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Production of VO2 thin films through post-deposition annealing of V2O3 and VOx films
Thin Solid Films 591 (2015) 143–148
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Production of VO2 thin films through post-deposition annealing of V2O3 and VOx films Bart Van Bilzen a,⁎, Pia Homm a, Leander Dillemans a, Chen-Yi Su a, Mariela Menghini a, Marilyne Sousa b, Chiara Marchiori b, Luman Zhang c, Jin Won Seo c, Jean-Pierre Locquet a a b c
Solid State Physics and Magnetism, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium IBM Research Laboratory — Zurich, Saumerstrasse 4, CH-8803, Ruschlikon, Switzerland Surface and Interface Engineered Materials, Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 bus 2450, 3001 Heverlee, Belgium
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Article history: Received 16 February 2015 Received in revised form 24 August 2015 Accepted 25 August 2015 Available online 28 August 2015 Keywords: Vanadium dioxide Metal-to-Insulator Treansition Annealing Oxidation Molecular Beam Epitaxy
a b s t r a c t VO2 thin films were produced on sapphire and silicon substrates through post-deposition ex-situ thermal treatment of V2O3 and VOx films. Thin epitaxial films of V2O3 on sapphire and amorphous VOx films on silicon substrates were grown using oxygen assisted molecular beam epitaxy. The post-deposition annealing was performed at different temperatures using an Ar flow. Structural, optical and electrical characterizations were performed to confirm the transformation of the films. The transformed films present a change in resistance across the metal to insulator transition of four orders of magnitude for annealed V2O3 on sapphire and around one order of magnitude in the case of annealed VOx on silicon. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Vanadium oxide (VyOx) can exist in many different phases like the Magnéli (VnO2n − 1, where 3 ≤ n ≤ 9) and Wadsley (VnO2n + 1, where n = 1–6) phases. The most common phases include: V2O3, VO2 and V2O5, with V2O3 being a Magnéli phase with n = 2 and VO2 with n → infinity [1]. Many vanadium oxides have a metal to insulator (or semiconductor) transition (MIT) at particular temperatures. Vanadium dioxide (VO2) is probably the most studied material showing a MIT at around 69 °C [2,3]. At the transition temperature (TMIT), VO2 undergoes a structural transition from a low temperature insulating (semiconducting) monoclinic phase to a high temperature metallic rutile phase [4]. Across this transition the resistivity of bulk VO2 changes by up to 4 orders of magnitude [5]. The transition temperature can be reduced towards room temperature by doping or strain [6,7], which makes VO2 an interesting candidate for all sorts of applications including bolometers [8–10], smart windows [11] and memristors for neuromorphic computing [12]. VO2 films on sapphire and silicon substrates have been prepared by sputtering [8,9,13], pulsed laser deposition [9], chemical vapor deposition [14,15], molecular beam epitaxy (MBE) [16–19] and sol– gel [6,20]. Due to the difference in structure between the sapphire substrate (corundum) and vanadium dioxide (high temperature rutile ⁎ Corresponding author. E-mail address: [email protected] (B. Van Bilzen).
http://dx.doi.org/10.1016/j.tsf.2015.08.036 0040-6090/© 2015 Elsevier B.V. All rights reserved.
and low temperature monoclinic [4]) it is difficult to achieve high quality epitaxial VO2 during deposition on sapphire substrates. Kozo et al. [21] showed that vanadium dioxide can be obtained from a VO/V2O3 layer by annealing it in oxygen. The authors, however, only used XRD to confirm this and no information of a MIT is presented. Yamaguchi et al. [22] proposed an alternative way to prepare VO2 thin films by epitaxial V2O3 deposition on sapphire and further annealing in an Ar–O2 mixture (P(O2) = 10 Pa) at 500 °C for different times. The films show a resistivity ratio at the MIT of three orders of magnitude. Okimura et al. [23] annealed their V2O3 thin films in a restricted oxygen flow (P(O2) = 500 Pa) at 450 °C for different times in order to transform the films to VO2. All of these works are restricted to sapphire substrates only. In this work we report epitaxial V2O3 grown on sapphire and amorphous VOx grown on Si substrates by MBE that underwent similar annealing treatment to transform them into VO2. 2. Experimental procedure Vanadium sesquioxide (V2O3) films were grown on 1 × 1 cm2 (0001) sapphire substrates and amorphous VOx films were grown on 2″ (100) silicon substrates using oxygen assisted MBE with a thickness ranging from 24 to 100 nm. The V2O3 deposition was performed at 700 °C and at an oxygen pressure in the order of 10− 4 Pa with a deposition rate of 0.1 Å/s. The VOx thin films deposited on silicon were grown at 200 °C and at an oxygen pressure of 6.2 · 10–4 Pa with a deposition rate of 0.1 Å/s. During the deposition vanadium was evaporated
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from an electron beam gun and molecular oxygen was introduced into the chamber. Finally, the deposited vanadium oxide films were annealed in a glass tube furnace in the presence of an Ar gas flow with a rate of 0.6 l/min. Residual oxygen in the Ar flow of the order of 200 Pa was determined by using a mass spectrometer, which is of the same order of magnitude as Okimura et al. [23]. The films were heated from room temperature to the different annealing temperatures at a rate of 10 °C/min. The annealing temperature was maintained for 1 or 2 h. Then, the films were naturally cooled down to room temperature inside the furnace to prevent the film to be in contact with air at elevated temperatures. During the entire procedure an Ar flow of 0.6 l/min inside the furnace was maintained. The annealing procedure is schematically represented in Fig. 1. The goal is to oxidize the vanadium oxide films to VO2. A low Ar flow inside the furnace allows a small O2 background pressure to oxidize the films. Further oxidation will cause V2O5 to form, which is unwanted. After deposition and annealing, the films were characterized by means of X-ray diffraction (XRD) and X-ray reflectivity (XRR) using a PANalytical X'Pert Pro diffractometer with Cu-Kα radiation (λ = 1.5418 Å) and optional monochromator. The X-rays impinge the films at an angle ω and the diffracted/reflected X-rays are collected at an angle 2θ with respect to the impinging radiation. For the XRD θ–2θ and XRR measurements, symmetric scans (ω = θ) are performed, also there is no rotation allowed around the normal of the sample stage (φ = 0) nor around the axis parallel to the sample stage (ψ = 0). For the XRD-φ-scans a ψ-offset was used for each measurement. Temperature dependent transport properties were assessed by performing resistivity measurements in the Van der Pauw configuration using a Keithley 4200 SCS and variable temperature SUSS probe station. Atomic force microscopy (AFM) was performed to extract the surface roughness using a Bruker Multimode 8 microscope. Finally, spectroscopic ellipsometry measurements were performed at room temperature using a VASE® from J.A. Woollam Co. to determine the optical constants of the films. These measurements have been carried out from 300 to 1680 nm at 3 angles of incidence (65°, 70° and 75°) on the VOx films grown on Si, both for as grown and annealed at 500 °C for 2 h. The data have been analyzed using the WVASE® software and implementing a 2 layer model (see inset Fig. 6b) with the VOx layer being simulated with a Generated Oscillator model. This Generated Oscillator model was based on 2 Gaussian peaks, 1 Tauc–Lorentz oscillator and 1 pole for the as grown films. For the annealed samples, 2 Gaussian peaks, 2 Tauc–Lorentz oscillators and 1 Drude peak were required.
Fig. 2. θ–2θ scans of as grown and annealed 24 nm V2O3 thin films on sapphire. The film annealed at 450 °C (blue curve) was annealed afterwards at 500 °C (red curve). The different scans are shifted in intensity to display them better.
diffraction peak corresponding to (0006) V2O3 in the as grown film has decreased in intensity and it has shifted from 2θ = 38.6° to 2θ = 39.3°. According to the (ICDD) database [24] we can associate this peak to the (−402) orientation of V3O5, corresponding to an intermediate oxidation between V2O3 and VO2.
3. Results and discussion Epitaxially grown V2O3 thin films on sapphire of 24 nm thickness were annealed in an Ar flow at 450 °C for 1 h. It was found that this procedure did not completely transform the films into VO2. From the symmetric XRD θ–2θ scan shown in Fig. 2 it can be observed that the
Fig. 1. Schematic representation of the annealing procedure. Times and temperatures correspond to one specific annealing condition.
Fig. 3. φ-Scan of the VO2 (011) (top) and Al2O3 (104) (bottom) orientations of the annealed 24 nm V2O3 thin films on sapphire, concluding the transformation into VO2 (020).
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Fig. 4. (a) XRR scans and (b) θ–2θ scans of as grown and annealed (at 500 °C for 1 h) 70 nm V2O3 films on sapphire.
Note that in the measured scan several peaks are related to the X-ray source and represent no extra information of the measured films. The Cu source emits characteristic X-rays with slightly different wavelengths (Kα1, Kα2, Kβ and Lα1 from tungsten (W) contamination [25]) leading to different observed peaks for the same substrate orientation. As reference, a scan of the bare substrate is shown in Fig. 2 where these peaks from the (0006) sapphire orientation are indicated. The film annealed at 450 °C for 1 h was then annealed at 500 °C for 1 h. After this further annealing the film peak completely shifts to 2θ = 39.8°, which can be attributed to both (002) and (020) VO2 reflections since they have both similar d-spacing. To discriminate between the two orientations, XRD φ-scans have been performed, see Fig. 3. The φ-scans on the VO2 (011) plane (2θ = 27.8°, ψ = 44.9°) and the Al2O3 (104) plane (2θ = 35.1°, ψ = 38.2°) have been carried out. There are 3 peaks observed for the sapphire substrate, due to the threefold symmetry of sapphire along the c-axis, and 6 peaks from the VO2. As discussed by Fan et al. [26] VO2 has a twofold symmetry along the [020] orientation and the (011), (−111) and (110) planes are crystallographically equivalent and rotated 120° from each other, due to the similar Bragg and pole angles (2θ011 = 27.8°, 2θ110 = 2θ− 111 = 26.87°, ψ011 = 44.9°, ψ110 = ψ− 111 = 42.9°). So all three planes
Fig. 6. θ–2θ scans of as grown and annealed 80 nm (a) and 100 nm (b) VOx films on silicon. None of the films underwent multiple annealing treatments.
generate two peaks in the φ-scan leading to 6 peaks as observed. Since the (011), (− 111) and (110) planes of the [002] orientation have different pole angles (ψ011 = 45.1°, ψ110 = 68.37°, ψ− 111 =
Fig. 5. Resistivity vs temperature of annealed (at 500 °C for 1 h) 70 nm V2O3 film on sapphire. The temperature has been swept at a rate of 0.3 °C/min. Both the heating and cooling curves are displayed.
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Fig. 7. Room temperature refractive index (a) and extinction coefficient (b) for VOx (as grown) and VO2 (annealed) films for both 80 nm (red) and 100 nm (blue) VOx films. VO2 data taken from [31] is used as a reference. Inset: the 2 layer model used to determine the optical dispersion index of the film from the ellipsometry measurement.
68.74°), these peaks could not appear on a φ-scan in this case. So we can attribute our film peak at 2θ = 39.8° to be (020) VO2. Note that the peak at 2θ = 39.8° of the film annealed at 450 °C is from W contamination [25], not of VO2. Although for the 500 °C annealed film the two peaks overlap (VO2 and W contamination), the broadening of the peak and the increase in intensity as compared to the as grown film indicate that it corresponds to the formation of VO2. Finally epitaxially grown V2O3 thin films on sapphire of 70 nm thickness were annealed in an Ar flow directly at 500 °C for 1 h. Fig. 4 shows respectively XRR and θ-2θ XRD scans of the film prior to and after the annealing. The as grown film shows a peak of V2O3 at 2θ = 38.5°, corresponding to the (0006) orientation, indicating epitaxial growth of the V2O3 on sapphire. After annealing the films for 1 h at 500 °C in Ar, the V2O3 (0006) peak disappeared and a new peak at 2θ = 39.8° emerged. This new peak is also attributed to VO2 (020), thus indicating that V2O3 was transformed into VO2. In this case, there is no peak associated with the W contamination since a monochromator was used during the measurements. Note that the second film is much thicker (70 nm vs. 24 nm), which leads to a sharper and narrower peak. From the XRR scan it can be seen that the critical angle decreases after annealing, indicating a decrease in density. V2O3, VO2 and V2O5 have bulk densities of 5.01, 4.39 and 3.37 g/cm3 respectively [24]. From XRR fitting densities of 4.86 and 4.11 g/cm3 were extracted for as grown and annealed films respectively, which are lower than the bulk densities, as is expected for thin films. Therefore the combination of XRD and XRR indicates that the films are transformed into VO2. From the strongly reduced amplitude of the oscillations in the XRR scans, it can also be inferred that the roughness significantly increased after annealing.
In Fig. 5 the resistivity (ρ) as a function of the temperature of the 70 nm film annealed at 500 °C for 1 h (Fig. 4) is shown. A clear transition from an insulating to a metallic state can be observed. The change in resistance along this MIT is about 4 orders of magnitude and the transition temperature is 72 °C for the heating curve and 66 °C for the cooling curve. These values correspond with values for bulk VO2 found in literature [5]. Semiconductor activation energies of 0.31 eV and 0.33 eV were extracted in the insulating state for the heating and cooling curves respectively. This is consistent with literature, where values between 0.1 eV and 0.65 eV have been reported, although slightly lower than the VO2 bulk value of 0.42–0.45 eV [27–29]. This is probably due to donor type stoichiometric defects [30]. In our case the annealing treatment most likely caused a transformation that almost completely transformed the V2O3 to VO2, creating oxygen vacancies which act as donor defects, leading to a lower activation energy. Next, the results on silicon substrates will be discussed. Amorphous VOx films grown on silicon have been annealed in Ar gas flow using different times and temperatures. Films of two different thicknesses have been investigated: 80 and 100 nm. In Fig. 6 the symmetric XRD θ–2θ scans of the 80 nm (a) and 100 nm (b) films are shown for various annealing treatments. The peak at 2θ = 32.99° corresponds to the (200) reflection of the substrate. The absence of any other peak in the as grown film indicates that the films were originally amorphous. For both films it is clear that the annealing causes a peak at 2θ = 28° to form, which corresponds to VO2 (011). It is important to mention that a peak at this position is also expected for V2O5. Therefore, the XRD data alone is not enough to conclude the formation of VO2 upon annealing. The intensity of the peak increased after annealing at 600 °C for 1 h.
Fig. 8. (a) Resistivity vs temperature of 80 nm VOx on silicon as grown and annealed with various thermal budgets. (b) Resistivity vs temperature of 80 and 100 nm VO2 films on silicon obtained by annealing at 500 °C for 2 h.
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Similar results were obtained in both films with different thicknesses. Annealings at 475 °C and 525 °C have been performed on the 100 nm film, but no significant changes in the XRD scans have been observed compared to the annealing at 500 °C for 1 and 2 h. With the increase of the annealing temperature or time an increase in surface roughness is observed indicated by the lower intensity fringes from the XRR scans (not shown here). AFM measurements show that the roughness of the film surface increased from 1.6 to 6.6 nm after annealing being consistent with the XRR measurements. The densities of these films obtained from XRR fittings varied between 4.1 and 4.6 g/cm3, in agreement with the reported values for VO2 [24]. After the films were annealed at 600 °C for 1 h, the density (3.15 g/cm3) is much lower indicating that the phase corresponds mainly to V2O5. As it can be seen in Fig. 7, there is a clear change in wavelength dependent behavior of the refractive index (a) and extinction coefficient (b) between the as grown and the annealed films (for both thicknesses) grown on Si. The observed wavelength behavior as well as the magnitude of the optical constants of the annealed films are comparable with reported data for VO2 thin films [31]. Therefore, these results further confirm the transformation to a VO2 phase in our films after annealing at 500 °C for 2 h. In Fig. 8a the resistivities of 80 nm VOx films on silicon annealed at various thermal budgets are shown. For the as grown film a slight increase of ρ is observed with increasing temperature, which indicates metallic behavior. On the other hand, when annealing at 500 °C for 1 h a slight decrease of the resistance with increasing temperature is observed. Finally, when annealing at the same temperature for 2 h, the resistivity decreased more than one order of magnitude when the temperature increased from 70 °C to 90 °C. This is a clear signature of a MIT as expected for VO2. Note that the transition temperature is slightly higher compared to bulk VO2. This difference can be due to the presence of other vanadium oxide phases in our films and the microstructure of the films. Resistivity measurements on the 600 °C annealed film did not show a clear MIT, but a more gradual change in resistivity with changing temperature. This can be due to the large increase in surface roughness and most probably due to the formation of V2O5 phase. Note that the resistivity of the as grown film and the film annealed at 500 °C for 1 h were only measured at discrete points and the other films were measured continuously with a temperature sweep rate of 0.3 °C/min. In Fig. 8b we show the comparison of the resistivity curve of 80 and 100 nm VOx films on silicon annealed at 500 °C for 2 h. 4. Conclusion V2O3 films grown epitaxially on sapphire substrates and VOx films grown amorphously on silicon substrates were annealed in an Ar environment with the aim to obtain VO2 thin films. Annealing at 500 C gave the best electrical properties both for the epitaxial V2O3 (annealed for 1 h) and for the amorphous VOx (annealed for 2 h) films. XRD θ–2θ scans, electrical Van der Pauw and ellipsometry measurements all confirmed the transformation into VO2. In the case of the epitaxial grown V2O3 film, the annealing step leads to VO2 with a MIT with four orders of magnitude change in resistance. This change in resistance is larger than what has been reported in literature for VO2 films obtained by post-deposition annealing [22,23]. In the case of the annealed amorphous VOx films grown on silicon the resistance ratio is more than one order of magnitude. It is demonstrated that low temperature grown amorphous VOx films can be oxidized and crystallized with this procedure to form (poly-)crystalline VO2 films on a silicon substrate showing a MIT. A first step to make this process CMOS compatible has been achieved by fabricating VO2 on silicon substrates. Lowering the annealing temperature is necessary to make the process completely CMOS compatible, which will also improve the roughness of the films. For this purpose, annealing in oxygen instead of argon at lower temperatures will be investigated.
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Acknowledgments This work is financially being supported by the European FP7-ICT2013-11-619456 Sitoga project and by FWO project G052010N. PH acknowledges support from Becas Chile — CONICYT.
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Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Production of VO2 thin films through post-deposition annealing of V2O3 and VOx films Bart Van Bilzen a,⁎, Pia Homm a, Leander Dillemans a, Chen-Yi Su a, Mariela Menghini a, Marilyne Sousa b, Chiara Marchiori b, Luman Zhang c, Jin Won Seo c, Jean-Pierre Locquet a a b c
Solid State Physics and Magnetism, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium IBM Research Laboratory — Zurich, Saumerstrasse 4, CH-8803, Ruschlikon, Switzerland Surface and Interface Engineered Materials, Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 bus 2450, 3001 Heverlee, Belgium
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 24 August 2015 Accepted 25 August 2015 Available online 28 August 2015 Keywords: Vanadium dioxide Metal-to-Insulator Treansition Annealing Oxidation Molecular Beam Epitaxy
a b s t r a c t VO2 thin films were produced on sapphire and silicon substrates through post-deposition ex-situ thermal treatment of V2O3 and VOx films. Thin epitaxial films of V2O3 on sapphire and amorphous VOx films on silicon substrates were grown using oxygen assisted molecular beam epitaxy. The post-deposition annealing was performed at different temperatures using an Ar flow. Structural, optical and electrical characterizations were performed to confirm the transformation of the films. The transformed films present a change in resistance across the metal to insulator transition of four orders of magnitude for annealed V2O3 on sapphire and around one order of magnitude in the case of annealed VOx on silicon. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Vanadium oxide (VyOx) can exist in many different phases like the Magnéli (VnO2n − 1, where 3 ≤ n ≤ 9) and Wadsley (VnO2n + 1, where n = 1–6) phases. The most common phases include: V2O3, VO2 and V2O5, with V2O3 being a Magnéli phase with n = 2 and VO2 with n → infinity [1]. Many vanadium oxides have a metal to insulator (or semiconductor) transition (MIT) at particular temperatures. Vanadium dioxide (VO2) is probably the most studied material showing a MIT at around 69 °C [2,3]. At the transition temperature (TMIT), VO2 undergoes a structural transition from a low temperature insulating (semiconducting) monoclinic phase to a high temperature metallic rutile phase [4]. Across this transition the resistivity of bulk VO2 changes by up to 4 orders of magnitude [5]. The transition temperature can be reduced towards room temperature by doping or strain [6,7], which makes VO2 an interesting candidate for all sorts of applications including bolometers [8–10], smart windows [11] and memristors for neuromorphic computing [12]. VO2 films on sapphire and silicon substrates have been prepared by sputtering [8,9,13], pulsed laser deposition [9], chemical vapor deposition [14,15], molecular beam epitaxy (MBE) [16–19] and sol– gel [6,20]. Due to the difference in structure between the sapphire substrate (corundum) and vanadium dioxide (high temperature rutile ⁎ Corresponding author. E-mail address: [email protected] (B. Van Bilzen).
http://dx.doi.org/10.1016/j.tsf.2015.08.036 0040-6090/© 2015 Elsevier B.V. All rights reserved.
and low temperature monoclinic [4]) it is difficult to achieve high quality epitaxial VO2 during deposition on sapphire substrates. Kozo et al. [21] showed that vanadium dioxide can be obtained from a VO/V2O3 layer by annealing it in oxygen. The authors, however, only used XRD to confirm this and no information of a MIT is presented. Yamaguchi et al. [22] proposed an alternative way to prepare VO2 thin films by epitaxial V2O3 deposition on sapphire and further annealing in an Ar–O2 mixture (P(O2) = 10 Pa) at 500 °C for different times. The films show a resistivity ratio at the MIT of three orders of magnitude. Okimura et al. [23] annealed their V2O3 thin films in a restricted oxygen flow (P(O2) = 500 Pa) at 450 °C for different times in order to transform the films to VO2. All of these works are restricted to sapphire substrates only. In this work we report epitaxial V2O3 grown on sapphire and amorphous VOx grown on Si substrates by MBE that underwent similar annealing treatment to transform them into VO2. 2. Experimental procedure Vanadium sesquioxide (V2O3) films were grown on 1 × 1 cm2 (0001) sapphire substrates and amorphous VOx films were grown on 2″ (100) silicon substrates using oxygen assisted MBE with a thickness ranging from 24 to 100 nm. The V2O3 deposition was performed at 700 °C and at an oxygen pressure in the order of 10− 4 Pa with a deposition rate of 0.1 Å/s. The VOx thin films deposited on silicon were grown at 200 °C and at an oxygen pressure of 6.2 · 10–4 Pa with a deposition rate of 0.1 Å/s. During the deposition vanadium was evaporated
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from an electron beam gun and molecular oxygen was introduced into the chamber. Finally, the deposited vanadium oxide films were annealed in a glass tube furnace in the presence of an Ar gas flow with a rate of 0.6 l/min. Residual oxygen in the Ar flow of the order of 200 Pa was determined by using a mass spectrometer, which is of the same order of magnitude as Okimura et al. [23]. The films were heated from room temperature to the different annealing temperatures at a rate of 10 °C/min. The annealing temperature was maintained for 1 or 2 h. Then, the films were naturally cooled down to room temperature inside the furnace to prevent the film to be in contact with air at elevated temperatures. During the entire procedure an Ar flow of 0.6 l/min inside the furnace was maintained. The annealing procedure is schematically represented in Fig. 1. The goal is to oxidize the vanadium oxide films to VO2. A low Ar flow inside the furnace allows a small O2 background pressure to oxidize the films. Further oxidation will cause V2O5 to form, which is unwanted. After deposition and annealing, the films were characterized by means of X-ray diffraction (XRD) and X-ray reflectivity (XRR) using a PANalytical X'Pert Pro diffractometer with Cu-Kα radiation (λ = 1.5418 Å) and optional monochromator. The X-rays impinge the films at an angle ω and the diffracted/reflected X-rays are collected at an angle 2θ with respect to the impinging radiation. For the XRD θ–2θ and XRR measurements, symmetric scans (ω = θ) are performed, also there is no rotation allowed around the normal of the sample stage (φ = 0) nor around the axis parallel to the sample stage (ψ = 0). For the XRD-φ-scans a ψ-offset was used for each measurement. Temperature dependent transport properties were assessed by performing resistivity measurements in the Van der Pauw configuration using a Keithley 4200 SCS and variable temperature SUSS probe station. Atomic force microscopy (AFM) was performed to extract the surface roughness using a Bruker Multimode 8 microscope. Finally, spectroscopic ellipsometry measurements were performed at room temperature using a VASE® from J.A. Woollam Co. to determine the optical constants of the films. These measurements have been carried out from 300 to 1680 nm at 3 angles of incidence (65°, 70° and 75°) on the VOx films grown on Si, both for as grown and annealed at 500 °C for 2 h. The data have been analyzed using the WVASE® software and implementing a 2 layer model (see inset Fig. 6b) with the VOx layer being simulated with a Generated Oscillator model. This Generated Oscillator model was based on 2 Gaussian peaks, 1 Tauc–Lorentz oscillator and 1 pole for the as grown films. For the annealed samples, 2 Gaussian peaks, 2 Tauc–Lorentz oscillators and 1 Drude peak were required.
Fig. 2. θ–2θ scans of as grown and annealed 24 nm V2O3 thin films on sapphire. The film annealed at 450 °C (blue curve) was annealed afterwards at 500 °C (red curve). The different scans are shifted in intensity to display them better.
diffraction peak corresponding to (0006) V2O3 in the as grown film has decreased in intensity and it has shifted from 2θ = 38.6° to 2θ = 39.3°. According to the (ICDD) database [24] we can associate this peak to the (−402) orientation of V3O5, corresponding to an intermediate oxidation between V2O3 and VO2.
3. Results and discussion Epitaxially grown V2O3 thin films on sapphire of 24 nm thickness were annealed in an Ar flow at 450 °C for 1 h. It was found that this procedure did not completely transform the films into VO2. From the symmetric XRD θ–2θ scan shown in Fig. 2 it can be observed that the
Fig. 1. Schematic representation of the annealing procedure. Times and temperatures correspond to one specific annealing condition.
Fig. 3. φ-Scan of the VO2 (011) (top) and Al2O3 (104) (bottom) orientations of the annealed 24 nm V2O3 thin films on sapphire, concluding the transformation into VO2 (020).
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Fig. 4. (a) XRR scans and (b) θ–2θ scans of as grown and annealed (at 500 °C for 1 h) 70 nm V2O3 films on sapphire.
Note that in the measured scan several peaks are related to the X-ray source and represent no extra information of the measured films. The Cu source emits characteristic X-rays with slightly different wavelengths (Kα1, Kα2, Kβ and Lα1 from tungsten (W) contamination [25]) leading to different observed peaks for the same substrate orientation. As reference, a scan of the bare substrate is shown in Fig. 2 where these peaks from the (0006) sapphire orientation are indicated. The film annealed at 450 °C for 1 h was then annealed at 500 °C for 1 h. After this further annealing the film peak completely shifts to 2θ = 39.8°, which can be attributed to both (002) and (020) VO2 reflections since they have both similar d-spacing. To discriminate between the two orientations, XRD φ-scans have been performed, see Fig. 3. The φ-scans on the VO2 (011) plane (2θ = 27.8°, ψ = 44.9°) and the Al2O3 (104) plane (2θ = 35.1°, ψ = 38.2°) have been carried out. There are 3 peaks observed for the sapphire substrate, due to the threefold symmetry of sapphire along the c-axis, and 6 peaks from the VO2. As discussed by Fan et al. [26] VO2 has a twofold symmetry along the [020] orientation and the (011), (−111) and (110) planes are crystallographically equivalent and rotated 120° from each other, due to the similar Bragg and pole angles (2θ011 = 27.8°, 2θ110 = 2θ− 111 = 26.87°, ψ011 = 44.9°, ψ110 = ψ− 111 = 42.9°). So all three planes
Fig. 6. θ–2θ scans of as grown and annealed 80 nm (a) and 100 nm (b) VOx films on silicon. None of the films underwent multiple annealing treatments.
generate two peaks in the φ-scan leading to 6 peaks as observed. Since the (011), (− 111) and (110) planes of the [002] orientation have different pole angles (ψ011 = 45.1°, ψ110 = 68.37°, ψ− 111 =
Fig. 5. Resistivity vs temperature of annealed (at 500 °C for 1 h) 70 nm V2O3 film on sapphire. The temperature has been swept at a rate of 0.3 °C/min. Both the heating and cooling curves are displayed.
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Fig. 7. Room temperature refractive index (a) and extinction coefficient (b) for VOx (as grown) and VO2 (annealed) films for both 80 nm (red) and 100 nm (blue) VOx films. VO2 data taken from [31] is used as a reference. Inset: the 2 layer model used to determine the optical dispersion index of the film from the ellipsometry measurement.
68.74°), these peaks could not appear on a φ-scan in this case. So we can attribute our film peak at 2θ = 39.8° to be (020) VO2. Note that the peak at 2θ = 39.8° of the film annealed at 450 °C is from W contamination [25], not of VO2. Although for the 500 °C annealed film the two peaks overlap (VO2 and W contamination), the broadening of the peak and the increase in intensity as compared to the as grown film indicate that it corresponds to the formation of VO2. Finally epitaxially grown V2O3 thin films on sapphire of 70 nm thickness were annealed in an Ar flow directly at 500 °C for 1 h. Fig. 4 shows respectively XRR and θ-2θ XRD scans of the film prior to and after the annealing. The as grown film shows a peak of V2O3 at 2θ = 38.5°, corresponding to the (0006) orientation, indicating epitaxial growth of the V2O3 on sapphire. After annealing the films for 1 h at 500 °C in Ar, the V2O3 (0006) peak disappeared and a new peak at 2θ = 39.8° emerged. This new peak is also attributed to VO2 (020), thus indicating that V2O3 was transformed into VO2. In this case, there is no peak associated with the W contamination since a monochromator was used during the measurements. Note that the second film is much thicker (70 nm vs. 24 nm), which leads to a sharper and narrower peak. From the XRR scan it can be seen that the critical angle decreases after annealing, indicating a decrease in density. V2O3, VO2 and V2O5 have bulk densities of 5.01, 4.39 and 3.37 g/cm3 respectively [24]. From XRR fitting densities of 4.86 and 4.11 g/cm3 were extracted for as grown and annealed films respectively, which are lower than the bulk densities, as is expected for thin films. Therefore the combination of XRD and XRR indicates that the films are transformed into VO2. From the strongly reduced amplitude of the oscillations in the XRR scans, it can also be inferred that the roughness significantly increased after annealing.
In Fig. 5 the resistivity (ρ) as a function of the temperature of the 70 nm film annealed at 500 °C for 1 h (Fig. 4) is shown. A clear transition from an insulating to a metallic state can be observed. The change in resistance along this MIT is about 4 orders of magnitude and the transition temperature is 72 °C for the heating curve and 66 °C for the cooling curve. These values correspond with values for bulk VO2 found in literature [5]. Semiconductor activation energies of 0.31 eV and 0.33 eV were extracted in the insulating state for the heating and cooling curves respectively. This is consistent with literature, where values between 0.1 eV and 0.65 eV have been reported, although slightly lower than the VO2 bulk value of 0.42–0.45 eV [27–29]. This is probably due to donor type stoichiometric defects [30]. In our case the annealing treatment most likely caused a transformation that almost completely transformed the V2O3 to VO2, creating oxygen vacancies which act as donor defects, leading to a lower activation energy. Next, the results on silicon substrates will be discussed. Amorphous VOx films grown on silicon have been annealed in Ar gas flow using different times and temperatures. Films of two different thicknesses have been investigated: 80 and 100 nm. In Fig. 6 the symmetric XRD θ–2θ scans of the 80 nm (a) and 100 nm (b) films are shown for various annealing treatments. The peak at 2θ = 32.99° corresponds to the (200) reflection of the substrate. The absence of any other peak in the as grown film indicates that the films were originally amorphous. For both films it is clear that the annealing causes a peak at 2θ = 28° to form, which corresponds to VO2 (011). It is important to mention that a peak at this position is also expected for V2O5. Therefore, the XRD data alone is not enough to conclude the formation of VO2 upon annealing. The intensity of the peak increased after annealing at 600 °C for 1 h.
Fig. 8. (a) Resistivity vs temperature of 80 nm VOx on silicon as grown and annealed with various thermal budgets. (b) Resistivity vs temperature of 80 and 100 nm VO2 films on silicon obtained by annealing at 500 °C for 2 h.
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Similar results were obtained in both films with different thicknesses. Annealings at 475 °C and 525 °C have been performed on the 100 nm film, but no significant changes in the XRD scans have been observed compared to the annealing at 500 °C for 1 and 2 h. With the increase of the annealing temperature or time an increase in surface roughness is observed indicated by the lower intensity fringes from the XRR scans (not shown here). AFM measurements show that the roughness of the film surface increased from 1.6 to 6.6 nm after annealing being consistent with the XRR measurements. The densities of these films obtained from XRR fittings varied between 4.1 and 4.6 g/cm3, in agreement with the reported values for VO2 [24]. After the films were annealed at 600 °C for 1 h, the density (3.15 g/cm3) is much lower indicating that the phase corresponds mainly to V2O5. As it can be seen in Fig. 7, there is a clear change in wavelength dependent behavior of the refractive index (a) and extinction coefficient (b) between the as grown and the annealed films (for both thicknesses) grown on Si. The observed wavelength behavior as well as the magnitude of the optical constants of the annealed films are comparable with reported data for VO2 thin films [31]. Therefore, these results further confirm the transformation to a VO2 phase in our films after annealing at 500 °C for 2 h. In Fig. 8a the resistivities of 80 nm VOx films on silicon annealed at various thermal budgets are shown. For the as grown film a slight increase of ρ is observed with increasing temperature, which indicates metallic behavior. On the other hand, when annealing at 500 °C for 1 h a slight decrease of the resistance with increasing temperature is observed. Finally, when annealing at the same temperature for 2 h, the resistivity decreased more than one order of magnitude when the temperature increased from 70 °C to 90 °C. This is a clear signature of a MIT as expected for VO2. Note that the transition temperature is slightly higher compared to bulk VO2. This difference can be due to the presence of other vanadium oxide phases in our films and the microstructure of the films. Resistivity measurements on the 600 °C annealed film did not show a clear MIT, but a more gradual change in resistivity with changing temperature. This can be due to the large increase in surface roughness and most probably due to the formation of V2O5 phase. Note that the resistivity of the as grown film and the film annealed at 500 °C for 1 h were only measured at discrete points and the other films were measured continuously with a temperature sweep rate of 0.3 °C/min. In Fig. 8b we show the comparison of the resistivity curve of 80 and 100 nm VOx films on silicon annealed at 500 °C for 2 h. 4. Conclusion V2O3 films grown epitaxially on sapphire substrates and VOx films grown amorphously on silicon substrates were annealed in an Ar environment with the aim to obtain VO2 thin films. Annealing at 500 C gave the best electrical properties both for the epitaxial V2O3 (annealed for 1 h) and for the amorphous VOx (annealed for 2 h) films. XRD θ–2θ scans, electrical Van der Pauw and ellipsometry measurements all confirmed the transformation into VO2. In the case of the epitaxial grown V2O3 film, the annealing step leads to VO2 with a MIT with four orders of magnitude change in resistance. This change in resistance is larger than what has been reported in literature for VO2 films obtained by post-deposition annealing [22,23]. In the case of the annealed amorphous VOx films grown on silicon the resistance ratio is more than one order of magnitude. It is demonstrated that low temperature grown amorphous VOx films can be oxidized and crystallized with this procedure to form (poly-)crystalline VO2 films on a silicon substrate showing a MIT. A first step to make this process CMOS compatible has been achieved by fabricating VO2 on silicon substrates. Lowering the annealing temperature is necessary to make the process completely CMOS compatible, which will also improve the roughness of the films. For this purpose, annealing in oxygen instead of argon at lower temperatures will be investigated.
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Acknowledgments This work is financially being supported by the European FP7-ICT2013-11-619456 Sitoga project and by FWO project G052010N. PH acknowledges support from Becas Chile — CONICYT.
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