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Correlation between oxidant concentrations, morphological aspects and etching kinetics of silicon nanowires during silver-assist electroless etching
Accepted Manuscript Title: Correlation between oxidant concentrations, morphological aspects and etching kinetics of silicon nanowires during silver-assist electroless etching Authors: Besma Moumni, Abdelkader Ben Jaballah PII: DOI: Reference:
S0169-4332(17)31765-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.110 APSUSC 36310
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-3-2017 5-6-2017 8-6-2017
Please cite this article as: Besma Moumni, Abdelkader Ben Jaballah, Correlation between oxidant concentrations, morphological aspects and etching kinetics of silicon nanowires during silver-assist electroless etching, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Correlation between oxidant concentrations, morphological aspects and etching kinetics of silicon nanowires during silver-assist electroless etching Besma Moumni1, Abdelkader Ben Jaballah*,1,2 1
Photovoltaic Laboratory, Research and Technology Centre of Energy (CRTEn), Borj Cedria Technopark, PB 95 Hammam Lif 2050, Tunisia. 2 Department of Physics, Faculty of Science and Arts in Samtha, Jazan University, Jazan-Kingdom of Saudi Arabia. *Corresponding author: e-mail: [email protected],
1
Graphical abstract
Cross section SEM images of silicon nanowires prepared in etchant solution composed of 4% HF and 4.5% H2O2 for different etching time: A) 30 min, B) 60 min,C) 90 mn, D) 180 min, E)120 min and F) 240 min. J) Height of silicon nanowire versus time for three different H2O2 concentration.
2
Highlights A correlation between oxidant concentration and the morphological changes of silicon nanowires is established. For 1% Molar concentration of H2O2, porous silicon (PS) is obtained. The dynamic and kinetics of silicon nanowires for different H2O2 concentration (3%, 4.5% and 7%) are studied by scanning electron microscopy. The spectra IR of SiNWs showing the formation of Si-O radicals becomes consistent when rising the concentration of H2O2 and the immersion duration in HF/ H2O2. More importantly, an efficient antireflective character for silicon solar cell (reflectance close to 2%) is realized at 8% H2O2. The luminescence of the prepared Si-nanostructures is claimed by photoluminescence which exhibit a large enhancement of the intensity and a blue shift for narrow and deep SiNWs.
3
Abstract Silicon porosification by silver assisted chemical etching (Ag-ACE) for a short range of H2O2 concentration is reported. We experimentally show that porous silicon (PSi) is obtained for 1% H2O2, whereas silicon nanowires (SiNWs) appeared by simply tuning the concentration of H2O2 to relatively high concentrations up to 8%. The morphological aspects are claimed by scanning electron microscopy proving that the kinetics of SiNWs formation display nonlinear relationships versus H2O2 concentration and etching time. A semiqualitative electrochemical etching model based on local anodic, Ic, and cathodic, Ia, currents is proposed to explain the different morphological changes, and to unveil the formation pathways of both PS and SiNWs. More importantly, an efficient antireflective character for silicon solar cell (reflectance close to 2%) is realized at 8% H2O2. In addition, the luminescence of the prepared Si-nanostructures is claimed by photoluminescence which exhibit a large enhancement of the intensity and a blue shift for narrow and deep SiNWs.
Keywords: Ag-assisted electroless etching, H2O2 concentration, Silicon nanowires, Porous silicon, Charge transport, Reflectivity, Photoluminescence.
4
1. Introduction The synthesis and fabrication of porous silicon (PSi) and silicon nanowires (SiNWs) have generated great impetus as PSi and SiNWs are advantageous in offering huge surface to volume ratios, favorable transport properties, tuned optical properties, and thermoelectric effects resulting from the nanoscale dimensions. The further importance of PSi and SiNWs lays on their enormous future applications in energy-related applications such as solar cells [1, 2, 3], thermoelectrics [4], negative electrodes for lithium ion batteries [5, 6, 7], as well as supercapacitors [8], field emitter [9], photodetectors, micromachining [10, 11] optical modulators and in a variety of sensing devices [12, 13]. Single- and double-step silver-assisted chemical etching of bulk silicon (Si) [10, 14] have been widely used as simple methods for fabricating highaspect ratio of PSi and SiNWs. Basically, it is well accepted that the whole wet etching of Si by HF and an oxidizing agent like H2O2 started at the interface between Si and a noble metal usually silver nanoparticles (AgNPs). There still a great debate concerning the detailed etching mechanism which leads to the formation of both structures [14-17]. It was, previously, established that many parameters are considered to affect metal assist anisotropic etching of silicon. These include metal nanoparticles type, size, shape and percentage coverage [18, 19, 20], and HF concentration [16, 14] (as main parameters), as well as [HF]/([HF]+ [H2O2]) molar concentration of HF acid [21]. However, the effects of H2O2 oxidative agent as a key etching parameter, on the morphology and etching kinetics of the SiNWs have not yet fully explored for a photovoltaic applications. Here, the changes in the nanostructure and etch rate of solar grade boron doped p-type silicon in a HF–H2O2–H2O etching system with electrolessly predeposited silver catalyst are systematically investigated. We develop a low cost 5
and simple methodology for silicon porosification. Indeed, solar grade silicon is processed. Moreover, H2O2 concentration is selected as fundamental and unique parameter which is adjusted to tune both the dissolution regimes (from porous silicon to silicon nanowires) and the etching rates. This allows controlling the morphologies of the prepared silicon nanostructures and etching depths for a short range of low H2O2 concentrations, while keeping fixed [HF] and silver concentration. Therefore, we explore the effects of [H2O2] in aqueous HF/H2O2 solution on the morphological aspects, kinetics growth and formation pathways of porous silicon and silicon nanowires. More importantly, a correlation between structural changes and reflectance/photoluminescence features of the fabricated silicon nanostructures is realized. It is recognized that
the
obtained
results
are
consistent
to
the
field
of
silicon
photovoltaics and photonics.
2. Experimental P-type boron doped solar grade silicon wafers (100) with a resistivity ranging from 1 to 5 Ω cm were used as starting material. The electroless chemical attack of silicon is realized by two-step silver assisted etching method (Ag-ACE2) at room temperature [16]. Prior to fabrication, Si samples 3x3 cm2 on size, were degreased successively in boiling acetone and ethanol both for 5 min. A subsequent clean following the RCA1 technique is processed to remove the organic deposits and metal contaminants from the surface, then immersed in a diluted hydrofluoric acid (HF) solution to dissolve oxides and finally dried with a flux of nitrogen. The Ag-ACE2 process involves in a first step the chemical deposition of silver metal on silicon substrates in a solution composed of HF and 0.02 M AgNO3 for 3 min. In a second step, the Ag covered samples are etched in a wet chemical solution containing various low concentrations of H2O2 with a fixed HF concentration without the application of an external potential. Finally, the prepared layers 6
were rinsed for 5 min with HNO3 solution to dissolve the excessive Ag nanoparticles, leaving behind traces of Ag for catalyzing the etching reaction. The morphologies and microstructures of etched silicon using AgACE2 process and the kinetic formation of the as-synthesized SiNWs were investigated by scanning electronic microscopy (SEM; HITACHI-S4800, Chiyoda-ku,
Japan)
and
Atomic
Force
Microscopy
(AFM). The
structural properties are evaluated by Fourier Transform Infrared spectroscopy. The antireflective character of silicon nanowires is evaluated by reflectivity measurement under UV-Visible light absorption. Photoluminescence (PL) measurements were performed by using a variable temperature (10-300 K) close-cycle cryostat under 480 nm line of argon ion Ar + laser as excitation source. The PL signal was dispersed by a single-grating monochromator and detected by a photomultiplier associated with a standard lock-in technique. A correlation between photoluminescence and morphology changes is established.
3. Results and discussions When silver nanoparticles are used as metal catalyst, it is evident that an increase in H2O2 concentration in Ag-ACE2 method is equivalent to an increase in the current density in electrochemical anodization based methods. By simply raising the H2O2 concentration which activates the oxidation rate, it resulted in nanostructures with varying morphological, formation dynamics (etching rates) and optical properties.
3-1. Effect of low H2O2 concentration on the formation of Si nanostructures Figure 1 shows SEM images of the silicon nanostructures prepared in an etching solution with different H2O2 concentrations. It can be obviously seen that as the concentration of H2O2 is increased from 1% to 8%, the 1% etched substrate in figure 1, clearly exhibited a morphology different to that evolved by 7
the wire array for 3%, 4.5% and 7%, which is independent of the etching rate. When the concentration of H2O2 takes 1%, the prepared layers do not show an expected morphology of silicon nanowire arrays but a porous structure. Moreover, in comparison with the 3% and 4.5% SiNWs, which show a bunched phenomenon and high nanowire density, 7% SiNWs possess a diffusion like configuration and low nanowire density with the nanowire space enlarged as presented in figure 1. The morphological features in figure 1 show that an appropriate improvement of the H2O2 concentration can accelerate the etching in depth and widen the space of the prepared bunched nanowires and amend their density (insets of figures 1 for H2O2 concentrations of 4.5% and 7%).
Figure 1: 45° tilted SEM top views of silicon nanostructures evolved in etchant solutions composed of 4% HF and different concentrations of H2O2, etchant lasts 120 min. The insets are SEM cross section images of vertical silicon nanowires.
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It is found that for etchant solution composed with relatively low H2O2 concentration, the oriented (001) etching in depth is limited by the formation of a thick porous silicon layer at the surface of the substrate and underneath silver particles. Eventually, the formation of PS promotes the development of highly depleted regions (DRs) [22]. Consequently, etching is being possible only by charge transfer like stain-etching/chemical vapor etching of Si in/from HF/HNO3, which means that the probability of local etching prevails and the oriented mass transfer induced by Ag sinking across bulk silicon is stopped. However, the nature of the unclear charge transport mechanism still remains a debate. The morphology changes along with H2O2 concentration variations are confirmed
by
three-dimensional atomic
force
microscopy (3D-AFM)
investigations realized in tapping mode, presented in figure 2. One can observe that for 1% H2O2 concentration, the 3D-AFM image of figure 2-A reveals the existence of irregular and randomly distributed nanocrystalline silicon pillars and voids over the entire surface. The etched silicon structure appears to be comparable to porous silicon which shows the surface roughness and pyramid like hillocks surface. Furthermore, when etching is realized with H2O2 concentration of 3%, 4.5% and 7%, figures 2-B, C and D show that the surface is segment like with increased roughness. As the immersion proceeds in etchant solution relatively rich on H2O2, the size of the islands increases due to the overlapping of adjacent ones as shown on AFM view of figure 2-C. Although, the Si nanostructures fabricated using an H2O2 concentration of 1% demonstrated a distinguished morphology compared to other conditions. Excessive H2O2 concentration can generate a stiff silicon nanowires and a moderately rough morphology which is not a prerequisite for effective solar cells performances and efficient optical switching in silicon modulator. Indeed, rougher structures are distinguished by high densities of surface defect 9
states creating step -stones for free carriers (traps for photo-generated carriers). Hence, proper
concentration
of
oxidant
is
required
to
produce
appropriate Si nanostructures, with a smooth and flat surface, by Ag-ACE-2 process for solar cell applications and silicon photonic structures.
B : 2%, rms = 120 nm
A : 1%, rms = 80 nm
D : 4.5 %, rms = 161 nm
C : 3%, rms = 94 nm
Figure 2: Three dimensional AFM images of silicon nanowires formed in etchant solution of 4% HF and H2O2 of variable concentrations through a twostep reaction for 30 min: (A) 1% H2O2,(B) 2% H2O2, (C) 3%H2O2 et (D) 4.5% H2O2.
In addition, it appears that when H2O2 concentration was 1%, SiNWs can grow by adjusting the concentration of the HF reactant up to 10%. Figure 3-A shows a top view SEM micrograph of regular, uniform and homogeneous large 10
area of silicon nanowires. As depicted in figure 3-B, for solution containing 10% HF and 0.6% H2O2 reactant concentrations, the silicon nanowires height exhibited a linear relationship as a function of the etching time. Commonly, total ionization is achieved at relatively low HF concentrations; there was not enough HF to dissolve the oxide formed beneath silver nanoparticles. This oxide layer hindered the electron transfer from Si to Ag+. Therefore, the sinking of AgNPs is retarded and restricted by HF. In this case, excess Ag+ ions could not be reduced. Further increasing the HF concentration could accelerate the etching reaction and enable mass transfer at the AgNPs/Si interface.
5 µm
Heigth of silicon nanowires (µm)
4,0 3,5 3,0 2,5 2,0 1,5 1,0 60
120
180
240
Etching time (min)
Figure 3: A) SEM top view of silicon nanostructures evolved in etchant solution composed of 10% HF and H2O2 concentration of 0.6%, etchant lasts 120 min. and B) the correspondent length evolution of silicon nanowires as a function of etching time.
3-2. Kinetic growth of silicon nanowires 3-2-1. Effect of H2O2 concentration The concentrations of H2O2 may affect not only the morphologies, but also the etching rate of the etched structures. Figure 4 shows the average height silicon nanowires (lSiNWs) depending on the oxidant concentration (H2O2) 11
which follows a nonlinear relationship. The H2O2 concentration was adjusted to be low as possible ranging from 2% to 10% in an aqueous solution while fixing the volume ratio of H2O2 and HF. As the etchant concentration increased from 2% estimated
by
to
8%,
the
average
SiNWs
height,
lSiNWs,
as
SEM investigations, increased from 1.21 µm to 14.8 µm and
attenuates at high concentrations. At the first increasing stage, that is, for H2O2 concentrations from 2% to 5%, the etching rate increased from 0.011 to 0.108 μm/min, indicating that the etching activity depended strongly on the H2O2 concentration. When the H2O2 concentration exceeds 5%, the etching rate exhibited a different dynamic of wire formation. Some further increase in H2O2 concentration to 8% as shown in Figure 4, the etching rate was slightly lower than the rates at H2O2 concentrations of 5% and 7%. 16
Heigth of silicon nanowires (µm)
14 12 10
5 µm
8
5 µm
6
5 µm
4 2 0 3
4
5 6 Concentration of H2O2 (%)
7
8
Figure 4: Estimated average height of silicon nanowires versus the H2O2 oxidant concentration (etching time 120 min). The insets show crosssection SEM images for etchant solution composed of 4% HF and different H2O2 concentrations.
12
With the increase of H2O2 concentration which acts as hole donor and oxidant in the etching process, the oxidation rate of the silicon surrounding the Ag nanoparticles which sink deeply in volume, increases, resulting in the increase of the vertical etching rate of silicon. When the H2O2 concentration exceeds 5% in the etching solution, as shown in the insets of figure 4, more silicon around Ag nanoparticles will be oxidized into SiO2 and then dissolved slowly by HF. When the etching solution is too poor on H2O2, the involved nucleating sites allow the formation of porous silicon characterized by a low etching rate. 3-2-2. Effect of the etching time The cross section view images of the as-prepared SiNWs at different reaction duration are shown in figure 5A−F at H2O2 concentration of 4.5%. The dependence of the SiNWs length on the reaction time was studied in the range from 30 to 360 min. As shown in figure 5J, for the solutions containing 4.5%, and 7% at room temperature, the SiNWs length exhibited a nearly linear law with the etching time. In contrast, for 8% H2O2 the height of formed silicon nanowires saturates at high etching time revealing a declining etching rate. Both top- and cross section views suggested that is possible to produce SiNWs with desired length and diameter by simply tuning the etching duration and H2O2 concentration.
13
B
C
D
E
F
Concentration of H2O2
Leigth of silicon nanowires (µm)
A
J
4.5 % 7% 8%
Etching time (min)
Figure 5: Cross section SEM images of silicon nanowires prepared in etchant solution composed of 4% HF and 4.5% H2O2 for different etching time: A) 30 min, B) 60 min,C) 90 mn, D) 180 min, E)120 min and F) 240 min. J) Height of silicon nanowire versus time for three different H2O2 concentration.
The surface composition of the different samples was characterized by Fourier Transform Infrared spectroscopy (FTIR) analysis. The results are illustrated in Figure 6. The spectra of SiNWs prepared for different etching time are closely similar, showing the characteristic asymmetric stretching signals of the Si-O-Si bridge between 1,000 and 1,300 cm−1, with a strong band at 1,100 cm−1 and a shoulder at 1,240 cm−1 [14] which increased with increasing etching 14
time. The characteristic asymmetric stretching signals of the Si-O-Si bridge between 1,000 and 1,300 cm−1 and the wagging and stretching points of O3Si-H at 847 and 2,258 cm−1 are too weak. The formation of Si-O radicals becomes consistent when rising the concentration of H2O2 and the immersion duration in HF/ H2O2. This implies that the charge transfer faces some difficulty in order to establish an electrical equilibrium. Hence, the etching rate of silicon decays and a mismatch can be observed in some areas at the tip of the wires.
Reference SiNWs 0.5 h SiNWs 1 h SiNWs 2 h SiNWs 3 h SiNWs 4 h SiNWs 19 h
0,10
Absorption (a.u)
0,08
0,06
0,04
0,02
0,00 1300
1200
1100
1000
Wave number (cm-1)
Figure 6: FTIR absorbance spectra between 1,000 and 1,300 cm−1 for silicon nanowires prepared with [H2O2]= 7% showing an increased intensities of the bands peaking at 1,100 cm−1 and 1240 cm-1 with increasing etching durations .
3-3. The SiNW array formation mechanism and kinetics Ag-ECE2 of Si essentially operates like an electrochemical cell, with the Ag coating acting as a cathode, Si acting as the anode, and the etching solution acting as the electrolyte [19, 22]. H2O2 is catalytically reduced by Ag and the resultant hole current is injected into the adjacent Si which reacts with HF to form a soluble compound (H2SiF6). This causes the Si underneath the Ag catalyst, up to some depth dAg/Si, to be removed over time. The Ag 15
catalyst then re-establishes contact with the new Si surface by moving through the distance of dAg/Si in the etching solution under the influence of some attractive
force between silver and silicon surfaces. The full process then
repeats itself. The ultimate cathodic and anodic separate redox mechanisms can be described by these two half equilibrium reactions [16, 22]: - Anodic reaction: H2O2 + 2H+ → 2H2O + 2H+
(E1)
-Cathodic reaction: 2Si + 12HF + 6H → H2SiF6 + 6H+ 𝐻2↑
(E2)
the associated local anodic, Ic, and cathodic, Ia, currents can be expressed by the fallowing equations, respectively [21, 22]: Ic = AAu Nc F k c [H2 O2 ]m [HF]n
(I)
Ia = ASi Na F k a [HF]p
(II)
where m and n are constants, ASi and AAg refer to the electrode area of silicon and silver, F refers to Faraday’s constant, kc and ka refer to the rate constant for the cathode and anode reactions, respectively, and Nc and Na refer to the number of holes produced/consumed per molecule of H2O2 and HF in the cathodic and anodic reactions, respectively. When silicon nanowires grow during etching, three phenomena are simultaneously operating which are: i) Sinking of silver in contact with silicon, ii) Diffusion of chemical species and iii) Dissolution reactions of silicon (chemical reactions). The study of SiNWs formation rates constitutes the subject of kinetics. In this case, kinetics can be subdivided into physical kinetics which deals with physical phenomena such as diffusion and sinking, and chemical kinetics dealing with the rates of chemical reactions. If considering equation (I), it appears that when raising the concentration of H2O2, more cathodic current is injected into Si. Under steady-state Ic and Ia are equivalent, to consume this increased current with the same level of [HF], a 16
larger dAg/Si is required by sinking of AgNPs and dissolution of Si. However, at higher [H2O2] levels, the cathodic reaction rate declines, consuming the injected holes at slower rate, leading to a lower formation rate of SiNWs as depicted in figure 4. Similarly, when [HF] is increased, a larger cathodic current is injected into Si, causing more etching per time unit (i.e., dAg/Si increases). The top-down etching process is preferential following the (001) direction yielding SiNWs with various lengths and diameter. In addition, it is also shown in section 3-1 that porous silicon is formed when the etching solution is poor with H2O2 relative to HF. By adopting equations (I) and (II), a higher [H2O2] will inject a higher current into the Si anode during Ag-ACE2, and if [HF] is small so that the anodic reaction is relatively slow, a large amount of charge will have to be distributed over a bigger anodic area to be consumed. Consequently, the separation between Ag and Si during each etch cycle will be larger, leading to a steep reduction in the attractive force of Van der Wall type (vdW) existing between Ag and Si nucleus [22], which will allow the metal catalyst to peel off more easily during Ag- ACE2. As a result a chaotic nanostructure such as porous silicon is formed. Other studies [15, 22, 23] have shown that porous silicon is an intermediate thin layer mediating the charge transfer for the formation of silicon nanowires or a separate layer.
4. Reflectance and Photoluminescence properties of SiNWs In comparison with silicon without nanowires, the surfaces of etched SiNWs are much tougher as revealed in the reflectance spectra of Fig. 7 taken for wavelengths ranging between 0 and 1,750 nm. We observed that the reflectivity decreases with increasing H2O2 concentration. For a concentration of [H2O2] =2% the reflectivity equal to 7% and equal 3% for [H2O2] = 8 %. The roughness increases as a result of the formation of an increasingly thick SiNW layer. Texturing by SiNWs is more efficient than alkaline texturing. 17
Reflectance (%)
Si bare
With SiNWs
Wavenumber (nm)
Figure 7: Reflectance spectra of silicon nanowires obtained for different H2O2 concentration. An excellent antireflective layer is formed with [H2O2] =8% For [H2O2] =8 %, the reflectivity increases with increasing etching time (Figure 8). For etching time less than two hours, the reflectivity is constant for the wavelength ranging between 300 and 1,000 nm. For etching time greater than 2 h, the reflectivity varies with the wavelength. This effect can be explained by the presence of an index gradient and the uniformity and homogeneity of the
Reflectance (%)
structure disappear.
Wavenumber (nm)
Figure 8: Reflectance spectra of silicon nanowires obtained for [H2O2] =8% for different etching time. 18
Figure 9 depicts the room-temperature PL spectra of silicon nanostructures
prepared
by
Ag-ACE2
in
solutions
containing
H2O2
concentration in the range from 1% to 8%. Clearly, with the increase of H 2O2 concentration, it appears that the PL intensity increases greatly. Five orders of magnitude enhancement of light intensity for the band centered at 553 nm, is observed for the SiNWs prepared at H2O2 concentration of 7% compared to that obtained for 2%, which only exhibits a very weak PL spectrum (as shown in the inset of Figure 9). One can also observe that when the H2O2 concentration reaches 8%, the peak intensities shift to high wavelengths (576 nm and 650 nm) with two Gaussian like profiles. In addition, the PL signal is broad, and lacks sharp excitations and fine structure in the tail of the PL that are associated to spatial confinements effects from silicon structures embedded in silica [24, 25]. However, the PL spectrum of SiNCs evolved with H2O2 concentration of 1%, presents a single band with relatively low intensity centered at around 553 nm.
Intensité de PL (u.a)
576 nm
intensity PL Intensité de (a.u) PL (u.a)
553 nm
450
500
550
600
650
700
Longeur d'onde (nm)
a b c d e f
487 nm
500
550
600
650
700
750
Wavelength (nm)(nm) Longeur d'onde
Figure 9: Photoluminescence spectra of silicon nanowires showing a blue shift and intensity enhancement due to the increase of H2O2 concentration: a) 1%, b) 2%, c) 3%, d) 4.5%, e) 7%, and f) 8%), etchant lasts 30 min. The inset shows PL spectra when [H2O2] is less than 7%.
19
From the SEM and AFM images of SiNWs in figures 1 and 2, we find that at low H2O2 concentration (1%), the etched film is composed of nanocrystalline silicon whereas at higher H2O2 concentration, they exhibit silicon nanowires with increasing depth (Figure 4). This trend is consistent with that found in the PL intensity in figure 7, and it indicates that the PL enhancement is related to the surface nanostructures of SiNWs [26, 27, 28]. This result clearly indicates that is possible to tune the intensity and the emission band PL by simply adjusting the concentration of H2O2.
4. Conclusions We have investigated the use of relatively low H2O2 on concentration for tuning the morphology, the formation rates, the reflectivity and the photoluminescence of silicon nanostructures formed during two-stage silver assisted electroless etching of Si. It was found that the nature of the obtained nanostructure depends strongly on the proportions of H2O2 and HF. Particularly, porous silicon is formed when H2O2 concentration is lower than 1%. However, silicon nanowires are being to grow by increasing H2O2 proportion. The etch rate of SiNWs, on the other hand, increases with increasing H2O2 concentration and declines for high concentrations. Based on the associated local anodic, and cathodic, currents model some features of AgACE2 are claimed especially kinetics and dynamics of wire formation and the nanostructure that can be fabricated. More importantly, an efficient antireflective character for silicon solar cell (reflectance close to 2%) is realized at 8% H2O2.
20
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22
Figure captions
Figure 1: 45° tilted SEM top views of silicon nanostructures evolved in etchant solutions composed of 4% HF and different concentrations of H2O2, etchant lasts 120 min. The insets are SEM cross section images of vertical silicon nanowires. Figure 2: Three dimensional AFM images of silicon nanowires formed in etchant solution of 4% HF and H2O2 of variable concentrations through a twostep reaction for 30 min: (A) 1% H2O2,(B) 2% H2O2, (C) 3%H2O2 et (D) 4.5% H2O2. Figure 3: A) SEM top view of silicon nanostructures evolved in etchant solution composed of 10% HF and H2O2 concentration of 0.6%, etchant lasts 120 min. and B) the correspondent length evolution of silicon nanowires as a function of etching time. Figure 4: Estimated average height of silicon nanowires versus the H2O2 oxidant concentration (etching time 120 min). The insets show cross-section SEM images for etchant solution composed of 4% HF and different H2O2 concentrations. Figure 5: Cross section SEM images of silicon nanowires prepared in etchant solution composed of 4% HF and 4.5% H2O2 for different etching time: A) 30 min, B) 60 min,C) 90 mn, D) 180 min, E)120 min and F) 240 min. J) Height of silicon nanowire versus time for three different H2O2 concentration. Figure 6: FTIR absorbance spectra between 1,000 and 1,300 cm−1 for silicon nanowires prepared with [H2O2]= 7% showing an increased intensities of the bands peaking at 1,100 cm−1 and 1240 cm-1 with increasing etching durations . Figure 7: Reflectance spectra of silicon nanowires obtained for different H2O2 concentration. An excellent antireflective layer is formed with [H2O2] =8%.
23
Figure 8: Reflectance spectra of silicon nanowires obtained for [H2O2] =8% for different etching time. Figure 9: Photoluminescence spectra of silicon nanowires showing a blue shift and intensity enhancement due to the increase of H2O2 concentration: a) 1%, b) 2%, c) 3%, d) 4.5%, e) 7%, and f) 8%), etchant lasts 30 min. The inset shows PL spectra when [H2O2] is less than 7%.
24
S0169-4332(17)31765-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.110 APSUSC 36310
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-3-2017 5-6-2017 8-6-2017
Please cite this article as: Besma Moumni, Abdelkader Ben Jaballah, Correlation between oxidant concentrations, morphological aspects and etching kinetics of silicon nanowires during silver-assist electroless etching, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Correlation between oxidant concentrations, morphological aspects and etching kinetics of silicon nanowires during silver-assist electroless etching Besma Moumni1, Abdelkader Ben Jaballah*,1,2 1
Photovoltaic Laboratory, Research and Technology Centre of Energy (CRTEn), Borj Cedria Technopark, PB 95 Hammam Lif 2050, Tunisia. 2 Department of Physics, Faculty of Science and Arts in Samtha, Jazan University, Jazan-Kingdom of Saudi Arabia. *Corresponding author: e-mail: [email protected],
1
Graphical abstract
Cross section SEM images of silicon nanowires prepared in etchant solution composed of 4% HF and 4.5% H2O2 for different etching time: A) 30 min, B) 60 min,C) 90 mn, D) 180 min, E)120 min and F) 240 min. J) Height of silicon nanowire versus time for three different H2O2 concentration.
2
Highlights A correlation between oxidant concentration and the morphological changes of silicon nanowires is established. For 1% Molar concentration of H2O2, porous silicon (PS) is obtained. The dynamic and kinetics of silicon nanowires for different H2O2 concentration (3%, 4.5% and 7%) are studied by scanning electron microscopy. The spectra IR of SiNWs showing the formation of Si-O radicals becomes consistent when rising the concentration of H2O2 and the immersion duration in HF/ H2O2. More importantly, an efficient antireflective character for silicon solar cell (reflectance close to 2%) is realized at 8% H2O2. The luminescence of the prepared Si-nanostructures is claimed by photoluminescence which exhibit a large enhancement of the intensity and a blue shift for narrow and deep SiNWs.
3
Abstract Silicon porosification by silver assisted chemical etching (Ag-ACE) for a short range of H2O2 concentration is reported. We experimentally show that porous silicon (PSi) is obtained for 1% H2O2, whereas silicon nanowires (SiNWs) appeared by simply tuning the concentration of H2O2 to relatively high concentrations up to 8%. The morphological aspects are claimed by scanning electron microscopy proving that the kinetics of SiNWs formation display nonlinear relationships versus H2O2 concentration and etching time. A semiqualitative electrochemical etching model based on local anodic, Ic, and cathodic, Ia, currents is proposed to explain the different morphological changes, and to unveil the formation pathways of both PS and SiNWs. More importantly, an efficient antireflective character for silicon solar cell (reflectance close to 2%) is realized at 8% H2O2. In addition, the luminescence of the prepared Si-nanostructures is claimed by photoluminescence which exhibit a large enhancement of the intensity and a blue shift for narrow and deep SiNWs.
Keywords: Ag-assisted electroless etching, H2O2 concentration, Silicon nanowires, Porous silicon, Charge transport, Reflectivity, Photoluminescence.
4
1. Introduction The synthesis and fabrication of porous silicon (PSi) and silicon nanowires (SiNWs) have generated great impetus as PSi and SiNWs are advantageous in offering huge surface to volume ratios, favorable transport properties, tuned optical properties, and thermoelectric effects resulting from the nanoscale dimensions. The further importance of PSi and SiNWs lays on their enormous future applications in energy-related applications such as solar cells [1, 2, 3], thermoelectrics [4], negative electrodes for lithium ion batteries [5, 6, 7], as well as supercapacitors [8], field emitter [9], photodetectors, micromachining [10, 11] optical modulators and in a variety of sensing devices [12, 13]. Single- and double-step silver-assisted chemical etching of bulk silicon (Si) [10, 14] have been widely used as simple methods for fabricating highaspect ratio of PSi and SiNWs. Basically, it is well accepted that the whole wet etching of Si by HF and an oxidizing agent like H2O2 started at the interface between Si and a noble metal usually silver nanoparticles (AgNPs). There still a great debate concerning the detailed etching mechanism which leads to the formation of both structures [14-17]. It was, previously, established that many parameters are considered to affect metal assist anisotropic etching of silicon. These include metal nanoparticles type, size, shape and percentage coverage [18, 19, 20], and HF concentration [16, 14] (as main parameters), as well as [HF]/([HF]+ [H2O2]) molar concentration of HF acid [21]. However, the effects of H2O2 oxidative agent as a key etching parameter, on the morphology and etching kinetics of the SiNWs have not yet fully explored for a photovoltaic applications. Here, the changes in the nanostructure and etch rate of solar grade boron doped p-type silicon in a HF–H2O2–H2O etching system with electrolessly predeposited silver catalyst are systematically investigated. We develop a low cost 5
and simple methodology for silicon porosification. Indeed, solar grade silicon is processed. Moreover, H2O2 concentration is selected as fundamental and unique parameter which is adjusted to tune both the dissolution regimes (from porous silicon to silicon nanowires) and the etching rates. This allows controlling the morphologies of the prepared silicon nanostructures and etching depths for a short range of low H2O2 concentrations, while keeping fixed [HF] and silver concentration. Therefore, we explore the effects of [H2O2] in aqueous HF/H2O2 solution on the morphological aspects, kinetics growth and formation pathways of porous silicon and silicon nanowires. More importantly, a correlation between structural changes and reflectance/photoluminescence features of the fabricated silicon nanostructures is realized. It is recognized that
the
obtained
results
are
consistent
to
the
field
of
silicon
photovoltaics and photonics.
2. Experimental P-type boron doped solar grade silicon wafers (100) with a resistivity ranging from 1 to 5 Ω cm were used as starting material. The electroless chemical attack of silicon is realized by two-step silver assisted etching method (Ag-ACE2) at room temperature [16]. Prior to fabrication, Si samples 3x3 cm2 on size, were degreased successively in boiling acetone and ethanol both for 5 min. A subsequent clean following the RCA1 technique is processed to remove the organic deposits and metal contaminants from the surface, then immersed in a diluted hydrofluoric acid (HF) solution to dissolve oxides and finally dried with a flux of nitrogen. The Ag-ACE2 process involves in a first step the chemical deposition of silver metal on silicon substrates in a solution composed of HF and 0.02 M AgNO3 for 3 min. In a second step, the Ag covered samples are etched in a wet chemical solution containing various low concentrations of H2O2 with a fixed HF concentration without the application of an external potential. Finally, the prepared layers 6
were rinsed for 5 min with HNO3 solution to dissolve the excessive Ag nanoparticles, leaving behind traces of Ag for catalyzing the etching reaction. The morphologies and microstructures of etched silicon using AgACE2 process and the kinetic formation of the as-synthesized SiNWs were investigated by scanning electronic microscopy (SEM; HITACHI-S4800, Chiyoda-ku,
Japan)
and
Atomic
Force
Microscopy
(AFM). The
structural properties are evaluated by Fourier Transform Infrared spectroscopy. The antireflective character of silicon nanowires is evaluated by reflectivity measurement under UV-Visible light absorption. Photoluminescence (PL) measurements were performed by using a variable temperature (10-300 K) close-cycle cryostat under 480 nm line of argon ion Ar + laser as excitation source. The PL signal was dispersed by a single-grating monochromator and detected by a photomultiplier associated with a standard lock-in technique. A correlation between photoluminescence and morphology changes is established.
3. Results and discussions When silver nanoparticles are used as metal catalyst, it is evident that an increase in H2O2 concentration in Ag-ACE2 method is equivalent to an increase in the current density in electrochemical anodization based methods. By simply raising the H2O2 concentration which activates the oxidation rate, it resulted in nanostructures with varying morphological, formation dynamics (etching rates) and optical properties.
3-1. Effect of low H2O2 concentration on the formation of Si nanostructures Figure 1 shows SEM images of the silicon nanostructures prepared in an etching solution with different H2O2 concentrations. It can be obviously seen that as the concentration of H2O2 is increased from 1% to 8%, the 1% etched substrate in figure 1, clearly exhibited a morphology different to that evolved by 7
the wire array for 3%, 4.5% and 7%, which is independent of the etching rate. When the concentration of H2O2 takes 1%, the prepared layers do not show an expected morphology of silicon nanowire arrays but a porous structure. Moreover, in comparison with the 3% and 4.5% SiNWs, which show a bunched phenomenon and high nanowire density, 7% SiNWs possess a diffusion like configuration and low nanowire density with the nanowire space enlarged as presented in figure 1. The morphological features in figure 1 show that an appropriate improvement of the H2O2 concentration can accelerate the etching in depth and widen the space of the prepared bunched nanowires and amend their density (insets of figures 1 for H2O2 concentrations of 4.5% and 7%).
Figure 1: 45° tilted SEM top views of silicon nanostructures evolved in etchant solutions composed of 4% HF and different concentrations of H2O2, etchant lasts 120 min. The insets are SEM cross section images of vertical silicon nanowires.
8
It is found that for etchant solution composed with relatively low H2O2 concentration, the oriented (001) etching in depth is limited by the formation of a thick porous silicon layer at the surface of the substrate and underneath silver particles. Eventually, the formation of PS promotes the development of highly depleted regions (DRs) [22]. Consequently, etching is being possible only by charge transfer like stain-etching/chemical vapor etching of Si in/from HF/HNO3, which means that the probability of local etching prevails and the oriented mass transfer induced by Ag sinking across bulk silicon is stopped. However, the nature of the unclear charge transport mechanism still remains a debate. The morphology changes along with H2O2 concentration variations are confirmed
by
three-dimensional atomic
force
microscopy (3D-AFM)
investigations realized in tapping mode, presented in figure 2. One can observe that for 1% H2O2 concentration, the 3D-AFM image of figure 2-A reveals the existence of irregular and randomly distributed nanocrystalline silicon pillars and voids over the entire surface. The etched silicon structure appears to be comparable to porous silicon which shows the surface roughness and pyramid like hillocks surface. Furthermore, when etching is realized with H2O2 concentration of 3%, 4.5% and 7%, figures 2-B, C and D show that the surface is segment like with increased roughness. As the immersion proceeds in etchant solution relatively rich on H2O2, the size of the islands increases due to the overlapping of adjacent ones as shown on AFM view of figure 2-C. Although, the Si nanostructures fabricated using an H2O2 concentration of 1% demonstrated a distinguished morphology compared to other conditions. Excessive H2O2 concentration can generate a stiff silicon nanowires and a moderately rough morphology which is not a prerequisite for effective solar cells performances and efficient optical switching in silicon modulator. Indeed, rougher structures are distinguished by high densities of surface defect 9
states creating step -stones for free carriers (traps for photo-generated carriers). Hence, proper
concentration
of
oxidant
is
required
to
produce
appropriate Si nanostructures, with a smooth and flat surface, by Ag-ACE-2 process for solar cell applications and silicon photonic structures.
B : 2%, rms = 120 nm
A : 1%, rms = 80 nm
D : 4.5 %, rms = 161 nm
C : 3%, rms = 94 nm
Figure 2: Three dimensional AFM images of silicon nanowires formed in etchant solution of 4% HF and H2O2 of variable concentrations through a twostep reaction for 30 min: (A) 1% H2O2,(B) 2% H2O2, (C) 3%H2O2 et (D) 4.5% H2O2.
In addition, it appears that when H2O2 concentration was 1%, SiNWs can grow by adjusting the concentration of the HF reactant up to 10%. Figure 3-A shows a top view SEM micrograph of regular, uniform and homogeneous large 10
area of silicon nanowires. As depicted in figure 3-B, for solution containing 10% HF and 0.6% H2O2 reactant concentrations, the silicon nanowires height exhibited a linear relationship as a function of the etching time. Commonly, total ionization is achieved at relatively low HF concentrations; there was not enough HF to dissolve the oxide formed beneath silver nanoparticles. This oxide layer hindered the electron transfer from Si to Ag+. Therefore, the sinking of AgNPs is retarded and restricted by HF. In this case, excess Ag+ ions could not be reduced. Further increasing the HF concentration could accelerate the etching reaction and enable mass transfer at the AgNPs/Si interface.
5 µm
Heigth of silicon nanowires (µm)
4,0 3,5 3,0 2,5 2,0 1,5 1,0 60
120
180
240
Etching time (min)
Figure 3: A) SEM top view of silicon nanostructures evolved in etchant solution composed of 10% HF and H2O2 concentration of 0.6%, etchant lasts 120 min. and B) the correspondent length evolution of silicon nanowires as a function of etching time.
3-2. Kinetic growth of silicon nanowires 3-2-1. Effect of H2O2 concentration The concentrations of H2O2 may affect not only the morphologies, but also the etching rate of the etched structures. Figure 4 shows the average height silicon nanowires (lSiNWs) depending on the oxidant concentration (H2O2) 11
which follows a nonlinear relationship. The H2O2 concentration was adjusted to be low as possible ranging from 2% to 10% in an aqueous solution while fixing the volume ratio of H2O2 and HF. As the etchant concentration increased from 2% estimated
by
to
8%,
the
average
SiNWs
height,
lSiNWs,
as
SEM investigations, increased from 1.21 µm to 14.8 µm and
attenuates at high concentrations. At the first increasing stage, that is, for H2O2 concentrations from 2% to 5%, the etching rate increased from 0.011 to 0.108 μm/min, indicating that the etching activity depended strongly on the H2O2 concentration. When the H2O2 concentration exceeds 5%, the etching rate exhibited a different dynamic of wire formation. Some further increase in H2O2 concentration to 8% as shown in Figure 4, the etching rate was slightly lower than the rates at H2O2 concentrations of 5% and 7%. 16
Heigth of silicon nanowires (µm)
14 12 10
5 µm
8
5 µm
6
5 µm
4 2 0 3
4
5 6 Concentration of H2O2 (%)
7
8
Figure 4: Estimated average height of silicon nanowires versus the H2O2 oxidant concentration (etching time 120 min). The insets show crosssection SEM images for etchant solution composed of 4% HF and different H2O2 concentrations.
12
With the increase of H2O2 concentration which acts as hole donor and oxidant in the etching process, the oxidation rate of the silicon surrounding the Ag nanoparticles which sink deeply in volume, increases, resulting in the increase of the vertical etching rate of silicon. When the H2O2 concentration exceeds 5% in the etching solution, as shown in the insets of figure 4, more silicon around Ag nanoparticles will be oxidized into SiO2 and then dissolved slowly by HF. When the etching solution is too poor on H2O2, the involved nucleating sites allow the formation of porous silicon characterized by a low etching rate. 3-2-2. Effect of the etching time The cross section view images of the as-prepared SiNWs at different reaction duration are shown in figure 5A−F at H2O2 concentration of 4.5%. The dependence of the SiNWs length on the reaction time was studied in the range from 30 to 360 min. As shown in figure 5J, for the solutions containing 4.5%, and 7% at room temperature, the SiNWs length exhibited a nearly linear law with the etching time. In contrast, for 8% H2O2 the height of formed silicon nanowires saturates at high etching time revealing a declining etching rate. Both top- and cross section views suggested that is possible to produce SiNWs with desired length and diameter by simply tuning the etching duration and H2O2 concentration.
13
B
C
D
E
F
Concentration of H2O2
Leigth of silicon nanowires (µm)
A
J
4.5 % 7% 8%
Etching time (min)
Figure 5: Cross section SEM images of silicon nanowires prepared in etchant solution composed of 4% HF and 4.5% H2O2 for different etching time: A) 30 min, B) 60 min,C) 90 mn, D) 180 min, E)120 min and F) 240 min. J) Height of silicon nanowire versus time for three different H2O2 concentration.
The surface composition of the different samples was characterized by Fourier Transform Infrared spectroscopy (FTIR) analysis. The results are illustrated in Figure 6. The spectra of SiNWs prepared for different etching time are closely similar, showing the characteristic asymmetric stretching signals of the Si-O-Si bridge between 1,000 and 1,300 cm−1, with a strong band at 1,100 cm−1 and a shoulder at 1,240 cm−1 [14] which increased with increasing etching 14
time. The characteristic asymmetric stretching signals of the Si-O-Si bridge between 1,000 and 1,300 cm−1 and the wagging and stretching points of O3Si-H at 847 and 2,258 cm−1 are too weak. The formation of Si-O radicals becomes consistent when rising the concentration of H2O2 and the immersion duration in HF/ H2O2. This implies that the charge transfer faces some difficulty in order to establish an electrical equilibrium. Hence, the etching rate of silicon decays and a mismatch can be observed in some areas at the tip of the wires.
Reference SiNWs 0.5 h SiNWs 1 h SiNWs 2 h SiNWs 3 h SiNWs 4 h SiNWs 19 h
0,10
Absorption (a.u)
0,08
0,06
0,04
0,02
0,00 1300
1200
1100
1000
Wave number (cm-1)
Figure 6: FTIR absorbance spectra between 1,000 and 1,300 cm−1 for silicon nanowires prepared with [H2O2]= 7% showing an increased intensities of the bands peaking at 1,100 cm−1 and 1240 cm-1 with increasing etching durations .
3-3. The SiNW array formation mechanism and kinetics Ag-ECE2 of Si essentially operates like an electrochemical cell, with the Ag coating acting as a cathode, Si acting as the anode, and the etching solution acting as the electrolyte [19, 22]. H2O2 is catalytically reduced by Ag and the resultant hole current is injected into the adjacent Si which reacts with HF to form a soluble compound (H2SiF6). This causes the Si underneath the Ag catalyst, up to some depth dAg/Si, to be removed over time. The Ag 15
catalyst then re-establishes contact with the new Si surface by moving through the distance of dAg/Si in the etching solution under the influence of some attractive
force between silver and silicon surfaces. The full process then
repeats itself. The ultimate cathodic and anodic separate redox mechanisms can be described by these two half equilibrium reactions [16, 22]: - Anodic reaction: H2O2 + 2H+ → 2H2O + 2H+
(E1)
-Cathodic reaction: 2Si + 12HF + 6H → H2SiF6 + 6H+ 𝐻2↑
(E2)
the associated local anodic, Ic, and cathodic, Ia, currents can be expressed by the fallowing equations, respectively [21, 22]: Ic = AAu Nc F k c [H2 O2 ]m [HF]n
(I)
Ia = ASi Na F k a [HF]p
(II)
where m and n are constants, ASi and AAg refer to the electrode area of silicon and silver, F refers to Faraday’s constant, kc and ka refer to the rate constant for the cathode and anode reactions, respectively, and Nc and Na refer to the number of holes produced/consumed per molecule of H2O2 and HF in the cathodic and anodic reactions, respectively. When silicon nanowires grow during etching, three phenomena are simultaneously operating which are: i) Sinking of silver in contact with silicon, ii) Diffusion of chemical species and iii) Dissolution reactions of silicon (chemical reactions). The study of SiNWs formation rates constitutes the subject of kinetics. In this case, kinetics can be subdivided into physical kinetics which deals with physical phenomena such as diffusion and sinking, and chemical kinetics dealing with the rates of chemical reactions. If considering equation (I), it appears that when raising the concentration of H2O2, more cathodic current is injected into Si. Under steady-state Ic and Ia are equivalent, to consume this increased current with the same level of [HF], a 16
larger dAg/Si is required by sinking of AgNPs and dissolution of Si. However, at higher [H2O2] levels, the cathodic reaction rate declines, consuming the injected holes at slower rate, leading to a lower formation rate of SiNWs as depicted in figure 4. Similarly, when [HF] is increased, a larger cathodic current is injected into Si, causing more etching per time unit (i.e., dAg/Si increases). The top-down etching process is preferential following the (001) direction yielding SiNWs with various lengths and diameter. In addition, it is also shown in section 3-1 that porous silicon is formed when the etching solution is poor with H2O2 relative to HF. By adopting equations (I) and (II), a higher [H2O2] will inject a higher current into the Si anode during Ag-ACE2, and if [HF] is small so that the anodic reaction is relatively slow, a large amount of charge will have to be distributed over a bigger anodic area to be consumed. Consequently, the separation between Ag and Si during each etch cycle will be larger, leading to a steep reduction in the attractive force of Van der Wall type (vdW) existing between Ag and Si nucleus [22], which will allow the metal catalyst to peel off more easily during Ag- ACE2. As a result a chaotic nanostructure such as porous silicon is formed. Other studies [15, 22, 23] have shown that porous silicon is an intermediate thin layer mediating the charge transfer for the formation of silicon nanowires or a separate layer.
4. Reflectance and Photoluminescence properties of SiNWs In comparison with silicon without nanowires, the surfaces of etched SiNWs are much tougher as revealed in the reflectance spectra of Fig. 7 taken for wavelengths ranging between 0 and 1,750 nm. We observed that the reflectivity decreases with increasing H2O2 concentration. For a concentration of [H2O2] =2% the reflectivity equal to 7% and equal 3% for [H2O2] = 8 %. The roughness increases as a result of the formation of an increasingly thick SiNW layer. Texturing by SiNWs is more efficient than alkaline texturing. 17
Reflectance (%)
Si bare
With SiNWs
Wavenumber (nm)
Figure 7: Reflectance spectra of silicon nanowires obtained for different H2O2 concentration. An excellent antireflective layer is formed with [H2O2] =8% For [H2O2] =8 %, the reflectivity increases with increasing etching time (Figure 8). For etching time less than two hours, the reflectivity is constant for the wavelength ranging between 300 and 1,000 nm. For etching time greater than 2 h, the reflectivity varies with the wavelength. This effect can be explained by the presence of an index gradient and the uniformity and homogeneity of the
Reflectance (%)
structure disappear.
Wavenumber (nm)
Figure 8: Reflectance spectra of silicon nanowires obtained for [H2O2] =8% for different etching time. 18
Figure 9 depicts the room-temperature PL spectra of silicon nanostructures
prepared
by
Ag-ACE2
in
solutions
containing
H2O2
concentration in the range from 1% to 8%. Clearly, with the increase of H 2O2 concentration, it appears that the PL intensity increases greatly. Five orders of magnitude enhancement of light intensity for the band centered at 553 nm, is observed for the SiNWs prepared at H2O2 concentration of 7% compared to that obtained for 2%, which only exhibits a very weak PL spectrum (as shown in the inset of Figure 9). One can also observe that when the H2O2 concentration reaches 8%, the peak intensities shift to high wavelengths (576 nm and 650 nm) with two Gaussian like profiles. In addition, the PL signal is broad, and lacks sharp excitations and fine structure in the tail of the PL that are associated to spatial confinements effects from silicon structures embedded in silica [24, 25]. However, the PL spectrum of SiNCs evolved with H2O2 concentration of 1%, presents a single band with relatively low intensity centered at around 553 nm.
Intensité de PL (u.a)
576 nm
intensity PL Intensité de (a.u) PL (u.a)
553 nm
450
500
550
600
650
700
Longeur d'onde (nm)
a b c d e f
487 nm
500
550
600
650
700
750
Wavelength (nm)(nm) Longeur d'onde
Figure 9: Photoluminescence spectra of silicon nanowires showing a blue shift and intensity enhancement due to the increase of H2O2 concentration: a) 1%, b) 2%, c) 3%, d) 4.5%, e) 7%, and f) 8%), etchant lasts 30 min. The inset shows PL spectra when [H2O2] is less than 7%.
19
From the SEM and AFM images of SiNWs in figures 1 and 2, we find that at low H2O2 concentration (1%), the etched film is composed of nanocrystalline silicon whereas at higher H2O2 concentration, they exhibit silicon nanowires with increasing depth (Figure 4). This trend is consistent with that found in the PL intensity in figure 7, and it indicates that the PL enhancement is related to the surface nanostructures of SiNWs [26, 27, 28]. This result clearly indicates that is possible to tune the intensity and the emission band PL by simply adjusting the concentration of H2O2.
4. Conclusions We have investigated the use of relatively low H2O2 on concentration for tuning the morphology, the formation rates, the reflectivity and the photoluminescence of silicon nanostructures formed during two-stage silver assisted electroless etching of Si. It was found that the nature of the obtained nanostructure depends strongly on the proportions of H2O2 and HF. Particularly, porous silicon is formed when H2O2 concentration is lower than 1%. However, silicon nanowires are being to grow by increasing H2O2 proportion. The etch rate of SiNWs, on the other hand, increases with increasing H2O2 concentration and declines for high concentrations. Based on the associated local anodic, and cathodic, currents model some features of AgACE2 are claimed especially kinetics and dynamics of wire formation and the nanostructure that can be fabricated. More importantly, an efficient antireflective character for silicon solar cell (reflectance close to 2%) is realized at 8% H2O2.
20
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Figure captions
Figure 1: 45° tilted SEM top views of silicon nanostructures evolved in etchant solutions composed of 4% HF and different concentrations of H2O2, etchant lasts 120 min. The insets are SEM cross section images of vertical silicon nanowires. Figure 2: Three dimensional AFM images of silicon nanowires formed in etchant solution of 4% HF and H2O2 of variable concentrations through a twostep reaction for 30 min: (A) 1% H2O2,(B) 2% H2O2, (C) 3%H2O2 et (D) 4.5% H2O2. Figure 3: A) SEM top view of silicon nanostructures evolved in etchant solution composed of 10% HF and H2O2 concentration of 0.6%, etchant lasts 120 min. and B) the correspondent length evolution of silicon nanowires as a function of etching time. Figure 4: Estimated average height of silicon nanowires versus the H2O2 oxidant concentration (etching time 120 min). The insets show cross-section SEM images for etchant solution composed of 4% HF and different H2O2 concentrations. Figure 5: Cross section SEM images of silicon nanowires prepared in etchant solution composed of 4% HF and 4.5% H2O2 for different etching time: A) 30 min, B) 60 min,C) 90 mn, D) 180 min, E)120 min and F) 240 min. J) Height of silicon nanowire versus time for three different H2O2 concentration. Figure 6: FTIR absorbance spectra between 1,000 and 1,300 cm−1 for silicon nanowires prepared with [H2O2]= 7% showing an increased intensities of the bands peaking at 1,100 cm−1 and 1240 cm-1 with increasing etching durations . Figure 7: Reflectance spectra of silicon nanowires obtained for different H2O2 concentration. An excellent antireflective layer is formed with [H2O2] =8%.
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Figure 8: Reflectance spectra of silicon nanowires obtained for [H2O2] =8% for different etching time. Figure 9: Photoluminescence spectra of silicon nanowires showing a blue shift and intensity enhancement due to the increase of H2O2 concentration: a) 1%, b) 2%, c) 3%, d) 4.5%, e) 7%, and f) 8%), etchant lasts 30 min. The inset shows PL spectra when [H2O2] is less than 7%.
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