阅读说明：本技术 具有厚缓冲层的半导体层结构 (Semiconductor layer structure with thick buffer layer ) 是由 L.范 于 2019-06-28 设计创作，主要内容包括：一种半导体层结构,可以包括衬底、形成在衬底上的缓冲层,以及形成在缓冲层上的一组外延层。所述缓冲层可以具有大于2微米(μm)的厚度。所述一组外延层包括量子阱层。量子阱混合区域可以与所述量子阱层和从半导体层结构的表面的区域扩散的材料相关联地形成。(A semiconductor layer structure may include a substrate, a buffer layer formed on the substrate, and a set of epitaxial layers formed on the buffer layer. The buffer layer may have a thickness greater than 2 micrometers (μm). The set of epitaxial layers includes a quantum well layer. The quantum well intermixed region can be formed in association with the quantum well layer and a material diffused from a region of a surface of the semiconductor layer structure.)

1. A semiconductor layer structure, comprising:

a substrate;

a buffer layer formed on the substrate; and

a set of epitaxial layers formed on the buffer layer;

wherein the buffer layer has a thickness greater than 2 micrometers (μm),

wherein the set of epitaxial layers includes a quantum well layer, an

Wherein a quantum well intermixed region is formed in association with the quantum well layer and a material diffused from a region of a surface of the semiconductor layer structure.

2. The semiconductor layer structure of claim 1, wherein the semiconductor layer structure is included in a laser device.

3. The semiconductor layer structure of claim 2, wherein the laser device has a laser wavelength in the Infrared (IR) or near-infrared range.

4. The semiconductor layer structure of claim 3 wherein the laser wavelength is substantially independent of a slicing position or a number of slices from the substrate of the ingot.

5. The semiconductor layer structure of claim 1, wherein the buffer layer comprises the same material as the substrate.

6. The semiconductor layer structure of claim 1, wherein the buffer layer has a thickness in a range of 2 μ ι η to 5 μ ι η.

7. A semiconductor laser comprising:

a substrate;

a buffer layer formed on the substrate; and

a set of epitaxial layers formed on the buffer layer;

wherein the buffer layer has a thickness in a range of 2 micrometers (μm) to 5 μm,

wherein the set of epitaxial layers includes a quantum well layer, an

Wherein a quantum well intermixing region is formed within the quantum well layer using a quantum well intermixing material diffused through a region from a surface of the semiconductor layer structure.

8. The semiconductor laser of claim 7, wherein the semiconductor laser and another semiconductor laser formed using another substrate of the same boule as the substrate have a laser wavelength variation of less than 20 nanometers (nm).

9. The semiconductor laser of claim 8, wherein the lasing wavelength of the semiconductor laser and the lasing wavelength of the another semiconductor laser are measured under the same operating conditions.

10. The semiconductor laser of claim 8, wherein the lasing wavelengths corresponding to the semiconductor laser and the another semiconductor laser are in the Infrared (IR) or near-infrared range.

11. The semiconductor laser of claim 10, wherein lasing wavelengths corresponding to the semiconductor laser and the another semiconductor laser occur when the semiconductor laser and the another semiconductor laser lase at room temperature.

12. The semiconductor laser of claim 7, wherein the buffer layer is an n-doped buffer layer.

13. The semiconductor laser of claim 12, wherein an n-doped cap layer of the set of epitaxial layers is formed on an n-doped buffer layer,

wherein the quantum well layer is formed on the n-doped cladding layer.

14. The semiconductor laser of claim 7, wherein the thickness averages 4 μ ι η across the buffer layer.

15. An optical device, comprising:

a substrate;

a buffer layer formed on the substrate; and

a set of epitaxial layers formed on the buffer layer;

wherein the buffer layer has an average thickness of 4 micrometers (μm) across the buffer layer,

wherein the set of epitaxial layers includes a quantum well layer, an

Wherein the quantum well intermixed region is formed within the quantum well layer by diffusion of a material from a region of the surface of the semiconductor layer structure using quantum well intermixing,

wherein the cover layer is formed on the buffer layer.

16. The optical device of claim 15, wherein the buffer layer is an n-doped buffer layer.

17. The optical device of claim 15, wherein the buffer layer is a gallium arsenide (GaAs) buffer layer.

18. The optical device of claim 15, wherein a difference between a laser wavelength of the optical device and another laser wavelength of another optical device is less than 20 μm,

wherein the substrate of the optical device and the further substrate of the further optical device are associated with the same ingot.

19. The optical apparatus of claim 18, wherein the laser wavelength and the another laser wavelength are in the Infrared (IR) or near-infrared range.

20. The optical apparatus of claim 18, wherein the laser wavelength and the further laser wavelength are at an operating current (I)_op) And occurs during lasing at room temperature.

Technical Field

The present disclosure relates to emitter arrays, and more particularly to semiconductor layer structures having thick buffer layers.

Background

Semiconductor lasers are formed from various epitaxial layers. Various epitaxial layers are grown on the substrate. When supplied with electric current, the semiconductor laser emits laser light. The semiconductor laser may comprise an edge emitting laser or a vertical emitting laser, such as a Vertical Cavity Surface Emitting Laser (VCSEL).

Disclosure of Invention

According to some embodiments, a semiconductor layer structure may include: a substrate; a buffer layer formed on the substrate; and a set of epitaxial layers formed on the buffer layer, wherein the buffer layer has a thickness greater than 2 micrometers (μm), wherein the set of epitaxial layers includes a quantum well layer, and wherein a quantum well intermixed region is formed in association with the quantum well layer and a material diffused from a region of a surface of a semiconductor layer structure.

According to some embodiments, a semiconductor laser may include: a substrate; a buffer layer formed on the substrate; and a set of epitaxial layers formed on the buffer layer, wherein the buffer layer has a thickness in a range of 3 micrometers (μm) to 5 μm, wherein the set of epitaxial layers includes a quantum well layer, and wherein a quantum well intermixed region is formed within the quantum well layer by material diffused from a region of a surface of the semiconductor layer structure using quantum well intermixing.

According to some embodiments, an optical device may include: a substrate; a buffer layer formed on the substrate; and a set of epitaxial layers formed on the buffer layer; wherein the buffer layer has a thickness of 4 micrometers (μm) on average across the buffer layer, wherein the set of epitaxial layers comprises a quantum well layer, and wherein a quantum well intermixed region is formed within the quantum well layer using quantum well intermixing through material diffused from a region of a surface of a semiconductor layer structure, wherein a capping layer is formed on the buffer layer.

According to some embodiments, a method may comprise: providing a substrate; forming a buffer layer on the substrate, wherein the buffer layer has a thickness greater than 2 micrometers (μm); and forming a set of epitaxial layers on the buffer layer, wherein the set of epitaxial layers includes a quantum well layer, and wherein a quantum well intermixed region is formed in association with the quantum well layer and a material diffused from a region of a surface of the semiconductor layer structure.

Drawings

Fig. 1 is a graph depicting the relationship between the laser wavelength and the number of slices of an ingot (boule) of a conventional semiconductor layer structure.

Fig. 2 is a diagram depicting an example embodiment of a semiconductor layer structure having a thick buffer layer as described herein.

Fig. 3 and 4 are diagrams depicting one or more graphs comparing various thicknesses of a buffer layer and the correspondence between the number of slices and the laser wavelength for various thickness ingots.

Fig. 5 is a flow chart depicting an example process for forming a semiconductor layer structure having a thick buffer layer as described herein.

Detailed Description

The following detailed description of example embodiments refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements.

Impurity-induced disorder (Impurity-induced disorder) can be used to produce high power diode lasers. During this process, the wafer is placed in a high temperature environment until the quantum wells (quantum wells) are mixed. However, quantum well intermixing occurs not only in the region where quantum well intermixing is expected to occur, but also in the active region of the semiconductor layer structure of the diode laser where it is detrimental. The result of this process is a significant difference in laser wavelength between different wafers, which can result in low yield for a given wavelength specification. Significant wavelength variation has been observed as the number of substrate slices from the same ingot varies (e.g., wafers from the same ingot may experience a wavelength variation of 30 nanometers (nm) or greater depending on the number of slices of wafers from the ingot). For example, the laser wavelength for wafers with low substrate slice numbers (e.g., in the range of 850nm to 865 nm) is typically much shorter than the laser wavelength for wafers with high substrate slice numbers (e.g., in the range of 875nm to 895 nm) in the same growth run. During the growth process and other wafer thermal treatments, particularly during impurity-induced disordering process steps, impurities or point defects (e.g., holes, where atoms are absent in the crystal lattice) are present in the substrate and tend to migrate toward the epitaxial layers of the semiconductor layer structure. This migration of impurities or point defects promotes quantum well intermixing, causing the laser wavelength to deviate from the design wavelength.

A barrier layer between the substrate and the epitaxial layer is required to block and/or reduce such migration. The buffer layer separating the epitaxial layer from the substrate plays a key role in epitaxial growth quality. The buffer layer facilitates a smooth interface for epitaxial growth since even the most careful substrate preparation does not provide an atomically smooth surface, which becomes more rough during the initial "oxide blow-off". After growing a thin GaAs buffer layer, the epitaxial structure of the near-infrared semiconductor laser is typically grown on a GaAs substrate. In the above semiconductor layer structure, the thickness of the buffer layer is generally about 0.4 micrometers (μm) (or about 400 nm).

Some embodiments described herein provide a semiconductor layer structure (e.g., for a semiconductor diode laser) that includes a thick buffer layer. For example, the thick buffer layer may separate the substrate of the semiconductor layer structure and various epitaxial layers of the semiconductor layer structure (e.g., when the semiconductor layer structure is included in a semiconductor laser, the various epitaxial layers may be associated with excitation light). The thick buffer layer may have a thickness of multiple micrometers (μm) that prevents or impedes migration of impurities or point defects from the substrate into the respective epitaxial layers. This provides improved control of the laser wavelength between semiconductor lasers generated from different wafers by reducing or eliminating quantum well intermixing in unintended regions of the semiconductor layer structure, thereby reducing or eliminating wavelength variability between different semiconductor lasers, and/or reducing the likelihood that the wavelength of a semiconductor laser formed from the semiconductor layer structure has a laser wavelength outside of the design wavelength range. Improved control of the laser wavelength improves the yield of semiconductor lasers produced from an ingot by providing improved control of the laser wavelength over a range of wavelengths across different semiconductor lasers formed from different wafers of the ingot. The improved yield reduces cost and eliminates waste that would otherwise occur through the use of a semiconductor layer structure that does not include a thick buffer layer.

Fig. 1 is a graph 100 depicting the relationship between the laser wavelength and the number of slices of an ingot for a prior art semiconductor layer structure. The graph shows this relationship for a conventional semiconductor laser that includes a conventional semiconductor layer structure (e.g., including a buffer layer having a thickness of about 0.4 μm). The chart is on the figureThe number of boule slices ("slice #" of boule) is shown on the first axis of the table (e.g., wafer or substrate slices) and the operating current in nanometers (I) is shown on the second axis_op) Wavelength of laser light at ('I')_op(nm) laser wavelength "). The graph shows various graphs (e.g., growth 1 through growth 3) of different growth processes of an existing semiconductor layer structure, wherein the graphs represent semiconductor laser wavelength and corresponding number of slices formed from various ingot slices for each growth process. In addition, the graph shows the trend of different growth curves for each growth process. As indicated by reference numeral 110, a low number of slices is associated with a low laser wavelength, regardless of the growth process. For example, for growth 1 associated with a black circle and a black trend line, approximately 230 slices are correlated with a laser wavelength in the range 860nm to 865 nm. As indicated by reference numeral 120, a high number of slices is associated with a high laser wavelength, irrespective of the growth process. For example, for growth 1, a number of about 315 slices is associated with a laser wavelength in the range of about 888nm to about 891 nm. As a result, there can be significant laser wavelength variation between higher slice numbers of slices and lower slice numbers of wafers even if the slices are from the same ingot. For example, for growth 1, the laser wavelength variation is greater than 20nm (for some growth processes and more than 30nm between the low end of the growth process and the high end of the growth process). Depending on design requirements, this wavelength variation can result in significant waste and can increase costs during the fabrication of semiconductor lasers. For example, referring to growth 1, if a laser wavelength in the range of 875nm to 895nm is required for a particular design, only wafers having a number of slices in the range of about 264 to about 312 may be used to fabricate a semiconductor laser for that particular design. Slices having a number of slices outside this range may produce semiconductor lasers that do not have sufficient laser wavelengths and therefore need to be discarded or repurposed for different designs. Some embodiments described herein reduce or eliminate this wavelength variability, thereby reducing or eliminating potential problems caused by wavelength variability, described below with reference to fig. 2-4.

As described above, fig. 1 is provided as an example. Other examples may differ from the example described with reference to fig. 1.

Fig. 2 is a diagram depicting an example embodiment of a semiconductor layer structure 200 having a thick buffer layer as described herein.

As shown in fig. 2, the semiconductor layer structure may include a substrate 210. For example, the substrate 210 may be a gallium arsenide (GaAs) substrate or the like. Additionally or alternatively, the substrate 210 may be doped. For example, the substrate 210 may be an n-doped substrate. Substrate 210 may be formed from a wafer (e.g., sliced) from an ingot, and various other layers of the semiconductor layer structure may be formed (e.g., grown) on substrate 210.

As further shown in fig. 2, the semiconductor layer structure may include a buffer layer 220. For example, the buffer layer 220 may be formed on the substrate 210. Buffer layer 220 may be a GaAs buffer layer or the like. Additionally or alternatively, buffer layer 220 may be doped. For example, the buffer layer 220 may be an n-doped buffer layer. In some embodiments, buffer layer 220 may comprise the same material as substrate 210 (e.g., buffer layer 220 and substrate 210 may both comprise GaAs).

Buffer layer 220 may have a thickness of multiple microns. For example, the buffer layer 220 may have a thickness greater than or equal to 2 μm. In some embodiments, as described below, buffer layer 220 may have a thickness of about 4 μm across buffer layer 220 (e.g., it may be in the range of 3 μm and 5 μm across buffer layer 220, it may have an average thickness of 4 μm across buffer layer 220, etc.).

Buffer layer 220 may be lattice matched to substrate 210. For example, substrate 210 and buffer layer 220 may have matching (e.g., nearly equal) lattice constants. This facilitates growth of buffer layer 220 on substrate 210 to the thicknesses described above, and/or facilitates creating resistance to migration of defects in substrate 210 into buffer layer 220 and/or into various epitaxial layers formed on buffer layer 220 described below.

Various epitaxial layers may be formed on the buffer layer 220. For example, the various epitaxial layers may include a first capping layer 230 (e.g., an n-capping layer) formed on the buffer layer 220. The first capping layer 230 may form an n-type double heterostructure layer. As further shown in fig. 2, the semiconductor layer structure may include a first waveguide layer 240 (e.g., an n-waveguide layer) formed on the first cladding layer 230. First waveguide layer 240 may guide electrons (or light) in a particular direction within a semiconductor laser formed from the semiconductor layer structure and/or may confine electrons (or light) to quantum well layer 250. As further shown in fig. 2, the semiconductor layer structure may include a quantum well 250 formed on the first waveguide layer 240. As shown, the second waveguide layer 260 may be formed on the quantum well layer 250. The quantum well layer 250 may function as an active region of the semiconductor laser. The quantum well intermixed region may be formed in association with the quantum well layer 250 in certain regions of a semiconductor chip (not shown). Quantum well intermixed regions can be intentionally formed in these regions by diffusing material (e.g., silicon atoms) from the wafer surface in these regions. During this intentional impurity diffusion, the wafer may be placed in a high temperature environment for a period of time until material may diffuse from some regions of the wafer surface to some regions of the quantum well layer 250. Other areas of the wafer surface may be protected so that material from the wafer surface does not diffuse into the quantum well layer 250. In some embodiments, these protected regions may serve as semiconductor laser active regions within the laser cavity.

As further shown in fig. 2, the semiconductor layer structure may include a second waveguide layer 260 (e.g., a p-waveguide layer) formed on the quantum well layer 250 and a second cladding layer 270 (e.g., a p-cladding layer) formed on the second waveguide layer 260. Second waveguide layer 260 may be similar to first waveguide layer 240 and first cladding layer 230, respectively, except that it is a p-type layer instead of an n-type layer. As further shown in fig. 2, the semiconductor layer structure may include a cap layer 280. The cap layer 280 may be highly doped.

The semiconductor layer structure shown in fig. 2 and described with reference thereto may be used to form various types of devices. For example, the semiconductor layer structure may be used to form a semiconductor laser (e.g., a semiconductor diode laser), a light emitting device, and the like. In some embodiments, the laser wavelength of the device can be in the Infrared (IR) or near-infrared (nir)A range (e.g., in the range of 700nm to 1000 nm). By including a thick buffer layer, the semiconductor layer structure shown in fig. 2 provides improved control of laser wavelength variability between different devices formed from different substrates 210 from the same ingot (e.g., while at I |)_opAnd room temperature, e.g., about 21 degrees celsius, or other temperature set by a thermoelectric cooler (TEC), when lasing occurs. ) In some embodiments, the laser wavelengths of laser devices formed from different substrates 210 in the same ingot may be measured at the same operating conditions (e.g., same current, same heat sink temperature, etc.) associated with making this determination. For example, improved control may maintain the laser wavelength within 20nm for devices formed from substrates with low numbers of slices and devices formed from substrates with high numbers of slices (e.g., when the laser wavelengths of devices formed from substrates with low numbers of slices and devices formed from substrates with high numbers of slices are measured under the same operating conditions). Thus, the laser wavelength may be substantially independent of the number of slices or the slice position of the substrate 210 in the ingot (e.g., laser wavelength control may facilitate the use of devices formed with low number of slices and/or high number of slices depending on the intended application). In this case, the laser wavelength is substantially independent of the slicing position or the number of slices, and can be interpreted as a change between the laser wavelength and that of another device formed of any other ingot substrate by less than a specific amount (e.g., 15nm, 30nm, etc.), regardless of the slicing position or the number of slices of the pair of substrates.

In this manner, some embodiments described herein provide a semiconductor layer structure that includes a thick buffer layer. The thick buffer layer may reduce or eliminate migration of defects from the substrate 210, which improves the performance of devices formed using semiconductor layer structures including the thick buffer layer. In addition, a semiconductor layer structure including a thick buffer layer provides improved control of wavelength variability between devices formed from substrates 210 having different numbers of slices from the ingot. This increases the throughput of ingots when used to produce devices that lase over a narrow wavelength range (e.g., 20nm range), thereby reducing the costs and/or waste associated with device production.

As described above, fig. 2 is provided as an example. Other examples may differ from the example described with reference to fig. 2.

Fig. 3 is a graph 300 depicting various thicknesses of a comparative buffer layer and the correspondence between laser wavelength and number of slices of the ingot for various thicknesses. For example, the graph shows a comparison of the relationship between laser wavelength and the number of slices for a semiconductor layer structure including a 0.4 μm thick buffer layer and a semiconductor layer structure including a 4.0 μm thick buffer layer (e.g., a semiconductor layer structure including a thick buffer layer). When the corresponding device is in operation current (I)_op) And room temperature, the laser wavelengths shown in the graph of fig. 3 occur.

Reference numeral 310 shows a curve of a semiconductor layer structure having a buffer layer of 0.4 μm thickness, which is generally used in the conventional semiconductor layer structure. As shown, the wavelength variation of a device formed from the semiconductor layer structure may be greater than 35nm (e.g., from a low end of about 855nm to a high end of about 892 nm). In some applications, this wavelength variation may be too large, or may result in a laser wavelength that is outside of an acceptable range.

Reference numeral 320 shows a graph of a semiconductor layer structure including a thick buffer layer (e.g., a 4.0 μm buffer layer). As shown, the wavelength variation of a device formed from this semiconductor layer structure including a thick buffer layer may be less than 20nm (e.g., from a low end of about 882nm to a high end of about 892 nm). As a result, by using a thick buffer layer, the difference between the laser wavelengths corresponding to two different devices formed from the substrate of the ingot may be less than 20 nm. In this manner, the semiconductor layer structure including the thick buffer layer provides improved control of the laser wavelength by reducing wavelength variability between devices formed from slices of the ingot, which reduces cost and waste, as described elsewhere herein.

As described above, fig. 3 depicts one or more examples. Other examples may differ from the example described in the opening fig. 3.

FIG. 4 is a drawingA graph 400 comparing various thicknesses of the buffer layer and the correspondence between the laser wavelength and the number of slices of the ingot for various thicknesses is plotted. For example, the graph shows a comparison of the relationship between the laser wavelength and the number of slices for a semiconductor layer structure including a 0.4 μm thick buffer layer (e.g., commonly used in existing semiconductor layer structures), a semiconductor layer structure including a 0.8 μm thick buffer layer, a semiconductor layer structure including a 2.0 μm thick buffer layer, a semiconductor layer structure including a 4.0 μm thick buffer layer. When the corresponding device is in operation current (I)_op) And room temperature, the laser wavelengths shown in the graph of fig. 4 occur similar to that described elsewhere herein.

Reference numeral 410 shows the same curve as that shown by reference numeral 310 of the open fig. 3, which is a curve of a semiconductor layer structure including a buffer layer 0.4 μm thick. Reference numeral 420 shows a curve of a semiconductor layer structure including a buffer layer of 0.8 μm thickness. As shown, the wavelength variability between devices sliced from different ingots is still greater than 20nm (e.g., from the low end of about 858nm to the high end of about 882 nm). Furthermore, for some applications requiring laser wavelengths in the 875nm to 895nm range (as a specific example), more than half of the devices associated with the 0.8 μm and 0.4 μm examples do not have laser wavelengths in this range, resulting in a loss rate of greater than 50% when using 0.4 μm or 0.8 μm thick buffer layers.

Reference numeral 430 shows a curve of a semiconductor layer structure including a buffer layer 2.0 μm thick. As shown, the wavelength variability between devices sliced from different boules is less than 20nm (e.g., from the lower end of about 870nm to the upper end of about 889 nm). In this example, and for the applications described above (e.g., requiring laser wavelengths in the range 875nm to 895 nm), two of the three devices may be suitable for those applications, resulting in only 33.33% loss. In this way, devices having a 2.0 μm thick buffer layer may provide improved wavelength control over devices including a 0.8 μm thick or 0.4 μm thick buffer layer. Reference numeral 440 shows a curve of a semiconductor layer structure including a buffer layer 4.0 μm thick. As shown, the 4.0 μm thick buffer layer provides improved wavelength variability control over the 2.0 μm thick buffer layer. For example, for applications requiring laser wavelengths in the range 875nm to 895nm, 9 of 10 devices have laser wavelengths in this range, resulting in a loss of only 10%. In this way, devices with thick buffer layers (e.g., multiple microns thick buffer layers) can reduce the laser wavelength difference between wafers. For example, the wavelength difference between wafers is reduced from 37nm for a 0.4 μm thick buffer layer to 14nm for a 4.0 μm thick buffer layer.

As noted above, fig. 4 is provided as an example only. Other examples may differ from the example described in connection with fig. 4.

Fig. 5 is a flow diagram depicting an example process 500 for forming a semiconductor layer structure with a thick buffer layer.

As shown in fig. 5, the process 500 may include providing a substrate (block 505). For example, the substrate 210 may be provided as described above.

As further shown in fig. 5, process 500 may include forming a buffer layer on a substrate (510). For example, buffer layer 220 may be formed on substrate 210, as described above. In some aspects, buffer layer 220 has a thickness greater than 2 μm.

As further shown in fig. 5, the process 500 may include forming a set of epitaxial layers on the buffer layer (block 515). For example, a set of epitaxial layers may be formed over buffer layer 220, as described above. In some aspects, the set of epitaxial layers includes a quantum well layer 250. In some aspects, the quantum well intermixed region is formed in association with the quantum well layer 250 and the material diffused from the surface region of the semiconductor layer structure.

Process 500 may include additional embodiments, such as any single embodiment or combination of embodiments described below and/or in conjunction with one or more other processes described elsewhere herein.

In some embodiments, the semiconductor layer structure is included in a laser device. In some embodiments, the laser device has a wavelength in the Infrared (IR) or near-infrared range. In some embodiments, the laser wavelength of the laser device is substantially independent of the slicing position of the substrate from the ingot.

In some embodiments, the buffer layer comprises the same material as the substrate. In some embodiments, the buffer layer has a thickness in the range of 2 μm to 5 μm. In some embodiments, the thickness averages 4 μm across the buffer layer. In some embodiments, the buffer layer is an n-doped buffer layer. In some embodiments, the n-doped cap layer of the set of epitaxial layers is formed on the n-doped buffer layer. In some embodiments, the quantum well layer is formed on the n-doped cladding layer.

Although fig. 5 shows example blocks of the process 500, in some implementations, the process 500 may include additional blocks, fewer blocks, different blocks, or blocks arranged differently than those depicted in fig. 5. Additionally or alternatively, two or more blocks of process 500 may be performed in parallel.

The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Modifications and variations are possible in light of the above disclosure or may be acquired from practice.

As used herein, the term "layer" is intended to be construed broadly as one or more layers and includes layers that are oriented horizontally, vertically, or at other angles.

Some embodiments are described herein in connection with a threshold.

As used herein, meeting a threshold may refer to a value being greater than the threshold, greater than or equal to the threshold, less than or equal to the threshold, and the like, depending on the context.

Although particular combinations of features are recited in the claims and/or disclosed in the specification, these combinations are not intended to limit the disclosure of the various embodiments. Indeed, many of these features may be combined in ways not specifically recited in the claims and/or disclosed in the specification. Although each dependent claim listed may directly depend on only one claim, the disclosure of the various embodiments includes each dependent claim with every other claim in the set of claims.

No element, act, or instruction used herein should be construed as critical or essential unless explicitly described as such. In addition, as used herein, the articles "a" and "an" are intended to include one or more items, and may be used interchangeably with "one or more". Further, as used herein, the article "the" is intended to include one or more items that are referenced in association with the article "the" and may be used interchangeably with "one or more.

Further, as used herein, the term "group" is intended to include one or more items (e.g., related items, unrelated items, combinations of related and unrelated items, etc.) and may be used interchangeably with "one or more. In the case of only one item, the phrase "only one" or similar language is used. Further, as used herein, the term "having" and the like are intended to be open-ended terms. Further, the phrase "based on" is intended to mean "based, at least in part, on" unless explicitly stated otherwise. Further, as used herein, the term "or" when used in succession is intended to be inclusive and may be used interchangeably with "and/or" unless specifically stated otherwise (e.g., if used in combination with "either" or "only one").