Method of fabricating a semiconductor optical device

A method of fabricating a semiconductor optical device is disclosed. This semiconductor optical device includes first and second optical semiconductor elements. This method comprises the steps of: growing, in a metal-organic vapor phase deposition reactor, plural semiconductor layers for the first semiconductor optical element on a primary surface of a substrate which has first and second areas for the first semiconductor optical element and the second optical semiconductor element, respectively; forming an insulating mask on the plural semiconductor layers and the first area; etching the plural semiconductor layers by use of the insulating mask to form a semiconductor portion having an end face; growing a layer of a first semiconductor on the second area and deposit of the first semiconductor on the end face in the reactor by use of the insulating mask; supplying etchant for etching the first semiconductor to remove at least a part of the deposit of the first semiconductor on the end face by use of the insulating mask; and after removing the deposit of the first semiconductor, growing a layer of a second semiconductor for the second optical element on the second area in the reactor by use of the insulating mask.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the method of fabricating a semiconductor optical device.

2. Related Background Art

Non-Patent Publication (K. Hinzer et al., Technical Digest of Optical Fiber Communication, 2003, FG7, p. 684-685) discloses a buried semiconductor laser. In the fabrication of this semiconductor laser, after forming a mesa-stripe SiO2mask on a semiconductor stack of a substrate, this substrate is loaded into a metal-organic vapor phase deposition (MOVPE) reactor. In order to perform mesa-etching, methyl-iodide as an etchant is supplied to this reactor and PH3gas is also supplied to the reactor. After this etching, the mesa stripe is buried by selective regrowth in the same reactor to form a pn block layer.

SUMMARY OF THE INVENTION

A semiconductor laser and an electro-absorption modulator are integrated to provide a semiconductor optical device. The semiconductor optical device has a butt joint structure to optically couple the semiconductor laser and the electro-absorption modulator with each other. This butt joint structure is formed as follows: A semiconductor stack structure including a number of semiconductor layers, such as an n-type cladding layer, an active layer and a p-type cladding layer, is formed on both an area for the modulator and an area for the semiconductor laser of a substrate. Then, an insulating mask is formed on the area for the semiconductor laser. The semiconductor stack structure is etched by use of the mask to remove semiconductor layers for an active layer located on the area for the modulator, and an end face of the semiconductor stack structure is formed. After this etching, plural semiconductor layers for the modulator are grown by use of the mask. In this growth, semiconductor is deposited on the above end face in addition to on the area the modulator. Due to an anomalous deposition, semiconductor deposit is grown upward along the end face of the semiconductor stack structure. The reason why this anomalous growth is caused is as follows. Raw material gas is supplied onto the substrate, and a portion of the raw material gas is supplied onto the insulating mask and is not deposited thereon. This unconsumed portion of the raw material gas flows along the insulating mask to the area for the modulator, and this material gas from the insulating mask is additionally deposited on the area for the modulator to cause anomalous semiconductor deposition. Accordingly, the raw material gas is supplied around the boundary between the areas for the modulator and the semiconductor laser to form the anomalous deposit on the end face as well as the desired semiconductor layer on the area for the modulator.

Thereafter, when subsequent semiconductor layers are formed thereon, the active layer for one of the modulator or laser is bent around the boundary of the above areas for the modulator and the laser because of the anomalous deposit. Accordingly, the height matching of the active layers for the semiconductor laser and modulator is not obtained at the boundary.

In contrast, Non-Patent publication discloses that methyl-iodide is supplied to the metal-organic-vapor-phase deposition reactor for mesa-etching, but does not disclose any integration of plural semiconductor devices.

It is an object to provide a method of fabricating a semiconductor optical device including first and second semiconductor optical elements that are integrated with each other, and this method permits the reduction of curve radius of semiconductor layers around the boundary of the first and second semiconductor optical elements of the semiconductor optical device.

One aspect of the present invention is a method of fabricating a semiconductor optical device, and this semiconductor optical device includes a first semiconductor optical element and a second optical semiconductor element. The method comprises the steps of: growing, in a metal-organic vapor phase deposition reactor, plural semiconductor layers for the first semiconductor optical element on a primary surface of a substrate, which has a first area for the first semiconductor optical element and a second area for the second optical semiconductor element; forming an insulating mask on the plural semiconductor layers and the first area; etching the plural semiconductor layers by use of the insulating mask to form a semiconductor portion having an end face; after etching the plural semiconductor layers, growing a layer of a first semiconductor for the second optical element on the second area and deposit of the first semiconductor on the end face in the metal-organic vapor phase deposition reactor by use of the insulating mask; after growing the layer of the first semiconductor, supplying etchant for etching the first semiconductor to remove at least a part of the deposit of the first semiconductor on the end face by use of the insulating mask; and after removing at least a part of the deposit of the first semiconductor, growing a layer of a second semiconductor for the second optical element on the second area in the metal-organic vapor phase deposition reactor by use of the insulating mask.

In the method according to the present invention, gas containing the etchant for etching the first semiconductor is supplied to the metal-organic vapor phase deposition reactor. Further, in the method according to the present invention, the gas containing the etchant for etching the first semiconductor includes HCl, and further includes at least one of AsH3and PH3.

In the method according to the present invention, the layer of the first semiconductor is formed for an optical guide layer of the second optical semiconductor element, and the layer of the second semiconductor is formed for an active layer of the second optical semiconductor element. Further, in the method according to the present invention, the semiconductor portion includes a semiconductor layer for an active layer of the first semiconductor optical element.

In the method according to the present invention, one of the first and second optical semiconductor elements includes a semiconductor laser and the other of the first and second optical semiconductor elements includes an electro-absorption modulator.

The method according to the present invention further comprises the steps of: after growing the layer of the second semiconductor, growing a layer of a third semiconductor on the second area and deposit of the third semiconductor on the end face in the metal-organic vapor phase deposition reactor by use of the insulating mask; and after growing the layer of the third semiconductor, supplying etchant for etching the third semiconductor to remove at least a part of the deposit of the third semiconductor by use of the insulating mask. Further, in the method according to the present invention, the layer of the first semiconductor is formed for an optical guide layer of the second optical element; the layer of the second semiconductor is formed for an active layer of the second optical element; and the layer of the third semiconductor is formed for another optical guide layer of the second optical element. Furthermore, in the method according to the present invention, gas containing the etchant for etching the third semiconductor is supplied to the metal-organic vapor phase deposition reactor, and may include HCl.

The method according to the present invention further comprises the steps of: prior to growing the layer the first semiconductor, growing a layer of a fourth semiconductor for the second optical element on the second area and deposit of the fourth semiconductor on the end face in the metal-organic vapor phase deposition reactor by use of the insulating mask; and prior to growing the layer of the first semiconductor and after growing the layer of the fourth semiconductor, supplying etchant for etching the fourth semiconductor to remove at least a part of the deposit of the fourth semiconductor by use of the insulating mask. Further, in the method according to the present invention, the layer of the first semiconductor is formed for an optical guide layer of the second optical element; the layer of the second semiconductor is formed for an active layer of the second optical element; the layer of the third semiconductor is formed for another optical guide layer of the second optical element; and the layer of the fourth semiconductor is formed for a buffer layer of the second optical element. Furthermore, in the method according to the present invention, gas containing the etchant for etching the fourth semiconductor is supplied to the metal-organic vapor phase deposition reactor, and may include HCl.

The method according to the present invention further comprises the steps of: prior to growing the layer the first semiconductor, growing a layer of a third semiconductor for the second optical element on the second area and deposit of the third semiconductor on the end face in the metal-organic vapor phase deposition reactor by use of the insulating mask; and prior to growing the layer of the first semiconductor and after growing the layer of the third semiconductor, supplying etchant for etching the third semiconductor to remove at least a part of the deposit of the fourth semiconductor by use of the insulating mask. Further, in the method according to the present invention, the layer of the first semiconductor is formed for an optical guide layer of the second optical element; the layer of the second semiconductor is formed for an active layer of the second optical element; and the layer of the third semiconductor is formed for a buffer layer of the second optical element.

In the method according to the present invention, the end face extends along a reference plane which intersects with the primary surface. Further, in the method according to the present invention, the layer of the second semiconductor has a multiple quantum well structure for an active layer of the second optical semiconductor element. Furthermore, the insulating mask is made of silicon oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, embodiments of the present invention will be explained. When possible, parts identical to each other will be referred to with symbols identical to each other.

FIGS. 1 to 5are cross sectional views for explaining a method of fabricating a semiconductor optical integrated device according to the embodiment of the present invention. In the subsequent explanation, a semiconductor laser (LD), as a first semiconductor optical element, and an electro-absorption (EA) modulator, as a second semiconductor optical element, both are formed on a substrate.

Referring to Part (a) ofFIG. 1, an InP semiconductor substrate1is shown. The primary surface1aof the semiconductor substrate1has a first area1b(hereinafter referred to as “LD area”) for the semiconductor laser and a second area1c(hereinafter referred to as “EA area”) for the EA modulator. In Part (A) ofFIG. 1and other parts of other figures, the boundary of the LD area and the EA area is indicated by dashed line “B.”

Referring to Part (b) ofFIG. 1, a buffer layer3, a first optical guide layer5, an active layer7, and a second optical guide layer9are subsequently grown in the whole of the primary surface1aof the semiconductor substrate1. The buffer layer3is, for example, Si-doped InP layer and its thickness is 550 nanometers. In the present example, the carrier density of the buffer layer3is, for example, 1.1×10+18cm−3. The first and second optical guide layers5and9are, for example, a GaInAsP layer corresponding to the wavelength of 1550 nanometers, and its thickness is 100 nanometers.

The active layer7is made of GaInAsP, for example. The active layer7has a multiple quantum well structure including barrier layers and well layers, for example, sixteen layers (eight pairs of barrier layers and well layers) which are alternately arranged. In the present example, each barrier layer has a bandgap wavelength of 1200 nanometers and a thickness of 10 nanometers, and each well layer has a bandgap wavelength of 1150 nanometers and a thickness of 5 nanometers. For example, 1.0%-strained active layer can be used.

Referring to Part (c) ofFIG. 1, a diffraction grating11is formed on the second optical guide layer9. The period of the diffraction grating11is, for example, 242 nanometers. A cladding layer13is formed on the diffraction grating11. The cladding layer13is made of InP layer doped with zinc, and has a thickness of 240 nanometers. The carrier density of the cladding layer13is 6.5×10+17cm−3. A cap layer15is formed on the cladding layer13. The cap layer15is, for example, InGaAs layer doped with zinc, and has a thickness of 100 nanometers. The carrier density of the cap layer15is 2.0×10+17cm−3.

The layers as explained above are grown on the whole of the primary surface1aof the InP semiconductor substrate by metal-organic-vapor-phase deposition method.

Referring to Part (a) ofFIG. 2, a mask17made of an insulating film is formed on the cap layer15. The insulating film mask17is located on the LD area1b. The insulating film mask17is made of silicon oxide, such as SiO2, or silicon nitride, such as SiN, and has a thickness of 200 nanometers, for example.

Referring to Part (b) ofFIG. 2, the above plural semiconductor stack is etched using the insulating film mask17to form a semiconductor stack. In this etching, the first optical guide layer5, the active layer7, the second optical guide layer9, the diffraction grating11, the cladding layer13and the cap layer15are removed from the EA area1c.

An etching, such as reactive ion etching (RIE), is used for this removal. Mixed gas of CH4and H2is supplied to the RIE apparatus. The flow rate of the mixed gas of CH4and H2is, for example, 25 sccm, and the RF power is, for example, 100 watts. The depth of the etching is, for example, 0.8 micrometers.

The reactive ion etching is carried out as above to form a first semiconductor portion19on the LD area1b. This first semiconductor portion19includes a buffer layer3b, a first optical guide layer5b, an active layer7b, a second optical guide layer9b, the diffraction grating11b, the cladding layer13band the cap layer15b. The first semiconductor portion19has an end face19a, which is formed by the above etching and is located at the boundary1dof the LD area1band BA area1c. The end face19aextends along a reference plane intersecting with the primary surface1aof the InP semiconductor substrate1.

In an example shown in Part (b) ofFIG. 2, a part of the buffer layer3is removed by RIE in the buffer layer3con the EA area1c. After this etching, the surface of the buffer layer3cis damaged to form, what is called, a damage layer, and it is preferable that this damage layer be removed. Sulfuric acid can be used as etchant for this removal.

As shown in Part (a) ofFIG. 3, a buffer layer3dis formed on the buffer layer3c. The buffer layer3dis made of the same material as the buffer layer3cand has a thickness of 50 nanometers. For example, the buffer layer3dis Si-doped InP layer, and the carrier density of the buffer layer3dis 1.1×10+18cm−3.

This buffer layer3dis grown using the insulating mask17by supplying raw material gas to the metal-organic vapor phase deposition reactor. Since the insulating film mask17is still left on the LD area1b, a part of the raw material gas supplied onto the LD area1bis not consumed for depositing semiconductor on the insulating film mask and this unconsumed raw material gas flows along the insulating film mask17to the EA area1c. The raw material gas from the LD area1bis consumed around the boundary1din the EA area1cto form semiconductor deposit. Accordingly, as shown in Part (a) ofFIG. 3, anomalous deposit21as well as the buffer layer3dis formed on the end face19aaround the boundary1din the EA area1c.

Referring to Part (b) ofFIG. 3, the anomalous deposit21is removed by etching. In this etching step, mixed gas containing HCl, PH3and AsH3is used as etchant and is supplied to the metal-organic vapor phase deposition reactor for the etching. The flow rate of HCl is, for example, 0.1 sccm. The flow rate of PH3is, for example, 50 sccm. The flow rate of AsH3is, for example, 1.0 sccm. If required, the etching may be performed in an RIE apparatus, and the above etching gas can be used therein.

This etching is performed in the metal-organic vapor phase deposition reactor by use of the insulating film mask17. The etching gas is supplied to the LD area1bas well as the EA area1cin the metal-organic vapor phase deposition reactor. The etching gas is not consumed on the insulating film mask17located on the LD area1b, and the unconsumed etching gas flows along the insulating film mask17to the EA area1c. The etching gas delivered to the EA area1cis consumed to etch the anomalous deposit21in the EA area1caround the boundary1d. As in the case of the formation of the anomalous deposit21explained above, the etching gas is supplied directly to the EA area1cand via the LD area1bto the boundary1d, thereby causing the etching of much more anomalous deposit as compared with the etching in the EA area1c.

As shown in Part (b) ofFIG. 3, the anomalous deposit21is removed in the etching. As a result of the etching, the surface of buffer layer3dis planarized. After growing the buffer layer3din the metal-organic vapor phase deposition reactor to form a semiconductor product including the anomalous deposit21and buffer layer3d, the same reactor is used for the subsequent etching of the anomalous deposit21without taking out the semiconductor product from the reactor.

Referring to Part (a) ofFIG. 4, a first optical guide layer23for the EA modulator is grown on the buffer layer3d. For example, the first optical guide layer23is made of GaInAsP having a bandgap corresponding to the wavelength of 1150 nm, and has a thickness of 60 nm. The first optical guide layer23is grown using the insulating film mask17by metal-organic vapor phase deposition method as in the case of the formation of the buffer layer3d. Accordingly, anomalous deposit25shown in Part (a) ofFIG. 4is formed on the end face19ain the EA area1caround the boundary1d. The reason why the anomalous deposit25is formed is the same as the formation of the anomalous deposit21.

Referring to Part (b) ofFIG. 4, the anomalous deposit25is removed by etching. The anomalous deposit25can be etched as in the case of the removal of the anomalous deposit21. After the etching, the surface of the first optical guide layer23is planarized.

Referring to Part (c) ofFIG. 4, an active layer27for the EA modulator is grown on the planarized surface of the first optical guide layer23. The active layer27is grown using the insulating film mask17by metal-organic vapor phase deposition method. The active layer27includes, for example, a GaInAsP layer. The active layer27has a quantum well structure which well layers and barrier layers are alternatively arranged, and its thickness is, for example, 105 nanometers. The active layer27is hardly bent because the surface of the first optical guide layer23is planarized and the well layers and barrier layers of the active layer27are thin in thickness.

Referring to Part (a) ofFIG. 5, a second optical guide layer29for the EA modulator is grown on the active layer27. The second optical guide layer29is made of, for example, GaInAsP of the bandgap corresponding to a wavelength of 1150 nanometers, and has a thickness of, for example, 60 nanometers. The second optical guide layer29is grown using the insulating film mask17by metal-organic vapor phase deposition method. Accordingly, anomalous deposit31shown in Part (a) ofFIG. 5is formed on the end face19ain the EA area1caround the boundary1d. The reason why the anomalous deposit31is formed is the same as the formation of the anomalous deposits21and25. Referring to Part (b) ofFIG. 5, the anomalous deposit31is removed by etching. The anomalous deposit31can be etched as in the case of the removal of the anomalous deposits21and25. As a result of the etching, the surface of the first optical guide layer29is planarized.

Referring to Part (c) ofFIG. 5, a cladding layer33and a cap layer35for the EA modulator is grown on the second optical guide layer29. The cladding layer33is, for example, an InP layer doped with Zn, and its thickness is, for example, 100 nanometers. The carrier concentration of the cladding layer33is, for example, 6.5×10+17cm−3. The cap layer35is formed on the cladding layer33. The cap layer35is, for example, an InGaAs layer doped with Zn, and its thickness is, for example, 100 nanometers. The carrier concentration of the cap layer35is, for example, 2.0×10+17cm−3.

The cladding layer33is grown using the insulating film mask17by metal-organic vapor phase deposition method. Thus, anomalous deposit is formed on the end face19ain the EA area1caround the boundary1d. After this formation of the cladding layer33, this anomalous deposit is removed. Etching by use of the insulating film mask17can be used as in the case of the removal of the anomalous deposits21,25and31.

The cap layer35is grown using the insulating film mask17by metal-organic vapor phase deposition method. Thus, anomalous deposit is formed on the end face19ain the EA area1caround the boundary1d. After growing the cap layer35, this anomalous deposit is removed. Etching by use of the insulating film mask17can be used as in the case of the removal of the anomalous deposits21,25and31. As a result of the above etching, the surface of the cap layer35is planarized.

As explained above, in the etching step and the semiconductor growing step for the EA modulator, the semiconductor portion37is formed on the EA area1c. The semiconductor portion37includes the buffer layer3c,3d, the first optical guide layer23, the active layer27, the second optical guide layer29, the cladding layer33, and the cap layer35. The semiconductor portion37abuts against the first semiconductor portion19at the boundary1dto form an optical coupling therebetween. The anomalous deposits that are created in growing each layer contained in the semiconductor portion37are removed as above by etching to make the surface of the semiconductor portion37planarized. Accordingly, the positional misalignment between the active layer7bfor the semiconductor layer and the active layer27for the EA modulator is made small.

Referring to Part (a) ofFIG. 6, the insulating film mask17is removed. Part (b) ofFIG. 6is a perspective view showing a part of a product in the manufacturing steps of the semiconductor laser. The cross sectional views inFIG. 1to Part (a) ofFIG. 6are taken along I-I line shown in Part (b) ofFIG. 6. Referring to Part (b) ofFIG. 6, optical waveguides39b,39care formed and extend along the axis directed from the LD area1bto the EA area1c. The optical waveguide39bis provided on the LD area1b, and the optical waveguide39cis provided on the EA area1c.

As shown in Part (b) ofFIG. 6, in order to form the optical waveguides39band39c, a mask41is formed on the first and second semiconductor portions19and37and extend in a direction of the above axis. The mask41is made of insulator and its shape is a stripe, for example. The insulating material for the mask41is made of silicon oxide, such as SiO2, or silicon nitride, such as SiN.

In order to form the optical waveguides39band39c, the first and second semiconductor portions19and37are etched using the mask41to expose the primary surface1aof the InP semiconductor substrate1. By this etching, a stripe mesa for acting as optical waveguides is obtained. The optical waveguide39bincludes a buffer layer3e, a first optical guide layer5e, an active layer7e, a second optical guide layer9e, a cladding layer13eand a cap layer15e, and these layers are located on the LD area1b. The optical waveguide39cincludes a buffer layer3fand3g, a first optical guide layer23f, an active layer27f, a second optical guide layer29f, a cladding layer33fand a cap layer35f, and these layers are located on the EA area1c.

Referring to Part (c) ofFIG. 6, the mesa of the optical waveguides39band39cis buried by a burying semiconductor layer43. In order to bury the mesa of the optical waveguides39band39c, the semiconductor layer43, such as Fe-doped InP, for burying the above mesa is regrown using the mask41on the InP semiconductor substrate1. After this growth, the mask41is removed.

Referring to Part (a) ofFIG. 7, a contact layer45is formed on the optical waveguides39band39cand the burying semiconductor layer43. The contact layer45, for example Zn-doped GaInAs, is formed on the whole surface of the optical waveguides39band39cand the burying semiconductor layer43.

Referring to Part (b) ofFIG. 7, a contact layer45bfor the semiconductor laser and a contact layer45cfor the EA modulator are formed. In order to form the contact layers45band45c, the part of the contact layer45on the boundary1dis removed to form a groove, and a part of the surface of the optical waveguides39band39cis exposed at the removed portion of the contact layer45. The groove extends in a direction perpendicular to the above axis. By this removal, the contact layers45band45cis separated from each other, and this removal permits the electrical separation of one electrode for the semiconductor laser and LD and another electrode for the EA modulator from each other.

Referring to Part (c) ofFIG. 7, an insulating film47is formed on the exposed surfaces of the optical waveguides39band39c, the contact layers45band45c, and the burying semiconductor layer43. The insulating film47can be made of, for example, insulating silicon compound. The insulating film47has openings47band47cto the contact layers45band45c, respectively.

Referring to Part (d) ofFIG. 7, a cathode electrode49is formed on the backside of the InP semiconductor substrate1. An anode electrode51bfor the semiconductor laser is formed on the insulating film47and opening47b, and another anode electrode51cfor the EA modulator is formed on the insulating film47and opening47c. The anode electrodes51band51care connected through the openings47band47cwith the contact layers45band45c, respectively. After the above steps, the semiconductor optical device53as shown in Part (d) ofFIG. 7is obtained.

As explained above, the present embodiment comprises the step of, before growing the active layer27for the EA modulator, removing material anomalously deposited on the end face19aof the first semiconductor portion19, i.e., at least a part of anomalous deposit which has been grown in forming the first optical guide layer23. This step reduces a portion of the active layer27curved by the anomalous deposit. Therefore, the present method prevents the optical coupling between the active layer7bfor the semiconductor laser and the active layer27for the EA modulator from decreasing due to the curved portion of the active layer27.

Since the present embodiment comprises an etching step provided just after growing each of the buffer layer3d, the second optical guide layer29, cladding layer33and the cap layer35, the curved portions of these layers formed by anomalous deposits in their growth are also made small.

Each of the above etching steps for removing the anomalous deposit follows the corresponding growing step of the layer for the EA modulator in the same reactor. These sequences prevent the oxidization of the semiconductor surfaces caused by taking out them from the reactor, and shorten the processing time for the repetition of the growing step and etching step.

Since the surfaces of the semiconductor layers are planarized in the etching step, optical performances of the semiconductor optical device53according to the present embodiment cannot be deteriorated due to the bending of semiconductor layers caused by the anomalous growth.

Having described and illustrated the embodiments of the semiconductor optical amplifiers according to the invention, the application of the present invention is not limited thereto. Details of structures of these devices can be modified as necessary. We therefore claim all modifications and variations coming within the spirit and scope of the following claims.