The paper presents a study on a series of porous silicon films of various thicknesses, prepared at 20 mA current density using a photoluminescence fitting model to determine the average crystallite size of sphe-rical shaped interconnected silicon quantum dots. Discrepancy in photoluminescence behavior of the samples is well explained with this model.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3097

Dhiman, Neeraj

Gill, Fateh Singh

Gupta, Himanshu

Kumar, R.

Mehra, R.M.

Pal, Pankaj K.

Purohit, L.P.

Sharma, Kiran

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 5 No 1, 01019(3pp) (2013) Том 5 № 1, 01019(3cc) (2013)   2077-6772/2013/5(1)01019(3) 01019-1  2013 Sumy State University Short Communication  Study of Photoluminescence Behaviour of Porous Silicon Samples Prepared at  20 mA Current Density  Fateh Singh Gill1, Himanshu Gupta2, L.P. Purohit2,*, Pankaj K. Pal, Kiran Sharma1, Neeraj Dhiman1, R. Kumar2, R.M. Mehra3  1 Graphic Era University, Clement Town, Dehradun, India 2 Department of Physics, Gurukula Kangri University, 249401 Haridwar, India 3 School of Engineering & Technology, Sharda University, 201306 Greater Noida, India  (Received 30 December 2012; published online 28 March 2013)  The paper presents a study on a series of porous silicon films of various thicknesses, prepared at 20 mA current density using a photoluminescence fitting model to determine the average crystallite size of sphe-rical shaped interconnected silicon quantum dots. Discrepancy in photoluminescence behavior of the sam-ples is well explained with this model.  Keywords: Photoluminescence, Quantum dots, Nanoparticles.  PACS numbers: 78.55.Mb, 81.07.Ta, 61.46.Df                                                                *lppurohit@gmail.com 1. INTRODUCTION  Porous silicon (PS) is considered to be composed ei-ther of spherical shaped interconnected silicon quan-tum dots or combination of quantum dots and columns. During the last decades, there has been a rapid re-search to introduce porous silicon [1] in optoelectronics and medical field [2]. Every field of applications, re-quire an optimized PS structures under experimental conditions. As PS-based solar cells require thin and highly porous structures [3] and the drug loaded in PS depends on the properties of both the micro particles and the loaded substances [2] therefore, it is the need to study the distribution of crystallites size in a PS sys-tem through existing theoretical models. Therefore, the main objectives of present work are to determine the average diameter of the crystallites from extend of the photoluminescence (PL) spectra broadening and to ver-ify the results and prediction of Islam-Kumar model, explaining the shape of the PL spectra. As PL signal of PS at room temperature is a recom-bination process of different radiative channels, two of them are dominant and size dependent; the excitonic transitions for the crystallite size range of 4-2.5 nm [4] and phonon assisted transition [5]. The phonon assist-ed transition dominates radiative recombination of excitons for crystallite greater than about 2 nm. The cause of such channels has been recognized in the form of surface states which arise due to natural oxidation or by surface passivation.   2. EXPERIMENTAL  Present series of PS samples in table 1 were fabri-cated from 100 oriented single side polished boron-doped silicon wafers (thickness 600-650 μm), with a resistivity ranging from 0.8 to 1.2 ohm-cm. An alumin-ium back contact was deposited with a vacuum coating machine. Nano-PS layers of different thicknesses were prepared by a computer (interface) controlled electro-chemical etching (anodization time was varied) of the front side of the silicon wafer at fixed 20 mA current density in a solution consisting of a mixture of hydro-fluoric acid (HF 48%) and pure ethanol (volume propor-tion HF:CH3COOH; with a volumetric ratio of 1:1). As-prepared samples were cintered at 5500C in nitrogen environment; in order to avoid evolution of the PS structure in the presence of oxygen.Thickness of the samples were determined by scanning electron micro-scope (SEM).The PL spectra were monitored using a FLSP920 Phosphorescence lifetime spectrometer set-up. A 300 nm line of a continuous Ar + laser of power 4 Wcm-2 was used as an excitation source.  The PL spec-tra were measured after stabilization of the PL intensi-ty, where no evolution of the PL peak energy with time was observed.  Table 1 – Optimized fitting parameters   3. RESULTS AND DISCUSSION  An analytical expression [6], explicitly included the surface states for low crystallite sizes with the quan-tum confinement effect, which assumed an ensemble of nanometer sized spherical particles having a well-defined size distribution. A quantitative agreement of this application model with the experimental PL data Sam-ple PL Peak (eV) Mean crystal-lite (nm) Varia. (nm) Con-fine. energy (eV) FWHM (eV) Osci. Stre. () PS1 1.83 3.16 0.25 0.83 0.59 3 PS2 1.91 2.96 0.23 0.91 0.55 3 PS3 1.88 3.03 0.24 0.88 0.56 3 PS4 1.88 3.04 0.27 0.88 0.63 4 PS5 1.89 3.02 0.27 0.89 0.63 3 Varia.  Variance, Confine.  confinement,  Osci.Stre.  Oscillator Strength  FATEH SINGH GILL, H. GUPTA, L.P. PUROHIT, ET AL. J. NANO- ELECTRON. PHYS. 5, 01019 (2013)   01019-2 is depended on the exactness of average size and its dispersion, exciton binding energy (Eb), on the models or band gap and oscillator strength calculations. This model proposed the equation for PL energy with the explicitly inclusion of an amount of localization of ener-gy (Es) of surface states. Therefore, the PL energy is given by   0 ( )g b sw E E E E     , (1)  where the confinement (energy up shift) associated with a wire of width d is given by   0 2cEd     Thus, the emitted photon energy from a nanocrys-tallite is lower than the band gap energy of the crystal-lite by an amount of localization energy Es of the sur-face states and the exciton binding energy Eb. Both, in general, are functions of crystallite size.  0.6 0.7 0.8 0.9 1.0 1.1 1.20.00.20.40.60.81.00.6 0.7 0.8 0.9 1.0 1.1 1.20.00.20.40.60.81.00.6 0.7 0.8 0.9 1.0 1.1 1.20.00.20.40.60.81.00.6 0.7 0.8 0.9 1.0 1.1 1.20.00.20.40.60.81.00.6 0.7 0.8 0.9 1.0 1.1 1.20.00.20.40.60.81.00 2 4 6 8 101.821.841.861.881.901.92Normalized PL Intensity (a.u.)Normalized PL Intensity (a.u.) E (eV)PS2 Theo. Exp. E (eV)PS 5 Theo. Exp. E (eV)PS 4 Theo. Exp. E (eV)PS 3 Theo.PL Exp.PLNormalized PL Intensity (a.u.)  E (eV)PS1 Theo. PLExp. PLPL Peak (eV)Thickness( micro meter)PL Peak (eV) Fig. 1 – Comparison of the experimental (—) and the theoreti-cal (- - -) PL spectra for the samples PS1, PS2, PS3, PS4 and PS5 respectively as well as the PL peak variation with thick-ness of each sample  Therefore, emitted photon energy from a crystallite is given as   1p g s bE E E E E      (2)  where ∆E is the amount of band gap upshift due to QCE in the nanocrystallites and Eg is the band gap corresponding to the bulk crystalline material. With a log-normal distribution the expression for PL intensity transforms to     21/5 /02ln / ln( )/( ) exp2nn n c E Lc EI Enc           (3)  The value of band gap i.e. Eg  1.12 eV for c-Si was taken at room temperature. The constant value of Eb  0.07 eV was taken over the mean crystallite size ranging from 2.5-4.0 nm and surface localization ener-gy Es was the order of optical phonon energy of 0.05 eV. In Eq. (2.3) the value of n was taken 1.39 and c has the value 4.122 eV (when d0 is in units of nm) calculated [7] (to retain the constraints imposed by model used). The comparison of the experimental and theoretical PL spectra for the samples PS1, PS2, PS3, PS4 and PS5 respectively is shown in figure 1. The fitting parameter obtained from PL model used (table 1) explains the same discrepancy. As the sample PS2 has maximum confinement energy (0.91) and lowest mean crystalline size (2.96 nm) with oscillator strength of 3, the reason for its shift at highest energy place is due to its lowest full width half maximum(FWHM)  0.55 eV and lowest variance (0.23) among samples. Generally, the decrease in crystallite size results in increase of the FWHM of PL curve but if the decrease in mean crystallite is fol-lowed by a decrease in the disorder (lowest variance) as well, it results in decrease of width of experimental PL. It supports the relevant explanation for shift of sample PS2. Further, the position of peaks of sample PS3 and PS4 at same energy (1.88 eV) is understood in terms of oscillator strength of samples. However, PL fitting data in table 1 shows that sample PS4 has greater value of mean crystallite (3.04 nm) and variance (0.27 eV) as compared to sample PS3, it is supposed to show red shift but it lies at same peak position as that of sample PS3. The reason for it lies in higher value of oscillator strength of 4 because as the oscillator strength increas-es, it causes blue shift [6]. The anomalous behavior of PL of PS sample could be explained on the basis of ob-servations obtained from the model.  4. CONCLUSION  Statistical study made on systematic series of sam-ples establish the significance of theoretical models to study PL behaviour which is  helpful for optimization of parameters to achieve desired PS for template and device purpose as well as to develop better model ex-plaining the behaviour of samples prepared at particu-lar conditions.  ACKNOWLEDGEMENT  One of the authors, F S Gill is very thankful to Dr. V. Agarwal, Sentro De Investigacion en in Ge-nieriay Ciencias Applikadas, Maxico for facilitating the research work by providing the systematic series of samples and Dr. R.K. Kotnala, Senior Scientist, Na-tional Physical laboratory, New Delhi for providing the facility of Photoluminescence spectroscopy measure-ments.   THEORETICAL STUDY OF PL BEHAVIOUR OF POROUS SILICON… J. NANO- ELECTRON. PHYS. 5, 01019 (2013)   01019-3 REFERENCES  1. L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990). 2. E.J. Anglin, M.P. Schwartz, V.P. Ng, L.A. Perelman, M.J. Sailor, Langmuir 20, 11264 (2004). 3. L.A. Balagurov, S.C. Baliss, A.F. Orlov, E.A. Petrova, B. Unal, D.G. Yarkin, J. Appl. Phys. 90, 4184 (2001). 4. Aaron Puzder, A.J.Williamson, Jeffrey C. Grossman,  Giulia Galli, Phys. Rev. Lett. 88, 097401 (2002). 5. M. Rosenbauer, S. Finkbeiner, E. Bustarret, J. Weber, M. Stutzmann, Phys. Rev. B 51, 10593 (1995). 6. Md.N. Islam, Satyendra Kumar, J. Appl. Phys. 93, 1753 (2003). 7. J.P. Proot, C. Delerue, G. Allan, Appl. Phys. Lett. 61, 1948 (1992).   

Study of Photoluminescence Behaviour of Porous Silicon Samples Prepared at 20 mA Current Density

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 5 No 1, 01019(3pp) (2013) Том 5 № 1, 01019(3cc) (2013) 
 
 
2077-6772/2013/5(1)01019(3) 01019-1  2013 Sumy State University 
Short Communication 
 
Study of Photoluminescence Behaviour of Porous Silicon Samples Prepared at  
20 mA Current Density 
 
Fateh Singh Gill1, Himanshu Gupta2, L.P. Purohit2,*, Pankaj K. Pal, Kiran Sharma1, Neeraj Dhiman1, 
R. Kumar2, R.M. Mehra3 
 
1 Graphic Era University, Clement Town, Dehradun, India 
2 Department of Physics, Gurukula Kangri University, 249401 Haridwar, India 
3 School of Engineering & Technology, Sharda University, 201306 Greater Noida, India 
 
(Received 30 December 2012; published online 28 March 2013) 
 
The paper presents a study on a series of porous silicon films of various thicknesses, prepared at 20 mA 
current density using a photoluminescence fitting model to determine the average crystallite size of sphe-
rical shaped interconnected silicon quantum dots. Discrepancy in photoluminescence behavior of the sam-
ples is well explained with this model. 
 
Keywords: Photoluminescence, Quantum dots, Nanoparticles. 
 PACS numbers: 78.55.Mb, 81.07.Ta, 61.46.Df 
 
 
                                                             
*lppurohit@gmail.com 
1. INTRODUCTION 
 
Porous silicon (PS) is considered to be composed ei-
ther of spherical shaped interconnected silicon quan-
tum dots or combination of quantum dots and columns. 
During the last decades, there has been a rapid re-
search to introduce porous silicon [1] in optoelectronics 
and medical field [2]. Every field of applications, re-
quire an optimized PS structures under experimental 
conditions. As PS-based solar cells require thin and 
highly porous structures [3] and the drug loaded in PS 
depends on the properties of both the micro particles 
and the loaded substances [2] therefore, it is the need 
to study the distribution of crystallites size in a PS sys-
tem through existing theoretical models. Therefore, the 
main objectives of present work are to determine the 
average diameter of the crystallites from extend of the 
photoluminescence (PL) spectra broadening and to ver-
ify the results and prediction of Islam-Kumar model, 
explaining the shape of the PL spectra. 
As PL signal of PS at room temperature is a recom-
bination process of different radiative channels, two of 
them are dominant and size dependent; the excitonic 
transitions for the crystallite size range of 4-2.5 nm [4] 
and phonon assisted transition [5]. The phonon assist-
ed transition dominates radiative recombination of 
excitons for crystallite greater than about 2 nm. The 
cause of such channels has been recognized in the form 
of surface states which arise due to natural oxidation 
or by surface passivation.  
 
2. EXPERIMENTAL 
 
Present series of PS samples in table 1 were fabri-
cated from 100 oriented single side polished boron-
doped silicon wafers (thickness 600-650 μm), with a 
resistivity ranging from 0.8 to 1.2 ohm-cm. An alumin-
ium back contact was deposited with a vacuum coating 
machine. Nano-PS layers of different thicknesses were 
prepared by a computer (interface) controlled electro-
chemical etching (anodization time was varied) of the 
front side of the silicon wafer at fixed 20 mA current 
density in a solution consisting of a mixture of hydro-
fluoric acid (HF 48%) and pure ethanol (volume propor-
tion HF:CH3COOH; with a volumetric ratio of 1:1). As-
prepared samples were cintered at 5500C in nitrogen 
environment; in order to avoid evolution of the PS 
structure in the presence of oxygen.Thickness of the 
samples were determined by scanning electron micro-
scope (SEM).The PL spectra were monitored using a 
FLSP920 Phosphorescence lifetime spectrometer set-
up. A 300 nm line of a continuous Ar + laser of power 4 
Wcm-2 was used as an excitation source.  The PL spec-
tra were measured after stabilization of the PL intensi-
ty, where no evolution of the PL peak energy with time 
was observed. 
 
Table 1 – Optimized fitting parameters 
 
 
3. RESULTS AND DISCUSSION 
 
An analytical expression [6], explicitly included the 
surface states for low crystallite sizes with the quan-
tum confinement effect, which assumed an ensemble of 
nanometer sized spherical particles having a well-
defined size distribution. A quantitative agreement of 
this application model with the experimental PL data 
Sam-
ple 
PL Peak 
(eV) 
Mean 
crystal-
lite 
(nm) 
Varia. 
(nm) 
Con-
fine. 
energy 
(eV) 
FWHM 
(eV) 
Osci. 
Stre. 
() 
PS1 1.83 3.16 0.25 0.83 0.59 3 
PS2 1.91 2.96 0.23 0.91 0.55 3 
PS3 1.88 3.03 0.24 0.88 0.56 3 
PS4 1.88 3.04 0.27 0.88 0.63 4 
PS5 1.89 3.02 0.27 0.89 0.63 3 
Varia.  Variance, Confine.  confinement,  
Osci.Stre.  Oscillator Strength 
 FATEH SINGH GILL, H. GUPTA, L.P. PUROHIT, ET AL. J. NANO- ELECTRON. PHYS. 5, 01019 (2013) 
 
 
01019-2 
is depended on the exactness of average size and its 
dispersion, exciton binding energy (Eb), on the models 
or band gap and oscillator strength calculations. This 
model proposed the equation for PL energy with the 
explicitly inclusion of an amount of localization of ener-
gy (Es) of surface states. Therefore, the PL energy is 
given by 
 
 0 ( )g b sw E E E E     , (1) 
 
where the confinement (energy up shift) associated 
with a wire of width d is given by 
 
 0 2
c
E
d
    
 
Thus, the emitted photon energy from a nanocrys-
tallite is lower than the band gap energy of the crystal-
lite by an amount of localization energy Es of the sur-
face states and the exciton binding energy Eb. Both, in 
general, are functions of crystallite size. 
 
0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.0
0.2
0.4
0.6
0.8
1.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.0
0.2
0.4
0.6
0.8
1.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.0
0.2
0.4
0.6
0.8
1.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.0
0.2
0.4
0.6
0.8
1.0
0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10
1.82
1.84
1.86
1.88
1.90
1.92
N
o
r
m
a
li
z
e
d
 P
L
 I
n
t
e
n
s
it
y
 (
a
.u
.)
N
o
r
m
a
li
z
e
d
 P
L
 I
n
t
e
n
s
it
y
 (
a
.u
.)
 E (eV)
PS2
 Theo.
 Exp.
 E (eV)
PS 5
 Theo.
 Exp.
 E (eV)
PS 4
 Theo.
 Exp.
 E (eV)
PS 3
 Theo.PL
 Exp.PL
N
o
r
m
a
li
z
e
d
 P
L
 I
n
t
e
n
s
it
y
 (
a
.u
.)
  E (eV)
PS1
 Theo. PL
Exp. PL
P
L
 P
e
a
k
 (
e
V
)
Thickness( micro meter)
PL Peak (eV)
 
Fig. 1 – Comparison of the experimental (—) and the theoreti-
cal (- - -) PL spectra for the samples PS1, PS2, PS3, PS4 and 
PS5 respectively as well as the PL peak variation with thick-
ness of each sample 
 
Therefore, emitted photon energy from a crystallite 
is given as 
 
 1p g s bE E E E E      (2) 
 
where ∆E is the amount of band gap upshift due to 
QCE in the nanocrystallites and Eg is the band gap 
corresponding to the bulk crystalline material. With a 
log-normal distribution the expression for PL intensity 
transforms to 
    
2
1/
5 /
0
2
ln / ln( )/
( ) exp
2
n
n n c E Lc E
I E
nc


 
 
 
   
  
 
 
 (3) 
 
The value of band gap i.e. Eg  1.12 eV for c-Si was 
taken at room temperature. The constant value of 
Eb  0.07 eV was taken over the mean crystallite size 
ranging from 2.5-4.0 nm and surface localization ener-
gy Es was the order of optical phonon energy of 0.05 eV. 
In Eq. (2.3) the value of n was taken 1.39 and c has the 
value 4.122 eV (when d0 is in units of nm) calculated 
[7] (to retain the constraints imposed by model used). 
The comparison of the experimental and theoretical PL 
spectra for the samples PS1, PS2, PS3, PS4 and PS5 
respectively is shown in figure 1. The fitting parameter 
obtained from PL model used (table 1) explains the 
same discrepancy. As the sample PS2 has maximum 
confinement energy (0.91) and lowest mean crystalline 
size (2.96 nm) with oscillator strength of 3, the reason 
for its shift at highest energy place is due to its lowest 
full width half maximum(FWHM)  0.55 eV and lowest 
variance (0.23) among samples. Generally, the decrease 
in crystallite size results in increase of the FWHM of 
PL curve but if the decrease in mean crystallite is fol-
lowed by a decrease in the disorder (lowest variance) as 
well, it results in decrease of width of experimental PL. 
It supports the relevant explanation for shift of sample 
PS2. Further, the position of peaks of sample PS3 and 
PS4 at same energy (1.88 eV) is understood in terms of 
oscillator strength of samples. However, PL fitting data 
in table 1 shows that sample PS4 has greater value of 
mean crystallite (3.04 nm) and variance (0.27 eV) as 
compared to sample PS3, it is supposed to show red 
shift but it lies at same peak position as that of sample 
PS3. The reason for it lies in higher value of oscillator 
strength of 4 because as the oscillator strength increas-
es, it causes blue shift [6]. The anomalous behavior of 
PL of PS sample could be explained on the basis of ob-
servations obtained from the model. 
 
4. CONCLUSION 
 
Statistical study made on systematic series of sam-
ples establish the significance of theoretical models to 
study PL behaviour which is  helpful for optimization 
of parameters to achieve desired PS for template and 
device purpose as well as to develop better model ex-
plaining the behaviour of samples prepared at particu-
lar conditions. 
 
ACKNOWLEDGEMENT 
 
One of the authors, F S Gill is very thankful to 
Dr. V. Agarwal, Sentro De Investigacion en in Ge-
nieriay Ciencias Applikadas, Maxico for facilitating the 
research work by providing the systematic series of 
samples and Dr. R.K. Kotnala, Senior Scientist, Na-
tional Physical laboratory, New Delhi for providing the 
facility of Photoluminescence spectroscopy measure-
ments. 
 
 THEORETICAL STUDY OF PL BEHAVIOUR OF POROUS SILICON… J. NANO- ELECTRON. PHYS. 5, 01019 (2013) 
 
 
01019-3 
REFERENCES 
 
1. L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990). 
2. E.J. Anglin, M.P. Schwartz, V.P. Ng, L.A. Perelman, 
M.J. Sailor, Langmuir 20, 11264 (2004). 
3. L.A. Balagurov, S.C. Baliss, A.F. Orlov, E.A. Petrova, 
B. Unal, D.G. Yarkin, J. Appl. Phys. 90, 4184 (2001). 
4. Aaron Puzder, A.J.Williamson, Jeffrey C. Grossman,  
Giulia Galli, Phys. Rev. Lett. 88, 097401 (2002). 
5. M. Rosenbauer, S. Finkbeiner, E. Bustarret, J. Weber, M. 
Stutzmann, Phys. Rev. B 51, 10593 (1995). 
6. Md.N. Islam, Satyendra Kumar, J. Appl. Phys. 93, 1753 
(2003). 
7. J.P. Proot, C. Delerue, G. Allan, Appl. Phys. Lett. 61, 1948 
(1992). 
 
 


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Study of Photoluminescence Behaviour of Porous Silicon Samples Prepared at 20 mA Current Density

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