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Epitaxial and electronic structures of ultra-thin copper films on MgO crystal surfaces
Surface
934
EPITAXIAL AND ELECTRONIC STRUCTURES COPPER FILMS ON MgO CRYSTAL SU~ACES Jian-Wei
HE * and Preben
l3epurtmentof P!tysitzrl .FUnrrersitelsparken, Received
20 March
Chemisty,
OF ULTRA-THIN
J. M0LLER Ff. C. Qrsted Instrtute.
DK-2100 Copenhagen 19X6: accepted
Science 17X (19X6)934-941 North-Holland. Amsterdam
Universi
of Copet7kaget7,
0, Dennwh
for publication
2 June 1986
The deposition of Cu on MgO(100) and MgO(ll1) surfaces by electron beam evaporation technique at room temperature in UHV was studied by LEED, AES and EELS. An epitaxial growth was seen. already from the initial deposition stage, of Cu(100) orientation both on freshly cleaved and on Ar-” bombarded MgO(100) surfaces and of Cu(ll1) orientation on freshly cleaved MgO( 111) surfaces. The changes in the electron energy loss spectra with copper coverage are discussed and a model for copper epitaxial growth on MgO crystal surfaces is proposed.
1. Introduction The surfaces of crystalline metal oxides have been studied quite extensively [l]. Due to experimental difficulties because of its highly insulating nature, only a limited number of studies have been carried out on MgO surfaces, however, and even less work on metal deposition on MgO crystal surfaces. Among these it is the (100) cleaved plane that has been the favoured target 12-71. Palmberg et al. [2] thus found epitaxial growth of Au and Ag, Kanagi et al. 13.51 and Hubert and Gilles [7] of Fe, Shigeta and Maki 161 of Ag, and Lord and Prutton [4] of Cu on MgO(100). The latter, whose studies also involved alkali halides, discussed the epitaxial growth in terms of the critical accommodation cluster centre theory that was deduced by considering the potential of lowest energy in a hard-sphere model of both substrate and deposited atoms [8]. The observed deviation, from the theoretical prediction, of Cu(ll0) epitaxial growth on MgO(l~) was explained as due to the high valency of the ions in the MgO(lO0) surface [4]. In the present paper experimental low energy electron diffraction (LEED). electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES) results of Cu thin layer deposition upon Mg(l00) and MgO(ll1) * Permanent address: Chemistry department of East China Institute Engineering, Shanghai. People’s Rep. of China.
0039-6028/~6/$03.50 (9 Elsevier Science Publishers (North-Holland Physics Publishing Division)
of Chemical
B.V.
Technology
and
J.-W.
He,P.J. MsIIer / Epitaxiai and electtonic sr~eturesof Cu on MgO
935
surfaces are reported and a disagreement with previous [4] results regarding the epitaxial relationships is found.
2. Experimental procedure The Cu deposition was carried out in a 6 X lo-l1 mbar base pressure ultra-high vacuum chamber upon (99.995% pure, W. & C. Spicer & Co. Ltd., UK) single crystals of MgO, which had been cleaved in air along the (100) and (111) planes and subsequently introduced into the vacuum after cleaving. Through heating to 340°C for 10 h clean and well-structured surfaces were obtained as indicated by their Auger spectra and excellent LEED patterns. By electron beam evaporation technique (model ESVZ evaporator, LeyboldHereaus GmbH) thin layers of Cu were deposited at a pressure of less than 2 x 10e9 mbar. The Cu source was of 99.995% purity (Johnson Matthey Ltd., UK). The thickness of the Cu deposits was monitored by a quartz crystal microbalance, located in the vapour beam next to the MgO target. The deposition was carried out at room temperature and at a rate of 1 A mm’. The EELS spectra were obtained by using an Auger cylindrical mirror analyzer with a primary energy of 97 eV, a modulation peak-to-peak voltage of 1 V and at perpendicular incidence of the primary electron beam. The argon bombardment was carried on for 0.5 h at a 0.5 keV acceleration voltage and a 10 PA cm-2 target current. The purity of the Ar and of the CO gas was 99.999% and 99.998% respectively.
3. Results and discussion Fig. 1 demonstrates the growth of Cu films as followed by LEED. During the film growth the sharp LEED pattern from the freshly cleaved Ago surface was gradually blurred until* the substrate pattern was completely eliminated at a layer thickness of 8 A (i.e. at a distribution of islands with a total average thickness of 8 A). At a deposit thickness of 15 A a weak Cu(100) pattern appeated, which upon further growth became sharper. Fig. la thus gives the 100 A deposit LEED pattern, which we find to be of good quality, considering the lack of an annealing process. Fig. lb shows the sharp LEED pattern of clean Mgql~) surface as a reference. The excellent LEED pattern is due to good stability of this surface. Even when the MgO(lOO) had been exposed to an argon ion bombardment, a MgO(100) LEED pattern still existed. This stability against Ar+ bombardment agrees with previous observations [9]. For the Ar+ bombarded surface we also observed Cu(100) epitaxial growth showing a picture similar to fig. la. On a freshly cleaved MgO(ll1) surface an epitaxial growth of Cu(ll1) was
Fig.
1. LEED patterns of (a) Cu(100) on MgO(100) with a Cu thickness of 100 A. primarc incident electron energy E, = 198.4 eV. (b) clean MgO(100) at I?, = 149.0 eV.
observed upon deposition of approximately 14 A of Cu. The Cu LEED pattern sharpened with increasing average layer thickness. Fig. 2a gives the Cu(ll1) structure after deposition of 64 A Cu. and the LEED pattern of clean MgO( 111) is inserted as fig. 2b. It is well known that MgO is a highly insulating material which is generally thought to be studied by electron spectroscopy only with difficulty. but on a well-ordered clean MgO surface the charging of the surface is saturated quickly when accompanied by secondary emission [lo].
Fig. 2. LEED patterns
of (a) Cu(lll) on MgO(ll1) with Cu thickness (b) clean MgO(ll1) at EP = 202 eV.
of 64 A, at E, = 31X.2 cV.
For neither of these three surfaces any superstructures were observed during the room temperature film growth without any annealing. In the case, where a 80 A Cu cpitaxially grown film was exposed to a poor vacuum (of
40 30 2a 10 0 Energy toss E, (eV! Fig. 3. EELS spectra of Cu/MgO(lOO) surfaces. Curve (1): clean MgO(100). Curves (2) to (4): 0.7, 8.4 and 60.4 A Cu deposits, respectively. Curve (5): CO oxidized, 60 .k film on MgQ(100). Curve (6): clean MgO(1.11).
038
i
I
0
1
I
5
10
A
15
Cu thickness(iI
Fig. 4. The change (0)
R(Cu/Mg)
=
of the ratios
(Ic,,,zu
of the Auger signal
cv,,‘&.)/(I,,,,,,,
,“,/sM&)
intensities and
(A)
with R(Cu/O)
Cu thxkntx = (1,
“,‘)a,
increaae. c\.)//.%.u)/’
I “,r,,?Lw/s3).
about 2 x lo-’ mbar) for several hours, no LEED pattern was observed upon 0 a subsequent further 100 A deposition. The LEED structure in this case apparently was destroyed through chemisorption of residual gases. The gradual changes in EELS structures as a function of copper coverage during Cu growth on the MgO(100) are shown in fig. 3. Fig. 4 gives the Auger ratios of the copper to magnesium and copper to oxygen intensities, W&” = ;U$cU , R(Cu/O) = I. 0 0 Mg % where Z is the peak-to-peak heights in the corresponding AES spectra and S the relative Auger sensitivities of Cu, Mg and 0. The AES spectrum for the MgO(l11) clean surface of the same crystal showed a Mg/O ratio of 0.414, which by comparison with the 0.34 ratio of the MgO(100) suggests a Mg surface ion enrichment in the Mg(ll1) surface layers. A comparison of the observed EELS spectral changes (fig. 3) with energy loss values of the clean MgO(100) agrees well with previous findings [93 as seen from table 1. which also includes a reference to a band structure R (Cu/Mg)
of electron
a) Band structure
calculation
2.0
4.1 5.1 4.3 4.5 4.9 4.5 4.3
2.9 2.5 2.8
60 A Cu/MgO(lOO) cu a’ Cu(lll) Cu(311) CO/Cu/MgO(lOO) CO/CU(311) cuo/cu(1oo) cu,o/cu(1cO)
2.8
4.1
4.5
2.9
2.2
2.6 2.3 2.4
Loss energies (eV)
8.4 A Cu/MgO(lOO)
0.7 A Cu/MgO(lOO)
MgO(ll1)
MgO(lW MgWW
Surface
5.9
5.9
5.7
6.5
6.2 6.1
6.8
7.0
6.8
6.7
7.7
7-9
8.9
9.2
8.9
10.3
10.5
9.8 11.6
11.5 10.4
9.2 9.1
11.5 11.4 11.2
9.5 9.0 8.3
- 12.5
12.2
13.5 13.7
14.4
14.3 14.4 14.1
17.5
17.4 17.6 17.6
1131 U41 [I41
[ill [I21 U31 Present study
Present study
Present study
Present study
22.4
Present study (91 Present study
Ref.
&posit (A) of CUon
22.4 22.2 22.2
energy loss spectroscopy (EELS) peak values E, (in eV) for surfaces of clean MgO( 100) and MgO( 111). and for surfaces of crystals and Cu(I) of CU(II) oxides
MgO(100).oxidized CU on MgO(l00)
Comparison
Table 1
calculation for Cu [ll] and to EELS literature values for Cu [12,13] and for Cu ,O and CuO [14]. Quite large changes of the observed EELS structure with increasing Cu deposition were seen (fig. 3). At approximately 0.7 A (average) Cu deposition the strong resonance peaks at 2.6 eV for the MgO(100) surface (which was also previously seen at 2.3 eV 191) and at 2.2 eV for the MgO(ll1) surfaces. both were eliminated and two new peaks arose at 2.2 eV and 4.5 eV from the Cu deposited MgO(100) surface (fig. 3, curve 2). This stage presumably shows that the first small Cu deposits may be bound to the oxygen ligands as ions and preferably stick to Mg*+ vacancy sites causing the surface defect peak in the spectrum to disappear and two new resonance peaks which have character of copper oxides to appear due to Cu impurity level related electronic transitions fl5]. As a detailed study on defect center structures has previously been published ]15] we shall only briefly discuss these here. Compared to the EELS spectrum of the clean MgO(100) (fig. 3, curves 1 and 6, respectively) the MgO(ll1) spectrum only shows a small difference for the 6.1 eV surface state related peak, whose intensity is a little stronger, and for the surface defect peak, whose intensity is a little weaker. Noting that the initial small 0.1 deposition has the same influence on the EELS structure as does the change from the (100) to the (111) surface, the latter being a polar surface richer in Mg, we further gain support for the suggestions that the initial copper deposits have a high possibility of interaction with V,- centers on the MgO surfaces. At an average coverage of about 3-5 A a drastic change in the EELS spectrum of MgO has occurred. This is perhaps due to the very high scattering cross section of the Cu3d valence electron level [16] and to the low primary incident energy with a very low electron mean free path. Already at an average coverage of about 5 A the spectra show some of the characteristic bulk copper loss features. At average thicknesses above 15 A, corresponding to a few monolayers of Cu. the EELS spectra just show the features of bulk copper, and by continued deposition the structures do not change much. We furthermore note that the spectrum of the Cu epitaxial film oxidized by CO (fig. 2, curve 5) agrees well with that of oxidized copper crystals [II]. The geometry of the atomic arrangement and misfit considerations are important in a epitaxial growth process, but not the only important parameters. Factors such as substrate temperature, contamination and film thickness are key parameters as well [17]. In the present study. however, both the experiment and recent theoretical calculations 1181 propose that magnesium vacancy sites on the MgO(100) surface are trapping centers for the initial copper deposits. It is reasonable, then, to suspect that the trapped copper ions are active sites for the subsequent nucleation process. The copper atoms may bind to the oxygen ions on the surface in a bridge configuration subsequently leading to a Cu(100) epitaxial growth.
J.-W. He, P.J. Mailer / E~~tux~uland electronic startles
of Cu on MgO
941
The simple parallel relations both for Cu(lOO)/Mg(lOO) and for Cu(lll)/MgO(lll), i.e. no relative rotations, have been found from these experiments through detailed analysis of the corresponding LEED patterns. A possible 30° rotation of the Cu(ll1) film relative to the MgO(ll1) substrate surface was ruled out by noticing the higher primary energy of the incident electrons on the Cu(ll1) thin film. The performance of the Auger signal ratios (fig. 4) as the Cu deposit thickness increases clearly indicates a two stage growth mechanism. It seems that the growth mechanism fits the Stranski-Krastanov model, so that the crystallites form after the completion of the first monolayer 0 in a two-dimensional growth process. The critical point being near the 5 A Cu thickness is seen in fig_ 4. Based uponOthe observed appearance of the 2.2 and 4.5 eV EELS peaks at the initial 0.7 A average Cu/Mgql~) spectrum and referring to the work of Benndorf et al. [14] on the spectroscopic identification of Cu(1) and Cu(I1) oxides we suggest that Cu(I1) is the corresponding state of the copper atoms fitting into the Vs- centers, and that the copper atoms forming the bridge configuration to the oxygen ions are in the Cu(1) state. Similarly cation vacancies will be occupied by deposited copper bounded to three anions on the MgO(ll1) surface, thus forming nucleation centers for Cu(ll1) epitaxial growth. The proposed mechanism for the epitaxial Cu growth on MgO crystal surfaces is thus as follows. Initially the arriving Cu deposits interact with magnesium surface vacancies and behave as active centers for the subsequent nucleation. This corresponds to the first stage of the growth mechanism. By further deposition copper forms clusters around the active center leading to an epjtaxial growth of a Cu film, whose structure can be detected by LEED. The clustering process yields the uniform changes of the Auger intensity ratios in fig. 4.
4. Conclusions
From the experimental studies on the Cu deposition on cleaved MgO(100) and MgO(ll1) surfaces at room temperature in UI-IV epitaxial growth of Cu has been observed on both the MgO(100) and the MgO(ll1) surface with parallel Cu(100) and Cu(ll1) o~entations, respectively. The initial growth follows the Stranski-Krastanov model. The submonolayer Cu deposits are supposed to interact with Mg2+ vacancies and subsequently to act as the active nucleation centers in the epitaxial growth of the ultra-thin film as elucidated from the corresponding electron energy loss spectra.
Acknowledgements The support of the Danish Natural Science Research Council, the Carlsberg Foundation, and the Thomas B. Thrige Foundation and valuable comments by Professor E.I. Solomon are gratefully acknowledged. One of us (J.-W. He) is indebted to the Education Committee of the People’s Republic of China for its support.
References [I] [2] [3] [4] [5] [6] [7] [X] [9] [IO] [ll] [12] [13] [14] (151 [16] [17] [1X]
V.E. Henrich, Rept. Progr. Phys. 48 (1985) 14X1. P.W. Palmberg, T.N. Rhodin and C.J. Todd. Appl. Phys. Letters 11 (1967) 33. T. Kanagi, K. Asamo and S. Nagata, Vacuum 23 (1973) 55. D.G. Lord and M. Prutton, Thin Solid Films 21 (1974) 341. T. Kanagi, T. Kagotani and S. Nagata. Thin Solid Films 32 (1976) 217. Y. Shigeta and K. Maki, Japan. .I. Appl. Phys. 16 (1977) 845. R.A. Hubert and J.M. Gilles. Appl. Surface Sci. 22/23 (1985) 631. C.A.O. Henning and J.S. Vermaak, Phil. Map. 22 (1970) 281. V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Phys. Rev. B22 (1980) 4764. C.C. Chang, in: Characterization of Solid Surfaces, Eds. P.F. Kane and C;.B. Larahcc (Plenum, New York, 1974). R. Lesser. N.V. Smith and R.L. Benbow, Phys. Rev. B24 (1981) 1895. L.H. Jenkins and M.F. Chuang, Surface Sci. 26 (1971) 151. H. Papp, Surface Sci. 63 (1977) 182. C. Benndorf. H. Caus. B. Egert, H. Seidel and F. Thieme, J. Electron Spectrox. Related Phenomena 19 (1980) 77. J.-W. He and P.J. Meller. Chem. Phys. Letters 129 (1986) 13. G. Rossi and I. Lindau. Phys. Rev. B2X (19X3) 3597. K.L. Chopra, Thin Film Phenomena (McGraw-Hill, New York. 1969) p_ 225. N.C. Bacalis and A.B. Kunz, Phys. Rev. 832 (1985) 4857.
934
EPITAXIAL AND ELECTRONIC STRUCTURES COPPER FILMS ON MgO CRYSTAL SU~ACES Jian-Wei
HE * and Preben
l3epurtmentof P!tysitzrl .FUnrrersitelsparken, Received
20 March
Chemisty,
OF ULTRA-THIN
J. M0LLER Ff. C. Qrsted Instrtute.
DK-2100 Copenhagen 19X6: accepted
Science 17X (19X6)934-941 North-Holland. Amsterdam
Universi
of Copet7kaget7,
0, Dennwh
for publication
2 June 1986
The deposition of Cu on MgO(100) and MgO(ll1) surfaces by electron beam evaporation technique at room temperature in UHV was studied by LEED, AES and EELS. An epitaxial growth was seen. already from the initial deposition stage, of Cu(100) orientation both on freshly cleaved and on Ar-” bombarded MgO(100) surfaces and of Cu(ll1) orientation on freshly cleaved MgO( 111) surfaces. The changes in the electron energy loss spectra with copper coverage are discussed and a model for copper epitaxial growth on MgO crystal surfaces is proposed.
1. Introduction The surfaces of crystalline metal oxides have been studied quite extensively [l]. Due to experimental difficulties because of its highly insulating nature, only a limited number of studies have been carried out on MgO surfaces, however, and even less work on metal deposition on MgO crystal surfaces. Among these it is the (100) cleaved plane that has been the favoured target 12-71. Palmberg et al. [2] thus found epitaxial growth of Au and Ag, Kanagi et al. 13.51 and Hubert and Gilles [7] of Fe, Shigeta and Maki 161 of Ag, and Lord and Prutton [4] of Cu on MgO(100). The latter, whose studies also involved alkali halides, discussed the epitaxial growth in terms of the critical accommodation cluster centre theory that was deduced by considering the potential of lowest energy in a hard-sphere model of both substrate and deposited atoms [8]. The observed deviation, from the theoretical prediction, of Cu(ll0) epitaxial growth on MgO(l~) was explained as due to the high valency of the ions in the MgO(lO0) surface [4]. In the present paper experimental low energy electron diffraction (LEED). electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES) results of Cu thin layer deposition upon Mg(l00) and MgO(ll1) * Permanent address: Chemistry department of East China Institute Engineering, Shanghai. People’s Rep. of China.
0039-6028/~6/$03.50 (9 Elsevier Science Publishers (North-Holland Physics Publishing Division)
of Chemical
B.V.
Technology
and
J.-W.
He,P.J. MsIIer / Epitaxiai and electtonic sr~eturesof Cu on MgO
935
surfaces are reported and a disagreement with previous [4] results regarding the epitaxial relationships is found.
2. Experimental procedure The Cu deposition was carried out in a 6 X lo-l1 mbar base pressure ultra-high vacuum chamber upon (99.995% pure, W. & C. Spicer & Co. Ltd., UK) single crystals of MgO, which had been cleaved in air along the (100) and (111) planes and subsequently introduced into the vacuum after cleaving. Through heating to 340°C for 10 h clean and well-structured surfaces were obtained as indicated by their Auger spectra and excellent LEED patterns. By electron beam evaporation technique (model ESVZ evaporator, LeyboldHereaus GmbH) thin layers of Cu were deposited at a pressure of less than 2 x 10e9 mbar. The Cu source was of 99.995% purity (Johnson Matthey Ltd., UK). The thickness of the Cu deposits was monitored by a quartz crystal microbalance, located in the vapour beam next to the MgO target. The deposition was carried out at room temperature and at a rate of 1 A mm’. The EELS spectra were obtained by using an Auger cylindrical mirror analyzer with a primary energy of 97 eV, a modulation peak-to-peak voltage of 1 V and at perpendicular incidence of the primary electron beam. The argon bombardment was carried on for 0.5 h at a 0.5 keV acceleration voltage and a 10 PA cm-2 target current. The purity of the Ar and of the CO gas was 99.999% and 99.998% respectively.
3. Results and discussion Fig. 1 demonstrates the growth of Cu films as followed by LEED. During the film growth the sharp LEED pattern from the freshly cleaved Ago surface was gradually blurred until* the substrate pattern was completely eliminated at a layer thickness of 8 A (i.e. at a distribution of islands with a total average thickness of 8 A). At a deposit thickness of 15 A a weak Cu(100) pattern appeated, which upon further growth became sharper. Fig. la thus gives the 100 A deposit LEED pattern, which we find to be of good quality, considering the lack of an annealing process. Fig. lb shows the sharp LEED pattern of clean Mgql~) surface as a reference. The excellent LEED pattern is due to good stability of this surface. Even when the MgO(lOO) had been exposed to an argon ion bombardment, a MgO(100) LEED pattern still existed. This stability against Ar+ bombardment agrees with previous observations [9]. For the Ar+ bombarded surface we also observed Cu(100) epitaxial growth showing a picture similar to fig. la. On a freshly cleaved MgO(ll1) surface an epitaxial growth of Cu(ll1) was
Fig.
1. LEED patterns of (a) Cu(100) on MgO(100) with a Cu thickness of 100 A. primarc incident electron energy E, = 198.4 eV. (b) clean MgO(100) at I?, = 149.0 eV.
observed upon deposition of approximately 14 A of Cu. The Cu LEED pattern sharpened with increasing average layer thickness. Fig. 2a gives the Cu(ll1) structure after deposition of 64 A Cu. and the LEED pattern of clean MgO( 111) is inserted as fig. 2b. It is well known that MgO is a highly insulating material which is generally thought to be studied by electron spectroscopy only with difficulty. but on a well-ordered clean MgO surface the charging of the surface is saturated quickly when accompanied by secondary emission [lo].
Fig. 2. LEED patterns
of (a) Cu(lll) on MgO(ll1) with Cu thickness (b) clean MgO(ll1) at EP = 202 eV.
of 64 A, at E, = 31X.2 cV.
For neither of these three surfaces any superstructures were observed during the room temperature film growth without any annealing. In the case, where a 80 A Cu cpitaxially grown film was exposed to a poor vacuum (of
40 30 2a 10 0 Energy toss E, (eV! Fig. 3. EELS spectra of Cu/MgO(lOO) surfaces. Curve (1): clean MgO(100). Curves (2) to (4): 0.7, 8.4 and 60.4 A Cu deposits, respectively. Curve (5): CO oxidized, 60 .k film on MgQ(100). Curve (6): clean MgO(1.11).
038
i
I
0
1
I
5
10
A
15
Cu thickness(iI
Fig. 4. The change (0)
R(Cu/Mg)
=
of the ratios
(Ic,,,zu
of the Auger signal
cv,,‘&.)/(I,,,,,,,
,“,/sM&)
intensities and
(A)
with R(Cu/O)
Cu thxkntx = (1,
“,‘)a,
increaae. c\.)//.%.u)/’
I “,r,,?Lw/s3).
about 2 x lo-’ mbar) for several hours, no LEED pattern was observed upon 0 a subsequent further 100 A deposition. The LEED structure in this case apparently was destroyed through chemisorption of residual gases. The gradual changes in EELS structures as a function of copper coverage during Cu growth on the MgO(100) are shown in fig. 3. Fig. 4 gives the Auger ratios of the copper to magnesium and copper to oxygen intensities, W&” = ;U$cU , R(Cu/O) = I. 0 0 Mg % where Z is the peak-to-peak heights in the corresponding AES spectra and S the relative Auger sensitivities of Cu, Mg and 0. The AES spectrum for the MgO(l11) clean surface of the same crystal showed a Mg/O ratio of 0.414, which by comparison with the 0.34 ratio of the MgO(100) suggests a Mg surface ion enrichment in the Mg(ll1) surface layers. A comparison of the observed EELS spectral changes (fig. 3) with energy loss values of the clean MgO(100) agrees well with previous findings [93 as seen from table 1. which also includes a reference to a band structure R (Cu/Mg)
of electron
a) Band structure
calculation
2.0
4.1 5.1 4.3 4.5 4.9 4.5 4.3
2.9 2.5 2.8
60 A Cu/MgO(lOO) cu a’ Cu(lll) Cu(311) CO/Cu/MgO(lOO) CO/CU(311) cuo/cu(1oo) cu,o/cu(1cO)
2.8
4.1
4.5
2.9
2.2
2.6 2.3 2.4
Loss energies (eV)
8.4 A Cu/MgO(lOO)
0.7 A Cu/MgO(lOO)
MgO(ll1)
MgO(lW MgWW
Surface
5.9
5.9
5.7
6.5
6.2 6.1
6.8
7.0
6.8
6.7
7.7
7-9
8.9
9.2
8.9
10.3
10.5
9.8 11.6
11.5 10.4
9.2 9.1
11.5 11.4 11.2
9.5 9.0 8.3
- 12.5
12.2
13.5 13.7
14.4
14.3 14.4 14.1
17.5
17.4 17.6 17.6
1131 U41 [I41
[ill [I21 U31 Present study
Present study
Present study
Present study
22.4
Present study (91 Present study
Ref.
&posit (A) of CUon
22.4 22.2 22.2
energy loss spectroscopy (EELS) peak values E, (in eV) for surfaces of clean MgO( 100) and MgO( 111). and for surfaces of crystals and Cu(I) of CU(II) oxides
MgO(100).oxidized CU on MgO(l00)
Comparison
Table 1
calculation for Cu [ll] and to EELS literature values for Cu [12,13] and for Cu ,O and CuO [14]. Quite large changes of the observed EELS structure with increasing Cu deposition were seen (fig. 3). At approximately 0.7 A (average) Cu deposition the strong resonance peaks at 2.6 eV for the MgO(100) surface (which was also previously seen at 2.3 eV 191) and at 2.2 eV for the MgO(ll1) surfaces. both were eliminated and two new peaks arose at 2.2 eV and 4.5 eV from the Cu deposited MgO(100) surface (fig. 3, curve 2). This stage presumably shows that the first small Cu deposits may be bound to the oxygen ligands as ions and preferably stick to Mg*+ vacancy sites causing the surface defect peak in the spectrum to disappear and two new resonance peaks which have character of copper oxides to appear due to Cu impurity level related electronic transitions fl5]. As a detailed study on defect center structures has previously been published ]15] we shall only briefly discuss these here. Compared to the EELS spectrum of the clean MgO(100) (fig. 3, curves 1 and 6, respectively) the MgO(ll1) spectrum only shows a small difference for the 6.1 eV surface state related peak, whose intensity is a little stronger, and for the surface defect peak, whose intensity is a little weaker. Noting that the initial small 0.1 deposition has the same influence on the EELS structure as does the change from the (100) to the (111) surface, the latter being a polar surface richer in Mg, we further gain support for the suggestions that the initial copper deposits have a high possibility of interaction with V,- centers on the MgO surfaces. At an average coverage of about 3-5 A a drastic change in the EELS spectrum of MgO has occurred. This is perhaps due to the very high scattering cross section of the Cu3d valence electron level [16] and to the low primary incident energy with a very low electron mean free path. Already at an average coverage of about 5 A the spectra show some of the characteristic bulk copper loss features. At average thicknesses above 15 A, corresponding to a few monolayers of Cu. the EELS spectra just show the features of bulk copper, and by continued deposition the structures do not change much. We furthermore note that the spectrum of the Cu epitaxial film oxidized by CO (fig. 2, curve 5) agrees well with that of oxidized copper crystals [II]. The geometry of the atomic arrangement and misfit considerations are important in a epitaxial growth process, but not the only important parameters. Factors such as substrate temperature, contamination and film thickness are key parameters as well [17]. In the present study. however, both the experiment and recent theoretical calculations 1181 propose that magnesium vacancy sites on the MgO(100) surface are trapping centers for the initial copper deposits. It is reasonable, then, to suspect that the trapped copper ions are active sites for the subsequent nucleation process. The copper atoms may bind to the oxygen ions on the surface in a bridge configuration subsequently leading to a Cu(100) epitaxial growth.
J.-W. He, P.J. Mailer / E~~tux~uland electronic startles
of Cu on MgO
941
The simple parallel relations both for Cu(lOO)/Mg(lOO) and for Cu(lll)/MgO(lll), i.e. no relative rotations, have been found from these experiments through detailed analysis of the corresponding LEED patterns. A possible 30° rotation of the Cu(ll1) film relative to the MgO(ll1) substrate surface was ruled out by noticing the higher primary energy of the incident electrons on the Cu(ll1) thin film. The performance of the Auger signal ratios (fig. 4) as the Cu deposit thickness increases clearly indicates a two stage growth mechanism. It seems that the growth mechanism fits the Stranski-Krastanov model, so that the crystallites form after the completion of the first monolayer 0 in a two-dimensional growth process. The critical point being near the 5 A Cu thickness is seen in fig_ 4. Based uponOthe observed appearance of the 2.2 and 4.5 eV EELS peaks at the initial 0.7 A average Cu/Mgql~) spectrum and referring to the work of Benndorf et al. [14] on the spectroscopic identification of Cu(1) and Cu(I1) oxides we suggest that Cu(I1) is the corresponding state of the copper atoms fitting into the Vs- centers, and that the copper atoms forming the bridge configuration to the oxygen ions are in the Cu(1) state. Similarly cation vacancies will be occupied by deposited copper bounded to three anions on the MgO(ll1) surface, thus forming nucleation centers for Cu(ll1) epitaxial growth. The proposed mechanism for the epitaxial Cu growth on MgO crystal surfaces is thus as follows. Initially the arriving Cu deposits interact with magnesium surface vacancies and behave as active centers for the subsequent nucleation. This corresponds to the first stage of the growth mechanism. By further deposition copper forms clusters around the active center leading to an epjtaxial growth of a Cu film, whose structure can be detected by LEED. The clustering process yields the uniform changes of the Auger intensity ratios in fig. 4.
4. Conclusions
From the experimental studies on the Cu deposition on cleaved MgO(100) and MgO(ll1) surfaces at room temperature in UI-IV epitaxial growth of Cu has been observed on both the MgO(100) and the MgO(ll1) surface with parallel Cu(100) and Cu(ll1) o~entations, respectively. The initial growth follows the Stranski-Krastanov model. The submonolayer Cu deposits are supposed to interact with Mg2+ vacancies and subsequently to act as the active nucleation centers in the epitaxial growth of the ultra-thin film as elucidated from the corresponding electron energy loss spectra.
Acknowledgements The support of the Danish Natural Science Research Council, the Carlsberg Foundation, and the Thomas B. Thrige Foundation and valuable comments by Professor E.I. Solomon are gratefully acknowledged. One of us (J.-W. He) is indebted to the Education Committee of the People’s Republic of China for its support.
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