Effect of Al2O3 Buffer Layers on the Properties of Sputtered VO2 Thin Films

VO2 thin films were grown on silicon substrates using Al2O3 thin films as the buffer layers. Compared with direct deposition on silicon, VO2 thin films deposited on Al2O3 buffer layers experience a significant improvement in their microstructures and physical properties. By optimizing the growth conditions, the resistance of VO2 thin films can change by four orders of magnitude with a reduced thermal hysteresis of 4 °C at the phase transition temperature. The electrically driven phase transformation was measured in Pt/Si/Al2O3/VO2/Au heterostructures. The introduction of a buffer layer reduces the leakage current and Joule heating during electrically driven phase transitions. The C–V measurement result indicates that the phase transformation of VO2 thin films can be induced by an electrical field. Electronic supplementary material The online version of this article (doi:10.1007/s40820-017-0132-x) contains supplementary material, which is available to authorized users.


Introduction
VO 2 thin films have generated a considerable interest among scientists over the past decades owing to their nearroom-temperature phase transformation [1]. VO 2 thin films undergo transition from a reversible monoclinic (M phase) to a rutile (R phase) structural phase at approximately 65°C, which is accompanied by a dramatic change in their electrical and optical properties [2][3][4][5][6]. The resistivity of Electronic supplementary material The online version of this article (doi:10.1007/s40820-017-0132-x) contains supplementary material, which is available to authorized users. VO 2 thin films changes by three to five orders of magnitude during this phase transition. The metal-to-insulator (MIT) transition has been utilized in a variety of electronic devices such as electromagnetic wave modulators, switches [7][8][9][10], and holographic storage [11]. So far, the applications of VO 2 thin films have been mainly explored for infrared and millimeter waves [10,12]. Recently, VO 2 thin films have been used for terahertz (THz)-wave devices [7]. In 2010, VO 2 nanowires were deposited on glass substrates using photolithography and magnetron sputtering and used for the thermal modulation of THz waves by Wen et al. [7]. The research results showed that the transmittance of THz waves reduced by 65% after traveling through VO 2 nanowire arrays during the MIT transition [7].
In 2013, VO 2 thin films with silver nanowire antenna arrays were deposited on a silicon substrate [13]. A reduction in the relative refractive index and a blue shift of the resonance frequency were observed, owing to changes in the dielectric constant of VO 2 thin films during heating [13]. Similarly, Seo et al. [8] created a rectangular hole array based THz wave antenna for controlling the transmittance of THz waves from 0.2 to 2 THz using the phase transition of VO 2 thin films. VO 2 thin films have exhibited good potential for use in THz wave devices. However, compared to the electrical modulation in graphene [14][15][16][17] and the optical modulation in silicon [18][19][20], it is difficult to obtain a comparable response speed via the thermal modulation of THz waves using the phase transition of VO 2 thin films. Consequently, it is important to explore the fast phase transition of VO 2 thin films induced by an electrical field.
In many applications, the integration of VO 2 thin films with silicon technology and processing is desired, which requires the growth of VO 2 thin films on a silicon substrate [21]. However, there is a large lattice mismatch between the VO 2 thin films and silicon/silicon oxide substrates. The direct deposition of VO 2 thin films on silicon/silicon oxide often yields an inferior crystalline texture, resulting in a small change of resistivity and a large thermal hysteresis during MIT [21,22]. Furthermore, the oxygen diffusion at the VO 2 /Si interface adversely affects the quality of the deposited VO 2 thin films [22].
It has been proposed that using yttrium-stabilized zirconia (YSZ) as a buffer layer aids the growth of VO 2 thin films on a silicon substrate [21]. A YSZ buffer layer of 30-145 nm can reduce the thermal hysteresis temperature to 6 K and result in a resistance change of three orders of magnitude [21]. However, owing to the instability of the microstructure of YSZ, it is necessary to study a new buffer layer for VO 2 thin film deposition on a silicon substrate to achieve a better performance.
Previously, we used a dense Al 2 O 3 thin film as the buffer layer to improve the quality of VO 2 thin films grown on a silicon substrate. This layer was grown via atomic layer deposition (ALD) [5]. The introduction of the Al 2 O 3 buffer layer significantly improved the resistivity changes and the thermal hysteresis of VO 2 thin films during the MIT transition. An electrical-field-induced phase transformation was observed owing to the insulating Al 2 O 3 layer.
However, it is challenging to obtain a pure phase of VO 2 in thin films, and much knowledge about the phase transformation of VO 2 is still unclear. Here, we have deposited VO 2 thin films under different conditions and investigated their optical and electrical properties. It was observed that the partial pressure of oxygen gas during sputtering has a significant impact on the phase of the deposited thin film. Pure VO 2 can be obtained on the Al 2 O 3 buffer layer by optimizing the growth conditions. In contrast, it is difficult to obtain a pure VO 2 phase on a naked silicon substrate. In addition, the phase transformation induced by an electrical field was also studied. The capacitance-voltage curves indicate that the phase transformation occurs through the voltage-driven nucleation and the growth of metallic regions in VO 2 thin films. The high-quality VO 2 thin films grown on silicon substrates have great potential for applications in many functional devices.

Experimental Methods
In this study, p-type silicon (001) (with a resistivity q of approximately 1 X cm) was used as substrate, and cleaned by acetone, ethylene, and deionized water before deposition. The pretreated silicon substrate was loaded into a chamber for Al 2 O 3 thin film deposition via plasma-enhanced atomic layer deposition (PEALD). Pure trimethylaluminum (TMA) was used as the precursor chemical. Al 2 O 3 thin films with thicknesses ranging from 25 to 50 nm were deposited and used as the buffer layers for the growth of VO 2 thin films. Subsequently, the Al 2 O 3 -coated silicon substrate was transferred into a magnetron sputtering machine. In addition, naked silicon substrates were loaded into the same batch for comparison. Pure 4-inch vanadium (99.99%) was used as a target, and pure oxygen and argon gases were used as the reaction and sputtering gases. The base pressure was \3 9 10 -4 Pa. During the deposition, the substrate temperature was maintained between 450 and 550°C with a sputtering power of 200 W. VO 2 thin films with thicknesses ranging from 80 to 200 nm were obtained after a deposition time of 10-30 min. The chamber pressure was approximately 1 Pa. The partial pressure of oxygen varied from approximately 4-5%.
The deposited Al 2 O 3 buffer layer and VO 2 thin films were evaluated using scanning electron microscopy (SEM, JSM-7600F), X-ray diffraction (XRD, DX-2700), and atomic force microscopy (AFM, SPA-300HV). Two gold electrodes of 1 mm separated by a distance of 6.5 lm were deposited to measure the in-plane resistivity. The silicon substrate was coated with platinum, and a gold pad with an area of 100 9 100 lm 2 was deposited on the VO 2 thin films for perpendicular measurement. The electrical resistivity was measured via a standard four-point measurement method using a Keithley 2400 Source Meter. The I-V curve and the capacitance as a function of voltage and frequency were measured using an Agilent 4156C system.

Results and Discussion
Al 2 O 3 thin films deposited via PEALD were dense, smooth, with a clear interface, and high dielectric constant (i.e., k of approximately 8.8). They were thermally and chemically stable. Furthermore, the quality of the VO 2 thin film was significantly improved by depositing it on the Al 2 O 3 buffer layer. A 3D AFM image of the Al 2 O 3 thin film is shown in Fig. S1. The roughness of the deposited Al 2 O 3 films was approximately 0.2 nm, which is comparable with that of the polished silicon substrates. In our study, Al 2 O 3 thin films with a thickness of 25 nm were used as the buffer layers for VO 2 thin film deposition. Figure 1 shows the SEM images of the sputtered VO 2 thin films on a silicon substrate and on the deposited Al 2 O 3 buffer layers at different magnifications. It was observed that both the VO 2 thin films were polycrystalline but with different microstructures. The grains of the VO 2 thin films grown on the silicon substrate were small, irregular, and nonuniform in size. In contrast, the grains of the VO 2 thin films grown on the Al 2 O 3 buffer were large, smooth, dense, and uniform in size. In the case of polycrystalline VO 2 thin films, the phase transformation can be affected by their crystallinity, grain size, density, and grain boundaries [23]. The discontinuous phase transformation is mainly caused by the energy barriers imposed by the grain boundaries [23]. Consequently, larger and denser grains benefit the phase transformation of the VO 2 thin films, and they were obtained in this work by introducing the Al 2 O 3 buffer layer, as shown in Fig. 1.
Vanadium has multiple valence states (?2, ?3, ?4, and ?5) in oxides, which is a challenge for obtaining a pure phase of VO 2 thin films. In our experiment, we observed that there was only a small window for the oxygen partial pressure to produce pure VO 2 thin films on the Al 2 O 3 buffer layer. A slight difference in oxygen pressure during magnetron sputtering can significantly affect the phase of the deposited VO x thin films. In this study, oxygen partial pressures of 4% and 5% were used to demonstrate their impact on the VO 2 thin films. Figure 2a, b show the XRD patterns of two sets of samples prepared under oxygen pressures of 5% and 4%, respectively. In the case of oxygen partial pressure of 5%, VO 2 thin films were obtained both on silicon and Al 2 O 3 buffer layer. On a naked silicon substrate, V 6 O 13 and V 2 O 5 phases were also observed and the growth of the VO 2 phase was significantly suppressed. In contrast, the growth of VO 2 thin films was enhanced on the Al 2 O 3 buffer layer, and the V 2 O 5 phase was indiscernible by XRD for oxygen partial pressures of 5% and 4%. However, a small amount of V 6 O 13 phase was still observed. The undesired phases of V 6 O 13 and V 2 O 5 were oxygen rich as compared to the desired VO 2 phase. Consequently, the oxygen partial pressure was reduced to 4% during the magnetron sputtering. On the naked silicon substrate, the V 2 O 5 phase was not discernible for the reduced oxygen partial pressure. Although a small amount of V 6 O 13 phase was still present in the deposited thin films, the growth of the VO 2 phase was highly enhanced compared to the deposition at the oxygen partial pressure of 5%. Moreover, a pure VO 2 phase was obtained for the thin films deposited on the Al 2 O 3 buffer layer at the oxygen partial pressure of 4%, and the growth of the VO 2 phase in the (011) direction was highly enhanced, which was evident from the high intensity of the (011) diffraction peak.
The enhanced VO 2 growth on the Al 2 O 3 buffer layer could be the result of the reduced lattice mismatches and the oxygen diffusion at the Al 2 O 3 /VO x interface. The XRD patterns did not show signatures of the ALD-deposited Al 2 O 3 thin films, indicating the amorphous nature of the buffer layers, which is consistent with previous research results [24,25]. The amorphous Al 2 O 3 buffer layer did not strain the lattice of the VO 2 thin films during growth, unlike naked silicon. Therefore, the VO 2 thin films grown on the Al 2 O 3 buffer layers were much more homogenous than those grown on the silicon substrate, as shown in Fig. 1. Further, the Al 2 O 3 buffer layers promoted texture growth along the (011) direction, which was also reported by other groups [26,27].
Further, the Al 2 O 3 buffer layer was chemically stable, preventing the interdiffusion of oxygen at the Al 2 O 3 /VO 2 interface. Oxygen atoms can easily diffuse at the Si/VO 2 interface at 450-550°C, which renders the control of the stoichiometric composition of the VO 2 thin films difficult. Therefore, by introducing the Al 2 O 3 buffer layer on a silicon substrate, the microstructure and stoichiometric composition were easier to control, resulting in higher quality VO 2 thin films. Notably, pure VO 2 thin films can also be deposited by magnetron sputtering from a vanadium target by carefully controlling the growth conditions [27]. However, the use of the Al 2 O 3 buffer layer renders the deposition window wider and the electrical properties stronger, which will be discussed later.
The thermally induced phase transformations were investigated under different deposition conditions and microstructures. The sheet resistance of the thin films was measured as a function of temperature. The results are shown in Fig. S2 (oxygen partial pressures of 5%) and Fig. S3 (oxygen partial pressures of 4%). Table 1 lists the sheet resistances of the samples at 25 and 85°C, and the transition temperatures during heating and cooling, which are calculated from Fig. S2 and Fig. S3. The resistivity ratio of the thin films before (T = 25°C) and after (T = 25°C) phase transition were significantly improved when deposited on the Al 2 O 3 buffer layers over a naked silicon substrate, owing to the high resistivity of the thin films at room temperature (25°C).
As shown in Table 1, the sheet resistance of the thin films deposited on the naked silicon was much smaller than that on the Al 2 O 3 buffer layer, which could be the result of the presence of the V 6 O 13 phase. The V 6 O 13 phase undergoes an MIT transition at -123°C, and it is in a metallic state at room temperature [28]. In Fig. 2, the thin films possessed a significant amount of V 6 O 13 phase when deposited on the naked silicon, whereas the growth of V 6 O 13 was suppressed when deposited on the Al 2 O 3 buffer layer. The resistivity changes in the VO 2 thin films were further increased during the phase transition when deposited at the oxygen partial pressure of 4%. In addition, the temperature of thermal hysteresis was reduced from 9°C 20   Electrodes were deposited on Si/Al 2 O 3 /VO 2 to investigate the electrically driven phase transformation. The resistance of the VO 2 thin films was measured along the inplane and perpendicular directions, as shown in the insets of Fig. 3a, b, respectively. Two gold electrodes of 1 mm, separated by 6.5 lm, were deposited via thermal evaporation for the in-plane measurement. The Al 2 O 3 buffer layers could prevent current leakage through the p-Si. At room temperature, the resistance had a magnitude of 10 6 X when no bias was applied to the electrodes. As the voltage bias was increased from 0 V, initially the resistance decreased gradually. However, when the voltage approached 8.5 V, an abrupt decrease of resistance was observed at room temperature, indicating a phase transformation.
Furthermore, an electrically driven phase transition was observed as the environmental temperature was increased to 40°C, but the thermally induced phase transformation still did not occur. However, the resistivity was small at 40°C than that at room temperature. Further, the switching voltage was reduced to approximately 7 V, which was lower than the value at room temperature. Therefore, thermal energy can assist the electrically driven phase transformation of VO 2 thin films. When the environmental temperature approached the value of the thermal phase transformation temperature at 60°C, the resistance change during the electrically driven phase transition was very    small, as shown by the green curves in Fig. 3a. At 80°C, the VO 2 thin films were in metallic states, and no abrupt change in resistance was observed. Notably, the switching voltage monotonically decreased as the temperature increased.
The electrically driven phase transformation could be a result of the applied electrical field or Joule heating. In this regard, we are prone to believe that the phase transformation was induced by the applied electrical field. For verification, we measured the resistance of the VO 2 thin films along the perpendicular direction, as shown in the inset of Fig. 3b. The insulating Al 2 O 3 buffer layer could effectively decrease the electrical current passing through the VO 2 thin films. The resistance was measured as a function of the applied voltage, as shown in Fig. 3b. The phase transformation occurred at 8.4 V for the Pt/Si/Al 2 O 3 / VO 2 /Au stack, and at 4.4 V for the Pt/Si/VO 2 /Au stack.
In Fig. 3a, the resistance of VO 2 is negligible after the phase transformation. Therefore, we estimated that the apparent resistances of the silicon substrate, Al 2 O 3 buffer layer, and VO 2 thin films immediately before the phase transformation were 2 9 10 5 , 8 9 10 5 , and 3 9 10 6 X, respectively. A switching electrical field of 3 9 10 7 V m -1 for the Pt/Si/Al 2 O 3 /VO 2 stack was estimated. When the VO 2 thin films were deposited on the silicon substrate, their resistance was estimated to be 2.45 9 10 6 X before the phase transformation, and the switching electrical field was 2 9 10 7 V m -1 . The decrease in the switching field of the Pt/Si/VO 2 /Au stack could be a result of the microstructure of the deposited thin films.
In order to further investigate the electrically driven phase transformation of the VO 2 thin films, the capacitances of the Pt/Si/Al 2 O 3 /VO 2 /Au and Pt/Si/VO 2 /Au stacks were measured under an alternating field at 100 Hz. The result is shown in Fig. 4a. As the voltage increased, the capacitance of both the stacks increased and subsequently decreased, with a maximum value observed at 1.5 V for Pt/ Si/VO 2 /Au and at 6.8 V for Pt/Si/Al 2 O 3 /VO 2 /Au. The capacitance-voltage curves were very different from those of typical MOS capacitors. We attribute this unusual behavior to the electrically driven phase transformation of the VO 2 thin films. As the voltage increased, some regions of the VO 2 thin films were transformed from insulating to metallic. Figure 5 shows the charge carriers were delocalized in the metallic region and oscillated with the AC field. It resulted in an increase in the dielectric constant of the VO 2 thin films. As the metallic region was percolated at high voltages, the capacitance decreased. The maximum capacitance during the phase transformation, which were triggered by temperature [29], was also observed in other MIT transitions. As shown in Fig. 4a, the nucleation and growth of the conducting regions can be induced gradually via a fast charge carrier injection during the AC oscillation [30]. These regions became conductive as percolation was achieved.
The capacitance of the Pt/Si/Al 2 O 3 /VO 2 /Au stack was also measured as a function of voltage at different frequencies, as shown in Fig. 4b. At 1 kHz, the maximum capacitance was observed at a lower voltage of 5.2 V compared to the corresponding voltage value observed at 100 Hz. As the frequency of the AC field increased to 10 kHz, the maximum capacitance disappeared as the voltage increased, indicating that the oscillation of the charge carriers in the metallic regions could not match the AC field. The electrically driven phase transition of the VO 2 thin films can occur within 10 ls [31,32]. The longer response time in our experiment could be a result of the large time constant RC owing to the presence of the Al 2 O 3 layer. However, the phase transformation, indicated by the drop in capacitance, shifted to lower voltages as the frequency increased. This could be the result of easier nucleation of the new phase at high frequencies.

Conclusions
Al 2 O 3 thin films were used as buffer layers to improve the growth of VO 2 thin films on silicon substrate. Pure VO 2 phases were obtained by depositing on the Al 2 O 3 buffer layer at an oxygen partial pressure of 4%. Accordingly, a large resistance change and a small hysteresis were obtained for the thermally induced phase transformation. Furthermore, the electrical-field-induced phase transformation of  5 Schematic mechanism of electrically driven phase transformation of VO 2 thin films VO 2 thin films was also investigated. By measuring the capacitance at different voltages and frequencies, we propose that the phase transformation was initialized by the formation of metallic regions in the insulating VO 2 matrix. Our work provides a facile method to deposit VO 2 thin film on silicon substrate, which paves the way for fabricating VO 2 -based devices such as fast electrical modulation of THz waves.