Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels
Low-temperature luminescence spectra under broad-scale variation of an excitation level Iexc are studied for ZnS/ZnSe single quantum wells (QW) and for CdS bulk crystals and epilayers. In the first case, the manifestations turn out to be of the heterointerface inhomogeneity - i.e. fluctuations of Q...
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irk-123456789-1205452017-06-13T03:05:36Z Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels Brodyn, M.S. Shevel, S.G. Tishchenko, V.V. Low-temperature luminescence spectra under broad-scale variation of an excitation level Iexc are studied for ZnS/ZnSe single quantum wells (QW) and for CdS bulk crystals and epilayers. In the first case, the manifestations turn out to be of the heterointerface inhomogeneity - i.e. fluctuations of QW thickness. The position of the mobility edge for excitons localized by fluctuations is determined. In the second case the effect of the increase of Iexc is systematically studied not only for excitonic but also for impurity-related edge luminescence. Contrary to the earlier and commonly assumed expectations, up to the highest Iexc close to damage threshold no saturation of edge luminescence intensity was observed in bulk CdS crystals, whereas in a few thick epilayers such saturation did occur. The suggested qualitative explanation takes into account diffusion (non-diffusive transport) of carriers beyond the excited near-surface layer. Дослiджено спектри низькотемпературної люмiнесценцiї при варiацiї рiвня збудження I у широкому дiапазонi для одиночних квантових ям (КЯ) ZnS/ZnSe та об’ємних кристалiв i епiтаксiйних шарiв CdS. У першому випадку знайдено прояви неоднорiдностi гетерограницi, тобто флуктуацiй товщини КЯ. Визначено енергетичну позицiю краю рухливостi для екситонiв, локалiзованих на флуктуацiях. У другому випадку систематично вивчений вплив зростання I не лише на екситонну, а й на пов’язану з домiшками крайову люмiнесценцiю. На вiдмiну вiд загальноприйнятих ранiше уявлень, при збiльшеннi I аж до порогу руйнування не спостерiгалося нiякого насичення iнтенсивностi крайової люмiнесценцiї в об’ємних кристалах CdS. У той же час в епiтаксiйних шарах товщиною декiлька мiкронiв таке насичення справдi мало мiсце. Пропонується пояснення на якiсному рiвнi, яке враховує дифузiю (недифузiйний транспорт) носiїв зi збудженого приповерхневого шару. 1999 Article Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels / M.S. Brodyn, S.G. Shevel, V.V. Tishchenko // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 531-542. — Бібліогр.: 26 назв. — англ. 1607-324X DOI:10.5488/CMP.2.3.531 PACS: 78.20.-e http://dspace.nbuv.gov.ua/handle/123456789/120545 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Low-temperature luminescence spectra under broad-scale variation of an
excitation level Iexc are studied for ZnS/ZnSe single quantum wells (QW)
and for CdS bulk crystals and epilayers. In the first case, the manifestations
turn out to be of the heterointerface inhomogeneity - i.e. fluctuations of QW
thickness. The position of the mobility edge for excitons localized by fluctuations is determined. In the second case the effect of the increase of Iexc
is systematically studied not only for excitonic but also for impurity-related
edge luminescence. Contrary to the earlier and commonly assumed expectations, up to the highest Iexc close to damage threshold no saturation of
edge luminescence intensity was observed in bulk CdS crystals, whereas
in a few thick epilayers such saturation did occur. The suggested qualitative
explanation takes into account diffusion (non-diffusive transport) of carriers
beyond the excited near-surface layer. |
| format |
Article |
| author |
Brodyn, M.S. Shevel, S.G. Tishchenko, V.V. |
| spellingShingle |
Brodyn, M.S. Shevel, S.G. Tishchenko, V.V. Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels Condensed Matter Physics |
| author_facet |
Brodyn, M.S. Shevel, S.G. Tishchenko, V.V. |
| author_sort |
Brodyn, M.S. |
| title |
Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels |
| title_short |
Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels |
| title_full |
Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels |
| title_fullStr |
Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels |
| title_full_unstemmed |
Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels |
| title_sort |
experimental studies of the recombination processes in ii-vi semiconductors (bulk crystals and epilayers) at variable excitation levels |
| publisher |
Інститут фізики конденсованих систем НАН України |
| publishDate |
1999 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120545 |
| citation_txt |
Experimental studies of the recombination processes in II-VI semiconductors (bulk crystals and epilayers) at variable excitation levels / M.S. Brodyn, S.G. Shevel, V.V. Tishchenko // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 531-542. — Бібліогр.: 26 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
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| first_indexed |
2025-07-08T18:06:37Z |
| last_indexed |
2025-07-08T18:06:37Z |
| _version_ |
1837103055609266176 |
| fulltext |
Condensed Matter Physics, 1999, Vol. 2, No. 3(19), pp. 531–542
Experimental studies of the
recombination processes in II-VI
semiconductors (bulk crystals and
epilayers) at variable excitation levels
M.S.Brodyn, S.G.Shevel, V.V.Tishchenko
Institute of Physics of the National Academy of Sciences of Ukraine,
46 Nauky Avenue, 252022 Kyiv, Ukraine
Received April 21, 1998
Low-temperature luminescence spectra under broad-scale variation of an
excitation level Iexc are studied for ZnS/ZnSe single quantum wells (QW)
and for CdS bulk crystals and epilayers. In the first case, the manifestations
turn out to be of the heterointerface inhomogeneity - i.e. fluctuations of QW
thickness. The position of the mobility edge for excitons localized by fluctu-
ations is determined. In the second case the effect of the increase of Iexc
is systematically studied not only for excitonic but also for impurity-related
edge luminescence. Contrary to the earlier and commonly assumed expec-
tations, up to the highest Iexc close to damage threshold no saturation of
edge luminescence intensity was observed in bulk CdS crystals, whereas
in a few thick epilayers such saturation did occur. The suggested qualitative
explanation takes into account diffusion (non-diffusive transport) of carriers
beyond the excited near-surface layer.
Key words: luminescence,quantum well,epilayer, exciton, ZnSe, CdS
PACS: 78.20.-e
1. Introduction
From the mid-sixties till early eighties much work was done towards the un-
derstanding of the behaviour of carriers (excitons) in bulk direct-gap II-VI crystals
with the increase of optical excitation level Iexc. The results are reviewed, e.g. in
[1,2]. With the later advance of growth techniques to fabricate high-quality epilay-
ers and quantum well (QW) structures the interest in the above-mentioned field has
grown anew, see, e.g. [3,4] and references therein. This interest is largely application-
motivated, especially after the demonstration of ZnCdSe-based laser diode operating
in the blue-green [5]. Besides, some relevant problems of fundamental importance
remained relatively unexplored and require a better insight.
c© M.S.Brodyn, S.G.Shevel, V.V.Tishchenko 531
M.S.Brodyn, S.G.Shevel, V.V.Tishchenko
In the numerous spectroscopic studies of high-excitation phenomena, the atten-
tion was mostly focused on the various collective processes with participation of
excitons as manifested in the occurrence of new bands in the excitonic part of the
near-edge emission spectrum with the increase of Iexc [1,2]. In the present work we
shall try to demonstrate that the studies of emission spectra under variable Iexc
could provide a valuable information regarding the intrinsic properties of carriers
(excitons) as well. The objects under study are the epitaxial layer structures with a
different confinement scale.
2. Results and discussion
2.1. ZnSe-ZnS QW structures
In the first part of the paper the results are discussed for ZnSe-ZnS single QW
on GaAs substrate. Single 11 Å – thick ZnSe QW’s sandwiched between wider-gap
ZnS barrier layers of 54 Å thickness were grown by a relatively simple, inexpensive
and flexible technique of photo-assisted vapour phase epitaxy – PAVPE. The details
of the growth procedure are reported elsewhere [6]. The obtained structures are
coherently strained since the thickness of the layers is smaller than the respective
critical values of about 100 Å when mismatch dislocations emerge. Consequently,
the small variations in QW thickness do not cause the change of deformation within
the layer plane.
In quite many earlier works carried out for QW structures grown by other epitax-
ial techniques like MBE and MOCVD it was shown that the heterointerface disorder
(fluctuations δLz in QW thickness) resulted in the inhomogeneous broadening of ex-
citonic photoluminescence (PL) band [7-10]. In the highest quality structures δLz
corresponds to a single atomic layer.
In our case the properties of excitonic PL were studied under excitation by 325
nm line of cw He-Cd laser (average power ( 10 mW). The intensity Iexc on the sample
surface varied over about three orders of magnitude. The spectra were registered by
photon-counting technique.
In figure 1 plot “a” represents an “integrated” (taken with the excitation spot
diameter d=2 mm to cover the entire sample surface) PL spectrum typical for all
QW under study. The emission is of excitonic nature as it is evident from the com-
parison with the low-temperature exciton reflection curve – plot “e” in figure 1. The
spectra are shifted by ∼ 188 meV to higher energies in comparison with their posi-
tions known for bulk ZnSe. This short-wavelength shift is caused by the combined
effect of size quantization and elastic stress deformation. The latter is due to lattice
mismatch between ZnSe/ZnS and GaAs substrate; the deformation contribution into
the observed shift does not exceed 5 % [11].
The peak position Em=2.980 eV (λm =415.9 nm) of the excitonic PL band
(plot “a” in figure 1) is only weakly sample-dependent whereas the band itself is
asymmetric with a number of shoulders. They are not equidistant in energy scale
and therefore cannot be assigned to phonon replicas. Taking also into account the
532
Experimental studies of the recombination processes
412 416
60
120
Wavelength (nm)
R
es
po
ns
e
(a
.u
.)
d
c
b
a
e
Ehh
Figure 1. Spectra of integrated (a) and local (b-d) photoluminescence and reflec-
tion curve (e) for ZnSe-ZnS QW at T=4.5 K. The spectra b to d were measured
at different excitation spot positions on the sample surface. Excitation level for
b-d is 400 times higher than for a. The different plots in this and in other figures
are arbitrarily shifted along Y-axis for clarity.
dependence of Em on the excitation spot position when the spot diameter was made
d=100 µ – cf. plots 1b, c and d – it is plausible to attribute the occurrence of
shoulders to the inhomogeneity of QW structures under study.
In figure 2 the dependencies of PL spectra taken from different excitation spot
positions on Iexc are illustrated. As a rule, with the increase of Iexc the peak Em
shifts to higher energies approaching a limit of 2.991 eV. Simultaneously, the band
acquires additional asymmetry – its low-energy tail becomes more extended. The
similar behaviour is observed with the increase of temperature. Noteworthy is the
absence of any new bands that could be associated with the collective processes.
The above-described findings are readily explained within the framework of the
model of localization of excitons by potential fluctuations resulting from the disorder
in the interface between ZnS and ZnSe layers. Disorder (fluctuations of QW thick-
ness) on the scale of exciton Bohr radius aB inevitably results in the emerging of 2D
potential relief and the respective density-of-states tail. The filling of localized tail
states is described by Fermi-Dirac statistics [12]. Though excitons behave like bosons
on the sufficiently large distances between them, the applicability of these statistics
reflects the fact that due to the internal structure of the quasiparticles under dis-
cussion every separate well of the relief that has emerged can localize not more than
a single exciton provided the characteristic size Rs of the well is comparable to aB.
In view of the above-stated, the behaviour represented in figure 2 is viewed as a
result of the increase of the value of chemical potential of the degenerate system of
localized excitons with the increase of its density. Clearly, at sufficiently high I exc the
533
M.S.Brodyn, S.G.Shevel, V.V.Tishchenko
4 0 5 4 1 0 4 1 5 4 2 0 4 2 5
a
2 s
0 .1 3 I
o
0 .3 6 I o
I
o
IN
T
E
N
S
IT
Y
(
a
.u
.)
W A V E L E N T H (n m )
�� �� ���
�����
����
E
m
(
eV
)
Iexc (W cm-2)
3
2
1
.
b
Figure 2. Examples of PL spectra of ZnSe-ZnS QW at different Iexc (a) and
the dependencies of the position Em of PL peak on Iexc (b). In (a) level Io=60
W/cm2. In (b) the dependencies 1-3 correspond, respectively, to the curves b-d
in figure 1. Points in (b) represent the experimental data whereas the solid lines
are the guides for the eye.
chemical potential level may reach the mobility edge E c. With the further increase
of Iexc the filling of delocalized states sets in. As a consequence, PL peak becomes
stabilized at Ec. The reason for this stabilization is a transition to the statistics with
a density-independent distribution function. In contrast to bulk crystals, for 2D QW
structures the excitation intensity necessary for such a transition is attainable in real
experimental conditions.
From the data in figure 2 we have determined that in our case the mobility
edge corresponds to the energy 2.991 eV. We also note that the width of PL band
(35 to 40 meV on 0.1 intensity level) indicates that δLz corresponds to an atomic
monolayer.
The key question is the position of Ec with respect to the free exciton energy.
To determine the latter value, a close comparison of the “integrated” PL band
(figure 1a) with the reflection curve (figure 1e) is helpful. We draw attention to the
weak PL peak at 2.997 eV (413.5 Å), labelled as Ehh in figure 1a. It is clearly
seen only in the PL spectrum integrated over the entire surface and its spectral
position falls in the vicinity of the reflection curve minimum (figure 1e). With the
precision of the order of the energy of longitudinal-transverse splitting (1.5 meV for
bulk ZnSe [13]) this minimum coincides with the position of free exciton resonance
[14]. Therefore we conclude that Ehh peak is due to free exciton recombination in
the sample areas of nominal thickness Lz=11 Å. The excitation of the entire surface
of the sample as well as the low excitation densities favour the observation of Ehh
peak.
It follows from the above attribution that in the samples under study, the mo-
bility edge is 6 meV below the ground state level of h-h exciton in the QW of 11 Å
534
Experimental studies of the recombination processes
thickness (the level of l-h exciton is about 200 meV higher in energy scale [11]).
As regards the nature of delocalized states responsible for the emission within
the spectral range from Ec to Ehh, they may be attributed to the exciton states in
those QW regions where Rs < aB [15]. Excitons move freely within any region of the
kind and they experience scattering only on its edges [16]. Then, plot d in figure 1
is thought to correspond to such areas on the surface where mostly Rs < aB.
Therefore the experiments at variable Iexc enabled us to conclude that the samples
under study are inhomogeneous in the sense that the heterointerfaces between QW
and barrier layers are disordered as well as the distribution of size R s over the
surface is not homogeneous. The latter feature is probably caused by the lateral
inhomogeneity of hydrogen flow that is used in PAVPE to carry the reagents to
GaAs substrate.
2.2. CdS epilayers and bulk crystals
In the second part of the paper we report on the experimental check of how the
increase of Iexc over 6 orders of magnitude affects the behaviour of not only excitonic
but also of the so-called “edge” luminescence involving donor-acceptor pairs (DAP).
The objects under study were vapour-grown CdS single platelet crystals with
the thicknesses of the order of 40 µ and over, as well as a few thick CdS epilayers
grown by hot wall epitaxy [17]. Luminescence was studied mostly at T = 20 K. The
excitation sources were conventional Hg lamp (365 nm line, Iexc 6 50 mW/cm2),
third harmonic of a Q-switched low-repetition rate (∼ 4 Hz) YAG:Nd3+ laser with
an amplifier stage and UV radiation of µs flash lamps. A computer-controlled two-
channel digital registration system featured the monitoring of I exc in each shot and
step-by-step spectral scanning. This approach offered us some advantages like the
possibility to adjust the sensitivity on-line when registering spectrally close emission
bands of very different intensities, to perform gated detection within the pre-set
limits of Iexc to compensate its variations, to measure directly not only spectra but
also the dependencies Ilum=f(Iexc) for the selected emission bands.
Before a further discussion it is helpful to briefly recall the well-established gen-
eral description of the near-edge radiative recombination channels in CdS (a model
object for the entire group of II-VI semiconductors) [2,18]. At low Iexc of the order
of 1 W/cm2 or below, typical features of the excitonic (“blue”) luminescence at
helium temperatures appear to be a weak free A-exciton band and narrow bands
I2 and I1 due to excitons bound on neutral donors and acceptors, respectively, to-
gether with their LO-phonon replicas. Non-excitonic recombination of electrons and
holes via impurity levels gives rise to the so-called edge (“green”) luminescence at
longer wavelengths. At helium temperatures, recombination “electron on donor –
hole on acceptor” prevails. It is manifested by a “long-wavelength (LW)” series -
zero-phonon band with satellites due to the interaction with LO phonons. At el-
evated temperatures, electrons escape from relatively shallow donor centers and
gradually “short-wavelength (SW)” series due to recombination “free exciton – hole
on the same acceptor” becomes dominant.
PL spectra under Hg-lamp excitation for CdS platelet crystal (figure 3) agree
535
M.S.Brodyn, S.G.Shevel, V.V.Tishchenko
•
490 500 510 520 530
70 KSW
LW
I 1-
2L
O
I 1-
L
O
LO
20 K
(x5)
I1
A
I2
v
c
I1
I2
A
ed
ge
lu
m
.
SW LW
EA
ED
ex
ci
to
ni
c
lu
m
.
I lu
m
, a
.u
.
Wavelength (nm)
Figure 3. Simplified scheme of the transitions responsible for excitonic and edge
luminescence (the insert) and experimental PL spectra at Hg-lamp excitation for
CdS platelet crystal at 20 K and 70 K. In this and in other figures, scaling factors,
if different from 1, are shown in parentheses.
with this recombination scheme (illustrated in the insert). In this case the (net)
intensity of the LW band ( 517 nm) of the DAP emission was about 2.7 times higher
than the intensity of the I1-LO excitonic component (496.3 nm) and almost 18 times
higher in comparison with I1-2LO (504.3 nm). Evidently, such ratios depend on the
content of impurities in a particular sample and sometimes were found to vary
slightly with the position of an excitation spot (of about 0.5 mm diameter) on the
surface. In the “high quality” CdS crystals [18] and epilayers [17], edge luminescence
and sometimes I1 (not I2!) excitonic band may not be present at all. In the usual,
not intentionally doped platelet CdS crystals used here, the edge luminescence was
often present. The basic findings to be discussed below were found to be qualitatively
similar in different crystals of the kind. Therefore, the examples in the figures to
follow will be given for the same platelet sample as in figure 1.
Our main aim was to check the behaviour of edge luminescence with the increase
of Iexc up to the highest possible values (a few MW/cm2 – close to the damage
threshold). As already mentioned in the introduction, for excitons in bulk CdS the
respective “high-excitation phenomena” were studied in great detail and rather well
comprehended. In contrast, the effect of the similar increase of Iexc on non-excitonic
edge luminescence remained quite unexplored. Already in one of the pioneering works
on high-excitation phenomena [19], where the model of rate equations had been for-
warded to describe the collective processes in highly excited CdS, the assumption
was made that the impurity centers involved in DAP emission soon became satu-
rated with the increase of Iexc and could be neglected. Since then, such a viewpoint
has become commonly accepted though, to our knowledge, the saturation of edge
emission intensity with the increase of Iexc , in particular, above few tens of kW/cm2,
had never been demonstrated experimentally for either of the II-VI semiconductors.
536
Experimental studies of the recombination processes
496 504 512 520 528
Wavelength, nm
N
or
m
al
ize
d
I lu
m
, a
.u
.
a
1
2
3
4
5
I1-LO
I1-2LO
(:16)
(x4)
(x4)
(x4)
(:4)
(x4)
102 103 104 105 106
102
103
104
105
106
b
SW-LO
I1-LO
2
Iexc (W/cm2)
I lu
m
, a
. u
.
3
1
Figure 4. PL spectra normalized with respect to I1-2LO component (a) and
dependencies Ilum=f(Iexc) for the exciton I1-LO band and the first phonon replica
SW-LO of edge emission (b) of CdS platelet crystal at T=20K under excitation by
the third harmonic of YAG:Nd3+ laser. Spectra in (a) are taken at the following
excitation levels in W/cm2 : 1 – 5x10−2 (excitation by Hg-lamp) , 2 – 0.9x103,
3 – 5x103, 4 – 3.2x104, 5 – 3x105. Points in (b) represent the experimental data
(after averaging and smoothing), straight lines 1, 2, 3 correspond to a power law
Ilum ∝ (Iexc)
γ with the slopes γ ( of 1.48, 0.46 and 0.86, respectively.
This problem has got wider implications if striving for the comprehensive description
of various relaxation channels of radiative and non-radiative recombination, see, e.g.
[1,19,20].
The behaviour of excitonic PL with the increase of Iexc was found to be quite
typical, namely, first I2 and I1 bands become broadened, than the new features due
to various exciton scattering processes emerge at different spectral positions until at
the highest Iexc close to damage threshold (few Mw/cm2) ill-structured broad-band
emission in the range from ∼ 490 nm to ∼ 493 nm dominates. The attributions of
the new bands to various collective processes – bIexcitons, exciton-exciton inelastic
scattering, electron-hole plasma emission at highest Iexc, etc. – are well established
though not completely free of controversies [1].
We should emphasize, however, that spectrally isolated phonon replicas I1-LO
and I1-2LO were found to survive in the spectra up to the highest Iexc. This confirms
the earlier conclusions [1,21] regarding the possibility for the various exciton-related
recombination processes to coexist in highly excited II-VI crystals. A possible expla-
nation will be discussed hereinafter. Concerning the edge luminescence, it was found
to exhibit a more complicated behaviour with the increase of Iexc than the simple
saturation figures 4a,b). Already at the initial Iexc of about 1 kW/cm2 (by the third
harmonic of YAG:Nd3+ laser), when excitonic I2 and I1 bands are not yet affected,
the relative intensity of edge luminescence drops drastically. Thus, the net intensity
of the zero-phonon band of edge emission (the SW-band in this case, see further dis-
537
M.S.Brodyn, S.G.Shevel, V.V.Tishchenko
cussion) is more than 20 times weaker in comparison with the excitonic band I 1-2LO
(e.g., plot 2 in figure 4a). This is in an apparent contrast to the situation with the
excitation by Hg lamp (Iexc 6 50 mW/cm2). In fact, without the above-mentioned
experimental option of adjusting the sensitivity on-line, we would quite probably
miss the weak edge emission whatsoever when registering the neighbouring intense
excitonic components.
Notably, no saturation occurs with the further increase of Iexc of over three or-
ders of magnitude up to a few MW/cm2. Instead, the intensity of the DAP emission
also grows. The growth rate is not very different from that for the excitonic compo-
nents. This is evident from figure 4a and – more clearly – from figure 4b where the
experimental dependencies Ilum=f(Iexc) (following the averaging and smoothing) are
presented. Details on the measurements and on the pre-processing algorithm can
be found in [22]). Within certain ranges of Iexc these dependencies are well approxi-
mated by a power law: Ilum ∝ (Iexc)
γ with γ < 2. In principle, this type of behaviour
is obtained (for excitonic bands) in the analytical [22] and in the numerical [23]
theoretical calculations as well. Without going into too much details we may note
that the obtained experimental results could become helpful when composing the
underlying sets of rate equations.
In the additional experiments with the selected microsecond flash lamps as exci-
tation sources we were able to obtain some data in the range of excitation intensities
below 1 kW/cm2. Remarkably, it was found that the initial drop of the relative inten-
sity of DAP emission with respect to excitonic PL becomes noticeable and develops
rather fast already at quite moderate – few W/cm2 – excitation levels, i.e. far below
the values when collective processes for excitons show up.
The revealed peculiarities of the edge luminescence behaviour in CdS bulk (pla-
telet) crystals with the increase of Iexc – fast initial decrease of relative intensity
compared to excitonic components but no saturation up to the highest Iexc – seem
to be of common nature. In fact, we have observed the qualitatively similar behaviour
for the other (bulk) direct-gap II-VI crystals such as CdSe and ZnxCd1−xS.
To explain the experimental findings we suggest the qualitative model that takes
into account the surface character of a single-photon excitation. Namely, the thick-
ness of the excited near-surface layer, roughly estimated as the inverse of a band-to-
band absorption coefficient, amounts to ∼ 1µ or even less [24]. Then the level, say,
10 W/cm2 of the UV 365 nm radiation would correspond to the bulk generation rate
of electron-hole pairs of about 2 · 1024 cm−3 · s−1. This is sufficient to saturate the
impurity centers of edge luminescence assuming their concentrations to be 1016-1017
cm−3 and the radiation lifetimes of the order of tens of nanoseconds or more [25].
Likewise, the rough estimations indicate the possibility of screening the Coulomb
interaction for impurity-bound carriers in the near-surface active layer in the same
conditions.
At the same time it should be taken into account that with the increase of Iexc
the relative number of carriers becomes larger and they migrate by a diffusion or
a non-diffusive [26] transport, driven by an enormous gradient of their chemical
potential, to the larger distances from the surface. Then, saturated and/or screened
538
Experimental studies of the recombination processes
103 104
103
104
105
LW
I2
I lu
m
, a
.u
.
Iexc (W/cm2)
480 500 520 540
Wavelength, nm
I2
LW
4
3
2
1
N
or
m
al
iz
ed
I
lu
m
, a
.u
.
Figure 5. Normalized (with respect to I2 band) luminescence spectra of CdS
epilayer at T=20 K under excitation by Hg-lamp (1) and the third harmonic
of YAG:Nd3+ laser (2,3,4). Excitation levels in W/cm2 are, respectively: 1 –
0.05 , 2 – 1x103, 3 – 17x103, 4 – 44x103 . The insert – dependencies Ilum=f(Iexc)
measured as described in the text for excitonic band I2 (crosses) and zero-phonon
DAP emission band at ∼550 nm (circles).
impurity centers of edge luminescence in the active near-surface layer coexist with
the deeper parts of the crystal where the concentration of the excited carriers is
smaller while edge luminescence (relatively weakened) as well as (not self-absorbed)
excitonic bands I1-LO and I1-2LO follow the increase of Iexc. Such a qualitative model
agrees with the experimental observations. It also provides the explanation for the
transition from the LW to the SW series with the increase of Iexc (cf. figure 4a and
figure 3) if a reasonable additional assumption is made that the donors involved are
accumulated mainly near the surface in the growth process whereas in the bulk their
content is smaller. The fast initial drop of the relative intensity of edge luminescence
finds a consistent explanation as the one resulting from the saturation within the
near-surface layer before the essential transport inside the crystal sets in.
To verify the suggested model we have performed additional experiments with
CdS epilayers. The motivation was obvious: if the above considerations are true,
then sufficiently thin (of the order of diffusion length, i.e. a few microns) crystalline
layers could be excited almost homogeneously and, in contrast to thick bulk crystals,
the saturation of edge luminescence with the increase of Iexc could be observed. As
far as excitons are concerned, they could reach the interface to the GaAs substrate
and recombine there.
We have, indeed, observed the expected behaviour for CdS epilayer of (4µ-
thickness – figure 5). The dependencies Ilum=f(Iexc) (the insert in figure 5) were
obtained in this case by monitoring the intensities of the respective bands – I2 peak
539
M.S.Brodyn, S.G.Shevel, V.V.Tishchenko
at ∼470 nm and sim515-nm zero-phonon band of DAP emission – in a sequence of
spectra measured at different Iexc with ±10 % gate. The difference in the spectral
position of the zero-phonon edge luminescence band (∼515 nm) in comparison with
the LW-band in bulk crystals (∼517.5 nm)could be connected with the different
type of donor or some strain in the layer. The possible origin of the emission (that
shifts appreciably to shorter wavelength with the increase of Iexc) in the “interme-
diate” spectral range between I2 and 515-nm bands is not clear yet. By an apparent
contrast to the bulk crystal, the unambiguous saturation of the zero-phonon band
of DAP luminescence was achieved (at Iexc above ∼2 kW/cm2).
3. Acknowledgements
This work was supported in part by INTAS program within the project 94–324
and by the Fundamental Research State Fund, Ukraine (grant 2.4/86).
We are grateful to A.Kovalenko, M.Vytrikhivski, M. Grün and M.Hetterich for
providing the samples for the present studies, to M.Bondar and V.Vozny for the
assistance in the experiments and to Prof. C.Klingshirn for stimulating discussions.
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541
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Експериментальні дослідження рекомбінаційних
процесів у напівпровідниках A2B6 (об’ємні кристали
і епітаксійні шари) при варіації рівня збудження
М.С.Бродин, С.Г.Шевель, В.В.Тищенко
Інститут Фiзики НАН України
пр. Науки 46, 252022, Київ 22, Україна
Отримано 21 квітня 1998 р.
Дослiджено спектри низькотемпературної люмiнесценцiї при варi-
ацiї рiвня збудження I у широкому дiапазонi для одиночних кван-
тових ям (КЯ) ZnS/ZnSe та об’ємних кристалiв i епiтаксiйних шарiв
CdS. У першому випадку знайдено прояви неоднорiдностi гетеро-
границi, тобто флуктуацiй товщини КЯ. Визначено енергетичну по-
зицiю краю рухливостi для екситонiв, локалiзованих на флуктуацiях.
У другому випадку систематично вивчений вплив зростання I не ли-
ше на екситонну, а й на пов’язану з домiшками крайову люмiнесцен-
цiю. На вiдмiну вiд загальноприйнятих ранiше уявлень, при збiльшен-
нi I аж до порогу руйнування не спостерiгалося нiякого насичення iн-
тенсивностi крайової люмiнесценцiї в об’ємних кристалах CdS. У той
же час в епiтаксiйних шарах товщиною декiлька мiкронiв таке наси-
чення справдi мало мiсце. Пропонується пояснення на якiсному рiвнi,
яке враховує дифузiю (недифузiйний транспорт) носiїв зi збудженого
приповерхневого шару.
Ключові слова: люмiнесценцiя, квантова яма, епiтаксiйний шар,
екситон, ZnSe, CdS
PACS: 78.20.-e
542
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