Electroabsorption optical modulator and fabrication method thereof

Abstract: An impurity diffusion preventing layer (7) is provided between a first upper clad layer (6) adjacent to an optical absorption layer (5) and a second upper clad layer (8) provided on the first upper clad layer (6) in order to prevent p type impurity from diffusing to the first upper clad layer (6) adjacent to the optical absorption layer (5).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical modulator, which is an important element in an optical communication or an optical information processing system, and, particularly, to a semiconductor electroabsorption optical modulator, in which electroabsorption rate is changed by applying an electric field thereto. 2. Description of the Prior Art In an optical modulator, which is a key device in an optical communication system whose bit rate and communication distance are increased more and more recently, specifically, an electroabsorption type optical modulator, which utilizes a phenomenon that the optical absorption edge of a semiconductor material is shifted toward a long wavelength side upon application of an electric field, a multiple quantum well (MQW) layer is usually used as an optical absorption layer, as shown in FIG. 1. The optical modulator shown in FIG. 1 has a striped multi-layered structure including an n type InP buffer layer 2, an n type InGaAsP optical guide layer 3, an n type InP clad layer 4, a non-doped MQW optical absorption layer 5 formed by alternately laminating InGaAsP well layers and InGaAsP barrier layers, a p type InGaAsP clad layer 6 and a p type InP clad layer 8, which are formed on an n type InP substrate in the order. Semi-insulating InP burying layers 9 are formed on both sides of the multi-layered structure and a p + InP clad layer 10 and a p + InGaAs contact layer 11 are further formed on the semi-insulating InP burying layer 9 and the p type InP clad layer 8. In the optical modulator having such structure, optical ON/OFF ratio (extinction ratio) per unit voltage application is large since the MQW layer is used as the optical absorption layer. However, there may be diffusion of p type impurity from the p + InGaAs contact layer 11 and the p + InP clad layer 10 on the optical absorption layer 5 to the InGaAsP clad layer 6 adjacent to the optical absorption layer 5, during a fabrication thereof. Therefore, strength of electric field applied to the optical absorption layer 5 becomes large in a portion of the optical absorption layer 5, so that electric field strength distribution in the optical absorption layer becomes uneven. Since the degree of absorption in the electroabsorption type optical modulator depends on square of electric field strength, the extinction ratio characteristics with respect to applied voltage becomes non-linear as shown by a curve 52 in FIG. 5 when there is an unevenness of electric field strength within the optical absorption layer. Therefore, when a high speed modulation higher than 2.5Gb/s is performed, Cross points of eye Pattern, which is a modulated waveform, are lowered as shown in FIG. 6B, so that there is a problem that an acceptable eye opening can not be obtained. Document EP 0 871 057 A discloses the features of the preamble of claim 1. The impurity diffusion inhibiting layer therein is formed from intrinsic AlGaInAs and the first upper clad layer is also made from intrinsic AlGaInAs. SUMMARY OF THE INVENTION An object of the present Invention is to provide a semiconductor electroabsorption type optical modulator capable of inhibiting impurity from diffusing to a vicinity of an optical absorption layer thereof and maintaining crosspoints in an ideal state. The invention achieves this object with the subject-matter of claim 1. Preferred embodiments are disclosed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS This above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, in which: FIG. 1 is a cross section of a conventional optical modulator; FIG. 2 is a cross section of an optical modulator according to an embodiment of the present invention; FIGS. 3A to 3E show fabrication steps of the optical modulator, according to the first embodiment of the present invention; FIGS. 4A to 4E shows fabrication steps of the optical modulator, according to the second embodiment of the present invention; FIG. 5 shows the extinction characteristics of the conventional optical modulator and the present optical modulator; and FIGS. 6A and 6B show modulation waveforms under NRZ modulation of the conventional and present optical modulators. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A semiconductor electroabsorption optical modulator includes a striped waveguide constructed with a multi-layered structure, in which an optical absorption layer is sandwiched between clad layers having band gap energy larger than that of the optical absorption layer, formed on a semiconductor substrate, so that absorption of light incident on one edge of the optical absorption layer is changed by changing strength of electric field applied to the optical absorption layer, wherein between a first upper clad layer adjacent to an upper surface of the optical absorption layer and a second upper clad layer, an impurity diffusion inhibiting layer for preventing p type impurity from diffusing from the second upper clad layer and a semiconductor layer provided above the second upper clad layer to the first upper clad layer is provided. When the impurity diffusion inhibiting layer is a semiconductor layer with carrier concentration lower than that of the second upper clad layer and p type impurity diffusion speed is lower than that of the first upper clad layer, the impurity diffusion preventing effect thereof becomes considerable. Furthermore, the waveguide may have a multi-layered structure constructed by laminating an optical guide layer of a first conductivity type, a lower clad layer of the first conductivity type having band gap energy larger than that of the optical guide layer, an MQW optical absorption layer having band gap energy smaller than that of the optical guide layer, a first upper clad layer of a second conductivity type having band gap energy smaller than that of the lower clad layer, an impurity diffusion inhibiting layer of the second conductivity type having carrier concentration lower than that of the first clad layer and a second upper clad layer of the second conductivity type having band gap energy larger than that of the first upper clad layer and carrier concentration higher than that of the impurity diffusion preventing layer, in the order. Moreover, when the striped waveguide is buried by high resistance semiconductor layers provided on both sides thereof, light confinement efficiency thereof becomes excellent, resulting in high modulation efficiency. Furthermore, in the electroabsorption type optical modulator the semiconductor substrate, the lower clad layer, the impurity diffusion inhibiting layer and the second upper clad layer may be formed of InP, the optical guide layer may be formed of InGaAsP, the optical absorption layer may be of InGaAsP MQW and the first upper clad layer may be formed of InGaAsP. The fabrication method for fabricating the semiconductor electroabsorption type optical modulator comprises the step of forming, on a semiconductor substrate of a first conductivity type, a multi-layered structure by sequentially forming at least a lower clad layer of the first conductivity type, a non-doped optical absorption layer having band gap energy smaller than that of the lower clad layer, a first upper clad layer having band gap energy larger than that of the optical absorption layer and smaller than that of the lower clad layer, an impurity diffusion inhibiting layer of a second conductivity type having band gap energy larger than that of the first clad layer and carrier concentration lower than that of the first upper clad layer and a second upper clad layer having band gap energy larger than that of the first upper clad layer and carrier concentration higher than that of the impurity diffusion inhibiting layer, the step of forming a waveguide having a striped multi-layered structure by etching the multi-layered structure, the step of forming a burying layer of high resistance semiconductor on both sides of the waveguide and the step of forming, on at least the waveguide, a semiconductor layer of the second conductivity type having carrier concentration higher than that of the second upper clad layer. A further fabrication method for fabricating the electroabsorption type optical modulator comprises the steps of forming a mask of dielectric material having a striped opening on a semiconductor substrate of a first conductivity type, forming a waveguide having a striped multi-layered structure by sequentially forming, on the opening of the mask by selective growth, at least a lower clad layer of the first conductivity type, a non-doped optical absorption layer having band gap energy smaller than that of the lower clad layer, a first upper clad layer having band gap energy larger than that of the optical absorption layer and smaller than that of the lower clad layer, an impurity diffusion inhibiting layer of a second conductivity type having band gap energy larger than that of the first clad layer and carrier concentration lower than that of the first upper clad layer and a second upper clad layer having band gap energy larger than that of the first upper clad layer and carrier concentration higher than that of the impurity diffusion preventing layer, forming a burying layer of high resistance semiconductor on both sides of the waveguide by selective growth and forming, on at least the waveguide, a semiconductor layer of the second conductivity type having carrier concentration higher than that of the second upper clad layer. In the above two fabrication methods, the waveguide includes an optical guide layer of the first conductivity type having band gap energy smaller than that of the lower clad layer of the first conductivity type and larger than that of the optical absorption layer below the lower clad layer. The semiconductor substrate, the lower clad layer, the impurity diffusion inhibiting layer and the second upper clad layer may be formed of InP, the optical guide layer may be formed of InGaAsP, the optical absorption layer may be of InGaAsP MQW and the first upper clad layer may be formed of InGaAsP. Furthermore, in view of the impurity diffusion prevention, one may reduce the growing time or the growing temperature of the semiconductor layer formed after the waveguide is formed, that is, to reduce the growing time or the growing temperature of the burying layer and the semiconductor layer formed on the waveguide. (First Embodiment) An embodiment of the present invention will be described with reference to FIGS. 2 and 3A to 3E. As shown in FIG. 3A, on an n type (100) InP substrate 1, an n type InP Buffer layer 2 having carrier concentration of 1X10 18 cm -3 and thickness of 0.8µm, an n type InGaAsP optical guide layer 3 having band gap wavelength of 1.13 µm, an n type InP clad layer 4 having carrier concentration of 5x10 17 cm -3 and thickness of 0.5 µm, an MQW optical absorption layer 5 composed of alternately laminated nine InGaAsP well layers each 7nm thick and having band gap wavelength of 1.55µm and 10 ten InGaAsP barrier layers each 5.2nm thick and having band gap wavelength of 1.2µm, a p type InGaAsP clad layer 6 having carrier concentration of 5×10 17 cm -3 , band gap wavelength of 1.05µm and thickness of 0.5µm, a p - type InP impurity diffusion preventing layer 7 having carrier concentration of 2×10 17 cm -3 and thickness of 0.1µm and a p type InP clad layer 8 having carrier concentration of 5×10 17 cm -3 and thickness of 0.5µm are grown sequentially in the order by metal organic vapor phase epitaxy (MOVPE) using trimethyl indium (TMIn), triethyl gallium (TEGa), diethyl zinc (DEZn), arsine (AsH 3 ), phosphine (PH 3 ) and disilane (Si 2 H 6 ) as raw materials under conditions of growing temperature of 625°C and growing pressure of 10hPa, resulting in a multi-layered structure. The impurity diffusion inhibiting layer 7 may have a carrier concentration ranging from 8×10 16 cm -3 to 7× 10 17 cm -3 . Then, as shown in FIG. 3B, a SiO 2 film is deposited on a whole surface of the p type InP clad layer 8 by thermal CVD and a SiO 2 mask 21 having a striped pattern 1.5µm wide is formed by using usual photolithography. Thereafter, the multi-layered structure is etched up to the n type InP substrate 1 by reactive ion beam etching (RIBE) method to form a waveguide 20 having a striped multi-layered structure. In this case, the etching of the multi-layered structure may be made up to a middle of the buffer layer 2. Then, as shown in FIG. 3C, the striped waveguide 20 is buried by semi-insulating InP burying layers 9 having thickness as large as a height of the striped waveguide 20 and formed on both sides of the striped waveguide 20 by MOVPE while the SiO 2 mask 21 on an upper portion of the waveguide 20 being left as it is. Then, as shown in FIG. 3D, the SiO 2 mask 21 on an upper portion of the waveguide 20 is removed and, then, a p + type InP clad layer 10 having carrier concentration of 1 ×10 18 cm -3 and thickness of 1µm and a p + type InGaAs contact layer 11 having carrier concentration of 1×10 19 cm -3 and thickness of 0.2µm are grown on whole surfaces of the InP clad layer 8 and the semi-insulating InP burying layer 9 by MOVPE. On a region of the p + type InGaAs contact layer 11, corresponding to an immediate above portion of the striped waveguide, a striped SiO 2 mask 22 is formed by photolithography and, then, the p + type InGaAs contact layer 11 and the p + type InP clad layer 10 are removed by chemical etching to form a striped p + type InP clad layer 10 and p + type InGaAs contact layer 11 immediately above the waveguide. Then, as shown in FIG. 3E, after the striped SiO 2 mask 22 on the p + type InGaAs contact layer 11 is removed, a SiO 2 protective layer 12 is deposited on a whole surface of the wafer by thermal CVD. Then, a portion of the SiO 2 protective layer 12, which includes the immediate above portion of the waveguide, is removed by usual photolithography to form a striped opening 12a in the SiO 2 protective layer 12. After a surface electrode 13 is vapor deposited on a whole surface of the wafer, an electrode pattern is formed by using usual photolithography. Finally, the InP substrate 1 is polished up to an appropriate thickness and the electroabsorption type optical modulator shown in FIG. 2 is completed by forming an electrode 14 on a rear surface of the InP substrate 1. Opposite edge faces of the optical modulator thus fabricated were coated with films each having reflectivity of 0.1% and the characteristics thereof was evaluated. The extinction characteristics of the optical modulator measured by inputting a laser light from a distributed feedback (DFB) laser having oscillation wavelength of 1.550 µm thereto is shown by a curve 51 in FIG. 5. As is clear from FIG. 5, the non-linearity of the extinction characteristics is improved. Further, the non-return-to-zero (NRZ) modulation waveform at 10Gb/s was measured. A result is shown in FIG. 6A. As is clear from FIG. 6A, it is clear that the cross point position is improved. (Second Embodiment) A second embodiment of the fabrication method of the present optical modulator is shown in FIGS. 4A to 4E. The second embodiment will be described with reference to FIGS. 2 and 4A to 4E. As shown in FIG. 4A, a SiO 2 film 100nm thick is deposited on an n type (100) InP substrate 1 by thermal CVD and a SiO 2 mask 23 is formed by patterning the SiO 2 film such that a striped opening extends in [011] direction of the InP substrate. Width of the opening of the mask 23 was 1.5µm in this case. Then, an n type InP buffer layer 2 having carrier concentration of 1×10 19 cm -3 and thickness of 0.8µm, an n type InGaAsP optical guide layer 3 having band gap wavelength of 1.13µm, an n type InP clad layer 4 having carrier concentration of 5×10 17 cm -3 and thickness of 0.5µm, an MQW optical absorption layer 5 composed of alternately laminated nine InGaAsP well layers each 7nm thick and having band gap wavelength of 1.55µm and ten InGaAsP barrier layers each 5.2nm thick and having band gap wavelength of 1.2µm, a p type InGaAsP clad layer 6 having carrier concentration of 5×10 17 cm -3 , band gap wavelength of 1.05µm and thickness of 0.5µm, a p - type InP impurity diffusion inhibiting layer 7 having carrier concentration of 2×10 17 cm -3 and thickness of 0.1µm and a p type InP clad layer 8 having carrier concentration of 5×10 17 cm -3 and thickness of 0.5µm are grown selectively and sequentially in the order on the opening portion of the SiO 2 mask 23 by MOVPE using trimethyl indium (TMIn), triethyl gallium (TEGa), diethyl zinc (DEZn), arsine (AsH 3 ), phosphine (PH 3 ), bis-cyclopentadienyl iron (ferrocene) and disilane (Si 2 H 6 ) as raw materials under conditions of growing temperature of 625°C and growing pressure of 100hPa, resulting in a striped waveguide 20 having a multi-layered structure. Then, as shown in FIG. 4B, a SiO 2 mask 21 is formed on only the waveguide by thermal CVD and photolithography after the SiO 2 mask 23 is removed. Thereafter, as shown in FIG. 4C, the striped waveguide 20 is buried by Fe doped high resistance burying layers 9a having thickness substantially the same as height of the striped waveguide 20 grown on both sides of the striped waveguide 20 by MOVPE with using the SiO 2 mask 21 as a selective growing mask. Then, as shown in FIG. 4D, the SiO 2 mask 21 on the waveguide 20 is removed and, then, a p + type InP clad layer 10 having carrier concentration of 1×10 18 cm -3 and thickness of 1µm and a p + type InGaAs contact layer 11 having carrier concentration of 1×10 19 cm -3 and thickness of 0.2µm are grown on whole surfaces of the InP clad layer 8 and the high resistance InP burying layers 9a by MOVPE. On a region of the p + type InGaAs contact layer 11, corresponding to an immediate above portion of the striped waveguide, a striped SiO 2 mask 22 is formed by photolithography and, then, the p + type InGaAs contact layer 11 and the p + type InP clad layer 10 are removed by chemical etching to making the p + type InP clad layer 10 and the p+ type InGaAs contact layer 11 immediately above the waveguide as striped. Then, as shown in FIG. 4E, after the striped SiO 2 mask 22 on the p + type InGaAs contact layer 11 is removed, a SiO 2 protective layer 12 is deposited on a whole surface of the wafer by thermal CVD. Then, a portion of the SiO 2 protective layer 12, which includes the immediate above portion of the waveguide, is removed by usual photolithography to form a striped opening 12a in the SiO 2 protective layer 12. After a surface electrode 13 is vapor deposited on a whole surface of the wafer, an electrode pattern is formed by using usual photolithography. Finally, the InP substrate 1 is polished up to an appropriate depth and the electroabsorption type optical modulator shown in FIG. 2 is completed by forming an electrode 14 on a rear surface of the InP substrate 1. Although, in any of the described embodiments, the buffer layer 2 is formed on the substrate, the buffer layer 2 may be omitted. Furthermore, the optical guide layer 3 may be also omitted. Moreover, although the burying layers 9 (9a) are selectively grown after the striped waveguide 20 having the multi-layered structure is formed, it may be possible that, after a burying layer is formed on the substrate, a groove is formed in the burying layer by etching and, then, the waveguide is formed in the groove by selective growing. In the latter case, the impurity diffusion preventing effect may be superior than those obtained by the described embodiments since the number of the heating steps (crystal growing steps) after the formation of the waveguide is smaller than that of any of the described embodiments by one. In the electroabsorption type optical modulator of the present invention, as is disclosed in the foregoing description, the impurity diffusion inhibiting layer 7 of p - type InP having low carrier concentration is provided between the p type InGaAsP clad layer 6 and the p type InP clad layer 8 on the MQW optical absorption layer. Furthermore, the diffusion speed of Zn is highest in InGaAs, and lowest in InP, that in InGaAsP being in between. Therefore, diffusion of Zn, which is a p type impurity of the p type InP clad layer 8, the p + type InP clad layer 10 and the p + type InGaAs contact layer 11 on the p - type InP impurity diffusion inhibiting layer 7, to the p type InGaAsP clad layer 6 adjacent to the MQW optical absorption layer 5 is prevented by the p - type InP impurity diffusion inhibiting layer 7. As a result, unevenness of electric field strength applied to the optical absorption is removed as shown by the extinction characteristics curve 51 in FIG. 5 and the modulation waveform is improved in cross point position as shown in FIG. 6A. Furthermore, the improvement of the cross point position corresponds to shorting of the rising time tr and the falling time tf of the optical signal waveform. Therefore, the modulation speed of the optical modulator is improved. Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments will become apparent to persons skilled in the art upon reference to the description of the invention.

Claims: A semiconductor electroabsorption type optical modulator comprising: a stripe waveguide (20) having a multi-layer structure which comprises an optical guide layer (3) of a first conductivity type, a lower cladding layer (4) of the first conductivity type provided on said optical guide layer (3) and having a band gap larger than that of an optical absorption layer, said optical absorption layer (5) provided on said lower cladding layer (4), a first upper cladding layer (6) provided on said optical absorption layer (5) and having a band gap larger than that of said optical absorption layer, an impurity diffusion inhibiting layer (7) provided on said first upper cladding layer (6), and a second upper cladding layer (8) of a second conductivity type provided on said impurity diffusion inhibiting layer (7), wherein the first and second conductivity types are different from each other, and a third upper cladding layer (10) of the second conductivity type provided on said second upper cladding layer (8); characterized in that each of said first upper cladding layer (6) and said impurity diffusion inhibiting layer (7) is doped with impurities to represent the second conductivity type, and that said impurity diffusion inhibiting layer (7) has a dopant impurity atom concentration lower than that of said first upper cladding layer (6). A semiconductor electroabsorption type optical modulator as claimed in claim 1, wherein said impurity diffusion inhibiting layer (7) is made of a compound semiconductor other than the compound semiconductor which constructs said first upper cladding layer (6), wherein the diffusion velocity of the impurity particle in said impurity diffusion inhibiting layer (7) is smaller than the one of said first upper cladding layer (6). A semiconductor electroabsorption type optical modulator as claimed in claim 1, further comprising a semi-insulating burying layer (9) disposed along the two sides of said stripe waveguide (20) which are substantially perpendicular to the plane of the optical guiding layer. A semiconductor electroabsorption type optical modulator as claimed in claim 3, wherein said lower cladding layer (4), said impurity diffusion inhibiting layer (7), and said second upper cladding layer (8) are formed of InP, said optical guide layer (3) is formed of InGaAsP, said optical absorption layer (5) is formed of InGaAsP Multiple Quantum Well material and said first upper cladding layer (6) is formed of InGaAsP.

Inventors: FURUSHIMA YUJI, SHIMIZU JUNICHI, FURUSHIMA, YUJI, SHIMIZU, JUNICHI
Applicants: NEC COMPOUND SEMICONDUCTOR, NEC COMPOUND SEMICONDUCTOR DEVICES, LTD.
Application year: 2000
Main classification: G02F1
IPC codes: G02F1