This application claims the priority of Japanese Patent Application No. 2001-263843 filed on Aug. 31, 2001, which is incorporated herein by reference.
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.
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 xcexcm to 150 xcexcm, both ends inclusive, and an inward depth is within a range from 0.5 xcexcm to 15 xcexcm, both ends inclusive. The diameter of curvature less than 5 xcexcm or the inward depth of less than 0.5 xcexcm 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 xcexcm will fail in ensuring a sufficient contact area with an electro-conductive paste. The inward depth of the concave curved surface exceeding 15 xcexcm 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 xcexcm to 150 xcexcm, and an inward depth thereof within a range from 0.5 xcexcm to 15 xcexcm. 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 the diameter of the convex curved surface can be adjusted so that the rough surface composed of a collective of such convex curved surfaces can function as a micro-lens. Probability of causing total reflection of light can further be reduced by making the rough surface into the micro-lens.
The GaP-base semiconductor substrate of the GaP-base semiconductor light emitting device of the present invention is preferably bonded to an electrode support member so that the entire portion of the second main surface thereof is covered with an electro-conductive paste. Since the GaP-base semiconductor light emitting device is designed to take the light out from the p-type layer side, the main surface on the n-type layer side, that is, the second main surface is eventually bonded to the electrode support member placing the electro-conductive paste in between. As described in the above, the second main surface has collectively formed thereon the specular concave curved surfaces, where each of which swelling inwardly into the semiconductor substrate, so that it is successful in ensuring a large area for bonding with the elector-conductive paste to thereby enhance the bonding strength of the semiconductor substrate to the electrode support member. This consequently prevents the semiconductor substrate from being undesirably inclined with regard to the electrode support member or from dropping out therefrom even when external force is applied thereto in a lateral direction.
The first and second main surfaces of the GaP-base semiconductor substrate have arranged thereon the electrodes for applying voltage for light emission, where it is necessary to compose the contact layers, which are located on the side of such electrodes to be brought into contact with the semiconductor substrate, with a material excellent in ohmic contact property with the semiconductor substrate, which is typified by Au-base alloy. FIGS. 6A and 6B schematically show finished status of electrode formation on the first and second main surfaces, respectively. FIG. 6A shows the electrode (first electrode 60) formed on the first main surface 10 on the p-type layer side, where the electrode comprises the contact layer (first contact layer 62) and a bonding pad layer 61. A material composing the first contact layer 62 is preferably an alloy of Au (gold) and Be (beryllium), or an alloy of Au and Zn (zinc), both of which being advantageous in establishing ohmic contact with p-type materials. Thus the first contact layer 62 excellent in ohmic contact property can be formed. The bonding pad layer 61 is preferably composed of Au in consideration of contact property with the first contact layer. On the surface of thus-formed first electrode 60, an Au wire for current supply is bonded to thereby allow taking of light mainly out from an area excluding the formation area for the first electrode 60.
FIG. 6B shows a status of the electrode (second electrode 63) formed on the second main surface 11 on the n-type layer side. Since the second main surface 11 is bonded to the electrode support member placing the electro-conductive paste in between as described in the above, the second electrode 63 has no bonding pad layer, unlike the first electrode 60 having the bonding pad 61, and instead the electrode per se functions as the contact layer (second contact layer 64). It is necessary that a material composing the second contact layer 64 is not only advantageous in establishing ohmic contact with n-type materials but in exhibiting only a low absorption of emitted light. After extensive experiments and discussion, the present inventors obtained a finding that composing the second contact layer 64 with an alloy of Au (gold), Si (silicon) and Ni (nickel) results in an excellent ohmic contact with n-type materials, and can desirably suppress absorption of the emitted light by the second contact layer 64. Composition of the second contact layer 64 with an alloy of Au, Si and Ni is thus successful in further raising luminance of the taken light.
The luminance can thus be improved by composing the contact layers of the electrodes to be formed on the first and second main surfaces using the foregoing materials.
The paragraphs in the above dealt with the morphology of the GaP-base semiconductor substrate and materials for composing the contact layers of being comprised the electrodes capable of raising emission luminance of the GaP-base semiconductor light emitting device of the present invention. Applying these constituents of the present invention especially to the GaP-base semiconductor light emitting device can improve the luminance of such device, which could not have been achieved to a desirable level by the prior art.
The GaP-base semiconductor light emitting device of the present invention thus comprises a GaP 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 GaP semiconductor substrate, wherein
a first main surface, which is defined as a main surface on the p-type layer side of the GaP semiconductor substrate, and side surface thereof have a form of rough surface which comprises a collective of outwardly-swelling convex curved surfaces,
a second main surface, which is defined as a main surface on the n-type layer side, is made into a specular surface which comprises a collective of specular concave curved surfaces,
the electrodes formed on the first main surface have a contact layer comprising an alloy of Au as combined with either of Be and Zn,
the electrodes formed on the second main surface has a contact layer comprising an alloy of Au, Si and Ni, and
the semiconductor substrate is bonded to an electrode support member so that the entire portion of the second main surface thereof is covered with an electro-conductive paste. As described in the above, the second main surface can readily be made into the specular surface which comprises a collective of specular concave surfaces simply by lapping of such second main surface and successive etching thereof using aqua regia.
It should now be noted that the GaP semiconductor substrate in the context of this specification conceptually includes both of that having luminescent center formed by nitrogen doping, and that having no luminescent center.