Vertical cavity surface emitting laser and manufacturing method thereof

Abstract

The invention discloses a vertical-cavity surface-emitting laser and a manufacturing method thereof, and relates to the technical field of semiconductor lasers, a resonant cavity layer of the vertical-cavity surface-emitting laser comprises at least one light-emitting unit, and the light-emitting unit comprises an active layer and a second type oxide layer located on the side, away from a substrate, of the active layer. An interface contact layer is arranged on a first surface, facing a substrate, of a second type oxide layer and / or a second surface, deviating from the substrate, of the second type oxide layer, so that the first surface and / or the second surface of the second type oxide layer are / is transited to other epitaxial structures from a non-doped oxide layer and a non-doped non-oxide layer in the interface contact layer in sequence; and the interface adjusting layer for adjusting the interface oxidation characteristic is arranged between the non-doped oxide layers, so that the stress in the second type oxide layer can be released or reduced, the adhesion capability of the second type oxide layer and the upper and lower epitaxial structures is enhanced, and the reliability of the device is improved.

Description

Technical Field

[0001] This application relates to the field of semiconductor laser technology, and in particular to a vertical cavity surface-emitting laser and its fabrication method. Background Technology

[0002] With the development of the Internet of Things (IoT), Artificial Intelligence (AI), and 5G technologies, 3D imaging and sensing technologies have experienced rapid growth, driving the development of multiple fields such as smartphones, smart cars, and augmented reality / virtual reality (AR / VR), and accelerating the arrival of the era of the Internet of Everything. Vertical-Cavity Surface-Emitting Lasers (VCSELs), as core components of 3D imaging and sensing systems, are at the pinnacle of the intelligent interconnected industry.

[0003] High-power vertical-cavity surface-emitting lasers (VCSELs) are currently a popular type of laser on the market. For CCSELs, high power, reaching up to the kW level, can be achieved through large apertures, back-emitting, array arrangements, or multi-junction structures. Among these, the multi-junction structure is an effective means of achieving high power through epitaxy. Multi-junction CCSELs specifically utilize tunnel junctions to stack multiple active layers in the vertical direction, connecting these active layers in series, thereby significantly improving the laser's power density and photoelectric conversion efficiency. Correspondingly, multi-junction CCSELs require multiple oxide layers. These oxide layers typically involve lateral oxidation of the epitaxial layer containing a high aluminum composition, transforming the oxidized portion into aluminum oxide with high insulation and low refractive index. Simultaneously, the unoxidized portion forms an oxide confinement window (also known as an oxide aperture) to confine the current and optical field. Furthermore, to meet performance requirements, the trend in oxide layer development is towards thinner layers, which necessitates increasingly higher aluminum compositions to ensure sufficient oxidation depth.

[0004] However, in existing vertical cavity surface-emitting lasers, whether single-junction or multi-junction structures, as the oxide layer becomes thinner and the aluminum content increases, the stress in the oxide layer accumulates and cannot be released in a short time after the chip undergoes the oxidation process. On the other hand, the adhesion between the oxide layer and the upper and lower epitaxial structures deteriorates. In subsequent product applications, the epitaxial layer is easily affected by environmental factors, resulting in product failure. This problem is more prominent in multi-junction vertical cavity surface-emitting lasers. Summary of the Invention

[0005] To address the aforementioned technical problems, this application provides a vertical-cavity surface-emitting laser and its fabrication method, which releases or reduces the stress in the oxide layer of the vertical-cavity surface-emitting laser, improves the crystal quality of the oxide layer, enhances the adhesion between the oxide layer and the upper and lower epitaxial structures, and improves the reliability of the vertical-cavity surface-emitting laser.

[0006] To achieve the above objectives, this application provides the following technical solution:

[0007] In a first aspect, this application provides a vertical cavity surface-emitting laser, including a substrate and a first type of reflective layer, a resonant cavity layer and a second type of reflective layer stacked on one side of the substrate;

[0008] The resonant cavity layer includes at least one light-emitting unit. The light-emitting unit includes an active layer and a second type oxide layer located on the side of the active layer away from the substrate. The second type oxide layer has a first surface facing the substrate and a second surface away from the substrate. An interface contact layer is disposed on at least one of the first surface and the second surface. The interface contact layer includes an undoped oxide layer and an undoped non-oxide layer stacked in a direction away from the second type oxide layer. An interface conditioning layer for adjusting interface oxidation characteristics is included between the undoped oxide layer and the undoped non-oxide layer.

[0009] Optionally, the thickness of the undoped oxide layer is no greater than 2 nm, and the thickness of the undoped non-oxide layer is no greater than 2 nm.

[0010] Optionally, when the interface contact layer is provided on the first surface, the interface conditioning layer in the interface contact layer is a phosphide non-oxidized layer.

[0011] When the interface contact layer is provided on the second surface, the interface conditioning layer in the interface contact layer is a phosphide oxide layer.

[0012] Optionally, the second type of oxide layer includes at least two sub-oxide layers, wherein the oxidation depth of the at least two sub-oxide layers in the second type of oxide layer is different.

[0013] Optionally, in the second type of oxide layer, at least one of the following is different: material, thickness, aluminum composition, and doping concentration of the two sub-oxide layers with different oxidation depths.

[0014] Optionally, the at least two sub-oxide layers include a first sub-oxide layer and a second sub-oxide layer alternately stacked along the thickness direction of the second type oxide layer, wherein the aluminum composition of the first sub-oxide layer and the second sub-oxide layer are different, resulting in different oxidation depths of the first sub-oxide layer and the second sub-oxide layer.

[0015] Optionally, the first sub-oxide layer and the second sub-oxide layer are alternately stacked along the thickness direction of the second type oxide layer to form a superlattice structure.

[0016] Optionally, the at least two sub-oxide layers include a third sub-oxide layer closest to the substrate, a fourth sub-oxide layer furthest from the substrate, and a fifth sub-oxide layer between the third and fourth sub-oxide layers. The aluminum composition of the third sub-oxide layer is less than that of the fifth sub-oxide layer, such that the oxidation depth of the third sub-oxide layer is less than that of the fifth sub-oxide layer. Furthermore, the aluminum composition of the fourth sub-oxide layer is less than that of the fifth sub-oxide layer, such that the oxidation depth of the fourth sub-oxide layer is less than that of the fifth sub-oxide layer.

[0017] Optionally, in the second type of oxide layer, along the thickness direction of the second type of oxide layer, the aluminum composition of each sub-oxide layer gradually decreases, thereby gradually reducing the oxidation depth of each sub-oxide layer; or, the aluminum composition of each sub-oxide layer gradually increases, thereby gradually increasing the oxidation depth of each sub-oxide layer.

[0018] Optionally, in the second type of oxide layer, the aluminum composition of one sub-oxide layer is greater than that of the other sub-oxide layer, and the thickness of the first sub-oxide layer is not less than twice the thickness of the other sub-oxide layer.

[0019] Optionally, the material of the sub-oxide layer includes AlGaAs, AlGaInAs, AlGaAsP, or AlGaInAsP.

[0020] Optionally, the aluminum content of the sub-oxide layer is greater than or equal to 90% and less than or equal to 100%.

[0021] Secondly, this application provides a method for fabricating a vertical-cavity surface-emitting laser, including:

[0022] Provide substrate;

[0023] A first-type reflective layer, a resonant cavity layer, and a second-type reflective layer are stacked on one side of the substrate. The resonant cavity layer includes at least one light-emitting unit, which includes an active layer and a second-type oxide layer located on the side of the active layer away from the substrate. The second-type oxide layer has a first surface facing the substrate and a second surface away from the substrate. An interface contact layer is disposed on at least one of the first surface and the second surface. The interface contact layer includes an undoped oxide layer and an undoped non-oxide layer stacked in a direction away from the second-type oxide layer. An interface conditioning layer for adjusting the interface oxidation characteristics is included between the undoped oxide layer and the undoped non-oxide layer.

[0024] Optionally, before forming the second type oxide layer, the formation process of the interface contact layer includes:

[0025] On the side of the active layer away from the substrate, an initial undoped and non-oxidized layer is first formed.

[0026] Then, phosphorus is doped onto the surface of the initial undoped and non-oxidized layer away from the substrate to form the undoped and non-oxidized layer and the interface conditioning layer, wherein the interface conditioning layer is a phosphide non-oxidized layer.

[0027] Subsequently, the undoped oxide layer is formed on the side of the interface conditioning layer opposite to the substrate;

[0028] After the formation of the second type oxide layer, the formation process of the interface contact layer includes:

[0029] On the side of the second type oxide layer away from the substrate, an initial undoped oxide layer is first formed.

[0030] Then, phosphorus is doped onto the surface of the initial undoped oxide layer away from the substrate to form the undoped oxide layer and the interface conditioning layer, wherein the interface conditioning layer is a phosphide oxide layer.

[0031] The undoped, non-oxidized layer is then formed on the side of the interface conditioning layer opposite to the substrate.

[0032] Compared with existing technologies, the above technical solution has the following advantages:

[0033] The vertical-cavity surface-emitting laser provided in this application includes a substrate and a first type reflective layer, a resonant cavity layer, and a second type reflective layer stacked on one side of the substrate. The resonant cavity layer includes at least one light-emitting unit, which includes an active layer and a second type oxide layer located on the side of the active layer away from the substrate. The second type oxide layer has a first surface facing the substrate and a second surface away from the substrate. By providing an interface contact layer on at least one of the first and second surfaces of the second type oxide layer, the interface contact layer specifically includes an undoped oxide layer and an undoped non-oxide layer stacked along a direction away from the second type oxide layer, such that at least one of the first and second surfaces of the second type oxide layer transitions sequentially from the undoped oxide layer and the undoped non-oxide layer in the interface contact layer to other epitaxial structures. Since both the oxide layer and the undoped oxide layer in the interface contact layer are undoped, and an interface adjustment layer for adjusting the interface oxidation characteristics is provided between the undoped oxide layer and the undoped oxide layer, it helps to release or reduce the stress in the second type oxide layer, improve the crystal quality of the second type oxide layer, and enhance the adhesion between the second type oxide layer and the upper and lower epitaxial structures, thereby improving the reliability of the vertical-cavity surface-emitting laser. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a cross-sectional structural diagram of a vertical cavity surface-emitting laser provided in an embodiment of this application;

[0036] Figure 2 This is a cross-sectional structural diagram of another vertical cavity surface-emitting laser provided in an embodiment of this application;

[0037] Figure 3 A cross-sectional structural schematic diagram of another vertical cavity surface-emitting laser provided in the embodiments of this application;

[0038] Figure 4 A cross-sectional structural schematic diagram of another vertical cavity surface-emitting laser provided in the embodiments of this application;

[0039] Figure 5 A cross-sectional structural schematic diagram of another vertical cavity surface-emitting laser provided in the embodiments of this application;

[0040] Figure 6 A cross-sectional structural schematic diagram of another vertical cavity surface-emitting laser provided in the embodiments of this application;

[0041] Figure 7 A cross-sectional structural schematic diagram of another vertical cavity surface-emitting laser provided in the embodiments of this application;

[0042] Figure 8 This is a cross-sectional structural diagram of another vertical cavity surface-emitting laser provided in the embodiments of this application.

[0043] Explanation of reference numerals in the attached figures:

[0044] 100 Substrate; 101 Buffer layer; 200 Type I reflective layer; 210 First refractive index layer; 220 Second refractive index layer; 300 Resonant cavity layer; 310 Light-emitting unit; 320 Tunnel junction; 10 Active layer; 11 Barrier layer; 12 Potential well layer; 20 Type II oxide layer; 21 Oxidized portion of Type II oxide layer; 22 Unoxidized portion of Type II oxide layer; 201 Sub-oxide layer; 201-1 First sub-oxide layer; 201-2 Second sub-oxide layer; 201-3 Third sub-oxide layer ; 201-4 Fourth sub-oxide layer; 201-5 Fifth sub-oxide layer; 30 Type I waveguide layer; 40 Type II waveguide layer; S1 First surface; S2 Second surface; 50 Interface contact layer; 51 Undoped oxide layer; 511 Oxidized portion of undoped oxide layer; 512 Unoxidized portion of undoped oxide layer; 52 Undoped non-oxide layer; 53 Interface conditioning layer; 400 Type II reflective layer; 410 Third refractive index layer; 420 Fourth refractive index layer; 500 Type II ohmic contact layer. Detailed Implementation

[0045] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0046] The terms "first," "second," etc., used in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate; this is merely a way of distinguishing objects with the same properties in the embodiments of this application. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0047] Secondly, this application provides a detailed description in conjunction with schematic diagrams. When detailing the embodiments of this application, for ease of explanation, the accompanying drawings illustrating the device structure may be partially enlarged, not to scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of this application. In addition, actual fabrication should include three-dimensional spatial dimensions of length, width, and depth.

[0048] As described in the background section, in existing vertical cavity surface-emitting lasers, whether single-junction or multi-junction structures, as the oxide layer becomes thinner and the aluminum content increases, the stress in the oxide layer accumulates and cannot be released in a short time after the chip undergoes the oxidation process. On the other hand, the adhesion between the oxide layer and the upper and lower epitaxial structures deteriorates. In subsequent product applications, the epitaxial layer is easily affected by environmental factors, resulting in product failure. This problem is more prominent in multi-junction vertical cavity surface-emitting lasers.

[0049] In view of this, embodiments of this application provide a vertical-cavity surface-emitting laser. Figure 1 and Figure 2 The following are schematic cross-sectional views of two vertical cavity surface-emitting lasers provided in embodiments of this application, as shown in the figure. Figure 1 and Figure 2 As shown, the vertical cavity surface-emitting laser includes a substrate 100 and a first type reflective layer 200, a resonant cavity layer 300 and a second type reflective layer 400 stacked on one side of the substrate 100; wherein, the resonant cavity layer 300 includes at least one light-emitting unit 310, and the light-emitting unit 310 includes an active layer 10 and a second type oxide layer 20 located on the side of the active layer 10 away from the substrate 100.

[0050] In this embodiment, the substrate 100 may be a GaAs substrate. A buffer layer 101 may be present between the substrate 100 and the first type reflective layer 200.

[0051] The first type of reflective layer 200 can be a first type of doped distributed Bragg reflector (DBR) layer, specifically including an alternately stacked first refractive index layer 210 and second refractive index layer 220, one of the first refractive index layer 210 and the other of the second refractive index layer 220 being a high refractive index layer and the other being a low refractive index layer, and the optical thickness of both the first refractive index layer 210 and the second refractive index layer 220 being 1 / 4 optical wavelength.

[0052] Similarly, the second type of reflective layer 400 can be a second type of doped DBR layer, specifically including alternating layers of a third refractive index layer 410 and a fourth refractive index layer 420, one of the third refractive index layer 410 and the fourth refractive index layer 420 being a high refractive index layer and the other being a low refractive index layer, and the optical thickness of both the third refractive index layer 410 and the fourth refractive index layer 420 being 1 / 4 optical wavelength.

[0053] The first type of reflective layer 200 and the second type of reflective layer 400 constitute a resonant cavity, which filters and reflects light of a specific wavelength to form laser oscillation.

[0054] The resonant cavity layer 300 between the first type reflective layer 200 and the second type reflective layer 400 includes at least one light-emitting unit 310; when the resonant cavity layer 300 includes one light-emitting unit 310, such as Figure 1 As shown, the vertical-cavity surface-emitting laser at this time has a single-junction structure; when the resonant cavity layer 300 includes at least two light-emitting units 310, as... Figure 2 As shown, the vertical cavity surface-emitting laser at this time has a multi-junction structure. Two adjacent light-emitting units 310 are connected by a tunnel junction 320, so that each light-emitting unit 310 is connected in series, thereby significantly improving the power density and photoelectric conversion efficiency of the laser.

[0055] In the embodiments of this application, such as Figure 1 and Figure 2 As shown, the light-emitting unit 310 includes an active layer 10 and a second type oxide layer 20 located on the side of the active layer 10 away from the substrate 100. In fact, the light-emitting unit 310 also includes a first type waveguide layer 30 located on the side of the active layer 10 close to the substrate 100 and a second type waveguide layer 40 located on the side of the active layer 10 away from the substrate 100. The second type oxide layer 20 is located on the side of the second type waveguide layer 40 away from the active layer 10. The active layer 10 includes alternating layers of barrier layer 11 and potential well layer 12. Under the action of injected current, electrons and holes recombine in the active layer 10, generating laser light through stimulated emission. The first type waveguide layer 30 and the second type waveguide layer 40 work together to confine the current and the optical field, precisely controlling the current flow to the active layer and effectively confining photons near the active layer, reducing optical loss and forming an optical waveguide. The second type oxide layer 20 is a second type doped oxide layer, which includes an oxidized portion 21 and an unoxidized portion 22. The oxidized portion 21 has high insulation and low refractive index, while the unoxidized portion 22 forms an oxide confinement window (also known as an oxide aperture), forcing the current to concentrate in the central channel and forming an optical waveguide to confine the optical field.

[0056] like Figure 1 and Figure 2As shown, the second-type oxide layer 20 has a first surface S1 facing the substrate 100 and a second surface S2 facing away from the substrate 100. Unlike existing vertical-cavity surface-emitting lasers, where the second-type oxide layer directly contacts the upper and lower epitaxial structures, resulting in stress buildup within the second-type oxide layer that cannot be released and poor adhesion between the second-type oxide layer and the upper and lower epitaxial structures, this embodiment of the application provides an interface contact layer 50 on at least one of the first surface S1 and the second surface S2 of the second-type oxide layer 20. That is, an interface contact layer 50 is provided on the first surface S1 of the second-type oxide layer 20, or on the second surface S2 of the second-type oxide layer 20, or on both the first surface S1 and the second surface S2 of the second-type oxide layer 20. It should be noted that... Figure 1 and Figure 2 The example shown is based on the provision of an interface contact layer 50 on both the first surface S1 and the second surface S2 of the second type oxide layer 20.

[0057] Regardless of whether the interface contact layer 50 is provided on the first surface S1 or the second surface S2 of the second type oxide layer 20, the interface contact layer 50 includes an undoped oxide layer 51 and an undoped non-oxide layer 52 stacked in a direction away from the second type oxide layer 20, and an interface adjustment layer 53 for adjusting the interface oxidation characteristics is included between the undoped oxide layer 51 and the undoped non-oxide layer 52.

[0058] It is understood that the vertical-cavity surface-emitting laser provided in this application embodiment, by providing an interface contact layer 50 on at least one of the first surface S1 and the second surface S2 of the second type oxide layer 20, the interface contact layer 50 specifically includes an undoped oxide layer 51 and an undoped non-oxide layer 52 stacked along a direction away from the second type oxide layer 20, such that at least one of the first surface S1 and the second surface S2 of the second type oxide layer 20 is sequentially transitioned from the undoped oxide layer 51 and the undoped non-oxide layer 52 in the interface contact layer 50 to other epitaxial structures. Since the oxide layer and the undoped non-oxide layer in the interface contact layer 50 are both undoped, and an interface adjustment layer 53 for adjusting the interface oxidation characteristics is provided between the undoped oxide layer and the undoped non-oxide layer, it helps to release or reduce the stress in the second type oxide layer 20, improve the crystal quality of the second type oxide layer 20, and enhance the adhesion ability of the second type oxide layer 20 to the upper and lower epitaxial structures, thereby improving the reliability of the vertical-cavity surface-emitting laser.

[0059] Optionally, in the interface contact layer 50, the thickness of the undoped oxide layer 51 is no greater than 2 nm, and the thickness of the undoped non-oxide layer 52 is no greater than 2 nm. That is, the thickness of the undoped oxide layer 51 and the undoped non-oxide layer 52 in the interface contact layer 50 are both very thin, so that the interface contact layer 50 can release the stress in the second type oxide layer 20 and enhance the adhesion between the second type oxide layer 20 and the upper and lower epitaxial structures.

[0060] Optional, such as Figure 1 and Figure 2 As shown, when an interface contact layer 50 is provided on the first surface S1 of the second type oxide layer 20, the interface adjustment layer 53 in the interface contact layer 50 can be a phosphide non-oxide layer. Specifically, the interface contact layer 50 is formed before the formation of the second type oxide layer 20. The formation process may include: first forming an initial undoped non-oxide layer on the side of the active layer 10 away from the substrate 100; then doping phosphorus on the surface of the initial undoped non-oxide layer away from the substrate 100 to form an undoped non-oxide layer 52 and an interface adjustment layer 53, wherein the interface adjustment layer 53 is a phosphide non-oxide layer; then forming an undoped oxide layer 51 on the side of the interface adjustment layer 53 away from the substrate 100; and subsequently forming the second type oxide layer 20, thereby forming the interface contact layer 50 on the first surface S1 of the second type oxide layer 20.

[0061] Optional, such as Figure 1 and Figure 2 As shown, when an interface contact layer 50 is provided on the second surface S2 of the second type oxide layer 20, the interface adjustment layer 53 in the interface contact layer 50 can be a phosphide oxide layer. Specifically, the interface contact layer 50 is formed after the second type oxide layer 20 is formed, and the formation process may include: first forming an initial undoped oxide layer on the side of the second type oxide layer 20 away from the substrate 100; then doping phosphorus on the surface of the initial undoped oxide layer away from the substrate 100 to form an undoped oxide layer 51 and an interface adjustment layer 53, wherein the interface adjustment layer 53 is a phosphide oxide layer; and then forming an undoped non-oxide layer 52 on the side of the interface adjustment layer 53 away from the substrate 100, thereby forming the interface contact layer 50 on the second surface S2 of the second type oxide layer 20.

[0062] It is understandable that in the interface contact layer 50, the interface adjustment layer 53 between the undoped oxide layer 51 and the undoped non-oxide layer 52 is thinner than the undoped oxide layer 51 and the undoped non-oxide layer 52. It is similar to doping phosphorus at the interface between the undoped oxide layer 51 and the undoped non-oxide layer 52. Since the oxidation degree of the material layer containing phosphorus is low, the phosphorus-doped interface adjustment layer 53 can adjust the interface oxidation characteristics between the undoped oxide layer 51 and the undoped non-oxide layer 52.

[0063] Optionally, the material of the undoped oxide layer 51 may include AlGaAs, AlGaInAs, AlGaAsP, or AlGaInAsP. That is, the undoped oxide layer 51 can be an AlGaAs layer, AlGaInAs layer, AlGaAsP layer, or AlGaInAsP layer, and the Al composition of the undoped oxide layer 51 is lower than that of the second-type oxide layer 20, so that the oxidation depth of the undoped oxide layer 51 is less than that of the second-type oxide layer 20, and the oxidation confinement window of the undoped oxide layer 51 is larger than that of the second-type oxide layer 20. However, this application does not limit the material and Al composition of the undoped oxide layer 51.

[0064] Furthermore, this application does not limit the material of the undoped and non-oxidized layer 52, as long as the undoped and non-oxidized layer 52 matches the lattice of other layers.

[0065] It should be noted that, as Figure 1 and Figure 2 As shown, the undoped oxide layer 51 in the interface contact layer 50 also includes an oxidized portion 511 and an unoxidized portion 512. The oxidized portion 511 has high insulation and low refractive index, and the unoxidized portion 512 forms an oxidation confinement window. However, since the thickness of the undoped oxide layer 51 is relatively thin, for example, no more than 2 nm, even if the oxidation confinement window of the undoped oxide layer 51 is smaller than the oxidation confinement window of the second type oxide layer 20, current can still pass through. Figure 1 and Figure 2 This illustration only demonstrates that the oxidation confinement window of the undoped oxide layer 51 and the oxidation confinement window of the second-type oxide layer 20 are of equal size, and does not constitute a limitation on the oxidation confinement windows of the undoped oxide layer 51 and the second-type oxide layer 20. The size of the oxidation confinement window of the undoped oxide layer 51 can be greater than, less than or equal to the size of the oxidation confinement window of the second-type oxide layer 20. Other figures in this application do not distinguish between the oxidized portion 511 and the unoxidized portion 512 of the undoped oxide layer 51. As previously known, when an interface contact layer 50 is provided on the second surface S2 of the second-type oxide layer 20, the interface adjustment layer 53 in the interface contact layer 50 can be a phosphide oxide layer. In this case, the oxidation depth of the interface adjustment layer 53 can be the same as the oxidation depth of its adjacent undoped oxide layer 51.

[0066] Optionally, in some embodiments of this application, such as Figure 1 and Figure 2 As shown, the second type oxide layer 20 is a single material layer, for example, the second type oxide layer 20 is a single high aluminum composition AlGaAs layer, AlGaInAs layer, AlGaAsP layer or AlGaInAsP.

[0067] However, the inventors found that when the second oxide layer 20 is made of a single high-aluminum material, as the thickness of the second oxide layer 20 becomes thinner and the aluminum content becomes higher, stress will still accumulate in the second oxide layer 20, and the adhesion between the second oxide layer 20 and the upper and lower epitaxial structures still needs to be improved.

[0068] Based on this, alternatively, in some embodiments of this application, such as Figure 3 As shown, Figure 3 This diagram illustrates a cross-sectional structure of another vertical-cavity surface-emitting laser provided in an embodiment of this application. It shows that the second-type oxide layer 20 includes at least two sub-oxide layers 201, and the oxidation depths of the at least two sub-oxide layers 201 in the second-type oxide layer 20 are different. For clarity, Figure 3 The oxidation depth w of only one sub-oxide layer 201 is marked.

[0069] It should be noted that the oxidation depth of the oxide layer refers to the distance that the oxidation reaction propagates laterally from the side of the oxide layer to the center during the selective wet oxidation of the aluminum-rich oxide layer in the fabrication process of a vertical cavity surface-emitting laser. The oxidation depth of the oxide layer determines the unoxidized area in the oxide layer, i.e., the oxidation confinement window (also known as the oxide aperture), which in turn determines the channels for current and optical field.

[0070] The material, thickness, aluminum composition, doping concentration, and oxidation process of the oxide layer all affect the oxidation depth of the oxide layer. Given a fixed oxidation process, in the second type oxide layer 20, at least two sub-oxide layers 201 have different oxidation depths. This means that at least one of the material, thickness, aluminum composition, and doping concentration of the at least two sub-oxide layers 201 is different. In other words, in the second type oxide layer 20, at least one of the material, thickness, aluminum composition, and doping concentration of the two sub-oxide layers 201 with different oxidation depths is different.

[0071] By setting the oxidation depth of at least two sub-oxide layers 201 in the second type oxide layer 20 to be different, firstly, it is beneficial to release or reduce the stress in the second type oxide layer 20; secondly, the oxidized and unoxidized portions of each sub-oxide layer 201 in the second type oxide layer 20 can form a cross structure or a stepped structure, making the oxidized and unoxidized portions of each sub-oxide layer 201 in the second type oxide layer 20 more stably bonded, and the oxidation depth of the sub-oxide layers 201 in contact with the upper and lower epitaxial structures can be adjusted to increase the contact area between the oxidation restriction window of the second type oxide layer 20 and the upper and lower epitaxial structures, thereby enhancing the adhesion ability of the second type oxide layer 20 to the upper and lower epitaxial structures.

[0072] Regarding the material selection for each sub-oxide layer 201 in the second type oxide layer 20, the material of the sub-oxide layer 201 can optionally be AlGaAs, AlGaInAs, AlGaAsP, or AlGaInAsP, that is, any sub-oxide layer 201 can be an AlGaAs layer, AlGaInAs layer, AlGaAsP layer, or AlGaInAsP layer.

[0073] To ensure oxidation depth, the aluminum composition of each sub-oxide layer 201 in the second type oxide layer 20 can be set to be greater than or equal to 90% and less than or equal to 100%.

[0074] The oxidation depth of the oxide layer is closely related to the size of the aluminum composition of the oxide layer. Under the same oxidation process conditions, the larger the aluminum composition of the oxide layer, the deeper the oxidation depth of the oxide layer, and the smaller the aluminum composition of the oxide layer, the shallower the oxidation depth of the oxide layer. Therefore, the aluminum composition of different sub-oxide layers 201 in the second type oxide layer 20 can be set to be different, so that the oxidation depth of different sub-oxide layers 201 is different.

[0075] The arrangement of sub-oxide layers 201 with different oxidation depths in the second type oxide layer 20 is described below.

[0076] Optionally, in some embodiments of this application, such as Figure 4 As shown, Figure 4 The diagram shows a cross-sectional view of another vertical cavity surface-emitting laser provided in this application embodiment. As can be seen, the at least two sub-oxide layers 201 in the second type oxide layer 20 include a first sub-oxide layer 201-1 and a second sub-oxide layer 201-2 alternately stacked along the thickness direction of the second type oxide layer 20. The aluminum composition of the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 is different, which makes the oxidation depth of the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 different.

[0077] It should be noted that the materials of the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 can be the same or different. For example, the first sub-oxide layer 201-1 / second sub-oxide layer 201-2 can be a combination of AlGaAs layers, AlGaAs layers, AlGaInAs layers, AlGaAs layers, AlGaAsP layers, AlGaAs layers, AlGaInAsP layers, AlGaInAs layers, AlGaInAs layers, AlGaInAsP layers, or AlGaInAs layers, etc.

[0078] Furthermore, the thicknesses of the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 can be equal or unequal.

[0079] Furthermore, the embodiments of this application do not limit the number of alternating logarithms of the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 in the second type oxide layer 20.

[0080] It is understandable that in the second type oxide layer 20, the aluminum composition of the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 are different. Therefore, one of the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 has a relatively high aluminum composition and a relatively deep oxidation depth, while the other has a relatively low aluminum composition and a relatively shallow oxidation depth. With this configuration, the second type oxide layer 20 is a composite structure of sub-oxide layers with high and low aluminum compositions and different oxidation depths. Firstly, this can release or reduce the stress within the second type oxide layer 20; secondly, the oxidized portion of the first sub-oxide layer 201-1... The unoxidized portion and the oxidized and unoxidized portions of the second sub-oxide layer 201-2 form an interdigitated structure, making the overall oxidized and unoxidized portions of the second type oxide layer 20 more stably bonded. Furthermore, the uppermost (and lowermost) layer of the second type oxide layer 20 can be set as the first sub-oxide layer 201-1 or the second sub-oxide layer 201-2 with a shallower oxidation depth to increase the contact area between the oxidation restriction window of the second type oxide layer 20 and the upper and lower epitaxial structures, thereby enhancing the adhesion ability of the second type oxide layer 20 to the upper and lower epitaxial structures.

[0081] Further optional, in some embodiments of this application, in the second type oxide layer 20, the first sub-oxide layer 201-1 and the second sub-oxide layer 201-2 are alternately stacked along the thickness direction of the second type oxide layer 20 to form a superlattice structure, so as to further release or reduce the stress in the second type oxide layer 20 and improve the crystal quality of the second type oxide layer 20.

[0082] Alternatively, in some embodiments of this application, such as Figure 5 As shown, Figure 5This illustration shows a cross-sectional structural diagram of another vertical-cavity surface-emitting laser provided in an embodiment of this application. It can be seen that the second-type oxide layer 20 includes at least two sub-oxide layers 201, one closest to the substrate 100 (third sub-oxide layer 201-3), one furthest from the substrate 100 (fourth sub-oxide layer 201-4), and one fifth sub-oxide layer 201-5 between the third and fourth sub-oxide layers 201-3 and 201-4. The aluminum composition of the third sub-oxide layer 201-3 is less than that of the fifth sub-oxide layer 201-5, resulting in an oxidation depth of the third sub-oxide layer 201-3 that is less than that of the fifth sub-oxide layer 201-5. Furthermore, the aluminum composition of the fourth sub-oxide layer is less than that of the fifth sub-oxide layer, resulting in an oxidation depth of the fourth sub-oxide layer that is less than that of the fifth sub-oxide layer. In other words, in the second-type oxide layer 20... The sub-oxide layer with the highest aluminum content and the deepest oxidation depth (i.e., the fifth sub-oxide layer 201-5) is located in the middle of the other sub-oxide layers with lower aluminum content and shallower oxidation depth (i.e., the third sub-oxide layer 201-3 and the fourth sub-oxide layer 201-4). With this configuration, the second type oxide layer 20 is also a composite structure of sub-oxide layers with high and low aluminum content and different oxidation depths. While reducing the stress in the second type oxide layer 20 and making the structure of the second type oxide layer 20 more stable, the oxidation depth of the third sub-oxide layer 201-3, which is closest to the substrate 100, and the fourth sub-oxide layer 201-4, which is furthest from the substrate 100, in the second type oxide layer 20 can be relatively shallow. This increases the contact area between the oxidation confinement window of the second type oxide layer 20 and the upper and lower epitaxial structures, and further enhances the adhesion ability of the second type oxide layer 20 to the upper and lower epitaxial structures.

[0083] It should be noted that the second type oxide layer 20 may include only the third sub-oxide layer 201-3, the fourth sub-oxide layer 201-4, and the fifth sub-oxide layer 201-5, or the second type oxide layer 20 may also include other sub-oxide layers besides the third sub-oxide layer 201-3, the fourth sub-oxide layer 201-4, and the fifth sub-oxide layer 201-5, wherein the aluminum composition of the other sub-oxide layers is less than that of the fifth sub-oxide layer 201-5, such that the oxidation depth of the other sub-oxide layers is less than that of the fifth sub-oxide layer 201-5.

[0084] Furthermore, in the second type oxide layer 20, the oxidation depth of each sub-oxide layer with an aluminum composition lower than that of the fifth sub-oxide layer 201-5 can be the same or different.

[0085] Alternatively, in some embodiments of this application, such as Figure 6 and Figure 7 As shown, Figure 6 and Figure 7This paper presents a cross-sectional structural schematic diagram of two more vertical cavity surface-emitting lasers provided in the embodiments of this application. It can be seen that in the second type oxide layer 20, along the thickness direction of the second type oxide layer 20, the aluminum composition of each sub-oxide layer 201 gradually decreases, resulting in a gradual decrease in the oxidation depth of each sub-oxide layer 201 (e.g., ...). Figure 6 (As shown), or, the aluminum composition of each sub-oxide layer 201 gradually increases, resulting in a gradual increase in the oxidation depth of each sub-oxide layer 201 (as shown). Figure 7 As shown in the figure), the second type oxide layer 20 is also a composite structure of sub-oxide layers with high and low aluminum composition and different oxidation depths. While reducing the stress in the second type oxide layer 20, the oxidized and unoxidized parts of each sub-oxide layer 201 in the second type oxide layer 20 can form a stepped structure, making the oxidized and unoxidized parts of each sub-oxide layer 201 in the second type oxide layer 20 more stably bonded. In addition, the oxidation depth of the uppermost or lowermost sub-oxide layer 201 in the second type oxide layer 20 is relatively shallow, making the contact area between the oxidation restriction window of the second type oxide layer 20 and the corresponding epitaxial structure larger, thereby enhancing the adhesion ability of the second type oxide layer 20 to the upper and lower epitaxial structures.

[0086] Regarding the thickness setting of each sub-oxide layer 201 in the second type oxide layer 20, based on any of the above embodiments, optionally, in some embodiments of this application, in the second type oxide layer 20, the aluminum composition of one sub-oxide layer 201 is greater than the aluminum composition of another sub-oxide layer 201, and the thickness of the first sub-oxide layer is not less than twice the thickness of the other sub-oxide layer. For example, such as Figure 8 As shown, Figure 8 A cross-sectional structural schematic diagram of another vertical cavity surface-emitting laser provided in an embodiment of this application is shown. It can be seen that the second type oxide layer 20 includes a first sub-oxide layer 201-1 and a second sub-oxide layer 201-2 alternately stacked along the thickness direction of the second type oxide layer 20. If the aluminum composition of the first sub-oxide layer 201-1 is greater than the aluminum composition of the second sub-oxide layer 201-2, then the oxidation depth of the first sub-oxide layer 201-1 is greater than the oxidation depth of the second sub-oxide layer 201-2. The thickness d1 of the first sub-oxide layer 201-1 can be set to be more than twice the thickness d2 of the second sub-oxide layer 201-2.

[0087] This is because the higher the aluminum content of the oxide layer, the faster the oxidation rate will be when selectively wet oxidizing the oxide layer. If the oxide layer is too thin, the oxidation reaction may consume the oxide layer before reaching the current pore size under high oxidation rate, resulting in over-oxidation. Therefore, for oxide layers with high aluminum content, a thicker material is needed as a buffer in order to accurately control the oxidation depth and obtain the designed pore size.

[0088] Figures 1-8The vertical cavity surface-emitting laser shown may also include a second type ohmic contact layer 500 located on the side of the second type reflective layer 400 facing away from the substrate 100, as well as other structures not shown, which will not be described in detail in this application.

[0089] Accordingly, embodiments of this application also provide a method for fabricating a vertical-cavity surface-emitting laser, referencing... Figures 1-8 As shown, the manufacturing method includes:

[0090] S100: Provides substrate 100;

[0091] S200: A first type reflective layer 200, a resonant cavity layer 300, and a second type reflective layer 400 are formed on one side of the substrate 100. The resonant cavity layer 300 includes at least one light-emitting unit 310. The light-emitting unit 310 includes an active layer 10 and a second type oxide layer 20 located on the side of the active layer 10 away from the substrate 100. The second type oxide layer 20 has a first surface S1 facing the substrate 100 and a second surface S2 away from the substrate 100. An interface contact layer 50 is disposed on at least one of the first surface S1 and the second surface S2. The interface contact layer 50 includes an undoped oxide layer 51 and an undoped non-oxide layer 52 stacked in a direction away from the second type oxide layer 20. An interface adjustment layer 53 for adjusting the interface oxidation characteristics is included between the undoped oxide layer 51 and the undoped non-oxide layer 52.

[0092] In this embodiment, the resonant cavity layer 300 between the first type reflective layer 200 and the second type reflective layer 400 includes at least one light-emitting unit 310; when the resonant cavity layer 300 includes one light-emitting unit 310, such as Figure 1 , Figures 3-8 As shown, the vertical-cavity surface-emitting laser at this time has a single-junction structure; when the resonant cavity layer 300 includes at least two light-emitting units 310, as... Figure 2 As shown, the vertical cavity surface-emitting laser at this time has a multi-junction structure. Two adjacent light-emitting units 310 are connected by a tunnel junction 320, so that each light-emitting unit 310 is connected in series, thereby significantly improving the power density and photoelectric conversion efficiency of the laser.

[0093] Optionally, in the interface contact layer 50, the thickness of the undoped oxide layer 51 is no greater than 2 nm, and the thickness of the undoped non-oxide layer 52 is no greater than 2 nm. That is, the thickness of the undoped oxide layer 51 and the undoped non-oxide layer 52 in the interface contact layer 50 are both very thin, so that the interface contact layer 50 can release the stress in the second type oxide layer 20 and enhance the adhesion between the second type oxide layer 20 and the upper and lower epitaxial structures.

[0094] Optional, such as Figure 1 and Figure 2As shown, when an interface contact layer 50 is provided on the first surface S1 of the second type oxide layer 20, the interface adjustment layer 53 in the interface contact layer 50 can be a phosphide non-oxide layer. Specifically, the interface contact layer 50 is formed before the formation of the second type oxide layer 20. The formation process may include: first forming an initial undoped non-oxide layer on the side of the active layer 10 away from the substrate 100; then doping phosphorus on the surface of the initial undoped non-oxide layer away from the substrate 100 to form an undoped non-oxide layer 52 and an interface adjustment layer 53, wherein the interface adjustment layer 53 is a phosphide non-oxide layer; then forming an undoped oxide layer 51 on the side of the interface adjustment layer 53 away from the substrate 100; and subsequently forming the second type oxide layer 20, thereby forming the interface contact layer 50 on the first surface S1 of the second type oxide layer 20.

[0095] Optional, such as Figure 1 and Figure 2 As shown, when an interface contact layer 50 is provided on the second surface S2 of the second type oxide layer 20, the interface adjustment layer 53 in the interface contact layer 50 can be a phosphide oxide layer. Specifically, the interface contact layer 50 is formed after the second type oxide layer 20 is formed, and the formation process may include: first forming an initial undoped oxide layer on the side of the second type oxide layer 20 away from the substrate 100; then doping phosphorus on the surface of the initial undoped oxide layer away from the substrate 100 to form an undoped oxide layer 51 and an interface adjustment layer 53, wherein the interface adjustment layer 53 is a phosphide oxide layer; and then forming an undoped non-oxide layer 52 on the side of the interface adjustment layer 53 away from the substrate 100, thereby forming the interface contact layer 50 on the second surface S2 of the second type oxide layer 20.

[0096] It is understandable that in the interface contact layer 50, the interface adjustment layer 53 between the undoped oxide layer 51 and the undoped non-oxide layer 52 is thinner than the undoped oxide layer 51 and the undoped non-oxide layer 52. It is similar to doping phosphorus at the interface between the undoped oxide layer 51 and the undoped non-oxide layer 52. Since the oxidation degree of the material layer containing phosphorus is low, the phosphorus-doped interface adjustment layer 53 can adjust the interface oxidation characteristics between the undoped oxide layer 51 and the undoped non-oxide layer 52.

[0097] It is understood that the method for fabricating a vertical-cavity surface-emitting laser provided in this application involves providing an interface contact layer 50 on at least one of the first surface S1 and the second surface S2 of the second type oxide layer 20. The interface contact layer 50 specifically includes an undoped oxide layer 51 and an undoped non-oxide layer 52 stacked along a direction away from the second type oxide layer 20. This allows at least one of the first surface S1 and the second surface S2 of the second type oxide layer 20 to transition sequentially from the undoped oxide layer 51 and the undoped non-oxide layer 52 in the interface contact layer 50 to other epitaxial structures. Since both the oxide layer and the non-oxide layer in the interface contact layer 50 are undoped, and an interface adjustment layer 53 for adjusting the interface oxidation characteristics is provided between the undoped oxide layer and the non-oxide layer, this helps to release or reduce the stress in the second type oxide layer 20, improve the crystal quality of the second type oxide layer 20, and enhance the adhesion between the second type oxide layer 20 and the upper and lower epitaxial structures, thereby improving the reliability of the vertical-cavity surface-emitting laser.

[0098] The specific structure of the vertical cavity surface-emitting laser (VCSEL) fabricated by the method provided in the embodiments of this application has been described in detail in the foregoing embodiments, and can be referred to the foregoing embodiments, and will not be repeated here.

[0099] The various parts of this manual are described in a combination of parallel and progressive methods. Each part focuses on the differences between the other parts, and the same or similar parts can be referred to each other.

[0100] The features described above regarding the disclosed embodiments can be substituted or combined with each other to enable those skilled in the art to implement or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A vertical-cavity surface-emitting laser, characterized in that, Includes a substrate and a first type reflective layer, a resonant cavity layer, and a second type reflective layer stacked on one side of the substrate; The resonant cavity layer includes at least one light-emitting unit. The light-emitting unit includes an active layer and a second type oxide layer located on the side of the active layer away from the substrate. The second type oxide layer has a first surface facing the substrate and a second surface away from the substrate. An interface contact layer is disposed on at least one of the first surface and the second surface. The interface contact layer includes an undoped oxide layer and an undoped non-oxide layer stacked in a direction away from the second type oxide layer. An interface conditioning layer for adjusting interface oxidation characteristics is included between the undoped oxide layer and the undoped non-oxide layer.

2. The vertical-cavity surface-emitting laser according to claim 1, characterized in that, The thickness of the undoped oxide layer is no greater than 2 nm, and the thickness of the undoped non-oxide layer is no greater than 2 nm.

3. The vertical-cavity surface-emitting laser according to claim 1, characterized in that, When the interface contact layer is provided on the first surface, the interface conditioning layer in the interface contact layer is a phosphide non-oxidized layer. When the interface contact layer is provided on the second surface, the interface conditioning layer in the interface contact layer is a phosphide oxide layer.

4. The vertical-cavity surface-emitting laser according to claim 1, characterized in that, The second type of oxide layer includes at least two sub-oxide layers, wherein the oxidation depth of the at least two sub-oxide layers in the second type of oxide layer is different.

5. The vertical-cavity surface-emitting laser according to claim 4, characterized in that, In the second type of oxide layer, at least one of the following is different: material, thickness, aluminum composition, and doping concentration of the two sub-oxide layers with different oxidation depths.

6. The vertical-cavity surface-emitting laser according to claim 4, characterized in that, The at least two sub-oxide layers include a first sub-oxide layer and a second sub-oxide layer that are alternately stacked along the thickness direction of the second type oxide layer. The first sub-oxide layer and the second sub-oxide layer have different aluminum compositions, which results in different oxidation depths between the first sub-oxide layer and the second sub-oxide layer.

7. The vertical-cavity surface-emitting laser according to claim 6, characterized in that, The first sub-oxide layer and the second sub-oxide layer are alternately stacked along the thickness direction of the second type oxide layer to form a superlattice structure.

8. The vertical-cavity surface-emitting laser according to claim 4, characterized in that, The at least two sub-oxide layers include a third sub-oxide layer closest to the substrate, a fourth sub-oxide layer furthest from the substrate, and a fifth sub-oxide layer between the third and fourth sub-oxide layers. The aluminum composition of the third sub-oxide layer is less than that of the fifth sub-oxide layer, such that the oxidation depth of the third sub-oxide layer is less than that of the fifth sub-oxide layer. The aluminum composition of the fourth sub-oxide layer is less than that of the fifth sub-oxide layer, such that the oxidation depth of the fourth sub-oxide layer is less than that of the fifth sub-oxide layer.

9. The vertical-cavity surface-emitting laser according to claim 4, characterized in that, In the second type of oxide layer, along the thickness direction of the second type of oxide layer, the aluminum composition of each sub-oxide layer gradually decreases, thereby gradually reducing the oxidation depth of each sub-oxide layer; or, the aluminum composition of each sub-oxide layer gradually increases, thereby gradually increasing the oxidation depth of each sub-oxide layer.

10. The vertical-cavity surface-emitting laser according to claim 4, characterized in that, In the second type of oxide layer, the aluminum composition of one sub-oxide layer is greater than that of the other sub-oxide layer, and the thickness of the first sub-oxide layer is not less than twice the thickness of the other sub-oxide layer.

11. The vertical-cavity surface-emitting laser according to any one of claims 2-10, characterized in that, The material of the sub-oxide layer includes AlGaAs, AlGaInAs, AlGaAsP, or AlGaInAsP.

12. The vertical-cavity surface-emitting laser according to any one of claims 2-10, characterized in that, The aluminum content of the sub-oxide layer is greater than or equal to 90% and less than or equal to 100%.

13. A method for fabricating a vertical-cavity surface-emitting laser, characterized in that, include: Provide substrate; A first-type reflective layer, a resonant cavity layer, and a second-type reflective layer are stacked on one side of the substrate. The resonant cavity layer includes at least one light-emitting unit, which includes an active layer and a second-type oxide layer located on the side of the active layer away from the substrate. The second-type oxide layer has a first surface facing the substrate and a second surface away from the substrate. An interface contact layer is disposed on at least one of the first surface and the second surface. The interface contact layer includes an undoped oxide layer and an undoped non-oxide layer stacked in a direction away from the second-type oxide layer. An interface conditioning layer for adjusting the interface oxidation characteristics is included between the undoped oxide layer and the undoped non-oxide layer.

14. The method for fabricating a vertical-cavity surface-emitting laser according to claim 13, characterized in that, Before the formation of the second type oxide layer, the formation process of the interface contact layer includes: On the side of the active layer away from the substrate, an initial undoped and non-oxidized layer is first formed. Then, phosphorus is doped onto the surface of the initial undoped and non-oxidized layer away from the substrate to form the undoped and non-oxidized layer and the interface conditioning layer, wherein the interface conditioning layer is a phosphide non-oxidized layer. Subsequently, the undoped oxide layer is formed on the side of the interface conditioning layer opposite to the substrate; After the formation of the second type oxide layer, the formation process of the interface contact layer includes: On the side of the second type oxide layer away from the substrate, an initial undoped oxide layer is first formed. Then, phosphorus is doped onto the surface of the initial undoped oxide layer away from the substrate to form the undoped oxide layer and the interface conditioning layer, wherein the interface conditioning layer is a phosphide oxide layer. The undoped, non-oxidized layer is then formed on the side of the interface conditioning layer opposite to the substrate.

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