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Optical investigation of the emission lines for Eu3+ and Tb3+ ions in the GaN powder host
ARTICLE IN PRESS
Journal of Luminescence 126 (2007) 219–224 www.elsevier.com/locate/jlumin
Optical investigation of the emission lines for Eu3+ and Tb3+ ions in the GaN powder host A. Podhorodeckia,, M. Nyka, J. Misiewicza, W. Strekb a
Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences P.O. Box 1410, 50-950 Wroclaw 2, Poland
b
Received 3 October 2005; received in revised form 20 April 2006; accepted 20 April 2006 Available online 22 August 2006
Abstract Gallium nitride (GaN) doped by Eu3+ and Tb3+ ions have been synthesized using powder technology. The emission and absorption spectra have been obtained by using photoluminesence technique and correlated with the local structural environments. The room temperature yellow emission from GaN grains as well as from Eu3+ and Tb3+ ions has been observed for nano- as well as for microGaN grains. Additionally, for GaN:1%Eu3 micrograins the blue emission from GaN nanocrystals has been observed. r 2006 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 78.66.Fd; 78.67.Bf; 78.60.b; 78.67.n Keywords: Photoluminescence; Rare-earth ions; Nanocrystals; Powders; GaN:Eu; GaN:Tb
1. Introduction Gallium nitride (GaN) has recently attracted extensive experimental and theoretical interest due to its physical properties, such as a wide and direct band gap (3.18 and 3.39 eV for cubic-zinc blende structure and hexagonalwurtzite structure symmetry, respectively), [1] low compressibility, and high thermal conductivity, which make them strong candidates for short-wavelength electroluminescent devices and light-temperature/high power diodes and transistors [2,3]. Recently, among different techniques commonly used to obtain GaN materials such as molecular beam epitaxy (MBE), [4,5] metaloorganic chemical vapor deposition (MOCVD) [6] or magnetron sputtering (MS) [7], methods based on powder technology became alternative, low-cost techniques which give possibility to incorporate dopands i.e, rare-earth (RE) ions , into the host material in an easy way. In this way, a combination of the electronic properties of GaN material with the unique optical properties of RE
ions, can be obtained. This combination can give red light sources based on the GaN materials, which is not possible by using typical techniques due to the difficulties associated with the synthesis processes. Thus such materials (GaN:RE) are interesting from the fundamental standpoint of structure– composition–properties of solids as well as from application perspectives. Especially, in case of the influence of the local environments on the optical properties of the incorporated RE ions there are a lot of open questions. Deeper understanding of this phenomenon is necessary for efficient applications in the optoelectronic devices. Thus, the influence of the local environment on the optical properties of Eu3+- and Tb3+-doped ions will be present. Also we will discuss more carefully a phenomena of the aggregation of nanometer grains of GaN:1%Eu3+, and its influence on the optical properties of the RE ions, to a micrometer conglomerate with the optical properties different from the optical properties of initial GaN:Eu3+ nanograins. 2. Experimental details
Corresponding author. Tel.: +48 713202358; fax: +48 713283696.
E-mail address: [email protected] (A. Podhorodecki). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.07.002
GaN nanocrystalline powder has been synthesized using a horizontal quartz reactor [8]. The portions of 2 0.5 g
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Ga2O3 (99.999%), 7.2 mg EuO2 (99.99%) and 7.6 mg terbium oxide (99.99%) were mixed and solubilized in hot concentrated nitric acid and evaporated to dryness. Next, the obtained powder was carefully dried in an oven by gradually increasing the temperature from 70 to 200 1C. Then, the powder, placed in an alumina crucible into quartz tube (24 mm ID) was calcined at 500 1C for 4 h in air flow (100 cm3/min) to convert Ga(NO3)3 into Ga2O3. The crushed powder sample was placed at room temperature into quartz tube in NH3 flow (120 cm3/min) and after purging (20 min) the sample was heated (10 1C/min) to the required temperature of 850 1C and then was held in it for 3.5 h. The NH3 used for nitridation (99.85 vol%) was additionally purified by passing over a zeolite trap. The samples were taken from the furnace after it was cooled to the room temperature under the ammonia flow. After sample preparation, cathodoluminescence (CL) was used as a post-growth treatment technique to obtained different structural (different grain sizes/defects densities) domains for the two samples (i.e. GaN:Eu3+ and GaN:Tb3+). The CL spectra were excited in vacuum (o106 Torr) in the column of a Tesla transmission electron microscope operating at 60 and 90 kV with current beam ranging from 10 to 120 mA. The powder was pressed into pellets at power press (80 kN) and then the luminescence measurements were performed. For photoluminescence (PL) measurements UV argon laser with excitation wavelength of 302 nm and output power around 20 mW has been used. The focused spot, used to scan different structural domains of investigated structures, had a diameter about 100 mm. Signals were has been detected by using Ocean Optics SR4000 spectrometer. 3. Results and discussion In our previous paper [8] it has been shown that investigated GaN powder contains aggregates of GaN crystallites of sizes of 20–100 nm with monodispersive size/ shape distribution and hexagonal wurtzite structure. From XRD patterns by using the Scherrer formula it has been determined that the average size of the nanocrystallite grain doped by lanthanide ions (Eu3+ and Tb3+) is about 20, 9, and 21 nm for GaN, GaN:1%Eu3+, and GaN:1%Tb3+, respectively. Free Eu3+ ion has six electrons in the 4f state. The spin–orbit (LS) coupling gives rise to the formation of 2S+1 LJ configurations, each of which is (2J+1)-fold degenerate. Intraconfigurational transitions between these levels are, in a Russel–Saunders sense, electric-dipole (ED) forbidden for the free ion (or, more strictly, in the presence of inversion symmetry) by the parity rule (for which DJ p 6, 020 forbidden, or DJ ¼ 2, 4, 6 if the initial state has J ¼ 0). They are also spin forbidden. Magnetic-dipole (MD) transitions are only spin forbidden. The spin selection rule is, however, partly lifted by spin–orbit mixing or different manifolds. In general, the parity selection rule may also be lifted by an external electric field of low
symmetry. These so-called forced ED transitions manifest themselves in a static field (like a low-symmetry crystal field) as well as in a dynamic field. The latter may for instance be due to vibrational excitations and then result in vibrational transitions. Moreover an external field causes Stark splitting of the (2J+1)-fold degenerate levels, the number of splitting lines being dependent on the symmetry of the field [10,11]. It has been shown previously [9,12], that the grains tend to aggregate to form bigger grains due to a post-growth treatment, e.g. a local heating caused by a focused electron beam, which was used in CL measurements. This phenomenon will reflect in the fact that signal from GaN micrograins will differ somehow from signal obtained from GaN nanograins. For these reason we scanned by UV laser spot all area of each sample after post growth processing, i.e. CL, to find differences in the optical properties between the domains which were heated by CL and domains which were not. During the scanning of the sample by the laser beam, three different natures of the emission signal have been found. PL signal from all of them (Fig. 1) exhibits the strongest emission peak corresponding to transition between the first excited state 5D0, and the 7F2 ground state of Eu3+ ion, which is a hypersensitive ED transition. In this case, according to the parity of selection rules, Eu3+ ion should occupy a site without inversion symmetry. The local symmetry of the RE3+ ions doped in GaN depends on the growth and annealing conditions present during sample preparation [13,14]. For RE GaN-doped materials usually RE3+ (Pr, Sm) ions substitute for Ga ions in catonic sites of C3v point group symmetry [15–17]. It has been reported that also Eu3+ ion occupies the Ga site in the wurtzite GaN host by substitution [18,19]. However, Monteiro et al. [17] found that lattice site location of lighter RE ions (i.e. Eu3+) ions imbedded in a hexagonal GaN, grown by MOCVD must be in site with lower symmetry than C3v. It was also reported that more than one local environment of Eu3+ ions may exist in the GaN:1%Eu3+ samples [20,21]. Wurtzite GaN has a trigonal C3v symmetry. Therefore, there is a lack of inversion symmetry, thereby relaxing the parity selection rule and leading to the strongest emission line at 613 nm of Eu3+ ion as is observed in Fig. 1 and it is in agreement with the suggested C3v local symmetry of Eu3+ ions. Other weak emission peaks correspond to the 4f–4f intrashell transitions between the first excited state 5 D0, and the 7F0,1,3,4 levels, i.e., prohibited ED and MD selection rules [21], are associated with 5D0-7F0 (at 580 nm), 5D0-7F3 (at 650 nm), 5D0-7F4 (at 700 nm) and MD transition between the lowest excited state 5D0 and the ground 7F1 (at 594 nm) in Eu3+ ion (for which DJ ¼ 0, 71, 020 forbidden) [22]. Luminescence from the higher excited states such as 5D1 is not observed, indicating a very efficient nonradiative (multiphonon) relaxation to the 5D0 level. However, for some GaN grains (Fig. 1c) besides Eu3+ ion-related transitions a broad yellow/red emission band
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7
(a)
5
D0 → 7 F J
n=3
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0.6
~19nm 610
620
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GaN:1%Eu3+ nano/micro grains
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F2
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0.3 J=0
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3+ F2 GaN:1%Eu nanograins n=3 7
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0.10 610
620
630
J=1
640 y-GaN
J=0
J=4
0.05 J=3
0.00 300
400
500 600 wavelength (nm)
700
Fig. 1. PL spectra for GaN:Eu3+ powder from different places of the sample.
(y-GaN) centered at 500 nm is observed, which in fact comprised of at least two overlapping emission peaks at 450 and 500 nm. The observed emission signal is associated with the recombination processes mostly through the surface states of GaN nanocrystals. According to other papers [23,24], nitrogen vacancies are a source of yellow luminescence in GaN and are most likely candidates for luminescence at longer wavelengths. Uedono et al. [4] suggest that the major species of defects in the RE-doped GaN films can be identified as vacancy clusters composed of two or more vacancies and these clusters are larger in case of Tb than Eu. These defects were introduced by replacing Ga with RE elements. Thus, it has been suggested that in this case, signal from nanocrystals is observed. In case of nanocrystals the surface/volume ratio is huge, thus surface processes should play a dominant role. For this reason, observed emission
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signal should be rather associated with the surface recombination processes. Also, the correlation between y-GaN-related PL intensity and PL intensity of Eu3+ transitions has been observed (Fig. 1c). For increasing of y-GaN PL intensity, the intensity of Eu3+-related emission peaks also increase, indicating that defect states play an important role in the excitation energy transfer processes and increasing its efficiency. There are also domains from which PL signal differ somehow from signal disscused above. Besides Eu3+-ion related transitions a new transition at 370 nm is observed and no emission related to GaN defects. This emission is associated with GaN band-to-band transition related to the crystal volume. The broadening of this band-to-band emission band is high (50 nm), indicating a significant concentration of defect states close to the conduction (valence) band minimum. The lower-energy part of this emission band is likely a result of the recombination of exciton band at impurity or defect sites. Other parameters that can affect the emission profile are the shape and size distribution of the grains. Also, from Fig. 1b it can be observed that there are also some domains of GaN grains in the sample from which the emission signal has properties observed in the two cases discussed above (i.e., nanograins, and micrograins). In this case the splitting of the emission lines related to Eu3+ ions is the strongest. This will be discussed in the next paragraph. This emission signal is associated with the situation when both kinds of grain coexist together or grains size is somewhere between nano- and micrometer. This will reflect both, bulk-like and surface-like optical properties of grains. Summarizing the discussion above, after aggregation of nanometer grains to micrometer conglomerates the surface/volume ratio decreased and hence the surface-related emission has decreased in favor of the bulk-like emission which is associated with band-to-band recombination. In addition, it has been observed that PL intensity for samples with micrometer grains is usually stronger than PL intensity for samples with nanometer grains. The factor of the enhancement of PL intensity changes from 2 to 10 depending on the postgrowth processing, suggesting more efficient excitation energy transfer in case of micrograins. Also existence of forbidden 7F2 transition confirms the possibility of C3v (or lower) symmetry site for Eu3+ ions, i.e. Ga substitution. Other differences observed from presented spectra are attributed (i) to different structural positions of Eu3+ ions (i.e., surface or volume positions, intersitials or substitutional positions) and can be observed as a different splitting of Eu3+-related emission peaks or (ii) to different chemical environments (i.e. local electric field intensities) observed as a differently shifted and splitted Eu3+-related emission peaks. Thus the number of peaks (optical transitions) is also dependent on the point symmetry of the RE ion site, that is, the higher the point symmetry, lesser number of
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peaks appear. Thus RE ions could be used as a fingerprint of different symmetry for ion sites in the system. To look more carefully on this phenomena, the main peak for each spectrum was deconvoluted with the Gaussian curves as seen in the insets of Fig. 1. The summed curve (thick line) of each deconvoluted curve is well fitted to the measured spectra (open circles). Thus, the number of Stark components identified for the 7F0,1,2,3,4 levels in the micrograins GaN:Eu3+ are: 3, 3, 3, 2, and at least 4 for the last one, respectively (inset of Fig. 1a). For the signal from domain concerning nano- as well as microGaN:1%Eu3+ grains (thin line) the number of Stark components identified for the 7F0,1,2,4 levels are 3, 4, 5 (degeneracy completely lifted), and at least 7 (quite complicated line shape), respectively. For transition to the 7F3 level, deconvolution was not possible due to the weak resolution. For nanoGaN grains, the number of Stark components identified for the 7F0,1,2,3,4 levels are 1, 3, 3, 3 and at least 6 (also quite complicated line shape), respectively (Fig. 1b). It means, that degeneracy, especially for nano/micrograins, is completely lifted, reflecting the fact that Eu3+ ions are in sites with lower symmetry than the suggested C3v. However, when permitted by the local symmetry, the 5 D0-7F0 transition is characterized by only one Stark component (both states are nondegenerate) since there are other transitions near this energy, indicating the presence of Eu3+ sites subject to distinctly different local field intensities [25,26]. Fig. 2 shows in detail the splitting of 5D0-7F0 transition for Eu3+ ions doped with GaN of different GaN grain sizes. Concerning this effect, it can be concluded that different chemical environments (local fields intensities) are responsible for a complex splitting of the emission lines rather than lower than C3v symmetry of the local Eu3+ ion environment.
D0 → 7F0
5
PL intensity
GaN micrograins
GaN nano/micrograins GaN nanograins
570
575
580 wavelength nm
585
590
Fig. 2. GaN:1%Eu3+-related 5D0-7F0 transition from different places of the sample.
It can be due to the fact that in case of ions being near the surface, due to the grain boundaries, rather asymmetric crystal field, with low symmetry (even for ions which are in the same position i.e., C3v), should split more the degenerate energy levels, and intensity for 5D0- 7F2 transition should increase due to more efficient the breaking of the forbidden selection rule for this transition. This will be even more efficient for micro- and nano-/ micrograins obtained in the structurally complicated agglomeration process. Thus, introducing the sites with lower symmetry than C3v for Eu3+ ions to explain this strong emission lines splitting is not needed. The ratio between the integrated intensity of the 5 D0-7F2 and 5D0-7F1 transitions, I02/I01, is used in lanthanide-based systems as a probe of changes on the nature of the local surroundings [25,27,28]. It is well known that the probability of the 5D0-7F2 transition is very sensitive to relatively small changes in the chemical surroundings of the Eu3+ ions [29]. The intensity of the ED 5D0-7F2 transition is significantly affected by the degree in the center of symmetry in the environment around Eu3+ ions. Conversely, the 5D0-7F1 emission around 592 nm is allowed by MD considerations, therefore being relatively insensitive to the local symmetry. This transition can thus be used as an internal standard in complex materials. When Eu3+ ions are situated at lowsymmetry sites, the ED transition has larger probability than the MD transition [28]. Therefore a greater I02/I01 ratio may correspond to more distorted (or asymmetric) local environment [27,28,30]. Base on these reasons, the observed decrease in the I02/I01 ratio for the nanograins (2.0), when compared to the values 4.0 and 4.5 for mixed sizes and micrograins, respectively, may thus be related to fact that intensity of the 5D0-7F2 transition increases with increasing distortion (on average) of the local-field symmetry around the Eu3+ cations, which confirms previous conclusions. Also, almost two times bigger broadening (19 and 18 nm) for the 5 D0-7F2 transition (Fig. 1a and b) suggest that rather different chemical surroundings (local field intensities) than lower-symmetry sites of Eu3+ ions are responsible for such a huge splitting in case of micrograins. The Tb3+ free ion possesses a 4f8 configuration, which gives rise to a 7F6 ground state and 7F5,4,3,2,1,0, 5D4, 5D3, 5 L10, 5G6, 5L9, 5G5, and 5D2 excited states. Lozykowski et al. [31] proposed that Tb3+ in GaN occupies relaxed substitutional Ga sites with trigonal C3v crystal symmetry. In C3v crystal symmetry, the states with J ¼ 0, 1, 2, 3, 4, 5, and 6, will split into 1(0), 1(1), 1(2), 3(2), 3(3), 3(4), and 5(4), single (doubly) degenerated crystal-field LSJ (Russell–Saunders) levels, respectively. On the other hand, Bang et al. [32] concluded that Tb3+ was incorporated into a substitutional Ga site with tetrahedral symmetry. Also Gruber et al. [16] proposed that Tb3+ ion resides in sites of D2 symmetry or lower in MOCVD-made GaN epilayers. Fig. 3 shows the PL spectra recorded at room temperature from different places on the GaN:1%Tb3+
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PL intensity (arb. u.)
5
GaN:1%Tb3+ λEXC = 302 nm nanograins
D4 → F5 7
~ 9nm
10
D4 → 7F6
5
D4 → 7F4
5
D4 → 7F3
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5
0 300
400
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associated with band-to-band recombination that was also observed in the PL spectra. In this case the emission related to RE ions is more efficient, suggesting more efficient excitation energy transfer through the defect states. In case of GaN:1%Eu3+ nanograins rather high local symmetry like C3v for Eu3+ ion (i.e. Ga substitution) site and only one dominating chemical environment is suspected. Similar situation can be observed for Tb3+doped GaN nanograins. For GaN:1%Eu3+ micrograins, more than one chemical environment for Eu3+ ions is suspected with local symmetry like C3v or lower, due to complicated agglomeration process. All these results mean, that GaN powder is a very suitable host for the RE elements.
800
Fig. 3. PL spectra for GaN:1%Tb3+ powder from different places of the sample.
sample (as a consequence of different heat treatment during CL). As can be observed, for all selected spectra, obtained results look similar. No emission band related to GaN band-to-band transition has been observed and only a broad yellow GaN defects-related emission band, which was discussed in previous sections, is observed. The presented spectra differ only slightly in the intensity of Tb3+-related emission observed clearly in all cases. Peaks at 620, 580, 550, and 480 nm are associated with the intrashell transitions between the first excited state 5D4, and the 7F3,4,5,6 levels of the ground multiplet, respectively. In these case no Stark splitting (or very weak at room temperature) has been observed, suggesting only one Tb3+ position, with rather high local symmetry, like C3v sites for Tb3+ ions. Also, a small broadening (9 nm) for the main peak confirms this expectation. All these results suggest that in case of heat treatment no micrograins have been obtained in case of Tb3+-doped GaN grains and only nanograins are present. It can be due to the fact that GaN:1%Tb3+ grains are two times larger than GaN:1%Eu3+ grains, and they need more energy to begin the agglomeration process. 4. Conclusion GaN powder doped by Eu3+ and Tb3+ ions has been synthesized and investigated optically. The origin of observed optical transitions has been explained. Also a phenomena of GaN:1%Eu3+ grain agglomeration due to the postgrowth treatment has been shown and investigated optically. For GaN:1%Tb3+ grains for similar postgrowth treatment condition no such a phenomena was observed. After the aggregation of the nanometer grains to the micrometer conglomerates the surface/volume ratio decreased and hence the surface-related emission has decreased in favor of the bulk-like emission which is
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Journal of Luminescence 126 (2007) 219–224 www.elsevier.com/locate/jlumin
Optical investigation of the emission lines for Eu3+ and Tb3+ ions in the GaN powder host A. Podhorodeckia,, M. Nyka, J. Misiewicza, W. Strekb a
Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences P.O. Box 1410, 50-950 Wroclaw 2, Poland
b
Received 3 October 2005; received in revised form 20 April 2006; accepted 20 April 2006 Available online 22 August 2006
Abstract Gallium nitride (GaN) doped by Eu3+ and Tb3+ ions have been synthesized using powder technology. The emission and absorption spectra have been obtained by using photoluminesence technique and correlated with the local structural environments. The room temperature yellow emission from GaN grains as well as from Eu3+ and Tb3+ ions has been observed for nano- as well as for microGaN grains. Additionally, for GaN:1%Eu3 micrograins the blue emission from GaN nanocrystals has been observed. r 2006 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 78.66.Fd; 78.67.Bf; 78.60.b; 78.67.n Keywords: Photoluminescence; Rare-earth ions; Nanocrystals; Powders; GaN:Eu; GaN:Tb
1. Introduction Gallium nitride (GaN) has recently attracted extensive experimental and theoretical interest due to its physical properties, such as a wide and direct band gap (3.18 and 3.39 eV for cubic-zinc blende structure and hexagonalwurtzite structure symmetry, respectively), [1] low compressibility, and high thermal conductivity, which make them strong candidates for short-wavelength electroluminescent devices and light-temperature/high power diodes and transistors [2,3]. Recently, among different techniques commonly used to obtain GaN materials such as molecular beam epitaxy (MBE), [4,5] metaloorganic chemical vapor deposition (MOCVD) [6] or magnetron sputtering (MS) [7], methods based on powder technology became alternative, low-cost techniques which give possibility to incorporate dopands i.e, rare-earth (RE) ions , into the host material in an easy way. In this way, a combination of the electronic properties of GaN material with the unique optical properties of RE
ions, can be obtained. This combination can give red light sources based on the GaN materials, which is not possible by using typical techniques due to the difficulties associated with the synthesis processes. Thus such materials (GaN:RE) are interesting from the fundamental standpoint of structure– composition–properties of solids as well as from application perspectives. Especially, in case of the influence of the local environments on the optical properties of the incorporated RE ions there are a lot of open questions. Deeper understanding of this phenomenon is necessary for efficient applications in the optoelectronic devices. Thus, the influence of the local environment on the optical properties of Eu3+- and Tb3+-doped ions will be present. Also we will discuss more carefully a phenomena of the aggregation of nanometer grains of GaN:1%Eu3+, and its influence on the optical properties of the RE ions, to a micrometer conglomerate with the optical properties different from the optical properties of initial GaN:Eu3+ nanograins. 2. Experimental details
Corresponding author. Tel.: +48 713202358; fax: +48 713283696.
E-mail address: [email protected] (A. Podhorodecki). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.07.002
GaN nanocrystalline powder has been synthesized using a horizontal quartz reactor [8]. The portions of 2 0.5 g
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Ga2O3 (99.999%), 7.2 mg EuO2 (99.99%) and 7.6 mg terbium oxide (99.99%) were mixed and solubilized in hot concentrated nitric acid and evaporated to dryness. Next, the obtained powder was carefully dried in an oven by gradually increasing the temperature from 70 to 200 1C. Then, the powder, placed in an alumina crucible into quartz tube (24 mm ID) was calcined at 500 1C for 4 h in air flow (100 cm3/min) to convert Ga(NO3)3 into Ga2O3. The crushed powder sample was placed at room temperature into quartz tube in NH3 flow (120 cm3/min) and after purging (20 min) the sample was heated (10 1C/min) to the required temperature of 850 1C and then was held in it for 3.5 h. The NH3 used for nitridation (99.85 vol%) was additionally purified by passing over a zeolite trap. The samples were taken from the furnace after it was cooled to the room temperature under the ammonia flow. After sample preparation, cathodoluminescence (CL) was used as a post-growth treatment technique to obtained different structural (different grain sizes/defects densities) domains for the two samples (i.e. GaN:Eu3+ and GaN:Tb3+). The CL spectra were excited in vacuum (o106 Torr) in the column of a Tesla transmission electron microscope operating at 60 and 90 kV with current beam ranging from 10 to 120 mA. The powder was pressed into pellets at power press (80 kN) and then the luminescence measurements were performed. For photoluminescence (PL) measurements UV argon laser with excitation wavelength of 302 nm and output power around 20 mW has been used. The focused spot, used to scan different structural domains of investigated structures, had a diameter about 100 mm. Signals were has been detected by using Ocean Optics SR4000 spectrometer. 3. Results and discussion In our previous paper [8] it has been shown that investigated GaN powder contains aggregates of GaN crystallites of sizes of 20–100 nm with monodispersive size/ shape distribution and hexagonal wurtzite structure. From XRD patterns by using the Scherrer formula it has been determined that the average size of the nanocrystallite grain doped by lanthanide ions (Eu3+ and Tb3+) is about 20, 9, and 21 nm for GaN, GaN:1%Eu3+, and GaN:1%Tb3+, respectively. Free Eu3+ ion has six electrons in the 4f state. The spin–orbit (LS) coupling gives rise to the formation of 2S+1 LJ configurations, each of which is (2J+1)-fold degenerate. Intraconfigurational transitions between these levels are, in a Russel–Saunders sense, electric-dipole (ED) forbidden for the free ion (or, more strictly, in the presence of inversion symmetry) by the parity rule (for which DJ p 6, 020 forbidden, or DJ ¼ 2, 4, 6 if the initial state has J ¼ 0). They are also spin forbidden. Magnetic-dipole (MD) transitions are only spin forbidden. The spin selection rule is, however, partly lifted by spin–orbit mixing or different manifolds. In general, the parity selection rule may also be lifted by an external electric field of low
symmetry. These so-called forced ED transitions manifest themselves in a static field (like a low-symmetry crystal field) as well as in a dynamic field. The latter may for instance be due to vibrational excitations and then result in vibrational transitions. Moreover an external field causes Stark splitting of the (2J+1)-fold degenerate levels, the number of splitting lines being dependent on the symmetry of the field [10,11]. It has been shown previously [9,12], that the grains tend to aggregate to form bigger grains due to a post-growth treatment, e.g. a local heating caused by a focused electron beam, which was used in CL measurements. This phenomenon will reflect in the fact that signal from GaN micrograins will differ somehow from signal obtained from GaN nanograins. For these reason we scanned by UV laser spot all area of each sample after post growth processing, i.e. CL, to find differences in the optical properties between the domains which were heated by CL and domains which were not. During the scanning of the sample by the laser beam, three different natures of the emission signal have been found. PL signal from all of them (Fig. 1) exhibits the strongest emission peak corresponding to transition between the first excited state 5D0, and the 7F2 ground state of Eu3+ ion, which is a hypersensitive ED transition. In this case, according to the parity of selection rules, Eu3+ ion should occupy a site without inversion symmetry. The local symmetry of the RE3+ ions doped in GaN depends on the growth and annealing conditions present during sample preparation [13,14]. For RE GaN-doped materials usually RE3+ (Pr, Sm) ions substitute for Ga ions in catonic sites of C3v point group symmetry [15–17]. It has been reported that also Eu3+ ion occupies the Ga site in the wurtzite GaN host by substitution [18,19]. However, Monteiro et al. [17] found that lattice site location of lighter RE ions (i.e. Eu3+) ions imbedded in a hexagonal GaN, grown by MOCVD must be in site with lower symmetry than C3v. It was also reported that more than one local environment of Eu3+ ions may exist in the GaN:1%Eu3+ samples [20,21]. Wurtzite GaN has a trigonal C3v symmetry. Therefore, there is a lack of inversion symmetry, thereby relaxing the parity selection rule and leading to the strongest emission line at 613 nm of Eu3+ ion as is observed in Fig. 1 and it is in agreement with the suggested C3v local symmetry of Eu3+ ions. Other weak emission peaks correspond to the 4f–4f intrashell transitions between the first excited state 5 D0, and the 7F0,1,3,4 levels, i.e., prohibited ED and MD selection rules [21], are associated with 5D0-7F0 (at 580 nm), 5D0-7F3 (at 650 nm), 5D0-7F4 (at 700 nm) and MD transition between the lowest excited state 5D0 and the ground 7F1 (at 594 nm) in Eu3+ ion (for which DJ ¼ 0, 71, 020 forbidden) [22]. Luminescence from the higher excited states such as 5D1 is not observed, indicating a very efficient nonradiative (multiphonon) relaxation to the 5D0 level. However, for some GaN grains (Fig. 1c) besides Eu3+ ion-related transitions a broad yellow/red emission band
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Fig. 1. PL spectra for GaN:Eu3+ powder from different places of the sample.
(y-GaN) centered at 500 nm is observed, which in fact comprised of at least two overlapping emission peaks at 450 and 500 nm. The observed emission signal is associated with the recombination processes mostly through the surface states of GaN nanocrystals. According to other papers [23,24], nitrogen vacancies are a source of yellow luminescence in GaN and are most likely candidates for luminescence at longer wavelengths. Uedono et al. [4] suggest that the major species of defects in the RE-doped GaN films can be identified as vacancy clusters composed of two or more vacancies and these clusters are larger in case of Tb than Eu. These defects were introduced by replacing Ga with RE elements. Thus, it has been suggested that in this case, signal from nanocrystals is observed. In case of nanocrystals the surface/volume ratio is huge, thus surface processes should play a dominant role. For this reason, observed emission
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signal should be rather associated with the surface recombination processes. Also, the correlation between y-GaN-related PL intensity and PL intensity of Eu3+ transitions has been observed (Fig. 1c). For increasing of y-GaN PL intensity, the intensity of Eu3+-related emission peaks also increase, indicating that defect states play an important role in the excitation energy transfer processes and increasing its efficiency. There are also domains from which PL signal differ somehow from signal disscused above. Besides Eu3+-ion related transitions a new transition at 370 nm is observed and no emission related to GaN defects. This emission is associated with GaN band-to-band transition related to the crystal volume. The broadening of this band-to-band emission band is high (50 nm), indicating a significant concentration of defect states close to the conduction (valence) band minimum. The lower-energy part of this emission band is likely a result of the recombination of exciton band at impurity or defect sites. Other parameters that can affect the emission profile are the shape and size distribution of the grains. Also, from Fig. 1b it can be observed that there are also some domains of GaN grains in the sample from which the emission signal has properties observed in the two cases discussed above (i.e., nanograins, and micrograins). In this case the splitting of the emission lines related to Eu3+ ions is the strongest. This will be discussed in the next paragraph. This emission signal is associated with the situation when both kinds of grain coexist together or grains size is somewhere between nano- and micrometer. This will reflect both, bulk-like and surface-like optical properties of grains. Summarizing the discussion above, after aggregation of nanometer grains to micrometer conglomerates the surface/volume ratio decreased and hence the surface-related emission has decreased in favor of the bulk-like emission which is associated with band-to-band recombination. In addition, it has been observed that PL intensity for samples with micrometer grains is usually stronger than PL intensity for samples with nanometer grains. The factor of the enhancement of PL intensity changes from 2 to 10 depending on the postgrowth processing, suggesting more efficient excitation energy transfer in case of micrograins. Also existence of forbidden 7F2 transition confirms the possibility of C3v (or lower) symmetry site for Eu3+ ions, i.e. Ga substitution. Other differences observed from presented spectra are attributed (i) to different structural positions of Eu3+ ions (i.e., surface or volume positions, intersitials or substitutional positions) and can be observed as a different splitting of Eu3+-related emission peaks or (ii) to different chemical environments (i.e. local electric field intensities) observed as a differently shifted and splitted Eu3+-related emission peaks. Thus the number of peaks (optical transitions) is also dependent on the point symmetry of the RE ion site, that is, the higher the point symmetry, lesser number of
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peaks appear. Thus RE ions could be used as a fingerprint of different symmetry for ion sites in the system. To look more carefully on this phenomena, the main peak for each spectrum was deconvoluted with the Gaussian curves as seen in the insets of Fig. 1. The summed curve (thick line) of each deconvoluted curve is well fitted to the measured spectra (open circles). Thus, the number of Stark components identified for the 7F0,1,2,3,4 levels in the micrograins GaN:Eu3+ are: 3, 3, 3, 2, and at least 4 for the last one, respectively (inset of Fig. 1a). For the signal from domain concerning nano- as well as microGaN:1%Eu3+ grains (thin line) the number of Stark components identified for the 7F0,1,2,4 levels are 3, 4, 5 (degeneracy completely lifted), and at least 7 (quite complicated line shape), respectively. For transition to the 7F3 level, deconvolution was not possible due to the weak resolution. For nanoGaN grains, the number of Stark components identified for the 7F0,1,2,3,4 levels are 1, 3, 3, 3 and at least 6 (also quite complicated line shape), respectively (Fig. 1b). It means, that degeneracy, especially for nano/micrograins, is completely lifted, reflecting the fact that Eu3+ ions are in sites with lower symmetry than the suggested C3v. However, when permitted by the local symmetry, the 5 D0-7F0 transition is characterized by only one Stark component (both states are nondegenerate) since there are other transitions near this energy, indicating the presence of Eu3+ sites subject to distinctly different local field intensities [25,26]. Fig. 2 shows in detail the splitting of 5D0-7F0 transition for Eu3+ ions doped with GaN of different GaN grain sizes. Concerning this effect, it can be concluded that different chemical environments (local fields intensities) are responsible for a complex splitting of the emission lines rather than lower than C3v symmetry of the local Eu3+ ion environment.
D0 → 7F0
5
PL intensity
GaN micrograins
GaN nano/micrograins GaN nanograins
570
575
580 wavelength nm
585
590
Fig. 2. GaN:1%Eu3+-related 5D0-7F0 transition from different places of the sample.
It can be due to the fact that in case of ions being near the surface, due to the grain boundaries, rather asymmetric crystal field, with low symmetry (even for ions which are in the same position i.e., C3v), should split more the degenerate energy levels, and intensity for 5D0- 7F2 transition should increase due to more efficient the breaking of the forbidden selection rule for this transition. This will be even more efficient for micro- and nano-/ micrograins obtained in the structurally complicated agglomeration process. Thus, introducing the sites with lower symmetry than C3v for Eu3+ ions to explain this strong emission lines splitting is not needed. The ratio between the integrated intensity of the 5 D0-7F2 and 5D0-7F1 transitions, I02/I01, is used in lanthanide-based systems as a probe of changes on the nature of the local surroundings [25,27,28]. It is well known that the probability of the 5D0-7F2 transition is very sensitive to relatively small changes in the chemical surroundings of the Eu3+ ions [29]. The intensity of the ED 5D0-7F2 transition is significantly affected by the degree in the center of symmetry in the environment around Eu3+ ions. Conversely, the 5D0-7F1 emission around 592 nm is allowed by MD considerations, therefore being relatively insensitive to the local symmetry. This transition can thus be used as an internal standard in complex materials. When Eu3+ ions are situated at lowsymmetry sites, the ED transition has larger probability than the MD transition [28]. Therefore a greater I02/I01 ratio may correspond to more distorted (or asymmetric) local environment [27,28,30]. Base on these reasons, the observed decrease in the I02/I01 ratio for the nanograins (2.0), when compared to the values 4.0 and 4.5 for mixed sizes and micrograins, respectively, may thus be related to fact that intensity of the 5D0-7F2 transition increases with increasing distortion (on average) of the local-field symmetry around the Eu3+ cations, which confirms previous conclusions. Also, almost two times bigger broadening (19 and 18 nm) for the 5 D0-7F2 transition (Fig. 1a and b) suggest that rather different chemical surroundings (local field intensities) than lower-symmetry sites of Eu3+ ions are responsible for such a huge splitting in case of micrograins. The Tb3+ free ion possesses a 4f8 configuration, which gives rise to a 7F6 ground state and 7F5,4,3,2,1,0, 5D4, 5D3, 5 L10, 5G6, 5L9, 5G5, and 5D2 excited states. Lozykowski et al. [31] proposed that Tb3+ in GaN occupies relaxed substitutional Ga sites with trigonal C3v crystal symmetry. In C3v crystal symmetry, the states with J ¼ 0, 1, 2, 3, 4, 5, and 6, will split into 1(0), 1(1), 1(2), 3(2), 3(3), 3(4), and 5(4), single (doubly) degenerated crystal-field LSJ (Russell–Saunders) levels, respectively. On the other hand, Bang et al. [32] concluded that Tb3+ was incorporated into a substitutional Ga site with tetrahedral symmetry. Also Gruber et al. [16] proposed that Tb3+ ion resides in sites of D2 symmetry or lower in MOCVD-made GaN epilayers. Fig. 3 shows the PL spectra recorded at room temperature from different places on the GaN:1%Tb3+
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associated with band-to-band recombination that was also observed in the PL spectra. In this case the emission related to RE ions is more efficient, suggesting more efficient excitation energy transfer through the defect states. In case of GaN:1%Eu3+ nanograins rather high local symmetry like C3v for Eu3+ ion (i.e. Ga substitution) site and only one dominating chemical environment is suspected. Similar situation can be observed for Tb3+doped GaN nanograins. For GaN:1%Eu3+ micrograins, more than one chemical environment for Eu3+ ions is suspected with local symmetry like C3v or lower, due to complicated agglomeration process. All these results mean, that GaN powder is a very suitable host for the RE elements.
800
Fig. 3. PL spectra for GaN:1%Tb3+ powder from different places of the sample.
sample (as a consequence of different heat treatment during CL). As can be observed, for all selected spectra, obtained results look similar. No emission band related to GaN band-to-band transition has been observed and only a broad yellow GaN defects-related emission band, which was discussed in previous sections, is observed. The presented spectra differ only slightly in the intensity of Tb3+-related emission observed clearly in all cases. Peaks at 620, 580, 550, and 480 nm are associated with the intrashell transitions between the first excited state 5D4, and the 7F3,4,5,6 levels of the ground multiplet, respectively. In these case no Stark splitting (or very weak at room temperature) has been observed, suggesting only one Tb3+ position, with rather high local symmetry, like C3v sites for Tb3+ ions. Also, a small broadening (9 nm) for the main peak confirms this expectation. All these results suggest that in case of heat treatment no micrograins have been obtained in case of Tb3+-doped GaN grains and only nanograins are present. It can be due to the fact that GaN:1%Tb3+ grains are two times larger than GaN:1%Eu3+ grains, and they need more energy to begin the agglomeration process. 4. Conclusion GaN powder doped by Eu3+ and Tb3+ ions has been synthesized and investigated optically. The origin of observed optical transitions has been explained. Also a phenomena of GaN:1%Eu3+ grain agglomeration due to the postgrowth treatment has been shown and investigated optically. For GaN:1%Tb3+ grains for similar postgrowth treatment condition no such a phenomena was observed. After the aggregation of the nanometer grains to the micrometer conglomerates the surface/volume ratio decreased and hence the surface-related emission has decreased in favor of the bulk-like emission which is
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