Vytautas Jakštas

The practical implementation of terahertz (THz) imaging and spectroscopic systems in real operational conditions requires them to be of a compact size, to have enhanced functionality, and to be user-friendly. This work demonstrates the single-sided integration of Fresnel-zone-plate-based optical elements with InGaAs bow-tie diodes directly on a semiconductor chip. Numerical simulations were conducted to optimize the Fresnel zone plate’s focal length and the InP substrate’s thickness to achieve constructive interference at 600 GHz, room-temperature operation and achieve a sensitivity more than an order of magnitude higher—up to 24.5 V/W—than that of a standalone bow-tie detector. Investigations revealed the strong angular dependence of the incident radiation on the Fresnel zone plate-integrated bow-tie diode’s response. These findings pave a promising avenue for the further development of single-sided integration of flat optics with THz detectors, enabling improved sensitivity, simplified manufacturing processes, and reduced costs for THz detection systems in a more compact design scheme.

Graphene has shown great potential for ultra-high frequency electronics. However, using graphene in electronic devices creates a requirement for electrodes with low contact resistance. Thermal annealing is sometimes used to improve the performance of contact electrodes. However, high-temperature annealing may introduce additional doping or defects to graphene. Moreover, an extensive increase in temperature may damage electrodes by destroying the metal–graphene contact. In this work, we studied the effect of high-temperature annealing on graphene and nickel–graphene contacts. Annealing was done in the temperature range of 200–800 °C and the effect of the annealing temperature was observed by two and four-point probe resistance measurements and by Raman spectroscopy. We observed that the annealing of a graphene sample above 300 °C increased the level of doping, but did not always improve electrical contacts. Above 600 °C, the nickel–graphene contact started to degrade, while graphene survived even higher process temperatures.

This paper reports on the AlGaN/GaN Schottky diodes (SDs) and high-electron-mobility transistors (HEMTs) grown on a semi-insulating SiC substrate. The electronic devices demonstrate an improved performance in comparison with the ones processed on a sapphire substrate. Both the SDs and HEMTs show much smaller leakage current density and a higher I_ON/I_OFF ratio, reaching values down to 3.0±1.2 mA/cm² and up to 70 dB under the reverse electric field of 340 kV/cm, respectively. The higher thermal conductivity of the SiC substrate leads to the increase of steady current and transconductance, and better thermal management of the HEMT devices. In addition, a successful detection of terahertz (THz) waves with the AlGaN/GaN HEMT is demonstrated at room temperature. These results open further routes for the optimization of THz designs which may result in development of novel plasmonic THz devices.

In this work we demonstrate the impact of the AlGaN/GaN superlattice (SL) on electrical properties of AlGaN/GaN heterostructures grown by MOCVD on sapphire substrate. Schottky diodes and high electron mobility transistors were processed of the AlGaN/GaN heterostructures with and without the SL. It was demonstrated that the presence of the SL increased the breakdown voltage and improved the saturation current and transistor transfer characteristics performance.