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8 III-Nitride LED Chip Fabrication Techniques
The characteristics of the transparent conductive oxide material are in good agreement with the above requirements, and thus can be used as a transparent electrode
of the group III nitride LED. This section briefly introduces the main theory of
transparent conductive oxide formation and its application in GaN-based LEDs
[27–30].
From the basic solid-electron gas model, the conductivity of solids is provided by
electrons in the conduction band. The conductivity is determined by the product of
carrier density, electron charge, and carrier mobility. Carrier mobility is controlled by
free carrier relaxation time, and effective mass. That is determined by σ = n • e • μ,
μ =
e·τ
m∗
, which shows the conductivity of the solid is proportional to the carrier
density and mobility. The mobility is proportional to the carrier lifetime and inversely
proportional to the carrier effective mass.
The transparent conductive material generally has a high electron density and
electron mobility, and thus has a high conductivity. It has a high transmittance in the
visible light band due to its wide bandgap. It is very important to study the microscopic electronic structure of transparent conductive oxide (TCO) and its formation
mechanism from the perspective of material physics and to understand its chemical
and structural origin from the perspective of material chemistry. For example, to
clarify the electronic structure, the principle of component-induced electron phase
transformation into simple and electronic gas, the mechanism of parasitic electron
scattering in degenerate electron gas, all forms of chemical engineering materialsoxides and super-oxides increase the potential performance. Higher conductivity
and lower absorption rate are the development direction of transparent conductive
oxides. Other functions based on TCO materials such as gas sensing characteristics
and piezoelectric characteristics have also been studied.
Taking ITO as an example, the main mechanism of the formation of transparent
conductivity of TCO materials is introduced. The ITO material has typical TCO
optical and electrical transport properties, and its conductivity is greater than 10
4
S/cm. The visible light transmittance is higher than 80%. This makes it an important
candidate in many applications, especially in optoelectronic components with high
efficiency.
It is generally believed that the conductivity of ITO is derived from the shallow
donors of the Sn
+
4 occupied ions and the oxygen vacancies in the indium oxide
structure. These shallow donor and impurity states are very close to the intrinsic
conduction band so that the donor electrons are thermally activated at room temperature. After entering the conduction band, by further mixing, a degenerate carrier
electron gas is formed, which is characterized by far-infrared absorption and high
conductivity, but at the same time the basic band gap is still intact, so the high visibility of the visible region is still maintained. The main advantage of TCO materials
applied to Group III nitride LEDs is their transparency and conductivity, especially
their transmittance is much higher than that of metal electrodes.
8 III-Nitride LED Chip Fabrication Techniques
The characteristics of the transparent conductive oxide material are in good agreement with the above requirements, and thus can be used as a transparent electrode
of the group III nitride LED. This section briefly introduces the main theory of
transparent conductive oxide formation and its application in GaN-based LEDs
[27–30].
From the basic solid-electron gas model, the conductivity of solids is provided by
electrons in the conduction band. The conductivity is determined by the product of
carrier density, electron charge, and carrier mobility. Carrier mobility is controlled by
free carrier relaxation time, and effective mass. That is determined by σ = n • e • μ,
μ =
e·τ
m∗
, which shows the conductivity of the solid is proportional to the carrier
density and mobility. The mobility is proportional to the carrier lifetime and inversely
proportional to the carrier effective mass.
The transparent conductive material generally has a high electron density and
electron mobility, and thus has a high conductivity. It has a high transmittance in the
visible light band due to its wide bandgap. It is very important to study the microscopic electronic structure of transparent conductive oxide (TCO) and its formation
mechanism from the perspective of material physics and to understand its chemical
and structural origin from the perspective of material chemistry. For example, to
clarify the electronic structure, the principle of component-induced electron phase
transformation into simple and electronic gas, the mechanism of parasitic electron
scattering in degenerate electron gas, all forms of chemical engineering materialsoxides and super-oxides increase the potential performance. Higher conductivity
and lower absorption rate are the development direction of transparent conductive
oxides. Other functions based on TCO materials such as gas sensing characteristics
and piezoelectric characteristics have also been studied.
Taking ITO as an example, the main mechanism of the formation of transparent
conductivity of TCO materials is introduced. The ITO material has typical TCO
optical and electrical transport properties, and its conductivity is greater than 10
4
S/cm. The visible light transmittance is higher than 80%. This makes it an important
candidate in many applications, especially in optoelectronic components with high
efficiency.
It is generally believed that the conductivity of ITO is derived from the shallow
donors of the Sn
+
4 occupied ions and the oxygen vacancies in the indium oxide
structure. These shallow donor and impurity states are very close to the intrinsic
conduction band so that the donor electrons are thermally activated at room temperature. After entering the conduction band, by further mixing, a degenerate carrier
electron gas is formed, which is characterized by far-infrared absorption and high
conductivity, but at the same time the basic band gap is still intact, so the high visibility of the visible region is still maintained. The main advantage of TCO materials
applied to Group III nitride LEDs is their transparency and conductivity, especially
their transmittance is much higher than that of metal electrodes.