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Contract No. DE-AC36-08GO28308

A Model for Electron-BeamInduced Current Analysis of 

mc-Si Addressing Defect 

Contrast Behavior in Heavily 

Contaminated PV Material

Preprint

Harvey Guthrey

Colorado School of Mines and National Renewable 

Energy Laboratory

Brian Gorman

Colorado School of Mines

Mowafak Al-Jassim

National Renewable Energy Laboratory

Gianluca Coletti

ECN Solar Energy

Presented at the 2012 IEEE Photovoltaic Specialists Conference

Austin, Texas

June 3–8, 2012

Conference Paper

NREL/CP-5200-54108

June 2012
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1

A Model for Electron-Beam-Induced Current Analysis of mc-Si 

Addressing Defect Contrast Behavior in Heavily Contaminated PV 

Material

Harvey Guthrey1,2, Brian Gorman1, Gianluca Coletti3, Mowafak Al-Jassim2

1

Colorado School of Mines, Golden, Colorado, United States

2

National Renewable Energy Laboratory, Golden, Colorado, United States

3

ECN Solar Energy, Petten, Netherlands

Abstract — Much work has been done to correlate electronbeam-induced current (EBIC) contrast behavior of extended 

defects with the character and degree of impurity decoration. 

However, existing models fail to account for recently observed 

contrast behavior of defects in heavily contaminated mc-Si PV 

cells. We have observed large increases in defect contrast with 

decreasing temperature for all electrically active defects,

regardless of their initial contrast signatures at ambient 

temperature. This negates the usefulness of the existing models 

in identifying defect character and levels of impurity decoration 

based on the temperature dependence of the contrast behavior. 

By considering the interactions of transition metal impurities 

with the silicon lattice and extended defects, we attempt to 

provide an explanation for these observations. Our findings will 

enhance the ability of the PV community to understand and 

mitigate the effects of these types of defects as the adoption of 

increasingly lower purity feedstocks for mc-Si PV production 

continues.

Index Terms — mc-Si, silicon defects, electron beam induced 

current, EBIC contrast, iron contamination.

I. INTRODUCTION

EBIC contrast behavior of various types of extended 

defects, including grain boundaries, stacking faults, and 

dislocations, have been studied extensively [1-7]. Particular 

attention has been given to the temperature dependence of the 

EBIC contrast, as plots of contrast at dislocations (Cd) vs. 

temperature reveal information about extended defect 

character and impurity decoration[8-11]. There are several 

classifications of Cd vs. T behavior that have been previously 

described[11]. Type 1 behavior is represented by a positive 

slope and high contrast at room temperature and is related to 

high concentrations of deep level defects related to impurities 

and dislocation cores. Type 2 behavior is characterized by a 

negative slope and light or undetected contrast at room 

temperature. Light contrast at 300K increasing with 

decreasing temperature represents defects decorated with 

metallic impurities in the 104 – 106 cm-1 range and shallow 

levels associated with strain. Type 2 defects showing little to 

no contrast at 300K are strain related or have concentrations 

of metallic impurities less than 104 cm-1

. Results of previous 

studies of the EBIC contrast of various extended defects have 

generally agreed with this model. One study however, 

revealed a large increase in contrast, for decreasing T, at grain 

boundaries, showing both high and low contrast at 300K[12]. 

This behavior was attributed to the increase of the reduced 

surface recombination velocity associated with the different 

boundaries. However, experimental data with this trend has 

only been reported for grain boundaries.

II. EXPERIMENTAL

To formulate an EBIC contrast model addressing the issues 

associated with heavily contaminated mc-Si material, several 

samples were selected that were known to have concentrations 

of impurities that would be higher than the concentrations 

associated with traditional processing methods. The subject of 

the study was EG-Si that was intentionally contaminated with 

metallic impurities in the melt and processed into mc-Si PV 

cells. An uncontaminated EG-Si mc-Si PV cell was the 

control. The EBIC measurements were performed on a JEOL 

5800 SEM equipped with a cold stage capable of achieving 

liquid nitrogen temperature.

III. RESULTS

EBIC images for the samples studied are shown in Fig. 1. 

The defects for which contrast measurement were taken are 

notated in both the images taken at 300K (a,c,e) and the 

images taken at 80K (b,d,f) and the results are summarized in 

Table 1. Figures 1a and 1b correspond to the EG-Si 

intentionally contaminated with 200ppm Fe in the melt, which 

is the most heavily contaminated specimen in this study. Two 

positions (1 and 2) were chosen, representing the traditional 

type 1 and type 2 room temperature contrasts. The contrast 

measurements at 80K reveal that both the areas measured 

exhibited large increases in contrast that deviate from the 

model. The contrast of the two defects measured from the 

sample contaminated with 50ppm Fe (Fig. 1c and 1d) does not 

have the same temperature dependent behavior. Mixed and 

lightly contaminated type 2 behavior fully describes the 
2

defects present in these images. Figures 1e and 1f are from 

EG-Si not contaminated with any known impurities. The 

contrast measurements represent type 2 behaviors as expected.

The 200ppm Fe sample also exhibits strong contrast 

increases in regions not associated with the typical line 

features. There is no detectable contrast at room temperature, 

but at 80K the contrast is at levels associated with type 1 

behavior. There is also a noticeable increase in contrast of the 

bulk material in the 50ppm Fe sample. The EG-Si sample did 

not have increased contrast in the bulk, suggesting that this 

effect is due to the Fe impurities.

Table 1. Summary of contrast measurements for the locations 

indicated in Figs. 1a-f.

Fig. 1. EBIC images for a) 200ppm Fe sample b) 50ppm Fe sample, 

and c) EG-Si sample at 300K and 80K with positions for contrast 

measurements indicated.

IV. DISCUSSION

The EBIC contrast measurements summarized in Table 1 

suggest a Cd vs. T relationship that is not fully described by 

the existing models. Using EG-Si and EG-Si with known 

concentrations of Fe added to the melt for the measurements 

suggests a strong dependence of the Cd vs. T behavior on 

contamination level. Both of the Fe-contaminated samples 

were taken from the top of a directionally solidified mc-Si 

ingot, so the concentration of Fe in each sample should be 

proportional to the amount added. However, the concentration 

of deep levels would be expected to increase with Fe 

contamination as the levels induced by Fe in interstitial 

positions, at grain boundaries and at dislocations, are deep in 

the bandgap[13]. If this is the case, then very strong contrast 

at all temperatures should be observed. However, the 

strongest contrasts are only observed at low temperatures. 

This behavior is typically understood as relating to the 

increased activity of shallow levels that become more active 

in the recombination process as temperatures decrease. 

However, the magnitude of the contrast increase suggests that

another mechanism is responsible.

V. SUMMARY

Based on these results and their reproducible nature, a 

modification of existing models or a new model of the 

temperature dependence of EBIC contrast for defects in mc-Si 

is necessary. This new approach should take into account the 

observed large increases in contrast at low temperatures in the 

context of high concentrations of impurities. As the mc-Si PV 

industry moves toward lower purity feedstock, an EBIC 

contrast theory tailored to high concentrations of impurities is 

necessary to provide mc-Si PV producers the opportunity to 

address the variation in defect behavior associated with

heavily contaminated material.

ACKNOWLEDGMENTS

Thanks to Manuel Romero, whose stimulating discussions on 

EBIC contrast made this work possible. This work was 

supported by the U.S. Department of Energy under Contract 

No. DE-AC36-08-GO28308 with the National Renewable 

Energy Laboratory.

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3

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