On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior
Some peculiarities in the rapid portion of the voltage dependences of luminance and transferred charge as well as in its aging behavior in ZnS:Mn thin film electroluminescent devices made by different deposition techniques have been revealed. The devices with nearly the same Mn concentration (~1 at....
Збережено в:
| Дата: | 2002 |
|---|---|
| Автори: | , , , , , , , , |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2002
|
| Назва видання: | Semiconductor Physics Quantum Electronics & Optoelectronics |
| Онлайн доступ: | http://dspace.nbuv.gov.ua/handle/123456789/121128 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior / N.A. Vlasenko, Z.L. Denisova, Ya.F. Kononets, L.I. Veligura, M.M. Chumachkova, Yu.A. Tsyrkunov, E.L. Soininen, R.O. Tornqvist, K.M. Vasame // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 58-62. — Бібліогр.: 12 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-121128 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1211282017-06-14T03:03:05Z On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior Vlasenko, N.A. Denisova, Z.L. Kononets, Ya.F. Veligura, L.I. Chumachkova, M.M. Tsyrkunov, Yu.A. Soininen, E.L. Tornqvist, R.O. Vasame, K.M. Some peculiarities in the rapid portion of the voltage dependences of luminance and transferred charge as well as in its aging behavior in ZnS:Mn thin film electroluminescent devices made by different deposition techniques have been revealed. The devices with nearly the same Mn concentration (~1 at. %) were deposited by electron-beam evaporation and two atomic-layer epitaxy processes based on chlorine (ZnCl₂, MnCl₂) or organic (diethyl Zn and Mn(thd)₃ ) precursors. It has been studied interrelation between these peculiarities and the differences observed in the photodepolarization spectra of the devices, which give data about defects in the ZnS:Mn films and the energy of corresponding local states in them. The obtained results are discussed as to physical processes responsible for the rapid portion of the above voltage dependences and for the causes of its change after short-time accelerated aging. 2002 Article On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior / N.A. Vlasenko, Z.L. Denisova, Ya.F. Kononets, L.I. Veligura, M.M. Chumachkova, Yu.A. Tsyrkunov, E.L. Soininen, R.O. Tornqvist, K.M. Vasame // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 58-62. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS: 71.55.Gs, 78.60.Fi, 78.66.Hf http://dspace.nbuv.gov.ua/handle/123456789/121128 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Some peculiarities in the rapid portion of the voltage dependences of luminance and transferred charge as well as in its aging behavior in ZnS:Mn thin film electroluminescent devices made by different deposition techniques have been revealed. The devices with nearly the same Mn concentration (~1 at. %) were deposited by electron-beam evaporation and two atomic-layer epitaxy processes based on chlorine (ZnCl₂, MnCl₂) or organic (diethyl Zn and Mn(thd)₃ ) precursors. It has been studied interrelation between these peculiarities and the differences observed in the photodepolarization spectra of the devices, which give data about defects in the ZnS:Mn films and the energy of corresponding local states in them. The obtained results are discussed as to physical processes responsible for the rapid portion of the above voltage dependences and for the causes of its change after short-time accelerated aging. |
| format |
Article |
| author |
Vlasenko, N.A. Denisova, Z.L. Kononets, Ya.F. Veligura, L.I. Chumachkova, M.M. Tsyrkunov, Yu.A. Soininen, E.L. Tornqvist, R.O. Vasame, K.M. |
| spellingShingle |
Vlasenko, N.A. Denisova, Z.L. Kononets, Ya.F. Veligura, L.I. Chumachkova, M.M. Tsyrkunov, Yu.A. Soininen, E.L. Tornqvist, R.O. Vasame, K.M. On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Vlasenko, N.A. Denisova, Z.L. Kononets, Ya.F. Veligura, L.I. Chumachkova, M.M. Tsyrkunov, Yu.A. Soininen, E.L. Tornqvist, R.O. Vasame, K.M. |
| author_sort |
Vlasenko, N.A. |
| title |
On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior |
| title_short |
On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior |
| title_full |
On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior |
| title_fullStr |
On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior |
| title_full_unstemmed |
On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior |
| title_sort |
on origin of rapid portion of luminance-voltage dependence of zns:mn tfel devices and its aging behavior |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2002 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121128 |
| citation_txt |
On origin of rapid portion of luminance-voltage dependence of ZnS:Mn TFEL devices and its aging behavior / N.A. Vlasenko, Z.L. Denisova, Ya.F. Kononets, L.I. Veligura, M.M. Chumachkova, Yu.A. Tsyrkunov, E.L. Soininen, R.O. Tornqvist, K.M. Vasame // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2002. — Т. 5, № 1. — С. 58-62. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT vlasenkona onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT denisovazl onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT kononetsyaf onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT veligurali onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT chumachkovamm onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT tsyrkunovyua onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT soininenel onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT tornqvistro onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior AT vasamekm onoriginofrapidportionofluminancevoltagedependenceofznsmntfeldevicesanditsagingbehavior |
| first_indexed |
2025-07-08T19:14:24Z |
| last_indexed |
2025-07-08T19:14:24Z |
| _version_ |
1837107321292980224 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics. 2002. V. 5, N 1. P. 58-62.
© 2002, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine58
1. Introduction
An important feature of ZnS:Mn AC thin-film
electroluminescent (TFEL) devices is the very rapid lumi-
nance-voltage (L-V) dependence. A section with a small
slope often preceds to the rapid dependence. The share of
the former and the slope of the latter are affected by depo-
sition techniques, the Mn concentration, and driving con-
ditions. In addition, they change after aging, which results
also in the essential change of the threshold voltage (Vth)
of the rapid portion of the dependence [1�3]. The character
of the L-V dependence corresponds to that of the voltage
dependence of the transferred charge (Q). The high steep-
ness of these dependences is due to the positive feedback
between three main physical processes: the tunnel genera-
tion of free electrons, the multiplication of them, and the
generation of a positive space charge (PSC) enhancing the
field in the cathodic part of the EL film. Two latter processes
result from the impact ionization of the lattice or some
centers. It is supposed in most published papers that the
lattice ionization and subsequent deep trapping of holes
are responsible for the multiplication of electrons and build-
PACS: 71.55.Gs, 78.60.Fi, 78.66.Hf
On origin of rapid portion of luminance-voltage
dependence of ZnS:Mn TFEL devices
and its aging behavior
N.A. Vlasenko1), Z.L. Denisova1), Ya.F. Kononets1), L.I. Veligura1), M.M. Chumachkova1), Yu.A.
Tsyrkunov2), E.L. Soininen3), R.O.Tornqvist3), K.M. Vasame3)
1)Institute of Semiconductor Physics, NAS of Ukraine, 45 prospekt Nauky, 03028 Kyiv, Ukraine
Phone: +380 (44) 265 6252; fax: +380 (44) 265 8342; e-mail: vlasenko@isp.kiev.ua
2)Special Technological and Design Office of ISP, NAS of Ukraine, 4 Lisogorskaya, 03028 Kyiv, Ukraine
Phone: +380 (44) 265 1836; fax: +380 (44) 265 0555; e-mail: tzyrk@sktb4.semicond.kiev.ua
3) Planar International, Ltd.Olarinluoma 9, P.O. Box 46, FIN �02201, Espoo, Finland
Fax: 358 942 2143; e-mail: runar_tornqvist@planar.com
Abstract. Some peculiarities in the rapid portion of the voltage dependences of luminance and trans-
ferred charge as well as in its aging behavior in ZnS:Mn thin film electroluminescent devices made by
different deposition techniques have been revealed. The devices with nearly the same Mn concentra-
tion (~1 at. %) were deposited by electron-beam evaporation and two atomic-layer epitaxy processes
based on chlorine (ZnCl2, MnCl2) or organic (diethyl Zn and Mn(thd)3 ) precursors. It has been
studied interrelation between these peculiarities and the differences observed in the photodepolarization
spectra of the devices, which give data about defects in the ZnS:Mn films and the energy of corre-
sponding local states in them. The obtained results are discussed as to physical processes responsible
for the rapid portion of the above voltage dependences and for the causes of its change after short-time
accelerated aging.
Keywords: electroluminescence, thin-films, ZnS:Mn, aging.
Paper received 26.11.01; revised manuscript received 22.01.02; accepted for publication 05.03.02.
ing-up of PSC (see, e.g. [4] and ref. [3,9-12] in it). How-
ever, in some papers the important role of the direct im-
pact ionization of some deep centers is pointed out. So,
in [5] it is concluded that the impact ionization of zinc
vacancies (VZn), which is a part of donor-acceptor (D �
A) complexes, causes the generation of PSC in ZnS:Mn,
Cl devices made by the atomic layer epitaxy (ALE) tech-
nique. In [1] the ionization of Mn isovalent traps
( [ ]0+2
ZnMn ) was supposed to be the predominant process
when the Mn concentration is rather high. The presence
of such centers has been confirmed by the study of the
photodepolarization (PDP) spectra [2,3]. There is no
special consideration in published papers of the physical
causes of the effect neither deposition technique nor ag-
ing on the rapid portion of the L - V dependence. In this
paper, this problem is considered for ZnS:Mn AC TFEL
devices made by the electron-beam evaporation (EBE)
and two ALE processes using the use of chlorine or or-
ganic precursors [3]. Different impurities and intrinsic
defects, whose local energy levels have been revealed in
the PDP spectra of these devices, are taken into account
under the consideration. The impact ionization coefficient
N.A. Vlasenko et al.: On origin of rapid portion of luminance-voltage...
59SQO, 5(1), 2002
of these deep centers is estimated relatively to that of the
lattice. The part of the revealed defects in the impact
formation of PSC as well as their role in an atomic rear-
rangement during aging are discussed as concerning the
observed features of the rapid portion of the L-V depend-
ence and its aging behavior in the above devices.
2. Experimental procedures
The devices under the study had the configuration
typical for AC TFEL devices with a ZnS:Mn film sand-
wiched between two insulator (I) layers. The devices were
made by EBE and ALE deposition techniques. In the case
of the former, the ZnS:Mn film ( ~ 650 nm) and I layers
(SiO2 / Al2O3, ~ 270 nm) were deposited at 1500C. Co-
evaporation of ZnS and Mn were used for doping. An-
nealing was performed in vacuum at ~ 350 0C for 1h. The
ALE devices had AlxTiyOz insulator layers with the thick-
ness of ~ 300 nm and the ~ 550 nm thick ZnS:Mn film
grown by two different processes based on chlorine pre-
cursors (ZnCl2, MnCl2 ) and organic precursors (diethyl
zinc and Mn(thd)3) [3]. Below the former and the latter
will be denoted ALE �Cl� and ALE �O�, respectively.
The Mn concentration was about 1 at. % in all above
devices.
The voltage dependence of the luminance and trans-
ferred charge of the devices as prepared and aged has been
measured by common methods [6]. Short-time (up to 10 h)
accelerated aging has been performed at 5 kHz sine wave
voltage.
The PDP spectrum of the devices has been measured in
the region from 1500 to 320 nm as it was described in detail
in [3, 7]. This spectrum represents the spectral dependence
of the photocurrent (Iph) arising in a precharged devices
in the residual polarization field under action of the mono-
chromatic probing light. The devices were charged by
low dc voltage ( ≅ Vth /2) of the �-Al� polarity. Illumina-
tion of them by probing light was from the ITO side. The
PDP spectra were normalized on the maximum
photocurrent, but were not corrected on the equal number
of probing photons. Interference extrema in the spectra
were averaged.
3. Results and discussion
The voltage dependences of the luminance and trans-
ferred charge of the devices before and after aging are
shown in Fig.1. The differences in the initial �soften� sec-
tion of the characteristics in the EBE, ALE �Cl�, and ALE
�O� devices as well as its changes after aging of these
devices and physical causes of different aging behavior
have been considered in detail in [2, 3]. As to the rapid
section of the L � V and Q - V dependencies the following
peculiarities should be noted.
In the EBE devices, this section begins before aging at
rather low luminance. After aging, it shifts to somewhat
higher voltages and lower L. This is accompanied by an
increase of its slope. Such a behavior suggests that the
energy depth of the filled electronic states, which take part
in the tunnel generation of free electrons, increase after
aging. This results from a decrease of the number of native
defects in the ZnS:Mn films, which follows from changes
observed in the PDP spectrum of these devices after aging
( Fig. 2a ). So, it is seen the significant decrease of Iph in the
region from ~3.4 eV to 2.2 eV. The photocurrent in this
region is due, as it has been shown earlier [2, 3, 8], to the
L
u
m
in
an
ce
, c
d/
m
2
1000
100
10
0 1.
L
u
m
in
an
ce
, c
d/
m
2
Vo lta ge rm s, V
Tr
an
sf
er
re
d
c
ha
rg
e,
C
/c
m
µ
2
T
ra
ns
fe
rr
ed
c
h
ar
ge
,
C
/c
m
µ
2
130 150 170 90 100 110 80 90 100
0,01
1
c
b
a
0 1.
1
10
1
10
100
1000
A g in g
0
2h
10h
Vo lta ge rm s, V
Fig. 1. Voltage dependences of luminance and transferred charge of ZnS:Mn TFEL devices made by EBE (a), ALE �chlorine� (b) and ALE
�organic� (c) deposition techniques before and after short-time (2h) aging f=5 kHz.
60 SQO, 5(1), 2002
N.A. Vlasenko et al.: On origin of rapid portion of luminance-voltage...
F0, MV/cm 4 5 6
Fs, MV/cm 1.3 1.6 2.0 1.3 1.6 2.0 1.3 1.6 2.0
Centers ec, eV C, at. %
[ ]0+2
ZnMn 3.4 1 0.6 0.18 0.07 4.6 0.65 0.18 55 3.5 0.50
-2
ZnV 2.7 10-2 0.025 0.4 0.03 10 0.26 0.02
( -2
ZnV - +
SV ) 2.1 10-2 0.10 2.8 0.10 136 1.6 0.08
( -2
ZnV - +
SCl ) 1.8 10-2 0.18 6.5 0.22 510 4.0 0.12
O-center 2.6 0.5 1.6 0.03 26 1.8 0.03 790 17 1.1
Table. Ratio of impact ionization coefficients of different centers and lattice at various F0 and Fs .
N
or
m
al
iz
ed
p
ho
to
cu
rr
en
t,
r
el
. u
n.
P h o ton en erg y, eV
A g in g
0
2h
1.0
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
1.5 2 2.5 3 3.5
1.5 2 2.5 3 3.5
1.5 2 2.5 3 3.5
3.65 e V
3.45 e V
3.78 e V
2.1 e V
~ 3 .4 5 e V
~ 2 .1 e V
1.8 e V
~ 3 .7 e V
2.1 e V
~ 2 .6 e V
1.0
0
(a )
(b )
(c )
Fig. 2. PDP spectra of ZnS:Mn TFEL devices made by EBE (a), ALE �chlorine� (b), and ALE �organic� (c) deposition techniques
unaged and after aging for 2 h.
N.A. Vlasenko et al.: On origin of rapid portion of luminance-voltage...
61SQO, 5(1), 2002
photoionization of single and double charged zinc va-
cancies ( -
ZnV , -2
ZnV ), the ionization energy of which is
equal to 0.55 �0.6 eV and ~ 2.7 eV, respectively. The
decrease of the concentration of -
ZnV and -2
ZnV acceptors
after aging causes lowering of the concentration of na-
tive donors, too (according to the principle of charge com-
pensation). As a consequence, the Fermi level drops in
the bulk and at the interfaces, i.e. the energy depth of the
source of free electrons, which tunnel from filled inter-
face states, increases after aging. In the PDP spectrum of
the aged EBE devices, there are, besides of the main peak
of the intrinsic photocurrent at ~ 3.65 eV, only a weak
peak at ~ 2.1 eV and the very intensive peak at 3.4 - 3.45
eV. The former has been attributed to complexes of na-
tive defects ( -2
ZnV - +
SV ) [2]. The latter, which increases
with the Mn concentration, is related to the isovalent traps
[ ]0+2
ZnMn [2,3]. Therefore only these centers can contrib-
ute markedly to the impact processes mentioned above in
the EBE aged devices.
In the ALE �Cl� devices, the rapid portion of the L � V
dependences begins before aging right at low luminance.
During the short-time aging its beginning shifts to higher L
and V (Fig.1b). In addition, the slope of this section de-
creases somewhat. These changes are accompanied by the
significant increase of the � soften � section of the charac-
teristics at V< Vth, which is explained by appearance of a
shallow electron source in the EL film [3]. The increase of
the charge transferred at low voltages is just the main cause
of the above changes in the rapid portion. The transferred
charge trapped at the anodic interface results in arising of a
polarization field opposite to the external one. Therefore,
the field Fs in the S layers is reduced at given voltage and,
as a consequence, the Vth increases. The formation of the
polarization field leads also to a slower increase of Fs with V
which can be a cause of the decrease of the slope of the
rapid portion after aging. The decrease of the slope may
result also from appearance of new centers with the lower
ionization energy, which take part in the formation of
PSC in addition to [ ]0+2
ZnMn traps. Indeed, it is seen in the
PDP spectra of the ALE �Cl� device (Fig. 2b) that the
very intensive band with the maximum at 1.8 - 2.1 eV
appears after aging. It has been related to donor-accep-
tor (D�A) complexes, namely Cls and native donors asso-
ciated with -2
ZnV [3].
In the ALE �O� devices, the rapid portion of the L � V
and of the Q � V dependences is preceded by the marked
�soften� section not only after aging, but also before it
(Fig.1c). The beginning of the rapid portion shifts after ag-
ing to lower voltages and remains at about the same lumi-
nance level (~ 8 � 10 cd/m2), on the contrary to its aging
behavior in the EBE and the ALE �Cl� devices. The slope of
this portion decreases significantly during initial stage of
aging ( 5 � 7 h). Taking into account the consideration given
above for the ALE �Cl� devices, one can suggest that
centers shallower significantly than [Mn2+]0 traps arise
in the ALE �O� devices during aging and these centers
are just responsible mainly for the impact formation of
PSC. This suggestion is confirmed by peculiarities in the
PDP spectra of these devices as compared with those of
the EBE and the ALE �Cl� devices (Fig. 2c). It is as
follows: (1) the weaker photocurrent related to the Mn
traps before aging; (2) its significant decrease after ag-
ing, which is accompanied by the appearance of new rather
intensive peaks at ~ 2.6 eV and ~ 2.1 eV. It has been
suggested in [3] that these peculiarities in the PDP spec-
tra of the ALE �O� devices result from the presence in the
ZnS:Mn film some oxygen centers which are originated
from non-dissociated fragments of used organic precur-
sor molecules. Dissociation of the fragments during ag-
ing leads to the increase of the concentration of oxygen
centers. It is known that oxygen incorporates into the
lattice of II-VI compounds as isovalent traps [O −2
s ]0 [9].
In addition, O2- ions can form complexes with other de-
fects, e.g. -2
ZnV and Mn2+ ions. The formation of (O2- -
Mn2+) complexes can result in the lowering of the con-
centration of [ ]0+2
ZnMn traps and, consequently, of the
decrease of their peak in the PDP spectrum. At the same
time, new centers with the lower ionization energy, such
as [O −2
s ]0, (O2- - VZn), (O2- - Mn2+), might arise and take
part in impact building-up of PSC. This may be just re-
sponsible for the new peaks in the PDP spectrum and the
decrease of both the slope and the threshold voltage of
the rapid portion of the L � V dependence. There is no
experimental evidence of existence of oxygen residues in
the ALE �O� ZnS: Mn film. However the above sugges-
tion is confirmed by the following. Firstly, the rather
high oxygen concentration (0.2 � 0.9 at. %) has been
revealed in ALE SrS: Ce films grown from thd-precursors
[10]. Secondly, the special dopping of ALE �Cl� ZnS:
Mn films with oxygen results in changes in the PDP spec-
trum analogous to the above mentioned peculiarities in
the PDP spectrum of the ALE �O� devices [11].
An estimation of the ratio of the impact ionization
coefficient of the lattice (αl) and the different deep centers
above mentioned (αc) has been carried out as follows.
The most often used expression for αl in the case of high
fields (> 1 MV /cm), at which TFEL devices operate usu-
ally, is [12]:
αl =
l
sFe
ε exp ( - 2
s
2
0
F
F
) (1)
where Fs and F0 are the field in the film and a character-
istic field, respectively; 2
0F = Aεl; A is a constant for the
given semiconductor, εl is the ionization energy of the
lattice.
The analogous expression can be used for αc :
αc = C
eF
c
s
ε exp ( - 2
s
c
F
Aε
) (2)
where C is the atomic concentration of the centers and εc
is their ionization energy.
After division (2) by (1) we obtain:
62 SQO, 5(1), 2002
N.A. Vlasenko et al.: On origin of rapid portion of luminance-voltage...
αc / αl =
c
C
l
ε
ε
exp [ 2
s
2
0
F
F
( 1 - εc / εl )] (3)
In semiconductors with parabolic bands εl≈1.5 εg (εg
is the band gap), that is it is equal to ~ 5.5 eV for ZnS. As
to the magnitude of εc, it is nearly equal to the energy
depth of the center level relatively to the conduction band.
This is due to the participation in the impact process of
an ion heavy as compared with free carriers, which pro-
vides conservation of momentum of particles before and
after impact of a hot electron with the center. The possi-
ble difference in the magnitude of the impact cross-sec-
tion for different centers was not taken into account al-
though in the case of the Mn traps and complex centers
the cross-section may be larger than for the lattice ions.
The results of such estimation carried out with variation
of Fs and F0 is given in Table. As seen, the impact ioniza-
tion of the Mn traps can be the dominant process if
CMn ≥ 1 at. %, Fs ≤ 1.6 MV/cm and F0 = (5 � 6) MV/cm.
In the ALE �Cl� devices an additional contribution of
the impact ionization of -2
ZnV in D�A complexes is possi-
ble too. In the case of the presence of oxygen centers with
ec ≤ 2.6 eV and Cc ≥ 0.5 at.%, their impact ionization
should be expected to play the major role.
3. Conclusions
The very rapid dependence of L and Q on applied
voltage is due to the positive feedback between three major
processes of the high-field electroluminescence: the tunnel
generation of free electrons, the impact multiplication of
them, and the formation of PSC, which result from impact
ionization of the lattice and / or some deep centers directly.
The threshold and the slope of the rapid section are af-
fected, on the one hand, by the presence of centers, whose
impact ionization coefficient is competitive with that of the
lattice. On the other hand, the existence of a shallow elec-
tron source effects on them, because of transferring of elec-
trons before the threshold of the impact processes causes
a reduction of the field in the EL film owing to field clamp-
ing. Aging behavior of the rapid section is different de-
pending upon the nature and energy depth of the defects.
In the case of only intrinsic defects and the Mn isovalent
traps (e.g., as in EBE ZnS:Mn devices), the number of na-
ture acceptors and donors decreases after aging because
of their annihilation and gettering to the interfaces or grain
boundaries during drift in the high field [2]. This results in
an increase of Vth and the slope of the rapid section. The
impact ionization of the Mn traps may be predominantly
responsible for the generation of the holes, when the Mn
concentration is higher than 1 at.%. If there are some impu-
rity defects, as in the ALE devices, their drift causes the
formation of the complexes with other impurity or native
defects. Their ionization energy is essentially lower than
that of the lattice and the Mn traps. Therefore, the slope
and Vth of the rapid section decreases after aging in such
devices.
References
1. N.A. Vlasenko.// Acta Polytechnica Scandinavica. Appl. Phys.
Ser., N 170, p.p. 27-39 (1990).
2. N.A. Vlasenko, A.I. Beletskii, Z.L. Denisova, Ya.F. Kononets,
L.I. Veligura. In: Inorganic and Organic Electroluminescence.
W&T Verlag, Berlin, p.p. 267-270,1996.
3. N.A. Vlasenko, Ya.F. Kononets, Z.L. Denisova, Yu.V.
Kopytko,L.I.Veligura, E.L. Soininen, R.O. Tornqvist, K.M.
Vasama. // Semiconductor Physics, Quantum Electronics and
Optoelectronics, 4 (1), p.p. 55-62 (2001).
4. W. M. Ang, S. Pennathur, L. Plam, J. W. Wager, S. M. Goodnick,
and A. A. Douglas. //J.Appl. Phys., 77, p.p. 2719-2725 (1995).
5. S. Shin, P.D. Keir, J.F. Wager, J. Viljanen.// J. Appl. Phys.,
78, p.p. 5775-5781 ,1995.
6. Y. Ono. Electroluminescent Displays. World Scientific. Singa-
pore-New Jersay-London-Hong Kong, 1995.
7. N. A. Vlasenko, A. I. Beletskii, Z. L. Denisova, Ya. F. Kononets,
and L. I. Veligura. In: Optical Diagnosis of Matirials and De-
vices for Opto-, Micro- and Quantum Electronics. Ed. by S.V.
Svechnikov and M.Ya. Valakh. Proc. of SPIE, p.p. 512-518,
1998.
8. N. A. Vlasenko, M.M. Chumachkova, Z.L. Denisova, L.I.
Veligura.// J. Crystal Growth, 216, p.p. 249-255 (2000).
9. N.K. Morozova, V.A. Kuznetsov. Sulfid Zinka: polucheniye i
opticheskiye svoistva. Nauka, Moscow, 1987, p.p. 65 �150.
10. W�M. Li, R. Lappalainen, J. Jokinen, M. Rutala, M. Leskela,
E.L. Soininen, B. Huttle. In: Inorganic and Organic
Electroluminescence, W&T Verlag, Berlin, p.p. 157-160 (1996).
11. N.A. Vlasenko, M.B. Kotlyarevskii, L.I. Veligura, Z.L. Denisova,
V.V. Kidalov, Y.F. Kononets, A.S. Revenko. //
Optoelectroniica i poluprovodnikovaya technika. 36, pp. 93 �
100 (2001).
12. I.K. Vereschagin. Electroluminescence of crystals, Nauka, Mos-
cow, 1974 (in Russian)
|