Depletion region surface effects in electron beam induced current measurements

TL;DR

This paper investigates how surface effects influence electron beam induced current (EBIC) measurements in semiconductor materials, revealing nonuniform charge collection in FIB-prepared samples and proposing a model that accounts for surface recombination effects.

Contribution

The paper introduces a new model of EBIC response that includes surface recombination effects, explaining deviations observed in FIB-prepared samples and highlighting the impact of surface charge states.

Findings

FIB-prepared Si samples show nonuniform EBIC lineshape.

Surface recombination significantly affects EBIC response.

Charged surface model aligns with experimental data.

Abstract

Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Current models of EBIC assume that excitations in the $p$-$n$ junction depletion region result in perfect charge collection efficiency. However we find that in CdTe and Si samples prepared by focused ion beam (FIB) milling, there is a reduced and nonuniform EBIC lineshape for excitations in the depletion region. Motivated by this, we present a model of the EBIC response for excitations in the depletion region which includes the effects of surface recombination from both charge-neutral and charged surfaces. For neutral surfaces we present a simple analytical formula which describes the numerical data well, while the charged surface response depends qualitatively on the location of the surface Fermi level relative to the bulk Fermi level. We find the experimental data on FIB-prepared Si solar cells is most consistent with a charged surface, and discuss the implications for EBIC experiments on polycrystalline materials.