Page 264 - Materials Chemistry, Second Edition

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4.2. Silicon-Based Applications

4.2. SILICON-BASED APPLICATIONS

Silicon is the second most abundant element in the earth’s crust, next to oxygen. Due
to the stability of silicon oxide compounds, elemental silicon does not occur in its
free state in nature, but occurs as oxide (e.g., sand, quartz, amethyst, flint, opal, etc.)
and silicate (e.g., granite, asbestos, feldspar, clay, mica, etc.) minerals. The wide-
spread availability of silicon-containing minerals is responsible for the ubiquitous
use of pottery, bricks, glass, and cement since the days of the earliest civilizations.
More recently, the applications for silicon now include electronics. A silicon-
based device is found in almost every consumer product available in our world
today. Even refrigerators now have extensive microprocessor controls, some fitted
with television screens! The popularity of flat-panel televisions has also employed
silicon-based technology, especially for thin-film transistor liquid crystal displays
(TFT LCDs). In addition to electronics, as the world looks for alternative sources of
energy due to dwindling petroleum reserves, silicon-based photovoltaic devices will
represent an increasingly important application for our society.

4.2.1. Silicon Wafer Production

The silicon employed for microelectronic and photovoltaic applications must first go
through extensive processing to ensure that the material is of utmost purity. This
section will describe these steps, with a discussion of perhaps the most intriguing
conversion in the realm of materials science: the synthesis of high-purity polished
silicon wafers from a naturally occurring form of silicon – sand.
Sea sand is primarily comprised of silicon dioxide (silica), which may be con-
verted to elemental silicon (96–99% purity) through reaction with carbon sources
such as charcoal and coal (Eq. 3). Use of a slight excess of SiO 2 prevents silicon
carbide (SiC) from forming, which is a stable product at such a high reaction
temperature. Scrap iron is often present during this transformation in order to
yield silicon-doped steel as a useful by-product.
The purity of silicon in this first step is only ca. 98%, and is referred to as
metallurgical grade silicon (MG-Si). In order for the silicon to be used for electron-
ics applications, additional steps are necessary to decrease the number of impurities.
Reaction of MG-Si with hydrogen chloride gas at a moderate temperature converts
the silicon to trichlorosilane gas (Eq. 4). When SiHCl 3 is heated to a temperature of

ca. 1,150 C, it decomposes into high-purity silicon and gaseous by-products (Eq. 5).
This reaction is typically performed in a bell-shaped Siemens-type reactor, where Si
is deposited onto heated U-rod silicon filaments. Excess hydrogen gas is also fed into
the reactor, which prevents homogeneous nucleation of Si dust within the reactor. [4]

2000 2500 c
ð3Þ SiO 2ðsÞ þ 2C ðsÞ ! Si ðsÞ þ 2CO ðgÞ
300 c
ð4Þ Si ðsÞ þ 3 HCl ðgÞ ! SiHCl 3ðgÞ þ H 2ðgÞ