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Semiconductor recycling plant case study of GaAs photovoltaic manufacturing - case study

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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 cell of GaAs can be produced using both bulk and thin film growth methods.

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]

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.

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.


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 Vapour Phase Epitaxy (VPE), Metalorganic Chemical Vapour Deposition (MOCVD), and Molecular Beam Epitaxy (MBE).

Vapour Phase Epitaxy (VPE)


Amount of material in a typical cell = 3100 g/m^2

Peak Power = 272.8 W/m^2

Amount of material per Watt Peak = 11.4 g/Wpeak[2]

Material utilization efficiency of MOCVD = 30%[3]

Material utilization efficiency of MBE for Ga = 40-70% Material utilization efficiency of MBE for As = 10-20% Average Material utilization efficiency of MBE = 35% [4]

Waste material rate MOCVD = 2170 g/m^2 and 7.98 g/Wpeak

Waste material rate MBE = 2015 g/m^2 and 7.41 g/Wpeak


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. http://www.spacequest.com/products/SP-X.pdf
  3. http://www.bnl.gov/pv/files/pdf/art_168.pdf
  4. http://www.bnl.gov/pv/files/pdf/art_168.pdf
Asi.png This page was part of a project for MY3701 -- an MTU class on semiconductors.

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