Interband Photodetector

Abstract

An interband photodetector that can be suitably applied to the detection of light at a desired detection wavelength in a wide wavelength range is provided. [Solution] The interband photodetector comprises a semiconductor substrate and a superlattice layer 30 including a unit laminate structure 31 having a type-I quantum well structure provided on the semiconductor substrate. The unit laminate structure 31 has an absorption region 32 including at least one quantum well layer and a relaxation region 33 including m quantum well layers. The absorption region 32 has a lower detection level due to the valence band level of the quantum well layer and an upper detection level due to the conduction band level, and the relaxation region 33 has m relaxation levels due to the conduction band levels of the m quantum well layers. The photodetector detects light by interband absorption from the lower detection level to the upper detection level in the absorption region 32, and electrons excited by light absorption are extracted via the m relaxation levels in the relaxation region 33.

Description

[Technical Field]

[0001] The present invention relates to an interband photodetector that utilizes optical absorption between bands in a quantum well structure. [Background technology]

[0002] In recent years, quantum cascade detectors (QCDs) using a cascade structure in which unit stacked layers each having a quantum well structure are stacked have been reported as photodetectors for the mid-infrared wavelength region and the like. A quantum cascade detector is a photodetector that detects incident light by absorbing light in the cascade structure and measuring the amount of current flowing due to carriers generated by the light absorption, and is characterized by its ability to operate at high speed without bias. Furthermore, a quantum cascade detector can more efficiently absorb and detect light by cascading semiconductor stacked structures, each including an absorption well layer that absorbs light, in multiple stages (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-88688 [Patent Document 2] U.S. Patent Application Publication No. 2007 / 0224721 [Non-patent literature]

[0004] [Non-Patent Document 1] LM Kruger et al., "High-speedinterband cascade infrared photodetectors: photo-response saturation by a femtosecondoscillator", Optics Express Vol.29, No.9 (2021) pp.14087-14100 Summary of the Invention [Problem to be solved by the invention]

[0005] The quantum cascade detector described above detects light by utilizing intersubband electron transitions in the subband level structure formed in the quantum well structure. Therefore, even if the degree of freedom in designing the quantum well structure is taken into consideration, the quantum cascade detector can only detect light of a wavelength corresponding to the energy difference between the subbands (e.g., mid-infrared light), making it difficult to apply the detector to the detection of light in a wide wavelength range.

[0006] On the other hand, apart from quantum cascade detectors, interband cascade detectors (ICDs) that utilize interband electron transitions in quantum well structures have been proposed (see, for example, Patent Document 2 and Non-Patent Document 1). Conventional interband cascade detectors mainly use type II quantum well structures.

[0007] However, the semiconductor materials that can be used for type-II quantum well structures are limited, and it may be difficult to obtain sufficient freedom in designing a quantum well structure that corresponds to the desired detection wavelength of light. For example, in the case of Sb-based semiconductor materials used in conventional interband cascade detectors, there is a limit to how short the detection wavelength can be, making it difficult to apply them to the detection of light over a wide wavelength range, as with quantum cascade detectors.

[0008] An object of the present invention is to provide a semiconductor photodetector that can be suitably applied to the detection of light at a desired detection wavelength within a wide wavelength range. [Means for solving the problem]

[0009] An interband photodetector according to a first aspect of the present invention comprises: (1) a semiconductor substrate; and (2) a superlattice layer provided on the semiconductor substrate and including a unit laminate structure having a Type I quantum well structure consisting of n quantum barrier layers (n is an integer of 3 or more) and n quantum well layers. (3) The unit laminate structure has an absorption region including at least one quantum well layer and a relaxation region including m quantum well layers (m is an integer of 2 or more and n-1 or less). The absorption region has, in its level structure, a detection lower level resulting from a valence band level in the quantum well layer that is included in the absorption region and functions as an absorption well layer, and an upper detection level resulting from a conduction band level. The relaxation region has, in its level structure, m relaxation levels resulting from the conduction band levels in each of the m quantum well layers included in the relaxation region. (4) Light to be detected is detected by interband absorption from the detection lower level to the detection upper level in the absorption region, and electrons excited by the interband absorption are extracted via the relaxation level structure of the m relaxation levels in the relaxation region.

[0010] In the above-described interband photodetector, the active layer for detecting light to be detected is configured by alternately stacking first to n-th barrier layers and first to n-th well layers, and uses a superlattice layer including unit laminates of a type-I quantum well structure having an absorption region used for absorbing and detecting light and a relaxation region used for relaxing and extracting electrons.The interband photodetector detects light to be detected by interband absorption between the lower detection level of the valence band and the upper detection level of the conduction band in the absorption region, and extracts electrons by relaxation via m relaxation levels in the relaxation region.

[0011] According to this configuration, by utilizing interband electronic transitions rather than intersubband electronic transitions to detect target light, it is possible to suitably set the detection wavelength of light by designing the band gap in the quantum well structure, etc. Furthermore, by using a type I quantum well structure as the quantum well structure in the unit laminate of the superlattice layer, the degree of freedom in selecting semiconductor materials and designing the quantum well structure is increased compared to when a type II quantum well structure is used, and it becomes possible to suitably apply the photodetector to the detection of light at a desired detection wavelength in a wide wavelength range.

[0012] In the interband photodetector of the second aspect, in the configuration of the first aspect, the band gap energy of each of the m quantum well layers included in the relaxation region may be set to be larger than the band gap energy of the quantum well layer included in the absorption region.

[0013] In the interband photodetector of the third aspect, in the configuration of the first or second aspect described above, the energy difference between the valence band level and the relaxation level in each of the m quantum well layers included in the relaxation region may be set to be larger than the energy difference between the detection lower level and the detection upper level in the quantum well layer included in the absorption region.

[0014] In the interband photodetector of the fourth aspect, in the configuration of the third aspect described above, the energy difference between the valence band level and the relaxation level in each of the m quantum well layers included in the relaxation region may be set to be larger than the detection energy of the light to be detected.

[0015] According to each of the above configurations, the occurrence of light absorption in the relaxation region including m relaxation levels can be suppressed by setting the energy difference, etc. This makes it possible to preferably realize interband light absorption in the absorption region and light detection operation due to electron relaxation in the relaxation region, thereby improving the light detection efficiency of the photodetector.

[0016] In the interband photodetector of the fifth aspect, in the configuration of any one of the above first to fourth aspects, in the quantum well layer included in the absorption region, the detection upper level may be a level resulting from the ground level in the subband level structure of the conduction band.

[0017] In the interband photodetector of the sixth aspect, in the configuration of any one of the first to fifth aspects, the relaxation level in each of the m quantum well layers included in the relaxation region may be a level resulting from the ground level in the subband level structure of the conduction band.

[0018] According to each of the above configurations, by appropriately setting the upper detection level in the subband level structure in the conduction band and the level structure of m relaxation levels, it is possible to suitably realize the interband light absorption in the absorption region and the light detection operation by electron relaxation in the relaxation region.

[0019] In the interband photodetector of the seventh aspect, in the configuration of any one of the above first to sixth aspects, in the unit laminate body, each of the n quantum barrier layers and the n quantum well layers may be configured to be made of an i-type semiconductor layer.

[0020] In this way, by using undoped i-type semiconductor layers for the quantum barrier layers and quantum well layers included in the unit laminate of the superlattice layer, it is possible to preferably realize detection of the target light by utilizing interband absorption.

[0021] In the interband photodetector of the eighth aspect, in the configuration of any one of the first to seventh aspects described above, the superlattice layer may be configured to include, as the unit laminate body, a plurality of unit laminate bodies, each having an absorption region and a relaxation region.

[0022] In this way, when the superlattice layer has a cascade structure in which a plurality of unit laminate bodies are stacked in multiple stages, the interband photodetector functions as an interband cascade detector. Note that the superlattice layer may also be configured to include a single unit laminate body.

[0023] In the interband photodetector of the ninth aspect, in the configuration of any one of the first to eighth aspects, the absorption region in the unit laminate structure may include a single quantum well layer.

[0024] In the interband photodetector of the tenth aspect, in the configuration of any one of the first to eighth aspects, the absorption region in the unit laminate structure may include a plurality of quantum well layers.

[0025] As in the above configurations, the quantum well layer included in the absorption region and functioning as the absorption well layer can be a single quantum well layer or multiple quantum well layers. When the absorption region includes a single quantum well layer, the configuration of the absorption region can be simplified. Furthermore, when the absorption region includes multiple quantum well layers, the light detection efficiency due to interband absorption can be improved.

[0026] In the interband photodetector of the eleventh aspect, in the configuration of any one of the first to tenth aspects described above, a carrier block layer may be provided in a region on the semiconductor substrate side of the superlattice layer and in a region on the opposite side of the superlattice layer from the semiconductor substrate that is in contact with the absorption region.

[0027] In the interband photodetector of the 12th aspect, in the configuration of any one of the above-mentioned first to 11th aspects, a p-type semiconductor layer may be provided in a region on the semiconductor substrate side of the superlattice layer and in a region on the opposite side of the superlattice layer from the semiconductor substrate, the region being in contact with the absorption region.

[0028] According to each of the above configurations, the carrier block layer or the p-type semiconductor layer provided on the absorption region side of the superlattice layer prevents electrons excited by interband absorption in the absorption region from migrating to the region opposite the relaxation region, thereby improving the efficiency of electron extraction by the m relaxation levels in the relaxation region.

[0029] The interband photodetector of the thirteenth aspect may be configured in such a way that, in the configuration of any one of the first to twelfth aspects, a low refractive index layer is provided in a region on the semiconductor substrate side of the superlattice layer.

[0030] The interband photodetector of the fourteenth aspect may be configured such that, in the configuration of any one of the first to thirteenth aspects, a low refractive index layer is provided in a region on the opposite side of the superlattice layer from the semiconductor substrate.

[0031] As in each of the above configurations, by providing a low refractive index layer that functions as a cladding layer for the superlattice layer in at least one of the region on the semiconductor substrate side of the superlattice layer and the region on the opposite side of the superlattice layer from the semiconductor substrate, it is possible to confine the light to be detected within the superlattice layer, thereby improving the light detection efficiency of the photodetector.

[0032] In the interband photodetector of the 15th aspect, in the configuration of any one of the above-mentioned 1st to 14th aspects, the energy difference between the detection lower level and the detection upper level in the absorption region may be set to be larger than the energy of the longitudinal optical phonon.

[0033] In the interband photodetector of the 16th aspect, in the configuration of any one of the above-mentioned 1st to 15th aspects, the energy difference between adjacent relaxation levels among the m relaxation levels in the relaxation region may be set to be larger than the energy of the longitudinal optical phonon.

[0034] According to each of the above configurations, in the relaxation level structure of m relaxation levels in the relaxation region, fast relaxation of electrons due to longitudinal optical phonon scattering can be used. In this case, electrons excited to the detection upper level by light absorption move to the relaxation level in the relaxation region by the resonant tunneling effect, and are further extracted at high speed in the relaxation level structure of m relaxation levels by a relaxation process including fast relaxation due to longitudinal optical phonon scattering.

[0035] In the interband photodetector of the 17th aspect, in the configuration of any one of the above-mentioned 1st to 16th aspects, in the unit laminate structure, the type I quantum well structure may be configured such that the upper end of the valence band in the quantum well layer is higher than the upper end of the valence band in the adjacent quantum barrier layer.

[0036] Specifically, for example, the type I quantum well structure constituting the unit laminated layer in the superlattice layer can be the structure described above. [Effects of the Invention]

[0037] The interband photodetector of the present invention provides a semiconductor photodetector that can be suitably applied to the detection of light at a desired detection wavelength within a wide wavelength range. [Brief explanation of the drawings]

[0038] [Figure 1] FIG. 1 is a diagram showing the basic configuration of a first embodiment of an interband photodetector. [Figure 2] FIG. 2 is a diagram showing a type I quantum well structure and a level structure in the superlattice layer of the interband photodetector. [Figure 3] FIG. 3 is a diagram showing a type-II quantum well structure in a conventional interband cascade detector. [Figure 4] FIG. 4 is a graph showing a specific example of the configuration of a unit laminate body that constitutes a superlattice layer. [Figure 5] FIG. 5 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 6] FIG. 6 is a diagram showing an example of a stacked structure in the superlattice layer of the interband photodetector shown in FIG. [Figure 7] FIG. 7 is a perspective view schematically showing an example of the configuration of a photodetector using an interband photodetector. [Figure 8] 8(a) and 8(b) are side views schematically showing an example of the configuration of a photodetector using an interband photodetector. [Figure 9] FIG. 9 is a graph showing a photodetection spectrum acquired by a photodetector using an interband photodetector. [Figure 10] FIG. 10 is a diagram showing a first modified example of the quantum well structure and the level structure in the superlattice layer of the interband photodetector. [Figure 11] FIG. 11 is a diagram showing a second modified example of the quantum well structure and the level structure in the superlattice layer of the interband photodetector. [Figure 12] FIG. 12 is a diagram showing the basic configuration of a second embodiment of an interband photodetector. [Figure 13] FIG. 13 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 14] FIG. 14 is a diagram showing the basic configuration of a third embodiment of an interband photodetector. [Figure 15] FIG. 15 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 16] FIG. 16 is a diagram showing the basic configuration of a fourth embodiment of an interband photodetector. [Figure 17] FIG. 17 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 18] FIG. 18 is a diagram showing the basic configuration of a fifth embodiment of an interband photodetector. [Figure 19] FIG. 19 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 20] FIG. 20 is a diagram showing the basic configuration of a sixth embodiment of an interband photodetector. [Figure 21] FIG. 21 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 22] FIG. 22 is a diagram showing the basic configuration of the seventh embodiment of the interband photodetector. [Figure 23]FIG. 23 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 24] FIG. 24 is a diagram showing the basic configuration of an eighth embodiment of an interband photodetector. [Figure 25] FIG. 25 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 26] FIG. 26 is a diagram showing the basic configuration of a ninth embodiment of an interband photodetector. [Figure 27] FIG. 27 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 28] FIG. 28 is a diagram showing the basic configuration of a tenth embodiment of an interband photodetector. [Figure 29] FIG. 29 is a diagram showing an example of a semiconductor laminated structure in the interband photodetector shown in FIG. [Figure 30] FIG. 30 is a diagram showing the basic configuration of an eleventh embodiment of an interband photodetector. [Figure 31] FIG. 31 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. [Figure 32] FIG. 32 is a diagram showing the basic configuration of a twelfth embodiment of an interband photodetector. [Figure 33] FIG. 33 is a diagram showing an example of a semiconductor laminate structure in the interband photodetector shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0039] Hereinafter, an embodiment of an interband photodetector will be described in detail with reference to the drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicated explanations will be omitted. Furthermore, the dimensional ratios of the drawings do not necessarily match those in the description.

[0040] 1 is a diagram showing a basic configuration of a first embodiment of an interband photodetector, schematically illustrating its semiconductor laminate structure. The interband photodetector 1A of this embodiment is a photodetector that detects light by utilizing optical absorption due to interband electronic excitation in a semiconductor quantum well structure. This interband photodetector 1A is configured to include a semiconductor substrate 10 and a superlattice layer 30 formed on the semiconductor substrate 10.

[0041] The superlattice layer 30 is configured to include one or more unit laminate structures 31, each having an absorption region (light absorption layer) used for absorbing and detecting light, and a relaxation region (electron relaxation layer) used for relaxing and extracting electrons, which are carriers. The number of stacked unit laminate structures 31 in the superlattice layer 30 is set appropriately depending on the light detection characteristics required of the photodetector 1A, etc. In this embodiment, the number of stacked unit laminate structures 31 constituting the superlattice layer 30 is set to one. Furthermore, the superlattice layer 30 is generally formed directly on the semiconductor substrate 10 or via another semiconductor layer.

[0042] 1, an n-type low refractive index layer 13, an n-type contact layer 12, and a carrier block layer 11 are provided in this order from the semiconductor substrate 10 side in a region between the semiconductor substrate 10 and the superlattice layer 30 on the semiconductor substrate 10 side of the superlattice layer 30, and the carrier block layer 11 is in contact with the bottom surface of the superlattice layer 30. An n-type contact layer 22 is provided in a region of the superlattice layer 30 opposite the semiconductor substrate 10, and the n-type contact layer 22 is in contact with the top surface of the superlattice layer 30.

[0043] Fig. 2 is a diagram showing the quantum well structure and the level structure in the superlattice layer 30 of the interband photodetector 1A shown in Fig. 1. As shown in Fig. 2, the unit laminate structure 31 included in the superlattice layer 30 is composed of n quantum barrier layers and n quantum well layers, where n is an integer of 3 or more.

[0044] These n barrier layers and well layers are formed in the following order from the left in the figure: first barrier layer 341, first well layer 351, second barrier layer 342, second well layer 352, third barrier layer 343, third well layer 353, ..., nth barrier layer, nth well layer. In unit laminate body 31, each of the n quantum barrier layers and n quantum well layers is preferably made of an i-type semiconductor layer.

[0045] The unit laminate 31 constituting the superlattice layer 30 is generally configured to have an absorption region 32 including at least one quantum well layer, and a relaxation region 33 including m quantum well layers, where m is an integer between 2 and n-1.

[0046] In the configuration shown in Fig. 2, among the semiconductor layers constituting the unit laminate structure 31, the first barrier layer 341 and the first well layer 351 form the absorption region 32, and the second barrier layer 342 to the n-th well layer form the relaxation region 33. Also, Fig. 2 shows the valence band upper edge A0 and the conduction band lower edge A1 for each semiconductor layer of the unit laminate structure 31. The energy difference between the valence band upper edge A0 and the conduction band lower edge A1 is called the band gap energy E g is.

[0047] In this embodiment, the unit laminate structure 31 of the superlattice layer 30 is configured to have a Type I quantum well structure as a whole, including the absorption region 32 and the relaxation region 33. In the configuration shown in Fig. 2, the conduction band minimum A1 in the quantum well layer is lower than the conduction band minimum in the adjacent quantum barrier layer, thereby forming a quantum well structure. On the other hand, the valence band maximum A0 in the above Type I quantum well structure is set higher than the valence band maximum in the adjacent quantum barrier layer.

[0048] In contrast to the type-I quantum well structure, in the type-II quantum well structure, the upper end of the valence band in the quantum well layer is set lower than the upper end of the valence band in the adjacent quantum barrier layer. These types of quantum well structures will be described later, along with the characteristics of interband photodetectors when each is used.

[0049] 2, as described above, absorption region 32 is configured to include, as one quantum well layer, first well layer 351. Relaxation region 33 is configured to include, as m=n−1 quantum well layers, second well layer 352 to n-th well layer.

[0050] The absorption region 32 has a level structure including a lower detection level L0 due to the valence band level in the first well layer 351 that functions as an absorption well layer included in the absorption region 32, and an upper detection level L1 due to the conduction band level. The relaxation region 33 has a level structure including n-1 relaxation levels L2 to L3 due to the conduction band levels in the second to n-th well layers included in the relaxation region 33. n It has the following characteristics.

[0051] In the first well layer 351 included in the absorption region 32, the detection upper level L1 is preferably a level resulting from the ground level in the subband level structure of the conduction band. Similarly, in each of the second to n-th well layers 352 to 353 included in the relaxation region 33, the relaxation level L k is preferably a level resulting from the ground level in the subband level structure of the conduction band.

[0052] In this configuration, the superlattice layer 30 in the photodetector 1A detects light hν incident on the photodetector 1A as light to be detected by interband absorption from the lower detection level L0 to the upper detection level L1 in the absorption region 32. Furthermore, electrons excited by this interband absorption are absorbed into n-1 relaxation levels L2 to L3 in the relaxation region 33. n The light to be detected is extracted via the relaxation level structure due to the photon beam, and the amount of current generated by this is measured to detect the light to be detected.

[0053] In the absorption region 32, the energy difference E1 between the detection lower level L0 and the detection upper level L1, which corresponds to the detection energy of the light to be detected hν, is, as shown in FIG. 2, the band gap energy E 1g and the energy difference E between the conduction band bottom A1 and the detection level L1 1a By adding E1=E 1g +E 1a In addition, the detection wavelength λ of the light to be detected can be calculated by λ=hc / E1.

[0054] The detection lower level L0 in the absorption region 32 usually substantially coincides with the upper edge A0 of the valence band in the first well layer 351. For this reason, in Fig. 2, for simplicity of illustration, the detection lower level L0 is illustrated by the upper edge A0 of the valence band. In addition, in the calculation of the detection energy E1 described above, the energy difference between the valence band level corresponding to the detection lower level L0 and the upper edge A0 of the valence band is ignored.

[0055] In addition, in the above configuration, the band gap energy E 1g is, for example, on the order of several hundred meV to eV. In addition, the energy difference E between the conduction band minimum A1 and the detection upper level L1 is 1a is, for example, about several hundred meV.

[0056] 1 , the absorption region 32 including the first well layer 351 is located on the semiconductor substrate 10 side, and the relaxation region 33 including the nth well layer is located on the opposite side of the semiconductor substrate 10. In this configuration, the carrier block layer 11 provided so as to be in contact with the absorption region 32 in the superlattice layer 30 has the function of suppressing the migration of electrons excited in the absorption region 32 by interband absorption to the region opposite the relaxation region 33.

[0057] In addition, the n-type low refractive index layer 13 provided between the n-type contact layer 12 and the semiconductor substrate 10 on the semiconductor substrate 10 side of the superlattice layer 30 functions as a cladding layer that confines the light to be detected within the superlattice layer 30.

[0058] The effects of the interband photodetector 1A according to this embodiment will be described.

[0059] 1 and 2, the interband photodetector 1A uses a superlattice layer 30 as an active layer for detecting light to be detected, which is configured by alternately stacking first to n-th barrier layers and first to n-th well layers, and includes a unit laminate structure 31 having an absorption region 32 used for absorbing and detecting light and a relaxation region 33 used for relaxing and extracting electrons. The light to be detected is detected by interband absorption between a lower detection level L0 of the valence band in the absorption region 32 and an upper detection level L1 of the conduction band, and m=n-1 relaxation levels L2 to L3 in the relaxation region 33. n The structure is such that electrons are extracted by relaxation via the

[0060] According to this configuration, by using interband electronic transitions instead of intersubband electronic transitions in the conduction band to detect target light, the detection wavelength of light can be suitably set within a wide wavelength range by designing the band gap in the quantum well structure, etc. For example, the interband photodetector 1A can detect light of shorter wavelengths than quantum cascade detectors that use intersubband electronic transitions.

[0061] In addition, the band gap energy in the quantum well structure of the unit laminate body 31 constituting the superlattice layer 30, the upper detection level L1 in the conduction band, and the relaxation levels L2 to L n The respective energies and the like can be controlled by the layer thickness of each semiconductor layer constituting the unit laminate structure 31, the composition of the semiconductor material, etc. Therefore, the detection wavelength and detection energy of light in the photodetector 1A can be set arbitrarily by designing the quantum well structure.

[0062] As described above, a type I quantum well structure in which the upper end of the valence band in the quantum well layer is set higher than the upper end of the valence band in the adjacent quantum barrier layer is used as the quantum well structure in the unit laminate structure 31 of the superlattice layer 30. This increases the degree of freedom in selecting semiconductor materials and designing the quantum well structure compared to when a type II quantum well structure is used, making it possible to suitably apply the photodetector 1A to detecting target light at a desired detection wavelength in a wide wavelength range.

[0063] Fig. 3 is a diagram showing a type-II quantum well structure used in a conventional interband cascade detector (see Patent Document 2). In the configuration shown in Fig. 3, among the semiconductor layers constituting the superlattice layer, the first barrier layer 541 and the first well layer 551 form an absorption region 52, and the semiconductor layers including the second barrier layer 542 and the second well layer 552 form a relaxation region 53.

[0064] In this configuration example, the superlattice layer used to detect the target light is configured to have a type II quantum well structure, including the absorption region 52 and the relaxation region 53, in which the upper end of the valence band in the quantum well layer is set lower than the upper end of the valence band in the adjacent quantum barrier layer.

[0065] Furthermore, the quantum well structure in the absorption region 52 has a structure in which the bottom of the conduction band in the quantum well layer is lower than the top of the valence band in the adjacent quantum barrier layer. Such a type II structure is sometimes called a type III quantum well structure. In such type II or type III quantum well structures, usable semiconductor materials are limited, and the degree of freedom in designing the quantum well structure for the desired detection wavelength of light is limited.

[0066] In contrast, in the interband photodetector 1A of the above embodiment, which uses a type-I quantum well structure in the entire superlattice layer 30, there are no restrictions on the semiconductor materials that can be used, and various semiconductor materials can be used, such as nitride-based, GaAs-based, InP-based, etc. Therefore, the interband photodetector 1A having the above configuration makes it possible to realize a photodetector that can operate at high speed without bias in a wide wavelength range, for example, from the ultraviolet region (e.g., wavelength 270 nm) to the near-infrared region (e.g., wavelength 2300 nm).

[0067] In the interband photodetector 1A of the above embodiment, the upper detection level L1 is preferably a level resulting from the ground level in the subband level structure of the conduction band in the quantum well layer included in the absorption region 32. Similarly, in each of the m quantum well layers included in the relaxation region 33, the relaxation level is preferably a level resulting from the ground level in the subband level structure of the conduction band.

[0068] In this way, in the subband level structure in the conduction band, by using the ground level instead of the excited level for light absorption and electron relaxation, the detection upper level L1 and m relaxation levels L2 to L n By appropriately setting the level structure based on the above, it is possible to suitably realize the detection operation of the target light in the photodetector 1A by interband light absorption in the absorption region 32 and relaxation of electrons in the relaxation region 33.

[0069] In the interband photodetector 1A of the above embodiment, each of the n quantum barrier layers and the n quantum well layers is preferably made of an i-type semiconductor layer in the unit laminate structure 31. In this way, by making the quantum barrier layers and quantum well layers included in the unit laminate structure 31 of the superlattice layer 30 undoped i-type semiconductor layers, it is possible to preferably realize detection of the target light utilizing interband absorption.

[0070] In quantum cascade detectors that utilize intersubband electronic transitions in the conduction band, semiconductor layers doped with n-type impurities are used in some quantum well layers to fill the base subband with charge. In contrast, in the interband photodetector 1A that utilizes interband electronic transitions, there is no need to fill the subband with charge, and by using i-type semiconductor layers as each semiconductor layer of the superlattice layer 30 as described above, the light detection efficiency of the photodetector 1A can be improved.

[0071] In the above embodiment, the superlattice layer 30 includes only a single unit laminate body 31. The superlattice layer 30 may include a plurality of unit laminate bodies 31, each having an absorption region 32 and a relaxation region 33. When the superlattice layer 30 has a cascade structure in which a plurality of unit laminate bodies 31 are stacked in multiple stages, the photodetector 1A functions as an interband cascade detector. In this case, the cascade structure of a plurality of unit laminate bodies 31 can improve the light detection efficiency of the photodetector 1A.

[0072] 2, in the interband photodetector 1A of the above embodiment, in the unit laminate structure 31, the absorption region 32 is configured to include a single quantum well layer 351. Furthermore, the absorption region 32 of the unit laminate structure 31 may be configured to include a plurality of quantum well layers, as will be described later.

[0073] As described above, the quantum well layers included in the absorption region 32 and functioning as absorption well layers may be single or multiple quantum well layers. When the absorption region 32 is composed of a single quantum well layer, the structure of the absorption region 32 can be simplified. When the absorption region 32 is composed of multiple quantum well layers, the light detection efficiency due to interband absorption can be improved.

[0074] Regarding the energy configuration of the valence band, conduction band, and each level used for light absorption and electron relaxation in the interband photodetector 1A, it is preferable that the band gap energy in each of the quantum well layers included in the relaxation region 33 is set larger than the band gap energy in the quantum well layer included in the absorption region 32.

[0075] Furthermore, it is preferable that the energy difference between the valence band level and the relaxation level in each of the quantum well layers included in relaxation region 33 is set to be larger than the energy difference between the detection lower level L0 and the detection upper level L1 in the quantum well layers included in absorption region 32. In this case, the energy difference between the valence band level and the relaxation level in each of the quantum well layers included in relaxation region 33 may be set to be larger than the detection energy of the light to be detected.

[0076] According to the energy configuration described above, by setting the band gap energy in relaxation region 33 or the energy difference between the valence band level and the relaxation level to be sufficiently large, it is possible to suppress the occurrence of unnecessary light absorption in relaxation region 33 and to cause light absorption to occur only in absorption region 32. This makes it possible to preferably realize interband light absorption in absorption region 32 and light detection operation due to electron relaxation in relaxation region 33, thereby improving the light detection efficiency of photodetector 1A.

[0077] When the valence band level is approximately equal to the valence band upper edge A0, the energy difference between the valence band level and the relaxation level in each quantum well layer included in relaxation region 33 can be calculated as the sum of the band gap energy corresponding to the energy difference between the valence band upper edge A0 and the conduction band lower edge A1 and the energy difference between the conduction band lower edge A1 and the relaxation level. The above-described configuration can be realized, for example, by using different semiconductor materials or compositions for the quantum well layers in absorption region 32 and those in relaxation region 33.

[0078] In the interband photodetector 1A of the above embodiment, the energy difference between the detection lower level L0 and the detection upper level L1 is preferably set larger than the energy of longitudinal optical (LO) phonons in the absorption region 32. Also, in the relaxation region 33, the energy difference between adjacent relaxation levels among the m relaxation levels is preferably set larger than the energy of LO phonons.

[0079] According to the above configuration, high-speed relaxation of electrons due to LO phonon scattering can be utilized in the relaxation level structure of electrons with m relaxation levels in the relaxation region 33. In this case, electrons excited to the detection upper level L1 by light absorption move to the relaxation level L2 of the relaxation region 33 by the resonant tunneling effect, and reach the relaxation levels L2 to L n In the relaxation level structure due to the LO phonon scattering, the LO phonon is rapidly extracted by the relaxation process.

[0080] 1 , in the interband photodetector 1A of the above embodiment, the absorption region 32 is located on the semiconductor substrate 10 side in the superlattice layer 30, and accordingly, the carrier block layer 11 is provided in a region on the semiconductor substrate 10 side of the superlattice layer 30. With this configuration, the carrier block layer 11 prevents electrons excited by interband absorption in the absorption region 32 from migrating to a region opposite the relaxation region 33, thereby improving the efficiency of electron extraction by the m relaxation levels in the relaxation region 33.

[0081] Conversely, when the absorption region 32 is located on the opposite side of the superlattice layer 30 from the semiconductor substrate 10, a carrier block layer may be provided in a region of the superlattice layer 30 on the opposite side of the semiconductor substrate 10. In general, it is preferable to provide a carrier block layer in a region of the superlattice layer 30 on the semiconductor substrate 10 side and in a region of the superlattice layer 30 on the opposite side of the semiconductor substrate 10 that is in contact with the absorption region 32.

[0082] Alternatively, a p-type semiconductor layer may be provided instead of the carrier block layer. In this case, it is preferable to provide the p-type semiconductor layer in a region on the semiconductor substrate 10 side of the superlattice layer 30 and in a region on the opposite side of the superlattice layer 30 from the semiconductor substrate 10 that contacts the absorption region 32. In this case, a p-type contact layer or a p-type low refractive index layer can be used as the p-type semiconductor layer, as will be described later.

[0083] 1, in the interband photodetector 1A of the above embodiment, a low refractive index layer 13 having a refractive index lower than that of the superlattice layer 30 is provided in a region on the semiconductor substrate 10 side of the superlattice layer 30. Alternatively, the photodetector 1A may be configured such that a low refractive index layer is provided in a region on the opposite side of the superlattice layer 30 from the semiconductor substrate 10.

[0084] In this way, by providing a low refractive index layer that functions as a cladding layer for the superlattice layer 30 in at least one of the region on the semiconductor substrate 10 side of the superlattice layer 30 and the region on the opposite side of the superlattice layer 30 from the semiconductor substrate 10, the light to be detected can be confined within the superlattice layer 30, thereby improving the light detection efficiency of the photodetector 1A.

[0085] The configuration of the interband photodetector 1A according to this embodiment will be described together with a specific example of a device structure including a quantum well structure in the superlattice layer 30. Here, the overall semiconductor stacked structure of the interband photodetector 1A uses the configuration shown in FIG. 1. Also, FIG. 4 is a graph showing a specific example of the configuration of a unit stacked layer 31 that constitutes the superlattice layer 30. In the graph of FIG. 4, the horizontal axis represents the position (nm) in the semiconductor stacking direction, and the vertical axis represents the energy (eV).

[0086] The quantum well structure of the superlattice layer 30 in this configuration example is designed for a light detection wavelength of 1550 nm. In Fig. 4, the superlattice layer 30 is assumed to include a single unit laminate structure 31, and the quantum well structure, valence band upper edge A0, conduction band lower edge A1, and level structure in the absorption region 32 and relaxation region 33 of the unit laminate structure 31 are shown. The device structures shown in Figs. 1 and 4 can be formed by crystal growth using, for example, molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE).

[0087] Fig. 5 is a diagram showing a specific example of a semiconductor laminated structure in the interband photodetector 1A shown in Fig. 1. Also, Fig. 6 is a diagram showing a specific example of a laminated structure in the superlattice layer 30 of the interband photodetector 1A shown in Fig. 1. The laminated structure shown in Fig. 6 corresponds to the quantum well structure shown in the graph of Fig. 4.

[0088] 5, a 3500 nm-thick n-type InP substrate is used as the semiconductor substrate 10 in the configuration shown in Fig. 1. Then, on this InP substrate 10, in order from the substrate 10 side, an n-type InP low refractive index layer 13 having a thickness of 500 nm, an n-type InGaAs contact layer 12 having a thickness of 50 nm, an InAlAs carrier block layer 11 having a thickness of 30 nm, a superlattice layer 30 including a single unit laminate body 31, and an n-type InGaAs contact layer 22 having a thickness of 50 nm are laminated, thereby forming the element structure of the interband photodetector 1A.

[0089] 4 and 6, the unit laminate structure 31 of the superlattice layer 30 in this configuration example is configured as a quantum well structure in which seven quantum barrier layers 341 to 347 and seven quantum well layers 351 to 357 are alternately stacked. In this configuration example, an additional quantum barrier layer 348 is provided outside the quantum well layer 357.

[0090] Of the semiconductor layers of the unit laminate body 31, the quantum barrier layers 341 to 348 are each made of In 0.52 Al 0.48The quantum well layers 352 to 357 are each made of an In layer. 0.51 Ga 0.49 The quantum well layer 351 is made up of an In layer, which has a different composition ratio from the other quantum well layers. 0.60 Ga 0.40 It is composed of As layers.

[0091] As a result, the superlattice layer 30 of this configuration example is configured with an InGaAs / InAlAs quantum well structure. The thicknesses of the quantum barrier layers and quantum well layers are as shown in Fig. 6. In Fig. 5, the thickness of the superlattice layer 30 includes the thickness of the additional quantum barrier layer 348.

[0092] In such a unit laminate structure 31, the first barrier layer 341 and the first well layer 351 constitute an absorption region 32 used for absorbing and detecting light. The second to seventh barrier layers 342 to 347 and the second to seventh well layers 352 to 357 constitute a relaxation region 33 used for relaxing and extracting electrons. All of these semiconductor layers are i-type semiconductor layers. In this configuration example, in the superlattice layer 30, the absorption region 32 is located on the semiconductor substrate 10 side, and the relaxation region 33 is located on the opposite side from the semiconductor substrate 10.

[0093] 4, the unit laminate structure 31 has the detection lower level L0 and the detection upper level L1 in the absorption region 32 described above with reference to Fig. 2, and relaxation levels L2 to L7 that constitute the relaxation level structure in the relaxation region 33. Such a level structure realizes the interband light absorption in the absorption region 32 and the light detection operation due to the relaxation of electrons in the relaxation region 33.

[0094] A photodetector element was fabricated and its light detection operation was measured for the interband photodetector 1A having the configuration shown in Figures 1, 2, and 4 to 6. Figures 7 and 8 are diagrams that schematically show an example of the configuration of a photodetector element 60 using the interband photodetector 1A, with Figure 7 showing a perspective view and Figures 8(a) and (b) showing side views, respectively.

[0095] As shown in Figures 7 and 8(a), the photodetector element 60 has a structure in which a ridge portion 62 including a superlattice layer 30 is formed on a base portion 61 including a semiconductor substrate 10. The element width in the base portion 61 is set to w = 500 µm, and the element length is set to l = 500 µm. In addition, the ridge width of the ridge portion 62 is set to w r = 50 μm, the ridge length is l = 500 μm, and the ridge height is h r =1.5 to 1.8 μm.

[0096] The light source used to supply the light to be detected was a benchtop, fiber-output, tunable laser light source (TLX1, manufactured by Thorlabs). The wavelength of the light to be detected was 1528 to 1566 nm, and the output was 8 mW. As shown by arrow B1 in Figure 7, the light to be detected was incident on the side of the photodetector element 60.

[0097] In addition, as shown by arrow B2 in Figure 7, when the light to be detected is incident on the surface of the photodetector 60, an opening can be formed in the metal electrode 63 formed on the surface of the element, and the light to be detected can be incident through this opening, as shown in Figure 8(b).

[0098] Fig. 9 is a graph showing a photodetection spectrum acquired by a photodetector element 60 using the interband photodetector 1A shown in Fig. 7 and Fig. 8. In the graph of Fig. 9, the horizontal axis represents the wavelength of light (nm), and the vertical axis represents the photocurrent (μA) output from the photodetector 1A. As shown in this graph, it can be confirmed that the target light is detected by using the interband photodetector 1A configured as described above.

[0099] The quantum well structure and the level structure in the superlattice layer 30 of the interband photodetector 1A will be further described.

[0100] Fig. 10 is a diagram showing a first modified example of the quantum well structure and level structure in the superlattice layer 30 of the interband photodetector 1A. In the configuration shown in Fig. 10, the configuration of the unit laminate body 31 in the superlattice layer 30 is the same as the configuration shown in Fig. 2, but in this modified example, the superlattice layer 30 is configured to include a plurality of unit laminate bodies 31. Fig. 10 shows, among the plurality of unit laminate bodies 31, a unit laminate body 31 having an absorption region 32 and a relaxation region 33, and a unit laminate body 31a having an absorption region 32a adjacent to the relaxation region 33 and a relaxation region 33a.

[0101] In this configuration, the superlattice layer 30 in the photodetector 1A detects the light to be detected that is incident on the photodetector 1A by interband absorption from the lower detection level L0 to the upper detection level L1 in the absorption region 32. Furthermore, the electrons excited by this interband absorption are absorbed by the relaxation levels L2 to L3 in the relaxation region 33. n The relaxation occurs via the relaxation level structure due to

[0102] Relaxation level L of the nth well layer n The electrons that have relaxed to the relaxation level L n to the valence band level L n0 The electrons then move to the detection lower level L0 in the absorption region 32a of the adjacent unit laminate structure 31a by the resonant tunneling effect. Then, in the unit laminate structure 31a, light detection operation is performed by light absorption and relaxation of the electrons, just like in the unit laminate structure 31.

[0103] In this way, by configuring the photodetector 1A as an interband cascade detector in which multiple unit laminate structures 31 are stacked in multiple stages in the superlattice layer 30, the light detection efficiency of the photodetector 1A can be improved.

[0104] 11 is a diagram showing a second modified example of the quantum well structure and level structure in the superlattice layer 30 of the interband photodetector 1A. In the configuration shown in Fig. 11, the absorption region 32 is configured to include two quantum well layers, that is, a first well layer 351 and a second well layer 352. The relaxation region 33 is configured to include m=n-2 quantum well layers, that is, a third well layer 353 to an n-th well layer.

[0105] The absorption region 32 has a level structure including two lower detection levels L 1 and L 2 resulting from the levels of the valence band in the first well layer 351 and the second well layer 352 that are included in the absorption region 32 and function as absorption well layers. 0a , L 0b and two upper detection levels L due to the conduction band level. 1a , L 1b In addition, relaxation region 33 has a level structure including n-2 relaxation levels L3 to L4 resulting from the levels of the conduction bands in third well layer 353 to n-th well layer included in relaxation region 33. n It has the following characteristics.

[0106] In this way, the absorption region 32 in the unit laminate structure 31 of the superlattice layer 30 may be configured to include two or more quantum well layers. In such a configuration, it is possible to improve the efficiency of detecting light due to interband absorption.

[0107] The basic configuration of the interband photodetector and its semiconductor laminate structure will be further described.

[0108] Fig. 12 is a diagram showing the basic configuration of a second embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 13 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 12.

[0109] The interband photodetector 1B according to this configuration example uses a 3500 nm-thick n-type InP substrate 10 as the semiconductor substrate in its semiconductor laminate structure. On this InP substrate 10, laminated in order from the substrate 10 side are an n-type InP low refractive index layer 13 having a thickness of 500 nm, an n-type InGaAs contact layer 12 having a thickness of 50 nm, a superlattice layer 30 including a unit laminate body 31, an InAlAs carrier block layer 21 having a thickness of 30 nm, and an n-type InGaAs contact layer 22 having a thickness of 50 nm.

[0110] In this configuration example, in unit laminate structure 31 of superlattice layer 30, absorption region 32 is located on the opposite side to semiconductor substrate 10, and relaxation region 33 is located on the semiconductor substrate 10 side. In this configuration, carrier block layer 21 provided in contact with absorption region 32 has the function of suppressing the migration of electrons excited by interband absorption in absorption region 32 to the region opposite relaxation region 33.

[0111] Fig. 14 is a diagram showing the basic configuration of a third embodiment of an interband photodetector in terms of its semiconductor laminate structure, and Fig. 15 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 14.

[0112] The interband photodetector 1C according to this configuration example uses a 3500 nm-thick n-type InP substrate 10 as the semiconductor substrate in its semiconductor laminate structure. On this InP substrate 10, laminated in order from the substrate 10 side are an n-type InP low refractive index layer 13 having a thickness of 500 nm, an n-type InGaAs contact layer 12 having a thickness of 50 nm, a superlattice layer 30 including a unit laminate body 31, and an n-type InGaAs contact layer 22 having a thickness of 50 nm.

[0113] In this configuration example, no carrier block layer is provided in the superlattice layer 30. Even with this configuration, the photodetector 1C can achieve light detection operation. Furthermore, with this configuration, the semiconductor laminate structure is simplified, making it easier to form the photodetector 1C by crystal growth. Note that in this configuration example, in the unit laminate structure 31 of the superlattice layer 30, the absorption region 32 may be located on the semiconductor substrate 10 side or on the opposite side from the semiconductor substrate 10.

[0114] Fig. 16 is a diagram showing the basic configuration of a fourth embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 17 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 16.

[0115] The interband photodetector 1D according to this configuration example uses a 3500 nm-thick p-type InP substrate 15 as the semiconductor substrate in its semiconductor laminate structure. Then, on this InP substrate 15, laminated in order from the substrate 15 side are a 500 nm-thick p-type InP low refractive index layer 17, a 50 nm-thick p-type InGaAs contact layer 16, a superlattice layer 30 including unit laminate structures 31, and a 50 nm-thick n-type InGaAs contact layer 22.

[0116] In this configuration example, the absorption region 32 is located on the semiconductor substrate 15 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, a depletion layer is formed at the interface between the superlattice layer 30 and the p-type contact layer 16, which is a p-type semiconductor layer, and this depletion layer, like the carrier block layer, has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to the region opposite to the relaxation region 33.

[0117] 16 and 17, the absorption region 32 may be located on the opposite side to the semiconductor substrate in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable that the semiconductor substrate, the low refractive index layer, and the contact layer provided below the superlattice layer 30 are each an n-type semiconductor layer, and that the contact layer provided above the superlattice layer 30 is a p-type semiconductor layer.

[0118] Fig. 18 is a diagram showing the basic configuration of a fifth embodiment of an interband photodetector in terms of its semiconductor laminate structure, and Fig. 19 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 18.

[0119] The interband photodetector 1E according to this configuration example uses a 3500 nm-thick n-type InP substrate 10 as the semiconductor substrate in its semiconductor laminate structure. On this InP substrate 10, laminated in order from the substrate 10 side are an n-type InP low refractive index layer 13 having a thickness of 500 nm, an InAlAs carrier block layer 11 having a thickness of 30 nm, a superlattice layer 30 including a unit laminate structure 31, an n-type InP low refractive index layer 23 having a thickness of 1000 nm, and an n-type InGaAs contact layer 22 having a thickness of 50 nm.

[0120] In this configuration example, the absorption region 32 is located on the semiconductor substrate 10 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the carrier block layer 11 has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to a region opposite the relaxation region 33.

[0121] Furthermore, in this configuration example, n-type low refractive index layers 13 and 23 are provided on the semiconductor substrate 10 side and the opposite side of the semiconductor substrate 10 with respect to the superlattice layer 30, respectively. By sandwiching the superlattice layer 30 between the low refractive index layers 13 and 23 in this manner, the light to be detected can be reliably confined within the superlattice layer 30, and the light detection efficiency of the photodetector 1E can be further improved.

[0122] 18 and 19, the absorption region 32 may be located on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable to provide a carrier block layer between the superlattice layer 30 and the n-type low refractive index layer 23. Alternatively, a configuration without a carrier block layer may be used. In this case, the absorption region 32 may be located on the semiconductor substrate 10 side or on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30.

[0123] Fig. 20 is a diagram showing the basic configuration of a sixth embodiment of an interband photodetector in terms of its semiconductor laminate structure, and Fig. 21 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 20.

[0124] The interband photodetector 1F according to this configuration example uses a 3500 nm-thick p-type InP substrate 15 as the semiconductor substrate in its semiconductor laminate structure. Then, on this InP substrate 15, laminated in order from the substrate 15 side are a 500 nm-thick p-type InP low refractive index layer 17, a superlattice layer 30 including a unit laminate body 31, a 1000 nm-thick n-type InP low refractive index layer 23, and a 50 nm-thick n-type InGaAs contact layer 22.

[0125] In this configuration example, the absorption region 32 is located on the semiconductor substrate 15 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the depletion layer formed at the interface between the superlattice layer 30 and the p-type low refractive index layer 17, which is a p-type semiconductor layer, has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to the region opposite to the relaxation region 33.

[0126] Furthermore, in this configuration example, a p-type low refractive index layer 17 and an n-type low refractive index layer 23 are provided on the semiconductor substrate 15 side and the opposite side of the superlattice layer 30 from the semiconductor substrate 15, respectively. By sandwiching the superlattice layer 30 between the low refractive index layers 17 and 23 in this manner, the light to be detected can be reliably confined within the superlattice layer 30, further improving the light detection efficiency of the photodetector 1F.

[0127] 20 and 21, the absorption region 32 may be located on the opposite side to the semiconductor substrate in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable that the semiconductor substrate and the low refractive index layer provided below the superlattice layer 30 are n-type semiconductor layers, and that the low refractive index layer and the contact layer provided above the superlattice layer 30 are p-type semiconductor layers.

[0128] Fig. 22 is a diagram showing the basic configuration of a seventh embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 23 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 22.

[0129] The interband photodetector 2A according to this configuration example uses a 3500 nm-thick n-type InP substrate 10 as the semiconductor substrate in its semiconductor laminate structure. On this InP substrate 10, laminated in order from the substrate 10 side are an n-type InP low refractive index layer 13 with a thickness of 500 nm, an InAlAs carrier block layer 11 with a thickness of 30 nm, a superlattice layer 30 including unit laminate structures 31, and an n-type InP low refractive index layer 23 with a thickness of 1000 nm.

[0130] In this configuration example, the absorption region 32 is located on the semiconductor substrate 10 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the carrier block layer 11 has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to a region opposite the relaxation region 33.

[0131] In this configuration example, n-type low refractive index layers 13 and 23 are provided on the semiconductor substrate 10 side and the opposite side of the superlattice layer 30 from the semiconductor substrate 10, respectively. By sandwiching the superlattice layer 30 between the low refractive index layers 13 and 23 in this manner, the light to be detected can be reliably confined within the superlattice layer 30, further improving the light detection efficiency of the photodetector 2A.

[0132] In this configuration example, an n-type contact layer is not formed above the n-type low refractive index layer 23, and the n-type low refractive index layer 23 is used as the n-type contact layer. In this configuration, it is possible to suppress the occurrence of absorption of the light to be detected by the contact layer.

[0133] 22 and 23, the absorption region 32 may be located on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable to provide a carrier block layer between the superlattice layer 30 and the n-type low refractive index layer 23. Alternatively, a configuration without a carrier block layer may be used. In this case, the absorption region 32 may be located on the semiconductor substrate 10 side or on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30.

[0134] Fig. 24 is a diagram showing the basic configuration of an eighth embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 25 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 24.

[0135] In the interband photodetector 2B according to this configuration example, a 3500 nm-thick p-type InP substrate 15 is used as the semiconductor substrate in its semiconductor laminate structure. Then, on this InP substrate 15, a 500 nm-thick p-type InP low refractive index layer 17, a superlattice layer 30 including unit laminate structures 31, and a 1000 nm-thick n-type InP low refractive index layer 23 are laminated in this order from the substrate 15 side.

[0136] In this configuration example, the absorption region 32 is located on the semiconductor substrate 15 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the depletion layer formed at the interface between the superlattice layer 30 and the p-type low refractive index layer 17, which is a p-type semiconductor layer, has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to the region opposite to the relaxation region 33.

[0137] In this configuration example, a p-type low refractive index layer 17 and an n-type low refractive index layer 23 are provided on the semiconductor substrate 15 side and the opposite side of the superlattice layer 30 from the semiconductor substrate 15, respectively. By sandwiching the superlattice layer 30 between the low refractive index layers 17 and 23 in this manner, the light to be detected can be reliably confined within the superlattice layer 30, further improving the light detection efficiency of the photodetector 2B.

[0138] In this configuration example, an n-type contact layer is not formed above the n-type low refractive index layer 23, and the n-type low refractive index layer 23 is used as the n-type contact layer. In this configuration, it is possible to suppress the occurrence of absorption of the light to be detected by the contact layer.

[0139] 24 and 25, the absorption region 32 may be located on the opposite side to the semiconductor substrate in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable that the semiconductor substrate and the low refractive index layer provided below the superlattice layer 30 are n-type semiconductor layers, and that the low refractive index layer provided above the superlattice layer 30 is a p-type semiconductor layer.

[0140] Fig. 26 is a diagram showing the basic configuration of a ninth embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 27 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 26.

[0141] The interband photodetector 2C according to this configuration example uses a 3500 nm-thick n-type InP substrate 10 as the semiconductor substrate in its semiconductor laminate structure. On this InP substrate 10, laminated in order from the substrate 10 side are an n-type InP low refractive index layer 13 having a thickness of 500 nm, an InAlAs carrier block layer 11 having a thickness of 30 nm, a superlattice layer 30 including a unit laminate body 31, and an n-type InGaAs contact layer 22 having a thickness of 50 nm.

[0142] In this configuration example, the absorption region 32 is located on the semiconductor substrate 10 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the carrier block layer 11 has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to a region opposite the relaxation region 33.

[0143] In this configuration example, an n-type contact layer is not formed between the n-type low refractive index layer 13 and the carrier block layer 11, and the n-type low refractive index layer 13 is used as the n-type contact layer. In this configuration, it is possible to suppress the absorption of the light to be detected by the contact layer.

[0144] 26 and 27, the absorption region 32 may be located on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable to provide a carrier block layer between the superlattice layer 30 and the n-type contact layer 22. Alternatively, a configuration without a carrier block layer may be used. In this case, the absorption region 32 may be located on the semiconductor substrate 10 side or on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30.

[0145] Fig. 28 is a diagram showing the basic configuration of a tenth embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 29 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 28.

[0146] The interband photodetector 2D according to this configuration example uses a 3500 nm-thick p-type InP substrate 15 as the semiconductor substrate in its semiconductor laminate structure. Then, on this InP substrate 15, a 500 nm-thick p-type InP low refractive index layer 17, a superlattice layer 30 including unit laminate structures 31, and a 50 nm-thick n-type InGaAs contact layer 22 are laminated in this order from the substrate 15 side.

[0147] In this configuration example, the absorption region 32 is located on the semiconductor substrate 15 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the depletion layer formed at the interface between the superlattice layer 30 and the p-type low refractive index layer 17, which is a p-type semiconductor layer, has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to the region opposite to the relaxation region 33.

[0148] 28 and 29, the absorption region 32 may be located on the opposite side to the semiconductor substrate in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable that the semiconductor substrate and the low refractive index layer provided below the superlattice layer 30 are n-type semiconductor layers, and that the contact layer provided above the superlattice layer 30 is a p-type semiconductor layer.

[0149] Fig. 30 is a diagram showing the basic configuration of an eleventh embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 31 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 30.

[0150] The interband photodetector 2E according to this configuration example uses a 3500 nm-thick n-type InP substrate 10 as the semiconductor substrate in its semiconductor laminate structure. On this InP substrate 10, laminated in order from the substrate 10 side are an n-type InGaAs contact layer 12 with a thickness of 50 nm, an InAlAs carrier block layer 11 with a thickness of 30 nm, a superlattice layer 30 including unit laminate structures 31, and an n-type InP low refractive index layer 23 with a thickness of 500 nm.

[0151] In this configuration example, the absorption region 32 is located on the semiconductor substrate 10 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the carrier block layer 11 has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to a region opposite the relaxation region 33.

[0152] 30 and 31 , the absorption region 32 may be located on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable to provide a carrier block layer between the superlattice layer 30 and the n-type low refractive index layer 23. Alternatively, a configuration without a carrier block layer may be used. In this case, the absorption region 32 may be located on the semiconductor substrate 10 side or on the side opposite to the semiconductor substrate 10 in the unit laminate structure 31 of the superlattice layer 30.

[0153] Fig. 32 is a diagram showing the basic configuration of a twelfth embodiment of the interband photodetector in terms of its semiconductor laminate structure, and Fig. 33 is a diagram showing a specific example of the semiconductor laminate structure in the interband photodetector shown in Fig. 32.

[0154] In the interband photodetector 2F according to this configuration example, a 3500 nm-thick p-type InP substrate 15 is used as the semiconductor substrate in its semiconductor laminate structure. Then, on this InP substrate 15, a 50 nm-thick p-type InGaAs contact layer 16, a superlattice layer 30 including unit laminate structures 31, and a 500 nm-thick n-type InP low refractive index layer 23 are laminated in this order from the substrate 15 side.

[0155] In this configuration example, the absorption region 32 is located on the semiconductor substrate 15 side in the unit laminate structure 31 of the superlattice layer 30. In this configuration, the depletion layer formed at the interface between the superlattice layer 30 and the p-type contact layer 16, which is a p-type semiconductor layer, has the function of suppressing the migration of electrons excited by interband absorption in the absorption region 32 to the region opposite to the relaxation region 33.

[0156] 32 and 33, the absorption region 32 may be located on the opposite side to the semiconductor substrate in the unit laminate structure 31 of the superlattice layer 30. In this case, it is preferable that the semiconductor substrate and the contact layer provided below the superlattice layer 30 are n-type semiconductor layers, and that the low refractive index layer provided above the superlattice layer 30 is a p-type semiconductor layer.

[0157] The interband photodetector is not limited to the above-described embodiment and configuration example, and various modifications are possible. For example, in the above-described configuration example, an InP substrate is used as the semiconductor substrate, and the superlattice layer is made of InGaAs / InAlAs. However, various configurations may be used as long as they are capable of absorbing and detecting light by interband transition in a quantum well structure and can realize the above-described level structure.

[0158] Furthermore, various structures other than the above may be used for the stacked structure in the superlattice layer of the interband photodetector and the semiconductor stacked structure of the entire photodetector element. In general, the interband photodetector may be configured to include a semiconductor substrate and a superlattice layer having the above-described structure provided on the semiconductor substrate. [Industrial Applicability]

[0159] The present invention can be used as an interband photodetector that can be suitably applied to the detection of light at a desired detection wavelength in a wide wavelength range. [Explanation of symbols]

[0160] 1A to 1F, 2A to 2F... Interband photodetector, 10... Semiconductor substrate, 11... Carrier blocking layer, 12... n-type contact layer, 13... n-type low refractive index layer, 15... Semiconductor substrate, 16... p-type contact layer, 17... p-type low refractive index layer, 21... Carrier blocking layer, 22... n-type contact layer, 23... n-type low refractive index layer, 30...superlattice layer, 31...unit stack, 32...absorption region, 33...relaxation region, 341-348...quantum barrier layers, 351-357...quantum well layers, L0...lower detection level, L1...upper detection level, L2-L7...relaxation level, A0...upper edge of valence band, A1...lower edge of conduction band, 60...photodetector element, 61...base portion, 62...ridge portion, 63...metal electrode.

Claims

1. a semiconductor substrate; a superlattice layer provided on the semiconductor substrate, the superlattice layer including a unit laminate having a Type I quantum well structure consisting of n quantum barrier layers (n is an integer of 3 or more) and n quantum well layers; Equipped with the unit laminate structure has an absorption region including at least one quantum well layer and a relaxation region including m quantum well layers (m is an integer of 2 or more and n-1 or less), the absorption region has, in its level structure, a detection lower level resulting from a valence band level in the quantum well layer included in the absorption region and functioning as an absorption well layer, and a detection upper level resulting from a conduction band level; the relaxation region has a level structure including m relaxation levels resulting from conduction band levels in the m quantum well layers included in the relaxation region; light to be detected is detected by interband absorption from the detection lower level to the detection upper level in the absorption region, and electrons excited by the interband absorption are extracted via a relaxation level structure formed by the m relaxation levels in the relaxation region. Interband photodetector.

2. 2. The interband photodetector according to claim 1, wherein the band gap energy of each of the m quantum well layers included in the relaxation region is set to be larger than the band gap energy of the quantum well layer included in the absorption region.

3. 2. The interband photodetector according to claim 1, wherein an energy difference between a valence band level and the relaxation level in each of the m quantum well layers included in the relaxation region is set to be larger than an energy difference between the detection lower level and the detection upper level in the quantum well layer included in the absorption region.

4. 4. The interband photodetector according to claim 3, wherein an energy difference between a valence band level and the relaxation level in each of the m quantum well layers included in the relaxation region is set to be larger than a detection energy of the light to be detected.

5. 5. The interband photodetector according to claim 1, wherein in the quantum well layer included in the absorption region, the upper detection level is a level resulting from a ground level in a subband level structure of the conduction band.

6. 5. The interband photodetector according to claim 1, wherein in each of the m quantum well layers included in the relaxation region, the relaxation level is a level resulting from a ground level in a subband level structure of the conduction band.

7. 5. The interband photodetector according to claim 1, wherein in said unit laminate structure, each of said n quantum barrier layers and said n quantum well layers is made of an i-type semiconductor layer.

8. 5. The interband photodetector according to claim 1, wherein the superlattice layer includes, as the unit laminate body, a plurality of unit laminate bodies, each of which has the absorption region and the relaxation region.

9. 5. The inter-band photodetector according to claim 1, wherein in the unit laminate structure, the absorption region includes a single quantum well layer.

10. 5. The inter-band photodetector according to claim 1, wherein in the unit laminate structure, the absorption region includes a plurality of quantum well layers.

11. An interband photodetector according to any one of claims 1 to 4, wherein a carrier blocking layer is provided in a region on the semiconductor substrate side of the superlattice layer and in a region on the opposite side of the superlattice layer from the semiconductor substrate, the region being in contact with the absorption region.

12. 5. The interband photodetector according to claim 1, wherein a p-type semiconductor layer is provided in a region on the semiconductor substrate side of the superlattice layer and in a region on the opposite side of the superlattice layer from the semiconductor substrate, the region being in contact with the absorption region.

13. 5. The interband photodetector according to claim 1, further comprising a low refractive index layer provided in a region of said superlattice layer on said semiconductor substrate side.

14. 5. The interband photodetector according to claim 1, further comprising a low refractive index layer provided in a region of said superlattice layer opposite to said semiconductor substrate.

15. 5. The interband photodetector according to claim 1, wherein in the absorption region, an energy difference between the detection lower level and the detection upper level is set to be larger than the energy of longitudinal optical phonons.

16. 5. The interband photodetector according to claim 1, wherein in the relaxation region, an energy difference between adjacent relaxation levels among the m relaxation levels is set to be larger than the energy of longitudinal optical phonons.

17. 5. The interband photodetector according to claim 1, wherein in the unit laminate body, the Type I quantum well structure is a structure in which the upper end of the valence band in the quantum well layer is higher than the upper end of the valence band in the adjacent quantum barrier layer.

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