Modelling dilute nitride 1.3 μm quantum well lasers: Incorporation of N compositional fluctuations

Summary form only given. Dilute nitride GaInNAs/GaAs quantum well (QW) lasers have been subject to intensive study since being first proposed by Kondow et al. [1]. Dilute nitride GaInNAs materials have a wide range of applications such as long wavelength infrared laser diodes, high efficient multi-junction solar cells, broad band semiconductor optical amplifiers (SOA) and tuneable lasers. The GaInNAs/GaAs material system has a large band-gap bowing which results in a large conduction band offset [2] and this system has the potential to cover a range of optical communication wavelengths by controlling the N composition. Also, the reduced temperature sensitivity and observed broad-band gain have made GaInNAs a promising candidate for un-cooled and tuneable communication lasers at 1.3 μm.Incorporation of N into GaInAs results in low photon luminescence (PL) intensities with wide line-widths [3], and thus for lasers, tuneability over a broad gain higher albeit with increased threshold current densities [3]. We have modelled the gain in GaInNAs/GaAs QW lasers this using a Band Anti-crossing (BAC) model [2] including the spatial compositional fluctuations of the N that lead to quantum dot (QD)-like fluctuations at the conduction band minimum. Therefore we use an array of inhomogeneous broadened QDs to represent the CBE fluctuations. We model this system using a rate equation approach. This gives us the population of the electrons in the QW energy level and within the energy levels of the inhomogeneous array of QD-like fluctuations which can be used to calculate the gain from the QW and the QD resulting photon output. Positive gain only occurs for levels with electron densities above transparency while absorption (negative gain) occurs below this electron density. At low nitrogen composition (N=1%), due to small density of states (DOS) of the QD-like fluctuations, the electron density is insufficient to reach the lasing threshold of the QD system. These fluctuatio- s act like defected-related non-radiative centres. We compare this rate equation analysis to one considering the monomolecular (defect-related) recombination process and find good agreement with the experimental increase in threshold current density. As the N composition increases we observe an increase in the lasing threshold as shown in Fig. 1 (a). For N=2% the density of the QD-like fluctuations is enough to allow lasing from electrons in these QD states. In this case we see simultaneous lasing occurs at both QW and QD energy states. We also observe carrier dynamics between the two systems which can result in short pulse lasing generation shown as Fig. 1 (b).The electron-photon dynamics can be used to calculate the material gain arising from both the QW confined level and from the QD-like fluctuations. It is observed to be broadened relative to the gain from the QW level only. We evaluate the gain for a single QW system as a function of input current. This model can be extended to a multi-quantum well system to further broaden the gain spectrum for use in comb generators.