Photodetector and usage thereof

Abstract

To provide a quantum well type photodetector capable of obtaining a large output photoelectric conversion current.SOLUTION: In a photodetector, first and second main surfaces have a multi-quantum well structure including one or more quantum well layers sandwiched between barrier layers, the quantum well layer is doped, when a bias voltage is applied in a direction perpendicular to the quantum well layer, the increment in photocurrent caused by light irradiation is detected using the intersubband transition that occurs in the conduction band of the quantum well layer, the photodetector is arranged on the outermost surface of the output extraction electrode barrier layer with a contact layer interposed therebetween, and a current is supplied to the multiple quantum well structure in the photodetection region through the plane of the quantum well layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a photodetector and a method for using the same. [Background technology]

[0002] Quantum well photodetectors have attracted attention as high-sensitivity, high-power photodetectors, and their disclosures can be found, for example, in Patent Document 1, Non-Patent Documents 1 and 2.

[0003] Patent Document 1 discloses a quantum well photodetector in which current is supplied from the electrode region to the photodetection region through metal wiring that interconnects metal patches, in combination with a patch antenna. Non-patent document 1 discloses a quantum well photodetector in which current is supplied from the electrode region to the photodetection region through an in-plane conductive layer (contact layer) outside the barrier layer. In this case, there is no need to use metal patches as conduction routes. Therefore, metal patches can be freely arranged without considering their interconnections, and this has the advantage of enabling highly flexible designs such as broadband detectors. Non-patent document 2 further discloses that increasing the total number of quantum wells from one to three reduces dark current and achieves high detection capability (S / N ratio).

[0004] Non-patent documents 1 and 2 both describe a method in which current is supplied from the electrode region to the photodetection region not through in-plane conduction via the quantum well layer, but through an in-plane conducting layer (contact layer) outside the barrier layer. Therefore, the current multiplication phenomenon due to in-plane conduction via the quantum well does not occur.

[0005] According to Non-Patent Documents 1 and 2, the sensitivity peaks of the detectors described in these documents occur at an average electric field of 37 kV / cm (bias voltage 0.70 V, multiple quantum well voltage imprinted thickness 192 nm) to 43 kV / cm (bias voltage 0.45 V, multiple quantum well voltage imprinted thickness 104 nm), and the resulting sensitivity, expressed as the external quantum efficiency, was at most about 62%. Even so, compared with conventional quantum well photodetectors, a greater sensitivity has been achieved by introducing a plasmon resonator, but it is not necessarily sufficient.

[0006] In other types of semiconductor detectors, detectors with an external quantum efficiency exceeding 100% have been put into practical use by utilizing the avalanche multiplication effect that occurs when a high electric field is applied. And when compared with the situation where it is applied to single photon measurement and the like, the sensitivity of the quantum well photodetector is not necessarily satisfactory, and further improvement in sensitivity has been desired.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Non-Patent Documents

[0008]

Non-Patent Document 1

Non-Patent Document 2

[0009] The object of the present invention is to solve the sensitivity problem of the quantum well type photodetector described above and to provide a quantum well type photodetector that can obtain a high-output photoelectric conversion current and a method for using the same. [Means for solving the problem]

[0010] The configuration of the present invention for solving the problem is shown below. (Composition 1) The multi-quantum well structure includes one or more quantum well layers in which the first and second main surfaces are sandwiched between barrier layers, The quantum well layer is doped, When a bias voltage is applied perpendicular to the quantum well layer, the increment in the photocurrent generated by light irradiation is detected using the inter-subband transitions occurring in the conduction band of the quantum well layer. An electrode for output extraction is arranged on the outermost surface of the barrier layer via a contact layer. A photodetector in which current is supplied to the multiple quantum well structure in the photodetection region through the in-plane of the quantum well layer. (Configuration 2) Island-shaped metal patches are formed adjacent to or above the multiple quantum well structure. The region including the aforementioned metal patch is defined as the photodetection region. The photodetector according to configuration 1, wherein the current supply to the region directly beneath the metal patch in the photodetection region is performed through the in-plane of the quantum well layer. (Composition 3) A quantum functional layer and an extraction electrode are sequentially stacked on a conductive layer. The quantum functional layer comprises one or more quantum well layers and barrier layers sandwiching the first and second main surfaces of the quantum well layers. A contact layer is formed between the electrode and the nearest barrier layer among the barrier layers. A photodetector in which current is supplied to the quantum functional layer in the photodetection region through the in-plane of the quantum well layer. (Composition 4) Island-shaped metal patches are formed adjacent to or above the quantum functional layer. The region including the aforementioned metal patch is defined as the photodetection region. The photodetector according to configuration 3, wherein the current supply to the region directly beneath the metal patch in the photodetection region is performed through the in-plane of the quantum well layer. (Composition 5) The photodetector according to any one of the configurations 1 to 4, wherein the quantum well layer to which the current is supplied is the quantum well layer located in the outermost layer of the quantum well layers. (Composition 6) The photodetector according to any one of the configurations 1 to 5, wherein the quantum well layer is a mixed crystal consisting of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. (Composition 7) The photodetector according to any one of the configurations 1 to 6, wherein the barrier layer is a mixed crystal consisting of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. (Composition 8) The photodetector according to any one of the configurations 1 to 7, wherein the number of quantum well layers is 1 or more and 200 or less. (Composition 9) The photodetector according to configuration 2 or 4, wherein the metal patches are arranged in a matrix. (Composition 10) The photodetector according to configuration 2 or 4, wherein the metal patches are arranged in an array. (Composition 11) The photodetector according to configuration 9 or 10, wherein the arrangement pitch of the metal patches is 0.5 μm or more and 200 μm or less. (Composition 12) The photodetector according to any one of the configurations 1 to 11, wherein the contact layer is a mixed crystal consisting of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. (Composition 13) A bias value V that gives a minimum value to the output current when a bias is applied to the output electrode for the photodetector described in any one of configurations 1 to 12. min Determine the bias value V min A method for using a photodetector, wherein a bias with a larger absolute value is applied to the extraction electrode. (Composition 14) A method for using the photodetector in configuration 13, wherein the bias sign is negative. [Effects of the Invention]

[0011] According to the present invention, a quantum well type photodetector that can obtain a highly sensitive and high-power photoelectric conversion current and a method for using the same are provided. [Brief explanation of the drawing]

[0012] [Figure 1] This is a structural diagram showing the cross-sectional structure of the main part of the quantum well photodetector of the present invention. [Figure 2]These are structural diagrams of conventional quantum well photodetectors, where the entire surface is covered with a thick contact layer. (a) shows an example of Brewster angle incidence, (b) a 45° back-facing incidence, (c) a back-facing irradiation using a diffraction grating, (d) an end-face incidence, and (e) an example using a plasmon resonator. In each figure, the upper panel is a plan view and the lower panel is a cross-sectional view. [Figure 3] This diagram illustrates the electron flow in a conventional quantum well photodetector, where the entire surface is covered with a thick contact layer, using a cross-sectional structure diagram. (a) shows a detector without a plasmon resonator, and (b) shows a detector with a plasmon resonator. [Figure 4] The image shows a cross-sectional structure of a quantum well photodetector without a contact layer in the photodetection region. (a) shows an example of Brewster angle incidence, (b) 45° back-side incidence, (c) back-side illumination using a diffraction grating, (d) end-face incidence, and (e) using a plasmon resonator. [Figure 5] These are cross-sectional diagrams of quantum well photodetectors in which the contact layer surface of the photodetector region has been removed to a depth that completely depletes it. (a) shows an example of Brewster angle incidence, (b) 45° back-side incidence, (c) back-side illumination using a diffraction grating, (d) end-face incidence, and (e) using a plasmon resonator. [Figure 6] This is a cross-sectional diagram of a quantum well photodetector in which there is no contact layer on the front surface where light is incident, and electrodes are directly connected to the quantum well layer in the electrode region. Examples are shown for Brewster angle incidence, (b) 45° back surface incidence, (c) back surface illumination using a diffraction grating, (d) end surface incidence, and (e) using a plasmon resonator. [Figure 7] This is a cross-sectional diagram of a quantum well photodetector, where (a) is a conventional comparative example in which the front surface is covered with a thick contact layer, and (b) is an example of the present invention in which the surface of the contact layer in the photodetection region is removed to a depth that completely depletes it. [Figure 8] This is a scanning electron microscope (SEM) image of a quantum well photodetector. [Figure 9]This diagram illustrates the electronic structural characteristics of the quantum well photodetector of the present invention, shown by the band structure of the conduction band and the wave function of the ground state of the quantum well. Here, (a) represents the conventional structure, and (b) represents the present invention. [Figure 10] This characteristic diagram shows the bias voltage dependence of the dark current of a quantum well photodetector, with (a) representing the conventional structure and (b) representing the present invention. [Figure 11] The characteristic diagrams show the sensitivity spectra of the quantum well photodetector, with (a1) and (a2) representing the conventional structure and (b1) and (b2) representing the present invention. [Figure 12] This characteristic diagram shows the bias voltage dependence of the peak sensitivity of a quantum well photodetector, with (a) representing the case of a conventional structure and (b) representing the case of the present invention. [Modes for carrying out the invention]

[0013] (Embodiment 1) The embodiments for carrying out the present invention will be described below with reference to the drawings. Note that A to B in the text refers to A and B, meaning A or greater and B or less.

[0014] <Detector structure> As one embodiment of the present invention, the quantum well photodetector 101 has a multiple quantum well structure 41 including one or more quantum well layers 14 in which the first and second main surfaces are sandwiched between barrier layers 13, as shown in Figure 1(a), the quantum well layers 14 are doped, and an electrode 15 for output extraction is arranged on the outermost surface of the barrier layer 13 via a contact layer 12. When a bias voltage is applied in a direction perpendicular to the quantum well layer 14, the increment of photocurrent generated by light irradiation is detected using the subband transitions occurring in the conduction band of the quantum well layer 14, and the current supply to the multiple quantum well structure 41 of the photodetection region 22 is performed through the plane of the quantum well layer 14. Here, the bias to the quantum well photodetector 101 is applied through the electrode 15 and the ground electrode 16. The multiple quantum well structure 41 is electrically connected to the ground electrode 16 via the lower contact layer 12d and the conductive semiconductor substrate 11a. Alternatively, a substrate with a conductive layer, such as metal, may be used instead of the conductive semiconductor substrate 11a.

[0015] As shown in Figure 1(b), one embodiment of the present invention, the quantum well photodetector 106 has a quantum functional layer 52 and an extraction electrode 15 stacked sequentially on a conductive layer 51. The quantum functional layer 52 has one or more quantum well layers 14 and barrier layers 13 sandwiching the first and second main surfaces of the quantum well layer 14. A contact layer 12 is formed between the electrode 15 and the nearest barrier layer 13a (located in the uppermost layer) of the barrier layers 13. The current supply from the photodetection region 22 to the quantum functional layer 52 is performed through the plane of the quantum well layer 14. In this case, it is preferable to bring the quantum functional layer 52 into ohmic contact with the conductive layer 51 via the lower contact layer 12d.

[0016] Furthermore, as shown in Figures 1(c) and 1(d), one embodiment of the present invention is a plasmon resonator type quantum well photodetector 105, 107 in which island-shaped metal patches 18 are formed on the quantum functional layer 52 of the photodetection region 22 or on the multiple quantum well structure 41. By using a plasmon resonator in combination, it is possible to further improve sensitivity and output.

[0017] A quantum well photodetector 105 using a plasmon resonator according to one embodiment of the present invention is characterized in that a quantum functional layer 52 and an extraction electrode 15 are sequentially stacked on a metal layer 17, the quantum functional layer 52 has one or more quantum well layers 14 and barrier layers 13 sandwiching the first and second main surfaces of the quantum well layer 14, a contact layer 12 is formed between the electrode 15 and the nearest barrier layer 13, and island-shaped metal patches 18 are formed on the quantum functional layer 52 of the photodetection region 22, and current is supplied to the quantum functional layer 52 directly beneath the metal patch 18 of the photodetection region 22 through the plane of the quantum well layer 14.

[0018] A metal layer 17 to which a ground electrode 16 is electrically connected is placed below the quantum functional layer 52, so that the quantum functional layer 52 is sandwiched between the metal patch 18 and the metal layer 17. Here, the metal layer 17 may be a metal substrate that constitutes the substrate itself, or it may be a metal layer 17 formed on a substrate 11b that has sufficient rigidity. The material for the metal layer 17 is not particularly limited as long as it is a conductor with a negative real dielectric constant, but it is preferable to use a material with a small imaginary dielectric constant. Gold (Au), silver (Ag), copper (Cu), aluminum (Al), platinum (Pt), tungsten (W), and molybdenum (Mo) can be used. Furthermore, transparent conductors such as ITO, AZO, and GZO can also be used, not necessarily metals. Since ohmic contact between the quantum functional layer 52 and the metal layer 17 is desirable for increasing the sensitivity and power output of the photodetector, it is preferable to place a lower contact layer 12d between the quantum functional layer 52 and the metal layer 17. Furthermore, it is preferable to set the thickness of the quantum functional layer 52 to a thickness that causes plasmon resonance. As a specific guideline, the thickness between the metal patch 18 and the metal layer 17, that is, the combined thickness of the quantum functional layer 52 and the lower contact layer 12d, is preferably greater than 0 and less than λ / (2n) when n is the average refractive index of the quantum functional layer 52 and the lower contact layer 12d with respect to the wavelength λ of the incident light. More specifically, it is preferable to set the thickness to one that sufficiently causes plasmon resonance for incident light of the desired wavelength based on Maxwell's equations.

[0019] Another embodiment of the present invention, a quantum well photodetector 107 using a plasmon resonator, has a multiple quantum well structure 41 including one or more quantum well layers 14 whose first and second main surfaces are sandwiched between barrier layers 13, wherein the quantum well layers 14 are doped, an electrode 15 for output extraction is placed on the outermost surface of the barrier layer 13 via a contact layer 12, and an island-shaped metal patch 18 is formed on the multiple quantum well structure 41 of the photodetection region 22. When a bias voltage is applied in a direction perpendicular to the quantum well layer 14, the increment of the photocurrent generated by light irradiation is detected using the subband transitions occurring in the conduction band of the quantum well layer 14, and current is supplied to the multiple quantum well structure 41 directly below the metal patch 18 of the photodetection region 22 through the plane of the quantum well layer 14. Here, the multiple quantum well structure 41 is placed on a substrate 11d made of a conductor having a negative real part of its dielectric constant via a lower contact layer 12d.

[0020] The material for the metal patch 18 is not particularly limited as long as it is a conductor with a negative real dielectric constant, but it is preferable to use one with a small imaginary dielectric constant. For example, gold (Au), silver (Ag), copper (Cu), aluminum (Al), platinum (Pt), tungsten (W), and molybdenum (Mo) can be used. Furthermore, it is not necessarily limited to metals; transparent conductors such as ITO, AZO, and GZO can also be used. In addition, as the arrangement of the metal patches 18, an array arrangement (linear arrangement, one-dimensional arrangement) or a matrix arrangement (lattice arrangement, two-dimensional arrangement) is preferable in order to maximize the utilization of the incident light for plasmon resonance and improve the sensitivity and output. The pattern pitch P and the pattern width L of the metal patch 18 are preferably set to sizes that can sufficiently cause plasmon resonance with respect to the incident light of the target wavelength based on Maxwell's equations. Examples of the pattern pitch P include 0.5 μm or more and 200 μm or less. For example, in order to detect light with a wavelength of 7.0 μm, it is preferable to set the pattern width L to 0.99 μm and the pitch P to 2.5 μm. The height of the metal patch 18 is preferably thick enough to sufficiently cause plasmon resonance and thin enough not to cause light shielding, and can be, for example, 30 nm or more and 300 nm or less.

[0021] Examples of the quantum well layer 14 include mixed crystals composed of one or two or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. Among these, GaAs, InAs, and InGaAs are particularly preferably used because they have a low bandgap and it is easy to obtain a paired barrier layer. In addition, the quantum well layer 14 is preferably doped at a high concentration so that it can supply current in its plane with sufficient conductivity. Examples of the dopant include Si, S, Sn, Ge, Te, Be, C, and Zn. The doping concentration is 1×10 17 cm -3 or more and 1×10<00,00005>cm -3 or less, preferably 5×10 17 cm -3 or more and 5×10 18 cm -3 or less, more preferably 1×10 18 cm -3 or more and 3×10<, 18 cm -3The following can be listed:

[0022] The quantum well layer to which the current is supplied is preferably the outermost quantum well layer among the quantum well layers 14, that is, the uppermost quantum well layer closest to the output electrode 15. When this current is supplied to the outermost quantum well layer 14, that quantum well layer 14, as well as all the quantum well layers 14 located below it, become capable of current amplification with high efficiency, making it possible to provide a highly sensitive, high-power photodetector.

[0023] Regarding the number of quantum well layers 14, sensitivity improves with increasing the number of layers, but as the number of layers increases, the amount of light reaching the quantum well layers 14 further away from the light irradiation location (for example, the lowest quantum well layer 14 when light is irradiated from directly above) decreases, causing the sensitivity and output improvement effect to saturate. Also, the number of manufacturing processes increases, leading to increased costs. Therefore, a number of quantum well layers 1 to 200 is preferable. For low-cost photodetectors, a number of quantum well layers 14 of 1 to 3 is preferable, while for high-sensitivity, high-output detectors, a number of quantum well layers 14 of 1 to 10 is preferable.

[0024] As the barrier layer 13, examples include mixed crystals consisting of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. Among these, AlGaAs, GaSb, AlSb, and InAlAs are particularly preferred because they have high band gaps and make it easy to obtain paired quantum well layers.

[0025] The contact layers 12 and 12d can be mixed crystals consisting of one or more selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si, and Ge. Among these, GaAs, InAs, and InGaAs are particularly preferred because they have low band gaps and make it easy to obtain quantum well layers and barrier layers to combine with them.

[0026] As will be explained in detail in the section on operating characteristics and effects, the above structure allows for electron multiplication by applying a high bias voltage without the need for a special avalanche layer, making it possible to provide a highly sensitive, high-power photodetector.

[0027] <Characteristics and effects of the operation> To explain the operational characteristics and effects of the detector of the present invention, we will first describe the structure of a conventional quantum well photodetector and its problems.

[0028] <<Conventional quantum well photodetector>> Figure 2 shows quantum well photodetectors 201-205 with a multi-quantum well structure that includes one or more (three layers in the figure) quantum well layers 14 sandwiched between conventional barrier layers 13. In this photoconductive quantum well photodetector structure, the increment of photocurrent generated by light irradiation is detected when a bias voltage is applied perpendicular to the quantum well layer, by utilizing the inter-subband transitions that occur in the conduction band of the quantum well containing free electrons due to doping. Examples of materials for the barrier layer 13 and quantum well layer 14 include AlGaAs and GaAs, respectively.

[0029] Outside the outermost barrier layer 13 are highly conductive layers called contact layers 12 and 12d on both sides. The outermost contact layer 12 has electrodes 15 patterned on it, to which wiring is connected. The area not covered by the electrodes 15 becomes the photodetection region. On the other hand, the lower contact layer 12d is generally directly connected to the conductive semiconductor substrate 11a and is connected to the other electrode, the ground electrode 16, which is placed on the substrate.

[0030] The contact layers 12 and 12d are made of, for example, GaAs, 1 × 10⁻¹⁴ 18 cm -3 Alternatively, it may be doped to an even higher concentration, resulting in high conductivity within the plane due to the high density of free electrons. When light 21 is irradiated onto the outermost surface with a bias voltage applied through the contact layers 12 and 12d, electrons are excited in the quantum well by inter-subband transitions. Due to the electric field of the bias voltage, the photocurrent flows perpendicular to the barrier layer 13 and the quantum well layer 14. Due to the conservation of current, the electrons that flow are supplied from the contact layer 12, and a current of the same magnitude flows perpendicular to the surface through the entire barrier layer 13 and the entire quantum well layer 14. This current flow is shown in Figure 3(a). Figure 3(a) illustrates the electron flow 31 when light 21 is incident on the photodetection region 22 at a Brewster angle in a conventional quantum well photodetector 201. The current supplied from the electrode 15 located in the electrode region 23 spreads uniformly through the contact layer 12 and flows perpendicular to the barrier layer 13 and the quantum well layer 14. Even when light is not irradiating the device, a finite current (dark current) flows due to the bias voltage, and a signal corresponding to the light intensity can be obtained as the current difference between the dark current and the optical current.

[0031] However, since an electric field perpendicular to the quantum well layer 14 is essential for inter-subband transitions, quantum well photodetectors are not sensitive to perpendicularly incident light (which only contains an electric field parallel to the quantum well layer). Special incidence modes such as Brewster angle incidence (Figure 2(a)), incidence from the 45° polished back surface (Figure 2(b)), diffraction by a diffraction grating (Figure 2(c)), and end-face incidence into the waveguide structure (Figure 2(d)) are required. Here, in the case of the diffraction grating type quantum well photodetector 203, light is incident perpendicularly from the substrate (conductive semiconductor substrate) 11a, creating an uneven structure on the uppermost layer of the detector (in the case of Figure 2, the uppermost thick-film diffraction grating-forming contact layer 12c) that diffracts light with a period larger than the wavelength of light in the semiconductor, and sensitivity is created by utilizing the perpendicular electric field component of the reflected diffracted light.

[0032] Figure 2(e) shows a quantum well photodetector 205 using a plasmon resonator, which has recently been studied. In this photodetector 205 structure, the contact layers 12 and 12d are sandwiched further outside by a metal layer 17 and a metal patch 18. Gold (Au) can be used as the material for the metal. The metal layer 17, the multiple quantum well layer 14, and the metal patch antenna 18 form a metal / dielectric / metal plasmon resonator, and when light of a resonant wavelength is incident perpendicularly, a large electric field perpendicular to the quantum well layer 14 is formed, resulting in high sensitivity. Details of this are disclosed in Non-Patent Documents 1 and 2. Furthermore, the materials and manufacturing process are described in detail in Patent Document 1. It is similar to the diffraction grating type quantum well photodetector 203 in Figure 2(c) in that it uses a periodic structure, but this one (plasmon resonator type) differs in that the period is small, about the same as or less than the wavelength of light in a semiconductor, and the metal patch 18 and metal layer 17 are paired together, sandwiching the multiple quantum well structure from above and below. Here, "metal" refers to any conductor whose real part of the dielectric constant has a negative value. Furthermore, it is not necessarily limited to metals; transparent conductors such as ITO, AZO, and GZO are also acceptable. Figure 3(b) shows the current flow in the case of the quantum well photodetector 205. This figure illustrates the electron flow 31 when light 21 is perpendicularly incident on the photodetection region in a conventional quantum well photodetector 205 using a plasmon resonator 24. The current supplied from the electrode 15 spreads uniformly through the contact layer 12 and flows perpendicularly to the barrier layer 13 and the quantum well layer 14.

[0033] <<The Quantum Well Photodetector of the Invention>> Next, we will explain the structural features of the quantum well photodetector of the present invention (Embodiment 1) and the quantum well in-plane conduction current multiplication phenomenon resulting from that structure.

[0034] Figures 4(a) to (d) show typical structural variations of the quantum well photodetectors 101 to 104 of the present invention. (a) is a Brewster angle incidence type, (b) is a 45° back-illumination type, (c) is a diffraction grating type, and (d) is an end-face illumination type. The difference from conventional quantum well photodetectors 201-204 is, as mentioned above, that the photodetector region does not have a contact layer 12. Furthermore, the quantum well photodetectors 101-104 of the present invention are not necessarily limited to the quantum well photodetectors of the present invention, but the quantum well layer 14 is 1 × 10⁻¹⁶ 18 cm -3 The above 3 x 10 18 cm -3 The quantum well layer 14 is doped at a high concentration as described below, resulting in a highly conductive quantum well layer. The electrode region has the same structure as conventional structures, but as shown in the current flow 31 in the figure, by applying a bias voltage, a dark current can flow perpendicularly through the barrier layer 13 and the quantum well layer 14. The quantum well layer 14 connects the photodetection region and the electrode region, and as mentioned above, the quantum well layer 14 has high conductivity, so each quantum well layer 14 is maintained at the same potential from the photodetection region to the electrode region.

[0035] In the structure of the present invention, when light in the wavelength range that excites electrons in the quantum wells by inter-subband transitions is incident on the light irradiation region of the quantum well photodetectors 101-104, the excited electrons conduct perpendicularly across the barrier layer 13 to the next quantum well layer 14. To replenish these photoelectrons, electrons are supplied to the quantum well layer 14 along the plane. The difference from the conventional type is that the quantum well layer 14 replaces the contact layer 12 in the conventional type.

[0036] In the conduction mode in which in-plane conduction occurs in this quantum well, a current multiplication phenomenon takes place. As a result, the quantum well photodetectors 101-104 of the present invention become highly sensitive and high-power detectors.

[0037] Basically, with the same bias voltage as conventional devices, the sensitivity (low field sensitivity) is the same as conventional devices. Further increasing the bias voltage initially reduces the sensitivity. However, when a bias voltage is applied such that the average field exceeds 75 kV / cm, causing the uppermost barrier layer 13 to have negative polarity, the sensitivity increases again, exceeding the low field sensitivity. We believe this current multiplication is due to the avalanche effect. In this case as well, Brewster angle incidence, 45° back-face incidence, diffraction grating, and end-face waveguide incidence are required for inter-subband transitions, just as in the conventional structure shown in Figures 2(a) to (d).

[0038] The same phenomenon occurs in the case of the vertical incidence detector 105 equipped with a plasmon resonator. Figure 4(e) shows this configuration. In this case, the gold patch antenna 18 is placed directly on the upper barrier layer 13. Figures 5 and 6 show different forms in which the same current multiplication phenomenon occurs.

[0039] Figure 5 shows a case where the surface of the contact layer in the photodetector region is engraved, leaving a thin contact layer as either a thin-film contact layer 12a or a thin-film diffraction grating contact layer 12b. When the thickness of the contact layer becomes thinner than a certain value, the surface becomes depleted and loses its conductivity. As a result, the remaining contact layer loses its function of supplying current within the plane, resulting in a situation that is virtually the same as that of the quantum well photodetectors 101-104 shown in Figure 4. The same applies when a plasmon resonator is placed (quantum well photodetector 115 shown in Figure 5(e)).

[0040] Figure 6 shows the case where all contact layers in the upper photodetector region are removed, and furthermore, in the electrode region where electrode 15 is formed, electrode 15 is directly connected to the uppermost quantum well layer 14 without going through the uppermost barrier layer. Due to measures taken to prevent depletion of the exposed quantum well surface, the electrical characteristics are virtually the same as in the case shown in Figure 4. The same is true when a plasmon resonator is placed (quantum well photodetector 125 shown in Figure 6(e)). In the conduction modes of the quantum well photodetectors 101-105, 111-115, and 121-125 shown in Figures 4-6, the current multiplication phenomenon due to in-plane conduction of the quantum well can be utilized when a high electric field is applied. This multiplication phenomenon does not occur in conventional conduction modes that do not depend on in-plane conduction of the quantum well, as shown in Figure 2.

[0041] As a result, although the details will be shown in the examples, it has been demonstrated that when the outermost barrier layer, which does not have an in-plane conductive layer (contact layer) on the outside, is oriented with negative polarity, and a high electric field of 75 kV / cm or more is applied, a photocurrent more than five times higher can be obtained compared to when this method is not used.

[0042] Avalanche detectors, which achieve high-sensitivity operation in high-electric-field regions by incorporating an avalanche multiplication layer in addition to the photodetector layer, have been used to detect small numbers of photons, even at the level of a few photons, in the infrared region. The quantum well photodetectors 101-105, 111-115, and 121-125 of the present invention have the effect of reproducibly achieving high sensitivity similar to that of an avalanche type by using a simple structure that omits a contact layer rather than providing a special avalanche layer. The vertically incident quantum well photodetector 125, combined with a plasmon resonator, is particularly effective. [Examples]

[0043] (Example 1) In Example 1, a detector with the above structure was fabricated and its characteristics were evaluated. Naturally, the present invention is not limited to such a specific form, and the technical scope of the present invention is defined by the claims.

[0044] <Sample Structure> As an example, a quantum well photodetector 116, as shown in Figure 7, and as a comparative example, a quantum well photodetector 205 were fabricated, and the characteristics of both were compared. Here, the structural difference between the example and the comparative example lies solely in the uppermost contact layer. In the example, the uppermost contact layer is a photodetector thinned contact layer 12e in which the portion other than the metal patch 18 in the photodetector area is thinned, whereas in the comparative example, it is a uniform, thick contact layer 12. As a result, the uppermost contact layer 12e in the example, unlike the comparative example, has lost its function of supplying current within its plane.

[0045] The basic structure of the multiple quantum well is the same in both the examples and the comparative examples, Al x Ga 1-x An undoped barrier layer 13 of As, x=0.30, thickness 50 nm, and GaAs, 3×10 18 cm -3 This involves Si doping and repeating a quantum well layer 14 with a thickness of 4 nm for 3 cycles. However, Al x Ga 1-x The As barrier layer 13 was made 40 nm thick, except for the outermost barrier layer.

[0046] In this embodiment, a 48 nm thick GaAs contact layer 12e was placed outside the outermost barrier layer. To achieve tunneling ohmic bonding with the electrode using a thin contact layer, the following Si doping density configuration was adopted: 3 × 10⁻¹⁶ on the side closer to the barrier layer 13. 18 cm -3 The outermost surface has 5 × 10⁶ layers over a thickness of 28 nm. 18 cm -3 It is highly doped, and furthermore, 3 × 10⁻¹⁶ doping is applied every 4 nm. 12 cm -2 Seven delta-doped layers are inserted. The total thickness of the multiple quantum well structure is 288 nm, of which the electric field is applied to a region with a thickness of 192 nm, excluding the contact layers 12 and 12e on both sides.

[0047] <Sample preparation method> The quantum well photodetector 116 in the example was fabricated in the same manner as the quantum well photodetector 205 in the comparative example, except that the uppermost contact layer 12e was etched to reduce the exposed surface of its photodetection region. Here, this multiple quantum well structure was fabricated using a molecular beam epitaxy (MBE) system (COMPACT21T, RIBER).

[0048] First, a first GaAs(100) substrate was prepared, and the oxide film on its surface was removed by heating at 580°C. Then, a GaAs buffer layer for surface planarization was grown to a thickness of 300 nm using an MBE apparatus. Subsequently, an AlGaAs sacrificial layer with an Al composition of 90% was formed to a thickness of 900 nm, and an AlGaAs sacrificial layer with an Al composition of 55% was formed to a thickness of 100 nm. The formation temperature at this time was 580°C. Next, the multi-layer quantum well structure shown in Figure 7(a) was formed, but this time in an inverted order compared to the structure in Figure 7(a). The layers were stacked in the order of top layer contact layer, top layer barrier layer, top layer quantum well layer, ..., with the bottom layer contact layer being the last layer.

[0049] A total of seven films containing Ga and As were formed as the top contact layer (12 or 12e). The top contact layer (12 or 12e) was a 4nm thick Si-doped GaAs (Si: 5 × 10⁻⁶). 18 cm -3 This is a Si-doped GaAs with a total thickness of 28 nm, consisting of seven stacked layers of ) with 3 × 10⁻¹⁶ Si added each time a layer is formed. 12 cm -2 The material was delta-doped a total of seven times. The formation temperature for this layer and subsequent layers was 530°C. Subsequently, a layer with a thickness of 15 nm and a Si volume content of 3 × 10⁻⁶ was formed. 18 cm -3 An undoped layer of GaAs with a thickness of 5 nm was formed. After the sacrificial layer, the portion with a thickness of 48 nm up to this point corresponds to the uppermost contact layer 12 in Figure 7(a).

[0050] Subsequently, to form the quantum well structure that plays a central role in the device, a 50nm thick Al 0.3 Ga 0.7 As barrier layer 13, thickness 4 nm, Si volume content 3 × 10 18 cm -3 GaAs quantum well layer 14 (only the last 0.85 nm is undoped), 40 nm thick Al 0.3 Ga 0.7 As barrier layer 13, GaAs quantum well layer 14 with a thickness of 4 nm (Si: 3 × 10⁻¹⁰ 18 cm -3 (Only the last 0.85nm is undoped), 40nm thick Al 0.3 Ga 0.7 As barrier layer 13, GaAs quantum well layer 14 with a thickness of 4 nm (Si: 3 × 10⁻¹⁰ 18 cm -3 (Only the last 0.85nm is undoped), 50nm thick Al 0.3 Ga 0.7 An As barrier layer 13 was formed.

[0051] Subsequently, the bottommost contact layer 12d in Figures 7(a) and (b) is defined as having a thickness of 15 nm and a Si volume content of 3 × 10⁻⁶. 18 cm -3 After forming GaAs, a 4nm thick Si-doped GaAs (Si: 5×10 18cm -3 A total of 7 layers are stacked, and immediately before forming each layer, Si is added in 3 × 10 12 cm -2 The material was delta-doped a total of seven times. The bottom contact layer 12d was then bonded to the Au substrate using wafer bonding technology. To do this, first, a 3nm thick Ti and a 150nm thick Au were formed as the Au substrate on the bottom contact layer located at the top surface of the quantum well structure by sputtering.

[0052] Furthermore, a metal layer consisting of 10 nm thick Ti and 500 nm thick Au was formed on the second GaAs(100) substrate, which would become the final semiconductor substrate 11b, by sputtering. The substrate with the quantum well was inverted, the two Au surfaces were brought into contact, and the substrate was heated at 250°C for 60 minutes in a nitrogen gas atmosphere while applying a load of 5 MPa. As a result, the two GaAs substrates were integrated via the Au. This integration formed a metal layer 17 consisting of Au and Ti.

[0053] Next, the first GaAs substrate was mechanically polished, leaving approximately 100 μm. The remaining GaAs portion and buffer layer of the first GaAs substrate were selectively removed at 38°C using a solution of 1 g / ml citric acid aqueous solution and hydrogen peroxide aqueous solution mixed in a 10:1 ratio, exposing the AlGaAs sacrificial layer. Finally, the AlGaAs sacrificial layer was selectively removed with hydrofluoric acid aqueous solution. In this way, a multi-quantum well structure with the top contact layer exposed on the surface was created on top of the Au substrate.

[0054] Metal patches 18 were fabricated using electron beam lithography, the lift-off method, and the dry etching method. An electron beam resist was applied, and a pattern was drawn within a 100 μm square area (which became the photodetection region) using an electron beam. Then, development exposed the top contact layer to form the shape of the metal patch. On top of this, a metal patch layer consisting of 3 nm thick Ti and 150 nm thick Au was formed. By dissolving the resist (lift-off method), metal patches 18 with the predetermined pattern were formed. In addition, an electrode pad 15 was formed on the uppermost contact layer using photolithography, and this area was designated as the electrode region.

[0055] Subsequently, in the case of the quantum well photodetector 116 of the example, the uppermost contact layer was etched by 35 nm using the metal patch 18 and electrode pad 15 as etching masks to form a thin-film contact layer 12e for the photodetector. A reactive dry etching apparatus (RIE-200NL, manufactured by SAMCO) was used for this etching, and a mixed gas of CHF3 and O2 was used as the etching gas. As a result, the metal patch 18 and the uppermost contact layer directly beneath the electrode pad 15 were not etched and remained at their original thickness of 48 nm. However, the remaining thickness of the exposed contact layer became 13 nm, causing that portion to become depleted and lose its conductivity. In this way, the contact layer 12e lost its function of supplying current within its plane. In the comparative example, this etching process was omitted, and the thickness of the uppermost contact layer 12 was set to 48 nm even in the exposed area. Therefore, in the comparative example, the contact layer 12 has the function of supplying current within its plane.

[0056] Finally, the necessary areas, including the detector region and electrode region, were cut out from the second GaAs substrate by cleavage, mounted onto an 8-pin ceramic package using conductive epoxy, and the electrodes were bonded to the package pins with Ag paste using Au wire.

[0057] In the comparative example quantum well photodetector 205, square Au patches with side length L = 0.93 μm and thickness 100 nm are arranged in a square lattice pattern with a period P = 2.5 μm in the photodetection region. A scanning electron microscope image of this arrangement is shown in Figure 8(a).

[0058] In the quantum well photodetector 116 of the embodiment, as shown in Figures 8(b1) and (b2), square Au patches with a side length L = 0.99 μm and a thickness of 100 nm are arranged in a square lattice pattern with a period P = 2.5 μm in the photodetection region.

[0059] The difference between the quantum well photodetector 116 of the example and the quantum well photodetector 205 of the comparative example is, as described above, that in the example, the GaAs layer in the photodetection region other than the metal patch 18 was removed to a depth of 35 nm by dry etching after the formation of the metal patch 18. The uppermost contact layer directly beneath the metal patch 18 was not etched and remained at its original thickness of 48 nm. The electrode region is also exactly the same as that of the quantum well photodetector 205 shown in Figure 7(a). Actual scanning electron microscope images are shown in Figures 8(b1) and (b2).

[0060] <Characteristic Evaluation> Figure 9 shows a comparison of the results of the conduction band structure calculations for the example and the comparative example when no bias is applied. Figure 9(a) shows the band structure for the quantum well photodetector 205 in the comparative example. The structure of the quantum well photodetector 116 is the same as that of the quantum well photodetector 205 in the electrode region, and the removed portion to a depth of 35 nm, excluding the metal patch 18 in the photodetection region, becomes the band structure of the quantum well photodetector 116. 35 nm dry etching of the GaAs layer means that the entire 28 nm high-doped region on the surface of the uppermost contact layer has been removed.

[0061] As a result, in Figure 9(b), the electron energy of the remaining contact layer increases (indicated by circles in the figure) and deviates significantly from the Fermi energy (0V). This means that the exposed contact layer region is depleted and no longer functions as an in-plane conductive layer. This realizes the situation in Figure 7(b). At this time, electrons are supplied from the electrode region to the photodetector region by in-plane conduction through the quantum well, so in Figure 9(b), the conditions are in place for the current multiplication phenomenon due to in-plane conduction by quantum wells to be realized. Furthermore, the band diagram when the contact layer corresponding to the quantum well photodetector 101 is completely removed is almost identical to that of Figure 9(b). In other words, if the surface portion of the contact layer is removed to the extent that the outermost surface becomes depleted, as in the quantum well photodetector 116 shown in Figure 7(b), there is no significant difference compared to when the contact layer is completely removed.

[0062] Figure 10 compares the dark current characteristics of the example and the comparative example. The temperature during measurement was 78K. The basic dark current characteristics are nearly equivalent, indicating that the quantum well layer has conductivity equivalent to that of the contact layer.

[0063] The sensitivity spectra of each detector in the examples and comparative examples are shown in Figure 11, and the bias voltage dependence of the peak sensitivity is shown in Figure 12. The measurement temperature was 78K in these cases as well. In the low bias voltage region (absolute value of 1.5V or less), the example and the comparison example show relatively similar characteristics. In both cases, the maximum sensitivity is approximately ±0.7V and about 2A / W. There is no significant difference depending on whether the bias is positive or negative. The wavelength shift is due to the difference in patch size L and is not essential. Furthermore, even in the high bias voltage region, when the polarity is positive, the sensitivity decreases unilaterally as the bias voltage increases, exhibiting a similar characteristic.

[0064] On the other hand, when the bias voltage increases in the negative direction, a significant difference arises between the example and the comparative example. In the comparative example, the quantum well photodetector 205, sensitivity decreases monotonically as the negative bias voltage increases. However, in the embodiment shown in Figure 7(b), which has a photodetector thin-film contact layer 12e with the contact layer surface removed to a depth of 35 nm, sensitivity decreases initially, but then increases sharply at a bias voltage of -1.5 V (electric field 75 kV / cm), reaching 17.8 A / W at -2.7 V. This sensitivity is more than eight times that of the low-bias region and corresponds to an external quantum efficiency of 308%. Note that there is a relationship between the sensitivity (A / W) and the external quantum efficiency in a photodetector: external quantum efficiency = 1.24 × sensitivity / wavelength (wavelength unit: μm). Furthermore, if the bias voltage is increased in the negative direction, sensitivity decreases again. This high-sensitivity phenomenon that appears in the high-electric-field region is the current multiplication phenomenon that uniquely occurs in quantum well photodetectors during in-plane conduction of the quantum well.

[0065] This electrical characteristic is common to the quantum well photodetector of the present invention, and the bias value V gives a minimum value to the output current when a bias is applied to the electrode for output extraction.min We find the bias value V min By applying a bias with a larger absolute value to the extraction electrode, this detector becomes a high-power, high-sensitivity device. Furthermore, the upper limit of the bias applied to the output electrode is such that increasing the bias too much will cause the sensitivity to decrease again, so the bias value V that gives the maximum sensitivity is important. max It is preferable to do so. Taking the above embodiment as an example, V at this time max The voltage is -2.7V.

[0066] One known current multiplication phenomenon caused by applying a high electric field is the avalanche multiplication phenomenon, which is widely used in conventional silicon and other photodetectors for single-photon measurements. However, in such cases, it was necessary to realize a unique and complex structure that required careful design, such as providing an avalanche layer in addition to the photodetector layer. However, this method is characterized by its ability to reproducibly realize a similar high-sensitivity detector with a relatively simple structure, which involves slightly removing or omitting the contact layer without providing a special avalanche layer. [Industrial applicability]

[0067] This invention provides a quantum well-type photodetector capable of generating a high-power photoelectric conversion current. High-power photodetectors have a wide range of applications, including sensors and cameras for civilian, industrial, and military use. Therefore, we believe that this invention will have a significant social impact and a major influence on industry. [Explanation of symbols]

[0068] 11a: Conductive semiconductor substrate 11b: Semiconductor substrate 11c: Conductive semiconductor substrate 11d: Substrate made of a conductor whose dielectric constant has a negative real part. 12: Contact layer (outermost contact layer) 12a: Thin-film contact layer for photodetector 12b: Thin film diffraction grating-forming contact layer 12c: Thick film diffraction grating-forming contact layer 12d: Lower contact layer 12e: Thin-film contact layer for the photodetector (contact layer) 13: Barrier layer 13a: Topmost barrier layer 13b: Diffraction grating-forming barrier layer 14: Quantum well layer 14a: Top layer of quantum wells 15: Electrodes (electrode pads) 16: Ground electrode 17: Metal layer 18: Metal patch 21: Ray of light 22: Photodetection region 23: Electrode area 24: Plasmon resonator 31: Electron flow 41: Multiple quantum well structure 51: Conductive layer 52:Quantum functional layer 101: Quantum well photodetector 102: Quantum well photodetector 103: Quantum well photodetector 104: Quantum well photodetector 105: Quantum well photodetector 106: Quantum well photodetector 107: Quantum well photodetector 111: Quantum well photodetector 112: Quantum well photodetector 113: Quantum well photodetector 114: Quantum well photodetector 115: Quantum well photodetector 116: Quantum well photodetector 121: Quantum well photodetector 122: Quantum well photodetector 123: Quantum well photodetector 124: Quantum well photodetector 125: Quantum well photodetector 201: Quantum well photodetector 202: Quantum well photodetector 203: Quantum well photodetector 204: Quantum well photodetector 205: Quantum well photodetector

Claims

1. the first and second main surfaces have a multiple quantum well structure including one or more quantum well layers sandwiched between barrier layers; the quantum well layer is doped; detecting an increment in photocurrent caused by light irradiation when a bias voltage is applied in a direction perpendicular to the quantum well layer, utilizing an intersubband transition occurring in a conduction band of the quantum well layer; an electrode for output extraction is disposed on the outermost surface of the barrier layer via a contact layer; A photodetector, wherein a current is supplied to the multiple quantum well structure in the photodetection region through the plane of the quantum well layer.

2. an island-shaped metal patch is formed in contact with the multiple quantum well structure or above the multiple quantum well structure; A region including the metal patch is defined as a light detection region, 2. The photodetector according to claim 1, wherein a current is supplied to the region of said photodetection region immediately below said metal patch through an in-plane of said quantum well layer.

3. A quantum function layer and an electrode for extraction are laminated in this order on a conductive layer, the quantum function layer includes one or more quantum well layers and barrier layers sandwiching first and second major surfaces of the quantum well layers; a contact layer is formed between the electrode and a most adjacent one of the barrier layers; A photodetector, wherein a current is supplied to the quantum function layer in the photodetection region through an in-plane of the quantum well layer.

4. An island-shaped metal patch is formed in contact with or above the quantum function layer, A region including the metal patch is defined as a light detection region, 4. The photodetector according to claim 3, wherein a current is supplied to the region of said photodetection region immediately below said metal patch through an in-plane of said quantum well layer.

5. 5. The photodetector according to claim 1, wherein the quantum well layer to which the current is supplied is an outermost quantum well layer among the quantum well layers.

6. 2. The photodetector according to claim 1, wherein the quantum well layer is a mixed crystal consisting of one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si and Ge.

7. 2. The photodetector according to claim 1, wherein the barrier layer is a mixed crystal consisting of one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si and Ge.

8. The photodetector according to claim 1 , wherein the number of said quantum well layers is 1 or more and 200 or less.

9. The photodetector according to claim 2 or 4, wherein the metal patches are arranged in a matrix.

10. The photodetector according to claim 2 or 4, wherein the metal patches are arranged in an array.

11. 10. The photodetector according to claim 9, wherein the arrangement pitch of the metal patches is not less than 0.5 [mu]m and not more than 200 [mu]m.

12. 2. The photodetector according to claim 1, wherein the contact layer is a mixed crystal consisting of one or more elements selected from the group consisting of GaAs, AlAs, InAs, InP, AlSb, GaP, AlP, GaN, AlN, ZnSe, CdSe, MgSe, GaSb, InSb, InN, PbS, PbSe, ZnTe, CdTe, HgTe, Si and Ge.

13. In the photodetector according to claim 1, a bias value V that gives a minimum value to the output current when a bias is applied to the electrode for output extraction is ｍｉｎ and the bias value V ｍｉｎ A method of using a photodetector, comprising applying a bias having a larger absolute value to the extraction electrode.

14. 14. The method of claim 13, wherein the sign of the bias is negative.

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