Gain spectra measurements in n-type modulation doped GaAs/AlGaAs quantum well lasers

The core of quantum well lasers provides a large parameter space for improving device performance due to the many variables available including well width, barrier and well compositions, and strain. Doping profiles have only recently been investigated primarily due to the difficulty in predicting where carriers will reside. Modulation doping refers to the doping of regions very near the quantum well but not the quantum well itself so as to populate the quantum well with carriers without reducing the radiative efficiency due to the presence of dopant atoms. Modulation doped quantum well lasers have been investigated for improved modulation performance in the case of p-type doping and reduced threshold current density in the case of n-type doping. For the n-type modulation doped lasers, shortened emission wavelength was also observed for heavily doped devices. This wavelength shift was attributed to the additional electron population altering the gain spectrum. As the number of electrons increases by changing the doping profile, the lasing wavelength moves to wavelengths corresponding to energies between the n=l and n=2 electron to heavy hole quantum well transitions. However, unlike previous n=2 lasers which have very high threshold current densities, there is either a relatively small increase or decrease in threshold current density with shortening laser wavelength in these modulation doped lasers. In this work, we present measured gain spectra of lasers having both undoped and modulation doped cores. The gain spectra clearly show the effects of modulation doping on both the position and shape of the gain curve