JP2015012221A - Quantum cascade laser element, method of driving the same, and electromagnetic wave generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve controllability and stability of laser output in a quantum cascade laser element.SOLUTION: A quantum cascade laser element 1 includes a GaAs substrate 11, an n-type GaAs layer 12, a quantum cascade layer 13, a p-type GaAs layer 14, an n-type AlGaAs layer 15, a first electrode 16, a second electrode 17, and a third electrode 18. The GaAs substrate 11 is a semi-insulating substrate. The n-type GaAs layer 12 is an n-doped semiconductor layer laminated on a surface of the GaAs substrate 11. For example, the n-type GaAs layer 12 has an electron concentration of 1×10cmby Sn doping and has a film thickness of 100 nm. The quantum cascade layer 13 is formed on a surface of the n-type GaAs layer 12 and constitutes a semiconductor superlattice.

Description

  The present invention relates to a quantum cascade laser element, and more particularly to a quantum cascade laser element that emits terahertz waves and infrared light, a driving method thereof, and an electromagnetic wave generator.

  Since a general semiconductor laser uses interband transition, the oscillation wavelength is determined by the semiconductor laser material forming the band. On the other hand, quantum cascade lasers with a light-emitting layer composed of superlattices use transitions between subband levels of the conduction band (or valence band), so the oscillation wavelength is the thickness of the well layer and barrier layer of the superlattice. And can be controlled freely by a hetero barrier (band discontinuity). Therefore, research and development has been actively conducted in recent years.

  For example, a terahertz band (1 to 3 THz) useful for fluoroscopy and analysis is difficult in the transition between bands, and even in Insb having the narrowest band gap among practically usable semiconductor materials, the band gap value is 170 meV. The oscillation frequency corresponding to the band gap is about 40 THz. Therefore, a quantum cascade laser has been proposed as a device that emits terahertz band electromagnetic waves (Non-Patent Document 1). Actually, oscillation of 1.6 THz (6.6 meV) is realized by light emission between subbands (Non-Patent Document 2).

  FIG. 6 is an example of a band structure diagram of a conventional terahertz band quantum cascade laser device. The vertical direction represents the energy level, and the horizontal direction represents the stacking thickness direction. A conventional quantum cascade laser element is composed of a semiconductor superlattice 600. In this structure, GaAs is used for the well layer and AlGaAs (Al15%) is used for the barrier layer as the semiconductor superlattice. The broken line portion in the figure is one unit, and one unit is constituted by AlGaAs, GaAs, AlGaAs, GaAs, and AlGaAs. The thicknesses are 4 nm, 2.4 nm, 4 nm, 3 nm, respectively (the fifth layer is included in the first layer). The first GaAs well is a light emitting well 601 and the second GaAs well is an injection well 603. Although three units are shown in this figure, in actuality, about 100 units are stacked.

  In this quantum cascade structure, the voltage source 610 is connected to both ends of the semiconductor superlattice and the voltage V is applied, so that the band is inclined. When a voltage is applied so that the lower subband level 601B of the light emitting well 601 and the upper subband level 603A of the injection well 603 are equal, electrons in the light emitting well 601 are injected into the injection well 603 and the injection well 603 due to the resonant tunneling phenomenon. Are transported to the light emitting well 601 one after another. As a result, electrons flow sequentially from top to bottom.

  In the light emitting well 601, when electrons move from the upper subband level 601A to the lower subband level 601B, the electrons lose their energy and emit terahertz light. The time constant τ3 related to this radiation is on the order of several psec.

  On the other hand, in the injection well 603, the energy difference between the upper subband level 603A and the lower subband level 603B is set to the same level as the LO (Longitudinal Optical) phonon energy of the material forming the semiconductor superlattice. deep. Specifically, in the case of GaAs, it is 35 meV. In this way, the energy transition from the upper subband level 603A to the lower subband level 603B in the injection well 603 resonates with the LO phonon, and the energy transition of electrons occurs with a time constant τ4 on the order of sub psec. .

  As a result, electrons having the energy of the upper subband level 603A (= lower subband level 601B of the light emitting well 601) of the injection well 603 are quickly lost. Due to the time constant relationship τ3> τ4 in these energy transitions, a so-called inversion distribution is generated in the light emitting well 601 in which the number of electrons in the upper subband level 601A is larger than that in the lower subband level 601B. Laser oscillation occurs at.

  On the other hand, the quantum cascade laser device is usefully operated even at wavelengths in the infrared region. In this case, a conduction band discontinuity large enough for subband transition is required, and an example using a nitride semiconductor material is shown (Patent Document 1). This document describes a quantum cascade laser element composed of a superlattice of GaN and AlN aiming at an oscillation wavelength of 1.5 μm.

  The above describes the basic principle. In an actual device, a plurality of well layers / barrier layers may be inserted to facilitate electron energy transition.

Nature 2002, 417, 156 (Nature, 2002, vol.417, p.156) Applied Physics Letters 2006, 89, 231121 (AppliedPhysics Letters, 2006, vol.89, p.231121)

JP 2004-311466 A

  However, the conventional quantum cascade laser device has problems that it is difficult to control the amount of emitted light and is not suitable for high output. Hereinafter, this problem will be described with reference to the drawings.

  FIGS. 7A, 7B, and 7C are band structure diagrams of conduction bands when voltages V1, V2, and V3 are applied to the above-described conventional quantum cascade laser elements, respectively. For the sake of simplicity, it is assumed that the quantum cascade laser element is composed of two wells (light emitting well 601 and injection well 603) and a barrier layer 602. Like FIG. 6, the light emitting well 601 has an upper subband level 601A and a lower portion. Assume that the subband level 601B and the injection well 603 are composed of an upper subband level 603A and a lower subband level 603B. Assume that the voltage is V1 <V2 <V3, and laser oscillation occurs near V = V2. That is, at V = V2, the lower subband level 601B and the upper subband level 603A, and the upper subband level 601A and the lower subband level 603B have substantially the same potential. Electrons pass through the barrier layer 602. Note that the upper subband level 601A and the lower subband level 603B have substantially the same potential in the adjacent well (to the well depicted in FIG. 7). Therefore, it is not depicted in FIG.

  At the stage of V = V1, the subband of the light emission well 601 and the subband of the injection well 603 do not coincide with each other as shown in FIG. The electrons in the upper subband level 601A have a higher probability of transition to the upper subband level 603A than the transition to the lower subband level 601B. As a result, a reactive current is generated.

  At the stage of V = V2, as shown in FIG. 7B, the subband of the light emitting well 601 and the subband of the injection well 603 have the same potential and a current flows.

  At the stage of V = V3, as shown in FIG. 7C, the potential of the subband of the light emission well 601 and the subband of the injection well 603 are shifted again, and the wave function of electrons in the light emission well 601 and the electrons in the injection well 603 are lost. The wave functions overlap each other and electrons do not pass (tunnel) through the barrier layer 602. As a result, no current flows.

  Therefore, to put it extremely, the quantum cascade laser element flows only in the state of V = V2. If the average resistance of the quantum cascade laser element at this time is R, a current of I = V2 / R flows. That is, the quantum cascade laser element is essentially a device that operates only at a specific voltage / current.

  FIG. 7D is a graph showing current-voltage characteristics and current-laser output characteristics of a conventional quantum cascade laser element. In an actual quantum cascade laser element, since each subband has a certain width, the injection current gradually increases as the applied voltage V increases (region (a) and region (shown in FIG. 7D)). b)). However, before and after the potentials of the subband of the light emitting well 601 and the subband of the injection well 603 coincide, a kink (bent portion) occurs in the IV characteristic. The lower current side than the kink corresponds to FIG. 7A, and a reactive current is flowing. On the other hand, the higher current side than the kink corresponds to FIG. 7B and is a state where no reactive current flows. On the higher current side than the kink, this laser element has a gain, and a laser output can be obtained from the point in time when the gain exceeds the resonator loss.

  Here, from the state of the region (b) shown in FIG. 7 (d), if the applied voltage is increased and V = V3 in order to increase the injection current for higher output, the subband is obtained. Are separated from each other and current does not flow rapidly (region (c) described in FIG. 7D). As a result, there is no gain and the output is reduced.

  As described above, in the conventional quantum cascade laser element, when the optimum applied voltage is determined, the injection current and the laser output amount corresponding to the injected current amount are simultaneously determined depending on the determined applied voltage. Therefore, the conventional quantum cascade laser element is difficult to control the amount of terahertz radiation, has poor laser output stability, and is not suitable for high output.

  In view of the above-described problems, an object of the present invention is to provide a quantum cascade laser element that has high controllability and stability of laser output and is capable of high output in a quantum cascade laser element, and a driving method thereof.

  In order to solve the above problems, a quantum cascade laser device according to the present invention is a quantum cascade laser device having a semiconductor superlattice that emits photons by intersubband transition, and voltage application to the semiconductor superlattice Independently, an electron injecting portion capable of injecting electrons into the semiconductor superlattice is provided.

  By adopting the above configuration, the amount of electron injection can be controlled independently with the voltage level applied to the semiconductor superlattice fixed and the potential level of each subband fixed, so the stability of laser oscillation output and high output Can be realized.

  The electron injection part preferably includes a p-type semiconductor layer in contact with the semiconductor superlattice.

  Thereby, for example, conduction band electrons generated in the p-type semiconductor layer by photoexcitation from the outside can be injected into the semiconductor superlattice independently of the voltage applied to the semiconductor superlattice. Therefore, the current flowing through the semiconductor superlattice can be controlled, and the controllability of the laser oscillation output is improved.

  The electron injection part further includes an n-type semiconductor layer in contact with the p-type semiconductor layer, and the p-type semiconductor layer and the n-type semiconductor layer are independent of voltage application to the semiconductor superlattice. It is preferable to inject electrons into the semiconductor superlattice by applying a voltage between the two.

  Thus, electrons injected from the n-type semiconductor layer into the p-type semiconductor layer by applying a voltage between the n-type semiconductor layer and the p-type semiconductor layer independently of the voltage applied to the voltage semiconductor superlattice. Can be implanted into the semiconductor superlattice. Therefore, the current flowing through the semiconductor superlattice can be controlled, and the controllability of the laser oscillation output is improved.

  The quantum cascade laser is preferably made of a nitride semiconductor. Nitride semiconductors have a large band discontinuity and increase the design freedom of the oscillation wavelength. In addition, the thermal conductivity is high, and heat generated by reactive power in the element can be easily released to the heat sink, so the laser operation is stabilized. Controllability of laser oscillation output is improved.

  Further, the resistance of the p-type layer can be reduced by about one digit compared to GaN by using InGaN. Therefore, by using InGaN for the p-type layer in the present invention, Joule heat generation in the element can be suppressed, and the controllability of laser oscillation output is improved.

  Further, it is desirable that the band gaps of the p layer and the n layer for injecting electrons are larger than the energy of the emitted photons of this quantum cascade laser.

  This eliminates absorption of outgoing photons by the p layer and n layer that inject electrons, and improves the controllability of the laser oscillation output.

  The thickness of the p-type semiconductor layer is preferably smaller than the electron diffusion distance in the p-type semiconductor layer.

  As a result, since the p-type semiconductor layer is thinner than the electron diffusion distance of the p-type semiconductor layer, it is possible to improve the efficiency of electron injection into the semiconductor superlattice, reduce the invalid power, and control the laser oscillation output. Improves.

  The band gap of the n-type semiconductor layer is preferably larger than the band gap of the p-type semiconductor layer.

  At the pn junction between the p-type semiconductor layer and the n-type semiconductor layer, mutual injection of electrons and holes occurs. With this configuration, since the band gap of the n-type semiconductor layer is larger than the band gap of the p-type semiconductor layer in the pn junction, electrons can be injected more than holes, and the semiconductor superlattice can be obtained. It becomes possible to improve the electron injection efficiency.

  Furthermore, a substrate, an n-doped semiconductor layer formed between the substrate and the semiconductor superlattice, a first electrode formed on the surface of the n-type semiconductor layer and electrically connected to the n-type semiconductor layer, A second electrode formed on the surface of the p-type semiconductor layer and electrically connected to the p-type semiconductor layer; and a third electrode formed on the surface of the n-doped semiconductor layer and electrically connected to the n-doped semiconductor layer. The voltage applied to the semiconductor superlattice is applied between the second electrode and the third electrode, and the voltage applied between the p-type semiconductor layer and the n-type semiconductor layer is It may be applied between two electrodes and the first electrode.

  As a result, the voltage application for electron injection into the semiconductor superlattice and the voltage application for adjusting the potential level of each subband of the semiconductor superlattice are made independent. Output can be realized. In addition, since the second electrode can be shared when the two voltages are applied, the quantum cascade laser element can be miniaturized.

  In addition, this invention is realizable as an electromagnetic wave generator which used the quantum cascade laser element provided with such a characteristic means as a light source. In addition, it cannot be overemphasized that an electromagnetic wave includes the frequency band which those skilled in the art treat as "light" here.

  In addition, the present invention can be realized not only as a quantum cascade laser element having such characteristic means but also as a driving method of the quantum cascade laser element.

  In order to solve the above problems, a driving method of a quantum cascade laser device according to the present invention is a driving method of a quantum cascade laser device having a semiconductor superlattice that emits photons by intersubband transition, and the semiconductor superlattice Optimum application in which the laser output from the semiconductor superlattice is maximized by sweeping the applied voltage to the semiconductor superlattice while keeping the amount of electron injection from the electron injection part constant so that electrons can be injected into the semiconductor superlattice. A voltage determining step for determining a voltage value; and while the applied voltage to the semiconductor superlattice layer is fixed to the optimum applied voltage value determined in the voltage determining step, from the electron injection portion to the semiconductor superlattice By sweeping the electron injection amount, an electron injection amount determination step for determining the optimum electron injection amount that maximizes the laser output from the semiconductor superlattice. Tsu, characterized in that it comprises a flop.

  Since this driving method can provide an optimum operating point of the quantum cascade laser element, highly efficient operation is possible.

  According to the quantum cascade laser element and the driving method thereof of the present invention, voltage application for adjusting the potential level of each subband of the semiconductor superlattice and current injection into the semiconductor superlattice can be controlled independently. . Therefore, the controllability and stability of the laser output amount can be improved, and high output can be realized.

It is a composition perspective view of the quantum cascade laser element concerning a 1st embodiment of the present invention. It is a figure which shows an example of the band structure figure of the quantum cascade laser element which concerns on the 1st Embodiment of this invention. 5 is a graph showing current-voltage characteristics and current-laser output characteristics of the quantum cascade laser device according to the first embodiment of the present invention. It is a flowchart showing the drive method of the quantum cascade laser element which concerns on the 1st Embodiment of this invention. (A) is a graph showing the current-voltage characteristics and current-laser output characteristics of the quantum cascade laser element in the case of low Vc, and (b) is the current-voltage characteristics and current of the quantum cascade laser element in the case of high Vc. -A graph representing laser output characteristics. It is a figure which shows an example of the band structure figure of the conventional quantum cascade laser element. (A) is a band structure diagram of the conduction band when the voltage V1 is applied to the conventional quantum cascade laser element, and (b) is a band structure of the conduction band when the voltage V2 is applied to the conventional quantum cascade laser element. FIG. 4C is a band structure diagram of a conduction band when a voltage V3 is applied to a conventional quantum cascade laser element. FIG. 4D is a current-voltage characteristic and a current-laser output characteristic of the conventional quantum cascade laser element. It is a graph showing. It is a structure perspective view of the quantum cascade laser element which concerns on the 2nd Embodiment of this invention. It is a figure which shows an example of the band structure figure of the quantum cascade laser element which concerns on the 2nd Embodiment of this invention.

  The quantum cascade laser device according to the embodiment of the present invention includes a semiconductor superlattice that emits photons by intersubband transition and an electron injection unit that can inject electrons into the semiconductor superlattice. The electron injection portion includes a p-type semiconductor layer in contact with the semiconductor superlattice and an n-type semiconductor layer in contact with the p-type semiconductor layer, independently of voltage application to the semiconductor superlattice. Electrons can be injected into the semiconductor superlattice by applying a voltage between the p-type semiconductor layer and the n-type semiconductor layer. Thereby, the electron injection amount can be controlled without changing the potential level of each subband of the semiconductor superlattice. Therefore, it is possible to realize stability and high output of the laser oscillation output.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a structural perspective view of a quantum cascade laser device according to a first embodiment of the present invention. The quantum cascade laser device 1 in FIG. 1 includes a GaAs substrate 11, an n ⁺ -type GaAs layer 12, a quantum cascade layer 13, a p-type GaAs layer 14, an n-type AlGaAs layer 15, a first electrode 16, and a second electrode. An electrode 17 and a third electrode 18 are provided.

  The GaAs substrate 11 is a semi-insulating substrate.

The n ⁺ -type GaAs layer 12 is an n-doped semiconductor layer stacked on the surface of the GaAs substrate 11. For example, the n ⁺ -type GaAs layer 12 has an electron concentration of 1 × 10 ¹⁸ cm ⁻³ by Sn doping and a film thickness of 100 nm.

The quantum cascade layer 13 is formed on the surface of the n ⁺ -type GaAs layer 12 and constitutes a semiconductor superlattice. The quantum cascade layer 13 is subjected to energy transition between subbands formed in the semiconductor superlattice by applying a predetermined voltage to both ends thereof. Due to this energy transition, the semiconductor superlattice emits photons and emits light. Unlike the general semiconductor laser, the quantum cascade laser element 1 can control the oscillation frequency not by the main material constituting the laser but by the thickness of the superlattice well layer, the barrier layer, and the heterobarrier.

  The quantum cascade layer 13 is made of AlGaAs / GaAs / AlGaAs / GaAs as one unit, and has a quantum cascade region composed of, for example, 170 units. The film thickness of each layer of the 1 unit is, for example, 4.8 nm / 8.2 nm / 3.7 nm / 9.3 nm in order, and the total film thickness is 26 nm. Moreover, the Al composition with respect to AlGaAs contained in the one unit is 15%. In addition, each layer of the 1 unit is undoped. As described above, the total number of layers of the quantum cascade layer 13 is 170 × 4 layers = 680 layers, and the total film thickness is 170 × 26 nm = about 4.4 μm.

The p-type GaAs layer 14 is stacked on the surface of the quantum cascade layer 13 and has a function as a p-type semiconductor layer. For example, the p-type GaAs layer 14 has a hole concentration of 3 × 10 ¹⁷ cm ⁻³ by Mg doping and a film thickness of 100 nm. The reason why Mg is used as the dopant of the p-type GaAs layer 14 is to prevent solid phase diffusion.

The n-type AlGaAs layer 15 is stacked on the surface of the p-type GaAs layer 14 and has a function as an n-type semiconductor layer. For example, the n-type AlGaAs layer 15 has an electron concentration of 1 × 10 ¹⁷ cm ⁻³ and a film thickness of 150 nm by Sn doping. The Al composition is 10%.

  The p-type GaAs layer 14 and the n-type AlGaAs layer 15 have a function as an electron injection part that injects electrons into the quantum cascade layer 13 by a pn junction formed at the junction interface between them.

  The first electrode 16 is formed on the surface of the n-type AlGaAs layer 15 and is connected to one end of the voltage source 19.

  The second electrode 17 is formed on the surface of the p-type GaAs layer 14 from which a part of the n-type AlGaAs layer 15 is removed, and is connected to the other end of the voltage source 19 and one end of the voltage source 20. In the present embodiment, the second electrode 17 is disposed at two positions with the n-type AlGaAs layer 15 interposed therebetween in order to reduce the resistance.

The third electrode 18 is formed on the surface of the n ⁺ -type GaAs layer 12 from which a part of the quantum cascade layer 13 has been removed, and is connected to the other end of the voltage source 20.

  The voltage source 19 is a first voltage source that can vary a supplied voltage value, and is inserted between the second electrode 17 and the first electrode 16, and the first electrode 16 becomes negative with respect to the second electrode 17. So connected.

  By applying a predetermined voltage from the voltage source 19, electrons are injected from the p-type GaAs layer 14 into the quantum cascade layer 13.

  The voltage source 20 is a second voltage source that can vary the voltage value to be supplied. The voltage source 20 is inserted between the second electrode 17 and the third electrode 18, and the third electrode 18 becomes positive with respect to the second electrode 17. So connected.

  By applying a predetermined voltage from the voltage source 20, the energy level of each band of the quantum cascade layer 13 is inclined in the stacking direction.

  With the above configuration, the voltage source 19 and the voltage source 20 can be set independently. Therefore, the inclination of the energy level of each band of the quantum cascade layer 13 and the amount of electrons injected from the p-type GaAs layer 14 into the quantum cascade layer 13 can be controlled independently. Further, since the second electrode 17 is shared when the voltages from the two voltage sources 19 and 20 are applied, the quantum cascade laser element can be reduced in size.

FIG. 2 is an example of a band structure diagram of the quantum cascade laser device according to the embodiment of the present invention. The vertical direction of the band structure diagram described in FIG. 2 represents the energy level of each band, and the horizontal direction represents the structure on the line XX ′ described in FIG. In FIG. 2, the description of the GaAs substrate 11 and the n ⁺ -type GaAs layer 12 in the structure on the XX ′ line is omitted, and only three units of the quantum cascade layer 13 are illustrated. .

  The quantum cascade laser device 1 according to the present invention uses GaAs as a well layer and AlGaAs as a barrier layer as the quantum cascade layer 13. A portion surrounded by a broken line in FIG. 2 is one unit described above. The first GaAs well is the light emitting well 132, and the second GaAs well is the injection well 134. In this figure, three units are shown, but actually about 170 units are stacked.

  In this quantum cascade structure, since the voltage source 20 is connected to both ends of the quantum cascade layer 13 and the variable voltage Vc is applied, the band is inclined. When a voltage is applied so that the lower subband level 132B of the light emitting well 132 is equal to the upper subband level 134A of the injection well 134, electrons in the light emitting well 132 are transferred to the injection well 134 and the injection well 134 due to the resonant tunneling phenomenon. Are transported to the light emitting well 132 one after another. As a result, electrons flow sequentially from top to bottom.

  At this time, in the light emitting well 132, when electrons move from the upper subband level 132A to the lower subband level 132B, the electrons lose their energy and emit terahertz light. The time constant τ1 related to this radiation is on the order of several psec.

  On the other hand, in the injection well 134, the energy difference between the upper subband level 134A and the lower subband level 134B is set to the same level as the LO (longitudinal optical) phonon energy of the material forming the quantum cascade layer 13. Keep it. Specifically, in the case of GaAs, it is 35 meV. By doing so, the energy transition from the upper subband level 134A to the lower subband level 134B in the injection well 134 resonates with the LO phonon, and the energy transition of electrons occurs with the time constant τ2 on the order of sub psec. .

  As a result, electrons having the energy of the upper subband level 134A of the injection well 134 (= lower subband level 132B of the light emitting well 132) are quickly lost. Due to the time constant relationship τ1> τ2 in these energy transitions, a so-called inversion distribution is generated in the light emitting well 132, in which the number of electrons in the upper subband level 132A is larger than that in the lower subband level 132B. Laser oscillation occurs at.

  As described above, the quantum cascade layer 13 matches the potentials of the lower subband level 132B of the light emission well 132 and the upper subband level 134A of the injection well 134 so that the upper subband level 132A of the light emission well 132 and By matching the potential of the injection well 134 with the lower subband level 134B, a current flows in the stacking direction (note that the upper subband level 132A and the lower subband level 134B match the adjacent wells. Yes, not shown in FIG. At the same time as this current flows, each light emitting well 132 emits a terahertz wave having a power corresponding to the amount of the current. The amount of current increases as the number of electrons injected into the light emitting well 132 increases. Therefore, it is possible to control the output amount of the terahertz wave by controlling the electron injection amount into the light emitting well 132.

  In the conventional quantum cascade laser element, the electron injection amount is controlled by a voltage applied to both ends of the quantum cascade layer. However, if the voltage applied to the quantum cascade layer is changed by this control method, the potential level of each subband of the semiconductor superlattice shifts and the emission of terahertz waves becomes unstable.

  On the other hand, the quantum cascade laser device 1 according to the embodiment of the present invention controls the amount of electrons injected into the quantum cascade layer 13 without changing the voltage Vc applied to the quantum cascade layer 13 once optimized. Is possible. Hereinafter, control of the electron injection amount independent of the applied voltage Vc to the quantum cascade layer 13 will be described.

  As described above, the p-type GaAs layer 14 and the n-type AlGaAs layer 15 are stacked on the surface of the quantum cascade layer 13 and connected to the voltage source 19 (described on the left side of the quantum cascade layer 13 in FIG. 2). Have been). In this configuration, a forward voltage is applied to the p-type GaAs layer 14 and the n-type AlGaAs layer 15, and as generally known, the following (Equation 1)

A current I represented by Here, e is the elementary electron content, k is the Boltzmann constant, T is the temperature, and I ₀ is a constant determined by the diffusion length of electrons in the p-type GaAs layer 14 and the diffusion length of holes in the n-type AlGaAs layer 15. It is. The n value is a value considering recombination at the interface and usually takes a value of 1 to 2. The rising voltage is approximately equal to the built-in voltage, and current starts to flow from Vi = about 1.5V.

  On the other hand, a reverse voltage is applied between the p-type GaAs layer 14 and the quantum cascade layer 13. Since the quantum cascade layer is undoped, most of the depletion layers are formed in the quantum cascade layer 13. That is, it can be considered that most of the applied voltage Vc is applied to the quantum cascade layer. By applying 50 meV per unit to the quantum cascade layer 13, the lower subband level 132 B of the light emitting well 132 coincides with the upper subband level 134 A of the injection well 134, and at the same time, the upper subband of the light emitting well 132. The level 132A coincides with the lower subband level 134B of the injection well 134, and electrons sequentially pass through the barrier layers 131 and 133 by resonance tunneling. In this embodiment, the quantum cascade layer 13 is 170 units, and Vc = 8.9 V is applied. The applied voltage Vc value is constant. As a result, the voltage applied to the quantum cascade layer 13 is also constant. Therefore, regardless of the amount of injected current, each subband of the quantum cascade layer is fixed in a state necessary for laser oscillation. On the other hand, the amount of electrons injected into the quantum cascade layer 13 is determined by I represented by the above-described (Equation 1), and takes a value corresponding to Vi set by the voltage source 19 independently of the applied voltage Vc. . The method for determining the Vc value will be described later.

  Note that the thickness of the p-type GaAs layer 14 is preferably smaller than the electron diffusion distance in the p-type GaAs layer 14. Thereby, the electron injection efficiency into the quantum cascade layer 13 can be increased. Here, the diffusion distance is described, and the electron diffusion distance in the p-type GaAs layer 14 in the present embodiment is obtained.

  Generally, the diffusion distance L is given by the following (Equation 2).

  Here, D is a diffusion constant, and τ is a minority carrier lifetime. Further, the diffusion constant D and the mobility μ are expressed by the following (Equation 3).

There is a relationship represented by Here, k is a Boltzmann constant, T is an absolute temperature, and q is an electron elementary quantity. In the present embodiment, the diffusion distance for electrons (minority carriers) passing through the p-type GaAs layer 14 (μ = 3000 cm ² / Vs) is a value to be obtained. Substituting room temperature (= 300K) and μ into Equation 3, D = 78 cm ² / s. In GaAs, the diffusion distance L = 8.8 μm is obtained from Equation 2 from τ to 10 ns. On the other hand, the thickness of the p-type GaAs layer 14 is 100 nm, and it is understood that the structure of the quantum cascade laser device 1 according to the present embodiment is sufficiently smaller than the obtained diffusion distance L.

  The band gap of the n-type AlGaAs layer 15 is preferably larger than the band gap of the p-type GaAs layer 14. Generally, mutual injection of electrons and holes occurs at the pn junction between the p-type semiconductor layer and the n-type semiconductor layer. With this configuration, at the pn junction between the n-type AlGaAs layer 15 and the p-type GaAs layer 14, it is possible to inject electrons larger than holes and increase the efficiency of electron injection into the quantum cascade layer 13. Can do.

  FIG. 3 is a graph showing current-voltage characteristics and current-laser output characteristics of the quantum cascade laser device according to the embodiment of the present invention. The horizontal axis represents the injection current Ic to the quantum cascade layer 13, and the vertical axis represents the voltage Vi (left axis in the figure) applied between the n-type AlGaAs layer 15 and the p-type GaAs layer 14 and the terahertz wave oscillation output. Represents power (right axis in the figure). The oscillation frequency is about 2.7 THz. The voltage Vc applied to the quantum cascade layer by the voltage source 20 is constant at 8.9 V as described above. The constant voltage value is such that the lower subband level 132B of the light emission well 132 matches the upper subband level 134A of the injection well 134, and at the same time, the upper subband level 132A of the light emission well 132 is lower than the lower subband level of the injection well 134. This is a voltage value that corresponds to the band level 134B and that allows electrons to sequentially pass through the barrier layers 131 and 133 by resonant tunneling. The current-voltage characteristics of the quantum cascade laser device 1 of the present invention shown in FIG. 3 are kink as shown in the current-voltage characteristics of the conventional quantum cascade laser device shown in FIG. (D) there is no bend between the region (a) and the region (b), and it is smooth over a wide range of injection current Ic. Further, in the conventional quantum cascade laser element, when an applied voltage is applied to increase the injection current Ic, the subband level is greatly shifted as shown in FIG. As in the area (c) in d), the output decreases.

  On the other hand, in the quantum cascade laser device 1 of the present invention, even if the injection current Ic is increased, the voltage Vc applied to the quantum cascade layer 13 does not change, so that the level of the subband level shifts. There is no. As a result, a wide range of injected current Ic (up to about 1 A) can be supplied, a terahertz wave output with a wide dynamic range can be obtained, and a high output state can be maintained.

  Next, a method for driving the quantum cascade laser device according to the first embodiment of the present invention will be described.

  FIG. 4 is a flowchart showing a driving method of the quantum cascade laser element according to the first embodiment of the present invention.

First, as an initial state, a non-bias state is set. That is, the applied voltage Vc to the quantum cascade layer 13 and the applied voltage Vi between the n-type AlGaAs layer 15 and the p-type GaAs layer 14 are both set to 0 (S10). Here, [Delta] Vc (= 0.1 V) and [Delta] Vi (= 0.1 V), respectively, is a step amount when increasing V _c and V _i gradually. The initial value of the parameter Pp is set to 0.

  Next, Vbi is given to Vi (S11). Vbi is called a rising voltage, and is a voltage at which current starts to flow through the pn junction. This value is approximately equal to the band gap. At this time, if the current is too small, the laser output Po is always zero. During this time, the parameter Pp = 0.

  In subsequent steps, optimization of Vc is performed by sweeping the applied voltage Vc to the quantum cascade layer 13.

  Next, the laser output Po is compared with the parameter Pp (S12). In the initial state, Po = Pp (= 0) (No in S12).

  Next, it is determined whether the number of increases in the Vc value has reached a predetermined number (S13). If the increase count of the Vc value reaches the predetermined count, Vi is increased by ΔVi, and step S12 is executed (Yes in S13).

  If the increase number of the Vc value is not the predetermined number, the parameter Pp is set to the laser output Po (S14), and Vc is increased by ΔVc (S15). Here, in the initial state where Vc = Vbi, no matter how much Vc is increased in step S15, since the current supplied from the pn junction is constant, Po = 0 remains. Therefore, Vi is slightly increased by increasing the number of increases in the Vc value in step S13, and Vc is further increased in step S15. The above steps S12 to S15 are repeated until the output of the laser beam is detected.

  Next, when the output of the laser beam is detected, the value of the laser output Po increases from zero to a certain value as Vc increases. At this time, in step S14, the parameter Pp is rewritten to a non-zero Po value.

  Next, in step S15, the laser output Po increases as Vc increases by ΔVc.

  Next, in step S12, Po> Pp (No in S12). As a result, in the next loop, Vc further increases and the laser output Po becomes larger and the No loop is repeated in step S12.

  Eventually, when Vc exceeds the optimum value, the laser output Po is decreased by increasing Vc in step S15. As a result, in step S12, Po <Pp (Yes in S12). This decrease in output indicates that the laser output Po has just exceeded the optimum value.

  Therefore, next, Vc is lowered by one step, and Vc is set to the optimum applied voltage value (S16).

  As described above, Vc is optimized by executing steps S12 to S16.

  At the stage of Vc optimization described above, it is assumed that the laser output is still small and has not reached the target set value.

  Therefore, in the subsequent steps, after the above-described optimization of Vc, by sweeping Vi, the optimization of Vi is executed and the laser output reaches the target set value.

  First, the laser output Po is compared with the laser output target value Pa (S17).

  If the laser output Po does not exceed the laser output target value Pa (No in step S17), Vi is increased by ΔVi (S18), and the laser output Po is compared with the laser output target value Pa again (S17).

  If the laser output Po exceeds the laser output target value Pa (Yes in step S17), it is determined that the electron injection amount from the p-type GaAs layer 14 has reached the optimum electron injection amount, and the adjustment of the laser output is finished (adjustment). End).

  As described above, in step S17 and step S18, the laser output can be optimized while Vc is optimal.

  According to the driving method of the quantum cascade laser device 1 according to the present invention described above, since Vc is in an optimum state, there is no reactive current and high-efficiency operation is possible.

  ΔVc and ΔVi are values determined in consideration of the material constituting the quantum cascade layer, the number of units, the laser output target value, and the like, and are not limited to the values described above.

  Further, the method of determining Vc and Vi is not limited to the driving method described above. For example, each time the Vc value is changed, the current-voltage characteristics and current-laser output characteristics of the quantum cascade laser device described in FIG. 5 are obtained, and these characteristics are monitored by a personal computer. It is possible to determine the Vc and Vi values at which a laser output is obtained at Ic and a high laser output is obtained.

  FIG. 5A is a graph showing current-voltage characteristics and current-laser output characteristics of the quantum cascade laser element in the case of low Vc, and FIG. 5B is a graph of the quantum cascade laser element in the case of high Vc. It is a graph showing a current-voltage characteristic and a current-laser output characteristic. In FIG. 5A where the Vc value is smaller than the optimum value, a current flows through the quantum cascade layer 13 with high resistance, but hardly contributes to the output of the terahertz wave. On the other hand, in FIG. 5B in which the Vc value is larger than the optimum value, the quantum cascade layer 13 has a very high resistance, and current hardly flows.

  As described above, the quantum cascade laser device according to the present embodiment has the following characteristics.

  (1) In the present invention, the output operating point of a quantum cascade laser element which has conventionally existed only in a pinpoint manner can be given linearly over a wide range of injection currents. As a result, controllability is facilitated.

  (2) Conventionally, in order to pass a current through the quantum cascade layer, a high driving voltage of about 10 V has been required to match the subband levels. On the other hand, in the structure of the present invention, the driving voltage for passing a current through the quantum cascade layer may be about the rising voltage of the p-type layer / n-type layer in contact with the quantum cascade layer, as in this embodiment. In the case where the p-type layer / n-type layer has a GaAs / AlGaAs structure, a current can be passed with a drive voltage of about 5 V, and a highly efficient light-emitting element can be realized.

  As described above, according to the quantum cascade laser device and the driving method thereof of the present invention, current injection into the quantum cascade layer is performed while the subband state of the quantum cascade layer is always fixed in a oscillatable state. It can be controlled independently of voltage application to the cascade layer. Since the amount of generated terahertz waves is proportional to the number of injected currents, according to the present invention, the amount of emitted light can be controlled independently, and the stability and high output of the laser oscillation output can be realized. It becomes.

(Second Embodiment)
FIG. 8 is a structural perspective view of a quantum cascade laser element 701 according to the second embodiment of the present invention. The quantum cascade laser element 701 in the figure includes a conductive GaN substrate 711, an n ⁺ -type GaN layer 712, a quantum cascade layer 713, a p-type InGaN layer 714, an n-type AlGaN layer 715, a first electrode 716, A second electrode 717 and a third electrode 718 on the back surface are provided.

  The wavelength of the outgoing infrared light is about 6 μm.

The n ⁺ -type GaN layer 712 is an n-doped semiconductor layer stacked on the surface of the GaN substrate 711. For example, the n ⁺ -type GaN layer 712 has an electron concentration of 1 × 10 ¹⁸ cm ⁻³ by Si doping and a film thickness of 500 nm.

The quantum cascade layer 713 is formed on the surface of the n ⁺ -type GaN layer 712 and constitutes a semiconductor superlattice. The quantum cascade layer 713 generates energy transition between subbands formed in the semiconductor superlattice by applying a predetermined voltage to both ends thereof. Due to this energy transition, the semiconductor superlattice emits photons and emits light. Unlike a general semiconductor laser, the quantum cascade laser element 701 can control the oscillation frequency not by the main material constituting the laser but by the thickness of the superlattice well layer, the barrier layer, and the heterobarrier.

  The quantum cascade layer 713 includes one unit of AlInN / GaN / AlInN / GaN, and has a quantum cascade region composed of, for example, 100 units. The film thickness of each layer of the 1 unit is, for example, 2.5 nm / 3.5 nm / 1.5 nm / 4.5 nm in order, and the total film thickness is 12 nm. In addition, the In composition relative to AlInN contained in one unit is about 17%, and AlInN and GaN are lattice-matched. Therefore, even if a large number of units are stacked, there is no risk of cracking due to lattice distortion, and a high-power quantum cascade layer can be realized. In addition, each layer of the 1 unit is undoped. From the above, the total number of layers of the quantum cascade layer 713 is 100 × 4 layers = 400 layers, and the total film thickness is 100 × 12 nm = about 1.2 μm.

The p-type InGaN layer 714 is stacked on the surface of the quantum cascade layer 713 and functions as a p-type semiconductor layer. For example, the p-type InGaN layer 14 has a hole concentration of 1 × 10 ¹⁸ cm ⁻³ by Mg doping and a film thickness of 50 nm.

The n-type AlGaN layer 715 is stacked on the surface of the p-type InGaN layer 714 and functions as an n-type semiconductor layer. For example, the n-type AlGaN layer 715 has an electron concentration of 5 × 10 ¹⁷ cm ⁻³ by Si doping and a film thickness of 150 nm. The Al composition is 10%.

  The p-type InGaN layer 714 and the n-type AlGaN layer 715 have a function as an electron injection part that injects electrons into the quantum cascade layer 713 by a pn junction formed at the junction interface between them.

  The first electrode 716 is formed on the surface of the n-type AlGaN layer 715 and is connected to one end of the voltage source 719.

  The second electrode 717 is formed on the surface of the p-type InGaN layer 714 from which a part of the n-type AlGaN layer 715 has been removed, and is connected to the other end of the voltage source 719 and one end of the voltage source 720. In the present embodiment, the second electrode 717 is disposed at two positions with the n-type AlGaN layer 715 interposed therebetween in order to reduce the resistance.

  The third electrode 718 is formed on the back surface of the substrate 711 and is connected to the other end of the voltage source 720.

  The voltage source 719 is a first voltage source that can vary the voltage value to be supplied, is inserted between the second electrode 717 and the first electrode 716, and the first electrode 716 becomes negative with respect to the second electrode 717. So connected.

  By applying a predetermined voltage from the voltage source 719, electrons are injected from the p-type InGaN layer 714 into the quantum cascade layer 713.

  The voltage source 720 is a second voltage source that can vary the voltage value to be supplied, and is inserted between the second electrode 717 and the third electrode 718, and the third electrode 718 becomes positive with respect to the second electrode 717. So connected.

  By applying a predetermined voltage from the voltage source 720, the energy level of each band of the quantum cascade layer 713 is inclined in the stacking direction.

  With the above configuration, the voltage source 719 and the voltage source 720 can be set independently. Therefore, the degree of inclination of the energy level of each band of the quantum cascade layer 713 and the amount of electrons injected from the p-type InGaN layer 714 to the quantum cascade layer 713 can be controlled independently. Further, since the second electrode 17 is shared when the voltages from the two voltage sources 719 and 720 are applied, the quantum cascade laser element can be reduced in size.

FIG. 9 is an example of a band structure diagram of the quantum cascade laser device according to the embodiment of the present invention. The vertical direction of the band structure diagram shown in FIG. 9 represents the energy level of each band, and the horizontal direction represents the structure on the YY ′ line shown in FIG. In FIG. 9, in the structure on the YY ′ line, the description of the GaN substrate 711 and the n ⁺ -type GaN layer 712 is omitted, and only three units of the quantum cascade layer 713 are illustrated. .

  The quantum cascade laser device 701 according to the present invention uses GaN as a well layer and AlGaN as a barrier layer as the quantum cascade layer 713. The portion surrounded by the one-dot chain line in the figure is one unit described above. The first GaN well is the light emitting well 732, and the second GaN well is the injection well 734. Although three units are shown in this figure, in actuality, about 100 units are stacked.

  In this quantum cascade structure, the voltage source 720 is connected to both ends of the quantum cascade layer 713 and the variable voltage Vc is applied, so that the band is inclined. When a voltage is applied so that the lower subband level 732B of the light emitting well 732 is equal to the upper subband level 734A of the injection well 734, electrons in the light emitting well 732 are transferred to the injection well 734 and the injection well 734 due to resonance tunneling. Are transported to the light emitting well 732 one after another. As a result, electrons flow sequentially from top to bottom.

  At this time, in the light emitting well 732, when electrons move from the upper subband level 732A to the lower subband level 732B, the electrons lose their energy and emit infrared light.

  On the other hand, in the injection well 734, a number of levels are formed between the upper subband level 734A and the lower subband level 734B (not shown). In this way, energy transition from the upper subband level 734A to the lower subband level 734B in the injection well 734 occurs smoothly (so-called Bound-to-Continum structure).

  As a result, electrons having the energy of the upper subband level 734A of the injection well 734 (= the lower subband level 732B of the light emission well 732) quickly disappear. As a result, an inversion distribution occurs and laser oscillation occurs in each light emitting well.

  As described above, the quantum cascade layer 713 matches the potentials of the lower subband level 732B of the light emission well 732 and the upper subband level 734A of the injection well 734 so that the upper subband level 732A of the light emission well 732 and By matching the potential with the lower subband level 734B of the injection well 734, a current flows in the stacking direction. At the same time as this current flows, each light emitting well 732 emits infrared light having a power corresponding to the amount of the current. The amount of current increases as the number of electrons injected into the light emitting well 732 increases. Accordingly, the amount of infrared light output can be controlled by controlling the amount of electrons injected into the light emitting well 732.

  Although the p-type InGaN layer 714 and the n-type AlGaN layer 715 are used as the second embodiment, a combination of p-type GaN and n-type AlGaN may be used.

  In addition, the GaN substrate can give the same effect to the quantum cascade laser regardless of whether it is a polar plane (c plane), a nonpolar plane (a plane, m plane), or a semipolar plane such as (20-21).

  Although the embodiments have been described above, the quantum cascade laser element and the driving method thereof according to the present invention are not limited to the above embodiments. Modifications obtained by various modifications conceived by those skilled in the art without departing from the spirit of the present invention with respect to the embodiments, and various apparatuses incorporating the quantum cascade laser element according to the present invention are also included in the present invention.

  For example, a terahertz generator using the quantum cascade laser element of the present invention as a light source can be mentioned. By using the quantum cascade laser element of the present invention as a light source, it is possible to realize a terahertz generator that is excellent in controllability and stability of the radiation amount of terahertz waves and is capable of high output.

  In the present invention, a GaAs / AlGaAs-based material or a GaN / AlInN-based material is used as the quantum cascade layer, but an InGaAlN-based material such as GaN / AlGaN or GaN / AlN, an InGaAs / AlInAs material, or a Ge / GeSiSn-based material is used. The quantum cascade laser device thus produced also has the same effect as this embodiment.

  Further, as the optical waveguide / resonator structure of the quantum cascade laser element, the Fabry-Perot structure described in FIGS. 1 and 8 is shown. However, a distributed feedback structure, a microdisk structure, a vertical radiation structure, or a composite resonator is shown. The present invention can also be applied to structures and the like.

  Further, the quantum cascade laser device of the present invention has no n-type AlGaAs layer 15 as an electron injection part in the first embodiment, for example, and the electrons from a p-type semiconductor layer in contact with the quantum cascade layer 13, for example. An injection part may be configured. In this case, conduction band electrons generated by irradiating the p-type semiconductor layer with light having energy larger than the energy gap of the p-type semiconductor layer can be injected into the quantum cascade layer 13. The same applies to the second embodiment.

  In this embodiment, the basic structure necessary for carrying out the present invention is described, and a plurality of well layers / barrier layers may be inserted in order to make the energy transition of electrons easier. . These insertions do not lose the essence of the present invention.

  The present invention is a laser oscillation for medical applications such as mail inspection in envelopes, food and belonging inspection, drug analysis, skin cancer inspection, semiconductor impurity amount inspection / complex dielectric constant evaluation, environmental measurement, engineering application, etc. It can be used as a source, and is particularly suitable for use as a quantum cascade laser element that requires control of high-power terahertz waves, infrared light, and output power, and a driving method thereof.

DESCRIPTION OF SYMBOLS 1,701 Quantum cascade laser element 11 GaAs substrate 12 n ⁺ type GaAs layer 13,713 Quantum cascade layer 14 p type GaAs layer 15 n type AlGaAs layer 16,716 First electrode 17,717 Second electrode 18,718 Third electrode 19, 20, 610 Voltage source 131, 133, 602 Barrier layer 132, 601, 732 Light emitting well 132A, 134A, 601A, 603A Upper subband level 132B, 134B, 601B, 603B Lower subband level 134, 603 Injection well 600 Semiconductor superlattice 711 GaN substrate 712 n ⁺ type GaN layer 714 p type InGaN layer 715 n type AlGaN layer 719, 720 Voltage source 732A, 734A Upper subband level 732B, 734B Lower subband level 734 Injection well

Claims (12)

A quantum cascade laser device having a semiconductor superlattice that emits photons by intersubband transition,
A quantum cascade laser device comprising: an electron injection unit capable of injecting electrons into the semiconductor superlattice independently of voltage application to the semiconductor superlattice. The electron injection part is
The quantum cascade laser device according to claim 1, further comprising a p-type semiconductor layer in contact with the semiconductor superlattice. The electron injection unit further includes:
An n-type semiconductor layer in contact with the p-type semiconductor layer;
Independently of voltage application to the semiconductor superlattice, a voltage is applied between the p-type semiconductor layer and the n-type semiconductor layer to inject electrons into the semiconductor superlattice. The quantum cascade laser device according to claim 2. The quantum cascade laser device according to claim 1, wherein a nitride semiconductor is used. 5. The quantum cascade laser element according to claim 1, wherein the p-type semiconductor layer uses InGaN. 6. 6. The quantum cascade laser device according to claim 1, wherein a band gap of the p-type semiconductor layer and the n-type semiconductor layer is larger than an energy of the photon. 7. 4. The quantum cascade laser device according to claim 3, wherein a thickness of the p-type semiconductor layer is smaller than an electron diffusion distance in the p-type semiconductor layer. 4. The quantum cascade laser device according to claim 3, wherein a band gap of the n-type semiconductor layer is larger than a band gap of the p-type semiconductor layer. A substrate,
An n-doped semiconductor layer formed between the substrate and the semiconductor superlattice;
A first electrode formed on a surface of the n-type semiconductor layer and electrically connected to the n-type semiconductor layer;
A second electrode formed on a surface of the p-type semiconductor layer and electrically connected to the p-type semiconductor layer;
A third electrode formed on a surface of the n-doped semiconductor layer and electrically connected to the n-doped semiconductor layer;
An applied voltage to the semiconductor superlattice is applied between the second electrode and the third electrode,
A voltage applied between the p-type semiconductor layer and the n-type semiconductor layer is applied between the second electrode and the first electrode,
The quantum cascade laser device according to claim 3. An electromagnetic wave generating device using the quantum cascade laser element according to any one of claims 1 to 9 as a light source. A method for driving a quantum cascade laser device having a semiconductor superlattice that emits photons by intersubband transition,
The laser output from the semiconductor superlattice is maximized by sweeping the applied voltage to the semiconductor superlattice while keeping the amount of electrons injected from the electron injection part capable of injecting electrons into the semiconductor superlattice constant. A voltage determining step for determining an optimum applied voltage value to become,
By sweeping the electron injection amount from the electron injection part to the semiconductor superlattice while fixing the applied voltage to the semiconductor superlattice layer at the optimum applied voltage value determined in the voltage determination step, An electron injection amount determining step for determining an optimum electron injection amount that maximizes the laser output from the semiconductor superlattice,
Driving method of quantum cascade laser element. The electron injection part is
A p-type semiconductor layer in contact with the semiconductor superlattice;
An n-type semiconductor layer in contact with the p-type semiconductor layer,
In the voltage determination step,
By making the voltage applied between the p-type semiconductor layer and the n-type semiconductor layer constant,
A constant electron injection amount from the electron injection part to the semiconductor superlattice,
In the electron injection amount determination step,
By sweeping a voltage applied between the p-type semiconductor layer and the n-type semiconductor layer,
12. The method of driving a quantum cascade laser element according to claim 11, wherein an electron injection amount from the electron injection portion to the semiconductor superlattice is swept.