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8 III-Nitride LED Chip Fabrication Techniques
the carrier concentration of n-GaN is high (on the order of 10
18 ). When fabricating
an n-type ohmic contact electrode, the work function ( mss ) of the selected metal
should be less than the work function of n-GaN [2]. At present, a lower specific
contact resistance of 10
–5 to 10
–8
cm
2 has been achieved on n-GaN materials,
which can meet the requirements of LED chips. There are many n-GaN ohmic contact
electrode materials. Commonly used for n-type ohmic contact materials include Al
( m = 4.28 eV), Ti ( m = 4.33 eV), Ta ( m = 4.25 eV), V ( m = 4.33 eV) and
other metal elements. At present, many metal alloys such as Ti, Ti/Al, Ti/Al/Ni/Au,
Ta/Ti/Ni/Au, V/Al/V/Au, V/Ti/Au, etc. can be also used [3–10].
8.5.2 p-type GaN Ohmic Contact
Since the ohmic contact electrode of p-GaN has a great influence on the performance
of the LED, it is necessary to form an ohmic contact electrode having low contact
resistance, high light transmittance or high reflectivity, and thermal stability on the
surface of p-GaN. However, for the ohmic contact of p-GaN materials, it is difficult to
form a low-resistance ohmic contact thereon. On one hand, heavy doping of p-GaN
has not made effective breakthrough. This is due to the high ionization energy of
the Mg-dopant in p-GaN (up to 170 meV). High ionization energy can result in low
ionization rate and low carrier concentration. On the other hand, metal with large
work function that is greater than of p-GaN (having a work function of approximately
6.12 eV) is not available, which makes it difficult to fabricate an ohmic contact of
low-contact resistance for p-GaN [11–15].
Usually the operating voltage of an LED is related to the series resistance (especially the contact resistance on p-type GaN) and the current distribution. The ptype GaN contact resistance directly affects LED operating voltage. To obtain a
low contact resistance, some high work function metal such as of Ni, Pt, Au, etc.
are typically used. Some surface treatment process is needed as well. The uneven
current distribution causes overload on part of the light-emitting area. Insufficient
injection on other active areas makes the total injection efficiency not high. Efforts
to homogenize the current usually cause more electrode absorption.
The contact between metal and semiconductor is one of the major topics that
began very early in semiconductor science. For an ideal metal and semiconductor
contact (regardless of the surface state effect), a barrier is formed if the work function
of the metal is greater than that of the n-type semiconductor. Such a contact exhibits
rectification characteristic. On the other hand, an ohmic contact will be formed if
the work of the metal is smaller than that of the semiconductor. The opposite is
true for p-type semiconductor materials. High-quality ohmic contacts are required
to have low resistance and thermal stability. High transmittance is required for lightemitting devices so these problems must be considered when selecting a metallization
solution. Generally, to obtain a low-resistance ohmic contact, the Schottky barrier
height between the metal and semiconductor interface should be small. The method
of lowering the barrier height without considering the influence of the surface state
8 III-Nitride LED Chip Fabrication Techniques
the carrier concentration of n-GaN is high (on the order of 10
18 ). When fabricating
an n-type ohmic contact electrode, the work function ( mss ) of the selected metal
should be less than the work function of n-GaN [2]. At present, a lower specific
contact resistance of 10
–5 to 10
–8
cm
2 has been achieved on n-GaN materials,
which can meet the requirements of LED chips. There are many n-GaN ohmic contact
electrode materials. Commonly used for n-type ohmic contact materials include Al
( m = 4.28 eV), Ti ( m = 4.33 eV), Ta ( m = 4.25 eV), V ( m = 4.33 eV) and
other metal elements. At present, many metal alloys such as Ti, Ti/Al, Ti/Al/Ni/Au,
Ta/Ti/Ni/Au, V/Al/V/Au, V/Ti/Au, etc. can be also used [3–10].
8.5.2 p-type GaN Ohmic Contact
Since the ohmic contact electrode of p-GaN has a great influence on the performance
of the LED, it is necessary to form an ohmic contact electrode having low contact
resistance, high light transmittance or high reflectivity, and thermal stability on the
surface of p-GaN. However, for the ohmic contact of p-GaN materials, it is difficult to
form a low-resistance ohmic contact thereon. On one hand, heavy doping of p-GaN
has not made effective breakthrough. This is due to the high ionization energy of
the Mg-dopant in p-GaN (up to 170 meV). High ionization energy can result in low
ionization rate and low carrier concentration. On the other hand, metal with large
work function that is greater than of p-GaN (having a work function of approximately
6.12 eV) is not available, which makes it difficult to fabricate an ohmic contact of
low-contact resistance for p-GaN [11–15].
Usually the operating voltage of an LED is related to the series resistance (especially the contact resistance on p-type GaN) and the current distribution. The ptype GaN contact resistance directly affects LED operating voltage. To obtain a
low contact resistance, some high work function metal such as of Ni, Pt, Au, etc.
are typically used. Some surface treatment process is needed as well. The uneven
current distribution causes overload on part of the light-emitting area. Insufficient
injection on other active areas makes the total injection efficiency not high. Efforts
to homogenize the current usually cause more electrode absorption.
The contact between metal and semiconductor is one of the major topics that
began very early in semiconductor science. For an ideal metal and semiconductor
contact (regardless of the surface state effect), a barrier is formed if the work function
of the metal is greater than that of the n-type semiconductor. Such a contact exhibits
rectification characteristic. On the other hand, an ohmic contact will be formed if
the work of the metal is smaller than that of the semiconductor. The opposite is
true for p-type semiconductor materials. High-quality ohmic contacts are required
to have low resistance and thermal stability. High transmittance is required for lightemitting devices so these problems must be considered when selecting a metallization
solution. Generally, to obtain a low-resistance ohmic contact, the Schottky barrier
height between the metal and semiconductor interface should be small. The method
of lowering the barrier height without considering the influence of the surface state