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Fabrication, characterization and electroluminescence studies of SrS: Ce3+ ACTFEL device
Materials Letters 198 (2017) 101–105
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication, characterization and electroluminescence studies of SrS: Ce3+ ACTFEL device Shubhra Mishra a, D.S. Kshatri b,⇑, Ayush Khare a, Sanjay Tiwari c, Prabhat K. Dwivedi d a
Department of Physics, National Institute of Technology, Raipur 492 010, India Department of Physics, Shri Shankaracharya Institute of Professional Management and Technology, Raipur 492 015, India c School of Studies in Electronics and Photonics, Pt. Ravishankar Shukla University, Raipur 492 010, India d Centre for Nanosciences, Indian Institute of Technology, Kanpur 208016, India b
a r t i c l e
i n f o
Article history: Received 23 February 2017 Received in revised form 2 April 2017 Accepted 3 April 2017 Available online 4 April 2017 Keywords: Thin films Deposition Optical materials Luminescence
a b s t r a c t The SrS: Ce3+ thin film active phosphor layer of different thickness based alternating current thin-film electroluminescent (ACTFEL) devices are fabricated by electron beam evaporation deposition (EBED) method. The morphology and chemical composition of the deposited films are investigated by field emission scanning electron microscopy (FESEM). The optical transmittance in visible range for the optimum film and optical band gap of SrS: Ce3+ film is described on the basis of Ce3+ doping concentration. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years, nanocrystalline semiconductor thin films have played an important role in research work due to their future applications in the diverse fields of science and technology. These kinds of films are used in various electronic devices because the optical, electrical and chemical properties of the material change significantly upon varying the grain size and thickness of the film [1]. Thus, it offers the possibility of materials enhancement device characteristics [2]. SrS: Ce is a promising phosphor material for blue and white alternating current thin-film electroluminescent (ACTFEL) devices [3]. The development of full-color ACTFEL flatpanel displays has motivated researchers to explore a variety of phosphor host/luminescent impurity combinations [4–6]. A simple ACTFEL device consists of a metal-insulator-semiconductor-insula tor-metal (MISIM) structure deposited on a substrate, usually glass. The primary application of ACTFEL technology is as a thin display in the flat-panel display (FPD) industry, which is driven by the demand for portable displays [7]. The leading FPD technology is the liquid-crystal display (LCD) found in a number of applications, such as watches, calculators, laptop, computer monitors and handheld electronic devices. SrS is a promising host material for thin film electroluminescent (TFEL) displays, and has been extensively
⇑ Corresponding author. E-mail address: [email protected] (D.S. Kshatri). http://dx.doi.org/10.1016/j.matlet.2017.04.013 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
studied as the blue component for multi- and full-color electroluminescence (EL) display panels [8]. The goal of the present study was to develop new sulfide processes for thin film phosphors for their use in ACTFEL devices and to study their effect on EL properties of the device. Sulfide materials were selected because the most successful phosphor materials used in ACTFEL devices have been sulfides [9]. In the present work, ACTFEL devices are fabricated using SrS: Ce3+ as the active emitting layer with different thicknesses (300 nm, 500 nm and 700 nm) on low temperature glass substrates.
2. Device fabrication We have earlier reported that SrS thin films with 0.5 mol% of Ce3+ doping exhibit highest luminescence intensity [10]. Therefore, the SrS: Ce3+ (0.5 mol%) based ACTFEL devices with different active phosphor layer thickness (300 nm, 500 nm and 700 nm) were prepared at 250 °C by EBED techniques. In this technique, the long range distortion is witnessed which triggers loss of long range order in as-prepared thin films, but it can be controlled by controlling the physical conditions like temperature, pressure, etc. The SrS phosphor layers of different thickness were deposited on ITO coated glass substrates. The SrS: Ce3+ phosphor is sandwiched between zinc sulfide (ZnS) buffer layers. The zinc sulfide buffer layers each of thickness 100 nm are also deposited at a temperature of 150 °C by EBED method. Aluminum (Al) served as the rear
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S. Mishra et al. / Materials Letters 198 (2017) 101–105
electrode in present case. Al2O3 layers of high dielectric strength and high dielectric constant each of 150 nm thick is deposited at a temperature of 200 °C and is applied as an insulating layer while ZnS layer is deposited on top and bottom of the SrS layer to protect the SrS layer from moisture and contamination. The pressure in the vacuum chamber prior to evaporation was 5.5 106 mbar. The device fabrication process is used to fabricate three ACTFEL devices with different active phosphor layer of SrS: Ce3+ (0.5 mol%) with varying thickness (300 nm, 500 nm and 700 nm).
3. Results and discussion 3.1. Field emission scanning electron microscopy From FESEM images, it can be clearly seen that pure SrS (Fig. 1 (a)) and Ce3+ (0.1, 0.5, 1 and 1.5 mol%) doped SrS (Fig. 1(b–e)) films present nearly same morphology. Grain growth in thin films is basically different from grain growth in bulk materials in several important ways [11]. One of the most important reasons for this fact is that the interface of the film with the glass substrate and the top surface of the film play an important role in suppressing normal growth of grains and encouraging secondary or abnormal grain growth due to this reason anisotropic behavior of energy of the surfaces or interfaces are exhibited. Secondary grains generally have restricted crystallographic orientations that are affected by the atomic arrangement of the substrate interface as well as the environment of the top surface of the film [12]. The crystal quality of the samples examined by XRD was explained in our previous paper [10]. It is also observed that (Fig. 1(a)) the granular surface of pure SrS thin film is smooth, uniformly dense and with tiny structures. Further, when SrS is doped with Ce3+ ions (0.1–1.5 mol%), smooth and uniform isolated nano-ice circle like structures with average diameter 30 nm are obtained (Fig. 1(b–e)). All the films have a
similar grain size distributed between 25 nm and 35 nm, and the increasing doping concentration has no noticeable effect on the grain size. 3.2. Transmittance spectral analysis Fig. 2(a) shows the optical transmission spectra of pure and Ce3+ (0.1%, 0.5%, 1% and 1.5 mol%) doped SrS thin films. The decrease in the transmission at about 300–350 nm is attributed to the absorption of glass substrate. The optical transmission of all the films is over 80% in the visible region of 400–700 nm, which reveals that the films are highly transparent in the visible region. Additionally, it seems that the influence of doping concentration on the transmittance spectra is random. However, there is a visible change in the band gap absorption edge of SrS: Ce3+ thin films. The absorption coefficient was calculated from Tauc’s plot [13] using the formula –
a¼
ðhm Eg Þ hm
2
ð1Þ
Thus, the experimental band-gap values for pure and Ce3+ (0.1%, 0.5%, 1% and 1.5 mol%) doped SrS as extracted from the Tauc’s plots [14] (Fig. 2(b)) are found to be 4.20 eV, 4.22 eV, 4.27 eV, 4.30 eV and 4.39 eV, respectively. The optical band gap values of the films increase with increasing Ce3+ concentration. As compared to pure SrS, the increased optical band gap value of different SrS: Ce thin films can be explained by the Burstein–Moss effect, which is about transition energy in degenerate semiconductors due to the partially filled conduction band. According to Burstein-Moss effect, the apparent band-gap of a semiconductor is increased as the absorption edge is pushed to higher energies as a result fall state close to the conduction band being populated [15]. CeSr-VSr defect complexes are believed to form as a consequence of selfcompensation [16] as explained in the following lines. Consider
Fig. 1. FESEM images of (a) pure SrS (b-e) SrS: Ce3+ (0.1, 0.5, 1.0, 1.5 mol%) thin films.
S. Mishra et al. / Materials Letters 198 (2017) 101–105
103
Fig. 2. (a) UV–vis diffuse reflectance spectra of pure SrS and SrS: Ce3+ (0.1, 0.5, 1.0 and 1.5 mol%) thin films (b) Plot of (aht)2 against energy band gap (hm).
an ideal situation that SrS is initially intrinsic, which means that its Fermi level is positioned at the middle of the bandgap with the phosphor having a negligible concentration of defects and impurities. Now assume that the phosphor is doped with an increasing concentration of Ce3+. This results in a modulation of the Fermi level towards the conduction band. Selfcompensation is envisaged to arise when the Fermi level position reaches a critical energy at which it is more energetically favorable to create an intrinsic self-compensating acceptor defect than to continue to modulate the Fermi level towards the conduction band [17]. 3.3. Electroluminescence spectra A model of MISIM structure of ACTFEL device is shown in Fig. 3 (a). The voltage dependence (50–100 V) of electroluminescence (EL) intensity of ACTFEL devices fabricated with different thicknesses (300 nm, 500 nm and 700 nm) of SrS: Ce3+ (0.5 mol%) phosphor are shown in Fig. 3(b–d). When excited with 1 kHz of frequency and above threshold voltage of 50 V, the EL spectra corresponding to the entire devices exhibit green and blue emission peaks at 442 nm and 536 nm, respectively in the wavelength range of 200–700 nm. The observed asymmetric spectra show a significant variation in intensity with active phosphor layer thickness. The EL intensity for three ACTFEL devices increases with increase in voltage from 50 V to 100 V while the peak positions remain unchanged respectively at 442 nm and 536 nm. Using EL spectra, one can know whether dpoant ions (Ce3+) are uniform distributed in the host lattice or not. If there is a considerable shift in the peak wavelength one could easily predict that Ce3+ distribution in the host lattice is non-uniform. In the present investigation, there was not observed any shift in the peak emission wavelength, which indicated that the Ce3+ ions were uniformly distributed in the SrS host. The electron-phonon interaction plays an important role in determining both the electron and phonon properties of thin films [18] and the Jahn-Teller splitting based on the temperature independence of the peak positions was well explained by Jones et al. [19]. In an ACTFEL device, the injected electrons are accelerated under the influence of high electric fields (2 MV/cm) in the phosphor. An understanding of high field carrier transport in the phosphor layer and the physics of the different threshold processes such as band to band impact ionization and impact ionization of luminescence impurities is essential for ACTFEL device designing
and fabrication [20]. In our study, we have neglected phononassisted impact ionization processes as well as deep level ionization. These effects may be important at high fields. In this electric field regime, the average drift velocity of electrons saturate at 105 m/s, a value which is almost independent of temperature and nature of semiconductor materials. Scattering with impurities, which controls the mobility at low field, can no longer stabilize the electron drift in this saturation region. This can be done by electron-phonon (electron-lattice) interaction [4]. The energy exchange between electron and phonon is described by the electron-phonon interaction Hamiltonian, where the electron can emit or absorb one phonon at a time. Kytik et al. [21] reported the role of electron-phonon interaction and demonstrated that chemical bonds may be effectively operated by external polarized field with large power densities. However, to achieve the large value of the effect, it is necessary to have large values of electron–phonon interactions. It is necessary to emphasize that this effect may be observed in relatively disordered materials like glasses and is not usually observed in perfect crystalline systems due to a relatively low total energy. The luminance-voltage curves of ACTFEL devices fabricated with different thicknesses (300 nm, 500 nm and 700 nm) of SrS: Ce3+ (0.5 mol%) phosphor layer are presented in Fig. 3(e). The devices show variation in threshold voltage as thickness is varied which is expected as there is a direct correlation between threshold voltage and thickness of the active phosphor layer. The device with thinnest (300 nm) active phosphor layer has a minimum threshold voltage (50 V) and it is maximum (110 V) for the device with thickest (700 nm) active phosphor layer. The L-V curve is also dependent on the ambient temperature and the dopant concentration of phosphor layer. When the temperature is increased, the probability of de-excitation through non-radiative recombination is significantly increased, which leads to reduced light output. Temperature also may affect the threshold voltage in an L-V curve since interface emission and space charge generation mechanisms are dependent on it. The luminance increases with impurity/ dopant level for low concentrations [22]. Increase in the density of luminescent centers raises the probability of an electron undergoing a collision with a luminescent center. This trend holds as long as the impurity concentration is low enough (up to 0.5 mol %) because phosphor crystallinity is maintained and concentration quenching does not occur.
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Fig. 3. (a) The MISIM structure of SrS: Ce3+ multilayer ACTFEL device (b–d) The EL spectra of SrS: Ce3+ (0.5 mol%) ACTFEL devices at different active phosphor layer thicknesses and (e) The L-V curves of SrS: Ce3+ (0.5 mol%) ACTFEL device with different active phosphor layer thicknesses.
S. Mishra et al. / Materials Letters 198 (2017) 101–105
4. Conclusions Results show that EL intensity decreases with increase in phosphor layer thickness while EL intensity increases with increase in applied voltage for each device. The L-V curves suggest the necessity of high threshold voltage to observe luminance with increasing thickness of active phosphor layer.
References [1] A. Tanaka, S. Onari, T. Arai, Phys. Rev. B 47 (1993) 1237–1243. [2] Biljana Pejova, Ivan Grozdanov, Mat. Chem. Phys. 99 (2006) 39–49. [3] K. Ohmi, K. Ishitani, S. Tanaka, H. Kobayashi, Appl. Phys. Lett. 67 (1995) 944– 946. [4] Y.A. Ono, Electroluminescent Displays, World Scientific, Singapore, 1995. [5] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, 1999. [6] C. Li, Z. Zang, W. Chen, Z. Hu, X. Tang, W. Hu, K. Sun, X. Liu, W. Chen, Opt. Express 24 (2016) 15071–15078. [7] R.H. Mauch, J. SID 5 (1997) 173–178.
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[8] S.S. Sun, E. Dickey, J. Kane, P. Yocom, International Display Research Conference, Toronto, 301, Society for Information Display, Santa Ana, CA, USA, 1997. [9] M. Ritala, M. Leskelä, Handbook of Thin Film Materials, vol. 1, Academic Press, San Diego, 2002, pp. 103–159. [10] S. Mishra, D.S. Kshatri, A. Khare, S. Tiwari, P.K. Dwivedi, Mater. Lett. 183 (2016) 191–196. [11] N. Moelans, B. Blanpain, P. Wollants, Acta Mater. 55 (2007) 2173–2182. [12] C.V. Thompson, Annu. Rev. Mater. Sci. 20 (1990) 245–268. [13] J. Tauc, Mater. Res. Bull. 5 (1970) 721–729. [14] J. Tauc, R. Grigorovici, A. Vancu, Phys. Stat. Sol. 15 (1966) 627–637. [15] M. Grundmann, The Physics of Semiconductors, second ed., Springer Berlin Heidelberg, New York, 2006. [16] F.A. Kroger, The Chemistry of Imperfect Crystals, second ed., North-Holland, Amsterdam, 1974. [17] J.F. Wager, Philos. Mag. A 67 (1993) 897–904. [18] R. Fivaz, D.E. Mooser, Phys. Rev. 136 (1964) A833–A836. [19] T.C. Jones, W. Park, E. Mohammed, B.K. Wagner, C.J. Summers, S.S. Sun, Mater. Res. Soc. Symp. Proc. 508 (1998) 281–287. [20] M. Dur, S.M. Goodnick, S.S. Pennathur, J.F. Wager, M. Reigrotzki, R. Redmer, J. Appl. Phys. 83 (6) (1998) 3176–3185. [21] I.V. Kityk, M. Guignard, V. Nazabal, X.H. Zhang, J. Troles, F. Smektala, B. Sahraoui, G. Boudebs, Physica B 391 (2007) 222–227. [22] M.M.A. Ahmed, New Trends Phys. (2004) 196–199.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication, characterization and electroluminescence studies of SrS: Ce3+ ACTFEL device Shubhra Mishra a, D.S. Kshatri b,⇑, Ayush Khare a, Sanjay Tiwari c, Prabhat K. Dwivedi d a
Department of Physics, National Institute of Technology, Raipur 492 010, India Department of Physics, Shri Shankaracharya Institute of Professional Management and Technology, Raipur 492 015, India c School of Studies in Electronics and Photonics, Pt. Ravishankar Shukla University, Raipur 492 010, India d Centre for Nanosciences, Indian Institute of Technology, Kanpur 208016, India b
a r t i c l e
i n f o
Article history: Received 23 February 2017 Received in revised form 2 April 2017 Accepted 3 April 2017 Available online 4 April 2017 Keywords: Thin films Deposition Optical materials Luminescence
a b s t r a c t The SrS: Ce3+ thin film active phosphor layer of different thickness based alternating current thin-film electroluminescent (ACTFEL) devices are fabricated by electron beam evaporation deposition (EBED) method. The morphology and chemical composition of the deposited films are investigated by field emission scanning electron microscopy (FESEM). The optical transmittance in visible range for the optimum film and optical band gap of SrS: Ce3+ film is described on the basis of Ce3+ doping concentration. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In recent years, nanocrystalline semiconductor thin films have played an important role in research work due to their future applications in the diverse fields of science and technology. These kinds of films are used in various electronic devices because the optical, electrical and chemical properties of the material change significantly upon varying the grain size and thickness of the film [1]. Thus, it offers the possibility of materials enhancement device characteristics [2]. SrS: Ce is a promising phosphor material for blue and white alternating current thin-film electroluminescent (ACTFEL) devices [3]. The development of full-color ACTFEL flatpanel displays has motivated researchers to explore a variety of phosphor host/luminescent impurity combinations [4–6]. A simple ACTFEL device consists of a metal-insulator-semiconductor-insula tor-metal (MISIM) structure deposited on a substrate, usually glass. The primary application of ACTFEL technology is as a thin display in the flat-panel display (FPD) industry, which is driven by the demand for portable displays [7]. The leading FPD technology is the liquid-crystal display (LCD) found in a number of applications, such as watches, calculators, laptop, computer monitors and handheld electronic devices. SrS is a promising host material for thin film electroluminescent (TFEL) displays, and has been extensively
⇑ Corresponding author. E-mail address: [email protected] (D.S. Kshatri). http://dx.doi.org/10.1016/j.matlet.2017.04.013 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
studied as the blue component for multi- and full-color electroluminescence (EL) display panels [8]. The goal of the present study was to develop new sulfide processes for thin film phosphors for their use in ACTFEL devices and to study their effect on EL properties of the device. Sulfide materials were selected because the most successful phosphor materials used in ACTFEL devices have been sulfides [9]. In the present work, ACTFEL devices are fabricated using SrS: Ce3+ as the active emitting layer with different thicknesses (300 nm, 500 nm and 700 nm) on low temperature glass substrates.
2. Device fabrication We have earlier reported that SrS thin films with 0.5 mol% of Ce3+ doping exhibit highest luminescence intensity [10]. Therefore, the SrS: Ce3+ (0.5 mol%) based ACTFEL devices with different active phosphor layer thickness (300 nm, 500 nm and 700 nm) were prepared at 250 °C by EBED techniques. In this technique, the long range distortion is witnessed which triggers loss of long range order in as-prepared thin films, but it can be controlled by controlling the physical conditions like temperature, pressure, etc. The SrS phosphor layers of different thickness were deposited on ITO coated glass substrates. The SrS: Ce3+ phosphor is sandwiched between zinc sulfide (ZnS) buffer layers. The zinc sulfide buffer layers each of thickness 100 nm are also deposited at a temperature of 150 °C by EBED method. Aluminum (Al) served as the rear
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S. Mishra et al. / Materials Letters 198 (2017) 101–105
electrode in present case. Al2O3 layers of high dielectric strength and high dielectric constant each of 150 nm thick is deposited at a temperature of 200 °C and is applied as an insulating layer while ZnS layer is deposited on top and bottom of the SrS layer to protect the SrS layer from moisture and contamination. The pressure in the vacuum chamber prior to evaporation was 5.5 106 mbar. The device fabrication process is used to fabricate three ACTFEL devices with different active phosphor layer of SrS: Ce3+ (0.5 mol%) with varying thickness (300 nm, 500 nm and 700 nm).
3. Results and discussion 3.1. Field emission scanning electron microscopy From FESEM images, it can be clearly seen that pure SrS (Fig. 1 (a)) and Ce3+ (0.1, 0.5, 1 and 1.5 mol%) doped SrS (Fig. 1(b–e)) films present nearly same morphology. Grain growth in thin films is basically different from grain growth in bulk materials in several important ways [11]. One of the most important reasons for this fact is that the interface of the film with the glass substrate and the top surface of the film play an important role in suppressing normal growth of grains and encouraging secondary or abnormal grain growth due to this reason anisotropic behavior of energy of the surfaces or interfaces are exhibited. Secondary grains generally have restricted crystallographic orientations that are affected by the atomic arrangement of the substrate interface as well as the environment of the top surface of the film [12]. The crystal quality of the samples examined by XRD was explained in our previous paper [10]. It is also observed that (Fig. 1(a)) the granular surface of pure SrS thin film is smooth, uniformly dense and with tiny structures. Further, when SrS is doped with Ce3+ ions (0.1–1.5 mol%), smooth and uniform isolated nano-ice circle like structures with average diameter 30 nm are obtained (Fig. 1(b–e)). All the films have a
similar grain size distributed between 25 nm and 35 nm, and the increasing doping concentration has no noticeable effect on the grain size. 3.2. Transmittance spectral analysis Fig. 2(a) shows the optical transmission spectra of pure and Ce3+ (0.1%, 0.5%, 1% and 1.5 mol%) doped SrS thin films. The decrease in the transmission at about 300–350 nm is attributed to the absorption of glass substrate. The optical transmission of all the films is over 80% in the visible region of 400–700 nm, which reveals that the films are highly transparent in the visible region. Additionally, it seems that the influence of doping concentration on the transmittance spectra is random. However, there is a visible change in the band gap absorption edge of SrS: Ce3+ thin films. The absorption coefficient was calculated from Tauc’s plot [13] using the formula –
a¼
ðhm Eg Þ hm
2
ð1Þ
Thus, the experimental band-gap values for pure and Ce3+ (0.1%, 0.5%, 1% and 1.5 mol%) doped SrS as extracted from the Tauc’s plots [14] (Fig. 2(b)) are found to be 4.20 eV, 4.22 eV, 4.27 eV, 4.30 eV and 4.39 eV, respectively. The optical band gap values of the films increase with increasing Ce3+ concentration. As compared to pure SrS, the increased optical band gap value of different SrS: Ce thin films can be explained by the Burstein–Moss effect, which is about transition energy in degenerate semiconductors due to the partially filled conduction band. According to Burstein-Moss effect, the apparent band-gap of a semiconductor is increased as the absorption edge is pushed to higher energies as a result fall state close to the conduction band being populated [15]. CeSr-VSr defect complexes are believed to form as a consequence of selfcompensation [16] as explained in the following lines. Consider
Fig. 1. FESEM images of (a) pure SrS (b-e) SrS: Ce3+ (0.1, 0.5, 1.0, 1.5 mol%) thin films.
S. Mishra et al. / Materials Letters 198 (2017) 101–105
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Fig. 2. (a) UV–vis diffuse reflectance spectra of pure SrS and SrS: Ce3+ (0.1, 0.5, 1.0 and 1.5 mol%) thin films (b) Plot of (aht)2 against energy band gap (hm).
an ideal situation that SrS is initially intrinsic, which means that its Fermi level is positioned at the middle of the bandgap with the phosphor having a negligible concentration of defects and impurities. Now assume that the phosphor is doped with an increasing concentration of Ce3+. This results in a modulation of the Fermi level towards the conduction band. Selfcompensation is envisaged to arise when the Fermi level position reaches a critical energy at which it is more energetically favorable to create an intrinsic self-compensating acceptor defect than to continue to modulate the Fermi level towards the conduction band [17]. 3.3. Electroluminescence spectra A model of MISIM structure of ACTFEL device is shown in Fig. 3 (a). The voltage dependence (50–100 V) of electroluminescence (EL) intensity of ACTFEL devices fabricated with different thicknesses (300 nm, 500 nm and 700 nm) of SrS: Ce3+ (0.5 mol%) phosphor are shown in Fig. 3(b–d). When excited with 1 kHz of frequency and above threshold voltage of 50 V, the EL spectra corresponding to the entire devices exhibit green and blue emission peaks at 442 nm and 536 nm, respectively in the wavelength range of 200–700 nm. The observed asymmetric spectra show a significant variation in intensity with active phosphor layer thickness. The EL intensity for three ACTFEL devices increases with increase in voltage from 50 V to 100 V while the peak positions remain unchanged respectively at 442 nm and 536 nm. Using EL spectra, one can know whether dpoant ions (Ce3+) are uniform distributed in the host lattice or not. If there is a considerable shift in the peak wavelength one could easily predict that Ce3+ distribution in the host lattice is non-uniform. In the present investigation, there was not observed any shift in the peak emission wavelength, which indicated that the Ce3+ ions were uniformly distributed in the SrS host. The electron-phonon interaction plays an important role in determining both the electron and phonon properties of thin films [18] and the Jahn-Teller splitting based on the temperature independence of the peak positions was well explained by Jones et al. [19]. In an ACTFEL device, the injected electrons are accelerated under the influence of high electric fields (2 MV/cm) in the phosphor. An understanding of high field carrier transport in the phosphor layer and the physics of the different threshold processes such as band to band impact ionization and impact ionization of luminescence impurities is essential for ACTFEL device designing
and fabrication [20]. In our study, we have neglected phononassisted impact ionization processes as well as deep level ionization. These effects may be important at high fields. In this electric field regime, the average drift velocity of electrons saturate at 105 m/s, a value which is almost independent of temperature and nature of semiconductor materials. Scattering with impurities, which controls the mobility at low field, can no longer stabilize the electron drift in this saturation region. This can be done by electron-phonon (electron-lattice) interaction [4]. The energy exchange between electron and phonon is described by the electron-phonon interaction Hamiltonian, where the electron can emit or absorb one phonon at a time. Kytik et al. [21] reported the role of electron-phonon interaction and demonstrated that chemical bonds may be effectively operated by external polarized field with large power densities. However, to achieve the large value of the effect, it is necessary to have large values of electron–phonon interactions. It is necessary to emphasize that this effect may be observed in relatively disordered materials like glasses and is not usually observed in perfect crystalline systems due to a relatively low total energy. The luminance-voltage curves of ACTFEL devices fabricated with different thicknesses (300 nm, 500 nm and 700 nm) of SrS: Ce3+ (0.5 mol%) phosphor layer are presented in Fig. 3(e). The devices show variation in threshold voltage as thickness is varied which is expected as there is a direct correlation between threshold voltage and thickness of the active phosphor layer. The device with thinnest (300 nm) active phosphor layer has a minimum threshold voltage (50 V) and it is maximum (110 V) for the device with thickest (700 nm) active phosphor layer. The L-V curve is also dependent on the ambient temperature and the dopant concentration of phosphor layer. When the temperature is increased, the probability of de-excitation through non-radiative recombination is significantly increased, which leads to reduced light output. Temperature also may affect the threshold voltage in an L-V curve since interface emission and space charge generation mechanisms are dependent on it. The luminance increases with impurity/ dopant level for low concentrations [22]. Increase in the density of luminescent centers raises the probability of an electron undergoing a collision with a luminescent center. This trend holds as long as the impurity concentration is low enough (up to 0.5 mol %) because phosphor crystallinity is maintained and concentration quenching does not occur.
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Fig. 3. (a) The MISIM structure of SrS: Ce3+ multilayer ACTFEL device (b–d) The EL spectra of SrS: Ce3+ (0.5 mol%) ACTFEL devices at different active phosphor layer thicknesses and (e) The L-V curves of SrS: Ce3+ (0.5 mol%) ACTFEL device with different active phosphor layer thicknesses.
S. Mishra et al. / Materials Letters 198 (2017) 101–105
4. Conclusions Results show that EL intensity decreases with increase in phosphor layer thickness while EL intensity increases with increase in applied voltage for each device. The L-V curves suggest the necessity of high threshold voltage to observe luminance with increasing thickness of active phosphor layer.
References [1] A. Tanaka, S. Onari, T. Arai, Phys. Rev. B 47 (1993) 1237–1243. [2] Biljana Pejova, Ivan Grozdanov, Mat. Chem. Phys. 99 (2006) 39–49. [3] K. Ohmi, K. Ishitani, S. Tanaka, H. Kobayashi, Appl. Phys. Lett. 67 (1995) 944– 946. [4] Y.A. Ono, Electroluminescent Displays, World Scientific, Singapore, 1995. [5] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, 1999. [6] C. Li, Z. Zang, W. Chen, Z. Hu, X. Tang, W. Hu, K. Sun, X. Liu, W. Chen, Opt. Express 24 (2016) 15071–15078. [7] R.H. Mauch, J. SID 5 (1997) 173–178.
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