Modelling Terahertz Wave Generation in InAs Quantum-Dot Containing GaAs-Based Photoconductive Antenna

InAs quantum-dot-containing GaAs-based photoconductive antenna (PCA) devices are promising for terahertz (THz) generation under below-bandgap excitation by ultrafast laser pulses. This paper describes the results of rate equation analyses of these devices by assuming the two-step photocarrier generation via midgap states in GaAs and carrier capture into the quantum dots. In the present simulation model, the Coulomb space charge screening effect and the radiated THz field screening effect are taken into consideration. The calculations indicate that, at low excitation powers, the radiated THz field is primarily governed by the fast photocurrent transient arising from rapid electron capture into the quantum dots. At higher photoexcitation power, the THz radiation is controlled predominantly by the carrier velocity reduction immediately after the excitation pulse due to the Coulomb space charge screening effect. As the photoexcitation power increases, the Coulomb screening effect causes saturation of the THz field amplitude, whereas the radiated THz field screening effect contributes little to the saturation of the THz field amplitude. In this study, the influence of key parameters, such as the density and electron capture time of the midgap states and quantum dots, are analyzed. Existing experimental data on THz emission characteristics are discussed on the basis of the present analysis. The observed features provide comprehensive understanding on the THz generation process involving many physical parameters in PCAs and can be applied to optimizing design for high-performance devices.