Interface texturing for light-emitting device

Details Interface texturing for light-emitting device Interface texturing for light-emitting device

2002-05-14

2003-12-09

Nhu, David (Department: 2818)

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

Incoherent light emitter structure

C257S918000

Reexamination Certificate

active

06661028

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a light-emitting device (LED), and more particularly to a LED with a textured interface, which can improve lightness of the LED.
BACKGROUND OF THE INVENTION
In recent years, some kinds of light-emitting device (LED) are developed and applied in flat-panel displayer (FPD). Among the LEDs, semiconductor light-emitting diodes are rapidly developed and generally used in indoor and outdoor displaying.
FIG. 1
shows a cross-sectional view of semiconductor light-emitting diodes. An epitaxial layer
20
having a p-n junction active layer
24
, a window layer
22
, and a window layer
26
is stacked on a semiconductor substrate
10
. The epitaxial layer
20
is usually made of gallium arsenide (GaAs) related compound, such as GaAs and AlGaAs, and the energy gap of the p-n junction active layer
24
is smaller than that of the window layer
22
, i.e. if a material of the p-n junction active layer
24
is Al
x
Ga
1−x
As and a material of the window layer
22
is Al
y
Ga
1−y
As, an aluminum content x of p-n junction active layer
24
is smaller than an aluminum content y of the window layer
22
and is greater than or equal to 0. Besides, the epitaxial layer
20
can also be made of GaInP or AlGaInP, and the energy gap of the p-n junction active layer
24
is smaller than that of the window layer
22
, i.e. if a material of the p-n junction active layer
24
is (Al
x
Ga
1−x
)
z
InP and a material of the window layer
22
is (Al
y
Ga
1−y
)
z
InP, an aluminum content x of p-n junction active layer
24
is smaller than an aluminum content y of the window layer
22
and is greater than or equal to 0. Electrodes
30
are formed on the top and the bottom of the stack layer. By injecting electric current, the p-n junction active layer
24
is “activized” and emitted, and thereby a light beam L is ejected out.
According to light refracting law, while a light beam in medium (I) ejects to medium (II), it must satisfy phase-matching condition to allow power transmission. That is, it must satisfy sin(&thgr;
1
)*n
1
=sin(&thgr;
2
)*n
2
, wherein &thgr;
1
and &thgr;
2
are the incident angle to the interface, and n
1
and n
2
are the material index of refraction. Otherwise, reflection will be occurred and the light beam cannot be transmitted into the medium (II). When the refraction index of medium (I) is greater than medium (II), the incident angle &thgr; must smaller than critical angle &thgr;
C
=arc sin(n
2


1
), or else the total internal reflection will be occurred and the light beam does not propagate into medium (II).
For semiconductor LED, the semiconductor material has refraction index (n~2.2-2.8) much greater than ambient, such as air (n~1) or transparent epoxy (n~1.5). When the light beam L from the semiconductor LED propagates to ambient and has an incident angle &thgr; greater than critical angle &thgr;
C
, total reflection is happened thereby limiting the external quantum efficiency of the semiconductor LED.
As shown in
FIG. 1
, the p-n junction active layer
24
is emitted and generates light beam L. For example, GaAs or GaInP related compound, such as GaAs (n
1
~3.65), and epoxy (n
2
~1.5) are used. Light beam L can be transmitted through the interface between GaAs and the epoxy layer if the incident angle &thgr; smaller than critical angle &thgr;c, else light beam L will be total reflected to light beam L′ and be again total reflected to light beam L″. Therefore, the light beam L will be continuously total reflected in the epitaxial layer
20
, and finally be absorbed under the reflection path or escaped from the sidewall.
For critical angle &thgr;c=24.27°, isotropic point source of light within the GaAs, the fraction of light emitted into the epoxy is only (1−cos &thgr;c)/2=4.4% of the available emitted light. For a cubic-shaped device having a completely reflective bottom surface, no top contact, and no internal absorption, there are six such interfaces and the fraction of total emitted light escaping the LED is 6×4.4%=26.4%. There is still a wide range for improving the extraction efficiency.
Hence, several methods for improving the light extraction from a LED have been proposed. One method is to change the macroscopic geometry of the LED to allow all or most of the light generated within the device to enter an escape cone at the interface with the ambient. Carr in Infrared Physics 6.1 (1996) observed that truncated cones, truncated pyramids, etc., can improve extraction efficiency. Dierschke, et al. in Applied Physics Letters 19.98 (1971) also noted large improvements in extraction efficiency for a hemispherical device. However, macroscopic shaping methods are costly and have associated manufacturability issues, such as inefficient material utilization and complicated fabrication processes and techniques.
In additional, Arpad, et al. in U.S. Pat. No. 3,739,217 described that another method is random texturing or roughening of the surfaces of the semiconductor LED, as shown in FIG.
2
. This randomization increases the overall probability that light L will enter an escape cone after many multiple passes through the device structure. But, each random texturing of the surfaces of the semiconductor LED is different, and much light is absorbed before escaping. These result in the extraction efficiency of each semiconductor LED is hardly controlled.
SUMMARY OF THE INVENTION
The present invention provides an interface texturing for a light-emitting device (LED). An ordered interface texturing is formed in the LED by using holographic lithographic techniques. The incident angle of the light total reflected in the textured interface can be changed in next time, and the probability of transmission in the interface can be improved. Therefore the total extraction efficiency can be increased.
The present invention provides a semiconductor epitaxial structure for a light-emitting device comprising: a first window layer; a p-n junction active layer stacked on the first window layer; and a second window layer stacked on the p-n junction active layer, and the second window layer having a textured surface, wherein the textured surface is caused by a plurality of interference lines formed by a plurality of overlaid coherent light beams.
The present invention further provides a light-emitting device comprising: a luminescent layer having a textured surface, wherein the textured surface of the luminescent layer is caused by at least one projection of light interference lines formed by a plurality of overlaid coherent light beams. The luminescent layer can be such as an epitaxial layer of a semiconductor LED.


REFERENCES:
patent: 3739217 (1973-06-01), Bergh et al.
patent: 4225380 (1980-09-01), Wickens
patent: 5135877 (1992-08-01), Albergo et al.
patent: 5349210 (1994-09-01), Ackley et al.
patent: 5601731 (1997-02-01), Hillmer
patent: 5779924 (1998-07-01), Krames et al.
patent: 6410348 (2002-06-01), Chen et al.

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