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Operated for the U.S. Department of Energy 

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by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 

A.H. Mahan, Y. Xu, L.M. Gedvilas, R.C. Reedy 

and H.M. Branz 

National Renewable Energy Laboratory

D.L. Williamson 

Colorado School of Mines

S. Datta and J.D. Cohen 

University of Oregon 

B. Yan 

United Solar Ovonics Corporation 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37475 

a-SiGe:H Materials and Devices 

Deposited by Hot Wire CVD 

Using a Tantalum Filament 

Operated at Low Temperature
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a-SiGe:H Materials and Devices Deposited by Hot Wire CVD Using a 

Tantalum Filament Operated at Low Temperature 

A.H. Mahan, Y. Xu, L.M. Gedvilas, R.C. Reedy, D.L. Williamson*, S. Datta**, J.D. Cohen**, B. Yan***, and H.M. Branz 

NREL, 1617 Cole Blvd., Golden, CO 80401 

*Colorado School of Mines, Golden, CO 80401 

** Physics Department, Univ. of Oregon, Eugene, OR 97403 

***United Solar Ovonics Corporation, 1100 W. Maple Road, Troy, MI 48084 

Abstract 

We report the deposition of improved hydrogenated 

amorphous silicon germanium (a-SiGe:H) films by the Hot Wire 

CVD (HWCVD) technique using a tantalum filament operating 

at a low temperature. We gauge the material quality of the aSiGe:H films by comparing infrared, small angle x-ray 

scattering (SAXS), photocapacitance and conductivity 

measurements to earlier results, and fabricate single junction ni-p solar cell devices using these i-layers. 

Introduction 

The incorporation of high Ge content a-SiGe:H into a 

low bandgap solar cell commonly involves the use of bandgap 

profiling, where the lowest bandgap, or the highest Ge content, 

material is placed near the middle of the i-layer to avoid an 

abrupt bandgap transition at the n/i and p/i interfaces (1,2). 

Since in this approach the a-SiGe:H must be profiled over a 

wide range of Ge content, all i-layer alloy compositions, 

including that of a-Si:H without Ge, must be of device quality 

and must be deposited using similar process conditions. 

Previously, these conditions were not met in HWCVD, as the 

process conditions to deposit the a-Si:H and a-SiGe:H films 

were quite different. In particular, our best quality a-Si:H was 

typically deposited at high filament temperatures using a 

tungsten (W) filament (3,4), but we could only deposit our 

device quality a-SiGe:H using a smaller diameter W filament 

operating at temperatures so low that deposition reproducibility 

and filament lifetime were significant issues (5). This work 

reports on deposition of optimum quality a-SiGe:H by the 

HWCVD technique using a tantalum (Ta) filament also 

operating at low temperatures (6). We gauge the material 

quality by comparing infrared, small angle x-ray scattering 

(SAXS), and photocapacitance results to those presented 

elsewhere, and conclude that the structural heterogeneity of the 

HWCVD a-SiGe:H films can be significantly reduced compared 

to those deposited by PECVD by using low filament 

temperatures. We show that this improved structural 

homogeneity translates into improved electronic properties, as 

measured by the sl/sd conductivity ratio, and we also report 

preliminary results on single junction n-i-p solar cell devices 

using these i-layers. 

Experimental 

The a-SiGe:H films were grown by HWCVD using a 

0.5-mm-diameter Ta filament located 5 cm from the substrate 

holder, in a tube reactor described elsewhere [7]. The filament 

a T

temperature (Tfil) was estimated from the filament current (8), 

and was also calibrated using a two wavelength pyrometer under 

vacuum conditions (9). Table I gives representative deposition 

parameters for a 1.25 eV bandgap a-SiGe:H film containing 65 

at.% Ge, deposited using a filament temperature of 1750°C and 

sub of 200°C; it exhibited a photo-to-dark conductivity ratio 

of 140. To change the film bandgap, the film Ge content was 

varied by changing the GeH4/SiH4 gas flow ratio while keeping 

the total (SiH4 + GeH4) gas flow equal to the H2 flow (25 sccm). 

Previous work (6) has shown that film reproducibility and 

filament lifetime are not problems when using a Ta filament 

operated at low Tfil, in that more than 10-12 μm of material can 

be deposited reproducibly without a filament change. 

SiH4 

(sccm) 

GeH4 

(sccm) 

H2 

(sccm) 

Pch 

(mT) 

Rd 

(Å/s) 

16 9 25 12.5 2.5 

Table I. 

Each film was deposited simultaneously on 1737F 

Corning glass and c-Si wafers, with evaporated coplanar (width 

to length ratio of 0.05) Cr contacts on the films for conductivity 

measurements. Optical measurements were performed on these 

films using an n&k 1280 Analyzer to determine the Tauc’s 

bandgap; this was obtained from fitting E vs. (ahn)

1/2, where a 

is calculated by the method of interference-free determination of 

a from the transmission and reflection data (10). Fourier 

transmission infrared (FTIR) absorption spectra were obtained 

from transmission measurements, for films deposited 

simultaneously on c-Si wafers, using a Nicolet 510 system -1 operating between 400 and 4000 cm . The film hydrogen 

content (CH) was determined by calculating the total integrated 

absorption of the SiH and GeH wagging modes located at about 

640 and 570 cm-1 respectively, while Gaussian deconvolution of 

the respective stretch modes gave information about the relative 

SiH and GeH magnitudes. Secondary ion mass spectroscopy 

(SIMS) measurements of the Ge content, for selected films 

deposited on c-Si substrates, enabled the preferential attachment 

ratio [PA=(SiH/Si)/(GeH/Ge)] to be calculated. In a separate 

series of depositions, 1.5-2.0 μm thick films covering a wide 

range of Ge contents were deposited on high purity Al foil for 

the small angle x-ray (SAXS) measurements, and on SS/n+ 

layers for the photocapacitance measurements. Finally, n-i 

structures were deposited on textured Ag/ZnO coated SS back 

reflectors supplied by United Solar Ovonics Corporation; these 

partial structures were then shipped to USOC, where the n-i-pITO solar cell structures were completed and analyzed. 

1 
Results and discussion 

Since filament lifetime and film reproducibility were 

previously discussed (6), we concentrate in this paper on aSiGe:H film and device properties. Figure 1 presents the IR 

stretch mode bonding configurations for representative 

HWCVD films deposited with Ge film contents ranging 

between 0 and 100% (11). Of interest are the virtual lack of 

Ge-H Si-H

1600 1800 2000 2200 2400

Wavelength (cm

-1

)

x = 0.102

x = 0

x = 0.149

x = 0.246

x = 0.40

x = 0.472

x = 0.635

x = 0.815

x = 1.0

CH=10.4 at.%

CH=14.7 at.%

CH=12.7 at.%

CH=12.3 at.%

CH=10.7 at.%

CH=9.6 at.%

CH=8.7 at.%

CH=5.2 at.%

CH=2.8 at.%

0

2.5

5.0

10

20

25

35

50

100

GeH4-%

Fig. 1. IR scans of a-SiGe:H sretch mode region for different 

film Ge contents. 

SiH2 and/or (SiH2) bonding appearing in the ‘shifted’ mode at n 

2090 cm-1, and the relative amount of H bonded to Ge atoms 

versus Si atoms. The 2090 cm-1 mode is linked to microvoid 

formation, while low GeH bonding can result in significant Ge 

dangling bond densities that may act as recombination centers. 

In Figure 2 we show the IR microstructure factor (R), or the 

SiH2/(SiH +SiH2) intensity ratio (12), for Ge film contents 

ranging from 12 to 80 at.%. Also included are results from two 

PECVD studies (13,14). As can be seen, the present HWCVD 

films have R < 0.1 and the 2090 cm-1 mode becomes difficult to 

0.5

0.4

0.3

0.2

0.1

0.0

Micro

s

tru

c

t

ure

F

a

c

t

or (R)

0 20 40 60 80 100

Film Ge Content (at.%)

PECVD (Julich)

PECVD (Kyoto)

HWCVD (NREL)

Fig. 2. Microstructure factor (R) versus film Ge content. 

detect for Ge film contents exceeding 50%. In contrast, for the 

PECVD films, R factors are a factor of 2-3 higher and, for one 

study, shows an opposite trend for higher Ge film contents (14). 

In yet another PECVD a-SiGe:H study (15), R is >1 for the 

same Tsub used in the present experiment 

We also find that deposition of films with small R 

values result in significantly lower SAXS microvoid densities. 

In an earlier publication highlighting HWCVD a-SiGe:H films 

containing ~ 60-70 at.% Ge (5), SAXS densities above 1 x 1025 

3 

T

eu/cm , corresponding to a 2-4% microvoid volume fraction 

(16), were reported for films deposited using either a high Tfil or 

a low Tsub. These films also exhibited high R values, as seen in 

Figure 3. It is only when R is significantly reduced (by reducing 

fil at moderate Tsub) that microvoid densities below 5 x 1024 

eu/cm3 were obtained. 

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Micro

s

tru

c

t

ure

F

a

c

t

or (R)

10

23

2 4 6 8

10

24

2 4 6 8

10

25

2 4 6 8

10

26

SAXS (eu/cm

3

)

Fig. 3. Microstructure factor (R) versus SAXS intensity. 

Figure 4 compares SAXS data for low Tfil (inverted 

triangles) to previously measured data for film Ge contents 

ranging from 0 to 87 at.%. While prior trends showed a 

significant increase in the SAXS signal with increasing film Ge 

content, starting with very low Ge contents (16), the present 

curve remains lower than the ‘standard’ curve over the whole 

composition region. The only films with lower SAXS values 

are those deposited using high bias microwave conditions, and 

Fig. 4. a-SiGe:H SAXS intensity versus film Ge content. 

these apparently have high defect densities due to the high 

energy bombardment conditions. The low SAXS densities for 

the present HWCVD films, achieved with minimal 

bombardment energies, may have a beneficial effect on film 

electronic performance. Figure 5 shows the film 

photoconductivity versus the SAXS intensity for the previous 

series of films deposited using varying Tfil and Tsub (5). It can be 

2 
seen that lowering the SAXS intensity by a factor of 10 to 20, by 

using our optimal low Tfil conditions, increases the film 

photoconductivity by a factor of 50. 

10

-8

10

-7

10

-6

10

-5

10

-4

s

L

10

23

2 3 4 5 6

10

24

2 3 4 5 6

10

25

2 3 4 5 6

10

26

SAXS

Eg = 1.20-1.35eV, Ge = 40-70%

Fig. 5. Film photoconductivity versus SAXS intensity. 

The other factor contributing to the improved structure 

of the present HWCVD a-SiGe:H films is the increase in the 

amount of GeH bonding. Assuming that the proportionality 

constants for the SiH and GeH stretch modes are similar, we 

determine the amount of SiH and GeH bonding by normalizing 

these stretch mode areas to the film H content. These results are 

shown in Figure 6, along with representative PECVD data (13). 

4

3

2

1

0

G

e

-

H

(

a

t

.

%

)

0 20 40 60 80

Ge content (%)

PECVD

HWCVD

Fig. 6. Total GeH bonding versus film Ge content. 

While the amount of SiH bonding is similar for the two film 

systems over the Ge content range studied, the amount of GeH 

bonding is higher in the HWCVD films by a factor of 2 for low 

Ge content films, and by a factor of 3 for higher Ge film 

contents. This results in a lower preferential attachment (PA) of 

H to Si for the HWCVD films, particularly in the higher Ge film 

content regime. Figure 7 compares the PA for our films to that 

of previous PECVD deposited a-SiGe:H films (13,17,18). 

12

10

8

6

4

2

0

P

A

0 20 40 60 80

Ge content (at.%)

PECVD

HWCVD

PECVD (Kyoto) 1999

HWCVD (NREL) 2004

PECVD (Harvard) 1990

PECVD (U. Nebr.) 2004

Fig. 7. Preferential attachment ratio versus film Ge content. 

Because of these improvements in film homogeneity 

and GeH bond content, we deposited a series of a-SiGe:H films 

for photocapacitance measurements of the film bandtails and 

defect densities. Preliminary results for the first film, containing 

~29% Ge, are compared to other films in Figure 8. As can be 

seen, the Urbach edge value of 42 mV is quite narrow, and is 

among the narrowest Urbach edges measured for any a-SiGe:H 

film (19). Measurements are in progress to determine the film 

defect density for this film, and to extend the results to other 

film Ge contents. 

Fig. 8. Urbach energy versus film Ge content. 

We also fabricated n-i-p solar cell devices using these 

i-layers. Two types of devices were fabricated, those with a 

single Ge profile, and those with a double profile (20). In both 

types of devices, the highest Ge content was placed near the top 

of the device near the p-layer in order to improve hole 

collection. Table II gives representative results under AM1 

illumination for both single profile (L1403) and double profile 

(L1415) cells, where a comparison is made to similar cells 

previously deposited using a 0.38 mm diameter W filament 

operated at ~ 1800C (20). In all these cells, the red quantum 

efficiency (QE) at a wavelength of 800 nm is measured to be ~ 

3 
Single 

(Ref. 20) 

Single 

L1403 

Double 

(Ref. 20) 

Double 

L1415 

Voc (mV) .556 .601 .620 .625 

Jsc (mA/cm2) 19.59 19.59 20.03 20.91 

FF (%) .56 .549 .48 .536 

h (%) 6.10 6.47 5.95 7.00 

Table II. 

40%. Table II shows an improvement in both types of grading 

structures. This has been achieved in spite of oxygen 

contamination in the L1403 and L1415 device i-layers. Figure 9 

shows a SIMS O depth profile for a film with ~ 65 at.% Ge 

which is likely similar to the other recent devices. Most likely, 

our house hydrogen has recently contained trace amounts of 

oxygen, a problem that did not exist when the devices of Ref. 20 

were made. At that earlier time, SIMS oxygen depth profiles 

indicated a very low oxygen level in our films (< 1 x 1018 cm-3), 

However, SIMS profiles taken on other films, deposited at other 

times using identical deposition conditions, can show 

significantly higher contamination levels. From an earlier study -3 relating to a-Si:H, oxygen levels exceeding 1-2 x 1019 cm 

would be expected to significantly affect device performance 

(21). We are currently installing purifiers on the H2 gas lines to 

the deposition system. This should allow us to better evaluate 

the device quality of the low Tfil Ta wire HWCVD material. 

Figure 9. SIMS depth profiles of selected elements for a film 

containing 65 at.% Ge, measured during the time 

device fabrication was ongoing. 

Acknowledgements 

The work at NREL is funded by the US DOE under 

subcontract number DE-AC36-99-GO10337. The authors thank 

Q. Wang for helpful discussions. 

References 

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Ross, and S.R. Ovshinsky, Appl. Phys. Lett. 54 (1989) 

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Thin Solid Films 430 (2003) 197-201. 

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A.H. Mahan, Proc. 3rd Intl. Conf. on Hot-Wire CVD 

(Cat-CVD) Process, Utrecht, The Netherlands, 2004, 

to be published in Thin Solid Films. 

12. A.H. Mahan, P. Raboisson, D.L. Williamson, and R. 

Tsu, Solar Cells 21 (1987) 117. 

13. A. Terakawa, Ph.D. Thesis, Kyoto University (1999). 

14. J. Folsch, F. Finger, T. Kulessa, F. Siebke, W. Beyer, 

and H. Wagner, MRS Symp. Proc. 377 (1995) 517. 

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4111. 

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37. 

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Exstrom. MRS Symp. Proc. 808 (2004) 359. 

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Proc. 808 (2004) 617. 

21. B. Yan, private communiction. 

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4. TITLE AND SUBTITLE

a-SiGe:H Materials and Devices Deposited by Hot Wire CVD Using 

a Tantalum Filament Operated at Low Temperature 

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A.H. Mahan, Y. Xu, L.M. Gedvilas, R.C. Reedy, D.L. Williamson, 

S. Datta, J.D. Cohen, B. Yan, and H.M. Branz 

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We report the deposition of improved hydrogenated amorphous silicon germanium (a-SiGe:H) films by the hot wire 

CVD (HWCVD) technique using a tantalum filament operating at a low temperature. We gauge the material quality of 

the a-SiGe:H films by comparing infrared, small angle X-ray scattering (SAXS), photocapacitance, and conductivity 

measurements to earlier results, and fabricate single-junction n-i-p solar cell devices using these i-layers.

15. SUBJECT TERMS

PV; hot-wire chemical vapor deposition (HWCVD); tantalum filament; low temperature; small-angle X-ray scattering 

(SAXS); photocapacitance; solar cells; 

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