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Energy transfer processes in yttrium oxide activated with europium
Journal of Luminescence 43 (1989) 261—274 North-Holland, Amsterdam
261
ENERGY TRANSFER PROCESSES IN YTUR1UM OXIDE ACI1VATED WITH EUROPIUM D.B.M. KLAASSEN
~,
R.A.M. van HAM
2
and T.G.M. van RuN
Philips Research Laboratories, P0 Box 80000, 5600 JA Eindhouen, The Netherlands Received 6 January 1989 Revised 3 April 1989 Accepted 14 April 1989
Energy transfer and non-radiative decay between excited states of europium in yttrium oxide are studied with dye-laser spectroscopy as a function of the europium concentration and the sample temperature. Combining the results with luminescence spectra measured under cathode-ray excitation, relative feeding rates for the excited states are obtained. These transfer, decay and feeding rates are used to interpret measurements on the sublinearity of the luminescence as a function of cathode-ray excitation density.
1. Introduction
Indications of complicating factors are the additional feeding process of the 5D0(C2) level:
The external radiant efficiency of phosphors under cathode-ray (CR) excitation decreases with increasing excitation density. television, For high density applications such as projection the linear-
via 5D~ (C a level-by-level cascade from the higher 2) levels [5—7]; 3 + ions at a differenergy transfer entvia crystallographic (S from Eu 6) site [8—15]. Consequently the method of the Leeuw and ‘t Hooft [4] cannot be3~applied. ions at Besides the C higher excited states Eu ± ions at S 2 sites, the excited statesofof Eu3 6 sites also have to be taken into account. 3 + ion in Y The luminescence of the Eu 203 has been widely and extensively investigated. 3~ions at CStudies relate to the spectra of single Eu 2 sites [16—21]and S6 sites [8,9,13,22—24],and also to the spectral features of ion 5D pairs [11,14,25]. Non5D radiative decay from the 1 to the 0 level of 3 ± ion at the C the Eu 2 site [5—7],energy transfer 3 + ions at sites of differbetween the levels of Euwell as between Eu3 + and ent symmetry [8—15]as other rare-earth ions has been investigated [5,12, 26—28]. Concentration quenching of Y 3~ 203 : Eu has been studied under UV excitation [29—31]and under cathode-ray excitation [18,30]. However, no data have yet been reported on the rates for non-radiative decay from the higher excited 5D 2 5D 3 ±ions at the C 3 + ions and 3 levels. Moreover, the studies on energy transfer 2 site to Eu at the S from Eu 6 site [8—15]did not yield a rate for this
ity of the luminescence intensity as a function of excitation density is a major selection criterion for the phosphor. At present3~) yttrium activated is theoxide red primary in with europium (Y203 : Eu these applications. The main emission of this phosphor originates from the lowest excited state (5 D0)(C2) of the europium ion. We found 5D (see also ref. [1]) that the sublinearity of the 0(C2) 3~in Y luminescence of Eu 203 cannot be explained by ground state depletion [2,3] only. In the analysis of the sublinearity of the emission of a particular level of the activator, the method proposed by overall the Leeuw andrate ‘t Hooft [4] can be applied if the feeding is (1) greater than the decay rate; (2) greater than the reciprocal excitation time in the application; and (3) constant as a function of activator concentration.
2
Author to whom correspondence should be directed, Present Océ The Research and Development, P0 Box 101, 5900address: MA Venlo, Netherlands.
—
—
0022-2313/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
262
D.B.M. Klaassen et aL
/
Energy transfer processes in yttrium oxide
process as a function of the Eu3 ± concentration, Consequently the feeding rates under cathode-ray excitation of the emitting levels of Eu3 + in Y 203 are not known. Yet knowledge of these feeding rates has proved to be vital for understanding the energy flow in a phosphor as a function of activator concentration or cathode-ray excitation density [32—34]. In the course of a detailed of the 3~, investigation we used dye-laser sublinearity of Y203 : Eu spectroscopy to determine the rates for non-radiative decay and energy transfer between the excited states of the Eu3~ions at the same and at different crystallographic sites, respectively, as a function of the Eu3 ± concentration and the sample temperature. These rates were used together with spectra measured under cathode-ray excitation, to determine the feeding rates of the emitting levels, Transfer, non-radiative and feeding rates were then used to interpret measurements on the sublinearity of the luminescence of Y 3 For clarity we present the results of 203 our : Eu investigations in three separate sections, each containing experimental results and their interpretation. First the dye-laser experiments on a large number of samples with different Eu31 concentration are presented, which yield the rates for energy transfer and non-radiative decay of the excited levels of Eu3 Next, a study of these rates as a function of sample temperature is presented together with a comparison with calculations using the Judd—Ofelt theory. Finally, the measurements under cathoderay excitation and their interpretation are presented. Part of the method and results were published before in a condensed form [35]. ~.
~.
dispersed with a 1 m Jarrell—Ash monochromator and detected with a photomultiplier using pulse counting techniques. The measurements as a function of sample temperature were performed using a Cryogenic Refrigerator System IIB of NMR Technologies. The spectra under CR excitation were measured in a demountable cathode-ray tube. The phosphors were excited withdensity a stationary of 20 keV electrons (current about beam 1 ~sA/ cm2). The luminescence was collected with a fibre and guided to a monochromator (Jarrell—Ash 1208; focal length 0.3 m; 147.5 grooves/mm grating blazed at 500 nm) equipped with a diode-array camera. This camera was part of an optical multichannel analyzer system (OMA-Ill of EG & G PAR; diode-array camera 1420BR). The wavelength indication of the system was calibrated with mercury and neon lamps. The sublinearity of the luminescence from the excited levels of Y 3 ± was measured in a 203 : Eu (Cameca) under stascanning electron microscope tionary CR excitation. The beam current ranged from 10 ~ to 10 6 A, while the spot diameter was varied between 20 and 200 tim; the accelerating voltage was 20 kV. The luminescence was collected from the bombarded side by a mirror systern, transmitted to a 0.75 m Spex monochromator and detected with a photomultiplier using pulse counting techniques. All spectra were corrected for the spectral response of the various detection systems.
3. Transfer and decay rates as a function of Eu3~ concentration We measured the decay curves of the 5D
2. Expenmental The phosphor powders with Eu concentra3± tions varying between 0.001 and 10 mol% were prepared in a solid state reaction directly from mixed yttnum and europium oxides as well as from precipitated yttnum and europium oxalates 1361 I J~
,
.
The expenmental laser set-up consisted of a Molectron DL 14P dye laser pumped with a Molectron UV-1000 nitrogen laser. The repetition rate of the laser was 10 Hz. The luminescence was
0(S6) emission upon excitation in the same level for 3 + concentrations at room temperature. For very samples with various Eu low and high concentra5 tions the intensity from the D 0(S6 ) emission was too weak to perform decay measurements. From the integrated decay curves the total de.
.
.
.
.
.
cay rate, [37]: k~01,under steady-state conditions was obtained =
f 0
1 1(t) dt
(1)
D.B.M. Klacssen et aL 5
,,,,-
~ i0
5D
i o~ ~
~D
3
;~‘~~
~--‘.
‘U
c
,/ /7 I ,,“
~
/
.~
/ 5D
-~
..-“
“
curves were exponential, the, total decay rates of these levels were easily obtained. These total de3 + concentration are cay rates given in fig. as 1. a function of Eu The rate for energy transfer from the 5D 0(S6) level to the D 0(C2) level, k56~~2, was obtained from the3~concentrations total decay rate as by the taking the value at radiative decay low Eu rate. This was done by fitting the following expression for the total decay rate to the experirnental data: 1 k~ 0~ + Cx , (2)
1
~-~—‘.-‘
~ 0~2i
~ io
102 10-2
263
perform decay measurements. As all the decay
..--
5D 2
>.
/ Energy transfer processes in yttrium oxide
‘T~ET~d 10-1 100
0(s6)
101 Europium concentration [%)
102
Fig. 1. Total decay rate (at room temperature) of the excited levels of Y 3+ as a function of the Eu3+ concentration. 203 : Eu 5D Diamonds indicate the results for the 5D0(S6) level; downward-directed triangles indicate 5D the 0(C2) level , and 5D 5D level; squares indicate upward-directed triangles the 1(C2) the 2(C2) level and circles the 3(C2) level. Dashed lines represent the results from model calculations,
=
where ~±
rad T rad
is the radiative decay time x is the
Eu concentration in mol%, and C and y are constants to be determined in the fit. The values obtained for ‘Trade C and y are given in table 1. Using eq. (2) one obtains for the activator ef1 [4]. ficiency flact = (ktotTi.ad)~’ = (1 + C;adx’~’) This concentration expression is identical tointhat usedoftopermainterpret quenching terms nent-multipole interaction (y 2, 2.7 and 3.3 for
where thetotal normalized decay curve, I( t = 0)1(t)1. is The decay rate of the 5D i.e. 0(S6) 3 ± concentration is given level in fig.as1. a function of Eu For the Eu3 ± ions at C 2 sites the 5D 5D 5Dwe measured 5D decay curves of the 0, 1, 2 and 3 emission upon excitation in the same level for samples 3 ± concentrations at room temperwith Eu high concentrations the intensity ature.various For the 5D from the 3 emission again was too weak to =
=
dip.—dip., dip.—quad. and quad.—quad. interaction, respectively; see refs. [29—31]and references cited therein), The results the fit forwhile k10~ are indicated in fig. 1 by the of dashed line, the results for k 56 -. ~, (i.e. Cx”) are indicated by the solid line in fig. 5.
Table 1 3 + in Y Experimental and theoretical radiative decay times for the excited levels of Eu 203 are given in the first two columns. In the last three columns 5D~, the+ rate CxT,for where non-radiative x is the Eu3+ decayconcentration to the next in lower mol% level at the C2 site is given; the rate can be obtained from knon.rad kPD~-. Level Radiative decay time Non-radiative decay rate This work (ms) 5D 5D3(C2) 2(C2) 5D 1(C2) 5D 0(C2)
5D a) b)
Theory (ms)
kPD~-. ~1)J
C
(S1)
(~_1)
3.0 0.69
1.35 b) 1.13 1.15 b) a)
1.1
1.02 1.03
a) b)
4 1.12x i04 1.66x10 3 9.62x i0
0.96
1.08 0.96
I,) a)
—
5.14x i03 1.52x104
0.970 0.527
4.94x103
0.744
4
4,47
1.28x10
0(s6) 6.9 9.27 b) — 51.7 Taken from ref. [7]. Calculated in our laboratory by KJ.B.M. Nieuwesteeg [38], whose contribution is gratefully acknowledged.
1.42
264
D.B.M. Klaassen et aL
/ Energy transfer processes in yttrium oxide from rates for non-radiative decay, via the relations
10~
>,
~
_.5D,,T~D1’i
(4a)
and I~ ~
~~.5D0T5D,hi.
(4b)
=
100
-
..
.7-—.y.
10~
-
102
-
and
I~ k~1~2 ~.5D0T5D,I2 + k5D
~.,5D,T5D2I2
=
~
k5D..5D
—
k5D
The observed relative integrated intensities of
-~
5D 5D 35D 5D 5D ± concentration. The radiative de0, 1, 2 and 3given levelslevel site) for excitation of the 3(C2) ~at 4the 1~,I~ and I~, respectively) are in(allfig. as C2 a function of Eu cay time of the 5D 3(C2) level, TSD3, can now be separated from rates for non-radiative decay, k5D~.~D, k5D,.SD and k5D~.,sD 0, via the relations
the
3
io
10-2
10.1
10°
101
-
Europium concentration [%]
3
10
Fig. 2. Relative 3’ under integrated selective intensities laser excitation from the of excited the 5D levels of Y203:Eu 3’ concentration. Downward-di1(C2) level astriangles rected a function indicate of the the Eu 5D 0(C2) level and upward-directed triangles the °D1(C2) level. Dashed lines represent the results from model calculations.
— —
k5D
_,5DT5D.’3
3
3
+ k5D_,5DT5DI2
-
+ k50
(5a)
_*5DT~DIJ~~
I~ k~1~ .5D,T5D~I3+ k5D =
_.~D1T5D2I2
—
(Sb)
k~D~5D0TsD1II,
101
To separate the total 3~ion at decay the C rates of the excited levels of the Eu 2 site into and non-radiative rates we measured the radiative emission spectra after selective excitation of each of these levels. The relative integrated intensities (i.e. their sum equals unity) of the 5D 5D 1(C2) 5D 0(C2) and levels for excitation of the 3 (C2) level (I~and I~,respectively) are given fig. 2 asdecay a function 3~concentration, The in radiative time of of Eu the 5D 1(C2) level, 5D and the rate for non-radia5D tive decay from the 3(C2) level to the 0(C2) 5D level, k5D -. 0’ can be obtained via the relation ~
5D (C ~
100
-~
1
~‘
5D ~
10
2
10-2
3
1~=k5D~D0T5D,1~.
(3)
The observed relative integrated intensities of the 5D 5D and 5D levels (all at the C 2 site) for 5D excitation of the 2(C2) level (Id, I~ and j2 respectively) are given in radiative fig. 3 as decay a function 3 + concentration. The time of Eu the 5D 1’~D 2(C2) level, 2, can now be separated -
.
1 .AA
i0
101
~0
101
Europium concentration [%]
Fig. 3. Relative integrated intensities from the excited levels of 3~under selective laser excitation of the 5D Y203: Eu 2(C2) 3 concentration. Downward-directed indicate the D level as tnangles a function of the 5Di (C Eu 5D 0(C2) level and upward-directed Dashed triangleslines the represent 2) level; indicate the calcula2(C2) level. the squares results from model tions.
D.B.M. Klaassen et a!. ~
101
c o
10
~
10’
/ Energy transfer processes in yttrium
oxide
265
tamed in the fit, are given in fig. 1 as a function of the Eu3 + concentration; the calculated relative integrated intensities are given in figs. 2, 3 and 4; ~Do (C
2)
-
.-
-
5D 1
-
~
• 10.2
5D
.~
~“.--
•~ ~--~
“
-
10-2
10-1
100
Europium concentration [%]
101 _____
Fig. 4. Relative Y 3~under integrated selective intensities laser excitation from the of excited the °D levels of 3~concentration. Downward-di203: Eu 3(C2) level astriangles rected a function indicate of the the Eu 5D 5D 0(C2) level and upward-directed triangles the 5D 1(C2) level; squares indicate the °D2(C2) level and circles the 3(C2) level. Dashed lines represent the results from model calculations.
greater is the experimental uncertainty in the radiative decay time, as the number of experimental data is smaller: r503 is contained in eq. (5),
I
and =
and rates the calculated and of transfer are shownnon-radiative in fig. 5 as adecay function the 3 + concentration. Eu The radiative decay times of the excited levels of Eu3~in Y 203 can also be predicted using the Judd—Ofelt theory (see e.g. ref. [38] and reference cited therein). The resulting radiative decay times, using the E 1, ~ and Q, parameters listed by Weber [7,39], are given in table 1. Comparing the Judd—Ofelt predictions and the experimental values obtained in the present investigation, it can be concluded that reasonable agreement has been obtained, especially for the lower levels. The difference 5D observed between experiment and theory fig. 1). for the 0(S6) level3~concentrations may be due to (see the lack of data at very low Eu It should be noted that the higher the level, the
—k5D2~5D0T5D2I~. (Sc) We fitted eqs. (3—5) simultaneously to the experimental data, using =
k~on,rad+
5D
5D 3-~ 2,
5D
io~
k5D3~5D2T5D3I~ — k5D2~5D,TsD2I~
knon,rad
106
Cx”’,
5D 2—
_/
1__—V,--’
,-.
-•—•—-—-——‘
,_.
5D
‘iii io~ C
~ io3
(6)
where x is the Eu3 ± concentration in mol% and C and y are constants to be determined in the fit (see also eq. (2)). In order to allow for concentration-dependent effects on the decay rate of the 5D 0(C2) level, the radiative decay rate, l/TSD0, is replaced by an expression similar to eq. (2). 1’his implies that all I~in eqs. (3—5) should be multiplied by by (Cx~’rsn0+ The best overall obtained allowing1).non-zero rates only fit for was the transitions the values next-lower level, i.e. fitonly 5D~, *to 0. The obtained in the for k50 the radiative decay times, T5D, and for kPD1 ‘13J i’ C and ~ are summarized in table 1. The total decay rates, calculated using the constants ob-.
-.
102
7FJ (C 2) 101 10-2
I
101
~
~
10°
101
Europium concentration [%]
102
a function of the between Eu3~concentration. Fig. 5. Rates for 3~as non-radiative transfer the excited levels of Y203 : Eu The dashed line indicates the rate from the 5D 3(C2) level to 5D the 2(C2) level; the chain-dotted line indicates the rate from the 5D 5D 5D 2(C2) level to the 1(C2) level; the chain-dashed line indicates the rate from the °D1(C2)level to the 0(C2) level; the full line indicates the rate from the °D0 (S6) level to 5D 5D the 0(C2) level and the7F~(C dotted line the rate from the 0(C2) level to the 2)ground-state multiplet.
266
‘
D.B.M. Klaassen et aL
•
65.
/ 5D
:
2 ..
>‘
D
~0
/ Energy transfer processes in yttrium oxide
2
/
•_•_• •
/
A
50
150
/
.
/
~
5D3/
/D1
~“
I
I
250
350
450
Temperature [K]
Once we have established that the excited levels of the Eu3~ion at the C2 site decay non-radiatively only the next level,ofwea can try to interpret thetodecay rateslower in terms multiphonon decay process [40]. This 5D was done until [7]. now 5D 3~by Weber only formeasured the decay from the Eu 1(C2)rates level of to the 0(C2) level of Y203 :0.1% We the total decay the 5D 5D 5D 3~as a function of sample (see fig. 1(C2), 2(C2) and 3(C2)temperature level of Y203 :0.5% Eu 6). The Eu3 chosen as high as possible, but± concentration low enough towas omit the concentra-
•
S
.4 0 7
:~
..
4. Decay rates as a function of sample temperature
550
~
Fig. 6. Total decay rates of the excited C levels ofindicate Y203 :0.5% 3 ± as a function of temperature. 2Squares the 5D 5D Eu 3(C2) 2(C2) level and triangles 5D level; dots indicate the the 1(C2) level. Solid lines represent the results from fits t theory (see text). 5D, is 2 is contained in eqs. (4) and (5), and T contained in eqs. (35) From the values of C and y, given in table 1 for the energy transfer from the S 6 site to the C2 site, a critical transfer distance of 8.1 A is found. This is in reasonable agreement 3~[10,11] andwith Gd published 3~ values for Y203: Eu 203 : Euof [15]. As far as the concentration dependence ‘T5D
the energy transfer from the S6 site to the C2 site is concerned (‘y 1.42), it should be noted that all transfer rates obtained by integration of theoretical decay curves for various types of interaction (see e.g. ref. [37]) yield about the same concentration dependence in the region of the critical concentration. For the non-radiative decay between the excited levels of the C2 site, the main conclusion is that to non-radiative decay from levels occurs only the next-lower level. Thethese concentration-de=
pendent term in these rates is attributed in the literature to cross-relaxation [15,21,31]. Further investigations on the concentration-independent part of the decay rates between the excited levels of the C2 site will be presented in the next section.
tion-dependent term (see previous section). Using the expression for the temperature-dependent multiphonon transition rate given by Riseberg and Moos [40], we fitted the experimental data: the following expression to kbot 5DJ 5D~~~”_+kTD° ‘Ei~T
(
exp(hw/kT) exp(hco/kT)—11~“ (7)
where ph~iis the energy gap to the next lower level and p is the number of optical phonons produced. In the fit we used the calculated values given in table 1 for the radiative decay times. A fit using the radiative decay rates obtained in this investigation should results, as the radiative decayyield ratealmost is onlythe 10%same of the total decay rate. The values for k~ 1~,5D,p and hw obtained in the fit are summarized in table 2. The total decay rates calculated with these constants are indicated by solid lines in fig. 6. The best fits were obtained by allowing for non-integer values for p. Fits using integer values for p yielded values for that were about 8% smaller Table 2 Values for the parameters in eq. (7) describing the3 +rate for at the non-radiative decay between the excited levels of Eu C 2 site of Y203 Transition p k~?~0~ 1) m~ (s 5D 5D 2 5.3 544 8980 5D3 —~ 5D 4,8 515 15370 5D 2 —~ 5D 1 1 -. 0 3.8 462 7520
D.B. M. Klaassen et a!.
/ Energy transfer processes in yttrium oxide
for the 5D1 ~D0 and 5D2 5D1 transition, and 5D 5D 6% larger for the 3 2 transition. These values are well within the experimental accuracy. The infrared absorption spectrum of Y2031 shows lattice at 468 561 400 cm”to combined withabsorptions a broad band fromand below well above 500 cm1 141]. Therefore the phonons produced in the relaxation process are easily absorbed by the lattice (see also ref. [7]). According to the simple theory of Riseberg and Moos [40] the multiphonon transition rates k~,?.5D 1, depend exponentially on the energy gap t~E=phw: —~
106
-~
267
~
—~
~
=
/3 e~~
-
a)
~ i0~ Z
•“....
:
-.
(8)
where /3 and a are constant for a particular host lattice. In fig. 7 we have reproduced of 3” the [40]results and the Riseberg Moos results of and Weber for for Y Y203 3: Er + and Y 3± 203 : Er 203 : Eu [7]. Taking into account the uncertainties mentioned above, our results agree quite well with those by Weber. Theobtained multiphonon transition rates for Y 203on: Er3 + show indeed an exponential dependence the energy gap between the levels involved in the transition. For Y 3 however, these rates 203 : Eu show a quite different behaviour as a function of the energy gap. As already pointed out by Weber [7], the deviation observed for the 5D 5D~transition can be explained by the fact1 that for this “,
—~
transition in The first-order the multiphonon emission rate is zero. discrepancy between our result for the 5D 5D 3 2 transition and the 3~found exponential by behaviour of covered the results Y203 range : Er in energy Weber, who thefor larger
102 1500
I
2000
I
I
2500 3000 - E~[cm~]
E,
3500
Fig. 7. Rates for non-radiative decay between excited levels of Er3~and Eu3±in Y 203 as a function of the energy difference ~E = E, — E, between the initial (i) and final (f) level, Open squares represent the results of Riseberg andofMoos for 3~ open circles represent the results Weber[40] [7] for Y203 Er3”; solid circles represent the results of Weber [7] for Y2O3 : Er 3 + and solid squares represent the results from the Y 2O3 : Euinvestigation for Y present ~. The upward and downward arrows indicate respectively3that only a lower and an Eu upper limit were obtained.203: Dashed lines (for the results of Weber [7]) and dotted lines (for the results of Riseberg and Moos [40]) represent the non-radiative rate following the energy gap law using the parameters as deduced by van Dijk [42] from these results. The cross indicates the ‘normalized’ result (from the present investigation) for the 5D 5D 3” (see text).3 —* 2 transition of Y203 : Eu
—~
gaps, is quite distinct. A more accurate theory developed by van Dijk and Schuurmans [42—44] takes into account the selection rules neglected by Riseberg and Moos [40]. A calculation using this theory (see e.g. eq. (2) of ref. [38]) yielded k~°502~
—E502)
kT~°5De~02~”~
=
(9)
The different values of a for the results of Weber and Riseberg and Moos [42] yield only a difference of 5% for the expression given in eq. (9). Therefore the averaged value is given. If we divide
our experimentally determined rate for the 5D 3 ~D~transition 5D by this5Dfactor, the 5D ratio 5D between the rates for the 2 1 and 3 2 transitions is in perfect agreement with the exponential energy-gap law (see fig. 7) using for a the value of Weber’s data. The absolute values for these transition rates calculated from eq. (2) of ref. [38] are a factor of 3.7 smaller than the experimental values. —~
—*
—
The theory used, however, predicts the ratios of non-radiative rates much more accurately than the absolute values. It may only be concluded that the introduction by Nieuwesteeg [38] of an extra factor of 6 with respect to the original derivation of the non-radiative rate by Schuurmans and van
268
D.B.M. K!aassen et aL
/ Energy transfer processes in yttrium oxide
Dijk [44], is certainly justified as far as Y203 is concerned. Summarizing the results of this section, we have given experimental evidence for the multiphonon character of the non-radiative 3~in Y decay process of the excited levels of Eu obtamed are in good agreement203. withThe bothresults published experiments and theory.
5. Experiments under cathode-ray excitation 5.1. Low excitation density Once we have quantified the rates for nonradiative decay between the excited levels of the Eu3~ion at the C 2 site,3”asions wellatasthe theS rate for energy 6 site to ions at transfer the C from Eu 2 site, we can use these rates for the interpretation of the cathodoluminescence spectra. We measured spectra of 3 underthe low luminescence excitation density in a deY203 : Eu mountable cathode-ray tube at room temperature. In fig. 8 the relative integrated intensities from the 3~ emitting levels are given as a function of the Eu ±
concentration. Over the whole 5Dconcentration region the emission from the 0(C2) level constitutes more than 85% of the total emitted intensity. To describe the energy flow in the phosphor we 3t use the phosphors model developed initially for Tbare activated [32—34].The activators excited into the charge transfer state of Eu3~at a rate aj, which is proportional to the excitation density. Upon excitation rapid, radiationless relaxation of energy takes place with rate y~ from the charge transfer state directly into the emitting 5D~levels. At low excitation densities the following relations are then obtained from the rate equations: (i (i
+ k5D~5DT5D)I5D 5D T5D )I~D =f5D3ac2Nc2, — k5D -‘5D T5D + k5D ~ 2
2
(1 +
ksD,
~D 1T5D,
=
2
3
2
3
3
(lob)
f3D2aC,NC,,
=
f3 2)
—
k~
)
‘~D1 —
k5D2
5D,T5D2’~D2
N~
a
1~D(C D, C2
—
t
I
(lOa)
ISD
(lOc)
2’
ksD, _.5D0T5DI5D, C2T5DO(SÔ)I5DO(S6)
=f5D0(c2)ac,Nc2, (lOd)
101 5D
and (1 + ks
0(C2)
10°
V
5D •~ 10~
0(S6)
-
- - -.
10 -2
.
~
~-
•
•
•
S.-.
-
•‘‘‘~‘.~.
102 I
101 I
10° I
Europium concentration [%]
=
(lOe)
asNs.
Here ‘~D are the integrated intensities under cathode-ray excitation; T5DJ and k5D~.,sD~ are the radiative decay times and rates for non-radiative decay determined in sect. 3 and given in table 3± 1; N~ and N~ are the concentrations of Eu ions at the C2 and S6 sites, respectively;7F~multiac2 and plet, which may for be different Eu3~ions at the as are the rates excitationfor from the C 2 and S6 sites; and is the branching ratio for 5D,: feeding of level f5 D (11) ‘
- - -.,
10~
~cT5D(s))I5D(s)
101
~
Fig. 8. Eu3” Relative integrated intensities the excited levels of under stationary 20 keV from cathode-ray excitation as a function of the Eu3 + concentration, Diamonds indicate the results for the 5D 5D 0(S6) level; downward-directed triangles mdicate the 5D 0(C2) level and upward-directed 5D triangles the 1(C2) 5D level’ squares indicate the 2 (C2) level and circles the 3(C2) level. Dashed lines represent the results from model calculations,
=
.
Fitting eq. (10) to the the measured relative 3 integrated intensities as a function of the Eu +
D.B.M. K!aassen et aL / Energy transfer processes in yttrium oxide
concentration, the following values for the branching ratios were obtained: f~D 5 f~~ 26 f~D3=0~38 2°° 1=O. and f~D 0(C2) 0.31. =
1.25 >‘ U
a) -0C
0.75
56/Nc2
equal to the most probable value 0.33, we find as 6/ac2 equal to 0.168. Summarizing this subsection, we have obtained the branching ratios for feeding of the emitting levels under cathode-ray excitation. It5Dshould be noted that the branching ratio for the 2 level is quite low, whereas the others are more or less equal. Secondly, the relative occupation of S6 and C2 sites is independent of the activator concentration. Although some statements are made about this occupation in the literature [11,15], this is the first experimental evidence. 5.2. High excitation density
At high excitation density the efficiency of phosphors decreases. In the ultimate limit the ground state is depleted: all the activators are in the excited states and it takes a period of one decay time before they can be excited again [2,3]. However, when the occupation of the excited states increases, interaction between activators which are both inloss an excited state can give rise to additional energy processes [4,32,33]. We measured the efficiency as a function of current density under 20 keY CR excitation for the emitting C 3”’ sample 2 levels of a Y203 : 1% Eu at room temperature. These results are given in fig. 9. From this figure it is clear to see that the efficiency of the 5D 0 level starts to decrease at a much lower current density than the efficiencies of the other levels (see also table 3). In combination with the large differences in total decay rates of the emitting levels (see fig. 1) this leads, as argued in the appendix, to the conclusion that at high
5D 5D ~.‘:-. 2 1 N~.
a a)
0.50
.~
Moreover, it turned out (see fig. 8) that the ratio as6Ns6/ac2Nc2 is equal This to 0.056, independent of 3’ concentration. implies that the rela the Eu tive occupation of ~6 and C 3” ions is 2 sites by Eu investiconstant over the concentration region gated. Taking the ratio of concentrations N
269
(6
~
0.25
C
0.00
-_________________________________ iO~
106
i0~
i0~
102
Current density [Acm2] Fig. 9. Experimental relative efficiency of the excited C2excitalevels 3” under stationary 20 keV cathode-ray of Y2O3 :1% Eu tion5D as a function of the current density. The full line indicates 5D 5D the (C2)dashed level;5Dthe the and1 (C2) level; 0the line chain-dotted indicates theline indicates 2(C2) level the dotted line the 3(C2) level, Shown are curves given by = (1+ (1/150% )~ — I, where j is the current density, and 150% and c were obtained from a fit of this formula to the experimental data.
current densities excitation from all emitting levels takes place. In order to give an accurate description at high excitation densities, eq. (10) has to be modified. For the case of excitation from the excited states, ac2 T5D~‘~D~ have to be added to the first term on the left-hand side. The relative efficiencies calculated using these modified relations are given in fig. 10. Comparison of figs. 9 and 10 shows a fair qualitative agreement between experiment and calculations. Also the (relative) current densities at which the efficiency is halved, J50%~are predicted in the right order for the different levels (see table 3L 3± we studied the saturation of Y203: Eu 11 as Next a function of activator concentration. In fig. the experimental values obtained for 150% of the Table 3 Experimental and calculated current densities at which the efficiency is halved ~ for a Y 3 + sample rela203 :1% Eu 5D tive to the value for the 3 level Level Experimental Calculated 5D 1 1 5D3 2 0.9 0.6 5D 1 5j)~
0.5 0.04
0.5 0.05
D. B. M. KIaa.ssen et a!.
270
/ Energy transfer processes inyttrium
1.25
oxide
1500
~i::o
:iO75~\\5D3
101
102
b-7
3
io
±
lation between 150% and the activator concentration. We note that these relations only contain terms that are linear in the occupation of the 3 ± ions at Sdifferent levels. Energy transfer from Eu 6 sites to those at C2 sites, as well as non-radia3” ions at tive decay between different levels of Eu the C2 site, both give rise to terms that contain a product of occupations. However, the S6 C2 ~~__—
0.2 0.1
~
1 Europium 2 concentration 3 5 [%l 7 — 10 Fig. 11. The current density, at which the relative efficiency has decreased to 0.5 ~ for the 5D 3~ as a function of the Eu concentration. 0(C2) The level error of Y2O3: bars Eu mdicate the errors as obtained from a fit of the experimental points to an analytical expression (see text). 0.3
0.5
0.7
io-~
io-~
iO-~
10-2
Current density [Acm-2]
5D 3 0 level are shown as a function of the Eu concentration. The current density 15Q% increased by less than a factor of three, while the Eu3 + concentration increased by a factor of twenty. Calculations using the modified equations mentioned above yielded, however, a proportional re-
00
io-6
Currentdensity [arb.un.[ —s’
Fig. 10. Theoretical relative efficiency of the excited C 3” under stationary cathode-ray excitation 2 levels as a of Y203:1% function of the Eu current density. The full line indicates the 5D 5D 0(C2) theline chain-dotted line 5D indicates the 1(C2) level; the level; dashed indicates the 5D 2 (C2) level and the dotted line the 3(C2) level.
03
‘~
10-
—~-
5D Fig. 12. Initial decay time of3” the as cathodoluminescence a function of the from current the density 0(C2)forlevel various of Y203: Eu3 + concentrations. Eu Solid circles represent 3” concentration of 0.03%, open circles 0.3%, open squares a Eu 1%, solid squares 5%, and open triangles 30%.
5D transfer can contribute the 0(C2) luminescence signal (seeonly sect.5.6% 5.1), to whereas crossrelaxation between the Eu3 levels only becomes important at very high concentrations. Therefore it can be concluded that additional energy loss processes give rise to the sublinear increase with the Eu3 + concentration of 150% for the 5D 0(C2) level. At high excitation density such additional energy processes to afrom decrease the initialloss decay time ofgive the rise levels, whichofthese ±
processes take decay times of place the 5D[4]. The experimental initial 0(C2) level are given in fig. 12 3 as a function of For current density for various + concentrations. all activator concentraEu tions the initial decay time is constant up to very high current densities. This leads to the conclusion that no interactions occur between Eu3~ions in the 5D 0(C2) level. As about 70% of the energy from the 5D 3” ionemitted at higher levels, energy 0(C2)loss level processes enters the in these Eu higher levels may be responsible for the saturation behaviour of the 5D 0(C2) level. To investigate this possibility, we 5D 3” sample as a function of current measured the initial decay time of the 3 level of a Y203 : 1% density. TheEu results are shown in fig. 13. At high current densities a clear decrease of the initial decay time is observed to about 50% of its value at .
‘
.
.
low current density. Consequently it can be concluded that an additional energy loss process emp-
D.B.M. Klaassen et aL / Energy transfer processes in yttrium oxide
consistent with sophisticated theory on non-radiative decay rates. From cathodoluminescence spectra at low excitation density the branching ratios for feeding of all the emitting levels were obtained. Moreover the experiments showed that the occupation of ~6 and 3”’ conC centration. 2 sites in Y203 The product is independent of excitation of the Eu rate and
100 80
~
T
60
-
40
-
20
-
1O-~
s”
- -~,,
“f
10-6
~-5
-~
~o-~
~
2]
271
_~
Current density [Acm Fig. 13. Initial decay time of the cathodoluminescence from the 5D 3” as a function of the current 3(C2) Typical level of errors Y203:1% density. are Eu indicated. The dashed line is for visual aid only.
concentration was found to be a factor of 18 smaller for the S sitethethan for the as C2a site. Measurements6 of efficiency function of cathode-ray excitation density led to the conclusion that at high excitation densities excitation takes place from all the emitting levels. Furthermore at moderate activator concentrations these measurements were quite well described by model calculations using the experimentally obtained information on the energy transfer and non-radia.
.
.
.
.
ties the 5D 3 level at high current densities. This process of the 5Dcertainly affects the saturation behaviour 0(C2) level. Additional measurements of the efficiency and initial decay time of all levels at several activator concentrations and as a function of current necessary to processes. establish whether theredensity are anyare other energy loss Moreover, the method of calculation presented in this section no longer suffices. The higher-order processes have to be included, which leads to a set of nonlinear equations.
tive decay. At high 3activator concentrations inter± ions both in high excited actions between Eu 5D levels (e.g. 3), cause additional energy losses. In conclusion it can be stated that we have developed3” awhich, modelin for the energy in combination with flow the exY203 : Eu perimentally determined rates and branching ratios for the feeding, describes all experimental data such as spectra and efficiencies at both low and high excitation densities in far detail.
Appendix 6. Conclusions In the present investigation the energy transfer processes in the phosphor Y 203 using activated with 3 + were extensively studied dye laser Eu spectroscopy. The rates for energy transfer from Eu3” ions at S 3” ions at C 6 sites to Eu 2 sites, as well as non-radiative decay rates between 3 ± ions at Cthe different energy levels of the Eu 2 sites, 3”’ conwere quantified as a function of lifetimes the Eu of the centration. The resultant radiative levels are in good agreement with theory. The non-radiative decay processes between the different energy levels of the Eu3 ions at C 2 sites were identified as multiphonon relaxation using measurements as a function of the sample temperature. Rates obtained for zero temperature are
In this appendix we will show that excitation from the emitting levels (under cathode-ray excitation) can determined by saturation ments. Thebeprecondition, however, is a measuredifferent total decay rate of the levels, For clarity we consider an activator with a ground level (0) and two emitting levels (1 and 2) in two cases: takes place from the ground level (1) Excitation only, and (2) excitation takes place from all levels with equal rate constants.
+
The excitation takes place to acharge high, transfer fast-decaying 3 and state level (Sd band for Th for Eu3 “-activated phosphors [32]). Using the model (and notation) for the energy flow in the +
D.B.M. K!aassen et a!.
272
/ Energy transfer processes
phosphor host lattice given by de Leeuw and ‘t Hooft [4], the rate equation for the density of
inyttrium oxide
and x
1
electron—hole (e—h) pairs n reads
x2 —
dn
2
g
=
—
a~x,n f3n,
~
(Al)
—
where g is the generation rate for the density of e—h pairs, x, is the concentration of activators in level i, a~ is the rate constant for excitation of activators in level i, and /3 is the rate for energy loss in the phosphor host lattice. The rate equation for the concentration of activators in the charge transfer state y yields 2
y,y,
—
dt
11
T2
a
(A8)
+~
c~
From these relations it can be seen that, independent of the excitation density (or g), the ratio between the total emitted intensities from level 1 (Ii) and from level 2 (‘2) is constant and equal to I~ f~+ k21T2 —
7
f2
—
‘
In other words, the efficiency is the same function of excitation density for both emitting levels. Consequently, the current density at which the ef-
2
V a~x1n
=
=
k2~1T2
T1
‘-~
(A2)
i~1
where ‘I’, is the feeding rate of level i. Under stationary conditions eqs. (Al) and (A2) yield
ficiency is halved is also the same for both levels. Case 2. a~=a~_—aE,=a”=/~O Using the following expression as a shorthand a~g
n
=
g ~ cx~x,+ /3 2
(A3)
,
i=0
(AlO) substitution of eqs. (A3) and (A4) into eq. (AS) yields for steady-state conditions a
acN + /3’
=
and
f2aN
I y
=
2
~y, i=1
(All)
‘
—+k21+a T2
~=o
2
1
x2=
2
(A4)
‘
The efficiency of level 2,
~a~x,+/3
~2’
is now given
by
I
i=0
—
The rate equations for the concentrations of activators in the two emitting levels, x1 and x2, read
~l2 =
dx2
From this equation it can be seen that the efficiency of level 2 is halved at a current density at which
x2
—~-~-- =
‘~2y
—
—
k2,1x2
—
a~nx2,
(AS)
1 +a
1+k
and a=
dx1 dt
(A12)
‘
+ k2 ~
—
+k2~1x2—a~nx1.
— =
(A6)
The following expression for the concentration of activators in level 1 is obtained by substitution of eqs. (A3) and (A4) into eq. (A6):
~
Case]. a’~=a~O,’a~=a~=O
Under stationary conditions the following expressions are obtained from eqs. (AS) and (A6): x2
(1 +k2.1T2)—
acxog =f2c+$
(Al3)
21. T2
(A7)
f1aN X1
=
1 —+a
+
k25f2aN / 1 \ (—-i-a)
\
T1
/
(
1
—+k2 T2
1+a
) (A14)
D.B. M, K!aassen et aL
/ Energy transferprocesses
The general expression for the efficiency of level 1 is quite complicated. Therefore we will consider here only two limiting cases: k2 -.1 = 0 and k2 -.1 >> 1/’r2. If k2 = 0 from eq. (14) it follows that the efficiency of level 1 is halved at a current density at which ,,~
1
(A15)
a=—.
If k2.~1>> given by
1/’r2,
~
+
the efficiency of level 1,
aft)
~,
is
1 T1
T1
in yttrium oxide
273
[8] H. Forest and 0. Ban, J. Electrochem. Soc. 116 (1969) [9] J. Heber, K.H. Hellwege, U. KObler and H. Murmann, Z. Physik 237 (1970) 189. [10] U. Köbler, Z. Physik 250 (1972) 217. [11] J. Heber and U. Köbler, in: Luminescence of Crystals, Molecules and Solids, ed. F. Williams (Plenum, New York, 1973) p. 379. [12] S. Qiang, C. Barthou, J.P. Dems, F. Pelle and B. Blanzat, J. Lumin. 28 (1983) 1. [13] R.G. Pappalardo and R.B. Hunt Jr.. J. Electrochem. Soc. 132 (1985) 721. [14] R.B. Hunt Jr. and R.G. Pappalardo, J. Lumin. 34 (1985) 133. [15] M. 9’ Buijs, A. Meyerink and G. Blasse, J. Lumin. 37 (1987)
(A16)
[16] N.C. Chang, J. AppI. Phys. 34 (1963) 3500. [17] NC. Chang and J.B, Gruber, J. Chem. Phys. 41 (1964) 3227. [18] R.A. Buchanan, K.A. Wickersheim, J.L. Weaver and E.E.
Consequently, the efficiency of level 1 is halved at a current density for which
Anderson, J. Appl. Phys. 39 (1968) 4342. [19] R.P. Leavitt, J.B. Gruber, NC. Chang and CA. Morrison, J. Chem. Phys. 76 (1982) 4775.
=
\T1
a
+
1 ~<
—,
a~(k2~1 + a) /
1
-i-a
(A17)
Generalizing the results obtained in this appenclix, we can state that, if the emitting levels have distinctly different total decay rates, excitation from the emitting levels can be determined by saturation measurements. If the total decay rates of the levels are different, one can conclude that: excitation takes place from the ground level only if the efficiency of both levels is halved at the same excitation density; excitation takes place from all levels if the efficiency of the levels is halved at different excitation densities. —
—
References [1] D.B.M. Klaassen, T.G.M. van Rijn and AT. Vink, J. Electrochem, Soc., to be published. [2] A. Bed, Physica 15 (1949) 361. [3] A. Br! and F.A. Kroger, Philips Techn. Rev, 12 (1950) 120. [4] D.M. de Leeuw and 0W. ‘t Hooft, J. Lumin. 28 (1983) 275. [5] iD, Axe and P.F. Weller, J, Chem. Phys. 40 (1964) 3066. [6] A. Bril, W.C. Nieuwpoort, W.L. Wanmaker, G. Blasse and C.DJ.C. de Laat, Proc. ICL (1966), Vol. 2, ed. G. Szigetti (Akad. Kiado, Budapest, 1968) p. 1689. [7] M.J. Weber, Phys. Rev. 171 (1968) 283.
[20] C.A. Morrison, R.P. Leavitt, J.B. Gruber and NC, Chang, J. Chem. Phys. 79 (1983) 4758. [21] AT. Rhys Williams and M.J. Fuller, Computer Enhanced Spectrosc. 1 (1983) 145. Zolin, A.M. Malova, V.M. Markushev and V.1. Tsaryuk, J. Appl. Spectrosc. 41(1984)1360. [23] V.M. Markushev, VI. Tsaryuk and V.F. Zolin, Opt. Spectrosc. 58 (1985) 356. [24] J.B. Gruber, R.P. Leavitt, C.A. Morrison and N,C. Chang.
[221V.F.
Phys. 82 (1985) [25] J. U.Chem. Köbler, Z. Physik 247 5373. (1971) 289. [26] L.G. van Uitert, E.F. Dearborn and J.J. Rubin, J. Chem. Phys. 46 (1967) 420. [27] N. Yamada, S. Shionoya and T. Kushida, J. Phys. Soc. Japan 32 (1972) 1577. [28] T. KimJ. Lumin. Ahn, T.39Ngoc, Thu Nga, V.T. Bich and P. Long, (1988)P.215. [29] L.G. van Uitert and L.F, Johnson, J. Chem. Phys. 44 (1966) 3514. [30] L. Ozawa, H. Forest, P.M. Jaffe and G. Ban, J, Electrochem. Soc. 118 (1971) 482. [31] L. Ozawa and P,M. Jaffe, J, Electrochem, Soc. 118 (1971) 1678. [32] W.F. van der Weg and M.W. van Tol, AppI. Phys. Lett, 38 (1981) 705. [33] W.F. van der Weg, J.M. Robertson, W,K. Zwicker and Th.J. Popma, J. Lumin. 24/25 (1981) 633. [34] W.F, van der Weg, Th.J. Popma and A.T. Vink, J. AppI. Phys. 57 (1985) 5450, [35] D.B.M. Klaassen, R.A.M. van Ham and T.G.M. van Rijn, J. Lumin. 40&41 (1988) 651. [36] The authors are indebted to J.M.E. Baken and H.T. Hmtzen (Philips Res, Lab.), to iL. van Koesveld (Philips TCDS), and to Philips Lighting for supplying the phosphor samples.
274
D.B.M. K!aassen et aL
/
Energy transfer processes in yttrium oxide
[37] B. di Bartolo, in: Energy Transfer Processes in Condensed Matter, ed. B. di Bartolo (Plenum, New York, 1984) ch. 2. [38] K.J.B.M. Nieuwesteeg, Philips J. Res. 44, Nos. 2/3 (1989). [39] G.S, Ofelt, J. Chem. Phys. 38 (1963) 2171. [40] L.A. Riseberg and H.W. Moos, Phys. Rev. 174 (1968) 429. [41] NT. McDevitt and A,D. Davidson, J, Opt. Soc. Am. 56 (1966) 636.
[42] J.M.F. van Dijk, J. Lumin, 24/25 (1981) 705. [43] J.M,F. van Dijk and M.F.H. Schuurmans, J. Chem. Phys. 78 (1983) 5317. [44] M,F.H. Schuurmans and J.M.F. van Dijk, Physica B 123 (1984) 131.
261
ENERGY TRANSFER PROCESSES IN YTUR1UM OXIDE ACI1VATED WITH EUROPIUM D.B.M. KLAASSEN
~,
R.A.M. van HAM
2
and T.G.M. van RuN
Philips Research Laboratories, P0 Box 80000, 5600 JA Eindhouen, The Netherlands Received 6 January 1989 Revised 3 April 1989 Accepted 14 April 1989
Energy transfer and non-radiative decay between excited states of europium in yttrium oxide are studied with dye-laser spectroscopy as a function of the europium concentration and the sample temperature. Combining the results with luminescence spectra measured under cathode-ray excitation, relative feeding rates for the excited states are obtained. These transfer, decay and feeding rates are used to interpret measurements on the sublinearity of the luminescence as a function of cathode-ray excitation density.
1. Introduction
Indications of complicating factors are the additional feeding process of the 5D0(C2) level:
The external radiant efficiency of phosphors under cathode-ray (CR) excitation decreases with increasing excitation density. television, For high density applications such as projection the linear-
via 5D~ (C a level-by-level cascade from the higher 2) levels [5—7]; 3 + ions at a differenergy transfer entvia crystallographic (S from Eu 6) site [8—15]. Consequently the method of the Leeuw and ‘t Hooft [4] cannot be3~applied. ions at Besides the C higher excited states Eu ± ions at S 2 sites, the excited statesofof Eu3 6 sites also have to be taken into account. 3 + ion in Y The luminescence of the Eu 203 has been widely and extensively investigated. 3~ions at CStudies relate to the spectra of single Eu 2 sites [16—21]and S6 sites [8,9,13,22—24],and also to the spectral features of ion 5D pairs [11,14,25]. Non5D radiative decay from the 1 to the 0 level of 3 ± ion at the C the Eu 2 site [5—7],energy transfer 3 + ions at sites of differbetween the levels of Euwell as between Eu3 + and ent symmetry [8—15]as other rare-earth ions has been investigated [5,12, 26—28]. Concentration quenching of Y 3~ 203 : Eu has been studied under UV excitation [29—31]and under cathode-ray excitation [18,30]. However, no data have yet been reported on the rates for non-radiative decay from the higher excited 5D 2 5D 3 ±ions at the C 3 + ions and 3 levels. Moreover, the studies on energy transfer 2 site to Eu at the S from Eu 6 site [8—15]did not yield a rate for this
ity of the luminescence intensity as a function of excitation density is a major selection criterion for the phosphor. At present3~) yttrium activated is theoxide red primary in with europium (Y203 : Eu these applications. The main emission of this phosphor originates from the lowest excited state (5 D0)(C2) of the europium ion. We found 5D (see also ref. [1]) that the sublinearity of the 0(C2) 3~in Y luminescence of Eu 203 cannot be explained by ground state depletion [2,3] only. In the analysis of the sublinearity of the emission of a particular level of the activator, the method proposed by overall the Leeuw andrate ‘t Hooft [4] can be applied if the feeding is (1) greater than the decay rate; (2) greater than the reciprocal excitation time in the application; and (3) constant as a function of activator concentration.
2
Author to whom correspondence should be directed, Present Océ The Research and Development, P0 Box 101, 5900address: MA Venlo, Netherlands.
—
—
0022-2313/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
262
D.B.M. Klaassen et aL
/
Energy transfer processes in yttrium oxide
process as a function of the Eu3 ± concentration, Consequently the feeding rates under cathode-ray excitation of the emitting levels of Eu3 + in Y 203 are not known. Yet knowledge of these feeding rates has proved to be vital for understanding the energy flow in a phosphor as a function of activator concentration or cathode-ray excitation density [32—34]. In the course of a detailed of the 3~, investigation we used dye-laser sublinearity of Y203 : Eu spectroscopy to determine the rates for non-radiative decay and energy transfer between the excited states of the Eu3~ions at the same and at different crystallographic sites, respectively, as a function of the Eu3 ± concentration and the sample temperature. These rates were used together with spectra measured under cathode-ray excitation, to determine the feeding rates of the emitting levels, Transfer, non-radiative and feeding rates were then used to interpret measurements on the sublinearity of the luminescence of Y 3 For clarity we present the results of 203 our : Eu investigations in three separate sections, each containing experimental results and their interpretation. First the dye-laser experiments on a large number of samples with different Eu31 concentration are presented, which yield the rates for energy transfer and non-radiative decay of the excited levels of Eu3 Next, a study of these rates as a function of sample temperature is presented together with a comparison with calculations using the Judd—Ofelt theory. Finally, the measurements under cathoderay excitation and their interpretation are presented. Part of the method and results were published before in a condensed form [35]. ~.
~.
dispersed with a 1 m Jarrell—Ash monochromator and detected with a photomultiplier using pulse counting techniques. The measurements as a function of sample temperature were performed using a Cryogenic Refrigerator System IIB of NMR Technologies. The spectra under CR excitation were measured in a demountable cathode-ray tube. The phosphors were excited withdensity a stationary of 20 keV electrons (current about beam 1 ~sA/ cm2). The luminescence was collected with a fibre and guided to a monochromator (Jarrell—Ash 1208; focal length 0.3 m; 147.5 grooves/mm grating blazed at 500 nm) equipped with a diode-array camera. This camera was part of an optical multichannel analyzer system (OMA-Ill of EG & G PAR; diode-array camera 1420BR). The wavelength indication of the system was calibrated with mercury and neon lamps. The sublinearity of the luminescence from the excited levels of Y 3 ± was measured in a 203 : Eu (Cameca) under stascanning electron microscope tionary CR excitation. The beam current ranged from 10 ~ to 10 6 A, while the spot diameter was varied between 20 and 200 tim; the accelerating voltage was 20 kV. The luminescence was collected from the bombarded side by a mirror systern, transmitted to a 0.75 m Spex monochromator and detected with a photomultiplier using pulse counting techniques. All spectra were corrected for the spectral response of the various detection systems.
3. Transfer and decay rates as a function of Eu3~ concentration We measured the decay curves of the 5D
2. Expenmental The phosphor powders with Eu concentra3± tions varying between 0.001 and 10 mol% were prepared in a solid state reaction directly from mixed yttnum and europium oxides as well as from precipitated yttnum and europium oxalates 1361 I J~
,
.
The expenmental laser set-up consisted of a Molectron DL 14P dye laser pumped with a Molectron UV-1000 nitrogen laser. The repetition rate of the laser was 10 Hz. The luminescence was
0(S6) emission upon excitation in the same level for 3 + concentrations at room temperature. For very samples with various Eu low and high concentra5 tions the intensity from the D 0(S6 ) emission was too weak to perform decay measurements. From the integrated decay curves the total de.
.
.
.
.
.
cay rate, [37]: k~01,under steady-state conditions was obtained =
f 0
1 1(t) dt
(1)
D.B.M. Klacssen et aL 5
,,,,-
~ i0
5D
i o~ ~
~D
3
;~‘~~
~--‘.
‘U
c
,/ /7 I ,,“
~
/
.~
/ 5D
-~
..-“
“
curves were exponential, the, total decay rates of these levels were easily obtained. These total de3 + concentration are cay rates given in fig. as 1. a function of Eu The rate for energy transfer from the 5D 0(S6) level to the D 0(C2) level, k56~~2, was obtained from the3~concentrations total decay rate as by the taking the value at radiative decay low Eu rate. This was done by fitting the following expression for the total decay rate to the experirnental data: 1 k~ 0~ + Cx , (2)
1
~-~—‘.-‘
~ 0~2i
~ io
102 10-2
263
perform decay measurements. As all the decay
..--
5D 2
>.
/ Energy transfer processes in yttrium oxide
‘T~ET~d 10-1 100
0(s6)
101 Europium concentration [%)
102
Fig. 1. Total decay rate (at room temperature) of the excited levels of Y 3+ as a function of the Eu3+ concentration. 203 : Eu 5D Diamonds indicate the results for the 5D0(S6) level; downward-directed triangles indicate 5D the 0(C2) level , and 5D 5D level; squares indicate upward-directed triangles the 1(C2) the 2(C2) level and circles the 3(C2) level. Dashed lines represent the results from model calculations,
=
where ~±
rad T rad
is the radiative decay time x is the
Eu concentration in mol%, and C and y are constants to be determined in the fit. The values obtained for ‘Trade C and y are given in table 1. Using eq. (2) one obtains for the activator ef1 [4]. ficiency flact = (ktotTi.ad)~’ = (1 + C;adx’~’) This concentration expression is identical tointhat usedoftopermainterpret quenching terms nent-multipole interaction (y 2, 2.7 and 3.3 for
where thetotal normalized decay curve, I( t = 0)1(t)1. is The decay rate of the 5D i.e. 0(S6) 3 ± concentration is given level in fig.as1. a function of Eu For the Eu3 ± ions at C 2 sites the 5D 5D 5Dwe measured 5D decay curves of the 0, 1, 2 and 3 emission upon excitation in the same level for samples 3 ± concentrations at room temperwith Eu high concentrations the intensity ature.various For the 5D from the 3 emission again was too weak to =
=
dip.—dip., dip.—quad. and quad.—quad. interaction, respectively; see refs. [29—31]and references cited therein), The results the fit forwhile k10~ are indicated in fig. 1 by the of dashed line, the results for k 56 -. ~, (i.e. Cx”) are indicated by the solid line in fig. 5.
Table 1 3 + in Y Experimental and theoretical radiative decay times for the excited levels of Eu 203 are given in the first two columns. In the last three columns 5D~, the+ rate CxT,for where non-radiative x is the Eu3+ decayconcentration to the next in lower mol% level at the C2 site is given; the rate can be obtained from knon.rad kPD~-. Level Radiative decay time Non-radiative decay rate This work (ms) 5D 5D3(C2) 2(C2) 5D 1(C2) 5D 0(C2)
5D a) b)
Theory (ms)
kPD~-. ~1)J
C
(S1)
(~_1)
3.0 0.69
1.35 b) 1.13 1.15 b) a)
1.1
1.02 1.03
a) b)
4 1.12x i04 1.66x10 3 9.62x i0
0.96
1.08 0.96
I,) a)
—
5.14x i03 1.52x104
0.970 0.527
4.94x103
0.744
4
4,47
1.28x10
0(s6) 6.9 9.27 b) — 51.7 Taken from ref. [7]. Calculated in our laboratory by KJ.B.M. Nieuwesteeg [38], whose contribution is gratefully acknowledged.
1.42
264
D.B.M. Klaassen et aL
/ Energy transfer processes in yttrium oxide from rates for non-radiative decay, via the relations
10~
>,
~
_.5D,,T~D1’i
(4a)
and I~ ~
~~.5D0T5D,hi.
(4b)
=
100
-
..
.7-—.y.
10~
-
102
-
and
I~ k~1~2 ~.5D0T5D,I2 + k5D
~.,5D,T5D2I2
=
~
k5D..5D
—
k5D
The observed relative integrated intensities of
-~
5D 5D 35D 5D 5D ± concentration. The radiative de0, 1, 2 and 3given levelslevel site) for excitation of the 3(C2) ~at 4the 1~,I~ and I~, respectively) are in(allfig. as C2 a function of Eu cay time of the 5D 3(C2) level, TSD3, can now be separated from rates for non-radiative decay, k5D~.~D, k5D,.SD and k5D~.,sD 0, via the relations
the
3
io
10-2
10.1
10°
101
-
Europium concentration [%]
3
10
Fig. 2. Relative 3’ under integrated selective intensities laser excitation from the of excited the 5D levels of Y203:Eu 3’ concentration. Downward-di1(C2) level astriangles rected a function indicate of the the Eu 5D 0(C2) level and upward-directed triangles the °D1(C2) level. Dashed lines represent the results from model calculations.
— —
k5D
_,5DT5D.’3
3
3
+ k5D_,5DT5DI2
-
+ k50
(5a)
_*5DT~DIJ~~
I~ k~1~ .5D,T5D~I3+ k5D =
_.~D1T5D2I2
—
(Sb)
k~D~5D0TsD1II,
101
To separate the total 3~ion at decay the C rates of the excited levels of the Eu 2 site into and non-radiative rates we measured the radiative emission spectra after selective excitation of each of these levels. The relative integrated intensities (i.e. their sum equals unity) of the 5D 5D 1(C2) 5D 0(C2) and levels for excitation of the 3 (C2) level (I~and I~,respectively) are given fig. 2 asdecay a function 3~concentration, The in radiative time of of Eu the 5D 1(C2) level, 5D and the rate for non-radia5D tive decay from the 3(C2) level to the 0(C2) 5D level, k5D -. 0’ can be obtained via the relation ~
5D (C ~
100
-~
1
~‘
5D ~
10
2
10-2
3
1~=k5D~D0T5D,1~.
(3)
The observed relative integrated intensities of the 5D 5D and 5D levels (all at the C 2 site) for 5D excitation of the 2(C2) level (Id, I~ and j2 respectively) are given in radiative fig. 3 as decay a function 3 + concentration. The time of Eu the 5D 1’~D 2(C2) level, 2, can now be separated -
.
1 .AA
i0
101
~0
101
Europium concentration [%]
Fig. 3. Relative integrated intensities from the excited levels of 3~under selective laser excitation of the 5D Y203: Eu 2(C2) 3 concentration. Downward-directed indicate the D level as tnangles a function of the 5Di (C Eu 5D 0(C2) level and upward-directed Dashed triangleslines the represent 2) level; indicate the calcula2(C2) level. the squares results from model tions.
D.B.M. Klaassen et a!. ~
101
c o
10
~
10’
/ Energy transfer processes in yttrium
oxide
265
tamed in the fit, are given in fig. 1 as a function of the Eu3 + concentration; the calculated relative integrated intensities are given in figs. 2, 3 and 4; ~Do (C
2)
-
.-
-
5D 1
-
~
• 10.2
5D
.~
~“.--
•~ ~--~
“
-
10-2
10-1
100
Europium concentration [%]
101 _____
Fig. 4. Relative Y 3~under integrated selective intensities laser excitation from the of excited the °D levels of 3~concentration. Downward-di203: Eu 3(C2) level astriangles rected a function indicate of the the Eu 5D 5D 0(C2) level and upward-directed triangles the 5D 1(C2) level; squares indicate the °D2(C2) level and circles the 3(C2) level. Dashed lines represent the results from model calculations.
greater is the experimental uncertainty in the radiative decay time, as the number of experimental data is smaller: r503 is contained in eq. (5),
I
and =
and rates the calculated and of transfer are shownnon-radiative in fig. 5 as adecay function the 3 + concentration. Eu The radiative decay times of the excited levels of Eu3~in Y 203 can also be predicted using the Judd—Ofelt theory (see e.g. ref. [38] and reference cited therein). The resulting radiative decay times, using the E 1, ~ and Q, parameters listed by Weber [7,39], are given in table 1. Comparing the Judd—Ofelt predictions and the experimental values obtained in the present investigation, it can be concluded that reasonable agreement has been obtained, especially for the lower levels. The difference 5D observed between experiment and theory fig. 1). for the 0(S6) level3~concentrations may be due to (see the lack of data at very low Eu It should be noted that the higher the level, the
—k5D2~5D0T5D2I~. (Sc) We fitted eqs. (3—5) simultaneously to the experimental data, using =
k~on,rad+
5D
5D 3-~ 2,
5D
io~
k5D3~5D2T5D3I~ — k5D2~5D,TsD2I~
knon,rad
106
Cx”’,
5D 2—
_/
1__—V,--’
,-.
-•—•—-—-——‘
,_.
5D
‘iii io~ C
~ io3
(6)
where x is the Eu3 ± concentration in mol% and C and y are constants to be determined in the fit (see also eq. (2)). In order to allow for concentration-dependent effects on the decay rate of the 5D 0(C2) level, the radiative decay rate, l/TSD0, is replaced by an expression similar to eq. (2). 1’his implies that all I~in eqs. (3—5) should be multiplied by by (Cx~’rsn0+ The best overall obtained allowing1).non-zero rates only fit for was the transitions the values next-lower level, i.e. fitonly 5D~, *to 0. The obtained in the for k50 the radiative decay times, T5D, and for kPD1 ‘13J i’ C and ~ are summarized in table 1. The total decay rates, calculated using the constants ob-.
-.
102
7FJ (C 2) 101 10-2
I
101
~
~
10°
101
Europium concentration [%]
102
a function of the between Eu3~concentration. Fig. 5. Rates for 3~as non-radiative transfer the excited levels of Y203 : Eu The dashed line indicates the rate from the 5D 3(C2) level to 5D the 2(C2) level; the chain-dotted line indicates the rate from the 5D 5D 5D 2(C2) level to the 1(C2) level; the chain-dashed line indicates the rate from the °D1(C2)level to the 0(C2) level; the full line indicates the rate from the °D0 (S6) level to 5D 5D the 0(C2) level and the7F~(C dotted line the rate from the 0(C2) level to the 2)ground-state multiplet.
266
‘
D.B.M. Klaassen et aL
•
65.
/ 5D
:
2 ..
>‘
D
~0
/ Energy transfer processes in yttrium oxide
2
/
•_•_• •
/
A
50
150
/
.
/
~
5D3/
/D1
~“
I
I
250
350
450
Temperature [K]
Once we have established that the excited levels of the Eu3~ion at the C2 site decay non-radiatively only the next level,ofwea can try to interpret thetodecay rateslower in terms multiphonon decay process [40]. This 5D was done until [7]. now 5D 3~by Weber only formeasured the decay from the Eu 1(C2)rates level of to the 0(C2) level of Y203 :0.1% We the total decay the 5D 5D 5D 3~as a function of sample (see fig. 1(C2), 2(C2) and 3(C2)temperature level of Y203 :0.5% Eu 6). The Eu3 chosen as high as possible, but± concentration low enough towas omit the concentra-
•
S
.4 0 7
:~
..
4. Decay rates as a function of sample temperature
550
~
Fig. 6. Total decay rates of the excited C levels ofindicate Y203 :0.5% 3 ± as a function of temperature. 2Squares the 5D 5D Eu 3(C2) 2(C2) level and triangles 5D level; dots indicate the the 1(C2) level. Solid lines represent the results from fits t theory (see text). 5D, is 2 is contained in eqs. (4) and (5), and T contained in eqs. (35) From the values of C and y, given in table 1 for the energy transfer from the S 6 site to the C2 site, a critical transfer distance of 8.1 A is found. This is in reasonable agreement 3~[10,11] andwith Gd published 3~ values for Y203: Eu 203 : Euof [15]. As far as the concentration dependence ‘T5D
the energy transfer from the S6 site to the C2 site is concerned (‘y 1.42), it should be noted that all transfer rates obtained by integration of theoretical decay curves for various types of interaction (see e.g. ref. [37]) yield about the same concentration dependence in the region of the critical concentration. For the non-radiative decay between the excited levels of the C2 site, the main conclusion is that to non-radiative decay from levels occurs only the next-lower level. Thethese concentration-de=
pendent term in these rates is attributed in the literature to cross-relaxation [15,21,31]. Further investigations on the concentration-independent part of the decay rates between the excited levels of the C2 site will be presented in the next section.
tion-dependent term (see previous section). Using the expression for the temperature-dependent multiphonon transition rate given by Riseberg and Moos [40], we fitted the experimental data: the following expression to kbot 5DJ 5D~~~”_+kTD° ‘Ei~T
(
exp(hw/kT) exp(hco/kT)—11~“ (7)
where ph~iis the energy gap to the next lower level and p is the number of optical phonons produced. In the fit we used the calculated values given in table 1 for the radiative decay times. A fit using the radiative decay rates obtained in this investigation should results, as the radiative decayyield ratealmost is onlythe 10%same of the total decay rate. The values for k~ 1~,5D,p and hw obtained in the fit are summarized in table 2. The total decay rates calculated with these constants are indicated by solid lines in fig. 6. The best fits were obtained by allowing for non-integer values for p. Fits using integer values for p yielded values for that were about 8% smaller Table 2 Values for the parameters in eq. (7) describing the3 +rate for at the non-radiative decay between the excited levels of Eu C 2 site of Y203 Transition p k~?~0~ 1) m~ (s 5D 5D 2 5.3 544 8980 5D3 —~ 5D 4,8 515 15370 5D 2 —~ 5D 1 1 -. 0 3.8 462 7520
D.B. M. Klaassen et a!.
/ Energy transfer processes in yttrium oxide
for the 5D1 ~D0 and 5D2 5D1 transition, and 5D 5D 6% larger for the 3 2 transition. These values are well within the experimental accuracy. The infrared absorption spectrum of Y2031 shows lattice at 468 561 400 cm”to combined withabsorptions a broad band fromand below well above 500 cm1 141]. Therefore the phonons produced in the relaxation process are easily absorbed by the lattice (see also ref. [7]). According to the simple theory of Riseberg and Moos [40] the multiphonon transition rates k~,?.5D 1, depend exponentially on the energy gap t~E=phw: —~
106
-~
267
~
—~
~
=
/3 e~~
-
a)
~ i0~ Z
•“....
:
-.
(8)
where /3 and a are constant for a particular host lattice. In fig. 7 we have reproduced of 3” the [40]results and the Riseberg Moos results of and Weber for for Y Y203 3: Er + and Y 3± 203 : Er 203 : Eu [7]. Taking into account the uncertainties mentioned above, our results agree quite well with those by Weber. Theobtained multiphonon transition rates for Y 203on: Er3 + show indeed an exponential dependence the energy gap between the levels involved in the transition. For Y 3 however, these rates 203 : Eu show a quite different behaviour as a function of the energy gap. As already pointed out by Weber [7], the deviation observed for the 5D 5D~transition can be explained by the fact1 that for this “,
—~
transition in The first-order the multiphonon emission rate is zero. discrepancy between our result for the 5D 5D 3 2 transition and the 3~found exponential by behaviour of covered the results Y203 range : Er in energy Weber, who thefor larger
102 1500
I
2000
I
I
2500 3000 - E~[cm~]
E,
3500
Fig. 7. Rates for non-radiative decay between excited levels of Er3~and Eu3±in Y 203 as a function of the energy difference ~E = E, — E, between the initial (i) and final (f) level, Open squares represent the results of Riseberg andofMoos for 3~ open circles represent the results Weber[40] [7] for Y203 Er3”; solid circles represent the results of Weber [7] for Y2O3 : Er 3 + and solid squares represent the results from the Y 2O3 : Euinvestigation for Y present ~. The upward and downward arrows indicate respectively3that only a lower and an Eu upper limit were obtained.203: Dashed lines (for the results of Weber [7]) and dotted lines (for the results of Riseberg and Moos [40]) represent the non-radiative rate following the energy gap law using the parameters as deduced by van Dijk [42] from these results. The cross indicates the ‘normalized’ result (from the present investigation) for the 5D 5D 3” (see text).3 —* 2 transition of Y203 : Eu
—~
gaps, is quite distinct. A more accurate theory developed by van Dijk and Schuurmans [42—44] takes into account the selection rules neglected by Riseberg and Moos [40]. A calculation using this theory (see e.g. eq. (2) of ref. [38]) yielded k~°502~
—E502)
kT~°5De~02~”~
=
(9)
The different values of a for the results of Weber and Riseberg and Moos [42] yield only a difference of 5% for the expression given in eq. (9). Therefore the averaged value is given. If we divide
our experimentally determined rate for the 5D 3 ~D~transition 5D by this5Dfactor, the 5D ratio 5D between the rates for the 2 1 and 3 2 transitions is in perfect agreement with the exponential energy-gap law (see fig. 7) using for a the value of Weber’s data. The absolute values for these transition rates calculated from eq. (2) of ref. [38] are a factor of 3.7 smaller than the experimental values. —~
—*
—
The theory used, however, predicts the ratios of non-radiative rates much more accurately than the absolute values. It may only be concluded that the introduction by Nieuwesteeg [38] of an extra factor of 6 with respect to the original derivation of the non-radiative rate by Schuurmans and van
268
D.B.M. K!aassen et aL
/ Energy transfer processes in yttrium oxide
Dijk [44], is certainly justified as far as Y203 is concerned. Summarizing the results of this section, we have given experimental evidence for the multiphonon character of the non-radiative 3~in Y decay process of the excited levels of Eu obtamed are in good agreement203. withThe bothresults published experiments and theory.
5. Experiments under cathode-ray excitation 5.1. Low excitation density Once we have quantified the rates for nonradiative decay between the excited levels of the Eu3~ion at the C 2 site,3”asions wellatasthe theS rate for energy 6 site to ions at transfer the C from Eu 2 site, we can use these rates for the interpretation of the cathodoluminescence spectra. We measured spectra of 3 underthe low luminescence excitation density in a deY203 : Eu mountable cathode-ray tube at room temperature. In fig. 8 the relative integrated intensities from the 3~ emitting levels are given as a function of the Eu ±
concentration. Over the whole 5Dconcentration region the emission from the 0(C2) level constitutes more than 85% of the total emitted intensity. To describe the energy flow in the phosphor we 3t use the phosphors model developed initially for Tbare activated [32—34].The activators excited into the charge transfer state of Eu3~at a rate aj, which is proportional to the excitation density. Upon excitation rapid, radiationless relaxation of energy takes place with rate y~ from the charge transfer state directly into the emitting 5D~levels. At low excitation densities the following relations are then obtained from the rate equations: (i (i
+ k5D~5DT5D)I5D 5D T5D )I~D =f5D3ac2Nc2, — k5D -‘5D T5D + k5D ~ 2
2
(1 +
ksD,
~D 1T5D,
=
2
3
2
3
3
(lob)
f3D2aC,NC,,
=
f3 2)
—
k~
)
‘~D1 —
k5D2
5D,T5D2’~D2
N~
a
1~D(C D, C2
—
t
I
(lOa)
ISD
(lOc)
2’
ksD, _.5D0T5DI5D, C2T5DO(SÔ)I5DO(S6)
=f5D0(c2)ac,Nc2, (lOd)
101 5D
and (1 + ks
0(C2)
10°
V
5D •~ 10~
0(S6)
-
- - -.
10 -2
.
~
~-
•
•
•
S.-.
-
•‘‘‘~‘.~.
102 I
101 I
10° I
Europium concentration [%]
=
(lOe)
asNs.
Here ‘~D are the integrated intensities under cathode-ray excitation; T5DJ and k5D~.,sD~ are the radiative decay times and rates for non-radiative decay determined in sect. 3 and given in table 3± 1; N~ and N~ are the concentrations of Eu ions at the C2 and S6 sites, respectively;7F~multiac2 and plet, which may for be different Eu3~ions at the as are the rates excitationfor from the C 2 and S6 sites; and is the branching ratio for 5D,: feeding of level f5 D (11) ‘
- - -.,
10~
~cT5D(s))I5D(s)
101
~
Fig. 8. Eu3” Relative integrated intensities the excited levels of under stationary 20 keV from cathode-ray excitation as a function of the Eu3 + concentration, Diamonds indicate the results for the 5D 5D 0(S6) level; downward-directed triangles mdicate the 5D 0(C2) level and upward-directed 5D triangles the 1(C2) 5D level’ squares indicate the 2 (C2) level and circles the 3(C2) level. Dashed lines represent the results from model calculations,
=
.
Fitting eq. (10) to the the measured relative 3 integrated intensities as a function of the Eu +
D.B.M. K!aassen et aL / Energy transfer processes in yttrium oxide
concentration, the following values for the branching ratios were obtained: f~D 5 f~~ 26 f~D3=0~38 2°° 1=O. and f~D 0(C2) 0.31. =
1.25 >‘ U
a) -0C
0.75
56/Nc2
equal to the most probable value 0.33, we find as 6/ac2 equal to 0.168. Summarizing this subsection, we have obtained the branching ratios for feeding of the emitting levels under cathode-ray excitation. It5Dshould be noted that the branching ratio for the 2 level is quite low, whereas the others are more or less equal. Secondly, the relative occupation of S6 and C2 sites is independent of the activator concentration. Although some statements are made about this occupation in the literature [11,15], this is the first experimental evidence. 5.2. High excitation density
At high excitation density the efficiency of phosphors decreases. In the ultimate limit the ground state is depleted: all the activators are in the excited states and it takes a period of one decay time before they can be excited again [2,3]. However, when the occupation of the excited states increases, interaction between activators which are both inloss an excited state can give rise to additional energy processes [4,32,33]. We measured the efficiency as a function of current density under 20 keY CR excitation for the emitting C 3”’ sample 2 levels of a Y203 : 1% Eu at room temperature. These results are given in fig. 9. From this figure it is clear to see that the efficiency of the 5D 0 level starts to decrease at a much lower current density than the efficiencies of the other levels (see also table 3). In combination with the large differences in total decay rates of the emitting levels (see fig. 1) this leads, as argued in the appendix, to the conclusion that at high
5D 5D ~.‘:-. 2 1 N~.
a a)
0.50
.~
Moreover, it turned out (see fig. 8) that the ratio as6Ns6/ac2Nc2 is equal This to 0.056, independent of 3’ concentration. implies that the rela the Eu tive occupation of ~6 and C 3” ions is 2 sites by Eu investiconstant over the concentration region gated. Taking the ratio of concentrations N
269
(6
~
0.25
C
0.00
-_________________________________ iO~
106
i0~
i0~
102
Current density [Acm2] Fig. 9. Experimental relative efficiency of the excited C2excitalevels 3” under stationary 20 keV cathode-ray of Y2O3 :1% Eu tion5D as a function of the current density. The full line indicates 5D 5D the (C2)dashed level;5Dthe the and1 (C2) level; 0the line chain-dotted indicates theline indicates 2(C2) level the dotted line the 3(C2) level, Shown are curves given by = (1+ (1/150% )~ — I, where j is the current density, and 150% and c were obtained from a fit of this formula to the experimental data.
current densities excitation from all emitting levels takes place. In order to give an accurate description at high excitation densities, eq. (10) has to be modified. For the case of excitation from the excited states, ac2 T5D~‘~D~ have to be added to the first term on the left-hand side. The relative efficiencies calculated using these modified relations are given in fig. 10. Comparison of figs. 9 and 10 shows a fair qualitative agreement between experiment and calculations. Also the (relative) current densities at which the efficiency is halved, J50%~are predicted in the right order for the different levels (see table 3L 3± we studied the saturation of Y203: Eu 11 as Next a function of activator concentration. In fig. the experimental values obtained for 150% of the Table 3 Experimental and calculated current densities at which the efficiency is halved ~ for a Y 3 + sample rela203 :1% Eu 5D tive to the value for the 3 level Level Experimental Calculated 5D 1 1 5D3 2 0.9 0.6 5D 1 5j)~
0.5 0.04
0.5 0.05
D. B. M. KIaa.ssen et a!.
270
/ Energy transfer processes inyttrium
1.25
oxide
1500
~i::o
:iO75~\\5D3
101
102
b-7
3
io
±
lation between 150% and the activator concentration. We note that these relations only contain terms that are linear in the occupation of the 3 ± ions at Sdifferent levels. Energy transfer from Eu 6 sites to those at C2 sites, as well as non-radia3” ions at tive decay between different levels of Eu the C2 site, both give rise to terms that contain a product of occupations. However, the S6 C2 ~~__—
0.2 0.1
~
1 Europium 2 concentration 3 5 [%l 7 — 10 Fig. 11. The current density, at which the relative efficiency has decreased to 0.5 ~ for the 5D 3~ as a function of the Eu concentration. 0(C2) The level error of Y2O3: bars Eu mdicate the errors as obtained from a fit of the experimental points to an analytical expression (see text). 0.3
0.5
0.7
io-~
io-~
iO-~
10-2
Current density [Acm-2]
5D 3 0 level are shown as a function of the Eu concentration. The current density 15Q% increased by less than a factor of three, while the Eu3 + concentration increased by a factor of twenty. Calculations using the modified equations mentioned above yielded, however, a proportional re-
00
io-6
Currentdensity [arb.un.[ —s’
Fig. 10. Theoretical relative efficiency of the excited C 3” under stationary cathode-ray excitation 2 levels as a of Y203:1% function of the Eu current density. The full line indicates the 5D 5D 0(C2) theline chain-dotted line 5D indicates the 1(C2) level; the level; dashed indicates the 5D 2 (C2) level and the dotted line the 3(C2) level.
03
‘~
10-
—~-
5D Fig. 12. Initial decay time of3” the as cathodoluminescence a function of the from current the density 0(C2)forlevel various of Y203: Eu3 + concentrations. Eu Solid circles represent 3” concentration of 0.03%, open circles 0.3%, open squares a Eu 1%, solid squares 5%, and open triangles 30%.
5D transfer can contribute the 0(C2) luminescence signal (seeonly sect.5.6% 5.1), to whereas crossrelaxation between the Eu3 levels only becomes important at very high concentrations. Therefore it can be concluded that additional energy loss processes give rise to the sublinear increase with the Eu3 + concentration of 150% for the 5D 0(C2) level. At high excitation density such additional energy processes to afrom decrease the initialloss decay time ofgive the rise levels, whichofthese ±
processes take decay times of place the 5D[4]. The experimental initial 0(C2) level are given in fig. 12 3 as a function of For current density for various + concentrations. all activator concentraEu tions the initial decay time is constant up to very high current densities. This leads to the conclusion that no interactions occur between Eu3~ions in the 5D 0(C2) level. As about 70% of the energy from the 5D 3” ionemitted at higher levels, energy 0(C2)loss level processes enters the in these Eu higher levels may be responsible for the saturation behaviour of the 5D 0(C2) level. To investigate this possibility, we 5D 3” sample as a function of current measured the initial decay time of the 3 level of a Y203 : 1% density. TheEu results are shown in fig. 13. At high current densities a clear decrease of the initial decay time is observed to about 50% of its value at .
‘
.
.
low current density. Consequently it can be concluded that an additional energy loss process emp-
D.B.M. Klaassen et aL / Energy transfer processes in yttrium oxide
consistent with sophisticated theory on non-radiative decay rates. From cathodoluminescence spectra at low excitation density the branching ratios for feeding of all the emitting levels were obtained. Moreover the experiments showed that the occupation of ~6 and 3”’ conC centration. 2 sites in Y203 The product is independent of excitation of the Eu rate and
100 80
~
T
60
-
40
-
20
-
1O-~
s”
- -~,,
“f
10-6
~-5
-~
~o-~
~
2]
271
_~
Current density [Acm Fig. 13. Initial decay time of the cathodoluminescence from the 5D 3” as a function of the current 3(C2) Typical level of errors Y203:1% density. are Eu indicated. The dashed line is for visual aid only.
concentration was found to be a factor of 18 smaller for the S sitethethan for the as C2a site. Measurements6 of efficiency function of cathode-ray excitation density led to the conclusion that at high excitation densities excitation takes place from all the emitting levels. Furthermore at moderate activator concentrations these measurements were quite well described by model calculations using the experimentally obtained information on the energy transfer and non-radia.
.
.
.
.
ties the 5D 3 level at high current densities. This process of the 5Dcertainly affects the saturation behaviour 0(C2) level. Additional measurements of the efficiency and initial decay time of all levels at several activator concentrations and as a function of current necessary to processes. establish whether theredensity are anyare other energy loss Moreover, the method of calculation presented in this section no longer suffices. The higher-order processes have to be included, which leads to a set of nonlinear equations.
tive decay. At high 3activator concentrations inter± ions both in high excited actions between Eu 5D levels (e.g. 3), cause additional energy losses. In conclusion it can be stated that we have developed3” awhich, modelin for the energy in combination with flow the exY203 : Eu perimentally determined rates and branching ratios for the feeding, describes all experimental data such as spectra and efficiencies at both low and high excitation densities in far detail.
Appendix 6. Conclusions In the present investigation the energy transfer processes in the phosphor Y 203 using activated with 3 + were extensively studied dye laser Eu spectroscopy. The rates for energy transfer from Eu3” ions at S 3” ions at C 6 sites to Eu 2 sites, as well as non-radiative decay rates between 3 ± ions at Cthe different energy levels of the Eu 2 sites, 3”’ conwere quantified as a function of lifetimes the Eu of the centration. The resultant radiative levels are in good agreement with theory. The non-radiative decay processes between the different energy levels of the Eu3 ions at C 2 sites were identified as multiphonon relaxation using measurements as a function of the sample temperature. Rates obtained for zero temperature are
In this appendix we will show that excitation from the emitting levels (under cathode-ray excitation) can determined by saturation ments. Thebeprecondition, however, is a measuredifferent total decay rate of the levels, For clarity we consider an activator with a ground level (0) and two emitting levels (1 and 2) in two cases: takes place from the ground level (1) Excitation only, and (2) excitation takes place from all levels with equal rate constants.
+
The excitation takes place to acharge high, transfer fast-decaying 3 and state level (Sd band for Th for Eu3 “-activated phosphors [32]). Using the model (and notation) for the energy flow in the +
D.B.M. K!aassen et a!.
272
/ Energy transfer processes
phosphor host lattice given by de Leeuw and ‘t Hooft [4], the rate equation for the density of
inyttrium oxide
and x
1
electron—hole (e—h) pairs n reads
x2 —
dn
2
g
=
—
a~x,n f3n,
~
(Al)
—
where g is the generation rate for the density of e—h pairs, x, is the concentration of activators in level i, a~ is the rate constant for excitation of activators in level i, and /3 is the rate for energy loss in the phosphor host lattice. The rate equation for the concentration of activators in the charge transfer state y yields 2
y,y,
—
dt
11
T2
a
(A8)
+~
c~
From these relations it can be seen that, independent of the excitation density (or g), the ratio between the total emitted intensities from level 1 (Ii) and from level 2 (‘2) is constant and equal to I~ f~+ k21T2 —
7
f2
—
‘
In other words, the efficiency is the same function of excitation density for both emitting levels. Consequently, the current density at which the ef-
2
V a~x1n
=
=
k2~1T2
T1
‘-~
(A2)
i~1
where ‘I’, is the feeding rate of level i. Under stationary conditions eqs. (Al) and (A2) yield
ficiency is halved is also the same for both levels. Case 2. a~=a~_—aE,=a”=/~O Using the following expression as a shorthand a~g
n
=
g ~ cx~x,+ /3 2
(A3)
,
i=0
(AlO) substitution of eqs. (A3) and (A4) into eq. (AS) yields for steady-state conditions a
acN + /3’
=
and
f2aN
I y
=
2
~y, i=1
(All)
‘
—+k21+a T2
~=o
2
1
x2=
2
(A4)
‘
The efficiency of level 2,
~a~x,+/3
~2’
is now given
by
I
i=0
—
The rate equations for the concentrations of activators in the two emitting levels, x1 and x2, read
~l2 =
dx2
From this equation it can be seen that the efficiency of level 2 is halved at a current density at which
x2
—~-~-- =
‘~2y
—
—
k2,1x2
—
a~nx2,
(AS)
1 +a
1+k
and a=
dx1 dt
(A12)
‘
+ k2 ~
—
+k2~1x2—a~nx1.
— =
(A6)
The following expression for the concentration of activators in level 1 is obtained by substitution of eqs. (A3) and (A4) into eq. (A6):
~
Case]. a’~=a~O,’a~=a~=O
Under stationary conditions the following expressions are obtained from eqs. (AS) and (A6): x2
(1 +k2.1T2)—
acxog =f2c+$
(Al3)
21. T2
(A7)
f1aN X1
=
1 —+a
+
k25f2aN / 1 \ (—-i-a)
\
T1
/
(
1
—+k2 T2
1+a
) (A14)
D.B. M, K!aassen et aL
/ Energy transferprocesses
The general expression for the efficiency of level 1 is quite complicated. Therefore we will consider here only two limiting cases: k2 -.1 = 0 and k2 -.1 >> 1/’r2. If k2 = 0 from eq. (14) it follows that the efficiency of level 1 is halved at a current density at which ,,~
1
(A15)
a=—.
If k2.~1>> given by
1/’r2,
~
+
the efficiency of level 1,
aft)
~,
is
1 T1
T1
in yttrium oxide
273
[8] H. Forest and 0. Ban, J. Electrochem. Soc. 116 (1969) [9] J. Heber, K.H. Hellwege, U. KObler and H. Murmann, Z. Physik 237 (1970) 189. [10] U. Köbler, Z. Physik 250 (1972) 217. [11] J. Heber and U. Köbler, in: Luminescence of Crystals, Molecules and Solids, ed. F. Williams (Plenum, New York, 1973) p. 379. [12] S. Qiang, C. Barthou, J.P. Dems, F. Pelle and B. Blanzat, J. Lumin. 28 (1983) 1. [13] R.G. Pappalardo and R.B. Hunt Jr.. J. Electrochem. Soc. 132 (1985) 721. [14] R.B. Hunt Jr. and R.G. Pappalardo, J. Lumin. 34 (1985) 133. [15] M. 9’ Buijs, A. Meyerink and G. Blasse, J. Lumin. 37 (1987)
(A16)
[16] N.C. Chang, J. AppI. Phys. 34 (1963) 3500. [17] NC. Chang and J.B, Gruber, J. Chem. Phys. 41 (1964) 3227. [18] R.A. Buchanan, K.A. Wickersheim, J.L. Weaver and E.E.
Consequently, the efficiency of level 1 is halved at a current density for which
Anderson, J. Appl. Phys. 39 (1968) 4342. [19] R.P. Leavitt, J.B. Gruber, NC. Chang and CA. Morrison, J. Chem. Phys. 76 (1982) 4775.
=
\T1
a
+
1 ~<
—,
a~(k2~1 + a) /
1
-i-a
(A17)
Generalizing the results obtained in this appenclix, we can state that, if the emitting levels have distinctly different total decay rates, excitation from the emitting levels can be determined by saturation measurements. If the total decay rates of the levels are different, one can conclude that: excitation takes place from the ground level only if the efficiency of both levels is halved at the same excitation density; excitation takes place from all levels if the efficiency of the levels is halved at different excitation densities. —
—
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274
D.B.M. K!aassen et aL
/
Energy transfer processes in yttrium oxide
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