JP2000156545A - Quantum cascade semiconductor laser system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser system that has a less expensive and simpler structure and can oscillate with long wavelength. SOLUTION: This quantum cascade semiconductor laser system has a superlattice semiconductor element having an intrinsic semiconductor layer 15, which is a superlattice structure layer obtained by laminating repetitively a plurality of layers, each of which consists of two barrier layers 21, 23 and two quantum well layers 22, 24. The thickness of each layer in the intrinsic semiconductor layer 15 is set so that the X level of the barrier layer 21 is substantially resonated with a second level of the quantum well layer 22, and a first level of the quantum well layer 22 is substantially resonated with a first level of the quantum well layer 24 to form an inverted population when bias voltage is applied. Injected electrons are moved from the X level of the barrier layer 21 to the second level of the quantum well layer 22, and then to the second level of the quantum well layer 22 to emit light. Further, they are transferred to the X level of the barrier layer 21 on the subsequent stage via the first level of the quantum well layer 24 to oscillate laser.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a quantum cascade laser device having a superlattice structure layer.

[0002]

2. Description of the Related Art In recent years, an optical communication network has been improved, and large-capacity, high-speed communication to a remote place has become possible. This was made possible by the realization of an optical fiber cable with low waveguide loss (0.2 dB / km) and the use of a compound semiconductor (indium phosphide) laser with a long wavelength (1.3 μm to 1.5 μm). This was because of the realization of The backbone optical network is now connected not only to Japan but also to overseas networks across the Pacific Ocean.

[0003]

However, in the case of indium phosphide compound semiconductors, a single crystal indium phosphide semiconductor substrate serving as a substrate is expensive, and a long wavelength laser light source is very expensive. For this reason, the maintenance of trunk line optical communication does not progress. In particular, Fiber to the Home (FT
In reality, the connection between each user called TH) is hardly realized because of the heavy burden on the users.

Further, the current waveguide loss characteristics of the silica-based optical fiber are almost equal to the theoretical limit, and little improvement can be expected in the future. However, it is known that there is a material such as a chalcogenite or fluorine glass optical fiber cable, whose waveguide loss is theoretically smaller than that of a silica-based optical fiber cable by about two orders of magnitude. In order to optimize the waveguide characteristics,
A longer wavelength light source of 3-4 μm is required. However, it is extremely difficult for a conventional semiconductor laser device to emit light of about 3 to 4 μm.

An object of the present invention is to solve the above problems,
It is an object of the present invention to provide a semiconductor laser device that can oscillate at a long wavelength with a simple structure at a lower cost than a conventional example.

[0006]

Generally, gallium arsenide-based materials and indium phosphide-based materials are widely used for light emitting devices. The reason is that since free electrons and holes in these materials have the same momentum, there is a high probability that so-called direct transition type electron-hole bonding is performed. In order to increase the emission wavelength, it is necessary to use a semiconductor having a small energy gap between electrons and holes.
This is why indium phosphide-based materials are used for laser light sources for communication.

[0007] The quantum cascade laser device is a light emitting element utilizing an electron level called a subband generated by a quantum effect in a semiconductor superlattice structure layer.
Suitable for long wavelength light source. Since the subband energy is determined by the quantum effect, the emission wavelength can be easily controlled even with the same material. Therefore, a long-wavelength light source can be manufactured using an inexpensive material such as GaAs. However, the structure of the conventionally proposed quantum cascade laser device is very complicated, and it is very difficult to design and grow crystals.

In view of the above, the present invention realizes a quantum cascade laser device with a simple structure by using the electron level at point X (hereinafter referred to as X level).

According to the quantum cascade laser device according to the first aspect of the present invention, in the quantum cascade laser device including a superlattice semiconductor element having a superlattice structure layer between two electrodes, The structural layer comprises a first
Of the first barrier layer, the first quantum well layer, the second barrier layer, and the second quantum well layer, which are repeated for a plurality of cycles as one cycle, and the first barrier layer and the first quantum well layer are formed. Layer and second
When a predetermined bias voltage is applied to the superlattice semiconductor device, the X level of the first barrier layer is changed to the first quantum well layer by changing the thicknesses of the barrier layer and the second quantum well layer. Substantially resonates with the second level of the first quantum well layer and the first level of the first quantum well layer.
Substantially resonates with the first level of the second quantum well layer, so that the number of carriers at the second level of the first quantum well layer is larger than that of the first quantum well layer. The population is set so as to form a population inversion that is larger than the number of carriers of the first level, and the electrons injected by applying the bias voltage to the superlattice semiconductor element are converted into X in the first barrier layer. After moving from the level to the second level of the first quantum well layer, light is emitted by transitioning to the first level of the first quantum well layer, and then the first quantum well layer is emitted. Laser oscillation is achieved by transitioning from the first level of the well layer to the X level of the first barrier layer in the next stage via the first level of the second quantum well layer. And

According to a quantum cascade laser device according to a second aspect of the present invention, in the quantum cascade laser device including a superlattice semiconductor element having a superlattice structure layer between two electrodes, The superlattice structure layer is formed by repeatedly stacking a barrier layer and a quantum well layer, and the thickness of each of the barrier layer and the quantum well layer is determined by applying a predetermined bias voltage to the superlattice semiconductor element. To
The X level of the barrier layer substantially resonates with the Nth level of the quantum well layer, and the (N-1) th level of the quantum well layer is equal to or higher than the X level of the next barrier layer. , And the X level of the next-stage barrier layer substantially resonates with the N-th level of the next-stage quantum well layer, and the number of carriers at the N-th level of the quantum well layer becomes The population is set so as to form a population inversion that is larger than the number of carriers of the (N-1) -th level in the quantum well layer, where N is a natural number of 3 or more. After the electrons injected by applying the bias voltage are moved from the X level of the barrier layer to the Nth level of the quantum well layer, the (N-1) th level of the quantum well layer is moved. To the next level, and then the amount of the next stage from the (N-1) level of the quantum well layer via the X level of the next barrier layer. By transition to level of the first N well layers, characterized in that a laser oscillation of.

Further, in the above quantum cascade semiconductor laser device, preferably, when a predetermined bias voltage is applied to the superlattice semiconductor element, the respective thicknesses of the barrier layer and the quantum well layer are changed to the quantum well layer. (N-1)
Is set so as to substantially resonate with the X level of the next barrier layer, and the electrons after the light emission are transferred from the (N-1) th level of the quantum well layer to the next barrier layer. Is transited to the Nth level of the next quantum well layer through the X level of
It is characterized by laser oscillation.

Further, in the above-described quantum cascade laser device, preferably, when a predetermined bias voltage is applied to the superlattice semiconductor element, the thickness of the barrier layer and the thickness of the quantum well layer are changed. (N-1)
Are set so as to substantially resonate with the X level of the barrier layer of the next stage, and the emitted electrons are separated from the (N-1) th level of the quantum well layer. The level lower than the (N-1) level of the quantum well layer and the X of the next barrier layer
The laser oscillation is achieved by transitioning to the N-th level of the next quantum well layer through the level.

Further, in the above quantum cascade laser device, N is preferably 3, 4, 5, or 6.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment> FIG. 1 is a perspective view and a partial sectional view showing the structure of a quantum cascade laser device according to a first embodiment of the present invention. The quantum cascade laser device of this embodiment includes a superlattice semiconductor light emitting device. The superlattice semiconductor light emitting device includes a first barrier layer 21 and a first quantum well layer 22, as shown in FIG. A heterojunction n− including an intrinsic semiconductor i-layer 15 that is a superlattice structure layer repeatedly stacked in a plurality of cycles with the second barrier layer 23 and the second quantum well layer 24 as one cycle.
It is an i-n type diode semiconductor element.

The details of the procedure for fabricating the superlattice semiconductor light emitting device of this embodiment will be described below. (1) n-type GaAs doped with n-type impurity ions of Si, for example, at a doping amount of 10 ¹⁸ / cm ³ (plane orientation of the upper surface (1, 0, 0)) and a 300 μm-thick n The following layers were sequentially crystal-grown on the type semiconductor substrate 10 using a molecular beam epitaxy apparatus. (A) n-type impurity ions made of Si have a doping amount of 2 × 1
0.2 μm thick n-type buffer layer 12 made of n-type GaAs doped with 0 ¹⁸ / cm ³ , and Si
Of n-type impurity ions of 1 × 10 ¹⁸ / cm
An n-type buffer layer 13 of n-type Al _0.5 Ga _0.5 As doped with ³ and having a thickness of 2 μm. (B) An i-type cladding layer 14 of i-type Al _0.5 Ga _0.5 As having a thickness of 500 °. (C) An intrinsic semiconductor i-layer 15 having a thickness of 1 μm and having a 20-period superlattice structure made of GaAs / AlAs. here,
First barrier layer 21 made of AlAs and having a thickness of 50.9 °
A first quantum well layer 22 made of GaAs having a thickness of 73.6 °, a second barrier layer 23 formed of AlAs having a thickness of 14.2 °, and a third barrier layer formed of GaAs having a thickness of 34.0 °. The two quantum well layers 24 are defined as one cycle, and are repeatedly stacked in 20 cycles to form an intrinsic semiconductor i-layer 15 having a thickness of 1 μm.
Get. (D) An i-type cladding layer 16 made of i-type Al _0.5 Ga _0.5 As and having a thickness of 500 °. (E) n-type impurity ions made of Si are doped at 1 × 1
N-type Al _0.5 Ga _0.5 As doped with 0 ¹⁸ / cm ³
And a 2 μm-thick n-type cap layer 17, and an n-type impurity ion similarly made of Si with a doping amount of 2 × 10 ¹⁸
A 0.2 μm thick n-type buffer layer 18 made of n-type GaAs doped with / cm ³ . (2) Then, AuGe / Ni /
The electrode 19 made of Au is connected to a photosensitive organic thin film (resist).
Was formed in a strip shape having a width of 10 μm by using a lift-off method using (3) Then, the back surface of the semiconductor substrate 10 was polished to a thickness of about 80 μm. (4) Further, AuGe / N is also provided on the back surface of the semiconductor substrate 10.
The electrode 11 made of i / Au (= 1000 ° / 300 ° / 1500 °) was formed. (5) Then, a heat treatment was performed on the entire device at 400 ° C. for 1.5 minutes to obtain an ohmic connection between the electrodes 11 and 19. (6) After the semiconductor substrate 10 was cut along the electrode lines, it was cleaved to a length of 300 μm. The inner surface of the cleavage plane (the surface on the near side and the other side in FIG. 1) has good flatness and is a mirror surface. In the present embodiment, the interval between the reflecting mirrors is 30.
It was set to 0 μm. However, the present invention is not limited to this, and may be 100 μm or 500 μm, that is, the interval can be set arbitrarily.

Further, the negative electrode of the variable voltage DC power supply 30 for applying a DC bias voltage is connected to the electrode 19 and the positive electrode thereof is connected to the electrode 11, so that a DC bias voltage is applied to the element. In the present embodiment, the electric field intensity in the intrinsic semiconductor i-layer 15 when the DC bias voltage is applied is about 300 to 400 kV / cm.

FIG. 2 is an energy band diagram showing the level energy with respect to the position in the thickness direction of the intrinsic semiconductor i-layer 15 of the quantum cascade laser device of FIG.

As shown in FIG. 2, by applying a predetermined bias voltage to the device, the Γ2 level (second level) of the first quantum well layer 22 is changed to the first barrier layer. 21
Slightly lower than the X level of the second quantum well layer 24 and substantially resonate with each other, while the Γ1 level (the first level) of the second quantum well layer 24.
Are slightly lower than the Γ1 level of the first quantum well layer 22 and substantially resonate with each other. Here, electrons injected by the application of the bias voltage and existing in the first barrier layer 21 (injection layer) having a larger effective mass of electrons are:
After moving to the Γ2 level of the first quantum well layer 22, the Γ
Light is emitted by transition to one level due to the quantum effect. Next, electrons in the Γ1 level of the first quantum well layer 22 move to the X level of the next barrier layer 21 at a high speed via the second quantum well layer 23, and the population inversion is changed. Once formed, the above process is repeated. Therefore, the first quantum well layer 2
2 becomes a light emitting layer. Here, the population inversion means that the number of carriers in the first quantum well layer 22 is smaller than the number of carriers in the first level by Γ2.
This is a state where the number of carriers in the level is large, which is a necessary condition for laser oscillation in the present embodiment.

That is, electrons are injected by applying a bias electric field to the intrinsic semiconductor i-layer 15, and light is obtained by moving through the step-like sub-band. The light obtained is
The upper and lower Al _0.5 Ga _0.5 As layers become the cladding layers 14 and 16 and light confinement occurs. Optical confinement occurs between the strip-shaped electrode 19 and the electrode 11 in the vertical direction of the element due to an increase in the refractive index due to current injection. In the direction parallel to the strip-shaped electrode 19, the cleavage mirror formed by the pair of opposed cleavage planes described above becomes a reflection mirror, and optical amplification by laser oscillation occurs. Light emitted by the laser oscillation is emitted from a side surface in a direction perpendicular to the direction of light reflection by the pair of reflecting mirrors.

As described above, in the present embodiment, the thickness of each of the layers 21 to 24 in the intrinsic semiconductor i-layer 15 is set such that the two quantum well layers 22 and 24 are asymmetrical to each other.
That is, when a predetermined bias voltage is applied, the first
X level of the barrier layer 21 substantially resonates with the Γ2 level of the first quantum well layer 22, and the Γ1 level of the first quantum well layer 22 is the Γ1 level of the second quantum well layer 24. Are set so as to substantially resonate with each other to form a population inversion. A light emission operation is caused by this population inversion, and further, a light confinement effect by the strip-shaped electrode 19 and a laser oscillation are generated by the pair of reflecting mirrors, so that a larger light emission can be obtained. The quantum cascade laser device including the superlattice semiconductor light emitting device configured as described above has a low-cost and simple structure as compared with the conventional example, and has, for example, 3 to 4
Laser oscillation can be performed at a long wavelength of μm.

<Second Embodiment> FIG. 3 is a perspective view and a partial sectional view showing the structure of a quantum cascade laser device according to a second embodiment of the present invention. In the device of the second embodiment, as shown in FIG. 3, the intrinsic semiconductor i-layer 15 is different from that of the first embodiment.
a. Other configurations are the same. Here, the intrinsic semiconductor i-layer 15a is
A barrier layer 31 made of AlAs and having a thickness of 50.9 °;
The quantum well layer 32 made of As and having a thickness of 101.9
The cycle is repeated in 20 cycles to form a laminate.
In a preferred embodiment, the electric field intensity in the intrinsic semiconductor i-layer 15a when the DC bias voltage is applied is about 350 kV / cm.

FIG. 4 is an energy band diagram showing the level energy with respect to the position in the thickness direction of the intrinsic semiconductor i-layer 15a of the quantum cascade laser device of FIG.

As shown in FIG. 4, by applying a predetermined bias voltage to the device, a quantum well layer 32 is formed.
Γ3 level (third level) is slightly lower than the X level of the barrier layer 31 and substantially resonates with each other, while the X level of the next-stage barrier layer 31 is Slightly lower than Γ2 (second level), and substantially resonate with each other. Here, the electrons injected by the application of the bias voltage and existing in the barrier layer 31 (injection layer) having a larger effective mass of electrons move to the Γ3 level of the quantum well layer 32, and then the Γ2 level. It emits light by transiting to a quantum effect.
Next, it moves to the X level of the next barrier layer 31 at high speed,
A population inversion is formed, and the above process is repeated. Also, light is emitted when electrons transition from the Γ2 level of the quantum well layer 32 to the Γ1 level due to the quantum effect. Note that light emission from the # 2 level to the # 1 level has a lower light emission intensity than light emission from the # 3 level to # 2. Therefore, the quantum well layer 32 becomes a light emitting layer. Here, the population inversion refers to a state where the number of carriers in the Γ3 level is larger than the number of carriers in the Γ2 level in the quantum well layer 32, and is a necessary condition for laser oscillation in the present embodiment.

That is, electrons are injected by applying a bias electric field to the intrinsic semiconductor i-layer 15a, and light emission is obtained by moving through the stepwise sub-band. The obtained light is formed by the upper and lower Al _0.5 Ga _0.5 As layers being clad layers 14 and 1.
It becomes 6 and light confinement occurs. Strip-shaped electrode 1
Light confinement occurs between the electrode 9 and the electrode 11 in the vertical direction of the element due to an increase in the refractive index due to current injection. In the direction parallel to the strip-shaped electrode 19, the cleavage mirror formed by the pair of opposed cleavage planes described above becomes a reflection mirror, and optical amplification by laser oscillation occurs.

As described above, in the present embodiment, a predetermined bias voltage is applied so that the thickness of each of the layers 31 and 32 in the intrinsic semiconductor i-layer 15a is symmetric with respect to the plurality of quantum well layers 32. Then, the X level of the barrier layer 31 substantially resonates with the Γ3 level of the quantum well layer 32, and the Γ2 level of the quantum well layer 32 substantially matches the X level of the next barrier layer 31. It is set to resonate to form a population inversion.
Due to this population inversion, from the Γ3 level of the quantum well layer 32 to Γ2
A light emission operation is caused by the transition to the level, and further, a light confinement effect by the strip-shaped electrode 19 and laser oscillation are generated by the pair of reflecting mirrors, so that larger light emission can be obtained. The quantum cascade laser device having the superlattice semiconductor light emitting device configured as described above has a low cost and simple structure as compared with the conventional example, and has a laser oscillation at a long wavelength of, for example, 3 to 4 μm. Can be done.

<Third Embodiment> FIG. 5 is an energy band showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15a of a quantum cascade semiconductor laser device according to a third embodiment of the present invention. FIG. This third
The device according to the second embodiment has the same structure as that of the second embodiment shown in FIG. 3 (this embodiment will be described with reference to the structural diagram of FIG. 3). Compared to,
By changing the thicknesses of the barrier layer 31 and the quantum well layer 32 and the applied bias voltage, the level at which the population inversion occurs in the quantum well layer 32 is changed to the Γ4 level (the fourth level) and the Γ3 level. (Third level), and the 設定 2 level lower than the Γ3 level and the X level of the barrier layer 31 at the next stage are set to substantially resonate. That is, a population inversion is generated such that the number of carriers in the # 4 level of the quantum well layer 32 is larger than the number of carriers in the # 3 level.

In this embodiment, the intrinsic semiconductor i-layer 15
a is a barrier layer 31 made of AlAs and having a thickness of 152.8 °.
And a quantum well layer 32 made of GaAs having a thickness of 45.3 °.
Are defined as one cycle, and are repeated in 20 cycles to form a laminate. In a preferred embodiment, the electric field intensity in the intrinsic semiconductor i-layer 15a when the DC bias voltage is applied is about 310 kV / cm.

As shown in FIG. 5, by applying a predetermined bias voltage to the device, a quantum well layer 32 is formed.
Γ4 level (fourth level) is slightly lower than the X level of the barrier layer 31 and substantially resonates with each other, while the X level of the next-stage barrier layer 31 is Slightly lower than Γ2 (second level), and substantially resonate with each other. Here, the electrons injected by the application of the bias voltage and existing in the barrier layer 31 (injection layer) having a larger effective mass of electrons move to the Γ4 level of the quantum well layer 32, and then move to the Γ3 level. After emitting light by transition to the level due to the quantum effect, the state transits to the Γ2 level. Next, the quantum well layer 32 moves at a high speed from the Γ2 level to the X level of the barrier layer 31 at the next stage to form a population inversion, and the above process is repeated. In addition, electrons are generated in the quantum well layer 32 by Γ2
Light is also emitted by transition from the level to the # 1 level due to the quantum effect. Therefore, the quantum well layer 32 becomes a light emitting layer.
Here, the population inversion means that in the quantum well layer 32, Γ4
This is a state where the number of carriers in the Γ3 level is larger than the number of carriers in the level, which is a necessary condition for laser oscillation in the present embodiment.

That is, electrons are injected by applying a bias electric field to the intrinsic semiconductor i-layer 15a, and light emission is obtained by moving through the stepwise sub-band. The obtained light is formed by the upper and lower Al _0.5 Ga _0.5 As layers being clad layers 14 and 1.
It becomes 6 and light confinement occurs. Strip-shaped electrode 1
Light confinement occurs between the electrode 9 and the electrode 11 in the vertical direction of the element due to an increase in the refractive index due to current injection. In the direction parallel to the strip-shaped electrode 19, the cleavage mirror formed by the pair of opposed cleavage planes described above becomes a reflection mirror, and optical amplification by laser oscillation occurs.

As described above, in the present embodiment, a predetermined bias voltage is applied so that the thickness of each of the layers 31 and 32 in the intrinsic semiconductor i-layer 15a is symmetric with respect to the plurality of quantum well layers 32. Then, the X level of the barrier layer 31 substantially resonates with the Γ4 level of the quantum well layer 32, and the Γ2 level of the quantum well layer 32 substantially matches the X level of the next barrier layer 31. It is set to resonate to form a population inversion.
Due to this population inversion, from the Γ4 level of the quantum well layer 32 to Γ3
A light emission operation is caused by the transition to the level, and further, a light confinement effect by the strip-shaped electrode 19 and laser oscillation are generated by the pair of reflecting mirrors, so that larger light emission can be obtained. The quantum cascade laser device having the superlattice semiconductor light emitting device configured as described above has a low cost and simple structure as compared with the conventional example, and has a laser oscillation at a long wavelength of, for example, 3 to 4 μm. Can be done.

<Fourth Embodiment> FIG. 6 is an energy band showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15a of a quantum cascade semiconductor laser device according to a fourth embodiment of the present invention. FIG. This fourth
The device according to the second embodiment has the same structure as that of the second embodiment shown in FIG. 3 (this embodiment will be described with reference to the structural diagram of FIG. 3). Compared to,
By changing the thicknesses of the barrier layer 31 and the quantum well layer 32 and the applied bias voltage, the level at which the population inversion occurs in the quantum well layer 32 is changed to the Γ5 level (the fifth level) and the Γ4 level. (Fourth level), and the Γ3 level lower than the Γ4 level and the X level of the barrier layer 31 at the next stage are set so as to substantially resonate. That is, a population inversion is generated such that the number of carriers in the # 5 level of the quantum well layer 32 is larger than the number of carriers in the # 4 level.

In this embodiment, the intrinsic semiconductor i-layer 15
a is a barrier layer 31 made of AlAs and having a thickness of 169.8 °.
And a quantum well layer 32 of GaAs having a thickness of 56.6 °
Are defined as one cycle, and are repeated in 20 cycles to form a laminate. In a preferred embodiment, the electric field intensity in the intrinsic semiconductor i-layer 15a when the DC bias voltage is applied is about 290 kV / cm. In the quantum cascade semiconductor laser device configured as described above,
As shown in FIG. 6, it operates in the same manner as the third embodiment,
It emits light and oscillates at a predetermined long wavelength.

<Fifth Embodiment> FIG. 7 is an energy band showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15a of a quantum cascade semiconductor laser device according to a fifth embodiment of the present invention. FIG. This fifth
The device according to the second embodiment has the same structure as that of the second embodiment shown in FIG. 3 (this embodiment will be described with reference to the structural diagram of FIG. 3). Compared to,
By changing the thicknesses of the barrier layer 31 and the quantum well layer 32 and the applied bias voltage, the level at which the population inversion occurs in the quantum well layer 32 is changed to the # 6 level (sixth level) and the # 5 level. (Fifth level), and a setting is made such that the Γ4 level lower than the Γ5 level and the X level of the barrier layer 31 at the next stage substantially resonate. That is, a population inversion is generated such that the number of carriers in the # 6 level of the quantum well layer 32 is larger than the number of carriers in the # 5 level.

In this embodiment, the intrinsic semiconductor i-layer 15
a is a barrier layer 31 made of AlAs and having a thickness of 184.0 °.
And a quantum well layer 32 of GaAs having a thickness of 56.6 °
Are defined as one cycle, and are repeated in 20 cycles to form a laminate. In a preferred embodiment, the electric field strength in the intrinsic semiconductor i-layer 15a when the DC bias voltage is applied is about 280 kV / cm. In the quantum cascade semiconductor laser device configured as described above,
As shown in FIG. 7, it operates in the same manner as the third embodiment,
It emits light and oscillates at a predetermined long wavelength.

<Modification> In the second to fifth embodiments described above, the level at which electrons are located after light emission in the quantum well layer 32 is at least higher than the X level of the barrier layer 31 in the next stage. It only has to be a place. That is, third to fifth
In the embodiment of the present invention, after the light emission, the electron transitions from the level at which the electron is located to the level lower by one stage and transits to the X level of the barrier layer 31 at the next stage, but the present invention is not limited to this. Alternatively, after the light emission, a transition may be made to the level (transition source) lower by a plurality of stages from the level at which the electron is located, and then to the X level of the barrier layer 31 at the next stage. In this case, it is preferable that the lower level of the transition source in the quantum well layer 32 is a level slightly higher than the X level of the barrier layer 31 of the transition destination and substantially resonates with each other. Furthermore, the Γ level before light emission in the quantum well layer 32 that substantially resonates with the X level of the barrier layer 31 at the preceding stage may be a Γ level where ΓN is a natural number of 3 or more.

[0037] In the above embodiment has been described superlattice semiconductor light emitting element which is a n-i-n diode semiconductor device, the present invention is not limited to this, n ⁺ -i
−n ⁺ type diode semiconductor element or n ⁺ −
It may be an ip ⁺ type diode semiconductor element. In the former case, the buffer layer 12, 13, 18 and the cap layer 17 can be formed by injecting Si at a doping amount of, for example, 10 ¹⁸ / cm ^{3. In} the latter case, the cap layer 17 and the cap layer 17 can be formed. The buffer layer 18 can be formed by injecting Be at a doping amount of, for example, 10 ¹⁸ / cm ³ . Further, an n-n-n type diode semiconductor element, an n-n-p type diode semiconductor element, n
-It may be a pp type diode semiconductor element.

[0038]

As described above in detail, according to the quantum cascade laser device according to the first aspect of the present invention, a quantum cascade including a superlattice semiconductor element having a superlattice structure layer between two electrodes is provided. In the semiconductor laser device, the superlattice structure layer is formed by repeatedly laminating a first barrier layer, a first quantum well layer, a second barrier layer, and a second quantum well layer in a plurality of cycles as one cycle. The thickness of the first barrier layer, the first quantum well layer, the second barrier layer, and the thickness of the second quantum well layer, when a predetermined bias voltage is applied to the superlattice semiconductor element, The X level of the first barrier layer is equal to the first level of the first barrier layer.
Substantially resonates with the second level of the quantum well layer, and the first level of the first quantum well layer substantially resonates with the first level of the second quantum well layer. The population is set so as to form a population inversion in which the number of carriers at the second level of the first quantum well layer is larger than the number of carriers at the first level of the first quantum well layer. The electrons injected by applying the bias voltage to the superlattice semiconductor element are moved from the X level of the first barrier layer to the second level of the first quantum well layer.
, The light is emitted by making a transition to the first level of the first quantum well layer, and then the second level from the first level of the first quantum well layer. The laser is oscillated by transitioning to the X level of the first barrier layer in the next stage through the first level of the quantum well layer. Therefore,
According to the present invention, a laser can be oscillated at a long wavelength with a simpler structure at a lower cost than the conventional example.

Further, according to the quantum cascade laser device according to the second aspect of the present invention, in the quantum cascade laser device provided with a superlattice semiconductor element having a superlattice structure layer between two electrodes, The superlattice structure layer is formed by repeatedly stacking a barrier layer and a quantum well layer, and the thickness of each of the barrier layer and the quantum well layer is determined by applying a predetermined bias voltage to the superlattice semiconductor element. To
The X level of the barrier layer substantially resonates with the Nth level of the quantum well layer, and the (N-1) th level of the quantum well layer is equal to or higher than the X level of the next barrier layer. , And the X level of the next-stage barrier layer substantially resonates with the N-th level of the next-stage quantum well layer, and the number of carriers at the N-th level of the quantum well layer becomes The population is set so as to form a population inversion that is larger than the number of carriers of the (N-1) -th level in the quantum well layer, where N is a natural number of 3 or more. After the electrons injected by applying the bias voltage are moved from the X level of the barrier layer to the Nth level of the quantum well layer, the (N-1) th level of the quantum well layer is moved. To the next level, and then the amount of the next stage from the (N-1) level of the quantum well layer via the X level of the next barrier layer. By transition to level of the first N well layer to laser oscillation. Therefore, according to the present invention, a laser can be oscillated at a long wavelength with a low cost and a simple structure as compared with the conventional example.

[Brief description of the drawings]

FIG. 1 is a perspective view and a partial cross-sectional view illustrating a structure of a quantum cascade laser device according to a first embodiment of the present invention.

FIG. 2 is an energy band diagram showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15 in the quantum cascade laser device of FIG.

FIG. 3 is a perspective view and a partial cross-sectional view illustrating a structure of a quantum cascade laser device according to a second embodiment of the present invention.

4 is an energy band diagram showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15a of the quantum cascade laser device of FIG.

FIG. 5 is an energy band diagram showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15a of a quantum cascade laser device according to a third embodiment of the present invention.

FIG. 6 is an energy band diagram showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15a of a quantum cascade laser device according to a fourth embodiment of the present invention.

FIG. 7 is an energy band diagram showing a level energy with respect to a position in a thickness direction of an intrinsic semiconductor i-layer 15a of a quantum cascade laser device according to a fifth embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 10: semiconductor substrate, 11, 19: electrode, 12, 13, buffer layer, 14: clad layer, 15, 15a: intrinsic semiconductor i layer, 16: clad layer, 17: cap layer, 18: buffer layer, 21: first layer 1 barrier layer, 22 first quantum well layer, 23 second barrier layer, 24 second quantum well layer, 30 DC power supply, 31 barrier layer, 32 quantum well layer.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Naoki Otani 5 Sanraya, Saniya-cho, Seika-cho, Soraku-gun, Kyoto, Japan AT-R Environmentally-friendly Communication Laboratory (72) Inventor Pablo Baccaro Kyoto 5 Shiragaya, Seiyacho, Seika-cho, Soraku-gun

Claims (8)

[Claims] 1. A quantum cascade laser device comprising a superlattice semiconductor element having a superlattice structure layer between two electrodes, wherein the superlattice structure layer comprises a first barrier layer and a first quantum well layer. , A second barrier layer, and a second quantum well layer are repeatedly formed in a plurality of cycles with one cycle being formed, and the first barrier layer, the first quantum well layer, the second barrier layer, and the second barrier layer are formed. When a predetermined bias voltage is applied to the superlattice semiconductor element, the X level of the first barrier layer is changed to the second level of the first quantum well layer. And the first level of the first quantum well layer substantially resonates with the first level of the second quantum well layer;
The first quantum well layer is set so as to form a population inversion such that the number of carriers at the second level in the first quantum well layer is larger than the number of carriers at the first level in the first quantum well layer; After the electrons injected by applying the bias voltage to the superlattice semiconductor element are moved from the X level of the first barrier layer to the second level of the first quantum well layer, the first Is emitted by making a transition to the first level of the quantum well layer, and then from the first level of the first quantum well layer via the first level of the second quantum well layer. By making a transition to the X level of the first barrier layer in the next stage,
A quantum cascade semiconductor laser device characterized by performing laser oscillation. 2. A quantum cascade laser device comprising a superlattice semiconductor element having a superlattice structure layer between two electrodes, wherein the superlattice structure layer is formed by repeatedly stacking a barrier layer and a quantum well layer. When a predetermined bias voltage is applied to the superlattice semiconductor device, the X level of the barrier layer is set to the Nth of the quantum well layer. Substantially resonates with the level, the (N-1) th level of the quantum well layer is located at or above the X level of the next barrier layer, and the X level of the next barrier layer is
The level substantially resonates with the Nth level of the next quantum well layer, and the number of carriers of the Nth level of the quantum well layer is the (N-1) th level of the quantum well layer. Is set so as to form a population inversion that is larger than the number of carriers of the order, where N is a natural number of 3 or more, and the electrons injected by applying the bias voltage to the superlattice semiconductor element are After moving from the X level of the barrier layer to the N-th level of the quantum well layer, the light is emitted by transitioning to the (N-1) -th level of the quantum well layer. From the (N-1) th level of the well layer to the Nth level of the next quantum well layer via the X level of the next barrier layer
Characterized in that a laser oscillates by making a transition to a level of. 3. The quantum cascade laser device according to claim 2, wherein the thickness of the barrier layer and the thickness of the quantum well layer are adjusted by applying a predetermined bias voltage to the superlattice semiconductor element. The (N-1) -th level of the layer is set so as to substantially resonate with the X-level of the next barrier layer, and the emitted electrons are transferred to the (N-1) -th level of the quantum well layer. A quantum cascade laser device wherein a laser oscillates by making a transition from a level to the Nth level of the next quantum well layer via the X level of the next barrier layer. 4. The quantum cascade laser device according to claim 2, wherein the thickness of the barrier layer and the thickness of the quantum well layer are adjusted by applying a predetermined bias voltage to the superlattice semiconductor element. The level lower than the (N-1) level of the layer is set so as to substantially resonate with the X level of the next-stage barrier layer. The Nth level of the next quantum well layer from the (N-1) level through the level lower than the (N-1) th level of the quantum well layer and the X level of the next barrier layer. A quantum cascade semiconductor laser device wherein a laser is oscillated by transition to a level. 5. The quantum cascade laser device according to claim 2, wherein said N is three. 6. The quantum cascade laser device according to claim 2, wherein N is 4. 7. The quantum cascade laser device according to claim 2, wherein said N is 5. 8. The quantum cascade laser device according to claim 2, wherein said N is 6.