Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular semiconductor material
Reexamination Certificate
2002-08-27
2004-03-02
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular semiconductor material
C257S095000, C257S099000, C257S618000
Reexamination Certificate
active
06700139
ABSTRACT:
RELATED APPLICATION
This application claims the priority of Japanese Patent Application No. 2001-263843 filed on Aug. 31, 2001, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a GaP-base light emitting device.
2. Description of the Related Art
GaP-base semiconductor light emitting device can emit a wide range of color from red to green in the visible light wavelength region when it is composed of GaP semiconductor, or of mixed crystal semiconductor material which comprises GaP semiconductor as a base material and substitutive component such as GaAs, InP or AlP. Indirect transition property of GaP semiconductor allows the resultant GaP-base semiconductor light emitting device to have the same property, where the emission efficiency of which can be improved by doping nitrogen or the like which can produce a luminescent center.
The doping of nitrogen for improving the emission efficiency, however, undesirably varies emission wavelength, which is typified by a fact that a nitrogen-doped GaP semiconductor emits yellowish-green light in contrast to the non-doped one which emits green light. Moreover, excessive doping of nitrogen is disadvantageous in that an excessive portion of nitrogen not contributable to luminescent center suppresses the emission efficiency.
Thus a countermeasure should be made not only from a viewpoint of raising the internal emission efficiency (internal quantum efficiency) but also from that of raising external taking out efficiency (external quantum efficiency). Various shapes of the light emitting device have been proposed in pursuit of raising external taking out efficiency. An exemplary light emitting device shown in
FIG. 8A
has a p-n junction between an n-type layer
21
and a p-type layer
22
, and an anode electrode
24
and cathode electrodes
25
so as to take the light out from the side of the p-type layer
22
, in which a strategy for raising the external taking out efficiency is found in slope portions
23
formed on the lateral planes by mesa etching, which successfully reduces total reflection of the emitted light. There is also known another example of light emitting device in which the main surface and slope portions
23
in the foregoing device are roughened so as to reduce total reflection of the emitted light, and the main surface opposite to the taking-out side is also roughened so as to, on the contrary, enhance the total reflection of light (disclosed in Japanese Patent No. 2907170).
FIG. 8B
shows a resultant case derived from the device shown in
FIG. 8A
after such roughening.
The morphology of the light emitting device shown in
FIG. 8B
is, however, still unsuccessful in achieving a satisfactory level of light emission efficiency when applied to the foregoing GaP-base semiconductor light emitting device based on indirect transition, and a device without doping of nitrogen which serves as a luminescent center will only results in more poorer luminance.
SUMMARY OF THE INVENTION
The present invention was proposed considering the aforementioned drawbacks. It is therefore an object of the present invention to provide a GaP-base semiconductor light emitting device having an improved luminance.
A GaP-base semiconductor light emitting device of the present invention comprises a GaP-base semiconductor substrate internally having a p-n junction formed between a p-type layer and n-type layer, and electrodes for applying drive voltage for light emission to such semiconductor substrate, wherein
a first main surface, which is defined as a main surface on the side of the p-type layer of the semiconductor substrate, and side surface thereof have a form of rough surface which comprises a collective of outwardly-swelling convex curved surfaces, and
a second main surface, which is defined as a main surface on the side of the n-type layer, has a form of specular surface finished by etching using aqua regia.
Since the emitted light is taken out from the p-type layer side of the GaP-base semiconductor substrate, the main surface on the n-type layer side (second main surface) is finished as a specular surface in order to enhance total reflection of light. On the contrary, the first main surface and side surface are finished by roughening so as to produce a rough surface which comprises a collective of outwardly-swelling convex curved surfaces in order to reduce total reflection of light. Such constitution of the GaP-base semiconductor light emitting device of the present invention can beneficially raise the take out efficiency of the emitted light, which results in an improved luminance as compared with that of conventional devices.
Specular finishing of the second main surface is accomplished by etching thereof using aqua regia. The etching with aqua regia can produce, on the second main surface, a specular surface which comprises a collective of specular concave curved surfaces each of which swells inwardly into the semiconductor substrate. This successfully allows the second main surface to fully exhibit an effect of total reflection of light. Smoothening by lapping and successive etching with aqua regia of the second main surface can further facilitate conversion of such second main surface into a specular surface which comprises a collective of specular concave curved surfaces.
By composing the second main surface with a specular surface which comprises a collective of specular concave curved surfaces, ratio of total reflection of light on the second main surface can be improved as compared with that on the conventional smooth surface. Further, according to the present invention, the diameter of curvature of the concave curved surface is within a range from 5 &mgr;m to 150 &mgr;m, both ends inclusive, and an inward depth is within a range from 0.5 &mgr;m to 15 &mgr;m, both ends inclusive. The diameter of curvature less than 5 &mgr;m or the inward depth of less than 0.5 &mgr;m will only result in an insufficient level of concave curved surface, which prevents the total reflection on the second main surface from being fully improved. On the contrary, the diameter of curvature exceeding 150 &mgr;m will fail in ensuring a sufficient contact area with an electro-conductive paste. The inward depth of the concave curved surface exceeding 15 &mgr;m will interfere the specular nature, and will thus prevent the total reflection of light from being improved.
In consideration of the foregoing situations, the concave curved surface can successfully enhance the total reflection of light and can also function as a bonding surface with an electro-conductive paste when the diameter of curvature thereof is adjusted within a range from 5 &mgr;m to 150 &mgr;m, and an inward depth thereof within a range from 0.5 &mgr;m to 15 &mgr;m. The specular surface which comprises a collective of such specular concave curved surfaces can readily be formed on the second main surface by using aqua regia.
On the contrary, the first main surface from which the light is taken out and the side surface have collectively formed thereon outwardly-swelling convex curved surfaces in order to reduce total reflection of light. Such collective formation of the convex curved surfaces will successfully reduce total reflection of light on the first main surface and side surface, where the light includes that reflected on the second main surface which comprises a collective of the concave curved surfaces. This raises the taken-out efficiency and thus improves the luminance.
The GaP-base semiconductor substrate for composing the GaP-base semiconductor light emitting device can be produced by stacking properly oriented crystal according to epitaxial growth method. The collective formation of the foregoing convex curved surface can be achieved by anisotropic etching, which proceeds at different etching rates depending on plane orientation of the stacked crystal, whereby a plurality of convex curved surfaces having a uniform morphology can readily be formed.
Employment of the anisotropic etching is also beneficial since
Ikeda Hitoshi
Kaneko Yasutsugu
Suzuki Kingo
Nelms David
Nguyen Dao H.
Shin-Etsu Handotai & Co., Ltd.
Snider Ronald R.
Snider & Associates
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