Method for manufacturing quantum cascade lasers

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 deposition apparatus. [Solution] 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 deposition apparatus. The optical device of this disclosure comprises an optical element and a reflective film provided on the end face of the optical element facing the direction of light guidance. 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] This disclosure relates to a quantum cascade laser equipped with a reflective film and a method for manufacturing the same. [Background technology]

[0002] Non-patent document 1 discloses a quantum cascade laser with a high-reflective (HR) coating to reduce losses in the cavity mirror. In this laser, SiO2, Ti (to improve adhesion to Au), and Au are layered in that order on the rear end face of the laser. The 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 Publication 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] M. A. 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 Problems to be Solved by the Invention

[0010] In the method of Non-Patent Document 1, since SiO2 is formed by the PECVD (Plasma Enhanced Chemical Vapor Deposition) method and Ti / Au is formed by the EB evaporation (Electron Beam Evaporation) method, two film-forming apparatuses are required to fabricate the reflective film.

[0011] An object of the present disclosure is to provide a quantum cascade laser having a reflective film that can be formed by a single film-forming apparatus in order to solve the above problems.

[0012] An object of the present disclosure is to provide a method for manufacturing a quantum cascade laser having a reflective film that can be formed by a single film-forming apparatus in order to solve the above problems. Means for Solving the Problems

[0013] Aspects of the present disclosure include an optical element, and a reflective film provided on a rear end surface of the optical element facing the light guiding direction of light, the reflective film has a first layer film provided on the end surface of the optical element and transmitting the light, and a second layer film provided on the first layer film and reflecting 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 directly in contact with the first layer film, and it is preferably a quantum cascade laser that oscillates mid-infrared light having a wavelength of 3 μm or more and 24 μm or less. Advantages of the Invention

[0014] In the quantum cascade laser of this disclosure, a first layer film and a second layer film, which is the outermost surface, are stacked in this order on the rear end face of a semiconductor laminate. The first layer film is a metal oxide or nitride, and the second layer film is the elemental metal. A reactive sputtering apparatus is used to manufacture the optical device. 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 deposited by supplying the raw material gas to the sample chamber in the reactive sputtering apparatus and sputtering a sputtering target made of the metal. The second layer film can be deposited by stopping the supply of the raw material gas and sputtering the sputtering target. This makes it possible to provide a quantum cascade laser having a reflective film that can be deposited with a single deposition apparatus, and a method for manufacturing the same. [Brief explanation of the drawing]

[0015] [Figure 1] This is a perspective view of the optical device according to Embodiment 1. [Figure 2] This is a cross-sectional view from ya-yb in Figure 1. [Figure 3] This is an enlarged cross-sectional view of the reflective film according to Embodiment 1. [Figure 4] This is the result of calculating the reflectance of the reflective film according to Embodiment 1 as a function of the film thickness of the second layer. [Figure 5] This is the result of calculating the reflectance of the reflective film in the comparative example as a function of the film thickness of the third layer. [Figure 6] This figure shows a method for manufacturing an optical device according to Embodiment 1. [Figure 7] This is a top view of the sample according to Embodiment 1. [Figure 8] This is a cross-sectional view taken along line xa-xb in Figure 7. [Figure 9] This is the result of calculating the reflectance of the reflective film according to Embodiment 2 as a function of the film thickness of the second layer. [Figure 10]This is the result of calculating the reflectance of the reflective film according to Embodiment 3 as a function of the film thickness of the second layer. [Figure 11] This is the result of calculating the reflectance of the reflective film according to Embodiment 4 as a function of the film thickness of the second layer. [Figure 12] This is the result of calculating the reflectance of the reflective film according to Embodiment 5 as a function of the film thickness of the second layer. [Figure 13] This is the result of calculating the reflectance of the reflective film according to Embodiment 6 as a function of the film thickness of the second layer. [Figure 14] This is an enlarged cross-sectional view of the reflective film according to Embodiment 7. [Figure 15] This is the result of calculating the reflectance of the reflective film according to Embodiment 7 as a function of the film thickness of the second layer. [Figure 16] This is an enlarged cross-sectional view of the reflective film according to Embodiment 8. [Figure 17] This is the result of calculating the reflectance of the reflective film according to Embodiment 8 as a function of the film thickness of the second layer. [Modes for carrying out the invention]

[0016] Embodiments of this disclosure will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repetition in the description may be omitted. Furthermore, the materials, dimensions, etc., of the semiconductor layer and reflective film described herein are examples only and may differ from those of actual components.

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

[0018] In this figure, the direction of the laser beam's guidance is defined as the positive y-axis direction. The thickness direction of the semiconductor layer is defined as the z-axis direction.

[0019] The optical device 100 has, in order, an n-type InP buffer layer 3 (thickness 1.0 μm), an n-type GaInAs photoconfinement layer 4 (thickness 230 nm), a core region 5 consisting of 30 to 40 stages, an n-type GaInAs photoconfinement 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), all stacked on an n-type InP substrate 2.

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

[0021] Core region 5 has a multi-quantum well (MQW) structure in which 30 to 40 stages, each consisting of alternating GaInAs quantum well layers and AlInAs barrier layers, are stacked. Inter-subband transitions in the quantum well structure generate mid-infrared light of approximately 3 to 24 μm, for example.

[0022] Furthermore, the number of stages constituting the core region 5 is not limited to 30-40; it can be adjusted as appropriate 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. During operation, a voltage is applied to the first electrode 1 and the second electrode 9, injecting current into the optical device 100 and causing the laser to oscillate.

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

[0025] Figure 2 is a cross-sectional view taken along the line ya-yb in Figure 1. A reflective film 11 is provided on the rear end surface of the semiconductor stack 12. Here, the rear end surface refers to the end surface opposite to the laser beam emission surface.

[0026] Figure 3 is an enlarged cross-sectional view of the reflective film 11 according to Embodiment 1. The reflective film 11 is formed by stacking a first layer film 13, a second layer film 14, and a third layer film 15 in that order on the rear end surface of the semiconductor laminate 12. In this embodiment, the first layer film 13 is Ta2O5 with a thickness of 450 nm, the second layer film 14 is Ta, and the third layer film 15 is Ta2O5 with a thickness of 100 nm.

[0027] The first layer film 13 provides insulation between the semiconductor laminate 12 and the second layer film 14, which is a metal. The first layer film 13 is an electrical insulator and is preferably made of a material with high transmittance to the wavelength of laser light; for example, a metal oxide can be suitably used. In this embodiment, Ta2O5 is used as the first layer film 13.

[0028] The second layer 14 is responsible for reflecting laser light. The second layer 14 is an element of the 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 plays a role in preventing solder from creeping up when the optical device 100 is joined to the submount. Generally, when joining the optical device 100 to the submount, the surface of the submount and the back surface of the substrate 2 are joined by solder such as AuSn provided on the surface of the submount. In this case, if the outermost surface of the reflective film 11 is a metal contained in the solder (Au in the case of AuSn solder), the wettability of the solder to the outermost surface increases. This causes the solder to creep up the surface of the reflective film 11, which is a problem.

[0030] From the viewpoint of preventing this defect, it is preferable that the third layer 15 is made of a material that does not contain the metal (Au in the case of AuSn solder) of the solder used to join the optical device 100 and the submount. By using, for example, a metal oxide as the third layer 15, the wettability of the solder can be reduced and creep can be prevented. In this embodiment, Ta2O5 is used as the third layer 15, the same as the first layer 13. Note that the third layer 15 may be any metal other than the metal contained in the solder.

[0031] <Calculation results of reflectance> The following section explains the results of calculating the reflectance of the reflective film 11. For the calculation, the effective refractive index of the semiconductor laminate 12 was set to nc. The complex refractive index of the first layer film 13 was set to n1. * The film thickness was defined as d1. Similarly, the complex refractive index of the second layer 14 was defined as n2. * Let the film thickness be d2, and the complex refractive index of the third layer 15 be n3 * The film thickness was set to d3.

[0032] Figure 4 shows the result of calculating the reflectance of the reflective film 11 according to Embodiment 1 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. However, 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 Literature 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 given by n²* = 10.68374 + i48.47897 was used. Note that there is a relationship between the complex permittivity and the complex refractive index described in Non-Patent Document 3.

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

[0034]

number

[0035] r * This is the complex amplitude reflectance of a multilayer film, obtained from the characteristic matrix of the multilayer film.

[0036] Figure 4 shows that in the region where the Ta film thickness d2 is 50 nm or more, the reflectivity tends to saturate, reaching 96% or more. This reflectivity is higher than the reflectivity calculated later based on Non-Patent Literature 1, indicating that the reflective properties of the reflective film 11 of this disclosure are excellent. Furthermore, from the reflectivity calculation results in Figure 4, it can be said that a film thickness d2 of 50 nm or more is particularly preferable for the second layer film 14 of this disclosure.

[0037] <Comparative Example> Below, as comparative examples, we will explain the results of calculating the reflectance in the following cases: (1) when there is no reflective film 11, and (2) when the reflective film 11 described in Non-Patent Document 1 is assumed.

[0038] (1) Reflectance when the reflective film 11 is absent The reflectance R0 in the absence of the reflective film 11 can be calculated using the following equation (2).

[0039]

number

[0040] If the effective refractive index of the semiconductor laminate 12 is set to nc = 3.15452, then R0 will be approximately 26.9%.

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

[0042] Here, although the first layer film 13 in Non-Patent Document 1 is SiO2, in this comparative example, the first layer film 13 is 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 at this wavelength. However, when targeting a wavelength of 9 μm as in this embodiment, Al2O3 with less light absorption than SiO2 is more suitable.

[0043] FIG. 5 shows the result 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 film 15. In the calculation, based on Non-Patent Document 2, the complex refractive index of Al2O3 at a wavelength of 9 μm was set as n1 * = 1.13309 + i0.08571. Also, based on Non-Patent Document 3, the complex refractive index of Ti at a wavelength of 9 μm was set as n2 * = 6.46976 + i16.50695. Further, based on Non-Patent Document 3, the complex refractive index of Au at a wavelength of 9 μm was set as n3 * = 9.42171 + i55.49599.

[0044] From FIG. 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%. It can be said that the reflectance of the comparative example is higher than the reflectance R0 when there is no reflective film 11, but does not reach the reflectance of the reflective film 11 of the present disclosure. <B

[0045] <Method for manufacturing a semiconductor laser device> FIG. 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, we will describe an example where the reactive sputtering apparatus 200 is an electron cyclotron resonance (ECR) plasma deposition apparatus. However, the plasma generation method is not limited.

[0047] In the plasma generation chamber 21, a plasma flow 26 is generated by microwave electron cyclotron resonance discharge against 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, the sputtering target 36 is positioned 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 commonly used for the first layer 13, the second layer 14, and the third layer 15. In this embodiment, Ta is used as the sputtering target 36.

[0049] In the sample chamber 22, the sample 27 is placed on the sample stage 28. A plasma stream 26, which incorporates ions generated from the target 36, is incident on the sample 27, causing a thin film containing the material of the target 36 to form 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. Ar, an inert gas, is particularly preferred 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, which is used to generate metal oxides for the first layer film 13 and the third layer film 15.

[0051] Figure 7 is a top view of sample 27 according to Embodiment 1, and Figure 8 is a cross-sectional view taken along the xa-xb line in Figure 7. Multiple bar-shaped semiconductor stacks 12, with their rear end faces facing upward (z direction), are placed on the base of a vise-shaped fixing device 44. Dummy bars 42 made of Si or stainless steel (SUS) are placed between each semiconductor stack 12 to prevent contact between the semiconductor stacks 12. The alternately arranged semiconductor stacks 12 and dummy bars 42 are sandwiched from both sides by retaining plates 43. The semiconductor stacks 12 and dummy bars 42 are fastened together by screws 45 of the fixing device 44 via the retaining plates 43.

[0052] In preparing sample 27, first, a retaining plate 43 is placed at one end of the base of the fixing device 44, and next to the retaining plate 43, dummy bars 42 and semiconductor laminates 12 with their rear end faces facing upwards are arranged alternately. Furthermore, a retaining plate 43 is placed at the ends of the alternately arranged semiconductor laminates 12 and dummy bars 42. Finally, sample 27 is completed by tightening the semiconductor laminates 12 and dummy bars 42 with screws 45 of the fixing device 44 via the retaining plates 43.

[0053] In the fabrication of the optical device 100 using the reactive sputtering apparatus 200, first, the sample 27 is placed on the sample stage 28. 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, Ta, which is the sputtering target 36, is sputtered to deposit a first layer film 13, Ta2O5, on the rear end surface of the semiconductor laminate 12.

[0054] Furthermore, while the supply of Ar from the first gas introduction system 32 is continued, the supply of O2 to the second gas introduction system 33 is stopped. In this state, the sputtering target 36, which is Ta, is sputtered to deposit the second layer film 14, which is Ta, on the first layer film 13.

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

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

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

[0058] <Variations> Furthermore, the end face on which the reflective film 11 is provided is not necessarily limited to the rear end face; it may also be provided on the laser beam emission surface. In other words, 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 direction of the laser beam guidance. This is common to all embodiments.

[0059] Embodiment 2 In this embodiment, we will describe a reflective film 11 in which the first layer 13 and the third layer 15 are metal oxides and the second layer 14 is the elemental metal, and in which case the metal is Al. The following will describe the changes from Embodiment 1.

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

[0061] Figure 9 shows the result of calculating the reflectance of the reflective film 11 according to Embodiment 2 as a function of the film thickness d2 of the second layer film 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in Embodiment 1. Also, based on Non-Patent Literature 2, the complex refractive index of Al2O3 at a wavelength of 9 μm was set to n1 * =n3 * = 1.13309 + i0.08571 was used. Also, based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is given by n2 * =19.66369+i77.26043 was used.

[0062] As shown in the figure, the reflectivity tends to saturate in the region where the film thickness d2 is 40 nm or more, reaching 96% or higher. In other words, a high reflectivity similar to that of Embodiment 1 can be obtained in this embodiment as well. Furthermore, as shown in the figure, a film thickness d2 of 40 nm or more for the second layer film 14 in this embodiment is particularly preferable.

[0063] Furthermore, the third layer 15 in this embodiment is Al2O3, which is a material that does not contain Au. Therefore, when the solder used to join the optical device 100 to the submount is AuSn solder, solder creep can be prevented.

[0064] <Method of manufacturing semiconductor laser devices> In the manufacturing of the optical device 100 of this embodiment, Al, which is commonly used in 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. In addition, O2 is used as the raw material gas to generate Al oxide.

[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, the Al sputtering target 36 is sputtered to deposit the first layer film 13, which is Al2O3, on the rear end surface of the semiconductor laminate 12. Furthermore, the supply of O2 from the second gas introduction system 33 is stopped and the sputtering target 36 is sputtered to deposit the second layer film 14, which is Al. Then, the supply of O2 from the second gas introduction system 33 is restarted and the sputtering target 36 is sputtered to deposit the third layer film 15, which is 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 with a single film deposition apparatus, and a method for manufacturing the same.

[0066] Embodiment 3 In this embodiment, we will describe a reflective film 11 in which the first layer 13 and the third layer 15 are metal oxides and the second layer 14 is the elemental metal, and in which case the metal is Ti. The following will describe the changes from Embodiment 1.

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

[0068] Figure 10 shows the result of calculating the reflectance of the reflective film 11 according to Embodiment 3 as a function of the film thickness d2 of the second layer film 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in Embodiment 1. Also, based on Non-Patent Literature 2, the complex refractive index of TiO2 at a wavelength of 9 μm was set to n1 * =n3 * = 1.64865 + i0.06818 was used. Also, based on Non-Patent Document 3, the complex refractive index of Ti at a wavelength of 9 μm is given by n² * =6.46976+i16.50695.

[0069] As shown in the figure, the reflectivity tends to saturate in the region where the film thickness d2 is 110 nm or more, reaching 80% or higher. In other words, a high reflectivity can be obtained in this embodiment as well. Furthermore, as shown in the figure, a film thickness d2 of 110 nm or more for the second layer film 14 in this embodiment is particularly preferable.

[0070] Furthermore, the third layer 15 in this embodiment is TiO2 and is a material that does not contain Au. Therefore, when the solder used to join the optical device 100 to the submount is AuSn solder, solder creep can be prevented.

[0071] <Method of manufacturing semiconductor laser devices> In the manufacturing of the optical device 100 of this embodiment, Ti, which is commonly used in 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. In addition, O2 is used as the raw material gas to generate the 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, the Ti sputtering target 36 is sputtered to deposit a first layer film 13, which is TiO2, on the rear end surface of the semiconductor laminate 12. Furthermore, the supply of O2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to deposit a second layer film 14, which is Ti, on top of the first layer film 13. Finally, the supply of O2 from the second gas introduction system 33 is restarted, and the sputtering target 36 is sputtered to deposit a third layer film 15, which is TiO2, on top of the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be deposited with a single film deposition apparatus, and a method for manufacturing the same.

[0073] Embodiment 4 In embodiments 1 to 3, the first layer 13 and the third layer 15 were described as metal oxides. However, the first layer 13 and the third layer 15 may also be metal nitrides.

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

[0075] Figure 11 shows the result of calculating the reflectance of the reflective film 11 according to Embodiment 4 as a function of the film thickness d2 of the second layer film 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in Embodiment 1. Also, based on Non-Patent Literature 2, the complex refractive index of AlN at a wavelength of 9 μm was set to n1 * =n3 * = 1.19630 + i0.04745 was used. Also, based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is given by n2 * =19.66369+i77.26043 was used.

[0076] As shown in the figure, in the region where the film thickness d2 is 30 nm or more, the reflectivity reaches approximately 96%, and a high reflectivity similar to that of Embodiment 1 is obtained. Furthermore, as shown in the figure, it can be said that a film thickness d2 of 30 nm or more for the second layer film 14 in this embodiment is particularly preferable.

[0077] Furthermore, the third layer 15 in this embodiment is AlN, which is a material that does not contain Au. Therefore, when the solder used to join the optical device 100 to the submount is AuSn solder, solder creep can be prevented.

[0078] <Method of manufacturing semiconductor laser devices> In the manufacturing of the optical device 100 of this embodiment, Al, which is commonly used in 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. In addition, N2 is used as the raw material gas to generate Al nitrides.

[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, the Al sputtering target 36 is sputtered to deposit AlN, which is the first layer film 13, on the rear end surface of the semiconductor laminate 12. Furthermore, the supply of N2 from the second gas introduction system 33 is stopped, and the sputtering target 36 is sputtered to deposit Al, which is the second layer film 14, on the first layer film 13. Furthermore, the supply of N2 from the second gas introduction system 33 is restarted, and the sputtering target 36 is sputtered to deposit AlN, which is 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 with a single film deposition apparatus, and a method for manufacturing the same.

[0080] <Variations> The combination of the metallic nitride and the metal may be TaN and Ta. In this case as well, the same effects as those described in Embodiment 4 can be obtained.

[0081] Embodiment 5 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 a metal oxide and the third layer 15 may be a metal nitride.

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

[0083] Figure 12 shows the result of calculating the reflectance of the reflective film 11 according to Embodiment 5 as a function of the film thickness d2 of the second layer film 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in Embodiment 1. Also, based on Non-Patent Literature 2, the complex refractive index of Al2O3 at a wavelength of 9 μm was set to n1 * = 1.13309 + i0.08571 was used. Also, based on Non-Patent Literature 2, the complex refractive index of AlN at a wavelength of 9 μm is given by n3 *= 1.19630 + i0.04745 was used. Also, based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is given by n2 * =19.66369+i77.26043 was used.

[0084] As shown in the figure, in the region where the film thickness d2 is 40 nm or more, the reflectivity reaches approximately 96%, and a high reflectivity similar to that of Embodiment 1 is obtained. Furthermore, as shown in the figure, it can be said that a film thickness d2 of 40 nm or more for the second layer film 14 in this embodiment is particularly preferable.

[0085] <Method of manufacturing semiconductor laser devices> In the manufacturing of the optical device 100 of this embodiment, Al, which is commonly used in 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. In addition, two types of gases are used as raw material gases: O2 for generating Al oxide and N2 for generating 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, the Al sputtering target 36 is sputtered to deposit the first layer film 13, Al2O3, on the rear end surface of the semiconductor laminate 12. Furthermore, after stopping the supply of O2 from the second gas introduction system 33, the sputtering target 36 is sputtered to deposit the second layer film 14, Al, on top of the first layer film 13. Furthermore, the raw material gas supplied to the second gas introduction system 33 is switched from O2 to N2, and the sputtering target 36 is sputtered to deposit the third layer film 15, AlN, on top of the second layer film 14. This makes it possible to provide an optical device 100 having a reflective film 11 that can be formed with a single film deposition apparatus, and a method for manufacturing the same.

[0087] Embodiment 6 Conversely to Embodiment 5, the first layer 13 may be a metal nitride and the third layer 15 may be a metal oxide.

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

[0089] Figure 13 shows the result of calculating the reflectance of the reflective film 11 according to Embodiment 6 as a function of the film thickness d2 of the second layer film 14. In the calculation, the wavelength of the laser light was set to 9 μm, as in Embodiment 1. Also, based on Non-Patent Literature 2, the complex refractive index of AlN at a wavelength of 9 μm was set to n1 * = 1.19630 + i0.04745 was used. Also, based on Non-Patent Literature 2, the complex refractive index of Al2O3 at a wavelength of 9 μm is given by n3 * = 1.13309 + i0.08571 was used. Also, based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is given by n2 * =19.66369+i77.26043 was used.

[0090] As shown in the figure, in the region where the film thickness d2 is 40 nm or more, the reflectivity reaches approximately 96%, and a high reflectivity similar to that of Embodiment 1 is obtained. Furthermore, as shown in the figure, it can be said that a film thickness d2 of 40 nm or more for the second layer film 14 in this embodiment is particularly preferable.

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

[0092] <Method of manufacturing semiconductor laser devices> In the manufacturing of the optical device 100 of this embodiment, Al, which is commonly used in 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. In addition, two types of gases are used as raw material gases: O2 for generating Al oxide and N2 for generating 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, the Al sputtering target 36 is sputtered to deposit AlN, which is the first layer film 13, on the rear end surface of the semiconductor laminate 12. Furthermore, after stopping the supply of N2 from the second gas introduction system 33, the sputtering target 36 is sputtered to deposit Al, which is the second layer film 14, on the first layer film 13. Furthermore, the raw material gas supplied to the second gas introduction system 33 is switched from N2 to O2, and the sputtering target 36 is sputtered to deposit Al2O3, which is 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 with a single film deposition apparatus, and a method for manufacturing the same.

[0094] Embodiment 7 The first layer film 13 may consist of multiple layers, including a layer of metal oxide and a layer of metal nitride.

[0095] Figure 14 is an enlarged cross-sectional view of the reflective film 11 according to Embodiment 7. The first layer film 13 of this embodiment includes a first layer film 13-1 provided on the rear end surface of the semiconductor laminate 12, and a first layer film 13-2 provided between the first layer film 13-1 and the second layer film 14. In this embodiment, the first layer film 13-1 on the rear end surface side is Al2O3 with a thickness of 100 nm, the first layer film 13-2 is AlN with a thickness of 300 nm, the second layer film 14 is Al, and the third layer film 15 is AlN with a thickness of 100 nm.

[0096] Hereafter, the complex refractive index of the first layer film 13-1 is n11 * The film thickness is set to d11. The complex refractive index of the first layer film 13-2 is set to n12. * The film thickness is set to d12.

[0097] Figure 15 shows the result of calculating the reflectance of the reflective film 11 according to Embodiment 7 as a function of the film thickness d2 of the second layer film 14.

[0098] In the calculation, the wavelength of the laser light was set to 9 μm, as in Embodiment 1. Also, based on Non-Patent Literature 2, the complex refractive index of Al2O3 at a wavelength of 9 μm was set to n11 * =1.13309+i0.08571 was used. Also, based on Non-Patent Literature 2, the complex refractive index of AlN at a wavelength of 9 μm is n12 * =n3 * = 1.19630 + i0.04745 was used. Also, based on Non-Patent Document 3, the complex refractive index of Al at a wavelength of 9 μm is given by n2 * =19.66369+i77.26043 was used.

[0099] As shown in the figure, in the region where the film thickness d2 is 30 nm or more, the reflectivity reaches approximately 96%, and a high reflectivity similar to that of Embodiment 1 is obtained. Furthermore, as shown in the figure, it can be said that a film thickness d2 of 30 nm or more for the second layer film 14 in this embodiment is particularly preferable.

[0100] <Method of manufacturing semiconductor laser devices> In the manufacturing of the optical device 100 of this embodiment, Al, which is commonly used in 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. In addition, two types of gases are used as raw material gases: O2 for generating Al oxide and N2 for generating nitride.

[0101] In the fabrication of 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, the Al sputtering target 36 is sputtered to deposit the first layer film 13-1, Al2O3, on the rear end surface of the semiconductor laminate 12.

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

[0103] Furthermore, after stopping the supply of N2 from the second gas introduction system 33, the sputtering target 36 is sputtered to deposit a second layer film 14, which is Al, on the first layer film 13-2. Then, the supply of N2 from the second gas introduction system 33 is restarted, and the sputtering target 36 is sputtered to deposit a third layer film 15, which is 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 deposited with a single film deposition apparatus, and a method for manufacturing the same.

[0104] Embodiment 8 The third layer 15 is not necessarily required. That is, the reflective film 11 may have a two-layer structure consisting of a first layer 13 and a second layer 14.

[0105] Figure 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 Ta2O5 with a thickness of 100 nm or 300 nm, and the second layer 14 is Ta.

[0106] Figure 17 shows the result 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 film 14. In the figure, the dashed line represents the reflectance when the film thickness d1 of the first layer film 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. Also, based on Non-Patent Literature 4, the complex refractive index of Ta2O5 at a wavelength of 9 μm was given by 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 given by n² * = 10.68374 + i48.47897.

[0108] As shown in the figure, when the film thickness d1 of the first layer 13 is 100 nm, the reflectance of the second layer 14 reaches approximately 94% in the region where the film thickness d2 is 60 nm or more, and a high reflectance similar to that of Embodiment 1 is obtained. Also, when the film thickness d1 of the first layer 13 is 300 nm, the reflectance of the second layer 14 reaches approximately 94% in the region where the film thickness d2 is 40 nm or more, and a high reflectance similar to that of Embodiment 1 is obtained. Therefore, it can be said that a film thickness d2 of the second layer 14 of 40 nm or more is particularly preferable in this embodiment.

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

[0110] In the fabrication of 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, the first layer film 13, Ta2O5, is deposited on the rear end surface of the semiconductor laminate 12 by sputtering the sputtering target 36. Furthermore, after stopping the supply of O2 from the second gas introduction system 33, the second layer film 14, Ta, is deposited on the first layer film 13 by sputtering the sputtering target 36.

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

[0112] Furthermore, the second layer 14 in this embodiment is Ta, which is a metal other than Au. Therefore, it can prevent the AuSn solder from creeping up and can perform the same role as the third layer 15.

[0113] This disclosure is not limited to the embodiments described above, and various modifications can be made during implementation without departing from its essence. Furthermore, each embodiment and its modifications may be combined as appropriate, and in that case, the combined effects can be obtained.

[0114] The metals used in this disclosure are not limited to Ta, Al, and Ti, but may also be other metals such as W (tungsten), Mo (molybdenum), and Ce (cerium). Specifically, by combining metal oxides with metals such as WO2 or WO3 and W, MoO3 and Mo, and CeO2 and 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 a reflective film 11, but may also be a photodiode (PD) or an electroabsorption (EA) modulator provided with a reflective film 11. In this case, the semiconductor laminate 12 only needs to include at least an optical waveguide layer.

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

[0117] In the claims, semiconductor laser devices, field absorption modulators, optical crystals, lenses, or prisms 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 apparatus

Claims

1. The optical element comprises an optical element and a reflective film provided on the rear end surface of the optical element facing the direction of light guidance. The reflective film comprises a first layer film provided on the end face of the optical element that transmits light, and a second layer film provided on the first layer film that reflects 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. A quantum cascade laser that emits mid-infrared light with a wavelength of 3 μm to 24 μm.

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

3. The invention further comprises a third layer film provided on the second layer film, The quantum cascade laser according to claim 1, wherein the third layer film is an oxide or nitride of the metal.

4. The quantum cascade laser according to claim 1 or 3, wherein the first layer film comprises a plurality of layers including a layer of the metal oxide and a layer of the metal nitride.

5. The quantum cascade laser according to claim 1 or 3, wherein the metal is one of Ta (tantalum), Al (aluminum), Ti (titanium), W (tungsten), Mo (molybdenum), and Ce (cerium).

6. The invention further comprises a third layer film provided on the second layer film, The quantum cascade laser according to claim 1, wherein the material of the third layer film is a metal-free material of the solder used when joining the quantum cascade laser to the submount.

7. A method for manufacturing a quantum cascade laser according to claim 1, If the first layer film is an oxide of the metal, oxygen is used as the raw material gas; if the first layer film is a nitride of the metal, nitrogen is used as the raw material gas. The optical element is placed in the sample chamber of a reactive sputtering apparatus, and the first layer film is formed on the end face of the optical element by sputtering a sputtering target made of metal while the raw material gas is supplied to the sample chamber. A method for manufacturing a quantum cascade laser, comprising depositing the second layer film on the first layer film by sputtering the sputtering target after stopping the supply of the raw material gas.

8. A method for manufacturing a quantum cascade laser according to claim 3 or 6, If the first layer film is an oxide of the metal, oxygen is used as the first raw material gas; if the first layer film is a nitride of the metal, nitrogen is used as the first raw material gas. The optical element is placed in the sample chamber of a reactive sputtering apparatus, and the first raw material gas is supplied to the sample chamber while sputtering a sputtering target made of the metal, thereby forming the first layer film on the end face of the optical element. After stopping the supply of the first raw material gas, the sputtering target is sputtered to form the second layer on the first layer. If the third layer film is an oxide of the metal, oxygen is used as the second raw material gas; if the third layer film is a nitride of the metal, nitrogen is used as the second raw material gas. A method for manufacturing a quantum cascade laser, comprising depositing the third layer film on the second layer film by sputtering the sputtering target while supplying the second raw material gas.

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