Growth and optical properties of strain-induced quantum dots

In recent years, high-quality quantum dots (QD) have been fabricated using self-organized island growth of strained layers, e.g., InAs on GaAs. In our approach, self-organized InP islands are used as stressors on top of a near-surface quantum well (QW), typically an InGaAs/GaAs QW. The strain field of the InP island causes a nearly parabolic lateral potential below the island. Vertical confinement is obtained by the QW potential. The QD structure can be easily tailored by changing the QW composition and thickness, the distance of the QW from the InP stressor, or the size of the stressors by varying the growth temperature. Furthermore, coupled QDs and QD superlattices have been fabricated by introducing two or more QWs into the structure. Narrow linewidth QD ground and excited state transitions are obtained by low-temperature photoluminescence (PL). The experimental transition energies agree well with the theoretical modeling based on the finite element method. Time-resolved luminescence experiments yield a radiative recombination time of 0.9 ns and an interlevel relaxation time of 0.6 ns for the electrons. PL up-conversion experiments show a fast rise time of ~ 1 ps for all QD transitions, which suggests that Coulomb scattering is the dominant scattering mechanism in the initial stage in agreement with the modeling. The effect of magnetic field on the optical properties of the QDs has been studied using a field up to 8 T, where a large Zeeman splitting of the excited QD states has been observed in agreement with a single-particle model.