The ultimate performance of semiconductor materials and devices relies on the ability to characterize and optimize their structural, chemical, and electronic properties at the nanoscale. While high-resolution structural characterization via aberration-corrected scanning transmission electron microscopy (STEM) is approaching routine, the same cannot be said yet of electronic structure/properties analysis. However, with drastic improvements in energy resolution via recent advances in monochromation, low-loss electron energy-loss spectroscopy (EELS) has the potential to fill this much-needed role. Here, we present work on the application of low-loss EELS, using a monochromated FEI Titan3 G2 STEM, toward two important electronic characterization thrusts in CuIn1-xGaxSe2 (CIGS) solar cells: spatially-resolved bandgap profiling using a newly-developed analysis approach, and detection of sub-gap defect states with high spatial and energy space accuracy.
Compositional nonuniformity can play a significant role in device performance due to resultant changes within a material’s bandgap. However, accurate characterization of such issues, especially in a complex, phase-rich material system like CIGS, is far from straightforward. A new, simplified bandgap extraction method, based on straightforward Gaussian fit model, was developed to enable more rapid and robust bandgap profiling. The applicability of this technique was demonstrated within the cross-section of a CIGS solar cell containing intentional Ga/(Ga+In) composition (and thus bandgap) gradients. Comparison of the EELS-based bandgap profile to the nominal profile calculated using STEM-based energy dispersive X-ray spectroscopic composition data shows excellent spatially-resolved agreement. While this approach sacrifices a small degree of absolute (systematic) accuracy, excellent internal precision is maintained, and the effectively intervention-free methodology improves analytical speed and robustness.
Electrically active defects, a well-known problem in CIGS (and many other semiconductor materials), limit achievement of maximum device performance. However, as before, correlating these defect levels with specific defect structures is exceptionally difficult. To this end, we present work on the energy- and spatially-resolved detection of sub-gap defect levels within two different CIGS samples with two different trap energies (EV + 0.43 eV and EV + 0.56 eV). Low-loss EELS is shown to not only enable spatially-resolved detection of these states, but is also found to provide identical energies to those obtained using conventional deep level transient spectroscopy (DLTS). Furthermore, correlation between a new scanned probe DLTS method and low-loss EELS show accurate correlation in both spatial localization and sub-gap energy position. Taken together, these results indicate the potential of high-resolution low-loss EELS for the accurate nanoscale characterization of important electronic structure details.