STEM EBIC as a Quantitative Probe of Semiconductor Devices

TL;DR

The paper addresses the challenge of nanoscale carrier transport characterization in thin device lamellae by deploying STEM-EBIC on silicon photodiode lamellae prepared with Ga-FIB and PFIB. It integrates STEM-EBIC imaging with I–V measurements and thickness/composition mapping to evaluate the quantitative potential of STEM-EBIC for nanoscale devices, highlighting the influence of surface damage and contact geometry. The authors find that diffusion lengths extracted from EBIC profiles are orders of magnitude shorter than bulk values due to pronounced surface recombination and FIB-induced artifacts, and that diode-like electrical behavior is not recovered in the lamellae due to high-resistance contacts. The study demonstrates STEM-EBIC as a quantitative probe of carrier transport in nanoscale devices while emphasizing the importance of artifact control and contact engineering for reliable metrology at sub-5 nm technology nodes.

Abstract

Electron beam-induced current (EBIC) imaging in the scanning transmission electron microscope (STEM), STEM-EBIC, provides direct access to carrier transport at the nanoscale. While well established in bulk SEM geometries, its application to thin TEM lamellae remains largely unexplored. Here, we present a systematic STEM-EBIC study of silicon photodiode lamellae prepared by gallium and xenon focused ion beam (FIB) milling. We directly visualize the p-n junctions in thin cross sections and extract effective diffusion lengths for electrons and holes as a function of local thickness. The values are orders of magnitude smaller than those obtained by SEM-EBIC on bulk silicon, reflecting pronounced surface recombination and FIB-induced surface modifications. Current-voltage measurements further reveal severe deviations from the expected diode-like behavior, which we attribute to ohmic metal-semiconductor contacts in the emasurement setup. Our analysis establishes STEM-EBIC as a quantitative probe of carrier transport in nanoscale devices.

Figures (5)

• Figure 1: Characteristics of the bulk silicon PD: (a) Light micrograph of the device, with the purple region marking the active area. (b) Secondary electron image of the front side (cf. red line in (a)) overlaid with EBIC contrast (red). (c) Current–voltage characteristics of the PD.
• Figure 2: STEM measurements. Left column: ADF images of the two TEM samples prepared using Ga-FIB (a) and PFIB (b). Right columns: Corresponding elemental maps of the regions highlighted by red rectangles in the left column.
• Figure 3: I–V characteristics of the TEM lamellae prepared using the Ga-FIB (a) and PFIB (b). The mean sample thicknesses, averaged over the whole silicon area between the slits, are indicated in the legend. Note that the current density range is larger by three orders of magnitude in (a) with respect to (b).
• Figure 4: STEM-EBIC maps of the Ga-FIB (b) - (e) and PFIB (g) - (j) samples, with the depletion region visible as red contrast. The white rectangles mark the positions of the linescans shown in Fig. \ref{['fig:Fig. 4']}. Sample thicknesses were determined at the points where the linescans intersect the depletion region. The ADF images of the corresponding sample areas for the Ga-FIB and PFIB are displayed in (a) and (f).
• Figure 5: Line profiles of the EBIC signal and effective diffusion lengths: the line profiles of the normalized EBIC signal for the indicated sample thicknesses measured on samples prepared with Ga-FIB (a) and PFIB (b).The curves were vertically offseted for better visibility. The gray and black dashed lines indicate fits of Eq. \ref{['eq:diffusion length']} to the rising and falling edges of the peaks (see text for details), i.e. on the p- and n-type side of the junction, respectively. (c) and (d) Effective diffusion for electrons (red) and holes (green) as obtained from fits of Eq. \ref{['eq:diffusion length']} in the p- and n-regions for the Ga-FIB (c) and PFIB (d) samples, respectively.