Quantum-well capture and interwell transport in semiconductor active layers

Abstract

Abstract. The dynamics of electrons and holes in multiquantum-well semiconductor gain media involves several different transport processes, such as diffusion and drift across the barrier region, as well as capture and escape transitions between the bound and the unbound states of the quantum wells. In addition to their fundamental interest, these processes are important because of their implications for the dynamic properties of multiquantum-well lasers and optical amplifiers. Experimentally, they have been studied with several time-domain optical techniques having (sub)picosecond resolution and, more recently, with frequency-domain techniques based on laser modulation measurements. This article gives a brief review of the work done in this area and then presents in detail a frequency-domain approach, four-wave mixing spectroscopy in semiconductor optical amplifiers, to investigate intrinsic capture and interwell equilibration. This technique allows one to extend the device modulation frequency to several hundreds of gigahertz, thus providing the required time resolution, and can be configured to isolate and directly study the transport process of interest.

Impact of Carrier Transport and Capture on VCSEL Dynamics

“…We refer to this as the nominal carrier effective QW capture time. The intrinsic carrier QW capture time is expected to be around 1 ps at room temperature [3]. However, the intrinsic carrier QW capture time is enhanced by the ratio of the volume of the SCH and the total QW volume [1], which in our case is 3.7.…”

“…We have chosen these values because our nominal τcap is 7 ps, meaning that 15 ps should represent a relatively long carrier effective QW capture time using the VCSEL design and parameters described in [9]. As aforementioned, 1 ps is at room temperature an estimate of the intrinsic carrier QW capture time [3], suggesting that we cannot go lower even if limitations by carrier diffusion and relaxation were eliminated, thus making this our lower limit.…”

“…(Corresponding author: Alexander Grabowski.) Alexander Grabowski, Johan Gustavsson, and Anders Larsson are with the Photonics Laboratory, Department of Microtechnology and Nanoscience Carrier transport and capture in the SCH and QWs is a complex process, including the relaxation of injected carriers within the band of unconfined states in the SCH region embedding the QWs, the diffusive carrier transport along the SCH, and the intrinsic capture of carriers into the confined states of the QWs [3]. The rate of this process is commonly quantified by a carrier effective capture time.…”

“…Carrier transport along the SCH is mainly governed by diffusion [1], with the associated speed set by the diffusion coefficients. Relaxation of carriers to the ground state of the QWs occurs through LO phonon emission [3], carrier-carrier scattering [4], and impurity scattering [5]. Therefore, active region design parameters that are expected to impact VCSEL dynamics include e.g.…”