Pulsed THz generation from InAs/GaAs quantum-dot structures

Summary form only given. Photoconductive devices for THz time-domain spectroscopy systems should ideally be based on materials with short carrier lifetimes, high breakdown voltages, low dark currents, and high carrier mobilities. One of the most widely used materials for such devices is low-temperature-grown (LTG) GaAs, which in its as-grown state is relatively conductive, and as such, must be annealed in order to increase its resistivity. However, a well-known trade-off is that the annealing process also increases LTG-GaAs´s carrier lifetime. In this paper, we demonstrate for the first time the use of an annealed surfactant-mediated-grown InAs/GaAs quantum-dot (QD) photoconductive antenna for pulsed THz generation, for which the annealing process did not display such trade-off. Previously, we had demonstrated THz emission from InAs/GaAs QD based photoconductive antennae using an autocorrelation interferometer setup [1]. Here, we also present the first coherent THz generation and detection results using a QD-based antenna as the emitter.The sample was grown by a surfactant-mediated technique. From double crystal X-ray diffraction measurements it has been demonstrated that QDs grown with this technique have less defects [2]. The structure was grown on a 612 μm-thick (100) GaAs substrate, atop of which a 170 nm GaAs buffer layer was grown at 580 °C, followed by 10 periods of InAs and GaAs grown at 450 °C (2.9 ML and 25 nm thick, respectively). The sample was then subject to rapid thermal annealing for 10 minutes at 550 °C. Annealing increased the resistivity of the InAs/GaAs QD sample from 1 G to 10 G, and lowered the dark current by around an order of magnitude (from 45 nA to 3 nA at a bias of 40 V). It also halved the carrier lifetimes for almost all pump power values, as measured by a photoreflective degenerate pump-probe technique, with a pump wavelength of 800 nm (e.g. 860 fs to 346 fs for a 100 mW-pump). The QDs act as traps for- carriers generated in the GaAs barriers, and we believe that annealing introduces additional defects and therefore more trapping centres, thus contributing to the observed reduction of the carrier lifetime.