Method of manufacturing green light emitting diode

Abstract: In a method of manufacturing a green light emitting diode, an included angle (ϑ] defined by a line extending along the longitudinal direction of light and dark stripes of an oxide region (7) formed on a cleavage surface (6) of a GaP single crystal wafer (1) and a (111) surface of the wafer (1) is set to be a predetermined angle, and a liquid phase epitaxial layer having a p-n junction is formed on the (111) surface (8) of the GaP single crystal wafer (1).

Description: The present invention relates to a method of manufacturing a green light emitting diode (to be referred to as an LED hereinafter). A green light emitting LED is manufactured by epitaxially forming a p-n junction layer on a GaP crystal substrate by LPE (Liquid Phase Epitaxy). Fig. 3 is a sectional view of a main part of a green light emitting LED. Referring to Fig. 3, reference numeral 1 denotes an n-type semiconductor substrate doped with sulfur (S), tellurium (Te), or the like. N- and p-type layers 2 and 3 respectively containing, e.g., Si and Zn as a donor and an acceptor are sequentially formed on an N-type region of a (111) major surface of the substrate 1. Subsequently, in order to obtain high luminous efficacy, nitrogen is doped in the N-type layer 2 to form a nitrogen (N) isoelectronic trap necessary for recombination light emission of excitons. In addition, an anode 4 is formed on the p-type layer 3, and a cathode 5 is formed on the lower surface of the semiconductor substrate 1. When a current flows between the anode 4 and the cathode 5 of the green light emitting LED having this arrangement, light emission occurs upon recombination of excitons trapped in the N isoelectronic trap as a luminescent center. In this case, the emission wavelength is about 565 µm, and the thicknesses of the n- and p-type layers 2 and 3 are set to be 25 µm to 50 µm. The luminous efficacy is influenced by a concentration of nitrogen to be doped, a carrier concentration distribution near a p-n junction, and the lifetime of minority carriers injected in a junction. It is known that there is a high correlation between the lifetime of minority carriers and the etch pit density (to be referred to as the EPD hereinafter) of an epitaxial layer. The lifetime is abruptly shortened at an EPD of 1 × 10⁵ /cm² or more, and the luminous efficacy is also decreased accordingly. Since a GaP crystal has a wide band gap, an impurity level serving as a non-emission recombination center is easily set. In addition, in a green light emitting LED, since the light emission level is shallow, excitons trapped in the N isoelectronic trap are thermally dissociated, and the possibility that non-emission recombination occurs after the dissociation of excitons is high. For this reason, it is important to suppress the EPD of an epitaxial layer as a non-emission recombination center to 1 × 10⁵ /cm². Since the EPD of an epitaxial layer tends to copy the EPD of a substrate, it is important to use a low-EPD substrate for a green light emitting LED. However, a GaP single crystal used as a substrate is manufactured by LEC (Liquid Encapsulated Czochralski) under a severe thermal environment of high temperature and pressure (1,467°C or more and 50 atm ; 1atm = 1,01325 bar). For this reason, it is difficult to manufacture a single crystal having an EPD of 1 × 10⁵ /cm² with a high yield, and demands for stable supply and a decrease in cost cannot be satisfactorily satisfied. As described above, in order to increase the luminous efficacy of a GaP green light emitting LED, it is very important to suppress the EPD of an epitaxial layer contained in the GaP substrate to a low value. In conventional techniques, however, since GaP crystals are grown under a severe environment of high temperature and pressure, it is difficult to stably manufacture an ingot having a sufficiently low EPD at a low cost. A strong demand has arisen on the market for a wider application range of green light emitting LEDs and higher external luminous efficacy. It is an object of the present invention to provide a method of manufacturing a green light emitting diode, by which green light emitting LEDs having high external luminous efficacies can be manufactured at a low cost with high reproducibility by using commercially available GaP substrates. According to the present invention, there is provided a method of manufacturing a green light emitting diode as specified in Claim 1. Further improvements thereof are specified in the Sub-claims. The present invention has been made on the basis of the following relationship between EPDs and included angles. Fig. 1 shows relationships between the EPDs and the included angles of n-type GaP single crystal wafers of different lots, which are manufactured by the LEC method, i.e., two types of wafers respectively having (111) major surfaces with EPDs of 2 × 10⁵ /cm² and of 1 × 10⁵ /cm². On the abscissa axis, an included angle defined by a line along the longitudinal direction of light and dark stripes obtained by anodization of a (110) cleavage surface of a wafer and the (111) major surface is plotted. On the ordinate axis, an EPD on a layer obtained by stacking n- and p-type layers by the LPE method is plotted. Curves a and b respectively represent the relationships between the included angles of the wafers respectively having EPDs of 2 × 10⁵ /cm² and of 1 × 10⁵ /cm² and the EPDs of the LPE layers. It is apparent from Fig. 1 that in the low-EPD substrates each having an EPD of 1 × 10⁵ /cm² and even in commercially available substrates each having an EPD of 2 × 10⁵ /cm², as the included angles are decreased, the EPDs of the LPE layers are decreased, and that desired low-EPD growth can be realized if the included angles are within 5°. 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 graph showing a relationship between included angles and the EPDs of LPE layers; Fig. 2 is a perspective view showing a GaP (110) cleavage surface, light and dark stripes, and included angles; and Fig. 3 is a sectional view showing a conventional green light emitting LED. An embodiment of the present invention will be described below with reference to the accompanying drawings. Fifty wafers are prepared from a sulfur-doped n-type GaP single crystal ingot having a diameter of 50 mm which is pulled/formed in a direction of the <111> axis by the LEC method. Each wafer has an EPD of 2 × 10⁵ /cm² and an electron concentration (substantially equivalent to an S concentration) of 2 × 10¹⁷ /cm³. Light and dark stripes of an oxide region are formed on a (110) cleavage surface of each wafer of this lot by performing anodization in, e.g., a 1N KOH aqueous solution. Anodization conditions are set such that a distinguishable oxide region can be formed by energization with, e.g., a current of several mA for several minutes. Fig. 2 shows light and dark stripes 7 obtained in this manner. A large number of stripes 7 are obliquely formed on a (110) cleavage surface 6 of an n-type GaP single crystal wafer 1. Upon examination, it was found that an included angle ϑ defined by a line B - B′ formed at a portion where the (110) cleavage surface 6 and a (111) p-type surface 8 cross each other and a line A - A′ extending along the longitudinal direction of the stripes 7 is uniformly set to be within 5°. After the oxide region is removed, n- and p-type layers in which Si and Zn are respectively doped at, e.g., average concentrations of 1 × 10¹⁶ /cm³ and of 1 × 10¹⁷ /cm³ by a slide board method are formed on the (111) n-type surface 8 of the major surface of the wafer by the LPE method. In addition, an anode 4 is formed on the p-type layer, and a cathode 5 is formed on the lower surface of the substrate, thereby obtaining a green light emitting LED having the same structure as that shown in Fig. 3. In this case, the EPD of the LPE layer was decreased to 1 × 10⁵ /cm² or less, i.e., 1/2 that of the conventional LED shown in Fig. 3, as shown by curve a in Fig. 1. In addition, the average external luminous efficacy of the obtained green light emitting LEDs was 0.5%, and a standard deviation δ was kept within 10%. In contrast to this, the average external luminous efficacy of the conventional green light emitting LEDs is 0.35%, and their luminous efficacies greatly vary from 0.20 to 0.50%. Wafers of the respective lots were extracted to check included angles defined by the (111) surfaces 8 and the light and dark stripes of the (110) surfaces 6 of anodized films, so that lots having included angles less than 5° were selected as materials for high-luminance LEDs, as in this embodiment. As a result, the yield of green light emitting LEDs was greatly increased. In the embodiment, an S-doped n-type GaP crystal is exemplified. However, the same effects can be obtained by using n-type crystals doped with other impurities such as Te and Si. As has been described above, according to the present invention, even by using commercially available GaP substrates each having an EPD of 2 × 10⁵ /cm², green light emitting LEDs having high external luminous efficacies for high luminance can be manufactured at a low cost with excellent reproducibility, and the yield can also be increased. If low-EPD substrates are used, green light emitting LEDs having higher external luminous efficacies can be obtained. Therefore, the present invention is very advantageous for industrial applications. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A method of manufacturing a green light emitting diode, comprising the steps of: (a) preparing GaP single crystal wafers (1) having a substantially (111) main surface (8) and a (110) cleavage surface (6); (b) forming by anodization light and dark stripes of an oxide region (7) on said (110) cleavage surface (6) of each wafer (1); (c) selecting from said wafers (1) a wafer having the light and dark stripes at an inclination angle (ϑ) which is not more than 5°, said inclination angle being defined by the angle between a line (A-A′) extending along a longitudinal direction of the light and dark stripes, and the main surface (8) of each wafer (1); (d) removing said oxide region (7); and (e) forming by liquid phase epitaxy a layer (2, 3) having a p-n junction on the main surface (8) of the selected wafer (1). The method as claimed in claim 1, wherein said oxide region (7) is formed on said cleavage surface (6) by anodization in a 1 N KOH aqueous solution. The method as claimed in claim 2, wherein the anodization is performed such that said oxide region (7) is formed on said cleavage surface (6) by energization with a current of several mA for several minutes. The method as claimed in anyone of the preceding claims, wherein said wafer (1) is doped with an element selected from a group consisting of S, Te and Si. The method as claimed in anyone of the preceding claims, wherein an etch pit density (EPD) of each wafer (1) is 2 x 10⁵/cm².

Inventors: NAKAJIMA MASAHIRO INTELLECTUAL, WATANABE MASAYUKI INTELLECTUAL, NAKAJIMA, MASAHIRO INTELLECTUAL PROPERTY DIVISION, WATANABE, MASAYUKI INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01L33
IPC codes: H01L33