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Tuning the phase transitions of VO2 thin films on silicon substrates using ultrathin Al2O3 as buffer layers
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys. D: Appl. Phys. 47 455304 (http://iopscience.iop.org/0022-3727/47/45/455304) View the table of contents for this issue, or go to the journal homepage for more
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Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 47 (2014) 455304 (5pp)
doi:10.1088/0022-3727/47/45/455304
Tuning the phase transitions of VO2 thin films on silicon substrates using ultrathin Al2O3 as buffer layers Ying Xiong1, Qi-Ye Wen1, Zhi Chen2, Wei Tian1, Tian-Long Wen1, Yu-Lan Jing, Qing-Hui Yang1 and Huai-Wu Zhang1 1
State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China 2 National Key Laboratory of Science and Technology of Communication, University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China E-mail:
[email protected] Received 6 August 2014, revised 16 September 2014 Accepted for publication 29 September 2014 Published 27 October 2014 Abstract
High quality VO2 thin films have been fabricated on silicon substrates using magnetron sputtering by introducing Al2O3 thin films as a buffer. The ultrathin Al2O3 deposited by plasma-assisted atomic layer deposition leads to a greatly improved crystallinity and textures in VO2 films. Dramatic change in electrical resistivity (4 orders of magnitude) and a small thermal hysteresis loop (~4 K) are obtained across the metal–insulator phase transition (MIT). Remarkably, by applying perpendicular voltage to a VO2/Al2O3 based metal/VO2/ semiconductor device, electrically driven MIT switching characteristics have been observed with a tiny tunneling leakage current of ~10 μA. These results show that an electric field alone is sufficient to trigger the MIT, and the realization of VO2 based ultrafast electrical switching devices on a silicon substrate is possible. Keywords: vanadium dioxide, phase transition, silicon substrate, electrical switching devices (Some figures may appear in colour only in the online journal)
1. Introduction
To fabricate high quality VO2 thin films, materials such as Al2O3 and TiO2 have been employed as substrates. The epitaxial growth of the VO2 thin films on sapphires render a change of the resistivity (ΔR) to more than four orders of magnitude due to the very small lattice mismatch [14–16]. To improve their performance in electronic and optical devices, it is of great demand to prepare high quality VO2 thin films on a silicon substrate, which is the mainstay substrate material in the microelectronics industry. Specifically, an electrical-driven MIT (E-MIT) can occur by applying a very small electric voltage in the perpendicular direction of the VO2 thin films, which have attracted great attention [4, 5, 17, 18]. Particularly, it is possible to miniaturize the VO2 based devices to submicron meters and to achieve an ultrafast switch (≤ 2 ns) between insulating and metallic states in the perpendicular metal/VO2/ semiconductor (MOS) structures [5]. The ultrafast E-MIT is believed to be induced by electronic correlation effects rather
Vanadium dioxide (VO2) thin films have evoked considerable interest since it was first discovered by Morin that they underwent a phase transition during heating or cooling [1]. Later it was found that VO2 thin films would reversibly transform from a monoclinic (M) to rutile (R) phase when exposed to thermal or voltage stimuli, accompanied by dramatic changes in their electrical and optical properties [2–6]. M (R) phase is the insulating (metallic) phase at the low (high) temperature. The VO2 thin films would undergo a so called metal-to-insulator transition (MIT) with a sheet resistivity change as large as three to five orders of magnitude, namely from a few ohms to several kilo-ohms. Due to their large variation in electrical resistance during the phase transition, the VO2 thin films may have numerous applications in thermal, electrical and optical devices [7–13]. 0022-3727/14/455304+5$33.00
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J. Phys. D: Appl. Phys. 47 (2014) 455304
than joule heating because the heat-driven MIT would have a much longer switch time [19]. However, the large crystal lattice mismatch and formation of silicides or native oxide layers set big obstacles for directly depositing VO2 on a silicon substrate [20, 21]. The direct deposition of VO2 on a Si substrate can only render a two-orders change in the resistivity of the VO2 thin film and a thermal hysteresis (ΔH) of more than 20 K [12]. Buffer layers such as a yttria-stabilized zirconia (YSZ) film were proposed to improve the growth of VO2 thin films on a silicon substrate [19]. It was reported that a 145 nm YSZ buffer layer can greatly decrease the thermal hysteresis (ΔH) to 6 K and increase the ΔR to three orders of magnitude [19]. However, due to the thermal instability in-phase and microstructure of the YSZ material [22, 23], novel techniques for fabricating high quality VO2 thin films on a silicon substrate are still highly desirable. Here we propose that an Al2O3 thin layer be used as buffer layers to grow high quality VO2 thin films on p-type silicon substrates (p-Si). Al2O3 is a high-κ material which is thermally, chemically and mechanically stable. It has thus been proposed as a promising and reliable candidate as a replacement for the conventional metal–oxide–semiconductor (MOS) transistors using SiO2 as a gate oxide material [24]. Furthermore, the Al2O3 thin films deposited by plasma-enhanced atomic-layerdeposition (PE-ALD) have sharp interfaces with silicon, low oxide trap charges, a high tunnel barrier and large dielectric permittivity (κ ~ 8.8 [25]). For this reason, we introduced an ultrathin Al2O3 film as a buffer deposited by the PE-ALD technique in this work. The E-MIT of the VO2 thin film grown on an Al2O3 buffer layer was investigated in a MOS structure. The results show that a 25 nm Al2O3 buffer layer can tremendously improve the quality of VO2 thin films on a silicon substrate. The VO2 thin film shows abrupt and reversible transitions during heating and cooling. The resistivity can change by four orders of magnitude, and a small hysteresis of 4 K can be achieved. The high-κ dielectric Al2O3 layers significantly reduce the gate tunneling leakage current to 10 μA before and after the MIT transitions. The reduced current through the MOS structure can greatly alleviate the joule heating, making an ultrafast switch possible. The integration of high-quality VO2 thin films with Si substrates is a significant development toward the realization of VO2-based ultrafast electrical switching devices in conventional silicon-based microelectronics and optoelectrics technology.
Figure 1. XRD patterns of VO2 thin films on Si substrates with and without a 25 nm Al2O3 buffer layer.
vacuum chamber was firstly evacuated to 10−4 Pa, and then a mixture (O2/Ar ratio = 5%) of high purity Ar (99.999%) and oxygen (99.999%) was introduced to keep the base pressure at ~1.0 Pa. The substrate temperature was kept at 600 °C and the RF source gun power was set at 200 W. Before the deposition, the surface of the vanadium target was cleaned by pre-sputtering for 10 min. For the measurement of E-MIT, 100 µm × 100 µm Au electrodes were finally deposited on top of the VO2 films to form MOS structures. The microstructures, electrical properties and phase transition of the thin films were then systemically studied. The x-ray diffractometer (XRD: DX-2700) using Cu Ka radiation was employed to characterize the crystal structures of the films. The cross-section and surface information were acquired by using an atomic force microscope (AFM: SPA-300HV) and field-emission scanning electron microscope (FE-SEM: JSM7600F). The temperature dependence of electric resistivity of the VO2 film was measured by a standard four-point measurement method using a Keithley 2400 sourcemeter, where the sample temperature was controlled by a thin Peltier heater/ cooler. The current–voltage (I–V) characteristics were measured by an Agilent 4156C system to investigate the electrical driven phase transitions. 3. Results and discussion Figure 1 shows the XRD patterns of VO2 films grown on Si substrates with and without Al2O3 buffer layers. A strong peak at 2θ = 28.0° and a weak peak at 2θ = 57.8° are observed for both samples, which are attributed to the (0 1 1) and (0 2 2) planes of the R-phase of VO2 thin films, respectively. The thin layer of Al2O3 does not alter the atomic lattice textures of the VO2 film on the silicon substrate. For the Al2O3 buffered VO2 thin film, no peaks can be attributed to other stoichiometries of vanadium oxide. The intensity of the (0 1 1) peak of the VO2 thin film grown on the Al2O3 layer is much stronger than that grown on silicon. It can be concluded that the Al2O3 buffer layers greatly promote the VO2 thin film growth along the (0 1 1) crystalline orientation.
2. Experiments An Al2O3 layer was deposited on top of the (1 0 0) p-Si substrate (ρ ~ 1 Ω cm) by the PE-ALD system at 120 °C using high purity trimethylaluminum (TMA) and O2 as the sources [25]. We controlled the deposition rate at a finite value to fabricate the Al2O3 film with a precise thickness of 25 nm. VO2 thin films were deposited on silicon substrates with and without an Al2O3 buffer, respectively, by resonant frequency (RF) magnetron reactive sputtering using vanadium as target (99.99%). The VO2 thin films directly deposited on silicon substrates serve as reference samples. Before deposition, the 2
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Figure 2. The cross-sectional micrograph of the VO2 film on silicon (a) without and (b) with the Al2O3 buffer. The surface morphology of the VO2 film on silicon (c) without and (d) with the Al2O3 buffer.
thin films on Si without a buffer layer have a resistance change of less than three orders during the MIT transition, which is consistent with previous reports [26]. In contrast, the resistance of VO2 grown on the Al2O3 buffer layer changes by four orders of magnitude. The phase transition of VO2 thin films on the Al2O3/Si substrate starts at 333.5 K and 329.5 K during heating and cooling, respectively, giving rise to a ΔH of 4 K, which is only half of that observed for the films grown on silicon (~8 K). The transition sharpness ΔT of the buffered film is 2.4 K, decreasing from 5.1 K for the film without the buffer. The MIT transitions are closely related to the microstructure of the thin films. The thermal hysteresis ΔH is due to the latent heat of the first order phase transitions, which is determined by microstructures such as crystallinity, grain orientation of thin films [27, 28]. ΔT is due to the presence of grains, defects and strains in the films [28]. The small values of ΔH and ΔT thus indicate improved quality of VO2 thin films by introducing Al2O3 as buffer layers, which is consistent with XRD results. It is worth pointing out that the ΔR, ΔH and ΔT of the as-grown VO2 film on the Al2O3/Si substrate is comparable to that of the epitaxial film grown directly on the sapphire substrate [16, 28]. Square gold electrodes were deposited on VO2 thin films, and a bias was applied in the perpendicular directions to characterize the E-MIT transitions. Doped p-silicon substrates act as the bottom contacts, as shown in the inset of figure 4. I–V curves for both samples were measured from 0 to 10 V
The cross-sections of the VO2 thin films with and without Al2O3 buffer layers were characterized by a FE-SEM machine. As shown in figures 2(a) and (b), both samples have sharp interfaces with obvious boundary lines. The ~25 nm Al2O3 thin film was clearly observed in figure 2(b) with no evidence of interfacial reaction. The thickness of the VO2 thin films is ~200 nm for both samples, suggesting Al2O3 thin layers have little effect on the growth rate of the VO2 thin films. The AFM images of VO2 thin films deposited on silicon and Al2O3 are shown in figures 2(c) and (d), respectively, which show very smooth surfaces consisting of continuous and compact nanoparticles. VO2 thin films directly deposited on a silicon substrate consist of spherical and uniformly arranged nanoparticles, while the VO2 thin films grown on Al2O3 buffer layers typically have tetragonal structures that are arranged in a compact way. The VO2 thin films grown on Al2O3 buffer layers have a slightly larger root mean squared roughness (~8.9 nm) than that grown on the silicon substrate (~7.3 nm). Figures 3(a) and (b) shows the temperature (T) dependent in-plane resistivity (R) of the VO2 thin films on the silicon substrate without and with Al2O3 buffer layers, respectively. MIT transitions are observed from the R-T curves for both samples. Here MIT is characterized by the transition temperatures (Tc), thermal hysteresis width (ΔH) and transition sharpness (ΔT). In our previous work [16], these parameters were obtained by fitting the derivatives of lnR versus T to a Gaussian distribution, as plotted in figures 3(c) and (d). VO2 3
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Figure 3. MIT behavior of VO2 thin films: resistivity versus temperature curves for the VO2 thin film (a) grown directly on Si and (b) with the buffer; Gaussian fitting of the differential curves of resistivity versus temperature for the VO2 thin film (c) grown directly on Si and (d) with the Al2O3 buffer.
d is the total thickness of the oxide layers. The calculated ET is in agreement with a previous report [26]. As a result of the large resistance and high dielectric constant of Al2O3, the threshold voltage of the Au/VO2/Al2O3/p-Si device is shifted to a higher bias voltage of 8.4 V with a critical electric field of 3.67 × 10−7 V m−1. Since the threshold voltage is elevated, the devices might have a higher energy consumption or easily fail. However, it is worth noticing that the high-κ Al2O3 layer greatly reduces the leakage current to less than 10 μA, even in the metallic state of the VO2 film. The measured current in the VO2 film grown on the Al2O3 layers is three orders smaller than the typical value measured in previous studies, which have an electrical current of ~ mA [4, 5, 17, 18]. A comparable small tunneling current was also observed in a special vertical structure measured by an AFM tip in another study [26], where the VO2/n-Si sample and AFM tip were separated by a distance of tens of nanometers. The gap between the AFM tip and VO2 thin films act as a potential barrier, with a function similar to the Al2O3 layer in the present work. The small tunneling current in our sample indicates that the phase transition of the VO2 film on the Al2O3 buffer is directly induced by the electric field rather than joule heating. For VO2 thin films grown on a silicon substrate, greater joule heating will be generated once the metallic state of VO2 is established, which will
Figure 4. I–V measured curves of (a) Au/VO2/Al2O3/p-Si and (b) Au/VO2/p-Si structures at RT.
at room temperature (RT), as shown in figure 4. Abrupt current jumps are observed in both I–V curves when the bias voltage reaches a threshold value, indicating the occurrence of the E-MIT of VO2 films [5, 18]. The threshold voltage (VT) of the Au/VO2/p-Si device is 4.5 V with a corresponding critical electric field of ET = VT/d = 2.14 × 10−7 V m−1, where 4
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adversely affect the on/Off switch of the devices. As a result, finite current is very favorable for ultrafast devices since the joule heating effect can be effectively depressed, which can be realized in our devices. 4. Conclusions A dense and smooth Al2O3 thin film was deposited by the PE-ALD technique to tune the phase transitions of the VO2 thin film on a silicon substrate. Much larger resistance change, smaller thermal hysteresis and sharper phase transitions were observed for the VO2 thin film grown on the Al2O3 buffer layer in comparison with their counterparts that were grown directly on silicon. Furthermore, The E-MIT of the VO2 thin film grown on Al2O3 buffer layers was investigated in a MOS structure. The high-κ dielectric Al2O3 layers significantly reduce the gate tunneling leakage current to 10 μA before and after the phase transition, indicating that the electric field alone is sufficient to trigger the MIT. Our work shows that it is possible to fabricate VO2 based ultrafast electrical switching devices on a silicon substrate, making these devices compatible with current silicon industries. Acknowledgment This work is financially supported by National Nature Science Foundation of China (No. 61131005), Keygrant Project of Chinese Ministry of Education (No. 313013), National High-tech Research and Development Projects (No. 2011AA010204), New Century Excellent Talent Foundation (No. NCET-11– 0068), Sichuan Youth S & T foundation (No. 2011JQ0001), and start-up research fund from the University of Electronic Science and Technology of China. References [1] Morin F J 1959 Phys. Rev. Lett. 3 34 [2] Zhi B W, Gao G Y, Xu H R, Chen F, Tan X L, Chen P F, Wang L F and Wu W B 2014 Appl. Mater. Interfaces 6 4603–8
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