Micro-led epitaxial wafer and preparation method thereof

Abstract

The application relates to the technical field of semiconductors, and particularly discloses a MicroLED epitaxial wafer and a preparation method thereof. The epitaxial wafer comprises a P-type regulation layer, the P-type regulation layer comprises a first superlattice layer, a second superlattice layer and a third superlattice layer arranged in sequence along an epitaxial direction; the first superlattice layer comprises periodically and alternately stacked first Mg-doped InAlGaN layers and first InGaN layers; the second superlattice layer comprises periodically and alternately stacked second Mg-doped InAlGaN layers, first AlGaN layers and h-BN layers; and the third superlattice layer comprises periodically and alternately stacked third Mg-doped InAlGaN layers, second AlGaN layers and AlN layers; and the Mg-doped concentration of the first, second and third Mg-doped InAlGaN layers increases in sequence along the epitaxial direction. The epitaxial wafer can effectively improve the radiation recombination efficiency of the MicroLED under a low current density.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and in particular to a MicroLED epitaxial wafer and its fabrication method. Background Technology

[0002] In recent years, LEDs have shown broad application prospects in many fields such as display, lighting, communication, and biomedicine due to their advantages of high brightness, long lifespan, and low power consumption. As application demands evolve towards miniaturization and integration, chip sizes continue to shrink to the micrometer level, thus giving rise to MicroLEDs.

[0003] However, the reduction in device size leads to a significant decrease in peak external quantum efficiency, while the optimal operating current density is forced to shift to higher levels. This change makes it difficult to achieve the low-power driving scenarios required for display applications (operating current density of 0.01 A / cm²). 2 ~0.5A / cm 2 MicroLEDs are clearly inefficient, and existing LED epitaxial structures are mostly designed for high current densities (such as 20A / cm²). 2 ~40 A / cm 2 Optimizing the efficiency peak at the current level, if directly applied to MicroLED, will not achieve effective efficiency improvement, thus affecting the widespread application of MicroLED. Summary of the Invention

[0004] The purpose of this invention is to provide a MicroLED epitaxial wafer and its preparation method, addressing the current state of existing technology.

[0005] This invention improves hole injection efficiency and reduces electron overflow by cooperating with the first, second, and third superlattice layers in the P-type control layer, thereby enabling more electrons and holes to recombine effectively in the light-emitting layer and reducing non-radiative recombination losses caused by dislocation defects, thus effectively improving the radiative recombination efficiency of MicroLEDs at low current densities.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a MicroLED epitaxial wafer, comprising a substrate and an epitaxial layer disposed on the substrate, wherein the epitaxial layer comprises a buffer layer, an undoped GaN layer, an N-type layer, a light-emitting layer, a P-type control layer and a P-type GaN layer disposed sequentially along the epitaxial direction; The P-type control layer includes a first superlattice layer, a second superlattice layer, and a third superlattice layer arranged sequentially along the epitaxial direction; The first superlattice layer comprises a first Mg-doped InAlGaN layer and a first InGaN layer that are periodically and alternately stacked; The second superlattice layer comprises a periodically alternating second Mg-doped InAlGaN layer, a first AlGaN layer, and an h-BN layer; The third superlattice layer comprises a periodically alternating stacked third Mg-doped InAlGaN layer, a second AlGaN layer, and an AlN layer; The Mg doping concentration of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer increases sequentially along the epitaxial direction.

[0007] In some embodiments, the number of periods of the first superlattice layer, the second superlattice layer and the third superlattice layer increases sequentially, and the number of periods of each layer is greater than 1. In the first superlattice layer, the Mg doping concentration of the first Mg-doped InAlGaN layer in each period generally increases from a1 to a2. In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer in each period generally increases from b1 to b2. In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer in each period generally increases from c1 to c2, wherein c2-c1 > b2-b1 > a2-a1.

[0008] In some embodiments, the Mg doping concentration of the first Mg-doped InAlGaN layer in the first superlattice layer is 1.0 × 10⁻⁶. 18 cm -3 ~5.0×10 18 cm -3 The number of cycles is 2 to 3; In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer is 5.0 × 10⁻⁶. 18 cm -3 ~1.0×10 19 cm -3 The number of cycles is 3 to 6; In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer is 1.0 × 10⁻⁶. 19 cm -3 ~8.0×10 19 cm -3 The number of cycles is 4 to 8.

[0009] In some embodiments, the Ga content of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer decrease sequentially, while the In content increases sequentially.

[0010] In some embodiments, the Ga content of the first Mg-doped InAlGaN layer is x1 and the In content is y1; the Ga content of the second Mg-doped InAlGaN layer is x2 and the In content is y2; the Ga content of the third Mg-doped InAlGaN layer is x3 and the In content is y3, wherein 0.5≤x3<x2<x1≤0.8 and 0.01≤y1<y2<y3≤0.1.

[0011] In some embodiments, the Al content in the first AlGaN layer and the second AlGaN layer increases sequentially.

[0012] In some embodiments, in the first superlattice layer, the thickness of a single Mg-doped InAlGaN layer is 1 nm to 5 nm, and the thickness of a single InGaN layer is 1 nm to 5 nm. In the second superlattice layer, the thickness of a single second Mg-doped InAlGaN layer is 1 nm to 5 nm, the thickness of a single first AlGaN layer is 1 nm to 5 nm, and the thickness of a single h-BN layer is 1 nm to 5 nm. In the third superlattice layer, the thickness of a single third Mg-doped InAlGaN layer is 3nm~10nm, the thickness of a single second AlGaN layer is 3nm~10nm, and the thickness of a single AlN layer is 3nm~10nm.

[0013] This invention also provides a method for preparing a MicroLED epitaxial wafer, comprising: Provide substrate; An epitaxial layer is deposited on the substrate; The epitaxial layer includes a buffer layer, an undoped GaN layer, an N-type layer, a light-emitting layer, a P-type control layer, and a P-type GaN layer arranged sequentially along the epitaxial direction; The P-type control layer includes a first superlattice layer, a second superlattice layer, and a third superlattice layer arranged sequentially along the epitaxial direction; The first superlattice layer comprises a first Mg-doped InAlGaN layer and a first InGaN layer that are periodically and alternately stacked; The second superlattice layer comprises a periodically alternating second Mg-doped InAlGaN layer, a first AlGaN layer, and an h-BN layer; The third superlattice layer comprises a periodically alternating stacked third Mg-doped InAlGaN layer, a second AlGaN layer, and an AlN layer; The Mg doping concentration of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer increases sequentially along the epitaxial direction.

[0014] In some embodiments, the number of periods of the first superlattice layer, the second superlattice layer, and the third superlattice layer increases sequentially, and the number of periods is greater than 1. In the first superlattice layer, the Mg doping concentration of the first Mg-doped InAlGaN layer in each period generally increases from a1 to a2. In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer in each period generally increases from b1 to b2. In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer in each period generally increases from c1 to c2, wherein c2-c1 > b2-b1 > a2-a1.

[0015] In some embodiments, the Ga content of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer decreases sequentially, while the In content increases sequentially; and / or, The first Mg-doped InAlGaN layer has a Ga content of x1 and an In content of y1; the second Mg-doped InAlGaN layer has a Ga content of x2 and an In content of y2; the third Mg-doped InAlGaN layer has a Ga content of x3 and an In content of y3, wherein 0.5 ≤ x3 < x2 < x1 ≤ 0.8, and 0.01 ≤ y1 < y2 < y3 ≤ 0.1.

[0016] The beneficial effects of this invention are as follows: In this invention, a P-type control layer composed of a first superlattice layer, a second superlattice layer, and a third superlattice layer is disposed between the light-emitting layer and the P-type GaN layer. Specifically, on the side of the P-type control layer closest to the light-emitting layer, a first superlattice layer is disposed, consisting of a first Mg-doped InAlGaN layer and a first InGaN layer stacked alternately. On the side of the P-type control layer closest to the P-type GaN layer, a third superlattice layer is disposed, consisting of a third Mg-doped InAlGaN layer, a second AlGaN layer, and an AlN layer stacked alternately. A second superlattice layer consisting of a second Mg-doped InAlGaN layer, a first AlGaN layer, and an h-BN layer stacked alternately is disposed between the first and third superlattice layers. The Mg doping concentration of the first, second, and third Mg-doped InAlGaN layers increases sequentially along the epitaxial direction. The multiple superlattice layers cooperate with each other. On one hand, this invention provides a first superlattice layer on the side of the P-type control layer near the light-emitting layer (using an InGaN / GaN superlattice structure), consisting of a first Mg-doped InAlGaN layer and a first InGaN layer stacked periodically, to improve the lattice matching between the light-emitting layer and the P-type control layer and reduce the upward extension of dislocation defects. Subsequently, a second superlattice layer is introduced on the first superlattice layer, consisting of a second Mg-doped InAlGaN layer, a first AlGaN layer, and an h-BN layer stacked periodically. The h-BN layer is bound by weak van der Waals forces, effectively blocking dislocations accumulated from the underlying growth layer. Then, a... The third superlattice layer, composed of a third Mg-doped InAlGaN layer, a second AlGaN layer, and an AlN layer stacked periodically, exhibits better AlN density, enabling the bending and merging of dislocations through a high-density interface. Simultaneously, the P-type control layer utilizes the periodic stacking of multiple superlattice layers to repeatedly bend, interact with, and annihilate residual penetrating dislocations, causing the dislocation density to decrease layer by layer during propagation. Thus, the P-type control layer of this invention effectively improves the overall crystal quality, reduces dislocation defects in the P-type control layer, and reduces nonradiative recombination of charge carriers at low current densities through the mutual cooperation between multiple superlattice layers, thereby improving the radiative recombination efficiency at low current densities.

[0017] On the other hand, this invention introduces Mg-doped InAlGaN material layers (first Mg-doped InAlGaN layer, second Mg-doped InAlGaN layer, and third Mg-doped InAlGaN layer) into each superlattice layer of the P-type control layer to replenish holes. Compared to AlGaN and GaN material layers, the InAlGaN material layer contains In, which is beneficial to improving the Mg doping efficiency and increasing the hole concentration. Combined with the increasing Mg doping concentration along the epitaxial direction, it reduces the valence band barrier spike caused by piezoelectric polarization, reduces hole injection resistance, and increases the number and efficiency of holes injected into the light-emitting layer. At the same time, the first superlattice layer on the side of the P-type control layer closest to the light-emitting layer has a lower barrier than other superlattice layers, further reducing the difficulty of hole injection and increasing the number and efficiency of holes injected into the light-emitting layer. In the third superlattice layer on the side of the P-type control layer closest to the P-type GaN layer, the polarization effect at the interface between the AlN layer and the third Mg-doped InAlGaN layer and the second AlGaN layer, together with the space charge region formed by the high concentration of Mg (the third superlattice layer has the highest Mg doping concentration in the P-type control layer) ionized acceptors, generates Coulomb repulsion force on electrons. Combined with the high potential barrier of the third superlattice layer compared to other superlattice layers, electrons are confined in the light-emitting layer. This allows the P-type control layer to increase the hole injection concentration while reducing electron overflow from the light-emitting layer. As a result, more electrons and more holes recombine effectively in the light-emitting layer, thereby improving the radiative recombination efficiency at low current densities.

[0018] Therefore, this invention improves hole injection efficiency and reduces electron overflow by cooperating with the first, second, and third superlattice layers in the P-type control layer, so that more electrons and more holes can recombine effectively in the light-emitting layer, and reduces non-radiative recombination loss caused by dislocation defects, thereby effectively improving the radiative recombination efficiency of MicroLED at low current density. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a MicroLED epitaxial wafer according to an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of the structure of the P-type control layer in an embodiment of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in further detail below.

[0022] See Figure 1 and Figure 2 As shown, the present invention discloses a MicroLED epitaxial wafer, including a substrate 1 and an epitaxial layer disposed on the substrate 1. The epitaxial layer includes a buffer layer 2, an undoped GaN layer 3, an N-type layer 4, a light-emitting layer 5, a P-type control layer 6 and a P-type GaN layer 7 disposed sequentially along the epitaxial direction. The P-type control layer 6 includes a first superlattice layer 61, a second superlattice layer 62 and a third superlattice layer 63 arranged sequentially along the epitaxial direction; The first superlattice layer 61 includes a periodically alternating first Mg-doped InAlGaN layer 611 and a first InGaN layer 612; The second superlattice layer 62 includes a periodically alternating second Mg-doped InAlGaN layer 621, a first AlGaN layer 622, and an h-BN layer 623; The third superlattice layer 63 includes a periodically alternating third Mg-doped InAlGaN layer 631, a second AlGaN layer 632, and an AlN layer 633. The Mg doping concentration of the first Mg-doped InAlGaN layer 611, the second Mg-doped InAlGaN layer 621, and the third Mg-doped InAlGaN layer 631 increases sequentially along the epitaxial direction.

[0023] In this invention, a P-type control layer 6, composed of a first superlattice layer 61, a second superlattice layer 62, and a third superlattice layer 63, is disposed between the light-emitting layer 5 and the P-type GaN layer 7. Specifically, on the side of the P-type control layer 6 closest to the light-emitting layer 5, a first superlattice layer 61, formed by periodically alternating layers of a first Mg-doped InAlGaN layer 611 and a first InGaN layer 612, is disposed. On the side of the P-type control layer 6 closest to the P-type GaN layer 7, a third Mg-doped InAlGaN layer 631, a second AlGaN layer 632, and an AlN layer 63 are disposed. A third superlattice layer 63, periodically alternatingly stacked, is formed between the first superlattice layer 61 and the third superlattice layer 63. This second superlattice layer 62 is composed of a second Mg-doped InAlGaN layer 621, a first AlGaN layer 622, and an h-BN layer 623, periodically alternatingly stacked. The Mg doping concentration of the first Mg-doped InAlGaN layer 611, the second Mg-doped InAlGaN layer 621, and the third Mg-doped InAlGaN layer 631 increases sequentially along the epitaxial direction. The multiple superlattice layers cooperate with each other. Specifically: On one hand, the present invention provides a first superlattice layer 61, composed of a first Mg-doped InAlGaN layer 611 and a first InGaN layer 612, periodically stacked alternately on the side of the P-type control layer 6 near the light-emitting layer 5 (which adopts an InGaN / GaN superlattice structure), to improve the lattice matching degree between the light-emitting layer 5 and the P-type control layer 6 and reduce the upward extension of dislocation defects. Subsequently, a second superlattice layer 62, composed of a second Mg-doped InAlGaN layer 621, a first AlGaN layer 622, and an h-BN layer 623, is introduced on the first superlattice layer 61. The h-BN layer 623 is bonded by weak van der Waals forces, which can effectively block dislocations accumulated from the bottom layer. Then, on the second superlattice layer 62... On lattice layer 62, a third superlattice layer 63 is deposited, which is formed by periodically alternating layers of a third Mg-doped InAlGaN layer 631, a second AlGaN layer 632, and an AlN layer 633. The AlN has better density and can bend and merge dislocations through the high-density interface. At the same time, the P-type control layer 6 uses the periodic stacking of multiple superlattice layers to repeatedly bend, interact with, and annihilate residual penetrating dislocations, so that the dislocation density decreases layer by layer during propagation. Thus, the P-type control layer 6 of the present invention effectively improves the overall crystal quality, reduces dislocation defects in the P-type control layer 6, reduces nonradiative recombination of charge carriers at low current densities, and thus improves the radiative recombination efficiency at low current densities through the mutual cooperation between multiple superlattice layers.

[0024] On the other hand, this invention introduces Mg-doped InAlGaN material layers (first Mg-doped InAlGaN layer 611, second Mg-doped InAlGaN layer 621, and third Mg-doped InAlGaN layer 631) into each superlattice layer of the P-type control layer 6 to replenish holes. Compared to AlGaN and GaN material layers, the InAlGaN material layers contain In, which is beneficial to improving the Mg doping efficiency and increasing the hole concentration. Combined with the increasing Mg doping concentration along the epitaxial direction, the valence band barrier peak caused by piezoelectric polarization is reduced, hole injection resistance is decreased, and the number and efficiency of holes injected into the light-emitting layer 5 are increased. At the same time, the first superlattice layer 61 on the side of the P-type control layer 6 closest to the light-emitting layer 5 has a lower barrier than other superlattice layers, further reducing the difficulty of hole injection and increasing the number and efficiency of holes injected into the light-emitting layer 5. In the third superlattice layer 63 on the side of the P-type control layer 6 close to the P-type GaN layer 7, the polarization effect at the interface between the AlN layer 633 and the third Mg-doped InAlGaN layer 631 and the second AlGaN layer 632, together with the space charge region formed by the ionized acceptors of high-concentration Mg (the Mg doping concentration of the third superlattice layer 63 is the highest in the P-type control layer 6), generates Coulomb repulsion force on electrons. Combined with the high potential barrier of the third superlattice layer 63 compared to other superlattice layers, electrons are confined in the light-emitting layer 5. This allows the P-type control layer 6 to increase the hole injection concentration while reducing the electron overflow from the light-emitting layer 5. As a result, more electrons and more holes recombine effectively in the light-emitting layer 5, thereby improving the radiative recombination efficiency at low current density.

[0025] Therefore, by cooperating with the first superlattice layer 61, the second superlattice layer 62 and the third superlattice layer 63 in the P-type control layer 6, the present invention improves the hole injection efficiency and reduces electron overflow, so that more electrons and more holes can be effectively recombine in the light-emitting layer 5, and reduces the non-radiative recombination loss caused by dislocation defects, thereby effectively improving the radiative recombination efficiency of MicroLED at low current density.

[0026] The substrate 1 can be any one of sapphire substrate, silicon carbide substrate, silicon substrate, gallium nitride substrate, and zinc oxide substrate, but is not limited to this.

[0027] Preferably, the Mg doping concentration of the first Mg-doped InAlGaN layer 611, the second Mg-doped InAlGaN layer 621, and the third Mg-doped InAlGaN layer 631 increases in a stepwise manner along the epitaxial direction.

[0028] In some embodiments, see Figure 1 and Figure 2 As shown, the number of periods of the first superlattice layer 61, the second superlattice layer 62 and the third superlattice layer 63 increases sequentially, and the number of periods is greater than 1 for each of them. In the first superlattice layer 61, the Mg doping concentration of the first Mg-doped InAlGaN layer 611 in each period generally increases from a1 to a2. In the second superlattice layer 62, the Mg doping concentration of the second Mg-doped InAlGaN layer 621 in each period generally increases from b1 to b2. In the third superlattice layer 63, the Mg doping concentration of the third Mg-doped InAlGaN layer 631 in each period generally increases from c1 to c2, wherein c2-c1 > b2-b1 > a2-a1.

[0029] Understandably, in the first superlattice layer 61, the Mg doping concentration of the first Mg-doped InAlGaN layer 611 in each period generally increases from a1 to a2. Taking a period of 2 in the first superlattice layer 61 as an example, the Mg doping concentration of the first Mg-doped InAlGaN layer 611 in the first period is a1, and the Mg doping concentration of the first Mg-doped InAlGaN layer 611 in the second period is a2. The corresponding explanations for the second Mg-doped InAlGaN layer 621 in the second superlattice layer 62 and the third Mg-doped InAlGaN layer 631 in the third superlattice layer 63 are similar and will not be repeated here.

[0030] By increasing the number of periods in the first superlattice layer 61, the second superlattice layer 62, and the third superlattice layer 63, and increasing the Mg doping concentration in each superlattice layer, combined with the design of c2-c1 > b2-b1 > a2-a1, on the one hand, it is beneficial to drive holes to be injected into the light-emitting layer 5 (along the direction of decreasing Mg doping concentration), so that holes can still be efficiently injected into the light-emitting layer 5 under low current density, ensuring that more electrons and more holes can recombine effectively in the light-emitting layer 5; on the other hand, it can avoid excessive changes in Mg doping concentration between adjacent superlattice layers or within superlattice layers (single-layer Mg-doped InAlGaN material layer or adjacent Mg-doped InAlGaN material layer), which would lead to an increase in defect density and reduce non-radiative recombination, thereby improving the overall radiative recombination efficiency.

[0031] In some embodiments, see Figure 1 and Figure 2 As shown, in the first superlattice layer 61, the Mg doping concentration of the first Mg-doped InAlGaN layer 611 is 1.0 × 10⁻⁶. 18 cm -3 ~5.0×10 18 cm -3 The number of cycles is 2 to 3; In the second superlattice layer 62, the Mg doping concentration of the second Mg-doped InAlGaN layer 621 is 5.0 × 10⁻⁶. 18 cm -3 ~1.0×10 19cm -3 The number of cycles is 3 to 6; In the third superlattice layer 63, the Mg doping concentration of the third Mg-doped InAlGaN layer 631 is 1.0 × 10⁻⁶. 19 cm -3 ~8.0×10 19 cm -3 The number of cycles is 4 to 8.

[0032] It is understandable that the Mg doping concentration ranges of the Mg-doped InAlGaN material layers (first Mg-doped InAlGaN layer 611, second Mg-doped InAlGaN layer 621, and third Mg-doped InAlGaN layer 631) in the above superlattice layers overlap. During fabrication, it is only necessary to ensure that the Mg doping concentration of the first Mg-doped InAlGaN layer 611, the second Mg-doped InAlGaN layer 621, and the third Mg-doped InAlGaN layer 631 increases sequentially along the epitaxial direction.

[0033] The doping concentration of the third Mg-doped InAlGaN layer 631 should not be too high, otherwise it will easily increase the defect density and thus lead to an increase in non-radiative recombination; the Mg doping concentration of the third Mg-doped InAlGaN layer 631 should not be too low, otherwise the hole injection efficiency improvement effect brought about by the design of increasing Mg doping concentration will be weak.

[0034] The Mg doping concentration of the first Mg-doped InAlGaN layer 611 should not be too high, otherwise it will easily increase the defect density, which will lead to an increase in non-radiative recombination and leakage current. The Mg doping concentration of the first Mg-doped InAlGaN layer 611 should not be too low, otherwise it will be difficult to form an effective concentration gradient driving force at the interface between the light-emitting layer 5 and the P-type control layer 6, which will affect the hole injection efficiency.

[0035] For example, the Mg doping concentration of the first Mg-doped InAlGaN layer 611 can be 1.0 × 10⁻⁶. 18 cm -3 1.2×10 18 cm -3 1.5×10 18 cm -3 1.8×10 18 cm -3 2.0×10 18 cm -3 2.2×10 18 cm -3 2.5×10 18 cm -3 2.8×10 18 cm -33.0×10 18 cm -3 3.2×10 18 cm -3 3.5×10 18 cm -3 3.8×10 18 cm -3 4.0×10 18 cm -3 4.2×10 18 cm -3 4.5×10 18 cm -3 4.8×10 18 cm -3 Or 5.0×10 18 cm -3 However, it is not limited to this; the number of cycles can be 2 or 3, but it is not limited to this.

[0036] For example, the Mg doping concentration of the second Mg-doped InAlGaN layer 621 can be 5.0 × 10⁻⁶. 18 cm -3 5.5×10 18 cm -3 5.8×10 18 cm -3 6.0×10 18 cm -3 6.5×10 18 cm -3 6.8×10 18 cm -3 7.0×10 18 cm -3 7.5×10 18 cm -3 7.8×10 18 cm -3 8.0×10 18 cm -3 8.5×10 18 cm -3 8.8×10 18 cm -3 9.0×10 18 cm -3 9.5×10 18 cm -3 9.8×10 18 cm -3 Or 1.0×10 19 cm -3 However, it is not limited to this; the number of cycles can be 3, 4, 5 or 6, but is not limited to this.

[0037] For example, the Mg doping concentration of the third Mg-doped InAlGaN layer 631 can be 1.0 × 10⁻⁶. 19 cm -3 1.5×10 19 cm -3 2.0×10 19 cm -3 2.5×10 19 cm -3 3.0×10 19 cm -3 3.5×10 19 cm -3 4.0×10 19 cm -3 4.5×10 19 cm -3 5.0×10 19 cm -3 5.5×10 19 cm -3 6.0×10 19 cm -3 6.5×10 19 cm -3 7.0×10 19 cm -3 7.5×10 19 cm -3 Or 8.0×10 19 cm -3 However, it is not limited to this; the number of cycles can be 4, 5, 6, 7, or 8, but is not limited to this.

[0038] Preferably, the Mg doping concentration of the third Mg-doped InAlGaN layer 631 is lower than the Mg doping concentration in the P-type GaN layer 7.

[0039] In some embodiments, the thickness of the p-type GaN layer 7 is 10 nm to 50 nm, and the doping concentration of Mg is 1.0 × 10⁻⁶. 20 cm -3 ~1.0×10 21 cm -3 .

[0040] In some embodiments, see Figure 1 and Figure 2 As shown, the Ga content of the first Mg-doped InAlGaN layer 611, the second Mg-doped InAlGaN layer 621, and the third Mg-doped InAlGaN layer 631 decreases sequentially, while the In content increases sequentially.

[0041] On the one hand, the decreasing Ga content in the corresponding Mg-doped InAlGaN material layers of the first superlattice layer 61, the second superlattice layer 62, and the third superlattice layer 63 is beneficial to improving the lattice matching degree between the corresponding Mg-doped InAlGaN material layer and other sublayers within the superlattice layer, thereby improving the overall crystal quality of the P-type control layer 6 and reducing non-radiative recombination. On the other hand, the decreasing Ga content combined with the increasing In content helps to improve the ionization efficiency of the Mg dopant layer by layer, ensuring that sufficient holes can still be efficiently ionized even at high doping concentrations (especially in the third Mg-doped InAlGaN layer 631 with a relatively high doping concentration), thereby improving the hole concentration and injection efficiency.

[0042] In some embodiments, the Ga content of the first Mg-doped InAlGaN layer 611 is x1 and the In content is y1; the Ga content of the second Mg-doped InAlGaN layer 621 is x2 and the In content is y2; the Ga content of the third Mg-doped InAlGaN layer 631 is x3 and the In content is y3, wherein 0.5≤x3<x2<x1≤0.8 and 0.01≤y1<y2<y3≤0.1.

[0043] In some embodiments, see Figure 2 As shown, the Al content in the first AlGaN layer 622 and the second AlGaN layer 632 increases sequentially. On the one hand, this effectively improves the lattice matching degree between adjacent sublayers in the second superlattice layer 62 and the third superlattice layer 63. On the other hand, it helps to suppress electron overflow and thus improve the overall radiative recombination efficiency.

[0044] In some embodiments, see Figure 1 and Figure 2 As shown, in the first superlattice layer 61, the thickness of the single-layer first Mg-doped InAlGaN layer 611 is 1nm~5nm, and the thickness of the single-layer first InGaN layer 612 is 1nm~5nm. In the second superlattice layer 62, the thickness of the single-layer second Mg-doped InAlGaN layer 621 is 1nm~5nm, the thickness of the single-layer first AlGaN layer 622 is 1nm~5nm, and the thickness of the single-layer h-BN layer 623 is 1nm~5nm. In the third superlattice layer 63, the thickness of the single-layer third Mg-doped InAlGaN layer 631 is 3nm~10nm, the thickness of the single-layer second AlGaN layer 632 is 3nm~10nm, and the thickness of the single-layer AlN layer 633 is 3nm~10nm.

[0045] For example, the thickness of the monolayer first Mg-doped InAlGaN layer 611 can be 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm or 5 nm, but is not limited thereto; the thickness of the monolayer first InGaN layer 612 can be 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm or 5 nm, but is not limited thereto.

[0046] For example, the thickness of the monolayer second Mg-doped InAlGaN layer 621 can be 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, or 5 nm, but is not limited thereto; the thickness of the monolayer first AlGaN layer 622 can be 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, or 5 nm, but is not limited thereto; the thickness of the monolayer h-BN layer 623 can be 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, or 5 nm, but is not limited thereto.

[0047] For example, the thickness of the monolayer third Mg-doped InAlGaN layer 631 can be 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm or 10nm, but is not limited thereto; the thickness of the monolayer second AlGaN layer 632 can be 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm or 10nm, but is not limited thereto; the thickness of the monolayer AlN layer 633 can be 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm or 10nm, but is not limited thereto.

[0048] In some embodiments, the buffer layer 2 includes an AlN buffer layer and a GaN buffer layer sequentially deposited along the epitaxial growth direction. The thickness of the buffer layer 2 is 10 nm to 50 nm. The AlN buffer layer can be formed by PVD sputtering, and the GaN buffer layer can be fabricated by MOCVD.

[0049] In some embodiments, the thickness of the undoped GaN layer 3 is 1 μm to 3 μm.

[0050] In some embodiments, the N-type layer 4 may be an N-type GaN layer with a thickness of 2 μm to 3 μm and a Si doping concentration of 1.0 × 10⁻⁶. 19 cm -3 ~5.0×10 19 cm -3 .

[0051] In some embodiments, the light-emitting layer 5 is a superlattice structure composed of alternating InGaN well layers and GaN barrier layers, with a period number of 5 to 20, a thickness of 2 nm to 5 nm for a single InGaN well layer, and a thickness of 5 nm to 15 nm for a single GaN barrier layer.

[0052] See Figure 1 and Figure 2 As shown, this invention also discloses a method for preparing a MicroLED epitaxial wafer, comprising: S100. Provide substrate 1; S200. Deposit an epitaxial layer on substrate 1; The epitaxial layer includes a buffer layer 2, an undoped GaN layer 3, an N-type layer 4, a light-emitting layer 5, a P-type control layer 6, and a P-type GaN layer 7 arranged sequentially along the epitaxial direction; The P-type control layer 6 includes a first superlattice layer 61, a second superlattice layer 62 and a third superlattice layer 63 arranged sequentially along the epitaxial direction; The first superlattice layer 61 includes a periodically alternating first Mg-doped InAlGaN layer 611 and a first InGaN layer 612; The second superlattice layer 62 includes a periodically alternating second Mg-doped InAlGaN layer 621, a first AlGaN layer 622, and an h-BN layer 623; The third superlattice layer 63 includes a periodically alternating third Mg-doped InAlGaN layer 631, a second AlGaN layer 632, and an AlN layer 633. The Mg doping concentration of the first Mg-doped InAlGaN layer 611, the second Mg-doped InAlGaN layer 621, and the third Mg-doped InAlGaN layer 631 increases sequentially along the epitaxial direction.

[0053] In some embodiments, see Figure 1 and Figure 2 As shown, the number of periods in the first superlattice layer 61, the second superlattice layer 62, and the third superlattice layer 63 increases sequentially, and the number of periods is greater than 1 for each layer. In the first superlattice layer 61, the Mg doping concentration of the first Mg-doped InAlGaN layer 611 in each period increases from a1 to a2. In the second superlattice layer 62, the Mg doping concentration of the second Mg-doped InAlGaN layer 621 in each period increases from b1 to b2. In the third superlattice layer 63, the Mg doping concentration of the third Mg-doped InAlGaN layer 631 in each period increases from c1 to c2, where c2-c1 > b2-b1 > a2-a1.

[0054] In some embodiments, see Figure 1 and Figure 2As shown, the Ga content of the first Mg-doped InAlGaN layer 611, the second Mg-doped InAlGaN layer 621, and the third Mg-doped InAlGaN layer 631 decrease sequentially, while the In content increases sequentially; and / or, The first Mg-doped InAlGaN layer 611 has a Ga content of x1 and an In content of y1; the second Mg-doped InAlGaN layer 621 has a Ga content of x2 and an In content of y2; the third Mg-doped InAlGaN layer 631 has a Ga content of x3 and an In content of y3, where 0.5 ≤ x3 < x2 < x1 ≤ 0.8 and 0.01 ≤ y1 < y2 < y3 ≤ 0.1.

[0055] The present invention will be further described below with reference to the accompanying drawings and embodiments: Example 1 This embodiment discloses a MicroLED epitaxial wafer, including a substrate and an epitaxial layer disposed on the substrate. The epitaxial layer includes a buffer layer, an undoped GaN layer, an N-type layer, a light-emitting layer, a P-type control layer and a P-type GaN layer disposed sequentially along the epitaxial direction. The P-type control layer includes a first superlattice layer, a second superlattice layer, and a third superlattice layer arranged sequentially along the epitaxial direction; The first superlattice layer comprises a periodically alternating first Mg-doped InAlGaN layer and a first InGaN layer; The second superlattice layer comprises a periodically alternating second Mg-doped InAlGaN layer, a first AlGaN layer, and an h-BN layer; The third superlattice layer comprises a periodically alternating stack of a third Mg-doped InAlGaN layer, a second AlGaN layer, and an AlN layer; The Mg doping concentration of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer increases sequentially along the epitaxial direction.

[0056] In this embodiment, the first superlattice layer has 3 periods, and the Mg doping concentration of the first Mg-doped InAlGaN layer in each period is 1.0 × 10⁻⁶. 18 cm -3 ; In the second superlattice layer, there are 3 periods, and the Mg doping concentration of the second Mg-doped InAlGaN layer in each period is 5.0 × 10⁻⁶. 18 cm -3 ; In the third superlattice layer, there are 5 periods, and the Mg doping concentration of the third Mg-doped InAlGaN layer in each period is 1.0 × 10⁻⁶. 19 cm -3 .

[0057] In this embodiment, the Ga content of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer is 0.6, and the In content is 0.05.

[0058] In this embodiment, the Al component content in the first AlGaN layer and the second AlGaN layer increases sequentially.

[0059] In this embodiment, in the first superlattice layer, the thickness of the single-layer first Mg-doped InAlGaN layer is 3nm, and the thickness of the single-layer first InGaN layer is 3nm. In the second superlattice layer, the thickness of the single second Mg-doped InAlGaN layer is 3 nm, the thickness of the single first AlGaN layer is 3 nm, and the thickness of the single h-BN layer is 3 nm. In the third superlattice layer, the thickness of the single-layer third Mg-doped InAlGaN layer is 4 nm, the thickness of the single-layer second AlGaN layer is 4 nm, and the thickness of the single-layer AlN layer is 4 nm.

[0060] Example 2 The difference between this embodiment and embodiment 1 is that, in this embodiment, the number of periods of the first superlattice layer, the second superlattice layer and the third superlattice layer increases sequentially, and the number of periods is greater than 1. In the first superlattice layer, the Mg doping concentration of the first Mg-doped InAlGaN layer in each period generally increases from a1 to a2. In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer in each period generally increases from b1 to b2. In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer in each period generally increases from c1 to c2, where c2-c1 > b2-b1 > a2-a1.

[0061] In this embodiment, the first superlattice layer has two periods, and the Mg doping concentration of the first Mg-doped InAlGaN layer in the first period is 1.0 × 10⁻⁶. 18 cm -3 The Mg doping concentration of the first Mg-doped InAlGaN layer in subsequent cycles increases by 1.0 × 10⁻⁶ per cycle. 18 cm -3 The growth rate is increasing; In the second superlattice layer, there are 3 periods, and the Mg doping concentration of the second Mg-doped InAlGaN layer in the first period is 5.0 × 10⁻⁶. 18 cm -3 The Mg doping concentration of the second Mg-doped InAlGaN layer in subsequent cycles increases by 1.0 × 10⁻⁶ per cycle. 18cm -3 The growth rate is increasing; In the third superlattice layer, there are 6 periods, and the Mg doping concentration of the third Mg-doped InAlGaN layer in the first period is 1.0 × 10⁻⁶. 19 cm -3 The Mg doping concentration of the third Mg-doped InAlGaN layer in subsequent cycles increases by 5.0 × 10⁻⁶ per cycle. 18 cm -3 The growth rate is increasing.

[0062] Example 3 The difference between this embodiment and Embodiment 1 is that the Ga content of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer decreases sequentially, while the In content increases sequentially.

[0063] In this embodiment, the Ga content of the first Mg-doped InAlGaN layer is x1, and the In content is y1; the Ga content of the second Mg-doped InAlGaN layer is x2, and the In content is y2; the Ga content of the third Mg-doped InAlGaN layer is x3, and the In content is y3, where x1 is 0.8, x2 is 0.6, x3 is 0.5, y1 is 0.02, y2 is 0.05, and y3 is 0.1.

[0064] Example 4 The difference between this embodiment and embodiment 1 is that, in this embodiment, the number of periods of the first superlattice layer, the second superlattice layer and the third superlattice layer increases sequentially, and the number of periods is greater than 1. In the first superlattice layer, the Mg doping concentration of the first Mg-doped InAlGaN layer in each period generally increases from a1 to a2. In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer in each period generally increases from b1 to b2. In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer in each period generally increases from c1 to c2, where c2-c1 > b2-b1 > a2-a1.

[0065] In this embodiment, the first superlattice layer has two periods, and the Mg doping concentration of the first Mg-doped InAlGaN layer in the first period is 1.0 × 10⁻⁶. 18 cm -3 The Mg doping concentration of the first Mg-doped InAlGaN layer in subsequent cycles increases by 1.0 × 10⁻⁶ per cycle. 18 cm -3 The growth rate is increasing; In the second superlattice layer, there are 3 periods, and the Mg doping concentration of the second Mg-doped InAlGaN layer in the first period is 5.0 × 10⁻⁶. 18 cm -3 The Mg doping concentration of the second Mg-doped InAlGaN layer in subsequent cycles increases by 1.0 × 10⁻⁶ per cycle. 18 cm -3 The growth rate is increasing; In the third superlattice layer, there are 6 periods, and the Mg doping concentration of the third Mg-doped InAlGaN layer in the first period is 1.0 × 10⁻⁶. 19 cm -3 The Mg doping concentration of the third Mg-doped InAlGaN layer in subsequent cycles increases by 5.0 × 10⁻⁶ per cycle. 18 cm -3 The growth rate is increasing.

[0066] In this embodiment, the Ga content of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer decrease sequentially, while the In content increases sequentially.

[0067] In this embodiment, the Ga content of the first Mg-doped InAlGaN layer is x1, and the In content is y1; the Ga content of the second Mg-doped InAlGaN layer is x2, and the In content is y2; the Ga content of the third Mg-doped InAlGaN layer is x3, and the In content is y3, where x1 is 0.8, x2 is 0.6, x3 is 0.5, y1 is 0.02, y2 is 0.05, and y3 is 0.1.

[0068] Comparative Example 1 The difference between this comparative example and Example 1 is that, in the P-type control layer, the Mg doping concentration of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer decreases sequentially.

[0069] In each period, the Mg doping concentration of the first Mg-doped InAlGaN layer is 1.0 × 10⁻⁶. 19 cm -3 The Mg doping concentration of the second Mg-doped InAlGaN layer in each period is 5.0 × 10⁻⁶. 18 cm -3 The Mg doping concentration of the third Mg-doped InAlGaN layer in each period is 1.0 × 10⁻⁶. 18 cm -3 .

[0070] Comparative Example 2 The difference between this comparative example and Example 1 is that the first superlattice layer is not provided in the P-type control layer.

[0071] Comparative Example 3 The difference between this comparative example and Example 1 is that no second superlattice layer is provided in the P-type control layer.

[0072] Comparative Example 4 The difference between this comparative example and Example 1 is that no third superlattice layer is provided in the P-type control layer.

[0073] Comparative Example 5 The P-type control layer was replaced with an electron blocking layer, which was an AlInGaN layer with a thickness of 40 nm, containing 0.05% Al and 0.1% In.

[0074] 1) Experimental group: Examples 1 to 4, Comparative Examples 1 to 5; 2) Test method: 10μm×10μm chips were prepared based on the epitaxial wafers of each experimental group. 300 LED chips were selected from each group and the luminous power was tested at a current of 0.1μA. The average value of each experimental group was taken, and the luminous efficiency improvement rate of the other experimental groups relative to Comparative Example 5 was calculated based on the data of Comparative Example 5.

[0075]

[0076] Experimental results show that, compared with the conventional epitaxial structure of Comparative Example 5, the epitaxial structures of Examples 1 to 4 of the present invention can effectively improve the radiative recombination efficiency of MicroLED at low current density.

[0077] Comparing Example 1 and Comparative Examples 1 to 4, it can be seen that in the P-type control layer of the present invention, the first superlattice layer, the second superlattice layer and the third superlattice layer are arranged sequentially and cooperate with each other, which can effectively improve the radiative recombination efficiency of MicroLED at low current density.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A MicroLED epitaxial wafer, characterized in that, The invention includes a substrate and an epitaxial layer disposed on the substrate. The epitaxial layer includes a buffer layer, an undoped GaN layer, an N-type layer, a light-emitting layer, a P-type control layer, and a P-type GaN layer disposed sequentially along the epitaxial direction. The P-type control layer includes a first superlattice layer, a second superlattice layer, and a third superlattice layer arranged sequentially along the epitaxial direction; The first superlattice layer comprises a first Mg-doped InAlGaN layer and a first InGaN layer that are periodically and alternately stacked; The second superlattice layer comprises a periodically alternating second Mg-doped InAlGaN layer, a first AlGaN layer, and an h-BN layer; The third superlattice layer comprises a periodically alternating stacked third Mg-doped InAlGaN layer, a second AlGaN layer, and an AlN layer; The Mg doping concentration of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer increases sequentially along the epitaxial direction.

2. The MicroLED epitaxial wafer according to claim 1, characterized in that, The number of periods in the first superlattice layer, the second superlattice layer, and the third superlattice layer increases sequentially, and the number of periods in each layer is greater than 1. In the first superlattice layer, the Mg doping concentration of the first Mg-doped InAlGaN layer in each period generally increases from a1 to a2. In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer in each period generally increases from b1 to b2. In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer in each period generally increases from c1 to c2, wherein c2-c1 > b2-b1 > a2-a1.

3. The MicroLED epitaxial wafer according to claim 1, characterized in that, In the first superlattice layer, the Mg doping concentration of the first Mg-doped InAlGaN layer is 1.0 × 10⁻⁶. 18 cm -3 ~5.0×10 18 cm -3 The number of cycles is 2 to 3; In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer is 5.0 × 10⁻⁶. 18 cm -3 ~1.0×10 19 cm -3 The number of cycles is 3 to 6; In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer is 1.0 × 10⁻⁶. 19 cm -3 ~8.0×10 19 cm -3 The number of cycles is 4 to 8.

4. A MicroLED epitaxial wafer according to claim 1, characterized in that, The Ga content of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer decreases sequentially, while the In content increases sequentially.

5. A MicroLED epitaxial wafer according to claim 4, characterized in that, The first Mg-doped InAlGaN layer has a Ga content of x1 and an In content of y1; the second Mg-doped InAlGaN layer has a Ga content of x2 and an In content of y2; the third Mg-doped InAlGaN layer has a Ga content of x3 and an In content of y3, wherein 0.5 ≤ x3 < x2 < x1 ≤ 0.8, and 0.01 ≤ y1 < y2 < y3 ≤ 0.

1.

6. A MicroLED epitaxial wafer according to claim 1, characterized in that, The Al content in the first AlGaN layer and the second AlGaN layer increases sequentially.

7. A MicroLED epitaxial wafer according to claim 1, characterized in that, In the first superlattice layer, the thickness of a single Mg-doped InAlGaN layer is 1 nm to 5 nm, and the thickness of a single InGaN layer is 1 nm to 5 nm. In the second superlattice layer, the thickness of a single second Mg-doped InAlGaN layer is 1 nm to 5 nm, the thickness of a single first AlGaN layer is 1 nm to 5 nm, and the thickness of a single h-BN layer is 1 nm to 5 nm. In the third superlattice layer, the thickness of a single third Mg-doped InAlGaN layer is 3nm~10nm, the thickness of a single second AlGaN layer is 3nm~10nm, and the thickness of a single AlN layer is 3nm~10nm.

8. A method for preparing a MicroLED epitaxial wafer, characterized in that, include: Provide substrate; An epitaxial layer is deposited on the substrate; The epitaxial layer includes a buffer layer, an undoped GaN layer, an N-type layer, a light-emitting layer, a P-type control layer, and a P-type GaN layer arranged sequentially along the epitaxial direction; The P-type control layer includes a first superlattice layer, a second superlattice layer, and a third superlattice layer arranged sequentially along the epitaxial direction; The first superlattice layer comprises a first Mg-doped InAlGaN layer and a first InGaN layer that are periodically and alternately stacked; The second superlattice layer comprises a periodically alternating second Mg-doped InAlGaN layer, a first AlGaN layer, and an h-BN layer; The third superlattice layer comprises a periodically alternating stacked third Mg-doped InAlGaN layer, a second AlGaN layer, and an AlN layer; The Mg doping concentration of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer increases sequentially along the epitaxial direction.

9. The preparation method according to claim 8, characterized in that, The number of periods in the first superlattice layer, the second superlattice layer, and the third superlattice layer increases sequentially, and the number of periods in each layer is greater than 1. In the first superlattice layer, the Mg doping concentration of the first Mg-doped InAlGaN layer in each period generally increases from a1 to a2. In the second superlattice layer, the Mg doping concentration of the second Mg-doped InAlGaN layer in each period generally increases from b1 to b2. In the third superlattice layer, the Mg doping concentration of the third Mg-doped InAlGaN layer in each period generally increases from c1 to c2, wherein c2-c1 > b2-b1 > a2-a1.

10. The preparation method according to claim 8, characterized in that, The Ga content of the first Mg-doped InAlGaN layer, the second Mg-doped InAlGaN layer, and the third Mg-doped InAlGaN layer decreases sequentially, while the In content increases sequentially; and / or, The first Mg-doped InAlGaN layer has a Ga content of x1 and an In content of y1; the second Mg-doped InAlGaN layer has a Ga content of x2 and an In content of y2; the third Mg-doped InAlGaN layer has a Ga content of x3 and an In content of y3, wherein 0.5 ≤ x3 < x2 < x1 ≤ 0.8, and 0.01 ≤ y1 < y2 < y3 ≤ 0.1.

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