SrF2 Films Research Articles

Optical constants of SrF2 thin films in the 25–780-eV spectral range

The transmittance and the optical constants of SrF2 thin films, a candidate material for multilayer coatings operating in the extreme ultraviolet and soft x-rays, have been determined in the spectral range of 25–780 eV, in most of which no experimental data were previously available. SrF2 films of various thicknesses were deposited by evaporation onto room-temperature, thin Al support films, and their transmittance was measured with synchrotron radiation. The transmittance as a function of film thickness was used to calculate the extinction coefficient k at each photon energy. A decrease in density with increasing SrF2 film thickness was observed. In the calculation of k, this effect was circumvented by fitting the transmittance versus the product of thickness and density. The real part of the refractive index of SrF2 films was calculated from k with Kramers-Krönig analysis, for which the measured spectral range was extended both to lower and to higher photon energies with data in the literature combined with interpolations and extrapolations. With the application of f- and inertial sum rules, the consistency of the compiled data was found to be excellent.

MOCVD of Epitaxial Alkali-Earth Fluorides Thin Films

Thin films of alkali-earth fluorides and multilayer structures including such layers are known as perspective materials for optics, photonics and chemical sensors. They can also be considered as a material for buffer layers for 2G HTSC wires. Here we report on growing epitaxial CaF2 and SrF2 films on different substrates by MOCVD. The films' properties were examined using RBS, SEM, EDX, XRD, EBSD and AFM. The obtained films revealed perfect in- and out-of-plane orientation and had a smooth, crack-free surface. Depending on the substrate and deposition conditions, some films required the recrystallization annealing to form the epitaxial layer or that of good texture. Additionally, a number of multilayer films including MF2 layers were grown. The obtained films were successfully used as substrates for HTS deposition.

Growth dynamics of insulating SrF2 films on Si(111)

Most high-resolution scanning tunnelling microscopy studies on insulating filmsthus far have been restricted to samples of less than one monolayer thickness onconducting substrates. On the other hand, insulating films with more than onemonolayer are crucial to electrically decouple the conducting substrate. In such acase, atomic force microscopy is required to explore the growth dynamics of theinsulating films. In this paper, we discuss the growth dynamics of an insulatingSrF2 film surface on an Si(111) substrate imaged by non-contact atomic forcemicroscopy (NC-AFM) at room temperature in ultrahigh vacuum. For∼0.7 monolayerSrF2 coverage, the coexistingsurfaces of the SrF2 rowstructure and Si(111)-(7 × 7) reconstruction are clearly observed by NC-AFM topography measurements,indicating evidence for reactive Si dangling bonds at the outermost tip apex. Wealso discuss the tip character obtained from two types of topographic images. Inaddition, we determined that the conformation changes from the row structure to(1 × 1) periodicityby further SrF2 evaporation.

Structural and electromagnetic characterizations of Fe–SrF2 granular films

We investigate composite Fe–SrF2 films to explore new materials showing the granular-type tunnelling magnetoresistance (TMR) effect. The films are composed of Fe nano-particles embedded in a SrF2 matrix. The electromagnetic properties of the ferromagnetic metal–insulator films are confirmed, including super-paramagnetism, tunnel conduction and TMR. We obtain a TMR of 3% at RT and 5% at 77 K. The film resistivity is much higher than that of oxide-based granular film. We also fabricate the nanostructures of granular films and aim at the application of the single-electron transistor. As a result, we observed a Coulomb blockade (CB) region and CB oscillation at low temperature.

Kinetics and Mechanisms of MOCVD Processes for the Fabrication of Sr-Containing Films From Sr(hfac)2Tetraglyme Precursor

The MOCVD process of Sr(hfac)2 tetraglyme has been studied in horizontal, low-pressure hot wall and cold wall reactors under various experimental conditions. In all the investigated conditions, the deposition process results in the formation of polycrystalline SrF2 films. The kinetics and the mechanism of film growth have been investigated by combining “in situ” FTIR techniques and “ex situ” techniques for chemical and structural analyses (energy-dispersive X-ray microanalysis, X-ray photoelectron spectroscopy, and X-ray diffraction). Two different reaction pathways have been proposed depending on deposition temperature. At low temperature a heterogeneous mechanism based on precursor adsorption, followed by the demolition of the β-diketonate framework, has been suggested; at high temperature, a homogeneous decomposition in which the fluorine transfer to the Sr atom leads to SrF2, CF2O, and acylketenes can occur.

Preparation and characterization of volatile alkaline-earth metal complexes with multiply coordinated aminoalkoxide ligands

The aminoalkoxide complexes Sr(amak)2 (1) and Ba(amak)2 (2) were prepared by treatment of a multiply-chelating fluorinated aminoalcohol ligand HOC(CF3)2CH2N(CH2CH2OMe)2, (amak)H, with the Group 2 metal reagents Sr(OPri)2 and BaH2 respectively. Single crystal X-ray diffraction studies indicate that complex 1 possesses an 8-coordinate distorted bicapped octahedral geometry with all O and N atoms coordinated to the central Sr cation. However, complex 2 adopts a unique 10-coordinate bicapped square antiprismatic structure, of which the coordination number is increased by two fluorine-to-barium dative interactions. Variable temperature 19F NMR studies show the existence of two inter-convertible isomers in solution and their possible molecular structures are proposed according to their structures in the solid-state. Preliminary investigation suggests that these complexes are good CVD source reagents for depositing SrF2 and BaF2 thin films.

Epitaxial Growth of SrF2 on ZnSe(100) Epitaxial Films

Epitaxial SrF2 films were grown on ZnSe(100) epitaxial substrates using molecular beam epitaxy (MBE), and their crystallinity dependent on growth temperature was characterized using X-ray diffraction and Rutherford backscattering spectroscopy. SrF2 film with good crystallinity was obtained at the growth temperature of 250°C.

Influence of the mismatch dislocations on the elastic properties of the insulating fluoride films deposited on Si(111)

Abstract CaF2 and SrF2 films grown on Si(111) by molecular beam epitaxy were studied by Brillouin spectoscopy method. The measured velocities of surface acoustic waves are in close agreement with those calculated within simple model of a layered structure. The only discrepancy was a decrease of the velocity in CaF2 films of about 100 nm thick and was attributed to the different distribution of the defects arizing at the early stages of the growth modes at the beginning of CaF2 and SrF2 epitaxy on Si(111).

Electrical properties of SrF2 on GaAs (100) interfaces

In situ rapid annealing, typically at 590 degrees C for 10 s, has been shown to be useful for improving the interface properties of strontium fluoride (SrF2) films grown epitaxially by a thermal evaporation technique on a GaAs (100) substrate. The electrical properties of the SrF2/GaAs heterojunction were studied by using capacitance-voltage and current-voltage measurements. Capacitance-voltage measurements on a metal-insulator-semiconductor structure show that the reduction in surface state density and the band bending may be controlled externally, indicating an unpinned Fermi level at the SrF2/GaAs interface. Current-voltage measurements for 114 AA thick SrF2 films were used to calculate barrier height, which was found to vary 0.6-0.67 eV for different diode and ideality factors approaching unity.

Residual strains in epitaxial fluorides on Si(111) substrates

Growth conditions for molecular beam epitaxy of SrF2 directly on Si(111) have been optimized for excellent crystal quality. A χmin of 2.6% was demonstrated. Lattice distortion measurements were carried out by ion channeling along off-normal channeling directions in the strained SrF2 layers. The measured residual tensile strain versus the film thickness demonstrated a special thickness: When films were thinner than this thickness, their strain dropped dramatically as thickness decreased, and when films were grown thicker than it, the strain gradually decreased with increasing film thickness. This unique behavior was successfully modeled as the result of competition between the large lattice mismatch between SrF2 and Si and the large difference in thermal coefficients of expansion. Residual strain remained even in SrF2 films thicker than 600 nm, unlike CaF2 on Si(111) for which CaF2 films thicker than 300 nm were observed to have no strain.

Barriers to Strain Relaxation in Epitaxial Fluorides on Si(111)

ABSTRACTThe mechanical stress of epitaxial SrF2 and CaF2 films on Si(111) substrates has been measured as a function of temperature by the substrate curvature technique. The temperature dependence of the stress in the SrF2 film is interpreted in terms of an energy barrier to dislocation motion. When the strain energy is smaller than the value needed to overcome the barrier, the change in stress is due mainly to elastic deformation. As the temperature change increases, the strain energy becomes large enough to overcome the barrier, at which point plastic deformation initiates. Unlike SrF2, the stress behavior of the CaF2 film for increasing temperature is quite different from its behavior for decreasing temperature. This unusual behavior is not understood at this time.

Epitaxial growth of lattice mismatched SrF2 films on (100) GaAs

For the first time insulating epitaxial SrF2 films on (100) GaAs substrates have been grown by thermal deposition followed by in situ annealing process. Structural properties of SrF2 films examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicate a very good crystalline quality. It is observed from the X-ray analysis that SrF2 layers thinner than 100 nm suffer two dimensional compressive stress due to the lattice misfit while those thicker than 100 nm suffer two dimensional tensile stress due to the difference in the thermal expansion coefficients.

Relationship between thermal stress and structural properties of SrF2 films on (100) InP

The measurement of thermal stress of SrF2 films on InP as a function of temperature is presented. The in situ and ex situ rapid isothermal annealed films have different values of thermal stress at room temperature and show entirely different behavior of thermal stress during heating and cooling cycles. X-ray photoelectron spectroscopy measurements were used to characterize the surface of the SrF2 films as well as the SrF2/InP interface for both the ex situ and in situ annealed films. It is shown that the difference in the microstructure of in situ and ex situ rapid isothermal annealed SrF2 films on InP is indeed reflected in the significant difference in the thermal stress.

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.

Role of i n s i t u rapid isothermal processing in the solid phase epitaxial growth of II-A fluoride films on (100) and (111) InP

Rapid isothermal processing based on incoherent sources of light is emerging as a reduced thermal budget processing technique for the fabrication of next generation of semiconductor devices and circuits. In this letter, we show that integration of the rapid isothermal processing unit and the ultrahigh vacuum deposition system provides an in situ rapid isothermal processing capability for the solid phase epitaxial growth of SrF2 and BaF2 films on (100) and (111)InP. We also show that neither as-deposited nor ex situ annealed films show solid phase epitaxial growth.

Role of in-Situ Rapid Isothermal Processing (Rip) in the Metallization and Passivation of Indium Phosphide Devices

ABSTRACTAs compared to a stand alone rapid isothermal annealing unit, the integration of deposition system and rapid isothermal processing unit is very attractive for the next generation of micro, opto and cryoelectronics. We have used in-situ rapid isothermal processor for in-situ rapid isothermal chemical cleaning of InP, solid phase epitaxial growth of II-A fluorides, and in-situ metallization of InP capacitors. As compared to ex-situ annealed films, SrF2 films deposited on InP by in-situ rapid isothermal processed films show less thermal stress and lower thermal hysteresis for the identical thermal budget. Similarly, as compared to ex-situ annealing, in-situ cleaning of InP before metallization followed by in-situ annealing results into improved high frequency capacitance-voltage (C-V) characteristics of Al-SrF2-InP capacitors.

Characterization of epitaxial group II fluoride films on (100)‐oriented GaAs substrates by ion backscattering and channeling

AbstractLattice matched (Ca, Sr)F2 and related fluoride films grown on (100)GaAs substrates were investigated by Rutherford backscattering/Channeling. This technique allows the characterization of the 30–250 nm thick heteroepitaxial layers with respect to chemical composition, film thickness, interface roughness and crystalline perfection. The first flash evaporated GaAs/fluoride/GaAs structure are briefly reported.

Growth and characterization of epitaxial SrF2 on InP(100)

Indium phosphide is an important substrate material for microwave metal–insulator–semiconductor field effect transistor (MISFET) devices; SrF2 is an attractive candidate for use as an insulating layer on it because of the relatively close lattice match (−1.2% mismatch). We have grown epitaxial SrF2 films on n-type and p-type InP(100) surfaces using a conventional UHV system. The combination of a brief ion bombardment at 350 °C using 0.5 keV argon ions and a subsequent anneal at the same temperature was found to be an effective cleaning technique for the InP(100) substrate that was chemicomechanically polished prior to insertion in the UHV chamber. The best quality epifilms were grown at 350 °C, using a slow (<1 Å/s) initial growth rate, with a postanneal at the same temperature. The substrate and the film surfaces were characterized in situ, using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), and ex situ, using reflection high-energy electron diffraction (RHEED) and Nomarski optical microscopy. In this paper we report these results, along with the results of capacitance–voltage (C–V) measurements on these films.

Structural and electrical properties of lattice-matched Ca0.44Sr0.56F2/GaAs structures grown by molecular beam epitaxy

Single-crystalline, lattice-matched (Ca, Sr)F2 films have been grown by molecular beam epitaxy (MBE) on GaAs(100) and (111) As substrates. The ratio of the Rutherford backscattering yields from the fluoride films in the aligned normal direction to a random direction, or χmin, is 0.03 for a (100) sample and 0.05 for a (111) sample, indicating excellent epitaxy. In fact, the χmin of 0.03 for the (100) sample is the best of all reported as-grown fluoride films on the (100) surface of a semiconductor. It is nearly the same as χmin from a bulk single-crystalline fluoride. For the (111) sample, channeling measurements show that the film has the same orientation as the substrate, i.e., type A. Plan-view transmission electron microscopy confirms these results. Electrical measurements show that the film has a resistivity of 3×1014 Ω cm and breakdown fields of 7×105 to 3×106 V/cm. At room temperature I–V and C–V curves show various amounts of hysteresis that is completely eliminated at low temperature, indicating the presence of positive ions.

Preparation of Oriented PbTiO3 Ferroelectric Thin Films on Silicon

A series of technical data on the preparation of oriented PbTiO3 ferroelectric thin films on Si wafer by rf sputtering have been presented. Two kinds of approaches have been tried to improve crystalline properties of the PbTiO3 films. Firstly, highly-oriented CaF2 and SrF2 films have been grown on Si wafer as a buffer layer by electron beam evaporation at 400–600°C. The PbTiO3 film on the SrF2/(100) Si and CaF2/(111) Si have been oriented in (100) and (110) & (101) directions, respectively. Secondly, negative DC bias voltage has been applied to the Si substrate during the sputtering of PbTiO3. The obtained films are highly-oriented and their dielectric properties are improved with increase of the bias voltage. Surface of the obtained film is very smooth.