Surface Review and Letters

La_0.67Ca_0.33MnO_{3 - δ} thin films were deposited using a high-pressure dc-sputtering process. Pure oxygen at a pressure of 3.8 mbar was used as sputtering gas. The films were grown on (001) LaAlO₃ and (001) SrTiO₃ substrates at heater temperature of 850° without any annealing treatment. The formation of highly a-axis-oriented films with sharp interface with substrate surface is demonstrated by X-ray diffraction, transmission electron microscope (TEM), and atomic force microscope (AFM) analysis. Electrical characterization revealed a metal–insulator transition at T_MI = 276 K, and magnetic characterization showed good magnetic properties with a PM–FM transition at T_C ≈ 262 K.

This work presents results concerning the development of n⁺-ZnO/i-ZnO and n⁺-ZnO/ZnSe bilayer structures with adequate properties to be used as optical windows in solar cells based on CuInSe₂ thin films. The optical and morphological properties of each of the layers constituting the bilayer structures were studied through spectral transmittance and atomic force microscopy (AFM) measurements. The studies revealed that windows of this type allow a significant increase of the spectral range of absorbed radiation by the solar cell in comparison with those using ZnO/CdS as optical window.

Coherent optical effects coming from two excited levels in an engineered asymmetric double quantum well have been detected with duration of picoseconds.¹ This short decoherence time is related with the dephasing time. Although dephasing is mainly due to electron–electron scattering and LO phonons in polar semiconductor provide the dominant inelastic scattering mechanism, a study of the elastic scattering with phonons during the coherent stage is also needed to understand the coherent electron dynamics. In this work is presented a microscopic study of the elastic scattering rates due to electron–LO-phonon interaction, which are obtained for each of the three conduction subbands on a tailored double asymmetric quantum well by using the imaginary part of the polaron self-energy within a many-body formalism. Both absorption and emission rates are calculated. The dephasing times for different electronic states in a given heterostructure can be very different, depending on the material, geometry, temperature, carrier density, etc. As an important geometric parameter is chosen the barrier width, which tunes the energy differences between the two upper levels or tunneling times. Electron density effect is studied through screening. Two limits are presented — unscreened and static screening. It is found that after an ultrashort pulse the absorption and emission scattering rates in each level are very different from the scattering rates in equilibrium and that screening effects can be used as coherent control mechanism.

The spectral variation of the effective dielectric constant in porous silicon thin films is modeled using effective medium theories. The pores in the material are supposed to be spheroid prolate inclusions in a homogeneous matrix. We employ modified Maxwell–Garnett and Bruggeman expressions obtained from previous work. Fitting to experimental reflectance spectra and the refractive index at the visible spectral range is satisfactory. Best results in both models are obtained using the spheroidal inclusions of 0.9 eccentricity. The best adjusted porosity values for two samples with different parameters of preparation are 47% with Bruggeman's expression and 67% with Maxwell–Garnett's expression, which are very close to the experimental ones of 44% and 67% respectively, obtained through gravimetric methods. Our model is useful for determining and for checking the influence of the preparation parameters on the optical and structural properties.

Electrical characteristics of high quality aluminum oxide thin films deposited by the spray pyrolysis technique on GaAs substrates are reported. The films were deposited using a spraying solution of aluminum acetylacetonate in N,N-dimethylformamide and an ultrasonic mist generator. The substrates were (100) GaAs wafers Si-doped (10¹⁸cm^-3). The substrate temperature during deposition was in the range of 300–600°C. The electrical characteristics of these films were determined by capacitance and current versus voltage measurements by the incorporation of these films into metal-oxide-semiconductor structures. The interface state density resulted in the order of 10¹² 1/eV-cm² and the films can stand electric fields higher than 5 MV/cm, without observing a destructive dielectric breakdown. The refractive index, measured by ellipsometry at 633 nm, resulted close to 1.64. The determination of the chemical composition of the films was achieved by energy dispersive X-ray spectroscopy; it resulted close to that of stoichiometric aluminum oxide (O/Al = 1.5) when films are deposited at substrate temperatures of 300–350°C.

First principles total energy calculations are performed to investigate the Si(001)-(3 × 4) reconstruction induced by the adsorption of group III metals (Ga, Al and In) when deposited at high temperatures. We have considered different models in our study and we have found that the pyramid-like structure proposed by Bunk et al. for In on Si(001) yields the most stable atomic configuration in all cases. We present a detailed description of this surface reconstruction, and compare the structural parameters for Al, Ga and In.

Tellurium-doped GaInArSb epitaxial layers with electron concentration in the range of 3 × 10¹⁷ – 2 × 10²⁰cm^-3 are grown at 530°C on (100) GaSb substares by liquid phase epitaxy (LPE). To dope the layers we used pellets of Sb₃Te₂ in preparing growth melts. The low temperature photoluminescence (PL) spectra (20 K) showed a dominant peak composed of three transitions associated to excitons bound to residual acceptor impurities. For highly Te-doped layers the excitonic transitions related to exciton bound to neutral acceptor, BE₂, disappears.

We calculate the states of an exciton formed by an electron confined in a 1D quantum wire (QW) and a hole confined in a 2D quantum layer (QL) perpendicular to the QW. The effective potential can be exactly calculated in the particular case where the confinement thicknesses are the same for the QW and QL. This effective potential can be used to calculate the exciton states for arbitrary confinement thicknesses, but here we show that in the limit of vanishing thickness the solution of our equation is very similar to the 3D bulk exciton equations, and therefore we can use these 3D exciton solutions as a basis for our system in a quantum perturbative approach.

Photoluminescence and optical absorption spectra from silicon nanoparticles embedded in SiO₂ are presented. The material was synthesized by RF cosputtering. The samples were prepared with a variety of Si volume concentrations and annealing temperatures. The results are discussed by correlating them with the preparation conditions, the nanostructure, and comparing them with the results obtained for porous silicon. The conductivity, photoconductivity and minority carriers diffusion length were also obtained for samples with different Si content and average size of the nanostructure. The study gives information about the percolation threshold and the electrical transport mechanisms in the composite. Eu- and Er-doped samples were finally prepared and studied for their PL response. A clear correlation between the enhancement of the Er⁺³ infrared emission and the presence of the silicon nanostructure is shown. In the case of Eu-doped samples, emissions from Eu⁺³ or Eu⁺² are observed depending on the characteristics of the host (when exciting with 514.5 nm laser radiation). Degenerate four-wave mixing signals suggest a strong interaction between the free carriers of the silicon nanostructure and the Eu ions that can promote the emission from Eu⁺² under illumination.

By means of pulsed laser deposition we prepared Co–Ni/p-Si thin films upon a Si(100) substrates. Samples were thermally treated in vacuum in order to promote silicide formation. From X-ray photo-electron spectroscopy (XPS) analysis, we detected chemical shifts of the Co2p and Ni2p transitions, characteristic of silicide binding energy, at the respective ranges of 778.3–778.6 and 853.2–853.6 eV. By means of high resolution transmission electron microscopy (HRTEM) we identified some nanocrystalline regions belonging to CoSi₂, Ni₂Si and NiSi₂ structures. We also appreciate that the resulting films are of a polycrystalline nature.

Ultrathin quantum wells (UTQWs) of CdTe within ZnTe barriers were successfully grown by atomic layer epitaxy (ALE) on GaAs(001) substrates. ALE growth of CdTe was performed by alternate exposure of the substrate surface to individual fluxes of Cd and Te. Two different samples with 2-monolayer (ML) (substrate temperature T_s= 270°C) and 4 ML (T_s = 290°C) CdTe QWs were grown. Low temperature photoluminescence (PL) experiments exhibited intense and sharp peaks associated to the 2 ML QWs at 2.26 eV. In the case of the nominally 4-ML-thick QW the PL spectrum presented an intense peak around 2.13 eV and two weak features around 2.04 and 1.91 eV. The first peak is attributed to ~ 3 ML QW and the second one to ~ 4 ML QW. The dominance of the 3 ML peak is mainly attributed to Cd loss in the QW due to its substitution by Zn atoms. Due to a high diffusion length of the photogenerated carriers in the barriers, quite weak signals from the ZnTe barriers were observed in both cases. Room temperature (RT) photoreflectance (PR) spectra showed contributions from the CdTe UTQWs, the ZnTe barriers, and the GaAs substrate.

CuInSe₂ thin films were deposited on soda lime glass substrates following a two-stage process which includes a chemical reaction between thin films of Cu and In_xSe_y, followed by thermal annealing in a Se environment. Initially, the Cu layer is deposited on the glass substrate by DC magnetron sputtering in the S-gun configuration, and subsequently the In_xSe_y layer is deposited by the closed spaced sublimation (CSS) method. The influence of the deposition method and of the main deposition parameters of the precursor layers (Cu and In_xSe_y) on the phases present in the resulting compound were studied by means of X-ray diffraction (XRD) and optical gap (E_g) measurements. The conditions for growing CuInSe₂ thin films in the chalcopyrite phase were determined through an exhaustive parameter study. The study revealed that the thickness of the precursor layers and the selenization conditions affect the phase in which the CuInSe₂ compound grows.

The purpose of this work is to investigate new materials which present good properties to be used as optical window and buffer layer in thin film solar cells based on CdTe and CuInSe₂. As potential candidates for optical windows, two different bilayer systems were studied: ZnO/In_xSe_y and ZnO/ZnSe. They are planned to be used later in the fabrication of solar cells with structures CuInSe₂/In_xSe_y/ZnO and ZnO/ZnSe/CdTe.

The methods used to prepare the different window materials, as well as the technological stages of the solar cell fabrication, will be described. Additionally, the optical and morphological properties of the window systems will be studied through transmittance and atomic force microscopy (AFM) measurements.

Preliminary results obtained with Cu(In,Ga)Se₂ solar cells, fabricated using ZnO/In_xSe_y as optical window, will be reported. They will be compared with those obtained using ZnO/CdS as optical window, which is regularly used in this type of cells.

CdTe thin films deposited by the CSS (close spaced sublimation) method, with adequate properties to be used as absorber layer in solar cells, were submitted to a chemical treatment in a saturated CdCl₂ solution, followed by thermal annealing in air at 400°C, in order to improve the electronic properties. The effect of chemical and thermal treatments on the morphological and crystallographic properties was studied through atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements.

The studies revealed that the CdTe grows in the cubic phase and that the postdeposition treatments affect the morphology as well as the crystallographic properties; the effect on the morphology is significantly stronger. Increase of the grain size and roughness was observed in samples treated chemically and thermally. On the other hand, no effects were identified on the crystalline structure as induced by the treatments, although recrystallization was observed after thermal annealing.

In_xSe_y thin films, grown on soda lime glass substrates in the In₆Se₇ phase by evaporation of the In₂Se₃ compound and in the In₂Se₃ phase by coevaporation of In and Se, were optically characterized through spectral transmittance measurements. A special procedure was developed to determine the thickness d and the optical constants (refractive index n, absorption coefficient α, and optical gap E_g). This includes experimental measurements of the spectral transmittance and the use of a model taking into account interference effects induced by internal reflections presented in the interfaces substrate/film and film/air and the fitting of the n vs. λ curve to the Cauchy equation (n = A + B/λ²). The optical constants of the In_xSe_y thin films were corrected comparing the experimental transmittance spectrum with the theoretical one.

The results revealed that the optical constants of the In_xSe_y films were significantly affected by the deposition method.

The purpose of this work is to find the conditions for fabricating SnO₂:F/CdS and ZnO/CdS bilayer structures with adequate properties to be used as optical windows in solar cells based on CdTe and CuInSe₂ thin films. In both types of structures the CdS layer is deposited by chemical bath deposition (CBD) and by the evaporation method; the SnO₂:F and ZnO layers are deposited by spray pyrolysis and reactive evaporation, respectively. The influence of the preparation methods on the optical and structural properties of each of the layers constituting the bilayer structures, was studied through spectral transmittance and X-ray diffraction (XRD) measurements.

The studies revealed that the spectral transmittance of the ZnO/CdS system is significantly greater than that of the SnO₂:F/CdS structure, indicating that the first type of structure is more suitable for using as optical window in solar cell fabrication.

Samples of Zn–22Al–2Cu (zinalco) alloys with and without a thin film of Y₂O₃ were submitted to corrosion by potentiodynamic polarization and analyzed by means of Tafel extrapolation. In addition, in-depth profiles by Ar ion sputtering and Auger electron spectroscopy were obtained. Samples with Y₂O₃-deposited film and subjected to potentiodynamic polarization present an improvement in corrosion resistance up to 75% in pH = 4.5 aerated water as compared to samples without the Y₂O₃ deposit. The variations in electrochemical parameters are explained by the spectroscopic in-depth profile measurements.

We suggest a new approach to theory of contact thermoelectric cooling (Peltier effect). The metal–metal, metal–n-type semiconductor, metal–p-type semiconductor, p–n junction contacts are analyzed. Both degenerate and nondegenerate electron and hole gases are considered. The role of recombination in the contact cooling effect is discussed for the first time.

The Al and Ge molar fraction of a few surface atomic layers in AlGe binary alloys was calculated using Auger electron spectroscopy (AES) and the evaluation of the Auger electron matrix factor or the matrix correction. The electron backscattering correction factor (R) and the inelastic mean path (IMFP) were taken into account to calculate the matrix correction. The IMFP was obtained from experimental optical data and elastic peak electron spectroscopy (EPES) measurements. The electron backscattering correction factor was calculated using Monte Carlo simulations. The main sources of the uncertainty of the Al and Ge molar fraction of a few surface atomic layers in AlGe binary alloys is the uncertainty of Al IMFP.

Sol–gel techniques are particularly useful for the formation of thin oxide films on various substrates, which can be important for many technological applications. In microelectronic technology these films can be used as passivation layers, diffusion and oxidation barriers, intermetal dielectrics (IMD) and so on. Sol–gel-derived oxides are obtained from tetraethoxysilane, which is used as precursor and a two-step process involving acid catalysts. Varying the composition of the sol–gel solution, optical and electrical properties of the resulted films can be changed. In the present work, HF and HNO₃ were used as catalyst in order to vary the electrical properties of the films. Oxides were deposited on Si substrates and MIS (metal–insulator–semiconductor) structures were fabricated. Electrical properties of the coatings were studied by the C–V (capacitance–voltage) method. Fourier transform infrared spectroscopy (FTIR) measurements ware employed to investigate the chemical bonding structure. The results show evidence of the existence of charge trapping in the TEOS/HNO₃ films when MIS capacitors are biased from accumulation to inversion condition. This fact can be applied in the semiconductor device technology.

We describe two new techniques for measuring the Hall effect in capped rare earth films during hydriding. In one, we simultaneously measure resistivity and the Hall coefficient for a rare earth film covered with four different thicknesses of Pd, recovering the charge transport quantities for both materials. In the second technique, we replace Pd with Mn as the covering layer. We will present results from both techniques.

Layers of 6 and 16 Cd–Te–Zn–Te periods were grown by atomic layer epitaxy (ALE) within ZnTe thin films. Different samples were grown at substrate temperatures of 260 and 290°C. Information about the kinetics of growth and surface reconstruction during the ALE growth of CdTe and ZnTe films, and Cd–Te–Zn–Te periods was obtained by means of reflection high-energy electron diffraction (RHEED) experiments and through the analysis of the temporal behavior of the intensities of several features of the RHEED patterns. The photoluminescence of the sample grown at 260°C presents two narrow and intense peaks corresponding to emission from quantum wells (QWs). However, the spectrum of the samples grown at 290°C does not show any feature associated with QWs, the spectrum resembling that of a ZnTe film. Cd replacement by Zn atoms explains the absence of the CdZnTe QWs at 290°C and a lower Cd content than expected at 260°C. The replacement of Cd atoms by Zn atoms in the CdTe surface was clearly demonstrated by Auger experiments.

Surface topography on epitaxial HoBa₂Cu₃O_7-δ (HBCO) and Bi₂Sr₂CaCu₂O_8+δ (BSCCO) thin films analysis by using the atomic force microscopy (AFM) technique was carried out. The films were deposited in situ on SrTiO₃ substrates with thicknesses ranging from 100 to 300 nm by a high-pressure sputtering process. Chemical etching with a nonaqueous solution of Br-ethanol was used to modify the surface of the samples. HBCO films showed spiral grains, while BSCCO samples exhibited a terraced growth. Etching with Br-ethanol has different effects on HBCO and BSCCO. In HBCO, etching produces clean surfaces with light changes on the surface morphology, whereas in BSCCO it conduces to strong changes in the structure and the roughness of the film surface.

Oxidation of carbon monoxide is one of the most extensively studied heterogeneous catalysis reactions, being important among other applications in automobile-emission control. Catalytic oxidation of carbon monoxide on platinum (111) surface was simulated by the Monte Carlo technique following an extended version of the model proposed by Ziff, Gulari and Barshad (ZGB). In the simulation, a simple square two-dimensional lattice of active sites replaces the surface of the catalyst. Finite reaction rates for (i) diffusion of the reactive species on the surface, (ii) reaction of a CO molecule with an oxygen atom in a nearest neighbor site, and (iii) desorption of unreacted CO molecules, have been taken into account. The produced CO₂ desorbs instantly. The average coverage of O, CO and the CO₂ production rate for a steady state configuration, as a function of the normalized CO partial pressure (P_CO), shows two kinetic phase transitions. In the ZGB model these transitions occur at P_CO ≈ 0.39 and P_CO ≈ 0.53. For 0.39 < P_CO < 0.53 a reactive (CO₂ production) steady state is found. Outside of the interval, the only steady state is a poisoned catalyst of pure CO or pure O. Our results show that finite reaction rates shift the values in which these phase transitions occur.

We report the results of a study of Al_0.065Ga_0.935Sb avalanche photodetectors grown on n-GaSb substrates. The devices have been fabricated from layers with the structure p-Al_0.13Ga_0.87Sb/n-Al_0.065Ga_0.935Sb:Te/n-GaSb (substrate) grown by liquid phase epitaxy (LPE) with a composition matched to detect light of 1.55 μm. The heterostructures were grown from Ga-rich solutions at 400°C. Just after their growth, the structures were subjected to baking processes inside the growth chamber. The baking time was varied and its influence on the breakdown voltages of the junctions was observed. Breakdown voltages up to 12 V, very low net donor concentration in the active layer (1E15 cm^-3), and avalanche multiplication factors of around 50 have been obtained. The carrier concentration was determined by the C–V method. Photoluminescence and X-ray diffraction measurements were carried out to investigate the properties of the LPE-grown ternary layers, to determine the band gap and to estimate the quality of epitaxial layers. The photoresponses of the detectors are also presented.

Local magnetic moments of Fe₁/Cr_N inclusions in an Fe matrix are calculated as a function of the Cr number of atoms N at zero temperature, and for N ≤ 64. We use a realistic spd-band Hamiltonian to determine their magnetic properties. The local magnetic moments μ(i) at the various cluster sites i are calculated self-consistently in the unrestricted Hartree–Fock approximation. The matrix Fe atoms couple always antiferromagnetically with the Cr atoms. The μ(i) of Cr atoms at the interface are enhanced by the presence of Fe atoms in their first nearest neighbor shell. In most cases the Fe moments close to the cluster are reduced. A remarkable interplay between the antiferromagnetism of Cr and the ferromagnetism of Fe is obtained.

The present work investigates the effects of the hydrostatic pressure and the external applied electric field on the binding energy for shallow donor impurities in GaAs–Ga_{1 - x}Al_xAs quantum wells. The effective mass approximation is used and a trial envelope wave function is adopted for the impurity carrier. For fixed well width and applied electric field, the binding energy of the shallow donor impurity is enhanced by increasing the external hydrostatic pressure, and for fixed well width and hydrostatic pressure, the binding energy decreases by increasing the external electric field.

Ultrathin films of (Bi–Pb)–Sr–Ca–Cu–O (2223) were produced by ex situ RF magnetron sputtering on MgO (100) substrates. Films with different thermal treatments and thickness varying between 30 nm and 300 nm were obtained and studied systematically. A structural characterization of these samples was carried out and correlated with their electrical properties and thickness.

The one-dimensional quantum Boltzmann equation in linear transport approximation was solved using the nonequilibrium Green's function method, and based on the lifetime approximation. As an example, the effect of interactions with electric fields were included, and the thermoelectric power coefficient (α) was evaluated, based on calculations of the statistical density current in a stationary regime over the nonequilibrium distribution function, and by the Green's function method. Results are compared in order to explore the advantages of each one, in evaluating the other kinetic transport coefficients.

Based on the solution of the quantum Boltzmann equation, in the linear transport approximation for an isotropic low-dimensional system, the thermoelectric power coefficient, Q, in a quantum well structure was determined. These calculations are based on variational methods, taking into account especial functions which include important parameters being varied at the time of evaluating the thermoelectric power, e.g. height and width barrier. Values of Q are carried out by using a computer code written in Mathematica, which allows one to depict them easily and in this way to compare different behaviors of Q against temperature according to the scattering mechanism (phonons).

Porous silicon layers were obtained by electrochemical etching on (111) plane surfaces of crystalline phosphorus-doped silicon in the presence of hydrofluoric acid. The photoluminescence of this kind of layers when illuminated with UV light is possibly explained by the quantum confinement model (QCM), which states the presence of nanometer-scale crystallites that enlarge the semiconductor band gap up to optical photon energies when the band-to-band recombination processes take place. In this study, the size determination of those proposed structures was performed by X-ray diffractiometry and by Raman spectroscopy. The obtained results suggest a consistency between the experimental work and the QCM.

Pulse measurements on the silicon/electrolyte interface have been used to study space-charge layers and surface states on the (100) faces of p-type silicon. The techniques used enable both the creation and study of space-charge layers at the semiconductor surface, ranging from large-depletion to strong-accumulation conditions. They also permit a straightforward separation of the different components of the induced charge at the silicon/electrolyte interface, so as to yield the variation of both the free-electron density in the space-charge layer and the density of occupied surface states with barrier height. The measured space-charge characteristics are in very good agreement with theory. The data in strong-accumulation layers indicate the presence of an insulating buffer layer (such as an oxide), 3–4 monolayers thick. As to surface states, we find that for CP-4-etched silicon, a distribution of states exists ~ 0.34 eV above the valence-band edge, with total density ~ 6 × 10¹¹cm^-2.

Bi₂MO₆ (M=Mo, W) compounds were prepared for low-temperature oxidation. The catalysts were prepared by solid-state reaction. The structural changes were followed by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy and BET nitrogen absorption. Auger analysis shows that Bi₂WO₆ catalysts have more bismuth on the surface than γ-Bi₂MoO₆, although both samples are bismuth deficient as compared to the stoichiometric compound. The results regarding catalytic activity show that Bi₂WO₆ prepared at 1073 K reaches total conversion of CO (100%) at a lower temperature when compared to γ-Bi₂MoO₆. This indicates that Bi₂WO₆ is a potential candidate to be used as catalyst in the CO to CO₂ oxidation.

We calculate the absorption spectrum of a single exciton, confined in an In_xGa_{1 - x}As/GaAs disk-shaped quantum dot, as a function of the strength of an applied magnetic field. Exciton eigenstates are obtained by numerical diagonalization of the Hamiltonian, within the effective mass approximation. A correction in the expression of the Coulomb interaction matrix elements is verified.

Systematic measurements of the Hall and longitudinal resistivities as a function of temperature in GdBa₂Cu₃O_{7 - δ} thin films were effectuated. These were performed in magnetic fields B = 0, 1, 2, 3, 4 and 5 T, applied perpendicular to the transport current orientation. Films were prepared by an in-situ high-pressure dc-sputtering technique in an atmosphere of pure oxygen. In the normal phase, the Hall coefficient is positive and varies inversely proportional to temperature. The Hall angle changes approximately as predicted by Anderson's formula. When the temperature approaches the mean-field critical temperature T_c from above, the Hall resistivity decreases abruptly in consequence of thermal fluctuations. Below T_c the Hall response changes signal, goes through a minimum and vanishes when the zero resistance is attained. We ascribe the sign-reversal feature to combined effects of thermal fluctuations and vortex motion. Close to the zero-resistance state, the Hall resistivity varies as a power law of the longitudinal resistivity, with a field-independent exponent. This result may be interpreted as evidence of a vortex-glass transition.

Hydroxyapatite coatings on metallic surfaces of orthopedic interest are achieved, with a chemical bond metal–hydroxyapatite established by the chemical process known as functionalizing. Two different functionalizing agents were used and the pH of the solutions was varied. Results show that crystal size and morphology depend on pH.

Silica–hydroxyapatite composites have been produced with a novel method. Particle size can be selected by the pH of the solution as well as crystallite morphology which also depends on pH. An ion reservoir is built by means of the complex solution Ca/EDTA (ethylenediaminetetra-acetic acid). The formation of hydroxyapatite (HAp) at low temperatures (boiling water) and in short periods of time (around 4 h) is achieved. It is thought that the production of silica promotes the nucleation of hydroxyapatite.

The question of the validity of the scaling ansatz in discrete deposition models and their connection with the scaling exponents of continuum differential equations is addressed. We specifically focus on the scaling properties of the Wolf–Villain type models and, as an extension of this model, on the influence of attractive and repulsive interactions up to second neighbors on the scaling relation. As an example of technological relevance, we present the evolution of steps in the vicinal (100) surface of Si during deposition at relatively low temperatures. We have found that, in general, one should not expect that discrete models, as well as real crystals, exhibit scaling.

Using scanning tunneling microscopy and first principles total energy calculations, we have studied the atomic structure of the low coverage phases of Pb on Ge(001): the (2 × 2) and the c(4 × 8)- α reconstructions. Based on the analysis of the experimental bias-dependent STM images and supported by the ab initio pseudopotential density-functional calculations, we present models for each structure. In both cases, there is a coexistence of Ge and Pb dimers. The (2 × 2) phase is formed by asymmetric Pb dimers parallel and in the trenches between the underlying Ge dimers. The Pb coverage is 0.5 ML and it corresponds to the maximum packing density of Pb dimers without breaking Ge dimers. The c(4 × 8) - α reconstruction has a Pb coverage of 0.75 ML and it is characterized by asymmetric Pb dimers forming chains along the [110] direction. However, in this case half of the Ge dimers are broken. The other half remain intact and they are perpendicular to the Pb dimers.

We have studied the adsorption of S on the Si(001) c(4 × 2) surface by first principles total energy calculations. It is found that the first S atom is adsorbed on top of a symmetric Si dimer. Following atoms occupy similar bridge sites up to half-monolayer coverage. Increasing the amount of S atoms leads to the breaking of the underlying Si dimers. Our calculations predict that from 1/2 ML up to a full ML the growth of S is highly anisotropic. There is a preference of the S atoms to grow along the Si dimers rows. At full monolayer, all the Si dimers are broken and the surface is completely de-reconstructed with all Si atoms near bulk ideal positions.

In this work a method is proposed to calculate the optical constants of porous silicon (PS) thin films. The method is based on the theoretical simulation of the experimental reflectance spectra. In the optical system of this method the PS is considered a homogeneous, absorbing thin film, deposited on a silicon substrate of semi-infinite dimension. The theoretical form of the systems reflectance is calculated using the summation method proposed by Airy. Light scattering is included in the model by introducing the Davies–Bennett relation. The refraction index of the material is fit with the simple harmonic oscillator, proposed by Wemple–DiDomenico. The model was tested on two samples fabricated with anodization times of 25 and 35 min; the values for the refraction indexes, absorption coefficients, thickness and roughness were calculated for both samples. The PS samples were fabricated by electrochemical anodization of single crystal p-type silicon substrates in HF (25%) + isopropyl alcohol solution.

We calculate the states of a one-dimensional hydrogen atom under the effect of an electric field (Stark effect) in the strong field regime being the field confined in a finite region of width 2a (capacitor region). We find numerically the solution inside the capacitor and match it to an analytical solution outside the capacitor. Although the electric field tends to separate the two opposite charges particles, the total energy of the system decreases with increasing electric field strength. Our results are useful as a guideline to study strong electric field effects in electronic states of impurities or excitons in 1D systems.