Effect of an electron beam on CaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> epitaxial layers on Si

Atomically smooth CaF₂ and BaF₂ layers have been sequentially grown on Si(111) substrates by molecular beam epitaxy. Pore macrodefects have been revealed at the points of the action of an electron beam from a diffractometer when analyzing the crystal structure of the surface during the growth with the subsequent observation using atomic force microscopy. The formation of these macrodefects is associated with the decomposition of fluorides by high-energy electrons, which is accompanied by the desorption of fluorine and the drift current of positive ions from the electron charge drains.     

Features of the surface morphology of CaF<Subscript>2</Subscript>/Si(100) films obtained by solid-phase epitaxy

The dielectric parameters of calcium fluoride films grown on the Si(100) surface by solid-phase epitaxy (group 2) and without it (group 1) are analyzed. It is established experimentally that the deposition mode of CaF₂ films immediately after growth of the Si buffer layer at a substrate temperature of 530°C is not suitable for producing high-quality dielectric layers. The use of solid-phase epitaxy at the initial stage of the nucleation of CaF₂ layers enables the production of single crystal uniformly thick films with high dielectric properties.     

Analysis of the oscillation intensity of RHEED specular reflection during the MBE growth of CaF<Subscript>2</Subscript>/Si/CaF<Subscript>2</Subscript> structures

The results of analysis of the oscillation intensity of RHEED specular reflection during the MBE growth of CaF₂/Si(111) structures in a wide temperature range from 100 to 600°С are presented. It is shown that the preliminary formation of a 2D Si buffer layer provides the two-dimensional growth of CaF₂ layers. Possible reasons which for the disruption of 2D growth at high substrate temperatures are discussed.     

Memory element based on Si/CaF<Subscript>2</Subscript> periodic nanostructures

A memory element based on a Si/CaF₂ periodic nanostructure is proposed. In this element, information is recorded through charge capture by trap states in a CaF₂ dielectric. The high and low signal levels correspond to the current in the maximum and minimum of the negative differential resistance region, which forms as a result of the resonant tunnel distribution of charge carriers over trap levels in the dielectric. The speed of such logical elements depends on the rate of activation carrier trapping and the rate of tunnel carrier transfer from one state to another. It is shown that both Si/CaF₂-based logical elements and memory elements proposed operate at temperatures from 77 to 300 K, have a switching time of 10⁻¹²–10⁻¹⁰ s, and are compatible with silicon IC technology.     

Self-assembling of Ge quantum dots in the CaF<Subscript>2</Subscript>/Ge/CaF<Subscript>2</Subscript>/Si heteroepitaxial system and the development of tunnel-resonance diode on its basis

A CaF₂/Ge/CaF₂/Si(111) heteroepitaxial structure with Ge quantum dots was grown by molecular-beam epitaxy. A negative differential conductivity and conductivity oscillations caused by resonant hole tunneling were observed at room temperature. The energy spacing between the levels in quantum dots, as determined from the oscillation period, is 40–50 meV depending on the Ge dot size.     

Resonant-Optical Detection of Nuclear Magnetization in the Si/CaF<Subscript>2</Subscript> Nanostructure

We have considered theoretically the characteristic features of optical detection of nuclear magnetization in the Si/CaF₂ structure under the conditions of EPR and NMR by measuring luminescence polarization. We show that application of EPR makes it possible to detect weak nuclear fields, but the time of spin relaxation of electrons imposes its constraint. The application of NMR is limited by nuclear fields of no less than 15–20 Gs. The possibility of using optical NMR for direct measurement of the nuclear field from changes in the luminescence polarization spectrum is shown. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 644–649, September–October, 2005.     

Spectral-kinetic characteristics of crystals and nanoceramics based on BaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript>: Ce

Optical characteristics of BaF₂ and BaF₂: Ce single crystals and nanoceramic materials prepared from these single crystals by uniaxial hot pressing have been studied. It has been shown that the introduction of Ce₃₊ ions into BaF₂ hardly affects the ultrafast (∼0.9 ns) luminescence component. The integrated luminescence intensity of the BaF₂: Ce nanoceramics is higher than that of the corresponding single crystal and considerably higher that the intensity of the undoped BaF₂ crystal. It has been demonstrated that the slow (several hundred nanoseconds) component of the luminescence decay of BaF₂: Ce is due to the energy transfer from excitons to Ce³⁺ ions.     

Comparative spectroscopic and laser properties of Yb<Superscript>3+</Superscript>-doped CaF<Subscript>2</Subscript>, SrF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> single crystals

We present the spectroscopic properties and room-temperature cw tunable laser operation of Yb³⁺-doped CaF₂, SrF₂ and BaF₂ single crystals grown and studied in the same conditions. Emission cross sections, lifetimes, laser thresholds, laser slope efficiencies and laser wavelength tuning ranges are compared. It appears that Yb³⁺-doped BaF₂ might be more promising for diode-pumped high power laser operation. PACS 42.55.Rz; 42.70.Hj     

Spin Polarization of Fluorine Nuclei in the Si/CaF<Subscript>2</Subscript> Nanostructure Under Optical Excitation

The laws of fluorine nuclear spin polarization in the Si/CaF₂ nanostructure under optical excitation of the charge carriers in it has been considered theoretically. It has been shown that maximum values (up to 3% of the concentration of nuclei in the lattice) are attained under a high rate of optical excitation (>10⁻⁹ sec⁻¹) when the nuclear spin diffusion process and the Auger recombination prevail. In this case, the nuclear relaxation time in an individual layer reaches 100–300 sec and the spin diffusion radius decreases to 0.3 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 397–403, May–June, 2005.     

Magnetic properties of arrays of cobalt nanoparticles on the CaF<Subscript>2</Subscript>(110)/Si(001) surface

The Co/CaF₂/Si(001) heterostructures with the corrugated (110) surface of the CaF₂ buffer layer have been grown by molecular beam epitaxy. The structures are nanoparticle arrays of single-crystal Co, mostly of the cubic fcc modification. The behavior of the magnetic hysteresis loops as a function of the density of coverage of the substrate by cobalt islands, the island size, and the temperature is studied using the magnetooptical technique. At low coverage densities, where the effective cobalt film thickness d _eff is less than the critical value d _eff ^c , the magnetic structure of the films at T = 294 K can be visualized as an ensemble of superparamagnetic, weakly interacting nanoparticles and is characterized by small values of the coercive field H _c and the remanent magnetization M _rem. A decrease in the temperature leads to a strong increase in H _c and M _rem, which is associated with the transition of the islands to the blocked state. The blocking temperature of the structures is T _b ～ 280 K. The magnetic anisotropy parameter K and the saturation magnetization M _s of the islands depend on the growth temperature of cobalt T _Co. An increase in the coverage density above the critical thickness d _iff ^c at T = 294 K brings about a strong increase in H _c and M _rem and the appearance of a hysteresis loop anisotropy originating from the corrugated structure of the CaF₂ buffer layer. The experimental results are compared with the model of an ensemble of noninteracting superparamagnetic particles.     

Defect entropies and enthalpies in BaF<Subscript>2</Subscript>

Various experimental techniques have revealed that the predominant intrinsic point defects in BaF₂ are anion Frenkel defects. Their formation, enthalpy and entropy, as well as the corresponding parameters for the fluorine vacancy and fluorine interstitial motion have been determined. In addition, low temperature dielectric relaxation measurements in BaF₂doped with uranium leads to the parameters τ₀, E in the Arrhenius relation τ = τ₀ exp(E/k _B T) for the relaxation time τ. For the relaxation peak associated with a single tetravalent uranium, the migration entropy deduced from the pre-exponential factor τ₀ is smaller than the anion Frenkel defect formation entropy by almost two orders of magnitude. We show that, despite their great variation, the defect entropies and enthalpies are interconnected through a model based on anharmonic properties of the bulk material which have been recently studied by employing density-functional theory and density-functional perturbation theory.     

Phosphorescence of CaF<Subscript>2</Subscript>-Dy crystals

The decay kinetics of phosphorescence excited by x-rays in a CaF₂-Dy crystal is investigated. It is found that localized charge carriers recombine through tunneling. The conclusion is drawn that, in the initial stages of x-ray irradiation, the accumulation of charge carriers occurs in pairs. This process is caused by the formation of excimer-like molecular states during excitation of the crystal.     

Photodesorption from ultra-thin metal films – a comparison of SO<Subscript>2</Subscript> and NO<Subscript>2</Subscript> on Ag/Si(100)

The photochemistry of SO₂ on thin epitaxial Ag films (5–60 nm) deposited on Si(100) has been studied using laser light with the wavelengths of 266, 355, and 532 nm. SO₂ desorbs with cross sections of 1.7×10^-19,1.7×10^-20 and 2.9×10^-21 cm², respectively. The average translation energy, 〈E_trans/2k〉, is 440 K for 266 and 355 nm light, and 270 K for 532 nm light. Cross sections for a 60 nm thick Ag film are practically identical to the ones for Ag(111) as the substrate. An increase by a factor of ∼3.5 is observed when the film thickness is reduced to 5 nm for 266 and 355 nm light. No significant change is observed for 532 nm excitation. The film thickness has no significant influence on the translational energy of the photodesorbed molecules. The data are discussed in connection with the change of absorptivity of the metal film–semiconductor system. A model is put forward which takes into account the light absorption in the Si substrate and the reduced relaxation of excited electrons in Si. Modelling indicates that electrons excited in the Si substrate with energies and parallel momenta not allowed in Ag contribute to the surface chemistry after crossing the gap in the projected band structure of Ag(111). PACS 82.45.MP; 73.63.-b; 82.50.Bc     

Polarization of Luminescence in the Nanostructure Si/CaF<Subscript>2</Subscript> Upon Polarization of the Nuclear Spins of Fluorine

The laws governing polarization of luminescence in the nanostructure Si/CaF₂ upon polarization of the spins of the fluorine nuclei by means of optical excitation of charge carriers are considered theoretically. The possibility of studying experimentally the properties of nuclear spins in analyzing luminescence is shown. The polarization of luminescence is most informative in the range of excitation rates of charge carriers from 3⋅10⁷ to 3⋅ 10⁸ sec⁻¹ with the CaF₂ layer of thickness from 0.6 to 0.8 nm and optical excitation polarization degree of 0.1. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 524–529, July–August, 2005.     

Additive coloring of CaF<Subscript>2</Subscript> optical ceramic

The specificity of additive coloring of CaF₂ optical ceramic (formation of color centers in it and photothermochemical transformation of these centers in colored ceramic samples) has been considered. Under the same coloring conditions, this process occurs more slowly in ceramics rather than in crystals; at the same time, the limiting concentration of color centers that can be introduced into ceramics is much higher. The photothermochemical transformations of color centers in crystals and ceramics, which occur under illumination at different wavelengths and upon heating, have been studied. The specific features of introduction of color centers into ceramic and their transformation under illumination and heating are likely to be related to the mass twinning of ceramic grains.     

Growth of AgInSe<Subscript>2</Subscript> on Si(100) substrate by thermal evaporation technique

AgInSe₂ films were prepared by a thermal evaporation technique onto Si(100) substrates at a pressure of 10⁻⁵ mbar. Structural and optical properties of films deposited at 300 and 473 K have been investigated. The film composition was studied by energy dispersive analysis through X-rays. X-ray diffraction patterns indicate that AgInSe₂ films have chalcopyrite structure with strong preferred orientation in the (112) direction. Average vertical crystallite size of 25 nm was observed. The optical energy gaps of 1.20 and 1.90 eV were obtained due to the fundamental absorption edge and a transition originating from crystal field splitting, respectively. Field emission scanning electron microscopy shows loosely packed grains of spherical symmetry with some facets.     

Magnetic and magneto-optical properties of epitaxial cobalt films grown on a corrugated CaF<Subscript>2</Subscript>/Si surface

The magnetic properties of CaF₂/Co/CaF₂(110)/Si(001) heterostructures fabricated by molecular-beam epitaxy and having a corrugated CaF₂ buffer surface were studied. The optical and magneto-optical properties of these structures reflect the C _2v symmetry of the corrugated structure surface. The studies of hysteresis loops using the longitudinal and transverse magneto-optical Kerr effects under oblique light incidence and of magneto-optical phenomena under near-normal light incidence demonstrate that the corrugated structure surface leads to optical and magneto-optical anisotropies. The magnetization of such structures occurs via coherent magnetization rotation over a wide magnetic-field range. The magnetic anisotropy of these structures is described using a Gaussian distribution of easy axes of magnetization in cobalt granules about the direction parallel to the groove direction. The asymmetry of hysteresis loops of the rotation of the plane of polarization detected under oblique and normal light incidence is shown to be related to the contributions to the effective film permittivity that are quadratic in the magnetic moment.     

Location of the energy levels of the rare-earth ions in BaF<Subscript>2</Subscript> and CdF<Subscript>2</Subscript>

The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF₂ and CdF₂ crystals is determined. The role of RE ³⁺ and RE ²⁺ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF₂: RE and CdF₂: RE is associated with the location of the excited energy levels in the band diagram of the crystals.     

Interfacial structures of LaAlO<Subscript>3</Subscript> films on Si(100) substrates

This paper investigates the interfacial characteristics of LaAlO₃ (LAO) and LaAlO_xN_y (LAON) films deposited directly on silicon substrates by the pulsed-laser deposition technique. High-resolution transmission electron microscopy (HRTEM) pictures indicate that an interfacial reaction between LAO and Si often exists. The interfacial layer thickness of LAO films deposited in a nitrogen ambient atmosphere is smaller than that of LAO films deposited in an oxygen ambient atmosphere. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to study the composition of the interfacial layer. The shift of the La 3d photoelectron peak to a higher binding energy compared to LaAlO₃, the shift of the Al 2p peak to a higher binding energy compared to LaAlO₃, the shift of the Si 2p peak to a lower binding energy compared to SiO₂ and the intermediate location of the O 1s peak compared to LaAlO₃ and SiO₂ indicate the existence of a La–Al–Si–O bonding structure, which was also proved by the AES depth profile of LAO films. It can be concluded that the interfacial layer is not simply SiO₂ but a compound of La–Al–Si–O. PACS 77.84.Bw; 77.84.-s; 77.55.+f