ISSN 1063-7850, Technical Physics Letters, 2016, Vol. 42, No. 2, pp. 124–126. © Pleiades Publishing, Ltd., 2016. Original Russian Text © R.G. Valeev, D.I. Petukhov, A.I. Chukavin, A.N. Bel’tyukov, 2016, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2016, Vol. 42, No. 3, pp. 23–28.
Electroluminescent Layers Based on ZnS:Cu Deposited into Matrices of Porous Anodic Al2O3 R. G. Valeeva*, D. I. Petukhova, b, A. I. Chukavina, and A. N. Bel’tyukova a
Physical Technical Institute, Ural Branch, Russian Academy of Sciences, Izhevsk, Russia b Moscow State University, Moscow, Russia * e-mail:
[email protected] Received September 2, 2015
Abstract—It is suggested to use a new nanocomposite material—nanostructures of copper-doped zinc sulfide in a matrix of porous aluminum oxide—as a light-emitting layer of electroluminescent sources of light. The material was deposited by thermal evaporation in a vacuum. The microstructure of the layers, impurity distribution in the electroluminescent-phosphor layer, and electroluminescence spectra at various copper concentrations in ZnS:Cu were studied. DOI: 10.1134/S1063785016020152
Light-emitting electroluminescent screens based on powders and thin films of electroluminescent phosphors have found wide use mainly in alphanumeric displays and lighting panels for advertising structures. Recent developments have made it possible to create thin-film DC panels fed from ordinary batteries, which enables their use in commercial applications [1]. The main materials for electroluminescent sources (ELSs) are powders and films of zinc sulfide doped with copper, chlorine, manganese, and other elements. The concentration and type of a doping element determine the wavelength and intensity of emission, i.e., there exist conditions in which a source of white light can be created [2]. However, together with their undeniable advantages, such as simple fabrication technology and the resulting low cost and possibility of obtaining large-size panels, the electroluminescent sources of light have important shortcomings: high power consumption and need for specific power sources, as well as the short service life (up to 1000 h, compared with the nearly “everlasting” light-emitting diodes). This is due to the degradation of the working layer, caused by the specific working principle of devices of this kind in extremely high electric fields (prebreakdown luminescence). The efficiency can be improved and the service life can be made longer, by using nanosize phosphor particles in electroluminescent layers [2]. The classical ELSs comprise a phosphor layer sandwiched between conducting plates and, as a rule, have two dielectric buffer layers [3]. Serving as a support for phosphor nanoparticles, the insulator can protect the material from external effects and, thereby, improve the performance of ELSs. It should also be noted that the formation of an ordered array of
phosphor nanostructures of the same size and shape makes it possible to represent each nanoparticle as a separate light emitter, with coherent addition of light from each source leading to a substantial rise in the intensity of light [4]. Thermal evaporation in a vacuum is widely used in fabrication of microelectronic devices. Previously, we have suggested forming semiconductor nanocomposites in dielectric matrices by deposition of materials onto the porous surface of anodic aluminum oxide (AAO) [5]. The experimental approaches used to form porous Al2O3 matrices make it possible to obtain films with unique porous structure, the parameters of which (pore diameter and length and distance between neighboring pores) can be varied in the course of synthesis [6]. In the case under consideration, the material reaching a porous AAO mounted on a substrate holder forms nanostructures, the size and shape of which are set by the geometric characteristics of the porous structure of the matrix (pore diameter and distance between pores). Thus, the goal of our study was to obtain an ELS phosphor layer on the basis of ZnS:Cu/Al2O3 nanocomposites and to examine the microstructure of the layers, the impurity distribution in the electroluminescent phosphor layer, and electroluminescence (EL) spectra at various copper concentrations in ZnS:Cu. The matrices of anodic aluminum oxide were synthesized by the method of double-stage anodic oxidation [6]. Copper-doped zinc sulfide was deposited onto the porous AAO surface by the method of discrete thermal evaporation of a mixture of ZnS (99.99%) and Cu (99.99%) powders in a high vacuum (with a residual pressure not exceeding 10–5 Pa). To
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ELECTROLUMINESCENT LAYERS BASED
ITO ZnS:Cu
ZnS:Cu@Al2O3 1 µm Fig. 1. Typical cross-sectional SEM image of an ELS sample.
form the top contact of the electroluminescent source, a layer of a transparent conductor ITO (indium-tin oxide, In2O3 × SnO2) was deposited onto the ZnS:Cu surface. ZnS:Cu with atomic percentage content of copper of 5 and 10% was deposited onto porous AAO films produced by anodic oxidation of aluminum plates at a fixed voltage of 80 V. The substrate temperature in the course of deposition was 200°C. It can be seen in the micrograph in Fig. 1 that, finding its way into the pores of the matrix in the course of deposition, the material heals the pores, with a nearly solid film formed on the AAO surface in the end. After a layerby-layer analysis of the chemical composition of the
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phosphor layer was carried out in order to examine the uniformity of the copper distribution across the thickness, a transparent conducting ITO layer was deposited on the samples. Figure 2 shows the concentration distributions of zinc, sulfur, and copper across a layer thickness of about 10 nm furnished by X-ray photoelectron spectroscopy (XPS) on a Specs spectrometer. The spectra were processed with a CasaXPS software package. It can be seen that the surface is enriched in copper, the content of which becomes constant after 2–3 min of etching and, on the whole, reproduces the stoichiometry set by the mixture of ZnS and Cu powders. The EL of the ELSs that we fabricated was analyzed on a Renishaw InVia spectrometer (United Kingdom) under ac excitation with a voltage of 220 V and frequency of 50 Hz. The spectra in Fig. 3 show an emission peak at 540 nm, which corresponds to the excitation of luminescence caused by the scattering of carriers drawn out from the conduction band on t2 Cu defect levels [7]. It can be seen here that the emission intensity grows with increasing copper concentration in the phosphor layer, which is also understandable: the number of t2 defect levels becomes larger. Thus, we have proposed a new material and method for using this material as the active layer of electroluminescent sources of light. The material is constituted by a nanocomposite of doped zinc sulfide deposited by thermal evaporation in a vacuum into pores of anodic aluminum oxide matrices. An analysis of the composition and structure of chemical bonds in the material by X-ray photoelectron spectroscopy demonstrated that the stoichiometry of ZnS:Cu nano-
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Relative concentration c, %
40 35 30 25
S 2p 10% Cu Zn 2p3/2 10% Cu
20
Cu 2p 10% Cu S 2p 5% Cu Zn 2p3/2 5% Cu
15
Cu 2p 5% Cu
10 5 0
1
2
3
4
5 6 Depth, nm
7
8
9
10
Fig. 2. Distribution of Zn, S, and Cu across the thickness of the phosphor layer for ZnS: 5% Cu (solid line) and ZnS : 10% Cu (dashed line) samples. TECHNICAL PHYSICS LETTERS
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Intensity, a.u.
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350
400
450
500 550 600 Emission wavelength, nm
650
700
Fig. 3. EL spectra of ELSs based on ZnS:Cu phosphor samples in a porous Al2O3 matrix: ZnS: 5% Cu (solid line) and ZnS:10% Cu (dashed line) samples.
structures formed in pores of the matrix in the course of deposition well corresponds to the stoichiometry calculated when preparing the mixture of ZnS and Cu powders. When luminescence is excited by an electric field at a voltage of 220 V and frequency of 50 Hz, the maximum emission intensity is reached in the green spectral range at a wavelength of 540 nm, which is due to the carrier scattering on t2 Cu defect levels. Acknowledgments. This study was supported by the Russian Science Foundation, project no. 15-1910002. REFERENCES 1. http://www.surelight.com/files/EL_Parallel_Panel_Technical_Data_sheet.pdf.
2. X. Fang, T. Zhai, U. K. Gautam, L. Li, L. Wu, Y. Bando, and D. Goldberg, Prog. Mater. Sci. 56, 175 (2009). 3. R. G. Valeev, A. N. Bel’tyukov, V. M. Vetoshkin, E. A. Romanov, and A. A. Eliseev, Tech. Phys. 56, 896 (2011). 4. H. J. Xu and X. J. Li, Semicond. Sci. Technol. 24, 075 008 (2009). 5. R. Valeev, E. Romanov, A. Beltukov, V. Mukhgalin, I. Roslyakov, and A. Eliseev, Phys. Status Solidi C 9, 1462 (2012). 6. M.-C. Jeong, B.-Y. Oh, M.-H. Ham, and J.-M. Myoung, Appl. Phys. Lett. 88, 202105 (2006). 7. G. Murugadoss, Particuology 11, 566 (2013).
Translated by M. Tagirdzhanov
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