2
1 Introduction
Group III nitride semiconductors, including AlN, InN, GaN and their alloys,
belong to all-component direct bandgap semiconductors. They have the advantages
of momentum conservation of radiation recombination and high radiation transition
efficiency. Their bandgap energy covers a wide spectrum, ranging from near infrared
to deep ultraviolet. They are ideal materials for realizing high efficiency light emitting
diodes and are also the most attractive in wide bandgap semiconductors [4]. However,
the luminous efficiency of light-emitting diodes will be affected by materials, devices,
packaging and other processing steps and technologies in actual production. This
series of problems has attracted extensive attention of many scientists and researchers
all over the world and become a hot field of scientific research.
Substrate material is the basis of the growth of group III nitride epitaxy film, and
also the main component of LED devices, which has an important impact on the
preparation and performance of LED devices. Taking GaN substrate as an example,
GaN homogeneous substrate is the most ideal substrate for growth of GaN epitaxial
layer. Its greatest advantage lies in the lattice match between the substrate and the
epitaxial layer, which can eliminate crystal defects caused by lattice mismatch to
the greatest extent. Furthermore, GaN substrate can improve the vertical conductivity and thermal conductivity, thus greatly enhance the performance of GaN-based
LED. However, the melting point of GaN is very high (2800 C) and the equilibrium vapor pressure is very high (4.5 GPa). It is very difficult to prepare GaN single
crystal [5]. Although some research institutes have prepared GaN single crystals
with good crystal quality through hydride vapor phase epitaxy (HVPE), Ammon
thermal method and low-pressure growth at lower pressure method, the high growth
cost and small size make it difficult to meet the needs of the market [6]. Therefore,
heteroepitaxy is widely used at present. Commonly used substrates are sapphire (aAl 2 O 3 ), silicon carbide (6H-SiC) and silicon (Si). However, heteroepitaxy has its
inherent shortcomings, such as lattice mismatch, thermal expansion mismatch and
poor chemical solubility. These problems lead to high dislocation density, mosaic
crystal structure, biaxial stress and warping of epitaxy sheets in epitaxy materials,
which affect the crystal quality of epitaxy materials and the performance of LED
devices [7]. In order to improve the crystal quality of heteroepitaxy materials, some
special methods and means are usually adopted. For example, epitaxial lateral overgrowth (ELOG), buffer layer or insertion layer, graphical substrate and non-polar
and semi-polar surface epitaxy growth have been explored [8, 9].
The epitaxy methods of group III nitrides mainly include metal organic
compounds vapor phase epitaxy (MOCVD), hydride vapor phase epitaxy (HVPE)
and molecular beam epitaxy (MBE). Due to the high growth rate of HVPE, it cannot
be used to grow low-dimensional structures such as quantum wells and superlattices, while MBE is quite expensive and has a slow growth rate. Considering the cost
and efficiency, MOCVD technology is used commercially for heteroepitaxial LED
structures along c-axis on sapphire substrates. Since wurtzite nitride semiconductors grown along the c-axis do not have central symmetry and the lattice mismatch
between nitride materials and substrates and nitride heterostructures, spontaneous
polarization and piezoelectric polarization occur in their crystals. The polarization effect generates built-in electric field at the interface and changes the energy
1 Introduction
Group III nitride semiconductors, including AlN, InN, GaN and their alloys,
belong to all-component direct bandgap semiconductors. They have the advantages
of momentum conservation of radiation recombination and high radiation transition
efficiency. Their bandgap energy covers a wide spectrum, ranging from near infrared
to deep ultraviolet. They are ideal materials for realizing high efficiency light emitting
diodes and are also the most attractive in wide bandgap semiconductors [4]. However,
the luminous efficiency of light-emitting diodes will be affected by materials, devices,
packaging and other processing steps and technologies in actual production. This
series of problems has attracted extensive attention of many scientists and researchers
all over the world and become a hot field of scientific research.
Substrate material is the basis of the growth of group III nitride epitaxy film, and
also the main component of LED devices, which has an important impact on the
preparation and performance of LED devices. Taking GaN substrate as an example,
GaN homogeneous substrate is the most ideal substrate for growth of GaN epitaxial
layer. Its greatest advantage lies in the lattice match between the substrate and the
epitaxial layer, which can eliminate crystal defects caused by lattice mismatch to
the greatest extent. Furthermore, GaN substrate can improve the vertical conductivity and thermal conductivity, thus greatly enhance the performance of GaN-based
LED. However, the melting point of GaN is very high (2800 C) and the equilibrium vapor pressure is very high (4.5 GPa). It is very difficult to prepare GaN single
crystal [5]. Although some research institutes have prepared GaN single crystals
with good crystal quality through hydride vapor phase epitaxy (HVPE), Ammon
thermal method and low-pressure growth at lower pressure method, the high growth
cost and small size make it difficult to meet the needs of the market [6]. Therefore,
heteroepitaxy is widely used at present. Commonly used substrates are sapphire (aAl 2 O 3 ), silicon carbide (6H-SiC) and silicon (Si). However, heteroepitaxy has its
inherent shortcomings, such as lattice mismatch, thermal expansion mismatch and
poor chemical solubility. These problems lead to high dislocation density, mosaic
crystal structure, biaxial stress and warping of epitaxy sheets in epitaxy materials,
which affect the crystal quality of epitaxy materials and the performance of LED
devices [7]. In order to improve the crystal quality of heteroepitaxy materials, some
special methods and means are usually adopted. For example, epitaxial lateral overgrowth (ELOG), buffer layer or insertion layer, graphical substrate and non-polar
and semi-polar surface epitaxy growth have been explored [8, 9].
The epitaxy methods of group III nitrides mainly include metal organic
compounds vapor phase epitaxy (MOCVD), hydride vapor phase epitaxy (HVPE)
and molecular beam epitaxy (MBE). Due to the high growth rate of HVPE, it cannot
be used to grow low-dimensional structures such as quantum wells and superlattices, while MBE is quite expensive and has a slow growth rate. Considering the cost
and efficiency, MOCVD technology is used commercially for heteroepitaxial LED
structures along c-axis on sapphire substrates. Since wurtzite nitride semiconductors grown along the c-axis do not have central symmetry and the lattice mismatch
between nitride materials and substrates and nitride heterostructures, spontaneous
polarization and piezoelectric polarization occur in their crystals. The polarization effect generates built-in electric field at the interface and changes the energy