Terahertz transmission characteristics across the

Terahertz transmission characteristics across the phase transition in VO2 films deposited on Si, sapphire, and SiO2 substrates Qiwu Shi, Wanxia Huang, Jing Wu, Yaxin Zhang, Yuanjie Xu et al. Citation: J. Appl. Phys. 112, 033523 (2012); doi: 10.1063/1.4746701 View online: http://dx.doi.org/10.1063/1.4746701 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i3 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 112, 033523 (2012)

Terahertz transmission characteristics across the phase transition in VO2 films deposited on Si, sapphire, and SiO2 substrates Qiwu Shi,1 Wanxia Huang,1 Jing Wu,1 Yaxin Zhang,2 Yuanjie Xu,1 Yang Zhang,1 Shen Qiao,2 and Jiazhen Yan1

1 College of Materials Science and Engineering, Sichuan University, Chengdu 610064, People’s Republic of China 2 Terahertz Science and Technology Research Center, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China

(Received 9 February 2012; accepted 11 July 2012; published online 14 August 2012) Vanadium dioxide (VO2) films were deposited on high-purity Si, sapphire, and SiO2 substrates by an organic sol-gel method. The effect of the substrate on the structure, morphology, and phase transition properties of the VO2 films was demonstrated. We proposed that the film-substrate interaction induced the differences in the fraction of the þ4 valence state vanadium oxide phase, surface morphology, and grain size for the VO2 films. The VO2 film on the Si substrate exhibited a switching property of about 2 orders of change in electrical resistivity. By contrast, the VO2 films on the sapphire and SiO2 substrates exhibited a switching property of about 3 orders of change in resistivity. The THz transmission across the phase transition in the VO2 films was quite different in the transmission modulation ratio, the width, and the slope of the hysteresis loop. In particular, the VO2 films on the sapphire and SiO2 substrates have the same reduction in THz transmission by about 46% comparing with about 35% in the VO2 film on the Si substrate. Furthermore, the VO2 C 2012 film on the SiO2 substrate exhibits the widest hysteresis loop with the steepest slope. V American Institute of Physics. [http://dx.doi.org/10.1063/1.4746701]

I. INTRODUCTION

Vanadium dioxide (VO2) has been studied extensively for its fascinating phase transition characteristics, that is, a reversible metal-insulator transition (MIT) at 340 K accompanied by dramatic and abrupt changes in optical and electrical properties.1,2 Especially, the MIT in VO2 could occur at the femtosecond scale,3–5 making it a promising candidate for novel electrical and optical applications, such as, but not limited to, ultrafast switches, sensors, memory and modulator.6–9 Recently, considerable efforts have been focused on exploring the phase transition phenomena in VO2 films and their application in the terahertz (THz) frequency range.9–12 The THz frequency range covers a wide bandwidth from the microwave to infrared regions of the spectrum (30–3000 lm), which is of particular significance for high bandwidth communication. Especially, VO2 films with giant and ultrafast phase transition properties provide for potential application in THz switches and modulators. Many researchers have focused on VO2 films and investigated the influence of their structural characteristics on the MIT parameters, including the switching efficiency, hysteresis, and phase transition temperature.13–16 The microstructure of the VO2 films is found to be highly substrate dependent. For instance, Yang et al. indicated that the VO2 films prepared by physical vapor deposition on Ge (100) substrates showed better crystallinity, slightly lower compressive strain, and a larger resistance change across the MIT than those grown on Si (100).17 Kovacs et al. described quite different growth modes for the VO2 films grown by pulsed laser deposition on sapphire (0001) and Si (001) substrates, which resulted in deviations in the switching amplitude and 0021-8979/2012/112(3)/033523/6/$30.00

hysteresis of the MIT.18 The diverse structures and phase transition characteristics of the VO2 films grown on TiO2, SiO2/Si, and Al2O3-coated Si substrates, caused due to different interface effects between the VO2 films and the substrates, have also been investigated.19–21 It is worth noting that the high-purity Si, sapphire, and SiO2 substrates are widely used as substrates for THz applications because of their high transparency in the THz frequency range.22–24 However, to the best of our knowledge, a comparison of the growth modes of the VO2 films on the three substrates and their THz transmission characteristics across the phase transition have been rarely studied so far. Nevertheless, the evolution of the orientation of the structure as well as the size and morphology of the VO2 films, particularly dominated by the film-substrate interactions, will further affect their phase transition properties. In this paper, we report the deposition of VO2 films on high-purity Si, sapphire, and SiO2 substrates by an organic sol-gel method and elucidate the differences in structures, morphologies, and the resulted phase transition properties in the THz frequency range, which is expected to provide a considerable insight into the preparation of VO2 films for applications in the THz frequency range. II. EXPERIMENTAL DETAILS

V2O5 powder 5 g (purity 99.5%) was first stirred in 200 ml of isobutyl alcohol (analytically pure) in a flask to afford a suspension. Then, 25 ml of benzyl alcohol (analytically pure) was added to the suspension. After stirring at around 110 C in an oil bath for 4 h, a brownish yellow sol resulted, which was subsequently filtered. The precursor sol was stabilized for 2 days resulting in the formation of a gel

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through a sol-gel transition. It has been suggested that under such conditions, a part of the þ5 valence vanadium is reduced with benzyl alcohol to form a þ4 valence state.25 The highpurity Si (100) substrates (0.5 mm thickness, 2000 X cm resistivity), sapphire (0001) substrates (0.5 mm thickness), and SiO2 (0001) substrates were cleaned in acetone and ethyl alcohol ultrasonically for 30 minutes, respectively. Precursor films were then prepared by spin-coating the as-prepared gel on the substrates at 1200 rpm for 15 s and dried at 60 C for 15 minutes. To further reduce the þ5 valence state and promote the crystallization, the VO2 films were annealed at 500 C for 1.5 h in a static atmosphere of nitrogen (purity 99.999%). The spin-coating parameters were stabilized to yield films of similar thickness, which were about 96, 89, and 101 nm on Si, sapphire, and SiO2 substrates, respectively, as measured using a stylus profiler (XP2, Ambios Technology). The crystalline structures of the VO2 films were determined by x-ray diffraction (XRD) (X’ Pert, Philips) using a 4 kW monochromatic CuKa (k ¼ 0.15406 nm) radiation source at an x-ray grazing angle of 1.5 . The surface morphologies were investigated using atomic force microscopy (AFM) (Nanoscope Multimode APM, Vecco Instrument, USA) in tapping mode under ambient conditions. Etched Si nanoprobe tips with a spring constant of 40 N m1 were used. The stoichiometry of the films was detected by x-ray photoelectron spectroscopy (XPS) (Xsam 800, Kratos) using a MgKa exciting source (h ¼ 1253.6 eV). The electrical resistivity measurements were carried out using the conventional four-point probe method combined with an electrically heated substrate holder, in which the four point probes were assigned in a straight line with an interval of 1 mm. The THz transmission of the VO2 films at 0.1-1.5 THz was investigated using a THz time domain spectroscopy (THz-TDS) system. For this experiment, a Ti:sapphire femtosecond laser source (Maitai HP, pulse width 80-100 fs, repetition frequency 80 MHz, average power >2.5 W) was applied to generate and detect terahertz radiation. The excited THz beam range from 0.1 THz to 3.0 THz was focused using a polyethylene lens on to the sample with a diameter of about 2 mm, and a second polyethylene lens recollimates the transmitted THz beam, which was directed to a photoelectric crystal. The time-varied electric field of the transmitted THz beam was recorded, and the electric field spectral amplitude was directly obtained by performing Fourier analysis. III. RESULTS AND DISCUSSION

The XRD patterns of the VO2 films are shown in Figure 1. All of the VO2 films formed on the Si, sapphire, and SiO2 substrates exhibit polycrystalline structures that match the monoclinic VO2 well (JCPDS card no. 72-0514). Herein, a strong XRD peak is found at the angle 2h 27.88 for the samples on different substrates, attributed to the VO2 phase with a (011) preferred orientation. Besides, several minor orientations appear in the XRD patterns for the three VO2 films, which are slightly different from each other. For the VO2 film on the Si substrate, the peaks at 2h ¼ 37.06 , 42.27 , 52.11 , 55.58 , 57.55 , and 65.06 are indexed to the diffractions from the (21 1), (111), (220), (221), (022), and (002) planes of the VO2

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FIG. 1. XRD patterns of the VO2 films on (a) Si, (b) sapphire, and (c) SiO2 substrates.

phase, respectively. The VO2 film on the sapphire substrate exhibits the same polycrystalline characteristic, but the intensity for the minor peaks mentioned above are slightly weaker than those on the Si substrate. In the XRD pattern of the VO2 film on the SiO2 substrate, although a peak at 2h ¼ 49.43 emerged due to diffractions from the (113) planes of the VO2 phase, the other minor peaks at the same locations as those observed for the VO2 film on the Si substrate are even weaker. The results indicate qualitatively that the VO2 film on the SiO2 substrate is best textured, and the VO2 film on the sapphire substrate is better textured than the one on the Si substrate. The difference in the crystalline structure of the films is possibly related with the changes in the grain size and grain crystallization, as shown by Brassard et al.26 However, the full-width-at-half maximum (FWHM) of the (011) peaks of the VO2 film on the Si, sapphire and SiO2 substrates is 0.421 , 0.424 , and 0.433 , respectively. According to Scherrer’s formula (d ¼ 0.9k/FWHM cos (h)), and, despite some tiny deviations due to measurement operation, the average grain size of the VO2 films on the Si, sapphire, and SiO2 substrates is close to each other. XPS was performed to understand the valence states of the vanadium in the films deposited on the Si, sapphire, and SiO2 substrates. As shown in Figure 2, the peak position of V2p3/2 was fitted using a Shirley function with the XPS PEAK 4.1 software. For all three VO2 films, two valence states of vanadium, þ4 valence (with a binding energy of 515.7516.2 eV) and þ5 valence (with a binding energy of 516.0517.2 eV)27 were detected. The relative fractions of the þ4 valence state in the VO2 film on the Si, sapphire, and SiO2 substrates have been evaluated as 70.53%, 76.4%, and 77.29%, respectively. The lower fraction of the þ4 valence state in the


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FIG. 2. High resolution XPS scans focusing on the V2p regime of the VO2 films on (a) Si, (b) sapphire, and (c) SiO2 substrates.

VO2 film on Si could be attributed to the interfacial reaction with the Si substrate owing to its instability to form Si oxides during the annealing process at 500 C.17 Then, the surface parameters of the VO2 films grown on the Si, sapphire, and SiO2 substrates were investigated by AFM, as shown in Figure 3. Although the average grain size of the three films was close to each other, as determined from the XRD analysis mentioned above, the size distribution of the grains and the average surface roughness (Ra) are quite different. Figures 3(a) and 3(d) exhibit the two-dimensional and three-dimensional morphology of the VO2 film on the Si substrate, respectively. The size distribution of the grains is quite nonuniform, where the diameter is between 40 and 150 nm, and the Ra value is 5.39 nm. Comparatively, the AFM morphology of the VO2 film on the sapphire substrate (Figures 3(b) and 3(e)) shows that the grain diameter varies

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from 40 to 100 nm, and the Ra value is 3.75 nm. For the VO2 film on the SiO2 substrate, the AFM morphology presents a relatively fine-grained microstructure (Figures 3(c) and 3(f)). The size distribution of the grains varies between 30 and 70 nm, and the Ra value decreases to 3.30 nm, indicating the most uniform surface morphology. It is well known that the growth of the thin films proceeds through nucleation, crystal growth, and grain growth, accompanied by the evolution of the morphology.28 Herein, the organic sol-gel derived precursor film was spin-coated on the substrate to form a uniform film thickness, resulting preferentially in a Zone II growth,18 in which the bulk diffusion forms the morphology and the surface and interfacial energy may be crucial. Therefore, we propose that the diverse morphologies of the VO2 films on different substrates are resulted from the different substrate surface energy, which would affect the density of the nucleation center and the further growth in the films. The diverse morphologies and grain sizes would result in the difference in the concentration of the structural defects and oxygen nonstoichiometry in the VO2 films on the different substrates, which will further affect the MIT properties.16,29 The electrical resistivity switching and THz transmission characteristics across the phase transition in the VO2 films grown on the three substrates are now discussed. Generally, the most straightforward and reliable evidence to determine whether the prepared VO2 films show MIT properties is the temperature-dependence resistance of the film. Here, we measured the electrical resistivity of the VO2 films on the Si, sapphire, and SiO2 substrates, respectively, in the temperature range from 25 C to 90 C. Figure 4 reveals that the MIT magnitude in electrical resistivity of the VO2 film is substrate dependent. The VO2 films on the sapphire and SiO2 substrates have the same switching property with about three orders of change in resistivity, but the VO2 film on the Si substrate has about two orders of change in resistivity. The lower MIT magnitude may originate from the lower fraction of the þ4 valence vanadium oxide phase in the VO2 film on the Si substrate. The other vanadium oxide phases do not show MIT in the studied temperature range, thereby resulting in an increase in the resistivity for the VO2 film at the high temperature metallic phase. A similar phenomenon has also been observed by Kovacs et al.18 The THz transmission property of the VO2 films on the Si, sapphire, and SiO2 substrates were compared in the temperature range of 25 C to 90 C. To avoid the effect of the substrates on the THz transmission spectrum, we normalized the THz transmission through the VO2 films by comparing the THz signals transmitted through the VO2 film on the substrates to that through the bare Si, sapphire and SiO2 substrates, as illustrated in Mandal et al.10 and Shi et al.30 As shown in Figure 5(a), the VO2 films are essentially transparent at the low temperature semiconductor phase at the studied range of 0.1 THz to 1.5 THz. However, the magnitude of the THz transmission through the VO2 films decreases considerably across the MIT triggered by temperature. Coincident with the electrical resistivity switching characteristic, the THz transmission property across the phase transition in the VO2 films varies according to the different substrate studied. Indeed, three differences can be identified:


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FIG. 3. Left: two-dimensional AFM photographs of the VO2 films on (a) Si, (b) sapphire, and (c) SiO2 substrates. Right: three-dimensional AFM images of the VO2 films on (d) Si, (e) sapphire, and (f) SiO2 substrates.

the transmission modulation ratio, the width, and the slope of the hysteresis loop. First, for the transmission modulation ratio, the VO2 films on the sapphire and SiO2 substrates have the same reduction in THz transmission by about 46%, but the VO2 film on the Si substrate has about a 35% reduction in the THz transmission. This result is consistent with the difference in resistivity switching characteristic of the VO2 films on the Si, sapphire, and SiO2 substrates, as mentioned above, because the decrease of the THz transmission is attributed to the increased absorption and reflection loss of the THz signals in the VO2 film, which is caused by the increase of electrical conductivity across the MIT in the VO2 film. The

lower resistivity corresponds to the higher conductivity and leads to the lower THz transmission. However, our earlier experiments showed that a VO2 film deposited by an inorganic sol-gel method exhibited a THz transmission modulation ratio of about 81% across the phase transition.30 Although the concentration of the þ4 valence vanadium oxide phase (79.85%) in that VO2 film was slightly higher than that in the VO2 film studied here, the reasons for the different THz modulation ratio between the VO2 films deposited by the inorganic and organic sol-gel method need further investigation. Jepsen et al. have proposed that the nonstoichiometric fraction or void in the VO2 films may be an explanation for the


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FIG. 4. Hysteresis loops of electrical resistivity against temperature for the VO2 films on Si (black circles), sapphire (red circles), and SiO2 (blue circles) substrates.

incomplete phase transition in the VO2 films.31 As illustrated in Figure 3, the VO2 films deposited by the organic sol-gel method exhibit a quite different compactness from the VO2 films deposited by the inorganic sol-gel method, as described in Ref. 30. With regard to the indistinctive difference in the stoichiometry, it can be suggested that the diverse compactness should play an important role in the THz transmission modulation properties of the VO2 films. The different compactness in the VO2 films on different substrates illustrated in this study should be derived from the different film-substrate interaction during the growth. The derivatives of the temperature dependence of transmission (dTr/dTemp) were extracted for the VO2 film on different substrates. As shown in Figures 5(b), 5(c), and 5(d), the MIT temperatures in the heating and cooling cycle

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(defined as Theating and Tcooling, respectively) for different films were evaluated and used to estimate the shape of the hysteresis loop quantitatively. We define the width of the hysteresis loop as Theating–Tcooling, and the intensity of the peak of [dTr/dTemp] corresponds to the slope of the hysteresis loop. With regard to the shape of the hysteresis loop, the VO2 film on the SiO2 substrate exhibits the widest hysteresis loop with the steepest slope. The observed differences in hysteresis could be attributed to the various grain sizes, grain size distribution, and the correlated concentrations of oxygen vacancies, dislocations, etc.26,29,32–34 Especially, the VO2 grains with comparable size have similar MIT temperatures, which will reduce the dispersion of MIT temperatures for different particles in the same VO2 film and result in a steeper slope of the hysteresis loop.34 According to the AFM analysis results, the size distribution of the VO2 film on the SiO2 substrate is the most uniform, which could be visible evidence for its steepest slope of the hysteresis loop. Moreover, the widest hysteresis loop of the VO2 film on the SiO2 substrate may be caused by the increased density of grain boundaries and associated defects.26

IV. CONCLUSIONS

VO2 films were deposited on high-purity Si, sapphire, and SiO2 substrates by an organic sol-gel method. All the films exhibit polycrystalline VO2 structures with a (011) preferred orientation. However, the texture of the VO2 films on different substrates varied slightly. The relative fractions of the þ4 valence state vanadium in the VO2 films on the Si, sapphire, and SiO2 substrates are evaluated to be 70.53%, 76.4%, and 77.29%, respectively. The relative fractions resulted in a switching property of about 2 orders of change in electrical resistivity in the VO2 film on the Si substrate, but about 3 orders of change in the VO2 films on the sapphire and SiO2 substrates. Correlated with the diversity in the surface morphology and grain size of the VO2 films on different substrates, the films on the sapphire and SiO2 substrates have the same reduction in THz transmission by about 46%, comparing with about 35% in the VO2 film on the Si substrate. Furthermore, the VO2 film on the SiO2 substrate exhibits the widest hysteresis loop with the steepest slope.35 ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Grants 61072036 and 61001031) and SRF. We thank the Analytical & Testing Center of Sichuan University for the XRD analysis. 1

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FIG. 5. (a) Hysteresis loop of the normalized transmission against temperature for the VO2 film at 0.1-1.5 THz. Derivatives of the temperature dependence of transmission (dTr/dTemp) extracted for the VO2 films on (b) Si, (c) sapphire, and (d) SiO2 substrates.


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