Amorphous silicon

  
Amorphous silicon (a-Si) is the non-crystalline form of silicon
Aluminum - Silicon - Phosphorus
C
Si
Ge  
 
 
Full table

General
Name, Symbol, Number Silicon, Si, 14
Series metalloid
Group, Period, Block 14 (IVA), 3, p
Density, Hardness 2330 kg/m3, 6.5
Appearance dark grey, bluish tinge
Atomic properties
Atomic weight 28.0855 amu

..... Click the link for more information. . Silicon is normally tetrahedrally

A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral," and is one of the Platonic solids.

The area A and the volume V of a regular tetrahedron of edge length a are:    A=\\sqrt a^2    V= \\sqrt a^3
..... Click the link for more information.  bonded to four neighboring silicon atoms. This is also the case in amorphous silicon, however, it does not form a continuous crystalline lattice as in crystalline silicon. Some atoms may actually have dangling bonds, which occur when it does not bond to four neighboring atoms. Since not all the atoms are four-fold coordinated, amorphous silicon is said to be under-coordinated. These dangling bonds are defects in the continuous random network, which can be passivated by introducing hydrogen
Hydrogen - Helium
 
H
Li  
 
 

Full table

General
Name, Symbol, Number Hydrogen, H, 1
Chemical series nonmetals
Group, Period, Block 1 (IA), 1 , s
Density, Hardness 0.0899 kg/m3, NA
Appearance colorless

..... Click the link for more information.  into the silicon. It then becomes hydrogenated amorphous silicon.

Applications

One of the main advantages of amorphous silicon over crystalline silicon is that it is much more uniform over large areas. Since amorphous silicon is full of defects naturally, any other defects, such as impurities, do not affect the overall characteristics of the material too drastically. Also, just the fact that it can be deposited over large areas using PECVD Chemical Vapor Deposition (CVD) is a chemical process for depositing thin films of various materials. In a typical CVD process the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile byproducts are also produced, which are removed by gas flow through the reaction chamber.

CVD is widely used in the semiconductor industry, as part of the semiconductor device fabrication process, to deposit various films including: polycrystalline, amorphous, and epitaxial silicon, SiO2, silicon germanium, tungsten, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics.
..... Click the link for more information.  in the first place gives it a huge advantage over crystalline silicon. Amorphous silicon is used as the active layer in thin-film transistors

A thin film transistor (TFT) is made by depositing thin films for the metallic contacts, semiconductor active layer, and dielectric layer. Their distinctive utilitarian property is transparency.

Its main application is in design of flat-panel displays, a kind of LCDs (liquid crystal displays). TFTs are the better variant of the LCD technology and more flexible one, since with them each pixel is controlled individually. This is achieved by means of to four transistors. In this kind of techology is provides today the highest resolution of LCDs. Due to its design TFTs are often called Active matrix LCDs.
..... Click the link for more information.  (TFTs) which are most widely used in large-area electronics applications, mainly for liquid-crystal displays
Reflective twisted nematic liquid crystal display

  1. Vertical filter film to polarize the light as it enters.
  2. Glass substrate with ITO electrodes. The shapes of these electrodes will determine the dark shapes that will appear when the LCD is turned on. Vertical ridges are etched on the surface so the liquid crystals are in line with the polarized light.
  3. Twisted nematic liquid crystals.

..... Click the link for more information.  (LCDs). Large-area solar cells

A solar cell is a device that uses the photoelectric effect to generate electricity from light. Solar cells are used to power many kinds of equipment, including satellites, calculators, remote radiotelephones, and advertising signs. They are generally linked into solar panels.

Types

The simplest type of solar cell is a silicon diode, but research is continuing into more exotic materials (see below) with greater efficiencies. Modern solar cells are encapsulated in glass-fronted plastic sheets. They have design lifetimes that exceed forty years. Sunlight provides about 1.36 kilowatts per square meter, and most solar cells are between 8 and 12 percent efficient. In desert areas, they can operate for an average of 6 hours per day when mounted in nonrotating brackets.
..... Click the link for more information.  are a new area for amorphous silicon, however, the small solar cells used in pocket calculators have been made with a-Si for many years. a-Si can also be deposited at very low temperatures, as low as 75 degrees Celsius, which allows for deposition on not only glass, but plastic as well. Amorphous silicon is receiving much more attention at the present time because of the potential for roll-to-roll processing Roll-to-roll processing, also known as 'web' processing, is the process of creating electronic devices on a roll of flexible plastic or metal foil. Large circuits made with thin-film transistors and other devices can be easily patterned onto these large substrates, which can be up to a few metres wide and 50 km long. Some of the devices can be patterned directly, much like an ink jet printer deposits ink. For most semiconductors, however, the devices must be patterened using photolithography techniques.
..... Click the link for more information. , whereby circuits are literally printed onto long sheets of plastic or metal foils. This processing technique is expected to be much cheaper than modern crystalline semiconductor manufacturing.

Crystalline silicon generally has better electrical properties than amorphous silicon, but in recent years researchers in the field have been able to close the gap somewhat.

See also

  • Nanocrystalline silicon Nanocrystalline silicon (nc-Si) is similar to amorphous silicon (a-Si), in that it has an amorphous phase. Where they differ, however, is that nc-Si has small grains of crystalline silicon within the amorphous phase. This is in contrast to polycrystalline silicon (poly-Si) which consists solely of crystalline silicon grains, separated by grain boundaries. nc-Si is sometimes also known as microcrystalline silicon (uc-Si). The difference comes solely from the grain size of the cystalline grains. Most materials with grains in the micrometre range are actually fine-grained polysilicon, so nanocrystalline silicon is a better term.
    ..... Click the link for more information.
  • Polycrystalline silicon Polycrystalline silicon or polysilicon or poly-Si has long been used as the conducting gate material in MOSFET and CMOS processing technologies. For these technologies it is deposited using LPCVD reactors at high temperatures and is usually heavily n or p-doped.

    More recently, intrinsic and doped polysilicon is being used in large-area electronics as the active and/or doped
    ..... Click the link for more information.

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