Cold white light generation from hafnium oxide films activated with Ce3+, Tb3+, and Mn2+ ions

Cold white light generation from hafnium oxide films activated

with Ce3+, Tb3+, and Mn2+ ions

Rafael Martınez-MartınezInstituto de Fısica y Matematicas, Universidad Tecnologica de la Mixteca, Huajuapan de Leon,Oaxaca 69000, Mexico

Enrique AlvarezDepartamento de Fısica, Universidad de Sonora (UNISON), Hermosillo, Sonora 83000, Mexico

Adolfo SpeghiniDiSTeMeV, Universita di Verona, and INSTM, UdR Verona, I-37029 San Floriano, Verona, Italy

Ciro FalconyCentro de Investigacion y de Estudio Avanzados del IPN, Departamento de Fısica, 07000 Mexico,D.F., Mexico

Ulises Caldinoa)

Departamento de Fısica, Universidad Autonoma Metropolitana-Iztapalapa, 09340 Mexico, D.F., Mexico

(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+ ionscan be generated upon ultraviolet (UV) excitation via a nonradiative energy transfer fromCe3+ to Tb3+ and Ce3+ to Mn2+. In the doubly doped film Ce3+! Tb3+ energy transfer viaan electric dipole–quadrupole interaction appears to be the most probable transfermechanism; the efficiency of this transfer is about 81% upon excitation at 270 nm. Inthe HfO2 films activated with Ce3+, Tb3+, and Mn2+ the efficiency of energy transfer fromCe3+ to Tb3+ and Mn2+ ions is enhanced by increasing the Mn2+ concentration, up to about76% for the film with the highest manganese content (1.6 at.%). In addition, it isdemonstrated that these triply doped films can generate cold white light emission uponexcitation at 270 nm (peak emission wave length of an AlGaN/GaN-based LEDs).

I. INTRODUCTION

The development of new phosphors that can bepumped through ultraviolet (UV) light emitting diodes(LEDs) has been the subject of intense research duringrecent decades because of the necessity of increasing theefficiency in white light emitting solid-state devices forapplication in new generation of lighting lamps.1 Whitelight generation through the simultaneous emission ofblue, green, and red emitting centers upon UV excitationwas reported for the first time in borate-based glassescontaining Ce3+, Tb3+, and Mn2+ as activators.2 Ce3+

ions exhibit broad absorption bands in the UV, so thatthey can be excited by LEDs.3 The advantage of usingcommercial diodes is their low-energy consumption, lon-gevity, and reduced size. The intense broad-band absorp-tion and luminescence of Ce3+ ions is a consequence oftheir 4f–5d parity allowed electric dipole transitions. Incodoped materials Ce3+ ions act as good sensitizers, so

that they can transfer a part of their excitation energyto activator ions such as Tb3+,2–5 Mn2+,2,3,6–11 andEu2+.12,13 Tb3+ ions in solids show allowed intense4f8 ! 4f75d absorptions in the UV and forbidden 4f8

! 4f8 absorptions at lower energy (near UV). On theother hand, Mn2+ ions in solids show forbidden 3d !3d absorption transitions. Nevertheless, both Tb3+ andMn2+ ions can be efficiently excited, through theirforbidden transitions, by Ce3+ ions, due to the strongoverlapping between the Ce3+ emission and someabsorptions of terbium5 and manganese.6–10

Hafnium oxide (HfO2) has attracted considerableattention because of its excellent physical and chemicalproperties, such as its high dielectric constant andinsulating characteristics,14 which allow its applicationas a dielectric material with relatively high refractiveindex and wide band gap, as well in the field of opticalcoatings and metal-oxide semiconductor devices of thenext generation.15

Considering that both Ce3+ and Tb3+ ions are impor-tant activators, as they can be widely used in phosphorsfor displays and fluorescent lamps,16 as well as the

a)Address all correspondence to this author.e-mail: [email protected]

DOI: 10.1557/JMR.2010.0065

J. Mater. Res., Vol. 25, No. 3, Mar 2010 © 2010 Materials Research Society484

importance of finding efficient luminescent materialsfor the design of optical devices based on hafnium oxide,in the present investigation a spectroscopic study of theTb3+ luminescence sensitized by Ce3+ ions in HfO2 filmsdeposited at 300 �C with the ultrasonic spray pyrolysistechnique is presented. Besides, it is demonstrated thatthis film activated with three dopant ions (Ce3+, Tb3+,and Mn2+) can generate cold white light emission uponexcitation at 270 nm, i.e., using an AlGaN/GaN-basedLEDs as excitation source.

II. EXPERIMENTAL

HfO2 films doped with Ce3+, Tb3+, and Mn2+ ionswere prepared using the ultrasonic spray pyrolysis tech-nique. The spraying solution (at a rate of 1 mL/min) wasa 0.07 M solution of HfOCl2�8H2O (Aldrich, 99+%)dissolved in deionized water (18 MO/cm), and CeCl3,TbCl3, and MnCl2 (Aldrich, 99.99+%) were addedas doping materials. The spray was produced using a0.8 MHz ultrasonic generator and deposited on Corning7059 glass slides at 300 �C. Filtered air (at a flow rate of10 L/min) was used as the transport gas. A film thicknessof about 5 mm (as measured with a profilometer, SloanDektak IIA) was attained with a deposition time of6 min. The surface roughness of the films, as measuredby the profilometer, was 0.65 � 0.03 mm. The crystallinestructure of the films was analyzed by x-ray diffraction(XRD) using a 1.540 A (Cu Ka) Siemens D5000 diffrac-tometer, operating at 30 keV. The chemical compositionof the films was measured by energy dispersive spectros-copy (EDS) using a Leica Cambridge Electron Micro-scope model Stereoscan 440 (equipped with a berylliumwindow x-ray detector).

Photoluminescence spectra were recorded using Perkin-Elmer LS50B and Jobin-Yvon Fluoro-Max-P spectropho-tometers. Lifetime data were obtained using a PTI GL300and GL302 Dye nitrogen pulsed laser, which producesa pulse of about 600 ps in duration and 0.1 nm bandwidthat 337.1 nm. The resulting transient fluorescence signalwas recorded through a Jobin-Yvon monochromator Triax550 and detected with HORIBA-Jobin Yvon i-SpectrumTwo ICCD.

All measurements were performed at room temperature.

III. RESULTS AND DISCUSSION

A. EDS and XRD measurements

The chemical composition of the studied films deter-mined by EDS are listed in Table I, which shows therelative atomic percentages of oxygen, chlorine, haf-nium, cerium, terbium, and manganese present in thefilms as a function of the CeCl3, TbCl3, and MnCl2contents in the spraying solution. The films studied:HfO2 97%–CeCl3 3%, HfO2 94%–TbCl3 6%, HfO2

91%–CeCl3 3%–TbCl3 6%, HfO2 90%–CeCl3 3%–TbCl3 6%–MnCl2 1%, HfO2 88%–CeCl3 3%–TbCl36%–MnCl2 3% and HfO2 86%–CeCl3 3%–TbCl36%–MnCl2 5% will be referred to hereafter as HOC,HOT, HOCT, HOCTM1, HOCTM3, and HOCTM5,respectively.

X-ray diffraction patterns displayed by the films de-posited at 300 �C exhibited a broad band without anyindication of crystallinity, so that they can be consideredas 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 elec-trical neutrality.17

B. Photoluminescence of HfO2:Tb3+

The emission spectrum recorded for the HOT filmexcited at 250 nm (into the 4f8! 4f75d absorption bandof Tb3+) exhibits several bands associated with 4f8! 4f8

transitions from the 5D4 level to the 7F6,7F5,

7F4, and7F3 multiplets, which are centered at 488, 543, 586, and622 nm, respectively (Fig. 1). A broad emission bandextending from 320 to 480 nm is also observed, whichmight be attributed to the intrinsic emission of the HfO2

host. This emission disappears when the spectrum isrecorded with a delay time of 0.1 ms (Fig. 1, dotted line),as expected from its decay time shorter than the one ofthe Tb3+ emissions. No emissions from the 5D3 level areobserved, which suggests that at 6 at.% concentration ofTbCl3 (1.6 at.% of Tb3+ as measured from EDS) there isa nonradiative relaxation from the 5D3 to the 5D4 level.Such relaxation is induced by the excitation from the 7F6to the 7F0 level by a cross-relaxation process.18 There-fore, in the HOT film only 5D4 to 7FJ emissions areobserved because the 5D3 to

7FJ transitions are quenchedby nonradiative energy transfer between identical cen-ters, described by 5D3 +

7F6! 5D4 +7F0.

The excitation spectrum of the HOT film monitored at544 nm, into the 5D4! 7F5 emission transition, consistsof Tb3+ excitation bands corresponding to transitionsfrom the 7F6(4f

8) ground state to higher-energy states of

TABLE I. Content (at.%) of oxygen, chlorine, hafnium, cerium, ter-

bium, and manganese in the CeCl3, TbCl3, and MnCl2 doped HfO2

films as measured by EDS.

Composition (at.%)

Film Oxygen Chlorine Hafnium Cerium Terbium Manganese

HOC 60.8 10.5 28.0 0.7 �� HOT 60.5 7.4 30.5 �� 1.6 ��HOCT 57.5 11.5 28.2 1.3 1.5 ��HOCTM1 58.5 11.5 27.4 0.9 1.5 0.4

HOCTM3 57.5 12.2 27.2 0.9 1.4 0.8

HOCTM5 58.4 12.1 25.6 0.7 1.6 1.6

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

J. Mater. Res., Vol. 25, No. 3, Mar 2010 485

the 4f8 and 4f75d configurations (Fig. 2). The excita-tion band associated with the 4f8 ! 4f75d electricdipole allowed transitions, located at around 227 nm,appears significantly more intense than those associ-ated with the 4f8 ! 4f8 forbidden transitions centeredat 317 nm (7F6 ! 5H7,

5D1), 338 nm (7F6 ! 5L7,8,5G3),

350 nm (7F6! 5L9,5D2,

5G5), 368 nm (7F6! 5L10), and377 nm (7F6! 5G6,

5D3).The decay time of the 5D4 level of Tb

3+ was monitoredat 544 nm wavelength upon 337 nm laser pulsed excita-tion into the 7F6! 5L7,8,

5G3 transition. The decay curveof the 5D4 level emission is single exponential, with alifetime value of 1.1 � 0.1 ms.

C. Photoluminescence of HfO2:Ce3+:Tb3+

Figure 3 shows emission spectra of HfO2 singly dopedwith 0.7 at.% of Ce3+ ions (HOC film) and doubly dopedwith 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 beenrecorded in the same experimental conditions for com-parison. The emission spectrum of HOCT exhibits the5D4! 7F6,5,4,3 green-yellow emissions of Tb3+ ions andthe broad band centered at around 380 nm associatedwith the 5d1 ! 4f1 transition of Ce3+ ions.17 From thespectra of Fig. 3 it can be noted that the addition of Tb3+

ions in the Ce3+ doped film causes a strong decrease ofthe cerium overall emission. This behavior provides evi-dence that energy transfer from Ce3+ to Tb3+ ions takesplace through a nonradiative process.

The global emission generated by the HOCT film(excited at 270 nm, i.e., using an AlGaN/GaN-basedLEDs as excitation source) was characterized by its chro-

maticity coordinates in a CIE diagram (Fig. 4), resultingin 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 tothe (4f8 ! 4f75d) Tb3+ absorption (excitation) transi-tions, a broad band in the 260 to 380 nm region (Fig. 5),which is similar to the 4f! 5d Ce3+ absorption (excita-tion) 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 consider-ing that the Ce3+ emission overlaps the 7F6! 5L7,8,

5G3,7F6 ! 5L9,

5D2,5G5,

7F6 ! 5L10, and7F6 ! 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 monitoringthe Tb3+ (488 and 543 nm) emissions after laser pulsed

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

FIG. 3. Emission spectra of the HOC and HOCT films excited at 270 nm.

The spectra were recorded in the same experimental conditions.

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).


J. Mater. Res., Vol. 25, No. 3, Mar 2010486

excitation at 337 nm. The decay curve of the 5D4 levelemission is single exponential, with a lifetime value of1.0 � 0.1 ms. Such lifetime value is the same, within theexperimental uncertainty, as the one measured for theTb3+ singly doped film (HOT), i.e., the same decay timesare obtained for the Tb3+ emissions under direct excita-tion and through Ce3+ ions, which suggests the absenceof Ce3+ Tb3+ back energy transfer.

The decay-time curve of the Ce3+ emission in theHOCT film after 337 nm laser pulsed excitation, withinthe Ce3+ 4f! 5d absorption transition, was analyzed todetermine some important quantitative information onthe Ce3+ ! Tb3+ energy transfer process, such as thenature of the Ce3+–Tb3+ coupling and the energy transfermicroparameter. According to previous models the time

dependence of the sensitizer luminescence, considering ashort-time excitation, follows the relation19,20:

IðtÞ ¼ I0 exp � t

t0� gSt

3=S � wt

� �; ð1Þ

where I0 is the intensity at t = 0, t0 is the decay time ofthe sensitizer in absence of the activator, gS (with S = 6,8, and 10 for dipole–dipole, dipole–quadrupole, andquadrupole–quadrupole electric interactions, respec-tively) is a time-independent constant related to thedirect Ce3+! Tb3+ energy transfer (static transfer), andw is the migration of excitation energy (dynamic trans-fer), which takes into account the sensitizer migration.The gS parameter can be written as:

gS ¼4p3G 1� 3

s

� �NAC

3=sDA ; ð2Þ

where G[1 � (3/s)] is the Euler integral, whose value isffiffiffipp

for s = 6, 1.43 for s = 8, and 1.30 for s = 10, NA is thenumber of (Tb3+) activator ions per cm3, and CDA is theenergy transfer microparameter, a useful parameter tocompare the ability of a given sensitizer-activator systemfor energy transfer independently of their distance.

The time dependence of the luminescence of Ce3+ ionsexcited 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 andEq. (1) is achieved for S = 8 (Fig. 7). Hence, it can beinferred that an electric dipole–quadrupole interactionmight be the dominant mechanism in the Ce3+ ! Tb3+

energy transfer. In fact, such interaction mechanism is

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

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 line-

shape functions. The Tb3+ absorption profile in the overlap region was

taken from the HOT film excitation spectrum (Fig. 2).



favored by the electric dipole allowed 4f! 5d transition ofCe3+ ions (given their short decay time) and the 4f ! 4fforbidden transitions of Tb3+ ions (given their long decaytime). From the fitting of the decay curve in Fig. 7 toEq. (1) with t0 = 32 ns (measured from the decay time ofthe Ce3+ emission in the HOC film) is obtained that g8 =23.1 � 1.0 s�3/8 and that the excitation energy migrationbetween Ce3+ ions is negligible (w� 0). Then, from Eq. (2)the energy transfer microparameter, CDA, was found tobe about 3.8 � 10�54 cm8/s using a Tb3+ ion concentrationNA � 4.14 � 1020 ions/cm3 (measured by EDS).

Assuming an electric dipole–quadrupole (d–q) interac-tion mechanism, the transfer rate Pdq

sa for this type ofinteraction between sensitizer (Ce3+) and activator (Tb3+)ions is given by21:

Pdqsa ¼

3h�4c4fql2sQa

4pn4tos fdR8sa

O ; ð3Þ

where O =R[Fs(E)Fa(E)/E

4]dE is the spectral overlap

integral between the normalized line-shape functions ofthe Ce3+ emission Fs(E) and Tb3+ absorption Fa(E), withE 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+ absorptiontransitions, fq and fd are the oscillator strengths of theTb3+ ion electric quadrupole and dipole transitions, re-spectively, and ls is the wave length position of the Ce3+

emission. The remaining symbols in Eq. (3) have theirusual meaning. In Eq. (3) the Qa integrated absorptioncoefficient of Tb3+ can be estimated using the relationderived by Blasse, Qa = 4.8 � 10�20 eV m2 � fd.22 In theregion of Ce3+ emission (330–560 nm) the fd electricdipole oscillator strength of the Tb3+ ion is low (3 �10�7 23). The overlap integral in Eq. (3) can be calculatedusing the normalized line-shape functions of the cerium

emission Fs(E) and terbium absorptions Fa(E) in theoverlap region (inset of Fig. 6).The Ro

sa critical interaction distance between Ce3+ andTb3+ ions for Ce3+ ! Tb3+ energy transfer through anelectric dipole–quadrupole interaction mechanism, i.e.,the one for which the probability of transfer equals theprobability of radiative emission of the Ce3+ (Pdq

sa tos = 1),

can be estimated from Eq. (3). Using n = 2.0, fq/fd� 10�2

to 10�3,23 and the values estimated for Qa (�1.44 �10�26 eV m2) and O (6 � 10�3 eV�5), it is found thatRosa � 9 A to 11 A, a reasonable value for energy transfer

involving rare-earth ions. Thus, the critical interactiondistance is smaller than the one assuming a random iondistribution (Drandom � 14 A). This behavior suggeststhat the Ce3+! Tb3+ energy transfer could take place inCe3+–Tb3+ clusters formed in the film instead of fromrandomly distributed ions. Drandom was estimated fromthe 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 estimatedfrom the emission intensities of the Ce3+ in the presence(Is) and absence (Ios ) of the Tb

3+ activator, i.e.5:

Z ¼ 1� IsIos

: ð4Þ

Thus, the efficiency of energy transfer from Ce3+ to Tb3+

ions, using Eq. (4) and the cerium emission spectrashown in Fig. 3, was around 81%.

D. Photoluminescence of HfO2:Ce3+:Tb3+:Mn2+

Mn2+ ions can also be sensitized by Ce3+ ions, sincethere exists an overlapping between the sensitizer emis-sion and 6A1(S) ! 4T1(G),

4T2(G),4A1(G),

4E(G),4T2(D),

4E(D), and 4T1(P) absorption (excitation) transi-tions of the activator.10

The emission spectrum of the HfO2 film triply dopedwith Ce3+, Tb3+, and Mn2+ ions recorded upon 270 nmexcitation within the Ce3+ 4f ! 5d absorption transitionexhibits a UV-blue broad band associated with the Ce3+

5d! 4f emission, four narrow green-yellow bands due tothe 5D4 ! 7F6,5,4,3 emissions of Tb3+ and a yellow-redbroad band due to the 4T1(G) ! 6A1(S) emission ofMn2+. Figure 8 shows emission spectra of the three triplydoped films under investigation after 270 nm excitation(peak emission wave length of an AlGaN/GaN-basedLEDs). As the Mn2+ content is increased, a decrease in theCe3+ emission intensity correlates with an increase in theMn2+ emission intensity. This enhancement of the Mn2+

emission intensity at the expense of the Ce3+ intensityrepresents an evidence of Ce3+!Mn2+ energy transfer.The excitation spectra of the triply doped films moni-

tored at 640 nm [inside the 4T1(G) ! 6A1(S) emissionof Mn2+] and 544 nm (inside the 5D4! 7F5 emission of

FIG. 7. Time dependence of the Ce3+ emission obtained for the

HOCT film. Solid line is the best fit to Eq. (1) for electric dipole-

quadrupole (S = 8) interaction.



Tb3+) show, in addition to the [6A1(S)! 4A1(G),4E(G),

6A1(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 sensitizedby Ce3+ ions. Such excitation spectra are shown in Fig. 9for 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 ofTb3+ and Mn2+ in the Ce3+ doped film produces a strongdecrease of the cerium overall emission, which repre-sents 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 theMn2+ content from around 70% (HOCTM1 film) up toaround 76% (HOCTM5 film), see Fig. 10. It can be notedthat such transfer efficiency tends to saturate at Mn2+

concentrations larger than 1.6 at.% (HOCTM5 film).The combination of the Ce3+, Tb3+, and Mn2+ emis-

sions 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 appreciatedfrom the CIE diagram portrayed in Fig. 11. The colortemperature associated to these chromaticity coordinates

FIG. 9. Excitation spectra of HOCTM3 monitored at 544 nm (solid

line) and 640 nm (dotted line).

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. 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.

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.



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+, doublydoped with Ce3+ and Tb3+, and triply doped with Ce3+,Tb3+, and Mn2+, were prepared using the ultrasonic spraypyrolysis technique. The crystalline structures of thefilms were monitored by XRD, and they exhibited a verybroad band without any indication of crystallinity, sothey can be considered as predominantly amorphous.

In the doubly doped film, energy transfer from Ce3+ toTb3+ ions could take place in Ce3+–Tb3+ clusters and anelectric dipole–quadrupole interaction appears to be themost probable energy transfer mechanism. The effi-ciency of this transfer was estimated to be about 81%when the film is excited at 270 nm (peak emission wavelength of an AlGaN/GaN-based LEDs).

In the triply doped films, both Tb3+ and Mn2+ ions canbe sensitized by Ce3+ ions, so that the simultaneousemission of these ions under UV excitation (270 nm)results in cold white light (6400–7200 K). The energytransfer efficiency from Ce3+ to Tb3+ and Mn2+ ions isenhanced by increasing the Mn2+ concentration, being upto about 76% for the film with the highest manganesecontent (1.6 at.%). This high efficiency of energytransfer, resulting in cold white light emission, makesthe Ce3+, Tb3+, and Mn2+ triply doped HfO2 film a ver-satile material for the design of efficient UV LEDspumped phosphors for generation of cold white light.

ACKNOWLEDGMENTS

This work was supported by the CONACyT underProject Contract 78802-2F and by the ICIT-DF. We wouldlike to thank CBI chemical laboratory of the UniversidadAutonoma Metropolitana-Iztapalapa for sharing its equip-ment. The technical assistances of Z. Rivera, A.B. Soto,and M. Guerrero (Departamento de Fısica, CINVESTAV),and Lic. O. Gutierrez V. (Facultad de Ciencias, UNAM)are also acknowledged.

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Cold white light generation from hafnium oxide films activated with Ce3+, Tb3+, and Mn2+ ions

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