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High saturation photocurrent THz waveguide-type MUTC-photodiodes reaching mW output power within the WR3.4 band

Marcel Grzeslo, Sebastian Dülme, Simone Clochiatti, Tom Neerfeld, Thomas Haddad, Peng Lu, Jonas Tebart, Sumer Makhlouf, Carlos Biurrun-Quel, José Luis Fernández Estévez, Jörg Lackmann, Nils Weimann, and Andreas Stöhr

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Abstract

In this paper, we report on waveguide-type modified uni-traveling-carrier photodiodes (MUTC-PDs) providing a record high output power level for non-resonant photodiodes in the WR3.4 band. Indium phosphide (InP) based waveguide-type 1.55 µm MUTC-PDs have been fabricated and characterized thoroughly. Maximum output powers of −0.6 dBm and −2.7 dBm were achieved at 240 GHz and 280 GHz, respectively. This has been accomplished by an optimized layer structure and doping profile design that takes transient carrier dynamics into account. An energy-balance model has been developed to study and optimize carrier transport at high optical input intensities. The advantageous THz capabilities of the optimized MUTC layer structure are confirmed by experiments revealing a transit time limited cutoff frequency of 249 GHz and a saturation photocurrent beyond 20 mA in the WR3.4 band. The responsivity for a 16 µm long waveguide-type THz MUTC-PD is found to be 0.25 A/W. In addition, bow-tie antenna integrated waveguide-type MUTC-PDs are fabricated and reported to operate up to 0.7 THz above a received power of −40 dBm.

© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

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Analysis of frequency response of high power MUTC photodiodes based on photocurrent-dependent equivalent circuit model

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Opt. Express 23(17) 21615-21623 (2015)

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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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Figures (14)
Fig. 1.
Fig. 1. Simulated electric field distribution in a) and the corresponding average electron velocity profile in b) at 1.2 V reverse bias and an optical intensity of 0.6 WM/cm2. The dashed line shows the initial electrical field distribution when no illumination is present.

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Fig. 2.
Fig. 2. Schematic process flow illustrating the fabrication of the waveguide photodiode.

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Fig. 3.
Fig. 3. Optical microscope picture a) and SEM-picture b) of fabricated CPW-integrated waveguide-type UTC-Photodiodes with length of 17 µm and 7 µm respectively.

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Fig. 4.
Fig. 4. DC-measurements of the diode characteristics for several photodiode areas.

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Fig. 5.
Fig. 5. Measured junction capacitances revealing the parasitic capacitance contributed by the tapered CPW-transition.

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Fig. 6.
Fig. 6. Equivalent circuit including transit time and RC characteristic at port 1 and port 2 respectively in a) and S-parameter characterization of several photodiode geometries in b) showing the corresponding fitting curves of port 2.

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Fig. 7.
Fig. 7. Obtained scattering parameters of the photodiode with a geometry of 4 × 18 µm2 in comparison with the fitted equivalent circuit and simulation in a) and b). The corresponding impedance values are shown in c) and d).

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Fig. 8.
Fig. 8. Experimentally extracted responsivities (circles) and the theoretical responsivity (dashed line) in dependency of the photodiode length obtained by optical FDTD simulations.

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Fig. 9.
Fig. 9. Measured and simulated relative RF-response until 320 GHz for several photodiode geometries in a). The absolute RF-response of the photodiode with 2 × 9 µm2 is shown in b). Here both a WR3.4-SBD (red squares) and a calorimeter (PM5B) (blue circles) from VDI were used to validate the accuracy of the absolute PD power within the WR3.4 band. The RC-limitation in the simulations (dashed lines) has been adjusted for the measured junction capacitances and series resistances obtained by fitting the equivalent circuit.

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Fig. 10.
Fig. 10. Measured cutoff frequencies obtained from response fitting in comparison with analytical calculations in dependency of the photodiodes’ length and widths for 4 µm (blue circles) and 2 µm (red circle). Additionally, the influence of a varying contact resistivity ρc is shown in the calculation with 2·10−6 Ωcm2 (dotted curves) and 5·10−6 Ωcm2 (dashed curves).

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Fig. 11.
Fig. 11. Measured (continuous) and simulated (dashed lines) RF-power in dependency of bias for several frequencies in a) and at 300 GHz in b). The PD features an area of 2 × 9 µm2 and was set to a photocurrent where no saturation was present. The EB-simulated voltage range for maximal output power matches well with the measurements using the while DD-simulations (dotted line) exhibit no optimal bias.

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Fig. 12.
Fig. 12. Measured saturation characteristics for several photodiode geometries at 240 GHz and 280 GHz. The corresponding simulations (dashed lines) are in good agreement with the saturation currents and saturated output powers. Highest output powers are provided by the PD with a geometry of 2 × 17 µm2 while achieving high photocurrents around 20 mA.

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Fig. 13.
Fig. 13. Optical microscope picture of a bowtie-antenna integrated photodiode (BT-PD) for THz-power measurements in a), using a UWB SBD detector. At a reverse bias of 1.2 V the photocurrent was set to 3 mA. The measured frequency response in b) (red squares) indicates an additional transit time limitation around 250 GHz. The simulated frequency response (dashed line) matches the experimental data below 0.7 THz. At higher frequencies the measured output power approaches the noise floor of the detector.

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Fig. 14.
Fig. 14. Relative RF-power for the whole frequency region (DC-THz) from both the bow-tie antenna integrated photodiode (red squares) and a CPW-integrated photodiode (blue circles) after their RC-characteristics have been deembedded. The fitted frequency response (dotted line) indicates a cutoff at 249 GHz, which confirms the simulated response with a transit time limited cutoff frequency of 254 GHz.

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Tables (2)

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Table 1. Epitaxial layers of waveguide MUTC-PD

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Table 2. Exemplary results from the fitted equivalent circuit

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