We demonstrate an electric control metamaterials-high electron mobility transistors (HEMTs) integrated terahertz (THz) modulator whose switching ability is developed by utilizing the symmetric quadruple-split-ring resonators (SRRs) metamaterial configuration and operating voltage is reduced by incorporating the HEMT elements. Meanwhile, the high switching speed of the HEMT implies that the THz modulator has a high potential in modulation speed. Under a reverse gate voltage of −4 V, the THz modulator exhibits a modulation depth of 80% at 0.86 THz and a phase shift of 0.67 rad (38.4°) at 0.77 THz, respectively. In addition, a modulation speed over 2.7 MHz is achieved and an improvement in the modulation speed of hundreds of MHz with optimum RC time constant is expected to achieve for the THz modulator.
© 2017 Optical Society of America
Over the past decades, terahertz (THz) waves have attracted worldwide attentions and led to intense theoretical and experimental studies for potential applications in spectroscopy [1,2], wireless communication [3–7], imaging , and other fields due to its unique properties such as excellent coherence, low photon energy, etc. As essential components of many THz systems, THz modulators used for active adaptive control of the THz waves are highly required. However, there are nearly no naturally occurring materials existing for the efficient construction of THz modulator. Thus, the development of metamaterials, a new class of composite artificial materials with the electromagnetic responses that are impossible in naturally occurring materials, offers a promising pathway for the realization of active THz modulators [9–12].
In recent years, the combination of metamaterials and different advanced semiconductor systems has been a popular scheme in the demonstration of THz modulators with different control methods including electric [13–23], optical [13, 14], and thermal  tunings. Obviously, electric control method has a dominant position in practical applications compared with other control methods. By fabricating metamaterials on a lightly doped semiconductor layer that functions as Schottky diodes, both intensity [17–21] and phase modulations [1, 22] for THz waves have been achieved by controlling the carrier concentration of the semiconductor electrically. However, most of such devices are limited by a high operating voltage (~15 V) and a low modulation speed (generally less than 1 MHz), which hinder their applications in practical THz systems. Recently, high electron mobility transistor (HEMT) is found to be a good candidate for design and engineering high speed THz modulators due to its high switching speed characteristic. Since first demonstrated by Shrekenhamer in 2011 , metamaterials-HEMTs integrated THz modulators have been achieved by multiple experts [23, 24]. Despite of their excellent performance in terms of modulation speed, most of such THz modulators possess a low resonance strength which means a limited modulation depth (typically less than 50%). Besides, the unitary metamaterial configurations including symmetry dual-split-ring resonators (SRRs) or dipoles are utilized in these reported THz modulators and there are no reported metamaterials-HEMTs integrated THz modulator schemes applying the multipole-SRRs to generate the LC resonance. Therefore, it is meaningful to incorporate the HEMTs into the multipole-SRRs who have higher resonance depth to achieve high modulation depth, high modulation speed and low operating voltage simultaneously.
In this paper, we demonstrate an electric control metamaterials-HEMTs integrated THz modulator in which quadruple-SRRs are utilized for higher modulation depth. The remarkable intensity modulation about 80% and phase modulation up to 0.67 rad can be achieved under a reverse gate voltage of −4 V. In addition, a modulation speed over 2.7 MHz is realized and is expected to reach hundreds of MHz with a lower RC time constant.
2. Structure design and fabrication
2.1 Structure design
The metamaterials-HEMTs integration is utilized for the electric control THz modulator, in which the HEMTs are implanted into the symmetric quadruple-SRRs based metamaterial structure to achieve the high modulation depth and high modulation speed simultaneously. As schematically depicted in Fig. 1(a), the HEMT is located beneath the split gap while source and drain contacts are shorted by the metamaterials. The length and width of the gate are 1 μm and 10 μm for all the HEMTs, respectively, and the relatively wider length in the gate stripes is chosen for avoiding the gates breaking in fabrication. In addition, the gates are arranged vertically or parallel to the crystal orientation of the GaAs substrate for being compatible with the gate-fabrication processes. Under the normal illumination of THz waves with an electric field as depicted in Fig. 1(a), the wires connecting individual metamaterial elements have little effect on the electromagnetic properties . Moreover, because of the symmetry of the metamaterial element, the THz planar metamaterial structure can cancel all magnetic response and reveal a strong purely electric response . As schematically depicted in Fig. 1(b), by tuning the conductivity (corresponds to RHEMT) beneath the spilt gaps electrically, the LC resonance strength of the matematerial elements can be controlled accordingly and thus the transmitted THz waves can be modulated. With no reverse gate voltage applied, the split gaps will be shorted sufficiently by the high concentration 2DEG, thus the metamaterials show no LC resonance behaviors. When two-dimensional electron gas (2DEG) beneath the gates depleted completely, a strong LC resonance occurs and the electric field is strongly constrained at the split gaps as shown in Fig. 1(c). In the schematic layout of the device (as shown in Fig. 1(d)), it can be observed that all the sources and drains are connected together to the SD pad, and all the gates are connected together to the G pad. The active area with size of 4 mm × 4 mm- consists of 5184 metamaterial elements and 20736 HEMTs in total.
Shown in Fig. 1(e) is the HEMT epitaxial structure employed in this work which was grown by molecular beam epitaxy (MBE) on a 625 μm semi-insulating (SI) (100)-oriented GaAs substrate. The delta-doped double pseudomorphic heterostructure, n-AlGaAs (20 nm)/i-AlGaAs (4 nm)/i-InGaAs (7 nm)/i-AlGaAs (4 nm)/n-AlGaAs (20 nm), is introduced to improve the sheet carrier concentration and electron mobility of 2DEG which are beneficial to the modulation speed and modulation depth of the device. As predicted by the band diagram in Fig. 1(e), the 2DEG is formed in the undoped InGaAs layer as a result of modulation doping effect. Therefore, due to the non-existence of the Coulomb scattering from ionized impurities, the 2DEG has an extraordinary high sheet concentration (2.5 × 1012 cm−2) and electron mobility (5000 cm2/V•s) at room temperature.
2.2 Device fabrication
A standard GaAs technology was used to construct the metamaterials-HEMTs integrated THz modulator which mainly composed of a metal metamaterial layer, a HEMTs array, and a benzocyclobutene (BCB) insulator. Figure 2(a) depicts the schematic cross-section of the fabricated device. Firstly, since only the areas beneath the spilt gaps are essential for the LC resonance of the matematerials, the definition of mesa isolation can greatly reduce the overall capacitance and thus increase the modulation speed of the device. Secondly, Ohmic contacts of the device were formed by electron-beam deposition (EBE) and rapid thermal annealing. Thirdly, Ti/Pt/Au (25/25/300 nm) metal stacks were deposited by EBE in sequence to form the HEMT gates and connect all the gates together. The SEM image shown in Fig. 2(b) indicates that the shapes and sizes of the Ohmic contacts and gates are consistent with expectations. Afterwards, a 1 μm-thick BCB insulator was spinned, followed by etching process to expose the source and drain Ohmic contacts. Subsequently, the metamaterial structure connected with the source and drain Ohmic contacts was patterned by the standard photolithography and then electroplated to be about 2 μm. Finally, to reduce the THz wave absorption, the SI-GaAs substrate thickness was thinned to be 225 μm. The SEM and optical microscope images presented in Fig. 2(c) and 2(d) demonstrate the success of the device fabrication. As schematically depicted in Fig. 2(e), the fabricated device was mounted on a printed circuit board (PCB) with wire connections from the PCB to the device realized by wire bonding. To monitor the fabrication process, the individual HEMT is designed in the mask and its electrical properties are shown in Fig. 3 which reveals a typical HEMT output and transfer characteristics. It is noticed that the drain to source current (Id) drops linearly and the 2DEG is depleted when the reverse gate voltage (Vg) approaches −1.6 V.
3. Results and discussion
3.1 THz transmission properties
The metamaterials-HEMTs device was characterized using a terahertz time domain spectrum (THz-TDS) system at room temperature in a dry air atmosphere. All the experiments were performed at normal incidence and the device was set in such a position that the electric field of incident THz wave was perpendicular to the connecting wires. The transmitted THz waves were first recorded in time-domain and then Fourier-transformed into frequency-domain. The transmission spectra in Fig. 4(b) were obtained by normalizing the transmission spectra with a reference spectrum free of device. As expected from our preliminary considerations, by tuning the 2DEG concentration beneath the gates of the HEMTs with gate voltage, the LC resonance strength of metamaterials varies accordingly, and thus the intensity and phase modulation of transmitted THz waves can be achieved. As depicted clearly in Fig. 4(b), for Vg greater than −0.5 V, there are no significant resonance behaviors for the device because the split gaps are shorted sufficiently by the high concentration 2DEG in the HEMTs. However, with Vg decreasing from −0.5 V to −3 V, the device exhibits a sharp transmission decrease near 0.86 THz, which agrees well with the designed resonance frequency of 0.85 THz. The 2DEG is completely depleted at approximately −3 V, and no distinct change in transmission can be observed with the further reduction of Vg. In order to characterize the switching ability of the device, the modulation depth is defined as:Fig. 4(c). Figure 4(d) shows the modulation depths at 0.86 THz and phase shifts at 0.77 THz under different Vg and it is observed that a maximum modulation depth of 80% and a maximum phase shift of 0.67 rad can be achieved by applying a reverse gate voltage of −4 V.
3.2 Modulation speed test
In order to characterize the modulation speed of the metamaterials-HEMTs integrated THz modulator, current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the device were measured and plotted in Fig. 5. As expected, the device shows a typical Schottky diode I-V characteristic. For the applied gate voltage of −4 V, the reverse saturation current is less than −1.5 mA which means that the depletion region beneath the gates with adequate insulation efficiency is formed. In addition, the equivalent Schottky forward resistance of the device is about 2 Ω. Since the modulation speed is strongly related to the overall capacitance of the device, we further analyzed the C-V characteristic of the device. It is noticed that the voltage-dependent capacitance of the device varies in a similar way to the voltage-dependent modulation depth and phase shift shown in Fig. 4(d) which indicates that operating characteristics of the device could be strictly controlled by varying the capacitance electrically. The modulation speed can be inferred by, where R is the sum of output impedance of the oscillator (50 Ω) and equivalent Schottky forward resistance of the device (2 Ω), and C is the overall capacitance of the device. During the calculation we take C = 1.15 nF, the capacitance of Vg = 0 V which corresponds to the largest capacitance and thereby the smallest modulation speed, and thus the modulation speed of the device is calculated to be 2.7 MHz.
A dynamic frequency response measurement as schematically depicted in Fig. 6 was also performed to characterize the modulation speed of the device. Since the modulation speed of the device is strongly related to the resistance and capacitance, we analyzed the frequency-dependent gain under different resistor R1 connected with the device in series. For this purpose we used the oscillator of FRA 5087 to generate a small frequency sweep sine voltage signal and then measured the voltage response. The voltage response of the device is normalized by voltage response of the whole circuit, thus the device gain with respect to frequency can be expressed as:Eq. (3), the 3-dB bandwidth of the device can be obtained when, thus, the 3-dB bandwidth can be given by. Figure 7(a) shows the device gain under different R1, and it is observed that the 3-dB bandwidth of the device decreases with the increase of R1 which is consistent with the preliminary derivation. The 3-dB bandwidth under different R1 which was extracted from Fig. 7(a) was schematically depicted in Fig. 7(b). Besides, the excellent agreement between the extracted 3-dB bandwidth under different R1 and the RC fitting demonstrates the stability of our measurement. In addition, it can be concluded that the modulation speed of the device is 2.75 MHz which corresponds to the 3-dB bandwidth when R1 = 50 Ω, and the consistency with the preliminary calculation of 2.7 MHz verifies the accuracy of our measurements. Therefore, the presented dynamic frequency response measurement can provides a simple and accurate method to measure the modulation speed of the metamaterials-HEMTs integrated THz modulator.
In conclusion, by the monolithically integration of symmetric quadruple-SRRs based metamaterials with HEMTs, we propose an electric control THz modulator with a remarkable modulation depth of 80% and a phase shift up to 0.67 rad under an applied reverse gate voltage of −4 V. The excellent THz wave modulation performances and low operating voltage make the device a promising candidate for the various applications of THz waves. So far, hindered by the large overall device capacitance from the large amount of HEMTs (over 2 × 104), the modulation speed of the presented device is limited to 2.7 MHz. Further improvement in the modulation speed of hundreds MHz is possible if we take measures to reduce the device capacitance such as reducing the active area and the length of HEMT gates which have little effect on the modulation depth and phase shift. However, the smaller active area and the shorter HEMT gate length increase the beam coupling difficulty and the fabrication complexity, respectively. These tradeoffs can be alleviated by applying the more advanced fabrication and integration technologies.
The authors wish to thank Peng Ding at Institute of Microelectronics of Chinese Academy of Science and Qing Sun at National Institute of Metrology, China for their fruitful discussion and technical assistance in the fabrication and test of the device.
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