Electro-thermal control of aluminum-doped zinc oxide/vanadium dioxide multilayered thin films for smart-device applications

We demonstrate the electro-thermal control of aluminum-doped zinc oxide (Al:ZnO) /vanadium dioxide (VO2) multilayered thin films, where the application of a small electric field enables precise control of the applied heat to the VO2 thin film to induce its semiconductor-metal transition (SMT). The transparent conducting oxide nature of the top Al:ZnO film can be tuned to facilitate the fine control of the SMT of the VO2 thin film and its associated properties. In addition, the Al:ZnO film provides a capping layer to the VO2 thin film, which inhibits oxidation to a more energetically favorable and stable V2O5 phase. It also decreases the SMT of the VO2 thin film by approximately 5–10 °C because of an additional stress induced on the VO2 thin film and/or an alteration of the oxygen vacancy concentration in the VO2 thin film. These results have significant impacts on technological applications for both passive and active devices by exploiting this near-room-temperature SMT.

Vanadium dioxide (VO 2 ) in thin-film and nanostructured forms has been intensely studied in recent years because of the presence of a metal-insulator (MIT) or semiconductor-metal transition (SMT) near room temperature (RT) at ~68 °C in bulk form, which is also accompanied by a structural phase transition [1][2][3][4][5][6][7] . The crystal symmetry of VO 2 changes from a P2 1 /c (monoclinic semiconducting phase) to a P4 2 /mnm (rutile metallic phase) space symmetry when VO 2 traverses from below to above the SMT. This transition can be temperature-1 , voltage- [8][9][10] , and photo-induced 11 in this strongly electron-correlated material. The successful growth and study of VO 2 thin films has been demonstrated using several deposition techniques, including reactive sputtering 12 , atomic layer deposition (ALD) 13 , pulsed laser deposition (PLD) 14 , chemical vapor deposition 15 , electron beam evaporation 16 , the sol-gel process 17 , and thermal evaporation 18 . These and many other studies have provided a better understanding of the SMT and VO 2 properties; the oxygen vacancy concentration 19 and strain 20 play important roles, but many aspects remain unclear. A more thorough understanding of the SMT and VO 2 properties will aid in the fine-tuning and control of the SMT so that it can be more reliably used for applications.
Technological applications in various passive and active devices have been developed based on this near-RT SMT. The most notable applications include low-loss plasmonics 21 , smart window coatings 22 , ultrafast optical switches and sensors 11 , new electronic devices such as Mott field effect transistors 23 , and uncooled bolometers for infrared imaging 24 . Smart devices, which use the large optical and electrical property changes of VO 2 , appear to be at the forefront of this field and are of particular interest here. For example, a smart energy-efficient window must satisfy a number of criteria, including high transmittance in the visible range (400-700 nm), low transmittance in the infrared range (3-50 μ m), and variable transmittance in the near-infrared range (700-3,000 nm). This variable transmittance depends on whether the building interior must be heated or cooled, i.e., the window transmits (reflects) the near-infrared light if there is a heating (cooling) demand 25 , and it must be actively or passively modulated. In this paper, we discuss a possible thin-film platform in which to electro-thermally control the VO 2 phase across its SMT, which enables modulation for smart window thin-film coatings and/or other technological devices.

Results
Epitaxial VO 2 thin films were grown on c-plane (0001) sapphire (Al 2 O 3 ) substrates using oxygen plasma-assisted pulsed laser deposition (PA-PLD). The SMT occurs near 50 °C, as confirmed with Raman spectroscopy and four-point probe electrical measurements, which are not shown here but can be found elsewhere in the literature 26 . The near-RT SMT of these VO 2 thin films can be induced using the aforementioned techniques, most of which are external to the VO 2 thin film and device. For example, the SMT is typically thermally induced using an external heater in contact with the VO 2 . However, the direct incorporation of a thin-film heater into a multilayered thin-film device has spatial and temporal advantages, low power requirements, and fast response times 27,28 . Therefore, a thick film of Al:ZnO, which has unique transparent heater qualities 29 , was grown on top of the VO 2 thin films using atomic layer deposition (ALD). Details on these Al:ZnO films can be found in the literature [29][30][31] . The ALD-grown Al:ZnO film is polycrystalline and the VO 2 thin film has an epitaxial relationship with the sapphire substrate of (010)[100]VO 2 (0001)  Electrical contacts were fabricated on top of the Al:ZnO film along opposite edges of the 10 mm × 10 mm sample [see the multilayered thin-film sample schematic in Fig. 1(c)], such that the transparent heating properties of the Al:ZnO film could be used to induce the SMT in the underlying VO 2 thin film. In addition to their transparency, the Al:ZnO films (with an Al:Zn ALD cycle ratio of 1:20) have a moderate thermal conductivity (4.2-4.3 W m −1 K −1 ), which facilitates heat transport to the VO 2 thin film 29 . An infrared (IR) camera was used to measure the temperature distribution over the 1 cm 2 area of the Al:ZnO/VO 2 multilayered thin-film samples at different applied voltages, and these measurements were corroborated via thermocouple measurements. Figure 2 shows the steady-state IR thermal images of the multilayered thin-film samples at applied voltages of 1-5 V. These steady-state temperatures were achieved within 2-5 minutes after applying the voltages, where the time rates of temperature increase are shown in Fig. 3(a); the final steady-state temperatures are plotted versus the applied voltage in Fig. 3(b). The temperature was measured at the center of the samples, where there is a fairly uniform temperature distribution across the entire surface, particularly at or below the SMT. Figure 4 shows this temperature gradient across the multilayered thin-film samples, which was measured between the electrodes from the upper left to the lower right [see Figs 1(c) and 2]. The SMT in these multilayered thin-film samples occurs at just under 3 V (discussed later), where the temperature gradient is ± 1 °C across the sample. Smaller temperature gradients are expected by improving the electrodes because of the noticeable defects in the upper and lower left corners of the sample (see Fig. 2), which cause the temperature to decrease at the 0 mm end (see Fig. 4).
The Al:ZnO film is heated because of the Joule heating effect, where the power converted to heat (and consequently the steady-state temperature) is proportional to the square of the applied voltage as shown in Fig. 3(b) 9 . This Joule heating effect can be reproducibly controlled using the Al:ZnO film properties, particularly the film thickness and amount of Al-doping (i.e., Al:Zn ALD cycle ratio), which govern the metallic behavior of the films based on the carrier concentration as we recently demonstrated 29 . These two parameters enable one to precisely tailor the amount of heat and temperature, where both thicker and more metallic-like films produce higher temperatures at lower applied voltages, an approach that has considerable advantages in device fabrication and operation.
The SMT of the VO 2 thin film below the Al:ZnO film layer was tracked using various characterization techniques. Micro-Raman spectroscopy is suitable to distinguish the SMT in VO 2 . Below the transition, sharp Raman peaks (modes) are observed, which signifies the monoclinic semiconducting phase, whereas only a broadband emission is observed for the rutile metallic phase above the transition 26,32 . Many, but not all, of the possible Raman modes for the monoclinic semiconducting phase of VO 2 were resolved in the RT spectra 26,[32][33][34][35] . The inset of Fig. 5 shows the Raman spectra, which highlights the 196 and 224 cm −1 modes, for both VO 2 phases: the semiconducting phase below the transition and the metallic phase above the transition. These two Raman modes were used to track the SMT in the VO 2 thin film to determine the transition temperature of the Al:ZnO/VO 2 device, which was found to be between 42 and 46 °C (see Fig. 5). Similar and corroborating results were obtained using an external heater below the sapphire substrate to increase the temperature of the VO 2 thin film (not shown).
XRD was also used to track the SMT in the VO 2 thin film as shown in Fig. 6 when voltages were applied to the Al:ZnO/VO 2 multilayered thin-film device. The XRD results show that there is a sudden shift in the VO 2 unit cell parameters at an SMT temperature of 43.5 °C, thus corroborating the Raman spectroscopy results. Below this temperature, the monoclinic VO 2 (020) reflection is centered at 2θ = 39.962° and yields a lattice parameter of 0.2254 nm. The VO 2 reflection suddenly shifts to a lower angle (2θ = 39.924°) above this temperature and yields an expanded lattice parameter of 0.2256 nm. These results are consistent with the bulk VO 2 x-ray powder diffraction files (PDFs) for the monoclinic (01-082-0661) and rutile (03-065-9786) phases from the International Centre for Diffraction Data (ICDD) and published XRD studies in the literature 36 . The shift in the bulk VO 2 structures amounts to ∆2θ = 0.09°, whereas only a shift of ∆2θ = 0.04° is observed in the multilayered thin-film device. This result is not surprising because the VO 2 thin film is under stress due to the mismatch with the sapphire substrate and the Al:ZnO film on top of it. Furthermore, thermal expansion of the VO 2 lattice cannot account for this shift. First, the peak shift is sudden and not gradual with increasing temperature. Second, the average thermal expansion of VO 2 is α ave = 5.70 × 10 −6 K −1 (monoclinic structure) and 13.35 × 10 −6 K −1 (rutile structure) at these temperatures 37 , which can only account for approximately 10% of the exhibited lattice expansion (i.e., α ave must be an order of magnitude larger to be responsible for the observed XRD peak shift in the data).
Electrical transport measurements in the form of electrical resistance versus temperature plots are typically used to study the SMT of VO 2 films. In this case, the VO 2 thin film in the Al:ZnO/VO 2 multilayered thin-film device is buried beneath the Al:ZnO film and therefore cannot be measured directly. However, the electrical resistance of the Al:ZnO film can be measured versus temperature using a four-point probe in the van der Pauw configuration and a resistive heater below the sapphire substrate. Figure 7 shows that indeed the SMT of the underlying VO 2 thin film is manifest in the measurements of the Al:ZnO resistance, with the expected hysteresis during heating and cooling. Furthermore, an identical Al:ZnO film without the underlying VO 2 thin film was measured for comparison and shows no hysteretic behavior.
Interestingly, the growth of an Al:ZnO film on top of the VO 2 thin film provides several advantages in addition to placing a thin-film heater in direct contact with the VO 2 (previously mentioned), which creates a mechanism to finely control the heat applied to VO 2 to induce the SMT. The thick Al:ZnO film also acts as a capping layer to the VO 2 thin film and prevents the oxidation of VO 2 to V 2 O 5 , which is the most energetically favorable and stable phase. Otherwise, over the lifetime of a device, the VO 2 thin film will oxidize to become V 2 O 5 and degrade the SMT and its associated properties. Another advantage of the thick Al:ZnO film is its ability to decrease the SMT of the VO 2 thin film by approximately 5-10 °C compared to a VO 2 thin film without an Al:ZnO capping film 26 . This phenomenon is thought to occur because of the additional stress on the VO 2 thin film and/or an alteration of the oxygen vacancy concentration, which pushes the SMT to a lower temperature 38 .
In summary, the electro-thermal control of a multilayered Al:ZnO/VO 2 thin-film device was demonstrated, where the SMT of the VO 2 thin film was induced by applying a small potential (< 3 V) across the Al:ZnO film. This electro-thermal energy provided by the Al:ZnO film can be finely tuned using its transparent conducting oxide properties to significantly control the VO 2 SMT and its associated electrical and optical properties. The Al:ZnO film acts as a transparent window and heater, serves as a protective capping layer to the VO 2 thin film, and aids in decreasing the VO 2 transition temperature. These results have important implications for the use of VO 2 and Al:ZnO in technological applications, particularly active smart devices.

Methods
Fabrication of the samples. Epitaxial vanadium dioxide (VO 2 ) thin films were grown on non-annealed c-plane (0001) sapphire (Al 2 O 3 ) substrates (10 mm × 10 mm) at 550 °C using a Neocera pulsed laser deposition (PLD) system, which operated at a base pressure of 10 −8 Torr. The V 2 O 5 target material was ablated with a KrF excimer laser (λ = 248 nm, pulse width = 25 ns, energy = 220 mJ/pulse) at an angle of 45° with a repetition rate of 3 Hz and a spot size of approximately 2 mm × 4 mm, which resulted in an energy density of ~3 J cm −2 . These 50 nm thick VO 2 thin films, confirmed with x-ray reflectivity and cross-sectional scanning electron microscopy (SEM) [ Fig. 1(d)], were grown at a rate of ~0.4 nm/min under a 150 W oxygen (O 2 ) radio frequency plasma using ultra-high purity (99.994%) O 2 gas at a working pressure of 3.0 × 10 −5 Torr (i.e., plasma-assisted PLD). Further details on these VO 2 thin films are discussed elsewhere 26 .
Aluminum-doped zinc oxide (Al:ZnO) thin films were grown on top of the epitaxial VO 2 (020) m thin films using a Cambridge NanoTech Ultratech atomic layer deposition (ALD) system, which operated at a base pressure in the mid-10 −3 Torr range. Ultra-high purity (99.999%) nitrogen (N 2 ) gas, which constantly flowed at 20 sccm, was used to purge the chamber and as the carrier gas for the precursors: diethyl zinc [DEZ, Zn(C 2 H 5 ) 2 ], water (H 2 O), and trimethylaluminum [TMA, Al(CH 3 ) 3 ]. The Al:ZnO films were grown at 250 °C by alternating between 15 ms pulses of DEZ and H 2 O (with 5 s purges in between each) for 20 times (cycles) to obtain a zinc oxide film. After the 20 th DEZ pulse, a 15 ms pulse of TMA was used for the aluminum dopant, which was followed by the 20 th H 2 O pulse. This 20 cycle sequence defines a 1:20 Al:Zn ratio and consequently constitutes the Al:ZnO film used herein. In total, 3000 cycles were performed, which yielded a growth rate of ~0.12 nm per cycle and thus a total thickness of 370 nm [ Fig. 1(d)]. Further details on these ALD pulsing sequences and Al:ZnO thin films are discussed elsewhere 30,31 . Fabrication of the electrical contacts. 45   10 mm × 10 mm sample [ Fig. 1(c)]. These contacts were deposited at RT with a growth rate of ~0.12 nm/sec in an AJA International electron beam evaporation system, which operated at a base pressure of 10 −8 Torr.

SEM imaging techniques.
A Hitachi SU8010 field-emission scanning electron microscope (FE-SEM) was used to obtain cross-sectional images of the multilayered Al:ZnO/VO 2 thin-film device after cleaving it in half.

Raman spectroscopy characterization techniques. A Horiba LabRAM HR Evolution Raman spec-
trometer with a laser excitation wavelength (λ = 785 nm) in the near-infrared range was used because it has been shown to produce the clearest spectrum of Raman active modes for VO 2 on sapphire compared to shorter wavelengths in the visible range 35 . This clearer spectrum is achieved because resonance fluorescence at these shorter visible wavelengths increases the background and overwhelms the signal from the VO 2 thin film. The spectra were acquired for the Al:ZnO/VO 2 thin-film device at various steady-state temperatures by applying a voltage across the electrical contacts on the Al:ZnO film and using an external resistive heater below the sapphire substrate to increase the temperature.

XRD characterization techniques.
A standard high-resolution (0.0001°) four-circle x-ray diffractometer (XRD) with Cu K α radiation and thin-film optics was used to characterize the microstructure of the Al:ZnO/VO 2  thin-film device. Diffraction scans were acquired for the Al:ZnO/VO 2 thin-film device at various steady-state temperatures by applying a voltage across the electrical contacts on the Al:ZnO film and using an external resistive heater below the sapphire substrate to increase the temperature.
Electrical transport characterization techniques. The temperature dependence of the electrical resistance of the multilayered Al:ZnO/VO 2 thin-film device was measured using a four-point probe in the van der Pauw configuration, where the SMT of the VO 2 was thermally induced with a resistive heater below the sapphire substrate. The four, 50 μ m diameter probes contacted the Al:ZnO film surface, where current-voltage (I-V) measurements were acquired versus temperature using a Keithley 6220 Current Source applying 21 mA of direct current along one edge of the sample and a Keithley 2182A Nanovoltmeter measuring the potential across the other edge.