1. Introduction

Modulation of near-infrared (IR) light has broad applications ranging from photoacoustic (PA) spectroscopic sensing, optical communication and to biomedical imaging [1–3]. Since visible light does not penetrate deep into tissue due to scattering and absorption, near-IR light is used for PA imaging such improved penetration depths and high-quality image resolutions [4,5]. Similarly, due to lower absorption of near IR light in fiber optic material and specific wavelength of absorption of mid-IR light by molecules, they are used extensively in optical communication and molecular spectroscopy. In order to reduce measurement noise, applications in communication, imaging, and sensing technologies typically utilize modulated near-IR light and a lock-in based detection [6–8]. At present, intensity modulation techniques include optical-chopper systems using a slotted wheel, or electronically pulsed lasers, requiring a separate pulsing circuit [7,8]. However, modulation rates of mechanical choppers are limited to ∼10 kHz [9], while infrared lasers with built-in pulsing circuits are expensive and often bulky [7]. Therefore, these IR modulation platforms are not suitable for applications in portable and miniaturized systems, especially if a broad-wavelength source of IR light is utilized (i.e. mercury lamp). In recent years, graphene based metasurfaces, plasmonic nano particles, electrochromic and thermochromic materials have been extensively studied to develop reliable, compact, low-power near-IR modulators [10,11]. Among these technologies, vanadium dioxide (VO₂), a well-known thermo- and electrochromic material, has emerged as one of the most promising candidates.

VO₂, an interesting member of oxovanadates family, exhibits unique insulator metal transition (IMT) properties as monoclinic insulating phase of VO₂ transforms into a rutile metallic phase near temperatures of 67-68 °C [12–15]. In addition to temperature triggered IMT, the phase transition of VO₂ can also be achieved with externally applied electric field and pressure pulses [12,16,17]. These IMT induced drastic resistance changes of VO₂ thin films, which can reach up to 3-4 orders of magnitude, and take place in nanoseconds [18,19], have opened up potential applications in field effect transistors [20,21], memristors [22], and radiofrequency switching devices [23,24]. In addition to the electrical property change of VO₂ films during IMT, optical transmittance and reflectance of the thin film, especially in near-IR and the IR regime, also changes drastically [25,26]. For the near and mid-IR part of the spectrum, the insulator phase of VO₂ is quite transparent, while it is much more reflective in the metallic phase [26–28]. Taking advantage of this behavior, optoelectronic devices utilizing IMT based modulation of light (caused by reversible phase change of VO₂ thin films), such as near and mid-IR modulators for imaging and sensing applications, can be developed. While Si, SiO₂, TiO₂, quartz, and sapphire are commonly used as substrates to realize VO₂ thin film based photonic devices [19,29–32], investigations on VO₂ thin film grown on wide-bandgap semiconducting materials is rather limited [33–37]. Compared to traditional substrates, GaN film for VO₂ growth offers several advantages stemming from the unique properties of III-Nitrides, including transparency at visible and infrared wavelengths, chemical inertness, and strong piezoelectric properties [34–36]. Especially, the possibility of integration of a VO₂ based solid-state optical modulator at infrared wavelengths with III-Nitride based surface acoustic wave (SAW) devices operating in GHz frequencies is very attractive, along with integration of III-Nitride microwave devices in the same substrate, offers interesting high frequency electro-optic modulation possibilities. Additionally, synthesis of high quality GaN films on Si (111) substrate is a mature technology (III-Nitride epilayers on 8-inch Si (111) substrate are routinely grown in high volume for light emitting diode applications), which also offers the possibility of easy integration with Si based integrated circuit in the same chip.

In this work, we have investigated for the first time electrically pulsed optical transmittance characteristics of a VO₂ thin film, grown by oxidizing 70 nm vanadium deposited on c-plane GaN (thickness of 1.3 µm) membrane. Using interdigitated electrodes patterned on the suspended membrane, electrical and optical characteristics of the VO₂ thin film with a radius of 500 µm were recorded at two near-IR wavelengths of 790 and 1550 nm. The IMT of the VO₂ thin film was triggered with temperature as well as, dc and pulsed voltages applied to the interdigitated fingers. For the latter, modulation IMT transitions and hence optical modulation could be achieved using voltage pulses down to 300 µs. COMSOL based finite element simulations were also performed with varying voltages applied to the interdigitated fingers to investigate the IMT in the membrane.

2. Materials and methods

The GaN epitaxial wafers used in this study were purchased from NTT advanced technology corporation, Japan. The wafer had 1.3 µm i-GaN buffer layer on 675-750 µm thick Si (111) substrate. The modulator devices utilizing VO₂ thin films on GaN membrane were fabricated using a traditional photolithographic fabrication process. First, GaN circular mesa structures with a diameter of 500 µm were defined on a substrate of 1.3 µm GaN epitaxial layer on Si(111) wafer, using an inductively coupled plasma (ICP) dry etching process. Then, 70 nm vanadium was deposited on top of the circular mesa using e-beam evaporation. A home-built low-pressure chemical vapor deposition system was used to oxidize the vanadium to VO₂ following an optimized synthesis process reported elsewhere [33,34,38]. After the VO₂ growth process, a set of 18 interdigitated metal fingers of Ti (20 nm) / Au (250 nm) stack were deposited on top of the membrane. The gap between the interdigitated fingers is 15 µm while the width of the metal electrodes is 10 µm. Finally, the GaN membrane was released from the Si substrate by through wafer etching of Si using an optimized Bosch process [34,38]. Schematic diagrams (top and side views) of the device structure are shown in Fig. 1(a), while a scanning electron micrograph (SEM) of the final fabricated device is shown in Figs. 1 (b). The x-ray diffraction (XRD) θ-2θ plot for the membrane is shown in Fig. 1(c), with inset showing a semi-log plot of the same. The peaks located at 2θ ≈ 28°, 52°, and 56° indicates the presence of VO₂ [39].

 

Fig. 1. (a) Schematic of the VO₂ thin film structure grown on epitaxial GaN membrane on Si (111) substrate. The top schematic illustrates a 3D view of the membrane with interdigitated metal fingers. The side view of the membrane is shown in the bottom schematic along with dimensions. (b) SEM image of VO₂ on GaN circular membrane and interdigitated fingers. The diameter of the membrane is 500 µm (equal to the scale bar shown in the image). The metal fingers were 10 µm wide, with a gap of 15 µm between them. (c) XRD characteristics of the VO₂ thin film on GaN membrane showing characteristics VO₂ peaks. The inset shows the XRD intensity values in log scale to

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To investigate electrical and optical characteristics of the fabricated VO₂ thin film membrane, an experimental setup as shown schematically in Fig. 2(a) was used. The chip containing VO₂ on GaN membrane was attached to a ceramic chip carrier that was carefully placed on top of a ceramic heater. The chip carrier (with a circular opening at the bottom of 10 mm diameter) was positioned on the heater, which also had a hole of 6 mm diameter, and they were aligned carefully with the GaN membrane to enable transmission of the laser beam. Two near-IR lasers with wavelengths of 1550 nm (Alphanov) and 790 nm (Worldstartech Inc.) were used in the experiments. The collimated laser beams, with spot sizes of 20 µm (for 790 nm) and 100 µm (for 1550 nm), were focused vertically on the VO₂ thin film using an x-y-z micropositioner. The transmitted laser power was measured using a photodiode (Newport 918D-IR) positioned underneath the heater, with its aperture properly aligned to capture the laser beam. Temperature measurements were carried using a data accusation system (Keysight 34972A) and a thermocouple (k type with a typical range of -200 to 1000 °C) attached to the chip carrier near the modulator chip. A computer-controlled source measurement unit (Keysight B2902A) was used to automatically apply the bias voltages to the interdigitated fingers through a 1 kΩ resistor to limit the current in the metallic state of VO₂.

 

Fig. 2. (a) Schematic of the experimental setup used to measure optical modulation by the VO₂ film using external voltage pulse. A series resistance of 1 kΩ was used in series with the voltage source to avoid high current related failures in the metallic state. (b) Variation in resistance and transmitted optical power near IMT in the VO₂ thin film. The left axis shows resistance changes as the temperature slowly rises and drops (solid and dotted red lines, respectively), while the transmitted optical power changes at 1550 nm wavelength, with temperature rise and drop (blue solid and dotted lines, respectively) are shown on the right axis. Inset presents temperature characteristics of large area 70 nm VO₂ thin film grown on GaN epitaxial film on Si substrate. (c) Effects of applied electric field on optical and electrical characteristics of the VO₂ thin film. A reduction by 7% and 14% in transmitted optical power is observed at wavelengths of 790 nm (red) and 1550 nm (blue) occurs around applied voltage of ∼15.7 V, respectively, which results in the IMT. The inset shows resistance variations of VO₂ thin film due to the applied field induced IMT.

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3. Results and discussion

At first, the temperature dependence of the electrical resistance and optical transmittance of the VO₂ film under 1550 nm near-IR laser exposure was determined. As the temperature was increased from room temperature to 75 °C and then cooled down to room temperature, the resistance of VO₂ membrane changed from 5 kΩ and 150 Ω (6 kΩ to 1.15 kΩ measured with the 1 kΩ series resistor R_S) and vice-versa. The temperature-induced resistance changes of VO₂ thin film are shown in Fig. 2(b), over the range of 30 to 75 °C are shown by red solid (for heating cycle) and dotted (for cooling cycle) lines on the left y-axis. A gradual resistance transition from insulating state to metallic state is observed around 67 °C for the heating process and around 62 °C for the cooling process. For comparison, we have included the resistance characterization results of a large area (1 cm × 1 cm) VO₂ thin film grown (from 70 nm V deposition) on GaN substrate in inset of Fig. 2(b). As shown in the inset, the large area VO₂ thin film resistance changes from 2 MΩ to 3 kΩ with much sharper transition rates, between 50-55 °C. The contrasting resistance characteristics between large and small area (current device) VO₂ thin films might be due to several factors, including structural defects in the VO₂ film grown on a small area, which can suffer from larger fluctuation of analyte concentrations over its surface [15,40,41].

Changes in the transmitted optical power, as the VO₂ film underwent MIT, was measured using the formula $Optical\; Power\; Change(\%)= \; {\raise0.7ex\hbox{${({T - {T_{RT}}} )}$} \!\mathord{\left/ {\vphantom {{({T - {T_{RT}}} )} {{T_{RT}}}}} \right.}\!\lower0.7ex\hbox{${{T_{RT}}}$}} \times 100\; $ where T and T_RT are the transmitted laser power at a given temperature, and at the room temperature, respectively. The results are shown as blue solid (heating cycle) and dotted (cooling cycle) lines on the right y-axis of Fig. 2(b). We find that the transmitted laser power (1550 nm) reduced approximately 25% when the temperature reached 75 °C. Unlike the gradual decreases in the resistance observed before and around the transition temperature of 67 °C, the optical transmittance remained relatively flat before the transition and changed sharply at the transition temperature. This is because the laser power heated the area on which it was focused, enabling quicker phase transition, unlike the electrical characteristics, which exhibited an aggregate of the transition of various areas (which could be non-uniform) over the entire membrane.

Following the initial characterization, we investigated electrical field induced IMT in the VO₂ thin film. A variable voltage from 0.5 V to 20 V was applied to the interdigitated fingers on the VO₂ thin film, while the resistance and relative optical transmittance changes were simultaneously measured. Figure 2(c) shows the percentage change in transmitted optical power of lasers with wavelengths of 790 nm (shown as red solid (forward sweep) and red dashed (reverse sweep) lines) and 1550 nm (displayed as blue solid (forward sweep) and blue dashed (reverse sweep) lines under applied electrical field. The change in transmittance was approximately 14% for 1550 nm laser and 7% for 790 nm laser. The 790 nm plot is somewhat noisier compared to the 1550 nm data as different responsivity values (chosen automatically based on input wavelength) was used by the photodetector to calculate incident optical power. Inset of Fig. 2(c) shows the resistance characteristics of VO₂ membrane under applied electric field during the 1550 nm laser transmittance measurements. IMT of the VO₂ thin film membrane was observed approximately at ∼15.7 V with upward bias sweeping configurations, whereas the film switched back from metal to insulator state at 12 V, under downward bias sweep. We note that the electrical and optical changes observed in Fig. 2(c) are much sharper compared to those in Fig. 2(b) near the IMT of VO₂. This can be attributed to faster and more uniform transition process, over the entire VO₂ film, enabled by application of electric field using the finger structure that was uniformly distributed, as compared to the heater pad, which raised the temperature of the various regions at a much slower rate causing various degrees of IMT over the various sections of the film.

After investigating IMT characterization experiments of the VO₂ thin film membrane, we performed COMSOL simulations to understand the surface temperature characteristics of the VO₂ thin film under application of electrical field. Figure 3(a) shows the temperature variation over the membrane when an electrical bias of 15 V is applied to one side of the electrodes while the other side was grounded. Figure 3(b) shows the temperature variation across a metal finger and a VO₂ gap as various voltages (5, 10, 15, and 20 V) are applied to the interdigitated fingers. For the simulations, the conductivity of the VO₂ film was taken as 13 S/m at the onset of phase transition, and the metal conductivity was 45.6×10⁶ S/m. The conductivity was determined from the film resistance (R_f) of ∼1 K Ω at the onset of transition (2 kΩ as read from Fig. 2(c)–1 kΩ series resistance), the thickness (t) of the VO₂ film of ∼70 nm (arising from a V deposition of 70 nm) [42], the finger gap (l) of 15 µm, and assuming an average width (w) of ∼400 µm. The conductivity σ is then calculated using the formula: σ $= l/({tw{R_{gap}}} )$, where R_gap is the resistance between one finger gap given as R_f × 17 (number of gaps). Putting in the numbers, we find σ = 13 S/m. From Fig. 3(b), we find the maximum temperature on the VO₂ thin film membrane at 15 V bias is ∼45 °C, which is slightly below the transition temperature of ∼67 °C [43]. The temperature reaches its maximum value of ∼45 °C at the middle of the VO₂ thin film section and reduces toward the metal contacts to ∼30 °C. The increase in temperature is not significant on the metal lines compared to the VO₂ thin film stripes due to much lower resistance (causing less heat dissipation) and much higher thermal conductivity (helping to carry away the heat faster).

 

Fig. 3. (a) COMSOL simulations showing surface temperature variations over the circular VO₂ on GaN thin film membrane with 500 µm diameter. The voltage applied across the metal fingers was 15 V. (b) Temperature variations across a VO₂ thin film stripe and a metal finger simulated for various applied voltages. (c) COMSOL simulations showing electric field variations over the circular VO₂ on GaN thin film membrane at 15 V applied bias. (d) Electric field variations across a VO₂ thin film stripe and a metal contact for various applied voltages. The results shown in (b) and (d) are displayed along the same cut line.

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The electric field mapping on the VO₂ thin film membrane at an applied bias of 15 V is shown in Fig. 3(c). The maximum value of the electric field on the top of the VO₂ thin film membrane is found to be ∼10⁶ V/m, which is very close to the average electric field expected from simply dividing the 15 V applied bias with the width of the VO₂ strip of 15 µm. The simulated electric fields at applied voltages of 5, 10, 15, and 20 V are shown in Fig. 3(d) across a VO₂ gap and a metal line. Noting that the critical electric field triggering phase transition process in VO₂ thin films is ∼10⁷ V/m [18,44], the experimentally observed phase transition of VO₂ thin film on GaN membrane at an applied bias of 15.7 V is not likely to be solely driven by the applied electric field. Also, the ∼45 °C temperature rise along the VO₂ thin film caused by Joule heating is not sufficient to trigger the phase transition, which occurs at ∼67 °C. Therefore, we conclude that a combination of electric field and Joule heating is responsible for phase transition in the VO₂ thin film, with more significant role played by the electric field. This is in agreement with earlier studies reporting that electric field assisted phase transition is generally a combination of electric field effect and temperature rise due to Joule heating [14,43,45].

Taking advantage of the abrupt drop and jumps in the optical transmittance data, the VO₂ membranes can be utilized to modulate near-IR light using a pulsed electric field. To experimentally demonstrate such optical modulation using applied electric field, pulsed voltage signals with varying pulse widths and magnitudes, and a constant duty cycle of 20%, were applied to the membrane, while the relative optical transmittance of the membrane under continuous near-IR laser exposure (790 or 1550 nm) was measured. Figures 4(a) – (c) show the optical transmittances of the VO₂ film under various applied voltage pulses with pulse widths of 100 ms, 1 ms, and 300 µs, respectively. Figure 4(d) displays the magnitude of the applied voltage pulses with respect to pulse width, while the relative change in transmission of the laser power at wavelengths of 1550 nm (blue) and 790 nm (green) was kept approximately constant, assuring the complete phase transition as shown in inset of Fig. 4(d). As demonstrated in Fig. 4(d), while ∼16 V bias was utilized in the pulse range of 1 s – 10 ms for IMT induced transmitted optical power changes, which resulted in 14% and 7% reduction for 1550 nm and 790 nm, respectively, the required voltages to trigger phase transition were increased to 17.5 V and 25 V for the pulse widths of 1 ms and 300 µs, respectively. Due to Joule heating effects during application of the externally applied electric field, both electronic and electrothermal mechanisms contribute to IMT, in agreement with our COMSOL simulations and previous reports [14,46,47]. As the duration of the applied voltage was decreased, the contribution of Joule heating on facilitating IMT in VO₂ thin film reduced. Consequently, the applied bias required to enable IMT in VO₂ thin film increased with a reduction in pulse width. Such an increase led to application of higher electric field and also led to higher Joule heating.

 

Fig. 4. Electric field induced modulation of near-IR (1550 nm shown as blue, 790 nm shown as green, with powers of 0.38 and 0.22 mW, respectively) transmitted light using voltage pulses applied across the metal fingers on the VO₂ on GaN thin film membrane, with pulse widths of (a) 100 ms, (b) 1 ms, (c) 300 µs (all with 20% duty cycle). (d) Voltage required for triggering IMT increases with reduction in voltage pulse width. Inset shows optical power changes ΔP_opt corresponding to laser wavelengths of 1550 nm (blue) and 790 nm (green) maintained approximately constant at various voltage pulses.

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In order to verify the contribution of temperature on the electric field necessary to facilitate IMT, we measured the applied voltage required to cause a phase change in VO₂ thin film at various temperatures ranging from 23 °C to 50 °C. As shown in Fig. 5, the voltage needed to trigger phase transition reduces as the membrane temperature increases. While the external voltage required for transition was 15.7 V at 23 °C displayed in Fig. 5(a), it decreased significantly to 12 V and 9.5 V when the film temperatures were increased to 40 °C, and then 50 °C as shown in Figs. 5 (b) and (c), respectively.

 

Fig. 5. Variation in optical power of 1550 nm laser transmitted through the VO₂ membrane while increasing (solid lines) and decreasing (dotted lines) applied voltage bias, applied across the interdigitated metal fingers on the VO₂ thin film on GaN membrane, at various temperatures of (a) 23 °C, (b) 40 °C, and (c) 50 °C. Increasing the VO₂ film temperature resulted in lowering of the critical voltage required for its IMT transition. While the transition voltage is ∼15.7 V at 23 °C, it reduces to 12 V, and further to 9.5 V, as the temperature is increased to 40 °C and then to 50 °C.

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In the literature, investigations on changes in transmittance properties of VO₂ thin films in near-IR and mid-IR regimes during IMT have been reported quite extensively [26–28,36,48,49]. Hiltunen et al [28] demonstrated temperature triggered IMT in 70 nm VO₂ thin film deposited on Al₂O₃ substrate causing light modulation from 49% to 14% at 1550 nm. Using VO₂ nanoparticle film on ITO glass substrate, Li et al. also reported comparable modulation of 38.8% at 1550 nm [48]. In the mid IR range at 2.5 µm, much larger transmittance change of ∼50% was demonstrated by Nakano et al. for 50 nm VO₂ thin film on TiO₂ substrate, utilizing electrically and thermally induced IMT [27]. After an extensive search, we found only a handful of studies reporting VO₂ thin film characterization on GaN film [35–37], and only one reported measurement of optical modulation (∼50% observed at 1.8 µm wavelength), through thermal effect using conductive GaN as the heating layer [36]. Although larger modulation on VO₂ films has been reported with superior electrical and optical modulation characteristics, most of the films are deposited on standard substrates like quartz, Si, SiO₂, TiO₂, glass or sapphire, or GaN/sapphire; using magnetron sputtering technique for film deposition [14,19,26,35]. However, for integration with mainstream Si devices and to reduce substrate costs, Si substrates are attractive, while growth on GaN entails its own favorable properties, including transparency to near-IR light, chemical inertness, and simple fabrication procedure of micro-size suspended membranes (by etching away the Si substrate under). In addition, direct oxidation of vanadium film in a CVD furnace is simpler and lower cost alternative to the magnetron sputtering technique. Indeed, this study is the first of its kind on demonstrating near-IR light modulation, with sub-ms response time, using VO₂ film grown on GaN epitaxial layer, through direct application of a pulsed voltage across the VO₂ layer to trigger IMT. The main reason behind for observing lower optical transmittance change in the VO₂ films used in this study can be attributed to poorer material quality of the VO₂ thin film grown over a microscale area. As pointed out before (inset of Fig. 2(b)), large area VO₂ thin film on GaN epitaxial layer, grown by the same CVD technique, provides 3-4 orders of magnitude change in resistance at IMT, which is comparable to the best results reported in the literature [33,35]. Microscale dimension films of comparable quality can be expected to exhibit much larger percentage change in transmitted optical light intensity upon phase transition.

We note here that although nanosecond switching rates of electrically driven IMT phase of VO₂ on various substrates has been demonstrated [18,19] using electrical bias, in our case, the IMT phase of the VO₂ thin film on GaN membrane was prompted utilizing both electric field and Joule heating, which likely limited it’s operational speed, since Joule heating triggered IMT tends to be slower than non-thermal switching methods [14,50]. We also note that the only report of optical modulation of VO₂ film on GaN/sapphire resulted in switching time on the order of seconds, which is much larger than the response time for our devices. The operating speed can be improved significantly by further improving the quality VO₂ film (leading to quicker and abrupt transition), reducing the spatial distance between fingers (which will enhance electric field for same applied voltage), doping the VO₂ film so that near room temperature operation, so that role of temperature change in the phase transition is minimized.

4. Conclusions

In conclusion, we have investigated electrical and near-IR optical transmittance characteristics near the insulator to metallic phase transition of VO2 thin film deposited on suspended GaN membrane on a Si substrate. IMT in the VO2 thin film, triggered by voltage pulses applied across interdigitated metal fingers, resulted in modulation of optical transmittances at multiple near-IR wavelengths, with sub ms time constant. COMSOL based finite element simulation indicated that the phase transition was triggered by a combination of Joule heating and applied electric field, which was corroborated by experimental observations.