8.5 Ohmic Contacts
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is to select a metal having a small work function for the n-type semiconductor and a
metal having a large work function for the p-type semiconductor.
Ohmic contact means that the resistance of the metal semiconductor contact is
very low, and the current–voltage characteristics of the device are mainly determined
by the bulk resistance of the semiconductor. In principle, it can be prepared from a
metal whose work function is lower than that of the n-type semiconductor or higher
than that of the p-type semiconductor. At present, most practical ohmic contacts are
prepared by preparing a highly doped semiconductor thin layer on the surface of
the semiconductor material to form an extremely thin depletion layer so that carriers
can easily tunnel through the barrier. However, the work function for p-GaN is about
7.5 eV which is much higher than that of the metal material. The carrier concentration
of p-GaN is low due to the activation energy of the acceptor with the formation of
the Mg and Mg-H complex. It is difficult to obtain a carrier concentration of more
than 10
18 cm
−3 .
For the design and fabrication of LED chips, a good low resistance ohmic contact
can reduce the threshold voltage of the chip. The composition of the ohmic contact
alloy is as follows: (1) Adhesive layer metal: Since some metal alloys and GaN semiconductor materials have insufficient adhesion properties, it is necessary to deposit a
metal material such as Cr, Ti, Ni as an adhesion layer to reinforce the metal adhesion
strength with GaN semiconductor materials; (2) Functional layer metal: important
for the formation of ohmic contacts, the selection of a suitable metal as a functional
layer is the key to forming a low-resistance ohmic contact; (3) Overlay metal: the
metal layer is to facilitate the wire bonding of the subsequent packaging process, and
generally requires the metal layer and the lower contact layer to have high adhesion
strength, stable performance, and easy soldering. The metals used to form ohmic
contact with the p-GaN material are mainly Ni, Pt, Au, and so on. At present, it is
common practice in the industry to use Cr as an adhesion layer metal, Pt as a functional layer metal, and Au as a cladding layer metal, that is, a Cr/Pt/Au alloy as an
ohmic contact metal material of p-GaN.
8.5.3 Specific Contact Resistivity
GaN-based material has excellent chemical and physical characteristics, exhibiting
immeasurable application prospects in the field of microelectronics and optoelectronics [16]. However, GaN-based devices present many difficulties during fabrication, wherein making an ohmic contact to the p-GaN is one of the difficulties. In
recent years, many research groups worldwide report various methods to improve
p-type GaN ohmic contact [17]. Ishikawa et al. found that the use of a metal with
a large work function as an electrode can reduce the Schottky barrier height of the
contact, thereby reducing the specific contact resistivity of the ohmic contact [18].
Specific contact resistivity (SCR) is an important parameter to quantitatively
reflect the quality of the ohmic contact between the electrode metal and the semiconductor material. Low resistance ohmic contacts are the basis for high quality devices.
165
is to select a metal having a small work function for the n-type semiconductor and a
metal having a large work function for the p-type semiconductor.
Ohmic contact means that the resistance of the metal semiconductor contact is
very low, and the current–voltage characteristics of the device are mainly determined
by the bulk resistance of the semiconductor. In principle, it can be prepared from a
metal whose work function is lower than that of the n-type semiconductor or higher
than that of the p-type semiconductor. At present, most practical ohmic contacts are
prepared by preparing a highly doped semiconductor thin layer on the surface of
the semiconductor material to form an extremely thin depletion layer so that carriers
can easily tunnel through the barrier. However, the work function for p-GaN is about
7.5 eV which is much higher than that of the metal material. The carrier concentration
of p-GaN is low due to the activation energy of the acceptor with the formation of
the Mg and Mg-H complex. It is difficult to obtain a carrier concentration of more
than 10
18 cm
−3 .
For the design and fabrication of LED chips, a good low resistance ohmic contact
can reduce the threshold voltage of the chip. The composition of the ohmic contact
alloy is as follows: (1) Adhesive layer metal: Since some metal alloys and GaN semiconductor materials have insufficient adhesion properties, it is necessary to deposit a
metal material such as Cr, Ti, Ni as an adhesion layer to reinforce the metal adhesion
strength with GaN semiconductor materials; (2) Functional layer metal: important
for the formation of ohmic contacts, the selection of a suitable metal as a functional
layer is the key to forming a low-resistance ohmic contact; (3) Overlay metal: the
metal layer is to facilitate the wire bonding of the subsequent packaging process, and
generally requires the metal layer and the lower contact layer to have high adhesion
strength, stable performance, and easy soldering. The metals used to form ohmic
contact with the p-GaN material are mainly Ni, Pt, Au, and so on. At present, it is
common practice in the industry to use Cr as an adhesion layer metal, Pt as a functional layer metal, and Au as a cladding layer metal, that is, a Cr/Pt/Au alloy as an
ohmic contact metal material of p-GaN.
8.5.3 Specific Contact Resistivity
GaN-based material has excellent chemical and physical characteristics, exhibiting
immeasurable application prospects in the field of microelectronics and optoelectronics [16]. However, GaN-based devices present many difficulties during fabrication, wherein making an ohmic contact to the p-GaN is one of the difficulties. In
recent years, many research groups worldwide report various methods to improve
p-type GaN ohmic contact [17]. Ishikawa et al. found that the use of a metal with
a large work function as an electrode can reduce the Schottky barrier height of the
contact, thereby reducing the specific contact resistivity of the ohmic contact [18].
Specific contact resistivity (SCR) is an important parameter to quantitatively
reflect the quality of the ohmic contact between the electrode metal and the semiconductor material. Low resistance ohmic contacts are the basis for high quality devices.