Journal of Structural Chemistry, Vol. 49, Supplement, pp. S124-S128, 2008 Original Russian Text Copyright 2008 by R. G. Valeev,. N. Deev, F. Z. Gilmutdinov, S. G. Bystrov,. I. Pivovarova, É.. Romanov, V. V. Kriventsov, M. R. Sharafutdinov, and.. Eliseev LOCAL ATOMIC STRUCTURE OF ZINC SELENIDE FILMS: EXAFS DATA R. G. Valeev, 1,2. N. Deev, 1 F. Z. Gilmutdinov, 1 S. G. Bystrov, 1. I. Pivovarova, 1, 2 É.. Romanov, 2 V. V. Kriventsov, 3 M. R. Sharafutdinov, 3 and.. Eliseev 4 UDC 538.9 This paper presents the results of our study of the structural state and local atomic structure of zinc selenide films obtained by thermal evaporation in supervacuum at condensation temperatures of 150 C, 0, and 150 C. Structure-sensitive methods such as X-ray diffraction, atomic force microscopy, and EXAFS spectroscopy were used. The parameters of the local atomic environment (interatomic distances, coordination numbers) of zinc and selenium atoms were obtained by Fourier transformation. Keywords: ZnSe, local atomic structure, EXAFS spectroscopy, X-ray diffraction, atomic force microscopy, nanocomposite, Fourier fitting, semiconductor. INTRODUCTION In modern materials science, emphasis is laid on studies of the physicochemical, optical, and electronic properties of new materials for various fields of technology. Recently, the tendencies toward miniaturization, which were especially strong for chip elements, have also become significant in optoelectronics because of the modern tendencies toward the use of optical waveguides for data exchange between chip elements and hence miniaturization of waveguides themselves and data exchange devices [1, 2]. Miniature and superminiature (nanosized and micron-sized) sources of coherent and incoherent radiation are of great interest for the development of several important fields in modern electronics such as the new generation of computers based on optical elements, superhigh-resolution displays with low response times, high-speed devices for ultradense optical data recording, etc. [3]. Special attention has been paid to the development and creation of efficient sources of radiation in the UV and visible (to near IR) regions of spectra, which is the primary demand of modern optoelectronics and photonics. One of the most actively developed directions is the creation of effective solid state sources of white light with low energy consumption and new effective coherent radiation sources based on A II B VI wide-band compounds. Recent achievements in the fields of UV, blue, and green lasers and light-emitting diodes were associated with this group of semiconductors [4-9]. The future of luminescent materials means raising their stability, simplification of synthetic procedures, and development of new methods for the synthesis of nanostructures with specified optical properties. At the same time, the possibility of control over the functional properties of nanomaterials depends on the size, structure, form, and dispersity 1 Physicotechnical Institute, Ural Division, Russian Academy of Sciences, Izhevsk; valeev@lasas.fti.udm.ru. 2 Udmurtia State University, Izhevsk. 3 G. K. Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk. 4 Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 49, Supplement, pp. S125-S129, 2008. Original article submitted June 20, 2007. S124 0022-4766/08/49 Supplement-0124 2008 Springer Science+Business Media, Inc.
of nanoparticles, the number of structural defects, and homogeneity of chemical composition. For example, even a minor deviation in the particle size of semiconductors leads to a pronounced change in the width of the electronic and hole band and an increase in the total energy of optical transitions (blue and red shifts of the edge of the absorption band) [10]. EXPERIMENTAL APPROACHES AND TECHNIQUES Zinc selenide films described in this work were obtained by vacuum evaporation of the powder of the material at 10 5 Pa, which provided high chemical purity of the samples. The films were prepared over a wide range of condensation temperatures (substrate temperatures). For structural studies we chose the films obtained at substrate temperatures of 150 C, 0 and 150 C. This choice was dictated by the fact that, according to the phase diagrams, at 150 C and below the films should be amorphous, at 0 they should consist of a mixture of the amorphous and crystalline phases, and at 150 C they should be polycrystalline phases. The film stoichiometry and the content of impurities (carbon and oxygen; their presence in the surface layers does not affect the electrophysical and optical properties) were controlled by electronic spectroscopy for chemical analysis (ESCA) on an ES-2401 X-ray photoelectron spectrometer at the Physicotechnical Institute, Ural Division, Russian Academy of Sciences, Izhevsk. The surface morphology was studied by atomic force microscopy (AFM) on a Solver atomic force microscope (NT-MDT, at the same institute). The structural state of the films was studied by time resolution X-ray diffraction on channel 5 of the VEPP-3 ring, Siberian Synchrotron Radiation Center (SSRC), Novosibirsk. The EXAFS spectra for local atomic structure studies were measured on an EXAFS station, channel 8 of VEPP-3, SSRC. RESULTS AND DISCUSSION For analysis of the chemical composition of ZnSe films, review spectra were obtained for each sample, and the C1s, O1s, Se3d, and Zn2p 3/2 spectra were studied in detail. The spectra were excited by MgK radiation (E = 1253.6 ev). To clean the surface from the adsorbed impurities and remove the 10 nm surface layer with a changed composition, we used etching with Ar + ions with energy 0.9 kev and current density 12 /cm 2. The results of this work showed that the free surface contained adsorbed hydrocarbon impurities and adsorbed oxygen. In the 1-3 nm surface layers, the carbon concentration reached 20-25 at.%; at depths of more than 8-10 nm, it was up to 3-5%. Oxygen was absent at the same depths. The ratios of selenium and zinc concentrations after the removal of the modified superthin surface layer are presented in Table 1. It follows that the Se and Zn concentrations correspond to the equiatomic composition ZnSe. The binding energy of the Se3d line corresponds to selenium compounds with the metal. The bonding energy of the Zn2p 3/2 line exceeds the value characteristic of pure zinc, and the chemical shift exceeds the value characteristic of the standard oxide ZnO. Consequently, the binding energies of Zn2p and Se3d can be attributed to ZnSe. Figure 1 presents the X-ray diffractograms of the films. The intensity of the first peak increases with the condensation temperature, which points to an increase in the coherent scattering blocks and hence the film grain sizes. Moreover, the peaks are rather diffuse at the base, and there is a weak amorphous halo, which points to the presence of an amorphous phase in the films. These data are confirmed by the results of atomic force microscopy studies (Fig. 2), which allows us to speak about the amorphous nanocrystalline (composite) composition of the films [11]. In the course of our EXAFS experiment, we obtained ZnK X-ray absorption spectra (E K = 9659 ev; range of scanning over energy 12,550-13,500 ev; step 1.5 ev) and Se (E K = 12,658 ev; range of scanning over energy 9550-10,450 ev; step 1.5 ev). The normalized oscillating parts of spectra after standard pretreatment and their Fourier images are presented in Fig. 3 in comparison with the model images calculated with the FEFF-7 program package [12]. Table 2 presents the parameters of the local environment of zinc and selenium atoms calculated by Fourier fitting using the Viper program [13, 14]. S125
TABLE 1. Se:Zn Ratio of Concentrations in Films Lying 10 nm Below the Surface Temperature, C Se concentration Zn concentration 150 51 49 0 50 50 150 48 52 Fig. 1. X-ray diffractograms of zinc selenide films. Fig. 2. AFM images of the surface of ZnSe films at condensation temperatures of ( ) 150 C, (b) 0, and (c) 150 C. S126
TABLE 2. Parameters of the Local Atomic Environment of Zn and Se Atoms Condensation R 1, Å N 1 R 2 temperature, C Zn Se Zn Se Zn Se Model 2.450 2.450 4.0 4.0 4.002 4.002 150 2.44(1) 2.45(1) 4.2(2) 3.3(2) 4.10(1) 4.03(1) 0 2.44(1) 2.45(1) 4.3(2) 3.4(2) 4.05(1) 4.03(1) 150 2.45(1) 2.45(1) 4.4(2) 3.3(2) 3.99(1) 4.03(1) Fig. 3. Normalized oscillating parts of EXAFS spectra of films and their Fourier images. Solid line model; condensation temperatures: 150 C, 0, and 150 C. From the form of the Fourier images of the normalized oscillating parts of the Se edge absorption spectra one can say that the second and third coordination spheres almost merge irrespective of the condensation temperature during film preparation. It is also evident that the peak corresponding to the third coordination sphere on the Fourier image of the model oscillating part is less pronounced than the peak for Zn. This is probably explained by the difference in the parameters of photoelectron scattering on Se and Zn atoms. As can be seen from Table 2, the coordination numbers for the first partial coordination sphere of zinc atoms are larger than for the environment of selenium. This increase in the coordination number in the first coordination sphere of zinc S127
and the decrease in the coordination number of selenium can be indicative of the fact that at the level of short-range ordering, there are pronounced deviations from the structure of the ideal ZnSe single crystal. This work was fulfilled under Agreement No. 02.513.11.3217 with the Federal Agency on Science and Innovations, Federal Target Program Priority studies and developments in science and technology of Russia for 2007-2012. REFERENCES 1. H. Babucke, P. Thiele, T. Prasse, et al., Semicond. Sci. Technol., 13, 200 (1998). 2. M. Straszburg, I. L. Krestnikov, Z. I. Alferov, et al., Physica E: Low-Dimensional Systems and Nanostructures, 2, 542 (1998). 3. S. Itoh, K. Nakano, and A. Ishibashi, J. Crystal Growth, 214/215, 1029 (2000). 4. M.-Ch. Jeong, B.-Y. Oh, M.-H. Ham, et al., Appl. Phys. Lett., 88, 202105 (2006). 5. J. Bao, M. A. Zimmler, F. Capasso, et al., Nano Lett., 6, 1719 (2006). 6. D.-K. Hwang, S.-H. Kang, J.-H. Lim, et al., Appl. Phys. Lett., 86, 222101 (2005). 7. Y. R. Ryu, T. S. Lee, J. A. Lubguban, et al., ibid., 87, 153504 (2005). 8. H. White and Y. Ryu, Comp. Semicond., 12, 16 (2006). 9. C. Yuen, S. F. Yu, S. P. Lau, et al., J. Crystal Growth, 287, 204 (2006). 10. A. L. Efros and M. Rosen, Ann. Rev. Mater. Sci., 30, 475-521 (2000). 11. R. G. Valeev, P. N. Krylov, and É. A. Romanov, Poverkhnost, No. 1, 41 (2007). 12. S. I. Zabinsky, J. J. Rehr, A. Ankudinov, et al., Phys. Rev. B, 52, 2995 (1995). 13. K. V. Klementiev, http://www.desy.de/ klmn/viper.html. 14. K. V. Klementiev, J. Phys. D: Appl. Phys., 34, 209 (2001). S128