Enhancement of Exciton Transition Probabilities in Ultrathin Films of Cadmium Telluride

Abstract

A great deal of interest is now focused on properties of thin semiconductor layers. It is caused by the success of film growth techniques, which are now capable of preparing films with thicknesses in the nanometer range. The two main techniques are molecular beam epitaxy (MBE) and metal-organic compound vapour deposition (MOCVD). At low film thickness, carrier parameters such as gap width, effective mass, and exciton binding energy differ from those of a bulk material. With the change of gap width, caused generally by the quantum size effect [1,2], the carrier effective masses begin to depend on the film thickness, thus changing the spectra of Wannier-Mott excitons. In addition, when the film thickness becomes smaller than the excitonic Bohr radius (a_B), the exciton wave function, which is initially spherical in bulk isotropic crystals, suffers deformation, caused by the confinement of motion along the z-direction, and changes completely to the 2D case at zero thickness. Theoretical studies predict that the wavefunction deformation due to the decrease of thickness results in an enhancement of exciton optical transition probabilities, i.e., in other words, in the enhancement of the oscillator strength [1,3]. The dependence of the positions of the excitonic peaks on film thickness was well studied both theoretically and experimentally [1–7].