Metalorganic chemical vapor deposition of highly oriented thin film composites of V 2 O 5 and V 6 O 13 : Suppression of
- PDF / 1,438,710 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 44 Downloads / 154 Views
Thin films of vanadium oxides were grown on fused quartz by metalorganic chemical vapor deposition using vanadyl acetylacetonate as the precursor. Growth at temperatures 艌560 °C results in composites of strongly (00l)-oriented V2O5 and V6O13. The dominant phase of the film changes from V2O5 to V6O13, and back to V2O5, as the growth temperature is raised from 560 to 570 °C, then to 580 °C, as evidenced by x-ray diffraction and Rutherford backscattering analyses. This reentrant-type growth trend was interpreted on the basis of the small difference in the free energy of formation of V2O5 and V6O13 and the presence of metal–oxygen bonds in the precursor. In contrast with single-crystalline V6O13, the film predominantly composed of highly oriented single-crystalline platelets of V6O13 did not undergo the semiconductor–metal transition at −123° K, despite the connectivity being well above the percolation threshold. Instead, a semiconductor-to-semiconductor transition was observed in this film, which is explained in terms of the observed relaxation of the edges of all the platelets of metallic V6O13 to semiconducting V2O5.
I. INTRODUCTION
The vanadium oxide system is rich and interesting because of the diversity of physical properties observed in apparently similar phases. For example, while vanadium pentoxide is a wide band gap semiconductor,1 V6O13, which has a very similar structure, is a metal at room temperature. V6O13 undergoes a metal–semiconductor (M-S) transition at −123° C, with a jump in resistivity by a factor of 105, though this is observed only in single-crystalline specimen.2 Sintered polycrystalline V6O13, by contrast, is reported to be a semiconductor at room temperature3 with activation energy for conduction Ea of 0.42 eV. It exhibits a semiconductor-to-semiconductor (S-S) transition3 at approximately 150 K, where the resistivity jumps by about an order of magnitude, the Ea at low temperatures being 0.21 eV. The M-S transition is accompanied by a crystallographic distortion.4 Furthermore, an antiferromagnetic ordering5 takes place in V6O13 at 50 K. While V2O5 is thermodynamically stable, the V6O13 phase, being an ordered defect structure closely related to V2O5, is metastable. Although accurate thermodynamic data (⌬G, the Gibbs free energy of formation) are not available for
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0394 J. Mater. Res., Vol. 19, No. 10, Oct 2004
http://journals.cambridge.org
Downloaded: 14 Mar 2015
V6O13, it may be expected from the V–O binary phase diagram6 that the free energy of V6O13 is likely to be close to that of V2O5. This is substantiated by the present study and is understandable given the intimate relationship between the crystal structures of V2O5 and V6O13. The V2O5 lattice in the c direction can be considered as stacking of alternately pure vanadyl oxygen (O) and mixed vanadium oxygen (V–O) layers [Fig. 1(a)]. Along the c axis, each vanadyl oxygen is bonded to two vanadium atoms, in one direction by a double bon
Data Loading...
