CaF2 Films Research Articles

Control of Epitaxial Relation of GaAs Film on Fluoride/Si(111) Structure

The X-ray diffraction measurements have revealed that the type B region, whose crystallographic orientation is rotated 180° about the surface normal with respect to the CaF2 film (rotational twin), partially exists in a GaAs film on CaF2(111). It is found that the fraction of the type B region increases as the GaAs growth temperature increases for good crystallinity in GaAs films. A decrease in temperature at the initial stage of growth followed by an increase in temperature, i.e., the two-step growth method, is found to be very effective for suppressing the generation of the rotational twins at the GaAs/CaF2 interface so as to grow high-quality GaAs on CaF2/Si(111).

Growth and characterization of calcium fluoride on (100) and (111) silicon substrates

Epitaxial films of CaF2 have been grown on Si(100) and Si(111). The substrate temperatures required for epitaxy, determined by Rutherford backscattering of 340 keV protons, are compared with those previously reported, as are the results of examination of the surface morphologies by scanning electron microscopy. Initial investigations of the electrical properties of the insulating films are also discussed.

Photoluminescence from submicron CaF2:Nd films grown epitaxially on Si(111) and Al(111)/Si(111)

Nd3+ doped CaF2 films have been grown epitaxially on Si(111) and Al(111)/Si(111) using CaF2 and NdF3 evaporation. Photoluminescence spectra from these samples show high intensity and narrow emission linewidth even when the thickness of the CaF2:Nd films is reduced to 0.2 μm. The 10 457 Å emission line is not quenched until the Nd concentration exceeds 3.8 wt.%. Moreover, while similar spectra are observed, the photoluminescence intensity from the CaF2:Nd films grown on Al/Si(111) is significantly higher than from the CaF2:Nd on Si(111).

Calcium Fluoride Films Assisting Diffusion Bonding of MgF2 Ceramics

The bonding of magnesium fluorides by hot pressing was studied. A layer of CaF2 with 2∼3 µm thickness was used as an agent to decrease the bonding temperature and avoid breaching. The main factors that influence the light scattering loss were the cellular growth of MgF2 films, voids caused by the lattice mismatch due to the interfaces of each layer, voids due to the films, grain growth, and recrystallization zone formation. By contrast, the grain growth of the MgF2 matrix and CaF2 films was not the main influence on the scattering loss. Another important factor that influences the scattering loss was the refractive index variations, which are dependent on the composition fluctuations. Measurements of the discontinuities in the concentration-distance profile describing interdiffusion of the components in the binary systems CaF2-MgF2 gave the crystalline solubility limits just below the eutectic temperature and under the 430 kg/cm2 pressure. These are 17 at.% Ca in MgF2 at 650°C, 5 h, and discontinuous values over the recrystallization zone at 700°C, 5 h.

Epitaxial growth of an Al/CaF2/Al/Si(111) structure

Despite their large lattice mismatch (∼25%), epitaxial CaF2 films have been grown on single crystal Al(111) on Si(111) by low temperature molecular beam epitaxy. X-ray diffraction shows that the orientations of the CaF2 are the same as those of the Al films, whether the orientations of the Al are the same rotated 180° or with respect to the underlying Si substrate. Furthermore, our successful fabrication of an epitaxial Al/CaF2/Al/Si(111) structure suggests that Al can be a useful conductor material in three-dimensional device integration.

Epitaxial relations and electrical properties of low-temperature-grown CaF2 on Si(111)

The temperature dependences of crystalline quality, epitaxial relations, and interface properties of CaF2 on Si(111) are studied by x-ray diffraction and electrical measurements. While the CaF2 films grown at temperatures above 400 °C exhibit B-type epitaxy, the films grown at temperatures below 400 °C show A-type epitaxy. Using low substrate temperatures, we also obtained A-type CaF2 on SiGe(111) and off-orientated Si(111). In addition to the change in epitaxial relations, the interface state density is significantly reduced when the growth temperature of the CaF2 films decreases. Although flat capacitance–voltage (C–V) curves are observed from capacitors made of CaF2 films grown at 600–800 °C, suggesting a pinned interface, unpinned C–V characteristics are observed from capacitors made of either A- or B-type epitaxial CaF2/Si(111) grown at 200–500 °C, and polycrystalline-CaF2/Si(111) grown at 25–100 °C. The C–V characteristics are also dependent on the post-annealing temperature. Our results indicate that the interface state density of CaF2/Si(111) is dominated by thermal history, rather than crystalline quality or orientation, of the CaF2 film.

Fabrication and characterization of Ti-Ba-Ca-Cu-O superconducting films on LaAlO3 substrates

A multi-step process is used to fabricate Tl2Ba2Ca1Cu2O8 films on (100) LaA103 substrates. Submicron thick precursor films of Ba-Ca-Cu-O are rf magnetron sputter deposited from a single target. Film stoichiometry is measured by ion beam backscattering spectroscopy. Deficiencies of the alkaline earths that are found in the precursor films are then compensated for by the addition of appropriate CaF2 and/or BaF2 films onto the surface of the precursor film. Post deposition annealing of the films is then done in an atmosphere of thallium oxide and oxygen in order to form the superconducting phases. The annealed films are examined using x-ray diffraction (XRD), an ac inductance technique, and critical current in an external magnetic field. XRD shows the c-axis length of the superconducting phase to increase as the overall film stoichiometry approaches 2212. The transition widths measured by inductive coupling weakly correlate with 77 K critical current measurements. Our best critical current results are 1.5*106 amps/ cm2 for a film measured at 4 K in an 8 T magnetic field (parallel to the films' c-axis).

New insight into the structure and growth of CaF2/Si(111)

We have used transmission electron microscopy and x-ray crystal truncation rod measurements to investigate thin (<50 Å) CaF2 films grown on Si(111) substrates by molecular beam epitaxy. The results indicate that CaF2/Si can be structurally as perfect as NiSi2/Si and CoSi2/Si, and that a reconstructed layer is present at the CaF2/Si(111) interface.

Electrical properties of low-temperature-grown CaF2 on Si(111)

While epitaxial CaF2 films grown on Si(111) at temperatures above 550 °C exhibited flat capacitance-voltage (C-V) curves, suggesting a pinned CaF2/Si(111) interface, we have observed unpinned C-V curves from as-deposited epitaxial CaF2 grown at 300 °C. Our results demonstrate that C-V characteristics of CaF2/Si(111) are determined by the thermal history, rather than the crystalline quality, of the CaF2 film. Correlations among CaF2/Si interface state density, thermal stress, and atomic bonding at the interface are discussed.

Epitaxial growth of alkaline earth fluoride films on HF-treated Si and (NH4)2Sx-treated GaAs without in situ cleaning

HF-treated Si and (NH4)2Sx-treated GaAs were used as substrates for epitaxial growth of CaF2 and BaF2 films without in situ cleaning. The fluoride layers grown on the chemical-treated substrates exhibit good crystalline quality and smooth surfaces. With fluorides on Si, minimum ion channeling yields are about 3%–5%, comparable with the best data reported in the literature using thermal etching or sputtering for substrate cleaning. CaF2 and BaF2 films grown on GaAs(100) exhibit the same (100)-lattice orientation as the underlying GaAs substrate and no misoriented crystallites are observed. Pole figure measurements on BaF2 show that the crystallites are in close registry with the principal axes of the GaAs.

Photoemission from thick overlying epitaxial layers of CaF2 on Si(111)

A tunable pulsed UV laser has been used to investigate photoemission from epitaxial films on Si(111). We find that internal photoemission from the Si surface provides significant electron emission through overlayers as thick as 8000 Å. The attenuation length in the epitaxial CaF2 films is found to be 2600 Å.

Nanocrystallites of Si Embedded in CaF2 by Molecular Beamepitaxy (MBE)

ABSTRACTThin films of CaF2 containing layers of Si nanocrystals were grown epitaxially on Si(111) substrates by MBE. When Si is deposited epitaxially on CaF2 it forms islands and, by choosing appropriate growth conditions, a composite of Si nanocrystals embedded in a CaF2 host crystal is obtained which we have observed with TEM. We varied the effective Si concentration in the composite region from 1% to 25%. The CaF2 could be grown with high crystal quality as demonstrated by ion channeling results which gave Ca xmin's as low as 4.7%. Raman spectra of the CaF2 containing Si nanocrystallites showed significant shifts toward lower energies. EUipsometry measurements were performed on a number of samples over a range of effective Si concentrations and Maxwell-Garnett effective medium theory was used to fit the data. A key technological advantage to this approach is that the epitaxial Si/CaF2 composite material is readily integrable into VLSI processing.

A Gamma‐ray Diffraction Study of MBE‐Grown CaF2/Si(111) Heterostructures

AbstractMBE grown epitaxial films of CaF2 onto Si(111) substrates were investigated by gamma ray diffraction to obtain assertions about the real structure and the strain situation in the epitaxial systems. It were measured the integral reflection coefficient Ri and the angle distribution of the reflected intensity of both (111) and (333) reflections. It was found that (i) a high temperature annealing step during the wafer preparation (1200 °C) causes a drastical increase of real structure defects in the substrate material, (ii) expitaxial layers of 18 nm thickness are grown pseudomorphically, layers having a thickness of 30 nm are relaxed, (iii) the misfit dislocation network formed during the relaxation process is localized not in the deposit but in the substrate material.

Effect of Substrate Off-Orientation on GaAs/CaF2/Si(111) Structure with Rotational Twin

The effect of off-orientation of substrate was investigated to improve the surface morphology and crystallinity of CaF2/Si(111) and GaAs/CaF2/Si(111) structures which were grown by molecular beam epitaxy. It was found that drastic improvement of surface morphology of the CaF2 films and the GaAs films was observed by using Si(111) substrates off-oriented toward not [110] but the [001] direction. Hall measurements showed that electrical properties of the GaAs films were also improved by using these substrates. This report pointed out that selection of the direction toward which the substrate is off-oriented is very important for growth of the GaAs/CaF2/Si(111) structure which has a rotational twin, the so-called “type-B” epitaxial relation.

Atomic Structure of CaF2/Si(111) Interface and Defect Formation on CaF2(111) Surface by Electron Irradiation

Atomic and electronic structures of the CaF2/Si(111) interface and the desorption mechanism of F from epitaxial CaF2(111) films produced by electron irradiation have been studied by low-energy ion scattering spectroscopy (ISS) and low-energy electron energy loss spectroscopy (EELS). The interface is formed by the desorption of F and becomes metallic. The F at the surface of epitaxial CaF2(111) films desorbs by electron irradiation, and the surface after F-desorption becomes metallic. The metallic surface reverts to the bulk electronic state before electron irradiation by annealing at about 400°C.

A Novel Growth Method for High Quality GaAs/CaF2/Si(111) Structures by Using “Type-A” CaF2 Film

ABSTRACTControl of epitaxial relationship of CaF2 films grown on Si(111) substrates was considered to be important to improve surface morphology and crystallini ty of GaAs films on CaF2/Si(111) structures. We successfully grew CaF2 films with the “type-A” epitaxial relationship on Si(111) substrates, that is, the crystallographic orientation of the CaF2 films were aligned in the same direction as that of the Si(111) substrates. These “type-A” CaF2 films were grown by a two step growth method. It was found that surface morphology of GaAs films on the CaF2/Si(111) structures was drastically improved by growth of the “type-A” CaF2 films.

Growth of CoSi2 on CaF2/Si (111) Heterostructures

ABSTRACTIn order to produce an epitaxial metal/insulator/semiconductor structure we investigated the growth of CoSi2 on CaF2/Si (111) heterostructures. We demonstrate the possibility to grow epitaxial CoSi2 on CaF2 films by utilization of the template technique. The fabrication of these structures will be presented. The properties of such heterostructures were determined by means of RHEED, LEED, AES, SIMS, RBS and resistivity measurements.

Low thermal budget solid phase epitaxial growth of CaF2 on Si (111) substrates

As a low thermal budget processing technique, we have used in-situ rapid isothermal processing for growing epitaxial CaF2 films on Si (111) substrates by solid phase epitaxy. Ex-situ rapid isothermal annealed films are polycrystalline in nature. Absorption of gases during ex-situ annealing inhibits solid phase epitaxial growth and highlights the importance of in-situ processing.

Microwave Plasma CVD of Oxide Films Relating to High-TcBi-Sr-Ca-Cu-O Superconductor

Abstract As a low temperature preparation method of Bi, Sr, Ca, and Cu oxide and fluoride films, microwave plasma of Ar and O2 gases was applied to decompose Bi(C6H5)3 and metal β-diketone complexes. As-deposited crystalline Bi2O3, CuO, SrF2, and CaF2 films were obtained at a substrate temperature of 400°C and below. The film thickness was nearly proportional to the deposition time for each film. These Bi2O3, SrF2, CuO, and CaF2 crystalline films designed to have a 2000A thickness each were stacked and annealed at 850°C for 30min to show a superconductivity with TC onset at 65K.

Planar stress relaxation in solid phase epitaxial CaF2 films grown on (111)Si by i n s i t u rapid isothermal processing

Planar strain in CaF2 films on (111) Si substrate has been measured by an x-ray double-crystal diffraction technique using rocking curves. The films grown by a solid phase epitaxial approach using in situ rapid isothermal processing are almost free of tensile planar strain, and free from defects as observed by the transmission electron microscope diffraction pattern.