Influence of In0.15Ga0.85As capping layers on the electron and hole energy levels of InAs quantum dots

Self-assembled InAs/(In,Ga)As quantum dots were embedded into n- or p-type Schottky diodes to investigate the conduction and valence band states, respectively. The samples were prepared by molecular beam epitaxy, and capacitance–voltage (C(V)) spectroscopy as well as photoluminescence (PL) measurements were performed and compared here. To study the influence of the capping layer, the quantum dots were overgrown either by GaAs or In0.15Ga0.85As. For the In0.15Ga0.85As cap, the ground state PL at 300 K is red-shifted to 1319 nm as compared to 1261 nm for the GaAs cap. This red-shift observed in PL is compared to the shift of the corresponding electron and hole energy levels obtained from C(V) spectroscopy. It is found that only 18% of the red-shift originate from the valence band states, while 82% originate from the conduction band states.