mismatch between the thermal expansion coefficients of the film material and substrate. Room temperature resistivity of Pt films is dependant on both the T_s and T_a of the films. Observed decrease in the film resistivity with T_a is discussed in terms of annihilation of film defects and grain-boundary. Scanning electron microscopic study reveals that as the annealing temperature increases film densification improves. But at an annealing temperature of ∼600 °C, pinholes appear on the film surface and the size of pinhole increases with further increase in the annealing temperature. From X-ray photoelectron spectroscopic analysis, existence of a thin layer of chemisorbed atomic oxygen is detected on the surfaces of the as deposited Pt films. Upon annealing, coverage of this surface oxygen increases.     

Thickness influence on surface morphology and ozone sensing properties of nanostructured ZnO transparent thin films grown by PLD

Transparent zinc oxide (ZnO) thin films with a thickness from 10 to 200 nm were prepared by the PLD technique onto silicon and Corning glass substrates at 350 °C, using an Excimer Laser XeCl (308 nm). Surface investigations carried out by atomic force microscopy (AFM) and X-ray diffraction (XRD) revealed a strong influence of thickness on film surface topography. Film roughness (RMS), grain shape and dimensions correlate with film thickness. For the 200 nm thick film, the RMS shows a maximum (13.9 nm) due to the presence of hexagonal shaped nanorods on the surface. XRD measurements proved that the films grown by PLD are c-axis textured. It was demonstrated that the gas sensing characteristics of ZnO films are strongly influenced and may be enhanced significantly by the control of film deposition parameters and surface characteristics, i.e. thickness and RMS, grain shape and dimension.     

Atomic layer deposition of PbZrO3 thin films

In this paper, we report on the preparation of lead zirconate films for the first time using atomic layer deposition in an attempt to investigate some of the film properties and also to evaluate possible use of the precursor combination to prepare more complex lead titanate zirconate. In the depositions tetraphenyl lead (Ph₄Pb) was used as the lead and zirconium 2,2,6,6-tetramethyl-3,5-heptadionato (Zr(thd)₄) as the zirconium precursor, while ozone was used as the oxygen source. Film growth, stoichiometry and quality were studied using different pulsing ratios at deposition temperatures of 275 and 300 °C. According to X-ray diffraction, the crystalline perovskite phase was observed when films deposited on SrTiO₃(1 0 0) were annealed at 600 °C. Surface roughness was reduced for lead deficient films as well as in annealed samples.     

Correlation between density and structure. in boron nitride thin films by X-ray diffraction

* _ion=100 eV. Above E^* _ion the average density (deduced from X-ray reflectivity) shows a strong increase, indicating the sudden appearance of the cubic boron nitride phase consistent with the sp³ concentration deduced from IR absorption spectroscopy. The in-plane X-ray diffraction shows that this cubic phase consists of small nanocrystals of 70 Å linear size. Received: 26 November 1996/Accepted: 27 January 1997     

Nanoscale domain switching mechanism in Pb(Zr,Ti)O<Subscript>3</Subscript> thin film

Pb(Zr,Ti)O₃ (PZT) ferroelectric thin film was prepared by the sol-gel technique and crystallized with a (111) preferred orientation. The domain structure and polarization reversal behavior were investigated by using scanning force microscopy (SFM) piezoresponse mode at the nanometer scale. A step structure of approximately 30 nm in width was directly observed, which was formed during the polarization reversal process. The presence of the step structure reveals that the forward domain-growth mechanism is the dominant domain-switching process in our PZT thin films. Received: 6 August 2002 / Accepted: 9 August 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/5241-3122, E-mail: huarongzeng@163.net     

Formation of the ferroelectric domains in epitaxial PbTiO3 thin films grown by metalorganic chemical vapor deposition

3 thin films have been prepared by metalorganic chemical vapor deposition under reduced pressure. The formation of ferroelectric domains in films grown on SrTiO₃ and LaAlO₃ substrates was investigated by synchrotron radiation and Rutherford backscattering spectroscopy. Single-domain (3000-Å thick) and multi-domain (4500-Å thick) PbTiO₃ films were produced on SrTiO₃. For multi-domain PbTiO₃ film, the c-domain presented epitaxial structure with its c-axis perpendicular to the substrate surface, while a-domains aligned four-fold symmetrically with c-domains by 2.79^° off the c-axis of c-domains. In the film, the measured lattice constants (a, b and c) of the a- and c-domains were different from each other, indicating that the films suffered a modulated strain during domain formation. In contrast, both the a and c domains of films on LaAlO₃ were alternatively aligned on substrate with the a-axis of the a-domain and the c-axis of c-domains perpendicular to the substrate surface. Two-dimensional distribution of these domains is proposed and the formation of these kinds of domains is discussed. The surface morphology and phase transition process of single and multi domain PbTiO₃ film on SrTiO₃ were studied by atomic force microscope (AFM) and high temperature X-ray diffraction, respectively. Received: 15 August 1996/Accepted: 21 January 1997     

Room temperature chemical synthesis of lead selenide thin films with preferred orientation

Room temperature chemical synthesis of PbSe thin films was carried out from aqueous ammoniacal solution using Pb(CH₃COO)₂ as Pb²⁺ and Na₂SeSO₃ as Se²⁻ ion sources. The films were characterized by a various techniques including, X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED), Fast Fourier transform (FFT) and UV-vis-NIR techniques. The study revealed that the PbSe thin film consists of preferentially oriented nanocubes with energy band gap of 0.5 eV.     

Size effect in optical spectra of microcrystalline diamond powders and CVD films

Received: 24 March 1997/Accepted: 27 May 1997     

Structural and ferroelectric properties of Bi4Ti3O12 thin films on IrO2 prepared by rf magnetron sputtering

Bismuth titanate, Bi₄Ti₃O₁₂, thin films were grown on IrO₂/SiO₂/Si substrates by radio-frequency magnetron sputtering. Crystallinity and microstructure of the films were characterized over a wide range of oxygen mixing ratio (OMR) during deposition. X-ray fluorescence spectra reveal that the cation content of the films is dependent upon the OMR, suggesting that control of Bi to Ti ratio is possible by the oxygen content in the sputtering atmosphere. Rutherford backscattering spectrometry measurements also show that oxygen content of the BTO film grown with an OMR = 0.5is close to a stoichiometric phase. In addition, Bi–O bonding chemistry is studied by X-ray photoelectron spectroscopy. Polarization vs. voltage loop shows that remnant polarization, P_r, is +12 μC/cm². Received: 26 November 1999 / Accepted: 26 November 1999 / Published online: 9 August 2000     

Properties of NiO thin films deposited by chemical spray pyrolysis using different precursor solutions

NiO thin films have been deposited by chemical spray pyrolysis using a perfume atomizer to grow the aerosol. The influence of the precursor, nickel chloride hexahydrate (NiCl₂·6H₂O), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), nickel hydroxide hexahydrate (Ni(OH)₂·6H₂O), nickel sulfate tetrahydrate (NiSO₄·4H₂O), on the thin films properties has been studied. In the experimental conditions used (substrate temperature 350 °C, precursor concentration 0.2-0.3 M, etc.), pure NiO thin films crystallized in the cubic phase can be achieved only with NiCl₂ and Ni(NO₃)₂ precursors. These films have been post-annealed at 425 °C for 3 h either in room atmosphere or under vacuum. If all the films are p-type, it is shown that the NiO films conductivity and optical transmittance depend on annealing process. The properties of the NiO thin films annealed under room atmosphere are not significantly modified, which is attributed to the fact that the temperature and the environment of this annealing is not very different from the experimental conditions during spray deposition. The annealing under vacuum is more efficient. This annealing being proceeded in a vacuum no better than 10⁻² Pa, it is supposed that the modifications of the NiO thin film properties, mainly the conductivity and optical transmission, are related to some interaction between residual oxygen and the films.     

Substrate and orientation influence on electrical properties of sputtered La-doped PZT thin films

Pb_1−2y/3La_yZr_xTi_1−xO₃ (PLZT) thin films have been prepared “in situ” by multi-target sputtering on Silicon, LaAlO₃ (LAO) and MgO substrates covered with a Pt bottom electrode. The purpose was to grow tetragonal PLZT films (Zr/Ti = 28/72 with different La contents) on these various substrates and to compare their electrical properties. To this aim, Pt was first deposited on the three different substrates to get (1 1 1)Pt/Si, (1 1 1)Pt/LAO and (2 0 0)Pt/MgO. Then PLZT was deposited in a same run on these three kinds of substrates and the influence of La contents and film orientation on electrical properties was investigated. The La content was varied from y = 0 to y = 32 in order to explore the phase transition between ferroelectric and paraelectric phases as a function of the substrate. For large amount of Lanthanum, relaxor behavior has been observed and studied.     

Influence of deposition conditions on the structural characteristics of sublimated CdTe thin films

In this paper the results obtained by X-ray diffraction studies on the structural characteristics of CdTe thin films deposited onto glass substrates by close-spaced sublimation technique are presented. Using different experimental arrangements and appropriate settings for growth parameters, the films with different polycrystalline structures were prepared. The geometry and the volume of the deposition chamber influence the size of film crystallites and also their preferential orientation. The role of deposition parameters such as the substrate temperature, the incidence angle, the film thickness, and the heat treatment in determination of the structural properties of the films are also investigated. Received: 26 November 1999 / Accepted: 7 January 2000 / Published online: 5 April 2000     

Elaboration of (1 1 1)-oriented La-doped PZT thin films on platinized silicon substrates

PLZT thin films with different thickness were deposited in situ on platinum coated silicon substrates using a multi-target sputtering system. The purpose was to grow (1 1 1)-textured PLZT films on Pt (1 1 1). To this aim, the role of some key parameters on both crystalline quality and electrical properties was investigated. An ultra-thin TiO₂ seeding layer was deposited, prior to PLZT, which strongly affected the crystallographic orientation of the films. The relation between temperature deposition and film crystallinity is analysed. TEM observations show the presence of some very small grains of Zr_0.9La_0.1O_1.95 at the film bottom interface. In the range of thickness investigated, the plot of the inverse capacitance as a function of the film thickness split up into two different curves, each with a linear shape, which however allows determination of a single value of interface capacitance. Above a thickness of 400-500 nm a saturation of the dielectric properties seems to be reached.     

Electric field dependence of topography in ferroelectric P(VDF/TrFE) films

The effect of electric fields on the topography and structures in P(VDF/TrFE) ferroelectric films was studied. The morphological changes after poling treatment at 25 °C, from previously chrysanthemum-like structures to regular and parallel slug-like domains, were observed by scanning probe microscope. Poling process at higher temperature induced even greater morphological changes. Imaging of local electrical properties showed that these changes could not be attributed to the influence of local charges injected into film surface during poling process. It was believed that this new morphology came from the reform of film structures induced by electric fields.     

Growth and structure of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 thin films prepared by pulsed laser deposition technique

Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ (BNKLT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition (PLD) technique. The films prepared were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effects of the processing parameters, such as oxygen pressure, substrate temperature and laser power, on the crystal structure, surface morphology, roughness and deposition rates of the thin films were investigated. It was found that the substrate temperature of 600 °C and oxygen pressure of 30 Pa are the optimized technical parameters for the growth of textured film, and all the thin films prepared have granular structure, homogeneous grain size and smooth surfaces.     

Growth of thin, crystalline oxide, nitride, and oxynitride films on NiAl and CoGa surfaces

At 300 K, an amorphous Al-oxide film is formed on NiAl(001) upon oxygen adsorption. Annealing of the oxygen-saturated NiAl(001) surface to 1200 K leads to the formation of thin well-ordered θ-Al₂O₃ films. At 300 K, and low-exposure oxygen atoms are chemisorbed on CoGa(001) on defects and on step edges of the terraces. For higher exposure up to saturation, the adsorption of oxygen leads to the formation of an amorphous Ga-oxide film. The EEL spectrum of the amorphous film exhibits two losses at ≈400 and 690 cm^-1. After annealing the amorphous Ga-oxide films to 550 K thin, well-ordered β-Ga₂O₃ films are formed on top of the CoGa(001) surface. The EEL spectrum of the β-Ga₂O₃ films show strong Fuchs-Kliewer (FK) modes at 305, 455, 645, and 785 cm^-1. The β-Ga₂O₃ films are well ordered and show (2×1) LEED pattern with two domains, oriented perpendicular to each other. The STM study confirms the two domains structure and allows the determination of the two-dimensional lattice parameters of β-Ga₂O₃. The vibrational properties and the structure of β-Ga₂O₃ on CoGa(001) and θ-Al₂O₃ on NiAl(001) are very similar. Ammonia adsorption at 80 K on NiAl(111) and NiAl(001) and subsequent thermal decomposition at elevated temperatures leads to the formation of AlN. Well-ordered and homogeneous AlN thin films can be prepared by several cycles of ammonia adsorption and annealing to 1250 K. The films render a distinct LEED pattern with hexagonal [AlN/NiAl(111)] or pseudo-twelve-fold [AlN/NiAl(001)] symmetry. The lattice constant of the grown AlN film is determined to be a_AlN= 3.11 Å. EEL spectra of AlN films show a FK phonon at 865 cm^-1. The electronic gap is determined to be E_g= 6.1±0.2 eV. GaN films are prepared by using the same procedure on the (001) and (111) surfaces of CoGa. The films are characterized by a FK phonon at 695 cm^-1 and an electronic band gap E_g= 3.5±0.2 eV. NO adsorption at 75 K on NiAl(001) and subsequent annealing to 1200 K leads to the formation of aluminium oxynitride (AlON). An oxygen to nitrogen atomic ratio of ≈2:1 was estimated from the analysis of AES spectra. The AlON films shows a distinct (2×1) LEED pattern and the EEL spectrum exhibits characteristic Fuchs-Kliewer modes. The energy gap is determined to be E_g= 6.6±0.2 eV. The structure of the AlON film is derived from that of θ-Al₂O₃ formed on NiAl(001). Received: 21 March 1997/Accepted: 12 August 1997     

Surface morphology of highly mismatched InSb films grown on GaAs substrates by molecular beam epitaxy

InSb films on GaAs(001) substrates with and without GaAs buffer layer have been grown by molecular beam epitaxy. Rather than surface undulations, aligned ripples and pyramidal hillocks along the orthogonal 〈110〉 directions were observed on the surface of InSb films. Both the preferential growth and the termination of ripples were proved to be related to strain‐driven mass transport. A model was proposed to elucidate the formation of the hillocks, which are more efficient to relax strain than ripples. Due to the strain relaxation through hillocks with small bases predominantly, the surfaces of the InSb films grown without a GaAs buffer layer are smoother than those of films grown with a GaAs buffer layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)