.

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 

B. Sopori, R. Reedy, K. Jones, Y. Yan, and 

M. Al-Jassim 

National Renewable Energy Laboratory 

Y. Zhang 

Bioarray Solutions 

B. Bathey and J. Kalejs 

RWE Schott Solar 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37477 

A Comprehensive Model of 

Hydrogen Transport into a 

Solar Cell During Silicon Nitride 

Processing for Fire-Through 

Metallization 
NOTICE 

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published 

form of this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States

government. Neither the United States government nor any agency thereof, nor any of their employees, 

makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, 

completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents 

that its use would not infringe privately owned rights. Reference herein to any specific commercial 

product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily 

constitute or imply its endorsement, recommendation, or favoring by the United States government or any 

agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect 

those of the United States government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy

Office of Scientific and Technical Information 

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste 
A COMPREHENSIVE MODEL OF HYDROGEN TRANSPORT INTO A SOLAR CELL DURING SILICON NITRIDE 

PROCESSING FOR FIRE-THROUGH METALLIZATION 

 

Bhushan Sopori1

, Yi Zhang2, Robert Reedy1, Kim Jones1, Yanfa Yan1, Mowafak Al-Jassim1, 

 Bala Bathey3

, and Juris Kalejs

3 

1

 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 2 Bioarray Solutions, Warren, NJ 07059 3

 RWE Schott Solar, Billerica, MA 01821 

ABSTRACT

A mechanism for the transport of H into a Si solar cell

during plasma-enhanced chemical vapor deposition

(PECVD) of a hydrogenated silicon nitride (SiN:H) layer

and its subsequent fire-through metallization process is 

described. The PECVD process generates processinduced traps, which “store” H at the surface of the solar

cell. This stored H is released and diffuses rapidly into the

bulk of Si during the high-temperature metallization-firing

process. During the ramp-down, the diffused H associates

with impurities and defects and passivates them. The 

firing step partially heals up the surface damage. The

proposed model explains a variety of observations and

experimental results. 

INTRODUCTION 

In the current technology for commercial Si solar cell 

fabrication, hydrogen passivation is a byproduct of two

other process steps—deposition of antireflection coating

and subsequent contact-metallization firing. In this “firethrough” process, a layer of hydrogenated silicon nitride

(SiN:H) is deposited on an N/P junction by a PECVD

technique [1,2]. Next, the front metallization pattern is

screen-printed and fired through the nitride layer. In some

cases, a back contact is also screen-printed prior to the 

firing step, and both contacts are co-fired. It has been

determined that the firing process is accompanied by a

large increase in the minority carrier lifetime of the cell, 

indicating that passivation of impurities and defects in the 

bulk occurs primarily during this step. Fourier transform 

infrared (FTIR) spectroscopic studies have shown that the 

firing step is also accompanied by a release of H from

SiN:H layer. These two factors have prompted

suggestions that the H for passivation of impurities and

defects in the bulk of the solar cell comes from SiN:H layer

and diffuses into Si. Thus, the H transport occurs primarily 

from plasma to SiN:H film and then to Si [3]. 

Some researchers have argued that H released from 

SiN:H cannot be the source of passivating H, because it is 

more favorable for H to evolve into air than make its way 

into the Si substrate [4]. These arguments are based on

kinetic studies of H-evolution from SiN:H, and from

theoretical modeling of chemical reactions in SiN:H during

high-temperature processing. From these arguments, it 

was thought that the SiN:H fire-through process cannot be

an efficient passivation scheme [5]. However, based on

theoretical modeling of trapping-assisted H diffusion in Si,

we have proposed a different hypothesis to explain 

efficient passivation by SiN:H processing [6,7]. It was 

predicted that H is introduced into Si during SiN:H

deposition itself. This introduction is facilitated by the 

process-induced defects (see discussion below), which

are generated primarily during the initial phase of the

nitride deposition or during a “pretreatment,” in which the

cell is exposed to a plasma without Si-bearing gases. 

In this paper, we explore the nature of processinduced traps (PITs), and address how these PITs

mediate H diffusion into Si. We show that PITs are

manifested as a damaged surface layer, which contains

very high concentrations of H. The surface damage can 

be observed by TEM and the SIMS profiles of H (near the

surface). A host of other analytical techniques is used to

understand association and detrapping of H from the PITs. 

We also show that H is released from PITs and a part of 

the damage is healed as a result of firing. Furthermore, 

some of the PITs can transform into other extended

defects through the firing process. 

PROCESS-INDUCED TRAPS AND THE DIFFUSION 

MECHANISM 

It is now well established that traps play a dominant 

role in controlling the effective solubility and the diffusivity

of H in Si containing defects and impurities [8,9]. Early 

studies on H diffusion show that the solubility of H in

relatively pure Si is very low [10]. However, in impure Si,

H has propensity to associate with impurities and defects,

thereby increasing the effective solubility and reducing the

diffusivity of H in Si [11]. In most cases, the primary traps 

of H in Si are the grown-in defects and impurities.

However, we have proposed that a very effective trapping

of H can be done by the process-induced defects. In a 

passivation process, process-induced defects are typically 

manifested as the surface damage produced by the

hydrogenation process itself (in this case by the plasma)

[12,13]. Detailed studies have been done on the defects

produced by plasma processing for a variety of cases

such as low temperature (< 400 °C) hydrogenation, ion

etching, and plasma-immersion doping. For example, a

long duration, low-temperature, plasma hydrogenation

(used in the past for impurity/defect passivation) results in

extensive surface damage in the form of extended defects

1
such as dislocation tangles, stacking faults, and platelets. 

This damage has a high solubility for H, and leads to

formation of a H-rich layer at the surface. Because in a

typical SiN:H deposition (for antireflection coating) it is

expected that the Si surface is exposed to the plasma for

only a very brief period, it is often believed that there is

little or no damage at the wafer or cell surface. Indeed, it

is difficult to observe such damage by standard

procedures. Concomitantly detailed information on the

effects of short exposures to low-energy plasma

processing is severely lacking. However, it is noteworthy 

that a nitride process, which is optimized for good

passivation of solar cells, requires a pretreatment wherein

the wafer encounters hydrogen plasma prior to initiation of 

a nitride deposition. Typically, this is accomplished by

blocking the flow of Si-bearing gases during the initial

phase of deposition. 

During the high-temperature (∼ 800 °C) metallization

firing, H is detrapped from the damage and diffuses into

the Si. At these temperatures, the detrapped H can

diffuse with nearly interstitial diffusivity. The hydrogen 

diffusion can be further improved in those situations where

impurity gettering can occur during firing process because

gettering is accompanied by a reduction in the bulk trap

density. However, annihilation of PITs is more difficult 

during short temperature pulses (at temperatures of about

800 °C). This behavior allows PITs to mediate H flow

across the SiN:H/Si interface during firing. 

It is important to point out that impurities near the

surface, introduced by other processes, can also influence

H diffusion because they can also act as traps (with 

temperature dependencies of trapping and detrapping

rates different from those of bulk traps). Of particular

interest in solar cell fabrication is the trapping in the

heavily doped n-type junction. 

THEORETICAL MODELING 

We have extended our previous H rapping/detrapping 

model to include contribution by the process-induced traps 

[13]. The surface trap density (T) for a plasma process 

can be represented by the following function:

where the first time-dependent term is due to process 

damage and Tb is a constant bulk trap density. The basic 

equations and some results of this model have been 

published earlier. Here, we simply mention some of the 

salient results of our theory. 

1. A high concentration of H is “stored” near the surface by

a PECVD process (which typically takes a few minutes for 

the deposition of about 750 Å of nitride) in a profile that 

corresponds to the Ttot profile.

2. For a good quality material (Tb < 1016 cm-3), H diffuses 

through the entire bulk in only a few seconds at the firing 

temperatures. 

3. The residual surface concentration of H, after firing,

corresponds to the residual trap density. If the surface

damage is fully healed, the H concentration will reach the

bulk trap density, typically, 1016 - 1017 cm-3. As pointed

out in this paper, a high residual concentration is 

associated with a poor firing process and /or formation of

platelets and stacking faults. The healing of damage is

important to achieve a low surface recombination velocity

of the solar cell.

4. Because surface damage, produced by different

PECVD methods (e.g., remote plasma, high frequency/low

frequency plasma) can be different, the performance of

solar cells with nitride layers deposited by these

techniques may vary. In principle, it should be possible to

adjust process conditions to minimize variations due to

different methods. However, in practice, it may be difficult

to achieve the same cell performance with different

PECVD methods, because H passivation and contact

formation occur in the same step. 

EXPERIMENTAL VERIFICATION 

Experiments were done on Si wafers and cells using

commercial SIN:H process and firing procedures.

Although observation of surface damage and

trapping/detrapping of H by the PITs can be facilitated by

performing plasma processing for extended periods and

substituting deuterium for H, it is important to use the

standard process conditions to satisfactorily verify the

proposed model. 

Here, we show experimental evidence of the

proposed mechanism. We first verify that indeed (i) a

surface damage is produced during the PECVD nitridation,

(ii) H is trapped near the surface below the nitride layer,

and (iii) and H diffuses into the bulk during firing. 

Trapping of hydrogen by the defects produced by ion

implantation and plasma processes used for bulk

passivation of solar cells have been well documented.

Thus, it is not surprising that one would expect surface

damage to play an important role in the hydrogenation

process. However, the PECVD process for deposition of

SiN:H is considerably more challenging because the bare

surface of Si is exposed to plasma for a very short

duration (only in the initial phase of the PECVD process).

Because the surface damage produced by low-energy

plasma is very mild, it is difficult to characterize it.

Through a variety of characterization techniques and using 

different substrate materials, we have identified the nature

of the damage produced by PECVD-SiN:H deposition for

AR coatings. Figure 1 is a cross-sectional TEM image of

an EFG ribbon, deposited with a SiN:H antireflection

coating by PECVD, prior to the firing step. This image

shows an unexpected feature: clusters of point defects are 

introduced near the Si surface. It may be pointed out that

tot 0 Tb ] a bt

x [T ] T exp [- +

+ =

2
other hydrogenation processes, such as plasma

processing and ion implantation, typically introduce very

strong (and easily identifiable) defects such as platelets, 

stacking faults and hydrogen “bubbles” that segregate at

dislocation nodes. 

Figure 2 shows a SIMS plot of H (dotted line) in an

EFG sample that was deposited with SiN:H coating

(before firing). The nitride was removed before SIMS

analysis; we have confirmed that the nitride removal

process did not alter the original H profile. Figure 2

shows existence of H near the surface and that the

surface concentration reaches about 1022cm-3. The depth 

of detectable H is about the same as that of the detectable

damage in Fig. 1. This clearly shows association of H with

PITs. The solid line in Figure 2 shows H profile of a

similar sample after firing. It can be seen that H has

diffused into the bulk of Si. However, there is still a high

concentration of residual H near the surface. Analyses of

many other samples have shown that there are significant

variations in the H profile from sample to sample. 

Variations also exist within each sample, particularly after

firing. This suggests that firing process may not “heal”

surface damage uniformly over the sample. This 

inference is also corroborated by TEM analyses. The 

majority of each sample has very little residual damage

after firing. However, we have found regions of samples

that have extended defects, which look very similar to

plasma hydrogenation but on a much smaller scale. 

Figure 3 shows a cross-sectional TEM of such a region in

a fired sample. The defects below the nitride layer consist 

of platelets and “bubbles,” as identified in the image.

Thus, some of the damage in the form of point-defect

clusters has developed into extended defects by the firing. 

SiN:H 

Figure 2. SIMS profiles of H before (dotted) and after 

firing (solid) the nitride-coated wafer. Nitride was 

removed prior to SIMS measurement. 

Platelet

CONCLUSION

We have verified that surface damage is produced in

a PECVD process used for SiN:H deposition. The

damage (manifested as process-induced traps) “stores” H, 

which is then driven into the bulk of Si during contact

firing. During the cool-down of the firing process, the

released H, with a nearly uniform distribution in the bulk of 

the material will face competing mechanisms for its 

Figure 1. A [011] cross-sectional TEM lattice image 

of a Si ribbon deposited with a commercial SiN:H 

(before alloying step) showing surface damage. 

SiN:H

Si

Figure 3. An X-TEM image of a fired, nitride-coated 

wafer showing a variety of extended defects generated

by the firing process step. 

3
association. A part of the H will interact with the lifetime

killing impurities—the most important from a passivation

stand point—and with other impurities like dopants and 

intrinsic traps. It is necessary to ensure: (i) sufficient H

available at the surface to passivate nearly all lifetime

killing impurities, (ii) that H does not associate with

dopants to change the resistivity of the material, and (iii) 

the surface damage is removed as much as possible.

These considerations need attention to factors such as the

nitride deposition method and pretreatment, firing

conditions, and cool-down, for effective passivation.

Some of these issues have already been reported in the

literature. In particular, two important reported

observations are: (a) low-frequency plasma nitride 

provides a better passivation than high-frequency plasma,

and (b) slow cool-down results in higher cell efficiencies. 

The role of the damaged layer is actually more

complex than appears from the brief explanation offered in

this paper. For example, the unhealed damaged layer can

also mediate H between the Si and the SiN:H layer.

Furthermore, other process-induced traps such as

dopants used for an N/P junction can influence the H

transfer process. Some of this evidence has been

observed experimentally. For example, PECVD

passivation is better in wafer with N/P junction at the

surface than in similar wafers where the N region has 

been etched off, leaving only the p-type substrate [14].

The present model explains these results. Figures 4 and

5 show SIMS plots of H, before and after firing, in two

types p-type ribbon wafers—with and without an n/p

junction. These diffusion profiles exhibit features that can

explain observed differences in the behavior of passivation

in wafers with and without n/p junctions. 

REFERENCES 

[1] P. Sana, A. Rohatgi, J. Kalejs and R. O. Bell, Appl. 

Phys. Lett. 64, 1994, p 111. 

[2] J. Nijs et al., Proc. of the 1st World Conference on

Photovoltaic Energy Conversion, Hawaii, 1994, p 1242. 

[3] V. Yelundur, A. Rohatgi, J-W. Jeong, A. M. Gabor, J.I. 

Hanoka, and R.L.Wallace, Procd. IEEE PVSC, 2000, p9. 

[4] C. Boehme and G. Lucovsky, J. Appl. Phys., 88, 2000 

p 6055. 

[5] Z. Lu, P. Santos-Filho, G. Stevens, M.J. Williams, and 

G. Lucovsky, J. Vac. Sci. Technol. A 13(3), 1995, p 606. 

[6] B.L. Sopori, Y. Zhang, R. Reedy, K. M. Jones, Y. Yan, 

M. M. Al-Jassim, J. Kalejs, and B. Bathey, On the 

Mechanism of Hydrogen Diffusion in Si Solar Cells Using 

PECVD SiN:H, MRS Spring 2004

[7] Bhushan Sopori, “Silicon Nitride Processing for Control 

of Optical and Electronic Properties of Silicon Solar Cells,” 

J. Electronic Mat., 32, 2003, p 1034. 

[8] J. P. Kalejs and S. Rajendran, App. Phys. Lett. 55, 

1989, p 25. 

[9] M. Capizzi and A. Mittiga, Appl. Phys. Lett. 50, 1987, p 

918. 

 [10] A. Van Wieringen and N. Warmholtz, Physica 22, 

1956, p 849. 

[11] B. L. Sopori, Kim Jones and Xiao Jun Deng, Appl. 

Phys. Lett. 61, 1992, p 2560. 

[12] N.M Johnson and C. Herring, Inst. Phys. Conf. Ser. 

95, 1989, p 415. 

[13] Bhushan Sopori, Y. Zhang, R. Reedy, K. Jones, N. 

M. Ravindra, S. Rangan, and S.Ashok, “Trapping and 

Detrapping of H in Si: Impact on Diffusion Properties and 

Solar Cell Processing, Proc. MRS Meeting, 719, 2002, pp 

125-133. 

 [14] L. Mittelstadt, A. Metz, and R. Hezel, Solar Energy 

Materials & Solar Cells, 72, 2002, pp 255-261. 

Figure 4. SIMS plots of H in p-type EFG ribbon 

before and after firing the nitride. 

Figure 5. SIMS plots of H in n/p EFG ribbon 

before and after firing the nitride. 

4
F1147-E(12/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

February 2005 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

3-7 January 2005 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

A Comprehensive Model of Hydrogen Transport into a Solar Cell 

During Silicon Nitride Processing for Fire-Through Metallization 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-520-37477 

5e. TASK NUMBER

PVA53101 

6. AUTHOR(S) 

B. Sopori, Y. Zhang, R. Reedy, K. Jones, Y. Yan, M. Al-Jassim, 

B. Bathey, and J. Kalejs 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 

80401 

Bioarray Solutions, Warren, NJ 07059 

RWE Schott Solar, Billerica, MA 01821 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-520-37477 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

A mechanism for the transport of H into a Si solar cell during plasma-enhanced chemical vapor deposition (PECVD) of a 

hydrogenated silicon nitride (SiN:H) layer and its subsequent fire-through metallization process is described. The 

PECVD process generates process-induced traps, which “store” H at the surface of the solar cell. This stored H is 

released and diffuses rapidly into the bulk of Si during the high-temperature metallization-firing process. During the 

ramp-down, the diffused H associates with impurities and defects and passivates them. The firing step partially heals up 

the surface damage. The proposed model explains a variety of observations and experimental results. 

15. SUBJECT TERMS

PV; hydrogen transport; solar cell; plasma-enhanced chemical vapor deposition (PECVD); hydrogenated silicon nitride 

(SiN:H); fire-through metallization; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18