Bandgap profiling in CIGS solar cells via valence electron energy-loss spectroscopy

Abstract

A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn 1-x Ga x Se 2 solar cell that possesses intentional Ga/(In þ Ga) composition variation. The EELSdetermined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., >100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconductor materials and devices.

Direct Nanoscale Characterization of Deep Levels in AgCuInGaSe2 Using Electron Energy‐Loss Spectroscopy in the Scanning Transmission Electron Microscope

“…Scanning-DLTS is a recently developed related technique, performed using a scanning Kelvin probe microscope (SKPM), that provides 2D trap concentration mapping with mesoscale (hundreds of nanometers) resolution. [33] Analysis of the EELS low-loss, or valence, range (<30 eV) can be used to identify interband transitions, like the bandgap, [30,34,35] and as we demonstrate here, sub-bandgap defect levels. Conversely, STEM-EELS, which probes the energy losses associated with characteristic, inelastic interactions between the incident electron beam and the specimen, [30][31][32] is confined to a thin sample cross-section, but can achieve sub-nanometer spatial resolution.…”

“…To isolate this anomalous spectral feature for further analysis, the zero‐loss peak (ZLP) was subtracted from the Region 2 EELS spectrum following the procedure detailed in the Supporting Information and shown in Figure S1 (Supporting Information) . After ZLP removal, the inelastic signal was fitted using two Gaussian functions between 0.0 and the ≈1.2 eV ACIGS bandgap, as shown in Figure 4 a, ostensibly providing delineation between the apparent sub‐bandgap peak and the continuous intensity at higher energies—here, presumably related to the onset of the bandgap itself, as well as potentially the E V + 0.98 eV shallow trap state that resides within the ACIGS material.…”

“…Cliff-Lorimer k-factors, which correlate X-ray count intensity to quantitative composition information in XEDS analysis, [35] were experimentally determined through a method that utilizes the linear relationship between X-ray counts and specimen thickness, [44] and data was processed using the Bruker ESPRIT software suite. EELS spectra were collected using an electron beam accelerating voltage of 60 kV, 30 pA probe current, probe convergence angle of 12 mrad, and a spectrometer collection angle of 22 mrad.…”

“…The same microscope is also fitted with an FEI SuperX XEDS system with a collection solid angle of 0.9 srad. Cliff‐Lorimer k ‐factors, which correlate X‐ray count intensity to quantitative composition information in XEDS analysis, were experimentally determined through a method that utilizes the linear relationship between X‐ray counts and specimen thickness, and data was processed using the Bruker ESPRIT software suite.…”

“…Conversely, STEM‐EELS, which probes the energy losses associated with characteristic, inelastic interactions between the incident electron beam and the specimen, is confined to a thin sample cross‐section, but can achieve sub‐nanometer spatial resolution . Analysis of the EELS low‐loss, or valence, range (<30 eV) can be used to identify interband transitions, like the bandgap, and as we demonstrate here, sub‐bandgap defect levels. When performed in an STEM, compositional and structural information can also be obtained via X‐ray energy‐dispersive spectroscopy (XEDS) and electron diffraction, respectively, allowing for direct correlation between electronic, compositional, and structural information.…”