4.4 Two-Step Growth Method for MOCVD Grown Nitride Materials
49
of this, film growth leads to a series of rich surface topography in a non-equilibrium
state, as well as the corresponding lattice relaxation problems.
In the epitaxial growth of a thin film, the deposited atoms fall on the substrate.
Firstly, they meet and combine in a certain way to form an atomic group, and then
new atoms are continuously added to these already formed atomic groups so that
they stably grow into larger particles family (often referred to as the “island”). As
the deposition continues, the atoms will fill the gaps between the islands, forming a
continuous film. During film growth, the size and distribution of the nucleation of
the deposited atoms, the islanding process, and the pattern of the initial growth will
greatly affect the quality of the entire film to be formed.
4.4.1.1 Island Size Effect and Distribution
Critical Island Size: In the process of nucleation, when an island contains more atoms
than the critical island size, the island is stable; when an island contains less atoms
than the critical island size, the island is unstable and will continue to absorb new
atoms for growth. The relationship between island density and critical island size is
as follows:
N =
D
F
−
i
i+2
exp
Ei
(i + 2)K B T
(4.12)
where N is the density of the total nucleation islands, D is the diffusion coefficient
of the atoms on the surface of the substrate, E i is the bond energy, and F is the
deposition flow rate. According to the Formula (4.12), the critical island size can be
derived by experimentally measuring the variation of the island density with respect
to the deposition flow rate.
4.4.1.2 Coarsening Mechanism
The surface of a real material usually has irregular steps, deposited atoms, atomic
groups, vacancies, and islands. Their existence indicates that the surface is in a
thermodynamic non-equilibrium state. Therefore, as long as conditions permit, such
surfaces will relax and go back to equilibrium. In many systems, surface relaxation
is achieved by atomic diffusion among islands. Such a process is commonly referred
to as Ostward Ripening, and it is also referred to as Coarsening due to the increase
in surface roughness accompanied with the process.
Ostwald Ripening exists in many physical and chemical systems. Under the framework of Ostwald Ripening, the atoms in the smaller islands have higher activity, so
the equilibrium vapor pressure is higher and the chemical potential is higher. Therefore, when two islands with different sizes are next to each other, there is a tendency
to evaporate atoms because the smaller-size island has a higher chemical potential.
49
of this, film growth leads to a series of rich surface topography in a non-equilibrium
state, as well as the corresponding lattice relaxation problems.
In the epitaxial growth of a thin film, the deposited atoms fall on the substrate.
Firstly, they meet and combine in a certain way to form an atomic group, and then
new atoms are continuously added to these already formed atomic groups so that
they stably grow into larger particles family (often referred to as the “island”). As
the deposition continues, the atoms will fill the gaps between the islands, forming a
continuous film. During film growth, the size and distribution of the nucleation of
the deposited atoms, the islanding process, and the pattern of the initial growth will
greatly affect the quality of the entire film to be formed.
4.4.1.1 Island Size Effect and Distribution
Critical Island Size: In the process of nucleation, when an island contains more atoms
than the critical island size, the island is stable; when an island contains less atoms
than the critical island size, the island is unstable and will continue to absorb new
atoms for growth. The relationship between island density and critical island size is
as follows:
N =
D
F
−
i
i+2
exp
Ei
(i + 2)K B T
(4.12)
where N is the density of the total nucleation islands, D is the diffusion coefficient
of the atoms on the surface of the substrate, E i is the bond energy, and F is the
deposition flow rate. According to the Formula (4.12), the critical island size can be
derived by experimentally measuring the variation of the island density with respect
to the deposition flow rate.
4.4.1.2 Coarsening Mechanism
The surface of a real material usually has irregular steps, deposited atoms, atomic
groups, vacancies, and islands. Their existence indicates that the surface is in a
thermodynamic non-equilibrium state. Therefore, as long as conditions permit, such
surfaces will relax and go back to equilibrium. In many systems, surface relaxation
is achieved by atomic diffusion among islands. Such a process is commonly referred
to as Ostward Ripening, and it is also referred to as Coarsening due to the increase
in surface roughness accompanied with the process.
Ostwald Ripening exists in many physical and chemical systems. Under the framework of Ostwald Ripening, the atoms in the smaller islands have higher activity, so
the equilibrium vapor pressure is higher and the chemical potential is higher. Therefore, when two islands with different sizes are next to each other, there is a tendency
to evaporate atoms because the smaller-size island has a higher chemical potential.