- Email: [email protected]
Auger electron spectroscopy of Si
SURFACE
SCIENCE 23 (1970) 347-362 Q Noah-Holl~d
AUGER EL~C~~N 5. T. GRANT
Publishing Co.
SPECTROSCOPY
OF Si
and T. W. HAAS
Aerospuce Research Laboratories (ARC) I ~r~~b~-P~~~er~onAir Force Base, Ohio 45433, U.S.A. Received 1 April 1970 Auger electron spectroscopy studies of Si have been carried out on Si (111) 2 x 1, Si (111) 7 x 7, Si (11 I)-Fe, Si (11 I)--Cu, Si (111)-Au, and Sic. Identification of the Auger transitions from clean Si surfaces is made, and it is found that some may involve multiple ionization. Chemical effects such as shifts in the energy of the main Si Auger peak have been observed due to oxidation or combination with carbon in Sic.
1. Introduction Several articles have appeared recently regarding the origin of the Si (111) 7 and ,/19 surface structures as determined by LEEDl-7). It is generally agreed that the Si (111) 4 19 structure is impurity stabiliz~l-*? 7~,and recent measurements indicate it is due to Ni 4*7)rather than Cs). Many studies have been made on the Si (111) 7 surface (referred to as Si 7) using a variety of methods for preparation: ion-bombardment and annealing”), heatings), epitaxys) and cleaving and annealinglo). Several workers have carried out Auger electron s~ctroscopy studies of the Si 7 surfaceb-sJ@Jr) in an attempt to find whether or not this structure is also impurity stabifized. From such measurements, Bauerrl) suggested that the Si 7 structure was due to Fe as he found that the size of one of the Auger peaks from the Si 7 surface increased when Fe was deposited and also that the Si 7 structure could be formed more easily from the ,/19 structure when Fe was present. However, the Auger spectrum of Fe also has peaks in the energy range 60%700 eV, but careful Auger measurements in this range failed to show any peaks*JO). Such peaks should have been detected if Fe was present, and it was concluded that there was no evidence that Fe produced the Si 7 structure, Recently, it was reportedlo) that the Auger spectrum from a cleaved Si (111) surface having a 2 x 1 surface structure is very similar to that obtained after annealing when a Si 7 structure was present, indicating that impurities are not responsible for this structure. On annealing small increases occurred in some of the Auger peaks relative to the main Si peak, but identification of these peaks was not made. This paper reports work that was 347
348
J. T. GRANT
AND
T. W. HAAS
carried out in an attempt to understand the behavior of the Auger peaks from Si. Auger measurements include effects of ion-bombardment, deposition of Fe, Cu and Au onto Si 7 surfaces, oxygen exposure and a study of Sic. Plasma losses were also measured. Marked changes were found in the Si Auger spectra showing that the transitions observed are very dependent on surface conditions. The above metals were selected for deposition because previous Auger spectroscopy studies have shown them to have low energy spectra which overlap some of the proposed Si peaksls). 2. Experimental The Si single crystal (100 ohm cm, p-type) was supplied by Dow Corning. Samples were cut on a diamond saw and ground with Sic such that after cleavage (111) surfaces approximately 5 mm square were obtained. The samples were about 30 mm long and were mounted in a Ta holder that allowed several cleavages to be made, fig. 1. Grooves 0.5 mm deep and about
GROOVES
Ta HOLDER
Fig. 1. Cleavage apparatus.
5 mm apart were cut to help initiate cleavage with the MO rod. The crystal could be advanced by undoing the insulated screws and pushing the crystal forward with the MO rod. Resistance heating (to produce the Si 7 structure) could be carried out by touching either the Si crystal or the top plate of the cleavage apparatus with the MO rod. Deposition of Fe, Cu or Au was carried out by evaporation from W or MO wires. The LEED-Auger system used was a four concentric grid system, the Auger measurements usually being made with a side gunls). 3. Results After cleavage, the Si (111) 2 x 1 structure was always observed, the half order beams being present in one or more of the three (112) direction@‘).
AUGER
ELECTRON
SPECTROSCOPY
349
OF si
w X40
Si(lll) 2 XI
ENERGY (eV)
2a I
I
I
I
1
2
dN(E:, dE
, 1
I
I
I
I
Si(l11)7
20
ENERGY (eV)
2b
Fig. 2. Auger spectra from Si, 20-120 eV; 1 keV beam 30” to surface, 1 V RMS suppressor grid modulation, (a) Si (111) 2 x 1 surface structure, (b) Si (111) 7 x 7 surface structure.
J.T.
350
GRANT
AND
T. W. HAAS
The Si 7 structure could be produced from the 2 x 1 by a short anneal (N 1000°C for 1 min). The Auger spectra in the range 20-120 eV from surfaces having these two structures are shown in fig. 2 (a), (b), and it can be seen that they are very similar. The minima of the peaks are used to identify their positions and they occur at 36, 45, 57, 75, 82, 92 and 108 eV. Other small peaks were found at 135 and 165 eV and are discussed by Taylor4). 3.1. DEPOSITIONOF Fe When Fe was depostied at room temperature on a surface having a Si 7 structure (producing large diffraction spots), an Auger peak appeared at 48 eV together with another Si Auger peak at 84 eV, fig. 3 (a). The main Auger transition from a pure Fe foil occurs at 48 eV12). After the crystal was heated to about 1000°C for a few minutes, the Auger peak at 48 eV was reduced in size considerably but was still at 48 eV (the nearest peak from the Si 7 surface being at 45 eV), fig. 3 (b). The LEED pattern from this surface [fig. 4 (a)] was not that of the Si 7 structure, fig. 4 (b). It can be seen that even when a small quantity of Fe is present at the surface, this Auger peak still occurs at 48 eV and that the Si 7 structure is not present. Further, when such small amounts of Fe are present, the high energy LMM Fe Auger peaks (600-700 ev), fig. 3 (c), are easily detected. It can be seen that small amounts of Fe do not produce a 7 x 7 structure and that contrary to work I
/
/
I
/
I
I
I
/
I
I
dN(E! dE
:i
‘ix,
I 20
I
Fe on Si(llO7
I
40
!
I
I
60
ENERGY kV) 3a
1
80
I
100
AUGER
ELECTRON
SPECTROSCOPY
OF
351
Si
Fe on Si (Ill)7
60
80
100
ENERGY (eV)
3b
I
I
I
I
I
1
I
I
I
I
x40 dN(E dE
Fe
‘
1
1
500
1
I
I
600
I
700
j
800
ENERGY (eV)
3c Fig. 3. (a), (b) Auger spectra (20-120 eV) from Si (111) with different amounts of Fe, 1.5 keV beam 30” to surface, 2 V RMS modulation. (c) High energy LMM Fe Auger peaks obtained with the amount of Fe shown in (a), 2 keV beam 30” to surface, + 300 V applied to crystal, 2 V RMS modulation.
352
.I.T.
GRANT
AND
T. W. HAAS
that was reported earlierll), the main Auger peak from Fe does not occur at the same energy as one from Si. 3.2. DEPOSITION OF Cu When Cu was deposited on the Si 7 surface, the Auger spectrum was
4a
4b Fig. 4.
Low energy electron diffraction pattern of (a) Fe on surface having Si (111) 7 structure and (b) Si (111) 7 structure, 32 V.
Cu on Si(lll)7
Fig. 5. Auger spectra from Si (lI1) with di%%entamaunts of Cu deposited, LB-1% eV* Beam energy 2 kc%, 30” to surface, X V RMS modulation. fl) Heavy deposition of Cu and (b) after annealing. Small C and 0 peaks were also detected.
354
l.T.GRANTANDT.
W. HAAS
quite different. Besides showing the Cu peaks at 58,61 and 106 eV, many new peaks appeared, fig. 5 (a), at 80, 86, 91 and 95 eV. After annealing, a 5 x 5 pattern was obtained, fig. 6, the corresponding Auger spectrum being shown in fig. 5 (b). [The LMM Cu peaks (750-950 eV) were also detectedls).] It had been suggested14) that Cu may have been responsible for the Si 7 structure, but these results show that it is not. 3.3. DEPOSITION OF Au The low energy (< 100 eV) Auger peaks from Au (52, 58 and 72 eV) are very close to some of those from Si (45,57 and 75 eV) and are also of similar relative sizesr2). However, when Au was deposited on a Si 7 surface, the Auger spectrum clearly shows the individual peaks of Au and Si, fig. 7, and after a mild anneal, a 5 x 5 diffraction pattern, fig. 8, was observed. This result is similar to that reported by Bishop and Rivi&rer5), except that they did not fully resolve the Au and Si Auger peaks. The detection of Au by Auger spectroscopy is also facilitated by its peaks above 100 eV (e.g. the NNN transitions)12). 3.4. OTHER MEASUREMENTS ON Si(ll1) Some ion-bombardment and oxygen adsorption studies were also made. When a surface having a Si 7 pattern was Ar ion bombarded (300 eV, - 100 PA/cm’ for 15 min), the heights of the 45,75 and 108 eV peaks relative to the main Si peak at 92 eV were not affected significantly, but the peak at
Fig. 6.
Low energy electron
diffraction pattern of Cu on Si (ill) 5 x 5-Cu structure, 42 V.
showing
Si (111)
AUGER ELECTRON
I
I
I
SPECTRGSCOPY
,
_
~~ K
AL
I
20
b
Au on
x2<
I
x40
AL
I 40
I
I
60
ENERGY
Fig.
?iJ-
Si
i
-
I
/
.
t Au
si
I
1
‘c
+ x4
355
Si
Si dNIEt dE
/
I
OF si
,
L
80
I
Si(li07
1
100
I
120
feV)
Auger spectrnm of Si (111) after deposition of Au, 20-120 eV. Beam energy 2 keV, 30” to surface, 1 V RMS modulation. Small C and 0 peaks were also detected.
7.
Fig. 8.
Low energy electron
diffraction pattem of Au on Si (111) showing 5 x ~-AU structure, 25 V.
Si (111)
356
J.T.GRANT
AND
T.W.HAA.9
57 eV was always smaller. The sizes of the small peaks at 135 and 165 eV seemed to vary somewhat during ion bombardment and annealing cycles but their small sizes4) do not allow accurate measurement. When a surface having a Si 7 structure was exposed to 0, changes similar to those reported by Taylore) were observed again indicating that the Auger spectrum of Si is very sensitive to surface conditions. The most obvious change in the Si Auger spectrum following oxidation was a marked enhancement of the 82 eV peak. After an O2 exposure at 1 x 10m7 Torr for 5 min, a shift in the position of the main Si Auger peak by 0.5 eV was also observed, the peak now occurring at a lower energy. This shift is similar to that observed on Ta (110) following oxidationls). I
I
I
I
I
Si (Ill)7
dN(E)_ dE
3
XI \ I
x4i
x40
L
+yf
-200
I
-150
-100 ENERGY
-50
0
LOSS (eV)
Fig. 9. Energy loss spectrum from Si (111) 7 surface using 500 eV beam at 30” to surface. Suppressor grid modulation was 2 V RMS. Loss minima at 100 and 151 eV are characteristic ionization losses.
Measurements were also made of the energy spectrum near the primary beam energy. Using a 500 eV electron beam, energy losses from a Si surface having a 7 x 7 structure were found at 10, 15, 29, 48, 65, 83, 100, 117 and 151 eV below the primary, fig. 9, the peak minima being used to identify their positions. The loss at 10 eV was markedly enhanced following oxidation
AUGER ELECTRON
SPECTRGSCGPY
OF
Si
357
as shown in fig. 10. In general, the relative heights of the loss peaks followed a complex behavior as a function of angle of incidence of electron beam, ion-bombardment and oxidation. 3.5. SILICON CARBIDE The Sic sample was cut from a Sylvania high purity sputtering source and was given several ion bombardment and annealing cycles to reduce the surface impurities detected (mainly S, K, Ca, and 0) to sufficiently low levels. The Auger spectrum in the range 40-120 eV is shown in fig. 11. A small peak was also observed near 135 eV but none was observed at 165 eV (a small amount of S was present and it may hinder detection). The main Si peak from Sic appears at a lower energy (about 90 eV) than that from clean
CLEAN
dN(E) dE
OXIDIZED
-4
x IO
I -60
1 -40
ENERGY
I -20
I 0
LOSS (eV)
Fig. 10. Energy loss spectrum from Si (111) 7 surface before and after oxidation (1 x lo-’ Torr 0~ for 13 min). Spectrum obtained using LEED electron gun at normal incidence, 190 eV beam energy, 2 V RMS modulation on suppressor grid.
358
J.T.GRANTANDT.W.HAA.3
I
1
/
I
I
!
I
t
I
I
dN(E) dE
280 I-LLi 40
2400
Sic
I
I
80
60
ENERGY
-I
100
120
ieV)
Fig. 11. Auger spectrum from Sic, 40-120 eV. Beam energy 2 keV, 30” to surface. Suppressor grid modulations 1 V RMS (solid line) and 2 V RMS (broken line).
Si (92 ev), probably due to the strong interaction with carbon and it can be seen that the shape of the Auger spectrum is quite different from clean Si. 4. Discussion The most important results in this work are the large changes observed in the relative sizes of the Auger peaks from Si when different materials are deposited. The electron binding energies for Si are given in table 1, and it TABLE 1
The electron binding energies in Si. Zero binding energy is taken at the Fermi level (data taken from ref. 17, Appendix 1) Shell
Binding energy (eV)
K Ll L2 L3 Ml M2, 3
1839 149 loo 99 8 3
AUGER
ELECTRON
SPECTROSCQPY
OF
359
Si
can be seen that the main Si peak at 92 eV is of the L,, 3 MM type, table 2. One cannot calculate the Auger energies exactly from the ground state binding energies because during the Auger process, an atom is initially ionized resulting in a shift in the energy levels of the remaining electrons. It has been TABLE
2
Calculated energies of Si Auger transitions using (a) ground state binding energies from table 1 and (b) using energy levels of phosphorus for ejected electron; transitions involving LS are not listed separately here as they occur only 1 eV below those involving L2 Transition L1LaM1 L1LzM2 LzMlMl LzMrMe LaMeM LlM1Ml L1M1M2 L1M2M2
Energy (eV) using ground state binding energies
Energy (eV) using phosphorus levels for ejected electron
41 46 84 89 94 133 138 143
33 39 76 82 87 125 131 136
suggestedls) that a good approximation to the Auger energy could be made by using energy levels for the atom of next higher atomic number (in this case phosphorus). Thus, the energy of the transition ,?&r(Z), where V is the energy level of the initial vacancy in an atom of atomic number Z, and the neutralizing and ejected electrons are in the X and Y levels respectively, is given by: &XY (Z) = & (Z) - & (Z) - Er (Z + 1).
(1)
More recently it has been suggestedla) that a correction factor given by: AE=AZ[E,(Z)-E,(Z+
l)]
(2)
be used, AZ to be determined by experiment. Eq. (1) then becomes: Evxr (Z) = E,(Z)
- E,(Z)
- Ey (Z) + AE.
(3)
The Auger energies for Si using AZ= 1 are listed in the right-hand column of table 2, and it can be seen that the correction is significant. Because of the uncertainty of AZ for each transition, it is difficult to label the observed Si transitions (fig. 2). Those observed at 36 and 45 eV are probably L,L,, 3M1 and L,L,, 3MZ respectively while those at 92 and 135 eV are of type L,, ,MM and L,MM respectively. The small Auger peaks at 75 and 82 eV could also be of type L,MM. The small peaks at 57, 108 and 165 eV, however, cannot
360
J.T.GRANT
AND
T.W.
HAAS
be identified from table 2. The peak at 57 eV is in a region of high background (fig. 2) and may be due to a plasma loss4~sO).It is also possible that the peak at 75 eV may involve a plasma 10~~4~2~).The peak at 108 eV was always detected from Si and was of constant height relative to the main peak, suggesting it is a true Si peak. A somewhat different ratio was observed in Sic. In the discussion so far, it has been assumed that a Si atom was in its ground state before initiation of the Auger process. However, if a Si atom were in a singly ionized state before the Auger process (due to a prior photoemission process), one would expect to observe Auger transitions at different energies. After this process, the atom would be in a triply ionized state rather than in a doubly ionized state. Assuming dZ= 1 again (therefore using energy levels of phosphorus when the Si atom is in the singly ionized state and those of sulfur when it is in the doubly ionized state) one might observe transitions at 109 eV (L,,,M,M,) and 165 eV (L,M,M& The observed 108 eV peak is bigger than that at 165 eV4), and one might expect this as the L,, 3MzM, (92 eV) transition from Si initially in its ground state is also the biggest, Such transitions involving multiply charged atoms have been predicted but were not previously observedsipss). The magnitudes of these possible Si transitions are not inconsistent with those predicted in nuclear experimentsss). Any further discussion of this effect, however, would require careful calculations of the relative transition rates involved. The energy loss spectrum shown in fig. 9 consists of plasma losses, and characteristic ionization lossesss), their identification being made in table 3. TABLE
3
Energy losses observed from Si (111) surface having a 7 x 7 structure, showing their identification: electron beam energy 500 eV. angle of beam to Si surface 30’ Peak
Energy loss (eV) 10 15
29 48 65 82 100 117 151
Ionization loss, MI level and/or surface plasma loss 1st order bulk plasma Ioss 2nd order bulk plasma loss 3rd order bulk plasma loss 4th order bulk plasma loss 5th order bulk plasma loss Ionization loss, La, L3 levels Plasma loss Ionization loss Lr level
The ionization losses measured sponding electron levels as can be seen that the bulk plasma intervals and that the higher
agree well with the energies of the correbe seen by comparison with table 1. It can losses are spaced at approximately 17 eV harmonics show a monotonic decrease in
AUGER
ELECTRON
SPECTROSCGPY
OF
Si
361
amplitude. Note that the ionization loss peaks at 100 and 151 eV (fig. 9) are larger than neighboring plasma loss peaks. The widths of the ionization loss peaks (about 5 eV, measured form peak-to-peak) are measureably smaller than the widths of the plasma losses (about 8 eV), in agreement with other worka4). It is possible that the 82 eV Si Auger peak might also involve an ionization loss (10 eV) from the main peak as well as a surface plasma loss. The changes observed in the Auger spectra following metal deposition could be due to chemical effects such as energy shifts or splitting of energy levels in Si which would result in Auger peaks at new energies and changes in their relative heights. Such effects have been observed in some metals following oxidationss). It should be noted that there is a more pronounced asymmetry in the main Si Auger peak from Sic than that from clean Si. This clipping of the peak is always observed when traces of carbon (e.g. from cracking of residual gases) were present on the Si 7 structure. The M electrons in Si are the valence electrons and one would expect changes in bonding to be reflected as changes in line shape of Auger transitions involving these electrons. Similar effects have been found for such Auger transitions in carbona6). 5. Summary It has been shown that all the Auger peaks obtained in the energy range 20 to 200 eV from a Si surface having a 2 x 1 or 7 x 7 structure may be identified as due to Si. The Auger spectrum probably involves (apart from simple Auger transitions) plasma and ionization losses and Auger transitions of multiply charged Si atoms. Deposition of Fe, Cu and Au onto Si (111) 7 surfaces markedly affects the Auger spectrum from Si, but the main Auger transitions from the deposited metals are easily identified. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
A. J. Van Bommel and F. Meyer, Surface Sci. 8 (1967) 467. H. K. Lintz, Surface Sci. 12 (1968) 390. A. J. Van Bommel and F. Meyer, Surface Sci. 12 (1968) 391. N. J. Taylor, Surface Sci. 15 (1969) 169. H. E. Bishop and J. C. Riviere, Surface Sci. 17 (1969) 462. N. J. Taylor, Surface Sci. 17 (1969) 466. J. M. Charig and D. K. Skinner, Surface Sci. 19 (1970) 283. F. Jona, Appl. Phys. Letters 6 (1965) 205. R. M. Broudy and H. C. Abbink, Appl. Phys. Letters 13 (1968) 212. J. T. Grant and T. W. Haas, Appl. Phys. Letters 15 (1969) 140. E. Bauer, Phys. Letters 26A (1968) 530. T. W. Haas, J. T. Grant and G. J. Dooley, Phys. Rev. B 1 (1970) 1449. P. W. Palmberg, Appl. Phys. Letters 13 (1968) 183. J. C. Riviere, private communication. H. E. Bishop and J. C. Riviere, Brit. J. Appl. Phys. [II] 2 (1969) 1635.
362
J.T.GRANT
AND
T.W.HAAS
16) T. W. Haas and J. T. Grant, Phys. Letters 30A (1969) 272. 17) K. Siegbahn et al., Electron Spectroscopy for Chemical Analysis, Air Force Materials Laboratory Technical Report AFML-TR-68-189 (1968). 18) E. Burhop, The Auger Effect and Other Radiationless Transitions (Cambridge Univ. Press, New York, 1952). 19) I. Bergstrom and R. D. Hill, Arkiv Fysik 8 (1954) 21. 20) W. M. Mularie and T. W. Rusch, Surface Sci. 19 (1970) 469. 21) M. A. Listengarten, Izv. Akad. Nauk SSSR 24 (1960) 1041. 22) Z. Sujkowski and N. Slatis, Arkiv Fysik 8 (1954) 21. 23) H. E. Bishop and J. C. Riviere, Appl. Phys. Letters 16 (1970) 21. 24) R. L. Gerlach, J. E. Houston and R. L. Park, Appl. Phys. Letters 16 (1970) 179. 25) T. W. Haas, J. T. Grant and G. J. Dooley, to be published. 26) T. W. Haas and J. T. Grant, Appl. Phys. Letters 16 (1970) 172.
SCIENCE 23 (1970) 347-362 Q Noah-Holl~d
AUGER EL~C~~N 5. T. GRANT
Publishing Co.
SPECTROSCOPY
OF Si
and T. W. HAAS
Aerospuce Research Laboratories (ARC) I ~r~~b~-P~~~er~onAir Force Base, Ohio 45433, U.S.A. Received 1 April 1970 Auger electron spectroscopy studies of Si have been carried out on Si (111) 2 x 1, Si (111) 7 x 7, Si (11 I)-Fe, Si (11 I)--Cu, Si (111)-Au, and Sic. Identification of the Auger transitions from clean Si surfaces is made, and it is found that some may involve multiple ionization. Chemical effects such as shifts in the energy of the main Si Auger peak have been observed due to oxidation or combination with carbon in Sic.
1. Introduction Several articles have appeared recently regarding the origin of the Si (111) 7 and ,/19 surface structures as determined by LEEDl-7). It is generally agreed that the Si (111) 4 19 structure is impurity stabiliz~l-*? 7~,and recent measurements indicate it is due to Ni 4*7)rather than Cs). Many studies have been made on the Si (111) 7 surface (referred to as Si 7) using a variety of methods for preparation: ion-bombardment and annealing”), heatings), epitaxys) and cleaving and annealinglo). Several workers have carried out Auger electron s~ctroscopy studies of the Si 7 surfaceb-sJ@Jr) in an attempt to find whether or not this structure is also impurity stabifized. From such measurements, Bauerrl) suggested that the Si 7 structure was due to Fe as he found that the size of one of the Auger peaks from the Si 7 surface increased when Fe was deposited and also that the Si 7 structure could be formed more easily from the ,/19 structure when Fe was present. However, the Auger spectrum of Fe also has peaks in the energy range 60%700 eV, but careful Auger measurements in this range failed to show any peaks*JO). Such peaks should have been detected if Fe was present, and it was concluded that there was no evidence that Fe produced the Si 7 structure, Recently, it was reportedlo) that the Auger spectrum from a cleaved Si (111) surface having a 2 x 1 surface structure is very similar to that obtained after annealing when a Si 7 structure was present, indicating that impurities are not responsible for this structure. On annealing small increases occurred in some of the Auger peaks relative to the main Si peak, but identification of these peaks was not made. This paper reports work that was 347
348
J. T. GRANT
AND
T. W. HAAS
carried out in an attempt to understand the behavior of the Auger peaks from Si. Auger measurements include effects of ion-bombardment, deposition of Fe, Cu and Au onto Si 7 surfaces, oxygen exposure and a study of Sic. Plasma losses were also measured. Marked changes were found in the Si Auger spectra showing that the transitions observed are very dependent on surface conditions. The above metals were selected for deposition because previous Auger spectroscopy studies have shown them to have low energy spectra which overlap some of the proposed Si peaksls). 2. Experimental The Si single crystal (100 ohm cm, p-type) was supplied by Dow Corning. Samples were cut on a diamond saw and ground with Sic such that after cleavage (111) surfaces approximately 5 mm square were obtained. The samples were about 30 mm long and were mounted in a Ta holder that allowed several cleavages to be made, fig. 1. Grooves 0.5 mm deep and about
GROOVES
Ta HOLDER
Fig. 1. Cleavage apparatus.
5 mm apart were cut to help initiate cleavage with the MO rod. The crystal could be advanced by undoing the insulated screws and pushing the crystal forward with the MO rod. Resistance heating (to produce the Si 7 structure) could be carried out by touching either the Si crystal or the top plate of the cleavage apparatus with the MO rod. Deposition of Fe, Cu or Au was carried out by evaporation from W or MO wires. The LEED-Auger system used was a four concentric grid system, the Auger measurements usually being made with a side gunls). 3. Results After cleavage, the Si (111) 2 x 1 structure was always observed, the half order beams being present in one or more of the three (112) direction@‘).
AUGER
ELECTRON
SPECTROSCOPY
349
OF si
w X40
Si(lll) 2 XI
ENERGY (eV)
2a I
I
I
I
1
2
dN(E:, dE
, 1
I
I
I
I
Si(l11)7
20
ENERGY (eV)
2b
Fig. 2. Auger spectra from Si, 20-120 eV; 1 keV beam 30” to surface, 1 V RMS suppressor grid modulation, (a) Si (111) 2 x 1 surface structure, (b) Si (111) 7 x 7 surface structure.
J.T.
350
GRANT
AND
T. W. HAAS
The Si 7 structure could be produced from the 2 x 1 by a short anneal (N 1000°C for 1 min). The Auger spectra in the range 20-120 eV from surfaces having these two structures are shown in fig. 2 (a), (b), and it can be seen that they are very similar. The minima of the peaks are used to identify their positions and they occur at 36, 45, 57, 75, 82, 92 and 108 eV. Other small peaks were found at 135 and 165 eV and are discussed by Taylor4). 3.1. DEPOSITIONOF Fe When Fe was depostied at room temperature on a surface having a Si 7 structure (producing large diffraction spots), an Auger peak appeared at 48 eV together with another Si Auger peak at 84 eV, fig. 3 (a). The main Auger transition from a pure Fe foil occurs at 48 eV12). After the crystal was heated to about 1000°C for a few minutes, the Auger peak at 48 eV was reduced in size considerably but was still at 48 eV (the nearest peak from the Si 7 surface being at 45 eV), fig. 3 (b). The LEED pattern from this surface [fig. 4 (a)] was not that of the Si 7 structure, fig. 4 (b). It can be seen that even when a small quantity of Fe is present at the surface, this Auger peak still occurs at 48 eV and that the Si 7 structure is not present. Further, when such small amounts of Fe are present, the high energy LMM Fe Auger peaks (600-700 ev), fig. 3 (c), are easily detected. It can be seen that small amounts of Fe do not produce a 7 x 7 structure and that contrary to work I
/
/
I
/
I
I
I
/
I
I
dN(E! dE
:i
‘ix,
I 20
I
Fe on Si(llO7
I
40
!
I
I
60
ENERGY kV) 3a
1
80
I
100
AUGER
ELECTRON
SPECTROSCOPY
OF
351
Si
Fe on Si (Ill)7
60
80
100
ENERGY (eV)
3b
I
I
I
I
I
1
I
I
I
I
x40 dN(E dE
Fe
‘
1
1
500
1
I
I
600
I
700
j
800
ENERGY (eV)
3c Fig. 3. (a), (b) Auger spectra (20-120 eV) from Si (111) with different amounts of Fe, 1.5 keV beam 30” to surface, 2 V RMS modulation. (c) High energy LMM Fe Auger peaks obtained with the amount of Fe shown in (a), 2 keV beam 30” to surface, + 300 V applied to crystal, 2 V RMS modulation.
352
.I.T.
GRANT
AND
T. W. HAAS
that was reported earlierll), the main Auger peak from Fe does not occur at the same energy as one from Si. 3.2. DEPOSITION OF Cu When Cu was deposited on the Si 7 surface, the Auger spectrum was
4a
4b Fig. 4.
Low energy electron diffraction pattern of (a) Fe on surface having Si (111) 7 structure and (b) Si (111) 7 structure, 32 V.
Cu on Si(lll)7
Fig. 5. Auger spectra from Si (lI1) with di%%entamaunts of Cu deposited, LB-1% eV* Beam energy 2 kc%, 30” to surface, X V RMS modulation. fl) Heavy deposition of Cu and (b) after annealing. Small C and 0 peaks were also detected.
354
l.T.GRANTANDT.
W. HAAS
quite different. Besides showing the Cu peaks at 58,61 and 106 eV, many new peaks appeared, fig. 5 (a), at 80, 86, 91 and 95 eV. After annealing, a 5 x 5 pattern was obtained, fig. 6, the corresponding Auger spectrum being shown in fig. 5 (b). [The LMM Cu peaks (750-950 eV) were also detectedls).] It had been suggested14) that Cu may have been responsible for the Si 7 structure, but these results show that it is not. 3.3. DEPOSITION OF Au The low energy (< 100 eV) Auger peaks from Au (52, 58 and 72 eV) are very close to some of those from Si (45,57 and 75 eV) and are also of similar relative sizesr2). However, when Au was deposited on a Si 7 surface, the Auger spectrum clearly shows the individual peaks of Au and Si, fig. 7, and after a mild anneal, a 5 x 5 diffraction pattern, fig. 8, was observed. This result is similar to that reported by Bishop and Rivi&rer5), except that they did not fully resolve the Au and Si Auger peaks. The detection of Au by Auger spectroscopy is also facilitated by its peaks above 100 eV (e.g. the NNN transitions)12). 3.4. OTHER MEASUREMENTS ON Si(ll1) Some ion-bombardment and oxygen adsorption studies were also made. When a surface having a Si 7 pattern was Ar ion bombarded (300 eV, - 100 PA/cm’ for 15 min), the heights of the 45,75 and 108 eV peaks relative to the main Si peak at 92 eV were not affected significantly, but the peak at
Fig. 6.
Low energy electron
diffraction pattern of Cu on Si (ill) 5 x 5-Cu structure, 42 V.
showing
Si (111)
AUGER ELECTRON
I
I
I
SPECTRGSCOPY
,
_
~~ K
AL
I
20
b
Au on
x2<
I
x40
AL
I 40
I
I
60
ENERGY
Fig.
?iJ-
Si
i
-
I
/
.
t Au
si
I
1
‘c
+ x4
355
Si
Si dNIEt dE
/
I
OF si
,
L
80
I
Si(li07
1
100
I
120
feV)
Auger spectrnm of Si (111) after deposition of Au, 20-120 eV. Beam energy 2 keV, 30” to surface, 1 V RMS modulation. Small C and 0 peaks were also detected.
7.
Fig. 8.
Low energy electron
diffraction pattem of Au on Si (111) showing 5 x ~-AU structure, 25 V.
Si (111)
356
J.T.GRANT
AND
T.W.HAA.9
57 eV was always smaller. The sizes of the small peaks at 135 and 165 eV seemed to vary somewhat during ion bombardment and annealing cycles but their small sizes4) do not allow accurate measurement. When a surface having a Si 7 structure was exposed to 0, changes similar to those reported by Taylore) were observed again indicating that the Auger spectrum of Si is very sensitive to surface conditions. The most obvious change in the Si Auger spectrum following oxidation was a marked enhancement of the 82 eV peak. After an O2 exposure at 1 x 10m7 Torr for 5 min, a shift in the position of the main Si Auger peak by 0.5 eV was also observed, the peak now occurring at a lower energy. This shift is similar to that observed on Ta (110) following oxidationls). I
I
I
I
I
Si (Ill)7
dN(E)_ dE
3
XI \ I
x4i
x40
L
+yf
-200
I
-150
-100 ENERGY
-50
0
LOSS (eV)
Fig. 9. Energy loss spectrum from Si (111) 7 surface using 500 eV beam at 30” to surface. Suppressor grid modulation was 2 V RMS. Loss minima at 100 and 151 eV are characteristic ionization losses.
Measurements were also made of the energy spectrum near the primary beam energy. Using a 500 eV electron beam, energy losses from a Si surface having a 7 x 7 structure were found at 10, 15, 29, 48, 65, 83, 100, 117 and 151 eV below the primary, fig. 9, the peak minima being used to identify their positions. The loss at 10 eV was markedly enhanced following oxidation
AUGER ELECTRON
SPECTRGSCGPY
OF
Si
357
as shown in fig. 10. In general, the relative heights of the loss peaks followed a complex behavior as a function of angle of incidence of electron beam, ion-bombardment and oxidation. 3.5. SILICON CARBIDE The Sic sample was cut from a Sylvania high purity sputtering source and was given several ion bombardment and annealing cycles to reduce the surface impurities detected (mainly S, K, Ca, and 0) to sufficiently low levels. The Auger spectrum in the range 40-120 eV is shown in fig. 11. A small peak was also observed near 135 eV but none was observed at 165 eV (a small amount of S was present and it may hinder detection). The main Si peak from Sic appears at a lower energy (about 90 eV) than that from clean
CLEAN
dN(E) dE
OXIDIZED
-4
x IO
I -60
1 -40
ENERGY
I -20
I 0
LOSS (eV)
Fig. 10. Energy loss spectrum from Si (111) 7 surface before and after oxidation (1 x lo-’ Torr 0~ for 13 min). Spectrum obtained using LEED electron gun at normal incidence, 190 eV beam energy, 2 V RMS modulation on suppressor grid.
358
J.T.GRANTANDT.W.HAA.3
I
1
/
I
I
!
I
t
I
I
dN(E) dE
280 I-LLi 40
2400
Sic
I
I
80
60
ENERGY
-I
100
120
ieV)
Fig. 11. Auger spectrum from Sic, 40-120 eV. Beam energy 2 keV, 30” to surface. Suppressor grid modulations 1 V RMS (solid line) and 2 V RMS (broken line).
Si (92 ev), probably due to the strong interaction with carbon and it can be seen that the shape of the Auger spectrum is quite different from clean Si. 4. Discussion The most important results in this work are the large changes observed in the relative sizes of the Auger peaks from Si when different materials are deposited. The electron binding energies for Si are given in table 1, and it TABLE 1
The electron binding energies in Si. Zero binding energy is taken at the Fermi level (data taken from ref. 17, Appendix 1) Shell
Binding energy (eV)
K Ll L2 L3 Ml M2, 3
1839 149 loo 99 8 3
AUGER
ELECTRON
SPECTROSCQPY
OF
359
Si
can be seen that the main Si peak at 92 eV is of the L,, 3 MM type, table 2. One cannot calculate the Auger energies exactly from the ground state binding energies because during the Auger process, an atom is initially ionized resulting in a shift in the energy levels of the remaining electrons. It has been TABLE
2
Calculated energies of Si Auger transitions using (a) ground state binding energies from table 1 and (b) using energy levels of phosphorus for ejected electron; transitions involving LS are not listed separately here as they occur only 1 eV below those involving L2 Transition L1LaM1 L1LzM2 LzMlMl LzMrMe LaMeM LlM1Ml L1M1M2 L1M2M2
Energy (eV) using ground state binding energies
Energy (eV) using phosphorus levels for ejected electron
41 46 84 89 94 133 138 143
33 39 76 82 87 125 131 136
suggestedls) that a good approximation to the Auger energy could be made by using energy levels for the atom of next higher atomic number (in this case phosphorus). Thus, the energy of the transition ,?&r(Z), where V is the energy level of the initial vacancy in an atom of atomic number Z, and the neutralizing and ejected electrons are in the X and Y levels respectively, is given by: &XY (Z) = & (Z) - & (Z) - Er (Z + 1).
(1)
More recently it has been suggestedla) that a correction factor given by: AE=AZ[E,(Z)-E,(Z+
l)]
(2)
be used, AZ to be determined by experiment. Eq. (1) then becomes: Evxr (Z) = E,(Z)
- E,(Z)
- Ey (Z) + AE.
(3)
The Auger energies for Si using AZ= 1 are listed in the right-hand column of table 2, and it can be seen that the correction is significant. Because of the uncertainty of AZ for each transition, it is difficult to label the observed Si transitions (fig. 2). Those observed at 36 and 45 eV are probably L,L,, 3M1 and L,L,, 3MZ respectively while those at 92 and 135 eV are of type L,, ,MM and L,MM respectively. The small Auger peaks at 75 and 82 eV could also be of type L,MM. The small peaks at 57, 108 and 165 eV, however, cannot
360
J.T.GRANT
AND
T.W.
HAAS
be identified from table 2. The peak at 57 eV is in a region of high background (fig. 2) and may be due to a plasma loss4~sO).It is also possible that the peak at 75 eV may involve a plasma 10~~4~2~).The peak at 108 eV was always detected from Si and was of constant height relative to the main peak, suggesting it is a true Si peak. A somewhat different ratio was observed in Sic. In the discussion so far, it has been assumed that a Si atom was in its ground state before initiation of the Auger process. However, if a Si atom were in a singly ionized state before the Auger process (due to a prior photoemission process), one would expect to observe Auger transitions at different energies. After this process, the atom would be in a triply ionized state rather than in a doubly ionized state. Assuming dZ= 1 again (therefore using energy levels of phosphorus when the Si atom is in the singly ionized state and those of sulfur when it is in the doubly ionized state) one might observe transitions at 109 eV (L,,,M,M,) and 165 eV (L,M,M& The observed 108 eV peak is bigger than that at 165 eV4), and one might expect this as the L,, 3MzM, (92 eV) transition from Si initially in its ground state is also the biggest, Such transitions involving multiply charged atoms have been predicted but were not previously observedsipss). The magnitudes of these possible Si transitions are not inconsistent with those predicted in nuclear experimentsss). Any further discussion of this effect, however, would require careful calculations of the relative transition rates involved. The energy loss spectrum shown in fig. 9 consists of plasma losses, and characteristic ionization lossesss), their identification being made in table 3. TABLE
3
Energy losses observed from Si (111) surface having a 7 x 7 structure, showing their identification: electron beam energy 500 eV. angle of beam to Si surface 30’ Peak
Energy loss (eV) 10 15
29 48 65 82 100 117 151
Ionization loss, MI level and/or surface plasma loss 1st order bulk plasma Ioss 2nd order bulk plasma loss 3rd order bulk plasma loss 4th order bulk plasma loss 5th order bulk plasma loss Ionization loss, La, L3 levels Plasma loss Ionization loss Lr level
The ionization losses measured sponding electron levels as can be seen that the bulk plasma intervals and that the higher
agree well with the energies of the correbe seen by comparison with table 1. It can losses are spaced at approximately 17 eV harmonics show a monotonic decrease in
AUGER
ELECTRON
SPECTROSCGPY
OF
Si
361
amplitude. Note that the ionization loss peaks at 100 and 151 eV (fig. 9) are larger than neighboring plasma loss peaks. The widths of the ionization loss peaks (about 5 eV, measured form peak-to-peak) are measureably smaller than the widths of the plasma losses (about 8 eV), in agreement with other worka4). It is possible that the 82 eV Si Auger peak might also involve an ionization loss (10 eV) from the main peak as well as a surface plasma loss. The changes observed in the Auger spectra following metal deposition could be due to chemical effects such as energy shifts or splitting of energy levels in Si which would result in Auger peaks at new energies and changes in their relative heights. Such effects have been observed in some metals following oxidationss). It should be noted that there is a more pronounced asymmetry in the main Si Auger peak from Sic than that from clean Si. This clipping of the peak is always observed when traces of carbon (e.g. from cracking of residual gases) were present on the Si 7 structure. The M electrons in Si are the valence electrons and one would expect changes in bonding to be reflected as changes in line shape of Auger transitions involving these electrons. Similar effects have been found for such Auger transitions in carbona6). 5. Summary It has been shown that all the Auger peaks obtained in the energy range 20 to 200 eV from a Si surface having a 2 x 1 or 7 x 7 structure may be identified as due to Si. The Auger spectrum probably involves (apart from simple Auger transitions) plasma and ionization losses and Auger transitions of multiply charged Si atoms. Deposition of Fe, Cu and Au onto Si (111) 7 surfaces markedly affects the Auger spectrum from Si, but the main Auger transitions from the deposited metals are easily identified. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)
A. J. Van Bommel and F. Meyer, Surface Sci. 8 (1967) 467. H. K. Lintz, Surface Sci. 12 (1968) 390. A. J. Van Bommel and F. Meyer, Surface Sci. 12 (1968) 391. N. J. Taylor, Surface Sci. 15 (1969) 169. H. E. Bishop and J. C. Riviere, Surface Sci. 17 (1969) 462. N. J. Taylor, Surface Sci. 17 (1969) 466. J. M. Charig and D. K. Skinner, Surface Sci. 19 (1970) 283. F. Jona, Appl. Phys. Letters 6 (1965) 205. R. M. Broudy and H. C. Abbink, Appl. Phys. Letters 13 (1968) 212. J. T. Grant and T. W. Haas, Appl. Phys. Letters 15 (1969) 140. E. Bauer, Phys. Letters 26A (1968) 530. T. W. Haas, J. T. Grant and G. J. Dooley, Phys. Rev. B 1 (1970) 1449. P. W. Palmberg, Appl. Phys. Letters 13 (1968) 183. J. C. Riviere, private communication. H. E. Bishop and J. C. Riviere, Brit. J. Appl. Phys. [II] 2 (1969) 1635.
362
J.T.GRANT
AND
T.W.HAAS
16) T. W. Haas and J. T. Grant, Phys. Letters 30A (1969) 272. 17) K. Siegbahn et al., Electron Spectroscopy for Chemical Analysis, Air Force Materials Laboratory Technical Report AFML-TR-68-189 (1968). 18) E. Burhop, The Auger Effect and Other Radiationless Transitions (Cambridge Univ. Press, New York, 1952). 19) I. Bergstrom and R. D. Hill, Arkiv Fysik 8 (1954) 21. 20) W. M. Mularie and T. W. Rusch, Surface Sci. 19 (1970) 469. 21) M. A. Listengarten, Izv. Akad. Nauk SSSR 24 (1960) 1041. 22) Z. Sujkowski and N. Slatis, Arkiv Fysik 8 (1954) 21. 23) H. E. Bishop and J. C. Riviere, Appl. Phys. Letters 16 (1970) 21. 24) R. L. Gerlach, J. E. Houston and R. L. Park, Appl. Phys. Letters 16 (1970) 179. 25) T. W. Haas, J. T. Grant and G. J. Dooley, to be published. 26) T. W. Haas and J. T. Grant, Appl. Phys. Letters 16 (1970) 172.