In this work, a systematic modeling study of polarization-induced internal field effects on the gain spectrum and threshold current density was performed. Two laser diodes of technological relevance: the In0.2Ga0.8N/GaN hexagonal nitride laser structures grown along the polar c-axis with internal field values up to 1.8MV/cm, and the In0.1Ga0.9As/Al0.15Ga0.85As (111)B laser structures with internal field values up to 100kV/cm were studied. The gain model is based on a self-consistent solution of Poisson-Schroedinger equations, and takes into account strain effects, free carrier screening, and the field dependence of gain and spontaneous emission rate.
In the nitride case, some of our main findings are: (a) assuming a laser structure with a single In0.2Ga0.8N/GaN quantum well (QW) and a modal gain Γg=30cm-1, the optimal QW width in terms of lowest current threshold is ≈3nm. (b) For a 3nm-wide QW and Γg=30cm-1, the presence of the internal field increases the threshold current over the zero-field value by at least a factor of three. This factor increases further for higher Γg's. (c) The optimal number of In0.2Ga0.8N/GaN QWs in the active region of a nitride laser with cavity losses of 30cm-1 is four, assuming homogeneous QW pumping.
In the arsenide case, however, our modeling shows that in some circumstances the internal field is beneficial and can lead to a substantial reduction of the threshold current, especially for cavities with low optical losses. This reduction was confirmed experimentally, by measuring systematically lower threshold current densities in In0.1Ga0.9As/Al0.15Ga0.85As (111)B laser diodes, compared to (100)-ones, carrying no internal field.