JP2017003301A - Photo detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photo detection device capable of detecting to-be-detected light such as infrared light at high speed with high sensitivity.SOLUTION: A photo detection device 1A is configured to comprise: a quantum cascade detector 30 provided on a substrate 10; and a light-receiving unit 20 having a metal layer 21 which is provided on the substrate 10 and on which a periodic structure 22 is formed and configured to generate a plasmon in response to incidence of to-be-detected light at a wavelength of λ. A first surface 30a of the detector 30 serves as an incidence surface for the plasmon from the light-receiving unit 20, and a trench structure is formed on a first surface side thereof, and this trench structure constitutes a reflection reducing structure 40 reducing reflectivity of the light at the wavelength of λ on the first surface 30a by stacking a first medium layer formed from a semiconductor material constituting the detector 30 and a second medium layer formed from a medium within a trench of the trench structure in a light incidence direction.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a photodetection device including a quantum well type semiconductor photodetection element using a conduction band subband level.

  In the middle to far infrared region (for example, a region having a wavelength of 3 μm or more), strong absorption is observed in many molecules, and such a region is known as a molecular fingerprint region. Further, in the wavelength range of 2 μm to 5 μm, there is a region having a high transmittance with respect to the atmosphere, and such a region is called an atmospheric window. Taking advantage of these characteristics, light in the mid-to far-infrared region is widely applied to detection and identification of substances.

  For example, an HgCdTe (MCT) detector is known and put into practical use as an infrared (mainly mid-infrared) photodetector that is essential for the use of infrared light. However, the MCT detector usually has a driving speed of about several hundreds of kHz, and cannot be used for applications that require a high speed of GHz or higher.

  In recent years, research and development of a quantum cascade detector (QCD) has been advanced as an infrared photodetector that solves such problems. Since the quantum cascade detector uses a high-speed (sub-picosecond) transition of electrons between conduction band subbands in a semiconductor, a high-speed response ranging from several GHz to several tens of GHz is possible, and a driving voltage is set. Because it is not necessary, no big noise is generated. Such a quantum cascade detector is promising as an ultra-high speed, low noise, high sensitivity infrared photodetector.

International Publication No. WO2008 / 077552 JP 2011-133472 A JP 2011-128162 A JP 2007-248141 A

A. Harrer et al., “Plasmonic lens enhanced mid-infrared quantum cascade detector”, Appl. Phys. Lett. Vol.105 (2014) pp.171112-1-171112-4 Z. Yu et al., "Design of midinfrared features enhanced by surface plasmons on gratingstructures", Appl. Phys. Lett. Vol.89 (2006) pp.151116-1-151116-3

  The quantum cascade detector described above has a great restriction on the light incident condition due to the optical transition selection rule between subbands. Specifically, the quantum cascade detector is sensitive only to polarized light (TM polarized light) that vibrates along the stacking direction (growth direction) of the semiconductor layers constituting the quantum well structure, and has a large light receiving area. Therefore, it is difficult to efficiently introduce light to be detected into the detector.

  For this reason, in the quantum cascade detector, the incident configuration of the light to be detected is a configuration in which light is collected on the cleaved end face and directly incident on the active layer, or the active layer is grown on the insulating substrate and the substrate is A configuration is used in which light is incident from a polished surface after being polished at 45 °, and light is oozed out from the substrate to the active layer by multiple reflection inside the substrate. However, in the above case, with the former configuration, the width of the light receiving surface is almost the same as the thickness of the active layer, making it difficult to efficiently enter light. Further, in the latter configuration, there are many components that do not contribute to sensitivity among the light incident on the substrate, and thus it is difficult to say that the configuration is an efficient light incidence configuration.

  Therefore, in the quantum cascade detector, the limit of the light receiving efficiency described above is a problem in improving sensitivity as a detector. In addition, such a problem of light reception efficiency is a high hurdle for arraying detectors and developing response to the image sensor. In addition to quantum cascade detectors, such problems include quantum well semiconductor photodetection using conduction band subband levels, such as quantum well infrared photodetectors (QWIP). The same occurs in the device.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a photodetection device capable of detecting light to be detected such as infrared light at high speed and with high sensitivity. .

In order to achieve such an object, a photodetector according to the present invention includes (1) a substrate, and (2) a quantum well type active layer provided on the substrate and using a conduction band subband level. And (3) a negative dielectric constant material layer formed of a material having a negative dielectric constant and having a periodic structure formed on the substrate together with the semiconductor photodetector element. And a light receiving unit configured to generate plasmons in response to incidence of light to be detected having a predetermined wavelength λ, and (4) the semiconductor photodetecting element is on one side of the plasmon propagation direction in the light receiving unit. The first surface is an incident surface for plasmons, and a groove structure is formed on the first surface side. (5) The groove structure is a refraction made of a semiconductor material constituting a semiconductor photodetector element. a first medium layer rate _{n 1,} Mizo構By the second medium layer having a refractive index n ₂ formed of a medium located in the groove are stacked on the light incident direction, constituting a reflection reducing structure for reducing the reflectance of light of wavelength λ in the first surface of the at It is characterized by that.

  In the above-described photodetector, a periodic structure of a negative dielectric constant material layer designed in consideration of the wavelength λ of light to be detected is applied to a semiconductor photodetector element such as a mesa type QCD or QWIP provided on a substrate. The light receiving portion is provided on the same substrate. Then, in the periodic structure of the light receiving unit, a surface plasmon is generated in response to the incident light to be detected, the plasmon is propagated to the semiconductor light detecting element, and the light caused by the plasmon is detected by the light detecting element. Yes.

  Further, in such a configuration, in the above-described photodetection device, a groove structure that functions as a reflection reducing structure is formed on the first surface side that is the light-receiving portion side of the semiconductor photodetection element and that is the incident surface for plasmons. Yes. In this way, for the quantum well type semiconductor photodetector using the conduction band subband level, a light receiving unit using the plasmon mechanism is provided on the same substrate, and a groove structure is formed on the incident surface side of the photodetector. According to the configuration that forms the reflection reduction structure using the light receiving area, the light receiving area for the detected light is expanded by the light receiving section, and the propagation and incidence efficiency of light from the light receiving section to the semiconductor photodetecting element is improved by the reflection reducing structure. Thus, it is possible to realize a light detection device capable of detecting light to be detected such as infrared light at high speed and with high sensitivity.

Here, regarding a specific configuration of the reflection reducing structure in the above-described photodetecting device, the reflection reducing structure is an M layer (M is an integer of 1 or more) including a first medium layer and a second medium layer. And the filling rate of the first medium layer in the M structure layer is q, and the average refractive index of the first medium layer and the second medium layer n _eff = {qn ₁ ² + (1 -Q) n ₂ ² } ^1/2
Is the optimum refractive index n _opt = (n ₁ n ₂ ) ^1/2 represented by the square root of the product of the refractive indices of the first medium layer and the second medium layer.
Is within a range of ± 20%, and the layer thickness l is the optimum layer thickness l _opt = λ / 4n _opt
In contrast, a configuration within a range of ± 20% can be used.

Alternatively, the reflection reducing structure includes a first medium layer and a second medium layer, respectively, and has a structure layer of M layers (M is an integer of 2 or more) having a filling rate of q _{1 to} q _M of the first medium layer. The M structural layers are stacked in the light incident direction from the first surface side in the order of the first structural layer to the Mth structural layer, and the mth structural layer included in the M structural layers (m is The average refractive index of the first medium layer and the second medium layer at 1 to M is an integer n _{eff, m} = {q _m n ₁ ² + (1-q _m ) n ₂ ² } ^1/2
There can be used a configuration approaching from the refractive index n ₂ of the stepwise second medium layer towards the first structural layer to the M structural layer to the refractive index n ₁ of the first medium layer.

Further, in this case, the reflection reduction structure has a target in which the filling rate q _m of the first medium layer in the m-th structure layer is set at an equal interval between 0 and 1 with an interval Δq = 1 / (M + 1). Filling rate q _{t, m} = m / (M + 1)
Was within the range of ± [Delta] q / 2 with respect to the layer thickness l _m of the m structure layer, filling rate q _m the target filling rate q _t, target layer thickness l t in the case of match the _{m, m =} lambda / 4n _{eff, t, m}
However, it may be configured to be within a range of ± 30%.

  According to the reflection reducing structure having these configurations, the light reflectance at the incident surface of the semiconductor photodetector element is suitably reduced, and the incident efficiency of light caused by the plasmon from the light receiving portion to the photodetector element is sufficient. Can be improved. In addition to the above, various configurations can be used as the specific configuration of the reflection reducing structure.

  The semiconductor photodetecting element in the above photodetecting device includes N (N is an integer of 3 or more) quantum well layers including the first well layer functioning as an absorption well layer, and N quantum barrier layers. By stacking unit laminates in multiple stages, an absorption region that includes the first well layer and detects light by intersubband absorption and a transport region that transports electrons excited by intersubband absorption alternately A quantum cascade detector (QCD) having an active layer formed with a stacked cascade structure can be used. Alternatively, as the semiconductor photodetector element, a photodetector element other than QCD such as a quantum well infrared photodetector (QWIP) may be used.

  Further, the above-described photodetection device has a waveguide layer made of a material having a negative dielectric constant, provided between the semiconductor photodetection element and the light-receiving unit on the substrate, and plasmons generated by the light-receiving unit. It is good also as a structure further provided with the waveguide part comprised so that it may propagate to a semiconductor photodetector element. Thus, according to the configuration in which the waveguide portion is provided between the light receiving portion and the semiconductor light detection element on the substrate, the plasmon generated in the light receiving portion is propagated to the light detection device with high efficiency. It becomes possible to make it enter.

  In this case, the waveguide part may be configured by a waveguide layer and a dielectric layer made of a dielectric material formed on the waveguide layer. The waveguide section may be formed in a tapered shape in which the waveguide width decreases from the light receiving section to the semiconductor light detection element. According to these configurations, it is possible to improve the plasmon propagation efficiency by the waveguide portion.

  As for the negative dielectric constant material layer in which the periodic structure is formed in the light receiving portion, specifically, for example, the negative dielectric constant material layer of the light receiving portion can be configured to be a metal layer made of a metal material. In addition, the negative dielectric constant material layer of the light receiving section has a concavo-convex structure (lattice structure) including a plurality of grooves (groove structures) periodically formed with the plasmon propagation direction to the semiconductor photodetector as an array direction as a periodic structure. ).

  In the above photodetector, the semiconductor photodetector element has a second groove structure formed on the second surface side opposite to the first surface, and the second groove structure includes a first medium layer, A reflection structure that increases the reflectance of light having a wavelength λ on the second surface may be configured by laminating the second medium layer. Thus, in the semiconductor light detection element, according to the configuration in which the reflection structure such as the Bragg reflection structure is formed on the second surface side in addition to the reflection reduction structure on the first surface side, the light detection efficiency in the light detection element Can be further improved.

  In addition, the above-described photodetecting device includes a plurality of photodetecting units that are arranged in a one-dimensional array or a two-dimensional array on a substrate, and each includes a light receiving unit and a semiconductor photodetecting element including a reflection reducing structure. It is also good. Such a light detection device can be used as, for example, a one-dimensional or two-dimensional imaging device.

  According to the photodetection device of the present invention, surface plasmons are generated in response to incidence of light to be detected in a light-receiving unit having a periodic structure of a negative dielectric constant material layer provided on a substrate together with a mesa-type semiconductor photodetection element. The plasmon is propagated to the light detection element so that the light caused by the plasmon is detected by the light detection element, and on the first surface side which is the light incident surface caused by the plasmon in the light detection element. By forming a groove structure that functions as a reflection reducing structure, it becomes possible to detect detected light such as infrared light at high speed and with high sensitivity.

It is a top view which shows the structure of 1st Embodiment of a photon detection apparatus. (A) Side surface sectional view taken along the line II-II showing the sectional configuration of the photodetecting device shown in FIG. 1, and (b) schematically showing incidence of detected light and plasmon propagation in the side sectional view. FIG. It is the (a) top view and (b) side surface sectional view which expand and show partially the structure of the quantum cascade detector and reflection reduction structure in the photodetector of 1st Embodiment. It is side surface sectional drawing which shows the structure of the modification of a photon detection apparatus. It is a graph which shows an example of the semiconductor laminated structure in a quantum cascade detector. It is a figure which shows an example of a structure of the unit laminated body which comprises the active layer in a quantum cascade detector. It is a graph which shows the wavelength dependence of the reflectance of the light in a reflection reduction structure. It is a graph which shows the dependence with respect to the filling rate q of the semiconductor material of the reflectance of the light in a reflection reduction structure. It is the (a) top view and (b) side surface sectional view which expand and show partially the structure of the quantum cascade detector and reflection reduction structure in the photodetector of 2nd Embodiment. 10 is a chart showing an example of a configuration of first to fourth structure layers in the reflection reducing structure shown in FIG. 9. It is a graph which shows the wavelength dependence of the reflectance of the light in a reflection reduction structure. It is a graph which shows the wavelength dependence of the reflectance of the light in a reflection reduction structure. It is a top view which shows the structure of 3rd Embodiment of a photon detection apparatus. It is a top view which shows the structure of 4th Embodiment of a photon detection apparatus. It is a top view which shows the structure of 5th Embodiment of a photon detection apparatus. It is side surface sectional drawing which shows the structure of 6th Embodiment of a photon detection apparatus. It is a graph which shows the wavelength dependence of the reflectance of the light in a reflective structure.

  Hereinafter, embodiments of a photodetection device according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.

  [First Embodiment]

  FIG. 1 is a plan view showing the configuration of the first embodiment of the photodetecting device according to the present invention. 2A is a side cross-sectional view taken along line II-II showing the cross-sectional configuration of the photodetecting device shown in FIG. 1, and FIG. 2B is a side cross-sectional view of the photodetecting device. It is a figure which shows typically about incidence | injection of to-be-detected light, propagation of a plasmon, etc. FIG. 3A is a partially enlarged view showing the configuration of the quantum cascade detector and the reflection reducing structure in the photodetector of the first embodiment shown in FIG. 1, and FIG. 3B is a side sectional view. It is.

  The photodetecting device 1A according to the present embodiment is a semiconductor photodetecting device including a quantum well type semiconductor photodetecting element 30 using a conduction band subband level. As described above, such a semiconductor photodetection element has a great restriction on the light incident condition due to the optical transition selection rule between subbands. Hereinafter, the configuration of the photodetecting device 1A will be described together with a specific configuration example thereof. In the following, the quantum well type semiconductor photodetector 30 using the above-described subband levels will be described by taking a quantum cascade detector (QCD) as an example.

  1 to 3 includes a substrate 10, a quantum cascade detector 30 that is a semiconductor light detection element, a light receiving unit 20, and a waveguide unit 50. The substrate 10 is made of, for example, a rectangular semiconductor substrate, and a quantum cascade detector 30 is provided on the detector formation region 11a on one surface (the upper surface in FIG. 2), and on the light receiving portion formation region 11b. A light receiving unit 20 and a waveguide unit 50 are provided. In this embodiment, as shown to Fig.2 (a), the light-receiving part formation area 11b is a surface part lower than the detector formation area 11a. Further, the lower electrode 12 used for driving the detector 30 is formed on the other surface (the lower surface in FIG. 2) of the substrate 10.

  As described above, the quantum cascade detector 30 includes the quantum well type active layer 35 using the conduction band subband level, and is formed in a mesa shape on the substrate 10. In the configuration example shown in FIG. 3, the quantum cascade detector 30 is configured by sequentially forming a lower contact layer 31, an active layer 35, a cladding layer 32, and an upper contact layer 33 on the substrate 10. On the upper contact layer 33, the upper electrode 13 used for driving the detector 30 is formed together with the lower electrode 12. In addition, illustration of the upper electrode 13 is abbreviate | omitted in Fig.3 (a).

  In the active layer 35 of the quantum cascade detector 30, a unit stack including N (N is an integer of 3 or more) quantum well layers including the first well layer functioning as an absorption well layer, and N quantum barrier layers. The body 36 (see FIG. 3B) is stacked in multiple stages, thereby transporting electrons excited by the intersubband absorption and an absorption region that includes the first well layer and detects light by intersubband absorption. A cascade structure is formed in which transport regions are alternately stacked. The configuration of the unit laminate 36 included in the active layer 35 will be specifically described later.

  The light receiving unit 20 is provided with a detector 30 on the substrate 10 and includes a negative dielectric constant material layer 21 made of a material having a negative dielectric constant. In the following, the negative dielectric constant material layer 21 is a metal layer made of a metal material such as Au (gold). The metal layer 21 is formed with a periodic structure 22 designed in consideration of the wavelength λ of light to be detected that is to be detected by the photodetecting device 1A. With the periodic structure 22 configured as described above, the light receiving unit 20 is configured to generate surface plasmons in response to incidence of the detected light A having a wavelength λ (see FIG. 2B). .

  In the configuration example shown in FIGS. 1 and 2, a plurality of grooves are periodically formed in the metal layer 21 with the propagation direction of plasmons toward the quantum cascade detector 30 as an array direction. As a result, the periodic structure 22 in this configuration example has a concavo-convex structure (lattice structure) in which a plurality of linear convex portions 22a and concave portions (groove portions) 22b having a longitudinal direction perpendicular to the plasmon propagation direction are alternately formed. Structure). A Bragg reflection structure 25 that reflects plasmons toward the detector 30 is provided on the opposite side of the light receiving unit 20 from the detector 30. In FIG. 1, the light receiving unit 20 and the reflecting structure 25 are shown separated by broken lines for easy viewing.

The quantum cascade detector 30 is arranged on one side of the plasmon propagation direction (arrangement direction of the convex portions 22a and the concave portions 22b in the periodic structure 22) in the light receiving unit 20, and has a first surface (left side in FIG. 2). The surface 30a is an incident surface for plasmons (light resulting from plasmons). Further, on the incident surface 30a side of the detector 30, a groove structure including one or a plurality of grooves formed from the upper surface of the detector 30 (the surface opposite to the substrate 10) is formed. The groove structure includes a first medium layer having a refractive index n ₁ made of a semiconductor material constituting the detector 30 and a second medium layer having a refractive index n _{2 made} of a medium (for example, air) in the groove in the groove structure. Are stacked in the light incident direction to constitute a reflection reduction structure 40 that reduces the reflectance of light having the wavelength λ on the first surface 30a. The light incident direction on the first surface 30 a of the quantum cascade detector 30 corresponds to the plasmon propagation direction in the light receiving unit 20.

  A plasmon waveguide unit 50 is provided on the substrate 10 between the quantum cascade detector 30 and the light receiving unit 20. The waveguide unit 50 includes a waveguide layer 51 made of a material having a negative dielectric constant, and the plasmon generated in the light receiving unit 20 is detected by a detector as indicated by the arrow A2 in FIG. 30 to propagate to 30.

  In the present embodiment, the waveguide layer 51 is composed of a metal layer made of a metal material such as Au, for example, like the negative dielectric constant material layer 21 of the light receiving unit 20. A dielectric layer 52 made of a dielectric material is formed. Further, the waveguide section 50 including the waveguide layer 51 and the dielectric layer 52 is formed in a tapered shape in which the waveguide width gradually decreases from the light receiving section 20 toward the quantum cascade detector 30 as shown in FIG. ing. In addition, the end face on the detector 30 side of the waveguide section 50 and the first face 30a that is the incident face of the detector 30 are arranged so as to be sufficiently close to each other.

  The effects of the photodetecting device 1A according to the present embodiment will be described.

  The photodetection device 1A shown in FIGS. 1 to 3 is designed in consideration of the wavelength λ of the detected light A with respect to the quantum cascade detector 30 which is a mesa type semiconductor photodetection element provided on the substrate 10. The light receiving unit 20 having the periodic structure 22 made of the metal layer 21 is provided on the same substrate 10. Then, in the periodic structure 22 of the light receiving unit 20, surface plasmons are generated in response to the incident light to be detected, the plasmons are propagated to the quantum cascade detector 30, and light caused by the plasmons is detected by the detector 30. It is configured to do.

  Further, in such a configuration, in the light detection device 1A, the first surface 30a side that is a side surface parallel to the semiconductor stacking direction on the light receiving unit 20 side in the detector 30 and serves as an incident surface for light caused by plasmons In addition, a groove structure is provided which is composed of one or a plurality of grooves formed substantially parallel to the first surface 30 a and functions as the reflection reducing structure 40.

  In this way, with respect to the quantum cascade detector 30 which is a quantum well type semiconductor photodetecting element using the conduction band subband level, the light receiving unit 20 using the plasmon mechanism is provided on the same substrate 10 and is detected. According to the configuration in which the reflection reducing structure 40 using the groove structure is formed on the incident surface 30a side of the vessel 30, the light receiving area for the detected light A incident on the light detection device 1A is enlarged by the light receiving unit 20, and Detection device capable of detecting the detected light A such as infrared light at high speed and high sensitivity by improving the propagation and incidence efficiency of light from the unit 20 to the quantum cascade detector 30 by the reflection reduction structure 40 1A can be realized.

  Here, regarding the light incident configuration to the quantum cascade detector by the light receiving unit using the plasmon mechanism, for example, Non-Patent Document 1 (A. Harrer et al., Appl. Phys. Lett. Vol.105 (2014) pp .171112-1-171112-4). However, in the configuration described in Non-Patent Document 1, the optical impedance is not matched between the termination surface of the plasmon waveguide and the incident surface of the detector, and thus loss due to light reflection occurs.

  In general, the refractive index of a semiconductor material constituting a semiconductor photodetector element such as a quantum cascade detector is, for example, about 3.2 to 3.3, and the reflectance when light is vertically incident from air is about 30%. It is. As a configuration for reducing such light reflection, generally, a configuration in which a low reflection coating with a dielectric multilayer film is applied to a light incident surface can be considered. However, in the configuration in which the semiconductor photodetecting element and the light receiving unit are formed on the same substrate as described above, the incident of the detector close to the end surface of the plasmon light receiving unit or the waveguide unit is caused by the structural feature. It is extremely difficult to form a dielectric multilayer film on the surface by vapor deposition or the like.

  On the other hand, in the photodetecting device 1A according to the above embodiment, a dielectric multilayer film or the like is not formed on the first surface 30a on the light receiving unit 20 side of the quantum cascade detector 30, but film deposition or the like is not used. An optical impedance adjusting mechanism having a groove structure that can be formed by etching is provided, and this groove structure is used as a reflection reduction structure 40 for light incident from the light receiving unit 20 and the waveguide unit 50. Thereby, the light caused by the plasmon from the light receiving unit 20 is efficiently incident on the inside of the detector 30, and the photodetector 1 </ b> A having high detection sensitivity with respect to the detected light A such as infrared light is obtained. It becomes possible to implement | achieve suitably.

  The quantum well type semiconductor photodetector 30 using the conduction band subband level used in the photodetector 1 </ b> A includes an N well including a first well layer that functions as an absorption well layer, as illustrated in the above embodiment. By stacking unit stacks composed of a single quantum well layer and N quantum barrier layers in multiple stages, an absorption region that includes the first well layer and detects light by intersubband absorption, and intersubband absorption A quantum cascade detector (QCD) having an active layer 35 in which a cascade structure in which transport regions for transporting electrons excited by is alternately stacked can be suitably used.

  Alternatively, as the semiconductor photodetection element, an element other than the quantum cascade detector, for example, a quantum well infrared photodetector (QWIP) may be used. Also in QWIP, the restriction on the light incident condition based on the selection rule of the optical transition between subbands is the same as that in QCD. Therefore, as described above with respect to the photodetector 1A, the plasmon is applied to the QWIP detector 30. Light that can detect light to be detected with high speed and high sensitivity by providing the light receiving unit 20 using the mechanism on the same substrate 10 and forming the reflection reducing structure 40 on the incident surface 30a side of the detector. A detection device can be realized.

  However, in the case of QWIP, since a high driving voltage is required to drive the element, a relatively large noise may be generated, and the SN ratio in light detection may deteriorate. On the other hand, when the QCD is used as the semiconductor photodetecting element as in the above-described embodiment, no driving voltage is required, so that no large noise is generated. Therefore, a low-noise photodetecting device can be obtained. There is an advantage that you can.

  In addition, as shown in FIGS. 1 and 2, the photodetecting device 1 </ b> A having the above configuration includes a waveguide layer 51 provided between the quantum cascade detector 30 and the light receiving unit 20 on the substrate 10. It is preferable to further include a waveguide unit 50 configured to propagate the plasmon generated by the light receiving unit 20 to the detector 30. According to the configuration in which the plasmon waveguide unit 50 is provided in this way, the plasmon generated by the light receiving unit 20 is propagated and incident on the detector 30 with high efficiency, and the detection light A is detected by the photodetector 1A. Efficiency can be improved. However, such a waveguide section 50 may be configured not to be provided if unnecessary.

  When the waveguide section 50 is formed in this way, the waveguide section 50 includes a waveguide layer 51 made of a metal layer and the like, and a dielectric layer made of a dielectric material formed on the waveguide layer 51. 52 is preferable. Thereby, plasmon propagation in the waveguide section 50 can be suitably realized. However, with respect to the waveguide portion 50, a plasmon waveguide may be configured by the waveguide layer 51 such as a metal layer and the air layer in contact with the upper surface of the waveguide layer 51 without providing the dielectric layer 52. good.

  Further, as shown in FIG. 1, the waveguide unit 50 may be configured to have a tapered shape in which the waveguide width decreases from the light receiving unit 20 to the quantum cascade detector 30. Even with such a configuration, the plasmon propagation efficiency to the detector 30 by the plasmon waveguide section 50 can be improved.

  Specifically, for example, as described above, a metal layer made of a metal material can be used as the negative dielectric constant material layer 21 in which the periodic structure 22 is formed in the light receiving unit 20. In addition, the negative dielectric constant material layer 21 of the light receiving unit 20 has a periodic structure 22 as a concavo-convex structure (lattice structure) formed by a plurality of grooves periodically formed with the propagation direction of plasmons to the quantum cascade detector 30 as an array direction. It can be set as the structure which has these. In this case, the shape of the plurality of grooves formed as the periodic structure 22 (the convex portions 22a and the concave portions 22b by the grooves) is preferably linear as shown in FIG. 1, or is configured to be concentric. Also good.

In addition, regarding the specific configuration of the reflection reducing structure 40 in the photodetecting device 1A, the reflection reducing structure 40 has a first medium layer (semiconductor layer) and a second medium layer (for example, a groove), where M is an integer of 1 or more. And the filling ratio of the first medium layer in the M structure layer is q, and the average of the first medium layer and the second medium layer Refractive index n _eff = {qn ₁ ² + (1-q) n ₂ ² } ^1/2
Is the optimum refractive index n _opt = (n ₁ n ₂ ) ^1/2 represented by the square root of the product of the refractive indices of the first medium layer and the second medium layer.
Is within a range of ± 20%, and the layer thickness l is the optimum layer thickness l _opt = λ / 4n _opt
In contrast, a configuration within a range of ± 20% can be used.

In such a configuration, the reflection reducing structure 40 has M structural layers including the first medium layer and the second medium layer, respectively, and the filling rate of the first medium layer in the M structural layer is q and the average refractive index of the first medium layer and the second medium layer n _eff = {qn ₁ ² + (1-q) n ₂ ² } ^1/2
Is the optimum refractive index n _opt = (n ₁ n ₂ ) ^1/2 represented by the square root of the product of the refractive indices of the first medium layer and the second medium layer.
And the layer thickness l is the optimum layer thickness l _opt = λ / 4n _opt
Is particularly preferable.

Alternatively, the reflection reducing structure 40 includes a first medium layer and a second medium layer, where M is an integer equal to or greater than 2, and includes M structure layers having a filling rate of the first medium layer of q _{1 to} q _M. The M structural layers are stacked in the order of the first structural layer to the Mth structural layer in the light incident direction from the first surface 30a side, and the mth structural layer included in the M structural layers. (m is an integer of 1 to M) average refractive index n _eff of the first medium layer and the second medium layer ^{_{in, m = {q m n 1}} 2 + (1-q m) n 2 2} 1 / ²
There can be used a configuration approaching from the refractive index n ₂ of the stepwise second medium layer towards the first structural layer to the M structural layer to the refractive index n ₁ of the first medium layer.

In this case, in the reflection reduction structure 40, the filling rate q _m of the first medium layer in the m-th structure layer is set at an equal interval between 0 and 1 with an interval Δq = 1 / (M + 1). Target filling rate q _{t, m} = m / (M + 1)
Was within the range of ± [Delta] q / 2 with respect to the layer thickness l _m of the m structure layer, filling rate q _m the target filling rate q _t, target layer thickness l t in the case of match the _{m, m =} lambda / 4n _{eff, t, m}
However, it may be configured to be within a range of ± 30%.

Further, in such a configuration, the reflection reducing structure 40 has the filling rate q _m of the first medium layer in the m-th structure layer at an equal interval between 0 and 1 with an interval Δq = 1 / (M + 1). Set target filling rate q _{t, m} = m / (M + 1)
Substantially coincides with, the layer thickness _{l m} of the m structure layer, the target filling rate filling rate _{q m q t,} target layer thickness when was consistent with _{m l t, m = λ /} 4n eff, t, m
Is particularly preferable.

  According to the reflection reduction structure 40 having these configurations, the reflectance of light at the incident surface 30a of the quantum cascade detector 30 is suitably reduced, and light caused by plasmons from the light receiving unit 20 to the detector 30 is reduced. Incidence efficiency can be sufficiently improved. In addition to the above, various configurations can be used as the specific configuration of the reflection reducing structure 40. A specific example of the configuration of the reflection reducing structure 40 and its configuration conditions will be described later.

  The specific configuration of each part in the photodetecting device 1A shown in FIGS. 1 to 3 will be further described. First, the configuration of the light receiving unit 20 that generates surface plasmons according to the incidence of the detected light A will be specifically described. In the following, the case where an Au layer made of Au (gold), which is a metal, is used as an example of the negative dielectric constant material layer 21 constituting the light receiving unit 20 will be described. However, for the negative dielectric constant material layer 21, a material such as a metal other than Au may be used.

ここで、Λは周期構造２２の周期、λは入射する被検出光Ａの波長、また、εは媒質の誘電率である。 The Au layer 21 of the light receiving unit 20 has a period corresponding to the wavelength λ of the detected light A incident on the photodetecting device 1A, as illustrated in FIG. 1 and FIG. A periodic structure 22 is formed. Such a period in the periodic structure 22 can be obtained by the following equation (1).

Here, Λ is the period of the periodic structure 22, λ is the wavelength of the incident light to be detected A, and ε is the dielectric constant of the medium.

ε ₁ and ε ₂ indicate real parts of dielectric constants of two different media constituting the periodic structure 22. For example, in the configuration shown in FIG. 2, when Au (gold) that is the first medium in the convex portion 22 a and air in the groove that is the second medium in the concave portion 22 b are used as the medium constituting the periodic structure 22. , Au has a dielectric constant of ε ₁ = −843.2, and air has a dielectric constant of ε ₂ = 1. Further, θ is the incident angle of the detected light A as viewed from the perpendicular to the surface (periodic structure surface) on which the periodic structure 22 is formed in the light receiving unit 20.

が得られる。以上から、例えば被検出光Ａの波長をλ＝４．５μｍとした場合、周期構造２２の最適な周期Λは、４．４９μｍと求められる。また、周期構造２２となる格子構造（凹凸構造）での凹凸のデューティ比は、例えば５０％に設定することができる。また、格子構造での溝深さは、例えば０．５μｍに設定することができる。 Here, consider a case in which the detected light A is vertically incident on the periodic structure 22 of the light receiving unit 20. In this case, since the incident angle is θ = 0 °, when the above equation (1) is solved for Λ under this condition, the following equation (2)

Is obtained. From the above, for example, when the wavelength of the detected light A is λ = 4.5 μm, the optimum period Λ of the periodic structure 22 is obtained as 4.49 μm. Further, the duty ratio of the unevenness in the lattice structure (uneven structure) that becomes the periodic structure 22 can be set to 50%, for example. The groove depth in the lattice structure can be set to 0.5 μm, for example.

ここで、ε_１’、ε_２’は、それぞれ第１媒質であるＡｕ、及び第２媒質である空気の複素誘電率であり、Ａｕについては、上記の実部と合わせてε_１＝−８４３．２＋２２９．１ｉ、空気については、ε_２＝１となる。ただし、ｉは虚数単位である。 Further, the propagation length (the distance at which the electric field strength becomes 1 / e due to the propagation loss) Lx on the periodic structure surface of the plasmon generated by the light receiving unit 20 by the incidence of the detected light A is expressed by the following equation (3). Can be sought.

Here, ε ₁ ′ and ε ₂ ′ are complex dielectric constants of Au as the first medium and air as the second medium, respectively, and ε ₁ = −843 together with the above real part for Au. .2 + 229.1i, for air, ε ₂ = 1. However, i is an imaginary unit.

  When the propagation length Lx is calculated by substituting each parameter into the equation (3), it is calculated to be about 2.4 mm. Therefore, in this configuration example, the maximum value of the number of periods in the periodic structure 22 having a period of 4.49 μm may be set to 500, for example, in consideration of propagation loss. On the other hand, regarding the minimum value of the number of periods, it is considered that the effect of generating and enhancing plasmons in the light receiving unit 20 can be sufficiently obtained by setting the number of periods in the periodic structure 22 to 5 or more (non-patent document). Reference 2 (see Z. Yu et al., Appl. Phys. Lett. Vol.89 (2006) pp.151116-1-151116-3)), it is preferable to set the minimum number of periods to 5. Moreover, there is no restriction | limiting in particular about the width | variety L3 (refer FIG. 1) of the periodic structure 22 in the light-receiving part 20, For example, it can set to L3 = 1mm.

  In the light receiving unit 20 having such a configuration, as shown schematically in FIG. 2B, the detected light A and the vibration direction A1 of the electric field, among the detected light A incident on the periodic structure 22, the period Due to the polarization component in the direction perpendicular to the stripe of the groove structure of the structure 22, an enhanced electric field is generated at the edge portion 22 c of the convex portion 22 a indicated by a dotted line. This electric field is caused by the fact that free electrons in the Au layer 21 vibrate due to the incidence of light (electromagnetic waves), and an electron density wave is formed. A coupled wave in which such an electron density wave and incident light that oozes onto the surface of a metal are combined is called a plasmon. As described above, the electric field enhanced on the periodic structure surface of the light receiving unit 20 is generated by substantially corresponding and matching the period Λ shown in Expression (2) with the incident wavelength λ of the detected light A. be able to.

  Plasmons generated in response to the incidence of the detected light A in the light receiving unit 20 propagate on the surface (periodic structure surface) of the periodic structure 22 of the Au layer 21 in a direction perpendicular to the stripes. Concomitantly, the electric field enhanced on the periodic structure surface propagates while vibrating in a direction perpendicular to the periodic structure surface. An electric field corresponding to the TM mode generated by such a mechanism is guided to the detector 30 via the Au / dielectric waveguide section 50, thereby detecting the detected light A incident on the photodetecting device 1A. Therefore, the light sensitivity can be obtained.

  The introduction of the electric field from the light receiving unit 20 to the quantum cascade detector 30 is performed by the waveguide unit 50 formed in a tapered shape toward the detector 30 as described above. As shown in FIG. 2, the termination surface of the waveguide unit 50 and the incident surface 30a of the detector 30 are arranged close to each other through an air layer with an interval of, for example, 1 μm to 10 μm. The resulting light can be propagated and incident on the detector 30. However, regarding the arrangement relationship between the waveguide unit 50 and the quantum cascade detector 30, as shown in a side cross-sectional view of the configuration of the modification of the photodetector in FIG. 4, the waveguide unit 50 does not pass through an air layer. It is good also as a structure which connects directly the terminal surface of this, and the entrance plane 30a of the detector 30. FIG.

  As shown in FIG. 1, for example, the waveguide portion 50 has an entrance side (light receiving portion 20 side) substantially the same width as the width L <b> 3 of the periodic structure 22, and an exit side (detector 30 side) is incident on the detector 30. It is formed to have substantially the same width as the surface 30a. In such a configuration, the plasmon from the light receiving unit 20 is narrowed toward the incident surface of the detector 30 and can be incident on the detector 30 in a state where the electric field on the periodic structure surface is reduced. Thereby, the quantum cascade detector 30 can be reduced in size while keeping the light receiving area (the area of the periodic structure 22 in the light receiving unit 20) in the photodetector 1A large. Moreover, the plasmon waveguide length D of the waveguide part 50 can be set to 300 micrometers, for example.

  Here, the electric field on the periodic structure surface propagating along with the plasmon attenuates exponentially as the distance from the periodic structure surface increases. In the waveguide unit 50, the spatial expansion of the electric field is suppressed, and the electric field is confined to the detector 30 in a state of being confined to the thickness of the active layer 35 in the quantum cascade detector 30. The incident efficiency of light can be improved.

  In order to realize such conditions, a material having a high refractive index may be used as the dielectric material of the dielectric layer 52 in the plasmon waveguide structure including the Au waveguide layer 51 and the dielectric layer 52 as described above. . An example of such a dielectric material is Ge (germanium). Further, the thickness of the dielectric layer 52 may be set to, for example, about the same as or less than the thickness of the active layer 35 of the detector 30, preferably less than half the thickness of the active layer 35.

  In addition, the plasmon generated in response to the incidence of the detected light A in the light receiving unit 20 isotropically propagates on the periodic structure surface in a direction perpendicular to the longitudinal direction of the stripe, and thus is opposite to the detector 30. Plasmon progresses in the direction as well. On the other hand, in the photodetecting device 1A of the above-described embodiment, the Bragg reflection structure 25 is provided on the side opposite to the detector 30 with respect to the light receiving unit 20, so that an electric field traveling in the direction opposite to the detector 30 is generated as light. The configuration contributes to sensitivity. The reflection structure 25 is constituted by, for example, an uneven structure of the Au layer 21 similarly to the periodic structure 22 of the light receiving unit 20, and a reflection structure having a duty ratio of 50% and two periods or more so that the period is ½ of the wavelength λ. Is preferably formed.

  Next, a specific example of the configuration of the quantum cascade detector 30 that detects light caused by plasmons incident from the light receiving unit 20 via the waveguide unit 50 will be described. FIG. 5 is a chart showing an example of a semiconductor stacked structure in the quantum cascade detector 30. FIG. 6 is a diagram illustrating an example of the configuration of the unit stacked body 36 (see FIG. 3B) constituting the active layer 35 in the quantum cascade detector 30.

  In the quantum well structure of the active layer 35 in this configuration example, an example is shown in which the absorption wavelength is designed as λ = 4.5 μm (energy 275 meV). In FIG. 6, the quantum well structure and the subbands due to the conduction band subband levels of a part of the multi-stage repeating structure of the unit stacked body 36 including the absorption region 37 and the transport region 38 in the active layer 35. The level structure is shown. The element structure of the semiconductor cascade structure in the quantum cascade detector 30 is, for example, crystal growth by molecular beam epitaxy (MBE) method, metal organic chemical vapor deposition (MOCVD) method, or the like. Can be formed.

In the semiconductor stacked structure of the quantum cascade detector 30 according to this configuration example, an n-type InP substrate is used as the semiconductor substrate 10 in the configuration shown in FIGS. Specifically, for example, an InP substrate doped with S at a concentration of 1 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ⁻³ can be used as the InP substrate 10. Then, on this InP substrate 10, as shown in FIG. 3B and FIG. 5, a lower contact layer 31 composed of an InGaAs layer having a thickness of 25 nm and an InAlAs layer having a thickness of 0.2 nm, an absorption region 37, and An active layer 35 in which unit stacks 36 including a transport region 38 are stacked in multiple stages, an InP cladding layer 32 having a thickness of 3000 nm, and an InP upper contact layer 33 having a thickness of 25 nm are sequentially stacked, thereby providing a quantum cascade detector. 30 element structures are formed.

A cladding layer 32 for efficiently confining light is provided above the active layer 35. The clad layer 32 has a function of preventing light entering the detector 30 from coming into contact with the electrode 13 at the top of the mesa structure and causing loss. Specifically, for example, an InP layer doped with Si at a concentration of 1 × 10 ¹⁷ cm ⁻³ can be used as the cladding layer 32. Further, the layer thickness of the cladding layer 32 is preferably set to a thickness equal to or more than ½ of the target wavelength λ and the wavelength λ.

  The active layer 35 in this configuration example is configured by repeatedly laminating unit laminated bodies 36 including an absorption region 37 and a transport region 38 at a plurality of periods. The number of lamination periods of the unit laminated body 36 in the active layer 35 can be set to 10 to 50 periods, for example. In addition, as shown in FIGS. 5 and 6, the unit stacked body 36 for one period includes a quantum well in which seven quantum well layers 361 to 367 and seven quantum barrier layers 371 to 377 are alternately stacked. It is structured as a structure.

  Among the semiconductor layers of these unit laminated bodies 36, the quantum well layers 361 to 367 are each composed of an InGaAs layer. The quantum barrier layers 371 to 377 are each composed of an InAlAs layer. Thereby, the active layer 35 of this structural example is comprised by the InGaAs / InAlAs quantum well structure. The layer thicknesses of the well layers and barrier layers constituting the active layer 35 are as shown in FIG.

In such a unit stacked body 36, the first barrier layer 371 and the first well layer 361 constitute an absorption region 37 that detects light by intersubband absorption. The second to seventh barrier layers 372 to 377 and the second to seventh well layers 362 to 367 have a transport region 38 that transports electrons excited by intersubband absorption to the absorption region 37b of the next period. It is composed. The first well layer 361 functioning as an absorption well layer used for light absorption is doped with Si as an n-type impurity at a concentration of 1 × 10 ¹⁸ cm ⁻³ in order to supply electrons as carriers. Yes.

In such a configuration, the unit laminate 36 has a detection lower level that contributes to light absorption in the absorption region 37 as a conduction band subband level used for light detection in the subband level structure shown in FIG. It has a detection lower level L ₁ , a detection upper level L ₂ , and a plurality of transport levels that contribute to electron transport in the transport region 38.

When light of the active layer 35 wavelength λ having such a unit stack 36 is incident, electrons present in the detection lower level L ₁ is excited to detect the level L ₂ by intersubband absorption. The electrons excited to the upper level L ₂ are transported and extracted to the detection lower level L ₁ in the subsequent absorption region 37 b through the transport level structure composed of a plurality of transport levels in the transport region 38. The By repeating such excitation of electrons by light absorption, relaxation of the excited electrons, transport, and extraction of electrons to the unit laminate of the next period, a plurality of unit laminates 36 constituting the active layer 35 can be used. Cascade light absorption occurs in layer 35. And the incident light is detected by taking out the electric current which generate | occur | produces by this as a signal, and measuring the electric current amount.

  The semiconductor stacked structure of the quantum cascade detector 30 including the active layer 35 is etched perpendicularly to the substrate 10 by, for example, reactive ion etching (dry etching), and thereby the mesa structure shown in FIGS. A detector 30 is produced. The etching depth d (refer to FIG. 2B) at this time is preferably set so as to be 200 nm to 300 nm deeper than the lower end of the active layer 35.

  On the other hand, the total layer thickness of the Au waveguide layer 51 and the dielectric layer 52 used as the waveguide unit 50 is set to be not less than 300 nm and not more than 1 μm. Thereby, the position of the dielectric layer 52 functioning as a plasmon waveguide can be set within a range corresponding to the active layer 35 of the detector 30. Here, the electric field propagating through the waveguide portion 50 exists mainly in the dielectric layer 52 and in the vicinity of the upper surface thereof. For this reason, by aligning the heights of the active layer 35 in the detector 30 and the dielectric layer 52 in the waveguide section 50, light can be efficiently incident on the detector 30.

With respect to the mesa size of the quantum cascade detector 30 having a mesa structure, for example, the detector width L2 (see FIG. 1) that is the width of the incident surface 30a can be set to 50 μm, and the detector length L1 can be set to 200 μm. The detector length L1 can be set to a length at which the intensity of incident light becomes 1 / e inside the detector 30, for example. Such a detector length can be obtained from, for example, an absorption coefficient (51 cm ⁻¹ ) for light having a wavelength λ = 4.5 μm when the doping amount to the active layer 35 is 1 × 10 ¹⁸ cm ^−1. .

  Next, a method for manufacturing the periodic structure 22 and the waveguide unit 50 in the light receiving unit 20 will be described. After the mesa structure of the quantum cascade detector 30 is formed as described above, Au to be the Au layer 21 is vapor-deposited on the substrate surface exposed by etching with a thickness of, for example, 0.6 μm or more and 1 μm or less. By performing the etching, the periodic structure 22 having the period Λ determined by the above equation (2) is produced. Further, the groove depth in the periodic structure 22 is, for example, 0.5 μm as described above.

  In the production of such a periodic structure 22, for example, a material other than Au, such as SiN, is vapor-deposited on the substrate surface in advance, a groove having a depth of 0.5 μm is produced at a desired period, and Au is deposited thereon. You may use the method of vapor-depositing.

  Between the light receiving unit 20 having the periodic structure 22 and the mesa detector 30, for example, a tapered waveguide unit 50 having a waveguide length D = 300 μm is manufactured. The Au waveguide layer 51 used for the plasmon waveguide is directly deposited on the substrate surface. At this time, all surfaces including the mesa side surface of the detector 30 are protected with a material such as SiN. In order to form the waveguide, photolithography and dry etching are performed in the same manner as the periodic structure 22. Further, if necessary, the dielectric layer 52 is formed on the Au waveguide layer 51 by vapor deposition or the like.

  Finally, SiN protecting the detector 30 is removed. At this time, as shown in FIG. 2, for example, a gap of several μm is formed between the end surface of the waveguide section 50 and the incident surface 30 a of the detector 30. Or as shown in FIG. 4, it is good also as a structure which connected the termination | terminus surface of the waveguide part 50, and the entrance plane 30a of the detector 30 directly. In such a configuration, there is no air gap between the waveguide portion 50 and the detector 30, so that light caused by plasmons can be incident on the detector 30 more efficiently. As a manufacturing method in this case, a resist is applied only to the upper part of the mesa structure of the detector 30 including the reflection reducing structure 40, Au and dielectrics constituting the waveguide portion 50 are deposited, and then the detector 30 is coated. A method of removing the resist on the top of the mesa can be used.

  Next, the reflection reducing structure 40 formed on the incident surface 30a side of the quantum cascade detector 30 will be described. The TM polarized light generated by the light receiving unit 20 propagates through the waveguide unit 50 and is guided to the incident surface 30 a of the detector 30. At this time, the electric field strength is highest near the interface between the dielectric layer 52 and air. Therefore, the incidence of light on the detector 30 may be considered as incidence on the detector 30 from the air.

In order to reduce the reflectance of light having a wavelength λ at the interface between media having different refractive indexes, the refractive index is refracted so that the thickness of the interface is l = λ / 4n (see FIG. 2B). A layer of rate n may be provided. Here, when the semiconductor material constituting the detector 30 is a first medium having a refractive index n ₁ = n _s and the air in the groove is a second medium having _a refractive index n ₂ = na, the reflectance is reduced. The optimum refractive index n _opt is the square root of the product of the refractive indices of the semiconductor layer (first medium layer) and the air layer (second medium layer) n _opt = (n ₁ n ₂ ) ^1/2 = (n _s n _a ) ^1/2
Sought by.

The reflection reducing structure 40 for suppressing the reflection of light under such conditions is configured by a semiconductor material constituting the detector 30 and air. Specifically, a groove structure is formed by etching on the incident surface 30a side of the detector 30, and from the first medium layer (semiconductor layer) made of a semiconductor material of the detector 30 and air in the groove in the groove structure. The second medium layer (air layer) that constitutes the reflection reducing structure 40 and adjusting the volume ratio between the semiconductor layer and the air layer, so that the average refractive index n _eff that satisfies the reflectance reduction condition is satisfied. To realize. The groove structure for the mesa quantum cascade detector 30 is preferably formed by vertical etching, and for example, it is preferably formed simultaneously with the mesa structure of the detector 30.

First, when the refractive index of the semiconductor layer is n _s = 3.3 and the refractive index of the air layer is n _a = 1, the optimum refractive index n _opt of the reflection reducing structure 40 is n _opt = (3.3 × 1) ^{1 / 2} = 1.8
Is required. Further, the optimum layer thickness l = l _opt of the reflection reducing structure 40 is such that the wavelength of the target light is λ = 4.5 μm, and l _opt = λ / 4n _opt = 0.63 μm
Is required. Therefore, the volume ratio between the semiconductor layer and the air layer may be set so that the average refractive index n _eff in the reflection reducing structure 40 matches the above-described optimum refractive index n _opt .

When the filling ratio of the semiconductor layer in the groove structure of the reflection reducing structure 40 (volume ratio of the semiconductor material) is q, the average refractive index _{n eff} is ^{_{n eff = {qn s 2 +}} (1-q) n a 2} 1 / ²
Sought by. In this equation, when the value of the filling rate q is determined so that n _opt = n _eff , q = 0.24 is obtained. Such a filling rate q is defined as an optimum filling rate q _opt .

In the reflection reduction structure 40 having a thickness l _opt = 0.63 μm, in order to realize the above-described filling factor q _opt , the total thickness of the semiconductor layers included in the reflection reduction structure 40 is 0.63 × 0.24. = 0.15 μm. Moreover, what is necessary is just to comprise the part of remaining layer thickness 0.63-0.15 = 0.48micrometer by the air layer in a groove | channel.

FIG. 7 is a graph showing the wavelength dependence of the light reflectance in the reflection reducing structure 40 configured using the layer thickness l _opt and the refractive index n _opt = n _eff set as described above. In this graph, the horizontal axis indicates the wavelength λ (μm) of the target light, and the vertical axis indicates the reflectance R (%) with respect to the light having the wavelength λ. According to the graph of FIG. 7, it can be seen that the reflectance R of light is reduced in a desired wavelength region by using the reflection reduction structure 40 having the above configuration.

  Further, such a reflection reducing structure 40 specifically includes an integer equal to or greater than 1 as the division number M for dividing the semiconductor layer made of the semiconductor material constituting the quantum cascade detector 30 by the air layer in the groove in the groove structure. As described above, it can be configured by M structural layers including a semiconductor layer (first medium layer) and an air layer (second medium layer).

  In the configuration example shown in FIG. 3, the reflection reducing structure 40 sets the division number for dividing the semiconductor layer of the detector 30 by the air layer in the groove to M = 2, and the first structure layer in order from the incident surface 30 a side. 41 and the second structure layer 42 are provided. The first structure layer 41 includes a semiconductor layer 41a and an air layer 41b having a groove structure. Similarly, the second structure layer 42 includes a semiconductor layer 42a and an air layer 42b. In such a configuration, the thicknesses of the semiconductor layers 41a and 42a are 0.15 / 2 = 0.075 μm, respectively. The layer thickness of the air layers 41b and 42b is 0.48 / 2 = 0.24 μm, respectively.

  In the above configuration, when the number of divisions is M = 1 and the reflection reducing structure 40 is configured by the single structural layer 41, the semiconductor layer 41a has a thickness of 0.15 μm, and the air layer 41b has a thickness of 0. 48 μm. As for the division number M in such a reflection reducing structure 40, it is desirable that the division number is large in terms of averaging the refractive index, but the division number M is set in consideration of the limit of processing accuracy in manufacturing the groove structure. Must be set. That is, the upper limit of the division number M depends on the wavelength λ of the target light and the limit of processing accuracy. In the present embodiment, the division number M in the reflection reducing structure 40 is preferably 1 or 2.

Next, the range of each parameter such as the refractive index and the layer thickness allowed in the above configuration will be described. The reflection characteristics of light in the reflection reducing structure 40 are determined by the total layer thickness l (see FIG. 2B) and the average refractive index n _eff obtained from the filling factor q of the semiconductor layer. Both change independently in production, but have the same effect in that they relate to the optical path length in the reflection reducing structure 40 that the incident light senses.

When the reflectance of light on the incident surface 30a of the detector 30 to be achieved in the above embodiment is 15% or less at the target wavelength λ, the layer thickness l allowed by the reflection reducing structure 40 and the average refractive index are set. The range of n _eff is, for example, 0.8 × l _opt <l <1.2 × l _opt
0.8 × n _opt <n _eff <1.2 × n _opt
And set. That is, the average refractive index n _eff is in a range of ± 20% with respect to the optimum refractive index n _opt , and the total layer thickness l of the reflection reducing structure 40 is ± 20 with respect to the optimum layer thickness l _opt . What is necessary is just to comprise the reflection reduction structure 40 so that it may become in the range of%. this is,
0.64 × n _opt l _opt <n _eff l <1.44 × n _opt l _opt
Is meant to be.

Further, since the refractive index of the reflection reducing structure 40 is determined by the filling factor q of the semiconductor material, an allowable filling factor q range may be set. Here, FIG. 8 is a graph showing the dependence of the light reflectance in the reflection reducing structure 40 on the filling rate q of the semiconductor material. In this graph, the horizontal axis represents the filling rate q of the semiconductor material in the reflection reducing structure, and the vertical axis represents the reflectance R (%) for light having a wavelength λ = 4.5 μm. In FIG. 8, a graph B1 shows the reflectance when the layer thickness of the reflection reducing structure 40 is l = l _opt , and a graph B2 shows the case where the layer thickness is l = 0.8 × l _opt. The reflectance is shown, and the graph B3 shows the reflectance when the layer thickness is 1 = 1.2 × l _opt .

From the graph of FIG. 8, the range of the filling rate q allowed in the above embodiment is 0.097 <q <0.371.
It becomes. Therefore, with respect to the optimum filling factor q _opt of the semiconductor layer obtained from the optimum refractive index n _opt and the average refractive index n _eff , the allowable range of the filling factor q is, for example, ± 50%,
0.5 × q _opt <q <1.5 × q _opt
And set.

  [Second Embodiment]

  9A is a plan view and FIG. 9B is a side cross-sectional view showing a partially enlarged configuration of the quantum cascade detector and the reflection reducing structure in the photodetector of the second embodiment. The photodetector 1B according to the present embodiment is different from the photodetector 1A according to the first embodiment in the configuration of the reflection reducing structure.

The reflection reducing structure 40 in the photodetecting device 1B of the present embodiment does not define an optimum average refractive index n _eff , and M is an integer of 2 or more, and a semiconductor layer (first medium layer) and an air layer (second layer), respectively. The reflection reduction structure 40 is configured by M structural layers having a filling rate of q _{1 to} q _M stacked in order from the first structural layer to the Mth structural layer from the incident surface 30a side. . In such a configuration, the reflection reducing structure 40 has an average refractive index n _eff in each structural layer so that the refractive index n ₂ = n of the air layer is gradually increased from the first structural layer toward the Mth structural layer. and it is configured so as to approach to the refractive index n _{1 =} n _s of the semiconductor layer from _a.

In the configuration example illustrated in FIG. 9, the reflection reducing structure 40 includes a first structure layer 46 including a pair of semiconductor layers and an air layer in order from the incident surface 30 a side, where the number of structure layers is M = 4. The structure includes four structural layers of the second structural layer 47, the third structural layer 48, and the fourth structural layer 49. When the filling rates of the semiconductor layers in the first to fourth structure layers 46 to 49 are q ₁ , q ₂ , q ₃ , and q ₄ , m is an integer of 1 or more and 4 or less, and the average in the m-th structure layer refractive index _{n eff, m} is ^{_{n eff, m = {q m}} n s 2 + (1-q m) n a 2} 1/2
Sought by.

FIG. 10 is a chart showing an example of the configuration of the first to fourth structure layers 46 to 49 in the reflection reducing structure 40 shown in FIG. In the present configuration example, the filling rate q _m of the semiconductor material in the m-th structure layer (m-th layer) constituting the reflection reducing structure 40 is set to an interval Δq = 1 / (M + 1) = 0. Target filling rate q _{t, m} set at equal intervals in 2
q _{t, m} = m / (M + 1) = 0.2 × m
Is set to match. At this time, the filling rates q _{1 to} q ₄ of the semiconductor material in the first to fourth structure layers 46 to 49 are 0.2, 0.4, 0.6, and 0.8, respectively, as shown in FIG. It becomes. Further, _n a = 1 the refractive index of the air layer, and the refractive index of the semiconductor layer and _n s = 3.3, the average refractive index in the first to fourth structural layer 46~49 _{n eff, 1 ~n} eff _{, 4} are 1.7, 2.2, 2.6, and 3.0, respectively.

Further, in the present configuration example, the layer thickness l _m in the entire of the m structure layer including the semiconductor layer and the air layer, the target layer thickness in the case where the filling rate q _m is equal to the target filling rate q _{t, m} l _{t, m} = λ / 4n _{eff, t, m
^{= Λ / 4 {q t,}} m n s 2 + (1-q t, m) n a 2} 1/2
Is set to match. At this time, the layer thickness _l 1 to l ₄ in the first to fourth structural layer 46 to 49 are made respectively 662 nm, 511 nm, 433 nm, and 375 nm. In addition, as shown in FIG. 10, the thicknesses of the semiconductor layer and the air layer in each structural layer can be obtained from the layer thickness l _m of the entire structural layer and the filling rate q _{m of the} semiconductor layer. . FIG. 11 is a graph showing the wavelength dependence of the reflectance of light in the reflection reducing structure 40 under the configuration conditions set in this way.

  In the configuration example shown in FIGS. 9 and 10, the groove requiring the most precise processing among the groove structures constituting the reflection reducing structure 40 is an air layer having a thickness of 75 nm in the fourth structure layer 49. Such a groove structure can be produced using, for example, a focused ion beam (FIB) apparatus. Specifically, when the mesa structure of the quantum cascade detector 30 is manufactured, the groove structure is not manufactured, and processing may be performed separately using FIB.

Further, the filling rates q _{1 to} q _M of the semiconductor layers in each of the M structural layers constituting the reflection reducing structure 40 are stepped at equal intervals between 0 and 1, as in the above configuration example. It is preferable to increase it. In such a configuration, when the number M of the structural layers is increased, the required processing accuracy of the groove structure is inevitably increased, but the upper limit of the number M of layers depends on the wavelength λ of the target light. The minimum number M of structural layers is two.

Next, the range of each parameter such as the filling rate and the layer thickness allowed in the above configuration will be described. In the first to M-th structure layers of the reflection reducing structure 40, the filling rate q _m of the semiconductor layer in the m-th structure layer is set to the target filling rate q _{t, m} set at equal intervals by the interval Δq as described above. On the other hand, it is preferable to set so as to be within a range of ± Δq / 2. For example, in the first structural layer 46 in the configuration example described above, the filling rate q ₁ is in the range of 0.1 to 0.3 with respect to the design value q _{t, 1} = 0.2 of the target filling rate at equal intervals. As long as it is set within. The layer thickness _{l m} of the m structural layer filling rate _{q m} the target filling rate _{q t,} target layer thickness _{l t} in the case of match the _{m, m = λ / 4n eff} , t, with respect to _m , It is preferable to set within a range of ± 30%.

  FIG. 12 is a graph showing the wavelength dependence of the reflectance of light in the reflection reducing structure 40 in which the configuration conditions are changed within a certain range from the configuration example of FIG. In FIG. 12, a graph C1 shows the reflectivity when the filling rate q is all increased by 0.1 and the layer thickness l is all increased by 30% from the configuration conditions of FIG. 10, and the graph C2 is the configuration of FIG. The reflectivity when the filling factor q is all reduced by 0.1 and the layer thickness l is all reduced by 30% is shown.

  [Third Embodiment]

  FIG. 13 is a plan view showing the configuration of the third embodiment of the photodetector. The photodetector 1C according to the present embodiment is different from the photodetector 1A according to the first embodiment in the configuration of the plasmon light receiving unit and the waveguide unit.

  In the present embodiment, the waveguide layer 50 (see FIG. 1) between the light receiving unit 20 and the quantum cascade detector 30 is not provided on the substrate 10, and the metal layer 21 in which the periodic structure 22 is formed is provided. The light receiving section 20 is expanded to the vicinity of the detector 30, and a predetermined portion on the detector 30 side is configured as a tapered section 23 that also functions as a plasmon waveguide. Also with the photodetecting device 1C having such a configuration, it becomes possible to detect detected light such as infrared light at high speed and with high sensitivity, similarly to the photodetecting device 1A of the first embodiment.

  [Fourth Embodiment]

  FIG. 14 is a plan view showing the configuration of the fourth embodiment of the photodetecting device. In the photodetector 1D according to the present embodiment, a mesa-type quantum cascade detector 30 is provided on the substantially square substrate 10 on one corner (lower right corner in the drawing) side. In addition, a first light receiving portion 201, a first waveguide portion 501, and a first Bragg reflection structure 251 are provided on a first incident surface (upper surface in the drawing) 30c of the detector 30. Similarly, a second light receiving portion 202, a second waveguide portion 502, and a second Bragg reflection structure 252 are provided for the second incident surface (left surface in the drawing) 30d. Further, on the incident surfaces 30c and 30d side of the detector 30, reflection reducing structures 401 and 402 each having a groove structure similar to the above-described embodiment are provided.

  As described above, the configuration in which the light receiving portions 201 and 202 are provided in two directions with respect to the quantum cascade detector 30 is effective in eliminating the polarization dependency of the photosensitivity (see FIG. 2B) in the photodetecting device 1D. It is. In particular, in the configuration shown in FIG. 14, the arrangement direction of the periodic structure in the first light receiving unit 201 and the arrangement direction of the periodic structure in the second light receiving unit 202 are orthogonal to each other on the substrate 10. Thereby, the polarization dependence of photosensitivity can be preferably eliminated.

  Further, in the configuration of FIG. 14, the entire light receiving portion 201 and the waveguide portion 501 are formed in a tapered shape with a width decreasing toward the detector 30 with respect to the first incident surface 30 c of the detector 30. Similarly, with respect to the second incident surface 30 d of the detector 30, the entire light receiving portion 202 and the waveguide portion 502 are formed in a tapered shape whose width decreases toward the detector 30. In the photodetector 1D of FIG. 14, space saving and miniaturization of the photodetector are achieved by such a configuration.

  Note that the periodic structures of the first and second light receiving portions 201 and 202 are formed separately from each other in FIG. 14, but these periodic structures are integrally connected. It is good also as a structure. Similarly to the configuration shown in FIG. 13, the waveguide portions 501 and 502 may be replaced by the tapered portions of the light receiving portions 201 and 202.

  [Fifth Embodiment]

  Since the light detection device 1D shown in FIG. 14 saves space as described above, an array of light detection devices including a quantum cascade detector, and response deployment to an image sensor and the like thereby. Is also effective.

  FIG. 15 is a plan view showing the configuration of the fifth embodiment of the photodetecting device. The photodetector 1E according to the present embodiment includes the quantum cascade detector 30 including the light receiving portions 201 and 202 and the reflection reducing structure, and the device portion configured similarly to the photodetector 1D illustrated in FIG. The detection unit 15 is configured such that a plurality of such light detection units 15 are arranged in a two-dimensional array. The photodetecting device 1E having such a configuration can be used as, for example, a two-dimensional image sensor.

  In such an array configuration in which a plurality of light detection units 15 are arranged in this way, it is desirable to reduce the size of the light detection units corresponding to the pixels as much as possible. In this case, the element size of the quantum cascade detector 30 can be set to, for example, 50 μm × 50 μm. Further, the pixel size of the entire light detection unit 15 can be set to, for example, 100 μm × 100 μm. The photodetection device 1E in FIG. 15 shows a configuration in which the photodetection units 15 are arranged in a two-dimensional array on the substrate. However, the photodetection units 15 are arranged in a one-dimensional array to obtain a one-dimensional image sensor. It is good also as a structure which can be used as.

  [Sixth Embodiment]

  FIG. 16 is a side sectional view showing the configuration of the sixth embodiment of the photodetecting device. In the photodetector 1F according to the present embodiment, in the quantum cascade detector 30 on the substrate 10, in addition to the reflection reducing structure 40 on the first surface 30a side, on the second surface 30b side opposite to the first surface 30a. A reflection structure 60 is provided.

  According to such a configuration, the light can be reciprocated inside the detector 30 due to the reflection of light by the reflection structure 60, and the light detection efficiency in the detector 30 can be improved. In this case, the detector length L1 (see FIG. 1) of the detector 30 is set to about ½ (for example, 100 μm) of the length at which the intensity of incident light having the wavelength λ becomes 1 / e in the detector 30. It becomes possible to set. As a result, the area of the mesa quantum cascade detector 30 can be reduced, and thermal noise can be reduced by increasing the element resistance. Further, when the detector 30 is miniaturized in this way, the capacitance for high-speed response can be reduced.

Specifically, a second groove structure is formed on the second surface 30 b side of the detector 30. This groove structure includes a first medium layer (semiconductor layer) having a refractive index n ₁ made of a semiconductor material constituting the detector 30 and a second medium layer having a refractive index n _{2 made} of a medium in the groove of the groove structure. A Bragg reflection structure 60 that increases the reflectance of light having the wavelength λ on the second surface 30b is configured by stacking (for example, an air layer). The method for forming the second groove structure constituting the reflection structure 60 is the same as the method for forming the groove structure in the reflection reduction structure 40 described above.

A suitable layer thickness of each of the first and second medium layers constituting the reflection structure 60 is obtained by λ / 4n where λ is the wavelength of light of interest and n is the refractive index of the medium. For example, when the wavelength of the light to be detected is λ = 4.5 μm and the first medium layer is a semiconductor layer having a refractive index n ₁ = 3.3, the preferred layer thickness of the semiconductor layer is 0.33 μm. When the second medium layer is an air layer having a refractive index n ₂ = 1, a preferable layer thickness of the air layer is 1.13 μm. The actual layer thickness of the first and second medium layers may be set within a range of, for example, ± 45% with respect to the ideal layer thickness described above. Thereby, the reflectance of the light in the reflective structure 60 can be 50% or more.

  FIG. 17 is a graph showing the wavelength dependence of the light reflectance in the reflective structure 60. In FIG. 17, a graph D <b> 1 shows the reflectance when the number of structural layers including the first medium layer and the second medium layer included in the reflection structure 60 is 1, and a graph D <b> 2 is included in the reflection structure 60. The reflectance is shown when the number of structural layers is 2. In this graph, for light having a wavelength λ = 4.5 μm, the reflectance is R = 90% in the configuration in which the number of structural layers of the reflective structure 60 is 1, and the reflectance is R in the configuration in which the number of layers = 2. = 99%. Therefore, it can be seen that if the number of structural layers constituting the reflective structure 60 (the number of periods of the first and second medium layers) is two, sufficient reflectance can be obtained.

  The light detection device according to the present invention is not limited to the above-described embodiments and configuration examples, and various modifications are possible. For example, in the above-described embodiment, the quantum cascade detector (QCD) is exemplified as the semiconductor photodetecting element. However, as described above, the quantum well type semiconductor photodetecting element using the conduction band subband level is used. If so, for example, a photodetection element other than QCD, such as a quantum well infrared photodetector (QWIP), may be used. Various configurations other than those exemplified in the above-described embodiment can be used as specific configurations of each unit of the device such as the semiconductor light detection element, the light receiving unit, and the waveguide unit.

  In addition, regarding the reflection reducing structure formed on the first surface side and the reflection structure formed on the second surface side of the semiconductor photodetector element, in the above-described embodiment, the first medium layer is a semiconductor layer made of a semiconductor material. Although the configuration in which the second medium layer is an air layer made of air in the groove is illustrated, the second medium layer may be a gas medium layer other than the air layer, for example. In addition, a layer in which a groove is filled with a predetermined material such as a dielectric material may be used as the second medium layer.

  INDUSTRIAL APPLICABILITY The present invention can be used as a light detection device that can detect light to be detected such as infrared light with high speed and high sensitivity.

DESCRIPTION OF SYMBOLS 1A-1F ... Photodetection apparatus, 10 ... Board | substrate, 11a ... Detector formation area, 11b ... Light-receiving part formation area, 12 ... Lower electrode, 13 ... Upper electrode, 15 ... Photodetection unit,
DESCRIPTION OF SYMBOLS 20 ... Light-receiving part, 21 ... Metal layer (negative dielectric constant material layer), 22 ... Periodic structure, 22a ... Convex part, 22b ... Concave part, 23 ... Tapered part, 25 ... Bragg reflection structure,
DESCRIPTION OF SYMBOLS 30 ... Quantum cascade detector (semiconductor light detection element), 30a ... 1st surface (incident surface), 30b ... 2nd surface, 31 ... Lower contact layer, 32 ... Cladding layer, 33 ... Upper contact layer, 35 ... Active layer 36 ... Unit laminated body, 37 ... Absorption region, 38 ... Transport region,
40: reflection reduction structure, 41: first structure layer, 42: second structure layer, 46: first structure layer, 47: second structure layer, 48: third structure layer, 49: fourth structure layer, 60: Reflective structure,
50: Waveguide portion, 51: Waveguide layer, 52: Dielectric layer.

Claims (12)

A substrate,
A semiconductor photodetecting element provided on the substrate and having a quantum well type active layer using a conduction band subband level and formed in a mesa type;
A negative dielectric constant material layer formed of a material having a negative dielectric constant and formed with a periodic structure is provided on the substrate together with the semiconductor photodetecting element, and in response to incident light to be detected having a predetermined wavelength λ A light receiving portion configured to generate plasmons;
The semiconductor photodetecting element is disposed on one side of the light receiving portion in the propagation direction of the plasmon, and the first surface is an incident surface with respect to the plasmon, and a groove structure is formed on the first surface side. And
The groove structure includes a first medium layer having a refractive index n ₁ made of a semiconductor material constituting the semiconductor photodetecting element, and a second medium layer having a refractive index n _{2 made} of a medium in the groove in the groove structure. Are arranged in the light incident direction to form a reflection reducing structure that reduces the reflectance of light of wavelength λ on the first surface. The reflection reducing structure has a structure layer of M layers (M is an integer of 1 or more) including the first medium layer and the second medium layer, respectively, and the first medium layer in the structure layer of the M layers The filling rate of is q,
Average refractive index of the first medium layer and the second medium layer n _eff = {qn ₁ ² + (1−q) n ₂ ² } ^1/2
Is the optimum refractive index n _opt = (n ₁ n ₂ ) ^1/2 represented by the square root of the product of the refractive indices of the first medium layer and the second medium layer.
Is within a range of ± 20%, and the layer thickness l is the optimum layer thickness l _opt = λ / 4n _opt
The photodetection device according to claim 1, wherein the photodetection device is configured to be within a range of ± 20% with respect to. Each of the reflection reduction structures includes the first medium layer and the second medium layer, and the first medium layer has a structure layer of M layers (M is an integer of 2 or more) having a filling factor of q _{1 to} q _M. And the M layer structure layer is laminated in the order of the first structure layer to the Mth structure layer from the first surface side,
The m structure layer (m is an integer of 1 to M) Average refractive index n _eff of the first medium layer and the second medium layer in included in the structural layers of the M _{layer, m = {q m} n ₁ ² + (1-q _m ) n ₂ ² } ^1/2
But it is configured so as to approach from the refractive index n ₂ of the stepwise the second medium layer toward the second M structure layer from said first structural layer to the refractive index n ₁ of the first medium layer The photodetection device according to claim 1. In the reflection reducing structure, the filling rate q _m of the first medium layer in the m-th structure layer is set to be equal to an interval of Δq = 1 / (M + 1) between 0 and 1. q _{t, m} = m / (M + 1)
It was within the range of ± [Delta] q / 2 with respect to the first thickness l _m of the m structure layer, filling rate q _m the target filling rate q _t, target layer thickness l t in the case of match the _{m, m} = λ / 4n _{eff, t, m}
The photodetection device according to claim 3, wherein the photodetection device is configured to be within a range of ± 30%.   The semiconductor photodetecting element includes a multi-layered unit stacked body including N (N is an integer of 3 or more) quantum well layers including a first well layer functioning as an absorption well layer, and N quantum barrier layers. Thus, a cascade structure in which an absorption region that includes the first well layer and detects light by intersubband absorption and a transport region that transports electrons excited by the intersubband absorption is alternately stacked. The photodetection device according to claim 1, wherein the photodetection device is a quantum cascade detector including the formed active layer.   A waveguide layer made of a material having a negative dielectric constant is provided between the semiconductor photodetecting element and the light receiving unit on the substrate, and the plasmon generated by the light receiving unit is detected by the semiconductor photodetection. The optical detection device according to claim 1, further comprising a waveguide portion configured to propagate to the element.   The photodetection device according to claim 6, wherein the waveguide section includes the waveguide layer and a dielectric layer made of a dielectric material formed on the waveguide layer.   The photodetection device according to claim 6 or 7, wherein the waveguide section is formed in a tapered shape in which a waveguide width decreases from the light receiving section to the semiconductor photodetector element.   The photodetection device according to claim 1, wherein the negative dielectric constant material layer of the light receiving unit is a metal layer made of a metal material.   The negative dielectric constant material layer of the light receiving unit has a concavo-convex structure including a plurality of grooves periodically formed with the propagation direction of the plasmon to the semiconductor photodetector as an array direction as the periodic structure. The photodetection device according to any one of claims 1 to 9, wherein The semiconductor photodetector element has a second groove structure formed on the second surface side opposite to the first surface,
The second groove structure constitutes a reflective structure in which the first medium layer and the second medium layer are stacked to increase the reflectance of light having a wavelength λ on the second surface. The photodetection device according to any one of claims 1 to 10, wherein   2. A plurality of light detection units arranged in a one-dimensional array or a two-dimensional array on the substrate and having the semiconductor light detection elements each including the light receiving unit and the reflection reducing structure. The photodetection device according to any one of 1 to 11.

Images