BLUE ELECTROLUMINESCENCE FROM MULTILAYERED BAS:EU AL2S3 THIN FILMS
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JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 1 1 JANUARY 2004
Blue electroluminescence from multilayered BaS:EuÕAl2 S3 thin films
P. F. Smet,a) D. Poelman, and R. L. Van Meirhaeghe
Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, 9000 Gent, Belgium
共Received 2 April 2003; accepted 10 October 2003兲
Electroluminescent 共EL兲 BaAl2 S4 :Eu devices were prepared with a multilayered phosphor,
composed of 10–20 alternating BaS:Eu and Al2 S3 thin films and a total phosphor thickness of 300
nm. Depending on evaporation and annealing conditions, pure blue EL was obtained 共CIE 1931
color coordinates x⫽0.120, y⫽0.098). The use of this multilayered BaS:Eu/Al2 S3 structure gave
access to additional information on the atomic composition and luminescent properties of
europium-doped barium thioaluminates. The composition of the phosphor layer 共diffusion
properties, oxygen impurities兲 was characterized using x-ray photoelectron spectroscopy as a
function of substrate temperature and annealing parameters. Optical and morphological properties of
BaS共:Eu兲 and Al2 S3 thin films are discussed as well. © 2004 American Institute of Physics.
关DOI: 10.1063/1.1630372兴
I. INTRODUCTION properties of the devices and the constituent layers will be
discussed.
In 1999 Miura et al. introduced europium-doped barium The approach that uses pulsed EBE with two targets can
thioaluminate (BaAl2 S4 :Eu) as a high-luminance, blue- almost be considered as a coevaporation technique, as the
emitting phosphor for thin-film electroluminescent electron beam is rapidly switched between the two pellets
devices.1,2 A specific evaporation technique was used to ob- 共with a duty cycle of only 10 ms兲. The final composition is
tain stoichiometric thin films: namely, pulsed electron then determined by the energy fed to the BaS:Eu and Al2 S3
beam evaporation using two targets. The electron beam was pellets during the evaporation process. In this way, the link
alternately switched between BaS:Eu and Al2 S3 targets. The with the properties of BaS:Eu and Al2 S3 thin films is lost as
use of a very short duty cycle 共10 ms兲 led to the deposition of a mixture of all elements is immediately obtained.
far less than a monolayer of BaS:Eu and Al2 S3 each cycle.1 BaAl2 S4 :Eu thin films prepared with pulsed EBE show
In this way stoichiometric BaAl2 S4 :Eu thin films could be a high oxygen content,3 significantly larger than in most
obtained, with a blue emission band peaking at 470 nm. other ACTFEL host materials, such as SrS 共Ref. 4兲 and CaS
The pulsed electron beam evaporation 共pulsed EBE兲 共Ref. 5兲. In our approach, depending on the evaporation con-
method with two targets was used by Miura et al. in 1997 for ditions, distinct BaS:Eu and Al2 S3 layers are present, which
the production of CaGa2 S4 :Ce thin films.2 Due to the differ- will allow us to pinpoint the origin of the oxygen contami-
ent vapor pressure of the elements of these thiogallates, the nation. Furthermore, Eu3⫹ emission is observed in BaS thin
films 共this work兲; in the BaAl2 S4 :Eu thin films only Eu2⫹
preparation of stoichiometric thin films is very difficult with
emission is present. We will present further information on
conventional techniques. Similar problems are experienced
the valence state of europium, which is easily accessible by
with thioaluminates: we were unable to produce high quality
using the multilayered structure.
BaAl2 S4 :Eu thin films when EBE of europium-doped
Although this multilayered approach yields additional
BaAl2 S4 pellets was tried. The thin films evaporated in this
information, the multilayer deposition technique will prob-
way were hardly transparant and did not show significant ably not yield the best thin films 共with the highest lumi-
electroluminescent emission. nances兲, when compared to the approach with pulsed EBE
In this paper, we propose an alternative deposition tech- using two targets. Indeed, it is perfectly understandable that
nique, which is very interesting from a fundamental scientific by using a multilayered approach, one cannot obtain the de-
point of view since it enables us to study different aspects of sired stoichiometry throughout the whole phosphor layer,
BaAl2 S4 :Eu thin films. The central phosphor layer is com- without the least deviation, even after considerable thermal
posed of alternating BaS:Eu and Al2 S3 layers, with an indi- annealing. This will have a serious impact on the efficient
vidual thickness varying from 15 to 50 nm. Due to the high acceleration of the electrons in the BaAl2 S4 :Eu phosphor
reactivity of the constituent layers, Bax Aly Sz :Eu can be layer and thus on the obtained luminance. To get a perfect
formed, and the thin films show bright electroluminescent mixture of BaS:Eu and Al2 S3 , one will tend to reduce the
emission, depending on the BaS/Al2 S3 ratio. For individual layer thickness, and in the end the multilayered
BaAl2 S4 :Eu, emission at 470 nm can be obtained, typical for approach will converge to a pulsed EBE method using two
these devices.1 The luminescent, optical, and morphological targets.
The peak wavelength of photoluminescent 共PL兲 emission
a兲
Author to whom correspondence should be addressed. Electronic mail: in europium-doped thioaluminate powder is known to be
philippe.smet@Ugent.be strongly depending on the BaS:Al2 S3 ratio 关 BaAl2 S4 共467
0021-8979/2004/95(1)/184/7/$22.00 184 © 2004 American Institute of Physics
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nm兲, Ba2 Al2 S5 共487 nm兲, Ba5 Al2 S8 共540 nm兲兴 共Ref. 6兲 due
to the strong influence of the crystal field, as the nearest
excited state of Eu2⫹ lies in the outer d shell. Furthermore,
the PL of BaAl2 S4 :Eu shows a higher efficiency and a better
thermal quenching behavior than other barium thioalumi-
nates. Miura described the influence of the BaS/Al2 S3 ratio
on the peak wavelength of the electroluminescent 共EL兲
emission.2 The emission shifts to longer wavelengths upon
an increase in this ratio, in correspondance with the results
obtained by Le Thi et al. on powder phosphors.6
II. EXPERIMENT
Sintered BaS:Eu and Al2 S3 pellets were used as the
source material for electron beam evaporation. BaS 共99.9%, FIG. 1. Transmission spectrum for 共a兲 BaS thin film 共850 nm thickness兲 and
共b兲 Al2 S3 thin film 共500 nm thickness⫹30 nm ZnS兲.
CERAC兲 and EuF3 共99.5%, CERAC兲 were mixed and
pressed into pellets, before sintering in H2 S at 800 °C for 1
h. Unless otherwise stated, europium concentration in BaS
was 2 mol %. Pressed pellets of Al2 S3 共99.9%, CERAC兲 nealing in H2 S (800 °C) pronounced this orientation. The 2
were sintered as well (H2 S, 600 °C, 1 h兲. To avoid contami- full width at half maximum decreased upon annealing 共from
nation of the source material due to the high reactivity and 0.33° to 0.24°), suggesting grain growth. XPS measure-
hygroscopic character of BaS and Al2 S3 , as many manipu- ments revealed only a slight oxygen contamination in the
lations as possible were performed under a protective nitro- BaS thin films. We can conclude that nearly stoichiometric
gen atmosphere. thin films can be evaporated at elevated substrate tempera-
Thin films were deposited on ITO-coated Corning 1737 ture.
substrates in H2 S atmosphere with a partial pressure of 2 Europium ions can generate luminescence in both diva-
⫻10⫺3 Pa. As insulating layers, electron beam evaporated lent and trivalent state.7 Eu3⫹ emission originates from for-
Al2 O3 共Merck兲 thin films were used, with an approximate bidden electric–dipole 4 f – 4 f ( 5 D 0 – 7 F J ) transitions, which
thickness of 300 nm. Here 200-nm-thick ZnS 共99.99%, are hardly affected by the crystal field. Therefore, the emis-
CERAC兲 thin films were introduced just above and below sion spectrum is characterized by narrow emission lines. On
the emitting layer, acting both as buffering and as electron the other hand, Eu2⫹ emission is based on electric–dipole
accelerating layers. allowed 5d4 f 6 – 4 f 7 transitions, which leads to a broad emis-
The central phosphor layer consists of an alternating sion band. When europium is introduced in a II-VI host as
structure of BaS:Eu and Al2 S3 layers, with a total thickness CaS or SrS, divalent broadband emission is usually ob-
of 300 nm. Thicknesses of the different layers are determined served. This emission is largely influenced by the crystal
from the molar ratio between BaS and Al2 S3 . Typical evapo- field as the nearest excited state of Eu2⫹ lies in the outer d
ration rates are 0.8 nm/s. The idle time between the evapo- shell: emission shifts to shorter wavelengths with decreasing
ration of two successive layers is kept short 共less than 10 sec兲 crystal field strength. When the host lattice is varied from
to reduce contamination from residual gasses in the vacuum CaS to SrS to BaS, the luminescence peak in lightly
system. europium-doped powder phosphors shifts from 651 nm to
Ex situ post-deposition annealing was performed in ni- 616 nm to 572 nm.8
trogen 关in a rapid thermal anneal 共RTA兲 system 共AST Super The present BaS:Eu thin films show a very weak orange
Heat System 1000兲兴. Emission spectra were recorded using a to nearly infrared electroluminescence, depending on the eu-
microchannel plate intensified optical multichannel analyzer ropium concentration 共see Fig. 2兲. BaS:Eu 关0.5 mol %兴
共EG&G OMA III兲. Optical transmission spectra were ob- shows a very broad emission band peaking at 740 nm 共CIE
tained using a Cary 500 spectrophotometer. X-ray photoelec- 1931 color coordinates x⫽0.61, y⫽0.33); the expected
tron spectroscopic 共XPS兲 measurements were performed broadband emission at 572 nm was not observed. Upon
with a Perkin Elmer PHI ESCA 5500 system. higher driving voltage the Eu3⫹ emission line at 616 nm,
corresponding with the 5 D 0 – 7 F 2 transition, emerges.
III. RESULTS AND DISCUSSION An increase in the europium concentration to 2 mol %
leads to higher luminances, and an emission spectrum which
A. BaS„:Eu… thin films
is dominated by 4 f – 4 f transitions superposed on broadband
Undoped BaS thin films were evaporated on Vycor emission 共see Fig. 2兲. At 5 mol % the emission is solely
Corning 7913 substrates at a substrate temperature of composed of narrow Eu3⫹ emission bands 共CIE 1931 color
500 °C. The optical transmission spectrum of a 850 nm thin coordinates x⫽0.66, y⫽0.34). The obtained luminance
film is shown in Fig. 1共a兲. Highly transparent thin films were (L 30) is low, 5 cd/m2 at 1 kHz. However, no optimization has
obtained with a refractive index n⫽2.22 at 500 nm. All been carried out on this phosphor.
peaks in the x-ray diffraction diagram 共not shown兲 could be The Eu3⫹ emission bands in Fig. 2 are assigned to the
attributed to BaS, with a preferential 关220兴 orientation. An- 5
D 0 – 7 F J transitions. If the Eu3⫹ ion occupies a site without
Downloaded 08 Jan 2004 to 192.58.150.40. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp186 J. Appl. Phys., Vol. 95, No. 1, 1 January 2004 Smet, Poelman, and Van Meirhaeghe
TABLE I. XPS results on Al2 S3 thin films.
T substrate 200 °C 500 °C 500 °C
Capping layer? 30 nm ZnS 30 nm ZnS None
Al 48.0 50.4 41.6
S 47.8 25.7 1.9
O 2.2 23.9 56.5
I(Al3⫹ )/I(Al0 ) 1.81 1.56 4.38
activity required relatively short measurement times 共less
than 10 min兲 to avoid oxidation of the Al2 S3 thin films, even
in the UHV ambient of the XPS equipment (10⫺7 Pa). The
FIG. 2. Influence of europium concentration (y⫽mol %) on EL of a Al 2p photoline in XPS consists of two peaks, the larger one
BaS:Eu device. Emission lines from Eu3⫹ originate from 5 D 0 – 7 F J transi- at 75.5 eV and the smaller one at 73.2 eV. 共Figure 4 shows
tions.
similar peak shapes in multilayered BaS/Al2 S3 devices兲.
Based on this energy difference, the smaller peak could be
inversion symmetry, the electric–dipole transitions are no attributed to metallic Al, while the larger one corresponds to
longer strictly forbidden and they appear as lines in the spec- the peak position for Al2 O3 or Al2 S3 , 10 hence aluminum
tra. The transitions with ⌬J⫽0, ⫾2 are hypersensitive to being in a trivalent bound state. Gaussian curves were fitted
this effect.7 The ⌬J⫽2 transition 共at 616 nm兲 is clearly to determine I(Al3⫹ )/I(Al0 ), being the ratio between alumi-
dominating over the other transitions, which indicates that num in a bound state and aluminum in a metallic state 共see
the Eu3⫹ ions occupy lattice positions without inversion Table I兲.
symmetry. The elimination of the inversion symmetry in the The oxygen content in thin films evaporated at a sub-
cubic BaS crystal lattice probably results from the proximity strate temperature of 200 °C is remarkably low. However, the
of europium ions 共given the rather high dopant concentra- Al:S ratio considerably differs from the desired 2:3 ratio.
tion兲. Taking into account the form of the Al 2p photoline, this
suggests that part of the thin film is composed of Al instead
B. Al2 S3 thin films of Al2 S3 . At 500 °C, about half of the sulfur is replaced by
oxygen, which will have its consequence for the composition
Undoped Al2 S3 thin films were evaporated on Vycor of the multilayered devices. At higher substrate temperatures,
Corning 7913 substrates at both low (200 °C) and high an increased sulfur deficiency can be expected for sulfide
(500 °C) substrate temperature. Al2 S3 thin films are very thin films, but apparently outgassing of the vacuum system
unstable, as they immediately react with moisture when also leads to increased oxygen incorporation. This can be
taken out of the vacuum system. A smell of H2 S is detected, linked to the XRD results discussed before, which indicated
suggesting the immediate oxidation of the thin films towards only a weak crystallinity in the thin films. A further optimi-
Al2 O3 . The thin films are initially yellowish-brown, but zation of certain evaporation parameters, such as H2 S partial
within minutes after exposure to the ambient atmosphere pressure and evaporation rate, will probably lead to thin
they become transparant, but slightly scattering. In order to films of higher stoichiometric and crystallographic quality.
enable ex situ characterization of the Al2 S3 layers, a 30-nm As an illustration, the atomic composition of a Al2 S3
ZnS capping layer was deposited on top of the Al2 S3 thin thin film without protective ZnS layer which was exposed for
films, which slowed down the oxidation process. 24 h to ambient air is given in Table I. The thin film was
This capping layer complicates the interpretation of op- almost entirely oxidized to Al2 O3 , as can be expected from
tical transmission measurements. Furthermore, the Al2 S3 ab- the Al:O ratio. Notice also that the XPS peak originating
sorption edge is not sharply defined; see Fig. 1共b兲 (T substrate from Al in a metallic state had almost disappeared.
⫽200 °C, measured immediately after deposition兲. After
oxidation of the thin film, the absorption edge shifts to
C. Multilayered BaS:EuÕAl2 S3
shorter wavelengths 共below 250 nm兲.
XRD data showed that Al2 S3 thin films evaporated at Besides experimental parameters such as dopant concen-
200 °C were completely amorphous, as no sharp diffraction tration, substrate temperature, growth rate and annealing
peaks were observed in the 10° – 60° 2 range. When evapo- temperature, etc., several other parameters are introduced
rated at 500 °C, two small but sharp diffraction peaks 共at when using a multilayered structure. The thickness of the
30.0° and 31.2° 2兲 are visible, which could be assigned to individual layers and their thickness ratio are the most im-
Al2 S3 . The diffraction angles match diffraction data we ob- portant as they will influence the luminescent spectrum and
tained from Al2 S3 powder and with literature data on Al2 S3 efficiency. This paper does not intend to present an optimi-
powder.9 zation of all these parameters 共and thus the achievement of
XPS data reveal quite some differences between low- high luminances兲; it is meant to provide additional informa-
and high-substrate-temperature films. After sputtering of the tion on the structure and optical properties of Bax Aly Sz :Eu
ZnS capping layer, approximately 40 nm of the Al2 S3 thin that can only be obtained by using this multilayered struc-
film was removed prior to XPS measurements. The high re- ture.
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FIG. 4. XPS photolines of Al and Eu in as-deposited multilayered devices
evaporated at 200 °C, with thick constituent layers. Al 2p photoline: 共a兲
mixed BaS/Al2 S3 region, 共b兲 pure Al2 S3 region. Each time, two components
were fitted to the Al 2p photoline. Eu 3d 5/2 photoline: 共c兲 pure BaS region,
共d兲 mixed BaS/Al2 S3 region.
measurement error typical for XPS. After a rapid thermal
anneal process at 800 °C, an almost homogeneous element
distribution is obtained 共see lower part of Fig. 3兲. The oxy-
gen concentration is lower than in devices prepared with the
two-target pulse-electron-beam evaporation technique by In-
oue et al.3 In the latter devices 共annealed at 920 °C during 2
min兲 the formation of an Al2 O3 -rich region was observed
FIG. 3. XPS depth profiles of a multilayered BaS:Eu/Al2 S3 device with
thick constituent layers 共30 nm BaS:Eu, 56 nm Al2 S3 ) evaporated at
between the phosphor layer and the top ZnS buffer layer.3,12
200 °C. Upper part, as-deposited. Lower part, RTA at 800 °C. In our devices, the outer thin films of the multilayered phos-
phor are composed of BaS:Eu, which can be seen in the XPS
profile in the as-deposited device 共upper part of Fig. 3兲. After
1. Low substrate temperature annealing, the interface layer shows an increased Al and O
a. XPS results. At relatively low substrate temperature content 共Ba:Al ratio⫽1:3.5), indicating the—premature—
(200 °C), the reactivity of the individual BaS共:Eu兲 and formation of an Al2 O3 interface layer. The annealing tem-
Al2 S3 layers is expected to be limited, which allows us to perature was limited to 800 °C 共determined by the annealing
obtain more or less distinct layers. The upper part of Fig. 3 point of the Corning substrates兲, which can explain the ab-
shows the atomic concentration as a function of sputter time sence of an almost pure Al2 O3 interface layer. In contrast
in an as-deposited multilayered structure, with relatively with the pulsed evaporation technique, which yields an al-
thick constituent layers 共30 nm for BaS:Eu, 56 nm for most continuous evaporation of BaS and Al2 S3 , the use of a
Al2 S3 ). The hypothesis of hardly interacting layers at low multilayered structure implies sites with higher aluminum
substrate temperature is indeed confirmed by XPS measure- and oxygen content 共as most of the oxygen contamination
ments, as the use of rather thick constituent layers clearly originates from the deposition of the Al2 S3 thin films兲, which
leads to unmixed regions. However, information from XPS can locally trigger the formation of Al2 O3 precipitates. This
depth profiling must be treated very carefully. During the can explain the increased Al and O concentration in the re-
sputter process, some elements can be ejected preferentially gion of 14 –17 min sputter time.
and thus apparently the atomic composition can be changed. Several features are seen in the aluminum and europium
Furthermore, the impact of the Ar ions can ‘‘push’’ certain photoline shapes. In Al2 S3 -rich regions, the Al 2p photoline
atoms deeper into the specimen 共‘‘knock-on’’ sputtering兲.11 consists of two peaks 关see Fig. 4共b兲兴. The same phenomenon
When thinner layers 共15 nm for BaS:Eu, 28 nm for Al2 S3 ) was observed in pure Al2 S3 thin films. As discussed above,
are used, no pure BaS or Al2 S3 layers can be discerned any- the peak at higher energy results from aluminum in a bound
more. The Ba:Al ratio changes from 1:1 in Ba-rich regions to state (Al2 S3 or Al2 O3 ), the other one from metallic Al. In
1:3 in Al-rich regions, indicating that considerable mixing regions where the Al2 S3 and BaS layers start to mix up 共even
has occurred in the as-deposited layers. The results given in with low Ba concentration兲, the peak at lower energy disap-
this section are based on structures with thick constituent pears, indicating a bound state for all Al 关see Fig. 4共a兲兴. This
layers 共30 nm for BaS:Eu, 56 nm for Al2 S3 ), as the discussed implies that a chemical reaction between the constituent lay-
features are more pronounced in these devices and unmixed ers takes place during the evaporation process, at least to
regions are still present. some extent.
The oxygen atomic concentration is approximately 13% Another feature is the valence state of Eu. In the section
when averaged over the measured depth; sulfur accounts for on BaS thin films we discussed the emission spectrum of
41%. The ratio Ba:Al:O⫹S approaches 1:2:4, within the BaS:Eu thin films. The presence of narrow emission lines
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indicated a trivalent state for europium. In BaAl2 S4 :Eu,
however, the emission spectrum is characterized solely by a
broad emission band. Based on the luminescent spectra of
BaS:Eu and BaAl2 S4 :Eu, we can conclude that a change in
the valence state of europium from 3⫹ to 2⫹ occurs.
In principle, XPS measurements are an ideal probe to
evaluate the valence state of Eu, as the 3d 5/2 photolines for
Eu3⫹ and Eu2⫹ are located at 1136 eV and 1125 eV,
respectively.4 However, the determination by means of XPS
measurements is difficult due to the rapid oxidation of Eu2⫹ .
This effect can be minimized by using only a short measure-
ment time 共of the order of 3 min兲 after each sputter
process.3,4 The combination of a relatively high europium
concentration and a high emission probability of photoelec-
FIG. 5. Elemental composition of as-deposited multilayered BaS:Eu/Al2 S3
trons allowed us to obtain sufficiently detailed XPS spectra,
devices. 共a兲 Ba:Al ratio in device with thin constituent layers evaporated at
even when using such a short measurement time. 550 °C. 共b兲 Ba:Al ratio and 共c兲 S:O ratio in device with thick constituent
In mixed BaS-Al2 S3 regions, both photolines from the layers evaporated at 500 °C.
divalent and trivalent state are present 关see Fig. 4共d兲兴. In pure
BaS regions, only photolines from Eu3⫹ are present 关see Fig.
4共c兲兴. Under the same measurement conditions, a clear dif- Al2 S3 . This is in correspondance with the results for pure
ference in the europium valence state is observed between BaS and Al2 S3 thin films 共see Secs. III A and III B兲 evapo-
BaS:Eu regions and 共premature兲 BaAl2 S4 :Eu regions. Ap- rated at higher substrate temperatures. These effects are more
parently, the europium valence state changes as soon as pronounced at a slightly lower substrate temperature, show-
Bax Aly Sz :Eu is formed. ing the critical formation temperature for thioaluminates.
It is reasonable to assume that the Eu3⫹ /Eu2⫹ ratio is Figures 5共b兲 and 5共c兲 illustrate the Ba:Al and S:O ratios for
exaggerated, due to the inevitable Eu oxidation during the an as-deposited device with thick constituent layers 共e.g.,
XPS measurements. Inoue et al. also suggested the possibil- thickness of BaS layer: 30 nm兲 at a substrate temperature of
ity of oxidation of the Eu ions by the oxygen impurities.3 500 °C.
The addition of EuF3 as the europium dopant source in BaS b. EL emission. Although XPS results indicate a rela-
might suggest a preferential Eu3⫹ incorporation. However, tively good mixed phosphor layer 关see Fig. 5共a兲兴, the limited
when CaS or SrS is doped with EuF3 , no Eu3⫹ emission is depth resolution of XPS does not allow us to state that a
observed.4,13 single BaAl2 S4 phase is present in the phosphor layer. In-
b. EL emission. As-deposited devices show no EL emis- deed, slight variations in the elemental composition 共over the
sion at all. A rapid thermal anneal process at 800 °C is nec- range of a few nanometer兲 will not be detected by XPS, due
essary to obtain blue EL emission at 470 nm (x⫽0.120, y to the reasons given in Sec. III C 1 a. If the electrolumines-
⫽0.098). Here 600 °C seems to be the critical temperature cent emission is characterized by emission bands other than
for the formation of BaAl2 S4 :Eu, as some devices annealed the desired emission at 470 nm, this indicates that other com-
at 600 °C already show the blue emission band. Other de- pounds than BaAl2 S4 are present. Generally, two emission
vices are characterized by a broad emission spectrum be- bands are observed in the as-deposited devices: one peaking
tween 450 and 650 nm. No traces of Eu3⫹ emission were at 470– 475 nm and another at 575 nm. Upon higher driving
found, even at higher driving voltages. voltage, the former emission band becomes more important.
Although the latter one is close to the position where
2. High substrate temperature BaS:Eu2⫹ is expected, this is contradictory to the results on
a. XPS results. At relatively high substrate temperature BaS:Eu EL devices, where only emission lines originating
(550 °C), a strong reactivity between BaS and Al2 S3 can be from Eu3⫹ emerged. Apparently, the mixing with a small
expected. In as-deposited devices composed of thin constitu- amount of Al2 S3 alters the emission characteristics 共and the
ent layers 共e.g., thickness of BaS layer: 15 nm兲, we find an valence state兲 of europium. In a study of luminescent
almost homogeneous distribution of the elements Ba, Al, S, Bax Aly Sz :Eu powders, Le Thi et al.6 found a shift to longer
O, and Eu over the XPS-measured thickness of 150 nm. The wavelengths when the ratio BaS/Al2 S3 increased. For in-
Ba:Al ratio shows little variation, as depicted in Fig. 5共a兲. stance, the PL of Ba5 Al2 S8 :Eu powder peaks at 540 nm.
The oxygen content is very high; the sulfur concentration The substrate temperature plays a critical role in the rela-
hardly reaches 10 at. %. During evaporation at elevated sub- tive importance of the EL emission band at 470 nm 关see Figs.
strate temperature, outgassing in the vacuum system cannot 6共a兲: T substrate⫽550 °C, 6共b兲: T substrate⫽600 °C, both samples
be neglected, and this leads to higher oxygen incorporation with thin constituent layers兴. An increase from 550 to 600 °C
in the phosphor layer. strongly enhances the blue emission band in the as-deposited
The use of thicker constituent layers leads to small varia- layers. The peak at longer wavelengths shifts as well 共from
tions in the Ba:Al ratio when sputtering through the phos- 575 to 560 nm兲, indicating a decrease in the Ba:Al ratio in
phor film. Generally, the S:O ratio follows the Ba:Al ratio, Ba-rich regions. Again, the critical temperature for the for-
which indicates a better stoichiometry for BaS than for mation of BaAl2 S4 :Eu is situated at 600 °C. Kawanishi
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due to the limited quality of the Al2 O3 insulator layers used
in our devices. Annealing at 800 °C largely deteriorated the
electrical properties of the insulator, leading to significant
leakage currents and poor breakdown characteristics; there-
fore, no electrical properties 共such as L-V characteristics or
efficiencies兲 are presented. In the near future, Ta2 O5 or
BaTiO3 insulators will be used, as their better thermal resis-
tance and higher dielectric constant, compared to Al2 O3 ,
should allow higher performances.
An optimization towards dopant concentration, evapora-
tion, and annealing conditions has not yet been performed.
However, it was not the purpose of the present work to ob-
tain higher luminances with the multilayered structure com-
FIG. 6. Normalized emission spectrum of multilayered BaS:Eu/Al2 S3 de- pared to the pulsed EBE method, but rather to focus on the
vices with thick constituent layers 共30 nm BaS:Eu, 56 nm Al2 S3 ). 共a兲 sub- composition, the morphology, etc., of BaAl2 S4 :Eu thin films.
strate temperature of 550 °C, as-deposited. 共b兲 Substrate temperature of
600 °C, as-deposited. 共c兲 Same as 共b兲, but with RTA at 800 °C.
3. XRD data
et al. noticed a similar critical temperature for the PL inten- In general, the multilayered thin films show little or no
sity and spectrum of BaAl2 S4 :Eu thin films evaporated at x-ray diffraction peaks originating from the central phosphor
different substrate temperatures.14 layer. The interpretation of the diffraction peaks that are
Miura described the influence of the BaS/Al2 S3 ratio on present is complicated by the lack of crystallographic data on
the peak wavelength of the EL emission.2 The emission barium thioaluminates. Peak positions and intensities have
shifted to longer wavelength upon an increase in this ratio. In only been calculated for BaAl2 S4 and BaAl4 S7 . 3
our as-deposited devices (T substrate⫽500– 600 °C), the blue No diffraction peaks besides those stemming from ZnS
emission band is always peaking at 470 nm, even with de- and Al2 O3 are visible in the as-deposited devices with indi-
viations of the BaS:Al2 S3 ratio from 1:1. This implies vidual layer thicknesses up to 30 nm for BaS 共56 nm for
BaAl2 S4 is preferentially formed over thioaluminates with Al2 S3 ), up to a substrate temperature of 600 °C.
other BaS:Al2 S3 ratio, which can be related to the fact that XPS results 共see Sec. III C 1 a兲 indicated that in devices
for the production of barium thioaluminate powders with a evaporated at low substrate temperature, distinct BaS and
ratio other than 1:2:4, temperatures higher than 1000 °C are Al2 S3 layers are present. We noticed already 共see Sec. III B兲
necessary.6 Only after annealing (800 °C) is the intended that Al2 S3 single layers were almost completely amorphous.
BaS:Al2 S3 ratio reflected in the peak wavelength of the EL Although BaS single layers 共see Sec. III A兲 yield sharp dif-
emission, indicating a further mixture of the elements in the fraction peaks, in multilayered devices diffraction peaks are
phosphor layer. It is reasonable to assume that the formation absent. This can be explained by a partial reaction of the BaS
temperature of these phases is lower in a thin film environ- with the Al2 S3 layers and by an inevitable amorphous region
nement than in macroscopic powder grains. at the beginning of each film deposition. It was only in an
In a PL study by Le Thi et al. of europium-doped as-deposited multilayered structure with very thick constitu-
Bax Aly Sz powders, only BaAl2 S4 :Eu showed a favorable ent layers 共60 nm for BaS兲 that rather strong BaS diffraction
thermal quenching behavior near room temperature.6 This peaks were observed. Using Scherrer’s formula, a grain size
implies that luminescent emission from certain Bax Aly Sz :Eu of 25 nm could be derived from the full width at half maxi-
thioaluminates might only be visible at lower temperatures. mum 共FWHM兲 of the 共200兲 peak. A strong preferential ori-
However, even when cooled to 100 K, our as-deposited mul- entation along the 共200兲 plane was observed.
tilayered devices showed no other strong EL emission bands In EL devices annealed at elevated temperatures
than the blue one at 470 nm and the orange one, supporting (800 °C), several small diffraction peaks become visible in
the idea of the preferential formation of BaAl2 S4 :Eu. the XRD spectra both at low and high substrate temperature.
After a rapid thermal anneal at 800 °C, only the blue EL Due to the limited knowledge of the crystallographic prop-
emission band remains 关see Fig. 6共c兲, thick constituent lay- erties and the rather complex diffraction pattern 共compared
ers兴, and the luminance increases strongly. The optical trans- to materials such as BaS or ZnS兲, the interpretation of the
mission of the devices increases as well. Apparently, a fur- diffraction data is difficult and will require further research.
ther mixing towards a homogeneous distribution occurs. Nevertheless, the 共311兲 diffraction peak of BaAl2 S4 can be
The luminance in annealed multilayered devices reaches detected in most annealed samples. According to the experi-
15– 20 cd/m2 at 1 kHz 共sine wave, 30 V above threshold兲. ments and the calculations by Inoue et al., this is the peak
This is significantly lower than the 180 cd/m2 reported by with the highest diffraction intensity.3
Miura et al. in 1999 共under similar measurement conditions The XRD spectra of 370-nm-thick pulsed EBE BaAl2 S4
and dopant concentration兲 and substantially lower than the devices, shown by Inoue et al., contain only small diffraction
reported maximum luminances of 2000 cd/m2 at 1 kHz in peaks, although the devices were annealed at 920 °C. 3
improved devices.1,2 However, in our devices the driving Tanaka et al.12 showed, with the use of cross-sectional trans-
voltage was limited to approximately 30 V above threshold, mission electron microscopy 共TEM兲, that the BaAl2 S4 layer
Downloaded 08 Jan 2004 to 192.58.150.40. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp190 J. Appl. Phys., Vol. 95, No. 1, 1 January 2004 Smet, Poelman, and Van Meirhaeghe
is almost amorphous and that only in a limited region could access to additional information on the elemental composi-
10-nm-wide crystals be detected. The probably less homoge- tion and luminescent properties of europium doped barium
neous elemental distribution in our devices, combined with thioaluminates. At low substrate temperatures, little mixing
the lower annealing temperature, can explain the lack of occurs between the constituent BaS:Eu and Al2 S3 layers. The
clear diffraction peaks. valence state of aluminum and europium in different regions
is discussed. Elevated substrate temperatures lead to 共1兲 a
4. Influence of ambient during evaporation considerable mixing of the constituent layers in as-deposited
As stated before, evaporation at higher substrate tem- devices, 共2兲 an increase in oxygen concentration, and 共3兲 EL
peratures leads to a replacement of sulfur by oxygen. To emission bands at 470 nm and 575 nm. Annealing at 800 °C
evaluate the influence of oxygen content in the EL devices, is sufficient to obtain pure blue EL emission.
phosphor layers were evaporated in an oxygen ambient, with
a partial pressure of 2⫻10⫺3 Pa. XPS measurements ACKNOWLEDGMENTS
showed the atomic ratio (Ba⫹Eu):Al:O to approach 1:2:4,
The authors wish to thank O. Janssens for XRD mea-
with only a weak sulphur signal 共atomic concentration
surements and U. Demeter for extensive XPS measurements.
⬍4%).
One of the authors 共P.F.S.兲 is a research assistant of the
As-deposited layers showed a strong EL emission band
Fund for Scientific Research–Flanders 共Belgium兲 共FWO-
at 465 nm and a smaller one at 575 nm. After annealing, only
Vlaanderen兲.
a single emission band peaking at 467 nm remains, which is
slightly blueshifted compared to the devices evaporated un-
1
der H2 S ambient. N. Miura, M. Kawanishi, H. Matsumoto, and R. Nakano, Jpn. J. Appl.
Given the very low sulfur signal, it seems unlikely that Phys., Part 2 38, L1291 共1999兲.
2
N. Miura, in Proceedings of the 21st International Display Research Con-
the EL emission still originates from BaAl2 S4 :Eu. Neverthe- ference in conjunction with the 8th International Display Workshops 共Asia
less, these devices show a rather high luminance compared Display/IDW’01兲, Nagoya, Japan, 2001, p. 1059.
to ‘‘standard’’ devices, up to 15 cd/m2 (L 30 , 1 kHz兲. Several 3
Y. Inoue, I. Tanaka, K. Tanaka, Y. Izumi, S. Okamoto, M. Kawanishi, D.
Barada, N. Miura, H. Matsumoto, and R. Nakano, Jpn. J. Appl. Phys., Part
data can be found on BaAl2 O4 :Eu powder phosphors, but
1 40, 2451 共2001兲.
the emission spectrum seems to be highly dependent on the 4
R. Vercaemst, D. Poelman, R.L. Van Meirhaeghe, L. Fiermans, W.H.
preparation technique, as emission bands at 410–530 nm are Laflère, and F. Cardon, J. Lumin. 63, 19 共1995兲.
5
reported.15–17 Only limited information is available on D. Poelman, R. Vercaemst, R.L. Van Meirhaeghe, W.H. Laflère, and F.
Cardon, J. Lumin. 75, 175 共1997兲.
BaAl2 O4 :Eu thin films. Lou et al. reported a cathodolumi- 6
K.T. Le Thi, A. Garcia, F. Guillen, and C. Fouassier, Mater. Sci. Eng., B
nescent emission band at 452 nm in BaAl2 O4 :Eu thin films 14, 393 共1992兲.
deposited with a spray pyrolysis method.18 Therefore it is 7
G. Blasse and B.C. Grabmaier, Luminescent Materials 共Springer-Verlag,
difficult to judge on the basis of the emission spectrum Berlin, 1994兲, p. 40.
8
S. Shionoya and W.M. Yen, Phosphor Handbook 共CRC Press, Boca Ra-
whether the observed emission of the EL devices is due to ton, 1999兲, pp. 193 and 224.
BaAl2 O4 :Eu or BaAl2 S4 :Eu. 9
W.-S. Jung and S.-K. Ahn, Mater. Lett. 43, 53 共2000兲.
10
Oxygen and sulfur are both group-VI elements. Possibly C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, and G.E. Muilen-
oxygen can substitute for a certain part of the sulfur in a berg, Handbook of X-ray Photoelectron Spectroscopy 共Perkin-Elmer Cor-
poration, Eden Prairie, 1979兲.
BaAl2 S4 crystal structure, before a separate BaAl2 O4 phase 11
L.C. Feldman and J.W. Mayer, Fundamentals of Surface and Thin Film
is formed. However, Inoue et al. assumed the oxygen in Analysis 共Elsevier Science, Amsterdam, 1986兲.
12
BaAl2 S4 thin films, deposited with the pulsed EBE, to be in I. Tanaka, Y. Inoue, K. Tanaka, Y. Izumi, S. Okamoto, M. Kawanishi, N.
an amorphous BaAl2 O4 form.3 Further research on this topic Miura, H. Matsumoto, and R. Nakano, J. Lumin. 96, 69 共2002兲.
13
J.E. Van Haecke, P.F. Smet, and D.Poelman 共unpublished兲.
is necessary for a good understanding of the BaAl2 S4 :Eu 14
M. Kawanishi, N. Miura, H. Matsumoto, and R. Nakano, in Proceedings
system. of the 21st International Display Research Conference in conjunction with
the 8th International Display Workshops 共Asia Display/IDW ’01兲,
IV. CONCLUSIONS Nagoya, Japan, 2001, p. 1075.
15
Y. Lin, Z. Zhang, Z. Tang, J. Zhang, Z. Zheng, and X. Lu, Mater. Chem.
Optical and morphological properties of BaS:Eu and Phys. 70, 156 共2001兲.
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S.H. Ju, U.S. Oh, J.C. Choi, H.L. Park, T.W. Kim, and C.D. Kim, Mater.
Al2 S3 thin films have been discussed. The electrolumines- Res. Bull. 35, 1831 共2000兲.
cent emission in BaS:Eu is dominated by Eu3⫹ emission. 17
S.H.M. Poort, W.P. Blokpoel, and G. Blasse, Chem. Mater. 7, 1547 共1995兲.
The approach of a multilayered BaS:Eu/Al2 S3 structure gave 18
Z. Lou, J. Hao, and M. Cocivera, J. Phys. D 35, 2841 共2002兲.
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