Influence of carrier transport on the turn-on delay in a separate confinement heterostructure quantum well laser

Abstract

We investigated theoretically the turn-on delay (t on ) of a SCH-QW-laser biased above the threshold current density. The dependence of this delay time on the geometrical laser structure is important for high-frequency characteristics. In our model the single QW is shifted from the center of the waveguide layer (1017 cm−3 p-doped) towards the n- or p-doped cladding regions with a doping concentration of 1018cm−3 (see inset in Fig. 1). We performed a one-dimensional simulation of the carrier dynamics with respect to carrier transport based on a drift diffusion model, including the influence of Fermi–Dirac statistics1,2 and Poissons equation for the electrostatic potential. For the photon number we use a rate equation.3 Changing the boundary conditions for the electrostatic potential, we increase the injected current on a timescale of 1 ps. To obtain comparable results the power ratio P on /P off is kept constant, as well as the difference of the injection currents (j on –j off ). Differential gain, transparency carrier concentration, and confinement factor are assumed to be equal for the three different structures. Figure 1 shows the light output power versus current density characteristics, revealing a change even in the stationary properties of the laser. This is a first hint that carrier transport becomes important. Looking at the response of the photon number we observe a significant dependence on the location of the QW. t on increases more than 20% if the QW is shifted towards the n-doped cladding layer and it decreases more than 20% due to the shift versus the p-doped cladding layer (see Fig. 2). The pulse shape was kept identical in all three cases. This allows us to compare the results.