Investigating dilute nitride materials for broad band SOAs for optical communications

The dilute nitride GaInNAs/GaAs quantum well material has been subject to intensive study since it was first proposed by Kondow et al. It has wide applications such as in long wavelength infrared laser diodes, high efficient multi-junction solar cells and broad band semiconductor optical amplifiers (SOA). Conventional materials for these emission applications based on GaInAsP/InP have poor temperature stability due to a small conduction band discontinuity resulting in poor electron confinement. The GaInNAs material system has a a bandgap that can be tuned, whilst remaining lattice matched to GaAs. In addition, it was found experimentally that a large band-gap bowing reduced the bandgap even further and resulted in a large conduction band offset. Thus this dilute nitride system has the potential to cover a range of optical communication wavelengths by controlling the small nitrogen concentration. Also, the reduced temperature sensitivity and observed broad-band gain have made GaInNAs a promising candidate for broad-band laser and SOA design. Dilute nitride has been found to be one of a class of such materials known as highly mismatched alloys (HMA) in which the addition of one constituent strongly affects the alloy properties such as band-gap and effective mass. Since the emergence of dilute nitride other such HMA have been discovered and these follow similar trends. Recent applications for this broader class of HMAs are as intermediate band solar cells and as Gunn-type electronic diodes. Incorporation of N into GaInAs results in low PL intensities with wide line-widths and the resulting lasers have high threshold current densities, which have been attributed to the difference between the N and As atoms in the lattice structure of GaInNAs. This has been successfully analysed using a Band Anti-crossing (BAC) model [7] in which the N acts as a defect on the GaInAs conduction band mixing with it and pushing it downwards. The N defect level also alters the effective mass of the conduction band. Spatial variation in the N composition leads to quantum dot (QD)-like fluctuations at the conduction band edge(CBE) as shown schematically in Fig 1. Therefore it is crucial to understand the effect of these QD-like fluctuations in GaInNAs material systems.