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Abstract
The performance of optical devices relying in vanadium dioxide (VO2) technology compatible with the silicon platform depends on the polarization of light and VO2 properties. In this work, optical switching in hybrid VO2/Si waveguides thermally triggered by lateral microheaters is achieved with insertion losses below 1 dB and extinction ratios above 20 dB in a broad bandwidth larger than 30 nm. The optical switching response has been optimized for TE and TM polarizations by using a homogeneous and a granular VO2 layer, respectively, with a small impact on the electrical power consumption. The stability and reversibility between switching states showing the possibility of bistable performance is also demonstrated.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The silicon photonics platform combined with disruptive materials will provide hybrid technologies able to overcome the challenging requirements in current photonic systems [1–3]. Several phase transition materials have been proposed as active materials to enhance the performance of optical devices based on silicon photonics. Among them, vanadium dioxide (VO2) has reached an outstanding performance due to its reversible and sharp insulator-metal transition (IMT) near room temperature [4–6]. Remarkable changes in electrical and optical properties of the active material can be obtained when controlling the phase transition by means of external stimuli like temperature [7–9], electric field [10–17] or optical pumping [18-20]. The change in the optical properties of the material is useful for the design of ultra-compact optical devices based on the hybrid VO2/Si technology [21–27]. Nevertheless, the phase transition in VO2 is still the subject of intense discussion. Several works in the literature have studied the IMT in VO2 induced by applying an external electric field to elucidate whether the mechanism of the transition is related to solely a Joule-heating process [8, 28, 29], to the electric field contribution [15, 30–32] or a combination of both effects. Furthermore, the structural properties of the VO2 layer seem to have a key role on the phase transition. It has been shown that crystalline quality, sample morphology and surface roughness are important characteristics influencing the hysteresis cycle [33, 34]. Polycrystalline and epitaxial layers have been grown using different deposition techniques [34–37], however, controlling process conditions is key to obtain good film quality.
In this work, optical switching is demonstrated in a hybrid VO2/Si waveguide by means of lateral resistive microheaters as shown in Fig. 1. The microheaters are based on a double metallization process to efficiently focus the generated heat on the hybrid waveguide [38]. Thus, the VO2 layer on top of the silicon waveguide is switched from the insulating to the metallic state, changing significantly the optical losses in a very short length. Furthermore, the effect of the morphology of the VO2 layers on the optical switching performance has also been experimentally analyzed and demonstrated to achieve a similar behavior for both light polarizations independently of the structural properties of the VO2 layer. An electrical power consumption for a complete VO2 phase transition ranging between 30 mW and 85 mW has been achieved, which is more than one order of magnitude lower compared to previous works based also on a Joule heating approach [26]. Lower power consumptions have been reported by triggering the VO2 phase transition by a carrier injection approach but at expenses of higher insertion losses and lower extinction ratios [14, 22].
Fig. 1 Concept art of the hybrid VO2/Si waveguide with a lateral Ti microheater based on a double metallization process. The inset shows the cross section of the hybrid VO2/Si waveguide. The silicon waveguide has a width of 480 nm and a thickness of 220 nm. The inset shows the spacer between the silicon waveguide and the VO2 layer that is made of a 10 nm-thick oxide layer plus a 50 nm-thick nitride (SiN) hardmask. The SiN layer is needed for planarization and protection of the silicon surface.
2. Description of the hybrid VO2/Si waveguide
A concept art of the hybrid VO2/Si waveguide with a lateral microheater is shown in Fig. 1. The silicon waveguide has a cross section of 480 nm x 220 nm. A VO2 patch with a length of 20 µm is deposited to form the hybrid waveguide. The spacer between the silicon waveguide and the VO2 layer is made of a 10 nm-thick oxide layer plus a 50 nm-thick nitride (SiN) hardmask. The SiN layer is needed for planarization and protection of the silicon surface. The VO2 layer is also extended below the heater as depicted in Fig. 1. The heater is based on a double metallization process with a lateral displacement of 550 nm to avoid optical losses due to the proximity of the metals to the hybrid waveguide. A voltage is applied on the low resistance AlCu pads, so that the current is mainly determined by the Ti section, which short-circuits the pads. Therefore, the heat is efficiently transferred to the VO2 patch placed over the waveguide, minimizing the electrical power consumption.
The optical losses of the hybrid VO2/Si waveguide depend on the state of the VO2. At steady conditions, the VO2 is in the insulating state and the material introduces a low level of optical losses. When an electrical power is applied to the heater, a temperature gradient determines the VO2 areas that will switch to the metallic state. By applying sufficient electrical power, the generated heat is high enough to induce the phase transition in the whole VO2 placed over the waveguide [26, 28]. Therefore, the optical losses are drastically increased and an optical switching with high extinction ratio is achieved.
The described hybrid VO2/Si waveguides have been fabricated on a SOI wafer with a buried oxide layer thickness of 2 µm. The silicon waveguide structures were first fabricated by a standard process. The wafer was then polished by chemical mechanical planarization (CMP) and diced for VO2 deposition and electrodes patterning. A 40-nm thick amorphous VOx layer has been grown by molecular beam epitaxy (MBE). After the deposition, a lift-off process was done to remove the VOx deposited on the undesired regions. Finally, an ex-situ annealing process is carried out to form a polycrystalline VO2 layer [39]. The electrodes have been fabricated by two e-beam positive resist exposure prior to metal evaporation and lift-off processes [38]. First, a 90 nm-thick Ti was processed and then 400 nm-thick AlCu pads were deposited.
Figure 2(a) shows a SEM image of the fabricated hybrid VO2/Si waveguide with the lateral microheater. Identical structures albeit with different grating couplers were fabricated to test the optical switching performance for both TE and TM polarizations. By changing the processing conditions, the morphology of the VO2 film could be modified such that a granular VO2 layer, instead of a homogeneous one, was obtained. The granular morphology was the result of an additional solvent bath step during the exposure/lift-off processes. Furthermore, the thickness and roughness of the VO2 layer was also measured by Atomic Force Microscopy (AFM) after the electrodes fabrication. A thickness of around 15 nm with a roughness of 10 nm was obtained in the sample with granulated VO2 compared to a thickness of 30 nm with a roughness of 6 nm obtained in the sample with homogeneous VO2. Figure 2(b) shows a zoom-in on the hybrid VO2/Si waveguide with a homogenous VO2 layer while Fig. 2(c) depicts the hybrid waveguide with a granular VO2 layer.
Fig. 2 (a) SEM image of the hybrid VO2/Si waveguide with a lateral microheater. The silicon waveguide contour has been outlined and the VO2 region has been highlighted with a false color for clarity. Zoom over the hybrid VO2/Si area of two different waveguides with (b) homogenous and (c) granular VO2 layer.
3. Experimental results
3.1 Insulator-metal transition of the VO2 active material
The electrical resistivity was first characterized as a function of temperature in VO2 test patches deposited on the same wafers having the hybrid VO2/Si waveguides after ex-situ annealing. The results obtained for the homogenous and granular VO2 are shown in Fig. 3. The insulator to metal phase transition is clearly observed for both cases. The transition occurs at around 75°C for the heating up process while the reversed transition (metal to insulator) occurs at 65°C along the cooling down process, which implies a hysteresis width of 10°C and a transition temperature of 70°C. The single-phase character of the VO2 layer is confirmed by the transition temperature being close to the reported value for bulk samples. However, a higher overall resistivity and a slightly lower resistivity change across the phase transition is achieved in the granular VO2.
Fig. 3 Electrical resistivity of the (a) homogenous and (b) granular VO2 layers as a function of temperature after ex-situ annealing. The one order of magnitude change in resistivity as the temperature increases, indicating an IMT at 70°C, confirms the transformation to VO2 in both samples.
3.2 Optical switching performance of the hybrid VO2/Si waveguides
The optical switching performance of the hybrid VO2/Si waveguides has been characterized by applying a voltage to the electrodes and measuring the variation of the optical power at the output. In addition, the applied electrical power was obtained by simultaneously measuring the electrical current flowing through the electrodes. The light from a continuous-wave (CW) laser was injected into the chip and extracted at the output through the grating couplers. The polarization of the injected light was adjusted with an external polarization controller. The output light was photodetected and measured with a power meter. The electro-optical responses for the different types of VO2 morphologies are shown in Fig. 4 for TE polarization and in Fig. 5 for TM polarization. For both polarizations, it can be clearly seen that the variation of the optical power has a hysteretic response in agreement with the change of the electrical resistivity with temperature observed in Fig. 3, which confirms the existence of the phase transition regardless the morphology of the VO2 film. The transition from the insulating to the metallic state is achieved by heating the VO2 (heating up process) while the VO2 is switched back from the metallic to the insulating state by decreasing the electrical power and therefore decreasing the heat applied to the hybrid waveguide (cooling down process).
Fig. 4 Variation of optical power as a function of the applied electrical power for TE polarization in the hybrid VO2/Si waveguide with (a) homogeneous and (b) granular VO2.
Fig. 5 Variation of optical power as a function of the applied electrical power for TM polarization in the hybrid VO2/Si waveguide with (a) homogeneous and (b) granular VO2.
Nevertheless, marked switching differences can be appreciated depending on the VO2 morphology and the polarization of the light. For TE polarization, the optical losses at steady conditions, i.e. without applying any electrical power, are similar for both hybrid VO2/Si waveguides with homogenous [Fig. 4(a)] and granular [Fig. 4(b)] VO2 layers. However, a much larger variation of the optical power when switching towards the metallic state (>20 dB) and a smaller power consumption is achieved for the case of the structure with the homogenous VO2 layer, which makes this waveguide configuration preferable for TE polarization. The smaller power consumption is attributed to a more efficient heat transfer through the homogenous VO2 film from the heater to the top of the optical waveguide and therefore independent of the light polarization.
The optimum switching structure for TM polarization is the opposite compared to TE polarization. In this case, optical losses at steady conditions are almost 10 dB higher for the hybrid waveguide with the homogenous VO2 layer, Fig. 5(a). Therefore, the waveguide with the granular VO2 layer is more attractive despite the larger power consumption. Furthermore, as there is a stronger interaction of light with the VO2 layer compared to TE polarization, a variation of optical losses above 20 dB is still achieved as it can be seen in Fig. 5(b).
The transmission spectrum of the hybrid VO2/Si waveguide is shown for TE polarization and homogeneous VO2 in Fig. 6(a) and for TM polarization and the granular VO2 in Fig. 6(b). A similar response is obtained for both polarizations showing that changing the VO2 morphology could be an easy and cost-effective approach to achieve hybrid VO2/Si structures with optimized performance for the target polarization. Insertion losses are almost negligible as it can be observed in Fig. 6 when comparing the response of the hybrid waveguide with VO2 in the insulating state with the response of a silicon waveguide without VO2 fabricated on the same sample. It should be noticed that the effective thickness of the VO2 layer is smaller in the hybrid waveguide with granulated VO2, which also contributes to reduce insertion losses. An extinction ratio higher than 20 dB is achieved for both polarizations when the VO2 is switched to the metallic state with a power consumption of ~45 mW for TE and of ~70 mW for TM. Furthermore, the optical switching bandwidth is maintained very broad (>30 nm) regardless of the morphology of the VO2. The wavelength range shown in Fig. 6 was determined by the bandwidth of the CW laser used in the measurements.
Fig. 6 Transmission spectra of a Si waveguide and the hybrid VO2/Si waveguide (a) with homogeneous VO2 layer and TE polarization and (b) with the granular VO2 and TM polarization. The VO2 metallic state is achieved by applying an electrical power of around (a) 45 mW for TE and (b) 70 mW for TM.
3.3 Temporal switching performance and bistability
The optical switching performance of the hybrid VO2/Si waveguide can be stabilized at any point of the hysteresis cycle just by changing the applied electrical power and, hence, film temperature. The performance of the hybrid waveguide with granular VO2 has been used to analyze if the morphology of the VO2 could have an influence on the temporal response. Measurements have been carried out by supplying electrical voltage pulses with a duration of several seconds on the electrodes. Figure 7(a) shows the switching operation for TM polarization between minimum and maximum optical levels of the hysteresis cycle exploiting the full VO2 phase transition. In this case, the smallest insertion losses and maximum extinction ratio are achieved. Figure 7(b) shows the temporal response of the electrical power (EP) applied to the electrodes and the corresponding normalized optical power (NOP). The switching between states is clearly stable and reversible.
Fig. 7 (a) Scheme of the switching process between maximum (Off) and minimum (On) optical levels and (b) electrical power (EP) of applied voltage pulses as a function of time and the corresponding response of the normalized optical power (NOP). (c) Scheme of the bistable switching performance between State 1 and State 2 and (d) temporal responses of EP and the corresponding NOP for successive reversible switching between both states.
On the other hand, different working points along the hysteresis curve can be chosen to take advantage of the VO2 bistability feature. An example is shown in Fig. 7(c) with the selected points marked in grey. Two different states in terms of optical losses, state 1 and state 2, can be achieved by using the same electrical signal applied to the electrodes (~25 mW). A short voltage pulse is applied to change between both states. The temporal response in terms of EP and NOP is depicted in Fig. 7(d). For switching from state 1 to state 2, the electrical power must be increased to complete the insulator to metal transition. This is achieved by applying a positive electric pulse and therefore increasing the power applied to the electrodes. On the other hand, the switching from state 2 to state 1 is achieved by applying a negative electric pulse such that the electrical power applied to the electrode is zero during the time of the pulse. The fastest switching speed was measured in the microsecond range consistent with a thermal based operation. Furthermore, the same temporal switching performance was also measured for the hybrid waveguide with homogeneous VO2.
4. Conclusions
Thermally triggered optical switching in hybrid VO2/Si waveguides has been proposed and demonstrated by using a lateral microheater based on a double metallization process. Furthermore, the impact of the VO2 layer morphology on the switching performance has been analyzed and exploited to have the best features depending on the light polarization. Therefore, insertion losses below 1 dB and extinction ratios above 20 dB have been achieved with an active length of only 20 µm and an electrical power consumption of around 45 mW for TE polarization and 70 mW for TM polarization. The electro-optical switching response has a broadband performance above 30 nm and is temporally stable and reversible. Furthermore, a bistable switching performance has also been shown by exploiting the hysteretic response of the VO2 phase transition. Such feature could be useful for implementing optical based memristors or microscale memories with optical readout functionality.
Funding
Funding from project TEC2016-76849 (MINECO/FEDER, UE) is acknowledged. The SOI samples were fabricated at IHP (we acknowledge Lars Zimmermann) in the framework of FP7-ICT-2013-11-619456 SITOGA project. Irene Olivares and Roberto Larrea also acknowledge respectively the Universitat Politècnica de València and the Ecuadorian Government for funding their grant. P.H. acknowledges support from Becas Chile-CONICYT.
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