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Subcontract Report 

NREL/SR-520-44088 

September 2008 

Precipitate Dissolution and 

Gettering under Vacancy 

Injection in Silicon 

Final Subcontract Report 

21 March 2006 – 15 January 2008 

T.Y. Tan and N. Li 

Duke University 

Durham, North Carolina 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Precipitate Dissolution and 

Gettering under Vacancy 

Injection in Silicon 

Final Subcontract Report 

21 March 2006 – 15 January 2008 

T.Y. Tan and N. Li 

Duke University 

Durham, North Carolina 

NREL Technical Monitor: Fannie Eddy 

Prepared under Subcontract No. XEJ-6-66132-01 

Subcontract Report 

NREL/SR-520-44088 

September 2008 

National Renewable Energy Laboratory

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ABSTRACT 

Using a deposited Al layer, optical processing at a temperature below the Si-Al eutectic temperature of Teu=577o

C for a few minutes and followed by T>Teu for a few more minutes are able 

to getter metallic precipitates out of multicrystalline Si. To accomplish the same, a few tens of hrs 

is needed when using thermal annealing at 700o

C. Possible mechanisms involved in opticalprocessing gettering are proposed. These mechanisms include vacancy injection, radiationenhanced solubility, and radiation-enhanced diffusion of vacancies and metal impurity atoms. 

Using FeSi2 as a model case for which the Si lattice expands concomitantly with the dissolution of 

the precipitates, physical modeling and numerical simulations are carried out to uncover and test 

the conditions for the mechanisms to be effective. The mechanisms are found to be effective provided that the injected Si vacancies due to alloy formation are nearly all retained inside and evenly 

distributed throughout the Si bulk at the lower temperature, and that the Fe atom migration energy 

barrier is reduced by ~0.15 eV by radiation at the higher temperature. On the other hand, for the 

gettering of a precipitate species for which the Si lattice will shrink concomitantly with the dissolution of the precipitates, the vacancy-injection mechanism will not only be ineffective but should 

also have a detrimental effect. 

INTRODUCTION 

 It is well known that gettering of metallic impurity precipitates from multicrystalline (mc-) Si 

using Al is not effective by thermal annealing at the relatively low temperatures of ~700oC for a 

short time, as is appropriate for solar cell processing. Using FeSi2 as a model precipitate species, 

we have shown that it will take more than 50 hrs at 700o

C to getter them out. We identified the 

limiting mechanism as the slow precipitate dissolution rate.1 In that study, the assumed conditions are that Fe was introduced at 900oC to its solubility of ~5.02x1013 cm-3 and then precipitated out to steady state at 700oC to the density of ρ=1011

 cm-3. On the other hand, it is also 

known that, using a deposited Al layer, gettering by optical-processing are able to getter metallic 

precipitates out of mc-Si with a total time of minutes.2

 Optical-processing gettering consists of 

two steps: (i) a low temperature step below the Si-Al eutectic temperature of Teu=577oC for several minutes; and (ii) a high temperature step with T>Teu for several minutes. In this report, the 

possible physical mechanisms responsible for the effectiveness in gettering under opticalprocessing conditions are proposed. Using FeSi2 as the model impurity precipitate, physical 

modeling and numerical simulations are carried out to test the effectiveness of these mechanisms 

by finding the appropriate conditions. The proposed mechanisms include vacancy injection, radiation-enhanced solubility, and radiation-enhanced diffusion of vacancies and metal impurity 

atoms 

 We have previously identified the involved physical processes and accordingly modeled the 

gettering of metals,1,3-6 including metal dissolution from the precipitates, diffusion of metal 

atoms to and their stabilization at the gettering sites, and the effect of the precipitate volume misfit with the Si matrix on the gettering effectiveness. In the present problem of optical-processing 

gettering of Fe and FeSi2 as the gettered metal and precipitates, we assume that the misfit due to 

precipitate dissolution is accommodated primarily by the point defect vacancies (V).1 As an initial test of the soundness of our model, numerical simulations were carried out for opticalprocessing gettering of Fe in Si wafers 200 μm in thickness by a 2 μm thick Al layer at the wafer 

1
backsurface (at the 200 μm position). It is assumed that Fe was introduced at 900oC to its solubility of ~5.02x1013 cm-3 and then precipitated out to steady state at 700oC to the density of ρ=1011

cm-3. For the FeSi2 precipitates the misfit parameter of y=-0.15 is used. The negative y value 

means that growth of FeSi2 precipitates is associated with a Si matrix volume contraction while 

their dissolution with a volume expansion. Thus, generation of V or consumption of I or both will 

result from FeSi2 precipitate growth, while generation of I or consumption of V or both will result from FeSi2 precipitate dissolution. Consequently, gettering of Fe will be faster if a Vsupersaturation exists. 

Optical processing occurs at two temperatures, a low temperature TL<Teu and a high temperature TH>Teu, where Teu is the Al-Si system eutectic temperature of 577o

C. Injection of V into Si 

takes place at TL, which is assumed to be 500o

C. At TL, a solid Al-Si alloy with 0.6 at% Si forms 

by the migration of Si atoms into Al (not the other way around, because the Al solubility in Si is 

negligibly small), with each Si atom went into Al leaving one V behind in Si. Assuming that alloy formation is complete at TL, then there will be CV = 3x1018

cm−3 V injected into Si for the 

case assuming an Al layer thickness of 2 μm and a Si wafer thickness of 200 μm. Dependent 

upon the V diffusivity in Si at TL under the optical processing condition, these injected V may 

either be essentially accumulated at the interface of the Al-Si alloy and the Si bulk, or distributed 

throughout the Si bulk. Now, the V contribution to Si self-diffusion is known to be7

DV

Si = DV CVeq ( ) /Co = 2~3x1022

exp −4eV / k ( BT)cm

2

s

−1

, (1) 

where C is the Si atom density. There are two sets of data in the literature of the 

separate 

o = 5x1022

cm−3

DV and CV

eq values:7 

 DV = 0.1exp −2eV / k ( ) BT cm

2

s

−1

, CV

eq = 2x1023

exp −2eV / k ( BT)cm−3 , (2) 

DV = 6.17x102

exp −0.5eV / k ( ) BT cm

2

s

−1

, CV

eq = 4.86x1023

exp −3.5eV / k ( BT)cm−3, (3) 

The values given by Eq. (2) did not allow to obtain the needed optical-processing gettering results, i.e., much longer times are needed. Hence, the values of Eq. (3) are used. If it is assumed 

that the injected V are accumulated at the Al-Si alloy and Si interface, our simulation results 

showed that it is not possible to complete th gettering process at 700o

C on a time scale of minutes. Thus, the injected V will be assumed as distributed throughout the Si bulk. This is justified 

using the DV value given by Eq. (3), which in fact predicts that the injected V will be essentially 

distributed throughout the Si bulk with the 500o

C optical annealing time exceeding a few minutes. This yields also that CV /CV

eq ~8.15x1012 , which is a huge V supersaturation. Furthermore, 

in order to simulate the complete gettering of the present case, the Fe atom migration energy 

needs to be reduced by ~0.15 eV (from 0.68 eV), which is assumed to be due to the radiation enhancement. In this simulation the radiation enhanced solubilities of V and Fe are not invoked, as 

it was not needed. As reported previously, Figure 1 shows the result of using thermal annealing 

only at 700o

C, and it is seen that more than 50 hrs is needed to complete the gettering process. 

Figure 2 shows the results of that of the optical-processing gettering, and it is seen that the getter2
ing process is complete with the processing time of only minutes at 500o

C and followed by also 

minutes at 700o

C. In addition, we have performed simulations for the case of a positive misfit y 

value, which means that precipitate growth is associated with a Si matrix volume expansion 

while precipitate dissolution is associated with a volume contraction. Figure 3 shows the simulation results of the case with y=+0.15 (instead of -0.15 for iron) under otherwise the same opticalgettering conditions as for the FeSi2 case. This shows that, when optical-gettering, the precipitate 

dissolution is slowed down from the gettering process of using thermal annealing. 

Fig. 1. Simulation results of gettering dissolved interstitial Fe (left) and FeSi2 precipitates (right) 

from the Si matrix by a 2m thick Al-Si liquid layer at the wafer backside (200 m position) by 

thermal annealing at 700o

C. 

Fig. 2. Simulation results of gettering dissolved interstitial Fe (left) and FeSi2 precipitates (right) 

for the case of Figure 1 by optical processing. 

3
Fig. 3. Simulation profiles of dissolved iron concentration (left) and the profiles of precipitate radii 

(right) of a hypothetical precipitate case that the misfit y is positive, for which the generation of V

or consumption of I or both will result from precipitate dissolution. Under the condition that optical processing injects V into the Si matrix, the needed gettering time is prolonged from that of the 

thermal annealing case. 

 It is obvious that optical-processing gettering involves hereto unconsidered physical mechanisms that introduce new challenges into the impurity precipitation/gettering modeling problem. In 

this initial modeling study, the mechanisms are proposed and used to demonstrate that they can 

give rise to improvement in the effectiveness of gettering metallic precipitates provided upon 

precipitate dissolution the matrix material will expand, and this is the case for FeSi2 which is 

most common in mc-Si. Should the precipitate species be a species that upon its dissolution the 

Si matrix material will contract, then the opposite effect can be expected, e.g., some Ni and Cu 

precipitate species. This is because the proposed mechanisms include Si vacancy injection into 

the Si matrix to cause a V supersaturation, such vacancies will release the compressive strain for 

the Si matrix expansion case. The opposite holds for the Si matrix contraction case, for which, 

owing to V-supersaturation due to injection, not only should the optical-gettering condition not 

effective, but also it might well be harmful. Verifications of the mechanism both experimentally 

and theoretically are needed. For instance the formation of the solid Al-Si alloy should be readily 

detectable after processing at TL. Also, if a buried dopant layer is used it should be possible to 

detect the injected vacancies in the TH processing step by observing an enhancement in the dopant diffusivity and dopant-V pair concentration. Examples of theoretical problems need to be 

addressed would include: whether the injected V did or did not substantially migrate out through 

the Al-Si alloy layer; obtaining a more precise physical picture of the migration energy barrier 

reduction through the radiation enhanced diffusion mechanism; and the role of radiation enhanced V and Fe solubilities. 

4
5

REFERENCES 

1. N. Li, H. Li and T. Y. Tan, Extended Abstracts and Papers, 14th Workshop on Crystalline 

Silicon Solar Cells & Modules: Materials and Processing, p. 238 (Winter Park, Co., Aug. 11-

14, 2004). 

2. B. L. Sopori, US Patent No. 6,852,371 B2 (Feb. 8, 2005). 

3. P. S. Plekhanov, R. Gafiteanu, U. M. Gösele, and T. Y. Tan, J. Appl. Phys. 86, 2453 (1999). 

4. S. M. Joshi, U. M. Gösele, and T. Y. Tan, Solar Energy Materials and Solar Cells 70, 231 

(2001). 

5. T. Y. Tan, "Impurity Gettering in Silicon" in Encyclopedia of Materials: Science and Technology, ed. Subhash Mahajan (Elsevier, Amsterdam, 2001) pp. 4031-4042. 

6. P. S. Plekhanov, M. D. Negoita, and T.Y. Tan, J. Appl. Phys. 90, 5388 (2001). 

7. T. Y. Tan and U. Gösele, "Point Defects, Diffusion, and Precipitation", in "Handbook of 

Semiconductor Technology, vol. 1: Electronic Structure and Properties of Semiconductors", 

eds. K. A. Jackson and W. Schröter (Wiley-VCH, New York, 2000) p. 231-290. 
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4. TITLE AND SUBTITLE

Precipitate Dissolution and Gettering under Vacancy Injection in 

Silicon: Final Subcontract Report, 21 March 2006 - 15 January 2008

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

T.Y. Tan and N. Li 

5d. PROJECT NUMBER

NREL/SR-520-44088 

5e. TASK NUMBER

PVA72401 

5f. WORK UNIT NUMBER

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Duke University - Office of Research Support 

334 North Building, Box 90077 

Durham, North Carolina 27708 

8. PERFORMING ORGANIZATION

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XEJ-6-66132-01 

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NREL Technical Monitor: Fanny Eddy 

14. ABSTRACT (Maximum 200 Words)

Using a deposited Al layer, optical processing at a temperature below the Si-Al eutectic temperature of Teu=577o

C for a 

few minutes and followed by T>Teu for a few more minutes are able to getter metallic precipitates out of multicrystalline 

Si. To accomplish the same, a few tens of hours are needed when using thermal annealing at 700o

C. Possible 

mechanisms involved in optical-processing gettering are proposed: vacancy injection, radiation-enhanced solubility, and 

radiation-enhanced diffusion of vacancies and metal impurity atoms. Using FeSi2 as a model case for which the Si lattice 

expands concomitantly with the dissolution of precipitates, physical modeling and numerical simulations uncover and test 

the conditions for the mechanisms to be effective. The mechanisms are effective if the injected Si vacancies due to alloy 

formation are nearly all retained inside and evenly distributed throughout the Si bulk at the lower temperature, and if the 

Fe atom migration energy barrier is reduced by ~0.15 eV by radiation at the higher temperature. On the other hand, for 

gettering a precipitate species for which the Si lattice will shrink concomitantly with the dissolution of precipitates, the 

vacancy-injection mechanism will not only be ineffective but should also have a detrimental effect. 

15. SUBJECT TERMS

PV; optical processing; gettering; multicrystalline silicon vacancy injection; radiation-enhanced solubility; radiationenhanced diffusion; metal impurity atoms; precipitate dissolution; 

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