Mechanism of Zn and Si diffusion from a highly doped tunnel junction for InGaP/GaAs tandem solar cells

Diffusion of impurities (Zn and Si) from a tunnel junction during epitaxial growth and the effects of impurity diffusion on InGaP/GaAs tandem cell pro perties have been investigated. Zn diffusion from the tunnel junction has b een found to deteriorate the effect of the back-surface field layer on mino rity carrier reflectance in the InGaP top cell and degrade the quantum effi ciency of the top cell. Furthermore, Zn diffusion has been found to be enha nced around the threading dislocations from a GaAs substrate and creates sh unt paths only in the top cell region. Si diffusion, which degrades the qua ntum efficiency of the GaAs bottom cell, has also been observed when a diff erent substrate with high etch pit density was used. Such anomalous diffusi on of Zn has been found to be suppressed by using a double-hetero structure InGaP tunnel junction sandwiched by AlInP layers. It has been found that t he Zn diffusion occurs as a layer highly doped with Si being formed nearby and Zn diffuses in the opposite direction from the Si-doped layer. The Zn d iffusion is thought to be caused by group III self-diffusion which originat es in the highly doped n-type layer. The direction of Zn diffusion is thoug ht to be due to Coulombic repulsion between the substitutional Zn on the Ga site and the substitutional Si on the As site. The large energies of the f ormation and migration of group III vacancies in the AlInP barrier layers a nd InGaP tunnel junction layers are thought to suppress Zn diffusion from t he tunnel junction. (C) 1999 American Institute of Physics. [S0021-8979(99) 04203-6].