Semiconductor laser element and method for manufacturing the same

Abstract

A first cladding layer (2) of a first conductivity type, an active layer (3), and a second cladding layer (4) of a second conductivity type are formed in this order on a semiconductor substrate (1) of the first conductivity type. A gettering layer (5) composed of an Al-based material is formed without a gap on the second cladding layer (4).

Description

Technical Field

[0001] This disclosure relates to semiconductor laser devices and methods for manufacturing the same. Background Technology

[0002] A scheme has been proposed to incorporate a diffusion suppression layer in a semiconductor laser, which suppresses the propagation of crystal defects or metal diffusion from the chip surface to the active layer (see, for example, Patent Document 1).

[0003] Patent Document 1: Japanese Patent No. 3785660

[0004] Semiconductor layers, such as an upper cladding layer and a diffraction grating layer, are formed on top of the active layer. When an insulating film is formed on these semiconductor layers using methods such as plasma CVD, fluorine or hydrogen can be introduced into the semiconductor layers. Furthermore, methane or halogen compounds used in the fabrication of the diffraction grating layer can also introduce fluorine or hydrogen into the surface of the semiconductor layer. The diffusion of residues from materials used in the manufacturing process of such semiconductor laser devices cannot be prevented by existing diffusion suppression layers. If these residues diffuse into the active layer, the characteristics of the semiconductor laser device change, leading to a decrease in reliability. Summary of the Invention

[0005] This disclosure was made to solve the aforementioned problems, and its purpose is to provide a semiconductor laser element and a method for manufacturing the same, which can prevent changes in characteristics and a reduction in reliability.

[0006] The semiconductor laser element disclosed herein is characterized by comprising: a semiconductor substrate of a first conductivity type; a first cladding layer of the first conductivity type, an active layer, and a second cladding layer of the second conductivity type sequentially formed on the semiconductor substrate; and a getter layer made of an Al-based material formed without gaps on the second cladding layer.

[0007] In this disclosure, a getter layer made of an Al-based material is formed seamlessly over the second coating layer. This prevents the diffusion of material residues used during manufacturing into the active layer. Therefore, it is possible to prevent variations in the characteristics of the semiconductor laser element and a decrease in its reliability. Attached Figure Description

[0008] Figure 1 This is a cross-sectional view showing the semiconductor laser element according to Embodiment 1.

[0009] Figure 2 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element according to Embodiment 1.

[0010] Figure 3 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element according to Embodiment 1.

[0011] Figure 4 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element according to Embodiment 1.

[0012] Figure 5 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element according to Embodiment 1.

[0013] Figure 6 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element according to Embodiment 1.

[0014] Figure 7 This is a cross-sectional view showing the semiconductor laser element involved in Embodiment 2.

[0015] Figure 8 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element according to Embodiment 2.

[0016] Figure 9 This is a cross-sectional view showing the semiconductor laser element involved in Embodiment 3.

[0017] Figure 10 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element involved in Embodiment 3.

[0018] Figure 11 This is a cross-sectional view showing the semiconductor laser element involved in Embodiment 4. Detailed Implementation

[0019] The semiconductor laser element and its manufacturing method according to the embodiments will be described with reference to the accompanying drawings. The same or corresponding components are labeled with the same reference numerals, and sometimes repeated descriptions are omitted.

[0020] Implementation Method 1

[0021] Figure 1 This is a cross-sectional view showing the semiconductor laser element according to Embodiment 1. This semiconductor laser element is a DFB laser (distributed feedback semiconductor laser) that performs single-mode oscillation. Although not shown, it has an optical waveguide structure and a mirror-based resonator structure formed to function as a semiconductor laser.

[0022] A first cladding layer 2 of the first conductivity type, an active layer 3, and a second cladding layer 4 of the second conductivity type are sequentially formed on a semiconductor substrate 1 of the first conductivity type. A getter layer 5 of the second conductivity type, made of an Al-based material, is formed on the second cladding layer 4. A third cladding layer 6 of the second conductivity type is formed on the getter layer 5. A diffraction grating 7 of the second conductivity type is formed on the third cladding layer 6. The diffraction grating 7 is embedded in a semiconductor layer 8 of the second conductivity type. An upper surface electrode 9 is formed on the semiconductor layer 8. A lower surface electrode 10 is formed on the lower surface of the semiconductor substrate 1. The diffraction grating 7 is composed of a plurality of patterns formed periodically with gaps between them. On the other hand, the getter layer 5 is formed without gaps on the entire upper surface of the second cladding layer 4.

[0023] One of the n-type and p-type is the first conductivity type, and the other is the second conductivity type. The semiconductor substrate 1, first cladding layer 2, second cladding layer 4, third cladding layer 6, and semiconductor layer 8 are, for example, made of InP. The getter layer 5 is, for example, made of a mixed crystal layer containing Al, such as AlGaInAs or AlAs. The diffraction grating 7 is made of a material that can obtain a refractive index difference with InP, such as a mixed crystal of InGaAsP or AlGaInAs. Alternatively, an InP buffer layer may be provided between the semiconductor substrate 1 and the first cladding layer 2.

[0024] Next, the manufacturing method of the semiconductor laser element involved in this embodiment will be described. Figures 2-6 This is a cross-sectional view showing the manufacturing process of the semiconductor laser element according to Embodiment 1.

[0025] First, such as Figure 2 As shown, a first cladding layer 2, an active layer 3, a second cladding layer 4, a getter layer 5, a third cladding layer 6, and a diffraction grating layer 11 are sequentially epitaxially grown on a semiconductor substrate 1. An insulating film 12 is formed on the diffraction grating layer 11 using methods such as plasma CVD. A photoresist 13 is coated on the insulating film 12. Next, as... Figure 3 As shown, a periodic pattern is formed in the photoresist 13 by photolithography.

[0026] The insulating film 12 is made of SiO2 (silicon dioxide) or SiN (silicon nitride), etc. Fluorine or hydrogen is introduced into the surface of the diffraction grating layer 11 by using fluorine compound gas, or material gas such as silane or ammonia, which is used to clean the film formation chamber of the CVD apparatus.

[0027] Next, as Figure 4 As shown, a photoresist 13 is used as a mask, and a fluorocarbon compound (fluorinated hydrocarbon compound) is used as a processing gas to pattern the insulating film 12. Fluorine or hydrogen is introduced into the surface of the diffraction grating layer 11 by the fluorocarbon compound. Subsequently, the photoresist 13 is removed using an organic solvent or the like.

[0028] Next, as Figure 5 As shown, the patterned insulating film 12 is used as a mask, and methane or a halogen compound is used as a processing gas to dry etch the diffraction grating layer 11, thereby forming the diffraction grating 7. Fluorine or hydrogen is introduced into the surface of the third coating layer 6 by the methane or halogen compound. Subsequently, the insulating film 12 is removed using hydrofluoric acid or buffered hydrofluoric acid.

[0029] Next, as Figure 6 As shown, a semiconductor layer 8 is epitaxially grown, and a diffraction grating 7 is embedded therein. Subsequently, a ridge formation process, an upper surface electrode 9, and a lower surface electrode 10 are formed to manufacture a semiconductor laser element.

[0030] As described above, residues from the materials used in the manufacturing process are introduced onto the surface of the semiconductor layer. If these residues diffuse and reach the active layer 3, non-luminescent recombination centers are formed in the active layer 3, affecting the charge state within the active layer 3 and causing changes in the characteristics of the semiconductor laser element.

[0031] In contrast, in this embodiment, a getter layer 5 made of Al-based material is formed seamlessly on the second coating layer 4. This prevents the diffusion of material residues used in the manufacturing process to the active layer 3. Therefore, it is possible to prevent variations in the characteristics of the semiconductor laser element and a decrease in its reliability.

[0032] Experiments confirmed that this effect can be achieved if the thickness of the getter layer 5 is 20 nm or more. Furthermore, the getter layer 5 can also prevent point defects introduced into the semiconductor layer surface during processing from diffusing to the active layer 3. The preferred composition of the getter layer 5 is a high mixed crystal ratio of Al, which has high reactivity with other elements.

[0033] If the getter layer 5 is epitaxially grown on the uneven surface, the crystallinity of the uneven portion will decrease. Fluorine or hydrogen may diffuse from the areas of reduced crystallinity in the getter layer 5 to the active layer 3. Therefore, the upper surface of the second coating layer 4 forming the getter layer 5 is preferably flat.

[0034] Implementation Method 2

[0035] Figure 7 This is a cross-sectional view showing the semiconductor laser element according to Embodiment 2. The difference from Embodiment 1 is that there is no third cladding layer 6, and the getter layer 5 is in contact with the diffraction grating 7.

[0036] Figure 8This is a cross-sectional view illustrating the manufacturing process of the semiconductor laser element according to Embodiment 2. Using a patterned insulating film 12 as a mask and methane or a halogen compound as a processing gas, the diffraction grating layer 11 is dry-etched to form the diffraction grating 7. Fluorine or hydrogen is introduced into the surface of the getter layer 5 by the methane or halogen compound. However, the getter layer 5 prevents these residues from diffusing to the active layer 3. Therefore, variations in the characteristics of the semiconductor laser element and a decrease in reliability can be prevented.

[0037] Implementation Method 3

[0038] Figure 9 This is a cross-sectional view showing the semiconductor laser element according to Embodiment 3. The difference from Embodiment 1 is that the getter layer 5 is formed in multiple layers within the third cladding layer 6. The total thickness of the plurality of getter layers 5 is 20 nm or more, but the thickness of each layer may be different.

[0039] Figure 10 This is a cross-sectional view illustrating the manufacturing process of the semiconductor laser element according to Embodiment 3. The multi-layered getter layer 5 prevents the diffusion of material residues used in the manufacturing process to the active layer 3. Therefore, variations in the characteristics of the semiconductor laser element and a decrease in reliability can be prevented.

[0040] The multiple getter layers 5 are compound semiconductors composed of three or more elements, and the composition and thickness of each getter layer 5 can be different. For compound semiconductors composed of three or more elements, the refractive index is changed by altering their composition. Since the laser is guided near the active layer 3, if a portion of the multiple getter layers 5 is disposed near the active layer 3, the light distribution of the laser can be altered by the influence of the refractive index of the getter layer 5. By changing the light distribution, the laser characteristics can be improved or it can be easily coupled to the fiber core. For example, by changing the light distribution, light output can be achieved at low power or the light output can be increased.

[0041] Implementation Method 4

[0042] Figure 11 This is a cross-sectional view showing the semiconductor laser element according to Embodiment 4. The difference from Embodiment 1 is that the getter layer 5 is a superlattice structure formed by alternating layers of two different compositions. Alternatively, some of the multiple getter layers 5 in Embodiment 3 may also be configured as a superlattice structure.

[0043] For example, a superlattice structure is formed by stacking 10 alternating layers of AlAs and AlInAs, each with a thickness of less than 10 nm. That is, both types of layers in the superlattice structure contain Al but have different compositions. However, it is also possible for one layer of the superlattice structure to contain Al while the other does not.

[0044] At the interface where different constituent materials are joined, band discontinuities are created, resulting in localized electric fields due to band bending. Since most impurities are charged particles, these localized electric fields enhance the impurity capture efficiency. The superlattice structure getter layer 5 has multiple interfaces that create band discontinuities, thus achieving a higher capture efficiency for charged particles such as hydrogen or fluoride ions compared to a single-layer getter layer 5.

[0045] Furthermore, if the lattice constant of the getter layer 5 differs from that of the semiconductor substrate 1, lattice relaxation and significant deterioration of crystallinity will occur if the single-layer getter layer 5 is grown above the critical thickness. On the other hand, for the superlattice structure getter layer 5, it is possible to grow it above the critical thickness by setting each layer below the critical thickness. Therefore, the impurity capture effect can be improved by increasing the thickness of the getter layer 5.

[0046] Furthermore, compressive strain occurs when epitaxially growing a material with a lattice constant greater than that of the substrate, while tensile strain occurs when growing a material with a small lattice constant. Therefore, by alternately growing materials with lattice constants greater and smaller than those of the semiconductor substrate 1 as the two layers of the superlattice structure, the overall strain of the getter layer 5 can be averaged (compensated). This allows the getter layer 5 to be grown to a thickness exceeding the critical film thickness. Moreover, even when both layers of the superlattice structure contain Al, as long as a quaternary mixed crystal containing Ga is used, it can be matched with the InP of the semiconductor substrate 1.

[0047] Explanation of reference numerals in the attached figures

[0048] 1…Semiconductor substrate; 2…First cladding layer; 3…Active layer; 4…Second cladding layer; 5…Getter layer; 6…Third cladding layer; 7…Diffraction grating; 11…Diffraction grating layer; 12…Insulating film.

Claims

1. A semiconductor laser element, characterized in that, have: First conductivity type semiconductor substrate; A first cladding layer of a first conductivity type, an active layer, and a second cladding layer of a second conductivity type are sequentially formed on the semiconductor substrate; and A getter layer made of Al-based material is formed seamlessly on top of the second coating layer.

2. The semiconductor laser element according to claim 1, characterized in that, It also has a diffraction grating formed on the getter layer.

3. The semiconductor laser element according to claim 2, characterized in that, It also has a third coating layer of a second conductive type formed on the getter layer. The diffraction grating is formed on the third coating layer.

4. The semiconductor laser element according to claim 2, characterized in that, The getter layer is in contact with the diffraction grating.

5. The semiconductor laser element according to any one of claims 1 to 4, characterized in that, The getter layer is formed by dividing the upper and lower layers into multiple layers.

6. The semiconductor laser element according to any one of claims 1 to 5, characterized in that, The getter layer has a superlattice structure.

7. The semiconductor laser element according to any one of claims 1 to 6, characterized in that, The thickness of the getter layer is 20 nm or more.

8. A method for manufacturing a semiconductor laser element, characterized in that, have: The process of epitaxially growing a first cladding layer of the first conductivity type, an active layer, a second cladding layer of the second conductivity type, a getter layer, and a diffraction grating layer on a semiconductor substrate of the first conductivity type in sequence. The process of forming an insulating film on the diffraction grating layer; The process of patterning the insulating film using fluorocarbon compounds as processing gases; as well as The process of forming a diffraction grating by using the patterned insulating film as a mask and dry etching the diffraction grating layer with methane or a halogen compound as a processing gas. The getter layer is formed seamlessly on the second coating layer and is made of an Al-based material.

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