Semiconductor quantum dots

Since the invention of semiconductor lasers, huge improvements in device performance have been achieved, and a large variety of specialized designs for different applications were conceived. Two major steps have played a key role in the improvement of device properties. The first step was the application of semiconductor heterostructures that allowed the separate optimization of optical and carrier confinement. The second step was the introduction of quantum films, also called quantum wells, in the carrier recombination zone (started in the 1980s). This permitted a strong reduction of threshold current density due to an increased density of states at the laser energy. This effect of increased density of states is related to the partial discretization of the allowed energy states of carriers, i.e., electrons and holes, and is based on quantum mechanical principles. One major advantage of quantum-dot structures results from the full three-dimensional carrier confinement on a nanometer scale. Therefore, a semiconductor quantum dots, InAs dots embedded in GaAs, behave like non- or weakly interacting single atoms. In addition, the realization of device-quality quantum dot structures became possible by the introduction of self-organized growth. Both, molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE) techniques, which are capable of the controlled deposition of a fraction of an atomic monolayer, can be used.