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Optical properties in Mn-doped ZnS thin films: Photoluminescence quenching
Materials Letters 163 (2016) 126–129
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical properties in Mn-doped ZnS thin films: Photoluminescence quenching A.I. Inamdar a, Sangeun Cho a, Yongcheol Jo a, Jongmin Kim a, Jaeseok Han a, S.M. Pawar a, Hyeonseok Woo a, R.S. Kalubarme b, ChanJin Park b, Hyungsang Kim a,n, Hyunsik Im a,n a b
Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, South Korea Department of Materials Science & Engineering, Chonnam National University, Gwangju 500-757, South Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 14 September 2015 Received in revised form 14 October 2015 Accepted 16 October 2015 Available online 17 October 2015
Mn-doped ZnS thin films are synthesized on soda-lime glass substrates using magnetron co-sputtering technique. X-ray diffraction and atomic force microscopy measurements indicate that of the as-obtained films including the highest Mn ( 11% relative to the Zn concentration) in the lattice of ZnS are amorphous with a granular morphology. X-ray photoelectron spectroscopy reveals the presence of Zn2 þ , Mn2 þ and S2 chemical states in the films. The undoped ZnS film exhibits photoluminescence (PL) peaks at energies around 3.26 eV (wavelength 379 nm) and 2.95 eV ( 420 nm), which originate from the interplay between excited electron, defect (sulfur vacancy) states and the valence band. For the Mndoped ZnS films, the band-to-band emission peak is quenched and shifts toward to higher energies at a rate of 11.77 2 meV/Mn%. We propose that Mn dopant-mediated structural phases and non-radiative deep traps in ZnS cause the modification in the optical transition. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mn-doped Zinc sulfide Photoluminescence Defects
1. Introduction ZnS has attracted an enormous amount of attention because it can be used in a wide variety of applications related to solar cells, electroluminescent devices, and optoelectronic device (such as in window layers for blue-light diodes), antireflection coatings, hydrogen production, displays, sensors, bio imaging and other nonlinear optical devices [1–3]. This material has a wide band gap and is therefore important for use in the violet and blue regions. ZnS is also suitable for use as a host material for various dopants, and it is non-toxic and possesses good quantum efficiency. It was demonstrated that Bhargava was the first to report that Mn-doping in ZnS can improve its luminescent efficiency [4]. The luminescence in Mn-doped ZnS has been proposed to be a result of the energy transfer from electron hole pairs delocalized inside of the ZnS host crystals [5]. Mn-doped ZnS can exhibit both blue defect-related emissions and orange Mn þ 2 ion-related emissions [1,6], and the Mn doping concentration affects the PL intensity of the film because the variation in the concentration changes the energy band, forming different luminescence centers, and also, surface passivators can contribute to the increase in the PL [4,7]. A maximum PL was reached at a certain Mn2 þ n
Corresponding authors. E-mail addresses: [email protected] (H. Kim), [email protected] (H. Im). http://dx.doi.org/10.1016/j.matlet.2015.10.074 0167-577X/& 2015 Elsevier B.V. All rights reserved.
concentration and a further increase led to a decrease in the PL intensity [8–10]. There have been multiple efforts for more than a decade to enhance and understand the PL properties of ZnS. Though various chemical synthesis techniques such as chemical precipitation, hydrothermal, sol–gel and chemical vapor deposition have been used to deposit Mn-doped ZnS, further improvement is still required. Furthermore, these synthetic methods are relatively more complicated than physical techniques because they require organic solvents and high reaction temperature. To the best of our knowledge, there have been rare reports of Mn-doped ZnS prepared by a magnetron co-sputtering method with its PL properties. In this work, we synthesize co-sputtered Mn-doped ZnS (ZnS:Mn) films and report on their photoluminescence properties to demonstrate that the Mn concentration plays an important role in tuning the PL properties of the ZnS:Mn films. Mninduced non-radiative defect-states and the formation of a MnS phase cause photoluminescence quenching in the ZnS:Mn films.
2. Experimental Mn-doped ZnS thin films (ZnS:Mn) were fabricated on sodalime glass substrates using conventional magnetron co-sputtering. Two different targets, ZnS and Mn, with a purity of 99.99% were simultaneously sputtered using RF and DC power supplies, respectively. Soda-lime glass substrates with an area of 1 cm2 were mounted approximately 4 cm away from the targets at a 45° angle.
A.I. Inamdar et al. / Materials Letters 163 (2016) 126–129
Prior to deposition, the substrates were consecutively cleaned ultrasonically in acetone, methanol, and deionized water. The chamber was initially evacuated using a rotary pump to 10 3 Torr and was then further evacuated to 7.5 10 6 Torr using a turbo molecular pump. Before sputtering, the chamber was filled with Ar gas at a flow rate of 75 sccm to reach a working pressure of 6.2 10 2 Torr. A fixed RF power of 90 W was applied to the ZnS target, and three different DC powers of 400 V, 475 V, and 550 V were applied to the Mn target to obtain different Mn concentrations. The targets were initially pre-sputtered for 5 min, and then deposition was carried out for 25 minutes. The substrate was rotated during deposition to improve the uniformity of the deposited films. The un-doped ZnS and ZnS:Mn grown at three different DC powers of 400 V, 475 V and 550 V are denoted as ZM0, ZM1, ZM2 and ZM3, respectively. The Mn concentration in the ZnS: Mn films are estimated using the X-ray photoelectron spectroscopy (XPS) measurements. The structural properties of the films were measured using high-resolution X-ray diffraction (XRD) with Ni-filtered Cu-Kα radiation (X’pert PRO, Philips, Eindhoven, Netherlands). The chemical binding states of the undoped ZnS and ZnS:Mn films were investigated using XPS (VG Multilab 2000, Thermo VG Scientific, UK) with an incident energy of 1400 eV. The PL measurements were carried out using a 75 cm monochromator equipped with an ultraviolet-sensitive photomultiplier tube at an excitation wavelength of 325 nm.
3. Results and discussion Fig. 1a and b shows the XRD pattern of the undoped ZnS and ZnS:Mn(ZM1) films. Both films exhibit two broad reflections in the 2θ range from 10° to 80°. The XRD patterns indicate that the particle size of the films is very small, which is quite obvious in the sputtered sample. A considerable broadening of the XRD spectra can be ascribed to the amorphous nature of the sample or to very tiny particles. The existence of fine particles in the amorphous ZnS:Mn films grown on soda lime glass substrates is directly observed in the atomic force microscopic (AFM) measurements as shown in Fig. 1(c–f). All of the samples exhibit a compact granular morphology with very fine particles below 10 nm, which is
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consistent with the XRD data. The un-doped ZM0 (see Fig. 1c) sample has a very compact morphology with a rough surface whereas the Mn co-sputtered samples exhibit a granular morphology(see Fig. 1d).From the surface roughness measurements, their surfaces become smooth as the Mn content increases (for example, ZM2 and ZM3). The XPS analysis of the undoped and Mn doped ZnS films is performed to determine the chemical binding state and to confirm the presence of the constituting element of the films. Fig. 2(a–c) show the Zn2p, Mn2p and S2p core level spectra for the ZM1 sample. The peaks related to Zn 2p3/2, Zn 2p1/2 and S 2p3/2 in the core level Zn2p and S2p spectra of the pure ZnS sample are detected at 1021.85, 1044.95, and 161.45 respectively, and these are attributed to the formation of the Zn–S bond [11]. For the ZnS:Mn samples, Zn 2p3/2, Zn 2p1/2 and S 2p3/2 peaks are observed in the core level spectra at the same positions, but additional peaks related to Mn 2p3/2 and Mn 2p1/2 (see Fig. 2c) are observed at 641.85 and 653.5 eV, respectively. These Mn 2p peaks are attributed to the existence of Mn2 þ states in the ZnS lattice [12], confirming the existence of the MnS phase. In addition, peaks related to O1s are also observed in the XPS spectra, and these are possibly due to unintentional oxygen doping during the growth. Since the concentration of Mn in the ZnS:Mn films is controlled by adjusting the DC power, the relative amounts of Zn2 þ and Mn2 þ are different in each of the ZM1, ZM2 and ZM3 samples, and their ratio can be deduced by comparing their XPS peak intensities. Fig. 2d shows the peak intensities for Zn2p and Mn2p for the films. The ZM1 film (which was grown at the lowest DC power) can be seen to have a nearly equal amount of Zn2 þ and Mn2 þ oxidation states while the ZM2 and ZM3 films (with a larger DC power) exhibit an increase in the amount of Mn2 þ and a decrease in Zn2 þ . From the XPS data, the Mn concentration in each Mn-doped ZnS film (relative to the Zn concentration) is estimated to be 3.1% for ZM1, 8.1% for ZM2, and 11.9% for ZM3. The PL properties of the ZnS and ZnS:Mn samples was studied at an excitation energy of 325 nm at room temperature. Fig. 3a displays the PL spectra of the samples. For the undoped sample, two emission peaks are observed at around 379 nm and 420 nm. The sharp PL emission peak in the NUV–Vis range is due to the radiative recombination between the conduction band and the valence band. The prominent blue self-activated emission
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band near 420 nm is associated with the radiative recombination of the electron–hole pairs between defect states and the valence band [13]. The donor-like defects are mainly due to sulfur vacancies in the ZnS lattice [14]. The observed optical bandgap of the un-doped ZnS film is smaller than bulk ZnS (3.7 eV), and this is
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A.I. Inamdar et al. / Materials Letters 163 (2016) 126–129
into two peaks that correspond to the direct band-to-band and defects-related emissions. However, the direct transition is dramatically quenched and appears to become saturated as the Mn concentration increases (see Fig. 3b and c). This dramatic reduction in the PL intensity in the Mn doped ZnS films can be attributed to the creation of non-radiative deep traps and the formation of the MnS phase. The MnS phase also reduces the amount of Mn2 þ ions that act as optically active luminescence centers in ZnS [15]. Similarly, the intensity of the luminescence of ZnS:Cu films decrease at a higher Cu concentration, and this is understood in terms of the formation of CuS [16]. Fig. 3d and e depicts the variation in the intensity and position of the PL peaks with the Mn concentration (relative to the Zn concentration). As the Mn concentration increases, the peaks shift towards higher energies (shorter wavelengths) with a rate of 11.77 2 meV/Mn%. This means that the effective optical bandgap increases, presumably because of the inclusion of Mn in the ZnS lattice. Note that the bandgap of γ-MnS ( 3.8 eV) can be larger than that of the undoped ZnS(O) film here ( 3.26 eV) [17]. The observed quench in the PL intensity and the peak shift towards higher energies are possibly because of alteration of cationic arrangement and phonon subsystem caused by Mn doping in ZnS species [18]. Mn dopant-mediated electron–phonon coupling interactions may induce the hybrid polaron-like states with resonant electron and phonon subsystems causing a significant modification of the energy spectra. Fig. 3f shows the proposed energy level diagram based on the photoluminescence of the ZnS thin films. The emission at around 3.26 eV is due to the direct band-to-band emission, and the emission at around 2.95 eV is due to the occurrence of donor-like sulfur vacancies near the surface whose energy level is formed at 0.3 eV below the conduction band. The increase in the Mn doping leads to the intensity reduction of the emission peaks. The PL quenching in the Mn-doped ZnS samples is mainly because of increased non-radiative defect concentration caused by Mn-doping. Another possible reason for this is the change in the crystal structure caused by the MnS phase. The decrease in the UV peak's intensity (Emission I) compared to the emission II peak intensity manifests itself that non-radiative defect density in the Mn-doped ZnS is higher compared to that of the undoped one.
4. Conclusions We synthesized Mn-doped ZnS thin films on soda-lime glass substrates by using magnetron co-sputtering. The films exhibit an amorphous nature with a granular morphology and a grain size of about 10 nm. X-ray photoelectron spectroscopy reveals the presence of Zn2 þ , Mn2 þ and S2 chemical binding states in the films. The undoped ZnS film shows two PL peaks: one is a direct bandto-band emission the other is a defects-mediated emission. When Mn is doped, the emission peaks are quenched and move towards higher energies with a rate of 11.772 meV/Mn%. The dramatic
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modifications in the optical transition of Mn-doped ZnS are attributed to the creation of Mn dopant-mediated structural phases such as MnS.
Acknowledgments This project was supported by the National Research Foundation (NRF) of Korea (Grant nos. 2015R1A2A2A01004782 and 2015M2A2A6A02045251).
References [1] T.Q. Hoa, N.D. The, S. McVitie, N.H. Nama, L.V. Vu, T.D. Canh, et al., Optical properties of Mn-doped ZnS semiconductor nanoclusters synthesized by a hydrothermal process, Opt. Mater. 33 (2011) 308–314. [2] N. Dixit, H. Soni, M. Chawda, D. Bodas, Study of electrical and optical properties of Mn doped ZnS clusters, Mater. Lett. 63 (2009) 2669–2671. [3] Q. Pan, D. Yang, Y. Zhao, Z. Ma, G. Dong, J. Qiu, Facile hydrothermal synthesis of Mn doped ZnS nanocrystals and luminescence properties investigations, J. Alloy. Comp. 579 (2013) 300–304. [4] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Optical properties of manganese-doped nanocrystals of ZnS, Phys. Rev. Lett. 72 (1994) 416–419. [5] M. Tanaka, Photoluminescence properties of Mn2 þ -doped II–VI semiconductor nanocrystals, J. Lumin. 100 (2002) 163–173. [6] P. Sajan, R. Vinod, M.J. Bushiri, High luminescent yield from Mn doped ZnS at yellow–orangeregion and 367 nm, J. Lumin 158 (2015) 110–115. [7] T. Toyoda, J. Matsuzawa, Q. Shen, Photoacoustic and Photoluminescence Spectra of ZnS:Mn Nanocrystals, Jpn. J. Appl. Phys. 41 (2002) 3383–3386. [8] E. Sotelo-Gonzalez, L. Roces, S. Garcia-Granda, M.T. Fernandez-Arguelles, J. M. Costa-Fernandez, A. Sanz-Medel, Influence of Mn2 þ concentration on Mn2 þ -doped ZnS quantum dot synthesis: evaluation of the structural and photoluminescent properties, Nanoscale 5 (2013) 9156–9161. [9] R. John, S.S. Florence, Effects of annealing temperature on structure/morphology and photoluminescence properties of Mn-doped ZnS nanoparticles, Mater. Lett. 107 (2013) 93–95. [10] M. Wei, J. Yang, Y. Yan, L. Yang, J. Cao, H. Fu, B. Wang, et al., Influence of Mn ions concentration on optical and magnetic properties of Mn-doped ZnS nanowires, Physica E 52 (2013) 144–149. [11] S.W. Shin, S.R. Kang, J.H. Yun, A.V. Moholkar, J.-H. Moon, J.Y. Lee, et al., Effect of different annealing conditions on the properties of chemically deposited ZnS thin films on ITO coated glass substrates, Sol. Energy Mater. Sol. Cells 95 (2011) 856–863. [12] C. Zhan, J. Lu, A.J. Kropf, T. Wu, A.N. Jansen, Y.K. Sun, et al., Mn(II) deposition on anodes and its effects on capacity fade in spinel lithium manganate–carbon systems, Nat. Commun. 4 (2013) 2437–2444. [13] H. Li, W.Y. Shih, W.H. Shih, Non-heavy-metal ZnS quantum dots with bright blue photoluminescence by a one-step aqueous synthesis, Nanotechnology 18 (2007) 205604–205609. [14] A.I. Inamdar, S. Lee, D. Kim, K.V. Gurav, J.H. Kim, H. Im, et al., Metal-doped ZnS (O) thin films on glass substrates using chemical bath deposition, Thin Solid Films 537 (2013) 36–41. [15] A. Divya, B.K. Reddy, S. Sambasivam, K.S. Kumar, P.S. Reddy, Structural and optical characterization of ZnS nanoparticles co-doped with Mn and Te, Physica E 44 (2011) 541–545. [16] G. Murugadoss, Structural and optical properties of monodispersed ZnS/CdS/ ZnO andZnO/ZnS/CdS nanoparticles, J. Lumin. 132 (2012) 2043–2048. [17] Y.C. Zhang, H. Wang, B. Wang, H. Yan, M. Yoshimura, Low-temperature hydrothermal synthesis of pure metastable γ-manganese sulfide (MnS) crystallites, J. Cryst. Growth 243 (2002) 214–217. [18] M.G. Brik, I.V. Kityk, Analysis of optical spectra of V2 þ centres in ZnS and ZnSe single crystals, Phys. Status Solidi (B) 245 (2008) 163–169.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical properties in Mn-doped ZnS thin films: Photoluminescence quenching A.I. Inamdar a, Sangeun Cho a, Yongcheol Jo a, Jongmin Kim a, Jaeseok Han a, S.M. Pawar a, Hyeonseok Woo a, R.S. Kalubarme b, ChanJin Park b, Hyungsang Kim a,n, Hyunsik Im a,n a b
Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, South Korea Department of Materials Science & Engineering, Chonnam National University, Gwangju 500-757, South Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 14 September 2015 Received in revised form 14 October 2015 Accepted 16 October 2015 Available online 17 October 2015
Mn-doped ZnS thin films are synthesized on soda-lime glass substrates using magnetron co-sputtering technique. X-ray diffraction and atomic force microscopy measurements indicate that of the as-obtained films including the highest Mn ( 11% relative to the Zn concentration) in the lattice of ZnS are amorphous with a granular morphology. X-ray photoelectron spectroscopy reveals the presence of Zn2 þ , Mn2 þ and S2 chemical states in the films. The undoped ZnS film exhibits photoluminescence (PL) peaks at energies around 3.26 eV (wavelength 379 nm) and 2.95 eV ( 420 nm), which originate from the interplay between excited electron, defect (sulfur vacancy) states and the valence band. For the Mndoped ZnS films, the band-to-band emission peak is quenched and shifts toward to higher energies at a rate of 11.77 2 meV/Mn%. We propose that Mn dopant-mediated structural phases and non-radiative deep traps in ZnS cause the modification in the optical transition. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mn-doped Zinc sulfide Photoluminescence Defects
1. Introduction ZnS has attracted an enormous amount of attention because it can be used in a wide variety of applications related to solar cells, electroluminescent devices, and optoelectronic device (such as in window layers for blue-light diodes), antireflection coatings, hydrogen production, displays, sensors, bio imaging and other nonlinear optical devices [1–3]. This material has a wide band gap and is therefore important for use in the violet and blue regions. ZnS is also suitable for use as a host material for various dopants, and it is non-toxic and possesses good quantum efficiency. It was demonstrated that Bhargava was the first to report that Mn-doping in ZnS can improve its luminescent efficiency [4]. The luminescence in Mn-doped ZnS has been proposed to be a result of the energy transfer from electron hole pairs delocalized inside of the ZnS host crystals [5]. Mn-doped ZnS can exhibit both blue defect-related emissions and orange Mn þ 2 ion-related emissions [1,6], and the Mn doping concentration affects the PL intensity of the film because the variation in the concentration changes the energy band, forming different luminescence centers, and also, surface passivators can contribute to the increase in the PL [4,7]. A maximum PL was reached at a certain Mn2 þ n
Corresponding authors. E-mail addresses: [email protected] (H. Kim), [email protected] (H. Im). http://dx.doi.org/10.1016/j.matlet.2015.10.074 0167-577X/& 2015 Elsevier B.V. All rights reserved.
concentration and a further increase led to a decrease in the PL intensity [8–10]. There have been multiple efforts for more than a decade to enhance and understand the PL properties of ZnS. Though various chemical synthesis techniques such as chemical precipitation, hydrothermal, sol–gel and chemical vapor deposition have been used to deposit Mn-doped ZnS, further improvement is still required. Furthermore, these synthetic methods are relatively more complicated than physical techniques because they require organic solvents and high reaction temperature. To the best of our knowledge, there have been rare reports of Mn-doped ZnS prepared by a magnetron co-sputtering method with its PL properties. In this work, we synthesize co-sputtered Mn-doped ZnS (ZnS:Mn) films and report on their photoluminescence properties to demonstrate that the Mn concentration plays an important role in tuning the PL properties of the ZnS:Mn films. Mninduced non-radiative defect-states and the formation of a MnS phase cause photoluminescence quenching in the ZnS:Mn films.
2. Experimental Mn-doped ZnS thin films (ZnS:Mn) were fabricated on sodalime glass substrates using conventional magnetron co-sputtering. Two different targets, ZnS and Mn, with a purity of 99.99% were simultaneously sputtered using RF and DC power supplies, respectively. Soda-lime glass substrates with an area of 1 cm2 were mounted approximately 4 cm away from the targets at a 45° angle.
A.I. Inamdar et al. / Materials Letters 163 (2016) 126–129
Prior to deposition, the substrates were consecutively cleaned ultrasonically in acetone, methanol, and deionized water. The chamber was initially evacuated using a rotary pump to 10 3 Torr and was then further evacuated to 7.5 10 6 Torr using a turbo molecular pump. Before sputtering, the chamber was filled with Ar gas at a flow rate of 75 sccm to reach a working pressure of 6.2 10 2 Torr. A fixed RF power of 90 W was applied to the ZnS target, and three different DC powers of 400 V, 475 V, and 550 V were applied to the Mn target to obtain different Mn concentrations. The targets were initially pre-sputtered for 5 min, and then deposition was carried out for 25 minutes. The substrate was rotated during deposition to improve the uniformity of the deposited films. The un-doped ZnS and ZnS:Mn grown at three different DC powers of 400 V, 475 V and 550 V are denoted as ZM0, ZM1, ZM2 and ZM3, respectively. The Mn concentration in the ZnS: Mn films are estimated using the X-ray photoelectron spectroscopy (XPS) measurements. The structural properties of the films were measured using high-resolution X-ray diffraction (XRD) with Ni-filtered Cu-Kα radiation (X’pert PRO, Philips, Eindhoven, Netherlands). The chemical binding states of the undoped ZnS and ZnS:Mn films were investigated using XPS (VG Multilab 2000, Thermo VG Scientific, UK) with an incident energy of 1400 eV. The PL measurements were carried out using a 75 cm monochromator equipped with an ultraviolet-sensitive photomultiplier tube at an excitation wavelength of 325 nm.
3. Results and discussion Fig. 1a and b shows the XRD pattern of the undoped ZnS and ZnS:Mn(ZM1) films. Both films exhibit two broad reflections in the 2θ range from 10° to 80°. The XRD patterns indicate that the particle size of the films is very small, which is quite obvious in the sputtered sample. A considerable broadening of the XRD spectra can be ascribed to the amorphous nature of the sample or to very tiny particles. The existence of fine particles in the amorphous ZnS:Mn films grown on soda lime glass substrates is directly observed in the atomic force microscopic (AFM) measurements as shown in Fig. 1(c–f). All of the samples exhibit a compact granular morphology with very fine particles below 10 nm, which is
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consistent with the XRD data. The un-doped ZM0 (see Fig. 1c) sample has a very compact morphology with a rough surface whereas the Mn co-sputtered samples exhibit a granular morphology(see Fig. 1d).From the surface roughness measurements, their surfaces become smooth as the Mn content increases (for example, ZM2 and ZM3). The XPS analysis of the undoped and Mn doped ZnS films is performed to determine the chemical binding state and to confirm the presence of the constituting element of the films. Fig. 2(a–c) show the Zn2p, Mn2p and S2p core level spectra for the ZM1 sample. The peaks related to Zn 2p3/2, Zn 2p1/2 and S 2p3/2 in the core level Zn2p and S2p spectra of the pure ZnS sample are detected at 1021.85, 1044.95, and 161.45 respectively, and these are attributed to the formation of the Zn–S bond [11]. For the ZnS:Mn samples, Zn 2p3/2, Zn 2p1/2 and S 2p3/2 peaks are observed in the core level spectra at the same positions, but additional peaks related to Mn 2p3/2 and Mn 2p1/2 (see Fig. 2c) are observed at 641.85 and 653.5 eV, respectively. These Mn 2p peaks are attributed to the existence of Mn2 þ states in the ZnS lattice [12], confirming the existence of the MnS phase. In addition, peaks related to O1s are also observed in the XPS spectra, and these are possibly due to unintentional oxygen doping during the growth. Since the concentration of Mn in the ZnS:Mn films is controlled by adjusting the DC power, the relative amounts of Zn2 þ and Mn2 þ are different in each of the ZM1, ZM2 and ZM3 samples, and their ratio can be deduced by comparing their XPS peak intensities. Fig. 2d shows the peak intensities for Zn2p and Mn2p for the films. The ZM1 film (which was grown at the lowest DC power) can be seen to have a nearly equal amount of Zn2 þ and Mn2 þ oxidation states while the ZM2 and ZM3 films (with a larger DC power) exhibit an increase in the amount of Mn2 þ and a decrease in Zn2 þ . From the XPS data, the Mn concentration in each Mn-doped ZnS film (relative to the Zn concentration) is estimated to be 3.1% for ZM1, 8.1% for ZM2, and 11.9% for ZM3. The PL properties of the ZnS and ZnS:Mn samples was studied at an excitation energy of 325 nm at room temperature. Fig. 3a displays the PL spectra of the samples. For the undoped sample, two emission peaks are observed at around 379 nm and 420 nm. The sharp PL emission peak in the NUV–Vis range is due to the radiative recombination between the conduction band and the valence band. The prominent blue self-activated emission
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ZM3
Sample
Binding energy (eV)
Fig. 2. X-ray photoelectron spectroscopy spectra for (a) core level Zn 2p, (b) core level S 2p, (c) core level Mn 2p, and (d) Comparison of Zn 2p and Mn 2p peak intensities.
band near 420 nm is associated with the radiative recombination of the electron–hole pairs between defect states and the valence band [13]. The donor-like defects are mainly due to sulfur vacancies in the ZnS lattice [14]. The observed optical bandgap of the un-doped ZnS film is smaller than bulk ZnS (3.7 eV), and this is
3k
sample: ZM0
Emission II
2k
ZM0 ZM1 ZM2
1k
ZM3
PL intensity (a.u.)
PL intensity (a.u.)
Emission I
0
2k Emission І Emission П
1k
400
400 450 Wavelength (nm)
500
Emission І Emission П
200 0
0 350
sample: ZM3
600 PL intensity (a.u.)
3k
possibly due to the effect of the unintentional oxygen doping [14]. In this regard, the actual chemical composition of the samples is ZnS(O). In contrast, the ZnS:Mn samples show a broad hump in the range from 350 to 500 nm. This broad PL band is deconvoluted
350
400 450 Wavelength (nm)
500
350
400 450 Wavelength (nm)
500
1500 Emission І Emission П
1000 500 0
3
6
9
Mn concentration (%)
12
450
Laser excitation 3.8 eV
2000
Peak Position (nm)
PL intensity (a.u.)
Excited states EC
400
Emission І
350
Emission П
0
3
6
9
Mn concentration (%)
12
Non-radiative recombination
3.26 eV Violet Emission I
2.95 eV Blue Emission II
EV Electron
Hole
Trap
Fig. 3. Room temperature photoluminescence (PL) spectra of (a) undoped ZnS and ZnS:Mn samples, (b) PL spectrum of undoped ZnS with deconvoluted curves, (c) PL spectrum of ZnS:Mn(ZM1) with deconvoluted curves, (d) PL peaks' intensities, (e) PL peaks' position, and (f) schematic energy band diagram explaining the measured PL characteristics.
A.I. Inamdar et al. / Materials Letters 163 (2016) 126–129
into two peaks that correspond to the direct band-to-band and defects-related emissions. However, the direct transition is dramatically quenched and appears to become saturated as the Mn concentration increases (see Fig. 3b and c). This dramatic reduction in the PL intensity in the Mn doped ZnS films can be attributed to the creation of non-radiative deep traps and the formation of the MnS phase. The MnS phase also reduces the amount of Mn2 þ ions that act as optically active luminescence centers in ZnS [15]. Similarly, the intensity of the luminescence of ZnS:Cu films decrease at a higher Cu concentration, and this is understood in terms of the formation of CuS [16]. Fig. 3d and e depicts the variation in the intensity and position of the PL peaks with the Mn concentration (relative to the Zn concentration). As the Mn concentration increases, the peaks shift towards higher energies (shorter wavelengths) with a rate of 11.77 2 meV/Mn%. This means that the effective optical bandgap increases, presumably because of the inclusion of Mn in the ZnS lattice. Note that the bandgap of γ-MnS ( 3.8 eV) can be larger than that of the undoped ZnS(O) film here ( 3.26 eV) [17]. The observed quench in the PL intensity and the peak shift towards higher energies are possibly because of alteration of cationic arrangement and phonon subsystem caused by Mn doping in ZnS species [18]. Mn dopant-mediated electron–phonon coupling interactions may induce the hybrid polaron-like states with resonant electron and phonon subsystems causing a significant modification of the energy spectra. Fig. 3f shows the proposed energy level diagram based on the photoluminescence of the ZnS thin films. The emission at around 3.26 eV is due to the direct band-to-band emission, and the emission at around 2.95 eV is due to the occurrence of donor-like sulfur vacancies near the surface whose energy level is formed at 0.3 eV below the conduction band. The increase in the Mn doping leads to the intensity reduction of the emission peaks. The PL quenching in the Mn-doped ZnS samples is mainly because of increased non-radiative defect concentration caused by Mn-doping. Another possible reason for this is the change in the crystal structure caused by the MnS phase. The decrease in the UV peak's intensity (Emission I) compared to the emission II peak intensity manifests itself that non-radiative defect density in the Mn-doped ZnS is higher compared to that of the undoped one.
4. Conclusions We synthesized Mn-doped ZnS thin films on soda-lime glass substrates by using magnetron co-sputtering. The films exhibit an amorphous nature with a granular morphology and a grain size of about 10 nm. X-ray photoelectron spectroscopy reveals the presence of Zn2 þ , Mn2 þ and S2 chemical binding states in the films. The undoped ZnS film shows two PL peaks: one is a direct bandto-band emission the other is a defects-mediated emission. When Mn is doped, the emission peaks are quenched and move towards higher energies with a rate of 11.772 meV/Mn%. The dramatic
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modifications in the optical transition of Mn-doped ZnS are attributed to the creation of Mn dopant-mediated structural phases such as MnS.
Acknowledgments This project was supported by the National Research Foundation (NRF) of Korea (Grant nos. 2015R1A2A2A01004782 and 2015M2A2A6A02045251).
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