Supporting Information for Gbps terahertz external modulator based on a composite metamaterial with a double-channel heterostructure Yaxin Zhang, Shen Qiao*, Shixiong Liang, Zhenhua Wu, Ziqiang Yang*, Zhihong Feng, Han Sun, Yucong Zhou, Linlin Sun, Zhi Chen, Xianbing Zou, Bo Zhang, Jianhao Hu, Shaoqian Li, Qin Chen, Ling Li, Gaiqi Xu, Yuncheng Zhao and Shenggang Liu I. Detailed descriptions of the unit cell Figure S1. Sectional axonometric drawing of the unit cell. The InAlN/AlN/GaN/AlN/GaN double channel heterostructures material is grown on SiC substrate. A thickness of 141 um is adopted for the SiC relative permittivity of 9.8 to ensure the well substrate transmittance at 0.34 THz. The dark red area is isolated by B + ion implantation to achieve independent active area. The source and drain Ohmic contact is prepared using electron-beam (e-beam) evaporated Ti/Al/Ni/Au (20 nm/120 nm/70 nm/100 nm) metallization, followed by rapid thermal annealing in N 2 environment. Then, Ni/Au (50 nm/150 nm) was deposited to form the Schottky gate with a gate length of 500 nm. The device has a gate width of 12 um and a source-drain spacing of 4 um. The metamaterial structure is a symmetrical structure consisting of two dipole units. Each unit is composed of a long rod, a shot rod and a 1
connected rod. The shot rod is aligned on the Ohmic contact electrode. The gate electrode is connected by two 4 um metal wires. The sectional axonometric drawing and dimension parameters of a unit cell are shown in Figure S1. II. MMTM fabrication processes. Fabrication of MMTM started with InAlN/AlN/GaN/AlN/GaN double channel heterostructures material, which was grown on SiC substrates by MOCVD. The InAlN/AlN/GaN/AlN/GaN structures consist of a 15 nm thick Al 0.82 In 0.18 N barrier, 2 nm AlN, 7 nm GaN, 1 nm AlN and about 1.2 um undoped GaN. Hall measurement results showed the 2DEG with sheet carrier concentration of 1.13 10 13 cm -2 and an electron mobility of 1570 cm 2 /Vˑs at room temperature, corresponding to a sheet resistance of 212 Ω/. Transistor fabrication began with the mesa isolation of InAlN/AlN/GaN/AlN/GaN using B + implantation technology. Secondly, source and drain Ohmic contact was formed by E-beam evaporating a multilayered Ti/Al/Ni/Au sequence, after lift-off process followed by a rapid thermal annealing at 800 C for 30 seconds in a N 2 ambient. Contact resistance of 0.41 Ωˑmm was extracted based on the transmission line method (TLM) measurements. Thirdly, the metamaterial array structures and contact pads were patterned by the alignment of optical lithography after the metal layer was evaporated. Fourthly, the Schottky gate was patterned by optical lithography, and the gate metallization was processed with evaporating Ni/Au double layer. Finally, in an effort to avoid current slump and degradation in properties of the fabricated devices as well as to increase performance, the Si 3 N 4 passivation layer was deposited by PECVD, followed by RIE dry etching process, which allowed removal of the dielectric layer existing on top of the PADs. III. Static experiment system setup Figure S2. Static experiment system. (a) THz time-domain spectroscopy (THz-TDS) in a transmission model. (b) 0.34 THz continuous wave (CW) system. Figure S2a shows the THz-TDS (Teraview TPS3000) which is applied in the static experiment of this work. In the THz-TDS, the broadband terahertz waves are generated by mode-locked Ti:sapphire laser which emits near-infrared radiation with 2
sub-100 femtosecond pulse duration at a repetition rate of 80 MHz, a wavelength of 800 nm and has a maximum average output power of 300 mw. The pulsed laser beam was focused onto a biased gallium arsenide antenna to generate THz radiation with a frequency range of 0.06 THz 3 THz. Coherent detection of the terahertz radiation is performed in a similar photoconductive antenna circuit. By gating the photoconductive gap with a femtosecond pulse synchronized to the terahertz emission, a current proportional to the terahertz electric field is measured. The terahertz time domain can be sampled by varying the optical path length to the receiver. Hence both the amplitude and phase of the incident terahertz wave can be obtained, and a dynamic range of >60 db demonstrated using time-gated detection. Figure S2b shows a 0.34 THz continuous wave (CW) system. In this system the THz transmitter produced by V.D.I Company composed of an amplifier multiplier chain and a 25 db gain antenna has been applied to provide a quasi-linear THz wave with 0.34 THz frequency. The Erickson Power Meter (PM4) is used as detector. Such power meter is a calibrated calorimeter-style power meter for 75 GHz to > 2000 GHz applications. The sensor head has an input of WR10 straight waveguide which results in a useful frequency response from 75 GHz to the visible. The modulator is driven by DC power supply GPS-4303C produced by Gwinstek Company to ensure enough current and voltage during the static test. IV. Dynamic experiment system setup Figure S3. Experimental setup of the dynamic test system. 3
Figure S4. The driving circuit of this modulator. The dynamic system is composed of 3 key components: THz transmitter, THz modulator and THz receiver. The system block diagram is shown in Figure S3. The THz transmitter consists of a 0.34 THz amplifier multiplier chain and a 25 db gain antenna which could provide 15 mw CW THz wave. In the real-time dynamic test system, this THz modulator is driven by a sinusoidal voltage which is generated by the amplified bias circuit. This circuity mainly composed of an amplifier (Mini-circuits TIA-1000-1R8-2, Freq., 50 khz-1000 MHz; Gain 35 db; Output Power at 1dB comp., 32 dbm.) and a DC bias for THz modulator. The input modulating signal (a typical sinusoidal baseband signal) is generated by Agilent E8267D. The power supply DC1 provides the DC bias for the amplifier and DC2 provides the DC bias for the modulator around 4 V. The receiver consists of a signal generator, a 0.34 THz mixer amplifier multiplier chain, an oscilloscope and a 25 db gain antenna. The multiplied LO signal is mixed with the RF terahertz wave. In this test system, we used zero intermediate frequency (IF) detection. The THz transmitter and receiver used the same local oscillator (LO) to make sure the IF=0 GHz. In the first step of the experiment, a 0.34 THz carrier wave with nearly linear polarization was generated from the THz transmitter. In the second step, 4 V V P-P sine voltage modulation signal with different modulation speed was loaded on the modulator to realize real-time dynamic control of this modulator. Meanwhile, the 0.34 THz wave is normal incident to the modulator, resulting in the sine amplitude modulation signal loaded on the carrier wave after the THz wave across the modulator. Next, the modulated THz waves were converged to parallel light by a super lens and received by another lens. In the last step, zero IF detection applied by the THz receiver is carried out to detect the sine amplitude modulation signal which is loaded on the 0.34 THz carrier wave. 4
V. MMTM semiconductor test Figure S5. Characteristics of single unit. (a) Output characteristics. (b) Transfer characteristics and transconductance characteristics at V ds =6V. (c) Leakage current at V ds =6V. All these data are measured by Keithley 4200. Table S1. Capacitances of the MMTM array measured with different voltages. U(V) 0.5 0-0.5-1 -1.5-2 -2.5-3 -3.5-4 -4.5-5 C(pF) 6.31 6.41 6.55 6.53 6.49 6.31 6.36 6.47 6.23 6.77 6.15 5.31 These CV data are measured by Agilent B1500A. The inverse of the calculated RC time constant corresponds to about 1.3 GHz with the average capacitance in Table S1 and 122 Ohm resistance, which is in good agreement with the dynamic test results. VI. 2DEG Simulation model In order to simulate the distribution of 2DEG, dispersive Drude model was carried out to characterize 2DEG with different carrier concentrations, which can be equivalent to the representation of loading gate-voltage in the device. The complex relative permittivity can be expressed analytically as 1 2 γω ε ( ω) = ε + iω p ω 2 γ 2 + The real part of permittivity is equal to epsilon infinity (9.8ε 0 for GaN). In the 2 2 * imaginary part, ω p = e Ns ε 0m d is the plasma frequency and the collision * frequency γ = 2π 1.4 THz is calculated with the relation γ = e / mµ where µ is * the electron mobility of the channel. Respectively, e and m are the electron charge and effective mass in 2DEG. At the plasma frequency, Ns is the two-dimensional carrier concentration and d is the simulated model thickness of the channel layer. 5