Semiconductor recycling plant case study of GaAs photovoltaic manufacturing: Difference between revisions

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General Information

General Facts about gallium Arsenide^[1]

• 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.^[2]

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 growth of semiconductors makes use of a temperature gradient through the horizontal freeze gradient technique.

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/

1. Modules are shredded to <5mm pieces
2. Semiconductor etched from film during at 4-6hr leaching processes
3. Glass separated from the semiconductor and cleaned
4. 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

Source: http://www.spectrolab.com/DataSheets/Panel/panels.pdf

Average utilization rate (MOCVD) = 40%

Spire Semiconductors, http://www.spirecorp.com/spire-semiconductor/downloadable_documents/10%20Concentrator%20PV%20Datasheet_5_20_10.pdf

• 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.

https://springerlink3.metapress.com/content/n574150274899336/resource-secured/?target=fulltext.pdf&sid=bluadn00adighl3fg4fguntl&sh=www.springerlink.com

• 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.

• 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.

• costs assessment

The major costs associated with the recycling of semiconductors are associated with the disposal of waste. 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.