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Core level spectroscopy and RHEED analysis of KGd(WO4)2 surface
Solid State Communications 133 (2005) 347–351 www.elsevier.com/locate/ssc
Core level spectroscopy and RHEED analysis of KGd(WO4)2 surface V.V. Atuchina,*, V.G. Keslerb, N.Yu. Maklakovac, L.D. Pokrovskya a
Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russian Federation b Technical Centre, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russian Federation c Laboratory of Crystallization of Oxide Materials, Institute of Mineralogy and Petrography, Novosibirsk 630090, Russian Federation Received 4 November 2004; received in revised form 29 November 2004; accepted 30 November 2004 by E.L. Ivchenko Available online 9 December 2004
Abstract Structural and electronic characterisation of mechanically polished (010) KGd(WO4)2 (KGW) has been produced by reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). With XPS analysis the original element binding energies, chemical composition and valence band structure of KGW have been determined. q 2004 Published by Elsevier Ltd. PACS: 42.70.Hj; 61.14.Hg; 82.65.KI; 82.80.Pv Keywords: A. Tungstates; C. RHEED; D. Surface structure; E. XPS
1. Introduction Monoclinic a-KGd(WO4)2 (KGW) is one from the best host materials is for laser active lanthanide RE3C (REZPr, Nd, Dy, Ho, Er, Tm, Yb) ions, possessing very high generation performance at low pumping energies [1–6]. KGW also has large cubic optical nonlinearity c(3) and can be used as an effective medium in lasers with ultra-low threshold stimulated Raman scattering [6,7]. From structural point of view, KGW is a representative of wide ARE(WO4)2 (AZK, Rb) binary tungstate family and strong modification of crystal properties with high doping by RE or substitution of Rb or Cs (low range) for K is possible without optical quality degradation [5,8,9]. Recently, an attempt to form KGW thin film optical waveguides has been reported [10]. Most of the previous works on KGW discussed the structure and spectroscopic properties of RE-doped crystals, the
* Corresponding author. Tel.: C7 3832 343889; fax: C7 3832 332771. E-mail address: [email protected] (V.V. Atuchin). 0038-1098/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ssc.2004.11.042
information on other properties of pure and doped KGW is very scare or absent. The present work is directed on the observation of structural and electronic characteristics of KGW polished surface.
2. Experimental The crystal of a-KGW has been grown by top-seeded solution method (TSSG) [6,11,12]. The sample with dimensions 10!12!1 mm3 and (010) big plane was prepared by mechanical polishing in water based suspension of diamond nanoparticles. To remove the residual contaminations the sample was cleaned in ammonia based solution and rinsed in deionized water. Crystallohraphic properties of the surface were investigated by reflection high energy electron diffraction (RHEED) at an electron accelerating voltage 50 kV. For charging effect elimination the chargeneutralization flood gun was utilized. Element composition and surface electronic structure of KGW was studied with X-ray photoelectron spectroscopy (XPS). Photoemission spectra were obtained with MAC-2
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V.V. Atuchin et al. / Solid State Communications 133 (2005) 347–351
Fig. 1. RHEED patterns for (010) KGW surface: (a) electron beam is along [100] and (b) electron beam is along [010]. Minor background signal detected implies the presence of some part of amorphous component.
(Riber) analyser using nonmonochromatic Mg Ka radiation (1253.6 eV). The energy resolution of the instrument was chosen to be 0.5 eV to have sufficiently small spreading of natural core level lines in combination with reasonable signal/noise ratio. Under the conditions the observed full width at half maximum (FWHM) of Cu 2p3/2 line was 1.4 eV. The binding enegry scale was calibrated with reference to the Cu 2p (932.7 eV) and Cu 3p (75.1 eV) lines, yielding an accuracy of G0.1 eV in any peak position determination in reference to the copper Fermi level. Surface charging effects were taken into account in reference to C 1s level (284.6 eV) of adventitious carbon. To remove the surface contaminations, the bombardment by ArC ions has been performed with an energy of 3 keV at sample current 100 nA. The ion beam was rastered over area 7!20 mm2 and the sputtering rate was estimated as ˚ /min. The sputtering for 50 min results in complete 0.25 A suppression of C 1s signal and to relate the energy scales for initial and bombarded surfaces the persistance of binding energies of W 4f7/2 core level in these spectra has been postulated.
3. Results and discussion RHEED observation of mechanically polished and chemically cleaned KGW surface reveals the presence of
Fig. 2. Survey XPS spectra from (1) as inserted and (2) cleaned by ArC bombardment KGW surface.
only some amorphous phase at the outer surface. To remove this modified layer, the surface was subjected to mechanochemical treatment on chamois–leather in glycerine– mannitum mixture with hand stirring for 20 min. No abrasive was added to the mixture. The period of 20 min was enough to expose the monocrystal surface of the substrate, and taking into account typical penetration depth of high-energy electrons into solids [13] the thickness of ˚. removed amorphous layer may be estimated as O50–100 A RHEED patterns, recorded after the procedure, are shown in Fig. 1. Intensive streak system displays the domination of monocrystal surface phase that is refined as KGW. Minor background signal detected implies the presence of some part of amorphous component. In this state of the sample surface the measurements of photoemission characteristics have been produced with XPS technique. Wide scan X-ray photoelectron spectra for as inserted and bombarded surfaces are presented in Fig. 2. Apart from the photoelectron and Auger transitions of principal KGW constituents only intensive C 1s line was detected in spectrum (1) among strong spectral features. Besides this, very weak signal of adsorbed nitrogen was also observed. Both these contaminations have been completely removed by ArC ion, 3 keV bombardment during 50 min. All other spectral components measured for initial and bombarded surfaces were attributed to basic elements K, Gd, W and O.
V.V. Atuchin et al. / Solid State Communications 133 (2005) 347–351
349
Table 1 Electron binding energies of the observed lines in photoemission spectra of KGW Binding energy (eV)
Core level
35.2 50.5 79.7 142.3 147.4 170.0 246.5 259.0 292.3 360–400 425.8 530.1 741.6 761.6 1003.1 1085.0–1144.5 1187.6
W 4f, Gd 5s W 5p1/2 W 5s Gd 4d3/2 Gd 4d5/2 Gd 4d satellite W 4d3/2 W 4d5/2 K 2p3/2 Gd 4s, Gd MNN W 4p3/2 O 1s O KLL O KLL K LMM W, Gd Auger Gd 3d
A very complex structure of photoelectron spectrum of KGW with several peak superpositions and fine structure appears due to great number of Gd and W core levels [14– 18]. A peculiarity of KGW photoelectron spectra is the presence of few wide peaks in the binding energy range 1060–1150 eV related to Auger transitions on the external core levels of Gd and W. The binding energies of the core levels and Auger transitions detected for KGW surface cleaned by bombardment are presented in Table 1. Some broadening of K 2p, W 4f, W 4p and W 4d peaks have been found in core level spectra for bombarded surface in reference to the lines recorded for as inserted one. In Fig. 3(a) and (b) this effect is shown for K 2p, W 4d and W 4f doublet components. Previously, worse differenciating of the K 2p1/2 and K 2p3/2 components for bombarded surface in comparison to that for started surface has been found also in ATiOPO4 (AZK, Tl) [19,20]. So, the interaction of middle energy ArC ions with oxide crystal surface generates noticeable variation of chemical environment of univalent cations. As it seems, the appearance of intensive components from the lower energy side of all tungsten lines may be governed by partial transition of W6C to low valency states induced by oxygen loss from top surface layers [21]. Similar transformation of W 4f level, caused by reduction of WO3 surface in hydrogen flow, has been detected in Ref. [22]. Furthermore, appearance of new spectral components in W 4f line with broadening to lower energies when chemical composition in WOx oxides shifts from xZ3.0 to 2.0 has been found in Ref. [18]. The spectral features related to Gd core levels are pronounced for bombarded surface and the binding energies measured are in good relation to these reported for Gd2O3 [16]. In valence band of KGW only a particular peak at 8.3 eV is detected. As it seems this dominating component is
Fig. 3. Photoelectron spectra of (a) the (K 2p–W 4d) range, (b) (Gd 5s C W 4f) envelope and valence band and (c) O 1s core level for (1) as-inserted and (2) bombarded (ArC, 3 keV, 50 min) KGW surfaces.
related to Gd 4f level [14,16,23]. In Fig. 3(c) the O 1s core level is shown. Before the bombardment the O 1s peak had a shoulder from higher binding energy side, supposedly due to presence of some carbon oxide species on the surface. After
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Table 2 Relative element composition of KGW surface Sample
O
K
W
Gd
As-inserted surface Bombarded for 100 min by ArC (3 kV) Nominal composition KGd (WO4)2
0.52 0.50 0.67
0.08 0.08 0.08
0.25 0.30 0.17
0.15 0.12 0.08
the bombardment the peak profile becomes nearly symmetric with FWHM equal to 1.9 eV (530.1 eV). The relative element contents were estimated by K 2p, O 1s, W 4f and Gd 4d peak areas and tabulated data on atomic sensitivity factors (ASF) [24]. This set of ASF was taken because double-pass cylindrical-mirror type analyzer was used in Ref. [24] for ASF determination and MAC-2 analyzer is of the same type. The results of the calculations presented in Table 2 are some different from nominal KGW chemical composition but no pronounced effect of bombardment on the constitutional element relative contents has been detected. We consider that the difference between the nominal and measured compositions is connected mainly with the accuracy of ASF used and probable dependence of its values on chemical state of the elements. Some overestimating of W content seems be a result of superposition of weak intensity Gd 5s feature and strong W 4f doublet. It is known that usual accuracy of routine estimation of relative element concentration using tabulated ASF is not better than w10% and more precise quantification is possible only by determination of ASF for special internal standards under the same experimental conditions.
4. Conclusions The present study concerns the electronic structure of KGd(WO4)2 (KGW) crystal surface. It has been shown by RHEED analysis that mechanically polished surface of this practically valuable optical material is covered by thick ˚ . This modified layer can be amorphous layer, O50–100 A removed by accurate hand treatment applied to expose the monocrystal surface. The observation of the surface with XPS yields a set of constitutional element binding energies and Auger parameters that are the characteristics of KGW. In practical applications of RE:KGW crystals for fabrication of laser elements it is reasonable to develop such polishing technology that avoids the formation of thick amorphous layer on the surface. The irreproducible chemical composition of the layer due to possible contaminations of polishing and cleaning agents is able to induce temporal degradation of optical parameters of the elements. The electronic parameters of monocrystal KGW surface defined in this study can be used as a reference and are valuable to control the chemical state of fine optical surfaces of RE:KGW crystals.
References [1] A.A. Kaminskii, A.A. Pavlyuk, P.V. Klevtsov, I.F. Balashov, V.A. Berenberg, S.E. Sarkisov, V.A. Fedorov, M.A. Petrov, V.V. Lyubchenko, Izv. Acad. Nauk SSSR, Neorg. Mater. 13 (1977) 582. [2] A.A. Kaminskii, A.A. Pavlyuk, I.F. Balashov, V.A. Berenberg, V.V. Lyubchenko, V.A. Fedorov, T.I. Butaeva, L.I. Bobovich, Izv. Acad. Nauk SSSR, Neorg. Mater. 13 (1977) 1541. [3] A.A. Kaminskii, A.A. Pavlyuk, I.F. Balashov, V.A. Berenberg, V.V. Lyubchenko, V.A. Fedorov, T.I. Butaeva, L.I. Bobovich, Izv. Acad. Nauk SSSR, Neorg. Mater. 14 (1978) 2256. [4] M.C. Pujol, M. Rico, C. Zaldo, R. Sole´, V. Nikolov, X. Solans, M. Aguilo´, F. Dı´az, Appl. Phys. B68 (1999) 187. [5] M.C. Pujol, R. Sole´, J. Gavalda`, J. Massons, M. Aguilo´, F. Dı´az, V. Nokolov, C. Zaldo, Mater. Res. 14 (1999) 3739. [6] A.A. Kaminskii, J.B. Gruber, S.N. Bagaev, K.-i. Ken-ichi Ueda, U. Ho¨mmerich, J.T. Seo, D. Temple, B. Zandi, A.A. Kornienko, E.B. Dunina, A.A. Pavlyuk, R.F. Klevtsova, F.A. Kuznetsov, Phys. Rev. 65 (2002) 125108. [7] I.V. Mochalov, J. Opt. Technol. 62 (1995) 746. [8] N.V. Klevtsov, R.F. Klevtsova, J. Struct. Chem. 18 (1977) 419 (in Russian). [9] M.T. Borowiec, A. Majchrowski, H. Szymczak, M. Zaleski, J. Zmija, Cryst. Res. Technol. 35 (2000) 1343. [10] P.A. Atanasov, R.I. Tomov, J. Perrie´re, R.W. Eason, N. Vainos, A. Klini, A. Zherikhin, E. Millon, Appl. Phys. Lett. 76 (2000) 2490. [11] R. Sole´, V. Nikolov, X. Ruiz, J. Gavalda`, X. Solans, M. Aguilo´, F. Dı´az, J. Cryst. Growth 169 (1996) 600. [12] C. Pujol, M. Aguilo´, F. Dı´az, C. Zaldo, Opt. Mater. 13 (1999) 33. [13] G. Schimmel, Elektronenmikroskopische Methodik, SpringerVerlag, Berlin, Heidelberg, New York, 1969. [14] D.F. Mullica, C.K.C. Lok, H.O. Perkins, G.A. Benesh, V. Young, J. Electron. Spectrosc. Relat. Phenom. 71 (1995) 1. [15] J. Szade, M. Neumann, J. Alloys Compd. 236 (1996) 136. [16] Yu.A. Teterin, A.Yu. Teterin, Russ. Chem. Rev. 71 (2002) 347. [17] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Eden Prairie, Minesota, 1978. p. 55374. [18] O. Yu, Khyzhun, J. Alloys Compd. 305 (2000) 1. [19] V.V. Atuchin, V.G. Kesler, N.Yu. Maklakova, L.D. Pokrovsky, V.N. Semenenko, Surf. Interf. Anal. 34 (2002) 320. [20] V.V. Atuchin, V.G. Kesler, N.Yu. Maklakova, L.D. Pokrovsky, V.I. Voronkova, V.K. Yanovskii, Surf. Rev. Lett. 11 (2004) 191.
V.V. Atuchin et al. / Solid State Communications 133 (2005) 347–351 [21] S.F. Ho, S. Contarini, J.W. Rabalais, J. Phys. Chem. 91 (1987) 4779. [22] J. Haber, J. Stoch, L. Ungier, J. Solid State Chem. 19 (1976) 113. [23] F.W. Kutzler, D.E. Ellis, D.J. Lam, B.W. Veal, A.P. Paulikas, A.T. Aldred, V.A. Gubanov, Phys. Rev. B29 (1984) 1008.
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[24] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (Eds.), Handbook of X-ray photoelectron spectroscopy, Perkin–Elmer Corporation, Physical Electronics Division, 6509 Flying Cloud Drive Eden Prairie, Minnesota, 1979.
Core level spectroscopy and RHEED analysis of KGd(WO4)2 surface V.V. Atuchina,*, V.G. Keslerb, N.Yu. Maklakovac, L.D. Pokrovskya a
Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russian Federation b Technical Centre, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russian Federation c Laboratory of Crystallization of Oxide Materials, Institute of Mineralogy and Petrography, Novosibirsk 630090, Russian Federation Received 4 November 2004; received in revised form 29 November 2004; accepted 30 November 2004 by E.L. Ivchenko Available online 9 December 2004
Abstract Structural and electronic characterisation of mechanically polished (010) KGd(WO4)2 (KGW) has been produced by reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). With XPS analysis the original element binding energies, chemical composition and valence band structure of KGW have been determined. q 2004 Published by Elsevier Ltd. PACS: 42.70.Hj; 61.14.Hg; 82.65.KI; 82.80.Pv Keywords: A. Tungstates; C. RHEED; D. Surface structure; E. XPS
1. Introduction Monoclinic a-KGd(WO4)2 (KGW) is one from the best host materials is for laser active lanthanide RE3C (REZPr, Nd, Dy, Ho, Er, Tm, Yb) ions, possessing very high generation performance at low pumping energies [1–6]. KGW also has large cubic optical nonlinearity c(3) and can be used as an effective medium in lasers with ultra-low threshold stimulated Raman scattering [6,7]. From structural point of view, KGW is a representative of wide ARE(WO4)2 (AZK, Rb) binary tungstate family and strong modification of crystal properties with high doping by RE or substitution of Rb or Cs (low range) for K is possible without optical quality degradation [5,8,9]. Recently, an attempt to form KGW thin film optical waveguides has been reported [10]. Most of the previous works on KGW discussed the structure and spectroscopic properties of RE-doped crystals, the
* Corresponding author. Tel.: C7 3832 343889; fax: C7 3832 332771. E-mail address: [email protected] (V.V. Atuchin). 0038-1098/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ssc.2004.11.042
information on other properties of pure and doped KGW is very scare or absent. The present work is directed on the observation of structural and electronic characteristics of KGW polished surface.
2. Experimental The crystal of a-KGW has been grown by top-seeded solution method (TSSG) [6,11,12]. The sample with dimensions 10!12!1 mm3 and (010) big plane was prepared by mechanical polishing in water based suspension of diamond nanoparticles. To remove the residual contaminations the sample was cleaned in ammonia based solution and rinsed in deionized water. Crystallohraphic properties of the surface were investigated by reflection high energy electron diffraction (RHEED) at an electron accelerating voltage 50 kV. For charging effect elimination the chargeneutralization flood gun was utilized. Element composition and surface electronic structure of KGW was studied with X-ray photoelectron spectroscopy (XPS). Photoemission spectra were obtained with MAC-2
348
V.V. Atuchin et al. / Solid State Communications 133 (2005) 347–351
Fig. 1. RHEED patterns for (010) KGW surface: (a) electron beam is along [100] and (b) electron beam is along [010]. Minor background signal detected implies the presence of some part of amorphous component.
(Riber) analyser using nonmonochromatic Mg Ka radiation (1253.6 eV). The energy resolution of the instrument was chosen to be 0.5 eV to have sufficiently small spreading of natural core level lines in combination with reasonable signal/noise ratio. Under the conditions the observed full width at half maximum (FWHM) of Cu 2p3/2 line was 1.4 eV. The binding enegry scale was calibrated with reference to the Cu 2p (932.7 eV) and Cu 3p (75.1 eV) lines, yielding an accuracy of G0.1 eV in any peak position determination in reference to the copper Fermi level. Surface charging effects were taken into account in reference to C 1s level (284.6 eV) of adventitious carbon. To remove the surface contaminations, the bombardment by ArC ions has been performed with an energy of 3 keV at sample current 100 nA. The ion beam was rastered over area 7!20 mm2 and the sputtering rate was estimated as ˚ /min. The sputtering for 50 min results in complete 0.25 A suppression of C 1s signal and to relate the energy scales for initial and bombarded surfaces the persistance of binding energies of W 4f7/2 core level in these spectra has been postulated.
3. Results and discussion RHEED observation of mechanically polished and chemically cleaned KGW surface reveals the presence of
Fig. 2. Survey XPS spectra from (1) as inserted and (2) cleaned by ArC bombardment KGW surface.
only some amorphous phase at the outer surface. To remove this modified layer, the surface was subjected to mechanochemical treatment on chamois–leather in glycerine– mannitum mixture with hand stirring for 20 min. No abrasive was added to the mixture. The period of 20 min was enough to expose the monocrystal surface of the substrate, and taking into account typical penetration depth of high-energy electrons into solids [13] the thickness of ˚. removed amorphous layer may be estimated as O50–100 A RHEED patterns, recorded after the procedure, are shown in Fig. 1. Intensive streak system displays the domination of monocrystal surface phase that is refined as KGW. Minor background signal detected implies the presence of some part of amorphous component. In this state of the sample surface the measurements of photoemission characteristics have been produced with XPS technique. Wide scan X-ray photoelectron spectra for as inserted and bombarded surfaces are presented in Fig. 2. Apart from the photoelectron and Auger transitions of principal KGW constituents only intensive C 1s line was detected in spectrum (1) among strong spectral features. Besides this, very weak signal of adsorbed nitrogen was also observed. Both these contaminations have been completely removed by ArC ion, 3 keV bombardment during 50 min. All other spectral components measured for initial and bombarded surfaces were attributed to basic elements K, Gd, W and O.
V.V. Atuchin et al. / Solid State Communications 133 (2005) 347–351
349
Table 1 Electron binding energies of the observed lines in photoemission spectra of KGW Binding energy (eV)
Core level
35.2 50.5 79.7 142.3 147.4 170.0 246.5 259.0 292.3 360–400 425.8 530.1 741.6 761.6 1003.1 1085.0–1144.5 1187.6
W 4f, Gd 5s W 5p1/2 W 5s Gd 4d3/2 Gd 4d5/2 Gd 4d satellite W 4d3/2 W 4d5/2 K 2p3/2 Gd 4s, Gd MNN W 4p3/2 O 1s O KLL O KLL K LMM W, Gd Auger Gd 3d
A very complex structure of photoelectron spectrum of KGW with several peak superpositions and fine structure appears due to great number of Gd and W core levels [14– 18]. A peculiarity of KGW photoelectron spectra is the presence of few wide peaks in the binding energy range 1060–1150 eV related to Auger transitions on the external core levels of Gd and W. The binding energies of the core levels and Auger transitions detected for KGW surface cleaned by bombardment are presented in Table 1. Some broadening of K 2p, W 4f, W 4p and W 4d peaks have been found in core level spectra for bombarded surface in reference to the lines recorded for as inserted one. In Fig. 3(a) and (b) this effect is shown for K 2p, W 4d and W 4f doublet components. Previously, worse differenciating of the K 2p1/2 and K 2p3/2 components for bombarded surface in comparison to that for started surface has been found also in ATiOPO4 (AZK, Tl) [19,20]. So, the interaction of middle energy ArC ions with oxide crystal surface generates noticeable variation of chemical environment of univalent cations. As it seems, the appearance of intensive components from the lower energy side of all tungsten lines may be governed by partial transition of W6C to low valency states induced by oxygen loss from top surface layers [21]. Similar transformation of W 4f level, caused by reduction of WO3 surface in hydrogen flow, has been detected in Ref. [22]. Furthermore, appearance of new spectral components in W 4f line with broadening to lower energies when chemical composition in WOx oxides shifts from xZ3.0 to 2.0 has been found in Ref. [18]. The spectral features related to Gd core levels are pronounced for bombarded surface and the binding energies measured are in good relation to these reported for Gd2O3 [16]. In valence band of KGW only a particular peak at 8.3 eV is detected. As it seems this dominating component is
Fig. 3. Photoelectron spectra of (a) the (K 2p–W 4d) range, (b) (Gd 5s C W 4f) envelope and valence band and (c) O 1s core level for (1) as-inserted and (2) bombarded (ArC, 3 keV, 50 min) KGW surfaces.
related to Gd 4f level [14,16,23]. In Fig. 3(c) the O 1s core level is shown. Before the bombardment the O 1s peak had a shoulder from higher binding energy side, supposedly due to presence of some carbon oxide species on the surface. After
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V.V. Atuchin et al. / Solid State Communications 133 (2005) 347–351
Table 2 Relative element composition of KGW surface Sample
O
K
W
Gd
As-inserted surface Bombarded for 100 min by ArC (3 kV) Nominal composition KGd (WO4)2
0.52 0.50 0.67
0.08 0.08 0.08
0.25 0.30 0.17
0.15 0.12 0.08
the bombardment the peak profile becomes nearly symmetric with FWHM equal to 1.9 eV (530.1 eV). The relative element contents were estimated by K 2p, O 1s, W 4f and Gd 4d peak areas and tabulated data on atomic sensitivity factors (ASF) [24]. This set of ASF was taken because double-pass cylindrical-mirror type analyzer was used in Ref. [24] for ASF determination and MAC-2 analyzer is of the same type. The results of the calculations presented in Table 2 are some different from nominal KGW chemical composition but no pronounced effect of bombardment on the constitutional element relative contents has been detected. We consider that the difference between the nominal and measured compositions is connected mainly with the accuracy of ASF used and probable dependence of its values on chemical state of the elements. Some overestimating of W content seems be a result of superposition of weak intensity Gd 5s feature and strong W 4f doublet. It is known that usual accuracy of routine estimation of relative element concentration using tabulated ASF is not better than w10% and more precise quantification is possible only by determination of ASF for special internal standards under the same experimental conditions.
4. Conclusions The present study concerns the electronic structure of KGd(WO4)2 (KGW) crystal surface. It has been shown by RHEED analysis that mechanically polished surface of this practically valuable optical material is covered by thick ˚ . This modified layer can be amorphous layer, O50–100 A removed by accurate hand treatment applied to expose the monocrystal surface. The observation of the surface with XPS yields a set of constitutional element binding energies and Auger parameters that are the characteristics of KGW. In practical applications of RE:KGW crystals for fabrication of laser elements it is reasonable to develop such polishing technology that avoids the formation of thick amorphous layer on the surface. The irreproducible chemical composition of the layer due to possible contaminations of polishing and cleaning agents is able to induce temporal degradation of optical parameters of the elements. The electronic parameters of monocrystal KGW surface defined in this study can be used as a reference and are valuable to control the chemical state of fine optical surfaces of RE:KGW crystals.
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