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

A new experimental framework for the characterization of defects in semiconductors is demonstrated. Through the direct, energy-resolved correlation of three analytical techniques spanning six orders of magnitude in spatial resolution, a critical mid-bandgap electronic trap level (E-V + 0.56 eV) within Ag0.2Cu0.8In1-xGaxSe2 is traced to its nanoscale physical location and chemical source. This is achieved through a stepwise, site-specific correlated characterization workflow consisting of device-scale (approximate to 1 mm(2)) deep level transient spectroscopy (DLTS) to survey the traps present, scanning probe-based DLTS (scanning-DLTS) for mesoscale-resolved (hundreds of nanometers) mapping of the target trap state's spatial distribution, and scanning transmission electron microscope based electron energy-loss spectroscopy (STEM-EELS) and X-ray energy-dispersive spectroscopy for nanoscale energy-, structure, and chemical-resolved investigation of the defect source. This first demonstration of the direct observation of sub-bandgap defect levels via STEM-EELS, combined with the DLTS methods, provides strong evidence that the long-suspected Cu-In/Ga substitutional defects are indeed the most likely source of the E-V + 0.56 eV trap state and serves as a key example of this approach for the fundamental identification of defects within semiconductors, in general.