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Photoemission study of hydrogenated and unhydrogenated amorphous SiNx 0 ⩽ x ⩽ 2)
Journal of Non-CrystallineSolids 59 & 60 (1983) 593-596 North-Holland Publishing Company
593
PHOTOEMISSION STUDY OF HYDROGENATEDAND UNHYDROGENATEDAMORPHOUSSiN x (o s x s 2) R. K~RCHER, R.L. JOHNSON, and L. LEY M a x - P l a n c k - l n s t i t u t f u r Festk~rperforschung, Heisenbergstrasse I , 7000 S t u t t g a r t 80, Federal Republic of Germany <
<
Valence and core level photoemission data of SiN× and SiN :H f o r 0 - x - 2 are presented. The valence bands are i n t e r p r e t e ~ in term~ of p a r t i a l densit i e s of states and the core level spectra in terms of Si-N i bonding configurations. 1. INTRODUCTION Amorphous s i l i c o n n i t r i d e (Si3N4) is a material w i d e l y used in m i c r o e l e c t r o nic c i r c u i t r y charges I .
as gate d i e l e c t r i c ,
d i f f u s i o n mask or f o r the storage of e l e c t r i c
In many of these a p p l i c a t i o n s Si-Si3N 4 i n t e r f a c e s form which e x h i b i t
a gradient in the nitrogen concentration.
As a f i r s t
step in the examination
of these i n t e r f a c e s we have studied by photoemission the e l e c t r o n i c s t r u c t u r e of a-SiN x over a wide range of nitrogen concentration and in the presence of i n corporated hydrogen (a-SiNx:H). 2. SAMPLE PREPARATION Hydrogen free specimens were prepared by sputtering a t a r g e t of c-Si in an argon-nitrogen mixture.
Samples of a-SiNx:H were prepared by sputtering with
a d d i t i o n a l H2 or in a SiH4/N 2 glow discharge. at room temperature.
All f i l m s were prepared in s i t u
The s t o i c h i o m e t r y was obtained from the i n t e n s i t y r a t i o of
the N i s / S i 2p core l e v e l s a f t e r c a l i b r a t i o n with SigN 4. 3. RESULTS I . Valence bands The valence band spectra of a-SiN1. 5 shown in Fig. 1 e x h i b i t two regions: the valence band proper and the c o r e - l i k e N 2s level at 20 eV binding energy.
The
two photon energies in Fig. I (1486.6 and 87.1 eV) were chosen so as to i d e n t i f y the atomic o r i g i n of the three structures A, B, and C on the basis of photon energy dependent cross sections 2 ' 3 .
Peak C is due to a mixture of Si 3s and
N 2p s t a t e s , peak B represents a combination of mainly N 2p and Si 3p derived states and peak A represents the N 2p lone p a i r electrons 4 ' 5 .
Fig. 2 demon-
s t r a t e s the e v o l u t i o n of the valence band structure in sputtered a-SiN x as a function of nitrogen content x.
The spectra f o r x ~ 0.37 e x h i b i t the three
0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
R. Kiircher et al. / Hydrogenated and unhydrogenated amorphous SiN x (0 ~ x <__O)
594
a- SiNts
F
a-SiNx SP =
I .
>hv
[eV]~/C~% ~
i
z
o
U3 tO ILl
o 1 o. Q
z
II
, ',Robertson -
-
/
q
.-1._J I,
~
Jl
"l I
J
.
~
l~
l
\
I
L.._
I \ Ren and LII { I ,Ching I
~ IIIj
I
,
O:E F 5 tO 15 BINDING ENERGY [eV]
I
V '~,
I t--~---
t
\.__
jl
Q
'L
I ~,~ ,s
L_-
L
FIGURE 2 E v o l u t i o n of valence bands in a-SiN x
O:E F 5 10 15 20 BINDING ENERGY [eV] FIGURE 1 Valence band spectra of a-SiN1. 5 and theoretical lines)
d e n s i t i e s of states of Si3N 4 (dashed
s t r u c t u r e s A, B, C as discussed above.
The pure a-Si valence band is dominated
by Si 3p emission at the top of the valence bands.
I t i s seen t h a t these states
p e r s i s t as a shoulder at E f - l . 9 eV up to x = 0.6 a t the valence band edge and only f o r x ~ 0.85 does the atomic c h a r a c t e r of the highest occupied states change over to N 2p. drastically
In f i l m s c o n t a i n i n g hydrogen (Fig. 3) t h i s shoulder is
reduced and v i r t u a l l y
absent a l r e a d y f o r x = 0.44.
bonding states show up at ~7 eV binding energy f o r 0 ~ x ~ 1.0.
Silicon-hydrogen With i n c r e a s i n g
n i t r o g e n content hydrogen r e l a t e d emission occurs also on top of peak C around 13 eV as is p a r t i c u l a r l y
evident in the spectrum of a-SiN2.0:H.
We a t t r i b u t e
t h a t to a change in bonding c o n f i g u r a t i o n s from Si-H to N-H. The valence band maximum (VBM) receds l i n e a r l y with x up to x = 1.2 by as much as 0.8 eV as shown in Fig. 4.
The gap between EF and VBM opens up a t the
R. Kiircher et al. / Hydrogenated and unhydrogenated amorphous SiNx (0 ~- x ~ O)
--=
595
a-SiNx:H
----oa-SiNx 1 eV
_= o r e , =
i1 o
o_o
o
~6.~-o~
o
o
" , ~ . ' -
i
t
0=E~
i
i
I
s
i
I
.
.
.
.
1;
.
.
,s
I
EF
o
.... . .......... -=~
01s
,.'0
•
~is
VBM
210
NITROGEN CONCENTRATION x
I
BINDING ENERGY [eV ]
FIGURE 3 Valence band spectra of hydrogenated a-SiNx :H
FIGURE 4 Energies of the valence band maximum (VBM) and the Fermi level (EF) as a function of nitrogen concentration
same time from 0.3 to 1.9 eV and stays constant t h e r e a f t e r .
With the a d d i t i o n
of hydrogen VBM receds another 0.3 eV and EF s h i f t s 0.5 eV f u r t h e r towards the conduction band edge.
From a comparison with the o p t i c a l absorption data 6 EF
l i e s w i t h i n 0.3 eV of the conduction band edge in a-SiNx:H f o r x > 1.2 whereas Eo increases r a p i d l y f o r higher nitrogen concentrations while EF and VBM remain unchanged. 2. Core l e v e l s The replacement of Si-Si bonds with the p a r t i a l l y
i o n i c Si+-N - bonds induces
a chemical s h i f t in the Si 2p binding energy t h a t s h i f t s the average p o s i t i o n of the Si 2p l i n e to higher binding energy with increasing x as shown in Fig. 5. The l i n e reaches i t s maximum width when a l l possible Si-N i ( i = 0 . . . 4 ) c o n f i g u r a tions c o n t r i b u t e to the spectrum around x=O.7.
F i t t i n g 5 equally spaced compo-
nents to the Si 2p l i n e s we obtain a chemical s h i f t per attached N-atom t h a t i n creases from 0.65 to 0.85 eV between 0 ~ x ~ 1.0 due to a diminishing screening of the core hole.
Following the arguments given in Ref. 7 we deduce from the
i n t e n s i t i e s of the s h i f t e d components nitrogen concentrations x(Si 2p) t h a t we compare in Fig. 6 with x(N i s ) , section 2.
the concentration determined as described in
We f i n d t h a t x(N is) agrees with x(Si 2p) f o r x ~ 0.6 in unhydroge-
nated films under the assumption t h a t nitrogen has always three s i l i c o n nearest neighbors (the s o l i d l i n e in Fig. 6).
The d e v i a t i o n from the s t r a i g h t l i n e
R. K?ircher et al. / Hydrogenated and unhydrogenated amorphous SiNx (0_< x ~- O)
596
Si 2p
, a_SiNx
I0~
,
/ ":
l
-
-
>
101 IOC / ° /
i
i
a-SiN x a-SiNx:H
o
I•5
°./
10
/
9E
/
Si2p
z
/"
o,/ o /
x 05
N Is
/ o!s
i
/
x (Si 2p]
,%
I
,s
x~
FIGURE 5 Position and width of the Si 2p and N ls core levels
FIGURE 6 Total nitrogen content, x(N i s ) , vs. nitrogen content deduced from analysis of Si 2p spectrum
implies an undercoordination of N or the formation of N-N and N-H (in a-SiNx:H ) bonds, respectively.
The changing bonding environment of N is responsible f o r
the increasing width of the N Is level (Fig. 4). 3. CONCLUSIONS The analysis of the valence and core level spectra of a-SiN x reveals a change in the nature of the states at the valence band edge from Si 3p to N 2p around x=O.7.
At the same concentration nitrogen atoms s t a r t to form N-N and N-H bonds
or remain undercoordinated instead of being threefold coordinated with only Si as nearest neighbors. ACKNOWLEDGEMENT This work was in part supported by Der Bundesminister fur Forschung und Technologie. REFERENCES I) C.-E. Morosanu, Thin Solid Films 65 (1980) 171. 2) S.M. Goldberg, C.S. Fadley, and S. Kono, J. Electron. Spectros. 21 (1981) 285 3) I.M, Band, Y . I . Kharitonov, and M.B. Trzhaskovskaya, Atomic Data and Nuclear Data Tables 23 (1979) 443. 4) S.-Y. Ren and W.Y. Ching, Phys. Rev. B 23 (1981) 5454. 5) J. Robertson, P h i l . Mag. B 44 (1981) 215. 6) H. Kurata, M. Hirose, and Y. Osaka, Jpn. J. Appl. Phys. 20 (1981) L811. 7) K.J. Gruntz, L. Ley, and R.L. Johnson, Phys. Rev. B 24 (1981) 2069.
593
PHOTOEMISSION STUDY OF HYDROGENATEDAND UNHYDROGENATEDAMORPHOUSSiN x (o s x s 2) R. K~RCHER, R.L. JOHNSON, and L. LEY M a x - P l a n c k - l n s t i t u t f u r Festk~rperforschung, Heisenbergstrasse I , 7000 S t u t t g a r t 80, Federal Republic of Germany <
<
Valence and core level photoemission data of SiN× and SiN :H f o r 0 - x - 2 are presented. The valence bands are i n t e r p r e t e ~ in term~ of p a r t i a l densit i e s of states and the core level spectra in terms of Si-N i bonding configurations. 1. INTRODUCTION Amorphous s i l i c o n n i t r i d e (Si3N4) is a material w i d e l y used in m i c r o e l e c t r o nic c i r c u i t r y charges I .
as gate d i e l e c t r i c ,
d i f f u s i o n mask or f o r the storage of e l e c t r i c
In many of these a p p l i c a t i o n s Si-Si3N 4 i n t e r f a c e s form which e x h i b i t
a gradient in the nitrogen concentration.
As a f i r s t
step in the examination
of these i n t e r f a c e s we have studied by photoemission the e l e c t r o n i c s t r u c t u r e of a-SiN x over a wide range of nitrogen concentration and in the presence of i n corporated hydrogen (a-SiNx:H). 2. SAMPLE PREPARATION Hydrogen free specimens were prepared by sputtering a t a r g e t of c-Si in an argon-nitrogen mixture.
Samples of a-SiNx:H were prepared by sputtering with
a d d i t i o n a l H2 or in a SiH4/N 2 glow discharge. at room temperature.
All f i l m s were prepared in s i t u
The s t o i c h i o m e t r y was obtained from the i n t e n s i t y r a t i o of
the N i s / S i 2p core l e v e l s a f t e r c a l i b r a t i o n with SigN 4. 3. RESULTS I . Valence bands The valence band spectra of a-SiN1. 5 shown in Fig. 1 e x h i b i t two regions: the valence band proper and the c o r e - l i k e N 2s level at 20 eV binding energy.
The
two photon energies in Fig. I (1486.6 and 87.1 eV) were chosen so as to i d e n t i f y the atomic o r i g i n of the three structures A, B, and C on the basis of photon energy dependent cross sections 2 ' 3 .
Peak C is due to a mixture of Si 3s and
N 2p s t a t e s , peak B represents a combination of mainly N 2p and Si 3p derived states and peak A represents the N 2p lone p a i r electrons 4 ' 5 .
Fig. 2 demon-
s t r a t e s the e v o l u t i o n of the valence band structure in sputtered a-SiN x as a function of nitrogen content x.
The spectra f o r x ~ 0.37 e x h i b i t the three
0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
R. Kiircher et al. / Hydrogenated and unhydrogenated amorphous SiN x (0 ~ x <__O)
594
a- SiNts
F
a-SiNx SP =
I .
>hv
[eV]~/C~% ~
i
z
o
U3 tO ILl
o 1 o. Q
z
II
, ',Robertson -
-
/
q
.-1._J I,
~
Jl
"l I
J
.
~
l~
l
\
I
L.._
I \ Ren and LII { I ,Ching I
~ IIIj
I
,
O:E F 5 tO 15 BINDING ENERGY [eV]
I
V '~,
I t--~---
t
\.__
jl
Q
'L
I ~,~ ,s
L_-
L
FIGURE 2 E v o l u t i o n of valence bands in a-SiN x
O:E F 5 10 15 20 BINDING ENERGY [eV] FIGURE 1 Valence band spectra of a-SiN1. 5 and theoretical lines)
d e n s i t i e s of states of Si3N 4 (dashed
s t r u c t u r e s A, B, C as discussed above.
The pure a-Si valence band is dominated
by Si 3p emission at the top of the valence bands.
I t i s seen t h a t these states
p e r s i s t as a shoulder at E f - l . 9 eV up to x = 0.6 a t the valence band edge and only f o r x ~ 0.85 does the atomic c h a r a c t e r of the highest occupied states change over to N 2p. drastically
In f i l m s c o n t a i n i n g hydrogen (Fig. 3) t h i s shoulder is
reduced and v i r t u a l l y
absent a l r e a d y f o r x = 0.44.
bonding states show up at ~7 eV binding energy f o r 0 ~ x ~ 1.0.
Silicon-hydrogen With i n c r e a s i n g
n i t r o g e n content hydrogen r e l a t e d emission occurs also on top of peak C around 13 eV as is p a r t i c u l a r l y
evident in the spectrum of a-SiN2.0:H.
We a t t r i b u t e
t h a t to a change in bonding c o n f i g u r a t i o n s from Si-H to N-H. The valence band maximum (VBM) receds l i n e a r l y with x up to x = 1.2 by as much as 0.8 eV as shown in Fig. 4.
The gap between EF and VBM opens up a t the
R. Kiircher et al. / Hydrogenated and unhydrogenated amorphous SiNx (0 ~- x ~ O)
--=
595
a-SiNx:H
----oa-SiNx 1 eV
_= o r e , =
i1 o
o_o
o
~6.~-o~
o
o
" , ~ . ' -
i
t
0=E~
i
i
I
s
i
I
.
.
.
.
1;
.
.
,s
I
EF
o
.... . .......... -=~
01s
,.'0
•
~is
VBM
210
NITROGEN CONCENTRATION x
I
BINDING ENERGY [eV ]
FIGURE 3 Valence band spectra of hydrogenated a-SiNx :H
FIGURE 4 Energies of the valence band maximum (VBM) and the Fermi level (EF) as a function of nitrogen concentration
same time from 0.3 to 1.9 eV and stays constant t h e r e a f t e r .
With the a d d i t i o n
of hydrogen VBM receds another 0.3 eV and EF s h i f t s 0.5 eV f u r t h e r towards the conduction band edge.
From a comparison with the o p t i c a l absorption data 6 EF
l i e s w i t h i n 0.3 eV of the conduction band edge in a-SiNx:H f o r x > 1.2 whereas Eo increases r a p i d l y f o r higher nitrogen concentrations while EF and VBM remain unchanged. 2. Core l e v e l s The replacement of Si-Si bonds with the p a r t i a l l y
i o n i c Si+-N - bonds induces
a chemical s h i f t in the Si 2p binding energy t h a t s h i f t s the average p o s i t i o n of the Si 2p l i n e to higher binding energy with increasing x as shown in Fig. 5. The l i n e reaches i t s maximum width when a l l possible Si-N i ( i = 0 . . . 4 ) c o n f i g u r a tions c o n t r i b u t e to the spectrum around x=O.7.
F i t t i n g 5 equally spaced compo-
nents to the Si 2p l i n e s we obtain a chemical s h i f t per attached N-atom t h a t i n creases from 0.65 to 0.85 eV between 0 ~ x ~ 1.0 due to a diminishing screening of the core hole.
Following the arguments given in Ref. 7 we deduce from the
i n t e n s i t i e s of the s h i f t e d components nitrogen concentrations x(Si 2p) t h a t we compare in Fig. 6 with x(N i s ) , section 2.
the concentration determined as described in
We f i n d t h a t x(N is) agrees with x(Si 2p) f o r x ~ 0.6 in unhydroge-
nated films under the assumption t h a t nitrogen has always three s i l i c o n nearest neighbors (the s o l i d l i n e in Fig. 6).
The d e v i a t i o n from the s t r a i g h t l i n e
R. K?ircher et al. / Hydrogenated and unhydrogenated amorphous SiNx (0_< x ~- O)
596
Si 2p
, a_SiNx
I0~
,
/ ":
l
-
-
>
101 IOC / ° /
i
i
a-SiN x a-SiNx:H
o
I•5
°./
10
/
9E
/
Si2p
z
/"
o,/ o /
x 05
N Is
/ o!s
i
/
x (Si 2p]
,%
I
,s
x~
FIGURE 5 Position and width of the Si 2p and N ls core levels
FIGURE 6 Total nitrogen content, x(N i s ) , vs. nitrogen content deduced from analysis of Si 2p spectrum
implies an undercoordination of N or the formation of N-N and N-H (in a-SiNx:H ) bonds, respectively.
The changing bonding environment of N is responsible f o r
the increasing width of the N Is level (Fig. 4). 3. CONCLUSIONS The analysis of the valence and core level spectra of a-SiN x reveals a change in the nature of the states at the valence band edge from Si 3p to N 2p around x=O.7.
At the same concentration nitrogen atoms s t a r t to form N-N and N-H bonds
or remain undercoordinated instead of being threefold coordinated with only Si as nearest neighbors. ACKNOWLEDGEMENT This work was in part supported by Der Bundesminister fur Forschung und Technologie. REFERENCES I) C.-E. Morosanu, Thin Solid Films 65 (1980) 171. 2) S.M. Goldberg, C.S. Fadley, and S. Kono, J. Electron. Spectros. 21 (1981) 285 3) I.M, Band, Y . I . Kharitonov, and M.B. Trzhaskovskaya, Atomic Data and Nuclear Data Tables 23 (1979) 443. 4) S.-Y. Ren and W.Y. Ching, Phys. Rev. B 23 (1981) 5454. 5) J. Robertson, P h i l . Mag. B 44 (1981) 215. 6) H. Kurata, M. Hirose, and Y. Osaka, Jpn. J. Appl. Phys. 20 (1981) L811. 7) K.J. Gruntz, L. Ley, and R.L. Johnson, Phys. Rev. B 24 (1981) 2069.