In present work, the effect of annealing on dark and photo conductivity as well as the various causes of
conduction mechanics in S doped amorphous hydrogenated silicon films (a-Si:H) is discussed. The variation
of the dark conductivity as a function of temperature has been carried out on unannealed and annealed
(annealed at an optimized temperature of 300 °C) thin film samples and the activation energy of dark conductivity
of respective samples was also calculated at different temperatures. The Study concludes that at
high temperatures, an activated type mechanism is responsible for conduction in the a-Si:H films

Fateh, Singh Gill

Gupta, H.

Kumar, R.

Mehra, R.M.

Sharma, S.K.

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 10 No 3, 03014(4pp) (2018) Том 10 № 3, 03014(4cc) (2018)   2077-6772/2018/10(3)03014(4) 03014-1  2018 Sumy State University Electronic Conduction in Annealed Sulfur-Doped a-Si:H Films   Himanshu Gupta1, Fateh Singh Gill2, S.K. Sharma3, R. Kumar4, R.M. Mehra5  1 Department of Physics, Gurukula Kangri University, Haridwar-249401, India  2 Graphic Era University, Clement Town, Dehradun, India 3 Semiconductor Device Laboratory, Department of Semiconductor Science, Dongguk University-Seoul, Jung-gu, Seoul 100-715, Korea 4 Garg Degree College, Luksar Road Haridwar, Uttrakhand, India 5 School of Engineering & Technology, Sharda University, Greater Noida -201306, India  (Received 26 March 2018; revised manuscript received 06 June 2018; published online 25 June 2018)  In present work, the effect of annealing on dark and photo conductivity as well as the various causes of conduction mechanics in S doped amorphous hydrogenated silicon films (a-Si:H) is discussed. The variation of the dark conductivity as a function of temperature has been carried out on unannealed and annealed (annealed at an optimized temperature of 300 °C) thin film samples and the activation energy of dark con-ductivity of respective samples was also calculated at different temperatures. The Study concludes that at high temperatures, an activated type mechanism is responsible for conduction in the a-Si:H films.    Keywords: a-Si:H, Dark and Photoconductivity, Annealing effect, S doped a-Si:H.   DOI: 10.21272/jnep.10(3).03014 PACS numbers: 71.55.Ak , 71.55.Jv, 73.61.Jc   1. INTRODUCTION  An a-Si:H film has advantage over a crystalline sili-con (c-Si) based film because of its property of absorb-ing more energy by a less quantity. Therefore, allowing materials to be deposited inexpensively at large areas. Hence, a-Si:H films is a promising  material to use in variety of fields as photo sensors [1] photovoltaic solar cells [2], gas sensors[3], photo detectors [4], optical im-aging[5], pixel detectors for high energy particles [6] and  micro-electro-mechanical systems [7] to deposition for gate metal in thin film transistors [8] and  so forth. However, at prolonged illumination there occurs a light induced degradation, known as Staebler Wronski Effect (SWE) [9]. This problem has been reported to overcome by applying a polymer coating on the film [10]. The SWE results in reduction of dark and photoconductivi-ty. The phenomena of light induced metastable Si dan-gling bond defects (DB) confined its use in optoelectron-ic. Therefore, it is required to drive off the introduced gap states raised due to structural disorder and defects which either can be reduced by variation of hydrogen contents during preparation, or with post preparation treatment as thermal annealing and light soaking. The hydrogen incorporation is known to reduce the number of deep lying localized states due to dangling bonds but light soaking is also known to introduce dangling bond defect states. Many researchers have shown that reduc-ing the dangling bond density reduces the recombina-tion rate. However, there are two ways to decrease the dangling bond density; as discussed either by annealing or by doping/alloying with halogen or with a tetrahe-drally coordinated element such as carbon and germa-nium or with a group III element. Annealing is an im-portant tool to tailor the disorder and defect level when annealed out below 250 C but are created above that temperature. Generally, annealing above the deposition temperature tends to drive off hydrogen leaving behind unsaturated dangling bonds. Heat treatment signifi-cantly influence the structure and the electrical proper-ties of a-Si:H films. The conductivity of doped a-Si:H is very sensitive to the rate at which the sample is cooled following a high temperature anneal hence, much em-phasis has been paid to the preparation conditions to improve photo conductivity.  In a work by Sharma et. al. [11], we presented a study on conduction mechanism in S and Se doped  a-Si:H films in low temperature range (less than 370  K). Now, in present work the effect of annealing on the dark and photoconductivity at high temperature of S doped a-Si:H (a-Si:H,S:H) films is investigated which were annealed at an optimized temperature of 300 °C.  2. EXPERIMENTAL    The a-Si:H,S:H thin film samples were prepared by RF plasma-enhanced chemical vapor deposition (PECVD) technique using a standard reactor configura-tion by decomposition of  H2S vapour  mixed with silane gas (SiH4) on 7059 corning glass at 230 °C. The thick-nesses of the films were found in the range 2.21-5.44 m (using a surface profiler (Dek TeK3)). The X-ray diffraction patterns of the films confirmed the films      HIMANSHU GUPTA, FATEN SINGH GILL, ET AL. J. NANO- ELECTRON. PHYS. 10, 03014 (2018)   03014-2 Fig. 1 – Variation of conductivity with inverse temperature for S-doped a-Si:H films annealed at 300 °C Table 1 – Activation energy of unannealed and annealed a-Si,S:H films prepared at different gas ratio by PECVD technique  Sample code RV (H2S/SiH4) Activation Energy (un-annealed) Activation Energy (annealed at 300 C) Ea (eV) Low Temp. range Ea (eV) High Temp. range Ea (eV) Low Temp. range Ea (eV) High Temp. range S-7 6.8  10 – 7 0.086 0.76 0.019 0.91 S-5 1.1  10 – 5 0.040 0.74 0.016 0.66 S-4 1.0  10 – 4 0.060 0.69 0.014 0.48  as amorphous. The aluminum contacts were made in coplanar configuration. The dark conductivity of sam-ples was observed in vacuum (10 – 3 torr) from room temperature to 200 °C at the rate of 2-3 °C/min using a Keithley source meter. After that the samples were annealed at the different temperature from 100 °C to 500 °C in a microprocessor controlled vacuum furnace (~ 10 – 6 torr) for an hour and again observed the photo conductivity under the illumination of white light at 100 mW/cm2 .    3. RESULTS AND DISCUSSION   Temperature dependent dark conductivity is affect-ed by doping concentration. Temperature dependence of dark conductivity of a-Si,S:H thin films for conduc-tion is assumed to follow Arrhenius equation   0 exp ,aEkT      (1)  where, Ea is the temperature dependent activation. The activation energy is determined from the linear fit of Arrhenius plot between ln σ versus 1000/T. The conductivity prefactor 0 in equation 1 can be deter-mined either by linear shift coefficients of Arrhenius plot between ln σ versus 1000/T or by Meyer-Neldel (MN) rule proposed as   0 00 exp .aMNEE       In a work with Sanjeev et. al. [14, 16, 19], a correla-tion between MN prefactor and activation has been observed and for low temperature range conductivity and conductivity is attributed to hopping of electrons between localized states [11].  In present work, we measured the conductivity up-on heating the samples from lower temperature to higher temperature (160 K to 473 K). The activation energy of the samples was found to decrease with dop-ing concentration after annealing (Fig. 4). The net de-crease in the number of dangling bonds after each an-neal is possibly due to some of the double donor impuri-ty introduced by S which have been effectively passiv-ated by hydrogen in annealed state [12]. Thereby, less number of localized states due to unsaturated hydrogen bonds (dangling bonds) after each anneals is possible. The observation concludes that optical activation seems to be more efficient; as a result, there occur a metasta-ble photo-induced increase in both the dark and photo conductivities and the Fermi level moves towards the conduction band edge. This metastable increase is simi-lar in some multilayer heterostructures of a-Si:H and other amorphous alloys [9]. The effect of doping concentration can also be real-ized by determining the intensity dependence of photo-conductivity ph. The photo intensity dependence of photoconductivity of a-Si:H thin films obey the power-law dependence    ph F   where F is the intensity and γ  is an empirical expo-nent. Comparing our published work [13] For Se doped a-Si:H, the  was found  to be 0.75 to 0.51 for gas ratio of 10 – 4 to 10 – 1 respectively. As For a-Si:H,S:H film, the  γ lies between  0.72 to 0.74 for gas ratio of 6.8  10 – 7to 1.0  10 – 4. This observation in S doped a-Si:H films also indicates that there occurs both the monomolecu-lar and bimolecular  recombination  process [14].    Fig. 2 – Variation of dark and photoconductivity with doping ratio for unannealed and annealed a-Si:H, S:H samples  The Fig. 2 shows the comparison of dark conductivity of annealed and unannealed films as well as of photo-conductivity with doping gas ratio for a-Si:H,S:H films respectively. The reason for decrease in photoconductivi-ty than dark conductivity of annealed films is due to the fact that absorption of light introduces effective recom-bination centers other than normal number of dangling bonds in dark at room temperature in annealed films. At temperature above room temperature, the concentration (N0) of neutral dangling bond is given by    00 0 ,tN n N    where nt is the number of trapped electron at conduc- ELECTRONIC CONDUCTION IN ANNEALED… J. NANO- ELECTRON. PHYS. 10, 03014 (2018)   03014-3 tion band tail and N00 number of dangling bonds in dark. As the N00 increases due to occupation function for charged states of defects with positive correlation energy [15], the net number of dangling bonds (N0) in-creases at above the room temperature. Moreover, de-fect model also [16] explains that the number of dan-gling bonds increases from their normal number at above the equilibrium temperature.    Fig. 3 – Variation of activation energy with gas ratio H2S/SiH4 for unannealed and annealed a-Si:H, S:H films samples in the low temperature range    Fig. 4 – Variation of activation energy with gas ratio for un-annealed and annealed S-doped a-Si:H films in high tempera-ture range  Fig. 3 shows the variation of activation energy with gas ratio for unannealed and annealed S doped films in the low temperature range. At low temperature range the increased metastable defects level affects hopping of electrons through localized states in the band gap from valence band to conduction level. Consequently, the photo conductivity is found less than the dark con-ductivity. In Fig. 4, the variation of activation energy with gas ratio for unannealed and annealed a-Si:H, S:H films in high temperature range are shown respectively. The activation energy is quite high than the activation en-ergy observed at low temperatures. At high tempera-tures the localized states positioned in mid gap proba-bly are shifted towards the valence band. Hence, at high temperatures, there occurs an activated type mechanism of conduction.    4. CONCLUSION  The effect of annealing on dc dark and photoconduc-tivity of a-Si:H, S:H films is studied. The concentration of dangling bonds in S doped a-Si:H films are effective-ly reduced by annealing process, hence reduce the non-radiative centers and defects states lying near midgap. It is because hydrogen breaks the weak bonds, hence passivate dangling bonds [17]. Hereby, it is to mention that some other worker, using Car-Parrinello molecu-lar dynamics has shown the hydrogen bonds with two silicon atoms only in Si2H, Si3H and Si5H clusters, while in other clusters (i.e. Si4H, Si6H, Si7H, Si8H, Si9H and Si10H) hydrogen is bonded to only one silicon atom and the calculation of the lowest energy fragmentation products of SinH clusters shows that hydrogen is easily removed from SinH clusters [18]. Our observation con-cludes that in the low temperature range the conduc-tivity is attributed to hopping of electrons through lo-calized states between valence band to conduction band; and at high temperatures, an activated type mechanism takes place.       REFERENCES  1. D. Caputo, G.D. Cesare, A. Nascetti, M. Tucci, Sens. Actu-at. A 153, 1 (2009). 2. J.J. Thevaril, S.K. O’ Leary, Solid State Commun. 150, 1851 (2010). 3. D.E. Williums, Adam Hilger, Conduction and gas response of semiconductor gas sensors, in: solid state gas sensors, Springer, 1987, p. 71. 4. H. Stiebig, E. Maoulin, B. Rech, Thin Solid Films 515, 7522 (2007). 5. A. Nathan, B.K. Park, Q. Ma, A. Sazonov, J.A. Rolands, Microelectron. Reliab. 42,735 (2002). 6. N. Wyrsch, C. Miazza, S. Dunand, C. Ballif, A. Shah, M. Despeisse, D. Moraes, F. Powolny, P. Jarron, J. Non Cryst. Solids 352, 1797 (2006). 7. M.A. Gosalvez, I. Zubel, E. Viiniika, Handbook of MEMS materials and technologies, 375 (2010). 8. S.P. Singh, P. Srivastava, S. Ghosh, S.A. Khan, C.J. Otonc, G.V.Prakasha, Scripta Mater. 63, 605-608 (2010). 9. R.A. Street, J. Kokalios, C.C. Tsai and T.M. Hayas, Phys. Rev. B 35, 1316 (1986). 10. N.P. Mandal, A. Kumar, S.C. Agarwal, J. Non-Cryst. Sol-ids 352, 2015 (2006). 11. S.K. Sharma, P. Sagar, Himanshu Gupta, Rajen-dra Kumar, R.M. Mehra, Solid State Electr. 51, 1124 (2007).  HIMANSHU GUPTA, FATEN SINGH GILL, ET AL. J. NANO- ELECTRON. PHYS. 10, 03014 (2018)   03014-4 12. Ian M. Brown, Phys. Rev B 56, 994 (1997). 13. Himanshu Gupta, L.P. Purohit, K. Pal Pankaj, R. Kumar, Ajendra Kumar, F.S. Gill, R.M. Mehra, Invert. J. Renew. Energy 4, No 2, 69 (2014). 14. Sanjeev Kumar Sharma, Himanshu Gupta, Rajen-dra Kumar, Ram Mohan Mehra, Turk. J. Phys. 29, 243 (2005). 15. R.M. Mehra, J. Baweja, L.P. Purohit, R. Kumar, A.V. Singh, P.C. Mathur, P.C. Taylor, J. Non- Cryst. Sol-ids 708-712, 266 (2000). 16. S.K. Sharma, J. Baweja, R.M. Mehra, Int. J. Electron. 90, 423 (2003). 17. V.I. Oliveria, J. Phys. Cond. r. 18, 709-716 (2000). 18. D. Balamurugan, R. Prasad, Phys. Rev. B 64, 205406 (2001). 19. S.K. Sharma, Deuk Young Kim, J. Korean Phys. Soc. 62, No. 9, 1269 (2013).  

Electronic Conduction in Annealed Sulfur-Doped a-Si:H Films

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 10 No 3, 03014(4pp) (2018) Том 10 № 3, 03014(4cc) (2018) 
 
 
2077-6772/2018/10(3)03014(4) 03014-1  2018 Sumy State University 
Electronic Conduction in Annealed Sulfur-Doped a-Si:H Films  
 
Himanshu Gupta1, Fateh Singh Gill2, S.K. Sharma3, R. Kumar4, R.M. Mehra5 
 
1 Department of Physics, Gurukula Kangri University, Haridwar-249401, India  
2 Graphic Era University, Clement Town, Dehradun, India 
3 Semiconductor Device Laboratory, Department of Semiconductor Science, Dongguk University-Seoul, Jung-gu, 
Seoul 100-715, Korea 
4 Garg Degree College, Luksar Road Haridwar, Uttrakhand, India 
5 School of Engineering & Technology, Sharda University, Greater Noida -201306, India 
 
(Received 26 March 2018; revised manuscript received 06 June 2018; published online 25 June 2018) 
 
In present work, the effect of annealing on dark and photo conductivity as well as the various causes of 
conduction mechanics in S doped amorphous hydrogenated silicon films (a-Si:H) is discussed. The variation 
of the dark conductivity as a function of temperature has been carried out on unannealed and annealed 
(annealed at an optimized temperature of 300 °C) thin film samples and the activation energy of dark con-
ductivity of respective samples was also calculated at different temperatures. The Study concludes that at 
high temperatures, an activated type mechanism is responsible for conduction in the a-Si:H films.   
 
Keywords: a-Si:H, Dark and Photoconductivity, Annealing effect, S doped a-Si:H.  
 
DOI: 10.21272/jnep.10(3).03014 PACS numbers: 71.55.Ak , 71.55.Jv, 
73.61.Jc 
 
 
1. INTRODUCTION 
 
An a-Si:H film has advantage over a crystalline sili-
con (c-Si) based film because of its property of absorb-
ing more energy by a less quantity. Therefore, allowing 
materials to be deposited inexpensively at large areas. 
Hence, a-Si:H films is a promising  material to use in 
variety of fields as photo sensors [1] photovoltaic solar 
cells [2], gas sensors[3], photo detectors [4], optical im-
aging[5], pixel detectors for high energy particles [6] 
and  micro-electro-mechanical systems [7] to deposition 
for gate metal in thin film transistors [8] and  so forth. 
However, at prolonged illumination there occurs a light 
induced degradation, known as Staebler Wronski Effect 
(SWE) [9]. This problem has been reported to overcome 
by applying a polymer coating on the film [10]. The 
SWE results in reduction of dark and photoconductivi-
ty. The phenomena of light induced metastable Si dan-
gling bond defects (DB) confined its use in optoelectron-
ic. Therefore, it is required to drive off the introduced 
gap states raised due to structural disorder and defects 
which either can be reduced by variation of hydrogen 
contents during preparation, or with post preparation 
treatment as thermal annealing and light soaking. The 
hydrogen incorporation is known to reduce the number 
of deep lying localized states due to dangling bonds but 
light soaking is also known to introduce dangling bond 
defect states. Many researchers have shown that reduc-
ing the dangling bond density reduces the recombina-
tion rate. However, there are two ways to decrease the 
dangling bond density; as discussed either by annealing 
or by doping/alloying with halogen or with a tetrahe-
drally coordinated element such as carbon and germa-
nium or with a group III element. Annealing is an im-
portant tool to tailor the disorder and defect level when 
annealed out below 250 C but are created above that 
temperature. Generally, annealing above the deposition 
temperature tends to drive off hydrogen leaving behind 
unsaturated dangling bonds. Heat treatment signifi-
cantly influence the structure and the electrical proper-
ties of a-Si:H films. The conductivity of doped a-Si:H is 
very sensitive to the rate at which the sample is cooled 
following a high temperature anneal hence, much em-
phasis has been paid to the preparation conditions to 
improve photo conductivity.  
In a work by Sharma et. al. [11], we presented a 
study on conduction mechanism in S and Se doped  
a-Si:H films in low temperature range (less than 
370  K). Now, in present work the effect of annealing on 
the dark and photoconductivity at high temperature of 
S doped a-Si:H (a-Si:H,S:H) films is investigated which 
were annealed at an optimized temperature of 300 °C. 
 
2. EXPERIMENTAL  
  
The a-Si:H,S:H thin film samples were prepared by 
RF plasma-enhanced chemical vapor deposition 
(PECVD) technique using a standard reactor configura-
tion by decomposition of  H2S vapour  mixed with silane 
gas (SiH4) on 7059 corning glass at 230 °C. The thick-
nesses of the films were found in the range 2.21-
5.44 m (using a surface profiler (Dek TeK3)). The X-
ray diffraction patterns of the films confirmed the films  
 
 
 
 HIMANSHU GUPTA, FATEN SINGH GILL, ET AL. J. NANO- ELECTRON. PHYS. 10, 03014 (2018) 
 
 
03014-2 
Fig. 1 – Variation of conductivity with inverse temperature for S-doped a-Si:H films annealed at 300 °C 
Table 1 – Activation energy of unannealed and annealed a-Si,S:H films prepared at different gas ratio by PECVD technique 
 
Sample 
code 
RV 
(H2S/SiH4) 
Activation Energy 
(un-annealed) 
Activation Energy 
(annealed at 300 C) 
Ea (eV) 
Low Temp. 
range 
Ea (eV) 
High Temp. range 
Ea (eV) 
Low Temp. range 
Ea (eV) 
High Temp. range 
S-7 6.8  10 – 7 0.086 0.76 0.019 0.91 
S-5 1.1  10 – 5 0.040 0.74 0.016 0.66 
S-4 1.0  10 – 4 0.060 0.69 0.014 0.48 
 
as amorphous. The aluminum contacts were made in 
coplanar configuration. The dark conductivity of sam-
ples was observed in vacuum (10 – 3 torr) from room 
temperature to 200 °C at the rate of 2-3 °C/min using a 
Keithley source meter. After that the samples were 
annealed at the different temperature from 100 °C to 
500 °C in a microprocessor controlled vacuum furnace 
(~ 10 – 6 torr) for an hour and again observed the photo 
conductivity under the illumination of white light at 
100 mW/cm2 .   
 
3. RESULTS AND DISCUSSION  
 
Temperature dependent dark conductivity is affect-
ed by doping concentration. Temperature dependence 
of dark conductivity of a-Si,S:H thin films for conduc-
tion is assumed to follow Arrhenius equation 
 
 0 exp ,
aE
kT
 
 
  
 
 (1) 
 
where, Ea is the temperature dependent activation. 
The activation energy is determined from the linear 
fit of Arrhenius plot between ln σ versus 1000/T. The 
conductivity prefactor 0 in equation 1 can be deter-
mined either by linear shift coefficients of Arrhenius 
plot between ln σ versus 1000/T or by Meyer-Neldel 
(MN) rule proposed as 
 
 0 00 exp .
a
MN
E
E
 
 
  
 
 
 
In a work with Sanjeev et. al. [14, 16, 19], a correla-
tion between MN prefactor and activation has been 
observed and for low temperature range conductivity 
and conductivity is attributed to hopping of electrons 
between localized states [11].  
In present work, we measured the conductivity up-
on heating the samples from lower temperature to 
higher temperature (160 K to 473 K). The activation 
energy of the samples was found to decrease with dop-
ing concentration after annealing (Fig. 4). The net de-
crease in the number of dangling bonds after each an-
neal is possibly due to some of the double donor impuri-
ty introduced by S which have been effectively passiv-
ated by hydrogen in annealed state [12]. Thereby, less 
number of localized states due to unsaturated hydrogen 
bonds (dangling bonds) after each anneals is possible. 
The observation concludes that optical activation seems 
to be more efficient; as a result, there occur a metasta-
ble photo-induced increase in both the dark and photo 
conductivities and the Fermi level moves towards the 
conduction band edge. This metastable increase is simi-
lar in some multilayer heterostructures of a-Si:H and 
other amorphous alloys [9]. 
The effect of doping concentration can also be real-
ized by determining the intensity dependence of photo-
conductivity ph. The photo intensity dependence of 
photoconductivity of a-Si:H thin films obey the power-
law dependence  
 
 ph F
 
 
 
where F is the intensity and γ  is an empirical expo-
nent. Comparing our published work [13] For Se doped 
a-Si:H, the  was found  to be 0.75 to 0.51 for gas ratio 
of 10 – 4 to 10 – 1 respectively. As For a-Si:H,S:H film, the  
γ lies between  0.72 to 0.74 for gas ratio of 6.8  10 – 7to 
1.0  10 – 4. This observation in S doped a-Si:H films 
also indicates that there occurs both the monomolecu-
lar and bimolecular  recombination  process [14]. 
 
 
 
Fig. 2 – Variation of dark and photoconductivity with doping 
ratio for unannealed and annealed a-Si:H, S:H samples 
 
The Fig. 2 shows the comparison of dark conductivity 
of annealed and unannealed films as well as of photo-
conductivity with doping gas ratio for a-Si:H,S:H films 
respectively. The reason for decrease in photoconductivi-
ty than dark conductivity of annealed films is due to the 
fact that absorption of light introduces effective recom-
bination centers other than normal number of dangling 
bonds in dark at room temperature in annealed films. At 
temperature above room temperature, the concentration 
(N0) of neutral dangling bond is given by  
 
 
0
0 0 ,tN n N   
 
where nt is the number of trapped electron at conduc-
 ELECTRONIC CONDUCTION IN ANNEALED… J. NANO- ELECTRON. PHYS. 10, 03014 (2018) 
 
 
03014-3 
tion band tail and N00 number of dangling bonds in 
dark. As the N00 increases due to occupation function 
for charged states of defects with positive correlation 
energy [15], the net number of dangling bonds (N0) in-
creases at above the room temperature. Moreover, de-
fect model also [16] explains that the number of dan-
gling bonds increases from their normal number at 
above the equilibrium temperature. 
 
 
 
Fig. 3 – Variation of activation energy with gas ratio H2S/SiH4 
for unannealed and annealed a-Si:H, S:H films samples in the 
low temperature range 
 
 
 
Fig. 4 – Variation of activation energy with gas ratio for un-
annealed and annealed S-doped a-Si:H films in high tempera-
ture range 
 
Fig. 3 shows the variation of activation energy with 
gas ratio for unannealed and annealed S doped films in 
the low temperature range. At low temperature range 
the increased metastable defects level affects hopping 
of electrons through localized states in the band gap 
from valence band to conduction level. Consequently, 
the photo conductivity is found less than the dark con-
ductivity. 
In Fig. 4, the variation of activation energy with gas 
ratio for unannealed and annealed a-Si:H, S:H films in 
high temperature range are shown respectively. The 
activation energy is quite high than the activation en-
ergy observed at low temperatures. At high tempera-
tures the localized states positioned in mid gap proba-
bly are shifted towards the valence band. Hence, at 
high temperatures, there occurs an activated type 
mechanism of conduction.   
 
4. CONCLUSION 
 
The effect of annealing on dc dark and photoconduc-
tivity of a-Si:H, S:H films is studied. The concentration 
of dangling bonds in S doped a-Si:H films are effective-
ly reduced by annealing process, hence reduce the non-
radiative centers and defects states lying near midgap. 
It is because hydrogen breaks the weak bonds, hence 
passivate dangling bonds [17]. Hereby, it is to mention 
that some other worker, using Car-Parrinello molecu-
lar dynamics has shown the hydrogen bonds with two 
silicon atoms only in Si2H, Si3H and Si5H clusters, 
while in other clusters (i.e. Si4H, Si6H, Si7H, Si8H, Si9H 
and Si10H) hydrogen is bonded to only one silicon atom 
and the calculation of the lowest energy fragmentation 
products of SinH clusters shows that hydrogen is easily 
removed from SinH clusters [18]. Our observation con-
cludes that in the low temperature range the conduc-
tivity is attributed to hopping of electrons through lo-
calized states between valence band to conduction 
band; and at high temperatures, an activated type 
mechanism takes place. 
 
 
 
 
 
 
REFERENCES 
 
1. D. Caputo, G.D. Cesare, A. Nascetti, M. Tucci, Sens. Actu-
at. A 153, 1 (2009). 
2. J.J. Thevaril, S.K. O’ Leary, Solid State Commun. 150, 
1851 (2010). 
3. D.E. Williums, Adam Hilger, Conduction and gas response 
of semiconductor gas sensors, in: solid state gas sensors, 
Springer, 1987, p. 71. 
4. H. Stiebig, E. Maoulin, B. Rech, Thin Solid Films 515, 
7522 (2007). 
5. A. Nathan, B.K. Park, Q. Ma, A. Sazonov, J.A. Rolands, 
Microelectron. Reliab. 42,735 (2002). 
6. N. Wyrsch, C. Miazza, S. Dunand, C. Ballif, A. Shah, 
M. Despeisse, D. Moraes, F. Powolny, P. Jarron, J. Non 
Cryst. Solids 352, 1797 (2006). 
7. M.A. Gosalvez, I. Zubel, E. Viiniika, Handbook of MEMS 
materials and technologies, 375 (2010). 
8. S.P. Singh, P. Srivastava, S. Ghosh, S.A. Khan, 
C.J. Otonc, G.V.Prakasha, Scripta Mater. 63, 605-608 
(2010). 
9. R.A. Street, J. Kokalios, C.C. Tsai and T.M. Hayas, Phys. 
Rev. B 35, 1316 (1986). 
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Electronic Conduction in Annealed Sulfur-Doped a-Si:H Films

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