**2. Properties of porous silicon**

Porous silicon structures, like other porous materials, are classified by their dominant pore dimensions. Structures with pore dimensions below 2 nm and above 50 nm are called microporous

> Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

and macroporous silicon, respectively; those lie between are called mesoporous silicon. Due to the extremely rich details with respect to the range of variations in pore size, shape, orientation, branching, interconnection, and distribution, morphology is the least quantifiable aspect of this material. **Figure 1** schematically demonstrates the four different morphological aspects of porous silicon and their variations, and **Figure 2** shows cross-sectional SEM micrographs of different porous silicon structures.

Various morphologies and different pore dimensions give porous silicon extremely diverse structural, mechanical, optical, electrical, thermal, emissive, physiochemical, and biochemical properties. **Table 1** compares the properties of mesoporous silicon with those of bulk silicon. As the structure and surface chemistry of porous silicon can be precisely controlled during properly chosen fabrication process and appropriate post-fabrication treatment, the material's properties can be tuned according to the desired application. Tuning of porous silicon properties can be performed by manipulating its structural parameters, altering its surface chemistry, or impregnating other materials [4].

**Figure 2.** Cross-sectional SEM micrographs of various porous silicon structures formed by anodic etching [6].

Structural Porosity — 20–95%

Mechanical Young's modulus 160 GPa 1–100 GPa

Optical Bandgap 1.1 eV 1.1–3.2 eV

**Property Bulk silicon Mesoporous silicon**

/g

Porous Silicon

5

http://dx.doi.org/10.5772/intechopen.72910

0.05–1 GPa (composite)

Brown-yellow (particle)

Density 2.33 g/cm3 0.12–1.9 g/cm3 Pore size (diameter) — 2–50 nm Surface area — 100–800 m2

Lattice structure Diamond Diamond

Yield strength 7 GPa — Fracture toughness 0.6 MPam½ —

Infrared refractive index 3.5 1.1–3.0

Reflectivity (500–1000 nm) 10–35% 0.1–10%

Color Gray All colors (layer)

Hardness 11.5 GPa 0.2–10 GPa (layer)

Figure 1. Morphological characteristics of porous silicon; Orientation: (a) aligned to <100> direction and source of holes, (b) roughly aligned to source of holes, (c) partially aligned to <100> direction and source of holes, (d) aligned only to <100> direction; branching: (a) smooth pore walls, (b) branches shorter than diameter, (c) second level branches only, (d) dendritic branches, (e) main pores with second and third level branches, (f) dense, random and short branches; fill of macropores: (a) unfilled, (b) partially filled with microporous Si, (c) fully filled with microporous Si; depth variation of the porous layer: (a) single layer of microporous Si, (b) single layer of macroporous Si with smaller pores near the surface, (c) a layer of microporous Si on top of macroporous Si (Macropores may be filled by microporous Si.) [5].

and macroporous silicon, respectively; those lie between are called mesoporous silicon. Due to the extremely rich details with respect to the range of variations in pore size, shape, orientation, branching, interconnection, and distribution, morphology is the least quantifiable aspect of this material. **Figure 1** schematically demonstrates the four different morphological aspects of porous silicon and their variations, and **Figure 2** shows cross-sectional SEM micrographs of different

Various morphologies and different pore dimensions give porous silicon extremely diverse structural, mechanical, optical, electrical, thermal, emissive, physiochemical, and biochemical properties. **Table 1** compares the properties of mesoporous silicon with those of bulk silicon. As the structure and surface chemistry of porous silicon can be precisely controlled during properly chosen fabrication process and appropriate post-fabrication treatment, the material's properties can be tuned according to the desired application. Tuning of porous silicon properties can be performed by manipulating its structural parameters, altering its surface

Figure 1. Morphological characteristics of porous silicon; Orientation: (a) aligned to <100> direction and source of holes, (b) roughly aligned to source of holes, (c) partially aligned to <100> direction and source of holes, (d) aligned only to <100> direction; branching: (a) smooth pore walls, (b) branches shorter than diameter, (c) second level branches only, (d) dendritic branches, (e) main pores with second and third level branches, (f) dense, random and short branches; fill of macropores: (a) unfilled, (b) partially filled with microporous Si, (c) fully filled with microporous Si; depth variation of the porous layer: (a) single layer of microporous Si, (b) single layer of macroporous Si with smaller pores near the surface, (c) a layer of microporous Si on top of macroporous Si (Macropores may be filled by microporous Si.) [5].

porous silicon structures.

4 Porosity - Process, Technologies and Applications

chemistry, or impregnating other materials [4].

**Figure 2.** Cross-sectional SEM micrographs of various porous silicon structures formed by anodic etching [6].



cell as the anode, a platinum electrode is utilized as the cathode, and hydrofluoric acid as the electrolyte. Passing electric current through the silicon wafer leads to dissolution of silicon atoms and removal of surface roughness if a critical current density (JPSL) is exceeded. In 1956, something went wrong during an electropolishing process at Bell Labs, and the current in the cell reduced leaving a matt black, brown, or red layer on the surface of the wafer [7]. For more than a decade, it was believed that the matt dark layer formed on the silicon surface was a sub-

was a dissolution/precipitation product resulted from a two-step disproportionation reaction. Finally, in 1969, it was discovered that the layer indeed has a porous structure formed by dis-

Anodic etching, which is also called electrochemical etching, has been the most common method for the fabrication of porous silicon over the last 60 years. During these years, three electrochemical cell configurations have been utilized for the formation of porous silicon: lateral cell, single cell, and double cell. Lateral cell, which is the simplest electrochemical cell used for anodic etching of silicon, is shown in **Figure 3(a)**. Silicon wafer about to be etched serves as the anode, platinum or any other conducting material resistant to hydrofluoric acid, like graphite, serves as the cathode electrode, and the cell body is made of acid-resistant polymers like PTFE. As the wafer is soaked in HF, any silicon surface that is exposed to the electro-

lyte is porosified as long as the current density remains below the critical value (J < J

to nonuniform current density and therefore nonuniform porosity and thickness [8].

main advantages of the lateral cell are its simplicity and ability to anodize silicon-on-insulator (SOI) wafers. Its drawback is the nonuniformity in both porosity and thickness of the resulting layer. This inhomogeneity is due to a lateral potential drop across the wafer which leads

The second configuration, single cell, shown in **Figure 3(b)**, is the most common electrochemical cell used for porosification of silicon wafers. In order to provide uniform current density inside the silicon wafer, a back-side contact is used for the anode and the wafer is sealed so that only its front-side could be exposed to the electrolyte. Using this single cell configuration, acceptable porosity and thickness uniformity can be achieved for low resistivity silicon wafers. However, high resistivity wafers need high dose B or P ion implantation and subsequent annealing on their back-side to provide appropriate electrical contact to the external circuit. This implantation and subsequent annealing steps might be even followed by deposition of a thin layer of metal. Single cell configuration provides simultaneous control over porosity and thickness of the porous silicon film. Moreover, illumination which is necessary for n-type silicon wafers, can be easily performed in this cell. Using chemical pumps to circulate the electrolyte further improves the uniformity and minimizes the attachment of hydrogen bubbles

The last configuration, double cell, is designed to optimize the uniformity of porous silicon layer. It is composed of two half-cells separated by the silicon wafer about to be etched as illustrated in **Figure 3(c)**. Large platinum electrodes which are immersed in both half-cells serve as anode and cathode. The electric current flows from one half-cell to the other through the wafer. Hence, the front-side and back-side of the wafer act as local anode and local cathode. Chemical pumps are used to circulate the electrolyte between the half-cells to prevent any decrease in the local concentration of the electrolyte and remove the hydrogen bubbles. Here, electrolytic contact to the wafer reduces the nonuniformities associated with the back-side metal contact in

solution of silicon atoms in an electrochemical etching process [5].

grown during the anodic dissolution. Later, it was proposed that the dark film

PSL). The

Porous Silicon

7

http://dx.doi.org/10.5772/intechopen.72910

fluoride (SiF<sup>2</sup>

)*x*

to the silicon surface [9].

**Table 1.** Tunable properties of mesoporous silicon in comparison with those of bulk silicon [4].
