Epitaxial CaF2 Films Research Articles

Epitaxial Growth of CaF2 Films on Si(111) Studied by Scanning Tunneling Microscopy

Using ultrahigh vacuum scanning tunneling microscopy, we have studied the early stages of the epitaxial growth of CaF2 films on Si(111) at 630°C at a slow deposition rate of 0.2 monolayer (ML) per minute. After formation of CaF interface layer, we observe the film grows in the sequence of step edge decoration, one-dimensional islands growth on the terrace, formation of clusters of the islands, coalescence of the island cluster and filling of gaps between the one-dimensional islands in the cluster and between the island clusters to complete a layer. By such a sequence, epitaxial CaF2 films with atomically flat surface could be grown in layer-by-layer fashion up to 3 ML. The growth mode is discussed in relation to kinetic phase diagram [M. A. Olmstead: Thin Films: Heteroepitaxial Systems, eds. W. K. Liu and M. B. Santos (World Scientific, Singapore, 1999) Chap. 5].

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.

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.

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.

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.

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.

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.

The Effect of Annealing on the Structure of Epitaxial CaF2 Films on Si(100)

ABSTRACTWe have studied the effect of different annealing conditions on the crystallinity and morphology of 1000Å epitaxial CaF2 films on Si(100). The crystallinity of the films is improved by the anneals, with the highest anneal temperatures giving the greatest improvement. The results are consistent with the model previously proposed to explain the success of rapid thermal annealing in improving thicker epitaxial CaF2 films.

Thermal mismatch biased rhombohedral structure of strained epitaxial CaF2 films on Si(111)

The capability of growing high-quality strained-layer systems from lattice-mismatched materials (semiconductors, dielectrics, metals in combination) offers a number of attractive opportunities in both basic and device research. The strain field inherent in pseudomorphic growth of dissimilar materials causes the bulk structure of the deposit to be changed, depending not only on the misfit value of the pair lattice constant but also on the mismatch of the thermal expansion coefficients. Epitaxial layers of CaF2 grown 30 nm thick by MBE on Si(111) wafers, are found to be strained of tensile type, resulting in a rhombohedral substitution of the otherwise cubic structure. This fact is discussed in terms of the difference in thermal expansion coefficients of deposit and substrate rather than by a simple model of pseudomorphic growth. An exact calculation is presented of both, the strain perpendicular to the interface and the angular misalignment of crystallographic rows. In order to obtain pseudomorphic growth in the system CaF2/Si(111), the critical epi-layer thickness must be smaller than 30 nm used in the experiments. Die Herstellung von hochwertigen verspannten Schichtsystemen aus gitterfehlangepaßten Ausgangswerkstoffen (Halbleiter, Dielektrika, Metalle) ermöglicht der Grundlagen- und angewandten Forschung interessante Möglichkeiten. Das bei pseudomorphem Wachstum verschiedener Werkstoffe aufeinander verursachte Spannungsfeld verändert die Gitterstruktur des Deposits in charakteristischer Weise. Neben Unterschieden in der Gitterkonstante der Partner spielen hierbei verschiedene thermische Ausdehnungskoeffizienten eine große Rolle. Mittels MBE gezüchtete 30 nm dicke CaF2-Schichten auf Si(111)-Unterlage weisen eine Dehnungsverspannung auf, die eine rhomboedrische Struktur gegenüber der im Volumen üblichen kubischen Struktur zur Folge hat. Dies wird auf der Basis unterschiedlicher thermischer Ausdehnungskoeffizienten der Partner diskutiert. Eine exakte Berechnung der Verspannung senkrecht zur Grenzfläche sowie von Winkelabweichungen kristallographischer Gitterrichtungen wird angegeben. Für pseudomorphes Wachstum von CaF2 auf Si(111) muß die kritische Schichtdicke offenbar unterhalb der gewählten 30 nm liegen.

The Electronic Properties of Epitaxial Calcium Fluoride-Silicon Structures

ABSTRACTThe characterization of electronic devices using epitaxial CaF2 on Si is described. In addition, the growth and annealing techniques used to obtain high quality epitaxial films are discussed. In particular, the results of using rapid thermal annealing to improve the epitaxial quality of CaF2 films are presented in detail.The electronic and electrical properties of these structures are very promising. Epitaxial CaF2 films with breakdown fields as high as 3 × 106 V/cm and interface trap densities as low as 7 × 1010cm-2eV-1 have been fabricated. In addition, minority carrier dominated trapping has been observed at the CaF2 /Si interface. Finally, the material properties of these structures, as determined by Rutherford backscattering, channeling, and electron microscopy, are discussed and correlated with their electronic properties.