Strain-engineered cladding for optimized active region strain and improved laser diode performance

Abstract

Some embodiments may include a laser diode having a strain engineered cladding layer for optimizing active region strain and improving laser diode performance. In one embodiment, the laser diode may include a semiconductor substrate having a material composition with a first lattice constant; and a plurality of epitaxial layers formed on the semiconductor substrate, the plurality of epitaxial layers including a waveguide layer and a cladding layer, wherein the waveguide layer includes an active region having a material composition associated with a target light wavelength, wherein the material composition of the active region has a second lattice constant different from the first lattice constant; wherein the material composition and / or thickness of a single cladding layer of the cladding layer is configured to impose a target stress field on the active region to optimize the active region strain. Other embodiments may be disclosed and / or claimed.

Description

Technical Field

[0001] The present invention relates to laser diodes. Background Art

[0002] Fiber lasers are widely used in industrial processes (e.g., cutting, welding, cladding, heat treatment, etc.). In some fiber lasers, the optical gain medium includes one or more active optical fibers having cores doped with rare earth elements. The rare earth elements can be optically excited ("pumped") by light from one or more semiconductor laser sources. There is a great demand for both high-power diode lasers to scale power and reduce price (measured in dollars per watt) and high-efficiency diode lasers to reduce energy consumption and extend lifetime. BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the technology disclosed herein, wherein like reference numerals represent like elements.

[0004] Figure 1 shows a cross-sectional view of an epitaxial layer structure of a laser diode according to some embodiments;

[0005] Figure 2 shows a cross-sectional view of another epitaxial layer structure of a laser diode according to some embodiments;

[0006] Figure 3A A cross-sectional view of an epitaxial layer structure of a laser diode according to some embodiments is shown, wherein the cladding layer is strain-engineered to optimize active region strain and improve laser diode performance;

[0007] Figure 3B A cross-sectional view of another epitaxial layer structure of a laser diode according to some embodiments is shown, wherein the cladding layer is strain-engineered to optimize active region strain and improve laser diode performance;

[0008] Figure 4A -C is according to some embodiments Figure 2 Block diagram of the construction of the cladding layer. DETAILED DESCRIPTION

[0009] As used in this application and the claims, the singular forms of "a", "an", and "the" include the plural forms unless the context clearly dictates otherwise. In addition, the term "comprising" means "including". In addition, the term "connected (coupled)" does not exclude the presence of intermediate elements between the items being connected (coupled). The systems, devices, and methods described herein should not be interpreted as limiting in any way. On the contrary, the present disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, individually and in various combinations and sub-combinations with each other. The term "or" means "and / or", not "exclusive or" (unless otherwise specified).

[0010] The disclosed systems, methods, and apparatus are not limited to any particular aspect or feature or combination thereof, nor do the disclosed systems, methods, and apparatus require the presence of any one or more particular advantages or problems to be solved. Any theory of operation is helpful for explanation, but the disclosed systems, methods, and apparatus are not limited to these theories of operation. Although the operations of some disclosed methods are described in a particular order for ease of presentation, it should be understood that such description includes rearrangement unless the specific language set forth below requires a particular order. For example, in some cases the operations described in sequence may be rearranged or performed simultaneously. In addition, for simplicity, the accompanying drawings may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus.

[0011] In addition, the specification sometimes uses terms such as "produce" and "provide" to describe the disclosed methods. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms will vary according to the specific implementation and are easily discernible by those of ordinary skill in the art. In some examples, values, processes, or devices are referred to as "minimum", "optimal", "minimum", etc. It should be understood that such descriptions are intended to indicate that a selection can be made among many functional alternatives used, and that such selections are not necessarily better, lesser, or more preferred than other selections.

[0012] Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” etc. These terms are used for convenience of description but do not imply any particular spatial orientation.

[0013] Strain engineering of active regions (e.g., quantum wells) is one approach to improving the performance of high-power laser diodes. Introducing strain in the active region can modify the band structure, resulting in a reduction in threshold current density and an increase in differential gain. Active regions with optimized strain levels can significantly improve the performance of high-power laser diodes, including power efficiency (compared to active regions without optimized strain levels). However, wavelength limitations may prevent known strain designs of active regions from achieving optimized strain levels in the active region alone. Providing optimized strain levels in the active region using strain-engineered cladding layers can improve diode performance.

[0014] Figure 1 A cross-sectional view of the epitaxial layer structure of a large optical cavity (LOC) laser diode is shown. The LOC laser diode includes an n-type doped GaAs substrate 5, an n-type cladding layer 11, an n-type waveguide layer 13, an active region 14 (e.g., a quantum well or quantum dot), a p-type waveguide layer 15, a p-type cladding layer 16, and a cap layer 17, e.g., a p-type cap layer (p-cap). In some 900 nm or larger LOC laser diodes, the p-type cladding layer 16 may include a thick Al 0.6 Ga 0.4 As material. Thick Al 0.6 Ga 0.4 The lattice mismatch between the As material and the GaAs substrate 5 may result in reduced compressive strain in the active region, which may negatively affect the observed threshold current and slope efficiency of the LOC laser diode.

[0015] In some LOC laser diodes, such as Figure 1 The p-side of the vertical waveguide may have a significant impact on the overall waveguide thickness, as shown. A diode laser design with a significantly reduced p-type waveguide thickness (e.g., a large quantum well offset toward the p-side of the waveguide) can mitigate the electrical series resistance generated in the p-type waveguide, the optical losses and leakage currents associated with carrier accumulation in the p-type waveguide at large forward biases, and can therefore result in lower voltages, higher slope efficiency and power, and higher wall plug efficiency. Figure 2 A cross-sectional view of one such embodiment is shown (wherein the thickness of the p-type waveguide in the laser diode may be significantly reduced and / or a dual n-type cladding layer structure may be used).

[0016] Similar to Figure 1In the LOC laser diode, the degree of lattice mismatch between the material of the p-type cladding layer 116 and the GaAs substrate 105 can result in a reduction in the compressive strain in the active region 114, especially when the active region 114 is very close to the p-type cladding layer 116, making the stress field of the p-type cladding layer stronger, thereby negatively affecting the observed threshold current and slope efficiency of the diode laser. The embodiment also includes a double n-type cladding layer structure, which includes an outer n-type cladding layer 111 and an inner n-type cladding layer 112. The other layers in this embodiment can be similar to any other layers of the LOC laser diode.

[0017] In one example, the p-type cladding layer 116 may include Al 0.6 Ga 0.4 As. Al 0.6 Ga 0.4 The room temperature lattice mismatch between As and GaAs is about 0.08%, so when the p-type cladding layer 116 is Al 0.6 Ga 0.4 In the case of As, it is under a compressive strain of about 0.08%. Although this lattice mismatch is small, Al 0.6 Ga 0.4 The bulk layer of As material may be thick enough to apply a stress field that may cause wafer bending during wafer manufacturing. In addition, the stress field from the strained p-type cladding layer 116 can propagate to the active region 114, partially relieving the compressive strain in the active region 114, which may be critical to achieving high performance of the diode laser (an appropriate level of strain in the active region 114 changes the band structure, resulting in a reduction in threshold current, an improvement in differential gain and slope efficiency).

[0018] Some embodiments described herein may strain compensate the cladding by changing the material composition and / or thickness of the cladding to change the degree of lattice mismatch, thereby applying an optimal strain on the active region. In some embodiments, the method is by including a strain compensating material into the cladding (e.g., including phosphorus into AlGaAs - replacing some of the arsenic atoms in the AlGaAs cladding with phosphorus atoms). Such a strain compensating material may change the lattice constant of the cladding to match the lattice constant of GaAs differently, thereby applying a stress field on the active region. In a diode laser design with a significantly reduced p-type waveguide thickness, its p-type cladding is fully strain compensated to suppress strain relaxation on the active region, and it is expected that the full performance potential in the thin p-type waveguide design under optimized quantum well strain can be recovered in the strain compensated design in the form of lower threshold current, higher slope efficiency, and higher power at larger drive currents.

[0019] The level of strain compensation in the AlGaAsP p-type cladding can be optimized for optimal active region strain. Due to wavelength limitations in some diode lasers, a predetermined composition, namely InGaAs, is present in the active region. The predetermined composition can impose a predetermined strain in the active region, which may not have an ideal active region strain level (e.g., the InGaAs active region compressive strain in a 915-nm diode is below the optimized value for best performance). An AlGaAsP p-type cladding with optimized strain can alleviate this limitation on diode performance. For example, for a 915-nm diode, a slightly tensile strained AlGaAsP p-type cladding can provide a desired active region strain level. 0.6 Ga 0.4 AsP (phosphorus content greater than 2.5%) can be used to exert a stress field on the active region to provide increased compressive strain in the InGaAs quantum wells.

[0020] Figure 3A 2 shows a cross-sectional view of an epitaxial layer structure having a strain-engineered cladding layer for optimizing active region strain and improving diode laser performance. The semiconductor substrate 205 and active layer 214 and any intermediate layers may be similar to the semiconductor substrate 105 and active layer 114 and any intermediate layers ( Figure 2 The cladding layer 216 may be strain engineered to apply a stress field on the active region 214, thereby optimizing the strain in the active region 214 to improve diode performance. In one example, the thickness of the cladding layer 216 is about the same as the cladding layer 116 ( Figure 2 ) is the same as that of the active region, but the cladding layer 216 includes a strain compensating material. In other embodiments, any strain-engineered cladding layer may have a strain-engineered thickness and / or a strain-engineered composition to exert a stress field on the active region.

[0021] Figure 3A The laser diodes of the present invention utilize a thin p-type waveguide design (wherein the thickness of the p-type waveguide portion is significantly less than the thickness of the n-type waveguide portion of the waveguide layer); however, this does not require the use of a strained cladding layer to optimize the active region strain and improve the laser diode performance. In some thin p-type waveguide design embodiments, the thickness of the p-type waveguide portion can be less than 30% of the thickness of the n-type waveguide portion (in other embodiments of the thin p-type waveguide design, other ratios are also possible and practical). Figure 3B Shown with Figure 1 2 is a cross-sectional view of an epitaxial layer structure similar to the epitaxial layer structure shown (e.g., a fiber laser with LOC), which has a strained cladding layer 216. The semiconductor substrate 305 and the active layer 314 and any intermediate layers can be similar to the semiconductor substrate 5 and the active layer 14 and any intermediate layers ( Figure 1The cladding layer 316 may be strain engineered to apply a stress field on the active region 314, thereby optimizing the strain in the active region 314 to improve diode performance. In one example, the thickness of the cladding layer 316 is about the same as the cladding layer 16 ( Figure 1 ) is the same as that of the active region, but the cladding layer 316 includes a strain compensating material. In other embodiments, any strain-engineered cladding layer may have a strain-engineered thickness and / or a strain-engineered composition to exert a stress field on the active region.

[0022] refer to FIG. 4A to FIG. 4C , incorporated into the cladding layers 216 and / or 316 (respectively FIG. 3A to FIG. 3B ) may be uniformly distributed in a portion of the cladding layer, or non-uniformly distributed in a portion of the cladding layer 216 and / or 316. Figure 4A An example is shown in which the strain adjustment material is uniformly distributed throughout the cladding layer 216 and / or 316. In the illustrated example, the strain adjustment material is phosphorus and the material composition of the cladding layer 216 and / or 316 is Al 0.6 Ga 0.4 As 0.98 P 0.02 . Slightly different material composition Al 0.6 Ga 0.4 As 0.975 P 0.025 It has zero lattice mismatch with GaAs at room temperature; using it as a p-type cladding layer can completely eliminate the stress field it applies to the active region. Using a material composition that is lattice mismatched with the substrate, such as Al 0.6 Ga 0.4 As 0.98 P 0.02 , a non-zero stress field can be applied across the active region, which can optimize the strain in the active region, which can provide improved laser diode performance.

[0023] Although it may be desirable to set the cladding layer to impose a certain stress field or a zero stress field on the active region at room temperature, there are additional constraints on the cladding layer strain design because there is a threshold strain-thickness product value that the diode epitaxial structure should not exceed at the growth temperature to prevent the formation of dislocations and wafer cracking. Therefore, in some embodiments, the thickness of the cladding layer can also be changed based on the threshold strain-thickness product value to impose the desired stress field without forming dislocations and wafer cracking. In addition, the strain adjustment material can be located closer to or farther away from the active region (e.g., non-uniformly distributed and / or uniformly distributed in one of multiple portions of the cladding layer). Figure 4C A non-uniform distribution using a gradient material composition is shown, wherein the concentration of the strain modulating material varies with increasing distance from the active region.

[0024] Figure 4B Another example is shown, where the strain accommodation material may still be uniformly distributed in a single portion (but the concentration in at least two portions of the single portion may be different). Figure 4B One embodiment may employ a superlattice cladding design with alternating layers of AlGaAsP of varying phosphorus composition and varying strain levels. The phosphorus composition and strain level of the alternating layers of AlGaAsP may be selected to have zero average strain or a desired average strain level. The lattice mismatch between the alternating layers of differently strained AlGaAsP may be localized such that there may be no (or almost no) dislocations, although the cumulative strain-thickness product may exceed a critical value. Similar to Figure 4C Embodiments may have an AlGaAsP p-type cladding layer with a varying phosphorus composition, with zero or less compressive strain closer to the waveguide and greater compressive strain farther away from the active region (e.g., active well). The active region is now farther away from the more strained AlGaAsP cladding layer and therefore experiences a smaller stress field. On the other hand, the gradient phosphorus composition in the cladding layer can reduce the cumulative strain-thickness product value at the growth temperature to below a critical value.

[0025] Material composition of strain-engineered cladding

[0026] The strain-engineered cladding of the laser diode may incorporate strain-accommodating materials into a composition that will be lattice-matched (or non-optimally lattice-mismatched with the semiconductor substrate) to apply a target stress field on the active region (e.g., on quantum wells, quantum dots, etc.). In some embodiments, the strain-accommodating material may be a periodic table Group III material or a periodic table Group V material. For example, phosphorus atoms may be included to replace some of the arsenic atoms in the AlGaAs cladding. The room temperature lattice constant of AlGaAsP decreases linearly with increasing phosphorus content. Al 0.6 Ga 0.4 The lattice mismatch between AsP and GaAs substrate is due to the Al 0.6 Ga 0.4 About 0.08% of As is compressed to Al 0.6 Ga 0.4 As 0.954 P 0.04 6 is stretched by about 0.08%, compared with Al 0.6 Ga 0.4 As 0.977 P 0.023 In some other embodiments, the material composition of the strain-engineered cladding layer can be a ternary alloy, a quaternary alloy, or a quinary alloy of the periodic table III-V group, and the material composition of the ternary, quaternary, or quinary alloy can be adjusted so that the cladding layer is compressively strained, lattice-matched, or tensilely strained relative to the GaAs substrate.

[0027] Examples of quaternary alloy cladding layers that may be lattice matched or slightly mismatched to GaAs include, but are not limited to, AlGaAsP, InGaAsP, and AlInGaP. 0.49 Ga 0.45 As 0.06 P has the same properties as Al used in the cladding layer. 0.6 Ga 0.4 As, but lattice matched to the GaAs substrate, and its material composition can be adjusted to be compressively strained, lattice matched, or tensile strained relative to the GaAs substrate. Examples of quinary alloy cladding layers include, but are not limited to, AlInGaAsP. Compared to quaternary alloys, quinary alloys can provide greater design flexibility in bandgap design and lattice matching.

[0028] The strain accommodation material may be included differently in different portions of the cladding. For example, the cladding may include two or more sublayers, wherein the average concentration of the strain accommodation material in one sublayer is different from the average concentration of the strain accommodation material in a different sublayer. The sublayer closest to the active region may have a different average concentration than another sublayer. In other examples, a portion of the cladding may have a gradient concentration of the strain accommodation material (e.g., the entire cladding or portions of the cladding).

[0029] In various embodiments, the laser diode may include more than one cladding layer, some or all of which may be strain engineered. In the case of more than one cladding layer in the strain engineered cladding layer, each strain engineered cladding layer may apply a single stress field, wherein the total stress field strains the active region.

[0030] In view of the many possible embodiments to which the disclosed technical principles may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the invention. We claim as our invention all that comes within the scope and spirit of the appended claims.

Claims

1. A laser diode, comprising: a semiconductor substrate having a material composition with a first lattice constant; and a plurality of epitaxial layers formed on the semiconductor substrate, the plurality of epitaxial layers comprising a waveguide layer and a cladding layer, wherein the waveguide layer comprises an active region having a material composition associated with a target light wavelength, wherein a second lattice constant of the material composition of the active region is different from the first lattice constant; wherein the material composition of a single cladding layer in the cladding layer exerts a stress field on the active region; wherein the single one of the cladding layers comprises a strain regulating material, and Wherein, the single one of the cladding layers comprises two or more sub-layers, and an average concentration of the strain adjustment material in one of the sub-layers is different from an average concentration of the strain adjustment material in a different one of the sub-layers.

2. The laser diode according to claim 1, wherein: The single one of the coating layers includes two or more sub-layers each having two or more uniform material components.

3. The laser diode according to claim 2, wherein: A first sublayer of the two or more sublayers is closer to the active region than a second sublayer of the two or more sublayers, and a degree of lattice mismatch with the first lattice constant in the first sublayer is different from a degree of lattice mismatch with the first lattice constant in the second layer.

4. The laser diode according to claim 1, wherein: The single one of the cladding layers includes alternating layers having different degrees of lattice mismatch with the first lattice constant.

5. The laser diode according to claim 1, wherein: The single one of the cladding layers comprises a gradient material composition and a gradient lattice constant.

6. The laser diode according to claim 1, wherein: The strain adjustment material includes a periodic table Group III material or a periodic table Group V material.

7. The laser diode according to claim 1, wherein: The material composition of the single cladding layer in the cladding layers includes any ternary alloy, quaternary alloy or quinary alloy of Group III-V of the Periodic Table.

8. The laser diode according to claim 1, wherein: The single one of the cladding layers includes a p-type cladding layer of the cladding layers.

9. The laser diode according to claim 1, wherein: The cladding layer includes a p-type cladding layer and an n-type cladding layer.

10. The laser diode according to claim 9, wherein: The strain adjustment material is located in the p-type cladding layer.

11. The laser diode according to claim 9, wherein: The strain accommodation material is located in the n-type cladding layer.

12. The laser diode according to claim 9, wherein: The cladding layer further includes a p-type cap for the p-type cladding layer.

13. The laser diode according to claim 9, wherein: The n-type cladding layer includes an inner n-type cladding layer and an outer n-type cladding layer.

14. The laser diode according to claim 1, wherein: The semiconductor substrate includes an n-type substrate.

15. The laser diode according to claim 1, wherein The n-type substrate includes a GaAs n-type substrate.

16. The laser diode according to claim 1, wherein The laser diode also includes a large optical cavity.

17. The laser diode according to claim 1, wherein: The waveguide layer further includes an n-type waveguide and a p-type waveguide, wherein a thickness of the p-type waveguide is less than 30% of a thickness of the n-type waveguide.

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