Aozasa, Masao

Kusabiraki, Minoru

Maekawa, Yoshiharu

Nitta, Atushi

Tanaka, Kenji

Umekawa, Satoshi

Osaka City University Repository

Mem. Fac. Eng., Osaka City Univ., Vol. 36,pp. 13-19.(1995)
Optical emission spectroscopy of rf-magnetron sputtering plasma
for fabrication of ZnS:Mn active layer in thin-film EL devices
by
Satoshi UMEKAWA*, Atushi NITTA**, Kenji TANAKA***,
Minoru KUSABlRAKI****, Yoshiharu MAEKAWA**** and Masao AOZASA*****
(Received September 29, 1995)
Synopsis
ZnS:Mn active layers in electroluminescent devices were fabricated by rf-magnetron
sputtering, and the luminous characteristics of the devices were examined. The optical emission
spectra of the sputtering plasma for active layers were examined, and the dependence of the
qualities of active layers and the luminous characteristics of the devices on the plasma was
investigated.
Keywords : electroluminescence, EL, ZnS:Mn, sputtering, optical emission spectroscopy, OES
1. Introduction
Ac thin -film electroluminescent (ACTFEL) 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
sputtering, electron- beam (EB) deposition, metalorganic chemical vapor deposition
(MOCVD) and atomic layer epitaxy (ALE). The sputtering is a convenient fabrication
method which enables easy concentration control of doping material and large area device
fabrication at low cost. However, the sputtering processes are not yet well understood and
they have been investigated by many workers.
In this study, we fabricated TFEL devices of the structure that an active layer is
sandwiched between two insulating layers by rf-magnetron sputtering. The optical
emission spectra of the sputtering plasma were examined when the active layer was
deposited and the effect of the plasma state on the qualities of active layer and the
luminous characteristics of the devices were investigated. 1)
* Student, Master Course of Dept. of Electrical Engineering
** Student, Doctor Course of Dept. of Electrical Engineering
*** Research Associate, Department of Electrical Engineering
**** Lecturer, Department of Electrical Engineering
***** Professor, Department of Electrical Engineering
13
14 Satoshi UMEKAWA, Atushi NITTA, Kenji TANAKA, Minoru KUSABIRAKI,
Yoshiharu MAEKAWA and Masao AOZASA
2. Device fabrication and experimentals
AI electrode
SWON
ZnS:Mn
~ SiAlON
......----------.a..-.tL....... I TO
glass substrate
Fig. 1. Structure of the EL device
Fig.l shows a schematic diagram of EL device structure used for investigating the
luminous characteristics. The substrate was an indium-tin-oxide (ITO) coated glass. An
insulating layer of about 200 nm was first deposited on the substrate and an active layer of
about 700 nm and a insulating layer of about 200 nm were successively deposited. An
aluminum electrode was deposited as the top electrode.
Both insulating and active layer were deposited by rf-magnetron sputtering, and Table 1
shows the deposition condition. The aluminum electrode was deposited by vacuum
evaporation.
Table 1. Deposition conditions
(a) Insulating layer
Target
Substrate temperature roC]
RF power [W]
Gas pressure [mTorr]
Sputtering gas flow rate[ccm]
Thickness [nm]
Si3N4 powder + Al203 powder
260
200
5.0
Ar:4 N2: 1
250
(b)Active layer
Ar:5 He:5
Target
Substrate temperature roC]
RF power [W]
Gas pressure [mTorr]
Sputtering gas flow rate[ccm]
Thickness [nm]
ABC
ZnS powder + MnS powder (lwt%)
200
50
2.0
Ar:5 He:5 Ar:8 He:5
700
In this study, the following three kinds of active layers were prepared. They are active
layer A with insufficient degassing and gas flow rate of 5 cern at deposition, active layer B
with sufficient degassing and gas flow rate of 5 cern and active layer C with sufficient
degassing and gas flow rate of 8 CcDl.
OES of sputtering plasma for fabrication of ZnS:Mn film 15
Fig. 2 shows an experimental system consisting of a rf-magnetron sputtering (Tokki,
Model SPK-301) and optical emission spectroscopy. While each active layer was deposited,
emission light from sputtering plasma was introduced through an optical fiber cable to a
monochrometer (Shimadzu Works, Model SPG-100S). The monochromatic light was
detected by a photomultiplier and the light intensity data was aquisited. and processed by a
personal computer.
Sputtering
chamber
Personal computer
GP-IB
interface
AID
converter
Spectrometer
Photomultiplier
Stepping motor
controller
Main amp.
Fig. 2. Measurement system of optical emission spectra
The luminance versus applied voltage characteristics of EL device was usually examined
at room temperature in atmosphere. The crystallinity and the stoichiometry of active layers
were evaluated by X-ray diffraction (Shimadzu Works, Model XD-610) and X-ray
photoelectron spectroscopy (Shimadzu Works, Model ESCA-750) respectively.
3. Experimental results and discussions
3.1 Optical emission spectra
Fig. 3 shows the optical emission spectra of the plasma in the case of active layer A, B
and C. Emission from Ar and He which are sputtering gases, and emission from Zn which is
the ingredient of the target were detected. Emission from Sand Mn, which are the other
ingredients of the target, was not detected. The maximum peak intensity of the spectra
from Ar at 750.3 nm was normalized as unity, and the spectra of active layer A, Band Care
compared.
16 Satoshi UMEKAWA, Atushi NITTA, Kenji TANAKA, Minoru KUSABIRAKI,
Yoshiharu MAEKAWA and Masao AOZASA
400 600
Wavelength [nm]
800
Fig. 3. Plasma emission spectra
Table 2. Peak intensity of plasma emission spectra
Zn(472.2nm)
Ar(763.5nm)
active layer A
0.282
0.721
active layer B active layer C
0.260 0.234
0.738 0.856
Ar emission peak at 750.3 nm is due to 4s' -4p' transition 2). The relative intensity of
Zn emission at 477.2 nm decreases in the order of A, B, and C, and the relative intensity of
Ar emission at 763.5 nm increases in the order of A, B, and C. The results are summarized
in Table 2. It suggests that the degassing of active layer and the gas flow rate at deposition
affects on the plasma state.
3.2 Luminous characteristics
Fig. 4 shows luminance versus applied voltage characteristics of the devices with active
layer Band C. The luminance of the device with active layer A was very low and extremely
unstable and available data could not be obtained. The device with an active layer C has
lower threshold voltage and larger maximum luminance than that with an active layer B.
Therefore the active layer quality is in the order of C, B and A.
DES of sputtering plasma for fabrication of ZnS:Mn film 17
f=5kHz
1kHz
1kHz
-------,,--.;1
10 3 Active layer B
----- Active layer C 1'-" 5kHZ
,
I
,
I
,
, ,
, ,
, ,
, ',
, ,
, ,
, ,
, I
"
"
"
"
"I,
"
"
"
"
"~
~
~I,
", "
10 0"'-.......'_'.._,_..a....- '.,;.'."I."I',~................A..._~___I_-----'L...--....&...- .......
100 200
Applied Voltage [V r m. ]
Fig. 4. Luminance versus applied voltage characteristics
3.3 Crystallinity
Fig. 5 shows the X-ray diffraction patterns of active layer A, Band C. The intensive
diffraction line at around 28.5 degrees arising from (111) plane of zinc-blende structure has
been observed and the peak at around 56 degrees is due to (311) plane of zinc-blende
structure. The crystal orientations of these films are almost the same. The peak near 30
degrees is due to ITO. Table 3 shows the peak intensity and the full width at half maxima
(FWHM). Each peak intensity and each peak FWHM shows little difference, which means
that the crystallinity of the three kinds of films is almost the same.
Thi ckncs~ IO!lOnll
Th i Cknp.S5 700"8
Arl ive la)'cr C
Active lay('r R
Adive layer A
80
28 [degJ
40
281dcgJ
60 80
Fig. 5. XR.D patterns of the active layer Fig. 6. XR.D patterns of the active layer
18 Satoshi UMEKAWA, Atushi NITTA, Kenji TANAKA, Minoru KUSABIRAKI,
Yoshiharu MAEKAWA and Masao AOZASA
Fig. 6 shows the X-ray diffraction patterns of active layers A and C with about 1050 nm
thickness. Table 4 shows the peak intensity and FWHM. Active layer C has higher peak
intensity at 56 and 28.5 degrees and smaller FWHM than active layer A. Therefore, active
Table 3. XRD Peak intensity and full width at half maxima (FWHM) of
active layers with 700 nm thickness
ZnS(111) intensity [cps]
ZnS(111) FWHM [deg]
ZnS(311) intensity [cps]
ZnS(311) FWHM [deg]
active layer A active layer B active layer C
903 1068 828
0.342 0.342 0.376
144 146 151
0.785 0.820 0.752
Table 4. XRD Peak intensity and full width at half maxima (FWHM) of
active layers with 1050 nm thickness
ZnS(lll) intensity [cps]
ZnS(lll) FWHM [deg]
ZnS(311) intensity [cps]
ZnS(311) FWHM [deg]
active layer A active layer C
1315 1675
0.273 0.205
333 410
0.478 0.478
layer C has better crystallinity than active layer A. The comparison between Table 3 and
Table 4 shows that active layers with the thinner thickness of about 700 nm thickness had
higher portion of dead layer, which is uncrystallized part of the layer, which results in little
difference in XRD peak intensity.
3.4 Stoichiometry
Table 5 shows the result of stoichiometry analysis by XPS. It was obtained after three
minutes etching by Ar ion at 500 V of acceleration voltage and 20 rnA of discharge current.
We assumed that the stoichiometry of Zn to S in ZnS powder is 1 to 1. The
stoichiometry of Zn to S in active layer C is the closest to 1 to 1 in the three kinds of active
layers.
Table 5. Stoichiometry of active layers
ZnS powder
active layer A
active layer B
active layer C
Zn
1.00
1.23
1. 15
1.01
S
1.00
1.00
1.00
1.00
OES of sputtering plasma for fabrication of ZnS:Mn film
4. Conclusion
19
The conclusions can be summarized as follows:
1. Sufficient degassing of active layer and higher Ar gas flow rate at sputtering of active
layers results in better films.
2. Degassing of active layers and Ar gas flow rate at sputtering change the plasma state.
3. In the optical emission spectra in sputtering plasma, when Ar (750.3 nm) peak intensity is
normalized as unity, emission from Zn (472.2 nm) and Ar (763.5 nm) can be good
guidelines for the deposition of the active layer of the ZnS:Mn electroluminescent devices.
References
1) Umekawa, S., Nitta, A., Tanaka, K., Kusabiraki, M., Maekawa, Y., and Aozasa, M.: Record of Kansai-Sect.
Joint Convention of IEE Japan, p.G214 (1994)
2) Gray, D. E. (ed.), "American Institute of Physics Handbook", 2nd ed., McGraw Hill Book Co., Inc., New
York (1963)


Optical emission spectroscopy of rf-magnetron sputtering plasma for fabrication of ZnS:Mn active layer in thin-film EL devices

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Optical emission spectroscopy of rf-magnetron sputtering plasma for fabrication of ZnS:Mn active layer in thin-film EL devices

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