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Waste As per Area = 9099 g/m<sup>2</sup>
 
Waste As per Area = 9099 g/m<sup>2</sup>
  
=====Waste as per Watt<sub>peak</sub>=====
+
=====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> = 11.4 g/Watt<sup>peak</sup>*74.922/(69.723+74.922) [(GaAs/Watt<sup>peak</sup>)*(Wt% As)]
  

Revision as of 23:32, 8 October 2011

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Introduction

Gallium arsenide (GaAs) is a semiconductor material that is used in a wide variety of applications ranging from circuits to solar cells. Solar cells of GaAs can be produced using both bulk and thin film growth methods.

Material Processing

Manufacturing process of GaAs and other semiconductors with the waste products produced

Bulk Growth

There are two common ways to produce GaAs using bulk growth, Liquid-Encapsulated Czochralski (LEC) growth, and Vertical Gradient Freeze (VGF) technology. [1]

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. [2]

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. [2]

Thin Film Growth

Thin films of GaAs have many advantages over large single crystals of GaAs when it comes to being used in solar cells. Thin films lack some of the impurities found in large crystals, and are capable of being used without requiring extensive slicing. The rest of the case study will be dedicated to thin film GaAs semiconductors.

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.[3] During MOCVD of GaAs, trimethylgallium Ga(CH3)3 and arsine AsH3 are consumed.[4]

Molecular Beam Epitaxy (MBE)

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. [2] During MBE, ultra pure gallium and arsenic are consumed.

Waste during Production

Preliminary Calculations

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/Wattpeak. The calculations are seen below.

Amount of Material per Area

Size of Panels = 10 inches x 9 inches[5]

Number of Cells per Panel = 18[5]

Area of Cell = (90 in2/18)*.00064516 m2/in2 [((Area Panel)/(Number Cells))*(conversion from in.2 to m2)]

Area of Cell = .0032258 m2

GaAs per Cell = 10 grams[5]

GaAs per Area = 10 g /.0032258 m2 [(GaAs/Cell)/(Area Cell)]


Amount of Material per Area = 3100 g/m2

Amount of Material per Watt

Peak Power per Cell = 880mW [5]

Peak Power per Area = 880mW / .0032258 m2 [(Power/Cell)/(Area Cell)]

Peak Power per Area = 272.8 W/m2

GaAs per Wattpeak = 3100 g/m2 / 272.8 W/m2 [(GaAs/Area)/(Power/Area)]


GaAs per Wattpeak = 11.4 g/Wpeak

Metalorganic Chemical Vapour Deposition Waste

Using the data from the preliminary calculations, and the utilization efficiency given below for MOCVD, the Waste GaAs per Area/Wattpeak of MOCVD can be determined.

Average Material Utilization Efficiency = 30%[6]

Waste GaAs per Area

Waste GaAs per Area = (3100 g/m2/.3)-3100g/m2 [(GaAs/Area)/(Percentage Waste)-(GaAs/Area)]


Waste GaAs per Area = 7233 g/m2

Waste GaAs per Wattpeak

Waste GaAs per Wattpeak = (11.4 g/Wpeak/.3)-11.4g/Wpeak [(GaAs/Wattpeak)/(Percentage Waste)-(GaAs/Wattpeak)]


Waste GaAs per Wattpeak = 26.6 g/Wpeak

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%[6]

Average Material utilization efficiency for Ga = 55%

Waste Ga per Area

Grams of Ga per Area = 3100 g/m2*69.723/(69.723+74.922) [(GaAs/Area)*(Wt% Ga)]

Grams of Ga per Area = 1494.3 g/m2

Waste Ga per Area = (1494.3 g/m2/.55)-1494.3g/m2 [(Ga/Area)/(Percentage Waste)-(Ga/Area)]


Waste Ga per Area = 1222.6 g/m2

Waste Ga per Wattpeak

Grams of Ga per Wattpeak = 11.4 g/Wattpeak*69.723/(69.723+74.922) [(GaAs/Wattpeak)*(Wt% Ga)]

Grams of Ga per Wattpeak = 5.495 g/Wattpeak

Waste Ga per Wattpeak = (5.495 g/Wpeak/.55)-5.495g/Wpeak [(Ga/Wattpeak)/(Percentage Waste)-(Ga/Wattpeak)]


Waste Ga per Wattpeak = 4.496 g/Wpeak

Waste As Calculations

Material utilization efficiency for As = 10-20%[6]

Average Material utilization efficiency for As = 15%

Waste As per Area

Grams of As per Area = 3100 g/m2*74.922/(69.723+74.922) [(GaAs/Area)*(Wt% As)]

Grams of As per Area = 1605.7 g/m2

Waste Asper Area = (1605.7 g/m2/.15)-1605.7g/m2 [(As/Area)/(Percentage Waste)-(As/Area)]


Waste As per Area = 9099 g/m2

Waste As per Wattpeak

Grams of As per Wattpeak = 11.4 g/Wattpeak*74.922/(69.723+74.922) [(GaAs/Wattpeak)*(Wt% As)]

Grams of As per Wattpeak = 5.9 g/Wattpeak

Waste As per Wattpeak = (5.9 g/Wpeak/.15)-5.9g/Wpeak [(As/Wattpeak)/(Percentage Waste)-(As/Wattpeak)]


Waste As per Wattpeak = 33.4 g/Wpeak

Recycling

Collecting Waste

Semiconductor waste from MBE and MOCVD occurs in two primary methods, solid waste coating the reactor walls and parts, and 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 1018[2] 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 would have to 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.[7] 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.

To determine the cost of collecting non-solid waste, we need to determine the cost of operating a water scrubber. The calculations can be seen below.

Yearly cost to run 250 Nm3/hr water scrubber = €110,000.

Cost to run 250 Nm3/hr water scrubber = €110,000*1.3391/(365*24) [(Cost to run in €)*(conversion € to $)/((days/year)*(hours/day))

Cost to run 250 Nm3/hr water scrubber = $16.82/hr

Process of recycling and re-purification

Proposed recycling process of GaAs semiconductor material

Water Waste recycling and re-purification

Solid Waste recycling and re-purification

The recycling of solid waste GaAs can be accomplished in three steps, Thermal Separation, Sublimation Refining of Arsenic, and Gallium Zone Refinement.

Thermal Separation

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, 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.

Thermal Separation Furnace
Sublimation Refining of Arsenic

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.

Purification of As
Gallium Zone Refining

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.

Purification of Ga

Downcycling

Large scale downcycling for GaAs waste currently does not exist, and will most likely not be examined in the near future. The reason that downcycling does not exist for GaAs is the limited utility of downcycled gallium and arsenic. GaAs semiconductor components account for 98% of the current gallium consumption[8]. With the narrow usage of gallium in other fields, there is no real market behind downcycling gallium. Arsenic, although more widely used in other fields than gallium, also does not have a market drive behind downcycling. Arsenic can be used to treat wood, preserve leather, kill insects, and harden lead for use in bullets, however it is extremely toxic to humans. There is a large push to reduce the usage of arsenic in industry, especially because companies that produce arsenic products are often responsible for the environmental clean up[2].

Producing a Recycling Plant

Part of the goal of this case study is to determine the technical viability of recycling GaAs. This can be accomplished by designing a Semiconductor Recycling Plant capable of supporting a 1 GW thin-film soar photovoltaic manufacturing facility.

Plant Capacity

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/Wpeak. 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/Wpeak *1 GW)/(1000 *365)[((Waste/Wpeak)*(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/Wpeak for Ga and 33.4 g/Wpeak 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/Wpeak *1 GW)/(1000 *365)[((Waste/Wpeak)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]

Required Ga Recycling Capacity = 12318 kg/day

Required As Recycling Capacity = (33.4 g/Wpeak *1 GW)/(1000 *365)[((Waste/Wpeak)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]

Required As Recycling Capacity = 91506.8 kg/day

Safety

MSDS file for Gallium

MSDS File for Arsenic

MSDS File for Trimethylgallium

MSDS File for Arsine

The manufacturing of semiconductors using gallium and arsenic does result in the handling of hazardous chemicals and needing to dispose of them. These waste materials can come on solid and liquid form posing many issues of containment and disposal.[6] Many PV manufacturers have a system for the management, handling, and disposal of the hazardous waste. Examples of these are secondary enclosures, ventilation systems, chemical detection, and neutralization systems, and automating delivery and process systems as much as possible.[6] The administration also deals with implementing employee training programs, and ensuring that proper emergency procedures are in place. Remotely operated cylinder valves have helped remove workers from possible contact with hazards by allowing them to operate cylinders from a distance, and also allowing them to shutdown remotely in case of an emergency.[6] Redundancy is also implemented in the critical systems that control pumps, flow regulators, valves, exhaust pumps, and any other equipment that could be critical in a spill prevention or hazard detection.

References

  1. R.L. Adams, Growth of high purity GaAs using low-pressure vapour-phase epitaxy, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 395, Issue 1, 1 August 1997, Pages 125-128, ISSN 0168-9002, 10.1016/S0168-9002(97)00624-4. (http://www.sciencedirect.com/science/article/pii/S0168900297006244) Keywords: Low-pressure vapour-phase epitaxy; LPVPE; GaAs
  2. 2.0 2.1 2.2 2.3 2.4 Swartzbaugh Joseph, Sturgill Jeffery. Reduction of Arsenic Wastes in the Semiconductor Industry, 1998 (http://www.epa.gov/nrmrl/pubs/600r02089/600R02089.pdf)
  3. http://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy
  4. http://en.wikipedia.org/wiki/Gallium_arsenide
  5. 5.0 5.1 5.2 5.3 GaAs Solar Panel; Product Data Sheet; 3554 Chain Bridge Road, Suite 103, Fairfax, VA 22030, http://www.spacequest.com/products/SP-X.pdf , (accessed September 2011).
  6. 6.0 6.1 6.2 6.3 6.4 6.5 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)
  7. 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)
  8. Deborah A. Kramer, Gallium, Mineral Resources Program USGS (http://minerals.usgs.gov/minerals/pubs/commodity/gallium/460303.pdf )