JP2011151249A - Quantum cascade laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum cascade laser capable of improving characteristics such as injection efficiency of electrons into a light emission upper level. <P>SOLUTION: This quantum cascade laser is formed by including a semiconductor substrate, and an active layer where unit laminated bodies 16 each composed of a luminescent layer 17 and an injection layer 18 are laminated in multiple tiers. A sub-band level structure of the unit laminated body 16 includes light emission upper level L<SB>up</SB>and lower level L<SB>low</SB>, an injection level L<SB>i</SB>on a high energy side, a relax level L<SB>r</SB>on a low energy side; electrons having passed through light emission transition from the upper level to the lower level are injected into the injection level of the luminescent layer at a rear stage from the injection layer 18 through the relax level, and electrons are supplied from the injection level to the upper level. Energy spacing between the injection level and the upper level is set larger than 10 meV and smaller than energy of an LO phonon, and light emission intensity from the injection level to the lower level is set smaller than the light emission intensity from the upper level to the lower level. <P>COPYRIGHT: (C)2011,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 (QCLs) have attracted attention as high-performance semiconductor light sources in such a wavelength region (see, for example, Patent Documents 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.

JP 2008-60396 A JP 2009-206340 A JP 2008-10733 A US Pat. No. 5,457,709

  In the quantum cascade laser, in order to increase the efficiency of light emission operation due to the transition between subbands, the efficiency of injection and supply of electrons from the level in the electron injection layer in the previous stage to the upper level in the light emission layer is improved. Is required. Here, the conventional quantum cascade laser level structure (see, for example, Patent Document 4) has a problem in that it is difficult to efficiently inject electrons into the emission upper level in the cascade structure in the active layer. .

  On the other hand, in the subband level structure in the unit stack composed of the quantum well light emitting layer and the injection layer, in addition to the upper emission level and the lower emission level, the energy higher than the upper emission level. A configuration of a four-level system in which a relaxation level having a lower energy than the injection level and the lower emission level is proposed (Patent Documents 1 and 2). In this four-level system configuration, electrons from the injection layer are injected directly into the injection level without direct injection of electrons from the previous relaxation level and the injection layer into the emission upper level (Direct Pump). Electrons are supplied from the injection level to the upper emission level by means of Longitudinal Optical (LO) phonon scattering (IndirectPump). However, even in such a configuration, sufficient characteristics may not be obtained with respect to laser element characteristics such as electron injection efficiency into the emission upper level.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a quantum cascade laser capable of improving laser element characteristics such as electron injection efficiency into the emission upper level. And

The inventor of the present application has examined the subband level structure in the quantum cascade laser in detail with respect to the above problem. As a result, the energy interval between the injection level and the emission upper level, which is set higher than the LO phonon energy E _LO in the above-described four-level system, is set smaller than the LO phonon energy E _LO. It has been found that even in the configuration described above, it is possible to improve the laser element characteristics such as the efficiency of electron injection into the emission upper level, and the present invention has been achieved.

  That is, the quantum cascade laser according to the present invention includes (1) a semiconductor substrate, and (2) a quantum well light emitting device that is provided on the semiconductor substrate and is stacked in multiple layers including a quantum well light emitting layer and an injection layer. And an active layer having a cascade structure in which layers and injection layers are alternately stacked, and (3) the unit stacked body has a light emitting upper level in the subband level structure in the quantum well light emitting layer. And (4) quantum well light emission, having an emission lower level, an injection level that is an energy level higher than the emission upper level, and a relaxation level that is an energy level lower than the emission lower level. Light is generated by the intersubband transition of electrons from the upper emission level to the lower emission level in the layer, and the electrons that have undergone the intersubband transition are relaxed from the lower emission level to the relaxation level, and relaxed. Subsequent quantum from the injection layer through the level The electrons are injected into the injection level in the light emitting layer, and electrons are supplied from the injection level to the emission upper level. (5) The energy interval between the injection level and the emission upper level is greater than 10 meV and is a longitudinal optical. (LO) The emission intensity from the injection level to the lower emission level is smaller than the emission intensity from the upper emission level to the lower emission level. And

  In the above quantum cascade laser, in the subband level structure in the unit laminate structure composed of the light emitting layer and the injection layer, the light emitting upper level (level 3) and the lower level (level 2) related to the light emission. In addition, an injection level (level 4) used for injection and supply of electrons with higher energy than the emission upper level, and a relaxation level (level) used for electron relaxation with lower energy than the emission lower level. The structure of a four level system provided with position 1) is used.

  By providing the injection level on the high energy side in this way, it is possible to improve the efficiency of supplying electrons to the emission upper level, and to realize an operation at a high temperature and a high output. In addition, by providing a relaxation level on the low energy side, electrons that have undergone light-emitting transition between subbands in the light-emitting layer are extracted from the lower light emission level due to relaxation to the relaxation level, and are inverted in the light-emitting layer. A distribution can be formed efficiently.

Furthermore, in such a configuration, for the injection level provided on the higher energy side than the emission upper level, specifically, the energy interval ΔE ₄₃ between the injection level and the emission upper level is larger than 10 meV, In addition, the emission intensity is set lower than the LO phonon energy E _LO (10 meV <ΔE ₄₃ <E _LO ), and the emission intensity due to the intersubband transition from the injection level to the emission lower level is reduced from the emission upper level to the emission level. The intensity is lower than the emission intensity to the level. With the above configuration, a quantum cascade laser capable of sufficiently improving the laser element characteristics such as the efficiency of electron injection into the emission upper level and the formation efficiency of the inversion distribution in the light emitting layer is realized. Note that the subband level structure in the unit laminate structure as described above can be controlled by designing the quantum well structure in the unit laminate structure constituting the active layer.

  Here, regarding the intensity of the luminescence transition of electrons to the lower emission level, the emission intensity from the injection level to the lower emission level is 1 / of the emission intensity from the upper emission level to the lower emission level. The strength is preferably 3 or less. Thus, the injection level suitably functions as an electron injection and supply level, and the light emission upper level functions as a light emission transition level, and between the light emission upper level and the lower level. An effective inversion distribution can be formed with high efficiency, and a cascaded light emission operation in the above-described four-level system can be suitably realized.

  Alternatively, the quantum well light-emitting layer includes n (n is an integer of 2 or more) well layers, and the ratio of the wave function of the injection level existing in the active layer other than the first well layer on the most upstream injection layer side Is preferably 30% or less. Thus, by localizing the wave function of the injection level in the first well layer, the injection level becomes the level for injection and supply of electrons, and the emission upper level becomes the level for emission transition. Each of the four level systems is preferably functioned, and an effective inversion distribution is formed between the upper level and the lower level with high efficiency, and the cascaded light emission operation in the above four level system is preferably realized. be able to.

Further, the energy interval ΔE ₄₃ between the injection level and the emission upper level is determined by the condition 10 meV <ΔE ₄₃ <34 meV.
It is preferable to set within a range that satisfies the above. Thereby, it is possible to suitably realize high-speed and high-efficiency supply of electrons from the injection level to the emission upper level.

  As for extraction of electrons from the lower emission level, it is preferable that electrons that have undergone intersubband transition in the unit stack are relaxed from the lower emission level to the relaxation level by longitudinal optical phonon scattering. Thus, in a configuration in which the energy difference between the lower emission level and the relaxation level is set to correspond to the energy of the LO phonon, electrons that have undergone the emission transition in the emission layer emit light via LO phonon scattering. It is pulled out from the lower level at high speed. Accordingly, it is possible to realize efficient inversion distribution formation in the light emitting layer and lower threshold of laser operation.

  Alternatively, the unit laminate body included in the active layer has a relaxation miniband functioning as a relaxation level in the subband level structure, and electrons that have undergone intersubband transition in the unit laminate body are longitudinal optical phonons. A configuration may be used in which the light emission lower level is relaxed to the relaxation miniband by scattering, and injection is performed from the injection layer to the injection level in the subsequent quantum well light emission layer via the relaxation miniband.

  In the above configuration, a relaxation miniband having an energy level lower than the lower emission level is provided in the subband level structure in the unit stack. The level structure is configured so that the energy difference between the emission lower level and the relaxation miniband corresponds to the energy of the LO phonon. In such a configuration, electrons that have undergone an emission transition 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. Accordingly, it is possible to realize efficient inversion distribution formation in the light emitting layer and lower the threshold value of the laser operation, and to particularly improve the laser operation performance. In addition, the use of minibands for electron relaxation facilitates the design of the electron relaxation structure from the lower emission level, stabilizes characteristics during laser device manufacturing, and improves yield. Can be realized.

  According to the quantum cascade laser of the present invention, in the subband level structure in the unit stack of the active layer, in addition to the emission upper level and the lower level, an injection level having a higher energy than the emission upper level, And a four-level system having a relaxation level with lower energy than the lower emission level, and the energy interval between the injection level and the upper emission level is larger than 10 meV and higher than the LO phonon energy. The light emission intensity from the injection level to the lower emission level is set to be smaller than the emission intensity from the upper emission level to the lower emission level. It is possible to sufficiently improve the laser device characteristics such as the electron injection efficiency and the inversion distribution formation efficiency.

It is a figure which shows schematically the basic composition of a quantum cascade laser. It is a figure shown about an example of the subband level structure in the active layer of a quantum cascade laser. It is a figure which shows typically about the supply of the electron from an injection | pouring level to the light emission upper level. It is a figure which shows typically about the leak path in the supply of the electron from an injection level to the light emission upper level. It is a graph which shows the EL spectrum obtained with a quantum cascade laser. It is a graph which shows the current density dependence of a voltage and peak output power. It is a graph which shows the temperature dependence of slope efficiency and threshold current. 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 dependence to the layer thickness of the 1st well layer of the energy interval of an injection | pouring level and the light emission upper level. It is a graph which shows the dependence to the layer thickness of the 1st well layer of the dipole moment of a light emission transition. It is a graph which shows the dependence to the layer thickness of the 2nd barrier layer of the anticrossing of an injection | pouring level and the light emission upper level.

  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.

  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 stacked bodies 16 included in the active layer 15 is configured by a quantum well light emitting layer 17 and an electron injection layer 18. The 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, as will be described later. Thereby, in the unit laminated body 16, the subband level structure which is an energy level structure by a quantum well structure is formed.

In the quantum cascade laser 1A according to the present embodiment, the unit stacked body 16 constituting the active layer 15 has a light emission upper level related to light emission due to intersubband transition, as shown in FIG. and (level _{3) L up,} the emission lower level in addition to the (level _{2) L low,} a higher energy level than the emission upper level _{L up} injection level (the level 4) _{L i} Have. The injection level L _i has an energy interval ΔE ₄₃ between the injection level L _i and the emission upper level L _{up of} greater than 10 meV and smaller than the energy E _{LO of} the longitudinal optical (LO) phonon ( 10 meV <ΔE ₄₃ <E _LO ).

Here, the energy E _{LO of} LO phonon is E _LO = 34 meV, for example, when InGaAs is assumed as the semiconductor material of the quantum well layer in which the subband level structure is formed. Moreover, the injection level L _i is the injection level emission intensity from L _i to the emission lower level L _low is, a smaller intensity than the light emission intensity from emission upper level L _up to the emission lower level L _{low Yes} , the emission transition from the upper emission level L _up to the lower emission level L _low is set to be dominant.

Further, in the subband level structure shown in FIG. 2, a relaxation level (level 1) _Lr is further provided as an energy level lower than the emission lower level L _low . The relaxation level L _r is a level for extracting electrons after the emission transition from the emission lower level L _low , and preferably the energy interval between the emission lower level L _low and the relaxation level L _r is LO. The phonon energy E _LO is set. The injection level L _i is set so as to substantially coincide with a predetermined level in the previous injection layer 18a, for example, the relaxation level L _r .

  Further, in the unit stacked body 16 shown in FIG. 2, an injection barrier for electrons injected from the injection layer 18 a to the light emitting layer 17 between the quantum well light emitting layer 17 and the injection layer 18 a in the previous unit stacked body. (Injection barrier) layer is provided. Further, an extraction barrier layer for electrons from the light emitting layer 17 to the injection layer 18 is provided between the light emitting layer 17 and the injection layer 18. These barrier layers are provided as necessary depending on the specific laminated structure and subband level structure of the active layer 15 including the light emitting layer 17 and the injection layer 18.

In such a subband level structure, electrons e ⁻ from the relaxation level L _{r in the} previous injection layer 18 a are injected into the injection level L _i of the light emitting layer 17 by the resonant tunneling effect through the injection barrier. Is done. Moreover, the injection was injected into the level L _i electrons are supplied at high speed from the injection level L _i in the light-emitting layer 17 to the emission upper level L _{Stay up-.} Furthermore, the electrons supplied to the emission upper level L _{up undergo an} emission transition to the emission lower level L _low , which corresponds to the energy difference between the subband levels of the emission upper level and the lower level. Light hν having a wavelength is generated and emitted.

The electrons that have transitioned to the emission lower level L _low are relaxed to the relaxation level L _r by a relaxation process such as LO phonon scattering. Thus, by using the relaxation level L _r to extract electrons from the emission lower level L _low , inversion for realizing laser oscillation between the upper level L _up and the lower level L _low. A distribution is formed. Further, the electrons relaxed from the emission lower level L _low to the relaxation level L _r are injected from the relaxation level L _r to the injection level L _i in the subsequent light emitting layer 17 b through the extraction barrier and the injection layer 18. Are injected in cascade.

  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 1 </ b> A shown in FIGS. 1 and 2, the upper level L _up and the lower level related to light emission in the subband level structure in the unit stacked body 16 composed of the light emitting layer 17 and the injection layer 18. in addition to the L _low, emission upper than level L _up electrons implanted at a high energy implantation level L _i to be used in feed, used in electronic relaxation at lower energy than the emission lower level L _low relaxation and using the configuration of four-level system provided with a level L _r.

Thus, by providing the injection level L _i on the higher energy side with respect to the emission upper level and the lower level, the efficiency of supplying electrons to the emission upper level L _up can be improved, and the quantum cascade can be improved. It is possible to realize the operation of the laser 1A at high temperature and high output. In addition, by providing the relaxation level L _r on the low energy side, electrons that have undergone an emission transition between subbands in the light emitting layer 17 are extracted from the lower emission level L _{low due} to the relaxation to the relaxation level L _r . As a result, an inversion distribution can be efficiently formed in the light emitting layer 17.

Further, in such a configuration, for the injection level L _i , specifically, the energy interval ΔE ₄₃ between the injection level L _i and the emission upper level L _up is larger than 10 meV, and the LO phonon energy E _LO The emission intensity due to the _intersubband transition from the injection level L _i to the emission lower level L _low is smaller than the emission intensity from the emission upper level L _up to the emission lower level L _low . The structure has a small strength. With the above configuration, the quantum cascade laser 1A capable of sufficiently improving the laser element characteristics such as the electron injection efficiency into the emission upper level L _up and the inversion distribution forming efficiency in the light emitting layer 17 is realized. The

Note that the subband level structure in the unit stacked body 16 as described above can be controlled by designing the quantum well structure in the unit stacked body 16 constituting the active layer 15. In the quantum cascade laser having the above configuration, the energy interval ΔE ₄₃ between the injection level L _i and the emission upper level L _up is set smaller than the energy E _{LO of the} LO phonon. Compared with a configuration in which ₄₃ is made equal to _ELO , the degree of freedom in element design is increased. In addition, the conditions for controlling the film thickness required during crystal growth are alleviated.

Here, the injection for level or intensity of electron emission transition from the emission upper level to the lower level, more specifically, the injection level emission intensity from L _i to the emission lower level L _low is The intensity is preferably 1/3 or less of the emission intensity from the emission upper level L _up to the emission lower level L _low . Accordingly, the injection level L _i preferably functions as a level for electron injection and supply, and at the same time, the light emission upper level L _up functions suitably as a level for light emission transition, and the light emission upper level L _An effective inversion distribution can be formed between the _{up level} and the lower level L _low with high efficiency, and a cascaded light emission operation in the 4-level system can be suitably realized.

The quantum well light emitting layer 17 includes n (n is an integer of 2 or more) well layers, and the injection level L _i existing in the active layer 15 other than the first well layer on the most upstream injection layer 18a side. It is preferable that the ratio of the wave function is 30% or less. As described above, by sufficiently localizing the wave function of the injection level L _i in the first well layer, the injection level L _i preferably functions as a level for electron injection and supply, The light emission upper level L _up is preferably functioned as a level for light emission transition, and an effective inversion distribution is formed between the light emission upper level L _up and the lower level L _low with high efficiency. A cascaded light emission operation in the coordinate system can be suitably realized.

Further, regarding the extraction of electrons from the emission lower level L _low , the electrons that have undergone intersubband transition in the unit laminate 16 change from the emission lower level L _low to the relaxation level L _{r due} to longitudinal optical phonon scattering. It is preferable to be relaxed. As described above, in the configuration in which the energy difference between the emission lower level and the relaxation level is set to correspond to the LO phonon energy E _LO , the electrons that have undergone the emission transition in the emission layer 17 exhibit LO phonon scattering. Via the lower level L _low . Therefore, it is possible to realize efficient inversion distribution formation in the light emitting layer 17 and lowering the threshold value of the laser operation.

Alternatively, the unit laminate 16 of the active layer 15 has a relaxation miniband MB that functions as a relaxation level L _r in the subband level structure, and the electrons that have undergone intersubband transition in the unit laminate 16 are , the longitudinal optical phonon scattering is relaxed to relaxation miniband MB from emission lower level L _low, is injected from the injection layer 18 through the relaxation miniband MB to injection level L _i in subsequent stage of the quantum well active layer 17b A configuration may be used.

In the above configuration, the relaxation miniband MB having an energy level lower than the emission lower level L _low is provided in the subband level structure in the unit stacked body 16. The level structure is configured such that the energy difference between the emission lower level L _low and the miniband MB corresponds to the LO phonon energy E _LO . In such a configuration, electrons that have undergone a light emission transition in the light emitting layer 17 are rapidly extracted from the lower level L _low through LO phonon scattering and relaxation within the miniband. Therefore, it is possible to improve the laser operation performance by realizing efficient inversion distribution formation in the light emitting layer 17 and thereby lowering the threshold value of the laser operation. In addition, the use of the mini-band MB for electron relaxation facilitates the design of the electron relaxation structure from the lower emission level, stabilizes the characteristics during the manufacture of the laser element, and improves the yield. Improvements can be realized.

Here, regarding the emission lower level L _low in this case, one subband (the subband of the highest energy in the miniband MB) in the relaxation miniband MB is transferred from other subbands to the energy E _{LO of the} LO phonon. For example, a configuration can be used in which the separated level is separated to the higher energy side and the separated level is the lower emission level. Thereby, a level structure including the lower emission level L _low and the relaxation miniband MB can be suitably realized.

  Further, regarding the quantum well structure of the active layer 15, it is preferable that an extraction barrier layer is provided between the light emitting layer 17 and the injection layer 18 in the unit stacked body 16. As a result, the seepage of the wave function of electrons from the injection layer 18 to the light emitting layer 17 can be suppressed, and the efficiency of light emission transition in the light emitting layer 17 can be improved. That is, by suppressing the leakage of the electron wave function in this way, the optical transition that contributes to laser oscillation is surely performed between the emission upper level and lower level subbands, and from the lower level. Carriers are relaxed to a relaxation level or a relaxation miniband by a relaxation process such as LO phonon scattering.

  Further, when the relaxation miniband MB is used in the subband level structure, the relaxation miniband MB preferably has a band structure in which the miniband in the 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 tunneling time of electrons from the light emitting layer 17 to the injection layer 18 can be made very short.

The effect of improving the laser element characteristics such as the efficiency of electron injection into the light emission upper level by the above-described subband level structure will be described more specifically. As described above, in a four-level quantum cascade laser having an injection level, an emission upper level, a lower level, and a relaxation level, conventionally, an injection level _Li and an emission upper level L _up are The energy interval ΔE ₄₃ is set to be _{equal to} or greater than the LO phonon energy E _LO .

With such injection level L _i of indirect excitation via structure, in theory, can be injected all previously placed carriers in the electron injection layer 18 by doping the light emitting layer 17 is there. In an actual laser element, loss (mainly free carrier absorption) is reduced by the quenching effect of carriers in the electron injection layer due to an increase in effective injection efficiency, its characteristic temperature (temperature dependence of threshold current _Jth ), and Improvements in device characteristics such as slope efficiency have been realized.

On the other hand, as a result of further examination of the subband level structure, the inventor of the present application has determined that the energy interval ΔE ₄₃ between the injection level L _i and the emission upper level L _{up in} the above four level system. Even in a configuration set smaller than the LO phonon energy E _LO, it is possible to improve the laser device characteristics such as the efficiency of electron injection into the emission upper level L _up and maintain a low threshold while maintaining a high characteristic temperature. It has been found that good characteristics such as high slope efficiency can be achieved up to a high temperature region.

As described above, in the configuration in which the energy interval ΔE ₄₃ between the injection level L _i and the emission upper level L _up is set to be smaller than E _LO , electrons increase from the injection level L _i to the emission upper level L _up . The reason why an excellent laser element characteristic can be obtained by the fact that the emission transition from the emission upper level L _up to the lower level L _low becomes dominant and the laser element characteristics can be obtained will be briefly discussed below.

The energy interval ΔE ₄₃ is the condition 10 meV <ΔE ₄₃ <E _LO
In the quantum cascade laser 1A of the above embodiment in which the energy of the injection level (level 4) L _i and the emission upper level (level 3) L _up are set so as to satisfy the above, electrons from the injection level L _i are One is supplied to the emission upper level L _up by electron-electron scattering occurring at high speed.

Further, as schematically shown in FIG. 3, a process in which carriers activated by an increase in electron temperature and equivalent to the energy of LO phonon relaxes by LO phonon scattering occurs. It is considered that electrons are supplied from the injection level L _i to the emission upper level L _up at a high speed and with a high efficiency by an effective relaxation process. Of the relaxation processes described above, the relaxation process by LO phonon scattering of thermally activated carriers is considered to be remarkable in the region near room temperature or higher.

In the configuration in which the energy interval ΔE ₄₃ between the injection level L _i and the emission upper level L _up is set to be smaller than E _LO , the leakage current is reduced compared to the configuration in which the energy interval ΔE ₄₃ is set to be _{equal to} or higher than E _LO. It is considered that the device characteristics may be improved. FIG. 4 is a diagram schematically showing a leak path in the supply of electrons from the injection level L _i to the emission upper level L _up .

In FIG. 4, FIG. 4A shows a subband level structure in the light emitting layer in the conventional structure in which the energy interval ΔE ₄₃ is set to E _LO or more. FIG. 4B shows a subband level structure in the light emitting layer in the embodiment in which the energy interval ΔE ₄₃ is set smaller than E _LO . In these level structure diagrams, in addition to the injection level L _i , the emission upper level L _up , the lower level L _low , and the relaxation level L _r , the injection level L _i is on the higher energy side. The high energy level L _h that is positioned and does not contribute to laser operation is also shown.

In the structure in which the energy interval ΔE ₄₃ is set to be _{equal to} or greater than E _LO , the energy of the injection level L _i becomes high. Therefore, as shown in FIG. 4A, the high energy level L _h from the injection level L _{i is obtained.} Energy interval ΔE _h until becomes relatively small. Therefore, in the laser light emitting operation using such a level structure, it is possible that the leakage current is generated becomes level L _h is a leakage path. When a leak current is generated in this way, the temperature characteristics and the like are excellent, but the threshold value tends to be higher due to the leak current compared to other structures.

On the other hand, in the structure in which the energy interval ΔE ₄₃ is set to be smaller than E _LO , as shown in FIG. 4B, the energy of the injection level L _i is lowered to reduce the injection level L. The energy interval ΔE _h from _i to the high energy level L _h can be increased. According to this structure, by suppressing the influence of the leakage path through the level L _h, it is possible to improve the device characteristics.

More specifically, for the energy interval ΔE ₄₃ between the injection level L _i and the emission upper level L _up , the condition 10 meV <ΔE ₄₃ <34 meV
It is preferable to set the energy interval ΔE ₄₃ within a range that satisfies the above. Thereby, it is possible to suitably realize high-speed and high-efficiency supply of electrons from the injection level to the emission upper level. Here, 34 meV, which is the upper limit in the above conditional expression, corresponds to the LO phonon energy E _LO when InGaAs is assumed as the semiconductor material of the quantum well layer. The LO phonon energy is 36 meV when the quantum well layer is made of GaAs, and 32 meV when it is made of InAs, which is substantially the same as the 34 meV described above.

Further, 10 meV, which was the lower limit in the above conditional expressions, strong with ground level of the previous cycle (e.g. relaxation level L _r), coupled with the injection level L _i in the light-emitting layer (anticrossing gap) If the energy interval ΔE ₄₃ is less than this value, it is considered that sufficient current cannot be carried. That is, the maximum current J _max that can flow in the quantum cascade laser is determined by the strength Etchiomega binding to the ground level of the previous cycle injected level L _i, the binding strength, for example hΩ = 5~ It is about 15 meV, typically 10 meV. When the energy interval ΔE ₄₃ is smaller than this, the thickness of the second barrier layer in the light emitting layer 17 is thicker than the thickness of the first barrier layer (injection barrier layer), and the current that can be carried becomes smaller.

Further, the energy interval ΔE _h between the injection level L _i and the high energy level L _h is more specifically set to the condition ΔE _h > E _LO (for example, when the semiconductor material of the quantum well layer is InGaAs) Preferably satisfies ΔE _h > 34 meV). When the energy interval ΔE _h is less than E _LO , it is considered that the influence of the leak path is increased.

FIG. 5 is a graph showing an example of an EL spectrum obtained by the quantum cascade laser having the configuration shown in the above embodiment. In this graph, the horizontal axis indicates the wave number (cm ⁻¹ ), and the vertical axis indicates the emission intensity (au). Here, in the following description, a laser element is configured with a resonator length of L = 4 mm, a ridge width in a ridge waveguide configuration of W = 14 μm, a single facet output without an end face coat, a pulse width of 100 ns, and a repetition frequency of 1 kHz. The laser element characteristics when the pulse operation is performed at. A specific configuration of the quantum cascade laser element will be described later.

In the EL spectrum shown in FIG. 5, a sufficiently narrow emission spectrum width of about 100 cm ⁻¹ or less is observed. This is because, in the level structure of the light emitting layer, electrons injected into the injection level L _i are supplied to the emission upper level L _up with high efficiency, and light emission between the upper level L _up and the lower level L _low. Is becoming dominant.

FIG. 6 is a graph showing the current density dependence of voltage and peak output power. In this graph, the horizontal axis represents current density (kA / cm ² ), and the vertical axis represents voltage (V) or peak output power (W). In the graph of FIG. 6, graph A1 shows the current density dependence of the peak output power at the operating temperature of 300K, and graph A2 shows the current density dependence (IV characteristic) of the voltage.

In the peak output power characteristic shown in the graph A1 of FIG. 6, the oscillation threshold current density J _th = 1.5 kA / cm ² and the maximum peak output power P _max = 1 W are obtained at the operating temperature of 300K. Further, in the IV characteristic shown in the graph A2, it is observed that the voltage suddenly rises due to the resonance tunnel path being cut off at a certain current density and the light emission is stopped, as indicated by A3 in the graph. The fact that the light emission stops in this manner indicates that there is no leak path that does not contribute to the light emission other than the light emission path. Therefore, as described above, the influence of the leakage path through a high energy level L _h is seen to have been suppressed.

FIG. 7 is a graph showing temperature dependence of slope efficiency and threshold current. FIG. 7A is a graph showing the temperature dependence of the slope efficiency. In this graph, the horizontal axis indicates temperature (K), and the vertical axis indicates slope efficiency (W / A). FIG. 7B is a graph showing the temperature dependence of the threshold current. In this graph, the horizontal axis indicates temperature (K), and the vertical axis indicates threshold current J _th (kA / cm ² ).

In the graph of FIG. 7 (a), the slope efficiency is 0.91 W / A at a temperature T = 77K, 0.89 W / A at 300K, and 0.75 W / A at 380K. Maintains over 80%. From this graph and the graph of FIG. 7B, it can be seen that an ideal state is achieved in which the slope efficiency is not lowered to the high temperature region of 400K while maintaining the low threshold. This is presumably because the carrier injection efficiency was improved by adopting the above-described level structure. Further, as shown in the graph of FIG. 7B, the characteristic temperature indicating the temperature dependence of the threshold is also T ₀ = 306 K, and 300 K or more is recorded (around 200 K in the normal structure).

  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. 8 is a diagram illustrating an example of a specific configuration of the quantum cascade laser. FIG. 9 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 this configuration example, an example is illustrated in which the oscillation wavelength is 6.6 μm and the operating electric field is 40 kV / cm. FIG. 9 shows the quantum well structure and the subband level structure of a part of the multistage repetitive structure of the active layer 15 including the quantum well light emitting layer 17 and the injection layer 18. The device structures shown in FIGS. 8 and 9 can be formed by crystal growth by, for example, molecular beam epitaxy (MBE) method or metal organic chemical vapor deposition (MOCVD) method.

  In the semiconductor stacked structure of the quantum cascade laser 1 </ b> B shown in FIG. 8, an n-type InP substrate 50 is used as the semiconductor substrate 10. Then, on this InP substrate 50, in order from the substrate side, an InP clad layer 51 having a thickness of 3.5 μm, an active layer 15 in which unit laminated bodies 16 are laminated in multiple stages, an InP clad layer 52 having a thickness of 3.5 μm, The element structure of the laser 1B is formed by sequentially laminating the high-concentration Si-doped InGaAs contact layer 53. For the entire structure of the quantum cascade laser 1B, for example, a ridge waveguide type laser chip structure formed by a normal process step can be used.

  The active layer 15 in this configuration example is configured by stacking unit laminates 16 including the quantum well light-emitting layers 17 and the electron injection layers 18 in, for example, 30 periods. In addition, as shown in FIG. 9, the unit stacked body 16 for one period includes 11 quantum well layers 161 to 164 and 181 to 187, and 11 quantum barrier layers 171 to 174 and 191 to 197 alternately. It is configured as a stacked quantum well structure.

  Among the semiconductor layers of these unit laminated bodies 16, the quantum well layer is composed of an InGaAs layer. The quantum barrier layer is composed of an InAlAs layer. Thereby, the active layer 15 is configured by an InGaAs / InAlAs quantum well structure lattice-matched to the InP substrate 50. Further, in the stacked structure of the unit stacked body 16 shown in FIG. 9, a stacked portion including the four well layers 161 to 164 and the barrier layers 171 to 174 is a portion that functions as the light emitting layer 17. Further, a laminated portion composed of seven well layers 181 to 187 and barrier layers 191 to 197 is a portion functioning as the injection layer 18.

  In addition, among the semiconductor layers of the light emitting layer 17, the first quantum barrier layer 171 is located between the previous injection layer and the light emitting layer 17, and electrons from the previous injection layer to the light emitting layer 17 are present. It is an injection barrier layer against the above. Similarly, among the semiconductor layers of the injection layer 18, the first-stage quantum barrier layer 191 is located between the light emitting layer 17 and the injection layer 18, and prevents electrons from the light emitting layer 17 to the injection layer 18. It is an extraction barrier layer. FIG. 10 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 unit stacked body 16 has an injection level L _i , a light emission upper level L _up , a light emission lower level L _low , and a relaxation level L _{r in the} subband level structure shown in FIG. It has a relaxation mini-band MB. A plurality of levels correspond to a relaxation miniband MB that functions as a relaxation level. The thicknesses of the well layer and the barrier layer in the light emitting layer 17 and the injection layer 18 are designed based on quantum mechanics.

In the level structure described above, the injection level _Li and the emission upper level L _up are mainly formed by the first well layer 161 and the second well layer 162 of the light emitting layer 17. The energy interval ΔE ₄₃ between the injection level _Li and the emission upper level L _up is mainly determined by the thickness of the second barrier layer 172 between the first well layer 161 and the second well layer 162. The In this configuration example, the thickness of the barrier layer 172 is slightly thick as 3.0 nm. The energy interval ΔE ₄₃ is smaller than the LO phonon energy E _LO = 34 meV and is about 21 meV.

Also, injection for the wave function of level L _i is the wave function of the injection level L _i present in the active layer 15 (the unit laminate 16) except the most preceding injection layer side of the first well layer 161 The ratio is 28%, which satisfies the condition of 30% or less. That is, in this configuration example, the wave function of the injection level L _i is sufficiently localized in the first well layer 161.

  The characteristics and the like of the quantum cascade laser 1B according to the above-described configuration example will be further described with reference to FIGS.

FIG. 11 is a graph showing the dependence of the energy interval between the injection level L _i and the emission upper level L _up on the thickness of the first well layer. In this graph, the horizontal axis indicates the layer thickness (nm) of the first well layer 161 in the light emitting layer 17, and the vertical axis indicates the energy interval ΔE ₄₃ (meV) between the injection level L _i and the emission upper level L _up. Show.

FIG. 12 is a graph showing the dependence of the dipole moment corresponding to the transition intensity of the luminescence transition on the thickness of the first well layer. In this graph, the horizontal axis indicates the thickness (nm) of the first well layer 161 in the light emitting layer 17 as in FIG. 11, and the vertical axis indicates the light emission transition from the light emission upper level L _up to the light emission lower level L _low . And the dipole moment of the emission transition from the injection level L _i to the emission lower level L _low .

9 and FIG. 10, the thickness of the first well layer 161 of the light emitting layer 17 is 2.2 nm. On the other hand, as shown in the graph of FIG. 11, when the layer thickness of the well layer 161 is changed, when the thickness is increased with respect to 2.2 nm of the above configuration example, the energy interval ΔE ₄₃ is minimized, It changes to increase again. On the other hand, when the layer thickness is made thinner than 2.2 nm, the energy interval ΔE ₄₃ changes so as to increase.

As shown in the graph of FIG. 12, at 2.2 nm in the above configuration example, the emission intensity from the emission upper level L _up to the emission lower level L _low is such that the emission level L _{i is} lower than the emission level L _{i. It} can be seen that the intensity of light emitted to _low is sufficiently large. Further, when the thickness of the well layer 161 is changed, when the layer thickness is reduced, the emission intensity from the emission upper level L _up to the emission lower level L _low is changed. Here, the emission intensity in an actual laser element is determined by the square of the dipole moment (the square of the transition intensity). In the configuration example described above, the emission intensity from emission upper level L _up to the emission lower level L _low, the injection and domination 3.5 times the level intensity of emission from L _i to the emission lower level L _low Strength. In consideration of other conditions, it is preferable that the thickness of the first well layer 161 in the above configuration example is set within a range indicated by R1 and R2 in FIGS.

FIG. 13 is a graph showing the dependence of the anti-crossing of the injection level L _i and the emission upper level L _up on the thickness of the second barrier layer. In this graph, the horizontal axis indicates the layer thickness (nm) of the second barrier layer 172 in the light emitting layer 17, and the vertical axis indicates anti-crossing that indicates the strength of the coupling between the injection level L _i and the emission upper level L _up. hΩ ₄₃ (meV) is shown.

9 and FIG. 10, the thickness of the second barrier layer 172 of the light emitting layer 17 is 3.0 nm. On the other hand, as shown in the graph of FIG. 13, when the layer thickness of the barrier layer 172 is changed, the anti-crossing at the two levels that gives the energy interval ΔE ₄₃ changes. This point is also an important factor in defining the structure of the quantum cascade laser. The thickness of the second barrier layer 172 is preferably set in consideration of such conditions for the anti-crossing hΩ ₄₃ and the energy interval ΔE ₄₃ .

  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 in which an InP substrate is used as the semiconductor substrate and the active layer is configured by InGaAs / InAlAs has been described, but light emission transition by intersubband transition in the quantum well structure is possible and described above. Specifically, various configurations may be used as long as the subband 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.

  In addition, regarding the stacked structure in the active layer of the quantum cascade laser and the semiconductor stacked structure as the entire laser device, various structures other than the structures shown in FIGS. 8 to 10 may be used. 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. 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 improving laser element characteristics such as electron injection efficiency into the emission upper level.

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 ... InP clad layer, 52 ... InP Cladding layer, 53 ... InGaAs contact layer, L _i ... injection level, L _up ... light emission upper level, L _low ... light emission lower level, L _r ... relaxation level, MB ... relaxation miniband.

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, which are provided on the semiconductor substrate. An active layer,
The unit laminate includes a light emission upper level, a light emission lower level, and an injection level that is an energy level higher than the light emission upper level in the subband level structure in the quantum well light emitting layer. And a relaxation level that is an energy level lower than the lower emission level,
Light is generated by the intersubband transition of electrons from the upper emission level to the lower emission level in the quantum well light emitting layer, and the electrons that have undergone the intersubband transition are transferred from the lower emission level to the Relaxed to the relaxation level, injected from the injection layer to the injection level in the subsequent quantum well light-emitting layer via the relaxation level, and electrons are supplied from the injection level to the emission upper level And
The energy interval between the injection level and the emission upper level is set to be larger than 10 meV and smaller than the energy of the longitudinal optical phonon, and the emission intensity from the injection level to the emission lower level is A quantum cascade laser characterized by having an intensity smaller than an emission intensity from an emission upper level to the emission lower level.   The emission intensity from the injection level to the lower emission level is 1/3 or less of the emission intensity from the upper emission level to the lower emission level. The quantum cascade laser described.   The quantum well light-emitting layer includes n well layers (n is an integer of 2 or more), and the wave function of the injection level existing in the active layer other than the first well layer on the frontmost injection layer side. The quantum cascade laser according to claim 1 or 2, wherein the ratio is 30% or less. The energy interval ΔE ₄₃ between the injection level and the emission upper level is defined by the condition 10 meV <ΔE ₄₃ <34 meV.
The quantum cascade laser according to claim 1, wherein the quantum cascade laser is set within a range that satisfies the above.   5. The electron that has undergone the intersubband transition in the unit laminate is relaxed from the emission lower level to the relaxation level by longitudinal optical phonon scattering. 6. The quantum cascade laser according to item. The unit laminate body has a relaxation miniband functioning as the relaxation level in the subband level structure,
In the unit stack, the electrons that have undergone the transition between the subbands are relaxed from the emission lower level to the relaxation miniband by longitudinal optical phonon scattering, and are separated from the injection layer through the relaxation miniband. The quantum cascade laser according to claim 1, wherein the quantum cascade laser is injected into the injection level in the well light emitting layer.

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