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easyshow
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| 作者：easyshow 发表时间：
2006/2/24 9:37:48 查看：
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Amorphous-Si Nanocrystalline-Si Polycrystalline-Si
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来源 http://www.bambooweb.com/
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.
nc-Si has many useful advantages over a-Si, one being that if grown properly it can have a higher mobility, due to the presence of the silicon crystallites. It also shows increased absorption in the red and infrared wavelengths, which make it an important material for use in a-Si solar cells. One of the most important advantages of nanocrystalline silicon, however, is that it has increased stability over a-Si, one of the reasons being because of its lower hydrogen concentration. Although it currently cannot attain the mobility that poly-Si can, it has the advantage over poly-Si that it is easier to fabricate, as it can be deposited using conventional low temperature a-Si deposition techniques, such as PECVD, as opposed to laser annealing or high temperature processes, in the case of poly-Si.
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Amorphous silicon (a-Si) is the non-crystalline form of silicon. Silicon is normally tetrahedrally 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 into the silicon. It then becomes hydrogenated amorphous silicon.
[Top]ApplicationsOne 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 in the first place gives it a huge advantage over crystalline silicon. Amorphous silicon is used as the active layer in thin-film transistors (TFTs) which are most widely used in large-area electronics applications, mainly for liquid-crystal displays (LCDs). Large-area solar cells 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, 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.
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Polycrystalline silicon or polysilicon or poly-Si is a material consisting of multiple small silicon crystals, and 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 layers in thin-film transistors. Although it can be deposited by low-pressure chemical-vapour deposition (LPCVD), plasma-enhanced chemical vapour deposition (PECVD), or solid-phase crystallization (SPC) of amorphous silicon in certain processing regimes, these processes still require relatively high temperatures of at least 300°C. These temperatures make deposition of polysilicon possible for glass substrates, but not for plastic substrates. Instead, a relatively new technique called laser crystallization can be used to crystallize a precursor amorphous silicon (a-Si) material on a plastic substrate without melting or damaging the plastic. Short, high-intensity ultraviolet laser pulses are used to heat the deposited a-Si material to above the melting point of silicon, without melting the entire substrate. The molten silicon will then crystallize as it cools. By precisely controlling the temperature gradients, researchers have been able to grow very large grains, of up to hundreds of micrometers in size in the extreme case, although grain sizes of 10 [nanometer]s to 1 micrometer are also common. In order to create devices on polysilicon over large-areas however, a crystal grain size smaller than the device feature size is needed for homogeneity of the devices.
The main advantage of polysilicon over a-Si is that the mobility can be orders of magnitude larger and the material also shows greater stability under electric field and light-induced stress. This allows more complex, high-speed circuity to be created on the glass substrate along with the a-Si devices, which are still needed for their low-leakage characteristics. When polysilicon and a-Si devices are used in the same process this is called hybrid processing. A complete polysilicon active layer process is also used in some cases where a small pixel size is required, such as in projection displays.
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easyshow于2006/2/24 9:41:00经过修改.
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回复者：easyshow 回复时间：2006/2/24 10:15:17 [第1评]
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An exciton is a bound state of an electron and a hole in an insulator (or semiconductor), or in other words, a Coulomb correlated electron/hole pair. It is an elementary excitation of a solid.
A vivid picture of exciton formation is as follows: a photon enters a semiconductor, exciting an electron from the valence band into the conduction band. The missing electron in the valence band leaves a hole behind, of opposite electric charge, to which it is attracted by the Coulomb force. The exciton results from the binding of the electron with its hole; as a result, the exciton has slightly less energy than the unbound electron and hole. The wavefunction of the bound state is hydrogenic (an "exotic atom" state akin to that of a hydrogen atom). However, the binding energy is much smaller and the size much bigger than a hydrogen atom because of the effects of screening and the effective mass of the constituents in the material.
Excitons can be treated in two limiting cases, which depend on the properties of the material in question. In semiconductors, the dielectric constant is generally large, and as a result, screening tends to reduce the Coulomb interaction between electrons and holes. The result is a Mott-Wannier exciton, which has a radius much larger than the lattice spacing. As a result, the effect of the lattice potential can be incorporated into the effective masses of the electron and hole, and because of the lower masses and the screened Coulomb interaction, the binding energy is usually much less than a hydrogen atom, typically on the order of 0.1 eV. This type of exciton was named for Sir Nevill Francis Mott and Gregory Wannier. In insulators, excitons tend to be much smaller, of the same order as the unit cell, so the electron and hole sit on the same site. This is a Frenkel exciton, named after J. Frenkel..
The probability of the electron falling into (annihilating with) the hole is limited by the difficulty of losing the excess energy, and as a result excitons can have a relatively long lifetime. (Lifetimes of up to several milliseconds have been observed in copper (I) oxide) Another limiting factor in the recombination probability is the spatial overlap of the electron and hole wavefunctions (roughly the probability for the electron to run into the hole). This overlap is smaller for lighter electrons and holes and for highly excited hydrogenic states.
The existence of exciton states may be inferred from the absorption of light associated with their excitation. Typically, excitons are observed just below the band gap. Alternatively, an exciton may be thought of as an excited state of an atom or ion, the excitation wandering from one cell of the lattice to another.
An exciton can bind with other excitons to form a biexciton, analogous to a hydrogen molecule. If a large density of excitons is created in a material, they can interact with one another to form an electron-hole liquid.
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