WO2010082405A1 - Laser à cascade quantique - Google Patents

Laser à cascade quantique Download PDF

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Publication number
WO2010082405A1
WO2010082405A1 PCT/JP2009/070181 JP2009070181W WO2010082405A1 WO 2010082405 A1 WO2010082405 A1 WO 2010082405A1 JP 2009070181 W JP2009070181 W JP 2009070181W WO 2010082405 A1 WO2010082405 A1 WO 2010082405A1
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layer
quantum
level
quantum well
emission
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PCT/JP2009/070181
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Japanese (ja)
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和上 藤田
忠孝 枝村
直大 秋草
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浜松ホトニクス株式会社
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/3401—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers
    • H01S5/3402—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers intersubband lasers, e.g. transitions within the conduction or valence bands
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals

Definitions

  • the present invention relates to a quantum cascade laser using intersubband transition in a quantum well structure.
  • the light in the mid-infrared wavelength region (for example, wavelength 5 to 30 ⁇ m) is an important wavelength region in the spectroscopic analysis field.
  • quantum cascade lasers (QCL: Quantum Cascade Laser) have attracted attention as high-performance semiconductor light sources in such a wavelength region (for example, Patent Documents 1 to 5 and Non-Patent Documents regarding quantum cascade lasers). 1 to 4).
  • Quantum cascade lasers are monopolar laser elements that use a level structure with subbands formed in a semiconductor quantum well structure and generate light by electronic transition between subbands.
  • High-efficiency and high-power operation can be realized by cascading the quantum well light-emitting layers serving as active regions in multiple stages.
  • the cascade coupling of the quantum well light-emitting layers is realized by alternately stacking the quantum well light-emitting layers and the injection layers using an electron injection layer for injecting electrons into the emission upper level.
  • Non-patent document 1 M. Beck et al., Science Vol. 295 (2002) pp. 301-305. Thereafter, room temperature CW operation was also achieved by the group of M. Razeghi et al. At an oscillation wavelength of 6 ⁇ m and 4.8 ⁇ m (Non-Patent Document 2: J. S. Yu et al., Appl. Phys. Lett. Vol. .83 (2003) pp. 2503-2505)
  • Non-Patent Document 3 A. Evans et al., Appl. Phys. Lett. Vol. 85 (2004) pp. 2166-2168).
  • Patent Document 1 US Pat. No. 5,457,709
  • an active layer including a triple quantum well light emitting layer is used, and an injection barrier immediately between the electron injection layer and the quantum well light emitting layer is provided.
  • a relaxation level (E1) that is lower by the energy of polar longitudinal optical (LO) phonons is provided as an energy level lower than the emission lower level (E2),
  • the carriers accumulated in the extracted level are thermally redistributed, which causes a deterioration in the temperature characteristics of the element.
  • the laser element described in Patent Document 2 US Pat. No. 5,745,516 uses a transition between minibands by a superlattice.
  • an inversion distribution can be easily formed by high-speed relaxation of carriers in the miniband in the lower emission level.
  • the carrier in the miniband of the lower emission level is used. The lifetime is estimated to be about 0.1 ps.
  • Patent Document 3 US Pat. No. 6,751,244, corresponding Japanese publication: JP-T-2004-521481
  • BTC Greenhouse Cipheral Component
  • Patent Document 4 US Pat. No. 6,922,427, corresponding Japanese publication: JP 2004-507903 A
  • quantum cascade lasers of these structures it cannot be said that sufficient performance is obtained under operating conditions such as room temperature.
  • the present invention has been made to solve the above problems, and provides a quantum cascade laser capable of efficiently forming an inversion distribution in a quantum well light-emitting layer and improving laser operating performance. For the purpose.
  • the quantum cascade laser according to the present invention includes (1) a semiconductor substrate, and (2) a unit laminate body provided on the semiconductor substrate and including a quantum well light emitting layer and an injection layer in multiple stages. And an active layer having a cascade structure in which quantum well light-emitting layers and injection layers are alternately stacked, and (3) each of the plurality of unit stacks included in the active layer includes
  • the band level structure has a light emission upper level, a light emission lower level, and a relaxation miniband consisting of energy levels lower than the light emission lower level and functioning as a relaxation level.
  • the energy width of the relaxation miniband Is set to be larger than the energy of the longitudinal optical phonon.
  • the quantum well light-emitting layer includes n first (n is an integer of 5 or more) quantum barrier layers and n first (n) -th quantum well layers in order from the previous unit stacked body side.
  • the first quantum barrier layer functions as an injection barrier layer from the previous injection layer to the quantum well light-emitting layer, and the layer thickness of the third quantum well layer is the same as that of the second quantum well layer. It is set within a range of 90% to 105% with respect to the layer thickness.
  • a relaxation miniband consisting of low energy levels is also provided in the subband level structure in the unit stack including the quantum well light emitting layer and the injection layer.
  • a relaxation miniband consisting of low energy levels is also provided in the subband level structure.
  • the subband level structure is configured so that the energy interval between the emission lower level and the relaxation miniband corresponds to the energy of the longitudinal optical phonon (LO phonon).
  • the emission transition between the upper and lower emission levels is a transition between subbands.
  • the light emission gain can be concentrated.
  • the use of minibands for relaxation of electrons that have undergone intersubband transition facilitates the design of electron relaxation structures from the lower emission level and stabilizes characteristics during laser device manufacturing. And improvement in yield can be realized.
  • the subband level structure as described above can be controlled by designing the quantum well structure in the unit laminate structure constituting the active layer.
  • the quantum well light-emitting layer is constituted by five or more quantum barrier layers and quantum well layers
  • the first barrier layer is used as an injection barrier layer
  • the layer thickness of the third well layer is set to be the second well layer. It is set within the range of 90% to 105% with respect to the layer thickness. This makes it possible to increase the dipole moment indicating the intensity of the transition by spreading the wave functions of the upper and lower emission levels throughout the emission layer, thereby improving the efficiency of emission transitions in the quantum well emission layer. It becomes.
  • a quantum cascade laser in which the inversion distribution in the light emitting layer is efficiently formed and the intensity of the light emission transition is improved is realized.
  • Such a configuration is effective in realizing, for example, a low threshold and high output laser operation at an operating temperature of room temperature or higher.
  • the energy level is lower than the lower level.
  • a relaxation miniband having an energy width larger than that of LO phonon energy is provided, and electrons having undergone intersubband transition are extracted at a high speed from the lower level by LO phonon scattering and relaxation within the miniband, and five layers
  • a light emitting layer is constituted by the above well layer and barrier layer, the first barrier layer is an injection barrier layer, and the layer thickness of the third well layer is 90% or more and 105% or less with respect to the layer thickness of the second well layer.
  • FIG. 1 is a diagram schematically showing a basic configuration of a quantum cascade laser.
  • FIG. 2 is a diagram showing a subband level structure in the active layer of the quantum cascade laser shown in FIG.
  • FIG. 3 is a diagram illustrating an example of the configuration of the quantum cascade laser.
  • FIG. 4 is a diagram showing an example of the configuration of the unit laminate structure constituting the active layer.
  • FIG. 5 is a chart showing an example of the structure of a unit laminate for one cycle in the active layer.
  • FIG. 6 is a graph showing the correlation between the thickness of the fifth barrier layer and the ratio of the dipole moment in the fifth well layer.
  • FIG. 7 is a graph showing a change in the layer thickness of the quantum well layer in the light emitting layer.
  • FIG. 8 is a graph showing the correlation between the thickness of the fifth well layer and the energy interval ⁇ E h .
  • FIG. 9 is a graph showing the evaluation results of the characteristics of the quantum cascade laser.
  • FIG. 1 is a diagram schematically showing a basic configuration of a quantum cascade laser according to the present invention.
  • the quantum cascade laser 1A of the present embodiment is a monopolar type laser element that generates light by utilizing electronic transition between subbands in a semiconductor quantum well structure.
  • This quantum cascade laser 1 ⁇ / b> A includes a semiconductor substrate 10 and an active layer 15 formed on the semiconductor substrate 10.
  • a mirror surface (not shown) constituting an optical resonator is formed on two predetermined opposing surfaces of the side surface of the quantum cascade laser 1A.
  • the active layer 15 has a cascade structure in which quantum well light-emitting layers used for light generation and electron injection layers used for injection of electrons into the light-emitting layers are alternately stacked in multiple stages.
  • a semiconductor multilayer structure composed of a quantum well light emitting layer and an injection layer is used as a unit laminated body 16 for one period, and the unit laminated body 16 is laminated in multiple stages, whereby an active layer 15 having a cascade structure is formed. It is configured.
  • the number of stacked unit stacked bodies 16 including the quantum well light emitting layer and the injection layer is appropriately set, and is about several hundreds, for example.
  • the active layer 15 is formed directly on the semiconductor substrate 10 or via another semiconductor layer.
  • FIG. 2 is a diagram showing a subband level structure in the active layer of the quantum cascade laser shown in FIG.
  • each of the plurality of unit laminated bodies 16 included in the active layer 15 is constituted by a quantum well light emitting layer 17 and an injection layer 18.
  • the quantum well light emitting layer 17 and the injection layer 18 are formed to have a predetermined quantum well structure including a quantum well layer and a quantum barrier layer, respectively.
  • the subband level structure which is an energy level structure by a quantum well structure is formed.
  • the light-emitting layer 17 includes n (n is an integer of 5 or more) quantum barrier layers of the first to n-th quantum barrier layers in order from the injection layer 18a side of the previous unit stack, and the first to n-th quantum barrier layers.
  • the nth quantum well layer has n quantum well layers, and the barrier layers and the well layers are alternately stacked.
  • the first quantum barrier layer is an injection barrier layer 171 for electrons injected from the injection layer 18 a in the previous stage to the light emitting layer 17.
  • the injection layer 18 includes m (m is an integer) quantum barrier layers of the first to mth quantum barrier layers and m quantum well layers of the first to mth quantum well layers in order from the light emitting layer 17 side.
  • the barrier layer and the well layer are alternately stacked.
  • the first quantum barrier layer is an extraction barrier layer 191 for electrons from the light emitting layer 17 to the injection layer 18.
  • the unit laminate 16 of the active layer 15 in the present embodiment has a light emission upper level L up and a light emission lower level L related to light emission due to intersubband transition in the subband level structure.
  • it has a relaxation miniband MB composed of a level that is lower than the emission lower level L low and functions as a relaxation level.
  • the miniband MB is set so that the energy difference between the emission lower level L low and the miniband MB becomes the energy E LO of the longitudinal optical (LO) phonon.
  • the mini-band MB is set so that its energy width ⁇ E MB is larger than the energy of LO phonon ( ⁇ E MB > E LO ).
  • E LO is about 34MeV.
  • the layer thickness of the third quantum well layer is set to be approximately the same as the layer thickness of the second quantum well layer. It has become. Specifically, the layer thickness of the third quantum well layer is set within a range of 90% to 105% with respect to the layer thickness of the second quantum well layer. As a result, the center of the wave function can be set near the center of the light emitting layer, and the wave functions of the light emitting upper level L up and the lower level L low are sufficiently spread over the entire light emitting layer 17.
  • the electrons that have transitioned to the lower emission level L low are relaxed at a high speed to the relaxation miniband MB by LO phonon scattering, and are further relaxed at a high speed within the miniband MB.
  • laser oscillation occurs between the emission upper level L up and the lower level L low by extracting electrons from the emission lower level L low at high speed via LO phonon scattering and relaxation in the miniband.
  • the inversion distribution for realizing is formed with high efficiency. Further, in the above configuration, since ⁇ E MB > E LO , fast relaxation of electrons occurs via LO phonon scattering even in the miniband MB.
  • the relaxation miniband MB has a band structure in which the miniband in the quantum well light emitting layer 17 and the miniband in the injection layer 18 are combined.
  • the electrons relaxed from the emission lower level L low to the relaxation miniband MB are emitted from the miniband MB via the extraction barrier layer 191 and the injection layer 18 and emitted from the subsequent emission layer 17b. Cascade injection to the upper level L up .
  • cascade light generation occurs in the active layer 15. That is, by stacking a large number of quantum well light-emitting layers 17 and injection layers 18, electrons move one after another in the cascade 16 in a cascade manner, and light h ⁇ at the time of transition between subbands in each laminate 16. Is generated. Further, such light is resonated in the optical resonator of the laser 1A, so that laser light having a predetermined wavelength is generated.
  • the emission upper level L up related to light emission and in addition to the emission lower level L low, there is provided a relaxation miniband MB consisting of lower energy level than the level L low emission lower.
  • the subband level structure is configured such that the energy interval between the lower emission level L low and the relaxation miniband MB corresponds to the LO phonon energy E LO .
  • the energy width ⁇ E MB of the relaxation miniband MB is set larger than the energy E LO of the LO phonon.
  • the emission transition between the emission upper level L up and the lower level L low is the mini-band ⁇ . It is not a transition between minibands or a transition between subbands and minibands, but a transition between subbands and subbands. Thereby, the gain of light emission in the light emission transition can be concentrated.
  • the use of the miniband MB for relaxation of electrons that have undergone intersubband transition facilitates the design of an electron relaxation structure from the emission lower level L low, and at the time of manufacturing a laser device. It is possible to stabilize the characteristics and improve the yield (see Patent Document 5).
  • the subband level structure as described above can be controlled by designing the quantum well structure in the unit laminate structure 16 constituting the active layer 15. Furthermore, in the above configuration, the quantum well light-emitting layer 17 is constituted by five or more quantum barrier layers and quantum well layers, the first barrier layer 171 is an injection barrier layer, and the thickness of the third well layer is It is set within the range of 90% to 105% with respect to the thickness of the two well layers.
  • the wave functions of the emission upper level L up and the lower level L low are spread over the entire light emitting layer 17.
  • the dipole moment indicating the intensity of the luminescence transition is determined by the spatial integration of the upper level and lower level wave functions. Therefore, the wave functions of the upper level and the lower level are spread over the entire light emitting layer 17, thereby increasing the dipole moment and improving the efficiency of light emission transition in the light emitting layer 17.
  • the quantum cascade laser 1A in which the inversion distribution in the light emitting layer 17 is efficiently formed and the intensity of the light emission transition is improved is realized.
  • Such a configuration is effective in realizing, for example, a low threshold and high output laser operation at an operating temperature of room temperature or higher.
  • the layer thicknesses of the fourth quantum well layer and the fifth quantum well layer in the quantum well light emitting layer 17 are respectively the second quantum well layers. It is preferably set within a range of 70% to 100% with respect to the thickness of the well layer. Thereby, the subband level structure in the unit laminated body 16 can be formed suitably.
  • the thicknesses of the second quantum barrier layer, the third quantum barrier layer, and the fourth quantum barrier layer are preferably set within a range of 2 to 5 atomic layers, respectively. As described above, by setting the thicknesses of the second to fourth barrier layers to be thin, the wave functions of the light emission upper level L up and the lower level L low spread over the entire light emitting layer 17 are sufficiently strongly coupled. Can do.
  • the layer thickness of the fifth quantum barrier layer is larger than any one of the second quantum barrier layer, the third quantum barrier layer, and the fourth quantum barrier layer, and in the injection layer 18.
  • the thickness is preferably set smaller than the thickness of the extraction barrier layer 191 from the light emitting layer 17 to the injection layer 18.
  • the wave functions of the emission upper level L up and the lower level L low can be distributed with a sufficient size to the fifth well layer. .
  • the quantum well structure of the light emitting layer 17 will be specifically described later.
  • the energy interval Delta] E h from the emission upper level L up to level L h is It is preferably set so as to be larger than the energy E LO of LO phonon ( ⁇ E h > E LO ).
  • one subband in the miniband MB (the subband with the highest energy in the miniband MB) is shifted from the other subband to the higher energy side by the LO phonon energy E LO.
  • the separated level can be a lower level L low .
  • a level structure including the emission lower level L low and the relaxation miniband MB separated from the lower level by the LO phonon energy E LO can be suitably realized.
  • an extraction barrier layer 191 for electrons from the light emitting layer 17 to the injection layer 18 is preferably provided between the quantum well light emitting layer 17 and the injection layer 18.
  • the relaxation miniband MB preferably has a band structure in which the miniband in the quantum well light emitting layer 17 and the miniband in the injection layer 18 are combined.
  • the tunnel time of electrons from the quantum well light emitting layer 17 to the injection layer 18 can be shortened, and the lower level. It is possible to prevent effective extraction of electrons from L low at a high speed.
  • FIG. 3 is a diagram illustrating an example of a specific configuration of the quantum cascade laser.
  • FIG. 4 is a diagram showing an example of the configuration of the unit laminate structure constituting the active layer in the quantum cascade laser shown in FIG.
  • FIG. 4 shows the quantum well structure and the subband level structure of a part of the multi-stage repetitive structure of the light emitting layer 17 and the injection layer 18 constituting the active layer 15.
  • 3 and 4 can be formed by crystal growth by, for example, molecular beam epitaxy (MBE) or metal organic vapor phase epitaxy (MOVPE).
  • MBE molecular beam epitaxy
  • MOVPE metal organic vapor phase epitaxy
  • the n-type InP single crystal substrate 50 is used as the semiconductor substrate 10 in the semiconductor stacked structure of the quantum cascade laser 1B shown in FIG. Then, on this InP substrate 50, in order from the substrate side, an InGaAs lower core layer 51 having a thickness of 300 nm, an active layer 15 in which unit laminated bodies 16 are stacked in multiple stages, an InGaAs upper core layer 52 having a thickness of 300 nm, An element structure of the quantum cascade laser 1B is formed by sequentially laminating an InP clad layer 53 having a thickness of 3.5 ⁇ m and an InGaAs contact layer 54 having a thickness of 10 nm.
  • the active layer 15 in this configuration example is configured by laminating unit laminated bodies 16 including the quantum well light emitting layer 17 and the electron injection layer 18 in 33 periods.
  • the unit stacked body 16 for one cycle includes 11 quantum well layers 161 to 165 and 181 to 186, and 11 quantum barrier layers 171 to 175 and 191 to 196 alternately. It is configured as a stacked quantum well structure. Specifically, these quantum well layers / quantum barrier layers are composed of In 0.53 Ga 0.47 As / In 0.52 Al 0.48 As lattice-matched with the InP substrate.
  • a laminated portion composed of the well layers 161 to 165 and the barrier layers 171 to 175 is a portion that functions as the light emitting layer 17.
  • a stacked portion composed of the well layers 181 to 186 and the barrier layers 191 to 196 is a portion that functions as the injection layer 18.
  • the first barrier layer 171 of the light emitting layer 17 is an injection barrier layer for electrons from the injection layer 18 a to the light emitting layer 17.
  • the first barrier layer 191 of the injection layer 18 is an extraction barrier layer for electrons from the light emitting layer 17 to the injection layer 18.
  • FIG. 5 shows an example of a specific structure of the unit laminate body 16 for one period in the active layer 15.
  • the subband level structure of the unit laminate structure 16 shown in FIG. 4 has 12 levels contributing to laser operation, and a plurality of levels correspond to the relaxation miniband MB. .
  • the level structure shown in FIG. 4 has a light emission upper level L up , a light emission lower level L low , and a relaxation miniband MB including a plurality of levels.
  • the lower level L low is separated from the miniband MB to the high energy side by about 34 meV corresponding to the energy of the LO phonon.
  • the level structure of this configuration example is designed so that the wave functions of the emission upper level L up and the lower level L low are sufficiently spread over the entire light emitting layer 17.
  • the energy interval ⁇ E h with respect to the level L h and the spread of the wave function of each level in the light emitting layer 17 should be designed by the combination of the thicknesses of the well layer and the barrier layer constituting the light emitting layer 17. Is possible.
  • a specific quantum well structure in the light emitting layer 17 and the injection layer 18 will be described together with a design method thereof.
  • the oscillation wavelength is 9 ⁇ m and the operating electric field is 45 kV / cm. This operating electric field is set based on the expected film thickness and voltage drop per cycle in the design of the quantum well structure.
  • the configuration of the quantum well light emitting layer 17 will be described.
  • the light emitting layer 17 is formed by alternately stacking first to fifth quantum barrier layers 171 to 175 and first to fifth quantum well layers 161 to 165 in order from the previous injection layer 18a side.
  • the first barrier layer 171 has a slightly thick layer thickness of 3.7 nm and serves as an injection barrier layer from the injection layer 18a to the light emitting layer 17.
  • the subband level structure in the light emitting layer 17 is determined by the layer thicknesses of the second to fifth barrier layers 172 to 175 and the first to fifth well layers 161 to 165 and the above-described operating electric field. Note that the layer thickness of each of these semiconductor layers cannot be determined independently because the wave function of each level is sensitive to the influence of the barrier layer and the well layer. Thus, the layer thickness of each layer is designed.
  • the energy width ⁇ E MB of the relaxation miniband MB is larger than the energy of the LO phonon, and the wave functions of the emission upper level L up and the lower level L low are spread over the entire emission layer 17. . Therefore, the layer thicknesses of the barrier layers 172 to 175, particularly the second barrier layer 172, the third barrier layer 173, and the fourth barrier layer 174 are as thin as 0.8 nm, 0.7 nm, and 0.8 nm, respectively. Is set. As described above, by reducing the thickness of the second to fourth barrier layers 172 to 174, the levels are sufficiently strongly coupled in the light emitting layer 17, and the energy width ⁇ E MB of the miniband MB is increased. Become.
  • the thicknesses of the second to fourth barrier layers 172 to 174 are preferably set within the range of 2 to 5 atomic layers, respectively, and within the range of 2 to 4 atomic layers, or 2 to 3 atomic layers. More preferably, it is set.
  • the layer thickness of the fifth barrier layer 175 is set to be larger than any of the second to fourth barrier layers 172 to 174 and smaller than the layer thickness of the extraction barrier layer 191 in the injection layer 18. It is preferable that the second, third and fourth barrier layers ⁇ the fifth barrier layer ⁇ the extraction barrier layer). In the present configuration example, the layer thickness of the fifth barrier layer 175 is set to 1.6 nm with respect to the layer thickness of the second to fourth barrier layers 172 to 174 and the layer thickness of the extraction barrier layer 191 described above. Has been.
  • the reason why the thickness of the fifth barrier layer 175 is set to be slightly thick in this way is to appropriately set the level interval in the miniband. That is, if the fifth barrier layer 175 is too thin, the coupling between levels in the miniband becomes strong and the level interval becomes too wide, so that the degree of freedom in laser manufacturing is reduced. On the other hand, if the fifth barrier layer 175 is too thick, the coupling between levels in the miniband becomes weak, and the electron transport efficiency is lowered.
  • FIG. 6 is a graph showing the correlation between the thickness of the fifth barrier layer 175 and the ratio of the dipole moment in the fifth well layer 165.
  • the horizontal axis represents the thickness (nm) of the fifth barrier layer
  • the vertical axis represents the ratio (%) of the dipole moment in the fifth well layer to the dipole moment in the entire light emitting layer 17.
  • the ratio of the dipole moment in the fifth well layer 165 is about 10%.
  • the wave functions of the light emission upper level and the lower level are sufficiently distributed to the fifth well layer 165.
  • a sufficiently large dipole moment of about 20% is obtained in the well layer 165.
  • the energy interval between the emission upper level and the lower level corresponding to the emission wavelength in the light emitting layer 17 and the spread of the wave function of those levels are combined with the setting of the layer thicknesses of the barrier layers 171 to 175 described above. It is determined by the layer thickness of the well layers 161-165.
  • the first well layer 161 on the most upstream injection layer 18a side is a relatively thin layer having a layer thickness of 1.7 nm. This has the effect of improving the injection efficiency of electrons from the injection layer 18a in the previous stage to the light emitting layer 17.
  • the thickness of the second to fifth well layers 162 to 165 determines the weight of the wave function in each well layer, and basically the layers from the second well layer to the fifth well layer. It is set so that the thickness decreases monotonously. In such a configuration, the wave functions of the upper and lower light emission levels are gradually attenuated from the second well layer 162 toward the fifth well layer 165. Further, by setting the layer thickness of each well layer in this way, the energy interval ⁇ E h between the emission upper level and the level above it can be sufficiently increased as described above. This energy interval ⁇ E h greatly affects laser characteristics particularly at high temperatures.
  • FIG. 7 is a graph showing a change in the layer thickness of the quantum well layer in the light emitting layer 17.
  • the horizontal axis indicates the number (1 to 5) of the quantum well layers in the light emitting layer
  • the vertical axis indicates the layer thickness ratio of each well layer when the layer thickness of the second well layer 162 is 1. Show. As shown in the graph of FIG. 7, in the configuration shown in FIGS. 4 and 5, the layer thickness ratio of the second to fifth well layers 162 to 165 is 1: 1: 0.93: 0.75. Yes.
  • the second and third well layers 162 and 163 determine the light emission wavelength in the subband level structure, and the light emission upper level.
  • the thicknesses of the second and third well layers 162 and 163 need to be approximately the same.
  • the layer thicknesses of the well layers 162 and 163 can be set so that one of them becomes larger by several percent. In consideration of such conditions, the layer thickness of these well layers is set such that the layer thickness of the third well layer 163 is within a range of 90% or more and 105% or less with respect to the layer thickness of the second well layer 162. It is preferable to set within a range of 95% to 105%.
  • the second, third, and third well layers 164 and 165 are set in order to set the energy interval ⁇ E h between the emission upper level and the level above it sufficiently large as described above. It is preferable that the fourth well layer 164, the fifth well layer 165, and the thickness of the well layers 162, 163 are sequentially reduced with respect to the thickness of the well layers 162, 163.
  • FIG. 8 is a graph showing the correlation between the layer thickness of the fifth well layer 165 and the energy interval ⁇ E h .
  • the horizontal axis indicates the layer thickness (nm) of the fifth well layer
  • the vertical axis indicates the energy interval ⁇ E h (meV) from the emission upper level to the level on the higher energy side than the emission upper level. Is shown.
  • the energy interval ⁇ E h is 34 meV, and the LO phonon It is about the same as the energy ELO .
  • the energy interval ⁇ E h is sufficiently large as about 53 meV. can do.
  • the layer thickness of the fifth well layer 165 is 100% or less, more preferably 95% or less with respect to the layer thickness of the second well layer 162 in order to keep the energy interval ⁇ E h larger than E LO. It is preferable that it is set within the range of. Also, the layer thickness of the fourth well layer 164 greatly affects the energy interval ⁇ E h , so that the layer thickness of the second well layer 162 is equal to or less than 100%, similarly to the layer thickness of the fifth well layer 165. More preferably, it is set within the range of 95% or less.
  • the thicknesses of the fourth well layer 164 and the fifth well layer 165 are preferably set within a range of 70% or more with respect to the layer thickness of the second well layer 162, respectively.
  • the injection layer 18 has a configuration in which first to sixth quantum barrier layers 191 to 196 and first to sixth quantum well layers 181 to 186 are alternately stacked in this order from the light emitting layer 17 side.
  • the first barrier layer 191 is an extraction barrier layer from the light emitting layer 17 to the injection layer 18. If the extraction barrier layer 191 is too thick, the flow of electrons from the light emitting layer 17 to the injection layer 18 is impaired, but if it is too thin, the wave function in the injection layer 18 is strongly coupled to the wave function in the light emitting layer 17. Therefore, it is preferable to set the thickness of the barrier layer 191 in consideration of these conditions.
  • a funnel injector (see Japanese Patent Laid-Open No. 10-4242) is used so that the energy width of the miniband MB becomes narrower as the light emitting layer of the next period approaches.
  • the efficiency of electron injection into the next emission upper level is increased.
  • Such a level structure of the injection layer 18 is realized by decreasing the thickness of the well layer and increasing the thickness of the barrier layer from the light emitting layer 17 side toward the light emitting layer side of the next period. be able to.
  • the layer thickness of the well layer and the barrier layer on the light emitting layer 17 side (extraction barrier layer 191 side) of the same period is such that all of the level electrons existing in the light emitting layer 17 are in the injection layer 18. It is preferably designed to be transportable to a miniband.
  • the layer thickness of the well layer and the barrier layer on the light emitting layer side (injection barrier layer side) in the next period is such that the energy width of the miniband is sufficiently narrowed, and the electrons from the injection layer 18 emit the emission upper level L up. It is preferable to design so that it is not injected into the level L h above it.
  • the thicknesses of the first to sixth barrier layers 191 to 196 are 2.0 nm, 1.6 nm, 1.8 nm, and 2. It is set to 1 nm, 2.7 nm, and 3.2 nm.
  • the layer thicknesses of the first to sixth well layers 181 to 186 are set to 3.4 nm, 3.1 nm, 3.0 nm, 2.9 nm, 3.0 nm, and 2.8 nm, respectively.
  • the layer thickness of the first barrier layer 171 of the light emitting layer 17 that becomes an injection barrier layer from the injection layer 18 to the light emitting layer of the next period is set.
  • the thickness of the barrier layer 171 is set to 3.7 nm as described above.
  • the injection barrier layer 171 determines the strength of coupling in each cycle of the unit stacked body 16 stacked in a plurality of cycles and the maximum current that can be supplied to the laser.
  • the strength of coupling of the wave functions is determined by the anti-crossing gap.
  • the anti-crossing gap is set to 9 meV, and the design is such that a sufficiently large current can be transported.
  • FIG. 9 is a graph showing the evaluation results of the characteristics of the quantum cascade laser manufactured based on the above-described embodiment, in which the horizontal axis indicates current (A) and the vertical axis indicates laser output (W). .
  • This evaluation result shows the current-output characteristics of the laser device during room temperature pulse operation.
  • the maximum output of the conventional quantum cascade laser was about 1.6 W, but by adopting the above structure, a very high maximum output of about 4 W was obtained. I understand.
  • the quantum cascade laser according to the present invention is not limited to the above-described embodiments and configuration examples, and various modifications are possible.
  • an InP substrate is used as the semiconductor substrate and the active layer is InGaAs / InAlAs.
  • emission transition between subbands in a quantum well structure is possible, and the above subbands are used.
  • various structures may be used as long as the level structure can be realized.
  • 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.
  • the quantum cascade laser may be configured to include a semiconductor substrate and the active layer having the above-described configuration provided on the semiconductor substrate.
  • the number of barrier layers and well layers that constitute the quantum well light-emitting layer is five in the above configuration example, but may be six or more.
  • the configuration in which lattice matching is performed with respect to the InP substrate has been described.
  • a configuration in which lattice mismatch is introduced into the InP substrate may be used. In this case, it is possible to increase the degree of freedom in device design, to efficiently confine carriers, and to shorten the oscillation wavelength.
  • a semiconductor substrate and (2) a unit stacked body provided on the semiconductor substrate and including a quantum well light-emitting layer and an injection layer are stacked in multiple stages.
  • the energy width of the relaxation miniband is that of the longitudinal optical phonon.
  • the first quantum barrier layer functions as an injection barrier layer from the previous injection layer to the quantum well light-emitting layer, and the layer thickness of the third quantum well layer is smaller than that of the second quantum well layer.
  • a configuration set within a range of 90% to 105% is used.
  • the thicknesses of the fourth quantum well layer and the fifth quantum well layer are respectively the same as those of the second quantum well layer. It is preferably set within a range of 70% to 100% with respect to the layer thickness. Thereby, the subband level structure in a unit laminated body can be formed suitably.
  • the thicknesses of the second quantum barrier layer, the third quantum barrier layer, and the fourth quantum barrier layer are each set in the range of 2 to 5 atomic layers. As described above, by setting the thicknesses of the second to fourth barrier layers to be thin, the wave functions of the light emission upper level and the lower level spread over the entire light emitting layer as described above can be coupled sufficiently strongly. it can.
  • the layer thickness of the fifth quantum barrier layer is larger than any of the second quantum barrier layer, the third quantum barrier layer, and the fourth quantum barrier layer, and in the injection layer
  • the thickness is preferably set smaller than the thickness of the extraction barrier layer from the quantum well light emitting layer to the injection layer.
  • the energy interval from the emission upper level to the level on the higher energy side than the emission upper level is set to be larger than the energy of the longitudinal optical phonon. Is preferred. Thereby, out of the electrons injected from the injection layer in the previous stage to the emission upper level, leakage of the electrons to a level having a higher energy than the emission upper level can be suppressed.
  • the relaxation miniband preferably has a band structure in which a miniband in the quantum well light-emitting layer and a miniband in the injection layer are combined.
  • the present invention can be used as a quantum cascade laser capable of efficiently forming an inversion distribution in the quantum well light emitting layer and improving the laser operation performance.
  • SYMBOLS 1A, 1B Quantum cascade laser, 10 ... Semiconductor substrate, 15 ... Active layer, 16 ... Unit laminated body, 17 ... Quantum well light emitting layer, 18 ... Injection layer, 50 ... InP substrate, 51 ... InGaAs lower core layer, 52 ... InGaAs upper core layer, 53... InP clad layer, 54... InGaAs contact layer, L up ... Emission upper level, L low .

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Abstract

La présente invention porte sur un laser à cascade quantique, qui est configuré à partir d'un substrat semi-conducteur et d'une couche active dans laquelle des corps stratifiés unitaires (16), chacun formé d'une couche électroluminescente (17) et d'une couche d'injection (18), sont empilés en de multiples étages. La structure de niveaux de sous-bande du corps stratifié unitaire (16) a un niveau supérieur Lup d'émission, un niveau inférieur Llow d'émission et une mini-bande de relaxation MB, et de la lumière est produite par transition entre sous-bandes d'électrons du niveau supérieur au niveau inférieur. Les électrons ayant subi une transition d'émission sont relâchés du niveau inférieur Llow vers la mini-bande MB par diffusion de phonon LO et injectés dans la couche électroluminescente d'étage suivant à travers la mini-bande MB. Une première couche barrière de la couche électroluminescente (17) fonctionne en tant que couche barrière d'injection, et l'épaisseur d'une troisième couche de puits est réglée dans la plage de 90 % à 105 % de l'épaisseur d'une seconde couche de puits. Par conséquent, l'invention concerne un laser à cascade quantique permettant une amélioration de la performance laser en provoquant efficacement une inversion de population dans la couche électroluminescente.
PCT/JP2009/070181 2009-01-19 2009-12-01 Laser à cascade quantique WO2010082405A1 (fr)

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US8325774B2 (en) * 2010-08-12 2012-12-04 Wisconsin Alumni Research Foundation High power, high efficiency quantum cascade lasers with reduced electron leakage
US9548590B2 (en) * 2011-11-29 2017-01-17 Thorlabs Quantum Electronics, Inc. Quantum cascade laser design with stepped well active region

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WO2013061655A1 (fr) * 2011-10-28 2013-05-02 浜松ホトニクス株式会社 Laser à cascade quantique
JP2013098251A (ja) * 2011-10-28 2013-05-20 Hamamatsu Photonics Kk 量子カスケードレーザ
US8958450B2 (en) 2011-10-28 2015-02-17 Hamamatsu Photonics K.K. Quantum cascade laser

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