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Optical absorption of a-SiNx:H films prepared by RF glow-discharge
Journal of Non-Crystalline Solids 59 & 60 (1983) 605-608 North-Holland Publishing Company
605
OPTICAL ABSORPTION OF a-SiN :H FILES PREPARED BY RF GLOW-DISCHARGE x
H. WATANABE, K. KATOH*, M. YASUI* and Y. SHIBATA** Sendai Radio Technical College, Miyagi 989-31, Japan * Research & Development Lab. Stanley Co. Ltd., Yokohama 227, Japan ** Department of Electronic Engineering, Tohoku University, Sendal 980, Japan
The films of a-SiN :H are prepared by RF glow-discharge of SiH4-N~-H~ gas x mixtures. The concentratlons of nitrogen and hydrogen are estima~ed~from the intensities of the IR absorption due to vibrations of Si-N bonds and Si-H bonds, respectively. The optical gap energy is obtained from absorption spectrum. Correlation of the optical gap with the amounts of nitrogen and hydrogen is discussed. I. INTRODUCTION The properties of hydrogenated amorphous silicon films containing nitrogen (a-SiN :H films) have been of considerable interest because of their application x as highly sensitive photodetectors I or photoreceptors of electrophotography. 2 Doped and undoped a-SINx:H films have been prepared by varying widely the deposition conditions; i.e. RF power density, substrate temperature, gas flow rates of N2, SIN 4 and dopant gases of PH 3 and B2H 6.
The electrical properties of 2 these films have been partly reported in the previous papers. ,3 In this article, the optical gap of a-SiN :H films has been measured as a function of the x deposition conditions and discussed in connection with composition.
2. EXPEPJYEh"YAL
All the a-SiN :H films of thickness around I ~m were prepared by RF glowx discharge of SiH4-N2-H 2 gas mixtures in a capacitive reactor system. The ratio of N 2 flow rate to Sill4 flow rate N2/SiH ~ was widely varied, while the total pressure was maintained at I to 2 Torr.
The RF power density Prf which was de-
fined by the absorbed RF power devided by the electrode area, and the substrate temperature du~.ing deposition Ts were also systematically varied.
Impurity
doping was carried out by introducing PH 3 or B2H 6 gas into the reaction chamber. Hydrogen concentration [HI (atomic % of hydrogen to silicon plus nitrogen) and nitrogen to silicon ratio N/Si in the films were obtained from IR absorption measurements as described in the previous papers. 2'3
The optical absorption co-
efficient a in the high absorption region was calculated as a function of photon energy from the transmission and reflection spectrum.
The optical gap Eg was
deterrmined graphically by using the square-root forraula of ~
= ~(hv
0022-3093/83/0000-0000[$03.00 © 1983 North-HoUand/Physical Society of Japan
- Eg).
H. Watanabe et al. / Optical absorption of a-SiNx.'H films
606
0.~
f
30
N/Si/ ~0 .o ~
~2 Z
z~°o z,
---
0.2
I0~,
1.9 (]
0 2.00
~1.85
1.95
1.8(3
1.90 Pff : 0.1 W;crn2 Ts : 250"C
~1.75
I 103
1.70
I
I
102 101 N21SiH~
30
0,6
0.~
l
I
100
101
~1.85
o~ 20 -~ O
O
I
O
I
I
I
I
0
-< • J Q~/
Ts :2~,0"C
•
1.80 el
I
]
I
I
0.2 0./* 0.6 0.8 1.0 RF POWERDENSITY (W/cmz)
FIGURE i Value of Eg, N/Si ratio and [H] as a function of molar ratio of N 2 to Sill4 in the gas phase.
FIGURE 2 Value of Eg, N/Si ratio and [H] as a function of RF power density.
3. RESULTS Figure I shows Eg, N/Si ratio and [HI versus N2/SiH4, while Prf is held at 0.I W/cm 2 and Ts at 250 °C.
Eg slightly changes with N2/SiH 4 ratio as far as
the ratio is less than i, and after~rax~d increases rapidly with increase of the ratio.
The IR absorption measurements show that N/Si ratio increases with
increase of N2/SiH 4 ratio, while [HI decreases from about 14 % to 8 %. Figure 2 shows Eg, N/Si ratio and [HI versus Prf, while N2/SiH 4 ratio is held at 0.5 and Ts at 240 °C.
Eg increases monotonously with increase of Prf.
ratio increases and [H] gradually decreases with increase of Prf.
N/SI
The bond
energy of N------Nis 225 kcal/mole and that of S i - H is 76 kcal/mole. 4
The
increase of Prf will increase the radical ratio between nitrogen and silane in the gas phase, and give a film of larger N/Si ratio. Figure 3 shows Eg, N/Si ratio and [HI versus Ts, while N2/SiH 4 ratio is held at 0.5 and Prf at 0.i W/em 2.
Eg gradually decreases with increase of Ts.
[H]
decreases and N/Si ratio also slightly decreases with increase of Ts. Figure 4 shows Eg, N/Si ratio and [HI versus doping gas ratio, while N2/SiH 4 ratio is held at i, Prf at 0.i W/cm 2 and Ts at 250 °C.
In the boron-doped films,
Eg increases slightly as the doping gas ratio increases to 5x10 -5, and afterwards
H. Watanabe et al. / Optical absorption of a-SiNx.'H /ilms
0.3
30
0.3
o~
20 "oE: -'6 10~"-r-
0.2 0.1-
I
0
1
I
I
I
o
30 O
O
O
0.2 ~ " ~
~0 "~o
~0.1
z~
0 I
0
t7 8
I
Z~
]
1.801 F "
O-
.... ;;
;
i
[HI
-
1.75
N21SiH~,: 0.5 Prf : 0.1Wlcm2 I I I 100 200 300
~_ LJ
o~
N/Si
607
1.7( 0
1.70
1 L,00
CL
500
SUBSTRATE TEMPERATURE ('C)
~1.6 I PIou~ 3 Value of F~, N/Si ratio and [H] as a function of substrate temperature.
N21SiH~: 1.0 Prf:O'I250"C WlCm2Ts:
1.5 [
I
[
~I
,,
102 103 10~ 10s B2H6/(SiH~*N2)
I
I
10~ 103 102 PH31ISiH~+N2}
decreases rapidly with increase of the ratio.
On the other hand, effects of
phosphorus-doping on Eg are rather s~ll.
N/Si ratio scarcely changes
FIGURE 4 Value of Eg, N/Si ratio and [HI as a function of doping gas ratio.
with doping gas ratio and [H] increases a little at high doping regions both in B-doped and P-doped films.
4. DISCUSSION The optical gap energy critically depends on the compositions and structure of samples.
The present amorphous silicon films contain nitrogen and hydrogen,
and can be expressed as a-SiNx:H films.
It is well established by many re-
searchers 5-7 that the optical gap of a-Si films is mainly affected by hydrogen concentration independent of deposition methods. present films.
This is also effective in the
The variation of the optical gap with substrate temperature
shown in Fig.3 is mostly due to change of the hydrogen content. The incorporation of nitrogen also changes the optical gap of a-Si films. Though some parts of nitrogen atoms in a-SiNx:H films are incorporated into the silicon matrix with four-fold coordination and act as donor i~purities, most of nitrogen atoms are incorporated with three-fold coordination and cause increase of the band gap energy.
In order to make clear the effect of nitrogen incorpo-
ration on the optical gap, the contribution of hydrogen has been subtracted by
tt. Watanabe et al. / Optical absorption of a-SiNx.H fitms
608
using the following equation, Eg(cor.)
= Eg + 0.02(14
2.2
- [HI )
21
where a correlation between Eg and [H] 2.0
is assumed to be linear with a slope dEg/d[H] of 0.02 eV/at.%. 7
~ 1.9
Eg(cor.)
o
corresponds to the optical gap of films
~o
which have the same hydrogen concen-
1.~
tration as that of the films without
1.1 _.e
nitrogen.
•A
• •
• : N21SiH~ • : Prf
o:Ts
Figure 5 sunmarizes the value
of Eg(cor. ) with N/Si ratio.
Though
1.6 0
0.1
there is a considerable scatter in the
I
I
I
I
I
I
0.2
0.3
0./*
05
0.6
0.7
CL8
NISi
results, it is obvious that the optical gap increases with the amount of nitrogen incorporated. Concerning the influence of boron
FIGURE 5 Correlation between Eg and N/Si ratio. The effect of hydrogen incorporation has been subtracted.
and phosphorus doping on the optical gap, similar results as shown in Figure 4 have been observed in a-Si:H films. 8"9
An anomalous drop of the optical gap
in heavily B-doped films has been considered to be due to decrease of the bonded hydrogen content in a-Si:H network. 9
This is not the present case and more
studies are neeessary to understand this variation. In conclusion, the optical gap of the a-SiNx:H films can be controlled from around I. 78 eV to I. 85 eV by nitrogen incorporation without causing a serious degradation of the electrical properties. 3
REFERENCES i) H. Kurata and M. Hirose et al. Jpn. J. Appl. Phys. 20 (1981) LSII. 2) H. Watanabe and K. Katoh et al. Jpn. J. Appl. Phys. 21 (1982) L341. 3) H. Watanabe and K. Katoh et al. Thin Solid Films, in print. 4) J.A.A. Ketelaar, Chemical Constitution (Maruzen Pub. Co. Tokyo, 1958) p.205. 5) J. Perrin and I. Solomon et al. Thin Solid Films 62 (1979) 327. 6) J. C. Bruyere and A. Deneuville et al. J. Appl. Phys. 51 (1980) 2199. 7) A. Matsuda and M. Matsumura et al. Jpn. J. Appl. Phys. 20 (1981) L183. 8) S. Nitta and S. Itoh et al. Solar Energy Materials 8 (1982) 249. 9) S. Yamasaki and A. Matsuda et al. Jpn. J. Appl. Phys. 21 (1982) L789.
605
OPTICAL ABSORPTION OF a-SiN :H FILES PREPARED BY RF GLOW-DISCHARGE x
H. WATANABE, K. KATOH*, M. YASUI* and Y. SHIBATA** Sendai Radio Technical College, Miyagi 989-31, Japan * Research & Development Lab. Stanley Co. Ltd., Yokohama 227, Japan ** Department of Electronic Engineering, Tohoku University, Sendal 980, Japan
The films of a-SiN :H are prepared by RF glow-discharge of SiH4-N~-H~ gas x mixtures. The concentratlons of nitrogen and hydrogen are estima~ed~from the intensities of the IR absorption due to vibrations of Si-N bonds and Si-H bonds, respectively. The optical gap energy is obtained from absorption spectrum. Correlation of the optical gap with the amounts of nitrogen and hydrogen is discussed. I. INTRODUCTION The properties of hydrogenated amorphous silicon films containing nitrogen (a-SiN :H films) have been of considerable interest because of their application x as highly sensitive photodetectors I or photoreceptors of electrophotography. 2 Doped and undoped a-SINx:H films have been prepared by varying widely the deposition conditions; i.e. RF power density, substrate temperature, gas flow rates of N2, SIN 4 and dopant gases of PH 3 and B2H 6.
The electrical properties of 2 these films have been partly reported in the previous papers. ,3 In this article, the optical gap of a-SiN :H films has been measured as a function of the x deposition conditions and discussed in connection with composition.
2. EXPEPJYEh"YAL
All the a-SiN :H films of thickness around I ~m were prepared by RF glowx discharge of SiH4-N2-H 2 gas mixtures in a capacitive reactor system. The ratio of N 2 flow rate to Sill4 flow rate N2/SiH ~ was widely varied, while the total pressure was maintained at I to 2 Torr.
The RF power density Prf which was de-
fined by the absorbed RF power devided by the electrode area, and the substrate temperature du~.ing deposition Ts were also systematically varied.
Impurity
doping was carried out by introducing PH 3 or B2H 6 gas into the reaction chamber. Hydrogen concentration [HI (atomic % of hydrogen to silicon plus nitrogen) and nitrogen to silicon ratio N/Si in the films were obtained from IR absorption measurements as described in the previous papers. 2'3
The optical absorption co-
efficient a in the high absorption region was calculated as a function of photon energy from the transmission and reflection spectrum.
The optical gap Eg was
deterrmined graphically by using the square-root forraula of ~
= ~(hv
0022-3093/83/0000-0000[$03.00 © 1983 North-HoUand/Physical Society of Japan
- Eg).
H. Watanabe et al. / Optical absorption of a-SiNx.'H films
606
0.~
f
30
N/Si/ ~0 .o ~
~2 Z
z~°o z,
---
0.2
I0~,
1.9 (]
0 2.00
~1.85
1.95
1.8(3
1.90 Pff : 0.1 W;crn2 Ts : 250"C
~1.75
I 103
1.70
I
I
102 101 N21SiH~
30
0,6
0.~
l
I
100
101
~1.85
o~ 20 -~ O
O
I
O
I
I
I
I
0
-< • J Q~/
Ts :2~,0"C
•
1.80 el
I
]
I
I
0.2 0./* 0.6 0.8 1.0 RF POWERDENSITY (W/cmz)
FIGURE i Value of Eg, N/Si ratio and [H] as a function of molar ratio of N 2 to Sill4 in the gas phase.
FIGURE 2 Value of Eg, N/Si ratio and [H] as a function of RF power density.
3. RESULTS Figure I shows Eg, N/Si ratio and [HI versus N2/SiH4, while Prf is held at 0.I W/cm 2 and Ts at 250 °C.
Eg slightly changes with N2/SiH 4 ratio as far as
the ratio is less than i, and after~rax~d increases rapidly with increase of the ratio.
The IR absorption measurements show that N/Si ratio increases with
increase of N2/SiH 4 ratio, while [HI decreases from about 14 % to 8 %. Figure 2 shows Eg, N/Si ratio and [HI versus Prf, while N2/SiH 4 ratio is held at 0.5 and Ts at 240 °C.
Eg increases monotonously with increase of Prf.
ratio increases and [H] gradually decreases with increase of Prf.
N/SI
The bond
energy of N------Nis 225 kcal/mole and that of S i - H is 76 kcal/mole. 4
The
increase of Prf will increase the radical ratio between nitrogen and silane in the gas phase, and give a film of larger N/Si ratio. Figure 3 shows Eg, N/Si ratio and [HI versus Ts, while N2/SiH 4 ratio is held at 0.5 and Prf at 0.i W/em 2.
Eg gradually decreases with increase of Ts.
[H]
decreases and N/Si ratio also slightly decreases with increase of Ts. Figure 4 shows Eg, N/Si ratio and [HI versus doping gas ratio, while N2/SiH 4 ratio is held at i, Prf at 0.i W/cm 2 and Ts at 250 °C.
In the boron-doped films,
Eg increases slightly as the doping gas ratio increases to 5x10 -5, and afterwards
H. Watanabe et al. / Optical absorption of a-SiNx.'H /ilms
0.3
30
0.3
o~
20 "oE: -'6 10~"-r-
0.2 0.1-
I
0
1
I
I
I
o
30 O
O
O
0.2 ~ " ~
~0 "~o
~0.1
z~
0 I
0
t7 8
I
Z~
]
1.801 F "
O-
.... ;;
;
i
[HI
-
1.75
N21SiH~,: 0.5 Prf : 0.1Wlcm2 I I I 100 200 300
~_ LJ
o~
N/Si
607
1.7( 0
1.70
1 L,00
CL
500
SUBSTRATE TEMPERATURE ('C)
~1.6 I PIou~ 3 Value of F~, N/Si ratio and [H] as a function of substrate temperature.
N21SiH~: 1.0 Prf:O'I250"C WlCm2Ts:
1.5 [
I
[
~I
,,
102 103 10~ 10s B2H6/(SiH~*N2)
I
I
10~ 103 102 PH31ISiH~+N2}
decreases rapidly with increase of the ratio.
On the other hand, effects of
phosphorus-doping on Eg are rather s~ll.
N/Si ratio scarcely changes
FIGURE 4 Value of Eg, N/Si ratio and [HI as a function of doping gas ratio.
with doping gas ratio and [H] increases a little at high doping regions both in B-doped and P-doped films.
4. DISCUSSION The optical gap energy critically depends on the compositions and structure of samples.
The present amorphous silicon films contain nitrogen and hydrogen,
and can be expressed as a-SiNx:H films.
It is well established by many re-
searchers 5-7 that the optical gap of a-Si films is mainly affected by hydrogen concentration independent of deposition methods. present films.
This is also effective in the
The variation of the optical gap with substrate temperature
shown in Fig.3 is mostly due to change of the hydrogen content. The incorporation of nitrogen also changes the optical gap of a-Si films. Though some parts of nitrogen atoms in a-SiNx:H films are incorporated into the silicon matrix with four-fold coordination and act as donor i~purities, most of nitrogen atoms are incorporated with three-fold coordination and cause increase of the band gap energy.
In order to make clear the effect of nitrogen incorpo-
ration on the optical gap, the contribution of hydrogen has been subtracted by
tt. Watanabe et al. / Optical absorption of a-SiNx.H fitms
608
using the following equation, Eg(cor.)
= Eg + 0.02(14
2.2
- [HI )
21
where a correlation between Eg and [H] 2.0
is assumed to be linear with a slope dEg/d[H] of 0.02 eV/at.%. 7
~ 1.9
Eg(cor.)
o
corresponds to the optical gap of films
~o
which have the same hydrogen concen-
1.~
tration as that of the films without
1.1 _.e
nitrogen.
•A
• •
• : N21SiH~ • : Prf
o:Ts
Figure 5 sunmarizes the value
of Eg(cor. ) with N/Si ratio.
Though
1.6 0
0.1
there is a considerable scatter in the
I
I
I
I
I
I
0.2
0.3
0./*
05
0.6
0.7
CL8
NISi
results, it is obvious that the optical gap increases with the amount of nitrogen incorporated. Concerning the influence of boron
FIGURE 5 Correlation between Eg and N/Si ratio. The effect of hydrogen incorporation has been subtracted.
and phosphorus doping on the optical gap, similar results as shown in Figure 4 have been observed in a-Si:H films. 8"9
An anomalous drop of the optical gap
in heavily B-doped films has been considered to be due to decrease of the bonded hydrogen content in a-Si:H network. 9
This is not the present case and more
studies are neeessary to understand this variation. In conclusion, the optical gap of the a-SiNx:H films can be controlled from around I. 78 eV to I. 85 eV by nitrogen incorporation without causing a serious degradation of the electrical properties. 3
REFERENCES i) H. Kurata and M. Hirose et al. Jpn. J. Appl. Phys. 20 (1981) LSII. 2) H. Watanabe and K. Katoh et al. Jpn. J. Appl. Phys. 21 (1982) L341. 3) H. Watanabe and K. Katoh et al. Thin Solid Films, in print. 4) J.A.A. Ketelaar, Chemical Constitution (Maruzen Pub. Co. Tokyo, 1958) p.205. 5) J. Perrin and I. Solomon et al. Thin Solid Films 62 (1979) 327. 6) J. C. Bruyere and A. Deneuville et al. J. Appl. Phys. 51 (1980) 2199. 7) A. Matsuda and M. Matsumura et al. Jpn. J. Appl. Phys. 20 (1981) L183. 8) S. Nitta and S. Itoh et al. Solar Energy Materials 8 (1982) 249. 9) S. Yamasaki and A. Matsuda et al. Jpn. J. Appl. Phys. 21 (1982) L789.