Gallium arsenide
Condensed from Wikipedia, the free encyclopedia
compound of the elements gallium and arsenic. It is a III-V direct bandgap semiconductor with a zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circuits, infrared light-emitting diodes, laser diodes, solar cells and optical windows.[2]
GaAs is often used as a substrate material for the epitaxial growth of other III-V semiconductors including: Indium gallium arsenide, aluminum gallium arsenide and others.
7.[5]
A pristine 2-inch single crystal gallium arsenide wafer with a (100) surface orientation. Purple features are a reflection of a nitrile glove.
This low carrier concentration is similar to an intrinsic (perfectly undoped) crystal, but much easier to achieve in practice. These crystals are called "semi-insulating", reflecting their high resistivity of 107–109 Ω·cm (which is quite high for a semiconductor, but still much lower than a true insulator like glass).[6]
Another advantage of GaAs is that it has a direct band gap, which means that it can be used to absorb and emit light efficiently. Silicon has an indirect bandgap and so is relatively poor at emitting light. Nonetheless, advances silicon LEDs and lasers may be possible.
As a wide direct band gap material with resulting resistance to radiation damage, GaAs is an excellent material for space electronics and optical windows in high power applications.
Because of its wide bandgap, pure GaAs is highly resistive. Combined with the high dielectric constant, this property makes GaAs a very good electrical substrate and unlike Si provides natural isolation between devices and circuits. This has made it an ideal material for microwave and millimeter wave integrated circuits, MMICs, where active and essential passive components can readily be produced on a single slice of GaAs.
One of the first GaAs microprocessors was developed in the early 1980s by the RCA corporation and was considered for the Star Wars program of the United States Department of Defense. Those processors were several times faster and several orders of magnitude more radiation hard than silicon counterparts, but they were rather expensive.[9] Other GaAs processors were implemented by the supercomputer vendors Cray Computer Corporation, Convex, and Alliant in an attempt to stay ahead of the ever-improving CMOS microprocessor. Cray eventually built one GaAs-based machine in the early 1990s, the Cray-3, but the effort was not adequately capitalized, and the company filed for bankruptcy in 1995.
Complex layered structures of gallium arsenide in combination with aluminium arsenide (AlAs) or the alloy AlxGa1-xAs can be grown using molecular beam epitaxy (MBE) or using metalorganic vapor phase epitaxy (MOVPE). Because GaAs and AlAs have almost the same lattice constant, the layers have very little induced strain, which allows them to be grown almost arbitrarily thick. This allows for extremely high performance high electron mobility, HEMT transistors and other quantum well devices.
In addition, a Si crystal has an extremely stable structure mechanically and it can be grown to very large diameter boules and can be processed with very high yields. It is also a decent thermal conductor, thus enabling very dense packing of transistors that need to get rid of their heat of operation, all very desirable for design and manufacturing of very large ICs. Such good mechanical characteristics also makes it a suitable material for the rapidly developing field of nanoelectronics.
The second major advantage of Si is the existence of a native oxide (silicon dioxide, SiO2), which is used as an insulator in electronic devices. Silicon dioxide can easily be incorporated onto silicon circuits, and such layers are adherent to the underlying Si. SiO2 is not only a good insulator (with a band gap of 8.9 eV), but the Si-SiO2 interface can be easily engineered to have excellent electrical properties, most importantly low density of interface states. GaAs does not have a native oxide and does not easily support a stable adherent insulating layer.[citation needed] Aluminum oxide (Al2O3) has been extensively studied as a possible gate oxide for GaAs (and InGaAs). However, at this point the electrical properties of the interfaces aren't comparable to those of the Si-SiO2 interface.
The third, advantage of silicon is that it possesses a higher hole mobility compared to GaAs (500 versus 400 cm2V-1s-1).[10] This high mobility allows the fabrication of higher-speed P-channel field effect transistors, which are required for CMOS logic. Because they lack a fast CMOS structure, GaAs circuits must use logic styles which have much higher power consumption; this has made GaAs circuits less able to compete with silicon logic circuits.
For manufacturing solar cells, silicon has relatively low absorptivity for the sunlight meaning about 100 micrometers of Si is needed to absorb most sunlight. Such a layer is relatively robust and easy to handle. In contrast, the absorptivity of GaAs is so high that only a few micrometers of thickness are needed to absorb all of the light. Consequently GaAs thin films must be supported on a substrate material.[11]
Silicon is a pure element, avoiding the problems of stoichiometric imbalance and thermal unmixing of GaAs.[citation needed]
Silicon has a nearly perfect lattice, impurity density is very low and allows very small structures to be built (currently down to 16 nm[12]). GaAs in contrast has a very high impurity density,[citation needed] which makes it difficult to build integrated circuits with small structures, so the 500 nm process is a common process for GaAs.
High-efficiency, triple-junction gallium arsenide solar cells covering the MidSTAR-1 satellite
In 1970, the first GaAs heterostructure solar cells were created by the team led by Zhores Alferov in the USSR.[13][14][15] In the early 1980s, the efficiency of the best GaAs solar cells surpassed that of silicon solar cells, and in the 1990s GaAs solar cells took over from silicon as the cell type most commonly used for Photovoltaic arrays for satellite applications. Later, dual- and triple-junction solar cells based on GaAs with germanium and indium gallium phosphide layers were developed as the basis of a triple-junction solar cell, which held a record efficiency of over 32% and can operate also with light as concentrated as 2,000 suns. This kind of solar cell powers the rovers Spirit and Opportunity, which are exploring Mars' surface. Also many solar cars utilize GaAs in solar arrays.
GaAs-based devices hold the world record for the highest-efficiency single-junction solar cell at 28.8%.[16] This high efficiency is attributed to the extreme high quality GaAs epitaxial growth, surface passivation by the AlGaAs,[17] and the promotion of photon recycling by the thin film design.[18]
Complex designs of AlxGa1−xAs-GaAs devices can be sensitive to infrared radiation (QWIP).
GaAs diodes can be used for the detection of X-rays.[19]
Band structure of GaAs. The direct gap of GaAs results in efficient emission of infrared light at 1.424 eV (~870 nm).
GaAs has been used to produce (near-infrared) laser diodes since 1962.[20]
GaAs is often used as a substrate material for the epitaxial growth of other III-V semiconductors including: Indium gallium arsenide, aluminum gallium arsenide and others.
Preparation and chemistry
In the compound, gallium has a +3 oxidation state. Gallium arsenide single crystals can be prepared by three industrial processes:[2]- The vertical gradient freeze (VGF) process. Most GaAs wafers are produced using this process.[3]
- Crystal growth using a horizontal zone furnace in the Bridgman-Stockbarger technique, in which gallium and arsenic vapors react, and free molecules deposit on a seed crystal at the cooler end of the furnace.
- Liquid encapsulated Czochralski (LEC) growth is used for producing high-purity single crystals that can exhibit semi-insulating characteristics (see below).
- VPE reaction of gaseous gallium metal and arsenic trichloride:
- 2 Ga + 2 AsCl
3 → 2 GaAs + 3 Cl
2
- MOCVD reaction of trimethylgallium and arsine:
- Ga(CH
3)
3 + AsH
3 → GaAs + 3 CH
4
- Molecular beam epitaxy (MBE) of gallium and arsenic:
- 4 Ga + As
4 → 4 GaAs
- 2 Ga + As
2 → 2 GaAs
7.[5]
_2%22_wafer.jpg)
Semi-insulating crystals
If a GaAs boule is grown with excess arsenic present, it gets certain defects, in particular arsenic antisite defects (an arsenic atom at a gallium atom site within the crystal lattice). The electronic properties of these defects (interacting with others) cause the Fermi level to be pinned to near the center of the bandgap, so that this GaAs crystal has very low concentration of electrons and holes.This low carrier concentration is similar to an intrinsic (perfectly undoped) crystal, but much easier to achieve in practice. These crystals are called "semi-insulating", reflecting their high resistivity of 107–109 Ω·cm (which is quite high for a semiconductor, but still much lower than a true insulator like glass).[6]
Etching
Wet etching of GaAs industrially uses an oxidizing agent such as hydrogen peroxide or bromine water,[7] and the same strategy has been described in a patent relating to processing scrap components containing GaAs where the Ga3+ is complexed with a hydroxamic acid ("HA"), for example:[8]- GaAs + H
2O
2 + "HA" → "GaA" complex + H
3AsO
4 + 4 H
2O
Comparison with silicon
GaAs advantages
Some electronic properties of gallium arsenide are superior to those of silicon. It has a higher saturated electron velocity and higher electron mobility, allowing gallium arsenide transistors to function at frequencies in excess of 250 GHz. Unlike silicon junctions, GaAs devices are relatively insensitive to heat owing to their wider bandgap. Also, GaAs devices tend to have less noise than silicon devices, especially at high frequencies. This is a result of higher carrier mobilities and lower resistive device parasitics. These properties recommend GaAs circuitry in mobile phones, satellite communications, microwave point-to-point links and higher frequency radar systems. It is used in the manufacture of Gunn diodes for generation of microwaves.Another advantage of GaAs is that it has a direct band gap, which means that it can be used to absorb and emit light efficiently. Silicon has an indirect bandgap and so is relatively poor at emitting light. Nonetheless, advances silicon LEDs and lasers may be possible.
As a wide direct band gap material with resulting resistance to radiation damage, GaAs is an excellent material for space electronics and optical windows in high power applications.
Because of its wide bandgap, pure GaAs is highly resistive. Combined with the high dielectric constant, this property makes GaAs a very good electrical substrate and unlike Si provides natural isolation between devices and circuits. This has made it an ideal material for microwave and millimeter wave integrated circuits, MMICs, where active and essential passive components can readily be produced on a single slice of GaAs.
One of the first GaAs microprocessors was developed in the early 1980s by the RCA corporation and was considered for the Star Wars program of the United States Department of Defense. Those processors were several times faster and several orders of magnitude more radiation hard than silicon counterparts, but they were rather expensive.[9] Other GaAs processors were implemented by the supercomputer vendors Cray Computer Corporation, Convex, and Alliant in an attempt to stay ahead of the ever-improving CMOS microprocessor. Cray eventually built one GaAs-based machine in the early 1990s, the Cray-3, but the effort was not adequately capitalized, and the company filed for bankruptcy in 1995.
Complex layered structures of gallium arsenide in combination with aluminium arsenide (AlAs) or the alloy AlxGa1-xAs can be grown using molecular beam epitaxy (MBE) or using metalorganic vapor phase epitaxy (MOVPE). Because GaAs and AlAs have almost the same lattice constant, the layers have very little induced strain, which allows them to be grown almost arbitrarily thick. This allows for extremely high performance high electron mobility, HEMT transistors and other quantum well devices.
Silicon advantages
Silicon has three major advantages over GaAs for integrated circuit manufacture. First, silicon is abundant and cheap to process. Si is highly abundant in the Earth's crust, in the form of silicate minerals. The economy of scale available to the silicon industry has also reduced the adoption of GaAs.In addition, a Si crystal has an extremely stable structure mechanically and it can be grown to very large diameter boules and can be processed with very high yields. It is also a decent thermal conductor, thus enabling very dense packing of transistors that need to get rid of their heat of operation, all very desirable for design and manufacturing of very large ICs. Such good mechanical characteristics also makes it a suitable material for the rapidly developing field of nanoelectronics.
The second major advantage of Si is the existence of a native oxide (silicon dioxide, SiO2), which is used as an insulator in electronic devices. Silicon dioxide can easily be incorporated onto silicon circuits, and such layers are adherent to the underlying Si. SiO2 is not only a good insulator (with a band gap of 8.9 eV), but the Si-SiO2 interface can be easily engineered to have excellent electrical properties, most importantly low density of interface states. GaAs does not have a native oxide and does not easily support a stable adherent insulating layer.[citation needed] Aluminum oxide (Al2O3) has been extensively studied as a possible gate oxide for GaAs (and InGaAs). However, at this point the electrical properties of the interfaces aren't comparable to those of the Si-SiO2 interface.
The third, advantage of silicon is that it possesses a higher hole mobility compared to GaAs (500 versus 400 cm2V-1s-1).[10] This high mobility allows the fabrication of higher-speed P-channel field effect transistors, which are required for CMOS logic. Because they lack a fast CMOS structure, GaAs circuits must use logic styles which have much higher power consumption; this has made GaAs circuits less able to compete with silicon logic circuits.
For manufacturing solar cells, silicon has relatively low absorptivity for the sunlight meaning about 100 micrometers of Si is needed to absorb most sunlight. Such a layer is relatively robust and easy to handle. In contrast, the absorptivity of GaAs is so high that only a few micrometers of thickness are needed to absorb all of the light. Consequently GaAs thin films must be supported on a substrate material.[11]
Silicon is a pure element, avoiding the problems of stoichiometric imbalance and thermal unmixing of GaAs.[citation needed]
Silicon has a nearly perfect lattice, impurity density is very low and allows very small structures to be built (currently down to 16 nm[12]). GaAs in contrast has a very high impurity density,[citation needed] which makes it difficult to build integrated circuits with small structures, so the 500 nm process is a common process for GaAs.
Other applications
Solar cells and detectors
Another important application of GaAs is for high efficiency solar cells. Gallium arsenide (GaAs) is also known as single-crystalline thin film and are high-cost high-efficiency solar cells.In 1970, the first GaAs heterostructure solar cells were created by the team led by Zhores Alferov in the USSR.[13][14][15] In the early 1980s, the efficiency of the best GaAs solar cells surpassed that of silicon solar cells, and in the 1990s GaAs solar cells took over from silicon as the cell type most commonly used for Photovoltaic arrays for satellite applications. Later, dual- and triple-junction solar cells based on GaAs with germanium and indium gallium phosphide layers were developed as the basis of a triple-junction solar cell, which held a record efficiency of over 32% and can operate also with light as concentrated as 2,000 suns. This kind of solar cell powers the rovers Spirit and Opportunity, which are exploring Mars' surface. Also many solar cars utilize GaAs in solar arrays.
GaAs-based devices hold the world record for the highest-efficiency single-junction solar cell at 28.8%.[16] This high efficiency is attributed to the extreme high quality GaAs epitaxial growth, surface passivation by the AlGaAs,[17] and the promotion of photon recycling by the thin film design.[18]
Complex designs of AlxGa1−xAs-GaAs devices can be sensitive to infrared radiation (QWIP).
GaAs diodes can be used for the detection of X-rays.[19]
Light-emission devices
GaAs has been used to produce (near-infrared) laser diodes since 1962.[20]
