KCl Films Research Articles

Interface structure of alkali-halide heteroepitaxial films studied by x-ray-absorption fine structure

We have investigated the interface structures of KCl and NaCl thin films grown on NaBr(100) by means of Cl K-edge x-ray-absorption fine structure. The x-ray-absorption near-edge structure of 1-ML-thick KCl on NaBr indicates that the mixed crystal does not form at the interface. Through the analysis of extended x-ray-absorption fine structure, it was revealed that with a decrease in film thickness, the K-Cl bond distance is slightly shortened in KCl/NaBr(100), while the Na-Cl bond is elongated in NaCl/NaBr(100). The 1-ML-thick film does not, however, form coherent bonds with the substrate. This implies that the in-plane bond distance of the film does not coincide with that of the substrate but is close to the bond distance of the bulk material. For further understanding of the bond character between the film and substrate, Monte Carlo calculations have been carried out for the KCl/KBr system which gave a similar conclusion in our previous study. The calculated results were found to agree with the experimental results reasonably. These findings are contradictory with the previous diffraction results that have suggested the formation of coherent bonding at the interface. Such a discrepancy implies that the short-range order (bond distance) does not always reflect the long-range order (lattice constant).

Defects in epitaxial insulating thin films

Defects on thin epitaxial insulator films of NaCl(100), KCl(100), and MgO(100) generated during growth and by electron bombardment are investigated by high-resolution spot profile analysis in low-energy electron diffraction (SPALEED), photoelectron spectroscopy (XPS, UPS), electron energy-loss spectroscopy (EELS), and thermal desorption spectroscopy (TDS). All three insulators contain morphological defects: NaCl overgrows the monatomic Ge(100) steps in a carpet-like mode, whereas KCl grown on NaCl(100) forms a regular array of stacking faults up to three monolayers of KCl, and the MgO film grown on Ag(100) reveals a broadened (1 × 1) pattern in LEED due to the formation of mosaics. In EELS, surface colour centres, produced by electron bombardment, on NaCl induce losses at 2.1 eV for FS centres and at 1.5 eV for MS centres and on KCl induce losses at 1.6 eV and 1.0 eV, respectively. Close to room temperature, high electron exposures result in additional losses in the band gap due to surface and bulk plasmons of Na and K clusters. A similar anion vacancy defect structure on MgO with losses at 2.1 eV and 3.3 eV can be produced by incomplete desorption of metallic Mg from the MgO(100) surface. TDS experiments show that colour centres increase the binding energy of Kr physisorbed on NaCl(100) by about 25%. The concentration of point defects was determined by titration with noble gases in thermal desorption.

Heteroepitaxial growth of KCl on a cleaved (001) face of KBr studied by extended X-ray-absorption fine structure

Thickness dependence of the local structure of epitaxially grown KCl films on KBr(001) was investigated by Cl K-edge EXAFS measurements at 150 K. The first-nearest neighbor KCl distance parallel to the surface was found to be elongated gradually with a decrease in the film thickness: 3.13, 3.16 and 3.17 Å for the films with KCl thicknesses of 25, 6 and 2 Å, respectively. The first one of 3.13 Å for the 25 Å film corresponds exactly to that of KCl bulk crystal. The present findings are contradictory to the previous diffraction results of the formation of coherent bonding at the KCl/KBr interface, which indicated that the KCl distance should be the bulk KBr distance of 3.28 Å. The present study revealed that even a very thin film is likely to maintain the natural KCl distance of bulk KCl, as in the case of solid solutions. The 6 Å film shows an anisotropy of the KCl distance (3.16 and 3.13 Å for the in-plane and out-of-plane distances, respectively), this resulting in the face-centered tetragonal structure of the KCl film.

Melt-quenched KCl:Eu2+ films for photostimulated luminescence applications

Using a conventional melt-quenching method, Eu2+-doped KCl films have been prepared. These films yield intense photostimulated luminescence (PSL) with the stimulated excitation maximum shifting to longer wavelengths as compared to the reports on the single-crystal analog. This will be advantageous for image reading by a He–Ne laser or red-emitting diode lasers when used in the PSL applications. Several types of centers have been found. Further, in the electron paramagnetic resonance spectrum, the hyperfine structure due to the Eu2+ centers has been observed to disappear at low temperatures (∼5 K). This can be attributed to the formation of complexes by the trapping of defect centers at the Eu2+ site(s). Crystallization effects and paramagnetic centers as well as optical properties of these films are discussed.

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.

F-aggregate centres in KCl films

Abstract The optical behaviour of colour centres is studied in thin films of KCl obtained by thermal evaporation onto amorphous substrates. Deposition conditions are optimized in order to minimize chemical and thermal effects on films. Concentration of colour centres even higher than in bulk crystals can be induced by low energy electron irradiation. Thermal bleaching of F centres is observed at room temperature and its kinetics, deduced from absorption measurements, consists of two exponential processes with very different time constants. Luminescence is obtained for the first time from aggregate colour centres in KCl films, and typical emissions of F2, F3, F4 defects (but not of F centres because of concentration quenching) are revealed.

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.

Optical and structural characterizations of Cu +-doped KCl films

The production of highly Cu +-doped KCl films and the properties of their 266 nm absorption band, which has an off-center property in doped single crystals, open the possibility of application of these films as ultra-violet optical filters. The investigated films, of approximately 1 μm thickness, were prepared by resistive co-evaporation of KCl and CuCl powders on different substrates of CaF 2, Al 2O 3, SiO 2, KCl and Si. The Cu + concentration, as determined by energy-dispersive X-ray, ranges from 10 20 to 10 21 cm −3, for 1–15% CuCl nominal mole percent concentration. Structural and optical properties were investigated through scanning electron microscopy, X-ray diffraction, ellipsometry, optical absorption and transmission. The films are polycrystalline, and the gain size decreases with increasing Cu + impurity concentration, yielding an increase of visible transmission to a limited CuCl concentration. These films show a 6.295 Å lattice parameter with a f.c.c. structure and an index of refraction of 1.53 at 266 nm. When the Cu + concentration is increased, the UV band position remains the same and no clusters are evidenced even to the high 15% CuCl concentration investigated, which differs very much from single crystals samples grown by the Kyropoulos-Czochralski method. For a Cu + concentration of 8×10 20 cm −3 the film shows a transmission better than 88% at 350 nm wavelength.

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.

Crystalline properties and colour centres in alkali halide thin films

Abstract Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage.

Transient and AC electrical conductivity of potassium chloride and potassium bromide thin films

The transient and AC electrical conduction in KCl and KBr thin films have been studied at room temperature between 0.4 and 2.0 V. The time in the transient study covered the range 10-5 s to 104 s following the application of a step voltage. The frequency range in the AC study was 1 Hz to 200 kHz. From the frequency dependence of the AC conductivity and the application of Fourier transforms a transient current is calculated numerically, which differs from the experimentally determined transient current. Taking into consideration the experimental fact that the systems studied respond linearly to harmonically varying voltages, the observed discrepancies between AC and transient electrical conductivities are attributed to different mechanisms. Specifically the AC conductivity is attributed to hopping anions, whereas the transient conductivity is due not only to hopping anions but also to the release of trapped electrons by overcoming the barrier of the trap (Poole-Frenkel effect) alone or assisted by tunnelling through the barrier.

Heteroepitaxial growth of KCl and LiF on a cleaved (001) face of KBr

Abstract An initial stage of heteroepitaxial growth of KCl and LiF on a KBr substrate was studied using RHEED, EELS and AES. Both KCl and LiF films grow heteroepitaxially on a KBr substrate with their crystallographic axes parallel to that of substrate. In the case of KCl growth occurs in a well ordered layer-by-layer fashion and it is found that thickness of an intermediate region between KCl and KBr is less than 2 nm. In contrast three-dimensional island growth occurs in the case of LiF. Differences in lattice mismatch and solid solubility seem to determine the mechanism of heteroepitaxy of KCl and LiF on a KBr substrate.

Heteroepitaxial growth of KCl and LiF on a cleaved (001) face of KBr

An initial stage of heteroepitaxial growth of KCl and LiF on a KBr substrate was studied using RHEED, EELS and AES. Both KCl and LiF films grow heteroepitaxially on a KBr substrate with their crystallographic axes parallel to that of substrate. In the case of KCl growth occurs in a well ordered layer-by-layer fashion and it is found that thickness of an intermediate region between KCl and KBr is less than 2 nm. In contrast three-dimensional island growth occurs in the case of LiF. Differences in lattice mismatch and solid solubility seem to determine the mechanism of heteroepitaxy of KCl and LiF on a KBr substrate.

Surface states on epitaxial thin films of NaCl and KCl

Thin epitaxial layers of NaCl and KC1 have been grown on clean Ge(100) in UHV and investigated in situ. It is shown that good quality films are produced with thicknesses up to 5 nm with no observable charging up. The electronic structure is measured both by UPS and EELS. On clean surfaces the well known bulk transitions are reproduced. Surface states within the forbidden gaps are indentified after water adsorption with a filled band at 3.4 and 2.2 eV above the valence band edge for NaCl and KC1 respectively. With EELS also an empty surface state band is found.

Surface states on epitaxial thin films of NaCl and KCl

Abstract Thin epitaxial layers of NaCl and KC1 have been grown on clean Ge(100) in UHV and investigated in situ. It is shown that good quality films are produced with thicknesses up to 5 nm with no observable charging up. The electronic structure is measured both by UPS and EELS. On clean surfaces the well known bulk transitions are reproduced. Surface states within the forbidden gaps are indentified after water adsorption with a filled band at 3.4 and 2.2 eV above the valence band edge for NaCl and KC1 respectively. With EELS also an empty surface state band is found.

Attenuated total reflection at a metal-salt interface

The attenuated total reflection (ATR) technique has been used to follow the reaction of films of Ag with films of KCl in air at room temperature. An intermediate mixed layer develops between salt and metal as metal diffuses into salt. Apparent growth rates are as low as 0.2 nm/h. The experimental results indicate that water plays a crucial role in such a reaction. The ATR technique is shown to be very sensitive to processes taking place at the interface.

CO adsorption at 77 K on KCl films. An infrared investigation

KCl films have been prepared by vapour deposition in vacuo on a cold (77 K) NaCl plate and then sintered at room temperature for 12 h. The electron micrographs of the sintered film show that the crystallites have cubic shape and prevalently expose low-index {100} faces. The i.r. spectra of CO adsorbed at 77 K on both unsintered (high surface area) and sintered (low surface area) films can be interpreted in terms of CO species adsorbed on edges and steps and species adsorbed on flat portions of extended faces. The molecules adsorbed on the flat parts of {100} faces form a parallel array of oscillators localized at the cation sites, which are mutually interacting via dynamic and static mechanisms. The dynamic and static effects have been estimated by using isotopic 13CO–12CO mixtures. The dynamic polarizability of CO adsorbed on {100} faces has also been calculated.

Infrared laser stimulated surface processes

Gas-surface interactions including the process of chemisorption, the reaction involving the adsorbate and the adsorbent, the formation of reaction products and the desorption of adsorbed species enhanced by infrared laser radiation have been investigated. Specifically, SF 6 and SiF 4 interactions with silicon affected by CO 2 laser pulses, and the infrared photon stimulated desorption of C 5H 5N and C 5D 5N molecules from KCl, Ag(110) and Ag island films have been studied to illustrate the many facets of the vibrationally activated surface processes. Aspects related to surface reaction dynamics and molecular selectivity in photodesorption are discussed.

Molecular dynamics computer simulation of thin films of LiCl, NaCl, KCl and RbCl possessing (100) crystal–vacuum surfaces

Equilibrium and dynamic properties of the (100) crystal–vacuum interfaces of LiCl, NaCl, KCl and RbCl have been investigated by the method of molecular dynamics computer simulation using model interionic potentials and for various temperatures between 298 K and the normal crystal melting points. The use of free laminae models as against a static lattice boundary condition is discussed. In either case at least 10 ionic layers are required in the simulations. Using a simple Born–Mayer–Huggins pair potential there was considerable expansion of the layer separations near the interface and significant buckling occurred for the surface layer of LiCl and of NaCl. The inclusion of polarisation forces using a modified Tosi–Doyama potential in the case of NaCl resulted in a slight contraction of the surface interlayer spacings.

Injection Mechanism for Transmission Secondary Emission from Porous KCl Films

The energy distribution of transmission secondary electrons emitted from a porous KCl film much thinner than a practical one was measured by means of the ac-retarding potential method. The energy distribution curve exhibited a subpeak near the potential of the Al substrate. For higher external fields the characteristic sharp peak corresponding to the tunnel emission from the Al film was found. The electron injection current from the Al film as a function of the field was calculated by a model based on the well-known field emission theory. The result showed that the injection of electrons was caused mainly by the tunnel emission of metal electrons in the case of practical dynodes. It is concluded that the work function of a metal film plays an important role in the characteristics of the transmission secondary emission from porous KCl films.