Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure
Effect of post-growth thermal annealing within the temperature range 200 to 430 ºC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD) heterostructure was studied. Annealing at lower temperatures (Tann <= 270 ºС) results in an increase by a factor of 2-3 of the inten...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2010
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| Цитувати: | Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure/ L.V. Borkovska, T.R. Stara, N.O. Korsunska, К.Yu. Pechers'ka, L.P. Germash, V.O.Bondarenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2010. — Т. 13, № 2. — С. 202-208. — Бібліогр.: 37 назв. — англ. |
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irk-123456789-1182182017-05-30T03:05:31Z Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure Borkovska, L.V. Stara, T.R. Korsunska, N.O. Pechers’ka, К.Yu. Germash, L.P. Bondarenko, V.O. Effect of post-growth thermal annealing within the temperature range 200 to 430 ºC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD) heterostructure was studied. Annealing at lower temperatures (Tann <= 270 ºС) results in an increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the first being caused by excitonic transitions in QDs and the second one being connected with the defect complex including a column II vacancy. The effect is supposed to be caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures (Tann > 270 ºС) stimulates a decrease of the QD photoluminescence band intensity and up to 100 meV blue shift of its peak position. The former is explained by generation of extended defects and reduction of the QD density. The blue shift observed at 370-430 ºС is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD density. It is found that the energy of excitonic transitions in the wetting layer does not change upon annealing. Lower thermal stability of QDs as compared to that of the wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via vacancies. The presence of column II vacancies in the wetting layer is proved by characteristics of defect-related PL band and its excitation spectra. 2010 Article Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure/ L.V. Borkovska, T.R. Stara, N.O. Korsunska, К.Yu. Pechers'ka, L.P. Germash, V.O.Bondarenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2010. — Т. 13, № 2. — С. 202-208. — Бібліогр.: 37 назв. — англ. 1560-8034 PACS 66.30.Pa, 78.55.Et, 78.67.Hc http://dspace.nbuv.gov.ua/handle/123456789/118218 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
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English |
| description |
Effect of post-growth thermal annealing within the temperature range 200 to
430 ºC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD)
heterostructure was studied. Annealing at lower temperatures (Tann <= 270 ºС) results in an
increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the
first being caused by excitonic transitions in QDs and the second one being connected
with the defect complex including a column II vacancy. The effect is supposed to be
caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures
(Tann > 270 ºС) stimulates a decrease of the QD photoluminescence band intensity and up
to 100 meV blue shift of its peak position. The former is explained by generation of
extended defects and reduction of the QD density. The blue shift observed at 370-430 ºС
is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD
density. It is found that the energy of excitonic transitions in the wetting layer does not
change upon annealing. Lower thermal stability of QDs as compared to that of the
wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via
vacancies. The presence of column II vacancies in the wetting layer is proved by
characteristics of defect-related PL band and its excitation spectra. |
| format |
Article |
| author |
Borkovska, L.V. Stara, T.R. Korsunska, N.O. Pechers’ka, К.Yu. Germash, L.P. Bondarenko, V.O. |
| spellingShingle |
Borkovska, L.V. Stara, T.R. Korsunska, N.O. Pechers’ka, К.Yu. Germash, L.P. Bondarenko, V.O. Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Borkovska, L.V. Stara, T.R. Korsunska, N.O. Pechers’ka, К.Yu. Germash, L.P. Bondarenko, V.O. |
| author_sort |
Borkovska, L.V. |
| title |
Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure |
| title_short |
Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure |
| title_full |
Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure |
| title_fullStr |
Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure |
| title_full_unstemmed |
Effect of thermal annealing on the luminescent characteristics of CdSe/ZnSe quantum dot heterostructure |
| title_sort |
effect of thermal annealing on the luminescent characteristics of cdse/znse quantum dot heterostructure |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2010 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118218 |
| citation_txt |
Effect of thermal annealing on the luminescent characteristics
of CdSe/ZnSe quantum dot heterostructure/ L.V. Borkovska, T.R. Stara, N.O. Korsunska, К.Yu. Pechers'ka, L.P. Germash, V.O.Bondarenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2010. — Т. 13, № 2. — С. 202-208. — Бібліогр.: 37 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-07-08T13:34:33Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 202-208.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
202
PACS 66.30.Pa, 78.55.Et, 78.67.Hc
Effect of thermal annealing on the luminescent characteristics
of CdSe/ZnSe quantum dot heterostructure
L.V. Borkovska1, T.R. Stara1, N.O. Korsunska1, К.Yu. Pechers’ka2, L.P. Germash2 , V.O. Bondarenko1
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
Phone: 38 (044)525-72-34; e-mail: bork@isp.kiev.ua; korsunska@ukr.net
2National Technical University of Ukraine “KPI”, 37, prospect Peremogy, 03056 Kyiv, Ukraine
Abstract. Effect of post-growth thermal annealing within the temperature range 200 to
430 ºC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD)
heterostructure was studied. Annealing at lower temperatures (Tann 270 ºС) results in an
increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the
first being caused by excitonic transitions in QDs and the second one being connected
with the defect complex including a column II vacancy. The effect is supposed to be
caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures
(Tann > 270 ºС) stimulates a decrease of the QD photoluminescence band intensity and up
to 100 meV blue shift of its peak position. The former is explained by generation of
extended defects and reduction of the QD density. The blue shift observed at 370-430 ºС
is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD
density. It is found that the energy of excitonic transitions in the wetting layer does not
change upon annealing. Lower thermal stability of QDs as compared to that of the
wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via
vacancies. The presence of column II vacancies in the wetting layer is proved by
characteristics of defect-related PL band and its excitation spectra.
Keywords: self-assembled quantum dots, thermal annealing, Cd diffusion, CdSe,
photoluminescence.
Manuscript received 03.02.10; accepted for publication 25.03.10; published online 30.04.10.
1. Introduction
In the recent decade, the processes of quantum dot (QD)
formation in CdSe/ZnSe heterostructures grown by
molecular beam epitaxy (MBE) as well as their
structural, optical and luminescent properties have been
extensively studied [1-4]. In particular, it was found that
self-organization of CdSe QDs via Stranski-Krastanow
growth mode is hindered by cadmium segregation [5, 6]
and Cd/Zn interdiffusion [2, 7, 8]. It was shown that
because of significant intermixing of CdSe and ZnSe
layers a CdSe sheet transforms into cadmium enriched
CdZnSe QDs of different sizes buried into 3-4 nm thick
two-dimensional CdZnSe wetting layer [2, 4, 7]. The
peculiarities of structural properties of epitaxial
CdSe/ZnSe QD heterostructures determine their optical
and luminescent characteristics in many respects [1, 3,
4].
An interest to CdSe QDs grown by MBE was
stimulated by their potential application in
optoelectronic devices, in particular in green laser diodes
[9-11] instead of CdZnSe quantum wells (QWs)
[12, 13]. Green laser diodes based on II-VI compound
low-dimensional structures are still of interest because of
both absence of commercially available alternatives and
high demands for such devices. Specifically, they can be
a new light source for plastic optical fibres with PMMA,
compact full colour projector screens, laser TV
projectors, etc.
The first injection lasers and optically pumped
lasers that used the sheets with CdSe QDs as an active
media demonstrated several advantages over QW-based
devices, namely a reduced threshold for optical pumping
and higher degradation stability [10, 11].
Heterostructures with CdSe QDs were found to be more
stable against photo-degradation as compared to CdZnSe
QWs [14]. These advantages were explained by effective
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 202-208.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
203
localization of carriers in QDs that hinders their
diffusion to relaxed QDs and other regions where
carriers can recombine nonradiatively and stimulate
defect multiplication in the active region. However,
degradation processes in CdSe QD heterostructures have
not been studied in details.
In particular, the peculiarities of Cd/Zn
interdiffusion stimulated by external influences in as-
grown CdSe QD heterostructures have not been studied
at all. At the same time, it is known that degradation of
light-emitting devices based on CdZnSe QWs is
accompanied not only by noticeable reduction of QW
emission caused by dislocation multiplication in active
region, but also by the shift of QW emission band
towards high energy spectral region (blue shift) due to
Cd/Zn interdiffusion across QW heterointerface
[12, 15, 16]. Study of the processes of Cd/Zn
interdiffusion in CdZnSe/Zn(S)Se QW heterostructures
by applying thermal annealing revealed that diffusion of
Cd from the QW is governed by column II vacancies
(VZn or VCd) [17] and the diffusion coefficient of Cd can
be varied by about two orders of magnitude by varying
the concentration of column II vacancies [18]. It was
shown also that intermixing of the materials of QW and
the barriers under thermal annealing occurs via the
vacancies generated at the surface of the sample and
diffuse into the structure [17]. In addition, we have
found earlier in CdSe/ZnSe QD heterostructures that
column II vacancies during the growth gather in the
CdSe layers and influence significantly the QD self-
organization process up to its full suppression [19]. It
can be supposed that presence of the vacancies in the
wetting layer will influence degradation of QD
luminescent characteristics, too.
In this paper, we report photoluminescence (PL)
study of CdSe/ZnSe QD heterostructure subjected to
thermal annealing with the aim to find a method for
improvement of QD luminescent characteristics and to
obtain additional information about their degradation
connected with Cd/Zn interdiffusion.
2. Experimental details
The studied structure was grown on (001) GaAs
substrate by MBE and contained 250-nm thick ZnSe
buffer layer, 12 vertically stacked CdSe inserts separated
by ZnSe spacers of about 15 nm thickness and 150-nm
thick ZnSe cap layer. Nominal thickness of CdSe inserts
was 5 monolayers.
The growth rate was 5 nm/min. The growth
temperature was 280 ºC for ZnSe buffer layer and
230 ºC for the rest of ZnSe layers as well as for CdSe
layers. To stimulate QD formation, after the deposition
of each CdSe layer the Cd beam was blocked, and the
structure was heated up to 340 ºC and then cooled down
to 230 ºC under Se flux. The duration of both steps was
4 min. The reflection high-energy electron diffraction
(RHEED) was used for in situ control of three-
dimensional island formation.
The PL spectra and PL excitation spectra were
measured at 77 K. The PL spectra were excited by the
light of 250-W halogen lamp at the excitation
wavelength exc = 440 nm and by 365-nm line of 500-W
Hg-lamp. The PL excitation spectra were measured
using a light of halogen lamp passing through a grating
spectrometer as an excitation source. The PL signal was
dispersed using a prism spectrometer (when the PL was
excited by the light of a halogen lamp) or a grating
spectrometer (when the PL was excited by the light of an
Hg-lamp) and collected by photoelectronic multiplier.
Samples cut from wafer were thermally treated for
15 min at 200, 220, 270, 300, 335, 370 and 430 °С in
nitrogen ambient to avoid surface oxidation.
3. Experimental results
The PL spectrum of the as-grown sample is shown in
Fig. 1a (curve 1). In the spectrum, the band IQD peaking
at 544 nm (2.277 eV) and caused by radiative
recombination of excitons in QDs dominates. The full
width at a half maximum (FWHM) of this band is
100 meV and is related with dispersion of QDs both in
composition and in size. In the PL spectrum, a defect
related band ID peaking at 670 nm (1.844 eV) and of
300 meV FWHM is also present. The intensity of ID
band is more than 10 times lower than that of IQD band.
Fig. 1a also shows the excitation spectra of both the
QD and defect related bands (curves 2 and 3,
correspondingly). The excitation spectrum of IQD band
was detected in the low energy tail of the band, while the
excitation spectrum of ID band was measured in the band
maximum. In the spectra, in addition to the region
caused by absorption of excitation light in ZnSe layers
(λ 445 nm) two peaks can be distinguished: (i) the
peak WL at 505 nm (2.455 еV), and (ii) the peak X at
470 nm (2.638 eV). Our previous investigations of
similar multistack QD structures have shown that the
peak WL is caused by ground state heavy-hole-like
exciton absorption in the wetting layer, while the peak X
can be ascribed to ground state light-hole-like exciton
absorption in the wetting layer [19-21]. We have found
earlier the linear dependence of the ID band maximum
position versus the spectral position of WL peak in ID
band excitation spectra [20,21]. Approximation of this
dependence to the value of the ZnSe energy gap revealed
that ID band is caused by defect complex including
column II vacancy and shallow donor. Thus, the
excitation spectra of ID band indicate the presence of
column II vacancies in the wetting layer of the as-grown
sample.
The changes introduced to the PL and PL excitation
spectra by thermal treatment at 270, 300 and 370 °С are
also depicted in Fig. 1b, c and d, correspondingly.
Fig. 1b shows that annealing at 270 °С results in a
noticeable increase in the intensity of both the PL bands
and in no change of their spectral position and excitation
spectra. However, in the sample annealed at 300 °С the
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 202-208.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
204
ID band intensity stops growing, while the intensity of
IQD band starts to decrease (Fig. 1c). These are
accompanied by the shift to shorter wavelengths of the
spectral position of the ID band maximum and the
decrease of WL peak intensity in its excitation spectrum.
At the same time, no change is found in the excitation
spectrum of the IQD band. In the sample subjected to
thermal annealing at 370 °С, the intensity of both PL
bands decreases and their spectral position shifts to
shorter wavelengths (blue shift) (Fig. 1d). The excitation
spectrum of IQD band still does not change, but in the
excitation spectrum of ID band the intensity of WL peak
keeps decreasing.
Fig. 2 shows the way in which both the spectral
position (a) and the intensity (b) of the PL bands change
under annealing in the whole range of the annealing
temperatures. This range can be divided by 2 regions:
(i) low annealing temperatures (Тann 270 °С), and
(ii) higher annealing temperatures (Тann > 270 °С).
At low annealing temperatures (Тann 270 °С), the
spectral position of both IQD and ID bands does not
change, while their intensity increases. It should be
noted that PL intensity starts growing already at 200 °С
and 2 to 3 times increases in the sample annealed at
270 °С.
At higher annealing temperatures (Тann > 270 °С),
the intensity of the IQD band falls down and decreases
more than by the order of the value after thermal
treatment at 430 °С. The intensity of the ID band
decreases too, but starting from 370 °С and more slowly
than that of IQD band.
Spectral position of the IQD band does not change
upon annealing up to 335 °С and is blue-shifted within
the range 370-430 °С. The shift amounts to 100 meV in
the sample annealed at 430 °С (Fig. 2a). The blue shift
of ID band position is found in the range of annealing
temperatures lower than that of the IQD band (300-
370 °С) and is accompanied by the changes in the
excitation spectrum mentioned above (Fig. 1c). The
spectral position of the ID band in the sample annealed at
370 °С comes to 630 nm (1.965 eV) and is not
influenced by annealing at 430 °С. The excitation
spectrum of the ID band does not change at 430 °С, too.
Fig. 2a also shows the dependence of the spectral
position for both WL and X bands on the annealing
temperature. The spectral positions were extracted from
the IQD band excitation spectrum. As one can see,
contrary to the IQD band position, they are not influenced
by annealing at least up to 335 °С. This is also true for
the spectral position of WL and X bands extracted from
the excitation spectrum of the ID band. However, in this
case the WL peak intensity decreases noticeably as
opposed to the X band (see Fig. 1c,d). This is the
evidence that the WL and X peaks are due to light
absorption in different parts of heterostructure. We
suppose that WL peak is indeed caused by ground state
450 500 550 600 650 700 750
0
20
40
60
80
X
X
11
2
3
WL
WL
I
D
I
QD
x 10
(a) аs-grown
PL
in
te
ns
ity
, a
rb
. u
n.
Wavelength, nm
450 500 550 600 650 700 750
0
20
40
60
80
100
120
X
X
1
1
2
3 WL
WL
I
D
I
QD
x 10
(b) T
ann
=270 0C
PL
in
te
ns
ity
, a
rb
. u
n.
Wavelength, nm
450 500 550 600 650 700 750
0
20
40
60
80
X
X
(c) T
ann
=300 0C
11
2
3 WL
WL
I
D
I
QD
x 10
P
L
in
te
ns
it
y,
a
rb
. u
n.
Wavelength, nm
450 500 550 600 650 700 750 800
0
2
4
6
8
10
X
X
(d) T
ann
=370 0C
1
2
3
WL
I
D
I
QD
PL
in
te
ns
it
y,
a
rb
. u
n.
Wavelength, nm
Fig. 1. PL (curves 1) and excitation spectra of ІQD band (curves 2) and of ІD band (curves 3) of the as-grown sample (a) and of
the sample annealed at 270 (b), 300 (c) and 370 0С (d). The excitation spectra of ІQD and ІD bands are detected in low energy
tail of the band and in the band maximum, respectively. Т=77 К, exc=440 nm.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 202-208.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
205
exciton absorption in the wetting layers, while the X
peak is probably due to light absorption in ZnSe layers
doped with cadmium during the growth. Specifically,
similar layers have been found by us in MBE-grown
CdZnSe/ZnSe quantum well heterostructures with high
resolution X-ray diffraction and low temperature PL
spectroscopy [22].
Unfortunately, WL peak was not detected in the IQD
band excitation spectrum of the sample subjected to
thermal annealing at 430 °С. Most likely, this is caused
by both the noticeable decrease of the IQD band intensity
and blue shift of its peak position. Therefore, to obtain
the energy of the ground state exciton transition in the
wetting layers, the PL spectra at higher excitation levels
were studied. Fig. 3 presents normalized PL spectra of
as-grown (curves 1, 2) and annealed at 430 °С (curves
1′, 2′) samples under excitation by 365-nm line of Hg-
lamp, where the increase of a curve number corresponds
to the ten-fold increase of the excitation power density.
As the excitation level rises, a FWHM of the IQD band
increases approximately up to 1.3 times in the sample
annealed at 430 °С, but does not change in as-grown
sample. In addition, in annealed sample a new band at
505 nm arises in the shortwave wing of the IQD band.
Generally, this band can be due to radiative transitions
through the QD excited states as well as through the
ground states in smaller QDs or wetting layer. Taking
into account a more than by an order of magnitude
decrease in the QD luminescence intensity as a result of
thermal treatment at 430 °С, this band has to be ascribed
to the wetting layer emission. Spectral position of this
emission is marked in Fig. 3 by the open triangle. The
total dependence presented in Fig. 3 by triangles
indicates that the energy of the ground state exciton
transition in the wetting layers is not modified by
thermal treatment in the whole range of annealing
temperatures.
4. Discussion
Thus, the post-growth thermal treatment of CdSe/ZnSe
QD heterostructures results in changes in the PL
intensity (at first the increase and then the decrease) and
in the shift of PL band position to the high-energy
spectral region (blue shift).
The increase of the PL intensity is observed at low
annealing temperatures (Тann270 0С) and is not
200 250 300 350 400
1.8
1.9
2.0
2.2
2.3
2.4
2.5
2.6
2.7
200 250 300 350 400
0.1
1
а)
PL
b
an
d
po
si
tio
n,
e
V
Annealing temperature, 0C
б)
N
or
m
al
iz
ed
P
L
in
te
ns
ity
Annealing temperature, 0C
Fig. 2. Spectral position (a) and intensity normalized to
that of the as-grown sample (b) of the ІQD (filled circles), ІD
(open circles), WL (triangles) and X (filled squares) bands
versus annealing temperature. The spectral position of the
WL and X bands is extracted from the excitation spectra of
ІQD band (filled triangles and squares) and PL spectrum
excited by 365-nm line of Hg-lamp (open triangle). Dashed
lines present spectral position of the ІQD, ІD, WL and X
bands of the as-grown sample. Т=77 К, exc=440 nm.
480 500 520 540 560 580
2' 2
1' 1P
L
in
te
ns
it
y,
a
rb
. u
n.
Wavelength, nm
Fig. 3. Normalized photoluminescence spectra of the as-
grown (curves 1, 2) and annealed at 430 0C (curves 1’, 2’)
samples excited by 365-nm line of 500-W Hg-lamp at the
excitation power density of P0 (curves 1, 1’) and 10P0
(curves 2, 2’). Т=77 К.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 202-208.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
206
accompanied by any change in the spectral position of
PL bands or in their excitation spectra. The effect of PL
intensity increase has been found earlier in
CdZnSe/ZnSe QW heterostructures subjected to post-
growth thermal annealing at 250-700 0C [23-27] and
explained by interfacial smoothing resulting from the
small-scale lateral diffusion. The increase in the intensity
of QW luminescence band was observed without any
changes in its spectral position [24-26] or with a
noticeable blue shift [23, 27]. A similar effect was also
found in the InGaAs/GaAs heterostructures with QWs
[28, 29] or QDs [30-32] subjected to thermal treatment
and was ascribed to QW interface smoothing [28] or
nonradiative defect annealing [29, 32]. We suppose that
the increase in intensities of both IQD and ID bands is the
result of the annealing of as-grown defects (point
defects, for example) that act as the centers of
nonradiative recombination and are located in different
layers of heterostructure.
The decrease in intensity of both PL bands
observed at higher temperatures (Тann>270-335 0С) is
probably caused by generation of the centers of
nonradiative recombination under thermal treatment. In
particular, it can be due to multiplication of extended
defects (dislocations) nearby the stacking faults at ZnSe
buffer layer/GaAs substrate interface and their following
growing into the active layers (QD layers). It was
proposed earlier to explain both quenching of CdZnSe
QW emission after rapid thermal annealing treatment
[33] and rapid degradation of blue-green laser diodes
based on CdZnSe QWs [12].
However, the only rise of nonradiative defect
concentration in the result of annealing can not explain
different rates of the decrease of the IQD and ID band
intensities. As it was mentioned above, quenching of the
QD emission occurs much sharply than that of defect-
related band. This can be due to the increase of
concentration of defects giving rise to ID band and/or the
decrease of QD concentration.
Of the two mechanisms, the former can be realized
if column II vacancies are generated during annealing at
the surface of the sample and then diffuse into the
structure as it was observed in [17]. This explanation
agrees with the blue shift of ID band position and the
decrease of WL peak in its excitation spectra. Both of
these are observed in the same range of annealing
temperatures (Тann=300-335 0С) and are very likely
caused by the increase of contribution of emission of
vacancy-related defects localized in the ZnSe layers to
the ID band (Fig. 1c, d). At the same time, the blue shift
of defect-related band is accompanied by the increase of
its FWHM, which in the sample annealed at 430 0C is
1.5 times larger than that in the as-grown one. In
addition, the WL peak decreases but not disappears in the
ID band excitation spectrum upon annealing. These
indicate that even after thermal treatment at 430 0C the
ID band remains mutlicomponent and the contribution of
defects localized in the wetting layers to the ID band is
large enough. The data obtained imply that the total
concentration of column II vacancies increases in the
structure.
On the other hand, a noticeable blue shift of the QD
emission band is found at higher annealing temperatures
(Тann=370-430 0С). The shift is apparently caused by Cd
outdiffusion from the QDs, which results in gradual
dissolution of QDs in the surrounding matrix. The latter
means a decrease in the QD concentration. Diffusion of
Cd from the QDs will also result in the growth of QD
sizes and further disappearance of spatial confinement
for carriers in the QDs. This also implies the decrease of
QD concentration. Reduction of the QD concentration
causes predominant decrease of the IQD band intensity as
well as the increase in the intensity of defect-related and
wetting layer PL bands as recombination channels
competing with the QD emission. This agrees well with
the supposition made above of the appearance of the
wetting layer PL band in the emission spectra after
annealing at 430 0C (Fig. 3 curve 2’).
Therefore, it can be assumed that a difference in the
degradation rates of the IQD and ID band intensities is
caused by the influence of two effects that dominate in
different ranges of the annealing temperatures: (i) the
increase of column II vacancy concentration (Тann=300-
430 0С), and (ii) the decrease of QD concentration
(Тann=370-430 0С).
The presented results make it possible to suppose
that just the column II vacancies, that diffuse from the
surface of the sample into the structure stimulate the
process of Cd/Zn interdiffusion across the ZnSe/QD
layer interface and cause a blue shift of the IQD band. But
in this case, not only Cd outdiffusion from the QDs but
also Cd outdiffusion from the wetting layers should
occur. However, we did not found any increase in the
energy of excitonic transition in the wetting layers as
opposed to the one in QDs (Fig. 2a). A similar result,
namely much larger blue shift of the QD emission band
as compared to that of the wetting layer and QW
emission bands, has been observed during rapid thermal
annealing of InGaAs/GaAs QD heterostructures [30, 31,
34, 35]. Structural investigations have proved fast
dissolution of the QDs in surrounding matrix during
annealing [31, 34] and predominant increase of the
lateral sizes of the QDs [34] that implies lateral
interdiffusion. We suppose that in our samples lateral
diffusion, i.e. diffusion of Cd from the QDs in
surrounding wetting layer, prevails over Cd/Zn
interdiffusion across the QD/ZnSe layer interface. The
diffusion occurs via vacancies generated during the
growth process in the wetting layers. This is the non-
Fickian strain-enhanced interdiffusion, which has been
proposed as an explanation of some results of rapid
thermal annealing treatment of InGaAs/GaAs QWs and
QDs [34, 36]. Since the lattice constant of CdSe exceeds
that of ZnSe, the CdSe layer is under compressive strains
that relax partially during QD formation [37]. The
compressive strain enhances the vacancy concentration
[36], for example by stimulating the process of vacancy
gettering in the QD layer. Thus, we can suppose that the
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© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
207
column II vacancy concentration in the CdZnSe wetting
layers should be much higher than that in the ZnSe
layers. It should be noted that in the InGaAs/GaAs QD
heterostructures the presence of cation vacancies in the
QD layer was only supposed to explain experimental
results but not proved directly [34, 36]. Our data, namely
such characteristics of defect-related band as its spectral
position, FWHM and its excitation spectrum indicate
directly the presence of column II vacancies in the
wetting layers and confirm proposed mechanism of
interdiffusion.
5. Conclusions
In conclusion, we have found that post-growth thermal
treatment of CdSe/ZnSe QD heterostructures influences
the QD luminescence intensity and results in up to 100
meV blue shift of the QD luminescence band position. It
is revealed that annealing of the samples at temperatures
up to 270 0C allows raising the QD luminescence
intensity by 2 to 3 times with no changes in other QD
luminescent characteristics. The effect is supposed to be
due to annealing of as-grown centers of nonradiative
recombination. The blue shift occurs at annealing
temperatures of 370-430 0C concurrently with the
decrease in the QD luminescence intensity and is not
accompanied by the changes in the energy of the ground
state excitonic transition in the wetting layer. This effect
is ascribed to strain-enhanced lateral Cd/Zn
interdiffusion in the QD layers through the vacancies
generated during the growth of the structure.
Acknowledgment
The authors would like to acknowledge Prof. Yu.G.
Sadofyev from P.N. Lebedev Physical Institute of RAS,
Moscow, Russia for growing the investigated structure.
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PACS 66.30.Pa, 78.55.Et, 78.67.Hc
Effect of thermal annealing on the luminescent characteristics
of CdSe/ZnSe quantum dot heterostructure
L.V. Borkovska1, T.R. Stara1, N.O. Korsunska1, К.Yu. Pechers’ka2, L.P. Germash2 , V.O. Bondarenko1
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
Phone: 38 (044)525-72-34; e-mail: bork@isp.kiev.ua; korsunska@ukr.net
2National Technical University of Ukraine “KPI”, 37, prospect Peremogy, 03056 Kyiv, Ukraine
Abstract. Effect of post-growth thermal annealing within the temperature range 200 to 430 ºC for 15 min on the luminescent characteristics of CdSe/ZnSe quantum dot (QD) heterostructure was studied. Annealing at lower temperatures (Tann ( 270 ºС) results in an increase by a factor of 2-3 of the intensity of two photoluminescence bands observed, the first being caused by excitonic transitions in QDs and the second one being connected with the defect complex including a column II vacancy. The effect is supposed to be caused by annealing of as-grown nonradiative defects. Annealing at higher temperatures (Tann > 270 ºС) stimulates a decrease of the QD photoluminescence band intensity and up to 100 meV blue shift of its peak position. The former is explained by generation of extended defects and reduction of the QD density. The blue shift observed at 370-430 ºС is ascribed to diffusion of cadmium from QDs that also results in reduction of the QD density. It is found that the energy of excitonic transitions in the wetting layer does not change upon annealing. Lower thermal stability of QDs as compared to that of the wetting layer has been explained by strain-enhanced lateral Cd/Zn interdiffusion via vacancies. The presence of column II vacancies in the wetting layer is proved by characteristics of defect-related PL band and its excitation spectra.
Keywords: self-assembled quantum dots, thermal annealing, Cd diffusion, CdSe, photoluminescence.
Manuscript received 03.02.10; accepted for publication 25.03.10; published online 30.04.10.
1. Introduction
In the recent decade, the processes of quantum dot (QD) formation in CdSe/ZnSe heterostructures grown by molecular beam epitaxy (MBE) as well as their structural, optical and luminescent properties have been extensively studied [1-4]. In particular, it was found that self-organization of CdSe QDs via Stranski-Krastanow growth mode is hindered by cadmium segregation [5, 6] and Cd/Zn interdiffusion [2, 7, 8]. It was shown that because of significant intermixing of CdSe and ZnSe layers a CdSe sheet transforms into cadmium enriched CdZnSe QDs of different sizes buried into 3-4 nm thick two-dimensional CdZnSe wetting layer [2, 4, 7]. The peculiarities of structural properties of epitaxial CdSe/ZnSe QD heterostructures determine their optical and luminescent characteristics in many respects [1, 3, 4].
An interest to CdSe QDs grown by MBE was stimulated by their potential application in optoelectronic devices, in particular in green laser diodes [9-11] instead of CdZnSe quantum wells (QWs) [12, 13]. Green laser diodes based on II-VI compound low-dimensional structures are still of interest because of both absence of commercially available alternatives and high demands for such devices. Specifically, they can be a new light source for plastic optical fibres with PMMA, compact full colour projector screens, laser TV projectors, etc.
The first injection lasers and optically pumped lasers that used the sheets with CdSe QDs as an active media demonstrated several advantages over QW-based devices, namely a reduced threshold for optical pumping and higher degradation stability [10, 11]. Heterostructures with CdSe QDs were found to be more stable against photo-degradation as compared to CdZnSe QWs [14]. These advantages were explained by effective localization of carriers in QDs that hinders their diffusion to relaxed QDs and other regions where carriers can recombine nonradiatively and stimulate defect multiplication in the active region. However, degradation processes in CdSe QD heterostructures have not been studied in details.
In particular, the peculiarities of Cd/Zn interdiffusion stimulated by external influences in as-grown CdSe QD heterostructures have not been studied at all. At the same time, it is known that degradation of light-emitting devices based on CdZnSe QWs is accompanied not only by noticeable reduction of QW emission caused by dislocation multiplication in active region, but also by the shift of QW emission band towards high energy spectral region (blue shift) due to Cd/Zn interdiffusion across QW heterointerface [12, 15, 16]. Study of the processes of Cd/Zn interdiffusion in CdZnSe/Zn(S)Se QW heterostructures by applying thermal annealing revealed that diffusion of Cd from the QW is governed by column II vacancies (VZn or VCd) [17] and the diffusion coefficient of Cd can be varied by about two orders of magnitude by varying the concentration of column II vacancies [18]. It was shown also that intermixing of the materials of QW and the barriers under thermal annealing occurs via the vacancies generated at the surface of the sample and diffuse into the structure [17]. In addition, we have found earlier in CdSe/ZnSe QD heterostructures that column II vacancies during the growth gather in the CdSe layers and influence significantly the QD self-organization process up to its full suppression [19]. It can be supposed that presence of the vacancies in the wetting layer will influence degradation of QD luminescent characteristics, too.
In this paper, we report photoluminescence (PL) study of CdSe/ZnSe QD heterostructure subjected to thermal annealing with the aim to find a method for improvement of QD luminescent characteristics and to obtain additional information about their degradation connected with Cd/Zn interdiffusion.
2. Experimental details
The studied structure was grown on (001) GaAs substrate by MBE and contained 250-nm thick ZnSe buffer layer, 12 vertically stacked CdSe inserts separated by ZnSe spacers of about 15 nm thickness and 150-nm thick ZnSe cap layer. Nominal thickness of CdSe inserts was 5 monolayers.
The growth rate was 5 nm/min. The growth temperature was 280 ºC for ZnSe buffer layer and 230 ºC for the rest of ZnSe layers as well as for CdSe layers. To stimulate QD formation, after the deposition of each CdSe layer the Cd beam was blocked, and the structure was heated up to 340 ºC and then cooled down to 230 ºC under Se flux. The duration of both steps was 4 min. The reflection high-energy electron diffraction (RHEED) was used for in situ control of three-dimensional island formation.
The PL spectra and PL excitation spectra were measured at 77 K. The PL spectra were excited by the light of 250-W halogen lamp at the excitation wavelength (exc = 440 nm and by 365-nm line of 500-W Hg-lamp. The PL excitation spectra were measured using a light of halogen lamp passing through a grating spectrometer as an excitation source. The PL signal was dispersed using a prism spectrometer (when the PL was excited by the light of a halogen lamp) or a grating spectrometer (when the PL was excited by the light of an Hg-lamp) and collected by photoelectronic multiplier.
Samples cut from wafer were thermally treated for 15 min at 200, 220, 270, 300, 335, 370 and 430 °С in nitrogen ambient to avoid surface oxidation.
3. Experimental results
The PL spectrum of the as-grown sample is shown in Fig. 1a (curve 1). In the spectrum, the band IQD peaking at 544 nm (2.277 eV) and caused by radiative recombination of excitons in QDs dominates. The full width at a half maximum (FWHM) of this band is (100 meV and is related with dispersion of QDs both in composition and in size. In the PL spectrum, a defect related band ID peaking at 670 nm (1.844 eV) and of (300 meV FWHM is also present. The intensity of ID band is more than 10 times lower than that of IQD band.
Fig. 1a also shows the excitation spectra of both the QD and defect related bands (curves 2 and 3, correspondingly). The excitation spectrum of IQD band was detected in the low energy tail of the band, while the excitation spectrum of ID band was measured in the band maximum. In the spectra, in addition to the region caused by absorption of excitation light in ZnSe layers (λ ( 445 nm) two peaks can be distinguished: (i) the peak WL at (505 nm (2.455 еV), and (ii) the peak X at (470 nm (2.638 eV). Our previous investigations of similar multistack QD structures have shown that the peak WL is caused by ground state heavy-hole-like exciton absorption in the wetting layer, while the peak X can be ascribed to ground state light-hole-like exciton absorption in the wetting layer [19-21]. We have found earlier the linear dependence of the ID band maximum position versus the spectral position of WL peak in ID band excitation spectra [20,21]. Approximation of this dependence to the value of the ZnSe energy gap revealed that ID band is caused by defect complex including column II vacancy and shallow donor. Thus, the excitation spectra of ID band indicate the presence of column II vacancies in the wetting layer of the as-grown sample.
The changes introduced to the PL and PL excitation spectra by thermal treatment at 270, 300 and 370 °С are also depicted in Fig. 1b, c and d, correspondingly. Fig. 1b shows that annealing at 270 °С results in a noticeable increase in the intensity of both the PL bands and in no change of their spectral position and excitation spectra. However, in the sample annealed at 300 °С the ID band intensity stops growing, while the intensity of IQD band starts to decrease (Fig. 1c). These are accompanied by the shift to shorter wavelengths of the spectral position of the ID band maximum and the decrease of WL peak intensity in its excitation spectrum. At the same time, no change is found in the excitation spectrum of the IQD band. In the sample subjected to thermal annealing at 370 °С, the intensity of both PL bands decreases and their spectral position shifts to shorter wavelengths (blue shift) (Fig. 1d). The excitation spectrum of IQD band still does not change, but in the excitation spectrum of ID band the intensity of WL peak keeps decreasing.
480
500
520
540
560
580
2'
2
1'
1
PL intensity, arb. un.
Wavelength, nm
Fig. 2 shows the way in which both the spectral position (a) and the intensity (b) of the PL bands change under annealing in the whole range of the annealing temperatures. This range can be divided by 2 regions: (i) low annealing temperatures (Тann ( 270 °С), and (ii) higher annealing temperatures (Тann > 270 °С).
At low annealing temperatures (Тann ( 270 °С), the spectral position of both IQD and ID bands does not change, while their intensity increases. It should be noted that PL intensity starts growing already at 200 °С and 2 to 3 times increases in the sample annealed at 270 °С.
At higher annealing temperatures (Тann > 270 °С), the intensity of the IQD band falls down and decreases more than by the order of the value after thermal treatment at 430 °С. The intensity of the ID band decreases too, but starting from 370 °С and more slowly than that of IQD band.
Spectral position of the IQD band does not change upon annealing up to 335 °С and is blue-shifted within the range 370-430 °С. The shift amounts to (100 meV in the sample annealed at 430 °С (Fig. 2a). The blue shift of ID band position is found in the range of annealing temperatures lower than that of the IQD band (300-370 °С) and is accompanied by the changes in the excitation spectrum mentioned above (Fig. 1c). The spectral position of the ID band in the sample annealed at 370 °С comes to (630 nm (1.965 eV) and is not influenced by annealing at 430 °С. The excitation spectrum of the ID band does not change at 430 °С, too.
Fig. 2a also shows the dependence of the spectral position for both WL and X bands on the annealing temperature. The spectral positions were extracted from the IQD band excitation spectrum. As one can see, contrary to the IQD band position, they are not influenced by annealing at least up to 335 °С. This is also true for the spectral position of WL and X bands extracted from the excitation spectrum of the ID band. However, in this case the WL peak intensity decreases noticeably as opposed to the X band (see Fig. 1c,d). This is the evidence that the WL and X peaks are due to light absorption in different parts of heterostructure. We suppose that WL peak is indeed caused by ground state exciton absorption in the wetting layers, while the X peak is probably due to light absorption in ZnSe layers doped with cadmium during the growth. Specifically, similar layers have been found by us in MBE-grown CdZnSe/ZnSe quantum well heterostructures with high resolution X-ray diffraction and low temperature PL spectroscopy [22].
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Unfortunately, WL peak was not detected in the IQD band excitation spectrum of the sample subjected to thermal annealing at 430 °С. Most likely, this is caused by both the noticeable decrease of the IQD band intensity and blue shift of its peak position. Therefore, to obtain the energy of the ground state exciton transition in the wetting layers, the PL spectra at higher excitation levels were studied. Fig. 3 presents normalized PL spectra of as-grown (curves 1, 2) and annealed at 430 °С (curves 1′, 2′) samples under excitation by 365-nm line of Hg-lamp, where the increase of a curve number corresponds to the ten-fold increase of the excitation power density. As the excitation level rises, a FWHM of the IQD band increases approximately up to 1.3 times in the sample annealed at 430 °С, but does not change in as-grown sample. In addition, in annealed sample a new band at (505 nm arises in the shortwave wing of the IQD band. Generally, this band can be due to radiative transitions through the QD excited states as well as through the ground states in smaller QDs or wetting layer. Taking into account a more than by an order of magnitude decrease in the QD luminescence intensity as a result of thermal treatment at 430 °С, this band has to be ascribed to the wetting layer emission. Spectral position of this emission is marked in Fig. 3 by the open triangle. The total dependence presented in Fig. 3 by triangles indicates that the energy of the ground state exciton transition in the wetting layers is not modified by thermal treatment in the whole range of annealing temperatures.
4. Discussion
Thus, the post-growth thermal treatment of CdSe/ZnSe QD heterostructures results in changes in the PL intensity (at first the increase and then the decrease) and in the shift of PL band position to the high-energy spectral region (blue shift).
The increase of the PL intensity is observed at low annealing temperatures (Тann(270 0С) and is not accompanied by any change in the spectral position of PL bands or in their excitation spectra. The effect of PL intensity increase has been found earlier in CdZnSe/ZnSe QW heterostructures subjected to post-growth thermal annealing at 250-700 0C [23-27] and explained by interfacial smoothing resulting from the small-scale lateral diffusion. The increase in the intensity of QW luminescence band was observed without any changes in its spectral position [24-26] or with a noticeable blue shift [23, 27]. A similar effect was also found in the InGaAs/GaAs heterostructures with QWs [28, 29] or QDs [30-32] subjected to thermal treatment and was ascribed to QW interface smoothing [28] or nonradiative defect annealing [29, 32]. We suppose that the increase in intensities of both IQD and ID bands is the result of the annealing of as-grown defects (point defects, for example) that act as the centers of nonradiative recombination and are located in different layers of heterostructure.
The decrease in intensity of both PL bands observed at higher temperatures (Тann>270-335 0С) is probably caused by generation of the centers of nonradiative recombination under thermal treatment. In particular, it can be due to multiplication of extended defects (dislocations) nearby the stacking faults at ZnSe buffer layer/GaAs substrate interface and their following growing into the active layers (QD layers). It was proposed earlier to explain both quenching of CdZnSe QW emission after rapid thermal annealing treatment [33] and rapid degradation of blue-green laser diodes based on CdZnSe QWs [12].
However, the only rise of nonradiative defect concentration in the result of annealing can not explain different rates of the decrease of the IQD and ID band intensities. As it was mentioned above, quenching of the QD emission occurs much sharply than that of defect-related band. This can be due to the increase of concentration of defects giving rise to ID band and/or the decrease of QD concentration.
Of the two mechanisms, the former can be realized if column II vacancies are generated during annealing at the surface of the sample and then diffuse into the structure as it was observed in [17]. This explanation agrees with the blue shift of ID band position and the decrease of WL peak in its excitation spectra. Both of these are observed in the same range of annealing temperatures (Тann=300-335 0С) and are very likely caused by the increase of contribution of emission of vacancy-related defects localized in the ZnSe layers to the ID band (Fig. 1c, d). At the same time, the blue shift of defect-related band is accompanied by the increase of its FWHM, which in the sample annealed at 430 0C is 1.5 times larger than that in the as-grown one. In addition, the WL peak decreases but not disappears in the ID band excitation spectrum upon annealing. These indicate that even after thermal treatment at 430 0C the ID band remains mutlicomponent and the contribution of defects localized in the wetting layers to the ID band is large enough. The data obtained imply that the total concentration of column II vacancies increases in the structure.
On the other hand, a noticeable blue shift of the QD emission band is found at higher annealing temperatures (Тann=370-430 0С). The shift is apparently caused by Cd outdiffusion from the QDs, which results in gradual dissolution of QDs in the surrounding matrix. The latter means a decrease in the QD concentration. Diffusion of Cd from the QDs will also result in the growth of QD sizes and further disappearance of spatial confinement for carriers in the QDs. This also implies the decrease of QD concentration. Reduction of the QD concentration causes predominant decrease of the IQD band intensity as well as the increase in the intensity of defect-related and wetting layer PL bands as recombination channels competing with the QD emission. This agrees well with the supposition made above of the appearance of the wetting layer PL band in the emission spectra after annealing at 430 0C (Fig. 3 curve 2’).
Therefore, it can be assumed that a difference in the degradation rates of the IQD and ID band intensities is caused by the influence of two effects that dominate in different ranges of the annealing temperatures: (i) the increase of column II vacancy concentration (Тann=300-430 0С), and (ii) the decrease of QD concentration (Тann=370-430 0С).
The presented results make it possible to suppose that just the column II vacancies, that diffuse from the surface of the sample into the structure stimulate the process of Cd/Zn interdiffusion across the ZnSe/QD layer interface and cause a blue shift of the IQD band. But in this case, not only Cd outdiffusion from the QDs but also Cd outdiffusion from the wetting layers should occur. However, we did not found any increase in the energy of excitonic transition in the wetting layers as opposed to the one in QDs (Fig. 2a). A similar result, namely much larger blue shift of the QD emission band as compared to that of the wetting layer and QW emission bands, has been observed during rapid thermal annealing of InGaAs/GaAs QD heterostructures [30, 31, 34, 35]. Structural investigations have proved fast dissolution of the QDs in surrounding matrix during annealing [31, 34] and predominant increase of the lateral sizes of the QDs [34] that implies lateral interdiffusion. We suppose that in our samples lateral diffusion, i.e. diffusion of Cd from the QDs in surrounding wetting layer, prevails over Cd/Zn interdiffusion across the QD/ZnSe layer interface. The diffusion occurs via vacancies generated during the growth process in the wetting layers. This is the non-Fickian strain-enhanced interdiffusion, which has been proposed as an explanation of some results of rapid thermal annealing treatment of InGaAs/GaAs QWs and QDs [34, 36]. Since the lattice constant of CdSe exceeds that of ZnSe, the CdSe layer is under compressive strains that relax partially during QD formation [37]. The compressive strain enhances the vacancy concentration [36], for example by stimulating the process of vacancy gettering in the QD layer. Thus, we can suppose that the column II vacancy concentration in the CdZnSe wetting layers should be much higher than that in the ZnSe layers. It should be noted that in the InGaAs/GaAs QD heterostructures the presence of cation vacancies in the QD layer was only supposed to explain experimental results but not proved directly [34, 36]. Our data, namely such characteristics of defect-related band as its spectral position, FWHM and its excitation spectrum indicate directly the presence of column II vacancies in the wetting layers and confirm proposed mechanism of interdiffusion.
5. Conclusions
In conclusion, we have found that post-growth thermal treatment of CdSe/ZnSe QD heterostructures influences the QD luminescence intensity and results in up to 100 meV blue shift of the QD luminescence band position. It is revealed that annealing of the samples at temperatures up to 270 0C allows raising the QD luminescence intensity by 2 to 3 times with no changes in other QD luminescent characteristics. The effect is supposed to be due to annealing of as-grown centers of nonradiative recombination. The blue shift occurs at annealing temperatures of 370-430 0C concurrently with the decrease in the QD luminescence intensity and is not accompanied by the changes in the energy of the ground state excitonic transition in the wetting layer. This effect is ascribed to strain-enhanced lateral Cd/Zn interdiffusion in the QD layers through the vacancies generated during the growth of the structure.
Acknowledgment
The authors would like to acknowledge Prof. Yu.G. Sadofyev from P.N. Lebedev Physical Institute of RAS, Moscow, Russia for growing the investigated structure.
References
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Fig. 3. Normalized photoluminescence spectra of the as-grown (curves 1, 2) and annealed at 430 0C (curves 1’, 2’) samples excited by 365-nm line of 500-W Hg-lamp at the excitation power density of P0 (curves 1, 1’) and 10P0 (curves 2, 2’). Т=77 К.
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Fig. 2. Spectral position (a) and intensity normalized to that of the as-grown sample (b) of the ІQD (filled circles), ІD (open circles), WL (triangles) and X (filled squares) bands versus annealing temperature. The spectral position of the WL and X bands is extracted from the excitation spectra of ІQD band (filled triangles and squares) and PL spectrum excited by 365-nm line of Hg-lamp (open triangle). Dashed lines present spectral position of the ІQD, ІD, WL and X bands of the as-grown sample. Т=77 К, (exc=440 nm.
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Fig. 1. PL (curves 1) and excitation spectra of ІQD band (curves 2) and of ІD band (curves 3) of the as-grown sample (a) and of the sample annealed at 270 (b), 300 (c) and 370 0С (d). The excitation spectra of ІQD and ІD bands are detected in low energy tail of the band and in the band maximum, respectively. Т=77 К, (exc=440 nm.
a)�b) �
Fig. 9. Spectral dependences of the photocurrent on wavelength for Au/GaAs structures with thick (a) and thin (b) Au contacts. Curve 1 in the figure b is for structure with Au NPs and curve 2 without them, dotted curve is transmittance of the light into the GaAs substrate through the continuous film of Au with 21 nm thickness.
a)� b)�
Fig. 10. a) Calculated transmittance spectra of the light into the GaAs substrate with Au NPs on the top. The angle of light incidence is 0º. The numbers in the figure are the distance between NP in nm. Parameters for calculations: outer NP diameter is 55 nm, Au core diameter is 15 nm, shell is SiO2 with the refractive index n = 1.47. NPs are placed in triangular cell on GaAs substrate. b) Corresponding Au NPs absorption spectra.
�� � �
a
Surface distance 14.082 nm
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Angle 19.189 degree
Surface distance 10.923 nm
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Surface distance 30.256 nm
Horiz distance 29.297 nm
Vert distance 3.780 nm
Angle 7.353 degree
b c
Fig. 1. Vertical film’s surfaces profile (a) with the indication of sizes between bench marks (b); the nuance of grey color corresponds to the nuance of bench marks; 3-D view of SnO2 film surface(c).
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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Fig. 1. Vertical film’s surfaces profile (a) with the indication of sizes between bench marks (b); the nuance of grey color
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