- Email: [email protected]
Characterization of MgO thin films grown by rf-sputtering
Vacuum 67 (2002) 577–581
Characterization of MgO thin films grown by rf-sputtering D. Ca! ceresa,*, I. Coleraa, I. Vergaraa, R. Gonza! leza, E. Roma! nb a
Departamento de F!ısica, Escuela Polit!ecnica Superior, Universidad Carlos III, Avda. de la Universidad, 30, 28911 Legan!es, Madrid, Spain b Instituto de Materiales de Madrid (ICMM-CSIC), Cantoblanco, 28049 Madrid, Spain
Abstract Thin films of MgO were grown on an Si(1 0 0) substrate at 1000 K by rf-sputtering using two different targets: Mg and MgO. The total pressure during the growing process was maintained at 7.0 103 mbar and the O2 to Ar percentage was varied from 5% to 50%. X-ray diffraction measurements indicate that two different crystalline structures are formed depending on whether the target was Mg or MgO. In MgO films grown using an Mg target, the diffraction peak corresponds to MgO(2 0 0) of the rock salt structure, and in MgO films grown with an MgO target the diffraction peak corresponds to MgO(4 0 0) of the cubic spinel structure. In both structures, the maximum intensity of the diffraction peak occurs when the oxygen to argon percentage is 20%. The surface stoichiometry of both types of films was determined by Auger electron and X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy corroborates the presence of OH radicals. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: MgO thin films; Sputtering; X-ray diffraction; Auger electron spectroscopy; X-ray photoelectron spectroscopy
1. Introduction Magnesium oxide is a simple ionic oxide with excellent properties such as electrical insulation, chemical inertness, optical transparency, thermal conductivity and high efficiency for secondary electron emission. These properties make MgO thin films useful in many applications such as optical films [1], insulating coatings for MHD electrodes [2], buffer layers for high Tc oxide superconductors [3], plasma display devices [4,5], and barriers in Josephson tunnel junctions [6–8]. Several techniques have been used for deposition of MgO films, including chemical vapor
deposition [9], thermal oxidation of evaporated metal films [10], electron beam evaporation [5,11], laser beam evaporation [3,12,13], ion-beam assisted deposition [14], and rf-sputtering [2,15–17]. In this study, MgO thin films were grown on a silicon substrate by reactive rf-sputtering using targets of either pure Mg metal or MgO powder. The effect of oxygen content (in the sputtering mixture of Ar+O2 during the deposition process) on the stoichiometry and structure of the grown films is investigated. In addition, the influence of target composition is studied.
2. Experimental procedure *Corresponding author. Tel.: +34-916249478; fax: +34916248749. E-mail address: dcaceres@fis.uc3m.es (D. C!aceres).
MgO films were deposited on Si (1 0 0) substrates by rf-magnetron sputtering operating at
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 2 5 1 - 8
! D. Caceres et al. / Vacuum 67 (2002) 577–581
25 W using either Mg or MgO targets, each with a 2 in diameter. The Mg target has a nominal purity of 99.97%. The MgO target was sintered by pressing MgO powder of 99.9% purity at 1573 K in air. During the growing process, the substrate temperature was maintained at 1000 K using a resistive heater stage. The temperature of the substrate was measured by a thermocouple mounted in contact with the heating stage. The chamber was first evacuated to a base pressure of E5 107 mbar, and then a mixture of argon (99.999%) and oxygen (99.999%) was introduced at a total pressure of 7.0 103 mbar. The total pressure was kept constant during the deposition while the oxygen to argon percentage varied from 5% to 50%. The crystallographic structure and orientation of the films were determined with a Phillips X-Pert X-ray diffractometer using the Cu Ka radiation. The Auger spectra were obtained at a primary energy of 3 keV and recorded in the N(E) mode. XPS experiments were performed using a conventional Al Ka X-ray source, 1486.6 eV, and the double-pass analyzer was operated at constant energy resolution with pass energy in the range of 50–200 eV. After installing the sample in the ultrahigh vacuum system, the surface was cleaned of carbon contamination by Ar+ sputtering with 3 keV and 1 mA ion current. No preferential sputtering was observed, in agreement with the previous works [18].
3. Results and discussion 3.1. X-ray diffraction measurements Fig. 1 shows the X-ray diffraction spectra of MgO films grown from either Mg (solid line) or MgO targets (dotted line). These spectra indicate that all the films have a preferred (1 0 0) orientation normal to the surface, regardless of both the types of target and the oxygen percentage during the growing process. However, the film structure depends on the target type. In films grown from the Mg target, the spectral peak occurs at 2y ¼ 431, which corresponds to the (2 0 0) diffraction peak from the rock salt crystal structure of the
%O 2
MgO target Mg target
50%
INTENSITY (a.u.)
578
20%
10%
5% 41
42
43
44
45
46
47
2θ (°) Fig. 1. X-ray diffraction spectra of MgO films grown with different percentages of O2 in the chamber from an Mg target (—) and an MgO target (– – –).
periclase. In those prepared using the sintered MgO target, the spectral peak occurs at 2y ¼ 44:41 associated with the (4 0 0) diffraction peak of a mixture of MgO and Mg(OH)2 with a cubic spinel structure [19]. This difference is attributed to the OH content of the sintered MgO target. MgO is hygroscopic and hydrogen is incorporated in the powder from the atmosphere. In both structures, the maximum of the diffraction peak occurs when the oxygen to argon percentage is 20%. The grain size of the films was calculated using the Scherrer formula as a function of the full-width at halfmaximum (FWHM) of the diffraction peaks [20]. ( for the The resulting values are E250 and 210 A films grown from the Mg and the MgO targets, respectively, and is independent of the oxygen percentage. Another important difference between films grown with different targets is observed in the growing rate. This parameter depends on the oxygen percentage in the chamber during the growing process because the sputtering is primarily made by the argon ions. When the oxygen percentage increases, and the total pressure is kept constant, the amount of argon diminishes and the growing rate decreases. Films grown with the same oxygen percentage but from different targets show
O(KLL)/Mg(KLL)
3.2. Surface characterization by AES and XPS The stoichiometry and homogeneity of the films were characterized by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The AES spectra of the MgO thin films grown from an Mg target are from a clean surface with the three main peaks at kinetic energies of 33, 502, and 1172 eV, which correspond to Mg2+(LMM), O2(KLL) and Mg2+(KLL) transitions, respectively (Fig. 2). The O(KLL) to Mg(KLL) peak-intensity ratio as a function of the oxygen percentage in the growing atmosphere is shown in Fig. 3 (top). Within the experimental uncertainties, these values are E5.4, indicating that at 5% of O2 percentage and with a total pressure of 7.0 103 mbar the surface is already oxygen saturated. Assuming a relative Auger sensitivity factor of 0.5 and 0.1 for O(KLL) and Mg(KLL) transitions, respectively [22], this 5.4 ratio indicates that the thin film surface is nearly stoichiometric. This value is higher than that of 3.8, obtained for bulk MgO after Ar+ sputtering and annealing in vacuum at 1000 K [23], where most likely some oxygen vacancies are present.
dN(E)/dE (a.u.)
AES 3KeV Mg(LMM)
O(KLL) Mg(KLL)
x 0.4
50
100
450 500 550 1100 1150 1200 KINETIC ENERGY (eV)
Fig. 2. Mg2+(LMM), O2(KLL) and Mg2+(KLL) AES transitions for MgO films grown with an Mg target in an atmosphere containing 20% of oxygen.
O1s/Mg2s
very different growing rates. For the films grown using the Mg target, the growing rate is two times greater than for those grown from an MgO target. This difference is attributed to the sputtering yield differences between the two types of targets [21].
O1s/Mg2p
! D. Caceres et al. / Vacuum 67 (2002) 577–581
579
6
5
8 7 5 4
0
10
20 30 40 O2 PERCENTAGE (%)
50
Fig. 3. O(KLL) to Mg(KLL) intensity ratio (top), O 1s to Mg 2p and O 1s to Mg 2s area ratio (bottom) as a function of the oxygen percentage for MgO films grown from an Mg target.
Additional information is obtained from the XPS analysis. Fig. 3 (bottom) shows both the O 1s to Mg 2p and O 1s to Mg 2s area ratios as a function of the oxygen percentage for MgO films grown from an Mg target. The areas of the O 1s, Mg 2s and Mg 2p peaks are measured after subtracting both the Xray satellites and the Shirley background. Similar intensity ratios are obtained for all the thin films, as was observed by AES. The stoichiometry of the thin films was determined by comparing the ratio (7.870.4) of the area of the O 1s peak at 531 eV to that of the Mg 2p at 50 eV with the corresponding ratio (7.7) from the MgO(1 0 0) single crystal surface [24]. The latter has a one to one stoichiometry. This result agrees with the stoichiometry obtained from the O 1s to Mg 2s area ratio (with a mean value of 4.7), using the corresponding sensitivity factors (1 for O 1s and 0.2 for Mg 2s), which gives a value of 0.9470.09. Fig. 4 shows the XPS spectra of the MgO films grown from an Mg target and in an atmosphere containing 20% oxygen. The energy scale was calibrated with the O 1s binding energy, associated with the O2 ions from MgO at 531 eV. The most intense peak corresponds to the Mg 1s core level at 1304 eV. Binding energies for the other levels (see inset of Fig. 4) are 89 eV (Mg 2s), 50 eV (Mg 2p), 24 eV (O 2s), and about 7 eV (O 2p). In MgO thin films grown from an MgO target, AES and XPS experiments indicate that some SiO2
! D. Caceres et al. / Vacuum 67 (2002) 577–581
580
XPS 1486.6eV
Mg2p O2s
-100 -80 -60 -40 -20 BINDING ENERGY (eV)
X 0.5
2-
0
Mg(KLL)
O1s O(KLL)
-1400-1200 -1000
-800
-600
2-
O (MgO)
O (SiO2 )
O2p
INTENSITY (a.u.)
INTENSITY (a.u.)
Mg2s
Mg1s
-400
-200
OH
b)
2-
O (MgO)
0
BINDING ENERGY (eV) Fig. 4. XPS spectrum of the MgO films grown using an Mg target and with 20% of oxygen. The inset shows the low energy levels.
-
a)
-538
OH
-
-536
-534
-532
-530
-528
-526
BINDING ENERGY (eV)
contamination from the target is present. This contamination was introduced in the additive used to press the MgO powder. Fig. 5 compares the O 1s spectra of MgO films grown from Mg and MgO targets, respectively. In the former films, the appearance of a shoulder in the high binding energy side of the photoemission peak (due to an energy shift of 2.3 eV from the oxide) indicates that some oxygen is present in the form of hydroxide [25]. A larger proportion of oxygen in the hydroxide configuration is expected in the MgO films grown from an MgO target, where the (4 0 0) diffraction peak of the cubic spinel structure of Mg(OH)2 was observed in the X-ray diffraction measurements. The O 1s XPS spectra of these films indeed exhibit a larger FWHM. The ratio between the areas associated with the OH radicals and the O2 ions is 0.70, while it is 0.25 for the thin film grown from an Mg target. MgO thin films grown from an MgO target also have a constant value for the O(KLL) to Mg(KLL) intensity ratio, regardless of the O2 percentage. XPS experiments in the films grown from an MgO target show that the O 1s/Mg 1s area ratio is E1.5, indicating that there is a mixture of MgO and Mg(OH)2, in agreement with the XRD measurements.
Fig. 5. O 1s photoemission spectra of MgO films grown from targets of (a) Mg and (b) MgO. The spectrum in (a) was fitted with two Gaussians associated with O2 ions and OH radicals. The spectrum in (b) was fitted with three Gaussians associated with O2 ions from MgO and SiO2 contamination, and OH radicals.
line structures depending on whether the target was Mg or MgO. In MgO films grown using an Mg target, the diffraction peak corresponds to MgO(2 0 0) of the rock salt structure, and in MgO films grown from an MgO target the diffraction peak corresponds to MgO(4 0 0) of the cubic spinel structure. In both structures, the maximum of the diffraction peak occurs when the oxygen to argon percentage is 20%. Auger electron and X-ray photoelectron spectroscopy show that the stoichiometry of the films is independent of the O2 percentage in the growing atmosphere. The resulting values of the O/Mg ratio are 1.0 and 1.5 for the films grown from Mg and MgO targets, respectively. Based on XRD and XPS results, the latter value was attributed to a mixture of MgO and Mg(OH)2.
4. Conclusions
Acknowledgements
MgO films grown on an Si(1 0 0) substrate at 1000 K by rf-sputtering exhibit two different crystal-
Research at the University Carlos III was supported by the Ministerio de Ciencia y
! D. Caceres et al. / Vacuum 67 (2002) 577–581
Tecnolog!ıa of Spain under contract nos. PB970087 and MAT1999-1703-E, and the Comunidad ! Autonoma de Madrid.
References [1] MacLean SG, Duley WW. J Phys Chem Solids 1984;45:223–5. [2] Vuoristo P, M.antyl.a T, Kettunen P. J Vac Sci Technol 1986;A4:2932–7. [3] Fork DK, Ponce FA, Tamontana JC, Geballe TH. Appl Phys Lett 1991;58:2294–6. [4] Yi W, Yu S, Lee W, Han T, Jeong T, Woo Y, Lee J, Jin S, Choi W, Heo J, Jeon D, Kim JM. J Appl Phys 2001;89:4091–5. [5] Lee J, Jeon T, Yu S, Jin S, Heo J, Yi W, Jeon D, Kim JM. Appl Surf Sci 2001;174:62–9. [6] Shoji A, Aoyagi M, Kosaka S, Shinoki F, Hayakawa H. Appl Phys Lett 1985;46:1098–100. [7] Thakoor S, Leduc HG, Stern JA, Thakoor AP, Khanna SK. J Vac Sci Technol 1987;A5:1721–5. [8] Tonouchi M, Sakaguchi Y, Kobayashi T. J Appl Phys 1987;62:961–6. [9] Maruyama T, Shionoya J. Jpn J Appl Phys 1990;29: L810–1. [10] Corneille JS, He JW, Goodman DW. Surf Sci 1994;306:269–78.
581
[11] Lee SM, Murakami H, Ito T. Appl Surf Sci 2001;175176:517–24. [12] Tarsa EJ, De Graef M, Clarke DR, Gossard AC, Speck JS. J Appl Phys 1993;73:3276–83. [13] Chen XY, Yang B, Liu ZG, Shi LJ. Appl Surf Sci 1998;135:233–7. [14] Dong L, Zepeda-Ruiz LA, Srolovitz DJ. J Appl Phys 2001;89:4105–12. [15] Hsu WY, Raj R. Appl Phys Lett 1992;60:3105–7. [16] Pinto R, Poothra SC, Purandare SP, Pai SP, D’Souza CP, Kumar D, Sharon M. J Vac Sci Technol 1991;A9:2670–4. [17] Misaki Y, Mikawa M, Ishiguro T, Hamasaki K. J Vac Sci Technol 1997;A15:48–51. [18] Henrich VE. Rep Prog Phys 1985;48:1481–541. [19] Powder Diffraction File Database. International Centre for Diffraction Data (ICDD), 1995. [20] Cullity BD. Elements of X-ray diffraction. Reading, MA: Addison-Wesley, 1978. p. 102. [21] Wasa K, Hayakawa S. Handbook of sputter deposition technology. New Jersey: Noyes Publications, 1992. p. 49. [22] Davis LE, MacDonald NC, Palmberg PW, Riach GE, Weber RE. Handbook of auger electron spectroscopy. Minnesota: Physical Electronics Industries, Inc., 1976. [23] Colera I, Gonz!alez R, Soria E, de Segovia JL, Rom!an EL, Chen Y. J Vac Sci Technol 1997;A15:1698–703. [24] Peng XD, Barteau MA. Surf Sci 1990;233:283–92. [25] Liu P, Kendelewicz T, Brown Jr GE, Parks A. Surf Sci 1998;412/413:287–314.
Characterization of MgO thin films grown by rf-sputtering D. Ca! ceresa,*, I. Coleraa, I. Vergaraa, R. Gonza! leza, E. Roma! nb a
Departamento de F!ısica, Escuela Polit!ecnica Superior, Universidad Carlos III, Avda. de la Universidad, 30, 28911 Legan!es, Madrid, Spain b Instituto de Materiales de Madrid (ICMM-CSIC), Cantoblanco, 28049 Madrid, Spain
Abstract Thin films of MgO were grown on an Si(1 0 0) substrate at 1000 K by rf-sputtering using two different targets: Mg and MgO. The total pressure during the growing process was maintained at 7.0 103 mbar and the O2 to Ar percentage was varied from 5% to 50%. X-ray diffraction measurements indicate that two different crystalline structures are formed depending on whether the target was Mg or MgO. In MgO films grown using an Mg target, the diffraction peak corresponds to MgO(2 0 0) of the rock salt structure, and in MgO films grown with an MgO target the diffraction peak corresponds to MgO(4 0 0) of the cubic spinel structure. In both structures, the maximum intensity of the diffraction peak occurs when the oxygen to argon percentage is 20%. The surface stoichiometry of both types of films was determined by Auger electron and X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy corroborates the presence of OH radicals. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: MgO thin films; Sputtering; X-ray diffraction; Auger electron spectroscopy; X-ray photoelectron spectroscopy
1. Introduction Magnesium oxide is a simple ionic oxide with excellent properties such as electrical insulation, chemical inertness, optical transparency, thermal conductivity and high efficiency for secondary electron emission. These properties make MgO thin films useful in many applications such as optical films [1], insulating coatings for MHD electrodes [2], buffer layers for high Tc oxide superconductors [3], plasma display devices [4,5], and barriers in Josephson tunnel junctions [6–8]. Several techniques have been used for deposition of MgO films, including chemical vapor
deposition [9], thermal oxidation of evaporated metal films [10], electron beam evaporation [5,11], laser beam evaporation [3,12,13], ion-beam assisted deposition [14], and rf-sputtering [2,15–17]. In this study, MgO thin films were grown on a silicon substrate by reactive rf-sputtering using targets of either pure Mg metal or MgO powder. The effect of oxygen content (in the sputtering mixture of Ar+O2 during the deposition process) on the stoichiometry and structure of the grown films is investigated. In addition, the influence of target composition is studied.
2. Experimental procedure *Corresponding author. Tel.: +34-916249478; fax: +34916248749. E-mail address: dcaceres@fis.uc3m.es (D. C!aceres).
MgO films were deposited on Si (1 0 0) substrates by rf-magnetron sputtering operating at
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 2 5 1 - 8
! D. Caceres et al. / Vacuum 67 (2002) 577–581
25 W using either Mg or MgO targets, each with a 2 in diameter. The Mg target has a nominal purity of 99.97%. The MgO target was sintered by pressing MgO powder of 99.9% purity at 1573 K in air. During the growing process, the substrate temperature was maintained at 1000 K using a resistive heater stage. The temperature of the substrate was measured by a thermocouple mounted in contact with the heating stage. The chamber was first evacuated to a base pressure of E5 107 mbar, and then a mixture of argon (99.999%) and oxygen (99.999%) was introduced at a total pressure of 7.0 103 mbar. The total pressure was kept constant during the deposition while the oxygen to argon percentage varied from 5% to 50%. The crystallographic structure and orientation of the films were determined with a Phillips X-Pert X-ray diffractometer using the Cu Ka radiation. The Auger spectra were obtained at a primary energy of 3 keV and recorded in the N(E) mode. XPS experiments were performed using a conventional Al Ka X-ray source, 1486.6 eV, and the double-pass analyzer was operated at constant energy resolution with pass energy in the range of 50–200 eV. After installing the sample in the ultrahigh vacuum system, the surface was cleaned of carbon contamination by Ar+ sputtering with 3 keV and 1 mA ion current. No preferential sputtering was observed, in agreement with the previous works [18].
3. Results and discussion 3.1. X-ray diffraction measurements Fig. 1 shows the X-ray diffraction spectra of MgO films grown from either Mg (solid line) or MgO targets (dotted line). These spectra indicate that all the films have a preferred (1 0 0) orientation normal to the surface, regardless of both the types of target and the oxygen percentage during the growing process. However, the film structure depends on the target type. In films grown from the Mg target, the spectral peak occurs at 2y ¼ 431, which corresponds to the (2 0 0) diffraction peak from the rock salt crystal structure of the
%O 2
MgO target Mg target
50%
INTENSITY (a.u.)
578
20%
10%
5% 41
42
43
44
45
46
47
2θ (°) Fig. 1. X-ray diffraction spectra of MgO films grown with different percentages of O2 in the chamber from an Mg target (—) and an MgO target (– – –).
periclase. In those prepared using the sintered MgO target, the spectral peak occurs at 2y ¼ 44:41 associated with the (4 0 0) diffraction peak of a mixture of MgO and Mg(OH)2 with a cubic spinel structure [19]. This difference is attributed to the OH content of the sintered MgO target. MgO is hygroscopic and hydrogen is incorporated in the powder from the atmosphere. In both structures, the maximum of the diffraction peak occurs when the oxygen to argon percentage is 20%. The grain size of the films was calculated using the Scherrer formula as a function of the full-width at halfmaximum (FWHM) of the diffraction peaks [20]. ( for the The resulting values are E250 and 210 A films grown from the Mg and the MgO targets, respectively, and is independent of the oxygen percentage. Another important difference between films grown with different targets is observed in the growing rate. This parameter depends on the oxygen percentage in the chamber during the growing process because the sputtering is primarily made by the argon ions. When the oxygen percentage increases, and the total pressure is kept constant, the amount of argon diminishes and the growing rate decreases. Films grown with the same oxygen percentage but from different targets show
O(KLL)/Mg(KLL)
3.2. Surface characterization by AES and XPS The stoichiometry and homogeneity of the films were characterized by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The AES spectra of the MgO thin films grown from an Mg target are from a clean surface with the three main peaks at kinetic energies of 33, 502, and 1172 eV, which correspond to Mg2+(LMM), O2(KLL) and Mg2+(KLL) transitions, respectively (Fig. 2). The O(KLL) to Mg(KLL) peak-intensity ratio as a function of the oxygen percentage in the growing atmosphere is shown in Fig. 3 (top). Within the experimental uncertainties, these values are E5.4, indicating that at 5% of O2 percentage and with a total pressure of 7.0 103 mbar the surface is already oxygen saturated. Assuming a relative Auger sensitivity factor of 0.5 and 0.1 for O(KLL) and Mg(KLL) transitions, respectively [22], this 5.4 ratio indicates that the thin film surface is nearly stoichiometric. This value is higher than that of 3.8, obtained for bulk MgO after Ar+ sputtering and annealing in vacuum at 1000 K [23], where most likely some oxygen vacancies are present.
dN(E)/dE (a.u.)
AES 3KeV Mg(LMM)
O(KLL) Mg(KLL)
x 0.4
50
100
450 500 550 1100 1150 1200 KINETIC ENERGY (eV)
Fig. 2. Mg2+(LMM), O2(KLL) and Mg2+(KLL) AES transitions for MgO films grown with an Mg target in an atmosphere containing 20% of oxygen.
O1s/Mg2s
very different growing rates. For the films grown using the Mg target, the growing rate is two times greater than for those grown from an MgO target. This difference is attributed to the sputtering yield differences between the two types of targets [21].
O1s/Mg2p
! D. Caceres et al. / Vacuum 67 (2002) 577–581
579
6
5
8 7 5 4
0
10
20 30 40 O2 PERCENTAGE (%)
50
Fig. 3. O(KLL) to Mg(KLL) intensity ratio (top), O 1s to Mg 2p and O 1s to Mg 2s area ratio (bottom) as a function of the oxygen percentage for MgO films grown from an Mg target.
Additional information is obtained from the XPS analysis. Fig. 3 (bottom) shows both the O 1s to Mg 2p and O 1s to Mg 2s area ratios as a function of the oxygen percentage for MgO films grown from an Mg target. The areas of the O 1s, Mg 2s and Mg 2p peaks are measured after subtracting both the Xray satellites and the Shirley background. Similar intensity ratios are obtained for all the thin films, as was observed by AES. The stoichiometry of the thin films was determined by comparing the ratio (7.870.4) of the area of the O 1s peak at 531 eV to that of the Mg 2p at 50 eV with the corresponding ratio (7.7) from the MgO(1 0 0) single crystal surface [24]. The latter has a one to one stoichiometry. This result agrees with the stoichiometry obtained from the O 1s to Mg 2s area ratio (with a mean value of 4.7), using the corresponding sensitivity factors (1 for O 1s and 0.2 for Mg 2s), which gives a value of 0.9470.09. Fig. 4 shows the XPS spectra of the MgO films grown from an Mg target and in an atmosphere containing 20% oxygen. The energy scale was calibrated with the O 1s binding energy, associated with the O2 ions from MgO at 531 eV. The most intense peak corresponds to the Mg 1s core level at 1304 eV. Binding energies for the other levels (see inset of Fig. 4) are 89 eV (Mg 2s), 50 eV (Mg 2p), 24 eV (O 2s), and about 7 eV (O 2p). In MgO thin films grown from an MgO target, AES and XPS experiments indicate that some SiO2
! D. Caceres et al. / Vacuum 67 (2002) 577–581
580
XPS 1486.6eV
Mg2p O2s
-100 -80 -60 -40 -20 BINDING ENERGY (eV)
X 0.5
2-
0
Mg(KLL)
O1s O(KLL)
-1400-1200 -1000
-800
-600
2-
O (MgO)
O (SiO2 )
O2p
INTENSITY (a.u.)
INTENSITY (a.u.)
Mg2s
Mg1s
-400
-200
OH
b)
2-
O (MgO)
0
BINDING ENERGY (eV) Fig. 4. XPS spectrum of the MgO films grown using an Mg target and with 20% of oxygen. The inset shows the low energy levels.
-
a)
-538
OH
-
-536
-534
-532
-530
-528
-526
BINDING ENERGY (eV)
contamination from the target is present. This contamination was introduced in the additive used to press the MgO powder. Fig. 5 compares the O 1s spectra of MgO films grown from Mg and MgO targets, respectively. In the former films, the appearance of a shoulder in the high binding energy side of the photoemission peak (due to an energy shift of 2.3 eV from the oxide) indicates that some oxygen is present in the form of hydroxide [25]. A larger proportion of oxygen in the hydroxide configuration is expected in the MgO films grown from an MgO target, where the (4 0 0) diffraction peak of the cubic spinel structure of Mg(OH)2 was observed in the X-ray diffraction measurements. The O 1s XPS spectra of these films indeed exhibit a larger FWHM. The ratio between the areas associated with the OH radicals and the O2 ions is 0.70, while it is 0.25 for the thin film grown from an Mg target. MgO thin films grown from an MgO target also have a constant value for the O(KLL) to Mg(KLL) intensity ratio, regardless of the O2 percentage. XPS experiments in the films grown from an MgO target show that the O 1s/Mg 1s area ratio is E1.5, indicating that there is a mixture of MgO and Mg(OH)2, in agreement with the XRD measurements.
Fig. 5. O 1s photoemission spectra of MgO films grown from targets of (a) Mg and (b) MgO. The spectrum in (a) was fitted with two Gaussians associated with O2 ions and OH radicals. The spectrum in (b) was fitted with three Gaussians associated with O2 ions from MgO and SiO2 contamination, and OH radicals.
line structures depending on whether the target was Mg or MgO. In MgO films grown using an Mg target, the diffraction peak corresponds to MgO(2 0 0) of the rock salt structure, and in MgO films grown from an MgO target the diffraction peak corresponds to MgO(4 0 0) of the cubic spinel structure. In both structures, the maximum of the diffraction peak occurs when the oxygen to argon percentage is 20%. Auger electron and X-ray photoelectron spectroscopy show that the stoichiometry of the films is independent of the O2 percentage in the growing atmosphere. The resulting values of the O/Mg ratio are 1.0 and 1.5 for the films grown from Mg and MgO targets, respectively. Based on XRD and XPS results, the latter value was attributed to a mixture of MgO and Mg(OH)2.
4. Conclusions
Acknowledgements
MgO films grown on an Si(1 0 0) substrate at 1000 K by rf-sputtering exhibit two different crystal-
Research at the University Carlos III was supported by the Ministerio de Ciencia y
! D. Caceres et al. / Vacuum 67 (2002) 577–581
Tecnolog!ıa of Spain under contract nos. PB970087 and MAT1999-1703-E, and the Comunidad ! Autonoma de Madrid.
References [1] MacLean SG, Duley WW. J Phys Chem Solids 1984;45:223–5. [2] Vuoristo P, M.antyl.a T, Kettunen P. J Vac Sci Technol 1986;A4:2932–7. [3] Fork DK, Ponce FA, Tamontana JC, Geballe TH. Appl Phys Lett 1991;58:2294–6. [4] Yi W, Yu S, Lee W, Han T, Jeong T, Woo Y, Lee J, Jin S, Choi W, Heo J, Jeon D, Kim JM. J Appl Phys 2001;89:4091–5. [5] Lee J, Jeon T, Yu S, Jin S, Heo J, Yi W, Jeon D, Kim JM. Appl Surf Sci 2001;174:62–9. [6] Shoji A, Aoyagi M, Kosaka S, Shinoki F, Hayakawa H. Appl Phys Lett 1985;46:1098–100. [7] Thakoor S, Leduc HG, Stern JA, Thakoor AP, Khanna SK. J Vac Sci Technol 1987;A5:1721–5. [8] Tonouchi M, Sakaguchi Y, Kobayashi T. J Appl Phys 1987;62:961–6. [9] Maruyama T, Shionoya J. Jpn J Appl Phys 1990;29: L810–1. [10] Corneille JS, He JW, Goodman DW. Surf Sci 1994;306:269–78.
581
[11] Lee SM, Murakami H, Ito T. Appl Surf Sci 2001;175176:517–24. [12] Tarsa EJ, De Graef M, Clarke DR, Gossard AC, Speck JS. J Appl Phys 1993;73:3276–83. [13] Chen XY, Yang B, Liu ZG, Shi LJ. Appl Surf Sci 1998;135:233–7. [14] Dong L, Zepeda-Ruiz LA, Srolovitz DJ. J Appl Phys 2001;89:4105–12. [15] Hsu WY, Raj R. Appl Phys Lett 1992;60:3105–7. [16] Pinto R, Poothra SC, Purandare SP, Pai SP, D’Souza CP, Kumar D, Sharon M. J Vac Sci Technol 1991;A9:2670–4. [17] Misaki Y, Mikawa M, Ishiguro T, Hamasaki K. J Vac Sci Technol 1997;A15:48–51. [18] Henrich VE. Rep Prog Phys 1985;48:1481–541. [19] Powder Diffraction File Database. International Centre for Diffraction Data (ICDD), 1995. [20] Cullity BD. Elements of X-ray diffraction. Reading, MA: Addison-Wesley, 1978. p. 102. [21] Wasa K, Hayakawa S. Handbook of sputter deposition technology. New Jersey: Noyes Publications, 1992. p. 49. [22] Davis LE, MacDonald NC, Palmberg PW, Riach GE, Weber RE. Handbook of auger electron spectroscopy. Minnesota: Physical Electronics Industries, Inc., 1976. [23] Colera I, Gonz!alez R, Soria E, de Segovia JL, Rom!an EL, Chen Y. J Vac Sci Technol 1997;A15:1698–703. [24] Peng XD, Barteau MA. Surf Sci 1990;233:283–92. [25] Liu P, Kendelewicz T, Brown Jr GE, Parks A. Surf Sci 1998;412/413:287–314.