KBr Films Research Articles

Orientation-controlled growth of molecular organic thin films

The control of the structure and preferential orientation of molecular organic thin films might be achieved by the selection of suitable substrates that, in general, have to be designed. Langmuir–Blodgett (LB) and self-assembled monolayers (SAMs) films are good candidates as substrates due to the chemical versatility of their surfaces. We illustrate an example of the selection of either of the two energetically most probable preferential orientations of polycrystalline neutral tetramethyltetraselenafulvalene (TMTSF) thin films by the choice of different substrates. The films (thickness ∼1 µm), obtained by sublimation of TMTSF microcrystals in high vacuum (∼10−6 mbar), are dominated by (0 −2 1)- and (100)-oriented microcrystals when using KBr (100) and methyl-terminated films, respectively.

Electron-stimulated desorption of thin epitaxial films of KBr grown on (100)InSb

We have measured time-of-flight (TOF) distributions of Br atoms desorbed from thin (less than 1000 Å) epitaxial films of KBr on (100) InSb with a 2 keV electron beam. Although the general structure of the TOF spectra was similar to that obtained previously for the thick crystals, both the fast and the slow (thermal) components of the distribution were strongly dependent on the film thickness. We argue that this dependence is due to two different diffusion processes involved in the transport of the primary excitation products from the bulk to the surface. By measuring the velocity resolved ESD yield for films of various thicknesses, we determined that a diffusion length of the carriers responsible for the thermal ESD component varied from 30 to 700 Å with temperature in the range 20–300°C. In contrast, for the non-thermal desorption we found the carrier diffusion length of about 140 Å which did not depend significantly on the temperature.

CU+ high doping effects in KCl and KBr films

Highly Cu + doped KCl and KBr polycrystalline films were investigated in the range of 10 20 -10 21 cm -3 Cu + concentration. Several techniques were used to obtain the optical and structural properties, as optical absorption and transmittance, electronic microscopy (SEM), X-ray diffraction and ellipsometry. The Cu + off-center effect is still present in the films, as determined by optical and thermally stimulated depolarization current measurements (TSDC). The films were obtained by thermal evaporation, and the corresponding doping Cu-halide is introduced in the same evaporating crucible. Samples with diverse concentrations, ranging from nominal 1 to 10% mole, were prepared. The increase of Cu + concentration results in an improved cyrstallinity of the grains, with higher density and a better transmittance of the film above 350 nm. Optical application as UV filter device can be considered.

Target current spectroscopy of the alkali halides KCl, CsCl and KBr

Thin films of KCl, CsCl and KBr deposited on Si(111) were investigated by means of target current spectroscopy (TCS). From the TCS initial peak position the surface potential changes during deposition were obtained. From the surface potential dependence on the alkali halide film thickness the critical island size in the initial stage of the film growth was estimated: for CsCl, 10 Å; for KCl, 18 Å; for KBr, 25 Å. In the target current spectra of the alkali halides an intensive structure in the energy region corresponding with the bandgap was discovered. One group of peaks was induced by electron irradiation and vanished at high temperatures. This fine structure was considered to be due to an electronic transition related to the defect states in the bandgap. From the evolution of the TCS maximum intensities the defect concentration changes caused by electron irradiation were estimated.

VUV Photoelectric Yield Spectra in Thin Films of KCl, KBr and RbCl at 80 K

Relative photoelectric yield spectra of thin films in KCl, KBr and RbCl are investigated in the photon energy 7 eV–40 eV range at 80 K as well as at room temperature. Several new characteristics of the spectra due to the thin film specimens and due to the lower temperature are observed. It is revealed that the yield spectra dominantly reflect the photo-absorptions modified by the inelastic electron-electron scattering in the valence excitation region and by the Auger process especially in the core excitation region. A large yield peak due to the next deeper core level K + 3 s or Rb + 4 s is newly identified in a photon energy range of 30–40 eV. Interband thresholds of the yield spectra are observed and the electron affinities are derived.

Surface dynamics of epitaxially grown potassium bromide overlayers on a sodium chloride substrate

The results of high-resolution helium atom scattering experiments on the dynamics of thin epitaxially grown films of KBr on NaCl(001) are presented and interpreted with a simple force constant model. The measurements reported are for two, three, four, and seven monolayers. The model treats the KBr overlayer with force constants obtained by fitting the bulk dispersion curves, the NaCl surface as a rigid wall, and the coupling between the two by a second set of force constants. The latter are determined by the fit to these data and are softer than the KBr force constants

Surface Defects of KBr Produced by Electron Irradiation

Low energy electron loss spectra (LEELS) of KBr single crystals and films were measured at several temperatures from room(RT) to liquid nitrogen temperatures(LNT). For single crystals, four loss peaks at 2.05( A ), 1.65( B ), 1.30( C ) and 1.00( D )eV were observed at -140°C and at 2.05( A ), 1.65( B ) and 0.5( E )eV at LNT in the band gap region where no loss peaks were observed at RT. Among them, the loss peak at 2.05 eV is ascribed to the bulk F -centers, and other three peaks are to defects at crystal surfaces produced by electron irradiation. For evaporated films, only three lower loss peaks, B , C and D were observed. Because evaporated films are polycrystaline and have large surface area, it is thought that LEELS show only loss peaks due to surface defects.

A study of high-temperature phonon anharmonicity in KBr films by the FT-IR emission technique

Polarised far-infrared spectral emission from KBr films is observed around the restrahlung band at an emission angle of 45 degrees . The spectra observed at elevated temperatures show an abnormal line shape around the longitudinal optical (LO) mode frequency. In order to discuss the effect of anharmonic phonon interaction on the spectra, these are analysed using a dielectric response function in which the empirical phonon self-energy function is considered. The results suggest that the shape of the damping function changes with temperature as well as with an increase of magnitude, which implies that the higher-order contribution to the self-energies becomes important at high temperatures as well as the lowest orders. The re-normalised quasi-harmonic LO mode frequency remains fairly constant close to the harmonic mode frequency for all temperatures. More than 10% of deviation from the Lyddane-Sachs-Teller relation is obtained at high temperatures.

マイカ上に蒸着されたＫＢｒ（１１１）面におけるＴｅ蒸着膜の成長

The growth of Te thin films deposited on thin evaporated films of KBr which were deposited on air-cleaved muscovite mica at residual gas pressures in the about 10-3 Pa range have been studied by transmission electron microscopy and reflection electron diffraction methods. The KBr films on mica at 200-250°C are entirely (111) oriented monocrystals which include some twins. The surfaces of KBr films are not smooth, and consisted of triangle-cone facets, square-plate-like facets and corrugated facet structures. Tellurium deposited on the KBr films has grown with the c-axis parallel to the ‹110› direction on the {100} face of each facets. This mechanism is similar to that of Te deposited on the cleaved KBr (001) surface. The points of intersection of each facets give rise to maximum nucleation densities of Te particles.

Electroluminescence in KBr and KI crystals and films at room temperature

Electroluminescence is observed at room temperature in KBr and KI crystals and films under alternating electric fields above 104 V/cm. The emission spectra show broad and unresolved bands in the visible region. The brightness waves and the brightness-voltage relation are studied and possible excitation models are examined.

An X-Ray Diffractometric Study of Hygroscopic Process of Alkali Halides

Surface structure changes of alkali halides crystals on adsorbing water vapor molecules in multilayer have been studied by means of X-ray powder diffraction. Effects of particle size and some surface properties of the salts on the intensities of X-ray diffraction by the concerned samples were examined. Three salts, KI, KBr and KC1, were adopted as samples, and their evaporated films, fine crystals and powders ground to 200 mesh were prepared and used. The multilayer adsorption of water molecules on the salt results in a formation of the hydrated ions. At that time it is found that both the su-rface state and the X-ray diffraction intensity change and caking of the salt appears. The intensity of X-ray diffraction by the salt with low crystallinity, such as evaporated films of KI and KBr, increased above the vapor pressures of water molecules forming the hydrated ions. This fact might be attributed to a rearrangement of the surface ions. On the contrary when the salt had a high crystallinity, the depression of the diffraction intensity appeared in the vapor pressure range. The value of the vapor pressure at which adsorbed water molecules act to form the hydrated ions was peculiar to the material. Their values are 40, 51 and 55%rH for KI, KBr and KC1, respectively. From the above results it may be expected that X-ray diffractometry is an effective method for detecting formation of hydrated ions on a salt and for studying its caking-process.

Thin film emission of KBr, RbCl, RbBr and NaCl in the far infra-red

The emission coefficient E of thin films (2–9μ) of KBr, RbCl, RbBr and NaCl has been measured between 50 and 250 cm −1. An expression for E is given. Values of ω 0 and Г for the TO oscillator between 350°K and 800°K have been derived from the spectra.

Intrinsic Photoconductivity in the Alkali Halides

Photoconductivity in thin films of RbI, KBr, and KCl has been measured at temperatures of 10 and 80\ifmmode^\circ\else\textdegree\fi{}K in the spectral range from 5.5 to 10 eV. The Schubweg of the photoelectrons and the yield of photoelectrons per incident quantum were determined. In the case of RbI the data tend to confirm the existence of exciton states lying between the two components of the familiar spin-orbit doublet, as was recently suggested by Fischer and Hilsch. It is estimated that the band gaps of RbI, KBr, and KCl are 6.37, 8.0, and 8.7 eV, respectively.

Fine Structure in the Exciton Bands of the Alkali Halides

This paper presents new data on the optical absorption spectra of evaporated films of KI, NaI, RbI, NaBr, and KBr in the region between 1700 A and 2500 A. The measurements were made at room temperature and ---180\ifmmode^\circ\else\textdegree\fi{}C. The absorption spectra of the iodides reveal hitherto unreported structure in the fundamental-band region of the salts.