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

Abstract

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 (EV + 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 (≈1 mm2) 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 CuIn/Ga substitutional defects are indeed the most likely source of the EV + 0.56 eV trap state and serves as a key example of this approach for the fundamental identification of defects within semiconductors, in general.

DLTS investigations on CIGS solar cells from an inline co-evaporation system with RbF post-deposition treatment

“…600 meV: regularly observed for different measurement parameters, but sometimes difficult to evaluate. Similar results are reported in literature [11,13].…”

“…They investigated the effect of RbF as well, and observed no reduction of the density of this trap, supporting the present results. Deitz et al show measurements of a 560 meV trap in a silver alloy of CIGS (Ag, Cu)(In, Ga)Se 2 (ACIGS) as well and assigned it to the Cu In/Ga substitutional defects as the most likely source [13]. Ab initio DFT calculations by Zhang et al yield a trap energy of 580 meV for Cu In traps in CuInSe 2 (CIS) which confirms this assignment even more.…”

Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se2 absorber materials

“…The lowering of the midgap trap concentration upon annealing is consistent with the increase in measured lifetime by TRPL after dark heat exposure (vide infra) and is likely associated with reduced atomic disorder from annealing, leading to a reduction of Cu M (M = Ga or In) substitutional defects. 26 The observed reduction in V oc with DH1000, which we attribute to a decrease in carrier concentration, occurs despite this reduction in midgap defects.…”

STEM Tools for Semiconductor Characterization: Beyond High-Resolution Imaging

“…As well as establishing the expected compositional depence of the band gap, these analyses may allow further correlations, such as possible band gap polymorphism depence [ 62 ]. Additionally, recent studies have shown the potential of the technique for the analyses of atomic defects in semiconductors, being able to detect and characterize sub-bandgap defect levels, which are key in the later performance of the material [ 66 ].…”