Study of Gallium Interaction with Metal-oxide Surfaces

Transcription

1 WDS'1 Proceedings of Contributed Papers, Part III, 72 77, 21. ISN MATFYZPRESS Study of Gallium Interaction with Metal-oxide Surfaces T. Zahoranová and V. Nehasil Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. In the present work, X-ray Photoelectron Spectroscopy (XPS) was used to study interaction of gallium deposited on different metal-oxide surfaces. Materials such as CeO x /Si, Si, γ-al 2 O 3, Al 2 O 3 /Al and Al foil and were used as substrates. Ga was deposited onto these substrates in number of steps up to a thickness of several nanometres. Prepared samples were further studied under different temperature conditions. Gallium oxidation state was determined by Auger parameter analysis. Interaction of gallium with highly oxidized substrates led to gallium oxidation. In general, interaction of gallium with surface was strongly influenced by oxidation state of particular substrate. In addition, intense interaction between gallium and CeO x support occurred. Introduction For many years, gallium has represented an important part in various technological applications. With its high sensitivity to oxygen, metallic gallium is easily oxidised to its most common oxide form Ga 2 O 3. Gallium oxide is electrically insulating at room temperature but exhibits semiconducting properties at high temperatures. The conductivity and also interaction with atmospheric oxygen were widely studied on both crystal and polycrystalline gallium oxide specimens [Lorenz et. al., 1967], [Harwig et. al., 1976]. Cerium oxide (CeO 2 - ceria) found use in electronic and optical applications due to its excellent properties (high index of refraction, good adhesion and stability against mechanical abrasion or high temperature resistance) [Xiao et al., 23]. Ceria represents considerable part in exhaust catalysts in car industry. Thanks to its ability to change the stoichiometry under reducing conditions, CeO 2 plays an important role in three-way catalysts. Reduced cerium oxide in the form of Ce 2 O 3 (cerium in the state of Ce 3+ ) can be easily oxidized back to CeO 2 in the presence of oxygen. Present research shows that catalytic activity of transmission metals supported on cerium oxide depends closely on the oxidation state of ceria [Mullins et al., 1998]. Several studies have dealt with gallium oxide cerium interaction. The performance of Ce doped Ga 2 O 3 gas sensors have been tested [Li et. al., 23]. In Ref. [Skála et.al., 28] authors have examined gallium-ceria model catalytic system. Reduction of ceria substrate has been observed after deposition of,35 nm of gallium. Simultaneously, immediate oxidation of gallium occured and also formation of mixed Ga-Ce-O oxide has been proposed. This study follows after our previous experiments regarding catalytic properties of cerium oxide. Interaction of deposited gallium (up to approximately 2 nm) with CeO x /Si is examined and this substrate is further compared with standard inert catalytic substrates like γ-al 2 O 3, natural oxide (Al 2 O 3 ) and Si or Al metallic foil. Additionally, the influence of subsequent heating at temperatures up to 62 C is discussed. Thanks to its reducing properties gallium could promote catalytic activity of cerium oxide by creating oxygen deficient sites on its surface. Experimental All experiments were performed in an ultra-high vacuum (UHV) chamber with background pressure below 2x1-1 Torr. The experimental UHV chamber was equipped with a five-channel hemispherical analyzer (Omicron EA 125), dual anode (Mg/Al) X-ray source and Ar + ion gun. Mg Kα (1253,6 ev) and also Al Kα line (1486,6 ev) were used for XPS measurements to avoid overlapping of photoelectron lines for different substrates. Mg Kα was used in case of γ-al 2 O 3, CeO x /Si, Si and Al 2 O 3 /Al, while for Al foil we used Al Kα. The spectrometer was calibrated using gold, silver and copper samples. 72

2 In the present study, gallium was deposited on following metal and metal-oxide substrates: CeO x /Si, γ-al 2 O 3, Al 2 O 3 /Al, Al foil and Si. The preparation of all these samples are described in following paragraph. Gallium was evaporated from sapphire crucible at temperatures C. We estimated deposition rate of gallium as,4 nm/min. Cerium oxide layer was prepared by magnetron sputtering on silicon substrate to a thickness of several tens of nanometres. efore gallium deposition, CeO x /Si sample was cleaned by heating for 3 minutes at 38 C. γ-al 2 O 3 substrate was prepared by thermal oxidation of Al foil which was annealed at 6 C for 24 hours in ambient atmosphere, according to Ref. [Stará et. al., 1995] and [Ealet et. al., 1994]. y this method polycrystalline films of alumina were formed with thickness of about 5-1 nm. In cited literature it was shown that layers are formed by fcc Al 2 O 3 crystals with size of,1 μm. This substrate was cleaned in situ by heating at 52 C for 5 min and subsequent Ar + ion sputtering for 2 min (E i = 1 kev). Natural oxide - Al 2 O 3 /Al was prepared as Al foil oxidised in air (thickness of oxidised layer was appoximately 15 2 Å). The sample was afterwards heated at 52 C for 1 min in UHV. Pure Al foil used as another substrate was cleaned by Ar + ion sputtering (3 min), heating at 52 C for 1 min and another Ar + sputtering (3 min). y this procedure we wanted to obtain a metal-aluminium surface with a minimum of oxygen. We also used silicon substrate cleaned by heating at 32 C for 1 min. Gallium was deposited onto cleaned substrates in several steps. In order to compare the amounts of gallium deposited on all samples, similar deposition time was used. The number of deposition steps was individual for particular samples. Studied samples were afterwards heated in steps from 12 C to 62 C. XPS measurements were performed after every deposition step and sample heating. Results and discussion The thickness t of deposited gallium layer was determined from the formula derived from Ref. [riggs et. al., 199]. In our case we considered expression for continuous overlayer: t cos ln I I t (1) where γ is angle of detection related to the surface normal (in our case cos γ = 1) and λ represents IMFP of photoelectrons from the atom in the substrate that pass through the layer of deposited metal. I is an intensity of photoelectron line of clean substrate and I t is an intensity of this line after deposition of metal layer with thickness t. The time of deposition of gallium on individual samples was compared to gallium layer thickness calculated from equation (1). It was proved, that thicknesses of Ga layers on different samples with approximately same time of deposition were consistent with each other. In Table 1 the thicknesses of gallium layers on all substrates after the first and last deposition are listed. We show also the development of thickness after the first and last heating. The values of oxygen amount on all materials determined from O 1s intensity (I O1S ) and respective surface material intensity (I sub ) using relation I O1s /(I O1s +I sub ) are also listed in the table. Table 1. Values of oxygen amount in used samples and thicknesses of overlayers on particular samples after the first and last gallium deposition and after the first and last heating. The heating temperatures are noted in parenthesis. t [nm] Sample amount of oxygen after 1 st Ga deposition after last Ga deposition after 1 st heating after last heating CeO x /Si,79,2 1,7 1,5 (22 C),4 (52 C) γ-al 2 O 3 /Al,62,5,3,4 (12 C),1 (52 C) Al 2 O 3 /Al,57,7 2,9 2,7 (22 C) 2,2 (52 C) Al,4,3,8,4 (8 C),3 (52 C) Si,45,2 1,6 1,3 (32 C) 1,1 (62 C) 73

3 Auger parameter Modified Auger parameter (MAP) was used to characterize the chemical state of gallium on all substrates after the deposition and subsequent annealing. The main advantage of using this parameter is its independence on charging effects. The value for modified Auger parameter can be obtained as a sum of binding energy of photoelectron line and kinetic energy of particular Auger line [riggs et. al., 199]: MAP = (i) + E K (jkl) (2) In our case, photoelectron line Ga 2p 3/2 and Auger line Ga LMM were used. Calculated values of MAP for all samples after the first and last deposition of Ga and after the first and last heating are presented in Table 2. These experimentally obtained values were compared with a database [NIST Standard Reference Database]. MAP in range 218,1-218,4 ev represent gallium oxide whereas values 2184,8 2185,1 ev imply presence of the metallic gallium. The results in Table 2 show that gallium grows as a metal on following samples: CeO x /Si, Al 2 O 3 /Al, Al and Si. On these substrates the metallic value of MAP is even more obvious after the last gallium deposition. In the case of Al 2 O 3 /Al, Al and Si, metallic character of Ga persists on the surface also after heating. On the other hand, we noticed that on the cerium oxide surface gallium became oxidised after heating the sample to 52 C. Gallium oxide value of MAP was observed on the inert γ- Al 2 O 3 support after Ga deposition and also after heating. In Fig. 1 we can see the development of Ga LMM Auger line in the process of deposition and heating. In left figure gallium on γ-al 2 O 3 is shown. In agreement with [utt et al., 1999] the shape of presented Auger spectra indicates presence of Ga 2 O 3. Compared to γ-al 2 O 3, Auger Ga LMM spectra from CeO x /Si (right picture) show metal character after deposition. Nevertheless heating of the sample at high temperature (42 C) caused gallium oxidation, which is apparent from last two spectra presenting Ga 2 O 3 shape. The discrepancy in values of binding energies of Ga 3+ compounds (approximately 1 ev) of these two surfaces (γ-al 2 O 3 and CeO x /Si) could be explained by the different interaction of gallium with individual substrates. The Auger spectra of other substrates (not displayed here) do not exhibit any change from the usual metallic shape. Table 2. Values of modified Auger parameters (MAP) on different samples after first and last gallium deposition and after first and last heating. Values for gallium oxide are indicated in bold font. Sample MAP after 1 st Ga deposition after last Ga deposition after 1 st heating after last heating CeO x /Si 2184,2 2184,9 2184, γ-al 2 O 3 /Al 2179, ,1 218,1 Al 2 O 3 /Al 2184, ,1 Al 2184,8 2184,9 2184,9 2185,1 Si 2184,2 2185,2 2185,3 2185, C 32 C 12 C E [ e V ] 4 th Ga deposition 3 rd Ga deposition 2 nd Ga deposition 1 st Ga deposition C 42 C 22 C 1,7 nm 1,2 nm Figure 1. Development of Auger spectra of Ga LMM on: γ-al 2 O 3 (left picture) and CeO x /Si (right picture) by gallium deposition and subsequent annealing.,2 nm 74

4 Ga 2p 3/2 changes Analysis of gallium 2p or 3d photoelectron line enables us to obtain additional information about its chemical state, as it was demonstrated in various publications ([Carli et. al., 1994], [Serykh et. al., 21]). Oxygen O 1s line was used for calibration needed because of charging of the samples. In Fig. 2, we can see the development of Ga 2p 3/2 spectra at particular preparation steps on γ Al 2 O 3 substrate. Only one peak with the binding energy approximately 1118,6 ev can be observed. Comparison of these energy values with literature [Trinchi et. al., 23] indicates the presence of Ga 2 O 3. In Fig. 3, Ga 2p 3/2 spectra after the deposition of approximately,35 nm of Ga on γ-al 2 O 3 and Al sample are presented. Value of gallium binding energy on aluminium substrate is 1116,7 ev, which according to [Chastain, 1992] suggests the presence of metal compound Ga. Gallium deposited on Si substrate exhibits according to MAP metallic character, however after deconvoluting the Ga 2p 3/2 spectra (Shirley background has been subtracted in all fitted spectra and Doniach-Sunjic line shapes were used for fitting metallic compounds), additional components occur at binding energy approximately 112 ev after heating to 32 C (Fig. 4). The difference between the metallic compound and the state in the picture marked with letter A suggests, according to [Serykh et. al., 21], that this new peak can be assigned to Ga 3+ state. Ga 2p 3/2 spectra on CeO x /Si substrate are presented. At the beginning, gallium grows like metal (1116,3-1116,8 ev). After heating to 42 C, oxidized state of Ga 3+ (1118,5 ev) became slightly subs C 22 C 12 C 4 th G a d e p o s i t i o n 3 rd G a d e p o s i t i o n 2 nd G a d e p o s i t i o n 1 st G a d e p o s i t i o n [e V] Figure 2 Development of Ga 2p 3/2 XPS spectra by gallium deposition and subsequent annealing on γ Al 2 O 3. 5 Al γ-al 2 O ,7 ev ,6 ev Figure 3. XPS spectra of Ga 2p 3/2 after deposition of approximately,35 nm of gallium on: γ-al 2 O 3 (dashed line) and Al substrate (straight line). 75

5 m e a s u r e d d a t a f i t t e d s p e c t r a A - G a G a 42 C - 1,1 nm 3 C - 1,3 nm 1,6 nm,2 nm A C 2 42 C C ,7 n m 1,2 n m,2 n m A m e a s u r e d d a t a f i t t e d s p e c t r a A - Ga 3+ - Ga Figure 4. Development of Ga 2p 3/2 XPS spectra by gallium deposition and subsequent annealing on: Si (left picture) and CeO x /Si (right picture) nm 1,2 nm A C D Fitted spectra Measured data A - Ce 4+ f 1 - Ce 3+ f 1 C - Ce 4+ f 2 D - Ce 3+ f C Figure 5. Development of the region of Ce 3d XPS with lower binding energies affected by gallium deposition and subsequent annealing of CeO x /Si substrate. Obtained data were deconvoluted to components respective to Ce 4+ and Ce 3+ states. tantial in the spectra at the expense of metallic compound. At 52 C, gallium oxide signal became dominant in Ga 2p 3/2 spectra. In Fig. 5 we display Ce 3d spectra. For better observation of changes in oxidation state we show only lower part of binding energies. The component marked with letter represents Ce 3+ state present in reduced cerium oxide. Stepwise reduction of cerium oxide with deposition of gallium and subsequent annealing is apparent on the spectra. Conclusion Interaction of deposited gallium with CeO x /Si, γ-al 2 O 3, Al 2 O 3 /Al, Al and Si substrates was studied by means of XPS. Changes after heating were further examined. After deposition of gallium on inert γ-al 2 O 3 catalytic substrate, immediate oxidation of Ga was observed. On the other hand, metallic gallium was proved on Al, natural oxide Al 2 O 3, Si and CeO x /Si supports. The results indicate that the oxidation state of gallium strongly depends on the nature of the support. In some cases (Al, natural Al 2 O 3, Si) with less amount of oxygen in surface layer interaction of deposited gallium with metal substrate through the thin oxide layer can occur. The difference from the oxidised gallium on CeO 2 (111) in Ref. [Skála et.al., 28] can be caused by higher amount of gallium (approximately 2 nm) in our case and also by different type of cerium oxide sputtered polycrystalline CeO x used in our experiment. 76

6 Heating at the high temperature did not affect the oxidation state of gallium on γ-al 2 O 3, Al and Al 2 O 3 /Al. Nevertheless, on Si and especially CeO x substrate, oxidation of metallic gallium after heating to 42 C occured. New peak at binding energy 1118,5 ev was assigned to Ga 3+ state. In the case of CeO x substrate strong interaction with deposited layer caused reduction of ceria. Subsequent heating to the high temperature caused oxidation of gallium and even higher CeO x reduction. This property can have an important role in activity toward catalytic reactions. References riggs D., Seah M.P (Ed.): Practical Surface Analysis (Second edition), John Wiley & Sons, England, 199 utt D. P., Park Y., Taylor T. N., Thermal vaporization and deposition of gallium oxide in hydrogen, J. Nucl. Mater., 264, 71-77, 1999 Carli R., ianchi C. L., XPS analysis of gallium oxides, Appl. Surf. Sci., 74, 99-12, 1994 Chastain J. (Ed.): Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer, USA, 1992 Ealet., Elyakhloufi M. H., Gillet E., Ricci M., Electronic and crystalographic structure of γ-alumina thin films, Thin Solid films, 25, 92-1, 1994 Harwig T., Wubs G. J., Dirksen G. J., Electrical properties of β-ga 2 O 3 single crystals, Solid State Commun., 18, 1223, 1976 Li X., Trinchi A., Wlodarski W., et. al., Investigation of the oxygen gas sensing performance of Ga 2 O 3 thin films with different dopants, Sens. Actuators, 93, , 23 Lorenz M. R., Woods J. F., Gambino R. J., Some electrical properties of the semiconductor β-ga 2 O 3, J. Phys. Chem. Solids, 28, 44, 1967 Mullins D. R., Overbury S. H., Huntley D. R., Electron spectroscopy of single crystal and polycrystalline cerium oxide surfaces, Surf. Sci., 49, , 1998 NIST Standard Reference Database 2, Version 3.5: Serykh A. I., Amiridis M. D., In-situ X-ray photoelectron spectroscopy study of supported gallium oxide, Surf. Sci., 64, 12-15, 21 Skála T., Šutara F., Cabala M., et. al., A photoemission study of the interaction of Ga with CeO 2 (111) thin films, Appl. Surf. Sci., 254, , 28 Stará I., Nehasil V., Matolín V., The influence of particle size on CO oxidation on Pd/alumina model catalyst, Surf. Sci., , , 1995 Trinchi A., Wlodarski W., Li Y. X., Study of thin film gas sensors, Proceedings of 2nd IEEE Sensors Conference, Toronto, Canada, October, 23 Xiao W., Guo Q., Wang E. G., Transformation of CeO 2 (1) films, Chem. Phys. Lett., 368, , 23 77