Concentration quenching of the luminescence from trivalent thulium,terbium, and erbium ions embedded in an AlN matrix

Felix Benz a,n, Andreas Gonser a, Reinhart Völker b, Thomas Walther c,Jan-Thomas Mosebach a, Bianka Schwanda a, Nicolas Mayer a, Gunther Richter b,Horst P. Strunk a

a Institute of Materials Science, University of Stuttgart, Heisenbergstr. 3, 70569 Stuttgart, Germanyb Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germanyc Kroto Centre for High Resolution Imaging & Spectroscopy, Department of Electronic & Electrical Engineering, University of Sheffield, Mappin Street,Sheffield S1 3JD, UK

a r t i c l e i n f o

Article history:Received 12 April 2013Received in revised form27 August 2013Accepted 5 September 2013Available online 13 September 2013

Keywords:LuminescenceConcentration quenchingTerbiumErbiumThuliumAluminium nitride

a b s t r a c t

The concentration quenching behaviour of erbium, terbium, and thulium ions embedded in sputterdeposited AlN films was investigated. For each of these three systems a series of specimens with differention concentrations was prepared. In all three cases the concentration for maximum of the luminescenceintensity (optimum concentration) at the selected excitation parameters was determined. Theseoptimum concentrations differ strongly for the different ions. A rate equation model based on transitionprobabilities can explain the observations.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Rare earth doped semiconductors offer very narrow emissionlines, as desired for lasing and data transfer [1]. This property iscaused by the shielded 4f orbitals from which the luminescenceoriginates [2]. This luminescence is, as long as the matrix has awide enough bandgap, hardly influenced by the matrix material[3,4]. Nanocrystalline or amorphous matrices incorporate rareearth ions in much larger amounts than do single crystallinematerials. However, the increased luminescence expected for thepossible high rare earth concentration finds its limits in the so-called concentration quenching. Fig. 1 schematically shows thatbeyond the maximum usable concentration, called here optimumconcentration, luminescence intensity falls drastically (quenchingregion). This concentration quenching is caused by an enhancedprobability for energy transfer between the rare earth ions thatleads to a high chance to reach a path of non-radiative decay. Sucha path can be provided by grain boundaries or other defects that

offer states that can be excited and then relax non-radiatively. Onthe other hand, at low dopant concentrations the luminescenceintensity increases with increasing concentration (accessionregion).

In the present paper we investigate and compare the concen-tration quenching effect in aluminium nitride (AlN) layers dopedwith one of the three different rare earth ions thulium (Tm),terbium (Tb), and erbium (Er).

2. Experimental

2.1. Sample deposition and characterisation of the microstructure

All samples were deposited on silicon (100) wafers by reactivemagnetron co-sputtering in an argon/nitrogen atmosphere. Wecould alter the rare earth concentration by adjusting the power ofthe respective rare earth target. After the deposition process allsamples were annealed at 1000 1C in a 1 bar nitrogen atmospherefor 30 min in order to activate the luminescent centres. Scanningtransmission electron microscopy (STEM, Fig. 2(a) and (b)) and X-ray diffraction (XRD, Fig. 2(c)) investigations showed that the filmsconsist of a columnar AlN grain structure without any otherphases. An energy dispersive X-ray (EDX) map (Tb L edge) of the

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0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jlumin.2013.09.014

n Corresponding author. Current address: NanoPhotonics Centre, CavendishLaboratory, University of Cambridge, Cambridge CB3 0HE, UK.Tel.: þ49 711 685 619 12; fax: þ49 711 689 3412.

E-mail address: felix.benz@imw.uni-stuttgart.de (F. Benz).

Journal of Luminescence 145 (2014) 855–858

same area is shown in Fig. 2(b), note that the varying intensityalong the vertical axis is due to the wedge shape of the sample asalso visible in Fig. 2(a). The mapping indicates a quasi-homo-geneous distribution of terbium ions on the scale of severalnanometres. Thus precipitates, as e.g. observed for rare earth ionsembedded in silica [5–7], can be ruled out within this notion. TheXRD data shown in Fig. 2(c) support this conclusion as they showAlN peaks only; no secondary phase was observed. Nevertheless,clustering on an atomic scale cannot be excluded from the presentexperimental data. The slight asymmetry of the XRD peaks and theobserved shift in the peak position of AlN:Er could indicate thatthere is a difference in the local atomic arrangement leading to asmall local change in the lattice parameter. This aspect can beneglected within the scope of the present paper, respective work iscurrently on the way [8].

2.2. Photoluminescence spectroscopy and concentrationmeasurements

The photoluminescence intensity was recorded in dependenceon the emission wavelength within the visible spectrum. Thephotoluminescence measurement was performed with lightmonochromated from a xenon lamp (450 W), the excitation powerwas 2:5 μW=mm2 (255 nm for Tm and Er) and 0:6 μW=mm2

(250 nm for Tb). Both excitation powers lie in a region in whichthe concentration quenching behaviour is hardly changed byvarying the excitation power. Here the sum of the relaxation rates(radiative and non-radiative) is much larger than the excitationrate, i.e. the population in the excited state can be neglected.Recent results indicate that this region extends up to an excitationpower of approximately 5 μW=mm2 [9]. Erbium and terbium showgreen luminescence, whereas thulium emits in the blue spectralregion. The integrated intensity values of the respective emissionpeaks were evaluated as a function of the average nearestneighbour distance ⟨r⟩ between the dopant ions. This average rareearth nearest neighbour interdistance ⟨r⟩ was calculated fromnumber densities n, which were determined by inductivelycoupled plasma optical emission spectroscopy (ICP-OES), byassuming a random distribution function ρðrÞ, as proposed byChandrasekhar [10]. Since this author considered planets far awayfrom each other, i.e. point elements, we have modified hisapproach for the finite diameter of the densely packed ions ofthe crystal by introducing a minimum distance d0 approximatelytwice the ionic radius for the respective rare earth ion(d0 � 0:206 nm, 0.212 nm, 0.204 nm for Er, Tb, Tm, respectively)

ρðrÞ ¼ ðr2�d20Þn� exp �43πnðr3�d30Þ

h ið1Þ

⟨r⟩¼R1d0

dr ρðrÞ � rRdr ρðrÞ : ð2Þ

inte

nsity

average interdistance

concentration

accession regionquenching region

optimum concentrationcritical distance

Fig. 1. Schematic of the concentration quenching effect: for low concentrations (i.e.large interdistances) the luminescence intensity increases with increasing concen-tration. If the concentration is increased beyond the optimum concentration theluminescence intensity decreases drastically.

Si

AlN:Tb

54525048464442403836343230

inte

nsity

(a.u

.)

diffraction angle 2θ (°)

100 002101

102

AlN:Tb

AlN:Er

AlN:Tm

Fig. 2. (a) Scanning transmission electron microscope bright field image representative for all samples (here: AlN:Tb layer with approx. 2:1� 1021 cm�3 Tb ions). (b) Energydispersive X-ray spectroscopy (Tb L edge) map of the same area as shown in (a) the size of the electron probe was around 2 nm, the sampling is 6.4 nm per pixel. Note thatthe vertical variation in intensity in the AlN region in (b) is due to thickness variations as clearly can be seen from (a). (c) X-ray diffractograms ðCu�KαÞ forAlN : Tb ð4:9� 1021 cm�3Þ, AlN : Er ð1:7� 1021 cm�3Þ, and AlN : Tm ð4:0� 1021 cm�3Þ. Reflectances indicate the sole presence of the AlN wurtzite phase. Note that theindividual curves are shifted in intensity. The shown data are representative for all investigated samples.

F. Benz et al. / Journal of Luminescence 145 (2014) 855–858856

3. Results and discussion

Fig. 3 shows the results obtained from all samples in theinvestigated interdistance region ð0:3 nmoro0:8 nmÞ. The fittingcurves in the quenching region (solid lines) are the result of a rateequation model (see below). For the accession region (dashedlines) intensity curves proportional to the number density n(converted into average interdistances) are shown. The criticaldistances rcrit, i.e. the distances at which the highest luminescenceintensity was observed, vary strongly among the different rareearth ions.

Table 1 lists the obtained critical distances and the correspond-ing (optimum) number densities nopt.

In a first attempt to explain these dissimilarities we use asimple rate equation model. As reported previously the emissionintensity can be expressed as [11]

I ¼N � A� pexc

pexcþAþCþD� pETð3Þ

with Einstein's A coefficient [12], the probability for excitation pexc,the total number of ions N, the quenching parameters C and D, andthe probability for energy transfer pET. The various interactionpossibilities with the rare earth surroundings are accounted for bythe respective probabilities p. For example pexc strongly dependson the realised excitation mechanism, like a charge transferexcitation [13], excitation by 4fn-4fn�15d1 transitions [13],excitation via bound excitons [14,15], or excitation by energytransfer from defects in the rare earth surroundings [16]. To keepthe mathematical description as simple as possible electrostaticinteraction will be assumed to be responsible for the energytransfer (pETpðr0=rÞm, r0: Förster radius [17]). The first derivativeof Eq. (3) with respect to r yields the critical distance rcrit (distancewith the largest intensity) that is proportional to

rcritpr0 �D

AþCþpexc

� �1=m

ð4Þ

with m a parameter corresponding to the mode of energy transfer(m¼6,8,10 for dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interaction, respectively).1 The quenching parameter Ccontains the total probability for other, concentration-indepen-dent, relaxation mechanisms such as thermal quenching or emis-sion of phonons; this probability thus depends, for large bandgapsemiconductors, on the temperature and on the quality (disloca-tion density, grain size, defect densities, etc.) of the matrix materialonly. On the other hand, D describes the enhancement of theprobability for a non-radiative relaxation and is therefore propor-tional to the defect density. (Note: D is equal to unity if eachenergy transfer leads to a non-radiative relaxation and, hypothe-tically, zero in the case that energy transfer does not lead toloosing the excitation intensity.) Since all investigated samples aredeposited, treated, and investigated under the same circumstancesand with the same parameters we can assume that both quench-ing parameters, C and D, do not strongly differ among the usedsamples. Likewise the ionic radii for all used ions are approxi-mately equal (note: r0pd0) and can therefore be assumed to beconstant. By using these assumptions one can conclude that thedifferences in the critical distances are either due to differentexcitation probabilities or due to different radiative recombinationprobabilities. Further investigations, like the measurements oflifetimes, would be necessary for a more detailed analysis of theresponsible process.

4. Conclusion

We have studied the visible luminescence properties of thethree rare earth ions Tm3þ , Tb3þ , and Er3þ in a polycrystalline AlNmatrix with special attention to the concentration quenchingbehaviour. The rare earth concentration at which the lumines-cence intensity that initially increases with concentration even-tually starts to decrease (optimum concentration) is different forthe three investigated rare earth ions. It lies in the 1021 cm�3

range (3.6, 1.2 and smaller than or equal to 0.42�1021 cm�3 forerbium, terbium, and thulium, respectively). We anticipate thatthese concentrations increase for higher excitation intensitiesuntil, at very high excitation intensities, concentration quenchingis completely absent [11].

Acknowledgements

The authors thank Y. Weng and H. Radenbach for carefulreading of the first draft and G. Maier (Max Planck Institute forIntelligent Systems, Stuttgart, Germany) for supporting the XRDmeasurements.

References

[1] For a review see, e.g., K. O'Donnell, V. Dierolf (Eds.), Rare-Earth Doped III-Nitrides for Optoelectronic and Spintronic Applications, Springer, Dordrecht,2010.

[2] B. Johansson, N. Mårtensson, Phys. Rev. B 21 (1980) 4427.[3] A.R. Zanatta, Appl. Phys. Lett. 82 (2003) 1395.[4] F. Benz, J.A. Guerra, Y. Weng, A.R. Zanatta, R. Weingärtner, H.P. Strunk, J. Lumin.

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1.00.90.80.70.60.50.40.30.20.10.0

0.800.700.600.500.40

norm

alis

ed in

tens

ity (-

)

average distance (nm)

AlN:ErAlN:TbAlN:Tm

Fig. 3. Quenching curves for AlN:Re (Re¼Tb, Tm, Er). The dashed curves corre-spond to a fit in the accession region, which is proportional to the number densityof ions. The solid lines are fits corresponding to the fraction in Eq. (3).

Table 1Critical distances rcrit and corresponding optimum number densities nopt for thedifferent samples.

Sample rcrit (nm) nopt (1021 cm�3)

AlN:Er 0.42 3.6AlN:Tb 0.57 1.2AlN:Tm Z0:78 r0:42

1 Note that the used distances are average nearest neighbour distances,interactions with rare earth ions that are farer away are neglected for the presentmodel. To be even precise one should use ⟨pETðrÞ⟩ instead of pETð⟨r⟩Þ. The usedformulae can be regarded as a first approximation since the Förster energy transferprobability decreases rapidly with increasing rare earth–rare earth interdistance.Preliminary simulation results suggest that this approximation is practicallyaccurate to within approximately 10% [18].

F. Benz et al. / Journal of Luminescence 145 (2014) 855–858 857

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