The AlGaN/GaN heterostructures are widely used as main elements of many optoelectronic devices, including detectors [1] and modulators [2] of THz radiation. The high electron mobility transistor (HEMT) structures based on AlGaN/GaN with a two-dimensional electron gas (2DEG) possessing mobility of 19 000 (for 77 K) and 2200 cm2/Vs (for 300 K) [3, 4] are promising for the development of resonant THz detectors and emitters based on the plasma waves phenomena [5, 6]. Despite the considerable interest, the electron effective mass contains significant uncertainties in these heterostructures with value increase from 0.19-0.25 me (me free electron mass) at cryogenic temperatures up to 0.33-0.36 me at room temperature [7].
Grating-coupled AlGaN/GaN HEMT structures exhibit resonant behaviour due to a 2D plasmons excitation in the THz range [3,8]. The position of the resonances depends on the electron concentration, effective mass, and the wave vector in the lattice in accordance to analytical models [8-9], while the full width at half maximum of the resonance peak can be related to the electron scattering time. The resonant 2D plasmon features can be observed in both amplitude and phase spectra measured by the THz time-domain spectroscopy (TDS) system [3]. Thus, THz TDS allows direct investigation of the electron effective mass dependence on carrier density or temperature in plasmonic structures [8, 10].
The AlGaN/GaN heterostructures were grown on a 500 μm thick semi-insulating SiC substrate. Periodical metal gratings for excitation of 2D plasmons were fabricated on the heterostructure surface in an area of 2x2 mm2. Four samples with grating parameters of the width/period of 350/600, 440/800, 530/1030, and 830/1000 in nanometers were investigated in transmission geometry using a commercial spectrometer T-SPEC 800 (TeraVil) in the frequency range 0.1-4 THz at various temperatures provided by a liquid nitrogen cryostat.
Plasmons were excited in all samples in the frequency range 1-3 THz. The resonant features of 2D plasmons under the grating were found as a pronounced minimum and an inflection point in the power and phase spectra, respectively. The position of the resonance redshifted with an increase of grating period in an agreement with theory [4, 8, 10], for example, the plasmons were excited at 2.2 and 1.4 THz in the samples with grating periods of 600 and 1000 nm, respectively. Resonance features remain visible in both power and phase spectra up to 220 K and 300 K for samples with the same grating period of 1000 nm but different filing factor 50% and 80%, respectively. Moreover, with the temperature increase, 2D plasmon resonances experienced the red-shift in the transmission spectra of all samples. For a case of 80% filling factor, the position changed was largest from 1.38 THz at 80 K to 1.13 THz at 300 K. It was noticed that the concentration of electrons at room temperature increased up to 10% of the low-temperature value which in turn should blue shift the resonance position. Thus, the phenomenon was attributed to the renormalization of electron effective mass, which deviation starts at 134K and at 295K increased up to 55% of its nominal value, following the phenomenological dependence of m*(T) = me [1+0.01(T/134)5].