Hyperspectral Imaging of Localized, Optically-Active Defects in GaAs Solar Cells

A novel hyperspectral imaging instrument with dual electroluminescence and photoluminescence capability was used to image and analyze localized radiative defects in rear-junction GaAs solar cells grown at 1 $\boldsymbol{\mu} \mathbf{m}/\boldsymbol{\min}$ . The absolute photon emission rates were measured over a wide spectral range encompassing both the sub-band gap region and the higher photon energies, resolving spatial features on the order of 1 $\boldsymbol{\mu}\mathbf{m}$ with complete spectral information. We find that some radiative defects are pinholes in the GaAs active layer, transmitting luminescence signal from other buried device layers. More notably, we observe the formation of a high-intensity, halo-like defect band in the vicinity of some of these processing-related defects. Such defects show a characteristic double-peak emission with maxima at 870 nm and 894 nm at room temperature corresponding to the band-to-band and the band-to-impurity optical transitions respectively. These sub-band gap radiative regions are likely gallium antisite defects and are thought to form shallow impurity bands within the band gap.