Black Silicon

Processing, Properties, and Applications

Nuggehalli M. Ravindra, Sita Rajyalaxmi Marthi, Suramya Sekhri

• Publisher: Momentum Press
• Year: 2016
• Print ISBN: 9781606507650
• eBook ISBN: 9781606507667
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The ability to tailor the properties of silicon, by controlling its surface morphology in the form of black silicon, is anticipated to revolutionize silicon photonics. Black Silicon (BSi), one of the approaches to counter the inherent disadvantages of Si, is a surface modification of crystalline Si (c-Si). This book presents a discussion on manufacturing BSi by various methods for morphing the surface of the material, which are economical and easily applicable for mass production. Comparisons of the surfaces of BSi fabricated by various techniques are presented and thermal, electrical, electronic, and optical properties of black silicon are described. Some major applications of BSi have been given and possible future directions in the research and development of BSi and its applications are identified.

CHAPTER 1

HISTORICAL PERSPECTIVES OF SILICON

1.1 INTRODUCTION

Silicon (Si) has become a household word since the Santa Clara Valley was renamed as the Silicon Valley. The “silicon” part of the name refers to the high concentration of companies involved in the design, processing, and manufacturing of semiconductors and also the computer-related industries that are concentrated in the area, with silicon being a major component in both the industries. Silicon continues to be the dominant material used in the semiconductor industry. It is one of the most useful elements to mankind.

Silicon is the eighth most common element in the universe by mass and the second most abundant element in earth’s crust. It very rarely occurs as a pure free element in nature. It is most widely distributed in dust, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. Over 90 percent of the earth’s crust is composed of silicate minerals. They are also responsible for the warm, white beaches in the form of silica, an oxide of silicon, which is the most common form of sand. Silicon is the main ingredient in very low-technology creations, including bricks and ceramics. The element, when ultrapure, is a solid with a blue-grey metallic sheen.

Two forms of silicon exist at room temperature: amorphous and crystalline. Amorphous silicon appears as a brown powder while crystalline silicon has a metallic luster and a grayish color. Single crystals of crystalline silicon can be grown by a process known as the Czochralski (Cz) process. Amorphous silicon was first prepared by J.J. Berzelius in 1824 by reducing potassium fluorosilicate (K₂SiF₆) with molten potassium. The crystalline form of silicon was first prepared by Deville in 1854. It was T. Thomson who named the element in 1831. It can be prepared commercially (96 to 99 percent pure) by heating silica (SiO₂) and pure coke in an electric arc furnace. Very pure silicon, which is used for semiconductor applications, can be obtained from silicon tetrachloride by reduction action with pure zinc or magnesium. It is melted and grown into cylindrical single crystals, and then purified by zone refining. Other methods include reduction of sodium fluorosilicate (Na₂SiF₆) by sodium metal for producing solar cells. The crystalline product can be doped with boron, gallium, arsenic, and so on, for use in transistors, rectifiers, and other solid-state devices. The use of silicon devices in real-time radiation monitoring systems can be attributed mainly to their high sensitivity and also to well-developed manufacturing techniques [1].

Silicon forms other useful compounds. Silicon carbide (SiC) is nearly as hard as diamond and is used as an abrasive. It also exists as quartz, rock crystal, amethyst, agate, flint, jasper, and opal. Silicon dioxide is extensively used in the manufacture of glass and bricks. Silica gel, a colloidal form of silicon dioxide, easily absorbs moisture and is used as a desiccant. Sodium silicate (Na₂SiO₃), also known as water glass, is used in the production of soaps, adhesives, and as an egg preservative. Silicon tetrachloride (SiCl₄) is used to create smoke screens. Silicon is also an important ingredient in silicone, a class of material that is used in applications such as lubricants, polishing agents, electrical insulators, and medical implants.

1.2 PHYSICAL PROPERTIES

Silicon is a solid at room temperature, with high melting and boiling points of 1,414°C and 3,265°C, respectively. It has a higher density in the liquid state than in the solid state. Unlike most materials, Si does not contract when it freezes, but expands. With a relatively high thermal conductivity of 149 W m^-1 K^-1, silicon conducts heat well. Silicon has a hardness of about 7, compared to that of diamond of 10, on the Mohs scale of mineral hardness.

In its crystalline form, pure silicon has a gray color and a metallic luster. Silicon is very brittle, and prone to chipping. Silicon crystallizes in a diamond cubic crystal structure, with a lattice spacing of 5.430710 Å.

The outer electron orbital of silicon, like that of carbon, has four valence electrons. The 1s, 2s, 2p, and 3s subshells are completely filled while the 3p subshell contains two electrons out of a possible six.

Table 1.1. Summary of physical properties of silicon [2, 3]

State	Solid
Color	Gray
Melting point	1,414°C
Boiling point	3,265°C
Thermal conductivity	149 W m^-1 K^-1
Hardness	7 Mohs
Lattice spacing	5.430710 A
Atomic number	14
Electronic configuration	1S2 2S2 2P6 3S2 3P2
Temperature coefficient of thermal expansion (at 20°C)	–70 × 10^−3 (°C)^−1
Resistivity	0.1–60 ohm m
Density (at 20°C)	2.33 × 10^3 kg m^−3
Specific heat capacity	712 J kg^−1 K^−1
Debye temperature	645 K
Young’s modulus	130–188 GPa
Shear modulus	51–80 GPa
Bulk modulus	97.6 GPa
Poissson’s ratio	0.064–0.28

Silicon has a negative temperature coefficient of resistance, since the number of free charge carriers increases with temperature. The resistivity of semiconductors depends strongly on the presence of impurities in the material, a fact that makes them useful in solid state electronics. The electrical resistance of single crystal silicon significantly changes under the application of mechanical stress due to the piezoresistive effect.

A summary of the physical properties of Si is presented in Table 1.1.

1.3 CHEMICAL PROPERTIES

Silicon is a metalloid, with four valence electrons. Like carbon, its four bonding electrons give it opportunity to combine with many other elements or compounds to form a wide range of compounds. Tetravalent silicon is relatively inert. Water, steam, and most acids have very little effect...

Table of contents

• Cover
• Title
• Copyright
• List of Figures
• List of Tables
• 1. Historical Perspectives of Silicon
• 2. Silicon Types and Physical Properties
• 3. Black Silicon—Introduction
• 4. Processing Methods
• 5. Thermal Properties
• 6. Electronic Properties
• 7. Optical Properties
• 8. Electrical Properties
• 9. Applications, Patents, and Future Directions
• Abbreviations
• References
• Index
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