www.elsevier.com/locate/sab

Spectrochimica Acta Part B

Cathodoluminescence study of the activator distribution

in SrS:Cu,Ag thin luminescent filmsB

D. Poelmana,*, J. Doerschelb, P.F. Smeta

aDepartment of Solid State Sciences, Ghent University, Krijgslaan 281 S1, B-9000 Gent, BelgiumbInstitut fur Kristallzuchtung (IKZ), Max-Born-Strasse 2, D-12489 Berlin (Adlershof), Germany

Received 1 November 2003; accepted 30 April 2004

Available online 1 September 2004

Abstract

The lateral distribution of the CL (cathodoluminescent) emission of electron beam deposited SrS:Cu,Ag thin films was studied using

SEM-CL (CL in a scanning electron microscope) spectroscopy. Both CL spectra and CL images at different wavelengths were acquired at

room temperature and at low temperature (80 K). Using this technique together with a deconvolution of the CL spectra, the contributions of

Cu and Ag activators to the overall emission could be separated. Differences in emission spectra between samples, annealed under different

conditions, could be interpreted from the different distribution of activators.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Cathodoluminescence; Thin films; SrS:Cu,Ag; Activator distribution

1. Introduction

During the last few years, there has been extensive

interest in SrS:Cu,Ag as a blue electroluminescent emitter

for flat panel display applications [1]. While there have been

several efforts to find a model explaining the emission

process in this phosphor, there is still quite some uncertainty

on the role played by the copper and silver activators and

their location in the SrS host. Recently, the lateral

distribution of the luminescent emission from such a

phosphor layer was monitored, using SEM-CL (cathodolu-

minescence using a scanning electron microscope) [2,3].

With this technique, it was shown that the light distribution,

usually seen from these layers (light mainly coming from

the grain boundaries) is due to optical outcoupling effects

and that the emission mainly originates from the bulk of the

phosphor grains.

0584-8547/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.sab.2004.04.016

B This paper was presented at the International Congress on X-Ray

Optics and Microanalysis (ICXOM XVII), held in Chamonix, Mont Blanc,

France, 22-26 September 2003, and is published in the special issue of

Spectrochimica Acta Part B, dedicated to that conference.

* Corresponding author. Tel.: +32 9 264 43 67; fax: +32 9 264 4996.

E-mail address: dirk.poelman@UGent.be (D. Poelman).

In the present work, spectroscopic SEM-CL was used in

order to separate the contributions due to Cu and Ag

emission. CL spectra and CL mappings at selected wave-

lengths were measured on electron beam deposited

SrS:Cu,Ag layers. Both layers annealed in N2 and in H2S

were studied. Since the thermal quenching of the lumines-

cence in these materials is considerable, room temperature

as well as low temperature (80 K) measurements were

performed. The penetration depth and the attainable lateral

resolution were estimated using CASINO Monte-Carlo

simulations [4].

2. Experimental

Thin films SrS:Cu [0.4 mol%], Ag [0.6 mol%] (activator

concentrations in the starting materials) were deposited by

electron beam deposition under a H2S atmosphere (partial

pressure of 5�10�3 Pa), at a substrate temperature of 500

8C. A Leybold Univex 450 vacuum system with a base

pressure of about 2�10�4 Pa was used. Sintered powder

was used as source material: appropriate amounts of SrS

(Cerac, Milwaukee, 99.99%), metallic Cu and Ag2S were

59 (2004) 1775–1780

D. Poelman et al. / Spectrochimica Acta Part B 59 (2004) 1775–17801776

thoroughly mixed. This powder was sintered in a H2S

atmosphere during 1 h at 1000 8C, which yielded material

that could be used as the electron beam evaporation source

without further treatment or pressing [5]. Films with a

thickness of about 1000 nm were deposited on Vycor

(Corning 7913) substrates.

Since these as-deposited SrS:Cu,Ag films are not

luminescent [1], they were subjected to a post-deposition

annealing treatment. Two types of annealing were per-

formed, which had already proven to yield good results for

this kind of samples [1,3]. A first type of post-deposition

annealing (further called specimens type A) consisted of a

rapid thermal anneal (RTA) in a nitrogen atmosphere at

atmospheric pressure during 30 s at 800 8C, using an AST

Super Heat System 1000. A second type of annealing

(specimens type B) was performed at 680 8C during 30 min

in a quartz tube furnace, in a pure H2S flow at atmospheric

pressure. Both kinds of annealing treatment closely repre-

sent maximum annealing times and temperatures for

common flat panel display substrates (typically Corning

borosilicate glass 7059 or 1737). Unfortunately, it is not

possible to perform annealing experiments in H2S in the

RTA equipment for technical reasons.

3. Results and discussion

On the subject of the exact location of the Cu and Ag

activator ions in luminescent SrS:Cu,Ag films, different

results have been published: Xin et al. [6] expected Cu-

diffusion into crystallite grains in SrS:Cu upon annealing for

thermodynamic reasons and also found experimental evi-

dence for this behaviour [7]. Vdovenkov et al. [8] reported

increased activator concentrations at the grain boundaries in

annealed layers, based on EDX mappings. In the present

work, it turned out to be that the activator concentrations

were below the detection limit of our EDX equipment.

However, luminescence measurements can be orders of

magnitude more sensitive for the detection of defects than

X-ray microanalysis [9]. Therefore, the optical emission

characteristics of the layers were used to study the activator

distribution.

Over the last decades, numerous articles have been

published, discussing the luminescent emission spectrum

(mainly photoluminescence and electroluminescence) of

SrS:Cu and SrS:Cu,Ag powders and thin films. At first,

the emission mechanism in SrS:Cu was thought to be the

same as in ZnS:Cu, namely the recombination of a donor

level to the Cu acceptor level [10]. Soon, however, it was

realised that a different kind of transition was responsible

for the luminescence in SrS:Cu [11], an internal transition in

the Cu+ ion (4s3d9Y3d10).

Part of the problem in identifying the peaks in the SrS:Cu

emission stems from its irreproducibility [12]: the spectrum

is strongly dependent on the preparation conditions of the

powders and the thin films [13] and on the activator

concentration. The latter effect was studied in detail on

SrS:Cu powders by Yamashita et al. [14]. These authors

distinguished the emission from Cu monomers, dimers,

aggregates and large aggregates, at 513, 543, 558 nm and at

higher wavelengths respectively (photoluminescence at

80 K). At room temperature, an additional peak in the

blue region is seen, and at moderate Cu concentrations,

two peaks are usually described, at 460 and 520 nm

[5,13,15]. These are attributed to emission from Cu-ions

in different site symmetries.

In 1997, SrS:Cu was proposed for the first time as the

blue emitter in AC thin film electroluminescent (ACTFEL)

display devices [16]. While quite high luminance could be

obtained from these devices, most of them could not provide

the saturated blue emission, necessary for full colour display

applications. SrS:Cu,Ag was then proposed (Ag acting as a

co-activator) to shift the emission spectrum to shorter

wavelengths and to obtain pure blue emission [17].

While improved colour purity and luminance could be

obtained using SrS:Cu,Ag, the interpretation of the emis-

sion spectrum and the interactions of the activators is much

more difficult than in SrS:Cu. SrS:Ag in itself only shows

weak luminescence (at about 360 nm—in the UV—and at

420 nm [17]), which is strongly quenched at room

temperature. The 360 nm peak has been attributed to

emission from single Ag-ions (monomers), while it was

thought that the 420 nm peak is correlated with Ag

dimers [17]. The photoluminescent emission spectrum of

SrS:Cu,Ag at low temperature shows both the peaks due to

Ag (360 and 420 nm) and a broad peak due to Cu at about

520 nm [18,19]. At room temperature, the 360 nm peak is

consistently quenched; the spectrum then consists of peaks

at about 420, 460–490 and 500–520 nm, again very much

dependent on preparation conditions [5]. While the peak

around 420 nm is clearly due to Ag and the one around

500–520 nm due to copper, there is some uncertainty about

the origin (being Ag-or Cu-related) of the 460–490 nm

emission [13].

When the photoluminescent, electroluminescent or cath-

odoluminescent emission of thin films is observed on a

microscopic scale, the observed light distribution is usually

entirely governed by optical outcoupling effects. In a

previous publication [3], these effects were studied in detail

and, at the same time, SEM-CL was proven to be a suitable

technique to avoid these optical outcoupling effects. There-

fore, SEM-CL allows observing the optical emission where

it is generated in the layer, not where it exits the layer after

(possibly) multiple reflections. The aim of the present work

was to separate the contributions of the Cu- and Ag-related

emission in SrS:Cu,Ag thin films, in order to improve on

our understanding of their interactions.

CL emission spectra and CL images were acquired using

a Zeiss DSM 962 scanning electron microscope, equipped

with an Oxford monoCL cathodoluminescence extension.

The acceleration voltage was 10 kV for all experiments,

while the beam current was chosen between 5 and 200 pA

Fig. 2. Cathodoluminescence emission spectra from the SrS:Cu,Ag layers

for both types of samples, at 80 K and at room temperature. Gaussian fits

were performed when the spectra were plotted on an energy scale.

D. Poelman et al. / Spectrochimica Acta Part B 59 (2004) 1775–1780 1777

as a function of the CL brightness at the wavelength

selected. In all cases, the beam diameter was well below

100 nm.

In order to get an estimate of the CL image resolution

under these experimental conditions, the beam penetration

in the SrS sample was calculated using a CASINO [4]

Monte-Carlo simulation. Fig. 1 shows the resulting energy

distribution of the incident electron beam. For primary

electron energies of the order of 10 kV, the phosphor

efficiency (in lumen per watt) is usually independent of

electron energy, while it rapidly decreases towards lower

energy [20]. From this fact, and from Fig. 1, it was

estimated that at least half of the CL emission emerges from

an apparent beam spot of 250 nm wide. Therefore, 250 nm

can also be used as an upper limit of the attainable CL

lateral image resolution.

The CL emission spectra, obtained at both low temper-

ature (80 K) and room temperature, are shown in Fig. 2, for

a specimen type A (rapid annealing in N2) and for type B

(slow annealing in H2S). In all cases, multiple Gaussian fits,

using a minimum number of peaks to get a decent match,

were performed on the spectra. In order to perform this

fitting, spectra were plotted on an energy scale [10].

At low temperature, four emission peaks can be

distinguished, at about 485, 440, 425 and 360 nm. At room

temperature, the UV emission (360 nm) is completely

quenched and the other peaks are somewhat shifted,

yielding peaks at about 450, 500–510 and 520–530 nm.

These spectra, both at low temperature and at room

temperature, are remarkably similar to the PL spectra of

sputtered films SrS:Ag,Cu,Ga from Li et al. [13]. We

tentatively ascribe the low temperature peaks to Ag (360

and 425 nm) and Cu (440 and 485 nm), the latter peaks

being clearly influenced by the presence of Ag. At room

temperature, only one Ag-peak remains (at 450 nm),

together with very broad peaks for Cu. As can be seen in

Fig. 2, there is so much overlap between the peaks at room

temperature that the peak fitting and the corresponding

identification becomes quite difficult.

Fig. 1. CASINO [4] Monte-Carlo simulation of the penetration of a 100 nm

wide electron beam (primary energy 10 kV) in SrS (density q=3.7 g/cm3).

Numbers indicate the average electron energy relative to the primary

energy.

The complete set of all fitted peaks is listed in Table 1,

together with the intensity ratio of Ag- to Cu-related

emission.

The corresponding CL-mappings (recorded at low

temperature) are depicted in Figs. 3 and 4, for specimens

A and B, respectively. Room temperature mappings are

not shown, since they essentially provide the same

information. Moreover, the overlap between different

emission peaks increases at higher temperature due to

an increase of the line width with temperature, making it

difficult to distinguish different peaks on single-wave-

length CL mappings. Secondary electron (SE) images

were added to show the actual surface morphology of

the films. As observed earlier [2,21], the type of

annealing has a marked influence on the morphology:

rapid thermal annealing (type A) leads to a cracked surface,

while slow annealing (type B) yields well-developed

grains.

Table 1

Emission peaks of both type A and type B films, at low temperature and at

room temperature (RT)

Specimen T (K) Cu-

peaks

(eV)

Ag-

peaks

(eV)

Cu-

peaks

(nm)

Ag-

peaks

(nm)

IAg/ICuratio

A (N2-anneal) 80 K 2.50 2.93 481 424 0.54

2.80 3.46 442 358

A (N2-anneal) RT 2.33 2.72 531 454 0.61

2.36 499

B (H2S-anneal) 80 K 2.52 2.91 486 425 1.00

2.78 3.45 444 360

B (H2S-anneal) RT 2.25 2.78 519 444 1.99

2.36 510

Fig. 3. Secondary electron (SE) and CL images at several wavelengths of a SrS:Cu,Ag layer, annealed in N2 (type A). Images were recorded at low temperature

(80 K).

D. Poelman et al. / Spectrochimica Acta Part B 59 (2004) 1775–17801778

The very low Cu-related emission in some samples

SrS:Cu,Ag, and the associated increase in Ag-emission,

have been interpreted as an energy transfer from Cu to Ag

[19]. This transfer requires the Cu and Ag activators to be in

close proximity, the maximum distance for efficient energy

transfer between centres being of the order of 5–10 nm for

multipolar interactions [22]. Thus, there should be a

correlation between the Ag to Cu intensity ratio and the

proximity of Cu and Ag ions.

Fig. 4. Secondary electron (SE) and CL images at several wavelengths of a S

temperature (80 K).

If we compare the fitted peaks in Fig. 2 and the selected

wavelengths in Figs. 3 and 4, and the above literature

overview of the assignment of peaks in SrS:Cu,Ag, it

becomes clear that we have to interpret the (low temperature)

CL mappings as follows: 359 and 426 nm peaks correspond

to pure Ag emission (monomers and—tentatively—dimers

respectively), the latter with some influence of the short-

wavelength tail of the 442–444 nm Cu peak, and the

emission at 520 or 535 nm is due to Cu-emission only.

rS:Cu,Ag layer, annealed in H2S (type B). Images were recorded at low

D. Poelman et al. / Spectrochimica Acta Part B 59 (2004) 1775–1780 1779

In samples annealed in H2S (type B), there is little

difference between the lateral distribution of the different

emission peaks. In samples, annealed using RTA (rapid

thermal annealing, type A) in N2, the emission due to Ag

is seen to emerge from the bulk of the crystallite grains,

while Cu emission is also coming from the grain

boundaries. The latter effect indicates Cu migrates to grain

boundaries upon annealing in N2. This observation is

consistent with the relative decrease of the Ag-related

emission to the Cu emission, as seen in the last column of

Table 1.

From the results of Figs. 3 and 4, one could interpret the

difference in lateral distribution between the 359 nm and the

426 nm emission as an experimental indication of the fact

that these peaks should arise from Ag monomers and dimers

respectively. While the Ag dimers and monomers could

indeed be located on slightly different positions in the

grains, this interpretation should be handled with caution,

especially since the emission at 426 nm also partly results

from Cu.

4. Conclusions

It has been argued that a strong energetic coupling exists

between Cu- and Ag-ions in SrS:Cu,Ag [23], based on the

fact that the emission in SrS:Ag (without Cu doping) is very

weak. Therefore, the proximity of Cu- and Ag-ions should

be crucial for the efficient luminescent emission from Ag-

ions and distributions of Ag and Cu should virtually overlap

in order to have high luminance from the Ag centres. This

interpretation is supported by the current results, since

migration of Cu towards the grain boundaries leads to a

physical separation of Cu and Ag ions and thus to a decrease

of the Ag-related emission. The latter effect is only observed

in specimens of type A, using rapid annealing in N2. In this

case, the cracks that are formed upon annealing, become

decorated with Cu. In specimens of type B, no outdiffusion

of Cu seems to take place.

Using SEM-CL spectroscopy, the lateral distribution of

Cu- and Ag-activators in SrS:Cu,Ag luminescent layers

could be determined. It was observed that the type of

post-deposition annealing of the layers has a marked

influence on the emission spectra and the diffusion of Cu.

This effect can in part explain the differences in emission

spectrum, found for SrS:Cu,Ag layers, produced by

different research groups. While the present results do

not allow to give a definitive answer to the problems with

the interpretation of the emission spectra of SrS:Cu,Ag, it

has been shown that SEM-CL spectroscopy is a powerful

technique to distribution of luminescent activators in such

complex systems. Future work will include the measure-

ment and analysis of photoluminescent excitation and

emission spectra of SrS:Cu,Ag layers as a function of

activator concentrations, in order to confirm and complete

the present results.

Acknowledgments

One of the authors (P.F.S.) is research assistant for FWO-

Vlaanderen (Fund for scientific research Flanders).

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