Epitaxial CaF2 Research Articles

Thick crack-free CaF2 epitaxial layer on GaAs (100) substrate by molecular beam epitaxy

The crystallographic orientation of low temperature (LT) grown CaF2 on GaAs (100) substrates is investigated. LT epitaxial (100) CaF2 layers are obtained on a thin (100) oriented CaF2 template at growth temperatures down to room temperature. This makes it possible to grow crack-free CaF2 (100) using a multiple-temperature-growth scheme at any desired temperature. The resulting CaF2 layers, with thickness up to 680 nm, can withstand temperature cycling from RT to 650°C without cracking. Based on these results, a four pair Ga0.5Al0.5As/CaF2 quarter-wavelength Bragg reflector was fabricated with center wavelength at 880 nm. The reflector, with a total CaF2 thickness of 615 nm, shows broadband high reflectivity with a crack-free surface. This crack-free surface can then be overgrown with further device layers.

Scanning tunneling microscopy and ballistic electron emission spectroscopy studies of molecular beam epitaxially grown Pt/CaF2/Si(111) structures

The electronic and morphological properties of ultrathin Pt/CaF2/Si(111) metal insulator semiconductor structures have been characterized in situ by scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM). Platinum thickness from 2 Å to 20 Å grown on 5 Å epitaxial CaF2 have been characterized. The STM images of the Pt/CaF2/Si(111) structures show the atomic steps of the underlying CaF2 morphology, as well as the formation of Pt nodules. These nodules have been observed to coalesce into bigger grains when left at room temperature under ultrahigh vacuum conditions for over 24 h. For ultrathin Pt coverages (2 Å) the STM images at different tip biases reveal different topography, depending on the value of the tip bias relative to the conduction band minimum (CBM) of the CaF2 intralayer (3.3 eV). The STM images at biases well above the CBM of the CaF2 show similar features to bare CaF2/Si(111), while images at the CBM show features of the deposited Pt. BEEM spectra of the 10 Å sample show a peak at ∼4.5 eV due to the density of states of the CaF2 intralayer and an additional peak at 2 eV, which has not been observed in previous studies of metal/CaF2/Si(111) structures.

ZnSe ionicity control layer inserted in GaAs/CaF 2(111) interface

For heteroepitaxial growth of covalent crystals on ionic crystals which have very different ionicities, we propose a new approach of introducing a material which has intermediate ionicity to the heterointerface for the purpose of accommodation of the large ionicity difference. By introduction of a thin ZnSe layer to the interface of GaAs/CaF 2(111) as a buffer layer, we successfully improved the crystallinity of the MBE-grown GaAs layer, which was examined by the double crystal X-ray diffraction method. The ZnSe layer was grown by a 2-step growth method in which a few nm ZnSe was deposited at low temperature on an epitaxial CaF 2 film grown on Si(111) substrate and it was annealed in situ before overgrowth of GaAs. The growth conditions of the ZnSe layer such as deposition temperature, thickness, and annealing temperature were studied to obtain good crystallinity of the GaAs overlayer.

Artificial Control of Dot Sites for Ga Droplet Arrays on CaF 2 Films by Surface Steps and Electron Beam Modifications

The control of formation sites for Ga droplets on epitaxial CaF2 films was investigated by two approaches: alignment on step edges, and local surface modification by focused electron beam exposure. Ga droplets 10 nm in diameter were formed with spacings of 5–30 nm on monolayer step edges on CaF2 surface. Straight and parallel multi-steps were formed using vicinal substrates, and the preferential formation of Ga droplets on these steps was observed. Site control by surface modifications of CaF2 using a focused electron beam exposure was also achieved. Two dimensional dot matrix arrays of Ga droplets were fabricated using this technique. Preliminary results indicating the formation of a GaAs quantum dot array from these site controlled Ga droplets by supplying a molecular beam of As4 were obtained.

Photo bleaching in Sm 2+ selectively doped epitaxial CaF 2 films

We present a study on the bleaching properties of Sm 2+ doped epitaxial CaF 2 films. Previously, the Sm 2+ emission in these films has been found to bleach partially under optical excitation. For a detailed investigation of the bleaching mechanism, single crystal CaF 2 films were grown on Si(111) surfaces by molecular beam epitaxy (MBE) and were selectively doped with Sm 2+ ions at various distances from the semiconductor/insulator interface. A correlation between the bleaching rate and the distance from the insulator-semiconductor interface could not been found. In addition, we find that the bleaching rate depends linearly on the intensity of the optical excitation and that the shape of the emission line is not altered during the bleaching process. Our results are discussed in the framework of photoionization due to excited-state absorption, tunneling of excited electrons into the semiconductor substrate, and the role of lattice defects.

Fabrication of site-controlled metal dot array by electron beam surface modification

Two dimensional arrays of Ga droplets on the CaF2 film were fabricated by the site control technique using electron beam surface modification. Spacing of each droplet can be close to each other to the effective beam spot size at the AsCaF2 interface. Epitaxial CaF2 film coated with a thin As film was exposed by focused electron beam to obtain local surface modified region on the CaF2 surface. It is considered that the surface potential for Ga adsorption becomes low in the modified region, where deposited Ga atoms migrate and aggregate to form a droplet at the minimum potential site.

Thermal imaging camera with linear Pb 1− xSn xSe-on-Si infrared sensor array and combined JFET/CMOS read-out electronics

A thermal imaging camera for the 8–12 μm wavelength range is described which employs a new infrared sensor device and read-out principle. A bilinear 2 × 128 element infrared sensor array is fabricated in a narrow gap Pb 1− x Sn x Se layer grown epitaxially on a Si-substrate. A ≈ 30 Å thick intermediate epitaxial CaF 2 buffer layer is used for compatibility reasons. The read-out electronics chips contain, for each sensor, an integrator with a low noise JFET input transistor, correlated multiple sampling, and a sample and hold amplifier. They are wire-bonded to the sensor array and operated at 80–120 K. The JFET input transistors allow to amplify from much lower source impedances (down to <10 kΩ) than with CMOS design without adding significant noise. Therefore, infrared sensors with lower impedances can be used, which allows operation at higher temperature, or to use sensors with longer cut-off wavelengths.

Interface and overlayer structure of epitaxial CaF 2 thin films on Si(111): An X-ray scattering study

Using X-ray scattering techniques we investigated a 50 Å thick CaF 2 film, epitaxially grown on a Si(111) substrate. The CaF 2 overlayer exhibits an incommensurately modulated structure parallel to the heterointerface, consisting of a triangular network of discommensurations, which separate domains with two different Ca site occupations H3 and T4. The 2D spatial modulation of the overlayer is induced by the competition between the strong covalent-like Ca-Si bonding at the interface and the ionic Ca-F bonding in the film. Remarkably, the in-plane periodic lattice distortion generated at the interface propagates many atomic layers through the CaF 2 film. Specular reflectivity and standing wave measurements suggest that the CaF 2-Si interface is unstable towards the formation of an intermediate CaSi 2 layer. In situ temperature-dependent strain measurements revealed that the strong interfacial Ca-Si bonding prevents a complete in-plane lattice relaxation up to 700°C while the overlayer expands considerably normal to the interface.

The Effect of Strain Relaxation Mechanisms on the Electrical Properties of Epitaxial CaF2/Si(111) Heterostructures

ABSTRACTA new technique for growth of exactly two monolayers of CaF2 on Si(111) substrates is demonstrated. This technique takes advantage of the tendency of CaF2 to form thick islands at Si step edges on vicinal substrates once a two-monolayer thick wetting layer is deposited. A comparison of I-V characteristics for epitaxial CaF2 layers grown on on-axis versus off-axis substrates demonstrates the advantages of this technique. In addition, preliminary results for electron tunneling through the CaF2 structure is shown using the ballistic electron emission microscopy (BEEM) technique.

Characterization of molecular beam epitaxy grown CdF2 layers by x-ray diffraction and CaF2:Sm photoluminescence probe

For studies of crystal quality and strain relaxation in heterostructures with epitaxial CdF2 and CaF2 layers on Si(111), differential double crystal x-ray diffractometry and impurity photoluminescence techniques were used. The crystalline quality of molecular beam epitaxy grown CdF2 layers can be high and comparable with that of III–V compound pseudomorphic heterostructures. The best results were obtained for CdF2 layers grown on CaF2 buffer layers which were coherent with the Si substrate.

Epitaxial growth of a metal(CoSi2)/insulator(CaF2) nanometer‐thick heterostructure and its application to quantum‐effect devices

Metal(CoSi2)/insulator(CaF2) nanometer‐thick heterostructures are grown on a Si(111) substrate, and applied to quantum‐effect electron devices. In the epitaxial growth of a CoSi2/CaF2 multilayer, an epitaxial CoSi2 layer on CaF2 was obtained by the two‐step growth technique, i.e., solid phase epitaxy with the epitaxial Si layer grown in the first step and Co deposited in the second step. An epitaxial CaF2 layer was formed on CoSi2/CaF2 at the low substrate temperature of 450 °C with a partially ionized and accelerated CaF2 beam. Co agglomeration in nanometer‐thick CoSi2 layer was avoided by using these techniques. A resonant tunneling transistor with double CoSi2 quantum wells was fabricated, and a transistor action with the negative differential resistance was observed at 77 K. The transfer efficiency of electrons from the emitter to the collector through the resonant energy levels is estimated to be close to unity. A resonant hot electron transistor structure was also fabricated, and multiple negative d...

IN SITU STUDIES WITH LOW-ENERGY ELECTRON MICROSCOPY

This paper gives a brief review of low-energy electron microscopy (LEEM) as used for in situ studies of surface dynamical processes. The capabilities of LEEM are illustrated with two examples. One is a kinetic instability observed during growth of the first layer of CaF 2 on Si (111). The second concerns the nucleation of misfit dislocations during the growth of thicker, epitaxial CaF 2 films on Si (111), as the critical thickness is exceeded. Both examples highlight the importance of real time, in situ observations of surface dynamical processes.

Dynamical properties of epitaxial CaF2 films deposited on an Si(001) substrate studied by Brillouin spectroscopy

Using Brillouin spectroscopy we have studied the dynamical properties of epitaxial CaF2 films, with thicknesses in the range h=61-300 nm, grown on Si(001) substrates. Theoretical calculations for the velocity values, based on a simple elastic continuum model, give results that agree quite well with experiment. A correlation between the dynamical properties of CaF2/Si(001) systems and their structure is found and discussed.

Thermal-mismatch-strain relaxation in epitaxial CaF2, BaF2/CaF2, and PbSe/BaF2/CaF2 layers on Si(111) after many temperature cycles.

The thermal mismatch strain in stacks containing PbSe, ${\mathrm{BaF}}_{2}$, and/or ${\mathrm{CaF}}_{2}$ on Si(111) substrates is relieved by the glide of dislocations in the principal 〈110〉{100} glide system. The strain in the ${\mathrm{BaF}}_{2}$(111) and ${\mathrm{CaF}}_{2}$(111) buffer layers is relaxed at room temperature regardless of whether they form the top layer in the stack or are overgrown by other layers. PbSe (as well as ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$Se and PbTe) top layers are capable of relieving the strain induced by the thermal-expansion mismatch even at 77 K, and after many temperature cycles between room temperature and 77 K. Even after 1400 such cycles, plastic relaxation still occurs on each cycle. The x-ray rocking curves, typically 150--190 arc sec wide, do not broaden on cycling. The total cumulative plastic deformation of the layer corresponds to as much as 400%. After the first few thermal cycles, no new thermal-strain-relieving dislocations are created, but the existing ones move back and forth on the same atomic glide planes with each cycle.

Radiation damage of epitaxial CaF2 overlayers on Si(111) studied by photon-stimulated desorption: Formation of surface F centers

The effects of radiation damage at the surfaces of CaF2 films grown on Si(111) substrates have been studied and the electronic transitions responsible for the formation of surface F centers have been identified by photon-stimulated desorption and soft x-ray photoelectron spectroscopy. The dominant desorption channel for F+ ions is a direct Auger-stimulated process following an electronic transition from Ca 3p to 3d-derived states above the CaF2 conduction band minimum. At the Si 2p edge, there is an indirect x-ray induced electron-stimulated desorption process that contributes only slightly to the desorption of F+. The changes in the kinetic energy distributions of the desorbing F+ ions as a function of film thickness are also discussed.

Mechanism of epitaxial growth of monolayer CaF on Si(111)-(7 x 7).

Using scanning tunneling microscopy, the epitaxial growth of a monolayer of CaF on the Si(111)-7\ifmmode\times\else\texttimes\fi{}7) surface has been studied. We observe how the Si atoms of the 7\ifmmode\times\else\texttimes\fi{}7 structure rearrange into a bulklike structure and find that four excess Si atoms per 7\ifmmode\times\else\texttimes\fi{}7 unit cell are produced in the transition to the bulklike structure. These excess Si atoms seem to remain in the epitaxial CaF layer, creating defects.

Surface modification of CaF2 on Si(111) by low-energy electron beam for over growth of GaAs films

Surface modifications of epitaxial CaF2 on Si(111) by low-energy electron beam, in order to get modified surfaces with reduced damage on which high quality GaAs films could be grown, were studied. By using x-ray photoelectron spectroscopy measurements, it was found that it is possible to modify the surface of CaF2(111) even though the energy of the electron beam was as low as ∼10 eV. The generation of defects in bulk CaF2 and the diffusion of As were found at an energy of 305 eV, whereas no such evidence of defect, but rather, a stable adsorption of As on the surface of CaF2, was observed at an energy of 40 eV.

Epitaxial bilayer growth of CaF2(111)/Pd(111) on CaF2(111) substrates

Epitaxial Pd(111) films were grown in UHV on air-cleaved CaF2(111) substrates. Subsequently, epitaxial CaF2(111) films were grown on top of the Pd layer. All films were grown by vacuum evaporation techniques. The growth conditions, structure, and surface morphology of the evaporated epitaxial bilayers of Pd and CaF2 were studied with reflection high energy electron diffraction, x-ray diffraction, and scanning electron microscopy. This epitaxial dielectric film/metal film/insulator substrate structure has possible important applications for monolithic microwave integrated circuit devices.

Multilayer structure for epitaxial growth of oxide films on Si with an underlying electrode

Multilayers of CaF2 , Pd, and Pt were epitaxially grown on HF treated (100)Si by sequential deposition of the desired materials in ultrahigh vacuum. This structure is suitable for epitaxial growth of oxide films on Si with an underlying electrode. The epitaxial layer CaF2 effectively acts as a barrier to impede metal-substrate reaction, and the metal bilayer Pd/Pt forms an epitaxial electrode with good adhesion to the underlying substrate and high resistance to oxidation. KNbO3 deposited on this multilayer structure showed epitaxial growth in a mixed (001)/(110) orientation.

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.