Research Article www.acsami.org
Infrared Response and Optoelectronic Memory Device Fabrication Based on Epitaxial VO2 Film Lele Fan,†,‡ Yuliang Chen,‡ Qianghu Liu,§ Shi Chen,‡ Lei Zhu,† Qiangqiang Meng,† Baolin Wang,∥,† Qinfang Zhang,† Hui Ren,‡ and Chongwen Zou*,‡ †
Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng 224051, China ‡ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, 230029, China § Science and Technology on Electro-optical Information Security Control Laboratory, Tianjin 300300, China ∥ School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, People’s Republic of China ABSTRACT: In this work, high-quality VO2 epitaxial films were prepared on high-conductivity n-GaN (0001) crystal substrates via an oxide molecular beam epitaxy method. By fabricating a two-terminal VO2/GaN film device, we observed that the infrared transmittance and resistance of VO2 films could be dynamically controlled by an external bias voltage. Based on the hysteretic switching effect of VO2 in infrared range, an optoelectronic memory device was achieved. This memory device was operated under the “electrical writing− optical reading” mode, which shows promising applications in VO2-based optoelectronic device in the future. KEYWORDS: vanadium dioxide, n-GaN, memory device, phase transition modulation, infrared transmission
1. INTRODUCTION As a typical metal−insulator transition (MIT) material, VO2 has attracted widespread interest. Across the phase transition, the resistance of VO2 undergoes a sharp change (up to 5 orders of magnitude) and the infrared transmittance decreases greatly, showing pronounced switching effects.1 These excellent characteristics of VO2 material make it suitable for many promising applications such as smart windows, ultrafast optical switches, infrared detectors, phase-change material, and thermal sensors.2−10 Normally, the MIT of VO2 will occur when the critical phase-transition temperature (Tc ≈ 340 K) is reached. Except the thermal stimuli, some other ways can also induce the MIT behavior, such as electric field (bias voltage/current) and electromagnetic radiation.11−13 Although the intrinsic Tc value of VO2 is close to room temperature, it is still relatively high, which seriously limits the practical applications. Thus, modulating the MIT process and decreasing the Tc to room temperature are necessary to satisfy its practical applications. In addition, realizing continuous phase transition modulation can deliver more potential applications based on its MIT process. Many approaches have been developed to modulate the phase transition behavior, such as element doping,14,15 oxygen vacancies,16−18 hydrogenation19,20 and interfacial strain.21−25 While controllable and continuous phase transition control is highly desirable, the Nakano group recently fabricated a VO2based electric double-layer transistor assisted by an ionic liquid and continuously regulated the MIT process.26 However, the complex device fabrication and long response time hindered its © XXXX American Chemical Society
further application. The electrothermal-controlled MIT of VO2 attracted more interest, since the phase transition behavior could be effectively tuned by external bias voltage. For example, the Li group covered Ag nanowires on VO2 film surface as electrodes and triggered the MIT process by the externalvoltage-induced Joule heating.27 The Skuza group deposited the highly conductive Al-doped ZnO film on VO2 film and examined the MIT process under different bias voltages.28 It was clear that the voltage-induced MIT modulation basically relied on the Joule heating from a conductive layer. Accordingly, direct deposition of the VO2 film onto a conductive substrate should be more suitable for this voltagecontrolled MIT process. Although some substrates such as Fdoped tin oxide (FTO) or In-doped tin oxide (ITO) showed excellent conductivity, the epitaxial growth of VO2 on them was difficult, because of the large lattice mismatching. In the current wok, we selected highly conductive n-type GaN/Al2O3 (0001) crystal as the growth substrate, since the Si-doped GaN/Al2O3 substrate showed the suitable lattice parameters, good transparency from visible to infrared, and excellent electric conductivity. In addition, much literature has reported on optical writing or electrical writing memory devices based on the hysteresis phase transition property4,29−31 or just based on the different Received: October 9, 2016 Accepted: November 15, 2016 Published: November 15, 2016 A
DOI: 10.1021/acsami.6b12831 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. XRD results of VO2/n-GaN in (a) θ−2θ scanning mode and (b) φ-scan mode of VO2, GaN, and Al2O3, which showing the epitaxial growth behavior of the films. 530 °C. More experimental details could be found in our previous work.17 High-resolution X-ray diffraction (XRD) measurements were performed at the Shanghai Synchrotron Radiation Facility (SSRF) 14B beamline. The BL14B shows the energy resolution (ΔE/E) of 1.5 × 10−4 at 10 keV and the beam size of 0.3 mm × 0.35 mm with the photo flux of up to 2 × 1012 photons/s at 10 keV. The ultravioletvisible-near-infrared (UV-vis-NIR) spectrometer (SolidSpec 3700) combined with Keithley sourcemeters (Models 2410 and 2450) were employed to measure the infrared transmittance and achieve the memory device in “electrical writing−optical reading” mode. Specifically, the Keithley Model 2410 sourcemeter supplied a constant DC voltage to heat the sample, while the Keithley Model 2450 sourcemeter produced the pulsed voltage necessary to achieve the memory function. The resistance of the film sample, as the function of temperature, was measured by a customized four-probe system.
phase-change structures.32 Compared with “optical writing” manner, the “electrical writing” mode could be easy to control and realize the integration. For example, the Basov group reported the capacitance memory features of VO2 controlled by pulse voltage,4 as well as a prototype of VO2 memristor through a two-terminal film device.29 However, this VO2 memristor could be only operated with an external heating cell, which made it difficult to realize the device integration. To avoid this shortage, the Wang group developed a two-voltage system for their VO2 nanowire-based memristor.30 First, the constant voltage was added on a VO2 nanowire and self-heated it to a stable temperature, and then the pulsed voltage manipulated the VO2 single nanowire for memory performance. While all the above memristor devices were operated by measuring the electric capacitance/resistance variation induced by pulse voltage, which was the so-called “electrical writing−electrical reading” mode. Since the VO2 material was sensitive to infrared light across the MIT process, it was possible to realize an infrared photonic memory device in an “electrical writing− optical reading” type, if the variation of infrared transmittance induced by pulse voltage could be effectively measured. In the present work, we prepared high-quality epitaxial VO2 film on a conductive GaN/Al2O3 substrate via the oxide molecular beam epitaxy (MBE) method. Synchrotron radiation-based φ-scan XRD was conducted to investigate the epitaxial growth feature. The continuous phase transition modulation of VO2 was achieved by changing the bias voltage added on a two-terminal device. The transmittance of infrared light at 1.8 μm with different bias voltages was dynamically analyzed. Furthermore, we demonstrated a VO2 infrared memory prototype with the “electrical writing−optical reading” mode.
3. RESULTS AND DISCUSSION Figure 1a shows the XRD pattern in normal θ−2θ scanning mode for the VO2/n-GaN/Al2O3 sample. The peak located at 2θ = 34.72° corresponds to the GaN layer (002) plane and the peak at 41.68° is attributed to Al2O3 (0006) diffraction. The peak located at 39.7° is assigned to the VO2 (020) diffraction, according to the JCPDS reference (File No. 82-0661) and previous reports.33,34 No other diffraction peaks existed in the XRD curve, indicating the high-oriented growth of the VO2 epitaxial film. To further examine the epitaxial growth behavior of VO2 on GaN/Al2O3 surface, the φ-scan XRD study is conducted. Figure 1b shows the φ-scan patterns for VO2 (011), GaN (102), and Al2O3 (102) planes. From the VO2 (011) plane-related φ-scan mode, six diffraction peaks with a separation of 120° can be clearly observed, which are quite consistent with our previous work, according to multidomain growth.35 In fact, from the φ-scan XRD pattern of GaN/Al2O3 substrate, it is observed that the Al2O3 crystal has 3-fold symmetry, whereas the n-GaN layer has the 6-fold symmetry, both of them are following the [0001] direction as the rotational axis. In addition, there exists a 30° deviation between the Al2O3 (102) peaks and the GaN (102) peaks in the φ-scan process, showing that the in-plane lattice matching is GaN [21̅1̅0]//Al2O3 [101̅0]. Furthermore, the VO2 (011) diffraction peaks are consistent with GaN (102) peaks exactly, suggesting that the a-axis of VO2 is aligned with the b-axis of GaN lattice. Accordingly, the entire in-plane lattice matching relations can
2. EXPERIMENTAL METHODS Commercial n-type GaN/Al2O3 (0001) crystal film grown by MOCVD was used in this study (Si-doped, carrier concentration of ∼1.0 × 1018 cm−3). Prior to the VO2 deposition, the GaN/Al2O3 (0001) substrates were ultrasonically cleaned consequently in acetone, isopropanol, and deionized water, for 10 min each. After the above treatments, the substrate was blown dry by N2 flux and then rapidly transferred into the vacuum chamber. The VO2 films were deposited on these n-GaN/Al2O3 substrates by using a radio-frequency (rf) plasma-assisted oxide MBE chamber with the base pressure of 3 × 10−7 Pa. During the deposition, the substrate temperature was kept at B
DOI: 10.1021/acsami.6b12831 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Scheme for bias-voltage-controlled VO2/n-GaN/Al2O3 optical device. (b) Visible−infrared (Vis-IR) transmittance of VO2/n-GaN/ Al2O3 sample. For comparison, the transmittance of the bare n-GaN/Al2O3 substrate was also plotted. (c) The transmittance of 1.8 μm infrared light, as the function of time when the device was heated by a steplike bias voltage. The voltage increased 0.5 V after each 50 s. (d) The dynamics response of IR transmittance at 1.8 μm under different bias voltages.
oscillation peaks, the VO2 film thickness was estimated to be ∼30 nm, which was quite consistent with the measurement by the quartz-crystal oscillator during the growth process. From Figure 2b, it was clear that, for this two-terminal device, if we added a bias voltage (Vdc) of 4.0 V, the transmittance curve for the VO2/GaN/Al2O3 (0001) sample showed little difference, compared to the situation of Vdc = 0.0 V. While if the bias voltage was up to 5.0 or 6.0 V, the transmittance at the infrared range decreased completely, indicating the occurrence of the MIT process. In order to examine the threshold voltage for triggering the MIT process, we measured the variations of the transmittance at 1.8 μm when a step-increasing DC voltage was added in Figure 2c. The bias voltage increased step by step from 0.0 to 6.0 V and each step was maintained for 45 s. When the bias voltage was lower than 4.0 V, little transmittance change was observed, whereas, when the bias voltage increased up to 5.0 V, the transmittance abruptly decreased, indicating the ∼5.0 V threshold bias voltage in our experiment to trigger the MIT process. In order to examine the infrared switching effect driven by bias voltages, we investigated the dynamics process of transmittance variation at 1.8 μm in Figure 2d. It was observed that, for a lower voltage, such as Vdc ≈ 4.5 V, the transmittance showed a slow variation as well as a very long response time. As the bias voltage increased, the transmittance switching effect
be written as VO2 [100]//GaN [21̅1̅0]//Al2O3 [101̅0], which agree well with the previous literature.33,34 The φ-scan XRD results confirm the epitaxial growth property of VO2/n-GaN/ Al2O3 (0001) samples with low structural defects36,37 and demonstrate the high quality of the film samples in our experiments. To test the electrothermal-controlled phase transition process, we fabricated a VO2/GaN/Al2O3 (0001) film device, as shown in Figure 2a. The VO2 epitaxial film was grown on the center area of GaN surface and two Au stripe-electrodes were sputtered on the edges of GaN layer to form a two-terminal device. Because of the excellent electric conductivity of the heavily doped n-GaN (
