2.2 Radiation and Non-radiation Recombination
11
can also be released in other forms (such as transferring excess energy to the lattice
to enhance the lattice vibration, or giving energy to other carriers to increase their
kinetic energy, also known as Auger recombination [6] without emitting photons.
This recombination is called nonradiative recombination. The LED luminescence is
a radiation recombination process, and the non-radiation recombination process is
not beneficial to the luminescence of LED [7].
In LED, the non-radiation compound effect can only be reduced and cannot be
eliminated. There is always a competitive relationship between radiation recombination and non-radiation recombination. The transition from band to bank and the
transition through the trap can be either radiative recombination or non-radiative
recombination. For instance, the inter-band recombination in indirect bandgap semiconductors is non-radiative recombination, while the recombination is radiative
recombination through traps of isoelectronic levels.
In the process of structure design of LED, there are many ways to increase the radiation recombination probability of unbalanced carriers. For example, the structure
of p–n junction is transformed from homojunction to heterojunction and/or double
heterojunction to enhance the radiation recombination efficiency of electron–hole in
the junction region [8]. In heterojunction structures, the region where electrons and
holes recombine is called active region. Thinning the active region can improve the
internal quantum efficiency of the LED and reduce the photon reabsorption in the
active region. As the thickness of the active region getting close to the de Broglie
wavelength of the electron, the energy of the carriers moving in the direction perpendicular to the junction is no longer continuous. This nanoscale active region is called
a quantum well. The thin films with multi-layer quantum well structure can be epitaxially grown by MOCVD. At present, the active region of high brightness LED chips
is composed of multiple-quantum-well (MQW) structure. The quantum well structure plays an important role in enhancing the radiation recombination probability of
unbalanced carriers in LED [9].
2.3 LED Optical and Electrical Characteristics
2.3.1 LED Quantum Efficiency
Quantum efficiency is an important parameter related to radiation in the characteristics of light emitting diodes. It reflects the efficiency of photon generation by
recombination of injected carriers. Quantum efficiency can refer to both internal
quantum efficiency and external quantum efficiency.
Internal quantum efficiency refers to the ratio of the number of photons produced
by the radiation recombination of semiconductors per unit time to the number of
injected carriers, which reflects the efficiency of converting carrier current into
photons. Ideally, all the photons emitted by the recombination of carrier radiation
in the active region can be propagated to the outside. However, due to the existence
11
can also be released in other forms (such as transferring excess energy to the lattice
to enhance the lattice vibration, or giving energy to other carriers to increase their
kinetic energy, also known as Auger recombination [6] without emitting photons.
This recombination is called nonradiative recombination. The LED luminescence is
a radiation recombination process, and the non-radiation recombination process is
not beneficial to the luminescence of LED [7].
In LED, the non-radiation compound effect can only be reduced and cannot be
eliminated. There is always a competitive relationship between radiation recombination and non-radiation recombination. The transition from band to bank and the
transition through the trap can be either radiative recombination or non-radiative
recombination. For instance, the inter-band recombination in indirect bandgap semiconductors is non-radiative recombination, while the recombination is radiative
recombination through traps of isoelectronic levels.
In the process of structure design of LED, there are many ways to increase the radiation recombination probability of unbalanced carriers. For example, the structure
of p–n junction is transformed from homojunction to heterojunction and/or double
heterojunction to enhance the radiation recombination efficiency of electron–hole in
the junction region [8]. In heterojunction structures, the region where electrons and
holes recombine is called active region. Thinning the active region can improve the
internal quantum efficiency of the LED and reduce the photon reabsorption in the
active region. As the thickness of the active region getting close to the de Broglie
wavelength of the electron, the energy of the carriers moving in the direction perpendicular to the junction is no longer continuous. This nanoscale active region is called
a quantum well. The thin films with multi-layer quantum well structure can be epitaxially grown by MOCVD. At present, the active region of high brightness LED chips
is composed of multiple-quantum-well (MQW) structure. The quantum well structure plays an important role in enhancing the radiation recombination probability of
unbalanced carriers in LED [9].
2.3 LED Optical and Electrical Characteristics
2.3.1 LED Quantum Efficiency
Quantum efficiency is an important parameter related to radiation in the characteristics of light emitting diodes. It reflects the efficiency of photon generation by
recombination of injected carriers. Quantum efficiency can refer to both internal
quantum efficiency and external quantum efficiency.
Internal quantum efficiency refers to the ratio of the number of photons produced
by the radiation recombination of semiconductors per unit time to the number of
injected carriers, which reflects the efficiency of converting carrier current into
photons. Ideally, all the photons emitted by the recombination of carrier radiation
in the active region can be propagated to the outside. However, due to the existence