We present an extensive study of resonant two-dimensional (2D) plasmon excitations in grating-gated quantum well heterostructures, which enable an electrical control of periodic charge carrier density profile. Our study combines theoretical and experimental investigations of nanometer-scale $\mathrm{AlGaN}/\mathrm{GaN}$ grating-gate structures and reveals that all terahertz (THz) plasmonic resonances in these structures can be explained only within the framework of the plasmonic crystal model. We identify two different plasmonic crystal phases. The first is the delocalized phase, where THz radiation is absorbed with the entire grating-gate structure that is realized at a weakly modulated 2D electron gas (2DEG) regime. In the second, the localized phase, THz radiation interacts only with the ungated portions of the structure. This phase is achieved by fully depleting the gated regions, resulting in strong modulation. By gate controlling of the modulation degree, we observe a continuous transition between these phases. We also discover that, unexpectedly, the resonant plasma frequencies of ungated parts (in the localized phase) still depend on the gate voltage. We attribute this phenomenon to the specific depletion of the conductive profile in the ungated region of the 2DEG, the so-called edge gating effect. Although we study a specific case of plasmons in $\mathrm{AlGaN}/\mathrm{GaN}$ grating-gate structures, our results have a general character and are applicable to any other semiconductor-based plasmonic crystal structures. Our work represents the first demonstration of an electrically tunable transition between different phases of THz plasmonic crystals, which is a crucial step toward a deeper understanding of THz plasma physics and the development of all-electrically tunable devices for THz optoelectronics.

Electrical and noise properties of graphene contacts to AlGaN/GaN heterostructures were studied experimentally. It was found that graphene on AlGaN forms a high-quality Schottky barrier with the barrier height dependent on the bias. The apparent barrier heights for this kind of Schottky diode were found to be relatively high, varying within the range of φ_b = (1.0–1.26) eV. AlGaN/GaN fin-shaped field-effect transistors (finFETs) with a graphene gate were fabricated and studied. These devices demonstrated ~8 order of magnitude on/off ratio, subthreshold slope of ~1.3, and low subthreshold current in the sub-picoamperes range. The effective trap density responsible for the 1/f low-frequency noise was found within the range of (1–5) · 10¹⁹ eV⁻¹ cm⁻³. These values are of the same order of magnitude as reported earlier and in AlGaN/GaN transistors with Ni/Au Schottky gate studied as a reference in the current study. A good quality of graphene/AlGaN Schottky barrier diodes and AlGaN/GaN transistors opens the way for transparent GaN-based electronics and GaN-based devices exploring vertical electron transport in graphene.

We report experimental results on the low-frequency noise in GaN/AlGaN transistors fabricated under different conditions and evaluate different methods to extract the effective trap density using the McWhorter model. The effective trap density is found to be below 10¹⁹ cm⁻³ for some of the wafers. This trap density is of the same order of magnitude as that reported in Si MOSFETs with a high-k dielectric. One of the structures manifested about two orders of magnitude higher noise level. These measurements correlate with the results of secondary ion mass spectroscopy and terahertz electroluminescence measurements which indicated a ∼30% higher concentration of uncompensated oxygen in this structure. Effective trap density extracted from noise measurements is proven to be a very sensitive figure of merit parameter for the GaN/AlGaN field effect transistors and material quality assessment.

We report on the study of the magnetic ratchet effect in AlGaN/GaN heterostructures superimposed with a lateral superlattice formed by a dual-grating gate structure. We demonstrate that irradiation of the superlattice with a terahertz beam results in the direct ratchet current, which shows giant magneto-oscillations in the regime of Shubnikov–de Haas oscillations. The oscillations have the same period and are in phase with the resistivity oscillations. Remarkably, their amplitude is greatly enhanced as compared with the ratchet current at zero magnetic field, and the envelope of these oscillations exhibits large beatings as a function of the magnetic field. We demonstrate that the beatings are caused by the spin-orbit (SO) splitting of the conduction band. We develop a theory which gives a good qualitative explanation of all experimental observations and allows us to extract the SO splitting constant ${\alpha }_{\mathrm{SO}}=7.5\pm 1.5$ meVÅ. We also discuss how our results are modified by plasmonic effects and show that these effects become more pronounced with decreasing the period of the grating gate structures down to submicrons.

We report on the investigations of AlGaN/GaN field effect transistors with two lateral Schottky barrier gates on the sides of the two-dimensional electron gas. This kind of 'EdgeFET' allowed us to efficiently control the current flow in the 2DEG conduction channel. Moreover, due to depletion, regions at a certain range of reverse biasing form a nanowire, which is beneficial for the adjustable resonant THz detection. Our studies of DC characteristics and photoresponse in the sub-terahertz frequency confirm the validity of the approach.

Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm⁻².