The radiative recombination process has nothing to do with the source of non-equilibrium carriers, but is closely related to the physical properties of the material. Radiation recombination can be produced directly by the recombination of electrons and holes between bands, or by intermediate energy levels formed by crystal defects, doped impurities and impurity polymers. In actual semiconductor materials, there is not only one type of radiation recombination, nor all types of radiation recombination, but there can be several types of radiation recombination.
(1) Interband radiation recombination
Inter-band radiation recombination is the direct transition of electrons in the conduction band to the valence band and the recombination of holes in the valence band. The energy of the emitted photons is close to the band gap Eg of the semiconductor material. Interband radiation recombination is the inverse process of intrinsic absorption. Interband radiation recombination is achieved by the recombination of electrons and holes at those energy levels close to the edge of the band. Due to the thermal distribution of carriers, electrons are not completely at the bottom of the conduction band, and holes are not completely at the top of the valence band, so the combined emission spectrum has a certain width.
According to the different energy band structures of semiconductor materials, inter-band recombination can be divided into direct radiation recombination and indirect radiation recombination (as shown in Figure 1). For semiconductors that undergo direct energy gap recombination, the conduction band minimum and valence band maximum occur at the same point in the Brillouin zone, that is, have the same k value. The quantum transition process requires that the electron must observe the energy conservation and Quasi-momentum conservation. In the process of direct transition, the quasi-momentum conservation of electrons is easy to satisfy, so the transition probability is high, and the luminous efficiency of direct radiation recombination is high. Most HE-V compound semiconductors have a band structure with a direct band gap and are important luminescent materials. For semiconductors with indirect energy gaps, the minimum conduction band and the maximum valence band do not occur at the same point in the Brillouin zone, but have different k values. Therefore, this transition is a non-vertical transition. In the transition process, the wave number of photons is much smaller than that of electrons. Therefore, the quasi-momentum conservation requirement
A third party must participate, that is, it must be accompanied by the absorption or emission of phonons during the transition. For non-vertical transitions, on the one hand, it involves the interaction of electrons and electromagnetic radiation, and on the other, it involves the interaction of electrons and crystal lattices. In theory, this is a two-level process, a process with a much lower probability than the vertical transition, so the luminous efficiency of indirect radiation recombination is much lower than that of direct radiation recombination. Ge, Si and GaP, AIAs and AlSb in Group III-V compounds belong to this category. Although the probability of indirect radiation recombination transition is very low, if appropriate impurities are added to this type of material, the recombination probability can also be changed and the luminous efficiency can be improved. For example, the doping of nitrogen or oxygen into gallium phosphide will form isoelectron traps, which greatly increases the probability of recombination of electrons and holes, and significantly improves the luminous efficiency of gallium phosphide, making it an important luminescent material .
(2) Recombination between shallow energy level and main band
The recombination between the main band and the shallow energy level is shown in Figure 2. It can be a recombination between a shallow donor and a valence band hole or a shallow acceptor and a conduction band electron. Because the ionization energy of the shallow donor (receiver) is very small (usually a few millielectron volts), it is often difficult to distinguish from the inter-band transition, but the experiment proves that the photon energy of this radiation is always smaller than the forbidden band width, so It is not caused by inter-band composite luminescence. It can generally be considered as the recombination of holes in the valence band and electrons trapped at the shallow donor level or the recombination of conduction band electrons and holes trapped at the shallow acceptor level. The light-emitting process is first conduction band electrons. It is trapped in a localized energy level, and then the electrons and valence band holes on this localized energy level recombine to emit light. This kind of light emission is also called edge luminescence. Experiments have shown that these localized energy levels may be physical defects (vacancies or gaps) in the crystal.
(3) Donor-acceptor pair (D-A pair) compound
The recombination between donor and acceptor is the recombination between electrons captured by the donor and holes captured by the acceptor. In the recombination process, photons are emitted, and the energy of the photons is less than the band gap. This is an important recombination luminescence mechanism in which the radiant energy is less than the forbidden band width. This recombination is also called D-A pair recombination. When the donor impurity and acceptor impurity enter the lattice point with substitution atoms at the same time and form close neighbors, these grouped pairs of donor and acceptor systems are relatively close, and the wave functions overlap each other, which makes the donor and acceptor independent of each other. The city field disappears to form a dipole potential field, which is combined into a donor-acceptor pair joint luminous center, which is called a DA pair. The energy level of D-A to the luminous center is shown in Figure 3. The donor captures the electrons, and the acceptor captures the holes, both of which are in an electrically neutral state. After the electrons on the donor recombine with the holes on the acceptor, the donor is again positively charged, and the acceptor is again negatively charged. Therefore, the D-A pair recombination process is a process of generating ionized donor-acceptor pairs in a neutral configuration, so recombination has a Coulomb effect. The strength of the Coulomb effect in the transition depends on the distance between the donor and the acceptor.
The spectrum of D-A’s composite emission is a series of discontinuous lines. The interval between the spectral lines depends on the distance between the donor and the acceptor. As the distance increases, the spectral lines move to the long wave, and as the distance increases, the spectrum becomes continuous into bands. Different impurity atoms and their substitution states will cause The spectrum of D-A’s composite emission is a series of discontinuous lines. The interval between the spectral lines depends on the distance between the donor and the acceptor. As the distance increases, the spectral lines move to the long wave, and as the distance increases, the spectrum becomes continuous into bands. Different impurity atoms and their substitution states will cause different ionization energies of D-A pairs. The luminescence of the D-A pair has a strong interaction with phonons at room temperature, and it is difficult to find the recombination line spectrum of the D-A pair. However, the line spectrum series of D-A pair emission can be clearly observed at low temperature.
4) Recombination through deep energy levels
When electrons and holes recombine through deep energy levels, the radiated photon energy is much smaller than the band gap, and the wavelength of the emitted light is far from the absorption edge. For narrow band gap materials, it is difficult to obtain visible light, but for wide band gap materials, this type of luminescence still has practical significance. For example, red luminescence in GaP belongs to this type of composite. In addition to the effects of deep-level impurities on radiation recombination, they are often the root cause of non-radiative recombination, especially in direct-pressing gland materials. Therefore, in actual work, it is often necessary to reduce the deep energy level as much as possible to improve the luminous efficiency.
(5) Exciton recombination
If the semiconductor absorbs photons with energy less than the band gap, electrons are excited from the valence band. However, due to the Coulomb effect, it is still connected with the holes left in the valence band, forming a bound state. This pair of electrons and holes bound together by Coulomb energy is called excitons. The excitons as a whole can move freely in the crystal. Since it is electrically neutral as a whole, the movement of excitons does not cause current. An exciton is an energy system, and this bound state can re-release energy in a radiant or non-radiative manner. If the energy is released in the form of radiation, a light-emitting process can be formed. The excitons may be bound during the movement in the crystal, and the bound excitons can no longer move freely in the crystal. Such excitons are called bound excitons. There are donors, acceptors, donor-acceptor pairs and isoelectronic traps in the centers that can bind excitons in the crystal. For free excitons, when electrons and holes recombine, they release energy to produce light.
In recent years, in the research of luminescent materials, it has been found that the emission of bound excitons plays an important role and has high luminous efficiency. For example, in GaP materials, bound excitons caused by Zn-O pairs cause red luminescence, and bound excitons caused by electron traps such as nitrogen cause green luminescence. These two light-emitting mechanisms greatly improve the light-emitting efficiency of GaP light-emitting diodes, and exciton emission has become the main light-emitting mechanism of this type of light-emitting diode. Therefore, the study of exciton luminescence has attracted more and more attention.
(6) Isoelectronic trap recombination
The so-called isoelectronic trap is the bound state produced by isoelectronic impurities replacing the lattice host atoms. Isoelectronic impurities refer to atoms in the same group as the semiconductor matrix atoms in the periodic table. Because the valence electrons of the atoms of the same group are equal, replacing the host atoms with isoelectronic impurities will not increase electrons or holes, but form an electrically neutral center, so it is called isoelectronic impurities. For example, nitrogen is the isoelectronic impurity of phosphorus atoms in gallium phosphide. Because the electronegativity and atomic radius between the isoelectronic impurity and the replaced atom are different, it causes distortion of the lattice potential, which can bind electrons or holes to form a charged center, just like in the position of the isoelectronic impurity. A trap is formed to trap electrons or holes, so it is called an isoelectronic trap. When the atomic radius of the isoelectronic impurity is very different from the radius of the substituted host atom, the crystal lattice deforms greatly to produce an effective bound state, thereby forming a strongly bound isoelectron trap.
When the waiting electron trap captures a certain type of carrier, it becomes a charged center. This charged center captures the carriers with the opposite charged sign by the Coulomb action, forming a bound exciton state, which is a bond bound to the isoelectronic impurity. Excitons. When the excitons recombine, they can release energy in the form of emitted photons.