JP2006310784A - Quantum cascade laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum cascade laser of an easily manufactured structure which oscillates at lower frequency than conventional one. <P>SOLUTION: A superlattice device has a configuration in which an AlSb layer or GaAlSb layer and a GaSb layer are repeatedly laminated with an electrode layer formed at both ends. The thickness of the GaSb layer constituting a quantum well for stimulated emission of light has such thickness as a difference between the base state of quantum level formed in the GaSb layer, and an intrinsic energy in a first excitation state becomes LO phonon energy of GaSb. The superlattice of which AlSb layer, GaSb layer, AlSb layer, GaSb layer, AlSb layer, n-type GaSb layer, AlSb layer, and GaSb layer are repeated unit, is sandwiched between two n-type semiconductor layers used as a contact layer. The AlSb layer may be replaced with a GaAlSb layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a quantum cascade laser that oscillates in the terahertz region.

  The terahertz region (1-10 THz) is in the frequency region between microwave and light. In this region, absorption by the atmosphere is large, and since there is no small solid-state signal source, it is difficult to use, and the use of this region has been limited. However, in 1994, quantum cascade lasers (QCL: Quantum Cascade Lasers) utilizing electron transition between subbands in a multiple quantum well structure of a compound semiconductor were invented, and this oscillation frequency was in the near infrared region. In 2002, a quantum cascade laser in the terahertz region was announced, which has recently attracted attention as a compact solid-state signal source in the terahertz region.

One example of a quantum cascade laser is one that uses an AlGaAs / GaAs multiple quantum well structure and uses phonon scattering, as shown in Non-Patent Document 1. This is to perform laser oscillation by forming an inversion distribution of electrons in the subband using scattering by GaAs LO (longitudinal optical) phonons. The range of the oscillation frequency is limited by the LO phonon energy. In the case of GaAs where the LO phonon energy is 36 meV, it was about 3 THz. As shown in FIG. 1, the quantum cascade laser has a structure in which a basic multiple quantum well structure is repeated a plurality of times. In the above-mentioned Non-Patent Document 1, the basic multiple quantum well structure is 5.4 / 7.8 / 2.4 / 6.5 / 3.8 / 14.9 / 3.0 / 9.5 ( nm). It has become. Here, the underlined layer is a barrier made of Al _0.15 Ga _0.85 As, and the non-underlined layer is a well made of GaAs. The GaAs layer with a thickness of 14.9 nm is n-type and is doped to a level of 1.9 × 10 16 per cubic centimeter.

  The operating principle of this quantum cascade laser will be briefly described below. First, the subband state of this multiple quantum well structure can be obtained by solving the Schrodinger equation and the Poisson equation in a self-consistent manner. FIG. 1 shows the subband states of the conduction bands of two basic multiple quantum well structures when an electric field of 12.0 kV / cm is applied. One basic multiple quantum well structure is divided into an injection region and an active region, as shown in FIG. First, electrons are injected from the injection region state 2 ′ or 1 ′ to the active region excited state n = 5. Next, the injected electrons transition in the active region while emitting terahertz waves from the excited state n = 5 to the ground state n = 4 or 3. This frequency is determined by the energy difference between the excited state and the ground state. The energy difference between the state n = 4 or 3 and the state n = 2 is designed to be almost the same value as the vertical optical (LO) phonon energy of the material to be a well. For example, the LO phonon energy of GaAs is 36 meV. Thus, electrons in state n = 4 or 3 are scattered to state n = 2 by LO phonons, and the number of electrons in state n = 4 or 3 decreases. As a result, an inversion distribution is formed between the state n = 5 and the state n = 4 or 3, and laser oscillation starts. The oscillation frequency of this laser is designed to be 3.6 THz (15 meV).

  However, when the electric field strength is lower than the value used in FIG. 1 (12.0 kV / cm), the energy of the state n = 4 is higher than that of the states n = 1 ′ and 2 ′, and the laser oscillation process described above is performed. No longer holds and no laser oscillation occurs.

BS Williams et al., In Non-Patent Document 2, have a structure similar to that of the above example and a thickness of 5.6 / 8.2 / 3.1 / 7.0 / 4.2 / 16.0 / 3 , respectively. Quantum cascade lasers with .4 / 9.6 nm (the one with an underline is an AlGaAs layer, the one without is a GaAs layer) have been announced. An oscillation of 2.1 THz is obtained by setting the thickness of the barrier in the center of the active layer to 3.1 nm and setting the energy difference between the state 5 and the state 4 to 9.0 meV. This is only that the thickness of the barrier of the above-described conventional example is increased to 0.6 nm, that is, only two atomic layers. The thickness of the well before and after that is only 0.4 to 0.5 nm thick.

However, the above example has the following limitations and problems.
First, in these examples, the thickness of the well or barrier in the active layer is less than 10 nm. Since the thickness of one atomic layer in GaAs or AlGaAs is about 0.3 nm, it is necessary to reduce the thickness distribution to be formed, and this limits the degree of freedom in design.

  The electric field strength at the start of laser oscillation is about 12 kV / cm, which is relatively high. For this reason, the generation of hot electrons and hot phonons increases, and the operation may be different from the design. Further, since the heat generation becomes large, there is a problem that the laser is easily deteriorated.

  Further, in general, this quantum cascade laser is manufactured by a molecular beam epitaxial (MBE) apparatus. Since the film thickness is about 10 μm, there is a problem that irregularities such as waviness are likely to occur. For this reason, electric field concentration occurs locally, and laser oscillation occurs in a place where the electric field is concentrated, but oscillation does not occur in other places, but rather the generated terahertz wave is absorbed, and the laser power is weak. There is a case.

In order to obtain a high laser output in a GaSb / AlSb (or AlGaSb) quantum cascade laser, it is necessary to confine a terahertz wave in the active region of the laser. Although it is known that the wavelength of electromagnetic waves in the terahertz band is longer than that of visible light and the active layer of the laser only needs to be thickened accordingly, this is difficult due to limitations of crystal growth technology. On the other hand, a metal-metal waveguide, that is, an active region of a quantum cascade laser sandwiched between upper and lower metal electrodes can achieve a very high confinement effect (confinement coefficient Γ to 0.99). It has been known. However, in this case, a technique requiring skill such as wafer bonding and removal of the substrate by selective etching is required, and the production is not easy.
BS Williams, et al., "3.4 THz quantum cascade laser based on longitudinal-optical phonon scattering for depopulation," Applied Physics Letters, 82: 1015 (2003) BS Williams, et al., Electronics Letters Vol. 40, No. 7, p. 431

  A thicker semiconductor layer can be used to form a necessary superlattice. In addition, the electric field intensity at the start of laser oscillation is lowered to suppress abnormal operation due to generation of hot electrons or hot phonons and laser degradation due to heat generation.

  According to the present invention, it is possible to realize a quantum cascade laser having a structure that oscillates at a lower frequency and is easier to manufacture than the conventional one.

  The quantum cascade laser of the present invention uses a GaSb layer having a LO phonon energy smaller than that of GaAs conventionally used, and an AlSb layer or a GaAlSb layer and a GaSb layer are repeatedly stacked, and electrodes are formed at both ends thereof. A superlattice element formed by forming a layer, wherein the thickness of a GaSb layer constituting a quantum well that performs stimulated emission of light is determined based on the ground state of the quantum level formed in the GaSb layer and the first excitation. The thickness is such that the difference between the intrinsic energy of the states becomes the LO phonon energy of GaSb.

  In particular, a superlattice having a repeating unit of an AlSb layer, a GaSb layer, an AlSb layer, a GaSb layer, an AlSb layer, an n-type GaSb layer, an AlSb layer, and a GaSb layer is used, and these are used as two contact layers. The structure is sandwiched between n-type semiconductor layers.

  Alternatively, the AlSb layer described above functions as a quantum cascade laser even when the structure is replaced with a GaAlSb layer.

  Further, the above-described quantum cascade laser is formed on the superlattice buffer, thereby effectively confining the terahertz wave.

  Further, by producing the above quantum cascade laser on the GaAs substrate via the buffer layer, it becomes possible to irradiate the reference light through the GaAs substrate.

  The above-described quantum cascade laser can be used for synchronous oscillation. In this configuration, a potential difference necessary for laser oscillation is applied between the electrode layers, and the superlattice portion is irradiated with reference light.

  In the above quantum cascade laser, a first metal electrode is provided above the quantum cascade structure via a first contact layer, and a substrate is provided below the quantum cascade structure via a second contact layer, By making the conductivity of the first contact layer higher than the conductivity of the second contact layer, the terahertz wave can be effectively confined.

  In order to effectively confine the terahertz wave as described above, as an example, the substrate is GaAs, and the quantum cascade structure includes a well made of GaSb and a barrier made of AlGaSb or AlSb. It is good to do.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason. First, an embodiment of the present invention will be described with reference to FIG.

FIG. 2 shows an embodiment of the present invention. A semi-insulating GaAs substrate is used as the semiconductor substrate. Using a molecular beam epitaxy (MBE) apparatus, as a buffer layer, for example, 100 nm GaAs, 10 nm AlAs, 100 nm AlSb, and 1000 nm GaSb layers are sequentially formed on the substrate. Further, as the buffer layer, a 2.5 nm AlSb layer and a 2.5 nm GaSb layer are alternately stacked 10 times. Next, an n + GaSb layer in which an n-type impurity is doped to 3 × 10 18 to the cm ³ / cm ³ is formed to a thickness of about 800 nm as the first contact layer. Next, 4.3 nm / 14.4 nm / 2.4 nm / 11.4 nm / 3.8 nm / 24.6 nm / 3.0 nm / 16.2 nm from the upper layer part (the underlined is the thickness of the barrier, that is, the AlSb layer, A basic multiple quantum well structure having a well, that is, a GaSb layer) is formed, that is, a 16.2 nm GaSb layer is formed on the substrate side. The GaSb layer with a thickness of 24.6 nm is n-type and is doped to a level of 1.9 × 10 16 sq. Cm ³ . In this n-type layer, electrons are supplied and scattering occurs. This basic multiple quantum well structure is formed repeatedly, for example, 200 times. Further, as the second contact layer, for example, a GaSb layer which is an n-type layer having a thickness of 60 nm and is doped with impurities at, for example, 5 × 10 18 power / cm ³ is formed. These compound semiconductor layers are formed by a molecular beam epitaxial (MBE) apparatus after the buffer layer.

  Here, since antimony-based compound semiconductors such as GaSb are likely to diffuse on the surface during crystal growth, surface irregularities can be suppressed. As a result, electric field concentration is less likely to occur when a voltage is applied. Therefore, the laser light is generated more uniformly, and the terahertz light is not easily absorbed at portions other than the electric field concentration portion, so that the laser intensity can be increased.

Next, a manufacturing process of the quantum cascade laser (QCL) having the above layer structure will be described.
1) FIG. 4A shows a cross-sectional view of a wafer immediately after MBE growth. First, a resist is selectively left on the portion of the quantum cascade laser that forms the ridge structure.
2) Thereafter, for example, reactive ion etching (RIE) using SiCl ₄ gas is used to remove the multiple quantum well structure using the resist as a mask, and the first contact layer is selectively exposed.
3) After removing the resist (FIG. 4B), a metal layer made of, for example, Pd / AuGe / Ni / Au is selectively formed on the exposed first contact layer using a lift-off method. FIG. 4C shows a cross-sectional view at this stage.
4) Thereafter, annealing is performed at 300 ° C. for 1 minute, for example.
5) Next, a metal layer made of, for example, Pd / Au is selectively formed on the upper portion of the mesa structure formed by RIE, that is, on the second contact layer, using a lift-off method.
6) Thereafter, the sample is cleaved to form a resonator having a length of about 2 mm. The width of the ridge structure is about 100 μm to 200 μm.
7) Connect two gold wires to the electrode on the first contact layer and the electrode on the second contact layer, respectively, to form a lead line. FIG. 5A and FIG. 5B show a cross-sectional view and a bird's-eye view at this stage, respectively.
8) In order to operate the device thus manufactured, a voltage is applied between the lead lines, that is, between the electrodes. At this time, if necessary, the quantum cascade laser is cooled with liquid nitrogen or liquid helium, and a pulsed voltage is applied.

  FIG. 3 shows subbands in the conduction band of two units of the basic structure of the QCL structure of the first embodiment. One scale on the vertical axis corresponds to 10 meV, and one scale on the horizontal axis corresponds to 10 nm. The subband was obtained by solving the Schrodinger equation and Poisson equation in one dimension in a self-consistent manner. In FIG. 3, the subband represents the energy of the state, the existence probability of the state, that is, the square of the wave function, multiplied by an appropriate multiple for convenience of display. The sum of the existence probabilities of each subband is normalized to 1. Wells a, b, c, and d represent GaSb layers having thicknesses of 16.2 nm, 24.6 nm, 11.4 nm, and 14.4 nm, respectively. The electric field strength in FIG. 3 is 5.45 kV / cm, which is ½ or less of the value in FIG. The energy difference between states 5 and 4 corresponds to 1.85 THz at 7.66 meV, and the energy difference between states 5 and 3 corresponds to 2.57 THz at 10.64 meV. Thus, although the thickness of the barrier between the well c and the well d is 2.4 nm, which is the same as that in the first conventional example, laser oscillation with a frequency lower than that can be obtained. Further, the thickness of the well is thicker than that of the conventional example, and the degree of freedom in design increases.

Hereinafter, a design guideline for the QCL structure will be described to clarify a desirable configuration of the present invention.
First, in the active layer, the ground state and the first excited state are generated in the well c and the well d, and the well c and the well d are set so that the energy difference is about the oscillation frequency of the quantum cascade laser at a predetermined electric field strength. And design the thickness of the barrier between them. The ground-state wave function is generated mainly from the well d and the excited-state wave function is generated from the well c so that the ground-state and excited-state wave functions exist in both the wells c and d. That is, the width of the well c is reduced.

  Next, with respect to the injection layer, in FIG. 3, the electrons need to be LO phonon scattered from state 4 or 3 to state 2 or 1. Therefore, the thickness of the well b is determined so that the difference between the intrinsic energy of the ground state and the first excited state in the well b becomes the GaSb LO phonon energy at a predetermined electric field strength, for example, the electric field strength at which laser oscillation is started. . The LO phonon energy of GaSb is about 28.9 meV, which is characterized by being smaller than the value of the conventional example described above.

  In addition, as shown in FIG. 3, the ground state energy of the well b and the ground state energy of the well a are shown in FIG. 3 so that phonon scattered electrons are efficiently extracted from the well b to the well a at a predetermined electric field strength. The thickness of the well a and the thickness of the barrier between the wells a and b are determined so as to be substantially the same. Since the ground state energy increases as the well becomes thinner, the thickness of the well a is made thinner than that of the well b.

  Further, regarding the relationship between the active layer and the injection layer, the ground state of the injection layer and the active state are obtained at a predetermined electric field strength so that electrons are efficiently injected from the well a ′ to the first excited state of the well d or the well c. The first excited state of the layer is at approximately the same energy level so that both wave functions are coupled.

  Further, at a predetermined electric field strength, electrons are efficiently extracted from the ground state of the active layer region to the injection region, that is, the ground state of the well c and the well d is coupled with the first excited state in the well b. To. The thickness of the well c and the well d is about half of the thickness of the well b.

The calculation is repeated, adjusting the field strength, well and barrier thicknesses so that these conditions are met. As a result, it can be seen that the sum of the thicknesses of the well a ′, the well c, and the well d is about twice or less the thickness of the well b.

  In addition, when a predetermined electric field strength is applied, the energy difference between the excited state and the ground state of the injection layer, that is, the LO phonon energy, and the excited state and ground state energy in the active layer, for one basic unit of the quantum cascade laser. The difference, that is, a potential difference equivalent to the sum of the energy corresponding to the oscillation frequency must be generated. As described above, since the thicknesses of the wells a to d are substantially determined, it is necessary to satisfy the condition by changing the electric field strength. As a result, by reducing the LO phonon energy as in the present invention, it is possible to reduce the electric field strength at which laser oscillation starts.

  Further, when the LO phonon energy is small as in the present invention, the width of the well b is increased, and as a result, the width of other wells and the thickness of the barrier can be increased. The effective mass of GaSb electrons is 0.0412, which is smaller than the effective mass of GaAs electrons, 0.067. Therefore, when GaSb is used as in the present invention, the well width can be further increased. As a result, the influence of the error of one atomic layer on the performance such as the oscillation frequency of the laser is reduced, the allowable film thickness range is increased during the thin film growth by MBE, and the MBE growth is facilitated.

  Furthermore, by using antimony-based MBE growth as described above, it is possible to suppress surface waviness and increase the laser power.

  FIG. 7 shows a cross section of the quantum cascade laser of the present invention. A buffer layer made of, for example, GaAs, GaSb, or AlSb is provided on the GaAs substrate, and a contact layer made of n-type doped GaSb is provided thereon. A quantum cascade structure (active region) made of GaSb, AlGaSb, or AlSb is provided thereon, and a contact layer made of n-type doped GaSb and a metal serving as an electrode are also provided above the quantum cascade structure.

  FIG. 8A shows the result of calculating the electric field distribution of the eigenmode in the structure of FIG. 7 using the two-dimensional finite element method. The frequency was 3.0 THz. The thickness of the GaSb / AlGaSb quantum cascade structure was 15 μm, and the relative dielectric constant was 15.1. The laser ridge width was 150 μm, and the distance between the edge of the ridge and the lower electrode was 25 μm. The thicknesses of the upper and lower n-type GaSb contact layers were 60 nm and 800 nm, the relative dielectric constant was 14.8, and the conductivity was 40000 S / m. The relative dielectric constant of the semi-insulating GaAs substrate was 13.3. The thickness of the buffer layer is 1 μm.

  In the calculation result of the electric field distribution of the natural mode shown in FIG. 8A, it can be seen that the electric field of the terahertz wave is well confined in the ridge region, that is, the active region of the quantum cascade laser. When the confinement factor Γ is obtained by dividing the integral of the square of the electric field strength in the active region by the integral of the square of the electric field strength in the entire region, it is 0.78 in the case of FIG.

  For comparison, FIG. 8B shows the electric field distribution of the eigenmode when a GaSb-based quantum cascade laser is provided on a GaSb substrate. The frequency is 3.0 THz. In the structure of FIG. 7, the calculation was performed by replacing the semi-insulating GaAs substrate and the buffer layer with a GaSb substrate. The confinement coefficient Γ was about 0.1.

  In this way, high confinement of terahertz waves can be realized by making the substrate GaAs having a lower dielectric constant than GaSb.

  In order to perform laser oscillation with a low threshold, it is also necessary to reduce the attenuation of the laser light along the propagation direction of the laser light in the quantum cascade laser. The attenuation constant α in the propagation direction of the laser beam, that is, in the direction perpendicular to the paper surface in FIG. 7, was calculated using the above-described two-dimensional finite element method. FIG. 9 shows a calculation result when the conductivity of the n-type GaSb contact layer in FIG. 7 is changed. The frequency is 3.0 THz. The smaller the attenuation constant α, the smaller the attenuation. The black circles (●) in FIG. 4 represent the attenuation constant when the conductivity of the upper n-type GaSb contact layer is constant, that is, 10000 S / m, and the conductivity of the lower n-type GaSb contact layer is changed. Square dots (■) represent attenuation constants when the conductivity of the lower n-type GaSb contact layer is constant, ie, 10000 S / m, and the conductivity of the upper n-type GaSb contact layer is changed.

  As shown in FIG. 9, the attenuation constant increases as the conductivity of the lower contact layer increases. This is presumably because the lower contact layer has a higher loss of electrons due to scattering and the like because the number of electrons increases as the conductivity increases. On the other hand, the higher the conductivity of the upper contact layer, the smaller the attenuation constant. This is because the loss due to scattering increases as in the case of the lower contact layer, but the plasmon propagation mode that occurs at the metal / dielectric interface is further generated, and the propagation of low loss is dominant. It is done.

  Thus, by increasing the conductivity of the GaSb contact layer in contact with the metal electrode and decreasing the conductivity of the GaSb contact layer in contact with the substrate, it is possible to reduce the loss associated with the propagation of terahertz light. However, it is a matter of course that the resistance increases as the conductivity decreases. Further, since the conductivity of the contact layer depends on temperature, the impurity concentration of the contact layer must be determined in consideration of the operating temperature of the quantum cascade laser.

  As described above, in the GaSb / AlGaSb-based quantum cascade laser provided on the GaAs substrate of the present invention, high optical confinement can be realized by a simple production process. Moreover, since a GaAs substrate is cheaper than a GaSb substrate, it is advantageous in terms of cost. Furthermore, by increasing the conductivity of the GaSb contact layer in contact with the metal electrode and decreasing the conductivity of the GaSb contact layer in contact with the substrate, it is possible to reduce propagation loss and perform laser oscillation efficiently.

  As described above, in the present invention, the degree of freedom in design can be further increased, and the oscillation frequency can be further decreased.

In the above embodiments, AlSb is used as the barrier layer. However, there is no reason to limit to this compound, and it is possible to use a material that forms a superlattice structure as a barrier against GaSb. In addition to the above, for example, Ga _x Al _1-x Sb (x is a value between 0 and 1) may be used. Moreover, since the AlSb layer or the GaAlSb layer is used as a barrier, it can be mixed.

  As for the waveguide structure, the present invention uses a surface plasmon mode on one side. However, the present invention is not limited to this, and a metal-metal waveguide may be used. Further, in order to confine the generated terahertz wave in the quantum cascade laser structure, contact layers with high impurity concentration are provided above and below the quantum cascade laser structure to control the refractive index. However, there is no reason to limit to this, and it is obvious that a layer exhibiting a low refractive index may be provided.

  The quantum cascade laser described above can be synchronously oscillated by injecting reference light as follows.

  For example, when it is desired to lock the oscillation of a quantum cascade laser by irradiating a 1.5 micron band semiconductor laser (injection locking, injection locking), pulse laser light is injected with the configuration shown in FIG. However, in the case of the above-described electrode, the semiconductor laser beam is reflected because the upper electrode of the quantum cascade laser is generally a metal such as gold. On the other hand, assuming that the substrate of the quantum cascade laser is GaSb, GaSb absorbs to the long wavelength side, so that injection locking with a 1.5 micron laser, for example, is not possible. In contrast to the fact that there is no absorption at the substrate, if GaAs is used as the substrate as in the embodiment, there is no such absorption, so a 1.5 micron band semiconductor laser is used, for example, a quantum cascade laser from the back of the substrate. It is possible to lock the oscillation. The semiconductor laser may be irradiated not only from the substrate side, but also from the side surface of the ridge structure or from the top if the electrode of the quantum cascade laser is a transparent electrode such as ITO.

It is a schematic diagram which shows the energy band structure of the conventional quantum cascade laser. It is a schematic diagram which shows the layer structure of the quantum cascade laser of this invention. It is a schematic diagram which shows the energy band structure of the quantum cascade laser of this invention. It is a figure which shows the outline of the manufacturing process of the quantum cascade laser of this invention. It is a figure which shows sectional drawing (a) and bird's-eye view (b) of the quantum cascade laser of this invention. It is a figure which shows the arrangement | positioning in the case of performing injection locking. It is a figure which shows the typical sectional drawing of the quantum cascade laser of this invention. It is a figure which shows the electric field distribution of the natural mode in a GaSb type | system | group quantum cascade laser on (a) GaAs substrate and (b) GaSb substrate. It is a figure which shows the relationship between the electrical conductivity of a contact layer, and an attenuation constant.

Explanation of symbols

1 State n = 1
2 State n = 2
1 'implantation region state n = 1
2 'implantation region state n = 2
3 Ground state n = 3
4 Ground state n = 4
5 Excited state n = 5

Claims (8)

  An AlSb layer or a GaAlSb layer and a GaSb layer are repeatedly stacked, and an electrode layer is formed on both ends of the superlattice device, and the thickness of the GaSb layer constituting the quantum well that performs stimulated emission of light A quantum cascade laser characterized by having a thickness at which the difference between the intrinsic energy of the quantum level formed in the GaSb layer and the first excited state becomes the LO phonon energy of GaSb.   The above superlattice element has two n layers in which a superlattice having an AlSb layer, a GaSb layer, an AlSb layer, a GaSb layer, an AlSb layer, an n-type GaSb layer, an AlSb layer, and a GaSb layer as a repeating unit is used as a contact layer. 2. The quantum cascade laser according to claim 1, wherein the quantum cascade laser has a structure sandwiched by a semiconductor layer of a type.   The above superlattice element has two n layers in which a superlattice having a GaAlSb layer, a GaSb layer, a GaAlSb layer, a GaSb layer, a GaAlSb layer, an n-type GaSb layer, a GaAlSb layer, and a GaSb layer as repeating units is used as a contact layer. 2. The quantum cascade laser according to claim 1, wherein the quantum cascade laser has a structure sandwiched by a semiconductor layer of a type.   4. The quantum cascade laser according to claim 2, wherein the quantum cascade laser is formed on a superlattice buffer.   5. The quantum cascade laser according to claim 2, wherein the quantum cascade laser is formed on a GaAs substrate via a buffer layer.   6. The quantum cascade laser according to claim 1, wherein a potential difference necessary for laser oscillation is applied between the electrode layers, and the superlattice portion is irradiated with reference light.   A first metal electrode is provided above the quantum cascade structure via a first contact layer, and a substrate is provided below the quantum cascade structure via a second contact layer. A quantum cascade laser characterized by having a conductivity higher than that of the second contact layer.   8. The quantum cascade laser according to claim 7, wherein the substrate is GaAs, and the quantum cascade structure is a multiple quantum well structure including a well made of GaSb and a barrier made of AlGaSb or AlSb.

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