Deep ultraviolet LED with a gradient transparent electrode contact layer and method for preparing the same

Abstract

The present invention discloses a deep ultraviolet LED with a graded transparent electrode contact layer and a preparation method thereof. The deep ultraviolet LED with a graded transparent electrode contact layer includes a sapphire substrate, an AlN intrinsic layer, an n-type AlGaN electron injection layer, a current spreading layer, a multi-quantum well active layer, an electron blocking layer, a p-type AlGaN hole injection layer, and a transparent electrode contact layer which are sequentially stacked; the transparent electrode contact layer is a Mg-doped AlGaN single-layer structure. Along the growth direction, the Mg doping concentration of the transparent electrode contact layer shows an increasing trend, and the percentage of Al component shows a decreasing trend. By introducing a graded transparent electrode contact layer to replace the existing p-type GaN contact layer, on the one hand, good ohmic contact can be obtained, the voltage can be reduced, and the hole injection ability can be improved; on the other hand, the problem of serious light absorption of the existing p-type GaN contact layer is solved, the absorption of the p-type semiconductor contact layer for the emitted light is significantly reduced, and the light emission efficiency of the device is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor optoelectronics, and particularly to a deep ultraviolet LED with a graded transparent electrode contact layer and a preparation method thereof. Background Art

[0002] The ultraviolet band can generally be divided into: long-wave ultraviolet, middle-wave ultraviolet, short-wave ultraviolet, and vacuum ultraviolet according to its biological effects. Although ultraviolet light cannot be perceived by the human eye, its applications are very extensive. Long-wave ultraviolet light sources have great application prospects in the fields of medical treatment, ultraviolet curing, ultraviolet lithography, information storage, plant lighting, etc.; while deep ultraviolet light, which includes middle-wave ultraviolet and short-wave ultraviolet, plays an irreplaceable role in sterilization and disinfection, water purification, biochemical detection, non-line-of-sight communication, etc.

[0003] At present, in ultraviolet epitaxial wafers based on AlGaN, on the one hand, as the Al component increases, the solid solubility of Mg gradually decreases, and the impurity energy level of Mg gradually increases, resulting in low doping and activation efficiencies of high-Al-component AlGaN. On the other hand, the bandgap width of the AlGaN material also gradually increases as the Al component increases, which causes relatively great difficulties in the fabrication of ohmic contacts. Therefore, considering the above two aspects, p-type GaN is still selected as the hole injection layer for mass-produced deep ultraviolet LEDs at present. However, the p-type GaN layer will absorb the emitted light from the quantum well active region, causing the deep ultraviolet LED device to lose about 50% of the light extraction efficiency. This phenomenon of light extraction loss due to the ultraviolet absorption of the p-type GaN layer is particularly severe in flip-chip devices. To solve the problem of ohmic contact, low-temperature p-type GaN is usually used as the contact layer to contact the metal. Although the thickness of this p-type GaN layer is relatively thin, it still absorbs a large proportion of the emitted light. Therefore, there is an urgent need to provide a new deep ultraviolet LED design method to solve the above problems. Summary of the Invention

[0004] The purpose of the present invention is to provide a deep ultraviolet LED with a graded transparent electrode contact layer and a preparation method thereof, which is used to solve the problem that the existing deep ultraviolet LED absorbs a large amount of emitted light when using low-temperature p-type GaN as the contact layer.

[0005] To solve the above technical problems, the first solution provided by the present invention is: a deep ultraviolet LED with a graded transparent electrode contact layer, which includes a sapphire substrate, an AlN intrinsic layer, an n-type AlGaN electron injection layer, a current spreading layer, a multi-quantum well active region, an electron blocking layer, a p-type AlGaN hole injection layer, and a transparent electrode contact layer arranged in a stacked manner in sequence; the transparent electrode contact layer is a Mg-doped AlGaN single-layer structure, and along the growth direction, the Mg doping concentration of the transparent electrode contact layer shows an increasing trend, and the percentage of the Al component shows a decreasing trend.

[0006] Preferably, the p-type AlGaN hole injection layer is a Mg-doped single AlGaN layer structure, with the Al component percentage being 20% to 60%, the thickness being 0.1 nm to 50 nm, and the Mg doping concentration being 1×10 18 to 5×10 20 cm -3 .

[0007] Preferably, the bandgap of the p-type AlGaN hole injection layer is greater than or equal to the photon energy of the emitted light in the multi-quantum well active layer.

[0008] Specifically, the deep ultraviolet LED with a graded transparent electrode contact layer further includes an n electrode and a p electrode; a stepped structure is formed between the n-type AlGaN electron injection layer and the current spreading layer, and the area of the n-type AlGaN electron injection layer is larger than the area of the current spreading layer; the p electrode is disposed on the transparent electrode contact layer, and the n electrode is disposed at the stepped structure of the n-type AlGaN electron injection layer and is located on one side of the current spreading layer.

[0009] Preferably, the Al component percentage on the side of the transparent electrode contact layer in contact with the p-type AlGaN hole injection layer is X 1 , the Al component percentage on the side of the transparent electrode contact layer in contact with the p electrode is X 2 , and the Al component percentage of the p-type AlGaN hole injection layer is X 3 , satisfying X 3 ≥X 1 >X 2 >0%.

[0010] More preferably, in the direction from the p-type AlGaN hole injection layer to the p electrode, the Al component percentage of the transparent electrode contact layer linearly changes from X 1 to X 2 .

[0011] Preferably, the Mg doping concentration on the side of the transparent electrode contact layer in contact with the p-type AlGaN hole injection layer is M 1 , the Mg doping concentration on the side of the transparent electrode contact layer in contact with the p electrode is M 2 , satisfying 1×10 21 cm -3 ≥M 2 >M 1 ≥1×10 16 cm -3 .

[0012] More preferably, in the direction from the p-type AlGaN hole injection layer to the p electrode, the Mg doping concentration of the transparent electrode contact layer linearly changes from M 1 to M 2 .

[0013] Preferably, the thickness of the transparent electrode contact layer is 0.1 - 10 nm.

[0014] To solve the above technical problems, the second solution provided by the present invention is: a method for preparing a deep ultraviolet LED with a graded transparent electrode contact layer, which is used to prepare the deep ultraviolet LED with a graded transparent electrode contact layer in the aforementioned first solution, and includes the following steps:

[0015] (1) Under the condition of 400 - 800 °C, grow a buffer layer in the AlN intrinsic layer on the sapphire substrate, with a thickness of 10 - 50 nm.

[0016] (2) Raise the temperature to 1200 - 1400 °C, and grow an AlN intrinsic layer on the buffer layer in the AlN intrinsic layer. The total thickness of the AlN intrinsic layer is 500 - 4000 nm.

[0017] (3) Lower the temperature to 800 - 1200 °C, and grow an n-type AlGaN electron injection layer on the AlN intrinsic layer, doped with SiH 4 , where the percentage of Al component is 20 - 90%, and the thickness is 500 - 4000 nm.

[0018] (4) Maintain the temperature in step (3), stop introducing SiH 4 doping, and grow a current spreading layer on the n-type AlGaN electron injection layer, with the percentage of Al component being 70 - 100% and the thickness being 10 - 300 nm.

[0019] (5) Lower the temperature to 700 - 1100 °C, and grow a multi-quantum well active layer on the current spreading layer. The thickness of the well layer in the multi-quantum well active layer is 0.1 - 5 nm and the percentage of Al component in the well layer is 30 - 80%, and the thickness of the barrier layer is 1 - 30 nm and the percentage of Al component in the barrier is 40 - 90%.

[0020] (6) Under the condition of 700 - 1100 °C, grow an electron blocking layer on the multi-quantum well active layer, with the percentage of Al component being 50 - 100% and the thickness being 1 nm - 200 nm.

[0021] (7) Under the condition of 700 - 1100 °C, grow a p-type AlGaN hole injection layer on the electron blocking layer, with the percentage of Al component being 20 - 60% and the thickness being 0.1 - 50 nm, using Mg as the p-type dopant, and the Mg doping concentration is 1×10 18 - 5×10 20 cm -3 .

[0022] (8) A transparent electrode contact layer is grown on the p-type AlGaN hole injection layer. The growth temperature is linearly decreased from the termination temperature in step (7) to 400 - 800 °C. Meanwhile, the import amount of the Al source is linearly decreased and the Mg doping amount is linearly increased. The thickness of the transparent electrode contact layer is 0.1 - 10 nm, and Mg is used as the p-type dopant.

[0023] (9) Etch to the n-type AlGaN electron injection layer to form a step structure. Deposit an n electrode at the step structure of the n-type AlGaN electron injection layer, and deposit a p electrode on the transparent electrode contact layer.

[0024] The beneficial effects of the present invention are as follows: Different from the prior art, the present invention provides a deep ultraviolet LED with a graded transparent electrode contact layer and a preparation method thereof. By introducing a graded transparent electrode contact layer between the p-type AlGaN hole injection layer and the p electrode to replace the existing p-type GaN contact layer, on the one hand, good ohmic contact can be obtained, the voltage can be reduced, and the hole injection ability can be improved; on the other hand, the problem of serious light absorption of the existing p-type GaN contact layer is solved, the absorption of the p-type semiconductor contact layer for the emitted light is significantly reduced, and the light emission efficiency of the device is greatly improved. Description of the Drawings

[0025] Figure 1 It is a schematic structural diagram of an embodiment of the deep ultraviolet LED with a graded transparent electrode contact layer in the present invention. Detailed Embodiments

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0027] For the first solution proposed in the present invention, please refer to Figure 1 , Figure 1It is a schematic structural diagram of an embodiment of a deep ultraviolet LED with a graded transparent electrode contact layer in the present invention. The deep ultraviolet LED with a graded transparent electrode contact layer in the present invention includes a sapphire substrate 1, an AlN intrinsic layer 2, an n-type AlGaN electron injection layer 3, a current spreading layer 4, a multi-quantum well active layer 5, an electron blocking layer 6, a p-type AlGaN hole injection layer 7, and a transparent electrode contact layer 8 which are sequentially stacked; the transparent electrode contact layer is a Mg-doped AlGaN single-layer structure. Along the growth direction, the Mg doping concentration of the transparent electrode contact layer shows an increasing trend, and the percentage of Al component shows a decreasing trend. By introducing a graded transparent electrode contact layer to replace the existing p-type GaN contact layer, on the one hand, good ohmic contact can be obtained, the voltage can be reduced, and the hole injection ability can be improved; on the other hand, the problem of serious light absorption of the existing p-type GaN contact layer is solved, the absorption of the p-type semiconductor contact layer for the emitted light is significantly reduced, and the light emission efficiency of the device is greatly improved. In addition, the above deep ultraviolet LED with a graded transparent electrode contact layer further includes an n electrode 9 and a p electrode 10. A stepped structure is formed between the n-type AlGaN electron injection layer and the current spreading layer, and the area of the n-type AlGaN electron injection layer is larger than that of the current spreading layer; the p electrode is disposed on the transparent electrode contact layer, and the n electrode is disposed at the stepped structure of the n-type AlGaN electron injection layer and on one side of the current spreading layer. The n electrode and the p electrode preferably use common transparent electrode materials, and metal electrode materials can also be selected, which are not limited herein.

[0028] Specifically, the p-type AlGaN hole injection layer is a Mg-doped AlGaN single-layer structure, the percentage of Al component is 20% - 60%, the thickness is 0.1 nm - 50 nm, and the Mg doping concentration is 1×10 18 ~5×10 20 cm -3 . In this embodiment, the band gap width of the p-type AlGaN hole injection layer is greater than or equal to the photon energy of the emitted light in the multi-quantum well active layer; the thickness of the transparent electrode contact layer is preferably 0.1 - 10 nm.

[0029] In this embodiment, the percentage of Al component on the side where the transparent electrode contact layer contacts the p-type AlGaN hole injection layer is X 1 , the percentage of Al component on the side where the transparent electrode contact layer contacts the p electrode is X 2 , and the percentage of Al component of the p-type AlGaN hole injection layer is X 3 , satisfying X 3 ≥X 1 >X 2 >0%; further preferably, in the direction from the p-type AlGaN hole injection layer to the p electrode, the percentage of Al component of the transparent electrode contact layer linearly changes from X 1 to X 2。

[0030] In this embodiment, the Mg doping concentration on the side of the transparent electrode contact layer in contact with the p-type AlGaN hole injection layer is M 1 , and the Mg doping concentration on the side of the transparent electrode contact layer in contact with the p electrode is M 2 , satisfying 1×10 21 cm -3 ≥M 2 >M 1 ≥1×10 16 cm -3 ; Further preferably, in the direction from the p-type AlGaN hole injection layer to the p electrode, the Mg doping concentration of the transparent electrode contact layer linearly changes from M 1 to M 2 .

[0031] For the second solution proposed by the present invention, the steps of the preparation method of the deep ultraviolet LED with a gradient transparent electrode contact layer include:

[0032] (1) Under the condition of 400-800 °C, grow a buffer layer in the AlN intrinsic layer on the sapphire substrate, with a thickness of 10-50 nm.

[0033] (2) Raise the temperature to 1200-1400 °C, and grow an AlN intrinsic layer on the buffer layer in the AlN intrinsic layer. The total thickness of the AlN intrinsic layer is 500-4000 nm.

[0034] (3) Lower the temperature to 800-1200 °C, and grow an n-type AlGaN electron injection layer on the AlN intrinsic layer, doped with SiH 4 , where the Al component percentage is 20-90%, and the thickness is 500-4000 nm.

[0035] (4) Maintain the temperature of step (3), stop introducing SiH 4 doping, and grow a current spreading layer on the n-type AlGaN electron injection layer, with an Al component percentage of 70-100% and a thickness of 10-300 nm.

[0036] (5) Lower the temperature to 700-1100 °C, and grow a multi-quantum well active layer on the current spreading layer. The well layer thickness of the multi-quantum well active layer is 0.1-5 nm and the Al component percentage in the well layer is 30-80%, and the barrier layer thickness is 1-30 nm and the Al component percentage in the barrier is 40-90%.

[0037] (6) Under the condition of 700-1100 °C, grow an electron blocking layer on the multi-quantum well active layer, with an Al component percentage of 50-100% and a thickness of 1 nm-200 nm.

[0038] (7) Under the condition of 700 - 1100 °C, a p-type AlGaN hole injection layer is grown on the electron blocking layer. The Al component percentage is 20 - 60%, the thickness is 0.1 - 50 nm, Mg is used as the p-type dopant, and the Mg doping concentration is 1×10 18 ~5×10 20 cm -3 .

[0039] (8) A transparent electrode contact layer is grown on the p-type AlGaN hole injection layer. The growth temperature linearly decreases from the termination temperature of step (7) to 400 - 800 °C. At the same time, the import amount of the Al source linearly decreases and the Mg doping amount linearly increases. The thickness of the transparent electrode contact layer is 0.1 - 10 nm, and Mg is used as the p-type dopant.

[0040] (9) Etch to the n-type AlGaN electron injection layer to form a step structure. Deposit an n electrode at the step structure of the n-type AlGaN electron injection layer, and deposit a p electrode on the transparent electrode contact layer.

[0041] Since the deep ultraviolet LED preparation method with a graded transparent electrode contact layer in the second solution is used to prepare the deep ultraviolet LED with a graded transparent electrode contact layer in the aforementioned first solution, the structures and functions of the deep ultraviolet LEDs with a graded transparent electrode contact layer in the two solutions should be consistent.

[0042] Next, the performance and effects of the above deep ultraviolet LED with a graded transparent electrode contact layer are characterized through specific examples, and analysis is carried out based on the characterization results.

[0043] Example 1

[0044] In this example, the steps for preparing a deep ultraviolet LED with a graded transparent electrode contact layer are as follows:

[0045] (1) Under the condition of 600 °C, a buffer layer in the AlN intrinsic layer is grown on the sapphire substrate, with a thickness of 20 nm.

[0046] (2) Heat up to 1200 °C, and grow an AlN intrinsic layer on the buffer layer in the AlN intrinsic layer. The total thickness of the AlN intrinsic layer is 800 nm.

[0047] (3) Cool down to 1000 °C, and grow an n-type AlGaN electron injection layer on the AlN intrinsic layer, doped with SiH 4 , where the Al component percentage is 50%, and the thickness is 1000 nm.

[0048] (4) Maintain 1000 °C, and stop introducing SiH 4Dope and grow a current spreading layer on the n-type AlGaN electron injection layer, where the Al component percentage is 70% and the thickness is 60 nm.

[0049] (5) Cool down to 900 °C and grow a multi-quantum well active layer on the current spreading layer, where the well layer thickness of the multi-quantum well active layer is 1 nm and the Al component percentage in the well layer is 65%, and the barrier layer thickness is 10 nm and the Al component percentage in the barrier is 80%.

[0050] (6) Grow an electron blocking layer on the multi-quantum well active layer at 1000 °C, where the Al component percentage is 60% and the thickness is 50 nm.

[0051] (7) Grow a p-type AlGaN hole injection layer on the electron blocking layer at 800 °C, with an Al component percentage of 45% and a thickness of 25 nm. Use Mg as the p-type dopant, and the doping concentration is 1×10 19 cm -3 .

[0052] (8) Grow a transparent electrode contact layer on the p-type AlGaN hole injection layer. The growth temperature linearly decreases from 800 °C to 700 °C. At the same time, the introduction amount of the Al source linearly decreases from 45% to 5% in terms of component percentage, and the Mg doping amount linearly increases from 1×10 19 cm -3 to 2×10 20 cm -3 . The thickness of the transparent electrode contact layer is 5 nm, and Mg is used as the p-type dopant.

[0053] (9) Etch to the n-type AlGaN electron injection layer to form a step structure. Deposit an n electrode at the step structure of the n-type AlGaN electron injection layer, and deposit a p electrode on the transparent electrode contact layer to obtain a deep ultraviolet LED with a graded transparent electrode contact layer.

[0054] Comparative Example 1

[0055] In this comparative example, based on the preparation steps of Example 1, replace step (8) above with: Grow a p-type GaN contact layer on the p-type AlGaN hole injection layer at 850 °C, where the Al component percentage is 45% and the thickness is 5 nm, and the doping concentration is 1×10 20 cm -3 , and the dopant is Mg; other steps are the same as those in Example 1.

[0056] Comparative Example 2

[0057] In this comparative example, based on the preparation steps of Example 1, in step (8) above, adjust the thickness of the transparent electrode contact layer to 20 nm, and other steps are the same as those in Example 1.

[0058] Comparative Example 3

[0059] In this comparative example, based on the preparation steps of Example 1, the above step (8) was replaced with: growing a transparent electrode contact layer on the p-type AlGaN hole injection layer, the growth temperature was linearly decreased from 800 °C to 700 °C. At the same time, the import amount of Al source increased linearly from 5% to 45% in terms of component percentage, and the Mg doping amount decreased linearly by 2×10 19 cm -3 linearly to 2×10 18 cm -3 . The thickness of the transparent electrode contact layer was 5 nm, and Mg was used as the p-type dopant.

[0060] Optical performance tests were carried out on the LED samples prepared in the above Example 1 and Comparative Examples 1 to 3, and the test current was 40 mA for all. The results are shown in Table 1.

[0061] Table 1

[0062] Output optical power / mW Operating voltage / V Example 1 12.8 5.5 Comparative Example 1 11.6 6.5 Comparative Example 2 12.6 6.2 Comparative Example 3 11.0 6.8

[0063] Based on the test data of the samples of Example 1 and Comparative Example 1 in Table 1, it can be seen that the optical power of the sample of Comparative Example 1 is lower than that of the sample of Example 1, and the operating voltage of the sample of Comparative Example 1 is higher than that of Example 1. And what Comparative Example 1 reflects is exactly the traditional LED structure with a p-type GaN contact layer. Thus, it shows that after replacing the traditional p-type GaN contact layer with a transparent electrode contact layer in this application, by using the gradual change design method of its Al component percentage and Mg doping concentration, a better ohmic contact effect can be obtained, thereby reducing the operating voltage; at the same time, the absorption of ultraviolet light by the p-type contact layer is reduced, and the light output power is increased.

[0064] Based on the test data of the samples of Example 1 and Comparative Example 2 in Table 1, it can be seen that compared with Example 1, the thickness of the transparent electrode contact layer in Comparative Example 2 is thicker. Although it has little influence on the light output power, the too thick transparent electrode contact layer in Comparative Example 2 will affect its ohmic contact effect, making the operating voltage of the sample of Comparative Example 2 slightly higher than that of Example 1. Therefore, to obtain a better ohmic contact effect, it is necessary to strictly control the thickness of the prepared transparent electrode contact layer.

[0065] Based on the test data of the samples in Example 1 and Comparative Example 3 in Table 1, it can be seen that compared with Example 1, Comparative Example 2 adjusted the gradual design method of the Al component percentage and the Mg doping concentration. However, after adjusting the gradual change trend, the light output power of the sample in Comparative Example 2 was significantly lower than that of the sample in Example 1, and the working voltage was significantly higher. This indicates that after adjusting the gradual change trend of the Al component percentage and the Mg doping concentration of the transparent electrode contact layer, both the light output effect and the ohmic contact effect will be significantly weakened. Therefore, it is necessary to adopt the method in Example 1 where the introduction amount of the Al source decreases linearly and the Mg doping amount increases linearly to obtain better light output effect and ohmic contact effect.

[0066] Different from the prior art, the present invention provides a deep ultraviolet LED with a gradually changing transparent electrode contact layer and a preparation method thereof. By introducing a gradually changing transparent electrode contact layer between the p-type AlGaN hole injection layer and the p-electrode to replace the existing p-type GaN contact layer, on the one hand, a better ohmic contact can be obtained, the voltage can be reduced, and the hole injection ability can be improved; on the other hand, the problem of serious light absorption of the existing p-type GaN contact layer is solved, the absorption of the p-type semiconductor contact layer for the emitted light is significantly reduced, and the light emission efficiency of the device is greatly improved.

[0067] The above embodiments only represent the implementation manners of the present invention, and the description is relatively specific and detailed, but it cannot be understood as a limitation to the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent should be subject to the appended claims.

Claims

1. A deep ultraviolet LED with a gradually changing transparent electrode contact layer, characterized in that, it includes a sapphire substrate, an AlN intrinsic layer, an n-type AlGaN electron injection layer, a current spreading layer, a multi-quantum well active layer, an electron blocking layer, a p-type AlGaN hole injection layer, and a transparent electrode contact layer which are stacked in sequence. The deep ultraviolet LED with the gradually changing transparent electrode contact layer further includes a p electrode, and the p electrode is disposed on the transparent electrode contact layer; the transparent electrode contact layer is a Mg-doped AlGaN single-layer structure. Along the growth direction, the Mg doping concentration of the transparent electrode contact layer shows an increasing trend, and the percentage of Al component shows a decreasing trend; The percentage of Al component on the side of the transparent electrode contact layer in contact with the p-type AlGaN hole injection layer is X 1 , and the percentage of Al component on the side of the transparent electrode contact layer in contact with the p electrode is X 2 , and the percentage of Al component of the p-type AlGaN hole injection layer is X 3 , satisfying X 3 ≥X 1 >X 2 >0%, in the direction from the p-type AlGaN hole injection layer to the p electrode, the percentage of Al component of the transparent electrode contact layer linearly changes from X 1 to X 2 ; The Mg doping concentration on the side of the transparent electrode contact layer in contact with the p-type AlGaN hole injection layer is M 1 and the Mg doping concentration on the side of the transparent electrode contact layer in contact with the p electrode is M 2 , satisfying 1×10 21 cm -3 ≥M 2 >M 1 ≥1×10 16 cm -3 , in the direction from the p-type AlGaN hole injection layer to the p electrode, the Mg doping concentration of the transparent electrode contact layer linearly changes from M 1 to M 2 .

2. The deep ultraviolet LED with the gradually changing transparent electrode contact layer according to claim 1, characterized in that, The p-type AlGaN hole injection layer is a Mg-doped single AlGaN layer structure, with an Al component percentage of 20% - 60%, a thickness of 0.1 nm - 50 nm, and a Mg doping concentration of 1×10 18 ~5×10 20 cm -3 .

3. The deep ultraviolet LED with the gradually changing transparent electrode contact layer according to claim 2, characterized in that, the bandgap width of the p-type AlGaN hole injection layer is greater than or equal to the photon energy of the emitted light in the multi-quantum well active layer.

4. The deep ultraviolet LED with the gradually changing transparent electrode contact layer according to claim 1, characterized in that, the deep ultraviolet LED with the gradually changing transparent electrode contact layer further includes an n electrode; a stepped structure is formed between the n-type AlGaN electron injection layer and the current spreading layer, and the area of the n-type AlGaN electron injection layer is larger than the area of the current spreading layer, and the n electrode is disposed at the stepped structure of the n-type AlGaN electron injection layer.

5. The deep ultraviolet LED with the gradually changing transparent electrode contact layer according to claim 1, characterized in that, the thickness of the transparent electrode contact layer is 0.1 - 10 nm.

6. A preparation method of a deep ultraviolet LED with a gradually changing transparent electrode contact layer as described in any one of claims 1 - 5, characterized in that, it includes the following steps: (1) Under the condition of 400 - 800 °C, grow a buffer layer in the AlN intrinsic layer on the sapphire substrate, and the thickness is 10 - 50 nm; (2) Raise the temperature to 1200 - 1400 °C, and grow the AlN intrinsic layer on the buffer layer in the AlN intrinsic layer, and the total thickness of the AlN intrinsic layer is 500 - 4000 nm; (3) Cool down to 800 - 1200 °C, and grow an n-type AlGaN electron injection layer on the AlN intrinsic layer, using SiH 4 doping, where the percentage of Al component is 20 - 90%, and the thickness is 500 - 4000 nm; (4) Maintain the temperature in step (3), stop introducing SiH 4 doping, grow a current spreading layer on the n-type AlGaN electron injection layer, with an Al component percentage of 70-100% and a thickness of 10-300 nm; (5) Lower the temperature to 700 - 1100 °C, and grow the multi-quantum well active layer on the current spreading layer, wherein the thickness of the well layer of the multi-quantum well active layer is 0.1 - 5 nm and the percentage of Al component in the well layer is 30 - 80%, and the thickness of the barrier layer is 1 - 30 nm and the percentage of Al component in the barrier layer is 40 - 90%; (6) Under the condition of 700 - 1100 °C, grow the electron blocking layer on the multi-quantum well active layer, and the percentage of Al component in it is 50 - 100%, and the thickness is 1 nm - 200 nm; (7) Under the condition of 700 - 1100 °C, a p-type AlGaN hole injection layer is grown on the electron blocking layer, with an Al component percentage of 20 - 60% and a thickness of 0.1 - 50 nm. Mg is used as the p-type dopant, and the Mg doping concentration is 1×10 18 ~5×10 20 cm -3 ; (8) A transparent electrode contact layer is grown on the p-type AlGaN hole injection layer, and the growth temperature is linearly reduced from the termination temperature in step (7) to 400 - 800 °C. At the same time, the introduction amount of the Al source decreases linearly and the Mg doping amount increases linearly. The thickness of the transparent electrode contact layer is 0.1 - 10 nm, and Mg is used as the p-type dopant; (9) Etch to the n-type AlGaN electron injection layer to form a stepped structure, deposit an n electrode at the stepped structure of the n-type AlGaN electron injection layer, and deposit a p electrode on the transparent electrode contact layer.

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