JP5801589B2 - Light modulation element - Google Patents

Description

  The present invention relates to a light modulation element, and more particularly to a light modulation element having multiple quantum wells.

  Many semiconductor lasers (LDs) are used in the field of high-speed optical fiber communication. In the case of direct modulation LD, if the transmission rate exceeds 10 gigabits per second (10 Gbit / s), the influence of chirp characteristics becomes significant, and the transmission distance is limited by the dispersion of the optical fiber. For this reason, a modulator integrated LD which is an external modulation LD and has excellent chirp characteristics has been put to practical use. In this modulator integrated LD, a semiconductor modulator and a distributed feedback laser diode (DFB-LD) are monolithically integrated on a semiconductor substrate.

  As the semiconductor modulator, for example, an electro-absorption optical modulator (EA modulator) using an absorption edge wavelength shift by applying an electric field and a Mach-Zehnder optical modulator (Mach-) using a refractive index change are used. Zehnder Optical Modulator).

  The EA modulator is an element having a Pin diode structure including a p-type and n-type InP (indium phosphide) cladding layer and a non-doped multiple quantum well (MQW) active layer sandwiched between them. An EA modulator is disclosed in, for example, Japanese Patent Laid-Open No. 9-243975 (Patent Document 1). In the EA modulator, the LD output light is modulated by changing the amount of absorption of light incident on the MQW active layer by applying a reverse bias voltage with the n-type electrode being positive and the p-type electrode being negative. The

  As a semiconductor material constituting the EA modulator for long wave optical fiber communication, a mixed crystal material such as InGaAsP (indium gallium arsenide phosphorus) epitaxially grown on an InP substrate is used. In this material system, the energy gap difference (ΔEg) at the heterointerface is divided into a ratio of 40:60 in the conduction band and the valence band, and there is a large barrier against heavy holes. For this reason, when strong light is absorbed in the MQW and carriers (electrons and holes) are generated, holes are less likely to flow as currents across the barrier than electrons. This phenomenon is called a hole pile-up phenomenon. As a result, there are problems such as absorption saturation and deterioration of high-speed response characteristics caused by accumulated carriers shielding an external electric field (screening effect).

  In particular, when the MQW layer is directly sandwiched between InP clad layers having a wide band gap and when a single InGaAsP separation confinement layer (SCH) is inserted between the MQW layer and the InP clad layer, There is a large band discontinuity at the edge of the valence band at the interface. For this reason, the residence time until the holes overcome the barrier by thermal excitation and are absorbed by the InP cladding layer is increased.

  Therefore, it has been proposed that a GRIN-SCH structure (Graded Index Separate Confinement Heterosutruture) with a continuously changing band gap is inserted between the MQW layer and the cladding layer to eliminate the influence of valence band discontinuity. ing. The GRIN-SCH structure is disclosed in, for example, Japanese Patent Laid-Open No. 9-171162 (Patent Document 2).

  In addition, by adopting a multilayer SCH structure, it has been attempted to eliminate the influence of valence band discontinuity. In this multi-layer SCH structure, the SCH layer is composed of a multi-layer film, and the band discontinuity between each layer of the multi-layer film is reduced by decreasing the band gap in order from the InP cladding layer to the MQW layer. It is comprised so that it may become.

Japanese Patent Laid-Open No. 9-243975 JP-A-9-171162

  In recent EA modulators, a structure in which a GRIN-SCH structure or a multi-layer SCH structure is inserted between the MQW layer and the InP clad layer is employed to suppress the pileup phenomenon of carriers (particularly holes) as described above. ing.

  On the other hand, in MOCVD (Metal Organic Chemical Vapor Deposition) method, which is often used for crystal growth of optical semiconductor elements, Zn (zinc) is mainly used as a p-type dopant. Zn tends to diffuse during crystal growth. In particular, when the doping concentration of the p-InP clad layer is increased for the purpose of reducing the voltage drop in the p clad layer and the carrier pileup phenomenon, the p-type impurity may diffuse to the SCH layer or the MQW layer. . As a result, the p-type impurity profile in the MQW layer becomes non-uniform, and the difference in electric field strength depending on the position of the quantum well increases.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an optical modulation element capable of reducing the difference in electric field strength depending on the position of the quantum well.

  The light modulation element of the present invention is sandwiched between a first conductivity type first semiconductor clad layer, a second conductivity type second semiconductor clad layer, and first and second semiconductor clad layers, and A multiple quantum well having a plurality of well layers and a plurality of barrier layers stacked alternately. The plurality of barrier layers include a second conductivity type barrier layer into which a second conductivity type dopant contained in the second cladding layer is introduced. The second conductivity type barrier layer is provided in a range where the first conductivity type dopant contained in the first semiconductor cladding layer is diffused in the multiple quantum well.

  According to the light modulation element of the present invention, since the second conductivity type barrier layer is provided in a range in which the first conductivity type dopant is diffused in the multiple quantum well, the first conductivity type of the second conductivity type barrier layer is provided. Insulating properties can be improved by offsetting the first conductivity type dopant of the multiple quantum well with the dopant. For this reason, the bending of a band can be suppressed. And the electric field concerning a multiple quantum well can be made uniform. Thereby, the electric field strength difference by the position of a quantum well can be made small.

It is a schematic perspective view which shows typically the structure of the modulator integrated DFB-LD in Embodiment 1 of this invention. It is a schematic sectional drawing in alignment with the II-II line of FIG. It is a figure which shows the structure and impurity profile of the P1 part of FIG. It is a figure explaining the principle of a quantum confined Stark effect, Comprising: The figure (a) which shows the state of an electric field zero, and the figure (b) which shows the state to which the reverse bias voltage was applied. It is a figure which shows the applied voltage dependence of EA modulator absorption spectrum, and an operating condition. It is a figure which shows typically the potential at the time of reverse bias application of the EA modulator in a comparative example. It is a figure which shows the p side non-doped InP film thickness dependence of the potential distribution inside the multiple quantum well layer in a comparative example. It is a figure which shows the p side non-doped InP film thickness dependence of the electric field distribution inside the multiple quantum well layer in a comparative example. It is a figure which shows the reverse bias voltage dependence of the electric field distribution inside the multiple quantum well layer in a comparative example. It is a figure which shows typically the potential at the time of reverse bias application of the light modulation element in Embodiment 1 of this invention. It is a figure which shows the barrier layer n-type dope position dependence of the potential distribution inside the multiple quantum well layer in Embodiment 1 of this invention. It is a figure which shows the 5th barrier layer n-type dope density | concentration dependence of the potential distribution inside the multiple quantum well layer in Embodiment 1 of this invention. It is a figure which shows the reverse bias voltage dependence of the electric field distribution inside the multiple quantum well layer in Embodiment 1 of this invention. It is a figure which shows the potential at the time of reverse bias application of the light modulation element of the modification in Embodiment 1 of this invention. It is a schematic top view which shows typically the structure of the modulator integrated DFB-LD in Embodiment 2 of this invention. It is a schematic sectional drawing in alignment with the XVI-XVI line of FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Since the light modulation element in the present embodiment includes not only a single light modulator but also a modulator integrated LD, the modulator integrated LD which is the best embodiment will be described below.

(Embodiment 1)
First, the configuration of the modulator integrated LD according to the first embodiment of the present invention will be described. In the present embodiment, an EA modulator integrated LD will be described as an example of a modulator integrated LD.

  Referring to FIGS. 1 and 2, the EA modulator integrated LD mainly includes an EA modulator section 14, a separation region 15, and a DFB-LD section 16. The separation region 15 is a region for electrically separating the EA modulator unit 14 and the DFB-LD unit 16. The EA modulator integrated LD is configured such that the oscillation light from the DFB-LD unit 16 is modulated by the EA modulator unit 14 and the emitted light 25 is extracted from the EA modulator unit 14 side.

  The EA modulator integrated LD includes an EA modulator absorption layer 17, an LD active layer 18, a p-type SCH layer 20, an n-side SCH layer 21, a p-InP cladding layer 22, and an n-InP cladding layer 23. The diffraction grating 24, the non-doped InP layer 26, the p-type electrode 31, and the n-type electrode 32 are mainly included. The n-InP clad layer 23 also serves as an n-InP substrate.

  In the EA modulator section 14, a gap between the p-InP clad layer 22 that is the first semiconductor clad layer of the first conductivity type and the n-InP clad layer 23 that is the second semiconductor clad layer of the second conductivity type. The EA modulator absorption layer 17 is sandwiched between the two. Between the p-InP cladding layer 22 and the EA modulator absorption layer 17, a p-side SCH layer 20 that is a first separation confinement layer and a non-doped InP layer 26 are sequentially formed from the EA modulator absorption layer 17 side. Are stacked.

  Between the n-InP clad layer 23 and the EA modulator absorption layer 17, an n-side SCH layer 21 as a second separation confinement layer is provided. The p-side SCH layer 20 is formed in multiple layers. The n-side SCH layer 21 is formed as a single layer. Note that the n-side SCH layer 21 may be formed in multiple layers. A p-type electrode 31 is provided on the side of the p-InP cladding layer 22 opposite to the non-doped InP layer 26 side. An n-type electrode 32 is provided on the opposite side of the n-InP cladding layer 23 from the n-side SCH layer 21.

  In the DFB-LD portion 16, the LD active layer 18 is sandwiched between the p-InP cladding layer 22 and the n-InP cladding layer 23. A p-side SCH layer 20 is provided between the p-InP cladding layer 22 and the LD active layer 18. A diffraction grating 24 is provided inside the p-side SCH layer 20. An n-side SCH 21 is provided between the n-InP clad layer 23 and the LD active layer 18. In the DFB-LD unit 16, a p-type electrode 31 is provided on the opposite side of the p-InP cladding layer 22 from the p-side SCH 20. An n-type electrode 32 is provided on the opposite side of the n-InP cladding layer 23 from the n-side SCH 21.

  In the separation region 15, the EA modulator absorption layer 17 and the LD active layer 18 are joined at the butt joint growth interface 19. In the isolation region 15, the p-type electrode 31 is not provided.

  The EA modulator section 14 and the DFB-LD section 16 have a ridge structure 20. The EA modulator section 14 and the DFB-LD section 16 are not limited to the ridge structure 20 and may be a widely used buried hetero structure. Even with this heterostructure, the same effect as in the present embodiment can be obtained.

  Referring to FIG. 3, the EA modulator absorption layer 17 includes a multiple quantum well 40. In other words, the multiple quantum well 40 constitutes an absorption layer that utilizes the electroabsorption effect. The multiple quantum well 40 has a plurality of well layers 1 and a plurality of barrier layers 2 that are alternately stacked. The plurality of barrier layers 2 have a second conductivity type barrier layer 2 a into which a second conductivity type dopant (n-type impurity) contained in the n-InP cladding layer 23 is introduced. The second conductivity type barrier layer 2 a is provided in a range where the first conductivity type dopant (p-type impurity) contained in the p-InP cladding layer 22 is diffused in the multiple quantum well 40.

  The second conductivity type barrier layer 2 a is located on the p-InP cladding layer 22 side from the intermediate point of the multiple quantum well 40 in the direction sandwiched between the p-InP cladding layer 22 and the n-InP cladding layer 23. The second conductivity type barrier layer 2a has a second conductivity type dopant concentration (n-type impurity concentration) in at least a part of the multiple quantum well 40 closer to the n-InP cladding layer 23 than the second conductivity type barrier layer 2a. ) Has a higher concentration. The second conductivity type barrier layer 2a has a concentration higher than the concentration (p-type impurity concentration) of the first conductivity type dopant in the second conductivity type barrier layer 2a.

  Subsequently, the impurity profile of each layer will be described. Hereinafter, the peak concentration is shown as the impurity concentration value.

Regarding the n-type impurity concentration, the n-type impurity concentration of the n-InP cladding layer 23 is, for example, 1 × 10 ¹⁸ cm ⁻³ . The n-type impurity concentration of the n-side SCH layer 21 decreases from the boundary between the n-side SCH layer 21 and the n-InP cladding layer toward the boundary between the n-side SCH layer 21 and the multiple quantum well 40, and the n-side SCH layer It becomes substantially zero at the boundary between 21 and the multiple quantum well 40. The n-type impurity concentration of the second conductivity type barrier layer 2a is, for example, 1 × 10 ¹⁷ cm ⁻³ . The n-type impurity concentration of the second conductivity type barrier layer 2a is preferably 1 × 10 ¹⁷ cm ⁻³ or less.

Regarding the p-type impurity concentration, the p-type impurity concentration of the p-InP cladding layer 22 is, for example, 1 × 10 ¹⁸ cm ⁻³ . The p-type impurity concentration of the non-doped InP layer 26 decreases from the boundary between the non-doped InP layer 26 and the p-InP cladding layer 22 toward the boundary between the non-doped InP layer 26 and the p-side SCH layer 20. The p-type impurity concentration at the boundary with the p-side SCH layer 20 is, for example, 4 × 10 ¹⁷ cm ⁻³ . The impurity concentration of the p-side SCH layer 20 decreases from the boundary between the p-side SCH layer 20 and the non-doped InP layer 26 toward the boundary between the p-side SCH layer 20 and the multiple quantum well 40, and is multiplexed with the p-side SCH layer 26. The n-type impurity concentration at the boundary with the quantum well 40 is, for example, 1 × 10 ¹⁷ cm ⁻³ .

Each layer may contain residual impurities or impurities slightly diffused from adjacent portions. Here, the net impurity concentration is a value obtained by subtracting the residual impurity concentration of the opposite polarity from the doping concentration of a certain polarity. For example, the p-type impurity concentration of the n-side SCH layer 21 and the n-InP cladding layer 23 is approximately the same as the background impurity in the non-doped layer. Further, the n-type impurity concentration of the p-side SCH layer 20 and the p-InP cladding layer 22 is approximately the same as the background impurity in the non-doped layer. Therefore, the concentration of these residual impurities or impurities slightly diffused from the adjacent portion is, for example, 1 × 10 ¹⁶ cm ⁻³ or less, and ideally, for example, 1 × of SIMS (secondary ion mass spectrometry) measurement limit. Since it is 10 ¹⁵ cm ⁻³ or less, it can be almost ignored.

Next, the operation principle of the EA modulator unit will be described.
The field effect in one quantum well will be described with reference to FIG. FIG. 4A is an energy band diagram when there is no electric field (electric field zero). FIG. 4B is an energy band diagram when a reverse bias electric field is applied.

  4A and 4B, the quantum well includes a well layer 1 having a small band gap and a barrier layer 2 having a large band gap. Form quantum levels. For simplicity, only the electron first quantum level (Ee1) 3 and the hole first quantum level (Eh1) 4 are considered, and the electron wave function 5 and hole wave function 6 corresponding thereto are shown. Has been.

  Electrons and holes are confined in the well, and are slowly combined by Coulomb attraction to form excitons. At this time, the transition energy 7 accompanying light absorption is determined by the energy difference between the electron and hole quantum levels as shown in the figure. When a reverse bias electric field is applied, the quantum well band gap 8 itself does not change, but the wave function of electrons and holes shifts in the opposite direction, so that the transition energy shifts to the low energy side (long wavelength side). This phenomenon is called the quantum confined stark effect (QCSE).

  Since the exciton absorption peak wavelength shift due to QCSE is approximately proportional to the fourth power of the quantum well thickness, a larger change in absorption coefficient is obtained when the well thickness is thicker. On the other hand, since the oscillator strength itself becomes weaker as the well becomes thicker, there is an optimum well thickness. When an electric field is applied, the oscillator strength (absorption peak strength) also decreases due to the shift of the wave function of electrons and holes.

The dependence of the absorption spectrum of the EA modulator on the applied voltage will be described with reference to FIG.
An EA absorption peak wavelength 9, an LD wavelength 10 that is modulated light, a detuning Δλ (wavelength difference between the EA absorption peak and the LD wavelength) 11, an absorption peak shift 12 due to an electric field, and an extinction ratio 13 are shown. In the zero bias (0 V) state, exciton absorption peaks are observed near wavelengths of 1.52 μm and 1.45 μm. They shift to the longer wavelength side with reverse voltage application (V application). Therefore, for example, when light having a wavelength of 1.585 μm is incident, the absorption increases with the applied voltage. At this time, the light can be modulated by making the zero bias correspond to the ON state and apply the V bias to the OFF state. The ON / OFF ratio is a characteristic parameter called the extinction ratio 13.

Next, the effect of this embodiment will be described in comparison with a comparative example.
Referring to FIG. 6, in the EA modulator of the comparative example, both the well layer 1 and the barrier layer 2 constituting the multiple quantum well 40 are non-doped layers. A non-doped InP layer 26 is inserted between the multiple quantum well 40 and the p-InP cladding layer 22, but p-type dopants are transferred from the p-InP cladding layer 22 to the p-side SCH layer during the crystal growth and fabrication process. 20 and the multiple quantum well 40. Specifically, if the impurity concentration of the p-InP cladding layer 22 is, for example, 1 × 10 ¹⁸ cm ⁻³ , it is diffused by about 100 nm and becomes about 1 × 10 ¹⁷ cm ⁻³ . Due to this influence, band bending occurs near the p-side SCH layer 20. That is, in the vicinity of the p-side SCH layer 20 having a high impurity concentration, it becomes difficult to apply a voltage compared to the multiple quantum well 40 having a low impurity concentration. Therefore, the potential is unlikely to decrease in the band diagram. As a result, band bending occurs near the p-side SCH layer 20.

The non-uniformity of the electric field distribution in the comparative example EA modulator will be described in more detail.
Referring to FIG. 7, for the EA modulator of the comparative example, the potential distribution in the built-in state (zero bias) is calculated using the thickness of the non-doped InP layer 26 as a parameter in consideration of the diffusion of the p-type dopant. explain.

  The EA modulator of this comparative example has a pin structure. A non-doped InP layer 26 and a p-side SCH layer 20 are disposed between the p-InP cladding layer 22 and the multiple quantum well 40. The p-side SCH layer 20 is composed of six multilayer films having a band gap wavelength of 1180 nm to 1000 nm. Between the n-InP cladding layer 23 and the multiple quantum well 40, a single n-side SCH layer 21 having a band gap wavelength of 1180 nm is disposed.

  In the multiple quantum well 40, the barrier layer 2 has a band gap wavelength of 1180 nm and a thickness of 8 nm. The well layer 1 has a thickness of 10 nm and an absorption edge peak wavelength of 1470 nm. The number of well layers 1 is ten.

  In FIG. 7, only the multiple quantum well 40 is shown in an enlarged manner, and the right side of FIG. 7 is at a position close to the p-side SCH layer 20, and the left side is at a position close to the n-side SCH layer 21. The film thickness of the non-doped InP layer 26 is set to 10 nm to 125 nm. As shown in FIG. 7, when the film thickness of the non-doped InP layer 26 is 30 nm or less, a potential curve is observed in a portion near the p-side SCH layer 20, and when the film thickness of the non-doped InP layer 26 is 50 nm or more, the potential curve is observed. You can see that it has improved.

  Subsequently, with reference to FIG. 8, the electric field (corresponding to the potential gradient in FIG. 7) in each well layer 1 of the multiple quantum well 40 is shown enlarged. In FIG. 8, the well layers 1 are numbered in order from the left side close to the n-side SCH layer 21 to the right side close to the p-side SCH layer 20. As shown in FIG. 8, the electric field distribution inside the multiple quantum well 40 in the built-in state is not uniform. When the film thickness of the non-doped InP layer 26 is increased to 125 nm, the uniformity is slightly improved, but the non-uniformity remains, and the improvement effect is saturated.

  Next, with reference to FIG. 9, an electric field distribution inside the multiple quantum well 40 when a reverse bias of 0 V to 2.4 V is applied is shown. In FIG. 9, it can be seen that there is a large difference in electric field strength between the right side close to the p-side SCH layer 20 and the left side close to the n-side SCH layer 21, and variations in absorption peak shift due to QCSE also appear.

  On the other hand, referring to FIG. 10, in the light modulation element of the present embodiment, the bending of the band near the p-side SCH layer 20 is suppressed by inserting the second conductivity type barrier layer 2a.

  According to the light modulation element of the present embodiment, since the second conductivity type barrier layer 2a is provided in the range in which the p-type impurity that is the first conductivity type dopant is diffused in the multiple quantum well 40, the second conductivity type By canceling the p-type impurity of the multiple quantum well 40 by the n-type impurity of the type barrier layer 2a, the insulation can be improved. For this reason, the bending of a band can be suppressed. The electric field applied to the multiple quantum well 40 can be made uniform. Thereby, the electric field strength difference by the position of a quantum well can be made small.

  Further, as a result of the electric field applied to the multiple quantum well 40 being made uniform, the absorption edge shift amount is made uniform in all the well layers 1, thereby improving the extinction ratio. Therefore, the number of quantum wells can be reduced, and low voltage driving can be realized, and both good extinction characteristics and chirping characteristics can be achieved. Furthermore, the variation in characteristics due to the in-plane non-uniformity of the p-type dopant diffusion amount is improved, thereby improving the production yield.

  When the well layer 1 is doped, the carrier density in the well layer 1 increases, the exciton absorption peak intensity decreases, and the absorption change decreases. It is possible to suppress the absorption change from being reduced by introducing the second conductivity type dopant into the second conductivity type barrier layer 2a. In addition, crystal defects may be introduced by doping. By introducing the second conductivity type dopant into the second conductivity type barrier layer 2a, it is possible to suppress introduction of crystal defects by doping to the minimum necessary level.

  Further, according to the light modulation element of the present embodiment, the second conductivity type barrier layer 2 a is located from the intermediate point of the multiple quantum well 40 in the direction sandwiched between the p-InP cladding layer 22 and the n-InP cladding layer 23. It is located on the p-InP cladding layer 22 side. The second conductivity type barrier layer 2a has a concentration higher than the concentration of the second conductivity type dopant in at least a part of the multiple quantum well 40 on the n-InP cladding layer 23 side than the second conductivity type barrier layer 2a. And a concentration higher than the concentration of the first conductivity type dopant in the second conductivity type barrier layer 2a. For this reason, the electric field applied to the multiple quantum well 40 can be made uniform. Thereby, the electric field strength difference by the position of a quantum well can be made small.

Further, according to the light modulation element of the present embodiment, the concentration of the second conductivity type dopant in the second conductivity type barrier layer 2a is 1 × 10 ¹⁷ cm ⁻³ or less. Thereby, the p-type impurities in the multiple quantum well 40 can be more appropriately offset by controlling the doping amount of the n-type impurities in the second conductivity type barrier layer 2a. For this reason, insulation can be raised more appropriately.

  As shown in FIG. 10, in the light modulation device of the present embodiment, the p-side SCH layer 20 is smaller than the band gap energy of the p-InP cladding layer 22 and the band gap of the well layer 1 of the multiple quantum well 40. It has a larger band gap energy than the energy. The n-side SCH layer 21 has a band gap energy that is smaller than the band gap energy of the n-InP cladding layer 23 and larger than the band gap energy of the well layer 1 of the multiple quantum well 40.

  Thereby, the amount of band discontinuity between the p-InP cladding layer 22 and the multiple quantum well 40 can be reduced. In addition, the amount of band discontinuity between the n-InP cladding layer 23 and the multiple quantum well 40 can be reduced.

  In addition, according to the light modulation element of the present embodiment, the multiple quantum well 40 constitutes an absorption layer that uses the electroabsorption effect, and therefore the EA modulator unit 14 emits light using the electroabsorption effect. Can be modulated.

  Also, as shown in FIG. 10, in the light modulation element of the present embodiment, the p-side SCH layer 20 has a band gap energy that gradually decreases from the p-InP cladding layer 22 side to the multiple quantum well 40 side. It is configured. The p-side SCH layer 20 includes a first separation confinement layer portion 20a and a second separation confinement layer portion 20b. The second separation confinement layer portion 20b has a band gap energy smaller than that of the first separation confinement layer portion 20a. A second separation confinement layer portion 20b is disposed closer to the multiple quantum well 40 than the first separation confinement layer portion 20a. Note that the n-side SCH layer 21 may be similarly configured.

  For this reason, in the p-side SCH layer 20, the band discontinuity can be reduced stepwise from the p-InP cladding layer 22 side to the multiple quantum well 40 side. As a result, the band discontinuity between the p-InP cladding layer 22 and the multiple quantum well 40 can be reduced in order.

The homogenization of the electric field distribution in the light modulation element of the present embodiment will be described in more detail.
Referring to FIG. 11, the calculation result of the potential distribution in the built-in state when any of the fourth to seventh barrier layers is subjected to n-type doping with an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ . An example is shown. The fourth barrier layer to the seventh barrier layer are the fourth to seventh barrier layers when the numbers from 1 to 9 are assigned in order from the side closer to the n-side SCH layer 20.

  As shown in FIG. 11, it can be seen that the potential of the portion doped with the barrier is lowered and the overall potential distribution is changed as compared with the case without barrier doping. For each barrier, the electric field inside the multiple quantum well 40 was calculated in the same manner as shown in FIG. 8, and it was found that the uniformity when the fifth barrier layer was doped was the best.

Referring to FIG. 12, the potential distribution when the doping concentration to the fifth barrier layer is changed is shown. For each doping concentration, the electric field inside the multiple quantum well 40 was calculated as in the case shown in FIG. 8, and as a result, it was found that the impurity concentration of 5 × 10 ¹⁶ cm ⁻³ was the best.

Referring to FIG. 13, in the optimum condition in which the fifth barrier layer is doped with an impurity concentration of 5 × 10 ¹⁶ cm ⁻³ , the inside of the multiple quantum well 40 when a reverse bias is applied from 0 V to 2.4 V is obtained. The electric field distribution is shown. Compared with the comparative example shown in FIG. 11, the uniformity of the electric field inside the multiple quantum well 40 is greatly improved, and the electric field strength is almost in the range of 0 V to 1.8 V, which is important for application to a modulator. It turns out that it is uniform. As a result, the structure of the present embodiment can maximize the effect of QCSE with the same number of well layers as compared to the structure of the comparative example in which the barrier layer 2 is not doped.

  In the above description, the case where the doping of the second conductivity type is performed only on one barrier layer 2 closer to the p-InP cladding layer 22 side than the central portion of the multiple quantum well 40 has been described. The layer may be doped.

  Referring to FIG. 14, second conductivity type barrier layer 2 a has a plurality of second conductivity type barrier layer portions 2 a 1 separated from each other by well layer 1. For this reason, an n-type impurity can be introduced into the separated position. Therefore, the insulating property can be improved by offsetting the p-type impurity of the multiple quantum well 40 by the n-type impurity of the second conductivity type barrier layer 2a at the separated position. For this reason, since a dope position can be controlled further finely, the bending of a band can be suppressed more effectively. In addition, the electric field applied to the multiple quantum well 40 can be made more effective. The doping concentration of the plurality of barrier layers may be adjusted according to the electric field distribution.

(Embodiment 2)
In the second embodiment of the present invention, an MZ modulator integrated LD will be described as an example of a modulator integrated LD.

  First, the basic principle of the MZ modulator will be described. In the MZ modulator, input light coupled to a waveguide is branched into a plurality of waveguides by a Y-branch element or a multi-mode interference optical coupler (MMI). Then, after a predetermined phase difference (0 or π in the case of a symmetric branch) is given between them, they are combined again to cause interference, and optical modulation is performed. Since the MZ modulator can further improve the chirping characteristics as compared with the EA modulator, it is mainly applied to long-distance trunk optical communication.

  The EA modulator according to the first embodiment uses the change in the absorption coefficient due to the application of an electric field, but the refractive index change is obtained by the Kramers-Kronig conversion. The MZ modulator of the present embodiment modulates light by giving a phase difference between the branched waveguides using this refractive index change.

Next, the configuration of the MZ modulator integrated LD in the present embodiment will be described.
Referring to FIGS. 15 and 16, the MZ modulator integrated LD of the present embodiment mainly includes an MZ modulator unit 27 and a DFB-LD unit 16. The MZ modulator integrated LD has a structure in which the DFB-LD unit 16 is monolithically integrated in the rear input waveguide of the MZ modulator unit 27.

  The MZ modulator unit 27 of the present embodiment mainly includes an MMI 28, a first phase modulator 29, and a second phase modulator 30. The first phase modulator 29 and the second phase modulator 30 are provided so as to be sandwiched between the MMI 28. The first phase modulator 29 and the second phase modulator 30 are provided in the branching waveguide.

  The first phase modulator 29 and the second phase modulator 30 have the same configuration. The first phase modulator 29 and the second phase modulator 30 have a refractive index modulation layer 33 composed of a multiple quantum well 40 (see FIG. 3) similar to that of the first embodiment. That is, the multiple quantum well 40 constitutes a refractive index modulation layer 33 that utilizes the electro-optic effect. The refractive index modulation layer 33 is sandwiched between the p-side SCH layer 20 and the n-side SCH layer 21. A p-InP cladding layer 22 is provided on the p-side SCH layer 20.

  A p-type electrode 31 is provided on the p-InP cladding layer 22. An n-InP cladding layer 23 is provided on the opposite side of the n-side SCH layer 20 from the refractive index modulation layer 33. An n-type electrode 32 is provided on the opposite side of the n-InP cladding layer 23 from the n-side SCH layer 21. The first phase modulator 29 and the second phase modulator 30 are provided with the n-side SCH layer 21, the n-InP clad layer 23, and the n-type electrode 32 in common.

Next, the function and effect of this embodiment will be described.
In the light modulation element of the present embodiment, a reverse bias voltage is applied between the p-type electrode 31 and the n-type electrode 32, thereby controlling the electric field applied to the refractive index modulation layer 33 including the multiple quantum well 40. The refractive index difference is adjusted.

  According to the light modulation element of the present embodiment, the second conductivity type barrier layer 2a is provided in the multiple quantum well 40 in the range where the p-type impurity which is the first conductivity type dopant is diffused. The electric field applied to 40 can be made uniform. For this reason, the change in the refractive index at the same driving voltage becomes large, and the device can be driven at a lower voltage and the element can be miniaturized.

  Further, according to the light modulation element of the present embodiment, the multiple quantum well constitutes the refractive index modulation layer 33 that utilizes the electro-optic effect, so that the MZ modulator unit 27 utilizes the electro-optic effect. The light can be modulated.

  In the first and second embodiments, the case where the first conductivity type is p-type and the second conductivity type is n-type has been described. However, the present invention is not limited to this, and the first conductivity type is n-type. The second conductivity type may be p-type. In this case, the first conductivity type can be changed to the n type and the second conductivity type can be changed to the p type by switching the relationship between the p type and the n type in the light modulation element.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  1 well layer, 2 barrier layer, 2a second conductivity type barrier layer, 3 electron first quantum level, 4 hole first quantum level, 5 electron wave function, 6 hole wave function, 7 transition Energy, 8 quantum well band gap, 9 EA absorption peak wavelength, 10 LD wavelength, 11 detuning (Δλ), 12 absorption peak shift, 13 extinction ratio, 14 EA modulator section, 15 DFB-LD section, 16 separation region , 17 EA modulator absorption layer, 18 LD active layer, 19 Butt joint growth interface, 20 p side SCH layer, 20a First separation confinement layer portion, 20b Second separation confinement layer portion, 21 n side SCH layer, 22 p-InP cladding layer, 23 n-InP cladding layer, 24 diffraction grating, 25 outgoing light, 26 non-doped InP layer, 27 MZ modulator section, 28 multimode interference optical coupler, 9 The first phase modulator, 30 second phase modulator, 31 p-type electrode, 32 n-type electrode, 33 the refractive index modulating layer, 40 a multiple quantum well.

Claims (8)

A first conductivity type first semiconductor clad layer;
A second semiconductor cladding layer of a second conductivity type;
A multi-quantum well having a plurality of well layers and a plurality of barrier layers sandwiched between the first and second semiconductor cladding layers and alternately stacked;
The plurality of barrier layers include a second conductivity type barrier layer into which a second conductivity type dopant contained in the second cladding layer is introduced,
The second conductivity type barrier layer is provided in a range where the first conductivity type dopant contained in the first semiconductor cladding layer is diffused in the multiple quantum well,
The position of the second conductivity type barrier layer and the concentration of the dopant are set so that the electric field applied to each quantum well is uniform ,
An optical modulation element, wherein the barrier layer into which the second conductivity type dopant is not introduced is disposed closer to the first semiconductor clad layer than the second conductivity type barrier layer .   The second conductivity type barrier layer is located closer to the first semiconductor clad layer than an intermediate point of the multiple quantum well in a direction sandwiched between the first and second semiconductor clad layers, and the second conductivity type A concentration higher than the concentration of the dopant of the second conductivity type in at least a part of the multiple quantum well on the second semiconductor clad layer side of the second barrier layer, and in the second conductivity type barrier layer The light modulation element according to claim 1, wherein the light modulation element has a concentration higher than that of the first conductivity type dopant.   The light modulation element according to claim 1, wherein the second conductivity type barrier layer includes a plurality of second conductivity type barrier layer portions separated from each other by the well layer. 4. The light modulation element according to claim 1, wherein a concentration of the second conductivity type dopant in the second conductivity type barrier layer is 1 × 10 ¹⁷ cm ⁻³ or less. A first isolation confinement layer provided between the first semiconductor cladding layer and the multiple quantum well;
A second isolation confinement layer provided between the second semiconductor cladding layer and the multiple quantum well;
The first confinement confinement layer has a band gap energy smaller than a band gap energy of the first semiconductor cladding layer and larger than a band gap energy of the well layer of the multiple quantum well;
2. The second isolation confinement layer has a band gap energy smaller than a band gap energy of the second semiconductor cladding layer and larger than a band gap energy of the well layer of the multiple quantum well. The light modulation element according to any one of? At least one of the first and second separate confinement layers is
The light modulation element according to claim 5, wherein the light modulation element is configured such that band gap energy gradually decreases from the first and second semiconductor clad layer sides to the multiple quantum well side.   The light modulation element according to claim 1, wherein the multiple quantum well constitutes an absorption layer using an electroabsorption effect. The light modulation element according to claim 1, wherein the multiple quantum well constitutes a refractive index modulation layer using an electro-optic effect.

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