Material properties and performance of ErAs:In(Al)GaAs photoconductors for 1550 nm laser operation

ErAs:In(Al)GaAs photoconductors have proven to be outstanding devices for photonic terahertz (0.1-10THz) generation and detection with previously reported sub-0.5ps carrier lifetimes. We present the so far most detailed material characterization of these superlattices composed of ErAs, InGaAs, and InAlAs layers grown by molecular beam epitaxy. The variation of the material properties as a function of the ErAs concentration and the superlattice structure is discussed with focus on source materials. Infrared spectroscopy shows an absorption coefficient in the range of 4700-6600cm - 1 at 1550nm, with shallow absorption edges toward longer wavelengths caused by absorption of ErAs precipitates. IV characterization and Hall measurements show that samples with only 0.8 monolayers of electrically compensated ErAs precipitates (p-delta-doped at 5 x 10 13cm - 2) and aluminum-containing spacer layers enable high dark resistance ( similar to 10-20M Omega) and high breakdown field strengths beyond 100kV/cm, corresponding to > 500V for a 50 mu m gap. With higher ErAs concentration of 1.6ML (2.4ML), the resistance decreases by a factor of similar to 40 (120) for an otherwise identical superlattice structure. We propose a theoretical model for calculation of the excess current generated due to heating and for the estimation of the photocurrent from the total illuminated current. The paper concludes with terahertz time-domain spectroscopy measurements demonstrating the strengths of the material system and validating the proposed model.