METHOD OF MANUFACTURING A COMPOUND SEMICONDUCTOR LIGHT-EMITTING DIODE

Abstract: A light emitting diode is disclosed which comprises an N-GaAs substrate (11), a double hetero-junction structure obtained by forming an N-GaAℓAs clad layer (13), a P-GaAs active layer (14) and a P-GaAℓAs clad layer (15) on the substrate, and a current narrowing structure obtained by selectively forming a contact metal (16) on the P-GaAℓAs clad layer (15) in the double hetero-junction structure with an insulating film formed around the contact metal. In the light emitting diode so constructed, the above-mentioned double hetero-junction structure is formed by a metal organic chemical vapor deposition method.

Description: This invention relates to a light emitting diode excellent in larger quantity production and, in particular, to a light emitting diode of a double hetero-junction structure using a compound semiconductor material. Recently, optical subscriber transmission systems are beginning to acquire a quick and wide acceptance in the art. Among them, a light emitting diode (LED) can be manufactured at low cost with high reliability as compared with a laser diode (LD) and does not encounter, for example, any modal noise resulting from a combination of the LD with multi-mode fiber. From this, it will be understood that the LED is a key device of the above-mentioned optical subscriber transmission system. Though inexpensive, the LED is very much higher in cost than general electric parts. Thus, in order for the optical subscriber transmission system to become generally accepted, it is essential that the LED's be inexpensive. A low yield in the manufacture of the LED's constitutes one of the major bars to the realization of the low-cost LED's. At present, during the manufacture of LED's, the respective layers are formed by a LPE (liquid phase epitaxy) method. However, the LPE method must be applied generally to small wafer areas, and a poor uniformity in the thickness of the respective films and in the composition is involved. Thus, it may be said that the LPE method is not suitable for a large quantity production of LED's. The use of the LPE method causes drooping at a layer-to-layer interface due to the lack of sharpness at the interface, resulting in the lowered light emission efficiency of the active layer. The use of the LPE method assures no excellent light emission characteristic. In order to increase input power to the optical fiber a lens is attached to the LED wafer. This step also presents a bar to the obtainment of low-cost LED's. In the LD, on the other hand, the use of a MOCVD (Metal Organic Chemical Vapor Deposition) method in the crystal growth step is now under study. When the MOCVD method is used for the manufacture of LED's, the light emission efficiency is markedly lowered. In the case of the LD, since light is emitted due to the induced emission, the lowered light emission efficiency at the active layer offers no great problem. For the LED, on the other hand, the lowered light emission efficiency produces a fatal defect. As appreciated from the above, the use of the MOCVD method has been believed unsuitable for the manufacture of LED's. From IEEE Journal of Quantum Electronics, volume QE 17 number 3, March 1981, pages 387-391, New York (US), T.P. Lee "Improved molecular beam epitaxial growth of AlGaAs/GaAs high radiance LEDs for optical communications" there is known a light emitting diode manufactured by molecular beam epitaxy having a substrate and a double heterostructure comprising a first clad layer of a first conductivity type, an active layer of the first conductivity type and a clad layer of the second conductivity type, on the latter there being a current narrowing structure comprising a metal contact around which an insulating film is formed (see Section II and Figure 1). This known light emitting diode has an etched window in the substrate which is formed to the depth of the first cladding layer in a position corresponding to the contact metal. The substrate is GaAs and the double heterostructure is formed from GaAlAs layers (see Figure 1). From Optical Communication, 9th European Conference on optical communication ECOC 83, Genev. 23-26 October 1983, (Switzerland); Edited by H. Melchior & A. Sollberger, Institute for applied physics, Zürich (CH), pages 471-474. M. Krakowski et al.: "AlGaAs/GaAs double heterostructure super luminescent diodes for fibre gyro" there is known a superluminescent diode having a semiconductor substrate of a first conductivity type, with, in sequence, a n-type getter-buffer layer, a n-type confinement layer and a contact layer having a current narrowing structure (see Introduction and Figures 1 and 2). The getter-buffer and n-type confinement layers both have a greater band width than that of the active layer and have different composition ratios, with the confinement layer having a wider band width than the getter-buffer layer, therefore the two layers may be regarded as providing a multilayer structure. It is the object of this invention to provide a light emitting diode which is higher in light emission efficiency than obtainable so far and excellent in quantity production. According to this invention the above-mentioned object can be attained by the specific arrangement as disclosed in the claims. According to this invention it is possible to obtain the following advantages: (1) It is possible to obtain a very uniform wafer of a larger diameter by the MOCVD method. This invention is, therefore, effective for the quantity production of LED's, assuring a low manufacturing cost. (2) Since a sharp layer-to-layer interface is obtained by the MOCVD method, it is possible to form an ideal double hetero-junction structure. For this reason, the complete containment of carriers is achieved, effectively serving to enhance the light emission efficiency. (3) The positive containment of injection carriers can be achieved by adequately enlarging the forbidden band of that N-type clad layer of a multi-layer structure which is located at the side of the active layer. Furthermore, the forbidden band of the clad layer distant from the active layer is made smaller, adequately enhancing the carrier concentrations and thus adequately decreasing the spreading resistance at the clad layer. The LED's of this invention, even if manufactured by, for example, the MOCVD or MBE method, involve no saturated or lowered light emission power and thus assure a higher quantity production and higher power output. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross-sectional view showing an arrangement of a Burrus type LED according to this invention; Figs. 2A to 2C are cross-sectional views showing the steps of manufacturing LED according to the embodiment of Fig. 1; and Fig. 3 is a cross-sectional view showing an arrangement of a Burrus type LED according to a second embodiment of this invention. First, the fundamental principle of this invention will be explained below before an explanation of the embodiments of this invention. The summary of this invention resides in that a MOCVD method is used for the crystal growth of a double hetero-junction structure and that the lowering of the light emission efficiency of LED's can be suppressed even if the MOCVD method is used. The inventors have conducted various experiments using the MOCVD method whereby LED's are manufactured, and found that the resultant lowered light emission efficiency of the LED as prepared by the MOCVD method is due to a defective element, such as a contaminated row material and to improper parameters. That is, the MOCVD method is still novel and in the MOCVD method which has conventionally been practiced, its merits could not be obtained to full satisfaction. When LED's are prepared with various associated parameters optimally set and without using any such defective element, it has been found possible to obtain the light emission efficiency substantially as high as, or higher than, that of LED's with the use of the LPE method. Here, the use of the MOCVD method assures an excellent uniformity in the thickness of the layers and in the composition as well as permiting a wider wafer area to be taken. Under the above-mentioned requirements, LED's of high efficiency can be implemented at low cost with the use of the MOCVD method. The embodiments of this invention will be explained below by referring to the accompanying drawings. Fig. 1 is a cross-sectional view showing an arrangement of a Burrus type LED according to an embodiment of this invention, and Figs. 2A to 2C are cross-sectional views showing the steps of manufacturing the above-mentioned LED. In the embodiment of Fig. 1, an N-GaAs buffer layer 52, an optional first clad layer 53 on N-GaAℓAs which serves as an etch stopper layer, a second clad layer 54 of N-GaAℓAs which serves as a spreading resistance reduction layer, a third clad layer 55 of N-GaAℓAs, an active layer 56 of P-GaAs and a clad layer 57 of P-GaAℓAs are grown by the MOCVD method on an N-GaAs substrate 51 in that order as shown in Fig 2A. Here, the mole fraction, carrier concentration and depth of the respective layers are as shown in Table 1 below. In this connection it is to be noted that Se and Zn were used as an N-type and a P-type dopant, respectively. The temperature for crystal growth was set at 750 [°C] under the condition that a better surface state is obtained. The above-mentioned numerical values are the most preferable examples, and can be properly varied according to the specification. For example, the AℓAs mole fraction X, carrier concentration n and thickness of the first and second N-type clad layers 53 and 55 are 0.3 to 0.45, 0.8 to 4 X 10¹⁷ [cm⁻³] and 1 to 3 [µm], respectively, noting that these numerical values can properly be varied within these numeral ranges. Similarly, the AℓAs mixed crystal ratio X, carrier concentration and thickness of the second clad layer 54 are 0.1 to 0.25, 4 to 8 × 10¹⁷ [cm⁻³] and 5 to 20 [µm] , respectively, noting that these numeral values can properly be varied within these ranges. As shown in Fig. 2B, after a 300 to 400 nm (3,000 to 4,000 [Å])-thick ohmic electrode (contact metal) 58 made of AuZn (5 % Zn) was formed on the P-type clad layer 57, it was etched, with a I₂-KI-H₂O (weight ratio: 1 : 1 : 4)-based etching solution, with a photoresist as a mask to leave a circular electrode 58 of 30 [µm] in diameter. Then, a 300nm (3,000 [Å])-thick SiO₂ film 59 was formed by the CVD method on the surface of the P-side layer, followed by etching the SiO₂ film 59 on the ohmic electrode 58 with a photoresist as a mask by using an ammonium fluoride solution. Then, as shown in Fig. 2C, a 100 nm (1,000 [Å]) Cr layer and 500 nm (5,000 [Å]) Au layer were formed on the surface of the P-side layer to form a P-side electrode 60. Then, an about 20 [µm] heat sink 61 of Au was formed by an electroplating method on the surface of the P-side electrode 60. Then, a 500 nm (5,000 [Å]) layer of AuGe (0.5 % Ge) and 100 nm (1,000 [Å]) layer of Au were evaporated on the rear surface of the substrate 51 to form an N-side electrode 62. Thereafter, using a photoresist as a mask, that circular portion of an N-side electrode 62 which had a diameter of 150 [µm] and which was opposite to the ohmic electrode 58 was removed. With the N-side electrode 62 as a mask, the substrate 51 and buffer layer 52 were etched, with an NH₄OH-H₂O-based etching solution, to a depth leading to the first clad layer 53 of the N-type. A light pick-up window 63 was formed by this etching step and thus a Burrus type LED as shown in Fig. 1 was completed. When the NH₄OH-H₂O (weight ratio: 1 : 30) is used as an etching solution for window formation, the etching rate of GaAs in the substrate 51 and buffer layer 52 is about 10 times as fast as that of the first clad layer 53 (GaAℓAs) of the N-type and, therefore, the first clad layer 53 acts as an etching stopper layer when the light pick-up window 63 is formed. In the LED which is formed as shown in Fig. 1, the heat sink 61 is mounted on a stem, not shown. In the LED so formed, the forbidden band width of the N-type clad layers 53, 54 and 55 is greater than the forbidden band width of the active layer 56 and, therefore, light emitted from the active layer 56 is taken out without absorption at the interior of the LED structure. In the three-layer structure of the N-type clad layers, the confinement of carriers due to the presence of a double hetero-junction structure is carried out through the third clad layer 55, the reduction of the series resistance in the LED is carried out through the second clad layer 54 and the optional first clad layer 53 serves as an etching stopper layer when the light pick-up window is formed. According to the disclosed embodiment of this invention, therefore, injection carriers can be positively confined through the action of the third clad layer 55 and the spreading resistance can be decreased through the action of the second clad layer 54. Furthermore, it is possible to beforehand prevent the light emission power from being dropped due to the influence of heat resulting from the series resistance at the LED. Therefore, a still higher output power can be assured due to such light emission characteristic. Fig. 3 is a cross-sectional view showing an arrangement of a Burrus type LED according to the second embodiment of this invention. In Fig. 3, the same reference numerals are employed to designate portions corresponding to those shown in Fig. 1, and, therefore, no further explanation is included for brevity. This embodiment is different from the embodiment of Fig. 1 in that the N-type clad layer is formed as three or more layers. That is, a number of N-GaAℓAs layers 73₁, ..., 73 N are formed, as the N-type clad layers, between the N-type buffer layer 52 and the P-type active layer. In this case, those layers near the active layer 56 should be formed so that, in order to assure a positive confinement of injection carriers, the AℓAs mole traction X is set to be higher than 0.3; the intermediate layer should be so formed that, in order to adequately raise the carrier concentration, the AℓAs mixed crystal ratio X is set to be adequately small; and the layer in contact with the buffer layer 52 should be so formed that, in order to act as a stopper layer when the etching step is carried out, the AℓAs mole traction X is set to be somewhat greater. In the embodiment of Fig. 3, it is also possible to produce the same effect as in the embodiment of Fig. 1 Furthermore, the AℓAs, mole traction X in the N-type clad layer can be varied in a step-like fashion to permit the mole fraction X to vary in a near-continuous state. For preparing the disclosed embodiments of light emitting diodes an MBE (Molecular Beam Epitaxy) method can be used in place of the MOCVD method. In place of the SiO₂ film (the insulating film) formed on the above-mentioned clad layer to permit current to be narrowed, use may be made of a semiconductor layer having a conductivity opposite to that of the above-mentioned clad layer. The semiconductor material which is used for the substrate and respective layers is not restricted to the GaAs/GaAℓAs-based material only. It is also possible to use an InP/InGaAsP-based material or the other semiconductor material belonging to Group III-V compounds.

Claims: A light emitting diode comprised of a compound semiconductor material, comprising: a semiconductor substrate (51) of a first conductivity type; a double hetero-junction structure obtained by sequentially forming a clad layer (55) of the first conductivity type, an active layer (56) of the first conductivity type or the second conductivity type and a clad layer (57) of the second conductivity type on the substrate (51) in that order; and a current narrowing structure obtained by sequentially forming a contact metal (58) on the clad layer (57) of the second conductivity type in the double hetero-junction structure with an insulating film (59) or a reverse junction formed around the contact metal, characterized in that a spreading resistance reduction layer (54) of the first conductivity type is provided between the substrate (51) and said clad layer (55) of the first conductivity type, said spreading resistance reduction layer (54) having a bandwidth greater than that of the active layer (56) but smaller than that of the clad layer (55) of the first conductivity type and having a higher doping concentration than that of the said clad layer (55) of the first conductivity type. A light emitting diode according to Claim 1, characterized in that an etch-stop layer (53) of the first conductivity type is provided between the substrate (51) and said resistance reduction layer (54), said etch-stop layer (53) having the same bandwidth as that of the said clad layer (55) of the first conductivity type.

Inventors: KOMATSUBARA, TADASHI PATENT DIVISION, NAKAMURA, MASARU PATENT DIVISION, NITTA, KOICHI PATENT DIVISION, OKAJIMA, MASAKI PATENT DIVISION, SADAMASA, TETSUO PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: H01L33
IPC codes: H01L33