Chromium (VI) oxide (chromium trioxide) CrO3
The crystal structure of CrO3 consists of infinite chains of corner-sharing CrO4 tetrahedra running parallel to the c axis. The bridging Cr-O bond length is 1.748 Ǻ, the terminal Cr-O length is 1.599 Ǻ. The angle at the brodging O atom is 143°. [858]
Chromium trioxide CrO3 with O2 as carrier gas (200sccm), has been applied as precursor for the selective atmospheric-pressre CVD growth of epitaxial CrO2 (110) thin films on MgO(001) substrates with TiO2(110) buffer layers obtained by oxidizing TiN(001) thin films upon annealing process in low-pressure oxygen. XRD and SEM revealed that CrO2(110) films are orthogonally twinned and formed with small grains of CrO2 of 3-5 µm size. Epitaxial relationship of the sample was CrO2(110)[001]∥TiO2(110)[001]∥MgO(001)[110]
Chromium trioxide CrO3 has been used for the deposition of CrO2 thin films on sapphire (0001) at growth temperature 390°C (just below decomposition temperature of CrO2 to Cr2O3), what allowed to obtain epitaxial CrO2 films. The precursor was evaporated at 260°C (melting point of CrO3) and trasported to the deposition area by the O2 carrier gas flow of 500sccm.
Highly oriented a-axis CrO2 films (containing highly oriented (0001)Cr2O3) were grown on Al2O3(0001) substrates by atmospheric pressure (AP) CVD at low deposition temperature (330 °C) using CrO3 as precursor (vaporised at 260°C), and oxygen as carrier gas. Deposition rate was significantly varying with the substrate temperature, whereas film surface microstructure depended mainly on film thickness. CrO2 growth kinetics was dominated by a surface reaction mechanism with activation energy of 121.0 ± 4.3 kJ/mol. Sharp magnetisation transition at 375K and saturation magnetisation of 1.92 µB/f.u., close to the bulk value of 2µB/f.u. were observed.The presence of a Cr2O3 layer at the CrO2 film/interface was explained by some structural effects during the study of the growth process as a function of the deposition time [861]
Epitaxial CrO2 films were grown by atmospheric pressure CVD using thermal decomposition of gaseous CrO3 onto rutile (TiO2) single crystals in air. Pure CrO2 epitaxial films were produced the optimum substrate temperature of 390°C; layers included Cr2O5 impurity at growth temperature of 380°C, and Cr2O3 appeared at 400°C. Magnetic domain patterns of these films were studied by longitudinal Kerr effect. [863]
Thin epitaxial CrO2 films (a thickness series 27-535 nm) were deposited on single-crystal rutile TiO2(100) substrates by CVD using CrO3 as a solid precursor; layer magnetic anisotropy was strongly thickness-dependent according to the ferromagnetic resonance (FMR) studies. In the thinnest films (27 nm), the strain-induced anisotropy was predominant and the easy magnetization axis switched from the [001] direction (characteristic of the bulk magnets) to the [010] direction. [864,865]
Solid CrO3 precursor was used for the CVD growth of epitaxial CrO2 (100) and CrO2 (110) films on TiO2 (100) and TiO2 (110) substrates, respectively. Layer-by-layer growth mode was observed on TiO2 (100) resulting in smooth surfaces but significant out-of-plane compressive stress. In contrast, films on TiO2 (110) follow an islandlike growth mode; even the thinnest films (∼35 nm) are practically strain free. CrO2 (100) films display strong change of magnetic anisotropy with increasing thickness due to the substrate-induced stress, in contrast to the CrO2 (110) films. [866]
CrO3 was apllied as the precursor for the atmospheric-pressure CVD growth assisted by AAO templatesof the high-density vertically aligned CrO2 nanowire arrays which presented significantly improved coercivity compared with CrO2 films or bulk. Nanowire lenght was strongly influenced by the AAO template pore diameter. [867]
CrO2 thin films were deposited using CrO3 precursor by laser-assisted CVD [868]
CrO3 powder as precursor was used for the preparation of CrO2 films by thermal CVD on sapphire at 320 - 410 ºC temperatures and 50 - 500 sccm O2 carrier gas flow. Highly oriented a-axis CrO2 films were prepared at temperatures as low as 330 ºC; they kept high quality magnetic and transport properties as those deposited at higher temperatures. [869]
High-quality epitaxial CrO2(100) films were prepared on TiO2(100) substrates by CVD using CrO3 as precursor material. The spin-resolved electronic structure of the films was investigated by means of x-ray absorption and spin-resolved photoemission spectroscopy. Near EF an energy gap was observed for spin-down electrons; a spin polarization of about +(90 ± 10)% was found at 293 K, in agreement with the half-metallic nature of CrO2. [870]
Epitaxial CrO2/RuO2 thin film heterostructures were grown on TiO2(100) substrates by atmospheric pressure CVD using CrO3 and Ru(thd)3 precursors. Current-in-plane and current-perpendicular-to-plane giant magnetoresistive stacks were fabricated with either Co or another epitaxial CrO2 layer as the top electrode, however, magnetoresistance was low, probably due to the appearance of a chemically and magnetically disordered layer at the CrO2 and RuO2 interfaces when Cr2O3 was transformed into rutile structures during its intermixing with RuO2. [871]
Decomposition of CrO3 on the substrate contained in a pressure vessel, which can be considered as a kind of CVD, was used for the epitaxial growth of oriented layers of CrO2 on on the {0001} planes of single crystal Al2O3 and Fe2O3{100}, and on the {110}, {210}, {001} surfaces of single crystal TiO2 (rutile). The CrO2 layer begins to form during the decomposition of Cr2O5 by nucleation of oriented CrO2 at many sites on the substrate surface, with the perfection of the oriented layer being limited mainly by the defects on the surface of the substrates. Film characterization by chemical analysis and XRD indicated that the epitaxial CrO2 is identical in all respects to the bulk material; the oriented CrO2 layers have been characterized by magnetic measurements [[i][PS1] ]
Octachromium unicosoxide Cr8O21 is the intermediate oxide between CrO3 and CrO2, it was possible to be prepare it ex situ, store and use directly for the CVD growth. Cr8O21 is formed during CrO3 decomposition (i.e. CrO3 does not decompose directly to CrO2 and oxygen, as it had been previously thought). Ivanov proposed the hypothesis that the role of Cr8O21 in the CVD process is to exude unstable molecules of CrO4, and that the reaction on the substrate is the decomposition CrO4→ CrO2 + O2.
Octachromium unicosoxide Cr8O21 has been applied for the CVD growth of highly ordered CrO2 films on TiO2(110), TiO2(100), and Al2O3(0001) substrates, as well as variety CrO2-containing heterostructures such as superconductor/insulator/CrO2 and Co(polycr.)/AlOx/CrOx/CrO2(100)/substrate-TiO2(100). [872]
[i] R.C. DeVries, Materials Research Bulletin, Volume 1, Issue 2, October 1966, Pages 83-93