Semiconductor laser

Abstract

The present invention comprises, on a substrate (101): an n-type layer (102) comprising an n-type compound semiconductor; a p-type layer (103) comprising a p-type compound semiconductor; and an active layer (104) having a multiple quantum well structure, comprising a compound semiconductor, and formed between the n-type layer (102) and the p-type layer (103), wherein the distortion of each quantum well layer (141) of the multiple quantum well structure constituting the active layer (104) is different.

Description

Semiconductor laser

[0001] The present invention relates to a semiconductor laser.

[0002] Many gases such as methane, nitrous oxide, carbon monoxide, and carbon dioxide have absorption with a narrow spectral width in a wavelength band of 1.6 μm to 2.5 μm corresponding to the vibration energy of molecules. For sensing of these gases, lasers in the long wavelength band on an InP substrate have been used (Non-Patent Document 1, Non-Patent Document 2).

[0003] Lasers for gas sensing applications are required to have 1. high-speed operation, 2. high optical output, and 3. a wide wavelength sweep range. For this reason, distributed feedback (Distributed Bragg Reflector: DBR) lasers have been used. The oscillation wavelength of the laser has been varied and wavelength swept by injecting current into the DBR region or by local heating with respect to the optical output (optical gain) from the active layer. Each gas has an absorption line specific to its type, and by using a laser with a wide wavelength sweepable wavelength range, sensing of many types of gases can be covered.

[0004] T. Shindo et al., "2.0-μm Wavelength Superstructure-Grating-(SSG-) Distributed Bragg Reflector Laser With Tuning Range of Over 50 nm", IEEE Photonics Technology Letters, vol. 33, no. 13, pp. 641-644, 2021.T. Sato et al., "InAs Quantum-well Distributed Feedback Lasers Emitting at 2.3 μm for Gas Sensing Applications", NTT Technical Review, vol. 7 no. 1, Special Feature: Light Source Technologies for Sensing Applications, 2009.

[0005] As mentioned earlier, to cover a wide range of gas sensing applications, it is crucial to use lasers with a broad wavelength range that can be swept. Currently, there is a demand for lasers with even broader wavelength ranges that can be swept to cover even more gas sensing applications.

[0006] This invention was made to solve the above-mentioned problems and aims to provide a laser with an even wider wavelength range that can be swept.

[0007] The semiconductor laser according to the present invention comprises an n-type layer made of an n-type compound semiconductor formed on a substrate, a p-type layer made of a p-type compound semiconductor formed on the substrate, an active layer of a multiple quantum well structure made of a compound semiconductor formed between the n-type layer and the p-type layer, and a resonator formed on the substrate, wherein the strain amount of the quantum well differs from layer to layer in the multiple quantum well structure.

[0008] As explained above, according to the present invention, since the amount of strain in the quantum well is made different for each layer, a laser with an even wider wavelength range that can be swept is provided.

[0009] Figure 1 is a cross-sectional view showing the configuration of a semiconductor laser according to an embodiment. Figure 2 is a characteristic diagram showing the calculation results of a simulation of the optical gain from the active layer 104 of the semiconductor laser according to the embodiment. Figure 3A is a cross-sectional view showing the state of the semiconductor laser in an intermediate step to explain the manufacturing method of the semiconductor laser according to the embodiment. Figure 3B is a cross-sectional view showing the state of the semiconductor laser in an intermediate step to explain the manufacturing method of the semiconductor laser according to the embodiment. Figure 3C is a cross-sectional view showing the state of the semiconductor laser in an intermediate step to explain the manufacturing method of the semiconductor laser according to the embodiment.

[0010] The semiconductor laser according to this embodiment will be described below with reference to Figure 1. This semiconductor laser first comprises an n-type layer 102 made of an n-type compound semiconductor and a p-type layer 103 made of a p-type compound semiconductor, both formed on a substrate 101. In this example, the n-type layer 102 is located on the substrate 101 side, and the p-type layer 103 is located above the n-type layer 102 when viewed from the substrate 101 side, but this is not the only configuration. The p-type layer 103 can be located on the substrate 101 side, and the n-type layer 102 can be located above the p-type layer 103 when viewed from the substrate 101 side.

[0011] Furthermore, this semiconductor laser includes an active layer 104 with a multiple quantum well structure made of a compound semiconductor formed between an n-type layer 102 and a p-type layer 103. The multiple quantum well structure is a structure in which quantum well layers 141 and barrier layers 142 are alternately stacked. In this embodiment, the multiple quantum well structure constituting the active layer 104 is such that the amount of strain in the quantum well layer 141 differs from layer to layer. For example, the multiple quantum well structure constituting the active layer 104 is such that the amount of strain in the quantum well layer 141 differs by 0.3% or more from layer to layer. The amount of strain in the quantum well layer 141 of the multiple quantum well structure constituting the active layer 104 can be in the range of 0.2% to 1.7%.

[0012] For example, the n-type layer 102 and the p-type layer 103 can function as cladding layers. In this example, a photo-confinement layer 105 made of a compound semiconductor is provided between the n-type layer 102 and the active layer 104. In addition, a photo-confinement layer 106 made of a compound semiconductor is provided between the p-type layer 103 and the active layer 104. These constitute a Separate Confined Heterostructure (SCH) structure. In this example, a semiconductor layer 107 is formed on the p-type layer 103. Note that Figure 1 shows the main part including the active layer 104, and resonator structures such as diffraction gratings necessary for functioning as a laser are omitted, but a resonator is formed on the substrate 101.

[0013] For example, the substrate 101 can be made of n-type InP, the n-type layer 102 can be made of n-type InGaAsP, and the photoconfinement layer 105 can be made of InGaAs. Also, for example, the active layer 104 can be made of any of InGaAs, InGaAsP, and InAs. Also, for example, the photoconfinement layer 106 can be made of InGaAs, the p-type layer 103 can be made of p-type InGaAsP, and the semiconductor layer 107 can be made of p-type InP.

[0014] According to the embodiment described above, since the amount of strain in the quantum well layer 141 differs from layer to layer, it is possible to effectively widen the full width at half maximum of the optical gain compared to the case where the amount of strain in a conventional quantum well is uniform. For example, the semiconductor laser according to the embodiment can have an oscillation wavelength of 1.6 μm or more and 2.5 μm or less. As a result, it becomes possible to detect a wider variety of gases.

[0015] Next, the calculation results of the simulation of the optical gain from the active layer 104 of the semiconductor laser according to the embodiment will be described. In the calculation, the thickness of the substrate 101 made of n-type InP was set to 120 μm, the thickness of the n-type layer 102 made of InGaAsP was set to 100 nm, and the thickness of the optical confinement layer 105 made of InGaAs was set to 30 nm.

[0016] Furthermore, the thickness of the photo-confinement layer 106, made of InGaAs, was set to 20 nm, the thickness of the p-type layer 103, made of InGaAsP, was set to 180 nm, and the thickness of the semiconductor layer 107 was set to 3.2 μm. The thickness of the quantum well layer 141 was set to 10 nm, and the thickness of the barrier layer 142 was set to 13 nm. The strain amount t of the barrier layer 142 was set to -1%. Here, a positive strain amount represents the strain in the compression direction, and a negative strain amount represents the strain in the tension direction.

[0017] The calculation results are shown in Figure 2. In the conventional structure (b), the strain amount c of the quantum well layer of the active layer was uniformly set to 1.7%. In contrast, in the embodiment (a), the strain amounts C1 to C6 of the quantum well layer 141 were set to 0.2%, 0.5%, 0.8%, 1.1%, 1.4%, and 1.7% from the substrate 101 side, with each layer having a different value. Furthermore, in the optical gain calculation, the current value for the active layer was normalized so that the current density was the same in the conventional structure and the embodiment.

[0018] As shown in Figure 2(b), the full width at half maximum (FWHM) in the conventional structure is FWHM = 479 nm. On the other hand, as shown in Figure 2(a), in the embodiment, the FWHM is FWHM = 745 nm.

[0019] In this embodiment, the amount of strain in the quantum well layer 141 differs from layer to layer. As a result, the optical gain peak wavelength corresponding to the energy gap between the valence band and the conductor differs from layer to layer (Reference 1). Consequently, the overall optical gain of the active layer 104 is a superposition of optical gains with different wavelengths, enabling an expansion of the full width at half maximum.

[0020] Furthermore, if the strain amount C differs from layer to layer by less than 0.3%, the change in the optical gain peak wavelength from layer to layer is small, and the effect of widening the full width at half maximum is limited. Also, if the strain amount c is less than 0.2%, the strain amount in the quantum well layer 141 approaches ~0%, making it difficult to secure an effective optical gain. The strain amount of the quantum well in a typical strained quantum well laser is 1 to 1.5% (Reference 1). Also, if the strain amount c is greater than 1.7%, it approaches the critical thickness of the crystal, making it difficult to ensure crystallinity and making it difficult to obtain an effective optical gain.

[0021] The above calculation results demonstrate that the full width at half maximum of the optical gain can be expanded by making the amount of strain in each quantum well layer 141 different. In the above example, the amount of strain in each quantum well layer 141 was increased monotonically from the substrate 101 side, but this is not the only method. It is sufficient that the amount of strain in each quantum well layer 141 constituting the active layer 104 of the multiple quantum well structure is different for each layer; it is not necessary to change it monotonically in the stacking direction.

[0022] Next, a method for manufacturing a semiconductor laser according to an embodiment will be briefly described. First, as shown in Figure 3A, an n-type layer 102 is formed on a substrate 101 by growing InGaAsP to a thickness of about 100 nm. Next, as shown in Figure 3B, a light confinement layer 105 is formed on the n-type layer 102 by growing InGaAs to a thickness of about 30 nm.

[0023] Next, as shown in Figure 3C, the barrier layer 142 and the quantum well layer 141 are alternately grown to form the active layer 104. The thickness of the barrier layer 142 is 13 nm, and the thickness of the quantum well layer 141 is 10 nm. The strain t of the barrier layer 142 is -1%. The strain C of the quantum well layer 141 differs by 0.3% from layer to layer, with the smallest strain being 0.2% and the largest strain being 1.7%. The number of quantum well layers 141 is 3 or more.

[0024] Next, InGaAs is grown on the active layer 104 to a thickness of approximately 20 nm to form a photoconfinement layer 106. Then, InGaAsP is grown on the photoconfinement layer 106 to a thickness of approximately 180 nm to form a p-type layer 103. Next, InP is grown to a thickness of approximately 3.2 μm to form a semiconductor layer 107. As a result, the main part of the semiconductor laser shown in Figure 1 is formed. Note that when the number of quantum well layers n=2, the overall optical gain peak shape of the active layer becomes unstable (two peaks exist), so it is preferable to set n≧3.

[0025] Furthermore, the light confinement layer 105 and the light confinement layer 106 can be composed of InGaAs, InGaAsP, or InAs, not limited to InGaAs. Also, the active layer 104 can be composed of InGaAs, InGaAsP, or InAs, not limited to InGaAs.

[0026] As described above, according to the embodiment, since the amount of strain in the quantum well is made different for each layer, it becomes possible to provide a laser with an even wider wavelength range that can be swept.

[0027] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented within the technical concept of the present invention by those with ordinary skill in the art.

[0028] [Reference 1] Ken Kamijo and Hideaki Horikawa, "Performance Improvement of Semiconductor Lasers by Strained Quantum Well Structure," Applied Physics, Vol. 62, No. 2, pp. 134-138, 1993. [Reference 2] Noriyuki Yokouchi, "Basic Course: Fundamentals of Optical Semiconductor Technology - Strained Quantum Well Optical Semiconductors," Applied Physics, Vol. 74, No. 9, pp. 1238-1241, 2005.

[0029] 101...Substrate, 102...n-type layer, 103...p-type layer, 104...active layer, 105...photoconfinement layer, 106...photoconfinement layer, 107...semiconductor layer, 141...quantum well layer, 142...barrier layer.

Claims

1. A semiconductor laser comprising: an n-type layer made of an n-type compound semiconductor formed on a substrate; a p-type layer made of a p-type compound semiconductor formed on the substrate; an active layer of a multiple quantum well structure made of a compound semiconductor formed between the n-type layer and the p-type layer; and a resonator formed on the substrate, wherein the multiple quantum well structure has a different amount of strain in each layer of the quantum well.

2. The semiconductor laser according to claim 1, wherein the multiple quantum well structure is a semiconductor laser in which the amount of strain of the quantum well differs by 0.3% or more from layer to layer.

3. The semiconductor laser according to claim 1, wherein the multiple quantum well structure is a semiconductor laser in which the amount of strain of the quantum wells is in the range of 0.2% to 1.7%.

4. A semiconductor laser according to claim 1, wherein the active layer is composed of any one of InGaAs, InGaAsP, and InAs.

5. A semiconductor laser according to any one of claims 1 to 4, wherein the oscillation wavelength is 1.6 μm or more and 2.5 μm or less.

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