Energy band structure for light-emitting devices

Abstract

Apparatus and methods are disclosed for reducing the quantum confinement Stark effect and electron leakage in the optical gain region of polarized compound semiconductors. The apparatus includes a continuous-gradient multiple quantum well (MQW) band structure, where the quantum wells (QWs) are linked, there are no quantum barriers (QBs) between them, and there is only one heterojunction interface per well. An alternative equivalent MQW band structure includes continuous-gradient QWs separated by continuous-gradient QBs with only one heterojunction interface per well. In both embodiments, the band structure lacks a constant bandgap layer. MQW structures with non-uniform periodicity and non-uniform bandgap ranges are considered. MQW structures with non-uniform QW / QB thickness ratios are also considered. A smooth-gradient electron barrier layer (EBL) having a tangential energy band on the MQW side and only one heterojunction interface on the p-injection side may be used to improve electron confinement and hole injection.

Description

Technical Field

[0001] Cross - reference to Related Applications This application claims priority to and the benefit of co - pending U.S. Patent Application No. 18 / 331,908, filed on June 8, 2023.

[0002] The present invention relates to an apparatus, system, and method for reducing the quantum confinement Stark effect and electron leakage in a light - emitting device of a polar compound semiconductor. More particularly, it relates to a multiple quantum well structure having an improved overlap of the wave functions of electrons and holes and an improved electron barrier layer with improved electron confinement.

Background Art

[0003] Compound semiconductors have achieved great success in realizing practical optoelectronic devices such as lasers and detectors. Compounds based on III - V elements such as InP and GaAs have created optoelectronic devices that emit light from far - infrared to visible, i.e., orange - colored wavelengths. For short wavelengths from green to ultraviolet, semiconductors with larger bandgaps, i.e., III - nitride compounds such as InN, GaN, and AlN, have been used. The former compounds have a non - polar zinc - blende structure, while the III - nitride material system has a wurtzite lattice structure and shows a very high intrinsic and naturally occurring piezoelectric field and a (strain - induced) piezoelectric field due to the broken inversion symmetry in the uniaxial crystal structure, which leads to a significant decrease in quantum efficiency at high current densities.

[0004] In the following disclosure, the energy bands of the light-emitting structures are shown in Figures 1A to 18B. In all figures, the epitaxial growth direction is from left to right. Light-emitting semiconductor devices typically use heterojunctions to confine electrons and holes within a common layer of the crystal lattice, called a quantum well, referred to herein as "QW," thereby increasing the probability of radiative recombination. Figures 1A and 1B show the effect of polarization on the energy bands of a single QW101. Figure 1A shows the conduction band 120 and valence band 130 of a single QW101, which includes a layer of narrow-bandgap material 200 surrounded by layers of larger bandgap 300a, 300b, also called a quantum barrier, referred herein as "QB." Without polarization, the conduction band 120 and valence band 130 are flat, i.e., horizontal, in the unbiased state, but the QW / QB interface has a stepwise, i.e., perpendicular change in the energy gap between the electron band and the hole band. The quantum levels of electron 210a and hole 210b, as well as the probability wave functions 220a and 200b, are shown, the latter being symmetric within QW and having a high degree of overlap. Referring to Figure 1B, under the influence of polarization, a downward step in the band gap, such as between layers 300a and 200, introduces negative polarization at the interface, which attracts positive sheet charges and causes a change in the band structure gradient. In the presence of sheet charges, the band gradients on either side of the heterointerface must have opposite signs, in this case, positive on the left and negative on the right. An upward step in the band gap, such as between layers 200 and 300b, does the opposite. Between interfaces, band gradients and / or bends must occur to satisfy these constraints. Figure 1B also shows the effect of this band distortion on quantum levels 210a and 210b, and wave functions 220a and 220b, where the quantum levels of electron 210a and hole 210b are attracted to each other more closely in energy, while the wave functions of electron 220a and hole 220b are pushed further apart. This is known as the quantum confinement Stark effect (QCSE).

[0005] QCSE results in weaker carrier transition intensity, longer carrier lifetimes, and a reduced radiative recombination rate. Longer carrier lifetimes are undesirable because they make it difficult to fabricate practical lasers in the green to ultraviolet wavelength range from these materials. Furthermore, lower recombination probabilities lead to higher carrier concentrations at high current densities. This is undesirable because, as carrier density increases, the growing portion of carriers recombines through non-radiative Auger processes, resulting in a further decrease in quantum efficiency. This is often cited as a major reason for the efficiency drop, also known as droop, in blue LEDs at high current densities. Another example in GaN / InGaN systems is that QCSE increases with the In concentration in the QW of InGaN. This has been found to be a major reason for the efficiency loss of III-N nitride devices in green LEDs and lasers, which require high In content to reach the green spectral region. This is particularly important because only III-N systems can be used to construct devices that emit green light.

[0006] Another cause of reduced efficiency is the non-uniform distribution of holes, which is related to low injection efficiency and the QW shape. In devices with standard rectangular QWs, polarization strain causes holes to accumulate on the p-side of the device, resulting in a highly non-uniform carrier distribution between QWs. One proposed solution is p-doping of the QW barrier, where a Mg-doped barrier is used to improve the hole distribution between multiple QWs (MQWs). However, in p-type MQWs, the Mg dopant diffuses easily into the well, consequently reducing the radiation efficiency.

[0007] Several methods have been proposed to increase electron-hole overlap in III-N QWs. Two main approaches are to reduce intrinsic polarization through either gradient or step-type growth of QWs and / or QBs, or to rely on growth in semipolar and nonpolar crystal orientations. Regarding gradient-type methods, research has focused on improving quantum efficiency through bandgap design of QW and QB structures. Regarding step-type methods, large, cost-effective semipolar and nonpolar substrates are not yet available on the market and therefore are unlikely to play a major role in increasing LED efficiency for the time being. Thus, the remaining major pathways include gradient growth of QWs and QBs, as well as ultra-broad QWs that exhibit dynamic carrier effects.

[0008] Certain QW structures are known as III-N material systems. Figures 2A–2O show some examples of proposed, simulated, and / or experimentally tested QW shapes. For clarity, only the conduction band 120 is shown, and the valence band 130 is omitted, which is a vertically mirrored and scaled version of the conduction band 120, as shown in Figure 1A. Also, the top flat portions located on the sides of QW200 represent QB300a and 300b, and polarization effects are omitted for clarity.

[0009] The gradient of a QW can be used to realize a wide variety of shapes that can be broadly classified as stepped, linear, or nonlinear, i.e., smooth. In all three classifications, the well 200 may be symmetric or asymmetric. Note that a horizontal mirror image of an asymmetric well may also be included in the list of possible profiles. Also note that various types of gradients may be combined within a single quantum well 200. The most common type of QW200 is a non-gradient rectangular well, shown for reference in Figure 2A, with a single stepped interface at each end. In the stepped gradient QW200 shown in Figure 2B, layers of constant composition that decrease the band gap are grown, followed by layers of constant composition that increase the band gap. Most published or disclosed schemes use between 2 and 7 layers, but the number and thickness of the layers and the step size are variable. Similarly, the linear gradient QW uses layers of constant gradient, and the number and thickness of the layers and the gradient of each layer may vary. An example of a simple symmetrical V-shaped well is shown in Figure 2C, while asymmetrical wells with forward and reverse linear gradients are shown in Figures 2D and 2E, respectively. Various combinations of step gradients and linear gradients are shown in Figures 2F to 2H. Two types of compound linear gradients in symmetric wells are shown in Figures 2I and 2J. In Figure 2K, a combination of compound linear gradients and step gradients forms an asymmetric well. Smooth gradients use layers with varying slopes, i.e., curvature, to create various shapes of QWs, such as semicircular, sinusoidal, parabolic, elliptic, or Fermi functions. Figures 2L and 2M show wells with parabolic and Fermi profiles, respectively. Figure 2N is a smooth QW with an arbitrary but symmetrical shape. Finally, Figure 2O shows combined sinusoidal and linear gradient QWs.

[0010] In a conventional MQW structure, each well is separated from adjacent wells, often from the carrier injection layer, by a QB. In this context, QW200 refers to one or more layers in which at least one layer contains the smallest band gap of the MQW structure, and the energy bands of all other QW layers increase monotonically in the band gap. QB300 refers to one or more layers in which at least one layer contains the largest band gap of the MQW structure, and the energy bands of all other QW layers decrease monotonically in the band gap. At the QW / QB interface, the energy bands may have one of three characteristics: there may be a step 160, as in Figure 2B; there may be a kink 161, as in Figure 2C; or the energy bands on either side of the interface may be in contact with each other 162, as in Figure 2M.

[0011] In conventional light-emitting devices, an electron barrier layer (EBL) 400 may be used to further confine electrons to the MQW structure and prevent undesirable leakage into the p-injection layer. In this context, EBL 400 refers to one or more layers in which at least one layer has a band gap larger than the maximum QB band gap. At the MQW / EBL interface, the energy bands may have one of three characteristics: there may be a step 160 as in Figure 2B, there may be a kink 161 as in Figure 2C, or the energy bands on either side of the interface may be touching each other 162 as in Figure 2M.

[0012] Similar to the case of QW200, the structures of specific QB300 and EBL400 are known in the III-N material system. Figures 3A–3L show some examples of proposed, simulated, and / or experimentally tested QB300 and / or EBL400 shapes. For clarity, only the conduction band 120 is shown in Figures 3A–3L, and the valence band 130 is understood to be a vertically mirrored and scaled version of the conduction band 120, as shown in Figure 1A. Also for clarity, polarization effects are omitted. In the case of QB (Figures 3A–3I), the bottom left and right flat portions represent QW200a–b. In the case of EBL400 (Figures 3A–3L), the leftmost flat portion represents the last QB of the MQW100 structure, while the rightmost flat portion represents the p-injection layer 180. It should be noted that the MQW100 region and the last QB of the p-injection layer 180 may have different band gaps, as is the case with ultraviolet (UV) light-emitting devices, and as illustrated in Figures 3J to 3L.

[0013] The most common type of QB300 and / or EBL400 is a non-gradient rectangular barrier with a single-step interface at each end, as shown in Figure 3A. Gradient of the QB300 can be used to achieve a wide variety of shapes, broadly classified as stepped, linear, or non-linear, i.e., smooth. In all three classifications, the barrier may be symmetrical or asymmetrical. In the case of asymmetrical barriers, the gradient in the growth direction may be forward or reverse, generally corresponding to a decrease or increase in the band gap, respectively. Note that a horizontal mirror image of an asymmetrical QB may also be included in the list of possible profiles. Also note that various types of gradients may be combined within a single QB.

[0014] The following description of QB300 is equally applicable to EBL400. In step gradient QB300, layers of constant composition grow, increasing and / or decreasing the band gap, as shown, for example, in Figure 3A. Figure 3B shows an example of a reverse asymmetric step gradient barrier. Most known step gradients use between one and three layers, but the number and thickness of layers and the step size may be variable. As with QW200, linear gradient QB300 uses layers of constant gradient, and the number and thickness of layers and the gradient of each layer may vary. An example of a simple symmetric inverted V-shaped barrier is shown in Figure 3C, while asymmetric forward and reverse linear gradient barriers are shown in Figures 3D and 3E, respectively. Various combinations of step gradient and linear gradient are shown in Figures 3F-3H. Smooth gradient processing uses one or more layers with varying gradients, i.e., a curved band gap, to create QB300s of various shapes, such as semicircular, sinusoidal, parabolic, elliptic, or Fermi function. Figure 3I shows a sinusoidal gradient type QB300. Examples of linear gradient EBLs, double linear gradient EBLs, and double inverted linear gradient EBLs are shown in Figures 3J to 3L, respectively.

[0015] From the examples in Figures 2-3I, those skilled in the art will understand that various MQW structures, such as multiple QW200s, can be realized, for example, by combining profiles of QW200 and QB300 with and without gradients. However, when polarization effects are incorporated, some embodiments of these structures present drawbacks. First and foremost, the step in the energy bands adds strain-induced polarization to the spontaneous polarization already present at the heterointerface. Decreasing the step in the band gap results in uncompensated negative charges that attract holes forming positive sheet charges at the interface. Conversely, increasing the step in the band gap results in uncompensated positive charges that attract free electrons forming negative sheet charges at the interface. Any constant bandgap energy band between such opposing sheet charges slopes proportionally to the surface density of the sheet charge, as shown in Figure 1B, generating an internal electric field. Similar, but less pronounced, effects occur in QB300s, which are conventionally 5-10 times thicker than QW200s. Since the carrier wavefunction penetrates the barrier, the asymmetry of the barrier causes the wavefunction to shift in the opposite direction. Secondly, the constant bandgap energy band is subjected to bending not only by the sheet charge induced by the step, but also by the volume charge induced by the kink or curvature, similar to the intrinsic region of a pin diode. The step heterojunction and constant bandgap layers thus contribute to 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 the uniformity of the overall QW carrier distribution in photogenerated MQW band structures. Finally, there has been a long-standing need to improve carrier confinement within photogenerated MQW band structures. Lastly, there has been a long-standing need to reduce electron leakage through improved EBL band structures. [Overview of the Initiative]

[0017] The present invention provides a system for reducing polarization effects, primarily the quantum confinement Stark effect (QCSE), in multiple quantum well optical gain structures of compound semiconductors. This is achieved by a continuously gradient band structure in which discontinuities in constant bandgap layers and steps are minimized or eliminated. In one embodiment, the MQW structure includes a continuously gradient QW without a QB and with no discontinuities at the QW / QW interface. In another embodiment, the MQW structure includes a continuously gradient QW and a continuously gradient QB with no discontinuities at the QW / QB interface. In yet another embodiment, the MQW structure includes a continuously gradient QW without a QB and with a single discontinuity at the QW / QW interface. Barrierless MQW structures with non-uniform QW / QB thickness and / or non-uniform minimum and / or maximum bandgap are also contemplated. An EBL is disclosed that has a smooth gradient on the MQW side and only one discontinuity on the p side.

[0018] The objective of this invention is to mitigate or eliminate the harmful effects of band slope on the overlap of carrier wave functions in MQW band structures.

[0019] The objective of this invention is to improve the uniformity of the overall carrier distribution in the photogenerated MQW band structure.

[0020] The objective of this invention is to improve carrier confinement and reduce electron leakage within photogenerating devices.

[0021] Other desirable features and characteristics will become apparent from the embodiments, drawings, abstract, and claims for subsequent inventions, given consideration of the outline of the present invention.

[0022] Non-limiting and non-exclusive embodiments of the present invention will be described with reference to the following drawings. In the drawings, similar reference numerals refer to the same parts throughout various figures unless otherwise specified.

[0023] To better understand the present disclosure, reference is made to the following embodiments for carrying out the invention. The embodiments for carrying out the invention are incorporated herein, form a part of this specification, show specific aspects of the subject matter disclosed herein, and are read in conjunction with the accompanying drawings which are useful for explaining a part of the principles associated with the disclosed embodiments.

Brief Description of the Drawings

[0024] [Figure 1A] The band diagram of a QW without a polarization effect according to the prior art is shown. [Figure 1B] The band diagram of a QW with a polarization effect according to the prior art is shown. [Figure 2A] The band diagram of a rectangular QW without a polarization effect and without a gradient according to the prior art is shown. [Figure 2B] The band diagram of a step-gradient type QW without a polarization effect according to the prior art is shown. [Figure 2C] The band diagram of a symmetric V-shaped linear gradient type QW without a polarization effect according to the prior art is shown. [Figure 2D] The band diagram of an asymmetric V-shaped forward linear gradient type QW without a polarization effect according to the prior art is shown. [Figure 2E] The band diagram of an asymmetric V-shaped reverse linear gradient type QW without a polarization effect according to the prior art is shown. [Figure 2F] The band diagram of an asymmetric V-shaped forward partially linear gradient type QW without a polarization effect according to the prior art is shown. [Figure 2G] The band diagram of an asymmetric V-shaped reverse partially linear gradient type QW without a polarization effect according to the prior art is shown. [Figure 2H] The band diagram of an asymmetric forward partially linear gradient type QW without a polarization effect according to the prior art is shown. [Figure 2I] The band diagram of a symmetric V-shaped composite linear gradient type QW having a negative kink and without a polarization effect according to the prior art is shown. [Figure 2J] [[ID= [Figure 2K] The band diagrams of the prior art for asymmetric V-shaped compound linear and step-gradient QWs with positive kinks and no polarization effect are shown. [Figure 2L] The band diagram for a nonlinear gradient type QW with a symmetric smooth parabolic shape without polarization effects, based on prior art, is shown. [Figure 2M] The band diagram of a nonlinear gradient type QW with a symmetric smooth Fermi function shape and no polarization effect, based on prior art, is shown. [Figure 2N] The band diagram for a symmetric smooth nonlinear gradient type QW without polarization effects, based on prior art, is shown. [Figure 2O] The band diagrams for asymmetric nonlinear gradient and linear gradient QWs without polarization effects, based on prior art, are shown. [Figure 3A] The band diagram of a rectangular QB without polarization effect and gradient, based on prior art, is shown. [Figure 3B] The band diagram for a step gradient type QB without polarization effect, based on prior art, is shown. [Figure 3C] The band diagram of a symmetrical V-shaped linear gradient QB with negative kinks and no polarization effect, based on prior art, is shown. [Figure 3D] The band diagram of a forward linear gradient type asymmetric V-shaped QB without polarization effect, based on prior art, is shown. [Figure 3E] The band diagram of a QB with an asymmetric V-shaped, inverse linear gradient and no polarization effect, based on prior art, is shown. [Figure 3F] The band diagram of a partially linear gradient QB with asymmetrical, reverse-direction, and no polarization effect, based on prior art, is shown. [Figure 3G] The band diagrams of partially linear and step-gradient QBs with asymmetric forward direction and no polarization effect, based on prior art, are shown. [Figure 3H] The band diagrams of symmetric, partially linear, and step-gradient QBs without polarization effects, based on prior art, are shown. [Figure 3I] The band diagram for a symmetric nonlinear (sinusoidal) gradient type QB without polarization effects, based on prior art, is shown. [Figure 3J]The band diagram for an asymmetric linear gradient EBL without polarization effects, based on prior art, is shown. [Figure 3K] The band diagram of a symmetrical, bilinear gradient EBL without polarization effects, based on prior art, is shown. [Figure 3L] The band diagram of a symmetrical, double-reverse linear gradient EBL without polarization effects, based on prior art, is shown. [Figure 4A] This shows a discontinuity in the interfacial conduction band in the form of a heterojunction without polarization effect, according to prior art. [Figure 4B] This shows the sheet charge induced by polarization in the downward step conduction band discontinuity, according to prior art. [Figure 4C] This shows a discontinuity in the interfacial conduction band in the form of a heterojunction with polarization effects, according to prior art. [Figure 5A] The prior art exhibits a downward linear gradient conduction band without polarization effects. [Figure 5B] The prior art demonstrates a uniform space charge induced by polarization within a layer having a downward linear band gap. [Figure 5C] The prior art demonstrates a downward linear gradient conduction band with polarization effects. [Figure 6A] This shows a downward nonlinear gradient conduction band without polarization effects, as demonstrated by prior art. [Figure 6B] The prior art demonstrates heterogeneous space charge induced by polarization within a layer having a downward nonlinear band gap. [Figure 6C] This shows a downward nonlinear gradient conduction band with polarization effects, as demonstrated by prior art. [Figure 7A] This shows a connected, symmetrical V-shaped linear gradient MQW band structure without polarization effects, according to one embodiment of the present invention. [Figure 7B] This shows a connected, symmetrical V-shaped linear gradient MQW band structure with polarization effects, according to one embodiment of the present invention. [Figure 8A] This shows a coupled, symmetrical Y-shaped, nonlinear gradient MQW band structure without polarization effects, according to one embodiment of the present invention. [Figure 8B] This shows a connected, symmetrical Y-shaped, nonlinear gradient MQW band structure with polarization effects, according to one embodiment of the present invention. [Figure 9A] This document shows a nonlinear gradient MQW band structure with connected, symmetrically smooth shapes and no polarization effect, according to one embodiment of the present invention. [Figure 9B] This shows a polarizing, coupled, symmetrically smooth, nonlinear gradient MQW band structure according to one embodiment of the present invention. [Figure 10A] This shows a connected, asymmetrical, sawtooth-shaped, forward-linear gradient MQW band structure according to one embodiment of the present invention, which has no polarization effect. [Figure 10B] This shows a polarizing, connected, symmetrical, sawtooth-shaped, forward-linear gradient MQW band structure according to one embodiment of the present invention. [Figure 11A] This shows an MQW band structure with connected, asymmetrical, sawtooth-shaped, reverse-direction linear gradients, free from polarization effects, according to one embodiment of the present invention. [Figure 11B] This shows a polarizing, connected, symmetrical, sawtooth-shaped, reverse-direction linear gradient MQW band structure according to one embodiment of the present invention. [Figure 12A] This invention illustrates a connected, asymmetrical, sawtooth-shaped, forward-nonlinear gradient MQW band structure without polarization effects, according to one embodiment of the present invention. [Figure 12B] This shows a polarizing, connected, symmetrical sawtooth-shaped, forward-nonlinear gradient MQW band structure according to one embodiment of the present invention. [Figure 13A] This exhibits an embodiment of the present invention, showing a connected, asymmetric, sawtooth-shaped, reverse-direction nonlinear gradient MQW band structure without polarization effects. [Figure 13B] This shows a polarizing, connected, asymmetric sawtooth-shaped, reverse-direction nonlinear gradient MQW band structure according to one embodiment of the present invention. [Figure 14A] One embodiment of the present invention shows a symmetric V-shaped linear gradient QW and a symmetric V-shaped linear gradient QB within an MQW band structure without polarization effects. [Figure 14B]One embodiment of the present invention shows a nonlinear gradient type QW and a nonlinear gradient type QB with a symmetrical smooth shape within an MQW band structure, without polarization effects. [Figure 14C] One embodiment of the present invention shows a symmetrically smooth, nonlinear gradient type QW within an MQW band structure without polarization effects, as well as an asymmetric nonlinear gradient type and a linear gradient type QB. [Figure 15A] The present invention shows a linear gradient type QW with symmetrical V-shaped 6 layers within an MQW band structure that is free from polarization effects. [Figure 15B] The present invention presents a linear gradient type QW with four symmetrical V-shaped layers and a linear gradient type QB with two symmetrically inverted V-shaped layers within an MQW band structure that does not exhibit polarization effects. [Figure 15C] The present invention presents a linear gradient type QW with two symmetrical V-shaped layers and a linear gradient type QB with four symmetrically inverted V-shaped layers within an MQW band structure, which are free from polarization effects. [Figure 15D] The present invention shows a linear gradient type QW with three asymmetric V-shaped layers and a linear gradient type QB with three asymmetric inverted V-shaped layers within an MQW band structure, without polarization effects. [Figure 16A] The present invention shows a linear gradient type QW with three asymmetric sawtooth-shaped layers within an MQW band structure that does not exhibit polarization effects. [Figure 16B] The present invention shows a linear gradient type QW with two asymmetric sawtooth-shaped layers and a linear gradient type QB with a single asymmetric sawtooth-shaped layer within an MQW band structure that does not exhibit polarization effects. [Figure 16C] The present invention shows a linear gradient type QW with a single asymmetric sawtooth-shaped layer and a linear gradient type QB with two asymmetric sawtooth-shaped layers within an MQW band structure that does not exhibit polarization effects. [Figure 16D] Prior art demonstrates linear gradient QW and linear gradient QB bands within an asymmetric monolayer MQW band structure, without polarization effects. [Figure 17A] An embodiment of the present invention shows a connected, asymmetric, sawtooth-shaped, reverse-direction nonlinear gradient MQW and a smooth / smooth gradient EBL band structure without polarization effects. The dashed lines separate the n-injection layer, MQW layer, EBL layer, and p-injection layer. [Figure 17B] An embodiment of the present invention shows a polarizing, connected, asymmetrical, sawtooth-shaped, reverse-direction nonlinear gradient MQW and a smooth / smooth gradient EBL band structure. The dashed lines separate the n-injection layer, MQW layer, EBL layer, and p-injection layer. [Figure 18A] An embodiment of the present invention shows connected, asymmetrical, sawtooth-shaped, reverse-direction linear gradient MQW and smooth / discontinuous gradient EBL band structures without polarization effects. The dashed lines separate the n-injection layer, MQW layer, EBL layer, and p-injection layer. [Figure 18B] An embodiment of the present invention shows a polarizing, connected, asymmetrical, sawtooth-shaped, reverse-direction linear gradient MQW and a smooth / discontinuous gradient EBL band structure. The dashed lines separate the n-injection layer, MQW layer, EBL layer, and p-injection layer. [Modes for carrying out the invention]

[0025] Non-limiting embodiments of the present invention are described below with reference to the drawings, and similar reference numerals throughout represent similar elements. While the present invention has been described in detail with respect to its preferred embodiments, it will be understood that certain variations to the preferred embodiments will become apparent upon reading and understanding the foregoing, and nevertheless, these variations will be understood to be within the spirit and scope of the present invention. The drawings characterized in the figures are provided for the purpose of illustrating some embodiments of the present invention and should not be considered limitations to the present invention. The drawings of band structure, carrier distribution, quantum levels, and wave functions are qualitative in nature and are intended to show general features of the non-polarized and polarized layers, interfaces, and MQW structures. The terms “a” or “an” are defined as one or more than one as used herein. The term “plural” is defined as two or more than two as used herein. The term “another” is defined as at least the second and subsequent as used herein. The terms “including” and / or “having” are defined as including (i.e., in the public language) as used herein. As used herein, the term "combined" is defined as being connected, but not necessarily directly or mechanically.

[0026] Throughout this Specimen, references to “some embodiments,” “one embodiment,” “certain embodiments,” and “an embodiment,” or similar terms, mean that any particular feature, structure, or characteristic described in relation to an embodiment is included in at least one embodiment of the present invention. Therefore, such phrases appearing in various places throughout this Specimen do not necessarily all refer to the same embodiment. Furthermore, any particular feature, structure, or characteristic may be combined in any preferred manner in one or more embodiments.

[0027] As used herein, the term “or” should be interpreted as an inclusive “or” meaning any one or any combination thereof. Thus, “A, B, or C” means “A, B, C, A and B, A and C, B and C, A, B, and C.” Exceptions to this definition arise only when any combination of elements, functions, steps, or actions is intrinsically contradictory to one another in any way.

[0028] The term “means” preceding the present molecule of an action indicates a desired function in which one or more embodiments exist, i.e., one or more methods, devices, or apparatus for achieving the desired function, and a person skilled in the art will select from these or their equivalents in light of the disclosure herein, and the use of the term “means” is not intended to be limiting.

[0029] The term "strictly increasing" (also called "monotonically increasing") is defined as always increasing, meaning it never remains constant or decreases. The term "strictly decreasing" (also called "monotonically decreasing") is defined as always decreasing, meaning it never remains constant or increases. The term "monotonically increasing" is defined as never decreasing at all. The term "monotonically decreasing" is defined as never increasing at all. All four definitions allow for steps that take the form of a vertical portion in the direction of the slope.

[0030] Figures 1A to 3I show specific conduction bands corresponding to various known layers and interfaces in non-polarized and polarized semiconductors. As will be detailed in the following paragraphs, a layer may refer to two interfaces separated by a thickness. Between the two interfaces, the composition of the layer may be constant or gradient. In the case of a gradient layer, the gradient may be linear or nonlinear, i.e., smooth. Examples of layer and interface types are shown in Figures 4A to 6C, where vertical solid or dashed lines indicate interfaces, while the space between interfaces corresponds to layers.

[0031] At an interface, the energy bands may be described as stepped 160, kinked 161, or tangent 162, with the tangent 162 energy band implying that at least one of the adjacent layers must be of the nonlinear gradient type. If both adjacent layers are of the nonlinear gradient type, the tangent interface 162 may form a maximum or minimum value. For example, the stepped 160 shown in Figure 4A refers to a sudden change in composition and a sudden change in band gap, and may be called a heterojunction or heterointerface. A heterojunction is a discontinuous interface. For example, the kinks 161a and 161b shown in Figure 5 refer to a sudden change in slope. The tangents 162a and 162b refer to a sudden change in curvature, as illustrated, for example, in Figure 6A, where the slope is continuous across the interface. Both the kinked 161 interface and the tangent 162 interface are continuous interfaces. The tangent interfaces 162a, 162b may have a plane 140a, 140b or a linear gradient layer on one side. Alternatively, the tangent interface 162c may have smooth gradient layers on both sides of the interface, which can be identified by a change in the sign of curvature, for example, from positive to negative or vice versa (also known as an inflection point). Alternatively, the tangent interface 162 may have smooth gradient layers with the same sign of curvature on both sides of the interface, which can be identified by the maximum or minimum value between them. Note that step 160 can be combined with a kink 161 or a tangent 162 interface to produce a step-kink interface or a step-tangent interface. A kink-tangent interface is possible, implying smooth gradient layers on either side of the interface with the same sign of curvature but different magnitudes. A kink-step-tangent interface is also possible. In summary, a 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 band gap can be either a constant band gap (140) or a band gap (150). A band gap (150) can be a linear band gap (151) or a nonlinear band gap (152), i.e., a smooth band gap. A smooth band gap refers to an energy band that does not have a constant section, a kinked section, or a stepped section. An example of a constant band gap layer is shown in Figure 4A, where two band gaps (140a and 140b) of constant but unequal layers are adjacent to each other, forming a heterointerface. An example of a linear band gap layer (151) is shown in Figure 5A, where layers of constant band gaps (140a and 140b) are positioned on its sides. Finally, an example of a smooth band gap layer is shown in Figure 6A, where two parabolic band gap layers of opposite signs share a tangent interface, i.e., an inflection point, with the first and last interfaces adjacent to a constant band gap layer, i.e., a flat portion. In general terms, a single smooth gradient layer may have both signs of band curvature, or conversely, a single sign of curvature including local minimum or maximum values; however, by convention, in this specification, a single layer may have only one sign of slope, or one sign of curvature with no change in slope. This convention eliminates any ambiguity in the definition of a layer. According to this definition, for example, Figure 4A shows two layers and one interface, Figure 5A shows three layers and two interfaces, Figure 5A shows three layers and two interfaces, and Figure 6A shows four identifiable layers and three interfaces.

[0033] To understand the origin of band distortion in polarized semiconductors, it is necessary to understand something about the polarization field induced by the distortion. For example, in the InGaN / GaN system used in the QW / QB structure of blue LEDs, growth typically occurs on the N-polar c-plane of the wurtzite lattice. Increasing the mole fraction of In increases the lattice constant, inducing compressive stress, lattice elongation along the 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 accompanying compensation depend on both the magnitude and sign of the change in In composition. Figures 4B, 5B, and 6B show the distribution of free holes in the steep interface and gradient layer of Figures 4A, 5A, and 6A, respectively. In the step junction 160 of Figure 4A, all of the distortion-induced polarization is concentrated within the interface plane. As a result, all free carriers, i.e., holes, are also located at the interface, as shown in Figure 4B, thereby forming a sheet charge 170 and lowering the Fermi level below the valence band. To accommodate this change, the band on the high bandgap side (i.e., the left side) tilts upward, while the band on the low bandgap side (i.e., the right side) tilts downward, as shown in Figure 4C. In the linear gradient bandgap layer 151 with kinked interfaces 161a, 161b shown in Figure 5A, the sheet charge 170 is diffused into a uniform space charge 180, as shown in Figure 5B. At the interface shown in Figure 5C, the bands tilt, albeit to a lesser degree, as in the previous example, due to the decrease in carrier concentration resulting from the diffusion of the sheet charge. In the case of smooth gradient bandgap layers 152a and 152b with tangent interfaces 162a and 162c as shown in Figure 6A, the hole space charge 180 with a concentration gradient exists within layers 152a and 152b, with the 3D hole concentration starting at the sparse left interface 162a, increasing towards the steeply sloped interface 162c between the smooth gradient layers 152a and 152b, and decreasing again towards the opposite tangent interface 162b. In this case, the band curvature follows a similar trajectory as shown in Figure 6C, with a small amount of curvature added to the left side of the gradient layer, a large amount of curvature added in the middle, and a small amount of curvature added to the right side of the gradient layers 152a and 152b.It should be noted that the slope of the flat band portions in energy bands 140a and 140b occurs in all three cases, implying sheet charges, volume charges, or gradient negative charges (invisible) on either side of the constant bandgap layer, where the electric field may terminate. It is also important to note that such flat band portions in polarized MQW structures always slope in this way and often contribute to the QCSE within the QW region. For this reason, one of the objectives of the present invention is to minimize or eliminate such flat portions from the band structure 120. The above example also applies to GaN / AlGaN material systems, and other systems, where the sign of the strain is opposite to that of GaN / InGaN systems.

[0034] In exemplary embodiments, Figures 7A and 7B show the energy bands of a connected linear gradient V-shaped MQW structure 100 according to the present invention. Figure 7A shows the conduction band 120 and valence band 130 of two non-polarized wells (indicated by vertical dashed lines) that are symmetric around the band gap minimum at the center of well 200. Alternatively, the linear downward and upward gradients may include multiple layers of different gradients and may be asymmetric, and the alternatives are not limited. The first quantum levels 210a, 210b and wave functions 220a, 220b for electrons and holes are also shown, and it can be seen that they have good overlap in Figure 7A. However, the overlap is never perfect because the effective mass of holes is greater than the effective mass of electrons. Thus, the hole quantum level 210b is closer to the bottom of well 200 than the electron quantum level 210a, and has a substantially narrower well. Furthermore, the evanescent portion of the hole wavefunction 220b penetrates the V well less than the electron wavefunction 220a, resulting in a narrower well for the hole wavefunction 220b compared to the electron wavefunction 220a. These effects are offset by the fact that the hole band offset is smaller than the electron band offset, making the slope of the valence band 130 shallower than the slope of the conduction band 120, and substantially widening the well for holes compared to electrons. The shallower slope of the valence band 130 allows the hole wavefunction 220b to diffuse more than the electron wavefunction 220a, improving the overlap between the two. The balance of these conflicting effects depends on the specific composition and gradient of the layer and is best determined numerically.

[0035] Figure 7B shows a V-well 100 with a polarization effect incorporated according to one embodiment of the present invention. A downward linear gradient on the left side of well 200 generates a positive space charge, inducing negative curvature in energy bands 120 and 130. Conversely, an upward linear gradient on the right side of well 200 generates a negative space charge, inducing positive curvature in energy bands 120 and 130. The result appears as a slope at the bottom of the well, with the conduction band 120 and valence band 130 tilted in opposite directions. The first quantum levels 210a, 210b and wave functions 220a, 220b for electrons and holes are also shown and can be seen to be shifted in the opposite direction, reducing the magnitude of the overlap integral. Furthermore, the shape of well 200 changes, becoming substantially wider for both electrons and holes. The quantum levels of the wider well are located closer to the band edge, shifting the emission wavelength towards red. Therefore, QCSE remains valid for this band structure.

[0036] In exemplary embodiments, Figures 8A and 8B show the energy bands of a connected linear gradient Y-shaped MQW structure 100 according to the present invention. Figure 8A shows the conduction band 120 and valence band 130 of a non-polarized well that is symmetric around the band gap minimum at the center of well 200. Alternatively, the nonlinear downward and upward gradients may include multiple layers of different curvatures and may be asymmetric, and the alternatives are not limited. The first quantum levels 210a, 210b of electrons and holes, and the wave functions 220a, 220b are also shown and can be seen to have excellent overlap. However, the overlap is by no means perfect for the same reasons shown with respect to Figure 7A.

[0037] Figure 8B shows a Y-well 100 incorporating a polarization effect according to one or more embodiments of the present invention. A downward nonlinear gradient on the left side of well 200 generates a non-uniform positive space charge, inducing negative curvature in energy bands 120 and 130. Conversely, an upward nonlinear gradient on the right side of well 200 generates a non-uniform negative space charge, inducing positive curvature in energy bands 120 and 130. The magnitude of the negative and positive changes in curvature depends on the slope of the gradient, as described above. The result manifests as a slope at the bottom of the well, where the conduction band 120 and valence band 130 are inclined in opposite directions. The first quantum levels 210a, 210b and wave functions 220a, 220b of electrons and holes are shifted in opposite directions, reducing the magnitude of the overlap integral. Furthermore, the shape of well 200 changes, becoming substantially wider for both electrons and holes. The quantum levels in the wider well are located closer to the band edge, shifting the emission wavelength towards red. Therefore, QCSE remains valid for this band structure.

[0038] In exemplary embodiments, Figures 9A and 9B show the energy bands of a connected nonlinear gradient smooth-shape MQW structure 100 according to the present invention. Figure 9A shows the conduction band 120 and valence band 130 of a nonpolarized well 200 that is symmetric around the band gap minimum at the center of the well 200. Alternatively, the nonlinear downward and upward gradients may include multiple layers of different curvatures and may be asymmetric, and the alternatives are not limited. The first quantum levels 210a, 210b of electrons and holes, and the wave functions 220a, 220b are also shown, and it can be seen that they have a favorable overlap in Figure 7A. However, the overlap is by no means perfect for the same reasons shown with respect to Figure 7A.

[0039] Figure 9B shows a smooth well 100 incorporating a polarization effect according to the present invention. A downward nonlinear gradient on the left side of well 200 generates a non-uniform positive space charge, inducing negative curvature in energy bands 120 and 130. Conversely, an upward nonlinear gradient on the right side of well 200 generates a negative space charge, inducing positive curvature in energy bands 120 and 130. The magnitude of the negative and positive changes in curvature depends on the slope of the gradient, as described above. The result appears 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 levels 210a, 210b and wave functions 220a, 220b of electrons and holes are also shown, and it can be seen that they are shifted in the opposite direction, reducing the magnitude of the overlap integral. However, unlike the V-shaped and Y-shaped wells 200, the shape of well 200 remains substantially the same, and quantum levels 210a and 210b are located at approximately the same distance from the band edge. Therefore, the QCSE can be reduced, at least partially, for this band structure.

[0040] The linked QWs 200 within the MQW band structure 100 may have a single heterointerface between them, as intended in one or more embodiments of the present invention. In exemplary embodiments, Figures 10A–11B show the energy bands of a linked linear gradient asymmetric sawtooth MQW structure 100. Figure 10A shows the conduction band 120 and valence band 130 of a non-polarized forward sawtooth well 200 that is asymmetric around the band gap minimum at the right end of the well 200. Alternatively, one or more linear increasing gradient layers may be used to form a reverse sawtooth, as shown in Figure 11A. Furthermore, the linear downward gradient may include multiple layers of different gradients and / or portions of nonlinear gradients, and the alternatives are not limited. The first quantum levels of electrons and holes 210a, 210b, and wave functions 220a, 220b are also shown in Figures 10A and 11A, and it can be seen that they have excellent overlap. However, the overlap is by no means perfect, for the same reasons shown with respect to Figure 7A.

[0041] Figure 10B shows a linear sawtooth-shaped QW200 with a polarization effect incorporated according to the present invention. In Figure 10B, the downward linear gradient on the left side of the well 200 generates a uniform positive space charge, inducing negative curvature in energy bands 120 and 130. Conversely, the upward step gradient on the right side of the well 200 generates a negative sheet charge, inducing a more positive kink in energy bands 120 and 130. The magnitude of the negative change in curvature varies depending on the slope of the gradient, as described above, while the magnitude of the positive kink is proportional to the magnitude of the band gap step. Advantageously, the right side of the QW200 formed by the step heterojunction does not slope, thereby reducing the influence of the deformation of energy bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a, 210b and wavefunctions of electron 220a and hole 220b are also shown and are found to be shifted in the opposite direction, reducing the magnitude of the overlap integral. The curvature of the conduction band 130 becomes more negative, and the notch becomes narrower. As a result, the energy of the conduction band quantum level 210a increases, and the peak of the wavefunction 220a shifts towards the nearest QW200 interface. Also, the curvature of the valence band 130 becomes more negative, the notch becomes flatter, and a minimum value is formed further away from the nearest QW200 interface. As a result, the energy of the valence band quantum level 210b increases and approaches the conduction band 120. However, overall, the distance between quantum levels 210a and 220b remains substantially the same or increases slightly, resulting in a shift towards blue at the emission wavelength. Thus, QCSE can be partially reduced for this band structure. Depending on the gradient applied, the overlap between the wave functions of electron 220a and hole 220b can be reduced.

[0042] Figure 11B shows a linear reverse sawtooth QW200 with a polarization effect incorporated according to the present invention. The upward linear gradient on the left side of the well 200 generates a uniform negative space charge, inducing positive curvature in energy bands 120 and 130. Conversely, the downward step gradient on the right side of the well 200 generates a positive sheet charge, inducing a more negative kink in energy bands 120 and 130. The magnitude of the positive change in curvature depends on the slope of the gradient, as described above, while the magnitude of the negative kink is proportional to the magnitude of the band gap step. Advantageously, the right side of the QW200 formed by the step heterojunction does not slope, thereby reducing the influence of the deformation of energy bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a, 210b and wavefunctions of electron 220a and hole 220b are also shown and are found to be shifted in the opposite direction, reducing the magnitude of the overlap integral. The curvature of the conduction band 130 starts flatter and the notch becomes wider. As a result, the energy of conduction band quantum level 210a decreases and the peak of wavefunction 220a moves away from the nearest QW200 interface. Also, the curvature of the valence band 130 becomes more positive and the notch becomes sharper, forming a minimum closer to the nearest QW200 interface. As a result, the energy of valence band quantum level 210b increases and moves away from the conduction band 120. However, overall, the distance between quantum levels 210a and 220b remains substantially the same or increases slightly, resulting in a shift towards blue at the emission wavelength. Thus, QCSE can be partially reduced for this band structure. Depending on the gradient applied, the overlap between the wave functions of electrons 220a and holes 220b can be reduced. To mitigate this effect, a sawtooth profile with a nonlinear gradient may be used.

[0043] In exemplary embodiments, Figures 12A and 12B show the energy bands of a connected nonlinear gradient asymmetric forward sawtooth MQW structure 100 according to the present invention. Figure 12A shows the conduction band 120 and valence band 130 of a non-polarized well 200 that is asymmetric around the band gap minimum at the right end of each well 200. Alternatively, the nonlinear downward gradient may include multiple layers of different curvatures and / or portions of linear gradients, and the alternatives are not limited. The first quantum levels 210a, 210b of electrons and holes, and the wave functions 220a, 220b are also shown and are found to have excellent overlap. However, the overlap is never perfect for similar reasons shown with respect to Figure 7A.

[0044] Figure 12B shows a nonlinear sawtooth-shaped QW200 with a polarization effect incorporated according to the present invention. The downward nonlinear gradient on the left side of the well 200 generates a non-uniform positive space charge, inducing a more negative curvature in energy bands 120 and 130. Conversely, the upward step gradient on the right side of the well 200 generates a negative sheet charge, inducing a more positive kink in energy bands 120 and 130. The magnitude of the negative change in curvature depends on the slope of the gradient, as described above, while the magnitude of the positive kink is proportional to the magnitude of the step. Advantageously, the right side of the QW200 formed by the step heterojunction does not slope, thereby reducing the influence of the deformation of energy bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a, 210b and wavefunctions of electron 220a and hole 220b are also shown and are found to be shifted in the opposite direction, reducing the magnitude of the overlap integral. The curvature of the conduction band 130 becomes more negative, and the notch becomes narrower. As a result, the energy of the conduction band quantum level 210a increases, and the peak of the wavefunction 220a shifts towards the nearest QW200 interface. Also, the curvature of the valence band 130 becomes more negative, the notch becomes flatter, and a minimum value is formed further away from the nearest QW200 interface. As a result, the energy of the valence band quantum level 210b increases and approaches the conduction band 120. However, overall, the distance between quantum levels 210a and 210b remains substantially the same or increases slightly, resulting in a shift towards blue at the emission wavelength. Depending on the gradient used, the overlap between the wavefunctions of electron 220a and hole 220b may remain substantially the same. Therefore, QCSE can be reduced with respect to this band structure.

[0045] In exemplary embodiments, Figures 13A and 13B show the energy bands of a coupled nonlinear gradient-type asymmetric reverse sawtooth MQW structure 100. Figure 13A shows the conduction band 120 and valence band 130 of a non-polarized well that is asymmetric around the band gap minimum at the left end of well 200. Alternatively, the nonlinear upward gradient may include multiple layers of different curvatures and / or portions of linear gradients, and the alternatives are not limited. The first quantum levels of electrons and holes 210a, 210b, and wave functions 220a, 220b are also shown and are found to have excellent overlap. However, the overlap is never perfect for similar reasons shown with respect to Figure 7A.

[0046] Figure 13B shows a reverse-sawtooth QW100 with a polarization effect incorporated according to the present invention. The upward nonlinear gradient on the left side of well 200 generates a non-uniform negative space charge, inducing a more positive curvature in energy bands 120 and 130. Conversely, the downward step on the right side of well 200 generates a positive sheet charge, inducing a more positive kink in energy bands 120 and 130. The magnitude of the positive change in curvature depends on the slope of the gradient, as described above, while the magnitude of the positive kink is proportional to the magnitude of the step. Advantageously, the right side of QW200 formed by the step heterojunction is not sloped, thereby reducing the influence of the deformation of energy bands 120 and 130 on the shift of wave functions 220a and 220b. Nevertheless, the first quantum levels 210a, 210b and wave functions 220a, 220b for electrons and holes are also shown and are found to be shifted in the opposite direction, reducing the magnitude of the overlap integral. The curvature of the conduction band 120 becomes more negative, and the notch becomes wider. As a result, the energy of the conduction band quantum level 210a decreases, and the peak of the wave function 220a moves away from the nearest QW200 interface. The curvature of the valence band 130 also becomes more positive, and the notch becomes sharper, slightly shifting the hole wave function 220b toward the QW200 interface. As a result, the energy of the valence band quantum level 210b decreases, moving away from the conduction band 120. However, overall, the distance between quantum levels 210a and 210b remains substantially the same or increases slightly, resulting in a shift toward blue at the emission wavelength. Depending on the gradient used, the overlap between the wave functions of electrons 220a and holes 220b may remain substantially the same. Therefore, the QCSE can be reduced for this band structure. This MQW band structure 100 may have the additional advantage of an electric field incorporated into the valence band that promotes right-to-left carrier drift, thereby improving the uniformity of the hole distribution.

[0047] According to the present invention, meta-gradients of the period or composition of the MQW structure 100 are also intended for the purpose of improving the uniformity of the hole distribution. 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. Either the period of the linked QW 100 or the period of the QW / QB pair can vary, for example, being smaller on the n side and larger on the p side of the MQW structure 100, or vice versa. Such a heterogeneous period can coincide with the hole drift and / or diffusion profile, thereby promoting uniformity of the distribution. For example, the heterogeneous period of the linked QW can vary by a value of about 5 nm to about 15 nm. As used herein, the word "about" used with respect to thickness values ​​means ±0.01 nm. If QB is present, the period of the QW / QB pair can be changed by changing the thickness of the QW and / or QB. For example, the period of the QW / QB pair can vary from approximately 8 nm to approximately 20 nm, where the thickness of QB remains constant at 3 nm, and the thickness of QW varies from approximately 5 nm to approximately 17 nm. Alternatively, the period of the QW / QB pair may remain constant, while the ratio of QW to QB thicknesses changes. For example, the period of the QW / QB pair may be 15 nm, while the thickness of QB changes from approximately 5 nm to approximately 10 nm, and simultaneously, the thickness of QB changes from approximately 10 nm to approximately 5 nm.

[0048] With respect to composition, the bandgap range of a QW or QW / QB combination can vary, for example, being smaller on the n side and larger on the p side in MQW structure 100, or vice versa. The bandgap range can be calculated for each QW or QW / QB pair as the difference between the maximum bandgap and the minimum bandgap. Variation in the bandgap range can be achieved by varying the minimum and / or maximum bandgaps. Such compositional gradients can facilitate hole drift from the p side to the n side, thereby providing an overall internal electric field that promotes uniformity of distribution. For example, in a heterogeneous composition MQW structure, the minimum bandgap varies from about 2.4 eV to about 2.6 eV in the growth direction, and 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 word "about" used with respect to bandgap values ​​means ±1 meV. III-N material systems (AlN, GaN, InN, and their ternary and quaternary alloys) can produce a band gap between approximately 0.65 and approximately 6.1 eV.

[0049] In an alternative but equivalent method, the continuous gradient QW100 may be separated by the continuous gradient QB300 according to the present invention, with only one interface discontinuous between them. The QW100 and / or QB300 may be symmetric or asymmetric. Figures 14A to 14C show some examples of multiple QW / QB band structures where the interface is indicated by a dashed vertical line. In Figure 14A, the symmetric linear gradient QW200a, 200b are separated by the symmetric linear gradient QB300a to 300c. There is a kink at the interface between QW200a, 200b and QB300a to 300c. In Figure 14B, the symmetric smooth gradient QW200a, 200b are separated by the asymmetric smooth gradient QB300a to 300c. The interface between QW200a and QB300b is tangential, while the interface between QB300b and QW200b has a kink. In Figure 14C, the asymmetric smooth gradient type QW200a and 200b are separated by the asymmetric smooth gradient type and the linear gradient type QB300a to 300c. The interface between QW200a and QB300b has a positive kink, while the interface between QB300b and QW200b includes a negative kink, a downward step, and a positive kink. Other combinations of these elements can be used without limitation. In all cases, the well / barrier pair has only one discontinuous interface per well.

[0050] Barrierless QW200s having three or more gradient layers have been previously described, for example, in Figures 2I-2J illustrating four linear gradient layers, and in Figures 2M and 2N illustrating four smooth gradient layers with inflection points between some layers. Such QW200s may be symmetric or asymmetric. Whenever a QW200 includes multiple gradient layers as defined herein, the possibility arises to interpret at least one layer as a QB200 or a sublayer thereof. Figures 15A-15C illustrate various possible interpretations of an exemplary multilayer linear gradient MQW structure 100 according to the present invention. It is helpful to recall that the QW layers must contain the minimum band gap, and the QB layers, if present, must contain the maximum band gap. In the figures, vertical dashed lines represent the QW / QB interface. In Figure 15A, the band structure can be interpreted as multiple symmetrical, barrierless connected QW200s with the left and right wells only partially shown. In Figure 15B, the same band structure can be interpreted as a symmetrical 4-layer linear gradient QW200 sandwiched between a pair of 2-layer linear gradient QB300a and 300b. In this case, both positive and negative kinks exist within QW200. In Figure 15C, a pair of 4-layer linear gradient QB300a and 300b are positioned on the side of a symmetric V-well 200. Note that both positive and negative kinks exist within the barrier. In Figure 15D, the same band structure can be interpreted as an asymmetrical 3-layer linear gradient QW200 sandwiched between a pair of 3-layer linear gradient asymmetric QB300a and 300b. In this case, both positive and negative kinks exist in both QW200 and QB300a and 300b. The same interpretation is possible for nonlinear gradient layers, or combinations of linear and nonlinear gradient layers. However, in all cases, the interpretation conforms to the disclosed embodiment of either a continuous gradient connected QW or a continuous gradient QW surrounded by continuous gradient QBs without discontinuities in between.

[0051] Another example of the QW / QB interpretation is shown in Figures 16A-16C, which illustrate an asymmetric multilayer linear gradient sawtooth MQW band structure 100 according to the present invention. In Figure 16A, the band structure can be interpreted as multiple asymmetric multilayer linear gradient sawtooth QW100. In Figure 116B, the same band structure can be interpreted as a two-layer linear gradient asymmetric QW200a sandwiched between pairs of single-layer linear gradient QB300a, 300b. In Figure 116C, the same band structure can be interpreted as a single-layer linear gradient asymmetric QW200a sandwiched between pairs of two-layer linear gradient QB300a, 300b. In summary, any continuous gradient periodic band structure, i.e., a band structure without a constant bandgap, can be interpreted as either a continuous gradient QW200 surrounded by connected QW200s, continuous gradient QB200s without discontinuities in between, or a combination of continuous gradient QW / QB pairs with only one discontinuity interface per pair, and is within the scope of this disclosure. Note that adding a second discontinuity interface to each QW / QB pair, as shown in Figure 16D, generates a more conventional gradient QW / QB pair, which is outside the scope of this disclosure.

[0052] QB300 may be placed between the n-type electron injection layer and the MQW100 active region, and between the MQW100 active region and the p-type electron blocking layer and / or p-type hole injection layer. However, other transition layers, including the same or different first and last QB300, may be used without limitation. In this specification, any type of transition layer between the MQW active region and adjacent layers is considered.

[0053] EBL layers can be used to reduce or eliminate electron leakage from the active region to the p-injection layer. EBL layers may have a shape similar to the QB in Figures 3A-3I, a larger bandgap than the QB, and a slightly larger thickness than the QB. In one embodiment, the EBL may include a constant-bandgap AlGaN layer, as shown in Figure 3A. However, EBLs with step discontinuities or kinks may suffer polarization effects, including a reduction in barrier height.

[0054] In an alternative embodiment, an EBL with a strictly increasing smooth gradient band gap grows on the MQW region. In the first embodiment shown in Figures 17A and 17B, the nonlinear reverse sawtooth QW100 of Figure 13A is located on an n-injection layer 170 and terminates at the horizontal tangential energy band on the p-type side. EBL400 is located on the MQW region 100, followed by a p-injection layer 180. In this case, the energy band of EBL400 is tangent to the MQW100 energy bands 120 and 130 at the MQW100 / BL400 interface 162a, as shown in Figure 17A. The gradient EBL band gap increases parabolic to an inflection point 162c 152a, and then decreases parabolic to a tangent point 162b with the maximum band gap of EBL162d 152b. Near the interface of the EBL400 / p injection layer 180, a similar smooth gradient band gap may be used so that energy bands 120 and 130 are in contact at the interface. Alternatively, a step or kink or otherwise unsmooth gradient may be used at or near the interface of the EBL400 / p injection layer 180. Note that, as with the reverse sawtooth QW described above, only one discontinuity is permitted in the energy band and must be located on the p side of the EBL. Figure 17B shows the MQW100 / EBL400 of Figure 17A with the polarization effect incorporated according to the present invention. The resulting energy bands 120 and 130 avoid notching at the MQW100 / EBL400 interface and reduction of the EBL height 162e. The EBL height 162e remains greater than or equal to the non-polarization barrier height 162d.

[0055] As a practical matter, when the MQW uses a composition profile in which the final part is rich in In, such as a forward sawtooth profile (see Figure 10A), it may be necessary to provide a capping layer to avoid In desorption before AlGaN EBL growth. The capping function can be provided by the transition layer described above. A typical capping layer may be 10 nm thick GaN or other alloy with a low In content. In this case, the energy band is horizontal and tangential at the capping layer / EBL interface, as in the case of Figure 17A.

[0056] In alternative embodiments, if the energy band of QW terminates with a positive gradient on the p side, for example, in a V profile (Figure 7A) or an inverted sawtooth profile (mirror image of Figure 11A), the EBL gradient may coincide with (i.e., be in contact with) the positive gradient of QW at the interface. In the exemplary embodiments shown in Figures 18A and 18B, the linear gradient inverted sawtooth QW100 of Figure 11A is located on an n-injection layer 170 and terminates with a positively sloped conduction band (negatively sloped valence band) on the p-type side. EBL400 is located on the MQW region 100, followed by a p-injection layer 180. In this case, the energy bands 120 and 130 of EBL400 are in contact with the MQW100 energy bands 120 and 130 at the MQW100 / BL400 interface, as shown in Figure 18A. The gradient EBL band gap increases parabolic to the inflection point 162c 152a, then parabolic to the tangent point 162b, which has the maximum band gap at EBL 162d. A discontinuous band gap may be used at the interface of EBL 400 / p injection layer 180. Alternatively, a kink or otherwise unsmooth gradient may be used near the interface of EBL 400 / p injection layer 180. Note that in this embodiment, only one discontinuity is permitted in the energy band, and it is on the p side of the EBL. Figure 18B shows the MQW 100 / EBL 400 of Figure 18A with the polarization effect incorporated according to the present invention. The resulting energy bands 120, 130 avoid a notch at the MQW 100 / EBL 400 interface and a reduction in the EBL height 162e. The EBL height 162e remains greater than or equal to the non-polarization barrier height 162d. Advantageously, the step junction on the p side moves the conduction band 120 upward, thereby increasing the EBL height 162e.

[0057] While specific configurations of the structure are shown for the purpose of presenting the basic structure of the present invention, those skilled in the art will understand that other variations are still possible and may still fall within the scope of the appended claims. Those skilled in the art will readily recall additional advantages and modifications. Therefore, in its broader aspects, the present invention is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications can be made without departing from the spirit or scope of the general inventive concept defined by the appended claims and their equivalents.

Claims

1. A light-emitting device, n-type semiconductor layer, An active region disposed on the n-type semiconductor layer, wherein the active region comprises a plurality of connected quantum wells, and each connected quantum well is A strictly increasing band gap that extends from the minimum value to the maximum value, Each connected quantum well has only one bandgap discontinuity, The active region having an energy band structure including, A p-type semiconductor layer arranged in the active region, The light-emitting device comprising the above.

2. The light-emitting device according to claim 1, further comprising 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.

3. The light-emitting device according to claim 1, wherein the 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 connected quantum wells has a period that varies between about 5 nm and about 20 nm, and at least one connected quantum well has a period that is non-uniform with respect to at least one other period.

5. The light-emitting device according to claim 1, wherein each of the plurality of connected quantum wells includes a band gap range that varies between about 0.1 eV and about 3.5 eV, and at least one connected quantum well has a band gap range that is non-uniform with respect to at least one other band gap range.

6. A light-emitting device, n-type semiconductor layer, An active region including a plurality of quantum wells arranged in the n-type semiconductor layer, An electron barrier layer having a first interface and a second interface arranged in the active region, A strictly increasing smooth gradient band gap extending from the first interface to the maximum value, There is only one band gap discontinuity, The electron barrier layer having an energy band structure including, A p-type semiconductor layer arranged in the active region, The light-emitting device comprising the above.

7. A light-emitting device, n-type semiconductor layer, An active region disposed on the n-type semiconductor layer, A plurality of quantum wells, each of which includes a strictly increasing band gap that increases from a minimum value, A quantum barrier that separates each quantum well, wherein each of the quantum barriers includes a band gap that decreases exactly from a maximum value, An interface that connects the strictly increasing band gap and the strictly decreasing band gap that form the energy band, Includes, The energy band at the interface is discontinuous at only one interface per quantum well. The active region and, A p-type semiconductor layer arranged in the active region, The light-emitting device comprising the above.

8. The light-emitting device according to claim 6, further comprising 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 connected quantum well and adjacent quantum barrier has a period that varies between approximately 5 nm and approximately 25 nm, and at least one connected quantum well and adjacent quantum barrier has a period that is non-uniform with respect to at least one other period.

10. The light-emitting device according to claim 6, wherein each connected quantum well and adjacent quantum barrier includes a quantum well thickness to quantum barrier thickness ratio that varies between about 0.05 and about 0.95, and at least one quantum well and adjacent quantum barrier have an uneven ratio with respect to at least one other ratio.

11. The light-emitting device according to claim 6, wherein each quantum well and adjacent quantum barrier includes a bandgap range that varies between approximately 0.1 eV and approximately 3.5 eV, and at least one quantum well and adjacent quantum barrier have a bandgap range that is non-uniform with respect to at least one other bandgap range.

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

Images