Theoretical investigation of direct and phonon-assisted tunneling currents in InAlGaAs/InGaAs bulk and quantum-well interband tunnel junctions for multijunction solar cells

Abstract

Direct and phonon-assisted tunneling currents in InAlGaAs-InGaAs bulk and double-quantum-well interband tunnel heterojunctions are simulated rigorously using the nonequilibrium Green's function formalism for coherent and dissipative quantum transport in combination with a simple two-band tight-binding model for the electronic structure. A realistic band profile and the associated built-in electrostatic field are obtained via self-consistent coupling of the transport formalism to Poisson's equation. The model reproduces experimentally observed features in the current-voltage characteristics of the devices, such as the pronounced current enhancement in the quantum-well junction as compared to the bulk junction and the structure appearing in the negative-differential resistance regime due to quantization of emitter states. Local maps of density of states and the current spectrum reveal the impact of quasibound states, electric fields, and electron-phonon scattering on the interband tunneling current. In this way, resonances appearing in the current through the double-quantum-well structure in the negative-differential resistance regime can be related to the alignment of subbands in the coupled quantum wells.