Aozasa, Masao

Kusabiraki, Minoru

Maekawa, Yoshiharu

Nitta, Atsushi

Tanaka, Kenji

Tonimoto, Noriyasu

Osaka City University Repository

Photoluminescence of ZnS:Mn Thin-film EL Device Active Layer
- Influence of H 2 Gas in Sputtering Atomsphere-
Noriyasu TANIMOTO* , Atsushi NITTA** , Kenji TANAKA*** ,
Yoshiharu MAEKAWA**** , Minoru KUSABIRAKI***** and Masao AOZASA******
(Received October 15, 2001)
Synopsis
Six kinds of thin-film electroluminescent (TFEL) devices with an active layer prepared at different
partial pressure of H2 gas in sputtering atmosphere were fabricated by RF magnetron sputtering
method and luminance versus applied voltage characteristic, EL spectrum and photoluminescent (PL)
spectrum were examined. We examined how a small amount of H2 gas gave an effect on the luminous
characteristics. PL and EL intensity increased with increasing partial H2 gas pressure up to 2.0x 10-8
Torr, but they decreased when it was increased further. PL spectrum showed strong luminescence at
longer wavelength, which is not observed in EL spectrum.
Keywords: Electroluminescent device, ZnS:Mn, Sputtering, Photoluminescence
1. Introduction
Alternative current TEFL devices have been considered as one of the promising flat-panel displays
because of their advantages such as completely solid state and emissive display with wide viewing
angle. The TFEL devices have been fabricated by sputteringl ), electron-beam (EB) deposition, metal
organic chemical vapor deposition (MOCVD) and atomic layer epitaxy (ALE). The sputtering is a
convenient fabrication method which enables large area device fabrication at low cost. In this study,
we fabricated samples with an active layer on the substrate to measure PL emission and TFEL devices
of the structure that an active layer is sandwiched between two insulating layers were fabricated to
measure EL emission. In sputtering, a small amount of impurity gases in sputtering atmosphere
give large influence to the luminous characteristics of fabricated thin film2,3). Here we paid a special
attention on the influence of H2 gas in the impurity gases. In this study we could easily evaluate
an active layer in a simple sample configuration without fabrication of actual EL device by using PL
measurement.
2. Device fabrication and experimental
Fig. 1 shows a schematic diagram of RF magnetron sputtering equipment (Tokki, Model SPK-301)
with a quadrupole mass spectrometer (QMS) attached for gas analysis. The data was acquisited into
a personal computer through RS232C interface. A partial pressure of H2 gas in sputtering atmosphere
was read by QMS.
Mn was doped in ZnS powder at the concentration of 0.5 wt. %. A mixed powder of ZnS and MnS
was fired at 550°C in an argon atmosphere and used as a target on the cathode. A mixture of 60 wt.
% Si3N4 powder and 40 wt. % Al30 3 powder was used as a target for SiAION insulating layer.
Fig. 2 shows a schematic diagram of EL device structure used for investigation of luminous char-
•Student, Master Course of Dept. of Electrical Engineering
··Student, Doctor Course of Dept. of Electrical Engineering
···Research Associate, Dept. of Electrical Engineering
····Lecturer, Dept. of Electrical Engineering
•••••Associate Professor, Dept. of Electrical Engineering
•••••• Professor, Dept. of Electrical Engineering
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~
Turbo moleculer
pump
Glass substrate
Shutter
rtl Sputtering gas
Exhaustion
.....- -.. pump
Fig. 1: RF magnetron sputtering equipment
acteristics. An insulating layer of about 250 nm was first deposited on an indium-tin-oxide (ITO)
coated glass (Corning #7059) where ITO serve~ as a transparent lower electrode. And an Mn doped
ZnS active layer (about 1600 nm thick) and an insulating layer (about 250 nm thick) were successively
deposited. An aluminum electrode was deposited as a top electrode. Both insul~ting and active layer
were deposited by RF magnetron sputtering. Table 1 shows the preparation conditions. The upper
aluminum electrode was deposited by vacuum evaporation method. It is noted that an active layer of
EL device and a single active layer on the substrate for PL sample were deposited in the same batch.
In this study, the following six kinds of active layers were prepared under the condition of differ-
ent partial pressure of H2 gas in sputtering atmosphere: (a) 1.0x10-8 Torr, (b) 1.5x10-s Torr, (c)
2.0x10-s Torr, (d) 2.5x10-8 Torr, (e) 6.0x10-8 Torr and (f) 1.2x10-7 Torr.
Table 1: Deposition conditions
Target
Base pressure
Gas pressure
Sputtering gas flow rate
Deposition time
Substrate temperature
RF power
Thickness
Active layer
ZnS:Mn (Mn : 0.5 wt. %)
1.2 X 10-6 Torr
2.0 X 10-3 Torr
Ar (10 ccrn) + He (10 ccm)
50 min.
200°C
50W
1600 nm
Insulating layer
Si3N4 + Al20 3 (3 : 2)
1.9 X 10-6 Torr
5.0 X 10-3 Torr
Ar (4 cern) + N2 (1 cern)
20 min.
260°C
200 W
250 nm
Luminance versus applied voltage characteristic and luminescent spectrum of EL device were exam-
ined under 5 kHz sinusoidal voltage application at room temparature in air. PL measurements were
also carried out at room temparature in air. Fig. 3 shows EL and PL spectrum measurement system.
PL spectrum was measured as follows. A 20 mW He-Cd laser (KIMMON Electric, Model IK3201R-F)
with a wavelength of 325 nm was used as an excitation source. The cw light of the He-Cd laser
through glass filter (HOYA U-340) and interferance filter (SIGMA KOKI 326.0 nm) was transformed
into 275 Hz alternating current signal by optical chopper (SCITEC INSTRUMENTS). PL under the
excitation light was monochromized and transformed into an electric signal by a combination of a
monochromator (JASCO, Model CT-25C) and photomultiplier (HAMAMATSU, Model R1387). The
electric signal was fed to a lock-in amplifier (NF, Model LI-572B) and synchronized with the reference
signal of optical chopper. At last the electric signal data was acquisited through an AID converter
(Interface, Model IBX-3120) and analyzed by a personal computer (IBM, Model Aptiva). EL spec-
trum was measured as follows. EL under 5 kHz sinusoidal voltage application was transformed into
an electric signal by monochromator and photomultiplier. The electric signal was acquisited through
an AID converter and analyzed by personal computer.
-12-
gla~s substrate
Epoxy resin
SiAION
ZnS:Mn
SiAlON
ITO
AC Power Supply (5kHz)
Fig. 2: EL device Structure Fig. 3: Spectrum measurement system
3. Experimental results and discussion
Fig. 4 shows the luminance versus applied voltage characteristic of EL devices with an active layer
(a) through (f). The threshold voltage is usually defined as the voltage at which the luminance is 1
cd/m2 • EL intensity L30 is defined as the luminance at the voltage which is 30 V higher than the
threshold voltage. L30 of EL devices with the six kinds of active layers were: (a) 3232 cd/m2 , (b) 3767
cd/m2 , (c) 4567 cd/m2 , (d) 2862 cd/m2 , (e) not observed and (f) not observed. The threshold voltage
of each device was about 170 through 180 V. EL intensity increased with increasing a small amount of
parcial pressure of H2 up to 2.0x 10-8 Torr and decreased when it was increased further. The results
were shown in Fig. 5. Fig. 5 also shows the dependence of PL intensity on partial pressure of H2 ,
where PL intensity is the integrated intensity of PL emission from 500 nm to 800 nm. The dependence
of PL intensity on partial pressure of H2 was similar to that of the EL intensity.
5000....----.......---.........----..300
-·2
:J
.ci
~
200 ~
.Ci)
c:
Q)
£
...J
CL
• EL Intensity
o PL Intensity
........a" .•
i l \
/ \
/ \
ZnS:Mn / 1
~
1 2 _ 3100
Partial Pressure of H2 [ x 10 ~orrl
N
E
:g 4000
j
~
.Ci)
c:
~ 3000
....J
LU
room temparature
in atmosphere
5kHz
sinusoidal 'r
voltage r;j H
• 2
• -8
• (a)1.0 x 10 Torr
~ (b)1.5 x 10-Sy-orr
• (c)2.0 x 10-8rorr
o (d)2.5 x 10-Srorr
ZnS:Mn
o1~ 0....0........---.....-...........-2....00.......-..-.-.-.-1....3-..00
Applied Voltage [Vrmsl
Fig. 4: Luminance versus applied voltage Fig. 5: PL and EL intensity against
partial pressure of H2
Fig. 6 shows EL and PL spectrum when the active layer was prepared at a partial pressure of
H2 gas 2.0x10-s Torr. EL and PL spectrum were normalized as the integration of spectrum was 1.
Broad luminescence at the wavelength longer than 650 nm can be observed in PL emission, while
such luminescence can not be observed in EL emission. This would probably be due to the different
excitation mechanism of EL and PL or the different interface structure of EL and PL sample.
-13-
0.02------r----~---,
ZnS:Mn
600 700 800
Wavelength [nm]
....
'e
:J
.c(ij
:;0.01
'en
c:::
Q)
C
-8
H2: 2.0 x 10 Torr
Fig. 6: EL and PL spectrum
To examine influences of H2 gas on the EL spectrum more in depth, peak separation was performed
with least squares method assuming gausian peaks. To solve least squares problem, we used a FOR-
TRAN program generated by ASNOP (Application System for Nonlinear Optimization Programs).
ASNOP is an application system which generates a FORTRAN program automatically. Fig. 7 shows
EL spectrum (solid line) and separated peaks (dotted lines) when partial pressure of H2 gas is 2.0x 10-8
Torr. In the EL spectrum of ZnS:Mn, it is known that the luminescence at around 580 nm originates
from single Mn2+ and luminescence at around 610 nrn originates from clustered Mn2+. Therefore
we first tried to separate EL spectrum into two peaks at 580 nm and 610 nm, but satisfactory curve
fitting could not be attained. For the satisfactory curve fitting, we had to increase the number of
peaks up to four. Two other peak wavelengths were determined to be around 630 nm and 660 nm.
Fig. 8 shows the ratio of the total integrated peak intensity except around 580 nm main peak against
parcial pressure of H2 gas. The figure shows that the total integrated peak intensity increases with
increasing the partial pressure of H2.
0.02.....--,..-...........------.------.
• total integrated peak intensity
except for 580nm main peak
0
0 1 2 8 3
Partial Pressure of H2 [x 10- Torr]
ELZnS:Mn
o
~ 0.5
a:
800
ZnS:Mn
600 700
Wavelength [nm]
....
'c
:J
.ci(ij
:;0.01
'en
c:::
Q)
E
Fig. 7: EL spectrum and separated peaks Fig. 8: Ratio of total integrated peak
intensity except for 580 nm main peak
Fig. 9 shows PL spectra when partial pressure of H2 gas are 1.OxlO-8 Torr, 2.0xl0-s Torr and
1.2xl0-7 Torr respectively. The results show that the contribution of the emission at the longer
wavelength in the total luminescence increases as the partial pressure of H2 gas is increased.
-14-
0.02 0.02 0.02
~ H2: 1.0 x 10-8Torr H2: 2.0 x 10-8Torr 0;::::" H2: 1.2 x 10-7Torr
·2
·2
·2::l ::l ::l
.e .ci
.ens (ij
~ 0.01 ns~ 0.01 ~ 0.01
·iii
"iii .Ci)c: c:Q) Q) c:£ Q)£ £
ZnS:Mn ZnS:Mn
eoo eoo eoo600 700 800 600 700 800 600 700 800
Wavelength [nm] Wavelength [nm] Wavelength [nm]
Fig .9: PL spectrum when the active layer was prepared
at different partial pressure of H2 gas
4. Conclusion
The conclusions can be summarized as follows:
• Both EL and PL intensities increased with increasing a small amount of parcial pressure of H2
gas up to 2.0x 10-8 Torr and decreased when parcial pressure of H2 was increased further.
• Broad luminescence at the wavelength longer than 650 nm can be observed in PL emission,
however such luminescence can not be observed in EL emission.
• EL and PL spectra show that the contribution of the emission at the longer wavelength in the
total luminescence increases as the partial pressure of H2 gas is increased.
5. References
1) M. Aozasa, et aI., Japanese Journal of Applied Physics, 29, 1997 (1990)
2) A. Nitta, et aI., Record of The 1998 Kansai-Section Joint Convention of Institutes of Electrical
Engineering, Japan, G 183
3) A. Nitta, et aI., Proceedings of The 1999 Electronics Society Conference of IEICE, C-6-3
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Photoluminescence of ZnS:Mn Thin-film EL Device Active Layer : Influence of H_2 Gas in Sputtering Atomsphere

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Photoluminescence of ZnS:Mn Thin-film EL Device Active Layer : Influence of H_2 Gas in Sputtering Atomsphere

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