Semiconductor light emitting device

Abstract: In a light emitting device comprising a first clad layer composed of a mixed crystal compound semiconductor of first type conductivity, an active layer composed of a mixed crystal compound semiconductor of first type conductivity which has the mixed crystal ratio required to emit the prescribed wavelength, and a second clad layer composed of a mixed crystal compound semiconductor of second type conductivity which has a mixed crystal ratio equivalent to that of the first clad layer, the active layer is sandwiched by the first and second clad layers and forms the double hetero structure with the first and second clad layers, and the carrier concentration in the first clad layer near the junction with the active layer was made to be 5 x 10¹⁶ cm⁻³ or less. The carrier concentration in the active layer is preferably 1 x 10¹⁷ cm⁻³ or less, and the carrier concentration in the second clad layer is preferably 5 x 10¹⁶cm⁻³ or more.

Description: FIELD OF THE INVENTION This invention relates to light emitting devices, and in particular to light emitting devices with a double hetero structure. BACKGROUND OF THE INVENTION A light emitting device is a semiconductor device which emits visible or infrared light when a forward direction current is applied to its pn junction. It is used as a point light source for a pilot lamp and such, and also as a light source in optical communications, and its applications are increasing. In general, a light emitting device has a three-layer double hetero structure in which an active layer is sandwiched by a set of clad layers, and electrodes are formed on both sides of this double hetero structure to obtain a light emitting device. Each of the three layers is composed of a semiconductor, for which mainly the III-V group compound semiconductors are currently used commercially. Of these, the GaAlAs semiconductors are used for light emitting devices for high luminance applications because they provide higher luminance. The light emitting devices described above are made, for example, by growing a three-layer double hetero structure on a GaAs substrate using the epitaxial growth method. Either the gas phase growth method or the liquid phase growth method may be employed. However, the liquid phase growth method is mostly used because it provides growth layers with better crystallinity. As the applications for light emitting devices broaden, light emitting devices with higher luminance are increasingly desired. However, the conventional light emitting devices described above cannot provide enough luminance, and development of light emitting devices with higher luminance has been desired. EP-A-0 356 037 describes a light-emitting device which comprises a first p-type clad layer composed of Ga 1-x Al x As with x between 0.6 and 0.9 and an impurity concentration between 10¹⁶ and 10¹⁸ cm⁻³. Furthermore it includes a p-type active layer of Ga 1-x Al x As with x between 0.35 and 0.45 and a dopant concentration in the same range as the first clad layer, and a second n-type clad layer composed of Ga 1-x Al x As with x between 0.6 and 0.9 and a dopant concentration between 5 x 10¹⁶ and 5 x 10¹⁷cm⁻³. This has the disadvantage that the relative luminance or brightness of the light emitting device is reduced. EP-A-0 301 893 describes a light-emitting semiconductor device of double hetero structure comprising a p-type Ga 0.2 Al 0.8 As first clad layer, an active Ga 0.65 Al 0.35 As layer and an n-type Ga 0.2 Al₈As second clad layer. The dopant concentration of the first clad layer is between 10¹⁶ and 3 x 10¹⁷ cm⁻³. The literature reference "Applied Physics Letter", Vol. 50, No. 8, (1987) pages 427 bis 429 discloses a twin-stripe laser diode wherein the active layer is not p-type. Accordingly, it is an object of this invention to provide a light-emitting device, comprising: a first clad layer composed of a mixed crystal compound semiconductor of first type conductivity: an active layer composed of a mixed crystal compound semiconductor of first type conductivity which has the mixed crystal ratio required to emit the prescribed wavelength; and a second clad layer composed of a mixed crystal compound semiconductor of second type conductivity which has a mixed crystal ratio substantially equal to that of said first clad layer; wherein said active layer is sandwiched between said first and second clad layers and forms a double hetero structure with said first and second clad layers; this device is characterized in that said first clad layer has a depth-wise carrier concentration profile in which the carrier concentration near the opposite end towards said active layer is more than 1 x 10¹⁸cm⁻³ and up to 3 x 10¹⁹cm⁻³, the carrier concentration decreases when approaching said active layer; and the carrier concentration in said first clad layer near the junction with said active layer is 5 x 10¹⁶ cm⁻³ or less. Preferred embodiments of the invention are recited in the dependent claims. The carrier concentration of the active layer is preferably 1 x 10¹⁷ cm⁻³ or less, and the carrier concentration in the second clad layer near the junction with the active layer is preferably 5 x 10¹⁶ cm⁻³ or more. Also, the lower limits of the carrier concentration in the first clad layer near the junction with the active layer and the concentration of the active layer are set such that the first type conductivity is maintained. The upper limit of the carrier concentration of the second clad layer is set such that the added impurity does not deteriorate the crystallinity. As the impurity for the active layer and the first clad layer, Zn or C, for example, is chosen. There may be carried out a plural doping in which more than two impurities are doped thereto. As the impurity for the second clad layer, Te or Sn, for example, is chosen. There may be also carried out the plural doping. The first and second clad layers may be composed of a mixed crystal type compound semiconductor represented as Ga 1-x Al x As (where 0 < x < 1). The active layer may be composed of a mixed crystal type compound semiconductor represented as Ga 1-y Al y As (where 0 < y < 1). Here, x is preferably between 0.6 and 0.85 and y is preferably between 0.3 and 0.45. In this invention, the luminance of the light emitting device is improved by making the carrier concentration in the first clad layer near the active layer 5 x 10¹⁶ cm⁻³ or less. In the past, the carrier concentration in this clad layer was usually 5 x 10¹⁶ cm⁻³ or more. However, as described above, the inventors of the invention have confirmed that the the luminance of a light emitting device increases under conditions in which the carrier concentration near the active layer is 5 x 10¹⁶ cm⁻³ or less. It is thought that the reason for this is that lowering the added impurity concentration near the active layer has greater positive effects from reduced crystal defects than negative effects from a reduction of the carriers and such, resulting in the increase in luminance. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be explained in more detail in conjunction with appended drawings, wherein: Fig. 1 is an explanatory view showing the depth-wise structure of the light emitting device in a preferred embodiment according to the invention; Fig. 2 is an explanatory view showing an example of methods of fabricating the light emitting device in the preferred embodiment according to the invention; Fig. 3 is a graph showing the depth-wise carrier concentration profile of the light emitting device in the preferred embodiment according to the invention; Fig. 4 is a graph showing the relationship between the carrier concentration in the p-type clad layer near the active layer and the relative luminance of the light emitting device; and Fig. 5 is a graph showing the frequency distribution by relative luminance for the concentration range in the preferred embodiment according to the invention (1 to 2 x 10¹⁶ cm⁻³) and the distribution for the conventional concentration range (1 to 2 x 10¹⁷ cm⁻³). DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of this invention is described below by referring to the attached drawings. Figure 1 is a cross-sectional view showing the depth-wise structure of an embodiment of the light emitting device of this invention. In this figure, the three layers of the epitaxial growth layers composing the light emitting device of this invention are formed on the p-type GaAs <100> substrate 10 with the added Zn concentration of approximately 1 to 3 x 10¹⁹ cm⁻³ as the impurity. The p-type clad layer 11 of 150 µ m thickness composed of Ga 0.25 Al 0.75 As mixed crystal compound semiconductor doped with Zn as the impurity to have the prescribed depth-wise profile, the p-type active layer 12 of 1 µ m thickness composed of Ga 0.62 Al 0.38 As mixed crystal compound semiconductor doped with Zn as the impurity to have the prescribed concentration, and the p-type clad layer 13 of 50 µ m thickness composed of Ga 0.25 Al 0.75 As mixed crystal compound semiconductor doped with Te as the impurity are consecutively formed on the p-type GaAs substrate 10 by the epitaxial growth method. The p-type GaAs substrate 10 will be removed by selective etching after the formation of the epitaxial growth layer described above. Now we will describe an example of the manufacturing methods of the light emitting device of this embodiment by referring to Figure 2. The light emitting device of this embodiment may be manufactured by growing each layer using the liquid phase growth method. Here, an example of the slide-boat method using a slide-boat is shown. Also shown is the gradual cooling method in which the growing is conducted by lowering the temperature gradually. In Figure 2, the p-type GaAs substrate 10 is secured on a boat body 20 in such way that its top surface is flush with the top surface of the boat body 20. The first solution reservoir 22 which contains the p-type clad layer growth solution 22a, the second solution reservoir 23 which contains the p-type active layer growth solution 23a, and the third solution reservoir 24 which contains the n-type clad layer growth solution 24a are set up on the sliding solution reservoir 21. Each solution reservoir does not have a bottom, so that each solution directly soaks the top surface of the boat body 20. The p-type and n-type clad layer growth solutions 22a and 24a are melted Ga 0.25 Al 0.75 As liquid with Zn added as the p-type impurity and Te added as the n-type impurity, respectively. The p-type active layer growth solution 23a is melted Ga 0.62 Al 0.38 As liquid with Zn added as the p-type impurity. The sliding solution reservoir 21 is moved by a handling rod 25. Now, we describe the process using the equipment described above to form the three epitaxial growth layers on the p-type GaAs substrate 10. First, using the handling rod 25, the sliding solution reservoir 21 is slidden, in the direction of the arrow, from the position indicated in Figure 2(a) to set the p-type clad layer growth solution 22a in the first reservoir 22 on the p-type GaAs substrate 10, and the p-type clad layer 11 is grown under the conditions where, for example, the temperature is lowered from 900°C to 800°C (Figure 2(b)). Then, the sliding solution reservoir 21 is slidden further in the direction of the arrow to set the p-type active layer growth solution 23a in the second reservoir 23 on the p-type GaAs substrate 10, and the p-type active layer is grown under the conditions where, for example, the temperature is lowered from 800°C to 795°C (Figure 2(c)). The sliding solution reservoir 21 is then slidden further in the direction of the arrow to grow the n-type clad layer 13 under the conditions where, for example, the temperature is lowered from 795°C to 650°C (Figure 2(d)). After this, the sliding solution reservoir 21 is slidden further in the direction of the arrow to complete the growth process (Figure 2(e)). In order to keep the carrier concentration in the p-type clad layer 11 near the active layer at 5 x 10¹⁶ cm⁻³ or less, the amount of the p-type impurity Zn added to the p-type clad layer growth solution 22a is very small, or Zn is not added at all. However, in the subsequent thermal process, the highly concentrated Zn added to the p-type GaAs substrate 10 diffuses into the clad layer, causing so-called "auto doping". As a result, the clad layer becomes the p-type clad layer 11 with the prescribed concentration profile, and an area with low carrier concentration (1 x 10¹⁶ cm⁻³) is formed near the p-type active layer 12. After forming the three layer double hetero structure on the p-type GaAs substrate 10, as described above, the substrate is removed by etching and electrodes are formed on both sides of the three layer structure to obtain the light emitting device. Figure 3 shows a depth-wise carrier concentration profile of the light emitting device of this embodiment which was made as described above. The carrier concentration in the p-type clad layer 11 near the p-type GaAs substrate 10 is approximately equivalent to the carrier concentration in the substrate (1 to 3 x 10¹⁹ cm⁻³). However, the closer to the p-type active layer 12, the lower the carrier concentration, and it is as low as 1 x 10¹⁶ cm⁻³ near the junction with the p-type active layer. On the other hand, the carrier concentration in the p-type active layer is 1 x 10¹⁷ cm⁻³. The carrier concentration in the n-clad layer 13 increases from 1 x 10¹⁷ cm⁻³, which is equivalent to the carrier concentration in the p-type active layer, to approximately 6 x 10¹⁷ cm⁻³ in the top region. Figure 4 shows the relationship between the carrier concentration in the p-type clad layer 11 near the p-type active layer 12 and the relative luminance of the light emitting device. It is shown that as the carrier concentration in the p-type clad layer 11 near the p-type active layer 12 decreases, the relative luminance of the light emitting device increases. Figure 5 shows the frequency distribution by relative luminance for the concentration range in the embodiment of this invention (1 to 2 x 10¹⁶ cm⁻³) and the distribution for the conventional concentration range (1 to 2 x 10¹⁷ cm⁻³). It is shown that the relative luminance for the concentration range in this embodiment (1 to 2 x 10¹⁶ cm⁻³) is very high, approximately 400. As described above, in a light emitting device comprising three layers, i.e., (a) a first clad layer composed of a mixed crystal compound semiconductor of first type conductivity, (b) an active layer composed of a mixed crystal compound semiconductor of second type conductivity which has the mixed crystal ratio required to emit the prescribed wavelength, and (c) a second clad layer composed of a mixed crystal compound semiconductor of second type conductivity which has a mixed crystal ratio equivalent to that of the first clad layer, wherein the active layer is sandwiched by the first and second clad layers and forms the double hetero structure with the first and second clad layers, it is possible to obtain higher luminance than that of conventional light emitting devices by keeping the carrier concentration in the first clad layer near the junction with the active layer at 5 x 10¹⁶ cm⁻³ or less.

Claims: A light emitting device, comprising: a first clad layer (11) composed of a mixed crystal compound semiconductor of first type conductivity: an active layer (12) composed of a mixed crystal compound semiconductor of first type conductivity which has the mixed crystal ratio required to emit the prescribed wavelength; and a second clad layer (13) composed of a mixed crystal compound semiconductor of second type conductivity which has a mixed crystal ratio substantially equal to that of said first clad layer (11); wherein said active layer (12) is sandwiched between said first (11) and second (13) clad layers and forms a double hetero structure with said first (11) and second (13) clad layers; characterized in that said first clad layer (11) has a depth-wise carrier concentration profile in which the carrier concentration near the opposite end towards said active layer (12) is more than 1 x 10¹⁸cm⁻³ and up to 3 x 10¹⁹cm⁻³, the carrier concentration decreases when approaching said active layer (12); and the carrier concentration in said first clad layer (11) near the junction with said active layer (12) is 5 x 10¹⁶ cm⁻³ or less. A light emitting device according to claim 1, wherein the carrier concentration of said active layer (12) is 1 x 10¹⁷ cm⁻³ or less. A light emitting device according to claims 1 or 2, wherein the carrier concentration in said second clad layer (13) near the junction with said active layer (12) is 5 x 10¹⁶ cm⁻³ or more. A light emitting device according to any one of claims 1 to 3 wherein the carrier concentration in said second clad layer (13) is 1 x 10¹⁷ cm⁻³ or less; and the carrier concentration in said second clad layer (13) near the junction with said active layer is 5 x 10¹⁶ cm⁻³ or more. A light emitting device according to any one of claims 1 to 4, wherein an impurity selected from zinc (Zn) and carbon (C) is added to said active layer (12) and said first clad layer (11). A light emitting device according to any one of claims 1 to 4, wherein an impurity selected from tellurium (Te) and tin (Sn) is added to said second clad layer (13). A light emitting device according to any one of claims 1 to 6, wherein said first clad layer (11) is composed of a mixed crystal compound semiconductor represented as Ga 1-x A1 x As (where 0 < x < 1); said active layer (12) is composed of a mixed crystal compound semiconductor represented as Ga 1-y A1 y As (where 0 < y < 1); and said second clad layer (13) is composed of a mixed crystal compound semiconductor which has a mixed crystal ratio substantially equal to that of said first clad layer (11). A light emitting device according to claim 7, wherein said x is between 0.6 and 0.85, and said y is between 0.3 and 0.45.

Inventors: TAKENAKA TAKUO C O HANDOTAI IS, YAMADA MASATO C O HANDOTAI ISO, TAKENAKA, TAKUO, C/O HANDOTAI ISOBE LAB, SHIN-ETSU, YAMADA, MASATO, C/O HANDOTAI ISOBE LAB., SHIN-ETSU
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1992
Main classification: H01L33
IPC codes: H01L33