5.1 Introduction to InGaN Material System
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of dislocation densities. Therefore, InGaN material is not suitable for other spectral
regions than the visible spectrum.
So far, the growth of InGaN/GaN multiple quantum wells structure is mainly
through MOCVD technology [4, 5]. In the growth process of multiple quantum
wells, the large change of temperature is very important, because the GaN barrier
layer is generally required to grow at a relatively high temperature (1050 °C), which
is most advantageous for growing high-quality GaN crystal. However, in order to
keep stability during growth, the InGaN layer must grow at a lower temperature
(800 °C). This requires the growth system to have a capacity to change the substrate
temperature by 100 °C per minute. For large-scale manufacturing, therefore, only
MOCVD can provide such capability at present.
5.2 Polarization Effects in InGaN/GaN Multiple Quantum
Wells Materials
5.2.1 Polarity of GaN-Based Materials
In sphalerite and wurtzite structures, cations and anions are produced due to the
charge transfer between atoms. For the wurtzite structure, the material exhibits polarization macroscopically due to the deviation of the positive and negative charging
centers of the unit cell. This material is called a polar material. GaN is a polar
material. Generally, if the epitaxial layer has a Ga terminated surface, the epitaxial
material has a Ga plane polarity. Conversely, if the epitaxial layer has an N plane
as the termination surface, the epitaxial material has an N plane polarity. It is worth
noting that polarity is an integral property of a material, not a surface property. For
example, covering a single atomic layer of N atoms on a Ga-polar material does
not change the properties of the Ga-plane polarity of the material. In addition, there
are cases where the termination surface of the epitaxial layer can include both Ga
atoms and N atoms. In this case there is generally a dominant polarity. In the III-V
nitride materials, the conventional epitaxial layer grown by MOCVD is along the
(0001) direction of sapphire. Generally, a high-quality GaN epitaxial layer grown by
a low-temperature buffer layer on a sapphire substrate by MOCVD has a Ga surface
polarity.
The polarity direction of GaN is the (0001) direction, and it can also be represented
by a GaN plane, a + c, a gallium polarity, or a Ga polar plane. The opposite direction
is the N polar plane. For the Ga polar plane, the Ga atoms exist with one bond facing
up and the three bonds facing downward. The N atoms are the opposite with three
bonds facing upward and one bond facing downward. The N polar plane is opposite
to Ga polar plane.
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of dislocation densities. Therefore, InGaN material is not suitable for other spectral
regions than the visible spectrum.
So far, the growth of InGaN/GaN multiple quantum wells structure is mainly
through MOCVD technology [4, 5]. In the growth process of multiple quantum
wells, the large change of temperature is very important, because the GaN barrier
layer is generally required to grow at a relatively high temperature (1050 °C), which
is most advantageous for growing high-quality GaN crystal. However, in order to
keep stability during growth, the InGaN layer must grow at a lower temperature
(800 °C). This requires the growth system to have a capacity to change the substrate
temperature by 100 °C per minute. For large-scale manufacturing, therefore, only
MOCVD can provide such capability at present.
5.2 Polarization Effects in InGaN/GaN Multiple Quantum
Wells Materials
5.2.1 Polarity of GaN-Based Materials
In sphalerite and wurtzite structures, cations and anions are produced due to the
charge transfer between atoms. For the wurtzite structure, the material exhibits polarization macroscopically due to the deviation of the positive and negative charging
centers of the unit cell. This material is called a polar material. GaN is a polar
material. Generally, if the epitaxial layer has a Ga terminated surface, the epitaxial
material has a Ga plane polarity. Conversely, if the epitaxial layer has an N plane
as the termination surface, the epitaxial material has an N plane polarity. It is worth
noting that polarity is an integral property of a material, not a surface property. For
example, covering a single atomic layer of N atoms on a Ga-polar material does
not change the properties of the Ga-plane polarity of the material. In addition, there
are cases where the termination surface of the epitaxial layer can include both Ga
atoms and N atoms. In this case there is generally a dominant polarity. In the III-V
nitride materials, the conventional epitaxial layer grown by MOCVD is along the
(0001) direction of sapphire. Generally, a high-quality GaN epitaxial layer grown by
a low-temperature buffer layer on a sapphire substrate by MOCVD has a Ga surface
polarity.
The polarity direction of GaN is the (0001) direction, and it can also be represented
by a GaN plane, a + c, a gallium polarity, or a Ga polar plane. The opposite direction
is the N polar plane. For the Ga polar plane, the Ga atoms exist with one bond facing
up and the three bonds facing downward. The N atoms are the opposite with three
bonds facing upward and one bond facing downward. The N polar plane is opposite
to Ga polar plane.