Epitaxial substrate for high-intensity led, and method of manufacturing same

An epitaxial layer having a double-hetero structure is forming using an MOCVD process or an MBE process, and an epitaxial substrate is formed using an LPE process, thereby forming a substrate which exploits the distinguishing features of both processes. Since the MOCVD process or MBE process exhibits mixed-crystal ratio and film thickness controllability, excellent reproducibility and uniformity are obtained when forming the double-hetero structure on a compound semiconductor substrate. Since the growth process takes place under thermal non-equilibrium, the amount of impurity doping is raised to more than 10.sup.19 cm.sup.3. This is advantageous in terms of forming an electrode contact layer. With the LPE process, the material dissolved in the melt is grown epitaxially on the substrate by slow cooling, and the rate of growth is high. This process is suitable for forming the substrate after removal of the compound semiconductor substrate. By virtue of this liquid phase epitaxy, an oxide film preventing layer is removed by raising the degree of unsaturation of the melt, the epitaxial layer is grown in a short period of time, and a high-quality, highly uniform epitaxial substrate can be quickly manufactured. It is also possible to reduce cost.

BACKGROUND OF THE INVENTION 
This invention relates to a high-intensity LED epitaxial substrate, which 
is of the type having a double-hetero structure, manufactured by joint use 
of vapor phase epitaxy (an MOCVD process or MBE process) and liquid phase 
epitaxy (an LPE process), and to a method of manufacturing the substrate. 
In order to epitaxially grow a substrate for, say, a red-light emitting 
high-intensity LED in the prior art, first a p-type layer of Al.sub.0.75 
Ga.sub.0.25 As is formed as a p-type cladding layer to a thickness of 200 
microns on a p-type GaAs substrate [(100) surface] by the LPE process. 
This is followed by forming a p-type Al.sub.0.35 Ga.sub.0.65 As layer as a 
p-type active layer to a thickness of 2-3 microns, and then an n-type 
Te-doped Al.sub.0.75 Ga.sub.0.25 As layer as an n-type cladding layer to a 
thickness of 50 microns. Next, a GaAs substrate-selective etchant (e.g., 
NH.sub.4 OH:H.sub.2 O.sub.2 =1.7) is used to remove the light-absorptive 
GaAs substrate, thereby providing a high-intensity LED chip. 
Though the LPE process exhibits a high growth rate and therefore is 
suitable for forming layers that are thick, it is difficult to control 
thickness and carrier concentration. Consequently, when an epitaxial 
substrate having a double-hetero structure is fabricated solely by the LPE 
process, a variance in thickness and carrier concentration within the 
wafer surface tends to occur when the active layer is formed. As a result, 
stable luminance cannot be obtained. 
Furthermore, attempting to grow a mechanically strong and thick (about 200 
microns) mixed-crystal substrate solely by the MOCVD or MBE process is 
impractical since it involves a prolonged period of time and high cost. 
SUMMARY OF THE INVENTION 
A principal object of the invention is to manufacture a high-quality and 
highly uniform epitaxial substrate in a highly efficient manner. 
Another object of the invention is to make joint use of the MOCVD process 
or MBE process and the LPE process to exploit the advantages of both 
processes. 
Still another object of the invention is to form a substrate, at a high 
rate of growth, in which the layer thickness and carrier concentration of 
an epitaxially grown layer are uniformalized. 
A further object of the invention is to manufacture an epitaxial substrate 
which is high in quality, highly uniform and low in cost. 
At the present time, epitaxial growth of a high-intensity LED substrate 
largely relies upon AlGaAs, and in most cases the LPE process is employed. 
The LPE process features a rapid rate of growth and therefore is 
advantageous in forming thick layers. Though it is effective to adopt a 
double-hetero structure in order to achieve a high-intensity LED, control 
of layer thickness involves considerable difficulties when relying upon 
the LPE process. 
On the other hand, the MOCVD process or MBE process is advantageous for the 
epitaxial growth of a high-intensity LED substrate since epitaxial layer 
thickness can be accurately controlled for uniformity within the wafer 
surface while carrier concentration is also controlled. 
In many cases, however, a GaAs substrate is used as the substrate for 
epitaxial growth. Therefore, when it is attempted to also obtain higher 
intensity light, it is necessary to remove the GaAs substrate owing to its 
visible light-absorbing property. Accordingly, attempting to grow a thick 
mixed-crystal substrate (e.g., a substrate having a thickness of about 200 
microns) solely by the MOCVD process or MBE process in order to furnish 
mechanical strength involves considerable time and high cost. This makes 
the effort impractical. 
The present invention combines the merit of the MOCVD or MBE process, 
namely the controllability of the layer thickness and carrier 
concentration of the epitaxially grown layer, with the merit of the LPE 
process, namely the high rate of growth. As shown in (a) of FIG. 1, an 
n.sub.+ -type contact layer 2, an n-type cladding layer 3, a p-type active 
layer 4, a p-type cladding layer 5 and an anti-oxidation layer 6 are 
formed on a compound semiconductor substrate 1 by vapor phase epitaxy, 
thereby forming a double-hetero structure. Next, as shown in (b) of FIG. 
1, a thick epitaxial layer 7 is formed on the oxidation-preventing layer 
(removed by melt-back) by liquid phase epitaxy, after which the compound 
semiconductor substrate, which is highly absorptive of the wavelength of 
emitted light, is removed. This makes it possible to reduce cost greatly. 
Other features and advantages of the present invention will be apparent 
from the following description taken in conjunction with the accompanying 
drawings, in which like reference characters designate the same or similar 
parts throughout the figures thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will now be described with reference 
to the drawings. 
FIG. 2 is a view illustrating an embodiment of the present invention. The 
structure shown in FIG. 2 includes a GaAs substrate 1, an n.sub.+ -type 
Al.sub.0.75 Ga.sub.0.25 As layer 2, an n-type Al.sub.0.75 Ga.sub.0.25 As 
cladding layer 3, a p-type Al.sub.0.35 Ga.sub.0.65 As active layer 4, a 
p-type Al.sub.0.6 Ga.sub.0.4 As cladding layer 5, and a p-type GaAs layer 
6. 
In a first step according to the present invention, an n.sub.+ -type 
Al.sub.0.75 Ga.sub.0.25 As layer having a carrier concentration of '33 
10.sup.19 cm.sup.-3 and a thickness of 5 microns, an n-type Al.sub.0.75 
Ga.sub.0.25 As cladding layer having a carrier concentration of 
2.times.10.sup.17 cm.sup.-3 and a thickness of 5 microns, a p-type 
Al.sub.0.35 Ga.sub.0.65 As active layer having a carrier concentration of 
5.times.10.sup.16 cm.sup.-3 and a thickness of 1 micron, a p-type 
Al.sub.0.75 Ga.sub.0.25 As cladding layer having a carrier concentration 
of 1.times.10.sup.18 cm.sup.-3 and a thickness of 10 microns, and a p-type 
GaAs layer having a carrier concentration of 1.times.10.sup.18 cm.sup.-3 
and a thickness of 1 micron are formed, in the order mentioned, on a 300 
micron-thick GaAs substrate which is 2.degree. off the (100) surface. 
These layers are formed by vapor phase epitaxy (an MOCVd process, MBE 
process, etc.) at a substrate temperature of 750.degree. C. using 
trimethyl gallium [Ga(CH.sub.3).sub.3 ], trimethyl aluminum 
[Al(CH.sub.3).sub.3 ], hydrogen selenide (H.sub.2 Se) and diethyl zinc 
[C.sub.2 H.sub.5).sub.2 ] as the gases. 
Next, a Zn-doped, p-type Al.sub.0.6 Ga.sub.0.4 As layer having a carrier 
concentration of 1.times.10.sup.18 cm.sup.-3 and a thickness of 120 
microns is formed on the substrate by an LPE process. The epitaxial 
conditions at this time are a temperature of 875.degree. C., with the melt 
being GaAs: 32.0 mg, Zn: 1.7 mg, Al: 6.3 mg in 1 g of Ga. At this time the 
p-type GaAs layer 6 (1 micron) formed by MOCVD is completely removed by 
melt-back, and a p-type Al.sub.0.75 Ga.sub.0.25 As layer is formed anew. 
Though the above embodiment is described in connection with an example in 
which a p-type Al.sub.0.6 Ga.sub.0.4 As layer is formed by the LPE 
process, it is not necessary to limit the invention to this material. The 
eptiaxial layer grown by the LPE process can be formed by any material 
that facilitates ohmic contact and that has a band gap less than that of 
the cladding layer of the double-hetero epitaxial layer and greater than 
that of double-hetero active layer so as not to absorb the emitted light. 
Example for Purpose of Comparison 
When the luminance of the LED chip fabricated in accordance with the 
present invention and the luminance of a conventional LED chip are 
compared in the wafer surface, the result is as shown in FIG. 3. 
FIG. 3(b) illustrates luminance (in units which are arbitrary) in the 
direction of the chip shown in FIG. 3(a). In the present invention, it 
will be understood that uniformity in the surface (especially at the wafer 
periphery) is improved by forming the active layer uniformly, and that the 
luminance level is also improved. 
Thus, in accordance with the invention as described above, uniformity of 
the double-hetero structure portion is improved by using the MOCVD process 
or MBE process, and a variance in luminance in the wafer surface can be 
reduced. In addition, by using the LPE process, the epitaxial substrate 
can be formed in a short period of time. Since the LPE-grown layer is only 
a single layer, the melt is of one type and it is possible to charge a 
number of sheets, as a result of which total cost can be reduced, in 
comparison with the conventional LPE process. Furthermore, since the GaAs 
substrate is removed an the n-type layer becomes the top layer, the 
requirement for mesa etching can be reduced to a thin 11 microns and yield 
can be raised when forming chips. 
Further, the GaAs substrate surface is polished and exhibits a mirror 
surface on which vapor phase epitaxy is performed. Therefore, the n.sup.+ 
-type AlGaAs surface is obtained as a mirror surface when the GaAs is 
removed, and handling as at the time of pattern formation is comparatively 
simple. Flattening as by polishing is unnecessary. 
As many apparently widely different embodiments of the present invention 
can be made without departing from the spirit and scope thereof, it is to 
be understood that the invention is not limited to the specific 
embodiments thereof except as defined in the appended claims.