Semiconductor laser element and manufacturing method thereof

Abstract

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

Description

Semiconductor laser element and its manufacturing method

[0001] The present disclosure relates to a semiconductor laser device and a method for manufacturing the same.

[0002] It has been proposed to provide a diffusion suppression layer in a semiconductor laser for suppressing the propagation of crystal defects or metal diffusion from the chip surface to the active layer (see, for example, Japanese Patent Application Laid-Open No. 2003-144998).

[0003] Japanese Patent No. 3785660

[0004] Semiconductor layers such as an upper cladding layer and a diffraction grating layer are formed on the active layer. Fluorine or hydrogen is introduced into the semiconductor layer when an insulating film is formed on the semiconductor layer using plasma CVD or the like. Furthermore, fluorine or hydrogen is introduced into the surface of the semiconductor layer by methane or a halogen compound used in processing the diffraction grating layer. The diffusion of residues from materials used in the manufacturing process of such semiconductor laser devices could not be prevented by conventional diffusion suppression layers. When the residues diffuse into the active layer, the characteristics of the semiconductor laser device change, resulting in a problem of reduced reliability.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its object is to provide a semiconductor laser element and a manufacturing method thereof that can prevent fluctuations in characteristics and deterioration in reliability.

[0006] The semiconductor laser device according to the present disclosure 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 a second conductivity type, which are formed in this order on the semiconductor substrate; and a getter layer made of an Al-based material and formed without any gaps on the second cladding layer.

[0007] In this disclosure, a getter layer made of an Al-based material is formed on the second cladding layer without any gaps. This prevents the diffusion of residues of materials used in the manufacturing process into the active layer. This prevents fluctuations in the characteristics of the semiconductor laser device and a decrease in reliability.

[0008] Fig. 1 is a cross-sectional view showing a semiconductor laser element according to a first embodiment. Fig. 2 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the first embodiment. Fig. 3 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the first embodiment. Fig. 4 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the first embodiment. Fig. 5 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the first embodiment. Fig. 6 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the first embodiment. Fig. 7 is a cross-sectional view showing a semiconductor laser element according to a second embodiment. Fig. 8 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the second embodiment. Fig. 9 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the third embodiment. Fig. 10 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the third embodiment. Fig. 11 is a cross-sectional view showing a manufacturing process for a semiconductor laser element according to the third embodiment. Fig. 12 is a cross-sectional view showing a semiconductor laser element according to a fourth embodiment.

[0009] A semiconductor laser device and a method for manufacturing the same according to an embodiment will be described with reference to the drawings. The same or corresponding components are designated by the same reference numerals, and repeated description may be omitted.

[0010] 1 is a cross-sectional view showing a semiconductor laser device according to a first embodiment. This semiconductor laser device is a DFB laser (distributed feedback semiconductor laser) that performs single-mode oscillation. Although not shown, an optical waveguide structure and a resonator structure using a reflecting mirror are formed so that the semiconductor laser functions.

[0011] A first conductivity type first cladding layer 2, an active layer 3, and a second conductivity type second cladding layer 4 are formed in this order on a first conductivity type semiconductor substrate 1. A second conductivity type getter layer 5 made of an Al-based material is formed on the second cladding layer 4. A second conductivity type third cladding layer 6 is formed on the getter layer 5. A second conductivity type diffraction grating 7 is formed on the third cladding layer 6. A second conductivity type semiconductor layer 8 embeds the diffraction grating 7. A top surface electrode 9 is formed on the semiconductor layer 8. A bottom surface electrode 10 is formed on the bottom surface of the semiconductor substrate 1. The diffraction grating 7 consists of a plurality of patterns formed periodically with gaps between them. Meanwhile, the getter layer 5 is formed without gaps over the entire top surface of the second cladding layer 4.

[0012] 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 made of, for example, InP. The getter layer 5 is made of an Al-containing mixed crystal layer such as AlGaInAs or AlAs. The diffraction grating 7 is made of a material that has a refractive index different from that of InP, such as a mixed crystal such as InGaAsP or AlGaInAs. An InP buffer layer may be provided between the semiconductor substrate 1 and the first cladding layer 2.

[0013] 2 to 6 are cross-sectional views showing the manufacturing process of the semiconductor laser device according to the first embodiment.

[0014] First, as shown in Fig. 2, 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 epitaxially grown in this order on a semiconductor substrate 1. An insulating film 12 is formed on the diffraction grating layer 11 by plasma CVD or the like. A photoresist 13 is applied onto the insulating film 12. Next, as shown in Fig. 3, a periodic pattern is formed in the photoresist 13 by photolithography.

[0015] The insulating film 12 is made of SiO ２ (silicon oxide) or SiN (silicon nitride), etc. Fluorine or hydrogen is introduced into the surface of the diffraction grating layer 11 by a fluorine compound gas used when cleaning the film-forming chamber of the CVD apparatus, or a material gas for the insulating film 12 such as silane or ammonia.

[0016] 4, the insulating film 12 is patterned using the photoresist 13 as a mask and fluorocarbon (a fluorinated hydrocarbon compound) as a processing gas. Fluorine or hydrogen is introduced from the fluorocarbon into the surface of the diffraction grating layer 11. The photoresist 13 is then removed with an organic solvent or the like.

[0017] 5, the patterned insulating film 12 is used as a mask, and the diffraction grating layer 11 is dry-etched using methane or a halogen compound as a processing gas to form the diffraction grating 7. Fluorine or hydrogen from the methane or halogen compound is introduced into the surface of the third cladding layer 6. Thereafter, the insulating film 12 is removed using hydrofluoric acid, buffered hydrofluoric acid, or the like.

[0018] 6, a semiconductor layer 8 is epitaxially grown and the diffraction grating 7 is embedded therein. Thereafter, a ridge forming step and steps for forming an upper electrode 9 and a lower electrode 10 are carried out to complete the semiconductor laser device.

[0019] As described above, residues of materials used in the manufacturing process are introduced onto the surface of the semiconductor layer. When these residues diffuse and reach the active layer 3, they form non-radiative recombination centers in the active layer 3, affecting the charge state within the active layer 3 and causing fluctuations in the characteristics of the semiconductor laser device.

[0020] In contrast to this, in this embodiment, the getter layer 5 made of an Al-based material is formed without any gaps on the second cladding layer 4. This makes it possible for the getter layer 5 to prevent residues of materials used in the manufacturing process from diffusing into the active layer 3. Therefore, it is possible to prevent fluctuations in the characteristics of the semiconductor laser device and a decrease in reliability.

[0021] It has been experimentally confirmed that this effect can be obtained if the thickness of the getter layer 5 is 20 nm or more. The getter layer 5 can also prevent point defects introduced into the surface of the semiconductor layer during processing or the like from diffusing into the active layer 3. The composition of the getter layer 5 preferably has a high mixed crystal ratio of Al, which has high reactivity with other elements.

[0022] If the getter layer 5 is epitaxially grown on the uneven surface, the crystallinity will be reduced in the uneven portions. Fluorine or hydrogen may diffuse into the active layer 3 from the portions of the getter layer 5 where the crystallinity is reduced. Therefore, it is preferable that the upper surface of the second cladding layer 4 on which the getter layer 5 is formed is flat.

[0023] 7 is a cross-sectional view showing a semiconductor laser device according to embodiment 2. The difference from embodiment 1 is that the third cladding layer 6 is absent and the getter layer 5 and the diffraction grating 7 are in contact with each other.

[0024] 8 is a cross-sectional view showing a manufacturing process of a semiconductor laser device according to the second embodiment. Using the patterned insulating film 12 as a mask, the diffraction grating layer 11 is dry-etched using methane or a halogen compound as a processing gas to form the diffraction grating 7. Fluorine or hydrogen is introduced into the surface of the getter layer 5 from the methane or halogen compound. However, the getter layer 5 can prevent these residues from diffusing into the active layer 3. This prevents fluctuations in the characteristics of the semiconductor laser device and a decrease in reliability.

[0025] 9 is a cross-sectional view showing a semiconductor laser device according to embodiment 3. The difference from embodiment 1 is that the getter layer 5 is formed in the third cladding layer 6 as a plurality of separate layers above and below. The total thickness of the plurality of getter layers 5 is 20 nm or more, but the thickness of each layer may be different.

[0026] 10 is a cross-sectional view showing a manufacturing process of a semiconductor laser device according to embodiment 3. The multiple getter layers 5 can prevent residues of materials used in the manufacturing process from diffusing into the active layer 3. This can prevent fluctuations in the characteristics of the semiconductor laser device and a decrease in reliability.

[0027] The multiple getter layers 5 are compound semiconductors composed of three or more elements, and the compositions and thicknesses of the multiple getter layers 5 may be different. The refractive index of a compound semiconductor composed of three or more elements can be changed by changing its composition. Since laser light is guided near the active layer 3, disposing some of the multiple getter layers 5 near the active layer 3 can change the light distribution of the laser light due to the influence of the refractive index of the getter layers 5. Changing the light distribution can improve laser characteristics or facilitate coupling with the core of an optical fiber. For example, changing the light distribution can enable light output at lower power or increase light output.

[0028] 11 is a cross-sectional view showing a semiconductor laser device according to a fourth embodiment. The difference from the first embodiment is that the getter layer 5 has a superlattice structure in which two layers with different compositions are alternately stacked. Note that some of the multiple getter layers 5 of the third embodiment may have a superlattice structure.

[0029] For example, a superlattice structure may be formed by stacking 10 alternating AlAs and AlInAs layers, each of which is 10 nm or less in thickness. That is, the two layers in the superlattice structure both contain Al but have different compositions. However, it is also possible for one layer in the superlattice structure to contain Al and the other not to contain Al.

[0030] At band discontinuities that occur at interfaces where materials of different compositions are bonded, local electric fields are generated due to band bending. Because impurities are often charged particles, this local electric field enhances the effect of capturing impurities. Because the getter layer 5 with a superlattice structure has many junction interfaces where band discontinuities occur, it is more effective at capturing charged particles such as hydrogen ions or fluorine ions than a single-layer getter layer 5.

[0031] Furthermore, if the material composition of the getter layer 5 differs from the lattice constant of the material of the semiconductor substrate 1, growing a single getter layer 5 beyond its critical thickness will cause lattice relaxation and significantly deteriorate its crystallinity. On the other hand, a getter layer 5 with a superlattice structure can be grown to a thickness greater than the critical thickness by setting each layer below its critical thickness. Therefore, increasing the thickness of the getter layer 5 can improve its impurity capture effect.

[0032] Furthermore, when a material with a larger lattice constant than the substrate is epitaxially grown, compressive strain is inherent, while when a material with a smaller lattice constant is grown, tensile strain is inherent. Therefore, by alternately growing materials with a larger and smaller lattice constant than the semiconductor substrate 1 as the two types of layers in the superlattice structure, the amount of strain throughout the getter layer 5 can be averaged (compensated). This also makes it possible to grow the getter layer 5 to a critical thickness or greater. Even if both types of layers in the superlattice structure contain Al, they can be matched with the InP of the semiconductor substrate 1 by making them quaternary mixed crystals containing Ga.

[0033] REFERENCE SIGNS LIST 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 device 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 a second conductivity type formed in that order on the semiconductor substrate; and a getter layer made of an Al-based material formed without any gaps on the second cladding layer.

2. The semiconductor laser device according to claim 1, further comprising a diffraction grating formed on said getter layer.

3. The semiconductor laser device according to claim 2, further comprising a third cladding layer of the second conductivity type formed on the getter layer, wherein the diffraction grating is formed on the third cladding layer.

4. The semiconductor laser device according to claim 2, wherein the getter layer and the diffraction grating are in contact with each other.

5. The semiconductor laser device according to any one of claims 1 to 4, wherein the getter layer is formed by dividing it into a plurality of upper and lower layers.

6. The semiconductor laser device according to any one of claims 1 to 5, wherein the getter layer has a superlattice structure.

7. The semiconductor laser device according to any one of claims 1 to 6, wherein the thickness of said getter layer is 20 nm or more.

8. A method for manufacturing a semiconductor laser element, comprising the steps of: epitaxially growing a first cladding layer of a first conductivity type, an active layer, a second cladding layer of a second conductivity type, a getter layer, and a diffraction grating layer in that order on a semiconductor substrate of a first conductivity type; forming an insulating film on the diffraction grating layer; patterning the insulating film using a fluorocarbon as a processing gas; and using the patterned insulating film as a mask, dry-etching the diffraction grating layer using methane or a halogen compound as a processing gas to form a diffraction grating, wherein the getter layer is formed without any gaps on the second cladding layer and is made of an Al-based material.

Images