Quantum cascade laser and method for manufacturing quantum cascade laser

Abstract

The present disclosure aims to provide a method for manufacturing an optical device having a reflective film that can be formed using a single film formation apparatus. The optical device of the present disclosure includes an optical element and a reflective film provided on an end face of the optical element that faces the light guide direction. The reflective film has a first layer film that transmits light and is provided on the end face of the optical element, and a second layer film that reflects light and is provided on the first layer film. The first layer film is a metal oxide or nitride, and the second layer film is the metal.

Description

[Technical Field]

[0001] The present disclosure provides a method for manufacturing a reflective film-equipped quantum cascade laser and a manufacturing method thereof. [Background technology]

[0002] Non-Patent Document 1 discloses a quantum cascade laser with a high-reflectivity (HR) coating to reduce loss in the cavity mirror. SiO2, Ti to improve the adhesion of the Au, and Au are layered in this order on the rear facet of the laser. Au ensures high reflectivity, and the insertion of SiO2 provides insulation between the laser body and the Ti / Au layer.

[0003] The formula for calculating the reflectance of a multilayer film is disclosed in Patent Document 1.

[0004] The complex refractive indices of Al2O3, TiO2 and AlN at a wavelength of 9 μm are disclosed in Non-Patent Document 2.

[0005] The complex refractive indices of Al, Ti and Au at a wavelength of 9 μm are disclosed in Non-Patent Document 3.

[0006] The complex refractive index of Ta2O5 at a wavelength of 9 μm is disclosed in Non-Patent Document 4.

[0007] The complex refractive index of Ta at a wavelength of 9 μm is disclosed in Non-Patent Document 5. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-219568 [Non-patent literature]

[0009] [Non-Patent Document 1] JS Yu et. al., “High-performance continuous-wave operation of λ~4.6μm quantum-cascade lasers above room temperature”, IEEE J. Quantum Electron., vol. 44, no. 8, pp. 747-754, 2008. [Non-patent document 2] J. Kischkat et. al., “Mid-infrared optical properties of thin films of aluminum oxide, titanium dioxide, silicon dioxide, aluminum nitride, and silicon nitride”, Appl. Opt., vol. 51, no. 28, pp. 6789-6798, 2012. [Non-patent document 3] AD Rakic ​​et. al., “Optical properties of metallic films for vertical-cavity optoelectronic devices”, Appl. Opt., vol. 37, no. 22, pp. 5271-5283, 1998. [Non-patent document 4] E. Franke et. al., “Dielectric function of amorphous tantalum oxide from the far infrared to the deep ultraviolet spectral region measured by spectroscopic ellipsometry”, J. Appl. Phys., vol. 88, no. 9, pp. 5166-5174, 2000. [Non-Patent Document 5] MA Ordal et. al., “Optical properties of Al, Fe, Ti, W, and Mo at submillimeter wavelengths”, Appl. Opt., vol. 27, no. 6, pp. 1203-1209, 1988. Summary of the Invention [Problem to be solved by the invention]

[0010] In the method of Non-Patent Document 1, SiO2 is deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition) and Ti / Au is deposited by EB (Electron Beam Evaporation), so two deposition devices are required to produce the reflective film.

[0011] In order to solve the above problem, the present disclosure provides a method for forming a reflective film using a single film forming apparatus. quantum cascade laser The purpose is to provide the following.

[0012] In order to solve the above problem, the present disclosure provides a method for forming a reflective film using a single film forming apparatus. quantum cascade laser The object of the present invention is to provide a method for producing the above-mentioned compound. [Means for solving the problem]

[0013] of the present disclosure 1st The aspect is an optical element and a light guide element facing the optical element in the waveguide direction; After a reflective film provided on the end surface, the reflective film includes a first layer film that is provided on the end face of the optical element and transmits the light, and a second layer film that is provided on the first layer film and reflects the light, The first layer film is Ta Oxidation of With things can be, The second layer film is provided in direct contact with the first layer film. Ta's It is a single layer film, It is preferable that the laser be a quantum cascade laser that oscillates mid-infrared light with a wavelength of 3 μm or more and 24 μm or less. The second aspect is as follows: an optical element; and a reflective film provided on a rear end surface of the optical element facing a waveguiding direction of light, the reflective film includes a first layer film that is provided on the end face of the optical element and transmits the light, and a second layer film that is provided on the first layer film and reflects the light, the first layer film is a metal oxide or nitride, the second layer film is a single layer film made of the metal provided in direct contact with the first layer film, and further includes a third layer film provided on the second layer film; the third layer film is an oxide or nitride of the metal, a combination of the first layer film, the second layer film, and the third layer film is Ta oxide, Ta, Ta oxide or Al oxide, Al, Al oxide or Al nitride, Al, Al nitride or Al oxide, Al, Al nitride or Al nitride, or Al, Al oxide; It is preferable that the laser be a quantum cascade laser that oscillates mid-infrared light with a wavelength of 3 μm or more and 24 μm or less. [Effects of the Invention]

[0014] of the present disclosure quantum cascade laser In the case of the above, a first layer film and a second layer film, which is the outermost surface, are laminated in this order on the rear end face of the semiconductor laminate. The first layer film is a metal oxide or nitride, and the second layer film is the metal itself. A reactive sputtering system is used to manufacture optical devices. When the first layer film is a metal oxide, oxygen is used as the raw material gas, and when the first layer film is a metal nitride, nitrogen is used as the raw material gas. The first layer film can be formed by supplying the raw material gas to the sample chamber in the reactive sputtering system and then sputtering a sputtering target of the metal. The second layer film can be formed by stopping the supply of raw material gas and then sputtering a sputtering target. This allows for the production of a reflective film that can be formed using a single film formation system. quantum cascade laser and a method for producing the same. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a perspective view of an optical device according to a first embodiment. [Figure 2] 2 is a cross-sectional view taken along the line ya-yb in FIG. 1. [Figure 3] FIG. 2 is an enlarged cross-sectional view of a reflective film according to the first embodiment. [Figure 4]10 shows the results of calculating the reflectance of the reflective film according to the first embodiment as a function of the film thickness of the second layer film. [Figure 5] 10 shows the results of calculating the reflectance of the reflective film according to the comparative example as a function of the film thickness of the third layer film. [Figure 6] 5A to 5C are diagrams illustrating a method for manufacturing the optical device according to the first embodiment. [Figure 7] FIG. 2 is a top view of a sample according to the first embodiment. [Figure 8] 8 is a cross-sectional view taken along the line xa-xb in FIG. 7. [Figure 9] 10 shows the results of calculating the reflectance of the reflective film according to the second embodiment as a function of the film thickness of the second layer film. [Figure 10] 10 shows the results of calculating the reflectance of the reflective film according to the third embodiment as a function of the film thickness of the second layer film. [Figure 11] 10 shows the results of calculating the reflectance of the reflective film according to the fourth embodiment as a function of the film thickness of the second layer film. [Figure 12] 10 shows the results of calculating the reflectance of the reflective film according to the fifth embodiment as a function of the film thickness of the second layer film. [Figure 13] 10 shows the results of calculating the reflectance of the reflective film according to the sixth embodiment as a function of the film thickness of the second layer film. [Figure 14] FIG. 13 is an enlarged cross-sectional view of a reflective film according to a seventh embodiment. [Figure 15] 13 shows the results of calculating the reflectance of the reflective film according to the seventh embodiment as a function of the film thickness of the second layer film. [Figure 16] FIG. 13 is an enlarged cross-sectional view of a reflective film according to an eighth embodiment. [Figure 17] 13 shows the results of calculating the reflectance of the reflective film according to the eighth embodiment as a function of the film thickness of the second layer film. DETAILED DESCRIPTION OF THE INVENTION

[0016] Embodiments of the present disclosure will be described with reference to the drawings. The same or corresponding components will be designated by the same reference numerals, and repeated description may be omitted. Furthermore, the materials, dimensions, etc. of the semiconductor layer and reflective film in the following description are merely examples and may differ from the actual ones.

[0017] Embodiment 1 1 is a perspective view of an optical device 100 according to a first embodiment. Here, as an example, a case will be described in which the optical device 100 is a quantum cascade laser (QCL). However, the optical device 100 of the present disclosure is not limited to a quantum cascade laser, and may be a semiconductor laser device such as a laser diode (LD).

[0018] In this figure, the guiding direction of the laser light is the positive y-axis direction, and the thickness direction of the semiconductor layer is the z-axis direction.

[0019] The optical device 100 has an n-type InP buffer layer 3 (thickness 1.0 μm), an n-type GaInAs optical confinement layer 4 (thickness 230 nm), a core region 5 consisting of 30 to 40 stages, an n-type GaInAs optical confinement layer 6 (thickness 230 nm), an n-type InP cladding layer 7 (thickness 3.5 μm), and an n-type GaInAs contact layer 8 (thickness 500 nm), which are stacked in this order on an n-type InP substrate 2.

[0020] Furthermore, on the surface of the buffer layer 3, a current blocking layer 10 is provided on both sides of the layers from the optical confinement layer 4 to the contact layer 8.

[0021] The core region 5 has a multi-quantum well (MQW) structure in which 30 to 40 stages are stacked, each of which is made up of alternating GaInAs quantum well layers and AlInAs barrier layers, and oscillates mid-infrared light, for example, at about 3 to 24 μm, due to intersubband transitions in the quantum well structure.

[0022] The number of stages constituting the core region 5 is not limited to 30 to 40, but may be adjusted appropriately depending on the desired laser characteristics.

[0023] An n-type second electrode 9 is provided on the contact layer 8. An n-type first electrode 1 is provided on the back surface of the substrate 2. When the optical device 100 is in operation, the first electrode 1 is negatively biased and the second electrode 9 is positively biased. When the optical device 100 is in operation, a voltage is applied to the first electrode 1 and the second electrode 9, which in turn injects a current into the optical device 100 and causes laser oscillation.

[0024] Hereinafter, the substrate 2, the buffer layer 3 to the contact layer 8 stacked in this order on the surface of the substrate 2, the current blocking layer 10, the first electrode 1, and the second electrode 9 will be collectively referred to as the semiconductor laminate 12. However, the semiconductor laminate 12 in the optical device 100 of the present disclosure only needs to include an active layer that generates and guides laser light. The active layer in a quantum cascade laser is the core region 5, but the active layer does not necessarily have to have a quantum cascade laser structure.

[0025] Fig. 2 is a cross-sectional view taken along the line ya-yb in Fig. 1. A reflective film 11 is provided on the rear end face of the semiconductor laminate 12. The rear end face here means the end face opposite to the laser light emission face.

[0026] 3 is an enlarged cross-sectional view of the reflective film 11 according to the first embodiment. The reflective film 11 includes a first layer 13, a second layer 14, and a third layer 15 stacked in this order on the rear facet of the semiconductor laminate 12. In this embodiment, the first layer 13 is made of Ta2O5 and has a thickness of 450 nm, the second layer 14 is made of Ta, and the third layer 15 is made of Ta2O5 and has a thickness of 100 nm.

[0027] The first layer 13 provides insulation between the semiconductor laminate 12 and the second layer 14, which is made of metal. The first layer 13 is an electrical insulator and is preferably made of a material that has high transmittance to the wavelength of the laser light, such as a metal oxide. In this embodiment, Ta2O5 is used as the first layer 13.

[0028] The second layer 14 is responsible for reflecting the laser light. The second layer 14 is a simple metal contained in the metal oxide of the first layer 13. In this embodiment, Ta is used as the second layer 14.

[0029] The third layer 15 serves to prevent solder from creeping up when the optical device 100 is bonded to the submount. Generally, when bonding the optical device 100 to the submount, the surface of the submount is bonded to the backside of the substrate 2 using solder such as AuSn provided on the surface of the submount. In this case, if the top surface of the reflective film 11 is made of a metal contained in the solder (Au in the case of AuSn solder), the wettability of the solder to the top surface is high. This causes the problem of the solder creeping up the surface of the reflective film 11.

[0030] To prevent this problem, the third layer 15 is preferably made of a material that does not contain the metal of the solder used to bond the optical device 100 to the submount (Au in the case of AuSn solder). For example, by using a metal oxide as the third layer 15, the wettability of the solder can be reduced, preventing creep-up. In this embodiment, the third layer 15 is made of Ta2O5, the same as the first layer 13. Note that the third layer 15 may be made of a metal as long as it is not a metal contained in the solder.

[0031] <Reflectance calculation results> The following describes the results of calculating the reflectance of the reflective film 11. In the calculation, the effective refractive index of the semiconductor laminate 12 is set to nc. The complex refractive index of the first layer 13 is set to n1. * Similarly, the complex refractive index of the second layer 14 is n2 * , the film thickness is d2, and the complex refractive index of the third layer film 15 is n3 * The film thickness was d3.

[0032] 4 shows the results of calculating the reflectance of the reflective film 11 according to the first embodiment as a function of the film thickness d2 of the second layer film 14. As an example, the reflectance at a wavelength of 9 μm was calculated. Here, the effective refractive index of the semiconductor laminate 12 at a wavelength of 9 μm was set to nc=3.15452. Also, based on Non-Patent Document 4, the complex refractive index of Ta2O5 at a wavelength of 9 μm was set to n1 * =n3 * = 1.50333 + i0.00000, where i is the imaginary unit. Also, based on Non-Patent Document 5, the complex refractive index of Ta at a wavelength of 9 μm is set to n2 * = 10.68374 + i48.47897. Note that there is a relationship between the complex dielectric constant and the complex refractive index described in Non-Patent Document 3.

[0033] In addition, in calculating the reflectance, formula (1) which is a formula for calculating the reflectance R of a multilayer film based on Patent Document 1 was used.

[0034]

number

[0035] Furthermore, * is the complex amplitude reflectance of the multilayer film determined from the characteristic matrix of the multilayer film.

[0036] 4, when the Ta film thickness d2 is 50 nm or more, the reflectance tends to saturate, reaching 96% or more. This reflectance is higher than the reflectance calculated based on Non-Patent Document 1, which will be described later, and indicates that the reflective film 11 of the present disclosure has excellent reflective properties. Furthermore, from the reflectance calculation results in FIG. 4, it can be said that a film thickness d2 of 50 nm or more for the second layer film 14 of the present disclosure is particularly suitable.

[0037] Comparative Example Below, as comparative examples, the results of calculating the reflectance when (1) there is no reflective film 11 and (2) the reflective film 11 of Non-Patent Document 1 is assumed will be described.

[0038] (1) Reflectance without the reflective film 11 The reflectance R0 in the absence of the reflective film 11 can be calculated using the following formula (2).

[0039]

number

[0040] When the effective refractive index of the semiconductor laminate 12 is nc=3.15452, R0 is approximately 26.9%.

[0041] (2) Reflectance when the reflective film 11 of Non-Patent Document 1 is assumed In the reflective film 11 of the comparative example, the first layer 13 was made of Al2O3 with a thickness of 450 nm, the second layer 14 was made of Ti with a thickness of 15 nm, and the third layer 15 was made of Au.

[0042] Here, the first layer 13 in Non-Patent Document 1 is made of SiO2, but in this comparative example, the first layer 13 is made of Al2O3. In Non-Patent Document 1, the wavelength of the laser light is 4.6 μm, and it can be said that SiO2 is suitable for this wavelength. However, when targeting a wavelength of 9 μm as in this embodiment, Al2O3, which has lower light absorption than SiO2, is more suitable.

[0043] 5 shows the results of calculating the reflectance of the reflective film 11 according to the comparative example as a function of the film thickness d3 of the third layer 15. In the calculation, the complex refractive index of Al2O3 at a wavelength of 9 μm is set as n1 * = 1.13309 + i0.08571. Based on Non-Patent Document 3, the complex refractive index of Ti at a wavelength of 9 μm is set to n2 * = 6.46976 + i16.50695. Furthermore, based on Non-Patent Document 3, the complex refractive index of Au at a wavelength of 9 μm is set to n3 * =9.42171+i55.49599.

[0044] 5, the reflectance of the comparative example saturates in the region where the film thickness d3 is 50 nm or more, but is at most about 95.5%. Although the reflectance of the comparative example is higher than the reflectance R0 in the absence of the reflective film 11, it can be said that it does not reach the reflectance of the reflective film 11 of the present disclosure.

[0045] <Method for manufacturing semiconductor laser device> 6 is a diagram showing a method for manufacturing the optical device 100 according to embodiment 1. The reflective film 11 of the optical device 100 of the present disclosure can be formed by a reactive sputtering apparatus 200.

[0046] Here, an example will be described in which the reactive sputtering apparatus 200 is an electron cyclotron resonance (ECR) plasma deposition apparatus, but the plasma generation method is not limited thereto.

[0047] In the plasma generation chamber 21, a plasma flow 26 is generated by electron cyclotron resonance discharge of microwaves with respect to the gas introduced from the first gas introduction system 32. The generated plasma flow 26 is introduced into the sample chamber 22 through the plasma extraction window 25. In the sample chamber 22, a sputtering target 36 is disposed near the plasma extraction window 25. The sputtering target 36 is sputtered by the plasma flow 26 introduced into the sample chamber 22, generating ions.

[0048] The material of the sputtering target 36 is a metal that is commonly used for the first layer film 13, the second layer film 14, and the third layer film 15. In this embodiment, Ta is used as the sputtering target 36.

[0049] In the sample chamber 22, a sample 27 is placed on a sample stage 28. A plasma flow 26 that incorporates ions generated from a target 36 is incident on the sample 27, thereby forming a thin film containing the material of the target 36 on the surface of the sample 27.

[0050] The reactive sputtering apparatus 200 has two gas introduction systems: a first gas introduction system 32 and a second gas introduction system 33. The first gas introduction system 32 is used to supply a gas for plasma generation to the plasma generation chamber 21. The inert gas Ar is particularly suitable as the gas for plasma generation, but other gases may also be used. The second gas introduction system 33 is used to supply a raw material gas for the reflective film 11 to the sample chamber 22. The raw material gas is O2 for generating metal oxides for the first layer film 13 and the third layer film 15.

[0051] FIG. 7 is a top view of sample 27 according to embodiment 1, and FIG. 8 is a cross-sectional view taken along the xa-xb line in FIG. 7. A plurality of bar-shaped semiconductor laminates 12 with their rear end surfaces facing upward (in the z direction) are placed on the base of a vice-like fixing device 44. Dummy bars 42 made of silicon or stainless steel (SUS) are placed between each semiconductor laminate 12 to prevent contact between the semiconductor laminates 12. The alternatingly arranged semiconductor laminates 12 and dummy bars 42 are sandwiched from both sides by pressure plates 43. The semiconductor laminates 12 and dummy bars 42 are fastened to each other via the pressure plates 43 by screws 45 of the fixing device 44.

[0052] In fabricating the sample 27, first, a pressure plate 43 is placed at one end of the base of a fixing device 44, and then dummy bars 42 and semiconductor stacks 12 with their rear end faces facing upward are alternately arranged next to the pressure plate 43. Then, a pressure plate 43 is placed at the end of the alternately arranged semiconductor stacks 12 and dummy bars 42. Then, the semiconductor stacks 12 and dummy bars 42 are fastened together with screws 45 of the fixing device 44 via the pressure plate 43, thereby completing the sample 27.

[0053] In fabricating the optical device 100 using the reactive sputtering apparatus 200, first, a sample 27 is placed on a sample stage 28. Next, Ar is supplied from a first gas introduction system 32, and O is supplied from a second gas introduction system 33. In this state, Ta, which is a sputtering target 36, is sputtered to form a TaO film, which is a first layer film 13, on the rear end face of the semiconductor laminate 12.

[0054] Furthermore, while continuing to supply Ar from first gas introduction system 32, the supply of O is stopped from second gas introduction system 33. In this state, Ta, which is sputtering target 36, is sputtered to deposit Ta, which is second layer film 14, on first layer film 13.

[0055] Furthermore, the supply of O2 from the second gas introduction system 33 is resumed. In this state, Ta, which is the sputtering target 36, is sputtered to form a Ta2O5 film, which is the third layer film 15, on the second layer film 14. This makes it possible to manufacture an optical device 100 in which the reflective film 11 is provided on the rear end face of the semiconductor laminate 12.

[0056] As described above, the optical device 100 of this embodiment has the first layer 13, the second layer 14, and the third layer 15, which is the outermost layer, stacked in this order on the rear end face of the semiconductor laminate 12. The first layer 13 and the third layer 15 are metal oxides, and the second layer 14 is a simple metal contained in the metal oxides of the first layer 13 and the third layer 15.

[0057] In addition, in manufacturing the optical device 100 of this embodiment, the metal commonly used for the first layer film 13, the second layer film 14, and the third layer film 15 is used as the material for the sputtering target 36. Furthermore, O2 is used as the raw material gas for generating the oxide of the metal. The first layer film 13 and the third layer film 15, which are metal oxides, can be formed in the reactive sputtering apparatus 200 by supplying the raw material gas to the sample chamber 22 and then sputtering the sputtering target 36. Furthermore, the second layer film 14 can be formed by sputtering the sputtering target 36 after stopping the supply of the raw material gas. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0058] <Variations> The end face on which the reflective film 11 is provided does not necessarily have to be the rear end face, but may be the laser light output face. That is, it is sufficient that the reflective film 11 is provided on at least one of the end faces of the semiconductor laminate 12 that face the waveguide direction of the laser light. This is common to all the embodiments.

[0059] Embodiment 2 In this embodiment, a case will be described in which the first layer 13 and the third layer 15 are made of a metal oxide, and the second layer 14 is made of the metal itself in the reflective film 11, where the metal is Al. The following describes changes from the first embodiment.

[0060] In this embodiment, the first layer 13 is made of Al2O3 and has a thickness of 300 nm, the second layer 14 is made of Al, and the third layer 15 is made of Al2O3 and has a thickness of 100 nm.

[0061] 9 shows the results of calculating the reflectance of the reflective film 11 according to the second embodiment as a function of the film thickness d2 of the second layer 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in the first embodiment. Also, based on Non-Patent Document 2, the complex refractive index of Al2O3 at a wavelength of 9 μm was calculated as n1 * =n3 * = 1.13309 + i0.08571. Based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is set to n2 * =19.66369+i77.26043.

[0062] As can be seen from the figure, the reflectance tends to saturate and reaches 96% or more in the region where the film thickness d2 is 40 nm or more. In other words, this embodiment also achieves a high reflectance similar to that of embodiment 1. Furthermore, from the figure, it can be said that a film thickness d2 of 40 nm or more of the second layer film 14 in this embodiment is particularly suitable.

[0063] In addition, the third layer 15 of this embodiment is made of Al2O3, a material that does not contain Au, which prevents the solder from creeping up when AuSn solder is used to bond the optical device 100 to the submount.

[0064] <Method for manufacturing semiconductor laser device> In the manufacture of the optical device 100 of this embodiment, Al, which is commonly used for the first layer film 13, the second layer film 14, and the third layer film 15, is used as the material of the sputtering target 36. O2 is used as the raw material gas for generating an oxide of Al.

[0065] In fabricating the optical device 100, first, a sample 27 of the semiconductor laminate 12 is placed on the sample stage 28 of the reactive sputtering apparatus 200. Next, Ar is supplied from the first gas introduction system 32, and O2 is supplied from the second gas introduction system 33. In this state, an Al sputtering target 36 is sputtered to form a first layer film 13 of AlO on the rear face of the semiconductor laminate 12. Then, the supply of O2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to form a second layer film 14 of Al on the first layer film 13. Then, the supply of O2 from the second gas introduction system 33 is resumed, and the sputtering target 36 is sputtered to form a third layer film 15 of AlO on the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0066] Embodiment 3 In this embodiment, a case will be described in which the first layer 13 and the third layer 15 are made of a metal oxide, and the second layer 14 is made of the metal itself in the reflective film 11, and the metal is Ti. The following describes changes from the first embodiment.

[0067] In this embodiment, the first layer 13 is made of TiO2 and has a thickness of 500 nm, the second layer 14 is made of Ti, and the third layer 15 is made of TiO2 and has a thickness of 100 nm.

[0068] 10 shows the results of calculating the reflectance of the reflective film 11 according to the third embodiment as a function of the film thickness d2 of the second layer 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in the first embodiment. Based on Non-Patent Document 2, the complex refractive index of TiO2 at a wavelength of 9 μm was calculated as n1* =n3 * = 1.64865 + i0.06818. Based on Non-Patent Document 3, the complex refractive index of Ti at a wavelength of 9 μm is set to n2 * =6.46976+i16.50695.

[0069] As can be seen from the figure, the reflectance tends to saturate and reaches 80% or more when the film thickness d2 is 110 nm or more. In other words, a high reflectance can be obtained in this embodiment as well. Furthermore, the figure indicates that a film thickness d2 of 110 nm or more for the second layer 14 in this embodiment is particularly suitable.

[0070] In addition, the third layer 15 of this embodiment is made of TiO2, a material that does not contain Au, so when the solder used to bond the optical device 100 to the submount is AuSn solder, it is possible to prevent the solder from creeping up.

[0071] <Method for manufacturing semiconductor laser device> In the manufacture of the optical device 100 of this embodiment, Ti, which is commonly used for the first layer film 13, the second layer film 14, and the third layer film 15, is used as the material of the sputtering target 36. O2 is used as the raw material gas for generating an oxide of Ti.

[0072] In fabricating the optical device 100, first, a sample 27 of the semiconductor laminate 12 is placed on the sample stage 28 of the reactive sputtering apparatus 200. Next, Ar is supplied from the first gas introduction system 32, and O2 is supplied from the second gas introduction system 33. In this state, a Ti sputtering target 36 is sputtered to form a first layer film 13 of TiO2 on the rear facet of the semiconductor laminate 12. Next, the supply of O2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to form a second layer film 14 of Ti on the first layer film 13. Next, the supply of O2 from the second gas introduction system 33 is restarted, and the sputtering target 36 is sputtered to form a third layer film 15 of TiO2 on the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0073] Embodiment 4 In the first to third embodiments, the first layer 13 and the third layer 15 are described as being made of a metal oxide. However, the first layer 13 and the third layer 15 may be made of a metal nitride.

[0074] In this embodiment, the first layer 13 is made of AlN and has a thickness of 450 nm, the second layer 14 is made of Al, and the third layer 15 is made of AlN and has a thickness of 100 nm.

[0075] 11 shows the results of calculating the reflectance of the reflective film 11 according to the fourth embodiment as a function of the film thickness d2 of the second layer 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in the first embodiment. Also, based on Non-Patent Document 2, the complex refractive index of AlN at a wavelength of 9 μm was calculated as n1 * =n3 * = 1.19630 + i0.04745. Based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is set to n2 * =19.66369+i77.26043.

[0076] As can be seen from the figure, in the region where the film thickness d2 is 30 nm or more, the reflectance reaches approximately 96%, which is as high as that of embodiment 1. Furthermore, it can be said from the figure that the film thickness d2 of second layer film 14 in this embodiment is particularly preferably 30 nm or more.

[0077] Furthermore, the third layer 15 of this embodiment is made of AlN, a material that does not contain Au, so that when AuSn solder is used to bond the optical device 100 to the submount, the solder can be prevented from creeping up.

[0078] <Method for manufacturing semiconductor laser device> In the manufacture of the optical device 100 of this embodiment, Al, which is commonly used for the first layer film 13, the second layer film 14, and the third layer film 15, is used as the material of the sputtering target 36. N2 is used as the raw material gas for generating Al nitride.

[0079] In fabricating the optical device 100, first, a sample 27 of the semiconductor laminate 12 is placed on the sample stage 28 of the reactive sputtering apparatus 200. Next, Ar is supplied from the first gas introduction system 32, and N2 is supplied from the second gas introduction system 33. In this state, an Al sputtering target 36 is sputtered to form an AlN film as the first layer film 13 on the rear face of the semiconductor laminate 12. Next, the supply of N2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to form an Al film as the second layer film 14 on the first layer film 13. Next, the supply of N2 from the second gas introduction system 33 is resumed, and the sputtering target 36 is sputtered to form an AlN film as the third layer film 15 on the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0080] <Variations> The combination of metal nitride and metal may be TaN and Ta, in which case the same effects as those described in the fourth embodiment can be obtained.

[0081] Fifth embodiment The first layer 13 and the third layer 15 do not necessarily have to be made of the same material; the first layer 13 may be made of a metal oxide, and the third layer 15 may be made of a metal nitride.

[0082] In this embodiment, the first layer 13 is made of Al2O3 and has a thickness of 450 nm, the second layer 14 is made of Al, and the third layer 15 is made of AlN and has a thickness of 100 nm.

[0083] 12 shows the results of calculating the reflectance of the reflective film 11 according to the fifth embodiment as a function of the film thickness d2 of the second layer 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in the first embodiment. Also, based on Non-Patent Document 2, the complex refractive index of Al2O3 at a wavelength of 9 μm was calculated as n1 * = 1.13309 + i0.08571. Based on Non-Patent Document 2, the complex refractive index of AlN at a wavelength of 9 μm is set to n3 * = 1.19630 + i0.04745. Based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is set to n2 * =19.66369+i77.26043.

[0084] As can be seen from the figure, in the region where the film thickness d2 is 40 nm or more, the reflectance reaches approximately 96%, which is as high as that of embodiment 1. Furthermore, from the figure, it can be said that a film thickness d2 of 40 nm or more of the second layer film 14 in this embodiment is particularly suitable.

[0085] <Method for manufacturing semiconductor laser device> In manufacturing the optical device 100 of this embodiment, Al, which is commonly used for the first layer 13, the second layer 14, and the third layer 15, is used as the material for the sputtering target 36. Two types of raw material gases are used: O2 for generating an oxide of Al, and N2 for generating a nitride.

[0086] In fabricating the optical device 100, first, a sample 27 of the semiconductor laminate 12 is placed on the sample stage 28 of the reactive sputtering apparatus 200. Next, Ar is supplied from the first gas introduction system 32, and O2 is supplied from the second gas introduction system 33. In this state, an Al sputtering target 36 is sputtered to form a first layer film 13 of Al2O3 on the rear face of the semiconductor laminate 12. Then, the supply of O2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to form a second layer film 14 of Al on the first layer film 13. Next, the source gas supplied to the second gas introduction system 33 is switched from O2 to N2, and the sputtering target 36 is sputtered to form a third layer film 15 of AlN on the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0087] Sixth embodiment Contrary to the fifth embodiment, the first layer 13 may be made of a metal nitride and the third layer 15 may be made of a metal oxide.

[0088] In this embodiment, the first layer 13 is made of AlN and has a thickness of 450 nm, the second layer 14 is made of Al, and the third layer 15 is made of Al2O3 and has a thickness of 100 nm.

[0089] 13 shows the results of calculating the reflectance of the reflective film 11 according to the sixth embodiment as a function of the film thickness d2 of the second layer 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in the first embodiment. Also, based on Non-Patent Document 2, the complex refractive index of AlN at a wavelength of 9 μm was calculated as n1 * = 1.19630 + i0.04745. Based on Non-Patent Document 2, the complex refractive index of Al2O3 at a wavelength of 9 μm is set to n3 * = 1.13309 + i0.08571. Based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is set to n2 * =19.66369+i77.26043.

[0090] As can be seen from the figure, in the region where the film thickness d2 is 40 nm or more, the reflectance reaches approximately 96%, which is as high as that of embodiment 1. Furthermore, from the figure, it can be said that a film thickness d2 of 40 nm or more of the second layer film 14 in this embodiment is particularly suitable.

[0091] Furthermore, since the first layer 13 of this embodiment is made of AlN, which has high thermal conductivity, it is possible to suppress a temperature rise in the vicinity of the rear end face of the semiconductor laminate 12 .

[0092] <Method for manufacturing semiconductor laser device> In manufacturing the optical device 100 of this embodiment, Al, which is commonly used for the first layer 13, the second layer 14, and the third layer 15, is used as the material for the sputtering target 36. Two types of raw material gases are used: O2 for generating an oxide of Al, and N2 for generating a nitride.

[0093] In fabricating the optical device 100, first, a sample 27 of the semiconductor laminate 12 is placed on the sample stage 28 of the reactive sputtering apparatus 200. Next, Ar is supplied from the first gas introduction system 32, and N2 is supplied from the second gas introduction system 33. In this state, an Al sputtering target 36 is sputtered to form a first layer film 13 of AlN on the rear facet of the semiconductor laminate 12. Next, the supply of N2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to form a second layer film 14 of Al on the first layer film 13. Next, the source gas supplied to the second gas introduction system 33 is switched from N2 to O2, and the sputtering target 36 is sputtered to form a third layer film 15 of Al2O3 on the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0094] Embodiment 7 The first layer 13 may be a multi-layer film including a layer of a metal oxide and a layer of a nitride of the metal.

[0095] 14 is an enlarged cross-sectional view of the reflective film 11 according to the seventh embodiment. The first layer 13 of this embodiment includes a first layer 13-1 provided on the rear end face of the semiconductor laminate 12, and a first layer 13-2 provided between the first layer 13-1 and the second layer 14. In this embodiment, the first layer 13-1 on the rear end face side is made of Al2O3 and has a thickness of 100 nm, the first layer 13-2 is made of AlN and has a thickness of 300 nm, the second layer 14 is made of Al, and the third layer 15 is made of AlN and has a thickness of 100 nm.

[0096] Hereinafter, the complex refractive index of the first layer film 13-1 is defined as n11 * The complex refractive index of the first layer film 13-2 is n12. * , the film thickness is d12.

[0097] FIG. 15 shows the results of calculating the reflectance of the reflective film 11 according to the seventh embodiment as a function of the film thickness d2 of the second layer 14. In FIG.

[0098] In the calculation, the wavelength of the laser light was set to 9 μm, as in the first embodiment. Based on Non-Patent Document 2, the complex refractive index of Al2O3 at a wavelength of 9 μm was set to n11 * = 1.13309 + i0.08571. Based on Non-Patent Document 2, the complex refractive index of AlN at a wavelength of 9 μm is set to n12 * =n3 * = 1.19630 + i0.04745. Based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is set to n2 * =19.66369+i77.26043.

[0099] As can be seen from the figure, in the region where the film thickness d2 is 30 nm or more, the reflectance reaches approximately 96%, which is as high as that of embodiment 1. Furthermore, it can be said from the figure that the film thickness d2 of second layer film 14 in this embodiment is particularly preferably 30 nm or more.

[0100] <Method for manufacturing semiconductor laser device> In manufacturing the optical device 100 of this embodiment, Al, which is commonly used for the first layer 13, the second layer 14, and the third layer 15, is used as the material for the sputtering target 36. Two types of raw material gases are used: O2 for generating an oxide of Al, and N2 for generating a nitride.

[0101] In fabricating the optical device 100, first, a sample 27 of the semiconductor laminate 12 is placed on a sample stage 28 of a reactive sputtering apparatus 200. Next, Ar is supplied from a first gas introduction system 32, and O is supplied from a second gas introduction system 33. In this state, an Al sputtering target 36 is sputtered to form an AlO film as a first layer film 13-1 on the rear end face of the semiconductor laminate 12.

[0102] Furthermore, the source gas supplied to the second gas introduction system 33 is switched from O2 to N2, and the sputtering target 36 is sputtered to form an AlN film as the first layer film 13-2 on the first layer film 13-1.

[0103] Furthermore, the supply of N2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to form an Al film as the second layer film 14 on the first layer film 13-2. Furthermore, the supply of N2 from the second gas introduction system 33 is restarted, and the sputtering target 36 is sputtered to form an AlN film as the third layer film 15 on the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0104] Embodiment 8 The third layer 15 does not necessarily have to be provided. That is, the reflective film 11 may have a two-layer structure consisting of a first layer 13 and a second layer .

[0105] 16 is an enlarged cross-sectional view of the reflective film 11 according to embodiment 8. The reflective film 11 of this embodiment has a two-layer structure consisting of a first layer 13 and a second layer 14. The first layer 13 is made of Ta2O5 and has a thickness of 100 nm or 300 nm, and the second layer 14 is made of Ta.

[0106] 17 shows the results of calculating the reflectance of the reflective film 11 according to embodiment 8 as a function of the film thickness d2 of the second layer 14. In the figure, the dashed line represents the reflectance when the film thickness d1 of the first layer 13 is 100 nm, and the solid line represents the reflectance when the film thickness is 300 nm.

[0107] In the calculation, the reflectance at a wavelength of 9 μm was calculated as an example. Based on Non-Patent Document 4, the complex refractive index of Ta2O5 at a wavelength of 9 μm was calculated as n1 * = 1.50333 + i0.00000, where i is the imaginary unit. Also, based on Non-Patent Document 5, the complex refractive index of Ta at a wavelength of 9 μm is set to n2 * =10.68374+i48.47897.

[0108] As can be seen from the figure, when the thickness d1 of first layer 13 is 100 nm, the reflectance reaches approximately 94% in the region where the thickness d2 of second layer 14 is 60 nm or more, thereby achieving a reflectance as high as that of embodiment 1. Furthermore, when the thickness d1 of first layer 13 is 300 nm, the reflectance reaches approximately 94% in the region where the thickness d2 of second layer 14 is 40 nm or more, thereby achieving a reflectance as high as that of embodiment 1. Therefore, it can be said that a thickness d2 of 40 nm or more of second layer 14 in this embodiment is particularly suitable.

[0109] <Method for manufacturing semiconductor laser device> In the manufacture of the optical device 100 of this embodiment, Ta, which is commonly used for the first layer film 13 and the second layer film 14, is used as the material of the sputtering target 36. O2 is used as the raw material gas for generating an oxide of Ta.

[0110] In fabricating the optical device 100, first, a sample 27 of the semiconductor laminate 12 is placed on the sample stage 28 of the reactive sputtering apparatus 200. Next, Ar is supplied from the first gas introduction system 32, and O2 is supplied from the second gas introduction system 33. In this state, a Ta sputtering target 36 is sputtered to form a first layer film 13 of Ta2O5 on the rear facet of the semiconductor laminate 12. Furthermore, the supply of O2 from the second gas introduction system 33 is stopped, and then the sputtering target 36 is sputtered to form a second layer film 14 of Ta on the first layer film 13.

[0111] As described above, in manufacturing the optical device 100 of this embodiment, the metal commonly used for the first layer film 13 and the second layer film 14 is used as the material for the sputtering target 36. Furthermore, oxygen is used as the source gas when the first layer film 13 is a metal oxide, and nitrogen is used as the source gas when the first layer film is a metal nitride. The first layer film 13, which is a metal oxide, can be formed in the reactive sputtering apparatus 200 by supplying the source gas to the sample chamber 22 and then sputtering the sputtering target 36. Furthermore, the second layer film 14 can be formed by sputtering the sputtering target 36 after stopping the supply of the source gas. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed using a single film formation apparatus, and a method for manufacturing the same.

[0112] In addition, the second layer 14 of this embodiment is made of Ta, not Au, and therefore can prevent the AuSn solder from creeping up, and can play the same role as the third layer 15.

[0113] The present disclosure is not limited to the above-described embodiments, and various modifications can be made in the implementation stage without departing from the spirit of the present disclosure. Furthermore, the embodiments and modifications may be implemented in appropriate combinations, in which case the combined effects can be obtained.

[0114] The metals used in the present disclosure are not limited to Ta, Al, and Ti, and may be other metals such as W (tungsten), Mo (molybdenum), Ce (cerium), etc. Specifically, by combining a metal oxide with a metal such as WO2 or WO3 with W, MoO3 with Mo, or CeO2 with Ce, the same effects as those described in each embodiment can be obtained.

[0115] The optical device 100 is not limited to a semiconductor laser device provided with the reflective film 11, but may also be a photodiode (PD) or an electroabsorption (EA) modulator provided with the reflective film 11. In this case, the semiconductor laminate 12 only needs to include at least an optical waveguide layer.

[0116] The optical device 100 of the present disclosure may also be an optical crystal such as lithium niobate (LiNbO3:LN) provided with a reflective film 11, a lens, a prism, or the like.

[0117] In the claims, semiconductor laser devices, electroabsorption modulators, optical crystals, lenses, prisms, etc. are collectively referred to as optical elements. [Explanation of symbols]

[0118] 1: first electrode, 2: substrate, 3: buffer layer, 4: optical confinement layer, 5: core region, 6: optical confinement layer, 7: cladding layer, 8: contact layer, 9: second electrode, 10: current blocking layer, 11: reflective film, 12: semiconductor laminate, 13: first layer film, 13-1: first layer film, 13-2: first layer film, 14: second layer film, 15: third layer film, 21: plasma generation chamber, 22: sample chamber, 25: plasma extraction window, 26: plasma flow, 27: sample, 28: sample stage, 32: first gas introduction system, 33: second gas introduction system, 36: sputtering target, 42: dummy bar, 43: retaining plate, 44: fixing device, 45: screw, 100: optical device, 200: reactive sputtering device

Claims

1. an optical element; and a reflective film provided on a rear end surface of the optical element facing a waveguiding direction of light, the reflective film includes a first layer film that is provided on the end face of the optical element and transmits the light, and a second layer film that is provided on the first layer film and reflects the light, the first layer film is an oxide of Ta, the second layer is a single layer of Ta provided in direct contact with the first layer, A quantum cascade laser that emits mid-infrared light with a wavelength of 3 μm or more and 24 μm or less.

2. 2. The quantum cascade laser according to claim 1, wherein the optical element is a semiconductor laminate including an optical waveguide layer.

3. an optical element; and a reflective film provided on a rear end surface of the optical element facing a waveguiding direction of light, the reflective film includes a first layer film that is provided on the end face of the optical element and transmits the light, and a second layer film that is provided on the first layer film and reflects the light, the first layer film is a metal oxide or nitride, the second layer film is a single layer film made of the metal provided in direct contact with the first layer film, and further includes a third layer film provided on the second layer film; the third layer film is an oxide or nitride of the metal; a combination of the first layer film, the second layer film, and the third layer film is Ta oxide, Ta, Ta oxide, or Al oxide, Al, Al oxide, or Al nitride, Al, Al nitride, or Al oxide, Al, Al nitride, or Al nitride, or Al, Al oxide; A quantum cascade laser that emits mid-infrared light with a wavelength of 3 μm or more and 24 μm or less.

4. Further comprising a third layer film provided on the second layer film, 2. The quantum cascade laser according to claim 1, wherein the material of the third layer is a material that does not contain metal of a solder used to bond the quantum cascade laser to a submount.

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