Some impurities and defects in the composite center semiconductor can promote carrier recombinationNonequilibrium carrier lifetime The length of impurities and defects that play a decisive role is called the composite center.Impurities and defects as recombination centers generally introduce one or several deep levels in the band gap, which can capture both electrons and holes, thus promoting the recombination process.[1]
In addition to donors and acceptors, there is also a kind of impurities that play a special role in semiconductors, called "recombination centers".It has a great influence on the working performance of semiconductor devices.
donor、acceptorThe energy level in the band gap is close to the bottom of the conduction band or the top of the valence band, usually about 10-2The order of magnitude of eV is generally called "shallow level".For the "recombination center" to be explained now, its energy levels are mostly located in the middle of the forbidden band, far from the bottom of the conduction band or the top of the valence band, which is usually called "deep level".The role of the recombination center in semiconductors is to promote the recombination of electrons and holes.After the recombination center captures an electron (that is, the electron in the conduction band falls into the recombination center), if another hole is captured (that is, the electron in the recombination center falls into the empty state in the valence band, this process is equivalent to the hole falling from the valence band to the recombination center), the electron and hole will recombine and disappear, and the recombination center will return to its original state.This effect of promoting the recombination of electrons and holes seems to be a "step".With this step, the recombination of electrons and holes is not a direct encounter between electrons in the conduction band and holes in the valence band and disappears, but a two-step process through the "step" of the recombination center. Therefore, this recombination process is also called "indirect recombination", which is different from the aforementioned direct recombination.In fact, the recombination process in semiconductors is often realized by indirect recombination.
On the other hand, the composite center also plays a role.The effect is the reverse process of recombination, that is, the electrons in the valence band are first excited to the recombination center, and then excited to the conduction band from the recombination center, leaving holes in the valence band.
The recombination and generation of the recombination center have a great influence on some characteristics of semiconductor devices.
Gold is a famous recombination center in semiconductor silicon. Gold is often doped into switching transistors to accelerate the recombination of electrons and holes and improve the switching speed of transistors.[2]
Composite center theory
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Impurities and defects can introduce localized energy levels into the band gap. These energy levels, especially those at the bottom of the conduction band and the top of the valence band, act as intermediate stations for the recombination of electrons and holes just like steps. They can promote the recombination of electrons and holes. These impurities and defects that can promote the recombination process are called recombination centers,Their energy levels are called composite central energy levels.Indirect compounding refers to compounding through the compounding center.
Only when the recombination effect of the recombination center is very weak, the life is determined by direct recombination.The life determined by direct composite is the maximum life value that the material can have.Because the general actual materials contain impurities and defects, the life is far less than the value determined by direct composite.
For a simple case where there is only one compound central energy level in the band gap, as shown in Figure 1 (the top horizontal line represents ECEnergy level, the middle represents EtEnergy level, the bottom horizontal line represents EvEnergy level), the composite central energy level E in the band gaptLike a step, the recombination of electrons and holes can be divided into two steps: the conduction band electrons fall into the central energy level of the recombination, and the electrons fall from the central energy level of the recombination to the valence band and recombine with the holes. The recombination center returns to the original state, and the next recombination process can be completed again.Of course, there is also the inverse process of the above two processes, so indirect recombination is still a statistical process, relative to the central energy level E of recombinationtThere are four micro processes:
A: electron capture process;The electrons fall from the conduction band into the empty impurity level.
B: Emission of electronic process;The electron is excited from the central energy level to the conduction band, which is the inverse process of A.
C: hole capture process;The electron falls into the valence band and recombines with the hole from the energy level of the recombination center, which is equivalent to that the hole is trapped by the valence band in the recombination center.
D: hole emission process;The valence band electrons are excited to the energy level of the empty recombination center, which is equivalent to that the recombination center emits holes to the valence band.
When the recombination center captures an electron or hole, it also needs to release excess energy. In principle, the specific process can be a radiative transition, a non radiative transition or an Auger process.[3]
Figure 1
Action mechanism of composite center
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The recombination through the recombination center is a kind ofIndirect recombinationThis recombination process is the basic process that determines the minority carrier lifetime in indirect band structure semiconductors such as Si, Ge, etc.The impurities in the composite center are often metal elements with small atomic radius, which are easy to enter the semiconductor;Therefore, in order to ensure that a few carriers have a long enough life, special attention should be paid to cleanliness in the process of making devices to ensure that the impurities in the composite center do not cause pollution.
The semiconductor surface itself is a big defect, so after the semiconductor devices and integrated circuits are fabricated, the device surface needs to be well protected to reduce the impact of the surface recombination center, which is actually one of the main purposes of the so-called surface passivation technology.
The indirect recombination process caused by the recombination center is usually much slower than the direct recombination process between the conduction band and the valence band. This is because the recombination process needs to meet both energy conservation and momentum conservation.For direct band semiconductors (such as GaAs), the minority carrier lifetime is mainly determined by the direct recombination process, so the minority carrier lifetime of such semiconductors is inherently short.While the direct recombination life of minority carriers in Si, Ge, etc. is longer (because the bottom of the conduction band and the top of the valence band are not at the same point in the Brillouin region), the life is mainly determined by the indirect recombination process, so it has a great relationship with the concentration of the recombination center.
Difference from other two important impurities and defect centers
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(1) Trap center: This is a deep level impurity or defect.The characteristic of the trap center is that the role of capturing one carrier is particularly strong, while the role of capturing another carrier is particularly weak, so the trap center has the role of storing one carrier.For example, electron traps play the role of storing electrons, and hole traps play the role of storing holes.In general, the energy level depth of the trap center is shallower than that of the composite center.
(2) Shallow level center: This is a shallow level impurity.Donor and acceptor impurity centers belong to this;The energy levels of these centers are very shallow (very close to the bottom of the conduction band or the top of the valence band), and they mainly play the role of providing carriers.
Various impurities and defect centers in semiconductors not only play different special roles, but also play a scattering center and affectCarrier mobilityRole of.
Isoelectronic recombination center
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A certain amount of impurity atoms equivalent to the main atoms are doped into the semiconductor to replace the atoms on the lattice.Due to the electrical difference between the impurity atom and the main atom, neutral impurity atoms can bind electrons or holes and become charged centers.The charged center attracts a carrier whose symbol is opposite to that of the bound carrier, forming an exciton bound state.This exciton bound state is called isoelectronic recombination center.
For example, a certain amount of pentavalent nitrogen atoms are doped into the GaP crystal to replace the pentavalent phosphorus atoms on the lattice.The nitrogen atom binds an electron to become a negative center, and then attracts a positively charged hole to form an exciton bound state with it, becoming an exciton composed of electrons and holes.The electrons in the exciton recombine with the holes, causing luminescence.
Isoelectron means that the impurity atom is equivalent to the main atom it replaces.Exciton is a neutral particle formed by the combination of electrons and holes.
The introduction of isoelectronic recombination centers in semiconductors is an indirect transition type such as gallium phosphideLuminescent materialIt is an important way to realize high efficiency luminescence.[4]
Non luminous composite center
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Generally speaking, the non luminous composite center is the composite center that does not emit light within the measurement wavelength range, and may emit light beyond the measurement wavelength.Defects in crystals generally form non luminescent composite centers, while donors and acceptors with shallow energy levels are usually not non luminescent composite centers.For example, there are energy levels near the center of the band gap that become the non luminescent recombination center, or there are many energy levels, and these energy levels undergo transitions in sequence.These centers emit long wave light even if they emit light, and multi phonon processes are easy to occur when the energy level spacing is small.
An interesting phenomenon related to this is that free electrons in high-purity GaP crystals are trapped by oxygen deep donors at low temperatures.The spectrum is the emission spectrum when the electron is captured by the excited state of the O donor and then falls to the ground state.This energy is about 17 times that of optical wave phonons, and it is difficult to generate multi phonon processes.Such luminescence disappears with the increase of temperature.When the donor concentration or acceptor concentration is greater than about 10seventeencm-3In the crystal of O donor, such luminescence will not occur, because the Auger process on the donor or acceptor near O donor causes ionization, so that the energy is transferred.
The crystal surface is also a non luminous center.The influence of surface can be estimated by introducing surface recombination velocity into the carrier diffusion equation.For GaP
In general, at room temperature, S>4X10fifteenCm/s, while S>2X10 on the corroded surfacesixCm/sec.The S value decreases at low temperatures.[5]
