Energy band structure for light emitting devices

Abstract

An apparatus and method for reducing quantum confinement stark effect and electron leakage in a polar compound semiconductor optical gain region is disclosed. The apparatus includes a continuously graded multi-quantum well (MQW) band structure in which quantum wells (QWs) are cascaded together with no quantum barrier (QB) in between and no more than one heterojunction interface per well. An alternative equivalent MQW band structure includes continuously graded QWs separated by continuously graded QBs, no more than one heterojunction interface per well. In both embodiments, there are no constant bandgap layers. Non-uniform periodicity and non-uniform bandgap range MQW structures are also presented. Non-uniform QW / QB thickness ratio MQW structures are also presented. Smoothly graded electron blocking layers (EBLs) with tangential band on the MQW side and no more than one heterojunction interface on the p-injection side can be used to improve electron confinement.

Description

[0001] Cross-reference to related applications This application claims priority and benefit to co-pending U.S. Patent Application No. 18 / 331,908, filed June 8, 2023. Technical Field

[0002] This application relates to an apparatus, system, and method for reducing the quantum confinement Stark effect and electron leakage in polarized compound semiconductor light-emitting devices, and more specifically, to a multi-quantum-well structure in which the electron-hole wavefunction has improved overlap and the electron blocking layer has improved electron confinement. Background Technology

[0003] Compound semiconductors have achieved great success in realizing practical optoelectronic devices such as lasers and detectors. Based on compounds of group III-V elements (such as InP and GaAs), optoelectronic devices emitting light from far-infrared to visible (i.e., orange) wavelengths have been fabricated. For shorter wavelengths from green to ultraviolet, semiconductors with larger band gaps, such as group III nitrides, namely InN, GaN, and AlN, have been used. Although the former class of compounds have nonpolar zinc blend structures, group III nitride material systems have wurtzite lattice structures and exhibit extremely high built-in spontaneous and (strain-induced) piezoelectric fields due to the destruction of inversion symmetry in the uniaxial crystal structure, leading to a significant decrease in quantum efficiency at high current densities.

[0004] In the following content, Figures 1A to 18B The energy band structure of the light-emitting structure is shown in the figures. In all the figures, the epitaxial growth direction is from left to right. Light-emitting semiconductor devices typically employ heterojunctions to confine electrons and holes in a common layer of a lattice called a quantum well (referred to herein as "QW"), thereby increasing the possibility of radiative recombination. Figure 1A and Figure 1B The effect of polarization on the band structure of a single QW 101 is shown. Figure 1A A diagram showing the conduction band 120 and valence band 130 of a single QW 101 is presented. This single QW comprises a narrow bandgap material layer 200 surrounded by larger bandgap layers 300a and 300b, also referred to as quantum barriers, and hereinafter referred to as "QB". In the unpolarized state, the conduction band 120 and valence band 130 are flat, i.e., horizontal in the unbiased state, while the QW / QB interface exhibits a step change in the bandgap between the electron and hole bands, i.e., a vertical change. The diagram also shows electron quantum levels 210a and 210b, as well as probability wave functions 220a and 200b, which are symmetrical within the QW and have a high degree of overlap. Reference Figure 1BUnder the influence of polarization, such as between layers 300a and 200, the downward step of the band gap leads to negative polarization at the interface. This attracts positive thin-layer charges, resulting in a change in the slope of the band structure. The presence of thin-layer charges requires that the band slopes on both sides of the heterojunction surface have opposite signs; in this case, the left side is positive and the right side is negative. For example, the upward step of the band gap between layers 200 and 300b is the opposite. Band tilting and / or bending must occur between the interfaces to satisfy these constraints. Figure 1B The effect of this band distortion on quantum energy levels 210a and 210b and wave functions 220a and 220b is also shown; electron quantum level 210a and hole quantum level 210b are closer to each other in energy, while electron wave functions 220a and hole wave functions 220b are pushed further apart. This is known as the Quantum Confined Stark Effect (QCSE).

[0005] The low quantum concentration (QCSE) results in weaker carrier transition strength and a longer carrier lifetime, leading to a reduced radiative recombination rate. This longer carrier lifetime makes it difficult to fabricate practical lasers from these materials across wavelengths from green to ultraviolet, and is therefore undesirable. Furthermore, the low recombination probability leads to high carrier concentrations at high current densities. This is also undesirable because as carrier density increases, more and more carriers recombine via a non-radiative Auger process, further reducing quantum efficiency. This is generally considered the primary reason for the sharp efficiency drop (also known as droop) in blue LEDs at high current densities. In another example, in GaN / InGaN systems, the QCSE increases with increasing indium (In) concentration in the InGaN QW. This has been found to be a major cause of efficiency loss in group III nitride devices in green LEDs and lasers, as these devices require high In content to achieve the green light spectral range. This is particularly important because only group III nitride systems can be used to fabricate devices emitting green light.

[0006] Another reason for the reduced efficiency is poor injection efficiency and non-uniform well distribution, in which the QW shape also plays a role. In devices with standard square QWs, polarization distortion tends to cause holes to accumulate on the p-side of the device, resulting in a highly non-uniform carrier distribution within the QW. One solution is p-doping of the QW barrier, where the Mg-doped barrier is used to improve the hole distribution among multiple QWs (MQWs). However, in p-type MQWs, the Mg dopant readily diffuses into the well, thus reducing radiation efficiency.

[0007] Several methods for increasing the overlap of electrons and holes in group III nitride quantum waves (QWs) have been proposed. The two main approaches rely on reducing intrinsic polarization by growing graded or staggered QWs and / or QBs, or by growing them in semi-polar and non-polar crystal orientations. Regarding the graded approach, research has focused on improving quantum efficiency through bandgap engineering of QW and QB structures. Regarding the staggered approach, large and cost-effective semi-polar and non-polar substrates are not yet commercially available and are unlikely to play a major role in improving LED efficiency for some time. Therefore, the remaining main pathways include graded QWs and QBs, and ultrawide QWs exhibiting dynamic carrier effects.

[0008] Some QW structures are referred to as group III nitride material systems. Figures 2A to 2O Examples of proposed, simulated, and / or experimentally tested QW shapes are shown. For clarity, only the conduction band 120 is shown, while the valence band 130 is omitted; as... Figure 1A As shown, valence band 130 is a scaled-down version of its vertical mirror image. Furthermore, the topmost flat portion of the side of QW 200 indicates QB 300a and 300b, and for clarity, the polarization effect has been omitted.

[0009] QW gradients can be used to realize a variety of shapes, broadly classified as step-shaped, linear, or nonlinear (i.e., smooth). In all three categories, the well 200 can be symmetric or asymmetric. It should be noted that horizontal mirror images of asymmetric wells can be included in the list of possible distributions. It should also be noted that various types of gradients can be combined within a single quantum well 200. Figure 2A As shown, the most common QW 200 type is a non-gradient square well with a single step interface at each end. Figure 2B In the step-gradient QW 200 shown, constant composition layers with increasing bandgap are grown, followed by another constant composition layer with increasing bandgap. The number and thickness of the layers, as well as the size of the step, are variable, although most publicly disclosed schemes use 2 to 7 layers. Similarly, linearly graded QWs use layers with a constant slope, where the number and thickness of the layers, as well as the slope of each layer, can vary. Figure 2C An example of a simple symmetrical V-shaped trap is shown, while Figure 2D and Figure 2E These represent an asymmetric forward linear tapered trap and an asymmetric reverse linear tapered trap, respectively. Figures 2F to 2H The diagram shows various combinations of step transitions and linear transitions. Figure 2I and Figure 2J The diagram illustrates two composite linear gradients in a symmetric well. For example... Figure 2KAs shown, the combination of composite linear gradients and step gradients forms an asymmetric trap. Smooth gradients use layers with varying slopes (i.e., curvatures) to produce QWs of various shapes, such as semicircles, sinusoidal waveforms, parabolic shapes, elliptical shapes, or Fermi functions. Figure 2L and Figure 2M The traps with parabolic and Fermi distributions are shown respectively. Figure 2N It is a smooth QW with an arbitrary but symmetrical shape. Finally, Figure 20 shows a QW combining a sine wave and a linear gradient.

[0010] In a conventional MQW structure, each well is separated from its adjacent well by a QB, which is typically a carrier injection layer. In this case, QW 200 refers to one or more layers, where at least one layer comprises the smallest bandgap of the MQW structure, and the band structures of all other QW layers monotonically increase within the bandgap. In this case, QB 300 refers to one or more layers, where at least one layer comprises the largest bandgap of the MQW structure, and the band structures of all other layers monotonically decrease within the bandgap. At the QW / QB interface, the band structure can exhibit one of three characteristics: such as... Figure 2B As shown, a step jump of 160° is possible; as Figure 2C As shown, a twist of 161 may exist; or, as... Figure 2M As shown, the energy bands on both sides of the interface can be tangent to each other 162.

[0011] In conventional light-emitting devices, an electron barrier layer (EBL) 400 can be used to further confine electrons within the MQW structure and prevent undesirable leakage into the p-injection layer. In this case, EBL 400 refers to one or more layers, at least one of which has a band gap greater than the maximum QB band gap. At the MQW / EBL interface, the band structure can exhibit one of three characteristics: such as... Figure 2B As shown, a step jump of 160° is possible; as Figure 2C As shown, a twist of 161 may exist; or, as... Figure 2M As shown, the energy bands on both sides of the interface can be tangent to each other 162.

[0012] Similar to the case of QW 200, certain QB 300 and EBL 400 structures are known in group III nitride material systems. Figures 3A to 3L Examples of proposed, simulated, and / or experimentally tested QB 300 and / or EBL 400 shapes are shown. For clarity, Figures 3A to 3L Only the conductor band 120 is shown. (Example) Figure 1A As shown, it can be understood that valence band 130 is a scaled version of its vertical mirror image. Furthermore, for clarity, polarization effects have been omitted. For the QB case ( Figures 3A to 3IThe flat sections at the bottom left and right indicate QW 200a-b. For EBL 400 (… Figures 3A to 3L The leftmost flat portion represents the last QB of the MQW structure 100, while the rightmost flat portion represents the p-injection layer 180. It should be noted that the last QB of the MQW region 100 and the p-injection layer 180 can have different band gaps, for example, Figures 3J to 3L The case of the ultraviolet (UV) light-emitting device is shown.

[0013] like Figure 3A As shown, the most common QB 300 and / or EBL 400 types are non-gradient square barriers with a single step interface at each end. Gradients in QB 300 can be used to achieve a variety of shapes, broadly categorized as step-shaped, linear, or nonlinear (i.e., smooth). In all three categories, the barrier can be symmetric or asymmetric. For asymmetric barriers, the gradient in the growth direction can be positive or negative, corresponding to roughly decreasing or increasing band gaps, respectively. It should be noted that horizontal mirror images of asymmetric QBs can be included in the list of possible distributions. It should be noted that various types of gradients can be combined within a single QB.

[0014] The following discussion of QB 300 also applies to EBL 400. In the step-gradient QB 300, for example, as Figure 3A As shown, layers with constant composition and increasing and / or decreasing band gaps were grown. Figure 3B An example of an inverse, asymmetric, step-gradient potential barrier is shown. The number and thickness of the layers, as well as the size of the step, can be variable, although most known step gradients use 1 to 3 layers. Similar to the QW 200, the linearly gradient QB 300 uses layers with a constant slope, where the number and thickness of the layers, as well as the slope of each layer, can vary. Figure 3C The example shown is a simple symmetrical inverted V-shaped barrier, while Figure 3D and Figure 3E These represent an asymmetric potential barrier with a forward linear gradient and an asymmetric potential barrier with a reverse linear gradient, respectively. Figures 3F to 3H Various combinations of step gradients and linear gradients are shown. Smooth gradients use one or more layers with varying slopes (i.e., curved band gaps) to produce various shapes of QB 300, such as semicircles, sine waves, parabolic shapes, ellipses, or Fermi functions. Figure 3I The QB 300 with a sine wave gradient is shown. Figures 3J to 3L Examples of EBL with linear gradient, bilinear gradient, and bi-reverse linear gradient are shown respectively.

[0015] from Figures 2A to 3IIn the examples, those skilled in the art will recognize that various MQW structures (e.g., multiple QW 200s) can be realized by combining various non-gradient and / or graded QW 200 and QB 300 profiles. However, some aspects of these structures suffer from drawbacks when combined with polarization effects. First and foremost, the band gap step adds strain-induced polarization to the spontaneous polarization already present at the heterojunction surface. A decrease in band gap step leads to uncompensated negative charges attracting holes, resulting in a thin layer of positive charge at the interface. Conversely, an increase in band gap step leads to uncompensated positive charges attracting free electrons, resulting in a thin layer of negative charge at the interface. Figure 1B As shown, any constant bandgap band between such opposing thin-layer charges will tilt proportionally to the areal density of the thin-layer charge, thus generating an internal electric field. QB 300 exhibits a similar, but less pronounced, effect; QB 300 is typically 5 to 10 times thicker than QW 200. Since the carrier wavefunction penetrates the barrier, the asymmetry of the barrier also causes the wavefunction to shift in the opposite direction. Second, the constant bandgap band is bent not only by step-induced thin-layer charge but also by volume charge induced by the twisting or curvature of the intrinsic region, similar to that of a pin diode. Step heterojunctions and constant bandgap layers are therefore the root of QCSE in polarized semiconductors.

[0016] Therefore, there has been a long-standing need to mitigate or eliminate the detrimental effects of band tilt on carrier wavefunction overlap in photogenerated MQW band structures. There has also been a long-standing need to improve carrier distribution uniformity across the QW in photogenerated MQW band structures. Furthermore, there has been a long-standing need to improve carrier confinement in photogenerated MQW band structures. Finally, there has been a long-standing need for improved EBL band structures to reduce electron leakage. Summary of the Invention

[0017] This application provides a system for reducing polarization effects, primarily the quantum confinement Stark effect (QCSE), in compound semiconductor multi-quantum-well optical gain structures. This is achieved through a continuously graded band structure in which a constant bandgap layer and step discontinuities are minimized or eliminated. In one embodiment, the MQW structure comprises a continuously graded QW without QB and without discontinuities at the QW / QW interface. In another embodiment, the MQW structure comprises a continuously graded QW without discontinuities at the QW / QB interface and a continuously graded QB. In yet another embodiment, the MQW structure comprises a continuously graded QW without QB and with a single discontinuity at the QW / QW interface. Barrier-free MQW structures with non-uniform QW / QB thicknesses and / or non-uniform minimum and / or maximum bandgap are also envisioned. An EBL with a smooth gradient on the MQW side and no more than one discontinuity on the p-side is disclosed.

[0018] One objective of this application is to mitigate or eliminate the detrimental effect of band tilt on carrier wavefunction overlap in the MQW band structure.

[0019] One objective of this application is to improve the uniformity of carrier distribution on the QW in the optically generated MQW band structure.

[0020] One objective of this application is to improve carrier confinement and reduce electron leakage within the light-generating device.

[0021] Other desired features and characteristics will become apparent when considering the content of this application, in light of the following detailed description, drawings, abstract, and claims. Attached Figure Description

[0022] Non-limiting and non-exhaustive embodiments of this application are described with reference to the following accompanying drawings. In the drawings, unless otherwise stated, the same reference numerals denote the same parts in the various views.

[0023] To better understand this disclosure, reference will be made to the accompanying drawings, which are incorporated in and form part of this specification, illustrating certain aspects of the subject matter disclosed herein and, together with the description, helping to explain some principles associated with the disclosed embodiments, wherein: Figure 1A The band structure of a prior art QW without polarization effect is shown; Figure 1B The band structure of a QW with polarization effect in the prior art is shown; Figure 2A The band structure of a non-gradient square QW without polarization effect in the prior art is shown; Figure 2B The band diagram of a step-gradient QW without polarization effect in the prior art is shown; Figure 2C The band diagram of a symmetrical, V-shaped, linearly graded QW without polarization effect in the prior art is shown. Figure 2D The band structure of an asymmetric, V-shaped, positively linearly graded QW without polarization effect is shown in the prior art. Figure 2E The band diagram of an asymmetric, V-shaped, inversely linearly graded QW without polarization effect is shown in the prior art. Figure 2F The band diagram of an asymmetric, V-shaped, positive, partially linearly graded QW without polarization effect is shown in the prior art. Figure 2GThe band diagram of an asymmetric, V-shaped, inverted, partially linearly graded QW without polarization effect is shown in the prior art. Figure 2H The band diagram of an asymmetric, positive, partially linearly graded QW without polarization effect in the prior art is shown. Figure 2I The band diagram of a symmetrical, V-shaped, composite linearly graded QW in the prior art is shown, which has negative kinks and no polarization effect. Figure 2J The band diagram of a symmetrical, V-shaped, composite linearly graded QW in the prior art is shown, which has positive kinks and no polarization effect. Figure 2K The band diagram of an asymmetric, V-shaped, composite linear and step-gradient QW in the prior art is shown, which has positive kinks and no polarization effect. Figure 2L The band diagram of a symmetric, smooth, parabolic, nonlinearly graded QW without polarization effect is shown in the prior art. Figure 2M The band diagram of a symmetric, smooth, Fermi function-shaped, nonlinearly gradient QW without polarization effect is shown in the prior art. Figure 2N The band structure of a symmetric, smooth, nonlinearly graded QW without polarization effect is shown in the prior art. Figure 2O The band diagrams of asymmetric nonlinear and linearly graded QWs without polarization effects in the prior art are shown. Figure 3A The band structure of a non-gradient square QB without polarization effect in the prior art is shown; Figure 3B The band structure of a step-gradient QB without polarization effect in the prior art is shown. Figure 3C A symmetrical, V-shaped, linearly graded band diagram of a QB in the prior art is shown, which has negative kinks but no polarization effect. Figure 3D The band structure of an asymmetric, V-shaped, positively linearly graded QB without polarization effect is shown in the prior art. Figure 3E The band diagram of an asymmetric, V-shaped, inversely linearly graded QB without polarization effect in the prior art is shown. Figure 3F The band diagram of an asymmetric, inverse, partially linearly graded QB without polarization effect in the prior art is shown. Figure 3G The band diagram of an asymmetric, positive, partially linear, and step-gradient QB without polarization effect is shown in the prior art. Figure 3H The band diagrams of symmetric partially linear and step-gradient QB without polarization effects in the prior art are shown. Figure 3I The band structure of a symmetrical, nonlinear (sinusoidal) gradient QB without polarization effect in the prior art is shown. Figure 3J The band diagram of an asymmetric linearly graded EBL without polarization effect in the prior art is shown. Figure 3K The band diagram of a symmetrical bilinearly graded EBL without polarization effect in the prior art is shown. Figure 3L The band diagram of a symmetrical, double-inverse linearly graded EBL without polarization effect in the prior art is shown. Figure 4A This illustrates the interface conduction band discontinuity in heterojunctions without polarization effects in the prior art; Figure 4B This illustrates the polarization-induced thin-layer charge at the downward step conduction band discontinuity in the prior art; Figure 4C This illustrates the interface conduction band discontinuity in heterojunctions with polarization effects in the prior art; Figure 5A This illustrates a downwardly linearly tapered conduction band in the prior art that does not exhibit polarization effects; Figure 5B The diagram illustrates a polarization-induced uniform space charge within a layer with a downwardly linearly tapered bandgap in the prior art. Figure 5C This illustrates a downwardly linearly tapered conduction band with polarization effect in the prior art; Figure 6A This illustrates a downwardly nonlinearly gradient conduction band in the prior art that does not exhibit polarization effects; Figure 6B This illustrates polarization-induced non-uniform space charge within a layer with a downwardly nonlinearly graded bandgap in the prior art; Figure 6C This illustrates a downwardly nonlinearly gradient conduction band with polarization effect in the prior art; Figure 7A This application illustrates a cascaded, symmetrical, V-shaped, linearly gradient MQW band structure without polarization effects, according to an embodiment of the present application. Figure 7BThis application illustrates a cascaded, symmetrical, V-shaped, linearly gradient MQW band structure with polarization effect according to an embodiment of the present application. Figure 8A This application illustrates a cascaded, symmetrical, Y-shaped, nonlinearly gradient MQW band structure without polarization effects, according to an embodiment of the present application. Figure 8B This application illustrates a cascaded, symmetrical, Y-shaped, nonlinearly gradient MQW band structure with polarization effect according to an embodiment of the present application. Figure 9A This application illustrates a cascaded, symmetrical, smooth, nonlinearly gradient MQW band structure without polarization effects, according to an embodiment of the present application. Figure 9B This application illustrates a cascaded, symmetrical, smooth, nonlinearly gradient MQW band structure with polarization effect according to an embodiment of the present application. Figure 10A This application illustrates a cascaded, asymmetric, sawtooth, positive, linearly graded MQW band structure without polarization effect, according to an embodiment of the present application. Figure 10B This application illustrates a cascaded, symmetrical, sawtooth, positively oriented, linearly gradient MQW band structure with polarization effect according to an embodiment of the present application. Figure 11A This application illustrates a cascaded, asymmetric, sawtooth, inverse, linearly gradient MQW band structure without polarization effect, according to an embodiment of the present application. Figure 11B This application illustrates a cascaded, symmetrical, sawtooth, inverse, linearly gradient MQW band structure with polarization effect according to an embodiment of the present application. Figure 12A This application illustrates a cascaded, asymmetric, sawtooth, positive, nonlinearly graded MQW band structure without polarization effect, according to an embodiment of the present application. Figure 12B This application illustrates a cascaded, symmetrical, sawtooth, positively oriented, nonlinearly gradient MQW band structure with polarization effect according to an embodiment of the present application. Figure 13A This application illustrates a cascaded, asymmetric, sawtooth, inverse, nonlinearly graded MQW band structure without polarization effects, according to an embodiment of the present application. Figure 13B This application illustrates a cascaded, asymmetric, sawtooth, inverse, nonlinearly graded MQW band structure with polarization effect according to an embodiment of the present application. Figure 14AThe present application illustrates a symmetrical, V-shaped, linearly graded QW and a symmetrical, V-shaped, linearly graded QB in an MQW band structure without polarization effect, according to embodiments of the present application. Figure 14B The present application illustrates a symmetrical, smooth, nonlinearly graded QW and a symmetrical, smooth, nonlinearly graded QB in an MQW band structure without polarization effect, according to embodiments of the present application. Figure 14C The present application illustrates a symmetrical, smooth, nonlinearly graded QW and an asymmetrical, nonlinearly and linearly graded QB in an MQW band structure without polarization effect, according to embodiments of the present application. Figure 15A The present application illustrates a symmetrical, V-shaped, six-layer linearly graded QW in an MQW band structure without polarization effects. Figure 15B The present application illustrates a symmetrical, V-shaped, four-layered, linearly graded QW and a symmetrical, inverted V-shaped, two-layered, linearly graded QB in the MQW band structure without polarization effect. Figure 15C The present application illustrates a symmetrical, V-shaped, two-layer linearly graded QW and a symmetrical, inverted V-shaped, four-layer linearly graded QB in the MQW band structure without polarization effect. Figure 15D The present application illustrates an asymmetric, V-shaped, three-layer linearly graded QW and an asymmetric, inverted V-shaped, three-layer linearly graded QB in the MQW band structure without polarization effect. Figure 16A An asymmetric, sawtooth, three-layered linearly graded QW in an MQW band structure without polarization effect is shown according to this application; Figure 16B The present application illustrates an asymmetric, serrated, two-layer linearly graded QW and an asymmetric, serrated, single-layer linearly graded QB in an MQW band structure without polarization effect. Figure 16C The present application illustrates an asymmetric, serrated, monolayer linearly graded QW and an asymmetric, serrated, two-layer linearly graded QB in an MQW band structure without polarization effect. Figure 16D The diagram illustrates an asymmetric monolayer linearly graded QW and an asymmetric monolayer linearly graded QB in the prior art MQW band structure without polarization effect. Figure 17AThis application illustrates cascaded, asymmetric, sawtooth, inverse, nonlinearly graded MQW and smooth / smoothly graded EBL band structures without polarization effects, according to embodiments of this application. Dashed lines separate the n-injection layer, MQW layer, EBL layer, and p-injection layer. Figure 17B This application illustrates cascaded, asymmetric, sawtooth, inverse, nonlinearly graded MQW and smooth / smoothly graded EBL band structures with polarization effects, according to embodiments of this application. Dashed lines separate the n-injection layer, MQW layer, EBL layer, and p-injection layer.

[0024] Figure 18A This application illustrates cascaded, asymmetric, sawtooth, inverse, linearly graded MQW and smooth / discontinuously graded EBL band structures without polarization effects, according to embodiments of this application. Dashed lines separate the n-type injection layer, MQW layer, EBL layer, and p-injection layer; and Figure 18B This application illustrates cascaded, asymmetric, zigzag, inverse, linearly graded MQW and smooth / discontinuously graded EBL band structures with polarization effects, according to embodiments of this application. Dashed lines separate the n-injection layer, MQW layer, EBL layer, and p-injection layer. Detailed Implementation

[0025] Non-limiting embodiments of this application will now be described with reference to the accompanying drawings, wherein the same reference numerals always denote the same elements. While this application has been described in detail with reference to its preferred embodiments, it should be understood that certain variations of the preferred embodiments will be apparent upon reading and understanding the foregoing, and these variations remain within the spirit and scope of this application. The figures shown in the accompanying drawings are provided for the purpose of illustrating some embodiments of this application and should not be considered as limitations on these embodiments. The plotting of band structures, carrier distributions, quantum levels, and wave functions is qualitative in nature and intended to illustrate the general characteristics of unpolarized and polarized layers, interfaces, and MQW structures. As used herein, the term “a” or “an” is defined as one or more (a kind). As used herein, the term “a plurality” is defined as two or more (a kind). As used herein, the term “another” is defined as at least a second or more (a kind). As used herein, the terms “comprising” and / or “having” are defined as including (i.e., open-ended language). As used herein, the term “coupled” is defined as a connection, although not necessarily a direct connection or a mechanical connection.

[0026] The use of terms such as “some embodiments,” “one embodiment,” “certain embodiments,” and “embodiment” or similar terms throughout this document means that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. Therefore, the appearance of such phrases throughout this specification does not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics can be combined in any suitable manner in one or more embodiments without limitation.

[0027] As used herein, the term "or" will be interpreted inclusively, meaning either one or any combination thereof. Therefore, "A, B, or C" means any of the following: "A; B; C; A and B; A and C; B and C; A, B, and C". Exceptions to this definition occur only when the combination of elements, functions, steps, or actions is inherently mutually exclusive in some way.

[0028] The term "method" preceding the current word segmentation of the operation indicates the desired function for which one or more embodiments exist, namely one or more methods, devices or apparatuses for implementing the desired function, and those skilled in the art can choose from these embodiments or their equivalents in light of the disclosure herein, and the use of the term "method" is not intended to be limiting.

[0029] The term "strictly increasing" (also known as "monotonically increasing") is defined as always increasing; never remaining constant or decreasing. The term "strictly decreasing" (also known as "monotonically decreasing") is defined as always decreasing; never remaining constant or increasing. The term "monotonically increasing" is defined as completely non-decreasing. The term "monotonically decreasing" is defined as completely non-increasing. All four definitions take into account step changes in the vertical portion of the slope.

[0030] Figures 1A to 3I Some conduction bands are shown, corresponding to various known layers and interfaces in unpolarized and polarized semiconductors. As will be detailed in the following paragraphs, a layer can refer to two interfaces separated by thickness. Between the two interfaces, the composition of the layer can be constant or gradient. For a gradient layer, the gradient can be linear or non-linear (i.e., smooth). Figures 4A to 6C Examples of layer and interface types are shown, where vertical solid or dashed lines correspond to interfaces, and the spaces between interfaces correspond to layers.

[0031] At the interface, the energy band can be described as a step 160, a kink 161, or a tangent 162, where tangency 162 means that at least one of the adjacent layers must be non-linearly graded. If two adjacent layers are non-linearly graded, the tangent interface 162 can form a maximum or minimum value. For example, Figure 4AThe step 160 shown refers to a sudden change in composition and a sudden change in band gap, and can be called a heterojunction or heterojunction surface. A heterojunction is a discontinuous interface. For example... Figure 5A The twists 161a and 161b shown refer to sudden changes in the slope. For example, Figure 6A The tangents 162a and 162b shown refer to abrupt changes in curvature, where the slope is continuous across the interface. Both the kink 161 interface and the tangent 162 interface are continuous interfaces. Tangent interfaces 162a and 162b may have flat layers 140a and 140b or linear gradient layers on one side. Alternatively, tangent interface 162c may have smooth gradient layers on both sides of the interface and be identified by changes in the sign of curvature, for example, from positive to negative or vice versa (also called inflection points). Alternatively, tangent interface 162 may have smooth gradient layers with the same curvature sign on both sides of the interface (these smooth gradient layers have the same curvature sign on both sides of the interface) and be identified by the maximum or minimum value between them. It should be noted that a step 160 can be combined with either the kink 161 interface or the tangent 162 interface to create a step-kink or step-tangent interface. A kink-tangent interface is possible, meaning that the smooth gradient layer has curvatures of the same sign but different magnitudes on both sides of the interface. A kink-step-tangent interface is also possible. In short, the layer must be defined by two interfaces, each of which can be identified by a step 160, a kink 161, or a tangent 162, or a valid combination thereof.

[0032] Within a layer, the bandgap can be either non-gradual (i.e., constant) at 140° or gradually 150°. A gradually gradual bandgap at 150° can be linearly gradual at 151° or non-linearly gradual at 152°, i.e., smoothly gradual. A smoothly gradual bandgap refers to a bandgap without constant, kinked, or step-like segments. Examples of constant bandgap layers are shown in... Figure 4A As shown, two layers 140a and 140b with constant but unequal band gaps are adjacent to form a heterojunction. An example of a linearly graded band gap layer 151 is given in... Figure 5A As shown, its sides are layers with constant band gaps 140a and 140b. Finally, Figure 6A An example of a smooth gradient layer is shown, in which two parabolic gradient layers with opposite signs share a tangent interface, i.e., an inflection point, and where the first and last interfaces are adjacent to a constant bandgap layer, i.e., a flat portion. While in common terminology a single smooth gradient layer can have two signs of band curvature, or alternatively a single curvature sign but also including local minima or maxima, in this paper, the convention is that a layer can have only one slope sign or one curvature sign without slope variation. This convention eliminates some ambiguity in the definition of a layer. According to this definition, for example, Figure 4A It shows two layers and one interface. Figure 5AIt shows three layers and two interfaces, and Figure 6A Four identifiable layers and three interfaces are shown.

[0033] To understand the origin of band distortion in polarized semiconductors, it is necessary to understand some strain-induced polarization fields. For example, in the InGaN / GaN system with QW / QB structures used in blue LEDs, growth typically occurs on the N-pole c-plane of the wurtzite lattice. Increasing the molar fraction of In increases the lattice constant, thereby causing compressive stress, lattice elongation along the

[0001] Miller index direction, and uncompensated negative polarization. The negative polarization is compensated by positive charge carriers (i.e., holes). The amount of polarization and the associated compensation depends on the magnitude and sign of the change in In composition. Figure 4B , Figure 5B and Figure 6B They are shown respectively Figure 4A , Figure 5A and Figure 6A The distribution of free holes at the abrupt interface and in the gradient layer. Figure 4A In the step junction 160 shown, all strain-induced polarization is concentrated on the plane of the interface. Therefore, as Figure 4B As shown, all free charge carriers (i.e., holes) are also located at the interface, forming a thin layer of charge 170 and causing the Fermi level to drop below the valence band. Figure 4C As shown, to accommodate this change, the band on the high bandgap side (i.e., the left side) tilts upwards, while the band on the low bandgap side (i.e., the right side) tilts downwards. Figure 5B As shown, in Figure 5A In the linearly graded bandgap layer 151 with kinked interfaces 161a and 161b shown, the thin-layer charge 170 has diffused into a uniform space charge 180. Figure 5C At the interface shown, the energy bands are tilted as in the previous example, although the degree of bending is slight. This is due to the lower carrier concentration caused by the thin-layer charge expansion. For Figure 6A The smooth, gradient bandgap layers 152a and 152b shown have tangential interfaces 162a-162c. Hole space charge 180 with a concentration gradient exists within layers 152a and 152b, starting at the left interface 162a with sparse 3D hole concentration, increasing towards the highly inclined interface 162c between the smooth, gradient layers 152a and 152b, and decreasing again towards the opposing tangential interface 162b. In this case, as... Figure 6CAs shown, band bending follows a similar trajectory, with a small amount of curvature added to the left side of the graded layer, a large amount of curvature added to the middle, and a small amount of curvature added to the right side of the graded layers 152a and 152b. It should be noted that in all three cases, the flat band portions of bands 140a and 140b are tilted, meaning that thin layers, volumes, or gradient negative charges (invisible) are removed from the left and right sides of the constant bandgap layer, thus terminating the electric field. It is also important to note that this flat band portion of the polarized MQW structure is always tilted in this manner, which generally contributes to QCSE in the QW region. Therefore, one of the objectives of this application is to minimize or eliminate this flat portion in the band structure 120. The previous example also applies to GaN / AlGaN material systems, where the sign of strain is reversed compared to the GaN / InGaN system, and also to other systems.

[0034] In an exemplary embodiment, Figure 7A and Figure 7B The band structure of the cascaded, linearly gradient, V-shaped MQW structure 100 according to this application is shown. Figure 7A The conduction band 120 and valence band 130 (indicated by vertical dashed lines) of two unpolarized traps are shown, symmetrical about the minimum bandgap at the center of trap 200. Alternatively, the linear down and up gradients may comprise multiple layers with different slopes and may be asymmetric, and such alternatives are non-limiting. Figure 7A The diagram also shows the first quantum energy levels 210a and 210b of electrons and holes, and wave functions 220a and 220b, which exhibit excellent overlap. However, the overlap is not perfect because the effective mass of the hole is greater than that of the electron. Therefore, the hole quantum energy level 210b is closer to the bottom of the well 200 than the electron quantum energy level 210a, and has an effectively narrower well. Furthermore, the evanescent portion of the hole wave function 220b penetrates the V-shaped well less than the electron wave function 220a, resulting in a further narrowing of the hole wave function 220b relative to the electron wave function 220a. These effects are offset by the smaller hole band offset compared to the electron band offset, resulting in a gentler slope in the valence band 130 than in the conduction band 120, thus effectively widening the hole well relative to the electron well. The gentler slope of the valence band 130 allows the hole wave function 220b to expand relative to the electron wave function 220a, thereby improving the overlap between the two. The balance between these opposing effects depends on the specific composition and gradation of each layer, and is best determined numerically.

[0035] According to the embodiments of this application, Figure 7BA V-shaped well 200 incorporating polarization effects is shown. A downward linear gradient on the left side of well 200 results in a positive space charge, thus inducing negative curvature in bands 120 and 130. Conversely, an upward linear gradient on the right side of well 200 results in a negative space charge, thus inducing positive curvature in bands 120 and 130. This manifests as a tilted bottom of the well, with the conduction band 120 and valence band 130 tilted in opposite directions. The first quantum energy levels 210a and 210b for electrons and holes, and wave functions 220a and 220b, are also shown, and it can be seen that they are shifted in opposite directions, resulting in a reduced amplitude of the overlap integral. Furthermore, the shape of well 200 has been altered, with both the electron and hole wells effectively becoming wider. The quantum energy levels in the wider wells are closer to the band edges, resulting in a redshift of the emission wavelength. Therefore, QCSE remains valid for this band structure.

[0036] According to an exemplary embodiment of this application, Figure 8A and Figure 8B The band structure of the cascaded, Y-shaped, linearly gradient MQW structure 100 is shown. Figure 8A The conduction band 120 and valence band 130 of an unpolarized well are shown, which are symmetric about the bandgap minimum at the center of well 200. Alternatively, the nonlinear down and up gradients can comprise multiple layers with different curvatures and can be asymmetric, and alternatives are non-limiting. First quantum energy levels 210a, 210b for electrons and holes and wave functions 220a, 220b are also shown, and their excellent overlap is evident. However, compared with the reference... Figure 7A The reasons given are similar, and the overlap is not complete.

[0037] According to one or more embodiments of this application, Figure 8B A Y-shaped well 100 incorporating polarization effects is shown. A downward nonlinear gradient on the left side of well 200 results in a non-uniform positive space charge, thus inducing negative curvature in bands 120 and 130. Conversely, an upward nonlinear gradient on the right side of well 200 results in a non-uniform negative space charge, thus inducing positive curvature in bands 120 and 130. As previously described, the magnitudes of the negative and positive curvature changes vary with the slope of the gradient. This results in a tilted bottom of the well, with the conduction band 120 and valence band 130 tilted in opposite directions. The first quantum levels 210a and 210b of electrons and holes, and the wave functions 220a and 220b, shift in opposite directions, leading to a reduction in the magnitude of the overlap integral. Furthermore, the shape of well 200 has been altered, effectively becoming wider for both electrons and holes. The quantum levels in the wider well are closer to the band edges, resulting in a redshift of the emission wavelength. Therefore, QCSE remains valid for this band structure.

[0038] According to an exemplary embodiment of this application, Figure 9A and Figure 9BThe band structure of the cascaded, smooth, nonlinearly gradient MQW structure 100 is shown. Figure 9A The conduction band 120 and valence band 130 of the unpolarized well 200 are shown, which are symmetric about the bandgap minimum at the center of the well 200. Alternatively, the nonlinear down and up gradients can comprise multiple layers with different curvatures and can be asymmetric, and alternatives are non-limiting. First quantum energy levels 210a, 210b of electrons and holes and wave functions 220a, 220b are also shown, and their favorable overlap can be observed. However, compared with the reference... Figure 7A The reasons given are similar, and the overlap is not complete.

[0039] According to the embodiments of this application, Figure 9B A smoothed well 100 incorporating polarization effects is shown. A downward nonlinear gradient on the left side of well 200 results in a non-uniform positive space charge, thus inducing negative curvature in bands 120 and 130. Conversely, an upward nonlinear gradient on the right side of well 200 results in a negative space charge, thus inducing positive curvature in bands 120 and 130. As previously described, the magnitudes of the negative and positive curvature changes with the slope of the gradient. This manifests as a shift at the bottom of well 200, where the conduction band 120 and valence band 130 shift in opposite directions. The first quantum energy levels 210a and 210b of electrons and holes and wave functions 220a and 220b are also shown, and it can be seen that their shift in opposite directions results in a reduction in the magnitude of the overlap integral. However, unlike the V-shaped and Y-shaped wells 200, well 200 maintains a substantially identical shape, and quantum energy levels 210a and 210b are located at approximately the same distance from the band edges. Therefore, for this band structure, QCSE can be reduced at least partially.

[0040] According to one or more embodiments of this application, the cascaded QWs 200 in the MQW band structure 100 may have a single heterojunction surface. In an exemplary embodiment, Figures 10A to 11B The band structure of the cascaded, asymmetric, zigzag, linearly gradient MQW structure 100 is shown. Figure 10A The conduction band 120 and valence band 130 of an unpolarized, positively serrated well 200 are shown, and the minimum band gap of the conduction and valence bands is asymmetric about the right edge of the well 200. Figure 11A As shown, alternatively, one or more linearly increasing gradient layers can be used to form a reverse zigzag pattern. Alternatively, the linear downward gradient can include multiple layers with different slopes and / or portions of non-linear gradients, and such alternatives are not limiting. Figure 10A and Figure 11A The first quantum energy levels 210a and 210b for electrons and holes, and wave functions 220a and 220b, are also shown, and their excellent overlap is evident. However, compared with the reference... Figure 7A The reasons given are similar, and the overlap is not complete.

[0041] According to the embodiments of this application, Figure 10B A linear sawtooth QW 200 incorporating polarization effects is shown. (Example) Figure 10B As shown, the downward linear gradient on the left side of well 200 results in a uniform positive space charge, thus inducing negative curvature in bands 120 and 130. Conversely, the upward step gradient on the right side of well 200 results in a negative thin-layer charge, thus inducing a more positive twist in bands 120 and 130. As previously mentioned, the magnitude of the negative curvature change varies with the slope of the gradient, while the magnitude of the positive twist is proportional to the magnitude of the bandgap step. Advantageously, the right side of the QW 200 formed by the step heterojunction is not tilted, thereby reducing the effect of the deformation of bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a and 210b of electrons and holes and the wave functions 220a and 220b are also shown, and it can be seen that they are shifted in opposite directions, resulting in a reduction in the magnitude of the overlap integral. The curvature of conduction band 130 becomes more negative, resulting in a narrower notch. Therefore, the energy of conduction band quantum level 210a increases, and the peak of wavefunction 220a shifts toward the nearest QW 200 interface. The curvature of valence band 130 also becomes more negative, causing the notch to flatten and form a minimum away from the nearest QW 200 interface. As a result, the energy of valence band quantum level 210b increases as it moves closer to conduction band 120. However, overall, the distance between quantum levels 210a and 210b remains substantially the same or slightly increases, causing a blue shift in the emission wavelength. Therefore, for this band structure, QCSE can be partially reduced. Depending on the gradient used, the overlap between electron wavefunction 220a and hole wavefunction 220b can be reduced.

[0042] According to the embodiments of this application, Figure 11BA linearly reverse-zigzag QW 200 incorporating polarization effects is shown. An upward linear gradient on the left side of the well 200 results in a uniform negative space charge, thus inducing positive curvature in bands 120 and 130. Conversely, a downward step gradient on the right side of the well 200 results in a positive thin-layer charge, thus inducing more negative twist in bands 120 and 130. As previously stated, the magnitude of the positive curvature change varies with the slope of the gradient, while the magnitude of the negative twist is proportional to the magnitude of the bandgap step. Advantageously, the right side of the QW 200 formed by the step heterojunction is not tilted, thereby reducing the effect of the deformation of bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a and 210b of electrons and holes and wave functions 220a and 220b are also shown, and it can be seen that they are shifted in opposite directions, resulting in a reduction in the amplitude of the overlap integral. The curvature of the conduction band 130 begins to flatten, causing the notch to widen. As a result, the energy of the conduction band quantum level 210a decreases, and the peak of the wavefunction 220a shifts away from the nearest QW 200 interface. The curvature of the valence band 130 also becomes more positive, causing the notch to sharpen and form a minimum closer to the nearest QW 200 interface. As a result, the energy of the valence band quantum level 210b increases as it shifts away from the conduction band 120. However, overall, the distance between quantum levels 210a and 210b remains essentially the same or slightly increases, causing a blue shift in the emission wavelength. Therefore, for this band structure, the QCSE can be partially reduced. Depending on the gradient used, the overlap between the electron wavefunction 220a and the hole wavefunction 220b can be reduced. To mitigate this effect, a nonlinearly gradiented sawtooth distribution can be used.

[0043] According to an exemplary embodiment of this application, Figure 12A and Figure 12B The band structure of the cascaded, asymmetric, forward zigzag, nonlinearly graded MQW structure 100 is shown. Figure 12A The conduction band 120 and valence band 130 of the unpolarized well 200 are shown, which are asymmetric about the band gap minimum at the right edge of each well 200. Alternatively, the nonlinear downgradation can include multiple layers of different curvatures and / or linearly graded portions, and such alternatives are not limiting. The first quantum energy levels 210a, 210b of electrons and holes and wave functions 220a, 220b are also shown, and they can be seen to have excellent overlap. However, with Figure 7A The reasons given are similar, and the overlap is not complete.

[0044] According to the embodiments of this application, Figure 12BA nonlinear sawtooth QW 200 incorporating polarization effects is shown. The downward nonlinear gradient on the left side of the well 200 results in a non-uniform positive space charge, thus inducing a more negative curvature in bands 120 and 130. Conversely, the upward step gradient on the right side of the well 200 results in a negative thin-layer charge, thus inducing a more positive twist in bands 120 and 130. As previously stated, the magnitude of the negative curvature change varies with the slope of the gradient, while the magnitude of the positive twist is proportional to the magnitude of the step. Advantageously, the right side of the QW 200 formed by the step heterojunction is not tilted, thereby reducing the effect of the deformation of bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a and 210b of electrons and holes and the wave functions 220a and 220b are also shown, and it can be seen that they are shifted in opposite directions, resulting in a reduction in the magnitude of the overlap integral. The curvature of the conduction band 130 becomes more negative, causing the notch to narrower. As a result, the energy of the conduction band quantum level 210a increases, and the peak of the wavefunction 220a shifts toward the nearest QW 200 interface. The curvature of the valence band 130 also becomes more negative, causing the notch to flatten and form a minimum further away from the nearest QW 200 interface. Therefore, the energy of the valence band quantum level 210b increases as it moves closer to the conduction band 120. However, overall, the distance between quantum levels 210a and 210b remains substantially the same or slightly increases, causing a blue shift in the emission wavelength. Depending on the gradient used, the overlap between the electron wavefunction 220a and the hole wavefunction 220b can remain substantially the same. Therefore, for this band structure, the QCSE can be reduced.

[0045] According to an exemplary embodiment, Figure 13A and Figure 13B The band structure of the cascaded, asymmetric, reverse-zigzag, nonlinearly graded MQW structure 100 is shown. Figure 13A The conduction band 120 and valence band 130 of an unpolarized well are shown, which are asymmetric about the band gap minimum at the left edge of the well 200. Alternatively, the nonlinear upward gradient can include multiple layers with different curvatures and / or linearly gradient portions, and such alternatives are not limiting. First quantum energy levels 210a, 210b of electrons and holes and wave functions 220a, 220b are also shown, and their excellent overlap is evident. However, with... Figure 7A The reasons given are similar, and the overlap is not complete.

[0046] According to the embodiments of this application, Figure 13BA reverse-zigzag QW 100 incorporating polarization effects is shown. The upward nonlinear gradient on the left side of well 200 results in a non-uniform negative space charge, thus inducing a more positive curvature in bands 120 and 130. Conversely, the downward step on the right side of well 200 results in a positive thin-layer charge, thus inducing a more positive twist in bands 120 and 130. As previously mentioned, the magnitude of the positive curvature change varies with the slope of the gradient, while the magnitude of the positive twist is proportional to the magnitude of the step. Advantageously, the right side of the QW 200 formed by the step heterojunction is not tilted, thereby reducing the effect of the deformation of bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a and 210b of electrons and holes and the wave functions 220a and 220b are also shown, and it can be seen that they are shifted in opposite directions, resulting in a reduction in the magnitude of the overlap integral. The curvature of the conduction band 120 becomes more negative, causing the notch to widen. As a result, the energy of the conduction band quantum level 210a decreases, and the peak of the wave function 220a shifts away from the nearest QW 200 interface. The curvature of the valence band 130 also becomes more positive, causing the notch to sharpen, which in turn causes the hole wave function 220b to shift slightly toward the QW 200 interface. As a result, the energy of the valence band quantum level 210b decreases as it shifts away from the conduction band 120. However, overall, the distance between quantum levels 210a and 210b remains substantially the same or slightly increases, causing a blue shift in the emission wavelength. Depending on the gradient used, the overlap between the electron wave function 220a and the hole wave function 220b can remain substantially the same. Therefore, for this band structure, the QCSE can be reduced. This MQW band structure 100 can have the additional advantage of an electric field built into the valence band that promotes right-to-left carrier drift, thereby improving hole distribution uniformity.

[0047] According to embodiments of this application, a meta-grading of the period or composition of the MQW structure 100 is proposed to improve hole distribution uniformity. The period of the MQW structure can be measured along the growth direction from the first substrate-side QW interface to the next substrate-side QW interface. The period of the cascaded QW 100 or the period of the QW / QB pair can vary, for example, smaller on the n-side and larger on the p-side, and vice versa. Such a non-uniform period can match the drift and / or diffusion distribution of holes, thereby promoting distribution uniformity. For example, the non-uniform period of the cascaded QW can vary from a value of about 5 nm to about 15 nm. As used herein, the term "about" when referring to thickness values ​​means ±0.01 nm. When QB is present, the period of the QW / QB pair can be changed by varying the thickness of the QW and / or QB. For example, the period of the QW / QB pair can vary from about 8 nm to about 20 nm, where the thickness of the QB remains constant at 3 nm, and the thickness of the QW varies from about 5 nm to about 17 nm. Alternatively, the period of the QW / QB pair can remain constant, while the ratio of the QW to QB thickness varies. For example, the period of the QW / QB pair could be 15 nm, while the thickness of the QW varies from about 5 nm to about 10 nm, and the thickness of the QB simultaneously varies from about 10 nm to about 5 nm.

[0048] Regarding composition, the bandgap range of the QW or QW / QB combination can vary, for example, being smaller on the n-side and larger on the p-side in the MQW structure 100, and vice versa. For each QW or QW / QB pair, the bandgap range can be calculated as the maximum bandgap minus the minimum bandgap. Variations in the bandgap range can be achieved by changing the minimum and / or maximum bandgap. This compositional gradient can provide a global internal electric field that promotes hole drift from the p-side to the n-side, thereby promoting uniform distribution. For example, in a non-uniformly composed MQW structure, the minimum bandgap can vary from about 2.4 eV to about 2.6 eV in the growth direction, while the maximum bandgap varies from about 3.0 eV to about 3.4 eV in the same direction, giving a bandgap range variation of 0.6 eV to 0.8 eV. As used herein, the term "about" when referring to bandgap values ​​means 1 meV. Group III nitride material systems (AlN, GaN, InN and their ternary and quaternary alloys) can produce band gaps between about 0.65 eV and about 6.1 eV.

[0049] According to embodiments of this application, in an alternative but equivalent method, the continuously gradient QW 100 can be separated by continuously gradient QB 300, with no more than one interface separating them. The QW 100 and / or QB 300 can be symmetrical or asymmetrical. Figures 14A to 14CSeveral examples of QW / QB band structures are shown, with their interfaces represented by vertical dashed lines. For example... Figure 14A As shown, the symmetrically linearly gradient QW 200a, 200b are separated by the symmetrically linearly gradient QB 300a-300c. The interface between QW 200a, 200b and QB 300a-300c is twisted. Figure 14B In the diagram, the symmetrical, smoothly gradient QW200a and 200b are separated by the asymmetrical, smoothly gradient QB300a-300c. The interface between QW200a and QB300b is tangent, while the interface between QB300b and QW200b is distorted. (See diagram for example.) Figure 14C As shown, the asymmetric, smoothly gradient QW200a and 200b are separated by the asymmetric, smoothly linearly gradient QB300a-300c. The interface between QW200a and QB300b has a positive kink, while the interface between QB300b and QW200b includes negative kinks, downward steps, and positive kinks. These elements can be combined in any other way without limitation. In all cases, each well / barrier pair has no more than one discontinuous interface.

[0050] Barrier-free QW 200 with more than two gradient layers has been described previously, for example, Figure 2I and Figure 2J Four linear gradient layers are shown. Figure 2M and Figure 2N Four smooth gradient layers are shown, with inflection points between some layers. Such a QW 200 can be symmetric or asymmetric. Whenever a QW 200 includes more than one gradient layer as defined herein, there is a possibility of interpreting at least one layer or its sublayers as a QB 200. According to the examples in this application, Figures 15A to 15C Various possible interpretations of the multi-layered linearly gradient MQW structure 100 are shown. Looking back, it is helpful to note that the QW layer must include the minimum bandgap, and the QB layer (if present) must include the maximum bandgap. In the accompanying figures, the vertical dashed line indicates the QW / QB interface. Figure 15A As shown, the band structure can be interpreted as multiple symmetrical, barrier-free cascaded QW 200s, with only the left and right wells partially shown. Figure 15B As shown, the same band structure can be interpreted as a symmetrical four-layer linearly graded QW 200 sandwiched between a pair of two-layer linearly graded QB 300a, 300b. In this case, both positive and negative kinks exist within the QW 200. Figure 15C As shown, the symmetrical V-shaped well 200 is flanked by a pair of four-layer linearly graded QBs 300a and 300b. Note that both positive and negative kinks exist within the barrier. (As shown...) Figure 15DAs shown, the same band structure can be interpreted as an asymmetric three-layer linearly graded QW 200 blocked by a pair of asymmetric three-layer linearly graded QB 300a, 300b. In this case, there are positive and negative kinks in both QW 200 and QB 300a, 300b. The same interpretation is possible for nonlinear graded layers, or combinations of linear and nonlinear graded layers. However, in all cases, this interpretation conforms to the disclosed embodiments of a continuously graded cascaded QW or a continuously graded QW surrounded by continuously graded QBs (with no discontinuities between them).

[0051] Another example of QW / QB interpretation can be found in Figures 16A to 16C As seen in the embodiments of this application, Figures 16A to 16C An asymmetric, multilayered, sawtooth-shaped, linearly graded MQW band structure 100 is shown. (e.g.) Figure 16A As shown, the band structure can be interpreted as multiple asymmetric, multilayered, sawtooth-shaped, linearly graded QW 100 bands. For example... Figure 16B As shown, the same band structure can be interpreted as an asymmetric two-layer linearly graded QW 200a sandwiched between a pair of single-layer linearly graded QB 300a, 300b. Figure 16C As shown, the same band structure can be interpreted as an asymmetric single-layer linearly graded QW 200a sandwiched between a pair of two-layer linearly graded QB 300a, 300b. In summary, any continuously graded periodic band structure (i.e., a band structure without a constant band gap) can be interpreted as cascaded QW200, continuously graded QW 200 surrounded by continuously graded QB 200 (with no discontinuities between them), or combinations of continuously graded QW / QB pairs, each having no more than one discontinuity interface, and within the scope of this disclosure. It should be noted that, as Figure 16D As shown, adding a second discontinuous interface to each QW / QB pair produces a more conventional gradient QW / QB pair, which is outside the scope of this disclosure.

[0052] QB 300 can be positioned between the n-type electron injection layer and the MQW 100 active region, and between the MQW 100 active region and the p-type electron blocking layer and / or the p-type hole injection layer. However, other transition layers can be used without limitation, including the same or different first QB and last QB 300. This paper envisions any type of transition layer between the MQW active region and adjacent layers.

[0053] An EBL layer can be used to reduce or eliminate electron leakage from the active region to the p-implantation layer. The EBL layer can have a similar structure to... Figures 3A to 3I The shape of QB, the band gap larger than QB, and the thickness slightly larger than QB. For example... Figure 3A As shown, in one embodiment, the EBL may comprise an AlGaN layer with a constant bandgap. However, EBLs with step discontinuities or kinks may suffer from polarization effects, including a reduction in barrier height.

[0054] In an alternative embodiment, an EBL with a strictly increasing, smoothly graded bandgap is grown on the MQW region. In the first embodiment, as... Figure 17A and Figure 17B As shown, Figure 13A A nonlinear, reverse-sawtooth QW 100 is disposed on the n-type injection layer 170 and terminates at the horizontal tangential band on the p-type side. An EBL 400 is disposed on the MQW region 100, followed by the p-injection layer 180. In this case, as... Figure 17A As shown, the EBL 400 band is tangent to the MQW 100 bands 120 and 130 at the MQW 100 / EBL 400 interface (162a). The EBL bandgap increases parabolically (152a) to an inflection point (162c), and then decreases parabolically (152b) to a tangent point (162b) with the maximum bandgap of the EBL 162d. A similar smooth gradient bandgap can be used near the EBL 400 / p injection layer 180 interface, such that bands 120 and 130 are tangent at the interface. Alternatively, a step, kink, or other non-smooth gradient can be used at or near the EBL 400 / p injection layer 180 interface. It should be noted that, similar to the previously described reverse sawtooth QW, only one discontinuity is allowed in the band structure, and this discontinuity must be located on the p-side of the EBL. According to embodiments of this application, Figure 17B It shows Figure 17A The combination incorporates the polarization effect of MQW 100 / EBL 400. The resulting bands 120 and 130 avoid the notch at the MQW100 / EBL 400 interface and the reduction of the EBL height 162e, which remains at or above the unpolarized barrier height 162d.

[0055] In fact, if MQW adopts a component distribution where the last part is In-rich, for example, a positive sawtooth distribution (see... Figure 10A If this is not the case, a capping layer may be needed to prevent In desorption prior to AlGaN EBL growth. The capping function can be provided by the transition layer mentioned earlier. A typical capping layer can be 10 nm GaN or other low-In-content alloys. In this case, such as... Figure 17A As shown, the band structure is horizontal and tangent at the overlay / EBL interface.

[0056] In an alternative embodiment, the band structure of QW terminates with a positive gradient on the p-side, such as a V-shaped distribution. Figure 7A ) or reverse zigzag distribution ( Figure 11A(The mirror image), the EBL gradient can match (i.e., be tangent) the positive gradient of QW at the interface. Figure 18A and Figure 18B In the exemplary embodiment shown, Figure 11A A reverse-zigzag, linearly tapered QW 100 is disposed on the n-type injection layer 170 and terminates at the positive slope conduction band (negative slope valence band) on the p-type side. EBL 400 is disposed on the MQW region 100, followed by the p-injection layer 180. In this case, the EBL 400 bands 120 and 130 are tangent to the MQW 100 bands 120 and 130 at the MQW 100 / EBL 400 interface 162a, as shown. Figure 18A As shown. The EBL bandgap increases parabolically from 152a to inflection point 162c, and then decreases parabolically to tangent point 162b with the maximum bandgap of EBL 162d. A discontinuous bandgap can be used at the interface of EBL 400 / p injection layer 180. Alternatively, a kink or other non-smooth gradient can be used near the interface of EBL 400 / p injection layer 180. It should be noted that in this embodiment, only one discontinuity is allowed in the bandgap, and this discontinuity is located on the p-side of the EBL. According to an embodiment of this application, Figure 18B It shows Figure 18A The resulting MQW 100 / EBL400 incorporates polarization effects. The resulting band structures 120 and 130 avoid the notch at the MQW 100 / EBL 400 interface and the reduction in the EBL height 162e, which remains at or above the unpolarized barrier height 162d. Advantageously, the step junction on the p-side causes the conduction band 120 to shift upwards, thereby increasing the EBL height 162e.

[0057] While certain constructions of the structure have been shown for the purpose of illustrating the basic structure of this application, those skilled in the art will understand that other variations are possible that still fall within the scope of the appended claims. Further advantages and modifications will readily occur to those skilled in the art. Therefore, this application is not limited in its broader aspects to the specific details and representative embodiments shown and described herein. Thus, various modifications may be made without departing from the spirit or scope of the overall application concept as defined by the appended claims and their equivalents.

Claims

1. A light-emitting device, comprising: n-type semiconductor layer; An active region is disposed on the n-type semiconductor layer, the active region comprising multiple cascaded quantum wells, each cascaded quantum well having a band structure, the band structure comprising: A strictly increasing bandgap extending from the minimum to the maximum; and each cascaded quantum well has no more than one bandgap discontinuity; and A p-type semiconductor layer is disposed on the active region.

2. The light-emitting device according to claim 1 further includes an electron blocking layer disposed between the active region and the p-type injection layer, wherein the electron blocking layer is characterized by having a band gap larger than that of the active region.

3. The light-emitting device according to claim 1, wherein, A transition layer is disposed between the plurality of quantum wells and quantum barriers and one or more of the adjacent layers.

4. The light-emitting device according to claim 1, wherein, Each of the plurality of cascaded quantum wells includes a period varying between 5 nm and 20 nm, and wherein at least one cascaded quantum well has a non-uniform period relative to at least one other period.

5. The light-emitting device according to claim 1, wherein, Each of the plurality of cascaded quantum wells includes a bandgap range varying between 0.1 eV and 3.5 eV, wherein at least one cascaded quantum well has a non-uniform bandgap range relative to at least one other bandgap range.

6. A light-emitting device, comprising: n-type semiconductor layer; The active region includes a plurality of quantum wells disposed on the n-type semiconductor layer; An electron blocking layer is disposed on the active region and has a first interface and a second interface. The electron blocking layer has a band structure, the band structure comprising: A strictly increasing, smoothly gradient bandgap extending from the first interface to the maximum value; and No more than one band gap discontinuity; and A p-type semiconductor layer is disposed on the active region.

7. A light-emitting device, comprising: n-type semiconductor layer; An active region is disposed on the n-type semiconductor layer, the active region comprising: Multiple quantum wells, each of which includes a band gap that strictly increases from a minimum value; Separate the quantum barriers of each quantum well, each quantum barrier comprising a band gap that strictly decreases from its maximum value; and An interface, coupled with the strictly increasing band gap and the strictly decreasing band gap, thereby forming an energy band; Wherein, the energy bands at no more than one interface of each quantum well are discontinuous; and A p-type semiconductor layer is disposed on the active region.

8. The light-emitting device according to claim 6 further includes an electron blocking layer disposed between the active region and the p-type injection layer, wherein the electron blocking layer is characterized by a band gap larger than that of the active region.

9. The light-emitting device according to claim 6, wherein, Each quantum well and adjacent quantum barrier comprises a period varying between 5 nm and 25 nm, wherein at least one quantum well and adjacent quantum barrier have a non-uniform period relative to at least one other period.

10. The light-emitting device according to claim 6, wherein, Each quantum well and adjacent quantum barrier includes a ratio of quantum well thickness to quantum barrier thickness varying between 0.05 and 0.95, wherein at least one quantum well and adjacent quantum barrier have unequal ratios relative to at least one other ratio.

11. The light-emitting device according to claim 6, wherein, Each quantum well and adjacent quantum barrier comprises a bandgap range varying between 0.1 eV and 3.5 eV, wherein a quantum well and adjacent quantum barrier have a non-uniform bandgap range relative to at least one other bandgap range.

12. The light-emitting device according to claim 6, wherein, A transition layer is disposed between the plurality of quantum wells and quantum barriers and one or more of the adjacent layers.

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