Method of growing epitaxial layers

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 (70, 90) comprises steps of forming at least a first crystal surface and a second crystal surface which are crystallographically non-equivalent from each other on the substrate, introducing particles comprising constituent elements of the epitaxial layers to a region in a vicinity of the substrates 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 element therein is released as a result of the decomposition, and depositing the 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 (76a, 92a, 94a, 96a) and a second epitaxial layer (76b, 92b, 94b, 96b), different in property from each other, are grown respectively on the first and second crystal surfaces, wherein the step of deposition is performed such that the growth of the first and second epitaxial layers is controlled by the decomposition of the metal-organic molecule.

Description: The present invention generally relates to a method of fabricating a semiconductor device, and more particularly to an epitaxial process for growing a plurality of epitaxial layers having different properties on a common substrate in a continuous 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, an efficient oscillation of 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 to 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 to 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 once 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 additional processes is undesirable as such a step and other processes performed outside the epitaxial growing system tend to introduce defects to 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, 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. J. Appl. Phys., Vol. 63, No. 3, 1.2.88., pp 743-748, discloses MBE of Si-doped GaAs simultaneously on (100), (311)A, and (311)B GaAs substrates. Si is incorporated primarily as a donor for growth on (100) and (311)B substrates, but primarily as an acceptor for growth on (311)B substrates, rendering the grown layers n-type and p-type respectively. Appl. Phys. Lett., Vol. 47, No. 12, 15.12.85., pp 1309-1311, discloses a similar MBE process using (100) and (111)A GaAs substrates deposited with Si-doped GaAs to form lateral n-p-n structures. Appl. Phys. Lett., Vol. 50, No. 17, 27.4.87., pp 1149-1151, concerns carbon incorporation in MOCVD of AlGaAs films on (100), (311)A and (311)B GaAs substrates. It is disclosed that minimum carbon contamination occurs with (311)B substrates. IBM T.D.B., Vol. 31, No. 2, July 1988, p. 240, discloses a laser structure in which an active QW GaAs layer is embedded between two AlGaAs cladding layers. The centre portion of the GaAs layer is on a (100) GaAs substrate surface, and end portions, acting as mirrors, are on (411) surfaces thereby causing a reduced thickness of the end portions. EP-A-0 261 262 discloses a transverse junction stripe laser having an MBE-deposited GaAs active layer formed over (111)A and (100) surfaces of a GaAs substrate, having p-n homojunctions at the intersections of those surfaces. WO-A-87/00694 discloses a heterostructure device with a mesa having (100) and (111)A surfaces of a group III-V substrate. The mesa is exposed to group III-V chemical reagents so as to form group III-V layers on one or both surfaces, depending on the group V material used. According to a first aspect of the present invention, there is provided a method of fabricating a semiconductor device comprising an epitaxial process for growing a plurality of epitaxial layers having properties different from each other simultaneously on a common substrate, said process comprising the steps of forming at least a first crystal surface and a second crystal surface which are crystallographically non-equivalent to each other on said substrate, and introducing particles comprising constituent elements of the epitaxial layers to a region in a vicinity of the substrate; characterized in that said particles comprise metal-organic molecules containing elements forming the epitaxial layers, the process including further steps of decomposing the metal-organic molecules to release the or each element therein, and depositing the released elements on said first and second crystal surfaces so that a first epitaxial layer and a second epitaxial layer, different from each other in chemical composition, excluding those chemical compositions that differ only in conductivity type, are grown respectively on the first and second crystal surfaces, and wherein in said decomposing and depositing steps, the growth rate of the first and second epitaxial layers is controlled by adjusting the temperature and/or the vapor pressure of said elements under which the epitaxial growth occurs, so as to regulate the rate of decomposition of the metal-organic molecules and hence the supply of elements for forming the epitaxial layers. According to a second aspect of the present invention, there is provided a method of fabricating an optical semiconductor device comprising an epitaxial process for growing a plurality of epitaxial layers having properties different from each other successively on an upper major surface of a substrate, said upper major surface being represented by a set of three Miller indices, said process comprising steps of:- introducing particles comprising constituent elements of the epitaxial layers to a region in a vicinity of the substrate, said particles comprising metal-organic molecules containing elements forming the epitaxial layers; decomposing the metal-organic molecules so as to release the or each element therein; and depositing the released elements on said substrate so that a first epitaxial layer and a second epitaxial layer, different in chemical composition from each other, are successively grown on the substrate; characterised in that in the depositing step, the growth rate of the first and second epitaxial layers is controlled by adjusting the temperature under which the epitaxial growth occurs, so as to regulate the rate of decomposition of the metal-organic molecules and hence the supply of elements for forming the epitaxial layers, whereby the first and second epitaxial layers having different chemical compositions are formed from the same particles. As described in detail later, by means of the present invention a plurality of epitaxial crystal layers different in chemical composition from each other can be grown on a common substrate simultaneously, the epitaxial crystal layers also having different conduction types. This can be achieved by controlling the temperature of the growth (e.g. substrate temperature) or vapor pressure of the deposited element(s). The present invention also provides 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 it out of the system during the process of manufacturing. By means of the present invention, the compositional profile of a graded layer can be controlled accurately at the time of growth of the layer. For example, a laser diode having a GRIN-SCH structure may be manufactured with 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 with an accurately controlled compositional profile. As explained below, in another method of the invention, the composition of a GaAlAs layer grown on a GaAs substrate exposing either a (311)A surface or a (311)B surface by metal-organic molecular beam epitaxy depositing at least Al and Ga, is controlled by controlling the temperature of the GaAs substrate. Similarly, a desired compositional profile can be obtained for graded layers of GaAlAs on a GaAs substrate by simply controlling the temperature of the GaAs substrate. Reference is made, by way of example, to the accompanying drawings, in which:- FIG.1 is a schematic view showing a metal-organic molecular beam epitaxial crystal growth system used in the present invention; FIG.2 is a graph showing a 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 showing 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; FIGS.6(A) - (D) 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 showing the structure of a laser diode manufactured according to a second embodiment of the present invention ; FIG.8 is a graph showing a relation between the impurity concentration level and the vapor pressure of As in various surfaces of an epitaxial layer grown on a substrate exposing various crystal surfaces; FIG.9 is a diagram summarizing the relation 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 showing a laser diode according to the third embodiment of the present invention. 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, and 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 solid beryllium acting as a source of Be, a p-type dopant, and the third cell 26 accommodates solid arsenic which is used as the source of As. Further, a pipe 25a for introducing triethylaluminium (TEA), a gaseous source material of Al, and a pipe 25b for introducing triethylgallium (TEG), a gaseous source material of Ga, are provided in the second cell 25. The amount of TEA and TEG in the reaction chamber 10 is adjusted by valves 25c and 25d respectively connected to the pipes 25a and 25b. Further, in the fourth cell 27, solid silicon (Si) is accommodated as a source of Si which acts usually as an n-type dopant. Further, each of the cells 24 through 26 are provided with shutters 28 through 31 for controlling the supply of the 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 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 2.26 mPa (1.7 x 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 a 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 for 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 for 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 for 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 for a thickness of about 3 to 10 nm. The temperature for the growth of the active layer may be different from the foregoing temperature 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 the graded layer 46 is grown on the p-type GaAs active layer 48 for 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 made on the layer 50 while maintaining the temperature of the substrate 40 at 630 °C for 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 for 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 the 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 another curve B also shown in FIG.5 is used instead. 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 °C, starting from the instance t1. Responsive to the epitaxial growth made as such for 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 °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 first embodiment enables an 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 (H₃PO₄), hydrogen peroxide (H₂O₂) and water (H₂O) with a mixing ratio of 50: 1: 50. The foregoing mixture acts anisotropically to 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 exposes the (100) surface and the (311)A surface, 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 exposes the (311)B surface in addition to the (100) surface, on the other hand, a GaAlAs crystal layer having a composition of Al 0.32 Ga 0.68 As 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 laser diode using the foregoing method of epitaxial growth will be described with reference to FIG.7. First, a GaAs substrate 70 defined with the (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 for a thickness of about 2 µm. In the buffer layer 72, the impurity concentration level is controlled to about 2 X 10¹⁸ 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 takes a composition of about Al 0.25 Ga 0.75 As. The impurity concentration level of the layer 74 is about 3 X 10¹⁷ cm⁻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 630 °C, the shutter 31 is closed and an epitaxial layer 76 of p-type GaAlAs is grown on the clad layer 74 for 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 76a 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 for a thickness of about 1.5 µm with an impurity concentration level of about 5 X 10¹⁷cm⁻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 X 10¹⁹ cm⁻³, is grown on the layer 78 for 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 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 out the partially completed laser diode from 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 76b acts as the active layer and the first part 76a acts as the optical confinement layer or clad layer. In other words, a laser diode having an identical structure to the foregoing embodiment is obtained. Next, a third embodiment of the present invention will be described together with results of 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 1.3 mPa (1 X 10⁻⁵ Torr) when the epitaxial layer is grown on the (100), (110) or (111)A surface while the transition occurs at about 4 mPa (3 X 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 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 a same substrate by properly choosing the vapor pressure of As and by processing the substrate such that the substrate exposes various crystal surfaces. The process to expose the (311)A or (311)B surface on the (100) surface of GaAs is 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 1.3 - 2.7 mPa (1 - 2 X 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 1.3 mPa (1 X 10⁻⁵Torr), 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 for a thickness of about 2 µm. During this step, the vapor pressure of As is maintained at about 2.3 mPa (1.7 X 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 2.3 mPa (1.7 X 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 for 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 2.3 mPa (1.7 X 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 2.3 mPa (1.7 X 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 6 mPa (4.5 x 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 6 mPa (4.5 X 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 out the partially completed laser diode from 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 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.

Claims: A method of fabricating a semiconductor device comprising an epitaxial process for growing a plurality of epitaxial layers having properties different from each other simultaneously on a common substrate (70, 90), said process comprising the steps of forming at least a first crystal surface and a second crystal surface which are crystallographically non-equivalent to each other on said substrate, and introducing particles comprising constituent elements of the epitaxial layers to a region in a vicinity of the substrate; characterized in that said particles comprise metal-organic molecules containing elements forming the epitaxial layers, the process including further steps of decomposing the metal-organic molecules to release the or each element therein, and depositing the released elements on said first and second crystal surfaces so that a first epitaxial layer (76a, 92a, 94a, 96a) and a second epitaxial layer (76b, 92b, 94b, 96b), different from each other in chemical composition, excluding those chemical compositions that differ only in conductivity type, are grown respectively on the first and second crystal surfaces, and wherein in said decomposing and depositing steps, the growth rate of the first and second epitaxial layers is controlled by adjusting the temperature and/or the vapor pressure of said elements under which the epitaxial growth occurs, so as to regulate the rate of decomposition of the metal-organic molecules and hence the supply of elements for forming the epitaxial layers. A method as claimed in claim 1, wherein said first (76a, 92a, 94a, 96a) and second (76b, 92b, 94b, 96b) epitaxial layers comprise gallium aluminium arsenide, and said substrate (70, 90) comprises gallium arsenide. A method as claimed in claim 2, wherein said first crystal surface is a (100) surface of gallium arsenide, and said second crystal surface is a (311)A or (311)B surface of gallium arsenide. A method as claimed in claim 2, wherein said metal-organic molecules comprise triethylaluminium and triethylgallium. A method as claimed in claim 1, wherein said step of exposing the first and second crystal surfaces comprises a step of applying an anisotropic etching to the substrate (60). A method as claimed in claim 5, wherein said anisotropic etching is performed by an etching solution comprising a mixture of phosphoric acid, hydrogen peroxide, and water. A method as claimed in claim 1, wherein said depositing step comprises controlling the temperature of the substrate (70) under which the epitaxial growth occurs. A method as claimed in claim 1, wherein said depositing step comprises controlling the vapor pressure of one of the elements under which the epitaxial growth occurs, and wherein the first epitaxial layer (92a, 94a) and the second epitaxial layer (92b, 94b) also have different conductivity types. A method as claimed in claim 3, wherein said depositing step comprises controlling the vapor pressure of arsenic which is introduced as one of the particles such that the first epitaxial layer (92a, 94a) grown on the (100) surface of the gallium arsenide substrate (90) is doped to become n-type and the second epitaxial layer (92b, 94b) grown on the (311)A surface of the gallium arsenide substrate is doped to become p-type. A method as claimed in claim 1, wherein said depositing step is achieved such that the elements released by the decomposition of the metal-organic molecules are deposited under a controlled pressure of one of the elements so as to determined the composition and conductivity type of the resulting epitaxial layer. A method as claimed in claim 10, wherein said substrate comprises gallium arsenide defined with one of (100), (110), (111)A, and (311)A crystal surfaces as said predetermined crystal surface, and said epitaxial layers comprise gallium aluminium arsenide. A method as claimed in claim 1, wherein said semiconductor device is a laser diode, wherein said step of forming the first and second crystal surfaces includes defining a stripe-like region in part of the substrate (70, 72) of gallium arsenide that is bounded by a (100) surface acting as the first crystal surface, such that the stripe-like region exposes a (311)A or (311)B surface acting as the second crystal surface, said decomposing and depositing steps comprising:- growing an initial epitaxial layer (74) of aluminium gallium arsenide having a composition Al x Ga 1-x As with a compositional parameter x set at a first value x1 on the substrate by metal-organic molecular beam epitaxy performed at a first predetermined temperature, said metal-organic molecular beam epitaxy depositing at least aluminium and gallium as a result of decomposition of metal-organic molecules supplied to a region in a vicinity of the substrate; growing said first epitaxial layer (76b) of gallium aluminium arsenide having a composition Al x Ga 1-x As with the compositional parameter x set at a second value x2 smaller than the foregoing first value x1 on a part of the initial epitaxial layer grown on said stripe-like region, by metal-organic molecular beam epitaxy performed at a second predetermined temperature different from the first predetermined temperature, and simultaneously growing a pair of said second epitaxial layers (76a) of gallium aluminium arsenide having the composition Al x Ga 1-x As with the compositional parameter x set at a third value x3 larger than the foregoing second value x2, on the initial epitaxial layer at a region excluding the stripe-like region such that the first epitaxial layer is sandwiched laterally by said pair of second epitaxial layers; and growing a further epitaxial layer (78) of gallium aluminium arsenide having a composition identical to the initial epitaxial layer on said first and second epitaxial layers by performing metal-organic molecular beam epitaxy at said first predetermined temperature. A method as claimed in claim 12, wherein said stripe-like region exposes the (311)B surface, and said second predetermined temperature is set higher than the first predetermined temperature. A method as claimed in claim 13, wherein the first predetermined temperature is set to about 520°C and the second predetermined temperature is set to about 630°C. A method as claimed in claim 12, wherein said stripe-like region exposes the (311)A surface, and said second predetermined temperature is set lower than the first predetermined temperature. A method as claimed in claim 15, wherein the first predetermined temperature is set to about 630°C and the second predetermined temperature is set to about 520°C. A method as claimed in claim 10, wherein said semiconductor device is a laser diode, wherein said step of defining said stripe-like region exposes the (311)A surface, and said decomposing and depositing steps comprise:- growing a first buffer layer (92b) of p-type gallium arsenide on said stripe-like region by performing a first metal-organic molecular beam epitaxy for depositing at least gallium, arsenic and an n-type dopant under a vapor pressure of arsenic set at a first predetermined level, and simultaneously growing a pair of second buffer layers (92a) of n-type gallium arsenide on a part of the substrate excluding the stripe-like region such that the second buffer layers sandwich the first buffer layer laterally; growing a first clad layer (94b) of p-type gallium aluminium arsenide having a composition Al x Ga 1-x As with a compositional parameter x at a first value x1 on the first buffer layer by performing a second metal-organic molecular beam epitaxy for depositing aluminium, gallium, arsenic and the n-type dopant while setting the vapour pressure of arsenic to the first predetermined level, and simultaneously growing a pair of second clad layers (94a) of n-type gallium aluminium arsenide having a composition identical to the first clad layer on said second buffer layers such that the second clad layers sandwich the first clad layer laterally; growing, as said first epitaxial layer, an active layer (96b) of p-type gallium aluminium arsenide having a composition Al x Ga 1-x As with the compositional parameter x at a second value x2 smaller than the foregoing first value x1 on the first clad layer by performing the second metal-organic molecular beam epitaxy, and simultaneously growing, as said second epitaxial layer, a pair of third clad layers (96a) of p-type gallium aluminium arsenide having a composition Al x Ga 1-x As with the compositional parameter set to a third value x3, larger than the second value x2, on said second clad layers such that the third clad layers sandwich the active layer laterally; growing a fourth clad layer (98) of n-type gallium aluminium arsenide having a composition identical to the first clad layers so as to cover said active layer and said third and fourth clad layers by performing the second metal-organic molecular beam epitaxy while setting the vapor pressure of arsenic to a second predetermined level. A method as claimed in claim 17, wherein said step of growing the active layer (96b) is performed under a vapor pressure of arsenic set to the first predetermined level. A method as claimed in claim 17, wherein said steps of growing the first and second buffer layers (92a, 92b), the first and second clad layers (94a, 94b), and the fourth clad layer (98) are performed at a first predetermined temperature, while said step of growing the active layer (96b) and the third clad layers (96a) is performed at a second predetermined temperature lower than the first predetermined temperature. A method of fabricating an optical semiconductor device comprising an epitaxial process for growing a plurality of epitaxial layers having properties different from each other successively on an upper major surface of a substrate (40), said upper major surface being represented by a set of three Miller indices, said process comprising steps of:- introducing particles comprising constituent elements of the epitaxial layers to a region in a vicinity of the substrate, said particles comprising metal-organic molecules containing elements forming the epitaxial layers; decomposing the metal-organic molecules so as to release the or each element therein; and depositing the released elements on said substrate so that a first epitaxial layer (46) and a second epitaxial layer (50), different in chemical composition from each other, are successively grown on the substrate; characterised in that in the depositing step, the growth rate of the first and second epitaxial layers is controlled by adjusting the temperature under which the epitaxial growth occurs, so as to regulate the rate of decomposition of the metal-organic molecules and hence the supply of elements for forming the epitaxial layers, whereby the first and second epitaxial layers having different chemical compositions are formed from the same particles excluding those chemical compositions that differ only in conductivity type. A method as claimed in claim 20, wherein the epitaxial process is applied to growing a gallium aluminium arsenide layer on a (311)A or (311)B surface of a gallium arsenide substrate (40), wherein the metal-organic molecules contain aluminium and gallium, and wherein the depositing step comprises depositing aluminium and gallium on the substrate whilst controlling the temperature of the substrate. A method as claimed in claim 21, wherein the temperature of the substrate is controlled in a range between 520°C and 630°C.

Inventors: FUJII TOSHIO, SANDHU ADARSH, FUJII, TOSHIO, SANDHU, ADARSH
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H01L21
IPC codes: H01S5, H01L21