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Surface states on thin SiO2 layers induced by oxygen adsorption
Thin Solicl Films, 36 (1976) 195-198 © Elsevier Sequoia S.A., Lausanne--Printed in Switzerland
195
SURFACE STATES ON THIN SiO2 LAYERS INDUCED BY OXYGEN ADSORPTION* N. JAKOWSKIAND H. GLAEFEKE Sektion Physik, University o f Rostock, R o s t o c k (G.D.R.)
(Received August 25, 1975) INTRODUCTION In recent years systematic investigations of surface states by means of thermally stimulated exoelectron emission (TSEE) have been reported by Krylova 1, Euler e t al. 2 and Jakowski e t al. 3 The investigated surface states were assumed to be caused by oxygen adsorption. Hiernaut e t al. 4 found that the excitation of TSEE-active surface states by electron bombardment showed a resonance-like behaviour. The present paper attempts a qualitative physical interpretation of the experimental results. EXPERIMENTAL The sample was a single-crystal silicon slice covered with an ultra-thin SiO2 layer. The adsorption of oxygen on the SiO2 layer was accomplished by passing oxygen into the ultrahigh vacuum vessel via diffusion valves. A special electron gun s generated very slow electrons for bombardment of the sample. An additional facility allowed observation of the electron stimulated desorption (ESD) of oxygen during the electron bombardment. After bombardment the temperature of the sample was increased linearly with time. The electrons emitted during heating were detected by a 17-stage open photomultiplier with copper beryllium dynodes. The energy of the bombarding electrons was the parameter for the recorded glow curves, i.e. the rate of electron emission in dependence on temperature. RESULTS On varying the energy of the bombarding electrons, it was found that the ESD yield of adsorbed oxygen became negligible below 20 eV. Thus electron energies below 20 eV were used for the study of surface states induced by oxygen adsorption. A clean silicon surface showed no TSEE. A Si surface covered by SiO2 displayed a TSEE intensity which increased with the time of exposure to oxygen at room temperature (Fig. 1). The presence of additional argon did not affect the emission intensity. The energy distribution of the emitted electrons showed different energies for different glow peaks at the maximum of the energy distribution curve. The activation energy I¢a and the frequency factor s in the relation p = S exp(-Wa/kT
)
* Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 9-74.
196
N. JAKOWSKI, H. GLAEFEKE
1,8
1,6 1,4 C
~,2
x
~_0,6
~o,4 0,2 0
40
80
120
160
200
240
2.80
temperature (aC) Fig. ]. Increase of the glow curve peak height after exposure of the Si02 layer to 02 (4 tort rain): curve a before and curve b after exposure to 02.
were determined from decay curves at elevated temperatures. For the 100 °C TSEE peak we obtained s ~ 1011 s -1 and Wa = 0.95 eV. It is of interest that a very low frequency factor of 102 s -1 was obtained for the transition indicated by the maximum at about 210 °C. The corresponding activation energy was 0.42 eV. The experiments were carried out with SiO2 layers of about 20-30 A thickness, ensuring that the charging of the insulating surface layer was small enough to allow measurements for energies Wexc of the bombarding electrons down to 1 eV. The dependence of the total number of emitted electrons on Wexc is shown in Fig. 2. Evidently resonances exist at Wexc ~ 2.5 eV, 7 eV and 9 eV. Further measurements revealed the existence of thresholds for the excitation of surface states induced by oxygen adsorption. Finally the dependence of the excitation of such states on the direction of the incident electrons indicated an anisotropic cross section. DISCUSSION
These adsorption experiments clearly point to the dominant role which oxygen plays in TSEE from surface states. The results of studies of oxygen molecules in molecular physics lead us to the conclusion that the measured results are mainly due to adsorbed 02-. This will be discussed in terms of a qualitative physical model (Fig. 3) which is consistent with the experimental results. The model reflects the essential features of different adsorbed molecules capable of causing TSEE-active centres, and is based on the position of the adsorbed molecule in the image force potential of the adsorbing solid. It will be described briefly.
SURFACE STATES OF THIN SiO2 LAYERS
197
140
/:
~oe
o
8o
k l
o
~ 40
/
"6
~ ~o ~
0
• o
/
: ;
lY
:
i internuctectf separation E o ~ ~
f\o/
2
I
i
i
,0
,'~
14
~ ~ J
x
, exc,tat,on ,
o.-----0 delocatisation and emission
electron energy (eV)
Fig. 2. Yield of thermally stimulated electrons as a function of the electron energy Wexcof the incident electrons. The main resonance peaks are at about 2.5 eV, 7 eV and 9 eV. Fig. 3. Potential model for the excitation and emission mechanism by TSEE from adsorbed molecules. W ~ - Wx, value of the image force potential at position x of the molecule; Wth threshold energy for the excitation of A + B-; Wr, resonance energy for the excitation of A + B-; Wa, activation energy for the transition from the excited state A + B- to the ground state A + B + e(Wx). A threshold value Wth exists for the excitation of a negatively ionized molecule A + B- via a Franck-Condon transition. A resonance for the excitation of this state is expected at Wr. The state A + B- is physically stable if, as in the case of special 0 2 states, the potential barrier between A + B-and A + B + e ( W x ) is large enough to exclude tunnelling6. The thermal detachment of the electrons occurs during the transition of the system from the excited A + B- state into the ground state A + B + e ( W x ) . This process is governed by the activation energy Wa and the frequency factor s. Wa is fixed by the height of the potential barrier. The frequency factor s = Kf decreases considerably if K, the transmission coe fficient, becomes very small because of non-adiabatic transitions An optical liberation is possible via a F r a n c k - C o n d o n transition. The maximum kinetic energy of the emitted electrons is W b -- (Woo -- W x ) , and increases with Wb if W ~ - W x is assumed to be constant. Our experimental results are discussed on the basis of this model and the potential curves of 02 published by Michels and Harris 7. Resonances of oxygen were measured in studies of electron impact in the gas phase by Craggs e t a l . 8 at 3 eV, by Rapp and Briglia 9 at 7 eV and by Wong e t a l . 1o at 9.5 eV. Our
198
N. JAKOWSKI, H. GLAEFEKE
analogous excitation resonance at 2.5 eV should correspond to the excitation of the 4Z u state. A subsequent heating causes a 4N u ~ 3Ng transition and results in the 100 °C TSEE peak. The measured activation energy of 0.95 eV and the large frequency factor of 1011 s-1 are in good agreement with calculated data 7 and the character of the transition. The very low frequency factor of about 102 s - I for the TSEE peak at about 210 °C is explained as a non-adiabatic transition. Our excitation resonance at 7 eV should correspond to the excitation of a II u or a Ag state. The thermally stimulated transition to the 3Zg ground state produces the 160 °C TSEE peak. Our 9.5 eV resonance could not be identified. A 2Z u resonance is likely to be involved. In the model introduced, the electrons liberated by the transitions should show different kinetic energies, the lower ones resulting from the 4~ u ~ 3~g transition. This was verified experimentally. In the gas phase the cross section of the 7 eV resonance is one order of magnitude larger than that of the 3 eV resonance. We noticed the same tendency in our measurements with adsorbed oxygen. The anisotropy of the cross section for the excitation of 0 2- states has been observed in the gas phase by Van Brunt and Kiefer 11 In analogy to their results, we measured an anisotropy for the cross section of the above mentioned II u or Ag state. This indicates an 02 axis perpendicular to the solid surface, in accordance with the Dunn selection rules. CONCLUSION The agreement between the experimental results and our model leads us to believe that tile method of TSEE after excitation by electrons is capable of yielding information about molecules in the adsorbed state, as far as these states are TSEE-active. This should be of some interest for basic research as well as for applications in catalysis for example. REFERENCES 1 I. V. Krylova, Phys. StatusSolidiA, 7 (1971) 359. 2 M. Euler, W. Kriegseisand A. Scharmann, Phys. Status Solidi A, 15 (1973) 431. ' 3 N. Jakowski, H. Glaefeke and W. Wild, Proc. Physik der Halbleiteroberfli~che, 4. Arbeitstagung, Binz, 1973, Akademie der Wissenschaften der DDK, ZIE, Berlin-Adlershof, 1973, Part 1, p. 109; 5. Arbeitstagung, Binz, 1974, Akademie der Wissenschaften der DDR, ZIE, Berlin-Adlershof,
1974, p. 157. 4 J.P. Hiernaut, R. P. Forier and J. Van Cakenberghe, Vacuum, 22 (1972) 471. 5 R. N. Lee, Rev. Sci. Instrum., 39 (1968) 1306. 6 R. P. Bell, Proc. R. Soc. London, Ser. A, 148 (1935) 241. 7 H. H. Michels and F. E. Harris, 7th lnt. Conf. on the Physics o f Electronic a n d A t o m i c Collisions, Vol. 2, North-Holland Publ. Co., Amsterdam, 1971, p. 1170. 8 J. D. Craggs, R. Thorburn and B. A. Tozer, Proc. R. Soc. London, Set. A, 240 (1957) 473. 9 D. Rapp and D. D. Briglia, J. Chem. Phys., 43 (1965) 1480. 10 S. F. Wong, M. J. W. Boness and G. J. Schulz,Phys. Rev. Lett., 31 (1973) 969. 11 R. J. Van Brunt and L. J. Kiefer, Phys. Rev. A, 2 (1970) 1899.
195
SURFACE STATES ON THIN SiO2 LAYERS INDUCED BY OXYGEN ADSORPTION* N. JAKOWSKIAND H. GLAEFEKE Sektion Physik, University o f Rostock, R o s t o c k (G.D.R.)
(Received August 25, 1975) INTRODUCTION In recent years systematic investigations of surface states by means of thermally stimulated exoelectron emission (TSEE) have been reported by Krylova 1, Euler e t al. 2 and Jakowski e t al. 3 The investigated surface states were assumed to be caused by oxygen adsorption. Hiernaut e t al. 4 found that the excitation of TSEE-active surface states by electron bombardment showed a resonance-like behaviour. The present paper attempts a qualitative physical interpretation of the experimental results. EXPERIMENTAL The sample was a single-crystal silicon slice covered with an ultra-thin SiO2 layer. The adsorption of oxygen on the SiO2 layer was accomplished by passing oxygen into the ultrahigh vacuum vessel via diffusion valves. A special electron gun s generated very slow electrons for bombardment of the sample. An additional facility allowed observation of the electron stimulated desorption (ESD) of oxygen during the electron bombardment. After bombardment the temperature of the sample was increased linearly with time. The electrons emitted during heating were detected by a 17-stage open photomultiplier with copper beryllium dynodes. The energy of the bombarding electrons was the parameter for the recorded glow curves, i.e. the rate of electron emission in dependence on temperature. RESULTS On varying the energy of the bombarding electrons, it was found that the ESD yield of adsorbed oxygen became negligible below 20 eV. Thus electron energies below 20 eV were used for the study of surface states induced by oxygen adsorption. A clean silicon surface showed no TSEE. A Si surface covered by SiO2 displayed a TSEE intensity which increased with the time of exposure to oxygen at room temperature (Fig. 1). The presence of additional argon did not affect the emission intensity. The energy distribution of the emitted electrons showed different energies for different glow peaks at the maximum of the energy distribution curve. The activation energy I¢a and the frequency factor s in the relation p = S exp(-Wa/kT
)
* Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 9-74.
196
N. JAKOWSKI, H. GLAEFEKE
1,8
1,6 1,4 C
~,2
x
~_0,6
~o,4 0,2 0
40
80
120
160
200
240
2.80
temperature (aC) Fig. ]. Increase of the glow curve peak height after exposure of the Si02 layer to 02 (4 tort rain): curve a before and curve b after exposure to 02.
were determined from decay curves at elevated temperatures. For the 100 °C TSEE peak we obtained s ~ 1011 s -1 and Wa = 0.95 eV. It is of interest that a very low frequency factor of 102 s -1 was obtained for the transition indicated by the maximum at about 210 °C. The corresponding activation energy was 0.42 eV. The experiments were carried out with SiO2 layers of about 20-30 A thickness, ensuring that the charging of the insulating surface layer was small enough to allow measurements for energies Wexc of the bombarding electrons down to 1 eV. The dependence of the total number of emitted electrons on Wexc is shown in Fig. 2. Evidently resonances exist at Wexc ~ 2.5 eV, 7 eV and 9 eV. Further measurements revealed the existence of thresholds for the excitation of surface states induced by oxygen adsorption. Finally the dependence of the excitation of such states on the direction of the incident electrons indicated an anisotropic cross section. DISCUSSION
These adsorption experiments clearly point to the dominant role which oxygen plays in TSEE from surface states. The results of studies of oxygen molecules in molecular physics lead us to the conclusion that the measured results are mainly due to adsorbed 02-. This will be discussed in terms of a qualitative physical model (Fig. 3) which is consistent with the experimental results. The model reflects the essential features of different adsorbed molecules capable of causing TSEE-active centres, and is based on the position of the adsorbed molecule in the image force potential of the adsorbing solid. It will be described briefly.
SURFACE STATES OF THIN SiO2 LAYERS
197
140
/:
~oe
o
8o
k l
o
~ 40
/
"6
~ ~o ~
0
• o
/
: ;
lY
:
i internuctectf separation E o ~ ~
f\o/
2
I
i
i
,0
,'~
14
~ ~ J
x
, exc,tat,on ,
o.-----0 delocatisation and emission
electron energy (eV)
Fig. 2. Yield of thermally stimulated electrons as a function of the electron energy Wexcof the incident electrons. The main resonance peaks are at about 2.5 eV, 7 eV and 9 eV. Fig. 3. Potential model for the excitation and emission mechanism by TSEE from adsorbed molecules. W ~ - Wx, value of the image force potential at position x of the molecule; Wth threshold energy for the excitation of A + B-; Wr, resonance energy for the excitation of A + B-; Wa, activation energy for the transition from the excited state A + B- to the ground state A + B + e(Wx). A threshold value Wth exists for the excitation of a negatively ionized molecule A + B- via a Franck-Condon transition. A resonance for the excitation of this state is expected at Wr. The state A + B- is physically stable if, as in the case of special 0 2 states, the potential barrier between A + B-and A + B + e ( W x ) is large enough to exclude tunnelling6. The thermal detachment of the electrons occurs during the transition of the system from the excited A + B- state into the ground state A + B + e ( W x ) . This process is governed by the activation energy Wa and the frequency factor s. Wa is fixed by the height of the potential barrier. The frequency factor s = Kf decreases considerably if K, the transmission coe fficient, becomes very small because of non-adiabatic transitions An optical liberation is possible via a F r a n c k - C o n d o n transition. The maximum kinetic energy of the emitted electrons is W b -- (Woo -- W x ) , and increases with Wb if W ~ - W x is assumed to be constant. Our experimental results are discussed on the basis of this model and the potential curves of 02 published by Michels and Harris 7. Resonances of oxygen were measured in studies of electron impact in the gas phase by Craggs e t a l . 8 at 3 eV, by Rapp and Briglia 9 at 7 eV and by Wong e t a l . 1o at 9.5 eV. Our
198
N. JAKOWSKI, H. GLAEFEKE
analogous excitation resonance at 2.5 eV should correspond to the excitation of the 4Z u state. A subsequent heating causes a 4N u ~ 3Ng transition and results in the 100 °C TSEE peak. The measured activation energy of 0.95 eV and the large frequency factor of 1011 s-1 are in good agreement with calculated data 7 and the character of the transition. The very low frequency factor of about 102 s - I for the TSEE peak at about 210 °C is explained as a non-adiabatic transition. Our excitation resonance at 7 eV should correspond to the excitation of a II u or a Ag state. The thermally stimulated transition to the 3Zg ground state produces the 160 °C TSEE peak. Our 9.5 eV resonance could not be identified. A 2Z u resonance is likely to be involved. In the model introduced, the electrons liberated by the transitions should show different kinetic energies, the lower ones resulting from the 4~ u ~ 3~g transition. This was verified experimentally. In the gas phase the cross section of the 7 eV resonance is one order of magnitude larger than that of the 3 eV resonance. We noticed the same tendency in our measurements with adsorbed oxygen. The anisotropy of the cross section for the excitation of 0 2- states has been observed in the gas phase by Van Brunt and Kiefer 11 In analogy to their results, we measured an anisotropy for the cross section of the above mentioned II u or Ag state. This indicates an 02 axis perpendicular to the solid surface, in accordance with the Dunn selection rules. CONCLUSION The agreement between the experimental results and our model leads us to believe that tile method of TSEE after excitation by electrons is capable of yielding information about molecules in the adsorbed state, as far as these states are TSEE-active. This should be of some interest for basic research as well as for applications in catalysis for example. REFERENCES 1 I. V. Krylova, Phys. StatusSolidiA, 7 (1971) 359. 2 M. Euler, W. Kriegseisand A. Scharmann, Phys. Status Solidi A, 15 (1973) 431. ' 3 N. Jakowski, H. Glaefeke and W. Wild, Proc. Physik der Halbleiteroberfli~che, 4. Arbeitstagung, Binz, 1973, Akademie der Wissenschaften der DDK, ZIE, Berlin-Adlershof, 1973, Part 1, p. 109; 5. Arbeitstagung, Binz, 1974, Akademie der Wissenschaften der DDR, ZIE, Berlin-Adlershof,
1974, p. 157. 4 J.P. Hiernaut, R. P. Forier and J. Van Cakenberghe, Vacuum, 22 (1972) 471. 5 R. N. Lee, Rev. Sci. Instrum., 39 (1968) 1306. 6 R. P. Bell, Proc. R. Soc. London, Ser. A, 148 (1935) 241. 7 H. H. Michels and F. E. Harris, 7th lnt. Conf. on the Physics o f Electronic a n d A t o m i c Collisions, Vol. 2, North-Holland Publ. Co., Amsterdam, 1971, p. 1170. 8 J. D. Craggs, R. Thorburn and B. A. Tozer, Proc. R. Soc. London, Set. A, 240 (1957) 473. 9 D. Rapp and D. D. Briglia, J. Chem. Phys., 43 (1965) 1480. 10 S. F. Wong, M. J. W. Boness and G. J. Schulz,Phys. Rev. Lett., 31 (1973) 969. 11 R. J. Van Brunt and L. J. Kiefer, Phys. Rev. A, 2 (1970) 1899.