The Annealing Conditions' Effect On The Electrical And Optical Properties Of The Vanadium Oxides Thin Film

Vanadium, a transition element, can compose multivalent vanadiumoxides with oxygen, which have been widely studied because of theirexcellent characters. Here, V₂O₅ thin films were deposited by vacuumevaporation method from V₂O₅ powder (purity≥99.99%), and were directlyannealed in the retrofitted vacuum coating machine. Then several pure andwell-crystallized vanadium oxides thin films, such as V₄O₉, V₆O₁₃, VO₂(A),VO₂(B), were prepared. And their crystal structure, valence, surfacemorphology, electrical properties and optical properties were studied, whichwere affected by the annealing conditions, especially for the properties ofthe VO₂(A) thin films and VO₂(B) thin films. The results were showed asfollows:(1)When the substrate was heated to 250℃, V₂O₅ thin films depositedwere pure and best-crystallized, in which the valence of the vanadium wassingle+5. In our experiment, they were selected and were directly annealedin the retrofired vacuum coating machine, and the lower valent vanadiumoxides thin films were gained. To our knowledge, this is a new method, formost of other researchers' method is that the thin films were annealed in theannealing furnace, which required two steps: to take the thin films out from the coating machine and to put them into the annealing furnace.(2)The effects of the annealing vacuum on the phase of the vanadiumoxides thin films were reported firstly. With the increase of the annealingtemperature, the films annealed in high vacuum changed from V₄O₉, toVO₂(B), to a mixture of VO₂(B) and V₆O₁₃, to V₆O₁₃, while the filmsannealed in low vacuum changed from a mixture of V₄O₉ and VO₂(B), toVO₂(B), to VO₂(A).(3)The phase of the thin films annealed in different conditions weredifferent, which resulted that their resistance and temperature coefficientresistance varied greatly, and that only the VO₂(A) thin films showed theproperty of the thermochromic phase transition.(4)The valence of the vanadium in the films decreased when annealedboth in high vacuum and in low vacuum, and with the increase of theannealing temperature, it became lower.(5)The vanadium oxides thin films were reduced more easily whenannealed in low vacuum than when annealed in high vacuum, but the filmscrystallized better when annealed in high vacuum than in low vacuum,which has not been reported.(6)The VO₂(B) thin films were gained when annealed in low vacuumfrom 400℃to 480℃, but they were gained when annealed in highvacuum only from 400℃to 440℃. The resistance and temperaturecoefficient resistance's absolute value of the thin films decreased with theincrease of the annealing temperature from 400℃to 440℃whenannealed both in both in high vacuum and in low vacuum. However, theydecreased more rapidly and the temperature coefficient resistance's absolutevalue of the films was bigger when annealed in low vacuum. And thetransmission of the VO₂ (B) thin film in the ultraviolet, visible and nearinfrared was lower when annealed in high vacuum than when annealed inlow vacuum. (7)The VO₂(A) thin films were gained when the thin films annealed inlow vacuum. When the annealing temperature varied, their properties of thethermo chromic phase transition of the VO₂(A) thin films, such as thecritical temperature, the order of magnitude of phase transition and thehysteresis loop width also changed. The transmission of the VO₂(A) thinfilm decreased with the decrease of its critical temperature, but only itstransmission in middle infrared changed corresponding to the variety of theorder of magnitude of phase transition.