38
4 Epitaxial of III-Nitride LED Materials
Corporation of Japan have used low-temperature GaN materials as buffer layers, and
further optimized the growth temperature and thickness of the buffer layer to produce
epitaxial GaN films capable of producing high-quality LEDs and LDs [5]. Epitaxial
high-quality optoelectronic devices such as LEDs and LDs on sapphire substrates
are mostly based on the two-step epitaxy method reported by Amano and Nakamura.
To further improve the crystal quality of GaN thin films on the basis of the two-step
epitaxy, some other modified processes such as epitaxial lateral overgrowth (ELOG)
[6], multi-buffer layer epitaxy [7], and suspension epitaxy [8] have been developed.
4.3 Group III Nitride LED Epitaxial Technology
4.3.1 LPE Method
Liquid phase epitaxy (LPE) is a method in which a solid phase material is precipitated
from a solution and deposited on a substrate to form a thin layer of a single crystal.
Liquid phase epitaxy was invented by Nielson in 1963 and became the main growth
method for thin layers of compound semiconductor single crystals. LPE is widely
used in the production of electronic devices. Epitaxial growth of a thin layer on
the substrate with the same material is called homoepitaxy. On the contrary, it is
called heteroepitaxy. Liquid phase epitaxy can be divided into tilting method, vertical
method and sliding boat method. The tilting method is to tilt the quartz container in
the quartz tube in a certain orientation before initiating the growth and loading the
solution and the substrate in the two ends of the container respectively. The vertical
method is to place the solution in the graphite crucible before starting the growth
and loading the substrate on the substrate holder above the solution. The sliding
method refers to the epitaxial growth process that is carried out in the graphite boat
with multiple solution tanks. In the epitaxial growth process, solution cooling can be
carried out by four methods: equilibrium method, rapid cooling method, supercooling
method and two-phase method.
Compared with other epitaxial methods; LPE has the following advantages: (1)
the growth equipment is relatively simple; (2) higher growth rate; (3) a wide range of
dopant selection; (4) good crystal integrity, The dislocation density of the epitaxial
layer is lower than the substrate; (5) The crystal has high purity. There are no highly
toxic and corrosive raw materials and products in the growth system, and the operation
is safe and simple.
The disadvantage of LPE is that the growth process cannot be performed well
if the difference in lattice constant between the epitaxial layer and the substrate is
greater than 1%, Secondly, it is difficult to control the uniformity of doping and
multi-component composition through the film thickness due to the difference in the
segregation coefficient, except for the epitaxial layer which is very thin. Furthermore,
the surface of the epitaxial layer grown by LPE is generally not as good as the epitaxial
layer grown by other techniques.
4 Epitaxial of III-Nitride LED Materials
Corporation of Japan have used low-temperature GaN materials as buffer layers, and
further optimized the growth temperature and thickness of the buffer layer to produce
epitaxial GaN films capable of producing high-quality LEDs and LDs [5]. Epitaxial
high-quality optoelectronic devices such as LEDs and LDs on sapphire substrates
are mostly based on the two-step epitaxy method reported by Amano and Nakamura.
To further improve the crystal quality of GaN thin films on the basis of the two-step
epitaxy, some other modified processes such as epitaxial lateral overgrowth (ELOG)
[6], multi-buffer layer epitaxy [7], and suspension epitaxy [8] have been developed.
4.3 Group III Nitride LED Epitaxial Technology
4.3.1 LPE Method
Liquid phase epitaxy (LPE) is a method in which a solid phase material is precipitated
from a solution and deposited on a substrate to form a thin layer of a single crystal.
Liquid phase epitaxy was invented by Nielson in 1963 and became the main growth
method for thin layers of compound semiconductor single crystals. LPE is widely
used in the production of electronic devices. Epitaxial growth of a thin layer on
the substrate with the same material is called homoepitaxy. On the contrary, it is
called heteroepitaxy. Liquid phase epitaxy can be divided into tilting method, vertical
method and sliding boat method. The tilting method is to tilt the quartz container in
the quartz tube in a certain orientation before initiating the growth and loading the
solution and the substrate in the two ends of the container respectively. The vertical
method is to place the solution in the graphite crucible before starting the growth
and loading the substrate on the substrate holder above the solution. The sliding
method refers to the epitaxial growth process that is carried out in the graphite boat
with multiple solution tanks. In the epitaxial growth process, solution cooling can be
carried out by four methods: equilibrium method, rapid cooling method, supercooling
method and two-phase method.
Compared with other epitaxial methods; LPE has the following advantages: (1)
the growth equipment is relatively simple; (2) higher growth rate; (3) a wide range of
dopant selection; (4) good crystal integrity, The dislocation density of the epitaxial
layer is lower than the substrate; (5) The crystal has high purity. There are no highly
toxic and corrosive raw materials and products in the growth system, and the operation
is safe and simple.
The disadvantage of LPE is that the growth process cannot be performed well
if the difference in lattice constant between the epitaxial layer and the substrate is
greater than 1%, Secondly, it is difficult to control the uniformity of doping and
multi-component composition through the film thickness due to the difference in the
segregation coefficient, except for the epitaxial layer which is very thin. Furthermore,
the surface of the epitaxial layer grown by LPE is generally not as good as the epitaxial
layer grown by other techniques.