Exciton quantum confinement in nanocones formed on a surface of CdZnTe solid solution by laser radiation

Article (PDF Available)inNanoscale Research Letters 7(1):514 · September 2012with26 Reads
DOI: 10.1186/1556-276X-7-514 · Source: PubMed
Abstract
The investigation of surface morphology using atomic force microscope has shown self-organizing of the nanocones on the surface of CdZnTe crystal after irradiation by strongly absorbed Nd:YAG laser irradiation at an intensity of 12.0 MW/cm2. The formation of nanocones is explained by the presence of a thermogradient effect in the semiconductor. The appearance of a new exciton band has been observed after irradiation by the laser which is explained by the exciton quantum confinement effect in nanocones.
N AN O E X P R E S S Open Access
Exciton quantum confinement in nanocones
formed on a surface of CdZnTe solid solution by
laser radiation
Artur Medvid'
1*
, Natalia Litovchenko
2
ˆ
, Aleksandr Mychko
1
and Yuriy Naseka
2
Abstract
The investigation of surface morphology using atomic force microscope has shown self-organizing of the
nanocones on the surface of CdZnTe crystal after irradiation by strongly absorbed Nd:YAG laser irradiation at an
intensity of 12.0 MW/cm
2
. The formation of nanocones is explained by the presence of a thermogradient effect in
the semiconductor. The appearance of a new exciton band has been observed after irradiation by the laser which
is explained by the exciton quantum confinement effect in nanocones.
Keywords: Nanocones, Exciton quantum confinement effect, Thermogradient effect, CdZnTe, Nd:YAG laser
Background
Nowadays, nanostructures are one of the most investi-
gated objects in semiconductor physics, especially the
quantum confinement effect (QCE) in such quantum
systems as quantum dots or 0D [1-5], quantum wires or
1D [6-8] and quantum wells or 2D [9-14]. In the case of
nanostructures, the energy band diagram of the semi-
conductor is strongly changed. This leads to a crucial
change of semiconductor properties, such as electrical
properties (the change of free charge carriers concentra-
tion and their mobility), optical properties (absorption
coefficient , reflectivity coefficient, and radiative recom-
bination efficiency), and mechanical and thermal proper-
ties [15]. Another possibility to change a property of a
semiconductor is by using solid solution, such as
Cd
1x
Zn
x
Te [16] and Si
1x
Ge
x
[17], which change the
component content. It wa s shown [18] tha t the shapes
and sizes of the mentioned quantum systems have more
influence on the properties of a semiconductor than its
component content. For example, the blue shift of
Si
0.7
Ge
0.3
photoluminescence (PL) spectrum of nano-
cones is up to 1.2 eV, but the possible maximal shift of
PL spectra due to change of x is only up to 0.33 eV.
Moreover, the band of PL spectrum is broader and more
intense due to QCE and graded band gap. Moreover,
nanocones enhance radiation hardness of CdZnTe de-
tector, as shown in [19].
In this paper, we report about the appearance of a new
band in PL spectra of Cd
1x
Zn
x
Te solid solution irra-
diated by Nd:YAG laser, which is explained by exciton
QCE in nanocones formed on the irradiated surface of
the sample.
Methods
The laser processing was performed on the samples of
Cd
1x
Zn
x
Te solid solution with x = 0.1 in ambient at-
mosphere at room temperature, pressure 1 atm, and
80% humidity. The surface of Cd
1x
Zn
x
Te sample was
irradiated by pulses of Nd:YAG laser with the wave-
length of λ = 532 nm, pulse duration of τ = 15 ns and
power p = 1 MW. The spot of laser beam with 3 mm
diameter was moved by 20 μm steps over the surface of
the sample. Atomic force microscope (AFM) was used
for the study of the irradiated surface morphology. The
low-temperature PL at 5 K was carried out to investigate
the optical properties of the nanostructur es forme d by
laser radiation (LR) on the samples. He-Ne laser with
λ = 632.8 nm was used as an excitation source.
Results and discussion
The formation of nanocones on a surface of Cd
1x
Zn
x
Te
solid solution with x = 0.1 after irradiation by the
* Correspondence: medvids@latnet.lv
ˆ
Deceased
1
Riga Technical University, Azenes Street 14, Riga 1048, Latvia
Full list of author information is available at the end of the article
© 2012 Medvid' et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
Medvid' et al. Nanoscale Research Letters 2012, 7:514
http://www.nanoscalereslett.com/content/7/1/514
strongly absorbed Nd:YAG laser radiation with intensity
of I = 12.0 MW/cm
2
was observed using AFM, as shown
in Figure 1.
While studying the optical properties of nanocones, a
new exciton band at energy up to 1.87 eV in PL
spectrum for the first time was found, as shown in
Figure 2. The PL spectrum is rather complex, consisting
of an intense line (A
0
, X) at 1.6362 eV, which is ascribed
to bound excitons to shallow acceptors (Cd vacancies,
V
Cd
) and its longitudinal op tical (LO)-phonon replica at
1.6181 eV, an intense line (D
0
, X) at 1.6475 eV ascribed
to bound excitons to shallow donors (Cd interstitial
atoms, I
Cd
). At the same time, the shift of A
0
X and D
0
X
exciton lines of 3.2 and 2.7 meV, correspondingly, to-
ward the higher energy of quantum, that is, the so-called
blue shift was observed, as shown in Figure 2. The ap-
pearance of a new PL band is explained by exciton
quantum confinement (EQC) effe ct in nanocones, and
the blue shift of A
0
X and D
0
X exciton bands - due to
photo-induced mechanical compressive stress of the top
layer.
This process takes place in the following way: the ir-
radiation of the Cd
1x
Zn
x
Te solid solution by the laser
leads to the drift of Cd atoms toward the irradiated sur-
face and of Zn atoms - in the opposite direction due to
high gradient of temperature. This is so-called thermo-
gradient effect (TGE) [20]. As a result, the formation of
CdTe/Cd
1x1
Zn
x1
Te heterostructure, where x
1
> x, takes
place due to the replacement of Zn atoms by Cd atoms
at the irradiated surface. At the same time, the opposite
process takes place under the top layer. In the buried
layer of the semiconductor, Zn atoms replace Cd atoms.
At least three factors determine A
0
X and D
0
X exciton
lines position in PL spectrum. They are a s follows : the
concentration of Zn atoms in the CdTe top layer and in
CdZnTe buried layer, 2D EQC effect in the CdTe layer
when its thickness is comparable with Bohr radius of the
exciton, and the mechanical compressive stress of the
CdTe top layer due to mismatch of CdTe and CdZnTe
crystalline lattice constant.
The decrease of Zn atoms concentration in the top
layer with increased intensity of LR, according to the
proposed model, leads to the red shift of the exciton
bands in PL spectra, as was shown in [21], but increase
of the Zn atoms concentration in the buried CdZnTe
layer manifests it self in the blue shift of PL spectrum, as
shown in Fig ure 3. These effects do not compensate
each other because they take place in different layers.
This unusual situation can be explained by different in-
put of these layers in the position and intensity of PL
spectrum. If the top layer is excited by short wavelength
light, then mostly the red shift of PL spectrum will be
observed; but if mainly the buried layer is excited, for
example, due to small thickness or transparency of top
layer, then the blue shift will be observed. Of course, it is
possible to observe both PL spectra simultaneously at
intermediate situation. Such situation is exactly observed
in the PL spectrum in Figure 2, after the destruction of
the CdTe top layer and formation of nanocones on the
irradiated surface of the sample. The relaxation of the
mechanical compressive stress in CdTe layer corre-
sponds to the decreasing part of the curves in Figure 3.
It is manifested as the self-assembly of nanocones on the
irradiated surface of the structure as explained by
Stransky-Krastanov' growth mode. A simultaneous ap-
pearance of a new exciton band at 1.872 eV in PL
spectrum at high intensity of LR takes place. The recon -
struction of this band according to Gaussian fitting
shows that it consists of three lines which look like A
0
X,
D
0
X, and A
0
X-LO lines (the distance between the lines
and their full width at half maximum are the same) in
the nonirradiated PL spectrum of the semiconductor.
Figure 1 Atomic force microscope 3D image of the Cd
1x
Zn
x
Te
(x = 0.1) surfaces after irradiation by the laser at intensity of
12 MW/cm
2
. The blue and red curly arrows show the different
surfaces with different luminescence properties.
1.620 1.647 1.674 1.8 1.9
0
200
400
600
800
1000
1200
2
A
0
XQC-LO
1.8462 eV
D
0
XQC
1.8836 eV
A
0
XQC
1.8636 eV
A
0
X-LO
1.6181 eV
A
0
X
1.6362 eV
h , eV
PL intensity, a r b . unit.
D
0
X
1.6475 eV
1
2
0
30
60
90
120
150
180
210
240
270
Figure 2 Photoluminescence spectra of the Cd
1x
Zn
x
Te (x = 0.1)
measured at temperature of 5 K before (curve 1) and after
(curve 2) the irradiation by the laser at I = 10.0 MW/cm
2
.
Medvid' et al. Nanoscale Research Letters 2012, 7:514 Page 2 of 4
http://www.nanoscalereslett.com/content/7/1/514
Therefore, we connect the appearance of the new lines
in PL spectrum with the nanocones' formation on the
irradiated surface of the semiconductor and with EQC
in the nanocones. We denote them as A
0
XQC and
D
0
XQC lines. An evidence of the mechan ical stress re-
laxation process in CdTe layer is a non-monotonic de-
pendence of the blue shift as a function of LR intensity,
as shown in Figure 3. Such non-monotonic dependence
have been obser ved in p- and n-type Si after studying of
mechanical micro hardness of Si after irradiation by Nd:
YAG laser [22]. The calculation of the mechanical com-
pressive stress in CdTe top layer using the maxim um of
the blue shift of A
0
X exciton line from Figure 3 and
dEg/dP = 10 eV/GPa [23], where Eg and P are band gap
of CdTe crystal and mechanical stress, correspondingly,
gives P = 4.62 × 10
5
Pa. This value corresponds to the ul-
timate strength limit of CdTe [24]. The calculation of
the quantum dot diameter using formula from [25] and
the blue shift of A
0
XQC in the PL spectrum of 0.27 eV
gives about 10.0 nm diameter of the quantum dot s.
These data correspond to the size of nanocones [26]
(height and diameter of the bottom of the cones are
10.0 nm) mea sured using 3D image of AFM as can be
seen in Figure 1. An evidence of the presence of EQC in
nanocones is the decrease of LO phonon energy by
0.7 meV in the PL spectrum. This is the so-called pho-
non quantum confinement effect [27]. Moreover, the
increase of Huang-Rhys factor for A
0
X-LO line up to
three times is a good evidence of EQC effect in the
nanocones. Our calculation of Zn atoms distribution de-
pending on the intensity of LR using the thermo-
diffusion equation has shown that the process of CdTe/
Cd
1x1
Zn
x1
Te heterostructure formation is characterized
by the gradual increase of Zn atoms concentration in
the buried layer with intensity of LR up to 8% (x
1
= 0.18).
The thickness of the CdTe layer after irradiation by the
laser with intensity of I = 12.0 MW/cm
2
becomes 10 nm.
The process of nanocones formation is characterized by
the LR threshold intensity of approximately I = 10.0 MW/
cm
2
, as can be seen in Figure 3: nanocones formation
starts at the maximums of the blue shift position. There-
fore, the process of the nanocones formation is character-
ized by two stages: the first stage is the formation of
CdTe/CdZnTe heterostructure, and the second stage is
the nanocones self-assembly due to laser annealing of the
mechanical compressive stress in CdTe layer. An evi-
dence of the presence of the first stage of nanocones for-
mation process is the redistribution of the intensity of
LO-ZnTe, TO-CdTe, and LO-CdTe phonon bands in
Raman back scattering spectra, as shown in Figure 4.
It means that before the irradiation of the sample by
the laser, the intensity of LO-ZnTe phonon band was
three to four tim es higher than the intensity of TO and
LO-CdTe phonon bands, but after irradiation the oppos-
ite situation in R aman spectra is observed.
Conclusions
The studies of the effect of highly absorbed laser radi-
ation on the optical properties of the Cd
1x
Zn
x
Te
(x = 0.1) compound have revealed the formation of nano-
cones on the surface of the semiconductor under
024681012
1,636
1,638
1,640
1,647
1,648
1,649
1,650
1,651
A
0
X
D
0
X
Maximal shift
is 3.2 meV
Maximal shift
is 2.7 meV
hv
max
,eV
I
laser
,MW/cm
2
Figure 3 Blue shift of A
0
X and D
0
X exciton lines in PL spectra
as function of the laser intensity.
100 150 200 250 300
0
10
20
30
40
50
60
70
I
ex
=100mW
T=80K
λ=4880A
I=0
I=11MW/cm
2
I=2.8MW/cm
2
⎯⎯ I=8.6 MW/cm
2
LO-ZnTe
LO-CdTe
TO-CdTe
Intensity , arb.unit.
Raman shift, cm
-1
Figure 4 Raman back scattering spectra of nonirradiated and
irradiated CdZnTe sample. Intensities of LR are as follows: I =0,
black curve; I = 2.8 MW/cm
2
, blue curve; I = 8.6 MW/cm
2
, green curve;
and I = 11.0 MW/cm
2
, red curve.
Medvid' et al. Nanoscale Research Letters 2012, 7:514 Page 3 of 4
http://www.nanoscalereslett.com/content/7/1/514
irradiation by the Nd:YAG laser within the intensity
range of 9.0-12.0 MW/cm
2
and the simultaneous ap-
pearance of a new PL ban d at 1.8 7 eV, which is explained
by the exciton quantum confinement effect in
nanocones.
The TGE has the main role in redistribution of Zn
atoms at the surface of Cd
1x
Zn
x
Te irradiated by the sec-
ond harmonic of Nd:YAG laser. The graded band gap
structure with open optical window is formed on the top
of nanocones after the irradiation by Nd:YAG laser at the
intensity 4.0-12.0 MW/cm
2
. Formation of a graded band
gap with a close of optical window in Cd
1x
Zn
x
Te crystal
is possible under irradiation by the second harmonic of
Nd:YAG laser at the intensity 0.2-2.0 MW/cm
2
. A two-
stage model of the nanocones formation on the surface
of the Cd
1x
Zn
x
Te (x = 0.1) under the irradiation by Nd:
YAG laser at the intensity 4.0-12.0 MW/cm
2
was
proposed.
Abbreviations
AFM: Atomic force microscope; 0D: Quantum dots; 1D: Quantum wires;
2D: Quantum wells; EQC: Exciton quantum confinement; I
Cd
: Cd interstitial
atoms; LR: Laser radiation; PL: Photoluminescence; QCE: Quantum
confinement effect; TGE: Thermogradient effect; V
Cd
: Cd vacancies.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
AM and LN conceived the studies and coordinated the experiment. All of
the authors participated to the analysis of the data and wrote the article.
AMy and YN carried out the sample preparation, the measurements for solid
solutions of CdZnTe. All the authors read and approved the manuscript.
Acknowledgment
The authors gratefully acknowledge the financial support in part by the
European Regional Development Fund within the projects Solgel and laser
technologies for the development of nanostructures and barrier structures
and 2010/0221/2DP/2.1.1.0/10/APIA/VIAA/145, RTU PVS ID 1535.
Author details
1
Riga Technical University, Azenes Street 14, Riga 1048, Latvia.
2
Institute of
Semiconductor Physics, NAS of Ukraine, Pr. Nauki 28, Kyiv 03028, Ukraine.
Received: 15 August 2012 Accepted: 6 September 2012
Published: 20 September 2012
References
1. Alivisatos A: Semiconductor clusters, nanocrystals, and quantum dots.
Science 1996, 271:933937.
2. Norris D, Bawendi M: Measurement and assignment of the size-
dependent optical spectrum in CdSe quantum dots. Phys Rev B 1996,
53:1633816346.
3. Kunz A, Weidman R, Collins T: Pressure-induced modifications of the
energy band structure of crystalline CdS. J Phys C: Solid State Phys 1981,
14:L581L584.
4. Lee C, Mizel A, Banin U, Cohen M, Alivisatos A: Observation of pressure-
induced direct-to-indirect band gap transition in InP nanocrystals.
J Chem Phys 2000, 113:20162020.
5. Yoffe A: Low-dimensional systems - quantum-size effects and electronic
properties of semiconductor micro crystallites (zero-dimensional
systems) and some quasi-2-dimensional systems. Adv Phys 1993,
42:173266.
6. Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Gates B, Yin Y, Kim F, Yan H: One-
dimensional nanostructures: synthesis, characterization, and applications.
Adv Mater 2003, 15:353389.
7. Krutarth T, Hyungsang Y, Herman C, Moon J, Walter H: Quantum
confinement induced performance enhancement in sub-5-nm
lithographic Si nanowire transistors. Nano Lett 2011, 11:14121417.
8. Reed M, Randall J, Aggarwal R, Matyi R, Moore T, Wetsel A: Observation of
discrete electronic states in a zero-dimensional semiconductor
nanostructure. Phys Rev Lett 1988, 60:535537.
9. Fowler A, Fang F, Howard W, Stiles P: Magneto-oscillatory conductance in
silicon surfaces. Phys Rev Lett 1966, 16:901903.
10. Tulkki J, Heinamaki A: Confinement effect in quantum well dot induced
by InP stressor. Phys Rev B 1995, 52:8239.
11. Xiao X, Liu C, Sturm J, Lenchyshyn L, Thewalt M, Gregory R, Fejes P:
Quantum confinement effects in strained silicon-germanium alloy
quantum wells. Appl Phys Lett 1992, 60:21352137.
12. Kuo Y, Lee Y, Ge Y, Ren S, Roth J, Kamins T, Miller D, Harris J: Strong
quantum-confined Stark effect in germanium quantum-well structures
on silicon. Nature 2005, 437:
13341336.
13. Dingle R, Wiegmann W, Henry C: Quantum states of confined carriers in
very thin Al
x
Ga
1x
As-GaAs-A
lx
Ga
1x
as heterostructures. Phys Rev Lett
1974, 33:827830.
14. Parsons C, Thacker B, Szmyd D, Peterson M, McMahon W, Nozik A:
Characterization and photocurrent spectroscopy of single quantum
wells. J Chem Phys 1990, 93:77067715.
15. Li J, Hong X, Liu Y, Li D, Wang Y, Li J, Bai Y, Li T: Highly photoluminescent
CdTe/poly(N-isopropylacrylamide) temperature-sensitive gels. Adv Mater
2005, 17:163166.
16. Reno J, Jones E: Determination of the dependence of the band-gap
energy on composition for Cd
1x
Zn
x
Te. Phys Rev B 1992, 45:14401442.
17. Sun K, Sue S, Liu C: Low-dimensional systems and nanostructures. Physica
E 2005, 28:525530.
18. Medvid' A: Nano-cones formed on a surface of semiconductors by laser
radiation: technology, model and properties.InNanowires Science and
Technology. Edited by Viena LN. Rijeka: INTECH; 2010:6182.
19. Medvid' A, Mychko A, Dauksta E, Naseka Y, Crocco J, Dieguez E: The effect
of laser radiation on CdZnTe radiation hardness. JINST 2011, 6:C11010.
20. Medvid' A: Redistribution of point defects in the crystalline lattice of a
semiconductor in an inhomogeneous temperature field. Defect Diffus
Forum 2002, 89102:210212.
21. Medvid' A, Fedorenko L, Korbutjak B, Kryluk S, Yusupov M, Mychko A:
Formation of graded band-gap in CdZnTe byYAG:Nd laser radiation.
Radiat Meas 2007, 42:701703.
22. Medvid' A, Onufrijevs P, Chiradze G, Muktapavela F: Impact of laser
radiation on microhardness of a semiconductor. AIP Conf Proc 2011,
1399:181182.
23. Thomas D, Hopfield J: Excitons and band splitting produced by uniaxial
stress in CdTe. J Appl Phys 1961, 32:22982304.
24. Yonenaga I: Hardness, yield strength, and dislocation velocity in
elemental and compound semiconductors. Mater Trans 2005,
46:19791985.
25. Kayanuma Y: Quantum-size effects of interacting electrons and holes in
semiconductor microcrystals with spherical shape. Phys Rev B 1988,
38:97979805.
26. Lee H, Park H, Lee I, Kim T: Formation and optical properties of CdTe/
ZnTe nanostructures with different CdTe thicknesses grown on Si (100)
substrates. Appl Phys Lett 2007, 102(103507):15.
27. Campbell H, Fauchet P: The effect of microcrystal size and shape on the
one phonon Raman-spectra of crystalline semiconductors. Solid State
Commun 1986, 58:739741.
doi:10.1186/1556-276X-7-514
Cite this article as: Medvid' et al.: Exciton quantum confinement in
nanocones formed on a surface of CdZnTe solid solution by laser
radiation. Nanoscale Research Letters 2012 7:514.
Medvid' et al. Nanoscale Research Letters 2012, 7:514 Page 4 of 4
http://www.nanoscalereslett.com/content/7/1/514
    • "Instead of the thermal annealing the pulsed laser annealing [6,7] provides a highly localized annealing effect. The irradiation of CdZnTe semiconductor crystal with nanosecond laser pulses has been used for the modification of structural , electrical and optical properties, as well as to induce changes in chemical composition891011. In general, the interaction of the laser radiation with semiconductors is usually described by the thermal model [12]. "
    [Show abstract] [Hide abstract] ABSTRACT: The present paper deals with the laser ablation in CdZnTe crystal irradiated by pulsed infrared laser. Two values of threshold intensities of the laser ablation were determined, namely of about 8.5 and 6.2 MW/cm2 for the incident and the rear surfaces, correspondingly. Lower intensity of the laser ablation for the rear surface is explained by thermal self-focusing of the laser beam in the CdZnTe crystal due to heating of Te inclusions with a following hydrodynamic expansion.
    Full-text · Article · Oct 2015
    A. MedvidA. MedvidA. MychkoA. MychkoE. DaukstaE. Dauksta+1more author...[...]
    • "This method is expensive; it requires vacuum, high temperature and a long annealing time ($ 100 h). Recently, we have shown compositional and structural modification of the CdTe and CdZnTe surfaces by a Nd:YAG laser (λ ¼ 532 nm)8910. After laser irradiation, the intensity of the D1X exciton line in photoluminescence (PL) spectrum caused by the Cd interstitials atoms increased, but the intensity of the A1X exciton line produced by V Cd on contrary, decreased. "
    Full-text · Article · Jul 2015
    • "This method is expensive; it requires vacuum, high temperature and a long annealing time ($ 100 h). Recently, we have shown compositional and structural modification of the CdTe and CdZnTe surfaces by a Nd:YAG laser (λ ¼ 532 nm)8910. After laser irradiation, the intensity of the D1X exciton line in photoluminescence (PL) spectrum caused by the Cd interstitials atoms increased, but the intensity of the A1X exciton line produced by V Cd on contrary, decreased. "
    [Show abstract] [Hide abstract] ABSTRACT: The resistivity of CdZnTe crystals significantly increases after irradiation with λ=1064 nm Nd:YAG laser. This effect is explained by the compensation of Cd vacancies with In interstitial atoms caused by the increased In solubility due to a laser-induced temperature gradient field around the Te inclusions in the bulk of the crystal. The temperature gradient is caused by the selective absorption of the laser radiation by the Te inclusions. Evidence supporting our explanation is both the increase of the optical transparency of the crystal in IR spectra as well as the decrease of A°X band and the increase of D°X band intensity in photoluminescence spectra after irradiation by the laser.
    Full-text · Article · Apr 2015
Show more