Zinc oxide-based quantum cascade laser element

Abstract

In order to provide a THz-QCL element which takes advantage of the characteristics of a ZnO-based semiconductor material, the present disclosure provides a quantum cascade laser element which has a semiconductor superlattice structure (a QCL structure 100) wherein each one of a plurality of unit structures 10U, which are repeatedly stacked upon each other, is composed of three well layers 10W1 to 10W3 that each have a composition of ZnO or ZnMgO, and barrier layers 10B1 to 10B3 that each separate the well layers from each other and have a higher MgO proportion than adjacent well layers on the both sides, while having a composition of ZnMgO or MgO.

Description

Zinc oxide quantum cascade laser device

[0001] This disclosure relates to quantum cascade laser (QCL) devices, and more particularly to QCL devices that utilize zinc oxide-based semiconductor materials.

[0002] Quantum cascade lasers (QCLs) have attracted considerable attention. QCLs typically have a semiconductor superlattice structure consisting of repeated unit structures. Electrons are the carriers responsible for electrical conduction within the QCL. In addition to a bias electric field due to an external voltage, potentials act on the electrons in each layer of each unit structure. The potential acting on the electrons generally has multiple wells and barriers in each unit structure. The potential wells and barriers for electrons form irregularities that reflect the conduction band edge of each material layer of each unit structure and the conduction band offset depending on the position in the thickness. When an external voltage is applied to operate the QCL, the overall potential is tilted due to the bias electric field. Electrons, the carriers, undergo repeated intersubband transitions (ISBTs) as they are transported through subbands, or quantum levels, formed by the tilted and uneven potential. Each transition couples with an electromagnetic field, generating stimulated emission, which results in laser oscillation. The semiconductor superlattice structure thus becomes the active region for light emission. The term "cascade" refers to the behavior of transporting electrons as they lose energy through intersubband transitions. QCLs can lasing wavelengths independent of the energy band gap of the material. The lasing wavelength or frequency can be controlled by the materials and design of the semiconductor superlattice structure. For this reason, QCLs are attracting attention as unipolar semiconductor lasers, providing coherent light sources in wavelength (frequency) ranges not previously available from solid-state light sources. Mid-infrared and far-infrared QCLs have been commercialized to date.

[0003] Quantum cascade lasers (THz-QCLs) for the terahertz range have been experimentally realized, with the most success achieved by QCLs based on GaAs / AlGaAs quantum well systems (GaAs-based THz-QCLs). However, GaAs-based THz-QCLs generally have the following challenges. First, they must operate at low temperatures, requiring cooling. Second, their oscillation frequency is limited to 5.4 THz or less. These two challenges are attributed to the low longitudinal optical (LO) phonon energy of GaAs, which is only 36 meV. However, the low operating temperature can be addressed to some extent by reducing the nonradiative channel due to phonon emission by thermally excited electrons at high temperatures. In contrast, the latter problem is due to the reststrahlen bands (30-50 meV) of GaAs LO-phonons, making it difficult to achieve oscillation frequencies above 5.4 THz with GaAs-based materials, which have low LO phonon energy. In fact, the oscillation frequency range demonstrated so far with GaAs-based THz-QCLs is 1.1 THz to 5.4 THz, with the highest operating temperatures achieved only in the narrow range of 3 THz to 4 THz. THz-QCLs have yet to achieve oscillation in the frequency range of 6-12 THz, a range known as the THz gap.

[0004] QCLs made of semiconductor materials with large LO phonon energy, particularly QCLs made of GaN / AlGaN-based semiconductor materials ("GaN-based THz-QCLs"), have been developed as QCL elements that show the possibility of oscillation in the THz gap and are expected to operate at high temperatures (see, for example, Patent Document 1).

[0005] JP 2016-042572 (Patent No. 6627309)

[0006] Enrico Bellotti, Kristina Driscoll, Theodore D. Moustakas, and Roberto Paiella, ``Monte Carlo simulation of terahertz quantum cascade laser structures based on wide-bandgap semiconductors'', Journal of Applied Physics 105, 113103 (2009); DOI:10.1063 / 1.3137203VPSirkeli, HLHartnagel, ``ZnO-based terahertz quantum cascade lasers'', Opto-Electronics Review, Volume 27, Issue 2, June 2019, Pages 119-122, DOI:10.1016 / j.opelre.2019.04.002

[0007] In addition to GaN / AlGaN-based semiconductor materials, THz-QCLs (hereinafter referred to as "ZnO-based THz-QCLs") using semiconductor materials ("ZnO-based semiconductors") with a composition of zinc oxide (ZnO) or a mixed crystal of ZnO and MgO (ZnMgO) have also attracted interest. ZnO has a large LO phonon energy of 72 meV, approximately twice that of GaAs, making it an attractive material for bridging the THz gap that would otherwise be the Liststrallen band for other materials. Advances in epitaxy technology for ZnO-based materials, particularly in nonpolar crystal orientations, have made ZnO-based THz-QCLs, which require the actual fabrication of a superlattice structure, increasingly feasible. ZnO-based THz-QCLs are disclosed, for example, in Non-Patent Documents 1 and 2.

[0008] However, the designs of conventional ZnO-based THz-QCLs have remained unchanged, applying the design techniques previously adopted for GaAs-based THz-QCLs (Non-Patent Documents 1 and 2). ZnO-based THz-QCLs have not fully utilized the inherent advantages of ZnO-based semiconductors, and there is still room for improvement in their designs.

[0009] The present disclosure aims to solve at least some of the above problems and is based on the idea of ​​a new operating mechanism for ZnO-based THz-QCLs that can fully utilize the properties of ZnO-based semiconductor materials, and proposes a specific configuration for a THz-QCL that can operate at high temperatures and in frequency ranges where oscillation has not been achieved before.

[0010] The inventors have utilized the NEGF (Non-Equilibrium Green's Function) method, a calculation method that enables highly accurate performance prediction, to explore an operating mechanism suitable for ZnO-based semiconductor materials and a design that utilizes this mechanism. As a result, they have discovered a specific configuration for a ZnO-based THz-QCL based on a new idea that can bring out the material properties of ZnO-based semiconductor materials.

[0011] That is, the present disclosure provides a quantum cascade laser element having a semiconductor superlattice structure sandwiched between a pair of conductive parts, the semiconductor superlattice structure having a plurality of unit structures that are repeatedly stacked, each unit structure comprising three well layers having a composition of ZnO or ZnMgO, and barrier layers that separate the well layers from each other and have a composition of ZnMgO or MgO with a higher MgO ratio than the adjacent well layers on both sides, and the semiconductor superlattice structure serves as an active region that emits electromagnetic waves of a certain wavelength when an external voltage is applied through the pair of conductive parts for operation.

[0012] In this application, electromagnetic waves in the terahertz range refer to electromagnetic waves in the frequency range of roughly 0.1 THz to 30 THz, i.e., a wavelength range of approximately 10 μm to 3 mm. Furthermore, in the explanations of this application, device structure and function may be described using technical terms adapted or borrowed from the fields of electronic devices and physics that deal with visible light and infrared light. For this reason, even when describing electromagnetic waves in wavelength or frequency ranges that do not qualify as visible light, terms such as "laser" and "emission" may be used to refer to quantum cascade laser devices or stimulated emission phenomena, or terms such as "light," "optical-," and "photo-" may be used. Examples include optical confinement and refraction. Wavelengths follow the convention of using values ​​in a vacuum, even in materials. Furthermore, the formation of a level in a well layer does not necessarily mean that the probability of electron existence, determined by the wave function of that level, is 100% contained within the range of that well layer. In explanations of semiconductor engineering fields such as quantum wells and semiconductor superlattices, terms from the field of quantum mechanics, such as ground state and first excited state, are also used as appropriate. For example, the ground state, first excited state, and second excited state of electrons in a quantum well, which are discrete energy states, refer to quantum mechanical levels of electrons bound and roughly discrete in a one-dimensional quantum well, and are also called subbands in a QCL. In addition, the carriers responsible for electrical conduction and optical transitions in the QCL of this application are electrons.

[0013] According to any one of the aspects of the present disclosure, a practical ZnO-based THz-QCL device capable of utilizing the properties of a ZnO-based semiconductor is provided.

[0014] FIGS. 1A-1C are a perspective view (FIG. 1A), an enlarged cross-sectional view (FIG. 1B), and a further enlarged cross-sectional view (FIG. 1C) showing the overall configuration of a ZnO-based THz-QCL according to an embodiment of the present disclosure. FIGS. 2A-2D show design examples of ZnO-based THz-QCLs based on conventional concepts (FIGS. 2A and 2B) and design examples according to an embodiment of the present disclosure (FIGS. 2C and 2D). FIGS. 3A-3E are graphs showing the results of numerical simulations for determining the optical gain at each frequency in the active region of a ZnO-based THz-QCL according to a design method of an embodiment of the present disclosure. FIG. 4 is a map showing the peak intensity of optical gain as a function of the oscillation frequency and the corresponding maximum operating temperature in a ZnO-based THz-QCL according to an embodiment of the present disclosure, expressed as a concentration at each position. FIGS. 5A-5E are maps showing the electron density distribution at room temperature (300 K) versus thickness direction and energy for different oscillation frequencies in a design example of an embodiment of the present disclosure. Figures 6A-B are maps showing the change in peak gain as a function of the detuning energy and anticrossing energy between the lower laser level and the injection level for 3.5 THz and 8.5 THz oscillation frequencies in a ZnO-based THz-QCL according to an embodiment of the present disclosure. Figures 7A-B are schematic diagrams and a flowchart illustrating the design strategy of an embodiment of the present disclosure. Figures 8A-B are calculated emission spectra (Figure 8A) for ZnO-based THz-QCLs operating at 8 THz designed by the presently disclosed method and a conventional method, and the peak gain at each temperature (Figure 8B) for these ZnO-based THz-QCLs and a THz-QCL made of GaAs-based semiconductor material operating at 4 THz. Figures 9A-B are graphs showing the calculated dependence of current density on the electric field for a ZnO-based THz-QCL according to an embodiment of the present disclosure, with oscillation frequencies designed for 3 THz and 8 THz, respectively.

[0015] 1. Embodiments Hereinafter, embodiments of QCL devices according to the present disclosure will be described with reference to the drawings. Common parts or elements are designated by common reference numerals throughout the drawings unless otherwise specified. Furthermore, in the drawings, elements of each embodiment are not necessarily drawn to scale.

[0016] 1-1. Operation mechanism and design guidelines compatible with the physical properties of ZnO-based semiconductors From the viewpoint of THz-QCL design, semiconductor materials (called "ZnO-based semiconductors") with a composition of zinc oxide (ZnO) or a mixed crystal of ZnO and magnesium oxide (MgO) (ZnMgO) differ from GaAs-based semiconductor materials in the following three respects: (1) In ZnO-based semiconductors, the strength of the Froehlich coupling, i.e., the coupling between electrons and LO phonons, is extremely strong, 21 times that of GaAs-based semiconductor materials. This is because the relative permittivity of ZnO at low and high frequencies (ε r ε ∞ ) is ε r = 8.9, ε ∞ = 3.7, and the value of GaAs (ε r = 12.93, ε ∞ This is because the ρ is much smaller than that of GaN (=10.89). (2) In ZnO-based semiconductor materials, the increase in interface roughness due to interdiffusion is serious. (3) In ZnO-based semiconductor materials, the effective mass of electrons that act as carriers in THz-QCL is large. Table 1 shows the physical properties of GaAs-based and ZnO-based semiconductor materials, along with those of GaN-based materials.

[0017] Due to the difference in (1) above, ZnO-based semiconductor materials are generally not suitable for THz-QCLs for low frequencies (below 6 THz). However, careful design is required even for higher frequencies of 6 THz or higher. In the ZnO-based THz-QCL of the present disclosure, the electron transport mechanism for electron depopulation from the lower laser level employs electron-electron scattering instead of the conventional method of combining resonant tunneling and electron-LO phonon scattering within the layer due to resonant phonons. This allows the ZnO-based THz-QCL of this embodiment to maintain a sufficiently high electron extraction efficiency from the lower laser level while suppressing an increase in the emission linewidth. As a specific method for enhancing electron extraction by electron-electron scattering, the ZnO-based THz-QCL of this embodiment carefully balances the detuning energy and anticrossing energy between the lower laser level and the injection level in the region where electron scattering occurs (the region sandwiching the first barrier layer, described below). Furthermore, even if the conventional design based on the idea adopted for GaAs-based THz-QCLs, i.e., the "RP method" design that utilizes electron-LO phonon scattering due to resonant phonons in a three-well structure, is applied to ZnO-based semiconductor materials, the level broadening occurs, the oscillation linewidth increases, and the gain becomes very small. This is related to the strength of the Fröhlich coupling in ZnO-based semiconductors.

[0018] Furthermore, in accordance with the characteristic (2) above, the ZnO-based THz-QCL of this embodiment avoids the use of resonant tunneling for electron transport, including injection. This is because the increase in interface roughness due to interdiffusion in ZnO materials is significant, which can hinder precise layer control when relying solely on resonant tunneling. Furthermore, the ZnO-based THz-QCL of this embodiment employs diagonal indirect injection instead of conventional resonant tunneling injection to enhance electron injection and maximize electron population inversion.

[0019] The large effective mass of electrons in the ZnO material (3) generally leads to a weakening of the coupling between the subbands belonging to the two wells. In this case, the doping level must be increased in the THz-QCL. However, this also increases the Coulomb interaction of electrons, directly leading to an increase in the linewidth. Again, the solution to this problem is to carefully balance the detuning energy and anticrossing energy.

[0020] To specifically design the ZnO-based THz-QCL of this embodiment, it is necessary to accurately handle Fröhlich coupling, electron-electron scattering, and other real-world electron transport phenomena. Therefore, an advanced numerical simulation model based on the NEGF (Non-Equilibrium Green's Function) method is employed as a design tool for the ZnO-based THz-QCL of this embodiment. In this embodiment, the gain value of the ZnO-based THz-QCL is first estimated by taking into account the level broadening described above, and then a strategy for suppressing the effects of this level broadening is proposed. Thus, this embodiment proposes a ZnO-based THz-QCL that takes into account the effects of level broadening caused by strong Fröhlich coupling.

[0021] 1-2. Device Configuration FIGS. 1A-1C are a perspective view (FIG. 1A), an enlarged cross-sectional view (FIG. 1B), and a further enlarged cross-sectional view (FIG. 1C) showing the overall configuration of a ZnO-based THz-QCL device according to this embodiment. A typical ZnO-based THz-QCL device 1000 (FIG. 1A) according to this embodiment generally comprises a pair of electrodes 20 and 30 (a pair of conductive portions) and a semiconductor superlattice QCL structure 100 sandwiched between them. The electrodes 20 and 30 are used to externally apply a voltage to the QCL structure 100 to form an electric field and a current to emit electromagnetic waves, i.e., light. Furthermore, the electrodes 20 and 30 are typically made of metal. When electromagnetic waves in the THz range act on the electrodes, surface plasmons are induced, and the electrodes 20 and 30 also exert an optical confinement effect due to their cavity structure. This structure is also known as a double-metal waveguide (DMW) structure. The electrodes 20 and 30 are not necessarily made of metal. For example, one of the electrodes may be a highly n-doped layer, forming a metal layer and the other a highly conductive semiconductor layer (a single-sided metal waveguide structure). The QCL structure 100 includes an active region 10. The THz-QCL device 1000 operates by passing electrons through a repeating structure of electron potentials formed in the active region 10 in the thickness direction when the voltage is applied. During this passage, the electrons transition between subbands, i.e., between levels, emitting electromagnetic waves 2000 in the THz range. The THz-QCL device 1000 shown in FIG. 1 is fabricated by forming the metal layer 30B of the electrode 30 on a receptor substrate 40 (hereinafter referred to as "receptor 40") and bonding it to the metal layer 30A formed on the QCL structure 100. In addition to these, the THz-QCL device 1000 of FIG. 1 also employs layers for operation and device fabrication, such as highly doped layers 120 and 140, a δ-doped layer 160, and an etching stopper layer 60, as appropriate.

[0022] The active region 10 (FIG. 1B) has a plurality of unit structures 10U of a certain thickness, each of which includes alternating well layers 10W and barrier layers 10B. Each unit structure 10U is repeatedly stacked in the thickness direction. In the semiconductor superlattice structure 100A shown in FIG. 1B, the active region 10 is composed of 10 to 200 identical unit structures 10U, each of which is repeatedly stacked. FIG. 1C shows an enlarged view of one unit (one period) of each unit structure 10U. Each unit structure 10U consists of three well layers 10W and three barrier layers 10B, with each well layer 10W separated from the others by its respective barrier layer 10B. The individual well layers 10W are distinguished, starting from the substrate 50 side, as a first well layer 10W1, a second well layer 10W2, and a third well layer 10W3. The first well layer 10W1, the second well layer 10W2, and the third well layer 10W3 are also referred to as an injection well layer, an upper light-emitting well layer, and a lower light-emitting well layer, respectively. Individual barrier layers 10B are also distinguished as necessary and are referred to as a first barrier layer 10B1, a second barrier layer 10B2, and a third barrier layer 10B3, respectively, from the substrate 50 side. The first barrier layer 10B1, the second barrier layer 10B2, and the third barrier layer 10B3 are also referred to as a depopulation barrier layer, an injection barrier layer, and a light-emitting barrier layer, respectively. The well layer 10W1 is disposed adjacent to the barrier layer 10B1, and the barrier layer 10B2 is disposed adjacent to the well layer 10W1, and so on. The barrier layer 10B4 becomes the barrier layer 10B1 of the next unit structure 10U. In this embodiment, the central range in the thickness direction of the first barrier layer 10B1 of a unit structure 10U and the central range in the thickness direction of the first barrier layer 10B1 (barrier layer 10B4) of the next unit structure 10U are described as one period of the unit structure 10U. However, the range selected as the unit structure 10U is a matter of convenience for the purpose of explanation.

[0023] Each unit structure 10U is Zn, where 0≦x<y≦1. 1-y Mg y Zn separated from each other by a barrier layer 10B having a composition of O 1-x Mg xThe semiconductor superlattice structure 100A is made up of at least three well layers 10W each having a composition of O. The semiconductor superlattice structure 100A serves as an active region that emits electromagnetic waves of a certain wavelength under an external voltage applied for operation through a pair of conductive portions such as electrodes 20 and 30.

[0024] The substrate 50 can be made of any material suitable for the epitaxial growth required for fabricating a ZnO-based THz-QCL. The substrate 50 can be a single-layer wafer or a wafer with an appropriate buffer layer formed thereon. To form a ZnO-based THz-QCL with both nonpolar and polar orientations, a sapphire substrate is typically used for the substrate 50. Any method, such as molecular beam epitaxy (MBE), can be used for epitaxial growth of the ZnO-based semiconductor. To generate electrons as carriers, gallium (Ga) is added to one of the well layers of the unit structure 10U as a dopant to achieve n-type conductivity.

[0025] 1-3. Electron transport mechanism Figures 2A to 2D show design examples of ZnO-based THz-QCLs, comparing those based on conventional concepts (Figures 2A and 2B) with those of this embodiment (Figures 2C and 2D). The symbols indicating each part of the potential profile are those for the corresponding layer. In both the design example based on conventional concepts and the design example of this embodiment, ZnO is used in the well layer and Zn is used in the barrier layer. 0.8 Mg 0.2 This is a non-polar ZnO-based THz-QCL with three quantum wells in a unit structure, each of which employs ZnO. As shown in the conduction band edge profiles CB in Figures 2A and 2C, the potential for electrons is low in the ZnO well layer, and high in the Zn 0.8 Mg 0.2 The barrier due to O becomes higher, and the three quantum wells within the unit structure generally show a downward sloping slope in response to the external electric field acting as a bias. The unit structure is shown by the dotted line in Figures 2B and 2D. Electrons generally flow from left to right on the page. Each level may be distinguished from the unit structure it belongs to by a suffix n, which increases toward the right.

[0026] The application of the conventional ideas developed for GaAs-based THz-QCLs to ZnO-based THz-QCLs is referred to as the "conventional approach." In the design example of a ZnO-based THz-QCL using the conventional approach (Figs. 2A and 2B), there are four main levels involved in electron transport and the resulting light emission. If these levels are shown in order so that the center of gravity of the electron existence probability is from upstream to downstream, the injection level i n , upper laser level u n , lower laser level l n , and the extraction level d n In the conventional method, the physical phenomenon responsible for electron transport between levels is as follows: Injection level i n From the upper laser level u n : Resonant tunneling Upper laser level u n From the lower laser level l n : Optical transition (intersubband transition of THz wave emission) Lower laser level l n Extraction level d n : Resonant tunneling Extraction level d n to injection level i n+1 : Vertical transition due to electron-LO phonon scattering

[0027] The combination of these electron transport modes has been adopted for GaAs-based THz-QCLs, and here we apply it as is to ZnO-based THz-QCLs made of ZnO-based semiconductors. Specifically, as shown in Figures 2A and 2B, the injection level i n From the upper laser level u nThe mechanism of electron injection into the well is resonant tunneling (RT). Resonant tunneling is a phenomenon related to the discretized energy levels within the well. In resonant tunneling, electrons tunnel resonantly across the barrier. In this case, if the wave functions of electrons in each well on both sides have energies close to the discretized energy levels such as the ground state, first excited state, and second excited state, they have vibration antinodes in the well, and the probability of existence is high. If the conditions of depth and bias electric field are met so that the energy levels are simultaneously at the discretized levels in both wells, the wave function has vibration antinodes in both wells, and there is a high probability of existence in both wells. There are not just one wave function but multiple wave functions that can have such energy values, and electrons tunnel between these wave functions even if both wells are separated by a barrier of finite thickness and height. Lower laser level l n and the extraction level d n The structure and operating conditions are set so as to satisfy this relationship.

[0028] Here, the lower laser level l n For electron extraction from n and resonant tunneling to the injection level i n+1 Two transport mechanisms are involved: a vertical transition with emission of one LO phonon quantum to the lower laser level l n is delocalized. Due to resonant tunneling, the lower laser level l n is the extraction level d n The energy value is almost the same as the extraction level d n Then, the extraction level d n downstream injection level i, which receives electrons via LO phonon emission from n+1 Also, the extraction level d n The lower laser level l whose energy is equal to n The lower laser level l n and the downstream injection level i n+1 The LO phonon-mediated coupling between nThis will result in a serious increase in linewidth. If the linewidth broadening is Γ, the gain of the optical transition is proportional to 1 / Γ, so laser oscillation cannot be expected with conventional methods. Note that although this design method was effective for GaAs-based semiconductor materials, it is not suitable for ZnO-based semiconductor materials because of differences in the interaction between electrons and LO phonons, i.e., Frohlich coupling. In ZnO-based semiconductor materials, where the strength of the Frohlich coupling is 21 times greater than that of GaAs-based semiconductor materials, LO phonons couple strongly to electrons, resulting in a significant increase in linewidth and a serious decrease in gain.

[0029] In contrast, in the design example of this embodiment (FIGS. 2C and 2D), three levels are mainly involved in the transport of electrons and the resulting light emission. For population inversion, the upper laser level u n It increases the efficiency of electron injection into the lower laser level l n In the example of the ZnO-based THz-QCL design of this embodiment (FIGS. 2C and 2D), there are three levels per unit structure 10U of the THz-QCL, namely, the injection level i n , upper laser level u n , and the lower laser level l n The phenomenon for electron transport employed in the design example of this embodiment is as follows: n-1 to injection level i n : Electron-electron scattering (ee) Injection level i n From the upper laser level u n : Diagonal transition due to electron-LO phonon scattering Upper laser level u n From the lower laser level l n : Optical transition (intersubband transition of THz wave emission)

[0030] More specifically, the lower laser level l n-1 to injection level i n For this reason, in the design of this embodiment, when an external electric field (bias electric field) for operation is applied, the lower laser level l n-1 and the injection level i nThe layers are designed so that the injection level i n From the upper laser level u n The supply of electrons (injection operation) to the well layer 10W is due to a diagonal transition caused by electron-LO phonon scattering, which emits one LO phonon quantum. Looking at the relationship between each well layer 10W and each level at this time, under an external voltage, the injection level i n is the ground state of the first well layer 10W1, and the upper laser level u n is the ground state of the second well layer 10W2, and the lower laser level l n-1 is the ground state of the third well layer 10W3. This relationship is due to the injection level i n has one maximum value of the electron existence probability in the first well layer 10W1, and the upper laser level u n has one maximum value of the electron existence probability in the second well layer 10W2, and the lower laser level l n-1 This can also be expressed as a state in which the maximum value of the electron existence probability is one in the third well layer 10W3.

[0031] Comparing the design of this embodiment (FIGS. 2C and 2D) with the design based on the conventional concept (FIGS. 2A and 2B), the ZnO-based THz-QCL of this embodiment has a lower laser level l n-1 and the injection level i n The laser level is detuned to have an appropriate energy difference, i.e., detuning energy ΔE. n-1 and the injection level i n The distribution of the existence probability is different for the third well layer 10W3 and the first well layer 10W1. The distribution is clearly shown as the difference in the amplitude of each level in FIG. 2D. In contrast, in the design based on the conventional concept, the lower laser level l n and the extraction level d n and each other, and the injection level i n and the upper laser level u nand have the same energy values ​​as much as possible, so that electrons are transported resonantly. In other words, the ZnO-based THz-QCL design example of this embodiment is in stark contrast to the design example based on the conventional concept in that resonant tunneling is not utilized. Also, although electron-LO phonon scattering that emits one quantum of LO phonon is involved, in the design example based on the conventional concept, electrons are transferred within the same well (vertical transfer), whereas in the design example of this embodiment, the injection level i n From the upper laser level u n The difference between diagonal and vertical transitions is whether electrons move spatially before and after the transition. n to injection level i n+1 A vertical transition is a transition in which the spatial position of high electron existence probability does not move much before and after the transition, such as the transition to . In contrast, the transition used in the design example of this embodiment is a transition in which the position of high electron existence probability shifts and the electrons move spatially. In fact, the injection level i shown in FIGS. 2C and 2D n From the upper laser level u n The transition to diagonal transition is accompanied by a shift across the barrier layer 10B in the left-right direction on the paper (the z direction in FIG. 1B , i.e., the thickness direction of the stack). In this way, in the design example of this embodiment, LO phonons are used for indirect injection by diagonal transition (diagonal indirect injection), whereas in the design example based on conventional concepts, LO phonons are used for vertical transition rather than injection. Note that the involvement of LO phonons in injection is also called indirect injection.

[0032] In both the conventional and present designs, ZnO-based semiconductor materials are used, and there is no difference in the fact that the strength of the Frohlich coupling is large. In addition, in both cases, electron-LO phonon scattering is involved in electron transport. Furthermore, in the design of this embodiment, the number of involved levels is rather smaller than in the conventional design. Nevertheless, the inventors believe that the reason why the involvement of LO phonons does not lead to an increase in the linewidth in the design of this embodiment is as follows. Injection level i nFrom the viewpoint of the injection level i, LO phonon scattering is involved in electron transport between the injection level i and the upper laser level u downstream thereof. However, the electron transition is a diagonal transition, and the electron transitions from the second well layer 10W2 to the third well layer 10W3, sandwiching the third barrier layer 10B3 therebetween. Therefore, unlike the conventional design in which LO phonons act on two levels coexisting in the same well, the magnitude of the Frohlich coupling strength is unlikely to directly lead to the formation of a state in which electrons and lattice vibrations are combined, and it is unlikely to affect the energy linewidth through the upper laser level u. In addition, the injection level i n From the viewpoint of the upstream lower laser level l n-1 As a result of the detuning, the injection level i n and the immediately preceding lower laser level l n-1 As shown in FIG. 2D, the existence probability of the injection level i n and the lower laser level l having a large value in the third well layer 10W3. n-1 , and so on. Therefore, the extraction level d n and the lower laser level l n (Fig. 2B) n and the lower laser level l n It is difficult to create a situation in which both electrons are scattered by the LO phonon. n is affected by LO phonons, which leads to the lower laser level l n-1 This is the reason why the use of LO phonon scattering in the design of the ZnO-based THz-QCL of this embodiment does not lead to an increase in the linewidth of the optical transition, i.e., a decrease in the optical gain.

[0033] 1-4. Numerical Simulation In this embodiment, a design example of a ZnO-based THz-QCL is provided, and the characteristics of the design example are predicted by numerical simulation. Exemplary detailed conditions for one of the design examples are shown in Table 2. The simulation focuses on the electron transport characteristics in the QCL structure 100 and the resulting optical characteristics.

[0034] In addition to these conditions, some assumptions are made in the numerical simulation using the NEGF method of this embodiment. The dipole matrix element between the upper and lower laser levels involved in the optical transition is assumed to be 1 nm. n to injection level i n+1 Since electron-electron scattering is involved in the depopulation of electrons into the 11 cm -2 , the condition is adopted.

[0035] 1-5. Simulation Results (Frequency Range and Optical Gain) Figures 3A to 3E are graphs showing the results of a numerical simulation to determine the optical gain at each frequency in the active region of a ZnO-based THz-QCL designed using the design method of this embodiment. Each graph in Figures 3A to 3E represents a different design frequency. The ZnO-based THz-QCL designed using this embodiment can cover a frequency range from 2 THz to 12 THz, and Figures 3A to 3E are representative examples within that range, with a design frequency of 3.5 to 10.5 THz. Note that Figure 3A indicates, with a double-headed arrow, the frequency range (5.4 THz to 14 THz) in which GaAs-based THz-QCLs cannot achieve oscillation due to the Liststrallen band.

[0036] The gain characteristics of Figures 3A to 3E were calculated as follows. First, the conditions shown in Table 2 were determined so that the frequency shown in each figure (3.5 THz in Figure 3A, and so on below) would be the peak of the optical gain of the designed ZnO-based THz-QCL when operating at room temperature (300 K). Since this frequency is a target frequency determined in advance, it is referred to as the design frequency. Note that, to determine each design frequency, the conditions (layer thickness of each layer, barrier height (i.e., composition), and bias voltage) were determined by numerical simulation based on the NEGF (Non-Equilibrium Green's Function) method. Therefore, although the configuration of the semiconductor superlattice structure 100A differs between Figures 3A to 3E, it is fixed in each figure. Then, using the determined specific conditions, the optical gain of the active region 10 was calculated by changing the frequency through numerical simulation.

[0037] The ZnO-based THz-QCL designed in this embodiment is advantageous for achieving high-temperature operation. Specifically, as shown in Figures 3A-3E, at frequencies from 3 THz to 12 THz, a peak gain that overcomes the cavity loss (e.g., 20 / cm) is maintained not only at low temperatures (10 K) but also at room temperature (300 K). In particular, at the low frequency of 3.5 THz, the ZnO-based THz-QCL exhibits extremely high temperature resistance, with the optical gain at 300 K decreasing only slightly from 110 / cm to 80 / cm compared to 10 K. This reflects the magnitude of the LO phonon energy in ZnO-based semiconductor materials. This demonstrates that the significant decrease in optical gain with increasing temperature, a problem in GaAs-based THz-QCLs, can be overcome in ZnO-based THz-QCLs.

[0038] Figure 4 is a map showing the peak intensity of optical gain as a function of the oscillation frequency and the corresponding operating temperature in the ZnO-based THz-QCL of this embodiment, expressed as a concentration at each position. Figure 4 was calculated as follows. The frequencies shown on the horizontal axis are the design frequencies, and the optical gain is calculated by changing the temperature in a ZnO-based THz-QCL constructed based on the design of this embodiment. Figure 4 provides insight into the ease of oscillation when a combination of each design frequency and operating temperature is determined, based on the gain.

[0039] 5A-E are maps showing the electron density distribution at room temperature (300 K) versus thickness position and energy for different frequencies in a design example of this embodiment. n The most electrons are present in the lower laser level l n In other words, it was confirmed that the population inversion is maintained over a wide frequency range at room temperature due to the efficient combination of indirect injection by LO phonon scattering and electron extraction by electron-electron scattering. n The electron occupancy rate of was 35% of the total.

[0040] 1-6. Optimization method for design parameters (design strategy) The structure of the ZnO-based THz-QCL according to the design method of this embodiment can be optimized according to the target oscillation frequency and the temperature conditions used by adjusting the details of each part, including the conditions shown in Table 2 etc. The optimization of the structure can be performed by creating conditions that make it easy to maintain the population inversion without increasing the linewidth of the optical transition. This is because if it becomes easy to maintain the population inversion, the possibility of oscillation at high temperatures increases. More specifically, the lower laser level l n to injection level i n+1 In order to make the electron extraction process as fast as possible using electron-electron scattering, n and the injection level i n+1 It is useful to adjust the detuning energy and anticrossing energy between the lower laser level l n and the injection level i n+1 is localized in the region sandwiching the first barrier layer 10B1, that is, the region where electron scattering occurs.

[0041] The detuning energy here is the lower laser level l n and the injection level i n+1 The difference between the energy values ​​of the two levels is the difference between the energy values ​​of the two levels. In the design example of this embodiment, the detuning energy when a bias electric field for oscillation operation is applied is an issue. Generally, levels with small detuning energy form coupling, and when the detuning energy is zero, this coupling is maximized. Tunneling occurs between two levels only when the two levels have sufficiently close energy values ​​and the detuning energy is small. To adjust the detuning energy, it is sufficient to adjust the relative value of the energy values ​​of the two levels. Generally, it is effective to tilt the potential using an external voltage or to adjust the thickness of a well layer, which affects the energy value of each level.

[0042] Also, the anticrossing energy here is the lower laser level l n and the injection level i n+1This parameter indicates the degree of coupling between the two states and the LO phonon. It is defined as 2*h-bar*Ω, using the coefficient h-bar*Ω of the interaction term that links electron transitions and LO phonon generation in the Hamiltonian describing the system for theoretical analysis (where h-bar = h / 2π, h is Planck's constant). When two levels are close enough in space and energy to couple with each other, the two levels couple with each other even if the detuning energy between them is not zero. As a result, electrons in the higher-energy level can be transported to the lower-energy level by electron-electron scattering. The electron transport process of the THz-QCL of this embodiment actively utilizes this electron-electron scattering. To achieve this, the inventors realized that the degree of coupling between the two levels, i.e., the anticrossing energy 2*h-bar*Ω, is a very important parameter, along with the detuning energy ΔE. When simulating the behavior of electrons based on the NEGF method, the anticrossing energy 2*h-bar*Ω is calculated according to the designed layer structure and operating conditions. Specifically, it can be calculated from the wave functions in the Wannier-Stark mode and the tight-binding mode.

[0043] Lower laser level l n and the injection level i n+1 The adjustment of the detuning energy and anticrossing energy in the case where the external electric field (bias electric field) is fixed will be explained. The applied bias is basically determined by the target oscillation frequency. n and the injection level i n+1 When the thickness of the barrier layer (first barrier layer 10B1, FIG. 2D) where electron-electron scattering occurs due to the lower laser level l is fixed at a value (e.g., 3 nm) at which coupling between these levels can occur, the coupling strength depends on the detuning energy. n and the injection level i n+1The smaller the detuning energy between the two levels, the stronger the coupling between the two levels becomes, and when the detuning energy is zero, the coupling is at its maximum. In this case, the two levels completely coincide. For example, the detuning energy is ΔE = 0 meV, and the anticrossing energy is 2 * h-bar * Ω = 12 meV. In this case, the lower laser level l in Figure 2B n and the extraction level d n The lower laser level l n and the injection level i n+1 As mentioned in Section 1-3, the conditions for resonant tunneling increase the linewidth, making it difficult to operate a THz-QCL with a ZnO-based semiconductor. However, the lower laser level l n and the injection level i n+1 By adjusting the detuning energy between θ and θ and the anticrossing energy appropriately, electron-electron scattering can function efficiently and the linewidth increase can be suppressed. In other words, the adjustment here involves striking a balance between (1) a nonzero value for the detuning energy so that it is not too small to prevent resonant tunneling, and (2) a nonzero value for the anticrossing energy so that it is not too high to prevent resonant tunneling and also so that it is not too small to prevent slowing down electron transport due to electron-electron scattering. Note that the requirement in (2) that the anticrossing energy is not too high is automatically satisfied by (1). Therefore, we show the gain as a function of the detuning energy and the anticrossing energy, and achieve a high gain by balancing these two energies.

[0044] This balance can be better understood by understanding the degree of gain achieved for each combination of detuning energy and anticrossing energy. Figures 6A-B show the lower laser level l for oscillation frequencies of 3.5 THz and 8.5 THz in the ZnO-based THz-QCL of this embodiment. n and the injection level i n+1This is a map showing how the peak gain changes as a function of the detuning energy and anticrossing energy between . Since the peak gain values ​​around 0 to 50 / cm are important, only the contours with values ​​around that range are shown with numbers. n and the injection level i n+1 are shown in Figures 2C-2D. Both the detuning energy and the anticrossing energy are n In each figure, the cavity loss, which is a measure of the oscillation threshold, is about 20 / cm, and the dotted line indicates the condition where 15 / cm is obtained.

[0045] Achieving a balance between the detuning energy and the anticrossing energy is important in the design of this embodiment. The combinations of detuning energy and anticrossing energy that result in a constant peak gain value are shown by contour lines in the parameter space shown in Figures 6A and 6B. That is, each contour line traces a curve that exhibits a gradual deviation from a downward-sloping straight line. In each figure, the line aligned with the positions indicated by the pair of arrows a pointing to each other satisfies the relationship ΔE + h-bar * Ω = 5.0 meV, while the line aligned with the pair of arrows b satisfies the relationship ΔE + h-bar * Ω = 5.5 meV. Here, ΔE is the detuning energy, and h-bar * Ω is 1 / 2 the anticrossing energy. Moving along these lines in the maps shown in each figure results in a gradual change in the peak gain value in accordance with the gradual deviation. In contrast, moving perpendicular to these lines results in a significant change in the peak gain. Therefore, the balance between the detuning energy and the anticrossing energy can be roughly determined by evaluating the sum of the detuning energy and half the anticrossing energy, i.e., excluding the deviation from the downward-sloping straight line. When the balance is constrained by the straight line aligned with the position indicated by the pair of arrows a, the maximum value of the peak gain is approximately 20 / cm. In other words, when ΔE+h-bar*Ω=5.0 meV is satisfied, the possibility of laser oscillation begins to arise. Satisfying ΔE+h-bar*Ω≧5.0 meV is a necessary condition for realizing laser oscillation. On the other hand, when the balance is constrained by the straight line aligned with the position indicated by the pair of arrows b, the minimum value of the peak gain is approximately 20 / cm. In other words, when ΔE+h-bar*Ω=5.5 meV is satisfied, the possibility of laser oscillation begins to arise. Satisfying ΔE+h-bar*Ω≧5.5 meV is a sufficient condition for realizing laser oscillation.

[0046] When achieving the above balance, the preferred range of the detuning energy ΔE for increasing the optical gain can be further limited. The above-mentioned gradual deviation appears as a deviation from a downward-sloping straight line not over the entire range of detuning energy but over a portion of it. Since good optical gain is achieved, the range in which this deviation appears is a preferred condition. That is, good optical gain can be achieved when the lower limit of the detuning energy ΔE is 2.0 meV or more, more preferably 2.5 meV or more, and most preferably 3.0 meV or more. Furthermore, good optical gain can be achieved when the upper limit of this range of detuning energy ΔE is 4.5 meV or less, more preferably 4.0 meV or less, and most preferably 3.5 meV or less. The numerical values ​​defining these lower and upper limits can be combined arbitrarily.

[0047] The specific configuration of the quantum wells and quantum barriers of the ZnO-based THz-QCL designed in this embodiment can be optimized based on the following design strategy. Figures 7A and 7B are schematic diagrams and a flowchart illustrating the design strategy in this embodiment. Figure 7A is a schematic diagram of the structure of a semiconductor superlattice structure 100A. Here, the thicknesses of the first barrier layer 10B1 to the third barrier layer 10B3 are B1 to B3, respectively, and the thicknesses of the first well layer 10W1 to the third well layer 10W3 are W1 to W3, respectively.

[0048] The design optimization of the ZnO-based THz-QCL in this embodiment is performed under appropriate constraints. The constraints include, in addition to the target oscillation frequency, constraints imposed on, for example, the external electric field, well depth, and dipole matrix elements. That is, the external electric field is related to the thickness of the QCL structure 100, i.e., the period and total number of stacked unit structures 10U, and the voltage applied between the conductive portion (TCO layer) 20 and the conductive portion 30, and is set, for example, from a range of 10 kV / cm to 100 kV / cm according to the oscillation frequency. The well depth is determined by the barrier height, i.e., the composition of each of the well layer and the barrier layer. In the general formula, 0≦x<y≦1, and the well is Zn 1-x Mg x O composition, barrier is Zn 1-y Mgy The composition is O. Here, the relationship between the y-x value and the well depth (barrier height) increases as the y-x value increases, and is set to a value such as 0.15. Because oscillation frequency and other operating conditions often allow only a very narrow adjustment range for the external electric field and well depth (barrier height) as parameters, we will explain the case where they are fixed. Furthermore, the dipole matrix element of the radiation, which gives the optical transition probability, is related to the emission intensity, but will be explained by fixing it to an appropriate value such as 1 nm.

[0049] In the optimization under these constraints, first, the thickness W3 of the third well layer 10W3 is set, and then the thickness W1 of the first well layer 10W1 and the thickness B1 of the first barrier layer 10B1 are set (step S02). n and the injection level i n+1 The detuning energy ΔE and anticrossing energy 2*h-bar*Ω between the levels are determined, and the detuning energy ΔE and anticrossing energy 2*h-bar*Ω between the levels can be calculated. In step S02, the thicknesses W3, W1, and B1 are optimized until the detuning energy ΔE and anticrossing energy 2*h-bar*Ω reach the target values. At this stage, not all layer designs are determined, but this optimization is possible. This is because, from FIGS. 6A and 6B, it can be determined whether the conditions are such that the peak gain is easily increased based on the detuning energy ΔE and the anticrossing energy 2*h-bar*Ω. Setting the detuning energy in (1) above to a value that is not too small typically involves adjusting the lower laser level l so that the detuning energy is greater than zero. n at the injection level i n+1 Generally, when the well is made thicker, the energy of the internal level measured from the bottom of the well becomes smaller. Also, from the distribution of probability amplitudes of each level in FIG. 2D, when the third well layer 10W3 and the first well layer 10W1 are made thicker, the injection level i n+1 is the thickness W3 of the third well layer 10W3, and the lower laser level l nis strongly affected by the thickness W1 of the first well layer 10W1. By utilizing this relationship, the detuning energy ΔE can be adjusted. As a result, for example, the thickness W1 of the first well layer 10W1 is made smaller than the thickness W3 of the third well layer 10W3. If the thicknesses W1 and W3 are the same value or if the thickness W1 is larger, the lower laser level l n and the injection level i n+1 However, it is difficult to adjust the detuning energy ΔE and the anticrossing energy 2*h-bar*Ω between the first well layer 10W1 and the third well layer 10W3 to appropriate values. Making the thickness W1 of the first well layer 10W1 smaller than the thickness W3 of the third well layer 10W3 is preferable for adjusting the detuning energy ΔE and the anticrossing energy 2*h-bar*Ω. In addition, to make the anticrossing energy (2) not too small, the structure of the first barrier layer 10B1 is adjusted. Even when the well depth is fixed, it is useful to adjust the thickness of the first barrier layer 10B1. Specifically, by reducing the thickness B1 of the first barrier layer 10B1, the lower laser level l n and the injection level i n+1 By reducing the thickness B1 as necessary, this coupling can be made sufficiently strong, and the resulting optimum values ​​of thicknesses W3, W1, and B1 are fixed.

[0050] Next, in step S04, the thickness W2 of the second well layer 10W2 is set, and the thickness B3 of the third barrier layer 10B3 is set. By doing so, the oscillator strength OS ul Since the oscillator strength is a measure of the optical gain, the thicknesses W2 and B3 that maximize this value are searched for and fixed as optimal values.

[0051] Finally, in step S06, the thickness W2 of the second well layer 10W2 and the thickness B2 of the second barrier layer 10 (injection barrier layer) are optimized again. This optimization is performed by optimizing the injection level i n and the upper laser level u n The thicknesses W2 and B2 are optimized so that the injection level in and the upper laser level u n Energy difference (injection energy) E iu and binding energies can be determined, which gives a measure of electron transport for the diagonal transitions.

[0052] 1-7. Comparison of Extraction (Depopulation) Methods In designs based on conventional concepts, electron extraction from the lower laser level for population inversion is due to RP (resonant phonon scattering), whereas the ZnO-based THz-QCL design of this embodiment employs electron-electron scattering. Figure 8A shows the calculated emission spectra for ZnO-based THz-QCLs designed to operate at 8 THz, based on the method of this embodiment and the conventional concept.

[0053] For each characteristic, the ZnO-based THz-QCL designed using the method of this embodiment assumes a structure with a design frequency of 7 THz, as shown in Figures 3C and 5C, while the one based on conventional concepts assumes a design utilizing RP, as shown in Figures 2A and 2B. Figure 8A clearly shows the quantum level widths for the design based on conventional concepts utilizing RP and the design of this embodiment utilizing electron-electron scattering. The scale in Figure 8A is consistent to enable comparison between this embodiment and the conventional approach utilizing resonant phonons. This calculation result suggests that even if the design utilizing resonant phonons, which is most commonly used in GaAs-based THz-QCLs, is applied directly to ZnO-based semiconductors, the lower laser level l itself will be directly affected by LO phonons, which have high energy. This is a consequence of relying on LO phonons to extract electrons from the lower laser level l in ZnO-based semiconductors. As a result, the energy linewidths of the upper laser level u and lower laser level l, which are responsible for the optical transition, will be excessively broad, resulting in insufficient gain. In fact, the increased linewidth in the design using resonant phonons shown in Fig. 8A corresponds to a very small optical gain of about 0.5 / cm at 300 K. In contrast, in the design example of the method using electron-electron scattering according to this embodiment, the increase in linewidth is sufficiently suppressed even at the same temperature of 300 K.

[0054] 1-8. Temperature Characteristics Figure 8B shows the calculated peak gains at each temperature for a ZnO-based THz-QCL designed to operate at 8 THz and a GaAs-based THz-QCL designed to operate at 4 THz, both of which were designed based on the method of this embodiment and conventional concepts. For a fair comparison, the peak gains for the three designs shown in Figure 8B are optical gains under the same doping level in the semiconductor superlattice layer, and Figure 8B shows their temperature dependence. The dotted line in the figure also indicates a cavity loss of approximately 20 / cm, which serves as a guideline for laser oscillation. The ZnO-based THz-QCL designed using the method of this embodiment is labeled "ZnO ee." The designs based on conventional concepts utilizing resonant phonon extraction are labeled "ZnO RP" for the ZnO-based THz-QCL and "GaAs RP" for the 4 THz GaAs-based THz-QCL. The resonant phonon extraction technique, often employed in GaAs-based THz-QCLs, exhibits high gain at low temperatures, as shown by the "GaAs RP" line, but the gain rapidly decreases as the temperature approaches room temperature (300 K). Even if the resonant phonon extraction mechanism employed in GaAs-based THz-QCLs were applied to a ZnO-based THz-QCL operating at 8 THz, as shown in Figure 2B, the gain would actually decrease, as shown by the "ZnO RP" line, and the upper temperature limit for oscillation would also be lower. This is because the conventional concept itself does not adequately address the strong Fröhlich coupling in ZnO-based semiconductors. In contrast, the design of this embodiment, which utilizes electron-electron scattering, achieves high gain at all temperatures, as shown by the "ZnO ee" line, and the gain decrease at high temperatures is significantly suppressed. Thus, the design example using the method of this embodiment clearly demonstrates the temperature tolerance of the gain.

[0055] 9A and 9B are graphs showing the dependence of current density on the electric field calculated for a ZnO-based THz-QCL in this embodiment. Calculations were performed for a unit structure equivalent to one period. Figures 9A and 9B show the results for oscillation frequency design values ​​of 3 THz and 8 THz, respectively. When the oscillation frequency design value is as low as 3 THz, the parasitic current near 56 kV / cm becomes significantly large due to the resonant tunneling process. Therefore, a ZnO-based THz-QCL designed to oscillate at 3 THz has a relatively small dynamic range at a temperature of 300 K. The dynamic range here is defined as the current range between the current at which the maximum gain is achieved and the current at which the gain is exactly equal to the resonator loss (oscillation threshold current). In other words, the dynamic range of a ZnO-based THz-QCL oscillating at 3 THz is 1300 A / cm 2 On the other hand, the dynamic range of a ZnO-based THz-QCL with a design oscillation frequency of 8 THz at 300 K is extremely large, reaching approximately 2600 A / cm 2 From these comparisons, it can be seen that the ZnO-based THz-QCL of this embodiment can be more easily operated at high frequencies than at low frequencies.

[0056] 2. Modifications Various modifications can be made to this embodiment.

[0057] 2-1. Material In the above description of the embodiment of the present disclosure, the same material (e.g., ZnO) is used in each well layer, and the same material (e.g., ZnO) is used in each barrier layer. 0.8 Mg 0.2 However, in the design of the present disclosure, it is also acceptable to use different materials for each well layer and for each barrier layer. Changing the material to appropriately adjust the energy value of each level for the purpose of controlling the electron transport process by forming quantum wells and quantum barriers using the potential acting on electrons according to the conduction band edge profile can be an effective method for optimizing ZnO-based THz-QCLs implemented by the design of the embodiments of the present disclosure. In addition, in the selection of preferred materials, ZnO is used for each well layer and Zn is used for each barrier layer. 1-y Mg y0 (where 0.1≦y≦0.2) is adopted. The MgO composition ratio y in the mixed crystal of ZnO and MgO is a parameter for directly controlling the well depth (barrier height).

[0058] 2-2. Polar ZnO / ZnMgO Quantum Well System The above description of the embodiments of the present disclosure was based on the potential profile realized in a quantum well made of a ZnO-based semiconductor grown in a nonpolar direction. ZnO-based semiconductors can also be grown in a polar direction, and even in this case, the Fröhlich coupling remains stronger than that of GaAs-based materials. Although the epitaxial growth conditions and the quality of the grown crystal generally differ depending on whether or not the crystal is polar, the dielectric properties and the resulting effective electron mass (Table 1) do not change significantly. Therefore, the above description of this embodiment also applies to ZnO-based semiconductors grown in a polar direction. In particular, the design concepts for each level and the underlying knowledge of the electron transport mechanism also apply to ZnO-based semiconductors grown in a polar direction.

[0059] 2-3. Laser Operational Frequency Range In the embodiments of the present disclosure, laser oscillation by a ZnO-based THz-QCL has been described using examples of low frequencies of 3 to 3.5 THz and high frequencies of 8 to 8.5 THz. Such operation can be achieved across the wider frequency range of 2 THz to 13 THz. In particular, the operating temperature range, although dependent not only on the frequency but also on the resonator performance at the operating oscillation frequency, includes high temperatures up to about room temperature (300 K). In particular, the frequency range of 6 THz to 12 THz is a THz gap that cannot be accommodated by GaAs-based semiconductor materials. Therefore, the THz-QCL device of this embodiment, which can operate in this frequency range by utilizing the characteristics of ZnO-based semiconductor materials, is advantageous.

[0060] 3. Summary The embodiments of the present disclosure have been specifically described above. The above-described embodiments, modifications, and examples have been described to explain the invention disclosed in this application, and the scope of the invention of this application should be determined based on the description of the claims. Modifications within the scope of the present disclosure, including other combinations of the embodiments, are also included in the scope of the claims.

[0061] 1000 QCL element 100 QCL structure (100A semiconductor superlattice structure) 10 active region 10B, 10B1 to 10B4 barrier layer 10W, 10W1 to 10W3 well layer 10U unit structure 20 conductive portion (TCO layer) 22, 32 metal contact 24 passivation film 30 conductive portion 40 receptor substrate 50 substrate 60 etching stopper layer 120, 140 highly doped layer 160 δ-doped layer 2000 electromagnetic wave

Claims

1. A quantum cascade laser element having a semiconductor superlattice structure sandwiched between a pair of conductive parts, wherein the semiconductor superlattice structure has a plurality of unit structures that are repeatedly stacked, and each unit structure is composed of three well layers having a composition of ZnO or ZnMgO, and barrier layers that separate each well layer from the others and have a composition of ZnMgO or MgO with a higher MgO ratio than the adjacent well layers on both sides, and wherein the semiconductor superlattice structure becomes an active region that emits electromagnetic waves of a certain wavelength when an external voltage is applied through the pair of conductive parts for operation.

2. 0≦x<y≦1, and the composition of each of the three well layers is Zn. 1-x Mg x O, and the composition of the barrier layer is Zn 1-y Mg y The quantum cascade laser device according to claim 1 , wherein the ZnO is O.

3. The three well layers all have a composition of ZnO, and the barrier layers all have a composition of Zn. 1-y Mg y 3. The quantum cascade laser device according to claim 2, wherein y is 0 (where 0.1≦y≦0.2).

4. The quantum cascade laser device according to claim 1, wherein the three well layers include a first well layer, a second well layer, and a third well layer in this order from upstream to downstream of the electron flow, and wherein the unit structures are configured such that, under the external voltage, an injection level having a maximum amplitude is formed in the first well layer, an upper laser level having a maximum amplitude is formed in the second well layer, and a lower laser level having a maximum amplitude is formed in the third well layer, and wherein, under the external voltage, electrons in the upper laser level of a certain unit structure optically transition to the lower laser level of the unit structure, and electrons in the lower laser level of the unit structure are extracted by electron-electron scattering to the injection level of another unit structure adjacent to the unit structure on the downstream side.

5. The quantum cascade laser device according to claim 4, wherein, under the external voltage, electrons at the injection level of a certain unit structure are injected into the upper laser level of that unit structure by electron-LO phonon scattering.

6. The quantum cascade laser device according to claim 4, wherein a detuning energy value between an injection level of a certain unit structure and a lower laser level of another unit structure adjacent to said unit structure on the upstream side is greater than zero, and an anticrossing energy value between said injection level and said lower laser level is greater than zero.

7. The quantum cascade laser device according to claim 6, wherein the sum of the detuning energy value and half of the anticrossing energy value is 5 meV or more.

8. The quantum cascade laser device according to claim 6, wherein the sum of the detuning energy value and half of the anticrossing energy value is 5.5 meV or more.

9. The quantum cascade laser device of claim 4, wherein, under the external voltage, the injection level is the ground state of the first well layer, the upper laser level is the ground state of the second well layer, and the lower laser level is the ground state of the third well layer.

10. The quantum cascade laser device according to claim 1, wherein the electromagnetic wave has a frequency of 6 THz or more and 12 THz or less.