The present invention relates to a light emitting diode which comprises a substrate having formed thereon an electrode on the top surface thereof, and at least one semiconductor lamina provided on the substrate and a non-ohmic electrode provided in each groove or through hole formed in the semiconductor lamina.
An InGaN based compound semiconductor light emitting element has a structure wherein an InGaN active layer is formed on a GaN substrate. The compound semiconductor light emitting element emits blue or white light, and has already been put to practical use in illumination apparatuses, large screen displays and traffic signals.
A conventional compound semiconductor light emitting element, shown in FIG. 3(a), is disclosed in “Proceedings of the Japan Society of Applied Physics”, Vol. 68, No. 4, 1999, pp. 593-596. FIG. 3(a) shows a section of the semiconductor element. The reference numeral 50 designates a n-GaN substrate having a crystal face of (0001) on one main surface thereof, 52 a laminate of an n-Al0.07Ga0.93N cladding layer and an n-GaN contact layer on the n-GaN substrate 50, 53 a light emitting layer of InGaN, 54 an electrode on the n-GaN contact layer 53, 55 an electrode on the n-AlGaN cladding layer 52, and 56 a silicon substrate having a crystal face of (111) on one main surface thereof, and 58 an electrode on the silicon substrate 56. As shown in FIG. 3(b), this compound semiconductor light emitting element has a cross sectional structure in which an n-Al0.07Ga0.93N cladding layer and an n-GaN contact layer which have different composition ratios are laminated.
FIG. 4 is a graph showing the luminous intensity of the conventional semiconductor light emitting element versus a current density. The vertical axis indicates the luminous intensity of light in relative intensity, and the horizontal axis indicates the current density in mA/cm2. As shown in FIG. 4, the luminous intensity of the compound semiconductor light emitting element abruptly decreases as the current density increases in a current region of 10 to 20 mA/cm2. This is because a so-called current-crowding phenomenon occurs. When the current density increases to 20 to 30 mA/cm2, the luminous intensity becomes an almost constant value, and this value is about 4.8 lm.
Japanese Unexamined Patent Publication No. Hei 8-185,747 discloses an ultraviolet emitting diode shown in FIG. 5. This diode comprises a sapphire substrate 1, an n-type GaN buffer layer 2, a non-doped GaN buffer layer 3, an InGaN active layer 4 and a p-type clad layer 5 laminated in order on the sapphire substrate 1. The active layer 4 comprises a MQW (multiple quantum well) structure of InGaN in which a plurality of InGaN quantum wells are layered. An n-type GaN contact layer 6 is further formed on the active layer 4. An etching stop layer 7 is provided between the n-type GaN contact layer 6 and the p-type clad layer 5. Furthermore, a transparent electrode 8 is provided on the n-type GaN contact layer 6. A p-side electrode 9 is provided on the p-type clad layer 5. The current-supplying layer 5 has a portion where a plurality of grooves are provided to be filled with the electrode 8.
In the case of the ultraviolet light emitting diode, the light emitted from the active layer 4 to the upper side thereof is absorbed in the electrode and the p-type clad layer, which are formed as opaque materials. Therefore, the ultraviolet light having a wavelength of around 370 nm having a sufficiently large transmittance on the sapphire substrate 1 is extracted to the outer side thereof, and the ultraviolet light having the remaining wavelengths, which have a lower transmittance on the sapphire substrate, is not absorbed in the p-type clad layer 5 and absorbed in the electrode and the transparent electrode 8.
On the other hand, Japanese Unexamined Patent Publication No. Hei 6-177,936 discloses a semiconductor light emitting element which has a structure similar to that of the ultraviolet light emitting diode. In this light emitting diode, the current-supplying layer 5 is formed of a single crystal layer. A groove is provided on a p-type clad layer 5 and an electrode 8 is provided in the groove.
In the semiconductor light emitting element having the structure shown in FIG. 3, a voltage is applied through an electrode 58 on the silicon substrate 56 and an electrode 55 on the n-Al0.07Ga0.93N cladding layer 52 and the n-GaN contact layer 53 which have different composition ratios. Therefore, the current concentrates on an interface 54 between the n-GaN contact layer 53 and the n-Al0.07Ga0.93N cladding layer 52 due to a current diffusion effect. Namely, a current-crowding phenomenon occurs. Therefore, the current density of about 15 to 25 mA/cm2 is required to maintain the luminous intensity at a predetermined value. The current density of about 20 to 30 mA/cm2 is required in order to obtain the same luminous intensity as that of the GaN light emitting diode.
On the other hand, the ultraviolet light emitting diode shown in FIG. 5 has the following problem: As the current flows to the electrode 8, the luminous intensity reduces because the current concentration occurs on the interface between the n-GaN contact layer 6 and the n-type GaN contact layer 6. This phenomenon is called a current-crowding effect. Namely, it is impossible to increase the luminous intensity at a predetermined current density which is required to obtain a predetermined luminous intensity. When the current density is too small, the luminous intensity reduces in principle. On the other hand, when the current density is too high, the luminous intensity is reduced due to the current-crowding effect as described above. Furthermore, the voltage drop occurs in the electrode 8 in the course of applying a voltage, and thus the current density in the light emitting layer 4 is reduced. The luminous intensity decreases, even if the light emitting layer 4 itself emits light at a predetermined luminous intensity. This is because the luminous intensity decreases as a whole, since a part of the light emitted from the light emitting layer 4 does not go through the electrode 8 due to the current-crowding effect, and does not contribute to increasing the luminous intensity of the light emitting layer 4.
Further, the ultraviolet light emitting diode disclosed in Japanese Unexamined Patent Publication No. Hei 6-177,936 has a problem in that the structure is complicated. Namely, since the current-supplying layer 5 is formed by a single crystal growth of the semiconductor layer, it is necessary to perform a crystal growth process in at least two times. Therefore, the crystal growth process is complicated.
In addition, the above two light emitting diodes have further problems in that the output light does not have a single peak wavelength, and in that the peak wavelengths change as the current density changes. Namely, the light emitting diodes disclosed in the above publications cannot be applied to optical communication apparatuses, which require the output light having a single wavelength. Furthermore, the luminous efficiency of the light emitting diodes disclosed in the above publications is poor in the region of high output power, because the amount of the light radiated in the direction of the perpendicular to the substrate is small. This causes a problem that it is impossible to obtain a sufficient luminous intensity.
It is a first object of the present invention to provide a light emitting element, wherein the light having a wavelength of 280 to 310 nm emitted from the active layer is distributed in the light having a longer wavelength, while the output light has a narrow half-width of less than 100 nm in a region of high current density.
It is a second object of the present invention to provide a light emitting element, wherein the current-crowding phenomenon and the current concentration phenomenon do not occur. Therefore, the luminous efficiency increases, and the output light has
