====Liquid-Encapsulated Czochralski Growth(LEC) ====
LEC growth is accomplished by melting high-purity arsenic and gallium in a high temperature vessel, and slowly cooling to produce a single crystal. The GaAs crystal produced using this method however has some impurities such as significant levels of carbon, and numerous dislocations. These impurities cause the semiconductor to be unusable for some applications.
====Vertical Gradient Freeze Technology (VGF) ====
VGF growth works by placing high purity arsenic and gallium in an enclosed quartz ampoule with a crystal of GaAs. The arsenic and gallium are melted, and then brought into contact with the GaAs crystal. When cooled slowly, a single crystal of GaAs is formed. The single crystal formed has many of the same impurities as LEC growth crystals, which restricts the utility of the crystals.
Typical dimensions of the semiconductor crystals are 1-6 inches in diameter and 2-30 inches long. The usual rate of crystal growth is 1-5mm per hour. <ref name=""> Swartzbaugh Joseph, Sturgill Jeffery. Reduction of Arsenic Wastes in the Semiconductor Industry, 1998 (http://www.epa.gov/nrmrl/pubs/600r02089/600R02089.pdf)</ref>
====Cutting, Polishing, and Etching ====
The bulk crystals (boule) produced are cylindrical with conical ends, thus they need to be cut, polished and etched before they can be used. During processing the conical ends are cut and wasted, the boule is ground to ensure a uniform diameter over the length, the crystals are aligned using x-ray diffraction, and the boule is cut into wafers. Just forming the correct shape results in nearly one third of the total mass being wasted. Even further processing is required to etch off the damaged layer of semiconductors from sawing and to polish the semiconductor. As a result of processing, nearly 50% of the semiconductor is wasted. <ref name=""></ref>
The most common thin film growth methods for producing GaAs films are Metalorganic Chemical Vapour Deposition (MOCVD) and Molecular Beam Epitaxy (MBE).
====Metalorganic Chemical Vapour Deposition (MOCVD) ====
MOCVD is a chemical vapour deposition method of depositing epitaxial films using surface reactions of organic compounds, metalorganics or metal hydrides. The growth of the epitaxial film is the result of a chemical reactions, and not physical deposition. MOCVD of GaAs requires a reaction chamber with gas injection units, a temperature control system, and a pressure control system.<ref>http://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy</ref> During MOCVD of GaAs, trimethylgallium Ga(CH<sub>3</sub>)<sub>3</sub> and arsine AsH<sub>3</sub> are consumed.<ref>http://en.wikipedia.org/wiki/Gallium_arsenide</ref>
====Molecular Beam Epitaxy (MBE) ====
[[File:MBE_Diagram.png|thumb|right|Diagram of MBE setup]]MBE is the process of depositing epitaxial films on a substrate under ultrahigh vacuum (UHV) conditions using atomic or molecular beams. The atomic or molecular beams are generated from elemental feedstocks in Knudsen-type effusion cells. The beams travel in straight paths to the substrate where they condense and grow under kinetically controlled conditions. <ref name=""></ref> During MBE, ultra pure gallium and arsenic are consumed.
Current Production methods of GaAs semiconductors produce a large amount of waste. To determine the amount of waste produced during production of GaAs solar cells, we must first determine the amount of GaAs per Area/Watt<sub>peak</sub>. The calculations are seen below.
====Amount of Material per Area ====
Size of Panels = 10 inches x 9 inches<ref name="">
GaAs Solar Panel; Product Data Sheet; 3554 Chain Bridge Road, Suite 103, Fairfax, VA 22030,
Number of Cells per Panel = 18<ref name=""></ref>
Area of Cell = (90 in<sup>2</sup>/18)*.00064516 m<sup>2</sup>/in<sup>2</sup> [((Area Panel)/(Number Cells))*(conversion from in.<sup>2</sup> to m<sup>2</sup>)]
Area of Cell = .0032258 m<sup>2</sup>
GaAs per Cell = 10 grams<ref name=""></ref>
GaAs per Area = 10 g /.0032258 m<sup>2</sup> [(GaAs/Cell)/(Area Cell)]
Amount of Material per Area = 3100 g/m<sup>2</sup>
====Amount of Material per Watt ====
Peak Power per Cell = 880mW <ref name=""></ref>
Peak Power per Area = 880mW / .0032258 m<sup>2</sup> [(Power/Cell)/(Area Cell)]
Peak Power per Area = 272.8 W/m<sup>2</sup>
GaAs per Watt<sub>peak</sub> = 3100 g/m<sup>2</sup> / 272.8 W/m<sup>2</sup> [(GaAs/Area)/(Power/Area)]
GaAs per Watt<sub>peak</sub> = 11.4 g/W<sub>peak</sub>
===Metalorganic Chemical Vapour Deposition Waste===
Average Material Utilization Efficiency = 30%<ref name="">V.M. Fthenakis, B. Bowerman, Environmental Health and Safety (EHS) Issues in III-V Solar Cell Manufacturing, National PV EHS Assistance Center (http://www.bnl.gov/pv/files/pdf/art_168.pdf)</ref>
====Waste GaAs per Area ====
Waste GaAs per Area = (3100 g/m<sup>2</sup>/.3)-3100g/m<sup>2</sup> [(GaAs/Area)/(Percentage Waste)-(GaAs/Area)]
Waste GaAs per Area = 7233 g/m<sup>2</sup>''' ====Waste GaAs per Watt<sub>peak</sub> ====
Waste GaAs per Watt<sub>peak</sub> = (11.4 g/W<sub>peak</sub>/.3)-11.4g/W<sub>peak</sub> [(GaAs/Watt<sub>peak</sub>)/(Percentage Waste)-(GaAs/Watt<sub>peak</sub>)]
'''Waste GaAs per Watt<sub>peak</sub> = 26.6 g/W<sub>peak</sub> '''
===Molecular Beam Epitaxy Waste===
Similar to the waste rates of MOCVD, the waste rates of MBE can be determined using the preliminary calculations and the MBE utilization efficiencies. However, the waste rates for MBE are given for Ga and As individually.
====Waste Ga Calculations ====
Material utilization efficiency for Ga = 40-70%<ref name =""></ref>
Average Material utilization efficiency for Ga = 55%
=====Waste Ga per Area =====
Grams of Ga per Area = 3100 g/m<sup>2</sup>*69.723/(69.723+74.922) [(GaAs/Area)*(Wt% Ga)]
Grams of Ga per Area = 1494.3 g/m<sup>2</sup>
Waste Ga per Area = (1494.3 g/m<sup>2</sup>/.55)-1494.3g/m<sup>2</sup> [(Ga/Area)/(Percentage Waste)-(Ga/Area)]
'''Waste Ga per Area = 1222.6 g/m<sup>2</sup>'''
=====Waste Ga per Watt<sub>peak</sub> =====
Grams of Ga per Watt<sub>peak</sub> = 11.4 g/Watt<sup>peak</sup>*69.723/(69.723+74.922) [(GaAs/Watt<sup>peak</sup>)*(Wt% Ga)]
Grams of Ga per Watt<sub>peak</sub> = 5.495 g/Watt<sup>peak</sup>
Waste Ga per Watt<sub>peak</sub> = (5.495 g/W<sub>peak</sub>/.55)-5.495g/W<sub>peak</sub> [(Ga/Watt<sub>peak</sub>)/(Percentage Waste)-(Ga/Watt<sub>peak</sub>)]
Ga per Watt<sub>peak</sub> = 4.496 g/W<sub>peak</sub>'''
==Waste As Calculations====
Material utilization efficiency for As = 10-20%<ref name =""></ref>
Average Material utilization efficiency for As = 15%
=====Waste As per Area=====
Grams of As per Area = 3100 g/m<sup>2</sup>*74.922/(69.723+74.922) [(GaAs/Area)*(Wt% As)]
Grams of As per Area = 1605.7 g/m<sup>2</sup>
Waste As per Area = (1605.7 g/m<sup>2</sup>/.15)-1605.7g/m<sup>2</sup> [(As/Area)/(Percentage Waste)-(As/Area)]
'''Waste As per Area = 9099 g/m<sup>2</sup>'''
=====Waste As per Watt<sub>peak</sub> =====
Grams of As per Watt<sub>peak</sub> = 11.4 g/Watt<sup>peak</sup>*74.922/(69.723+74.922) [(GaAs/Watt<sup>peak</sup>)*(Wt% As)]
Grams of As per Watt<sub>peak</sub> = 5.9 g/Watt<sup>peak</sup>
Waste As per Watt<sub>peak</sub> = (5.9 g/W<sub>peak</sub>/.15)-5.9g/W<sub>peak</sub> [(As/Watt<sub>peak</sub>)/(Percentage Waste)-(As/Watt<sub>peak</sub>)]
'''Waste As per Watt<sub>peak</sub> = 33.4 g/W<sub>peak</sub>'''
[[File:Recycling process flowchart.jpg|thumb|left|Proposed process for recycling GaAs waste]]
Semiconductor waste from MBE occurs mainly as a solid waste coating the reactor walls and parts, while the MOCVD process creates non solid waste in the form of exhaust vapors drawn off of the epitaxial reactors.
====Solid Waste ====
The coating of the reactor walls and parts forms a solid waste material, which can be collected by simply scraping it off off reactor components. The solid waste material collected in this manner may be contaminated with dopants such as Si, Zn, C and Cr, as well as with GaAsP, arsenic oxides, and phosphorous oxides. These dopants however usually only have concentration levels of only 10<sup>18</sup><ref name=""></ref> atoms/cc.
The energy collection cost for this solid waste is assumed to be negligible, as the solid waste needs to be removed in current production processes.
====Non-solid Waste ====
The collection of semiconductor waste from exhaust vapors is not as straight forward as the collection of solid waste material. In order to collect the waste from the exhaust, the exhaust vapors undergo a series of "scrubbing" processes, in which cooled water would be sprayed into the vapor, cooling it off until the relative temperature of the desired components in the exhaust reach a temperature where it changes from a gas to either liquid or solid phase.<ref name="">Potter, G. U.S. Patent 4,008,056, 1977 (http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F4008056)</ref> This material would then be collected and, depending upon the contaminants (which would include the dopants used to create the thin films), would undergo multiple purification process in order to reach the desired purity for reuse.
Energy Cost = .003182 kW-hrs/g
====Collection Efficiency ====
Since the primary methods of material loss are accounted for above, it can be assumed that, while a certain loss of materials will always occur, most of the materials will be captured and re-purified. Materials will be lost in the water scrubbing method, as not all of the waste materials will be condensed into liquid and so some reusable gases will escape into the atmosphere despite the taken precautions. The solid wastes will undoubtedly be captured entirely, with little to no loss occurring. However, when the purification process takes place, some Ga and As will undoubtedly be lost, as defects and impure materials will have to be removed from the system in order to purify the elements to their required level of purity. It can be assumed that about 80-90% of the materials will be reusable though, depending upon the standards of quality enacted in processing the waste materials.
[[File:GaAs recycling process.jpg|thumb|left|Proposed recycling process of GaAs semiconductor material]]
====Non-Solid Waste recycling and re-purification ====
Before the non-solid waste produced from the MOCVD process can be recycled and re-purified, it must first be turned into solid waste.
=====Non-Solid Waste to Solid Waste Conversion ===== The waste water obtained from the water scrubbing process is adjusted to a pH between 11.5 and 12 with the use of sodium hydroxide. After the pH is correctly adjusted, a soluble calcium salt is added to the mixture. This soluble calcium salt reacts with the arsenate, and produces a calcium arsenate precipitate which can be removed using a centrifuge. After the calcium arsenate precipitate is removed the pH is readjusted to between 6 to 8 using sulfuric acid, which causes gallium hydroxide to precipitate out, which can be collected and filtered<ref name=""></ref>.
=====Purifying Gallium Hydroxide=====The solid waste gallium hydroxide can be redissolved in a caustic solution, and then electrolyzed to produce gallium metal<ref>Recycling-Metals. Web. 9 Oct. 2011. <http://minerals.usgs.gov/minerals/pubs/commodity/recycle/recycmyb02r.pdf>.</ref>.
The energy cost to retrieve the gallium through
electrolyzing is shown below.
This gallium metal can be processed further using the gallium zone refinement described in the next section.
=====Purifying Calcium Arsenate =====
The calcium arsenate can be further processed using the sublimation refining process described in the next section.
====Solid Waste recycling and re-purification ====
The recycling of GaAs solid waste can be accomplished in three steps, Thermal Separation, Sublimation Refining of Arsenic, and Gallium Zone Refinement.
=====Thermal Separation =====
[[File:Thermal Separation Furnace schematic.jpg|thumb|right|Thermal Separation Furnace]]
Thermal separation is the process of separating gallium and aresenic using elevated temperatures and reduced pressures. To begin separation solid GaAs waste is placed into a graphite or SiC crucible inside of a thermal separation furnace. The furnace is heated to above 1050 degrees C for 2-3 hours, and the pressure inside is reduce to less than 1 torr. At these elevated temperatures and reduced pressures, arsenic is able to be separated out as a condensable vapor, leaving a gallium-rich residue in the crucible. Due to the low melting temperature of gallium compared to other elements, the gallium-rich residue left in the crucible is composed of a liquid gallium fraction, and a slag fraction. Filtering out the slag from the gallium-rich liquid before cooling, and condensing the arsenic vapors produces fairly pure gallium and arsenic solids.
Energy cost thermal separation per gram = .001663 kW-hrs/g
=====Sublimation Refining of Arsenic =====
[[File:Purification of Arsenic.jpg|thumb|left|Purification of As]]
Although thermal separation separates gallium and arsenic fairly effectively, the arsenic can still be contaminated with more volatile contaminants like carbon. To further purify, the arsenic is put through through the process of sublimation refining. During sublimation refining, the arsenic is heated slightly above 610 degrees C, the sublimation temperature of arsenic. This heating is performed in an inert gas stream such as nitrogen, and the arsenic is transferred to and recondensed in a second chamber. To obtain higher purity arsenic, this process can be accomplished multiple times. This method works because arsenic and the impurities within will differ in partial pressures and volatility.
The calculation for the energy for sublimation refining can be estimated by determining the energy cost to heat arsenic to 610 degrees C.
Energy cost of As purification per gram = .25 (kW-hrs/hr)/pound * 2 hours / 453.6 grams/pound [(Energy Cost/hr)*(Number of hours run)/(Grams to pound conversion)]
Energy cost of
furnace per gram = .001102 kW-hrs/g ===== Gallium Zone Refining =====
[[File:Purification of Gallium.jpg|thumb|right|Purification of Ga]]
Gallium zone refining is a purification technique for gallium based upon low expectations of impurities. The process only works due to the near pure gallium obtained from thermal separation. To further purify, gallium is placed in a gallium refining pan with a spiral groove in the pan bottom, and cooled to 0 degrees C. A heat lamp is used to heat specific zones of the gallium sample, and the pan is slowly rotated. Based upon the temperature difference of solid and liquid in the sample, impurity segregation will occur, and impurities will segregate to both ends of the spiral groove in the gallium refining pan.
== = = =
Purity = = = = =
The contamination level of Gallium and Arsenic after the above processes are completed can be as low as 5-50 ppb, or in other words the material is 99.99999% pure<ref>Bautista, R. G. 2003. Processing to Obtain High-Purity Gallium. JOM., 55: 23–26.(http://atmsp.whut.edu.cn/resource/pdf/1994.pdf)</ref>. This purity meets the requirements for both semiconductor grade, 99.99%, and solar grade, 99.9999%.
Using the material waste values produced through various methods of GaAs thin-film production, we can determine the necessary capacity of a feasible recycling plant.
====Supporting a MOCVD plant ====
The amount of material per Watt Peak wasted using MOCVD was determined to be 26.6 g/W<sub>peak</sub>. Using this value, and the size of the supported manufacturing plant, we can determine the daily required capacity of a recycling plant.
Required Recycling Capacity = (26.6 g/W<sub>peak</sub> *1 GW)/(1000 *365)[((Waste/W<sub>peak</sub>)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]
'''Required Recycling Capacity = 72876.7 kg/day '''
= = = = Supporting a MBE plant
The amount of material per Watt Peak wasted using MBE was determined to be 4.496 g/W<sub>peak</sub> for Ga and 33.4 g/W<sub>peak</sub> for As. Using these values, and the size of the supported manufacturing plant, we can determine the daily required capacity of a recycling plant.
Required Ga Recycling Capacity = (4.496 g/W<sub>peak</sub> *1 GW)/(1000 *365)[((Waste/W<sub>peak</sub>)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]
'''Required Ga Recycling Capacity = 12318 kg/day'''
Required As Recycling Capacity = (33.4 g/W<sub>peak</sub> *1 GW)/(1000 *365)[((Waste/W<sub>peak</sub>)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]
'''Required As Recycling Capacity = 91506.8 kg/day'''
Depending upon the solar production technique supported, the equipment requirements will vary slightly for recycling.
====Supporting a MOCVD plant ====
The following equipment is necessary to recycle waste from a MOCVD plant.
-Gallium Zone Refiner
====Supporting a MBE plant ====
The following equipment is necessary to recycle waste from a MBE plant.
-Gallium Zone Refiner