CA1322040C - Epitaxial gallium arsenide semiconductor wafer and method of producing the same - Google Patents

Abstract

A semiconductor wafer having an epitaxial GaAs layer, including a monocrystalline Si substrate and at least one intermediate layer epitaxially grown on the major surface of the monocrystalline Si substrate, as a buffer layer for accommodating a lattice mismatch between the Si substrate and the epitaxial GaAs layer which is formed epitaxially on a major surface of a top layer of the at least one intermediate layer. The at least one intermediate layer may comprise a GaP layer or one or more GaP/GaAsP, GaAsP/GaAs superlattice layers. The wafer may be used to produce a semiconductor light emitting element which has a plurality of crystalline GaAs layers including a light emitting layer epitaxially grown on the GaAs layer on the intermediate layer. The wafer may also be used to produce a compound semiconductor device such as amplifying and switching elements, light emitting and receiving elements and photovolataic elements. Methods for producing the semiconductor wafer, light emitting element and compound semiconductor devices are also disclosed.

Description

TITLE OF THE INVENTION
~PITAXIAL GALLIUM ARS~NIDE SEMICONDUCTOR WAFER
AND M~T~OD OF PROD~CIhG T~E SAM~

BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to semiconductor wafers having an epitaxially grown gallium arsenide layer, useful for fabricating various GaAs compound semiconducting devices, and methods suitable for producing such semiconductor wafers and devices.
Discussion of the Related Art Group III-V compound semiconductors with a gallium arsenide (GaAs) layer have high carrier mobility, direct-gap band structure, and variable nature of band gap and lattice parameter or constant in the case of the compounds consisting of three or four elements. Owing to these characteristics, the Group III-V compound semiconductors have been enjoying expanding industrial applications for fabricating not only a variety of semiconductor elements such as high-speed transistors, laser diodes, light emitting diodes (LED), phototransistors, photodiodes and solar cells, but also integrated circuits incorporating these semiconductor elements. An active layer providing such an active element is formed in a monocrystalline substrate of gallium arsenide, by preferential diffusion or ion implantation 1322~4~

techni~ue, or alternatively as an epitaxial GaAs layer formed by epitaxy on the GaAs substrate crystal.
However, a monocrystalline gallium arsenide (GaAs) substrate suffers from some disadvantages over a monocrystalline silicon (Si) substrate, e.g., more difficulty in obtaining an ingot of a relatively large diameter, higher cost of production, and lower mechanical strength. In the light of these disadvantages of the monocrystalline gallium arsenide substrate, there has been an attempt to use a monocrystalline substrate of silicon, since the technologies for producing and processing its ingot are most advanced in the field of semiconductor materials. In this case, an active layer of gallium arsenide is epitaxially grown or formed by epitaxy on the surface of the monocrystalline silicon substrate. However, the thus obtained semiconductor wafer tends to have a lattice mismatch or dislocation of the epitaxial gallium arsenide active layer and the silicon substrate, and is not satisfactory in crystallinity of the active layer.

It is therefore a primary object of the present disclosure to provide a semiconductor wafer having a gallium arseni~e layer of excellent crystallinity formed by epitaxy on a monocrystalline silicon substrate, suitable for economical, efficient large-scale production of gallium arsenide semiconductor elements or devices including newly 13220~0 developed optical IC arranqements.
In an effort to an improved semiconductor wafer with a gallium arsenide layer epitaxially grown on a major surface of a monocrystalline silicon substrate, the present inventors have conducted extensive studies and investigations on an intermediate buffer layer to be interposed between the silicon substrate and the epitaxial gallium arsenide layer for accommodating a lattice mismatch therebetween, and on an off angle at which the major surface of the silicon substrate is inclined with respect to a given plane (100) of the crystal.
The studies and investigations revealed that the crystallinity of an epitaxially grown gallium arsenide layer was enhanced if the major surface of the monocrystalline silicon substrate was inclined to (100) at an angle within a selected optimum range. Further, it was found that the crystallinity of the gallium arsenide layer was improved with selected materials of the intermediate layer, or where the intermediate layer includes a ; 20 superlattice layer or layers of a selected combination or combinations of materials.
In the meantime, the inventors' studies and investigations indicated an improvement in the crystallinity of the epitaxial gallium arsenide layer, where the intermediate buffer layer consists of a plurality :: :
of layers including a layer of gallium phosphide (GaP), at least one gallium phosphide/gallium arsenide-phosphide '' .

,:

1322~0 (GaP/GaAsP) superlattice layer, and at least one gallium .arsenide-phosphide/gallium arsenide (GaAsP/GaAs) superlattice layer, such that the GaP layer and the GaP/GaAsP and GaAsP/GaAs superlattice layers are superposed on each other in the order of description as seen in the direction from the major surface of the silicon substrate. The inventors also recognized a further improvement in the crystallinity of the epitaxial gallium arsenide layer where the off angle of the major surface of the silicon substrate relative to (100) is held within a given range.
The present invention was developed as a result of the above findings of the inventors.
The crystallinity of the epitaxial gallium arsenide (GaAs) layer formed on the silicon (Si) substrate was evaluated by measuring the surface roughness of the GaAs layer by a contact-type surface roughness tester, and the value (meV) of photoluminescence full-width at half maximum. The specified optimum range (0.5 to 5 degrees) of the off angle of the Si substrate permits the epitaxial GaAs layer embodying the invention to have a higher degree of crystallinity, and a smaller, than those of a GaAs layer to be grown on the Si substrate of (100) orientation (commonly used as a major surface on which the GaAs layer is formed). The off angle range is preferably between 1 and 3, and more preferably in the neighborhood of 2.
The novel epitaxially grown gallium arsenide layer is S given an enhanced level of crystallinity, thanks to the selected off angle range between 0.5 and 5 of the Si substrate to (100), and to the provision of at least one intermediate buffer layer which is formed by epitaxy on the major surface of the Si substrate, and on which the gallium arsenide layer is formed by epitaxy. Accordingly, the economical large-scale production of semiconductor wafers is permitted using a silicon substrate and a gallium arsenide active layer, which are suitable for fabricating various high-performance semiconductor elements, integrated circuits, and semiconductor devices such as optical integrated circuits, and other hybrid integrated circuits which are combinations of conventional silicon ICs and Ground III-V
compound gallium arsenide semiconductor ICs.
According to one advantageous feature, the at least one intermediate layer consists of a single gallium phosphide (GaP) layer. The at least one intermediate layer may include at least one superlattice layer.
According to another advantageous feature, the at least one intermediate layer consists of at least one GaP/GaAsP
superlattice layer formed on the Si substrate, 13220~0 and at least one GaAsP/GaAs superlattice layer which is formed on the at least one GaP/GaAsP superlattice layer and on which the epitaxial GaAs layer is formed.
According to a further advantageous feature, there is formed a single GaP layer, or there are formed an aluminum phosphide (AlP) layer and an aluminum-gallium phosphide (AlGaP) layer, between the Si substrate and the above-indicated at least one GaP/GaAsP superlattice layer.
In the semiconductor wafer, where the second GaAs layer is foxmed on the intermediate buffer layers which consist of the GaP layer, the GaP/GaAsP and GaAsP/GaAs superlattice layers, all of these intermediate layers are superposed on each other in the order of description as seen in the direction from the major surface of the Si substrate. The provision of these intermediate layers contribute to improved crystallinity of the epitaxial GaAs layer formed on top of the intermediate layers. Therefore, this semiconductor wafer using a silicon substrate and an epitaxial gallium arsenide active layer is suitably applied for the fabrication of various high-quality semiconductor elements, integrated circuits, and optical integrated circuits and other hybrid integrated circuits which are combinations of conventional silicon ICs and Ground III-V compound gallium arsenide ICs. For example, the semiconductor wafer is used for bi-polar transistors, field effect ~322~40 transistors, light emitting diodes, laser diodes, light receiving elements, solar cells, and other semiconductor elements, and is further used for integrated circuits incorporating such semiconductor elements in combination.
In accordance with a first aspect of the present invention there is provided a semiconductor wafer having an epitaxial GaAs layer, comprising:
a monocrystalline Si substrate; and at least one intermediate layer epitaxially grown on a major surface of said monocrystalline Si substrate, as a buffer layer for accommodating a lattice mismatch between said Si substrate and said epitaxial GaAs layer:
said at least one intermediate layer consisting of: a GaP
layer formed on said Si substrate; at least one GaP/GaAsP
superlattice layer formed on said GaP layer; and at least one GaAsP/GaAs superlattice layer formed on said at least one GaP/GaAsP superlattice layer, said epitaxial GaAs layer being formed on said at least one GaAsP/GaAs superlattice layer; and said at least one GaP/GaAsP superlattice layer and said at least one GaAsP/GaAs superlattice layer having an average lattice constant between the lattice constant of said Si substrate and the lattice constant of said GaAs layer.
In accordance with a second aspect of the present invention there is also provided a method of producing a semiconductor wafer, comprising the steps of:
preparing a monocrystalline Si substrate;
,~ .

forming a GaP layer by epitaxy on a major surface of said monocrystallic Si substrate;
forming at least one GaP/GaAsP superlattice layer by epitaxy on said GaP layer;
forming at least one GaAsP/GaAs superlattice layer by epitaxy on said at least one GaP/GaAsP superlattice layer; and forming a GaAs layer by epitaxy on said at least one GaAsP/GaAs superlattice layer, wherein said at least one GaP/GaAsP superlattice layer and said at least one GaAsP/GaAs superlattice layer has an average lattice constant between the lattice constant of said Si substrate and the lattice constant of said GaAs layer.
In accordance with a third aspect of the present invention there is provided a semiconductor light emitting element having an epitaxial GaAs layer, comprising:
a monocrystalline Si substrate;
at least one intermediate layer epitaxially grown on a major surface of said monocrystalline Si substrate, as a buffer layer for accommodating a lattice mismatch between said Si substrate and said epitaxial GaAs layer, said epitaxial GaAs layer being epitaxially grown on a ma~or surfaae of a top layer of said at least one intermediate layer;
said at least one intermediate layer consisting of: a GaP
: layer formed on said Si substrate: at least one GaP/GaAsP
superlattice layer formed on said GaP layer: and at least one A GaA~P/GaAs superlattice layer formed on said at least one .~ .

~ -1322Q~
- 8a -GaP/GaAsP superlattice layer, said epitaxial GaAs layer being formed on said at least one GaAsP/GaAs superlattice layer:
said at least one GaP/GaAsP superlattice layer and said at least one GaAsP/GaAs superlattice layer having an average lattice constant between the lattice constant of said Si substrate and the lattice constant of said GaAs layer: and a plurality of semiconductor crystalline layers containing GaAs as base elements, which are epitaxially grown on said epitaxial GaAs layer and superposed on each other, said - 10 crystalline layers including a light emitting layer.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 is a cross sectional view of a novel semiconductor wafer;

: .....
~ h ,.... .

g Fig. 2 is a graphical representation of measurements indicating the surface roughness of an epitaxial GaAs layer of the wafer versus the off angle of a Si substrate of the wafer;
Fig. 3 is a cross sectional view of another semiconductor ~wafer embodying the invention;
Fig. 4 is a graphical representation of measurements indicating the value (meV) of photoluminescence full-width at half maximum of an epitaxial GaAs layer of the wafer of Fig. 3 versus the off angle of a Si substrate of the wafer;
Fig. 5 is a cross sectional view of a further novel semiconductor wafer;
Fig. 6 is a cross sectional view of an embodiment of the invention in the form of a laser diode;
Fig. 7 is a graph showing an input-output characteristic curve of the laser diode of Fig. 6;
Fig. 8 is a graph showing a spectrum of a laser beam produced by the laser diode;
Fig. 9 is a cross sectional view of an embodiment of the invention in the form of a photodiode;
Fig. lO is a graph showing the quantum efficiency versus wavelength of a light beam emitted by the photodiode of Fig. 9;
Fig. 11 is a cross sectional view of a further embodiment of the invention in the form of a solar cell; and Fig. 12 is a graph showing spectral sensitivity curves of the solar cell of Fig. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following examples are given for illustrative purpose only, to further clarify the principle$ of the present invention, referring to the accompanying drawings.
Examle 1 Referring first to Fig. 1, there is shown in cross section one inventi~e embodiment of a semiconductor wafer. In the figure, reference numeral 10 designates an n-type monocrystalline silicon (Si) substrate. On this Si substrate are formed an intermediate gallium phosphide (GaP) layer 30 and an epitaxial gallium arsenide (GaAs) layer 50. The monocrystalline Si substrate 10 has a thickness of 300 microns.
The GaP layer 30 is 0.5 micron thick, while the GaAs layer 50 is

2 microns thick. The GaP and GaAs layers 30, 50 were formed continuously by epitaxy on the Si substrate, in a metalorganic chemical vapor deposition process (MOCVD), in a reaction furnace in the form of a horizontal induction-heating furnace under the ~ 25 atmospheric pressure. As the materials for the GaP and : .

11- 1322~0 GaAs layers 30, 50, trimethylgallium (TMGa, Ga(Ch3)3), arsine (AsH3) and phosphine (PH3) were used. The flow rates of these gases were controlled accurately by a flow control device so that the epitaxial growth of the GaP and GaAs layers may occur at a suitable rate. No pre-cracking of the Group V elements was effected. Prior to starting the epitaxy, the Si subctrate 10 was first annealed at 1000C
in hydrogen for ten minutes, to remove an oxide layer from its surface. Subsequently, the Si substrate 10 was ,10 maintained at 900C to permit an epitaxial growth of GaP
into the intermediate GaP layer 30, and thereafter maintained at 650C to allow an epitaxial growth of GaAs into the GaAs layer 50.
In the above-described manner, a number of semiconductor wafers were prepared, with different off angles to (100) of the major surface 11 of their Si substrates 10.
- The roughness of an exposed surface 51 of the epitaxial GaAs layer 50 of each wafer was measured by a contact type surface roughness tester. The measurements of the individual wafers are shown in Fig. 2, wherein the off angle ~ (degree) of the major surface 11 of the Si substrate 10 to (100) is taken along the abscissa, while the peak-to-peak roughness (Anqstrom) of the surface 51 of the GaAs layer is taken along the ordinate~ As is apparent from the graph in Fig. 2, the smoothness of the surface 51 of the wafers with the substrate off angle within a range between 0.5 to 5 is improved over that with the substrate - 12 _ 1322040 off angle of o. The best result is obtained with the off an~le range from 1 to 2, within which the roughness of the surface 51 :is less than 50 Angstroms, more than three times as small as 150 Angstroms in the case of the off angle of 0.
ExamPle 2 Referring next to Fig. 3, there is shown in cross section another example of a semiconductor wafer wherein reference numeral 10 designates an n-type monocrystalline Si substrate on which a plurality of intermediate layers 30 are formed, so as to accommodate a lattice mismatch bet~een the Si substrate 10 and an epitaxial GaAs layer 50 formed on the intermediate layers 30. This Example 2 is different from Example 1 in that the four intermediate layers 30 are used in Example 2, in place of the single intermediate GaP layer 30 used in Example 1. Described more specifically, the intermediate layers 30 of Example 2 consist of: a first intermediate layer in the form of an aluminum phosphide (AlP) layer 31 which is easily epitaxially gro~n on the Si substrate 10, with a high ; degree of adherence thereto; a second intermediate layer in the form of an aluminum gallium phosphide (Alo 5GaO 5P) layer 32 formed on the first intermediate AlP layer 31;
a third intermediate layer in the form of ten superlattice layers 33 of gallium phosphide (GaP) and gallium arsenide-phosphide (GaAsO 5Po 5), formed on the second intermediate Alo 5GaO 5P layer 32; and a fourth intermediate layer in the form of ten suerlattice layers 34 of gallium arsenide (GaAsO 5Po 5) and gallium arsenide (GaAs), formed on the third intermediate GaP/GaAsO 5P0 5 superlattice layer 33. The Si substrate 10 is 300 microns thick, and the epitaxial GaP layer 50 is 2 microns thick. The four intermediate layers 30 have a total thickness of 0.42 micron. Each of the first and second intermediate layers 31, 32 is 100 Angstroms thick. Each of the ten third intermediate GaP/GaAsO 5P0 5 superlattice layers 33, and each of the ten fourth intermediate GaAsO 5P0 5/GaAs superlattice layers 34 have a thickness of 200 Angstroms.
As in Example 1, a metalorganic chemical vapor deposition process (MOCVD) was employed to form the intermediate layers 30 and the GaAs layer 50. In addition to the materials used in Example 1, trimethylaluminum (TMAl.Al(CH3)3~ was used. The temperature during the epitaxial growth of the first through fourth intermediate layers 31-34 was 830C, and the temperature during the 20 epitaxial growth of the GaAs layer 50 was 730 C. The other conditions were the same as in Example 1.
In the above-indicated manner, a number of ;
semiconductor wafers were prepared, with different off ~ angles to (100) of the major surface 11 of t~eir Si -~ 25 substrates 10.

; The photoluminescense full-width at half maximum ~ImeV) was measured for each wafer prepared. The . , .
" .

. ~ , .
' . , . :

.. . .

- lq - i322~40 measurements of the individual wafers are shown in Fig. 4.
As indicated in the figure, the meV value is lower with the substrate off angle within a range between 0.5 to 5, than that with the substate off angle of O. This means improved crystallinity of the epitaxial GaAs layer 50 of the present wafers. Comparatively good results were obtained with the Si substrates 10 having an off angle within a range from 1 to 4, and the best results were obtained with the Si substrate 10 with an off angle in the neightborhood of 2.
ExamPle 3 Semiconductor wafers were prepared according to Example 2 of Fig. 3, but without the first and second intermediate layers 31 and 32. Photoluminescense tests conducted on these wafers showed substantially the same results as in Example 2. That is, the tests revealed improvement in the crystallinity of the epitaxial GaAs layer 50 where the off angle of the Si substrate 10 is within the range between 0.5 and 5 .
Example 4 In Bxample 4, the first and second intermediate layers 31 and 32 of the wafer of Fig. 3 were replaced by a single GaP layer having a thickness of 0.5 micron.
Photoluminescense tests showed similar results, i.e., improved crystallinity of the GaAs layer 50 on the wafers ; 25 whose Si substrates 50 have an off angle between 0.5 and 5, - 15 ~ 1322~
Example 5 This example was given to manifest that the crystallinity of the epitaxial GaAs layer 50 may be improved even when the off angle of the Si substrate 10 is 0.
A wafer prepared according to Example 5 is depicted in Fig. 5. The wafer includes a plurality of intermediate layers 30 to accommodate a lattice mismatch bêtween an n-type monocrystalline (100)-oriented Si substrate 10 (with an off angle of 0) and an epitaxial GaAs layer 50. The intermediate layers 30 consist of: a first intermediate layer in the form of a GaP layer formed on the Si substrate 10; a second intermediate layer in the form of ten superlattice layers 32 of gallium phosphide (GaP) and gallium arsenide-phosphide (GaAsO 5Po 5), formed on the GaP layer 31; and a third intermediate layer in the form of ten superlattice layers 33 of gallium arsenide (GaAsO 5Po 5~ and gallium arsenide (GaAs), formed on the second intermediate GaP/GaAsO 5Po 5 superlattice layer 33. The Si substrate lO is 30Q microns thick, and the epitaxial GaP layer 50 is 2 microns thick. The three 20 intermediate layers 30 (31, 32, 33) have a total thickness of 0.9 micron. The first intermediate layer 31 is 0.5 micron thick. Each of the ten second intermediate GaP/GaAsO 5Po 5 superlattice layers 32, and each of the ten third intermediate GaAsO 5Po 5/GaAs superlattice layers 33 have a thickness of 200 Angstroms.
The intermediate layers 30 and the epitaxial GaAs 16 - 13220~
layers were formed continuously by epitaxy on the Si substrate 10, in a metalorganic chemical vapor deposition process (MOCVD), within a horizontal induction-heating furnace under the atmospheric pressure. As the materials for the vapor deposition, trimethylgallium (TMGa, Ga(Ch3)3~, arsine (AsH3) and phosphine (PH3) were employed.
The overall flow rate of these gases was 2.5 liters/min., and the flow rate of each individual gas was controlled accurately by a flow control device so as to achieve an epitaxial growth rate of 0.1 micron/min. No pre-cracking of the Group V elements was effected. Prior to starting the epitaxy, the Si substrate 10 was first annealed at 950 C
with a flow of PH3 at lOOcc/min. Thereafter, the Si substrate 10 was maintained at 900C to permit an epitaxial growth of GaP into the first intermediate GaP layer 31, and then at 830C to allow epitaxial growth of the second and third intermediate superlattice layers 32, 33.
Subsequently, the epitaxial growth temperature was lowered down to 650C, to grow the GaAs layer 50 on the GaAsO 5Po 5/GaAs superlattice layer 33. The annealing temperature of the Si substrate 10 may be selected within a range from 600 C to 1100C, and the annealing period during which the Si substrate 10 is exposed to PH3 may be selected within a range from 1 to 60 minutes.
In the above-described manner, the semiconductor wafer of Fig. 5 (Sample No. 1) was prepared, and its photolumi escense characteristic was compared with those ot ' :

:

- 17 - 1322~0 Comparative Samples Nos. 2-4. The measured photoluminescense full-width at half maximum (meV) at 150K
are indicated in the following table.

Samples Composition P~ Full-width at half maximum (meV) No. 1 GaAs/(GaAsP/GaAs)/ 45 (Example 5) (GaP/GaAsP)/GaP/Si No. 2 GaAs/(GaAsP/GaAs)/ 52 (GaP/GaAsP)/Si _ _ 10 No. 3 GaAs/Ge/Si 62 No. 4 GaAs/GaAs 33 It will be understood from the above table that Comparative Sample No. 3 using a single intermediate layer of Ge had the largest measurement of 62 meV, and that the second largent measurement (52 meV) was obtained on Comparative Sample No. 2 which uses GaP/GaAsO 5Po 5 superlattice layers and GaAsO.5PO.5/GaAs superlattice layers. Sample No. 1 of the present Example 5 had a value of 45 meV. This value is considered to indicate a relatively high degree of crystallinity of the GaAs layer - 50 of Sample No. l, in view of the fact that Comparative ~; Sample No. 4 which uses a GaAs substrate for an epitaxial GaAs layer had a value of 33 meV.
Example 6 Referring next to Fig. 6, there is shown another :

embodiment of the present invention in the form of a semiconductor laser diode. In the figure, reference numeral 10 designates an n-type monocrystalline Si substrate having a major surface which is inclined at 2 with respect to the 5 ~100) orientation. Reference numeral 30 generally indicates a plurality of intermediate layers, while reference numeral 50 indicates an n-type epitaxially grown GaAs layer. On the epitaxial GaAs layer 50, the following layers are superposed on each other in the order of description:
n-GaO 7Alo 3As cladding layer 52; i-GaAs active layer 54;
p-GaO 7Alo 3As cladding layer 56; P-GaAs cap layer 58; Sio2 insulating layer 60; and Au-Zn electrode 62 (anode). An Au-Sn electrode 64 (cathode) is also formed on the epitaxial n-GaAs layer 50. The thicknesses of the layers 10, 30, 50, 52, 54, 56 and 58 are as follows:
Si substrate 10: 300 microns Intermediate layers 30: 0.25 micron (total) GaAs layer 50: 2 microns Cladding layer 52: 1.5 microns 20 Active layer 54: 0.1 micron Cladding layer 56; 1.5 microns Cap layer 58: 0.5 micron The intermediate layers 30 consist of: a first intermediate layer in the form of a single GaP layer 31 ; 25 formed on the Si substrate 10; a second intermediate layer -~ in the form of five Gap/GaAs0 5Po 5 superlattice layers 32 formed on the GaP layer 31 and a third intermediate layer ;`
.

' , in the form of five GaAsO 5P0 5/GaAs superlattice layers 33 formed on the second intermediate layer 32. The first intermediate GaP layer 31 has a thickness of 500 Angstroms, and each superlattice layer of the second and third intermediate layers 32, 33 has a thickness of 200 Angstroms.
The layers 30, 50, 52, 54, 56 and 58 were formed continuously on each other by epitaxy on the Si substrate 10, in a metalorganic chemical vapor deposition process (MOCV~), in a horizontal induction-heating furnace under the atmospheric pressure. As the materials for the vapor deposition, trimethylgallium (TMGa, Ga(Ch3)3), trimethylaluminum (T~Al.Al(CH3)3), arsine (AsH3) and phosphine (PH3) were employed. As p-type and n-type dopants, dietylzinc (DEZn) and hydrogen serenide (H2Se) were used. The flow rates of these gases were controlled accurately by a flow control device so as to achieve a suitable rate of epitaxial growth of crystals. No pre-cracking of the Group V elements was effected. Prior to starting the epitaxy, the Si substrate 10 was first annealed at 950C with a flow of PH3 at lOOcc/min., to remove the oxide layer from its surface. Thereafter, the Si substrate 10 was maintained at 900C to permit an epitaxial ;~ growth of a GaP crystal into the first intermediate GaP
layer 31, and then at 700 C to allow epitaxial growth of `~ the second and third intermediate superlattice layers 32, 33. The epitaxial gr~wth temperat~re of 700 was f~rther .

.

.
.~ , , . .. ' ~ .

--: ' ' ' . ., ', 1322~l10 maintained to grow the n-GaAs layer, n-GaO 7Alo 3As cladding layer 52, Ga~s active layer 54, p-GaO 7Alo 3As cladding layer 56 and p-GaAs cap layer 58. Subsequently, the surface of the p-GaAs cap layer 58 was coated with a photoresist or light-sensitive coating. A selected area of the applied photoresist coating was covered with a mask, and the non-covered area of the photoresist coating was exposed to light, to provide a mask for a subsequent .
etching process. The portions of the layers 52, 54, 56, 58 corresponding to the exposed area of the photoresist were removed in the etching process, whereby a portion of the surface of the n-GaAs layer 50 was exposed, as shown in Fîg. 6. Then, Sio2 was applied to the surface of the p-GaAs cap layer 58, to form the Sio2 insulating layer Ç0. A
recess was formed in the insulating layer 60 in a known photolithographic manner, and the Au-Zn electrode 62 (anode) was formed by vapor deposition on the insulating layer 60. Similarly, the Au-Zn electrode 64 (cathode) was formed by vapor deposition and diffusion on the exposed surface of the n-GaAs layer. The thus prepared multilayered structure was cut into a plurality of laser diodes, such that each diode had a laser emitting face Tbe laser diodes were activated to produce a lasing action. The input-output characteristic curve is shown in Fig. 7. The threshold value of current was 460 mA
The spectrum of the emitted light is given in Fig. 8.

1322~40 Example 7 Referring further to Fig. 9, there is shown a further embodiment of the present invention in the form of a photodiode. In the figure, reference numeral 13 indicates an n-type monocrystalline Si substrate having a major surface which is inclined at 2 with respect to (100). On this Si substrate 13, there are formed an intermediate n Gap layer 34, an epitaxial n GaAs layer 70, an epitaxial n GaAs layer 71, and an epitaxial p-GaAs layer 72. A Au-Zn electrode 73 and a Au-Sn electrode 74 are formed on the top surface of the p-GaAs layer 72 and the bottom surface of the Si substrate 13, respectively. The layers 70, 71 and 72 constitute a PI~ diode. The Si substrate 13 which has an initial thickness of 300 microns, was ground to 50 microns 15 after the layers 34, 70-72 had been formed thereon by epitaxy. Thicknesses of the epitaxially grown layers 34, 71, 72 and 73 are 1 micron, 0.5 micron, 2.5 microns and 0.5 micron, respectively. These layers were formed in an MOCVD
process as practiced in the preceding Examples. The 20 intermediate layer 34 was grown at 900C, and the layers 71-73 were grown at 650C. After these layers were formed by epitaxy, the electrodes 73, 74 were vapor deposited, and the prepared multilayered structure was cut into a plurality of photodiodes, such that each photodiode had a light receiving face. The quantum efficiency (emitted energy/incident energy) of the thus produced photodiodes, in relation to the wavelength, is shown in Eig. 10.

-~

- 22 - 1322~40 Example 8 Referring next to Fig. 11, there is illustrated yet another embodiment of the invention in the ~orm of a solar cell. In the figure, reference numeral 12 designates an n-type monocrystalline Si substrate having a major surface which is inclined at 2 to (100). On this Si substrate 12, there are epitaxially formed the following epitaxial layers: a first intermediate GaP layer 35; a second intermediate layer in the form of GaP/GaAsO 5Po 5 superlattice layers 36; a third intermediate layer in the form of GaAsO 5Po 5/GaAs superlattice layers 37; an n-GaAs layer 80; a p-GaAs layer 81; and p-GaO 2Alo 8As layer 82.
Reference numeral 84 designates a non-reflection SiO2 coating, and numerals 85 and 86 indicate Au-Zn and Al J~ 15 electrodes, respectively. The Si substrate 12 which has an initial thickness of 300 microns, was ground to 50 microns after the intermediate layers 35-37 and the layers 80-82, 84 had been epitaxially grown thereon. The first intermediate GaP layer 35 is 500 Angstroms thick. The 20 thicknesses of the layers 80, 81, 82 and 84 are 0.5 micron, 0.5 micron, 0.1 micron and 80 nanometers, respectively.
These epitaxial layers 35-37, 80-82 were formed in a metalorganic chemical vapor deposition process ~MOCVD~ as in the preceding Examples. The ~rowth temperature was 900 C
25 for the intermediate layer 35, and 650C for the layers ~; 80-82. After the layer 82 had been formed, Sio2 was applied by sputtering to form the non-reflection coating 84 on the - 23 - 1 322 ~40 layer 82. By photolithogray and etching techniques, recesses were formed in the selected areas of the coating 84 and the layer 82, in order to accommodate the Au-Zn electrodes 85 which were formed by vapor deposition. ~hen, the Si substrate 12 was ground to 50 microns as previously mentioned. The Al electrode 86 was subsequently formed by vapor deposition. The thus prepared solar cell using the silicon substrate 12 exhibited very excellent spectral sensitivity as shown in solid line in Fig. 12. Dasked line in the same figure represents a spectral sensitivity curve of a conventional solar cell using a GaAs substrate. The spectral sensitivity of the present Example 8 using the Si substrate 12 is comparable to that of the conventional cell using the GaAs substrate.
Although the gallium arsenide wafers employed for the semiconductor devices of Examples 6-8 use a silicon ; substrate whose major surface is inclined at 2 to ~100), the off angle of the substrates of these examples may be zero. In this case, too, the GaAs layers are given a satisfactory level of crystallinity. Further, the semiconductor devices of Examples 6-8 may use the semiconductor wafers of Examples 2 and 3.
As another modification, the GaAs light emitting layer 54 of Example 6 maty be replaced by a AlGaP layer.
While the present invention has been described in its preferred embodiments with a certain degree of particularity, it is to be understood that the invention is - 24 _ 1322~40 by no means limited thereto, but various changes, modifications and improvements may be made in the invention, in the light of the foregoing teachings, without departing from the spirit and scope of the invention defined in the appended claims.

~. -. , :; , , ' , .
, ............ . .

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A semiconductor wafer having an epitaxial GaAs layer, comprising:
a monocrystalline Si substrate; and at least one intermediate layer epitaxially grown on a major surface of said monocrystalline Si substrate, as a buffer layer for accommodating a lattice mismatch between said Si substrate and said epitaxial GaAs layer:
said at least one intermediate layer consisting of: a GaP
layer formed on said Si substrate; at least one GaP/GaAsP
superlattice layer formed on said GaP layer; and at least one GaAsP/GaAs superlattice layer formed on said at least one GaP/GaAsP superlattice layer, said epitaxial GaAs layer being formed on said at least one GaAsP/GaAs superlattice layer; and said at least one GaP/GaAsP superlattice layer and said at least one GaAsP/GaAs superlattice layer having an average lattice constant between the lattice constant of said Si substrate and the lattice constant of said GaAs layer.

2. A method of producing a semiconductor wafer, comprising the steps of:

preparing a monocrystalline Si substrate;
forming a GaP layer by epitaxy on a major surface of said monocrystallic Si substrate;
forming at least one GaP/GaAsP superlattice layer by epitaxy on said GaP layer;
forming at least one GaAsP/GaAs superlattice layer by epitaxy on said at least one GaP/GaAsP superlattice layer; and forming a GaAs layer by epitaxy on said at least one GaAsP/GaAs superlattice layer, wherein said at least one GaP/GaAsP superlattice layer and said at least one GaAsP/GaAs superlattice layer has an average lattice constant between the lattice constant of said Si substrate and the lattice constant of said GaAs layer.

3. A method according to claim 2, further comprising annealing said monocrystalline Si substrate by subjecting the Si substrate at a temperature between 600°C and 1100°C, to a flow of PH3 gas for a period ranging from 1 to 60 minutes.

4. A semiconductor light emitting element having an epitaxial GaAs layer, comprising:
a monocrystalline Si substrate;
at least one intermediate layer epitaxially grown on a major surface of said monocrystalline Si substrate, as a buffer layer for accommodating a lattice mismatch between said Si substrate and said epitaxial GaAs layer, said epitaxial GaAs layer being epitaxially grown on a major surface of a top layer of said at least one intermediate layer;
said at least one intermediate layer consisting of: a GaP
layer formed on said Si substrate; at least one GaP/GaAsP
superlattice layer formed on said GaP layer; and at least one GaAsP/GaAs superlattice layer formed on said at least one GaP/GaAsP superlattice layer, said epitaxial GaAs layer being formed on said at least one GaAsP/GaAs superlattice layer;
said at least one GaP/GaAsP superlattice layer and said at least one GaAsP/GaAs superlattice layer having an average lattice constant between the lattice constant of said Si substrate and the lattice constant of said GaAs layer: and a plurality of semiconductor crystalline layers containing GaAs as base elements, which are epitaxially grown on said epitaxial GaAs layer and superposed on each other, said crystalline layers including a light emitting layer.

5. A semiconductor light emitting element according to claim 4, wherein said major surface of said monocrystalline Si substrate is inclined at an off angle between 0.5° and 5° with respect to (100).

6. A semiconductor light emitting element according to claim 4, wherein said at least one intermediate layer consists of at least one GaP/GaAsP superlattice layer formed on said Si substrate, and at least one GaAsP/GaAs superlattice layer formed on said at least one GaP/GaAsP superlattice layer, said epitaxial GaAs layer being formed on said at least one GaAsP/GaAs superlattice layer.

7. A semiconductor light emitting element according to claim 4, wherein said at least one intermediate layer consists of: an AlP layer formed on said Si substrate; an AlGaP layer formed on said AlGaP layer; at least one GaP/GaAsP superlattice layer formed on said AlGaP layer; and at least one GaAsP/GaAs superlattice layer formed on said at least one GaP/GaAsP
superlattice layer, said epitaxial GaAs layer being formed on said at least one GaP/GaAsP superlattice layer.

8. A semiconductor light emitting element according to claim 4, wherein said at least one intermediate layer consists of: a GaP layer formed on said Si substrate; at least one GaP/GaAsP
superlattice layer formed on said GaP layer; and at least one GaAsP/GaAs superlattice layer formed on said at least one GaP/GaAsP superlattice layer, said epitaxial GaAs layer being formed on said at least one GaAsP/GaAs superlattice layer.

9. A semiconductor light emitting element according to claim 4, wherein said light emitting layer consists of a semiconductor crystalline layer containing AlGaP as base elements.