Study of VO2 thin film synthesis by atomic layer deposition

Abstract

Vanadium dioxide emerges as an appealing material for smart thermal management and electrical/optical switching owing to its abrupt semiconductor-to-metal transition (SMT) at near room temperature. The application potential of this material should be leveraged by the implementation of the atomic layer deposition (ALD), a method that enables an unequaled control over the thickness and film conformality on complex substrate structures by virtue of self-limiting growth mechanism. Here, the vanadium oxide thin films are synthesized from vanadyl (V) triisopropoxide (VTOP) as the vanadium precursor and H2O as the reactant. The thermal decomposition threshold of VTOP was observed at 90 °C, and temperature window with a constant growth rate was observed at 50–90 °C. The saturation behavior of the ALD half reactions was demonstrated at 80 °C, yielding the optimized ALD cycle conditions of 8 s VTOP/15 s purge/6 s H2O/15 s purge with a growth rate of 0.047 nm/cycle. The as-grown amorphous films convert into VO2 via a 4-h heat treatment under 10−5 mbar at 550 °C. The formed VO2 features a sudden and reversible change in the total hemispherical reflectance in UV-VIS-NIR region at 68 °C, confirming its SMT behavior. The conversion process induces, however, a surface roughening, which was considerably suppressed by a postdeposition treatment in plasma environment prior the annealing treatment.