Method of manufacturing optical semiconductor device

A refractive index of the active layer is obtained by a photoluminescence inspection and an equivalent refractive index of the optical semiconductor element is computed. A refractive index of the optical waveguide layer is obtained by a photoluminescence inspection and an equivalent refractive index of the optical waveguide is computed. A film thickness of the refractive index adjustment layer is adjusted by etching the refractive index adjustment layer so that the equivalent refractive index of the optical semiconductor element and the equivalent refractive index of the optical waveguide are matched to each other. After adjusting the film thickness of the refractive index adjustment layer, a contact layer is formed on the second cladding layer and the refractive index adjustment layer. The optical waveguide is a passive waveguide to which no electrical field is applied and no current is injected.

BACKGROUND OF THE INVENTION

Field

The present invention relates to a method of manufacturing an optical semiconductor device wherein an optical semiconductor element and an optical waveguide are monolithically integrated.

Background

With the increase in optical communication traffic in recent years, optical semiconductor devices capable of high-speed operation have been developed. A substantial number of such optical semiconductor devices is of a structure in which an optical semiconductor element such as a semiconductor laser or an optical modulator and an optical waveguide are monolithically integrated (see, for example, JP 2014-82411 A).

SUMMARY

If an equivalent refractive index of the optical semiconductor element and an equivalent refractive index of the optical waveguide are not matched to each other in the integrated device, a mode mismatch loss of propagating guided wave light occurs and an end surface emergent light output characteristic degrades. Instability of laser operation, appearing, for example, as a kink due to reflected return light, is also caused.

Conventionally, the composition of the active layer of the optical semiconductor element and the composition of the optical waveguide layer of the optical waveguide are controlled to match the equivalent refractive indices to each other. However, there is a possibility of failure to make the composition of the active layer and the composition of the optical waveguide layer equal to each other due to manufacturing variation and, hence, a mismatch between the equivalent refractive index of the optical semiconductor element and the equivalent refractive index of the optical waveguide occurs. Also, each of the equivalent refractive indices is determined at the point in time at which the crystal growth of the corresponding structure is completed, and it is difficult to thereafter correct the determined refractive index.

In view of the above-described problem, an object of the present invention is to provide a method of manufacturing an optical semiconductor device capable of correcting a mismatch in equivalent refractive index due to manufacturing variation.

According to the present invention, a method of manufacturing an optical semiconductor device includes: successively laying a first cladding layer, an active layer and a second cladding layer on a semiconductor substrate to form an optical semiconductor element; obtaining a refractive index of the active layer by a photoluminescence inspection and computing an equivalent refractive index of the optical semiconductor element; successively laying a third cladding layer, an optical waveguide layer jointed to the active layer and having a refractive index smaller than that of the active layer, and a refractive index adjustment layer on the semiconductor substrate to form an optical waveguide; obtaining a refractive index of the optical waveguide layer by a photoluminescence inspection and computing an equivalent refractive index of the optical waveguide; adjusting a film thickness of the refractive index adjustment layer by etching the refractive index adjustment layer so that the equivalent refractive index of the optical semiconductor element and the equivalent refractive index of the optical waveguide are matched to each other; and after adjusting the film thickness of the refractive index adjustment layer, forming a contact layer on the second cladding layer and the refractive index adjustment layer, wherein the optical waveguide is a passive waveguide to which no electrical field is applied and no current is injected.

In the present invention, the optical semiconductor element and the optical waveguide are formed; their equivalent refractive indices of the optical semiconductor element are thereafter obtained by photoluminescence inspection; and the film thickness of the refractive index adjustment layer is adjusted by etching the refractive index adjustment layer so that the equivalent refractive indices are matched to each other. A mismatch in equivalent refractive index due to manufacturing variation can be corrected thereby.

DESCRIPTION OF EMBODIMENTS

A method of manufacturing an optical semiconductor device according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted.

First Embodiment

FIG. 1is a flowchart of a method of manufacturing an optical semiconductor device according to a first embodiment of the present invention.FIGS. 2 to 15are sectional views showing the method of manufacturing an optical semiconductor device according to the first embodiment of the present invention.FIGS. 2 to 9, and 13are sectional views taken along a resonator direction, whileFIGS. 10 to 12, and 14are sectional views perpendicular to the resonator direction.FIG. 16is a perspective view of the optical semiconductor device according to the first embodiment of the present invention.FIG. 17is a see-through perspective view of internal portions of the optical semiconductor device according to the first embodiment of the present invention. The method of manufacturing an optical semiconductor device according to the present embodiment will be described with reference to these figures.

First, referring toFIG. 2, an n-type InP substrate1is housed in a chamber (step S1), and an n-type InP cladding layer2, an active layer3and a p-type InP cladding layer4are successively laid on the n-type InP substrate1by metal organic chemical vapor deposition (MOCVD) to form an optical semiconductor element5(step S2). The optical semiconductor element5in the present embodiment is a laser diode.

Subsequently, the refractive index of the active layer3is obtained by evaluating the wavelength of emission from the active layer3by a photoluminescence inspection (step S3). In photoluminescence measurement, light of a particular wavelength is applied to the substrate on which the active layer3is formed and the emission therefrom is measured. Measurement results are used, for example, to evaluate the bandgap and composition. Independently of whether the material of the active layer3is InGaAsP or AlGaInAs, the refractive index can be calculated by ascertaining the composition (see, for example, Semiconductor Lasers, Applied Physics Series, Ohmsha, p. 35, and Refractive Indexes of (Al, Ga, In) As Epilayers on InP for Optoelectronic Applications, M. J. Mondry, et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 4, NO. 6, June 1992).

Subsequently, as shown inFIG. 3, SiO2film6is formed and an unnecessary portion of the SiO2film6is removed (step S4) while leaving a portion of the SiO2film6only in a place where the optical semiconductor element5is to be left. Subsequently, as shown inFIG. 4, unnecessary portions of the n-type InP cladding layer2, active layer3and p-type InP cladding layer4are removed by performing dry etching or wet etching with the SiO2film6used as a mask (step S5).

Subsequently, as shown inFIG. 5, an n-type InP cladding layer7, an optical waveguide layer8of InGaAsP, a 20 nm-thick InP layer9, a refractive index adjustment layer10and a p-type InP cladding layer11are successively laid on the n-type InP substrate1, with the SiO2film6used as a selective growth mask, thereby forming an optical waveguide12. The optical waveguide layer8may alternatively be AlGaInAs. The optical waveguide layer8has its side surface jointed to a side surface of the active layer3and has a refractive index smaller than that of the active layer3.

The refractive index adjustment layer10includes a plurality of InGaAsP layers10a,10b,10c, and10deach having a layer thickness of 20 nm and having component ratios graded in steps of 20 nm in the wavelength decreasing direction from the optical waveguide layer8toward the p-type InP cladding layer11. Since InGaAsP is a material having a smaller refractive index in a shorter-wavelength composition, the InGaAsP layers10a,10b,10c, and10dhave refractive indices in decreasing order from the optical waveguide layer8side. This arrangement enables preventing the distribution of guided light from expanding to the p-type InP cladding layer11side. Each of the compositions of the layers in the refractive index adjustment layer10can be set in such a range that guided light is not influenced by band end absorption, and each of the layer thickness and the number of layers can be freely set.

Subsequently, the refractive index of the optical waveguide layer8is obtained by evaluating the wavelength of emission from the optical waveguide layer8by photoluminescence inspection (step S7). The same measuring method as that in the case of obtaining the refractive index of the active layer3is used.

Subsequently, an equivalent refractive index of the optical semiconductor element5and an equivalent refractive index of the optical waveguide12are computed by numeric computation. Then a film thickness of the refractive index adjustment layer10with which the two equivalent refractive indices are matched to each other is computed (step S8).

Subsequently, the film thickness of the refractive index adjustment layer10is adjusted by etching the refractive index adjustment layer10on the basis of the computation result so that the equivalent refractive index of the optical semiconductor element5and the equivalent refractive index of the optical waveguide12are matched to each other (step S9).

More specifically, as shown inFIG. 6, the p-type InP cladding layer11is removed by wet etching using, for example, a solution of a mixture of hydrochloric acid and phosphoric acid, with the SiO2film6left as a mask. Subsequently, as shown inFIG. 7, the InGaAsP layer in the refractive index adjustment layer10is etched by using a solution of a mixture of tartaric acid and hydrogen peroxide solution.

FIG. 18is a diagram showing an example of the layer construction of the optical waveguide.FIG. 19is a diagram showing the relationship between the equivalent refractive index and the number of removed layers of the refractive index adjustment layer necessary for equivalent refractive index matching. Computation is performed with respect to a case where the active layer3is an AlGaInAs-based material, the equivalent refractive index of the optical semiconductor element5is 3.267 and the compositions of the optical waveguide layer8and the refractive index adjustment layer10are such that the wavelength is reduced by 20 nm through the entire layer. It can be understood that matching of the equivalent refractive index of the optical semiconductor element5and the equivalent refractive index of the optical waveguide12can be achieved by removing the InGaAsP layers10b,10cand10d.

Particular ones of the InGaAsP layers can be etched by adjusting the mixture ratio of tartaric acid and hydrogen peroxide solution. The etching rate changes depending on the compositions of the InGaAsP layers while the mixture ratio is fixed. Selective etching can therefore be performed. Also, etching can be performed with sufficiently high accuracy because the etching rate is about 10 nm per minute.

Forming the refractive index adjustment layer10of the InGaAsP-based material enables use of an etchant of a low etching rate in etching of the refractive index adjustment layer10. The refractive index adjustment layer10may be formed of an AlGaInAs-based material. However, it is difficult to prepare an etchant of a low etching rate for etching on AlGaInAs. The tartaric acid-hydrogen peroxide mixture solution is not exclusively used. A solution of a mixture of a different organic acid and hydrogen peroxide solution may alternatively be used. Also, wet etching is not exclusively used. Dry etching may alternatively be used.

The amount of etching of the refractive index adjustment layer10is adjusted by controlling the etching time. The InP layer9functions as an etching stopper on the tartaric acid-hydrogen peroxide mixture solution but is not necessarily required in the case where the etching time is controlled.

The optical waveguide layer8in the optical waveguide12is designed so that the equivalent refractive index of the optical waveguide12is smaller than the equivalent refractive index of the optical semiconductor element5in the case of the conventional structure not provided with the refractive index adjustment layer10. That is, the refractive index of the optical waveguide layer8in the optical waveguide12is set to a value smaller than the refractive index of the active layer3and is adjusted with the refractive index adjustment layer10on the optical waveguide layer8. By this adjustment, the equivalent refractive index of the optical waveguide12can be increased but cannot be reduced. Therefore such a setting is made. Also, it is necessary that the equivalent refractive index of the optical waveguide12before refractive index adjustment be larger than the equivalent refractive index of the optical semiconductor element5.

Subsequently, as shown inFIG. 8, the SiO2film6is removed by using an etchant such as hydrofluoric acid (step S10). Subsequently, as shown inFIGS. 9 and 10, SiO2film13is newly formed and worked into the shape of a stripe having a width of about 1 to 2 μm. Subsequently, as shown inFIG. 11, a ridge is formed by performing dry etching using the SiO2film13as a mask (step S11). Wet etching may be performed instead of dry etching.

Subsequently, as shown inFIG. 12, a p-type InP layer14, an n-type InP layer15and a p-type InP layer16are successively laid along ridge side surfaces by MOCVD using as a selective growth mask the SiO2film13used at the time of ridge forming, thereby forming a current constriction structure17(step S12). The current constriction structure17may use a semi-insulating semiconductor.

Subsequently, as shown inFIGS. 13 and 14, the SiO2film13is removed, for example, by hydrofluoric acid, and a p-type contact layer18is formed by MOCVD on the p-type InP cladding layer4, the refractive index adjustment layer10and the current constriction structure17(step S13). Subsequently, as shown inFIG. 15, a p-side electrode19and an n-side electrode20are formed (step S14). The p-side electrode19is formed not on the optical waveguide12but only on the optical semiconductor element5. Also, a band discontinuity occurs in the refractive index adjustment layer10, so that the current injection is impeded. Accordingly, the optical waveguide12is a passive waveguide having no current injected thereinto. The n-side electrode20is formed on the entire back surface of the n-type InP substrate1. However, the n-side electrode20may alternatively be formed only on the optical semiconductor element5. A structure, such as shown inFIGS. 16 and 17, in which the optical semiconductor element5and the optical waveguide12are integrated, is manufactured by the above-described process steps.

In the present embodiment, as described above, the optical semiconductor element5and the optical waveguide12are formed; equivalent refractive indices of the optical semiconductor element5and the optical waveguide12are thereafter obtained by photoluminescence inspection; and the film thickness of the refractive index adjustment layer10is adjusted by etching the refractive index adjustment layer10so that the two equivalent refractive indices are matched to each other. A mismatch in equivalent refractive index due to manufacturing variation can be corrected thereby. Since the adjustment is made by etching after crystal growth, matching can easily be achieved in the wafer process.

FIG. 20is a diagram showing a current-optical output characteristic of the optical semiconductor device when the equivalent refractive indices are matched to each other and a current-optical output characteristic of the optical semiconductor device when the equivalent refractive indices are not matched to each other. By matching the equivalent refractive indices, degradations of I-L characteristics such as a kink due to reflected return light and a reduction in slope efficiency due to scattering loss can be inhibited.

In the present embodiment, the refractive index adjustment layer10is formed of a plurality of InGaAsP layers differing in composition from each other. The refractive index adjustment layer10may alternatively be one InGaAsP layer uniform in composition. The InGaAsP layers differing in composition from each other, however, have different etching rates and therefore make easier the adjustment of the film thickness of the refractive index adjustment layer10performed by etching.

Second Embodiment

FIG. 21is a sectional view showing a method of manufacturing an optical semiconductor device according to a second embodiment of the present invention. In the first embodiment, the refractive index distribution center of the optical waveguide12is offset relative to the refractive index distribution center of the optical semiconductor element5. To correct this, in the present embodiment, the film thickness of the optical waveguide layer8is reduced relative to the film thickness of the active layer3so that the refractive index distribution center of the optical waveguide12does not offset relative to the refractive index distribution center of the optical semiconductor element5. An AlGaInAs material is used for the active layer3, thereby preventing the active layer3from being etched when the refractive index adjustment layer10is etched. In other respects, the construction and advantages of the second embodiment are the same as those in the first embodiment.

Third Embodiment

FIGS. 22 to 24are sectional views showing a method of manufacturing an optical semiconductor device according to a third embodiment of the present invention.

The steps shown inFIGS. 1 to 4are performed, as in the first embodiment. Subsequently, as shown inFIG. 22, an n-type InP cladding layer7, an optical waveguide layer8of InGaAsP, a refractive index adjustment layer21and a p-type InP cladding layer11are successively laid on an n-type InP substrate1, with SiO2film6used as a selective growth mask, thereby forming an optical waveguide12.

The refractive index adjustment layer21in the present embodiment differs from the refractive index adjustment layer10in the first embodiment in that two different types of semiconductor layers: an InP layer21aand an InGaAsP layer21bare alternately laid one on another. The film thickness of each of these layers is 20 nm. The composition of the InGaAsP layer21bis selected in such a range that guided light is not influenced by band end absorption, as in the first embodiment. The layer contacting the InGaAsP optical waveguide layer8may be the InP layer21aor the InGaAsP layer21b. The layer thickness of each layer and the number of pairs of the layers can be freely set.

Subsequently, the refractive index of the optical waveguide layer8is obtained by evaluating the wavelength of emission from the optical waveguide layer8by photoluminescence inspection, as in the first embodiment. An equivalent refractive index of the optical semiconductor element5and an equivalent refractive index of the optical waveguide12are computed by numeric computation. A film thickness of the refractive index adjustment layer10with which the two equivalent refractive indices are matched to each other is then computed.

Subsequently, as shown inFIG. 23, the p-type InP cladding layer11is removed by wet etching using, for example, a solution of a mixture of hydrochloric acid and phosphoric acid. The film thickness of the refractive index adjustment layer21is adjusted by etching the refractive index adjustment layer21so that the equivalent refractive index of the optical semiconductor element5and the equivalent refractive index of the optical waveguide12are matched to each other. It is desirable to use an etchant of a low etching rate, e.g., a solution of a mixture of hydrochloric acid and phosphoric acid for etching of the InP layer21a. A mixture of tartaric acid and hydrogen peroxide solution is used for etching of the InGaAsP layer21b. While etching is performed by controlling the etching time in the first embodiment, the InP layer21aand the InGaAsP layer21balternately function as an etching stopper layer in the present embodiment and, therefore, etching time control is not necessary in the present embodiment.

The same process steps as those in the first embodiment are thereafter performed, thereby manufacturing the optical semiconductor device according to the present embodiment, as shown inFIG. 24.

In the present embodiment, as described above, the optical semiconductor element5and the optical waveguide12are formed; equivalent refractive indices of the optical semiconductor element5and the optical waveguide12are thereafter obtained by photoluminescence inspection; and the number of pairs of InP layer21aand InGaAsP layer21bis adjusted by etching the refractive index adjustment layer21so that the two equivalent refractive indices are matched to each other, thus obtaining the same advantages as those of the first embodiment.

In the present embodiment, the refractive index adjustment layer21has two types of semiconductor layers formed of different materials and alternately laid one on another. However, the refractive index adjustment layer21may alternatively have two types of semiconductor layers formed of the same material, having different compositions and alternately laid one on another.

Fourth Embodiment

FIG. 25is sectional view showing a method of manufacturing an optical semiconductor device according to a fourth embodiment of the present invention. In the third embodiment, the refractive index distribution center of the optical waveguide12is offset relative to the refractive index distribution center of the optical semiconductor element5. To correct this, in the present embodiment, the film thickness of the optical waveguide layer8is reduced relative to the film thickness of the active layer3. Making the refractive index distribution center of the optical waveguide12offset relative to the refractive index distribution center of the optical semiconductor element5is avoided thereby. An AlGaInAs material is used for the active layer3, thereby preventing the active layer3from being etched when the refractive index adjustment layer21is etched. In other respects, the construction and advantages of the fourth embodiment are the same as those in the third embodiment.

FIG. 26is a plan view showing a modified example of the optical semiconductor devices according to the first to fourth embodiments of the present invention. The optical waveguide12may be a bent waveguide, such as illustrated.

While the devices according to the first to fourth embodiments are each an optical waveguide integrated semiconductor laser formed on the n-type InP substrate1, the same advantages can also be obtained with an optical waveguide integrated structure made on a p-type substrate. Also, in the case of an integrated optical semiconductor device in which the optical waveguide12is a non-current-injection passive optical waveguide, the same advantages can be obtained. The optical semiconductor element5is not limited to the laser diode. The optical semiconductor element5may be an array-type laser or optical modulator.

The entire disclosure of Japanese Patent Application No. 2015-252124, filed on Dec. 24, 2015 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.