Effect of W addition on the electrical switching of VO 2 thin films

Vanadium Oxide has been a frontrunner in the field of oxide electronics because of its metal-insulator transition (MIT). The interplay of different structures of VO2 has played a crucial role in deciding the magnitude of the first order MIT. Substitution doping has been found to introduce different polymorphs of VO2. Hence the role of substitution doping in stabilizing the competing phases of VO2 in the thin film form remains underexplored. Consequently there have been reports both discounting and approving such a stabilization of competing phases in VO2. It is reported in the literature that the bandwidth of the hysteresis and transition temperature of VO2 can be tuned by substitutional doping of VO2 with W. In this work, we have adopted a novel technique called, Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM) to deposit VO2 and W- doped VO2 as thin films. XRD and Raman spectroscopy were used to investigate the role of tungsten on the structure of VO2 thin films. Morphology of the thin films was found to be consisting of globular and porous nanoparticles of size similar to 20nm. Transition temperature decreased with the addition of W. We found that for 2.0 at % W doping in VO2, the transition temperature has reduced from 68 degrees C to 25 degrees C. It is noted that W-doping in the process of reducing the transition temperature, alters the local structure and also increases room temperature carrier concentration. (c) 2016 Author(s).


I. INTRODUCTION
Vanadium dioxide is a model strongly correlated system that undergoes a first order metalinsulator transition (MIT) displaying both structural and Mott transition.MIT occurs when the carrier density in the material exceeds the critical carrier density i.e., electron-electron interaction.During the transition, the crystal structure changes from (P21/C) monoclinic at room temperature to (P42/mnm) tetragonal at high temperature.The monoclinic structure has the V-V pairs along the a mono = 2 c rutile axis with alternate V-V separations of 2.65 Å and 3.12 Å .The tetragonal phase above the transition temperature has the V-V separations of 2.87 Å. 1 In addition to the MIT, band structure also changes from the metallic phase to the insulating phase.Across the MIT, the 3d I I band splits into two bands: a) a lower energy, filled bonding configuration (3d I I band) and b) a higher energy, empty anti bonding configuration (3d I I * band).The anti-bonding 3d Π * band is pushed to a higher energy.4][5] Starting with M1 (monoclinic), it undergoes intermediate phase changes such as M2 (C2/m centered monoclinic), T(triclinic) and R (rutile) phases. 6Where M2 is reported as an intermediate between M1 and R, T occurs between M1 and M2 7,8 i.e., M1 −→ T −→ M2 −→ R. Interplay between the intermediate a Electronic mail: rajeswaran.bharathi@gmail.com,rbharathi@mrc.iisc.ernet.instructures and competition between the different mechanisms are also affected by the stresses caused by substrates in thin films, interactions with the substrates, 9 doping [10][11][12] and defects. 13n the literature, attempts have been made to understand the mechanisms behind the effect of dopants on the structure and MIT of VO 2 .It was demonstrated that doping of Al and hydrogen alter the MIT characteristics.Al stabilizes the M2 phase or when hydrogen is used in reversible doping process, the MIT characteristics differ. 14Reports also suggest the formation of M2 phase under uniaxial strain. 15his variation in resistivity during M1 −→ R transition, can be huge, upto 5 orders of magnitude, in the case of single crystals 16 with the transition temperature of 68 o C.However, polycrystalline thin films of VO 2 exhibit a change in resistivity of 4 orders of magnitude. 98][19] Characteristics of the transition is influenced by the nature of the phases (M1, M2, T, R), microstructure (i.e.morphology and relative orientation of the grains and the grain boundaries) 20 and doping. 21hile some reports have shown emergence of M2 phase with W, there are also others that discount the possibility of stabilization of M2 phase. 22Interestingly doping VO 2 with W also causes huge changes in the transition characteristics by charge compensation mechanisms, like the introduction of extra electrons into the V-d bands via the substitution of W 6+ in the place of V 4+ .Thus, the V sublattice doping with W has resulted in a pronounced reduction in the transition temperature by 20 K/at % W for the bulk and by 50 K/at % W for nanostructures. 23At higher concentrations of W, metallic state of VO 2 results.Thus the experiments on the system W x V 1−x O 2 would not only be interesting in terms of applications but also in enriching our understanding on role of carrier concentration and on competition between different structural phases.
Weakening of V-V pairs occurs either by charge transfer 24 or by the expansion of crystal axes. 25n order to understand the mechanism induced by W-doping on VO 2 , we have tried to systematically study W x V 1−x O 2 (x=0.2 at % to 2 at %) by I-V characterization, Raman Spectroscopy etc. by depositing the W-doped films of VO 2 on LaAlO 3 substrates using Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM).Earlier this technique was demonstrated as a cost effective and robust method to synthesize polycrystalline VO 2 on LaAlO 3 substrates. 26

II. EXPERIMENTAL
Polycrystalline W x V 1−x O 2 thin films studied in this work were synthesized by UNSPACM 27 with W varied between x=0.2 at % to 2 at %. Stoichiometric ratios of Ammonium metavanadate (NH 4 VO 3 )and Ammonium hexa tungstate hydrate (H 26 N 6 O 40 W 12 .xH 2 O)were dissolved in water.Drops of nitric acid were added to it to form a colourless solution.Urea was added to this aqueous combustion mixture (ACM).The calculations involved in these are based on propellant reactions.The ACM was taken in a special glassware set up which is nebulized by a ultrasonic medical nebulizer (mystic air sep USA)of frequency (2.5MHz).Lanthanum Aluminate, LaAlO 3 (LAO) substrates were precleaned with dilute HCl followed by ethanol, acetone, distilled water and finally with trichloroethylene.The ACM redox mixture taken in the glassware is nebulized ultrasonically.The vapors were carried by N 2 gas to fall on top of the substrate which is preheated to 600 0 C. The flow rate of N 2 gas is kept at 1000 SCCM.As shown in the equation, the combustion reaction yields W-V 2 O 5 product.

W
W-doped V 2 O 5 formed by the above reaction is reduced by treating the product in N 2 atmosphere saturated with hydrocarbon as reported in our earlier study. 26Structural characterization was carried out by Raman spectroscopy (Horiba Jobin Yvon HR-Raman-123 micro PL spectrometer with a wavelength of 519 nm) and X-ray diffraction (Cu-Kα-1.5418Å-PANALYTICAL and for glancing angle XRD, RIGAKU was used).Carrier density was measured by Hall Effect measurements.The compositions of these films were analyzed using X-ray Photoelectron spectroscopy (XPS) measurements.XPS measurements were made on an Axis Ultra DLD (from Kratos) high resolution instrument with automatic charge neutralization equipped with Mg K radiation (1253.5 eV).The films were milled to analyze the chemical composition of the bulk using 4 eV Ar + ion etching.XPS Peak41 software was used to fit the data.The electrical measurements were carried out on a DC probe station equipped with an ATT thermal controller coupled with a B1500A semiconductor device analyzer.

III. RESULTS AND DISCUSSION
Fig. 1 shows the glancing angle X-ray diffraction data, for undoped VO 2 and W 0.004 V 0.996 O 2 thinfilms.The (011) peak of the above samples were located at 27.92 o for the undoped and 27.82 o for W 0.004 V 0.996 O 2 thinfilm .The effective lattice strain and crystallite size for these two films were calculated using Williamson Hall (W-H) equation as given in supplementary information(SI). 28he patterns were indexed to be belonging to the M1 (P21/c) phase of VO 2 (JCPDS no: 82-0661) and is shown in SI. 28 The (011) peak for W 0.020 V 0.980 O 2 is located at 27.77 o cannot be associated with the emergence of M2 phase as the (-201) peak of M2 is located at 27.37 o as per JCPDS no: 76-0674.
Fig. 2 represents the room temperature Raman spectra of (representative compositions) W x V 1−x O 2 thin films.Raman Spectroscopy was done to verify the interplay of M1 and M2 phases 2. Raman spectra of representative W-doped VO 2 thin films to show the effect of W on the M1 phase. of VO 2 , as pronounced differences in the local structure can give rise to significant change in Raman signatures.It was noticed that for monophasic undoped VO 2 synthesized by UNSPACM, the presence of M1 (P21/C) phase 26 was confirmed by Raman spectroscopy.The phase can be identified with the shifting and disappearance of peaks.The confirmation of M1 or M2 phase can be observed from the shift of the peak 613 (cm −1 ) 7 or by splitting of 225 (cm −1 ), by the shifts in peaks 189 (cm −1 ) and also in 444 (cm −1 ).W x V 1−x O 2 thin films show no effective change in the Raman peak of 614 cm −1 .Hence the shifting of peaks is not significant enough to claim that M2 transition has taken place and instead can be attributed to the incorporation of W in the lattice.There is no emergence of M2 phase from the M1 phase as the shifting associated with M1 −→ M2 transition is very high from 613 cm −1 to 649 cm −1 .This figure also illustrates that with the addition of W upto 1.6 at %, M1 phase did not change, while simultaneously reducing the transition temperature.Tracking the peaks position with the increase in W content suggests no evidence of such transition at room temperature.Table 1 28 given in the SI illustrates that the effect of W doping has not changed the M1 phase of VO 2 .
Temperature dependent electrical resistance of W x V 1−x O 2 thin films grown on LAO substrates was measured in the in-plane geometry using two probe DC probe station varying the temperature as shown in Fig. 3. Inset in the figure shows the derivative plot of the resistance versus temperature.Metal-Insulator Transition for all other compositions can be found in the SI. 28Inter digital gold electrodes were deposited on the film by using an aluminum mask and contact between them was measured to be linear and ohmic.Current-Voltage (I-V) characteristics of the thin film were tested for reproducibility by performing the experiment several times on the undoped sample.
Thermal hysteresis (T h ), defined as the difference between the critical temperatures during heating and cooling of the VO 2 (undoped) film was found to be 4 o C. Hysteresis width did not seem to follow an increase or a decrease and remained between 4-5 o C upto 1.6 at % W. But it considerably reduced to 2 o C and then 1 o C for 1.8 at % W and 2.0 at % W respectively as shown in Table2 in the SI. 28ig. 4 shows the activation energy variation of W x V 1−x O 2 thin films before and after the transition.The activation energies were obtained from the expression R(T)=R o e E a /K B T , where the activation energies are the slopes of ln[R(T)] versus 1/K B T plot.In the semiconducting phase for the undoped thinfilms, activation energy was relatively higher at 300 meV as compared to that of the metallic phase at 114 meV.With W doping, the activation energies of both the phases fell FIG. 3. Temperature dependence of the normalized resistance of W x V 1−x O 2 thin films grown on LAO substrates.R-T curves for undoped, 1.0 at % W and 2 at% W thin films.In the figure, h and c refer to heating and cooling respectively.Inset shows the corresponding derivative plot.gradually and after 1.0 at % W, the metallic phase activation energy could not be measured from the ln[R(T)] versus 1/K B T plot.With W addition, VO 2 became progressively metallic, more so at higher temperatures.It can be understood as, the volume of metallic phases increases, with the injection of W into the system.Transition width (∆T), defined as the full width at half maximum of the derivative curve varied between 7.93 o C and 13.00 o C for the undoped sample.However with the increase in W content,the transition width increased,thereby reducing the sharpness of the transition as shown in Fig. 5(a).Resistance ratio is another useful quantity (∆A) (R (20 o C) /R (100 o that indicates MIT strength as shown in the equation. With W addition, Resistance ratio also came down alleviating the MIT strength as seen in Fig. 5(a).Fig. 5(b).shows the reduction in transition temperature on the addition of W. T MIT decreased from 61 o C for undoped VO 2 to 25 o C for 2.0 at % W. The surfaces of W x V 1−x O 2 thin films grown on LAO substrates were studied by X-ray Photoelectron spectroscopy.Fig. 6 shows the XPS spectra of few chosen samples from the batch of W x V 1−x O 2 thin films.Chemical states of the samples were identified.The carbon C 1S peak was assumed to have a binding energy of 284.8 eV.Calibration of V, W and O were done based on the adventitious C 1S peak.Shirley background subtraction was employed and V 2p 3/2 peak was deconvoluted into two peaks.The peak positions at 516.5 eV, 523.6 eV, 516.4 eV, 523.4eV 516.3 eV, 523.6 eV corresponded to V 2+ and V 4+ of 0.8 at % W, 1.0 at % W and 1.6 at % W respectively.These phase fractions of V 4+ and V 2+ were calculated from the respective areas under the peaks of 4+ and 2+ and it is found out to be 72 % of V 4+ and 28 % of V 2+ in the undoped VO 2 thin film with x=0 at % W. For x=0.8 at % W, V 4+ was measured to be 73 % and V 2+ was 27 %.For the composition of x= 1.0 at % W in W x V 1−x O 2 thin films ,they were 79 % and 21 % respectively.With further increase in the W content for x= 1.6 at % W, V 4+ was 81 % and V 2+ was 19 %.This suggested that the increase in W content stabilized the V 4+ state.The addition of W, stabilized the 4+ phase of Vanadium and is visible from Fig. 7.
Room temperature Hall effect measurements were made with a home-built system on W x V 1−x O 2 thin films with a magnetic field of 2.6 KGauss.Carrier density was calculated as seen in Fig. 8 for all compositions of doped VO 2 .Room temperature carrier density was found to be 3 x10 19 cm −3 which is similar to that reported by Ruzmetov et al. 29 It was observed that with an increase in tungsten content, carrier concentration increased from 3x10 19 cm −3 for the undoped sample to 1.7x10 22 cm −3 for 2.0 at %. Tungsten addition not only controlled the transition by altering the structure as seen from the shifts in XRD and Raman peaks but also altered the electron FIG. 7. at % W Vs % fraction of V4+ and area under the peak of W. concentration.Tungsten with W 6+ valence when substituting for the V 4+ gives electrons per atom thereby increasing the number of charge carriers.The absence of M2 phase in our XRD and Raman measurements adds an additional insight into the nature of W x V 1−x O 2 system.Previous report by Tselev et al 30 suggested that both the monoclinic phases of VO 2 resolve the instability of the rutile phase.Tweaking the characteristics of the system by manipulating the strain, inducing chemical pressure by doping, etc can make one phase more thermodynamically favorable over other.This was seen when VO 2 was doped with Cr 3+ and Al 3+ +, where M2 was stabilized because of the presence of V 5+ which in turn, promoted hole doping. 14,31But in contrast with W doping, W 6+ substitutes for V 4+ , adds two electrons for each W atom to the system, disrupting V 4+ -V 4+ and creates V 3+ .Evidences of the addition of electrons to the system can be seen from our room temperature Hall-effect measurements and our activation energy analysis.Thus W addition, does not lead to the stabilization of M2 phase but introduces charge carriers to the system.

IV. SUMMARY
W x V 1−x O 2 films have been synthesized by using UNSPACM with x=0.0 at % W to 2.0 at % W in steps of 0.2 at % W. With W addition, the transition temperature and the strength of the transition reduced.Transition temperature of 25 o C was achieved for 2.0 at % of W with less than 2 orders of change in resistance.While the structural characterization accounts for M1 to R transition,Hall Effect measurements and activation energy analysis show that there is a change in carrier concentration with the addition of W.
FIG.1.Glancing angle incidence XRD of undoped and 0.4 at % W doped samples.
and A. M. Umarji AIP Advances 6,035215 (2016) FIG. 4. Activation energy variation of Wx V 1−x O 2thin films before and after the transition.

FIG. 5
FIG. 5. (a) shows the variation of Transition width and resistance ratio with W content and (b) shows the variation of Transition temperature with W content.

035215- 7 B
FIG. 8. Variation of carrier concentration with W content.