Structural characterization of heteroepitaxial films of CdSe and ZnTe

The effects of composition, orientation and temperature of substrate on the structure and morphology of epitaxial CdSe and ZnTe films are analyzed. Single crystal wafers of zinc telluride and zinc selenide oriented along (110) and natural cleaved mica were used as substrates. This paper presents a complex investigation of the growth mechanism and crystal perfection of the films by optical microscopy, X-ray and electron diffraction, and by electron microscopy. The structure of the transient region of the film substrate interface was studied by X-ray microanalyzer and by scanning electron microscope. Cadmium selenide layers grown on mica had hexagonal structure and were oriented in the (0001) plane, those grown on zinc telluride crystals had cubic structure and orientation along (110). Zinc telluride layers were cubic and were oriented parallel the (111) plane when deposited on mica, and parallel to (110) when deposited on zinc selenide substrates. It is shown that the layers grew by a layer mechanism. Epitaxial growth of cubic cadmium selenide layers on zinc telluride single crystals, as well as of ZnTe layers on ZnSe substrates, made it possible to grow heterojunctions with a low density of misfit dislocations at the interface; this gave rise to intensive injection electroluminescence and effective separation of non-equilibrium charge carriers in such structures.     

To the phenomenological theory of ferroelectric phase transitions

Some insufficiently studied aspects of proper ferroelectric phase transitions are considered. These are the role played by higher-degree terms of the expansion of a thermodynamic potential in polarization, the universal temperature dependences of physical quantities represented in terms of dimensionless variables, the soft mode (which, along with other modes, gives the contribution to polarization) and the polarization considered as an order parameter, and also the phase transition from piezoelectric phases.     

Pseudosymmetry and ferroelectric phase transitions in the KTP structure type

The characteristic pseudosymmetry observed in the crystal structures of KTiOPO₄-type ferroelectrics have been analyzed. The low-symmetry phases of all the crystals under consideration are characterized by a pronounced pseudosymmetry. The transition temperature monotonically decreases with an increase of the pseudosymmetry parameter of the crystal structure of the ferroelectric phase. The approach suggested to evaluate the transition temperature is more precise and universal than the Abrahams-Jamieson-Kurtz criterion.     

Structural phase transitions in Cu2–xFeyS crystals

Crystals with Cu_2−xFe_yS compositions (x = 0.30, 0.25, 0.20, 0.15; y = 0.05) have been synthetized and obtained by the method of slow cooling. X-ray diffractometric studies have shown that at room temperature the above crystals consist of an orthorhombic phase (anilite), a monoclinic phase (djurlite), and their metastable high-temperature facecentered cubic phases (FCC)₁ and (FCC)₂, respectively. A crystallographic relation has been found between the lattices of the orthorhombic, monoclinic and (FCC)₁ phase. It is shown that all these phases transform in the final analysis to a single high-temperature fcc phase. All the transitions are reversible, i.e. in the course of inverse cooling to room temperature, the diffraction patterns are restored completely in reverse order.     

Effect of NaCl filler on ferroelectric phase and polaron configurations of PVDF films

Polyvinyldene fluoride (PVDF) films filled with NaCl of mass fraction range 1 ≤ W ≤ 6 % were prepared by casting technique. Their crystalline structure, thermal, optical properties and Electron spin resonance (ESR) were examined. X‐ray diffraction (XRD) and differential thermal analysis (DTA) measurements indicated a maximum ferroelectric β‐phase increment at 4%. DTA was used to identify the phase transition temperatures, the order of reaction and the activation energy of melting. The UV‐Visible optical absorption implied a minimum value of the estimated optical energy gap at W = 4%. ESR spectra contained a Lorentizian signal exhibiting a minimum value of the symmetry factor at W = 4%. The energy levels of the optical gap boundaries were though to contribute to ESR transitions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

Structural phase transitions in the LaMnO3+λ system

Structural phase transitions in the LaMnO_3+λ system are studied at various temperatures and with different Mn⁴⁺ ion concentrations. Two structural phase transitions are established. The O′-orthorhombic – O-orthorhombic transition (O′ O transition) is due to removing the cooperative Jahn-Teller distortions and occurs in the Mn⁴⁺ concentration range from 10 to 14% at room temperature. The temperature of this transition varies from 710 K for the stoichiometric LaMnO₃ to room temperature for LaMnO_3.07. The O-orthorhombic – rhombohedral transition (O R transition) is due to the change of the rotation axis of the undistorted MnO₆ octahedra from [100] and [110] to [11 1]. The temperature of this transition varies from 1010 K for LaMnO₃ to 230 K for LaMnO_3.135. The structural phase diagram of temperature vs composition is plotted, indicating the existence of regions of the phase with different structures.     

Epitaxial growth of ferroelectric oxide films

This review paper begins with a brief overview of the most common ferroelectric materials, the perovskites and the Aurivillius families. The epitaxial growth of ferroelectric epitaxial films can be a viable approach to improve the ferroelectric properties, in particular for the layered perovskites. Defects related to a polycrystalline structure, which lead to a degradation of ferroelectric properties like remanent polarisation, piezoelectric coefficient, charge retention, and may cause time-dependent fatigue problems, can be prevented. However, it is also to be considered that effects connected with thin films like substrate clamping, strain or finite thickness may limit the film properties. Substitution of elements allows the adjustment of the film characteristics to the device function. Additionally, the orientation of the films can be controlled by the appropriate choice of the substrate, which is important due to the anisotropy of the ferroelectrics.The deposition methods commonly used for ferroelectric oxide layers are reviewed, particularly with regard to epitaxial growth. The conditions under which stoichiometric, crystalline growth can be obtained are described. The paper primarily focuses on the MO-CVD technique.Furthermore whether epitaxial growth of ferroelectric films occurs or not depends on several conditions like lattice mismatch between film and substrate, surface orientation and crystal symmetry of the substrate, thermal expansion of film and substrate. The influence of these parameters on epitaxial growth is discussed. Local epitaxial growth of ferroelectric layers on metallic electrodes is also mentioned due to its importance in device fabrication. The site-engineering concept is shortly reviewed as the substitution of elements constitutes a simple way to modify film properties in thin film technology.     

The heteroepitaxial growth of vacuum-deposited GaP thin films on Si substrates

Heteroepitaxial GaP thin films were grown on (100) and (111)Si substrates by vacuum evaporation of the compound GaP and their structural characteristics were compared with homoepitaxial GaP films grown by the same technique on (100)GaP substrates. Nearly monocrystalline GaP thin films were deposited reproducibly on (100)Si substrates at the optimum substrate temperature of about 680–700°C and the deposition rate of 0.1 nms⁻¹. The structure of such films was comparable in quality to homoepitaxial GaP/GaP films. No monocrystalline films could be deposited on (111)Si substrates.     

Improper ferromagnetic phase transitions in crystals

A complete set of possible improper phase transitions leading to polarization and magnetization of crystals is found from tensor bases of representations of the black-and-white point group of crystal symmetry by using the multiplication table of irreducible representations. The dependences of the secondary order parameter on the primary order parameter are presented for different classes of the symmetry group.     

Laser-induced phase transitions in PbI2

Pure crystals of PbI₂ have been grown using zone-refining system. The structural phase transition 12 R → 2 H has been observed when exposed to ruby laser light. The results have been explained on the basis of structural considerations of PbI₂ and compared with the earlier existing findings in literature.     

Polymorphism and phase transitions in TlS

The conditions for synthesis and the range of existence of the monoclinic TlS modification in the Tl-S system have been determined by means of differential thermal, X-ray diffraction, and microstructural analyses. It was shown that the monoclinic phase is a structural analogue of TlGaSe₂. Possible variants of formation of structural sheets and the nature of phase transitions in the monoclinic TlS modification are discussed.     

Specific features of dielectric response in PZT ferroelectric films

Crystallography Reports - Reverse dependences of the permittivity ε′(E =) of Pb(Ti0.45Zr0.53W0.01Cd0.01)O3 ferroelectric films prepared by high-frequency magnetron sputtering on a...     

The change of the lattice constant in heteroepitaxial silver films with increase of thickness

Deposition of silver on NaCl, KCl, and LiF with 20 Å · s⁻¹ in vacuum (5 · 10⁻⁶ torr) and electronographical determination of changes of lattice constant. Precision of measurement better than ± 0.004 Å (reflections {400} and {220}). Thickness of films from 50 to 500 Å with intervals of 10 Å. The lattice parameter of films till to 115 Å in thickness did'nt differ from the parameter of bulk silver (4.086 Å). At about 175 Å the parameter reached minimum (about 4.00 Å) and took again the parameter of bulk material with the film thickness of 215 Å. With polycrystals no changes of lattice parameters occurred. – The phenomenon is connected to growth mechanism, two-dimensional and three-dimensional nucleation and to the differences in lattice parameters of substrate and deposit.     

Synchrotron X-ray diffraction studies of heteroepitaxial ZnO films grown by pulsed laser deposition

Heteroepitaxial ZnO films were grown by pulsed laser deposition on various substrates such as GaN-buffered C-Al₂O₃, C-Al₂O₃, A-Al₂O₃, and R-Al₂O₃. The epitaxy nature of the films was investigated mainly by synchrotron X-ray diffraction. The results showed that the GaN interlayer plays a positive role in growing an unstrained, well-aligned epitaxial ZnO film on the basal plane of Al₂O₃. Importantly, the ZnO film grown on R-Al₂O₃ has two differently aligned domains. The dominant (1 1 0) oriented domain has much better alignment in the in-plane direction than the minor portion of (0 0 1) oriented domain, while in the out-of-plane direction the two domains have almost the same mosaic distribution.     

Phase transitions and orientational order in thin oxygen films

The structure of condensed oxygen films has been studied by the transmission electron diffraction method. Specimens were prepared directly in situ by depositing 99.99% pure gas on amorphous carbon and polycrystalline Al substrates. Measurements were made in the temperature range between 5 K and the sublimation point. It was established that the as-grown oxygen films had a short-lived α′ structure, which was transformed into an α-phase or a β modification depending on the temperature range. The temperature behavior of the structural characteristics of oxygen deposits has been investigated. An unusual character of the α–β transition was manifested in the appearance of α′ reflections on the diffraction pattern during the transformation. A few possible mechanisms of the transition are discussed in the paper. The influence of quenching conditions on crystals’ growth has also been investigated. The new conditions for the formation of a magnetically distorted α′-phase were established. An unknown phase of solid oxygen was observed in samples deposited under certain conditions. One of its possible origins is an altered orientational arrangement of molecules in closely packed layers. The conditions required for the disordered phase of solid oxygen to form have been determined. Amorphous oxygen was also observed as a result of rapid cooling of crystalline samples.     

Metallic phase formation in oxide films

Metallic particles embedded in the oxide film play an important role in film’s optical property. In this paper it is demonstrated that metallic phases of Ag, Ti, and Pb, can be formed in different oxide films under heating, X-ray photon, and electron radiation. The metallic phase separated from metal oxide film was investigated by X-ray photoelectron spectroscopy technique. Metal oxygen bond breaking and total energy reduction in the film result in the formation of metal in the film. It is necessary to fully understand the formation mechanism of metallic particles so their shapes and distributions can be tailored to achieve the desired film’s properties.     

Energy band-gap in elastic-strained heteroepitaxial layers

The effect of internal elastic stress on energy band-gap shift in the A³B⁵ heteroepitaxial layers was investigated. The possibilities to get higher accuracy of divergent beam X-ray stress determination method, designed in the scanning electron microscope (SEM), was presented. X-ray topography method in the SEM with X-ray magnification from 2 up to 10 was for the first time described. The epilayer photoluminescence (PL) peak energy shift as a function of internal elastic stress was measured on the GaInP/GaAs, GAInAsP/InP heterostructures and the influence of different factors on the accuracy of the results was discussed.