Importance of growth direction in mid-infrared quantum cascade lasers

We report on the effect of growth direction on the performance of mid-infrared Quantum Cascade lasers. The design used has a symmetric active core, capable of operating under both negative and positive polarities, which allows to test for residual growth asymmetries such as interface roughness and dopant migration. Calculations of scattering lifetimes from interface roughness and ionized impurities suggest a dominant contribution from the former, with devices biased positively averaging ∼15% larger broadening and ∼50% shorter upper state lifetime than negatively biased devices. Experimental results for positively biased devices show at least 30% larger broadening and 35% lower electroluminescence peak intensity than those biased negatively, in good agreement with the modeling results.