Light emitting device

Abstract: A light emitting device with higher luminance than that of a conventional light emitting device is provided. According to the present invention, a light emitting device comprises the following three layers of: a first cladding layer 11 made of a p-type mixed crystal compound semiconductor; a p-type active layer 12 made of a p-type mixed crystal compound semiconductor which has the mixed crystal ratio required to emit the light having a predetermined wavelength; and a second cladding layer 13 made of a mixed crystal compound semiconductor, the conductivity type of which is n throughout the second cladding layer 13 or except the region in the vicinity of the heterojunction with the p-type active layer 12, wherein the p-type active layer is sandwiched by the first cladding layer 11 and the second cladding layer 13, and forms the double hetrojunction structure with said both cladding layers, and is characterized in that: the second cladding layer 13 is doped with an n-type impurity or impurities and a p-type impurity or impurities; the total concentration of a p-type impurity or impurities in the second cladding layer 13 is in the range of 1 x 10¹⁷ cm⁻³ and more; and a pn junction is formed in the second cladding layer 13 of a position within 1 µm from the heterojunction with the p-type active layer 12.

Description: The present invention relates to a light emitting device, and in more particular to a light emitting diode with a double heterojunction structure. Light emitting devices are semiconductor devices which emit visible or infrared light when forward currents are supplied across the pn j unction thereof. Some of them are used as point light sources for pilot lamps and the like, and others are also used as light sources in optical communication systems. Their applications are expanding more and more. In general, a light emitting device has a three-layer double heterojunction structure, in which an active layer is sandwiched by a set of cladding layers. The device is completed by forming electrodes on both sides of this three-layer double heterojunction structure. Each of the three layers is made of semiconductor, for which III-V group compound semiconductors are mainly used for current commercial purposes. Among the III-V group compound semiconductors, GaAlAs semiconductors are selectively used for light emitting devices of high luminance applications because they provide especially higher luminance. The light emitting devices mentioned above are produced, for example, by forming a three-layer double heterojunction structure on a GaAs substrate by means of a epitaxial growth method. As the epitaxial growth method, either a vapor phase epitaxial growth method or a liquid phase epitaxial growth method can be used. The liquid phase epitaxial growth method is more often used because it provides growth layers with better crystallinity. As the applications of light emitting devices broaden, those with higher luminance are increasingly demanded. The conventional light emitting device mentioned above, however, cannot provide enough luminance. Development of a light emitting device with higher luminance has therefore been desired. Attention is also drawn to the disclosures of US-A-5 214663, EP-A-0 516 162 and US-A-5 264 397. It is an object of the present invention, accordingly, to provide a light emitting device, in particular a light emitting diode, with higher luminance than that of a conventional light emitting device. The present invention provides a light emitting diode comprising the following three layers of: a first cladding layer made of a p-type mixed crystal compound semiconductor represented as Ga 1-x Al x As (where 0<x <1); a p-type active layer made of a p-type mixed crystal compound semiconductor represented as Ga 1-y Al y As(where 0< y <1), which has a mixed crystal ratio required to emit light having a predetermined wavelength, and which is doped with a p-type impurity or impurities; and a second cladding layer made of a mixed crystal compound semiconductor represented as Ga 1-z Al z As(where 0< z < 1), the conductivity type of which is n type except a region in the vicinity of the heterojunction with said p-type active layer or which is entirely n type, wherein said p-type active layer is sandwiched by said first cladding layer and said second cladding layer and forms a double heterojunction structure with both said cladding layers, characterized in that: said second cladding layer is doped throughout with Te and Zn; the concentration of Zn in the second cladding layer is at least 1x10 17 cm -3 ; and a pn junction is formed in the second cladding layer at a position within 1 µm from the heterojunction with the p-type active layer. Preferably, the p-type im purity concentration of said p-type active layer is at least 1 x 10 17 cm -3 . The p-type impurity concentration P P of said first cladding layer in a region adjacent to the heterojunction with said p-type active layer is suitably equal to or less than the p-type impurity concentration P A of said p-type active layer. A p-type impurity or impurities do ed in said p-type active layer and said first cladding layer preferably include at least Zn. Said x and z are preferably selected in the range of 0.5 to 0.85 and said y is preferably selected in the range of 0.25 to 0.45. According to the present invention, the luminance of a light emitting diode is improved by controlling the Zn concentration as p-type impurity concentration of the n-type second cladding layer to be at least 1 x 10 17 cm -3 . Conventionally, in this n-type second cladding layer, n-type impurity or impurities such as Te and the like were doped in order to attain n-type conductivity therein but p-type impurity or impurities have never been doped intentionally. The level of the p-type impurity concentration in the second cladding layer, therefore, is of the order of spontaneous contamination from carbonaceous parts except the diffusion of p-type impurities from the p-type active layer. In general, the concentration was less than 1 x 10 17 cm -3 . However, it has been confirmed by the present inventors that the luminance of a light emitting diode is unexpectedly improved under the conditions that the concentration of the p-type impurity or impurities in the n-type second cladding layer is 1 x 10 17 cm -3 and more. Regarding the reasoning for this fact, it is considered that the size effect of Zn atoms in the crystal lattice shows up by increasing the concentration of a p-type impurity or impurities inclusive of Zn in the second cladding layer and the positive effects such as decreasing crystallographic defects prevail due to the size effect of Zn, so that the effect of improving the luminance results. The present invention will be further described with reference to the accompanying drawings , wherein: FIG.1 is a view showing the depth-wise structure of an embodiment of a light emitting diode according to the present invention; FIG.2 is an explanatory view showing an example of a process for producing a light emitting diode according to the present invention; FIG.3 is a graph showing the depth-wise impurity concentration profile of an embodiment of a light emitting diode according to the present invention; FIG.4 is a graph showing the relationship between the Zn concentration of the n-type cladding layer and the relative luminance of the light emitting diode according to the present invention; and FIG.5 comprises graphs showing the frequency distribution of each set of the relative luminances of an embodiment of a light emitting diode according to the present invention, where the Zn concentration of the n-type cladding layer is 1 x 10 17 cm -3 to 5 x 10 17 cm -3 , and of a conventional light emitting device, where the Zn concentration of the n-type cladding layer is 1 x 10 16 cm -3 to 5 x 10 16 cm -3 . An embodiment of the present invention will be described below in reference to the accompanying drawings. FIG.1 shows the depth-wise structure of an embodiment of a light emitting diode according to the present invention. In this figure, three epitaxial growth layers constituting a light emitting diode according to the present invention, are formed on a p-type < 100 > GaAs substrate 10 which is doped with Zn as p-type impurity at a concentration of approximately 1 x 10 19 cm -3 to 3 x 10 19 cm -3 . More particularly, the following three layers are sequentially grown on the p-type GaAs substrate 10 by means of an epitaxial growth method, which are: a p-type cladding layer 11 of 150 µm thickness made of Ga 0.25 Al 0.75 As mixed crystal compound semiconductor doped with Zn as p-type impurity in a manner such that the concentration is distributed in a predetermined depth-wise profile; a p-type active layer 12 of 1 µm thickness made of Ga 0.62 Al 0.38 As mixed crystal compound semiconductor doped with Zn as p-type impurity to have the concentration of 1 x 10 17 cm -3 and more; and an n-type cladding layer 13 of 50 µm thickness made of Ga 0.25 Al 0.75 As mixed crystal compound semiconductor doped with both Te as n-type impurity and Zn as p-type impurity. The p-type GaAs substrate 10 is removed by selective etching, if necessary, after forming the three epitaxial growth layers. Next, an example of a process for producing a light emitting diode of an embodiment according to the present invention will be explained referring to FIG.2. The light emitting diode of the embodiment can be produced by growing each of the layers by means of a liquid phase epitaxial growth method. Here, an example of the slide boat method, in which a slide boat is used, is shown and the gradual cooling method is adopted, in which the temperature is gradually lowered for growing. In FIG.2, the p-type GaAs substrate 10 is fixed in a recess of the upper surface of a boat body 20 in such a manner that the upper surface of the substrate 10 is flush with the upper surface of the boat body 20. A sliding solution reservoir 21, which is slidable on the boat body 20, has: a first solution reservoir 22 holding a Ga solution 22a for growing the p-type cladding layer, a second solution reservoir 23 holding a Ga solution 23a for growing the p-type active layer, and a third solution reservoir 24 holding a Ga solution 24a for growing the n-type cladding layer. Each reservoir has no bottom and thus the solution in each reservoir directly contacts the upper surface of the boat body 20. The Ga solution 22a for growing the p-type cladding layer is a Ga solution resolving Ga 0.25 Al 0.75 As with Zn added as p-type impurity. The Ga solution 24a for growing the n-type cladding layer is a Ga solution resolving Ga 0.25 Al 0.75 As with both Zn as p-type impurity and Te as n-type impurity. The Ga solution 23a for growing the p-type active layer is a Ga solution resolving Ga 0.62 Al 0.38 As with Zn as p-type impurity. The sliding solution reservoir 21 is adapted to be moved by operating an actuating rod 25. A process for forming three epitaxial growth layers on the p-type GaAs substrate 10 according to the aforementioned apparatus will be described below. The sliding solution reservoir 21 is slid by moving the actuating rod 25 in the arrowed direction from the position shown in FIG.2(a) to set the Ga solution 22a for growing the p-type cladding layer 11 held in the first solution reservoir 22 on the p-type GaAS substrate 10, and the p-type cladding layer 11 is grown on the substrate 10 under a condition that the temperature is lowered, for example, from 900 °C to 800 °C ( FIG.2(b)). Then, the sliding solution reservoir 21 is further slid in the arrowed direction to set the Ga solution 23a for growing the p-type active layer 12 held in the second solution reservoir 23 on the p-type GaAs substrate 10, and the p-type active layer 12 is grown on the p-type cladding layer 11 under a condition that the tem perature is lowered, for example, from 800 °C to 795 °C( FIG.2(c)). The sliding solution reservoir 21 is still further slid in the arrowed direction to set the Ga solution 24a for growing the n-type cladding layer 13 held in the third solution reservoir 24 on the p-type GaAs substrate 10, and the n-type cladding layer 13 is grown on the p-type active layer 12 under a condition that the temperature is lowered, for example, from 795 °C to 650 °C(FIG.2(d)). Thereafter, the sliding solution reservoir 21 is still further slid in the arrowed direction to complete the whole process for growth (FIG.2(e)). In the above-mentioned process, Te and Zn are both added in the Ga solution 24a for growing the n-type cladding layer wherein Zn is doped to such an extent that the Zn concentration doped in the n-type cladding layer 13 is at least 1 x 10 17 cm -3 and the conductivity type of the n-type cladding layer 13, however, remains n-type. In a subsequent heat treatment, the highly doped Zn in the p-type active layer 12 diffuses into the side of the n-type cladding layer 13 and thus the region thereof adjacent to the p-type active layer 12, which has a thickness of 1 µm or less, may become p-type. After preparation of the three-layer double heterojunction structure on the p-type GaAs substrate 10 as described above, the substrate 10 is removed by selective etching and electrodes are formed on both sides of the three-layer structure to complete a light emitting diode. FIG.3 shows the depth-wise impurity concentration profile of the light emitting diode produced in the manner described above. The concentration of the intentionally-doped Zn in the n-type cladding layer 13 at least is 1 x 10 17 cm -3 , and the concentration of Te in the n-type cladding layer 13 is approximately 1 x 10 18 cm -3 in the vicinity of the surface, and, in the region adjacent to the p-type active layer 12, is lowered to approximately 7 x 10 17 cm- 3 , but the entire n-type cladding layer 13 remains n-type. The Zn concentration of the p-type active layer 12 is at least 1 x 10 17 cm -3 . The concentration of Zn in the p-type cladding layer 11 is profiled in such a manner that the concentration is lower in the region adjacent to the p-type active layer 12 than that of Zn in the p-type active layer 12 and becomes higher with increase in distance from the p-type active layer 12. FIG.4 shows the relationship between the Zn concentration of the n-type cladding layer and the relative luminance of the light emitting diode according to the present invention. It is seen from the figure that the relative luminance of the light emitting diode becomes larger with increase in the Zn concentration of the n-type cladding layer 13. FIG.5 comprises graphs showing the frequency distribution of each set of the relative luminances of an embodiment of the light emitting diode according to the present invention, where the Zn concentration of the n-type cladding layer 13 is 1 x 10 17 cm -3 to 5 x 10 17 cm -3 , and of a conventional light emitting device, where the Zn concentration of the n-type cladding layer 13 is 1 x 10 16 cm -3 to 5 x 10 16 cm -3 . In the embodiment, it is found that the light emitting diode according to the present invention shows typically an extremely high value of approximately 400 in relative luminance. It is to be noted that the luminance in mcd of the product operated at 20mA is approximately equal to the value of the relative luminance multiplied by 8. In the above description, an embodiment according to the present invention has been explained. It should be understood, however, that the present invention is not limited to the embodiment, but various modifications thereof can be made within the scope of the present invention as defined by the appended claims.

Claims: A light emitting diode comprising the following three layers of: a first cladding layer (11) made of a p-type mixed crystal compound semiconductor represented as Ga 1-x Al x As (where 0<x <1); a p-type active layer (12) made of a p-type mixed crystal compound semiconductor represented as Ga 1-y Al y As(where 0< y <1), which has a mixed crystal ratio required to emit light having a predetermined wavelength, and which is doped with a p-type impurity or impurities; and a second cladding layer (13) made of a mixed crystal compound semiconductor represented as Ga 1-z Al z As(where 0< z < 1), the conductivity type of which is n type except a region in the vicinity of the heterojunction with said p-type active layer (12) or which is entirely n type, wherein said p-type active layer (12) is sandwiched by said first cladding layer (11) and said second cladding layer (13) and forms a double heterojunction structure with both said cladding layers, characterized in that: said second cladding layer (13) is doped throughout with Te and Zn; the concentration of Zn in the second cladding layer (13) is at least 1x10 17 cm -3 ; and a pn junction is formed in the second cladding layer (13) at a position within 1 µm from the heterojunction with the p-type active layer (12). A light emitting diode as claimed in claim 1, characterized in that the p-type impurity concentration of said p-type active layer (12) is at least 1x10 17 cm -3 . A light emitting diode as claimed in claim 1 or 2, characterized in that the p-type impurity concentration P p of said first cladding layer (11) in a region adjacent to the heterojunction with said p-type active layer (12) is equal to or less than the p-type impurity concentration P A of said p-type active layer (12). A light emitting diode as claimed in any of claims 1 to 3, characterized in that a p-type impurity or impurities doped in said p-type active layer (12) and said first cladding layer (11) include at least Zn. A light emitting diode as claimed in any of claims 1 to 4, characterized in that said x and z are selected in the range of 0.5 to 0.85 and said y is selected in the range of 0.25 to 0.45.

Inventors: KAWASAKI MAKOTO, YAMADA MASATO, KAWASAKI, MAKOTO, YAMADA, MASATO
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1995
Main classification: H01L33
IPC codes: H01L33