Young’s modulus of VO2 thin films as a function of temperature including insulator-to-metal transition regime Nelson Sepúlveda, Armando Rúa, Rafmag Cabrera, and Félix Fernández Citation: Appl. Phys. Lett. 92, 191913 (2008); doi: 10.1063/1.2926681 View online: http://dx.doi.org/10.1063/1.2926681 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v92/i19 Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 92, 191913 共2008兲
Young’s modulus of VO2 thin films as a function of temperature including insulator-to-metal transition regime Nelson Sepúlveda,1,a兲 Armando Rúa,2 Rafmag Cabrera,1 and Félix Fernández2 1
Department of Electrical and Computer Engineering, University of Puerto Rico, Mayaguez, Puerto Rico 00681, USA 2 Department of Physics, University of Puerto Rico, Mayaguez, Puerto Rico 00681, USA
共Received 11 March 2008; accepted 22 April 2008; published online 15 May 2008兲 Young’s modulus of VO2 thin films has been measured for the first time through the material’s insulator-to-metal transition. The resonant frequency of silicon VO2 coated cantilevers was measured in the temperature range 30– 90 ° C. It has been found that during the semiconductor to metallic transition of VO2 thin films, which occurs at a temperature of 68 ° C, Young’s modulus changes most dramatically with temperature, abruptly reversing its declining trend with increasing temperature. The film is stiffened through the transition and, as the temperature is further raised, the declining trend is reasserted at a similar rate. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2926681兴 Materials, which undergo insulator-to-metal transitions 共IMTs兲, are very interesting due in part to the relationship of this phenomenon with many others such as high TC superconductivity and colossal magnetoresistance. The first order IMT phase transition 共PT兲 of vanadium dioxide 共VO2兲 can be induced by ordinary heating,1 pressure,2 electric field,3 or ultrashort optical radiation.4–6 The unique properties of this material make it very attractive for temperature, pressure, and optical sensors, which can be used in microelectromechanical system 共MEMS兲 applications. However, there is very little information about the mechanical properties of VO2 thin films in the semiconductor state7 and, to our knowledge, there are no reported experimental results for these properties of VO2 films in its metallic state. This letter reports the first measurements on Young’s modulus of VO2 thin films as a function of temperature. The findings show that the most dramatic change occurs during the material’s transition at 68 ° C. The previously reported results on the material’s semiconductor state use micro- and nanoindentation and the results show an elastic modulus of VO2 in the semiconductor state in the range of 140 GPa for sputtered VO2 thin films on fused-silica or polycrystalline silicon substrates, with decreasing values for regions close to the substrate.7,8 The studies reported in this letter show the first experimental measurement of the elastic modulus of VO2 thin films in temperatures ranging from 30 to 90 ° C. A 350 Å thick VO2 thin film was deposited on commercially available 共MicroMasch兲 single crystal silicon 共SCS兲 micromechanical cantilevers by pulsed laser deposition technique. The surface of the chip on which the cantilevers are fabricated corresponds to a silicon 共100兲 plane. The VO2 films were fabricated in an oxygen and argon atmosphere under a total pressure of 50 mtorr, as measured with a diaphragm capacitance sensor, and with Ar and O2 gas flows independently adjusted with mass flow controllers. Background pressure before depositions was of order 10−6 torr. A metallic vanadium target was ablated by a pulsed KrF excimer laser 共Lambda Physik Compex 110, wavelength = 248 nm, 20 ns pulse duration, and 4 J / cm2 fluence兲. During deposition, the substrate was held a兲
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at a temperature of 550 ° C by a feedback controler. Film thickness was measured after deposition with a Tencor profilometer and scanning at various places were VO2 steps were intentionally created along the chip. Three cantilevers, with lengths ranging from 100 to 235 m were tested. All cantilevers were 35 m wide and 1 m thick 共before VO2 deposition兲. The growth conditions employed are essentially the same as used in previous work,5,6 and have produced nearly pure-phase monoclinic 共M 1兲 VO2. In order to verify the phase and orientation of the deposited VO2 films, x-ray diffraction 共XRD兲 measurements were performed for a sample simultaneously grown on a similar substrate. A Bruker D8 Discover diffractometer 共BrukerAXS, Karlsruhe, Germany兲 was used for these measurements. Figure 1 shows the XRD scans for the VO2 / Si test sample in both of the VO2 phases: monoclinic 共taken at room temperature兲 and tetragonal 共taken at a temperature above the 68 ° C transition temperature兲. The peaks observed in this region of the scans correspond to the monoclinic 共011兲 and tetragonal 共110兲 reflections, respectively, as expected. No other peaks are observed for the film, which indicates that the crystallites are strongly oriented with the 共011兲 planes parallel to the sample surface.
FIG. 1. 共Color online兲 XRD scans for VO2 / Si thin film, taken at room temperature 共monoclinic phase兲 and above transition temperature 共tetragonal phase兲.
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FIG. 2. 共Color online兲 Experimental testing setup.
Therefore, Young’s modulus values obtained from the experiment described below correspond to a direction perpendicular to these planes. The exact angular position of the monoclinic 共011兲 reflection is shifted up by 0.1° 共2-theta兲 with respect to the bulk value given in Powder Diffraction File 82-0661. This shift may be indicative that the film is under tensile stress after deposition. An XRD scan was also directly obtained for the cantilever chip by using a 0.5 mm tube collimator in the x-ray beam and long counting time. This confirmed that the film deposited was VO2 共M 1兲. Also, the electrical resistance of the VO2 films was measured as a function of temperature. A decrease of three orders of magnitude and a transition temperature around 68 ° C was observed, as expected. The experimental method used consists of a piezoelectric actuation and light scattering detection technique similar to those that have been described in the past.9,10 In this technique, a substrate containing micrometer sized cantilever beams is attached to a piezoelectric actuator and a laser beam is reflected over its surface. The reflected laser signal is directed into a photodetector, which converts the oscillating light signal into a voltage signal. That voltage signal is sent to a low-noise preamplifier 共Stanford Research Systems Model SR560兲, and then to a spectrum analyzer 共Agilent 9320A兲, and also to a rf filter that sends back the signal to the piezoelectric actuator, completing a feedback configuration. The filter is used to suppress signals from higher order modes, which have higher frequency components. This feedback configuration would require an initial perturbation, which can be achieved by simply tapping on the chamber or playing with the filter gain values. Once the feedback system settles, the spectrum analyzer will show a maximum in the voltage signal at the mechanical resonant frequency of the cantilever. In order to induce the PT in the VO2 film, a Peltier heater and a temperature controller were used to set the temperature at the sample. Figure 2 shows a schematic of the testing system used. The pressure level inside the vacuum chamber was about 10−4 torr. The VO2 thin film had a thickness of 350 Å, which is about 28 times smaller than the silicon cantilever thickness. The resonant frequency of a coated cantilever beam, where the coating is much thinner than the uncoated cantilever, is given by
FIG. 3. 共Color online兲 Resonant frequency shifts with increasing temperature for the 235 m coated cantilever.
f 2c = K
再
冋
冉
冊 册冎
1 ␦ ␦2 t + + + 2 t 2t2 6 1tw + 22␦共w + t + 2␦兲
E1共wt + 12兲 + E2␦ 共w + 2␦兲
, 共1兲
where l, t, w, 1, and E1 are the length, thickness, width, density, and Young’s modulus of the uncoated cantilever, respectively; E2, ␦, and 2 are the elastic modulus, thickness, 12.36t2 11 and density of the coating, respectively, and K⫽ 共2 . 兲 2l 4 Equation 共1兲 has been used in the past for measuring the elastic modulus of nanostructured gold and platinum thin films12 by combining it with the commonly known equation for calculating the resonant frequency of an uncoated cantilever beam:
fu =
1.02t 2l2
冑
E1 , 1
共2兲
where l, t, E1, and 1 are the cantilever’s length, thickness, Young’s modulus, and density, respectively. The combination leads to the following ratio of resonant frequencies:
FIG. 4. 共Color online兲 VO2 thin film Young’s modulus with varying temperature.
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TABLE I. Slopes for the three regions of interest in Fig. 4: below, during, and above the transition temperature. Slope 共MPa/°C兲
Cantilever length 共m兲
Below transition
During transition
Above transition
100 120 235
−28.85 −27.98 −40.91
127.75 115.93 116.08
−21.79 −19.93 −35.66
f 2c f 2u
再 冉 冊冋
1 wt + 12 =
冉
冊 册冎
1 ␦ ␦2 E2 t ␦ 共w + 2␦兲 + + 2 + 2 t 2t E1 6 1tw + 22␦共w + t + 2␦兲
.
共3兲 In the analysis, densities of 2330 and 4670 kg/ m3 were used for SCS and VO2. It has been previously reported that the density of VO2 is essentially the same for both phases of interest here.13 The values for f u and f c were experimentally obtained by testing the uncoated and coated SCS cantilevers. Since the major contribution of the work is to report the change in the transition region 共⬃68 ° C兲, the f u and f c values were measured in the 30– 90 ° C temperature range. The elastic modulus for the uncoated SCS cantilevers 共E1兲 in the 30– 90 ° C temperature range was obtained by using the measured f u values for that temperature range, and Eq. 共2兲. Once this is done, the only variable left in Eq. 共3兲 is Young’s modulus of the VO2 thin film 共E2兲. In order to verify the testing setup and method, a different set of identical silicon cantilevers was coated with a 0.1 m thick sputtered gold layer and the obtained elastic modulus for gold was 79 GPa, which is the commonly known value. Figure 3 shows the resonant peaks 共first resonant mode兲 for different temperature values for the 235 m long coated cantilever beam. Starting at room temperature, it can be seen how the resonant peak shifts to lower frequencies as the temperature is increased. Once the temperature approaches the IMT region, the peaks abruptly moves to higher frequencies. Subsequently, after the transition region has passed, it gradually shifts again to lower frequencies. It was observed that the resonant peak of the 235 m long SCS uncoated cantilever gradually shifted to lower frequencies during the entire range. This highlights the effect of the VO2 coating on the cantilever resonant frequency. The resonant frequencies of two other cantilevers were measured 共coated and uncoated兲 as the temperature was increased. Again, the same behavior was observed through the transition region. This change in resonant frequency is caused by a change in Young’s modulus for the film, which can be calculated by using Eq. 共3兲 and the measured resonant frequency of the coated and uncoated cantilevers 共f c and f u兲. Figure 4 shows how the calculated value for Young’s modulus changed from 30 to 90 ° C for the three cantilevers. From 30 to 62 ° C and from 80 to 90 ° C, the modulus for
the 100 m long cantilever decreases with temperature with slopes of approximately −28.85 and −21.79 MPa/ ° C, respectively, while for the 62– 80 ° C temperature range, the slope is about +127.75 MPa/ ° C. Table I has the details for the other cantilevers, which showed a very similar behavior. It should be stressed that the behavior observed was entirely reversible and repeatable. The response observed for each cantilever was repeated as they underwent numerous heating and cooling cycles. Another identical set of SCS cantilevers with a thicker coating of 70 nm was tested and the results were very similar. In conclusion, in this experiment, an abrupt change in the value of Young’s modulus for VO2 thin films has been observed as the material undergoes its IMT. The value of the modulus was measured as a function of temperature in a direction normal to the film’s surface, which was strongly oriented with 共011兲 planes of its monoclinic structure parallel to the sample surface. The variation of the modulus through the IMT dramatically contrasts with the linear relationship between Young’s modulus and temperature of most materials, such as that of silicon in the uncoated cantilevers.14 The value of the modulus for VO2 increases on heating through the transition, signifying that in the crystallographic direction pertaining to the experiment the material stiffens as the temperature is increased. At temperatures higher than those near the transition, the material, now in its metallic state, resumes the elastic behavior of the insulating phase before the IMT. These results show that VO2 thin films can be of interest for applications in MEM devices, due to their remarkable elastic response at conditions near room temperature. This project has been supported by the NSF-PREPSCoR Start-Up funds. 1
V. S. Vikhnin, S. Lysenko, A. Rua, F. Fernandez, and H. Liu, Phys. Lett. A 343, 6 共2005兲. 2 E. Arcangeletti, L. Baldassarre, D. Di Castro, S. Lupi, L. Malavasi, C. Marini, A. Perucchi, and P. Postorino, Phys. Rev. Lett. 98, 196406 共2007兲. 3 H. T. Kim, B. G. Chae, D. H. Youn, S. L. Maeng, G. Kim, K. Y. Kang, and Y.-S. Lim, New J. Phys. 6, 52 共2004兲. 4 A. Cavalleri, Cs. Tóth, H. C. W. Siders, J. A. Squier, F. Raksi, P. Forget, and J. C. Kieffer, Phys. Rev. Lett. 87, 237401 共2001兲. 5 S. Lysenko, V. Vikhnin, F. Fernández, A. Rua, and H. Liu, Phys. Rev. B 75, 075109 共2007兲. 6 S. Lysenko, A. Rúa, V. Vikhnin, F. Fernández, and H. Liu, Phys. Rev. B 76, 035104 共2007兲. 7 K.-Y. Tsai, T.-S. Chin, and H.-P. D. Shieh, Jpn. J. Appl. Phys., Part 1 43, 9A 共2004兲. 8 P. Jin, S. Nakao, S. Tanemura, T. Bell, L. S. Wielunski, and M. V. Swain, Thin Solid Films 343, 134 共1999兲. 9 D. A. Czaplewski, J. P. Sullivan, T. A. Friedmann, D. W. Carr, B. E. N. Keeler, and J. R. Wendt, J. Appl. Phys. 97, 023517 共2005兲. 10 N. Sepúlveda, D. M. Aslam, and J. P. Sullivan, Diamond Relat. Mater. 15, 2 共2006兲. 11 R. E. D. Bishop and D. C. Johnson, The Mechanics of Vibration 共Cambridge University Press, Cambridge, 1960兲. 12 M. C. Salvadori, I. G. Brown, A. R. Vaz, L. L. Melo, and M. Cattani, Phys. Rev. B 67, 153404 共2003兲. 13 Ch. Leroux, G. Nihoul, and G. Van Tendeloo, Phys. Rev. B 57, 5111 共1998兲. 14 N. Ono, K. Kitamura, K. Nakajima, and Y. Shimanuki, Jpn. J. Appl. Phys., Part 1 39, 2A 共2000兲.
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