Buried interface and luminescent coupling analysis with time-resolved two-photon excitation microscopy in II-VI and III-V semiconductor heterostructures

Semiconductor heterostructures are used in high-efficiency solar cells and in other electronic devices. Solar cells can’t reach thermodynamic efficiency limits in part due to the charge carrier recombination, and efforts are applied to understand and reduce recombination. We describe a novel experimental approach to identify and quantify recombination losses at semiconductor interfaces. Using time-resolved two-photon excitation microscopy, we generate carriers at well-defined absorber depths and find that the red spectral shift of the photoluminescence (PL) emission can be used as a “spectroscopic ruler” to identify recombination depth up to 30 μm. We apply this analysis to quantify Shockley-Read-Hall recombination at the buried CdTe/CdTe interface, where 15 μm thick epitaxial CdTe is grown by the molecular beam epitaxy on the single crystal CdTe substrate. We also measure luminescent coupling between the GaInP and GaAs layers in heterostructures grown by the metal-organic chemical vapor deposition. Our results resolve important limitations for accurate 3D charge carrier lifetime tomography. Earlier we analyzed recombination due to extended defects and grain boundaries with the lateral resolution sufficient to resolve such features (approximately 0.5 μm), but interpretation of the carrier lifetime microscopy data for buried interfaces and buried semiconductor layers was a challenge. Using methods described here, the axial (z) coordinate for the PL microscopy measurements becomes as well defined as the lateral (x, y) coordinates, enabling accurate 3D identification and analysis of the charge carrier recombination locations in semiconductor heterostructures.