JP2015023214A - Semiconductor light emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor light emitting element which can achieve a light emitting operation using ISB polariton with high efficiency.SOLUTION: A semiconductor light emitting element 1A comprises: an active layer 20 having a luminescent layer and an injection layer in a former stage of the luminescent layer; and first and second metal layers 11, 12 which form a plasmon structure. The luminescent layer has a subband electron level structure including an upper level and a lower level and a polariton structure including an upper polariton level and a loser polariton level, and transition from the lower polariton level to the lower level produces light. A thickness T of the resonator is set at not greater than a light wavelength in the resonator and a width W of the resonator is set at when assuming that a width of the resonator when an energy spacing between the upper and lower levels and optical cutoff energy in the resonator coincide with each other is W, within a range of not less than Wand not more than 1.15×W.

Description

  The present invention relates to a semiconductor light emitting device that generates light by light emission transition using a subband level structure in a quantum well structure.

  In recent years, Bola-Einstein condensation (BEC) of polariton (material-photon interaction state) generated in a semiconductor microresonator realizes laser oscillation with very few electron injections without forming an inversion distribution. The possibility of this is shown, and attempts to generate and agglomerate polaritons using excitons have been energetically performed.

  In addition, generation of ISB polaritons by collective electronic excitation between intersubbands (ISBs) formed in a semiconductor quantum well structure has been experimentally confirmed, and light emission phenomenon via polaritons by current injection has been observed. (For example, see Non-Patent Document 1: Phys. Rev. Lett. Vol. 100 (2008) 136806).

JP 2011-35138 A

L. Sapienza et al., “Electrically Injected Cavity Polaritons”, Phys. Rev. Lett. Vol.100 (2008) pp.136806-1-136806-4 P. Jouy et al., "Transition from strong to ultrastrong coupling regime in mid-infraredmetal-dielectric-metal cavities", Appl. Phys. Lett. Vol.98 (2011) pp.231114-1-231114-3 Benjamin S. Williams et al., "Terahertz quantum-cascade laser at λ = 100μm using metal waveguide for mode confinement", Appl. Phys. Lett. Vol. 83 (2003) pp. 2124-2126

  Polaritons in the resonator are generated by the interaction between photons (electromagnetic waves) in the resonator and substances having electric bias (polarization waves). The polariton state is obtained by quantizing the interaction state between the electromagnetic wave and the polarized wave as described above, and represents the eigenstate derived from the interaction Hamiltonian (here, the interaction Hamiltonian between the photon system and the electron system). Occupy. Such a polariton state has, for example, two eigenstates in a two-level system and three eigenstates in a three-level system.

  Polaritons generated in the resonator become quasiparticles having a boson property, and the polaritons can be aggregated in the lower polariton level among the plurality of polariton levels. In this case, strong emission with high coherence can be obtained by oscillating these macroscopic numbers of polariton quasiparticles by repeating excitation and relaxation all at once. Further, in such a light emission process, it is possible to realize a semiconductor light emitting element that does not require an inversion distribution and consumes a very small amount of power with a small injection current.

  However, in the above-described light emission phenomenon between subbands, light emission occurs in the TM mode, and absorption also occurs for light in the TM mode. In the TM mode that requires light confinement in the in-plane direction perpendicular to the semiconductor stacking direction (growth direction), for example, an existing technology such as a distributed Bragg reflector (DBR) type resonator is used. I can't. Therefore, it is difficult to construct an effective resonator, and it is difficult to realize efficient generation and aggregation of ISB polaritons.

  The present invention has been made to solve the above-described problems, and provides a semiconductor light-emitting device capable of realizing the generation and aggregation of ISB polaritons and the light emission operation thereby by high efficiency. Objective.

In order to achieve such an object, the semiconductor light emitting device according to the present invention is used for (1) a light emitting layer including at least one quantum well layer, and an electron injection provided in the light emitting layer provided in the preceding stage of the light emitting layer. An active layer having an injection layer, (2) a first metal layer provided on one side of the semiconductor stacking direction with respect to the active layer, and (3) provided on the other side of the active layer with respect to the semiconductor stacking direction. And a second metal layer that forms a plasmon resonator structure with the first metal layer, and (4) the light emitting layer has at least an upper level and a lower level in the subband level structure, Due to the band level structure, a polariton level structure including an upper polariton level and a lower polariton level is formed, and light is generated by transition from the lower polariton level to the lower level, (5) Both plasmons In vessel structure, cavity thickness T of the semiconductor stacking direction, while being set to less than the wavelength lambda _e of light in the resonator (T ≦ lambda _e), the resonator width W, the upper level and lower and energy gap of level, as the resonator width W _c of when the light cut-off energy in the resonator are matched, or W _c 1.15 × W _c within the range (W _c ≦ W ≦ 1.15 × W _c ).

  In the semiconductor light emitting device described above, an active layer is constituted by a light emitting layer including a quantum well layer and an injection layer provided in the preceding stage. In the light-emitting layer, a subband electron level structure including an upper level (upper electron level) and a lower level (lower electron level), and an upper polariton level by ISB polariton resulting from the subband level structure. And a polariton level structure including a lower polariton level, and light is generated by a transition from the lower polariton level to the lower electron level. Thereby, the light emission operation using ISB polaritons in the active layer can be suitably realized.

Further, in such a configuration, the first and second metal layers are respectively provided on both sides of the active layer in the semiconductor stacking direction (thickness direction), and the quantum well active layer structure is sandwiched between the upper and lower metal layers, thereby plasmon A resonator structure is configured. Further, in this plasmon resonator structure, specifically, the resonator thickness (thickness of the active layer sandwiched between the first and second metal layers) T is set to the wavelength of light in the resonator (constituting the active layer). The effective wavelength of light in the substance to be emitted) is set to λ _e or less. In such a configuration, as will be described in detail later, electromagnetic waves exist as standing waves inside the resonator, so that light is confined in the width direction (in-plane direction) of the resonator, and efficient ISB polaritons. Can be produced and agglomerated.

Further, in the plasmon resonator structure described above, the resonator width W is determined by the energy interval between the upper level and the lower level in the subband level structure of the light emitting layer, and the cutoff energy of light in the resonator. the resonator width when matching as W _c, is set to be within a range of W _c or 1.15 × W _c. According to such a configuration, the subband electronic level structure and the polariton level structure can suitably realize a light emission operation by a transition from the lower polariton level to the lower electron level with high efficiency. .

Here, the resonator thickness T, more specifically, in the plasmon resonator structure, the resonator thickness T is in the range of 1/4 or less of the light wavelength lambda _e in the 60nm or more resonators in It is preferable to set (60 nm ≦ T ≦ λ _e / 4). According to such a configuration, electromagnetic waves cannot exist as standing waves in the thickness direction of the resonator, and it is possible to reliably eliminate light cut-off energy for light confinement in the thickness direction.

  In addition, regarding a specific configuration of the active layer, the light emitting layer includes an injection barrier layer formed closest to the injection layer, and a downstream of the light emitting layer includes an extraction barrier layer formed closest to the light emitting layer. An extraction layer may be provided, and the thickness of the extraction barrier layer may be set within a range of 70% to 150% with respect to the thickness of the injection barrier layer. Thus, by setting the layer thickness of the extraction barrier layer to be thick, the tunneling time of electrons from the lower level in the light emitting layer to the level in the extraction layer becomes longer, which is necessary for light emitting operation using ISB polaritons. Electrons are present at a high density in the lower electron level in the light emitting layer.

In addition, for the injection layer before the light emitting layer, the injection layer is preferably doped so that the carrier density is in the range of 3 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁹ cm ⁻³ or less. . Thereby, electrons serving as carriers can be sufficiently supplied to the quantum well structure of the active layer, and the generation of ISB polaritons in the resonator and the efficiency of the light emission operation using the ISB can be further improved. . Further, when an extraction layer is provided after the light emitting layer, the carrier density of the extraction layer is similarly in the range of 3 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁹ cm ⁻³ or less. It is preferable that doping is performed.

  In a state where no operating voltage is applied, the lower level in the light emitting layer is preferably an energy level that is lower by 5 meV or more than the ground level in the injection layer. This makes it possible to stably generate the polariton state by concentrating electrons serving as carriers on the lower electron level, which is the ground level in the light emitting layer.

  The active layer may have a cascade structure in which a plurality of injection layers and light emitting layers are alternately stacked. Even with such a configuration, a light emitting operation using ISB polaritons in the active layer can be suitably realized.

  As for the level structure in the light-emitting layer, the light-emitting layer has a second upper level, which is an energy level higher than the upper level in the subband level structure, and is generated in the second upper level. The second upper level is an energy level that is higher than the minimum point of the lower polariton level by the energy of longitudinal optic (LO) phonons. It is good also as a structure which is a rank. According to such a configuration, polaritons can be efficiently supplied to the lower polariton levels related to light emission via the second upper level.

  In this case, the energy interval between the second upper level and the (first) upper level is preferably set smaller than the energy of the longitudinal optical phonon. Thereby, fast relaxation of electrons from the second upper level to the first upper level is prohibited, and the supply of polaritons to the lower polariton level can be preferably realized.

According to the semiconductor light emitting device of the present invention, an active layer is constituted by a light emitting layer including a quantum well layer and an injection layer preceding the quantum well layer, and in the light emitting layer, a subband level including an upper level and a lower level. The structure and the polariton level structure including the upper polariton level and the lower polariton level due to the subband level structure are formed, and light is generated by the transition from the lower polariton level to the lower electron level. In addition, a plasmon resonator structure is configured by providing first and second metal layers on both sides of the active layer in the thickness direction, and the resonator thickness T is determined by the light in the resonator. And the resonator width W when the energy interval between the upper level and the lower level and the cutoff energy of light in the resonator coincide with each other, W _{c Wc} or more 1.15 × W By setting within the range of _c or less, it is possible to realize a light emission operation using ISB polaritons with high efficiency.

It is a perspective view which shows roughly an example of the element structure of a semiconductor light-emitting device. It is a figure which shows the 1st example of the subband level structure in an active layer. It is a perspective view which shows the element structure of the conventional semiconductor light-emitting device. It is a graph which shows the wave number dependence of the energy of the photon in the active layer of the conventional semiconductor light-emitting device, and the energy of an electron. It is a graph which shows the dispersion curve of the polariton in the active layer of the conventional semiconductor light-emitting device. It is a figure which shows typically about the plasmon resonator structure in the semiconductor light-emitting device shown in FIG. 2 is a graph showing the wave number dependence of photon energy and electron energy in the active layer of the semiconductor light emitting device shown in FIG. 1. 2 is a graph showing a polariton dispersion curve in an active layer of the semiconductor light emitting device shown in FIG. 1. It is a graph which shows the 1st example of the semiconductor laminated structure in an active layer. FIG. 10 is a diagram showing a subband level structure in an unbiased state in the active layer shown in FIG. 9. It is a figure which shows the carrier distribution in the active layer shown in FIG. It is a figure which shows the subband level structure in the bias application state in the active layer shown in FIG. It is a graph which shows the dispersion curve of the polariton in an active layer. It is a graph which shows the wave number dependence of the Hopfield coefficient (alpha) with respect to a photon. It is a graph which shows the wave number dependence of the Hopfield coefficient (beta) with respect to an electron. It is a graph which shows the dispersion curve of the polariton in an active layer. It is a graph which shows the wave number dependence of the Hopfield coefficient (alpha) with respect to a photon. It is a graph which shows the wave number dependence of the Hopfield coefficient (beta) with respect to an electron. It is a figure which shows the 2nd example of the subband level structure in an active layer. It is a graph which shows the 2nd example of the semiconductor laminated structure in an active layer. It is a figure which shows the 3rd example of the subband level structure in an active layer. It is a graph which shows the 3rd example of the semiconductor laminated structure in an active layer. It is a figure which shows the subband level structure in the active layer shown in FIG. It is a graph which shows the dispersion curve of the polariton in an active layer. It is a perspective view which shows roughly the other example of the element structure of a semiconductor light-emitting device.

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

  FIG. 1 is a perspective view schematically showing an example of an element structure in a semiconductor light emitting element according to the present invention. The semiconductor light emitting device 1A according to the present embodiment is a monopolar type light emitting device that generates light by light emission transition using a subband level structure in a semiconductor quantum well structure. In particular, the light emitting element 1A is configured as a semiconductor light emitting element that generates light using the above-described ISB (Intersubband) polaritons.

  A semiconductor light emitting device 1A shown in FIG. 1 includes a semiconductor substrate 10 and an active layer 20 having a semiconductor quantum well structure. As the semiconductor substrate 10, for example, an InP substrate can be used. The active layer 20 is provided on the substrate 10 with a mesa structure. Here, in the following, as shown in the xyz orthogonal coordinate system in FIG. 1, the semiconductor lamination direction (thickness direction) in the active layer 20 is the z-axis direction, the mesa-shaped width direction is the x-axis direction, and the length direction is the length direction. The y-axis direction is assumed.

  A first metal layer 11 is provided on one side (upper side in the drawing) of the active layer 20 in the semiconductor stacking direction. A second metal layer 12 is provided between the active layer 20 and the substrate 10 on the other side (lower side in the drawing) of the active layer 20 in the semiconductor stacking direction. These first and second metal layers 11 and 12 are made of, for example, a metal material such as gold (Au) or copper (Cu). The second metal layer 12 is formed on the entire upper surface of the substrate 10 wider in the width direction than the mesa-shaped active layer 20.

  In the above configuration, a plasmon resonator (plasmon cavity) structure is formed by the first and second metal layers 11 and 12 sandwiching the active layer 20. Further, a lower electrode layer 13 made of metal, for example, is formed on the entire lower surface of the substrate 10 on the opposite side of the substrate 10 from the second metal layer 12. The plasmon resonator structure is a microresonator using the plasmon effect for confining light in the in-plane direction, and details thereof will be described later.

  FIG. 2 is a diagram showing a first example of the semiconductor stacked structure of the active layer 20 and the subband level structure in the active layer 20 of the semiconductor light emitting device 1A shown in FIG. Here, FIG. 2A shows a subband level structure in a state where an operating voltage is applied to the semiconductor light emitting element 1A (bias applied state). FIG. 2B shows a subband level structure in a state where no operating voltage is applied (no bias state).

  The active layer 20 in this configuration example includes a quantum well light-emitting layer 30, an injection layer 31 provided before the light-emitting layer 30 and used to inject electrons into the light-emitting layer 30, and a light-emitting layer provided after the light-emitting layer 30. 30 and an extraction layer 35 used for extraction of electrons from 30. Among these, the light emitting layer 30 is a region that contributes to the generation of ISB polaritons in the light emitting element 1A and the light emitting operation using the ISB polaritons. In addition, the injection layer 31 and the extraction layer 35 are regions that contribute to the transport of electrons in the upstream and downstream of the light emitting layer 30.

  The light emitting layer 30, the injection layer 31, and the extraction layer 35 are each formed with a predetermined semiconductor quantum well structure including a quantum well layer and a quantum barrier layer. Thereby, in each semiconductor layer 30, 31, and 35 of the active layer 20, the subband level structure which is an energy level structure by a quantum well structure is formed. A contact layer 38 is provided on the opposite side of the injection layer 31 from the light emitting layer 30. A contact layer 39 is provided on the opposite side of the extraction layer 35 from the light emitting layer 30.

  The light emitting layer 30 includes at least one quantum well layer. An injection barrier layer 301 for electrons from the injection layer 31 to the light emitting layer 30 is formed on the light emitting layer 30 closest to the injection layer 31. Further, an extraction barrier layer 351 for electrons from the light emitting layer 30 to the extraction layer 35 is formed on the most light emitting layer 30 side of the extraction layer 35.

The light emitting layer 30 has, in its subband level structure (electronic level structure), an upper level L2 and a lower level that is an energy level lower than the upper level L2 and serves as a ground level in the light emitting layer 30. And L1. Further, in the light emitting layer 30, a polariton level structure including an upper polariton level and a lower polariton level is formed due to the subband level structure by the electron levels L2 and L1, and the lower polariton level is formed. Light is generated by the transition from the position to the lower electron level L1. The light generated in the light emission transition between the polariton level and the electron level becomes light having energy slightly smaller than the energy interval ΔE ₂₁ between the upper level L2 and the lower level L1. In FIG. 2, the polariton level structure is not shown.

  The injection layer 31 has a single or a plurality of injection levels Li used for electron transport in the subband level structure. The extraction layer 35 has a single or a plurality of extraction levels Le used for electron transport in the subband level structure. In FIG. 2, for simplicity of explanation, the injection layer 31 shows a plurality of injection levels Li, and the extraction layer 35 shows a single extraction level Le.

  The element structure of the semiconductor light emitting element 1A shown in FIG. 1 can be manufactured by wafer bonding in the following manner, for example, with the semiconductor material of the substrate 10 being InP and the metal material of the metal layers 11 and 12 being Au. it can. Specifically, first, an active layer 20 is formed on an InP substrate, and a first member in which an Au film is deposited thereon, and an Au film is deposited on an n-type InP substrate to be the semiconductor substrate 10. Two members are prepared, and the first and second members are bonded together by applying an appropriate pressure with the Au film facing each other. The Au film formed on the bonded portion of the first and second members becomes the second metal layer 12.

  Next, the InP substrate of the first member on which the active layer structure has been grown is removed by etching, and an Au film serving as the first metal layer 11 is deposited on the surface of the active layer 20 from which the substrate has been removed. A plasmon resonator structure is formed. Finally, the active layer 20 is etched together with the first metal layer 11 to form a mesa active layer having a resonator width W in the x-axis direction, a resonator length L in the y-axis direction, and a resonator thickness T in the z-axis direction. Create the structure.

  In such a configuration, in the mesa structure in which the active layer 20 is etched in accordance with the width of the first metal layer 11, both side surfaces 21, 22 in the width direction formed by etching are used to increase the efficiency of the resonator. It is preferable that a highly reflective film such as a dielectric multilayer film is formed thereon, and the light reflectance at the side surfaces 21 and 22 is 90% or more, for example.

  As illustrated in FIG. 2, the light emitting element 1 </ b> A includes contact layers 38 and 39 above and below the active layer 20, and is configured to be able to inject current into the active layer 20. In the configuration shown in FIG. 1, the lower electrode layer 13 of the element 1A is electrically connected to the second metal layer 12 through the semiconductor substrate 10 doped with a sufficient carrier density. Alternatively, the electrode layer 13 may not be provided under the substrate 10 and the metal layer 12 may function as the lower electrode layer. Further, the first metal layer 11 functions as an upper electrode layer in the upper part of the element 1A. FIG. 1 shows a configuration in which a voltage source 15 is connected between the first metal layer 11 and the lower electrode layer 13.

  In the resonator structure formed by the active layer 20 sandwiched between the metal layers 11 and 12, when the light emitting element 1A is configured as an LED, the resonator length L is set to 100 μm so that the photon lifetime is less than the relaxation time of the polariton. It is preferable to set the following. Further, when the light emitting element 1A is configured as a laser element, it is preferable to set the resonator length L to 100 μm or more. Both end faces 23 and 24 in the length direction of the element 1A in such a resonator structure can be formed by a method such as cleavage or dry etching. When the element 1A is configured as a laser element, a high reflection film made of a dielectric multilayer film or a metal film may be formed on both end faces 23 and 24 of the element as well as the side faces 21 and 22. .

Further, in this semiconductor light emitting device 1A, in order to ensure the conditions necessary for the light emission operation using ISB polariton, in the plasmon resonator structure, the resonator thickness T in the z-axis direction is set to the wavelength of light in the resonator. The following are set. Further, the resonator width W in the x-axis direction is defined as W _{c where} the resonator width when the energy interval between the upper level L2 and the lower level L1 matches the cutoff energy of light in the resonator is W _c . W _c is set within the range of more than 1.15 × _{W c.}

  The effects of the semiconductor light emitting device 1A according to the present embodiment will be described.

  In the semiconductor light emitting device 1A shown in FIGS. 1 and 2, the active layer 20 includes the light emitting layer 30 including the quantum well layer and the injection layer 31 provided in the preceding stage. The light emitting layer 30 includes a subband electron level structure including the upper level L2 and the lower level L1, an upper polariton level due to the ISB polariton caused by the subband level structure, and a lower polariton level. A polariton level structure is formed, and light is generated by a transition from the lower polariton level to the lower electron level. Thereby, the light emission operation using ISB polaritons in the active layer 20 can be suitably realized.

In such a configuration, the first and second metal layers 11 and 12 are provided on both sides of the active layer 20 in the z-axis direction, and the quantum well active layer structure is sandwiched between the upper and lower metal layers 11 and 12. Thus, a plasmon resonator structure is configured. Further, in this plasmon resonator structure, specifically, the resonator thickness (the thickness of the active layer 20 sandwiched between the first and second metal layers 11 and 12) T is set to be the active layer 20 in the resonator. The wavelength of light to be generated (effective wavelength of light in the material constituting the active layer 20) is set to λ _e or less (T ≦ λ _e ). In such a configuration, electromagnetic waves exist as standing waves inside the resonator, and light confinement in the x-axis direction of the resonator and efficient generation and aggregation of ISB polaritons are possible.

Further, in the plasmon resonator structure described above, the resonator width W is set so that the energy interval ΔE ₂₁ between the upper level L2 and the lower level L1 in the subband level structure of the light emitting layer 30 and the light intensity in the resonator. as the resonator width _{W c,} by setting the above _{W c} 1.15 × _{W c} within the range _{(W c ≦ W ≦ 1.15 ×} W c) has at the time when the cutoff energy _{E c} that match . According to such a configuration, the light emission operation by the transition from the lower polariton level to the lower electron level can be suitably and efficiently realized by the subband electron level structure and the polariton level structure of the light emitting layer. It becomes possible.

  Currently, a quantum cascade laser (QCL: Quantum Cascade Laser) is known as a useful light source in the middle to far infrared region. QCL can secure sufficient light emission intensity in a wavelength region with low energy by stacking quantum well light emitting layers in multiple layers in the active layer. On the other hand, in QCL, a high drive voltage is essentially required due to the cascade structure, and high power consumption may be a problem. On the other hand, according to the semiconductor light emitting device having the above configuration, it is possible to efficiently generate and agglomerate polaritons, and to realize a light emitting device with low power consumption, high luminance, and high coherence in the mid to far infrared region. Can do. For example, since a polariton laser via BEC does not require an inversion distribution, it can oscillate at an extremely low threshold and cannot be achieved with a QCL that requires high driving power. Power consumption drive is possible.

  The operation of the semiconductor light emitting device 1A having the above configuration will be described in detail while comparing with a conventional semiconductor light emitting device. As a conventional semiconductor light emitting element using ISB polaritons, for example, there is one disclosed in Non-Patent Document 1: Phys. Rev. Lett. Vol.100 (2008) 136806. In this light emitting device, an active layer having a quantum cascade (QC) structure is used to generate ISB polaritons using collective excitation of electrons between subbands (ISB), and light emission through the polaritons is performed by current injection. Observing.

  In such a semiconductor light emitting device, in order to obtain light emission with high brightness and high coherence by polariton, polariton is aggregated at the lower polariton level (Bose-Einstein condensation: BEC), and a macroscopic number of polaritons are simultaneously produced. Must be excited and relaxed. However, it is very difficult to cause such a phenomenon in the conventional structure disclosed in the above document.

  FIG. 3 is a perspective view schematically showing an element structure of a conventional semiconductor light emitting element using ISB polaritons. In this semiconductor light emitting device 80, an active layer 85 having a QC structure for generating polaritons is sandwiched between resonators composed of a metal layer 86 and a low refractive index layer 87, and light can be confined in the z-axis direction. It is said.

FIG. 4 is a graph showing the wave number dependence of photon energy and electron energy in the active layer 85 of the semiconductor light emitting device 80 shown in FIG. In this graph, the horizontal axis indicates the wave number k _// in the in-plane direction, and the vertical axis indicates the energy E. In FIG. 4, a graph G0 shows the wave number dependence of photon energy. Further, the graph G2, shows a wave number dependency of electron energy corresponding to the upper level L2 of the energy E _2. The horizontal axis of the graph corresponds to the energy E ₁ of the lower level L1.

FIG. 5 is a graph showing a polariton dispersion curve in the active layer 85 of the semiconductor light emitting device 80 shown in FIG. In this graph, the horizontal axis indicates the wave number k _// in the in-plane direction, and the vertical axis indicates the energy E. In FIG. 5, P1 represents a lower polariton level in a polariton level structure formed due to a subband electron level structure composed of levels L2 and L1, and P2 represents a higher polariton. (UpperPolariton) level. The graphs G0 and G2 are the same as those in FIG.

  In the dispersion relationship shown in FIGS. 4 and 5, the interaction is strongest at the point where the photon system and the electron system intersect (region R surrounded by a broken-line circle in FIG. 5). Polaritons with almost the same contribution are generated. However, the polaritons generated in such a resonance region R are easy to relax toward the low energy side along this dispersion curve, as shown by the solid arrow along the graph of the lower polariton level P1 in FIG. For this reason, in such a level structure, polaritons cannot be aggregated in the vicinity of the resonance point, and it is extremely difficult to realize a light emitting element using BEC. As the distance from the resonance point increases, the polariton approaches one of the properties of a photon or an electron and loses the other property, and cannot excite a macroscopic number of particles by BEC, and cause fast oscillation of relaxation.

  Such a problem stems from the fact that electrons only interact with TM mode light (an electric field oscillating along the z-axis direction) according to the selection rule in ISB. In the TM mode, since light does not propagate in the z-axis direction, the confinement of light in the z-axis direction cannot control the interaction energy of photons in the resonator. Therefore, since the configuration shown in FIG. 3 cannot effectively confine light in the x-axis direction or the y-axis direction, the energy of photons in the resonator cannot be controlled, and ISB polaritons are efficiently generated and aggregated. Difficult to do.

  In contrast, the semiconductor light emitting device 1A shown in FIG. 1 has a plasmon resonance in which an active layer 20 having a quantum well structure is sandwiched between first and second metal layers 11 and 12, as schematically shown in FIG. By confining light in the x-axis direction (in-plane direction) by the resonator structure, the energy of the photons in the resonator is increased, and the formation of a dispersion relationship that is advantageous for the generation and aggregation of polaritons used in the light emission operation It is what makes it possible.

  When light is incident on the resonator constituted by the metal layers 11 and 12, free electrons in the metal move in the metal accompanying the electric field and vibrate. At this time, at the interface between the metal layer and the medium of the active layer inside the resonator, an electric field remarkably enhanced as compared with incident light is generated due to the plasmon effect. Further, the enhanced electric field that oozes out from the metal interface into the resonator is localized in a range of approximately the wavelength of the incident light.

Accordingly, when the resonator thickness T is set to be equal to or less than the wavelength of light in the resonator as described above, the metal layers 11 and 12 constituting the resonator are brought close to the range where the enhanced electric field is localized. A strong electric field is induced, interacting with each other and showing the enhanced electric field in the resonator by arrows F in FIG. Due to such an electric field, an electromagnetic wave exists as a standing wave S inside the resonator, and its wavelength (plasmon wavelength: λ _p ) is determined by the structure of the resonator.

  By using such a plasmon resonator structure, light can be confined in the x-axis direction or the y-axis direction, and a strong electric field that vibrates in the thickness direction necessary for interaction can be created. . Therefore, the quantum well active layer 20 is inserted into the plasmon resonator structure of the first and second metal layers 11 and 12 having the resonator thickness T set as described above, and light is confined in the in-plane direction. In addition, by appropriately setting the resonator width W, an appropriate dispersion relationship of polaritons is formed, and efficient generation and aggregation of polaritons by an enhanced electric field can be realized. The setting of the resonator width W will be further described later.

FIG. 7 is a graph showing the wave number dependence of photon energy and electron energy in the active layer 20 of the semiconductor light emitting device 1A shown in FIGS. In this graph, the horizontal axis represents the wave number k _y in the y-axis direction, and the vertical axis represents the energy E. In FIG. 7, as in FIG. 4, the graph G <b> 0 shows the wave number dependency of the photon energy. Further, the graph G2, shows a wave number dependency of electron energy corresponding to the upper level L2 of the energy E _2. The horizontal axis of the graph corresponds to the energy E ₁ of the lower level L1.

Here, as an example of a preferable configuration of the dispersion relationship, the energy interval ΔE ₂₁ between the upper level L2 and the lower level L1 and the light in the resonator by the confinement of light in the x-axis direction by the plasmon resonator structure. This shows a case where the cut-off energy E _c of the two coincides.

FIG. 8 is a graph showing polariton dispersion curves in the active layer 20 of the semiconductor light emitting device 1A shown in FIGS. In this graph, the horizontal axis represents the wave number k _y in the y-axis direction, and the vertical axis represents the energy E. In FIG. 8, P1 indicates the lower polariton level in the polariton level structure formed due to the subband electron level structure composed of the levels L2 and L1, and P2 indicates the upper polariton level. ing. The graphs G0 and G2 are the same as those in FIG.

In the dispersion relations shown in FIGS. 7 and 8, as described above, the energy interval ΔE ₂₁ between the levels L2 and L1 and the light cutoff energy E in the resonator due to light confinement in the x-axis direction. _The resonance region R where _c and the photon system intersect with the electron system and the interaction is the strongest is located at k _y = 0. Further, at this time, the conceptual diagram of the splitting of the intrinsic energy of the polariton is as shown in FIG. 8, and the lower polariton level P1 has the energy E at k _y = 0 where the interaction between the light and the ISB transition is strongest. Has a minimum point.

  In such a configuration, polaritons generated by current injection can be accumulated as they are at the bottom (minimum point) of the polariton level energy. Thereby, Bose-Einstein condensation (BEC) is expressed by increasing the density of polaritons, and high luminance due to oscillation of macroscopic number of polaritons due to emission transition from the lower polariton level P1 to the lower electron level L1. Luminescence can be obtained.

  The plasmon resonator structure using the metal layers 11 and 12 described above is, for example, a terahertz band quantum disclosed in Non-Patent Document 3: Appl. Phys. Lett. Vol. 83 (2003) pp. 2124-2126. It differs in function and configuration from a so-called metal waveguide utilized in cascade lasers. In other words, the metal waveguide enhances the confinement of light in the active layer by sandwiching the entire active layer with the metal layer, prevents loss due to light leaking into the substrate, etc., and lowers the threshold and improves the output. Realized. On the other hand, in the semiconductor light emitting device 1A having the above configuration, the active layer 20 is sandwiched between the metal layers 11 and 12, and the thickness T and the width W of the resonator are appropriately set. The light is confined in the light.

Here, in the semiconductor light emitting device 1A shown in FIG. 1, for the resonator thickness T of the plasmon resonator structure, more specifically, 1/4 or less of the light wavelength lambda _e in the 60nm or more resonators in (60 nm ≦ T ≦ λ _e / 4). According to such a configuration, the electromagnetic wave cannot exist as a standing wave in the thickness direction (semiconductor stacking direction) of the resonator, and the light cut-off energy for the light confinement in the thickness direction is surely eliminated. be able to.

  As for the specific configuration of the active layer 20, as shown in FIG. 2, the light emitting layer 30 includes an injection barrier layer 301 formed closest to the injection layer 31. It is preferable that an extraction layer 35 including an extraction barrier layer 351 formed closest to the light emitting layer 30 is provided. However, the extraction layer 35 may not be provided if not necessary.

  In the configuration having the extraction layer 35 as described above, the thickness of the extraction barrier layer 351 may be set within a range of 70% to 150% with respect to the thickness of the injection barrier layer 301. . In this way, by setting the layer thickness of the extraction barrier layer 351 to be larger than the normal layer thickness in a quantum cascade laser or the like, for example, from the lower level L1 in the light emitting layer 30 to the level Le in the extraction layer 35. The electron tunneling time becomes longer. At this time, electrons necessary for the light emission operation using the ISB polariton are present at a high density in the lower electron level L1 in the light emitting layer 30, and formation of an inversion distribution can be prevented.

In addition, with respect to the injection layer 31 in the preceding stage of the light emitting layer 30, the injection layer 31 is doped so that the carrier density is in the range of 3 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁹ cm ⁻³ or less. It is preferable. Thereby, electrons serving as carriers are sufficiently supplied to the quantum well structure of the active layer 20 to further improve the efficiency of generation of ISB polaritons in the resonator and light emission operation using the ISB polaritons. it can. As shown in FIG. 2, when the extraction layer 35 is provided after the light emitting layer 30, the carrier density of the extraction layer 35 is similarly 3 × 10 ¹⁷ cm ⁻³ or more and 1 ×. It is preferable to dope so that it is in the range of 10 ¹⁹ cm ⁻³ or less.

As for the subband level structure in the active layer 20, as shown in FIG. 2B, the lower level L <b> 1 in the light emitting layer 30 is in the injection layer 31 in an unbiased state where no operating voltage is applied. The energy level is preferably 5 meV or more lower than the ground level Li (see the energy interval ΔE ₀₁ shown in FIG. 2B). This makes it possible to stably generate the polariton state by concentrating electrons serving as carriers on the lower electron level L1, which is the ground level in the light emitting layer 30.

  The quantum well structure in the semiconductor light emitting device 1A shown in FIG. 1, the level structure for light emission operation, and the like will be further described together with specific examples thereof.

  FIG. 9 is a chart showing a first example of a semiconductor stacked structure in the active layer 20 of the semiconductor light emitting device 1A. In this configuration example, among the semiconductor layers constituting the active layer 20, the quantum well layer is configured by an InGaAs layer, and the quantum barrier layer including the injection barrier layer and the extraction barrier layer is configured by an InAlAs layer. Further, the thickness of each semiconductor layer and the doping concentration (carrier density by doping) in each layer are as shown in FIG.

  FIG. 10 is a diagram showing a subband level structure (see FIG. 2B) in an unbiased state (0 kV / cm) in the active layer 20 shown in FIG. In FIG. 10, focusing on the injection layer 31 and the light emitting layer 30 in the active layer 20, the quantum well structure and the subband level structure are shown. In this configuration example, the light emitting layer 30 includes an injection barrier layer 301 and a well layer 302. In the well layer 302, an upper level L2 and a lower level L1 that are subband electron levels are formed. ing.

The structure of the active layer 20 shown in FIG. 9 is an example in which the energy difference ΔE ₂₁ between the upper level L2 and the lower level L1 is set to 160 meV (wavelength 7.8 μm). However, the semiconductor light emitting element 1A having the above configuration can be applied to, for example, the mid-infrared region (wavelength 4 to 12 μm), the far-infrared region (terahertz region), and the like, and is limited only to the wavelengths exemplified above. It is not something. In addition, the semiconductor stacked structure of FIG. 9 is formed on an InP substrate by using a method such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), or the like on an InP substrate. And it can produce by crystal-growing an InAlAs layer.

In order to efficiently generate polaritons in the active layer 20, it is necessary to distribute a large amount of electrons in the lower level L1 and to prevent the formation of an inversion distribution. In order to create such a situation, in the configuration of FIG. 9, doping is performed so that the carrier density is 5 × 10 ¹⁷ / cm ³ in a part of the semiconductor layers constituting the injection layer 31 and the extraction layer 35. ing. Further, the thickness of the extraction barrier layer 351 is set to 111% with respect to the thickness of the injection barrier layer 301 so that the inversion distribution in the levels L1 and L2 is difficult to be formed.

  Furthermore, in order to concentrate the carrier distribution on the lower level L1, which is the ground level of the light emitting layer 30, in an unbiased state, the level L1 is set to be lower in energy level than all other levels. To do. In addition, the lower level L1 in the light emitting layer 30 is set to an energy level that is 5 meV or lower than the ground level Li in the injection layer 31. In the level structure shown in FIG. 10, the energy difference between the lower level L1 and the ground level Li of the injection layer is set to 21.8 meV.

  FIG. 11 is a diagram showing a carrier distribution (electron distribution) in the active layer shown in FIGS. 9 and 10. As shown in the graph of FIG. 11, in this configuration example, most of the carriers are concentrated in the lower level L1 in the well layer 302 of the light emitting layer 30, which makes it possible to generate a stable polariton state. Become.

  FIG. 12 is a diagram showing a subband level structure (see FIG. 2A) in the bias application state (45 kV / cm) in the active layer 20 shown in FIG. In such a configuration, a part of the carrier becomes a polariton and is injected into the lower polariton level P1 (see FIG. 8) formed at a lower energy position than the upper level L2, and this lower polariton level P1. Light emission can be obtained by the transition from 1 to the lower electron level L1.

  As described above, in the semiconductor light emitting device 1A having the above-described configuration, the light emission transition from the upper level L2 to the lower level L1 is not performed, but the lower polariton level P1 having lower energy than the upper level L2 is changed to the lower level L1. Utilize luminescence transition. The structure for preventing the formation of the inversion distribution as described above is completely different from the structure in a normal quantum cascade laser or the like that realizes laser oscillation by using an emission transition from the upper level L2 to the lower level L1. Yes. Further, as shown in FIG. 9, a contact layer made of a highly doped InGaAs layer is provided in the lowermost layer and the uppermost layer of the active layer 20.

  Next, the design of the plasmon resonator structure in the semiconductor light emitting device 1A using ISB polaritons will be described. In the design of the resonator in the semiconductor light emitting device 1A shown in FIG. 1, the parameters affecting the resonator system are the resonator thickness T in the z-axis direction and the resonator width W in the x-axis direction in the mesa shape ( (resonator length in the x-axis direction).

ここで、ｈ−ｂａｒはプランク定数、ｃは真空中の光速、ｎは活性層の実効的な屈折率、ｋ_ｘは波数ベクトルのｘ成分である。 First, the setting of the resonator width W will be described. In the plasmon resonator structure shown in FIGS. 1 and 6, the light cutoff energy E _c in the resonator due to light confinement in the x-axis direction (width direction) is expressed by the following equations (1) and (1): It is represented by (2).

Here, h-bar is the Planck constant, c is the speed of light in vacuum, n is the effective refractive index of the active layer, and k _x is the x component of the wave vector.

図９では、ΔＥ_２１を１６０ｍｅＶとした構成を例示したが、この数値を式（３）に代入し、また、活性層２０の屈折率をｎ＝３．３とすると、条件Ｅ_ｃ＝ΔＥ_２１が成立するときの共振器幅はＷ_ｃ＝１．１７μｍとなる。このとき、プラズモンの効果によって、共振器幅Ｗの２倍を波長とする定在波として、プラズモン共振器構造中に光を閉じ込めることができる。 Therefore, by appropriately setting the resonator width W in the above formula, an appropriate wave number k _x is created, and the light cut-off energy E _c is matched with the energy interval ΔE ₂₁ between the levels L2 and L1. As described above with reference to FIGS. 7 and 8, an ideal dispersion relation of polaritons can be formed. Assuming that the resonator width when the condition E _c = ΔE ₂₁ is satisfied is W _c , the resonator width W _c is expressed by the following equation (3).

FIG. 9 illustrates a configuration in which ΔE ₂₁ is set to 160 meV. However, if this numerical value is substituted into Equation (3) and the refractive index of the active layer 20 is n = 3.3, the condition E _c = ΔE ₂₁ The width of the resonator when is established is W _c = 1.17 μm. At this time, light can be confined in the plasmon resonator structure as a standing wave having a wavelength of twice the resonator width W by the effect of plasmon.

Next, the setting of the resonator thickness T will be described. In the semiconductor light emitting device 1A of FIG. 1, the energy of light in the resonator is controlled by confining light in the x-axis direction as described above. That is, by adjusting the cutoff energy E _c of the optical confinement of light in the x-axis direction (width direction) of the plasmon resonator structure, whereby efficient production of ISB polariton, realized level structure capable of aggregation To do.

  In the above configuration, if there is light cut-off energy due to light confinement in the z-axis direction (thickness direction), the energy of photons in the resonator becomes too high due to confinement in two directions. The deviation from the transition energy becomes large and the interaction becomes impossible. In order to avoid such a situation, the resonator thickness T may be made sufficiently small with respect to the wavelength of light in the resonator.

  Specifically, as described above, the resonator thickness T is set to be equal to or less than the wavelength of light in the resonator, and more preferably, the resonator thickness T is set to 1 of the wavelength of light in the resonator. By setting it to / 4 or less, a standing wave cannot exist in the z-axis direction. This eliminates the light cut-off energy for confinement in the z-axis direction, and the light energy in the resonator depends only on the wave number in the in-plane direction.

  Such a configuration condition with respect to the resonator thickness T is important from the viewpoint that the interaction between the first and second metal layers 11 and 12 constituting the plasmon resonator structure can be strengthened. That is, if the resonator thickness T is sufficiently small with respect to the wavelength of light in the resonator, an interaction occurs between the metal layers 11 and 12 such as an Au layer constituting the resonator and is generated by the plasmon effect. Further, the electric field oscillating in the z-axis direction can be made stronger. In the case of the above configuration example, the value of the resonator thickness T which is calculated by using the setting wavelength 7.8 μm and the refractive index n of the active layer n = 3.3, which is ¼ of the wavelength of light in the resonator. T = 590 nm.

ここで、σは金属表皮深さであり、例えば金（Ａｕ）の場合、波長〜７．８μｍに対する表皮深さは７．０９ｎｍとなる。 On the other hand, the relationship between the resonator thickness T and the plasmon wavelength λ _p in the resonator is expressed by the following equation (4).

Here, σ is the metal skin depth. For example, in the case of gold (Au), the skin depth for a wavelength of ˜7.8 μm is 7.09 nm.

Equation (4), in accordance with the resonator thickness T becomes smaller, indicating that the plasmon wavelength lambda _p decreases. However, when considering the thickness of the realistic active layer structure in a semiconductor light emitting device 1A, the resonator width W, may be regarded as corresponding to 1/2 of the plasmon wavelength lambda _p. The lower limit of the resonator thickness T may be set to 60 nm, for example. In the above example, when the resonator thickness T is 60 nm, the effective refractive index calculated in the resonator (the value of the denominator in Equation (4)) is 1.2n. In the active layer structure shown in FIG. 9 including the injection layer, the light emitting layer, the extraction layer, and the contact layer, the active layer thickness T is 102.1 nm.

ここで、上記式において、Ｅ_１、Ｅ_２は電子のエネルギー、ｈ−ｂａｒωは光子のエネルギー、ｃ^†、ｃは電子の生成消滅演算子、ａ^†、ａは光子の生成消滅演算子、Ωはラビ振動周波数である。 Next, the allowable range of the resonator width W in the plasmon resonator structure will be described. The dispersion relationship in the interaction state described above can be obtained as the eigenstate of the interaction Hamiltonian represented by the following equations (5) to (7).

In the above equation, E ₁ and E ₂ are electron energies, h-barω is photon energy, c ^† and c are electron production / annihilation operators, a ^† and a are photon production / annihilation operators, Ω Is the Rabi vibration frequency.

この方程式を解き、光子のエネルギーを波数に変換することによって、分散曲線を得ることができる。また、式（６）のHopfield係数α、βは、以下の式

によって求めることができる。 Equation (6) is a state vector representing the ground state of polaritons, and α and β represent Hopfield coefficients for photon and electronic excitation, respectively (see Patent Document 1: Japanese Patent Application Laid-Open No. 2011-35138). Further, when the equation (7) is arranged, the following eigenvalue equation is obtained.

By solving this equation and converting the photon energy into wavenumbers, a dispersion curve can be obtained. In addition, the Hopfield coefficients α and β in the equation (6) are as follows:

Can be obtained.

13 to 15 show the dispersion relation and the Hopfield coefficients α and β when the resonator width W _c = 1.17 μm where the condition E _c = ΔE ₂₁ is satisfied. FIG. 13 is a graph showing a dispersion curve of polaritons in the active layer when the resonator width W _{c is} 1.17 μm. In this graph, the horizontal axis represents the wave number k _y (cm ⁻¹ ) in the y-axis direction, and the vertical axis represents energy E (meV). 13, the photon energy graph G0, the electron energy graph G2, the upper polariton level P2, and the lower polariton level P1 corresponding to the upper level L2 are the same as those in FIG.

FIG. 14 is a graph showing the wave number dependence of the square | α | ² of the absolute value of the Hopfield coefficient α for a photon. FIG. 15 is a graph showing the wave number dependency of the square | β | ² of the absolute value of the Hopfield coefficient β for electrons. In FIGS. 14 and 15, graphs A2 and A4 indicated by solid lines indicate Hopfield coefficient values corresponding to the upper polariton level P2. In addition, graphs A1 and A3 indicated by broken lines indicate Hopfield coefficient values corresponding to the lower polariton level P1, respectively.

In the dispersion relation of FIG. 13, in a region having a large absolute value of the wave number k _y in the y-axis direction, the upper polariton level P2 is the energy of the photon, also lower polariton level P1 is the electron energy, asymptotic respectively ing. Therefore, as shown in FIG. 14, the upper polariton level P2, the region having a larger absolute value of the wave number k _y | α | has a ^two approximately 1, and most exist as photons, whereas the lower In the polariton level P1, | α | ^{2 is} almost 0 in the same wavenumber region, and has almost no component as a photon. This relationship is the same for | β | ² shown in FIG.

On the other hand, the illustrated FIG. ^{14,15 | α | 2, | β} | 2 together, in the vicinity of the wave number _{k y} in the y-axis direction is 0 takes a value close to 0.5 to each other. This indicates that the wave number k _y interaction at zero is the strongest, the photon system and contribution of the electronic system are both the same level. In such a situation, since the property as a polariton appears strongly, the generated polariton can be aggregated at the minimum point of the energy of the lower polariton level P1.

Also, in the wave number region that is greatly deviated from the resonance point at which the interaction is the strongest, the lower polariton level P1 loses the property of the photon and approaches the property of the electron as described above, and the aggregation of the polariton to the same energy state is caused. It becomes impossible. Considering this point, the square of the absolute value of the Hopfield coefficient of the photon system or the electron system for the lower polariton level P1 and the upper polariton level P2 at the wave number k _y = 0 in the _y -axis direction | α | ² , | Β | ² is preferably at least 0.2 and at most 0.8 with 0.5 as the center.

When the resonator width W is increased, the energy of the photons in the resonator decreases, and the resonance point between the photon system and the electron system moves from the position of wave number k _y = 0 to a region with a large k _y . For this reason, the minimum point of the energy of the lower polariton level P1 at the wave number k _y = 0 is away from the resonance point. At this time, when the upper limit of the resonator width W for making the value of the Hopfield coefficient within the above range is obtained by calculation, the resonator width with respect to the resonator width W _c when the condition E _c = ΔE ₂₁ is satisfied. If the increased 15% is the upper limit, therefore, the resonator width W is preferably set within 1.15 × W _c below the range of W _c.

Specifically, for example, in the above-described example where W _c = 1.17 μm, 1.15 × W _c = 1.35 μm. At this time, the energy of the photon in the resonator is 145 meV. The dispersion relation and the Hopfield coefficients α and β at this time are shown in FIGS. FIG. 16 is a graph showing a dispersion curve of polaritons in the active layer when the resonator width W = 1.15 × W _c = 1.35 μm.

FIG. 17 is a graph showing the wave number dependency of the square | α | ² of the absolute value of the Hopfield coefficient α with respect to photons. FIG. 18 is a graph showing the wave number dependence of the square | β | ² of the absolute value of the Hopfield coefficient β for electrons. In FIGS. 17 and 18, graphs B2 and B4 indicated by solid lines indicate Hopfield coefficient values corresponding to the upper polariton level P2. Further, graphs B1 and B3 indicated by broken lines indicate Hopfield coefficient values corresponding to the lower polariton level P1, respectively.

In the semiconductor light emitting device 1A, if the resonator width W of the plasmon resonator structure is smaller than the resonator width W _c where the condition E _c = ΔE ₂₁ is satisfied, the photon system and the electron system do not cross each other. , You will not be able to obtain large splitting energy due to anti-crossing. Therefore, the lower limit of the resonator width W, the resonator width W _c described above.

  Here, regarding the specific active layer structure in the semiconductor light emitting element 1A using ISB polaritons, the level structure in the active layer, and the like, various configurations other than the above configuration examples can be used. For example, the active layer may be configured to have a cascade structure in which a plurality of injection layers and light emitting layers are alternately stacked. Even with such a configuration, a light emitting operation using ISB polaritons in the active layer can be suitably realized. For the active layer having such a configuration, for example, a quantum cascade laser can be referred to.

  As for the level structure in the light-emitting layer, the light-emitting layer has a second upper level, which is an energy level higher than the upper level in the subband level structure, and is generated in the second upper level. The second upper level is an energy level that is higher than the minimum point of the lower polariton level by the energy of longitudinal optic (LO) phonons. It is good also as a structure which is a rank. According to such a configuration, polaritons can be efficiently supplied to the lower polariton levels related to light emission via the second upper level.

  In this case, the energy interval between the second upper level and the (first) upper level is preferably set smaller than the energy of the longitudinal optical phonon. Thereby, fast relaxation of electrons from the second upper level to the first upper level is prohibited, and the supply of polaritons to the lower polariton level can be preferably realized. Hereinafter, other configuration examples such as an active layer and a level structure in the active layer in the semiconductor light emitting device 1A will be described.

  FIG. 19 is a diagram illustrating a second example of the semiconductor stacked structure of the active layer 20 and the subband level structure in the active layer 20 of the semiconductor light emitting device 1A illustrated in FIG. Here, a subband level structure in a state where an operating voltage is applied to the semiconductor light emitting element 1A (bias applied state) is shown.

  The active layer 20 in this configuration example includes a light emitting layer 30 similar to that in FIG. 2, an injection layer 31 preceding the light emitting layer 30, an extraction layer 35 subsequent to the light emitting layer 30, and contact layers 38 and 39. In addition, a second injection layer 33 including an extraction barrier layer 331 and a second light emission layer 32 including an injection barrier layer 321 are further provided between the (first) light emission layer 30 and the extraction layer 35. Configured. That is, the active layer 20 shown in FIG. 19 has a cascade structure in which injection layers 31 and 33 and light emitting layers 30 and 32 are alternately stacked, and an extraction layer 35 is provided at the final stage. It has become.

  FIG. 20 is a chart showing a second example of the semiconductor stacked structure in the active layer 20 of the semiconductor light emitting device 1A, and shows an example corresponding to the configuration shown in FIG. The active layer thickness T in this configuration example is 144 nm, and similarly to the first example, the resonator thickness T = ¼ nm, which is ¼ of the wavelength of light in the resonator. The resonator width W may be 1.17 μm as in the first example. According to such a configuration, the emission intensity can be increased by forming the active layer 20 in a cascade structure (QC structure).

  FIG. 21 is a diagram showing a third example of the semiconductor stacked structure of the active layer 20 and the subband level structure in the active layer 20 of the semiconductor light emitting device 1A shown in FIG. Here, a subband level structure in a state where an operating voltage is applied to the semiconductor light emitting element 1A (bias applied state) is shown.

The active layer 20 in the present configuration example includes a light emitting layer 30, an injection layer 31 preceding the light emitting layer 30, an extraction layer 35 subsequent to the light emitting layer 30, and contact layers 38 and 39. . The light emitting layer 30 has (first) upper level L2, lower level L1, and second upper level L3, which is an energy level higher than upper level L2, in the subband level structure. As described later, the polariton generated at the second upper level L3 is injected into the lower polariton level P1. The second upper level L3 is an energy level that is higher by the LO phonon energy than the minimum point of the lower polariton level P1. The energy interval ΔE ₃₂ between the second upper level L3 and the first upper level L2 is preferably set smaller than the energy of the LO phonon.

  FIG. 22 is a chart showing a third example of the semiconductor stacked structure in the active layer 20 of the semiconductor light emitting device 1A, and shows an example corresponding to the configuration shown in FIG. FIG. 23 is a diagram showing a subband level structure in a bias application state (70 kV / cm) in the active layer 20 shown in FIG. In the present configuration example, the light emitting layer 30 includes an injection barrier layer 301, a well layer 304, a barrier layer 305, and a well layer 306 in order from the injection layer 31 side. L3 is formed, and the first upper level L2 and the lower level L1 are formed in the well layer 306.

FIG. 24 is a graph showing a polariton dispersion curve in the active layer 20 of this configuration example of the semiconductor light emitting device 1A. In this graph, the horizontal axis represents the wave number k _y in the y-axis direction, and the vertical axis represents the energy E. In FIG. 24, a graph G0 shows the wave number dependence of photon energy. Further, the graph G2, shows a wave number dependency of electron energy corresponding to the first upper level L2 of energy E _2. A graph G3 shows the wavenumber dependence of electron energy corresponding to the second upper level L3 of energy E _3. The horizontal axis of the graph corresponds to the energy E ₁ of the lower level L1.

  In FIG. 24, P1 represents a lower polariton level in a polariton level structure formed due to a subband electron level structure composed of levels L3 to L1, and P2 represents a first upper polariton level. P3 represents the second upper polariton level.

  In the ISB transition in which the upper level and the lower level have a dispersion relationship parallel to the wave vector, many dark polaritons are generated by electron-electron scattering that do not contribute to light emission because they are not coupled with photons. In ISB, such dark polaritons are more present than bright polaritons that combine with photons, which may be a factor that impedes the realization of a highly efficient light-emitting element.

Therefore, as shown in FIG. 24, the second upper level L3 is prepared at a position higher by the LO phonon energy E _LO than the minimum point of the lower polariton level P1. As a result, the dark polariton existing in the vicinity of the level L3 generated by electron-electron scattering can be relaxed and aggregated at high speed into the bright polariton of the lower polariton level P1 by LO phonon scattering. For example, when the quantum well structure is composed of InGaAs / InAlAs, the energy of LO phonon is E _LO = 34 meV.

In this case, it is preferable to design the subband level structure so that electrons from the previous injection layer 31 are injected into the newly prepared second upper level L3. At this time, as described above, the energy interval ΔE ₃₂ between the levels L3 and L2 is set to be smaller than the energy E _LO of the LO phonon, and the high-speed transition due to the LO phonon scattering from the level L3 to the level L2 It is important to prohibit Thereby, the dark polariton generated in the vicinity of the level L3 can be efficiently relaxed to the minimum point of the energy of the lower polariton level P1, and the polaritons can be aggregated with high efficiency. Note that Patent Document 1 can be referred to for a configuration that relaxes dark polariton to bright polariton via LO phonons. By introducing an active layer having such a structure into the plasmon resonator structure, polaritons that interact strongly can be aggregated more efficiently.

  FIG. 25 is a perspective view schematically showing another example of the element structure in the semiconductor light emitting element. The semiconductor light emitting device 1B shown in FIG. 25 has the same basic configuration as the semiconductor light emitting device 1A shown in FIG. 1, but has a single active layer as a mesa type active layer structure provided on the substrate 10. Instead of the element 20, it differs from the element 1A in that three active layers 20a, 20b, 20c having the same configuration are arranged side by side in the x-axis direction. In addition, first metal layers 11a, 11b, and 11c are provided on the active layers 20a, 20b, and 20c, respectively. The second metal layer 12 is provided as a common metal layer for the three active layers. According to such a configuration, the light emission luminance as a whole of the semiconductor light emitting element 1B can be increased. In the configuration shown in FIG. 25, a plurality of active layer structures having the same structure are periodically arranged in the x-axis direction, but may be arranged aperiodically.

  The semiconductor light emitting device according to the present invention is not limited to the above-described embodiments and configuration examples, and various modifications are possible. For example, in the above configuration example, an InP substrate is used as the semiconductor substrate and the active layer is configured by InGaAs / InAlAs. However, the above semiconductor stacked structure, level structure, and light emitting operation can be realized. Specifically, various configurations may be used.

  As such a semiconductor material system, various material systems such as GaAs / AlGaAs, InAs / AlSb, GaN / AlGaN, and SiGe / Si can be used in addition to the above InGaAs / InAlAs. Various methods may be used for the semiconductor crystal growth method. Further, the first and second metal layers constituting the plasmon resonator structure are also exemplified by the Au layer in the above-described configuration example. However, for such a metal layer, a metal material other than Au may be used. good.

  INDUSTRIAL APPLICABILITY The present invention can be used as a semiconductor light emitting device capable of realizing ISB polariton generation, aggregation, and light emission operation thereby with high efficiency.

DESCRIPTION OF SYMBOLS 1A, 1B ... Semiconductor light emitting element, 10 ... Board | substrate, 11, 11a-11c ... 1st metal layer, 12 ... 2nd metal layer, 13 ... Lower metal electrode layer, 15 ... Voltage source, 20, 20a-20c ... Active layer , 21, 22 ... side surfaces in the width direction, 23, 24 ... end faces in the length direction,
30, 32 ... Quantum well light emitting layer, 31, 33 ... Injection layer, 35 ... Extraction layer, 38, 39 ... Contact layer, 301, 321 ... Injection barrier layer, 305 ... Quantum barrier layer, 302, 304, 306 ... Quantum well Layers, 331, 351 ... extraction barrier layers,
L1 ... lower level, L2 ... upper level, L3 ... second upper level, Li ... injection level, Le ... extraction level, P1 ... lower polariton level, P2 ... upper polariton level, P3 ... second Upper polariton level.

Claims (8)

An active layer having a light emitting layer including at least one quantum well layer, and an injection layer provided in a preceding stage of the light emitting layer and used for injecting electrons into the light emitting layer;
A first metal layer provided on one side in the semiconductor stacking direction with respect to the active layer;
A second metal layer provided on the other side of the semiconductor lamination direction with respect to the active layer and forming a plasmon resonator structure together with the first metal layer;
The light emitting layer has at least an upper level and a lower level in its subband level structure, and a polariton including an upper polariton level and a lower polariton level due to the subband level structure. A level structure is formed, and light is generated by the transition from the lower polariton level to the lower level,
In the plasmon resonator structure, the resonator thickness T in the semiconductor stacking direction is set to be equal to or less than the wavelength of light in the resonator, and the resonator width W is set to the upper level and the lower level. and energy intervals, and characterized in that the resonator width when the light cut-off energy in the resonator is matched as W _c, is set within the range of W _c or 1.15 × W _c A semiconductor light emitting device.   2. The semiconductor light emitting element according to claim 1, wherein in the plasmon resonator structure, the resonator thickness T is set within a range of 60 nm or more and 1/4 or less of a wavelength of light in the resonator. . The light emitting layer includes an injection barrier layer formed closest to the injection layer, and an extraction layer including an extraction barrier layer formed closest to the light emitting layer is provided at a subsequent stage of the light emitting layer,
3. The semiconductor light emitting device according to claim 1, wherein a thickness of the extraction barrier layer is set in a range of 70% to 150% with respect to a thickness of the injection barrier layer. The injection layer may include one any of claims 1 to 3, characterized in that it is doped to the carrier density is in the range of 3 × ¹⁰ 17 ^{cm -3} or more 1 × ¹⁰ 19 ^{cm -3} or less The semiconductor light-emitting device according to item.   The state according to claim 1, wherein the lower level in the light emitting layer is an energy level that is lower by 5 meV or more than the ground level in the injection layer when no operating voltage is applied. The semiconductor light emitting element according to any one of claims.   6. The semiconductor light emitting device according to claim 1, wherein the active layer has a cascade structure in which a plurality of the injection layers and the light emitting layers are alternately stacked. The light emitting layer has a second upper level which is an energy level higher than the upper level in the subband level structure, and polariton generated at the second upper level is converted to the lower polariton level. Configured to inject
7. The semiconductor light emitting device according to claim 1, wherein the second upper level is an energy level higher than a minimum point of the lower polariton level by an energy of a longitudinal optical phonon. element.   8. The semiconductor light emitting device according to claim 7, wherein an energy interval between the second upper level and the upper level is set to be smaller than energy of a longitudinal optical phonon.

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