Hydrogenated amorphous silicon germanium alloy thin films prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD) with varying Germanium concentrations have been investigated in both the annealed and the light soaked state. Samples were characterized using steady state photoconductivity and dual beam photoconductivity (DBP). The Staebler-Wronski effect has been investigated by monitoring the changes in the photoconductivity, σ ph, and the increase in the sub-bandgap absorption coefficient, α. The kinetics of defect creation for different germanium contents has also been compared with those for unalloyed hydrogenated amorphous silicon films. It is found that for the films with low Ge fraction, both a decrease in the photoconductivity and an increase in α (1.0eV) show similar time dependences to those observed in a-Si:H films. However, as the Ge content increases, σ ph degrades faster and the same time dependence is not seen in the increase of α(1.0eV)

Dönertaş, M. Elif

Güneş, Mehmet

English

DSpace@IZTECH Institutional Repository

Journal of Optoelectronics and Advanced Materials Vol. 7, No. 1, February 2005, p. 503 - 506 
 
 
 
 
 
 
LIGHT INDUCED DEGRADATION OF HYDROGENATED AMORPHOUS 
SILICON – GERMANIUM ALLOY (a-SiGe:H) THIN FILMS 
 
 
M. E. Dönerta, M Güne* 
 
Department of Physics, Izmir Institute of Technology, Izmir, TR – 35430, Turkey 
 
 
Hydrogenated amorphous silicon germanium alloy thin films prepared by Plasma Enhanced 
Chemical Vapor Deposition (PECVD) with varying Germanium concentrations have been 
investigated in both the annealed and the light soaked state. Samples were characterized 
using steady state photoconductivity and dual beam photoconductivity (DBP). The Staebler-
Wronski effect has been investigated by monitoring the changes in the photoconductivity, 
σph, and the increase in the sub-bandgap absorption coefficient, . The kinetics of defect 
creation for different germanium contents has also been compared with those for unalloyed 
hydrogenated amorphous silicon films. It is found that for the films with low Ge fraction, 
both a decrease in the photoconductivity and an increase in  (1.0eV) show similar time 
dependences to those observed in a-Si:H films. However, as the Ge content increases, σph 
degrades faster and the same time dependence is not seen in the increase of (1.0eV).  
 
 (Received December 9, 2004; accepted January 26, 2005) 
 
Keywords: Hydrogenated silicon germanium alloys, Staebler-Wronski effect,  
                  Absorption coefficient, Photoconductivity 
 
 
 1. Introduction 
 
Hydrogenated amorphous silicon-germanium (a-SiGe:H) alloy thin films are potential 
candidates to replace the lower bandgap absorber layer in multi junction solar cells [1]. Increasing 
the Ge concentration from pure amorphous silicon (0%Ge) decreases the bandgap and increases the 
Ge-related defect states in the alloy. Both silicon- and germanium-related defects are detected by 
electron spin resonance (ESR) [2]. However, sub-bandgap absorption measurements carried out 
using the constant photocurrent method (CPM) and photothermal deflection spectroscopy (PDS) do 
not show a distinction between Si and Ge related defects [3]. It is known that illumination with 
intense light leads to the creation of additional metastable states in the mobility gap of a-Si:H, i.e. 
the Staebler-Wronski effect [4]. This effect is also observed in hydrogenated amorphous silicon 
germanium alloy thin films [5]. In early studies carried out during the 1980s, a negligible SWE was 
reported in those lower quality alloy films [6,7]. More detailed investigations [8-11] have since been 
carried out after the material quality improved, and the issue of light induced degradation in these 
alloys has become a truly significant concern [12-15]. One of the major investigations of the SWE in 
a-SiGe:H alloys was reported in 1992, where the degradation in photoconductivity exhibits roughly 
a cube root dependence on the exposure time, which is similar to the degradation kinetics of a-Si:H 
films [16]. In addition, the degradation kinetics investigated using the sub-bandgap absorption 
resulted in no qualitative difference between a-SiGe:H alloys and pure a-Si:H films [17]. Another 
detailed investigation of deep defect creation in the alloys was carried out by using the drive–level 
capacitance profiling method [18]. It was found that as the Ge concentration increases, less deep 
defects are created.  
In this study, the Staebler-Wronski effect for intrinsic a-SiGe:H alloy thin films with 
different Ge concentrations was investigated using the steady-state photoconductivity and dual beam 
                                         
*
 Corresponding author: mehmetgunes@iyte.edu.tr 
M. E. Dönerta, M. Güne 
 
 
504 
photoconductivity (DBP) techniques, to understand the effect of rhe Ge content on the kinetics of 
defect creation, using 15-sun white light illumination. 
 
 
 2. Experimental procedure 
 
 Intrinsic hydrogenated amorphous silicon-germanium alloy thin films with different 
germanium concentrations were deposited by Plasma Enhanced Chemical Vapor Deposition 
(PECVD) [5], on glass substrates. The thicknesses of the samples, obtained from transmission 
measurements, were between 0.42 and 1 m. All samples were annealed in vacuum at 180 °C for  
2½ hours, before annealed state characterization. Steady state photoconductivity, σph, measurements 
in the annealed and light soaked states were made using a 690 nm interference filter and a RG - 610 
filter, together with a calibrated ENH type white light source. The intensity of the light was reduced 
using neutral density filters. The applied dc bias was in the ohmic region of the contacts. The light 
soaking procedure was carried out by illuminating the film at an intensity of 15 Suns, for determined 
time intervals. The temperature was kept constant at about 45 °C, using fans. Measurements were 
carried out after each light soaking period, and then the samples were re-annealed before beginning 
the next light soaking step. The sub-bandgap absorption spectrum, α(hν), was obtained from the 
dual beam photoconductivity (DBP) measurements (see [19] for a detailed description). In this 
study, the DBP yield spectrum, YDBP, measured at high and low bias light intensities, and the 
simultaneously measured transmission signal from the substrate side of the sample were used to 
calculate the absolute α(hν) spectrum, using a procedure [20] based on the Ritter-Weiser formula 
[21]. The Staebler-Wronski effect was characterized by monitoring the changes in the steady-state 
photoconductivity and sub-bandgap absorption coefficient. 
 
 
3. Results and discussion 
 
In Fig. 1a, the steady state photoconductivity, σph, results are shown in the annealed state for 
a-SiGe:H samples with Ge contents of 0, 15 and 25%, respectively. Higher photoconductivity values 
were obtaıned as the Ge content in the silicon network increases. The exponent γ of the 
photoconductivity was less than unity, and changed between 0.70 and 0.90. The degradation of 
samples began from these annealed state values as they were exposed to 15 Suns of white light 
illumination, filtered with a water filter to eliminate heating during the light soaking procedure. The 
degradation of σph as a function of time is shown in Fig. 1b, for the same a-SiGe:H samples. It is 
clearly seen that σph degrades as a power law function of the exposure time. Pure a-Si:H (0%Ge) and 
a a-SiGe:H alloy thin film with 15% Ge content exhibit similar degradation kinetics and show a 
characteristic t-1/3 rule. However, a-SiGe:H alloy thin film with 25% Ge content degrades faster and 
shows a slope of approximately -2/3. 
G (cm-3s-1)
1016 1017 1018 1019 1020 1021
σ
ph
 
(Ω
-
cm
)-1
10-9
10-8
10-7
10-6
10-5
10-4 (a)
25% Ge, γ = 0.70
15% Ge, γ = 0.83
0% Ge, γ = 0.89
Time (hr)
10-1 100 101 102
σ
ph
 
(@
G
=
10
20
 
cm
-
3 s
-
1 )
10-9
10-8
10-7
10-6
10-5
10-4(b)
25% Ge
15% Ge
0% Ge
t-1/3
t-1/3
t-0.54
 
 
 
Fig.1. (a)  Photoconductivity vs generation rate for a-SiGe alloys in the annealed state,  
(b) Degradation of the photoconductivity measured at  G = 1020 cm-3s-1,  as a  function  
                                                     of the illumination time. 
Light induced degradatıon of hydrogenated amorphous silicon – germanium alloy … 
 
 
505
E (eV)
0.8 1.2 1.6 2.0 2.4
Y
 
D
B
P 
(a.
u
)
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
25% Ge
15% Ge
E(eV)
0.8 1.2 1.6 2.0 2.4
Ph
a
se
 
( o
 
)
-40
-20
0
E(eV)
0.8 1.2 1.6 2.0 2.4
α
 
(cm
-
1 )
10-2
10-1
100
101
102
103
104
105
106
25% Ge
15% Ge
0.8 1.2 1.6 2.0 2.4
T 
(a.
u
)
2
4
6
8
E(eV)
0% Ge 0% Ge(a) (b)
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2. (a) Raw YDBP spectra of a-SiGe:H alloy thin films with different Ge contents. The 
inset shows the phases of the corresponding DBP signal. (b) Absolute α (hν) spectra of 
samples,  calculated  from  the  YDBP  spectrum  and  simultaneously measured transmission   
                                   signals. The inset shows the transmission spectra. 
 
 
The degradation of a-SiGe alloys can also be also characterized using sub-bandgap 
absorption methods, since the absorption coefficient at such energies increases with light soaking 
time. The sub-bandgap absorption coefficient spectra of these samples were measured in the 
annealed and light soaked states, using the dual beam photoconductivity (DBP) method. In Fig. 2a, 
raw DBP yield spectra, YDBP, are shown for films with three different Ge contents, in the annealed 
state. The absolute absorption coefficient spectrum, α(hν), for each sample was calculated from 
YDBP and the corresponding transmission signals, as shown in the inset of Fig. 2b. It is clearly seen 
that as the Ge content increases from 0% (a-Si:H) to 25%, the bandgap at α(hν) = 104 cm-1 shifts to 
lower energies, as is consistent with previous observations. In addition, there are also fringe patterns 
left on the spectrum after carrying out the fringe free calculation of the α(hν) spectrum for 0% and 
15% Ge content films. This indicates that a defective inhomogeneous layer is present in these 
samples.  
Time (hr)
10-1 100 101
1/
µτ
 
(@
G
=
10
16
cm
-
3 s
-
1 )
105
106
107
108
109
25% Ge
15% Ge
0% Ge
Time (hr)
10-1 100 101 102
α
 
(0.
90
eV
/1
.
0 
eV
)
10-1
100
101
102
103(a) (b)
t1/3
t0.58
t1/3
t1/3
25% Ge
15% Ge
0% Ge
t
1/3
 
Fig. 3. (a) Degradation of 1/µτ measured at G = 1016 cm-3s-1 for three different Ge content 
films and (b) the increase of the sub-bandgap absorption coeffient at 1.0 eV for 0% and 15% 
Ge content samples and at 0.90 eV for a 25% Ge  content  sample.  The  error margins  
                                      in (b) are comparable to the symbol sizes. 
 
 
After each light soaking step, the calculated α(hν) spectrum showed that there is an increase 
only in the sub-bandgap region, and no change was detected in the exponential absorption part of the 
M. E. Dönerta, M. Güne 
 
 
506 
spectrum. The values of α(hν) at 1.0 eV for 0% and 15% Ge content films, and at  
0.90 eV for 25% Ge films were taken as a representation of the change in α(hν) after light induced 
degradation. The results of the degradation are summarized in Fig. 3 for both α(0.90 eV/1.0 eV) and 
1/µτ versus illumination time. For all samples, α (hν) shows no initial degradation up to a few hours, 
then obeys an approximately t1/3 time dependence similar to that observed for intrinsic  
a-Si:H thin films [2]. Finally, it tends to reach a degraded steady state value after 30 hours of light 
soaking. In contrast, 1/µτ shows a different time dependence, with a slope close to 1/3 for 0% and 
15% Ge films and 0.58 for the 25% Ge content sample. Even though α(hν) tends to saturate, 1/µτ 
still follows its time dependence. This difference between the time dependences of the  
sub-bandgap absorption and 1/µτ was also observed in a-Si:H films, indicating that the increase of 
the α(hν) and degradation of the µτ-product are not controlled by the same types of defect [22]. 
 
 
4. Conclusions 
 
Preliminary results on the Staebler-Wronski effect for a-SiGe:H alloy thin films with 
different Ge contents were investigated using the steady-state photoconductivity and dual beam 
photoconductivity methods. It was found that for low Ge content films, the degradation of the 
photoconductivity and the increase in α(1.0eV) followed similar time dependences to those found 
for a-Si:H films [2]. As the Ge content increased, σph degraded faster. However, this did not exhibit 
the same time dependence as that for the increase in α(0.90 eV). These results cannot identify the 
types of defect causing the degradation of both parameters in a-SiGe:H alloys. More detailed 
investigation is necessary to fully understand the degradation kinetics in the alloy thin films. 
 
 
References 
 
  [1] J. Yang, A. Banerjee, S. Guha, Appl. Phys. Lett. 70, 2975, (1997). 
  [2] M. Stutzmann, R. A. Street, C. C. Tsai, J. B. Boyce, S. E. Ready, J. Appl. Phys. 
        66, 569, (1989). 
  [3] R. Carius, H. Steibeg, F. Siebke, J. Folsch, J. Non-Cryst. Solids 227–230, 432, (1998). 
  [4] D.L. Staebler, C. R. Wronski, Appl. Phys. Lett. 31, 292, (1977). 
  [5] J. David Cohen, Solar Energy Materials & Solar Cells 78, 399 (2003). 
  [6] J. Bullot, M. Galin, M. Gauthier, B. Bourdon, J. Phys. 44, 713 (1983). 
  [7] G. Nakamara, K. Sato, Y. Yukimoto, Solar Cells 9, 75, (1983). 
  [8] S. Aljishi, Z. E. Smith, S. Wagner, Amorph. Silicon & Related Mats., Ed. H. Fritzsche, World  
        Scientific, Singapore, 887, (1989). 
  [9] S. Guha, J. S. Payson, S. C. Argawal, S. R. Ovshinsky, J. Non-Cryst. Solids 
        97–98, 1455, (1988). 
[10] M. Stutzmann, R. A. Street, C. C. Tsai, J. B. Boyce, S. E. Ready, J. Appl. Phys.  
        66, 569, (1989). 
[11] C. E. Nebel, H. C. Weller, G. H. Bauer, Mater. Res. Soc. Symp. Proc. 118, 507, (1988). 
[12] W. Paul, J.H. Chen, E.Z. Liu, A.E.Wetsel, P.Wickboldt, J. Non-Cryst. Solids 
       164–166, 1, (1993). 
[13] S. Guha, J. Yang, S. J. Jones, Y. Chen, D. L. Williamson, Appl. Phys. Lett.  
        61, 1444, (1992). 
[14] T. Unold, J.D. Cohen, C.M. Fortmann, Appl. Phys. Lett. 64, 1714, (1994). 
[15] J. D. Cohen, in T. Searle (Ed.), Properties of Amorphous Silicon and its Alloys, EMIS,    
       Datareview Series No. 19, INSPEC, London, 1998, pp. 180–187. 
[16] G. Schumm, C. D. Abel, G. H. Bauer, Mater. Res. Soc. Symp. Proc. 258, 505, (1992). 
[17] N.W. Wang, P.A. Morin, V. Chu, S. Wagner, Mat. Res. Soc. Symp. Proc. 258, 589, (1992). 
[18] C. E. Michelson, A. V. Gelatos, J. D. Cohen, Appl. Phys. Lett. 47, 412, (1985). 
[19] C. R. Wronski, B. Abeles, T. Tiedje, G. Cody, Solid State Commun. 44, 1423, (1982). 
[20] R. Carius. Private Communication, Research Center Julich, Germany. 
[21] D. Ritter, K. Weiser, Optics Commun. 57, 336, (1986). 
[22]  M. Gunes, C. R. Wronski, J. Appl. Phys. 81, 3526, (1997). 


Light induced degradation of hydrogenated amorphous silicon - Germanium alloy (a-SiGe:H) thin films

http://openaccess.iyte.edu.tr/xmlui/bitstream/11147/2029/1/2029.pdf

Light induced degradation of hydrogenated amorphous silicon - Germanium alloy (a-SiGe:H) thin films

Abstract

Similar works

Full text

Available Versions

DSpace@IZTECH Institutional Repository