1. Introduction

Vanadium oxides (VO_x) 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 VO_x phase can be obtained [1–8]. 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 [9,10]. In our pre-work, VO_x thin films with −2%/K TCR is fabricated by reactive ion beam sputtering deposition method [11]. N.Fieldhouse deposited VO_x 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]. C. Venkatasubramanian reported that low resistivity VO_x thin films with the TCRs in the range of −1.6% to −2.2%/K were deposited by pulsed-dc sputtering [13]. He also performed ion implantation followed by annealing to improve the trade-off between TCR and resistivity. The resistivities of the VO_x thin films ranged from 0.05 Ωcm to 100 Ωcm and the TCR values varied from −1.1% to-2.7% [14]. 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]. 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]. 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], and the other is optional for optical switches [18,19]. 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 . 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.

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

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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 Kα 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 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], 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.

 

Fig. 1 Sheet resistance vs. temperature for VO₂(B) thin films made by magnetron controlled sputtering.

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Figure 2 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.

 

Fig. 2 Sheet resistance vs. temperature for VO₂(B) thin films after annealing at different temperature.

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From the x-ray diffraction measurements shown in Fig. 3 and Fig. 4 , 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), ($\overline{4}$ 01), and ($\overline{5}$11) of VO₂ (B), and (211) of Si respectively, from which we can conclude that the thin film is vanadium oxide composed of VO₂ (B) by magnetron sputtering deposition method. The structure parameters are$a_B=$12.03Ǻ, $b_B=$3.693 Ǻ, $c_B=$6.42 Ǻ, ${\beta }_B=$106.6 [10]. 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).

 

Fig. 3 XRD pattern of VO₂ (B) thin films.

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Fig. 4 XRD pattern of VO₂ thin films fabrication by post annealing for 60min at 425°C.

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Figure 5 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.

 

Fig. 5 Micrographs of VO₂ (B) thin film: (a) SEM and (b) AFM micrographs.

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Figure 6 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.

 

Fig. 6 The SEM micrograph of vanadium oxide thin film by post annealing process for 60min at 425°C.

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The speed of phase transition of VO₂ has been experimentally measured to be in the order of nanoseconds [20] and even 100 femtosecond [21] under photon excitation. Figure 7 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.

 

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.

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As shown in Fig. 8 , 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.

 

Fig. 8 Schematic diagram of the experimental apparatus used to measure vanadium oxide thin film switching time.

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The test result is shown in Fig. 9 . 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.

 

Fig. 9 The output voltage of the optical switch.

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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.