Abstract:
A method of growing a plurality of epitaxial layers each having a property which is different from each other simultaneously on a common substrate comprises steps of forming at least a first crystal surface and a second crystal surface which are crystallographically non-equivalent to each other on the substrate, introducing particles comprising constituent elements of the epitaxial layers into a region in the vicinity of the substrate, the particles including at least metal-organic molecules containing one of the elements constituting the epitaxial layers, decomposing the metal-organic molecules such that the layer constituting element therein is released as a result of the decomposition, and depositing the aforesaid particles including the element released by the decomposition of the metal-organic molecules on the first and second crystal surfaces so that a first epitaxial layer and a second epitaxial layer, respectively differing in properties from each other, are grown on respective the first and second crystal surfaces, the step of deposition being performed such that the growth of the first and second epitaxial layers is controlled by the decomposition of the metal-organic molecule.

Description:
This application is a continuation, of application number 08/262,356, filed Jun. 20, 1994, now abandoned, and a continuation of application Ser. No. 07/442,907 filed Nov. 28, 1989 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to growth of epitaxial layers and more particularly to a method of growing a plurality of epitaxial layers having different properties on a common substrate in a continuous epitaxial process, suitable for manufacturing a semiconductor device such as a stripe-type laser diode or a so-called graded-index waveguide separate-confinement heterostructure (GRIN-SCH) laser diode in which an active layer is surrounded by clad layers. 
     Laser diodes are currently used extensively in various fields such as optical telecommunications, optical information storage and reproduction on and from optical recording media such as compact disks or laser disks, optical measurement of various quantities, control of various systems and the like. 
     In most of these applications, a so-called stripe-type laser diode is used currently in which light is confined in a stripe-shaped narrow active layer surrounded vertically and laterally by clad layers. By constructing the laser diode as such, efficient oscillation of the laser is realized as a result of confinement of light into such a narrow active region. 
     In order to confine the light in the active layer, it is necessary to form the clad layers having a low refractive index such that the clad layers surround the active layer. This means that the active layer is not only sandwiched by a pair of clad layers, one at the top and one at the bottom but has to be bounded also laterally by a pair of clad layers. Further, in order to improve the efficiency of carrier injection, it is desired that the electric current, or carriers, injected to the active layer is confined laterally such that the carriers are concentrated in a part of the clad layer in contact with the active layer. For this purpose, it is necessary to form the clad layer into regions having different conductive types. 
     Conventionally, formation of such clad layers laterally bounding the active layer cannot be made simultaneously with the formation of the active layer. Similarly, formation of a clad layer laterally divided into a plurality of regions having different conductive types cannot be made immediately before or after the formation of the active layer without interrupting the process of crystal growth. Thus, when forming a structure having an active layer surrounded vertically and laterally by clad layers in an epitaxial growing system for growing crystal layers epitaxially, such as a molecular beam epitaxial (MBE) crystal growth system, a partially completed laser diode is taken out from the system and is subjected to various processes such as mesa formation, insulator film deposition, ion implantation, impurity doping and diffusion, and so on. After these processes are completed, the laser diode is returned to the epitaxial growing system for further formation of epitaxial layers. As will be easily understood, the step of taking out the partially completed laser diode from the epitaxial growing system for performing additional processes is undesirable as such a step, and the other processes performed outside the epitaxial growing system, tend to introduce defects in the laser diode. Thereby, the yield of the product is decreased. 
     Meanwhile, there is a so-called GRIN-SCH laser diode having a thin active layer of gallium arsenide (GaAs) forming an isolated quantum well structure. In this laser diode, a pair of clad layers of gallium aluminium arsenide (GaAlAs) are provided so as to sandwich the active layer and the composition of the clad layer is graded such that the content of aluminium (Al) is low at a side of the clad layer in contact with the active layer while the content is increased gradually as the distance from the active layer is increased. As a result of such a construction, the decrease of the optical confinement factor, indicative of the efficiency of confinement of light in the active layer, is prevented and associated therewith, the construction of a laser diode having a low threshold current density becomes possible. 
     Conventionally, such a graded clad layer used in the GRIN-SCH laser diode has been formed by changing the flow rate of a source gas of Al with respect to that of gallium (Ga) together with the progress of crystal growth of the clad layer when the clad layer is grown by a metal-organic chemical vapor deposition (MOCVD) technique. When the clad layer is grown by MBE, on the other hand, the temperature of a cell for holding a source material of Al and the temperature of a cell for holding a source material of Ga are changed such that a desired compositional profile is obtained. 
     In the presently available technique of MOCVD or MBE, however, there is a problem in that, although these techniques provide a satisfactory control of the Al content as long as the composition is constant throughout the clad layer, accurate control of the flow rate of the source gases or the temperature of the source materials, to a degree sufficient to provide a satisfactory compositional profile, is extremely difficult as such a control of the growth is indirect and not performed directly at the substrate where the growth is made. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful method of manufacturing a semiconductor device wherein the aforementioned problems are eliminated. 
     Another and more specific object of the present invention is to provide a method of growing epitaxial crystal layers wherein a plurality of epitaxial crystal layers different from each other are grown on a common substrate simultaneously. 
     Another object of the present invention is to provide a method of growing epitaxial crystal layers wherein a plurality of epitaxial crystal layers having different conduction types are grown on a common substrate simultaneously. 
     Another object of the present invention is to provide a method of manufacturing a laser diode having an active layer surrounded laterally and vertically by clad layers wherein the laser diode can be produced in an epitaxial growth system without taking the laser diode out of the system during the process of manufacturing. 
     Another object of the present invention is to provide a method of manufacturing a semiconductor device having a graded layer wherein the compositional profile of the graded layer is controlled accurately during the growth of the layer. 
     Another object of the present invention is to provide a method of manufacturing a laser diode having a GRIN-SCH structure, comprising an active layer of GaAs sandwiched by a pair of graded layers having a composition of Al x  Ga 1-x  As, wherein the content of Al is changed gradually through the graded layers in accordance with an accurately controlled compositional profile. 
     Another object of the present invention is to provide a method of growing epitaxial crystal layers comprising the steps of preparing a substrate having at least a first surface exposing a first crystal plane and a second surface exposing a second crystal plane which is different from the first crystal plane, releasing atoms forming the epitaxial crystal layers to be grown by decomposition of metal-organic molecules, and growing a first epitaxial layer and a second epitaxial layer simultaneously on said first surface and on said second surface by depositing the atoms. According to the present invention, different epitaxial layers can be grown on the substrate simultaneously by exposing different crystal planes at the surface of the substrate. 
     Another object of the present invention is to provide a method of growing epitaxial crystal layers having different conduction types by the steps of preparing a substrate having at least a first surface portion at which a first crystal plane is exposed and a second surface portion at which a second crystal plane different from the first crystal plane is exposed releasing atoms forming the epitaxial crystal layers to be grown by decomposition of metal-organic molecules, and growing a first epitaxial layer and a second epitaxial layer simultaneously on said first surface portion and on said second surface portion by depositing the atoms thus released, wherein the conduction type of the first and second epitaxial layers are made different by controlling the vapor pressure of a constituent element of the epitaxial layers at a predetermined value when the element is supplied in a form of the atoms. According to the present invention, the conduction type of various epitaxial layers grown simultaneously on the substrate can be controlled easily by controlling the vapor pressure of the foregoing constituent element. 
     Another object of the present invention is to provide a method of manufacturing a stripe-type laser diode comprising an active layer vertically bounded by first and second clad layers and laterally bounded by third and fourth clad layers, comprising the steps of processing a GaAs substrate defined by a (100) surface such that the substrate is further defined with a stripe-shaped surface portion exposing a selected one of a (311)A plane and a (311)B plane at a part of the (100) surface, growing the first clad layer having a composition Al x  Ga 1-x  As with a compositional parameter x set at a first value x1 on the substrate by performing a metal-organic molecular beam epitaxy for depositing at least Al and Ga at a first predetermined temperature, growing the active layer having a composition Al x  Ga 1-x  As with a compositional parameter x set at a second value x2 smaller than the foregoing first value x1 on a part of the first clad layer covering said stripe-shaped surface portion and simultaneously growing the third and fourth clad layers both having the composition Al x  Ga 1-x  As with a compositional parameter x set at a third value x3 larger than the foregoing second value x2 on the first clad layer, laterally and respectively on both opposite sides of the active layer by performing the foregoing metal-organic molecular beam epitaxy at a second predetermined temperature, and growing the second clad layer having a composition Al x  Ga 1-x  As with the compositional parameter x set at said first value x1 on said active layer and further on said third and fourth clad layers by performing the foregoing metal-organic molecular beam epitaxy at said first predetermined temperature. According to the present invention, the laser diode, having the active layer surrounded laterally and vertically by said first through fourth clad layers, can be manufactured in a metal-organic molecular beam epitaxial growth system without taking out the laser diode from the system during the step of manufacturing. 
     Another object of the present invention is to provide a method of manufacturing a stripe-type laser diode comprising an active layer vertically bounded by first and second clad layers and laterally bounded by third and fourth clad layers, comprising steps of processing a GaAs substrate defined by a (100) surface such that the substrate is further defined with a stripe-shaped surface portion exposing a selected one of a (311)A plane and a (311)B plane at a part of the (100) surface, growing a first buffer layer of p-type GaAs on said stripe-shaped surface portion of the substrate and simultaneously growing second and third buffer layers of n-type GaAs on the substrate at both sides on the first buffer layer by performing a metal-organic molecular beam epitaxy for depositing at least Ga, As and an n-type dopant while setting a vapor pressure of As at a first predetermined level, growing the first clad layer comprising a first portion comprised of p-type GaAlAs and second and third portions comprised of n-type GaAlAs, each of the portions having a composition Al x  Ga 1-x  As with a compositional parameter x at a first value x1 such that the first portion of the first clad layer is grown on the first buffer layer and the second and third portions are grown on the second and third buffer layers by performing a metal-organic molecular beam epitaxy for depositing Al, Ga, As and the an n-type dopant while setting the vapor pressure of As to the first predetermined level, growing the active layer of p-type GaAlAs having a composition Al x  Ga 1-x  As with a compositional parameter x at a second value x2 smaller than the foregoing first value x1 and simultaneously growing the second and third clad layers of p-type GaAlAs both having the composition Al x  Ga 1-x  As with a compositional parameter x at a third value x3 larger than the foregoing second value x2 by the second metal-organic molecular beam epitaxy such that the active layer is grown on the first portion of the first clad layer and such that the third and fourth clad layers are grown on the second and third portions of the first clad layer respectively, and growing the second clad layer of n-type GaAlAs having the composition Al x  Ga 1-x  As with the compositional parameter x at said first value x1 on said active layer and on said third and fourth clad layers by performing the foregoing metal-organic molecular beam epitaxy for depositing at least Ga, Al, As and the n-type dopant while setting the vapor pressure of As to a second predetermined level. According to the present invention, the laser diode has a structure which confines the current to the active layer. This laser diode can be manufactured without removing it from the apparatus used for growing crystal layers during the step of manufacturing. 
     Another object of the present invention is to provide a method of controlling the composition of a GaAlAs layer, grown on a GaAs substrate having either an exposed (311)A surface or an exposed (311)B surface by a metal-organic molecular beam epitaxy depositing at least Al and Ga, comprising a step of controlling the temperature of the GaAs substrate. 
     Another object of the present invention is to provide a method of manufacturing a semiconductor device comprising an active layer sandwiched by first and second graded layers of GaAlAs, comprising steps of preparing a GaAs substrate having either an exposed (311)A surface or an exposed (311)B surface, growing the first graded layer of GaAlAs having a composition Al x  Ga 1-x  As with a compositional parameter x for Al on the substrate by a metal-organic molecular beam epitaxy while changing the temperature of the substrate such that the compositional parameter x is decreased with the growth of the first graded layer, growing the active layer on the first graded layer, and growing the second graded layer on the active layer by the foregoing metal-organic molecular beam epitaxy such that the compositional parameter is increased with the growth of the second graded layer. According to the present invention, a desired compositional profile is obtained for the graded layers by simply controlling the temperature of the GaAs substrate. 
     Other objects and further features of the present invention will become apparent from the following detailed description of preferred embodiments when read in conjunction with attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematical view of a metal-organic molecular beam epitaxial crystal growth system used in the present invention; 
     FIG. 2 is a graph showing the compositional change of an epitaxial layer grown by the system of FIG. 1 on various crystal surfaces of a GaAs crystal while changing the temperature of growth; 
     FIG. 3 is a cross sectional view of a laser diode manufactured according to a first embodiment of the present invention; 
     FIG. 4 is a diagram showing a compositional profile of the laser diode of FIG. 3; 
     FIG. 5 is a graph showing a temperature curve employed in the system of FIG. 1 for realizing the compositional profile of FIG. 4 for the laser diode of FIG. 3; 
     FIG. 6, which has subports (A)-(D) which are diagrams showing steps of exposing a crystal surface such as (311)A or (311)B on a part of a major surface of a GaAs substrate which is defined by the (100) crystal plane; 
     FIG. 7 is a cross sectional view of the structure of a laser diode manufactured according to a second embodiment of the present invention; 
     FIG. 8 is a graph showing the relationships between the impurity concentration level and the vapor pressure of As in various corresponding surfaces of an epitaxial layer grown on a substrate having various, exposed crystal surfaces; 
     FIG. 9 is a diagram summarizing the relationships of FIG. 8 and showing a relation between the conduction type and As vapor pressure on various crystal surfaces of the substrate; and 
     FIG. 10 is a cross sectional view of a laser diode according to the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a MBE crystal growth system employed in the present invention for growing crystal layers. Referring to the drawing, the system comprises a reaction chamber 10 which is evacuated by a diffusion pump (not illustrated) through an exhaust conduit 12. In the reaction chamber 10, there is provided a holder 14 of molybdenum, a GaAs substrate 16, on which the growth of epitaxial layers is to be made, is held on the holder 14. The holder 14 is embedded with a heater 18 and the temperature of the substrate 16 is controlled by controlling electrical current flowing through the heater 18. Further, a reflection high energy electron diffraction (RHEED) gun 20 and a corresponding RHEED screen 22 are provided in the chamber 10, respectively in an upper part and a lower part, for monitoring the state of crystal layers grown on the substrate 16. 
     In order to introduce elements to be deposited on the substrate for epitaxial growth, the chamber 10 is provided with cells 24, 25, 26 and 27 wherein the first cell 24 accommodates a solid mass of beryllium acting as a source of Be, a p-type dopant, and the third cell 26 accommodates a solid mass of arsenic which is used as the source of As. Further, a pipe 25a for introducing triethylaluminium (TEAl), a gaseous source material of Al, and a pipe 25b for introducing triethylgallium (TEGa), a gaseous source material of Ga, are provided in the second cell 25. The amounts of TEAl and TEGa in the reaction chamber 10 are adjusted by valves 25c and 25d respectively connected to the pipes 25a and 25b. Further, in the fourth cell 27, a solid mass of silicon (Si) is accommodated as a source of Si which acts usually as an n-type dopant. Further, the cells 24 through 26 are provided with respective shutters 28 through 31 for controlling the supply of the associated source materials or elements into the reaction chamber 10. 
     Further, the inner surface of the reaction chamber 10 is covered with a liquid nitrogen shroud 32 except for the exhaust conduit 12, the RHEED gun 20, the RHEED screen 22, and the cells 24 through 27. 
     Using the crystal growth system of FIG. 1, the applicants of the present invention have conducted a research for growing GaAlAs epitaxial layers having a composition Al x  Ga 1-x  As on the GaAs substrate 16 with interesting results as will be described with reference to FIG. 2. 
     First, the experiments conducted in the research will be described. In this experiment, a GaAs substrate having a (100) surface and two other GaAs substrates, one having a (311)A surface and one having a (311)B surface, were prepared. Each of the substrates was held on the holder 14 of the system of FIG. 1 as the substrate 16 and heated to a predetermined temperature by the heater 18. While heating, the shutters 29 and 30 were opened and the valves 25c and 25d were opened together. Responsive thereto, an epitaxial layer of GaAlAs having a composition of Al x  Ga 1-x  As was grown on the substrate 16. FIG. 2 shows the compositional parameter x representing the content of Al as a function of the temperature used for growing the GaAlAs layer. Note that the growth was made with the vapor pressure of As set to 1.7×10 -5  Torr. 
     FIG. 2 shows the result of the experiment for each of the crystal surfaces (100), (311)A and (311)B. As can be seen in the drawing, the compositional parameter x of Al does not change significantly with the temperature of growth when the epitaxial layer is grown on the (100) surface of the substrate. On the other hand, when the epitaxial layer is grown on the (311)A surface, there appears a strong correlation between the compositional parameter x and the temperature of growth such that the compositional parameter x increases from about 0.1 to about 0.27 responsive to the increase of the temperature of growth from about 520° C. to about 630° C. Further, when the epitaxial layer is grown on the (311)B surface, the compositional parameter x decreases from about 0.22 to 0.05 responsive to the temperature increase from about 520° C. to about 630° C. 
     The result of FIG. 2 suggests that, by suitably controlling the temperature of growth during the epitaxial growth of GaAlAs layer on the (311)A or (311)B surface of a GaAs substrate, one can construct a semiconductor device having a graded layer of GaAlAs with a desired compositional profile. 
     Next, a first embodiment of the present invention will be described for a GRIN-SCH laser diode in which the foregoing discovery is applied. 
     FIG. 3 shows a cross sectional view of the GRIN-SCH laser diode manufactured according to the present embodiment. Referring to FIG. 3, the laser diode is formed first by holding a GaAs substrate 40 on the holder 14 of the system of FIG. 1 and heating the substrate to 630° C. In this embodiment, the GaAs substrate 40 exposes the (311)A surface. Next, the shutters 29 through 31 are opened and at the same time the valve 25d is opened. Responsive thereto, Ga and As are supplied into the reaction chamber together with Si acting as the n-type dopant and an n-type GaAs buffer layer 42 is grown on the substrate epitaxially, to a thickness of about 3 μm. Further, the valve 25c for introducing TEA is opened and an n-type clad layer 44 of Al 0 .3 Ga 0 .7 As is formed on the buffer layer 42 at the same temperature, to a thickness of about 1.3 μm. 
     After the formation of the clad layer 44, the temperature of the substrate 40 is controlled according to a curve A shown in FIG. 5 for a time interval ranging from an instance t1 to an instance t2 while continuing the epitaxial growth. Note that FIG. 5 shows a temperature profile used for the epitaxial growth of the graded layers in the laser diode, Thus, at the first instance t1, the temperature of the substrate 40 is set to 630° C. and the temperature is gradually lowered according to the curve A as the growth proceeds until the instance t2 is reached at which the temperature is set to 560° C. By choosing the time scale between the instance t1 and the instance t2 to 40 minutes, a graded layer 46 of n-type Al x  Ga 1-x  As is grown, to a thickness of 0.2 μm with a compositional profile in which the parameter x changes gradually from 0.30 to 0.18. 
     After the graded layer 46 is grown, the shutter 31 and the valve 25c are closed. As a result, a thin undoped GaAs active layer 48 is grown on the graded layer 46, to a thickness of about 3 to 10 nm. The temperature for the growth of the active layer may be different from the foregoing temperature of 560° C. 
     Next, the valve 25c is opened again and the shutter 28 is opened for introducing the P-type dopant, Be. Further, while gradually increasing the temperature from 560° C. to 630° C. along the curve A from an instance t3 to an instance t4, a p-type graded layer 50 of AlGaAs having the composition Al x  Ga 1-x  As with a reversed compositional profile to that of the graded layer 46 is grown on the p-type GaAs active layer 48 for, to a thickness of about 0.2 μm. Note that the time interval between the instance t3 and the instance t4 is identical to that between the instance t1 and the instance t2 and thus, the temperature curve used for growing the graded layer 50 is the reversal of the part of the curve A between the instance t1 and the instance t2. 
     Further, the growth of another clad layer 52 of p-type GaAlAs with a composition of Al 0 .3 Ga 0 .7 As is performed on the layer 50 while maintaining the temperature of the substrate 40 at 630° C., to a thickness of about 1.3 μm. Further, the valve 25c is closed and a p-type GaAs layer 54 is grown on the p-type layer 52, to a thickness of about 3 μm and the GRIN-SCH laser diode is completed. 
     FIG. 4 shows the energy band structure of the GRIN-SCH laser diode manufactured as described. As can be seen from the drawing, the energy gap between the conduction band Ec and the valence band Ev is changed gradually in correspondence to the gradual compositional change of the graded layers 44 and 52. Together with the gradual change of the band structure as such, the refractive index is also changed gradually and an effective confinement of photons as well as carriers in the active layer is achieved. 
     The foregoing procedure is applicable similarly to the case when the GaAs substrate 40 has an exposed (311)B surface and the growth of the epitaxial layers is made on this (311)B surface, except that the temperature curve used for growing the graded layers 46 and 50 is not the curve A but instead the curve B also shown in FIG. 5 is used. Thus, at the first instance t1, the temperature of the substrate 40 is set to 520° C. and the temperature is gradually increased with the growth of the layer 46 to 620°, starting from the instance t1. Responsive to the epitaxial growth made as such, to a thickness of about 0.2 μm, the compositional parameter x of the graded layer 46 is changed gradually from 0.23 to 0.05. When growing the graded layer 50 on the active layer 48, on the other hand, the temperature of the substrate 40 or the temperature of the epitaxial growth is decreased along the curve B from 620°60 C. at the instance t3 to 520° C. at the instance t4. Responsive to the growth, a compositional profile ranging from 0.05 to 0.23 is obtained for the graded layer 50. 
     As described heretofore, the present invention enables accurate control of the compositional profile of the epitaxial layer by controlling the temperature of the substrate. Note that the temperature of the substrate can be controlled accurately by controlling the current flowing through the heater 18. 
     Next, a second embodiment of the present invention will be described. According to the second embodiment, the (100) surface of a GaAs substrate is grooved by etching such that a (311)A surface or a (311)B surface of GaAs crystal is exposed additionally to the (100) surface, and an epitaxial growth of GaAlAs layers having different compositions is made simultaneously on the various crystal surfaces. 
     FIGS. 6(A) through (D) show a series of steps for exposing the (311)A surface or the (311)B surface by etching. Referring to the drawing, a GaAs substrate 60 defined by the (100) surface is prepared in a step of FIG. 6(A), and the substrate 60 thus prepared is covered entirely with a mask layer 62 which may comprise silicon oxide or silicon nitride. Next, the mask layer 62 is patterned such that a part of the layer 62, corresponding to the region in which a groove exposing the (311)A or (311)B surface is to be formed, is removed photolithographically. As a result, a structure shown in FIG. 6(B) is obtained. Further, the structure of FIG. 6(B) is subjected to anisotropic etching using a mixture of a phosphoric acid (H 3  PO 4 ), hydrogen peroxide (H 2  O 2 ) and water (H 2  O) with a mixing ratio of 50: 1: 50. The foregoing mixture acts anisotropically an the exposed (100) surface and as a result, a structure shown in FIG. 6(C), in which a groove exposing the (311)A or the (311)B surface is formed, is obtained as shown in FIG. 6. After the surface (311)A or (311)B is exposed, the mask layer 62 is removed and the substrate 60 as shown in FIG. 6(D) is obtained. 
     The substrate 60 thus obtained is brought into the metal-organic molecular beam epitaxial crystal growth system of FIG. 1 as the substrate 16, and the deposition of GaAlAs is made similarly to the first embodiment. Thereby, the composition of the GaAlAs layers grown on the (100) surface and the composition of that grown on the (311)A or (311)B surface become different because of the reason already described with reference to FIG. 2. 
     Thus, when the substrate 60 has the (100) surface and the (311)A surface exposed, a GaAlAs crystal layer having a composition of Al 0 .25 Ga 0 .75 As is grown on the (100) surface of the substrate while a GaAlAs crystal layer having a composition of Al 0 .1 Ga 0 .9 As is grown on the (311)A surface by setting the temperature of the substrate at 520° C. When the substrate 60 has the (311)B surface in addition to the (100) surface exposed, on the other hand, a GaAlAs crystal layer having a composition of Al 0 .32 Ga0.68As is grown on the (100) surface and a GaAlAs crystal layer having a composition of Al 0 .05 Ga 0 .95 As is grown on the (311)B surface simultaneously provided that the temperature of the substrate 60 is set to 630° C. 
     Next, a method of manufacturing a stripe-type layer diode using the foregoing method of epitaxial growth will be described with reference to FIG. 7. 
     First, a GaAs substrate 70 defined with an exposed (100) surface is subjected to anisotropic etching similarly to the processes described with reference to FIGS. 6(A)-(D) such that the (311)A or (311)B surface is exposed along a stripe-like portion extending on the (100) surface. Hereinafter, description will be given for a case in which it is the (311)B surface that is exposed along the stripe-like portion on the (100) surface of the substrate 70. 
     The substrate 70 prepared as such is first brought into the epitaxial growth system of FIG. 1 where the substrate 70 is held on the holder 14 similarly to the substrate 16. Next, the heater 18 is energized such that the temperature of the substrate 70 is maintained at 520° C. and the shutters 29 through 31 are opened. Simultaneously, the valve 25d is opened and a buffer layer 72 of n +  -type GaAs is grown epitaxially on the substrate 70, to a thickness of about 2 μm. In the buffer layer 72, the impurity concentration level is controlled to about 2×10 18  cm -3 . Note that the buffer layer 72 thus grown on the substrate 70 exposes a (100) top surface and a (311)B top surface in correspondence to the substrate 70. 
     Next, the temperature of the substrate 70 is maintained at 520° C. and the valve 25c is opened additionally. As a result, a clad layer 74 of n-type GaAlAs is grown on the buffer layer 72 for about 1.5 μm. As the temperature of the substrate is set to 520° C., the composition of the GaAlAs layer 74 is not substantially different between a part of the layer 74 grown on the (100) surface and a part grown on the (311)B surface (see FIG. 2). Thus, the composition of the layer 74 is uniform and particularly is a composition of about Al 0 .25 Ga 0 .75 As. The impurity concentration level of the layer 74 is about 3×10 17cm   -3  As the layer 74 is grown epitaxially, the layer 74 is defined by a (100) top surface corresponding to the (100) top surface of the buffer layer 72 and by a (311)B top surface corresponding to the (311)B top surface of the buffer 72. 
     Next, the temperature of the substrate 70 is increased to about 630° C. by increasing the current flowing through the heater 18. When the temperature has reached to 630° C. the shutter 31 is closed and an epitaxial layer 76 of p-type GaAlAs is grown on the clad layer 74, to a thickness of about 0.1 μm. As the temperature is set to 630° C., the composition of the epitaxial layer 76 is different between a part 76a covering the (100) top surface and a part 76b covering the (311)B top surface of the clad layer 74 (see FIG. 2). Thus, the part 76a formed on the (100) surface of the clad layer 74 has a composition Al 0 .32 Ga 0 .68 As while the part 76b formed on the (311)B surface of the clad layer 74 has a composition Al 0 .05 Ga 0 .95 As. As the part 76a, having an increased Al content as compared to the part 76b, has a reduced refractive index as compared to the part 76b, the part 76 a acts as the optical confinement layer for confining optical radiation in the part 76b which in turn acts as the active layer. Note that the active layer 76b has a limited width and extends along the stripe-like (311)B surface defined on the substrate 70. Thus, a structure in which the active layer 76b is bounded laterally by a pair of optical confinement layers 76a is obtained by a single step of growing the layer 76. 
     Next, the temperature of the substrate 70 is reduced to the foregoing temperature of 520° C. and a p-type GaAlAs layer 78 is grown on the epitaxial layer 76 with a composition Al 0 .25 Ga 0 .75 As. Note that there appears no substantial difference in the composition between a part of the layer 78 covering the optical confinement layer 76a and a part of the layer 78 covering the active layer 76b, as the growth of the layer 78 is made at 520° C. The layer 78 is grown to a thickness of about 1.5 μm with an impurity concentration level of about 5×10 19  cm -3 . 
     Next, the valve 25c is closed while maintaining the temperature of the GaAs substrate 70 at 520° C. and a p +  -type GaAs layer 80 having the impurity concentration level of about 1×10 19  cm -3  is grown on the layer 78 to a thickness of about 0.5 μm. Thus, the laser diode in which the active layer 76a is surrounded vertically and laterally by clad layers 74, 78 and 76a is obtained. 
     The laser diode thus obtained has an advantage in that the light is effectively confined in the active layer which is surrounded vertically and laterally by the clad layers and the efficiency of oscillation is increased. The particularly important feature of the present invention is that the laser diode having such an advantageous feature is manufactured without taking the partially completed laser diode out of the epitaxial crystal growth system of FIG. 1 during the step of manufacturing. 
     When constructing the stripe-type laser diode on the substrate 70 in which the (311)A surface is exposed instead of the (311)B surface, basically an identical process is applicable except for the control of the temperature of the substrate 70. Thus, the growth of the layers 72, 74, 78 and 80 is made at 630° C. while the growth of the p-type epitaxial layer 76 is made at 520° C. As will be understood from FIG. 2, the n-type GaAlAs clad layer 74 and the p-type GaAlAs clad layer 78 thus grown are homogeneous and the composition is represented as Al 0 .3 Ga 0 .7 As irrespective of whether the layer is grown on the (100) surface or on the (311)A surface while the p-type epitaxial layer 76 grown on the clad layer 74 is heterogeneous such that the first part 76a thereof on the (100) surface of the underlying clad layer 74 takes the composition Al 0 .1 Ga 0 .9 As and the second part 76b on the (311)A surface of the layer 74 takes the composition Al 0 .05 Ga 0 .95 As similarly to the foregoing embodiment. Thus, the second part 76 b acts as the active layer and the first part 76a acts as the optical confinement layer or clad layer. In other words, the laser diode having exactly an identical structure to the foregoing embodiment is obtained. 
     Next, a third embodiment of the present invention will be described together with result of a research conducted by the applicants for growing an epitaxial layer of GaAs doped with Si on various crystal surfaces of GaAs substrate while changing the vapor pressure of As. In the present research, substrates exposing the (100) surface, (110) surface, (111)A surface, (111)B surface, (311)A surface and the (311)B surface were used. 
     In the experiments, each of these substrates was held on the holder 14 of the crystal growth system of FIG. 1 as the substrate 16 and the shutters 29, 30 and 31 were opened. At the same time, the valve 25a was opened and the cells 26 and 27 were heated. As a result, an epitaxial layer of GaAs was grown on the substrate 16. During the growth, the temperature of the substrate 16 was held at 580° C. and the flow rate of TEG was fixed constant. 
     FIG. 8 shows a relation between the impurity concentration level of the epitaxial layer thus grown on the substrate and the As vapor pressure employed for the growth. In FIG. 8, the epitaxial layers grown on different crystal surfaces are distinguished by changing the symbols and the conduction type of the obtained epitaxial layer is also represented. As will be apparent from FIG. 8, the conduction type of the epitaxial layer is changed by the vapor pressure of As employed at the time of growth of the epitaxial layers. Such a change of the conduction type occurs as a result of Si entering into the site of Ga or into the site of As. Thus, when Si enters into the site of Ga, Si is charged negatively and acts as the donor while when Si enters into the site of As, Si is charged positively and acts as the acceptor. 
     FIG. 9 summarizes the result of FIG. 8 in a form which may be easier for understanding. Similarly to FIG. 8, the relation shown in this drawing is for the case in which the growth is made at 580° C. As will be seen in FIG. 9, the epitaxial layer tends to exhibit the p-type conduction in a relatively low vapor pressure region of As while there is a tendency that the n-type appears when the vapor pressure of As is increased. The critical vapor pressure of As at which the transition between the p-type and the n-type occurs is changed depending on the crystal surface of the substrate on which the growth is made. Thus, the transition from the p-type to the n-type occurs at the As pressure of about 1×10 -5  Torr when the epitaxial layer is grown on the (100), (110) or (111)A surface while the transition occurs at about 3×10 -5  Torr when the epitaxial layer is grown on the (311)A surface. When the epitaxial layer is grown on the (111)B surface or on the (311)B surface, on the other hand, the conduction type remains the n-type throughout the foregoing vapor pressure range. 
     It is believed, further, that the change of the conduction type is not only dependent on the vapor pressure of As alone but is further dependent on the rate of supply of Ga. Thus, the conduction type of the epitaxial GaAs layer grown on the GaAs substrate is ultimately determined by a ratio of the arsenic vapor pressure and the rate of supply of Ga. 
     Using the foregoing discovery, it is possible to grow epitaxial layers having different conduction types on the same substrate by properly choosing the vapor pressure of As and by processing the substrate such that the substrate has various different exposed crystal surfaces. The processes to expose the (311)A or (311)B surface on the (100) surface of GaAs are already described with reference to FIGS. 6(A)-(D). Thus, one can grow a p-type epitaxial layer of GaAs on the (311)A surface and simultaneously an n-type epitaxial layer of GaAs on the (100), (110), (111)A, (111)B or (311)B surface of the substrate by setting the vapor pressure to about 1-2×10 -5  Torr. In this case, a high resistivity layer may be formed on the (111)A surface. When the arsenic vapor pressure is set lower than the foregoing level of about 1×10 -5  Torr, on the other hand, the p-type GaAs epitaxial layer is grown on the (100), (110), (111)A or (311)A surface and at the same time the n-type GaAs layer is grown on the (111)B or (311)B surface. 
     Further, by combining the result of FIG. 2 showing the compositional change of the epitaxial layer grown on a substrate as a function of the temperature of the substrate, it is possible to grow an epitaxial layer of GaAlAs having different compositions and different conduction types on a single GaAs substrate by controlling the temperature of the crystal growth and the vapor pressure of As. 
     Next, the process of manufacturing a stripe-like laser diode in which the foregoing findings are employed will be described with reference to FIG. 10. 
     In a first step, a p +  -type substrate 90 is prepared such that the substrate is defined by the (100) surface except for a narrow stripe-like region in which the (311)A surface is exposed by etching similarly to the case of the laser diode of FIG. 7. The substrate 90 is incorporated into the metal-organic molecular beam epitaxial growth system of FIG. 1 and is placed on the holder 14 as the substrate 16. Further, the temperature of the substrate 16 is controlled to about 630° C. by the heater 18 and the shutters 29, 30 and 31 are opened. Further, the valve 25d is opened while keeping the valve 25c closed. As a result, Ga, As and Si are introduced into the reaction chamber 10 and a buffer layer 92 of GaAs is grown epitaxially on the substrate 90 to a thickness of about 2 μm. During this step, the vapor pressure of As is maintained at about 1.7×10 -5  Torr and a part 92a of the buffer layer 92 grown on the (100) surface of the substrate 90 is doped to the n-type while a part 92b grown on the (311)A surface is doped to the p-type. Note that the buffer layer part 92a is defined with a (100) top surface and the buffer layer part 92b is defined with a (311)A top surface. In the foregoing step, it should be noted that the buffer layer part 92a and the buffer layer part 92b are grown simultaneously at the time the buffer layer 92 is grown. 
     Next, the vapor pressure of As is maintained at about 1.7×10 -5  Torr and the valve 25c is opened while maintaining the temperature of the substrate 90 at 630° C. Responsive thereto, a clad layer 94 of GaAlAs having a composition Al x  Ga 1-x  As is grown on the underlying buffer layer 92 to a thickness of about 1.5 μm. As the temperature of the substrate is held at 630° C., the composition of the clad layer 94 is uniform and is represented as Al 0 .32 Ga 0 .68 As. Further, as the growth is made at the arsenic vapor pressure of 1.7×10 -5  Torr, it will be understood from FIG. 9 that a part 94a grown on the (100) top surface of the buffer layer part 92a is doped to the n-type while a part 94b of the clad layer 94 grown on the (311)A top surface of the buffer layer part 92b is doped to the p-type. Note that the clad layer part 94a is defined by a (100) top surface and the clad layer part 94b is defined by a (311)A top surface. The clad layer part 94a and the clad layer part 94b are grown simultaneously when the clad layer 94 is grown on the buffer layer 92. 
     Next, the shutter 28 is opened and Be acting as the p-type dopant is introduced into the reaction chamber 10 while keeping the vapor pressure of arsenic at 1.7×10 -5  Torr. Further, the temperature of the substrate 90 is reduced to 520° C. As a result, an epitaxial layer 96 of GaAlAs having a composition Al x  Ga 1-x  As is grown on the clad layer 94 for a thickness of about 0.1 μm. The epitaxial layer 96 thus grown is doped to the p-type throughout but the composition is different in a region 96a grown on the clad layer part 94a exposing the (100) surface in which the region 96a has a composition of Al 0 .25 Ga 0 .75 As and in a region 96b grown on the clad layer part 96b exposing the (311)A surface in which the region 96b has a composition Al 0 .1 Ga 0 .9 As. Similarly to the foregoing embodiment shown in FIG. 7, the region 96a and the region 96b are formed simultaneously when the epitaxial layer 96 is grown and the regions 96a at the both sides of the region 96b act as the optical confinement layer having a low refractive index and the region 96b acts as the active layer having a high refractive index. 
     Next, the shutter 28 is closed, the vapor pressure of As is increased to about 4.5×10 -5  Torr, and the temperature of the substrate 90 is again increased to about 630° C. Under this condition, another clad layer 98 of p-type GaAlAs having a composition Al 0 .32 Ga 0 .68 As is grown on the epitaxial layer 96 for a thickness of about 1.5 μm irrespective of whether the layer 98 is grown on the region 96a defined with a (100) top surface or on the region 96b defined with a (311)A top surface as will be understood from the relation shown in FIGS. 2 and 9. 
     Further, the valve 25c is closed while keeping the temperature of the substrate 90 at 630° C. and maintaining the vapor pressure of As at the foregoing value of 4.5×10 -5  Torr, and an n-type GaAs cap layer 100 is grown on the clad layer 98 for a thickness of about 1 μm. Responsive to the deposition of the cap layer 100, the stripe-type laser diode is completed. 
     The laser diode thus constructed has various advantageous features particularly in that the injection of the carrier to the active layer 96b surrounded by clad layers is made preferentially through the clad layer part 94b adjacent to the active layer 96b by concentrating the current to this region. According to the present invention, the laser diode having such a structure can be manufactured without taking the partially completed laser diode out of the epitaxial crystal growth system during the step of manufacturing. As a result, the yield of the laser diode is increased and the cost of the laser diode is decreased. 
     Further, it should be noted that the source of As is not limited to the foregoing solid arsenic material but a gaseous source material such as metal-organic molecule of As may also be used for supplying As. 
     Further, the present invention is not limited to the manufacturing of the stripe-type laser diode but is applicable to the manufacturing of any type of semiconductor devices in which a layer of GaAlAs comprising a plurality of sub-regions in which the composition is made different from each other is included. 
     Further, the present invention for manufacturing the semiconductor devices is applicable in general to epitaxial growth of crystal layers on a substrate as long as the crystal growth is controlled by the decomposition of metal-organic molecules at the surface of the substrate. 
     The gaseous source material for Al and Ga is not limited to TEA and TEG but other materials such as trimethyl aluminium and triisobutyl aluminium may also be used for the source of Al and trimethyl gallium may also be used for the source of Ga. 
     Further, the present invention is not limited to these embodiments described heretofore but various variations and modifications may be made without departing from the scope of the invention.