8.4 Evaporation and Sputtering
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The PECVD (plasma enhanced chemical vapor deposition) process is also used to
deposit a layer of SiO 2 or SiN x on the surface of the wafer after the epitaxial process is
completed, where SiO 2 or Si 3 N 4 serves as a hard mask layer for electrode etching to
increase the etch selectivity ratio of the mask and the GaN epitaxial layer that gives a
better etch profile. At present, the mainstream for LED fabrication is generally a flatplate PECVD (13.56 MHz). The film is formed at a temperature of about 250–300 °C.
The capacity can reach more than 40 wafers (2-in. substrate) at one time. The PECVD
system for LED production in China is still mainly imported. The key technologies
are still temperature control, plasma technology, vacuum system, software system and
so on. It is noted that the flat-panel PECVD equipment for passivation layer has been
developed in China. This equipment has great compatibility with PECVD equipment
for LED. After further hardware improvement, it is easier to realize high-efficiency
PECVD equipment that can be used for LED production.
8.5 Ohmic Contacts
The driving current of the III-nitride LED chip needs to be injected into the chip
through the metal electrodes, thus requiring a good low-resistance ohmic contact
between the metal electrode and the GaN material. Ohmic contact refers to the
fact that the contact of the metal with the semiconductor material does not produce
significant additional impedance and does not significantly alter the equilibrium
carrier concentration within the semiconductor material.
8.5.1 n-type GaN Ohmic Contact
Forming a low-resistance ohmic contact electrode on the III-nitride LED chip can
effectively improve the optical and electrical performance of the LED chip. The
basic principles of n-GaN ohmic contact electrodes are as follows: (1) selecting a
metal material with a certain work function to reduce the barrier height to enhance
the thermal electron emission mechanism; (2) selecting the metal material of the
electrode reacting with GaN occurring at the interface of the semiconductor material
that will lead to the increase of the carrier concentration at the interface and reduction
of the thickness of the space charge region to achieve the purpose of enhancing the
tunneling mechanism; (3) providing desired bonding property between the metal
and the GaN semiconductor to prevent electrode pad falling off during the wire bond
process; (4) providing excellent thermal stability and reliability of the LED chip at
high temperature [1].
Since the concentration of carriers of the n-GaN material in the chip is high, the
ohmic contact is easy to realize. For horizontal structure LED chips, it is necessary to
fabricate ohmic contact electrodes on Ga-plane of n-GaN. For the ohmic contact of
the Ga-plane n-type GaN, it is relatively easy to form an ohmic contact thereon since
163
The PECVD (plasma enhanced chemical vapor deposition) process is also used to
deposit a layer of SiO 2 or SiN x on the surface of the wafer after the epitaxial process is
completed, where SiO 2 or Si 3 N 4 serves as a hard mask layer for electrode etching to
increase the etch selectivity ratio of the mask and the GaN epitaxial layer that gives a
better etch profile. At present, the mainstream for LED fabrication is generally a flatplate PECVD (13.56 MHz). The film is formed at a temperature of about 250–300 °C.
The capacity can reach more than 40 wafers (2-in. substrate) at one time. The PECVD
system for LED production in China is still mainly imported. The key technologies
are still temperature control, plasma technology, vacuum system, software system and
so on. It is noted that the flat-panel PECVD equipment for passivation layer has been
developed in China. This equipment has great compatibility with PECVD equipment
for LED. After further hardware improvement, it is easier to realize high-efficiency
PECVD equipment that can be used for LED production.
8.5 Ohmic Contacts
The driving current of the III-nitride LED chip needs to be injected into the chip
through the metal electrodes, thus requiring a good low-resistance ohmic contact
between the metal electrode and the GaN material. Ohmic contact refers to the
fact that the contact of the metal with the semiconductor material does not produce
significant additional impedance and does not significantly alter the equilibrium
carrier concentration within the semiconductor material.
8.5.1 n-type GaN Ohmic Contact
Forming a low-resistance ohmic contact electrode on the III-nitride LED chip can
effectively improve the optical and electrical performance of the LED chip. The
basic principles of n-GaN ohmic contact electrodes are as follows: (1) selecting a
metal material with a certain work function to reduce the barrier height to enhance
the thermal electron emission mechanism; (2) selecting the metal material of the
electrode reacting with GaN occurring at the interface of the semiconductor material
that will lead to the increase of the carrier concentration at the interface and reduction
of the thickness of the space charge region to achieve the purpose of enhancing the
tunneling mechanism; (3) providing desired bonding property between the metal
and the GaN semiconductor to prevent electrode pad falling off during the wire bond
process; (4) providing excellent thermal stability and reliability of the LED chip at
high temperature [1].
Since the concentration of carriers of the n-GaN material in the chip is high, the
ohmic contact is easy to realize. For horizontal structure LED chips, it is necessary to
fabricate ohmic contact electrodes on Ga-plane of n-GaN. For the ohmic contact of
the Ga-plane n-type GaN, it is relatively easy to form an ohmic contact thereon since