Patent Publication Number: US-2003235224-A1

Title: Strained quantum-well structure having ternary-alloy material in both quantum-well layers and barrier layers

Description:
BACKGROUND  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to semiconductor lasers and, in particular, to semiconductor lasers having strained barrier layers and strained quantum-well (QW) layers.  
       [0003] 2. Background Information  
       [0004] Semiconductor lasers are of considerable importance in many applications, particularly in fiberoptic communication networks where the lasers may be used as transmitters. In fiberoptic communication networks, semiconductor lasers having long-wavelength emission, i.e., on the order of 1-1.6 μm, are of interest. Wavelengths of 1.3 μm and 1.55 μm are particularly important for silica-based optical-fiber networks.  
       [0005] InP-based semiconductor lasers operating near 1.3 μm are expected to have considerable importance in future fiberoptic communication networks. However, before the potential for these lasers can be fully realized, InP-based lasers having low threshold current, good high-temperature performance, and good reliability (long-life) must be developed.  
       [0006] Methods for obtaining low threshold current and good reliability in InP-based lasers have been proposed in the literature. For example, as noted in the article “Low-Threshold (3.2 mA per Element) 1.3 μm InGaAsP MQW Laser Array on a p-Type Substrate” by Yamashita et al. (IEEE Photonics Tech. Lett. Vol. 4, No. 9, pp. 954-957 (1992)), threshold currents may be improved by utilizing short cavity lengths, high-reflection coatings, and multiple-quantum-well (MQW) structures, particularly MQW structures having strained QW layers.  
       [0007] If strained MQW structures are utilized in a semiconductor laser, it can be beneficial to engineer the QW layers and barrier layers with opposite strains to obtain good reliability (long life) as noted in the article “Long-term reliability of strain-compensated InGaAs(P)/InP MQW BH lasers” by Seltzer et al. (Electronics Lett. Vol. 30, No. 3, pp. 227-229 (1994)). Such strain compensation can prevent relaxation of the layer structure, which can otherwise cause crystal defects and device failure. Strain compensation can allow numerous strained QW layers and strained barrier layers to be utilized in an MQW laser while maintaining a reliable device.  
       [0008] Though the above-noted approaches may improve low threshold current and reliability in InP-based lasers, InP-based lasers continue to be plagued with poor performance at elevated temperatures. For example, the optical gain can decrease significantly with increasing temperature, and the threshold current can increase significantly with increasing temperature. As noted in the articles “Analysis of Temperature Dependent Optical Gain of Strained Quantum Well Taking Account of Carriers in the SCH Layer” by Ishikawa et al. (IEEE Photonics Tech. Lett. Vol. 6, No. 3, pp. 344-347 (1994)) and “Effect of Thermionic Electron Emission from the Active Layer on the Internal Quantum Efficiency of InGaAsP Lasers Operating at 1.3 μm” by Andrekson et al. (IEEE Journ. Quant. Elec. Vol. 30, No. 2, pp. 219-221 (1994)), the above-mentioned poor temperature characteristics are believed to be caused in part by poor carrier confinement (i.e., by carrier leakage). Improving carrier confinement in InP-based lasers by using layers with large barrier heights may improve the high-temperature performance of such lasers, as noted in the article “High-Temperature Operation of InGaAs/InGaAsP Compressive-Strained QW Lasers with Low Threshold Currents” by Nobuhara et al. (IEEE Photonics Tech. Lett. Vol. 5, No. 9, pp. 961-962 (1993)).  
       [0009] In “Temperature dependence of threshold current density J th  and differential efficiency η d  of high-power InGaAsP/GaAs (λ=0.8 μm) lasers” by Yi et al. (Appl. Phys. Lett. Vol. 66, No. 3, pp. 253-255 (1995)), an experimental and theoretical study on the temperature dependence of threshold current density and differential efficiency was described for certain GaAs-based lasers. Yi et al. disclosed that the major reason for increased threshold current density and decreased differential efficiency at high temperatures was thermal broadening of the gain spectrum, which results in reduction of the gain peak. However, Yi et al. further disclosed that the observed temperature dependence of the threshold current density and the differential efficiency could not be fully explained by thermal broadening of the gain spectrum. For the InGaAsP/GaAs lasers discussed therein, Yi et al. found that an increase in the momentum relaxation rate  /τ also contributed to the observed temperature dependence of the threshold current density and the differential efficiency and that the increase in the momentum relaxation rate  /τ mainly originated from alloy scattering. Alloy scattering refers to the scattering of carriers in alloys due to randomness in the placement of component atoms among the available lattice sites, such as noted in the article “Alloy scattering potential in p-type Ga 1−x Al x As” by Masu et al. (Jour. App. Phys. Vol. 54, No. 10, pp. 5785-5792 (1983)).  
       SUMMARY  
       [0010] Applicant has recognized that alloy scattering may be detrimental to the high-temperature performance of InP-based lasers and that increased disorder in the placement of component atoms on the available lattice sites can result in increased alloy scattering. The conventional InP-based lasers noted above utilize quaternary-alloy materials in the QW layer or the barrier layers (or both) and, and the use of such quaternary-alloy materials may exacerbate alloy scattering. It would be desirable to have an InP-based laser that operates at long wavelength, that incorporates a strained MQW structure to provide a low threshold current, that incorporates strain compensation to provide good reliability, and that provides good carrier confinement while simultaneously having reduced alloy scattering to enhance high-temperature performance. It would also be desirable to have a semiconductor laser with the above characteristics that is less complex than conventional long-wavelength semiconductor lasers and, accordingly, less costly to fabricate.  
       [0011] According to the present invention, there is provided a layer structure for use in a long-wavelength semiconductor laser that provides for low-threshold, long-wavelength operation with high reliability and good high-temperature performance. In addition, the present invention combines the benefits of strain compensation with reduced alloy scattering and can provide for less complexity and lower fabrication costs than are encountered for conventional long-wavelength semiconductor lasers.  
       [0012] In one aspect of the present invention, a structure in a semiconductor laser is provided. The structure comprises at least one quantum-well layer of a first semiconductor ternary-alloy material comprising two elements categorized in the same column of the periodic table of the elements. The structure further comprises a plurality of barrier layers of a second semiconductor ternary-alloy material comprising the same two elements. The two elements are provided at the same composition ratio in both the first and second semiconductor ternary-alloy materials. Each quantum-well layer is disposed between adjacent barrier layers. The two elements can be group-V elements.  
       [0013] In an exemplary aspect, the two elements can be As and P. In addition, the first ternary alloy can include In, and the second ternary alloy can include Ga. The quantum-well layer can comprise InAs 0.45 P 0.55 , and the barrier layers can comprise GaAs 0.45 P 0.55 .  
       [0014] In addition, the barrier layers and the quantum-well layer can be strained with opposite signs such that the structure is strain compensated. In particular, the quantum-well layer can be compressively strained such that the total strain from the quantum-well layer and the barrier layers is substantially zero. Further, the structure can be incorporated into a semiconductor laser having a substrate comprising InP. The structure can provide for light emission at a wavelength of substantially 1.3 μm.  
       [0015] In another aspect of the present invention, a multiple-quantum-well structure in a semiconductor laser is provided. The structure comprises a plurality of quantum-well layers of a first semiconductor ternary-alloy material and a plurality of barrier layers of a second semiconductor ternary-alloy material. The first and second semiconductor ternary alloy materials both comprise a first element and a second element, and the first and second elements are categorized in the same column of the periodic table of the elements. The first and second elements are provided at the same composition ratio in both the first and second semiconductor ternary-alloy materials.  
       [0016] The first and second elements can be group-V elements and in particular can be As and P. The quantum well layers can further comprise In, and the barrier layers can further comprise Ga. The quantum-well layers can comprise InAS 0.45 P 0.55 , and the barrier layers can comprise GaAs 0.45 P 0.55 . The barrier layers and quantum-well layers can be strained with opposite signs. In particular, the quantum-well layers can be compressively strained and the structure can be strain compensated. Further, the total strain from the quantum-well layers and barrier layers can be substantially zero. In addition, the structure can be incorporated into a semiconductor laser having an InP substrate. The structure can provide for light emission at a wavelength of substantially 1.3 μm.  
       [0017] In another aspect of the present invention, the above-mentioned multiple-quantum-well structure further comprises a plurality of spacer layers. Each quantum-well layer is disposed between a pair of adjacent barrier layers to form a plurality of three-layer structures wherein each spacer layer is disposed between a pair of adjacent three-layer structures.  
       [0018] In another aspect of the present invention, there is provided a method of fabricating a layer structure in a semiconductor laser. The method comprises providing a substrate, and forming at least one quantum-well layer of a first semiconductor ternary-alloy material and a plurality of barrier layers of a second semiconductor ternary-alloy material on the substrate. The first and second semiconductor ternary alloy materials both comprise a first element and a second element, the first and second elements being categorized in a same column of the periodic table of the elements. The first and second elements are provided at the same composition ratio in both the first and second semiconductor ternary-alloy materials.  
       [0019] It should be emphasized that the terms “comprises” and “comprising”, when used in this specification, are taken to specify the presence of stated features, integers, steps or components. However, the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0020] The foregoing and other objects, features and advantages of the present invention will be understood by reading the following detailed description in conjunction with the drawings.  
     [0021]FIG. 1 shows an exemplary layer structure comprising one QW layer and two barrier layers according to one aspect of the present invention.  
     [0022]FIG. 2 is an energy-level diagram for the structure shown in FIG. 1.  
     [0023]FIG. 3 shows an exemplary layer structure comprising a plurality of QW layers and a plurality of barrier layers according to another aspect of the present invention.  
     [0024]FIG. 4 is an energy-level diagram for the structure shown in FIG. 3.  
     [0025]FIG. 5 shows an exemplary layer structure comprising a plurality of QW layers and a plurality of barrier layers with intervening spacer layers according to another aspect of the present invention.  
     [0026]FIG. 6 shows a flow diagram for an exemplary method of making a layer structure according to the present invention. 
    
    
     DETAILED DESCRIPTION  
     [0027] Various aspects of the invention will now be described with respect to the Figures. The invention can be used, for example, in semiconductor lasers of a fiberoptic communication network. However, the invention is not limited to this use, but can instead be used in a wide range of applications.  
     [0028]FIG. 1 illustrates an exemplary layer structure  10  for use in a semiconductor laser according to one aspect of the present invention. The structure  10  comprises at least one quantum-well (QW) layer  11  made of a first semiconductor ternary-alloy material and a plurality of barrier layers  12  made of a second semiconductor ternary-alloy material. Each QW layer  11  is disposed between adjacent barrier layers  12 . The barrier layers  12  formed at opposing sides of the QW layer  11  confine carriers within the QW layer  11 . The QW layer  11  and barrier layers  12  are formed on a substrate  14 , which can be, for example, an InP substrate. Additional layers  13  (e.g., waveguide layers in which the laser radiation propagates) can be formed at outer surfaces of the barrier layers  12 . For example, a bottom additional layer  13  can be formed between a bottom barrier layer  12  and the substrate  14 . Not shown are additional layers such as cladding layers and electrodes, which can be conventionally provided in ways known to those skilled in the art. By utilizing ternary-alloy materials, the QW layer  11  and the barrier layer  12  have fewer elemental constituents than layers comprised of quaternary-alloy materials. In other words, the QW layer  11  and the barrier layers  12  have a reduced alloy number compared to layers comprising quaternary-alloy materials.  
     [0029] The first semiconductor ternary-alloy material and the second semiconductor ternary-alloy material each comprise two elements from the same column of the periodic table of the elements. Such elements may hereinafter be referred to as “same-group” elements. Further, the two elements are provided at the same composition ratio in both the first and second semiconductor ternary-alloy materials. The two elements can be group-V elements where group-V refers to the fifth column of the periodic table.  
     [0030] In the exemplary structure  10  illustrated in FIG. 1 the QW layer  11  and the barrier layers  12  can have opposite strains (relative to the substrate  14 ) and appropriate thicknesses such that effective strain compensation is achieved. In other words, though the QW layer  11  and the barrier layers  12  are strained, the amount of strain and the layer of thicknesses can be engineered such that the layer structure  10  does not relax (form dislocations). For example, according to one aspect of the present invention, the QW layer  11  can be compressively strained, and the barrier layers  12  can be strained in tension to balance the compressive strain in the QW layer  11 . The strain of a layer multiplied by the thickness of that layer gives the total strain vector of that layer. Effective strain compensation can be achieved by alternating compressive and tensile strains between adjacent layers and by choosing appropriate strain magnitudes and layer thicknesses such that the total strain (sum of the strain vectors of the QW layer  11  and the barrier layer  12 ) of the layer structure  10  is substantially zero or low enough to prevent relaxation. Further, the thickness of the QW layer  11  and the barrier layers  12  can be chosen below an appropriate critical thickness for each layer such that relaxation of the layers does not occur during fabrication of the layer structure  10 . Such strain compensation is known to those skilled in the art as described, for example, in “Design criteria for structurally stable, highly strained multiple quantum well devices” by D. C. Houghton et al. (Appl-Phys. Lett. Vol. 64, No. 4, pp. 505-507 (1994)).  
     [0031] Such strain compensation provides for good device reliability (long life) and allows numerous quantum-well layers and barrier layers to be utilized, such as in an exemplary layer structure comprising multiple quantum wells according to the present invention as described below in relation to FIG. 3. Further, compressively straining the QW layer  11  can reduce the threshold current of an associated laser by decreasing the density of hole states within the QW layer. Lowering the density of hole states can allow a population inversion and lasing to be obtained at, a lower applied current, i.e., the threshold current can be reduced.  
     [0032] In an exemplary aspect relating to the exemplary layer structure  10  shown in FIG. 1, the additional layers  13  (e.g., waveguide layers) can be formed of InP. The additional layers  13  can be disposed at outer surfaces of two 1 nm thick ternary-alloy barrier layers  12  made of GaAs 0.45 P 0.55 . The barrier layers  12  can have a tensile strain (e.g., 5.5%) relative to an InP substrate (not shown). The barrier layers  12  are disposed at opposing surfaces of a 7 nm thick, compressively strained (e.g., 1.5%) ternary-alloy QW layer  11  made of InAs 0.45 P 0.55 . The As/P composition ratio of 0.45/0.55 is the same for both the QW layer  11  and barrier layers  12 . Calculations indicate that transitions between heavy hole states and electron states can provide for output light emission at a wavelength of approximately 1.29 μm for a laser incorporating such a layer structure and fabricated on an InP substrate. Such light emission at substantially 1.3 μm is beneficial for optical communications.  
     [0033] As used in this specification, the term “ternary-alloy material” does not preclude the addition of dopants of other elements to make the ternary-alloy material p-type or n-type. Such doped ternary-alloy materials are considered ternary-alloy materials and as such can be used in making the layer structure according to the present invention.  
     [0034] According to another aspect of the present invention, there is provided a method of fabricating a layer structure of a semiconductor laser. FIG. 6 illustrates an exemplary method  40  of fabricating a layer structure. Referring to FIG. 1 and FIG. 6, the method comprises providing a substrate  14  (step  42 ), and forming at least one QW layer  11  of a first semiconductor ternary-alloy material and a plurality of barrier layers  12  of a second semiconductor ternary-alloy material on the substrate  14  (step  44 ). The substrate  14  can be, for example, a p-doped or n-doped substrate and can be, for example, an InP substrate. The QW and barrier layers  11  and  12  can be formed using any appropriate technique such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and liquid phase epitaxy (LPE) to name a few. These and other techniques are well known in the art and do not require further description. Multiple repeats of the QW layer  11  and the barrier layers  12  can be provided to form a MQW structure, such as illustrated with regard to the example of FIG. 3 described below. The QW layer(s)  11  and barrier layers  12  are formed such that the first and second semiconductor ternary alloy materials both comprise a first element and a second element, the first and second elements being categorized in the same column of the periodic table of the elements (step  44 ). In addition, the QW layer(s)  11  and barrier layers  12  are formed such that the first and second elements are provided at the same composition ratio in both the first and second semiconductor ternary-alloy materials (step  44 ).  
     [0035]FIG. 2 shows an energy-band diagram for the exemplary structure  10  shown in FIG. 1 according to the present invention. In FIG. 2, BE denotes the energy-band edge (band edge) for electrons, BH denotes the band edge for heavy holes, BL denotes the band edge for light holes, WE denotes a ground-state wave function of an electron, WH denotes a ground-state wave function of a heavy hole, and WL denotes a ground-state wave function of a light hole. In FIG. 2, the barrier height is denoted by reference character  14 . By virtue of barrier height  14 , carriers (electrons and holes) can be predominantly contained within the QW layer. However, it is evident from FIG. 2 that the wave function WE of an electron and the wave functions WH and WL of holes can extend into the barrier layers  12 . The significance of this observation will be further described below in relation to the design of the layer structure  10  to provide for reduced alloy scattering and improved high-temperature performance.  
     [0036] The present invention has advantages compared to conventional InP-based semiconductor lasers that use quaternary QW layers and barrier layers. First, by utilizing ternary-alloy material for the QW layer, the present invention reduces alloy scattering of carriers in the QW layer and thus provides for improved high-temperature performance compared to conventional quaternary InP-based lasers. For example, by reducing the number of atom types in the QW layer, the crystal lattice of the QW layer can more easily be fabricated with fewer defects, i.e., the imperfections in the crystal lattice can be reduced compared to conventional quaternary structures. With fewer imperfections in the crystal lattice, electrons and holes propagating through the lattice are less likely to be scattered, i.e., alloy scattering is reduced. By reducing alloy scattering, the exemplary structure  10  according to the present invention can provide improved high-temperature performance of an associated laser.  
     [0037] In addition, the present invention further reduces alloy scattering by forming the barrier layers  12  from ternary-alloy materials. As noted above with regard to FIG. 2, the wave functions of electrons and holes predominantly confined to the QW layer  11  can nevertheless penetrate into the barrier layers  12 . Thus, the barrier layers  12  can, therefore, contribute to alloy scattering. Accordingly, the present invention further reduces the likelihood of such scattering by forming the barrier layers  12  of ternary-alloy materials. Thus, the present invention further provides for improved high-temperature performance of associated lasers.  
     [0038] Also, by utilizing ternary-alloy materials having two elements from the same column of the periodic table at the same composition ratio in both the QW layer  11  and the barrier layers  12 , the present invention further reduces the likelihood of alloy scattering. In this manner, interdiffusion between the QW layer  11  and the barrier layers  12  can be minimized. For example, interdiffusion of group-V elements such as As and P can be substantially reduced compared to layer structures used in conventional InP-based lasers. Such interdiffusion is reduced in the present invention because the layer structure possesses no composition gradient of same-group elements to drive interdiffusion of those elements between the QW layer  11  and the barrier layers  12 . In contrast, conventional InP-based lasers utilizing InGaAs QW layers and InGaAsP barrier layers, for example, show strong interdiffusion of the group-V elements As and P at elevated temperatures as reported in “Vacancy controlled interdiffusion of the group V sublattice in strained InGaAs/InGaAsP quantum wells” by Gillin et al. (Appl. Phys. Lett. Vol. 63, No. 6, pp. 797-799 (1993)). The exemplary layer structure  10  described above minimizes such interdiffusion by providing As and P at the same composition ratio in both the QW layer  11  and the barrier layers  12 , thereby minimizing degradation in crystal quality and minimizing alloy scattering. Accordingly, the present invention is believed to have substantial advantages over structures utilized in conventional long-wavelength lasers including InP-based lasers.  
     [0039] In addition, minimizing interdiffusion improves the material quality and reliability of the QW layers  11  and barrier layers  12 . Further, by utilizing QW layers  11  and barrier layers  12  made of ternary-alloy materials having two same-group elements at the same composition ratio, the present invention can facilitate strain compensation, for example, because the complexity of the layer structure is reduced.  
     [0040]FIG. 3 illustrates an exemplary multiple-quantum-well (MQW) structure  20  for use in a semiconductor laser according to another aspect of the invention. The QW layers  21  are disposed between barrier layers  22 , which act to confine carriers in the QW layers  21 . Also shown are additional layers  23  (e.g., waveguide layers). The QW layers  21  and the barrier layers  22  are made of first and second semiconductor ternary-alloy materials, respectively. The first and second semiconductor ternary-alloy materials include two elements from the same column of the periodic table (same-group elements), the two elements being provided at the same composition ratio in the QW layers  21  and the barrier layers  22 . The two elements can be group-V elements.  
     [0041] The exemplary structure  20  of FIG. 3 can have opposite strains in the QW layers  21  and in the barrier layers  22  relative to a substrate (not shown). For example, the QW layers  21  can have a compressive strain. Compressive strain removes the energy degeneracy between light hole and heavy hole states, thereby reducing the density of hole states and thus reducing the threshold current of the laser. The overall structure  20  can be strain compensated as described in relation to the exemplary layer structure  10  illustrated in FIG. 1. As a result, a structure  20  with a large number of layers can be reliably produced and utilized.  
     [0042] The exemplary MQW structure  20  illustrated in FIG. 3 provides advantages compared to conventional quaternary InP-based semiconductor lasers such as discussed above in relation to the exemplary layer structure  10  illustrated in FIG. 1. In addition, the MQW structure  20  can also provide for increased optical gain compared to a comparable SQW structure.  
     [0043] In an exemplary aspect of the present invention relating to the exemplary layer structure  20  shown in FIG. 3, the additional layers  23  (e.g., waveguide layers) can comprise InP. The additional layers  23  can be disposed at outer surfaces of a superlattice of six QW layers  21  and corresponding barrier layers  22 . The outermost barrier layers  22  adjacent to the top and bottom additional layers  23  can be made of 1 nm thick GaAs 0.45 P 0.55 . The inner barrier layers  22  can be made of 2 nm thick GaAs 0.45 P 0.55 . The barrier layers  22  can have a tensile strain (e.g., 5.5%) relative to an InP substrate. Pairs of barrier layers  22  can be disposed at opposing surfaces of 7 nm thick, compressively strained (e.g., 1.5%) QW layers  21  made of InAS 0.45 P 0.55 . The As/P composition ratio of 0.45/0.55 is the same for both the QW layers  21  and the barrier layers  22 .  
     [0044]FIG. 4 shows an energy-band diagram for the exemplary MQW structure  20  of FIG. 3. In FIG. 4, BE denotes the band edge for electrons, BH denotes the band edge for heavy holes, and BL denotes the band edge for light holes. The barrier height is denoted by reference character  24 . In addition, FIG. 4 also illustrates wave functions of electrons, heavy holes, and light holes, denoted as WE, WH and WL, respectively. The narrow separation between QW layers  21  provided by barrier layers  22  results in strong quantum-mechanical coupling between carriers in adjacent wells. This coupling is reflected by the shapes of the wave functions as illustrated in FIG. 4. Though there is strong coupling, the barrier height and thickness of the barrier layers  22  provide for substantial confinement of electrons and heavy holes, as evidenced by their corresponding wave functions. In addition, this coupling between carriers in adjacent wells causes light emission at a longer wavelength of 1.31 μm compared to 1.29 μm for the exemplary layer structure  10  illustrated in FIG. 1. The light emission at substantially 1.3 μm is advantageous for optical communication as noted previously. The extent of this coupling can be adjusted by including additional spacer layers as described below in relation to FIG. 5.  
     [0045]FIG. 5 illustrates a portion of an exemplary MQW structure  30  for use in a semiconductor laser according to another aspect of the invention. The exemplary MQW structure  30  is similar to that illustrated in FIG. 3 but provides additional spacer layers  33  to adjust the separation between adjacent QW layers  31 . The QW layers  31  are disposed between barrier layers  32 , which act to confine carriers in the QW layers  31 . The QW layers  31  and the barrier layers  32  are made of first and second semiconductor ternary-alloy materials, respectively, as described in relation to the layer structure  20  illustrated in FIG. 3. The first and second ternary-alloy materials include two elements from the same column of the periodic table (same-group elements), the two elements being provided at the same composition ratio both the first and second ternary-alloy materials (i.e., in the QW layers  31  and the barrier layers  32 ). The two elements can be group-V elements. The structure  30  illustrated in FIG. 5 can be viewed as a plurality of three-layer structures separated by spacer layers  33  wherein each three-layer structure comprises a barrier layer  32 , a QW layer  31  and another barrier layer  32 .  
     [0046] If the spacer layer  33  is of sufficient thickness (e.g., 5-10 nm), it can substantially decouple the wave functions of carriers in adjacent QW layers  31 . The extent of coupling between carriers of adjacent QW layers  31  can affect the densities of states of the carriers and can also affect the wavelength of the emitted light.  
     [0047] In an exemplary aspect of the present invention relating to the exemplary layer structure  30  shown in FIG. 5, a superlattice of six QW layers  31  and corresponding barrier layers  32  can be arranged between two additional InP layers (not shown), which can function, for example, as waveguide layers. The barrier layers  32  can be made of 2 nm thick GaAs 0.45 P 0.55  and can have a tensile strain (e.g., 5.5%) relative to an InP substrate (not shown). Pairs of barrier layers  32  can be disposed at opposing surfaces of 7 nm thick, compressively strained (e.g., 1.5%) QW layers  31  made of InAs 0.45 P 0.55 . The As/P composition ratio of 0.45/0.55 is the same for both the QW layers  31  and the barrier layers  32 . In this exemplary aspect, the spacer layers  33  can be made of InP and can be 5-10 nm in thickness. Of course, other thicknesses for the spacer layers  33  can be used. Spacer layers  33  approximately 5-10 nm in thickness can substantially decouple the carriers in adjacent QW layers  31  given barrier layers  32  with thicknesses of 2 nm. Those skilled in the art will recognize that the thicknesses of the barrier layers  32  can also affect the coupling between carriers in adjacent QW layers  31 .  
     [0048] The invention has been described with reference to particular embodiments. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those described above. This can be done without departing from the spirit of the invention. The embodiments described herein are merely illustrative and should not be considered restrictive in any way. The scope of the invention is given by the appended claims, rather than the preceding description, and all variations and equivalents that fall within the range of the claims are intended to be embraced therein.