We demonstrate true-blue 450 nm tunnel junction (TJ) laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy (PAMBE). The absence of hydrogen during PAMBE growth allows us to achieve TJs with low resistance. We compare TJ LDs with LDs of standard construction with p-type metal contact. For both types of LD, the threshold current density is around 3 kA/cm² and the slope efficiency is 0.5 W/A. We do not observe any significant changes in optical losses and differential gain in TJ LDs compared with standard LDs. The differential resistivity of the TJs for current densities higher than 2 kA/cm² is below 10⁻⁴ Ωcm².

We present the concept, the fabrication processes and the experimental results for materials and optics that can be used for terahertz field-effect transistor detector focal plane arrays. More specifically, we propose 3D printed arrays of a new type – diffractive multi-zone lenses of which the performance is superior to that of previously used mono-zone diffractive or refractive elements and evaluate them with GaN/AlGaN field-effect transistor terahertz detectors. Experiments performed in the 300-GHz atmospheric window show that the lens arrays offer both a good efficiency and good uniformity, and may improve the signal-to-noise ratio of the terahertz field-effect transistor detectors by more than one order of magnitude. In practice, we tested 3 × 12 lens linear arrays with printed circuit board THz detector arrays used in postal security scanners and observed significant signal-to-noise improvements. Our results clearly show that the proposed technology provides a way to produce cost-effective, reproducible, flat optics for large-size field-effect transistor THz-detector focal plane arrays.

We report on a design of fin-shaped channel GaN/AlGaN field-effect transistors developed for studying resonant terahertz plasma oscillations. Unlike common two dimensional FinFET transistor design, the gates were deposited only to the sides of the two dimensional electron gas channel, i.e., metal layers were not deposited on the top of the AlGaN. This side gate configuration allowed us to electrically control the conductivity of the channel by changing its width while keeping the carrier density and mobility virtually unchanged. Computer simulations and analytical model describe well the general shape of the characteristics. The side gate control of the channel width of these transistors allowed us to eliminate the so-called oblique plasma wave modes and paves the way towards future terahertz detectors and emitters using high quality factor plasma wave resonances.

We report on the photoresponse of AlGaN/GaN high electron mobility transistors to the THz radiation of low (15 mW/cm²) and high (up to 40 kW/cm²) intensities. We show that the response can be described by the Dyakonov-Shur theory in the whole range of radiation intensity. At low intensities, the photoresponse is linear in radiation intensity. Under intense laser radiation, we observe a saturation at intensities >20 kW/cm². We explain our results by the change of the channel conductivity under the influence of strong THz field. This mechanism of photoresponse saturation, which is due to the mobility decrease in high ac electric field, should exist for any type of field effect transistor detectors.

Properties of tunnel junctions (TJs) grown by plasma-assisted molecular beam epitaxy are investigated. Examined TJs consist of a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum well (QW) sandwiched between ${\mathrm{In}}_{0.02}{\mathrm{Ga}}_{0.98}\mathrm{N}$ barriers. The influence of piezoelectric polarization, doping, and width of a TJ QW on resistivity of TJs is studied. A decrease of TJ resistivity is observed for high doping and high ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ composition. Additionally, the crystal quality of TJs as a function of magnesium and silicon doping in a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ QW is discussed. Generation of defects in highly doped layers is observed in transmission electron microscopy and by defect selective etching. High doping leads to deterioration of surface morphology, which is corroborated by atomic force microscopy imaging. TJs with low resistance and high crystalline quality, without formation of structural defects, are demonstrated. A theoretical model for calculation of tunneling currents is proposed based on Kane tunneling theory, which considers a realistic electric field distribution in a TJ. A good agreement with the experimental data is obtained. The model is used to predict electrical parameters in a wide range of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ compositions (x = 0–0.40), doping ($n_{\mathrm{Mg},\mathrm{Si}}={10}^{17}-5\times {10}^{21}{\mathrm{cm}}^{-3}$), and TJ QW width (d = 2 – 12 nm).

The authors report the results of studies of lateral and vertical Schottky barrier diodes (SBD) based on molecular beam epitaxy (MBE) grown GaN/AlGaN heterostructures with two dimensional electron gas (2DEG) present at the heterostructure interface. The epilayers were grown by plasma assisted molecular beam epitaxy under metal-rich conditions on commercially available freestanding (0001) GaN substrates. Here, the authors present detailed results for two structures with electron sheet density (N2DEG) of 4.6 × 1012 and 1 × 1013 cm−2 and room temperature mobility of μ2DEG = 1925 cm2/V s and μ2DEG = 1760 cm2/V s, respectively. The processing of lateral and vertical Schottky barrier diodes was performed by laser writer using shallow mesas and Ni/Au metallization for Schottky barriers. The direct current electrical tests performed on lateral and vertical Schottky barrier diodes yielded a turn-on voltage of 0.6 and 1 V, respectively. For both cases, the reverse current registered in the experiment was very low and did not exceed 1 μA/mm for reverse voltage VR = −5 V while the breakdown voltage for both structures was approximately VB = −90 V, which is a good result for nonpassivated surface and anode/cathode distance of the order of a few micrometers. The comparison of current–voltage characteristics for lateral devices with different electron sheet densities shows significant disparity in differential resistances at on-state, which is attributed to the increase of ohmic contact resistance with aluminum mole fraction increase in the AlGaN epilayer. Lateral Schottky diodes demonstrated the low frequency noise amplitude comparable with that reported for regular GaN-based Schottky diodes of much larger area. Low frequency noise characterization on our SDBs is presented. Additionally, the authors present as an example our first result of 140 GHz radiation detection/rectification experiment done on the SBD grown by MBE on (0001) GaN/sapphire. The similar experiments on the SBD freestanding samples will be the subject of forthcoming studies.