The roles of carrier transport in determining the modulation of semiconductor quantum well lasers

Applications such as optical interconnects have made the modulation response of semiconductor laser diodes of great interest. Details of the modulation response have been attributed to different carrier transport mechanisms. Two imporlaxit features of the modulation response are the resonant frequency and the amount of gain saturation, often referred to as the low frequency roll-off. There has been some disagreement conceriiing which carrier transport mechanisms are most important in determining these features, particularly the gain saturation. One view of gain saturation concentrates on the ca,pture of carriers in the bound states of the quantum well.' Althougli carrier ca.pture is a relatively fast process, Kan et. al. feel it may be slow enough to cause some accumulation of carriers in tlie continuum states above the quantum well. This accumulation could then form a diffusive barrier to the transport of free caxriers to the active region. Since electrons usually have a slower capture rate lhan holes, it was concluded that the slow difk'usion of electrons to tlie quantum well may be to blame €or gain saturation. An alteriiative view is that the capture rneclianism is too fast to limit the modulation response.2 Instead, the holes, with their low mobilities, are slow in moving to the quantum well, and, therefore, are the cause of poor modulation. To test this idea, Nagarajan el. el. measured the modulation responses of different strained InGaAs quantum well lasers. The devices differed in the width of the separate confinement region (SCIt) and the location of the quantum well within this region. They showed that a SCR that is wide on both the n and p sides has significant gain saturation. Furthermore, they showed that when the n side of the SCR is narrow but the p side is still wide, the amount of gain saturation is comparable to the case in which both sides are wide. Thus, it was concluded thak it is hole transport, and not electron tra#nsport, that causes a poor modulation response. We present an irivestigation into this issue that was conducted with the Minilase laser sirnulator. The simulation includes all of the principal read space transport mechanisms, including driftdiffusion in bulk regions, thermionic emission at heterojunctions, and carrier capture into bound quantum states. The simulation was used to calculate modulation responses for GaAsIAZGaAs lasers similar ixi geometry to tliose measured by Nagarajan et. al. These responses are shown in figure 1, and the results show lhe same trends observed in the experimental measurements. We will present similar calculations on strained InGaAs/AlGaAs lasers together with computer experinients that manipulate carrier mobilities, thermionic emission rates, and capture times. Our results show tliat it is neither the transport of electrons or holes tu the quantum well that results in gain saturation. Ilalher, low frequency roll-off is primarily due to electrons that fail to get captured by the quantum well and, instead, are injected into the p side of the device and diffuse away from the active region.