Abstract:
An optical modulator comprises a first waveguide layer and a barrier layer, and a quantum well layer sandwiched between the first waveguide layer and the barrier layer, where the quantum well layer has a graded composition that varies the bandgap energy of the quantum well layer between a minimum bandgap energy and the bandgap energy of at least one of the first waveguide layer and the barrier layer.

Description:
BACKGROUND OF THE INVENTION 
     Semiconductor-based optical modulators have been developed using many different technologies and are used for modulating light for optical communications. One such semiconductor-based optical modulator is referred to as an electro-absorption modulator. In one example, a semiconductor electro-absorption optical modulator is used to modulate light in the 1550 nanometer (nm) wavelength for use in a long range fiber-optic communication system. A typical semiconductor electro-absorption optical modulator is fabricated using wafer processing technology. The optical modulator is typically reverse biased by an external electrical circuit so that when an input light source is coupled into the optical modulator, the input light is converted by one or more quantum wells in the optical modulator into a photocurrent. The wavelength of light at which the quantum wells in the optical modulator absorb photons and convert the photons to a photocurrent is dependent upon the material used to fabricate the layers of the optical modulator and the electrical bias applied to the optical modulator. The wavelength of light at which the quantum wells in the optical modulator absorb photons and convert the photons to a photocurrent can also be influenced by exploiting what is referred to as the Quantum Confined Stark Effect. The Quantum Confined Stark Effect is a phenomenon that allows the ground state subband energy separation of the material used to form the quantum well of the modulator to be reduced using a reverse electrical bias applied to the optical modulator. The reduction of the subband energy creates what is referred to as a “field effect” optical modulator. By employing the Quantum Confined Stark Effect, the speed at which an optical modulator can operate greatly exceeds the speed at which a conventional directly-modulated semiconductor laser can transmit data. 
     Generally, the bandgap of the material used to form the quantum well layer is lower than the bandgap of the material used to form the barrier layers that sandwich each quantum well layer. When the optical modulator is appropriately electrically biased, input light directed toward the quantum well is absorbed to generate electrical charge carriers, i.e., electrons in the conduction band and holes in the valence band, in the quantum well. The electron-hole pairs are then extracted from the quantum well to develop a photocurrent. The material used to form the quantum well layer and the electrical bias applied to the optical modulator greatly influences the absorption coefficient of the quantum well in the optical modulator. The absorption coefficient is a measure of the ability of the quantum well to absorb light and generate electron-hole pairs. 
     The material used to form the quantum well and the material used to form the barrier layers greatly influences the ability of the quantum well layer to release the photogenerated electron-hole pairs to generate the photocurrent. For example, a high energy barrier at the junction of the quantum well layer and the barrier layer provides a well-defined quantum state that exhibits a high absorption coefficient. However, a high energy barrier at the junction of the quantum well layer and the barrier layer also makes it difficult to extract the electron-hole pairs and generate a large photocurrent. If the photogenerated carriers are not efficiently extracted, an internal e-field will be created causing the response of the optical modulator to be slowed and causing the absorption characteristic to saturate with respect to incident power. 
     Therefore, it is desirable to provide an optical modulator that exhibits a high absorption coefficient and that generates a large photocurrent, while minimizing saturation and maintaining fast response at high optical power. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention provides an optical modulator, comprising a first waveguide layer, a barrier layer, and a quantum well layer sandwiched between the first waveguide layer and the barrier layer, where the quantum well layer has a graded composition that varies the bandgap energy of the quantum well layer between a minimum bandgap energy and the bandgap energy of at least one of the first waveguide layer and the barrier layer. 
     The invention also provides a method for generating a photocurrent comprising providing a quantum well structure having at least one element that forms a graded composition within the quantum well structure, the graded composition varying the bandgap energy of the quantum well structure, directing light onto the quantum well structure, converting the light into electron-hole pairs in the quantum well, and extracting the electron-hole pairs from the quantum well to generate a photocurrent. 
     The semiconductor optical modulator having a quantum well structure for increasing effective photocurrent generating capability substantially aligns the peaks of the wave functions of the electrons in the conduction band and the holes in the valence bands, respectively, in the quantum well. In one embodiment, the quantum well structure is formed with a quantum well layer that has a graded bandgap energy profile. In another embodiment, the quantum well structure is formed with an additional layer of material having a graded composition located adjacent the quantum well layer. The alignment of the electron and hole wave functions increases the absorption coefficient of the quantum well. The quantum well structure provides a high energy barrier at the interface of the quantum well and the barrier layer, while allowing efficient extraction of the photogenerated carriers to provide a large photocurrent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1A  is a schematic diagram illustrating a portion of a conventional semiconductor optical modulator  10 . 
         FIG. 1B  is an energy band diagram showing the conduction band profile and the valence band profile for the optical modulator of  FIG. 1A  under reverse electrical bias. 
         FIG. 2A  is a schematic diagram illustrating a portion of a semiconductor optical modulator fabricated in accordance with an embodiment of the invention. 
         FIG. 2B  is an energy band diagram showing the conduction band profile and the valence band profile for the optical modulator of  FIG. 2A  under zero or a modest forward electrical bias, also referred to as a “flat band” condition. 
         FIG. 3A  is a schematic diagram illustrating a portion of the semiconductor optical modulator of  FIG. 2A  under a reverse electrical bias. 
         FIG. 3B  is an energy band diagram showing the conduction band profile and the valence band profile for the optical modulator of  FIG. 3A  under reverse electrical bias. 
         FIG. 4A  is a schematic diagram illustrating a portion of a semiconductor optical modulator  200  fabricated in accordance with an alternative embodiment of the invention. 
         FIG. 4B  is an energy band diagram showing the conduction band profile and the valence band profile for the optical modulator of  FIG. 4A  under zero or a modest forward electrical bias. 
         FIG. 5A  is a schematic diagram illustrating a portion of the semiconductor optical modulator of  FIG. 4A  under a reverse electrical bias. 
         FIG. 5B  is an energy band diagram showing the conduction band profile and the valence band profile for the optical modulator  200  of  FIG. 5A  under reverse electrical bias. 
         FIG. 6A  is a schematic diagram illustrating a portion of a semiconductor optical modulator fabricated in accordance with another embodiment of the invention. 
         FIG. 6B  is an energy band diagram showing the conduction band profile and the valence band profile for the optical modulator of  FIG. 6A  under reverse electrical bias. 
         FIG. 7  is a flowchart illustrating a method of generating photocurrent by light absorption and subsequent extraction of carriers in a quantum well of an optical modulator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The semiconductor optical modulator having a quantum well structure for increasing photocurrent generating capability will be described below as being implemented in an indium phosphide (InP) material system, and specifically in an optical modulator fabricated using indium gallium arsenide phosphide (InGaAsP) on an InP substrate. However, the semiconductor optical modulator having a quantum well structure for increasing photocurrent generating capability can be implemented in devices fabricated using other materials, such as aluminum gallium indium arsenide (AlGaInAs). 
       FIG. 1A  is a schematic diagram illustrating a portion of a conventional semiconductor optical modulator  10 . The optical modulator  10  includes a p-type material layer  12  over which a waveguide layer  14  is formed. A quantum well layer  16  is formed over the waveguide layer  14 . A barrier layer  18  is formed over the quantum well layer  16 . Another quantum well layer  22  is formed over the barrier layer  18  and another waveguide layer  26  is formed over the quantum well layer  22 . An n-type material layer  28  is formed over the waveguide layer  26 . The material that forms the layers of optical modulator  10  can be chosen from the indium phosphide material system, or from other material systems, depending on the desired operating characteristics of the optical modulator  10 . 
     The quantum well layer  16  forms a quantum well  11  and the quantum well layer  22  forms a quantum well  14 . In this example, the waveguide layer  14  also functions as a barrier layer for the quantum well  11  and the waveguide layer  26  functions as a barrier layer for the quantum well  14 . The waveguide layer  14 , quantum well layer  16  and the barrier layer  18  form a quantum well structure  20 . The barrier layer  18 , the quantum well layer  22  and the waveguide layer  26  form a quantum well structure  22 . The material of the quantum well layers  16  and  22  has a lower bandgap than the material of the waveguide layers  14  and  26  and the material of the barrier layer  18 . While two quantum well structures are shown in  FIG. 1A , a greater or lesser number of quantum well structures may be formed in the optical modulator  10 . 
       FIG. 1B  is an energy band diagram  15  showing the conduction band profile and the valence band profile for the optical modulator  10  of  FIG. 1A  under reverse electrical bias. The optical modulator  10  typically operates under a reverse electrical bias. The profile of the conduction band  52  and the profile of the valence band  54  show the variation of band energy with distance. 
     In the example shown, the conduction band energy of the material of the quantum well layers  16  and  22  is less than the energy of the waveguide layers  14  and  26  and the barrier layer  18 . The valence band profile is generally a mirror image of the conduction band profile, in which the valence band energy of the material of the quantum well layers  16  and  22  is greater than the energy of the waveguide layers  14  and  26  and the barrier layer  18 . The conduction band and valence band energy changes abruptly at the interface of the quantum well layer  16  and the waveguide layer  14  and the barrier layer  18 . Similarly, the conduction band and valence band energy changes abruptly at the interface of the quantum well layer  22  and the waveguide layer  26  and the barrier layer  18 . 
     When reverse biased, the optical modulator  10  operates to convert light at the wavelength directed onto the quantum well structures  20  and  22  into electron-hole pairs in the quantum wells  11  and  14 . For example, light absorption will cause a bound electron  62  to be generated in the quantum well  11  in the conduction band  52  and a bound hole  64  to be generated in the quantum well  11  in the valence band  54 . In order for the optical modulator  10  to respond rapidly and not suffer from saturation due to light absorption at high optical powers, the photogenerated electrons and holes must be extracted from the quantum wells and swept toward the contacts (not shown, but in electrical contact with the p-type material layer  12  and the n-type material layer  28 ). However, to be extracted, the electrons and holes must overcome the energy barrier at the interface of the quantum well layer and the adjacent barrier layers, then drift in the applied field toward the surrounding n- and p-type layers, respectively, and be collected at the contacts to become a photocurrent. 
     The wave function of the electrons in the conduction band is illustrated at  66  and the wave function of the holes in the valence band is illustrated at  68 . As shown in  FIG. 1B , the peak of the electron wave function  66  is displaced by a distance, “d”, from the peak of the hole wave function  68 . This displacement between the peaks of the wave functions  66  and  68  occurs due to the electric field across the quantum well structures  20  and  22 . As a result of the reverse bias, the electron distribution in the conduction band is concentrated toward the n-type side of the optical modulator, and the hole distribution in the valence band is concentrated toward the p-type side of the optical modulator  10 . This concentration of electrons and holes is illustrated in the displacement between the peaks of the respective wave functions  66  and  68 . The concentration of electrons and holes as described means the spatial overlap between the electron and hole wavefunctions is reduced and results in an absorption coefficient that is lower than the absorption coefficient possible if the peaks of the respective wave functions  66  and  68  were more closely aligned. The misalignment of the peaks limits the absorption coefficient of the optical modulator  10 . 
       FIG. 2A  is a schematic diagram illustrating a portion of a semiconductor optical modulator  100  fabricated in accordance with an embodiment of the invention. The optical modulator  100  includes a p-type material layer  102  over which a waveguide layer  104  is formed. A quantum well layer  106  is formed over the waveguide layer  104 . In this embodiment of the invention, the quantum well layer  106  is formed to have a graded composition that is not uniform throughout the thickness of the layer. In this example, the quantum well layer  106  has a thickness of approximately 5–10 nanometers (nm) and has a minimum bandgap energy at the quantum well layer-waveguide layer interface and a maximum bandgap energy that approximates the bandgap energy of the barrier layer  108  at the quantum well layer-barrier layer interface. In this example, the quantum well layer  106  is linearly graded having a bandgap energy that increases in the direction from the waveguide layer  104  toward the barrier layer  108 , but other grading can also be employed. 
     A barrier layer  108  is formed over the quantum well layer  106 . Another quantum well layer  112  with graded composition is formed over the barrier layer  108  and another waveguide layer  116 , similar in structure to the waveguide layer  104 , is formed over the quantum well layer  112 . In this example, the quantum well layer  112  has a thickness of approximately 5–10 nanometers (nm), but can have a thickness different than the thickness of the quantum well layer  106 . The quantum well layer  112  has a minimum bandgap energy at the quantum well layer-barrier layer interface and a maximum bandgap energy that approximates the bandgap energy of the waveguide layer  116  at the quantum well layer-waveguide layer interface. In this example, the quantum well layer  112  is linearly graded having a bandgap energy that increases in the direction from the barrier layer  108  toward the waveguide layer  116 , but other grading can also be employed. Further, the minimum bandgap energy at the interface of the quantum well layer  106  and the waveguide layer  104  can be different than the minimum bandgap energy at the interface of the quantum well layer  112  and the barrier layer  108 . 
     An n-type material layer  118  is formed over the waveguide layer  116 . The material that forms the layers of optical modulator  100  can be chosen from the indium phosphide material system, or from other material systems, depending on the desired wavelength and operating characteristics of the optical modulator  100 . Further, the layers of the optical modulator  100  can be formed using known semiconductor processing techniques, such as organo-metallic vapor phase epitaxy (OMVPE), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or other known processing techniques. 
     In this example, the waveguide layer  104  also functions as a barrier layer for the quantum well  110  and the waveguide layer  116  functions as a barrier layer for the quantum well  114 . The waveguide layer  104 , quantum well layer  106  and the barrier layer  108  form a quantum well structure  120 . The barrier layer  108 , the quantum well layer  112  and the waveguide layer  116  form a quantum well structure  122 . The quantum well structures  120  and  122  respectively define quantum wells  110  and  114 . While two quantum well structures are shown in  FIG. 2A , a greater or lesser number of quantum well structures may be formed in the optical modulator  100 . 
     The alloy material of the quantum well layer  106  includes the same elements as the waveguide layer  104  and barrier layer  108 , but the composition is adjusted to reduce the bandgap energy of the quantum well layer  106  relative to that of the waveguide layer  104 . In this embodiment, the alloy composition in the quantum well layer  106  is progressively changed in the direction away from the waveguide layer  104  resulting in an increase in the bandgap energy of the quantum well layer  106  as the distance away from the waveguide layer  104  is increased. The alloy composition can also be changed using multiple stepped changes. Similarly, the alloy material of the quantum well layer  112  includes the same elements as the waveguide layer  116  and barrier layer  108 , but the composition is adjusted to reduce the bandgap energy of the quantum well layer  112  relative to that of the barrier layer  108 . In this embodiment, the alloy composition in the quantum well layer  112  is progressively changed in the direction away from the barrier layer  108  resulting in an increase in the bandgap energy of the quantum well layer  112  as the distance away from the barrier layer  108  is increased. 
     The graded composition of the quantum well layers  106  and  112  alter the shape of the quantum wells  110  and  114 , respectively, to enhance the extraction of photogenerated carriers from the quantum well structures  120  and  122 , and increase the absorption coefficient. The grading applied to the composition of quantum well layers  106  and  112  alters the energy profile of the quantum wells  110  and  114  in the conduction band and the valence band so that the electrons and holes in the conduction band and valence band, respectively, concentrate at the interface of the quantum well  106  and the waveguide layer  104 , and at the interface of the quantum well layer  112  and the barrier layer  108 , promoting the formation of electron-hole pairs in those areas. This results in a reduction of the distance “d” between the peak of the electron wave function and the hole wave function as compared to a conventional optical modulator. 
     In the example, a modulator designed to operate at 1550 nm wavelength is shown. The material of the p-type layer  102  is indium phosphide (InP), and the material of the waveguide layer  104  is indium gallium arsenide phosphide (InGaAsP) having the exemplary composition In 0.85 Ga 0.15 As 0.33 P 0.67 . The material of the quantum well layer  106  is In x Ga 1−x As y P 1−y , where 0&lt;x&lt;1 and 0&lt;y&lt;1, and where the indium fraction x and the arsenic fraction y vary through the thickness of the quantum well layer. In this example, the indium fraction progressively increases with distance from the waveguide layer  104  and the arsenic fraction progressively decreases with distance from the waveguide layer  104 . In this example, the indium fraction is about 0.61 and the arsenic fraction is about 0.84 at the interface of the quantum well layer  106  and the waveguide layer  104 . The indium fraction increases progressively to a maximum of about 0.85 and the arsenic fraction progressively decreases to about to 0.33 at the interface of the quantum well layer  106  and the barrier layer  108 . Here, the composition value x is defined as the ratio of the concentration of indium atoms in the material to the concentration of all Group III atoms (in this example, indium plus gallium) in the material. Likewise, the composition value y is defined as the ratio of the concentration of arsenic atoms in the material to the concentration of all Group V atoms (in this example, arsenic plus phosphorous) in the material. For example, a phosphorous fraction of one percent means that, on average, in every 100 Group V atoms in the material there is one atom of phosphorous. For InGaAs 0.99  P 0.01  (InGaAsP with a phosphorous fraction of 0.01 and an arsenic fraction of 0.99) of every 100 Group V atoms in the material, on average there is one atom of phosphorous and 99 atoms of arsenic. 
     The material of the barrier layer  108  is (In 0.85 Ga 0.15 As 0.33  P 0.67 ). The material and fabrication of the quantum well layer  112  is typically similar to the material and the fabrication of the quantum well layer  106 . The material of the waveguide layer  116  is (In 0.85 Ga 0.15 As 0.33  P 0.67 ) and the material of the n-type layer  118  is InP. 
     The fabrication of the optical modulator  100  described above can also be applied to an optical modulator fabricated using the aluminum indium gallium arsenide (AlInGaAs) material system. The AlInGaAs material system is also lattice matched to InP and may be used in modulators designed to operate in the 1300–1350 nm wavelength communication band. 
       FIG. 2B  is an energy band diagram  150  showing the conduction band profile and the valence band profile for the optical modulator  100  of  FIG. 2A  under zero or a modest forward electrical bias, also referred to as a “flat band” condition. 
     The quantum well having a graded bandgap energy profile is shown at  110  in the conduction band  152  and in the valence band  154 . The electrons  162  in the conduction band  152  and the holes  164  in the valence band  154  are now concentrated on the p-side of the quantum well  110 . Correspondingly, the peak of the electron wave function  166  closely aligns with the peak of the hole wave function  168  near the interface of the quantum well layer  106  and the waveguide layer  104 , substantially reducing the distance between the peaks of the wave functions  166  and  168 . This results in a greater spatial overlap between the photogenerated carriers and results in a larger absorption coefficient and a greater extinction ratio. The extinction ratio is the ratio of the light absorption between the modulator&#39;s “on” and “off” states. 
     The graded energy profile of the quantum well layers  106  and  112  also enhances the ability to extract the electron-hole pairs from the quantum wells  110  and  114 . The graded energy profile of the quantum well  110  provides a lower energy barrier to the electron  162 , thus maximizing the response and limiting saturation effects of the optical modulator  100 . 
       FIG. 3A  is a schematic diagram illustrating a portion of the semiconductor optical modulator  100  of  FIG. 2A  under a reverse electrical bias. The semiconductor optical modulator  100  is shown for reference with respect to the energy band diagram  150  of  FIG. 3B  and a description thereof will not be repeated. 
       FIG. 3B  is an energy band diagram  155  showing the conduction band profile and the valence band profile for the optical modulator  100  of  FIG. 3A  under reverse electrical bias. 
     In operation, a reverse electrical bias is applied to the optical modulator  100  to exploit the Quantum Confined Stark Effect. As mentioned above, the Quantum Confined Stark Effect is a phenomenon that allows the subband energy separation of the material used to form the quantum wells  110  and  114  of the optical modulator  100  to be shifted using a reverse electrical bias applied to the optical modulator  100 . With a given applied reverse bias voltage, if the shift of the subband energy separation is sufficient, light with a particular wavelength will be absorbed and converted to a photocurrent by the optical modulator  100 . The shift of the subband energy separation changes proportionally with the applied reverse bias voltage. The energy at which the optical modulator  100  absorbs light is referred to as the absorption edge. Increasing the reverse electrical bias lowers the energy corresponding to the absorption edge of the optical modulator. The shifting of the bandgap edges creates what is referred to as a “field effect.” A field effect optical modulator can operate at speeds on the order of 40–50 gigabits per second (Gb/sec). 
     The quantum well having a graded bandgap energy profile is shown at  110  in the conduction band  152  and in the valence band  154 . As mentioned above, the electrons  162  in the conduction band and the holes  164  in the valence band are concentrated on the p-side of the quantum well  110 . Correspondingly, the peak of the electron wave function  166  closely aligns with the peak of the hole wave function  168 , substantially reducing the distance between the peaks of the wave functions  166  and  168 . This results in a greater spatial overlap between the photogenerated carriers and results in a larger absorption coefficient and a greater extinction ratio. 
     The graded energy profile of the quantum well layers  106  and  112  also reduces the energy required to extract the electrons from the quantum wells  110  and  114 . The graded energy profile of the quantum well  110  provides a lower energy barrier to the electron  162  in the quantum wells  110  and  114 , thus maximizing the photocurrent that can be generated by the optical modulator  100  and the speed at which the optical modulator  100  can operate. In an alternative embodiment in which the quantum well layers are graded in the opposite direction such that the bandgap energy at the interface of the quantum well layer  106  and the barrier layer  108  is higher in the valence band than the bandgap energy at the interface of the quantum well layer  106  and the waveguide layer  104  in the valence band, then the energy required to extract holes from the quantum wells  110  and  114  would be reduced. 
       FIG. 4A  is a schematic diagram illustrating a portion of a semiconductor optical modulator  200  fabricated in accordance with an alternative embodiment of the invention. The optical modulator  200  includes a p-type material layer  202  over which a waveguide layer  204  is formed. A quantum well layer  206  is formed over the waveguide layer  204 . In this embodiment of the invention, the quantum well layer  206  is formed to have a graded composition that is not uniform throughout the thickness of the layer. In this example, the quantum well layer  206  has a thickness of approximately 5–10 nanometers (nm) and has a maximum bandgap energy that approximates the bandgap energy of the waveguide layer  204  at the quantum well layer-waveguide layer interface and a minimum bandgap energy at the quantum well layer-barrier layer interface. In this example, the quantum well layer  206  is linearly graded having a bandgap energy that decreases in the direction from the waveguide layer  204  toward the barrier layer  208 , but other grading can also be employed. 
     A barrier layer  208  is formed over the quantum well layer  206 . Another graded composition quantum well layer  212  is formed over the barrier layer  208  and another waveguide layer  216 , similar in structure to the waveguide layer  204 , is formed over the quantum well layer  212 . In this example, the quantum well layer  212  has a thickness of approximately 5–10 nanometers (nm), but can have a thickness different than the thickness of the quantum well layer  206 . The quantum well layer  212  has a maximum bandgap energy that approximates the bandgap energy of the barrier layer  208  at the quantum well layer-barrier layer interface and a minimum bandgap energy at the quantum well layer-waveguide layer interface. In this example, the quantum well layer  212  is linearly graded having a bandgap energy that decreases in the direction from the barrier layer  208  toward the waveguide layer  216 , but other grading can also be employed. Further, the minimum bandgap energy at the interface of the quantum well layer  206  and the barrier layer  208  can be different than the minimum bandgap energy at the interface of the quantum well layer  212  and the waveguide layer  216 . 
     An n-type material layer  218  is formed over the waveguide layer  216 . The material that forms the layers of optical modulator  200  can be chosen from the same group of materials as the optical modulator  100 . Further, the layers of the optical modulator  200  can be formed using the same techniques as those used to form the optical modulator  100 . 
     In this example, the waveguide layer  204  also functions as a barrier layer for the quantum well  210  and the waveguide layer  216  functions as a barrier layer for the quantum well  214 . The waveguide layer  204 , quantum well layer  206  and the barrier layer  208  form a quantum well structure  220 . The barrier layer  208 , the quantum well layer  212  and the waveguide layer  216  form a quantum well structure  222 . The quantum well structures  220  and  222  respectively define quantum wells  210  and  214 . While two quantum well structures are shown in  FIG. 4A , a greater or lesser number of quantum well structures may be formed in the optical modulator  200 . 
     The alloy material of the quantum well layer  206  includes the same elements as the waveguide layer  204  and barrier layer  208 , but the composition is adjusted to reduce the bandgap energy of the quantum well layer  206  relative to that of the barrier layer  208 . In this embodiment, the alloy composition in the quantum well layer  206  is progressively changed in the direction away from the waveguide layer  204  resulting in a decrease in the bandgap energy of the quantum well layer  206  as the distance away from the waveguide layer  204  is increased. The alloy composition can also be changed using multiple stepped changes. Similarly, the alloy material of the quantum well layer  212  includes the same elements as the waveguide layer  216  and barrier layer  208 , but the composition is adjusted to reduce the bandgap energy of the quantum well layer  212  relative to that of the waveguide layer  216 . In this embodiment, the alloy composition in the quantum well layer  212  is progressively changed in the direction away from the barrier layer  208  resulting in a decrease in the bandgap energy of the quantum well layer  212  as the distance away from the barrier layer  208  is increased. 
     The graded composition quantum well layers  206  and  208  alter the shape of the quantum wells  210  and  214 , respectively, to enhance the extraction of photogenerated carriers from quantum well structures  220  and  222 , and increase the absorption coefficient. The grading applied to the composition of the quantum well layers  206  and  212  alters the energy profile of the quantum wells  210  and  214  in the conduction band and the valence band so that the electrons and holes in the conduction band and valence band, respectively, concentrate at the interface of the quantum well  206  and the barrier layer  208 , and at the interface of the quantum well layer  212  and the waveguide layer  216 , promoting the formation of electron-hole pairs in those areas. The grading of the material of the quantum well layers  206  and  212  in the embodiment shown in  FIG. 4A  causes the carriers to concentrate toward the n-type side of the respective quantum well layers  206  and  212 . This results in a reduction of the distance between the peak of the electron wave function and the hole wave function as compared to a conventional square-well optical modulator. 
     In the example shown in  FIG. 4A , the material of the layers can be similar to the corresponding material layers of the optical modulator  100  shown in  FIG. 2A , the detail of which will not be repeated. In this example, the indium fraction in the quantum well layer  206  progressively decreases with distance from the waveguide layer  204  and the arsenic fraction in the quantum well layer  206  progressively increases with distance from the waveguide layer  204 . In this example, the indium fraction is about 0.85 and the arsenic fraction is about 0.33 at the interface of the quantum well layer  206  and the waveguide layer  204 . The indium fraction decreases progressively to about 0.61 and the arsenic fraction increases to about 0.84 at the interface of the quantum well layer  206  and the barrier layer  208 . The material and fabrication of the quantum well layer  212  can be similar to the material and the fabrication of the quantum well layer  206 . 
       FIG. 4B  is an energy band diagram  250  showing the conduction band profile and the valence band profile for the optical modulator  200  of  FIG. 4A  under zero or a modest forward electrical bias. 
     The quantum well having a graded bandgap energy profile is shown at  210  in the conduction band  252  and in the valence band  254 . The electrons  262  in the conduction band  252  and the holes  264  in the valence band  264  are concentrated on the n-side of the quantum well  210 . Correspondingly, the peak of the electron wave function  266  closely aligns with the peak of the hole wave function  268  near the interface of the quantum well layer  206  and the barrier layer  208 , substantially reducing the distance between the wave functions  266  and  268 . This results in a greater spatial overlap between the photogenerated carriers and results in a larger absorption coefficient and a greater extinction ratio. 
     The graded energy profile of the quantum well layers  206  and  212  also enhances the ability to extract holes from the quantum wells  210  and  214 . The graded energy profile of the quantum well  210  provides a lower energy barrier to the hole  264  toward the p-type side of the quantum wells  210  and  214 , thus improving response and limiting saturation effects of the optical modulator  200 . 
       FIG. 5A  is a schematic diagram illustrating a portion of the semiconductor optical modulator  200  of  FIG. 4A  under a reverse electrical bias. The semiconductor optical modulator  200  is shown for reference with respect to the energy band diagram  255  of  FIG. 5B  and a description thereof will not be repeated. 
       FIG. 5B  is an energy band diagram  255  showing the conduction band profile and the valence band profile for the optical modulator  200  of  FIG. 5A  under reverse electrical bias. 
     As described above, in operation, a reverse electrical bias is applied to the optical modulator  200  to exploit the Quantum Confined Stark Effect. The quantum well having a graded bandgap energy profile is shown at  210  in the conduction band  252  and in the valence band  254 . In this embodiment, the electrons  262  in the conduction band and the holes  264  in the valence band are concentrated on the n-side of the quantum well  210 . Correspondingly, the peak of the electron wave function  266  closely aligns with the peak of the hole wave function  268 , substantially reducing the distance between the peaks of the wave functions  166  and  168 . This results in a greater spatial overlap between the photogenerated carriers and results in a larger absorption coefficient and a greater extinction ratio. 
     The graded energy profile of the quantum well layers  206  and  212  also reduces the energy required to extract holes from the quantum wells  210  and  214 . The graded energy profile of the quantum well  210  provides a lower energy barrier to the hole  264  toward the p-type side of the quantum wells  210  and  214 , thus maximizing the photocurrent that can be generated by the optical modulator  200 . 
       FIG. 6A  is a schematic diagram illustrating a portion of a semiconductor optical modulator  300  fabricated in accordance with another embodiment of the invention. The optical modulator  300  includes a p-type material layer  302  over which a waveguide layer  304  is formed. In this embodiment of the invention, a graded-composition layer  325  is formed over the waveguide layer  304  and a fixed-composition layer  306  is formed over the graded-composition layer  325 . The graded-composition layer  325  and the fixed-composition layer  306  form a quantum well layer  330 . In this example, the fixed-composition layer  306  has a thickness of approximately 5–10 nanometers (nm). The graded-composition layer  325  is formed having an alloy composition that is not uniform throughout the thickness of the layer. In this example, the graded-composition layer  325  has a thickness of approximately 1–10 nanometers (nm) and has a minimum bandgap energy at the graded-composition layer-fixed-composition layer interface and a maximum bandgap energy that approximates the bandgap energy of the waveguide layer  304  at the graded-composition layer-waveguide layer interface. In this example, the graded-composition layer  325  is linearly graded having a bandgap energy that decreases in the direction from the waveguide layer  304  toward the fixed-composition layer  306 , but other grading can also be employed. Further, the term “fixed” does not preclude small variations in composition due to process variations, etc. 
     A barrier layer  308  is formed over the fixed-composition layer  306 . Another graded-composition layer  327  is formed over the barrier layer  308  and another fixed-composition layer  312  is formed over the graded-composition layer  327 . The graded-composition layer  327  and the fixed-composition layer  312  form a quantum well layer  334 . Another waveguide layer  316 , similar in structure to the waveguide layer  304 , is formed over the fixed-composition layer  312 . In this example, the fixed-composition layer  312  has a thickness of approximately 5–10 nanometers (nm), but can have a thickness different than the thickness of the fixed-composition layer  306 . 
     The graded-composition layer  327  has a minimum bandgap energy at the graded-composition layer-fixed-composition layer interface and a maximum bandgap energy that approximates the bandgap energy of the barrier layer  308  at the graded-composition layer-barrier layer interface. In this example, the graded-composition layer  327  is linearly graded having a bandgap energy that decreases in the direction from the barrier layer  308  toward the fixed-composition layer  312 , but other grading can also be employed. Further, the minimum bandgap energy at the interface of the graded-composition layer  325  and the fixed-composition layer  306  can be different than the minimum bandgap energy at the interface of the graded-composition layer  327  and the fixed-composition layer  312 . 
     An n-type material layer  318  is formed over the waveguide layer  316 . The material that forms the layers of optical modulator  300  can be chosen from the indium phosphide material system, or from other material systems, depending on the desired operating characteristics of the optical modulator  300 . Further, the layers of the optical modulator  300  can be formed using known semiconductor processing techniques as described above. 
     In this example, the waveguide layer  304  also functions as a barrier layer for the quantum well layer  330  and the waveguide layer  316  function as a barrier layer for the quantum well layer  334 . The waveguide layer  304 , quantum well layer  330  and the barrier layer  308  form a quantum well structure  320 . The barrier layer  308 , quantum well layer  334  and the waveguide layer  316  form a quantum well structure  322 . The quantum well structure  320  defines a quantum well  310  and the quantum well structure  322  defines a quantum well  314 . While two quantum well structures are shown in  FIG. 6A , a greater or lesser number of quantum well structures may be formed in the optical modulator  300 . 
     The alloy material of the fixed-composition layers  306  and  312  are of a composition that reduces the bandgap energy of the fixed-composition layers  306  and  312  relative to that of the waveguide layers  304  and  316  and the barrier layer  308 . In this embodiment, the fixed-composition layers  306  and  312  have a fixed composition resulting in a constant bandgap energy. The graded-composition layer  325  is formed from the same elements as the waveguide layer  304  and the barrier layer  308 , but its alloy composition is progressively changed to reduce the bandgap energy of the graded-composition layer  325  relative to that of the waveguide layer  304  and the barrier layer  308 . In this embodiment, the graded-composition layer  325  is formed using InGaAsP in which the indium content is progressively decreased and the arsenic content is progressively increased as the distance from the waveguide layer  304  increases, resulting in a reduction in the bandgap energy of the graded-composition layer  325  as the distance from the waveguide layer  304  increases. The alloy composition can also be changed using multiple stepped changes. Similarly, the graded-composition layer  327  is formed from the same elements as the waveguide layer  316  and the barrier layer  308 , but its alloy composition is progressively changed to reduce the bandgap energy of the graded-composition layer  327  relative to that of the waveguide layer  316  and the barrier layer  308 . In this embodiment, the graded-composition layer  327  is formed using InGaAsP in which the indium content is progressively decreased and the arsenic content is progressively increased as the distance from the barrier layer  308  increases, resulting in a reduction in the bandgap energy of the graded-composition layer  327  as the distance from the barrier layer  308  increases. 
     The graded-composition layers  325  and  327  alter the shape of the quantum wells  310  and  314 , respectively, to enhance the extraction of photogenerated charge carriers from the quantum well structures  320  and  322  and increase the absorption coefficient, while providing well defined quantum wells  310  and  314 . The grading applied to the graded-composition layers  325  and  327  alters the energy profile at the fixed-composition layer-graded-composition layer interfaces in the conduction band and the valence band so that the electrons and holes in the conduction band and valence band, respectively, are generated in well-defined quantum wells  310  and  314 , while providing a reduced energy barrier at the fixed-composition layer-graded-composition layer interfaces, so that holes can be easily extracted from the quantum wells  310  and  314 . In an alternative embodiment, reversing the grading direction of the graded-composition layers  325  and  327  would improve the extraction of electrons from the optical modulator  300 . 
     In the example shown, the material of the p-type layer  302 , the waveguide layers  304  and  316  and the barrier layer  308  is similar to the respective layers of the optical modulators  100  and  200  described above. 
     In an example, the material of the fixed-composition layers  306  and  312  is In 0.61 Ga 0.39 As 0.84 P 0.16 . The material of the graded-composition layer  325  is indium gallium arsenide phosphide (In x Ga 1−x As y P 1−y ), where 0&lt;x&lt;1 and 0&lt;y&lt;1, and where the indium content x and the arsenic content y vary through the thickness of the graded-composition layer. In this example, the indium fraction progressively decreases and the arsenic fraction progressively increases with increasing distance from the waveguide layer  304 . In this example, the indium content is about 0.7 and the arsenic content is about 0.7 at the interface of the graded-composition layer  325  and the fixed-composition layer  306 . The indium content increases to about 0.85 and the arsenic content decreases to about 0.33 at the interface of the graded-composition layer  325  and the waveguide layer  304 . 
     The fabrication of the optical modulator  300  described above can also be applied to an optical modulator fabricated using the aluminum indium gallium arsenide (AlInGaAs) material system. The AlInGaAs material system is also lattice matched to InP and may be used in modulators designed to operate in the 1300–1350 nm wavelength communication bands. 
     The material of the graded-composition layer  327  is similar to that of the graded-composition layer  325 . 
       FIG. 6B  is an energy band diagram  355  showing the conduction band profile and the valence band profile for the optical modulator  300  of  FIG. 6A  under reverse electrical bias. 
     As described above, in operation, a reverse electrical bias is applied to the optical modulator  300  to exploit the Quantum Confined Stark Effect. The quantum well energy profile is shown at  310  in the conduction band  352  and in the valence band  354 . The graded-composition layer energy profile provides well-defined quantum wells  310  and  314 , while still allowing the carriers to be easily extracted from the quantum wells  310  and  314  so that the optical modulator  300  may be rapidly modulated and operated with minimal saturation. 
     The graded energy profile of the graded-composition layers  325  and  327  provides a lower energy barrier to the hole  364  toward the p-type side of the quantum wells  310  and  314 , thus maximizing the response and limiting saturation effects of the optical modulator  300 . In an alternative embodiment, reversing the grading direction of the graded-composition layers  325  and  327  would provide a lower energy barrier to the electron  362  toward the n-type side of the quantum wells  310  and  314 , providing similar benefit as described above. 
       FIG. 7  is a flowchart  400  illustrating a method of generating photocurrent by light absorption and subsequent extraction of carriers in a quantum well of an optical modulator. Although specific operations are disclosed in the flowchart  400 , such operations are exemplary. Embodiments of the present invention are suited to performing various other operations or variations of the operations recited in the flowchart  400 . Further, the operations in the flowchart  400  can be performed in an order different that that described. In block  402 , a graded-composition quantum well structure is provided. In block  404 , light is absorbed in an optical modulator. In block  406 , electron-hole pairs are generated in the graded quantum well structure of an optical modulator. In block  408 , carriers are efficiently extracted from the graded quantum well structure. 
     This disclosure describes the invention in detail using illustrative embodiments. However, it is to be understood that the invention defined by the appended claims is not limited to the precise embodiments described.