Dilute Nitride III-V Superlattices Lattice-Matched to Silicon

Dilute nitrides quaternary alloys of GaP_1-x-yAs_y N_x have attracted much attention since they exhibit a direct bandgap and their lattice constant matches the one of silicon for y=4.7x-0.1. These alloys offer interesting perspectives for the monolithic integration of III-V photovoltaics with the silicon technology. However, for practically achievable nitrogen composition (xles0.04), the band gap of GaP_1-x-yAs_yN_x alloys lattice matched to Si is limited to about 1.75 to 1.8 eV. In an attempt to expand the operation wavelength of these dilute nitrides further toward the infrared a short period GaP_1-xN _x/GaAs_1-yN_y superlattice strain-balanced and lattice matched to silicon is devised. An eight-band Kane Hamiltonian modified to account for the strain effect and the band anti-crossing model is used to describe the electronic states of the highly strained GaP_1-xN_x and GaAs_1-yN _y ternaries. A transfer matrix method is applied to determine the electron and hole minibands of the superlattice structure, and the evolution of the band edge transition energies for different nitrogen compositions and alloying/thickness combinations. It is shown that lattice matched superlattice design, proposed here, allows for an additional bandgap shrinkage of 50-150 meV compared to quaternary alloys of similar average N concentration. The approach thus expands the direct bandgap photovoltaic material palette and offers new opportunities for two and three-bandgap lattice matched tandems operating in conjunction with Si bottom cells