QUANTUM CASCADE DETECTOR

Abstract

Quantum cascade detector (1A) comprising: a semiconductor substrate (10); an active layer (15) arranged on the semiconductor substrate (10) and having a cascade structure in which absorption regions and transport regions are arranged in the form of a multi-stage lamination of unit laminate structures, each comprising n, where n is an integer of 3 or more, quantum well layers (161-167) with a first quantum well layer (161) serving as an absorption quantum well layer and n quantum barrier layers (171-177), alternately stacked, wherein the absorption region (17, 17b) comprises the first quantum well layer (161) and detects the light to be detected by intersubband absorption, wherein the transport region transports electrons excited by the intersubband absorption; a lower cladding layer (21) arranged between the active layer and the semiconductor substrate (10) and comprising a has a lower refractive index than the active layer (15);a lower metal layer (23) arranged between the lower cladding layer and the semiconductor substrate (10); an upper cladding layer (26) arranged on a side opposite the semiconductor substrate (10) with respect to the active layer (15) and having a lower refractive index than the active layer (15); and an upper metal layer (28) arranged on a side opposite the active layer (15) with respect to the upper cladding layer (26); a first end face (20a) and a second end face (20b) in a waveguide direction in a waveguide structure comprising the active layer (15), the lower cladding layer (21) and the upper cladding layer (26), wherein the first end face (20a) is an entrance face for the light to be detected; and wherein no external voltage is applied to the quantum cascade detector (1A) to couple out photoelectric current;and both the lower mantle layer (21) and the upper mantle layer (26) have a dopant density of 5 × 1016 cm-3 or more and 2 × 1017 cm-3 or less.;

Description

BACKGROUND OF THE INVENTION Subject of the invention

[0001] The present invention relates to a quantum cascade detector that uses intersubband light absorption in a quantum well structure. State of the art

[0002] Since absorption lines exist that correspond to the fundamental vibrations of a number of gas molecules in a mid-infrared region (for example, in a wavelength range of 4 to 10 μm), mid-infrared light has been used in infrared absorption spectroscopy. In particular, when a laser light source is used as a mid-infrared light source, only one absorption line of a specific gas type is observed using its narrow emission line bandwidth, thus enabling highly sensitive absorption spectroscopy without interference from components of other gas types.

[0003] Photodetectors in the mid-infrared wavelength range include MCT (HgCdTe) detectors, InSb detectors, quantum well infrared photodetectors (QWIPs), and similar devices. For spectroscopic experiments measuring weak mid-infrared light, liquid nitrogen-based MCT detectors or InSb detectors are primarily used. However, both MCT and InSb detectors require an external voltage for operation, which introduces noise (dark current) due to the voltage application. Therefore, suppressing this noise to improve the signal-to-noise ratio and sensitivity of the spectroscopic measurement is challenging with these detectors. Furthermore, the MCT detector has the drawback of containing toxic material, making its use generally difficult.

[0004] In recent years, a quantum cascade laser (QCD) using a cascade structure has been proposed as a mid-infrared photodetector. The quantum cascade detector is capable of extracting a photoelectric current without applying an external voltage by controlling the intersubband transitions through the construction of a quantum well structure in an active layer. For this reason, the quantum cascade detector does not generate any noise component caused by the external voltage and is therefore expected to be used as an extremely low-noise photodetector. Furthermore, the quantum cascade detector can be constructed from ordinary semiconductor materials, thus avoiding the problem of toxic materials, as seen in the MCT detector (see, for example, non-patent documents 1 to 3). Non-patent document 1: FR Giorgetta et al., “Quantum Cascade Detectors”, IEEE Journal of Quantum Electronics Vol. 45 No. 8 (2009) pp. 1039-1052 Non-patent document 2: A. Harrer et al., “Plasmonic lens enhanced mid-infrared quantum cascade detector,” Appl. Phys. Lett. Vol. 105 (2014) pp. 171112-1-171112-4 Non-patent document 3: B. Schwarz et al., “Monolithically integrated mid-infrared lab-on-achip using plasmonics and quantum cascade structures,” Nat. Commun. Vol. 5 Art. 4085 (2014) pp. 1–7 SUMMARY OF THE INVENTION

[0005] The previously described quantum cascade detector exhibits a polarization dependence in response to the incident light, due to a selection rule of the optical intersubband transition, which represents a limitation in light detection. Since the quantum cascade detector is sensitive to light specifically only for polarized light (TM-polarized light) that oscillates along a lamination direction (growth direction) of the semiconductor layers forming the quantum well structure, and does not have a large light-receiving area, it is difficult for light to efficiently strike the detector and for high-sensitivity light detection to be achieved.

[0006] To solve this problem, a substrate end face is polished to 45° in a setup described in non-patent document 1, and light is directed to penetrate the polished surface and utilize the multiple reflections within the substrate to capture the light in the active layer. However, because this setup includes many components that do not contribute to the detection sensitivity of the light penetrating the substrate, achieving efficient light capture is difficult.

[0007] In a setup described in non-patent document 2, light is directed onto a metal-periodic structure formed on the same substrate as the quantum cascade detector. This excites and propagates plasmons, and the light generated by the plasmons is then detected by the quantum cascade detector. However, this setup suffers from the problem that the entire device structure, including the quantum cascade detector and the metal-periodic structure, is complex, making the fabrication of the detector difficult.

[0008] In a setup described in non-patent document 3, a quantum cascade laser and a quantum cascade detector are arranged such that they face each other on the same substrate. However, since this setup is intended to perform the functions of both the laser and the detector, the structure of the active layer cannot be optimized for either the laser or the detector.

[0009] The present invention was conceived in view of the above problems, and it is an object of the present invention to provide a quantum cascade detector that is capable of detecting the light to be detected with high efficiency.

[0010] To solve the above problem, a quantum cascade detector according to the present invention comprises: (1) a semiconductor substrate; (2) an active layer formed on the semiconductor substrate and having a cascade structure in which the absorption regions and the transport regions are alternately stacked in the form of a multi-stage lamination of unit laminate structures, each of these comprising n (where n is an integer of 3 or more) quantum well layers with a first quantum well layer serving as an absorption quantum well layer and n quantum barrier layers, wherein the absorption region comprises the first quantum well layer and detects the light to be detected by intersubband absorption, and wherein the transport region transports the electrons excited by the intersubband absorption;(3) a lower cladding layer provided between the active layer and the semiconductor substrate, having a lower refractive index than the active layer; (4) a lower metal layer provided between the lower cladding layer and the semiconductor substrate; (5) an upper cladding layer provided on the opposite side to the semiconductor substrate, with respect to the active layer, having a lower refractive index than the active layer; and (6) an upper metal layer provided on the opposite side to the active layer, with respect to the upper cladding layer, wherein (7) a first end face and a second end face extending in a waveguide direction in a waveguide structure comprising the active layer, the lower cladding layer, and the upper cladding layer, the first end face being an entry face for the light to be detected.

[0011] In the previously described quantum cascade detector, the active layer, used to detect the light to be detected, consists of a lower cladding layer located below the active layer and between the active layer and the substrate, and an upper cladding layer above the active layer. The first end face is the entrance face for the light to be detected and forms an end in the waveguide structure comprising the active layer and the lower and upper cladding layers. According to this configuration, the light to be detected, entering from the first end face, can be guided along the active layer by the waveguide structure, and the light to be detected can be efficiently absorbed and detected in the active layer.

[0012] Furthermore, in this setup, the waveguide structure described above is configured such that the lower metal layer is positioned beneath the lower cladding layer and between the lower cladding layer and the substrate, while the upper metal layer is positioned above the upper cladding layer. In this way, the waveguide structure, comprising the active layer and the lower and upper cladding layers, is sandwiched between the lower and upper metal layers. This limits the light reception area within the detector and improves its specific detectability. Therefore, according to the quantum cascade detector with the described setup, the light to be detected can be efficiently captured in a suitable manner.

[0013] According to the quantum cascade detector of the present invention, the active layer used to detect the light to be detected is configured such that the lower cladding layer is provided between the active layer and the substrate and the upper cladding layer is provided above the active layer, the first end face is defined as the entrance surface for the light to be detected in the waveguide structure with the active layer and the lower and upper cladding layers, wherein the detector is configured such that the lower metal layer is provided between the lower cladding layer and the substrate and the upper metal layer is provided above the upper cladding layer, whereby the light to be detected can be detected in a suitable manner with high efficiency.

[0014] The present invention will become clearer from the following detailed description and the accompanying drawings, which serve only as illustrations and do not limit the present invention in any way.

[0015] The further scope of application of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, although they represent preferred embodiments of the present invention, serve only for illustrative purposes, since various changes and modifications within the spirit and scope of the invention will be apparent to the person skilled in the art from the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Fig. Figure 1 shows a cross-sectional front view illustrating the setup of an embodiment of a quantum cascade detector.

[0017] Fig. Figure 2 shows a side view of the setup of the quantum cascade detector of the Fig. 1.

[0018] Fig. Figure 3 shows a table that presents an example of a semiconductor laminate structure in the quantum cascade detector.

[0019] Fig. Figure 4 shows a diagram illustrating an example of the construction of a unit laminate structure that forms an active layer in the quantum cascade detector.

[0020] Fig. Figure 5 shows a cross-sectional front view of a setup of another embodiment of the quantum cascade detector.

[0021] Fig. Figure 6 shows a diagram illustrating the dependence of the refractive indices of the active layer and the cladding layer on a dopant density.

[0022] Fig. Figure 7 shows a diagram illustrating the dependence of a limiting factor of light in the mantle layer on the thickness of the mantle layer.

[0023] Fig. Figure 8 shows a diagram illustrating a waveguide mode for light when the total thickness of the cladding layer is 4 μm.

[0024] Fig. Figure 9 shows a diagram of a waveguide mode of light when the total thickness of the cladding layer is 20 μm.

[0025] Fig. Figure 10 shows a diagram of a waveguide mode of light in a waveguide structure with the active layer and the cladding layer.

[0026] Fig. Figure 11 shows a diagram representing a sensitivity spectrum in the quantum cascade detector and an oscillation spectrum in the quantum cascade laser.

[0027] Fig. Figure 12 shows a diagram illustrating an example of the setup of an optical system with the quantum cascade laser and the quantum cascade detector.

[0028] Fig. Figure 13 shows a diagram representing a light condenser state through a lens of the light emitted from the quantum cascade laser.

[0029] Fig. Figure 14 shows a diagram of another example of the setup of the optical system with the quantum cascade laser and the quantum cascade detector.

[0030] Fig. Figure 15 shows a diagram representing carbon monoxide (CO) absorption lines.

[0031] Fig. Figure 16 shows a diagram illustrating a change in the wavenumber of the light due to a temperature of the quantum cascade laser and an input current.

[0032] Fig. Figure 17 shows a diagram illustrating a spectrometry method using the optical system of the Fig. 14 represents.

[0033] Fig. Figure 18 shows a diagram that displays a spectrometry result using the optical system of the Fig. 14 represents.

[0034] Fig. Figure 19 shows a cross-sectional front view, which is an example of a detector array setup using the quantum cascade detector. DESCRIPTION OF THE EXECUTION FORMS

[0035] An embodiment of a quantum cascade detector according to the present invention is described in detail below with reference to the drawings. In the description of the drawings, the same elements are identified by the same reference numerals, and repetition of descriptions is omitted. The dimensions in the drawings do not always correspond to those in the description.

[0036] Fig. Figure 1 shows a cross-sectional front view of a setup of an embodiment of a quantum cascade detector. Fig. Figure 2 shows a side view of the setup of the quantum cascade detector of the Fig. 1. A quantum cascade detector 1AAccording to the present embodiment, a photodetector is used to detect light by means of light absorption due to intersubband electron excitation in a semiconductor quantum well structure. The quantum cascade detector 1A It is constructed in such a way that it is a semiconductor substrate. 10 and an active layer 15 , which are on the substrate 10 is formed, includes.

[0037] The active layer 15 It exhibits a cascade structure in which an absorption region (light absorption layer), used for the absorption and capture of light, and a transport region (electron transport layer), used for the transport of electrons as a carrier, are alternately stacked in several stages. In particular, the active layer 15 , if n is an integer of 3 or more, a unit laminate structure 16 a period (see Fig. 1) a semiconductor laminate structure comprising n quantum well layers with a first quantum well layer serving as an absorption quantum well layer and n quantum barrier layers, forming a cascade structure in which the absorption regions and the transport regions alternate in the form of a multi-stage lamination of the unit laminate structures 16 They are stacked, with the absorption region comprising the first quantum well layer and the light to be detected being captured via intersubband absorption, and the transport region transporting the electrons excited by the intersubband absorption. The structure of the unit laminate. 16 , which are in the active layer 15 What is included will be described in more detail later.

[0038] The active layer 15 is with a lower mantle layer 21 , which are next to the active layer 15is formed and has a lower refractive index than the active layer 15 exhibits below the active layer 15 and between the active layer 15 and the semiconductor substrate 10 formed. A lower contact layer 22 is below the lower mantle layer 21 provided. A lower metal layer. 23 is between the lower mantle layer 21 , the lower contact layer 22 and the semiconductor substrate 10 planned.

[0039] The active layer 15 is with an upper mantle layer 26 , which are next to the active layer 15 is formed and has a lower refractive index than the active layer 15 exhibits above the active layer 15 and on an opposite side of the semiconductor substrate 10 provided. An upper contact layer 27 is above the upper mantle layer26 provided. An upper metal layer 28 is on a side opposite the active layer 15 , referring to the upper mantle layer 16 and the upper contact layer 27 , planned.

[0040] Based on the above, the semiconductor substrate 10 through the lower metal layer 23 , the lower contact layer 22 , the lower mantle layer 21 , the active layer 15 , the upper mantle layer 26 , the upper contact layer 27 and the upper metal layer 28 a laminate structure is formed from a first end face. 20a and a second front face 20b (see Fig. 2), which are oriented in a waveguide direction in a waveguide structure of the light with the active layer 15 , the lower mantle layer 21 and the upper mantle layer 26are arranged, the first end face 20a an entry surface through which the light to be detected enters, wherein the light detects a detection object in the detector 1A is. For the first end face in each case 20a and the second frontal surface 20b A slit plane is preferably used. A lower electrode layer 12 A metal layer is transformed into an active layer on the opposite side of a back surface. 15 of the semiconductor substrate 10 educated.

[0041] In the present embodiment, the quantum cascade detector 1A in the form of a mesa structure with a basal section 30 , which is the semiconductor substrate 10 includes, and a measuring section 20 , which is based on the base section 30 The active layer is intended to 15It comprises and extends in a strip shape in the waveguide direction within the waveguide structure described above. Fig. The configuration example shown in 1 forms the semiconductor substrate. 10 and the lower metal layer 23 the basic section 30 , and the lower contact layer 20 , the lower mantle layer 21 , the active layer 15 , the upper mantle layer 26 and the upper contact layer 27 form the measuring section 20 .

[0042] On the base section 30 and the measuring section 20 An insulating layer will be applied. 11 designed to measure both side surfaces and one top surface of the measuring section 20 and an upper surface of the lower metal layer 23 , which are on the substrate 10 It is exposed, to cover. The upper metal layer 28 is on the insulating layer 11trained. In the insulating layer 11 will create an opening (contact hole) 11a on the upper surface of the measuring section 20 formed, and the upper metal layer 28 is about the opening 11a in contact with the upper contact layer 27 With this structure, the upper metal layer serves a purpose. 28 as an upper electrode layer.

[0043] In the Fig. 2 shown configuration example for the waveguide structure with the active layer 15 and the lower and upper mantle layers 21 , 26 is an anti-reflective film (low-reflection film) 31 , which is designed to reduce the reflectivity for light of the wavelength of the light to be detected, on the first end face 20a , which is the entrance surface in the waveguide structure. Furthermore, a reflective film (film with high reflectivity) is formed. 32, which is designed to increase the reflectivity for light of the wavelength of the light to be detected, on the second front surface 20b , which is the opposite side to the first end face 20a in the waveguide structure, provided for, with the second end face 20b a reflective surface for reflecting the light to be captured.

[0044] The effects of the quantum cascade detector will be described below. 1A as described in the present embodiment.

[0045] In the Fig. 1 and Fig. 2 quantum cascade detectors shown 1A is the active layer 15 , which is used to detect the light to be detected, with the lower mantle layer 21 below the active layer 15 and between the active layer and the substrate 10 and the upper mantle layer 26 above the active layer 15formed, and furthermore, the first frontal surface 20a the entrance surface for the light to be detected, wherein the first end face has an end in the waveguide structure with the active layer and the lower and upper cladding layers 21 , 26 is. According to this configuration, the light to be detected, which originates from the first end face, can 20a enters, along the active layer 15 in the waveguide structure using the cladding layers 21 , 26 are guided, whereby the light to be detected is efficiently directed into the active layer 15 is absorbed and captured.

[0046] Furthermore, in this setup, the waveguide structure above is combined with the lower metal layer. 23 below the lower mantle layer 21 and between the lower mantle layer and the substrate 10 provided, and the upper metal layer 28 is above the upper mantle layer 26The waveguide structure is thus designed with the active layer. 15 and the lower and upper mantle layers 21 , 26 designed in such a way that they are between the lower metal layer 23 and the upper metal layer 28 is arranged in a sandwich-like manner, with one area being a light-receiving section of the detector. 1A on the entrance area 20a through the metal layers 23 , 28 This is limited, thereby improving specific detectability. Thus, according to the quantum cascade detector... 1A The above setup allows the light to be detected in a suitable manner with high efficiency. The quantum cascade detector 1A This setup can be used in a suitable way to detect light in a mid-infrared range with a wavelength of, for example, 4 to 10 μm.

[0047] In the above embodiment, the quantum cascade detector 1A formed in the form of a mesa structure, which forms the base section 30 with the semiconductor substrate 10 and the measuring section 20 , which is based on the base section 30 The active layer is intended to 15 It has a strip-like shape and extends in the waveguide direction within the waveguide structure. According to this configuration, the waveguide structure can be combined with the active layer. 15 and the mantle layers 21 , 26 in a suitable manner along a direction in which the measuring section 20 extends, are formed.

[0048] In the above embodiment, the anti-reflective film 31 to reduce the reflectivity of the light to be detected (for example, the reflectivity for light of the wavelength at which the detection sensitivity of the detector is 1A(is highest) on the first end face 20a , which forms the entry surface for the light to be captured in the waveguide structure. According to this design, the incident efficiency of the light to be captured from the entry surface is improved. 20a to the inside of the quantum cascade detector 1A , which increases the detection efficiency of the light in the detector 1A Improved. The anti-reflective film. 31 is preferably designed with a reflectance of 28% or less for light of the wavelength of the light to be detected.

[0049] In the above embodiment, the reflective film 32 to increase the reflectivity of the light to be captured on the second end face 20b, which forms the reflective surface for the light to be detected within the waveguide structure. According to this setup, the light to be detected, which is guided from the entrance surface through the waveguide structure and the reflective surface, is 20b reached, once again inside the quantum cascade detector 1A attributed to this, thereby reducing the detection efficiency of the light in the detector. 1A improved. The reflection film 32 is preferably designed with a reflectance of 95% or more for light of the wavelength of the light to be detected.

[0050] Regarding the layer thickness of each semiconductor layer that forms the quantum cascade detector 1A forms, exhibits both the lower mantle layer 21 as well as the upper mantle layer 26 preferably a layer thickness of 2 μm or more and 10 μm or less. The active layer 15preferably has a layer thickness of 1 μm or more. According to these configurations, it is possible to suitably penetrate the active layer. 15 and the mantle layers 21 , 26 to achieve the waveguide structure for the light to be detected, as well as a light limiting structure in the waveguide, and the like.

[0051] In both the lower mantle layer 21 as well as the upper mantle layer 26 The dopant density (n impurities) is preferably 5 × 10 16 cm –3 or more and 2 × 10 17 cm –3 or less. According to this structure, the dopant density in the shell layers can be adjusted. 21 , 26 the series resistance in the detector 1A and a loss of light in the mantle layers 21 , 26 be suppressed.

[0052] In the active layer 15The dopant density (n impurities) is preferably 1 × 10 17 cm –3 or more and 9 × 10 17 cm –3 or less. According to this structure, it is possible to deal with the active layer in a suitable manner. 15 and the mantle layers 21 , 26 the waveguide structure, the light limiting structure in the waveguide, and a light detection structure in the active layers 15 to form. The configuration conditions of the quantum cascade detector 1A , such as the layer thickness of each of the semiconductor layers and the dopant density, will be described in more detail later.

[0053] The following describes the structure of the quantum cascade detector. 1A according to the above embodiment, using a specific example of a device structure that incorporates the quantum well structure in the active layer 15 has been described. Fig. Figure 3 shows a table that provides an example of the semiconductor laminate structure in the quantum cascade detector. 1A represents. Fig. Figure 4 shows a diagram illustrating an example of the structure of the unit laminate. 16 , which is the active layer 15 in the quantum cascade detector 1A forms, represents.

[0054] The present configuration example shows an example where the quantum well structure is located in the active layer. 15 It is designed to have an energy difference of 290 meV between the lower and upper detection levels, which represents a peak in the light detection sensitivity spectrum, and to have detection sensitivity for light with a wavelength λ = 4.5 μm, located in the mid-infrared range. However, the detection wavelength in the quantum cascade detector is 1AThe above embodiment is not limited to 4.5 μm and can be set to any value as required. For example, the detection wavelength can be set to a wavelength range of 4 to 10 μm in the mid-infrared region.

[0055] Fig. Figure 4 shows the quantum well structure and a subband level structure in part of the multilevel repeating structure of the unit laminate structures. 16 , each with an absorption range 17 and a transport area 18 in the active layer 15 exhibiting the device structure with the semiconductor laminate structure in the quantum cascade detector. 1A can be formed by crystal growth using the molecular beam epitaxy (MBE) process or the metal-organic chemical vapor deposition (MOCVD) process.

[0056] In the semiconductor laminate structure of the quantum cascade detector1A According to the present configuration example, an n-InP substrate is used as the semiconductor substrate. 10 in which Fig. 1 and Fig. The setup shown in section 2 is used. As shown in Fig. 1 and Fig. Figure 3 shows the device structure of the quantum cascade detector. 1A on the InP substrate 10 by sequentially stacking the lower contact layer 22 , which is formed by an InGaAS layer with a thickness of 250 nm and an InAlAs layer with a thickness of 0.2 nm, the lower InP sheath layer 21 with a thickness of 3000 nm, the active layer 15 , in which the unit laminate structures 16 each with the absorption range 17 and the transport sector 18 stacked with 45 periods, the upper InP mantle layer 26 with a thickness of 3000 nm and the upper InGaAs contact layer 27 formed with a thickness of 250 nm.

[0057] The active layer 15 In the present configuration example, this is achieved through a multi-period lamination of the unit laminate structures. 16 each with the absorption range 17 and the transport sector 18 trained. The number of lamination repetitions / periods of the unit laminate structure. 16 in the active layer 15 It can be set to, for example, 10 to 50 periods, although in the present configuration example, as described above, it is set to 45 periods. The unit laminate structure 16 One period is formed as a quantum well structure in which there are seven quantum well layers. 161 until 167 and seven quantum barrier layers 171 until 177 , as in Fig. 3 and Fig. 4 are shown, stacked alternately.

[0058] These semiconductor layers form the unit laminate structure 16is each of the quantum well layers 161 until 167 formed from an InGaAs layer. Each of the quantum barrier layers 171 until 177 is formed from an InAlAs layer. Thus, the active layer 15 In the present configuration example, it is formed by an InGaAs / InAlAs quantum well structure.

[0059] The layer thicknesses and the like of the quantum well layers and the barrier layers that form the active layer 15 form, are in Fig. 3 shown.

[0060] In this unit laminate structure 16 form the first barrier layer 171 and the first quantum well layer 161 the absorption range 17 for detecting light via intersubband absorption. The second to seventh barrier layers. 172 until 177 and the second to seventh quantum well layer 162 until 167 form the transport sector 18for the transport of electrons excited by intersubband absorption into an absorption region 17b of the next period. It will be Si, which represents an n-impurity, with a dopant density of 5 × 10 17 cm –3 , into the first quantum well layer 161 , which serves as an absorption quantum well layer for absorbing the light to be detected, is doped to supply electrons as carriers.

[0061] In the present configuration example, as shown in Fig. 3 shown, Si, which forms the n-impurity, in the same way with the dopant density of 1 × 10 17 cm –3 in both the lower mantle layer 21 as well as the upper mantle layer 26 doped. Si, which forms the n-impurity, is doped with a dopant density of 3 × 10 18 cm –3 in both the InGaAs layer of the lower contact layer 22 as well as the contact layer27 endowed.

[0062] In this construction, the unit laminate structure exhibits 16 a lower detection level L1 and an upper detection level L2, which are used for light absorption in the absorption range 17 contribute, and a variety of transport levels L1 to L7, which are involved in electron transport in the transport region 18 contribute as conduction band sub-band levels to light detection in the in Fig. The subband level structure shown in section 4 is shown.

[0063] When light with a wavelength λ enters the active layer 15 with the unit laminate structures 16 The electrons present in the lower detection level L1 are excited to the upper detection level L1 via intersubband absorption. The electrons excited to the upper level L2 are then excited to the lower detection level L1 in the absorption range. 17bthe subsequent stage with the help of a transport level structure that accommodates the multitude of transport levels in the transport area 18 encompasses, transports, and extracts. Electron excitation by light absorption, relaxation and transport of the excited electrons, as well as the extraction of electrons in the unit laminate structure of the next period, are represented in the multitude of unit laminate structures. 16 , which is the active layer 15 This process is repeated, resulting in a cascade of light absorption in the active layer. 15 occurs. Subsequently, a current generated by this process is extracted as a signal and the magnitude of the current is measured in order to detect incident light.

[0064] The following is an example of a method for manufacturing the quantum cascade detector. 1A as described above in the embodiment. The quantum cascade detector 1Awith the laminate structure, in which the active layer 15 with the mantle layers 21 , 26 and the metal layers 23 , 28 , as previously described, can be produced, for example, by substrate lamination.

[0065] The laminate structure and the active layer structure in the quantum cascade detector 1A The layers are formed by sequential epitaxial growth of each layer using the MBE process, the MOCVD process, or similar techniques. First, a Si-doped InGaAs contact layer, a Si-doped InP sheath layer, an active layer with an InGaAs / InAlAs quantum well structure, a Si-doped InP sheath layer, and a Si-doped InGaAs contact layer are grown on an InP substrate (first substrate), and a first metal layer of Au (gold) with a thickness of 0.5 μm to 1.0 μm is deposited on top.

[0066] Subsequently, a second metal layer of Au with a thickness of 0.5 μm to 1.0 μm is deposited on an n-InP substrate (second substrate), which is the semiconductor support substrate. 10 The first metal layer on the first substrate and the second metal layer on the second substrate are formed and brought into contact with each other and subjected to moderate heat treatment to bond the two substrates together. The bonded first and second metal layers become the lower metal layer. 23 on the substrate 10 . In this case, the metal material used for the lower and upper metal layers is not limited to the previously described Au and other metal materials, such as Cu (copper) or Al (aluminum), which can be deposited, can be used.

[0067] Subsequently, the first substrate used for the growth of the semiconductor laminate structure is removed by selective chemical etching, and then, for example, a strip-shaped mesa structure is created (see Fig. 1) Formed with a width of 50 μm by wet or dry etching. Etching is carried out under the condition that the etching continues until the lower metal layer is reached. 23 , which forms a connecting section of the substrate lamination, takes place or in the lower contact layer 22 ends. The insulating layer is then... 11 made of an insulating material, such as SiN, the opening 11a and the upper metal layer 28 made of gold or the like, and the upper electrode layer is formed by vapor deposition and plating.

[0068] The back surface of the InP substrate is then 10 polished, so that the thickness of the substrate 10for example, 150 μm, and it is the lower electrode layer. 12 formed for the extraction of current on the substrate's rear surface. The lower electrode layer 12 This is formed, for example, by performing a vapor deposition process and producing an alloy of Ti and Au / Ge / Au. Finally, a slitting step is performed so that the device length is, for example, 500 μm to accommodate the quantum cascade detector. 1A to produce. The slit planes are at this point the first end face (entry face) in each case. 20a and the second end face (reflecting surface) 20b , which in the waveguide direction in the waveguide structure with the active layer 15 and the mantle layers 21 , 26 run (see Fig. 2).

[0069] On the second end face 20b The waveguide structure is fitted with a reflective film (high-reflection coating) 32formed, whose reflectivity is 95% or more for light of the wavelength of the light to be detected. Thus, the light to be detected, which is reflected from the first end face 20a into the active layer 15 penetrates and spreads through the interior, through the reflection film 32 reflected, thus increasing the propagation path of the light within the detector. 1A It can be formed that is twice as long as the device length.

[0070] Furthermore, on the first end face 20a To suppress reflection at the slit plane, an antireflection film (antireflection coating) is used. 31 formed, whose reflectance is 28% or less for light of the wavelength of the light to be detected. If the refractive index of the semiconductor material is denoted as n1 and the refractive index of air as n0, the reflectance for light at the slit plane is given by the expression (n1 – n0)2 / (n1 + n0) 2 obtained. In the previously described configuration example, the reflectance at the slit plane is 28.6% when the refractive index of InGaAs / InAlAs n1 = 3.3 and the refractive index of air n0 = 1. Thus, the antireflection film 31 formed, whose reflectivity, as previously described, is 28% or less, thereby suppressing the reflection of light at the slit plane.

[0071] For the reflection film 32 on the second end face 20b For example, a configuration can be used in which material, such as insulating Al2O3, SiO2 or CeO2, and Au are placed on the end face. 20b to be separated. As a reflective film. 32A dielectric multilayer film can be used by alternately stacking a material with a low refractive index, such as Al₂O₃, SiO₂, CeO₂, or ZnS, and a material with a high refractive index, such as Ge. For the antireflection film 31 on the first end face 20a For example, a configuration can be used in which Al₂O₃ is deposited with a thickness of 0.78 μm. The thickness mentioned above is a value where one-quarter of the wavelength of 4.5 μm is further divided by the refractive index of Al₂O₃, which is 1.44. As an antireflection film 31 Can a single layer film made of a different dielectric material or a dielectric multilayer film made of similar materials be used as a reflective film? 32 be used.

[0072] The following section describes the structure of the quantum cascade detector in more detail. Fig. Figure 5 shows a cross-sectional front view illustrating the setup of another embodiment of the quantum cascade detector. A quantum cascade detector 1B According to the present embodiment, it has the same structure as the quantum cascade detector. 1A , who in Fig. 1 and Fig. 2 shown is, on, with the difference that in addition to the opening 11a in the insulating layer 11 an opening 11b in the lower metal layer 23 is provided for and that instead of the lower electrode layer 12 on the back surface of the substrate 10 the lower electrode layer 13 on the insulating layer 11 is planned.

[0073] In the Fig. In the configuration example shown in section 5, the opening (contact hole) 11b on the upper surface of the lower metal layer 23 in the insulating layer 11formed. Part of the upper metal layer 28 , which are on the insulating layer 11 The metal layer is electrically isolated from the metal layer. 28 isolated by partially removing the metal material, and the isolated metal layer section is across the opening. 11b with the lower metal layer 23 in contact and becomes part of the lower electrode layer 13 . For the formation of the contact hole in this structure and for the partial removal of the metal layer of Au or the like, a process can be used in which a photoresist is structured by photolithography, a vapor deposition of SiN, Au and the like is carried out and then the photoresist is removed.

[0074] In the following, certain configuration conditions and the like of the quantum cascade detector according to the above embodiment are described in more detail. First, a relationship between the number of lamination periods of the unit laminate structure is established. 16 in the active layer 15 , the noise in the detector 1A , the limitation of light in the active layer 15 , and the like. In a noise stream i N in the quantum cascade detector 1A thermal noise, which depends only on the device resistance, is dominant, as indicated by the following formula (1).

[0075] In the formula above, k denotes BThe Boltzmann constant, T a device temperature, Δf a bandwidth (where Δf = 1), and R a device resistance. From formula (1), it can be seen that the device resistance must be increased to suppress the noise current and improve the signal-to-noise ratio.

[0076] The device resistance in the detector 1A is essentially proportional to the number of lamination periods of the cascade structure in the active layer 15 For this reason, the resistance of the device can be increased by increasing the number of periods of the unit laminate structures. 16 in the active layer 15 This will be increased. Increasing the number of lamination cycles in the active layer. 15 This also contributes to increasing the light-receiving area on the first end face. 20a in the waveguide structure with the active layer 15, to improve the limiting characteristics of the light when the light to be detected is within the detector 1A is directed to reduce light loss and the like.

[0077] Limiting the light in the active layer 15 This will be described in detail below. The limitation of the light to the active layer. 15 in the quantum cascade detector 1A The above setup depends on the configuration conditions, such as the refractive index of each active layer. 15 and the mantle layers 21 , 26 , the layer thicknesses of the mantle layers 21 , 26 and the layer thickness of the active layer 15 ab. When forming the waveguide structure, it is important that, of all these conditions, the refractive index of the active layer 15 higher than the refractive index of the mantle layers 21 , 26The refractive index of each semiconductor layer depends on the amount of dopant in each layer and can be obtained from a complex dielectric constant calculated on the basis of the Drude model.

[0078] Fig. Figure 6 shows a diagram illustrating the dependence of the refractive indices of the active layer (InGaAS / InAlAS layer) and the cladding layer (InP layer) on the dopant density. In the diagram of Fig. 6 gives the horizontal axis a dopant density (cm³). –3 ) the impurities in each layer and the vertical axis a refractive index. In Fig. 6. The plotted points A1 show a relationship between the refractive index of the active layer. 15 and the dopant density, and the plotted points A2 show a relationship between the refractive index of the mantle layers 21 , 26 and the dopant density.

[0079] In the waveguide structure with the active layer 15 and the mantle layers 21 , 26 To reduce light loss in the waveguide, the dopant density in the cladding layers is 21 , 26 preferably 2 × 10 17 cm –3 or less. However, it is in the quantum cascade detector 1A , to which no external voltage is applied, since the electromotive force is small, it is necessary to determine the series resistance except in the active layer 15 The resistance forming a photoelectric conversion section is kept low to extract photoelectric current from the electrode. If such a series resistance condition is taken into account, the dopant density in the cladding layers is... 21 , 26 preferably 5 × 10 16 cm –3 or more. It is evident from the above that the dopant density in each mantle layer 21 , 26preferably 5 × 10 16 cm –3 or more and 2 × 10 17 cm –3 or less.

[0080] From the diagram of Fig. 6 shows that the highest value of the refractive index of the mantle layers 21 , 26 Under the above dopant density condition, a refractive index value of 3.091 is obtained when the dopant density is 5 × 10 16 cm –3 is, as seen through the straight line in Fig. Figure 6 shows the upper limit of the dopant density in the active layer. 15 determined by the condition under which the refractive index of the active layer 15 exceeds the refractive index of the mantle layer, as previously described. From the in Fig. The 6 points shown in the diagram A1 show that the upper limit of the dopant density in the active layer 15 preferably 9 × 10 17 cm –3The refractive index value of the active layer is... 15 At this dopant density, it is 3.095.

[0081] However, the lower limit of the dopant density in the active layer is 15 From the perspective of waveguide formation, there are no limitations; however, a reduction in the number of free electrons contributing to the photoelectric current leads to a decrease in the detection signal intensity generated and output by the light detection. If such a signal intensity condition is taken into account, the lower limit of the dopant density in the active layer is... 15 preferably 1 × 10 17 cm –3At lower dopant density than the lower limit, it is advisable to define a lower limit, since the change in the refractive index is small and a significant improvement in the limiting characteristic is not to be expected. From the above, it follows that the dopant density in the active layer should preferably be 1 × 10 17 cm –3 or more and 9 × 10 17 cm –3 or less.

[0082] The following section describes the thickness of the mantle layers, 21 , 26 described. A function of the mantle layers 21 , 26 The above setup consists of capturing the light that is emitted from the first end face. 20a into the waveguide structure with the active layer 15 and the mantle layers 21 , 26 to penetrate, limit and reduce the absorption loss of light in the metal layers 23 , 28and the contact layers 22 , 27 to reduce with a high carrier density.

[0083] As previously described, the dopant density in the mantle layers is 21 , 26 5 × 10 16 cm –3 and the dopant density in the active layer 15 9 × 10 17 cm –3 , is the refractive index difference between the active layer 15 and the mantle layers 21 , 26 minimal. At this point in the waveguide structure, the configuration condition becomes one where limiting light in the active layer is most difficult and outputting light to the cladding layers is minimal. 21 , 26 is highest. with regard to the thickness of the mantle layers. 21 , 26 Thus, under this configuration condition, a state can be maintained that prevents light from reaching the contact layers. 22 ,27 and the metal layers 23 , 28 achieved, and which is able to confine the light within the waveguide structure. Regarding the wavelength of the light to be detected when adjusting the layer thickness, assuming that the wavelength range 4 The case of a wavelength of 10 μm, where limiting the light is most difficult, is considered up to 10 μm in the mid-infrared range.

[0084] Fig. Figure 7 shows a diagram illustrating the dependence of a limiting factor of the light in the cladding layer (InP layer) with respect to the contact layer (InGaAS layer) on the thickness of the cladding layer. In the diagram of the Fig. Figure 7 shows the horizontal axis as the thickness of the mantle layer (μm) and the vertical axis as the limiting factor of the light. Fig. 7 is the limiting factor of the light defined by a ratio of an integrated value of the electric field strength in the InP cladding layer to an integrated value of the total electric field strength for the case where a one-dimensional waveguide mode is obtained in the waveguide structure.

[0085] Fig. Figure 8 shows a diagram illustrating a simulation result of a waveguide mode of light when the total thickness of the InP cladding layers is 21 , 26 4 μm, for the InGaAs contact layers 22 , 27 as an example of light limitation. Similarly, it shows Fig. 9 a diagram representing a simulation result of a waveguide mode of light when the total thickness of the cladding layers 21 , 26 20 μm. In the diagrams of the Fig. 8 and the Fig. Figure 9 shows the horizontal axis as a position (μm) and the vertical axis as a normalized intensity of the light to be conducted or a refractive index of each layer. For the calculation, the thickness of the InGaAs contact layer is 3 μm and the dopant density in the contact layer is 3 × 10 18 cm –3 and the refractive index 2 , 5 The active layer 15 , which are located between the mantle layers 21 , 26 Its location is not taken into account here.

[0086] According to the diagram of Fig. 7 is the total thickness of the InP sheath layers 21 , 26 For layers 4 μm or thicker, the limiting factor of light for the cladding layer is 90% or more. Therefore, the thickness of each cladding layer is... 21 , 26 preferably 2 μm or more. In fact, since the active layer 15with a higher refractive index than that of the mantle layer between the mantle layers 21 , 26 The presence of a waveguide structure increases the limitation of light to the waveguide structure, thereby increasing the absorption loss of light in the contact layers. 22 , 27 and the metal layers 23 , 28 can be sufficiently suppressed.

[0087] The upper limit of the thickness of the mantle layers 21 , 26 is not limited to any specific one; however, the thickness of the mantle layer is 11 μm or more in the diagram of Fig. At point 7, the limiting factor reaches 99% and no longer changes. Taking this point into account, the layer thickness of each mantle layer is... 21 , 26 preferably 10 μm or less. It is evident from the above that the layer thickness of each cladding layer 21 , 26preferably 2 μm or more and 10 μm or less.

[0088] The following section describes the thickness of the active layer. 15 described, In the structure where the layer thickness of the mantle layers 21 , 26 In the structure described above, where the layer is thinnest, the total thickness of the lower mantle layer is 21 and the upper mantle layer 26 4 μm. However, due to the diffraction limit, a point of light can only be focused to approximately half a wavelength. Therefore, if the wavelength of the light to be detected is 10 μm, the sum of the layer thicknesses of the active layer is... 15 and the mantle layers 21 , 26 as a light-receiving surface on the first end face 20a preferably 5 μm or more. If such an incident light condition is taken into account, the thickness of the active layer is 15preferably 1 μm or more. If the wavelength of the light to be detected is less than 10 μm, since the incident light condition and the limiting light condition in the waveguide structure are relaxed, the above condition for light with a wavelength of 10 μm can be applied.

[0089] Since the layer thickness of the active layer 15 of the number of lamination periods of the unit laminate structures 16 in the active layer 15 Depending on the number of periods of the unit laminate structure 16 be adjusted so that the layer thickness of the active layer 15 1 μm or more. An average thickness of the unit laminate structure. 16 a period in the active layer 15 of the quantum cascade detector 1AThe thickness is approximately 50 nm, although it varies depending on the wavelength of the light being detected, the specific construction, or similar factors, in which case the number of periods 20 periods or more to determine the thickness of the active layer 15 to set to 1 μm or more. As previously described with reference to formula (1), this configuration condition is also used to increase the resistance in the active layer. 15 and suitable for suppressing thermal noise.

[0090] Since the upper limit of the layer thickness of the active layer 15 Since the layer thickness is not limited to any specific value, it can be suitably set to 1 μm or more. In the configuration example above, for instance, the number of periods of the unit laminate structure is... 16 in the active layer 15 45 periods, with the layer thickness set to 1.65 μm. As in Fig. As shown in 3, the layer thickness of each mantle layer is 21 , 26 3 μm. The dopant density of the impurities in the jacket layers. 21 , 26 is on 1 × 10 17 cm –3 determined, and the dopant density in the active layer 15 is on 5 × 10 17 cm –3 determined.

[0091] Fig. Figure 10 shows a diagram of a waveguide mode of light in the waveguide structure with the active layer. 15 and the mantle layers 21 , 26 in the configuration example above. In the configuration example, the limiting factor of the light in the active layer indicates 15 corresponding to the ratio of the integrated value of the electric field strength in the active layer to the integrated value of the total electric field strength, a value of 0.75.

[0092] This can be done in the quantum cascade detector1A with the in Fig. 1 and Fig. The setup shown in section 2 depicts the incidence of the light to be detected onto the waveguide structure within the detector. 1A by collecting the light with a lens on the first end face 20a This can occur, for example, in a slit plane on which the anti-reflective film is applied. 31 The above device structure is formed using the active layer. 15 , the mantle layers 21 , 26 and the metal layers 23 , 28 used to thereby reduce the light coming from the first end face 20a enters, to efficiently convert into electricity.

[0093] That is, with the mantle layers 21 , 26 , between which the active layer 15 When recorded from above and below, a spatial distance between the active layer can occur. 15 , which forms the photoelectric circumferential section, and the contact layers22 , 27 , which are doped with impurities at high density. This prevents light penetrating the interior of the waveguide structure from passing through the free carriers in the contact layers. 22 , 27 is absorbed, and thus the light to be detected cannot be captured within the active layer. 15 , which are located between the mantle layers 21 , 26 is arranged, can be directed, thereby increasing the amount of light that contributes to the photoelectric conversion.

[0094] The light that enters the active layer 15 is directed and the second front surface 20b This is achieved through the reflection film. 32 reflected and generates a photoelectric current while inside the active layer 15 is forwarded. This means that the detector device length 1AThe detection signal intensity can be reduced to 1 / 2. Such a reduction in device length also contributes to an increase in device resistance due to the reduction in device size and also proves to be efficient in reducing the noise current, as shown in equation (1).

[0095] In the previously described configuration example, the number of periods of the unit laminate structure is 16 in the active layer 15 set to 45 periods, with the layer thickness of the active layer 15 1.65 μm. Thus, the limiting factor for the light entering the active layer can be 15 penetration, increased, and the thermal noise can be increased due to the increase in device resistance in the detector. 1A be suppressed.

[0096] According to the above structure, which consists of the metal layers 23 , 28exhibiting features formed by substrate lamination or the like, the light-receiving section of the first end face is 20a only on the active layer 15 and the mantle layers 21 , 26 , which includes the active layer in between. Thus, the light-receiving area in the detector can be limited. 1A sufficiently reduced and a specific detective capability D* increased.

[0097] The following describes a relationship between the light-receiving area in the quantum cascade detector 1A and the specific detectivity D* is described. The specific detectivity D* of the detector 1A can be represented by the following formula (2).

[0098] In the formula above, v denotes s an output signal voltage, v na noise voltage (in the quantum cascade detector, the product of the noise current represented in formula (1) and the device resistance), P an energy of the incident light per unit area, A the light receiving area and Δf the bandwidth of the noise (where Δf = 1).

[0099] From the formula (2) above, it can be seen that if every voltage v s , v n and the energy P of the incident light is constant, the specific detectivity D* depends on the light-receiving area A, with the specific detectivity D* increasing as the light-receiving area A decreases.

[0100] In the device structure of the previously described quantum cascade detector 1A The photoelectric conversion section forms the active layer. 15 , and an entrance section of the light is originally directed onto a cross-section of the active layer 15 in the first end face 20aand its surroundings are limited. Thus, the mere fact that the light-receiving area is limited by the arrangement of the active layer leads to 15 and the mantle layers 21 , 26 between the metal layers 23 , 28 to limit, to no significant reduction of the output signal voltage v2 in formula (2).

[0101] For this reason, eliminating the influence of the substrate is beneficial. 10 and the like through the metal layers 23 , 28 and the limitation of the light-receiving area to only the cross-sectional area of ​​the active layer 15 and the mantle layers 21 , 26 in the first end face 20a This contributes to an improvement in the specific detectability D*. The reason is that sufficient signal intensity can be maintained even when the light-receiving area for the light to be detected is reduced to a size of, for example, a few tens of micrometers. 2The limitation lies in the fact that the light to be detected can be used for light detection with high efficiency, while it is located within the active layer. 15 with the waveguide structure using the cladding layers 21 , 26 , as previously described, spreads.

[0102] Fig. Figure 11 shows a diagram illustrating a sensitivity spectrum in the quantum cascade detector. 1A and represents an oscillation spectrum in the quantum cascade laser. In the diagram of the Fig. Figure 11 shows the horizontal axis as a wavelength (μm) and the vertical axis as a normalized intensity. Fig. Figure 11 shows diagram B1 a sensitivity spectrum in the quantum cascade detector (QCD) and diagram B2 shows an oscillation spectrum in the quantum cascade laser (QCL).

[0103] As in Fig. As shown in Figure 11, in the sensitivity spectrum of the quantum cascade detector, the sensitivity wavelength range around a wavelength of 4.5 μm, which forms the sensitivity peak, is approximately ±0.5 mm. Fig. Figure 11 also shows the oscillation spectrum of the distributed feedback quantum cascade laser (DFB-QCL) with an oscillation wavelength of 4.6 μm.

[0104] The following describes the setup of an optical system and the like, which uses the quantum cascade detector with the above-mentioned configuration. The quantum cascade detector is preferably used in conjunction with the quantum cascade laser. In this case, the detector's sensitivity wavelength is configured according to the oscillation wavelength of the laser used. Since the quantum cascade detector exhibits detection sensitivity only in a wavelength range determined by energy fluctuations of intersubband transitions, it is possible to suppress the influence of unwanted background light without the use of a filter or similar device.

[0105] Laser light supplied by a laser light source, such as a quantum cascade laser, can be easily focused by a lens to a spot size of approximately a few tens of micrometers. Thus, most of the light to be detected can be concentrated by the lens itself and then directed into the detector. 1A are directed when the light receiving section is on the active layer 15 and the mantle layers 21 , 26 , as previously described, is limited.

[0106] Fig. Figure 12 shows a diagram illustrating a configuration example of an optical system comprising the quantum cascade laser and the quantum cascade detector. In the Fig. The configuration example shown in 12 is that of the quantum cascade laser. 2A emitted light through a lens 36 collimated, and the collimated light is passed through a lens 37on the light-receiving surface of the quantum cascade detector 1A collected. In this case, the focal lengths of the lenses are 36 , 37 the same or different from each other.

[0107] In the configuration example above, the anti-reflective film is preferably applied to each of the lenses. 36 , 37 The lens is designed to have a reflectivity of 5% or less for light with a wavelength of 4 to 10 μm. Any material with a permeability to mid-infrared light, such as ZnSe, CaF2, or Ge, can be used. The diameter, focal length, and other parameters of the lens are not limited to any specific value; however, since the radiation light is emitted from the quantum cascade laser with a strong propagation of approximately 50° from one side, the lens's diameter and focal length are not limited. 2A The output should preferably have a numerical aperture of 0.5 or more.

[0108] Fig. Figure 13 shows a diagram representing a light-collecting state through a lens of the light emitted from the quantum cascade laser. In the diagram of Fig. Figure 13 shows the horizontal axis as a position (μm) and the vertical axis as a light-collecting intensity (au). Fig. 13 The plotted points indicate measured values, and the solid line represents a curve fitting using a Gaussian function.

[0109] In Fig. 13. An aspherical lens made of ZnSe with a focal length of 50.8 mm and a numerical aperture of 0.5 is used as the two lenses in each case. 36 , 37 used, whereby Fig. Figure 13 represents the light-collecting diameter when the radiation light from the DFB-QCL is collected with a wavelength of 4.6 μm. A knife-edge method is used for the measurement, and the measurement is performed at a specific position on the light-receiving surface within the quantum cascade detector. According to the results of a Gaussian fit, the full width at half maximum (FWHM) is approximately 10 μm.

[0110] In the quantum cascade detector 1A In the previously described configuration example, the sum of the layer thicknesses of the active layer is 15 and the mantle layers 21 , 26 , which are between the metal layers 23 , 28 are arranged, 7.65 μm. Thus, with regard to the thickness direction of the active layer, 15The reception area must be 70% or more of the full width at half maximum (FWHM) of the collected light to be detected. With respect to a direction perpendicular to the thickness direction, the entire collected light to be detected can be determined, since the edge width of the measurement section is... 20 50 μm, into the interior of the detector 1A be directed in that direction.

[0111] The radiation light from the quantum cascade laser 2A It exhibits linear polarization, in which the electric field oscillates in a direction perpendicular to the crystal growth surface. For this reason, at the moment the light to be detected strikes the quantum cascade detector, 1A the polarization direction is aligned such that the electric field is perpendicular to the crystal growth surface of the detector. 1A oscillates in the same way, so that the polarization dependence in the quantum cascade detector 1AThis poses no problem and the light from the quantum cascade laser 2A can be effectively captured.

[0112] This includes the information related to the quantum cascade detector. 1A The light source used with the above setup is not limited to the quantum cascade laser, and any light source can be used as long as it emits mid-infrared light and produces radiant light with a polarization component that the quantum cascade detector can detect. 1A addresses, for example, a gas laser, a carbon dioxide laser, a free electron laser, an infrared semiconductor LED light source, or a blackbody light source.

[0113] The following measurement example uses the quantum cascade detector. 1A The above setup was used to describe carbon monoxide (CO) spectrometry. Fig. Figure 14 shows a diagram that represents another configuration example of the optical system, which includes the quantum cascade laser and the quantum cascade detector, and that shows a configuration example of a measurement system for performing CO spectrometry.

[0114] The in Fig. The spectrometry system shown in section 14 comprises a light source unit. 50 with the quantum cascade laser 2A , a power supply 52 for the laser, and a lens 51 , an absorption unit 55 with a reusable cell 56 , a recording unit 60 with the quantum cascade detector 1A , a lens 61 , and a power amplifier 62 , and a signal processing unit 65 with a measuring device 66 , and a control device (PC) 67 They are mirrors. 57 , 58 between the reusable cell 56 and the lens 61planned

[0115] In the present configuration example, the light with a wavelength of 4.6 μm, which comes from the quantum cascade laser with distributed feedback, is used. 2A is emitted through the collimator lens 61 made to parallel light and caused to enter the reusable cell 56 , which is a gas cell, to penetrate. The light that passes through the gas cell and through the mirrors 57 , 58 The reflected image passes through the lens. 61 collected and brought into the quantum cascade detector 1A as light to be detected, followed by a detection signal output from the detector 1A is detected.

[0116] The through the detector 1A The detected signal is processed by the measuring device. 66 , such as an oscilloscope, using the current amplifier 62The presence of CO light absorption, measured by the attenuation of the detection signal intensity, is observed. In the present configuration example, the reusable cell 56 , in which an optical path length is, for example, 100 m, as a gas cell is used for the measurement; however, the setup is not limited to this, and depending on the condition, such as the type of gas to be measured or its concentration, a gas cell with one path or a resonator setup in which high-reflection mirrors are arranged opposite each other (for example, CRDS: Cavity Ring-Down Spectroscopy, ICOS: Integrated Cavity Output Spectroscopy), and the like, can be used.

[0117] Fig. Figure 15 shows a diagram representing carbon monoxide (CO) absorption lines. In the diagram of Fig. 15 shows the horizontal axis a wave number (cm). –1) and the vertical axis represents an absorption line intensity. The absorption lines specified in the HITRAN database are shown here. From the in Fig. The data shown in the 15 images forms 12 C 16 O with an absorption at a wavenumber of 2186.639 cm –1 (a wavelength of 4.573 μm) an object of observation for light absorption. In the present example, a gas cell with a volume of 0.35 m³ is used. 3 The gas cell is emptied and CO is trapped inside, resulting in a pressure of 0.3 Torr. This is related to the quantum cascade laser. 2A whose temperature is preferably kept constant by a temperature control function. The quantum cascade laser 2A is a single-mode oscillation DFB-QCL, and a laser with an oscillation wavelength of 4.6 μm is used in the present example.

[0118] Fig. Figure 16 shows a diagram illustrating the change in the wavenumber of light due to a change in the temperature of the quantum cascade laser and an input current. In the diagram of Fig. Figure 16 shows the horizontal line as a current (mA) and the vertical axis as a wavenumber of the light (cm). –1 ). In Fig. Figure 16 shows the current dependence of the wavenumber at a temperature of 20°C, the current dependence of a wavenumber at a temperature of 25°C, and the current dependence of a wavenumber at a temperature of 30°C.

[0119] The in Fig. The straight line shown in Figure 16 represents the CO absorption line, which depicts the previously described object of observation. From the diagram of the Fig. 16 shows that the driver temperature of the quantum cascade laser is, for example, 20°C and the feed current is changed in a range of 830 to 804 mA and the wavelength is continuously sampled so that the previously described absorption of CO can be observed.

[0120] Fig. Figure 17 shows a diagram illustrating a spectrometric method using the one described in Fig. The optical system shown in section 14 is represented. As in Fig. As shown in 17, a direct current is applied to the quantum cascade laser. 10 The signal is fed in with a slightly lower value than the threshold value It, and a modulation with a duration of, for example, ΔT = 5 ms is performed externally using a function generator in order to perform the wavelength sampling as described previously. Fig. In step 17, the current ΔI is adjusted such that the oscillation wavelength in the quantum cascade laser passes through the CO absorption line formed by the object being observed. The laser's oscillation is then synchronized with the oscilloscope, and an attenuation dip due to CO absorption can be observed in the pulse signal measured by the oscilloscope.

[0121] Fig. Figure 18 shows a diagram illustrating a CO spectrometry result using the [method / method] described in [reference]. Fig. The optical system shown in 14 is represented in the diagram of the Fig. Figure 18 shows the horizontal axis as time (ms) and the vertical axis as normalized signal intensity. In spectrometry using the oscilloscope above, the quantum cascade detector is used for measurements at room temperature, and an integration of 100 measurement results is shown. As in Fig. As shown in Figure 18, a sufficient signal intensity can be obtained to observe the CO absorption line even at room temperature operation using the quantum cascade detector. 1A is used with the above setup.

[0122] In the case of the quantum cascade detector with the above setup, it is possible, for example, to form the detector array by arranging several quantum cascade detectors in a one-dimensional array along a predetermined array direction.

[0123] Fig. Figure 19 shows a cross-sectional front view of an example of a detector array setup using the quantum cascade detector. Fig. The configuration example shown in section 19 is the quantum cascade detector. 16 with the in Fig. The setup shown in section 5 is used as the quantum cascade detector. A detector array 1Cis designed such that the array direction is a direction perpendicular to the waveguide direction in the waveguide structure in the detector 1B is, whereby a multitude of (in the Fig. 3 units) quantum cascade detectors 1B in a one-dimensional array along an array direction on the semiconductor substrate 10 are arranged. This detector array 1C It can be used, for example, as a line sensor.

[0124] The following is an example of a method for manufacturing the detector array. 1C using quantum cascade detectors 1B The configuration example above is briefly described below. First, the same steps are taken as in the procedure for manufacturing the previously described quantum cascade detector. 1AThe contact layer, the cladding layer, the active layer, the cladding layer, and the contact layer are grown on the InP substrate (first substrate), and then the first metal layer of Au is deposited on top. Subsequently, the second metal layer of Au is deposited on the semi-insulating InP substrate (second substrate) to complete the semiconductor substrate. 10 to form, whereby the first metal layer on the first substrate and the second metal layer on the second substrate are brought into contact with each other and subjected to a medium-stress heat treatment to bond the two substrates together. The first and second metal layers that are bonded together become the lower metal layer. 23 on the substrate 10 .

[0125] The first substrate used for growing the semiconductor laminate structure is then removed by selective chemical etching, and a strip-shaped mesa structure is formed by wet or dry etching. Furthermore, the insulating layer is... 11 made of an insulating material, such as SiN, the openings 11a , 11b (see Fig. 5) and the upper metal layer 28 from Au or the like to form the upper electrode layer and the lower electrode layer 13 formed from gold or the like.

[0126] Then, as in Fig. 19 shows the metal layer and the like being etched down to the substrate. 10 at a predetermined position between the adjacent detectors 1B removed, so that the multitude of quantum cascade detectors 1B , which are on the semiconductor substrate 10are arranged and electrically separated from one another. Finally, a splitting is performed so that the predetermined device length is reached, whereby the device is divided while a state in which the multitude of detectors 1B are arranged in an array, is maintained in order to thereby maintain the detector array 1C , which serves as a line sensor.

[0127] On the second end face (reflection surface) of the waveguide structure in each of the quantum cascade detectors 1B , which the detector array 1CFor example, a reflective film (high-reflectivity coating) is formed on the first surface, with a reflectivity of 95% or more for light of the wavelength to be detected. Conversely, on the second surface (entrance surface), an anti-reflective film (anti-reflectivity coating) is formed to suppress reflection at the slit plane, with a reflectivity of 28% or less for light of the wavelength to be detected.

[0128] The strip width, strip spacing, and similar parameters of each detector 1B in the detector array 1C depend on the mask structure in the photolithography step. The configuration conditions of the detector array. 1C can be adjusted appropriately depending on the spatial resolution required, for example, for the line sensor. Furthermore, each of the quantum cascade detectors... 1B, which are on the semiconductor substrate 10 are arranged, the necessary wiring is formed, and the detectors are connected to readout circuits, so that the detector array 1C It can be used as a line sensor.

[0129] The line sensor according to the configuration example above can, for example, be used as a photodetector for a spectroscope. In this case, the light components for each wavelength are detected using the line sensor with the above setup. The light is then subjected to spectral decomposition by a spectroscopy element, such as a diffraction grating or a prism, thus enabling the optical spectrum to be obtained in a simple manner.

[0130] When applied to the spectrometer of the line sensor, a high spatial resolution in the line sensor as a photodetector is required to obtain continuous spectral information. In the detector array 1CWith the above setup, the strip width, strip spacing, and similar parameters of each detector can be determined. 1B The mask structure in the photolithography step, as previously described, can be controlled, allowing the spatial resolution of the light detection to be easily increased by reducing the stripe width and spacing.

[0131] Increasing the spatial resolution by narrowing the fringe width in the quantum cascade detectors 1B This corresponds to a reduction in the light-receiving area and leads to a reduction in signal intensity in each of the separate detectors. 1B . On the other hand, according to the above setup, in which the light to be detected passes through the waveguide structure in the detector 1B guided and through the active layer 15 The photoelectric current is absorbed, and even with low incident light levels, it can be efficiently obtained for the light to be detected.

[0132] The quantum cascade detector according to the present invention is not limited to the embodiments and configuration examples described above and can be modified accordingly in various ways. For example, the configuration example above shows an example in which the InP substrate is used as the semiconductor substrate and the active layer is formed from InGaAs / InAlAs, although a different structure, particularly for the active layer, can be used. With regard to the semiconductor material systems, different material systems besides the previously described InGaAs / InAlAs, such as AlGaAs / GaAs or InGaN / GaN, can be used. A suitable material corresponding to the semiconductor material system and the like of the active layer can be used as the semiconductor material forming the cladding layer.

[0133] In the previously described configuration example, the quantum cascade detector is configured as a mesa structure comprising the base section with the semiconductor substrate and the measurement section with the active layer. However, the setup is not limited to the above and a different structure can be used instead of the mesa structure. For the antireflection film on the first end face and the reflection film on the second end face in the waveguide structure, a setup can be used in which the films are omitted if they are not required, or a setup can be used in which only one of the films is included.

[0134] In the quantum cascade detector with the above configuration, the active layer is formed with the lower and upper cladding layers, and the antireflection film, which increases the reflectivity for the detected light, is formed on the second end face opposite the first end face, which forms the entrance surface for the detected light in the waveguide structure with the active layer and the upper and lower cladding layers. This configuration also proves effective in a setup where the lower and upper metal layers are omitted. In this case, the antireflection film is even more preferably formed on the first end face to reduce the reflectivity for the detected light.

[0135] The quantum cascade detector according to the above embodiment comprises: (1) a semiconductor substrate; (2) an active layer provided on the semiconductor substrate and having a cascade structure in which absorption regions and transport regions are alternately stacked in the form of a multi-stage lamination of unit laminate structures, each comprising n (where n is an integer of 3 or more) quantum well layers with a first quantum well layer serving as an absorption quantum well layer and n barrier layers, wherein the absorption region comprises the first quantum well layer and detects the light to be detected by intersubband absorption, wherein the transport region transports electrons excited by the intersubband absorption; (3) a lower cladding layer provided between the active layer and the semiconductor substrate and having a lower refractive index than the active layer;(4) a lower metal layer provided between the lower cladding layer and the semiconductor substrate; (5) an upper cladding layer provided on a side of the semiconductor substrate opposite the active layer and having a lower refractive index than the active layer; and (6) an upper metal layer provided on the side of the active layer opposite the upper cladding layer; and (7) a first end face and a second end face in a waveguide direction in a waveguide structure comprising the active layer, the lower cladding layer, and the upper cladding layer, the first end face being an entrance face for the light to be detected.

[0136] With regard to the thickness of each semiconductor layer forming the quantum cascade detector, both the lower and upper cladding layers preferably have a thickness of 2 μm or more and 10 μm or less, respectively. The active layer preferably has a thickness of 1 μm or more. According to these configurations, it is possible to suitably achieve the waveguide structure for the detected light with the active layer and the cladding layers, as well as the light-limiting structure in the waveguide, and the like.

[0137] The dopant density in both the lower and upper layers of the jacket is preferably 5 × 10 16 cm –3 or more and 2 × 10 17 cm –3or less. According to this setup, by adjusting the dopant density in the cladding layer, the series resistance in the detector can be reduced and the loss of light in the cladding layer suppressed.

[0138] In the active layer, the dopant density is preferably 1 × 10 17 cm –3 or more and 9 × 10 17 cm –3 or less. According to this structure, it is possible to appropriately achieve the waveguide structure with the active layer and the cladding layers, the light limiting structure, the light detection structure in the active layer, and the like.

[0139] The quantum cascade detector can be configured as a mesa structure comprising a base section with the semiconductor substrate and a measurement section located on the base section. This measurement section includes the active layer and extends in a strip-like fashion along the waveguide direction. According to this configuration, the waveguide structure, with its active layer and cladding layers, can be suitably arranged along the measurement section.

[0140] The quantum cascade detector can have a design in which an antireflection film is formed on the first end face of the waveguide structure to reduce the reflectivity of the light being detected. According to this design, the incident efficiency of the light being detected is improved from the entrance surface to the inside of the detector, thereby increasing the detection efficiency of the light within the detector.

[0141] The quantum cascade detector can have a configuration in which a reflective film is formed on the second end face of the waveguide structure to increase the reflectivity for the light to be detected, with the second end face acting as a reflective surface for the light to be detected. According to this configuration, the light to be detected, which is guided from the entrance face through the waveguide structure and reaches the reflective surface, is redirected back into the interior of the detector, thereby improving the detection efficiency for the light within the detector.

[0142] The present invention can be used as a quantum cascade detector capable of detecting the light to be detected with high efficiency.

[0143] It is evident from the invention described above that the invention can be modified in many ways. Such modifications do not constitute a departure from the spirit and scope of the invention and are intended to include all modifications that are obvious to a person skilled in the art and fall within the scope of protection of the following claims. QUOTES INCLUDED IN THE DESCRIPTION

[0144] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature

[0145] FR Giorgetta et al., “Quantum Cascade Detectors”, IEEE Journal of Quantum Electronics Vol. 45 No. 8 (2009) pp. 1039-1052

[0004] A. Harrer et al., ”Plasmonic lens enhanced mid-infrared quantum cascade detector”, Appl. Phys. Lett. Bd. 105 (2014) S. 171112-1-171112-4

[0004] B. Schwarz et al., ”Monolithically integrated mid-infrared lab-on-achip using plasmonics and quantum cascade structures”, Nat. Commun. Bd. 5 Art. 4085 (2014) S. 1–7

[0004]

Claims

[1] Quantum cascade detector, comprising: a semiconductor substrate; an active layer provided on the semiconductor substrate and having a cascade structure in which absorption regions and transport regions are alternately stacked in the form of a multi-stage lamination of unit laminate structures, each comprising n (where n is an integer of 3 or more) quantum well layers with a first quantum well layer serving as an absorption quantum well layer and n quantum barrier layers, wherein the absorption region has the first quantum well layer and detects the light to be detected by intersubband absorption, wherein the transport region transports electrons excited by the intersubband absorption; a lower cladding layer that is provided between the active layer and the semiconductor substrate and has a lower refractive index than the active layer; a lower metal layer that is provided between the lower cladding layer and the semiconductor substrate; an upper cladding layer that is provided on a side of the semiconductor substrate opposite the active layer and has a lower refractive index than the active layer; and an upper metal layer provided on a side opposite the active layer in relation to the upper mantle layer, a first end face and a second end face in a waveguide direction in a waveguide structure comprising the active layer, the lower cladding layer and the upper cladding layer, wherein the first end face is an entry face for the light to be detected. [2] Quantum cascade detector according to claim 1, wherein both the lower cladding layer and the upper cladding layer have a layer thickness of 2 μm or more and 10 μm or less. [3] Quantum cascade detector according to claim 1 or 2, wherein the active layer has a layer thickness of 1 μm or more. [4] Quantum cascade detector according to one of claims 1 to 3, wherein both the lower cladding layer and the upper cladding layer have a dopant density of 5 × 10 16 cm –3 or more and 2 × 10 17 cm –3 or less. [5] Quantum cascade detector according to one of claims 1 to 4, wherein the active layer has a dopant density of 1 × 10 17 cm –3 or more and 9 × 10 17 cm –3 or less. [6] Quantum cascade detector according to one of claims 1 to 5, which is designed in the form of a mesa structure comprising a base section with the semiconductor substrate and a mesa section which is provided on the base section, the active layer and extends in a strip-like manner in the waveguide direction in the waveguide structure. [7] Quantum cascade detector according to one of claims 1 to 6, wherein an antireflection film is formed on the first end face in the waveguide structure to reduce the reflectivity for the light to be detected. [8] Quantum cascade detector according to one of claims 1 to 7, wherein a reflection film is formed on the second end face in the waveguide structure to increase the reflectivity for the light to be detected, and the second end face is a reflection surface for the light to be detected.