Synthesis, microstructure and visible luminescence properties of vertically aligned lightly doped porous silicon nanowalls

Accepted Manuscript Synthesis, microstructure and visible luminescence properties of vertically aligned lightly doped porous silicon nanowalls Anil K. Behera, R.N. Viswanath, C. Lakshmanan, K.K. Madapu, M. Kamruddin, T. Mathews PII:

S1387-1811(18)30362-7

DOI:

10.1016/j.micromeso.2018.06.052

Reference:

MICMAT 9005

To appear in:

Microporous and Mesoporous Materials

Received Date: 20 April 2018 Revised Date:

15 June 2018

Accepted Date: 28 June 2018

Please cite this article as: A.K. Behera, R.N. Viswanath, C. Lakshmanan, K.K. Madapu, M. Kamruddin, T. Mathews, Synthesis, microstructure and visible luminescence properties of vertically aligned lightly doped porous silicon nanowalls, Microporous and Mesoporous Materials (2018), doi: 10.1016/ j.micromeso.2018.06.052. 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.

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Anil K. Behera1, R. N. Viswanath1,2, C. Lakshmanan1, K. K. Madapu1, M. Kamruddin1, T. Mathews1 1 Materials Science Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603 102, Tamil Nadu, India 2 Centre of Excellence for Nanotechnology Research, Aarupadai Veedu Institute of Technology, Vinayaka Mission’s Research Foundation, Chennai - 603 104, TamilNadu, India

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Synthesis, microstructure and visible luminescence properties of vertically aligned lightly doped porous silicon nanowalls

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Abstract: Silicon nanowall templates in nonporous and porous forms have been prepared by 4.8 M HF

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and 0.2 M H2O2 etching of lightly boron doped silicon (100) wafers at room temperature and their

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morphology, microstructure and visible photoluminescence properties were studied. Metal assisted

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chemical and metal assisted anodic polarization etching methods were used respectively to produce these

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morphologically distinct silicon nanowalls. It is shown from the analysis of photoluminescence results

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that porous silicon nanowalls emit red and orange radiation signals. This observation is in contrast with

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nonporous silicon nanowalls that did not show any visible luminescence features. The microscopy results

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revealed that pores and inter-pore Si regions in porous SiNWs are interconnected in the form of skeletal

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morphology with quantum dimensions. Based on the temperature dependent luminescence results and the

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structure dimensions evaluated from the microscopy results, it is concluded that the red and orange

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emission in porous silicon nanowalls originates from silicon quantum structures and Si-O-Si bonded

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amorphous structures respectively. The effect of Si nanostructures and its dimensional dependence on

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photoluminescence properties in porous silicon nanowalls are also discussed. The synthesis and

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photoluminescence mechanism discussed in this work is very much useful for the development of silicon

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based functional porous materials for nanoscale devices application.

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Keywords: silicon nanowalls; visible luminescence; low temperature photoluminescence; metal

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assisted anodic polarization etching; quantum size effect

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Introduction: Porous silicon nanostructures have received tremendous attention in

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semiconductor technology due to their increased height specific surface area and structure size

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dependent optical and electronic properties [1-4]. Because of the evolution of vertically aligned

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porous silicon walls and owing to the variation in visible radiation emission with structure size,

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porous silicon nanostructures have also received considerable attention in the present

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semiconducting technology for developing different kinds of devices such as diode in

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photovoltaic cells, sensor for the detection of biological, chemical and nuclear species and field

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effect transistors to diagnose diseases [5-7]. It has been shown that the photoluminescence (PL)

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emission in porous silicon is blue shifted with respect to the Si structure size d and the energy

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shift follows approximately a d-1.39 law dependence [3]. Although many reports have indicated

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that the size of the Si quantum structures evolved in porous Si are numerically comparable to the

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free-exciton Bohr radius value 4.3 nm of crystalline silicon, bottlenecks still exist in

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understanding the role of microstructure (defects, chemical interface and its surface states, etc.)

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in porous silicon to the PL properties. Wang et al., [8] suggested from the analysis of PL results

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combined with structural characterization by high resolution transmission electron microscopy

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that the factors influencing the visible emission radiation in porous silicon is not solely by the

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quantum confinement effect as predicted by Canham [9] where the photoexcitation and

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photoemission occurs in single energy state in the Si crystallites, but by surface processes viz.,

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the interface state effect and surface chemical effect.

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Several methods, utilising both bottom-up and top down approaches, such as reactive ion

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etching, laser induced etching, anodic polarization etching and metal-assisted chemical (MAC)

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etching etc., have been developed for the fabrication of porous Si nanostructures. Among them,

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MAC etching [10, 11] is of special interest owing to its simplicity, versatility and the ability to

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produce different kind of Si nanostructures like porous Si film, Si nanowires/nanowalls (SiNWs)

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and porous Si nanowire/nanowalls (porous SiNWs) etc. The porous SiNWs is a new kind of

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nanostructured material with the capability of exhibiting the combined physical features of both

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Si nanowires/nanowalls and nanopores. Therefore, it has been pointed out that the porous SiNWs

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may open up new opportunities for the development of multifunctional optoelectronic and photo-

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electrochemical devices [12, 13]. However, it has been reported that the evolved microstructure

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in MAC etched Si surface depends highly on their doping concentration in addition to the type

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and crystallographic orientation of Si wafers used for the etching [11, 14]. For instance, MAC

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etched highly doped (1 - 5 mΩ - cm) Si wafers generate SiNWs with high density of porosity on

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compared to that obtained on lightly doped (1 - 3 Ω-cm) Si wafers. Since anodic polarization

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etching has the ability to make porous Si consisting of quantum structures irrespective of the

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doping concentration of Si wafer, it is worth and useful to combine MAC etching with anodic

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polarization etching to synthesize porous SiNWs. On combining so, one would also expect high

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visible luminescence properties in lightly doped Si wafers. Recently Lai et al [14] have

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combined MAC etching with anodic polarization etching and fabricated lightly doped porous

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SiNWs. However, their studies focus mainly on the mechanisms and the kinetics involved in the

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fabrication of SiNWs. So the present study reports the morphology, microstructure and

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luminescence properties of lightly doped porous SiNWs fabricated by metal assisted anodic

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polarization etching (MAAP) which is a combination of MAC and anodic polarization etching.

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In addition, the present paper discusses qualitatively the correlation between the interior quantum

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structures in porous SiNWs and the luminescence properties.

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Experimental Procedure: Single crystalline (100) oriented p-type (boron doped) Si wafers with

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resistivity ~ 2 Ω-cm were used as the starting material for the preparation of SiNWs. At the

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beginning, these wafers were ultrasonically cleaned by ultrapure water (18.2 MΩ-cm), acetone

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and ethanol. In order to remove the residual organic contaminants, the ultrasonically cleaned Si

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wafers were immersed into freshly prepared piranha solution (3:1 ratio of 98 % H2SO4 and 30 %

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H2O2) for 15 minutes, rinsed in 18.2 MΩ-cm grade water and dried with nitrogen gas. The dried

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wafers were then dipped in dilute HF solution to remove the native oxide layer. Subsequently,

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the oxide free Si wafer is housed into a home made teflon electrochemical cell with the polished

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Si wafer surface being exposed to the etching solution using screw tightened viton O-ring

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assembly as shown in Fig. 1. Silver deposition was made on the exposed polished surface of the

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Si wafer in a solution mixture of HF and AgNO3 (5% HF + 0.02M AgNO3) for 60 seconds and

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subsequently rinsed with 18.2 MΩ-cm grade water to remove the extra Ag. Then etching of Ag

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deposited Si wafer was carried out in a solution mixture of 4.8 M HF and 0.2 M H2O2 at 298 K

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for 15 minutes. The etched Si wafers were immersed in conc. HNO3 to remove the diffused Ag

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particles and washed with ultrapure water, dried with ultrapure nitrogen gas and finally stored in

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a vacuum desiccator for further study. It is worth to note that Si wafers were handled in dark

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conditions during both Ag deposition and subsequent etching to avoid any influence of

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photogenerated charges in the silicon wafers. Figures 1a) and 1 b) illustrate the schematic of the

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cell set-ups used for MAC and MAAP etching, respectively. The MAAP etching experiments

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were carried out in a two electrode cell assembly by applying a constant current flow of 5 mA

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between Ag deposited Si wafer and graphitic carbon for 15 minutes. Indium-Gallium eutectic

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alloy (99.99% purity, Sigma-Aldrich) was used to obtain ohmic electrical contact on silicon

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wafer surface.

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Fig. 1: Schematic illustration of Teflon cell assembly for a) metal assisted chemical (MAC) and b) metal assisted

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anodic polarization (MAAP) etching for the fabrication of SiNWs. Note that only polished Si surface has been

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exposed to the electrolyte. The Si wafer is housed in the cells in a stable configuration using the screw tightened O-

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ring assembly.

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Scanning electron microscope (Carl-Zeiss Supra 55) was used to investigate the microstructure

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of the as-prepared SiNWs. A portion of the MAC and MAAP etched SiNWs were dispersed

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separately onto the TEM Cu grids coated with ultra thin carbon films and studied in a high

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resolution transmission electron microscope (LIBRA 200FE Zeiss) operated at an accelerating

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voltage of 200 kV. The photoluminescence and Raman experiments were carried out using a

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micro-Raman spectrometer (Renishaw inVia) with an excitation light source of wavelength 514

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nm using Ar+ ion laser with a 10 s data acquisition time. The Raman scattered light and

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luminescence radiation signals were collected in a backscattering geometry using CCD detector.

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The laser power was kept at 0.02 mW during the measurements. The temperature dependent PL

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measurement was also carried out on the porous SiNWs in the temperature range 80 - 375 K

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using the liquid N2 cooled low temperature stage (Linkam T95, UK). Resistive heating filament

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housed underneath the sample in the cryostat was used to maintain the sample temperature

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during the data collection. Temperature stability during the measurement was typically ± 1 K.

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The surface chemical bonding of the prepared samples were investigated using the attenuated

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total reflection (ATR) - FTIR spectrometer (Bruker Tensor II) in the spectral range from 700 to

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4000 cm-1 using sixteen scans at 4 cm−1 spectral resolution.

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Results and Discussion: Figure 2 summarizes the microscopy results of MAC and MAAP

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etched SiNWs. Figures 2 a) and 2 b) show the top view of scanning electron microscopy (SEM)

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images of the MAC and MAAP etched SiNWs, respectively. The cross-section view of the SEM

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images as shown in the inset in Figs. 2 a, b) illustrate that the Si wall structure in both MAC and

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MAAP etched SiNWs is homogeneous and the walls are vertically aligned to the Si wafer. It is

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seen that the Si walls extrude ~ 4 µm in depth from the bulk Si. The low magnification TEM

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images of MAC and MAAP etched SiNWs as shown in Figs. 2 c, d) illustrate that the Si walls

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are assembled in the form of bundle-like structure. Figures 2 e. f) show that the TEM image of an

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isolated Si nanowall from the MAC and MAAP etched SiNW bundles, respectively. The average

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width and wall thickness estimated from the TEM and SEM images are 80 and 40 nm,

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respectively. Figures 2 g, h) show the HRTEM images of MAC and MAAP etched SiNWs

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respectively. The analysis of these HRTEM images reveals that the Si wall structure in both

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MAC and MAAP etched SiNWs is single crystalline all along their length. The mono-crystalline

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nature of the SiNWs was further confirmed by the grazing incidence X-ray diffraction (GIXRD)

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experiments [15]. The analysis of the GIXRD data (figure not shown) reveals that the diffraction

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pattern of both MAC and MAAP etched SiNWs shows (311) Bragg reflection, which is 25.3o

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tilted from the Si (100) wafer surface. This observation can be easily understood because in

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GIXRD configuration with Cu Kα radiation, the (311) crystal plane is the most optimal plane

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that satisfies the Bragg’s diffraction condition. It is worth to note here that there is negligible

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evidence for the formation of granular or any oxide crystalline phase in both MAC and MAAP

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etched SiNWs.

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Fig. 2: SEM and TEM results of MAC and MAAP etched SiNWs. a, b) Top view SEM images of MAC and MAAP

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etched SiNWs, respectively. The inset in sub figures a) and b) shows the SEM images in their cross-section view. c,

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d) Low magnification TEM images of MAC and MAAP etched bundles of SiNWs, respectively. e, f) TEM images

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of isolated nanowall from MAC and MAAP etched SiNWs bundles, respectively. g, h) HRTEM images of the

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selected area in sub figures e) and f), respectively. It must be noted that a thin amorphous layer (indicated by arrows

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in white colour) with mean thickness of 0.6 nm formed on Si wall surfaces. The fast Fourier transform (FFT)

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patterns of selected areas in sub figures g) and h) show a typical Si (111) arrangement.

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Typical fast Fourier transform (FFT) patterns obtained from selected area in Figs. 2 g, h) are

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shown in the inset. The analysis of these FFT patterns yields inter-planar spacing of 3.2 ± 0.1 Å

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which corresponds to Si (111) [16]. The morphological and structural results as summarized in

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Figs. e - h) thus suggest that the MAC and MAAP etching processes do not distort the

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crystallographic orientation of the etched SiNWs. Further, the morphological studies by the SEM

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and TEM indicate that the surface of MAC etched SiNWs is smooth and free of pores, whereas,

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pores and inter-pore Si regions evolved on the Si walls in MAAP etched SiNWs. The mean

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diameter of the pores and the inter-pore Si regions estimated from the analysis of the TEM

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images gives ~ 5 nm. It is seen that the porous structure formed in the Si nanowalls in MAAP

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etched SiNWs fairly resembles to porous silicon nanostructures obtained by the anodic

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polarization etching [17, 18]. However, microstructure results discussed in details in later part of

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the manuscript reveal the exact resemblances and deviations of the present MAAP etched SiNWs

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with that reported in Ref. 17 and 18.

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Furthermore, a thin amorphous type layer with mean thickness of 0.6 nm also has been formed

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on the Si wall surfaces (indicated by white colour arrows in Figs. 2 g, h)) in both MAC and

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MAAP etched SiNWs. The electron energy loss spectra recorded close to oxygen K-edge in the

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selected area of the HRTEM images provide reliable information that oxygen species present in

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both MAC and MAAP etched SiNWs. FTIR spectroscopy in the so called ATR mode is well

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known for the identification of chemical and structural information on silicon surfaces [19]. In

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order to get further insight about the thin amorphous type layer, the ATR-FTIR studies have been

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performed on both MAC and MAAP etched SiNWs in the spectral range 600 - 4000 cm-1. Figure

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3 shows the ATR-FTIR signals of both MAC and MAAP etched SiNWs obtained from 700 to

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1800 cm-1 where the features corresponding to the surface bonding appear. It is seen that both

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MAC and MAAP etched SiNWs have similar vibrational properties with a dominant broad peak

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at 1050 cm-1 followed by a shoulder peak at 1200 cm-1 and a peak at 800 cm-1. These observed

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three peaks correspond to the characteristic vibrational modes of Si-O-Si bridge structure [12,

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19, 20]. The low intense peak appeared at 873 cm-1 is assigned to the vibrational modes of

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OnSiHx structures [21]. We infer from the electron energy loss and ATR-FTIR spectroscopy

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studies that the evolution of amorphous structure in the surfaces of SiNWs, as observed from the

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HRTEM images (c.f. Figs. 2 g, h)), is due to the existence of Si-O-Si bridge structures.

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1 2 Fig. 3: ATR-FTIR spectra of both MAC and MAAP etched SiNWs recorded in the range 700 - 1800 cm-1 where the

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features corresponding to the surface bonding appear.

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Figure 4 a) shows the room temperature PL spectrum of the MAAP etched porous SiNWs,

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compared with that of the MAC etched porous free SiNWs and the planer Si wafer. Curve fitting

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with Gaussian function reveals that the room temperature PL spectrum of the MAAP etched

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SiNWs can be de-convoluted into two Gaussian profiles with high intense peak maxima EPL1 ~

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1.71 eV and low intense peak maxima EPL2 at 2.01 eV. In contrast, the visible emission peaks

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that exist in the MAAP etched SiNWs are found to be absent in both porous-free MAC etched

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SiNWs and planar Si wafers. Sham et al., [22] have studied the origin of visible emission in

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porous silicon deduced from the synchrotron-light-induced optical luminescence and observed

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that quantum size effect originated from the Si nanostructures gives optical transition, and hence

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luminescence. As obtained from the HRTEM results (c.f. Figs. 2 f, h)), the mean dimension of

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the distributed pores and inter-pore Si regions in MAAP etched SiNWs is close to the reported

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Bohr radius value ~ 4.3 nm for free-exciton in Si crystals [3]. Since the dimension of pores and

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inter-porous Si region evolved in MAAP etched SiNWs is found to be closer to the theoretical

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quantum size limit and the PL behaviour in HF-treated (H-terminated) and as-prepared porous

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SiNWs is nearly identical (Figure not shown), it can be considered that majority of the visible PL

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emission signals in the MAAP etched porous SiNWs originate from the confinement of charge

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carriers in Si quantum structure. If it so happens, the intrinsically emitted luminescence radiation

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at 1.12 eV in bulk silicon wafer is blue shifted in MAAP etched porous SiNWs according to a

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quantum confinement model [2, 3],

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gap energy of Si nanostructures in porous silicon, dc is the diameter of the Si nanostructure, E0

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denotes the energy band gap value in bulk silicon and -1.39 is the power law exponent.

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=



+ 3.73⁄

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, where EPL denotes the band

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.

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Fig. 4: a) Representative room temperature PL spectra of MAAP etched porous SiNWs (red line), MAC etched

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SiNWs (dark yellow line) and planar Si (blue line). The fitted PL spectrum of MAAP etched porous SiNWs has two

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Gaussian profiles with peak maxima 1.71 and 2.01 eV. b) Typical room temperature first-order Raman line for

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MAAP etched porous SiNWs (red line) and planar Si (black line). The vertical dash-dot lines in sub figures a, b)

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indicate the location of PL and Raman peaks, respectively.

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Figure 4 b) shows the Raman spectrum of MAAP etched porous SiNWs and bulk Si in the range

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470 – 550 cm-1. The notable feature present in the Fig. 4 b) is a single strong first-order Raman

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scattering profile of Si [23]. It is seen that the Raman line in MAAP etched porous SiNWs is

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down shifted by 2.8 cm-1 in comparison to the Raman line position obtained for bulk Si wafer. In

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addition, the full width at half maximum (FWHM) value 9.62 cm-1 deduced from the fitting of

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the Raman spectrum of porous SiNWs using pseudo-Voigt function is ~ 2 cm-1 larger compared

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to 7.56 cm-1 obtained for bulk Silicon (c.f. Fig. 4 b)). Similar red-shift and asymmetry

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broadening in the Raman line were observed previously on porous Si [24, 25]. As observed from

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the HRTEM results, the crystallographic orientation is identical in all the tested regions in the Si

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walls, including the pores (c.f. Figs. 2f, h)). This confirms that similar to the MAC etched

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nonporous SiNWs, the entire frame of observation in the MAAP etched porous SiNWs is single

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crystalline. Moreover, no Raman line was observed in the range 470 - 490 cm-1, indicating the

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absence of amorphous Si structure in MAAP etched porous SiNWs [25]. Review of the literature

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reports indicates that the observed downward shift and asymmetric broadening in the Raman

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scattering profile (cf. Fig. 4 b)) may have different origin viz., strain in the lattice,

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inhomogeneous laser heating and electron - phonon interaction (also called Fano interaction) and

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phonon confinement in nanostructures [24, 26]. The results obtained from the analysis of FFT

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pattern from HRTEM images (cf. Fig. 2 h)) indicated that porous SiNWs are single crystalline

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without any measurable lattice strain. The low laser power (0.02 mW) used for the Raman

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measurements neglect the inhomogeneous laser heating effect. Additionally, electron - phonon

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interaction does not exist in porous SiNWs prepared from a lightly doped Si wafer [26]. Due to

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these facts, on referring to the reports of Ghosh et al., [24] and Saxena et al.,[26], it is concluded

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that the observed downward shift and asymmetric broadening in the Raman scattering profile are

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due to the dominance of phonon confinement in Si nanostructures. Moreover, the bond

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, where ω

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polarizability model for phonon confinement [27], ∆

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denotes the Raman phonon wavenumber in a nanocrystal of diameter d, ω0 denotes the optical

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phonon wavenumber at the zone centre, a is the lattice constant of crystalline Si and the phonon

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confinement parameters A (= 47.41 cm-1) and γ (=1.44), yields the d value of 3.9 nm for the

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presently observed downward Raman line shift of 2.8 cm-1 (cf. Fig. 4 b)). This further supports

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that the visible luminescence radiation as summarized in Fig. 4 a) arise due the quantum

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confinement of charge carries in MAAP etched porous SiNWs.

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Now, it is worth to discuss how the inter-pore Si quantum structures that are responsible for the

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visible luminescence emission has been evolved in MAAP etched porous SiNWs. The porous

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SiNWs by MAAP etching was obtained in two steps. In the first step, Ag deposition onto the Si

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wafer takes place by the given cathodic reduction and anodic oxidation reaction: Si + 6HF +

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AgNO3 → H2SiF6+ Ag↓+ 2H2O+NO↑. The simultaneous reduction of Ag and oxidation of

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nearby Si leads to the formation of an interconnected Ag cluster network assembly onto the Si

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wafer [28]. The second step is the etching of Ag deposited Si wafer whose formation mechanism

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is schematically described in Fig. 5. As indicated earlier, the MAAP etching involves a

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combined etching of electrolytic anodic polarization and galvanic chemical etchings. In the

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electrolytic anodic polarization etching, the external current drives charge transfer redox reaction

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between graphitic carbon and Si wafer and in the case of galvanic chemical etching, the redox

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reaction occurs between the oxidant H2O2 and Si beneath the Ag cluster deposits. The

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independent electrolytic and galvanic charge transfer reactions that occur during the MAAP

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etching are listed on the right side of Fig. 5 [29-31]. These independent reactions create distinct

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current paths I1 and I2. The current I1 generated from the electrolytic process flows from the

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graphitic carbon to the Si wafer and the current I2 produced from the galvanic process flows from

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the Ag clusters exposing to the etching solution to the Si beneath the Ag clusters. The arrows in

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the Fig. 5 indicate the direction of current flow which is opposite to their respective electron flow

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direction. It should be noted that the fundamental requirement for the etching of Si is the removal

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of bonding electron from its valance band (or equivalently injecting holes into its valance band).

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This requirement can be achieved in the macroscopic electrolytic interface by means of the

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external current source and in the microscopic galvanic chemical interface by oxidant H2O2. The

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dissolution of Si that occurs at the microscopic galvanic interface contributes for the etching of

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Si underneath the Ag clusters whereas the anodic reaction that happens at the electrolytic

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interface is the basis for the formation of pores in the Si walls. The occurrence of the repeated

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site selective Si etching underneath the Ag deposits leads to the vertical intrusion of Ag clusters

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deep into the Si wafers and the formation of resulting vertical aligned porous SiNWs.

3 4 Fig. 5: Schematic description of charge transports in MAAP etching during the formation of porous SiNWs. The

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diagram shows two independent current paths: I1 flows from graphitic carbon to Si wafer by electrolytic charge

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transfer reaction and I2 flows from Ag clusters exposing to the etching solution to the Si underneath the Ag deposits

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by a galvanic charge transfer reaction. The charge transfer redox reactions that take place during the electrolytic and

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galvanic charge transfer processes are noted on the right side of the figure.

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The combined HRTEM, PL and Raman results as summarized in Figs. 2, 4 are fairly agreeing

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that the deviation in optical properties farther away from the bulk Si arise mainly from the Si

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quantum structures. However, according to the HRTEM results (c.f. Figs. 2 g, h)), a thin

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amorphous layer has been formed on the Si wall surfaces. The ATR-FTIR studies as shown in

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Fig. 3 reveal that the thin amorphous layer is mostly the Si-O-Si bonding structure. As the Si

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walls and the oxygen coupled amorphous surface layer play a nontrivial role in the luminescence

19

properties, the interplay that occurs at their interface structure may complicate to conclude that

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the visible emission in MAAP etched porous SiNWs is solely due to the quantum confinement

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effect. It is well known that the luminescence emission peak energy corresponding to the band to

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band emission in Si quantum structures is sensitive to sample temperature, whereas that

23

corresponds to the surface chemical effects are considered to be temperature insensitive [32, 33].

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Therefore, on studying the PL properties of porous SiNWs as a function of sample temperature,

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the confirmation of quantum confinement effect and surface chemical effect on the visible

26

emission in porous SiNWs can be verified. Figure 6 a) shows the overall PL behaviour of MAAP

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etched porous SiNWs in the visible region recorded at various temperatures between 80 and 375

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K. All the PL spectra depicted in Fig. 6 a) are de-convoluted into two Gaussian peak profiles.

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Figure 6 b) shows representative fitted PL results at 80 K. The resolved luminescence peaks EPL1

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and EPL2 obtained from the fit of each PL curve in Fig. 6 a) are shown in Figs. 6 c, d),

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respectively. It should be noted that the luminescence curves in Figs. 6 c, d) are normalized to

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their respective maximum peak intensity.

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1 Fig. 6: Results of temperature dependent photoluminescence behaviour in MAAP etched porous SiNWs. a) Visible

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PL behaviour in MAAP etched porous SiNWs at temperatures between 80 and 375 K. b) A fitted PL spectrum

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obtained at 80 K. c, d) Intensity normalized PL spectra for peak energies EPL1 and EPL2, respectively with sample

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temperatures 80, 120, 160, 200, 240, 260, 280, 325 and 375 K. Note that in Figs. a, c and d), the PL spectrum shown

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in different colours represents different sample temperature.

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The two most important luminescence parameters often considered in the analysis of temperature

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dependent luminescence behaviour in quantum semiconducting structures are luminescence

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emission peak energy and luminescence intensity. Figures 7 a, b) show the luminescence peak

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energies EPL1 and EPL2 with sample temperature. It is observed that there is a discrepancy in the

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behaviour between the emission peaks EPL1 and EPL2 with temperature. The peak energy EPL1 (red

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emission) shifts towards lower energy when the sample temperature increases from 80 to 375 K,

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whereas, the position of the peak energy EPL2 (orange emission) remains almost unchanged.

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Therefore, on assuming the emission peaks EPL1 is solely due to quantum confinement effect in

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porous SiNWs, the observed variation of EPL1 with temperature is fitted by using the well known

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equation,

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to band emission in bulk semiconductors like Si [32, 34]. Eg, A, Eg,0, Ω and kB denote the energy

19

band gap, temperature independent constant that describes the strength of electron-phonon

20

interaction, energy band gap at zero Kelvin, average phonon energy and Boltzmann constant

21

respectively. The solid line in Fig. 7 a) shows the best fit of the experimental data, which yields

22

1.887 ± 0.04 eV, 0.137 ± 0.05 eV and 0.060 ± 0.005 eV for Eg, A, Ω, respectively. Similar

23

temperature dependent photoluminescence results have been reported previously on Si

24

nanocrystals in the size range 1 - 5 nm [32, 35].

26

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−

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+ 1+ which is used routinely for studying the band

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=

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8

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Fig. 7: a, b) Variation of luminescence peak energies EPL1 and EPL1 as a function of sample temperature, respectively.

28

Solid line in sub figure a) shows best fit through the data points. c) Arrhenius - type plot obtained from the estimated

29

integrated PL intensity of emission peak EPL1 with inverse sample temperature. Solid line in sub figure c) illustrates

30

best fit to the linear region. The Ea value determined from the slope of fitted straight line yields 65.3 meV.

31

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1

As indicated earlier, PL intensity is another important temperature dependent parameter

3

discussed frequently to validate the luminescence properties of Si quantum structures. The

4

variation of PL intensity in porous SiNWs with temperature is important from a practical point of

5

view since the temperature dependent luminescence properties in Si quantum structures are

6

directly related to the density of electron excitation-emission states involved in the luminescence

7

process. Figure 7 c) shows the integrated photoluminescence intensity estimated from the

8

emission peak energy EPL1 in logarithmic scale as a function of inverse sample temperature. It is

9

seen that the integrated PL intensity of EPL1 decreases with temperature and this decreasing trend

10

is in good quantitative agreement with previous experimental reports [36]. However, the

11

decreasing trend of PL intensity with temperature obtained from different experimental reports

12

including the present one is showing contrast behaviour to the theoretical predictions. Hartel et

13

al., [35] from their computer simulation of PL intensity in Si nanocrystals reported that, on

14

neglecting the contribution of temperature influenced non-radiative transitions in the calculation,

15

the PL intensity value of porous Si reached saturation at temperatures beyond 80 K. As can be

16

seen in Fig. 7 c), the variation of integrated PL intensity is in the form of activation-type

17

temperature dependence, therefore we fitted the data in the Fig. 7 c) with an Arrhenius type

18

equation , - = , .1 + / exp 3−

19

temperature dependent non-radiative luminescence process in MAAP etched porous SiNWs [36].

20

The Ea value determined from the slope of the linear region, marked by a solid line in Fig. 7 c),

21

yields 65.3 meV, which is higher than the typical value of < 20 meV reported for porous silicon

22

where photoexcitation-emission occurs in a single energy transition [36].

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56 78 to determine the activation energy for the

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2

24

Based on the higher Ea value obtained from the integrated PL intensity versus inverse

25

temperature plot in Fig. 7 c) and the red-shift of EPL1 with temperature (c. f. Fig. 7 a)), we

26

summarize that though the red emission in MAAP etched SiNWs originates from the Si quantum

27

structure, its behaviour is also sensitive to the temperature dependent atomistic changes in the

28

inter-pore Si region interfaced to the Si-O-Si bonded amorphous layer. This is quite possible

29

because, in an incompletely oxidized Si, the oxygen atoms coupled with Si plays an important

30

role in the modification of electronic properties of Si crystals.

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1

Despite the results discussed above show the evidence that the red luminescence peak at 1.71 eV

3

is dominantly due to the presence of quantum structures in MAAP etched porous SiNWs, one

4

has to provide more and detail information regarding the effect of nanostructures in porous

5

SiNWs and their dimensional dependency to the photoluminescence properties. In order to

6

provide more insights, microstructure and luminescence experiments have been carried out on

7

another batch of MAAP etched porous SiNWs and the results obtained are summarized in Fig. 8

8

and Fig. 9, respectively. Figures 8 a) and 8 b - e) show the typical low and high magnification

9

cross-section SEM images of porous SiNWs, respectively. Similar to the microstructure results

10

of Fig. 2, the cross-section SEM images of the porous SiNWs as shown in Figs. 8 a - e) illustrate

11

that pores in a larger density have been formed in SiNWs and majority of them penetrate into the

12

Si wall surfaces. The diameter of the pores measured in 250 regions was considered to obtain the

13

pore size distribution histogram. The pore size distribution histogram as depicted in the inset of

14

Figs. 8 d, e) yields an average pore diameter of 4.9 ± 0.1 nm. TEM micrograph of an isolated

15

MAAP etched porous SiNW shows the bi-continuous microstructure of pores and Si solid

16

fraction (Fig. 8 f)). The phase contrast in the high resolution TEM image depicted in Fig. 8 g)

17

confirms that the inter-pore Si region in porous SiNWs is in the form of a skeleton-like structure.

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Similar Si structures have been reported by Hochbaun et al.[37] and To et al. [38] in their

19

prepared mesoporous SiNWs.

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Fig. 8: Microstructure results of MAAP etched porous SiNWs. a) Low magnification SEM image and b – e) high

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magnification SEM images of MAAP etched porous SiNWs. It is seen from sub figures a - e) that pores are formed

24

randomly in SiNWs and most of them penetrate into Si wall surfaces. Pore size distribution histogram shown in the

25

inset in sub figures d, e) yields an average pore diameter, Sd = 4.9 ± 0.1 nm. f) TEM image of a MAAP etched

26

porous SiNWs. g) High resolution TEM image of selected area in sub figure f). The phase contrast in sub figures g)

27

shows that pores and inter-pore Si regions are interconnected in the form of skeleton-like structure. The inter-pore Si

28

region in this skeletal architecture consists of Si cylinders (indicated by dotted yellow curve) with uneven spherical

29

interconnects (indicated by dotted red curve).

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In order to discuss the correlation between quantum structures in the MAAP etched porous

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SiNWs and the luminescence peak energy, let us look intuitively the SEM and TEM micrographs

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depicted in Figs. 2 and 8. It is seen that the Si skeleton consisting of continuous cylindrical solid

2

fraction with uneven spherical interconnects is of quantum dimensions. In this perspective, the

3

visible luminescence peak at 1.71 eV emerged in the MAAP etched porous SiNWs can be

4

thought of due to quantum confinement effect in the monolithic quantum wire-like cylindrical Si

5

structure and quantum dot-like uneven spherical Si interconnects. The existence of Si quantum

6

wire and Si quantum dot structures has been observed previously by Cullis et al [18] and

7

Kanemitsu et al [17] in their respective TEM studies of porous Si prepared by anodic

8

polarization etching. We have pointed out earlier from out high resolution TEM images that

9

there is no deviation in the lattice coherency between the wire-like cylindrical and dot-like

10

uneven spherical Si structures (Fig. 2 h)). Therefore there are difficulties in correlating the

11

observed photoluminescence peak energy with quantum structures using the existing theoretical

12

models. For simplicity, let us apply the established equation for the isolated Si quantum dot and

13

the quantum wire structures for the present case. If EPL denotes the band gap energy of Si

14

quantum dot and ds is the corresponding diameter of the Si quantum dot, then denoting the room

15

temperature band gap of Si as E0 (1.12 eV), the equation relating EPL and ds for Si quantum dot is

16

given by,

17

yields 4 nm for the experimentally obtained EPL value of 1.71 eV for MAAP etched SiNWs.

18

Similarly, the band gap relation,

19

[39] yields 3 nm for the diameter of Si nanowire dc. Based on the above analysis, we are

20

summarizing that though we have correlated the structures with luminescence behaviour

21

qualitatively, it is worth to note that the estimated

22

agreeing with the structure size obtained from the phase contrast HRTEM images of Si skeleton

23

structures in MAAP etched porous SiNWs.

25 26 27 28

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+ 3.73⁄

9

.

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=

[2, 3]. The ds value calculated using the above equation =

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+ 2.4⁄

.<

9 and

for Si nanowire derived by Yan et al

values from the models are fairly

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Fig. 9: PL spectra of MAAP etched porous SiNWs with etching time. Note that the PL spectrum shown in different colours represents different etching time.

29 30

In addition, as shown in Fig. 9, photoluminescence studies have been performed on MAAP

31

etched porous SiNWs as a function of etching time in order to know the pore length dependency

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on photoluminescence properties. It is seen that as the etching time increases the luminescence

2

peak intensity increases, whereas that of peak energy starts decreasing. The increase of

3

luminescence intensity is expected because as the etching time increases, the pore length and

4

hence the length of quantum confined Si skeleton structures increases, as a result, more quantum

5

structures take part in the luminescence process, which gives rise to the increase of luminescence

6

intensity. The gradual red-shift of the luminescence peak energy with increasing time is

7

unexpected. In general, as the etching time increases, the size of Si nanostructure gradually

8

decreases due to the continuous dissolution of Si in the quantum structures which should result in

9

the gradual blue-shift of luminescence peak energy. This suggests that besides that of the

10

structure diameter as we discussed, geometry of the quantum ensemble (assembly of Si

11

interconnects, Si cylinders and pore surfaces) also strongly influence on the luminescence

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properties of MAAP etched porous SiNWs. Understanding this subject further at a deeper level

13

may open up more opportunities in the future for making fundamentally new type of functional

14

luminescence materials for optically tuned micro-electromechanical devices.

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Conclusion: In conclusion, we have demonstrated that the silicon nanowall bundles produced

18

from the lightly doped silicon wafer by MAC and MAAP etchings are single crystalline and

19

vertically aligned to the substrate surface. It is found that the MAC etched silicon nanowall

20

surfaces are smooth and free of pores, whereas the MAAP etched silicon nanowall surfaces are

21

highly porous with inter-pore Si regions of quantum dimensions. The PL studies reveal that the

22

MAAP etched porous silicon nanowalls emit red and orange luminescence signals. The

23

combined studies by HRTEM, FTIR and Raman along with the temperature dependent

24

downward shift of red PL emission peak energy (EPL1) and invariance in the orange

25

luminescence peak energy (EPL2) with temperature confirmed that the red emission originates

26

from the quantum sized inter-pore Si regions and the orange emission results from the Si-O-Si

27

bonded amorphous structure. Moreover, the variation of temperature dependent PL intensity

28

indicates that, though the visible luminescence activity in MAAP etched porous SiNWs is

29

originated from the quantum sized inter-pore Si structures, its overall functional behaviour

30

depends on the atomistic changes at the Si-(Si-O-Si) interface. So, the above results suggest that

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MAAP etching is a suitable method to fabricate light emitting lightly doped porous SiNWs for

2

optoelectronic devices and also shed light on the visible luminescence behaviours at nanoscale.

3

Acknowledgement: The authors thank the DAE, Government of India for the research support.

5

We thank S. R. Polaki and S. Amirthapandian for SEM and TEM measurements. UGC-DAE

6

CSR is also kindly acknowledged. RNV would like to thank Vinayaka Mission Research

7

Foundation, Chennai 603 104 for the support and encouragement.

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[3] [4] [5] [6]

[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

[18]

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Figures and Figure Captions

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Synthesis, microstructure and visible luminescence properties of vertically aligned lightly doped porous silicon nanowalls

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Anil K. Behera1, R. N. Viswanath1,2, C. Lakshmanan1, K. K. Madapu1, M. Kamruddin1, T. Mathews1 1 Materials Science Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603 102, Tamilnadu, India 2 Centre of Excellence for Nanotechnology Research, Aarupadai Veedu Institute of Technology, Vinayaka Mission’s Research Foundation, Chennai - 603 104, Tamilnadu, India

Fig. 1: Schematic illustration of Teflon cell assembly for a) metal assisted chemical (MAC) and b) metal assisted anodic polarization (MAAP) etching for the fabrication of SiNWs. Note that only polished Si surface has been exposed to the electrolyte. The Si wafer is housed in the cells in a stable configuration using the screw tightened Oring assembly.

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Fig. 2: SEM and TEM results of MAC and MAAP etched SiNWs. a, b) Top view SEM images of MAC and MAAP etched SiNWs, respectively. The inset in sub figures a) and b) shows the SEM images in their cross-section view. c, d) Low magnification TEM images of MAC and MAAP etched bundles of SiNWs, respectively. e, f) TEM images of isolated nanowall from MAC and MAAP etched SiNWs bundles, respectively. g, h) HRTEM images of the selected area in sub figures e) and f), respectively. It must be noted that a thin amorphous layer (indicated by arrows

2

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in white colour) with mean thickness of 0.6 nm formed on Si wall surfaces. The fast Fourier transform (FFT)

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patterns of selected areas in sub figures g) and h) show a typical Si (111) arrangement.

Fig. 3: ATR-FTIR spectra of both MAC and MAAP etched SiNWs recorded in the range 700 - 1800 cm-1 where the

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features corresponding to the surface bonding appear.

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Fig. 4: a) Representative room temperature PL spectra of MAAP etched porous SiNWs (red line), MAC etched SiNWs (dark yellow line) and planar Si (blue line). The fitted PL spectrum of MAAP etched porous SiNWs has two

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Gaussian profiles with peak maxima 1.71 and 2.01 eV. b) Typical room temperature first-order Raman line for MAAP etched porous SiNWs (red line) and planar Si (black line). The vertical dash-dot lines in sub figures a, b)

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indicate the location of PL and Raman peaks, respectively.

.

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Fig. 5: Schematic description of charge transports in MAAP etching during the formation of porous SiNWs. The diagram shows two independent current paths: I1 flows from graphitic carbon to Si wafer by electrolytic charge

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transfer reaction and I2 flows from Ag clusters exposing to the etching solution to the Si underneath the Ag deposits by a galvanic charge transfer reaction. The charge transfer redox reactions that take place during the electrolytic and

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galvanic charge transfer processes are noted on the right side of the figure.

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Fig. 6: Results of temperature dependent photoluminescence behaviour in MAAP etched porous SiNWs. a) Visible

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PL behaviour in MAAP etched porous SiNWs at temperatures between 80 and 375 K. b) A fitted PL spectrum obtained at 80 K. c, d) Intensity normalized PL spectra for peak energies EPL1 and EPL2, respectively with sample temperatures 80, 120, 160, 200, 240, 260, 280, 325 and 375 K. Note that in Figs. a, c and d), the PL spectrum shown

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Fig. 7: a, b) Variation of luminescence peak energies EPL1 and EPL1 as a function of sample temperature, respectively. Solid line in sub figure a) shows best fit through the data points. c) Arrhenius - type plot obtained from the estimated integrated PL intensity of emission peak EPL1 with inverse sample temperature. Solid line in sub figure c) illustrates best fit to the linear region. The Ea value determined from the slope of fitted straight line yields 65.3 meV.

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Fig. 8: Microstructure results of MAAP etched porous SiNWs. a) Low magnification SEM image and b – e) high magnification SEM images of MAAP etched porous SiNWs. It is seen from sub figures a - e) that pores are formed randomly in SiNWs and most of them penetrate into Si wall surfaces. Pore size distribution histogram shown in the inset in sub figures d, e) yields an average pore diameter, Sd = 4.9 ± 0.1 nm. f) TEM image of a MAAP etched porous SiNWs. g) High resolution TEM image of selected area in sub figure f). The phase contrast in sub figures g) shows that pores and inter-pore Si regions are interconnected in the form of skeleton-like structure. The inter-pore Si region in this skeletal architecture consists of Si cylinders (indicated by dotted yellow curve) with uneven spherical interconnects (indicated by dotted red curve).

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Fig. 9: PL spectra of MAAP etched porous SiNWs with etching time. Note that the PL spectrum shown in different

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colours represents different etching time.

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Highlights Synthesis, microstructure and visible luminescence properties of vertically aligned lightly doped porous silicon nanowalls

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Anil K. Behera1, R. N. Viswanath1,2, C. Lakshmanan1, K. K. Madapu1, M. Kamruddin1, T. Mathews1 1 Materials Science Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603 102, Tamil Nadu, India 2 Centre of Excellence for Nanotechnology Research, Aarupadai Veedu Institute of Technology, Vinayaka Mission’s Research Foundation, Chennai - 603 104, TamilNadu, India

Porous silicon nanowalls from lightly doped silicon wafer have been fabricated.

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Lightly doped porous silicon nanowalls emit red and orange luminescence radiation. The interpore silicon region in porous silicon nanowall is in the form of skeleton of quantum dimensions. Temperature dependence photoluminescence confirms that red emission is due to quantum confinement in skeleton structures.

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A qualitative correlation between photoluminescence and quantum structures is obtained.