Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2002-09-19
2004-10-26
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S013000, C257S082000, C257S089000, C257S094000, C257S098000, C257S099000, C257S103000
Reexamination Certificate
active
06809340
ABSTRACT:
The present application claims the Convention priority from Japanese Patent Application No. 2001-304033, the complete disclosures of which are hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting element. More particularly, it relates to a light emitting element in which surface irregularities are formed on at least a side surface of a light emitting region formed on a substrate, and light is radiated from the side surface so as to increase the external quantum efficiency of the light emitting element.
The present invention can be applied to a light emitting element such as an LED which requires an increased external quantum efficiency.
2. Related Art
From in the past, there have existed semiconductor light emitting elements in which irregularities are formed on the top surface of a light emitting element to increase the light emitting efficiency. Examples of such light emitting elements are the semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 5-167101 and the semiconductor light emitting element and manufacturing method therefor disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-196152. The semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 5-167101 is shown in FIG.
11
. The semiconductor light emitting element shown in
FIG. 11
comprises a compound semiconductor substrate
1
made of n-GaAs or the like, a light reflecting layer
6
, light emitting layers
20
, a current dispersing layer
3
, a light scattering layer
10
, and electrodes
4
and
5
. The light emitting layers
20
comprise an n-InAlP layer
21
, an InGaAlP layer
23
, and a p-InAlP layer
22
.
The invention disclosed in that publication is characterized in that the light scattering layer
10
, which is made of GaP or the like, is formed on the current dispersing layer
3
. The light scattering layer
10
, which is formed by incomplete growth, has a different lattice constant from the current dispersing layer
3
. As a result, total reflection by the surface (the interface with the air) does not occur, and thus light is output with approximately two times the efficiency compared to when light is output from the glossy surface of the current dispersing layer
3
.
The semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-196152 is shown in FIG.
12
. It has a structure in which an n-GaN layer
52
, an InGaN light emitting layer
53
, and a p-GaN layer
54
are stacked atop a sapphire substrate
51
, an electrode
55
is formed atop a portion of the p-GaN layer
54
, and an electrode
56
is formed atop a portion of the n-GaN layer
52
. The light emitting element is characterized in that the surface
54
a
of the p-GaN layer
54
is formed to have a plurality of cylindrical lenses. If the surface of the p-GaN layer
54
were made flat in the conventional manner, of the light emitted by the InGaN light emitting layer
53
, only the light which fulfilled certain conditions (i.e., light incident within a critical angle of about 21.9° centered on a line normal to the surface) would be radiated to the exterior, and the other incident light would be confined by total reflection and attenuated.
However, in that invention, a plurality of cylindrical lenses are formed on the surface of the p-GaN layer
54
, so the conditions determining which light can be radiated are relaxed. Thus, the radiation efficiency is increased by making the surface cylindrical.
However, neither of the above prior art examples takes any step with respect to light which is propagated lateral within the light emitting layers. As shown in FIG.
13
A and
FIG. 13B
which are respectively a schematic vertical cross-sectional view and a plan view of a typical semiconductor light emitting element having a rectangular shape, no matter how many times light is reflected within the layer, there is a large amount of light which has an angle of incidence larger than the critical angle, and this light continues to be internally reflected, causing a decrease in the external quantum efficiency of the light emitting element.
In the semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-196152, forming cylindrical irregularities on the surface of a light emitting layer does in fact increase the external quantum efficiency, but the layer having the cylindrical irregularities is thin, and therefore, it is difficult to accurately form the irregularities thereon. In addition, with a stable material such as GaN, it is not possible to form random irregularities on the top surface of the material by surface treatment such as chemical etching, which was conventionally carried out for GaP and similar materials. On the other hand, physical methods of forming the cylindrical irregularities are difficult and suffer from poor productivity.
SUMMARY OF THE INVENTION
The present invention was made in order to solve the above-described problems. One object of the present invention is to increase the area of a crystal interface of a light emitting element without changing the light density within the crystal; i.e., without changing the size of the crystal, and to thereby increase the external radiation area with respect to emitted light present at random locations and having random orientations within the crystal, and to increase the efficiency of light radiation; i.e., to increase the external quantum efficiency. Another object of the present invention is to achieve such an increase in external quantum efficiency by a simple method so as to permit mass production of an improved light emitting element.
Yet another object of the present invention is to provide a light emitting element having surface irregularities on a side surface and on a top surface of a light emitting element so as to further increase the external quantum efficiency of the light emitting element.
A still further object of the present invention is to provide a light emitting element having a tapered side formed with surface irregularities so as to increase the light emitting efficiency of the light emitting element in the direction perpendicular to a substrate of the light emitting element.
While the various aspects of the present invention can collectively achieve all of the above objects, it should be understood that a single aspect does not necessarily achieve all of the above objects.
According to one form of the present invention, a light emitting element comprises a solid light emitting element having a light emitting region comprising at least one layer, with at least a portion of a side surface of the light emitting region having surface irregularities thereon.
The light emitting region may comprise a single layer or a plurality of layers. When the light emitting region comprises a semiconductor, the semiconductor may be either a p-type or an n-type. When the light emitting region comprises a plurality of layers, the different layers may contain the same compositional proportions having different impurity concentrations in each other, or the layers may differ in compositional proportion or in constituent element. In the latter case, the concentration of added impurities may differ among the plurality of layers. The light emitting region may be non-doped, or it may be an n-type or p-type semiconductor. Layers having various functions may be provided above or below the light emitting region. Such layers can be n-layers, p-layers, or non-doped layers. When the light emitting element is a semiconductor light emitting element, it may employ various structures, such as a homo pn structure, a single hetero structure, or a double hetero structure. The light emitting region can employ a single quantum well structure, a multiple quantum well structure, or the like.
The present invention can be applied not only to an injection type LED but to an intrinsic EL.
In this form of the invention, surface irregulari
Kato Hideaki
Suehiro Yoshinobu
Louie Wai-Sing
McGinn & Gibb PLLC
Pham Long
Toyoda Gosei Co,., Ltd.
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