Pulsed Laser Deposition Of High-quality Ru_xV_1-xO₂ Thin Films And Their Properties

VO₂is an excellent thermochromic material based on its insulator-to-metal transition(IMT)at?68 ?.The IMT is accompanied by a structural transition from(low temperature)monoclinic(M)to(high temperature)rutile(R)crystal structure.MIT and structural transition are reversible and occur on very short time scales(about 10^-12s).The electrical resistivityrchanges significantly at the transition temperature(or critical temperature,T_c)exhibiting an abrupt change of?rby 5 and 3 orders of magnitude for bulk single crystals and sputtered thin films,respectively.The optical properties are also affected by the MIT,e.g.,in the infrared(IR)region,a high optical transmittance in the insulating phase contrasts with a low optical transmittance in the metallic phase,providing a strong optical switching.The MIT may also be triggered by other external stimuli,e.g.light pulses or external electric fields.These switchable physical properties of VO₂have attracted much research attention.Ru doped VO₂(Ru_xV_1-xO₂)thin films were prepared on c-surface sapphire(c-Al₂O₃)and TiO₂(110)substrates by pulsed laser deposition(PLD),The effects of deposition oxygen pressure on the structure and MIT properties of VO₂thin films grown on c-Al₂O₃,the structure and MIT properties of Ru_xV_1-xO₂thin films grown on c-Al₂O₃and TiO₂(110)were systematically studied.The main contents and results of this paper are as follows:1.High-quality single crystal VO₂thin films were prepared on c-Al₂O₃substrate by adjusting oxygen pressure during pulsed laser deposition.The effect of oxygen pressure on the properties of VO₂films were studied.The results show that:These films are highly oriented along the(010)crystal plane with the full width at half maximum of rocking curves in the range of 0.050-0.091°,indicating a high crystal quality of the films.The composition of Ru_xV_1-xO₂film prepared under low oxygen pressure(1.5 Pa)deviates severely from the stoichiometry and contains a lot of O vacancy defects.As the oxygen pressure increases,the composition of the films approaches the stoichiometric ratio of2:1,and all the films prepared under oxygen pressures above 2.4 Pa show significant MIT characteristics.The phase transition temperatures of these films are between 50-55?,and the resistivity changes by three orders of magnitude before and after the phase transition.At the same time,the films show high modulation ability for infrared light up to 17%.If the oxygen pressure is too high,the film deposition process is degraded with a reduced growth rate.2.Thin films of Ru_xV_1-xO₂with various Ru contents were deposited on Al₂O₃(0001)substrates by pulsed laser deposition.Ru inlayed V metal targets were used as the ablation sources,and the substrate temperature and O₂pressure were fixed at 600? and 4.0 Pa,respectively,during the deposition of all the films.Along with the 2?-scans,it reveals the following epitaxial relationship:VO₂(010)//Al₂O₃(001),and VO₂[100]//Al₂O₃[10-10].At higher Ru contents,we observed the formation of Ru metallic inclusions.We found that Ru incorporation decreases the MIT temperature of VO₂considerably,and determined a value of42? for Ru_0.04V_0.96O₂.It is noteworthy that there is only a small narrowing of the band gap with increasing Ru content whilst the modulation capacity of the IR transmittance fairly persists.It should be noted that,because of the same+4 valence state and nearly the same ionic radii of Ru and V,the solubility of Ru in VO₂,i.e.,the formation of a Ru_xV_1-xO₂alloy should be possible for x higher than 4%,when the growth conditions are further optimized.This suggests that incorporating of something like 8%Ru in VO₂should allow one to bring down the MIT temperature close to room temperature.3.Thin films of Ru_xV_1-xO₂with various Ru contents were deposited on TiO₂(110)substrates by pulsed laser deposition.Ru inlayed V metal targets were used as the ablation sources,and the substrate temperature and O₂pressure were fixed at 600? and 6.3 Pa,respectively,during the deposition of all the films.From the in situ temperature dependent reciprocal space map,the MIT of Ru_xV_1-xO₂thin film is accompanied by a structural transition from(low temperature)monoclinic(M)to(high temperature)rutile(R)crystal structure;at the same time,it can also be found that the(011)plane spacing of the film increases with the Ru content gradually increasing,T_cdecreases with increasing Ru content,which is mutually confirmed with the results of electrical tests.When the Ru content is 7%,the electrical thermal hysteresis of the film disappeared,and the T_cdrops to 38?,while the optical and electrical MIT still exists.The TCR value can reach-5.6%/?,which is higher than that reported in recent VO₂studies TCR value.This result is conducive to promoting the application process of Ru_xV_1-xO₂on infrared detectors.The efficiency of Ru in reducing the Tc of the Ru_xV_1-xO₂film is 4.05?/mol%,which is slightly lower than that of the Ru_xV_1-xO₂alloy films grown on c-Al₂O₃.The reason may be that the diffusion of Ti in the substrate into the film reduces the modulation effect of Ru on T_c.Ru doping has little effect on the visible light modulation ability of the film(?T_lum?3.7%),and the infrared light modulation ability(?T_IR)gradually weakens as the Ru content increases.