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Reflection high-energy electron loss spectroscopy (RHEELS): a new approach in the investigation of epitaxial thin film growth by reflection high-energy electron diffraction (RHEED)
Vacuum 71 (2003) 59–64
Reflection high-energy electron loss spectroscopy (RHEELS): a new approach in the investigation of epitaxial thin film growth by reflection high-energy electron diffraction (RHEED) K. Mas$ek*, V. Moroz, V. Matol!ın ! 2, Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University, V Hole$sovi$ckach Prague 8 18000, Czech Republic Received 16 June 2002; received in revised form 11 September 2002; accepted 24 September 2002
Abstract Reflection high-energy electron diffraction (RHEED) is attractive as a technique for structural surface determination. The RHEED facility was equipped with a novel energy analyser permitting to measure energy loss spectra of diffracted high-energy electrons (RHEELS). The combination of RHEED and RHEELS is especially useful in the early stages of thin film growth when the intensity of the diffraction pattern is too low. The use of RHEED and RHEELS is demonstrated by the examples of the heteroepitaxial growth of ultrathin Pd layers on (0 0 0 1) alumina single-crystalline and natural (1 1 0) tin dioxide (cassiterite) substrate surfaces. The particle structure and epitaxial orientation was controlled by RHEED. It was shown that Pd formed three-dimensional clusters with the Pd (1 1 1) plane parallel to the substrate surface. Two different growth modes of Pd overlayers were found according to the nature of the substrate surface. The Pd surface plasmon peaks were found at rather low energy of 5 eV. It is probably a consequence of the very small size of Pd particles. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: RHEED; RHEELS; Pd; Growth; Al2O3; SnO2
1. Introduction Thin films of different materials play an important role in many fields of modern technologies. Structure and growth investigations of noncontinuous thin films belong to the fundamental studies in the fields of heterogeneous catalysis and gas sensors. Small clusters of metals like Pd, Rh, Pt deposited on different substrates form active *Corresponding author. E-mail address: [email protected] (K. Ma$sek).
phases in these devices. Because of the great complexity of such systems, fundamental studies are performed on so-called model systems consisting of small well-defined overlayers prepared on a single-crystalline substrate by means of vacuum evaporation. The model studies contribute to understanding of phenomena observed on laboratory systems as well as on real catalysts and gas sensors. In recent years, great effort has been made to investigate the structural, electronic and chemical properties of Pd-based catalysts and sensors.
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00714-5
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
60
involved in RHEED, surface and bulk plasmon losses (10–25 eV) and atomic core-shell excitations (a few hundred to a few thousand eV) are the most easily accessible to in situ studies. The combination of RHEED and reflection high-energy electron loss spectroscopy (RHEELS) is especially useful in the early stages of thin film growth when the intensity of the diffraction pattern is too low. The use of these techniques is demonstrated by examples of the growth of ultrathin Pd layers on alumina and cassiterite ((1 1 0) single-crystal SnO2) single-crystalline substrates. The results show different behaviour of these systems due to the strong difference in the metal–substrate interaction.
Alumina and tin dioxide are frequently used as substrates. Although aluminium oxide is generally considered as an inert substrate in the case of Pd/ alumina system, some reports indicate the MSI (metal–substrate interaction) effect [1–6]. Therefore, whilst metals like Cr, Ni and Cu are bonded to the oxygen sites, Pd forms bonds with the aluminium sites, causing stronger MSI in the case of reduced alumina surface [7]. It was pointed out that the surface structure and morphology depended strongly on the substrate preparation, especially on the cleaning procedure [7]. The strong interaction between Pd and Sn is significant also in the case of gas sensors based on SnO2 [8]. The enhanced sensitivity of Pd-doped SnO2 gas sensor was found [9,10]. This effect has not so far been successfully explained. Reflection high-energy electron diffraction (RHEED) is a powerful tool for surface structure investigation. Because of a favourable geometry (grazing angles of incidence and exit), RHEED makes it possible to investigate in situ epitaxial thin film growth in real time. The diffraction pattern analysis allows distinguishing between different modes of overlayer formation to be identified. The energy losses are characteristic for the surface structure and chemical composition of the sample. Adding energy loss spectroscopy capabilities to the RHEED detector can access the information contained in the electron loss spectrum. Among the several energy loss processes
2. Experimental The studies were performed in a specially designed UHV system. The UHV chamber was equipped with a RHEED facility permitting simultaneous observation of the substrate surface during deposition. The instrument was operated at an acceleration voltage of 25 kV. Schematic representation of the RHEED—RHEELS experimental set-up is shown in Fig. 1. The monoenergetic and low-divergence electron beam touches the sample surface at an angle that is typically in the range of 0.5–31. Diffracted electrons pass through the RHEA-100 energy analyser (STAIB INSTRUMENTS). The diffracted intensity RHEA
ELECTRON GUN
screen
SAMPLE
semi transparent mirror CAMERA
CCD
focusing and retarding lens
grid
PHOTO MULTIPLIER
lock-in
RHEED-VISION COMPUTER SYSTEM
Fig. 1. RHEED–RHEELS experimental set-up.
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
(a)
a.u.
Pd deposition
0
20
(b)
1.5 1.0 0.5 0.0 0
3.1. Pd growth on Al2O3 (0 0 01) substrate The Pd layer with a thickness of approximately 4 monolayers was deposited at a substrate temperature of 4201C. The final diffraction pattern is presented in Fig. 2a. The interpretation of a similar pattern can be found in Ref. [12]. The presence of relatively large circular spots in the
60
2.0
(c)
3. Results and discussion
40 energy [eV]
2.5
I(Pd)/I(Al2O3)
pattern is then converted to visible light by a fluorescent screen and recorded by the RHEEDVision Computer System. The RHEA-100 retarding-field analyser consists of electron optics and a single suppressor grid. A semi-transparent mirror reflects part of the light emitted by the screen to a photo-multiplier. Differentiation of the intensity signal using the lock-in technique with respect to the suppressor-grid potential yields the energy spectrum of the diffracted electrons. The Pd overlayers were prepared by heteroepitaxial growth using a special evaporation cell—micro electron beam evaporation source (MEBES) [11] operating at very low evaporation rates (the evaporation rate in these experiments was estimated to be approximately 0.2 monolayers per minute). The deposition was carried out at a background pressure of about 5 107 Pa. Liquid nitrogen cooled cryopanel, surrounding the MEBES cell, kept the stable pressure conditions during the deposition. The Pd layers of approximately 4 and 8 monolayers were deposited on alumina and tin dioxide substrates, respectively. The sapphire (0 0 0 1) and natural tin dioxide (cassiterite) (1 1 0) surfaces were used as substrates. The alumina substrate was chemically polished, cleaned and annealed in air at 12001C for 2 h and in UHV at 3001C for 1 h before the deposition. The (1 1 0) cut of cassiterite was mechanically polished and introduced into the vacuum chamber. The well-defined plane surface was obtained after repeated cycles of ion bombardment at an energy of 1 keV and heating in oxygen at a pressure of 1 105 Pa and a temperature of 6301C for 4 h.
61
5
10
15
20
25
deposition time [min]
Fig. 2. (a) RHEED diffraction pattern from a Pd deposit on an Al2O3 (0 0 0 1) substrate representing the (2 1 1) Pd crystallographic planes. The primary electron beam was parallel to the ½2 1 1 0 Al2O3 crystallographic direction. (b) A series of RHEELS spectra taken during the Pd deposition. (c) Evolution of the ratio of the characteristic loss intensities of Pd and Al2O3 during the deposition.
diffraction pattern indicates the formation of small three-dimensional particles. The orientation relationship between the substrate and the deposited
62
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
lattices was found as follows: ð1 1 1ÞPdjjð0 0 0 1ÞAl2 O3 ; ½2 1 1Pdjj½2 1 1 0Al2 O3 ðdouble positioningÞ:
(a)
Pd
Sn
SnO2
RHEELS spectra
20
a.u.
14 13 12 11 5 4 0 0
20
(b)
40
60
Energy [eV] 1.6 1.4 1.2
I(Pd)/I(SnO2)
Because of the FCC structure of Pd, these epitaxial parameters correspond well to an accommodation of two crystallographic planes having hexagonal symmetry (Pd (1 1 1) and Al2O3 (0 0 0 1)). A series of RHEELS spectra taken in regular time period of 3 min during the Pd deposition are presented in Fig. 2b. The spectra were treated by the following procedure. At first, the Shirley background and the elastic peak were subtracted. To compare the spectra a normalisation procedure was performed (area under the curve being equal to 1). The lowest spectrum corresponding to the clean alumina substrate shows the wide peak at energy about 24 eV. The interpretation of the EELS alumina spectrum can be found in Ref. [13]. The Pd deposition resulted in appearance of a Pd electron loss signal (Pd surface plasmon) at energy of 4.5 eV. This value is smaller than the one observed for the bulk material. It is probably due to the size dependence of the energy position of plasmon peaks in the case of small clusters [14]. In Fig. 2c, the intensity ratio of Pd and alumina signals during the growth is plotted. The intensities were obtained by deconvolution of RHEELS spectra. Due to the extreme surface sensitivity of this method, the asymptotic increase of Pd/Al2O3 ratio during the Pd deposition corresponds to the increase of the amount of Pd surface atoms. In addition, if we take into account the threedimensional features of the Pd diffraction patterns and that no intensity oscillation of specular spot was observed, Volmer–Weber growth mode (threedimensional without growth of an interfacial continuous layer) can be deduced.
1.0 0.8
2nd deposition
0.6
3rd deposition 1st deposition
0.4
annealing
0.2 0.0 0
(c)
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21
No. of step in experiment
3.2. Pd growth on SnO2 (1 1 0) substrate The Pd deposit with a total nominal thickness of approximately 8 monolayers was deposited at a substrate temperature of 3051C during three consecutive depositions. The diffraction pattern obtained at the end of the third depositions is presented in Fig. 3a. It consists of two diffraction
Fig. 3. (a) RHEED diffraction pattern from a Pd deposit on a SnO2 (1 1 0) substrate. The primary electron beam was parallel to the ½1 1 0 SnO2 crystallographic direction. (b) A series of RHEELS spectra taken at different steps of the experiment. The labels correspond to the x-axis values in Fig. 3c. (c) Evolution of the ratio of the characteristic loss intensities of Pd and SnO2 during the experiment.
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
features corresponding to two phases of the deposit. (1) Relatively large circular diffraction spots indicate three-dimensional Pd particles having (1 1 1) epitaxial plane. The interpretation of a similar diffraction pattern is published in Ref. [15]. The deposit is composed of two cluster populations rotated by 901 to each other along the substrate surface normal. It corresponds well to the rectangular symmetry of the SnO2 (1 1 0) substrate surface. (2) Diffuse diffraction rings indicate the presence of a polycrystalline phase on the surface. It arises from a Pd/Sn alloy overlayer covering the surface of the Pd particles. This effect is well known as ‘‘encapsulation’’, e.g. see Ref. [16] and its formation will be discussed in the following section. Fig. 3b represents a series of RHEEL spectra taken at different steps of the experiment. Time period between the spectra was 3 min during the Pd deposition. The spectra were treated in the same way as in the previous case. The spectrum marked 0 corresponds to the clean SnO2 substrate surface after the preparation procedure described in the experimental section. The EEL spectrum from a stoichiometric SnO2 surface consists mainly of a characteristic loss at 18 eV, which corresponds to the interband transitions in oxygen atoms [17,18]. Against these spectra spectrum 0 in Fig. 3b contains in addition, an important loss peak indicated by continuous line at 8 eV which corresponds to the plasmon peak of free metallic Sn. We conclude that our preparation procedure leads to the formation of a non-stoichiometric Snrich substrate surface. The dashed lines indicate other possible SnO2 characteristic losses. After the Pd deposition, the Pd plasmon peak at an energy of 5 eV appeared (curves 4, 11, 20). Fig. 3c shows the evolution of the intensity ratio of the Pd and SnO2 (at 18 eV) RHEELS signals. Completely different behaviour, in comparison to the previous case, can be seen. The slow decrease of this ratio during the first deposition can be explained by formation of three-dimensional Pd– Sn alloy particles by taking Sn atoms from the
63
substrate surface. This effect was also observed on the same system by XPS and ion scattering [19]. Curves 12–14 in Fig. 3b represent the evolution of RHEELS spectra during the thermal annealing at a temperature of 5101C for 45 min. The decrease of Pd signal against the increase of Sn signal can be seen. This effect is caused by reconstruction of the Pd–Sn particles. The driving force of this reconstruction is the effort of the system to reach the minimum free surface energy. Because of Sn having much lower free surface energy than Pd, this leads to the formation of Pd particles covered by an Sn or Pd–Sn layer [19]. Thermal annealing supplies the energy necessary for this reconstruction process. The peak of the Pd signal at the first stage of the subsequent depositions (see Fig. 3c) is probably caused by the growth of a two-dimensional Pd layer on the free surface of the SnO2 substrate and the subsequent reconstruction to three-dimensional particles. If we take into account a further slow increase of the Pd/SnO2 signal ratio, we can deduce that the growth continues by the formation of three-dimensional Pd particles.
4. Conclusion The growth and structure of a Pd deposit on aAl2O3 (0001) and SnO2 (1 1 0) substrate surfaces was investigated by means of RHEED and RHEELS. It was found that the deposit structure depends strongly on the nature of the substrate. The results showed that Pd formed three-dimensional particles with the Pd (1 1 1) crystallographic plane parallel to the substrate surface. Two different growth modes of Pd overlayers were found according to the substrate nature. While the three-dimensional Volmer–Weber growth was found on a Al2O3 substrate, the formation of a Pd–Sn alloy and the encapsulation of Pd particles by Sn atoms were observed in the case of the SnO2 substrate. The latter effect was explained by an effort of the system to minimise its surface free energy. The Pd surface plasmon peaks were found at an extremely low energy of 5 eV. This is probably a consequence of very small size of Pd particles and Mie interaction with the insulating substrate [14].
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K. Ma$sek et al. / Vacuum 71 (2003) 59–64
The presented results show that the combination of RHEED and RHEELS is a powerful tool for the investigation of structure and growth of heteroepitaxial systems. It is especially useful in the early stages of thin film growth when the intensity of the diffraction pattern is too low. RHEELS is able to provide additional information on the growth of thin films and, in this way, to recognise growth effects indistinguishable by standard RHEED. Acknowledgements This work was supported by grant no. 169/2001/ B-FYZ/MFF of the Grant Agency of Charles University by the research program MSM 113200002 that is financed by the Ministry of Education of the Czech Republic.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
References $ [1] Sarapatka TJ. J Phys Chem 1993;97:11247. $ [2] Sarapatka TJ. Thin Solid Films 1993;226:219.
[18] [19]
Ealet B, Gillet E. Surf Sci 1993;281:91. Ealet B, Gillet E. Surf Sci 1996;367:221. Ogawa S, Ichikawa S. Phys Rev B 1995;51:17231. Tsud N, Joh!anek V, Star!a I, Veltrusk!a K, Matol!ın V. Surf Sci 2000;467:169. Gunter PLJ, Niemantsverdreit JW, Riberio FH, Somorjai GA. Catal Rev Sci Eng 1997;39:77. . Barsan N, Schweizer-Berberich M, Gopel W. Fresenius J Anal Chem 1999;365:287. Lee AF, Baddeley CJ, Tikhov MS, Lambert RM. Surf Sci 1997;373:195. Lee AF, Lambert RM. Phys Rev B 1998;58:4156. Nehasil V, Ma$sek K, Moreau O, Matol!ın V. Czech J Phys 1997;47:261. Ma$sek K, Matol!ın V. Eur Phys J 1999;9(1-4):557–60. Star!a I, Zeze D, Matol!ın V, Pavluch J, Gruzza B. Appl Surf Sci 1997;115:46. Liebsch A. Phys Rev B 1993;48(15):11317. Ma$sek K, Matol!ın V. Vacuum 1998;50:151. Belton DN, Sun YM, White JM. J Phys Chem 1984; 88:5172. Moroz V, Ma$sek K. In: Proceedings of WDS’01, Prague: Matfyz Press, 2001. p. 650. Hoflund GB, Corallo GR. Phys Rev B 1993;46(11): 7110. Tsud N, Joh!anek V, Star!a I, Veltrusk!a K, Matol!ın V. Thin Solid Films 2001;391:204.
Reflection high-energy electron loss spectroscopy (RHEELS): a new approach in the investigation of epitaxial thin film growth by reflection high-energy electron diffraction (RHEED) K. Mas$ek*, V. Moroz, V. Matol!ın ! 2, Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University, V Hole$sovi$ckach Prague 8 18000, Czech Republic Received 16 June 2002; received in revised form 11 September 2002; accepted 24 September 2002
Abstract Reflection high-energy electron diffraction (RHEED) is attractive as a technique for structural surface determination. The RHEED facility was equipped with a novel energy analyser permitting to measure energy loss spectra of diffracted high-energy electrons (RHEELS). The combination of RHEED and RHEELS is especially useful in the early stages of thin film growth when the intensity of the diffraction pattern is too low. The use of RHEED and RHEELS is demonstrated by the examples of the heteroepitaxial growth of ultrathin Pd layers on (0 0 0 1) alumina single-crystalline and natural (1 1 0) tin dioxide (cassiterite) substrate surfaces. The particle structure and epitaxial orientation was controlled by RHEED. It was shown that Pd formed three-dimensional clusters with the Pd (1 1 1) plane parallel to the substrate surface. Two different growth modes of Pd overlayers were found according to the nature of the substrate surface. The Pd surface plasmon peaks were found at rather low energy of 5 eV. It is probably a consequence of the very small size of Pd particles. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: RHEED; RHEELS; Pd; Growth; Al2O3; SnO2
1. Introduction Thin films of different materials play an important role in many fields of modern technologies. Structure and growth investigations of noncontinuous thin films belong to the fundamental studies in the fields of heterogeneous catalysis and gas sensors. Small clusters of metals like Pd, Rh, Pt deposited on different substrates form active *Corresponding author. E-mail address: [email protected] (K. Ma$sek).
phases in these devices. Because of the great complexity of such systems, fundamental studies are performed on so-called model systems consisting of small well-defined overlayers prepared on a single-crystalline substrate by means of vacuum evaporation. The model studies contribute to understanding of phenomena observed on laboratory systems as well as on real catalysts and gas sensors. In recent years, great effort has been made to investigate the structural, electronic and chemical properties of Pd-based catalysts and sensors.
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00714-5
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
60
involved in RHEED, surface and bulk plasmon losses (10–25 eV) and atomic core-shell excitations (a few hundred to a few thousand eV) are the most easily accessible to in situ studies. The combination of RHEED and reflection high-energy electron loss spectroscopy (RHEELS) is especially useful in the early stages of thin film growth when the intensity of the diffraction pattern is too low. The use of these techniques is demonstrated by examples of the growth of ultrathin Pd layers on alumina and cassiterite ((1 1 0) single-crystal SnO2) single-crystalline substrates. The results show different behaviour of these systems due to the strong difference in the metal–substrate interaction.
Alumina and tin dioxide are frequently used as substrates. Although aluminium oxide is generally considered as an inert substrate in the case of Pd/ alumina system, some reports indicate the MSI (metal–substrate interaction) effect [1–6]. Therefore, whilst metals like Cr, Ni and Cu are bonded to the oxygen sites, Pd forms bonds with the aluminium sites, causing stronger MSI in the case of reduced alumina surface [7]. It was pointed out that the surface structure and morphology depended strongly on the substrate preparation, especially on the cleaning procedure [7]. The strong interaction between Pd and Sn is significant also in the case of gas sensors based on SnO2 [8]. The enhanced sensitivity of Pd-doped SnO2 gas sensor was found [9,10]. This effect has not so far been successfully explained. Reflection high-energy electron diffraction (RHEED) is a powerful tool for surface structure investigation. Because of a favourable geometry (grazing angles of incidence and exit), RHEED makes it possible to investigate in situ epitaxial thin film growth in real time. The diffraction pattern analysis allows distinguishing between different modes of overlayer formation to be identified. The energy losses are characteristic for the surface structure and chemical composition of the sample. Adding energy loss spectroscopy capabilities to the RHEED detector can access the information contained in the electron loss spectrum. Among the several energy loss processes
2. Experimental The studies were performed in a specially designed UHV system. The UHV chamber was equipped with a RHEED facility permitting simultaneous observation of the substrate surface during deposition. The instrument was operated at an acceleration voltage of 25 kV. Schematic representation of the RHEED—RHEELS experimental set-up is shown in Fig. 1. The monoenergetic and low-divergence electron beam touches the sample surface at an angle that is typically in the range of 0.5–31. Diffracted electrons pass through the RHEA-100 energy analyser (STAIB INSTRUMENTS). The diffracted intensity RHEA
ELECTRON GUN
screen
SAMPLE
semi transparent mirror CAMERA
CCD
focusing and retarding lens
grid
PHOTO MULTIPLIER
lock-in
RHEED-VISION COMPUTER SYSTEM
Fig. 1. RHEED–RHEELS experimental set-up.
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
(a)
a.u.
Pd deposition
0
20
(b)
1.5 1.0 0.5 0.0 0
3.1. Pd growth on Al2O3 (0 0 01) substrate The Pd layer with a thickness of approximately 4 monolayers was deposited at a substrate temperature of 4201C. The final diffraction pattern is presented in Fig. 2a. The interpretation of a similar pattern can be found in Ref. [12]. The presence of relatively large circular spots in the
60
2.0
(c)
3. Results and discussion
40 energy [eV]
2.5
I(Pd)/I(Al2O3)
pattern is then converted to visible light by a fluorescent screen and recorded by the RHEEDVision Computer System. The RHEA-100 retarding-field analyser consists of electron optics and a single suppressor grid. A semi-transparent mirror reflects part of the light emitted by the screen to a photo-multiplier. Differentiation of the intensity signal using the lock-in technique with respect to the suppressor-grid potential yields the energy spectrum of the diffracted electrons. The Pd overlayers were prepared by heteroepitaxial growth using a special evaporation cell—micro electron beam evaporation source (MEBES) [11] operating at very low evaporation rates (the evaporation rate in these experiments was estimated to be approximately 0.2 monolayers per minute). The deposition was carried out at a background pressure of about 5 107 Pa. Liquid nitrogen cooled cryopanel, surrounding the MEBES cell, kept the stable pressure conditions during the deposition. The Pd layers of approximately 4 and 8 monolayers were deposited on alumina and tin dioxide substrates, respectively. The sapphire (0 0 0 1) and natural tin dioxide (cassiterite) (1 1 0) surfaces were used as substrates. The alumina substrate was chemically polished, cleaned and annealed in air at 12001C for 2 h and in UHV at 3001C for 1 h before the deposition. The (1 1 0) cut of cassiterite was mechanically polished and introduced into the vacuum chamber. The well-defined plane surface was obtained after repeated cycles of ion bombardment at an energy of 1 keV and heating in oxygen at a pressure of 1 105 Pa and a temperature of 6301C for 4 h.
61
5
10
15
20
25
deposition time [min]
Fig. 2. (a) RHEED diffraction pattern from a Pd deposit on an Al2O3 (0 0 0 1) substrate representing the (2 1 1) Pd crystallographic planes. The primary electron beam was parallel to the ½2 1 1 0 Al2O3 crystallographic direction. (b) A series of RHEELS spectra taken during the Pd deposition. (c) Evolution of the ratio of the characteristic loss intensities of Pd and Al2O3 during the deposition.
diffraction pattern indicates the formation of small three-dimensional particles. The orientation relationship between the substrate and the deposited
62
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
lattices was found as follows: ð1 1 1ÞPdjjð0 0 0 1ÞAl2 O3 ; ½2 1 1Pdjj½2 1 1 0Al2 O3 ðdouble positioningÞ:
(a)
Pd
Sn
SnO2
RHEELS spectra
20
a.u.
14 13 12 11 5 4 0 0
20
(b)
40
60
Energy [eV] 1.6 1.4 1.2
I(Pd)/I(SnO2)
Because of the FCC structure of Pd, these epitaxial parameters correspond well to an accommodation of two crystallographic planes having hexagonal symmetry (Pd (1 1 1) and Al2O3 (0 0 0 1)). A series of RHEELS spectra taken in regular time period of 3 min during the Pd deposition are presented in Fig. 2b. The spectra were treated by the following procedure. At first, the Shirley background and the elastic peak were subtracted. To compare the spectra a normalisation procedure was performed (area under the curve being equal to 1). The lowest spectrum corresponding to the clean alumina substrate shows the wide peak at energy about 24 eV. The interpretation of the EELS alumina spectrum can be found in Ref. [13]. The Pd deposition resulted in appearance of a Pd electron loss signal (Pd surface plasmon) at energy of 4.5 eV. This value is smaller than the one observed for the bulk material. It is probably due to the size dependence of the energy position of plasmon peaks in the case of small clusters [14]. In Fig. 2c, the intensity ratio of Pd and alumina signals during the growth is plotted. The intensities were obtained by deconvolution of RHEELS spectra. Due to the extreme surface sensitivity of this method, the asymptotic increase of Pd/Al2O3 ratio during the Pd deposition corresponds to the increase of the amount of Pd surface atoms. In addition, if we take into account the threedimensional features of the Pd diffraction patterns and that no intensity oscillation of specular spot was observed, Volmer–Weber growth mode (threedimensional without growth of an interfacial continuous layer) can be deduced.
1.0 0.8
2nd deposition
0.6
3rd deposition 1st deposition
0.4
annealing
0.2 0.0 0
(c)
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21
No. of step in experiment
3.2. Pd growth on SnO2 (1 1 0) substrate The Pd deposit with a total nominal thickness of approximately 8 monolayers was deposited at a substrate temperature of 3051C during three consecutive depositions. The diffraction pattern obtained at the end of the third depositions is presented in Fig. 3a. It consists of two diffraction
Fig. 3. (a) RHEED diffraction pattern from a Pd deposit on a SnO2 (1 1 0) substrate. The primary electron beam was parallel to the ½1 1 0 SnO2 crystallographic direction. (b) A series of RHEELS spectra taken at different steps of the experiment. The labels correspond to the x-axis values in Fig. 3c. (c) Evolution of the ratio of the characteristic loss intensities of Pd and SnO2 during the experiment.
K. Ma$sek et al. / Vacuum 71 (2003) 59–64
features corresponding to two phases of the deposit. (1) Relatively large circular diffraction spots indicate three-dimensional Pd particles having (1 1 1) epitaxial plane. The interpretation of a similar diffraction pattern is published in Ref. [15]. The deposit is composed of two cluster populations rotated by 901 to each other along the substrate surface normal. It corresponds well to the rectangular symmetry of the SnO2 (1 1 0) substrate surface. (2) Diffuse diffraction rings indicate the presence of a polycrystalline phase on the surface. It arises from a Pd/Sn alloy overlayer covering the surface of the Pd particles. This effect is well known as ‘‘encapsulation’’, e.g. see Ref. [16] and its formation will be discussed in the following section. Fig. 3b represents a series of RHEEL spectra taken at different steps of the experiment. Time period between the spectra was 3 min during the Pd deposition. The spectra were treated in the same way as in the previous case. The spectrum marked 0 corresponds to the clean SnO2 substrate surface after the preparation procedure described in the experimental section. The EEL spectrum from a stoichiometric SnO2 surface consists mainly of a characteristic loss at 18 eV, which corresponds to the interband transitions in oxygen atoms [17,18]. Against these spectra spectrum 0 in Fig. 3b contains in addition, an important loss peak indicated by continuous line at 8 eV which corresponds to the plasmon peak of free metallic Sn. We conclude that our preparation procedure leads to the formation of a non-stoichiometric Snrich substrate surface. The dashed lines indicate other possible SnO2 characteristic losses. After the Pd deposition, the Pd plasmon peak at an energy of 5 eV appeared (curves 4, 11, 20). Fig. 3c shows the evolution of the intensity ratio of the Pd and SnO2 (at 18 eV) RHEELS signals. Completely different behaviour, in comparison to the previous case, can be seen. The slow decrease of this ratio during the first deposition can be explained by formation of three-dimensional Pd– Sn alloy particles by taking Sn atoms from the
63
substrate surface. This effect was also observed on the same system by XPS and ion scattering [19]. Curves 12–14 in Fig. 3b represent the evolution of RHEELS spectra during the thermal annealing at a temperature of 5101C for 45 min. The decrease of Pd signal against the increase of Sn signal can be seen. This effect is caused by reconstruction of the Pd–Sn particles. The driving force of this reconstruction is the effort of the system to reach the minimum free surface energy. Because of Sn having much lower free surface energy than Pd, this leads to the formation of Pd particles covered by an Sn or Pd–Sn layer [19]. Thermal annealing supplies the energy necessary for this reconstruction process. The peak of the Pd signal at the first stage of the subsequent depositions (see Fig. 3c) is probably caused by the growth of a two-dimensional Pd layer on the free surface of the SnO2 substrate and the subsequent reconstruction to three-dimensional particles. If we take into account a further slow increase of the Pd/SnO2 signal ratio, we can deduce that the growth continues by the formation of three-dimensional Pd particles.
4. Conclusion The growth and structure of a Pd deposit on aAl2O3 (0001) and SnO2 (1 1 0) substrate surfaces was investigated by means of RHEED and RHEELS. It was found that the deposit structure depends strongly on the nature of the substrate. The results showed that Pd formed three-dimensional particles with the Pd (1 1 1) crystallographic plane parallel to the substrate surface. Two different growth modes of Pd overlayers were found according to the substrate nature. While the three-dimensional Volmer–Weber growth was found on a Al2O3 substrate, the formation of a Pd–Sn alloy and the encapsulation of Pd particles by Sn atoms were observed in the case of the SnO2 substrate. The latter effect was explained by an effort of the system to minimise its surface free energy. The Pd surface plasmon peaks were found at an extremely low energy of 5 eV. This is probably a consequence of very small size of Pd particles and Mie interaction with the insulating substrate [14].
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K. Ma$sek et al. / Vacuum 71 (2003) 59–64
The presented results show that the combination of RHEED and RHEELS is a powerful tool for the investigation of structure and growth of heteroepitaxial systems. It is especially useful in the early stages of thin film growth when the intensity of the diffraction pattern is too low. RHEELS is able to provide additional information on the growth of thin films and, in this way, to recognise growth effects indistinguishable by standard RHEED. Acknowledgements This work was supported by grant no. 169/2001/ B-FYZ/MFF of the Grant Agency of Charles University by the research program MSM 113200002 that is financed by the Ministry of Education of the Czech Republic.
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