GaP light emitting device and method for fabricating the same

Abstract: A semiconductor substrate for GaP type light emitting devices includes an n-type GaP single crystal substrate, and an n-type GaP layer and a p-type GaP layer formed on the n-type GaP single crystal substrate. The carbon concentration in the n-type GaP single crystal substrate is not more than 1.0 × 10¹⁷ atom/cc. The n-type GaP single crystal substrate is obtained from an n-type GaP single crystal grown by the Liquid Encapsulation Czochralski method wherein B₂O₃ containing water corresponding to 200 ppm or more is used as an encapsulation liquid.

Description: FIELD OF THE INVENTION This invention relates to a semiconductor substrate for GaP type light emitting devices and a method for fabricating the same, and in particular to a semiconductor substrate for GaP type light emitting devices, in which a plurality of GaP layers are formed, used for fabricating GaP type light emitting devices which emit red light, and a method for fabricating the same. BACKGROUND OF THE INVENTION Light emitting devices are fabricated generally by the steps of forming a plurality of semiconductor layers on a semiconductor substrate to fabricate a multi-layer semiconductor substrate having a pn junction and fabricating the substrates to devices. Among these light emitting devices, GaP type light emitting diodes which emit red light are fabricated from a semiconductor substrate for light emitting device, consisting of a n-type GaP single crystal substrate on which n-type and p-type GaP layers are formed, as is described in US-A-4 180 423. The compound semiconductor GaP does not emit red light by itself even if a pn junction is formed in a substrate consisting thereof, so that zinc (Zn) and oxigen (O) are doped into a p-type layer, and pairs of Zn-O which become light emitting centers are formed in the p-type layer. GaP type light emitting diodes fabricated from the semiconductor substrate for GaP type light emitting devices emit red light having a peak wavelength of about 700 nm. However, according to the conventional light emitting devices fabricated from the semiconductor substrate for GaP type light emitting devices processed by the above conventional method, there is a disadvantage in that substantially high luminance can not be obtained. SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a GaP light emitting device with higher luminance than the conventional light emitting devices and a method for fabricating the same. According to this invention, a GaP light emitting devices as set out in Claim 1 includes an n-type GaP single crystal substrate and an n-type GaP layer and a p-type GaP layer formed on the n-type GaP single crystal substrate, wherein concentration of carbon contained in the n-type GaP single crystal substrate is equal to or lower than 1.0 × 10 17 atoms/cm 3 . A method for its fabrication is specified in Claim 5. If the n-type GaP single crystal is grown by the Liquid Encapsulation Czochralski (LEC) method, an n-type GaP single crystal containing carbon of the concentration as stated above is obtained by growing the single crystal using boron oxide (B 2 O 3 ), as liquid for encapsulation, containing at least 200 ppm of water, and then an n-type GaP single crystal substrate containing carbon of the concentration as stated above. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be explained in more detail in conjunction with appended drawings. Fig. 1 is a schematic cross-sectional view illustrating the cross-sectional structure of the semiconductor substrate for light emitting devices in an embodiment according to the invention. Fig. 2 is a structural view of an apparatus used for growing an n-type GaP single crystal in the embodiment according to the invention. Fig. 3 is a graph showing the relation between the carbon concentration in the n-type GaP single crystal substrate and the luminance of the light emitting device fabricated therefrom. Fig. 4 is a graph showing the ralation between amount of water in B 2 O 3 used for growing the n-type GaP single crystal by the LEC method and the carbon concentration in the n-type GaP single crystal substrate obtained from the n-type GaP single crystal. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of this invention will be explained by referring to the attached drawings. Figure 1 shows a GaP light emitting device. In the GaP light emitting device of this embodiment, an n-type GaP layer 2 and a p-type GaP layer 3 are successively formed on an n-type GaP single crystal substrate 1. Sulfur (S) and zinc (Zn) are used as dopants as n-type and p-type sources, for example. The p-type GaP layer 3 is doped with oxide (O) as well as Zn. A method for fabricating the GaP light emitting device mentioned above is described below, for example. First, the n-type GaP single crystal substrate 1 is fabricated from the n-type GaP single crystal grown by the LEC method, for example. In more detail, the growth step of the GaP single crystal by the LEC method is carried out using an apparatus shown in Fig. 2. In Fig. 2, a crucible 12 supported by a susceptor 11 is provided in a pressure vessel 10, and a GaP melt 13 is included in the crucible 12. The GaP melt 13 is covered on the top surface with B 2 O 3 17 which is lighter than the GaP melt 13 and is transparent. In order to avoid dissociation of GaP, the inside of the pressure vessel 10 is pressurized with an inert gas such as nitrogen 16. A GaP single crystal 14 is grown as being pulled out of the GaP melt 13 by a rotating rod 15. Further, the GaP melt 13 is heated by a heater 18 so that an interface 19 between the fused GaP 13 and the GaP single crystal 14 is kept at 1470°C, corresponding to the melting point of GaP. The inside of the pressure vessel 10 is monitored through a window 20, for example, by a TV monitor. The GaP single crystal 14 grown by the process explained above is sliced to obtain GaP single crystal substrates 1, and the epitaxial layers are formed on the substrate 1 by the liquid phase epitaxy (LPE). In more detail, a solution which is prepared by solving polycrystalline GaP in a Ga melt is positioned on the substrate at 1000°C, for example. Next, the solution is cooled to make GaP in the solution precipitate and grow a GaP layer on the substrate. In the process, n-type dopants such as sulfer are added to the solution for growing the n-type GaP layer 2, and p-type dopants such as zinc or oxide are added thereto for growing the p-type GaP layer 3. Thus, a semiconductor substrate for light emitting devices, which is a multi-layer GaP substrate, is obtained, wherein the n-type GaP layer and the p-type GaP layer are successively formed on the n-type GaP single crystal substrate. Then, n-electrodes are formed on the GaP single crystal substrate side of the semiconductor substrate for light emitting devices and p-electrodes are formed on the p-type GaP layer side thereof, respectively. Then, after being cut into single chips, each chip is mounted on a frame and wire-bonded, and encapsulated in epoxy resin, for example. Thus, light emitting diodes which emit red light are obtained. Fig. 3 shows the relation between the carbon concentration in the n-type GaP single crystal substrate and relative luminance of the light emitting devices which emit red light, fabricated from the semiconductor substrate for light emitting devices. As understood by Fig. 3, the luminance becomes higher as the carbon concentration decreases, and the carbon concentration in the n-type GaP single crystal substrate is required to be equal to or lower than 1.0 × 10 17 atoms/cm 3 to obtain light emitting devices having relative luminance higher than 45. In order to obtain the n-type GaP single crystal substrate with carbon concentration equal to or lower than 1.0 × 10 17 atoms/cm 3 , the n-type GaP single crystal grown from GaP polycrystal with originally low carbon concentration is preferably used as a starting material, for example. In this method, it is confirmed that the carbon concentraion in the GaP single crystal substrate is equal to or lower than 1.0 × 10 17 atoms/cm 3 when a GaP polycrystal with a carbon concentration lower than 1.0 × 10 18 atoms/cm 3 is used as the starting material. On the other hand, when the n-type GaP single crystal is grown by the LEC method, it is possible to control the carbon concentration by controlling the amount of water contained in B 2 O 3 used in the growth process as the encapsulation liquid. This connection is investigated in the article of J. Nishio and K. Terashima in Journal of Crystal Growth, Vol. 96, pages 605-608 (1989). The fabrication of a GaAs single crystal with reduced variation of the carbon concentration is described in JP-A-1 290 597. Fig. 4 shows the relation between amount of water contained in B 2 O 3 used in the growth process and the carbon concentration in the GaP single crystal. As understood by Fig. 4, the carbon concentration in the n-type GaP single crystal becomes low as amount of water in B 2 O 3 increases. Especially, the carbon concentration decreases drastically and becomes equal to or lower than 1.0 × 10 17 atoms/cm 3 when the amount of water in B 2 O 3 becomes larger than 200 ppm. Therefore, the carbon concentration in the n-type GaP single crystal can be kept low by controlling the amount of water in B 2 O 3 , contributing to the improvement of luminance of the light emitting devices. Although the amount of water in B 2 O 3 is not limited as far as the amount of water is not less than 200 ppm, there may be disadvantages in scope of growing GaP single crystal if the amount of water in B 2 O 3 is larger than 1000 ppm. For example, chemical reactions may occur and the B 2 O 3 layer may become opaque or crystal growth may be affected adversely. As described above, according to the invention, a GaP single crystal substrate with the carbon concentration equal to or lower than 1.0 × 10 17 atoms/cm 3 can be obtained, and it is possible to fabricate light emitting devices of high luminance by a semiconductor substrate for light emitting devices including the GaP single crystal substrate. Although the invention has been described with respect to a specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited.

Claims: A GaP light emitting device, comprising: an n-type GaP single crystal substrate; an n-type GaP layer formed on said n-type GaP single crystal substrate; and a p-type GaP layer formed on said n-type GaP layer, characterised in that the carbon concentration in said n-type GaP single crystal substrate is not more than 1.0 × 10 17 atoms/cm 3 . A GaP light emitting device, according to Claim 1, wherein said n-type GaP layer is doped with sulfur. A GaP light emitting device, according to Claim 1, wherein said p-type GaP layer is doped with zinc. A GaP light emitting device, according to Claim 1, wherein said p-type GaP layer is doped with oxide as well as Zn. A method for fabricating a GaP light emitting device according to any one of Claims 1 to 4, comprising the steps of: growing an n-type GaP single crystal by the Liquid Encapsulation Czochralski method and forming an n-type GaP layer and a p-type GaP layer successively on a semiconductor substrate obtained from said n-type GaP single crystal, characterised in that B 2 O 3 containing water corresponding to 200 ppm or more is used as an encapsulation liquid, so that the carbon concentration in said n-type GaP single crystal is not more than 1.0 × 10 17 atoms/cm 3 .

Inventors: HIGUCHI SUSUMU, NAKAMURA AKIO, OTAKI TOSHIO, TAMURA YUKI, YANAGISAWA MUNEHISA, HIGUCHI, SUSUMU, NAKAMURA, AKIO, OTAKI, TOSHIO, TAMURA, YUKI, YANAGISAWA, MUNEHISA, HIGUCHI, SUSUMU C/O ISOBE KOJOSHIN-ETSU, NAKAMURA, AKIO C/O ISOBE KOJO, SHIN-ETSU, OTAKI, TOSHIO C/O ISOBE KOJO, SHIN-ETSU, TAMURA, YUKI C/O ISOBE KOJO, SHIN-ETSU, YANAGISAWA, MUNEHISA C/O ISOBE KOJO SHIN-ETSU
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L33
IPC codes: H01L21, C30B29, C30B27, H01L33