Higher efficiency InGaN laser diodes with an improved quantum well capping configuration

A thin AlGaN cap is typically employed above the multiple quantum wells of InGaN-based lasers to prevent electron overflow from the active region and to protect the InGaN active region from the high temperature growth of subsequent $\mathrm{p-}\mathrm{type}$ layers. The growth conditions and placement of this cap can significantly affect the efficiency and operating characteristics of laser diodes. A 200 Å ${\mathrm{Al}}_{\text{0.2}}{\mathrm{Ga}}_{\text{0.8}}\mathrm{N:Mg}$ cap was placed above the last barrier of a three quantum well (QW) laser diode, as well as directly above the last QW. Lasers with the cap above the last QW exhibit a lower threshold current and a higher internal quantum efficiency than lasers with the cap above the last barrier. The internal quantum efficiency nearly doubles from 16.6% for lasers with the cap above the last barrier to 34.7% for the cap above the last QW. The improvement in efficiency from moving the cap to the last QW is attributed to electrons falling into the last QW and participating in radiative recombination, instead of nonradiative recombination at dislocation sites of the last barrier, or recombining radiatively at the barrier wavelength, rather than at the laser emission wavelength.