Changes in trap parameters in various mixed oxide garnets

Accepted Manuscript Changes in trap parameters in various mixed oxide garnets V. Khanin, I. Venevtsev, P. Rodnyi, C. Ronda PII:

S1350-4487(16)30036-1

DOI:

10.1016/j.radmeas.2016.02.001

Reference:

RM 5562

To appear in:

Radiation Measurements

Received Date: 25 October 2015 Revised Date:

5 January 2016

Accepted Date: 1 February 2016

Please cite this article as: Khanin, V., Venevtsev, I., Rodnyi, P., Ronda, C., Changes in trap parameters in various mixed oxide garnets, Radiation Measurements (2016), doi: 10.1016/j.radmeas.2016.02.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Changes in trap parameters in various mixed oxide garnets V. Khanina,b, I. Venevtseva, P. Rodnyia, C. Rondab

Peter the Great Saint-Petersburg Рolytechnic University, Polytekhnicheskaya 29, 195251 St.Petersburg, Russia

b

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a

Philips Research Eindhoven, High Tech Campus 34, 5656 AE, Eindhoven, the Netherlands

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Corresponding author: V. Khanin, e-mail address [email protected]

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Highlights

Thermally stimulated luminescence of mixed oxide garnet ceramics was studied

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TSL of Cr-, Yb- related impurity defects was identified in many garnet compositions

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Table of Cr-related trap positions in energy gap for mixed garnets was created

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Cr-related trap was found at 0.4-1.4 eV bellow the CB, depending on garnet composition

Keywords

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•

Abstract

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Scintillator, garnet, thermally stimulated luminescence, defect, Cr impurity

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Shallow and deep electronic traps in various mixed oxide garnet ceramics (Y,Lu,Gd)3(Al,Ga)5O12 have been studied by thermally stimulated luminescence spectroscopy in the 80 – 550 K temperature range. It is shown that the substitution of Al ions by Ga and Y by Gd or Lu in YAG:Ce affects the properties of the traps. It is established that the studied ceramics contain residual impurities of chromium and ytterbium. On the base of the obtained and literature data, a table of the Cr-related trap position relative to the bottom of the conduction band in RE3(Gax,Al5-x)5O12:Ce garnets (RE = Lu, Y, Gd and combinations thereof) has been constructed.

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1. Introduction In the search for new efficient scintillators for medical and industrial applications, cerium-doped oxide garnets have been investigated extensively during recent years. Y3Al5O12:Ce (YAG:Ce) and Lu3Al5O12:Ce

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(LuAG:Ce) were shown to have relatively low light yields (< 30000 ph/MeV [Moszynski et al., 1994]), whereas more than 60000 ph/MeV can be expected [Rodnyi et al., 1995]. Relatively high light yields, up to 60000 ph/MeV were found in mixed oxide garnets (e.g. (Y,Gd)-garnets [Kamada et al., 2011] and

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Gd3(Ga,Al)-garnets [Prusa et al., 2013, Zorenko et al., 2015 ]). However, many of the mixed oxide garnets as well as YAG:Ce and LuAG:Ce are characterized by a multi-exponential decay and a severe

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afterglow [Mihokova et al., 2013]. The afterglow is due to trapping of both electrons and holes. Many investigations have been devoted to both intrinsic and extrinsic traps in garnets [Nikl et al., 2008 and references therein]. The presence of shallow defects related to anti-sites was shown with thermally stimulated luminescence (TSL) [Nikl et al., 2007] as well as with EPR [Laguta et al., 2007, Asatryan et al., 2014] techniques. Recently it was shown also that some of the deeper defects in YAG:Ce can be

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associated with impurities, in particular with lanthanide impurities [You et al., 2012] or some of the transition metals such as Cr3+ [Xu et al., 2011, Ueda et al., 2015]. In this paper we study TSL of mixed oxide garnets containing Cr and Yb impurities as they are expected

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to be present even in high purity starting materials (based on their chemical similarity with starting

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materials), aiming at identifying the related trap depths. 2. Experimental

All the (Y,Lu,Gd)3(Al,Ga)5O12:Ce 0.2mol. % garnet ceramic samples in this study were prepared at the Philips Research Eindhoven facility by sintering of a mix of base oxides of 4N-purity in air at around 17000C in the form of pills of 14 mm diameter and 0.7-1.4 mm thickness. The supplier lists of starting materials state that Cr and Yb as impurities are present in total in amounts less than 2 and 5 ppm

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respectively. On the basis of the X-ray diffraction patterns it was concluded that all the samples consists of a single garnet phase. The list of samples studied is shown in Table 1.

Changes in Ga/Al, x

Y3(GaxAl5-x)O12:Ce

x = 0,1,2,3,4

Gd3(GaxAl5-x)O12:Ce

x = 2,3

Lu3(GaxAl5-x)O12:Ce

x = 0,2,3

Lu1Gd2(GaxAl5-x)O12:Ce

x = 1,2,4

abbreviation YAGG:Ce

GdAGG:Ce LuAGG:Ce

LuGdAGG:Ce

y = 0.15,0.45,0.75,1.2 YGdAG:Ce

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Y3-yGdy Al5O12:Ce

Changes in RE/Gd, y

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Composition

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Table 1. The description of multicomponent garnet ceramic samples under study

LuyGd3-y(Ga3Al2)O12:Ce

y = 0,0.3,3

LuGdAGG:Ce

The thermally stimulated luminescence (TSL) curves were obtained in the 80-550 K temperature range

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after irradiation with X-rays (Tube parameters were 55 kV acceleration voltage and 10mA current) detected with PMT R6357 that is sensitive in the range of 200-900 nm. The irradiation took place during 5 minutes; the samples were positioned 3 cm away from the tube. The waiting time on between irradiating

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the samples and the start of the measurements was 10 minutes, all of the TSL curves shown in the present work were recorded with β = 15 K/min heating rate. Black-body emission was subtracted from the TSL

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curve. The curves were also corrected for thermal quenching of the luminescence, measured with a UC920 Edinburg Instruments spectrofluorimeter under excitation with light with wavelength 450 nm in the range RT-550 K. The operating range of the spectrometer’s emission arm was 470 - 800 nm, the resolution was set at 2 nm. The equipment was calibrated to correct for the wavelength dependent transmission of the monochromators and the spectral sensitivity of the PMT. 3. Experimental Results

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Firstly, we wanted to see if there will be an impact from variations in cation stoichiometry of a garnet composition on their TSL curve structure. One can expect formation of intrinsic defects to be greatly influenced by the type of non-stoichiometry imposed upon e.g. YAG:Ce garnet either by excess of Y2O3

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or Al2O3 starting oxides [Kuklja, 2000] during synthesis. As opposed to that, impurity-related TSL peaks are not expected to be influenced significantly by slight cation stoichiometry changes.

TSL curves of YAG:Ce samples with several different stoichiometries are shown in Fig. 1, the curves are

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normalized to the TSL peak at 397 K. One can see that indeed samples with varying cation concentration ratios show a very different TSL peak structure at 100-300 K: for YAG:Ce with excess of Y3+ ions (Fig 1,

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curve 1) the TSL peak at 180 K can be seen much clearer as compared to two other samples (curves 2 and 3), in which TSL peaks at 200 and 230 K also have significant intensity. The samples with intended exact stoichiometry formula and with excess of Al look very similar (Fig 1, curve 2 and 3 respectively), it is highly probable that the intended exact-stoichiometry sample had a (unintended) slight excess of Al too. As opposed to that, the shape of the TSL curves above RT (Fig 1) in all the samples is very similar and

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seems not to be influenced by stoichiometry at all.

Generally, the low-temperature TSL peaks are attributed to anti-site defects [Nikl et al., 2005, Zorenko et al., 2005], or other structural defects [Varney, Selim 2014]. It was also shown that some of the peaks

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above RT in YAG:Ce can be related to Cr (384 K, 9 K/min [Ueda et al., 2015]), Yb (468 K, 120 K/min [You et al., 2012]) or Eu (618 K, 300 K/min [Milliken et al., 2012]) impurities. From this literature data

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and experimental results in Fig. 1 the tentative conclusion is drawn that TSL peaks that are not influenced by stoichiometry of the garnets can be associated with impurities.

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Fig. 1 TSL curves of YAG:Ce ceramic, with various stoichiometry of cations. β = 15 K/min For perovskites [Vedda at al., 2009] it was proposed that defect complexes of O- or oxygen vacancies spatially correlated to a RE-impurity can be the ones capturing the carriers and generating TSL process. Moreover, for YAGG:Ce,Cr [Ueda et al., 2015], the authors have concluded that Cr3+/2+ ions on octahedral sites are responsible for storage of electrons and creation of the TSL peak. We do not have yet

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enough experimental data to make a final conclusion on the carrier capture mechanism and the defects involved, therefore in this work we will simply refer to TSL peaks associated with impurities as “Cr- or Yb- related peaks”.

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As the next step we checked the influence of changes in garnet composition on the TSL peak structure, like substitution of Al ions by Ga and substitution of Y by Gd or Lu in YAG:Ce. Our main focus was at

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TSL around and above RT.

The dependences of TSL curves on Ga addition into YAGG:Ce or LuAGG:Ce crystals and polycrystals were investigated very thoroughly by the authors in [Fasoli et al., 2011, Ueda et al., 2015]. For all these compositions a shift to lower temperatures of all the TSL peaks with increasing Ga content was observed. It was also found [Xu et al., 2011] that co-doping with 0.05 mol. % Cr greatly enhances one of the TSL peaks in YAGG:Ce. With further TSL investigations for Y(Al5-xGax)G:Ce, x = 0 – 5 authors of [Ueda et

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al., 2015] have traced this peak from 384 K for YAG:Ce down to 161 K for YGG:Ce, measured with β = 10 K/min heating rate. A very similar approach of tracing Cr-related TSL peak was conducted in our work (not shown for

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briefness), for YAG:Ce the peak was found at 397 K (see e.g. Fig. 1), β = 15 K/min and for Y(Al1Ga4)G:Ce at 238 K, β = 15 K/min. The discrepancy in the TSL peak maximums between our results and the results of [Ueda et al 2015] work is attributed to a small difference in the heating rates in the TSL

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measurements.

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Fig. 2 TSL curves of Y3-yGdyAG:Ce, numbers are put above Cr-induced TSL peak. β = 15 K/min After studying Ga influence on garnets and tracing Cr-related TSL peak, we investigated the effect of Gd

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addition to YAG:Ce. In Fig. 2 the consequences of substitution of Y by 5, 15, 30, 40 mol. % of Gd are shown. The Cr-related TSL peak is shifting to lower temperatures the more Y3+ is substituted by Gd3+. On the right shoulder of the main studied Cr-related peak at 397 K, an additional smaller peak is seen at 450 K ( curve 1), in the work [You et al., 2012] for YAG:Ce,Yb it was shown that a TSL peak at this temperature can be attributed to an Yb trap. If the peak encountered in our samples at f.e. 450 K for curve 1, Fig 2 indeed belongs to an Yb-related trap, very likely Y2O3 is the source of this impurity.

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One can see that the TSL peak probably connected with Yb3+ impurity is present in most of the curves (f.e. 420 K for curve 2). The temperature difference between Yb- and Cr- related peaks is not varying much; it is approximately 70 ± 30 K for all the studied mixed garnets.

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When the Gd content is reaching 40% (curve 5), the TSL curves are becoming very complicated to such an extent that it is not possible to distinguish Cr- or Yb- related peaks in this composition, without additional co-doping with these ions. Further substitution by Gd (beyond 40%) in the YAG system was

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not conducted as it becomes very hard to make single garnet phase ceramics [Wu, Pelton, 1992] for Albased ceramic garnet compositions containing large amounts of Gd.

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Next, a commercially available Furukawa Co. Ltd. crystal of Gd3Ga3Al2O12:Ce 0.2% composition together with LuyGd3-y(Ga3Al2)O12:Ce, y = 0.3,3 ceramics were investigated, see Fig 3, curves 1,2 and 3 (higher Ga substitution leads to significant thermal quenching of the Ce emission [Mihokova et al., 2013], thus is difficult to study). Two TSL peaks can be seen at 188 K and 261 K for the Gd3Ga3Al2O12:Ce

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Furukawa crystal. One can also see the appearance of an additional TSL peak at around 300 K for Lucontaining garnet ceramics, which is increasing in intensity with increasing Lu content, a slight shift of this peak from 300 K (curve 2) to 315 K (curve 3) with Lu3+ fully substituting Gd3+ ions is also observed,

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similar to [Mihokova et al., 2013].

As we are not able to directly trace TSL associated with the Cr-impurity from YAG:Ce to LuyGd3compositions, a check if one of these peaks is related to Cr-impurity is necessary. For

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y(Ga3Al2)O12:Ce

this reason Gd3Ga3Al2O12:Ce,Cr ceramics with 50 ppm of co-doped Cr was sintered and TSL of it was measured (Fig 3, curve 4). In this ceramic sample the TSL of 261K-peak increased in intensity by nearly 30 times, so we assign this peak to a Cr-related trap. Yb3+

is

also

a

common

impurity

for

Lu-based

materials,

and

a

ceramic

sample

of

Lu0.3Gd2.7Ga3Al2O12:Ce,Yb composition with 20 ppm of co-doped Yb was made, see Fig 3, curve 5.

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Indeed, the TSL peak at 300 K increased in intensity by 20 times, and similarly to YAG:Ce the Yb-related

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TSL peak [Ueda et al., 2015] is slightly higher in temperature than the Cr-related one [You et al., 2012 ].

Fig. 3 TSL curves of (Gd3-yLuy)Ga3Al2O12:Ce, (y = 0;0.3;3) samples; solid line – single crystal, other lines – ceramics; curve 4 – Gd3Ga3Al2O12:Ce, Cr and curve 5 – (Lu0.3Gd2.7)Ga3Al2O12:Ce,Yb. β = 15 K/min

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In separate experiments afterglow spectra of some of our garnet samples singly doped with Ce were measured in the range of 200-700 nm with a PMT (shown in [Romanov et al., 2015]) and 200-1100 nm with an Ocean Optics QE65 CCD spectrometer (not shown here). For samples containing 0.2 mol% Ce or

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more, only Ce3+ luminescence with emission band maxima at wavelengths around 520-600 nm depending on the garnet composition was observed in the 2-300 K temperature range.

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4. Discussion

For most of the compositions investigated it was possible to find the position of the TSL peak that is related to Cr ions, whether by careful assessment or by co-doping with 10-50 ppm of Cr2O3. In cases of complicated TSL structure the temperature of the TSL maxima were extracted by the curve fitting method [Chen, Pagonis, 2011, Kitis et al., 1998]. The gathered TSL maxima (Tm, K) associated with Cr-related trap for the mixed garnet compositions are shown in table 2.

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As clearly, the TSL peak maximum is dependent on the heating rates used during the measurements, one may consider adding value to the data and calculating an absolute scale of defect energy depth from the bottom of CB (see columns (E, eV) in table 2).

Ga

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Table 2. Experimental data on TSL peak maxima (Tm) and calculations of energy depth (E) with mixed order kinetic equations for a Cr-related trap in various mixed oxide garnets. Cr-related TSL peak for (Lu3-y-z,Yy,Gdz)(Al5,Ga5-a)G:Ce Lu3-y-z,Yy,Gdz

Ga

y=0 z=0 y=3 z=0 y=0 z=3 y=1 z=2 Tm, K E, eV Tm, K E, eV Tm, K E, eV Tm, K E, eV

y,z=

a=0 Tm, K E, eV

0

518

y=2.85 z=0.15

385

y=2.55 z=0.45

365

y=2.25 z=0.75

336

0.9 0.8

y=0 z=2.7

266

a=3 0.56

1

1.39

0.68

404

1.03

297

326

0.77

Lu3-y-z,Yy,Gdz content

397

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1

0.6*

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a=

1*

335

0.79

340

0.65

364

0.89

291

0.81

321

0.75

3

263

0.67

294

0.67

261

0.57

265

0.56

4

0.4*

228

0.45

0.5*

180

0.29

5

0.4*

0.5*

0.4*

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2

0.97

0.4*

* Estimations with band gap schemes (±0.15eV)

Difficulties in determination of energy trap depths and frequency factors from the TSL curves are

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described in e.g. [Sidorenko et al., 2006]. By authors of [Ueda et al., 2015, You et al., 2012] the frequency factors were calculated to be 1011-1013 sec-1 for Yb- and Cr-related TSL traps in YAGG:Ce

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with the varying heating rates method [Chen, Winer, 1970]. With the assumption that frequency factors of these impurity defects are constant for all the oxide garnets under study, we assumed the value of 1012 sec1

for the frequency factor. TSL kinetics were calculated with the curve fitting method [Kitis et al., 1998]

and were found to be of a mixed order b = 1.2 or 1.5, (where b = 1 corresponds to 1st order and no retrapping and b = 2 to 2nd and a lot of re-trapping). Together with Tm extracted from the experimental data, the energy depths of the Cr-related defect were calculated with mixed order equations [Chen et al., 1981], see Table 2.

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A similar table as table 2 can be created for Yb-related trap levels in oxide garnets with the experimental data presented in this work. Recently, it was shown for YPO4:Ce,Ln [Bos et al., 2011] that identification of one TSL peak related to specific lanthanide impurity allows to calculate the TSL peak positions

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associated with any other lanthanide impurity in this composition, given that the mechanism of charge storage stays the same. So, using the models [Dorenbos, 2013] it is possible to draw similar tables as table 2 for any other RE-impurity related electron trap in oxide garnets.

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As a continuation of this work, the next step will be calculations of the lifetime of carriers on Cr-, Ybrelated traps at RT with McKeever’s expression 1/τ = s*exp(-E/kT) [McKeever, 1985]. Such lifetimes

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should be a characteristic of afterglow generated by thermal release from these electron traps via conduction band. These results will be published later. 5. Conclusion

In this paper, the thermoluminescence properties in the 80-550 K range of various multicomponent oxide

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garnets doped with Ce3+ and in some cases co-doped with Cr3+, Yb3+ ions have been studied. It was shown for YAG:Ce ceramics that TSL peaks for shallow intrinsic defects are greatly influenced by slight excess of different cations and as such could be distinguished from the impurity related defects.

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The TSL peaks generated from the release of electrons from Cr-related traps were traced through many multicomponent garnets, and a table of the peak maximums dependence on garnet chemical composition

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was created for specific heating rate β = 15 K/min. The experimental data was extended with calculations of the trap energy depths from the bottom of the CB and estimations of such values for some of the garnets not studied. It was found that Cr-related trap can be located at 0.4-1.4 eV below the C.B., strongly depending on the composition. References

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