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==Processing== | ==Processing== | ||
GaAs is grown on a substrate to produce a bulk semiconductor. The bulk semiconductor is cut, etched and polished. Next epitaxial layers are grown followed by doping and metalization to yield the final product. | |||
*Bulk crystal growth. | *Bulk crystal growth. | ||
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The semiconductor is grown from a seed crystal to ensure a dislocation free lattice. The GaAs is melted and placed into a quartz crucible. The melt is not in contact with the air but instead with boron oxide to prevent volatile gases from escaping. A seed is placed at the top of the quartz container at a cooler temperature than the melt. The crucible spins in a circle while the melt is pulled upward to the cooler temperature, thus solidifying as it is pulled. | The semiconductor is grown from a seed crystal to ensure a dislocation free lattice. The GaAs is melted and placed into a quartz crucible. The melt is not in contact with the air but instead with boron oxide to prevent volatile gases from escaping. A seed is placed at the top of the quartz container at a cooler temperature than the melt. The crucible spins in a circle while the melt is pulled upward to the cooler temperature, thus solidifying as it is pulled. | ||
Two popular growing techniques | |||
1. Molecular Beam Epitaxy | 1. Molecular Beam Epitaxy | ||
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The advantage of using MOCVD is that very fast changes in composition of epitaxial structures. Phosphorus poses no problem like it does with molecular beam epitaxy. | The advantage of using MOCVD is that very fast changes in composition of epitaxial structures. Phosphorus poses no problem like it does with molecular beam epitaxy. | ||
==Process for Recycling PV Cells== | ==Process for Recycling PV Cells== | ||
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The epitaxial process operates at about 25% efficiency. Much of the material is lost as solids on the reactor walls. With this estimated efficiency, $1,687,500 of gallium and $118,100 of arsenic are lost each year. | The epitaxial process operates at about 25% efficiency. Much of the material is lost as solids on the reactor walls. With this estimated efficiency, $1,687,500 of gallium and $118,100 of arsenic are lost each year. | ||
==Safety== | |||
As mentioned previously, Arsenic is highly toxic to the human body and must be handled with care. The following safety regulations should be acknowledged when handling recycled Arsenic: | |||
Ventilation: To keep particulate matter from being inhaled and to keep recycled material from being contaminated before reuse, ventilation systems should be installed. Clean air should be continuously circulated to keep air breathable for all employees. <ref>http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9734</ref> | |||
==References== | ==References== | ||
<references/> | <references/> | ||
Revision as of 23:39, 4 October 2011
General Information
General Facts about Gallium Arsenide[1][2]
- Gallium = rarer than gold
- Arsenic = poisonous
- 5 cents per gram, $45,000 per ton.
- Product highly efficient photovoltaic cells
- Band gap = 1.43 eV
- Density = 5.316 g/cm^3
- 10x cost of Si at 50 cents to $1.50 a gram. $900,000 per ton
- Lower strength than Si
- Relatively insensitive to heat
- Resistant to radiation damage
GaAs crystallizes into a cubic zinc blend called a sphalerite with is similar to a diamond cubic structure.
Processing
GaAs is grown on a substrate to produce a bulk semiconductor. The bulk semiconductor is cut, etched and polished. Next epitaxial layers are grown followed by doping and metalization to yield the final product.
- Bulk crystal growth.
Bulk growth of GaAs can be done through the Liquid-encapsulated Czochralski method(LEC). The semiconductor is grown from a seed crystal to ensure a dislocation free lattice. The GaAs is melted and placed into a quartz crucible. The melt is not in contact with the air but instead with boron oxide to prevent volatile gases from escaping. A seed is placed at the top of the quartz container at a cooler temperature than the melt. The crucible spins in a circle while the melt is pulled upward to the cooler temperature, thus solidifying as it is pulled.
Two popular growing techniques
1. Molecular Beam Epitaxy
Epitaxy it the process of depositing a mono-crystalline film onto a mono-crystalline substrate. The substrate acts as a seed crystal because the deposited film takes on the lattice structure and orientation of the substrate. It is important that the film has a coherent crystal structure. Molecular beam epitaxy is an easy method of depositing solid films through vacuum evaporation. Molecular beams are used to heat a substrate in a vacuum. The beams contain the components of that will become the epitaxial film. The temperature of the beams is controlled to insure the correct intensity of the beams. The beams escape and condense on the substrate and grow into the epitaxial film. The process is kinetically controlled by adsorption of the constituent molecules and surface migration and dissociation of the adsorbed molecules, and by incorporation of the atoms to the substrate resulting in growth.
2. Metal-organic chemical vapor deposition (MOCVD)
MOCVD uses metal alkyls and hydrieds as a source of materials in a cold walled reactor. Some of the metal alkyls are TMGa (trimethylgallium), TEGa (Triethylgallium), TMAl (Trimethylaluminum), TMIn (Trimethylindiumn), and TEIn (Triethylindium). The methyl compounds are used more often because they have a higher vapor pressure that facilitates their attachment to the substrate.
The GsAs crystals are grown by placing a susceptor (material used to converted electromagnetic energy to heat re-emitted as infrared thermal radiation) inside of a quartz reactor. A TMGa and TMAl gas flows through a growth tube by bubbling H2.Gas molecules diffuse through layers in the substrate. The gasis mixed with AsH3 and the hot surface causes the gas to decompose in the absence of oxygen due to high temperatures above the substrate. The following reaction takes place producing GaAs because the decomposition products move over the surface of the substrate and find available lattice sites where they are incorporated into the lattice.
The advantage of using MOCVD is that very fast changes in composition of epitaxial structures. Phosphorus poses no problem like it does with molecular beam epitaxy.
Process for Recycling PV Cells
Source: http://www.renewableenergyfocus.com/view/3005/endoflife-pv-then-what-recycling-solar-pv-panels/
- Modules are shredded to <5mm pieces
- Semiconductor etched from film during at 4-6hr leaching processes
- Glass separated from the semiconductor and cleaned
- Metals are precipitated out and reprocessed into raw material
Amount of material in typical cell = 1760 g/m^2 (5.5mm thick cell)
Power rating = 320 W/m^2
Amount of material needed per Watt of power = 5.5 g/W [3]
Average utilization rate (MOCVD) = 40% [4]
It is estimated the only 8.5% of the input materials from the semiconductor fabrication process above will be used in the final product. The reason for such little output from the semiconductor creation is that the final product must be 99.999% pure to be functional.
- Identification of Impurities
To identify imperfections in semiconductors scanning electron microscopy is used along with energy dispersive xray spectroscopy to determine the the size of the imperfection.
- Purification of Waste GaAS
The process for making pure GaAs for the fabrication of semiconductors involves several steps with little tolerance for impurities and mistakes. It is estimated that only 20% of the GaAs is actually usable for commercial grade semiconductors. In the processing of GaAs the following waste products are developed: liquids from etching, defective single crystals, wafers, epitaxial structures, and powders from the slicing process.S.A Kozlov et. all found through titration of semiconductor waste the weight percent of GaAs that can be recovered from the semiconductor fabrication process. Growth of epitaxial structures is 20-45%, crystal growth waste from rejected crystals is approximately 70% or less, rejected wafers are 6% or less, and powder waste after slicing is 40% or less.
Dopants added to the GaAs during semiconductor fabrication are extremely hard to remove from waste products. Some of the added dopants include: Sn, Ge, Pb, Cu, Ag, and Au. Processes do exist however, and are listed as follows: thermal dissociation, oxidation with oxygen, nitriding with ammonia, and chlorination with chlorine gas.
- Vacuum heat treatment
Vacuum heat treatment is used to remove moisture, dissolved gasses, and volatile compounds GaAs. The volatile compounds have a high vapor pressure in atmospheric air and a low boiling point. Such compounds include Cd, Zn, Mg, and alkali metals, all of which are more volatile than Ga. The process takes place in a quarts container with the Ga placed in a graphite well. The dynamic vacuum is held at a pressure of 0.013 Pa at a temperature of 900C for two hours.
Characterization of Gallium Arsenide
- Downcycling
As mentioned above, gallium and arsenic are separated using a thermal process and an aqueous waste process. As is usually not recycled because it is a low cost material compared to Ga. Gallium is an abundant element in rocks, however it is not highly concentrated with and average of 19 ppm. No concentrated sources of gallium exist. The gallium used is obtained from processing of other ores such as aluminum or zinc that have a low percentage of gallium which is later concentrated. Aluminum ores contain approximately 0.003 to 0.01% Ga.
Arsenic is currently not recycled in the semiconductor industry. Is is considered a low cost material and is usually discarded. However, it is possible to recycle As from both solid waste and from polishing waste water. To recycle As from solid waste, a zone refining method is used to separate arsenic compounds such as arsine. Zone refining works by causing a molten zone to travel along a solid rod. This process is carried out at 814 C and a pressure of 36 Atm. Impurities are more soluble in liquids than they are in solids and are carried away with the molten zone. The recrystallized material is purified where the zone starts. After the zone refining, the As must be reduced to an elemental state. This is done by reducing agents such as Hydrogen are aluminum. A negative aspect of handling arsine is that it is a toxic gas and has the possibility to spontaneously ignite in air.
- costs assessment
The major costs associated with the recycling of semiconductors are associated with the disposal of waste. 50% of solid GaAs wastes made through semiconductor foundries are disposed of and 50% are sent to off-site facilities for gallium recovery.[2] It is estimated that approximately 10 tons of GaAs contains 5 tons of Ga and 5 tons of As. This means that there is a 1:1 ratio by weight for Ga and As. Approximately 100 tons per year of bulk GaAs crystals are produced in the United States, and roughly 75% of this is waste material. Estimates show that 37.5 tons of Ga and As each are waste. With Ga costing $900,000/ton this would total $33,750,000 in gallium waste. The waste for As would be 37.5*$45,0000/ton equal to $1,678,500 in arsenic waste. Combined waste for GaAs totals $35,437,500 per year.
Etching the GaAs in the production line accounts for a loss of 1 ton each of gallium and arsenic and a total loss of $945,000.
The epitaxial process operates at about 25% efficiency. Much of the material is lost as solids on the reactor walls. With this estimated efficiency, $1,687,500 of gallium and $118,100 of arsenic are lost each year.
Safety
As mentioned previously, Arsenic is highly toxic to the human body and must be handled with care. The following safety regulations should be acknowledged when handling recycled Arsenic:
Ventilation: To keep particulate matter from being inhaled and to keep recycled material from being contaminated before reuse, ventilation systems should be installed. Clean air should be continuously circulated to keep air breathable for all employees. [5]
References
- ↑ Gallium Arsenide Solar Cells http://photovoltaics.sandia.gov/docs/PVFSCGallium_Arsenide_Solar_Cells.htm
- ↑ 2.0 2.1 Josepth T. Swartzbaugh, Jeffrey A. Sturgill, Reduction of Arsenic Wastes in the Semiconductor Industry, EPA, http://epa.gov/nrmrl/pubs/600r02089/600R02089.pdf
- ↑ http://www.spectrolab.com/DataSheets/Panel/panels.pdf
- ↑ Spire Semiconductors, http://www.spirecorp.com/spire-semiconductor/downloadable_documents/10%20Concentrator%20PV%20Datasheet_5_20_10.pdf
- ↑ http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9734