A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Si Passivation and Chemical 

Vapor Deposition of Silicon Nitride 

Final Technical Report 

March 18, 2007 

H.A. Atwater 

California Institute of Technology 

Pasadena, California 

Subcontract Report 

NREL/SR-520-42325 

November 2007 

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

Subcontract Report 

NREL/SR-520-42325 

November 2007 

Si Passivation and Chemical 

Vapor Deposition of Silicon Nitride 

Final Technical Report 

March 18, 2007 

H.A. Atwater 

California Institute of Technology 

Pasadena, California 

NREL Technical Monitor: R. Matson/F. Posey-Eddy 

Prepared under Subcontract No. AAT-2-31605-01 
This publication was reproduced from the best available copy 

Submitted by the subcontractor and received no editorial review at NREL

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Table of Contents

Subcontract Executive Overview.........................................................................................1 

1. Silicon Nitride Synthesis by Hot Wire Chemical Vapor Deposition:..............................1 

Introduction..................................................................................................................... 1 

Summary of Technical Accomplishments:..................................................................... 1 

Experimental Approach .................................................................................................. 2 

Radical Measurements – NH3 decomposition ................................................................ 2 

Film Growth Results....................................................................................................... 3 

Variation of SiH4/NH3 Flow Ratio ................................................................................. 4

Rutherford Back Scattering and Hydrogen Forward Scattering Analysis...................... 5

Annealing of Hydrogenated SiNx ................................................................................... 5

2. Solar Cell and Si Defect Passivation with HWCVD Silicon Nitride...............................7 

Silicon Nitride Passivation.............................................................................................. 7 

Emitter Passivation ......................................................................................................... 7 

Experiments.................................................................................................................... 7

Results............................................................................................................................. 8

Pt-H Complex Formation.............................................................................................. 10 

Summary of Defect and Emitter Passivation................................................................ 11 

3. Thin Film Silicon Passivation in HWCVD Growth ......................................................12 

Overview of Thin Film Si by HWCVD........................................................................ 12 

HWCVD Gas Phase Species Characterization ............................................................. 12 

Rates for Surface Kinetic Processes ............................................................................. 13

A Phase Diagram for Morphology and Properties of Low Temperature Deposited 

Polycrystalline Silicon Grown by Hot-wire Chemical Vapor Deposition.................... 15 

Minority Carrier Lifetimes of HWCVD Films............................................................. 19 

HWCVD Si Film Passivation ....................................................................................... 22 

Microstructure and Passivation Effects on Open Circuit Voltage................................ 22 

HWCVD Growth and Morphology .............................................................................. 23

Open-Circuit Voltage.................................................................................................... 23

Surface Passivation ....................................................................................................... 24

References..........................................................................................................................26 

Personnel Associated with Subcontract.............................................................................28 

Researchers:.................................................................................................................. 28 

Collaborators:................................................................................................................ 28 

Publications under subcontract ..........................................................................................29 

PhD Theses: .................................................................................................................. 29 

Journal Publications:..................................................................................................... 29 

Presentations under Subcontract........................................................................................31 

Honors and Awards under Subcontract .............................................................................33 

iii
List of Figures 

Figure 1. FTIR transmission spectra of films grown under different SiH4/NH3 

flow ratios (indicated in %)..............................................................................4 

Figure 2. FTIR spectrum at various annealing temperature for a film grown with 

8% SiH4 in NH3 ...............................................................................................9 

Figure 3. Creation of Pt-H complexes in bulk Si by annealing of hydrogentated 

SiN3 layer.......................................................................................................10 

Figure 4. Photomicrograph of silicon nitride-passivated String Ribbon solar cell; 

white line depicts position of the wire array during hot wire CVD growth ..11 

Figure 5. Rates of interest for wire surface kinetic processes .......................................14 

Figure 6. Phase diagram of HWCVD films grown at 50:1 hydrogen dilution with 

4% SiH4 in He................................................................................................17 

Figure 7. Plan-view TEM of HWCVD epitaxial film (T=3000C) on SNSPE 

template..........................................................................................................18 

Figure 8. LLI and HLI minority carrier lifetimes of HWCVD films on Si(100) as 

measured by RCPCD .....................................................................................19 

Figure 9. LLI and HLI minority carrier lifetimes on SNSPE templates .......................19 

Figure 10. Silicon to oxygen ratio in the first monolayer of growth as a function of 

substrate temperature for dilute silane growth at 50:1 hydrogen dilution 

and epitaxial thickness ...................................................................................20 

Figure 11. Silicon to oxygen ratio in the first monolayer of growth as a function 

of R for pure silane growth using the deposition conditions in Table 4 

along with the Raman crystalline % ..............................................................21 

Figure 12. Schematic of proposed photovoltaic device incorporating epitaxial Si 

growth on a large-grained polycrystalline template fabricated by SNSPE....22 

Figure 13. Cross-sectional TEM of growth morphology at various hydrogen 

dilutions..........................................................................................................23 

Figure 14. Voc with hydrogen dilution immediately after deposition and after one 

week in ambient air........................................................................................24 

Figure 15. Open-circuit voltage with hydrogen dilution after various post-deposition 

treatments.......................................................................................................25 

List of Tables 

Table 1. Film GROWTH COnditions and Results ........................................................3 

Table 2. Summary and Comparison of RBS, HFS, FTIR Film Analysis ......................6 

Table 3. Comparison of Electrical Properties for Hot-Wire Versus Plasma 

Nitride Cells...................................................................................................10 

Table 4. Results of HWCVD growth with pure silane at 380 o

C substrate temperature 

and at 2100 o

C graphite filament temperature with 3.5cm spacing................18 

iv
Subcontract Executive Overview 

This research program investigated chemical and physical methods for Si surface 

passivation for application in crystalline Si and thin Si film photovoltaic devices. 

Overall, our efforts during the project were focused in three areas: i) synthesis of silicon 

nitride thin films with high hydrogen content by hot wire chemical vapor deposition ii) 

investigation of the role of hydrogen passivation of defects in crystalline Si and Si solar 

cells by out diffusion from hydrogenated silicon nitride films iii) investigation of the 

growth kinetics and passivation of hydrogenated polycrystalline Si grown by hot wire 

chemical vapor deposition. 

Work on hydrogen-passivated silicon nitride synthesis was applied to solar cell 

passivation in collaboration with industrial partners at Evergreen Solar and thin film 

silicon passivation in collaboration with BP Solar. By way of enumeration, under this 

contract 3 PhD theses were published, along with 16 journal publications. Researchers 

working under this subcontract have also been actively serving the photovoltaics 

technical community -- the Principal Investigator served as a co-organizer for the 

Materials Research Society Symposium A in 2006 and served as a subpanel chair for the 

DOE Basic Energy Sciences Workshop on Basic Research in Solar Energy Utilization in 

2005. 

1. Silicon Nitride Synthesis by Hot Wire Chemical Vapor Deposition: 

Introduction 

 A major focus of our subcontract was on silicon nitride (SiNx) films, which have been 

widely used in the semiconductor industry for use as a lithographic mask and gate 

dielectric. Particularly for gate dielectric applications, silicon nitride offers the advantage 

of lower deposition temperature (300oC) as compared with other materials, avoiding the 

problems of impurity diffusion that can occur at conventional processing temperatures of 

900oC1. For photovoltaic applications, silicon nitride acts as an effective anti-reflection 

(AR) coating due to its high refractive index (2.0-2.5). These films may also serve as 

passivation coatings for surface and bulk defects in the underlying silicon, due to the 

large fraction of hydrogen that may be incorporated (up to 25 atomic %)2. 

 A promising technique for low temperature SiNx growth is hot-wire CVD (HWCVD), 

also known as catalytic CVD (Cat-CVD)3. Recent work on SiNx growth by HWCVD has 

focused gate dielectric applications1, with one group fabricating a thin-film transistor 

made entirely by this technique4. Our subcontract research focused on use of HWCVD 

SiNx for photovoltaic applications, with particular attention paid to the refractive index 

and hydrogen content of the films. 

Summary of Technical Accomplishments: 

 Silicon nitride films were grown by hot-wire chemical vapor deposition and film 

properties have been characterized as a function of SiH4/NH3 flow ratio. It was 

demonstrated that hot wire chemical vapor deposition leads to growth of SiNx films with 

controllable stoichiometry and hydrogen. The following findings resulted from our 

subcontract research: 

1
• The decomposition rate of NH3 on tungsten is low relative to SiH4, explaining the 

large excess flow of NH3 needed to produce stoichiometric silicon nitride films. 

• The NH3 decomposition reaction is catalyzed, however, with an activation energy 

of 31 kcal/mole. 

• Varying the ratio of SiH4/NH3 from 1-8% produced films ranging in refractive 

index from 1.8-2.5, with hydrogen content ranging from 9-18 atomic %. 

• Transmission measurements using FTIR revealed an abrupt transition in the 

bonding of H (going from N to Si) in SiNx as the flow ratio was increased beyond 

6% SiH4/NH3. As this ratio was increased, the overall H-content increased in the 

film, suggesting that SiH4 is the primary source of H under these conditions. 

• Films deposited with a low flow ratio of SiH4/NH3 of 1% showed a prominent SiO-Si feature suggesting post-growth oxidation, a result further confirmed with 

RBS, indicating 23% oxygen incorporation in the film. 

• Post-deposition H2 treatments were found to have little impact on the overall 

atomic percentage of H in the film, but did appear to have an etching effect with 

the Si-rich samples used. 

• Annealing studies revealed different kinetics for H release from Si versus N, 

consistent with a mechanism involving bond dissociation, followed by molecular 

diffusion9. 

Experimental Approach 

 The system used in the deposition of SiNx is a high vacuum chamber with a base 

pressure of order 10-9 Torr. Source gases, consisting of SiH4 (diluted to 1% in He), NH3

(and H2 for post-deposition treatments) are introduced through a gas inlet and 

decomposed on a W wire (0.5 mm diameter, 12 cm length, 1800o

C temperature). Flow 

rates of the various gases range from 4-48 sccm, with SiH4/NH3 ratios ranging from 1-

8%, and pressures in the range of 20-100 mTorr. A substrate heater is located 

approximately 5 cm from the wire, and a shutter is used to protect substrates from the 

evaporation of impurities from the wire during its initial heating; growth temperatures 

were approximately 300o

C for this study. The substrates used were lightly doped p-type 

(350 Ω-cm), double-side polished, float-zone Si. A quadrupole mass spectrometer is put 

in place of the substrate heater during experiments aimed at measuring radicals desorbed 

from the filament. 

Radical Measurements – NH3 Decomposition

 As a means to probe the kinetics of NH3 decomposition on the wire, low-pressure 

radical species measurements were made using a quadrupole mass spectrometer with the 

capability of tunable electron energy for selective radical ionization. This method is 

2
detailed in another study by these authors5. Mass spectra were acquired of the NHx

species desorbed from the wire under conditions of 3x10-6 Torr NH3, at a wire 

temperature of 2100oC. Even at such a high temperature, there is an extremely small 

yield of NH2 produced from NH3 decomposition. Relative to the decomposition 

probability of SiH4 at a similar temperature, NH3 is (at minimum) a factor of 10 less. 

This suggests that to produce stoichiometric (Si3N4) films, the feed gas must contain SiH4

in a large dilution of NH3, a common observation in the growth of these films1,4. The 

intensity of the NH2 yield was also measured as a function of wire temperature, from 

1550-2100oC. These data yield an approximate activation energy for the NH3

decomposition reaction: 

 NH3 + W = NH2 + W-H (1) 

where “W” represents a bare surface site on the W wire, and “W-H” is a hydrogenated 

surface site on the wire. The value of 31 kcal/mole obtained is significantly smaller than 

the known N-H bond dissociation energy of 93 kcal/mole6, indicating that as with the 

reaction of SiH4 to SiH3

5, the reaction in Eqn. 1 is catalyzed. 

Film Growth Results

FR QS 

(sccm) 

QN

(sccm) 

T 

(nm) n 

1% 8 8 62 1.8 

2% 16 8 128 1.8 

4% 32 8 287 1.9 

8% 32 4 129 2.5 

Table 1. Film growth conditions and results. 

FR refers to diluted SiH4 (1% in He)/NH3 flow 

rate ratio; QS and QR refer to SiH4(1% in He) 

and NH3 flow rates in sccm, respectively; T 

refers to SiNx film thickness in nm; n refers to 

film index of refraction 

 SiNx films were initially prepared under conditions of 1.8% SiH4 in NH3 (16 sccm 

1% SiH4 mixture, 8.7 sccm NH3), 20 mTorr total pressure, and a substrate temperature of 

280o

C, with a growth time of 60 minutes. Resulting films were analyzed with a single 

wavelength (633 nm) ellipsometer, yielding a thickness of 185 nm and refractive index of 

1.8. Subsequent analysis was performed using X-ray Photoelectron Spectroscopy (XPS) 

to gain insight into the bonding structure of the film. A HWCVD and standard 

(stoichiometric nitride) grown by 

conventional PECVD (50 nm Si3N4

on Si substrate) were examined with 

this technique. Results indicated a 0.8 

eV core level shift in the N 1s

spectrum for the hot-wire film, 

relative to the spectrum of the plasmagrown film. These shifts to higher 

binding energy could be associated 

with an N-Si2 bond structure7

(unsaturated bond on N), as opposed 

to the N-Si3 structure expected of 

stoichiometric silicon nitride. Larger 

percentages of oxygen were present in 

the HWCVD film, suggestive of SiO2, 

and an examination of the Si signal 

yielded a Si/N ratio of approximately 

1.3. These results suggest that the 

film was a combination of SiO2 and 

SiNx, possibly a silicon oxynitride 

(SixOyNz). 

3
Variation of SiH4/NH3 Flow Ratio 

Prompted by the results indicating that the initial growth conditions led to a film of low 

index (1.8), a series of different SiH4/NH3 flow ratios were used to examine the effects 

on film properties. The films to be described were grown at a substrate temperature of 

300oC for a period of 40 min, with a wire temperature of 1800oC and a total pressure of 

100 mTorr. Fig. 1 provides the Fourier Transform Infrared (FTIR) transmission spectra 

for a series of four SiNx films grown under different SiH4/NH3 flow ratios: 1, 2, 4, and 

8%. Table 1 provides the flow rates used for film growth (FR = flow ratio, QS = SiH4 

mixture flow rate, QN = NH3 flow rate), as well as the resulting film thickness (‘t’) and 

refractive index (‘n’). Peak assignments were based on data by Lanford and Rand2. The 

most prominent feature in the transmission spectra is the Si-N absorption around 860 cm1; the breadth of this feature suggests that the SiNx films are 

4000 3000 2000 1000

20

30

40

50

60

1% 2% 4% 8%

--Si-N

860 cm-1

Si-O-Si

1100cm-1

--Si-H

2160cm-1

--N-H

3340cm-1

transmission (%)

wavenumber (cm-1)

Fig. 1. FTIR transmission spectra of films grown under different SiH4/NH3 flow ratios 

 (indicated in %). 

amorphous. The features at 2160 cm-1 and 3340 cm-1 were assigned to Si-H and N-H 

stretches, respectively. Evident from Fig. 1 also is that the majority of H is bound to N 

for the 1, 2, and 4% flow ratios and there is an abrupt transition at 8%, at which point H 

becomes mostly bound to Si. For the 1, 2, and 4% cases, it is likely that the H observed 

comes from NH2 species produced at the wire. For the 8% case, the H of the film likely 

results from atomic H (produced by SiH4 decomposition) passivating Si dangling bonds 

present in the growing film. Using absorption cross sections provided by Lanford and 

Rand2, an estimate of the hydrogen concentration in the various films could be obtained. 

The range of values obtained by this method was 0.9-1.5x1022cm-3, although the degree 

of uncertainty associated with these measurements prevents the emergence of a clear 

trend in hydrogen content with respect to flow ratio. Other trends are noticeable from 

Table 1. The growth rate is proportional to the product of the SiH4 and NH3 flow rates. 

4
Also, the refractive index increases from 1.8 up to 2.5, with the 4% flow ratio film being 

the closest to stoichiometric silicon nitride (2.0). Finally, Fig. 1 reveals that the 1% film 

has a prominent Si-O-Si feature that is diminished or absent in films grown at higher flow 

ratios.

Rutherford Back Scattering and Hydrogen Forward Scattering Analysis 

 To complement these FTIR measurements, Rutherford Backscattering (RBS) and 

Hydrogen Forward Scattering (HFS) measurements were made to determine the 

stoichiometry and hydrogen content of a select number of SiNx films. The films chosen 

for analysis were the 1% and 8% flow ratio films, in addition to two films grown at a 

flow ratio of 6% (all other conditions identical), one of which was subjected to a postdeposition H2 treatment; FTIR analysis of the 6% flow ratio films revealed that H was 

bonded predominantly to Si, as in the 8% case. The post-deposition H2 treatment was 

carried out under conditions identical to SiNx deposition, but with the replacement of 

SiH4 and NH3 with H2 at the same 100 mTorr total pressure. The rationale for this 

treatment was to determine whether additional H (produced by H2 decomposition on the 

wire) could be incorporated into the as-grown SiNx. 

 Table 2 provides a summary of the RBS, HFS, and FTIR film analysis. As expected, 

the RBS data reveal an increase in the Si/N ratio in the film as the SiH4/NH3 flow ratio 

increases. All values are greater than the value of 0.75 expected of stoichiometric silicon 

nitride, although the value of ~1 obtained with FR = 1% is attributed to the presence of 

SiO2. There is a slight decrease in the Si/N ratio after the post-deposition H2 treatment, 

suggesting an etching effect of H on the Si of these Si-rich films. The HFS data reveal 

that the FTIR method of determining H-content is suitable for approximate 

determinations, but can be in error by as much as a factor of 2. Also revealed by HFS is 

an increase in the overall atomic percentage of H as the flow ratio (or Si/N) increases, 

supporting the idea that SiH4 is the primary source of this H at these flow ratios. 

 Interestingly, the post-deposition H2 treatment appeared to have little effect on the 

atomic percentage of H in the film, and may only have served to etch the Si-rich film. 

The films also exhibited a decrease in atomic density (as determined with RBS, using the 

known film thickness) as the proportion of Si/N increased. The film subjected to the H2

treatment showed an increase in atomic density, consistent with the idea that H etched 

excess Si from the film, followed by densification. Finally, it is noteworthy that the 1% 

film contains a large percentage of oxygen (23%), while it is absent (within detection 

limits) in films grown at higher flow ratios. This result is consistent with the earlier FTIR 

measurements showing a prominent Si-O-Si feature in the 1% film. It has also been 

observed by Stannowski et al.4, using similar techniques, that nitride films deposited with 

a small flow ratio of SiH4/NH3 (<2%) undergo post-deposition oxidation, likely due to 

absorption of H2O molecules. The authors infer that films grown under these conditions 

must be porous to allow for H2O absorption. 

Annealing of Hydrogenated SiNx

 For applications where hydrogenated SiNx is to be used as a passivation coating, the 

mobility of bound hydrogen is critical as it is thought that the degree of passivation (as 

measured in minority carrier lifetime) depends the amount of hydrogen released from the 

5
SiNx film8. As a means to study hydrogen release from SiNx, two of the films described 

previously were chosen. In order to observe the effects of H release from N versus Si, 

the 2% and 8% flow ratio films were selected. By analogy with the study by Yelundur8, 

we chose to anneal each of these films for 5 minutes at temperatures of 400o

C, 600oC and 

800o

C. Between each anneal, the sample was allowed to cool to room temperature, after 

which FTIR measurements were made to monitor to release of H from the SiNx film. 

Table 2. Summary and comparison of RBS, HFS, FTIR film analysis 

Flow 

Rate 

Ratio 

Film 

thickness 

(nm) 

Index 

n 

Si/N Ratio 

(RBS)

Bonded H 

Concentrat’n 

(cm-3) 

(FTIR)

Total H 

Concentrat’n 

[H](cm-3)/(at. %) 

(RBS-FS)

Atomic 

Density 

(at/cc) 

(RBS) 

Oxygen Concentr’n 

(at. %) 

 (RBS) 

1% 62 1.8 0.99 1.1x1022 0.77x1022 / 9.3% 8.2x1022 23% 

6% 209 2.2 1.3 0.94x1022 1.2x1022 / 16% 7.5x1022 0% 

6% 

+H2 

185 2.1 1.1 0.68x1022 1.3x1022 / 16% 8.4x1022 0% 

8% 129 2.5 2.2 1.5x1022 1.1x1022 / 18% 6.2x1022 0% 

Figure 2 shows the FTIR spectra at the various annealing temperatures for each of these 

films, along with an estimate of the H concentration, determined by the aforementioned 

method and Lanford and Rand2. Fig. 2a shows a steady reduction in the H bonded to N 

as the anneal temperature is increased, with an overall reduction of 70% after the 800oC 

anneal. Fig. 2b reveals only a slight reduction in the H bound to Si up to 600oC, with a 

drastic reduction of 80% after the 800o

C anneal. These results are in qualitative 

agreement with results and a H loss mechanism proposed by Boehme et al.9. 

 The dominant loss mechanism for H in N-rich films (all H bonded to N) is suggested 

to be one involving dissociation of Si-N and N-H bonds to form NH3, which may then 

diffuse through the SiNx network; this reaction is calculated to be downhill in energy by 

0.43 eV. We also observed only a slight decay in the H bonded to Si for annealing 

temperatures up to 600oC, followed by a rapid decay in Si-H for higher temperatures. 

Such results suggest a hydrogen loss reaction having a high activation energy (as 

compared with N-H loss) that is also highly exothermic (considering the rapid decay 

upon crossing this threshold temperature). Some of the reactions proposed that would 

satisfy the latter criteria involve: 1) dissociation of Si-H and N-H to form H2 and Si-N, 

ΔE = -1.86 eV (unlikely due to negligible amounts of N-H in the Si-rich film), 2) 

recombination of Si-H to form H2 and Si-Si, ΔE = -0.49 eV, and 3) reactions involving 

larger numbers of Si-H that are increasingly exothermic, forming SiH4 (ΔE = -0.52 eV) 

and Si2H6 (ΔE = -3.6 eV) as by-products. 

6
2. Solar Cell and Si Defect Passivation with HWCVD Silicon Nitride 

In this section, we describe results of research performed under the subcontract 

connecting the growth and hydrogen content of HWCVD silicon nitride films to defect 

passivation in Si and in Si solar cells. First, the stoichiometry and hydrogen content of 

hot-wire CVD-grown silicon nitride was examined as a function of SiH4/NH3 flow ratio. 

Then, the effect of post-deposition hydrogenation treatment on overall film hydrogen 

content was determined. The hydrogen release properties in Si-rich and N-rich nitride 

layers were characterized by annealing treatments. Hot-wire nitride layers were 

deposited onto Evergreen Solar diffused emitter String Ribbon silicon solar cells, and 

such passivation resulted in cells with comparable JSC, VOC, FF, and efficiency to those 

fabricated using plasma CVD nitride layers. Defect hydrogenation in Si was studied 

using attenuated total reflectance FTIR spectroscopy on platinum-diffused silicon 

substrates, in collaboration with Lehigh University. 

Silicon Nitride Passivation 

Silicon nitride acts as an effective anti-reflection (AR) coating for solar cells due to its 

high refractive index, tunable in the range from 1.8-2.5. These films may also serve as 

passivation coatings for surface and bulk defects in the underlying silicon, due to the 

large fraction of hydrogen that may be incorporated (up to 25 atomic %).2

 The 

conventional means for depositing silicon nitride films uses plasma enhanced chemical 

vapor deposition (PECVD). Another promising technique for low temperature SiNx

growth is hot-wire CVD (HWCVD), also known as catalytic CVD (Cat-CVD).10 As 

compared with PECVD, HWCVD offers the advantages of high deposition rate,10 as the 

process can take place at pressures higher than those at which a plasma can be sustained. 

In addition, it has been demonstrated that the process is compatible with large area 

deposition by careful design of gas delivery and filament geometry.11 The deposition of 

high hydrogen content nitride films by HWCVD for photovoltaic applications has been 

recently demonstrated.12,13 

Emitter Passivation 

Experiments 

The system used in the deposition of SiNx is a high vacuum chamber with a base 

pressure of order 10-9 Torr. Source gases, consisting of SiH4 (diluted to 1% in He) and 

NH3 are introduced through a gas inlet and decomposed on a W wire (0.5 mm dia, 12 cm 

length, 1800o

C temperature). Flow rates of the various gases range from 4-48 sccm, with 

SiH4/NH3 ratios ranging from 1-8%, and pressures in the range of 20-100 mTorr (higher 

pressures are possible, but were not explored in this study). Under these conditions, 

growth rates range from 16-52 Å/min, limited largely by the use of dilute SiH4; rates of 

up to 0.14 μm/min have been demonstrated with the use of pure SiH4 and NH3

ambients.13 A substrate heater is located approximately 5 cm from the wire, and a shutter 

is used to protect substrates from the evaporation of impurities from the wire during its 

initial heating; growth temperatures were approximately 300o

C for this study. The 

substrates used were lightly-doped p-type (350 Ω-cm), double-side polished, float-zone 

Si, except where otherwise indicated. 

7
Results 

A series of different SiH4/NH3 flow ratios (FR) were used to examine the effects on 

film properties. The films to be described were grown at a substrate temperature of 

300oC for a period of 40 min, with a wire temperature of 1800oC and a total pressure of 

100 mTorr. Fig. 1 provides the Fourier Transform Infrared (FTIR) transmission spectra 

for a series of four SiNx films grown under SiH4/NH3 flow ratios of 1, 2, 4, and 8%. The 

most prominent feature in the transmission spectrum is the Si-N absorption around 860 

cm-1. Also evident from Fig. 1 is that the majority of H is bound to N for the 1, 2, and 

4% flow ratios, while at 8%, H is mostly bound to Si. The refractive index, as 

determined by single wavelength (633 nm) ellipsometry, increases from 1.8 up to 2.5, 

with the 4% flow ratio film being the closest to stoichiometric silicon nitride (n = 2.0). 

To complement these FTIR measurements, Rutherford Backscattering (RBS) and 

Hydrogen Forward Scattering (HFS) measurements were made to determine the 

stoichiometry and hydrogen content of a select number of SiNx films. The films chosen 

for analysis were the 1% and 8% flow ratio films, in addition to two films grown at a 

flow ratio of 6% (all other conditions identical)†, one of which was subjected to a postdeposition H2 treatment. The post-deposition H2 treatment was carried out under 

conditions identical to SiNx deposition, but with the replacement of SiH4 and NH3 with 

H2 at the same 100 mTorr total pressure. As expected, an increase in the Si/N ratio in the 

film was observed as the SiH4/NH3 flow ratio increased. All values are greater than the 

value of 0.75 expected of stoichiometric silicon nitride, although the value of ~1 obtained 

with FR = 1% is attributed to the presence of SiO2. There is a slight decrease in the Si/N 

ratio after the post-deposition H2 treatment, suggesting an etching effect of H on the Si of 

these Si-rich nitride layers. Also revealed by HFS is an increase in the overall atomic 

percentage of H as the flow ratio (or Si/N) increases, supporting the idea that SiH4 is the 

primary source of this H at these flow ratios. It is noteworthy that the 1% film contained 

a large percentage of oxygen (23%), while it was absent in films grown at higher flow 

ratios. This has been attributed to post-deposition absorption of water by the nitride 

film.4

 

†

FTIR analysis of the 6% flow ratio films revealed that H was bonded predominantly to Si, as in the 8% 

case. 

8
For applications where hydrogenated SiNx is to be used as a passivation coating, the 

mobility of bound hydrogen is critical as it is thought that the degree of passivation (as 

measured in minority carrier lifetime) depends the amount of hydrogen released from the 

SiNx film.

8

 As a means to study hydrogen release from SiNx, two of the films described 

previously were chosen. In order to observe the effects of H release from N versus Si, 

the 2% and 8% flow ratio films were selected. Each film was annealed for 5 minutes at 

temperatures of 400o

C, 600oC, and 800oC. 

Between each anneal, the sample was 

allowed to cool to room temperature, after 

which FTIR measurements were made to 

monitor to release of H from the SiNx film. 

Fig. 2 shows the FTIR spectra at the various 

annealing temperatures for the 8% (Si-rich) 

film, along with an estimate of the H 

concentration, determined by the use of 

absorption cross sections.2 This figure 

reveals only a slight reduction in the H 

bound to Si up to 600oC, with a total 

reduction of 80% after the 800o

C anneal. 

For the 2% (N-rich) film, a steady reduction 

in the H bonded to N is seen as the anneal 

temperature is increased, with an overall 

reduction of 70% after the 800o

C anneal. 

This difference in hydrogen release kinetics can have important implications for the 

choice of annealing treatments (temperature, time). 

2300 2250 2200 2150 2100 2050 2000 1950

55.2

55.4

55.6

55.8

56.0

56.2

56.4

56.6

56.8

57.0

57.2

-SiH at 2164cm-1

film properties:

n=2.4, thickness=102nm

unchanged upon annealing

transmission (%)

wavenumber (cm-1)

 as-grown, 1.2x1022cm-3

 400oC, 1.2x1022cm-3

 600oC, 1.0x1022cm-3

 800oC, 0.22x1022cm-3

Fig. 2. FTIR spectrum at various annealing 

temperatures for a film grown with 8% SiH4 in 

NH3; annealing times of 5 min were used. 

To evaluate the quality of hot-wire-deposited silicon nitride layers relative to their 

plasma CVD counterparts, films were deposited onto String Ribbon substrates provided 

by Evergreen Solar (Marlboro, MA). Film growth conditions were 3% SiH4/NH3 at a 

total pressure of 70 mTorr for a growth period of 1 hour. The hydrogen content was 

estimated to be, at minimum, 10 at.% by comparison with previously grown films. The 

samples were p-type (resistivity of 3 Ω-cm), with a thin phosphorous-diffused n-type 

layer on top. Due to the large area of the substrates (15 cm x 8 cm), a filament array had 

to be used to improve thickness uniformity, as shown in Fig. 4. It has been observed11

that if the filament spacing is at most half the filament-to-substrate distance, then nonuniformity associated with the filament array is eliminated. With this design criterion in 

mind, the filaments were spaced 2 cm apart, with a wire-to-substrate separation of 5 cm. 

Samples grown using this wire array exhibited a thickness variation of approximately 15 

nm from the center to edge. 

Three nitride samples with an average center thickness of 83 nm were chosen for 

subsequent processing by Evergreen Solar. The nitrided samples had a thick aluminum 

layer deposited on back by a screen-printing process using a commercially available 

aluminum paste. The samples were then annealed in a belt furnace to form the back 

contact as well as release H from the nitride layer. Upon fabricating front contacts, the 

cell’s electrical properties were measured. Table 3 provides the short circuit current 

density (JSC), open circuit voltage (VOC), fill-factor (FF), and efficiency for a 

9
representative hot-wire (HW) nitride cell versus a similarly processed plasma nitride cell 

produced by Evergreen. The hot-wire nitride cell is comparable in electrical properties 

(with the exception of JSC) to the plasma nitride cell. Further improvements in JSC 

might be expected with the use of more uniform nitride coatings. 

Table 3. Comparison of electrical properties for hot-wire versus plasma nitride cells. 

Cell type 

JSC

(mA/cm2) 

VOC

(mV) 

FF Efficiency 

(%) 

HW nitride 29.99 579 0.712 12.4 

Plasma nitride 30.92 584 0.709 12.8 

Pt-H Complex Formation 

Although it is clear that large fractions of hydrogen can be liberated from nitride films 

upon annealing, questions remain about whether this hydrogen is simply released into the 

environment or is driven into the bulk c-Si beneath. 

To investigate this issue, the technique of 

Attenuated Total Reflectance FTIR (ATRFTIR) was used, which provides for a 

longer absorption path length that can 

enable trace vibrational features to be 

resolved. The first sample analyzed 

consisted of a Pt-diffused (to 1017cm-3) pSi ([B] = 2x1015 cm-3) ATR prism, onto 

which a SiNx film of 80 nm (n = 2.3, Sirich) was deposited. Pt was chosen given 

that it is a representative transition metal 

impurity that acts as a sink for H, and the 

resulting Pt-H complexes have been well 

studied, with their vibrational features 

assigned.14 Figure 3 shows the FTIR 

spectra at various annealing temperatures 

(10 min anneal) for Pt-H and Si-H (from 

the nitride layer). Pt-H features are not 

observed until a temperature of 500oC, 

consistent with the H-desorption kinetics 

previously observed for the nitride film. 

Previous calibration14 of the Pt-H line 

Fig. 3. Creation of Pt-H complexes in 

bulk Si by annealing of hydrogenated 

SiNx layer; annealing times of 10 min 

were used. 

1875 1880 1885 2000 2200 2400

0.00

0.02

0.04

0.06

0.08

SiNx: Si-H

Si: PtH

4 0 0o C

5 0 0o C

6 0 0o C

7 0 0o C

Absorbance

Frequency (cm-1)

SiNx / p-Si:Pt 

10
intensities enable an estimate of the concentration of H introduced into the bulk Si. For 

the 700oC anneal, [Pt-H] is estimated to be 5x1012 cm

-2. Through successive thinning of 

this sample to monitor H introduction, the Pt-H concentration is found to drop to 3.1x1012

cm

-2 at a depth of 390 μm. This, in turn, leads to a bulk Pt-H concentration of 5x1013 cm3

. Assuming a uniform distribution of H throughout the sample, a hydrogenated layer 

thickness of 1.1 mm is calculated. A similar Pt-diffused n-type ([P] = 3x1016 cm-3) 

sample revealed a comparable Pt-H concentration. Two undiffused samples (n, p) were 

also analyzed, revealing no vacancy-H or dopant-H complexes; both diffused samples 

also did not reveal such complexes. 

Fig. 4. Photomicrograph of silicon nitride-passivated String Ribbon solar cell; white line 

depicts position of the wire array during hot wire CVD growth 

Summary of Defect and Emitter Passivation 

Varying the SiH4/NH3 ratio from 1-8% produced silicon nitride films ranging in 

refractive index from 1.8-2.5, with hydrogen content ranging from 9-18 atomic %. 

Transmission measurements using FTIR revealed a transition in the bonding of H (N-H 

to Si-H) as the flow ratio was increased beyond 6% SiH4/NH3. Annealing studies 

revealed different kinetics for H release from Si versus N. String Ribbon silicon cells 

produced with hot-wire-grown silicon nitride layers show comparable electrical 

11
properties to those produced with plasma nitride layers. The appearance of Pt-H 

complexes (~1014 cm-3) was observed in Pt-diffused Si samples at annealing temperatures 

consistent with H-release from the nitride top layer 

3. Thin Film Silicon Passivation in HWCVD Growth 

Overview of Thin Film Si by HWCVD 

HWCVD epitaxial growth on large-grained templates is one strategy for the fast, 

low-temperature growth of large-grained films with hydrogen-passivated low-angle grain 

boundaries. In this section, we detail results under the subcontract on the role gas phase 

kinetics in HWCVD growth, film microstructural evolution and passivation of 

polycrystalline thin films in HWCVD growth. 

HWCVD Gas Phase Species Characterization

A knowledge of the primary radicals produced on the wire in a hot-wire chemical vapor 

deposition (HWCVD) reactor is critical to optimization of film microstructure and 

quality, as well as for modeling gas-phase chemistry. In year 2, we devoted considerable 

effort to understanding the changes in the catalytic properties of tungsten hot wires with 

time during operation. Wire catalytic properties were correlated with changes in surface 

and bulk microstructure in the wire. The results suggest that there are significant changes 

in the radical species generated by the wire during extended operation relative to those 

generated by a new wire. This point is of significant practical importance, since it 

implies that the radical species generated in film growth by hot wire chemical vapor 

deposition are a strong function of the frequency with which the wire is replaced (due to 

breakage or routine service operation). 

 The following findings resulted from our activities in this area: 

• With the use of a new wire, Si is found to be the predominant radical produced in 

a HWCVD reactor for wire temperatures in excess of 1500 K. 

• For temperatures below 1500 K, the SiH3 radical becomes predominant. The small 

activation energy (8 kcal/mole) observed for SiH3 formation suggests the process 

is catalyzed with the use of these wires. These results are in qualitative agreement 

with previous studies of radical chemistry at the wire. 

• Radical measurements performed on aged wires show high temperature activation 

energies for all SiHx species, suggesting a non-catalyzed process for radical 

formation. 

• Scanning electron microscopy of aged wires revealed a surface both rougher and 

more irregular than seen with new or heat-treated wires; this morphology is 

thought to be characteristic of Si deposition (either as free Si or a silicide). 

• Auger electron spectroscopy revealed surface Si concentrations as high as 15%, 

suggesting a two-phase equilibrium between W5Si3 and W (at a Si solubility of 

4%). Concentrations of Si in the interior of the wire (2-5%) are of order the 

solubility limit and reveal that Si diffusion into the wire is significant. 

12
• Radical measurements added further evidence of Si diffusion, as Si was detected 

in a silane-free ambient following an aging treatment. 

• Examining rates for various surface kinetic processes reveals that bulk diffusion 

of Si through a silicide is the slowest, followed by Si evaporation and then surface 

decomposition. 

• The high rate of surface decomposition supports the idea that Si is in fact the 

predominant evaporating and diffusing species in and on the wire. In light of the 

low rate of diffusion through the silicide, the diffusion mechanism in the initial 

stages of SiH4 exposure must either consist of bulk diffusion through elemental W 

or large cracks that develop on the wire surface and propagate to the interior. 

• Finally, experimentally observed evaporation kinetics suggest that Si desorption 

from a new wire comes from direct Si-W bond breakage, as opposed to 

evaporation from Si. 

Rates for Surface Kinetic Processes 

A comparison of the relative rates for the various surface kinetic processes on the wire, 

namely, diffusion, evaporation, and decomposition allows further insight into the nature 

of the wire aging process. At the highest temperatures investigated in this study (~ 2400 

K), the solid solubility of Si in W is a few atomic percent, dropping to less than 1 at.% at 

the lowest temperature (1273 K). To our knowledge, there have been no studies that 

examined the diffusion of Si into elemental W. A study by Kharatyan et al., however, 

investigated the diffusion of Si into various silicides of tungsten and molybdenum. For 

the diffusion of Si into W5Si3 (the only silicide for which data were available), a diffusion 

coefficient of 

was obtained. Using this diffusion coefficient, the rate for diffusion (using the wire 

radius as a lower limit for the characteristic length) as a function of wire temperature is 

given in Fig. 5. 

Data on the evaporation rate of Si from W were unavailable, but the study by 

Ehrlich does provide an evaporation rate for H from W, which is relevant to the SiH4 

decomposition process; this rate is plotted in Fig. 5. The evaporation rate of Si from 

liquid Si could be determined from the vapor pressure data of Margrave. As liquid Si can 

form at some point during the aging process (T = 2000oC, W<33%/Si>66%), this rate has 

relevance to the aged wires of this study. This rate of evaporation, as a function of wire 

temperature, is also plotted in Fig. 5. Finally, rates of SiH4 decomposition on W were not 

directly available, but a study by Yang et al.24 suggests the timescale for Si-H bond 

breaking is of order a few picoseconds (in a liquid environment), establishing a lower 

bound to the decomposition rate for the significantly higher temperatures in the present 

study; this rate is included for comparison in Fig. 5. 

Examining these rates in Fig. 5, bulk diffusion of Si through the silicide is clearly 

the slowest process, followed by Si evaporation, and then decomposition. These rates, 

coupled with experimental observations, give a picture of the relative rates of surface 

kinetic processes on the wire. First, the high rate of decomposition compared with 

13
evaporation and diffusion suggests that for the vast majority of wire temperatures, Si (as 

opposed to other SiHx) is the predominant evaporating/diffusing species; radical 

measurements in particular support the idea that it is the dominant evaporating species. 

At sufficiently low temperatures, however, this process becomes decompositionlimited, and a spectrum of other species is observed. The Auger and radical 

measurements described earlier provide clear evidence that diffusion through the wire is 

occurring, and at rates faster than would be predicted by bulk diffusion (through a 

silicide) alone. This suggests that the majority of Si that diffuses into the wire does so 

before a thick silicide has formed. Whether this higher effective diffusivity is due to a 

higher intrinsic rate of Si diffusion into W (rather than W5Si3) or diffusion occurring 

primarily through cracks that develop on the surface of the wire is unclear. 

Fig. 5. Rates of interest for wire surface kinetic processes. 

Finally, it appears that the rate of evaporation of Si from Si(l) is several orders of 

magnitude below that of decomposition, even at the highest wire temperatures. It was 

previously observed, however, that for wire temperatures in excess of 2000 K, a 

saturation in the Si signal occurs with the use of a new wire that does not occur with an 

aged wire. This observation is consistent with a competition between evaporation (of Si 

and/or SiH4) and decomposition. This, in turn, may suggest that the mechanism of Si 

desorption from a new wire does not consist of Si evaporation from Si(l), but a different 

mechanism, namely, the direct desorption of Si from a W surface. The rate for this 

14
process is likely to be of a similar magnitude as H evaporation from W, depicted in 

Fig. 5. It is also possible that the rate of SiH4 evaporation becomes comparable to its rate 

of decomposition at high temperatures, explaining the Si saturation. 

A picture then emerges of the hot-wire CVD decomposition process. First, SiH4 is 

adsorbed and then rapidly decomposed to Si and H on the surface of a new wire. This 

surface Si will then either evaporate or, if the surface concentration is high enough, 

diffuse to the interior of the wire up to its solubility limit. Once the thermodynamic 

solubility has been reached, excess Si can contribute to the formation of the W5Si3 phase, 

or at higher concentrations, WSi2. At the highest Si concentrations (> 67%), liquid Si can 

form at the surface and then Si evaporation from Si(l) becomes the dominant mechanism 

of Si production. The observation that a new wire readily absorbs Si, but retains it for 

a long period of time can be explained as a silicide diffusion-limited process. The silicide 

that forms at the surface acts as a diffusion barrier to Si in the interior of the wire (as well 

as to further diffusion of surface Si into the wire), and Fig. 5 shows that the characteristic 

diffusion time at the wire temperature used (~ 1750 C) is, at minimum, several hours. 

A Phase Diagram for Morphology and Properties of Low Temperature 

Deposited Polycrystalline Silicon Grown by Hot-wire Chemical Vapor 

Deposition 

The fabrication of low temperature polycrystalline silicon with internal surface 

passivation and with lifetimes close to single crystalline silicon is a promising direction 

for thin film polycrystalline silicon photovoltaics. To achieve high lifetimes, large grains 

with passivated low-angle grain boundaries and intragranular defects are required. We 

investigate the low-temperature (300-475oC) growth of thin silicon films by hot-wire 

chemical vapor deposition (HWCVD) on Si (100) substrates and on large-grained 

polycrystalline silicon template layers formed by selective nucleation and solid phase 

epitaxy (SNSPE). Phase diagrams for dilute silane deposition varying substrate 

temperature and for pure silane varying hydrogen dilution are shown. We will discuss the 

relationship between the microstructure and photoconductive decay lifetimes of these 

undoped layers on Si (100) and SNSPE templates as well as their suitability for use in 

thin-film photovoltaic applications. 

HWCVD epitaxial growth on large-grained templates is one strategy for the fast lowtemperature growth of large-grained films with hydrogen-passivated low-angle grain 

boundaries. We propose a structure by which a template with grains on the order of 10–

100 μm is fabricated on ITO-coated glass or another low cost conductive substrate by a 

solid-phase crystallization process called SNSPE15. This layer serves as the n+ back 

surface field, and the n and p+ layers are grown epitaxially on this template by HWCVD, 

using phosphine and tri-methyl boron as dopants. In order to achieve this goal, 

investigation of the phase diagrams that lead to epitaxial growth in HWCVD are 

necessary. Preliminary work towards the fabrication of this cell and the necessary 

intermediate goals are discussed. 

15
Dilute Silane Growth 

In order to promote crystalline growth under dilute silane conditions, we used a high 

hydrogen partial pressure dilution ratio of 50:1, using a mixture of 4% SiH4 in He at a 

partial pressure of 25 mTorr and H2 at a partial pressure of 50 mTorr. The 0.5mm 

diameter tungsten wire was positioned 2.5 cm from the substrate for a growth rate of 

1Å/s. The wire temperature was set to 1800oC as measured by optical pyrometry, and 

substrate temperatures ranged from 300oC to 475oC. 

Silicon (100) substrates and large-grained polycrystalline layers formed by SNSPE 

were used as templates for epitaxial growth. The formation of the large-grained 

templates is described in the Chen PhD thesis 15. The resulting polycrystalline templates 

have grain sizes on the order of 10 to 100 μm with low-angle grain boundaries. Before 

growth, both surfaces were cleaned with UV ozone for 10 minutes followed by an HF 

dip. Once in the chamber, they are heated at 200oC at a pressure of less than 10–6 Torr to 

desorb any residual hydrocarbons. 

Pure Silane Growth 

Using pure silane, the influence of dilution ratio was studied. The substrate 

temperature was held constant at 380 o

C while graphite filaments set to 2100 oC by optical 

pyrometry were placed 3.5 cm from the substrate. The H2 to SiH4 ratio ranged from 10 

to 50, total pressure also had to be adjusted for the higher H2 flow rates and ranged from 

20 to 50 mTorr. Substrates were cleaned as described above. 

Dilute Silane Growth - Substrate Temperature Effects 

We used TEM and RHEED to characterize the crystallinity of films grown at 50:1 

hydrogen dilution and temperatures between 300-4750C in the 50 nm to 2 μm thickness 

regime and observed four phases of growth. A completely defect-free epitaxial phase 

was observable by TEM at thicknesses below 50nm; the twinned epitaxial, mixed and 

polycrystalline phases were observable by TEM and RHEED16. Twinned epitaxial 

silicon denotes silicon material containing twin boundaries, but no other dislocation or 

grain boundary defects. From this data, we derived the phase diagram in Fig. 6. At 

3000C, the predominant phases are epitaxial and twinned, with a transition to mixed 

phase or polycrystalline growth occurring somewhere between 1-2 μm of growth. As 

temperature increases, the epitaxial and twinned phases no longer persist and the 

transition to mixed phase or polycrystalline growth occurs at smaller film thicknesses. 

This is in direct contrast to work done by Thiesen17 and Watahiki18 where epitaxial 

thickness increased with substrate temperature. We believe that the decrease in epitaxial 

thickness with the increase in substrate temperature is due to an interplay between surface 

hydrogenation at low temperatures and surface oxidation at high temperatures that 

reduces the epitaxial thickness. This is possibly related to the higher hydrogen content in 

HWCVD as compared to PECVD or MBE along with oxygen contamination in the 

deposition chamber19. 

16
Fig. 6: Phase diagram of HWCVD films grown at 50:1 hydrogen dilution with 4% SiH4

in He. 

Silicon films 300 nm thick grown on SNSPE templates under the same conditions 

showed epitaxial growth results consistent with local low temperature epitaxy on each of 

the 100 μm grains in the SNSPE templates, as shown in Fig. 7. Epitaxial breakdown is 

observed in the diffraction pattern of the HWCVD film, but some of the underlying loworder diffraction spots are visible. The underlying film therefore likely has morphology 

similar to that of the HWCVD films on Si(100). The effect of the orientation of the 

underlying grain structure of the SNSPE template on the morphology of the HWCVD 

film is shown in Figure 7. 

Cross-sectional analysis of these films reveals some areas of epitaxial growth as well 

as some areas of columnar growth. Before HWCVD growth, the SNSPE templates were 

cleaned in a solution of 3:7 HNO3: H2O, which has been shown by Auger spectroscopy to 

remove elemental Ni from the template surface20. The lack of epitaxy in some areas is 

thus more likely to have been caused by the presence of ubiquitous surface contaminants, 

such as carbon and oxygen, than by nickel nanoparticles, since a similar microstructure 

can be seen in deposition on Si(100) substrates as well. 

17
Pure Silane Growth – Hydrogen Dilution Effects 

We used TEM and Raman Spectroscopy to determine the structure of Si thin films 

grown at 380o

C with graphite wires at 2100oC. We found that the crystallinity did not 

increase with hydrogen dilution as expected21. Instead, an amorphous/protocrystalline 

structure is observed at dilution ratios of 20 to 30, while at both the 40 and 10 H2/SiH4

dilution ratios we observe epitaxial growth with polycrystalline breakdown. This 

suggests a complex interaction between pressure and dilution, which will be discussed 

later. 

 

Fig. 7. Plan-view TEM of HWCVD epitaxial film (T=3000C) on SNSPE template. (a) 

Selected area diffraction pattern from underlying SNSPE template. (b) Selected area 

diffraction pattern from HWCVD film on SNSPE template. (c) Bright-field image 

indicating selected area diffraction regions. Inset: diffraction from entire area. (d) Brightfield image of HWCVD film (T=3000

C) on SNSPE template showing selected area 

diffraction region. Inset: selected area diffraction pattern showing areas of large-grained 

polycrystalline growth. 

Table 4. Results of HWCVD growth with pure silane at 380 o

C substrate temperature and 

at 2100 o

C graphite filament temperature with 3.5cm spacing. 

Hydrogen 

Dilution Ratio 

Pressure 

(mTorr) 

Deposition 

Rate (nm/min)

Raman Crystallinity 

% Phase 

50:1 50 4 83 Poly 

40:1 27 5 84 Epi/poly 

30:1 20 3 9 a-Si/Proto 

20:1 20 6 10 a-Si/Proto 

10:1 20 9 96 Epi/poly 

18
Minority Carrier Lifetimes of HWCVD Films 

The minority carrier lifetimes of 1.5–15 μm thick films grown at 300 °C on Si(100) 

and SNSPE templates were determined through resonant-coupled photoconductive decay 

(RCPCD) measurements22. These films began with epitaxial growth which broke down 

to microcrystalline growth at thicknesses between 1 – 2 μm. Although the microstructure 

of these films is mostly microcrystalline, the lifetimes are microseconds long. The 

lifetimes for films on Si(100) range from 5.7 to 14.8 μs while those for films on SNSPE 

templates range from 5.9 to 19.3 μs under low level injection conditions, as summarized 

in Figures 8 and 9. Residual nickel present in the SNSPE templates does not significantly 

affect the lifetime of films grown on SNSPE templates, making the growth of epitaxial 

layers by HWCVD on SNSPE templates a viable strategy for the fabrication of thin film 

photovoltaics. 

C 

d 

Fig. 8. LLI and HLI minority carrier lifetimes of HWCVD films on Si(100) as measured 

by RCPCD. The dashed and dotted lines represent the LLI and HLI lifetimes, 

respectively, of the bulk Si(100) substrate. 

Fig. 9. LLI and HLI minority carrier lifetimes on SNSPE templates. 

19
Hot Wire CVD – A Simple Model 

The epitaxial growth and growth breakdown trends for HWCVD growth are 

consistent with a simple model correlating epitaxial growth breakdown with surface 

oxidation. Starting with an initial hydrogen surface coverage dependent only on the 

substrate temperature determined from temperature-programmed desorption data23, the 

model determines the steady-state surface hydrogen coverage by balancing thermal 

desorption of surface hydrogen with adsorption and abstraction of surface hydrogen by 

atomic hydrogen produced by the hot wire. Oxygen atom can be incorporated into the 

film at any empty sites. We used the model to determine the amount of oxygen deposited 

during the growth of the first monolayer of silicon for a given growth temperature as a 

function of dilution ratio R (R=H2/SiH4) at constant pressure, assuming that all silicon 

atoms incident on the substrate contribute to growth.19

Figure 10 shows that the maximum silicon to oxygen ratio decreases with temperature 

for between 311°C and 520°C at a hydrogen dilution ratio of 50:1. This may explain the 

decrease in epitaxial thickness with temperature in Fig. 6. 

300 350 400 450 500 550

6.0x103

8.0x103

1.0x104

1.2x104

1.4x104

1.6x104

1.8x104

2.0x104

2.2x104

2.4x104

2.6x104

Substrate Temperature (0C)

Si/O Ratio

0

200

400

600

800

1000

1200

Epitaxial Thickness (nm)

Fig. 10. Silicon to oxygen ratio in the first monolayer of growth as a function of 

substrate temperature for dilute silane growth at 50:1 hydrogen dilution and epitaxial 

thickness. 

The dependence of the epitaxial thickness on the silicon to oxygen ratio is difficult to 

quantify. However, it is known that, during MBE crystal growth, impurities at the 

growing interface can lead to surface roughening and subsequent epitaxial breakdown 

through the formation of voids which may lead to twinning and surface facets24. For our 

dilute silane experiments, a decrease in the maximum silicon to oxygen deposition ratio 

with temperature, as predicted by the model, may explain the observed decrease in 

epitaxial thickness with temperature. 

20
Figure 10 may also give some qualitative insight into the amorphous/protocrystalline 

peak at intermediate dilution for growth with pure silane. In this simulation the 

deposition pressure is changed along with the dilution ratio in order to match the 

deposition conditions in Table 4. The Si/O ratio is highest at the two extremes of the 

deposition conditions: low dilution ratio, low pressure; and high dilution ratio, high 

pressure. The high Si/O ratio correlates with a higher degree of crystallinity. A low Si/O 

ratio leads to the amorphous and ultimately protocrystalline film. Through these 

experiments and simulations we are working toward determining the optimal deposition 

parameters for epitaxial growth with polycrystalline breakdown. 

10 20 30 40 50

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

R=H2/SiH4

Si/O Ratio

0

20

40

60

80

100

Raman Crystalline %

Fig. 11. Silicon to oxygen ratio in the first monolayer of growth as a function of R for 

pure silane growth using the deposition conditions in Table 4 along with the Raman 

crystalline %. 

We have explored the phase space of HWCVD growth with dilute and pure silane at 

various substrate temperatures and dilution ratios. The complex interaction between 

deposition parameters can be qualitatively explained by an oxidation model. Twinned 

epitaxial growth of Si to thicknesses over 1 μm is observed with polycrystalline 

breakdown under dilute conditions. 

The minority carrier lifetimes of nearly-intrinsic epitaxial/microcrystalline films 

grown on Si (100) by HWCVD range from 5.7 to 7.5 μs. The lifetimes of films grown 

under the same conditions on SNSPE templates range from 5.9 to 19.3 μs, making them 

suitable for incorporation into photovoltaic devices. In particular, residual nickel from the 

SNSPE templates does not appear to be significantly detrimental to the lifetime of films 

grown on these templates. If the mobilities in these films are also high, it is possible that 

21
HWCVD epitaxy on large-grained SNSPE templates could be a viable strategy for the 

fabrication of thin-film photovoltaics. 

HWCVD Si Film Passivation 

Under this subcontract, we investigated thin film cell structures fabricated on 

polycrystalline silicon templates with grain sizes on the order of ten microns on glass 

coated with a transparent, conductive oxide (TCO). Templates were formed by a solidphase crystallization process called selective nucleation solid phase epitaxy (SNSPE)15. 

The template layer is designed to serve as the n+ layer of the device, and as the epitaxial 

template for n and p+ layers grown by HWCVD, using phosphine and trimethyl boron as 

dopants (Fig. 12). As a simple test structure for the passivation studies, we fabricated, ntype HWCVD Si films on p-type Si(100) substrates, in order to study both the 

microstructure and electrical properties of the film using a single crystal template, 

without the complications created by the large-grained polycrystalline template seed 

layer. 

Grain Boundaries

SNSPE Poly-Si n+template 

HWCVD epitaxy 

1-10 μm n-type 

 layer 

Glass 

p

10-100 μm

Conductive 

Passivation and Metal

Contacts

Oxide

Fig. 12. Schematic of proposed photovoltaic device incorporating epitaxial Si growth on 

a large-grained polycrystalline template fabricated by SNSPE. 

Microstructure and Passivation Effects on Open Circuit Voltage 

Hydrogen-passivated silicon films grown via hot wire chemical vapor deposition 

(HWCVD) are promising candidates for polycrystalline Si solar cell absorber layers. We 

studied the correlation of film microstructure and passivation with open-circuit voltage, a 

key cell electrical characteristic, for varying growth conditions and post-deposition 

treatments. We examined the role of hydrogen-dilution on growth morphology and Voc. 

Importantly, the results indicate effective hydrogen passivation in the ‘bulk’ of HWCVDdeposited Si films to the extent that the Voc is highly sensitive to surface passivation. 

Surface passivation, rather than bulk passivation, is thus identified as the limiting factor 

in achieving higher Voc; hence we have investigated the effect of several post-deposition 

passivation treatments on surface passivation. Our goal in this work was to elucidate the 

role of hydrogen in HWCVD Si film bulk passivation, crystalline fraction, growth 

22
morphology, and its relation to film electrical properties. Varying growth conditions and 

observing the subsequent effect on microstructure and open-circuit voltage enables us to 

optimize these characteristics for polycrystalline Si thin film solar cell absorber 

applications. 

 In these experiments, silicon films were grown on CZ-grown Si(100) via HWCVD, 

with either undoped or n-type with phosphine (5% in SiH4) as the dopant gas, at substrate 

temperatures varying from 230 C to 350 C. Silane gas (1% in Ar) is diluted with 

hydrogen in various ratios of R = H2/SiH4. We used tungsten filaments operated at 

T~1700 ° C to catalyze SiH4 decomposition. Open circuit voltage measurements were 

performed for n-type films with phosphorus doping of 1 x 1015 cm-3 grown on CZ-grown 

p-type Si (100) with boron doping of 1 x 1016 cm-3. Steady-state Voc25 measurements 

were performed on unmetallized HWCVD n-Si films/p-Si (100), and growth morphology 

was observed by cross-sectional transmission electron microscopy. 

HWCVD Growth and Morphology 

 The structure of as a function of hydrogen dilution was investigated by electron 

microscopy, which reveals an increase in porosity for growth in the range 230 °C < T < 

350 °C as dilution is decreased from R = 480 to R = 30, for a total pressure of P = 120 

mTorr. The resulting images reveal a more rapid breakdown to a porous microstructure 

as R decreases, as shown in Fig. 13. The pore-permeated microstructure originates 

during film growth and pores are oriented perpendicular to the surface. Although films 

exhibiting this porous microstructure would be suspected to be prone to oxidation, 

Raman measurements taken after ambient exposure for ~10 weeks did not indicate oxide 

formation. 

R=30 

100 nm 

R=30

20 nm 

R=240 

Fig. 13. Cross-sectional TEM of growth morphology at various hydrogen dilutions 26. 

Open-Circuit Voltage 

 An increase in open-circuit voltage with hydrogen dilution is observed for 1000 nm 

thick films grown with a hydrogen dilution in the range 0 < R < 244, as shown in Fig. 14. 

For R = 244, an initial Voc of 400 mV was observed, but Voc decreased to 290 mV after 

one week of ambient exposure. Another set of films 885nm thick were grown with a 

hydrogen dilution ranging from 45 < R < 180, with a similar increase in Voc observed 

23
with H-dilution. The highest stabilized Voc of 350 mV was observed for 885 nm thick 

films deposited at R=90. We attribute the higher Voc to lower film porosity compared to 

films grown at lower dilution, which leads to less surface recombination due to the 

smaller surface area. As the thickness of the film increases, dense epitaxial growth is 

observed to transition into a porous microstructure. For this reason, we do not observe a 

maximum stabilized Voc for the thickest film. 

The open-circuit voltage of both the 125 nm and 885 nm films was measured initially 

and after 1 week in ambient air, and significant changes are observed27. The rise in Voc

over time in ambient air indicates sufficient passivation of the bulk of the film due to high 

hydrogen content during growth. We attribute this change in Voc to a change in surface 

passivation during ambient oxidation, suggesting that hydrogen passivation of defects in 

the ‘bulk’ of the film is sufficient to prevent bulk recombination from limiting Voc. 

Fig. 14. Voc with hydrogen dilution immediately after deposition and after one week in 

ambient air. 

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300

Initial Voc 885nm

Initial Voc 125nm

Initial Voc 1000nm

Voc after 1 week

885nm

Voc after 1 week

125nm

Voc after one week

1000nm

Voc (mV) 

R= H2/SiH4 

Surface Passivation 

 The results of various surface treatments are shown in Fig 15. Annealing at 900 C 

causes a dramatic decrease in Voc that we attribute to hydrogen evolution from the film. 

Chemical oxidation by liquid immersion in H2O: H2O2: NH4OH during an RCA II clean 

results Voc comparable to those for films that undergo ambient air oxidation. 

We investigated the correlation between microstructure and surface passivation on 

Voc in low temperature HWCVD polycrystalline silicon thin films. Film porosity 

increases with decreasing hydrogen dilution for films grown under high hydrogen 

dilution. In general, epitaxial growth and low porosity correlate with higher Voc. Results 

from post-deposition treatments show promising improvements in Voc with RCA 

oxidation. A decrease in Voc is seen after high temperature anneals, suggesting loss of 

bulk hydrogen passivation during high temperature annealing. These results clearly 

demonstrate the ability to achieve high Voc for well-passivated films, and future work 

24
will address achieving stable passivation for HWCVD Si polycrystalline films with high 

initial Voc. 

0

50

100

150

200

250

300

0 50 100 150 200

V

As deposited

oc (mV) 

One week

Heat treatment (900 C

for 5 min.)

RCA oxidation (80 C

for 10 min.)

H treatment (200 C for

5 min.)

R= H2/SiH4 

Fig. 15. Open-circuit voltage with hydrogen dilution after various post-deposition 

treatments. 

25
References 

[1] H. Sato, A. Izumi, and H. Matsumura, Appl. Phys. Lett. 77, 2752 (2000). 

[2] W. Lanford and M. Rand, J. Appl. Phys. 49, 2473 (1978). 

[3] H. Matsumura, Jpn. J. Appl. Phys. 37, 3175-3187 (1998). 

[4] B. Stannowski, M. van Veen, and R.E.I. Schropp, Mat. Res. Soc. Symp. Proc. 664, 

A17.3.1 (2001). 

[5] J. Holt, M. Mason, D. Goodwin, and H. Atwater, unpublished. 

[6] R. Sanderson, Chemical Bonds and Bond Energy (Academic Press, New York, 

(1976). 

[7] S. Kaluri and D. Hess, Appl. Phys. Lett. 69, 1053 (1996). 

[8] V. Yelundur, A. Rohatgi, A. Ebong, A. Gabor, J. Hanoka, and R. Wallace, J. 

Electron. Mater. 30, 526 (2001). 

[9] C. Boehme and G. Lucovsky, J. Appl. Phys. 88, 6055 (2000). 

[10] B. Nelson, E. Iwaniczko, A. Mahan, Q. Wang, Y. Xu, R. Crandall, and H. Branz, 

Thin Solid Films 395, 292 (2001). 

[11] A. Ledermann, U. Weber, C. Mukherjee, and B. Schroeder, Thin Solid Films 395,

61 (2001). 

[12] J. Holt, M. Swiatek, D. Goodwin, and H. Atwater, Mat. Res. Soc. Symp. Proc. 715,

10.2.1 (2002). 

[13] J. Moschner, J. Schmidt, and R. Hezel, 29th IEEE Photovoltaic Specialists 

Conference, New Orleans, LA (2002). 

[14] M. Weinstein, M. Stavola, K. Stavola, S. Uftring, J. Weber, J. Sachse, and H. 

Lemke, Phys. Rev. B 65, 035206-1 (2001). 

[15] C. M. Chen, "Polycrystalline Silicon Thin Films for Photovoltaics," Ph.D. thesis in 

Applied Physics, California Institute of Technology, 2001. 

[16] M. S. Mason, Christine E. Richardson and Harry A. Atwater, MRS Proceedings 

A8.11, San Francisco, 2004. 

[17] J. Thiesen, E. Iwaniczko, K.M. Jones, A. Mahan, and R. Crandall, Appl. Phys. 

Letters, vol. 75, pp. 992, 1999. 

[18] T. Watahiki, A. Yamasa, and M. Konagai, J. Crys. Growth, vol. 209, pp. 335, 2000. 

[19] Christine E. Richardson, M. S. Mason, and Harry A. Atwater Thin Solid Films, to 

be published. 

[20] R.A. Puglisi, H. Tanabe, C.M. Chen, and Harry A. Atwater, Mat. Sci. Eng. B, vol. 

73, pp. 212, 2000. 

[21] M. S. Mason, "Synthesis of Large-Grained Polycrystalline Silicon by Hot-Wire 

Chemical Vapor Deposition for Thin Film Photovoltaic Applications," California 

Institute of Technology, 2004. 

[22] R. K. Ahrenkiel and S. Johnston, Sol. Energy Mat. Sol. Cells, vol. 55, pp. 59, 1998. 

[23] M.C. Flowers, N. B. H. Jonathon, Y. Liu, and A. Morris, Journal of Chemical 

Physics, vol. 99, pp. 7038, 1993. 

[24] D. J. Eaglesham, Journal of Applied Physics, vol. 77, pp. 3597, 1995. 

[25] R.A. Sinton and A. Cuevas, “A Quasi-Steady-State Open-Circuit Voltage Method 

for Solar Cell Characterization”, Proceedings of the 16th European Photovoltaic Solar 

Energy Conference, Glasgow, United Kingdom (2000). 

26
[26] C. E. Richardson, “Low-Temperature Hot-Wire Chemical Vapor Deposition of 

Epitaxial Films for Large-Grained Polycrystalline Photovoltaic Devices”, Ph. D. Thesis, 

California Institute of Technology, (2006). 

[27] C. E. Richardson, K. Langeland, and H. A. Atwater, “Electronic Properties of Low 

Temperature Epitaxial Silicon Thin Film Photovoltaic Devices Grown by HWCVD”, 

Proceedings of the 4th International Conference on Hot-Wire CVD Process, Takayama, 

Japan, (2006). 

27
Personnel Associated with Subcontract 

Researchers: 

1. Krista Langeland, graduate student 

2. Christine E. Richardson, graduate student 

3. Maribeth S. Mason, graduate student 

4. Jason Holt, graduate student 

5. Youngbae Park, postdoctoral research 

6. Harry A. Atwater, principal investigator 

Collaborators: 

1. Andrew Gabor, Evergreen Solar, Inc. 

2. David Carlson, BP Solar 

3. Michael Stavola, Lehigh University 

4. David Goodwin, Caltech 

5. William A. Goddard, Caltech 

6. Richard Ahrenkiel, NREL 

28
Publications under subcontract 

PhD Theses: 

1. Jason Knowles Holt, PhD. Chemical Engineering, 2002. 

2. Maribeth Swiatek Mason, PhD, Materials Science, 2004. 

3. Christine Esber Richardson, PhD, Applied Physics, 2006. 

Journal Publications: 

1. J.K. Holt, M. Swiatek, D.G. Goodwin, R.P. Muller, W.A. Goddard, III and H.A. 

Atwater. Thin Solid Films 395 (2001), pp. 29–35. 

2. “Gas Phase and Surface Kinetic Processes in Polycrystalline Silicon Hot-Wire 

Chemical Vapor Deposition”, J. K. Holt, M. Swiatek, D. G. Goodwin, and H.A. 

Atwater, Thin Solid Films 395, 29-35 (2001). 

3. “High Hydrogen Content Silicon Nitride For Photovoltaic Applications Deposited 

By Hot-Wire Chemical Vapor Deposition”, Jason K. Holt, Maribeth Swiatek 

Mason, David G. Goodwin, and Harry A. Atwater, Mat. Res. Soc. Symp. Proc., 

Materials Research Society, Pittsburgh, PA, (2002). 

•

4. Holt, JK; Swiatek, M; Goodwin, DG; Atwater, HA, The aging of tungsten 

filaments and its effect on wire surface kinetics in hot-wire chemical vapor 

deposition, Journal of Applied Physics, 92 (8): 4803-4808 OCT 15 2002. 

•

5. Mason, MS; Chen, CM; Atwater, HA, Hot-wire chemical vapor deposition for 

grained polycrystalline epitaxial silicon growth on large-silicon templates, Thin 

Solid Films, 430 (1-2): 54-57 APR 22 2003 

6. Jiang, F; Stavola, M; Rohatgi, A; Kim, D; Holt, J; Atwater, H; Kalejs, J, 

Hydrogenation of Si from SiNx(H) films: Characterization of H introduced into 

the Si, Applied Physics Letters, 83 (5): 931-933 AUG 4 2003 

7. Holt, JK; Goodwin, DG; Gabor, AM; Jiang, F; Stavola, M; Atwater, HA, Hotwire chemical vapor deposition of high hydrogen content silicon nitride for solar 

cell passivation and anti-reflection coating applications, Thin Solid Films, 430 (1-

2): 37-40 APR 22 2003 

8. Mason, MS; Holt, JK; Atwater, HA, Quantitative modeling of nucleation kinetics 

in experiments for poly-Si growth on SiO2 by hot wire chemical vapor 

deposition, Thin Solid Films, 458 (1-2): 67-70 JUN 30 2004 

29
9. Mason, MS; Holt, JK; Atwater, HA, Quantitative modeling of nucleation kinetics 

in experiments for poly-Si growth on SiO2 by hot wire chemical vapor deposition, 

Thin Solid Films, 458 (1-2): 67-70 JUN 30 2004 

10. Christine E. Richardson, Maribeth S. Mason, and Harry A. Atwater. "A Phase 

Diagram for Morphology and Properties of Low Temperature-Deposited 

Polycrystalline Silicon Grown by Hot-wire Chemical Vapor Deposition" in 

Amorphous and Nanocrystalline Silicon Science and Technology— 2004, (Mater. 

Res. Soc. Symp. Proc. 808, Warrendale, PA, 2004). 

11. Christine Esber Richardson, B.M. Kayes, M.J. Dicken, and Harry A. Atwater. "A 

Phase Diagram of Low Temperature Epitaxial Silicon Grown by Hot-wire 

Chemical Vapor Deposition for Photovoltaic Devices" in Amorphous and 

Nanocrystalline Silicon Science and Technology-2005, (Mater. Res. Soc. Symp. 

Proc. 862, Warrendale, PA, 2005). 

12. Christine Esber Richardson, M.S. Mason and Harry A. Atwater. “A Phase 

Diagram for Morphology and Properties of Low Temperature Deposited 

Polycrystalline Silicon Grown by Hot-wire Chemical Vapor Deposition.” 

Proceedings of the 31st IEEE Photovoltaics Specialists Conference. Lake Buena 

Vista, FL. (2005) 

13. Mason, MS; Richardson, CE; Atwater, HA; Ahrenkiel, RK, Microsecond 

minority carrier lifetimes in HWCVD-grown films and implications for thin film 

solar cells, Thin Solid Films, 501 (1-2): 288-290 APR 20 2006 

14. Richardson, CE; Mason, MS; Atwater, HA, Hot-wire CVD-grown epitaxial Si 

films on Si(100) substrates and a model of epitaxial breakdown, Thin Solid Films, 

501 (1-2): 332-334 APR 20 2006 

15. Richardson, CE; Park, YB; Atwater, HA; Surface evolution during crystalline 

silicon film growth by low-temperature hot-wire chemical vapor deposition on 

silicon substrates, Physical Review B, 73 (24): Art. No. 245328 JUN 2006 

16. C. E. Richardson, Y.-B. Park, and H. A. Atwater, “Surface Evolution during 

LowTemperature Epitaxial Silicon Growth by Hot-wire Chemical Vapor 

Deposition: Structural and Electronic Properties.” Proceedings of the 4th IEEE 

Photovoltaics Specialists Conference.Waikoloa, HI. (2006) 

30
Presentations under Subcontract 

1. “Hot-Wire Chemical Vapor Deposition of High Hydrogen Content Silicon Nitride 

for Crystalline Silicon Solar Cells”, J. K. Holt, D. G. Goodwin, A. M. Gabor, M. 

Stavola, and Harry A. Atwater, 12th Workshop on Crystalline Silicon Solar Cell 

Materials and Processes, Breckenridge, CO, August 12th, 2002. 

2.“Hot-Wire Chemical Vapor Deposition of High Hydrogen Content Silicon Nitride 

for Solar Cell Passivation and Anti-Reflection Coating Applications”, J. K. Holt, 

D. G. Goodwin, A. M. Gabor, M. Stavola, and Harry A. Atwater, 2nd International 

Conference on CAT-CVD (Hot Wire Chemical Vapor Deposition), Denver, CO, 

September 10th, 2002. 

3.“Hot-Wire Chemical Vapor Deposition of Silicon and Silicon Nitride”, Harry 

Atwater, invited talk at PolySE, International Conference on Polycrystalline 

Semiconductors, Nara City, Japan, September 10th, 2002. 

4. “Hot-Wire Chemical Vapor Deposition of Silicon and Silicon Nitride”, Harry 

Atwater, invited talk, MRS Symposium A: Amorphous and Nanocrystalline 

Silicon-Based Films 2003, Spring 2003 Materials Research Society Meeting, San 

Francisco, CA, April 21st – 25th, 2003. 

5.“Hot-Wire Chemical Vapor Deposition for Epitaxial Silicon Growth on LargeGrained Polycrystalline Silicon Templates” M.S. Mason, C.M. Chen, and H.A. 

Atwater, MRS Symposium A: Amorphous and Nanocrystalline Silicon-Based 

Films 2003, Spring 2003 Materials Research Society Meeting, San Francisco, CA, 

April 21st – 25th, 2003. 

6.“Microstructure and Passivation in Silicon-Based Thin Film Polycrystalline and 

Nanostructured Solar Cells. Harry Atwater, invited talk, Symposium V “Critical 

Interfacial Issues in Thin-Film Optoelectronic and Energy Conversion Devices”, 

Fall 2003 Materials Research Society Meeting, Boston MA, December 1st – 5th, 

2003. 

7. “A Phase Diagram for Morphology and Properties of Low Temperature-Deposited 

Polycrystalline Silicon Grown by Hot Wire Chemical Vapor Deposition”, 

Christine Esber Richardson, Maribeth Swiatek Mason and Harry A Atwater, 

Symposium A, Amorphous and Nanocrystalline Silicon Science and Technology2004, Spring 2004 Materials Research Society Meeting, San Francisco, CA, 

April 12 - 16, 2004. 

8. C. E. Richardson, Y.-B. Park, and H. A. Atwater, “Surface Evolution during Low 

Temperature Epitaxial Silicon Growth by Hot-wire Chemical Vapor Deposition: 

Structural and Electronic Properties.” Proceedings of the 4th IEEE Photovoltaics 

Specialists Conference. Waikoloa, HI. (2006) 

31
9. Christine E. Richardson, M.S. Mason and Harry A. Atwater. "Hot-wire CVD 

Grown Epitaxial Films and a Model of Epitaxial Breakdown." 3rd International 

Conference on CAT-CVD (Hot Wire Chemical Vapor Deposition), Utrecht 

Netherlands, August 23 – 27, 2004. 

10. Maribeth S. Mason, C. E. Richardson, Harry A. Atwater, R.K. Ahrenkiel. 

"Microsecond Minority Carrier Lifetimes in HWCVD-Grown Films and 

Implications for Thin Film Solar Cells." 3rd International Conference on CATCVD (Hot Wire Chemical Vapor Deposition), Utrecht Netherlands, August 23 – 

27, 2004. 

11. Christine E. Richardson, J. K. Holt, and Harry A. Atwater. "Epitaxial Silicon and 

Silicon Nitride Growth by Hot-wire Chemical Vapor Deposition for Photovoltaic 

Devices." 15th Workshop on Crystalline Si Solar Cells. Vail, CO. August 8th - 

12th, 2005 

12. “Large-Grained Polycrystalline Films for Photovoltaic Devices”, Christine Esber 

Richardson and Harry A. Atwater, Amorphous and Nanocrystalline Silicon 

Science and Technology-2005, Spring 2005 Materials Research Society Meeting, 

San Francisco, CA, March 28th – April 1st, 2005. 

13. “Morphology and Properties of Low Temperature Deposited Polycrystalline 

Silicon Grown by Hot-wire Chemical Vapor Deposition.” 31st IEEE 

Photovoltaics Specialists Conference. Lake Buena Vista, FL. January 3-7, 2005. 

14. Christine E. Richardson, Maribeth S. Mason, and Harry A. Atwater. "A Phase 

Diagram of Hot-wire CVD Epitaxial Si Films on Si (100) Substrates and Largegrained Polycrystalline Templates." 14th Workshop on Crystalline Si Solar Cells. 

Winterpark, CO. August 8 – 11, 2004 

15. Christine E. Richardson, Maribeth S. Mason, and Harry A. Atwater. "A Phase 

Diagram for Morphology and Properties of Low Temperature-Deposited 

Polycrystalline Silicon Grown by Hot-wire Chemical Vapor Deposition" in 

Amorphous and Nanocrystalline Silicon Science and Technology— 2004, Spring 

2004 Materials Research Society Meeting, San Francisco, CA, April 12th – 16th, 

2004. 

16. “Silicon Thin Film Photovoltaic Devices” Harry Atwater, NSF Workshop on 

Nanotechnology for Energy Sciences, November 21st, 2005, Arlington, VA, 

17. “Surface Evolution During Epitaxial and Polycrystalline Silicon Film Growth by 

Low Temperature Hot-Wire Chemical Vapor Deposition on Silicon Substrates” 

Christine Esber Richardson, Young-bae Park and Harry A Atwater, Symposium 

A: Amorphous and Polycrystalline Thin-Film Silicon Science and Technology, 

32
Spring 2006 Materials Research Society Meeting, San Francisco, CA, April 17th – 

21st, 2006. 

Honors and Awards under Subcontract 

1. Harry Atwater, Howard Hughes Chaired Professorship, California Institute of

Technology, 2002.

2. Christine M. Richardson, Best Young Researcher Award, 3rd International 

Conference on CAT-CVD (Hot Wire Chemical Vapor Deposition), Utrecht 

Netherlands, August 23 – 27, 2004. 

3. Harry Atwater Joop Los Award and Fellowship, Dutch Foundation for

Fundamental Research on Matter, 2004.

4. Christine M. Richardson, National Physical Sciences Consortium Fellowship,

2003.

5. Christine M. Richardson, Graduate Student Award, Spring 2005 Materials 

Research Society Meeting, San Francisco, CA, March 28th – April 1st, 2005. 

33
F1146-E(09/2007) 

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1. REPORT DATE (DD-MM-YYYY)

November 2007 

2. REPORT TYPE

Subcontract Report 

3. DATES COVERED (From - To)

March 18, 2007 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Si Passivation and Chemical Vapor Deposition of Silicon Nitride: 

Final Technical Report, March 18, 2007 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-42325 

5e. TASK NUMBER

PVA72101 

6. AUTHOR(S)

H.A. Atwater 

5f. WORK UNIT NUMBER

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

Thomas J. Watson Laboratory of Applied Physics 

California Institute of Technology 

Pasadena, California 91125 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

AAT-2-31605-01 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

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National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-42325 

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National Technical Information Service 

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Richard Matson/Fannie Posey-Eddy 

14. ABSTRACT (Maximum 200 Words)

This report investigated chemical and physical methods for Si surface passivation for application in crystalline Si and 

thin Si film photovoltaic devices. Overall, our efforts during the project were focused in three areas: i) synthesis of 

silicon nitride thin films with high hydrogen content by hot-wire chemical vapor deposition; ii) investigation of the role 

of hydrogen passivation of defects in crystalline Si and Si solar cells by out diffusion from hydrogenated silicon nitride 

films; iii) investigation of the growth kinetics and passivation of hydrogenated polycrystalline. Silicon nitride films 

were grown by hot-wire chemical vapor deposition and film properties have been characterized as a function of 

SiH4/NH3 flow ratio. It was demonstrated that hot-wire chemical vapor deposition leads to growth of SiNx films with 

controllable stoichiometry and hydrogen. 

15. SUBJECT TERMS

PV; silicon; hot-wire chemical vapor deposition; nitride; solar cells; stoichiometry; defects; surface passivation; 

crystalline Si; thin Si film; hydrogen passivation; growth kinetics; 

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