Shou-Yi Kuo and Wen-Feng Hsieh
Citation: Journal of Vacuum Science & Technology A 23, 768 (2005); doi: 10.1116/1.1938979 View online: http://dx.doi.org/10.1116/1.1938979
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/23/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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0.7 0.3 3
Shou-Yi Kuoa兲
Instrument Technology Research Center, National Applied Research Laboratories, Taiwan
Wen-Feng Hsieh
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Taiwan
共Received 4 October 2004; accepted 25 April 2005; published 23 June 2005兲
Er-doped Ba0.7Sr0.3TiO3 共BST:Er兲 thin films prepared by the sol–gel technique have been
investigated by means of x-ray diffraction共XRD兲, Raman, spectroscopic ellipsometry, Capacitance– voltage, and photoluminescence共PL兲 measurements. XRD results indicate that the film possess the highest degree of crystallinity at the annealing temperature of 700 ° C. The dependence of the refractive index on erbium concentration was also analyzed. In addition, the excitation-dependent PL studies indicate that the green emission peaks do not shift with the change in excitation power, while the integrated intensity increases monotonically with the increase in excitation power. The quenching mechanism of the green emission due to dopant concentrations and annealing temperatures was discussed in detail. All experimental results indicate that BST:Er thin films might be a potential candidate for optoelectronics devices. © 2005 American Vacuum Society. 关DOI: 10.1116/1.1938979兴
I. INTRODUCTION
Recently, the study of the luminescent properties of the rare-earth 共RE兲 doped materials is strongly motivated be-cause of their various applications in optoelectronic devices and next-generation flat-panel displays.1–4Erbium-doped ox-ides are of special interest due to their characteristic emission at 1.54m, corresponding to minimum loss in the silica-based fiber. Except for the distinguishing feature mentioned above, other Er3+ transitions in the blue 共2H
9/2→ 4I
15/2兲,
green 共4S3/2,2H11/2→4I15/2兲, and red 共4F9/2→4I15/2兲 spectral
regions were also found to have application potential.5–8Due to the 4f shell of RE ions being well shielded by the outer electrons, the energy of the transitions is relatively indepen-dent of the host matrix and the ambient temperature. Never-theless, the relative intensity of their photoluminescence, chemical stability, and sensitivity to the operation environ-ments are affected by the nature of the matrix, which will also influence their commercialization. In comparison with the traditional sulfide luminescent phosphors, oxide film phosphors offer potential advantages because of their atmo-spheric stability, reduced degradation under applied voltages, and anticorrosive properties required for practical use.9–12 The perovskite BaxSr1−xTiO3 共BST兲 has been attracting
much attention because of its unique combination of superior dielectric properties, high permittivity, and low leakage cur-rent. In addition, Er3+ ions can be readily incorporated into
barium titanate to form an extended solid solution.13,14 Therefore, BST:Er will be a promising candidate for opto-electronic devices.
BST thin films have previously been fabricated by differ-ent methods such as metalorganic chemical vapor deposition, molecular beam epitaxy, rf sputtering, thermal evaporation,
sol–gel and laser ablation.15–20Despite the crystalline quality being inferior to other vacuum deposition techniques, the sol–gel processing is still a relatively easy and economical way for making uniform large size, high quality stoichio-metric thin film phosphors. So far, there have been only a few reports on the optical properties of Er3+-doped BaxSr1−xTiO3films. The investigations of the optical
proper-ties are important to optimize and improve the performance of devices. In this article, we report and discuss the structural and optical properties in the visible spectral region of sol–gel derived erbium-doped Ba0.7Sr0.3TiO3thin films. As a conse-quence, the influence of annealing temperature and dopant concentration can also be investigated.
II. EXPERIMENT
The Er-doped thin films on Pt/ TiO2/ SiO2/ Si 共100兲 sub-strates were prepared by the sol–gel method. The precursor solutions were synthesized using barium acetate, strontium acetate, titanium isopropoxide, and erbium acetate as the starting materials. Barium acetate and strontium acetate with a molar ratio of 7:3 and erbium acetate were dissolved in heated glacial acetic, and an appropriate amount of ethylene glycol was added to stabilize the solution. Then equimolar amounts of titanium isopropoxide were also added into the solution. Finally, formamide was selected as an additive to adjust the solution viscosity in order to reduce the crack of BST:Er thin films.21 The prepared solution was clear and transparent.
The BST:Er thin films were deposited by spinning the diluted solution at a speed 3000 rpm for 30 s. Each layer of the films was dried at 200 ° C for 10 min to dry the gel and then pyrolyzed at 500 ° C 30 min in a furnace to remove residual organic compounds, followed by a suitable heating rate to obtain crack-free films. Post-annealing of the a兲Author to whom correspondence should be addressed; electronic mail:
multilayer BST:Er thin films was carried out for 1 h at tem-peratures ranging from 600 to 900 ° C under oxygen atmo-sphere.
The crystalline structure of the thin films was analyzed by x-ray diffraction共XRD兲 共Siemens D5005兲 with Cu K␣ radia-tion. For photoluminescence共PL兲 and Raman measurements, the 488 nm line of an Ar+laser was used as the excitation
source. The signal was analyzed using a SPEX 1877C triple spectrograph equipped with a cooled charge coupled device at 140 K. The ellipsometric spectra of the films were mea-sured by the spectroscopic ellipsometer共Sopra兲 with a fixed incident angle of 75°. The incident light beam is polarized before reflecting from the sample. Then the reflected beam, after passing through an analyzer, is dispersed by a mono-chromator and detected by a photomultiplier. The wave-length range of our measurement is 300– 800 nm with 1 nm steps. Capacitance–voltage共C–V兲 analyses were performed using a HP8142 impedance analyzer.
III. RESULTS AND DISCUSSION
Figure 1共a兲 shows are the XRD patterns of BST:Er films deposited on Pt/ TiO2/ SiO2/ Si substrates at various
anneal-ing temperatures of 600– 800 ° C. The thin films annealed at 600 ° C show weak perovskite phases such as 共100兲, 共110兲, 共111兲, and 共211兲. Meanwhile, the secondary phases originat-ing from共Bs,Sr兲2Ti2O5CO3and Er2O3were also found.22,23 The diffraction lines of Er2共Si,Ti兲2O7were recognized when
annealing temperatures were above 700 ° C.24–26 It is be-lieved that the secondary phase appeared in the thin films during the process of the high annealing temperature. The results indicate that the films annealed at 700 ° C have a single and crystalline perovskite phase, consistent with pre-vious reports.21,27 The XRD patterns of BST films doped with various Er concentrations are shown in Fig. 1共b兲. It was found that more erbium incorporated into the BST films, the worse the crystallinity appeared as determined by the full width half maximum of XRD spectra. We might attribute the deterioration to the Er3+substitution in the films.
Figure 2 shows the Raman spectrum of 3 mol % Er3+-doped BST film, annealed at 700 ° C, taken at room
temperature. From this figure, we found a broadband cen-tered at 260 cm−1corresponds to the A
1共TO2兲 phonon mode,
a peak at 300 cm−1 is attributed to the B
1 and E共TO+LO兲
modes, and the asymmetric broadband near 520 cm−1
corre-sponds to E共TO兲 and A1共TO3兲 modes. In addition, the
Ra-man peak at 300 cm−1 is specific to the tetragonal phase of
the polycrystalline BaxSr1−xTiO3.28It is known that the Curie
temperature Tc linearly increases with Ba concentration. A
structural change from centrosymmetric cubic to noncen-trosymmetric tetragonal phase is observed at room tempera-ture when x⬃0.75.28 However, the characteristic Raman peaks persist for all BST:Er films, implying these films are in the tetragonal phase. In order to identify the structural phase, expanded XRD data were investigated as shown in Fig. 2共b兲. The nondegeneracy of the 兵200/002其 reflection is obvious evidence of the tetragonality, which is consistent with an earlier report.29
A Pt top electrode was deposited by rf sputtering for a metal/insulator/metal structure to perform the C – V measure-ment. The dielectric constants 共not shown here兲 for the 3 mol % doped BST thin film capacitors annealed at differ-ent temperatures are 230, 320, and 440 for 600, 700, and 800 ° C, respectively. Compared with earlier reports, the re-sults demonstrate that the addition of an Er dopant did not reduce the dielectric properties of BST films.19,21
In spectroscopic ellipsometry, one deals with the measure-ments of the relative changes in the amplitude and phase of a linearly polarized monochromatic incident light upon an ob-lique reflection from the sample surface. The measured quan-tities are the traditional ellipsometric angles⌿ and ⌬, which are related to the ratio of the complex Fresnel reflection co-efficients defined by30
⬅rp rs
= tan⌿ exp共i⌬兲, 共1兲
where rpand rsare the Fresnel reflection coefficient for light
polarized parallel and perpendicular to the plane of
inci-FIG. 1. XRD patterns of BST:Er thin films annealed at various temperatures 共a兲 and doped with various Er concentrations at sintering temperature 700 ° C共b兲. Secondary phases are marked by various symbols.
dence, respectively. The refractive index 共n兲 was derived from the ellipsometric parameters and analyzed by a three-phase model. The refractive index of BST:Er films was de-scribed using the Cauchy dispersion model, given by
n共兲 = A + B
2+ C
4, 共2兲
where the parameters A, B, and C are determined from fits to the experimental spectra. The evaluated optical constant n of the BST:Er films is shown in Fig. 3. From the figure, we found that the index of refraction increase slightly as the Er3+-dopant concentration increase from 1 to 5 mol %. It has
been known that the BO6 octahedron significantly governs
the optical properties, and other ions in the ABO3 structure
have only a small effect on the optical properties.31Although the Er3+ ion is likely to take the position of共Ba2+, Sr2+兲 due to similar ionic radius rather than that of Ti4+, we might
attribute the increase of refractive index with increasing Er3+
doping to the changes of the electronic polarizability. Figure 4 shows the room-temperature PL spectra of Ba0.7Sr0.3TiO3 films doped with 3 mol % of Er annealed at
600, 700, 800, and 900 ° C. The green peaks at 530 and 550 nm are attributed to the Er3+ inner shell 4f 2H
11/2 and 4S
3/2to the15I15/2ground level, respectively. In addition, the
weak red emission centered at 660 nm is ascribed to the
4F
9/2→15I15/2. At the annealing temperature of 600 ° C, there
are only few broad peaks in the main emission wavelength. When the annealing temperature is above 700 ° C, the 550 nm emission peak becomes sharper and splits into sev-eral fine peaks attributed to the Stark splitting of the degen-erate 4f levels under the crystalline field. For BST:Er films annealed at 600 ° C, the weak emission intensity and broad spectrum indicated the films possess worse crystallinity, which is consistent with the XRD analysis.
For the 3 mol % Er3+-doped BST films annealed at 700 ° C, the excitation-dependent PL spectra are shown in Fig. 5共a兲. It indicated that these emission peaks do not show any apparent shift with the change in excitation power. In addition, the integrated green emission intensity as a function of excitation power has been plotted in Fig. 5共b兲 in log–log scale. The integrated PL intensity has a nearly linear depen-dence on the excitation power. This result confirms that green emission belongs to a one-photon interband transition and no other nonradiative mechanism or nonlinear processes developed from the high excitation condition.
Figure 6 shows the integrated green emission intensities of the BST films doped with various molar ratios of Er at different annealing temperatures of 600, 700, 800, and 900 ° C. Obviously, the PL emission at 550 nm reaches its maximum at 700 ° C and the emission was partially quenched for other annealing temperatures. The quenching of the green emission has occurred in the BST:Er films, which is directly related to microstructural changes caused by the crystallinity. According to the results of XRD mea-surements, the crystallization occurred at 700 ° C and other formation phase共Er2Si2O7, Er2Ti2O7兲 forms above this
tem-FIG. 2. Raman spectra of BST thin films annealed at temperature 700 ° C共a兲 and typical expanded XRD data around 45.5°共b兲. An extra peak comes from the laser source and is denoted by共*兲. The dotted and solid lines represent
the experimental and fitting results, respectively.
FIG. 3. Refractive index of BST:Er thin films annealed at 700 ° C with various Er3+concentrations.
perature. When the BST:Er films are amorphous, the emis-sion intensity depends on the Er3+ concentration, as can be seen in Fig. 6. In addition, as the films annealed at 700, 800, and 900 ° C, the spectrum is dominated by a main green emission around 550 nm, and we can observe that the emis-sion intensity of the BST:Er film increases as the doping concentration of Er changing from 1 to 3 mol %. When the Er doping concentration exceeds 3 mol %, the PL intensity diminishes. The concentration dependence of the films in the amorphous phase can be explained because the emission in-tensity is proportional to the solubility of Er in the amor-phous matrix. Meanwhile, the presence of clusters in the polycrystalline phase BST:Er films due to high Er concentra-tion will decrease the luminescence efficiency by energy transfer processes due to ion–ion interactions. As shown in Fig. 6, the Er-doped BST thin film with 3 mol % of Er dop-ants has the strongest PL intensity, while the PL intensity decreased greatly for the Er-doped BST thin film containing 5 mol % Er. The quenching mechanism is thought to be a cross-relaxation process between two closely placed Er3+
ions.32Very efficient cross relaxation can occur when two or more ions are sited closely together or form a pair or cluster, which results in almost immediate interaction between the ions. The short distance between ions results in enhanced luminescence quenching probability. Accordingly, the emis-sion intensity degrades while the Er concentration exceeds
3 mol %. The mean distances between Er3+ ions of the BST
films doped with various erbium concentrations are 1.2, 0.8, and 0.7 nm for 1, 3, and 5 mol % Er.33
Next, we will focus on the relationship between the crys-tallinity and emission intensity. In Fig. 6, the emission inten-sity of the BST film doped with 3 mol % has a maximum value at the annealing temperature of 700 ° C. Furthermore, the decrease in luminescence intensity is observed when the annealing temperature is above 700 ° C. From the XRD data, we have concluded that the films annealed at 600 ° C are amorphous and are polycrystalline when annealed at 700 ° C.
FIG. 4. Room temperature photoluminescence spectra of BST thin films doped with 3 mol % Er under various annealing temperatures. Appropriate offset has been made to highlight the variations in the line shapes of PL spectra.
FIG. 5. 共a兲 Excitation-dependent photoluminescence spectra of BST:Er thin film. The arrow indicates the direction of adding excitation power.共b兲 Inte-grated intensities of green emission vs the excitation power.
When the films are amorphous, the luminescence efficiency will be low due to the lower degree of crystallization and higher defect density. By increasing the annealing tempera-ture to 700 ° C, the green emission becomes stronger than those annealed at 600 ° C. In light of the XRD results, it implicitly indicates that the improvement of the crystallinity will enhance the luminescence efficiency. However, the emission intensities decrease while the annealing tempera-ture is above 700 ° C. The quenching mechanism of the emission intensity may arise from the formation of other phases and crystal field as described in earlier literature.
IV. CONCLUSION
In conclusion, Ba0.7Sr0.3TiO3 共BST兲 ferroelectric thin films with various erbium concentrations were grown by the sol–gel method. The crystallization of the films strongly de-pends on the annealing temperature and dopant concentra-tion. The effects of Er3+concentration on the refractive index
of BST thin films have been studied by spectroscopic ellip-sometry in the visible region, and a three-phase fitting model was employed to describe the dispersion relation. Our studies show that the index of refraction increase slightly as the Er3+-dopant concentration increases from 1 to 5 mol %.
Fur-thermore, we also found that the addition of Er dopant does not deteriorate the dielectric properties of BST. Photolumi-nescence measurements indicated that the green band emis-sion intensities vary with various Er concentrations and an-nealing temperatures. The emission efficiency of BST:Er thin films was found to be dominated by the mean distance be-tween Er3+ ions and solubility of Er in polycrystalline and
amorphous phases, respectively. With the doping of Er 3 mol %, the emission intensities of the films reach its maxi-mum at an annealing temperature of 700 ° C. While the Er concentration exceeds 3 mol %, the emission intensity di-minishes due to the presence of clusters. Moreover, the im-provement of the crystallinity of BST:Er films also results in the green emission enhancement.
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