Hot carrier type exchange in inorganic electroluminescent thin films

Article abstract of DOI:10.1063/1.2388942

The authors have observed the hot carrier type (holes or electrons) exchange in rare-earth-ion-activated strontium thiogallate (Sr Ga2 S4) thin films by measuring the transient electroluminescent wave forms of the devices having a single insulating thin film. Measured wave forms revealed that the green electroluminescence of europium activated Sr Ga2 S4 thin film occurs due to hot hole excitation. In contrast, the blue electroluminescence of cerium activated Sr Ga2 S4 thin film occurs due to hot electron excitation. Hence, the hot carrier type is exchanged by the different rare-earth-ion doping.

Full text of DOI:10.1063/1.2388942

Hot carrier type exchange in inorganic electroluminescent thin films
Katsu Tanaka and Shinji Okamoto
Citation: Applied Physics Letters 89, 203508 (2006); doi: 10.1063/1.2388942
View online: http://dx.doi.org/10.1063/1.2388942
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APPLIED PHYSICS LETTERS 89, 203508 ͑2006͒
Hot carrier type exchange in inorganic electroluminescent thin films
Katsu Tanakaa͒ and Shinji Okamoto
NHK Science & Technical Research Laboratories, 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan
͑Received 13 April 2006; accepted 5 October 2006; published online 15 November 2006͒
The authors have observed the hot carrier type ͑holes or electrons͒ exchange in
rare-earth-ion-activated strontium thiogallate ͑SrGa2S4͒ thin films by measuring the transient
electroluminescent wave forms of the devices having a single insulating thin film. Measured wave
forms revealed that the green electroluminescence of europium activated SrGa2S4 thin film occurs
due to hot hole excitation. In contrast, the blue electroluminescence of cerium activated SrGa2S4
thin film occurs due to hot electron excitation. Hence, the hot carrier type is exchanged by the
different rare-earth-ion doping. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2388942͔
Recently, inorganic phosphors of ternary compounds
have shown drastically improved photoluminescence,1,2
cathodoluminescence,3,4 and electroluminescence5,6 ͑EL͒
properties. Primary colors, blue, green, and red phosphors
were developed using by rare-earth-ion-activated ternary
compounds.7,8 As yet, however, the effect of rare-earth-ion
doping on the luminescent excitation process of these com-
pounds, especially for EL, has not been reported, because of
the difficulty to elucidate the hot carrier type ͑holes or elec-
trons͒ for the excitation. Of these ternary compounds, we
examined strontium thiogallate ͑SrGa2S4͒. Pure green emit-
ting europium activated SrGa2S4 has used for fabrication of a
full-color EL display9 and has also used as a blue-to-green
color conversion material.2
For the excitation process of luminescent centers in in-
organic EL thin film, the hot electron has long been known to
be the dominant carrier. Many theoretical analyses have as-
sumed that the excitations are due only to hot electrons.10–14
However, there has been no report of hot hole excitation in
impurity-ion activated phosphor thin film. Moreover, there
has been no report of hot carrier type exchange between hot
electrons and hot holes in the same host material. In this
letter, we report on the observation of the hot hole excitation
in rare-earth ͑Re͒-ion activated SrGa2S4 thin film and show
that the dominant hot carrier type is exchanged by the differ-
ent types of Re-ion doping. Up to now, there is no method to
elucidate the hot carrier type in impurity-ion activated phos-
phor thin film experimentally. To find out which carrier type
is dominant, we measured the transient EL wave form of
SrGa2S4:Re thin-film EL devices having a single insulating
thin film. Furthermore, we investigated the relationship be-
tween the dominant carrier type and the Re-ion doping ma-
terial by measuring the transient EL wave form.
We fabricated thin-film EL devices with an indium tin oxide
͑ITO͒–Ta2O5–SrGa2S4:Re–Al ͑MISM͒ multilayer structure
having a single insulating ͑Ta2O5͒ thin film. As a reference,
we also fabricated a ZnS:Mn thin-film EL device with the
same MISM structure. EL spectra and the transient EL wave
forms of the devices were measured by applying a rectangu-
lar pulse voltage biased above the threshold voltage with an
average electrical field on the order of 108 V/m.
The XRD pattern of the as-grown SrGa2S4 thin films
was reported in our previous paper.1 All peaks corresponded
to diffraction lines in the orthorhombic crystal structure of
SrGa2S4. No extraneous peaks, such as peaks corresponding
to GaS and SrS binary phases, were observed.
Figures 1͑a͒–1͑c͒ show the relationship between the ap-
plied voltage and the EL wave form from the SrGa2S4:Ce,
SrGa2S4:Eu, and ZnS:Mn thin films of the single insulating
thin-film EL devices. Asymmetric EL wave forms were ob-
served under an applied rectangular pulse voltage. Figures
1͑d͒–1͑f͒ show the EL spectra of SrGa2S4:Ce, SrGa2S4:Eu,
and ZnS:Mn thin films. The Commission Internationale
de’Eclarirage color coordinates of the spectra were ͑0.14,
0.14͒, ͑0.24, 0.69͒, and ͑0.54, 0.45͒, and these are in the pure
blue, green, and orange regions. These EL spectra originate
from intra-atomic transitions of Ce3+, Eu2+, and Mn2+, re-
spectively. As shown in Fig. 1͑b͒, the EL in the SrGa2S4:Eu
device occurred only when the bias condition was ITO posi-
tive and Al negative. Regarding the voltage–EL wave form
relationships for the SrGa2S4:Ce and ZnS:Mn thin-film EL
devices shown in Figs. 1͑a͒ and 1͑c͒, EL only occurs on the
bias condition of ITO negative and Al positive. The EL in
these cases occurs under a bias condition opposite to the one
under which EL occurs in the SrGa2S4:Eu device. In addi-
tion, EL of SrGa2S4:Re thin films shown in Figs. 1͑a͒ and
1͑b͒ were observed at the leading edge of rectangular applied
voltage. Leading-edge emission is usually observed in
ZnS:Mn thin films as shown in Fig. 1͑c͒. The leading-edge
emission suggests that Re ion doped in SrGa2S4 thin films
are directly excited by the impact of hot carriers similar to
that in ZnS:Mn thin films.
We grew SrGa2S4:Re thin films by multisource deposi-
tion using a molecular beam epitaxy system. The four Knud-
sen effusion cells were mounted in the system for Sr, Ga2S3,
Eu, and CeCl3. For the thin-film growth of the SrGa2S4 host,
we used Sr and Ga2S3 sources and fixed the Ga2S3/Sr flux
ratio at 70. A 470 °C substrate temperature and 1 h deposi-
tion time resulted in a film thickness of 1 ␮m. Re-ion acti-
vated thin films were obtained by the codeposition of Eu or
CeCl3 dopant materials. The crystallinity of the thin films
was analyzed with x-ray diffraction ͑XRD͒ measurements.
Here, we try to explain the excitation process of the EL
in SrGa2S4:Eu thin film when we assume hot electron exci-
tation. Figure 2 shows the relationship between EL wave
forms and energy band diagrams assuming that there is hot
electron excitation. As shown in Fig. 2͑c͒, under the bias
condition of ITO positive and Al negative, electrons are in-
jected from the Al electrode and accumulate in the interface
a͒
Electronic mail: tanaka.k-ob@nhk.or.jp
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0003-6951/2006/89͑20͒/203508/3/$23.00 89, 203508-1 © 2006 American Institute of Physics
206.196.184.84 On: Tue, 16 Dec 2014 19:24:23

203508-2
K. Tanaka and S. Okamoto
Appl. Phys. Lett. 89, 203508 ͑2006͒
FIG. 1. ͑Color online͒ Applied voltage and EL wave
form relationship observed in ͑a͒ SrGa2S4 :Ce ͑b͒
SrGa2S4 :Eu, and ͑c͒ ZnS:Mn thin film EL devices with
a single insulating thin film. The EL spectra corre-
sponding to the EL wave forms of ͑a͒–͑c͒ are also
shown in ͑d͒–͑f͒.
region between the Ta2O5 and SrGa2S4:Eu thin films. Under
the reverse bias condition shown in Fig. 2͑d͒, ITO negative
and Al positive, the accumulated electrons in the interface
region are accelerated to become hot electrons and EL results
from the excitation of hot electrons. Therefore, a strong
emission peak should be observed under this bias condition
shown in Fig. 2͑b͒. However, we did not observe the emis-
sion peak under this bias condition shown in Fig. 2͑a͒. Thus,
hot electron excitation cannot explain the experimental re-
sults for this bias condition in the SrGa2S4:Eu thin film.
We propose a new carrier type for the excitation of
SrGa2S4:Eu thin film. Under the bias condition of ITO nega-
tive and Al positive shown in Fig. 3͑a͒, holes are injected
from the Al electrode and accumulate in the interface region
between the Ta2O5 and SrGa2S4:Eu thin film. Under the
reverse bias condition, ITO positive and Al negative shown
in Fig. 3͑b͒, the accumulated holes in the interface region are
accelerated to become hot holes and EL results from the
excitation of hot holes. This process explains the observed
polarity of applied voltage. This indicates that Eu2+ ions are
excited by hot holes in the excitation process of green EL in
the SrGa2S4:Eu thin film.
Hot hole excitation is also supported by the electrical
field in the SrGa2S4:Eu thin film. The electrical field induced
by an externally applied pulse voltage has the same value for
the positive and negative polarities of the external voltage.
However, the additional field induced by accumulated
charges for the case of the positive polarity is drastically
different that of the negative polarity. When a rectangular
pulse voltage is applied to SrGa2S4:Eu thin film, a large
number of electrical charges accumulate in the interface re-
FIG. 2. ͑Color online͒ The relationship between EL wave form ͓͑a͒ and ͑b͔͒
and energy band diagram ͓͑c͒ and ͑d͔͒ assuming that the hot electron exci-
tation in SrGa2S4 :Eu thin film EL device with a single insulating thin film.
͑a͒ The hatched region represents the bias condition of ITO positive and Al
negative. ͑c͒ In this bias condition, the electrons are injected from the Al
electrode. ͑b͒ The nonhatched region represents the bias condition of ITO
negative and Al positive. ͑d͒ The electrons are accelerated to become hot
electrons and EL occurs due to hot electron excitation. Note that the strong
FIG. 3. ͑Color online͒ Schematic energy diagram of the excitation process
of SrGa2S4 :Eu thin-film EL devices. ͑a͒ In the bias condition of ITO nega-
tive and Al positive, holes are injected from the Al electrode. ͑b͒ In the bias
condition of ITO positive and Al negative, the holes become hot holes and
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emission peak should be observed under this bias condition.
EL occurs due to the hot hole excitation.
206.196.184.84 On: Tue, 16 Dec 2014 19:24:23

203508-3
K. Tanaka and S. Okamoto
Appl. Phys. Lett. 89, 203508 ͑2006͒
TABLE I. Conduction type and the hot carrier type in EL materials. Paren-
theses represent the conduction types, which were derived from the polarity
of asymmetric EL wave forms as shown in Fig. 1.
centers and cause green EL. Due to the constant valency
when Eu2+ ions are substituted at the Sr sites of the SrGa2S4
host, we conclude that hole conduction is related to accep-
torlike defects of the host, such as Sr vacancies. The majority
carriers under the high field in the SrGa2S4:Ce thin film are
electrons, suggesting n-type conductivity. The hot electrons
excite the Ce3+ centers and cause blue EL. Due to the in-
crease in valency from 2+ to 3+ when Ce3+ ions are substi-
tuted at the Sr sites of the SrGa2S4 host, we conclude that
electron conduction is related to donorlike states induced by
Ce3+ ion doping. Additional investigation, especially of the
barrier heights between the Al contacts for Ce- and Eu-doped
SrGa2S4 thin films would help to confirm our excitation
model.
In summary, we have investigated the EL excitation pro-
cess in SrGa2S4:Re ͑RevEu or Ce͒ by measuring the tran-
sient EL wave forms of thin film EL devices having a single
insulating thin film. Measured EL wave forms revealed that
the green EL of SrGa2S4:Eu device occurs due to hot hole
excitation. In contrast, the blue EL of SrGa2S4:Ce device
occurs due to hot electron excitation. Thus, the hot carrier
type of SrGa2S4:Re thin film was exchanged by changing
the type of rare-earth ions ͑Eu2+ or Ce3+͒ used for doping.
Our results indicate that hot holes are also effective for thin-
film EL excitation.
EL material
Conduction type
Hot carrier type
SrGa2S4 :Ce
SrGa2S4 :Eu
ZnS:Mn
͑n͒
͑p͒
n
Hot electron
Hot hole
Hot electron
gion between the Ta2O5 and SrGa2S4:Eu thin films. On the
opposite side of the thin film, the interface region between
the Al and SrGa2S4:Eu thin films cannot accumulate electri-
cal charges because the trapped charges are easily released to
the Al electrode. Therefore, the electrical field of the
SrGa2S4:Eu thin film can be strengthened ͑or weakened͒ by
the presence ͑or shortage͒ of the accumulated charges in the
interface region between the Ta2O5 and SrGa2S4:Eu thin
films.
When the majority carriers of the accumulated charges
are holes, the electrical field of the SrGa2S4:Eu thin film is
transitionally strengthened under which the pulse bias volt-
age is applied on the bias condition of ITO positive and Al
negative. Under the reverse bias condition of ITO negative
and Al positive, the electrical field of SrGa2S4:Eu thin film
becomes much weaker because of the shortage of the accu-
mulated charges.
On the other hand, when the majority carriers of the
accumulated charges are electrons, the electrical field of the
SrGa2S4:Eu thin film is strengthened ͑or weakened͒ under
the opposite bias conditions.
For the SrGa2S4:Eu thin film, we observed EL under the
bias condition of ITO positive and Al negative as shown in
Fig. 1͑b͒. This can be explained as follows: holes are accu-
mulated in the interface region between the Ta2O5 and
SrGa2S4:Eu thin films and high-field EL occurs through hot
hole excitation.
In contrast to this, the blue EL of the SrGa2S4:Ce thin-
film EL device occurs due to hot electron excitation because
the EL of SrGa2S4:Ce thin film shown in Fig. 1͑a͒ occurs at
the same polarity as that of the applied voltage in the
ZnS:Mn thin film shown in Fig. 1͑c͒ which has hot electron
excitation.10–14 In addition, the EL decay time of the ZnS:Mn
thin film in Fig. 1͑c͒ which is of the order of milliseconds is
longer than the SrGa2S4:Ce and SrGa2S4:Eu thin films’ or-
der of microseconds in Figs. 1͑a͒ and 1͑b͒. The longer decay
time reflects the nature of the forbidden transition of Mn2+
ions.15
Table I summarizes the conduction types and excitation
carriers in these phosphor materials. Since the majority car-
riers in ZnS:Mn are electrons because of its n-type
conductivity,16 the majority carriers in SrGa2S4:Eu are holes,
suggesting p-type conductivity. The hot holes excite the Eu2+
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