- Email: [email protected]
Thin film luminescence of ZnGa2O4:Mn deposited by PLD
Scripta Materialia 54 (2006) 237–240 www.actamat-journals.com
Thin film luminescence of ZnGa2O4:Mn deposited by PLD Imteyaz Ahmad Md. a,*, M. Kottaisamy b, N. Rama M.S. Ramachandra Rao b,c, S.S. Bhattacharya a
b,c
,
a
b
Materials Testing Facility, Materials Forming Laboratory, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Adyar, Chennai 600 036, Tamil Nadu, India Materials Science Research Centre, Indian Institute of Technology Madras, Adyar, Chennai 600 036, Tamil Nadu, India c Department of Physics, Indian Institute of Technology Madras, Adyar, Chennai 600 036, Tamil Nadu, India Received 13 July 2005; accepted 20 September 2005 Available online 13 October 2005
Abstract Manganese doped zinc gallate powder was synthesised using a citrate gel method and subsequently deposited as a thin film by pulsed laser deposition technique at different temperatures on quartz and glass substrates. The luminescence behaviour of the deposited film was studied and the results reported. Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Luminescence; Thin film; Laser deposition
1. Introduction Zinc gallate phosphor (with an appropriate dopant) has attracted enormous attention due to its potential application in field emission displays [1] and thin film electroluminescent devices [2]. These phosphors exhibit much better chemical stability in comparison to sulphide phosphors. Different dopants in zinc gallate cause emission at different wavelengths. Mn2+ doped ZnGa2O4 exhibits an intense green emission, Co3+ doped Zinc gallate shows a reddish orange emission [3], whereas, doping with Cr3+ results in a red emission [4]. Luminescence of ZnGa2O4:Mn has been studied by various researchers [5–7] and the emission peak of this material has been reported to be from 501 nm [5] to 506 nm [6,7]. In these phosphors activation of Mn2+ ions causes luminescence due to 4T1–6A1 transitions between 3 d electrons within the Mn2+ ion. It is assumed that Mn2+ ions replace Zn2+ ions in the host lattice as they have the same valency. However, during the doping of ZnGa2O4 phosphors, Mn2+ can get oxidized to Mn4+, which leads to *
Corresponding author. Tel.: +919840554150. E-mail address: [email protected] (I. Ahmad Md.).
the emission of faint orange–green light at 666 nm [8]. In order to prevent the oxidation of Mn2+, firing/sintering is generally done in a protective atmosphere of Ar [9] or in vacuum [7]. Initial sintering in air followed by reduction in H2 atmosphere has also been successfully used [8]. Solid-state reaction methods are commonly used for the preparation of such Mn doped zinc gallate oxide phosphors. However, these solid-state routes require fairly high processing temperatures (1200–1300 °C) for prolonged periods (up to 12 h). It is surprising to note that very little work has been done in the area of synthesising this material by low temperature routes, such as sol–gel, co-precipitation, etc. [9]. In this work, Mn2+ doped Zinc gallate powder was prepared by a citrate gel method. In addition to greater homogeneity and purity of the product, the citrate gel method allowed for the utilization of low processing temperature and greater morphological control. This method involved the formation of a mixed ion citrate that resulted in a three-dimensional network upon drying (gel). The subsequent pyrolysis of the gel yielded a homogeneous mixed oxide [10]. Using a pulsed laser deposition (PLD) route these phosphor materials were deposited in thin film form.
1359-6462/$ - see front matter Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.09.029
238
I. Ahmad Md. et al. / Scripta Materialia 54 (2006) 237–240
Although thin film phosphors have several advantages when compared to powders, such as higher lateral resolution, better thermal stability, reduced outgassing, and better adhesion to the solid surface, it is also essential that the substrate on which the film is deposited should possess good optical properties in terms of low absorption and high transmittance of the emitted light. There have been few studies involving epitaxial luminescent films [11]. In this study, optimization of the substrate temperature was also done during the PLD growth of ZnGa2O4:Mn phosphor. 2. Experimental
3. Results and discussion 3.1. ZnGa2O4:Mn powder preparation Thermogravimetric analysis of the citrate complex obtained after drying at 110 °C showed exothermic decomposition of the citrate complex at around 500 °C as shown in Fig. 1. The citrate complex on decomposition left a homogeneously mixed, partially reacted oxide mixture that on subsequent heating reacted to form the desired compound. As can be seen from the XRD pattern of the powder calcined at 600 °C, shown in Fig. 2(b), broad peaks indicated that ZnGa2O4 had begun to form even at a temperature as low as 600 °C; however, crystallization was not complete. This is clearly an advantage of the citrate gel method wherein crystallization is initiated at a temperature much lower than the case for other methods. No emission was observed in this sample because of incomplete crystallization. When the powder was ground and sintered at
Fig. 1. TGA analysis of the precursor dried at 110 °C showing citrate complex decomposition at around 400–500 °C.
o
(220)
400 200
(400)
reducing atmosphere.
(311)
(b) Sintered at 1050 C by
(222)
600
Intensity (a.u)
Ga2O3 (99.999% pure) was dissolved in a 3:1 mixture of HCl and HNO3. Following this step, ZnO (99.999% pure) and MnSO4 Æ H2O were added and dissolved under continuous stirring to obtain a clear solution. An appropriate amount of citric acid was added such that the molar ratio of metal ions to the citric acid was 1:1.2. The mixture was heated to 130 °C with stirring to obtain a yellowish-green semitransparent gel that was subsequently dried at a temperature of 180 °C. The obtained mass was heated to a temperature of 600 °C in air for 5 h. The resultant powder was fired in a carburising atmosphere at a temperature of 1050 °C and was characterised for its luminescence property. The powder was pelletized and sintered in air at 950 °C for 5 h. Thin films were deposited using a PLD unit (Nd:YAG laser, k = 350 nm, pulse width 19 ns) on quartz and glass substrates at different temperatures and at an oxygen partial pressure of 100 mTorr. These films were annealed at the deposition temperature for 30 min. The deposited films were characterised using various techniques, such as X-ray diffraction (XRD) and the photoluminescence (PL) fluorescence spectrophotometry was used to obtain photoluminescence (PL) spectra at room temperature.
0 100
(a) Gel calcined at 600 oC
in air
50
0 10
20
30 40 2θ (degrees)
50
60
Fig. 2. XRD of ZnGa2O4:Mn0.01 prepared by citrate gel method: (a) calcined at 600 °C in air, showing that ZnGa2O4:Mn0.01 forms even on calcining the citrate complex at a temperature as low as 600 °C; however, proper crystallization is attained only after heating it to 1050 °C and (b) in carburising atmosphere.
1050 °C in a carburising atmosphere, perfect crystallinity was observed (Fig. 2(a)). AUTOXÒ refinement of the XRD data gave a good fit and a lattice parameter of ˚ was obtained as reported earlier [7]. 8.316 A 4. Excitation and emission spectra of ZnGa2O4:Mn0.01 powder The excitation spectra of the powder are shown as Fig. 3. The peak excitation wavelength was seen at 283 nm (and lies in the UV range). The emission spectra of two powders of 1:1 and 1.1:1 molar ratios of ZnO/ Ga2O3 are shown in Fig. 4 (when excited by the wavelength of 283 nm). In both the cases, luminescence was obtained at 504 nm (green region). This green emission was attributed to d–d transition (4T1–6A1) in the Mn2+ ion at tetrahedral sites [12]. Sintering at higher temperatures led to ZnO loss (as indicated by the presence of excess Ga2O3 peak in the XRD pattern) and was compensated by an
I. Ahmad Md. et al. / Scripta Materialia 54 (2006) 237–240
excess molar ratio of ZnO. This was the reason that the 1.1:1 molar ratio of ZnO/Ga2O3 showed higher emission intensity compared to that of uncompensated 1:1 molar ratio of ZnO/Ga2O3 (Fig. 4).
1000
λ =283 nm
ZnGa2O4:Mn0.01
800 Intensity (a.u)
239
600
5. Thin film luminescence
400
The thickness of the film deposited by PLD was esti˚ using ellipsometry. The refractive index mated to be 1000 A of the ZnGa2O4:Mn0.01 film was 3.4. At substrate temperatures below 450 °C, the crystallinity of the deposited film was relatively poor. The XRD patterns of films deposited at temperatures of 450, 550 and 700 °C are shown in Fig. 5.
200 0 260
280
300 320 340 360 Wavelength (λ) (nm)
380
400
Fig. 3. Excitation spectra of ZnGa2O4:Mn0.01 powder prepared by citrate gel method showing maximum excitation at a wavelength of 283 nm.
Intensity (a.u)
λ excitation= 283 nm
475
500 525 Wavelength (λ) (nm)
o
350 C o 450 C o 550 C o 700 C
Intensity (a.u)
Emission Spectra of ZnGaO 4Mn0.01 Emission Spectra of Zn1.1GaO4Mn0.01
λ Emission= 503 nm
250
550
Fig. 4. Emission spectra of ZnGa2O4:Mn0.01 and Zn1.1Ga2O4:Mn0.01 powders shows the enhancement of luminescence intensity when extra Zn2+ ions are added (about 10%). This is because of the fact that Zn evaporates.
300
350 400 Wavelength (λ) (nm)
450
500
Fig. 6. Excitation spectra of ZnGa2O4:Mn0.01 thin films deposited and annealed at different temperatures. This shows the highest excitation intensity for the film deposited at a temperature of 450 °C.
3+
Ga at tetrahedral site
#
Deposited and annealed
o
350 C o 450 C o 550 C o 700 C
λ Excitation= 283nm
# (-213) Ga2O3
o
at 700 C
#
Deposited and annealed o
*
at 550 C
* (311) ZnGa2O4 # (-213) Ga2O3
Mn
Intensity (a.u)
Intensity (a.u)
(a)
2+
(b) Deposited and annealed
*
o
at 450 C
* (311) ZnGa2O4 (c)
10
20
30
40 50 2θ (degrees)
60
300
350
400 450 500 Wavelength (λ) (nm)
550
600
70
Fig. 5. XRD patterns of the films deposited at (a) 700 °C, (b) 550 °C and (c) 450 °C, showing that as the deposition temperature is increased, presence of Ga2 O# 3 phase is seen, which indicates loss of Zn from the ZnGa2 O4 matrix.
Fig. 7. Emission spectra of ZnGa2O4:Mn0.01 thin films deposited and annealed at different temperatures showing that maximum intensity of luminescence at a wavelength (503 nm) is achieved for the film deposited at 450 °C. On the other hand there is another peak observed at 360 nm for the film deposited at 700 °C which is due to Ga3+ going to the vacant Zn2+ sites.
240
I. Ahmad Md. et al. / Scripta Materialia 54 (2006) 237–240
Excitation Spectra o
(a)
(b)
o
450 C on quartz plate o 450 C on ITO coated glass plate
λ excitation = 283 nm
Intensity (a.u)
Intensity (a.u)
450 C on Quartz plate o 450 C on ITO coated glass plate
250
300
350
400
450
300
400
500
600
700
800
Wavelength (λ) (nm)
Wavelength (λ) (nm)
Fig. 8. (a) Excitation and (b) emission spectra of thin film ZnGa2O4:Mn0.01 deposited at 450 °C on different substrates, showing that for the same deposition temperature the film deposited on the glass substrate shows better luminescence compared to that of the quartz substrate.
It is seen that the film deposited at 450 °C showed the (3 1 1) peak of ZnGa2O4. On the other hand, at higher temperature, there was an excess of Ga2O3 present, which was indicative of the fact that ZnO evaporated progressively with the increase in temperature. Loss of ZnO affected the luminescence adversely while good crystallinity and a proper ZnO/Ga2O3 ratio of the compound was essential for luminescence in ZnGa2O4:Mn. Higher temperature (>500 °C) resulted in good crystallinity, but poor luminescence as a result of Zn evaporation from the lattice. The excitation and emission spectra of the deposited films are shown in Figs. 6 and 7. The peak excitation was seen at a wavelength of 283 nm in all the films deposited at different temperatures. Emission at 503 nm was highest in the case of the film deposited at 450 °C and poorest in the case of that deposited at 700 °C. On the other hand, the emission intensity at 365 nm (in UV range) became more pronounced with increase in deposition temperature and was maximum for the film deposited and annealed at 700 °C. It has been observed that when ZnGa2O4 was synthesised in an oxygen deficient atmosphere, loss of ZnO led to vacant Zn2+ sites and Ga3+ did not necessarily remain confined to the octahedral sites; some of Ga3+ went to the tetrahedral sites that led to fluorescence peaks at 360 nm [13]. In this study, the excess Ga3+ ions led to emission at 365 nm. In the case of quartz and indium tin oxide (ITO) coated glass substrate, there was no difference in the excitation and emission spectra (Fig. 8(a) and (b)). However, a slight reduction in intensity was observed in the film deposited on the quartz substrate. 6. Conclusions ZnGa2O4:Mn0.01 powder was successfully synthesised by a citrate gel method at temperatures lower than that
required by traditional solid-state reaction methods. Emission from the powder sample was obtained at 504 nm and the luminescence depended strongly on the Zn:Ga molar ratio present in the compound. The optimum substrate temperature for thin film deposition of ZnGa2O4:Mn by PLD was determined to be 450 °C (at an oxygen partial pressure of 100 mTorr) with an annealing time of 30 minutes. Loss of ZnO at temperature above 450 °C and poor crystallinity at temperature below this resulted in decreased luminescence emission. Under the same conditions of deposition, the luminescence intensity of film deposited on ITO coated glass substrate was better than that deposited on quartz plate. References [1] Shea LE, Datta Jr RK, Brown JJ. J Electrochem Soc 1994;141: 2198. [2] Minami T, Maeno T, Kuroi Y, Takata S. J Vacuum Sci Technol 1996;A14:1736. [3] Abritta T, Blak FH. J Luminescence 1991;48–49:558. [4] Yu CT, Pang Lin. J Appl Phys 1991;79:7191. [5] Tran TK, Park W, Tomm JW, Wagner BK, Jacobsen SM, Summers CJ, et al. J Appl Phys 1995;78:5691. [6] Hsu Kai-Hung, Chen Ko-Shao. Ceram Int 1999;25:339. [7] Kim JS, Park HL, Kim GC, Hwang YH. Solid State Commun 2003; 126:515. [8] Toshihoto O, Noriyuki S, Sakata Tadayoshi. Chem Phys Lett 1998; 298:395. [9] Kho Joong-con, Park Hee-Dong, Kim Dong-Pyo. Bull Korean Chem Soc 1999;20:1035. [10] Yu M, Lin J, Zhou YH, Wang SB. Mater Lett 2002;56:1007. [11] Yi SS, Kim IW, Bae JS, Moon BK, Kim SB, Jeong JH. J Crystal Growth 2003;247:213. [12] Hsiesh IJ, Chu KT, Yu CF, Feng MS. J Appl Phys 1994;76(9): 3735. [13] Jeong In-keun, Park Hong Lee, Mho Sun-il. Solid State Commun 1998;105:179.
Thin film luminescence of ZnGa2O4:Mn deposited by PLD Imteyaz Ahmad Md. a,*, M. Kottaisamy b, N. Rama M.S. Ramachandra Rao b,c, S.S. Bhattacharya a
b,c
,
a
b
Materials Testing Facility, Materials Forming Laboratory, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Adyar, Chennai 600 036, Tamil Nadu, India Materials Science Research Centre, Indian Institute of Technology Madras, Adyar, Chennai 600 036, Tamil Nadu, India c Department of Physics, Indian Institute of Technology Madras, Adyar, Chennai 600 036, Tamil Nadu, India Received 13 July 2005; accepted 20 September 2005 Available online 13 October 2005
Abstract Manganese doped zinc gallate powder was synthesised using a citrate gel method and subsequently deposited as a thin film by pulsed laser deposition technique at different temperatures on quartz and glass substrates. The luminescence behaviour of the deposited film was studied and the results reported. Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Luminescence; Thin film; Laser deposition
1. Introduction Zinc gallate phosphor (with an appropriate dopant) has attracted enormous attention due to its potential application in field emission displays [1] and thin film electroluminescent devices [2]. These phosphors exhibit much better chemical stability in comparison to sulphide phosphors. Different dopants in zinc gallate cause emission at different wavelengths. Mn2+ doped ZnGa2O4 exhibits an intense green emission, Co3+ doped Zinc gallate shows a reddish orange emission [3], whereas, doping with Cr3+ results in a red emission [4]. Luminescence of ZnGa2O4:Mn has been studied by various researchers [5–7] and the emission peak of this material has been reported to be from 501 nm [5] to 506 nm [6,7]. In these phosphors activation of Mn2+ ions causes luminescence due to 4T1–6A1 transitions between 3 d electrons within the Mn2+ ion. It is assumed that Mn2+ ions replace Zn2+ ions in the host lattice as they have the same valency. However, during the doping of ZnGa2O4 phosphors, Mn2+ can get oxidized to Mn4+, which leads to *
Corresponding author. Tel.: +919840554150. E-mail address: [email protected] (I. Ahmad Md.).
the emission of faint orange–green light at 666 nm [8]. In order to prevent the oxidation of Mn2+, firing/sintering is generally done in a protective atmosphere of Ar [9] or in vacuum [7]. Initial sintering in air followed by reduction in H2 atmosphere has also been successfully used [8]. Solid-state reaction methods are commonly used for the preparation of such Mn doped zinc gallate oxide phosphors. However, these solid-state routes require fairly high processing temperatures (1200–1300 °C) for prolonged periods (up to 12 h). It is surprising to note that very little work has been done in the area of synthesising this material by low temperature routes, such as sol–gel, co-precipitation, etc. [9]. In this work, Mn2+ doped Zinc gallate powder was prepared by a citrate gel method. In addition to greater homogeneity and purity of the product, the citrate gel method allowed for the utilization of low processing temperature and greater morphological control. This method involved the formation of a mixed ion citrate that resulted in a three-dimensional network upon drying (gel). The subsequent pyrolysis of the gel yielded a homogeneous mixed oxide [10]. Using a pulsed laser deposition (PLD) route these phosphor materials were deposited in thin film form.
1359-6462/$ - see front matter Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.09.029
238
I. Ahmad Md. et al. / Scripta Materialia 54 (2006) 237–240
Although thin film phosphors have several advantages when compared to powders, such as higher lateral resolution, better thermal stability, reduced outgassing, and better adhesion to the solid surface, it is also essential that the substrate on which the film is deposited should possess good optical properties in terms of low absorption and high transmittance of the emitted light. There have been few studies involving epitaxial luminescent films [11]. In this study, optimization of the substrate temperature was also done during the PLD growth of ZnGa2O4:Mn phosphor. 2. Experimental
3. Results and discussion 3.1. ZnGa2O4:Mn powder preparation Thermogravimetric analysis of the citrate complex obtained after drying at 110 °C showed exothermic decomposition of the citrate complex at around 500 °C as shown in Fig. 1. The citrate complex on decomposition left a homogeneously mixed, partially reacted oxide mixture that on subsequent heating reacted to form the desired compound. As can be seen from the XRD pattern of the powder calcined at 600 °C, shown in Fig. 2(b), broad peaks indicated that ZnGa2O4 had begun to form even at a temperature as low as 600 °C; however, crystallization was not complete. This is clearly an advantage of the citrate gel method wherein crystallization is initiated at a temperature much lower than the case for other methods. No emission was observed in this sample because of incomplete crystallization. When the powder was ground and sintered at
Fig. 1. TGA analysis of the precursor dried at 110 °C showing citrate complex decomposition at around 400–500 °C.
o
(220)
400 200
(400)
reducing atmosphere.
(311)
(b) Sintered at 1050 C by
(222)
600
Intensity (a.u)
Ga2O3 (99.999% pure) was dissolved in a 3:1 mixture of HCl and HNO3. Following this step, ZnO (99.999% pure) and MnSO4 Æ H2O were added and dissolved under continuous stirring to obtain a clear solution. An appropriate amount of citric acid was added such that the molar ratio of metal ions to the citric acid was 1:1.2. The mixture was heated to 130 °C with stirring to obtain a yellowish-green semitransparent gel that was subsequently dried at a temperature of 180 °C. The obtained mass was heated to a temperature of 600 °C in air for 5 h. The resultant powder was fired in a carburising atmosphere at a temperature of 1050 °C and was characterised for its luminescence property. The powder was pelletized and sintered in air at 950 °C for 5 h. Thin films were deposited using a PLD unit (Nd:YAG laser, k = 350 nm, pulse width 19 ns) on quartz and glass substrates at different temperatures and at an oxygen partial pressure of 100 mTorr. These films were annealed at the deposition temperature for 30 min. The deposited films were characterised using various techniques, such as X-ray diffraction (XRD) and the photoluminescence (PL) fluorescence spectrophotometry was used to obtain photoluminescence (PL) spectra at room temperature.
0 100
(a) Gel calcined at 600 oC
in air
50
0 10
20
30 40 2θ (degrees)
50
60
Fig. 2. XRD of ZnGa2O4:Mn0.01 prepared by citrate gel method: (a) calcined at 600 °C in air, showing that ZnGa2O4:Mn0.01 forms even on calcining the citrate complex at a temperature as low as 600 °C; however, proper crystallization is attained only after heating it to 1050 °C and (b) in carburising atmosphere.
1050 °C in a carburising atmosphere, perfect crystallinity was observed (Fig. 2(a)). AUTOXÒ refinement of the XRD data gave a good fit and a lattice parameter of ˚ was obtained as reported earlier [7]. 8.316 A 4. Excitation and emission spectra of ZnGa2O4:Mn0.01 powder The excitation spectra of the powder are shown as Fig. 3. The peak excitation wavelength was seen at 283 nm (and lies in the UV range). The emission spectra of two powders of 1:1 and 1.1:1 molar ratios of ZnO/ Ga2O3 are shown in Fig. 4 (when excited by the wavelength of 283 nm). In both the cases, luminescence was obtained at 504 nm (green region). This green emission was attributed to d–d transition (4T1–6A1) in the Mn2+ ion at tetrahedral sites [12]. Sintering at higher temperatures led to ZnO loss (as indicated by the presence of excess Ga2O3 peak in the XRD pattern) and was compensated by an
I. Ahmad Md. et al. / Scripta Materialia 54 (2006) 237–240
excess molar ratio of ZnO. This was the reason that the 1.1:1 molar ratio of ZnO/Ga2O3 showed higher emission intensity compared to that of uncompensated 1:1 molar ratio of ZnO/Ga2O3 (Fig. 4).
1000
λ =283 nm
ZnGa2O4:Mn0.01
800 Intensity (a.u)
239
600
5. Thin film luminescence
400
The thickness of the film deposited by PLD was esti˚ using ellipsometry. The refractive index mated to be 1000 A of the ZnGa2O4:Mn0.01 film was 3.4. At substrate temperatures below 450 °C, the crystallinity of the deposited film was relatively poor. The XRD patterns of films deposited at temperatures of 450, 550 and 700 °C are shown in Fig. 5.
200 0 260
280
300 320 340 360 Wavelength (λ) (nm)
380
400
Fig. 3. Excitation spectra of ZnGa2O4:Mn0.01 powder prepared by citrate gel method showing maximum excitation at a wavelength of 283 nm.
Intensity (a.u)
λ excitation= 283 nm
475
500 525 Wavelength (λ) (nm)
o
350 C o 450 C o 550 C o 700 C
Intensity (a.u)
Emission Spectra of ZnGaO 4Mn0.01 Emission Spectra of Zn1.1GaO4Mn0.01
λ Emission= 503 nm
250
550
Fig. 4. Emission spectra of ZnGa2O4:Mn0.01 and Zn1.1Ga2O4:Mn0.01 powders shows the enhancement of luminescence intensity when extra Zn2+ ions are added (about 10%). This is because of the fact that Zn evaporates.
300
350 400 Wavelength (λ) (nm)
450
500
Fig. 6. Excitation spectra of ZnGa2O4:Mn0.01 thin films deposited and annealed at different temperatures. This shows the highest excitation intensity for the film deposited at a temperature of 450 °C.
3+
Ga at tetrahedral site
#
Deposited and annealed
o
350 C o 450 C o 550 C o 700 C
λ Excitation= 283nm
# (-213) Ga2O3
o
at 700 C
#
Deposited and annealed o
*
at 550 C
* (311) ZnGa2O4 # (-213) Ga2O3
Mn
Intensity (a.u)
Intensity (a.u)
(a)
2+
(b) Deposited and annealed
*
o
at 450 C
* (311) ZnGa2O4 (c)
10
20
30
40 50 2θ (degrees)
60
300
350
400 450 500 Wavelength (λ) (nm)
550
600
70
Fig. 5. XRD patterns of the films deposited at (a) 700 °C, (b) 550 °C and (c) 450 °C, showing that as the deposition temperature is increased, presence of Ga2 O# 3 phase is seen, which indicates loss of Zn from the ZnGa2 O4 matrix.
Fig. 7. Emission spectra of ZnGa2O4:Mn0.01 thin films deposited and annealed at different temperatures showing that maximum intensity of luminescence at a wavelength (503 nm) is achieved for the film deposited at 450 °C. On the other hand there is another peak observed at 360 nm for the film deposited at 700 °C which is due to Ga3+ going to the vacant Zn2+ sites.
240
I. Ahmad Md. et al. / Scripta Materialia 54 (2006) 237–240
Excitation Spectra o
(a)
(b)
o
450 C on quartz plate o 450 C on ITO coated glass plate
λ excitation = 283 nm
Intensity (a.u)
Intensity (a.u)
450 C on Quartz plate o 450 C on ITO coated glass plate
250
300
350
400
450
300
400
500
600
700
800
Wavelength (λ) (nm)
Wavelength (λ) (nm)
Fig. 8. (a) Excitation and (b) emission spectra of thin film ZnGa2O4:Mn0.01 deposited at 450 °C on different substrates, showing that for the same deposition temperature the film deposited on the glass substrate shows better luminescence compared to that of the quartz substrate.
It is seen that the film deposited at 450 °C showed the (3 1 1) peak of ZnGa2O4. On the other hand, at higher temperature, there was an excess of Ga2O3 present, which was indicative of the fact that ZnO evaporated progressively with the increase in temperature. Loss of ZnO affected the luminescence adversely while good crystallinity and a proper ZnO/Ga2O3 ratio of the compound was essential for luminescence in ZnGa2O4:Mn. Higher temperature (>500 °C) resulted in good crystallinity, but poor luminescence as a result of Zn evaporation from the lattice. The excitation and emission spectra of the deposited films are shown in Figs. 6 and 7. The peak excitation was seen at a wavelength of 283 nm in all the films deposited at different temperatures. Emission at 503 nm was highest in the case of the film deposited at 450 °C and poorest in the case of that deposited at 700 °C. On the other hand, the emission intensity at 365 nm (in UV range) became more pronounced with increase in deposition temperature and was maximum for the film deposited and annealed at 700 °C. It has been observed that when ZnGa2O4 was synthesised in an oxygen deficient atmosphere, loss of ZnO led to vacant Zn2+ sites and Ga3+ did not necessarily remain confined to the octahedral sites; some of Ga3+ went to the tetrahedral sites that led to fluorescence peaks at 360 nm [13]. In this study, the excess Ga3+ ions led to emission at 365 nm. In the case of quartz and indium tin oxide (ITO) coated glass substrate, there was no difference in the excitation and emission spectra (Fig. 8(a) and (b)). However, a slight reduction in intensity was observed in the film deposited on the quartz substrate. 6. Conclusions ZnGa2O4:Mn0.01 powder was successfully synthesised by a citrate gel method at temperatures lower than that
required by traditional solid-state reaction methods. Emission from the powder sample was obtained at 504 nm and the luminescence depended strongly on the Zn:Ga molar ratio present in the compound. The optimum substrate temperature for thin film deposition of ZnGa2O4:Mn by PLD was determined to be 450 °C (at an oxygen partial pressure of 100 mTorr) with an annealing time of 30 minutes. Loss of ZnO at temperature above 450 °C and poor crystallinity at temperature below this resulted in decreased luminescence emission. Under the same conditions of deposition, the luminescence intensity of film deposited on ITO coated glass substrate was better than that deposited on quartz plate. References [1] Shea LE, Datta Jr RK, Brown JJ. J Electrochem Soc 1994;141: 2198. [2] Minami T, Maeno T, Kuroi Y, Takata S. J Vacuum Sci Technol 1996;A14:1736. [3] Abritta T, Blak FH. J Luminescence 1991;48–49:558. [4] Yu CT, Pang Lin. J Appl Phys 1991;79:7191. [5] Tran TK, Park W, Tomm JW, Wagner BK, Jacobsen SM, Summers CJ, et al. J Appl Phys 1995;78:5691. [6] Hsu Kai-Hung, Chen Ko-Shao. Ceram Int 1999;25:339. [7] Kim JS, Park HL, Kim GC, Hwang YH. Solid State Commun 2003; 126:515. [8] Toshihoto O, Noriyuki S, Sakata Tadayoshi. Chem Phys Lett 1998; 298:395. [9] Kho Joong-con, Park Hee-Dong, Kim Dong-Pyo. Bull Korean Chem Soc 1999;20:1035. [10] Yu M, Lin J, Zhou YH, Wang SB. Mater Lett 2002;56:1007. [11] Yi SS, Kim IW, Bae JS, Moon BK, Kim SB, Jeong JH. J Crystal Growth 2003;247:213. [12] Hsiesh IJ, Chu KT, Yu CF, Feng MS. J Appl Phys 1994;76(9): 3735. [13] Jeong In-keun, Park Hong Lee, Mho Sun-il. Solid State Commun 1998;105:179.