JP5264764B2 - Etched facet ridge laser with etch stop - Google Patents

Description

    The present invention generally relates to an etched facet photonic device, and more particularly, “High Reliability Etched Facet Photonic Devices” (Attorney Docket BIN20) filed on Feb. 17, 2006. Improved etched facet ridge laser device of the type disclosed in co-pending US patent application Ser. No. 11 / 356,203, assigned to the assignee of the present application, and such device Related to the process.

  Semiconductor lasers typically grow an appropriate layered semiconductor material on a substrate by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) to form an epitaxy structure having an active layer parallel to the substrate surface. By doing so, it is manufactured on the wafer. The wafer is then processed with various semiconductor processing tools to provide a laser optical resonator that incorporates an active layer and metal contacts attached to the semiconductor material. Laser mirror facets are typically formed at both ends of the laser resonator by cleaving the semiconductor material along its crystal structure to define the end face, i.e., the end of the laser resonator, and thus the contact ends. When a bias voltage is applied in between, the resulting current flow through the active layer causes photons to be emitted out of the faceted end face of the active layer in a direction perpendicular to the current flow. However, for most semiconductor devices, the aforementioned cleavage process is inaccurate because it depends on the position and angle of the crystal plane of the semiconductor material. For some materials, for example, there are cleaved surfaces of approximately equal strength that orient themselves to each other at an acute angle such that the minute perturbations that occur during cleavage can redirect the fracture interface from one cleaved surface to another. There is. Furthermore, the cleavage process results in fragile bars and tiny chips that are difficult to handle during testing. Furthermore, mechanical cleavage tends to be incompatible with subsequent processing of individual chips, such as required for monolithic integration of components on a chip. This is because, to obtain a fully functional laser, the wafer must be physically broken, and the wafer typically forms a small piece after it has been cleaved, so conventional lithographic techniques can be This is because it cannot be easily used for further processing.

  The aforementioned and other difficulties resulting from the use of cleaved facets have led to the development of processes for forming semiconductor laser mirror facets by etching. This process, for example as described in US Pat. No. 4,851,368, also allows the laser to be monolithically integrated on the same substrate along with other photonic devices. The process described in this patent is described in “Monolithic AlGaAs-GaAs Single Quantum-Well Ridge Lasers Fabricated with Dry-Etched Facets and Ridges”, A. Behfar-Rad and SSWong, IEEE Journal of Quantum Electronics, 28, 5 No. 1227-1231, May 1992, and further as described in the aforementioned US patent application Ser. No. 11 / 356,203, for fabricating ridge lasers with etched facets. Extended to provide a process. However, if consistent results should be obtained during the fabrication process, the depth of the ridge in such a device and the resulting position relative to the active region in the laser structure must be accurate, As well, it has proven very difficult to control the dry etch process sufficiently to provide a consistent ridge depth.

  Because it is difficult to control the ridge etch depth during dry etch, etched faceted ridge laser devices fabricated by conventional processes have low single transverse mode yield and a wide distribution of threshold currents I know that there is. In order to obtain usable devices with yields on the order of 30-40% using such a process, it is necessary to use a multi-step etch procedure to prevent the ridge etch from spreading too deeply into the epitaxial structure. there were. To do this, it is necessary to repeat the dry etch 3 or 4 times and measure the ridge etch depth after each etch to obtain the proper dimensions. This approach is time consuming and greatly increases the cost of such devices.

  Because the high uniformity and yield of etched faceted ridge devices are highly desirable, an improved process for fabricating etched faceted ridge laser devices is provided in accordance with the present invention.

  In this process, a ridge laser with etched facets is formed in an epitaxial structure that incorporates a wet etch stop layer where the bottom surface of the ridge should be. The ridge is partially formed using prior art lithography and dry etch processes, but the dry etch is stopped before the bottom of the ridge. Lithography defines a wet etch window that overlaps the partially etched ridge and keeps its end cross section and its end facets protected by a resist layer. The structure is then wet etched to complete ridge formation and to remove residual material from above the etch stop layer. The stop layer in the epitaxial structure stops wet etch accurately and reliably at the desired depth at the bottom of the ridge structure, thereby increasing the yield of the fabrication process to about 98-99.8%.

  Briefly, according to the present invention, a substrate or wafer is formed from, for example, certain types of III-V compounds or alloys thereof that can be appropriately doped. A series of layers is deposited on the top surface of the substrate, such as by an epitaxial deposition process such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). These layers that form the optical waveguide in the transverse direction typically include an active region that can be formed using AlInGaAs-based quantum wells and barriers, as well as adjacent upper and lower cladding regions. In one example, the semiconductor laser photonic device structure layer can be epitaxially formed on an InP substrate, and the upper cladding region and the lower cladding region are made of a semiconductor material such as InP having a refractive index lower than that of the active region. It is formed. An InGaAs cap layer is provided on the upper surface of the upper cladding layer to enable ohmic contact.

In accordance with the present invention, a wet etch stop layer is epitaxially deposited on the substrate at the level of the bottom surface of the ridge to be formed in the structure. Thus, for example, a stop layer made of indium gallium phosphide (GaInAsP) is deposited in the cap layer immediately above the upper cladding layer. This stop layer is about 20 nm thick and is lattice matched to the InP cladding layer. The resulting wafer is processed using standard photolithography steps and chemical assisted ion beam etching (CAIBE) dry etching steps to form one or more laser cavities and facets. Thereafter, a second photolithography step is used to define one or more ridges on the previously formed resonator, and CAIBE is used to etch the ridges. Since the CAIBE process is difficult to control accurately and is therefore not very accurate, this dry etch step is designed to be incomplete, i.e., before reaching the stop layer, just before the desired depth of the ridge. Designed to do. Thereafter, another photolithography step is performed to cover the edge cross section of the ridge and facet with a protective resist layer, and a wet etch step using a selective wet etchant is performed. A selective wet etchant, such as a mixture of HCl and H ₃ PO ₄ , removes the residual top cladding material and stops on the etch stop layer or etches the etch stop layer very slowly. Thus, the wet etch completes the ridge formation and stops at the stop layer. A wet etch etches a small amount laterally at the bottom of the ridge, but this is easily compensated and the process serves to form a ridge with exactly the desired dimensions. This wet etch also removes any residual artifacts left by the dry etch step, resulting in a ridge structure with a high degree of accuracy, resulting in a higher device yield.

  The foregoing example is based on providing a photonic device consisting of a laser device on an InP substrate, but the wet etch stop layer etches the etched features of other photonic devices having an active region. It will be appreciated that a combination of dry and wet etching can be fabricated with these limits defined, and that such devices can be formed on other substrates. Examples of such photonic devices are electroabsorption modulators and semiconductor optical amplifiers. Conventional ridge lasers whose facets are formed by cleavage are compatible with the use of an etch stop layer.

  The foregoing and further objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings.

FIG. 1 illustrates a prior art cleavage facet process for fabricating a plurality of photonic devices, such as a laser, on a wafer. FIG. 2 illustrates a prior art etching facet process for fabricating a plurality of photonic devices, such as a laser, on a wafer. 1 is a partial perspective view of a prior art etching facet laser on a wafer. FIG. 1 shows in cross section a series of layers forming a wafer according to the invention. FIG. 2 is a perspective view of a faceted ridge laser produced by etching according to the present invention with an etch stop before providing electrical contact. FIG. 5 shows as a cross-sectional view in the x-direction the fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 according to the present invention. FIG. 5 shows, as a cross-sectional view in the y direction, fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 in accordance with the present invention. FIG. 5 shows as a cross-sectional view in the x-direction the fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 according to the present invention. FIG. 5 shows, as a cross-sectional view in the y direction, fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 in accordance with the present invention. FIG. 5 shows the fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 as a cross-sectional view in the x direction according to the present invention. FIG. 5 shows, as a cross-sectional view in the y direction, fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 in accordance with the present invention. FIG. 5 shows as a cross-sectional view in the x-direction the fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 according to the present invention. FIG. 5 shows, as a cross-sectional view in the y direction, fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 in accordance with the present invention. FIG. 5 shows as a cross-sectional view in the x-direction the fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 according to the present invention. FIG. 5 shows, as a cross-sectional view in the y direction, fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 in accordance with the present invention. FIG. 5 shows as a cross-sectional view in the x-direction the fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 according to the present invention. FIG. 5 shows, as a cross-sectional view in the y direction, fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 in accordance with the present invention. FIG. 5 shows as a cross-sectional view in the x-direction the fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 according to the present invention. FIG. 5 shows, as a cross-sectional view in the y direction, fabrication steps for forming an etched facet ridge laser with etch stop using the wafer of FIG. 4 in accordance with the present invention.

  As indicated generally at 10 in FIG. 1, mechanical cleavage of the semiconductor epi-wafer 12 is typically used to define a reflective mirror, or facet, at the cavity end of an edge emitting diode laser fabricated on the wafer. Process. In this process, a plurality of waveguides 14 are fabricated on a wafer substrate, a metal contact layer is provided, and the wafer is cleaved mechanically along the cleavage lines 16 to form the bars 18 of the laser device 20. . Bars 18 are then laminated as indicated at 22 and the cleaved end facets of the laser device are coated to provide the desired reflection and emission characteristics. Individual laser devices 20 can then be tested by applying a bias voltage 26 across the individual lasers and detecting the resulting output light beam 28, as indicated at 24. The bars of the laser device can then be separated, i.e., separated as shown at 30, to produce individual chips 32, each containing one or more laser devices, as shown at 34. Can be properly packaged in a known manner.

  As discussed above, for most semiconductor devices, the cleavage process described above is not useful because it depends on the position and angle of the crystal plane of the semiconductor material to place the laser facets along the waveguide. Is accurate. In some materials, for example, small perturbations that occur during cleavage can cause the fracture interface to redirect from one cleavage plane to another, thereby placing facets in the wrong position or at the wrong angle. Thus, there may be cleaved surfaces of approximately equal intensity that are oriented at an acute angle such that the lasers can become inoperable. Further, the cleavage process shown in FIG. 1 results in the formation of fragile bars 18 that are difficult to handle during mirror coating and testing. Furthermore, mechanical cleavage tends to be incompatible with subsequent processing of individual chips, such as the process steps required to perform monolithic integration of components on the chip. This is because the wafer must be physically broken in order to fabricate a laser facet and obtain a fully functional laser.

  An alternative technique for fabricating a laser is shown generally at 40 in FIG. 2, where a plurality of waveguides 42 are fabricated on a suitable wafer substrate 44 as a first step. Preferably, these are parallel waveguides extending across the wafer as shown. A process based on photolithography and chemical assisted ion beam etching (CAIBE) is then used to fabricate individual laser waveguide resonators by forming facets at desired locations along the waveguide. These facets are accurately placed regardless of the crystal structure of the material and have a quality and reflectivity comparable to those obtained by cleavage. This process makes the laser monolithic on a single chip along with other photonic devices because the laser resonator and facets are fabricated on the wafer in much the same way that integrated circuits are fabricated on silicon. And can be inexpensively tested while still on the wafer, as shown at 46. Thereafter, as shown at 48, the wafer can be separated and the chip 50 can be separated, and then, as shown at 52, the chip can be packaged. This process has a relatively high yield and low cost compared to the cleavage process described above, and allows the fabrication of lasers with variously shaped resonators independent of the cleavage plane in the wafer material. The prior art fabrication process of FIG. 2 is described in more detail in the above IEEE Journal of Quantum Electronics paper.

  In fabricating an edge-emitting version of a faceted ridge laser by such etching, four photolithography patterning steps are required, and in the first step, one or more, each having spaced end facets. A pattern mask that defines the laser body is fabricated. This pattern is transferred into the wafer structure by etching. Thereafter, in a second lithography step, a pattern is formed to form a ridge on the structure, and the pattern is again transferred into the structure by etching using a CAIBE process. In the third lithography, a contact hole is formed, and in the fourth lithography, a pattern for p-contact metal coating is formed. The resulting edge-emitting ridge laser structure is shown in FIG. 3, in which ridges 60 and 62 are formed on the corresponding laser waveguides 42, and each laser resonator, such as resonator 72, for example, is formed. , With etched end facets 74 and 76.

  As noted above, etched facet ridge laser devices fabricated by the above procedure may have yields lower than desired, with wide variations in threshold current to establish lasing, and low singles. One side mode yield may occur. It has been found that the cause of these problems is the variation that occurs in the height of the laser ridge during the fabrication process. If consistent results are to be obtained during the fabrication process, the depth of each ridge etch and the resulting position of the bottom surface of the ridge relative to the active region in the laser structure must be accurate. However, it has been found very difficult to control the dry etch process enough to provide a consistent ridge etch depth.

  The foregoing problems are addressed by providing a layered wafer 98 in which the photonic device 100 is fabricated, as shown schematically in FIGS. 4 and 5, and a process for fabricating the photonic device on the substrate 102. This is overcome by the present invention as shown schematically in FIGS. 6 (a and b) to 12 (a and b). It will be appreciated that the dimensions and ratios shown in these figures are not necessarily proportional to the original dimensions, but are used to clearly illustrate the salient features of the structure and process. The present invention will be described in terms of an edge emitting ridge laser having an etched ridge 104, as shown in FIG. 5, but co-pending US patent application Ser. No. 10 / 958,069 and US patent application Ser. No. 10/963. It will be appreciated that other types of lasers or other photonic devices can be fabricated using the etch control process of the present invention, such as the ridge horizontal cavity surface emitting laser described in US Pat.

  As is conventional, the wafer 98 includes a substrate 102 formed, for example, from some type of III-V compound or alloy thereof that can be appropriately doped. As shown in FIG. 4, a series of layers 106 can be deposited on top surface 108 of substrate 102, such as by epitaxial deposition such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). These layers 106 form a laser structure or optical waveguide in the transverse direction that typically includes an active region 112 and an upper cladding region 114 and a lower cladding region 116.

  In one example, a structural layer 106 of a semiconductor laser or other photonic device is epitaxially formed on the InP substrate 102. The upper cladding region 114 and the lower cladding region 116 having a photonic structure are each formed from a semiconductor material such as InP having a refractive index lower than that of the active region 112. These cladding regions are adjacent to the active region, which can be formed using AlInGaAs-based quantum wells and barriers. In order to enable ohmic contact, an InGaAs cap layer 118 is provided on the upper surface of the upper cladding layer 114.

  As shown in FIG. 5, a wet etch stop layer 119 is incorporated into the cladding layer 114, and the wet etch stop layer 119 is placed at a height where the bottom surface 120 of the ridge should be at the completion of the fabrication process. The stop layer 119 is a layer in which GaInAsP is epitaxially deposited, has a thickness of about 20 nm, and divides the upper cladding layer 114 into a bottom segment 114 (a) and an upper segment 114 (b). The stop layer 119 is close to, but spaced above, the bottom surface 121 of the upper cladding layer 114, and this surface is also the top surface of the active region 112 at the same time.

In the process of the present invention, as shown in FIG. 4, a masking layer 122 such as a 200 nm thick layer of SiO _{2 is} deposited on the epitaxially grown laser structure 106 by plasma enhanced chemical vapor deposition (PECVD). For example, a first lithography step defining at least one laser body and facet in the photoresist layer is performed on a wafer 98 equivalent to wafer 44 shown in FIGS. 2 and 3, and the photoresist pattern is Transfer to the SiO ₂ mask layer 122 using reactive ion etching (RIE). Each photolithography step of spinning a photoresist layer on the mask layer 122, exposing the photoresist through a lithographic mask to produce a pattern, and then transferring the pattern to the mask layer 122 is conventional and is known in the art. It is not shown because it is known. After the photoresist is removed by oxygen plasma, the SiO ₂ pattern in layer 122 is transferred to the laser structure using chemical assisted ion beam etching (CAIBE) and laser guided as indicated generally at 42 in FIG. 6A and 6B of a photonic device such as a waveguide is formed, and a laser as indicated by reference numeral 100 in FIG. 5 is formed along the waveguide. Forming a plurality of discrete laser resonators. The body 123 is formed with etched sidewalls 124 and 125 and etched end facets 126 and 128, as shown in FIGS. 6 (a) and 6 (b). For HCSELs, two separate photolithography steps and two separate CAIBE steps are used, rather than the single step shown here for edge emitting lasers.

  6A is a cross section along the direction of the arrow at the xx axis of the waveguide 100 in FIG. 5, and FIG. 6B is the cross section of the arrow at the yy axis in FIG. 2 is a cross section of a waveguide along a direction.

7 (a) and 7 (b), for producing a pattern defining one or more ridges, such as ridges 104, on a previously defined laser body 123 on a wafer. A second photoresist lithography is performed and the photoresist pattern is transferred to the PECVD SiO ₂ mask layer 122 using RIE. After removing the photoresist with oxygen plasma, ridges 104 for each laser body 123 are formed in the laser structure using CAIBE as shown. In contrast to conventional fabrication processes for facet devices with ridge-type etching, this ridge CAIBE dry etch step prevents the etch from continuing unintentionally beyond the desired depth of the ridge bottom surface 120. In addition, it is designed to stop before the etch depth necessary to complete the ridge 104. In addition, dry etching may leave residual materials in the form of roughness or glass-like layers that can interfere with the precise depth control necessary to achieve high yields. Thus, as shown in FIGS. 7 (a) and 7 (b), this etch process always involves an etch floor 130 in the cladding layer 114 (b) that is at or above the upper surface 132 of the layer 119. Designed to finish with. This prevents the CAIBE etch from spreading below the desired height of the bottom surface 120 of the ridge 104 shown in FIG.

  8 (a) and 8 (b), the top surface of the device is covered with a photoresist masking layer 136 to complete the ridge structure, and then on the ridge 104 and on each side of the ridge 104. The masking layer 136 is patterned by photolithography so that a wet etch window 137 including regions 138 and 139 along its length is exposed after development. A photoresist masking layer 136 extends over the ridge ends 140 and 142 and end facets 126 and 128 to protect them from subsequent wet etches, preferably for lateral etching by a wet etch step. In order to do so, the outer edge portions 144 and 146 of the etch floor 130 overlap with each other by a small amount.

In order to remove residual top cladding material on layer 119, wet etch window 137 is exposed to a selective wet etch solution, such as a mixture of HCl and H ₃ PO ₄ . As shown in FIGS. 9 (a) and 9 (b), the wet etch stops at or substantially stops at the top surface 132 of the stop layer 119, which is the desired location of the bottom surface 120 of the ridge, causing the ridge to Leave at the desired height, then remove the photoresist mask 136 using oxygen plasma. Any roughness or glass-like features that may be present are removed by a wet etch process, thereby allowing precise depth control.

  Although shown in FIGS. 5-12 (a) and 12 (b) for a single laser resonator having a single ridge 104, multiple photonic devices are preferably It will be understood that it is fabricated on a wafer. For example, as shown in FIG. 2, a plurality of spaced apart waveguides 42, each of which can incorporate several ridge lasers, are typically fabricated on a single wafer, with the remaining process steps described below. After completion, as discussed above, they are separated for packaging, ie separated by dicing, to produce individual photonic devices.

After forming the laser body, facets, and ridges as described above with respect to FIG. 5 and FIGS. 6 (a) and 6 (b) -9 (a) and 9 (b), the disconnecting step of FIG. Earlier, as shown in FIGS. 10 (a) and 10 (b) for a single ridge 104, a 120 nm thick passivation layer 150 of dielectric material such as SiO _{2 is} deposited using PECVD. And cover the entire wafer including the photonic device. Thereafter, as shown in FIGS. 11 (a) and 11 (b), a third lithography is performed to define a p-contact opening in the photoresist mask layer on the photonic structure, and the RIE through the mask is performed. Is used to open a contact window 152 in the SiO ₂ layers 150 and 122. The photoresist is then removed using oxygen plasma.

A fourth lithography is performed in the photoresist mask layer to define a metallized lift-off pattern for the p-contact, where the lift-off structure 154 is lithographically provided to provide a contact opening 156 that surrounds the contact window 152. Defined. The undercut that a typical lift-off structure has is not explicitly shown in the metallized lift-off pattern 154, but it will be understood that it exists. An e-beam evaporator is then used to deposit p-contact metal 160 (FIGS. 12 (a) and 12 (b)) over the metallization lift-off pattern 154 and into the opening 156 to form the contact window 152. cover. A lift-off step that removes the metallization lift-off pattern 154 removes the undesired metallization, thereby leaving the p-contact 160 for the device. As shown, the p-contact extends beyond the edge of the contact window 152 and seals the contact opening that opens in the SiO ₂ layers 122 and 150. A laser n-contact 162 is also deposited on the backside of the wafer using electron beam evaporation. Another metallization lift-off step can be used to provide n-contact to the front side of the wafer. As described above, it will be appreciated that a plurality of photonic devices are typically fabricated on a substrate.

  As discussed above, the etched facets must be protected during the wet etch (see FIGS. 8 (a) and 8 (b)), so the remaining features such as walls or shoulders 163 Is formed on the upper surface of the wet etch stop layer below the mask 136 extending over the edges 144 and 146. This wall causes a change in the ridge depth at and near the facet, resulting in a shallower ridge depth at and near the facet by etching than at the bottom of the ridge. . The performance and mode behavior of edge-emitting ridge lasers and ridge HCSELs fabricated through this process will be investigated and any adverse effects on the remaining features 163 (see FIG. 5) while the yield rose to 98-99.8%. It turns out that it does not cause.

  While the invention has been illustrated in terms of preferred embodiments, it will be understood that changes and modifications can be made without departing from the true spirit and scope of the invention as set forth in the appended claims.

Claims (11)

A substrate,
At least a first epitaxial semiconductor structure comprising an active layer and a wet etch stop layer on the substrate;
An etched ridge formed in the structure, having a bottom surface at the stop layer;
At least a first etched facet fabricated in the structure;
An etch floor that surrounds the ridge at the bottom of the ridge and extends to the facet by the etching;
During the wet etch of the etch floor, the etching by being formed through a wet etch windows of the patterned masking layer to protect the facets on the etch floor, there wall portion adjacent to the facet by the etching Thus, the wall portion where the ridge is shallower at the etched facet than at the bottom, and
At least a first electrode on the structure;
Photonic device equipped with.   The device of claim 1, wherein the etch stop layer is an epitaxially deposited layer of GaInAsP.   The device of claim 2, wherein the etched ridge is partially dry etched and partially wet etched.   The device of claim 2, wherein the substrate is InP.   The device of claim 2, wherein the device is a laser.   The device of claim 1, wherein the wet etch stop layer is disposed above and adjacent to the active layer. The epitaxial semiconductor structure is
A lower cladding layer on the substrate;
It said active layer on said lower cladding layer,
A first upper cladding layer overlying the active layer;
Said wet etch stop layer on the first upper cladding layer,
A second upper cladding layer overlying the wet etch stop layer;
The device of claim 1, comprising a contact layer on the upper cladding layer. Forming an epitaxial semiconductor structure incorporating an active layer on a substrate;
Forming a wet etch stop layer over the active layer in the epitaxial structure;
Dry-etching the structure to form a laser having at least one etched facet;
Dry etching the structure to form a portion of the ridge for the laser;
Forming a patterned masking layer on the epitaxial structure, the patterned masking layer protecting the at least one etched facet during the wet etch, the masking layer comprising: Including a partially formed ridge and an area of an etch floor on each side of the partially formed ridge along the length of the partially formed ridge and excluding the at least one etched facet Defining a wet etch window; and
The wet and the structure via etch windows were wet etched to complete the formation of the ridge, a same time the wall portion a step is formed adjacent to the facet by the etching on the etching floor, whereby The ridge is shallower at the etched facet than at the bottom; and
Covering the laser and the etched facets with a protective conformal layer;
Opening a contact window in the protective layer;
Depositing a metal layer to cover the window and to make electrical contact with the laser. Forming the epitaxial structure further comprises:
Depositing a lower cladding layer under the active layer on the substrate;
9. The process of claim 8, comprising depositing an upper cladding layer on the active layer, wherein the wet etch stop layer is incorporated into the upper cladding layer.   The process of claim 9, wherein forming the epitaxial structure further comprises depositing a contact layer on the upper cladding layer. Forming the wet etch stop layer comprises:
Partially depositing the upper cladding layer on the active region;
Depositing the wet etch stop layer;
And then depositing the remainder of the upper cladding layer on the wet etch region.

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