Zinc oxide-based quantum cascade laser element

Abstract

In order to provide an IR-QCL element which utilizes the characteristics of a ZnO-based semiconductor material, the present disclosure discloses a quantum cascade laser element 1000 that has a plurality of unit structures 10U in each of which a well layer of ZnO or ZnMgO and a barrier layer of ZnMgO or MgO are alternately and repeatedly stacked. Each unit structure includes a light emitting layer 10E and a continuum layer 10C. The light emitting layer includes: an upstream well layer 10W1 that is composed of a step quantum well having a first layer and a second layer, which have different MgO composition ratios from each other; and a downstream well layer 10W2 that has a smaller MgO composition ratio than any of the first layer and the second layer.

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 and operate in the infrared region.

[0002] Quantum cascade lasers (QCLs) have been attracting attention. Generally, QCLs 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 the 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 entire potential is tilted due to the bias electric field. Electrons, the carriers, undergo repeated intersubband transitions (ISBTs) while being transported through subbands, or quantum levels, formed by the tilted and uneven potential. Each transition couples with an electromagnetic field, resulting in stimulated emission, which then leads to lasing. Therefore, the semiconductor superlattice structure serves as the active region for light emission. The term "cascade" refers to the behavior of electrons transported through intersubband transitions, losing energy as they undergo transitions. QCLs can lasing wavelengths independent of the energy band gap of the material. The lasing wavelength or frequency can be controlled by varying the materials and design of the semiconductor superlattice structure. Therefore, QCLs are attracting attention as unipolar semiconductor lasers, providing coherent light sources in wavelengths (frequency ranges) not previously available from solid-state light sources.

[0003] Quantum cascade lasers (IR-QCLs) for the infrared region have been experimentally realized. One of them is a QCL in which a superlattice of InGaAs and InAlAs materials is formed on an InP substrate with lattice matching (Non-Patent Document 1, hereafter referred to as a GaAs-based IR-QCL).

[0004] Yao, Y., Hoffman, A. & Gmachl, C. "Mid-infrared quantum cascade lasers", Nature Photon 6, 432-439 (2012). DOI: 10.1038 / nphoton.2012.143

[0005] However, GaAs-based IR-QCLs generally have the problem of lower efficiency at around room temperature (e.g., 300 K) compared to their efficiency at low temperatures (e.g., liquid nitrogen temperature (-196°C = 77 K) or lower). This disclosure is based on the idea of ​​a new operating mechanism for IR-QCLs, and proposes a specific configuration for an IR-QCL that enables operation at higher temperatures than previously possible and operates with high efficiency around room temperature.

[0006] The inventors have used the NEGF (Non-Equilibrium Green's Function) method, a calculation technique 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 IR-QCL based on a new idea that can bring out the material properties of ZnO-based semiconductor materials.

[0007] 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 in which well layers having a composition of ZnO or ZnMgO and barrier layers that separate the well layers and have a composition of ZnMgO or MgO higher than the well layers on both sides of the adjacent well layers are alternately stacked, each unit structure having a light-emitting layer and a continuum layer, the light-emitting layers having different MgO composition ratios from each other and positioned in order from the upstream side in the flow of electrons during operation. and a downstream well layer having a lower MgO composition ratio than either the first or second layer of the upstream well layer and located downstream of the upstream well layer in terms of electron flow during operation, wherein the continuum layer comprises a plurality of well layers and a plurality of barrier layers separating them from one another, and the semiconductor superlattice structure serves as an active region that emits electromagnetic waves of a certain wavelength under an external voltage applied through the pair of conductive portions for operation.

[0008] In this application, infrared electromagnetic waves refer to electromagnetic waves with wavelengths ranging from approximately 1 μm to 10 μm. Furthermore, in the description of this application, device structure and function may be explained using technical terms adapted or borrowed from fields such as electronic devices and physics that deal with visible light and infrared light. Therefore, even when describing electromagnetic waves in wavelength or frequency ranges that are not considered 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 "photon." Examples of such terms 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 electrons, determined by the wave function of that level, is 100% contained within the range of that well layer. Furthermore, in the QCL devices of this application, electrons are the carriers responsible for electrical conduction and optical transitions.

[0009] According to any one of the aspects of the present disclosure, a practical ZnO-based IR-QCL element capable of utilizing the properties of a ZnO-based semiconductor is provided.

[0010] 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 IR-QCL device according to an embodiment of the present disclosure. FIG. 2 is a band diagram showing the active layer design of a conventional IR-QCL using GaAs-based semiconductor materials, as disclosed in Non-Patent Document 1. FIGS. 3A-3B are schematic diagrams showing optical transitions according to an embodiment of the present disclosure, respectively showing a perpendicular transition (FIG. 3A) and a diagonal transition (FIG. 3B). FIG. 4 is a graph showing the LO phonon scattering rate on the vertical axis versus the oscillation wavelength on the horizontal axis in an embodiment of the present disclosure. FIGS. 5A-5B are schematic diagrams showing the electrons passing through interfaces while being scattered in an embodiment of the present disclosure. FIG. 6 is a graph showing the optical gain calculated for each wavelength for an embodiment of the present disclosure with a large and a small number of interfaces. 7A to 7D are explanatory diagrams showing specific configurations of the light-emitting layer design when diagonal transitions are employed in an embodiment of the present disclosure. These configurations include two quantum wells of the same depth for the light-emitting layer ( FIG. 7A ), and two quantum wells with an ingenious configuration employed in an embodiment of the present disclosure for the light-emitting layer ( FIG. 7C ). FIG. 8 is an explanatory diagram summarizing the configuration of a ZnO-based IR-QCL device according to the present disclosure. FIG. 9 is an explanatory diagram showing the band structure and electronic energy levels of an IR-QCL made of a ZnO-based semiconductor according to an embodiment of the present disclosure, grown with a polar crystal plane orientation. FIGS. 10A and 10B are contour plots showing the dynamic electron transport near the light-emitting layer of an IR-QCL made of a ZnO-based semiconductor with a polar crystal plane orientation, calculated using NEGF. These figures show the electron carrier density ( FIG. 10A ) and the electron carrier current (current) density ( FIG. 10B ). Fig. 11 is a two-dimensional map of optical gain calculated for each wavelength and operating temperature for a polar plane orientation in an embodiment of the present disclosure. Fig. 12 is an explanatory diagram showing the band structure and electronic energy levels of an IR-QCL made of a ZnO-based semiconductor according to an embodiment of the present disclosure grown with a nonpolar plane orientation. Figs. 13A and 13B are contour maps showing dynamic electron transport calculated by NEGF near the light-emitting layer of an IR-QCL made of a ZnO-based semiconductor with a nonpolar plane orientation, showing the electron carrier density (Fig. 13A) and the electron carrier current (current) density (Fig. 13B).Figure 14 is a two-dimensional map of optical gain calculated for each wavelength and operating temperature for a nonpolar plane orientation in an embodiment of the present disclosure. Figures 15A to 15C are graphs plotting the minimum MgO composition ratio that allows emission at 300 K versus wavelength for the barrier layers 10B1 to 10B5 (Figure 15A), the first layer 10W11 (Figure 15B), and the second layer 10W12 (Figure 15C) in an embodiment of the present disclosure. Figure 16 is a chart explaining a procedure for optimizing parameters in an embodiment of the present disclosure.

[0011] 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.

[0012] 1-1. Designing an IR-QCL Compatible with the Physical Properties of ZnO-Based Semiconductors The present inventors focused on semiconductor materials (referred to as "ZnO-based semiconductors") composed of zinc oxide (ZnO) and a mixed crystal of ZnO and magnesium oxide (MgO) (ZnMgO) in an attempt to improve the operating temperature of IR-QCLs. The construction of an IR-QCL using a ZnO-based semiconductor requires consideration of its physical properties and the corresponding design issues. Considering the physical properties of ZnO-based semiconductors, the following two points must be noted when comparing them with GaAs-based semiconductor materials: (1) ZnO-based semiconductor materials have a large effective mass for the electrons that act as carriers in the QCL. (2) ZnO-based semiconductors have a large Fröhlich coupling strength. In ZnO-based semiconductors, the coupling between electrons and LO phonons is 21 times stronger than in GaAs-based semiconductor materials. This is because the relative dielectric constant (ε ｒ , ε ∞ ) is ε ｒ = 8.9, ε ∞ = 3.7, and the value of GaAs (ε ｒ = 12.93, ε ∞ This is because it is much smaller than the σ (σ = 10.89).

[0013] Table 1 shows the physical properties of GaAs-based and ZnO-based semiconductor materials, along with those of GaN-based materials.

[0014] In addition, there are two design considerations for IR-QCLs that use ZnO-based semiconductors: (3) When a QCL is fabricated using ZnO-based semiconductor materials, interface scattering increases, and (4) With ZnO-based semiconductors, it is difficult to store electrons in the upper laser level due to thermal backfilling.

[0015] 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 IR-QCL device according to this embodiment. A typical ZnO-based IR-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 QCL structure 100, which is a semiconductor superlattice structure, 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. The electrodes 20 and 30 are typically made of metal. The electrodes 20 and 30 also exert an optical confinement effect due to the cavity structure at the oscillation wavelength. The electrodes 20 and 30 are not necessarily made of metal, and a structure in which one is a metal layer and the other is a highly conductive semiconductor layer (single-sided metal waveguide structure) can be adopted, for example, by making one of the electrodes a highly n-doped layer. To achieve this, for example, ZnO can be used for the substrate and ZnMgO that is highly n-doped for the electrode 30.

[0016] The QCL structure 100 includes an active region 10. The IR-QCL device 1000 operates by passing electrons through a repeating electron potential structure 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 infrared region. The IR-QCL device 1000 shown in FIG. 1 is fabricated by forming a metal layer 30B of an electrode 30 on a receptor substrate 40 (hereinafter referred to as "receptor 40") and bonding it to a metal layer 30A formed on the QCL structure 100. In addition to these, the IR-QCL device 1000 shown in FIG. 1 also includes layers for operation and device fabrication, such as heavily doped layers 120 and 140, a delta-doped layer 160, and an etching stopper layer 60, as appropriate.

[0017] The active region 10 (FIG. 1B) includes 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 unit structures 10U, typically 10 to 200 periods of identical unit structures 10U, stacked one after the other. FIG. 1C shows an enlarged view of the structure of one unit (one period) of each unit structure 10U. Each unit structure 10U includes a light-emitting layer 10E and a continuum layer 10C. This light-emitting layer 10E includes a well layer 10W1 and a well layer 10W2. The well layer 10W1 is an upstream well layer, and the well layer 10W2 is a downstream well layer, in terms of the flow of electrons during operation. Note that the flow of electrons during operation is along the +z axis in FIGS. 1A-C. The well layer 10W1, which is the upstream well layer, is a stack of a first layer 10W11 and a second layer 10W12. The first layer 10W11 and the second layer 10W12 are located in this order from the upstream side in terms of the flow of electrons during operation, and each layer has a different MgO composition ratio. Therefore, the well layer 10W1 has a stepped quantum well structure. The well layer 10W2, which is the downstream well layer, has a lower MgO composition ratio than either the first layer 10W11 or the second layer 10W12 of the well layer 10W1, which is the upstream well layer. The continuum layer 10C includes well layers 10W3 to 10W5. The light-emitting layer 10E and the well layers of the continuum layer 10C are separated by barrier layers 10B1 to 10B6. In FIG. 1C, only the barrier layers are hatched for ease of identification. The barrier layer 10B6 is the same layer as the barrier layer 101 in the next unit structure 10U. The remaining units are stacked in the same manner. The range that is designated as the unit structure 10U is selected for convenience in the explanation.

[0018] The ZnO-based IR-QCL device 1000 of this embodiment has the following typical configuration. Specifically, the barrier layers 10B1 to 10B5 included in each unit structure 10U have the same composition, with only the thickness adjusted. In contrast, the well layers 10W1 to 10W5 included in each unit structure 10U have, as described above, a stepped quantum well structure in which the well layer 10W1 (upstream well layer) of the light-emitting layer 10E is a stack of a first layer 10W11 and a second layer 10W12, and the well layer 10W2 (downstream well layer) has a different MgO composition ratio from that of the well layer 10W1. Furthermore, the three well layers 10W3 to 10W5 of the continuum layer 10C have the same composition. The thickness of each well layer is adjusted individually. This semiconductor superlattice structure 100A becomes 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.

[0019] The substrate 50 can be made of any material that can undergo the epitaxial growth required for fabricating a ZnO-based IR-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 IR-QCL with a polar or nonpolar plane orientation, a ZnO substrate is typically used as 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.

[0020] 1-3. Design Guidelines for IR-QCLs in ZnO-Based Semiconductor Materials The large effective mass of electrons in ZnO-based semiconductor materials (see (1) above) directly necessitates narrowing the quantum well, which directly leads to problems with unavoidable fluctuations in film thickness (or deviations from the design). For example, a quantum well thickness of 1 nm would be approximately 4 monolayers (ML) for ZnO-based semiconductor materials. Designs that make the film thickness too thin require precision in crystal growth. In particular, a problem can arise in that it becomes difficult to adjust the energy of the emission level to the desired value when the film thickness is thin.

[0021] Regarding the large Frohlich coupling in ZnO-based semiconductor materials in the above (2), increasing the rate of diagonal transitions of the laser level (details will be described later) can be a solution, but in that case, the film thickness fluctuations in the thickness of the layer where the diagonal transition occurs may also become a problem.

[0022] However, we believe that the above two issues can be overcome to some extent by advances in crystal growth technology that suppresses film thickness fluctuations.

[0023] The interface scattering in the ZnO-based semiconductor material in (3) above can be addressed by reducing the number of interfaces themselves, i.e., it is preferable to reduce the number of quantum well layers per unit structure.

[0024] As for the thermal backfilling in (4) above, a solution can be found by selecting the energy value of the injection level that performs the injection operation to the upper laser level.

[0025] Figure 2 is a band diagram showing the active layer design of a conventional IR-QCL using GaAs-based semiconductor materials, as disclosed in Non-Patent Document 1. This structure is a representative example of a QCL structure adopted for mid-infrared radiation. This structure is fabricated by forming a superlattice of InGaAs and InAlAs on an InP substrate. The horizontal axis of Figure 2 represents the distance in the z direction (thickness direction), and the vertical axis represents the electron energy. Conventional IR-QCLs, although made of different materials, have a similar structure to Figures 1A-B. In QCLs, including those disclosed herein, the energy at the conduction band edge acts on electrons as a potential. In the explanations of this disclosure, including Figure 2, this potential is depicted as sloping downward to the right and having multiple wells and barriers. In Figure 2, each curve, which is depicted as having multiple peaks depending on the position, represents the probability of electrons existing at each level, with each peak position having a high probability of existence. This is the square of the absolute value of the wave function representing the electron level. The curves are drawn with the baseline shifted so that the energy value of the electrons at that level is located along the vertical axis. The reason why the potential slopes downward is due to the gradient of the potential, i.e., the electric field, during operation when a voltage is applied.

[0026] Two electron levels, connected by a downward arrow and offset vertically, are the upper laser level (U) and the lower laser level (L). When population inversion occurs between these upper laser level (U) and lower laser level (L), stimulated emission becomes possible, enabling laser oscillation. Conventionally, the photon emission between these levels is indicated by a wavy arrow. The level 19 meV higher in energy than the upper laser level U is the injection level i. Electrons are injected from the injection level i into the upper laser level U. If the upper laser level U has a higher electron population than the lower laser level L, population inversion occurs, creating the possibility of laser oscillation. As shown in Figure 2, the transition (optical transition) from the upper laser level U to the lower laser level L is designed so that the centers of gravity of the existence probabilities before and after the transition are within the same well. This is called a vertical transition. A vertical transition is employed in IR-QCLs using GaAs-based semiconductors.

[0027] The above-mentioned thermal backfilling is caused by the existence of a level having an energy value within the range in which electrons in the upper laser level U can be thermally excited, that is, within the range in which they can be thermally excited higher than the upper laser level U. Since thermal excitation of electrons can occur in a range of about 26 meV at room temperature (300 K), in a design in which the injection level i is located 19 meV higher than the upper laser level U as shown in FIG. 2, it is difficult to increase the population of the upper laser level U, and it is difficult to expect operation near room temperature.

[0028] 3A and 3B are schematic diagrams showing the optical transitions in this embodiment, respectively showing vertical transitions (FIG. 3A) and diagonal transitions (FIG. 3B). In the vertical transition shown in FIG. 3, the optical transition occurs within the same potential, and the center of gravity of the electron existence probability before and after the optical transition hardly changes. In contrast, in the diagonal transition, the wells on the upstream and downstream sides are designed to have the center of gravity at the upper and lower laser levels, respectively. Therefore, the center of gravity of the electron existence probability moves before and after the optical transition. In the diagonal transition, the overlap integral of the electron wave function tends to be small, but separate wells can be used for each level, allowing for greater design freedom.

[0029] In particular, in ZnO-based semiconductors, where the probability of LO phonon scattering is high, it is advantageous to employ diagonal transitions. Figure 4 is a graph in which the horizontal axis represents the oscillation wavelength and the vertical axis represents the LO phonon scattering rate, and plots vertical transitions in GaAs-based semiconductor materials (vertical_GaAs) as shown in Figures 2 and 3A, vertical transitions in ZnO-based semiconductors (vertical_ZnO), diagonal transitions with weak diagonal transitions as shown in Figure 3B (diagonal (weak)_ZnO), and diagonal transitions with strong diagonal transitions (diagonal (strong)_ZnO). In GaAs-based semiconductor materials, even if vertical transitions are employed, the LO phonon scattering rate is sufficiently small, at 0.5 ps. －１ On the other hand, in ZnO-based semiconductors, the rate of LO phonon scattering is extremely high, and in particular, the vertical transition rate of ZnO is 20 ps. －１The rate of this LO phonon scattering exceeds 5 ps. －１ It is preferable to suppress the LO phonon scattering rate to less than 1 / 2. The LO phonon scattering rate also depends on the difference in effective mass and Fröhlich interaction. Therefore, adopting a diagonal transition and increasing (strengthening) its ratio to reduce the LO phonon scattering rate helps maintain the population inversion. Note that adopting a diagonal transition can reduce the overlap integral, making it difficult to emit light, so the reduction in the LO phonon scattering rate is adjusted taking this into consideration. If population inversion is achieved and stimulated emission is realized, laser oscillation will be realized, so a small overlap integral is less of a problem once laser oscillation is realized. As mentioned above, the ability to separately set the upper and lower laser levels in different wells also facilitates such adjustment.

[0030] Furthermore, in ZnO-based semiconductors, where scattering due to interface roughness is significant, it is advantageous to reduce the number of layers per unit structure 10U. Figures 5A and 5B are schematic diagrams illustrating the manner in which electrons pass through interfaces while scattering in this embodiment. Because the unit structure 10U includes the light-emitting layer 10E and the continuum layer 10C, it includes multiple well and barrier interfaces. A large number of interfaces, as in Figure 5A, increases the probability that electrons will be scattered as they pass through due to interface roughness caused by imperfect film formation. Furthermore, a small number of interfaces simplifies the design of quantum levels, facilitating a design that increases efficiency. Figure 6 is a graph of optical gain calculated by wavelength for a large number of interfaces (many interfaces) and a small number of interfaces (less interfaces) in this embodiment. This graph was calculated under conditions in which the number of interfaces was increased or decreased by changing the number of wells in the continuum layer 10C from 6 to 3. It was confirmed that, at all wavelengths, a smaller number of interfaces results in higher optical gain and changes the ease of laser oscillation.

[0031] The ZnO-based IR-QCL device 1000 of this embodiment incorporates additional innovations in the design of the light-emitting layer when employing diagonal transition. Figures 7A to 7D are explanatory diagrams showing specific configurations of the light-emitting layer 10E when employing diagonal transition. These configurations include a configuration with two quantum wells of the same depth for the light-emitting layer 10E (Figure 7A) and a configuration with two quantum wells of an innovative configuration employed in the embodiment of the present disclosure for the light-emitting layer 10E (Figure 7C). Figures 7B and 7D show the electron transport states in the configurations of Figures 7A and 7C, respectively. The configuration of Figure 7A is intended to realize a light-emitting layer 10E with a relatively small number of layers while achieving diagonal transition, and is a two-level design in which two levels are directly involved in light emission. As shown in Figure 7B, in this configuration, electrons from the layer corresponding to the continuum layer 10C, where the miniband is formed, are directly injected into the upper laser level U by tunneling. Light emission from the upper laser level U to the lower laser level L is achieved by diagonal transition. Electrons are extracted from the lower laser level U by LO phonon scattering and are then extracted to the downstream continuum layer 10C. This two-level design has three problems. First, the layer thickness of the quantum well that determines the energy of the upper laser level U becomes too thin. For example, a thickness of 1 nm results in a thin crystal of about 4 ML. However, if the thickness fluctuates to 3 ML or 5 ML, the energy value of the upper laser level U fluctuates significantly, making the oscillation wavelength unstable. Second, direct injection into the upper laser level U by tunneling does not provide a sufficient injection speed. Third, multiple levels with slightly higher energy than the upper laser level U are formed in the continuum layer 10C, resulting in thermal backfilling. Both the second and third problems hinder the formation of a population inversion.

[0032] Figure 7C shows the potential structure of a ZnO-based IR-QCL according to an embodiment of the present disclosure. In Figures 7C and 7D, the layer reference numbers in Figure 1C are added to the various portions of the potential. The well layer 10W1 is an upstream well layer and is a stack of a first layer 10W11 and a second layer 10W12. The light-emitting layer 10E also includes a downstream well layer, the well layer 10W2. Although the well layer 10W1 has a stepped quantum well structure internally, it is a single quantum well. Therefore, the number of quantum wells in the light-emitting layer 10E is two. In other words, the number of interfaces through which electrons pass is the same as in the configuration of Figure 7A, as far as the light-emitting layer 10E is concerned. The well layer 10W1 includes a first layer 10W11 and a second layer 10W12, each with a tailored composition and film thickness, and is thicker than that of Figure 7A. Therefore, high manufacturing precision is not required for the thickness of the well layer 10W1. As shown in Figure 7D, this configuration operates as follows. Electrons from the continuum layer 10C, where the miniband is formed, are injected into the injection level i by tunneling. From the injection level i, electrons are injected into the upper laser level U by LO phonon scattering. This injection occurs within the well layer 10W1, so it is a vertical transition, but it is not an optical transition involved in light emission. Light emission from the upper laser level U to the lower laser level L is achieved by diagonal transition. The well layer 10W1 has a stepped quantum well structure, and the first layer 10W11 is a deeper quantum well than the second layer 10W12. Therefore, the existence probability at the upper laser level U is shifted upstream (toward the left on the drawing). Therefore, the center of gravity shift due to the diagonal transition is large, and the degree of diagonal transition can be said to be strong. Electrons are extracted from the lower laser level U by LO phonon scattering, and are extracted to the downstream continuum layer 10C. The design of the ZnO-based IR-QCL of the present disclosure, shown in Figures 7C and 7D, is a three-level design. This overcomes the problems of the two-level design shown in Figures 7A and 7B. In other words, because the well layer 10W1 is relatively thick, the fabrication precision of the well layer 10W1 is unlikely to have a significant effect on the emission wavelength. Furthermore, injection into the upper laser level U involves the injection level i, and injection is not due to direct tunneling, so injection at a sufficient rate is achieved.Furthermore, there is no level in the energy range where thermal backfilling can occur, as viewed from the upper laser level U. Since the injection level i typically has an energy 26 meV or more higher than the upper laser level U, it is believed that thermal backfilling will not occur. In this way, the ZnO-based IR-QCL of this embodiment can overcome the problems caused by the two-level design shown in FIGS. 7A and 7B.

[0033] FIG. 8 is an explanatory diagram summarizing the configuration of the ZnO-based IR-QCL device of the present disclosure described with reference to FIGS. 7C and 7D. In FIG. 8, electron levels are indicated by bars representing energy values. The well layer 10W1, which employs a stepped quantum well structure consisting of the first layer 10W11 and the second layer 10W12, allows the wave functions of the injection level i and the upper laser level U to be ideally positioned. Because the compositions of the first layer 10W11 and the second layer 10W12 are different from those of the well layer 10W2, the energy values ​​of the upper laser level U and the lower laser level L can be adjusted according to the purpose. Furthermore, because the number of interfaces between the well layer and the barrier layer can be reduced, the effects of scattering at the interfaces are suppressed. These features combine to provide the ZnO-based IR-QCL of this embodiment with a useful configuration as an IR-QCL using a ZnO-based semiconductor. The injection through injection level i includes injection i1 from the miniband into injection level i and injection i2 from injection level i into upper laser level U.

[0034] In actual IR-QCLs using ZnO-based semiconductors, the potential structure differs depending on the plane orientation of the grown crystal, whether it is a polar plane orientation or a nonpolar plane orientation. Figure 9 is an explanatory diagram showing the band structure and electron levels of an IR-QCL using a ZnO-based semiconductor of this embodiment grown with a polar plane orientation. The injection level i, upper laser level U, and lower laser level L are the same as those described in Figures 7C and 7D, and are indicated with the same symbols as in Figure 1C, as is the electron conduction band edge potential. Crystals formed with a polar plane orientation generate a piezoelectric field, and a potential gradient reflecting this piezoelectric field is added in each portion. Even in this case, the stepped quantum well structure in which the well layer 10W1 is provided with a first layer 10W11 and a second layer 10W12, as described above, and the configuration of well layers and barrier layers are effective. Figure 9 also shows the existence probability of each level based on the wave function calculated by band calculation. A slight modification from the above description is that, due to the overlap of potential gradients in the first layer 10W11 and the second layer 10W12, the probability of electrons existing at the upper laser level U tends to be biased to the right (downstream in the flow of electrons during operation). However, by fabricating the first layer 10W11 and the second layer 10W12 with different compositions, and in particular by making the MgO composition ratio of the first layer 10W11 smaller than that of the second layer 10W12, this bias can be suppressed and the degree of diagonal transition can be increased. In this respect, the IR-QCL using a / ZnO-based semiconductor according to the embodiment of the present disclosure can appropriately control the diagonal transition.

[0035] In the well layer 10W1, which is the upstream well layer, the thickness of the first layer 10W11 is thinner than the thickness of the second layer 10W12. This allows the degree of diagonal transition to be appropriately set. Furthermore, the thickness of the barrier layer 10B1 located upstream of the well layer 10W1, which is the upstream well layer, is thicker than the thickness of the barrier layer 10B2 separating the well layer 10W1, which is the upstream well layer, from the well layer 10W2, which is the downstream well layer. This allows the injection operation to be appropriately set.

[0036] 10A and 10B are contour maps showing dynamic electron transport calculated by NEGF near the light-emitting layer 10E shown in FIG. 9 in an IR-QCL using a ZnO-based semiconductor with a polar plane orientation. These contour maps show the electron carrier density ( FIG. 10A ) and the electron carrier flow (current) density ( FIG. 10B ). The calculations are for a design example for oscillation at a wavelength of 6.5 μm. FIG. 10A shows that population inversion occurs, particularly in the upper laser level U and the lower laser level L. Furthermore, FIG. 10B shows that the current in the miniband in the continuum layer 10C is large, and that a clear current flows at an energy level near the upper laser level U (around 3.58 eV in FIGS. 10A and 10B ).

[0037] Figure 11 is a two-dimensional map of optical gain calculated for each wavelength and operating temperature for the polar plane orientation designed as shown in Figure 9. For each wavelength, an IR-QCL using a ZnO-based semiconductor with a polar plane orientation as shown in Figure 9 was designed, and the optical gain was calculated using the NEGF method, taking into account the operating temperature. Three arrows are shown at the top of the figure, indicating the wavelength ranges of 1.5 μm to 2 μm, 2 μm to 4 μm, and 4 μm to 10 μm. The compositions shown in Table 2 were used for these wavelength ranges. In zinc oxide (ZnO) and magnesium oxide (MgO), the higher the composition ratio of MgO, the higher the energy value of the conduction band edge, and the higher the electron potential at that composition. Because the well is made of ZnO except for the first layer 10W11 and the second layer 10W12, the higher the composition ratio of MgO in the barrier layer, the higher the barrier and the deeper the well. The reason for using the composition shown in Table 2 is, in short, that the shorter the wavelength, the easier it is to achieve a difference between the upper laser level U and the lower laser level L by ensuring a sufficient difference in height between the barrier and the well, i.e., the barrier height and well depth.

[0038] As shown in Figure 11, the possibility of oscillation at room temperature (300K) was confirmed over a wide wavelength range from 1.5 to 10 μm. To shorten the wavelength within this range, it is necessary to increase the composition ratio of MgO, particularly in the barrier layer, as shown in Table 2, which makes it difficult to grow good crystals. However, as exemplified in this embodiment, the prediction of good oscillation possibility and improved luminous efficiency at high temperatures can be said to serve as a catalyst for further development of crystal growth technology.

[0039] FIG. 12 is an explanatory diagram similar to FIG. 9 showing the band structure and electronic energy levels of an IR-QCL made of a ZnO-based semiconductor according to an embodiment of the present disclosure grown with a nonpolar crystal orientation, and FIGS. 13A and 13B are examples of calculations of electron transport by the NEGF in a crystal with a nonpolar crystal orientation. FIG. 14 shows the results of calculations of the optical gain similar to that shown in FIG. 11 for a nonpolar crystal orientation. As shown in FIG. 12, the band structure and electronic energy levels of an IR-QCL made of a ZnO-based semiconductor according to this embodiment grown with a nonpolar crystal orientation are generally similar to those shown in FIG. 9. Because a piezoelectric field is not generated in a crystal formed with a nonpolar crystal orientation, the structure is closer to the configurations shown in FIGS. 7C and 7D and 8 than to that shown in FIG. 9. Even in IR-QCLs using ZnO-based semiconductors with a nonpolar plane orientation, a stepped quantum well structure in which a first layer 10W11 and a second layer 10W12 are provided in the well layer 10W1, or a configuration of a well layer and a barrier layer, is effective. Thus, by fabricating the first layer 10W11 and the second layer 10W12 with different compositions, and particularly by making the MgO composition ratio of the first layer 10W11 smaller than that of the second layer 10W12, this bias can be suppressed and the degree of diagonal transition can be increased. In this respect, the IR-QCLs using ZnO-based semiconductors of the present disclosure can appropriately control diagonal transition. Furthermore, as shown in FIG. 14 , even in the case of a nonpolar plane orientation, the possibility of oscillation over a wide wavelength range from 1.5 μm to 10 μm and the possibility of efficient operation at high temperatures were confirmed. Note that the same wavelength-specific material composition adjustments as those shown in Table 2 for polar plane orientations are also employed for nonpolar plane orientations.

[0040] Next, in order to clarify the conditions for manufacturing an actual IR-QCL using a ZnO-based semiconductor, the minimum value for the MgO composition ratio in the barrier layers 10B1 to 10B5 and the first layer 10W11 and second layer 10W12 in the well layer 10W1 that gives rise to the possibility of oscillation was searched for for each oscillation wavelength. The crystal plane orientation was set to polar and nonpolar, and the wavelength was changed in the range of 1.5 to 8 μm, and designs were attempted in which the MgO composition ratio was adjusted for each wavelength, and an optical gain of 8 cm at 300 K was obtained. －１ This revealed the minimum MgO composition ratio that exceeds this limit. Figures 15A to 15C are graphs plotting the minimum MgO composition ratio that can emit light at 300 K versus wavelength for the barrier layers 10B1 to 10B5 (Figure 15A), the first layer 10W11 (Figure 15B), and the second layer 10W12 (Figure 15C). Each graph shows curves for configurations that employ a polar plane orientation (polar) and a nonpolar plane orientation (nonpolar). Note that the composition of all well layers 10W2 to 10W5 is assumed to be ZnO (i.e., the MgO composition ratio is zero).

[0041] As of the filing date of this application, advanced crystal growth technology may be required to form barrier layers with an MgO composition ratio of 0.5 when most well layers are made of ZnO, thereby producing high-quality crystals with numerous superlattice structures. However, if the usefulness of this technology is confirmed, especially through calculations, we can expect to see the development of experimental technology. Furthermore, all of the results, including those shown in Figures 15A-C, show similar curves for both polar and nonpolar plane orientations. This also suggests that the polar or nonpolar plane orientation can be selected based on the ease of crystal growth.

[0042] At the end of the explanation of the design guidelines, a method for optimizing the details of the IR-QCL using a ZnO-based semiconductor according to this embodiment will be described. Figure 16 is a chart illustrating the procedure for optimizing parameters in this embodiment. The reference numerals indicating the various components in Figure 16 are abbreviated as W1 for the well layer 10W1, B1 for the barrier layer 10B1, etc.

[0043] First, parameters to be fixed or optimized are selected in advance. For example, the compositions of the well layers other than the first layer 10W11 and the second layer 10W12 of the well layer 10W1 and the compositions of each of the barrier layers 10B1 to 10B5 are fixed to the values ​​shown in Table 2. The compositions of the first layer 10W11 and the second layer 10W12 of the well layer 10W1 may be fixed or optimized. However, the thicknesses of each well layer and barrier layer can be optimized. Furthermore, the bias electric field generated by the externally applied voltage may be a variable parameter or may be fixed.

[0044] Next, the continuum layer 10C is optimized (FIG. 16(a)). At this time, the well layers 10W3 to 10W5 and the barrier layers 10B4, 10B5, and 10B3 included in the continuum layer 10C are optimized. The unfixed thicknesses of these layers are optimized under a bias electric field. The success of the optimization can be evaluated by whether a miniband is properly formed and whether electron transport through the miniband can be performed. Next, the light-emitting layer 10E is optimized (FIG. 16(b)). The first layer 10W11 and the second layer 10W12 of the well layer 10W1 included in the light-emitting layer 10E, and the well layer 10W2 are optimized. The success of optimization can be determined by whether the vertical transition from the injection level i to the upper laser level U due to LO phonon scattering is achieved as intended, whether the energy difference between the upper laser level U and the lower laser level L determined between the well layers 10W1 and 10W2 is appropriate, and whether the degree of diagonalization determined by the balance of the thicknesses of the first layer 10W11 and the second layer 10W12 is appropriate. Next, the barrier layer 10B2 between the well layers 10W1 and 10W2 is optimized. The barrier layer 10B2 is as thin as possible, which also affects the degree of diagonalization. Finally, the entire structure is optimized ( FIG. 16( c) ). This adjusts the barrier layer 10B1, which acts as an injector barrier. The success of optimization can be determined by confirming whether electrons properly flow from the miniband of the continuum layer 10C to the light-emitting layer 10E.

[0045] To specifically design the ZnO-based IR-QCL of this embodiment, it is necessary to accurately handle real electron transport phenomena such as Fröhlich coupling and electron-electron scattering. Because 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 IR-QCL of this embodiment, theoretical predictions can be made with high accuracy to provide experimental guidelines, and a ZnO-based IR-QCL that takes various realistic conditions into account can be proposed.

[0046] 2. Modifications Various modifications can be made to this embodiment.

[0047] 2-1. Optimal Composition Range The materials for each layer shown in Table 2 are examples used in calculations, and a certain amount of deviation is allowed for the composition of each layer that can become an IR-QCL using a ZnO-based semiconductor with a plane orientation in the polar direction in this embodiment, so that similar performance can be expected. Including this deviation, preferred exemplary compositions for each layer are shown in Table 3.

[0048] 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.

[0049] 1000 QCL element 100 QCL structure (100A semiconductor superlattice structure) 10 active region 10U unit structure 10E light-emitting layer 10C continuum layer 10B, 10B1 to 10B5 barrier layers 10W, 10W1 to 10W5 well layer 10W11 first layer (in well layer 10W1) 10W12 second layer (in well layer 10W1) 20 conductive portion 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, the semiconductor superlattice structure having a plurality of unit structures in which well layers having a composition of ZnO or ZnMgO and barrier layers separating the well layers from each other and having a composition of ZnMgO or MgO higher in MgO than the well layers on both sides of the adjacent well layers are alternately stacked, each unit structure comprising: a light-emitting layer; and a continuum layer, the light-emitting layer including: an upstream well layer of a stepped quantum well having first and second layers with different MgO composition ratios and positioned in that order from the upstream side in terms of the flow of electrons during operation; and a downstream well layer having a lower MgO composition ratio than either the first or second layer of the upstream well layer and positioned downstream of the upstream well layer in terms of the flow of electrons during operation; the continuum layer comprising a plurality of well layers and a plurality of barrier layers separating them from each other, The quantum cascade laser device, wherein the semiconductor superlattice structure serves as an active region that emits electromagnetic waves of a certain wavelength under an external voltage applied through the pair of conductive portions for operation.

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

3. The quantum cascade laser device according to claim 2, wherein the thickness of the first layer of the upstream well layer is thinner than the thickness of the second layer of the upstream well layer.

4. The quantum cascade laser device according to claim 2, wherein the thickness of the barrier layer upstream of the upstream well layer in the flow of electrons during operation is thicker than the thickness of the barrier layer separating the upstream well layer from the downstream well layer.

5. The quantum cascade laser device according to claim 2, wherein each layer is formed with a plane orientation in the polar direction.

6. The quantum cascade laser device according to claim 2, wherein each layer is formed with a plane orientation in a nonpolar direction.

7. The quantum cascade laser device according to claim 1, wherein the number of well layers included in the continuum layer is three or less.

8. The quantum cascade laser element according to claim 1, wherein the unit structures are configured such that, under the external voltage, a plurality of levels forming a miniband are formed in the continuum layer, an injection level with a maximum amplitude is formed in the upstream well layer, an upper laser level with a maximum amplitude is formed in the upstream well layer, and a lower laser level with a maximum amplitude is formed in the downstream well layer; and under the external voltage, electrons are transported from the plurality of levels of the continuum layer of a certain unit structure to the injection level, electrons at the injection level are injected into the upper laser level, electrons at the upper laser level optically transition to the lower laser level of the unit structure, and electrons at the lower laser level of the unit structure are transported to any of a plurality of levels forming a miniband of a continuum layer of another unit structure adjacent to the unit structure on the downstream side.

9. The quantum cascade laser device according to claim 8, wherein under said external voltage, said upper laser level has an energy value at least 26 meV lower than said injection level.

10. The quantum cascade laser device according to claim 8, wherein, under the external voltage, electrons in the injection level of a certain unit structure are injected into the upper laser level of that unit structure by electron-LO phonon scattering.

11. The quantum cascade laser device of claim 8, wherein, under the external voltage, the injection level is a first excited state of the upstream well layer, the upper laser level is a ground state of the upstream well layer, and the lower laser level is a ground state of the downstream well layer.

12. The quantum cascade laser device according to claim 5 or 6, wherein the electromagnetic wave has a wavelength of 1.5 μm or more and 10 μm or less.

13. The quantum cascade laser device according to claim 12, wherein the electromagnetic wave has a wavelength of 2 μm or more and 4 μm or less.

14. The electromagnetic waves have a wavelength of 1.5 μm or more and 2 μm or less, and the composition of each of the barrier layers is Zn. 0.48+δ0 Mg 0.52-δ0 O (where -0.02≦δ0≦0.02), and the composition of the first layer of the upstream well layer among the well layers is Zn 0.88+δ1 Mg 0.12-δ1 O (where -0.02≦δ1≦0.02), and the composition of the second layer of the upstream well layer among the well layers is Zn 0.81+δ2 Mg 0.19-δ2 7. The quantum cascade laser device according to claim 5, wherein the composition of the other well layers is ZnO (where -0.02≦δ2≦0.02).

15. The electromagnetic waves have a wavelength of 2 μm or more and 4 μm or less, and the composition of the barrier layers is Zn. 0.69+δ0 Mg 0.31-δ0 O (where -0.02≦δ0≦0.02), and the composition of the first layer of the upstream well layer among the well layers is Zn 0.92+δ1 Mg 0.08-δ1 O (where -0.02≦δ1≦0.02), and the composition of the second layer of the upstream well layer among the well layers is Zn 0.84+δ2 Mg 0.16-δ2 7. The quantum cascade laser device according to claim 5, wherein the composition of the other well layers is ZnO (where -0.02≦δ2≦0.02).

16. The electromagnetic wave has a wavelength of 4 μm or more and 10 μm or less, and the composition of the barrier layer is Zn. 0.69+δ0 Mg 0.31-δ0 O (where -0.02≦δ0≦0.02), and the composition of the first layer of the upstream well layer among the well layers is Zn 0.92+δ1 Mg 0.08-δ1 O (where -0.02≦δ1≦0.02), and the composition of the second layer of the upstream well layer among the well layers is Zn 0.84+δ2 Mg 0.16-δ2 7. The quantum cascade laser device according to claim 5, wherein the composition of the other well layers is ZnO (where -0.02≦δ2≦0.02).

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