Growth of 3D photonic crystals by photolysis of ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$

Abstract

A method of three-dimensional (3D) holography is used for the growth of photonic crystals of sub-micron cell parameters by gas decomposition in a 3D interference field of UV laser light. Either amorphous or crystalline chromium oxides are obtained by photolysis of chromyl chloride ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$ for different experimental conditions. The most interesting product is obtained for a photolysis of ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$ at low pressure, on a cooled ${\mathrm{TiO}}_2$ single-crystal substrate and for a higher beam energy. The growth begins with the formation of epitaxial ${\mathrm{CrO}}_2$ metastable phase which is partly transformed into ${\mathrm{Cr}}_2{\mathrm{O}}_3$ under UV irradiation. Due to crystallographic orientational relationships between ${\mathrm{CrO}}_2$ and ${\mathrm{Cr}}_2{\mathrm{O}}_3$, the growth of a well organized 3D photonic crystal of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ phase goes on according to the 3D periodic modulations of electromagnetic energy of the interference field. In the present case, the ${\mathrm{Cr}}_2{\mathrm{O}}_3$ phase exhibits four sets of equivalent crystallographic orientations with respect to the single-crystal substrate. From such a result, one can expect to produce in a further work photonic crystals constituted of single-crystalline ${\mathrm{Cr}}_2{\mathrm{O}}_3$ phase in epitaxy onto a ${\mathrm{Cr}}_2{\mathrm{O}}_3$ or sapphire ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate.

Introduction

In 2000, Campbell et al. [1] demonstrated a fabrication of 3D photonic crystals through three-dimensional holographic lithography (3D HL). Thick layers of a commercial epoxy photo-resist have been patterned by these authors within the duration of single optical pulses of 10 ns by the intensity modulations that arise from the interference of four non-coplanar UV laser beams. Both face- and body-centered cubic structures with sub-micron lattice periodicity required for band gaps at visible wavelengths have been created. Later on, periodic structuring of organic–inorganic hybrid materials have also been performed [2] and a practical route to large-scale 3D photonic crystal device fabrication has recently been proposed by combining a direct two-photon laser writing with the 3D HL method [3].

From such results we have envisaged a possibility to obtain a direct growth of matter of pseudo-fcc organization ($a=922\mathrm{nm}$) by chemical vapor decomposition within a similar 3D interference field of UV laser light. An experimental setup, as reported in Duneau et al. [4] has been designed on the basis of a theoretical study in order to optimize the geometry and the contrast of the interference field. An interferometer adapted to a 10 Hz pulsed UV laser source at 355 nm has been realized and connected to a CVD reactor. A major difference with the single laser pulse irradiation of photo-resists or inorganic–organic hybrid compounds is, however, that a CVD growth requires series of laser pulses. Preliminary studies on the interference stability and on its accuracy have therefore been carried out both by video camera and by irradiation of organic–inorganic hybrids [5], [6].

Among several gaseous chemical precursors which can be decomposed by photolysis at a photon energy of 3.5 eV (i.e. corresponding to a wavelength of 355 nm) the chromyl chloride ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$ was chosen [7], [8], [9] . It has been shown that ${\mathrm{CrO}}_2$ and ${\mathrm{Cr}}_2{\mathrm{O}}_3$ are obtained by photolytic decomposition of this product in a state of adsorbed phase at wavelengths lower than 514 nm [10], [11]. The chromyl chloride has an extensive absorption spectrum in UV–visible and does absorb UV light at 355 nm [10], [12]. It is a red liquid at room temperature with a high vapor pressure (about 12 Torr at 20 °C) [13]. Arnone et al. [11] have extensively studied the photodeposition of ${\mathrm{CrO}}_2$ and ${\mathrm{Cr}}_2{\mathrm{O}}_3$ from ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$ and patented a method [14], [15]. As main results, these authors have shown that the rate of ${\mathrm{CrO}}_2$ formation is directly related to the amount of adsorbed chromyl chloride molecules in a pressure range of ${10}^{-3}$–$4\times {10}^{-2}\mathrm{Torr}$. This rate varies linearly with the laser power as long as the desorption remains negligible with increasing temperature. As the deposit of ${\mathrm{CrO}}_2$ increases, the UV absorption increases and a transition from photolysis to pyrolysis occurs. Indeed, ${\mathrm{CrO}}_2$ (a metastable phase of black color) decomposes into the stable ${\mathrm{Cr}}_2{\mathrm{O}}_3$ compound at about 400 °C according to the reaction ${\mathrm{CrO}}_2\to {\mathrm{Cr}}_2{\mathrm{O}}_3+\frac{1}{2}{\mathrm{O}}_2$ [16]. Besides, for large laser power and under a ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$ pressure of 0.1 Torr, a very high growth rate of about $3\mathrm{\mu }\mathrm{m}{\mathrm{s}}^{-1}$ has been observed by Arnone et al. [11], [15] as the result of a combined photolytic/pyrolytic reaction.

Results on the growth of chromium oxides obtained by photolytic decomposition of chromyl chloride ${\mathrm{CrO}}_2{\mathrm{Cl}}_2$ in a 3D interference field of UV laser light are reported in the present article. We show that interesting products are obtained at relatively high energy and low pressure on cooled single-crystal substrates. Besides, from a characterization of these products by transmission electron microscopy (TEM) and X-ray diffraction on a texture goniometer, we conclude to a further possibility to realize a growth of photonic crystal whose matter will be a metallic oxide in epitaxy onto a single-crystal substrate.