Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2002-05-21
2003-11-18
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S022000, C257S097000, C257S013000
Reexamination Certificate
active
06649942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device, and more specifically, it relates to a nitride-based semiconductor light-emitting device including a nitride-based semiconductor layer represented by a group III nitride-based semiconductor.
2. Description of the Background Art
An ultraviolet LED, a blue LED or a green LED employing a nitride-based semiconductor consisting of In
X
Al
Y
Ga
1-Y
N (0≦X, 0≦Y, X+Y≦1) is recently put into practice. Such an LED basically has a double hetero structure obtained by successively stacking an n-type nitride-based semiconductor layer consisting of n-type Al
Y
Ga
1-Y
N (0≦Y≦1), an emission layer consisting of In
X
Ga
1-X
N (0≦X≦1) and a p-type nitride-based semiconductor layer consisting of p-type Al
Y
Ga
1-X
N (0≦Z≦1), for example, on a transparent insulating substrate.
In relation to the nitride-based semiconductor light-emitting device having the aforementioned double hetero structure, a structure obtained by providing a light-transmitting p-side electrode consisting of a metal on a p-type nitride-based semiconductor layer defining an emission observation surface for externally extracting emission from the emission layer is known in general. This structure is disclosed in Japanese Patent Laying-Open No. 6-314822 (1994), for example.
A p-side electrode and an n-side electrode employed for an LED having the aforementioned structure must be in excellent ohmic contact with a p-type nitride-based semiconductor layer and an n-type nitride-based semiconductor layer in contact with the p-side electrode and the n-side electrode respectively, in order to reduce a forward voltage. In general, therefore, the n-side electrode contains Ti and Al exhibiting excellent ohmic contact with the n-type nitride-based semiconductor layer. Further, the p-side electrode having light transmittance contains Ni and Au exhibiting excellent ohmic contact with the p-type nitride-based semiconductor layer.
When forming the p-side electrode having light transmittance on the aforementioned conventional p-type nitride-based semiconductor layer, the thickness of the p-side electrode must be increased for reducing electric resistance, in order to reduce the forward voltage of the LED. When the thickness of the p-side electrode is increased as described above, however, the transmittance of the p-side electrode is disadvantageously reduced with respect to blue light and green light. Thus, luminous intensity of light extracted from the p-side electrode is disadvantageously reduced.
When the thickness of the p-side electrode is reduced in order to improve the transmittance of the p-side electrode, on the other hand, the sheet resistance of the p-side electrode is increased to increase the contact resistance between the p-side electrode and the p-type nitride-based semiconductor layer. Therefore, a current hardly homogeneously flows from the p-side electrode to the p-type nitride-based semiconductor layer, and hence it is difficult to attain homogeneous emission. The luminous intensity is disadvantageously reduced also in this case. In order to prevent such reduction of the luminous intensity, the driving voltage for the LED must disadvantageously be increased.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a nitride-based semiconductor light-emitting device capable of attaining homogeneous emission with a low driving voltage.
Another object of the present invention is to reduce the contact resistance between an electrode and a nitride-based semiconductor layer (contact layer) in the aforementioned nitride-based semiconductor light-emitting device.
Still another object of the present invention is to further reduce the sheet resistance of a light-transmitting electrode in the aforementioned nitride-based semiconductor light-emitting device.
A further object of the present invention is to reduce height of discontinuity of band gaps of the contact layer and a cladding layer in the aforementioned nitride-based semiconductor light-emitting device.
A nitride-based semiconductor light-emitting device according to an aspect of the present invention comprises a first conductivity type first nitride-based semiconductor layer formed on a substrate, an emission layer, consisting of a nitride-based semiconductor, formed on the first nitride-based semiconductor layer, a second conductivity type second nitride-based semiconductor layer formed on the emission layer, a second conductivity type intermediate layer, consisting of a nitride-based semiconductor, formed on the second nitride-based semiconductor layer, a second conductivity type contact layer, including a nitride-based semiconductor layer having a smaller band gap than gallium nitride, formed on the intermediate layer, and a light-transmitting electrode formed on the contact layer.
The nitride-based semiconductor light-emitting device according to the first aspect is provided with the second conductivity type contact layer including the nitride-based semiconductor layer having a smaller band gap than gallium nitride as hereinabove described, whereby the contact layer including the nitride-based semiconductor layer having a smaller band gap than gallium nitride can attain a higher carrier concentration than a contact layer (nitride-based semiconductor layer) consisting of gallium nitride and hence the thickness of a barrier formed on the interface between the contact layer and the electrode can be reduced. Thus, the contact resistance between the contact layer and the light-transmitting electrode can be reduced. Consequently, homogeneous emission can be attained while a driving voltage can be reduced. The nitride-based semiconductor layer having a smaller band gap than gallium nitride has higher electric conductivity than gallium nitride, and hence a current readily homogeneously spreads in the nitride-based semiconductor layer having a smaller band gap than gallium nitride. Thus, homogeneous emission can be attained also when the light-transmitting electrode is formed in a small thickness. When the second conductivity type intermediate layer is so formed as to substantially have an intermediate band gap between those of the second conductivity type contact layer and the second conductivity type second nitride-based semiconductor layer (cladding layer), the intermediate layer can reduce height of discontinuity of the band gaps of the contact layer and the cladding layer. Thus, resistance against a current flowing from the contact layer to the cladding layer can be reduced, thereby obtaining a nitride-based semiconductor light-emitting device having high luminous efficiency.
In the nitride-based semiconductor light-emitting device according to the aforementioned aspect, the carrier concentration of the second conductivity type contact layer is preferably at least 5×10
18
cm
−3
. According to this structure, the thickness of the barrier formed on the interface between the contact layer and the electrode can be so reduced that the contact resistance between the contact layer and the light-transmitting electrode can be readily reduced. Consequently, homogeneous emission can be attained and the driving voltage can be reduced.
In the aforementioned case, the second conductivity type contact layer preferably contains gallium indium nitride. According to this structure, the band gap of the contact layer can be readily reduced below that of gallium nitride.
In the aforementioned case, the light-transmitting electrode preferably contains at least one material selected from a group consisting of nickel, palladium, platinum and gold. According to this structure, excellent ohmic contact can be attained between the light-transmitting electrode and the contact layer.
In the aforementioned case, the light-transmitting electrode is preferably formed in a thickness capable of transmitting light. According to this structure, the electrode can be readily provided with light transmittance.
In the af
Hata Masayuki
Kano Takashi
Nomura Yasuhiko
Takeuchi Kunio
Yamaguchi Tsutomu
Huynh Andy
McDermott & Will & Emery
Sanyo Electric Co,. Ltd.
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