Patent ID: 12222546

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

First Embodiment

A first embodiment of the present invention will be described with reference to the drawings.FIG.1is a sectional view of an optical waveguide10according to the present embodiment.FIG.2indicates bandgap wavelength variation in a wave guiding direction of the optical waveguide10according to the present embodiment. The wave guiding direction is indicated by arrow X in the figures.

In the optical waveguide10according to the present embodiment, a light propagation layer formed of a quantum well structure in interposed between cladding layers102formed of a material that is the same as that of a substrate101is formed on the substrate101. An active region no has the quantum well structure iii that is not disordered and a passive region130has a quantum well structure131, all of which is disordered. An intermediate region120has a quantum well structure121disordered in such a manner that a bandgap wavelength continuously varies from a bandgap wavelength of the active region no to a bandgap wavelength of the passive region130.

Here, a bandgap wavelength λg refers to a wavelength corresponding to a bandgap Eg and has the below relationship.
λg(μm)=1.24/Eg(eV)

A bandgap wavelength can be approximated by a peak wavelength of PL light emission, the peak wavelength being obtained by photoluminescence (hereinafter referred to as “PL”) measurement, and can easily be evaluated, and thus, the below description is provided with a peak wavelength of PL light emission as a bandgap wavelength.

In the present embodiment, MOVPE using a reacting furnace with a pressure reduced to 50 Torr is used for crystal growth. For a group-III raw material, trimethylindium (TMIn) or triethylgallium (TEGa) is used, and for a group-V raw material, phosphine (PH3) or arsine (AsH3) is used. For a raw material of Zn, which functions as a p-type impurity, diethylzinc (DEZn) is used and for a raw material of Si, which functions as an n-type impurity, monosilane (SiH4) is used.

For evaluation of structural characteristics of a grown crystal, an X-ray diffraction device manufactured by Philips is used. For evaluation of optical characteristics, PL measurement using a laser having a wavelength of 532 nm as a light source is performed at room temperature (25° C.).

FIG.3is a sectional view of a crystal100used for manufacturing the waveguide according to the present embodiment. An n-type InP cladding layer102(film thickness: 500 nm) with Si doped at a concentration of 1×1018cm3, a non-doped InGaAsP optical confinement layer (film thickness: 100 nm)103having a bandgap wavelength of 1.3 μm, an InAs/InGaAs multi-quantum well structure (MQW)111consisting of six InAs well layers (film thickness: 5 nm) and seven InGaAs barrier layers (film thickness: 20 nm), a non-doped InGaAsP optical confinement layer (film thickness: 100 nm)104having a bandgap wavelength of 1.3 μm, a non-doped InGaAsP optical confinement layer (film thickness: 100 nm)105having a bandgap wavelength of 1.1 μm, and a p-type InP capping layer106(film thickness: 50 nm) with Zn doped at a concentration of 5×1017cm3are sequentially stacked on an n-type InP substrate101.

An intermediate region120and a passive region130are formed by the crystal100being subjected to quantum well disordering. In usual quantum well disordering, when impurity diffusion or ion implantation, which induces quantum well disordering, is performed, a SiO2film is formed only on a surface of the active region no. As a result, in the active region no, the quantum well is not disordered, and in a region in which the SiO2film is not formed, the quantum well structure111is disordered and becomes a passive region130. Therefore, no region (intermediate region120) having a bandgap wavelength between those of the active region110and the passive region130can be formed.

In the present embodiment, the intermediate region120whose bandgap wavelength varies between those of the active region no and the passive region130is formed by controlling crystal defect diffusion, which induces quantum well disordering, using an InP layer formed by selective growth. The details will be described below.

First, SiO2selective growth masks141are formed on a surface of the crystal.FIGS.4and5are a top view of the crystal with the SiO2masks141formed thereon, the SiO2masks141being used for quantum well disordering, in the present embodiment and is a sectional view along line V-V′ inFIG.4, respectively. The SiO2masks141are formed by stacking a SiO2film on the surface of the crystal using a usual method and processing the SiO2film using photolithography.

A width Ws of each SiO2mask141is 250 μm in the active region no and is continuously varied from 250 μm to 10 μm in the intermediate region120. On the other hand, no mask is formed in the passive region130. Also, a length of the intermediate region120(distance between the active region no and the passive region130) is 20 μm.

A width (Wg) of a region (hereinafter referred to as “opening portion”)142interposed between the SiO2masks141is 40 μm.

Next, a p-type InP layer with Zn doped at a concentration of 5×1017cm3is selectively grown in the opening portion142on the crystal surface with the SiO2masks141formed thereon. In this selective growth, InP is grown only in the opening portion142.

Here, the doping concentration of Zn is not limited to 5×1017cm3and may be another concentration. Also, an n-type InP layer may be selectively grown by doping e.g., Si or Se other than Zn or a non-doped InP layer may be selectively grown.

FIG.6indicates dependency of a growth speed of the InP selectively grown layer on the width (Ws) of the SiO2masks141. In selective growth of InP, raw material species obtained by, e.g., decomposition of a raw material gas, the raw material species coming onto the SiO2masks141, migrates on the SiO2masks141and contributes to the growth of InP in the opening portion142.

As a result, an epitaxial growth speed of InP selectively grown in the opening portion142increases in comparison with a case of crystal growth on a surface with no SiO2mask141. Therefore, the growth speed of the selective growth of InP increases along with an increase in width (Wm) of the SiO2masks141.

FIGS.7and8are a top view of the crystal subjected to the selective growth and a sectional view along line VIII-VIII′ inFIG.7, respectively. Thicknesses of an InP layer151selectively grown in the active region110, the intermediate region120, and the passive region130are 250 nm, 250 to 50 nm, and 50 nm, respectively. Here, the thickness of the InP layer151in the intermediate region120continuously decreases from one end on the active region110side to the other end on the passive region130side.

Next, after removal of the SiO2selective growth masks141used for the selective growth, a SiO2film (thickness: 300 nm) for annealing is formed on the entire crystal surface via sputtering.

Next, a SiO2annealing mask161is formed by removing a part of the SiO2film, the part being in a region not to be subjected to quantum well disordering (active region110).FIG.9is a sectional view of the crystal with the SiO2annealing mask161formed thereon.

Next, this crystal is subjected to ten-minute thermal treatment (rapid thermal annealing, which is hereinafter referred to as “RTA”) at 550° C. under a nitrogen atmosphere.

Through the above process, quantum well structure disordering is performed in the regions provided with the SiO2annealing mask161. Quantum well disordering is considered to occur due to crystal defects, such as vacant lattice points or interstitial atoms, occurring at an interface between the SiO2annealing mask161and the p-type InP capping layer106due to the RTA diffusing from the interface to the multi-quantum well structure (MQW)111.

FIG.10illustrates a relationship between a layer thickness of the InP (corresponding to the InP capping layer106and the InP selectively grown layer151) above the quantum well structure in and an amount of variation in bandgap wavelength due to quantum well disordering. In this way, the amount of wavelength variation due to quantum well disordering depends on the thickness of the InP layer above the quantum well structure in, and when the InP layer is thin, the wavelength variation amount is large, and when the InP layer is thick, the wavelength variation amount is small.

A reason for the above is as follows. When the InP layer is thin, there are many lattice defects occurring at the aforementioned interface and diffusing, and reaching the MQW111. As a result, the effect of quantum well disordering due to the lattice defects is large, and thus, the wavelength variation amount is large.

On the other hand, when the InP layer is thick, there are few lattice defects reaching the MQW111. As a result, the effect of quantum well disordering due to the lattice defects is small, and thus, the wavelength variation amount is small.

FIGS.11and12illustrate a section of the crystal with the quantum well structure111disordered and variation in bandgap wavelength, respectively. The bandgap wavelength of the active region110is constantly 2350 nm and the bandgap wavelength of the passive region130is constantly 2100 nm.

On the other hand, the bandgap wavelength of the intermediate region120decreases from 2300 nm at the one end on the active region110side (B in the graph) to 2100 nm at the other end on the passive region130side (C in the graph). In this way, the bandgap wavelength of the intermediate region120can continuously be varied in the direction from the active region no toward the passive region130.

Here, a bandgap wavelength discontinuity part occurs at a boundary between the active region no and the intermediate region120; however, such bandgap wavelength discontinuity is around one fifth of the wavelength variation amount in the passive region130and thus light loss caused by this discontinuity part can be ignored.

In the optical waveguide according to the present embodiment, the InAs/InGaAs multi-quantum well structure (MQW)111including InAs well layers and seven InGaAs barrier layers is used.

In a case where an optical waveguide is formed in the MQW111via conventional butt-joint growth, a growth surface (side surface) of the MQW is exposed to a PH3atmosphere, which is a growth gas, at a high temperature immediately before a start of crystal growth. At this time, the MQW111contains only As as a group-V material, and thus, crystal quality may deteriorate because of, e.g., replacement of As in the InAs/InGaAs growth surface and P in PH3with each other.

If an optical waveguide is formed (grown) in this deteriorated InAs/InGaAs-MQW growth surface, quality of an interface between the MQW and the optical waveguide deteriorates, resulting in an increase in light loss when light propagates.

As described above, in a case where an optical waveguide is formed in an active layer of, e.g., an MQW containing only As as a group-V material via conventional butt-joint growth, light loss may increase.

The optical waveguide according to the present embodiment and the method for manufacturing the same enable avoidance of light loss occurring when the aforementioned butt-joint growth is used.

Although in the present embodiment, the layer thickness of the selectively grown InP is varied by varying the width Ws of the selective growth masks141with the width Wg of the opening portion142kept constant, the layer thickness of the selectively grown InP can be varied by varying the width Wg of the opening portion142with the width Ws of each selective growth mask141kept constant. Also, the layer thickness of the selectively grown InP can be varied by varying both the width Ws of each selective growth mask141and the width Wg of the opening portion142.

In this way, the layer thickness of the selectively grown InP can be varied by varying a ratio between the width Ws of each selective growth mask141and the width Wg of the opening portion142. However, in consideration of uniformity (flatness) of the layer thickness of the selectively grown InP, varying the width Ws of each selective growth mask141with the width Wg of the opening portion142kept constant is more desirable for ease of control.

Although in the present embodiment, each selective growth mask141is formed in such a manner that a distal end of the selective growth mask141in the intermediate region120reaches a boundary with the passive region130, each selective growth mask141may be formed in such a manner that a distal end of the selective growth mask141falls inside the intermediate region120. Each selective growth mask141just needs to be formed in a part of the intermediate region120.

Also, in the present embodiment, the selective growth masks141are formed to grow InP thick in the active region no. Here, since no quantum well disordering occurs unless the annealing mask161is formed, there is no need to form the selective growth masks141to grow InP thick.

However, since the bandgap wavelength of the active region no may be affected by annealing, in consideration of bandgap wavelength continuity with the intermediate region120, it is desirable to form the selective growth masks141to grow InP thick in the active region110.

Also, although in the present embodiment, the annealing mask161is formed in the entire intermediate region120, even in a case where the annealing mask161is formed not in the entirety but a part of the intermediate region120, effects that are substantially the same as those of the present embodiment can be exerted as long as the bandgap wavelength in the intermediate region120can be varied. However, in consideration of bandgap wavelength continuity with the passive region130, it is desirable that the annealing mask161in the intermediate region120be formed in contact with the annealing mask161in the passive region130.

As above, the method for manufacturing the optical waveguide according to the present embodiment enables forming an optical waveguide in which a bandgap wavelength continuously varies in a direction from an active region toward a passive region in an intermediate region, by forming selective growth masks in parts of the intermediate region on a semiconductor crystal including a semiconductor quantum well structure and performing selective growth and then forming an annealing mask in a part of the intermediate region and the passive region and performing annealing.

Here, in a case where a region in which a semiconductor quantum well structure is not disordered (for example, an active region) is a first region and a region in which the semiconductor quantum well structure is disordered is a second region (for example, a passive region), an optical waveguide in which a bandgap wavelength continuously decreases from a first bandgap wavelength to a second bandgap wavelength between the first region and the second region can be formed by forming a selective growth mask and an opening portion in a region (for example, a part of an intermediate region) between the first region and the second region in a surface of a semiconductor crystal including the semiconductor quantum well structure and performing selective growth and then forming an annealing mask in the second region (for example, the passive region) and a region between the first region and the second region (for example, a part of the intermediate region) and performing annealing.

Therefore, in a case where light propagates in the optical waveguide according to the present embodiment, it is possible to curb light loss caused by the refractive index difference between the active region and the passive region.

Second Embodiment

A second embodiment of the present invention will be described with reference to the drawings. In the optical waveguide10according to the first embodiment, a bandgap wavelength discontinuity part occurs at the boundary between the active region110and the intermediate region120. An optical waveguide20according to the present embodiment and a method for manufacturing the same are intended to avoid the bandgap wavelength discontinuity.

In the optical waveguide20according to the present embodiment and the method for manufacturing the same, manufacturing conditions such as configurations of a crystal, selective growth masks and an annealing mask used for manufacturing are substantially similar to those of the first embodiment. In the optical waveguide20according to the present embodiment, a substrate201, an n-type InP cladding layer202, a non-doped InGaAsP optical confinement layer203, a non-doped InGaAsP optical confinement layer204, a non-doped InGaAsP optical confinement layer205, a p-type InP capping layer206, an InAs/InGaAs multi-quantum well structure (MQW)211, and an InP layer251correspond to the substrate101, the n-type InP cladding layer102, the non-doped InGaAsP optical confinement layer103, the non-doped InGaAsP optical confinement layer104, the non-doped InGaAsP optical confinement layer105, the p-type InP capping layer106, the InAs/InGaAs multi-quantum well structure (MQW)111, and the InP layer151, respectively, of the optical waveguide10of the first embodiment. The present embodiment is different from the first embodiment in a configuration of a part, around a boundary between an active region and an intermediate region, of an annealing mask.

FIG.13is a sectional view of a crystal with a SiO2annealing mask261formed thereon, which is used in the present embodiment, (corresponding to the V-V sectional view inFIG.5).

The SiO2annealing mask261is formed in such a manner as to cover a boundary between an active region210and an intermediate region220. The SiO2annealing mask261is formed in such a manner as to cover the boundary in a range of 5 μm to the active region210side (E in the figure) from the boundary between the active region210and the intermediate region220(B in the figure).

As in the first embodiment, disordering of a quantum well structure is performed using the SiO2annealing mask261.

FIG.14illustrates variation in bandgap wavelength of the crystal in which the quantum well structure is disordered in the present embodiment. A bandgap wavelength of the active region210is 2350 nm in a range up to 5 μm away from the boundary between the active region210and the intermediate region220(from A to E in the figure).

The bandgap wavelength decreases from the position (E in the figure) that is 5 μm away from the boundary with the intermediate region220to the boundary between the active region210and the intermediate region220(B in the figure).

Furthermore, as in the first embodiment, the bandgap wavelength decreases from the boundary between the active region210and the intermediate region220(B in the figure) to a boundary between the intermediate region220and a passive region230(C in the figure) and becomes 2100 nm in the intermediate region220.

A bandgap wavelength of the passive region230is 2100 nm.

In this way, the method for manufacturing the optical waveguide according to the present embodiment enables continuously varying the bandgap wavelength with no point of bandgap wavelength discontinuity at the boundary between the active region and the intermediate region, by forming an annealing mask in a part of the active region, the intermediate region and the passive region.

Here, in a case where a region in which a semiconductor quantum well structure is not disordered (for example, an active region) is a first region and a region in which the semiconductor quantum well structure is disordered is a second region (for example, a passive region), an optical waveguide in which a bandgap wavelength continuously decreases from a first bandgap wavelength to a second bandgap wavelength between the first region and the second region can be formed by forming a selective growth mask and an opening portion in a region (for example, a part of an intermediate region) between the first region and the second region in a surface of a semiconductor crystal including the semiconductor quantum well structure and performing selective growth and then forming an annealing mask in the second region (for example, the passive region) and a region between the first region and the second region (for example, a part of the active region and a part of the intermediate region) and performing annealing.

Therefore, in a case where light propagates in the optical waveguide according to the present embodiment, it is possible to curb light loss caused by bandgap wavelength discontinuity at the interface between the active region and the intermediate region in addition to light loss caused by a refractive index difference between the active region and the passive region.

In the present embodiment, the active region210is subjected to quantum structure disordering, and thus, light emission performance of the active region210may deteriorate. This deterioration can be curbed by increasing a thickness of a selectively grown layer because the thickness increase results in an increase in distance from the SiO2annealing mask261to the quantum well structure and thus results in a decrease of crystal defects contributing to quantum well disordering.

On the other hand, the decrease of crystal defects contributing to quantum well disordering results in a decrease in amount of wavelength variation due to the quantum well disordering. Therefore, in a case where the thickness of the selectively grown layer is increased in the present embodiment, the thickness of the selectively grown layer is adjusted in consideration of the light emission performance and the wavelength variation amount of the active region210.

Third Embodiment

Next, a semiconductor device according to a third embodiment of the present invention will be described.

As the semiconductor device according to the present embodiment, a distributed Bragg reflector (DBR) laser30using the waveguide10according to the first embodiment will be described.FIG.15illustrates the DBR laser30using a crystal subjected to quantum well structure disordering.

The DBR laser3oincludes an optical amplification region301, a DBR reflection region (preceding stage)302, an active layer region303, a phase adjustment region304and a DBR reflection region (subsequent stage)305. The active layer region303is a region in which light is emitted via current injection and the optical amplification region301is a region in which light oscillated via current injection is amplified.

In the quantum well disordered structure, as active regions310, the active layer region303and the optical amplification region301are provided, and as passive regions330, the phase adjustment region304and the distributed Bragg reflection regions302,305are provided. Also, an intermediate region320is provided at each of a boundary between the optical amplification region301and the DBR reflection region (preceding stage)302, a boundary between the DBR reflection region (preceding stage)302and the active layer region303, and a boundary between the active layer region303and the phase adjustment region304.

A method for manufacturing a DBR reflection structure in the DBR laser30will be described. First, in the crystal with the SiO2annealing mask161removed after quantum well disordering being performed in the first embodiment, only the InP selectively grown layer151and the InP capping layer106are selectively removed via wet etching.

Next, a diffraction grating is formed in a surface of the InGaAsP optical confinement layer with a bandgap wavelength of 1.1 μm via electron beam exposure and wet etching.

Next, a p-type InP cladding layer and a p-type InGaAsP contact layer are sequentially formed via crystal growth using MOVPE growth.

Next, a stripe structure is formed using both dry etching and wet etching using SiO2used for a mask.

Next, SiO2is formed on sides of the stripe, and a p-type electrode is formed on the active layer, the optical amplification region, the phase adjustment region, and the distributed Bragg regions.

Next, an n-type electrode is formed on a back surface of the substrate.

Last, the crystal is cleaved and opposite end surfaces are coated with a non-reflective film. In this way, a ridge-type DBR laser structure is manufactured.

For the DBR laser30, which is an optical semiconductor device according to the present embodiment, favorable laser characteristics such that in a room-temperature continuous operation, a single-mode oscillation wavelength is 2.340 μm, an amount of wavelength variation via current injection to the DBR regions is 5 nm or more, a side-mode suppression ratio in each of all oscillation wavelengths is 40 dB or more and a light output is 1 mW or more are obtained.

As above, the present embodiment enables curbing light loss when light propagates from the active region310to the passive region330, by introducing the intermediate region320and thus enables provision of favorable laser characteristics.

Although in the present embodiment, the waveguide10according to the first embodiment is used, effects that are similar to or exceed the above are exerted using the waveguide20according to the second embodiment.

Although the present embodiment has been described in terms of a DBR laser, the optical waveguide according to embodiments of the present invention is applicable to a structure including an active region and a passive region in the same waveguide, and thus, is clearly applicable even to an SSG-DBR laser using a superstructure grating (SSG) for a diffraction grating or a sampled grating (SG)-DBR laser in which diffraction gratings are periodically disposed. Also, the optical waveguide according to embodiments of the present invention is applicable to optical semiconductor devices in which an optical modulator, an optical switch, etc., are integrated other than lasers.

Also, the present embodiment indicates a laser with a ridge structure and it is clear that the present invention does not depend on the waveguide structure but is applicable to lasers with a pn buried structure formed by processing each of an active region, an intermediate region, and a passive region into a stripe shape and burying opposite sides of such regions in p-type InP and n-type InP alternately or a buried structure formed by burying such regions in semi-insulation InP.

Also, although in the embodiments of the present invention, an InAs/InGaAs quantum well structure that emits light with a wavelength of 2.3 μm is disordered, it is clear that quantum well disordering is applicable not only to this material but also to quantum well structures formed of, e.g., InGaAsP or InGaAlAs, each quantum well structure emitting light with a 1.3 μm band, a 1.55 μm band or a 2 μm band.

Also, in the embodiments of the present invention, SiO2is used for the selective growth masks and the annealing mask, but another material such as SiNxor TiO2may be used.

Although the dimensions of constituent units, components and the like in the optical waveguide, the method for manufacturing the optical waveguide, and the optical semiconductor device according to the first to third embodiments of the present invention have been described, the dimensions in the present invention are not limited to these dimensions but may be any dimensions that allow the respective constituent units, components and the like to function.

INDUSTRIAL APPLICABILITY

Embodiments of the present invention are applicable to semiconductor devices and semiconductor integrated devices such as semiconductor lasers for, e.g., optical communication, environment measurement or medical purposes.

REFERENCE SIGNS LIST

10optical waveguide101substrate102cladding layer110active region120intermediate region130passive region111(non-disordered) semiconductor quantum well structure (first region)121semiconductor quantum well structure disordered in such manner that bandgap wavelength varies131disordered semiconductor quantum well structure (first region)