- Email: [email protected]
Oxide formation on the silicon (111) surface studied by Auger electron spectroscopy and by low energy electron loss spectroscopy
Thin Solid Films, 61 (1979) 197-202 © Elsevier Sequoia S.A., Lausanne~Printed in the Netherlands
197
OXIDE FORMATION ON THE SILICON (111) SURFACE STUDIED BY AUGER ELECTRON SPECTROSCOPY AND BY LOW ENERGY ELECTRON LOSS SPECTROSCOPY N. LIESKE AND R. HEZEL Institut ffir Werkstoffwissenschafien V1, UniversitgttErlangen-Niirnberg, D-8520 Erlangen ( F.R.G.) (Received December 14, 1978; accepted January 31, 1979)
Core electron and valence electron excitation spectra measured using low energy electron loss spectroscopy in combination with Auger electron spectroscopy were used to study oxide formation on clean crystalline silicon. The chemical bonds formed in the various oxidation stages are described by localized molecular states. S i = O double bonds, Si--O bonds of the type found in SiO4 tetrahedra, Si--Si bonds and broken Si--O bonds were detected.
1. INTRODUCTION Thermal oxidation of crystalline silicon surfaces has been studied extensively in order to obtain a better understanding of the silicon surface and of the Si-SiO2 interface properties. The growth kinetics have been investigated 1-3 and spectroscopic methods have been used to obtain an atomic model for the bonding mechanisms4-8. Because of their sensitivity to surfaces Auger electron spectroscopy (AES) and low energy electron loss spectroscopy (ELS) have been used in more recent studies9-12. However, there is still insufficient knowledge of the bonding states formed during the initial stages of oxidation. It has been shown recently10,11 that the electronic states of amorphous silicon dioxide can be interpreted in terms of localized molecular states. In the present work an atomic description of the oxidation process is suggested. 2. EXPERIMENTAL The experiments were performed in an ultrahigh vacuum (UHV) apparatus equipped with a four-grid low energy electron diffraction (LEED) system, a 3 keV cylindrical mirror analyser with a coaxial electron gun and a special fiat-beamprofile ion gun. The apparatus has been described in detail in ref. 11. The clean crystalline silicon (111) surface was obtained by argon ion bombardment followed by a heat treatment (1100 °C for 10 min) and the well-known LEED diffraction pattern with a clear (7 x 7) superstructure was obtained. The oxidation of the surface was carried out in situ using oxygen of 99.995~ purity admitted through an UHVpumped gas inlet system. The residual gas composition was controlled with a quadrupole mass analyser. The sample temperature was measured using an optical pyrometer.
198 3.
N. LIESKE, R. HEZEL
RESULTS AND DISCUSSION
The interpretation of the chemical bonding effects during the oxidation of silicon will be based on the fact that the electronic states in amorphous silicon dioxide can be properly described by localized molecular states 5' l o, 11,13. This is demonstrated in Fig. 1 where core electron and valence electron spectra are given for amorphous silicon dioxide both as grown and after argon ion bombardment. The electron energy level scheme shows the filled and empty electron states together with the transitions corresponding to the measured excitations (see ref. 11). Figure 2 shows, for the clean silicon surface and for three different stages of oxidation performed in the UHV chamber, the Auger spectra, the valence electron excitation spectra and the Si(2p) and O(ls) core electron excitation spectra. Simplified models of the chemical bonds derived from the spectra are also given. For the clean crystalline silicon (111) (7 × 7) surface the Auger spectrum shows the typical elemental SiLvv signal. The valence electron spectrum is in good agreement with results obtained previously 9' 12.14, 1 5. It is characterized by three peaks (S 1, $2 and S 3) caused by surface state transitions including dangling-bond and back-bond surface states 9'14' 16,1v, by two bulk interband transitions (B 1 and B2) and by the bulk plasmon excitation hma and the surface plasmon excitation h~os 9, 14. The Si(2p) core spectrum is in excellent agreement with that obtained by Koma and Ludeke 14. Using their interpretation it is characterized by transitions into empty danglingbond surface states (s) and into empty bulk states (b 1 and bE). The simplified model shows the crystalline bulk silicon and the unsaturated bonds at the surface. After exposure to an oxygen partial pressure of 10-6 Torr at 600 °C for 10 min the Auger spectrum shows the elements oxygen and silicon. The presence of some chemical bonding is indicated by the chemical shift of the O~LL signal to 508 eV (512 eV for elemental oxygen) and by the slight change in the SiLvv line shape. The ELS spectra have changed substantially. The valence electron spectrum still shows the silicon bulk plasmon excitation at 16.9 eV. For excitation energies greater than 10 eV no structure corresponding to S i - - O bonds in SiO4 tetrahedra are observed (for comparison see Fig. 1 and ref. 11). In the lower energy range there are a number of characteristic excitations at 3.0, 3.9, 5.0, 7.0, 8.3 and 10.0 eV in good agreement with the results obtained by other authors 9' 12,15, 18, 19. From a comparison with the excitation bands calculated theoretically 2° and measured optically 21' 22 for gaseous SiO this structure has been attributed to the excitation of Si---~O double bonds ~8. In this work a different interpretation of the prominent peaks at 3, 5 and 7 eV is proposed. The 3 eV peak is present in elemental silicon (crystalline and amorphous) and in all of the oxidation stages. In addition there is a corresponding strong Si(2p) excitation. Therefore this 3 eV peak is attributed to the excitation of Si--Si bonds (see ref. 11). The valence electron excitations at 5.0 and 7.0 eV and the corresponding O(ls) excitations appear at exactly the same energies as those in argon-ionbombarded silicon dioxide (Fig. 1). This structure has been attributed to localized oxygen states caused by broken S i - - O bonds 11. In the present work the origin of such states can be explained by oxygen atoms which are not yet fully bonded to silicon atoms. The 5.0 and 7.0 eV transitions are assumed to occur from the O(2p) level, whilst for broken S i - - O bonds in silicon dioxide the initial state may be either the O(2p) atomic level or the Si(3s, 3p)--O(2p) non-bonding molecular level (which
I
"0
N
Z
-
UJ "0
N...J
CE Sj(2pl
E~RGY
LOSS EL leV)
~CEO(1s1
Si (2pl
ENERGY LOSS EL ~l
~CEO(151
531..0 536.2
VE
VE
------= -
. . . . . .-.
'T'_"
r-.
CE 0(15)
-94.5
-4-17.5-
0-
50t 3.3
70 6:5
Lrled SiO, etraheao
w
I I "'I
)1
ro-txnJong
Electron OrbItals n 51-0-51 Chan
~txJrdo<"<:j
0r.~ong
0(15)
Si (2p)
Sj(35,3p)bondorq0(2p) 0(25)
\0 \0
-
' -"'
-~
~
~
~
I
~
I I I II I I
I I
"
III ii
.-.=::.:..-:
Si (35, 3p lartlbondongO(2p1 0'51-51' .'"1-._._._ 0' 1=--- 51- 5I S.(3s I , 3p1 --0(2p) }
§ Iil
,
Si (3d)'
.
SI! 45 I _}
'7'Oi;j~~~~~~~~~E~~
14.0
i8:~i~ H .'h-'-'-' L HT!±-.i::=.-=..-:: mn1J J:mr---II
-524.0
ev
H
CE Si (2p)
Fig. l. Core electron and valence electron excitation spectra of amorphous silicon dioxide both as grown and after argon ion bombardment. The corresponding electron energy level scheme, the basic Si0 4 tetrahedra and the valence orbitals of the Si-O bond are also shown. For further details see ref. 1l.
#+ bombarded Si02
as grown 502
20.5
VE (- ',.--"--j
200
N. LIESKE, R. HEZEL
BONDS
ELS
AES 9' ',..j
.1
I
Si(111)
7×7
92
508
~i~'~
t]~
Si + 0 2 10-6 Tort, 02 600°C, 10r~n
-@-®-@ I
%
~,,,
V
'~
Si(2p)
!
I
-®-®-® I I I -@ -®-@
-6-6-6
,-....
-® -®
Sd2p)
• .(9_(9_<9 I I I
®,,,®-®-(9
92
-6-6-6
O(ls) s~-o
106.9 Si02 104,Torr' 02 900°C, 'lOm~
xl 7~2--592
",, . SI 2 ~ s~
"----tu,~
--
("---
-
® i
.-...(~)
o.
- Oils)
5o2
Si02 1Tort, 02 1000 °C, 10rn~
@,.®
?, r,
®
®'%-d-®
75 502 SiLVV
OKLL
.,' " ,, [ ENERGY
(eV)
"
O(Is)
VE EXCITATIONENERGY (eV)
Fig. 2. AES and ELS spectra of an o x i d a t i o n series performed in an U H V apparatus. The AES spectra were measured in the first derivative mode. The ELS spectra show valence electron (VE) and core electron (Si(2p) and O(1 s)) excitation spectra measured in the s e c o n d derivative mode (note the different energy ranges). The principal m o d e l s for the c h e m i c a l b o n d s derived from the spectra are also given.
is of similar energy to the O(2p) state). In contrast to the argon-ion-bombarded silicon dioxide, excitations of the Si(2p) core electrons into the O* final states are observed. This indicates a stronger interaction between the silicon and oxygen atomic states of the broken S i - - O bonds in the adsorption layer than in the bulk oxide. Excitation of the S i l O double bonds leads to the peaks at 3.9 and 8.3 eV and might also contribute to the 3, 5 and 7 eV energy losses. The 10.0 eV peak is
OXIDE FORMATION ON THE Si
(111) SURFACE
201
attributed 9 to surface plasmon excitations of a silicon surface covered with a dielectric layer of adsorbed oxygen. The model (Fig. 2) shows an adsorbed oxygen layer on bulk crystalline silicon containing S i = O double bonds and broken Si--O bonds. After exposure to an oxygen partial pressure of 10 -4 Torr at 900 °C, the OKLL signal in the Auger spectrum shows the complete shift typical of oxide formation 11'23'24. Although the SiLvv signal also indicates oxide formation, some excess silicon is still present. In the valence electron spectrum excitations due to Si--O bonds in SiO4 tetrahedra appear at 10.2, 12.0, 13.8, 17.0, 20.5, 22.5, 27.5 and 30.0 eV (see Fig. 1 and ref. 11). In the lower energy range the peaks due to Si--Si bonds (3.0 and 6.5 eV, see ref. 11) and those corresponding to the broken bonds are decreased, whilst the peaks due to S i = O double bonds have nearly vanished. The Si(2p) core spectrum is very similar to that of stoichiometric silicon dioxide1 o, 11 and the O(ls) core spectrum is identical to that of argon-ion-bombarded silicon dioxide. Therefore the ELS spectra clearly indicate that a layer of oxides of silicon containing SiO 4 tetrahedra has been formed. However, it is still imperfect because of the presence of Si--Si bonds and broken Si--O bonds. In the last oxidation stage, after exposure to an oxygen partial pressure of 1 Torr at 1000 °C, the typical silicon dioxide Auger spectrum is seen11,24. The valence electron excitation spectrum is in good agreement with the results of other authors 9,1 o, 12,15 and is characterized by the fully developed structure corresponding to Si--O bonds in SiO4 tetrahedra (see Fig. 1). The peaks due to Si--Si bonds (3.0 and 6.5 eV) are further decreased and those corresponding to broken Si--O bonds have vanished. The Si(2p) and O(ls) core electron spectra are in excellent agreement with previous results (see Fig. 1 and refs. 10 and 11) and also indicate the disappearance of broken Si--O bonds and the existence of SiO4 tetrahedra 11. The model in Fig. 2 shows a nearly perfect oxide network with a few remaining Si--Si bonds. 4. CONCLUSIONS The oxide formation on single-crystalline silicon was studied by measuring Auger electron spectra, valence electron and Si(2p) and O(ls) core electron excitation spectra. The chemical bonds formed during the oxidation process are described by localized molecular states based on a method which has been successfully applied to stoichiometric silicon dioxide. Whilst AES indicates only the elemental composition and the possible existence of chemical bonding, an interpretation in terms of specific bonding states can be given by ELS. S i = O double bonds, Si--O bonds of the type found in SiO4 tetrahedra, Si--Si bonds and broken Si--O bonds were detected. The onset of oxidation (in contrast to adsorption), i.e. the beginning of the formation of SiO4 tetrahedra, was identified clearly in the ELS spectra. During the initial oxidation stages electronic states emerge which are also characteristic of stoichiometric silicon dioxide after argon ion bombardment. Thus the combination of AES and ELS is a powerful tool for the investigation of all kinds of imperfect oxides of silicon (e.g. native oxide, oxides in the interface regions of MOS and MNOS structures, tunnel oxide, chemically vapour-deposited oxides and ion-implanted oxides) which are of technological importance.
202
N. LIESKE, R. HEZEL
REFERENCES
1 2
A . S . Grove, Physics and Technology oJ Semiconductor Devices, Wiley lnterscience, New York, 1967. B.E. Deal and A. S. Grove, J. Appl. Phys.; 36 (1965) 3770.
3 G.A. HaasandH. F. Gray, J. Appl. Phys.,46(1975) 3885. R. Flitsch and S. J. Raider, J. Vac. Sek Technol., 12 (1975) 305. T . H . DiStefano and D. E. Eastman, Phys. Rev. Lett., 27 ( 1971 ) 1560. H . R . Philipp, J. Phys. Chem. Solids, 32 (1971) 1935. G. Klein and H. U. Chun, Phys. Status Solidi A, 49 (1972) 167. K. Hiibner, Phys. Status Solidi A, 42 (1977) 501. H. Ibach and J. E. Rowe, Phys. Rev. B, 9 (1974) 1951. A. K o m a and R. Ludeke, Phys. Rev. Lett., 35 (1975) 107. N. Lieske and R. Hezel, Thin Solid Film,s', 6 ! (1979) 217. K. Fujiwara and H. Ogata, J. AppI. Phys., 4,'¢(1977) 4360. J . A . Tossel, D. J. Vaughan and K. H. Johnson, C#em. Phys. Lett., 20 (1973) 329. A. K o m a a n d R. Ludeke, Sur/. Sci.,55(1976) 735. Y . W . Chung, W. Siekhaus and G. A. Somorjai Sur/. Sci.. 5,'~(1976) 34t. K . C . Pandey and J. C. Phillips, Phys. Rev. Lett., 32 (1974) 1433. K . C . Pandey and J. C. Phillips, Phys. Rev. B, 13 (1976) 750. R. Ludeke and A. Korea, Phys. Rev. Lett., 3'/(1975) I 170. J.C. Phillips, Phys. Rev. B, 9 (1974) 2775. T . G . Heil and H. F. Schaefer, d. Chem. Phys., 56 (1972) 958. R . F . Barrow and H. C. Rowlinson, Proc. R. Soc. London. Ser. A, 224 (1954) 374. R. Cornet and I. Dubois, Can. J. Phys., 50 (1972) 630. T . W . Haas, J. T. Grant and G. J. Dooley, J. Appl. Phys., 43 (1972) 1853. B.A. Joyce and J. H. Neave, SurL Sei., 27 ( 1971 ) 499.
4 5 6 7 8 9 10 1I 12 13 14 15 16 17 18 19 20 21 22 23 24
197
OXIDE FORMATION ON THE SILICON (111) SURFACE STUDIED BY AUGER ELECTRON SPECTROSCOPY AND BY LOW ENERGY ELECTRON LOSS SPECTROSCOPY N. LIESKE AND R. HEZEL Institut ffir Werkstoffwissenschafien V1, UniversitgttErlangen-Niirnberg, D-8520 Erlangen ( F.R.G.) (Received December 14, 1978; accepted January 31, 1979)
Core electron and valence electron excitation spectra measured using low energy electron loss spectroscopy in combination with Auger electron spectroscopy were used to study oxide formation on clean crystalline silicon. The chemical bonds formed in the various oxidation stages are described by localized molecular states. S i = O double bonds, Si--O bonds of the type found in SiO4 tetrahedra, Si--Si bonds and broken Si--O bonds were detected.
1. INTRODUCTION Thermal oxidation of crystalline silicon surfaces has been studied extensively in order to obtain a better understanding of the silicon surface and of the Si-SiO2 interface properties. The growth kinetics have been investigated 1-3 and spectroscopic methods have been used to obtain an atomic model for the bonding mechanisms4-8. Because of their sensitivity to surfaces Auger electron spectroscopy (AES) and low energy electron loss spectroscopy (ELS) have been used in more recent studies9-12. However, there is still insufficient knowledge of the bonding states formed during the initial stages of oxidation. It has been shown recently10,11 that the electronic states of amorphous silicon dioxide can be interpreted in terms of localized molecular states. In the present work an atomic description of the oxidation process is suggested. 2. EXPERIMENTAL The experiments were performed in an ultrahigh vacuum (UHV) apparatus equipped with a four-grid low energy electron diffraction (LEED) system, a 3 keV cylindrical mirror analyser with a coaxial electron gun and a special fiat-beamprofile ion gun. The apparatus has been described in detail in ref. 11. The clean crystalline silicon (111) surface was obtained by argon ion bombardment followed by a heat treatment (1100 °C for 10 min) and the well-known LEED diffraction pattern with a clear (7 x 7) superstructure was obtained. The oxidation of the surface was carried out in situ using oxygen of 99.995~ purity admitted through an UHVpumped gas inlet system. The residual gas composition was controlled with a quadrupole mass analyser. The sample temperature was measured using an optical pyrometer.
198 3.
N. LIESKE, R. HEZEL
RESULTS AND DISCUSSION
The interpretation of the chemical bonding effects during the oxidation of silicon will be based on the fact that the electronic states in amorphous silicon dioxide can be properly described by localized molecular states 5' l o, 11,13. This is demonstrated in Fig. 1 where core electron and valence electron spectra are given for amorphous silicon dioxide both as grown and after argon ion bombardment. The electron energy level scheme shows the filled and empty electron states together with the transitions corresponding to the measured excitations (see ref. 11). Figure 2 shows, for the clean silicon surface and for three different stages of oxidation performed in the UHV chamber, the Auger spectra, the valence electron excitation spectra and the Si(2p) and O(ls) core electron excitation spectra. Simplified models of the chemical bonds derived from the spectra are also given. For the clean crystalline silicon (111) (7 × 7) surface the Auger spectrum shows the typical elemental SiLvv signal. The valence electron spectrum is in good agreement with results obtained previously 9' 12.14, 1 5. It is characterized by three peaks (S 1, $2 and S 3) caused by surface state transitions including dangling-bond and back-bond surface states 9'14' 16,1v, by two bulk interband transitions (B 1 and B2) and by the bulk plasmon excitation hma and the surface plasmon excitation h~os 9, 14. The Si(2p) core spectrum is in excellent agreement with that obtained by Koma and Ludeke 14. Using their interpretation it is characterized by transitions into empty danglingbond surface states (s) and into empty bulk states (b 1 and bE). The simplified model shows the crystalline bulk silicon and the unsaturated bonds at the surface. After exposure to an oxygen partial pressure of 10-6 Torr at 600 °C for 10 min the Auger spectrum shows the elements oxygen and silicon. The presence of some chemical bonding is indicated by the chemical shift of the O~LL signal to 508 eV (512 eV for elemental oxygen) and by the slight change in the SiLvv line shape. The ELS spectra have changed substantially. The valence electron spectrum still shows the silicon bulk plasmon excitation at 16.9 eV. For excitation energies greater than 10 eV no structure corresponding to S i - - O bonds in SiO4 tetrahedra are observed (for comparison see Fig. 1 and ref. 11). In the lower energy range there are a number of characteristic excitations at 3.0, 3.9, 5.0, 7.0, 8.3 and 10.0 eV in good agreement with the results obtained by other authors 9' 12,15, 18, 19. From a comparison with the excitation bands calculated theoretically 2° and measured optically 21' 22 for gaseous SiO this structure has been attributed to the excitation of Si---~O double bonds ~8. In this work a different interpretation of the prominent peaks at 3, 5 and 7 eV is proposed. The 3 eV peak is present in elemental silicon (crystalline and amorphous) and in all of the oxidation stages. In addition there is a corresponding strong Si(2p) excitation. Therefore this 3 eV peak is attributed to the excitation of Si--Si bonds (see ref. 11). The valence electron excitations at 5.0 and 7.0 eV and the corresponding O(ls) excitations appear at exactly the same energies as those in argon-ionbombarded silicon dioxide (Fig. 1). This structure has been attributed to localized oxygen states caused by broken S i - - O bonds 11. In the present work the origin of such states can be explained by oxygen atoms which are not yet fully bonded to silicon atoms. The 5.0 and 7.0 eV transitions are assumed to occur from the O(2p) level, whilst for broken S i - - O bonds in silicon dioxide the initial state may be either the O(2p) atomic level or the Si(3s, 3p)--O(2p) non-bonding molecular level (which
I
"0
N
Z
-
UJ "0
N...J
CE Sj(2pl
E~RGY
LOSS EL leV)
~CEO(1s1
Si (2pl
ENERGY LOSS EL ~l
~CEO(151
531..0 536.2
VE
VE
------= -
. . . . . .-.
'T'_"
r-.
CE 0(15)
-94.5
-4-17.5-
0-
50t 3.3
70 6:5
Lrled SiO, etraheao
w
I I "'I
)1
ro-txnJong
Electron OrbItals n 51-0-51 Chan
~txJrdo<"<:j
0r.~ong
0(15)
Si (2p)
Sj(35,3p)bondorq0(2p) 0(25)
\0 \0
-
' -"'
-~
~
~
~
I
~
I I I II I I
I I
"
III ii
.-.=::.:..-:
Si (35, 3p lartlbondongO(2p1 0'51-51' .'"1-._._._ 0' 1=--- 51- 5I S.(3s I , 3p1 --0(2p) }
§ Iil
,
Si (3d)'
.
SI! 45 I _}
'7'Oi;j~~~~~~~~~E~~
14.0
i8:~i~ H .'h-'-'-' L HT!±-.i::=.-=..-:: mn1J J:mr---II
-524.0
ev
H
CE Si (2p)
Fig. l. Core electron and valence electron excitation spectra of amorphous silicon dioxide both as grown and after argon ion bombardment. The corresponding electron energy level scheme, the basic Si0 4 tetrahedra and the valence orbitals of the Si-O bond are also shown. For further details see ref. 1l.
#+ bombarded Si02
as grown 502
20.5
VE (- ',.--"--j
200
N. LIESKE, R. HEZEL
BONDS
ELS
AES 9' ',..j
.1
I
Si(111)
7×7
92
508
~i~'~
t]~
Si + 0 2 10-6 Tort, 02 600°C, 10r~n
-@-®-@ I
%
~,,,
V
'~
Si(2p)
!
I
-®-®-® I I I -@ -®-@
-6-6-6
,-....
-® -®
Sd2p)
• .(9_(9_<9 I I I
®,,,®-®-(9
92
-6-6-6
O(ls) s~-o
106.9 Si02 104,Torr' 02 900°C, 'lOm~
xl 7~2--592
",, . SI 2 ~ s~
"----tu,~
--
("---
-
® i
.-...(~)
o.
- Oils)
5o2
Si02 1Tort, 02 1000 °C, 10rn~
@,.®
?, r,
®
®'%-d-®
75 502 SiLVV
OKLL
.,' " ,, [ ENERGY
(eV)
"
O(Is)
VE EXCITATIONENERGY (eV)
Fig. 2. AES and ELS spectra of an o x i d a t i o n series performed in an U H V apparatus. The AES spectra were measured in the first derivative mode. The ELS spectra show valence electron (VE) and core electron (Si(2p) and O(1 s)) excitation spectra measured in the s e c o n d derivative mode (note the different energy ranges). The principal m o d e l s for the c h e m i c a l b o n d s derived from the spectra are also given.
is of similar energy to the O(2p) state). In contrast to the argon-ion-bombarded silicon dioxide, excitations of the Si(2p) core electrons into the O* final states are observed. This indicates a stronger interaction between the silicon and oxygen atomic states of the broken S i - - O bonds in the adsorption layer than in the bulk oxide. Excitation of the S i l O double bonds leads to the peaks at 3.9 and 8.3 eV and might also contribute to the 3, 5 and 7 eV energy losses. The 10.0 eV peak is
OXIDE FORMATION ON THE Si
(111) SURFACE
201
attributed 9 to surface plasmon excitations of a silicon surface covered with a dielectric layer of adsorbed oxygen. The model (Fig. 2) shows an adsorbed oxygen layer on bulk crystalline silicon containing S i = O double bonds and broken Si--O bonds. After exposure to an oxygen partial pressure of 10 -4 Torr at 900 °C, the OKLL signal in the Auger spectrum shows the complete shift typical of oxide formation 11'23'24. Although the SiLvv signal also indicates oxide formation, some excess silicon is still present. In the valence electron spectrum excitations due to Si--O bonds in SiO4 tetrahedra appear at 10.2, 12.0, 13.8, 17.0, 20.5, 22.5, 27.5 and 30.0 eV (see Fig. 1 and ref. 11). In the lower energy range the peaks due to Si--Si bonds (3.0 and 6.5 eV, see ref. 11) and those corresponding to the broken bonds are decreased, whilst the peaks due to S i = O double bonds have nearly vanished. The Si(2p) core spectrum is very similar to that of stoichiometric silicon dioxide1 o, 11 and the O(ls) core spectrum is identical to that of argon-ion-bombarded silicon dioxide. Therefore the ELS spectra clearly indicate that a layer of oxides of silicon containing SiO 4 tetrahedra has been formed. However, it is still imperfect because of the presence of Si--Si bonds and broken Si--O bonds. In the last oxidation stage, after exposure to an oxygen partial pressure of 1 Torr at 1000 °C, the typical silicon dioxide Auger spectrum is seen11,24. The valence electron excitation spectrum is in good agreement with the results of other authors 9,1 o, 12,15 and is characterized by the fully developed structure corresponding to Si--O bonds in SiO4 tetrahedra (see Fig. 1). The peaks due to Si--Si bonds (3.0 and 6.5 eV) are further decreased and those corresponding to broken Si--O bonds have vanished. The Si(2p) and O(ls) core electron spectra are in excellent agreement with previous results (see Fig. 1 and refs. 10 and 11) and also indicate the disappearance of broken Si--O bonds and the existence of SiO4 tetrahedra 11. The model in Fig. 2 shows a nearly perfect oxide network with a few remaining Si--Si bonds. 4. CONCLUSIONS The oxide formation on single-crystalline silicon was studied by measuring Auger electron spectra, valence electron and Si(2p) and O(ls) core electron excitation spectra. The chemical bonds formed during the oxidation process are described by localized molecular states based on a method which has been successfully applied to stoichiometric silicon dioxide. Whilst AES indicates only the elemental composition and the possible existence of chemical bonding, an interpretation in terms of specific bonding states can be given by ELS. S i = O double bonds, Si--O bonds of the type found in SiO4 tetrahedra, Si--Si bonds and broken Si--O bonds were detected. The onset of oxidation (in contrast to adsorption), i.e. the beginning of the formation of SiO4 tetrahedra, was identified clearly in the ELS spectra. During the initial oxidation stages electronic states emerge which are also characteristic of stoichiometric silicon dioxide after argon ion bombardment. Thus the combination of AES and ELS is a powerful tool for the investigation of all kinds of imperfect oxides of silicon (e.g. native oxide, oxides in the interface regions of MOS and MNOS structures, tunnel oxide, chemically vapour-deposited oxides and ion-implanted oxides) which are of technological importance.
202
N. LIESKE, R. HEZEL
REFERENCES
1 2
A . S . Grove, Physics and Technology oJ Semiconductor Devices, Wiley lnterscience, New York, 1967. B.E. Deal and A. S. Grove, J. Appl. Phys.; 36 (1965) 3770.
3 G.A. HaasandH. F. Gray, J. Appl. Phys.,46(1975) 3885. R. Flitsch and S. J. Raider, J. Vac. Sek Technol., 12 (1975) 305. T . H . DiStefano and D. E. Eastman, Phys. Rev. Lett., 27 ( 1971 ) 1560. H . R . Philipp, J. Phys. Chem. Solids, 32 (1971) 1935. G. Klein and H. U. Chun, Phys. Status Solidi A, 49 (1972) 167. K. Hiibner, Phys. Status Solidi A, 42 (1977) 501. H. Ibach and J. E. Rowe, Phys. Rev. B, 9 (1974) 1951. A. K o m a and R. Ludeke, Phys. Rev. Lett., 35 (1975) 107. N. Lieske and R. Hezel, Thin Solid Film,s', 6 ! (1979) 217. K. Fujiwara and H. Ogata, J. AppI. Phys., 4,'¢(1977) 4360. J . A . Tossel, D. J. Vaughan and K. H. Johnson, C#em. Phys. Lett., 20 (1973) 329. A. K o m a a n d R. Ludeke, Sur/. Sci.,55(1976) 735. Y . W . Chung, W. Siekhaus and G. A. Somorjai Sur/. Sci.. 5,'~(1976) 34t. K . C . Pandey and J. C. Phillips, Phys. Rev. Lett., 32 (1974) 1433. K . C . Pandey and J. C. Phillips, Phys. Rev. B, 13 (1976) 750. R. Ludeke and A. Korea, Phys. Rev. Lett., 3'/(1975) I 170. J.C. Phillips, Phys. Rev. B, 9 (1974) 2775. T . G . Heil and H. F. Schaefer, d. Chem. Phys., 56 (1972) 958. R . F . Barrow and H. C. Rowlinson, Proc. R. Soc. London. Ser. A, 224 (1954) 374. R. Cornet and I. Dubois, Can. J. Phys., 50 (1972) 630. T . W . Haas, J. T. Grant and G. J. Dooley, J. Appl. Phys., 43 (1972) 1853. B.A. Joyce and J. H. Neave, SurL Sei., 27 ( 1971 ) 499.
4 5 6 7 8 9 10 1I 12 13 14 15 16 17 18 19 20 21 22 23 24