Bias-Dependent Carrier Dynamics and Terahertz Performance of ErAs:In(Al)GaAs Photoconductors
Uttam Nandi, Moritz Scheer, Hong Lü, Justin Norman, A. C. Gossard, Sascha Preu
Abstract
In this article, we present the investigation of bias-dependent carrier dynamics of ErAs:In(Al)GaAs superlattice (SL) photoconductors and compare the results to a theoretical model. The carrier dynamics for materials without InAlAs layer can be modeled by a monoexponential decay that features a linear decrease in the carrier lifetime with bias. For a 10 mW laser power, the bias-free lifetime for this material is 440 fs and decreases to 97 fs at 72 kV/cm. For materials consisting of an SL with an absorbing InGaAs and a high-barrier InAlAs layer, the carrier lifetime determined by differential transmission features a bias-dependent fast decay ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 100 fs) followed by a bias-independent slower decay ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 2.5 ps). The share of charges being trapped with the fast decay can go up to 83% at 80 kV/cm. Terahertz (THz) spectra for different biasing conditions were recorded in order to confirm the decrease of carrier lifetime. The reduction of the lifetime shifts the lifetime rolloff toward higher frequencies. The data show a maximum of 6 dB improvement in power for the high-frequency components, in agreement with theoretical expectations. The optimized time domain spectroscopy spectrum shows a peak dynamic range of 110 dB with more than 6 THz bandwidth. The maximum emitted THz power outcoupled through an uncoated silicon-lens with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 30% reflection loss was 472 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 35 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> W.