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Preparation and properties of glow-discharge deposited amorphous Si:N:H alloys from SiH4, N2, and H2 gases
Journal of Non-CrystallineSolids 59 & 60 (1983) 597-600 North-Holland Publishing Company
597
PREPARATION AND PROPERTIES OF GLOW-DISCHARGE DEPOSITED AMORPHOUSSi:N:H ALLOYS FROM SiH4, N2, AND H2 GASES Goro SASAKI, Toshiyuki TANAKA, Masaya OKAMOTO, Shizuo FUJITA, and Akio SASAKI Department of E l e c t r i c a l Engineering, Kyoto University, Kyoto 606, Japan Amorphous Si:N:H alloys were deposited by glow discharge of SiH 4, N~, and Hp gases, and the dependence of t h e i r properties on the deposition-conditions was investigated. The deposition rate is increased by Np gas incorporation. Nitrogen incorporation increases the number of bonded hydrogen, which widens the optical gap. Their e l e c t r i c a l properties are also described. I . INTRODUCTION Although amorphous silicon-nitrogen-hydrogen alloys (a-Si:N:H) are interested in purposes f o r control of t h e i r optical gap~ ~ '2 nitrogen incorporation into aSi:H not only widens t h e i r optical gap but also varies remarkably t h e i r e l e c t r i cal properties.
However, the dependence of t h e i r e l e c t r i c a l properties on the
number of nitrogen atoms is not well understood
because of i t s complication~
Although the nitrogen incorporation into a-Si could introduce donors anddefects~ t h e i r structural configurations or energetic levels in the m o b i l i t y gap are not understood.
E l e c t r i c a l properties of a-Si:H alloys are much influenced by the
incorporated hydrogen, and thus those of a-Si:N:H could also be influenced.
In
this study, we investigate the dependence of properties of a-Si:N:Hondeposition conditions, taking the r o l e of bonded hydrogen into account. 2. PREPARATION Amorphous Si:N:H alloys were deposited by r f (13.56MHz) glow discharge of SiH4, N2, and H2 gas mixture in a capacitive coupling reactor in which the spacing and diameter of the electrodes are 2 cm and 20 cm, r e s p e c t i v e l y . c i t a t i o n power was 40 W.
Substrates were set on the anode.
perature was 250°C or 350°C.
The r f ex-
The substrate tem-
The t o t a l gas flow of N2 and H2 was I00 sccm, and
SiH 4 gas flow was I0 or 20 sccm.
The t o t a l gas pressure was 1 T o r r .
Figure 1 shows the deposition rate of f i l m s which increases with the N2 gas flow.
These results would imply that the N2 gas incorporation enhances the
d i s s o c i a t i o n of SiH4, which could be understood by an increase in the concentration of f a s t electrons with the N2 gas incorporation due to the d i f f e r e n t ioni z a t i o n e f f i c i e n c y between N2 and H2 molecules. 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
G. Sasaki et al, / Glow-discharge deposited amorphous Si.'N.'H alloys
598
3. NITROGEN AND HYDROGENCONTENTS
H2 GAS FLOW RATE(SCCM) I.c 100 I
Figure 2 shows the incorporated N
80
60
I
40
I
20
I
0
v
I
TS=250"C
content CN evaluated by the integrated absorption c o e f f i c i e n t around 820 cm-l. 5
O
The CN increases with an N2 f l o w , a n d is saturated with the N2 flow exceeding 50 sccm.
The CN is l i t t l e
influenced by
the substrate temperatures of 250°C and 350°C, which suggests t h a t the CN would
i' O
be primely r e l a t e d to the concentrations of a c t i v e species in the plasma.
~ ~o ~o ~o ~ N2 ~
FIGURE 1 Deposition r a t e of a-Si:N:H films
Figure 3 shows the incorporated H content CH, of which the dependence on the N2 gas flow r a t e is s i m i l a r to t h a t of CN.
i~o
FLOW RATE (SCCM)
H2 GAS FLOW RATE(SCCM) 8O 60 4O 20 ~)
tO0 l
The films containing more N atoms
l
i
l 0
"Q
tend to contain more H atoms at the subs t r a t e temperature of 250°C, which sug-
/
gests t h a t the nitrogen i n c o r p o r a t i o n
C, ~:,'°'=
r e t a i n s the H effusion from the s o l i d phase during d e p o s i t i o n .
r.
4. OPTICAL GAP The o p t i c a l gap of films derived from the ~ - h v
p l o t is shown in F i g . 4 .
The
o p t i c a l gap is increased by the N2 gas i n t r o d u c t i o n i n t o the reactant gas.
?
o 6
~o ~o eb ~
6o
Nz GAS F10W RATE (5(x;lwO
FIGURE 2 Nitrogen content in a-Si:N:H films
In
films deposited a t the SiH 4 gas flow of 20 sccm, the CN is almost independent of
H2 GAS FLOWRATE (SCCM)
~3'°°
80
2
a
60 4o
20
o
the substrate temperature, whereas the CH o f f i l m s a t 250°C is l a r g e r than t h a t o f films a t 350°C, as shown in Figs.2 and 3.
Such increase in the CH would r e s u l t
the wider o p t i c a l gap of films at 250°C than t h a t of films at 350°C.
flow of lOsccm cont~n more H and N atoms than the films at 20sccm, and consequentl y the former has the wider o p t i c a l gap.
a
a / f
In films
a t 250°C, the films deposited at SiH 4
j |[SiH4]=10sc,:m
~L.-250'C.,0scorn A
~
A
A
A
t-350~, 20 sccm &-
i 0
t i i i l 20 40 60 80 100 N2 ~ FLOW RATE (SCCM)
FIGURE 3 Hydrogen content in a-Si:N:H films
G. Sasaki et al. / Glow-discharge deposited amorphous Si.N.H alloys H2 GAS FLOW RATE(SOCM)
5. DARK CONDUCTIVITY Figure 5 shows the dark c o n d u c t i v i t y of films a t room temperature, the a c t i -
Z0 ¢
o
v a t i o n energy, and the preexponential f a c t o r in the temperature dependence of the dark c o n d u c t i v i t y .
A little
~1.8
°~
250"C, I0 l m ~
F
o
i . ~ 250"C, 20 scu'n
/
o
incor-
poration of N atoms increases the dark
|~.~cm
1.6
c o n d u c t i v i t y and f u r t h e r i n c o r p o r a t i o n
I
decreases i t .
599
I t is noted t h a t the films
i
0
I
i
I
I
20 40 60 80 1OO N2 GAS FLOW RATE (SO~)
deposited at SiH 4 flow of lOsccm cont~n
FIGURE 4 Optical gap o f a-Si:N:H films
more N atoms than the films a t 20sccm. From the r e s u l t s shown in F i g . 5 , the i n crease in the dark c o n d u c t i v i t y by a little
i n c o r p o r a t i o n of N atoms is re-
lated to the decrease in the a c t i v a t i o n
H2 GAS FLOW RATE (SCCM) 8O 60 40 2O 0
IO0 !
i
1
!
6
energy, which suggests an i n t r o d u c t i o n
i
i
Ts=250"C
A
e o~.o------oT--~ o_ ~ . . o u ~ '-"~'5i~1: 20sco'n
of donors by the nitrogen i n c o r p o r a t i o n . On the other hand, the reduction in the dark c o n d u c t i v i t y could be due to the increase in the a c t i v a t i o n energy or
A0
~--~10sccm O"'O J,.JD
~,-.-20sccrn O~
0
oo
o
the decrease of the preexponential fact o r . I t could be considered t h a t the i n crease in the a c t i v a t i o n energy r e s u l t s
/
'
~
1--20~¢m
from the increase of the m o b i l i t y gap, and the decrease of the preexponential f a c t o r r e s u l t s from the decrease of the
u"°'~" =r~-~......a .~FI0 sccm I
0
mobility.
I
I
I
l
I
20 60 60 80 100 N2 GAS FLOW RATE (SCC:M)
FIGURE 5 Dark c o n d u c t i v i t y , a c t i v a t i o n energy, and preexponential f a c t o r
6. PHOTOCONDUCTIVITY Figure 6 shows the normalized photoc o n d u c t i v i t y of the r~T products a t the
100
H2 GAS FLOW RATE (SCCM) 8O 6O 40 20 0
i n c i d e n t photon energy of 2 eV, which is d r a s t i c a l l y increased by a small amount of N2 gas i n t r o d u c t i o n and the f u r t h e r i n t r o d u c t i o n decreases i t . that electrical
I t is noted
properties of f i l m s a t
SiH 4 flow of 20 sccm and N2 flow exceeding 50 sccm depend on the N2 f l o w , a l though t h e i r H and N contents are inde-
0
N2 GAS FLOW RATE (sco~
FIGURE 6 TI~z product o f a-Si:N:H films
G. Sasaki et al. / Glow-discharge deposited amorphous Si.'N.'H alloys
600
pendent of i t .
The reason might be as-
sociated with variations of H configurations.
The r e l a t i o n between the r~zT
/
product and the dark conductivity is shown in Fig.7.
The films of which the
dark conductivity is higher show larger TII~T product. These results suggest that the N incorporation strongly influences the m o b i l i t y . However, the results on
u
/o
Ts= 250.C
the ~ T products can be interpreted by a v a r i a t i o n of the l i f e t i m e which could be strongly influenced by the shallow
I
-II
I
i
I
-10 -g "8 i..OG(~.l{Ocm) ~)
_.~
trap of the t a i l states and the recombination center of the deep gap states. Further study on these states w i l l claMfy the influence of N incorporation on
FIGURE 7 Relation between dark conductivity and T~zz product in a-Si:N:H films
the e l e c t r i c a l properties. 7. SUMMARY Amorphous Si:N:H alloys were prepared by glow discharge of SiH4, N2, and H2 gases, and t h e i r optical and e l e c t r i c a l properties were investigated. The results can be summarized as follows. ( i ) The deposition rate is enhanced by the N2 gas introduction. ( i i ) The N incorporation increases the bonded H atoms in films.
(iii)
The optical gap is increased by the N2 introduction, and i t s de-
pendence on the deposition conditions can beinterpreted by not only the N incorporation but also the increase of bonded H atoms. ( i v ) The dark- and photoconductivity are d r a s t i c a l l y varied by the N incorporation, and t h e i r variations would be resulted by donor introduction and variations of the m o b i l i t y gap, the m o b i l i t y of free carriers in the extended states, and the gap states. REFERENCES I) D.A. Anderson and W.E. Spear, Philos. Mag. 35 (1977) I . 2) H. Kurata, M. Hirose, and Y. Osaka, Jpn. J. Appl. Phys. 20 (1981) L811. 3) T. Noguchi, S. Usui, A. Sawada, Y. Kanoh, and M. Kikuchi, Jpn. J. Appl. Phys. 21 (1982) L485. 4) G. Sasaki, S. Fujita, and A. Sasaki, J. Appl. Phys. 54 (1983) 2696. 5) G. Sasaki, M. Kondo, S. F u j i t a , and A. Sasaki, Jpn. J. Appl. Phys. 21 (1982) 1394.
597
PREPARATION AND PROPERTIES OF GLOW-DISCHARGE DEPOSITED AMORPHOUSSi:N:H ALLOYS FROM SiH4, N2, AND H2 GASES Goro SASAKI, Toshiyuki TANAKA, Masaya OKAMOTO, Shizuo FUJITA, and Akio SASAKI Department of E l e c t r i c a l Engineering, Kyoto University, Kyoto 606, Japan Amorphous Si:N:H alloys were deposited by glow discharge of SiH 4, N~, and Hp gases, and the dependence of t h e i r properties on the deposition-conditions was investigated. The deposition rate is increased by Np gas incorporation. Nitrogen incorporation increases the number of bonded hydrogen, which widens the optical gap. Their e l e c t r i c a l properties are also described. I . INTRODUCTION Although amorphous silicon-nitrogen-hydrogen alloys (a-Si:N:H) are interested in purposes f o r control of t h e i r optical gap~ ~ '2 nitrogen incorporation into aSi:H not only widens t h e i r optical gap but also varies remarkably t h e i r e l e c t r i cal properties.
However, the dependence of t h e i r e l e c t r i c a l properties on the
number of nitrogen atoms is not well understood
because of i t s complication~
Although the nitrogen incorporation into a-Si could introduce donors anddefects~ t h e i r structural configurations or energetic levels in the m o b i l i t y gap are not understood.
E l e c t r i c a l properties of a-Si:H alloys are much influenced by the
incorporated hydrogen, and thus those of a-Si:N:H could also be influenced.
In
this study, we investigate the dependence of properties of a-Si:N:Hondeposition conditions, taking the r o l e of bonded hydrogen into account. 2. PREPARATION Amorphous Si:N:H alloys were deposited by r f (13.56MHz) glow discharge of SiH4, N2, and H2 gas mixture in a capacitive coupling reactor in which the spacing and diameter of the electrodes are 2 cm and 20 cm, r e s p e c t i v e l y . c i t a t i o n power was 40 W.
Substrates were set on the anode.
perature was 250°C or 350°C.
The r f ex-
The substrate tem-
The t o t a l gas flow of N2 and H2 was I00 sccm, and
SiH 4 gas flow was I0 or 20 sccm.
The t o t a l gas pressure was 1 T o r r .
Figure 1 shows the deposition rate of f i l m s which increases with the N2 gas flow.
These results would imply that the N2 gas incorporation enhances the
d i s s o c i a t i o n of SiH4, which could be understood by an increase in the concentration of f a s t electrons with the N2 gas incorporation due to the d i f f e r e n t ioni z a t i o n e f f i c i e n c y between N2 and H2 molecules. 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
G. Sasaki et al, / Glow-discharge deposited amorphous Si.'N.'H alloys
598
3. NITROGEN AND HYDROGENCONTENTS
H2 GAS FLOW RATE(SCCM) I.c 100 I
Figure 2 shows the incorporated N
80
60
I
40
I
20
I
0
v
I
TS=250"C
content CN evaluated by the integrated absorption c o e f f i c i e n t around 820 cm-l. 5
O
The CN increases with an N2 f l o w , a n d is saturated with the N2 flow exceeding 50 sccm.
The CN is l i t t l e
influenced by
the substrate temperatures of 250°C and 350°C, which suggests t h a t the CN would
i' O
be primely r e l a t e d to the concentrations of a c t i v e species in the plasma.
~ ~o ~o ~o ~ N2 ~
FIGURE 1 Deposition r a t e of a-Si:N:H films
Figure 3 shows the incorporated H content CH, of which the dependence on the N2 gas flow r a t e is s i m i l a r to t h a t of CN.
i~o
FLOW RATE (SCCM)
H2 GAS FLOW RATE(SCCM) 8O 60 4O 20 ~)
tO0 l
The films containing more N atoms
l
i
l 0
"Q
tend to contain more H atoms at the subs t r a t e temperature of 250°C, which sug-
/
gests t h a t the nitrogen i n c o r p o r a t i o n
C, ~:,'°'=
r e t a i n s the H effusion from the s o l i d phase during d e p o s i t i o n .
r.
4. OPTICAL GAP The o p t i c a l gap of films derived from the ~ - h v
p l o t is shown in F i g . 4 .
The
o p t i c a l gap is increased by the N2 gas i n t r o d u c t i o n i n t o the reactant gas.
?
o 6
~o ~o eb ~
6o
Nz GAS F10W RATE (5(x;lwO
FIGURE 2 Nitrogen content in a-Si:N:H films
In
films deposited a t the SiH 4 gas flow of 20 sccm, the CN is almost independent of
H2 GAS FLOWRATE (SCCM)
~3'°°
80
2
a
60 4o
20
o
the substrate temperature, whereas the CH o f f i l m s a t 250°C is l a r g e r than t h a t o f films a t 350°C, as shown in Figs.2 and 3.
Such increase in the CH would r e s u l t
the wider o p t i c a l gap of films at 250°C than t h a t of films at 350°C.
flow of lOsccm cont~n more H and N atoms than the films at 20sccm, and consequentl y the former has the wider o p t i c a l gap.
a
a / f
In films
a t 250°C, the films deposited at SiH 4
j |[SiH4]=10sc,:m
~L.-250'C.,0scorn A
~
A
A
A
t-350~, 20 sccm &-
i 0
t i i i l 20 40 60 80 100 N2 ~ FLOW RATE (SCCM)
FIGURE 3 Hydrogen content in a-Si:N:H films
G. Sasaki et al. / Glow-discharge deposited amorphous Si.N.H alloys H2 GAS FLOW RATE(SOCM)
5. DARK CONDUCTIVITY Figure 5 shows the dark c o n d u c t i v i t y of films a t room temperature, the a c t i -
Z0 ¢
o
v a t i o n energy, and the preexponential f a c t o r in the temperature dependence of the dark c o n d u c t i v i t y .
A little
~1.8
°~
250"C, I0 l m ~
F
o
i . ~ 250"C, 20 scu'n
/
o
incor-
poration of N atoms increases the dark
|~.~cm
1.6
c o n d u c t i v i t y and f u r t h e r i n c o r p o r a t i o n
I
decreases i t .
599
I t is noted t h a t the films
i
0
I
i
I
I
20 40 60 80 1OO N2 GAS FLOW RATE (SO~)
deposited at SiH 4 flow of lOsccm cont~n
FIGURE 4 Optical gap o f a-Si:N:H films
more N atoms than the films a t 20sccm. From the r e s u l t s shown in F i g . 5 , the i n crease in the dark c o n d u c t i v i t y by a little
i n c o r p o r a t i o n of N atoms is re-
lated to the decrease in the a c t i v a t i o n
H2 GAS FLOW RATE (SCCM) 8O 60 40 2O 0
IO0 !
i
1
!
6
energy, which suggests an i n t r o d u c t i o n
i
i
Ts=250"C
A
e o~.o------oT--~ o_ ~ . . o u ~ '-"~'5i~1: 20sco'n
of donors by the nitrogen i n c o r p o r a t i o n . On the other hand, the reduction in the dark c o n d u c t i v i t y could be due to the increase in the a c t i v a t i o n energy or
A0
~--~10sccm O"'O J,.JD
~,-.-20sccrn O~
0
oo
o
the decrease of the preexponential fact o r . I t could be considered t h a t the i n crease in the a c t i v a t i o n energy r e s u l t s
/
'
~
1--20~¢m
from the increase of the m o b i l i t y gap, and the decrease of the preexponential f a c t o r r e s u l t s from the decrease of the
u"°'~" =r~-~......a .~FI0 sccm I
0
mobility.
I
I
I
l
I
20 60 60 80 100 N2 GAS FLOW RATE (SCC:M)
FIGURE 5 Dark c o n d u c t i v i t y , a c t i v a t i o n energy, and preexponential f a c t o r
6. PHOTOCONDUCTIVITY Figure 6 shows the normalized photoc o n d u c t i v i t y of the r~T products a t the
100
H2 GAS FLOW RATE (SCCM) 8O 6O 40 20 0
i n c i d e n t photon energy of 2 eV, which is d r a s t i c a l l y increased by a small amount of N2 gas i n t r o d u c t i o n and the f u r t h e r i n t r o d u c t i o n decreases i t . that electrical
I t is noted
properties of f i l m s a t
SiH 4 flow of 20 sccm and N2 flow exceeding 50 sccm depend on the N2 f l o w , a l though t h e i r H and N contents are inde-
0
N2 GAS FLOW RATE (sco~
FIGURE 6 TI~z product o f a-Si:N:H films
G. Sasaki et al. / Glow-discharge deposited amorphous Si.'N.'H alloys
600
pendent of i t .
The reason might be as-
sociated with variations of H configurations.
The r e l a t i o n between the r~zT
/
product and the dark conductivity is shown in Fig.7.
The films of which the
dark conductivity is higher show larger TII~T product. These results suggest that the N incorporation strongly influences the m o b i l i t y . However, the results on
u
/o
Ts= 250.C
the ~ T products can be interpreted by a v a r i a t i o n of the l i f e t i m e which could be strongly influenced by the shallow
I
-II
I
i
I
-10 -g "8 i..OG(~.l{Ocm) ~)
_.~
trap of the t a i l states and the recombination center of the deep gap states. Further study on these states w i l l claMfy the influence of N incorporation on
FIGURE 7 Relation between dark conductivity and T~zz product in a-Si:N:H films
the e l e c t r i c a l properties. 7. SUMMARY Amorphous Si:N:H alloys were prepared by glow discharge of SiH4, N2, and H2 gases, and t h e i r optical and e l e c t r i c a l properties were investigated. The results can be summarized as follows. ( i ) The deposition rate is enhanced by the N2 gas introduction. ( i i ) The N incorporation increases the bonded H atoms in films.
(iii)
The optical gap is increased by the N2 introduction, and i t s de-
pendence on the deposition conditions can beinterpreted by not only the N incorporation but also the increase of bonded H atoms. ( i v ) The dark- and photoconductivity are d r a s t i c a l l y varied by the N incorporation, and t h e i r variations would be resulted by donor introduction and variations of the m o b i l i t y gap, the m o b i l i t y of free carriers in the extended states, and the gap states. REFERENCES I) D.A. Anderson and W.E. Spear, Philos. Mag. 35 (1977) I . 2) H. Kurata, M. Hirose, and Y. Osaka, Jpn. J. Appl. Phys. 20 (1981) L811. 3) T. Noguchi, S. Usui, A. Sawada, Y. Kanoh, and M. Kikuchi, Jpn. J. Appl. Phys. 21 (1982) L485. 4) G. Sasaki, S. Fujita, and A. Sasaki, J. Appl. Phys. 54 (1983) 2696. 5) G. Sasaki, M. Kondo, S. F u j i t a , and A. Sasaki, Jpn. J. Appl. Phys. 21 (1982) 1394.