We report on a novel-designed superlattice (SL) InGaAs/InAlAs with artificially introduced epitaxial stresses into functional layers. The optimized and fabricated strained SL demonstrates a sub-picosecond photocarrier lifetime of 1.7 ps nevertheless featuring a rather moderate mobility. By means of numerical simulation we observe a decrease in the energy band gap of strained photoconductive layer InGaAs. In addition, the timedomain spectroscopic measurements reveal an increase in the spectrum amplitude of surface THz emission in the strained SL compared to lattice-matched one. We associate the decrease in photocarrier lifetime as well as the increase in the spectrum amplitude with residual strain in the SL caused by epitaxial stresses. The obtained results are of specific interest to THz science community since they open a way toward fabrication of cost-effective THz photoconductive devices for biomedical applications.
This review highlights recent and novel trends focused on metallic (plasmonic) and dielectric metasurfaces in photoconductive terahertz (THz) devices. We demonstrate the great potential of its applications in the field of THz science and technology, nevertheless indicating some limitations and technological issues. From the state-of-the-art, the metasurfaces are, by far, able to force out previous approaches like photonic crystals and are capable of significantly increasing the performance of contemporary photoconductive devices operating at THz frequencies.
We propose a novel technology for fabricating plasmonic photoconductive antennas (PCAs) based on superlattice (SL) with increased height of the plasmonic gratings up to 100 nm. We passivate the surface of the SL by Si3N4, etch there windows and deposit Ti/Au antenna metallization. The plasmonic gratings are formed by electron-beam lithography with Ti/Au metallization followed by lift-off. Then an Al2O3 anti-reflection coating layer for reduction of the Fresnel reflection losses is used on the top of the plasmonic gratings, which also serves for maintaining its mechanical stability and providing the excitation of guided modes at the resonant wavelengths of the subwavelength slab waveguide formed by the metal gratings. Current-voltage measurements under femtosecond laser illumination reveal strong increase of the transient photocurrent generated by the fabricated plasmonic PCA which is 15 times higher than for conventional one (i.e. without the plasmonic gratings). The obtained terahertz (THz) power spectra demonstrate 100-times increase of the THz power in the plasmonic PCA. The results might be of interest to the needs of THz spectroscopy and imaging systems, in particular, operating with low-power lasers.
We have investigated the influence of indium content (x) increase on spectral characteristics of InxGa1-xAs photoconductor. To avoid the mismatch between crystalline parameters of InxGa1-xAs and GaAs wafer we proposed to incorporate a step-graded metamorphic buffer layer. We showed that x increase strongly enhances THz emission and broadens THz spectrum of InxGa1-xAs. Since no polarity rehearsal of the THz waveform occurs and electron diffusion mobility increases up to 90% with x increase we attribute the increase of THz intensity to photo-Dember effect contribution. The maximum efficiency of optical-to-THz conversion was obtained for In0.72Ga0.28 As at optical fluence ~0.01 μJ=cm2. The fabricated photoconductors can be used as promising photo-Dember or lateral photo-Dember THz emitters in pulsed THz spectroscopy and imaging, in particular, operating with long wave optical pump.
We present the results of numerical and experimental study of the photoconductive antennas (PCAs) based on GaAs and its ternary compounds. We produced three photoconductive materials with different indium content, which then were applied for fabrication of the THz PCAs. These PCAs were used as emitters of the THz pulsed spectrometer. We evaluated the stationary transient current generated by the PCAs, simulated their I-V characteristics, and compared them with the experimental ones. Using the finite integration method, we studied the thermal properties of the PCAs and demonstrated significant influence of the heat-sink on the leakage currents of the InGaAs-based PCA. We showed that the heat-sink reduces the operation temperature of the InGaAs-based PCAs by 40-64 % depending on the indium content. The observed results might be interesting for applications of the PCAs in THz pulsed spectroscopy and imaging.
The gated GaAs structures like the field-effect transistor with the array of the Sn nanothreads was fabricated via delta-doping of vicinal GaAs surface by Sn atoms with a subsequent regrowth. That results in the formation of the chains of Sn atoms at the terrace edges. Two device models were developed. The quantum model accounts for the quantization of the electron energy spectrum in the self-consistent two-dimensional electric potential, herewith the electron density distribution in nanothread arrays for different gate voltages is calculated. The classical model ignores the quantization and electrons are distributed in space according to 3D density of states and Fermi-Dirac statistics. It turned out that qualitatively both models demonstrate similar behavior, nevertheless, the classical one is in better quantitative agreement with experimental data. Plausibly, the quantization could be ignored because Sn atoms are randomly placed along the thread axis. The terahertz hot-electron bolometers (HEBs) could be based on the structure under consideration.