Electronic processes in rare earth activated wide gap materials

phys. stat. sol. (a) 202, No. 2, 185–194 (2005) / DOI 10.1002/pssa.200460103

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

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Electronic processes in rare earth activated wide gap materials

Christian Pédrini*

Laboratoire de Physico-Chimie des Matériaux Luminescents, Unité Mixte de Recherche CNRS 5620, Université Lyon 1, bât. A. Kastler-10 rue Ampère, Domaine Scientifique de la Doua, 69622 Villeurbanne, France

Received 11 July 2004, revised 1 September 2004, accepted 17 September 2004 Published online 18 January 2005

PACS 7155.Ht, 72.40.+w, 78.55.Hx, 78.90.+t, 79.60.Bm

This paper is a review of charge exchange processes in rare earth doped insulating materials. After a de-scription of experimental approaches for the location of impurity ions in the host gap, charge transfer and photoionization phenomena are presented. Experimental data are discussed and interpreted using simple models.


1 Introduction

It is well-known that impurity ions embedded in host crystals give rise to localized states in the band gap. In the case of rare earths, transitions between these states often lead to interesting luminescence proper-ties and such spectroscopy, similar to atomic spectroscopy, is widely and easily studied. Another kind of transitions may occur between the bands of the host lattice and the dopant ions leading to charge exchange processes. They are more difficult to study because they often occur in far UV region and the starting or final states are delocalized and are more difficult to describe. These processes were under-estimated too long. Because they are of most importance in many technological applications of optical materials, they are now the object of detail investigations, for example to understand the energy loss processes in solid state lasers, phosphors, scintillators, optical fibers, but also to study new fast luminescence or selective manipulation of optical properties in optical memories and optical proces- sors. Charge exchange processes may occur between the host valence band and the dopant rare earth. Then, the so-called charge transfer transitions are of electron acceptor type for the rare earth impurity. When the charge exchange is between the rare earth impurity and the host conduction band, photoionization processes are involved through electron donor transitions of the rare earth impurity. The purpose of this presentation is to review charge exchange processes in rare earth doped insulating materials. Location of the energy of localized rare earth impurity levels relative to host band structure is of most importance for understanding and modeling charge exchange mechanisms. It is therefore worth-while first to present the most important experimental techniques allowing to get reliable information on this point. Then, charge transfer and photoionization processes will be studied through various systems, and experimental data will be successfully interpreted using simple models.

* e-mail: [email protected], Phone: +33 472 44 83 22, Fax: +33 472 43 11 30

186 Ch. Pédrini: Electronic processes in rare earth activated wide gap materials


Host CB

Host VB

Egap g

Egc

Evg

2 Location of rare earth energy levels in the gap. Experimental approach

Two kinds of approach can be used. One is based on the measurement of kinetic energy of emitted pho-toelectrons, leading to the determination of Evg, the energy difference between the ground localized state (g) of the rare earth dopant ion and the top of the valence band (Fig. 1). The other one is based on the measurement of photocurrent or of electron trapping effects on luminescence. In this case, the energy difference Egc between g and the bottom of the conduction band can be deduced.

2.1 Photoelectron spectroscopy

The scheme in Fig. 2 shows the excitation mechanisms involved in the photoelectron spectroscopy. The knowledge of the valence band maximum and 4fn binding energies allows to determine the 4fn ground state energy of the rare earth relative to the top of the valence band. Convential X-ray or UV photoelec-tron spectroscopy (XPS or UPS) can provide a clear information about the binding energy of core levels and of valence band of the host crystal, but the rare earth 4fn signal is usually weak and difficult to dis-tinguish, because of the overlap with the intense 2p valence band signal [1]. Resonant photoemission spectroscopy (RPES) using synchrotron radiation is able to extract the lanthanide component from the

Host CB

Host VB

4fn ground state

4fn bindingenergy

Kinetic E

E vacuum

VBMbindingenergy

ValenceBandMaximum(VBM)

hν

Fig. 1 (online colour at: www.pss-a.com) Location of the ground state of rare earth impurity ion in the gap of the host crystal.

Fig. 2 (online colour at: www.pss-a.com) Elec-tronic transitions involved in photoelectron spectros-copy.

phys. stat. sol. (a) 202, No. 2 (2005) / www.pss-a.com 187


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Fig. 3 (online colour at: www.pss-a.com) Extraction of the lanthanide component from the spectrum.

spectrum by using a “giant resonance” in the 4f electron photoemission when the photon energy is tuned into resonance with the ion’s 4d104f n to 4d94f n+1 transitions [2 and references therein]. In the case of Y2O3:Ce3+, the resonance energy is 121 eV and the difference between spectra obtained under excitation at the resonance energy and outside the resonance give a clear indication of the Ce 4f binding energy (Fig. 3). An important problem of this technique is the charging effects occurring in insulating materials, in-ducing shift, broadening and distortion of spectra. Fortunately, these effects can be strongly minimized [2]. A solution to suppress charging effects is to work on thin films deposited on conductive substrate. For example, RPES has been used to study the binding energy of 4f of trivalent rare earth ions (from Ce to Yb)-doped sol-gel films of Y2O3 deposited on silicon. Experimental results [3] are in excellent agree-ment with models recently developed [2, 4, 5].

2.2 Photoconductivity measurements

This kind of measurement is difficult because it is dealing with photoconductivity of impurity ions in insulating materials. Photocurrents to be detected are very weak, typically between 10–12 and 10–16 A/cm2, and polarization problems occur. Conventional photoconductivity technique uses blocking electrodes and a sandwich configuration in which light comes through the transparent front electrode and is collinear with the applied field [6]. This technique was then applied by many groups on rare earth doped crystals in order to measure the photo-conductivity thresholds. Two-step photoconductivity technique was also used in order to shift the direct excitation energy to lower indirect excitation energy through excited state absorption process. This technique is particularly useful to determine the location of the ground state of lanthanides when this state lies below the top of the valence band [7]. The microwave resonator technique, previously applied to semi-conductors for photoconductivity measurements, was recently applied to rare earth doped dielectric crystals [8]. This “contactless” tech-nique was able to detect the dielectric response of a rare earth doped crystal inserted in a resonant mi-crowave cavity and irradiated by laser pulses. This technique may be used not only for finding the loca-tion of the ground state of impurity ions with respect to the host conduction band, but also for detail studying of the photoionization dynamics.



2.3 Measurements of trapping effects on luminescence

The photoionization threshold can be measured by studying the excitation wavelength dependence for trap filling. Such methods, based on luminescence detection, are very sensible, applicable to both bulk and powder samples, and detected signals are impurity specific. Thermally stimulated luminescence excitation spectroscopy is a technique based on the detection of thermo-stimulated luminescence [9]. Another method is dealing with afterglow measurements through the fluorescence decay profiles. It works very well in case of fast fluorescence of the rare earth ion and when the detrapping is fast enough to give rise to a slow component of the decay. It was successfully applied to cerium doped compounds like LuAlO3:Ce3+, LaF3:Ce3+ and CeF3 [10, 11].

3 Charge transfer processes

Charge transfer transitions occur between ligand and rare earth dopant ions. They are allowed transitions (no selection rules), exhibit strong absorption and may exhibit intense and fast luminescence. Energies of lowest charge transfer absorption transitions can be estimated by the simple Jorgensen model [12, 13]:

σ = [χ(X) – χ(M)] ¥ 30.000 cm–1 (1)

where χ(X) and χ(M) are optical electro-negativity of the ligand ion and of the central rare earth ion, respectively. Electro-negativity of fluoride is high (χ(F) = 3.9) and absorption bands are expected in the far UV region. On the other hand, χ is much lower for oxygen (χ(O) = 3.2) and for sulfur (χ(S) = 2.8) so that absorption bands are expected in the near UV region in oxides and oxy-sulfides. The latter are there-fore more propitious host lattices for observing charge transfer absorption and emission transitions. Among all the trivalent rare earth ions embedded in oxides, Eu3+ and Yb3+ are ions which exhibit charge transfer absorption transitions at lowest energies [4].

3.1 Case of Eu3+-doped crystals

Charge transfer absorption transitions are often observed in Eu3+-doped oxides and oxysulfides, but not charge transfer emissions because of the presence of many underlying f levels to which non-radiative relaxation arises. In their early work on oxysulfide: Eu phosphors, Struck and Fonger [14] described charge transfer processes and showed their important role played in storage – loss mechanism in phosphors. In the con-figurational coordinate diagram of Fig. 4, the lowest charge transfer state (CTS) can be described as Eu2+ + hbound where the hole is delocalized on the ligands but still bounded to the divalent europium ion. This state feeds the 5D states through non-radiative relaxation and leads to sequential thermal quench-ing of the 5D emissions. No CT emission can occur. Another charge transfer state CTS* must be consid-ered for which the hole hfree is now free in the valence band. Its energy curve has its minimum a hole-binding energy ∆E above the CTS minimum, and its equilibrium configuration lies intermediate between those of CTS and 7FJ curves because of intermediate europium – ligand attraction. Such representation can explain energy loss through thermal hole ionization observed in some systems considered later.

3.2 Case of Yb3+-doped crystals

The systems are very interesting because the electronic structure of Yb3+ gives rise to only one 4f excited state close to the ground state and well below the CTS so that observation of CT emission transi-tions is possible [15–17]. A simple diagram (Fig. 5) shows that in case of radiative relaxation of CTS, two broad emission bands are expected separated by roughly 10 000 cm–1 which is the energy difference between the 2F5/2 and 2F7/2 spin–orbit states. A detailed study of charge transfer process have been performed in Yb3+-doped gar-



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45CTSCTS Eu + h2+

boundEu + h2+bound

CTS*CTS* Eu + h2+freeEu + h2+free

5D0

5D0

5D0

5D0

7F67F57F47F37F27F17F0

Configurational coordonate

Ene

rgy,

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m3

-1

∆E

Q- 0 +lattice

capt

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of a

n el

ectr

on

hνLUM

2F7/2

2F5/2

ECT

Yb2+ + hv

Yb3+

nets and perovskites [17–21]. CT fluorescence thermal quenching processes have been elucidated, due to trapping effects under X-ray excitation, and to both non-radiative relaxation of CTS to the lower states and thermal hole ionization under direct UV excitation of CTS. The latter thermal quenching was dem-onstrated through the investigation of correlated charge transfer and IR Yb 4f luminescences. The temperature dependence of the charge transfer and IR luminescence yield under excitation in the CT band (Fig. 6 up) shows that the feeding of 2F5/2 and 2F7/2 through radiative and non-radiative transi-tions from CTS can explain both CT and IR luminescences temperature dependence at temperatures above 150 K. However, at lower temperatures, the behaviour of the IR emission intensity cannot be simulated. Another mechanism must be involved. Taking into account CT ionization with creation of CTS* formed after thermal dissociation (Fig. 6 down), a good simulation is obtained. It seems that this process plays an important role in the CT fluorescence mechanism in most of Yb3+-doped oxide systems.

4 Photoionization processes

Photoionization of rare earth impurity ions is most often studied through photoconductivity measure-ments. The photoconduction thresholds observed are correctly interpreted as the energies between the ground state of the impurity ion in the band gap and the edge of the conduction band of the host crystal. It was demonstrated [6, 22] that a simple electrostatic model is a reasonably good guide to the values of the photoionization energies:

EPI = I – Em – ∆Em – Epol – EA (2)

where I is the ionization potential of the impurity ion,

Fig. 4 (online colour at: www.pss-a.com)Configurational coordinate diagram for the Eu center in La2O2S host.

Fig. 5 (online colour at: www.pss-a.com)Configurational coordinate diagram for the Yb center in YAG host.



0 50 100 150 200 250 3000

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,a.u

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0

5000

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35000

IR luminescenceCT luminescence

Fig. 6 (up) Temperature dependence of the charge transfer and IR luminescence yield under excitation in the CT band (6.2 eV). (down) Charge transfer states and emission transitions.

Em is the negative electrostatic potential energy at the metal ion site in the pure crystal (Madelung energy), ∆Em is the correction due to the distortion introduced by the impurity, Epol is the polarization due to removal of an electron from the metal site, EA is the electron affinity of the crystal (small). The most interesting rare earth ions for the study of photoionization of impurity ions are those which exhibit 4f ⇔ 5d transitions. 5d levels are often located close to the bottom of the conduction band, and photoionization processes are often involved in the 5d → 4f fluorescence mechanisms. The autoioniza-tion rate for a 5d state is usually large. Autoionization leads to larger photocurrents than direct photoionization. So the rare earth ions of importance for such a study are divalent rare earths and Ce3+.

4.1 Case of divalent rare earth ions in alkaline earth fluorides

Rare earth doped alkaline earth fluorides are model systems to study photoionization of impurity ions in crystals and they were the object of early investigations [6, 22–25]. The photoionization thresholds are expected at low energies because the energy of ionization (≈25 eV) is reduced by the high negative elec-trostatic potential (≈20 eV) to few eV. Divalent rare earth ions are stable in alkaline earth fluorides and exhibit 4f ⇔ 5d transitions in near UV – visible region. These systems are well-known and many data are available [26].



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Table 1 Photoionization energies (eV) of divalent rare earths in CaF2, SrF2 and BaF2.

CaF2 SrF2 BaF2

EPI calc EPI exp EPI calc EPI exp EPI calc EPI exp

Sm2+ 3.63 3.3 3.06 2.06 Eu2+ 5.25 3.8 4.45 3.8 3.79 2.9 Tm2+ 2.99 2.75 2.09 2.08 1.07 1.5 Yb2+ 4.22 4.34 3.34 3.10 2.27

In Table 1 are summarized photoionization energies of few divalent rare earths in CaF2, SrF2 and BaF2. The values calculated by the relation (2) are close to experimental data and the trends are well described for Sm2+, Tm2+ and Yb2+. The experimental photoionization energies of Eu2+ are actually much lower than expected from the electrostatic model. This discrepancy can be explained by the fact that excited state absorption is very strong in this case [27] and two-step photoionization process is dominant. In photoconductivity spectra, a strong modulation of the photoconductivity curves by the absorption of rare earth ions is often observed. It is for example the case of SrF2:Tm2+ [22] and SrF2:Yb2+ [24]. This shows that electron ejection occurs from the 5d state prior relaxation and proves that the autoionization rate from a 5d state is large. From a study of autoionization and Fano effect of 5d and 4f levels of Eu2+ in BaF2 [28–30], both levels being degenerated in the conduction band, it was deduced for 5d a relaxation time T1 = 25 fs while no broadening was observed for the 4f7(6P7/2) state after two-photon excitation. The autoionization rate from a 5d state is therefore much larger than from a 4f state. So-called “anomalous luminescences” (broad red-shifted emission bands) are observed in a few sys-tems like CaF2:Yb2+, SrF2:Yb2+, BaF2:Eu2+. A careful, detailed and complete spectroscopic investiga-tions [23–25] lead to assign these emissions to radiations from an impurity centered exciton which is the lowest excited state of the impurity-crystal system. Conditions for the formation of such an exciton is that the lowest excited states of the dopant ion are close or in the conduction band. The bound electron–hole pair can be described as a hole localized on the rare earth and the electron on nearly lattice sites. All the spectroscopic characteristics of “anomalous luminescences” can be interpreted with the impurity-trapped exciton model. It is interesting to establish a parallel between impurity-trapped exciton and charge transfer fluorescences through their representation in configurational coordinate diagrams (Fig. 7).

Fig. 7 Parallel between impurity-trapped exciton and charge transfer fluorescences.



Q- 0 + - 0 + Q

lattice

4f

5d

h�

Ce4+

2

2

h�

1

1

E =E (0)+E (0)g CT PI

E (0)CT

E (0)CT

ECT

ECT

E (0)PI

E (0)PI

EPI

EPI

(Ce )3+ *

Ce3+

Ce +h3+

v

Ce +e +h3+

c v

h ev c�

Ce +e4+

c

Ce +h +e4+

v c

Fig. 8 Born cycle model of the localized levels of Ce in the gap of the host. Single configuration coordi-nate diagrams.

Charge transfer fluorescence may occur in trivalent rare earth doped crystals and involves the transi-tion RE2+ + hbound → RE3+ while impurity-trapped exciton may occur in divalent rare earth doped crystals and involves the transition RE3+ + ebound → RE2+. With respect to the ground state, the CTS potential curve is shifted towards larger Q while the impurity-trapped exciton potential curve is shifted towards smaller Q. Both emitting states can ionize with the creation of a free hole in the valence band for charge transfer and of a free electron in the conduction band for impurity trapped exciton.

4.2 Case of Ce3+-doped crystals

These systems are of most interest because they often exhibit efficient and fast luminescence. But the location of the lowest 5d emitting state may be close or even degenerated in the conduction band and photoionization process may be an important limiting factor for the light yield. Photoionization and charge transfer mechanisms can be described using the Born cycle model involv-ing localized levels of Ce in the gap of the host (Fig. 8). In the case of Ce4+, CT absorption is often de-tected but no CT emission. The 5d → 4f fluorescence of Ce3+ is usually observed when the 5d is well below the bottom of the conduction band described in the diagram as the state Ce4+ + ec. In fact, three cases must be considered: – Case 1: all the 5d states are degenerated in the conduction band. This is for example the case of Ce-doped sesquioxides La2O3, Y2O3, Lu2O3. The cerium emission is totally quenched and the onset of the RT and LNT photoconductivity coincides with the onset of the lowest absorption band [31]. – Case 2: the lowest 5d state is close to the conduction band Examples: Ce-doped Lu2SiO5(LSO), LaI3 [31, 32]. Thermal quenching is governed mainly by thermo-stimulated photoionization. In LSO:Ce, the thermal activation energy is few tenths of eV, so that the luminescence intensity is high at room tempera-



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ture and rapidly decreases upon rising temperature. The onset of the RT photoconductivity coincides with the onset of the lowest absorption band [31] but at LNT, the photocurrent starts at energies corre-sponding to the high-energy side of the second absorption band. In LaI3 :Ce, the thermal activation en-ergy is still smaller (<0.1 eV). The crystal exhibits efficient luminescence only at T < 100 K. – Case 3: the lowest 5d state is well below the conduction band. This is the case of most Ce-doped fluorides, chlorides and oxides. Thermal quenching is governed by non-radiative relaxation to the ground state. High luminescence efficiency may be observed at high temperature.

5 Conclusions

Besides localized processes, charge exchange processes must be well described in order to understand fully the electronic properties of a rare earth impurity-crystal system. Photoionization and charge transfer of impurity ions in crystals are the electron donor and electron acceptor transitions of the impurity, respectively. The knowledge of the location of rare earth energy levels in wide band-gap crystals is of most impor-tance. Experimental determination can be performed using various techniques. Simple models are avail-able to evaluate the photoionization thresholds and charge transfer energies. Charge exchange processes are often involved in fluorescence quenching mechanisms, but may lead as well to new fast and intense luminescence like CT fluorescences and rare earth excitonic emissions.

Acknowledgements I would like to warmly thank Professor Donald S. McClure with whom I started in Princeton University early works on charge exchange processes in rare-earth doped ionic crystals. Professor McClure gave a major contribution to luminescence and spectroscopy [33] and he is a wonderful model for me. My collaborators in this field are strongly acknowledged as well, in particular Christophe Dujardin, Bernard Moine, Irina Kamenskikh and Ashot Petrosyan.

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Electronic processes in rare earth activated wide gap materials