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Title: Influence of etching parameters on optoelectronic properties of c-Si/porous silicon heterojunction – application to solar cells
Author: Fatiha Bechiri, Mokhtar Zerdali, Ilham Rahmoun, Saad Hamzaoui, Mohamed Adnane, and Taoufik Sahraoui

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Eur. Phys. J. Appl. Phys. (2013) 61: 30102

DOI:

10.1051/epjap/2013120152

Influence of etching parameters on optoelectronic properties
of c-Si/porous silicon heterojunction – application to solar cells
Fatiha Bechiri, Mokhtar Zerdali, Ilham Rahmoun, Saad Hamzaoui, Mohamed Adnane, and Taoufik Sahraoui

Eur. Phys. J. Appl. Phys. (2013) 61: 30102
DOI: 10.1051/epjap/2013120152

THE EUROPEAN
PHYSICAL JOURNAL
APPLIED PHYSICS

Regular Article

Influence of etching parameters on optoelectronic properties
of c-Si/porous silicon heterojunction – application to solar cells
Fatiha Bechiria , Mokhtar Zerdali, Ilham Rahmoun, Saad Hamzaoui, Mohamed Adnane, and Taoufik Sahraoui
Laboratoire de Microscopie Electronique & Sciences des Mat´eriaux (LME&SM),
Universit´e des Sciences et de la Technologie d’Oran (USTO), BP 1505, El MNaouer, 31100 Oran, Algeria
Received: 23 April 2012 / Received in final form: 30 September 2012 / Accepted: 15 January 2013
c EDP Sciences 2013
Published online: 8 March 2013 –
Abstract. Thin layers of nanoporous silicon PS were synthesized by anodic etching, in order to develop
photovoltaic cells. We proposed a diluted concentration of hydrofluoric acid with different etching current
densities (1, 3, 5 mA/cm2 ) on a fairly short time anodization. Observations by scanning electron microscope,
electrical measurements and optical measurements revealed that the structural properties of PS layers
depended on strong conditions of prints. The reverse and forward component of the I-V characteristics
showed an appropriate method to explore and extract the parameters of the diode ideality factor n. The
optimum conditions of formation of PS were: HF concentration of 1% and an etching current density of
1 mA/cm2 . Unlike silicon, which has a low absorption of short visible wavelengths, it was shown that the
PS had wide energy gap of ≈ 2 eV, and a marked improvement in the absorption between 400 and 600 nm.
This property has been used to optimize the response of the solar cell Ni/PS/c-Si. Efficiency performance
close to 4.2% was obtained with a Voc of 400 mV, and fill factor of 46%. The solar cell exhibited better
response than the reference cell Ni/c-Si. These results show that PS/c-Si heterojunction has a potential
for photovoltaic applications.

1 Introduction
Porous silicon (PS) films improve the performance of solar
cells by trapping light [1], as antireflection coating (ARC)
and significantly increase the efficiency of solar cells [2].
PS thin films are usually made by electrochemical etching, taking silicon as an anode. The solution is composed
of hydrofluoric acid (HF) and an organic solvent [3,4].
This method has the advantage of forming porous silicon to achieve low manufacturing cost to be later used for
mass production of commercial solar cells [5]. After achieving improvement in the parameters and the photovoltaic
cell efficiency, various technologies were proposed such as
the nanostructured antireflection layers [6], textured layers [7], porous silicon (IG-PS structure) [8] and hybrid solar cells [9]. These techniques generally require more than
two different materials and many manufacturing technology steps in several stages. Many authors have reported
various studies on both types of silicon, p and n, for the
synthesis of PS by both anodic etching in HF solution and
as a result of high etching current density [10]. Generally,
these conditions lead to a PS layer thickness of several
microns to ten microns (1–50 microns) [11]. These layers are manufactured and used as antireflection coatings
(ARC) in photovoltaic devices. The physical phenomenon
a

e-mail: FatihaBechiri@gmail.com

involved in the trapping of light is the change in refractive
index that ranges from 1.9 to 3.4 for the PS [12].
On the other hand, the PS layer was identified as
a semiconductor material, including the presence of the
nanoporous structure of silicon that leads to the formation
of nanocrystalline silicon elements. Therefore, an offset of
the two conduction bands and valence band was observed.
In a narrow space, the quantum confinement leads to the
shift of the energy gap of silicon ranging from 1.12 eV
to 2 eV for PS film [13]. A direct band transition was
observed [14,15] along with the presence of photoluminescence properties [16].
In this study, we focus on the production of layers of
nanoporous PS whose property is to improve the absorption of photons of short wavelengths near the blue, unlike
the effect of trapping by the antireflection layers [17].
Our interest was focused on the n-type silicon, as it
was reported that the anodic etching was driving at or
before the formation of nano- and macroporous silicon,
and this mixture improves the current density in the solar
cell [18]. To this end, we propose a concentration of diluted hydrofluoric acid to etch silicon surface with different current densities (1, 3, 5 mA/cm2 ) in order to obtain
optimum conditions to form a nanoporous layer.
We have demonstrated in this study that the optimum
current density of etch is 1 mA/cm2 when considering a
short anodizing time of 10 min to adjust the thickness

30102-p1

The European Physical Journal Applied Physics

of PS to about ≈ 100 nm. A range in thickness has the
advantage of having a PS layer whose resistance is lower
for the contribution to the series resistance of the solar
cell [19,20].
The presence of PS film as a nanostructure is inferred
indirectly from the shift of the photoluminescence peak
observed; the measurements indicate a high energy gap of
2 eV for the sample PS, in contrast to silicon where the
energy gap is 1.12 eV. Measurements of photoconductivity
confirmed that the short wavelengths in the visible region
exhibited strong absorption and a maximum absorption
close to PL peaks.
These measurements were carried out with a metal
comb structure to take into account only the current picture of the thin PS.
In this work, we propose that the solar cell
Ni/PS/c-Si require only two manufacturing steps: electrochemical etching followed by deposition of metal contacts.
The origin of the photovoltaic effect is attributed to
the effect in terms of the Schottky junction between the
metal and the PS layer, in addition to the contribution of
PS with Si heterostructure.
The photovoltaic cell based on nanoporous silicon
works in tandem, thus promoting the absorption of short
wavelengths, in contrast to silicon where the absorption
of long wavelengths is favorable [21].

2 Experimental

Fig. 1. SEM micrographs of PS layers prepared by etching;
(a) observation taken by a tilt angle of 30◦ for sample prepared by anodic current density of 1 mA/cm2 ; (b) top surface
profile for sample prepared with 1 mA/cm2 ; (c) 3 mA/cm2 ;
(d) 5 mA/cm2 .

3 Results and discussions

Layers of porous silicon were prepared using anodic etching in HF electrolyte solution. The electrolyte solution was
held in the electrochemical cell with a molybdenum foil as
a cathode and polished silicon, whose properties are as
follows: n-type oriented (1 0 0) and resistivity 1 Ω cm.
To ensure electrical contact with the substrate, we introduced nickel on the front using radio frequency sputtering
(RF) (ULVAC RFS-200). The etching solution consisted
of 1:1:50 parts HF (50%), C2 H5 OH (99%) and H2 O respectively.
PS layers were then etched by a DC voltage under different current densities 1, 3 and 5 mA/cm2 . The anodizing
time was adjusted in order to keep the thickness of the PS
layer to 100 nm.
The porous silicon surface was observed by scanning
electron microscope (SEM-S2500C Hitachi). I-V characteristics were recorded by an Advantest TR8652 electrometer connected to the devices. Photoconductivity measurements were performed using a 50 W halogen lamp, coupled
with optical interference filters as a source of monochromatic light. The photoluminescence spectra of PS films
were examined at room temperature. Photo-excitation for
the PL experiment was done with a (325 nm) He-Cd laser,
incident power of 22.3 mW/cm2 , with a Hamamatsu Photonics spectrophotometer, and a multi-Channel C10027
analyzer as a detector in the range of 350–1100 nm. The
photovoltaic characteristics were studied under the AM1.5
condition, using a halogen lamp (Osram, 24 V/250 W) as
the light source.

3.1 Surface morphology
Figure 1a shows a view of PS layer under a view angle of
30◦ for PS samples realized for 1 mA/cm2 etching current
densities.
The etching time was fixed at 10 min. We have used
SEM observations to estimate the PS film thickness. We
set the thickness of 100 nm PS layers to keep a close comparison between the PS samples produced with different
current densities. Figures 1b–1d show the surface morphology of PS layers anodically biased with different current densities of 1, 3 and 5 mA/cm2 . The best arrangement
of pores is obtained for J = 1 mA/cm2 . In the anodizing
process, the formation of mesoporous silicon (pore size
of about 100 nm or less) is recorded. Samples biased for
3 and 5 mA/cm2 showed inhomogeneous distribution of
pores. We distinguish the presence of both mesoporous,
and macroporous silicon (pore size greater than 100 nm).
However, a low current density appears to be favorable
for uniform distribution and homogeneous structure of the
PS.
3.2 Optical characterization
Figure 2 shows typical curves of photoluminescence (PL)
of PS layers. A current density of 1 mA/cm2 and a concentration of HF (1%) appear to be sufficient to induce an
intensive PL.

30102-p2

F. Bechiri et al.: Influence of etching parameters on optoelectronic properties of c-Si/porous silicon heterojunction

Fig. 2. Photoluminescence spectra of PS samples prepared
under different anodic current densities.

We note that the energy of PL peak correlated with
the energy gap for the PS material. The gap of silicon
changes from 1.12 to a higher value in the presence of
porous silicon. Figure 2 shows that, when the current density increases, the PL peak shifts to longer wavelengths.
We observe from the figure that the value of the
changed energy gap of PS can be from 1.71 eV (728 nm)
to 1.98 eV (628 nm) by simply adjusting the current density J from 5 to 1 mA/cm2 . PS layers made at different
current density must have different sizes of nanocrystallites leading to a shift in PL peaks [22]. On the other
hand, the samples made with current densities of 3 and
5 mA/cm2 have rather broad peaks around wavelengths
645 and 728 nm, respectively. A wide band around the
peak is generally attributed to the presence of nanocrystallites of different sizes in which they modulate the width
of the strips. It has been reported through the concept
of quantum confinement that the conduction band and
the valence of porous silicon are inversely proportional to
the rays of nanoelements [23]. The SEM measurements
revealed that the PS etched by 1 mA/cm2 presented a
homogeneous distribution of pores; the samples prepared
with 3 and 5 mA/cm2 presented a mixture of mesoporous
and macroporous silicon structure.

Fig. 3. Semilogarithmic I-V characteristics of PS samples prepared by current density of 1 mA/cm2 , 3 mA/cm2 , 5 mA/cm2
and reference sample Ni/c-Si structure.

Fig. 4. Logarithmic plots of I/[1 − exp(−qV /kT )] characteristic as a function of bias voltage at room temperature.

3.3 Electricals measurements

The same behavior is observed for Ni deposited on porous
silicon samples. For reverse bias, the current reaches the
saturation considering the limitation by thermionic effect.
The I-V relationship of the Schottky contact including
series resistance is conventionally written as [24]:


(1)
I = Is e(q(V −IRs)/(nkT )) 1 − e−q(V −IRs)/kT ,

Figure 3 shows the experimental semi-logarithmic I-V
characteristics of the reference structure Ni/c-Si and the
structures Ni/PS/c-Si PS layers etched with currents densities of 1, 3 and 5 mA/cm2 .
The electrical bias is applied to the diodes in either direction, reverse, forward direction. The forward bias corresponds to the case where the electrode metal is connected
to the positive output voltage source.
The reference diode exhibits Schottky contact behavior between the nickel contact and the silicon substrate.

where Is is the saturation current, Rs is the series resistance of the device, k is the Boltzmann constant, T (K) is
the absolute temperature, n is the ideality factor and V is
the bias voltage supply.
Figure 4 shows the characteristic of ln(I/[1−
exp(−qV /kT )]) versus V for different diodes. For all devices, the measured current-voltage I-V characteristics at
room temperature have two distinct regions, −2 V ≤ V
≤ 0.5 V and V ≥ 0.5, indicating different conduction
mechanisms.

30102-p3

The European Physical Journal Applied Physics

Current-voltage characteristics
In the interval region of −2 V ≤ V ≤ 0.5 V, the current
I should be expressed with a negligible series resistance
Rs. This approximation is sufficient to find an adequate
ideality factor n.
Such an interval shows a straight line from V = −2 V
to V = 0.5 V, covering a wider range of the curve from
which the ideality factor n can be determined.
The ideality factor n is determined from the experimental plot of ln(I/[1 − exp(−qV /kT )]) versus V and taking the slope of the linear region. The ideality factor is
given by:
n = q/kT dV /d ln(I).
(2)
The n value is equal to 1.01 and confirms thermionicdiffusion mechanisms in reverse bias (−(−2 V ≤ V ≤
0 V)) and small interval for forward directions (0 ≤ V ≤
0.5 V).
On the other hand, the diodes exhibit a rectification
behavior for bias voltage V ≥ 0.5 V; the curves deviate
considerably from linearity due to the effect of series resistance Rs, interface states, and interlayer film. The series resistance Rs is significant in the downward curvature
(nonlinear region) of the forward bias I-V characteristics
and cannot be neglected unlike the linear region.
The voltage V d = V − IRs across the Schottky diode
can be expressed in terms of the total voltage drop V
across the series combination of the Schottky diode and
the series resistance. Therefore, the ideality factor and
the series resistance are calculated from the forward bias
I-V data using the method developed by Cheung and
Cheung [25]. The forward bias current-voltage characteristics due to thermionic emission of Schottky contact with
series resistance can be expressed as [26]:
dV /d ln(I) = n(kT /q) + IRs.

(3)

Figure 5 shows the experimental dV /d ln(I) versus I plot
for various diodes.
Equation (3) should give straight line for the data of
downward curvature region in the forward bias I-V characteristics. Thus, a plot will give Rs as the slope, and
n(kT /q) as the y-axis intercept.
The ideality factor n obtained using equation (3) is
3.83, 4.08, 4.62. We observed that the n value increases
with increasing anodic current density.
The obtained n value is higher than unity; this is attributed to the series resistance. Beside the presence of the
porous silicon layer, the Ni/c-Si interface causes a nonideal
behavior and an ideality factor n higher than unity.
The obtained series resistances values Rs are quite
close; with Rs values of 120, 122 and 126 Ω respectively
at 300 K.

Fig. 5. Experimental dV /d ln(I) versus I plot for different PS
diodes structures.

Fig. 6. Photoconductivity spectra ratio of typical PS films
under different DC bias.

The spectral response of PC had maxima at ∼546 nm
equivalent to 2.27 eV. We clearly see that the response of
the cell in the presence of PS layer significantly improves
the absorption in the short wavelength.
However, the reference device based on silicon shows
only a small absorption at short wavelengths. The interest
in the PS layer is to improve the absorption spectrum toward the blue. It should be noted that the peak at 2.27 eV
of PC is quite close to the photoluminescence peak of
1.98 eV for the same PS layer produced at a drive current
of 1 mA/cm2 . According to Ozaki et al. [27], the difference
between the peak in photoluminescence and photoconductivity is the fluctuation of the electric potential along the
direction of the PS layer depth during etching.

3.4 Photoconductivity properties
3.5 Photovoltaic properties
Figure 6 shows measurements of photoconductivity (PC)
of the PS layer formed by the current densities 1, 3 and
5 mA/cm2 . We have deposited on the PS layer, two combshaped electrodes using micro-photolithography.

Figure 7 shows the I-V characteristics of solar cells made
by different etching current densities 1, 3 and 5 mA/cm2 ,
and the solar cell formed by only bulk silicon.

30102-p4

F. Bechiri et al.: Influence of etching parameters on optoelectronic properties of c-Si/porous silicon heterojunction

ment with the I-V characteristics of the cell. The presence
of the PS layer improves the absorption of photons and
hence an increase in cell efficiency. Table 1, shows the different characteristics of the cells in comparison with the
results already reported [28–30].
The increase in current density, Jsc , for the PS cell
could be explained by the decrease in surface recombination velocity due to the properties of the passivation
of PS layer [31]. This means that many of the localized
states in the PS layer formed are effective in capturing
charge carriers, thus acting as a barrier of repulsion for
the photo-excited carriers generated under solar cell illumination.

4 Conclusion
Fig. 7. I-V characteristics of Ni/PS/c-Si and Ni/c-Si solar
cells fabricated in this study, under AM1.5 illumination. Figure
inset shows grid lines made on PS’s top side.

The photocurrent is generated by a nickel metal gate
developed by lift-off technology, as shown in the inset in
Figure 7.
The structure of the solar cell is similar to a Schottky
diode formed by the Ni/PS/c-Si. Solar cell measurements
are performed under illumination and carried out under
standard conditions of AM1.5G. The surface of the cell
does not exceed 3 × 10−3 cm2 .
Solar cells based on PS layer made with current densities of 3 and 5 mA/cm2 are characterized by short-current
density Jsc , and an open-circuit voltage Voc that is moderately low compared to the cell obtained by PS at
1 mA/cm2 The cell density achieved with etching at
1 mA/cm2 is characterized by a Voc of 400 mV, a current density Jsc of 8.0 mA/cm2 , a fill factor (FF) of 46.5%
and a yield of conversion (η) of 4.2%. However, photoconductivity measurements have confirmed an improvement
in the absorption of photons of shorter wavelengths when
the PS layer was present in the cell. This result is in agree-

We have demonstrated the ability of PS film in their applications in the realization of solar cells. Different current
densities: 1, 3 and 5 mA/cm2 have been used to study
the effect of this parameter on the structural and optoelectronic properties of cells composed of Ni/PS/c-Si. The
electron microscopy observations indicated the presence of
mesoporous PS layers when the density is 1 mA/cm2 , and
a mixture between meso-porous and macrospores when
the current density is higher.
Electrical measurements confirm that a current density
of 1 mA/cm2 is sufficient to obtain a thin film of 100 nm
thick porous silicon. Electrical measurements show a rectifying effect in cells Ni/PS/c-Si-based PS thin film, similar
to the diode Ni/c-Si.
Photoconductivity measurements confirm that the absorption of photons of short wavelengths (400–600 nm)
is enhanced in the presence of PS layers instead of silicon. The etching current density influences the structural
properties of PS layers and consequently the absorption
properties. We recorded a significant absorption for PS
sample realized by 1 mA/cm2 , due to the pore reduction
in the case of meso-porous silicon, and consequently the
presence of fewer defects and traps that capture the photoelectrons. The smallest size of meso-porous structure is
confirmed by SEM measurements.

Table 1. Comparison of PS layer used as semiconductor material for solar cell applications.
Samples
n(PS)/p-type Si [28]
Hybrid solar cells [29]
CuPc-PS/Si/Al
In-grain porous [30]
silicon solar cell (IG-PS)
Ni/PS/c-Si(n)
J = 1 mA/cm2
J = 3 mA/cm2
J = 5 mA/cm2
Ni/c-Si

Jsc (mA/cm2 )


Voc (mV)
285

FF (%)
12

Efficiency (η(%))
8.2


7.66

190
520

32


1.22
2.76

8.5
6.9
5.8
2.5

400
350
350
140

46
36.5
30
20

4.2
2.7
2.3
0.5

η = 0.0015%; CuPC: copper phthalocyanine; J: etching current density; PS: porous silicon;
FF: fill factor; Jsc : short-circuit current density; Voc : open-circuit voltage.

30102-p5

The European Physical Journal Applied Physics

The study of cells (Ni/PS/C-Si) indicates the presence
of photovoltaic effect in all PS segments, in addition to
the reference sample (Ni/Si). The best performance is obtained at 4.2% for the PS layer prepared with an etching
current density of 1 mA/cm2 .
A higher value of short-circuit current density, Jsc , is
attributed to the layered PS probably due to a decrease
in the rate of surface recombination due to the passivation properties of the PS layer. The current density, Jsc ,
is higher for the sample obtained with an etching current
density of 1 mA/cm2 that shows the best structural and
optical characteristics. The proposed cell works in tandem in that the PS layer absorbs short wavelengths (400–
600 nm), while the silicon absorbs radiation below 1.12 eV.
The system, Ni/PS/c-Si, allows to harvest a larger number
of photons and increases the efficiency of the cell.
The authors wish to acknowledge their thanks to JICA organization for providing the experimental support and government
cooperation.

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