Method of manufacturing a GaP light emitting element

Abstract: A GaP light emitting element substrate comprising an n-type GaP layer (12), a nitrogen-doped n-type GaP layer (13) and a p-type GaP layer (14) layered one after another on a multi-layer GaP substrate built by forming an n-type GaP buffer layer(s) (11) on an n-type GaP single crystal substrate (10), wherein the sulfur (S) concentration in said n-type GaP buffer layer (11) is made to be 5 x 10¹⁶ [atoms/cc] or less. The method of manufacturing it is as follows: an n-type GaP buffer layer(s) (11) is formed on an n-type GaP single crystal substrate (10) to prepare a multi-layer GaP substrate, then an n-type GaP layer (12), a nitrogen doped n-type GaP layer (13) and a p-type GaP layer (14) are layered on said multi-layer GaP substrate by means of the melt-back method to obtain a GaP light emitting element substrate, wherein the sulfur (S) concentration in said n-type GaP buffer layer (11) is made to be 5 x 10¹⁶ [atoms/cc] or less when the multi-layer GaP substrate is prepared.

Description: This invention relates to a method of manufacturing a GaP light emitting element, and more specifically of manufacturing a GaP light emitting element comprising a substrate with a plurality of GaP layers on it and which is used when manufacturing GaP light emitting diodes which emit green light. Light emitting elements for light emitting diodes are obtained normally by layering a plurality of semiconductor layers on a semiconductor substrate to prepare a multi-layer semiconductor system with a pn junction(s), and making it into elements. Of these, green light emitting diodes can be obtained by using a light emitting element prepared by forming one or more layers of both n-type and p-type GaP layers, one after another, on an n-type GaP single crystal substrate. GaP is an indirect transition type semiconductor, and therefore the brightness is very low when a pn junction is simply formed. Therefore nitrogen (N), which would be the luminescence center, is added to the n-type GaP layer in the proximity of the pn junction to increase the light emitting output. A light emitting diode prepared from the GaP light emitting element which has the nitrogen-doped n-type GaP layer, as described above, will produce a yellow-green light emission with a peak wavelength of about 567 nm. Figure 1 shows an example of the cross-sectional structure of a GaP light emitting element which produces a green light emission. For this light emitting element, an n-type GaP buffer layer 11, an n-type GaP layer 12, a nitrogen-doped n-type GaP layer 13 and a p-type GaP layer 14 are formed one after another on an n-type GaP single crystal substrate 10. For the method of forming each GaP layer on the n-type GaP single crystal substrate 10, the liquid phase epitaxial growth method, for example, can be used. For the liquid phase epitaxial growth , two methods are normally employed. In the first liquid phase epitaxial growth method, for example, a Ga solution prepared by dissolving GaP poly-crystals in fused Ga at 1060°C is placed on a GaP substrate, and the GaP layer is grown by gradually lowering the temperature to precipitate GaP in the Ga solution on the GaP substrate. In the other liquid phase epitaxial growth method(hereafter referred to as melt-back liquid epitaxial growth method or melt-back method for short), fused Ga is placed on a GaP substrate, and then, for example, the temperature is raised to 1060°C so that the upper portion of the GaP substrate is dissolved into the fused Ga to prepare the Ga solution, and then the GaP layer is grown by gradually lowering the temperature to precipitate GaP in the Ga solution on the GaP substrate. Recently, the progress of GaP light emitting diodes which emit yellow-green light has been remarkable, and each year diodes with higher light emission outputs are developed. Along with this trend toward higher light emission output, the application range of the GaP light emitting diodes has spread to a wider range. However, development of light emitting diodes with even higher light emission outputs is desirable in order to further expand the application range. In order to obtain a light emitting element for manufacturing light emitting diodes with high light emission outputs, JP-A-54 133 093, concerning the fabrication by melt-back epitaxy of a green LED, already suggests to control the S content of the GaP substrate, and it has become common, as described, for example, in Japanese examined patent publication (Tokko) Hei-2-18319, to prepare a multi-layer Gap structure and use it when growing the GaP layers with the melt-back method as described above. In this method, a multi-layer Gap structure is previously prepared by forming an n-type GaP buffer layer 11 on an n-type GaP single crystal substrate 10, and, for the next step, the melt-back method is used to dissolve the upper portion of the n-type GaP buffer layer 11 on said multi-layer GaP substrate, then GaP is precipitated again to form the n-type GaP layer 12, the nitrogen-doped n-type GaP layer 13 and the p-type GaP layer 14, one after another. SUMMARY OF THE INVENTION However, when the method described above is used, and if sulfur (S), which is generally used as an n-type dopant, is involved, there is a problem in that the higher light emission output cannot be achieved because of the effect of S. For example, during the formation of the n-type GaP buffer layer 11, if the S concentration in the n-type GaP buffer layer 11 exceeds 5 x 10 16 [atoms/cm 3 ], then the light emission output of the light emitting diodes, made from this light emitting element , drops sharply. Therefore, the object of this invention is to provide a method of manufacturing a GaP light emitting element which contains a nitrogen-doped n-type GaP layer and produces a high light emission output. According to the inventive method a Gap light emitting element is obtained comprising an n-type GaP layer, a nitrogen-doped n-type GaP layer and a p-type GaP layer layered one after another on a multi-layer Gap structure built by forming an n-type GaP buffer layer(s) on an n-type GaP single crystal substrate, wherein the sulfur (S) concentration in said n-type GaP buffer layer is made to be 5 x 10 16 [atoms/cm 3 ] or less, and the effective donor concentration in said nitrogen doped n-type GaP Layer is made to be 2 x 10 16 [atoms/cm 3 ] or less. In order to meet the standards for the forward direction voltage of the green light emitting GaP light emitting diodes in operation (normally 2.30 V or lower at 20 mA), it is necessary to keep the n-type dopant concentration in said n-type GaP buffer layer 11 at 5 x 10 16 [atoms/cm 3 ] or higher. Therefore, it is desirable to dope said GaP buffer layer with another n-type dopant(s) in addition to S so that the n-type dopant concentration in the n-type GaP buffer layer is 5 x 10 16 - 3 x 10 17 [atoms/cm 3 ]. For this other n-type dopant used for doping along with S, tellurium (Te) would be an example. Even when the doping with S as an n-type dopant is not conducted intentionally, the n-type GaP buffer layer is naturally doped with S which comes from sulfur or sulfur compounds contained in the environment or in the raw materials. The concentration is about 5 x 10 16 [atoms/cm 3 ] or lower when the doping sources (contamination sources) mentioned above are kept out as much as possible. As already mentioned, in the Gap light emitting element of this invention, the sulfur (S) concentration in said n-type GaP buffer layer is made to be 5 x 10 16 [atoms/cm 3 ] or less. In order to increase the light emission output of a green light emitting GaP light emitting diode, it is necessary to keep the effective donor concentration [(donor concentration (ND)) - (Acceptor concentration (NA))] in the nitrogen-doped n-type GaP layer (i.e. the light emitting area) low (2 x 10 16 [atoms/cm 3 ] or less). Since NA, mentioned above, is usually in the range of 1 x 10 16 [atoms/cm 3 ], the corresponding ND would be 3 x 10 16 [atoms/cm 3 ] or less. The inventors found out that, in the method of manufacturing a light emitting element by using the melt-back method to layer a plurality of GaP layers on a multi-layer GaP structure obtained by forming an n-type buffer layer on an n-type GaP single crystal substrate, the n-type dopant S contained in the n-type GaP buffer layer is transferred, without a significant reduction in the concentration (approximately down to 1/2), to the nitrogen-doped n-type GaP layer which is grown by the melt-back method. On the other hand, in the case of other dopants, Te for example, the Te concentration in the nitrogen-doped n-type GaP layer after the process described above is reduced to about 1/50 of the Te concentration in the n-type GaP buffer layer. Therefore, in order to make the effective donor concentration [ND - NA] in the nitrogen-doped GaP layer, which is the light emission region, 2 x 10 16 [atoms/cm 3 ] or less (ND would be 3 x 10 16 [atoms/cm 3 ] or less), and thus manufacture a light emitting element with a high light emission output, it is necessary to make the S concentration in the n-type GaP buffer layer 5 x 10 16 [atoms/cm 3 ] or less. The total concentration of the n-type dopants in said buffer layer can therefor be 5 x 10 16 - 3 x 10 17 [atoms/cm 3 ] in case of another n-type dopant in addition to sulfur. Fig. 1 shows across-sectional view of an example of a GaP light emitting element. Figure 2 shows a process diagram which shows an example of the GaP layer growth method in the method of manufacturing the GaP light emitting element of this invention. Figure 3 shows the relationship between the light emission output (relative values) of the light emitting diodes and the effective donor concentrations when the GaP light emitting element from which these light emitting diodes are made are prepared with various effective donor concentrations in the nitrogen-doped n-type GaP layer (light emission region). Figure 4 shows the relationship between the S concentration in the nitrogen-doped n-type layer and the S concentration in the n-type GaP buffer layer when corresponding light emitting elements are manufactured with the melt-back method with various S concentrations in the n-type GaP buffer layers. Figure 5 shows the relationship between the light emission output (relative values) of the light emitting diodes and the S concentrations in the n-type GaP buffer layers when the GaP light emitting elements from which these light emitting diodes are made are prepared with the melt-back method with various S concentrations in the n-type GaP buffer layers. An example of a method according to the invention for manufacturing a GaP light emitting element is described next, by referring to the process diagram shown in Figure 2. First, as shown in Figure 2 (a), a Ga solution 16 at 1060°C into which GaP poly-crystals and Te, which is going to be an n-type dopant, are dissolved is placed on a GaP single crystal substrate 10. This Ga solution 16 is a GaP saturated solution at 1060°C into which Te is added in such a way that the total n-type dopant concentration in the n-type GaP buffer layer 11, which will be described later, becomes 5 x 10 16 - 3 x 10 17 [atoms/cm 3 ]. Then, the temperature of the system which includes said Ga solution 16 is gradually lowered in the presence of the flow of hydrogen sulfide (H 2 S), which is going to be an n-type dopant, and the carrier gas, hydrogen (H 2 ), so that GaP dissolved in the Ga solution 16 precipitates on the n-type GaP single crystal substrate 10. Thus, the multi-layer Gap structure 15 comprising the GaP buffer layer 11 doped with Te and S formed on the n-type GaP single crystal substrate 10 is obtained (Figure 2 (b)). Next, as shown in Figure 2 (c), fused Ga 17 without an n-type dopant is placed on said multi-layer GaP structure 15. At this time, the temperature is set at 600°C. Then, the temperature of the system which contains said fused Ga 17 is raised to 1000°C. The upper portion of the n-type GaP buffer layer 11 gradually dissolves into fused Ga 17, and this fused Ga becomes a saturated solution of GaP at 1000°C (Ga solution 17a). Here, Te and S which are contained in the n-type GaP buffer layer 11 also dissolve in said Ga solution 17a (Figure 2 (d)). Next, the temperature is lowered to grow the n-type GaP layer 12 doped with the n-type dopants (Te and S) (Figure 2 (e)), and, in the presence of the flow of NH 3 along with the carrier gas H 2 in said system, the temperature is further lowered to grow the nitrogen-doped n-type GaP layer 13 which is doped with nitrogen (N) and the n-typed dopants (Te and S) on the n-type GaP layer 12 (Figure 2 (f)). Then, the NH 3 flow is stopped, the temperature of the system, to which Zn is added here, is raised to about 700°C, and then the temperature is lowered. Thus Zn vapor flows along with the carrier gas H 2 , and the p-type GaP layer 14 doped with Zn is formed on the nitrogen-doped n-type GaP layer 13 (Figure (g)). In the manner described above, the n-type GaP buffer layer 11, the n-type GaP layer 12, the nitrogen-doped n-type GaP layer 13 and the p-type GaP layer 14 are formed one after another on the n-type GaP single crystal substrate 10 to prepare the light emitting element, and this light emitting element is made into pieces to obtain light emitting diodes with green light emission. Figure 3 shows the relationship between the light emission output (relative values) of the light emitting diodes and the effective donor concentrations when the GaP light emitting element from which these light emitting diodes are made are prepared with various effective donor concentrations [ND - NA] in the nitrogen-doped n-type GaP layer 13 (light emitting area). As shown in the figure, in order to obtain a green light emitting GaP light emitting diode with a high light emission output (light emission output (relative value) of 10 or more), it is necessary to make the effective donor concentration in said nitrogen-doped n-type GaP layer 2 x 10 16 [atoms/cm 3 ] or lower (corresponding to ND of 3 x 10 16 [atoms/cm 3 ] or lower). Figure 4 shows the relationship between the S concentration in the nitrogen-doped n-type GaP layer 13 and the S concentration in the n-type GaP buffer layer 11 when the light emitting elements are manufactured with the melt-back method by using said multi-layer GaP structures 15 with various S concentrations in the n-type GaP buffer layers 11. The similar relationship for Te is also shown in the same figure. As shown in the figure, the n-type dopant S contained in the n-type GaP buffer layer 11 is transferred, without a significant reduction in the concentration (approximately down to 1/2), to the nitrogen-doped n-type GaP layer 13 (light emitting area). On the other hand, the Te concentration in the nitrogen-doped n-type GaP layer 13 is reduced to about 1/50 of the Te concentration in the n-type GaP buffer layer 11. (For example, when the Te concentration in the n-type GaP buffer layer 11 is 2.5 x 10 17 [atoms/cm 3 ], the Te concentration in the nitrogen-doped n-type GaP layer 13 is about 5 x 10 15 [atoms/cm 3 ].) In order to make the total concentration of the n-type dopants in the nitrogen-doped n-type GaP layer 13, i.e. ND, 3 x 10 16 [atoms/cm 3 ] or lower, it is necessary to keep the S concentration in said n-type GaP buffer layer 11 at 5 x 10 16 [atoms/cm 3 ] or lower so that the total concentration of the n-type dopants in this buffer layer 11 (for example, a total of the S concentration and the Te concentration) is 5 x 10 16 - 3 x 10 17 [atoms/cm 3 ]. Figure 5 shows the relationship between the light emission output (relative values). of the light emitting diodes and the S concentrations in the n-type GaP buffer layers 11 when the GaP light emitting elements from which these light emitting diodes are made are prepared with the melt-back method by using said multi-layer Gap structures with various S concentrations in the n-type GaP buffer layers 11. As shown in figure 5, a green light emitting GaP light emitting diode with a high light emission output (light emission output (relative value) of 10 or more) can be obtained by keeping the S concentration in said n-type GaP buffer layer 11 at 5 x 10 16 [atoms/cm 3 ] or lower. As described thus far, the effect of this invention lies in that, by using this invention, a GaP light emitting element containing a nitrogen-doped n-type GaP layer with a sufficiently low concentration of n-type dopants can be obtained so that green light emitting GaP light emitting diodes manufactured by using this element produce a sufficiently high light emission output.

Claims: A method of manufacturing a GaP light emitting element wherein an n-type GaP buffer layer(s) (11) is formed on an n-type GaP single crystal substrate (10) to prepare a multi-layer GaP structure, then an n-type GaP layer (12), a nitrogen doped n-type GaP layer (13) and a p-type GaP layer (14) are layered on said multi-layer GaP structure by means of the melt-back method, said method being characterized in that the sulfur concentration in said n-type GaP buffer layer (11) is 5 x 10 16 [atoms/cm 3 ] or less, and the effective donor concentration in said nitrogen doped n-type GaP layer (13) is 2 x 10 16 atoms/cm 3 or less. The method of manufacturing a GaP light emitting element according to Claim 1, wherein said n-type GaP buffer layer (11) is doped with an n-type dopant(s) in addition to said sulfur and the total concentration of the n-type dopant(s) in said n-type GaP buffer layer (11) is 5 x 10 16 - 3 x 10 17 atoms/cm 3 .

Inventors: HIGUCHI SUSUMU, NAKAMURA AKIO, OOTAKI TOSHIO, TAMURA YUUKI, YANAGISAWA MUNEHISA, HIGUCHI, SUSUMU, NAKAMURA, AKIO, OOTAKI, TOSHIO, TAMURA, YUUKI, YANAGISAWA, MUNEHISA, HIGUCHI, SUSUMU, C/O ISOBE KOJO, NAKAMURA, AKIO, C/O ISOBE KOJO, Ootaki, Toshio, c/o Isobe Kojo, TAMURA, YUUKI, C/O ISOBE KOJO, Yanagisawa, Munehisa, c/o Isobe Kojo
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L33
IPC codes: H01L33