WO1998056090A1

WO1998056090A1 - Red-light semiconductor laser including gradient-composition layers

Info

Publication number: WO1998056090A1
Application number: PCT/US1998/005773
Authority: WO
Inventors: Mihail Dumitrescu; Marko U. Jalonen; Markus V. Pessa; Arto K. Salokatve; Antti P. Savolainen; Mika A. Toivonen
Original assignee: Coherent, Inc.
Priority date: 1997-06-02
Filing date: 1998-03-23
Publication date: 1998-12-10

Abstract

A semiconductor laser includes a quantum-well (QW) layer having semiconductor multilayer structures on each side thereof. Each of the semiconductor multilayer structures includes a waveguide (WG) layer for providing optical confinement, and at least one barrier layer between the QW layer and the WG layer for providing carrier confinement in the QW layer. The barrier layer includes a semiconductor material composition having a higher conduction-band energy (Ec) than the Ec of the QW layer and higher than the lowest Ec of the WG layer.

Description

RED-LIGHT SEMICONDUCTOR LASER INCLUDING GRADIENT-COMPOSITION LAYERS

BACKGROUND OF THE INVENTION The present invention relates in general to semiconductor lasers. It relates in particular to a semiconductor laser for providing light at a wavelength between about 630 and 690 nanometers (nm) including a quantum-well layer and multilayer structures on each side of the quantum-well layer, each multilayer including a waveguide layer and a barrier layer located between the waveguide layer and the quantum-well and a having a higher conduction-band energy (E_c) than the waveguide layer Semiconductor laser structures are designed to provide an optimum combination of optical and electrical aspects of the lasers . Electrical aspects include provision of sufficient carrier concentration, retention and recombination in an active layer or layers to provide optical gain. Optical aspects include confinement of laser light generated by carrier concentration and optimization of the distribution of light in an output beam.

Electrical aspects are influenced by conduction- band energy level and bandgap energy of semiconductor layer materials relative to each other. Optical aspects are influenced by the refractive index (n) of layer materials relative to each other. Electrical and optical aspects are influenced by layer thickness . Bandgap energy, conduction-band energy, and refractive index are all related to the chemical composition, thickness and doping of the semiconductor layers .

In prior art lasers this co-relationship or coupling of optical and electrical aspects of the lasers with material composition leads to what may be termed "contradictory coupling" issues between four key preferred properties of a semiconductor laser. These properties are: high carrier confinement in a quantum-well (QW) layer; high carrier confinement in waveguide (WG) layers adjacent the QW layer; high optical confinement of light in an active region comprising the QW and WG layers; and small far-field distribution of output laser radiation. Contradictory coupling means that any modification of one or more structural parameters which leads to an improvement of any one of the properties will lead to a deterioration of at least one other of the properties.

In red-light (for example 630-690 nm wavelength) lasers including WG and cladding layers formed from semiconductor compounds in the system {Al_(y)Ga₍₁._y) } (o.5i)^In(_o.₄₉)^p# where y <= 0.0 < 1.0, and a QW layer formed from a compound in the system Ga_(x)In₍₁._x)P, a layer parameter important to all four properties is the aluminum proportion y of the cladding and WG layers. This is illustrated schematically in FIG. 1, which illustrates, for example, that improving QW carrier confinement (by increasing Al proportion y in the WG layer) causes deterioration (all else being equal) of optical confinement. This is caused by an increase in conduction-band energy and decreasing refractive index as a result of the aluminum proportion increase in the WG layer.

It is believed that the above-discussed contradictory coupling issues have restricted performance improvements in prior art lasers . There is a need for a semiconductor laser structures in which the above-discussed key properties are significantly decoupled, to allow further improvement of laser performance.

SUMMARY OF THE INVENTION The present invention is directed to providing a semiconductor laser structure in which important properties, such as optical confinement and electrical confinement, may be individually optimized without a detrimental effect on another. This is accomplished in a semiconductor laser including a quantum-well layer and first and second semiconductor multilayer structures on each side of the quantum-well layer.

In one aspect of the invention, each of the multilayer structures comprises first, second, and third layers numbered in order of increasing distance from the quantum-well layer. The first layer adjoins the quantum-well layer and is one of a homogeneous composition (HC-) layer having a first conduction-band energy and a first refractive index, and a graded composition (GC-) layer having a conduction-band energy graded from a first conduction-band energy to a second conduction-band energy and a refractive index graded from a first refractive index to a second refractive index.

The second layer is a GC-layer having a conduction-band energy graded from said second conduction-band energy to a third conduction-band energy. The third layer is an HC-layer having a fourth conduction-band energy. The conduction-band energy grading and refractive index grading of the GC-layers is specified in a direction of increasing distance from the quantum-well layer.

The first conduction-band energy is greater than the conduction-band energy of the quantum-well layer and greater than the second conduction-band energy. The first refractive index is less than the second refractive index. The second conduction-band energy is greater than the conduction-band energy of the quantum-well layer. The third conduction-band energy is greater than the first conduction-band energy.

In another aspect, the invention is further characterized in that at least one of the multilayer structures further includes a fourth layer. The fourth layer is located on a side of the third layer furthest from the quantum-well layer and is one of: a GC-layer having a conduction-band energy graded from the fourth conduction-band energy at a junction thereof with the third layer to a fifth conduction- band energy, and an HC-layer having the fifth conduction-band energy.

In one preferred embodiment of a laser in accordance with the present invention, all layers other than the quantum-well layer have a general formula {Al_(y)Ga_{1._y) } ₍₀₅₁₎In₍₀₄₉₎P where 0.0 <= y < 1.0 and the relative magnitudes of the first, second, third, fourth, and fifth conduction band energies and the first, second, third, fourth, and fifth refractive indices are determined by the value of y in a corresponding composition. Conduction-band energy increases and refractive index decreases with increasing y.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention.

FIG. 1 is a diagram schematically illustrating contradictory coupling between four key preferred properties of prior-art semiconductor lasers having cladding and WG layers made from semiconductor compounds in the {Al_(y)Ga₍₁__y) } ₍₀₅₁₎In_{049)P system.

FIGS 2a and 2b are diagrams schematically illustrating relative conduction-band energy and refractive index of semiconductor layers in a first, preferred embodiment of a laser structure in accordance with the present invention.

FIGS 3a and 3b are diagrams schematically illustrating relative conduction-band energy and refractive index of semiconductor layers in a second embodiment of a laser structure in accordance with the present invention.

FIGS 4a and 4b are diagrams schematically illustrating relative conduction-band energy and refractive index of semiconductor layers in a third embodiment of a laser structure in accordance with the present invention.

FIGS 5a and 5b are diagrams schematically illustrating relative conduction-band energy and refractive index of semiconductor layers a fourth embodiment of a laser structure in accordance with the present invention.

FIGS 6a and 6b are diagrams schematically illustrating relative conduction-band energy and refractive index of semiconductor layers a fifth embodiment of a laser structure in accordance with the present invention.

FIGS 7a and 7b are diagrams schematically illustrating relative conduction-band energy and refractive index of semiconductor layers in a sixth embodiment of a laser structure in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Turning now to the drawings, FIG. 2a and FIG. 2b show the structure of one embodiment of a multilayer semiconductor laser 10 in accordance with the present invention. In FIG. 2a, the structure of laser 10 is illustrated in the form of a conduction- band energy diagram wherein relative conduction-band energy of the materials of layers of the structure is plotted as a function of approximate relative thickness of the layers. Junctions or interfaces between layers are indicated by discontinuities or singularities (changes of slope) in the plot. FIG. 2b depicts the relative refractive index of the layers as a function of approximate relative thickness of the layers. Here again, junctions or interfaces between layers are indicated by discontinuities or singularities (changes of slope) in the plot . Each value of E_c or n corresponds to a particular material composition.

The diagrams of FIGS 2a and 2b are types of diagrams commonly used in the art to which the present invention pertains to describe semiconductor lasers structures. Accordingly, these diagram types are used throughout this description.

Laser 10 includes two multilayer structures 12C and 12S in contact with a quantum-well layer QW. The layers of multilayer structures 12C and 12S are preferably, but not necessarily, symmetrically organized about the QW layer. Some relatively small variation in layer thickness and absolute values of conduction-band energy of layers may exist, however, the relative ordering of conduction band energies of corresponding layers must be the same in each multilayer structure. Further, as discussed hereinbelow, some variation of layer structure is possible in each of the multilayers while still including a minimum number of layers having conduction-band energy relationships therebetween which are important in the present invention.

As is usual in semiconductor laser art, all layers of laser 10 are assumed to be epitaxially- grown on a single crystal substrate or wafer. All layers of the structure except the QW layer are assumed to be lattice-matched to the substrate. The QW layer may be lattice mismatched to cause strain in the layer which, as is well known in the semiconductor laser art, can provide advantageous lasing properties. A contact layer, preferably of the material of the substrate, completes the structure. The substrate may

include one or more buffer layers (not shown) , grown thereon to promote epitaxy of the laser structure.

Typically, but not necessarily, laser 10 would be grown on an n-type substrate, with structures 12S and 12C including respectively n-doped and p-doped layers and forming opposite sides of a diode with the QW sandwiched therebetween. Structures 12C and 12S may be defined respectively contact and substrate side (of the QW) structures.

It can be seen from FIG. 2b that the refractive index relationship between layers of laser 10 is the inverse of the conductio -band energy relationship. This is characteristic of material systems which cause contradictory coupling problems in lasers made therefrom. Typically in such material systems, the relative conduction-band energy and refractive index relationships are determined primarily by the concentration or proportion of one element in the material system.

Continuing now with reference to FIGS 2a and 2b, a description of the characteristics and functions of individual layers in laser 10 is presented. This is done only for the layers of structure 12C, beginning with the layer adjacent the quantum-well. Corresponding layers of structure 12S have about the same characteristics and equivalent functions .

Layer Cl, a homogeneous composition layer or HC- layer, is a barrier layer which has a higher E_c and a lower refractive index than the QW layer. The term homogeneous composition, used and throughout this description and appended claim, means that the composition is homogeneous within the limits of manufacturing process control. Layer C3 , also a HC- layer, is a waveguide (WG) layer for providing optical confinement. Layer C3 has a lower E_c and a higher n than layer Cl . Layer C2 , located between layers Cl and C3 is a GC-layer having a composition graded (in a direction perpendicular to the plane of layers of the laser) from the composition of layers Cl the junction therewith to the composition of layer C3 at the junction therewith.

Barrier layer Cl is a layer which enables uncoupling of optical confinement from carrier confinement in the QW. The layer preferably has a thickness of about 10 nanometers (nm) . A disadvantage of this layer is that it decreases the efficiency of carrier injection into the QW. GC- layer C2 is provided to ease this problem. Layer C2 also preferably has a thickness of about 10 nm or less and is also considered a barrier layer. While layer C2 is effective in easing the carrier injection efficiency problem, it does present a potential secondary disadvantage, inasmuch as too great a total thickness of layers Cl and C2 would increase the distance of the layer C3 (the WG layer) from the QW and reduce optical confinement efficiency. Depending on the method of layer growth, providing a GC-layer having a thickness much less than 10 nm may prove impractical. In such a case, one or the other, but not both, of layers Cl and C2 may be omitted. Omitting layer Cl would allow layer C2 to have sufficient thickness to overcome the aforementioned potential practical difficulties in growth. Clearly, even if layer Cl were omitted, the E_c of barrier layer C2 at the QW interface could be made high enough to provide the desired carrier confinement thereby allowing layer C3 to have a lower Ec and a higher n.

It is emphasized here, that the provision of a thin barrier layer or layers adjacent the QW and having a higher E_c than an adjacent waveguide layer is considered a particularly novel aspect of the present invention. This arrangement transfers a major portion of the QW carrier confinement task to the barrier layers which allows a waveguide layer to have a lower E_c, and correspondingly a higher n for improving optical confinement.

While thin layers between a QW layer and a WG layer are not unknown in prior-art semiconductor lasers, these prior-art layers have a lower E_c than the WG layer, are less effective in providing electrical confinement than the WG layer, and are simply transition layers, provided to mitigate certain adverse effects which may exist at a QW-WG interface having a substantial difference in composition. In these prior-art lasers, the WG layer, in addition to providing optical confinement, still has the function of carrier confinement in the QW, and one of these properties can only be improved at the expense of the other.

Continuing now with the description of laser 10, it should be noted that, notwithstanding the decoupling of optical and carrier confinement properties, there remain some fundamental considerations which determine what the composition of WG layer C3 can be. For example, the composition of layer C3 is selected such that it has an E_c which is not less than the E_c of the QW-layer. This avoids electron tunneling out of the QW layer. Further, the composition of layer C3 must be selected such that it is above the Fermi level associated with the desired working regime, to avoid unwanted recombination of carriers in the WG layer. These considerations are known to those skilled in the art to which the present invention pertains but are presented briefly here for completeness of description.

Layer C5 is an electrical confinement layer which provides a barrier to prevent leakage of carriers out of WG layer C3 , and reduces coupling of the WG carrier confinement to the optical confinement and to far field distribution. Composition of layer C5 is selected such that the E_c of layer 5 is significantly higher than the E_c of layer Cl . A GC-layer C4 is graded in composition from the composition of layer C5 to the composition of layer C3, with E_c and n graded accordingly. This layer increases carrier injection efficiency into the WG region, at least partially offsetting some loss of carrier injection efficiency due to the high E_c of layer C5.

Layer C7 is a cladding layer. The presence of layer C5 permits that this layer can have lower E_c than would be necessary if layer C5 were not present . Layer C7 has a lower E_c than the E_c of layer C5. A lower E_c provides a higher refractive index, which improves far field distribution. Layer C7, however, must have an E_c higher than the E_c of layer Cl, and of course have a lower n than the n of layer Cl to provide good optical confinement. Layer C7 has a higher n index than the n of layer C5 this is advantageous in providing a narrow far-field distribution, but disadvantageous inasmuch as optical intensity in the QW is somewhat reduced.

Layer C6 is a GC-layer which is graded in composition, and accordingly in E_c and n, between the composition of layers C5 and C7. Layer C6 increases carrier injection efficiency into WG layer.

Layer C9 is an HC-layer which is provided for reducing the potential-barrier between the contact layer and cladding layer C7. This layer preferably has a composition selected to provide an E_c which is the average of the E_c values for the contact layer and cladding layer C7. Layer C8 is a GC-layer having a composition, and accordingly an E_c and n, graded from the composition of layer C9 to the composition of layer C7.

Structure 10 is described above primarily in terms of a relative ordering of Ec for layers of the structure, where the selection of a particular E_c (a particular composition) is at least partly motivated by selection of a desired n, the relationship of the desired n to the n of other layers is explicitly mentioned.

In general of course relative ordering n for the layers is the inverse of the relative ordering of E_c. Accordingly those refractive index relationships not mentioned will be implicit. In this regard it should be noted that the refractive index of layer C9 is unimportant since the cladding layer thickness is selected such that the optical field does not extend beyond cladding layer C7. The composition of layer C9 is selected entirely on the basis of above discussed E_c considerations

Further regarding the refractive index relationship between layers, barrier layers and waveguide layers should have a refractive index higher than, and confinement and cladding layers preferably have a refractive index less than a value n_eff, which may be summarily defined as the average of the refractive index distribution in the cladding, confinement, barrier and waveguide layers weighted by the respective values of the modal field distribution in the layers. These are well-known considerations for effective waveguiding and are mentioned only briefly here for completeness of description.

As discussed above, one material system which provides contradictory coupling problems is the system {Al_(y)Ga₍₁__y) } _(0.51)In₍₀₄₉₎P where 0.0 <= y < 1.0. TABLE 1 shows preferred thickness ranges, and compositions for layers Cl-9 in one example of laser 10 having a QW layer of Ga_(x)In₍₁._x)P where 0.35 <= x <= 0.46, for lasing at a wavelength between about 630 and 690 nm.

Approximate refractive index values corresponding to suggested composition ranges are also shown in TABLE 1 These represent what are believed to be the most reliable values derived from a number of sources which are sometimes contradictory. E_c values for the {Al_(y)Ga₍₁. _y) } (0.5_D ^In(o.₄9)^p system are well-documented in semiconductor literature and, accordingly, are not shown in TABLE 1.

It should be noted that the specification of composition ranges in TABLE 1 merely reflects that selection of a specific set of compositions for the any structure depends among other considerations on the desired lasing wavelength of a structure which in turn is dependent on the composition and Ec of the QW layer. This leads to overlapping ranges being specified for certain layers. For any given lasing wavelength, however, selection of a specific layer compositions must be consistent with above- discussed relative ordering of Ec ( and n) for the layers .

TABLE 1

The layers of TABLE 1 would of course be grown on a GaAs substrate and be furnished with a contact layer of GaAs. GaAs has a refractive index of about 3.806. The QW layer preferably has a thickness between about 5.5 and 9.0 nm and would have a refractive index of between about 3.35 and 3.50 in the preferred gallium concentration (x) range. The composition and thickness ranges, while specified in TABLE 1 for contact-side layers C1-C9 are equally applicable to substrate side layers S1-S9.

Those skilled in the semiconductor laser art will recognize that certain layers in structure 10 are doped layers. Preferably layers Cl, C2 and C3 , and corresponding substrate-side layers SI, S2 and S3 are undoped. All other layers are doped layers. When the substrate is n-type GaAs, substrate-side doped-layers are n-doped and contact-side doped- layers are p-doped. The doping profile is arranged to assure as smooth as possible a majority carrier flow into the active region and as high as possible barriers to minority carrier leakage out of the active region.

Continuing with reference to TABLE 1, it can be seen that certain layers are indicated as being optionally zero in thickness. This indicates that a particular one or more of these layers may be omitted from the structure of laser 10. This may be desired for example to simplify growth of the structure. Some loss of performance, of course, is to be expected. It should not be construed, however, that all of these layers can be optionally zero. It is emphasized in particular ( by asterisks in TABLE 1) that if any one of layers Cl and C2 is omitted from structure 10, or any other laser structure in accordance with the present invention, the other must be included. This is required, as discussed above to ensure that WG layer C3 can have a lower E_c and higher n, and be at least partly relieved of the responsibility for providing QW carrier confinement .

Similarly if any one of layers C3 and C4 is omitted from structure 10, or any other laser structure in accordance with the present invention, the other must be included. This is because there must be a layer providing a waveguide/optical confinement function.

Referring now to FIGS 3a and 3b, one simplified laser structure 20 (including multilayer structures 14C and 14S) in accordance with the present invention is illustrated. Numerals designating layers of structure 20 designate corresponding layers of structure 10. Thus it can be readily appreciated which of the layers of structure 10 have been omitted to arrive at structure 20. In the material system of TABLE 1 these corresponding layers of structures 10 and 20 may have about the same parameters .

In structure 20, layer C2 has the functions of layers Cl and C2 of structure 10, and layer C4 has the functions of Layers C3 and C4 of structure 10, becoming in effect the waveguide layer. Layers C5, C6, and C7 have precisely the same function as in structure 10, and layer C9 has the functions of layers C8 and C9 in structure 10. All of the GC- layers of structure 10 have been preserved in structure 20 for optimizing carrier injection and carrier flow in the structure. Structure 20 is but one example of a simplified semiconductor laser structure in accordance with the present invention. Based on the teachings herein, one skilled in the art may devise many combinations which embody principles of the present invention constituting simplifications or elaborations of structure 10 or elaborations of the relatively simple structures which

embody the inventive principles. A description of such relatively simple structures is set forth below.

Referring now to FIGS 4a and 4b, FIGS 5a and 5b, and FIGS 6a and 6b, structures 30, 40 and 50 (including multilayer structures 16C and 16S, 18C and 18S, and 19C and 19S) respectively are illustrated. These structures represent the simplest laser structures in accordance with the invention. In structure 30, HC-layer Cl alone provides barrier properties, while in structure 40, GC-layer C2 alone provides these properties. Layer C4 combines the functions of layers C3 and C4 in structure 10 thereby providing a waveguiding function while easing carrier injection into the waveguide region. Cladding layer C7 provides electrical confinement. In structure 50, GC-layer C4 is omitted and waveguide layer C3 is present. The barrier layer in structure 50 is depicted as being layer Cl alone, but could also be layer C2 alone .

The relative simplicity of structures 30, 40 and 50 notwithstanding, the novel aspect of the present invention (that which is important to decoupling the optical and electrical confinement properties of a waveguide layer) is maintained whether barrier layer Cl or barrier layer C2 adjoins the QW layer, and whether layer C3 or layer C4 provides a waveguide function for optical confinement. In each case, the E_c of the barrier layer at closest point to the QW is higher than the lowest E_c value in the waveguide layer, or simply the Ec of the waveguide layer when that layer is an HC-layer.

Layer structures in accordance with the present invention have been described with reference to a semiconductor laser having only a single quantum- well layer. Principles of the invention, however, are equally applicable to a laser including multiple quantum-wells (QW layers) separated by spacer layers, provided that the spacer layers separating the quantum-wells have a lower Ec than the highest E_c of layer C2 , or the E_c of layer Cl . By way of example such an embodiment of the present invention is illustrated in FIGS 7a and 7b. Here, a laser structure 60 (including multilayer structures 12C and 12S) in accordance with the present invention is similar to structure 10 with the exception that the QW layer of FIG. 10 is replaced by a QW structure including three QW layers QW1, QW2, and QW3 separated by two spacer layers Bl and B2. Spacer layers Bl and B2 are often referred to by practitioners of the semiconductor art as barrier layers. The term spacer layer is used here, however, to avoid confusion with barrier layers Cl and C2 of the present invention. These spacer layers must have an E_c less than the highest value of E_c in a barrier layer Cl or C2. The QW structure of structure 50 is but one example of a multiple QW structure which can be described generally as including N quantum-wells spaced apart by N-l spacer layers. The structure is bounded on each side thereof by a quantum-well layer.

General descriptions and specific example of laser structures in accordance with the present invention given above, the present invention is discussed with reference to vertical (perpendicular to the plane of layers) aspects of a laser structure. Those skilled in the art will recognize that consideration of lateral or longitudinal aspects of a laser in accordance with the present invention may lead to modifications of above presented structures without departing from the spirit and scope of the invention.

The present invention has been described in terms of a preferred and other embodiments . The invention is not limited however to the embodiments described and depicted. Rather, the invention is defined by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A semiconductor laser including a quantum- well layer and first and second semiconductor multilayer structures on each side of the quantum- well layer, the invention characterized in that each of the multilayer structures comprises: first, second, and third layers, said first, second, and third layers numbered in order of increasing distance from the quantum-well layer; said first layer adjoining the quantum-well layer and being one of a homogeneous composition (HC-) layer having a first conduction-band energy and a first refractive index, and a graded composition (GC-) layer having a conduction-band energy graded from a first conduction-band energy to a second conduction-band energy and a refractive index graded from a first refractive index to a second refractive index; said second layer being a GC-layer having a conduction-band energy graded from said second conduction-band energy to a third conduction-band energy; said third layer being an HC-layer having a fourth conduction-band energy; said conduction-band energy grading and refractive index grading of said GC-layers being specified in a direction of increasing distance from the quantum-well layer; said first conduction-band energy being greater than the conduction-band energy of said quantum-well layer and greater than said second conduction-band energy, said second conduction-band energy being greater than the conduction-band energy of said quantum-well layer, said third conduction- band energy being greater than said first conduction-band energy, said fourth conduction-band energy being less than or equal to said third conduction-band energy, and said first refractive index being less than said second refractive index.

2. The semiconductor laser of claim 1, the invention further characterized in that at least one of said multilayer structures further includes a fourth layer, said fourth layer on a side of said third layer furthest from the quantum-well layer and being one of a GC-layer having a conduction-band energy graded from the fourth conduction-band energy, at a junction thereof with said third layer, to a fifth conduction-band energy, and an HC-layer having the fifth conduction-band energy, said fifth conduction-band energy being less than the conduction-band energy of the quantum-well.

3. The semiconductor laser of claim 2, the invention further characterized in that said at least one of said multilayer structures further includes a fifth layer, said fifth layer being an HC-layer deposited on said fourth layer and having said fifth conduction-band energy.

4. The semiconductor laser of claim 1, the invention further characterized in that at least one of the multilayer structures further includes a fourth layer, said fourth layer being an HC-layer located between said second and third layers and having said third conduction-band energy, and further characterized in that in said at least one the multilayer structures said fourth conduction- band energy is less than said third conduction-band energy .

5. The semiconductor laser of claim 4, the invention further characterized in that said at least one of said multilayer structures further includes a fifth layer, said fifth layer being a GC- layer joining said fourth layer and said third layer and having a conduction-band energy graded from said third conduction-band energy at a junction with said fourth layer to said fourth conduction-band energy at a junction with said third layer.

6. The semiconductor laser of claim 1, the invention further characterized in that at least one of said multilayer structures further includes a fourth layer, said fourth layer located between said first and second layers and being an HC-layer having said second conduction-band energy and said second refractive index.

7. A semiconductor laser including a quantum- well region and first and second semiconductor multilayer structures on each side of the quantum- well region, the invention characterized in that: the quantum-well region includes N quantum- well layers and N-l spacer layers alternately arranged such that the quantum-well region is bounded on each side thereof by a quantum-well layer and said spacer layers having a higher conduction- band energy than the conduction-band energy of said quantum-wells ; each of the semiconductor multilayer structures includes first, second, and third layers, said first, second, and third layers numbered in order of increasing distance from the quantum-well region; said first layer closest a bounding one of said quantum-well layers and being one of a homogeneous composition (HC-) layer having a first conduction-band energy and a first refractive index, and a graded composition (GC-) layer having a conduction-band energy graded from a first conduction-band energy to a second conduction-band energy and a refractive index graded from a first refractive index to a second refractive index; said second layer being a GC-layer having a conduction-band energy graded from said second conduction-band energy to a third conduction-band energy; said third layer being an HC-layer having a fourth conduction-band energy; said conduction-band energy grading and refractive index grading of said GC-layers being specified in a direction of increasing distance from said quantum- ell region; and said first conduction-band energy being greater than the conduction-band energy of said spacer layers and greater than said second conduction-band energy, said second conduction-band energy being greater than the conduction-band energy of said quantum-well layer, said third conduction- band energy being greater than said first conduction-band energy, said fourth conduction-band energy being less than or equal to said third conduction-band energy, and said first refractive index being less than said second refractive index.

8. The semiconductor laser of claim 7, the invention further characterized in that at least one of said multilayer structures further includes a fourth layer, said fourth layer on a side of said third layer furthest from said quantum-well region and being one of a GC-layer having a conduction-band energy graded from the fourth conduction-band energy, at a junction thereof with said third layer, to a fifth conduction-band energy, and an HC-layer having the fifth conduction-band energy, said fifth conduction-band energy being less than the conductio -band energy of the quantum-well.

9. The semiconductor laser of claim 8, the invention further characterized in that said at least one of said multilayer structures further includes a fifth layer, said fifth layer being an HC-layer deposited on said fourth layer and having said fifth conduction-band energy.

10. The semiconductor laser of claim 7, the invention further characterized in that at least one of the multilayer structures further includes a fourth layer, said fourth layer being an HC-layer located between said second and third layers and having said third conduction-band energy, and further characterized in that in said at least one the multilayer structures said fourth conduction- band energy is less than said third conduction-band energy.

11. The semiconductor laser of claim 10, the invention further characterized in that said at least one of said multilayer structures further includes a fifth layer, said fifth layer being a GC- layer joining said fourth layer and said third layer and having a conduction-band energy graded from said third conduction-band energy at a junction with said fourth layer to said fourth conduction-band energy at a junction with said third layer.

.

12. The semiconductor laser of claim 7, the invention further characterized in that at least one of said multilayer structures further includes a fourth layer, said fourth layer located between said first and second layers and being an HC-layer having said second conduction-band energy and said second refractive index.

13. A semiconductor laser including a quantum- well layer and first and second semiconductor multilayer structures on each side of the quantum- well layer, the invention characterized in that each of the multilayer structures comprises: first, second, third, fourth, gradient composition (GC-) layers , said GC-layers numbered in order of increasing distance from the quantum- well layer; said first GC-layer having a composition graded from a first composition to a second composition, said second GC-layer having a composition graded from said second composition to a third composition, said third GC-layer having a composition graded from said third composition to a fourth composition, and said fourth GC- layer having a composition graded from said fourth composition to a fifth composition, said graded compositions of said GC-layers specified in a direction of increasing distance from the quantum-well layer; first and second homogeneous composition (HC-) layers, said first HC-layer located between said second and third GC layers and having said third composition, and said second HC-layer located between said third and fourth GC-layers and having said fourth composition; said first, second, third, fourth, and fifth compositions having respectively first, second, third, fourth, and fifth conduction band energies, and first second third, fourth and fifth refractive indices; and said first conduction-band energy greater than the conduction-band energy of said quantum-well layer, said second conduction-band energy less than said first conduction-band energy and greater than the conduction-band energy of the quantum-well, said third conduction-band energy greater than said first conduction-band energy, said fourth conduction-band energy less than said third conduction-band energy but greater than said first conduction-band energy, and said fifth conduction-band energy less than said second conduction-band energy, and said second refractive index greater than said first refractive index.

14. The semiconductor laser of claim 13, the invention further characterized in that at least one of the multilayer structures includes at least one of a group of HC-layers consisting of a third HC- layer located between the quantum-well layer and said first GC-layer and having said first composition, a fourth HC-layer located between said first and second GC-layers and having said second composition, and a fifth HC-layer located on the side of said fourth GC-layer furthest from the quantum-well layer and having said fifth composition.

15. The semiconductor laser of claim 14 wherein said first, second, fourth, and fifth compositions have a general formula {Al_(y)Ga₍₁__y) } ₍₀₅₁₎In₍₀₄₉₎P where 0.0 <= y < 1.0 and the relative magnitudes of said first, second, third, fourth, and fifth, conduction band energies and the relative magnitudes of said first, second, third, fourth, and fifth refractive indices are determined by the value of y in the corresponding composition, conduction band energy and refractive index respectively increasing and decreasing with increasing y.

16. The semiconductor laser of claim 15 wherein said first, second, fourth, and fifth compositions have y values of respectively yl, y2, y3 , y4 , and y5 , where y3 > y4 > yl > y2 > y5.

17. The semiconductor laser of claim 13, wherein said first, second, fourth, and fifth compositions have a general formula {Al_(y)Ga₍₁._y) } ₍₀₅₁₎In₍₀₄₉₎P where 0.0 <= y < 1.0 and the relative magnitudes of said first, second, third, fourth, and fifth, conduction band energies and the relative magnitudes of said first, second, third, fourth, and fifth refractive indices are determined by the value of y in the corresponding composition, conduction band energy and refractive index respectively increasing and decreasing with increasing y

18. The semiconductor laser of claim 17 wherein said first, second, fourth, and fifth compositions have values of y of respectively yl, y2, y3 , y4, and y5, where y3 > y4 > yl > y2 > y5.

19. A semiconductor laser including a quantum- well region and first and second semiconductor multilayer structures on each side of the quantum- well region, the invention characterized in that: the quantum-well region includes N quantum- well layers and N-l spacer layers alternately arranged such that the quantum-well region is bounded on each side thereof by a quantum-well layer, said spacer layers having a higher conduction-band energy than the conduction-band energy of said quantum-wells; each of the semiconductor multilayer structures includes first, second, third, fourth, gradient composition (GC-) layers , said GC-layers numbered in order of increasing distance from the quantum-well layer; said first GC-layer having a composition graded from a first composition to a second composition, said second GC-layer having a composition graded from said second composition to a third composition, said third GC-layer having a composition graded from said third composition to a fourth composition, and said fourth GC- layer having a composition graded from said fourth composition to a fifth composition, said graded compositions of said GC-layers specified in a direction of increasing distance from the quantum-well layer; each of the semiconductor multilayer structures includes first and second homogeneous composition (HC-) layers, said first HC-layer located between said second and third GC layers and having said third composition, and said second HC- layer located between said third and fourth GC- layers and having said fourth composition; said first, second, third, fourth, and fifth compositions having respectively first, second, third, fourth, and fifth conduction band energies, and first second third and fourth refractive indices; said first conduction-band energy greater than the conduction-band energy of said quantum-well layer, said second conduction-band energy less than said first conduction-band energy and greater than the conduction-band energy of the quantum-well, said third conductio -band energy greater than said first conduction-band energy, said fourth conduction-band energy less than said third conduction-band energy but greater than said first conduction-band energy, and said fifth conduction-band energy less than said second conduction-band energy, and said second refractive index greater than said first refractive index.

20. The semiconductor laser of claim 19, the invention further characterized in that at least one of the multilayer structures includes at least one of a group of HC-layers consisting of a third HC- layer located between the quantum-well layer and said first GC-layer and having said first composition, a fourth HC-layer located between said first and second GC-layers and having said second composition, and a fifth HC-layer located on the side of said fourth GC-layer furthest from the quantum-well layer and having said fifth composition.

21. The semiconductor laser of claim 20 wherein said first, second, fourth, and fifth compositions have a general formula {Al_(y)Ga₍₁__y) } ₍₀₅₁₎In₍₀₄₉₎P where 0.0 <= y < 1.0 and the relative magnitudes of said first, second, third, fourth, and fifth, conduction band energies and the relative magnitudes of said first, second, third, fourth, and fifth refractive indices are determined by the value of y in the corresponding composition, conduction band energy and refractive index respectively increasing and decreasing with increasing y.

22. The semiconductor laser of claim 21 wherein said first, second, fourth, and fifth compositions have y values of respectively yl, y2, y3 , y4 , and y5 , where y3 > y4 > yl > y2 > y5.

23. The semiconductor laser of claim 19, wherein said first, second, fourth, and fifth compositions have a general formula {Al_(y)Ga₍₁._y) } ₍₀₅₁₎In₍₀₄₉₎P where 0.0 <= y < 1.0 and the relative magnitudes of said first, second, third, fourth, and fifth, conduction band energies and the relative magnitudes of said first, second, third, fourth, and fifth refractive indices are determined by the value of y in the corresponding composition

24. The semiconductor laser of claim 23 wherein said first, second, fourth, and fifth compositions have values of y of respectively yl, y2, y3 , y4, and y5 , where y3 > y4 > yl > y2 > y5.

25. In a semiconductor laser including a quantum-well layer and first and second semiconductor multilayer structures on each side of the quantum-well layer, the invention characterized in that at least one of said multilayer structures comprises : first, second, third, fourth, fifth, sixth seventh, eighth, and ninth layers numbered in order of increasing distance from the quantum-well layer; said first layer adjacent the quantum-well layer, and having a first homogeneous composition providing a conduction-band energy greater than the conduction-band energy of the quantum-well layer; said third layer having a second homogeneous composition providing a conduction-band energy greater than the conduction-band energy of the quantum-well layer but less than the conduction- band energy of said first layer; said second layer having a gradient composition varying from said first composition at a junction with said first layer to said second composition at a junction with said third layer said fifth layer having a third homogeneous composition providing a conduction-band energy greater than the conduction-band energy of said first layer; said fourth layer having a gradient composition varying from said second composition at a junction with said third layer to said third composition at a junction with said fifth layer; said seventh layer having a fourth homogeneous composition providing a conduction-band energy greater than the conduction-band energy of said first layer but less than the conduction-band energy of said fifth layer; said sixth layer having a gradient composition varying from said third composition at a junction with said fifth layer to said fourth composition at a junction with said seventh layer; said ninth layer having a fifth homogeneous composition providing a conduction-band energy less than the conduction-band energy of said third layer; and said eighth layer having a gradient composition varying from said fourth composition at a junction with said seventh layer to said fifth composition at a junction with said ninth layer.

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