Abstract
We present a model for lasing oscillation due to bi-excitons and localized bi-excitons in wide-gap II–VI semiconductor quantum wells. We introduce the condition for population inversion in bi-exciton–exciton optical transition, and show the importance of studying the thermodynamic equilibrium between bi-exciton and exciton states. The Saha equation shows that bi-excitons decompose into two free excitons at fairly low temperatures due to the small binding energy (10 to 30 meV) in II–VI quantum wells, making the population inversion possible only at low temperatures. We point out that bi-excitons localized at certain potential minima in the quantum-well plane will achieve the population inversion even at room temperature if the local potential is deep enough to prevent thermal activation (about 100 meV). By simple theoretical calculation, we show that the localized bi-exciton–exciton optical transition will produce sufficient optical gain for lasing oscillation and has potential application in low-threshold-current blue-light emitting lasers.