Thermally-induced optical modulation in a vanadium dioxide-on-silicon waveguide

: In this paper, we report phase-pure vanadium dioxide (VO 2 ) deposition on silicon-on-insulator and demonstrate switching/modulation exploiting the phase-change property. We present electrical and optical properties of VO 2 during phase transition. Exploiting the phase change property, optical modulation is achieved by thermally tuning the VO 2 phase using a lateral micro-heater beside the waveguide. We achieve an optical modulation extinction of 25 dB and a low insertion loss of 1.4 dB using a ring resonator with a VO 2 patch. We also demonstrate the switching performance of a symmetric Mach-Zehnder interferometer and present a detailed discussion on the optimal operating point to achieve maximum modulation, higher speed, and lower insertion loss.


Introduction
Recently, phase-change materials (PCMs) have shown great promise for applications in integrated optics platform [1,2] due to their broadband operation and CMOS compatibility.Of all the PCMs, VO 2 has gained particular attention because of its transition from insulating, optically transmissive monoclinic phase to metallic and optically opaque tetragonal rutile phase, at very near to the room temperature (68 • C).The phase transition in VO 2 can be achieved by applying thermal [3,4], optical [5][6][7][8] or electrical stimuli [9][10][11].These two phases have high contrast in the electrical resistivity (> 3 order change) and optical constants (∆n and ∆k > 1 order change) in the telecommunication wavelength region [12,13].The refractive index of VO 2 changes from 3.21 + i0.17 to 2.15 + i2.79 (at 1550 nm) between the dielectric and metallic phase, respectively [14].The larger change in refractive index is exploited in the implementation of energy-efficient devices with smaller footprint, such as optical modulators [14][15][16][17][18] and photonic memories [19][20][21][22].The Oxygen stoichiometry plays a crucial role in governing the transport and optical properties of VO 2 [23,24].Oxygen vacancies can enhance the conductivity and deteriorate the switching properties of VO 2 .So, the phase purity and quality of the thin film are very crucial to incorporate VO 2 in these devices.Due to the multiple oxidation states of vanadium, optimization of the deposition and fabrication condition of VO 2 modulator are very critical and challenging [25].To overcome these problems various techniques such as sputtering [11], chemical vapor deposition [26], pulsed-laser deposition [27], nebulized spray pyrolysis [28] has been reported.
Recently there have been many demonstrations on optic modulation using VO 2 -Si waveguide across the metal-insulator transition of VO 2 .Briggs et al., achieved extinction of 6.5 dB and an insertion loss of 2 dB using 2 µm long device [14].An interesting geometry using embedded PCM along the waveguide was demonstrated with an extinction of 10 dB [15].However, the insertion loss was 6 dB, which is on the high-side.Whereas, Joushaghani et al. enhanced the extinction to 12 dB with a higher insertion loss of 5 dB with an active length of 1 µm [18].The authors also demonstrated a bandwidth of 1.3 MHz limited by the thermal dissipation.To exploit VO 2 , it is essential that the three performance parameters; extinction, insertion loss and speed, are optimised through the material and device design.
In this work, we demonstrate an absorption type optical modulator on silicon-on-insulator (SOI) platform by thermally tuning the refractive index of VO 2 .Pulsed-laser deposition (PLD) technique is used to grow the phase pure VO 2 thin film on SOI substrate.The phase purity and the evolution of insulating to the metallic phase of as grown VO 2 thin film were characterized using X-Ray Diffraction (XRD) and Raman spectroscopy.We also perform temperature-dependent electrical resistance and ellipsometry measurements to characterize the change in electrical resistance and optical constants of the material across the phase transition.Using VO 2 -on-silicon, we demonstrate phase transition using micro-heater and substrate heater.We achieved a maximum optical transmission extinction of ∼25 dB in both local heating and global heating with a low insertion loss of 1.4 dB.We also show that the refractive index change measured is ≈1.2 that agrees with spectroscopic ellipsometry on a thin film.We also analyzed the switching performance of the device and optimized the operation condition for the VO 2 -based Mach-Zehnder modulators.

Material characterization
Phase pure VO 2 is deposited on SOI using V 2 O 5 as the target.The details of thin-film depositions protocols were reported earlier in [27].The phase purity and insulator-metal transition (IMT)/metal-insulator transition (MIT) of the fabricated VO 2 device are characterized using XRD, Raman spectrometer, spectroscopic ellipsometer, and temperature-dependent electrical resistivity measurement.XRD patterns of the deposited films are shown in Fig. 1(a).It is evident from Fig. 1(a) that VO 2 film deposited on SOI is in pure monoclinic (insulating) phase.No sign of any parasitic phase has been observed within the XRD limit [29].
The room temperature Raman spectrum of VO 2 films deposited on SOI substrate is shown in Fig. 1(b).All the main peaks of the Raman spectrum are indexed well with Raman active mode of the monoclinic (insulating) phase of VO 2 without any other impurity phase in the system [28].The observed room temperature Raman spectrum is in full agreement with previous reports in the literature.The peak at 532 cm −1 corresponds to substrate peak.It should be noted that the Raman modes slightly shift to the higher frequency (1 cm −1 ), which can be attributed to the strain induced by the significant lattice mismatch between film and substrate.To check the evolution of the VO 2 phase, we have carried out Raman measurement at various temperature across the metal-insulator transition.The representative temperature-dependent Raman spectra for VO 2 thin film is shown in Fig. 1(c).The two low-frequency peak 194 cm −1 (Ag(1)) and 225 cm −1 (Ag(2)) are ascribed to the V-ions motion within the V-V chains, and the high-frequency peak 617 cm −1 (Ag (7)) corresponds to the V-O vibration mode.All the Raman modes broaden and weaken with increasing temperature until reaching the transition temperature.Above the transition temperature, all the Raman peaks disappear, which indicates the transition of VO 2 thin film from monoclinic M1 phase to rutile phase.The transition temperature obtained by Raman and electrical transport is the same.
The temperature-dependent electrical resistance of the VO 2 layer during heating and cooling are shown in Fig. 2(a).The resistance measurements are performed using a two-point probe method and the resistance of VO 2 thin films changes from 8.3 MΩ (insulating phase) to 7.3 kΩ (metallic phase).As clearly seen from Fig. 2(a), the film shows a three order change in electrical resistance across the transition.A thermal hysteresis behaviour has been observed upon heating and cooling, which is a typical feature of VO 2 .The metal-insulator transition temperature (T IMT or T MIT ) is obtained from the first derivative of electrical resistivity with respect to temperature, shown in Fig. 2(b).The transition temperature of VO 2 layer during heating and cooling cycle is 82 • C and 70 • C, respectively.The width of thermal hysteresis is around 12 • C. The slightly larger hysteresis is attributed to the polycrystalline nature of the thin film.The metal-insulator transition in VO 2 is accompanied by a substantial change in the refractive index.The refractive index of the VO 2 layer is measured using spectroscopic ellipsometry (250 to 1000 nm).The measurements are done at various angles (65 • -75 • ) to build a material model to extract the optical constants.Temperature dependent ellipsometer measurement is performed to analyze the optical constant evolution during heating and cooling cycle.In the dielectric phase, the optical constants are fit to a Lorentzian model whereas, in the metallic phase, Drude contribution is also included with Lorentz to model VO 2 absorption spectrum [30].Figure 3 shows the refractive index (n and k) of a 70 nm thick film on SOI substrate.Refractive index profile exhibits hysteresis of phase transition similar to electrical conductivity.When T>T MIT , n reduces and k increases and vice-versa when T<T IMT .The change in temperature-dependent optical parameters ∆n and ∆k at 1000 nm is 1.93 and 0.77, respectively.The thickness for the VO 2 film is verified using step height measurement.

Design and fabrication overview
The device is fabricated on an SOI substrate with 220 nm thick silicon and 2 µm buried oxide.A planarized shallow etched rib configuration is used to define waveguides, ring resonator, and Mach-Zehnder interferometer.
Figure 4(a-d) shows the fabrication process flow overview.Figure 4(a) shows the cross-section schematic of a planarized rib waveguide [31].On top of the rib waveguide, a 40 nm thick SiO 2 is deposited using plasma-enhanced chemical vapor deposition process (Fig. 4(b)).Following SiO 2 deposition, a 70 nm thick VO 2 film is deposited using V 2 O 5 as PLD target material.A substrate temperature of 580 • C is maintained during deposition at 10 mTorr oxygen pressure [27] (Fig. 4(c)).The sample is then spin-coated with photoresist and patterned using direct laser writer.VO 2 patches on top of the device are etched using reactive ion etching process by a mixture of Ar and Cl gas followed by wet etching using a mixture of perchloric acid and ceric ammonium nitrate to soft-land on SiO 2 (Fig. 4)).Combination of dry etch followed by a wet etch process avoids damage to underlying device.Furthermore, lateral etch of VO 2 due to wet etch process is reduced by this two-step etch.The oxide liner on top of the planarized waveguide also help in protecting the underlying device and also helps in preventing metal diffusion into silicon that could potentially result in higher insertion loss due to absorption in the waveguide.For light in and out-coupling focused grating couplers were used.Figure 5(a) shows temperature-dependent transmission spectrum of a ring resonator with VO 2 -patch.The transmission through the device is normalized to reference waveguide to extract the insertion loss.As explained in Section. 2 with increasing in temperature, VO 2 undergoes transition from dielectric to metallic phase.The transition increases loss in the ring cavity resulting in a resonance extinction reduction.In the initial dielectric phase, the ring is observed to be in a critical-coupled regime, the transition to the metallic phase pushes the ring into under-coupled regime reducing the quality factor and extinction of the resonance.The evolution is measured for both heating and cooling cycle which is presented in Fig. 5(b).The loss in the system is calculated from the spectral measurement using the following relation,

Substrate heating
where n g is the group index, L is the geometric path length, λ res is the resonance wavelength, and Q ring is the quality factor of the ring resonator.The temperature-dependent extinction ratio and loss are plotted in Fig. 5(b) and 5(c), respectively.It is apparent that the extinction ratio change is 25 dB and change in loss is around 0.4 dB across the phase transition.Similar to temperature-dependent electrical conductivity measurement, a thermal hysteresis has been observed in extinction ratio and loss during the heating and cooling cycle.Figure 5 shows the hysteresis and transition temperature .We observe a hysteresis of ≈25 • extracted from the device spectral response while thin-film hysteresis measured form the electrical resistance and refractive index change (Fig. 2 and 3) shows a narrow hysteresis width of ≈ 12 • , which is 13 • lower than the hysteresis width measured using a ring resonator.A broader hysteresis width can be attributed to the thermo-optic interplay between Si and VO 2 .Silicon has a positive thermo-optic coefficient dn/dT = 1.8 × 10 −4 /K.For T>T MIT , shift in spectrum due to VO 2 and silicon is opposite in nature.It can be observed in Fig. 5(a) that the spectrum is only red-shifted dominated by silicon layer.In order to negate the substrate effect and probe the effect of VO 2 layer alone, a local heated configuration is necessary, which is presented in the following section.

Lateral heating
Figure 6(a) and 6(b) shows the schematic of the cross-section and microscope image of a final device with lateral heaters.The lateral micro-strip heater would allow local heating of VO 2 .The length of the VO 2 patch is 19 µm long.The heater is designed to be 3 µm wide and 20 µm long placed 5 µm away from the waveguide.Titanium/Platinum (10/90 nm) is used as the heater material stack.The heater stack is deposited using sputter deposition.A similar experimental setup as explained in Section.3.2 is used while the VO 2 is locally heated by applying a current through the micro-strip heater.
Figure 6(c) shows the transmission spectrum of the micro-ring resonator with increasing heater power.We measure an insertion loss of 1.4 dB across IMT regime, which is one of the lowest reported [18].The IMT is achieved by locally heating the VO 2 using lateral micro-heaters.As the temperature increases, the change in refractive index of the material results in a spectral change in the ring response across the phase transition.We observe a blue shift in the spectrum when the heater power exceeds 20 mW accompanied with reduction in extinction ratio.Figure 6(d) shows the extinction ratio and wavelength shift extracted from the transmission spectrum.The change in refractive index in VO 2 can be calculated by using effective index change from Eq. ( 2) [32], since the equivalent thermo-optic coefficient (TOC) of a waveguide system (dn eff /dT) is a superposition of the TOC of the individual materials, which is given by, Here, Γ i is the confinement factor in i th material and dn i /dT is the TOC of the material.A TOC of 1.8 × 10 −4 / • C is used for Si in the calculation.The confinement factor in silicon and VO 2 is obtained using finite difference modal simulation.Since the refractive index of the VO 2 changes across T MIT , the confinement factor in the waveguide and VO 2 changes.The change in the effective index of the propagating mode in the waveguide, ∆n eff is calculated from the spectral shift using Eq. 3 [33].
where n eff is the effective index of the waveguide, R is the radius of the ring (35 µm), ∆λ is the measured wavelength shift from the spectrum, λ 0 is the centre wavelength of the ring without VO 2 , and L is the length of the VO 2 patch (19 µm).
From the spectral characteristics, the ring resonator with dielectric VO 2 in critically coupled regime; high-quality factor and extinction.As the temperature is increased using strip-heater, at lower heater power, we observe a slight red-shift in the spectrum due to thermo-optic shift from Si.However, at the onset of transition, the loss in the cavity increases due to metallic phase of VO 2 that reduces the extinction along with appreciable blue-shift in the spectrum due to reduction in the effective refractive index as shown in Fig. 3. From the measurements, we estimate a refractive index change of 1.2, which agrees well with the literature report [14].Furthermore, we estimate cavity loss increase from 0.96 dB to 2.79 dB during the insulator to metallic transition, respectively.

Time domain measurement
The switching behaviour and performance of the device are evaluated using a symmetric Mach-Zehnder Interferometer (MZI).Figure 7(a) shows a microscope image of a symmetric MZI with a 30 µm VO 2 patch on the balanced arms.The phase transition is driven by a micro-strip heater as mentioned in the previous section 3.3.Figure 7(b) shows the transmission spectrum of the MZI at various heater power.For heater powers >126 mW, the phase transition from insulating-to-metallic state starts.Figure 7(c) shows the insertion loss at various heater power.A maximum loss of 25 dB is achieved at a heater power of 168 mW.
The dynamic response of the device is characterized by driving both the arms using a 10 KHz square waveform.Though a maximum transmission extinction of 25 dB is achieved through DC drive, the maximum speed of switching depends on the operating point of the material.Furthermore, the size of the VO 2 patch also plays a crucial role in switching speed.In order to obtain the best performance of the device, it is biased near the dielectric phase, and the amplitude of the voltage swing is varied from 1 V pp to 4 V pp .Figure 7(d) shows the modulated optical output for a fixed heater power (DC) with variable voltage swing (AC).
The raising edge represents MIT, and falling edge indicates IMT.With an increase in the drive voltage, we observe an increase in the modulation extinction until 3.5 V pp and falls beyond.Furthermore, beyond 3 V pp the insertion loss increases.The change in the modulation extinction and insertion loss can be attributed to the phase transition due to the drive voltage swing.At lower drive voltages the transition is not fully metallic resulting in lower modulation and insertion loss.However, at higher drive voltages, the transition is pushed deep into metallic resulting in higher insertion loss and transition as well.The transition characteristics also affect the phase transition time as well.With increasing drive voltage, the rise time and fall time increases from 2.15 µs to 13.2 µs and 1.39 µs to 7.49 µs, respectively.It can also be seen that IMT is quicker than MIT transition.The experiment is repeated by fixing the voltage swing of 1.5 V pp , and the bias voltage is varied between 1 V and 2.5 V.For a particular bias voltage, the material's phase is fixed and the time constants related to each bias point is calculated.Figure 7(e) shows the modulated optical output for fixed voltage swing and variable heater power.It can be seen that as the bias voltage increases, the insertion loss of the device increases.After the phase transition (DC>2 V) the insertion loss increases and the optical modulation reduces which clearly shows that bias point has to be chosen near the dielectric phase.Considering the metrics of insertion loss and optical modulation, bias voltage of 1.5 V is the optimum operation point and the obtained rise time and fall time are 4.06 µs and 2.92 µs, respectively.
The optimal operating point depends on various factors such as material quality, hysteresis width, VO 2 patch dimensions and heater efficiency.However, in this work, we have demonstrated a schema to identify optimal operating point to achieve desired performance from a phase-change material based optical switch.

Conclusion
We presented an electrical and optical phase transition of PLD deposited VO 2 film on SOI.We demonstrated complete phase change behaviour of VO 2 and utilized it as an optical switch.We have achieved a maximum transmission extinction of 25 dB.We also measured and verified the refractive index change due to the phase transition.By using a micro-strip heater we demonstrated a maximum transmission extinction of 25 dB with an insertion loss 1.4 dB.We also analyzed the switching performance of an MZI-based device by obtaining the optimum heater power and the voltage swing.We presented a procedure to identify a suitable operating point to achieve maximum switching speed with maximum extinction and minimum insertion loss.We report a maximum switching speed of 118 kHz, which can be improved by reducing the size the VO 2 interaction length, however, with a trade-off of achievable modulation extinction.

Fig. 1 .
Fig. 1.Material characterization summary of VO 2 .(a) XRD pattern of VO 2 thin film on SOI substrate, (b) Raman spectrum of VO 2 at room temperature, and (c) Raman spectrum at various temperature.

Fig. 2 .
Fig. 2. Summary of electrical characterization of VO 2 , (a) Temperature dependent electrical resistance measurement during heating and cooling cycle across the transition of the studied device, (b) First derivative of electrical resistance with respect to temperature during the heating and cooling cycle.

Fig. 3 .
Fig. 3. Temperature dependent refractive index measurement of VO 2 at wavelength of 1000 nm using spectroscopic ellipsometry (a) n and (b) k.

Fig. 4 .
Fig. 4. (a-d) Fabrication process flow of VO 2 modulator, and (e) Microscope image of the ring resonator with VO 2 tab.

Figure 4 (
Figure4(e) shows the optical microscope image of the fabricated device.Optical characterization of the fabricated device is done using a tunable laser source (1510-1630 nm) and InGaAs photodetector.The light is coupled in and out through focused grating coupler fabricated on the Si device layer of SOI substrate.To perform temperature dependant measurement, a temperature-controlled sample stage was used.All the measurements were performed after the temperature is stabilized at a set temperature.Figure5(a) shows temperature-dependent transmission spectrum of a ring resonator with VO 2 -patch.The transmission through the device is normalized to reference waveguide to extract the insertion loss.As explained in Section. 2 with increasing in temperature, VO 2 undergoes transition from dielectric to metallic phase.The transition increases loss in the ring cavity resulting in a resonance extinction reduction.In the initial dielectric phase, the ring is observed to be in a critical-coupled regime, the transition to the metallic phase pushes the ring into under-coupled regime reducing the quality factor and extinction of the resonance.The evolution is measured for both heating and cooling cycle which is presented in Fig.5(b).The loss in the system is calculated from the spectral measurement using the following relation,

Fig. 5 .
Fig. 5. Summary of spectral characteristics of a substrate heated ring resonator with VO 2 patch.(a) Transmission spectrum of the ring resonator at various temperature, (b) Extinction ratio, (c) Cavity loss, and (d) d(Extinction)/dT during heating and cooling cycle of the substrate.The solid line through the points are provided to guide the eyes.

Fig. 6 .
Fig. 6.Summary of spectral characteristics of a ring resonator with a lateral heater.a) Schematic of a device with lateral heaters, b) Microscope image of a ring resonator with VO 2 patch, c) Transmission spectrum of a resonator with increasing heater power, and d) Wavelength shift and extinction ratio evolution with increasing heater power.The solid line through the points are provided to guide the eyes.

Fig. 7 .
Fig. 7. (a) Microscope image of the Mach-Zehnder interferometer with VO 2 tabs in both the arms, (b) Transmission spectrum of the symmetric Mach-Zehnder interferometer with varying heater power, (c) Insertion loss of the device with varying heater power, and Time domain measurement of the VO 2 based optical modulator with (d) fixed heater power and variable voltage swing and (e) fixed voltage swing and variable heater power.The solid line through the points are provided to guide the eyes.