Semiconductor Heterojunction Device Made by an Epitaxial Growth Technique

1. Field of the Invention

The present invention relates to a semiconductor device and a method for making the same, and in particular to a semiconductor device including one or more semiconductor layers formed as one or more functional layers on a semiconductor substrate and composed of a compound semiconductor and a method for making the same. The present invention is extensively employed for a functional device including an electronic device such as a diode and transistor, an optical device such as an LED, LD and optical waveguide, and a sensor.

2. Description of the Related Art

Generally, the technique for making a semiconductor layer or layers on a semiconductor substrate is broadly classified into a planer technique principally using the diffusion, ion implantation, oxidation, etc., and an epitaxial growth technique principally using a liquid phase epitaxy (LPE) and vapor phase epitaxy (VPE and CVD). In the formation of a compound semiconductor, in particular, the epitaxial growth technique is employed as a technique for forming semiconductor crystal or mixed crystal layers of a different composition on a semiconductor substrate. This technique has been extensively employed to fabricate an electronic device such as a hetero-bipolar transistor (HBT) and high electron mobility transistor (HEMT), and an optical device such as a light emitting diode (LED), laser diode (LD). photodiode (PD) and optical waveguide, all these devices requiring a hetero-junction structure of a different composition.

The epitaxial growth technique includes, in addition to a liquid phase epitaxy (LPE) method and vapor phase epitaxy (VPE) method using a halogen transport, a metal-organic vapor phase epitaxy (MOVPE) and a molecular beam epitaxy (MBE) method (including an MOMBE method), as capable of a greater material selectivity as well as ready making of a fine structure, an atomic layer epitaxy (ALE) method, a photochemical VPE method using light, as a decomposition energy for a material, in place of heat, and so on.

The conventional semiconductor process technique can readily deposit layers of a different composition on a semiconductor substrate in a direction vertical to the substrate surface, but it has been difficult to deposit layers having a different composition area in a direction parallel to the substrate surface.

Forming a simple optical waveguide on a semiconductor will be explained below by way of example.

In order to provide an optical waveguide over a semiconductor substrate, a semiconductor area acting as the optical waveguide is formed such that it is surrounded with a clad area of a lower refractive index than that of the semiconductor area. FIGS. 1A to 1C are perspective views showing various forms of an optical waveguide formed on a semiconductor substrate. FIG. 1A is an example in which an optical conducting layer 2 is formed on a substrate 1 such that it is surrounded with cladding layers 3 in a direction parallel to the surface of the substrate. FIG. 1B is an example in which an optical conducting layer 2 is formed on a substrate 1 such that it is surrounded with upper and lower cladding layers 3, in a stacking fashion, in a direction vertical to the surface of the substrate 1. FIG. 1C is an example in which an optical conducting layer 2 and cladding layers 3' are formed as one unit on the surface of a substrate 1 in a direction parallel to the surface of the substrate in which case the unit is sandwiched with cladding layers 3 in that vertical direction except for opposite end faces of the optical conducting layer 2. The structure as shown in FIG. 1C allows light to be conducted to the inner region of the optical conducting layer 2. It will be readily understood that the structure of FIG. 1C is composed of a combination of the structures of FIGS. 1A and 1B.

Usually in the case where an optical waveguide is provided on the substrate with the use of a semiconductor crystal, the following conditions have to be met:

(a) The refractive index of the optical conducting layer, upon being compared with that of the cladding layer, is great enough to confine light therein.

(b) Both the optical conducting layer and cladding layer can be formed as a single crystal on the semiconductor substrate.

As a method for satisfying these conditions (a) and (b), use has usually been made of a mixed crystal of, for example, III - V or II - VI compound semiconductors. This is because the lattice constant and band gap energy (Eg) can be controlled by varying the composition of constituent elements of which the mixed crystal is formed and because the refractive index has a correlation to the band gap energy, that is, an increase in the band gap energy (Eg) leads to a decrease in the refractive index.

The method for the making of a structure as shown in FIGS. 1A and 1B will be explained below in more detail.

The structure of FIG. 1B can readily be made by an epitaxial growth technique for semiconductors, that is, a cladding layer 3, optical conducting layer 2 and cladding layer 3 are sequentially grown over a semiconductor substrate 1 by an epitaxial growth method, such as a liquid phase epitaxy (LPE) or vapor phase epitaxy (VPE). If, at this time, the thickness of these layers has to be exactly controlled on the order of a few .mu.m or below, it is advantageous to employ a molecular beam epitaxy (MBE) or a metalorganic vapor phase epitaxy (MOVPE) method.

On the other hand, the structure of FIG. 1A is made as shown in FIG. 2. That is, an optical conducting layer 2 is grown as a crystal on a semiconductor substrate 1, by the epitaxial growth technique, as shown in FIG. 2A. Then a mask layer, such as an SiO.sub.2 film, is patterned, by a photoetching process (PEP), on the optical conducting layer 2 as shown in FIG. 2B and the optical conducting layer 2 is removed, by the etching process, at an area not covered with the mask 4 as shown in FIG. 2C. Cladding layers 3 are selectively grown, by the epitaxial growth technique, on the resultant exposed surface of the semiconductor substrate 1 as shown in FIG. 2C and, finally, the mask 4 on the optical conducting layer 2 is removed to expose the latter as shown in FIG. 2E.

However, the problems as will be set forth below arise in the making of the optical waveguide shown in FIG. 2A. First, the exposed side surfaces of the optical conducting layer 2 are injured or "side etched", by the etching process, during the making step as shown in FIG. 2C, or the exposed surfaces of the optical conducting layer are oxidized depending upon the material used at that process. It is, therefore, difficult to obtain a pure, exposed crystal face. Second, it is usually difficult to obtain flat boundaries relative to the side surfaces of the optical conducting layer 2 during the formation of the cladding layers relative to the optical conducting layer 2 by the selective epitaxial growth as shown in FIG. 2D, that is, to obtain buried cladding layers 3 of a flat boundary relative to the layer 2. To put it in another way, discontinuous surfaces are liable to be produced between the optical conducting layer and the cladding layer. For the reasons as set out above, such discontinuous surfaces and defects occur at the boundaries between the optical conducting layer and the cladding layers 3, thus causing the absorption or scattering of light at the boundaries of the optical waveguide. Third, it is generally difficult to form an epitaxial layer, by a selective epitaxial growth method, on the surface of the mask and remaining surface of the resultant structure, as shown in FIG. 2D, and it is possible to perform such a selective epitaxial growth under the specific conditions only.

Although forming a semiconductor layer of a different crystal composition in the direction parallel to the surface of the substrate has thus far bee explained, as a typical example (FIG. 1A), in connection with the optical waveguide, it is very difficult to employ a conventional process in the formation of such a semiconductor layer.

It is to be noted that the structure of the optical waveguide as shown in FIG. 1C is obtained by a combination of the process methods shown in FIGS. 1A and 1B.

For a semiconductor device in which semiconductor layers are formed as functional layers on a semiconductor substrate, in general, it is easier to sequentially form layers of different compositions on the semiconductor substrate in stacked fashion as shown in FIG. 1B, but it becomes very difficult to form such layers of different crystal compositions in a lateral direction, that is, in a direction parallel to a substrate surface as shown in FIG. 1A. The aforementioned wafer process often includes, for example, a selective etching and selective epitaxial growth process. Further, the semiconductor layers as stacked in a complex process over the semiconductor substrate contain discontinuous surfaces or crystal defects which are liable to occur at the boundaries between these layers of different crystal compositions. This is a cause for the degraded characteristics of a semiconductor device obtained.

It is accordingly an object of the present invention to provide a method for making a semiconductor layer or layers of less crystal defect which can readily deposit adjacent layer areas of a different crystal composition in a direction parallel to a substrate surface.

Another object of the present invention is to provide a laser diode which can readily form a window structure, by the aforementioned method, capable of suppressing a catastrophic optical damage (COD).

According to the present invention, there is provided a method for making a semiconductor device in which semiconductor layers of a different band gap and/or a different composition are formed by an epitaxial growth technique on first and second areas formed adjacent the semiconductor surfaces, said method comprising the steps of:

forming, adjacent a first area, a second area of a different composition or different electrical conduction, said first area being a portion of a major surface of a semiconductor substrate, to make the first and second area have a different surface temperature each at a heating step of a subsequent epitaxial growth process; and

deposing, by the epitaxial growth technique on exposed surfaces of the first and second areas, compound semiconductor layers of a different band gap energy and/or a different composition each, while being lattice-matched to both the areas.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIGS. 1A to 1C are perspective views showing a conventional optical waveguide;

FIGS. 2A to 2E are cross-sectional views showing the steps of making a conventional optical waveguide;

FIG. 3 is a cross-sectional view showing an optical waveguide according to a first embodiment of the present invention;

FIGS. 4A to 4C are cross-sectional views showing the steps of making an optical waveguide as shown in FIG. 3;

FIG. 5 is a cross-sectional view showing an optical waveguide according to a second embodiment of the present invention;

FIGS. 6A to 6E are cross-sectional views showing the steps of making the optical waveguide shown in FIG. 5;

FIG. 7 is a cross-sectional view showing an optical waveguide according to a third embodiment of the present invention;

FIGS. 8A to 8D are cross-sectional views showing the steps of making the optical waveguide shown in FIG. 7;

FIG. 9 is a perspective view showing a laser diode according to a fourth embodiment of the present invention;

FIG. 10 is a perspective view showing a laser diode according to a fifth embodiment of the present invention; and

FIGS. 11 and 12, each, are a characteristic curve showing a band gap energy to a growth temperature dependency relation of InGaP.

It has been known that when, of compound semiconductor materials, a III-V ternary compound such as InGaP or a III-V quaternary compound such as InGaAlP is grown by a vapor phase growth method, such as an MOVPE, on a semiconductor substrate such as GaAs, a different crystal structure or different crystal composition layer is formed depending upon growth conditions, such as the growth temperature, growth pressure, doping and so on.

FIG. 11 shows a growth temperature dependence, upon a band gap energy (Eg), of In.sub.0.5 Ga.sub.0.5 P grown as a crystal layer on a (100) GaAs substrate by an MOVPE method.

From a graph of FIG. 11 it will be seen that the band gap energy (Ep) of In.sub.0.5 Ga.sub.0.5 P varies to an extent of about 0.05 eV at a growth temperature of 650.degree. C. to 750.degree. C. Further, even if a feed supply rate is fixed in a crystal growth of InGaP, a resultant mixed crystal varies in composition depending upon the growth temperature, that is, the higher the growth temperature, the greater the value of In.sub.1-x Ga.sub.x P and hence the band gap energy (Eg). FIG. 11 shows a variation of the band gap energy (Eg) for the same composition, that is, In.sub.0.5 Ga.sub.0.5 P, but an amount of variation of Eg against the growth temperature is further increased, considered together with an effect of a variation of the mixed crystal's composition by the growth temperature. FIG. 12 is a graph showing the temperature dependence of Eg for In.sub.0.5 Ga.sub.0.5 P actually prepared by a low pressure MOVPE method, the effect including a lattice mismatching effect resulting from the growth temperature. It is to be noted that the band gap energy Eg is a value determined by a photoluminescence measurement.

On the other hand, it has been found that, when a semiconductor substrate, such as GaAs, is heated during a vapor phase growth process such as an MOVPE, the substrate surface's temperature varies depending upon the electric conductivity as well as the material of which the semiconductor substrate is formed. In actual practice, when an epitaxial layer is grown by the vacuum.

MOVPE method on a (100) GaAs substrate 350 .mu.m thick, then it is so formed as to have an area of different crystal composition and different band gap energy (Eg) in accordance with a substrate surface's temperature difference at that time.

According to the present invention it is possible to form those areas of different crystal composition and different band gap energy by giving a temperature difference to the substrate surface at different areas.

A semiconductor device according to a first embodiment of the present invention will be explained below in connection with an optical waveguide as an example.

FIG. 3 is cross-sectional view showing a modeled optical waveguide in the embodiment of the present invention. In FIG. 3, a semiconductor substrate 11 is composed of an n-type GaAs substrate 11a and crystal growth layer 14 formed on the substrate 11a at a given area.

An optical conducting layer 12 is formed, as a first epitaxial layer of InGaP, on the exposed surface of an n type GaAs substrate at an area (first region) not covered with the crystal growth layer 14. Cladding layers 13 are formed, as second epitaxial layer of InGaP, on the exposed surface of the crystal growth layer (second area) 14. A cap layer 15 which is formed over the optical conducting layer 12 and cladding layers 3 is formed of a material, such as GaAlAs, GaAs, which can be grown without depending upon the surface temperature. The crystal growth layer 14 is formed of, for example, GaAlAs, InGaAlP or ZnSe, which is lattice-matched to GaAs and In.sub.0.5 Ga.sub.0.5 P, and can be formed as a thin insulating layer if a lattice matching is obtained.

FIGS. 4A, 4B, and 4C are cross-sectional views showing making steps of an optical waveguide according to the first embodiment of the present invention. As shown in FIG. 4A, a thin InGaAlP crystal growth layer 14 of above 0.1 .mu.m is formed by an MOVPE method on an n-type GaAs substrate 11a . Then the growth layer 14 is partially removed by a PEP technique to expose the GaAs substrate 11A at selected areas as shown in FIG. 4B. As shown in FIG. 4C, InGaP layers 12 and 13 are grown, as a crystal layer of 0.5 to a few .mu.m thick, on the substrate, by virtue of the MOVPE method, under the same condition using a reaction gas at a feed supply rate In.sub.0.5 /Ga.sub.0.5 /P. At that time, the surface temperature of the crystal growth layer 14 becomes a higher level than that of the n type GaAs substrate 11a owing to a difference in composition between the n type GaAs substrate 11a and InGaAlP crystal growth layer 14. The surface temperature difference is maintained during the layer growth because an upper layer being grown is relatively thin. In spite of the fact that the layers 12 and 13 are grown over the semiconductor substrate under the same growth conditions, they are formed as growth layers of different Eg and crystal composition resulting from their surface temperature difference, that is, the InGaP layer 12 grown on the n type GaAs substrate 11a is made smaller in Eg, and greater in refractivity, than the layers 13 grown on the crystal growth layer 14. Subsequent to forming the InGaP layer 12 and 13 of a predetermined thickness over the substrate, an epitaxial GaAlAs layer 15 is formed as a cap layer of a few .mu.m to several tens of .mu.m on the resultant structure by means of an MOVPE method to provide an optical waveguide as shown in FIG. 3.

The present embodiment is directed to selectively forming a semiconductor layer on a substrate with their compositions made different from each other and forming upper semiconductor layers of different Eg and composition, utilizing a surface temperature difference between an area covered with the semiconductor layer and an area not covered with the semiconductor layer.

FIG. 5 is a cross-sectional view showing a model of an optical waveguide according to a second embodiment of the present invention. In FIG. 5, an n-type GaAs substrate 31a has a mesa-like projection and GaAs crystal growth layers 34 of high resistance are buried in the mesa-like recess of the substrate. The crystal growth layer 34 may be of such a type as to have a high resistance layers or a different electric conductivity from that of a substrate 31a, such as p type GaAs, or is formed of a different material from that of a substrate 31a, such as GaAlAs, InGaAlP or ZnSe. In the present embodiment, a liquid phase epitaxy (LPE) method is employed to obtain a planarized substrate surface in which case it is desirable to use a material for LPE. First and second epitaxial layers 32 and 33 of InGaP are formed on the exposed surface of the substrate 31a and crystal growth layer 34, respectively. A cap layer 35 of GaAlAs (or GaAs) is formed on the epitaxial layers 32 and 33.

FIGS. 6A-6E are cross-sectional views showing the making steps of an optical waveguide according to the second embodiment of the present invention. As shown in

FIG. 6A, a mask 30, such as an SiO.sub.2 film, is formed on the portion of an n type GaAs substrate 31a and, as shown in FIG. 6B, the GaAs substrate 31a is removed, by the PEP technique, at an area not covered with the mask 30, to are extent of about 0.5 to 1 .mu.m to provide mesa-like recesses 36. Then high resistance GaAs layers 34, are buried, in the mesa-like recess, by a selective liquid phase epitaxy method, as shown in FIG. 6C. The layers 34 may be formed of GaAlAs, InGaAlP or ZnSe. Then the mask 30 is etched away as shown in FIG. 6D and InGaP layers 32 and 33 are grown, by the MOVPE method, on the resultant structure, as shown in FIG. 6E, under the same growth conditions using a reaction gas at a molar ratio In.sub.0.5 /Ga.sub.0.5 /P.

It has been observed that, when an n-type Si-doped GaAs substrate of a carrier concentration 2.times.10.sup.18 to 4.times.10.sup.18 cm.sup.-3 and resistivity of 2.times.10.sup.-3 .OMEGA.cm, as well as semi-insulating, Cr-doped GaAs substrate of a resistivity 4.times.10.sup.18 to 7.times.10.sup.8 .OMEGA.cm, is placed on a susceptor heated to about 720.degree. C. in a carrier gas H.sub.2 atmosphere, the surface temperature of the semi-insulating substrate is about 20.degree. to 30.degree. C. higher than that of the n type GaAs substrate. It has also been found that, upon comparing the aforementioned GaAs substrate covered with a 50 .ANG. In.sub.0.5 Ca.sub.0.5 P layer and that not covered with the latter layer, the covered one is about 10.degree. to 15.degree. C. higher in its surface temperature under the same conditions than the one not covered with the In.sub.0.5 Ga.sub.0.5 P layer. Thus the exposed surface (first area) of the mesa-like substrate 31a is lower in its surface temperature than the grown crystal layers (second areas) 34. Therefore, the epitaxial layer 32 deposited on the exposed surface of the substrate 31a is made lower in Eg, and greater in light refractive index, than the epitaxial layers 33 deposited on the exposed surface cf the crystal growth layer 34.

Then the cap layer 35 of GaAlAs is formed on the layers 32 and 33 to provide a device as shown in FIG. 5.

This embodiment is directed to an example epitaxial growth layers of a different Eg are obtained due to a surface temperature difference relative to the substrate by forming mesa-like recess on the portion of the semiconductor substrate, burying the mesa-like recess with a material different in composition or in electric conductivity from the semiconductor substrate and forming the InGaP or InGaAlP layers on the corresponding exposed surface under the same conditions as set forth below.

FIG. 7 is a cross-sectional view showing a model of an optical waveguide according to a third embodiment of the present invention and FIGS. 8A-8D are cross-sectional views showing a making method of the optical waveguide shown in FIG. 7. As shown in FIG. 8A, a mask 50, such as an SiO.sub.2 film, is formed on a corresponding portion of an n type, (100) GaAs substrate 51a. Then as shown in FIG. 8B a proton (H.sup.+) is implanted into the substrate at those areas not covered with the mask 50 to provide high resistance layers 54. The mask is removed as shown in FIG. 8C and InGaP layers 52 and 53 are grown as crystal layers or the resultant structure by means of an MOVPE method under the same growth conditions using a reaction gas at a molar ratio of In.sub.0.5 /Ga.sub.0.5 /P--FIG. 8D. A cap layer 55 of GaAlAs (or GaAs) is formed on the layers 52 and 53 to obtain a optical waveguide as shown in FIG. 7.

Even in this embodiment, a surface temperature difference exists between the exposed surface of the n type GaAs substrate 51a and that of the high resistance layer 54 and, therefore, the InGaP epitaxial layers 52 and 53 over the underlying structure are different in Eg from each other.

Although, in the first to third embodiments as set out above, In.sub.x Ga.sub.1-x P layers have been explained as being grown as the first and second epitaxial layers on the GaAs substrate, the same result is also obtained even when In.sub.1-y (Ga.sub.1-x Al.sub.x).sub.y P layers are formed in place of the In.sub.x Ga.sub.1-x P layers. For a multi-layer structure composed of a combination of mixed crystals In.sub.x Ga.sub.1-x P and In.sub.1-y (Ga.sub.1-x Al.sub.x).sub.y P, the same result as set out above in connection with the preceding embodiment is obtained for the respective layers.

As the material for the n type GaAs substrate (11a , 31a, 51a) of the first to third embodiments, use may be made of a material for allowing In.sub.x Ga.sub.1-x P and In.sub.1-y (Ga.sub.1-x Al.sub.x).sub.y P to be epitaxially grown, such as Ga.sub.1-x Al.sub.x As, GaAs.sub.x P.sub.1-x, ZnSe, ZnS.sub.1-x Se.sub.x, Si, GaP, ZnS and InP.

For the semiconductor material other than InGaP and InGaAlP, the present invention can be applied to a material of such a nature as to allow its Eg or its crystal composition to vary in accordance with the substrate surface temperature at a time of epitaxial growth. The present invention is not restricted to the aforementioned layer thickness.

In order to relax a lattice mismatching between the exposed surfaces of the first and second areas of the substrate on one hand and the overlying epitaxial layers on the other hand, an underlying layer, such as a semi-conductor or other proper material, can be interposed at that in-between area. In this connection it is to be noted that the underlying layer needs to have such a thickness as to create a desired temperature difference, that is, a temperature difference necessary to form epitaxial layers of a different Eg or different crystal composition.

Applying the present invention to a laser diode will be explained below with reference with FIGS. 9 and 10. FIGS. 9 and 10 are perspective views showing fourth and fifth embodiments of the present invention, respectively. In these embodiments, a visible laser diode is shown which uses a double hetero-structure as an InGaP structure or an InGaAlP structure.

FIG. 9 shows an inner-strip (IS) laser diode of 670 nm band, comprising an n type GaAs substrate 71'; n type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P cladding layers 72, 72'; undoped In.sub.0.5 Ga.sub.0.5 P active layers 73, 73'; p type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P cladding layers 75, 75'; an n type GaAs current block layer 76; and a p type GaAs ohmic contact layer 77. In FIG. 9, the solid (black) areas indicate a laser emission plane and a laser beam is emitted in a direction as indicated by an arrow F in FIG. 9. When a high resistance area 74 is formed near the laser beam emission end face of the n type GaAs substrate, the n type cladding layer, active layer and p type cladding layer are grown over the underlying structure under a surface temperature difference between the substrate 71 and the high area 74 and, at the same time, are divided into those layers of a greater Eg and those layers of a smaller Eg in accordance with the high resistance area pattern on the substrate. Although the layers 73, 73' and layers 75, 75' are not in direct contact with the substrate 71 and high resistance area 4, the surface temperature difference is maintained because these interposing layers are very thin relative to the substrate. It is thus possible to grow these layers 73 and 73' and 75 and 75' of different Eg in a side-by-side relation. The layers of a greater Eg are formed near the laser beam emission end face of the device and are transparent to a laser beam of the inner layer of a smaller Eg. Thus a "window" structure can readily be provided which inhibits a catastrophic optical damage (COD) at the laser beam emission end face of the device.

The laser diode of the fifth embodiment as shown in FIG. 10 comprises a p type GaAs substrate 81; p type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P cladding layers 82, 82'; undoped In.sub.0.5 Ga.sub.0.5 P active layers 83, 83'; n type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P cladding layers 85, 85'; and an n type GaAs ohmic contact layer 87 in which case high resistance areas 84 are provided in the p type GaAs substrate 81 as shown in FIG. 10. By so doing, the p type cladding layer 82', active layer 83' and n type cladding layer 85' corresponding to a current injection area of a double hetero-structure have their Eg reduced relative to the surrounding structure. Stated in another way, the device is of a horizontal light conduction type as shown in FIG. 3 such that, relative to the light emission area at the current injection area and active layer, a surrounding area is made to have a small refractive index. It is, therefore, possible to effectively confine light emitted at the active layer. Since, in the fifth embodiment, the high resistance areas 84 on the substrate 81 serve also as current block layers, the present embodiment is advantageous in that it is simpler to construct and manufacture. From the above it will be appreciated that it is possible to readily obtain a laser diode from a combination of the fourth and fifth embodiments.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.