Impact of strain on periodic gain structures in vertical external cavity surface-emitting lasers

Abstract

In this article, the impact of strain relaxation on the emission properties of InGaAs/GaAs multiple quantum wells without strain compensation was examined. Structures consisting of different numbers of quantum wells, namely 4, 8, 12 and 16, on top of distributed Bragg reflectors were grown by molecular beam epitaxy as a typical vertical external cavity surface-emitting laser (VECSEL). The relation between emission parameters in the lasing regime and strain relaxation were investigated. A two-step control of the growth rate allowed for obtaining fixed spectral detuning in all structures regardless of the number of quantum wells. The heterostructures varied in its strain and the microcavity length. The other parameters remained unchanged. In consequence, for the first time a unique set of VECSEL-like heterostructures was investigated. The strain was analyzed by reciprocal space mapping using high-resolution X-ray diffractometry. It was found that the degree of structure relaxation caused by misfit dislocation generation depends linearly on the number of quantum wells. By fitting numerical simulations to the experimental results, we have quantitatively determined the extent to which output power was suppressed by increase in non-radiative recombination arising from misfit dislocations. The non-radiative coefficients were determined. Taking output power as a criterion, we determined the optimal number of QWs to be 12 and the maximum tolerable relaxation value of 0.27 for InGaAs/GaAs VECSEL structures with uniformly distributed quantum wells in microcavity. The dependence of the monomolecular recombination coefficient on structure relaxation has been determined.