JP2010165994A - Quantum cascade laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum cascade layer capable of enhancing the operational performance of the laser by efficiently forming inverted distribution in a light-emitting layer. <P>SOLUTION: The quantum cascade laser is composed of: a semiconductor substrate; and an active layer configured by laminating multiple stages of unit laminates 16 each of which includes a light-emitting layer 17 and an injection layer 18. A subband level structure of the unit laminate 16 includes an upper emission level L<SB>up</SB>, a lower emission level L<SB>low</SB>and relaxation miniband MB and light is generated by transition of electrons from the upper level to the lower level, between subbands. Further, electrons transited among the subbands are relaxed from the lower level L<SB>low</SB>to the miniband MB by LO phonon scattering and injected into the subsequent light-emitting layer through the miniband MB. A first barrier layer of the light-emitting layer 17 functions as an injection barrier layer, and the layer thickness of a third well layer is set within a range of 90 to 105% of the layer thickness of a second well layer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a quantum cascade laser using intersubband transition in a quantum well structure.

  Light in the mid-infrared wavelength region (for example, wavelength 5 to 30 μm) is an important wavelength region in the spectroscopic analysis field. In recent years, 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). 1-4).

  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.

US Pat. No. 5,457,709 US Pat. No. 5,745,516 US Pat. No. 6,751,244 US Pat. No. 6,922,427 JP 2008-177366 A

M. Beck et al., "Continuous Wave Operation of a Mid-Infrared Semiconductor Laser at RoomTemperature", Science Vol.295 (2002) pp.301-305 J. S. Yu et al., "High-Power Continuous-Wave Operation of a 6μm Quantum-Cascade Laser atRoom Temperature", Appl. Phys. Lett. Vol.83 (2003) pp.2503-2505 A. Evans et al., "Continuous-Wave Operation of λ to 4.8μm Quantum-Cascade Lasers at RoomTemperature", Appl. Phys. Lett. Vol.85 (2004) pp.2166-2168 A. Tredicucci et al., "High Performance Interminiband Quantum Cascade Lasers with Graded Superlattices", Appl. Phys. Lett. Vol.73 (1998) pp.2101-2103

  With regard to the quantum cascade laser described above, the element operating temperature was limited to an extremely low temperature at the time of successful laser oscillation, but in 2002, room temperature CW operation at an oscillation wavelength of 9.1 μm was achieved by M. Beck et al. (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 oscillation wavelengths of 6 μm and 4.8 μm (Non-Patent Document 2: JS 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).

  In order to realize the CW operation of the quantum cascade laser under an operating condition at a temperature higher than room temperature, it is necessary to improve the heat dissipation of the element and lower the threshold of the laser operation by forming an efficient inversion distribution. As described above, in the light emission operation by the electronic transition between the subbands of the active layer, in order to efficiently form the inversion distribution between the light emission upper level and the light emission lower level, Efficient electron injection and suppression of carrier distribution at the lower emission level (shortening of carrier lifetime) are important.

  For example, in the laser device described in 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. The structure in which a thin quantum well layer is provided next to the structure improves the efficiency of electron injection into the emission upper level (E3) (η = 0.87).

Further, in the laser element of Patent Document 1, 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), By extracting electrons from the lower emission level at high speed via LO phonon scattering, the carrier lifetime at the lower emission level (τ _E2 = about 0.4 ps) is realized. However, in this structure, the tunnel time from the quantum well light-emitting layer to the injection layer is relatively long as τ _esc = 2 to 3 ps, and the extraction of electrons at high speed due to LO phonon scattering is effectively limited. In this case, the carriers accumulated in the extracted level are thermally redistributed, which causes a deterioration in the temperature characteristics of the element.

  On the other hand, the laser element described in Patent Document 2: US Pat. No. 5,745,516 uses a transition between minibands by a superlattice. In such a structure, an inversion distribution can be easily formed by high-speed relaxation of carriers in the miniband in the lower emission level. For example, in the structure of A. Tredicucci et al. (Non-Patent Document 4: A. Tredicucci etal., Appl. Phys. Lett. Vol. 73 (1998) pp. 2101-2103), the carrier lifetime in the mini-band of the lower emission level. Is estimated to be about 0.1 ps. However, since this structure uses transitions between minibands, there are a number of levels that contribute to light emission, and there is a problem that the light emission gain is dispersed and the half-value width is widened. There are also problems such as an increase in layer thickness per cycle in the active layer and a low efficiency of injection of electrons into the emission upper level (η = 0.76).

  In addition, to solve these problems, as the structure of the active layer, a double phonon resonance structure (Patent Document 3: US Pat. No. 6,751,244, corresponding Japanese publication: JP-A-2004-521814), and BTC (Bound to Continuum) ) Structure (Patent Document 4: US Pat. No. 6,922,427, corresponding Japanese publication: JP-T-2004-507903) has been proposed. However, even with 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.

  In order to achieve such an object, 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. (4) Light is generated by intersubband transition of electrons from the upper emission level to the lower emission level in the quantum well emission layer. Electrons that have undergone inter-transition are relaxed from the lower emission level to the relaxation miniband by longitudinal optical phonon scattering, and are injected from the injection layer into the quantum well emission layer of the subsequent unit stack through the relaxation miniband, The quantum well light-emitting layer includes n quantum barrier layers (n is an integer greater than or equal to 5) of the first to nth quantum barrier layers and n pieces of the first to nth quantum well layers in order from the unit stacked body side in the previous stage. The first quantum barrier layer has a quantum well layer, and 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 layer of the second quantum well layer It is set within a range of 90% to 105% with respect to the thickness.

  In the quantum cascade laser described above, in the subband level structure in the unit stack including the quantum well light emitting layer and the injection layer, in addition to the light emitting upper level and the light emitting lower level related to light emission, A relaxation miniband consisting of low energy levels is also provided. 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).

  In such a configuration, electrons that have undergone an emission transition between subbands in the light emitting layer are extracted at a high speed from the lower emission level through LO phonon scattering and relaxation in the miniband. Therefore, it is possible to improve the laser operation performance by realizing efficient inversion distribution formation in the light emitting layer and lowering the laser operation threshold. In the above configuration, the energy width of the relaxation miniband is set larger than the energy of the LO phonon. Thereby, it is possible to improve the efficiency of relaxation of electrons in the miniband and the formation of the inversion distribution in the light emitting layer.

  In the above configuration using LO phonon scattering to extract electrons from the lower emission level to the relaxation miniband, the emission transition between the upper and lower emission levels is a transition between subbands. The light emission gain can be concentrated. In addition, the use of minibands for relaxation of electrons that have undergone intersubband transitions facilitates the design of electron relaxation structures from the lower emission level and stabilizes the 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. Further, in the above configuration, 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, and 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. As described above, according to the above configuration, 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.

  Here, for the well layers other than the second and third well layers in the light emitting layer, in the quantum well light emitting layer, 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.

  In the quantum well light-emitting layer, the layer 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, it is possible to couple the wave functions of the light emission upper level and the lower level spread over the entire light emitting layer sufficiently strongly as described above. it can.

  In the quantum well light-emitting layer, 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. As described above, by appropriately setting the layer thickness of the fifth barrier layer, the wave functions of the emission upper level and the lower level can be distributed with a sufficient size up to the fifth well layer.

  Also, in the sub-band level structure of the unit stack, 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. Thereby, the tunneling time of electrons from the quantum well light emitting layer to the injection layer can be shortened, and it is possible to prevent the electron extraction at a high speed from the emission lower level from being effectively limited.

  According to the quantum cascade laser of the present invention, in the subband level structure in the unit laminate structure constituting the active layer, in addition to the emission upper level and the lower level, 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. By setting within this range, it is possible to achieve efficient inversion distribution formation in the light emitting layer and improvement in the intensity of the light emission transition.

It is a figure which shows schematically the basic composition of a quantum cascade laser. It is a figure shown about the subband level structure in the active layer of the quantum cascade laser shown in FIG. It is a figure which shows an example of a structure of a quantum cascade laser. It is a figure which shows an example of a structure of the unit laminated body which comprises an active layer. It is a graph which shows an example of the structure of the unit laminated body for 1 period in an active layer. It is a graph which shows the correlation with the layer thickness of a 5th barrier layer, and the ratio of the dipole moment in a 5th well layer. It is a graph which shows the change of the layer thickness of the quantum well layer in a light emitting layer. And the layer thickness of the fifth well layer is a graph showing the correlation between the energy interval Delta] E _h. It is a graph which shows the evaluation result of the characteristic of a quantum cascade laser.

  Hereinafter, preferred embodiments of the quantum cascade laser 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 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. In addition, a mirror surface (not shown) constituting an optical resonator is formed on two predetermined opposing surfaces of the side surfaces 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 layer are alternately stacked in multiple stages. Specifically, 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. As shown in FIG. 2, each of the plurality of unit laminated bodies 16 included in the active layer 15 is configured 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. Thereby, in the unit laminated body 16, the subband level structure which is an energy level structure by a quantum well structure is formed.

  Specifically, 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 stacked body, The n-th quantum well layer includes n quantum well layers, and the barrier layers and the well layers are alternately stacked. Of these semiconductor layers, the first quantum barrier layer is an injection barrier layer 171 for electrons injected from the injection layer 18 a in the previous stage into the light emitting layer 17. The injection layer 18 has 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. Among these semiconductor layers, the first quantum barrier layer is an extraction barrier layer 191 for electrons from the light emitting layer 17 to the injection layer 18.

As shown in FIG. 2, 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 by intersubband transition in the subband level structure. _In addition to _low , 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 miniband MB is set such that its energy width ΔE _MB is larger than the energy of LO phonon (ΔE _MB > E _LO ). Note that the energy of LO phonons, for example in the case of using an InGaAs as semiconductor _{material, E LO} is about 34MeV.

Furthermore, in the unit stacked body 16 of this embodiment, in the semiconductor stacked structure in the quantum well light emitting layer 17, 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 throughout the light emitting layer 17.

In such a subband level structure, electrons e ⁻ from the miniband MB in the previous injection layer 18 a are injected into the emission upper level L _{up of the} quantum well light emitting layer 17 through the injection barrier layer 171. The The electrons injected into the light emission upper level L _{up undergo a} light emission transition to the light emission lower level L _low , which corresponds to an energy difference between the subband levels of the upper level L _up and the lower level L _low. Light hν having a wavelength is generated and emitted. Further, as described above, the wave functions of the levels L _up and L _low are spread over the entire light emitting layer 17, so that the dipole moment indicating the intensity of the light emission transition is increased, and the efficiency of the light emission transition is improved. .

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 further relaxed at a high speed within the miniband MB. Thus, 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 through LO phonon scattering and relaxation in the miniband. The inversion distribution for realizing is formed with high efficiency. In the above configuration, since ΔE _MB > E _LO , fast relaxation of electrons occurs via LO phonon scattering even in the miniband MB.

In the example of the level structure shown in FIG. 2, 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. In such a configuration, the electrons relaxed from the emission lower level L _low to the relaxation mini-band MB are emitted from the mini-band MB via the extraction barrier layer 191 and the injection layer 18 and emitted from the subsequent emission layer 17 b. Cascade injection into the upper level L _up .

  By repeating such electron injection, light emission transition, and relaxation in the plurality of unit laminated bodies 16 constituting the active layer 15, 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 hν 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 effect of the quantum cascade laser 1A according to the present embodiment will be described.

In the quantum cascade laser 1A shown in FIG. 1 and FIG. 2, in the subband level structure in the unit stacked body 16 composed of the quantum well light emitting layer 17 and the injection layer 18, 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 so that the energy interval between the emission lower level L _low and the relaxation miniband MB corresponds to the LO phonon energy E _LO .

In such a configuration, electrons that have undergone an emission transition between subbands in the light emitting layer 17 emit light through LO phonon scattering from the emission lower level L _low to the miniband MB, and relaxation in the miniband MB. It will be pulled out from the lower level L _{low at} high speed. Therefore, an efficient inversion distribution can be formed in the quantum well light-emitting layer 17 and a laser operation threshold value can be reduced thereby, and a laser element with high temperature, CW, and high output operation with improved laser operation performance can be realized. It can be realized.

In the above configuration, the energy width ΔE _MB of the relaxation miniband MB is set larger than the energy E _{LO of the} LO phonon. Thereby, it is possible to improve the efficiency of relaxation of electrons in the miniband MB and the formation of the inversion distribution in the light emitting layer 17 thereby. That is, in such a configuration, electrons relaxed in the miniband MB from the lower emission level L _low are efficiently relaxed to the ground level L _g in the injection layer 18 by relaxation in the miniband MB. Is done. At this time, as described above, by setting the energy width of the mini-band MB sufficiently larger than the energy of the LO phonon, the mini-band MB can be relaxed through LO phonon scattering, so that it is more efficient. Easy electron relaxation.

Further, in the above configuration in which LO phonon scattering is used for extraction of electrons from the emission lower level L _low to the relaxation miniband MB, the emission transition between the emission upper level L _up and the lower level L _low is a miniband − 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. In addition, the use of the mini-band 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 stacked body 16 constituting the active layer 15. Furthermore, in the above configuration, the quantum well light-emitting layer 17 is configured 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.

In such a quantum well structure, the wave functions of the 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. As described above, according to the above configuration, 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.

  Here, for the well layers other than the second and third quantum well layers in the light emitting layer 17, 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.

Moreover, in the light emitting layer 17, it is preferable that the layer thickness of a 2nd quantum barrier layer, a 3rd quantum barrier layer, and a 4th quantum barrier layer is each set in the range of 2-5 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 L _up and the lower level L _low spread over the entire light emitting layer 17 are sufficiently coupled. Can do.

In the light emitting layer 17, 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. As described above, by appropriately setting the layer thickness of the fifth barrier layer, the wave functions of the emission upper level L _up and the lower level L _low can be distributed in a sufficiently large size up to the fifth well layer. . The quantum well structure of the light emitting layer 17 will be specifically described later.

Further, as shown in FIG. 2, when there is a level L _h to a higher energy side than the emission upper level L _up 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 the} LO phonon (ΔE _h > E _LO ). As a result, out of the electrons injected from the upstream injection layer 18a into the emission upper level L _up , leakage of electrons to the level L _{h having a} higher energy than the emission upper level can be suppressed. . Further, in such a configuration, even if there are electrons that have entered the level L _h , such electrons are immediately relaxed to the emission upper level L _{up due} to LO phonon scattering. High injection efficiency of electrons only into L _up can be realized.

For the lower emission level L _low , 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. Can be used, and the separated level can be a lower level L _low . Thus, 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.

In the unit laminate body 16, 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. Thereby, it is possible to suppress the seepage of the wave function of electrons from the injection layer 18 to the light emitting layer 17 and to improve the efficiency of light emission transition in the light emitting layer 17. That is, by suppressing the leakage of the electron wave function in this way, the optical transition contributing to laser oscillation is reliably performed between the upper level L _up and the lower level L _low subbands, and the lower level. Carriers from L _low are relaxed into the miniband MB by one-stage LO phonon scattering.

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. Thus, by strongly coupling the miniband of the light emitting layer 17 and the miniband of the injection layer 18, 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.

  The configuration of the quantum cascade laser according to the present invention will be further described together with a specific example of an element structure including a quantum well structure in an active layer. 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.

  In the quantum well structure of the active layer 15 in the present embodiment, an example is shown in which the oscillation wavelength is 9 μm (energy 140 meV) and the operating electric field is 45 kV / cm. 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).

  In the semiconductor stacked structure of the quantum cascade laser 1 </ b> B shown in FIG. 3, an n-type InP single crystal substrate 50 is used as the semiconductor substrate 10. 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 μm 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. In addition, as shown in FIG. 4, the unit stacked body 16 for one period 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.

  Further, in the stacked structure of the unit stacked body 16 shown in FIG. 4, a stacked portion including the well layers 161 to 165 and the barrier layers 171 to 175 is a portion that functions as the light emitting layer 17. In addition, a stacked portion including 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. Similarly, 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.

In such a configuration, 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. . Specifically, 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. In this configuration example, 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 LO phonon. Further, the miniband MB is a miniband in the light emitting layer 17, with a mini-band having a band structures attached at the injection layer 18, the energy width is greater than the energy E _LO of LO phonon Delta] E _MB = It is set to 60 meV.

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. For the level L _h on the higher energy side than the emission upper level L _up, the energy interval from the upper level L _up to the level L _h is larger than the LO phonon energy and is set to ΔE _h = 55 meV. Has been.

The energy interval between the emission upper level L _up and the lower level L _low , the energy interval between the lower level L _low and the miniband MB, the energy width ΔE _MB of the miniband _MB , the upper level L _up and the level The energy interval ΔE _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 respective layer thicknesses of the well layer and the barrier layer constituting the light emitting layer 17. Is possible. Hereinafter, 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. In this configuration example, as described above, the oscillation wavelength is 9 μm 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.

  First, the configuration of the quantum well light emitting layer 17 will be described. In the configuration example shown in FIG. 4, 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 18 a side. Configured. Among the barrier layers 171 to 175, the first barrier layer 171 has a thickness of 3.7 nm, which is 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 operating electric field described above. 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.

In this configuration example, the energy width ΔE _MB of the relaxation mini-band MB is larger than the LO phonon energy, 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. Thus, by reducing the layer 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 ΔE _MB of the miniband _MB is increased. Become. In general, the thicknesses of the second to fourth barrier layers 172 to 174 are preferably set within a range of 2 to 5 atomic layers, respectively, and within a range of 2 to 4 atomic layers or 2 to 3 atomic layers. More preferably, it is set.

  The thickness of the fifth barrier layer 175 is set to be thicker 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 this configuration example, the 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 of 2.0 nm. 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.

  In addition, when the fifth barrier layer 175 is thick, in addition to the reduction in the electron transport efficiency described above, the wave functions of the light emission upper level and the lower level do not exist in the fifth well layer 165, so that the intensity of the light emission transition is increased. The dipole moment which shows is also reduced. 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. In this graph, the horizontal axis represents the thickness (nm) of the fifth barrier layer, and 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.

  As shown in the graph of FIG. 6, for example, when the thickness of the fifth barrier layer 175 is set to 2.0 nm, which is the same as that of the extraction barrier layer 191, the ratio of the dipole moment in the fifth well layer 165 is about 10%. On the other hand, in the above embodiment in which the thickness of the fifth barrier layer 175 is 1.6 nm, the wave functions of the light emission upper level and the lower level are sufficiently distributed to the fifth well layer 165. Thus, 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 thickness of the barrier layers 171 to 175 described above. It is determined by the layer thickness of the well layers 161-165. Of the first to fifth well layers 161 to 165, the first well layer 161 on the most upstream side of the injection layer 18a 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 ΔE _h between the emission upper level and the level above it can be sufficiently increased as described above. This energy interval ΔE _h greatly affects laser characteristics particularly at high temperatures.

  In this configuration example, the thicknesses of the second well layer 162, the third well layer 163, the fourth well layer 164, and the fifth well layer 165 are 5.6 nm, 5.6 nm, 5.2 nm, and 4.2 nm, respectively. Is set to Here, FIG. 7 is a graph showing a change in the layer thickness of the quantum well layer in the light emitting layer 17. In this graph, the horizontal axis indicates the number (1 to 5) of the quantum well layers in the light emitting layer, and 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.

  In the configuration of the second to fifth well layers 162 to 165 in the light emitting layer 17, the second and third well layers 162 and 163 determine the emission wavelength in the subband level structure and the upper emission level. In order to distribute the wave function of the lower level to the entire light emitting layer 17, the thicknesses of the second and third well layers 162 and 163 need to be approximately the same. However, 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%.

Next, with respect to the fourth and fifth well layers 164 and 165, as described above, the second, third, and third well layers 164 and 165 are set in order to set the energy interval ΔE _h between the upper emission level and the upper level sufficiently large. 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 ΔE _h . In this graph, the horizontal axis indicates the layer thickness (nm) of the fifth well layer, and the vertical axis indicates the energy interval ΔE _h (meV) from the emission upper level to the level on the higher energy side than the emission upper level. Is shown.

As shown in the graph of FIG. 8, for example, when the thickness of the fifth well layer 165 is set to 5.2 nm corresponding to 93% of the thickness of the second well layer 162, the energy interval ΔE _h is 34 meV, and the LO phonon The energy _ELO is about the same. On the other hand, in the above embodiment in which the thickness of the fifth well layer 165 is 4.2 nm corresponding to 75% of the thickness of the second well layer 162, the energy interval ΔE _h is sufficiently large as about 53 meV. can do.

Generally, 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 ΔE _h larger than E _LO. It is preferable that it is set within the range of. Further, the layer thickness of the fourth well layer 164 also greatly affects the energy interval ΔE _h , so that the layer thickness of the second well layer 162 is not more 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.

  On the other hand, regarding the lower limit of the layer thickness of the fourth and fifth well layers 164 and 165, when the layer thickness of the well layers 164 and 165 is less than 70% of the layer thickness of the second well layer 162, The wave function is strongly localized. Accordingly, 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.

  Next, the configuration of the electron injection layer 18 will be described. In the configuration example shown in FIG. 4, the injection layer 18 is configured by alternately stacking first to sixth quantum barrier layers 191 to 196 and first to sixth quantum well layers 181 to 186 in order from the light emitting layer 17 side. Has been. Of the barrier layers 191 to 196, 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.

  In the electron injection layer 18 in the present embodiment, a funnel injector (see Japanese Patent Application 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.

Of the injection layer 18, 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. On the other hand, the 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 upper emission level L _up. is injected only, it is preferable to design so as not to be injected into the level L _h thereon.

  As a result of designing the injection layer 18 in consideration of the above conditions, in the present configuration example, the layer 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.

  Finally, 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. In the present configuration example, 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. In this embodiment, 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). . Further, this evaluation result shows the current-output characteristics of the laser element during the room temperature pulse operation. As shown in this graph, 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. For example, in the configuration example described above, an example is shown in which an InP substrate is used as the semiconductor substrate and the active layer is InGaAs / InAlAs. However, emission transition between subbands in a quantum well structure is possible, and the above subbands are used. Specifically, various structures may be used as long as the level structure can be realized. 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, various structures other than the structure shown in FIG. 3 may be used for the semiconductor stacked structure as the entire laser element of the quantum cascade laser. In general, 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. In the above configuration example, the configuration in which lattice matching is performed with respect to the InP substrate has been described. However, for example, 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.

  INDUSTRIAL APPLICABILITY The present invention can be used as a quantum cascade laser capable of efficiently forming an inversion distribution in a quantum well light emitting layer and improving laser operating performance.

DESCRIPTION OF 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 ... Luminescence upper level, L _low .

Claims (6)

A semiconductor substrate;
A cascade structure in which the quantum well light-emitting layers and the injection layers are alternately stacked is formed by stacking unit stacks of quantum well light-emitting layers and injection layers in multiple stages provided on the semiconductor substrate. An active layer,
Each of the plurality of unit laminates included in the active layer is relaxed by a light emission upper level, a light emission lower level, and an energy level lower than the light emission lower level in the subband level structure. A relaxation miniband functioning as a level, the energy width of the relaxation miniband is set larger than the energy of the longitudinal optical phonon,
Light is generated by an intersubband transition of electrons from the upper emission level to the lower emission level in the quantum well emission layer, and the electrons that have undergone the intersubband transition are emitted by longitudinal optical phonon scattering. It is relaxed from the lower level to the relaxation miniband, and is injected from the injection layer to the quantum well light emitting layer of the subsequent unit stack through the relaxation miniband,
The quantum well light emitting layer includes n (n is an integer of 5 or more) quantum barrier layers of the first to nth quantum barrier layers and n pieces of the first to nth quantum well layers in order from the unit stacked body side in the previous stage. 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 thickness of the third quantum well layer is the second quantum well layer. The quantum cascade laser is characterized by being set within a range of 90% or more and 105% or less with respect to the layer thickness.   In the quantum well light emitting layer, the thicknesses of the fourth quantum well layer and the fifth quantum well layer are set within a range of 70% to 100% with respect to the thickness of the second quantum well layer, respectively. The quantum cascade laser according to claim 1.   The thickness of the second quantum barrier layer, the third quantum barrier layer, and the fourth quantum barrier layer in the quantum well light emitting layer is set within a range of 2 to 5 atomic layers, respectively. Item 3. The quantum cascade laser according to Item 1 or 2.   In the quantum well light emitting layer, 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 The quantum cascade laser according to any one of claims 1 to 3, wherein the quantum cascade laser is set to be smaller than a layer thickness of an extraction barrier layer from the quantum well light emitting layer to the injection layer.   In the subband level structure of the unit laminate, an energy interval from the emission upper level to a level on the higher energy side than the emission upper level is set larger than the energy of the longitudinal optical phonon. The quantum cascade laser according to any one of claims 1 to 4, wherein   The quantum according to claim 1, wherein the relaxation miniband has a band structure in which a miniband in the quantum well light emitting layer and a miniband in the injection layer are combined. Cascade laser.

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