Chapter 2 - Strained Layer Quantum Well Lasers

Abstract

This chapter reviews the manner in which the properties of semiconductor lasers can be considerably improved by the introduction of either compressive or tensile strain. The changes observed in visible lasers can be explained well in terms of the influence of strain on the band structure and gain processes. For long-wavelength lasers, high-pressure measurements of the quantum efficiency indicate that intervalence band absorption is effectively eliminated, whereas measurements of the threshold current show that Auger recombination has been reduced significantly in the devices investigated to date. Theoretical analysis, together with the measured pressure dependence of the threshold current, indicates that despite these improvements, the Auger process still remains the dominant recombination path; this behavior is reflected in the fact that the lasers remain temperature sensitive as predicted by a simple theoretical analysis. The increased rate of change of gain with carrier concentration in strained-layer lasers decreases the linewidth enhancement factor α , resulting in narrower linewidth and reduced chirp, while the modulation bandwidth is also enhanced in p-doped strained structures. We also have discussed briefly how the relaxation of the constraint of lattice matching allows new materials combinations and novel device applications, such as polarization-insensitive amplifiers that contain both tensiley and compressively strained QW layers. Strained layers retain the benefits of lattice-matched QW structures, and are also able to take advantage of new physical effects and materials combinations.