Epitaxial structure for improving internal quantum efficiency of green-light Micro LED

Abstract

The invention relates to the technical field of semiconductor light-emitting devices, and particularly discloses an epitaxial structure for improving the internal quantum efficiency of a green Micro LED. The epitaxial structure comprises a substrate, and a buffer layer, an undoped GaN layer, an n-type GaN layer, a stress release layer, an active layer, a low-temperature Mg heavily-doped GaN layer, an electron barrier layer, a p-type GaN layer and a p-type GaN contact layer which are sequentially arranged on the substrate. The active layer comprises a plurality of periods of InGaN quantum well layers and an InAlGaN / GaN superlattice quantum barrier layer alternating structure, and a CAP layer formed by an AlGaN / GaN superlattice is arranged behind each quantum well layer. By adopting a wide-well narrow-barrier structure, a superlattice CAP layer for neutralizing a polarization electric field and a quaternary InAlGaN barrier structure for lattice matching and hole injection enhancement, multiple regulation and control of a polarization field, dislocation density and hole transport are realized, the internal quantum efficiency of a green-light Micro LED is remarkably improved, and the green-light Micro LED epitaxial structure is suitable for epitaxial preparation of a high-performance Micro LED chip.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor light-emitting devices, and in particular to an epitaxial structure for improving the internal quantum efficiency of green light Micro LEDs. Background Art

[0002] With the rapid development of Micro LED display technology, the efficiency bottleneck of green LED chips in full-color Micro displays has become a key obstacle to their commercialization. GaN-based green LEDs typically use InGaN with a high indium content as the quantum well material. This results in a large lattice mismatch strain (Δa / a of approximately 2% to 3%) between the GaN barrier and the quantum well. This further induces a significant piezoelectric polarization effect (i.e., the quantum confined Stark effect, QCSE) in the c-plane polar structure, leading to band tilting, carrier wave function separation, and a significant reduction in internal quantum efficiency.

[0003] In addition, high-density threading dislocations (TDs) are easily generated in InGaN / GaN heterostructures, with a density often as high as 10 8 ~10 9 cm -2 , forming non-radiative recombination centers, further reducing the device's luminescence efficiency. At the same time, due to the low hole mobility and poor p-side injection efficiency, the quantum wells farther away from the p-region have limited luminescence contributions and insufficient carrier recombination.

[0004] To address the above issues, multi-level optimization design of the quantum well / barrier structure is required to synergistically improve the luminous efficiency of green light Micro LEDs. Summary of the Invention

[0005] The purpose of the present invention is to overcome the technical problems of low quantum efficiency, strong polarization field, high dislocation density and poor hole injection in the prior art green light LEDs, and to provide an epitaxial structure that improves the internal quantum efficiency of green light Micro LEDs.

[0006] To achieve the above object, the present invention provides the following technical solutions: A green Micro LED epitaxial structure is provided on a substrate and includes a buffer layer, an undoped GaN layer, an n-type GaN layer, a stress relief layer, an active layer, a low-temperature heavily Mg-doped GaN layer, an electron blocking layer, a p-type GaN layer, and a p-type GaN contact layer, all grown in sequence on the substrate. The structure is characterized by: The active layer includes a structure of multiple periods of alternating quantum well layers and quantum barrier layers, wherein a CAP layer (capping layer) is provided behind each quantum well layer; The quantum well layer is an InGaN layer, the CAP layer is an AlGaN / GaN superlattice structure, and the quantum barrier layer is an InAlGaN / GaN superlattice structure.

[0007] The active layer includes an alternating structure of 8 to 11 periods of quantum well layers and quantum barrier layers.

[0008] The thickness of the InGaN quantum well layer is 3.5-4 nm.

[0009] The CAP layer is 5 to 10 cycles of Al x Ga 1-x N / GaN superlattice structure, x is 0.1~0.2, wherein the Al x Ga 1-x The thickness of the N single layer is 0.2~0.4 nm, and the thickness of the GaN single layer is 0.2 nm.

[0010] The InAlGaN / GaN superlattice has a 10-period structure, and the composition of InAlGaN is In 0.05 Al 0.15 Ga 0.80 N, wherein the thickness of the InAlGaN single layer is 0.5-0.6 nm, and the thickness of the GaN single layer is 0.3-0.4 nm.

[0011] Through the coordinated optimization of multiple structural designs, the present invention effectively solves the problems of polarization field strength, high defect density and low hole injection efficiency existing in green light Micro LEDs. Its mechanism for improving internal quantum efficiency is reflected in the following three aspects: First, a wide-well and narrow-barrier structure is adopted, the thickness of the InGaN quantum well is widened to 3.5-4nm, and the GaN barrier layer is thinned to 8-10nm. This can release strain and weaken the piezoelectric polarization effect, thereby reducing the inhibition of carrier recombination by the quantum confined Stark effect (QCSE); secondly, an AlGaN / GaN superlattice transition layer (CAP layer) is introduced after the quantum well. This layer can induce dislocation bending or termination through interface stress, significantly reducing the threading dislocation density in the active region. At the same time, its anti-polarization direction helps to further neutralize the electric field in the well; thirdly, the barrier region adopts an InAlGaN / GaN superlattice structure. By adjusting the ratio of In and Al, the lattice constant of InAlGaN is matched with GaN, thereby suppressing the further accumulation of interface strain and utilizing the higher valence band top of InAlGaN to form a hole barrier, effectively improving the hole injection efficiency.

[0012] Through the above design, the present invention realizes the synergistic regulation effect of multiple structures, which has the following beneficial effects: Weakening the polarization electric field effect: By adopting a wide-well narrow-barrier design and an AlGaN / GaN superlattice CAP layer, the piezoelectric polarization field in the InGaN quantum well is effectively reduced, significantly alleviating the quantum confined Stark effect (QCSE), increasing the spatial overlap probability of electrons and holes, and improving the radiative recombination efficiency.

[0013] Reduce defect density: By introducing an AlGaN / GaN superlattice transition layer with a periodic stress interface and a lattice-matched InAlGaN / GaN barrier layer, dislocation bending and termination are induced, significantly inhibiting the extension of threading dislocations into the active layer, reducing the number of non-radiative recombination centers, and improving the device crystal quality.

[0014] Enhanced hole injection capability: The quantum barrier layer uses quaternary InAlGaN material, which can form a hole barrier below the quantum well, preventing holes from leaking to the n-type region and promoting more holes to be injected into the quantum well area, thereby increasing the probability of holes participating in recombination and improving the overall luminous efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 : Schematic diagram of the green light Micro LED epitaxial structure described in the present invention. DETAILED DESCRIPTION

[0016] The technical solutions of the present invention are described in detail below with reference to the embodiments. However, it should be understood by those skilled in the art that these embodiments are only used to illustrate the present invention and do not constitute a limitation on the protection scope of the present invention.

[0017] Example

[0018] This embodiment provides a green Micro LED epitaxial structure capable of improving internal quantum efficiency. The structure and parameters are as follows: On the sapphire substrate 600, the following epitaxial layers are grown in sequence using a metal organic chemical vapor deposition (MOCVD) method: 1. Buffer layer 100: A buffer layer of AlN or GaN is grown to improve the lattice matching between the substrate and the GaN main epitaxial layer. The thickness is 25-50 nm.

[0019] 2. Undoped GaN layer 200: has a thickness of 1 to 2 µm and is used to provide a high-quality template layer to improve crystal quality.

[0020] 3. n-type GaN layer 300: silicon is used as n-type dopant with a doping concentration of 8×10 18 ~ 5×10 19 atoms / cm³, thickness is 1.5~3 µm, and resistivity is less than 0.01 Ω·cm.

[0021] 4. Stress release layer 400: A cyclic structure of InGaN and GaN is used, with a cycle number of 3 to 9, which effectively alleviates the lattice strain transfer between the lower and upper layers. The InGaN layer thickness is 2 to 3 nm, and the GaN layer thickness is 30 to 60 nm.

[0022] 5. Active layer 500: It consists of 8 to 11 periods of alternating InGaN quantum well layers 510 and InAlGaN / GaN quantum barrier layers 520, with a CAP layer 511 disposed behind each quantum well layer. The structure is as follows: Quantum well layer (QW): InGaN single layer, 3.5-4 nm thick, with an In composition of approximately 0.25; compared to existing structures, the width of the quantum well InGaN layer in this invention is increased to 3.5-4 nm. The widened well allows for more strain release, reduces the piezoelectric charge density, and thus weakens the built-in electric field; CAP layer: 5-10 period AlGaN / GaN superlattice structure, where: The thickness of AlGaN single layer is 0.2~0.4 nm, and its composition is Al x Ga 1-x N, x is 0.1~0.2, and the lattice constant is 3.15~3.17 Å; The thickness of GaN single layer is 0.2 nm and the lattice constant is 3.189 Å; The lattice constant of the AlGaN layer is smaller than that of the GaN layer, resulting in the polarization direction of the AlGaN being opposite to that of the InGaN well layer, partially neutralizing the electric field in the well and further weakening the QCSE. The high hardness of the AlGaN layer and the interface stress difference cause threading dislocations (TDs) to bend or terminate, reducing the extension of dislocations to the upper layer and reducing the defect density by one to two orders of magnitude. Quantum Barrier (QB): 10 periods of In 0.05 Al 0.15 Ga 0.80 N superlattice structure, where: The thickness of the InAlGaN single layer is 0.5~0.6 nm; The thickness of GaN single layer is 0.3~0.4 nm; Inside the superlattice structure, InAlGaN is formed by adjusting the ratio of In and Al (In 0.05 Al 0.15 Ga 0.80 N), so that the lattice constant of the quaternary alloy matches that of GaN (a≈3.189 Å), eliminating the interface lattice mismatch; the valence band top of InAlGaN is higher than that of GaN, forming a hole barrier, reducing hole leakage to the n-type layer, and promoting hole injection into the quantum well; The 10-period superlattice structure controls the overall thickness of the quantum barrier layer to 8~10nm, reducing the strain accumulation between adjacent quantum wells, preventing stress from being transferred to subsequent epitaxial layers, reducing the effective electric field strength within the quantum well, and improving the overlap of electron-hole wave functions.

[0023] In the entire active area, the above structures are alternately stacked to form a multi-quantum well structure with 8 to 11 periods.

[0024] 6. Low-temperature heavily Mg-doped GaN layer 600: A heavily Mg-doped GaN layer was grown at a low temperature (approximately 750°C) with a doping concentration of 5×10 19 ~ 1.5×10 20 atoms / cm 3 , in order to improve crystal quality, increase hole injection efficiency, and reduce Mg diffusion contamination to the quantum well. The thickness is about 10~20 nm.

[0025] 7. Electron blocking layer 700: Al 0.25 Ga 0.75 N material, with a thickness of 20~30 nm, is used to block electrons from overflowing into the p region and helps improve carrier recombination efficiency.

[0026] 8. p-type GaN layer 800: p-type GaN layer doped with Mg, with a doping concentration of 1×10 19 ~ 5×10 19 atoms / cm 3 , with a thickness of about 150~200 nm, which is used to form a pn junction and provide a hole injection channel.

[0027] 9. p-type GaN contact layer 900: p-type GaN layer with a high Mg doping concentration of >1×10 20 atoms / cm 3 , with a thickness of about 3~10 nm, which is used to form a good ohmic contact.

[0028] In the present invention, although the substrate itself is not a structural layer formed by the epitaxial process, it serves as the starting point for the growth of the entire epitaxial structure and has a critical impact on the lattice matching, stress regulation, dislocation density, and overall device performance of the subsequent epitaxial layers. Therefore, to clearly define the structural composition of the present invention, this article includes the substrate in the hierarchical description of the epitaxial structure to accurately reflect the layer sequence and structural relationship of each functional layer. The substrate can be a common base material such as sapphire, Si, or SiC, and its selection can be adjusted according to the specific application requirements.

Claims

1. An epitaxial structure for improving the internal quantum efficiency of green Micro LEDs. The epitaxial structure is disposed on a substrate and includes a buffer layer, an undoped GaN layer, an n-type GaN layer, a stress relief layer, an active layer, a low-temperature heavily Mg-doped GaN layer, an electron blocking layer, a p-type GaN layer, and a p-type GaN contact layer, grown sequentially on the substrate. The structure is characterized by: The active layer includes a structure of multiple periods of alternating quantum well layers and quantum barrier layers, wherein a CAP layer is arranged behind each quantum well layer; The quantum well layer is an InGaN layer, the CAP layer is an AlGaN / GaN superlattice structure, and the quantum barrier layer is an InAlGaN / GaN superlattice structure.

2. The epitaxial structure according to claim 1, wherein: The active layer includes an alternating structure of 8 to 11 periods of quantum well layers and quantum barrier layers.

3. The epitaxial structure according to claim 1, wherein: The thickness of the InGaN quantum well layer is 3.5-4 nm.

4. The epitaxial structure according to claim 1, wherein: The CAP layer is 5 to 10 cycles of Al x Ga 1- x N / GaN superlattice structure, x is 0.1~0.

2.

5. The epitaxial structure according to claim 4, wherein: The Al x Ga 1-x The thickness of the N single layer is 0.2~0.4nm, and the thickness of the GaN single layer is 0.2 nm.

6. The epitaxial structure according to claim 1, wherein: The InAlGaN / GaN superlattice has a 10-period structure, and the composition of InAlGaN is In 0.05 Al 0.15 Ga 0.80 N.

7. The epitaxial structure according to claim 6, wherein: The thickness of the InAlGaN single layer is 0.5-0.6 nm, and the thickness of the GaN single layer is 0.3-0.4 nm.

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