Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
2000-01-14
2002-09-10
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S098000, C257S103000, C257S201000, C257S613000, C257S615000, C257S043000
Reexamination Certificate
active
06448584
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting diode with high luminance and method for making the same, and more particularly to a light emitting diode having a transparent window layer which is formed by a semiconductor film of nitrogen-containing compounds.
2. Description of the Related Art
Researches on nitrides of Group III or V attract people's attention recently, particularly when the blue light emitting diode with high luminance was mass produced by a Japanese company in 1993, many companies become engaged in the development of blue and green light emitting diodes with high luminance and laser diodes.
In a semiconductor made of nitrogen-containing compounds of Group III or V, for an InGaAlP which energy gap less than that of nitride, when the ratio of aluminum to gallium in the active region of the light emitting element is changed, the light wavelength varies from 680 nm to 550 nm. Further, since the lattice constant of InGaAlP can match the GaAs substrate perfectly, InGaAlP is suitable for the light emitting element used for visible light region.
As shown in
FIG. 1
, the structure of a conventional InGaAlP light emitting diode is formed by growing a multi-layered epitaxial layer on a n-type GaAs light-absorbing substrate
10
. First, a n-InGaAlP confining layer
11
is grown on the GaAs substrate
10
, then a InGaAlP active layer
12
is grown on the n-InGaAlP confining layer
11
and then a p-InGaAlP confining layer
13
is grown on the active layer
12
such that a double heterostructure is formed. Finally, the light emitting surface of the diode is coated with a front surface metal electrode
14
and the surface of the GaAs substrate
10
on which the epitaxial layer is not formed is coated with a back surface metal electrode
15
. The light wavelength of the light emitting diode is determined by the ratio of aluminum to gallium in the active layer
12
. An increase in the amount of aluminum in the active layer may shorten the light wavelength of the light emitting diode. Meanwhile, the amount of aluminum in the confining layers
11
,
13
outside the active layer
12
must be greater than that of aluminum in the active layer
12
such that the carriers may be not only effectively injected into the active layer
12
but also prevent the light emitted from the active layer
12
from being absorbed by the confining layers
11
and
13
.
Generally speaking, with the exception of the combination efficiency of the electron and the electric hole, the major factor for determining the light emitting efficiency of a light emitting diode is whether the current on the front surface electrode
14
can be effectively distributed to the boundary of grains such that the light can be uniformly produced from the p-n junction. If the p-type confining layer
13
is too resistant to effectively distribute the current, the current will flow from the front surface electrode
14
to the back surface electrode
15
, causing current jamming such that the current can not be spread effectively and that most of the generated light can not emit due to the blocking of the opaque front surface electrode
14
or the light emitting efficiency thereof is reduced because the generated light is reflected by the metal electrode
14
and absorbed by the substrate. The conventional InGaAlP light emitting diode has the above drawbacks. This is because the concentration of the p-type InGaAlP confining layer
13
is generally below 1×10
18
cm
−3
and the hole mobility thereof is merely 10~20 cm
2
·V/sec, which forms the resistivity 0.5 &OHgr;-cm. The resistivity is so high that the lateral current can not be spread over the whole grain. To solve this problem, different structures, as shown in
FIGS. 2 and 3
, have been developed. The structure as illustrated in
FIG. 2
is formed by growing a semiconductor window layer
16
differing from the InGaAlP layer on the p-type InGaAlP confining layer
13
. The window layer is characterized by its low resistivity, perfect conductivity, and energy gap which is greater than that of the active layer
12
. Thus, the semiconductor window layer
16
will not absorb the light emitted from the p-n junction. Generally speaking, the materials suitable for the semiconductor window layer
16
include GaAlAs, GaAsP, and GaP, etc. The optimum thickness for the window layer
16
ranges from five to tens of micrometers. However, the lattice constants of GaAsP and GaP do not match with those of the GaAs substrate and the InGaAlP layer
13
, thus a large amount of dislocation on the growing interface is occurred and opto-electronic characteristics of the light emitting diode are adversely effected.
Another structure of the conventional technique is illustrated in FIG.
3
and is characterized in that a current blocking layer
17
and a Bragg reflector layer
18
are additionally formed and the current spreading layer or window layer
19
is made of GaAlAs. The current flowed from the front surface electrode
14
is thus influenced by the current blocking layer
17
so as to effectively distribute to the current spreading layer
19
. In addition, the added Bragg reflector layer
18
between the GaAs substrate
10
and InGaAlP confining layer
11
can reduce the amount of light emitted from the InGaAlP active layer
12
to be absorbed by the GaAs substrate
10
. Such a structure can double the light emitting efficiency of the light emitting diode. However, such a structure has drawbacks of complicated process and long processing time. The current blocking layer
17
is defined after two times of organic metal chemical vapor phase deposition and one time of masking and etching process. In addition, the aluminum ratio in the InGaAlP confining layer
13
is very high and thus oxidization is easily occurred and the growth is hard to control. Furthermore, the growth for the composition and the thickness of the Bragg reflector layer
18
should be precisely controlled and the thickness is about a few micrometers. Therefore, the manufacturing process will incur a much longer time.
As mentioned above, nitride has been applied to the manufacture of blue and green light emitting diodes. The emitting wavelength can be adjusted from a beam of violet light to a beam of green-blue light and even a beam of orange light by adjusting the metal components of Group III in the active layer. The conventional nitride researches are confined to the adjustment of the metal components of Group III and the manufacture of the blue and green light emitting diodes. Recently, nitride researches regarding the adjustment of the metal components of Group V, such as GaPN, GaAsN, GaPAsN, have been engaged.
Although the energy gaps of GaN and GaP are 3.4 eV and 2.3 eV respectively, the energy gap of is not increased with the increase amount of nitrogen but is increased with a bow-like curve. Therefore, when GaP
1−x
N
x
contains a few amount of nitrogen, it possesses an energy gap less than that of GaP. As far as GaP
1−x−y
As
x
N
y
is concerned, the lattice constant of compound is reduced when the amount of nitrogen is increased and is increased when the amount of arsenic is increased. Such a characteristic can be used to adjust the lattice constant of GaP
1−x−y
As
x
N
y
to be equal to, greater or smaller than that of GaP. Therefore, GaP
1−x−y
As
x
N
y
can be used to replace the GaP window layer of a light emitting diode with high luminance so as to reduce dislocation and to increase light emitting efficiency.
SUMMARY OF THE INVENTION
To avoid the above-mentioned problems encountered in the prior art, the object of the present invention is to provide a novel high luminance InGaAlP light emitting diode having nitrogen-containing compounds and method for making the same. The present invention is mainly directed to growing a window layer of a light emitting diode with a nitrogen-containing compound on the double heterostructure of InGaAlP. Since the energy gap of the nitrogen-containing compound is gr
Chan Shih-Hsiung
Guo Jan-Dar
Sze Simon M.
Tsang Jian-Shihn
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Lee Eddie
Nguyen Joseph
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