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Journal of Physics Conference Series

High-Tcmicro and nano-constrictions modeling: hot spot approach for DC characteristics and HEB THz mixer performance

Romain G. Ladret, Annick F. Degardin, Alain Kreisler

Published May 12, 2014• 2 citations
Publisher

Abstract

High- T C hot electron bolometers (HEB) are promising THz mixers due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. We have simulated the characteristics of YBaCuO HEBs with a hot spot model usually dedicated to low- T C phonon cooled devices. With respect to e.g., NbN, the high- T C specificities are mainly arising from the large values of YBaCuO phonon thermal conductivity, and the film to MgO substrate phonon escape time. The consequent effects on the electron temperature profiles along the YBaCuO constriction and the current voltage DC characteristics are considered. The conversion gain G and noise temperature T N were computed for two constriction dimensions. For a 100 nm long × 100 nm wide × 10 nm thick constriction at 9 microwatt local oscillator (LO) power, the expected double sideband (DSB) T N = 1520 K ( G = −13.7 dB). For a larger (but more realistic according to YBaCuO aging effects) 400 nm long × 400 nm wide × 35 nm thick constriction, minimum T N = 1210 K DSB at 35 microwatt LO power ( G = −13.1 dB).

DOI
10.1088/1742-6596/507/4/042020

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High- T C hot electron bolometers (HEB) are promising THz mixers due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. We have simulated the characteristics of YBaCuO HEBs with a hot spot model usually dedicated to low- T C phonon cooled devices. With respect to e.g., NbN, the high- T C specificities are mainly arising from the large values of YBaCuO phonon thermal conductivity, and the film to MgO substrate phonon escape time. The consequent effects on the electron temperature profiles along the YBaCuO constriction and the current voltage DC characteristics are considered. The conversion gain G and noise temperature T N were computed for two constriction dimensions. For a 100 nm long × 100 nm wide × 10 nm thick constriction at 9 microwatt local oscillator (LO) power, the expected double sideband (DSB) T N = 1520 K ( G = −13.7 dB). For a larger (but more realistic according to YBaCuO aging effects) 400 nm long × 400 nm wide × 35 nm thick constriction, minimum T N = 1210 K DSB at 35 microwatt LO power ( G = −13.1 dB).
Hot electron bolometers (HEB) made from high-TC superconducting YBa2Cu3O7–x (YBCO) oxide nano-constrictions are promising THz mixers, due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. The challenge for YBCO resides, however, in the chemical reactivity of the material and the related aging effects. In this paper, we model and simulate the frequency dependent performance of YBCO HEBs operating as THz mixers. We recall first the main hypotheses of our hot spot model taking into account both the RF frequency effects in the YBCO superconducting transition and the nano-constriction impedance at THz frequencies. The predicted performance up to 4 THz is given in terms of double sideband noise temperature TDSB and conversion gain G. At 2.5 THz for instance, TDSB ≅ 1000 K and G ≅ − 6 dB could be achieved at 12.5 μW local oscillator power. We then consider a standoff target detection scheme and examine the feasibility with YBCO devices. For instance, detection at 3 m through cotton cloth in passive imaging mode could be readily achieved in moderate humidity conditions with 10 K resolution.
High-TCsuperconductor (YBaCuO) hot electron bolometers (HEB) are promising THz mixers due to the large predicted bandwidth and the low local oscillator (LO) power requirement. Recently, we introduced the hot-spot model, initially dedicated to low-TCNbN devices, to predict YBaCuO mixer performance. Major improvements to the model have been introduced: local description of the power absorption along the HEB constriction, THz dependence of the resistive transition, impedance matching between the THz antenna and the constriction. Mixer noise temperature TNand conversion gain G were predicted up to 2.5 THz. For instance, TN= 2700 K (DSB) and G = -13 dB are obtained with 9 μW of LO power at 1 THz.
To investigate the THz mixing performance of YBCO hot electron bolometers (HEB), the influence of the local oscillator (LO) radiofrequency (RF) radiation has been considered in detail. As opposed to the usual hot spot modeling approach, where the LO power is assumed to be uniformly distributed over the HEB constriction length, a uniform RF current has been assumed. The local electron temperature-as obtained by solving the coupled electron and phonon thermal reservoir equations-could then be used to determine the local YBCO complex resistivity, hence the locally dissipated LO power. Besides, the use of a modified two-fluid description allowed to determine the RF-dependent temperature shift (~-1% THz-1) and broadening (~+20% THz-1) of the resistive transition. Finally, the impedance matching to the THz antenna was considered. For a typical constriction, the conversion loss and noise temperature TDSBwere computed, with TDSB× exp(0.32f) behavior up to 2.5 THz.
The performance of a NbN based phonon-cooled Hot Electron Bolometric (HEB) quasioptical mixer is investigated in the 0.65-3.12 THz frequency range. The device is made from a 3 nm thick NbN film on high resistivity Si and integrated with a planar spiral antenna on the same substrate. The in-plane dimensions of the bolometer strip are 0.2/spl times/2 /spl mu/m. The best results of the DSB noise temperature at 1.5 GHz IF frequency obtained with one device are: 1300 K at 650 GHz, 4700 K at 2.5 THz and 10000 K at 3.12 THz. The measurements were performed at 4.5 K ambient temperature. The amount of local oscillator (LO) power absorbed in the bolometer is about 100 nW. The mixer is linear to within 1 dB compression up to the signal level 10 dB below that of the LO. The intrinsic single sideband conversion gain measured at 650 GHz is -9 dB, the total conversion gain is -14 dB.
The performance of a NbN based phonon-cooled Hot Electron Bolometric (HEB) quasioptical mixer is investigated in the 0.65-3.12 THz frequency range. The device is made from a 3 nm thick NbN film on high resistivity Si and integrated with a planar spiral antenna on the same substrate. The in-plane dimensions of the bolometer strip are 0.2/spl times/2 /spl mu/m. The results of the DSB noire temperature are: 1300 K at 650 GHz, 4700 K at 2.5 TBz and 10000 K at 3.12 THz. The RF bandwidth of the receiver is at least 2.5 THz. The amount of LO power absorbed in the bolometer is about 100 nW. The mixer is linear to within 1 dB compression up to the signal level 10 dB below that of the LO. The intrinsic single sideband conversion gain is measured to be -9 dB, the total conversion gain -14 dB.