Hole transport through an isotype amorphous-crystalline Ge2Sb2Te5 heterojunction under forward bias

The transport of holes through a representative isotype amorphous-crystalline Ge2Sb2Te5 heterojunction under forward bias is explored for the first time. An approximate analytic model, based on the exact solution to Poisson's equation, the Continuity equation, and the Transport equation, is proposed to describe the forward current-voltage characteristic and hole quasi-Fermi level distribution across the heterojunction with a reduced set of material and device parameters. The proposed model incorporates thermionic emission across the heterojunction interface as well as drift and diffusion across the quasi-neutral regions of the heterojunction layers, but neglects drift and diffusion across the space-charge regions of the heterojunction layers, as well as recombination. Solar cell capacitance simulation results demonstrate general agreement between the approximate and exact solutions. Therefore, the approximate model is effective in capturing the physics of thermionic emission-limited transport at low applied bias and drift-diffusion-limited transport through the quasi-neutral region of the amorphous layer at larger applied biases. However, simulation results also show that an extremely narrow subregion of the space-charge region within the amorphous layer, which has been neglected within the proposed model, limits the transport of holes at very low bias and inhibits transport at all other biases. Nevertheless, the proposed model provides improved accuracy across the entire bias range compared to the individual thermionic emission or drift-diffusion models.