ISSN 0020-1685, Inorganic Materials, 2017, Vol. 53, No. 1, pp. 39–44. © Pleiades Publishing, Ltd., 2017. Original Russian Text © M.A. Jafarov, E.F. Nasirov, R.S. Jafarli, 2017, published in Neorganicheskie Materialy, 2017, Vol. 53, No. 1, pp. 15–20.
Growth and Optical Properties of Nanostructured ZnS:Mn Films M. A. Jafarov*, E. F. Nasirov, and R. S. Jafarli Baku State University, ul. Khalilova 23, Baku, AZ1143 Azerbaijan *e-mail:
[email protected] Received January 12, 2016; in final form, February 20, 2016
Abstract⎯Nanostructured ZnS:Mn films have been grown and their structure, optical properties, and photoluminescence have been studied. The nanostructured ZnS:Mn films have been grown on silicon and glass substrates via hydrochemical deposition from solution. The crystal structure and microstructure of the films have been studied by X-ray diffraction and atomic force microscopy. The band gap of the nanostructured ZnS:Mn films has been determined. The intensity of their photoluminescence bands has been shown to increase with decreasing nanoparticle size. Keywords: nanostructured films, nanoparticles, hydrochemical deposition, photoluminescence, optical properties DOI: 10.1134/S0020168517010058
materials with the sphalerite structure. According to Elidrissi et al. [10], bright electroluminescence is always due to a mixed structure of the host material. The highest emission intensity is offered by electroluminescent materials whose crystal lattice has certain imperfections resulting from the wurtzite-to-sphalerite phase transition [11]. The higher efficiency of ZnS:Mn2+ films produced using xanthates in comparison with their analogs grown using dithiocarbamates was attributed by Bhattacharjee et al. [12] to the hexagonal structure of the former. According to Zdyb et al. [13], the wurtzite (hexagonal) phase is often responsible for reduced electroluminescence brightness. At the same time, it is not yet fully understood how the crystal lattice influences photoluminescence efficiency. In this paper, we describe the growth of nanostructured ZnS:Mn films, analyze their crystal structure, and examine the influence of nanoparticle size, activator and coactivator concentrations, and heat treatment conditions on their optical and photoluminescent properties.
INTRODUCTION Research into quantum size effects in II–VI semiconductors opens up great possibilities for designing novel, II–VI based devices with a wide range of functional capabilities. A special place is held by nanostructured materials for nanoelectronics. A spherical shape of nanocrystals is then of great importance for the ability to optimize the discrete energy level spectrum of quantum dots. Chemical precipitation from aqueous solutions allows one to obtain semiconductor nanocrystals far smaller than those produced by molecular beam epitaxy or lithography. Nucleation and nucleus growth processes in solution in the case of chemical precipitation lead to a nearly spherical shape of nanocrystallites, whereas film growth using molecular beam epitaxy or electrochemical deposition processes yields nonspherical particles. On the other hand, the II–VI semiconductors can be used to produce quantum dots [1], grow polymers, and fabricate photodetectors for the ultraviolet, visible, and near-IR regions of the electromagnetic radiation spectrum [2] and solar cells [3]. X-ray detectors [4], optical converters [5], and phosphors [6] based on the II–IV compounds offer the advantages of high radiation resistance and increased photosensitivity [7]. Characteristically, ZnS-based phosphors possess high brightness and luminous efficiency in the visible range. Doped ZnS crystals emit blue (ZnS:Ag), green (ZnS:Cu), or orange (ZnS:Mn) light [8]. The efficiency of emitters is determined by the properties of films, which in turn depend on the film growth technique. As shown by Afifi et al. [9], efficient electroluminescent phosphors can be produced from
EXPERIMENTAL ZnS films were produced by chemical bath deposition (CBD). Glass substrates were first thoroughly cleaned in a potassium dichromate + sulfuric acid solution. Next, the substrates were rinsed in hydrochloric acid and acetone and repeatedly in distilled water and then dried in a vacuum drying oven. The ZnS film growth process was based on reaction between zinc chloride (ZnCl2) as a Zn2+ source, thiourea (SC(NH2)2) as a S2– source, and aqueous 39
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Fig. 1. X-ray diffraction patterns of (a, b) ZnS films and (c, d) nanostructured ZnS:Mn films on (a, c) silicon and (b, d) glass substrates.
ammonia (NH4OH) as a complexing agent used to vary the pH of the solution, thereby controlling the Zn2+ concentration. Thin ZnS:Mn films on silicon and glass substrates were grown by chemical bath deposition from a solution containing thiourea (SC(NH2)2), zinc chloride (ZnCl2), manganese chloride (MnCl2), aqueous ammonia (NH4OH), and dimethyl sulfoxide ((CH3)2SO). An aqueous 0.1 M ZnCl2 solution was mixed with an aqueous 0.1 M SC(NH2)2 solution and then a MnCl2 solution was added with constant stirring in the ratio 99 : 1 in the presence of different dimethyl sulfoxide concentrations (0.5–4 g/L). In the resultant solution, the substrates were held for 1 to 10 min. Nanostructured ZnS:Mn films were grown via hydrochemical deposition with the use of four vessels containing liquids. The first vessel contained a solution of zinc and manganese chlorides with ammonia additions, the second and fourth vessels contained distilled water, and the third vessel contained a solution of thiourea and dimethyl sulfoxide. The substrates were held sequentially in the vessels for 20, 10, 20, and 10 s,
respectively. The process was repeated 30 times without interruption. The structure of the resultant films was studied by X-ray diffraction and atomic force microscopy (AFM). Their absorption spectra were measured using a Jobin-Yvon grating monochromator and detected by an FEU-100 photomultiplier tube. The output signal was fed to a transient digitizer system, which comprised a storage oscilloscope (LeCroy 9400) and diagnostic system (ABI Electronics BoardMaster 8000 Plus). Photoluminescence (PL) spectra were obtained on a Cary Eclipse spectrophotometer, using a xenon lamp as an excitation source. RESULTS AND DISCUSSION The phase composition of the ZnS:Mn films was determined by X-ray diffraction, which showed that the films consisted of a cubic, equilibrium phase (sphalerite) and a hexagonal, nonequilibrium phase (wurtzite) [13]. Figure 1 shows X-ray diffraction patterns of thin and nanostructured ZnS:Mn films on silicon and glass substrates. The peaks at 2θ = 29.50°, 48.80°, and 57.8° in the X-ray diffraction patterns (111, INORGANIC MATERIALS
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Fig. 2. AFM images of (a, c) ZnS films and (b, d) nanostructured ZnS:Mn films on (a, b) silicon and (c, d) glass substrates.
220, and 311 reflections, respectively) indicate that the samples have the zinc blende structure. The lattice parameter of the films was determined to be 5.36 Å, which is very close to a standard value (5.42 Å), and interplanar spacings are dhkl = 3.150 Å. After heat treatment at 200°C for 20 min, the nanostructured ZnS:Mn films on the silicon and glass substrates consisted of β-ZnS:Mn (hexagonal structure). Their lattice parameters are 3.86 and 6.26 Å, and interplanar spacings are dhkl = 3.31 Å. Cubic ZnS films are known to have a tendency toward the formation of a hexagonal phase at deposition temperatures well below the sphalerite–wurtzite phase transition temperature because of the presence of stacking faults, which lead to twinning. The fact that the hexagonal phase has stronger reflections indicates that it prevails, which is typical of the nanostructured ZnS:Mn films. The observed peak broadening suggests a decrease in nanoparticle size. Using the Debye–Scherrer formula, the nanoparticle size was estimated at 7–20 nm. INORGANIC MATERIALS
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Figure 2 shows AFM images of the conventional (Figs. 2a, 2c) and nanostructured (Figs. 2b, 2d) ZnS:Mn films on glass and silicon substrates. The thin ZnS films grown on silicon substrates consist of much thinner nanocrystallites in the form of isosceles triangles, whereas the ZnS films grown on glass substrates consist of oval nanocrystallites. As seen in Fig. 2, the nanoparticles in the ZnS:Mn films have a nonuniform distribution (Figs. 2b, 2d). On the surface of the thin ZnS films grown on silicon substrates, crystallites with a nearly hexagonal habit prevail. This leads us to conclude that annealing improves the structural perfection of the films. The surface topography of the nanostructured ZnS:Mn films grown on glass substrates is characterized by the presence of oval nanoparticles. Annealing such films also leads to coalescence of individual crystallites, but no crystal faceting was detected. The films consist of spherical nanocrystals ranging in size from 7 to 20 nm. Figure 3a shows transmission spectra of the ZnS:Mn films on glass substrates. The transmission data were used to determine the band gap of the
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nanoparticles. For comparison, Fig. 3b shows absorption spectra of the ZnS:Mn films. The position of the optical absorption edge indicates that the particle size decreases with increasing manganese concentration. The observed blue shift of the absorption edge is caused by the increase in band gap due to the quantum size effect.
then much larger than the Eg of the ZnS films. The Eg of the ZnS:Mn films was determined to be 3.63 eV. It is worth noting that the larger band gap of the nanostructured films in comparison with the conventional films is a characteristic feature of semiconductor nanoparticles. The band gap of nanoparticles is known to depend on the crystal, its reduced mass, and the
As seen in Fig. 3b, short-wavelength absorption corresponds to an energy of 3.85–3.88 eV. We believe that this is the band gap of the nanostructured ZnS:Mn films. The band gap of the nanoparticles is
nanoparticle size: Eg(nano) = Eg(cr) + π 2 . Our esti2m r a mates taking into account parameters of ZnS:Mn indicate that the calculated band gap of the nanoparticles is in satisfactory agreement with the value extracted from their absorption spectrum.
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Fig. 4. PL spectra of (1) ZnS films and (2–5) nanostructured ZnS:Mn films on (1, 3) glass and (2, 4, 5) silicon substrates: dimethyl sulfoxide concentrations of (1–3) 0.5, (4) 2, and (5) 4 g/L; ZnS : Mn = (2) 99 : 1 and (3–5) 99 : 2.
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We studied PL spectra of thin ZnS films and nanostructured ZnS:Mn films and assessed the influence of nanoparticle size and dimethyl sulfoxide content on the PL intensity. Figure 4 shows PL spectra of the ZnS:Mn films on glass (spectra 1, 3) and silicon (spectra 2, 4, 5) substrates. It is seen that the PL spectra cover a wavelength range far exceeding that of edge luminescence and lie in the continuous absorption range. The PL spectrum of the ZnS films contains bands corresponding to so-called intrinsic emission (λ = 460 nm) and a weak peak arising from point defects (Fig. 4, spectrum 1). The PL spectrum of ZnS:Mn contains two bands, which arise from intrinsic emission (λ = 460 nm) and Mn2+ luminescence (λ = 585 nm) (Fig. 4, spectrum 2). With increasing manganese concentration, the intensity of the longer wavelength band increases (Fig. 4, spectra 2, 3). With increasing dimethyl sulfoxide concentration, the peak intensity of the longer wavelength luminescence band INORGANIC MATERIALS
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Fig. 5. PL spectra of nanostructured ZnS:Mn films on silicon substrates after annealing at (a) (1) 100, (2) 150, (3) 200, and (4) 250°C for (b) (1) 10, (2) 20, and (3) 30 min.
increases, indicating that the nanoparticle size decreases (Fig. 4, spectra 4, 5). The intensity of the longer wavelength ZnS:Mn PL band (λ = 585 nm) is a nonmonotonic function of annealing temperature (Fig. 5a). With increasing annealing time, the intensity of the longer wavelength band first increases (Fig. 5b, spectra 1, 2) and then drops (Fig. 5b, spectrum 3). It is known that the intensity of PL related to point defects can be increased by raising their number density or reducing the nanocrystallite size. However, the concentration of an impurity is limited by its solubility and the emission efficiency as a function of decreasing nanoparticle size depends on two competing processes. On the one hand, reducing the nanoparticle size increases the overlap of electron and hole wave functions, thereby increasing the radiative recombination probability. On the other, the role of nonradiative recombination channels becomes more important. Clearly, ZnS films grown at low deposition temperatures contain a high density of nonequilibrium defects, which form a broad spectrum of states. Manganese in ZnS acts as an efficient luminescence center, substituting for Zn in the cation sublattice, and the Mn incorporation process is more efficient in the presence of zinc vacancies. The observed manganese dopinginduced decrease in intrinsic emission intensity is due to the decrease in the concentration of vacancies, which become filled with manganese to give efficient Mn2+ luminescence centers. The intensity of the orange PL in the ZnS:Mn nanoparticles increases by a factor of 3.5 with increasing manganese and dimethyl sulfoxide concentrations. It seems likely that this effect INORGANIC MATERIALS
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is associated with the ability of dimethyl sulfoxide to partially change the crystallographic symmetry of zinc sulfide and increase its lattice parameter, thus facilitating manganese incorporation into octahedral sites. CONCLUSIONS We have studied the structure, optical properties, and PL of ZnS films and nanostructured ZnS:Mn films grown on glass and silicon substrates via hydrochemical deposition from a solution containing thiourea, zinc chloride, manganese chloride, ammonia, and dimethyl sulfoxide. AFM images demonstrate that the films consist of nanocrystals ranging in size from 7 to 20 nm. The band gap of the nanostructured ZnS:Mn films far exceeds the Eg of the ZnS films. The PL intensity in the nanostructured ZnS:Mn films increases with decreasing crystallite size, which can be controlled by varying manganese and dimethyl sulfoxide concentrations. REFERENCES 1. Naumov, A.V., Blogova, T.G., Semenov, V.N., et al., Luminescence and photoconductivity of alkali-metaldoped cadmium sulfide films, Inorg. Mater., 2006, vol. 42, no. 5, pp. 463–469. 2. Jafarov, M.A., Mamedova, S.A., Mekhtiev, R.F., and Nasirov, E.F., Photoconductivity of solution-grown films of II–VI based solid solutions, Inorg. Mater., 2013, vol. 49, no. 11, pp. 1081–1085. 3. Schrier, J., Demchenko, D.O., and Wang, L.-W., Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications, Nano Lett., 2007, vol. 7, no. 8, pp. 2377–2382.
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4. Ichino, K., Ueyama, K., Yamamoto, M., Kariya, H., Miyata, H., Misasa, H., Kitagawa, M., and Kobayashi, H., High temperature growth of ZnS and ZnMgS by molecular beam epitaxy under high sulfur beam pressure, J. Appl. Phys., 2000, vol. 87, p. 4249. 5. Durrani, S.M.A., Al-Shukri, A.M., Iob, A., and Khawaja, E.E., Optical constants of zinc sulfide films determined from transmittance measurements, Thin Solid Films, 2000, vol. 379, pp. 199–202. 6. Dimitrova, V. and Tate, J., Synthesis and characterization of some ZnS-based thin film phosphors for electroluminescent device applications, Thin Solid Films, 2000, vol. 365, pp. 134–138. 7. Cheng, J., Fan, D.B., Wang, H., Liu, B.W., Zhang, Y.C., and Yan, H., Chemical bath deposition of crystalline ZnS thin films, Semicond. Sci. Technol., 2003, vol. 18, pp. 676–679. 8. Roy, P., Ota, J.R., and Srivastava, S.K., A new route for preparing crystalline ZnS thin films by chemical bath deposition method and its characterization, Thin Solid Films, 2006, vol. 515, pp. 1912–1917.
9. Afifi, H.H., Mahmoud, S.A., and Ashour, A., Structural study of ZnS thin films prepared by spray pyrolysis, Thin Solid Films, 1995, vol. 263, pp. 248–251. 10. Elidrissi, B., Addou, M., Regragui, M., Bougrine, A., Kachouane, A., and Bernede, J.C., ZnS:Cu, prepared by spray pyrolysis, Mater. Chem. Phys., 2001, vol. 68, pp. 175–179. 11. Svechnikov, S.V., Zavgelova, L.V., Rogina, N.N., Radionov, V.E., et al., Luminescent ZnS:Cu films prepared by chemical methods, Semiconductors, 2000, vol. 34, no. 10, pp. 1178–1182. 12. Bhattacharjee, B., Ganguli, D., Iakoubovskii, K., Stesmans, A., and Chaudhuri, S., Synthesis and characterization of sol–gel derived ZnS:Mn2+ nanocrystallites embedded in a silica matrix, Bull. Mater. Sci., 2002, vol. 25, no. 3, pp. 175–180. 13. Zdyb, A., Cieślak, K., and Olchowik, J.M., Properties of films fabricated from ZnS/Mn2+ nanoparticles, Mater. Sci. Pol., 2008, vol. 26, no. 2, pp. 389–394.
Translated by O. Tsarev
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