Sumitomo Electric Industries (SEI) announces improved manufacturing of Gallium
Nitride Laser Diode substrates
Semiconductor
lasers are used for recording and writing of data from and to CDs and DVDs.
A semiconductor laser is a single crystal substrate with
single crystal epitaxial layer grown on it. This device structure emits laser
light when an electric current passes through it.
In the case of ordinary CDs and DVDs, gallium arsenide
(GaAs) is used as the layer semiconductor material. However, for violet lasers
used in the next-generation DVD players, GaN that has shorter laser wavelength
than GaAs will be used. Sapphire substrates are commonly used for fabrication of
GaN devices, but since the use of sapphire substrate causes various problems, it
has been said that GaN substrates are inevitable in commercial production of
violet lasers.
<Advantages Over Sapphire
Substrate>
When sapphire substrates are used for epitaxial growth of
GaN, since the physical properties of sapphire and GaN are different, sapphire
substrates cause GaN epitaxial layer to have many crystal defects. Other
problems of sapphire substrate include inferior cleavage and nonconductive
substrate.
Using low-dislocation GaN substrate, these problems can be
solved.
< SEI's Proprietary GaN
Substrate Production Method >
Normally, when semiconductor crystal such as silicon and GaAs
is melted at high temperature and then cooled and coagulated, it becomes a
single crystal. However, GaN evaporates and decomposes at high temperature,
which means that the standard single crystal growth process cannot be used. SEI
has developed a new method for production of GaN substrate. This method uses
vapor phase preparation technique to grow GaN crystal on a substrate made of a
material other than GaN, and then removes the substrate to obtain GaN single
crystal. SEI's experiences in vapor phase preparation of GaAs semiconductors are
exploited in this method. SEI is the first company to mass-produce the
semiconductor substrates using this method, and is striving toward establishment
of mass-production process.
< DEEP Technique >
Elimination of dislocation (crystal defect) is the most significant
challenge in the fabrication of GaN substrates. In order to accomplish this, SEI
has established its own unique crystal growth process, called the Dislocation
Elimination by Epitaxial growth with inverse-pyramidal Pits, or 'DEEP'
technique, which reduces dislocations by forming inverse-pyramidal pits on the
surface of the grown crystal.
Each pit is made up of facets and its diameter is 100ĵ
m or larger. When a pit grows while keeping its inverse-pyramidal shape, as
facets grow, dislocations that exist within the pit region propagate in
horizontal direction and concentrates to the pit's bottom, thus resulting to the
reduction of dislocations around the pit.
The density per square centimeter of low dislocation areas
of GaN substrate fabricated using the DEEP technique is around 1,000,000, that
is 10,000 times less than the dislocation density of conventional GaN epitaxial
layers on sapphire substrates.
<Advantages of SEI GaN
Substrate>
GaN substrate manufactured by SEI is conductive and achieves
practical use size of 2-inch diameter. Moreover, as mentioned previously, the
use of DEEP technique had concentrated the dislocations and thus provided large
low-dislocation areas. The characteristics of SEI substrate are shown in the
table on the right.
< Future
Development >
SEI has managed to develop a technique that controls the area where
dislocations are concentrated using the DEEP technique. Responding to the
requests from violet laser manufacturers, SEI will strive toward the improvement
of crystal quality and the establishment of commercial production system.
[Contact] Sumitomo Electric Industries, Epi Solution
Division: Tel. +81-727-72-4582
