Interlevel optical transitions and many-body effects in a dense array of Ge/Si quantum dots

We have investigated experimentally the mid-infrared normal-incidence respo nse of holes confined in array of Ge/Si self-assembled quantum dots. The do ts have a lateral size of approximately 15 nm and a density of 3 x 10(11) c m(-2). An in-plane polarized absorption in the 70-90 meV energy range is ob served and attributed to the transition between the first two states in the dots. As the hole concentration in the dot ground state is increased, the absorption peak shifts to higher energies, its linewidth is reduced, and th e lineshape is changed from an asymmetric to symmetric one. We attribute al l features to a depolarization-type effect caused by collective interlevel excitations. (C) 2000 Elsevier Science B.V. All rights reserved.