Generation of THz radiation by photoconductive antennas on based thin films InGaAs and InGaAs/InAlAs
EPJ Web of Conferences
Epitaxial low-temperature grown (LT) semiconductor arsenides (Al, Ga, In)As are widely used as materials for photoconductive antennas (PCA) generators and detectors of pulsed radiation in the terahertz (THz) frequency range    . It is the combination of subpicosecond carrier lifetime, relatively high mobility and high resistivity that makes LTmaterials suitable for PCA applications. Lately, In-GaAs has been investigated as a potential candidate for THz-PCA photoconductive material due
... ctive material due to roomtemperature band gap of 0.74 eV, which allows for 1.56 μm optical excitation with Er 3+ fiber laser femtosecond pulses    . The low substrate temperatures result in a nonstoichiometric growth with the incorporation of excess arsenic in the crystal structure. The most common non-stoichiometry-related point defects in LTarsenides are arsenic antisites with concentrations in the range 10 17 -10 19 cm -3 depending on the substrate temperature and arsenic overpressure [7-10]. Antisiterelated defect band in the semiconductor energy bandgap play a significant role in carrier dynamics. Fast non-radiative recombination of photogenerated electrons and holes through antisite centers results in sub-picosecond carrier lifetimes in LT-materials at optimized growth and annealing conditions [11, 12] . It is generally agreed that main traps of photo-excited electrons are ionized antisite defects    . A possible approach to increase the resistivity of LT-InGaAs structures is to employ LT-InGaAs/InAlAs superlattices [6, 13, 16, 17] . LT-InAlAs layers have a higher dark resistivity as compared to LT-InGaAs and exhibit deep trap states that are situated energetically below the antisite defect levels of adjacent InGaAs layers that results in a reduction of residual carrier concentration. The Fig.1 shows the amplitude of THz radiation in time domain. It is seen that the signal from an In-GaAs /InAlAs-based structure is 5-6 times higher due to a higher bias voltage, which is possible (without sample breakdown) due to higher sample resistance and lower dark current. Fig.2 shows a comparison of the Fourier amplitude according to the materials of the antennas LT-InGaAs/InAlAs and LT-InGaAs. It is seen that the spectrum of the LT-InGaAs / InAlAs sample is slightly wider in the range from 0.1 THz to 0.6 THz than that of the LT-InGaAs sample. We explain this effect by the difference between the characteristic relaxation times of electrons in the transition from the conduction band to the antisites. Fig.1. Time-domain waveforms detected for the following antenna materials: A) LT-InGaAs B) LT-InGaAs/InAlAs. Fig. 2. Compare spectral amplitudes by antenna materials LT-InGaAs(Blue line), LT-InGaAs/InAlAs (Red line). We determined the characteristic times of electron relaxation by the "pump-probe" spectroscopy method. Fig.3 shows the dependence of the normalized transmission in time domain for the samples of LT-InGaAs and LT-InGaAs / InAlAs.We used 2-exponentional model for description experimental curves. On figures τ1 is an electron capture time (capture by charge AsGa defects) [18, 19] , τ2 is a recombination time of captured electrons and holes  .