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Abstract
Active metamaterials (MTMs) are artificially engineered structures with tunable and exceptional properties that are absent in natural materials. Recently, InGaZnO (IGZO), a widely used semiconductor for large-area and flexible display backplane drivers, has gained interest for active control of MTMs due to its large-area uniformity, ease of thin film deposition, and low cost. In this paper, IGZO Schottky barrier diodes (SBDs) are proposed to reconfigure electric-field-coupled inductor-capacitor (ELC) MTMs and actively control terahertz (THz) waves for the first time. The SBDs are designed to bridge the capacitors of the ELC resonators so that the average conductivity within the capacitor gap can be modulated by bias voltage while keeping the capacitance value almost unchanged. To precisely simulate this mechanism, two U-shaped resistive sheet models beside the gap are built for IGZO SBD in 3-D simulation for maintaining the same capacitance and resonant frequency. Furthermore, a device with 14400 MTM cells is fabricated and characterized using frequency-domain spectroscopy. The measured transmission shows a continuous modulation from -14.2 to -9.4 dB at 0.39 THz, which corresponds to a modulation depth of 14.3%. This work paves a new way for active THz MTMs using industrial compatible thin-film technology.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
Metamaterials (MTMs) are artificial composite materials with arrays of strongly scattering subwavelength “meta-atoms”, which can manipulate the amplitude, frequency, phase and polarization of electromagnetic (EM) waves [1–3]. With the advantage of the light-matter interaction enhancement and abnormal refractive index that natural materials hardly have, MTMs have unlocked some unprecedented optical phenomena such as electromagnetically-induced-transparency [4], perfect absorption [5], flat-lens [6], invisibility cloak [7], etc. However, the optical properties of most MTMs are not tunable due to their fixed structures, which prevents their development in real-time controllable devices. To dynamically manipulate EM waves, various active MTMs have been reported by incorporating passive MTMs with tunable components in the microwave and optical regions [8,9], enabling advances in holography [10], augmented reality [11], stealth radome [12], and communications [13].
Terahertz (THz) band, falling in the gap between microwave and optical spectra, holds great promise in advanced imaging and communication systems due to its unique properties, such as high penetration, non-destruction, and large bandwidth [14]. Recently, active MTMs at THz frequencies have gained great interest for wireless applications [15–26]. Typically, electrical [15–23], optical [25], and magnetic [26] approaches can be used to actively control THz MTMs, the first of which is most favorable for programmable MTMs and meta-surface due to the compact size and monolithic fabrication as well as multiple and versatile stimuli [15]. Several electro-thermal materials with extreme physical tunability have been studied for active control, such as phase-change materials [16], liquid crystals [17], and superconductors [18], however, whose modulation speed is typically less than several kilohertz. For improved modulation speed, III-V semiconductors with high electron mobility have been proposed to control MTMs with the compensation of high cost in material growth and device fabrication [19–21]. In addition, graphene can provide high modulation speed and relatively low cost for active MTMs [22,23]. However, it is challenging to obtain inch-scale uniform film for MTM arrays. Complementary metal oxide semiconductors (CMOS) have also been investigated to actively modulate THz MTMs with high uniformity in inch-scale size. However, their modulation depth is limited to several percent, and the CMOS fabrication is expensive [24]. In sum, novel approaches with high uniformity, reasonable modulation depth, and low cost need to be developed for several inch-scale and flexible active MTMs at THz frequencies.
InGaZnO (IGZO) is an optical transparent metal-oxide semiconductor widely used for large-area displays and flexible electronics due to its low cost, high yield, high uniformity, flexibility, and room-temperature process [27]. Typically, the operating frequency of IGZO devices, such as transistors [28] and diodes [29], is less than 6 GHz due to its low electron mobility. In 2016, IGZO transistor was reported for active MTMs with a modulation range from -25 dB to -22 dB at THz frequencies [30]. The corresponding modulation depth is as low as 0.3%. In 2019, our previous work demonstrates that IGZO Schottky barrier diodes (SBDs) significantly modulate spoof surface plasmon polaritons at 49 GHz [31], revealing the potential of IGZO SBD for active MTMs at THz frequencies.
In this paper, we first demonstrate reconfiguration of MTMs with IGZO SBDs at 0.388 THz, which possess a continuous amplitude modulation of 13% for the transmission of incident waves. The proposed device is composed of an electric-field-coupled inductor-capacitor (ELC) resonator array, IGZO SBDs bridging the ELC capacitors, and bias lines. The SBD actively controls the conductivity of IGZO film within the capacitor gap, which typically is equivalent to a tunable resistor parallel to the capacitor in the ELC gap. Thus, a resistive sheet model is carefully designed for the IGZO film without changing the capacitance of the MTMs in finite-element-method (FEM) simulation, achieving active modulation of permittivity and consistent resonant frequency. To verify this design, 14400 MTM cells were fabricated and characterized under various bias. The resonant magnitude is modulated by 14.3% at 0.39 THz, showing good agreement with the simulation.
2. Design and simulation
ELC resonator is composed of two spilt ring resonators sharing the same capacitor, which achieves negative permittivity at its resonant frequency [32]. Figure 1(a) illustrates the proposed MTM cell, which consists of a conventional ELC resonator and two parallel metallic lines for anode and cathode bias. To mitigate the dielectric loss at THz frequencies, the ELC resonator is designed on a 200-μm thick silicon substrate with a high resistivity of 10000 Ω·mm. Ansys High Frequency Structural Simulator (HFSS) was employed to simulate and optimize the ELC MTMs. The detailed dimensions of an ELC resonator at 0.39 THz are listed in the caption of Fig. 1. The width of the anode line and its distance to the ELC resonator are carefully designed to minimize bias voltage for SBDs with little impact to ELC resonance. As illustrated in Fig. 1(b), the resonant frequency red shifts from 0.393 to 0.388 THz with the bias lines due to the parasitic inductance. The ELC resonator with bias lines has a maximum transmission attenuation of 20 dB and a Q-factor of 46 at 0.388 THz. The other ripples in the spectrum are induced by the interference of the silicon substrate [33]. Figure 1(c) illustrates the H-field and E-field distributions of the MTM cell at the resonant frequency, i.e., 0.388 THz, and non-resonant frequencies, e.g., 0.35 and 0.43 THz. It can be seen that the electrical fields are mainly localized in the capacitor, and the magnetic fields are mainly distributed along the metallic ring at the resonant frequency. A weak capacitive coupling between the ELC resonator and the anode line is also observed, which is negligible at the resonant frequency. The surface current density vector for resonance are depicted in Fig. 1(d), where the induced circulating current from the bias line is not significant.
Fig. 1. (a) A passive ELC MTM cell (Wsub=120 μm, Tsub=200 μm, Lring=66 μm, Wgap=3 μm, Lgap=24 μm, Ws1=7 μm, Ws2=10 μm, Wd=7 μm). (b) Simulated S-parameters of the MTM cell with and without bias lines. The inset shows a conventional ELC MTM cell without bias lines. (c) H-field and E-field distributions and (d) surface current density vector of the proposed MTM cell.
Figure 2(a) illustrates an active ELC resonator with an IGZO SBD and its equivalent circuit. Typically, the mechanism of the ELC resonator can be explained as a simplified inductive-capacitive circuit model [19]. As can be found in Fig. 2(a), L is the cumulative inductance of a single split ring resonator, C is the capacitance of the capacitor, and Rg is the active resistance between the two electrodes of the capacitor. The corresponding resonant frequency can be expressed as
The SBD exhibits asymmetric current-voltage response at positive and negative bias and large on-off ratio due to the presence of Schottky contact [35]. In the proposed device, it can be equivalent as a tunable resistor parallel to the ELC capacitor by using Rg in Fig. 2(a). Rg decreases as the positive bias increases, which implies the average conductivity of the SBD enlarges. As the conductivity enlarges, the free carriers with increasing density gradually short out the ELC capacitor and thus damp the resonance [31]. Our previous work reports that a planar IGZO SBD has quasi-constant capacitance under a forward bias, and the average conductivity of an IGZO film can be tuned from 7.9×10−6 to 0.23 S/m [31]. To precisely model the device with a fixed capacitance, two U-shaped resistive sheets are designed to connect the capacitor electrodes from both lateral sides in the HFSS simulation, as depicted in Fig. 2(b). By changing the film conductivity σ, the SBD reconfigures the ELC resonator and maintains the same resonant frequency, as shown in Fig. 2(c). When the average conductivity is 7.9 ×10−6 S/m, the resonant magnitude is -14.5 dB, which corresponds to an “off” state for the SBD. As the conductivity increases, the resistive sheets gradually short out the ELC capacitor and thus damp the resonance. The transmission can be modulated from -14.5 to -9.9 dB at 0.388 THz with little change of the resonant frequency.Fig. 2. (a) An active MTM cell with an IGZO SBD and its equivalent circuit model. (b) The simulation model with little capacitance change (Wr1=12 μm, Wr2=4.8 μm, Lr1=7.2 μm, Lr2=2.4 μm). (c) Simulated transmission of the active MTMs with different film conductivities. (d) Real effective permittivity of the proposed MTMs cell.
To gain further insight, the effective permittivity is extracted using S-parameters inversion method [34], the real of which is illustrated in Fig. 2(d). For the conventional ELC MTMs without bias line, the effective permittivity sharply switches from 4.513 to -6.291 at 0.388 THz, which corresponds to the ELC resonance. Due to the parasitic inductance of the cathode line, the proposed MTM device red-shifts the switching frequency from 0.388 THz to 0.385 THz. The minimum permittivity decreases to -12.06. With the IGZO films of various conductivity, the maximum negative permittivity gradually changes from -8.559 to -3.456 at 0.388 THz without any frequency shifting. The permeability shows little change at 0.388 THz.
3. Experiment and discussion
To verify the aforementioned design, an active MTM device was fabricated on a high-impedance silicon substrate with a thickness of 200 μm and a 100-nm SiO2 insulating layer, as shown in Fig. 3(a). 14400 MTM cells are patterned in an area of 14.4×14.4 mm2. The corresponding fabrication process is depicted in Fig. 3(b). First, the MTM array with cathode lines and bias pad are fabricated with 10 nm titanium, 190 nm gold and 50 nm palladium, which is the Schottky electrodes for the IGZO SBDs. Then, a row of anode lines connected to another bias pad are fabricated with 10 nm titanium and 100 nm gold as the ohmic contact. Before growing IGZO film, the exposed palladium film was oxidized with oxygen plasma for 20 minutes to improve the quality of Schottky contact [35]. Finally, radio-frequency magnetron sputtering was used to deposit a 500-nm thick IGZO film at room temperature with a mixed gas of O2/Ar (1:39) and a ceramic target of In:Ga:Zn=1:1:1 in atomic ratio.
Fig. 3. (a) Top view of the fabricated active MTM device with IGZO SBDs. (b) Fabrication process of the MTM device.
The fabricated device was characterized with Toptica THz frequency-domain spectroscopy (FDS) TeraScan 1550 driven by two continuous wave lasers [36], as shown in Fig. 4(a). The device under test (DUT) was placed in the middle of the collimated optical path with a diameter of about 2 mm. The measured envelope photocurrents of air, silicon wafer, and our passive MTM devices are illustrated in Fig. 4(b) with a resolution of 30 MHz. By comparing the envelope photocurrents of the DUT to the air, the measured transmission of the silicon wafer and our MTM device are obtained in Fig. 4(c). The simulated and measured data have a good agreement for both the silicon and the proposed MTMs. The resonance of the passive MTM is at 0.405 THz, which induces a significant attenuation of 18.5 dB for the transmission. Compared to the simulation, the resonant frequency has a blue-shift of 17 GHz due to the manufacturing tolerance.
Fig. 4. (a) Experiment setup. (b) Measured envelope photocurrents as a function of frequency. (c) Measured and simulated transmission curves of a silicon wafer and the passive MTMs.
The measured transmission curves of the fabricated active MTMs at various bias are illustrated in Fig. 5(a). At zero bias, a strong resonance occurs at 0.39 THz, and the transmission attenuation is as high as 14.2 dB, which agrees well with the simulated data at a conductivity of 7.9 ×10−6 S/m for the IGZO film. Compared to the device without SBDs, the IGZO film slightly reduces the resonant magnitude and red-shifts the resonant frequency by 15 GHz, which is mainly attributed to the permittivity of the IGZO film. As the forward bias varies from 3 to 10 V, the transmission increases from -13.5 to -9.4 dB, showing reasonable match with the simulated data in Fig. 2(c). It should be noted that no frequency shifting is observed at different bias, which demonstrates the accuracy of our circuit model and 3-D simulation. At a reverse bias of 5 V, the transmission is almost the same as that at zero bias, revealing little conductivity increase for the IGZO film. The normalized transmission is illustrated in Fig. 5(b), where applying forward bias increases the transmission by a factor of 14.3% at 0.39 THz. The inset of Fig. 5(b) depicts an asymmetric diode response in terms of transmission amplitude versus bias, which is consistent with the I-V curve of an SBD.
Fig. 5. Amplitude modulation of the fabricated active THz MTMs device. (a) Measured transmission curves at different bias. The inset shows the measurement setup. (b) Normalized transmission curves at different bias. The inset shows the transmission versus bias voltage and the current-voltage relation of an SBD. (c) Simulated and Measured differential transmission. (d) Compare with the reported large-area active MTMs.
To further clarify the modulation capability, the differential transmission defined as ΔT/T0 are depicted in Fig. 5(c). Here, T0 is the transmission at zero bias, and ΔT is the transmission discrepancy with respect to T0. As the forward bias increases from 3 V to 10 V, the differential transmission enhances from 0.107 to 0.65 at 0.39 THz, showing good consistency between the measured and simulated data. Figure 5(d) compares the modulation depth and relative frequency shift of our device with the reported active MTMs potentially for large-area applications [24,30]. The relative frequency shift is defined as ΔF/F, where ΔF is the shifting frequency, and F is the resonant frequency with zero bias. Our device presents the largest modulation range and smallest frequency shifting, which is suitable for amplitude modulation of active MTMs at THz frequencies. We anticipate reasonable improvement for the modulation depth by increasing the In:Ga ratio in the sputtering target [29,30]. Additionally, annealing and passivation may be utilized to improve the average conductivity of the IGZO film, thus further increasing the modulation capability [35].
4. Conclusion
In summary, monolithic IGZO SBDs have been demonstrated to actively modulate ELC MTMs at 0.39 THz. The SBD fabricated in the MTM capacitor can actively tune the average conductivity of the IGZO film, and thus reconfigures the ELC resonator. A modulation range of 14.3% was demonstrated for the transmission with little frequency shifting. This work provides a new path for active and programmable MTMs and meta-surface at THz frequencies, particularly for large-area and flexible applications.
Funding
China Postdoctoral Science Foundation (2017M622201, 2018T110689); National Key Research and Development Program of China (2016YFA0201800, 2016YFA0301200); National Natural Science Foundation of China (61701283); Engineering and Physical Sciences Research Council (EP/N021258/1); Key Technology Research and Development Program of Shandong (2019JZZY020109); Postdoctoral Innovation Program of Shandong Province (20171006).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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