Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2003-01-16
2004-03-30
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S082000, C257S088000, C257S117000, C257S184000, C257S103000
Reexamination Certificate
active
06712478
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to light emitting device. More particularly, the present invention relates to a structure of light emitting diode having a superlattices contact layer. The light emitting diode is a nitride-base III-N group compound semiconductor device.
2. Description of Related Art
In recent years, gallium nitride-based III-N group compound semiconductor device, such as GaN, GaAlN, and GaInN, has been greatly taken as a light emitting device.
FIG. 1
is a cross-sectional view, schematically illustrating structure of a conventional light emitting diode made of III-N group compound.
In
FIG. 1
, the light emitting diode is formed on a substrate
10
, such as an Al
2
O
3
substrate. A nucleation layer
12
and an N-type conductive buffer layer
14
are sequentially formed over the substrate
10
. The buffer layer
14
includes GaN doped with N-type dopant, so as to ease the crystal growth for the subsequent crystal growing process. There is a light-emitting active layer
18
over the buffer layer
14
. Usually, the active layer
18
is confined by a confinement layer, that is, cladding layers
16
,
20
. The cladding layers
16
,
20
are doped with opposite conductive type. For example, if the lower cladding layer
16
is the GaN layer doped with N-type dopants, the upper cladding layer
20
is the GaN layer doped with P-type dopants. Then, a contact layer
22
is formed on the upper cladding layer
20
. The contact layer
22
a P-type GaN layer. A transparent electrode layer
24
is formed on the contact layer
22
, where the transparent electrode layer usually includes a N-type material layer, such as indium tin oxide (ITO), cadmium tin oxide (CTO), or ultra-thin metal. The transparent electrode serves as an anode of the diode. Moreover, an electrode layer
26
, serving as a cathode of the diode, is also formed on the buffer layer
14
but is separated from the cladding layers
16
,
20
and the active layer
18
.
FIG. 2
is a cross-sectional views, schematically illustrating a light emitting region for the light emitting diode in FIG.
1
. In
FIG. 2
, when the electrodes
24
,
26
are applied with a forward bias, the diode is conducted. At this situation, current can flow from the electrode
24
to the active layer
18
. In the conventional manner, the P-type contact layer
22
of GaN cannot have high carrier concentration and has large contact resistance. This results in a poor quality of current spreading. The p-type electrode layer
24
also only covers a portion of the contact layer
22
. As shown in
FIG. 2
, the area having current flow is about the width L of the electrode layer
24
. This limits the light emitting area for the diode. The function of the active layer cannot be fully performed. The light emitting efficiency of the diode is then greatly reduced.
The conventional LED using AlGaInP compound semiconductor can also be a structure as shown in FIG.
6
. In
FIG. 6
, a substrate layer
602
is used. An n-electrode layer
600
is formed on the back side of the substrate layer
602
while a confinement layer
604
is formed on the upper side of the substrate layer
602
. An active layer
606
is formed on the confinement layer
604
. Another confinement layer
608
is formed on the active layer
606
. Another substrate layer
610
is formed on the confinement layer
608
. Then, a p-electrode
612
is form on the substrate layer
610
. In this convention structure of LED, the p-electrode
612
directly contacts with the substrate
610
by only a portion. In this structure, the electrode contact is poor. As a result, the light emitting efficiency is poor either. It should be noted that the substrate layer
610
is a semiconductor layer in general.
In summary, the conventional light emitting diode is restricted by the physical properties of the contact layer. The contact layer cannot be grown with a high hole concentration. This also causes the high fabrication cost and also causes low yield. Further still, the conventional structure cannot provide a diode with high light emitting efficiency. A large portion of the active layer
18
of the diode is not well utilized. Moreover, the conductive doping types for the electrode layer and the contact layer are different. It could cause a junction between them at the contact region, and affecting the operation of the diode.
SUMMARY OF THE INVENTION
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained-layer supperlatices (SLS), so that the contact layer can easily have a high carrier concentration, resulting in high conductivity.
The invention provides a structure of light emitting diode, which uses strained-layer supperlatices structure to serve as a contact layer associating with the transparent electrode layer, so as to improve the light emitting efficiency and reduce the operation voltage.
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained layer supperlatices (SLS). The dopant type in the contact layer is therefore not necessary to be restricted. The transparent electrode and the contact layer can even be the same conductive type, so that the junction between the transparent electrode and the contact layer is avoided.
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained-layer supperlatices (SLS). The transparent electrode has better contact quality. The area of the transparent electrode can be about equal to the area of the active layer. This can allow the current to flow through the larger area of the active layer, so that the effective light emitting area is increased, and the light emitting efficiency is accordingly increased.
The invention provides a structure of light emitting diode, which is formed on a substrate, wherein an SLS structures is formed over a second confinement layer.
In the foregoing, the transparent electrode layer and the SLS layer can have different conductive types. The transparent electrode layer can have the conductive type of P-type or N-type.
The invention provides a structure of light emitting diode, having an SLS structure formed on the substrate.
In the foregoing, the transparent electrode layer and the SLS layer can be the same conductive type with all P-type or all N-type. The transparent electrode layer and the SLS layer can also be different conductive type.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 6093965 (2000-07-01), Nakamura et al.
patent: 6233265 (2001-05-01), Bour et al.
Hsu Samuel
Kuo Daniel
Sheu Jinn-Kong
Forde Remmon R.
J. C. Patents
South Epitaxy Corporation
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