Aozasa, Masao

Kusabiraki, Minoru

Maekawa, Yoshiharu

Nitta, Atsushi

Tanaka, Kenji

Tsuda, Iwao

Osaka City University Repository

Effects of H 2 Gas in Sputtering Ambient on Pulse Voltage Response
of ZnS:Mn Electroluminescent Device
Iwao TSUDA* , Atsushi NITTA**, Kenji TANAKA***,
Yoshiharu MAEKAWA****, Minoru KUSABIRAKI* ****
and Masao AOZASA· *****
(Received September 29, 2000)
Synopsis
Electroluminescent (EL) devices were prepared by RF magnetron sputtering method at different
partial pressure of H2 gas in sputtering ambient. The effects of H2 gas in sputtering ambient on
pulse voltage response were investigated for preparing high-quality EL device active layer. Luminance
versus applied voltage characteristics, XRn pattern, pulse voltage response and ESR spectrum were
examined. When a small amount of H2 gas was introduced in sputtering ambient, contribution of
single Mn2+ to pulse voltage response changed. It strongly suggests that small amount of H2 gas in
sputtering ambient affects the concentration of single Mn2+.
KEYWORDS: Electroluminescent device, ZnS:Mn, Sputtering, Pulse voltage response, ESR
Introduction
In recent years, various emissive devices were developed. Cathode-ray tube (CRT) is excellent in
luminance and contrast, but its size, weight and power consumption are large and it is not a proper
device for small equipment. Therefore light, small and low power consumption device is needed. Thin-
film electroluminescent (TFEL) device is one of the devices which is thin and light. TFEL display
is resistant to vibration and its viewing angle is wide, because TFEL is completely solid state and
emissive. Therefore TFEL devices have been considered as one of the candidate of the fiat-panel
displays. The TFEL devices have been prepared by sputteringl ), electron-beam evaporation2) and
atomic layer epitaxy(ALE)3), etc. The sputtering method has a characteristics that a large area
device can be prepared at low cost. However, it is important to control gas impurities in sputtering
ambient in order to prepare a high quality thin film by sputtering method4,S).
In this study, we paid attention to H2 gas in gas impurities (H2, H20, N2, 02) and examined its
influence on pulse voltage response of EL device.
Device fabrication and experimental
Fig. 1 shows a schematic structure diagram of EL device used for investigation of the luminous
characteristics. An insulating layer of about 250 nm was first deposited on an indium-tin-oxide (ITO)
coated substrate glass(Corning #7059) . ITO layer serves as a transparent lower electrode. A Mn
doped ZnS active layer (about 1600 nm) and an insulating layer (about 250 nm) were successively
deposited. Aluminum electrodes were deposited as a top electrode. Both insulating and active layer
were deposited by RF magnetron sputtering. The upper aluminum electrodes were deposited by
• Student, Master Course of Dept. of Electrical Engineering
•• Lecturer of Osaka Butsuryo College
• • • 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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vacuum evaporation method.
Epoxy re.in
SiAION
ZnS:Mn
Fig. 1: Schematic diagram of the EL device
(2)_ Jo
oo B1(t)dt
Ql - J~ B(t)dt '
In this study, the following four kinds of active layers were prepared. That is, they were active layer
(a), (b), (c) and (d) whose partial pressure of H2 gas in sputtering ambient are (a)I.O X 10-sTorr (Not
intentionally introduced), (b)I.5 x IO-8Torr, (c)2.0 x IO-sTorr and (d)2.5 x IO-8Torr. Luminance
versus applied voltage characteristics of EL device were examined under 5 kHz sinusoidal voltage
application condition at room temperature in atmosphere. The crystallinity of the active layer and
the state of luminescent center were examined by X-ray diffraction(Shimadzu Works, Model XD-
610) and Electron Spin Resonance(BRUKER Works, ESP-300), respectively. Electroluminescent light
decay B(t) was measured, applying the pulse voltage (pulse width is about 250 p,s, voltage is 150 V
'""200 V) to the sample.
Fig. 2 shows the measurement system. Pulse voltage with both polarity(4 ms intervals) was
applied to the device. Under these pulses, electroluminescent light of EL device is converted to an
electric signal by photomultiplier, and the signal is taken in a personal computer through AID board.
B(t) is supposed to be the sum of two components B1(t) and B2(t) with decay time constant Tl and
T2 respectively as
B(t) = B1 (t) +B2(t) = B~ exp (-tIT1) +B; exp (-tIT2)' (1)
where B~, B;, T1 and T2 are constant. The ratio of the two components Ql and Q2 are defined by the
following equations,
B~, B;, Tl and T2 were calculated by least squares method. To solve least squares problems, we used a
FORTRAN program which was created by ASNOP (Application System for Nonlinear Optimization
Problems). ASNOP is an application system which creates a FORTRAN program, and this FORTRAN
program is used for solving optimization problems. Because FORTRAN program is created by ASNOP
automatically, an user doesn't need writing long FORTRAN program and has only to prepare a file in
ASNOP's format. In this file, type of the problem, method, constraint functions, measurement data,
and etc. are written. According to this file, ASNOP assembles necessary parts into a FORTRAN
program. These parts are built in ASNOP system in advance. The FORTRAN program is compiled
and linked and an execution file is created. By executing this program, the values of B~, B;, Tl and
T2 are obtained. To solve a nonlinear least squares problem, the problem type was NLS(Nonlinear
Least Squares problems) and the method was NLSSQP(Sequential Quadratic Programming method
-14-
for Nonlinear Least Squares Problems}. To compile FORTRAN program, g77(FORTRAN77 compiler
of GNU) was used.
Pulse
Generator
Fig. 2: Measurement system of pulse voltage response
Experimental results and discussion
Fig. 3 shows the luminance versus applied voltage characteristics of EL devices with active layers
(a) through (d). Maximum luminance of each device were (a)3447 cd/m2 , (b)4600 cd/m2 , (c)5333
cd/m2 and (d)3119 cd/m2 •
Partial Pressure of H2 [Torr]
(a) H2:1.0 X 1O~
(b) H2:1.5 X 1O~
(c) H2:2.0 X 1O~
(d) H2 :2.5X 10~
(d)
(c)
(b)
(8)
20 40 60 80 100
Scattering Angle 29 [deg.]
Fig. 4 XRD patterns of the active layers
300
H2
.(a)1.0X 10-sTorr
~(b)1.5X 10-8Torr
.(c)2.0)( 10-8Torr
O(d)2.5X 10-8Torr
room temperature
in atmosphere
5kHz
sinusoidal
voltage
200
Applied Voltage [VrmJ
Fig. 3 Luminance versus applied
voltage characteristics
Fig. 4 shows the X-ray diffraction patterns of active layer (a) through (d). An intensive diffraction
line at a scattering angle of around 28.5 degrees arising from (111) plane of zinc-blende structure is
observed and a line which peaks at around 56 degrees due to (311) plane of zinc-blende structure
is also observed. The peak intensity arising from (111) plane of zinc-blende structure becomes the
strongest in the condition of (b) (H2:1.5 X 10-8 Torr).
-15-
Fig. 5 shows pulse voltage response of the device with active layer (a). Table 1 shows Tl, T2, Ql
and Q2. Decay time constants are almost constant, Tl =0.22 ms and T2 = 1.2 ms, respectively. These
values are close to the decay time constants of pair Mn2+ and single Mn2+ that were already reported,
respectively6) .
Table 1. Decay time constants and ratio of the components
H2[Torr] Tl [ms] T2 [ms] Ql Q2
1.OxlO-8 0.2797 1.2962 0.10224 0.89776
1.5xl0-8 0.25616 1.11534 0.07585 0.92415
2.0xl0-8 0.2171 1.1934 0.05786 0.94214
2.5xl0-8 0.2316 1.1152 0.29916 0.70085
H2 : 1.0X 10-8 [Torr]
-B(t)
........ B1(t)
--- B2(t)
B(t)=B1(t)+B2(t)
Time [ms]
Fig. 5 Pulse voltage response of active layer (a)
.9
o
0.8
Partial pressure of H2 [X 10~0"]
o
0.8
0.9
.7
"'-------_...-._~--&._......---I
o 1 2 3
Partial pressure of H2 [X 1O~orr]
• Maximum luminance
N-6000 • O2
E
~
B5000
c:
co
c:
·e
..2 4000
E
::s
E0)(
~3000
Fig. 6 shows maximum luminance and Q2 against partial pressure of H2. Q2 is the largest in the
case of (c). And maximum luminance takes its maximum also in the case of (c) corresponding to that
of Q2. It suggests that H2 gas in sputtering ambient strongly affects the density of single Mn2+.
._.300..---........--......-----.
~ [] Intensity of single Mn2+
~ .°2
~
Fig. 6 1tlaximum luminance and Q2
against partial pressure of H2
Fig. 7 Peak intensity of single Mn2+ and Q2
against partial pressure of H2
Fig. 7 shows peak intensity of single Mn2+ and Q2 against partial pressure of H2. The peak
intensity of single Mn2+ was obtained by analyzing ESR signal. Results on ESR observation were
reported in the previous work7). Both the peak intensity of single Mn2+ and Q2 were the largest in
the case of (c). The quantity of H2 gas in sputtering ambient affects the concentration of Mn2+, and
thus on the luminance characteristics of the device.
-16-
Conclusion
The conclusions can be summarized as follows:
1. Partial pressure of small amount of H2 gas in sputtering ambient strongly affects the crystallinity
of the active layer.
2. Partial pressure of H2 gas in sputtering ambient also affects the concentration of single Mn2+
which determine the luminance characteristics of the device.
References
1) M. Aozasa, et al., Japanese Journal of Applied Physics, 29, 1997 (1990)
2) T. Matsuoka, et al., Japanese Journal of Applied Physics, 27, 1430 (1988)
3) Runar O. Tornqvist, et al.~ IEEE Transactions on Electron Devices, ED-30, 468 (1983)
4) A. Nitta, et al., Record of The 1998 Kansai-Section Joint Convention of Institutes of Electrical
Engineering, Japan, G 183
5) A. Nitta, et al., Proceedings of The 1999 Electronics society Conference of IEICE, C-6-3
6) E. Walentynowicz, et al., Journal of Luminescence, 17, 109 (1978)
7) T. Fujita, et aI., Memoirs of the Faculty of Engineering, Osaka City University, 40, 19 (1999)
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Effects of H_2 gas in Sputtering Ambient on Pulse Voltage Response of ZnS:Mn Electroluminescent Device

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Effects of H_2 gas in Sputtering Ambient on Pulse Voltage Response of ZnS:Mn Electroluminescent Device

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