ISSN:
2181-3906
2024
International scientific journal
«MODERN SCIENCE АND RESEARCH»
VOLUME 3 / ISSUE 2 / UIF:8.2 / MODERNSCIENCE.UZ
1275
EFFECT OF TEMPERATURE ON THE OPTICAL PROPERTIES OF CDSE/ZNSE
QUANTUM DOTS
Sharibaev M.B.
Xojaxmetova G.A.
Karakalpak state univessity named after Berdakh.
Saparniyazova G.
Karakalpak Institute of Agriculture and agricultural technologies, Nukus, Uzbekistan.
https://doi.org/10.5281/zenodo.10726640
Abstract
.
The emission spectra of quantum-well CdSe/ZnSe heterostructures with quantum
dots were determined by an optical method. CdSe/ZnSe heterostructures with quantum dots were
grown at different temperatures; therefore, surface and deep-level minibands corresponding to
the compositions of the structure were determined in the emission spectra. In the
photoluminescence spectra, the shift of the radiative spectra is determined depending on the
temperature of the grown quantum-size structures.
Keywords:
Photoluminescence, photo reflections, quantum dot, relaxation.
ВЛИЯНИЕ ТЕМПЕРАТУРЫ НА ОПТИЧЕСКИЕ СВОЙСТВА КВАНТОВЫХ
ТОЧЕК CDSE/ZNSE
Аннотация.
Спектры излучения квантоворазмерных гетероструктур CdSe/ZnSe с
квантовыми точками определены оптическим методом. Гетероструктуры CdSe/ZnSe с
квантовыми точками выращивались при разных температурах; поэтому в спектрах
излучения определены поверхностные и глубокие мини-зоны, соответствующие составу
структуры. В спектрах фотолюминесценции определяется сдвиг спектров излучения в
зависимости от температуры выращиваемых квантоворазмерных структур.
Ключевые слова:
Фотолюминесценция, фотоотражения, квантовая точка,
релаксация.
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 instead of CdZnSe quantum wells (QWs). 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
ISSN:
2181-3906
2024
International scientific journal
«MODERN SCIENCE АND RESEARCH»
VOLUME 3 / ISSUE 2 / UIF:8.2 / MODERNSCIENCE.UZ
1276
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. Heterostructures with CdSe QDs were found to be more stable against
photo-degradation as compared to CdZnSe QWs. 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 asgrown 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. 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 (VZnor VCd) and the
diffusion coefficient of Cd can be varied by about two orders of magnitude by varying the
concentration of column II vacancies. [9-10]. 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. 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 selforganization process up to its full
suppression. 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.
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 threedimensional island formation.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.
Experimental results
ISSN:
2181-3906
2024
International scientific journal
«MODERN SCIENCE АND RESEARCH»
VOLUME 3 / ISSUE 2 / UIF:8.2 / MODERNSCIENCE.UZ
1277
The PL spectrum of the as-grown sample is shown in Fig. 1a (curve 1). In the spectrum,
the band I
QD
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 I
D
peaking at 670 nm (1.844 eV) and of ~300 meV FWHM is also
present. The intensity ofID band is more than 10 times lower than that of I
QD
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 I
QD
band was detected in the low energy tail of
the band, while the excitation spectrum of I
D
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 (2.455еV), 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. We have found earlier the linear dependence of the
I
D
band maximum position versus the spectral position of WL peak in I
D
band excitation
spectra. Approximation of this dependence to the value of the ZnSe energy gap revealed that I
D
band is caused by defect complex including column II vacancy and shallow donor. Thus,
the excitation spectra of I
D
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 I
D
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 I
D
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 I
QD
band.
ISSN:
2181-3906
2024
International scientific journal
«MODERN SCIENCE АND RESEARCH»
VOLUME 3 / ISSUE 2 / UIF:8.2 / MODERNSCIENCE.UZ
1278
Fig. 1.PL (curves 1) and excitation spectra of І
QD
band (curves 2) and of І
D
band (curves 3)
of the as-grown sample (a) and of the sample annealed at 270 (b), 300 (c) and 370
0
С (d).
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 I
QD
band still does not change, but in the excitation spectrum
of I
D
band the intensity of WL peak keeps decreasing.
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 (Т
ann
~270
0
С) 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 postgrowth
thermal annealing at 250-700 0C 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 or with a noticeable blue shift. A similar effect was also found
in the InGaAs/GaAs heterostructures with QWs or QDs subjected to thermal treatment and
was ascribed to QW interface smoothing or nonradiative defect annealing. We suppose that
the increase in intensities of both I
QD
and I
D
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>270-3350С) 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 and rapid degradation of blue-green laser diodes based on CdZnSe QWs.
However, the only rise of nonradiative defect concentration in the result of annealing
can not explain different rates of the decrease of the I
QD
and I
D
band intensities. As it was
mentioned above, quenching of the QD emission occurs much sharply than that of
defectrelated band. This can be due to the increase of concentration of defects giving rise to
I
D
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.
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 (Т
ann
=300-335
0
С) and are very likely caused by the increase of contribution of
emission of vacancy-related defects localized in the ZnSe layers to the I
D
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
0
C is 1.5 times larger than that in the as-
grown one. In addition, the WLpeak decreases but not disappears in the ID band excitation
ISSN:
2181-3906
2024
International scientific journal
«MODERN SCIENCE АND RESEARCH»
VOLUME 3 / ISSUE 2 / UIF:8.2 / MODERNSCIENCE.UZ
1279
spectrum upon nnealing. These indicate that even after thermal treatment at 430 0C the I
D
band
emains mutlicomponent and the contribution of defects localized in the wetting ayers to the I
D
band is large enough. The data obtained imply that the total concentration of column II
vacancies increases in the structure.
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
0
C 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
0
C 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.
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«MODERN SCIENCE АND RESEARCH»
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