Polycrystalline or polysilicon silicon is a material made of misaligned (polycrystalline) silicon glass. It occupies an intermediate position between amorphous silicon, in which there is no long-range order, and monocrystalline silicon.
This type of material is very important for the construction of photovoltaic panels and solar energy in general. The combination of silicon, together with solar radiation, makes it possible to take advantage of the photovoltaic effect and generate electricity.
This material has discrete metallic characteristics if strongly n-type doped. It often replaces aluminum for the production of metal parts within semiconductor electronic devices due to the improved mechanical strength of the integrated circuit production process. For example, the gate electrode of MOSFET transistors is often made of polysilicon. It is also used for the realization of capacitors in an integrated environment: with the polysilicon metal plates are manufactured, while with silicon oxide the dielectric interposed between the plates is made. However, the capacities of these capacitors are very low, for example, with a technology of 0.35μ, capacities of only 0.7 f F / μ m².
For electronic applications, polycrystalline silicon can be obtained with less sophisticated and less expensive techniques than those required for silicon deposition, such as chemical vapor deposition (CVD). Polycrystalline silicon can also be obtained during silicon manufacturing processes, for example, with the Siemens process. Polycrystalline silicon has an impurity level of 1 part per billion or less.
Polycrystalline Photovoltaic Panels
Polycrystalline silicon is also used in particular applications, such as photovoltaic panels. The polycrystalline silicon cells have an efficiency that varies from 12 to 21%. They are manufactured by recycling discarded electronic components, that is, the so-called "silicon remnants", which are remelted to obtain a compact crystalline composition.
This silicon waste melts into a crucible to create a homogeneous compound that is then cooled in such a way that it generates a crystallization that develops vertically. Then you get a bread of approximately 150-200 kg which is then cut vertically into parallelepiped ingots. With another cut, this time horizontal, cuts of a thickness similar to the single glass wafers are obtained. Also in this case, the wafers are cleaned with a soda connection, and then they are doped with phosphorus for the realization of the PN junctions,
Appearance and quality of polycrystalline silicon
Most of the polycrystalline silicon in the world is produced in the form of gray cylindrical rods with a rough dendritic surface. The rods themselves are not always sold. In general, the rods are divided into fragments ("pieces"), which are packed in clean polyethylene bags (5-10 kg), measured. The chipped rods have a shell-like shape, similar to the twists of amorphous materials. The cutting (polished section) of a polysilicon rod is usually studied by controlling the quality of the silicon produced and analyzing the course of the technological process.
In the center of the bar there is a "seed" of monkey or polysilicon. Previously, the seeds were obtained by pulling the electron quality polysilicon atmosphere (so-called oxygen bars). With the development of wire and tape cutting technologies, steel crystals were obtained by longitudinal cutting of ingots of mono and polysilicon bars in square rods (5 × 5, 7 × 7, 10 × 10 mm, etc.) .
The purity and, consequently, the electrical resistivity of the seed have a decisive influence on the purity of the final polycrystalline bar. This is due to the fact that the hydrogen reduction process of the silanes is carried out at temperatures of 900-1100 ° C for a long period of time, which leads to the active diffusion of impurities from the seed crystal to the material deposited in the seed. On the other hand, the decrease in the content of impurities and, consequently,
From the seed perpendicular to the generatrix, compact crystallites sprout in the form of short needles, with a cross section of less than 1 mm. With a high sedimentation rate, polysilicon grains often begin to grow dendritically (like "popcorn"), during an emergency process, the dendrites can even crust flakes. The quality and purity of this polysilicon is usually lower.
A small part of the polycrystalline silicon is produced from monosilane in a fluidized (boiling) layer in the form of dark gray granules with a diameter of 0.1 to 8 mm (MEMS). The production in a fluidized bed is more advantageous due to the orders of magnitude of greater surface deposition and, consequently, a more complete consumption of the reaction mixture; Due to the possibility of continuous removal of the reaction zone from particles that have reached a certain size limit.
On the other hand, said silicon contains a certain amount of amorphous material and fine particles of the reactor coating (including those coated with precipitated silicon). Due to the developed surface, the granular silicon easily gets dirty and absorbs a large amount of water and air gases. In general, granular silicon has a notably lower degree of purity than silicon obtained by fixed-bar deposition, and is used more often for the less demanding production of solar-grade crystals.