Aozasa, Masao

Nitta, Atsushi

Takamichi, Kazuhiro

Tanaka, Kenji

Osaka City University Repository

Mem. Fac. Eng., Osaka City Univ., Vol. 43, pp. 1 - 5 (2002)
Mn2+ centers in ZnS:Mn Thin-film EL Device Active Layer
Kazuhiro TAKAMICHI* , Atsushi NITTA** , Kenji TANAKA***
and Masao AOZASA****
(Received October 15, 2002)
Synopsis
It is very important to understand the state of incorporated Mn2+ centers to fabricate bright ZnS:Mn
thin-film electroluminescent (TFEL) devices. As the concentration of Mn is much less than that of
ZnS, it is very difficult to investigate its influence on luminescence. Five kinds of ZnS:Mn active layers
with different Mn concentration were fabricated by RF magnetron sputtering method, and X-ray
photoelectron spectroscopy (XPS) and Photoluminescent (PL) spectrum were examined to determine
Mn 3d orbit level and to decide the preferable weight percentage of Mn. Mn 3d orbit level were 2.6 eV
and 4.7 eV below vacuum level, and 0.5 wt. % was most efficient.
Keywords: Electroluminescence, ZnS:Mn, Sputtering, XPS, Photoluminescence
1. Introduction
Alternating current TFEL devices have been investigated as one of the hopeful flat-panel displays
because of the advantages such as completely solid state, emissive display with wide viewing angle
and rapid response time. Among the TFEL device fabrication methods such as sputtering l ), electron-
beam (EB) deposition, metal organic chemical vapor deposition (MOCVD) and atomic layer epitaxy
(ALE), we selected sputtering method. The sputtering method is a convenient fabrication method
which enables large area device fabrication at low cost. In this study, we fabricated five kinds of active
layers on the substrate and investigated the state of incorporated Mn2+ centers.
2. Device fabrication and experimental
ZnS:Mn active layer was fabricated on indium-tin-oxide (ITO)(MATSUNAMI GLASS IND., Corn-
ing #7059) coated glass substrate by RF magnetron sputtering method. Sputtering target was pre-
pared in the following procedures. ZnS powder (5N, 300 mesh, FURUUCHI CHEMICAL CORPO-
RATION) and MnS powder (2N, KOJUNDO CHEMICAL LABORATORY CO.,LTD.) were mixed
together and fired at 550°C in an argon atmosphere for 4 hours. There are two types of crystal
structures, zincblende-type and wurtzite-type. Films used in this study were zincblende-type, shown
in Fig. 1. It is known that a wurtzite-type structure is obtained when annealed at a temperature
higher than 1100 °C.
*Student, Master Course of Department of Physical Electronics and Information
··Student, Doctor Course of Department of Physical Electronics and Information
··*Research Associate, Department of Physical Electronics and Information
•••• Professor, Department of P hysical Electronics and Information
1
Table 1: Deposition conditions
• z~ ')···1 0')·11- s-----
Fig. 1: Zincblende crystal
structure of ZnS
Target
Base pressure
Gas pressure
Sputtering gas flow rate
Deposition time
Substrate temperature
RF power
Annealing temperature
Annealing time
ZnS+MnS powder
1.2 X 10-6 Torr
2.0 X 10-3 Torr
Ar (10 cern) + He (10 cern)
50 min.
200°C
50W
200°C
40 min.
Table 1 shows the deposition conditions. The following five kinds of active layers were prepared using
different sputtering targets: (a)active layer prepared with ZnS powder without Mn, (b)active layer
prepared with ZnS powder with 0.5 wt. %Mn, (c)active layer prepared with ZnS powder with 1.0 wt. %
Mn, (d)active layer prepared with ZnS powder with 2.0 wt. % Mn and (e)active layer prepared with
ZnS powder with 20.0 wt. % Mn. Figure 2 and Table 2 shows the X-ray diffraction(XRD) patterns
and thickness of the samples, respectively. Though the sputterings were performed under the same
conditions, the thickness of each sample was not the same. It would probably be due to the difference
in sputtering yields of ZnS and MnS.
The usual structure of ZnS:Mn EL device is /Glass substrate/ITO/Insulating layer/ZnS:Mn(Active
layer)/Insulating layer!Al electrode/. However in this study, the deposition of insulating layer and
electrode was not performed, because we are interested just in the investigation of the state of Mn2+
centers in active layer by X~S and PL method.
20 40 60 80
2 e(deg)
Fig. 2: XRD patterns
Table 2: Thickness of active layer
Active layer Thickness(A)
(b) 15000
(c) 12900
(d) 8700
(e) 2700
Table 3: XPS analysis conditions
Start Energy
Stop Energy
Step Energy
15 eV
oeV
0.05 eV
Sampling Time
Repeat Time
X-ray Source
1.0 sec
10 times
Mg
XPS spectra were obtained by X-ray photoelectron spectroscopy(ESCA-750, SHIMADZU CORPO-
RATION) and data processing system(ESPACIOO, SHIMADZU CORPORATION). As ZnS:Mn EL
peak wavelength is 585 nm due to the Mn2+ 3d levels, we decided to analyze the valence band region of
the samples and decided the Mn2+ levels below vacuum level. Table 3 shows analysis conditions. We
2
performed XPS investigation only on samples (a) without Mn and (e) with the highest concentration
ofMn.
Figure 3 shows a PL spectrum measurement system. PL spectrum was measured as follows2). A
150 W Xenon lamps (WACOM R&D CORPORATION, Model XD-151S) was used as an excitation
source. The light of the Xenon lamps through two glass filters (HOYA U-340, U-360) and interference
filter (334.0 nm) was chopped into 275 Hz signal with an optical chopper (SCITEC INSTRUMENTS).
Figure 4 shows the incident light. PL under the excitation light was monochromized and transformed
into an electric signal by a combination of a monochromator (JASCO, Model CT-25C) and photomul-
tiplier (HAMAMATSU, Model RI387). 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 AjD converter (Interface, Model IBX-3120) and analyzed by a personal
computer. We measured PL spectra of samples (b), (c), (d), (e), and four kinds of powder. When we
measured powder PL, the same mass of powder placed on the ITO glass was examined.
Optical Chopper Lens Optical Fiber
--=:::IC:::=+=::I1 --M-on-o---"""
chromator
Computer
300 400 500 600
Wavelength(nm)
Fig. 3: Spectrum measurement system Fig. 4: The excitation light for
PL measurement
3. Experimental results and discussion
3.1 Analysis of valence band of ZnS:Mn by XPS method
Figure 5 shows XPS spectra of two samples (a) and (e) after charging, smoothing and background
correction. Because the differences of two spectra are caused by the existence of Mn, we can find out
the levels of Mn from these spectra. To find them, peak separation of the spectrum was performed with
least squares method assuming Gaussian peaks. To solve least squares problem, we used a FORTRAN
program generated by ASNOP (Application System for Nonlinear Optimization Programs)3). ASNOP
is an application system which generates a FORTRAN program automatically.
Firstly, we separated spectrum of sample (a) into four peaks (dotted line), as shown in Fig. 6.
Secondly, we separated spectrum of sample (e) into six peaks, the previous four peaks (dotted line)
and other two more peaks(solid line) as shown in Fig. 7. Dashed lines in Fig. 6 and Fig. 7 are measured
data. These two peaks at 2.6 eV and 4.7 eV below vacuum level are the levels of Mn2+ 3d orbit, and
electrons transit from 2.6 eV level to 4.7 eV level on EL emission. The energy difference corresponds
to the wavelength of 590 nrn which, is pretty close to the EL peak wavelength of 585 nrn.
3
1\
J \
/ \/ : ....
Ii'
I i \
I . : \
I ..... .. \
I :I"~ ~.~
I ... .'\.... . :.. ~: \
..: · l, ~. :. \
..... ... .: ..•.
10 5 0
Binding Energy (eV)
Fig. 7: Peak separation of
sample (e) XPS spectrum
I/,.\
/:1I: \( i \
1\: \
I "~I: ~.'
-'I J, ..,
{:., I, ~ I' \
I 'I"~ : ~ 'I \__ I, 1-I ,", :', I~! " , .,', \
,! =,' ·, ....': •••••"t•••·:•••
10 5 0
Binding Energy (eV)
Fig. 6: Peak separation of
sample (a) XPS spectrum
10 5 0
Binding Energy (eV)
Fig. 5: XPS Spectrum
of valence band
3.2 The measurement of PL intensity
Figure 8 and 9 show the PL spectra of the thin films and powder, respectively. In the case of thin
film, the luminance decreased with increasing Mn concentration and sample (e) showed no emission.
These PL intensity were the original value divided by each thickness at the concentration region
examined. PL dependence and EL dependence on the Mn concentration is the same. It suggests that
we can use PL investigation for optimizing EL device active layer. On the other hands, powder PL
showed quite opposite results. That is, PL intensity increases with increasing Mn concentration up to
20 wt. %. Probably, Mn2+ state in the thin film is considerably different from that in the powder. It
is expected that much more isolated Mn2+ exist in the powder than in the thin film.
400 500 600
Wavelength(nm) 400 500 600Wavelength(nm)
Fig. 8: PL spectra of thin film Fig. 9: PL spectra of powder
4
4. Conclusion
The conclusions can be summarized as follows:
• Mn2+ 3d levels were estimated to be 2.6 eV and 4.7 eV below vacuum level. The enegy difference
between two levels of 2.1 eV corresponds to wavelength of 590 nm, which is very close to the EL
emission wavelength of 585nm.
• PL of thin film decreased with increasing Mn concentration, but PL of powder increased with
increasing Mn concentration up to 20 wt. %.
• The fact that many isolated Mn2+ can exist in the powder up to 20 wt. % suggests that such
high concentration of isolated Mn2+ can be realized also in thin film. And if some effective
energy transfer mechanism is introduced, very efficient device would be fabricated.
5. References
1) M. Aozasa, et aI., Japanese Journal of Applied Physics, 29, 1997 (1990)
2) N. Tanimoto, et aI., Memoirs of Faculty of Engineering, Osaka City University, 42, 11 (2001)
3) I. Tsuda, et aI., Memoirs of Faculty of Engineering, Osaka City University, 41, 13 (2000)
Acknowledgement
The authors would like to thank professors Minoru Kusabiraki and Yoshiharu Maekawa for their
encouragement and helpful discussions.
5


Mn^2^+ centers in Zns:Mn thin-film EL device active layer

http://dlisv03.media.osaka-cu.ac.jp/infolib/user_contents/kiyo/DB00010938.pdf

Mn^2^+ centers in Zns:Mn thin-film EL device active layer

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