Dual-wavelength luminous LED epitaxial structure and preparation method thereof

Abstract

The invention provides a dual-wavelength luminous LED epitaxial structure and a preparation method thereof. The LED epitaxial structure comprises two light-emitting active regions which are distributed up and down, respectively emit light with different wavelengths and are separated by a middle layer. By designing an electrode connection mode, only one active region is directly driven by current to emit light, and the generated light excites the other active region to generate second wavelength light, so that dual-wavelength simultaneous light emission is realized. The corresponding epitaxial growth method comprises the following steps: sequentially growing two groups of quantum well structures with different wavelengths on the same substrate in two stages, and reasonably controlling the doping layer to form a single electric injection P-N junction. Compared with the prior art, the LED device disclosed by the invention has the advantages that two light output proportions are stable and consistent under different driving currents, a wider spectrum can be covered, and the manufacturing cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of light emitting diode (LED) epitaxial growth and semiconductor lighting devices, and in particular to an LED epitaxial structure capable of emitting dual-wavelength light and a preparation method thereof. Background Art

[0002] LEDs efficiently convert electrical energy into light, releasing energy through the recombination of electrons and holes. They are widely used in lighting, display, and other fields. With the development of LED technology, the demand for full-spectrum white light output continues to increase. Integrating multiple emission wavelengths on the same chip can effectively reduce the manufacturing cost of full-spectrum devices. Currently, common dual-wavelength LED devices typically employ two sets of quantum wells emitting different wavelengths, grown sequentially within the same active region. Upon current injection, light output of both wavelengths is generated simultaneously. However, with this structure, the relative intensity ratio of the two wavelengths varies with driving current. This is because the spatial distribution of carriers within the active region shifts as current density increases from low to high. Due to the presence of V-pits in the gallium nitride material, carriers recombine primarily in the quantum wells near the N-type GaN side at lower current densities, while at higher current densities, the recombination position shifts toward the quantum wells near the P-type GaN side. Consequently, the color temperature of the light output from dual-wavelength LEDs varies inconsistently at different currents.

[0003] The existing dual-wavelength LED luminous intensity ratio is not only sensitive to current, making it difficult to precisely control, but also limiting its application range. To address these shortcomings of the existing technology, it is necessary to provide a new dual-wavelength LED epitaxial structure and growth method to achieve an LED device with a stable dual-wavelength luminous intensity ratio and controllable spectrum under varying driving current. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiency of existing dual-wavelength LEDs in that the luminous ratio varies with current, and to provide a dual-wavelength LED epitaxial structure and a preparation method thereof, so as to achieve stable luminous output of two wavelengths on a single chip.

[0005] To achieve the above objectives, the present invention vertically separates two active light-emitting regions emitting at different wavelengths within an LED epitaxial wafer. By adjusting the epitaxial structure and electrode arrangement, only one active region emits light directly through current injection, while the other active region is stimulated to emit light by absorbing photons emitted by the first active region. Specifically, the LED epitaxial structure of the present invention comprises several semiconductor layers, formed sequentially from bottom to top: an N-type semiconductor layer, a first active region, an intermediate semiconductor layer, a second active region, an electron blocking layer, and a P-type semiconductor layer. The first and second active regions each comprise a multi-quantum well structure, emitting light with peaks at different wavelengths. The two active regions are vertically separated by the intermediate semiconductor layer, and an effective PN junction for current injection is formed only in the region containing the second active region. When current is applied, the second active region at the PN junction generates spontaneous emission light. This light passes through the intermediate layer and is absorbed by the first active region, thereby stimulating the first active region to also generate spontaneous emission light. By means of the above-mentioned light excitation mechanism, the structure of the present invention can simultaneously output light of two different wavelengths and avoid the chromaticity drift caused by uneven carrier distribution in the direct double injection structure.

[0006] The present invention also provides a method for preparing the aforementioned LED epitaxial structure, comprising the following steps: first, growing a first N-type semiconductor layer and a first active region (optionally, a first electron blocking layer and a first P-type semiconductor layer) on a substrate; then, after the first active region is completed, growing an intermediate semiconductor layer and a second active region thereon, and growing an electron blocking layer and a P-type semiconductor layer on the second active region. By adjusting the doping type and hierarchical structure during the first and second growth stages, the final epitaxial wafer contains only one PN junction for electroluminescence, while the other active region is not included in the current path of the PN junction. Electrodes are then prepared using conventional chip manufacturing processes, with the P electrode contacting the P-type semiconductor layer and the N electrode contacting the N-type semiconductor layer, to produce a dual-wavelength LED chip.

[0007] Compared with the prior art, the dual-wavelength LED structure of the present invention adopts the design of light-excited secondary luminescent region, and only one active region is electrically driven. Therefore, the output ratio of the two colors of light under different currents remains basically constant, greatly improving the stability of the luminous color changing with the current. By growing the two luminous quantum wells separately, the present invention also allows the materials and structures of each active region to be independently optimized, improving the overall luminous efficiency and having the flexibility to combine different luminous wavelengths as needed. Furthermore, the single-chip integrated dual-wavelength light emission is conducive to achieving broader spectral coverage (for example, achieving white light illumination with high color rendering) and reducing the cost of multi-chip combination to produce a full spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1Schematic diagram of the cross-section of the epitaxial structure of the dual-wavelength light-emitting LED according to Example 1 of the present invention.

[0009] Figure 2 It is a schematic cross-sectional view of the epitaxial structure of the dual-wavelength light-emitting LED according to Example 2 of the present invention.

[0010] Figure 3 This is a schematic diagram of the light-emitting principle of the dual-wavelength LED structure of Example 1 of the present invention.

[0011] Figure 4 This is a schematic diagram of the light-emitting principle of the dual-wavelength LED structure of Example 2 of the present invention.

[0012] Figure 5 This is a comparison chart of the spectral output of the structure in Example 1 of the present invention and the comparative example "conventional structure" under different driving currents. DETAILED DESCRIPTION

[0013] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be noted that, in the absence of conflict, the technical features in the following embodiments can be combined with each other to obtain new implementation plans.

[0014] Example 1:

[0015] like Figure 1 As shown, the present embodiment provides a dual-wavelength LED epitaxial structure in which the top layer of the epitaxial wafer is a P-type semiconductor layer. The preparation method is as follows:

[0016] First, a patterned sapphire substrate (PSS) 100 is provided, on which a low-temperature GaN buffer layer 101 and an undoped GaN layer 102 are sequentially epitaxially grown. Next, a first N-type GaN layer 103 with a thickness of approximately 2 to 3 μm is grown on the undoped GaN layer 120. This layer is doped with silicon to enhance its n-type conductivity.

[0017] Next, a first stress-relieving layer 104 is grown on the first N-type GaN layer 103, and a first active region 105 is grown on the first stress-relieving layer 104. Specifically, the first active region 105 employs an InGaN / GaN multi-quantum well structure, for example, comprising several periods of InGaN well layers and GaN barrier layers, and can be designed to emit green light (with a first wavelength of approximately 520 nm). After the growth of the first active region 105 is completed, in this embodiment, an electron blocking layer and a P-type layer are not grown thereon (i.e., a complete PN junction is not yet formed), and the next step of growth is continued.

[0018] A second N-type GaN layer 106 (i.e., an intermediate semiconductor layer) is epitaxially grown above the first active region 105. This layer also uses silicon-doped GaN material, with a thickness of, for example, 1 to 2 μm, and serves as an n-type carrier transport layer and electrode extraction layer in the upper light-emitting region. To alleviate stress accumulation during the continuous growth of the heterostructure, after the second N-type GaN layer 106 is grown, a second stress release layer 107 is grown thereon in this embodiment. This stress release layer can use, for example, an InGaN / GaN superlattice structure or a thin GaN layer grown at low temperature to relax some of the lattice stress.

[0019] Subsequently, a second active region 108 is grown on the second stress release layer 107. The second active region 108 also adopts an InGaN / GaN multi-quantum well structure, but its luminescence peak is different from that of the first active region 105. In this embodiment, the second active region 108 can be designed as a quantum well light-emitting region that emits blue light (the second wavelength is about 450nm). After the growth of the second active region 108 is completed, an electron blocking layer (EBL) 109 is grown thereon. The EBL can be a magnesium-doped AlGaN layer to block electrons from escaping from the second active region 108. Finally, a P-type GaN layer 110 (i.e., the top P-type semiconductor layer) is grown on the EBL. It is given p-type conductivity by magnesium doping, and a higher doping concentration can be formed on the top layer to facilitate ohmic contact.

[0020] Next, the epitaxial wafer is subjected to chip production. First, a window is opened on the epitaxial wafer through photolithography and etching processes to remove the top P-type GaN layer 110, EBL 109 and second active area 108 in the local area until the second N-type GaN layer 106 below is exposed. Then, an N electrode 111 (metal electrode, such as Ti / Al stacking) is deposited and alloyed on the surface of the exposed second N-type GaN layer 106. At the same time, a P electrode 112 (such as a Ni / Au transparent electrode) is deposited and alloyed on the top P-type GaN layer 110 of the chip. In the LED chip prepared in this way, the P electrode is located on the top P-type GaN layer 110, and the N electrode is in ohmic contact with the middle second N-type GaN layer 106, and the two electrodes are isolated from each other. A positive structure can be used for the final packaging so that both the P electrode and the N electrode are drawn out from the top surface of the chip.

[0021] like Figure 3As shown in the figure, when the chip is powered on, electrons are injected from the N electrode into the middle N-type GaN layer, passing through the upper PN junction region (the junction formed by the middle N-type layer and the top P-type layer) to generate electron-hole recombination, mainly realizing electroluminescence in the second active region 108, emitting blue light. Since the first active region 105 is not in the direct path of the current, it itself hardly generates light from current injection. However, part of the blue light emitted by the second active region 108 is transmitted downward and is absorbed by the first active region 105 below when passing through the middle layer. After absorbing high-energy blue light photons, the InGaN quantum well in the first active region 105 is excited to generate electron-hole pairs and undergo radiative recombination, thereby emitting green light.

[0022] In this way, the chip simultaneously generates blue and green light outputs. The luminous intensity of the first active area 105 depends on the photon flux provided by the luminescent excitation of the second active area 108, while the luminous intensity of the second active area 108 is determined by the driving current. Therefore, the relative intensities of the two lights are primarily determined by factors such as the absorption efficiency of the first active area 105 of the light emitted from the second active area 108 during structural design. Unlike existing dual-wavelength co-injection structures, which vary significantly with current, this achieves stable dual-wavelength luminescence output.

[0023] Example 2:

[0024] like Figure 2 As shown, another dual-wavelength LED epitaxial structure provided in this embodiment includes a P-type semiconductor layer located at the bottom of the epitaxial wafer. The preparation method is as follows:

[0025] First, the LED basic structure layer, that is, the complete single-wavelength LED epitaxial structure, is epitaxially grown on a patterned sapphire substrate (PSS) 200. Specifically, a GaN buffer layer 201, an undoped GaN layer 202, and a first silicon-doped N-type GaN layer 203 about 2 μm thick are sequentially grown on the PSS100. Then, a first stress release layer 204 is grown, and a first active area 205 is grown on the first stress release layer 204. An InGaN / GaN multi-quantum well structure can be used, for example, designed to emit blue light (wavelength of about 450 nm). After the growth of the first active area 205 is completed, a layer of AlGaN electron blocking layer, namely the first electron blocking layer 206, is continued to be grown to block electrons from entering the subsequent P-type layer. Then, a first P-type GaN layer 207 is grown on the first electron blocking layer 206, and p-type conductivity is achieved by magnesium doping. Preferably, a thin layer of highly doped p++GaN contact layer, i.e., a highly doped P-type GaN contact layer (not shown) can be grown on the first P-type GaN layer 207 to reduce the ohmic contact resistance. Thus, the first LED structure at the bottom is formed.

[0026] Next, the second light-emitting structure is grown on the structure already containing the p++GaN contact layer. First, a silicon-doped N-type GaN layer, i.e., the second N-type GaN layer 208, is epitaxially grown on the surface of the first P-type GaN layer 207 or the p++GaN contact layer by a re-filming process. Since there may be problems such as lattice polarity and doping diffusion when growing N-type GaN directly on the first P-type GaN layer 207, the interface quality can be ensured by optimizing the growth conditions or inserting a transition layer at the interface. In this embodiment, a second stress release layer 209 is grown after the second N-type GaN layer 208 is grown to relieve the stress of subsequent layers. Then, a second active region 210 is grown on the second stress release layer 209, for example, using an InGaN / GaN quantum well structure and designed to emit green light (wavelength of approximately 520 nm). A second electron blocking layer 211 (such as a Mg-doped AlGaN layer) is grown above the second active region 210, and finally a top second P-type GaN layer 212 (magnesium-doped) is grown.

[0027] Subsequently, the epitaxial wafer is processed into a chip. This embodiment can adopt a flip-chip structure to facilitate the removal of the electrode of the lower layer P1. Specifically, the second P-type GaN layer 212, the second electron blocking layer 211, the second active area 210, the second stress release layer 209, the second N-type GaN layer 208, the first P-type GaN layer 207, the first electron blocking layer 206, the first active area 205 and the first stress release layer 204 in the local area can be removed on the top surface of the wafer by photolithography and etching, exposing the first N-type GaN layer 203 below, and then metal is deposited to form the N electrode 213. Next, metal is deposited on the first P-type GaN layer 207 or the p++GaN contact layer to form the P electrode 214. After bonding and packaging, the dual-wavelength LED chip has a P electrode connected to the first P-type GaN layer 207 or the p++GaN contact layer, and an N electrode connected to the first N-type GaN layer 203; the second active area 210 is located on the top of the chip and is not directly connected to the electrode.

[0028] like Figure 4 As shown, when voltage is applied to the chip, the PN junction formed by the first P-type GaN layer 207 and the first N-type GaN layer 203 turns on, and current is injected into the first active area 205, causing the first active area 205 (blue light region) to generate electroluminescent blue light. At this time, no current is directly injected into the upper second active area 210, but a portion of the blue light emitted by the first active area 205 is transmitted upward to the second active area 210. The quantum well in the second active area 210 absorbs blue light photons from below and is excited, producing green light emission.

[0029] Because the light emitted by the upper second active area 210 is entirely stimulated by the light from the lower first active area 205, changes in the intensity of the blue light from the first active area 205 drive proportional changes in the green light from the second active area 210 as the current is adjusted. The relative intensity ratio between the two remains essentially constant, unlike conventional dual-wavelength LEDs, which experience ratio imbalances with current changes. This structure is particularly suitable for lighting devices requiring stable dual-color output or wide-spectrum output.

[0030] like Figure 5 As shown, the left figure is the emission spectrum of the structure in Example 1 of the present invention. In the current range of 10mA to 150mA, the peak positions of blue light and green light remain basically unchanged, and the green / blue intensity ratio (G / B) is maintained between about 30% and 44%, with little change, indicating that the dual-wave output ratio of the structure of the present invention is stable under different currents. The right figure is the emission spectrum of a traditional dual-wave structure LED sample. As the driving current increases from 20mA to 300mA, its G / B intensity ratio drops sharply from about 69% to below 10%, indicating that its emission ratio is seriously affected by the current. This figure clearly shows the significant improvement of the dual-wave spectrum stability of the structure of the present invention compared to the traditional structure.

[0031] It should be noted that those skilled in the art can select an appropriate electrode formation method according to packaging and application requirements, and these changes all fall within the scope of protection of the present invention.

Claims

1. A dual-wavelength LED epitaxial structure, comprising a first N-type semiconductor layer, a first active region, an intermediate semiconductor layer, a second active region, an electron blocking layer, and a P-type semiconductor layer, sequentially formed on a substrate, characterized in that: The first active region and the second active region respectively emit light of different wavelengths, and the intermediate semiconductor layer is provided between the first active region and the second active region to separate the two active regions from each other; only one PN junction for current injection is formed in the LED epitaxial structure, and light generated by current passing through the PN junction is emitted by one of the active regions and absorbed by the other active region to stimulate the latter to emit light.

2. The LED epitaxial structure according to claim 1, characterized in that: No P-type semiconductor layer is provided on the first active region, and the P-type semiconductor layer is located above the second active region; the PN junction for current injection is located between the middle semiconductor layer and the P-type semiconductor layer.

3. The LED epitaxial structure according to claim 1, wherein: A first P-type semiconductor layer is arranged above the first active area, and a second P-type semiconductor layer is arranged above the second active area, wherein the first P-type semiconductor layer is located below the middle semiconductor layer; the PN junction for current injection is formed between the first N-type semiconductor layer and the first P-type semiconductor layer.

4. The LED epitaxial structure according to claim 1, wherein: The first active region and the second active region are both InGaN / GaN multiple quantum well (MQW) structures; the light emitted by the first active region is blue light or green light, and the light emitted by the second active region is visible light with a different wavelength from that of the first active region.

5. The LED epitaxial structure according to claim 1, characterized in that: The first N-type semiconductor layer and the intermediate semiconductor layer are both silicon-doped N-type GaN layers, the P-type semiconductor layer is a magnesium-doped P-type GaN layer, and the electron blocking layer is a magnesium-doped AlGaN layer.

6. The method for preparing an LED epitaxial structure according to any one of claims 1 to 5, characterized in that: The steps include: a) epitaxially growing a first N-type semiconductor layer on a substrate, and epitaxially growing a first active region on the first N-type semiconductor layer; b) after the growth of the first active region is completed, continuing to epitaxially grow an intermediate semiconductor layer and a second active region thereon; c) epitaxially growing an electron blocking layer and a P-type semiconductor layer on the second active region to form a PN junction structure including the second active region; d) making electrodes, and connecting a P electrode and an N electrode to the P-type semiconductor layer and the N-type semiconductor layer respectively, so that current is only injected into the second active area and the first active area emits light by absorbing light from the second active area.

7. The preparation method according to claim 6, characterized in that In the step a), after the growth of the first active area is completed, no P-type semiconductor layer is formed, and the intermediate semiconductor layer and the second active area are directly grown in step b); in the step d), the N electrode contacts the intermediate semiconductor layer through an etching window, and the P electrode is arranged on the surface of the P-type semiconductor layer.

8. The preparation method according to claim 6, characterized in that In step a), the first P-type semiconductor layer is continued to be grown after the growth of the first active area is completed; in step b), a second N-type semiconductor layer is grown on the first P-type semiconductor layer, and then a second active area is grown; in step c), a second P-type semiconductor layer is grown on the second active area; in step d), the P electrode is electrically connected to the first P-type semiconductor layer and the N electrode is electrically connected to the first N-type semiconductor layer by a flip-chip method.

9. The preparation method according to claim 6, characterized in that Before growing the second active region in step b), a stress release layer is first grown on the middle semiconductor layer.

10. The preparation method according to claim 6, characterized in that The quantum well structures of the first active region and the second active region are InGaN / GaN multiple quantum wells respectively. The light emission wavelength of the first active region is in the blue light band, and the light emission wavelength of the second active region is in the green light band.

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