Semiconductor light-emitting devices

Abstract: The present invention provides a semiconductor light-emitting device including a first clad layer (103) comprising a first conductive type of AlGaAsP compound, a second clad layer (104) that is located next to the first clad layer, comprises a first conductive type of AlGaInP compound and has a thickness of up to 0.5 µm, an active layer (105) that is located next to the second clad layer and comprises a first or second conductive type AlGaInP or GaInP, a third clad layer (106) that is located next to the active layer, comprises a second conductive type of AlGaInP compound has a thickness of up to 0.5 µm and a fourth clad layer (107) that is located next to the third clad layer and comprises a second conductive type of AlGaAsP compound, and/or a light-extracting layer that comprises a second conductive type AlGaP or GaP and has a thickness of 1 µm to 100 µm.

Description: BACKGROUND OF THE INVENTION The present invention relates to semiconductor light-emitting diodes which are obtained using AlGaInP semiconductor materials. One exemplary prior art semiconductor light-emitting diode (LED for short) using an AlGaInP semiconductor material is schematically illustrated in FIG. 1. Referring now to FIG. 1, reference numeral 201 represents an n-type GaAs substrate, and 202 a clad layer comprising an n-type AlGaInP, which is formed on the substrate 201. Reference numeral 203 represents an active layer comprising AlGaInP, and 204 a clad layer comprising a p-type AlGaInP. That is, the Al concentration is preset so that the energy gap of the AlGaInP active layer 203 is narrower than those of the AlGaInP clad layers 202 and 204. A p-type AlGaAs current-spreading layer 205 is provided to expand the electric current injected from electrodes, thereby expanding the light-emitting region and so increasing the efficiency of light extraction. Reference numeral 206 stands for a contact layer, and 207 and 208 indicate electrodes. AlGaInP or AlInP (which may hereinafter be collectively called an AlGaInP compound) have some defects; they are higher in resistivity and thermal resistance than AlGaAsP or AlGaAs (which may hereinafter be collectively called an AlGaAsP compound). Such defects lead to a problem that the operating voltage of the device becomes too high, resulting in an increase in the amount of the heat generated. This problem must be solved so as to improve the characteristics and reliability of the device, and becomes serious especially when the light-emitting density becomes high. Zinc (Zn) is generally used as a dopant for a clad layer comprising a p-type AlGaInP compound, but doping should be done at high concentration so as to lower resistivity, because Zn is low in the rate of activation. In this case, however, unactivated Zn diffuses so rapidly through the AlGaInP compound crystal body that the pn junction position is often largely shifted from the light-emitting layer toward the n-type layer. This, in turn, gives rise to abnormality in the current-voltage characteristics, a drop of light output power, and an increase in the threshold current of laser. Such diffusion of Zn from the p-type AlGaInP compound layer becomes noticeable as its thickness increases. When the light-emitting layer is relatively thin as in the case of laser, the current-voltage characteristics are liable to disorder due to the diffusion of Zn. Silicon (Si) having a low diffusion coefficient is effective for an n-type AlGaInP compound. In some cases, a light-extracting layer is located next to the clad layer. The transparency of the light-extracting layer to the light-emitting wavelength is important for improving the light-extraction efficiency of LED. However, it is probable that some light-extracting layer with high transparency has high resistivity depending on composition and that the surface current spreading area is small, as a result. US-A-5 008 718 describes semiconductor light-emitting diodes having active layers directly on a substrate. The active layer of n-type AlGaInP is sandwiched between a lower n-type confining layer of AlGaInP and an upper p-type confining layer of AlGaInP. On the upper confining layer of AlGaInP there is directly a p-type window layer transparent to the emitted light, which may be of AlGaAs, GaAsP or GaP. SUMMARY OF THE INVENTION An object of the present invention is to optimize the structure of clad layers of a semiconductor light-emitting diode including an active layer comprising AlGaInP or GaInP. Another object of the present invention is to reduce the thickness of a Zn-doped clad layer comprising a p-type AlGaInP compound. A further object of the present invention is to substantially eliminate or reduce the diffusion of Zn into an active layer and an n-type clad layer. A still further object of the present invention is to prevent a shift of the pn junction position due to the diffusion of Zn. A still further object of the present invention is to improve the characteristics and reliability of a light-emitting diode. A still further object of the present invention is to expedite mass production, reduce growth time, cut down material cost, and remove defects. According to the present invention, there is provided a semiconductor light-emitting diode as set out in claim 1. In this semiconductor light-emitting diode including an active layer comprising AlGaInP or GaInP, the diffusion of Zn into the active layer or an n-type clad layer can be reduced, if the thickness of a Zn-doped clad layer comprising a p-type AlGaInP compound is reduced to such an extent that the characteristics of the light-emitting device does not degrade. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of one exemplary prior art light-emitting diode. FIG. 2 is a schematic view of an embodiment of the semiconductor light-emitting diode including a light-extracting layer that is located next to the third clad layer and comprises a second conductive type AlGaP or GaP. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will now be explained at greater length. The present invention provides a semiconductor light-emitting diode characterized by including a first clad layer comprising a first conductive type of AlGaAsP or AlGaAs compound, a second clad layer that is located next to the first clad layer, comprises a first conductive type of AlGaInP or AlInP compound and has a thickness of up to 0.5 µm, an active layer that is located next to the second clad layer and comprises a first or second conductive type AlGaInP or GaInP, a third clad layer that is located next to the active layer, comprises a second conductive type of AlGaInP or AlInP compound and has a thickness of up to 0.5 µm, and a light-extracting layer that comprises a second conductive type AlGaP or GaP. It is here to be noted that the contents of the constitutional elements of the layers thereof may be determined while lattice matching between the substrate and each layer is taken into consideration. Shown in FIG. 2 is one embodiment of the present invention. This device is produced according to Example 1, given later. For production, the device of the present invention is usually formed on a single crystal substrate. For this substrate 101, a GaAs substrate is usually but not exclusively used. To decrease the aluminum composition of the active layer depending on light-emitting wavelength, etc., it is preferable to use a GaAsP substrate. It is preferable that a buffer layer 102 of up to about 2 µm is formed on the substrate, thereby preventing transfer of substrate defects into epitaxially grown layers. Then, a first clad layer 103, a second clad layer 104, the active layer 105, the third clad layer 106 and the light-extracting layer 107' are stacked on the buffer layer in the described order according to the present invention. The first clad layer 103 is formed of a first conductive type of AlGaAs compound, and it is preferable that its thickness is usually up to 15 µm. More preferably, the lower and upper limits of the thickness of the first clad layer are 0.1 µm and 2 µm, respectively. The first clad layer should preferably have a carrier concentration lying in the range of 5x10 16 cm -3 to 3x10 18 cm -3 , particularly with the lower and upper limits being 1x10 17 cm -3 and 1x10 18 cm -3 , respectively Stacked on the first clad layer is the second clad alyer 104 comprising a first conductive type of AlGaInP compound and having a thickness of up to 0.5 µm. More preferably, the lower and upper limits of the thickness of this layer are 0.01 µm and 0.3 µm, respectively. The second clad layer should preferably have a carrier concentration lying in the range of 5x10 16 cm -3 to 3x10 18 cm -3 , particularly with the lower and upper limits being 1x10 17 cm-3 and 1x10 18 cm -3 , respectively. Stacked on the second clad layer is the active layer 105 comprising a first or second conductive type AlGaInP or GaInP. The active layer should preferably have a thickness of 0.1 µm to 1 µm, particularly with the lower and upper limits being 0.2 µm and 0.5 µm, respectively. The active layer should preferably have a carrier concentration of up to 1x10 18 cm -3 , particularly with the lower and upper limits being 1x10 17 cm -3 and 5x10 17 cm -3 , respectively. Stacked on the active layer is the third clad layer 106 comprising a second conductive type of AlGaInP compound and having a thickness of up to 0.5 µm. The third clad layer should preferably have a carrier concentration lying in the range of 5x10 16 cm -3 to 3x10 18 cm -3 , particularly with the lower and upper limits of 1x10 17 cm -3 and 1x10 18 cm-3, respectively. Stacked on the third clad layer is the light-extracting layer 107' next to the third clad layer, said layer 107' comprising a second conductive type AlGaP or GaP. This light-extracting layer 107' should preferably have a thickness of at least 1 µm. The thicker, the more preferable, because light absorption on the electrodes is reduced. However, too thick a layer is of no substantial use if an efficiency drop ascribable to the absorption of light by the light-extracting layer perse and electrical resistance is taken into account. Thus, the light-extracting layer should have a thickness of up to 300 µm, more particularly up to 100 µm. It is here to be noted that a light-extracting layer of about 10 µm to about 100 µm in thickness should preferably be formed by hydride vapor phase growth techniques. In this case, the carrier concentration should preferably lie in the range of 1x10 17 cm -3 to 1x10 20 cm -3 , particularly with the lower and upper limits of 5x10 17 cm -3 and 5x10 18 cm -3 , respectively. GaP is an indirect gap material having a wide band gap; so it provides a light-extracting layer very effective in a visible range of wavelengths (550 nm to 690 nm) achievable by AlGaInP compounds. Even with Al x Ga 1-x As (x = 0.6 to 0.8) having a high Al concentration, some transparency is achievable. However, a GaP layer is by far lower in resistivity and wider in surface current spreading area than such an AlGaAs layer. For the light-extracting layer it is also preferable to use an n-type Gap because it is better in transparency and lower in resistivity than a p-type GaP. In the embodiment shown in Fig. 2, when the p-type layer is an AlGaAs layer, carbon is preferably used as the dopant or doping impurity for the above clad layers, etc., and when the p-type layer is an AlGaInP compound, beryllium and/or magnesium are preferably used as the doping impurities. Furthermore in this case, silicon is preferably used as an n-type doping impurity. In a more preferable embodiment of the present invention, a light-reflecting layer 108 is interleaved between the substrate and the first clad layer. For the light-reflecting layer various known substances capable of reflecting light may be used. Among these substances, the AlGaAsP compound is preferable in view of composition while a Bragg reflection film is preferable in view of structure. Other layers, e.g., a cap layar and a contact layer 109 may be prepared in conventional manners. For LED, it is usually required to grow the light-extracting layer having a relatively large thickness so as to increase external quantum efficiency of light output power. However, growth of this light-extracting layer takes a long time to complete. Accordingly, when the impurity having a high diffusion coefficient, like zinc or selenium, is used as the dopant, the dopant in the clad layers diffuses into the active layer or through the active layer to the opposite layer, so offering some problems such as considerable deterioration of LED characteristics or of reproducibility. If the thickness of the AlGaInP clad layer is reduced as much as possible as practiced in the present invention, any excessive doping can then be dispensed with for the purpose of prevention of diffusion, because the total amount of the impurities to be diffused can be reduced. This in turn enables the carrier concentration of the n-type clad layer to be reduced to 1x10 18 cm -3 or lower, resulting in improvements in crystal quality and device characteristics. In particular, the present invention is very effective for regrowth of the GaP light-extracting layer having a large thickness by hydride VPE. When the active layer is provided in the form of a very thin film such as a quantum well structure, it is increasingly required to prevent the above-mentioned diffusion of the impurities. In the present invention, carbon is used as the doping impurity for the p-type AlGaAsP compound clad layer, beryllium or magnesium as the doping impurities for the p-type AlGaInP compound clad layer, and silicon as the doping impurity for the clad layers comprising an n-type AlGaInP compound, so that the diffusion of the impurities can be further reduced, so making a great contribution to improvements in device yield and reproducibility. Moreover, it is effective to use a first conductive type of GaAs substrate which is inclined at an angle of 5 to 25 degrees from the plane (100) in the [011] direction, so that a reduction in the band gap due to the ordering of the atomic alignment inherent in the AlGaInP compound or, in another parlance, a reduction in the light-emitting wavelength can be inhibited, or the surface morphology can be improved or stabilized. The reduction in the Al composition of the light-emitting layer is important for improving the reliability and lifetime of the device, and this is easily achievable by use of a GaAsP substrate. For lattice matching, it is here advantageous to use an AlGaAsP clad layer or an AlGaAsP light-reflecting layer. According to the present invention, it is thus possible to achieve reductions in growth time and material cost, and considerable reductions in loads on production equipment by removal of defects, etc., and hence to mass produce a multiplicity of devices at the same time and in a stable manner by using production equipment of large size. The present invention will now be explained more specifically with reference to an example. However, it is to be noted that the present invention is in no sense limited to the following examples. Set out below is an example of the present invention wherein an MOVPE technique known to be excellent in terms of thickness/composition controllability and mass productivity was used for crystal growth. The feed gases used were trimethylaluminum (TMA), trimethylgallium (TMG), trimethylindium (TMI), phosphine (PH 3 ), and arsine (AsH 3 ), while the carrier gas used was highly purified hydrogen (H 2 ) gas. Example 1 The LED of the present invention, as shown in FIG. 2, is produced in the form of an epitaxial wafer according to the following procedures. Successively grown on a p-type GaAs (100) substrate 101 are a p-type GaAs buffer layer 102 (of 0.5 µm in thickness), a Bragg reflection film 108 made up of alternately laminated 15 p-type AlGaAs/AlAs laminae, a first clad layer 103 of a p-type Al 0.7 Ga 0.3 As(of 1.0 µm in thickness), a second clad layer 104 of a p-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P (of 0.2 µm in thickness), a (Al 0.4 Ga 0.6 ) 0.5 In 0.5 P active layer 105 (of 0.3 µm in thickness), a third clad layer 106 of an n-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P (of 0.2 µm in thickness), and an n-type Gap light-extracting layer 107' (of 4 µm in thickness). Referring here to the conditions for crystal growth in this example, the growth temperature is 650°C to 800°C, the pressure is 10 2 hPa, the V/III ratio is 25 to 50 for GaAs and AlGaAs, 500 to 750 for AlGaInP abd GaInP and 50 to 200 for GaP, and the growth rate is 1 to 5 µm/hr for GaAs and AlGaAs, 0.5 to 2 µm/hr for AlGaInP and GaInP and 0.5 to 5 for GaP. An LED lamp is. prepared from the LED wafer grown by the above-mentioned procedures. It is found that this lamp emits light at 580 nm to 590 nm with a luminance of about 1.5 cd observed at a driving current of 20 mA. This luminance is nearly twice as high as that obtained with a prior art structure using a p-type AlGaInP clad layer of about 1 µm thick. According to the present invention, the characteristics and reliability of the light-emitting device can be improved with a reduction in the material cost by the structural optimization of the clad layers between which the active layer comprising AlGaInP or GaInP is interposed.

Claims: A semiconductor light-emitting diode including the following layers arranged in the following order without any intervening layers between them: a first clad layer (103) comprising a first conductivity type AlGaAsP or AlGaAs, a second clad layer (104) comprising a first conductivity type AlGaInP or AlInP and having a thickness of up to 0.5 µm, an active layer (105) comprising a first or second conductivity type AlGaInP or GaInP, a third clad layer (106) comprising a second conductivity type AlGaInP or AlInP and having a thickness of up to 0.5 µm, and a light-extracting layer (107') that comprises a second conductivity type AlGaP or GaP. The semiconductor light-emitting diode as recited in claim 1, wherein said active layer has a thickness of 0.1 µm to 1 µm. The semiconductor light-emitting diode as recited in claim 1 or 2, wherein the first clad layer is a p-type layer comprising AlGaAsP or AlGaAs and containing carbon as a doping impurity. The semiconductor light-emitting diode as recited in claim 1 or 2, wherein one of the second and third clad layers (104, 106) is a p-type layer comprising AlGaInP or AlInP and containing beryllium and/or magnesium as a doping impurity. The semiconductor light-emitting diode as recited in any one of claims 1 to 4, which includes a light-reflecting layer (108) between a substrate (101) and the first clad layer (103). The semiconductor light-emitting diode as recited in claim 5, wherein said light-reflecting layer (108) comprises AlGaAsP or AlGaAs. The semiconductor light-emitting diode as recited in any one of claims 1 to 6, wherein said active layer has a thickness of 0.1 µm to 1 µm and wherein said light-extracting layer (107') has a thickness of 1 µm to 100 µm. The semiconductor light-emitting diode as recited in claim 7, wherein the first and second clad layers (103, 104) are of p-type, and the third and light-extracting layers (105,107') are of n-type. The semiconductor light-emitting diode as recited in claim 7 or 8, wherein the light-emitting wavelength is 550 nm to 690 nm. The semiconductor light-emitting diode as recited in any one of claims 7 to 9, wherein silicon is used as an n-type doping impurity. A process of making the semiconductor light-emitting diode as recited in any one of claims 1 to 10, wherein said light-extracting layer (107') has a thickness of 10 µm to 100 µm and is formed by a hydride vapor phase growth technique.

Inventors: FUJII KATSUSHI, GOTOH HIDEKI, HOSOI NOBUYUKI, SATO YOSHIHITO, SHIMOYAMA KENJI, YAMAUCHI ATSUNORI, FUJII, KATSUSHI, GOTOH, HIDEKI, HOSOI, NOBUYUKI, SATO, YOSHIHITO, SHIMOYAMA, KENJI, YAMAUCHI, ATSUNORI, Fujii, Katsushi, c/o Mitsubishi Chemical Corp., Gotoh, Hideki, c/o Mitsubishi Chemical Corp., HOSOI, NOBUYUKI, C/O MITSUBISHI CHEMICAL CORP., Sato, Yoshihito, c/o Mitsubishi Chemical Corp., SHIMOYAMA, KENJI, C/O MITSUBISHI CHEMICAL CORP., Yamauchi, Atsunori, c/o Mitsubishi Chemical Corp.
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1995
Main classification: H01L33
IPC codes: H01L33, H01S5