Method for producing a gallium phosphide epitaxial wafer

This disclosure herein pertains to a method for producing a GaP epitaxial wafer used for fabrication of light emitting diodes having higher brightness than light emitting diodes fabricated from a GaP epitaxial wafer produced by a conventional method have. The method comprises the steps of: preparing a GaP layered substrate 15 with one or more GaP layers on a GaP single crystal substrate 10 in the first series of liquid phase epitaxial growth; obtaining a layered GaP substrate 15a by eliminating surface irregularities of said GaP layered substrate 15 by mechano-chemical polishing to make the surface to be planar; and then forming a GaP light emitting layer composite 19 on said layered GaP substrate 15a in the second series of liquid phase epitaxial growth.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for producing electroluminescent 
materials used for fabricating GaP light emitting devices, which materials 
have a structure of a plurality of GaP layers epitaxially grown on a GaP 
single crystal substrate (hereafter referred to as GaP epitaxial wafer). 
2. Description of the Prior Art 
In general, GaP light emitting diodes are fabricated in such a manner that 
first one or more GaP crystal layers of both n-type and p-type are 
consecutively one by one laminated on an n-type GaP single crystal 
substrate to produce a GaP epitaxial wafer including a pn junction and 
then said GaP epitaxial wafer is processed into the devices. In the mean 
time, GaP light emitting diodes are largely divided into three kinds in 
regard to the colors of emitted light, such as a green GaP light emitting 
diode (or yellowish green GaP light emitting diode), a red GaP light 
emitting diode and a pure green GaP light emitting diode. 
A green GaP light emitting diode 2 is fabricated from a GaP epitaxial wafer 
as shown in FIG. 2, in which Nitrogen atoms (N) serving as luminescence 
centers are doped in the portion of the n-type GaP layer in the vicinity 
of the pn junction and emits yellowish green light with the peak 
wavelength of about 567 nm. 
A red GaP light emitting diode 4 is fabricated from a GaP epitaxial wafer 
as shown in FIG. 3, in which Zinc (Zn) and Oxygen (O) are both doped in 
the p type layer to form Zn-O pairs serving as luminescence centers and 
emits red light with the peak wavelength of about 700 nm. 
A pure green GaP light emitting diode 6 is fabricated from a GaP epitaxial 
wafer as shown in FIG. 4, which has no dopant as luminescence centers 
therein and emits pure green light with the peak wavelength of about 555 
nm. 
The GaP epitaxial wafer 2 used for fabricating green light emitting diodes, 
the GaP epitaxial wafer 4 used for fabricating red light emitting diodes 
and the GaP epitaxial wafer 6 used for fabricating pure green light 
emitting diodes are illustrated in sectional structure respectively in 
FIGS. 2, 3 and 4, as an example for each. The GaP epitaxial wafer 2 for 
green light diodes has a structure in which an n-type GaP buffer layer 
11a, an n-type GaP layer 12a, a Nitrogen doped n-type GaP layer 13a and a 
p-type GaP layer 14a are consecutively in that order formed on an n-type 
GaP single crystal substrate 10a; the GaP epitaxial wafer 4 for red light 
emitting diodes has a structure in which an n-type GaP layer 11b and a 
p-type GaP layer 14b including Zn-O pairs therein are consecutively formed 
on an n-type GaP single crystal substrate 10b; and the GaP epitaxial wafer 
6 for pure green light emitting diodes has a structure in which an n-type 
GaP buffer layer 11c, an n-type GaP layer 12c, an n-type GaP layer or a 
p-type GaP layer with a lower carrier density and a p-type GaP layer 14c 
are consecutively in that order formed on an n-type GaP single crystal 
substrate 10c. 
Among methods by which GaP layers are consecutively formed on a GaP single 
crystal substrate or a GaP layer(s) which has been formed on the GaP 
single crystal substrate in the preceding step(s) as mentioned above, an 
established method is a liquid phase epitaxial method by cooling a 
saturated Ga solution of GaP (hereinafter referred to as liquid phase 
epitaxial method or liquid phase epitaxial growth). The liquid phase 
epitaxial method is further divided into two methods, which consist of a 
melt-back method and a non-melt-back method. The non-melt-back method is 
performed in the following way: GaP polycrystal is dissolved into Ga melt 
at, for example, 1060.degree. C. to prepare Ga solution saturated with GaP 
as solute at 1060.degree. C. Then a GaP substrate is contacted with the Ga 
solution and both of them are gradually cooled so that the GaP solute in 
the Ga solution is deposited on the GaP substrate as a grown GaP layer. On 
the other hand the melt-back method is performed in the following way: a 
GaP substrate is contacted with Ga melt. Then both of them are together 
heated up to, for instance, 1060.degree. C. so that the upper portion of 
the GaP substrate is dissolved into the Ga melt to form Ga solution 
saturated with GaP as solute at 1060.degree. C., and thereafter the 
substrate and the Ga solution are both gradually cooled to have a GaP 
layer grown on the GaP substrate in the same manner as in the 
non-melt-back method. 
A GaP epitaxial wafer for light emitting diodes is usually produced by way 
of a liquid phase epitaxial method consisting of two steps (hereinafter 
specially referred to as two step method). 
Referring to FIG. 2, the two step method will be described first taking as 
an example a GaP epitaxial wafer 2 used for fabricating green light 
emitting diodes. In the first step, an n-type GaP buffer layer 11a is 
formed on an n-type GaP single crystal substrate 10a by a non-melt-back 
method or a melt-back method (hereinafter the thus processed substrate is 
referred to as a layered substrate). In the second step, the melt-back 
method is applied to the layered GaP substrate. That is, an n-type GaP 
layer 12a, a Nitrogen doped n-type GaP layer 13a and a p-type GaP layer 
14a are further consecutively in that order formed on the n-type GaP 
buffer layer 11a of the layered GaP substrate the melt-back method. 
In the cases of a two step method GaP epitaxial wafer for red light 
emitting diodes and a two step method GaP epitaxial wafer for pure green 
light emitting diodes, layered GaP substrates are an n-type GaP layer 11b 
on an n-type GaP single crystal substrate 10b and an n-type GaP buffer 
layer 11c on an n-type GaP single crystal substrate 10c respectively. 
Hereinafter a plurality of GaP layers formed by a liquid phase epitaxial 
growth on a layered GaP substrate is generically named a GaP light 
emitting layer composite. For example, in case of a GaP epitaxial wafer 2 
for green light emitting diodes as shown in FIG. 2, three layers of an 
n-type GaP layer 12a, a Nitrogen doped n-type GaP layer 13a and a p-type 
GaP layer 14a are called a GaP light emitting layer composite 19a of the 
GaP epitaxial wafer 2 for green light emitting diodes, while in case of a 
GaP epitaxial wafer 6 for the pure green light emitting diodes as shown in 
FIG. 4, three layers of an n-type GaP layer 12c, an n-type GaP layer or a 
p-type GaP layer 13c with a lower carrier concentration and a p-type GaP 
layer 14c are called a GaP light emitting layer composite 19c of the GaP 
epitaxial wafer 6 for pure green light emitting diodes. A p-type GaP layer 
14b, which is of a single layer, of a GaP epitaxial wafer 4 for red light 
emitting diodes as shown in FIG. 3 is also called a GaP light emitting 
layer composite 14b of the GaP epitaxial wafer 4 for red light emitting 
diodes. 
In the first step of the two step method, more than one GaP layers may be 
formed on a GaP single crystal substrate in order to further improve a 
characteristic(s) of a light emitting diode, though a monolayered GaP 
substrate is taken up as an example for the purpose of illustration only 
in the description above. 
A two step method with a layered GaP substrate having more than one layers 
therein is conventionally considered useful for achieving higher 
brightness. However, even a thus produced GaP epitaxial wafer for light 
emitting diodes has still a problem by which said GaP epitaxial wafer can 
not achieve desired brightness of the light emitting diodes therefrom 
under the present requirement for increasingly higher brightness. 
As the results of the research conducted by the inventors in regard to the 
factors lowering brightness of a light emitting diode, the following 
discoveries have been obtained that surface irregularities accompanying 
crystallographic defects are generated on the surface of a GaP layer 
formed on a GaP single crystal substrate and the surface irregularities 
give an ill influence to deteriorate the crystallinity of a following 
light emitting layer composite, which is formed on said layered GaP 
substrate in the following step of liquid phase epitaxial growth. 
Deterioration of said composite necessarily results in lowering the 
brightness of light emitting diodes. 
SUMMARY OF THE INVENTION 
The present invention was made based on the above discoveries and knowledge 
to solve the problem and therefore it is an object of the present 
invention to provide a method for producing a GaP epitaxial wafer used for 
fabricating light emitting diodes with higher brightness than conventional 
light emitting diodes. 
The method according to the present invention comprises the following steps 
of: a layered GaP substrate being prepared by forming one or more GaP 
layers on a GaP single crystal substrate in the first liquid phase 
epitaxial growth and a GaP light emitting layer composite being formed on 
said layered GaP substrate in the second liquid phase epitaxial growth, 
where surface irregularities of said layered GaP substrate, which are 
generated on the surface during the first liquid phase epitaxial growth, 
are eliminated to make the surface to be planar before the second liquid 
phase epitaxial growth gets started. 
As for making the surface to be planar, a mirror polishing means may be 
applied to the surface of a layered GaP substrate to eliminate the surface 
irregularities from the surface and thereby obtain the planar surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Below will an embodiment of the method according to the present invention 
be described in the case of production of a GaP epitaxial wafer used for 
fabricating green light emitting diodes, as an example, in reference to 
the accompanying drawings. 
FIGS. 1(A) to 1(I) show a process diagram in a producing method of a GaP 
epitaxial wafer used for green light emitting diodes. 
As shown in FIG. 1(A), a Ga solution 16, which is prepared by dissolving 
GaP polycrystal and Te as n-type dopant into Ga melt at 1060.degree. C., 
is brought into contact with an n-type GaP single crystal substrate 10, 
where the Ga solution 16 is saturated with GaP at 1060.degree. C. Then, 
the Ga solution 16 and the GaP substrate 10 are both gradually cooled in a 
stream of Hydrogen as carrier gas so that the GaP solute deposits partly 
on the n-type GaP single crystal substrate 10. In such a manner, an n-type 
GaP buffer layer 11 of about 150 .mu.m thickness is formed on the n-type 
GaP single crystal substrate 10 to make a GaP layered substrate 15 (FIG. 
1(B)). 
The layered GaP substrate 15 is taken out of a growth furnace (FIG. 1(c)), 
and the surface portion of the n-type GaP buffer layer 11 is 
mechano-chemically polished off by about 20 .mu.m in depth with GaP 
polishing slurry (Fujimi Incorp. made, Trade Mark: INSEC P) and thereby a 
mirror-polished and planar surface is achieved on the surface of the 
layered GaP substrate 15a (FIG. 1(D)). 
As shown in FIG. 1(E), the layered GaP substrate 15a, which as mentioned 
above is obtained by polishing off in part the surface portion of the 
n-type GaP buffer layer 11, is arranged to be contacted with Ga melt 17 
free of both n-type dopant and GaP polycrystal, when the temperature of 
the Ga melt 17 and the substrate 15a are both set to 600.degree. C. 
In the following stage, the Ga melt 17 and the substrate 15a are both 
heated up to 1000.degree. C. to dissolve the upper portion of the n-type 
GaP buffer layer 11 (doped with Te) into the Ga melt 17, which becomes a 
Ga solution 17a saturated with GaP at 1000.degree. C. (FIG. 1(F)). 
After an n-type GaP layer 12 doped with Te as n-type dopant is grown as the 
temperature is adjusted downward (FIG. 1(G)), the temperature is adjusted 
further downward in a stream of NH.sub.3 and Hydrogen as carrier gas to 
form a Nitrogen doped n-type GaP layer 13, which is doped with Nitrogen 
(N) as luminescence centers and Te as n-type dopant (FIG. 1(H)). 
Then the NH.sub.3 flow is stopped and the temperature of Zn source is 
raised to about 700.degree. C. to evaporate Zn. And after that the 
temperature of the Ga solution 17a and the substrate 10 with the layer(s) 
formed thereon are still further adjusted downward. In this condition, 
vapor Zn is carried on the Hydrogen stream to form a p-type GaP layer 14 
doped with Zn on the Nitrogen doped n-type GaP layer 13 (FIG. 1(I)). As 
shown in FIG. 1(I), the GaP light emitting layer composite 19 of the GaP 
epitaxial wafer for green light emitting diodes consists of the three 
layers of the n-type layer 12, the Nitrogen doped n-type GaP layer 13 and 
the p-type GaP layer 14. 
In such a manner, 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 
consecutively in that order formed on the n-type GaP single crystal 
substrate 10 and thereby the GaP epitaxial wafer for green light emitting 
diodes is produced. The thus produced GaP epitaxial wafer is further 
processed into green light emitting diodes in a device fabrication 
process. 
GaP epitaxial wafers for green light emitting diodes produced by the method 
according to the present invention and those produced by a conventional 
method, where the same steps of processing are adopted except for the 
elimination of surface irregularities of a layered GaP substrate prior to 
the following growth of a GaP light emitting layer composite, were both 
evaluated to compare the respective characteristics, as shown in Table 1, 
which is described below, 
TABLE 1 
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Mechano- Defect Density in Light 
chemical Emitting layer 
Polishing 
Relative Composite 
Step Brightness 
Defects/cm.sup.2 
______________________________________ 
Embodiment 
yes 93.0 1 .about. 3 .times. 10.sup.4 
of Invention 
Comparison 
no 73.8 1 .about. 5 .times. 10.sup.6 
______________________________________ 
where each pair of the brightness and defect density shown in the table 1 
is the respective averages computed from the data of 100 wafers produced 
in five batches at 20 wafers a batch according to the method of the 
present invention or the conventional method. 
As clearly understood from the values of the brightness and defect density 
in the table 1, the brightness achieved by the method according to the 
present invention is improved by about 26% over that by the conventional 
method. This improvement is inferred to be realized due to betterment in 
the crystallinity of a light emitting layer composite or in other words 
preventing it from being deteriorated. 
Further the method according to the present invention was applied for 
production of a GaP epitaxial wafer for red emitting diodes and a GaP 
epitaxial wafer for pure green light emitting diodes and as a result about 
25% respective improvements on the average in brightness were achieved in 
both groups of the light emitting diodes over the brightness on the 
average from the comparative samples thereof.