Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2001-01-11
2002-06-04
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S096000, C257S097000, C257S190000, C257S191000
Reexamination Certificate
active
06399965
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light emitting device. More particularly, the invention relates to a semiconductor light emitting device using AlGaInP compound semiconductor material.
AlGaInP compound semiconductor is used as material for a device emitting light in a 0.5-0.6 &mgr;m wavelength band. In particular, a light emitting diode (LED) which uses GaAs as a substrate and has a light emitting section composed of AlGaInP material which is lattice-matched with GaAs is capable of emitting high-intensity light in a wavelength range from red to green with lower power than a conventional one using indirect transition semiconductor material such as GaP, GaAsP or the like.
To achieve high brightness light emitting with low power consumption, it is important that light emitting efficiency in a light emitting section is improved and that efficiency of taking out light from the light emitting section is also improved. It is also important that an operating voltage is lowered.
FIG. 16
is a cross sectional view showing a LED having a conventional current diffusion layer and an intermediate layer (Japanese Patent Laid-Open Publication No. 9-260724). This LED has an n-type AlGaInP lower clad layer
2
, an AlGaInP active layer
3
and a p-type AlGaInP upper clad layer
4
successively laminated on an n-type GaAs substrate
1
. Then, on top of these layers, a p-type AlGaInP intermediate layer
5
and a p-type GaP current diffusion layer
6
are successively laminated. Further, a p-type electrode
7
and an n-type electrode
8
are formed by deposition.
A composition of the p-type AlGaInP intermediate layer
5
is selected such that its lattice matching rate &Dgr;a/a is a median value of those of the p-type AlGaInP upper clad layer
4
and the p-type GaP current diffusion layer
6
and that the lower edge of its conduction band is between the lower edge of the conduction band in the upper clad layer
4
and that of the conduction band in the current diffusion layer
6
and the upper edge of its valence band is between the upper edge of the valence band in the upper clad layer
4
and that of the valence band of the current diffusion layer
6
in energy levels before junctions are formed so that heterobarriers are lowered in an energy band profile.
Since the conventional LED has the p-type GaP current diffusion layer
6
, current can be injected not only into a region directly below the electrode
7
, but all over the active layer
3
.
FIG. 17
shows an energy band profile from the upper clad layer
4
to the current diffusion layer
6
in the conventional LED. Since this LED has the p-type AlGaInP intermediate layer
5
between the upper clad layer
4
and the current diffusion layer
6
, energy discontinuity level can be divided and decreased as compared with an energy band profile when an intermediate layer is not used shown in FIG.
18
. Therefore, heterobarriers
9
,
10
referred to as “notches” occurring at the p-type AlGaInP upper clad layer
4
interface and the p-type GaP current diffusion layer
6
interface can be lowered.
Further, in the conventional LED, a lattice constant of the p-type AlGaInP upper clad layer
4
is 5.65 Å and a lattice constant of the p-type GaP current diffusion layer
6
is 5.45 Å. The composition of the p-type AlGaInP intermediate layer
5
is selected such that its lattice constant is 5.55 Å, which is a median value of the aforementioned constants, and thereby lattice mismatch is relieved. Consequently, interface levels occurring at the upper clad layer
4
interface and the current diffusion layer
6
interface can be lowered and the degree of bending in the energy band profile occurring due to the interface levels can be reduced. Therefore, the energy barrier of the interface can be lowered as shown in FIG.
17
.
Thus, the operating voltage of this conventional LED can be substantially reduced by the above-described effect of lowering the energy barrier.
However, this conventional LED has problems described below. That is, as described above, the reduction of the operating voltage is achieved by selecting a composition in which the lower edge of the conduction band of the p-type AlGaInP intermediate layer
5
and the upper edge of its valence band are between those of the upper clad layer
4
and the current diffusion layer
6
in the energy level relationship before their junctions are formed. Further, the interface levels are lowered by setting the lattice constant of the intermediate layer
5
to be a median value of those of the upper clad layer
4
and the current diffusion layer
6
and thereby further reduction of operating voltage is achieved.
However, an experiment reveals that an effect of reducing the operating voltage and an effect of reducing crystal defects in the crystal surface are not sufficiently obtained depending on the lattice matching rate &Dgr;a/a of the p-type AlGaInP intermediate layer
5
with the GaAs substrate
1
.
When the effect of reducing the operating voltage and the effect of reducing crystal defects are not sufficiently obtained, current spread and light transmittance in the current diffusion layer
6
are deteriorated. The light taking out efficiency and the current injection efficiency are also deteriorated. Consequently, sufficient brightness cannot be obtained. Another problem is that power consumption is not sufficiently low since the operating voltage is not sufficiently reduced. Further, there is an adverse effect that adhesion of the electrode
7
formed on the current diffusion layer
6
is degraded due to crystal defects in the crystal surface and thereby the electrode
7
peels. Thus, productivity is decreased because the yield is lowered.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a low-priced semiconductor light emitting device with high brightness intensity and low power consumption by reducing operating voltage to reduce power consumption and reducing defects in the crystal surface to improve adhesion of an electrode so as to improve the yield.
To achieve the above object, the present invention provides a semiconductor light emitting device comprising:
a compound semiconductor substrate;
a laminated structure provided on the compound semiconductor substrate and including at least an active layer for emitting light, a first clad layer and a second clad layer sandwiching the active layer from both sides thereof;
an intermediate layer formed on the laminated structure;
a current diffusion layer formed on the intermediate layer;
at least one of a structure for enhancing a yield by improving adhesion of an electrode and a structure for reducing power consumption by reducing an operating voltage.
In one embodiment, a value of a lattice matching rate &Dgr;a/a of the intermediate layer with the compound semiconductor substrate is set such that the number of crystal defects observed in a crystal surface is 20 or less after crystal growth finishes.
According to the above embodiment, the number of crystal defects in the crystal surface is reduced to 20 or less after crystal growth finishes. Consequently, adhesion of an electrode formed on the crystal surface is improved and thereby the yield is enhanced. That is, a high brightness intensity semiconductor light emitting device can be provided at a low cost.
In one embodiment, a value of a lattice matching rate &Dgr;a/a of the intermediate layer with the compound semiconductor substrate is set such that an operating voltage increase when a driving current is 20 mA is 0.5 V or lower at interfaces in the intermediate layer.
According to the above embodiment, an operating voltage rise at interfaces in the intermediate layer when the driving current is 20 mA is suppressed to 0.5 V or lower. Thus, the operating voltage is reduced and thereby power consumption is reduced. That is, a high brightness semiconductor light emitting device with low power consumption can be provided.
In one embodiment, a value of a lattice matching rate &Dgr;a/a of the intermediate layer
Nakamura Jun-ichi
Ohyama Shouichi
Sasaki Kazuaki
Ho Tu-Tu
Nelms David
Nixon & Vanderhye P. C.
Sharp Kabushiki Kaisha
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