In this article, our results on the epitaxial crystallization of ion-bombarded crystalline silicon dioxide (α quartz) are reviewed. The epitaxial recrystallization of amorphized layers was achieved after alkali irradiation and annealing in air in the temperature range 650–875 °C. The systematic behavior of alkali ions in enhancing the regrowth rate both with decreasing ion size and increasing concentration is shown. The role of oxygen in the recrystallization was investigated by means of nuclear reaction analysis, by performing thermal treatments of the samples in 18O. A large amount of 18O diffuses inside the amorphous layer in the alkali-ion implanted samples at 600–800 °C. From the strong correlation between the migration of 18O and implanted alkali, it was possible to gain further insights into the recrystallization mechanism.

Epitaxial crystallization of keV-ion-bombarded α quartz

Epitaxial films of piezoelectric α-quartz could enable the fabrication of sensors with unprecedented sensitivity for prospective applications in electronics, biology and medicine. However, the prerequisites are harnessing the crystallization of epitaxial α-quartz and tailoring suitable film microstructures for nanostructuration. Here, we bring new insights into the crystallization of epitaxial α-quartz films on silicon (100) from the devitrification of porous silica and the control of the film microstructures: we show that by increasing the quantity of devitrifying agent (Sr) it is possible to switch from an α-quartz microstructure consisting of a porous flat film to one dominated by larger, fully dense α-quartz crystals. We also found that the film thickness, relative humidity and the nature of the surfactant play an important role in the control of the microstructure and homogeneity of the films. Via a multi-layer deposition method, we have extended the maximum thickness of the α-quartz films from a few hundreds of nm to the μm range. Moreover, we found a convenient method to combine this multilayer approach with soft lithography to pattern silica films while preserving epitaxial crystallization. This improved control over crystallization and the possibility of preparing patterned films of epitaxial α-quartz on Si substrates pave the path to future developments in applications based on electromechanics, optics and optomechanics.

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Tailoring the crystal growth of quartz on silicon for patterning epitaxial piezoelectric films

The migration of oxygen in ion-beam-amorphized c-SiO2 (α-quartz) was investigated by means of nuclear reaction analysis using the resonant reaction 18O(p,α)15N for oxygen depth profiling. Only very small amounts of oxygen were observed to diffuse in crystalline or in Xe+-ion beam-amorphized α-quartz after high-temperature annealing. However, a dramatic migration of oxygen occurs in Cs+-implanted α-quartz in the same temperature range (600–900 °C), where Cs diffuses out of the amorphized layer and epitaxial recrystallization occurs. These results point out to a strong correlation of all these processes. A mechanism to explain the observed indiffusion of 18O is proposed and is related to the Cs migration and the topological modification to achieve epitaxial regrowth of the SiO2 matrix.

Oxygen migration during epitaxial regrowth in Cs + -irradiated α-quartz investigated by means of nuclear reaction analysis

Optical functionality of materials used in devices is the basis of modern photonics. It depends on selected photoactive impurities or low-dimensional structures, which can be tailored by ion implantation. The present survey covers cathode-luminescence spectroscopy performed after Ge, Ba, Na, Rb and Cs ion implantation in α-quartz, in connection with dynamic, laser-induced and chemical epitaxy of the surface layers amorphized during the ion irradiation. The correlations, which emerged for various luminescence bands, ion species and thermal processing methods, allows one to classify the bands into ion-specific and intrinsic ones. The microstructural properties measured by ion beam analysis and transmission electron microscopy will be combined with the luminescence data, and the role of photoactive defects in quartz and nanoparticles of the implants will be discussed. The technologically most attractive case of double Rb/Ge implantation will be highlighted, which combines the achievement of full chemical epi...

Luminescence of ion-irradiated α-quartz

Laser-induced epitaxial recrystallization after alkali-ion implantion into α-quartz

A very strong exchange of oxygen between $\ensuremath{\alpha}$-quartz ${(}^{16}\mathrm{O})$ and annealing atmosphere ${(}^{18}\mathrm{O})$ observed during solid-phase epitaxial growth of ${\mathrm{Li}}^{+}$- and ${\mathrm{Cs}}^{+}$-ion-beam-amorphized single-crystal $\ensuremath{\alpha}$-quartz is reported. Epitaxial regrowth was observed in Li-irradiated samples after $700\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ annealing and in Cs-irradiated samples after $870\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ annealing, by means of Rutherford backscattering spectrometry in channeling geometry. The ${}^{18}\mathrm{O}{/}^{16}\mathrm{O}$ exchange and the outdiffusion of Li were investigated by the use of time-of-flight elastic recoil detection analysis. Our experiments show that alkali-ion implantation strongly enhances the exchange of ${}^{16}\mathrm{O}$ in ${\mathrm{SiO}}_{2}$ with ${}^{18}\mathrm{O}$ of the annealing atmosphere. The exchange accompanies the loss of alkali atoms, thus favoring the recrystallization of the lattice. Mechanisms of epitaxial regrowth in Li- and Cs-implanted $\ensuremath{\alpha}$-quartz are discussed.

J. Keinonen

Papers

Oxygen-activated epitaxial recrystallization of Li-implanted α − SiO 2