Cold white light generation from hafnium oxide films ...

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Instituto de Fısica y Matema´ticas, Universidad Tecnolo´gica de la Mixteca, Huajuapan de ... Ulises Caldin˜oa) ... tion and luminescence of Ce3+ ions is a consequence of ..... U. Caldin˜o G, C. de la Cruz, G. Mun˜oz H, and J. Rubio O: Ce3+ !

Cold white light generation from hafnium oxide films activated with Ce3+, Tb3+, and Mn2+ ions Rafael Martı´nez-Martı´nez Instituto de Fı´sica y Matema´ticas, Universidad Tecnolo´gica de la Mixteca, Huajuapan de Leo´n, Oaxaca 69000, Me´xico

´ lvarez Enrique A Departamento de Fı´sica, Universidad de Sonora (UNISON), Hermosillo, Sonora 83000, Me´xico

Adolfo Speghini DiSTeMeV, Universita` di Verona, and INSTM, UdR Verona, I-37029 San Floriano, Verona, Italy

Ciro Falcony Centro de Investigacio´n y de Estudio Avanzados del IPN, Departamento de Fı´sica, 07000 Me´xico, D.F., Me´xico

Ulises Caldin˜oa) Departamento de Fı´sica, Universidad Auto´noma Metropolitana-Iztapalapa, 09340 Me´xico, D.F., Me´xico (Received 15 September 2009; accepted 9 December 2009)

Hafnium oxide films doubly doped with CeCl3 and TbCl3 and triply doped with CeCl3, TbCl3, and MnCl2 were deposited at 300  C with the ultrasonic spray pyrolysis technique. The green and yellow emissions of Tb3+ ions and the yellow-red emission of Mn2+ ions can be generated upon ultraviolet (UV) excitation via a nonradiative energy transfer from Ce3+ to Tb3+ and Ce3+ to Mn2+. In the doubly doped film Ce3+ ! Tb3+ energy transfer via an electric dipole–quadrupole interaction appears to be the most probable transfer mechanism; the efficiency of this transfer is about 81% upon excitation at 270 nm. In the HfO2 films activated with Ce3+, Tb3+, and Mn2+ the efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions is enhanced by increasing the Mn2+ concentration, up to about 76% for the film with the highest manganese content (1.6 at.%). In addition, it is demonstrated that these triply doped films can generate cold white light emission upon excitation at 270 nm (peak emission wave length of an AlGaN/GaN-based LEDs).

I. INTRODUCTION

The development of new phosphors that can be pumped through ultraviolet (UV) light emitting diodes (LEDs) has been the subject of intense research during recent decades because of the necessity of increasing the efficiency in white light emitting solid-state devices for application in new generation of lighting lamps.1 White light generation through the simultaneous emission of blue, green, and red emitting centers upon UV excitation was reported for the first time in borate-based glasses containing Ce3+, Tb3+, and Mn2+ as activators.2 Ce3+ ions exhibit broad absorption bands in the UV, so that they can be excited by LEDs.3 The advantage of using commercial diodes is their low-energy consumption, longevity, and reduced size. The intense broad-band absorption and luminescence of Ce3+ ions is a consequence of their 4f–5d parity allowed electric dipole transitions. In codoped materials Ce3+ ions act as good sensitizers, so a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0065

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that they can transfer a part of their excitation energy to activator ions such as Tb3+,2–5 Mn2+,2,3,6–11 and Eu2+.12,13 Tb3+ ions in solids show allowed intense 4f8 ! 4f75d absorptions in the UV and forbidden 4f8 ! 4f8 absorptions at lower energy (near UV). On the other hand, Mn2+ ions in solids show forbidden 3d ! 3d absorption transitions. Nevertheless, both Tb3+ and Mn2+ ions can be efficiently excited, through their forbidden transitions, by Ce3+ ions, due to the strong overlapping between the Ce3+ emission and some absorptions of terbium5 and manganese.6–10 Hafnium oxide (HfO2) has attracted considerable attention because of its excellent physical and chemical properties, such as its high dielectric constant and insulating characteristics,14 which allow its application as a dielectric material with relatively high refractive index and wide band gap, as well in the field of optical coatings and metal-oxide semiconductor devices of the next generation.15 Considering that both Ce3+ and Tb3+ ions are important activators, as they can be widely used in phosphors for displays and fluorescent lamps,16 as well as the © 2010 Materials Research Society

R. Martı´nez-Martı´nez et al.: Cold white light generation from hafnium oxide films activated with Ce3+, Tb3+, and Mn2+ ions

importance of finding efficient luminescent materials for the design of optical devices based on hafnium oxide, in the present investigation a spectroscopic study of the Tb3+ luminescence sensitized by Ce3+ ions in HfO2 films deposited at 300  C with the ultrasonic spray pyrolysis technique is presented. Besides, it is demonstrated that this film activated with three dopant ions (Ce3+, Tb3+, and Mn2+) can generate cold white light emission upon excitation at 270 nm, i.e., using an AlGaN/GaN-based LEDs as excitation source. II. EXPERIMENTAL

HfO2 films doped with Ce3+, Tb3+, and Mn2+ ions were prepared using the ultrasonic spray pyrolysis technique. The spraying solution (at a rate of 1 mL/min) was a 0.07 M solution of HfOCl28H2O (Aldrich, 99+%) dissolved in deionized water (18 MO/cm), and CeCl3, TbCl3, and MnCl2 (Aldrich, 99.99+%) were added as doping materials. The spray was produced using a 0.8 MHz ultrasonic generator and deposited on Corning 7059 glass slides at 300  C. Filtered air (at a flow rate of 10 L/min) was used as the transport gas. A film thickness of about 5 mm (as measured with a profilometer, Sloan Dektak IIA) was attained with a deposition time of 6 min. The surface roughness of the films, as measured by the profilometer, was 0.65  0.03 mm. The crystalline structure of the films was analyzed by x-ray diffraction ˚ (Cu Ka) Siemens D5000 diffrac(XRD) using a 1.540 A tometer, operating at 30 keV. The chemical composition of the films was measured by energy dispersive spectroscopy (EDS) using a Leica Cambridge Electron Microscope model Stereoscan 440 (equipped with a beryllium window x-ray detector). Photoluminescence spectra were recorded using PerkinElmer LS50B and Jobin-Yvon Fluoro-Max-P spectrophotometers. Lifetime data were obtained using a PTI GL300 and GL302 Dye nitrogen pulsed laser, which produces a pulse of about 600 ps in duration and 0.1 nm bandwidth at 337.1 nm. The resulting transient fluorescence signal was recorded through a Jobin-Yvon monochromator Triax 550 and detected with HORIBA-Jobin Yvon i-Spectrum Two ICCD. All measurements were performed at room temperature.

91%–CeCl3 3%–TbCl3 6%, HfO2 90%–CeCl3 3%– TbCl3 6%–MnCl2 1%, HfO2 88%–CeCl3 3%–TbCl3 6%–MnCl2 3% and HfO2 86%–CeCl3 3%–TbCl3 6%–MnCl2 5% will be referred to hereafter as HOC, HOT, HOCT, HOCTM1, HOCTM3, and HOCTM5, respectively. X-ray diffraction patterns displayed by the films deposited at 300  C exhibited a broad band without any indication of crystallinity, so that they can be considered as predominantly amorphous. The Ce3+, Tb3+, and Mn2+ ion substitution into Hf4+ cations might generate an incorporation of chlorine ions, which could act as charge compensators to preserve electrical neutrality.17 B. Photoluminescence of HfO2:Tb3+

The emission spectrum recorded for the HOT film excited at 250 nm (into the 4f8 ! 4f75d absorption band of Tb3+) exhibits several bands associated with 4f8 ! 4f8 transitions from the 5D4 level to the 7F6, 7F5, 7F4, and 7 F3 multiplets, which are centered at 488, 543, 586, and 622 nm, respectively (Fig. 1). A broad emission band extending from 320 to 480 nm is also observed, which might be attributed to the intrinsic emission of the HfO2 host. This emission disappears when the spectrum is recorded with a delay time of 0.1 ms (Fig. 1, dotted line), as expected from its decay time shorter than the one of the Tb3+ emissions. No emissions from the 5D3 level are observed, which suggests that at 6 at.% concentration of TbCl3 (1.6 at.% of Tb3+ as measured from EDS) there is a nonradiative relaxation from the 5D3 to the 5D4 level. Such relaxation is induced by the excitation from the 7F6 to the 7F0 level by a cross-relaxation process.18 Therefore, in the HOT film only 5D4 to 7FJ emissions are observed because the 5D3 to 7FJ transitions are quenched by nonradiative energy transfer between identical centers, described by 5D3 + 7F6 ! 5D4 + 7F0. The excitation spectrum of the HOT film monitored at 544 nm, into the 5D4 ! 7F5 emission transition, consists of Tb3+ excitation bands corresponding to transitions from the 7F6(4f8) ground state to higher-energy states of TABLE I. Content (at.%) of oxygen, chlorine, hafnium, cerium, terbium, and manganese in the CeCl3, TbCl3, and MnCl2 doped HfO2 films as measured by EDS.

III. RESULTS AND DISCUSSION A. EDS and XRD measurements

Composition (at.%)

The chemical composition of the studied films determined by EDS are listed in Table I, which shows the relative atomic percentages of oxygen, chlorine, hafnium, cerium, terbium, and manganese present in the films as a function of the CeCl3, TbCl3, and MnCl2 contents in the spraying solution. The films studied: HfO2 97%–CeCl3 3%, HfO2 94%–TbCl3 6%, HfO2

Film HOC HOT HOCT HOCTM1 HOCTM3 HOCTM5

Oxygen Chlorine Hafnium Cerium Terbium Manganese 60.8 60.5 57.5 58.5 57.5 58.4

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10.5 7.4 11.5 11.5 12.2 12.1

28.0 30.5 28.2 27.4 27.2 25.6

0.7  1.3 0.9 0.9 0.7

 1.6 1.5 1.5 1.4 1.6

   0.4 0.8 1.6

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FIG. 1. Emission spectra of the HOT film excited at 250 nm. The spectra were recorded with a delay time of 0 ms (solid line) and 0.1 ms (dotted line).

FIG. 2. Excitation spectrum monitored at 544 nm for the HOT film.

the 4f8 and 4f75d configurations (Fig. 2). The excitation band associated with the 4f8 ! 4f75d electric dipole allowed transitions, located at around 227 nm, appears significantly more intense than those associated with the 4f8 ! 4f8 forbidden transitions centered at 317 nm (7F6 ! 5H7,5D1), 338 nm (7F6 ! 5L7,8,5G3), 350 nm (7F6 ! 5L9,5D2,5G5), 368 nm (7F6 ! 5L10), and 377 nm (7F6 ! 5G6,5D3). The decay time of the 5D4 level of Tb3+ was monitored at 544 nm wavelength upon 337 nm laser pulsed excitation into the 7F6 ! 5L7,8,5G3 transition. The decay curve of the 5D4 level emission is single exponential, with a lifetime value of 1.1  0.1 ms. C. Photoluminescence of HfO2:Ce3+:Tb3+

FIG. 3. Emission spectra of the HOC and HOCT films excited at 270 nm. The spectra were recorded in the same experimental conditions.

Figure 3 shows emission spectra of HfO2 singly doped with 0.7 at.% of Ce3+ ions (HOC film) and doubly doped with 1.3 at.% of Ce3+ ions and 1.5 at.% of Tb3+ ions (HOCT film), after 270 nm excitation within the Ce3+ 4f ! 5d absorption transition. Both spectra have been recorded in the same experimental conditions for comparison. The emission spectrum of HOCT exhibits the 5 D4 ! 7F6,5,4,3 green-yellow emissions of Tb3+ ions and the broad band centered at around 380 nm associated with the 5d1 ! 4f1 transition of Ce3+ ions.17 From the spectra of Fig. 3 it can be noted that the addition of Tb3+ ions in the Ce3+ doped film causes a strong decrease of the cerium overall emission. This behavior provides evidence that energy transfer from Ce3+ to Tb3+ ions takes place through a nonradiative process. The global emission generated by the HOCT film (excited at 270 nm, i.e., using an AlGaN/GaN-based LEDs as excitation source) was characterized by its chro-

maticity coordinates in a CIE diagram (Fig. 4), resulting in yellowish-green light, with x = 0.31 and y = 0.48. The excitation spectrum monitored at 545 nm (inside the 5D4 ! 7F5 emission of Tb3+) exhibits, in addition to the (4f8 ! 4f75d) Tb3+ absorption (excitation) transitions, a broad band in the 260 to 380 nm region (Fig. 5), which is similar to the 4f ! 5d Ce3+ absorption (excitation) band observed in the excitation spectrum of HfO2 doped with 1.9 at.% of Ce3+.17 The presence of this Ce3+ band indicates that Tb3+ ions can be excited through Ce3+ ions. This energy transfer is expected to occur considering that the Ce3+ emission overlaps the 7F6 ! 5L7,8,5G3, 7 F6 ! 5L9,5D2,5G5, 7F6 ! 5L10, and 7F6 ! 5G6,5D3 absorption (excitation) transitions of Tb3+ (Fig. 6). Lifetime measurements of the 5D4 ! 7F6,5 emissions of Tb3+ in the HOCT film were performed monitoring the Tb3+ (488 and 543 nm) emissions after laser pulsed

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FIG. 4. Chromaticity coordinates characteristic of the emissions observed in the HOCT film excited at 270 nm.

FIG. 6. Overlap region between the Ce3+ emission (solid line) and Tb3+ absorption (dotted line). The inset shows the normalized lineshape functions. The Tb3+ absorption profile in the overlap region was taken from the HOT film excitation spectrum (Fig. 2).

dependence of the sensitizer luminescence, considering a short-time excitation, follows the relation19,20:   t 3=S ; ð1Þ IðtÞ ¼ I0 exp   gS t  wt t0

FIG. 5. Excitation spectrum monitored at 545 nm for the HOCT film.

excitation at 337 nm. The decay curve of the 5D4 level emission is single exponential, with a lifetime value of 1.0  0.1 ms. Such lifetime value is the same, within the experimental uncertainty, as the one measured for the Tb3+ singly doped film (HOT), i.e., the same decay times are obtained for the Tb3+ emissions under direct excitation and through Ce3+ ions, which suggests the absence of Ce3+ Tb3+ back energy transfer. The decay-time curve of the Ce3+ emission in the HOCT film after 337 nm laser pulsed excitation, within the Ce3+ 4f ! 5d absorption transition, was analyzed to determine some important quantitative information on the Ce3+ ! Tb3+ energy transfer process, such as the nature of the Ce3+–Tb3+ coupling and the energy transfer microparameter. According to previous models the time

where I0 is the intensity at t = 0, t0 is the decay time of the sensitizer in absence of the activator, gS (with S = 6, 8, and 10 for dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole electric interactions, respectively) is a time-independent constant related to the direct Ce3+ ! Tb3+ energy transfer (static transfer), and w is the migration of excitation energy (dynamic transfer), which takes into account the sensitizer migration. The gS parameter can be written as:   4p 3 3=s gS ¼ G 1  NA CDA ; ð2Þ 3 s where p ffiffiffi G[1  (3/s)] is the Euler integral, whose value is p for s = 6, 1.43 for s = 8, and 1.30 for s = 10, NA is the number of (Tb3+) activator ions per cm3, and CDA is the energy transfer microparameter, a useful parameter to compare the ability of a given sensitizer-activator system for energy transfer independently of their distance. The time dependence of the luminescence of Ce3+ ions excited at 337 nm for the HOCT film was fitted to Eq. (1) by assuming dipole–dipole (S = 6), dipole–quadrupole (S = 8), and quadrupole–quadrupole (S = 10) electric couplings. The best agreement between the experimental decay and Eq. (1) is achieved for S = 8 (Fig. 7). Hence, it can be inferred that an electric dipole–quadrupole interaction might be the dominant mechanism in the Ce3+ ! Tb3+ energy transfer. In fact, such interaction mechanism is

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FIG. 7. Time dependence of the Ce3+ emission obtained for the HOCT film. Solid line is the best fit to Eq. (1) for electric dipolequadrupole (S = 8) interaction.

favored by the electric dipole allowed 4f ! 5d transition of Ce3+ ions (given their short decay time) and the 4f ! 4f forbidden transitions of Tb3+ ions (given their long decay time). From the fitting of the decay curve in Fig. 7 to Eq. (1) with t0 = 32 ns (measured from the decay time of the Ce3+ emission in the HOC film) is obtained that g8 = 23.1  1.0 s3/8 and that the excitation energy migration between Ce3+ ions is negligible (w  0). Then, from Eq. (2) the energy transfer microparameter, CDA, was found to be about 3.8  1054 cm8/s using a Tb3+ ion concentration NA  4.14  1020 ions/cm3 (measured by EDS). Assuming an electric dipole–quadrupole (d–q) interaction mechanism, the transfer rate Pdq sa for this type of interaction between sensitizer (Ce3+) and activator (Tb3+) ions is given by21: Psadq ¼

3h  4 c4 fq l2s Qa O 4pn4 tos fd R8sa

;

ð3Þ

R where O = [Fs(E)Fa(E)/E4]dE is the spectral overlap integral between the normalized line-shape functions of the Ce3+ emission Fs(E) and Tb3+ absorption Fa(E), with E being the average energy of the overlapping transition, tos is the Ce3+ intrinsic lifetime in absence of the Tb3+ ion, Qa is the oscillator strength of the Tb3+ absorption transitions, fq and fd are the oscillator strengths of the Tb3+ ion electric quadrupole and dipole transitions, respectively, and ls is the wave length position of the Ce3+ emission. The remaining symbols in Eq. (3) have their usual meaning. In Eq. (3) the Qa integrated absorption coefficient of Tb3+ can be estimated using the relation derived by Blasse, Qa = 4.8  1020 eV m2  fd.22 In the region of Ce3+ emission (330–560 nm) the fd electric dipole oscillator strength of the Tb3+ ion is low (3  107 23). The overlap integral in Eq. (3) can be calculated using the normalized line-shape functions of the cerium 488

emission Fs(E) and terbium absorptions Fa(E) in the overlap region (inset of Fig. 6). The Rosa critical interaction distance between Ce3+ and Tb3+ ions for Ce3+ ! Tb3+ energy transfer through an electric dipole–quadrupole interaction mechanism, i.e., the one for which the probability of transfer equals the o probability of radiative emission of the Ce3+ (Pdq sa ts = 1), can be estimated from Eq. (3). Using n = 2.0, fq/fd  102 to 103,23 and the values estimated for Qa (1.44  1026 eV m2) and O (6  103 eV5), it is found that ˚ to 11 A ˚ , a reasonable value for energy transfer Rosa  9 A involving rare-earth ions. Thus, the critical interaction distance is smaller than the one assuming a random ion ˚ ). This behavior suggests distribution (Drandom  14 A 3+ 3+ that the Ce ! Tb energy transfer could take place in Ce3+–Tb3+ clusters formed in the film instead of from randomly distributed ions. Drandom was estimated from the amounts of Ce3+ (2.5  1020 ions/cm3) and Tb3+ (4.1  1020 ions/cm3) measured by EDS. The Z energy transfer efficiency to evaluate whether Ce3+ ion is a good sensitizer to Tb3+ ion was estimated from the emission intensities of the Ce3+ in the presence (Is) and absence (Iso ) of the Tb3+ activator, i.e.5: Z¼1

Is Iso

:

ð4Þ

Thus, the efficiency of energy transfer from Ce3+ to Tb3+ ions, using Eq. (4) and the cerium emission spectra shown in Fig. 3, was around 81%. D. Photoluminescence of HfO2:Ce3+:Tb3+:Mn2+

Mn2+ ions can also be sensitized by Ce3+ ions, since there exists an overlapping between the sensitizer emission and 6A1(S) ! 4T1(G), 4T2(G), 4A1(G), 4E(G), 4 T2(D), 4E(D), and 4T1(P) absorption (excitation) transitions of the activator.10 The emission spectrum of the HfO2 film triply doped with Ce3+, Tb3+, and Mn2+ ions recorded upon 270 nm excitation within the Ce3+ 4f ! 5d absorption transition exhibits a UV-blue broad band associated with the Ce3+ 5d ! 4f emission, four narrow green-yellow bands due to the 5D4 ! 7F6,5,4,3 emissions of Tb3+ and a yellow-red broad band due to the 4T1(G) ! 6A1(S) emission of Mn2+. Figure 8 shows emission spectra of the three triply doped films under investigation after 270 nm excitation (peak emission wave length of an AlGaN/GaN-based LEDs). As the Mn2+ content is increased, a decrease in the Ce3+ emission intensity correlates with an increase in the Mn2+ emission intensity. This enhancement of the Mn2+ emission intensity at the expense of the Ce3+ intensity represents an evidence of Ce3+ ! Mn2+ energy transfer. The excitation spectra of the triply doped films monitored at 640 nm [inside the 4T1(G) ! 6A1(S) emission of Mn2+] and 544 nm (inside the 5D4 ! 7F5 emission of

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Tb3+) show, in addition to the [6A1(S) ! 4A1(G), 4E(G), 6 A1(S) ! 4T2(G), and 6A1(S) ! 4T1(G)] Mn2+, and (4f8 ! 4f75d) Tb3+ absorption (excitation) transitions, also the 4f ! 5d Ce3+ absorption (excitation) band, which shows that Tb3+ and Mn2+ ions can be sensitized by Ce3+ ions. Such excitation spectra are shown in Fig. 9 for the HOCTM3 film. The emission spectrum of the HOC film (with 0.7 at.% of Ce3+), recorded in the same experimental conditions, is also shown in Fig. 8 for comparison. The addition of Tb3+ and Mn2+ in the Ce3+ doped film produces a strong decrease of the cerium overall emission, which represents an evidence that energy transfer from Ce3+ to Tb3+ and Mn2+ ions occurs through a nonradiative process. The efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions, using Eq. (4) and the cerium emission

spectra shown in Fig. 8, was found to increase with the Mn2+ content from around 70% (HOCTM1 film) up to around 76% (HOCTM5 film), see Fig. 10. It can be noted that such transfer efficiency tends to saturate at Mn2+ concentrations larger than 1.6 at.% (HOCTM5 film). The combination of the Ce3+, Tb3+, and Mn2+ emissions in the three triply doped films results in white light, with chromaticity coordinates x = 0.29 and y = 0.38 (HOCTM1), x = 0.31 and y = 0.38 (HOCTM3), and x = 0.32 and y = 0.23 (HOCTM5), as can be appreciated from the CIE diagram portrayed in Fig. 11. The color temperature associated to these chromaticity coordinates

FIG. 10. Energy transfer efficiency from Ce3+ to Tb3+ and Mn2+ as a function of manganese content obtained from emission spectra (Fig. 8) and Eq. (4). The dotted line is drawn to guide the eyes. FIG. 8. Emission spectra of HfO2 singly doped with Ce3+ ions and codoped with Tb3+ and Mn2+ ions after 270 nm excitation. All the spectra have been obtained in the same experimental conditions.

FIG. 9. Excitation spectra of HOCTM3 monitored at 544 nm (solid line) and 640 nm (dotted line).

FIG. 11. Chromaticity coordinates characteristic of the emissions observed in the HOCTM1 (solid circle symbol), HOCTM3 (square symbol) and HOCTM5 (circle symbol) films excited at 270 nm.

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are 7200 K (HOCTM1), 6400 K (HOCTM3), and 6500 K (HOCTM5), which correspond to cold light. IV. SUMMARY

Films of hafnium oxide doped with only Ce3+, doubly doped with Ce3+ and Tb3+, and triply doped with Ce3+, Tb3+, and Mn2+, were prepared using the ultrasonic spray pyrolysis technique. The crystalline structures of the films were monitored by XRD, and they exhibited a very broad band without any indication of crystallinity, so they can be considered as predominantly amorphous. In the doubly doped film, energy transfer from Ce3+ to Tb3+ ions could take place in Ce3+–Tb3+ clusters and an electric dipole–quadrupole interaction appears to be the most probable energy transfer mechanism. The efficiency of this transfer was estimated to be about 81% when the film is excited at 270 nm (peak emission wave length of an AlGaN/GaN-based LEDs). In the triply doped films, both Tb3+ and Mn2+ ions can be sensitized by Ce3+ ions, so that the simultaneous emission of these ions under UV excitation (270 nm) results in cold white light (6400–7200 K). The energy transfer efficiency from Ce3+ to Tb3+ and Mn2+ ions is enhanced by increasing the Mn2+ concentration, being up to about 76% for the film with the highest manganese content (1.6 at.%). This high efficiency of energy transfer, resulting in cold white light emission, makes the Ce3+, Tb3+, and Mn2+ triply doped HfO2 film a versatile material for the design of efficient UV LEDs pumped phosphors for generation of cold white light.

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ACKNOWLEDGMENTS

This work was supported by the CONACyT under Project Contract 78802-2F and by the ICIT-DF. We would like to thank CBI chemical laboratory of the Universidad Auto´noma Metropolitana-Iztapalapa for sharing its equipment. The technical assistances of Z. Rivera, A.B. Soto, and M. Guerrero (Departamento de Fı´sica, CINVESTAV), and Lic. O. Gutie´rrez V. (Facultad de Ciencias, UNAM) are also acknowledged.

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REFERENCES 1. P.D. Rack, A. Naman, P.H. Holloway, S.S. Sun, and R.T. Tuenge: Materials used in electroluminescent displays. MRS Bull. 21, 49 (1996). 2. J.C. Zhang, C. Parent, G. Le Flem, and P. Hagenmuller: White light emitting glasses. J. Solid State Chem. 93, 17 (1991). 3. N. El Jouhari, C. Parent, and G. Le Flem: Photoluminescence of Ce3+, Tb3+, and Mn2+ in glasses of base composition LaMgB5O10. J. Solid State Chem. 123, 398 (1996). 4. J.A. Gonza´lez-Ortega, E.M. Tejeda, N. Perea, G.A. Hirata, E.J. Bosze, and J. McKittrick: White light emission from rare

490

16.

20.

21. 22. 23.

earth activated yttrium silicate nanocrystalline powders and thin films. Opt. Mater. 27, 1221 (2005). U. Caldin˜o, A. Speghini, and M. Bettinelli: Optical spectroscopy of zinc metaphosphate glasses activated by Ce3+ and Tb3+ ions. J. Phys. Condens. Matter 18, 3499 (2006). U. Caldin˜o G.: On the Ce–Mn clustering in CaF2 in which the Ce3+ ! Mn2+ energy transfer occurs. J. Phys. Condens. Matter 15, 3821 (2003). R. Martı´nez-Martı´nez, M. Garcı´a-Hipo´lito, F. Ramos-Brito, J.L. Herna´ndez-Pozos, U. Caldin˜o, and C. Falcony: Blue and red photoluminescence from Al2O3:Ce3+:Mn2+ films deposited by spray pyrolysis. J. Phys. Condens. Matter 17, 3647 (2005). U. Caldin˜o, J.L. Herna´ndez-Pozos, C. Flores, A. Speghini, and M. Bettinelli: Photoluminescence of Ce3+ and Mn2+ in zinc metaphosphate glasses. J. Phys. Condens. Matter 17, 7297 (2005). R. Martı´nez-Martı´nez, M. Garcı´a Hipo´lito, L. Huerta, J. Rickards, U. Caldin˜o, and C. Falcony: Studies on blue and red photoluminiscence from Al2O3:Ce3+:Mn2+ coatings synthesized by spray pyrolysis technique. Thin Solid Films 515, 607 (2006). R. Martı´nez-Martı´nez, M. Garcı´a, A. Speghini, M. Bettinelli, C. Falcony, and U. Caldin˜o: Blue-green-red luminescence from CeCl3 and MnCl2 doped hafnium oxide layers prepared by ultrasonic spray pyrolysis. J. Phys. Condens. Matter 20, 395205 (2008). R. Martı´nez-Martı´nez, A. Speghini, M. Bettinelli, C. Falcony, and U. Caldin˜o: White light generation through the zinc metaphosphate glass activated by Ce3+, Tb3+ and Mn2+ ions. J. Lumin. 129, 1276 (2009). U. Caldin˜o G, C. de la Cruz, G. Mun˜oz H, and J. Rubio O: Ce3+ ! Eu2+ energy transfer in CaF2. Solid State Commun. 69, 347 (1989). U. Caldin˜o G: Energy transfer in CaF2 doped with Ce3+, Eu2+ and Mn2+ ions. J. Phys. Condens. Matter 15, 7127 (2003). J. Sundqvist, A. Harsta, J. Aarik, K. Kukli, and A. Aidla: Atomic layer deposition of polycrystalline HfO2 films by the HfI4–O2 precursor combination. Thin Solid Films 427, 147 (2003). G.D. Wilk, R.M. Wallace, and J.M. Anthony: High-kappa gate dielectrics: Current status and materials properties considerations. J. Appl. Phys. 89, 5243 (2001). L.H. Huang, X.J. Wang, H. Lin, and X.R. Liu: Luminescence properties of Ce3+ and Tb3+ doped rare earth borate glasses. J. Alloys Compd. 316, 256 (2001). M. Garcı´a-Hipo´lito, U. Caldin˜o, O. Alvarez-Fragoso, M.A. AlvarezPe´rez, R. Martı´nez-Martı´nez, and C. Falcony: Violet-blue luminescence from hafnium oxide layers doped with CeCl3 prepared by spray pyrolysis process. Phys. Status Solidi 204, 2355 (2007). D. de Graaf, S.J. Stelwagen, H.T. Hintzen, and G. de With: Tb3+ luminescence as a tool to study clustering of lanthanide ions in oxynitride glasses. J. Non-Cryst. Solids 325, 29 (2003). M.J. Weber: Optical properties of Yb3+ and Nd3+–Yb3+ energy transfer in YAlO3. Phys. Rev. B 4, 3153 (1971). M. Louis, S. Hubert, E. Simoni, and J.Y. Gesland: Energy transfer between lanthanide and actinide ions in LiYF4. Opt. Mater. 6, 121 (1996). D.L. Dexter: A theory of sensitized luminescence in solids. J. Chem. Phys. 21, 836 (1953). G. Blasse: Energy transfer in oxidic phosphors. Philips Res. Rep. 24, 131 (1969). J.M.P.J. Verstegen, J.L. Sommerdijk, and J.G. Verriet: Cerium and terbium luminescence in LaMgAl11O19. J. Lumin. 6, 425 (1973).

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