Optical semiconductor equipment

Abstract

To obtain an optoelectronic device that can prevent a decrease in frequency bandwidth. [Solution] A first pad 8 connected to the first electrode 6 of the optical modulator 2 and a second pad 9 connected to the second electrode 7 of the optical modulator 2 are arranged on the first terrace 11 via an insulating film 22. On the first terrace 11, the first conductive layers 13 and 14 are removed between the first pad 8 and the second pad 9.

Description

Technical Field

[0001] The present disclosure relates to an optical semiconductor device.

Background Art

[0002] An optical semiconductor device in which a laser section and an optical modulator are monolithically integrated has been proposed (see, for example, Patent Document 1). The optical modulator operates differentially by a differential voltage applied between an anode pad and a cathode pad. By arranging the anode pad and the cathode pad of the optical modulator on the same-side terraces with respect to the waveguide, the lengths of the wires connected to both pads can be made the same.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] =]] When operating differentially, a leakage current flows between the two pads of the optical modulator, so that the voltage applied to the absorption layer of the optical modulator decreases. Since the leakage current flows through the capacitance under the electrodes of the optical modulator, the leakage current increases as the frequency increases and the extinction ratio decreases. As a result, there has been a problem that the frequency band in which the optical modulator can operate normally decreases.

[0005] The present disclosure has been made to solve the above-described problems, and an object thereof is to obtain an optical semiconductor device capable of preventing a decrease in the frequency band.

Means for Solving the Problems

[0006] The optical semiconductor device according to this disclosure comprises a substrate, a semiconductor layer including a first conductivity type layer, an absorption layer, and a second conductivity type layer formed in order on the substrate, an optical modulator having a first electrode connected to the first conductivity type layer and a second electrode connected to the second conductivity type layer, a first pad connected to the first electrode and a second pad connected to the second electrode, wherein the semiconductor layer has a waveguide and a first terrace and a second terrace arranged on opposite sides of the waveguide, the first pad and the second pad are arranged on the first terrace with an insulating film in between, and the first conductivity type layer is removed between the first pad and the second pad on the first terrace. [Effects of the Invention]

[0007] In this disclosure, the first conductive layer is removed between the first pad and the second pad in the first terrace. This interrupts the leakage current path between the first pad and the second pad, reducing the leakage current and improving the response, especially in the high-frequency range. As a result, a decrease in frequency bandwidth can be prevented. [Brief explanation of the drawing]

[0008] [Figure 1] This is a top view showing an optical semiconductor device according to Embodiment 1. [Figure 2] This is a cross-sectional view of the laser section along AA' in Figure 1. [Figure 3] This is a cross-sectional view of the optical modulator along BB' in Figure 1. [Figure 4] This is a cross-sectional view of the optical modulator along CC' in Figure 1. [Figure 5] This is a cross-sectional view of the waveguide along DD' in Figure 1. [Figure 6] This is a cross-sectional view of the first terrace along EE' in Figure 1. [Figure 7] This is a top view showing an optical semiconductor device related to a comparative example. [Figure 8] This is a cross-sectional view of the waveguide along AA' in Figure 7. [Figure 9]This is a cross-sectional view of the first terrace along BB' in Figure 7. [Figure 10] This figure shows the frequency response characteristics of Embodiment 1 and the comparative example. [Figure 11] This is a top view showing an optical semiconductor device according to Embodiment 2. [Figure 12] This is a cross-sectional view of the optical modulator along AA' in Figure 11. [Figure 13] This is a cross-sectional view of the optical modulator along BB' in Figure 11. [Modes for carrying out the invention]

[0009] The optical semiconductor device according to the embodiment will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repetition of the description may be omitted.

[0010] Embodiment 1 Figure 1 is a top view showing an optoelectronic device according to Embodiment 1. This optoelectronic device is a modulator-integrated laser diode in which a laser unit 1 and an optical modulator 2 are monolithically integrated on a semi-insulating InP substrate 3. The laser unit 1 is a distributed-feedback laser diode (DFB-LD). The optical modulator 2 is an electro-absorption modulator.

[0011] The laser unit 1 has a cathode electrode 4 and an anode electrode 5. The optical modulator 2 has a cathode electrode 6 and an anode electrode 7. A cathode pad 8 is connected to the cathode electrode 6. An anode pad 9 is connected to the anode electrode 7. The optical modulator 2 operates differentially due to the differential voltage applied between the cathode pad 8 and the anode pad 9.

[0012] The first terrace 11 and the second terrace 12 are arranged on opposite sides of the waveguide 10. The cathode pad 8 and the anode pad 9 are arranged on the first terrace 11. Accordingly, the lengths of the wires connected to the anode pad 9 and the wires connected to the cathode pad 8 can be made the same.

[0013] Region V in the figure is the region between the cathode pad 8 and the anode pad 9. Region W is the region between the laser section 1 and the optical modulator 2. Region X is the region where the cathode pad 8 is formed. Region Y is the region where the anode pad 9 is formed. Region Z is the region where the second terrace 12 of the optical modulator 2 exists. The region surrounded by the wavy line in the figure is the region where the n-type cladding layer and the n-type contact layer described later exist.

[0014] Figure 2 is a cross-sectional view of the laser section along A-A' in Figure 1. An n-InGaAs contact layer 13, an n-InP cladding layer 14, an active layer 15, a p-InP cladding layer 16, and a p-InGaAs contact layer 17 are sequentially stacked on the semi-insulating InP substrate 3. The active layer 15 has a multi-quantum well (MQW) structure of InGaAsP.

[0015] The active layer 15 is patterned in a stripe shape in plan view, and both sides are embedded with Fe-InP block layers 18. A diffraction grating 19 is formed in the p-InP cladding layer 16.

[0016] On both sides of the active layer 15, a first groove 20 and a second groove 21 are formed in the p-InGaAs contact layer 17, p-InP clad layer 16, Fe-InP block layer 18, and n-InP clad layer 14. The second groove 21 is formed down to under the n-InGaAs contact layer 13. The upper surface of the p-InGaAs contact layer 17 and the inner surfaces of the first groove 20 and the second groove 21 are covered with an insulating film 22. An opening is formed in the insulating film 22 at the upper part of the mesa structure between the first groove 20 and the second groove 21, and the anode electrode 5 is connected to the p-InGaAs contact layer 17 through this opening. An opening is formed in the insulating film 22 at the bottom surface of the first groove 20, and the cathode electrode 4 is connected to the p-InGaAs contact layer 17 through this opening. An n electrode 23 is formed on the lower surface of the semi-insulating InP substrate 3.

[0017] In order to make a low-resistance connection between the cathode electrode 4 of the laser section 1 and the active layer 15 of the laser section 1, a low-resistance n-InP clad layer 14 and n-InGaAs contact layer 13 are required below the active layer 15 of the waveguide 10 of the laser section 1 and at the connection part of the cathode electrode 4 of the laser section 1. In particular, the n-InGaAs contact layer 13 has a very low resistance.

[0018] FIG. 3 is a cross-sectional view of the optical modulator taken along B-B' of FIG. 1. On the semi-insulating InP substrate 3, as the semiconductor layer 24, an n-InGaAs contact layer 13, an n-InP clad layer 14, a p-InP clad layer 16, and a p-InGaAs contact layer 17 are laminated in order. A first groove 20 and a second groove 21 are formed separately from each other in the p-InGaAs contact layer 17, p-InP clad layer 16, and n-InP clad layer 14. The first groove 20 and the second groove 21 function as a waveguide 10 by restricting the lateral width of the absorption layer 25. An absorption layer 25 is formed between the n-InP clad layer 14 and the p-InP clad layer 16 within the mesa structure between the first groove 20 and the second groove 21. The absorption layer 25 has a multiple quantum well structure of InGaAsP.

[0019] The semiconductor layer 24 has a waveguide 10 and a first terrace 11 and a second terrace 12 positioned opposite each other with respect to the waveguide 10. In the first terrace 11 and the second terrace 12, an Fe-InP block layer 18 is formed between the n-InP cladding layer 14 and the p-InP cladding layer 16 instead of an absorption layer 25. An opening is formed in the insulating film 22 at the bottom of the first groove 20, and the cathode electrode 6 is connected to the n-InGaAs contact layer 13 through this opening. The cathode pad 8 is positioned on the first terrace 11 via the insulating film 22 and is connected to the cathode electrode 6. The depth of the second groove 21 is made deeper than the n-InGaAs contact layer 13 in order to electrically isolate the capacitance C2 from the lower part of the absorption layer 25 of the optical modulator 2. On the other hand, in order to connect the cathode electrode 6 of the optical modulator 2 with the n-InGaAs contact layer 13, the depth of the first groove 20 at the connection portion between the cathode electrode 6 and the n-InGaAs contact layer 13 is made to extend to the top of the n-InGaAs contact layer 13. These grooves of different depths need to be formed in a separate process.

[0020] To achieve a low-resistance connection between the absorption layer 25 of the optical modulator 2 and the cathode electrode 6 of the optical modulator 2, a low-resistance n-InP cladding layer 14 and an n-InGaAs contact layer 13 are required at the connection point between the lower part of the absorption layer 25 of the waveguide 10 of the optical modulator 2 and the cathode electrode 6 of the optical modulator 2. In particular, the n-InGaAs contact layer 13 has extremely low resistance.

[0021] Figure 4 is a cross-sectional view of the optical modulator along CC' in Figure 1. The anode pad 9, like the cathode pad 8, is positioned on the first terrace 11 via an insulating film 22. An opening is formed in the insulating film 22 at the top of the waveguide 10. The anode electrode 7 is connected to the p-InGaAs contact layer 17 via the opening in the insulating film 22 at the top of the waveguide 10 and to the anode pad 9 through the sides and bottom of the first groove 20.

[0022] Capacitance C1 is generated between the n-InGaAs contact layer 13 and the n-electrode 23, with the semi-insulating InP substrate 3 in between, below the anode pad 9 on the first terrace 11. Capacitance C2 is generated between the n-InGaAs contact layer 13 and the n-electrode 23, with the semi-insulating InP substrate 3 in between, on the second terrace 12. To electrically isolate capacitances C1 and C2 from the lower part of the absorption layer 25 of the optical modulator 2, the depth of the first groove 20 and the depth of the second groove 21 in the portion through which the anode electrode 7 passes are made deeper than the n-InGaAs contact layer 13.

[0023] Figure 5 is a cross-sectional view of the waveguide along DD' in Figure 1. The active layer 15 and the absorption layer 25 are connected by a transparent waveguide layer 26. A transparent waveguide layer 26 is also formed between the absorption layer 25 and the exit end face. The transparent waveguide layer 26 consists of a single layer of InGaAsP. In the waveguide 10 of region W between the optical modulator 2 and the laser unit 1, the low-resistance n-InP cladding layer 14 and n-InGaAs contact layer 13 are removed and filled with an Fe-InP block layer 18. As a result, the n-InGaAs contact layer 13 and n-InP cladding layer 14 of the laser unit 1 are electrically insulated from the n-InGaAs contact layer 13 and n-InP cladding layer 14 of the optical modulator 2.

[0024] Figure 6 is a cross-sectional view of the first terrace along EE' in Figure 1. The cathode pad 8 and anode pad 9 are each connected to a differential power supply 27. The differential power supply 27 operates in a push-pull configuration by alternately applying voltage to one end as positive and the other as negative. However, the voltage on the anode side may be varied, and the potential on the cathode side may be set to zero. A load resistor R is connected in parallel to the pn junction 28 of the optical modulator 2. The load resistor R is provided outside the device for impedance matching.

[0025] In the first terrace 11, the n-InP cladding layer 14 and n-InGaAs contact layer 13 of the low-resistance layer are removed and embedded with an Fe-InP block layer 18 in region V between the cathode pad 8 and the anode pad 9. Also, in region W between the cathode pad 8 of the optical modulator 2 and the cathode electrode 4 of the laser unit 1, the n-InP cladding layer 14 and n-InGaAs contact layer 13 of the low-resistance layer are removed and embedded with an Fe-InP block layer 18.

[0026] Next, the effects of this embodiment will be explained in comparison with a comparative example. Figure 7 is a top view showing an optical semiconductor device according to a comparative example. Figure 8 is a cross-sectional view of the waveguide along AA' in Figure 7. In the comparative example, the n-InGaAs contact layer 13 and the n-InP cladding layer 14 are formed on the entire surface of the chip and are not removed. Because the low-resistance n-InP cladding layer 14 and n-InGaAs contact layer 13 are present in the waveguide 10 between the optical modulator 2 and the laser unit 1, a leakage current flows between the optical modulator 2 and the laser unit 1. Consequently, the voltage amplitude applied to the optical modulator 2 from the cathode side of the differential power supply 27 decreases, and the load impedance seen from the anode side and the cathode side of the differential power supply 27 becomes different. Furthermore, a portion of the leakage current flows into the active layer 15 of the laser unit 1, causing the amount of current flowing into the active layer 15 to fluctuate and the optical output of the laser unit 1 to fluctuate. When the optical output of the laser unit 1 fluctuates, the optical output after modulation by the optical modulator 2 also fluctuates accordingly. Furthermore, very large wavelength chirping (wavelength fluctuations) occurs. Therefore, suppressing leakage current is important for improving communication quality.

[0027] Figure 9 is a cross-sectional view of the first terrace along BB' in Figure 7. The cathode pad 8 and anode pad 9 are connected at high frequency via an insulating film 22, a p-InP cladding layer 16, a Fe-InP block layer 18, an n-InP cladding layer 14, and an n-InGaAs contact layer 13. Therefore, a resistor R1, which is a leakage current path, exists in the semiconductor layer 24 between the cathode pad 8 and anode pad 9.

[0028] Similarly, the cathode pad 8 of the optical modulator 2 and the cathode electrode 4 of the laser unit 1 are also connected via high frequency. Therefore, a resistor R2, which is a leakage current path, exists in the semiconductor layer 24 between the anode pad 9 and the cathode electrode 4. Note that the positions of the cathode pad 8 and anode pad 9 of the optical modulator 2 may be reversed. In this case, a potential difference will occur between the anode electrode 7 of the optical modulator 2 and the cathode electrode 4 of the laser unit 1, resulting in leakage current.

[0029] The current due to the anode voltage of the differential power supply 27 flows through two paths: one through the load resistor R and resistor R2, and the other through the capacitance C of the insulating film 22 and resistors R1 and R2. The current due to the cathode voltage flows through resistor R2 to the cathode electrode 4 of the laser unit 1.

[0030] As the frequency increases, the impedance of capacitance C decreases, and the leakage current flowing through resistor R1 increases. This increase in leakage current reduces the current flowing through the load resistor R, thus decreasing the voltage applied to the optical modulator 2. Consequently, as the frequency increases, the leakage current increases and the extinction ratio decreases, resulting in a reduced frequency bandwidth.

[0031] In contrast, in this embodiment, by removing the n-InP cladding layer 14 and the n-InGaAs contact layer 13 in the region V between the cathode pad 8 and the anode pad 9, the leakage current path is interrupted, reducing the leakage current and improving the response, especially in the high-frequency region. As a result, a decrease in frequency bandwidth can be prevented. It is possible to remove either the n-InP cladding layer 14 or the n-InGaAs contact layer 13, but it is preferable to remove both. Furthermore, a similar effect can be obtained in a semiconductor device consisting only of an optical modulator without the laser unit 1 by removing the n-InP cladding layer 14 and the n-InGaAs contact layer 13 between the pads.

[0032] Furthermore, in the comparative example, resistors R1 and R2 are present, so the current paths due to the cathode voltage and the current paths due to the anode voltage are different, and their impedances are also different. In contrast, in this embodiment, by removing the n-InP cladding layer 14 and the n-InGaAs contact layer 13 in regions V and W, the current paths through resistors R1 and R2 are divided, so that the current due to both voltages flows only through the load resistor R. As a result, the impedances of the anode and cathode sides become equal. Consequently, the frequency response characteristics of anode modulation and cathode modulation become almost the same, enabling ideal differential operation. In addition, by making the impedances of the anode and cathode sides equal, the amplitude and phase of the noise on the anode and cathode sides can be matched, maximizing the noise reduction effect of differential operation.

[0033] Figure 10 shows the frequency response characteristics of Embodiment 1 and the comparative example. In this embodiment, it can be seen that both the frequency response to anode modulation and the frequency response to cathode modulation are improved compared to the comparative example.

[0034] Embodiment 2 Figure 11 is a top view showing an optical semiconductor device according to Embodiment 2. Figure 12 is a cross-sectional view of the optical modulator along AA' in Figure 11. Figure 13 is a cross-sectional view of the optical modulator along BB' in Figure 11. In this embodiment, the n-InP cladding layer 14 and the n-InGaAs contact layer 13 are removed from the entire area of ​​the first terrace 11 and the entire area of ​​the second terrace 12 of the optical modulator 2. The n-InP cladding layer 14 and the n-InGaAs contact layer 13 are present in other areas. In this embodiment as well, the n-InP cladding layer 14 and the n-InGaAs contact layer 13 may be removed in area W, similar to Embodiment 1. The other configurations are the same as in Embodiment 1.

[0035] In the first terrace 11, capacitance C1 is generated between the p-InP cladding layer 16, which sandwiches the semi-insulating InP substrate 3 and the Fe-InP block layer 18, and the n-electrode 23. However, the n-InP cladding layer 14 and the n-InGaAs contact layer 13 are removed in the first terrace 11. As a result, the resistance between the n-InP cladding layer 14 and the n-InGaAs contact layer 13 of the waveguide 10 of the optical modulator 2 and capacitance C1 increases significantly. Therefore, it is not necessary to make the bottom surface of the first groove 20 shown in Figure 13 deeper than the n-InGaAs contact layer 13. Similarly, capacitance C2 is generated between the p-InP cladding layer 16, which sandwiches the semi-insulating InP substrate 3 and the Fe-InP block layer 18, and the n-electrode 23 in the second terrace 12. However, the n-InP cladding layer 14 and the n-InGaAs contact layer 13 are removed in the second terrace 12. Therefore, the resistance between the n-InP cladding layer 14 and the n-InGaAs contact layer 13 of the waveguide 10 of the optical modulator 2 and the capacitance C2 increases significantly. Consequently, it is not necessary to make the bottom of the second groove 21 deeper than the n-InGaAs contact layer 13. By removing the n-InP cladding layer 14 and the n-InGaAs contact layer 13 in the first terrace 11 and the second terrace 12 of the optical modulator 2 in this way, the problem of increased capacitance does not occur even if the depths of the first groove 20 and the second groove 21 are made the same.

[0036] Furthermore, in Embodiment 1, as shown in Figure 3, the depths of the first groove 20 and the second groove 21 at the connection portion between the cathode electrode 6 and the n-InGaAs contact layer 13 are different. As a result, the distribution of light guiding the optical modulator 2 is asymmetrical. This causes scattering loss when laser light with a symmetrical distribution is guided to or within the optical modulator 2. In contrast, in this embodiment, the depths of the first groove 20 at the connection portion between the cathode electrode 6 and the n-InGaAs contact layer 13, the first groove 20 in the portion through which the anode electrode 7 passes, and the second groove 21 are all the same. As a result, the distribution of light guiding the optical modulator 2 is symmetrical. This makes it possible to suppress scattering loss when the laser light guides the optical modulator 2. [Explanation of symbols]

[0037] 1 Laser section, 2 Optical modulator, 3 Semi-insulating InP substrate (substrate), 4 Cathode electrode (electrode), 6 Cathode electrode (first electrode), 7 Anode electrode (second electrode), 8 Cathode pad (first pad), 9 Anode pad (second pad), 10 Waveguide, 11 First terrace, 12 Second terrace, 13 n-InGaAs contact layer (first conductivity layer), 14 n-InP cladding layer (first conductivity layer), 16 p-InP cladding layer (second conductivity layer), 20 First groove, 21 Second groove, 22 Insulating film, 24 Semiconductor layer, 25 Absorption layer

Claims

1. circuit board and A semiconductor layer comprising a first conductivity layer, an absorption layer, and a second conductivity layer formed sequentially on the substrate, and an optical modulator having a first electrode connected to the first conductivity layer and a second electrode connected to the second conductivity layer, A first pad connected to the first electrode, The device comprises a second pad connected to the second electrode, The semiconductor layer has a waveguide and a first terrace and a second terrace arranged on opposite sides of the waveguide. The first pad and the second pad are arranged on the first terrace with an insulating film in between. An optoelectronic semiconductor device characterized in that the first conductive layer is removed between the first pad and the second pad in the first terrace.

2. The aforementioned substrate further comprises the optical modulator and a monolithically integrated laser unit, The optical semiconductor device according to claim 1, characterized in that the first conductivity layer is removed between the optical modulator and the laser unit.

3. In the semiconductor layer of the optical modulator, a first groove is formed between the waveguide and the first terrace, and a second groove is formed between the waveguide and the second terrace. The first electrode is connected to the first conductive layer at the bottom of the first groove. The second electrode is connected to the second conductive layer at the top of the waveguide and to the second pad through the first groove, The first conductive layer is removed from the first terrace and the second terrace of the optical modulator. The optical semiconductor device according to claim 1 or 2, characterized in that the depth of the first groove at the connection portion between the first electrode and the first conductive layer, the depth of the first groove at the portion through which the second electrode passes, and the depth of the second groove are the same.

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