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Bright visible luminescence from highly textured, transparent Dy3+ doped RF sputtered zinc oxide films
Accepted Manuscript Bright visible luminescence from highly textured, transparent Dy sputtered zinc oxide films
3+
doped RF
R. Sreeja Sreedharan, R. Reshmi Krishnan, G. Sanal Kumar, V.S. Kavitha, S.R. Chalana, R. Jolly Bose, S. Suresh, R. Vinodkumar, S.K. Sudheer, V.P. Mahadevan Pillai PII:
S0925-8388(17)31983-7
DOI:
10.1016/j.jallcom.2017.06.010
Reference:
JALCOM 42078
To appear in:
Journal of Alloys and Compounds
Received Date: 1 March 2017 Revised Date:
19 May 2017
Accepted Date: 1 June 2017
Please cite this article as: R. Sreeja Sreedharan, R. Reshmi Krishnan, G. Sanal Kumar, V.S. Kavitha, S.R. Chalana, R. Jolly Bose, S. Suresh, R. Vinodkumar, S.K. Sudheer, V.P. Mahadevan Pillai, Bright 3+ visible luminescence from highly textured, transparent Dy doped RF sputtered zinc oxide films, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Bright visible luminescence from highly textured, transparent Dy3+ doped RF sputtered zinc oxide films. R. SreejaSreedharana, R.Reshmi Krishnana, G. Sanal Kumara, V.S. Kavithaa, S. R. Chalanaa,
Department of Optoelectronics, University of Kerala, Thiruvananthapuram-695581, Kerala,
India. b
Department of Physics,University College,Thiruvananthapuram,Kerala,India
*For correspondance:e-mail- [email protected]
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Ph.No:91-471-2308167
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a
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R.JollyBosea, , S.Suresha, R. Vinodkumar a,b, S .K .Sudheer a, V.P.MahadevanPillai a *
Abstract
Doping of rare earth elements such as Dy3+ into ZnO lattice can modify the luminescence properties. Dy3+ ions exhibit emissions in the visible region. The preparation of Dy3+ doped ZnO
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films with bright visible luminescence is of importance for luminescent device applications. In this study, highly textured, c-axis oriented, transparent, luminescent Dy3+ doped ZnO films are prepared using RF magnetron sputtering. The structural, morphological, optical and luminescent properties of the as-deposited films are investigated as a function of Dy3+ doping concentration.
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The structural analysis of the films carried out using X-ray diffraction and micro-Raman studies reveal the formation of hexagonal wurtzite ZnO phase in the films. All the films present smooth
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surface morphology consisting of dense distribution of grains with well-defined grain boundaries. The elemental analysis carried out using energy dispersive X-ray (EDX) spectra confirms the incorporation of Dy3+ ions in the ZnO lattice. The high transmittance of Dy3+doped ZnO films in the visible range with a sharp absorption edge shows good optical quality of the films. The visible luminescence observed ~ 580 nm in the Dy3+ doped ZnO films can be attributed to the 4 F9 2 → 6 H 13 2 transition of Dy3+ ions, suggesting its suitability of these films for luminescent device applications.
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1. Introduction ZnO being a semiconductor with wide-direct band gap (Eg=3.3 eV) [1] and large exciton binding energy (60 meV) [2], finds wide range of applications in the optoelectronic industry
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such as photoelectronic material in the blue-UV region [ 3-4], electrodes for solar cells [5], flat panel displays, energy efficient windows, smart windows [6], optical waveguides [7-8 ], light emitting diodes, solar cells, bioimaging [9-11] electroluminescent display devices, transparent random access memories [5] etc. Doping of semiconductor nanostructures with
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selected dopants is an effective method to tailor the physical and chemical properties of the host material [4]. ZnO has some advantages such as non-toxicity, low cost, easy fabrication, etc. over the commonly used host materials such as In2O3, SnO2, CdO, etc. [12]. Different
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dopants have different effects on the physical and chemical properties of the semiconductor host material [13]. Elements of group III, IV, V and VI are the commonly used dopants in ZnO [14]. Because of the long emission life time and narrow emission band width of luminescent emission, rare earth ions can act as photoactive centres when doped into a suitable host lattice [15-17]. Hence, the study on the luminescence properties of rare earth doped ZnO is of particular interest for the development of phosphor material with the three
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fundamental colours [18]. The 4f electron transitions at different energy levels in the rare earth ions make them luminescence centres for phosphor applications [19-20]. The 5s2 and 5p6 outer filled shells of the rare-earth ions shield the partially filled 4f shells from external fields [18, 21]. The 4f intrashell transitions from the partially filled inner shells can give rise
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to narrow emissions in the IR, visible and UV regions of the spectrum [22-24]. Since the f-f transitions of rare earth ions are parity forbidden, the direct excitation is usually insufficient
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to observe luminescence from these ions [25]. Hence, energy transfer from excited host lattice to rare earth ion can lead to characteristic luminescence from rare earth ions. The efficiency of rare earth emission depends on the dynamics of rare earth ion determined by the interaction of rare earth ion with the host lattice [18]. Doping of ZnO with rare earth ions is of key consideration for display applications. Among the rare earth elements, Eu3+ is well known for its red emission, Tb3+ for its green emission and Dy3+ for white light emission by suitably adjusting the yellow and blue emissions [12,18]. Dysprosium is one of the rare earth elements yielding bright visible luminescence. Its band gap energy is 4.9 eV and atomic weight is 162.5 amu. The electronic configuration of Dy3+ is [Xe]4f10 6s2 . Dy3+ ions usually
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exhibit three well-known red (660 nm), yellow (~580 nm) and blue (~480 nm) emissions which correspond to the
4
F9 2 → 6 H 11 2 ,
4
F9 2 → 6 H 13 2 and
4
F9 2 → 6 H 15 2 transitions respectively
[4,13] and hence Dy3+ ions are recognized as potential candidates in developing white LEDs.
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The reports on the properties of Dy:ZnO nanostructures are scanty. Wu et al. reported a weak visible emission ~575 nm from Dy-doped ZnO nanowires embedded in anodic alumina membranes fabricated by sol-gel template technique which they have attributed to the 4
F9 2 → 6 H 13 2
transition from the Dy3+ ion [4]. Yang et al. synthesized Dy-doped ZnO
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nanowires using high temperature and high pressure pulsed laser ablation [13] and reported the absence of characteristic emission lines of Dy3+ in the photoluminescence (PL) spectra.
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They suggested that the low concentration of Dy in the synthesized ZnO nanowires results in the lack of efficient energy transfer from ZnO host to Dy3+ ions [13]. Ajimsha et al. deposited Dy:ZnO thin films using high temperature buffer assisted PLD and observed room temperature luminescence ~575 nm [26]. Lo et al. also synthesized Dy:ZnO films by PLD and observed luminescence from zinc related defects. No emission line from Dy3+ 4f sub band was observed in the PL spectra [27].
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Dysprosium doped ZnO thin films are prepared on quartz substrate using RF sputtering technique. The as-deposited films are characterized using different techniques such as XRD, AFM, FESEM, spectroscopic ellipsometry, Raman, UV-Visible and photoluminescence (PL)
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spectroscopy. The analysis showed that the prepared films are highly crystalline, textured and transparent and they produce bright visible luminescence.
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2. Experimental
Zinc oxide films doped with dysprosium were prepared by sputtering technique:
details of the film preparation method have been previously reported [28-30]. The target used for sputtering was prepared from the ZnO and Dy2O3 powders (Aldrich – purity 99.99%) with required doping concentrations. The proportions of Dy2O3 used for doping in ZnO were 0, 0.5, 1, 3 & 5 wt%. The films were deposited for a deposition time of 45 min., on cleaned quartz substrates kept at a distance 5 cm from the target. The Dy3+ doped ZnO films thus prepared with doping concentrations 0, 0.5, 1, 3 & 5 wt% are designated as D0, D0.5, D1, D3 and D5 respectively.
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The structural properties of the undoped and Dy3+ doped films were analyzed using X-ray diffraction measurement using Brucker AXS D8 Advance X-ray diffractometer equipped with X-ray source KRISTAOFLXE 780, KF.4KE with λ ~ 1.5406 Aο in BraggBrentano geometry with step size of 0.0203º at a scan speed 2º min-1. For the vibrational
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spectroscopic analysis of the samples, Raman spectral measurements were performed with Labram-HR 800 micro-Raman spectrometer. The spectra were recorded with a laser radiation of wavelength 514.5 nm (argon ion laser). The surface morphological analysis of the prepared films are done using AFM and FESEM measurements AFM (Digital Instruments
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Nanoscope III) measurements were carried out in contact mode (Si3N4 100µ cantilever having force constant 0.58 N/m) and the data were analyzed using WSxM 5.0 Develop 6.4
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software. FESEM micrographs were recorded using Nano SEM-450 (FEI-Nova Model No.1027647) equipped with XFlash detector 6/10 (Bruker). Energy dispersive X-ray spectrometer (Quantax 200) attached to the FESEM instrument was used to carry out elemental analysis of the films. Thickness of the films were measured using three different techniques; lateral FESEM micrographs, spectroscopic ellipsometry (Horiba Jobin YvonMM16) and stylus profiler (Dektak 6M). The optical properties of the samples were probed
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using spectroscopic ellipsometry and uv-visible spectroscopy (JASCO V-550 UV-visible double beam spectrophotometer). The laser PL measurements were performed with Flourolog III modular spectroflourometer (Horiba Jobin Yvon, Japan). The spectra were
Koha).
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recorded with an excitation radiation of wavelength of 325 nm from a He-Cd laser (Kimmon-
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3. Results and Discussion 3.1 Structural analysis
Figure 1(a)-(e) shows the X-Ray diffractograms of the as-deposited pure and Dy3+ doped
ZnO films. All the films are highly textured along c-axis, perpendicular to the substrate surface exhibiting a sharp XRD peak corresponding to the (002) and a very weak peak corresponding to (103) directions of the hexagonal wurtzite ZnO phase (JCPDS 75-0576). The (002) plane has the lowest surface free energy in ZnO due to the minimum of internal stress [31] and this can be the reason for c-axis orientation of the nanocrystals leading to
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preferred growth along (002) plane. The absence of any peak corresponding to Dy2O3 within
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the detection limit indicates that Dy3+ ions are well-dissolved in the ZnO lattice.
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Figure 1. (a)-(e) XRD patterns of pure and Dy3+ doped ZnO films and (f) variation of FWHM and intensity of (002) diffraction peak with Dy3+doping
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The intensity and the full width at half maximum (FWHM) of the (002) peak is estimated for all the films (Table 1). The variation of the FWHM and intensity of the (002) diffraction peak as a function of Dy3+ doping concentration is shown in Figure 1(f). The intensity of the D0.5 film is almost same as that of the undoped film. All the other Dy3+ doped films exhibit intensity values
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higher than that of D0 film. Among the doped films, the D1 film shows the highest value for intensity of (002) peak, which is almost double that of the intensity of the undoped film. This film (D1) also shows least value for FWHM of (002) diffraction peak. These findings suggest the better crystalline quality of the D1 film over others. The variation of FWHM of the (002)
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diffraction peak with Dy3+ doping concentration is not regular. All the films present moderate values for FWHM of (002) diffraction peak ~ 0.25 - 0.35 degrees. Hence, it can be inferred that Dy3+ doping introduces enhancement in the crystalline quality of the films. The undoped film
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itself shows the preferred direction of crystal growth along <001>. Due to Dy3+ doping as well, the enhancement of XRD intensity is observed only for (002) plane. This suggest that Dy3+ doping also promotes preferred orientation of crystal growth along (002) plane. The intensity of the XRD peak is related to the electron density along that plane. Dysprosium being a bigger atom compared to zinc, its doping introduces more electrons. The enhancement of the XRD intensity
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along (002) plane indicates more electron dendity along <001> direction. Huang et al. also observed best crystallinity for moderate doping of Dy3+ in the ZnO lattice and a reduction in
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crystallinity at higher doping concentration[12].
Figure 2. (a) The shift in the (002) peak position and (b) Variation of average size of the crystallites and strain in the films as a function of Dy3+doping concentration.
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The position of the (002) peak shows a regular shift towards lower 2θ values compared to the undoped film (Figure 2(a)). This indicates that the films are under uniform stress [26, 32]. The ionic radius of Dy3+ ion (0.91Aº) is much larger than that of Zn2+ ion (0.74Aº) [12]. Hence, the substitution of Dy3+ ion into ZnO lattice can cause the expansion of the lattice leading to
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enhanced values for lattice parameter. The dhkl values are calculated from the experimental data (Table.1) using Bragg's law [33] and lattice constants are calculated using well-known equation for hexagonal crystal lattice [28]. It can be seen that the value of the lattice constant
c of the
Dy3+ doped ZnO films increases with increase in Dy3+ doping concentration (Table 1.). When
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dysprosium is doped into the ZnO lattice, the Dy3+ ions may either occupy the Zn2+ sites or occupy interstitial positions. According to Vegard’s rule [34], the substitutional incorporation of a cation having larger ionic radius than the host cation will lead to the expansion of the lattice.
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Hence, the lattice cell dimension is expected to increase linearly with increase in Dy3+ doping concentration. The systematic increase in the value of ‘c’ lattice constant with increase in Dy3+ doping concentration suggest the possibility of incorporation of Dy3+ ions as substituent at the Zn2+ sites, rather than at the interstitial positions[34]. The shift in the position of (002) peak towards lower diffraction angles and the progressive increase in the value of lattice constant ‘c’
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indicates the possibility of substitution of Dy3+ ions into Zn2+ sites [34]. This may create the deformation of the lattice producing strain in the films. The shift in the position of the (002) peak towards lower 2θ values and increase in the value of the c-lattice parameter with increase in Dy3+ doping concentration are also observed by Huang et al. in Dy doped ZnO nanocrystalline
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thin films prepared by PLD [12] and Ajimsha et al., in Dy : ZnO films deposited by buffer assisted PLD [26]. Zhang et al. also observed slight expansion of the lattice and increase in
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lattice parameter with Dy3+ doping in RF magnetron sputtered ZnO films [35]. In addition to the ionic size mismatch between Dy3+ and Zn2+ cations; the difference in thermal expansion coefficients of ZnO film and quartz substrate, lattice mismatch between ZnO film and substrate and the presence of defects in the film can also produce varying degrees of stress during deposition of the film [34]. The implantation of particles sputtered from the oxide target into the growing film also produces stress in the deposited film [36]. The thermal expansion coefficient (TEC) of ZnO ( 2.9 ×10 −6 K −1 ) is much larger than that of fused quartz ( 0.59 ×10−6 K −1 ) which can also produce a residual stress in the film during deposition process
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due to the rise in temperature while sputtering [34, 37-38]. The strain along the c-axis and the biaxial stress are calculated using the biaxial strain model [39]. The strain in the films is found to be increasing with increase in Dy3+ doping concentration (Figure 2(b)). The negative values for
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the calculated stress (Table 1) indicate that the films are under compressive stress [39, 33]. Table 1. Structural and morphological parameters of the as-deposited pure and Dy3+ doped ZnO films prepared by RF Sputtering technique. Size of
Lattice
Biaxial
nm
the
constant
Stress
crystallite
‘c’
( nm)
(nm)
RMS
surface
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d002
(GPa)
roughness (nm)
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Films
0.2618
31
5.237
-3.670
2.34
D0.5
0.2618
31
5.237
-3.672
1.44
D1
0.2625
33
5.250
-4.821
2.42
D3
0.2627
24
5.254
-5.193
2.21
D5
0.2628
30
5.256
-5.314
1.44
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D0
Scherer’s formula is used to determine the average crystallite’s size in the films [33] and the values obtained for average crystallite size are in the range 24-33 nm (Table 1), indicating
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their nanostructured nature. The variation of average size of the crystallites and strain in the films with increase in Dy3+ doping concentration is shown in Figure 2(B). The micro-Raman spectra of the undoped and Dy3+ doped ZnO films recorded in the
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spectral range 70 – 700 cm-1 are shown in Figure 3. Earlier studies show that the fundamental optical phonon modes in ZnO can be assigned to E 2 ( Low ) ~100 cm-1, E2 ( High ) ~438cm-1, -1 -1 -1 A1 (TO) ~ 381 cm , E1 (TO) ~ 412 cm and E1 ( LO) ~ 586 cm [29-30]. All the films show an
intense narrow Raman band around 100 cm-1 corresponding to the E2 (Low) mode and fairly narrow Raman band around 437 cm-1 corresponding to the E2 (High) mode of ZnO. These two non–polar optical phonon modes are the characteristic features of the wurtzite structure of ZnO [40]. The vibration of Zn sub-lattice causes the origin of E2 (Low) mode while the vibration of
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oxygen atoms results in the origin of E2 (High) mode [40]. The spectral assignments of the different Raman bands in the Raman spectra of the pure and Dy3+ doped ZnO films are given in Table 2. The appearance of sharp E2 (High) mode in the films indicates the preservation of typical
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wurtzite structure of ZnO and good crystallization in the films [41-42].
Figure 3. Micro Raman spectra of the pure & Dy3+ doped ZnO films with Dy3+ doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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The position and profile of the E2 (High) mode is affected by the residual stress, crystallization, crystal defects and structural disorder in the sample [43]. In the dysprosium doped films, the position of E2 (High) band shows a blue shift. Serrano et al. reported a strong dependence of the
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E2 (High) band on the isotopic composition of ZnO indicating that the shift in this mode can be
induced by the presence of intrinsic defects in the sample [44]. Huang et al. reported that Raman bands can reflect the distortion of the lattice, excursion of the components, crystal defects and phase transformation [45]. The stress induced in the crystal affect the E2 (High) frequency of the wurtzite crystal structure. A blue shift in the E2 (High) phonon frequency may be attributed to
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biaxial compressive stress where as a red shift in the E2 (High) phonon frequency may be attributed to tensile stress [45-46]. In the undoped film, E2 (High) mode is observed at 437 cm-1.
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With Dy3+ doping the E2 (High) mode shifts from 437 to 440 cm-1 which can be attributed to the presence of defects introduced due to doping and the compressive stress in the films as observed in the XRD analysis.
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Table 2. Raman spectral data and band assignments.
Raman spectral Data (cm-1)
Spectral Assignments
D0.5
D1
D3
D5
100 s
100 s
100m
100 m
118 w
E2 (Low)
275 w
275 w
278 m
277 w
276 m
B1 (Low) (activated)
330 w
-
342 sh
-
E2 (High) − E2 (Low)
437s
438s
439m
440m
E2 (High)
582w
583s
582s
582s
E1 (LO)
335 w
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437s
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D0
581w
Abrevations: s – strong; m-medium intense; w –weak; sh – shoulder
In the undoped film, the E1(LO) mode is observed with less intensity ~581 cm-1. An enhancement in the intensity of E1(LO) mode can be observed with increase in Dy3+ doping concentration. The origin of E1 (LO) mode is associated with the formation of defects such as
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oxygen vacancies, interstitial zinc or other defect states in the films [47]. This is an indication that the Dy3+ doping introduces defects in the ZnO lattice. The Raman band observed in the 330342 cm-1 region for the films may be assigned to the band due to E2 (High) − E2 (Low) mode [48]. The Raman spectra of all the films present a band ~ 275 cm-1 which is not observed in the Raman
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spectra of bulk ZnO (spectra not shown). Some authors attribute the Raman bands in the 250300 cm-1 region to the activation of silent modes [49-50]. Bundesmann et al. [51] and Du et al. [52] also reported additional mode at 277 cm-1 in doped ZnO films and the origin of this mode is attributed to the intrinsic host lattice defects [51-52]. An enhancement in the intensity of the
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that Dy3+ doping produces defects in the lattice.
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band ~ 275 cm-1 with increase in Dy3+ doping concentration also supports the earlier inference
3.2. Surface morphology and Compositional Analysis
Figure. 4 shows 3D AFM images of ZnO and Dy3+ doped ZnO films. All the films present dense distribution of grains with well- defined grain boundaries indicating their good crystalline quality. The rms surface roughness of the films is calculated using WSxM 5.0
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Develop 6.4 software and is in the range 1.44 - 2.42 nm which reveals a smooth surface
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morphology for these films (Table 1).
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Figure 4. The 3D AFM micrographs of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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FESEM micrographs of the Dy3+ doped ZnO films are shown in Figure. 5 and they also present dense distribution of grains with leaf like structures of more or less uniform size with
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well defined grain boundary.
Figure 5. The FESEM micrographs of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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The elemental analyses of the films are carried out using EDX spectra (Figure. 6). The EDX spectrum of the undoped film shows the presence of Zn and O elements in the appropriate ratio 1:1 supporting the formation of ZnO phase in this film. The EDX analysis also confirms the
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systematically with increase in Dy3+ doping concentration.
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incorporation of Dy3+ ions in the doped films. The atomic percentage of Dy is found to increase
Figure. 6 EDX spectra of undoped and Dy doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% showing the elemental analysis.
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The lateral FESEM micrographs of the films are shown in Figure. 7 and the thickness of the films are estimated from them. The thickness values are also estimated using stylus profiler and the values are given in the table (Table 3). Thickness of the films presents no systematic variation with Dy3+ doping concentration.
All the films, except
D0.5, exhibit thickness
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~ 435 nm. But the D0.5 film shows a slightly lower value of thickness ~410 nm. Hence, this reduction in thickness for D0.5 film cannot be linked with Dy3+ doping, rather it can be due to some unexpected variation in the preparation conditions such as argon flow rate during the
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sputtering process.
Figure 7. Lateral SEM micrographs of pure and Dy doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% .
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5.3.3. Optical Properties
The optical constants and thickness of the films are extracted by fitting the measured ellipsometric data to a three-layer model using Cauchy dispersion relation in such a way that
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the difference between the calculated spectra generated from the fitting model and measured spectra is minimum, using the Delta-Psi software [53-54] . Very good fits are obtained between the experimental data and the fitted spectra for the entire measured wavelength region for all the films characterized by the very low values of χ2, which is less than one. The thickness and
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optical constants are extracted from the best fitted data and are given in Table 3. The values of the thickness obtained for different films from the ellipsometric analysis is in close agreement
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with those obtained from stylus profilometry and lateral FESEM measurements. Table 3. Thickness and optical data of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%. Thickness (nm)
Film
Lateral
Stylus
metry
FESEM
profiler
Refractive
Average
Band
constant
index at
transmi-
gap
at λ=550
λ=550 nm
ttance
energy
(%)
(eV)
nm
D0
424
439
430
3.22
1.53
76
3.22
D0.5
410
405
410
3.22
1.53
82
3.22
D1
437
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Ellipso-
Dielectric
440
3.26
1.63
86
3.26
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443
D3
428
439
435
3.27
1.68
91
3.27
D5
427
438
425
3.28
1.83
91
3.28
The variation of refractive index and dielectric constant as a function of incident wavelength shows normal dispersion behavior (Figure 8(a) and (b) respectively). The D0 and D0.5 films exhibit similar values for refractive index. The D5 film shows the highest values for refractive index and dielectric constant (Table 3). The atomic mass and density of dysprosium
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are 162.5 amu and 8550 Kg/m3 respectively and that of zinc are respectively 65.38 amu and 7134 Kg/m3. The higher values of refractive index exhibited by the Dy3+ doped ZnO films can be attributed to the higher value of atomic mass and atomic density of Dy3+ ion compared to the
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Zn2+ ion [55].
Figure 8. The variation of (a) refractive indices and (b) real part of dielectric constants of the undoped and Dy3+ doped ZnO films (c) Transmittance spectra and (d) Reflectance spectra of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
The transmittance and reflectance spectra of the pure and Dy3+ doped ZnO films in the wavelength region 200-900 nm is shown in Figure 8 (c) and (d) respectively. Both the pure and Dy3+ doped ZnO films present very high value of transmittance in the wavelength region 400-
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900 nm (Table 3). A systematic enhancement in the transmittance with increase in Dy3+ doping concentration (Table 3) can be observed. The XRD analysis shows that Dy3+ doping enhances texturing of the films and improves the crystalline quality. The reflectance spectra (Figure 8.d) show low values of reflectance for doped films. The enhanced transmittance exhibited by the
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doped films can be attributed to the reduction in reflectance, enhanced texturing and crystalline quality of the films . The sharp fall of transmittance of the films in the UV region, is due to the fundamental absorption edge of ZnO around 380 nm [56]. The optical quality of the films can be understood from the high values of transmittance and sharp absorption edge shown by the films.
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The optical quality of the films may be influenced by different factors such as film thickness, structural defects, surface roughness etc. The XRD analysis indicates the highly textured and
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good crystalline nature of the films. AFM and SEM analyses present smooth surface with dense distribution of grains. This reduces the scattering effects resulting in the improved transmittance
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Figure 9. Tauc-plots of undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% and the variation of average transmittance and band gap with Dy3+ doping concentrations.
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Band gap energy values (Eg) of the direct band gap semiconductor can be obtained by fitting the relation
α (hν ) = A(hν − E g )
1 2
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(1)
where A is the band edge constant, α is the optical absorption coefficient, ν is the frequency of the incident light and h is the Planck’s constant [57-58]. The optical absorption coefficient (α) can be obtained from the transmittance data using the following equation, 1 1 t T
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α = ln
(2)
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where t is the thickness of the film and T is the transmittance.
The band gap energy of the films can be estimated from the extrapolation of the linear portion of the plot of α2 vs hν to zero absorption (Table 3). The plot of α2 vs hν for these films is shown in Figure 9 (a)-(e). The band gap energy shows a systematic increase with increase in Dy3+ doping concentration (Figure 9(f)). B-M effect can widen the band gap of doped semiconductors [59]. The increase in the carrier concentration and blocking of lowest states in
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the conduction band can be one of the reasons for the observed increase in Eg in the Dy3+ doped films in comparison with to the undoped one [60-61]. Crystallinity and strain in the films can also influence the band gap energy of the films. The XRD analysis shows that the incorporation of Dy3+ in ZnO lattice results in the expansion of the lattice. The expansion of lattice and the
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consequent enhancement in the tensile strain can also contribute to the increase in band gap with Dy3+ doping. Zhavo et al. observed a linear relationship of band gap energy with residual stress
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in the film [62]. Huang et al. also reported that the tensile strain along c-axis causes an increase in d002, shift of the peak position of (002) plane towards lower angles and increase in band gap in Dy3+ doped ZnO films [12].
The exponential dependence of absorption coefficient on photon energy near the band
edge is given by the equation [12]
α (λ ) = α 0 exp(hEν ) u
(3)
where Eu represents Urbach energy and α0 a constant. Figure 10 (a) shows the Urbach plots of the Dy3+ doped films for various doping concentrations. The values of Urbach energy can be
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estimated as the reciprocal gradient of Urbach plots in the linear portion [63]. The Urbach
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energy slightly decreases with increase in Dy3+ doping concentration as shown in Figure 10 (b).
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Figure 10 (a) Urbach plots of undoped and Dy3+ doped ZnO films and (b) variation of Urbach energy with Dy doping concentration.
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5.3.3.3 Photoluminescence Spectra
Figure 11. Photoluminescence spectra of (a) undoped and (b) Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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The photoluminescence spectra of the films were recorded using a laser radiation of wavelength 325nm from a He-Cd laser. The PL spectra were recorded in the wavelength region 350-600 nm. Even though the red emission from Dy3+ is expected ~ 660 nm, it will be masked by the double excitation peak (2λ) at 650 nm. Hence, we have not attempted for measuring the
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red emission. Figure 11 (a) shows the PL emission spectrum of the pure ZnO film (D0) exhibiting an intense near band edge (NBE) emission ~ 397 nm related to the
excitonic
transition in ZnO and feeble visible emissions in the blue and green regions. The visible emission in ZnO originates from the deep level (DL) defects in ZnO such as oxygen vacancies, zinc
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interstitials, antisite oxygen etc. [64-70]. In addition to a broad and weak NBE in the UV region, all the Dy3+ doped films exhibit an intense, narrow emission peak ~572 nm and a broad emission
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band ~ 435 nm (Figure 11 (b)). Similar observations were reported a few authors. Wu et al., also observed NBE emission and visible emission ~ 575 nm corresponding to 4f9/2 to 6H13/2 transition of Dy3+ ion [4].Characteristic and sharp emission lines of Dy3+ ions superimposed on the ZnO defect related emission band are also detected upon excitation above band gap by Liu et al., in the Dy3+ doped hexagonal wurtzite ZnO nanocrystals fabricated by Sol-gel method [25]. The FWHM of the NBE emission peak is found to be broadened with the increase in Dy3+
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concentration in Dy:ZnO films by Ajimsha et al., which they attributed to the random distribution of Dy impurity atoms which will lead to local potential fluctuation and tail states of band edges resulting in peak broadening [26].The peak positions of NBE in the doped films are at lower wavelength regions compared to the undoped film. The shifting of NBE emission
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towards higher energy regions with Dy3+ doping is an indication of the increase in the band gap energy of the doped films due to the B-M effect, improved crystalline quality, lattice expansion
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due to doping and strain in the films. It is found that the luminescence character of the host lattice changes as soon as the rare-earth ion (Dy3+) is introduced into the lattice. The visible emission in Dy3+ doped ZnO films may arise either from the 4f intrashell transitions in Dy3+ ion or from the deep level defects in ZnO lattice. The emission due to transition between 4f levels in the rare earth ions usually present sharp peaks compared to that originating from defect states. The yellow emission observed ~ 572 nm in the visible region present sharp emission peaks corresponding to the 4 F9 2 → 6 H 13 2 transition of Dy3+ ion. It is reported that the 4 F9 2 → 6 H 13 2 transition in Dy3+ ion is a forced electric dipole
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transition and it is allowed only in structures with low symmetries having no inversion centre [4]. The intensity of this luminescent peak is found to be increasing with increase in Dy3+ doping concentration up to 3 wt% of Dy3+ doping. There after this emission peak shows a reduction in the intensity. The concentration quenching due to cross relaxation in close Dy3+ - Dy3+ pairs may
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be a possible reason for reduction in intensity of visible luminescence at higher doping concentrations [26, 71-72]. Quenching of PL intensity due to cross relaxation may be expected in the rare earth doped films having higher dopant incorporation. The XRD, Raman and EDX spectral analysis reveals that the D5 film has larger incorporation of dysprosium. This suggests
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D5 film may be due to the cross-relaxation process.
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the possibility that the reduction in the intensity of the yellow emission observed ~ 572 nm in the
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Figure 12. Schematic representation of the possible energy transfer mechanism from ZnO host toDy3+ ions.
The appearance of characteristic rare-earth emission due to transitions arising from the 4f levels of the Dy3+ ions in the ZnO lattice indicates that efficient energy transfer takes place from the host ZnO lattice to Dy3+ ions. Figure 12 shows a schematic diagram explaining the possible energy transfer mechanism from ZnO host lattice to Dy3+ ions resulting in the visible emissions in the red, yellow and blue regions of the spectrum. Since the ionic radius (0.91Aº) and charge (3+) of the Dy3+ ion are different from that of the host cation (Zn2+ ion: radius0.74Aº and charge 2+), the substitution of Dy3+ ion into Zn2+ sites will create distortion of the lattice [12]. The charge compensation from the local defect sites can also results in the lattice deformation. The
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lattice deformation due to Dy3+ doping in ZnO is clearly reflected in the Raman spectra of these films. The systematic increase in the intensity of the defect related E1 (LO) mode and the activation of the mode observed ~275 cm-1 with Dy3+ doping concentration can be correlated to introduction of defects in the ZnO lattice due to Dy3+ doping. Hence, the origin of the broad
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visible emission observed ~ 430 nm in the Dy3+ doped films can be attributed to the deep level defect emission from ZnO lattice. The blue emission observed in ZnO films is generally attributed to transitions involving zinc interstitial defect states [73-75]. Peng et al., observed blue emission ~ 430 nm in the Cu doped RF sputtered ZnO films. The origin of this blue emission is
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assigned to zinc interstitial (Zni) and zinc vacancy (Vzn) level transitions [76]. Fang et al. also observed blue emission centered at 430 nm in ZnO films deposited by RF reactive sputtering
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technique. They attributed its origin to electron transition from the level of interstitial zinc to valance band [77]. Zhang et al., also reported blue emission ~2.78 eV and they attributed its origin to the electron transition from the shallow doner level of oxygen vacancies to the valance band and electron transition from shallow donor level of zinc interstitials to valance band. [78]. Zeng et al., observed blue emission ~440 nm in ZnO nanoparticles and suggested that it could be
Conclusion
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originating from transitions involving zinc interstitial defects states. [79]
Nanocrystalline ZnO films doped with dysprosium are prepared using RF magnetron sputtering. XRD analysis suggests the formation of highly textured c-oriented films with
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hexagonal wurtzite structure. It is found that moderate doping of dysprosium enhances the crystalline quality of the ZnO films. The formation of ZnO wurtzite phase in the films can be
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identified by the presence of prominent E2 (Low) and E2 (High) modes in the Raman spectra. AFM and SEM analyses reveal the formation of films with dense distribution of grains having welldefined grain boundaries and smooth surface morphology. All the films exhibit high transmittance in the visible range. The transmittance and band gap energy of the films is found to increase with Dy3+ doping. All the films present an intense yellow emission ~572 nm originating from the 4 F9 2 →6H132 transition of Dy3+ ions. The quenching of this luminescence at higher doping concentrations is attributed to the cross-relaxation process. The bright luminescence observed in the visible region suggests the suitability of these films for luminescent applications.
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ACCEPTED MANUSCRIPT Highlights Highly c-oriented transparent, luminescent Dy3+doped ZnO films are prepared. Structural, morphological, optical and luminescent properties are studied. Structural quality of the films is analysed using XRD and Raman spectra. Bright visible emission ~580 nm due to 4 F9 2 → 6 H 13 2 transition of Dy3+ ions is
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obtained.
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• • • •
3+
doped RF
R. Sreeja Sreedharan, R. Reshmi Krishnan, G. Sanal Kumar, V.S. Kavitha, S.R. Chalana, R. Jolly Bose, S. Suresh, R. Vinodkumar, S.K. Sudheer, V.P. Mahadevan Pillai PII:
S0925-8388(17)31983-7
DOI:
10.1016/j.jallcom.2017.06.010
Reference:
JALCOM 42078
To appear in:
Journal of Alloys and Compounds
Received Date: 1 March 2017 Revised Date:
19 May 2017
Accepted Date: 1 June 2017
Please cite this article as: R. Sreeja Sreedharan, R. Reshmi Krishnan, G. Sanal Kumar, V.S. Kavitha, S.R. Chalana, R. Jolly Bose, S. Suresh, R. Vinodkumar, S.K. Sudheer, V.P. Mahadevan Pillai, Bright 3+ visible luminescence from highly textured, transparent Dy doped RF sputtered zinc oxide films, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Bright visible luminescence from highly textured, transparent Dy3+ doped RF sputtered zinc oxide films. R. SreejaSreedharana, R.Reshmi Krishnana, G. Sanal Kumara, V.S. Kavithaa, S. R. Chalanaa,
Department of Optoelectronics, University of Kerala, Thiruvananthapuram-695581, Kerala,
India. b
Department of Physics,University College,Thiruvananthapuram,Kerala,India
*For correspondance:e-mail- [email protected]
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Ph.No:91-471-2308167
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a
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R.JollyBosea, , S.Suresha, R. Vinodkumar a,b, S .K .Sudheer a, V.P.MahadevanPillai a *
Abstract
Doping of rare earth elements such as Dy3+ into ZnO lattice can modify the luminescence properties. Dy3+ ions exhibit emissions in the visible region. The preparation of Dy3+ doped ZnO
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films with bright visible luminescence is of importance for luminescent device applications. In this study, highly textured, c-axis oriented, transparent, luminescent Dy3+ doped ZnO films are prepared using RF magnetron sputtering. The structural, morphological, optical and luminescent properties of the as-deposited films are investigated as a function of Dy3+ doping concentration.
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The structural analysis of the films carried out using X-ray diffraction and micro-Raman studies reveal the formation of hexagonal wurtzite ZnO phase in the films. All the films present smooth
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surface morphology consisting of dense distribution of grains with well-defined grain boundaries. The elemental analysis carried out using energy dispersive X-ray (EDX) spectra confirms the incorporation of Dy3+ ions in the ZnO lattice. The high transmittance of Dy3+doped ZnO films in the visible range with a sharp absorption edge shows good optical quality of the films. The visible luminescence observed ~ 580 nm in the Dy3+ doped ZnO films can be attributed to the 4 F9 2 → 6 H 13 2 transition of Dy3+ ions, suggesting its suitability of these films for luminescent device applications.
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1. Introduction ZnO being a semiconductor with wide-direct band gap (Eg=3.3 eV) [1] and large exciton binding energy (60 meV) [2], finds wide range of applications in the optoelectronic industry
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such as photoelectronic material in the blue-UV region [ 3-4], electrodes for solar cells [5], flat panel displays, energy efficient windows, smart windows [6], optical waveguides [7-8 ], light emitting diodes, solar cells, bioimaging [9-11] electroluminescent display devices, transparent random access memories [5] etc. Doping of semiconductor nanostructures with
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selected dopants is an effective method to tailor the physical and chemical properties of the host material [4]. ZnO has some advantages such as non-toxicity, low cost, easy fabrication, etc. over the commonly used host materials such as In2O3, SnO2, CdO, etc. [12]. Different
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dopants have different effects on the physical and chemical properties of the semiconductor host material [13]. Elements of group III, IV, V and VI are the commonly used dopants in ZnO [14]. Because of the long emission life time and narrow emission band width of luminescent emission, rare earth ions can act as photoactive centres when doped into a suitable host lattice [15-17]. Hence, the study on the luminescence properties of rare earth doped ZnO is of particular interest for the development of phosphor material with the three
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fundamental colours [18]. The 4f electron transitions at different energy levels in the rare earth ions make them luminescence centres for phosphor applications [19-20]. The 5s2 and 5p6 outer filled shells of the rare-earth ions shield the partially filled 4f shells from external fields [18, 21]. The 4f intrashell transitions from the partially filled inner shells can give rise
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to narrow emissions in the IR, visible and UV regions of the spectrum [22-24]. Since the f-f transitions of rare earth ions are parity forbidden, the direct excitation is usually insufficient
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to observe luminescence from these ions [25]. Hence, energy transfer from excited host lattice to rare earth ion can lead to characteristic luminescence from rare earth ions. The efficiency of rare earth emission depends on the dynamics of rare earth ion determined by the interaction of rare earth ion with the host lattice [18]. Doping of ZnO with rare earth ions is of key consideration for display applications. Among the rare earth elements, Eu3+ is well known for its red emission, Tb3+ for its green emission and Dy3+ for white light emission by suitably adjusting the yellow and blue emissions [12,18]. Dysprosium is one of the rare earth elements yielding bright visible luminescence. Its band gap energy is 4.9 eV and atomic weight is 162.5 amu. The electronic configuration of Dy3+ is [Xe]4f10 6s2 . Dy3+ ions usually
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exhibit three well-known red (660 nm), yellow (~580 nm) and blue (~480 nm) emissions which correspond to the
4
F9 2 → 6 H 11 2 ,
4
F9 2 → 6 H 13 2 and
4
F9 2 → 6 H 15 2 transitions respectively
[4,13] and hence Dy3+ ions are recognized as potential candidates in developing white LEDs.
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The reports on the properties of Dy:ZnO nanostructures are scanty. Wu et al. reported a weak visible emission ~575 nm from Dy-doped ZnO nanowires embedded in anodic alumina membranes fabricated by sol-gel template technique which they have attributed to the 4
F9 2 → 6 H 13 2
transition from the Dy3+ ion [4]. Yang et al. synthesized Dy-doped ZnO
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nanowires using high temperature and high pressure pulsed laser ablation [13] and reported the absence of characteristic emission lines of Dy3+ in the photoluminescence (PL) spectra.
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They suggested that the low concentration of Dy in the synthesized ZnO nanowires results in the lack of efficient energy transfer from ZnO host to Dy3+ ions [13]. Ajimsha et al. deposited Dy:ZnO thin films using high temperature buffer assisted PLD and observed room temperature luminescence ~575 nm [26]. Lo et al. also synthesized Dy:ZnO films by PLD and observed luminescence from zinc related defects. No emission line from Dy3+ 4f sub band was observed in the PL spectra [27].
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Dysprosium doped ZnO thin films are prepared on quartz substrate using RF sputtering technique. The as-deposited films are characterized using different techniques such as XRD, AFM, FESEM, spectroscopic ellipsometry, Raman, UV-Visible and photoluminescence (PL)
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spectroscopy. The analysis showed that the prepared films are highly crystalline, textured and transparent and they produce bright visible luminescence.
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2. Experimental
Zinc oxide films doped with dysprosium were prepared by sputtering technique:
details of the film preparation method have been previously reported [28-30]. The target used for sputtering was prepared from the ZnO and Dy2O3 powders (Aldrich – purity 99.99%) with required doping concentrations. The proportions of Dy2O3 used for doping in ZnO were 0, 0.5, 1, 3 & 5 wt%. The films were deposited for a deposition time of 45 min., on cleaned quartz substrates kept at a distance 5 cm from the target. The Dy3+ doped ZnO films thus prepared with doping concentrations 0, 0.5, 1, 3 & 5 wt% are designated as D0, D0.5, D1, D3 and D5 respectively.
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The structural properties of the undoped and Dy3+ doped films were analyzed using X-ray diffraction measurement using Brucker AXS D8 Advance X-ray diffractometer equipped with X-ray source KRISTAOFLXE 780, KF.4KE with λ ~ 1.5406 Aο in BraggBrentano geometry with step size of 0.0203º at a scan speed 2º min-1. For the vibrational
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spectroscopic analysis of the samples, Raman spectral measurements were performed with Labram-HR 800 micro-Raman spectrometer. The spectra were recorded with a laser radiation of wavelength 514.5 nm (argon ion laser). The surface morphological analysis of the prepared films are done using AFM and FESEM measurements AFM (Digital Instruments
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Nanoscope III) measurements were carried out in contact mode (Si3N4 100µ cantilever having force constant 0.58 N/m) and the data were analyzed using WSxM 5.0 Develop 6.4
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software. FESEM micrographs were recorded using Nano SEM-450 (FEI-Nova Model No.1027647) equipped with XFlash detector 6/10 (Bruker). Energy dispersive X-ray spectrometer (Quantax 200) attached to the FESEM instrument was used to carry out elemental analysis of the films. Thickness of the films were measured using three different techniques; lateral FESEM micrographs, spectroscopic ellipsometry (Horiba Jobin YvonMM16) and stylus profiler (Dektak 6M). The optical properties of the samples were probed
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using spectroscopic ellipsometry and uv-visible spectroscopy (JASCO V-550 UV-visible double beam spectrophotometer). The laser PL measurements were performed with Flourolog III modular spectroflourometer (Horiba Jobin Yvon, Japan). The spectra were
Koha).
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recorded with an excitation radiation of wavelength of 325 nm from a He-Cd laser (Kimmon-
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3. Results and Discussion 3.1 Structural analysis
Figure 1(a)-(e) shows the X-Ray diffractograms of the as-deposited pure and Dy3+ doped
ZnO films. All the films are highly textured along c-axis, perpendicular to the substrate surface exhibiting a sharp XRD peak corresponding to the (002) and a very weak peak corresponding to (103) directions of the hexagonal wurtzite ZnO phase (JCPDS 75-0576). The (002) plane has the lowest surface free energy in ZnO due to the minimum of internal stress [31] and this can be the reason for c-axis orientation of the nanocrystals leading to
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preferred growth along (002) plane. The absence of any peak corresponding to Dy2O3 within
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the detection limit indicates that Dy3+ ions are well-dissolved in the ZnO lattice.
.
Figure 1. (a)-(e) XRD patterns of pure and Dy3+ doped ZnO films and (f) variation of FWHM and intensity of (002) diffraction peak with Dy3+doping
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The intensity and the full width at half maximum (FWHM) of the (002) peak is estimated for all the films (Table 1). The variation of the FWHM and intensity of the (002) diffraction peak as a function of Dy3+ doping concentration is shown in Figure 1(f). The intensity of the D0.5 film is almost same as that of the undoped film. All the other Dy3+ doped films exhibit intensity values
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higher than that of D0 film. Among the doped films, the D1 film shows the highest value for intensity of (002) peak, which is almost double that of the intensity of the undoped film. This film (D1) also shows least value for FWHM of (002) diffraction peak. These findings suggest the better crystalline quality of the D1 film over others. The variation of FWHM of the (002)
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diffraction peak with Dy3+ doping concentration is not regular. All the films present moderate values for FWHM of (002) diffraction peak ~ 0.25 - 0.35 degrees. Hence, it can be inferred that Dy3+ doping introduces enhancement in the crystalline quality of the films. The undoped film
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itself shows the preferred direction of crystal growth along <001>. Due to Dy3+ doping as well, the enhancement of XRD intensity is observed only for (002) plane. This suggest that Dy3+ doping also promotes preferred orientation of crystal growth along (002) plane. The intensity of the XRD peak is related to the electron density along that plane. Dysprosium being a bigger atom compared to zinc, its doping introduces more electrons. The enhancement of the XRD intensity
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along (002) plane indicates more electron dendity along <001> direction. Huang et al. also observed best crystallinity for moderate doping of Dy3+ in the ZnO lattice and a reduction in
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crystallinity at higher doping concentration[12].
Figure 2. (a) The shift in the (002) peak position and (b) Variation of average size of the crystallites and strain in the films as a function of Dy3+doping concentration.
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The position of the (002) peak shows a regular shift towards lower 2θ values compared to the undoped film (Figure 2(a)). This indicates that the films are under uniform stress [26, 32]. The ionic radius of Dy3+ ion (0.91Aº) is much larger than that of Zn2+ ion (0.74Aº) [12]. Hence, the substitution of Dy3+ ion into ZnO lattice can cause the expansion of the lattice leading to
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enhanced values for lattice parameter. The dhkl values are calculated from the experimental data (Table.1) using Bragg's law [33] and lattice constants are calculated using well-known equation for hexagonal crystal lattice [28]. It can be seen that the value of the lattice constant
c of the
Dy3+ doped ZnO films increases with increase in Dy3+ doping concentration (Table 1.). When
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dysprosium is doped into the ZnO lattice, the Dy3+ ions may either occupy the Zn2+ sites or occupy interstitial positions. According to Vegard’s rule [34], the substitutional incorporation of a cation having larger ionic radius than the host cation will lead to the expansion of the lattice.
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Hence, the lattice cell dimension is expected to increase linearly with increase in Dy3+ doping concentration. The systematic increase in the value of ‘c’ lattice constant with increase in Dy3+ doping concentration suggest the possibility of incorporation of Dy3+ ions as substituent at the Zn2+ sites, rather than at the interstitial positions[34]. The shift in the position of (002) peak towards lower diffraction angles and the progressive increase in the value of lattice constant ‘c’
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indicates the possibility of substitution of Dy3+ ions into Zn2+ sites [34]. This may create the deformation of the lattice producing strain in the films. The shift in the position of the (002) peak towards lower 2θ values and increase in the value of the c-lattice parameter with increase in Dy3+ doping concentration are also observed by Huang et al. in Dy doped ZnO nanocrystalline
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thin films prepared by PLD [12] and Ajimsha et al., in Dy : ZnO films deposited by buffer assisted PLD [26]. Zhang et al. also observed slight expansion of the lattice and increase in
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lattice parameter with Dy3+ doping in RF magnetron sputtered ZnO films [35]. In addition to the ionic size mismatch between Dy3+ and Zn2+ cations; the difference in thermal expansion coefficients of ZnO film and quartz substrate, lattice mismatch between ZnO film and substrate and the presence of defects in the film can also produce varying degrees of stress during deposition of the film [34]. The implantation of particles sputtered from the oxide target into the growing film also produces stress in the deposited film [36]. The thermal expansion coefficient (TEC) of ZnO ( 2.9 ×10 −6 K −1 ) is much larger than that of fused quartz ( 0.59 ×10−6 K −1 ) which can also produce a residual stress in the film during deposition process
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due to the rise in temperature while sputtering [34, 37-38]. The strain along the c-axis and the biaxial stress are calculated using the biaxial strain model [39]. The strain in the films is found to be increasing with increase in Dy3+ doping concentration (Figure 2(b)). The negative values for
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the calculated stress (Table 1) indicate that the films are under compressive stress [39, 33]. Table 1. Structural and morphological parameters of the as-deposited pure and Dy3+ doped ZnO films prepared by RF Sputtering technique. Size of
Lattice
Biaxial
nm
the
constant
Stress
crystallite
‘c’
( nm)
(nm)
RMS
surface
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d002
(GPa)
roughness (nm)
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Films
0.2618
31
5.237
-3.670
2.34
D0.5
0.2618
31
5.237
-3.672
1.44
D1
0.2625
33
5.250
-4.821
2.42
D3
0.2627
24
5.254
-5.193
2.21
D5
0.2628
30
5.256
-5.314
1.44
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D0
Scherer’s formula is used to determine the average crystallite’s size in the films [33] and the values obtained for average crystallite size are in the range 24-33 nm (Table 1), indicating
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their nanostructured nature. The variation of average size of the crystallites and strain in the films with increase in Dy3+ doping concentration is shown in Figure 2(B). The micro-Raman spectra of the undoped and Dy3+ doped ZnO films recorded in the
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spectral range 70 – 700 cm-1 are shown in Figure 3. Earlier studies show that the fundamental optical phonon modes in ZnO can be assigned to E 2 ( Low ) ~100 cm-1, E2 ( High ) ~438cm-1, -1 -1 -1 A1 (TO) ~ 381 cm , E1 (TO) ~ 412 cm and E1 ( LO) ~ 586 cm [29-30]. All the films show an
intense narrow Raman band around 100 cm-1 corresponding to the E2 (Low) mode and fairly narrow Raman band around 437 cm-1 corresponding to the E2 (High) mode of ZnO. These two non–polar optical phonon modes are the characteristic features of the wurtzite structure of ZnO [40]. The vibration of Zn sub-lattice causes the origin of E2 (Low) mode while the vibration of
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oxygen atoms results in the origin of E2 (High) mode [40]. The spectral assignments of the different Raman bands in the Raman spectra of the pure and Dy3+ doped ZnO films are given in Table 2. The appearance of sharp E2 (High) mode in the films indicates the preservation of typical
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wurtzite structure of ZnO and good crystallization in the films [41-42].
Figure 3. Micro Raman spectra of the pure & Dy3+ doped ZnO films with Dy3+ doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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The position and profile of the E2 (High) mode is affected by the residual stress, crystallization, crystal defects and structural disorder in the sample [43]. In the dysprosium doped films, the position of E2 (High) band shows a blue shift. Serrano et al. reported a strong dependence of the
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E2 (High) band on the isotopic composition of ZnO indicating that the shift in this mode can be
induced by the presence of intrinsic defects in the sample [44]. Huang et al. reported that Raman bands can reflect the distortion of the lattice, excursion of the components, crystal defects and phase transformation [45]. The stress induced in the crystal affect the E2 (High) frequency of the wurtzite crystal structure. A blue shift in the E2 (High) phonon frequency may be attributed to
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biaxial compressive stress where as a red shift in the E2 (High) phonon frequency may be attributed to tensile stress [45-46]. In the undoped film, E2 (High) mode is observed at 437 cm-1.
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With Dy3+ doping the E2 (High) mode shifts from 437 to 440 cm-1 which can be attributed to the presence of defects introduced due to doping and the compressive stress in the films as observed in the XRD analysis.
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Table 2. Raman spectral data and band assignments.
Raman spectral Data (cm-1)
Spectral Assignments
D0.5
D1
D3
D5
100 s
100 s
100m
100 m
118 w
E2 (Low)
275 w
275 w
278 m
277 w
276 m
B1 (Low) (activated)
330 w
-
342 sh
-
E2 (High) − E2 (Low)
437s
438s
439m
440m
E2 (High)
582w
583s
582s
582s
E1 (LO)
335 w
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437s
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D0
581w
Abrevations: s – strong; m-medium intense; w –weak; sh – shoulder
In the undoped film, the E1(LO) mode is observed with less intensity ~581 cm-1. An enhancement in the intensity of E1(LO) mode can be observed with increase in Dy3+ doping concentration. The origin of E1 (LO) mode is associated with the formation of defects such as
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oxygen vacancies, interstitial zinc or other defect states in the films [47]. This is an indication that the Dy3+ doping introduces defects in the ZnO lattice. The Raman band observed in the 330342 cm-1 region for the films may be assigned to the band due to E2 (High) − E2 (Low) mode [48]. The Raman spectra of all the films present a band ~ 275 cm-1 which is not observed in the Raman
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spectra of bulk ZnO (spectra not shown). Some authors attribute the Raman bands in the 250300 cm-1 region to the activation of silent modes [49-50]. Bundesmann et al. [51] and Du et al. [52] also reported additional mode at 277 cm-1 in doped ZnO films and the origin of this mode is attributed to the intrinsic host lattice defects [51-52]. An enhancement in the intensity of the
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that Dy3+ doping produces defects in the lattice.
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band ~ 275 cm-1 with increase in Dy3+ doping concentration also supports the earlier inference
3.2. Surface morphology and Compositional Analysis
Figure. 4 shows 3D AFM images of ZnO and Dy3+ doped ZnO films. All the films present dense distribution of grains with well- defined grain boundaries indicating their good crystalline quality. The rms surface roughness of the films is calculated using WSxM 5.0
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Develop 6.4 software and is in the range 1.44 - 2.42 nm which reveals a smooth surface
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morphology for these films (Table 1).
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Figure 4. The 3D AFM micrographs of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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FESEM micrographs of the Dy3+ doped ZnO films are shown in Figure. 5 and they also present dense distribution of grains with leaf like structures of more or less uniform size with
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well defined grain boundary.
Figure 5. The FESEM micrographs of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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The elemental analyses of the films are carried out using EDX spectra (Figure. 6). The EDX spectrum of the undoped film shows the presence of Zn and O elements in the appropriate ratio 1:1 supporting the formation of ZnO phase in this film. The EDX analysis also confirms the
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systematically with increase in Dy3+ doping concentration.
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incorporation of Dy3+ ions in the doped films. The atomic percentage of Dy is found to increase
Figure. 6 EDX spectra of undoped and Dy doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% showing the elemental analysis.
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The lateral FESEM micrographs of the films are shown in Figure. 7 and the thickness of the films are estimated from them. The thickness values are also estimated using stylus profiler and the values are given in the table (Table 3). Thickness of the films presents no systematic variation with Dy3+ doping concentration.
All the films, except
D0.5, exhibit thickness
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~ 435 nm. But the D0.5 film shows a slightly lower value of thickness ~410 nm. Hence, this reduction in thickness for D0.5 film cannot be linked with Dy3+ doping, rather it can be due to some unexpected variation in the preparation conditions such as argon flow rate during the
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sputtering process.
Figure 7. Lateral SEM micrographs of pure and Dy doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% .
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5.3.3. Optical Properties
The optical constants and thickness of the films are extracted by fitting the measured ellipsometric data to a three-layer model using Cauchy dispersion relation in such a way that
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the difference between the calculated spectra generated from the fitting model and measured spectra is minimum, using the Delta-Psi software [53-54] . Very good fits are obtained between the experimental data and the fitted spectra for the entire measured wavelength region for all the films characterized by the very low values of χ2, which is less than one. The thickness and
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optical constants are extracted from the best fitted data and are given in Table 3. The values of the thickness obtained for different films from the ellipsometric analysis is in close agreement
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with those obtained from stylus profilometry and lateral FESEM measurements. Table 3. Thickness and optical data of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%. Thickness (nm)
Film
Lateral
Stylus
metry
FESEM
profiler
Refractive
Average
Band
constant
index at
transmi-
gap
at λ=550
λ=550 nm
ttance
energy
(%)
(eV)
nm
D0
424
439
430
3.22
1.53
76
3.22
D0.5
410
405
410
3.22
1.53
82
3.22
D1
437
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Dielectric
440
3.26
1.63
86
3.26
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443
D3
428
439
435
3.27
1.68
91
3.27
D5
427
438
425
3.28
1.83
91
3.28
The variation of refractive index and dielectric constant as a function of incident wavelength shows normal dispersion behavior (Figure 8(a) and (b) respectively). The D0 and D0.5 films exhibit similar values for refractive index. The D5 film shows the highest values for refractive index and dielectric constant (Table 3). The atomic mass and density of dysprosium
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are 162.5 amu and 8550 Kg/m3 respectively and that of zinc are respectively 65.38 amu and 7134 Kg/m3. The higher values of refractive index exhibited by the Dy3+ doped ZnO films can be attributed to the higher value of atomic mass and atomic density of Dy3+ ion compared to the
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Zn2+ ion [55].
Figure 8. The variation of (a) refractive indices and (b) real part of dielectric constants of the undoped and Dy3+ doped ZnO films (c) Transmittance spectra and (d) Reflectance spectra of the undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
The transmittance and reflectance spectra of the pure and Dy3+ doped ZnO films in the wavelength region 200-900 nm is shown in Figure 8 (c) and (d) respectively. Both the pure and Dy3+ doped ZnO films present very high value of transmittance in the wavelength region 400-
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900 nm (Table 3). A systematic enhancement in the transmittance with increase in Dy3+ doping concentration (Table 3) can be observed. The XRD analysis shows that Dy3+ doping enhances texturing of the films and improves the crystalline quality. The reflectance spectra (Figure 8.d) show low values of reflectance for doped films. The enhanced transmittance exhibited by the
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doped films can be attributed to the reduction in reflectance, enhanced texturing and crystalline quality of the films . The sharp fall of transmittance of the films in the UV region, is due to the fundamental absorption edge of ZnO around 380 nm [56]. The optical quality of the films can be understood from the high values of transmittance and sharp absorption edge shown by the films.
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The optical quality of the films may be influenced by different factors such as film thickness, structural defects, surface roughness etc. The XRD analysis indicates the highly textured and
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good crystalline nature of the films. AFM and SEM analyses present smooth surface with dense distribution of grains. This reduces the scattering effects resulting in the improved transmittance
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Figure 9. Tauc-plots of undoped and Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt% and the variation of average transmittance and band gap with Dy3+ doping concentrations.
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Band gap energy values (Eg) of the direct band gap semiconductor can be obtained by fitting the relation
α (hν ) = A(hν − E g )
1 2
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(1)
where A is the band edge constant, α is the optical absorption coefficient, ν is the frequency of the incident light and h is the Planck’s constant [57-58]. The optical absorption coefficient (α) can be obtained from the transmittance data using the following equation, 1 1 t T
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α = ln
(2)
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where t is the thickness of the film and T is the transmittance.
The band gap energy of the films can be estimated from the extrapolation of the linear portion of the plot of α2 vs hν to zero absorption (Table 3). The plot of α2 vs hν for these films is shown in Figure 9 (a)-(e). The band gap energy shows a systematic increase with increase in Dy3+ doping concentration (Figure 9(f)). B-M effect can widen the band gap of doped semiconductors [59]. The increase in the carrier concentration and blocking of lowest states in
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the conduction band can be one of the reasons for the observed increase in Eg in the Dy3+ doped films in comparison with to the undoped one [60-61]. Crystallinity and strain in the films can also influence the band gap energy of the films. The XRD analysis shows that the incorporation of Dy3+ in ZnO lattice results in the expansion of the lattice. The expansion of lattice and the
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consequent enhancement in the tensile strain can also contribute to the increase in band gap with Dy3+ doping. Zhavo et al. observed a linear relationship of band gap energy with residual stress
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in the film [62]. Huang et al. also reported that the tensile strain along c-axis causes an increase in d002, shift of the peak position of (002) plane towards lower angles and increase in band gap in Dy3+ doped ZnO films [12].
The exponential dependence of absorption coefficient on photon energy near the band
edge is given by the equation [12]
α (λ ) = α 0 exp(hEν ) u
(3)
where Eu represents Urbach energy and α0 a constant. Figure 10 (a) shows the Urbach plots of the Dy3+ doped films for various doping concentrations. The values of Urbach energy can be
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estimated as the reciprocal gradient of Urbach plots in the linear portion [63]. The Urbach
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energy slightly decreases with increase in Dy3+ doping concentration as shown in Figure 10 (b).
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Figure 10 (a) Urbach plots of undoped and Dy3+ doped ZnO films and (b) variation of Urbach energy with Dy doping concentration.
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5.3.3.3 Photoluminescence Spectra
Figure 11. Photoluminescence spectra of (a) undoped and (b) Dy3+ doped ZnO films with doping concentrations 0, 0.5, 1, 3 and 5 wt%.
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The photoluminescence spectra of the films were recorded using a laser radiation of wavelength 325nm from a He-Cd laser. The PL spectra were recorded in the wavelength region 350-600 nm. Even though the red emission from Dy3+ is expected ~ 660 nm, it will be masked by the double excitation peak (2λ) at 650 nm. Hence, we have not attempted for measuring the
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red emission. Figure 11 (a) shows the PL emission spectrum of the pure ZnO film (D0) exhibiting an intense near band edge (NBE) emission ~ 397 nm related to the
excitonic
transition in ZnO and feeble visible emissions in the blue and green regions. The visible emission in ZnO originates from the deep level (DL) defects in ZnO such as oxygen vacancies, zinc
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interstitials, antisite oxygen etc. [64-70]. In addition to a broad and weak NBE in the UV region, all the Dy3+ doped films exhibit an intense, narrow emission peak ~572 nm and a broad emission
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band ~ 435 nm (Figure 11 (b)). Similar observations were reported a few authors. Wu et al., also observed NBE emission and visible emission ~ 575 nm corresponding to 4f9/2 to 6H13/2 transition of Dy3+ ion [4].Characteristic and sharp emission lines of Dy3+ ions superimposed on the ZnO defect related emission band are also detected upon excitation above band gap by Liu et al., in the Dy3+ doped hexagonal wurtzite ZnO nanocrystals fabricated by Sol-gel method [25]. The FWHM of the NBE emission peak is found to be broadened with the increase in Dy3+
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concentration in Dy:ZnO films by Ajimsha et al., which they attributed to the random distribution of Dy impurity atoms which will lead to local potential fluctuation and tail states of band edges resulting in peak broadening [26].The peak positions of NBE in the doped films are at lower wavelength regions compared to the undoped film. The shifting of NBE emission
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towards higher energy regions with Dy3+ doping is an indication of the increase in the band gap energy of the doped films due to the B-M effect, improved crystalline quality, lattice expansion
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due to doping and strain in the films. It is found that the luminescence character of the host lattice changes as soon as the rare-earth ion (Dy3+) is introduced into the lattice. The visible emission in Dy3+ doped ZnO films may arise either from the 4f intrashell transitions in Dy3+ ion or from the deep level defects in ZnO lattice. The emission due to transition between 4f levels in the rare earth ions usually present sharp peaks compared to that originating from defect states. The yellow emission observed ~ 572 nm in the visible region present sharp emission peaks corresponding to the 4 F9 2 → 6 H 13 2 transition of Dy3+ ion. It is reported that the 4 F9 2 → 6 H 13 2 transition in Dy3+ ion is a forced electric dipole
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transition and it is allowed only in structures with low symmetries having no inversion centre [4]. The intensity of this luminescent peak is found to be increasing with increase in Dy3+ doping concentration up to 3 wt% of Dy3+ doping. There after this emission peak shows a reduction in the intensity. The concentration quenching due to cross relaxation in close Dy3+ - Dy3+ pairs may
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be a possible reason for reduction in intensity of visible luminescence at higher doping concentrations [26, 71-72]. Quenching of PL intensity due to cross relaxation may be expected in the rare earth doped films having higher dopant incorporation. The XRD, Raman and EDX spectral analysis reveals that the D5 film has larger incorporation of dysprosium. This suggests
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D5 film may be due to the cross-relaxation process.
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the possibility that the reduction in the intensity of the yellow emission observed ~ 572 nm in the
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Figure 12. Schematic representation of the possible energy transfer mechanism from ZnO host toDy3+ ions.
The appearance of characteristic rare-earth emission due to transitions arising from the 4f levels of the Dy3+ ions in the ZnO lattice indicates that efficient energy transfer takes place from the host ZnO lattice to Dy3+ ions. Figure 12 shows a schematic diagram explaining the possible energy transfer mechanism from ZnO host lattice to Dy3+ ions resulting in the visible emissions in the red, yellow and blue regions of the spectrum. Since the ionic radius (0.91Aº) and charge (3+) of the Dy3+ ion are different from that of the host cation (Zn2+ ion: radius0.74Aº and charge 2+), the substitution of Dy3+ ion into Zn2+ sites will create distortion of the lattice [12]. The charge compensation from the local defect sites can also results in the lattice deformation. The
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lattice deformation due to Dy3+ doping in ZnO is clearly reflected in the Raman spectra of these films. The systematic increase in the intensity of the defect related E1 (LO) mode and the activation of the mode observed ~275 cm-1 with Dy3+ doping concentration can be correlated to introduction of defects in the ZnO lattice due to Dy3+ doping. Hence, the origin of the broad
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visible emission observed ~ 430 nm in the Dy3+ doped films can be attributed to the deep level defect emission from ZnO lattice. The blue emission observed in ZnO films is generally attributed to transitions involving zinc interstitial defect states [73-75]. Peng et al., observed blue emission ~ 430 nm in the Cu doped RF sputtered ZnO films. The origin of this blue emission is
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assigned to zinc interstitial (Zni) and zinc vacancy (Vzn) level transitions [76]. Fang et al. also observed blue emission centered at 430 nm in ZnO films deposited by RF reactive sputtering
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technique. They attributed its origin to electron transition from the level of interstitial zinc to valance band [77]. Zhang et al., also reported blue emission ~2.78 eV and they attributed its origin to the electron transition from the shallow doner level of oxygen vacancies to the valance band and electron transition from shallow donor level of zinc interstitials to valance band. [78]. Zeng et al., observed blue emission ~440 nm in ZnO nanoparticles and suggested that it could be
Conclusion
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originating from transitions involving zinc interstitial defects states. [79]
Nanocrystalline ZnO films doped with dysprosium are prepared using RF magnetron sputtering. XRD analysis suggests the formation of highly textured c-oriented films with
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hexagonal wurtzite structure. It is found that moderate doping of dysprosium enhances the crystalline quality of the ZnO films. The formation of ZnO wurtzite phase in the films can be
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identified by the presence of prominent E2 (Low) and E2 (High) modes in the Raman spectra. AFM and SEM analyses reveal the formation of films with dense distribution of grains having welldefined grain boundaries and smooth surface morphology. All the films exhibit high transmittance in the visible range. The transmittance and band gap energy of the films is found to increase with Dy3+ doping. All the films present an intense yellow emission ~572 nm originating from the 4 F9 2 →6H132 transition of Dy3+ ions. The quenching of this luminescence at higher doping concentrations is attributed to the cross-relaxation process. The bright luminescence observed in the visible region suggests the suitability of these films for luminescent applications.
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ACCEPTED MANUSCRIPT Highlights Highly c-oriented transparent, luminescent Dy3+doped ZnO films are prepared. Structural, morphological, optical and luminescent properties are studied. Structural quality of the films is analysed using XRD and Raman spectra. Bright visible emission ~580 nm due to 4 F9 2 → 6 H 13 2 transition of Dy3+ ions is
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