8.5 Ohmic Contacts
167
indium tin oxide (ITO) transparent conductive film has been widely studied in the
past 60 years due to its excellent properties. Currently, typical transparent conductive
oxide (TCO) materials are fluorine-doped tin oxide (FTO), aluminum-doped zinc
oxide (AZO) and ITO. Although the performance of CdO: In is very good, it is not
widely used because of its toxicity. These materials are all made by increasing the
carrier concentration by heavy doping in the semiconductor of the wide band gap to
reduce the resistivity. Research on further improvement of TCO performance is still
an interesting research topic in the field.
With the development of nanomaterials since 1990s, it has been found that nanomixtures based on conducting carbon nanotubes can also form transparent conductive
materials [21]. Although nanocomposite materials have many issues at present, they
have new characteristics at the nanometer scale, and have attracted a lot of attention
due to their interesting characteristics. For example, such transparent conductive
materials like graphene have extremely high mobility. So far, the carbon nanotubes
are all disorderly arranged. The nanowire structure of various transparent conductive
materials has made considerable progress. The cost is low, the raw materials are rich,
and it is safe and harmless. There is still a large room for development. The properties of this broad class of transparent conductive materials are quite different from
commonly used transparent conductive oxides and have some special applications.
Since 2000, people began to fabricate GaN-based LEDs using the ITO [22, 23]
as the transparent conductive material. Since the ITO material and the p-GaN are
difficult to form an ohmic contact, the development is generally in the early stage, A
thin metal layer is inserted between ITO and the p-GaN which is used as a transparent
electrode. CS Chang et al. found that when epitaxial LEDs are placed, the top layer
is inserted into the heavily doped cap layer to effectively improve the contact characteristics [24]. Indium tin oxide materials (ITO) have been widely used in GaN-based
LEDs in industrial production to replace conventional nickel-gold transparent electrodes. At the same time, zinc oxide (ZnO)-based transparent conductive materials
with excellent performance and low cost have also received extensive attention as
substitutes for ITO [25, 26].
The improvement in the conductivity and low absorption of the transparent electrode can simultaneously improve the current injection efficiency and the extraction
efficiency. The transparent electrode material also plays an important role as an
optical extraction layer for improving the light extraction efficiency. Therefore, the
requirements of the performance of the transparent electrode for the III-nitride LEDs
are mainly in the following aspects:
(1) Good electrical conductivity, which can make the current of the device expand
well;
(2) Having the ability to form good ohmic contact with a GaN material;
(3) The transmittance of the transparent electrode is closely related to the extraction
efficiency, and it has a good transmittance;
(4) The refractive index value can be modulated to form an anti-reflection film or a
highly reflective film;
(5) The ability to form a structured light-emitting surface.
167
indium tin oxide (ITO) transparent conductive film has been widely studied in the
past 60 years due to its excellent properties. Currently, typical transparent conductive
oxide (TCO) materials are fluorine-doped tin oxide (FTO), aluminum-doped zinc
oxide (AZO) and ITO. Although the performance of CdO: In is very good, it is not
widely used because of its toxicity. These materials are all made by increasing the
carrier concentration by heavy doping in the semiconductor of the wide band gap to
reduce the resistivity. Research on further improvement of TCO performance is still
an interesting research topic in the field.
With the development of nanomaterials since 1990s, it has been found that nanomixtures based on conducting carbon nanotubes can also form transparent conductive
materials [21]. Although nanocomposite materials have many issues at present, they
have new characteristics at the nanometer scale, and have attracted a lot of attention
due to their interesting characteristics. For example, such transparent conductive
materials like graphene have extremely high mobility. So far, the carbon nanotubes
are all disorderly arranged. The nanowire structure of various transparent conductive
materials has made considerable progress. The cost is low, the raw materials are rich,
and it is safe and harmless. There is still a large room for development. The properties of this broad class of transparent conductive materials are quite different from
commonly used transparent conductive oxides and have some special applications.
Since 2000, people began to fabricate GaN-based LEDs using the ITO [22, 23]
as the transparent conductive material. Since the ITO material and the p-GaN are
difficult to form an ohmic contact, the development is generally in the early stage, A
thin metal layer is inserted between ITO and the p-GaN which is used as a transparent
electrode. CS Chang et al. found that when epitaxial LEDs are placed, the top layer
is inserted into the heavily doped cap layer to effectively improve the contact characteristics [24]. Indium tin oxide materials (ITO) have been widely used in GaN-based
LEDs in industrial production to replace conventional nickel-gold transparent electrodes. At the same time, zinc oxide (ZnO)-based transparent conductive materials
with excellent performance and low cost have also received extensive attention as
substitutes for ITO [25, 26].
The improvement in the conductivity and low absorption of the transparent electrode can simultaneously improve the current injection efficiency and the extraction
efficiency. The transparent electrode material also plays an important role as an
optical extraction layer for improving the light extraction efficiency. Therefore, the
requirements of the performance of the transparent electrode for the III-nitride LEDs
are mainly in the following aspects:
(1) Good electrical conductivity, which can make the current of the device expand
well;
(2) Having the ability to form good ohmic contact with a GaN material;
(3) The transmittance of the transparent electrode is closely related to the extraction
efficiency, and it has a good transmittance;
(4) The refractive index value can be modulated to form an anti-reflection film or a
highly reflective film;
(5) The ability to form a structured light-emitting surface.