The nano-composite nature of vanadium oxide thin films for use in infrared microbolometers

The current generation of portable, un-cooled infrared imaging devices utilizes thin-film materials with large thermal coefficients of resistivity. Incoming photons are absorbed by the material, converted into heat, and result in a decrease in the resistivity of the thermal sensing layer. Vanadium oxide thin films are used in the majority of these devices as they typically have very large thermal coefficients of resistivity with low noise characteristics.;In the work reported here, reactive pulsed DC sputtering was used to grow a systematic series of vanadium oxide thin films with resistivity ranging from 1 x 10-3 to 6.8 x 104 Ohm cm and TCR varying from 0 to 4% K-1. Throughout the parameter space studied, a transition from amorphous to nano-crystalline growth was observed. Films in the range of interest for a microbolometer, i.e. 1 x 10-3 to 10 Ohm cm, contain the face-centered cubic (FCC) VO x (0.8 < x < 1.3) phase. Films with larger resistivity were found to be amorphous. Stoichiometry measurements via Rutherford backscattering spectroscopy place many of the nano-crystalline films outside of the FCC VO x phase field according to the bulk phase diagram. Electron diffraction in the transmission electron microscope confirmed the presence of a secondary oxygen-rich amorphous vanadium oxide phase. The oxygen-rich amorphous phase explains the discrepancy between the observed oxygen content, which is outside of the FCC VOx phase field, and the presence of FCC VOx, which is limited to a maximum oxygen content of x = 1.3.;The resulting microstructure can be described as a nano-composite material composed of a low resistivity crystalline phase embedded in a high resistivity amorphous matrix. A mechanism has been proposed wherein the nano-composite structure of the films results from film growth in alternating oxygen deficient and oxygen rich regions in the chamber. While in the oxygen deficient region the nano-crystalline phase grows preferentially, and while in the oxygen-rich region the amorphous phase grows preferentially. The resulting microstructure is a mixture of the amorphous and nano-crystalline phases, both of which can be controlled by altering the sputtering geometry. A general mixing rule was used to simulate the observed transport. The resulting electrical properties vary from equivalent circuits of resistors in parallel to resistors in series. The observed data can only be represented by a structure between the two extreme cases, suggesting that neither a percolative network nor isolated grains are desired for application in microbolometers. This work also suggests that the use of vanadium oxide is not specifically necessary, and nanocomposites of other conductors and insulators could result in materials with large negative thermal coefficients of resistivity suitable for use in un-cooled infrared microbolometers.