SPICE-Aided Design of Low Threshold, Wide Bandwidth 1.55 µm Strained MQW Lasers

Abstract

Carrier transport properties have been demonstrated to strongly affect the static and dynamic behavior of quantum well lasers [1]. We have developed a tree level rate equation model (figure 1), which treats each well independently, in order to distinguish between space and state transport and to take nonuniform injection and carrier dependent gain for each well into account. Starting from the level scheme reported in figure 1, a complete set of rate equations can be derived and implemented on a SPICE circuit emulator thanks to their formal analogy with current balance equations at a capacitor node. By using SPICE, static and dynamic simulation can be run without any change in the model. The active region simulation can be integrated as well with parasitic, package equivalent circuit and other electrical components. Model parameters have been extracted from measurements on real devices. The model has been used to analyze the device behavior, identifying P-I curve bending in short devices as an heterojunction leakage effect (less efficient capture at higher injection currents). An improvement was found changing the number of wells: figure 2 shows the optimum well number, that gives the best static properties. Similar investigations have been done concerning the modulation bandwidth, suggesting the use of a larger number of wells. On the basis of these results we have grown structures with different well numbers (5, 9, 13 and 17 wells) using MOCVD (Metallo Organic Chemical Vapor Deposition). Wells were compressive strained (+0.9%) and barriers were tensile strained (0.5%) and respectively 8 and 9 nm wide. Threshold current densities of broad area lasers were measured, showing an optimum number of well around 9 (figure 3). Using this last structure Semi Insulating Buried Heterostructure Fabry-Perot laser have been then realized, obtaining 6-8 mA threshold currents, up to 22% external efficiency and very good temperature behavior. Due to parasitic effects the bandwidth was limited to 15 GHz. The intrinsic material bandwidth was then evaluated both by direct measurement with optical modulation, and by analytical processing of electrical bandwidth. As reported in figure 4, both ways lead to the same result which is an intrinsic material bandwidth in excess of 20 GHz.