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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 26 — Dec. 21, 2009
  • pp: 24153–24161
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Characterization of nanostructured VO2 thin films grown by magnetron controlled sputtering deposition and post annealing method

Sihai Chen, Jianjun Lai, Jun Dai, Hong Ma, Hongchen Wang, and Xinjian Yi  »View Author Affiliations


Optics Express, Vol. 17, Issue 26, pp. 24153-24161 (2009)
http://dx.doi.org/10.1364/OE.17.024153


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Abstract

By magnetron controlled sputtering system, a new nanostructured metastable monoclinic phase VO2 (B) thin film has been fabricated. The testing result shows that this nanostructured VO2 (B) thin film has high temperature coefficient of resistance (TCR) of −7%/K. Scanning electron microscopy measurement shows that the average grain diameter of the VO2 (B) crystallite is between 100 and 250 nm. After post annealed, VO2 (B) crystallite is changed into monoclinic (M) phase VO2 (M) crystallite with the average grain diameter between 20 and 50nm. A set up of testing the thin film switching time is established. The test result shows the switching time is about 50 ms. With the nanostructured VO2 (B) and VO2 (M) thin films, optical switches and high sensitivity detectors will be presented.

© 2009 OSA

1. Introduction

Vanadium oxides (VOx) show different electronic behavior depending on their valence configuration. The electronic and optical performance of vanadium oxide is relative to the structure of the thin films. By different deposition methods, different VOx phase can be obtained [1

1. G. L. Zhao and G. R. Han, “Study of the electrical properties of sol-gel-derived titanium-vanadium oxide films,” Int. J. Mod. Phys. B 16(28 & 29), 4465–4468 (2002). [CrossRef]

8

8. H. Miyazaki, M. Kamei, and I. Yasui, “Vanadium oxide thin films depostied onto Cu buffer layer by RF magnetron sputtering,” Thin Solid Films 343–344, 168–170 (1999). [CrossRef]

]. Many methods have been fulfilled by researchers to fabricate either vanadium oxide with relative low temperature coefficient of resistance (TCR) or vanadium dioxide with phase transition [9

9. M. B. Sahana, G. N. Subbanna, and S. A. Shivashankar, “Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metal organic chemical vapor deposition,” J. Appl. Phys. 92(11), 6495–6504 (2002). [CrossRef]

,10

10. S. D. Hansen and C. R. Aita, “Low temperature reactive sputter deposition of vanadium oxide,” J. Vac. Sci. Technol. A 3(3), 660–663 (1985). [CrossRef]

]. In our pre-work, VOx thin films with −2%/K TCR is fabricated by reactive ion beam sputtering deposition method [11

11. S. H. Chen, H. Ma, J. Dai, and X. J. Yi, “Nanostructured vanadium dioxide thin films with low phase transition temperature,” Appl. Phys. Lett. 90(10), 101117 (2007). [CrossRef]

]. N.Fieldhouse deposited VOx thin films with the TCRs in the range of −1.1% to −2.4%/K by reactive pulse direct current (dc) magnetron sputtering process [12

12. N. Fieldhouse, S. M. Pursel, R. Carey, M. W. Horn, and S. S. N. Bharadwaja, “Vanadium oxide thin films for bolometric applications deposited by reactive pulsed dc sputtering,” J. Vac. Sci. Technol. A 27(4), 951–955 (2009). [CrossRef]

]. C. Venkatasubramanian reported that low resistivity VOx thin films with the TCRs in the range of −1.6% to −2.2%/K were deposited by pulsed-dc sputtering [13

13. C. Venkatasubramanian, O. M. Cabarcos, D. L. Allara, M. W. Horn, and S. Ashok, “Correlation of temperature response and structure of annealed VOx thin films for IR detector applications,” J. Vac. Sci. Technol. A 27(4), 956–961 (2009). [CrossRef]

]. He also performed ion implantation followed by annealing to improve the trade-off between TCR and resistivity. The resistivities of the VOx thin films ranged from 0.05 Ωcm to 100 Ωcm and the TCR values varied from −1.1% to-2.7% [14

14. C. Venkatasubramanian, M. W. Horn, and S. Ashok, “Ion implantation studies on VOx films prepared by pulsed dc reactive sputtering,” Nucl. Instrum. Methods Phys. Res. B 267(8-9), 1476–1479 (2009). [CrossRef]

]. B. D. Gauntt increased the TCR to the value of −3.5%/°C by increasing oxygen content using pulsed dc magnetron sputtering in an atmosphere containing argon and oxygen [15

15. B. D. Gauntt, E. C. Dickey, and M. W. Horn, “Stoichiometry and microstructural effects on electrical conduction in pulsed dc sputtered vanadium oxide thin films,” J. Mater. Res. 24(4), 1590–1599 (2009). [CrossRef]

]. Dr.Hubert Jerominek reported the value of the TCR in some of the VO2 films in their semiconducting phase was 5.2% per degree Celsius [16

16. H. Jerominek, F. Picard, and D. Vincent, “Vanadium oxide films for optical switching and detection,” Opt. Eng. 32(9), 2092–2099 (1993). [CrossRef]

]. In this paper, we fabricated a new nanostructured VO2 (B) which has relatively higher TCR by exploring magnetron controlled deposition method. And by further experiments, we have found that the thin film structure can be changed by post annealing method. The electrical properties show that the square sheet resistance of the thin films is 20~50kΩ at room temperature and the TCR is as high as −7%/K before annealing. After annealing, the VO2 (B) is changed into VO2 (M) accompanied a phase transition performance at 68 °C. With the annealing temperature increasing, the phase transition degree is higher. By the two steps growth method, we can fabricate two kinds of vanadium oxide, one has high TCR which is promotional for the application on uncooled microbolometer with high sensitivity [17

17. L. A. L. de Almeida, G. S. Deep, A. M. N. Lima, H. F. Neff, and R. C. S. Freire, “A Hysteresis Model for a Vanadium Dioxide Transition-Edge Mirobolometer,” IEEE Trans. Instrum. Meas. 50(4), 1030–1035 (2001). [CrossRef]

], and the other is optional for optical switches [18

18. A. Leone, A. M. Trione, and F. Junga, “Alteration in electrical and infrared switching properties of vanadium oxides due to proton irradiation,” IEEE Trans. Nucl. Sci. 37(6), 1739 (1990). [CrossRef]

,19

19. F. Guinneton, L. Sauques, J. C. Valmalette, F. Cros, and J. R. Gavarri, “Optimized infrared switching properties in thermochromic vanadium dioxide thin films: role of deposition process and microstructure,” Thin Solid Films 446(2), 287–295 (2004). [CrossRef]

]. The switching time is about 50ms by laser induced heating testing system.

2. Experimental

The films were deposited in a diffusion pumped chamber which gave a base pressure less than 1 × 10−3 Pa using dc magnetron sputtering from a vanadium metal target (120 mm dia., 99.9% purity). The water-cooled vanadium target was sputtered in an Ar–O discharge to form oxide films. Gas flow meters controlled precisely the flow rates of oxygen and argon at about 145~175 and 18~25 SCCM (SCCM denotes standard cubic centimeters per minute) respectively. A radiant heater was used to heat the samples to between 200 and 250°C. The main experimental parameters are described in Table 1

Table 1. The parameters of the magnetron controlled sputtering deposition.

table-icon
View This Table
. The vanadium oxide materials were grown on silicon substrates with a 200-300 nm thickness of Si3N4 buffer layer. The uniform of the thin film reaches to 1%. After deposition by magnetron method, the thin films were annealed for 60 min in the temperature range from 400 to 475 °C under a flowing Ar atmosphere.

Electrical switching devices were made via deposition gold electrodes at the surface of a VO2 film. These electrodes were sputtered through a mask so that the geometry of the inter-electrode space can be accurately controlled.

3. Measurements and analysis

In order to investigate their electrical properties, the thin films were tested by a four probe station with a temperature controller. During testing, the film temperatures were raised from 20 to 80 °C and subsequently reduced to 20 °C.

The crystalline phase precipitated in the deposited film was analyzed by x-ray diffraction (XRD) measurements through x’Pert PRO x-ray diffractometer (PANalytical BV) with Cu radiation operated at 40 kV and 40 mA.

The scanning electron microscopy (SEM) images were taken using Sirion 200 microscope (FEI) operated at 5 kV. The atomic force microscopy (AFM) investigations of the thin film surfaces were carried out through an XE-100E microscope (PSIA).

The switching response property of the thin film was measured by using a SYNRAD J48-5W CO2 laser with wavelength of 10.6µm and a two-channel color digital phosphor oscilloscope.

Figure 1
Fig. 1 Sheet resistance vs. temperature for VO2(B) thin films made by magnetron controlled sputtering.
shows the experimental sheet resistance–temperature characteristic curves underoptimized deposition conditions. As shown in Fig. 1, the sheet resistance–temperature curve is nearly linear and the TCR of the thin film is −7%/K, which is larger than most TCR value of vanadium oxide thin film deposited by reactive ion deposition method. The high TCR value may result from the possibility that this film is not in a stable VOx phase. The following XRD measurement proves that it is metastable monoclinic phase VO2 (B). Further investigation will be made to understand about the relationship between VOx thin films structures and electrical properties. In some military surveillance and civilian electro-optical (EO) systems, it is necessary to produce high TCR vanadium oxide thin film. Since high TCR values are usually associated with high resistivities [14

14. C. Venkatasubramanian, M. W. Horn, and S. Ashok, “Ion implantation studies on VOx films prepared by pulsed dc reactive sputtering,” Nucl. Instrum. Methods Phys. Res. B 267(8-9), 1476–1479 (2009). [CrossRef]

], we will try to find optimized deposition conditions that provide VOx thin films with high TCR values and low resistivities, which leads to high sensitivity for example in infrared detectors application.

Figure 2
Fig. 2 Sheet resistance vs. temperature for VO2(B) thin films after annealing at different temperature.
shows the experimental sheet resistance–temperature characteristic curves of the annealed thin film. As shown in Fig. 2, at the relative low post annealing temperature (400°C), the phase transition happens. While the resistance ratio of low temperature over high temperature is very low, which means the transition process happened partially. The reason is that at temperature of 400°C only part of VO2(B) changes into VO2(M) which has the phase transition property. With the increasing of annealing temperature, this resistance ration is becoming larger. As shown in Fig. 2, this resistance ratio reaches 1500 at the temperature of 475°C. Although the resistance ratio is different, all the phase transition happened at Tc: 68°C.

From the x-ray diffraction measurements shown in Fig. 3
Fig. 3 XRD pattern of VO2 (B) thin films.
and Fig. 4
Fig. 4 XRD pattern of VO2 thin films fabrication by post annealing for 60min at 425°C.
, tow sets of diffraction pattern are observed. In Fig. 3, the spectrum shows peaks 1(2θ = 15.3°), 2 (2θ = 30.1°), 3(2θ = 45.1°), and 4 (2θ = 56.8°) those are related to the reflections from planes of (110), (4¯ 01), and (5¯11) of VO2 (B), and (211) of Si respectively, from which we can conclude that the thin film is vanadium oxide composed of VO2 (B) by magnetron sputtering deposition method. The structure parameters areaB=12.03Ǻ, bB=3.693 Ǻ, cB=6.42 Ǻ, βB=106.6 [10

10. S. D. Hansen and C. R. Aita, “Low temperature reactive sputter deposition of vanadium oxide,” J. Vac. Sci. Technol. A 3(3), 660–663 (1985). [CrossRef]

]. In Fig. 4, the spectrum shows peaks 2(2θ = 27.8°), 3 (2θ = 37.1°), 4(2θ = 55.5°) those are related to the reflections from planes of (110), (200), and (220) of VO2 (M) growing at the direction of (110). While there is only a small peak of 1 (2θ = 15.3) that is related the rest phase of VO2 (B).

Figure 5
Fig. 5 Micrographs of VO2 (B) thin film: (a) SEM and (b) AFM micrographs.
shows the micrographs of the VO2 thin film. It can be seen in the micrographs that the VO2 (B) thin films deposited on the silicon substrate materials have fine nanostructure with nanosized grains uniformly spreading over the entire substrate surfaces. The average height of the crystallite is 50nm and the grain diameter of the crystallite is between 100 and 250 nm. VO2 (B) thin film is composed of scalelike crystallite with a loose structure.

Figure 6
Fig. 6 The SEM micrograph of vanadium oxide thin film by post annealing process for 60min at 425°C.
shows the micrographs of the VO2 thin film processed by post annealing. As shown in Fig. 6, the crystalline structures change greatly. The scalelike crystallite changes into tiny crystallite piled in 3 dimensions and the size of the crystallite shrinks. There still shows a few single scalelike VO2 (B) breaks into multiple small VO2 (M) crystallites (marked in the squares). The cracking happed because at high temperature, the irreversible process of VO2 (B) reconstructing into tetragonal rutile(R)structure, VO2(R), once cooled at room temperature, the VO2 (R) finally changed into monoclinic phase VO2 (M). Annealed at 425°C for 60min, the grain diameter of the crystallite is between 25 and 50 nm, showing nanostructure.

The speed of phase transition of VO2 has been experimentally measured to be in the order of nanoseconds [20

20. G. Stefanovich, A. Pergament, and D. Stefanovich, “Electrical switching and Mott transition in VO2,” J. Phys. Condens. Matter 12(41), 8837–8845 (2000). [CrossRef]

] and even 100 femtosecond [21

21. A. Cavalleri, M. Rini, H. H. W. Chong, S. Fourmaux, T. E. Glover, P. A. Heimann, J. C. Kieffer, and R. W. Schoenlein, “Band-selective measurements of electron dynamics in VO2 using femtosecond near-edge X-ray absorption,” Bull. Am. Phys. Soc. 95, 067405 (2005).

] under photon excitation. Figure 7
Fig. 7 Transient thermal analysis result of the temperature distribution of the thin film irradiated for different period: (a) 1ms; (b) 2ms; (c) 5ms; (d) 10ms.
shows the simulated transient temperature distribution on the surface of the thin film irradiated for different time period. The output power of the laser is 42W. As shown in Fig. 7, the temperature gradient distribution is in circles. Once irradiated for 1ms, the surface temperature is 58.5°C, which is lower than the transition temperature. With the time increased, the temperature increased too. Irradiated for 10ms, the temperature is 124.0°C, which is lower than the VO2 carbonization temperature of 600°C. At this temperature, the phase transition is finished. By the simulation, 42 W can be chosen as the irradiation power which can produce the phase transition heat, and not lead to carbonization.

As shown in Fig. 8
Fig. 8 Schematic diagram of the experimental apparatus used to measure vanadium oxide thin film switching time.
, we established a platform for testing the switching time of the switches based on vanadium dioxide thin film. A dc voltage is applied to the device and the voltage–time (V–T) curve is recorded onto an oscilloscope. The output highest power of the laser is 50W, and can be tuned continuously; the incident light wavelength is 10.6μm, which is easily absorbed by VO2. The output power, the spot size and the radiation time are controlled by the computer. The film remains in the insulating ‘OFF’ state below the transition temperature. However, as the irradiating time increases, the film is heated via the laser heating effect and the vanadium dioxide turns metallic when its temperature becomes larger than Tc. The device remains in the ‘ON’ state as long as the intensity keeps continuously.

The test result is shown in Fig. 9
Fig. 9 The output voltage of the optical switch.
. From Fig. 9, the transition time from high voltage at semiconductor to low voltage at metal phase is about 80ms. Based on the definition of time constant, the response time is about 63% of the time reaching to stabilization state, we can get that the real thermal response time is 50.4 ms, which is much higher than ns. The reasons are following: first, the electrical interconnect metal is indium, which is apt to induce electrical noise in the testing. Second, the switch is 3 layers structure; the thermal capacity is large which therefore produces a slow thermal response. The third reason is when heating the thin film by the laser; the substrate is heated at the same time, which also increases the thermal capacity of the switch. The following work will focus on microbridge structure to reduce the thermal capacity by MEMS technology.

4. Conclusion

In this paper, we have obtained nanostructured VO2 (B) by magnetron controlled sputtering technique. Through the measurement results, we found that the deposition conditions are key parameters to form the structure and characteristics of the thin films. The testing result also shows that the VO2 (B) has TCR as high as −7%/K. By post annealing method, the VO2(B) is changed into VO2(M), which has a phase transition property at 68 °C and with switching time of about 50.4ms. These results indicate that VO2 (B) thin films can be promising high sensitivity uncooled infrared detector films and VO2 (M) thin films can be efficient electrical and optical switches with microbridge structure.

Acknowledgments

This research is supported by National Natural Science Foundation of China (Grant No. 60671004) & the Program for New Century Excellent Talents in University (No. NCET-07-0319).

References and links

1.

G. L. Zhao and G. R. Han, “Study of the electrical properties of sol-gel-derived titanium-vanadium oxide films,” Int. J. Mod. Phys. B 16(28 & 29), 4465–4468 (2002). [CrossRef]

2.

S. Deki, Y. Aoi, and A. Kajinami, “A novel wet process for the preparation of vanadium dioxide thin film,” J. Mater. Sci. 32(16), 4269–4273 (1997). [CrossRef]

3.

M. Melzer, J. Urban, H. Sack-Kongehl, K. Weiss, H.-J. Freund, and R. Schlögl, “Preparation of vanadium and vanadium oxide clusters by means of inert gas aggregation,” Catal. Lett. 81(3/4), 219 (2002). [CrossRef]

4.

S. Sakata, P. O. Vaccaro, S. Yamaoka, I. Umezu, and A. Sugimura, “Selective oxidation of vanadium thin film surfaces using an atomic force microscope,” Conference on Optoelectronic & Microelectronic Materials and Devices, Proceedings, COMMAD, 419–421(1999).

5.

K. Inumaru, M. Misono, and T. Okuhara, “Structure and catalysis of vanadium oxide overlayers on oxide supports,” Appl. Catal. A Gen. 149(1), 133–149 (1997). [CrossRef]

6.

J. P. Schreckenbach and P. Strauch, “Microstructure study of amorphous vanadium oxide films,” Appl. Surf. Sci. 143(1-4), 6–10 (1999). [CrossRef]

7.

Q. H. Wu, A. Thissen, W. Jaegermann, and M. L. Liu, “Photoelectron spectroscopy study of oxygen vacancy on vanadium oxides surface,” Appl. Surf. Sci. 236(1-4), 473–478 (2004). [CrossRef]

8.

H. Miyazaki, M. Kamei, and I. Yasui, “Vanadium oxide thin films depostied onto Cu buffer layer by RF magnetron sputtering,” Thin Solid Films 343–344, 168–170 (1999). [CrossRef]

9.

M. B. Sahana, G. N. Subbanna, and S. A. Shivashankar, “Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metal organic chemical vapor deposition,” J. Appl. Phys. 92(11), 6495–6504 (2002). [CrossRef]

10.

S. D. Hansen and C. R. Aita, “Low temperature reactive sputter deposition of vanadium oxide,” J. Vac. Sci. Technol. A 3(3), 660–663 (1985). [CrossRef]

11.

S. H. Chen, H. Ma, J. Dai, and X. J. Yi, “Nanostructured vanadium dioxide thin films with low phase transition temperature,” Appl. Phys. Lett. 90(10), 101117 (2007). [CrossRef]

12.

N. Fieldhouse, S. M. Pursel, R. Carey, M. W. Horn, and S. S. N. Bharadwaja, “Vanadium oxide thin films for bolometric applications deposited by reactive pulsed dc sputtering,” J. Vac. Sci. Technol. A 27(4), 951–955 (2009). [CrossRef]

13.

C. Venkatasubramanian, O. M. Cabarcos, D. L. Allara, M. W. Horn, and S. Ashok, “Correlation of temperature response and structure of annealed VOx thin films for IR detector applications,” J. Vac. Sci. Technol. A 27(4), 956–961 (2009). [CrossRef]

14.

C. Venkatasubramanian, M. W. Horn, and S. Ashok, “Ion implantation studies on VOx films prepared by pulsed dc reactive sputtering,” Nucl. Instrum. Methods Phys. Res. B 267(8-9), 1476–1479 (2009). [CrossRef]

15.

B. D. Gauntt, E. C. Dickey, and M. W. Horn, “Stoichiometry and microstructural effects on electrical conduction in pulsed dc sputtered vanadium oxide thin films,” J. Mater. Res. 24(4), 1590–1599 (2009). [CrossRef]

16.

H. Jerominek, F. Picard, and D. Vincent, “Vanadium oxide films for optical switching and detection,” Opt. Eng. 32(9), 2092–2099 (1993). [CrossRef]

17.

L. A. L. de Almeida, G. S. Deep, A. M. N. Lima, H. F. Neff, and R. C. S. Freire, “A Hysteresis Model for a Vanadium Dioxide Transition-Edge Mirobolometer,” IEEE Trans. Instrum. Meas. 50(4), 1030–1035 (2001). [CrossRef]

18.

A. Leone, A. M. Trione, and F. Junga, “Alteration in electrical and infrared switching properties of vanadium oxides due to proton irradiation,” IEEE Trans. Nucl. Sci. 37(6), 1739 (1990). [CrossRef]

19.

F. Guinneton, L. Sauques, J. C. Valmalette, F. Cros, and J. R. Gavarri, “Optimized infrared switching properties in thermochromic vanadium dioxide thin films: role of deposition process and microstructure,” Thin Solid Films 446(2), 287–295 (2004). [CrossRef]

20.

G. Stefanovich, A. Pergament, and D. Stefanovich, “Electrical switching and Mott transition in VO2,” J. Phys. Condens. Matter 12(41), 8837–8845 (2000). [CrossRef]

21.

A. Cavalleri, M. Rini, H. H. W. Chong, S. Fourmaux, T. E. Glover, P. A. Heimann, J. C. Kieffer, and R. W. Schoenlein, “Band-selective measurements of electron dynamics in VO2 using femtosecond near-edge X-ray absorption,” Bull. Am. Phys. Soc. 95, 067405 (2005).

OCIS Codes
(310.3840) Thin films : Materials and process characterization
(160.4236) Materials : Nanomaterials

ToC Category:
Thin Films

History
Original Manuscript: November 16, 2009
Revised Manuscript: December 10, 2009
Manuscript Accepted: December 10, 2009
Published: December 17, 2009

Citation
Sihai Chen, Jianjun Lai, Jun Dai, Hong Ma, Hongchen Wang, and Xinjian Yi, "Characterization of nanostructured VO2 thin films grown by magnetron controlled sputtering deposition and post annealing method," Opt. Express 17, 24153-24161 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-26-24153


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References

  1. G. L. Zhao and G. R. Han, “Study of the electrical properties of sol-gel-derived titanium-vanadium oxide films,” Int. J. Mod. Phys. B 16(28 & 29), 4465–4468 (2002). [CrossRef]
  2. S. Deki, Y. Aoi, and A. Kajinami, “A novel wet process for the preparation of vanadium dioxide thin film,” J. Mater. Sci. 32(16), 4269–4273 (1997). [CrossRef]
  3. M. Melzer, J. Urban, H. Sack-Kongehl, K. Weiss, H.-J. Freund, and R. Schlögl, “Preparation of vanadium and vanadium oxide clusters by means of inert gas aggregation,” Catal. Lett. 81(3/4), 219 (2002). [CrossRef]
  4. S. Sakata, P. O. Vaccaro, S. Yamaoka, I. Umezu, and A. Sugimura, “Selective oxidation of vanadium thin film surfaces using an atomic force microscope,” Conference on Optoelectronic & Microelectronic Materials and Devices, Proceedings, COMMAD, 419–421(1999).
  5. K. Inumaru, M. Misono, and T. Okuhara, “Structure and catalysis of vanadium oxide overlayers on oxide supports,” Appl. Catal. A Gen. 149(1), 133–149 (1997). [CrossRef]
  6. J. P. Schreckenbach and P. Strauch, “Microstructure study of amorphous vanadium oxide films,” Appl. Surf. Sci. 143(1-4), 6–10 (1999). [CrossRef]
  7. Q. H. Wu, A. Thissen, W. Jaegermann, and M. L. Liu, “Photoelectron spectroscopy study of oxygen vacancy on vanadium oxides surface,” Appl. Surf. Sci. 236(1-4), 473–478 (2004). [CrossRef]
  8. H. Miyazaki, M. Kamei, and I. Yasui, “Vanadium oxide thin films depostied onto Cu buffer layer by RF magnetron sputtering,” Thin Solid Films 343–344, 168–170 (1999). [CrossRef]
  9. M. B. Sahana, G. N. Subbanna, and S. A. Shivashankar, “Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metal organic chemical vapor deposition,” J. Appl. Phys. 92(11), 6495–6504 (2002). [CrossRef]
  10. S. D. Hansen and C. R. Aita, “Low temperature reactive sputter deposition of vanadium oxide,” J. Vac. Sci. Technol. A 3(3), 660–663 (1985). [CrossRef]
  11. S. H. Chen, H. Ma, J. Dai, and X. J. Yi, “Nanostructured vanadium dioxide thin films with low phase transition temperature,” Appl. Phys. Lett. 90(10), 101117 (2007). [CrossRef]
  12. N. Fieldhouse, S. M. Pursel, R. Carey, M. W. Horn, and S. S. N. Bharadwaja, “Vanadium oxide thin films for bolometric applications deposited by reactive pulsed dc sputtering,” J. Vac. Sci. Technol. A 27(4), 951–955 (2009). [CrossRef]
  13. C. Venkatasubramanian, O. M. Cabarcos, D. L. Allara, M. W. Horn, and S. Ashok, “Correlation of temperature response and structure of annealed VOx thin films for IR detector applications,” J. Vac. Sci. Technol. A 27(4), 956–961 (2009). [CrossRef]
  14. C. Venkatasubramanian, M. W. Horn, and S. Ashok, “Ion implantation studies on VOx films prepared by pulsed dc reactive sputtering,” Nucl. Instrum. Methods Phys. Res. B 267(8-9), 1476–1479 (2009). [CrossRef]
  15. B. D. Gauntt, E. C. Dickey, and M. W. Horn, “Stoichiometry and microstructural effects on electrical conduction in pulsed dc sputtered vanadium oxide thin films,” J. Mater. Res. 24(4), 1590–1599 (2009). [CrossRef]
  16. H. Jerominek, F. Picard, and D. Vincent, “Vanadium oxide films for optical switching and detection,” Opt. Eng. 32(9), 2092–2099 (1993). [CrossRef]
  17. L. A. L. de Almeida, G. S. Deep, A. M. N. Lima, H. F. Neff, and R. C. S. Freire, “A Hysteresis Model for a Vanadium Dioxide Transition-Edge Mirobolometer,” IEEE Trans. Instrum. Meas. 50(4), 1030–1035 (2001). [CrossRef]
  18. A. Leone, A. M. Trione, and F. Junga, “Alteration in electrical and infrared switching properties of vanadium oxides due to proton irradiation,” IEEE Trans. Nucl. Sci. 37(6), 1739 (1990). [CrossRef]
  19. F. Guinneton, L. Sauques, J. C. Valmalette, F. Cros, and J. R. Gavarri, “Optimized infrared switching properties in thermochromic vanadium dioxide thin films: role of deposition process and microstructure,” Thin Solid Films 446(2), 287–295 (2004). [CrossRef]
  20. G. Stefanovich, A. Pergament, and D. Stefanovich, “Electrical switching and Mott transition in VO2,” J. Phys. Condens. Matter 12(41), 8837–8845 (2000). [CrossRef]
  21. A. Cavalleri, M. Rini, H. H. W. Chong, S. Fourmaux, T. E. Glover, P. A. Heimann, J. C. Kieffer, and R. W. Schoenlein, “Band-selective measurements of electron dynamics in VO2 using femtosecond near-edge X-ray absorption,” Bull. Am. Phys. Soc. 95, 067405 (2005).

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