Semiconductor light emitting element

Details Semiconductor light emitting element Semiconductor light emitting element

1999-04-21

2002-02-26

Jackson, Jr., Jerome (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Incoherent light emitter structure

With particular dopant material

C257S097000, C257S099000, C372S045013

Reexamination Certificate

active

06350997

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a semiconductor light emitting element. More specifically, the invention relates to a semiconductor light emitting element made of InGaAlP materials on an n-type GaAs substrate and including a transparent electrode on its emission surface, which is reduced in operative voltage and increased in optical output by inserting one or both of a carbon-doped p-type GaAlAs layer and p-type GaAs layer between the transparent electrode and a cladding layer.
Semiconductor light emitting elements are widely expanding their field of application to indoor/outdoor displays, railway/traffic signals, compartment/cabin lamps, and so on, because of a number of advantage they have, such as compactness, low power consumption, reliability, for example. Especially, those using as the light emitting layer an InGaAlP material, which is a quaternary compound semiconductor, can be adjusted in composition to emit light in wide wavelength bands from red to green.
In the present application, “InGaAlP” pertains to semiconductors of any composition made by changing mole fractions x and y in the composition formula In
x
Ga
y
Al
1−x−y
P within the range satisfying 0≦x≦1, 0≦y≦1, and (x+y)≦1. That is, mixed crystals such as InGaP, InAlP, InGaAlP, GaP and GaAlP are also grouped into “InGaAlP”. Additionally, there are also involved mixed crystals containing arsenic (As) in addition to phosphorus (P) as group V elements.
For years, n-type GaAs substrates using silicon (Si) as the impurity have been typically used as substrates of InGaAlP light emitting diodes (LED).
FIGS. 10 through 12
are cross-sectional views schematically showing InGaAlP semiconductor light emitting elements as comparative examples, which were experimentally made by the Inventor in the course of researches toward the present invention.
The light emitting element
100
A shown in
FIG. 10
includes an n-type GaAs buffer layer
102
, n-type InGaAlP cladding layer
103
, InGaAlP active layer
104
, p-type InGaAlP cladding layer
105
, and ITO (indium tin oxide) transparent electrode
106
sequentially stacked on an n-type GaAs substrate
101
, and further formed are a p-side electrode
107
and an n-side electrode
108
. The semiconductor layers
101
through
105
are epitaxially grown by metal organic chemical vapor deposition (MOCVD), for example.
In the light emitting element
100
B shown in
FIG. 11
, a p-type GaAlAs current diffusion layer
109
on the p-type cladding layer
105
so that a current injected from the p-side electrode
107
disperses and spreads in a direction parallel to the element surface. The same components as those of the light emitting element shown in
FIG. 10
are labeled with common reference numerals, and their explanation is omitted here.
In the light emitting element
100
C shown in
FIG. 12
, a p-type GaAs low-resistance contact layer
110
and the transparent electrode
106
are stacked on the p-type cladding layer
105
.
InGaAlP LEDs shown in
FIGS. 10 and 12
, however, involve serious problems in their operative characteristics. Problems are particularly serious in the structure shown in
FIG. 10
, and it has not been brought into practice to date. One of reasons of the problems lies in the use of the transparent electrode
106
. The purpose of the transparent electrode
106
is to ensure uniform extension of a current along the emission surface and make a uniform light emitting intensity profile. However, the transparent electrode
106
is an n-type semiconductor like ITO (indium tin oxide), for example, and the cladding layer
105
in contact therewith is a p-type semiconductor. Therefore, when a forward voltage is applied to LED
100
A, a reverse-biased state is formed between the transparent electrode
106
and the p-type cladding layer
105
. As a result, almost no current flows as shown in
FIG. 2
as “Comparative Example (1)”.
The light emitting element
100
C shown in
FIG. 12
includes, between the p-type cladding layer
105
and the transparent electrode
106
, the p-type GaAs low-resistance contact layer
110
doped with a plenty of zinc (zn) (~1×10
20
cm
−3
). As a result, the contact resistance decreases, and relatively good current-voltage characteristics are obtained as shown in
FIG. 2
as “Comparative Example (3)”.
However, here arises another problem caused by doping of a large amount of Zn. Zinc tends to diffuse under heat or current. The large amount of zinc doped into the p-type GaAs layer
110
diffuses not only during crystal growth but also during operation of the element (when a current is supplied), and deteriorates the quality of the active layer
104
for emission, and adversely affects initial characteristics and lifetime characteristics of the element. As a result, as shown in
FIG. 7
as “Comparative Example (3)”, the element is low in optical output, and rapidly deteriorates toward reducing its lifetime. If the dope amount of zinc is reduced (1×10
19
cm
−3
or less), then the p-type GaAs layer
110
loses its function as the low-resistant contact layer, and the element results in involving the same problem of bad current-voltage characteristics.
In the light emitting element
100
B shown in
FIG. 11
, the p-type GaAlAs current diffusion layer
109
is provided instead of a transparent electrode on the emission surface. In the p-type GaAlAs layer
109
, mole fraction of Al is increased to transmit light from the active layer
10
, and zinc is doped to decrease the resistance. Since a dope amount of zinc of approximately 1×10
18
cm
−3
is sufficient therefor, the problem malfunction caused by diffusion of zinc, involved in he light emitting element
100
C shown in
FIG. 12
, need not be worried about so much. Also, since the transparent electrode
106
is not used, here is not the problem involved in the light emitting element shown in FIG.
10
. That is, the light emitting element
100
B shown in
FIG. 11
has good current-voltage characteristics as shown in
FIG. 2
as “Comparative Example (2)”.
However, this element also involves the problem that, since specific resistance of the p-type GaAlAs layer
109
is not as low as that of the transparent electrode, the current injected through the electrode
107
does not spread uniformly over the entire emission surface of the element. To solve the problem, it is necessary to increase the thickness of the p-type GaAlAs layer
109
as thick as approximately 4 &mgr;m or more so as to reduce the current-spreading resistance. However, the thicker layer requires a longer time for the crystal growth, and invites the problem of a higher manufacturing cost.
As reviewed above, in comparative semiconductor light emitting elements, it was difficult to obtain a sufficiently low resistance at the p-side, various approaches for overcoming it invited various problems, such as deterioration of characteristics caused by diffusion of zinc and an increase of the manufacturing cost, for example.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an InGaAlP semiconductor light emitting element operative with a lower voltage and ensuring higher output than conventional elements.
According to the invention, there is provided a semiconductor light emitting element comprising: an emission layer made of InGaAlP generating a light; a p-type contact layer made of a semiconductor doped with carbon as a p-type dopant; and a transparent electrode layer in contact with said p-type contact layer, said light generated at said emission layer being emitted through said transparent electrode.
In the present invention, a contact layer doped with a predetermined amount of carbon may be provided to decrease the contact resistance at the contact with the ITO electrode. Unlike zinc, carbon does not diffusion and does not deteriorate the property of the element.
An intermediate band gap layer having an intermediate band gap between those of the contact layer and the cladding layer may be interposed between the contact layer and t

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