Electronic structure of KTiOAsO 4, a novel material for non-linear optical applications

Transcription

1 Electronic structure of KTiOAsO 4, a novel material for non-linear optical applications Victor V. Atuchin* a, Oleg Y. Khyzhun b, V. L. Bekenev b, A. K. Sinelnichenko b, Ludmila I. Isaenko c, S. A. Zhurkov c a Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 90, , Russia; b Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, UA Kyiv, Ukraine; c Laboratory of Crystal Growth, Institute of Geology and Mineralogy, SB RAS, Novosibirsk 90, , Russia ABSTRACT A high-quality KTiOAsO 4 (KTA) single crystal has been successfully synthesized employing the high temperature solution growth technique with rotating and pooling. The XPS valence-band spectra have been measured for pristine and the 1.5 kev Ar + ion-bombarded thin (001)KTA plate cut from the crystal part that was without any optical inhomogeneities or domain boundaries. The present XPS measurements have revealed the existence of two O 2s subbands on the XPS spectrum of the pristine (001)KTA surface. It has been established that 1.5 kev Ar + ion bombardment of the (001)KTA surface causes the complete elimination of the O 2s sub-band related to oxygen atoms involved in the formation of Ti O As bonds in KTA. In addition, the XPS results reveal that such a treatment leads to a significant decrease of the relative intensity of the XPS As 5+ 3d core-level spectrum and causes the formation of the additional As 0 3d core-level spectrum in the topmost layer of the (001)KTA surface. The experimental data are compared to the results of the first-principles band-structure calculations of KTA. Keywords: KTiOAsO 4, electronic structure, band-structure calculations, X-ray photoelectron spectroscopy 1. INTRODUCTION Potassium titanyl arsenate, KTiOAsO 4 (KTA), is a representative member of the KTiOPO 4 (KTP) crystal family, crystallizes in a complex orthorhombic cell (space group Pna2 1 ) with the following lattice constants: a = nm, b = nm, and c = nm (Z = 8) [1,2]. KTA crystals possess pronounced nonlinear optical properties [3]. Furthermore, KTA reveals high chemical and thermal stability, appropriate birefringence, wide range of transparency, high nonlinear optical coefficients [4-7]. Additionally, KTA is considered as excellent material for the applications in the infrared spectral range, e.g., in optical frequency conversion, optical waveguides and frequency conversion. It is worth mentioning that KTA reveals a higher nonlinear figure of merit and electro-optical coefficient by 60 and 30% (for the second harmonic generation under pumping at λ = 1.32 μm), respectively, and lower absorption in the 3 5 µm spectral region as compared to those of KTP [8-14]. Planar KTA-based optical waveguides were successfully fabricated by the implantation of 2.6 MeV He +, 3.0 MeV O + and 3.0 MeV Si + ions ar room temperature [15-17]. Two abnormalities in the temperature dependence of the KTA pyroelectric coefficient have been detected recently above 200 K [18]. The abnormalities are explained by Shaldin et al. [18] in terms of the superionic conductivity and structural changes associated with the temperature increase and isomorphic ion substitutions. The crystal structure of KTA is presented in Fig. 1. Two nonequivalent types of potassium, titanium and arsenic atoms per cell exist in this structure. They are labeled as K1, K2, Ti1, Ti2, As1 and As2 in Fig. 1. Additionally, the existence of ten different positions for oxygen atoms (labeled as O1 to O10) is a characteristic of the crystal structure of KTA: there are eight Ti O As bonds and two Ti O Ti bonds. One can consider the structure of KTA as the one consisting of short string chains of distorted TiO 6 octahedra and chains of distorted AsO 4 tetrahedra with K ions occupying sites in the open channels. Such open structure promotes the ionic conduction of K ions in the polar z direction of KTA. It is worth pointing that the AsO 4 octahedra are highly distorted in KTA in comparison with the PO 4 tetrahedra in KTP. This peculiarity was suggested to be the origin of the increased optical coefficients in KTA as compared to those in KTP [8]. Nonlinear Optics and Applications VII, edited by Mario Bertolotti, Joseph Haus, Alexei M. Zheltikov, Proc. of SPIE Vol. 8772, 87721I 2013 SPIE CCC code: X/13/$18 doi: / Proc. of SPIE Vol I-1

2 In the present work we report the measurements results of X-ray photoelectron valence-band spectra both for pristine and for Ar + ion-bombarded (001)KTA surface. This work is a continuation of our previous studies [19,20], where the results of binding energy measurements of the core-level photoelectrons in a KTA crystal, as well as the influence of 1.5 kev Ar + ion bombardment on the X-ray photoelectron core-level spectra features and reflection high-energy electron diffraction (RHEED) patterns for the (001)KTA surface, were reported. Generally, the behavior of (001)KTA surface is similar to that of other members of the KTP crystal family [21-25]. Both X-ray photoelectron spectroscopy (XPS) and RHEED results [19,20] allow one to assume a high sensitivity of the (001)KTA surface to the Ar + ion bombardment: the long-range order of the KTA surface is lost during irradiation, leading to the formation of completely amorphous surface layers at the ion dose of ions/cm 2. The formation of unstable layers with chemically active and passive arsenic states observed at the (001)KTA surface [19] was supposed to be a factor reducing the optical parameters and durability of nonlinear devices involving potassium titanyl arsenate. Therefore, the aim of the present work is to test the influence of 1.5 kev Ar + ion bombardment on the XPS valence-band spectra features. To elucidate the peculiarities of electronic state occupations in the near valence-band region of KTA, we performed the full potential linearized augmented plane wave (FP-LAPW) band-structure calculations for this outstanding crystal. Figure 1. Crystal structure of KTiOAsO 4 : K - blue, Ti - green, As - brown and O - red spheres. 2. EXPERIMENTAL A KTA single crystal was specially grown for the present experimental studies. The details of specimens preparation were reported previously [19]. Briefly, a KTA crystal was grown on the oriented seed by the high temperature solution growth (TSG) technique with rotating and pooling [12]. The high-purity К 2 СО 3 and ТiО 2 oxides were used as starting reagents and КН 2 AsO 4 was synthesized using high-purity metallic arsenic as a precursor. A photo image of the as-grown KTA single crystal studied in the present work is presented in Fig. 2. For the present XPS measurements, a thin (001)KTA plate was prepared with the following dimensions: length 7 mm, width 5 mm and height 1 mm. The (001)KTA plate was cut from the crystal part without any optical inhomogeneities or domain boundaries. At the finishing stage of polishing the (001)KTA plate, the nanodiamond powder, 0.1 grade, lubricated by water was applied. Measurements of the XPS valence-band spectra of the (001)KTA plate, both pristine and Ar + ion-bombarded, were carried out in an ion-pumped chamber of an ES-2401 spectrometer. A base pressure during the measurements was less than Pa. The Mg Kα radiation (E= ev) was used as a source of XPS valence-band spectra excitation. The Proc. of SPIE Vol I-2

3 binding energy of 84.00±0.05 ev of the XPS Au 4f 7/2 core-level spectrum was used as a reference. The energy drift due to charging effects was calibrated using the procedure described in details in Ref. [19]. Figure 2. As-grown KTA single crystal used in the present experimental studies (highlighting from below with white light). 3. CALCULATION METHOD For calculations of the electronic structure of KTA, the first-principles self-consistent FP-LAPW method with the WIEN97 code [26] has been used. The positions of the atoms constituting KTA lattice have been chosen in accordance with the crystallography data determined for the crystal in Ref. [1] and their muffin-tin sphere radii were assumed to be the following: 1.70 a.u. (K), 1.75 a.u. (Ti), 1.75 a.u. (As), and 1.35 a.u. (O) (1 a.u. = nm). For calculations of the exchange-correlation potential, the generalized gradient approximation by Perdew et al. has been adopted [27]. The Rk max parameter determining a number of linearized augmented plane waves equals to 6.0. The basis function consisted the same atomic orbitals of K, Ti, As and O atoms as it has been used earlier in Ref. [28]. The Brillouin zone sampling has been done using 72 k-points within the irreducible part of the zone. The calculations were interrupted when, for three following iterations, the change of total energy was less than ev. 4. RESULTS AND DISCUSSION The results of XPS valence-band spectra measurements of pristine and Ar + ion-irradiated (001)KTA surfaces are presented in Fig. 3. It is obvious that the valence band of KTA consists of three pronounced peculiarities, namely A, B, and C. The shape and the energy positions of the above fine-structure peculiarities of the XPS valence band spectrum recorded for pristine (001)KTA surface are in fair agreement with the data of the FP-LAPW band-structure calculations of KTA. The curve of total DOS of KTA and curves of its smothering employing different Lorenz parameter γ (γ is the full width at half-maximum of the Lorenz curve) is presented in Fig. 4. To obtain the smothered curves presented in Fig. 4, we followed the technique described in detail in Ref. 28. It is worth mentioning that such a technique allows one to obtain a theoretical XPS valence-band spectrum for the compounds. From the comparison of Figs. 3 and 4, it is obvious that the main Proc. of SPIE Vol I-3

4 XPS KTiOAsO41 As!' 3d Pristine surface Ar ' -bombarded surface Ti 3s 70 i. i. AsII3d As13d. I. I Binding energy (ev) I I Figure 3. XPS valence-band spectra (including some upper core-levels) recorded for (1) pristine and (2) Ar + ion-bombarded (001)KTA surface. Total DOS y =0.20 ev y =0.25 ev y =0.30 ev y =0.40 ev KTiOAsO4` B C -o Energy (ev) Figure 4. FP-LAPW calculations (within the valence band region) of the total DOS and (2) its broadening with the Lorenz parameters γ = 0.2 to 0.4 ev using the technique described in detail in Ref. [28]. Proc. of SPIE Vol I-4

5 Density of states [electrons/(unit cell)] Density of states [electrons /(unit cell)] 5 o Density of states [electrons /(unit cell)] Density of states [electrons/(unit cell)] 5 0 Density of states [electrons /(unit cell)] Density of states ]electrons /(unit cell)] Density of states [electrons /(unit cell)] 000 Density of states [electrons/(unit cell)[ O9_tot 09_ _p Density of states Ielectrons /(unit cell)] a L o 5 1 Energy ev] Figure 5. Total density of oxygen atoms states and partial O 2p and O 2s densities of states in KTA. Proc. of SPIE Vol I-5

6 peculiarities of the theoretical XPS valence-band spectrum calculated for KTA adopting the parameter γ = 0.4 ev, the estimated spectrometer energy resolution employed for the XPS measurements, represent reasonably those visible on the shape of the experimental spectrum of pristine (001)KTA surface. Surfaces of oxygen-bearing crystals, alloys and films are very sensitive with respect to the influence of Ar + ion bombardment, nonstoichiometry and additives as it is evidenced by XPS measurements [19,29-35]. In particular, the long-range order of the oxygen-bearing crystal surface is lost during Ar + irradiation, leading to the formation of completely amorphous surface layers at an ion dose of ~ ions/cm 2 [19,29,31]. Additionally, the above Ar + ion irradiation causes a partial loss of some kinds of atoms, and partial transformation of chemical valence state of the atoms to a lower valence state, inducing electronic states at the top of the valence band [19,29,31]. Regarding the influence of 1.5 kev Ar + ion bombardment on the (001)KTA surface, it is revealed in Fig. 3 that such a treatment leads to a significant decrease of the relative intensity of the XPS As II 3d core-level spectrum formed by As 5+ ions and causes the formation of the XPS As I 3d core-level spectrum (due to the appearance of As 0 atoms in the topmost layer). In addition, the O 2s band splitting on two sub-bands in the XPS spectrum of the pristine (001)KTA surface, mainly O I 2s and O II 2s (see Fig. 3), reveals dramatic changes of its shape as a result of 1.5 kev Ar + ion bombardment. As one can see from Fig. 3, the above treatment of the (001)KTA surface causes the complete elimination of the O II 2s sub-band. In order to explain the above experimental data, the results of our FP-LAPW calculations of the total density of states of oxygen atoms, and partial O 2p and O 2s densities of states in KTA are shown in Fig. 5. It is obvious that, dominant contributions of the 2s states of the O1 O4 and O7 O10 atoms (Fig. 5, panels from a to d and from g to j) occur in the band centred at 19.5 ev, whilst the oxygen 2s states associated with the O5 and O6 atoms (Fig. 5, panels e and f) contribute almost exclusively to the band centred at about 18.0 ev. Therefore, from comparison of Figs. 3 and 5, one can conclude that the oxygen 2s sub-band detected at higher binding energies (O II 2s sub-band) of the XPS spectrum of the pristine (001)KTA surface is formed by the contributions of the O1 O4 and O7 O10 atoms taking part in generation of the Ti O As bonds in KTiOAsO 4. In addition, the O I 2s sub-band detected at lower binding energies of the XPS spectra of the (001)KTA surface, both pristine and Ar + ion-bombarded, is formed by the contributions of the O5 and O6 atoms which create Ti O Ti bonds in potassium titanyl arsenate. From the results presented in Fig. 3, it is obvious that Ti O Ti bonds are much stronger in KTA as compared to Ti O As bonds. 5. CONCLUSIONS A high-quality KTA single crystal was successfully synthesized in the present work employing the high temperature solution growth (TSG) technique with rotating and pooling. For such a synthesis, high-purity К 2 СО 3 and ТiО 2 oxides were used as reagents and КН 2 AsO 4 was synthesized using metallic arsenic as a precursor. The present XPS valenceband measurements, which have been made using a thin (001)KTA plate cut from the crystal part without any optical inhomogeneities or domain boundaries with following polishing by the nanodiamond powder (0.1 grade), reveal the existence of two O 2s sub-bands on the XPS spectrum of the pristine (001)KTA surface. The theoretical FP-LAPW calculations of the total density of oxygen atoms states and partial O 2p and O 2s densities of states in KTA, allows one to conclude that the 2s states of the O1 O4 and O7 O10 atoms form the sub-band centred at higher binding energies, whilst the oxygen 2s states associated with the O5 and O6 atoms form the sub-band at lower binding energies. Regarding the influence of 1.5 kev Ar + ion bombardment of the (001)KTA surface, our XPS data reveal that such a treatment leads to a significant decrease of the relative intensity of the As 5+ 3d core-level spectrum and causes the formation of the additional As 0 3d core-level spectrum in the topmost layer. In addition, 1.5 kev Ar + ion bombardment of the (001)KTA surface causes the complete elimination of the O 2s sub-band positioned at higher binding energies. From our XPS results, it is obvious that Ti O Ti bonds are much stronger in KTA as compared to Ti O As bonds. Acknowledgements: This study was partially supported by SB RAS (Grant 28.13). Proc. of SPIE Vol I-6

7 REFERENCES [1] Mayo, S. C., Thomas, P. A., Teat, S. J., Loiacono, G. M., Loiacono, D. N., Structure and non-linear optical properties of KTiOAsO 4, Acta Cryst. B 50, (1994). [2] Novikova, N. E., Verin, I. A., Sorokina, N. I., Alekseeva, O. A., Tseitlin, M., Roth, M., Structure of KTiOAsO 4 single crystals at 293 and 30 K, Crystallog. Reports 55(3), (2010). [3] Loiacono, G. M., Loiacono, D. N., Zola, J. J., Stolzenberger, R. A., McGee, T. and Norwood, R. G., "Optical properties and ionic conductivity of KTiOAsO 4 crystals," Appl. Phys. Lett. 61(8), (1995). [4] Cheng, L. K. and Bierlein, J. D., "KTP and isomorphs recent progress in device and material development," Ferroelectrics 142, (1993). [5] Fève, J. P., Boulanger, B., Pacaud, O., Rousseau, I., Menaert, B., Marnier, G., Villeval, P., Bonnin, C., Loiacono, G. M. and Loiacono, D. N., "Phase matching measurements and Selmeier equations over the complete transparency range of KTiOAsO 4, RbTiOAsO 4, and CsTiOAsO 4," J. Opt. Soc. Am. B 17, (2000). [6] Hansson, G., Karlsson, H., Wang, S. and Laurell, F., "Transmission measurements in KTP and isomorphic compounds," Appl. Opt. 39, (2000). [7] Pack, M. V., Armstrong, D. J. and Smith, A. V., "Measurement of the χ (2) tensors of KTiOPO 4, KTiOAsO 4, RbTiOPO 4, and RbTiOAsO 4 crystals," Appl. Opt. 43(16), (2004). [8] Bierlein, J. D., Vanherzeele, H. and Ballman, A. A., "Linear and nonlinear optical properties of flux-grown KTiOAsO 4,"Appl. Phys. Lett. 54, (1989). [9] Chu, D. K. T., Hsiung, H., Cheng, L. K. and Bierlein, J. D., "Curie temperatures and dielectric properties of doped and udoped KTiOPO 4 isomorphs," IEEE Trans. Ultrasonics, Ferroelect. Freq. Cont. 40(6), (1993). [10] Atuchin, V. V., Plotnikov, A. E., Ziling, C. C., Isaenko, L. I. and Tjurikov, V. I., Cs:KTiOAsO 4 optical ionexchanged waveguides, Proc. SPIE 2969, (1996). [11] McGowan, C., Reid, D. T., Ebrahimzadeh, M. and Sibbett, W., "Femtosecond pulses tunable beyond 4 μm from a KTA-based optical parametric oscillator," Opt. Commun. 134, (1997). [12] Isaenko, L. I., Merkulov, A. A., Tjurikov, V. I., Atuchin, V. V., Sokolov, L. V. and Trukhanov, E. M., Growth and real structure of KTiOAsO 4 crystals from self-fluxes, J. Cryst. Growth 171(1-2), (1997). [13] Kraemer, D., Hua, R., Cowan, M. L., Franjic, K. and Dwayne Miller, R. J., "Ultrafast noncollinear optical parametric chirped pulse amplification in KTiOAsO 4," Opt. Lett. 31, (2006). [14] Gao, Z. L., Sun, Y. X., Yin, X., Wang, S. P., Jiang, M. H. and Tao, X. T., "Growth and electric-elastic properties of KTiOAsO 4 single crystal," J. Appl. Phys. 108, (2010). [15] Wang, F. X., Chen, F., Wang, X. L., Lu, Q. M., Wang, K. M., Shen, D. Y., Ma, H. J. and Nie, R., "Fabrication of optical waveguides in KTiOAsO 4 by He or Si ion implantation, "Nucl. Instr. and Meth. Phys. Res. B, 215, (2004). [16] Chen, F., Wang, X.-L. and Wang, K.-M., Development of ion-implanted optical waveguides in optical materials: a review, Opt. Mater. 29, (2007). [17] Jiao, Y. and Wang, L., Optical channel waveguide in KTiOAsO 4 crystals produced by O + ion implantation, J. Lightwave Technol. 30(10), (2012). [18] Shaldin, Y. V., Matyjasik, S., Novikova N. E., Tseitlin, M., Mojaev, E. and Roth, M., "Pyroelectric properties of KTiOAsO 4 crystals in the K temperature range," Physica B 405, (2010). [19] Ramana, C. V., Atuchin, V. V., Becker, U., Ewing, R. C., Isaenko, L. I., Khyzhun, O. Y., Merkulov, A. A., Pokrovsky, L. D., Sinelnichenko, A. K. and Zhurkov, S. A., "Low-energy Ar + ion-beam-induced amorphization and chemical modification of potassium titanyl arsenate (001) crystal surfaces," J. Phys. Chem. C 111, (2007). [20] Atuchin, V. V., Isaenko, L. I., Khyzhun, O. Y., Pokrovsky, L. D., Sinelnichenko, A. K. and Zhurkov, S. A., "Structural and electronic properties of the KTiOAsO 4 (001) surface," Opt. Mater. 30, (2008). [21] Atuchin, V. V., Kesler, V. G., Maklakova, N. Yu., Pokrovsky, L. D., Semenenko, V. N., Study of KTiOPO 4 surface by X-ray photoelectron spectroscopy and reflection high-energy electron diffraction, Surf. Interface Anal. 34, (2002). [22] Atuchin, V. V., Maklakova, N. Yu., Pokrovsky, L. D., Semenenko, V. N., Restoration of KTiOPO 4 surface by annealing, Opt. Mater. 23, (2003). Proc. of SPIE Vol I-7

8 [23] Atuchin, V. V., Pokrovsky, L. D., Kesler, V. G., Maklakova, N. Yu., Voronkova, V. I. and Yanovskii, V. K., Superstructure formation and X-ray photoemission properties of the TlTiOPO 4 surface, Surf. Rev. Lett. 11(2), (2004). [24] Atuchin, V. V., Litvinov, M. A., Maklakova, N. Yu., Pokrovsky, L. D., Semenenko, V. N. and Sheglov, D. V., Formation of TiO 2 and KTiOPO 4 nanoclusters on the (001) surface of KTiOPO 4 crystal upon annealing, J. Struct. Chem. 45, S84-S87 (2004). [25] Atuchin, V. V., Kesler, V. G., Meng, G. and Lin, Z. S., The electronic structure of RbTiOPO 4 and the effects of the A-site cation substitution in KTiOPO 4 -family crystals, J. Phys.: Condens. Matter 24, (2012). [26] Blaha, P., Schwarz, K. and Luitz, J. [WIEN97, A Full Potential Linearized Augmented Plane Wave Package for Calculating Crystal Properties], Technical University, Vienna, (1999). [27] Perdew, J. P., Burke, S. and Ernzerhof, M., "Generalized Gradient Approximation Made Simple," Phys. Rev. Lett. 77, (1996). [28] Khyzhun, O. Y., Bekenev, V. L., Atuchin, V. V., Sinelnichenko, A. K. and Isaenko, L. I., "Electronic structure of KTiOAsO 4 : A comparative study by the full potential linearized augmented plane wave method, X-ray emission spectroscopy and X-ray photoelectron spectroscopy," J. Alloys Compd. 477, (2009). [29] Atuchin, V. V., Pokrovsky, L. D., Khyzhun, O. Y., Sinelnichenko, A. K. and Ramana, C. V., "Surface crystallography and electronic structure of potassium yttrium tungstate," J. Appl. Phys. 104, (2008). [30] Bekenev, V. L., Khyzhun, O. Y. and Atuchin, V. V., "Electronic structure of monoclinic α-ky(wo 4 ) 2 tungstate as determined from first-principles FP-LAPW calculations and X-ray spectroscopy studies," J. Alloys Compd. 485, (2009). [31] Atuchin, V. V., Galashov, E. N., Khyzhun, O. Y., Kozhukhov, A. S., Pokrovsky, L. D. and Shlegel, V. N., "Structural and Electronic Properties of ZnWO 4 (010) Cleaved Surface," Cryst. Growth Des. 11, (2011). [32] Shtepliuk, I., Lashkarev, G., Khyzhun, O., Kowalski, B., Reszka, A., Khomyak, V., Lazorenko, V. and Timofeeva, I., "Enhancement of the Ultraviolet Luminescence Intensity from Cd-Doped ZnO Films Caused by Exciton Binding," Acta Phys. Polonica A 120, (2011). [33] Khyzhun, O. Y., Bekenev, V. L., Karpets, M. V. and Zavaliy, I. Y., "First-principles FP-LAPW calculations and X-ray spectroscopy studies of the electronic structure of Zr 3 V 3 O and Zr 3 V 3 O 0.6 oxides," J. Phys. Chem. Solids 73, (2012). [34] Shtepliuk, I., Khyzhun, O., Lashkarev, G., Khomyak, V. and Lazorenko, V., "XPS and Raman characterizations of Zn 1 x Cd x O films grown at the different growth conditions," Acta Phys. Polonica A 122, (2012). [35] Lavrentyev, A. A., Gabrelian, B. V., Shkumat, P. N., Nikiforov, I. Y., Zavaliy, I. Y. and Khyzhun, O. Y., "Electronic structure of Zr 4 Fe 2 O: ab initio APW+LO calculations and X-ray spectroscopy studies," J. Phys. Chem. Solids 74, (2013). Proc. of SPIE Vol I-8