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X-ray photoelectron spectroscopy study of ultrathin oxide layers on Al and Si substrates
Thin Solid Films, 226 (1993) 219-223
219
X-ray photoelectron spectroscopy study of ultrathin oxide layers on Al and Si substrates T. J. Sarapatka J. Heyrovskj Institute of Physical 182 23 Prague 8 (Czechoslovakia) (Received
October
Chemistry
16, 1992; accepted
and Electrochemistry,
November
Czechoslovak
Academy
of Sciences, Dolejtkova
3,
25, 1992)
Abstract The initial stages of oxide growth on Al and Si are studied by X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy methods. The change from initial Si-0 and Al-O bonding through interface layer formation to SiO, and Al,O, oxide growth are characterized in terms of charge transfer, relaxation energy and particular bonding together with oxide Fermi level shifts. The relation between the Auger parameter and hole localization or extra-atomic relaxation energies is discussed, taking into account the influence of excessive oxygen atoms presented in the oxide layer. The advantages of using these substrates for the preparation of a model catalyst and for investigating the metal-oxide support interaction are discussed. While the diffusion of deposited metal prevails for oxide thicknesses up to 1.2 nm, charge transfer and tunnelling processes dominate in the interaction of metal deposited on thicker oxide interlayers.
1. Introduction Thin insulating layers have attracted considerable attention in investigating supported dispersed metal catalysts. This is because these substrates do not usually exhibit a charging effect upon X-ray irradiation. Many studies have been done on surface and interface investigation of ultrathin oxide layers formed on Si [l-6] and Al [ 7- 121 supports. The growth, morphology and electron structure of the oxides have been determined from various electron spectroscopy results. Extra-atomic relaxation and charge transfer effects were considered as origins of oxidation-induced binding energy & shifts of substrate core levels [ 1, 2, 5, 131. However, the certain instability of these substrates in the gas adsorption experiments [ 141 and significant shifts of their core levels after metal deposition [ 15, 161 need further studies. Auger parameters [ 171 are widely used for characterizing particular chemical states of elements [2, 31 as they are independent of sample surface charging. Recently, detailed discussions on the Auger parameter have shown its connection with the local geometry, polarization energy and refractive index of the bulk materials [ 18-231. In this study, X-ray photoelectron spectroscopy (XPS) and X-ray-induced Auger electron spectroscopy (XAES) were used to characterize the initial stages of Al and Si oxide growth. The evolution of the spectral characteristics is compared with those of bulk oxide
0040-6090/93/%6.00
samples to determine the variations in relaxation energy and the effect of oxide Fermi level shifts. The influence of the oxide overlayer thickness on its interaction with the deposited metal is discussed.
2. Experimental
details
Experiments were carried out in a VG ESCA 3 Mk II spectrometer using non-monochromatic Al Ka excitation at a pressure lower than 2 x lo-’ Pa. Calibration of the spectrometer was set up to an Eb for Au 4f,,z of 84.0 eV (full width at half-maximum (FWHM), 1.2 eV). The estimated error in binding energy determination was fO.l eV. We used n-Si( 111) doped with 6 x lOi P atoms cmm3 and polycrystalline Al foil (Hicol; purity, 99.999%) substrates. SiOz( 1340) and A&0,( lOi2) samples were used as bulk standards. They were cleaned by argon sputtering and subsequent annealing at 1000 K. Si oxides were prepared in situ at 900 + 50 K upon oxygen exposures from 4 to 4 x lo5 langmuirs. Room temperature or elevated temperatures up to 700 K were used for the growth of A&O,. The X-ray photoelectron spectra were recorded 15 min after cooling the samples to the room temperature. The Si(A1) 2p spectra after subtraction of a linear background were fitted by three Gauss-Lorentz curves (Fig. 1). The particular peaks correspond to elemental Si or Al (peak P,), an interface Si suboxide or a
0
1993 -
Elsevier
Sequoia.
All rights
reserved
T. J. Sarapatka
Si
d ox=l.l
2~
dox
/ XPS of‘ultrathin
oxide
layers on Al and SI
[26] agreed for the thickness measured oxides with the determined do, = d, + d2 to within + 10%):
nm
SioI
d,
Si
_;__ b pz
'1
1
I
I
I
100
01
1
s
BINDING
1
I
I
I
I
110
105
ENERGY
I
I
I 115
(eV)
Fig. I Illustration of the peak fitting into three contributions for the Si 2p spectrum. For this fit, the resulting FWHMS of the particular peaks P,, P, and P, were 1.5 eV, 1.7 eV and 1.9 eV respectively. The meaning of the particular oxide thicknesses is demonstrated in the inset
precursor chemisorbed phase of Al (peak P, ) [ 9, 241 and SiO, or Al,O, on the top (peak Pz) [6, 9, 241. For Si oxides thicker than 0.6 nm, the observed energy separation between the Si 2p peaks P, and P, of about 2 eV agrees with the Eb shift between the emissions from SiO and SiO, [6]. The Al 2p peak P, was shifted by 1.4 eV towards a higher Eb and was observed only at preparation temperatures below 450 K. A decrease in the P,-to-P, peak intensity ratio for more glancing angles of photoelectron collection reflects the predominant SiO, and/or A&O3 growth above the interface layer. Assuming layer-by-layer growth, we calculated the mean thickness d, and d2 of the particular oxide layers, in Fig. 1, from the following equations [24, 251 (the values of the oxide thickness obtained by an alternative method based on the angle-dependent XPS peak areas
TABLE
Si Al
1. Material
constants
used in eqns.
PO ( x IO1 kgm-‘)
PI
2.4 2.7
2.3 2.8
K2+
K3
+
0
I
Here I is the integral intensity of the particular Al 2p or Si 2p peaks, a denotes the signal from the bulk samples and 8 is the take-off angle of photoelectrons measured from the sample surface. The inelastic mean free paths 2 and densities p used in eqns. ( 1) -( 3) are listed in Table 1 together with the values of K, and K, [24]. We did not detect surface static charging on our measured oxide layers.
3. Results and discussion The dependences of the energy separation AEb(P2PO) between the Si(A1) 2p peaks in the elemental and oxide form and the energy separation A&(0 Is-P,) between the 0 1s and Si or Al P, peaks, on the oxide thickness are shown in Figs. 2(a) and 2(b) respectively. Different build-up processes occur during the initial stages of Si and Al oxide formation [2, 9, 10, 131. The continuous transfer of electrons from Si to oxygen atoms [ 1, 131 associated with an increase in the Si-0 bond length (from 1.39 to 1.62 A [ 3 11) leads both to the initial increase in A&,(Pz-PO) and to the decrease in AEb(O Is-P,). For the thinnest Al oxides, the dependences in Fig. 2 reflect the gradual change from the initial formation of Al-O resonance-type bonds [9] to those found in Al,O, with reduction in the Al-O bond length from 2.2 to 1.9 8, [32]. The curve shapes in Fig. 2(b) suggest that the above initial processes are finished for Si and Al oxide thicknesses of 0.8 nm and of 1.2 nm respectively. This agrees
(l-3) [27-301
yi 2.2 2.9
lo3 kg m-‘)
*0
i .I
i”,
(nm)
(nm)
(nm)
2.36” 2.58
2.53” 2.67
2.70” 2.75
‘Note that using significantly higher I values for Si species [30], i.e. i, = 3.15 nm, L, = 3.44 nm and I., = 3.73 nm, increases 1.32 times. For detailed discussion see refs. 24 and 25.
K,
KZ
0.61 0.65
0.43 0.56
the calculated
d,.
T. J. Sarapatka
4.2
1042.4
-
SiO2
-
8 3.2
~*-%%_
-
N2.2 & D
1041.6
d
;
: a0 I
-5
d
‘%.q
1
ul
.\ ‘\
a” I,.,,,,,.III.......,I,,,,.,.,II,,,.,..,,I,,I......, 1 2 0 oxide
(a) 457.2
thickness
.z
456.6
1040.0
3Lxyx
\ 4
3
5
‘:x
1039.2
(nm)
1038.4
/F#
B
3_c-A’2o3
i,~.......,........,,.........,.........,......... 0
3-
3
2
1 oxide
_+--*X$-x-r-______,
:
*a-~~-*---__*
A;
3
-Al20 9
z 8
x
-Al,0 1.2
221
oxide layers on Al and Si
1 Omw-o
% -
1 XPS of ultrathin
thickness
5
4 (nm)
Fig. 3. The dependence of the clo IS values on the oxide thickness for SiO,/Si ( 0) and AI,O,/AI ( x) samples. The corresponding dependences between the oxide stoichiometry and a, ,E are shown in the inset. 510
509
9
.A! Fig. 2. Dependences of (a) the energy separation A&(Pz-PO) between the Si(AI) 2p peaks in the elemental and oxide form and (b) the energy separation A.!&(0 Is-P,) between the 0 1s and Si(AI) 2p peaks P(2), on the oxide thickness for SiOz/Si (0) and Al,O,/AI ( X) samples.
g
506
9 LA+
well with the 50, interface determined to be about 1 nm in other experiments [l, 331. The final reduction in AEb(P2-PO) at the SiO,-Si interface (see Fig. 1) does not occur if an Si support of a low dopant concentration is used [2, 13, 241. For our Si sample, X-ray-induced minority carriers [ 341 increase the positive potential of the oxide layer. With increasing d,,,, their effective influence is reduced and saturates for d,,, > 1.6 nm. The continuous decrease in AEb( P,--P,) with increasing Al oxide thickness above 2 nm reflects the change in the Fermi level location within the oxide gap EF(AlzO,) due to a decrease in donor-like interface states [ 121. &cording to the published correlation [lo], the initial values of AE,(P,-P,) in Fig. 2(a) correspond to the reactive surface with an E,(A1103) of 4.6 eV above the conduction band (CB) edge. With d,, increasing from 2 to 4 nm, E,(Al,O,) decreases to the final value of 3.2 eV above the CB. The evolution of the Auger parameter ao ,s = Eb(O 1s) + Ek(Ocvv) with increasing oxide thickness is shown in Fig. 3. Three oxidation steps [9] with the a0 ,s values of about 1040.6, 1039.9 and 1039.2 eV can be seen for the Al*O,/Al system. The initial dissociative
507
1 533
532 Eb (0 1s)
531 (ev)
Fig. 4. The Wagner chemical state plot derived from the oxygen energies for SiOJSi (0) and AI,O,/Al ( x ). &(O Is) and &(Ocvv) The significant values of the oxide thickness are indicated on the curves.
chemisorption of oxygen on Si [35] is characterized by an a0 ,s of 1041.8 eV. On completion of the first monolayer, the continuous Si suboxide growth is manifested by the decrease in a0 Is to 1041.1 eV. This is followed by the formation of SiOp as d,, exceeds 0.9 nm which results in an a0 ,s of 1040.2 eV. The transition from single Al-O bonding to A&O3 growth and a rather continuous process going through the particular suboxides to SiOZ can be seen also from the plotted dependence between the oxide stoichiometry [24, 361 and a0 Is, shown in Fig. 3. Additional information can be obtained from the Wagner chemical state plot presented in Fig. 4 which is
T. J. garapatka / XPS of ultrathin oxide layers on Al and Si
1
1
b- c
I
+_,c”-w
, I
,
500 KINETIC
510 ENERGY
I
520 (eV)
Fig. 5. The X-ray induced O(KLL) Auger transitions for Al oxide layers 0.7 nm (curve a) and 3.2 nm (curve b) thick. The transition for the bulk AI,O, sample (curve c) is added for comparison. The parts of the difference spectra (curves b-a and b-c) indicate the variation in the shoulder of the 0( KLL) main transition.
based on the &( 0 1s) and Ek( Ocvv) data [ 371. In a similar way to Fig. 2, particular regions of prevailing chemical states are visible (the values of d,, are given in parentheses). This map also reflects the initial decrease in the extra-atomic relaxation energy with increasing d . The spectral shifts upon constant s~o ,s for d:: > 0.9 nm (SiO,) and 1.2 nm (Al,O,) represent the discussed Fermi level shifts within the oxide gaps. The excess oxygen content in the Si (Al) oxides for d,, > 1.2 (2.1) nm (see Fig. 2) was not observed with bulk oxides. We also did not detect a systematic decrease in the intensity of 0 1s photoelectron losses with increasing d,,. According to ref. 2, the observed oxygen excess is caused by the additional unresolvable signal of oxygen atoms which do not reach the interaction region during oxidation. With increasing d,, , we also observed a slight increase in the relative intensity of the high energy shoulder located 6-7 eV above the O(KL,,L,,) main transition (Fig. 5). This band-like part of the Auger spectra represents interatomic charge transitions between two neighbouring oxygen ions [22]. Therefore the excessive oxygen atoms might electronically influence the surrounding oxide. These charge transitions can thus explain the differences between the saturated values of A&,(0 Is-P,) and ~~ Is in Fig. 2(b) and Fig. 3 and those of the bulk reference oxides. That is, the effectively lower amount of a charge per oxygen atom transferred from Si or Al
atoms leads to a reduced A&(0 IsAP,). Screening and polarization effects are enhanced by the excessive oxygen atoms (an increase in the electronic part of the polarizability of the oxygen first neighbour [32]). This leads to an increase in our saturated values of cl0 ,s [22]. The above results agree with the d,,,-dependent evolution of the localization energy of holes on the oxygen atom [19]. For the O(KL,,L,,) Auger transition, we calculated the spin-dependent repulsion energies U(2p2p) between two holes in the final state using the model published in ref. 22. For the thinnest measured Al (Si) oxides, we obtained a U(2p2p) value of 6.4 eV (5.3 eV). With increasing d,, , the U( 2~2~) values increase (which reflects the stronger localization of the holes on the oxygen atom) and they reach saturated values of 7.8 eV for A&O, 1.5 nm thick and of 6.9 eV for SiOz 1.0 nm thick. For the bulk oxide samples, we determined U(2p2p) values of 8.0 eV(Al,O,) and of 7.2 eV (SiO*), which agree with the published data [19]. As in the case of a, ,s, with further increasing d,, the U(2p2p) values remain unchanged. This means that they are independent of subsequent oxide E, shifts which continue up to the Si and Al oxide thicknesses of 1.6 nm and 4 nm respectively. Thus the process of the decrease in the extra-atomic relaxation energy with increasing d,, is not affected by support-to-oxide potential shifts. Note that the excessive oxygen atoms cause charge exchange processes within the oxide layer. For a precise description of the oxide formed, the influence of these atoms on the calculated characteristics (a0 ,s and U(2p2p)) has to be considered. We shall describe now the d,, regions of the oxide support for which different interaction processes can be expected after metal deposition (see regions A, B and C in Fig. 3). The following survey is consistent with ours [38-401 and other recent studies of the metal-insulator-semiconductor (or metal) (M-I-S(M)) systems [ 15, 41-461. Region A. The thinnest oxide layers cannot hinder metal diffusion into the underlying support [40-421. Direct detection of this process (in the case of Pd by the chemical shift with Pd,Si formation [43]) can be limited for some metal deposits (e.g. Ni [40]). The penetrating metal atoms change the charge distribution and the chemical composition of the oxygen’s surroundings. This manifests itself in a change in the a0 ,s value. Region B. The charge transfer between the deposited metal and insulating substrate via M-I interface dipole formation can be studied by using the AEb(P2-PO) values. Their variations reflect changes in the charge balance within the oxide layer and the potential shift of the oxide with respect to the substrate [38, 39, 441.
T. J. Sarapatka
/ XPS of ultrathin
Region C. The metal deposited on these oxide supports is not affected by the electronic influence (field effect and charge tunnelling) of elemental Si or Al because of the thick oxide interlayer [38, 461. Thus it affords investigating growth processes and interactions of the deposited metals on insulating substrates with the help of still observable reference levels of elemental Si or Al under the oxide layers [15, 40, 45, 461.
oxide layers on Al and Si
7 8 9 IO 11 I2
4. Summary The initial stages of the Al,03/Al and SiO,/Si system formation were studied by XPS and XAES methods. The spectral shifts during the interface transition layer build-up are caused by the change in the charge transfer and relaxation effects. This process is finished for the Si (Al) oxide thickness of 0.9 nm (1.2 nm). With further growth of SiO, and A1203, the influence of the above effects is hindered while the oxide EF changes to thicknesses of 1.6 nm and 4 nm respectively. This is a result of variations in the density and population of the I-S(M) interface states. This effect should be considered when the charge transfer model is used for the description of the oxidation-induced Eb shifts. The relation between the Auger parameter and the hole localization and extra-atomic relaxation energies evaluated for bulk oxides is useful also for characterizing thin layers. These substrates eliminate difficulties caused by surface charging and can be successfully used for investigating metal-support interaction. While the diffusion of the deposited metal prevails for oxide thicknesses up to 1.2 nm, the charge transfer and tunnelling processes dominate in the interaction of the metal deposited on the thicker oxide interlayers.
Acknowledgment
I would like to thank Dr. Z. Bastl for critical comments and for careful reading of the manuscript.
I3 14 IS 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
References 1 T. Hattori, Thin Solid Films, 206 (1991) I. 2 G. Hollinger, Y. Jugnet, P. Petrosa and T. M. Due, Chem. Phys. Lett., 36 (1975) 441. 3 J. S. Johannessen, W. E. Spicer and Y. E. Strausser, J. Appl. Phys., 47 (1976) 3028. 4 F. Lutz, J. L. Bischoff, D. Bolmont and L. Kubler, Rev. Phys. Appl., 25 (1990) 923. 5 M. Oshima, H. Sugahara, N. Kawamura, N. Yabumoto and K. Minegishi, Jpn. J. Appl. Phys., 30 (1991) 116. 6 M. Niwano, H. Katakura, Y. Takeda, Y. Takakuwa, N.
39 40 41
42 43 44 45 46
223
Miyamoto, A. Hiraiwa and K. Yagi, J. Vat. Sci. Technol. A, 9 (1991) 195. K. Hirokawa, F. Honda and M. Oku, J. Electron Spec@osc. Relat. Phenom., 6 (1975) 333. M. Wlsche, E. Linke and K. Richter, Surf. Interface Anal., 4 (1982) 178. 1. P. Batra and L. Kleinman, J. Electron Spec@osc. Relal. Phenom., 33 (1984) 175. W. M. Mullins and B. L. Averbach, Surf. Sci., 206 (1988) 52. G. B. Frederick, G. Apai and T. N. Rhodin, J. Electron Spectrosc. Relat. Phenom., 54-55 (1990) 415. A. Bianconi, R. Z. Bachrach, S. B. M. Hangstrom and S. A. 6. Flodstriim, Phys. Rev. B, 19 (1979) 2837. F. J. Grunthaner, P. J. Grunthaner, R. P. Vasquez, B. F. Lewis, J. Maserjian and A. Madhukar, Phys. Rev. Letr., 43 (1979) 1683. L. H. Dubois, P. K. Hansma and G. A. Somorjai, Appl. Surf. Sci., 6 (1980) 173. G. B. Frederick, G. Apai and T. N. Rhodin, J. Am. Chem. Sot., 109 (1987) 4797. D. N. Belton and S. J. Schmieg, Appl. Surf. Sci., 32 ( 1988) 173. C. D. Wagner, Anal. Chem., 47(1975) 1201. H. Kuroda, T. Ohta and Y. Sato, J. Electron Spectrosc. Relat. Phenom., 15 ( 1979) 21. C. D. Wagner, D. A. Zatko and R. H. Raymond, Anal. Chem., 52 (1980) 1445. C. D. Wagner, H. A. Six, W. T. Jansen and J. A. Taylor, Appl. Surf: Sci., 9 (1981) 203. R. H. West and J. E. Castle, Surf. Interface Anal., 4 (1982) 68. G. Moretti, J. Electron Specfrosc. Relar. Phenom., 58 (1992) 105. P. Streubel, R. Franke, T. Chasse, R. Fellenberg and R. Szargan, J. Electron Spectrosc. Relar. Phenom., 57 ( 1991) I. J. Finster and D. Schulze, Phys. Status Solidi A, 68 (1981) 505. B. R. Strohmeier, Surf. Interface Anal., 15 (1990) 51. R. Jisl, Surf. Interface Anal., 15 (1990) 719. R. C. Weast (ed.), Handbook of Chemistry and Physics, 54th edn., CRC Press, Cleveland, OH, 1973, p. B03. D. R. Penn, J. Electron Spectrosc. Relar. Phenom., 9 (1976) 29. H. Ebel, C. Piihn, R. Svagera, M. E. Wernle and M. F. Ebel, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 109. S. Tanuma, C. J. Powell and D. R. Penn, Surf. Interface Anal., II (1988) 577. U. Hijfner, P. Morgen, W. Wurth and E. Umbach. Phys. Rev. B, 40(1989) 1130. G. Moretti, Surf: Interface Anal., 17 (1991) 352. T. L. Barr, Appl. Surf. Sci., 15 (1983) 1. M. H. Hecht, Phys. Rev. B, 41 (1990) 7918. A. Stockhausen, T. U. Kampcn and W. Miinch, Appl. Surf, Sci., 56-58 (1992) 795. J. H. Scofield, J. Electron Spectrosc. Relat. Phenom., 8 ( 1976) 129. C. D. Wagner, J. Electron Spectrosc. Relat. Phenom., 47 (1988) 283. T. J. Sarapatka, J. Electron Spectrosc. Relat. Phenom., 58 (1992) 233. T. J. Sarapatka, Surf. Sci., 275 (1992) 443. T. J. Sarapatka, J. Electron Specwosc. Relat. Phenom. submitted for publication. R. T. Dillingham, A. R. Chourasia, D. R. Chopra, S. R. Martin, K. L. Peterson and C. Z. Hu, J. Vat. Sci. Technol. A, 5 (1987) 3340. T. Asakawa, K. Tanaka and I. Toyoshima, Langmuir, 4 ( 1988) 521. B. Schleich, D. Schmeissec and W. GBpel, Surf. Sci., 191 (1987) 367. X. D. Li and L. Z. Zeng, Phys. Status Solidi A, 121 (1990) K185. Z. Bastl, Commun Dep. Chem. Bulg. Acad. Sci., 22 (1989) 173. H.-J. Anklam, R. Mattheis and F. Thrum, Surf. Interface Anal., 14 (1989) 433.
219
X-ray photoelectron spectroscopy study of ultrathin oxide layers on Al and Si substrates T. J. Sarapatka J. Heyrovskj Institute of Physical 182 23 Prague 8 (Czechoslovakia) (Received
October
Chemistry
16, 1992; accepted
and Electrochemistry,
November
Czechoslovak
Academy
of Sciences, Dolejtkova
3,
25, 1992)
Abstract The initial stages of oxide growth on Al and Si are studied by X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy methods. The change from initial Si-0 and Al-O bonding through interface layer formation to SiO, and Al,O, oxide growth are characterized in terms of charge transfer, relaxation energy and particular bonding together with oxide Fermi level shifts. The relation between the Auger parameter and hole localization or extra-atomic relaxation energies is discussed, taking into account the influence of excessive oxygen atoms presented in the oxide layer. The advantages of using these substrates for the preparation of a model catalyst and for investigating the metal-oxide support interaction are discussed. While the diffusion of deposited metal prevails for oxide thicknesses up to 1.2 nm, charge transfer and tunnelling processes dominate in the interaction of metal deposited on thicker oxide interlayers.
1. Introduction Thin insulating layers have attracted considerable attention in investigating supported dispersed metal catalysts. This is because these substrates do not usually exhibit a charging effect upon X-ray irradiation. Many studies have been done on surface and interface investigation of ultrathin oxide layers formed on Si [l-6] and Al [ 7- 121 supports. The growth, morphology and electron structure of the oxides have been determined from various electron spectroscopy results. Extra-atomic relaxation and charge transfer effects were considered as origins of oxidation-induced binding energy & shifts of substrate core levels [ 1, 2, 5, 131. However, the certain instability of these substrates in the gas adsorption experiments [ 141 and significant shifts of their core levels after metal deposition [ 15, 161 need further studies. Auger parameters [ 171 are widely used for characterizing particular chemical states of elements [2, 31 as they are independent of sample surface charging. Recently, detailed discussions on the Auger parameter have shown its connection with the local geometry, polarization energy and refractive index of the bulk materials [ 18-231. In this study, X-ray photoelectron spectroscopy (XPS) and X-ray-induced Auger electron spectroscopy (XAES) were used to characterize the initial stages of Al and Si oxide growth. The evolution of the spectral characteristics is compared with those of bulk oxide
0040-6090/93/%6.00
samples to determine the variations in relaxation energy and the effect of oxide Fermi level shifts. The influence of the oxide overlayer thickness on its interaction with the deposited metal is discussed.
2. Experimental
details
Experiments were carried out in a VG ESCA 3 Mk II spectrometer using non-monochromatic Al Ka excitation at a pressure lower than 2 x lo-’ Pa. Calibration of the spectrometer was set up to an Eb for Au 4f,,z of 84.0 eV (full width at half-maximum (FWHM), 1.2 eV). The estimated error in binding energy determination was fO.l eV. We used n-Si( 111) doped with 6 x lOi P atoms cmm3 and polycrystalline Al foil (Hicol; purity, 99.999%) substrates. SiOz( 1340) and A&0,( lOi2) samples were used as bulk standards. They were cleaned by argon sputtering and subsequent annealing at 1000 K. Si oxides were prepared in situ at 900 + 50 K upon oxygen exposures from 4 to 4 x lo5 langmuirs. Room temperature or elevated temperatures up to 700 K were used for the growth of A&O,. The X-ray photoelectron spectra were recorded 15 min after cooling the samples to the room temperature. The Si(A1) 2p spectra after subtraction of a linear background were fitted by three Gauss-Lorentz curves (Fig. 1). The particular peaks correspond to elemental Si or Al (peak P,), an interface Si suboxide or a
0
1993 -
Elsevier
Sequoia.
All rights
reserved
T. J. Sarapatka
Si
d ox=l.l
2~
dox
/ XPS of‘ultrathin
oxide
layers on Al and SI
[26] agreed for the thickness measured oxides with the determined do, = d, + d2 to within + 10%):
nm
SioI
d,
Si
_;__ b pz
'1
1
I
I
I
100
01
1
s
BINDING
1
I
I
I
I
110
105
ENERGY
I
I
I 115
(eV)
Fig. I Illustration of the peak fitting into three contributions for the Si 2p spectrum. For this fit, the resulting FWHMS of the particular peaks P,, P, and P, were 1.5 eV, 1.7 eV and 1.9 eV respectively. The meaning of the particular oxide thicknesses is demonstrated in the inset
precursor chemisorbed phase of Al (peak P, ) [ 9, 241 and SiO, or Al,O, on the top (peak Pz) [6, 9, 241. For Si oxides thicker than 0.6 nm, the observed energy separation between the Si 2p peaks P, and P, of about 2 eV agrees with the Eb shift between the emissions from SiO and SiO, [6]. The Al 2p peak P, was shifted by 1.4 eV towards a higher Eb and was observed only at preparation temperatures below 450 K. A decrease in the P,-to-P, peak intensity ratio for more glancing angles of photoelectron collection reflects the predominant SiO, and/or A&O3 growth above the interface layer. Assuming layer-by-layer growth, we calculated the mean thickness d, and d2 of the particular oxide layers, in Fig. 1, from the following equations [24, 251 (the values of the oxide thickness obtained by an alternative method based on the angle-dependent XPS peak areas
TABLE
Si Al
1. Material
constants
used in eqns.
PO ( x IO1 kgm-‘)
PI
2.4 2.7
2.3 2.8
K2+
K3
+
0
I
Here I is the integral intensity of the particular Al 2p or Si 2p peaks, a denotes the signal from the bulk samples and 8 is the take-off angle of photoelectrons measured from the sample surface. The inelastic mean free paths 2 and densities p used in eqns. ( 1) -( 3) are listed in Table 1 together with the values of K, and K, [24]. We did not detect surface static charging on our measured oxide layers.
3. Results and discussion The dependences of the energy separation AEb(P2PO) between the Si(A1) 2p peaks in the elemental and oxide form and the energy separation A&(0 Is-P,) between the 0 1s and Si or Al P, peaks, on the oxide thickness are shown in Figs. 2(a) and 2(b) respectively. Different build-up processes occur during the initial stages of Si and Al oxide formation [2, 9, 10, 131. The continuous transfer of electrons from Si to oxygen atoms [ 1, 131 associated with an increase in the Si-0 bond length (from 1.39 to 1.62 A [ 3 11) leads both to the initial increase in A&,(Pz-PO) and to the decrease in AEb(O Is-P,). For the thinnest Al oxides, the dependences in Fig. 2 reflect the gradual change from the initial formation of Al-O resonance-type bonds [9] to those found in Al,O, with reduction in the Al-O bond length from 2.2 to 1.9 8, [32]. The curve shapes in Fig. 2(b) suggest that the above initial processes are finished for Si and Al oxide thicknesses of 0.8 nm and of 1.2 nm respectively. This agrees
(l-3) [27-301
yi 2.2 2.9
lo3 kg m-‘)
*0
i .I
i”,
(nm)
(nm)
(nm)
2.36” 2.58
2.53” 2.67
2.70” 2.75
‘Note that using significantly higher I values for Si species [30], i.e. i, = 3.15 nm, L, = 3.44 nm and I., = 3.73 nm, increases 1.32 times. For detailed discussion see refs. 24 and 25.
K,
KZ
0.61 0.65
0.43 0.56
the calculated
d,.
T. J. Sarapatka
4.2
1042.4
-
SiO2
-
8 3.2
~*-%%_
-
N2.2 & D
1041.6
d
;
: a0 I
-5
d
‘%.q
1
ul
.\ ‘\
a” I,.,,,,,.III.......,I,,,,.,.,II,,,.,..,,I,,I......, 1 2 0 oxide
(a) 457.2
thickness
.z
456.6
1040.0
3Lxyx
\ 4
3
5
‘:x
1039.2
(nm)
1038.4
/F#
B
3_c-A’2o3
i,~.......,........,,.........,.........,......... 0
3-
3
2
1 oxide
_+--*X$-x-r-______,
:
*a-~~-*---__*
A;
3
-Al20 9
z 8
x
-Al,0 1.2
221
oxide layers on Al and Si
1 Omw-o
% -
1 XPS of ultrathin
thickness
5
4 (nm)
Fig. 3. The dependence of the clo IS values on the oxide thickness for SiO,/Si ( 0) and AI,O,/AI ( x) samples. The corresponding dependences between the oxide stoichiometry and a, ,E are shown in the inset. 510
509
9
.A! Fig. 2. Dependences of (a) the energy separation A&(Pz-PO) between the Si(AI) 2p peaks in the elemental and oxide form and (b) the energy separation A.!&(0 Is-P,) between the 0 1s and Si(AI) 2p peaks P(2), on the oxide thickness for SiOz/Si (0) and Al,O,/AI ( X) samples.
g
506
9 LA+
well with the 50, interface determined to be about 1 nm in other experiments [l, 331. The final reduction in AEb(P2-PO) at the SiO,-Si interface (see Fig. 1) does not occur if an Si support of a low dopant concentration is used [2, 13, 241. For our Si sample, X-ray-induced minority carriers [ 341 increase the positive potential of the oxide layer. With increasing d,,,, their effective influence is reduced and saturates for d,,, > 1.6 nm. The continuous decrease in AEb( P,--P,) with increasing Al oxide thickness above 2 nm reflects the change in the Fermi level location within the oxide gap EF(AlzO,) due to a decrease in donor-like interface states [ 121. &cording to the published correlation [lo], the initial values of AE,(P,-P,) in Fig. 2(a) correspond to the reactive surface with an E,(A1103) of 4.6 eV above the conduction band (CB) edge. With d,, increasing from 2 to 4 nm, E,(Al,O,) decreases to the final value of 3.2 eV above the CB. The evolution of the Auger parameter ao ,s = Eb(O 1s) + Ek(Ocvv) with increasing oxide thickness is shown in Fig. 3. Three oxidation steps [9] with the a0 ,s values of about 1040.6, 1039.9 and 1039.2 eV can be seen for the Al*O,/Al system. The initial dissociative
507
1 533
532 Eb (0 1s)
531 (ev)
Fig. 4. The Wagner chemical state plot derived from the oxygen energies for SiOJSi (0) and AI,O,/Al ( x ). &(O Is) and &(Ocvv) The significant values of the oxide thickness are indicated on the curves.
chemisorption of oxygen on Si [35] is characterized by an a0 ,s of 1041.8 eV. On completion of the first monolayer, the continuous Si suboxide growth is manifested by the decrease in a0 Is to 1041.1 eV. This is followed by the formation of SiOp as d,, exceeds 0.9 nm which results in an a0 ,s of 1040.2 eV. The transition from single Al-O bonding to A&O3 growth and a rather continuous process going through the particular suboxides to SiOZ can be seen also from the plotted dependence between the oxide stoichiometry [24, 361 and a0 Is, shown in Fig. 3. Additional information can be obtained from the Wagner chemical state plot presented in Fig. 4 which is
T. J. garapatka / XPS of ultrathin oxide layers on Al and Si
1
1
b- c
I
+_,c”-w
, I
,
500 KINETIC
510 ENERGY
I
520 (eV)
Fig. 5. The X-ray induced O(KLL) Auger transitions for Al oxide layers 0.7 nm (curve a) and 3.2 nm (curve b) thick. The transition for the bulk AI,O, sample (curve c) is added for comparison. The parts of the difference spectra (curves b-a and b-c) indicate the variation in the shoulder of the 0( KLL) main transition.
based on the &( 0 1s) and Ek( Ocvv) data [ 371. In a similar way to Fig. 2, particular regions of prevailing chemical states are visible (the values of d,, are given in parentheses). This map also reflects the initial decrease in the extra-atomic relaxation energy with increasing d . The spectral shifts upon constant s~o ,s for d:: > 0.9 nm (SiO,) and 1.2 nm (Al,O,) represent the discussed Fermi level shifts within the oxide gaps. The excess oxygen content in the Si (Al) oxides for d,, > 1.2 (2.1) nm (see Fig. 2) was not observed with bulk oxides. We also did not detect a systematic decrease in the intensity of 0 1s photoelectron losses with increasing d,,. According to ref. 2, the observed oxygen excess is caused by the additional unresolvable signal of oxygen atoms which do not reach the interaction region during oxidation. With increasing d,, , we also observed a slight increase in the relative intensity of the high energy shoulder located 6-7 eV above the O(KL,,L,,) main transition (Fig. 5). This band-like part of the Auger spectra represents interatomic charge transitions between two neighbouring oxygen ions [22]. Therefore the excessive oxygen atoms might electronically influence the surrounding oxide. These charge transitions can thus explain the differences between the saturated values of A&,(0 Is-P,) and ~~ Is in Fig. 2(b) and Fig. 3 and those of the bulk reference oxides. That is, the effectively lower amount of a charge per oxygen atom transferred from Si or Al
atoms leads to a reduced A&(0 IsAP,). Screening and polarization effects are enhanced by the excessive oxygen atoms (an increase in the electronic part of the polarizability of the oxygen first neighbour [32]). This leads to an increase in our saturated values of cl0 ,s [22]. The above results agree with the d,,,-dependent evolution of the localization energy of holes on the oxygen atom [19]. For the O(KL,,L,,) Auger transition, we calculated the spin-dependent repulsion energies U(2p2p) between two holes in the final state using the model published in ref. 22. For the thinnest measured Al (Si) oxides, we obtained a U(2p2p) value of 6.4 eV (5.3 eV). With increasing d,, , the U( 2~2~) values increase (which reflects the stronger localization of the holes on the oxygen atom) and they reach saturated values of 7.8 eV for A&O, 1.5 nm thick and of 6.9 eV for SiOz 1.0 nm thick. For the bulk oxide samples, we determined U(2p2p) values of 8.0 eV(Al,O,) and of 7.2 eV (SiO*), which agree with the published data [19]. As in the case of a, ,s, with further increasing d,, the U(2p2p) values remain unchanged. This means that they are independent of subsequent oxide E, shifts which continue up to the Si and Al oxide thicknesses of 1.6 nm and 4 nm respectively. Thus the process of the decrease in the extra-atomic relaxation energy with increasing d,, is not affected by support-to-oxide potential shifts. Note that the excessive oxygen atoms cause charge exchange processes within the oxide layer. For a precise description of the oxide formed, the influence of these atoms on the calculated characteristics (a0 ,s and U(2p2p)) has to be considered. We shall describe now the d,, regions of the oxide support for which different interaction processes can be expected after metal deposition (see regions A, B and C in Fig. 3). The following survey is consistent with ours [38-401 and other recent studies of the metal-insulator-semiconductor (or metal) (M-I-S(M)) systems [ 15, 41-461. Region A. The thinnest oxide layers cannot hinder metal diffusion into the underlying support [40-421. Direct detection of this process (in the case of Pd by the chemical shift with Pd,Si formation [43]) can be limited for some metal deposits (e.g. Ni [40]). The penetrating metal atoms change the charge distribution and the chemical composition of the oxygen’s surroundings. This manifests itself in a change in the a0 ,s value. Region B. The charge transfer between the deposited metal and insulating substrate via M-I interface dipole formation can be studied by using the AEb(P2-PO) values. Their variations reflect changes in the charge balance within the oxide layer and the potential shift of the oxide with respect to the substrate [38, 39, 441.
T. J. Sarapatka
/ XPS of ultrathin
Region C. The metal deposited on these oxide supports is not affected by the electronic influence (field effect and charge tunnelling) of elemental Si or Al because of the thick oxide interlayer [38, 461. Thus it affords investigating growth processes and interactions of the deposited metals on insulating substrates with the help of still observable reference levels of elemental Si or Al under the oxide layers [15, 40, 45, 461.
oxide layers on Al and Si
7 8 9 IO 11 I2
4. Summary The initial stages of the Al,03/Al and SiO,/Si system formation were studied by XPS and XAES methods. The spectral shifts during the interface transition layer build-up are caused by the change in the charge transfer and relaxation effects. This process is finished for the Si (Al) oxide thickness of 0.9 nm (1.2 nm). With further growth of SiO, and A1203, the influence of the above effects is hindered while the oxide EF changes to thicknesses of 1.6 nm and 4 nm respectively. This is a result of variations in the density and population of the I-S(M) interface states. This effect should be considered when the charge transfer model is used for the description of the oxidation-induced Eb shifts. The relation between the Auger parameter and the hole localization and extra-atomic relaxation energies evaluated for bulk oxides is useful also for characterizing thin layers. These substrates eliminate difficulties caused by surface charging and can be successfully used for investigating metal-support interaction. While the diffusion of the deposited metal prevails for oxide thicknesses up to 1.2 nm, the charge transfer and tunnelling processes dominate in the interaction of the metal deposited on the thicker oxide interlayers.
Acknowledgment
I would like to thank Dr. Z. Bastl for critical comments and for careful reading of the manuscript.
I3 14 IS 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
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