Fabrication of Multi-layer Self-assembled InAs Quantum Dots for High-Efficiency Solar Cells

We have investigated the characteristics of quantum dot (QD) solar cells with stacked multilayers of self-organized QDs. The GaAs-based p-i-n solar cells with 20 stacked InAs QD layers were fabricated by atomic hydrogen-assisted molecular beam epitaxy (H-MBE) on GaAs (001) substrates, and multiple stacking of InAs QDs was achieved by strain-compensation growth technique, in which the tensile strain with respect to GaAs substrate induced by InAs QDs is compensated by a compressive strain induced by GaNAs spacer layers. By controlling the net average lattice strain to a minimum by covering each QD layer with a 40 nm-thick GaN_0.005As_0.995 strain compensating layer (SCL), we were successful in obtaining a superior QD stacked structure with no degradation in size uniformity. Further, no dislocations were observed even after 30 layers of stacking, and the area density of QDs amounted to the order of 10¹² cm^-2 . The external quantum efficiency characteristics show a spectral response up to 1150 nm. The enhanced absorption performance of the cell in the lower photon energies is attributed to InAs QDs inserted in the i-layer