We present the results of design, fabrication, and characterization of the room-temperature, low electron heat capacity
hot-electron THz microbolometers based on two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. The
2DEG sensor is integrated with a broadband THz antenna and a coplanar waveguide. Devices with various patterning of
2DEG have been fabricated and tested. Optimizing the material properties, geometrical parameters of the 2DEG, and
antenna design, we match the impedances of the sensor and antenna to reach strong coupling of THz radiation to 2DEG
via the Drude absorption. Testing the detectors, we found that the THz-induced photocurrent, ΔI, is proportional to the
bias current, I, and the temperature derivative of the resistance and inversely proportional to the area of 2DEG sensor, S.
The analysis allowed us to identify the mechanism of the 2DEG response to THz radiation as electron heating. The
responsivity of our sensors, normalized to the bias current and to unit area of 2DEG, R*= ΔI•S/ (I∙P), is ~ 103 W-1 μm2.
So, for our typical sensor with an area of 1000 μm2 and bias currents of ~ 10 mA, the responsivity is ~ 0.01 A/W. The
measurements of mixing at sub-terahertz frequencies showed that the mixing bandwidth is above 2 GHz, which
corresponds to a characteristic electron relaxation time to be shorter than 0.7 ps. Further decrease of the size of 2DEG
sensors will increase the responsivity as well as allows for decreasing the local oscillator power in heterodyne
applications.
We present results on design, fabrication, and characterization of hot-electron bolometers based on low-mobility
two-dimensional electron gas (2DEG) in AlInN/GaN and AlGaN/GaN heterostructures. Electrical and optical
characterization of our Hot Electron Bolometers (HEBs) show that these sensors combine (i) high coupling to incident
THz radiation due to Drude absorption, (ii) significant electron heating by the THz radiation due to small value of the
electron heat capacity, (iii) substantial sensitivity of the device resistance to the heating effect. A low contact resistance
(below 0.5 Ω·mm) achieved in our devices ensures that the THz voltage primarily drops across the active region. Due to
a small electron momentum relaxation time, the inductive part of the impedance in our devices is large, so these sensors
can be combined with standard antennas or waveguides. In the capacity of the THz local oscillator (LO) for heterodyne
THz sensing, we fabricated AlGaAs/GaAs quantum cascade lasers (QCLs) with a stable continuous-wave single-mode
operation in the range of 2.5-3 THz. Spectral properties of the QCLs have been studied by means of Fourier transform
spectroscopy. It has been demonstrated that the spectral purity of the QCL emission line doesn't exceed the spectrometer
resolution limit at the level of 0.1 cm-1 (3 GHz). Discrete spectral tuning can be achieved using selective devices; fine
tuning can be done by thermally changing the refractive index of the material and by applied voltage. Compatibility of
the low-mobility 2DEG microbolometers with QCLs in terms of LO power requirements, spectral coverage, and cooling
requirements makes this technology especially attractive for THz heterodyne sensing.
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