Structure-functional property relationships in rf-sputtered vanadium dioxide thin films

Abstract

The study of metal-insulator transition (MIT) in VO2 thin films synthesized by means of rf sputtering from a VO2 target is presented. A comparison with conventional reactive sputtering from a V target is also given. Detailed x-ray diffraction analysis, electrical resistance switching, and infrared optical reflectance measurements confirm that our sputtering technique yields high-quality VO2 films. We discuss in depth how synthesis conditions affect MIT parameters derived from temperature dependence of electrical resistance. Sharp MIT is observed in films sputtered on technologically important Si substrates. The choice of Si (or sapphire) substrates results in the transition temperature above (below) the values obtained for single VO2 crystals. The MIT becomes narrower and stronger in thinner films. This is consistent with the assumption that the increased width of the MIT in thin films with respect to single crystals is the result of averaging of the transition parameters over a distribution of crystallites in the film. The measurements of MIT in VO2 patterned into devices do not reveal a noticeable lateral size effect down to 20 μm devices, encouraging use of the phase transition in switching electronic devices. The effect of substrate temperature and ambient during the sputtering on MIT is discussed. While VO2 films are found to be stable in ambient environment with time, the additional exposure to UV radiation near room temperature is shown to enhance the oxidation kinetics and produce changes in film resistance. Using UV radiation as an additional tool to control the oxidation process during VO2 synthesis may allow the synthesis temperature to be lowered and an improvement in material quality. We anticipate these results may be of relevance to synthesizing functional oxide films with potential applications in electronics and sensors.

Synthesis of vanadium dioxide thin films on conducting oxides and metal–insulator transition characteristics

“…The sputtering was performed in an Ar environment at set pressure of 10 mTorr from a V 2 O 5 target for 6 hours, with the substrate temperature and RF source power set at 550 o C and 120 W, respectively. Detailed description of VO 2 film growth by sputtering and deposition condition effects on properties can be found in previous reports [4,8].…”

Nanoscale imaging and control of resistance switching in VO2 at room temperature

“…Epitaxial films on insulating Al 2 O 3 may have RR up to 10 412 . As a route to interface with existing electronics, recent effort has been devoted to film growth on Si substrates 13 , where voltage-controlled switching of VO 2 in capacitor geometry has been demonstrated at room temperature 14 . However, the lattice mismatch results in polycrystalline VO 2 films with RR so far limited to ∼ 10 3 .…”

Three-terminal field effect devices utilizing thin film vanadium oxide as the channel layer

“…fabricate such a device we have found that a high-quality VO 2 film sputtered on a sapphire substrate would not conduct if its thickness is below d min =45nm. More details of our VO 2 synthesis technique and the relationship between film morphology and its electrical properties are described elsewhere [8]. The absence of conductance is attributed to a discontinuous morphology in the VO 2 film, consisting of isolated VO 2 islands, as confirmed by topographic scans using atomic force microscopy.…”

“…In our tests, the initial deposition of under metallic leads to provide low resistivity contacts. The large contact pads are necessary to eliminate the distortion of the R vs. T curve above the transition temperature due to the contact resistance as was shown in our previous study [8]. The VO 2 stripe size between the gold electrodes is 110μm x 8μm.…”

“…Published in JOURNAL OF APPLIED PHYSICS 107, 114516 (2010) 3/30 In conventional observations of the MIT in VO 2 , the electrical resistance or an optical parameter, such as reflectance, is measured while the temperature of the sample is ramped over the transition temperature (T MIT ) near 70ºC [7,8]. A drop in resistance, or a jump in reflectance, is observed once the sample is heated across T MIT .…”