What is the main purpose of gallium arsenide

Gallium arsenide is a compound semiconductor material with a molecular formula of GaAs. The cubic zinc blende structure, that is, a compound lattice composed of face-centered cubic lattice sets of two kinds of atoms, As and Ga, has a lattice constant of 5.6419A. The band gap at room temperature is 1.428eV, which is a direct bandgap semiconductor with a melting point of 1238°C, a mass density of 5.307g/cm3, and a permittivity of 13.18. The conduction band of gallium arsenide single crystal has a dual-energy valley structure, and its lowest energy valley is located in the center of the first Brillouin zone, the effective electron mass is 0.068m0 (m0 is the electron mass, see carrier), and the second lowest energy valley is in the direction Point L is about 0.29eV higher than the lowest energy valley, and its effective electron mass is 0.55m0. The top of the valence band is about the center of the Brillouin zone. The effective masses of light holes and heavy holes in the valence band are 0.082m0, respectively. And 0.45m0. The electron and hole mobility of relatively pure gallium arsenide crystals are 8000cm2/(V·s) and 100～300cm2/(V·s) respectively, and the minority carrier lifetime is 10-2～10-3μs. Doping with group VI elements Te, Se, S, etc. or group IV element Si can obtain N-type semiconductors, doping with group II elements Be, Zn, etc. to prepare P-type semiconductors, doping with Cr or increasing the purity can be made Semi-insulating material with resistivity as high as 107～108Ω·cm. In the past ten years, due to the development of molecular beam epitaxy and metal organic chemical vapor deposition (MOCVD) technology, heterojunction and superlattice structures can be fabricated on GaAs single crystal substrates. These structures have been used to make new semiconductors. Devices such as high electron mobility transistors (HEMT), heterojunction bipolar transistors (HBT) and lasers have developed a broader prospect for the application of GaAs materials.

Background and overview [1]

Gallium arsenide is a compound semiconductor material with a molecular formula of GaAs. The cubic zinc blende structure, that is, a compound lattice composed of face-centered cubic lattice sets of two kinds of atoms, As and Ga, has a lattice constant of 5.6419A. The band gap at room temperature is 1.428eV, which is a direct bandgap semiconductor with a melting point of 1238°C, a mass density of 5.307g/cm3, and a permittivity of 13.18. The conduction band of gallium arsenide single crystal is a dual-energy valley structure, and its lowest energy valley is located in the center of the first Brillouin zone, the effective mass of electrons is 0.068m0 (m0 is the mass of electrons, see carrier), and the second lowest energy valley is located at < The point L in the direction of 111> is about 0.29eV higher than the lowest energy valley, and its effective electron mass is 0.55m0. The top of the valence band is about the center of the Brillouin zone. The effective masses of light holes and heavy holes in the valence band are respectively It is 0.082m0 and 0.45m0. The electron and hole mobility of relatively pure gallium arsenide crystals are 8000cm2/(V·s) and 100～300cm2/(V·s) respectively, and the minority carrier lifetime is 10-2～10-3μs. Doping with group VI elements Te, Se, S, etc. or group IV element Si can obtain N-type semiconductors, doping with group II elements Be, Zn, etc. to prepare P-type semiconductors, doping with Cr or increasing the purity can be made Semi-insulating material with resistivity as high as 107～108Ω·cm. In the past ten years, due to the development of molecular beam epitaxy and metal organic chemical vapor deposition (MOCVD) technology, heterojunction and superlattice structures can be fabricated on GaAs single crystal substrates. These structures have been used to make new semiconductors. Devices such as high electron mobility transistors (HEMT), heterojunction bipolar transistors (HBT) and lasers have developed a broader prospect for the application of GaAs materials.

Structure and properties [2]

The gallium arsenide lattice is composed of two face-centered cubic (fcc) sub-lattices (two sub-lattices of arsenic and gallium on the lattice points) shifted by 1/4 sets along the diagonal of the space body. This crystal structure is called the sphalerite structure in physics. Figure 1 shows a schematic diagram of the gallium arsenide unit cell structure, and Table 1 shows the physical and electrical parameters of currently known gallium arsenide semiconductor materials at room temperature.

Regarding the chemical composition of gallium arsenide, the covalent bond model of group III-V compounds believes that this type of compound forms a tetrahedral covalent bond. When the bond is formed, group III atoms provide three valence electrons in the s2p1 configuration, while group V atoms Provide 5 valence electrons in s2p3 configuration. There are four valence electrons per atom on average between them, which can be used to form tetrahedron covalent bonding. This type of compound is mainly covalently bound, but is mixed with some ionic binding properties. This is because the electronegativity of group V elements is greater than that of group III elements. When forming a crystal, part of the electrons will be transferred from atoms with low electronegativity (group III elements) to atoms with higher electronegativity (group V elements) To go, this transfer (polarization) of charge makes the group III element positively charged and the group V element negatively charged. If the concept of effective charge Z*e is used to describe the extent of this charge transfer, the "covalent bond" model can consider that gallium arsenide crystals are mainly covalently bonded, but are mixed with some ionic bonding properties, and each ion has Effective charge Z*e

Purpose [3]

Because GaAs has a high electron mobility, it can be used to prepare high-speed or microwave semiconductor devices. Gallium arsenide is also used to make high-temperature, radiation-resistant or low-noise devices, as well as near-infrared light-emitting and laser devices, as well as photocathode materials. More importantly, it will become the basic material for the development of ultra-high-speed semiconductor integrated circuits in the future.

1. Development and utilization of gallium arsenide electronic devices

According to the high electron mobility of gallium arsenide material, it is an ideal device material for the development of ultra-high-speed computers. Its electron mobility is about 5 times higher than that of silicon, and its operation speed is much higher than that of silicon devices. In the 1970s and 1980s, people predicted and optimistic that gallium arsenide materials will play an important role in the development of ultra-high-speed computers, and invested considerable human and financial resources in research. However, due to some technical and cost issues, and the sudden emergence of silicon materials, complementary metal oxide semiconductor circuits have been developed, which have low operating voltage, low power consumption, high speed, and low cost, which can meet the needs of the devices at that time. As a result, competition pressure for gallium arsenide has been formed, causing the development of ultra-high-speed computers by gallium arsenide to temporarily slow down. In recent years, with the end of the Cold War, many military technologies have been transferred to civilian use. Due to the unique high-frequency, high-speed, low-noise, and low-voltage characteristics of gallium arsenide materials, it plays an important role in high-frequency and high-speed transmission of information and digital processing. Electronic devices developed with gallium arsenide materials: such as metal semiconductor field effect transistors, high mobility transistors, microwave monolithic integrated circuits, heterojunction bipolar transistors (CDB), etc. in mobile communications, optical fiber communications, satellite broadcast communications Silicon devices play an irreplaceable role in silicon devices, newspaper processing, and other areas. The promotion of these applications has greatly promoted the development of gallium arsenide materials.

2. Application of gallium arsenide optoelectronic devices

Application aspects of gallium arsenide optoelectronic devices: visible light emitting diodes. Because of its small size, energy saving, fast response and long life, it is widely used in home appliances, office equipment, billboards, traffic lights and car tail lights, etc.; infrared light-emitting diodes. Used as remote control, optical isolator, encoder and 34 machine, wireless connection of office equipment, short-distance information transmission, etc.; laser. Widely used in CD, MD, DVD, medical and other industrial fields; GaAs solar cells are used in satellite communications and other fields. These widespread military and civilian applications of gallium arsenide devices have greatly promoted the development of doped conductive gallium arsenide.

Defects [4]

Technicians are working hard to improve the growth quality of crystals, but the crystals still contain a large number of point defects, dislocations and impurities. These crystal defects always affect the performance of gallium arsenide devices. The formation of these defects is mainly determined by the way of doping into the material and the condition of growth.

1. Point defect

Defects within the atomic scale of crystals are called point defects. Can exist in perfect crystals or imperfect crystals. Point defects include vacancies in the crystal lattice, interstitial atoms, dislocation atoms, and impurity atoms introduced intentionally or unintentionally during the crystal growth process. It is of great significance to study the behavior of these point defects in crystals, because the behavioral effects and defect types of these defects have a greater impact on the scattering of carriers in the material and the number of carriers, which will directly affect the electrical properties of the material. Stable performance. When these mechanisms are clear, we can deliberately introduce beneficial defects during the growth of the crystal material, so as to achieve the purpose of improving the electrical properties of the crystal material.

2. Dislocation

A series of continuous point defects penetrate a certain area of the crystal, forming dislocations. When the stress on the crystal lattice exceeds the maximum elastic force required for elastic deformation of the crystal lattice, this continuous defect can be produced. If the crystal is made into a thin sheet, and then ground and polished, the dislocation outcrop on the thin sheet can show some unique corrosion pit morphology under the action of chemical reagent corrosion. The morphology of these corrosion pits only exists at a certain distance near the dislocation. In the gallium arsenide wafer processing technology of the semiconductor industry, the number of etch pits per unit area is specified as the dislocation density of the wafer (EtchPitDensity-EPD). The existence of dislocations is equivalent to forming a scattering channel inside the semiconductor, which will accelerate the scattering of carriers in the semiconductor. If described by energy band theory, it is equivalent to introducing a capture center in the forbidden band, which will change the performance effect of the wafer during etching, and the direct consequence is to change the electrical performance of the device. Studies have shown that in field-effect transistors, due to the dislocation effect, it will adversely affect the source channel current, gate voltage, carrier concentration, and substrate resistivity.

3. Impurities in Gallium Arsenide

During the crystal growth process, impurities are introduced intentionally or unintentionally. Under normal circumstances, the introduced impurities are electrically active, but some introduced pollution will form vacancies in the crystal, which will not be electrically active. It is stipulated that the doped impurities are either donor atoms or acceptor atoms in the semiconductor. A donor atom has one or more electrons more than the atom it replaces, and these extra electrons can move freely in the crystal to form an electric current; on the contrary, an acceptor atom has one or more electrons less than the atom it replaces. Therefore, acceptor atoms can trap freely moving electrons in the crystal. No matter what type of impurity is doped in the semiconductor, it will cause the change of the electrical properties of the semiconductor material. The energy level of the shallow donor impurity and the energy level of the shallow acceptor impurity are respectively located in the energy range of 3kT near the conduction band and the valence band in the forbidden band. Since the energy required to make the carriers in the impurity energy level transition to its corresponding higher energy level is very small, it is generally considered that the impurities in the semiconductor are completely ionized at room temperature.

Because the Fermi level is the level of the electron filling level in the energy band, the Fermi level moves from the middle position to the vicinity of the impurity level with the type of dopant incorporated. In other words, when the donor impurity is doped, the Fermi level will move closer to the conduction band, and the energy level difference between the Fermi level and the bottom of the conduction band decreases with the increase of the doping concentration; the acceptor impurity is in the conduction band The behavior in is the opposite of the donor impurity. The purpose of doping in gallium arsenide is to introduce shallow donor or shallow acceptor impurities. If the energy level of the introduced impurity is located in the central region of the band gap, the impurity is called a deep level impurity. In general, deep-level impurities will affect device performance due to reduced carrier lifetime. Two types of impurities, whether they are shallow-level impurities or deep-level impurities, exist in gallium arsenide crystals through complex combinations with arsenic atoms or gallium atoms. Silicon is currently the most widely studied dopant. This Group IV element can form p-type materials with gallium arsenide at low temperatures, and n-type materials with gallium arsenide at high temperatures. Chromium is the dominant atom in gallium arsenide, and its impurity energy level is close to the center of the band gap. Using this feature, the shallow n-type gallium arsenide material can be compensated by doping chromium to obtain a semi-insulating material. The behavior of other elements, such as copper, oxygen, selenium, tellurium, tin, etc. in gallium arsenide has also been extensively studied. In this way, we can dope to obtain n-type or p-type gallium arsenide according to the requirements of device design. .

Preparation [5]

The method of preparing GaAs single crystal includes zone melting method and liquid-sealed Czochralski method. Diffusion, ion implantation, gas or liquid phase epitaxy and evaporation can be used to make PN junctions, heterojunctions, Schottky junctions and ohmic contacts.

Main reference materials

[1] Encyclopedia of China Electric Power·Basic Volume of Electrical Engineering Technology

[2] Gallium arsenide material

[3] The development and prospects of gallium arsenide materials