Compound semiconductor single crystal

Abstract: A compound semiconductor single crystal which has an Si concentration of not more than 1x10¹⁷ atoms/cm³ and/or an O concentration of not more than 7x10¹⁶ atoms/cm³ is particularly suitable for use as substrate for epitaxial deposition of a high luminance light-emitting device which emits green light.

Description: The present invention relates generally to a composite of a GaP single crystal used as a substrate and an epitaxial deposit thereon of a light emitting device which emits green light and to a method of emitting light, including green light from a light emitting diode. JP-A-60210599 disloses a method for growing semiconducting GaAs crystal. In particular, it is described that when similar conditions are applied during ion implantation to all crystal substrates obtained by slicing the ingot crystal, ion implanted layers having the same free electron concentration can be formed with high reproducibility. JP-A-60210599 suggests that a controlled amount of oxygen is added to grow a semiconducting GaAs crystal. This controlled amount of oxygen (3 × 10¹⁶ cm⁻³) then leads to a uniform C concentration over the whole crystal. Journal of Applied Physics, Vol. 45, No. 1, January 1974, pages 243 to 245 is only a basic study of the total oxygen content of gallium phosphide grown by the Czochralski technique using liquid encapsulation. In particular, it is diclosed that the more substitutional oxygen that is contained in the gallium phosphide, the higher the red luminescence intensity is. Starting from this known fact, the oxygen concentration in crystals grown by various growth techniques has been studied in a range from 2 x 10⁷ to 5 x 10¹⁷ atoms/cm³. Finally, GB-A-2006043, discloses gallium phosphide single crystals with low defect density and their method of manufacture. The teaching of this reference is based upon the fact that the structural quality of the epitaxial layer is influenced by the substrate (page 1, lines 28 to 30, page 2, lines 40 to 43). The demand for a high luminance GaP green light light emitting diode (LEDs) has increased year after year. However, since the luminance reduction is caused by various reasons, a satisfactory solution to the foregoing demand has not been proposed. For example, when a liquid-phase epitaxial layer is deposited on a GaP single crystal substrate to fabricate an epitaxial wafer for a green light LED, a problem occurs in that the luminance at a particular portion of the substrate becomes inadequate. Through an investigation on the impurity concentration of GaP single crystal substrates involving the inadequate luminance problem and of GaP single crystal substrates free from the inadequate luminance problem, the present inventors have found that a definite correlation exists between the luminance and the concentrations of particular impurities. Based on this finding, the present inventors have completed the present invention. It is an object of this invention to provide a composite of a GaP single crystal as a substrate and an epitaxial deposit thereon of a light emitting device which emits green light and is free from the inadequate luminance problem and to provide a method of emitting light including green light from a light emitting diode. The object is achieved by a composite of a GaP single crystal as a substrate and an epitaxial deposit thereon of a light emitting device which emits green light, wherein said substrate has an impurity concentration limit of both not more than 1 x 10¹⁷ atoms/cm³ of silicon and not more than 7 x 10¹⁶ atoms/cm³ of oxygen, said impurity concentration limit being insufficient to cause precipitation of sufficient silica to increase the amount of defects generated by a subsequent epitaxial growth process. The inventive method is defined in claim 3. Many other objects, advantages an features of the present invention will be better understood from the following descrition taken in conjunction with the accompanying drawings. Figure 1 is a graph showing a correlation between the concentrations of Si and O in compound semiconductor single crystals and the luminance of LEDs using the compound semiconductor single crystals; and FIG. 2 is a diagrammatical side view of a liquid-phase epitaxial growth system used in Example 1 of the present invention. One embodiment of this invention will now be described with reference to the accompanying drawings. FIG. 2 shows a liquid-phase epitaxial growth system 2 used in Example 1 described later. The liquid-phase epitaxial growth system 2 includes a reaction tube 4 made of quartz in which a growth boat 6 is disposed. The growth boat 6 includes a substrate holder 8 for holding therein a plurality (five in the illustrated embodiment) of GaP substrates W1 - W5 arranged in the longitudinal direction thereof, and a solution holding boat 12 disposed on the substrate holder 8 and having a plurality of growth solution pits 10a - 10e each provided for a corresponding one of the GaP substrates W1 - W5 and holding therein a solution used for the liquid-phase epitaxy. The substrate holder 8 and the solution holding boat 12 are slidable relative to one another. A dopant evaporation source 14 comprised of Zn, for example, is disposed at a position spaced a distance from the growth boat 6. The reaction tube 4 has at its one end a gas supply port 16 from which a gas is supplied into the reaction tube 4 and, at the opposite end, a gas discharge port 18 from which the gas is discharged from the reaction tube 4. Reference characters 20a and 20b are heaters disposed around the reaction tube 4, and numeral 21 is a pull rod for reciprocating the growth boat 6. The invention will be further described by way of the following example which should be construed as illustrative rather than restrictive. Example 1 Using 14 samples of GaP single crystal substrates fabricated by the LEC method with different concentrations of silicon and oxygen as shown in Table 1, liquid-phase epitaxial layers were grown in the liquid-phase epitaxial growth system 2 shown in FIG. 2 in order to fabricate epitaxial wafers for a green light LED. The liquid-phase epitaxial growth was performed in the manner described below. (1) In an H₂ atmosphere, the temperature was elevated to 1000°C, thus causing GaP polycrystal to melt down into a Ga melt. (2) The pull rod 21 was manipulated to bring the substrates W1 - W5 into contact with the Ga melt. (3) The temperature was elevated to 1010°C, thus causing a portion of each substrate to melt back into the Ga melt. (4) The system 2 was gradually cooled down at the rate of 1 - 3°C per minute during which time an n-type layer was grown first. (5) While supplying H₂ and NH₃ from the gas supply port 16, the system 2 is continuously cooled so that an n-type layer added with N was grown. (6) The dopant evaporation source 14 was heated to generate a Zn vapor and while supplying the Zn vapor and H₂, the system 2 was continuously cooled so that a p-type epitaxial layer was grown. (7) Finally, the pull rod 21 was manipulated to separate the Ga melt and the substrates W1 - W5, thereby terminating the epitaxial growth. To the epitaxial wafers thus produced, a p-type electrode of an Au alloy and an n-type electrode of an Au alloy were formed by vapor deposition. Subsequently, the wafers were cut into 0.3 mm-square pellet-like, green light LEDs. Then, the luminance of the LEDs thus fabricated were measured under the following conditions. Header: TO-18 header Epoxy coating: none DC forward current (I f ): 10 mA Using a criterion of 4.5 mcd, the measured LEDs were classified into two groups depending on their luminance levels, i.e., non-defective LEDs having luminance levels not less than 4.5, and defective LEDs having luminance levels less than 4.5 mcd. Then, the measurements on the luminance were analyzed in respect of a relation to the conjunction with the concentrations of impurities Si and O. The results thus obtained are shown in FIG. 1. In FIG. 1, the mark "o" represents an LED which was free from precipitation of SiO₂ and exhibited a luminance of 4.5 mcd or more, and the mark "x" represents an LED which involved a local precipitation of SiO₂ and, due to this crystal defect, were only able to exhibit a luminance less than 4.5 mcd. A substrate which is suitable for use in epitaxial deposition of a luminance-reduction-free green light LED, has a specified range of impurity concentration, that is, an Si concentration of not more than 1x10¹⁷ atoms/cm³, and an O concentration of not more than 7x10¹⁶ atoms/cm³. In other words, an LED which is capable of emitting green light with an adequate luminance can be obtained by using a compound semiconductor substrate which has an Si concentration of not more than 1x10¹⁷ atoms/cm³ and an O concentration of more than 7x10¹⁶ atoms/cm³. As described above, a GaP single crystal of this invention is particularly suitable for use as a substrate for epitaxial deposition of a light-emitting device which is able to emit green light without involving an inadequate luminance problem. No. Si Concentration O Concentration 1 0.63 0.56 2 1.12 0.56 3 2.22 0.63 4 2.31 0.69 5 2.07 0.91 6 1.20 0.93 7 1.02 0.93 8 0.63 0.93 9 0.89 1.07 10 0.10 1.08 11 0.89 1.40 12 1.35 2.00 13 1.10 2.10 14 2.00 1.50 (Concentration: x10¹⁷ atoms/cm³)

Claims: A composite of a GaP single crystal substrate and an epitaxial deposit thereon of a light emitting device which emits green light, wherein said substrate has an impurity concentration limit of both not more than 1 x 10¹⁷ atoms/cm³ of silicon and not more than 7 x 10¹⁶ atoms/cm³ of oxygen, said impurity concentration limit being insufficient to cause precipitation of sufficient silica to increase the amount of defects generated by a subsequent epitaxial growth process. A composite as claimed in claim 1, wherein the impurity concentration limit of silicon and oxygen contained in said substrate is insufficient to impair the luminance of the light emitting device. A method of emitting light, including green light, from a light emitting diode, which diode comprises: a GaP single crystal semiconducting substrate; and an epitaxial deposit on at least one surface of said substrate comprising a green light emitting device, wherein said method comprises: forming said diode while limiting impurity concentration of both silicon and oxygen in said GaP single crystal substrate so that the green light emission enhancing amount of silicon is not more than 1 x 10¹⁷ atoms/cm³, and the green light emission enhancing amount of oxygen is not more than 7 x 10¹⁶ atoms/cm³; and energizing said diode to an extent sufficient to produce light, with enhanced green light, therefrom.

Inventors: IKEDA JUN, KAISE TSUNEYUKI, OTAKI TOSHIO, IKEDA, JUN, KAISE, TSUNEYUKI, OTAKI, TOSHIO
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1992
Main classification: C30B19
IPC codes: H01L29, H01L21, C30B15, C01G15, C30B19, H01L33