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Gallium arsenide: structure, properties, uses, risks

Nanostructures of this compound have been obtained with potential for various uses in many fields of electronics. It belongs to a group of materials called compounds III-V due to the location of their elements in the chemical periodic table.

GaAs nanostructures. Яна Сычикова, Сергей Ковачёв / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

It is a semiconductor material, which means that it can conduct electricity only under certain conditions. It is widely used in electronic devices, such as transistors, GPS, LED lights, lasers, tablets, and smart phones.

It has characteristics that allow it to easily absorb light and convert it into electrical energy. For this reason it is used in solar cells of satellites and space vehicles.

However, it is a toxic compound and can cause cancer in humans and animals. Electronic equipment that is disposed of in landfills can release dangerous arsenic and be harmful to the health of people, animals and the environment.

Structure

Gallium arsenide has a 1: 1 ratio between an element of Group III of the periodic table and an element of Group V, which is why it is called compound III-V.

It is considered to be an intermetallic solid composed of arsenic (As) and gallium (Ga) with oxidation states ranging from Ga (0) As (0) to Ga (+3) As (-3) .

Gallium arsenide crystal. W. Oelen / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0). Source: Wikimedia Commons.

Nomenclature

  • Gallium arsenide
  • Gallium monoarsenide

Properties

Physical state

Dark gray crystalline solid with blue-green metallic luster or gray powder. Its crystals are cubic.

GaAs crystals. Left: polished side. Right: rough side. Materialscientist at English Wikipedia / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0). Source: Wikimedia Commons.

Molecular weight

144.64 g / mol

Melting point

1238 ºC

Density

5.3176 g / cm 3 at 25 ° C.

Solubility

In water: less than 1 mg / mL at 20 ° C.

Chemical properties

It has a hydrate that can form acid salts. It is stable in dry air. In humid air it darkens.

It can react with steam, acids and acid gases, emitting the poisonous gas called arsine, arsan or arsenic hydride (AsH 3 ). Reacts with bases emitting hydrogen gas.

It is attacked by concentrated hydrochloric acid and by halogens. When molten it attacks the quartz. If it gets wet it gives off a garlic odor and if it is heated to decomposition it emits very toxic arsenic gases.

Other physical properties

It is a semiconductor material which means that it can behave as a conductor of electricity or as an insulator depending on the conditions to which it is subjected, such as the electric field, pressure, temperature or radiation it receives.

Gap between electronic bands

It has an energy gap width of 1,424 eV (electron volts). The width of the energy gap, forbidden band or bandgap (from English bandgap ) is the space between the electron shells of an atom.

The wider the energy gap, the greater the energy required by the electrons to “jump” to the next shell and cause the semiconductor to change to a conductive state.

GaAs has an energy gap width greater than that of silicon and this makes it highly resistant to radiation. It is also a direct gap width, so it can emit light more effectively than silicon, whose gap width is indirect.

Obtaining

It can be obtained by passing a gaseous mixture of hydrogen (H 2 ) and arsenic over gallium (III) oxide (Ga 2 O 3 ) at 600 ° C.

It can also be prepared by the reaction between gallium (III) chloride (GaCl 3 ) and arsenic oxide (As 2 O 3 ) at 800 ° C.

Use in solar cells

Gallium arsenide has been used in solar cells since the 1970s, as it has outstanding photovoltaic characteristics that give it an advantage over other materials.

It performs better than silicon in converting solar energy into electricity, delivering more energy under high heat or low light conditions, two of the common conditions that solar cells endure, where there are changes in lighting and temperature levels.

Some of these solar cells are used in solar powered cars, space vehicles, and satellites.

GaAs solar cells on a small satellite. United States Naval Academy / Public domain. Source: Wikimedia Commons.

Advantages of GaAs for this application

It is resistant to humidity and ultraviolet radiation, which makes it more durable against environmental conditions and allows it to be used in aerospace applications.

It has a low temperature coefficient, so it does not lose efficiency at high temperatures and resists high accumulated doses of radiation. Radiation damage can be removed by tempering at just 200 ° C.

It has a high coefficient of absorption of photons of light, so it has a high performance in low light, that is, it loses very little energy when there is poor illumination from the sun .

GaAs solar cells are efficient even in low light. Author: Arek Socha. Source: Pixabay.

It produces more energy per unit area than any other technology. This is important when you have a small area such as aircraft, vehicles or small satellites.

It is a flexible and low- weight material , being efficient even when applied in very thin layers, which makes the solar cell very light, flexible and efficient.

Solar cells for space vehicles

Space programs have used GaAs solar cells for more than 25 years.

The combination of GaAs with other compounds of germanium, indium and phosphorus has made it possible to obtain very high efficiency solar cells that are being used in vehicles that explore the surface of the planet Mars .

Artist’s version of the Curiosity rover on Mars. This device has solar cells of GaAs. NASA / JPL-Caltech / Public domain. Source: Wikimedia Commons.

Disadvantage of GaAs

It is a very expensive material compared to silicon, which has been the main barrier to its practical implementation in terrestrial solar cells.

However, methods for their use in extremely thin layers are being studied, which will reduce costs.

Use in electronic devices

GaAs has multiple uses in various electronic devices.

In transistors

Transistors are elements that serve to amplify electrical signals and open or close circuits, among other uses.

Used in transistors, GaAs has a higher electronic mobility and a higher resistivity than silicon, so it tolerates higher energy and higher frequency conditions, generating less noise.

GaAs transistor used to amplify power . Epop / CC0. Source: Wikimedia Commons.

On GPS

In the 1980s the use of this compound allowed the miniaturization of the Global Positioning System or GPS ( Global Positioning System ) receivers .

This system makes it possible to determine the position of an object or person on the entire planet with an accuracy of centimeters.

Gallium arsenide is used in GPS systems. Author: Foundry Co. Source: Pixabay.

In optoelectronic devices

GaAs films obtained at relatively low temperatures have excellent optoelectronic properties, such as high resistivity (requires high energy to become conductive) and rapid electron transfer.

Its direct energy gap makes it suitable for use in this type of device. They are devices that transform electrical energy into radiant energy or vice versa, such as LED lights, lasers, detectors, light-emitting diodes, etc.

LED flashlight. May contain gallium arsenide. Author: Hebi B. Source: Pixabay.

In special radiation

The properties of this compound have prompted its use to generate radiation with frequencies of terahertz, which are radiation that can penetrate all types of materials except metals and water.

Terahertz radiation, because it is non-ionizing, can be applied in obtaining medical images, since it does not damage the tissues of the body or cause changes in the DNA like X-rays.

These radiations would also make it possible to detect hidden weapons in people and luggage, can be used in spectroscopic analysis methods in chemistry and biochemistry, and could help uncover hidden works of art in very old buildings.

Potential medical treatment

A type of GaAs laser has been shown to be useful in enhancing the regeneration of muscle mass damaged by a type of snake venom in mice. However, studies are required to determine its effectiveness in humans.

Various teams

It is used as a semiconductor in magnetoresistance devices, thermistors, capacitors, photoelectronic fiber optic data transmission, microwaves, integrated circuits used in satellite communication devices, radar systems, smartphones (4G technology) and tablets.

Electronic circuits in smartphones can contain GaAs. Author: Arek Socha. Source: Pixabay.

Risks

It is a highly toxic compound. Prolonged or repeated exposure to this material causes damage to the body.

Symptoms of exposure can include hypotension, heart failure, seizures, hypothermia, paralysis, respiratory edema, cyanosis, liver cirrhosis, kidney damage , hematuria, and leukopenia, among many others.

It can cause cancer and damage fertility. It is toxic and carcinogenic also for animals.

Hazardous waste

The increasing use of GaAs in electronic devices has raised concerns regarding the fate of this material in the environment and its potential risks to public and environmental health.

There is a latent risk of arsenic release (a toxic and poisonous element) when devices containing GaAs are disposed of in municipal solid waste dumps.

Studies show that pH and redox conditions in landfills are important for GaAs corrosion and arsenic release. At pH 7.6 and under a normal oxygen atmosphere , up to 15% of this toxic metalloid can be released.

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