Green light-emitting diode (LED)

Abstract

The invention relates to a green light LED. The green light LED comprises an n-type layer, a multi-quantum well layer, a superlattice electron blocking layer, a p-type layer and a p-type electrode which are sequentially stacked on a substrate, the multi-quantum well layer comprises n periods of quantum well structures, each period of quantum well structure is formed by sequentially stacking an InGaN layer, a first GaN layer, an AlGaN insertion layer and a second GaN layer, and the Al component of the AlGaN insertion layer increases layer by layer in the direction from the substrate to the p-type layer; the superlattice electron barrier layer is composed of an AlGaN / GaN laminated layer with m periods and an AlGaN thick layer arranged on the laminated layer; due to the arrangement of the multi-quantum well layer and the superlattice electron barrier layer, the barrier height is effectively increased, electron leakage is reduced, the moving speed of hot electrons is relieved, faster diffusion of current carriers is promoted, the current congestion effect is improved, and better radiation luminescence performance and higher modulation bandwidth are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of optical communication, and in particular to a MINI green light LED with high modulation bandwidth and high light output efficiency. Background Art

[0002] The emergence of LED optical communication technology has alleviated the problem of lack of RF communication spectrum. Optical communication has quickly become a hot topic of current research due to its advantages of high speed, high bandwidth, low loss and low interference. MINI LED refers to LED devices with a size of less than 200μm. Since the current expansion is better than that of large-size LEDs, as the size of the LED decreases, the lateral propagation of the LED light beam decreases, and higher luminous efficiency can be obtained; and the horizontal resistance decreases, which reasonably improves the current distribution, greatly improves the current expansion effect, improves the recombination efficiency of carriers, and further improves the bandwidth. The advantages of high modulation bandwidth and high current density of MINI LED make it often selected as the signal source in LED visible light communication, but due to the strong polarization caused by the increase in mismatch stress between AlGaN and GaN, it leads to a serious quantum confined Stark effect (QCSE), which aggravates electron leakage and non-radiative recombination, especially Ogier recombination. Affected by non-radiative recombination, the current level of MINI LED light output power LOP is very small, less than 1mW, and the wall plug efficiency WPE is less than 1%, which cannot meet the requirements of long transmission distance. The modulation bandwidth and light output efficiency are both limited, making it difficult to meet high-speed communication. Summary of the invention

[0003] In view of the technical problems existing in the prior art, the primary purpose of the present invention is to provide a green light LED, which has a high modulation bandwidth and high light output efficiency. The carrier concentration distribution in the active area is more uniform, which accelerates the diffusion movement of carriers, greatly improves the probability of carrier radiation recombination, reduces the lifetime of radiation recombination, and further improves the light output efficiency and modulation bandwidth.

[0004] In one aspect, the present invention provides a green light LED, comprising an n-type layer, a multi-quantum well layer, a superlattice electron blocking layer, a p-type layer and a p-type electrode stacked in sequence on a substrate;

[0005] The multi-quantum well layer includes a quantum well structure of n periods, and a single-period quantum well structure is formed by sequentially stacking an InGaN layer, a first GaN layer, an AlGaN insertion layer, and a second GaN layer. In the multi-quantum well layer, the Al component of the AlGaN insertion layer increases layer by layer in a direction from the substrate to the p-type layer, 2≤n≤5; the setting of the AlGaN insertion layer increases the effective barrier height, reduces electron leakage, effectively alleviates the movement speed of hot electrons, promotes faster diffusion of carriers, and improves the current congestion effect;

[0006] The superlattice electron blocking layer is composed of an AlGaN / GaN stack of m periods and an AlGaN thick layer arranged on the stack, 2≤m≤5; the setting of the superlattice electron blocking layer ultimately increases the effective barrier height of electrons to the maximum, reduces the effective barrier height of holes, improves the injection efficiency of carriers, and improves the radiation luminescence performance.

[0007] Furthermore, in the multi-quantum well layer, the Al composition of the AlGaN insertion layer in the quantum well structure of the first period is 0.04, the Al composition of the AlGaN insertion layer in the quantum structure of the last period is 0.2, and the difference in the Al composition of the AlGaN insertion layer in the quantum well structure of adjacent periods is constant.

[0008] Furthermore, in a single period quantum well structure, the thickness of the InGaN layer is 3 nm, the thickness of the first GaN layer is 3 nm, the thickness of the AlGaN insertion layer is 1-2 nm, and the thickness of the second GaN layer is 3 nm.

[0009] Furthermore, in the AlGaN / GaN stack, the thickness of the AlGaN layer is 1-2 nm, and the thickness of the GaN layer is 1 nm.

[0010] Furthermore, in the AlGaN / GaN stack, the Al composition of the AlGaN layer is 0.2.

[0011] Furthermore, the thickness of the AlGaN thick layer is 3-6 nm, and the Al component of the AlGaN thick layer is 0.2.

[0012] Furthermore, the p-type layer is a p-type GaN layer.

[0013] Furthermore, the n-type layer is an n-type GaN layer, and an n-type electrode is also disposed on the n-type GaN layer.

[0014] Furthermore, the substrate is a sapphire or silicon carbide substrate.

[0015] Furthermore, the green LED is a MINI green LED.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects:

[0017] The present invention forms a single period structure of InGaN / GaN / AlGaN / GaN by inserting an AlGaN insertion layer into a GaN barrier layer of a multi-quantum well structure, and at the same time, the Al component of the AlGaN insertion layer increases layer by layer as the period increases, and a superlattice electron blocking layer consisting of an AlGaN / GaN stacking period and an AlGaN thick layer arranged on the stacking period is arranged on the multi-quantum well structure, thereby increasing the effective barrier height, reducing electron leakage, effectively alleviating the movement speed of hot electrons, promoting faster diffusion of carriers, improving current congestion, and ultimately increasing the effective barrier height of electrons to the maximum, and reducing the effective barrier height of holes. In one embodiment, the effective barrier height of electrons is increased to 462.4 meV, and the effective barrier height of holes is reduced to 338.2 meV, both of which are superior to conventional structures and inserted barrier structures with unchanged Al components, and the electron concentration and hole concentration of each quantum well are increased.

[0018] The present invention proposes to insert an AlGaN insertion layer with gradually increasing composition into the GaN barrier layer of the multi-quantum well structure, and the AlGaN / GaN stacked periodic structure has the greatest benefit for carrier injection, and has better radiative luminescence performance and higher modulation bandwidth. In one embodiment, at 40mA, the light output power reaches 222.93mW, and the efficiency decreases from 87% to 48.6%. The more composite carriers there are inside the active region, the greater the probability of radiative recombination of carriers, the shorter the lifetime of radiative recombination, and the higher the 3dB modulation bandwidth of the corresponding device. In one embodiment, the modulation bandwidth of the green light LED is increased from 76.9MHz to 93.8MHz at a turn-on voltage of 3.86V and 60mA. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 FIG. 1 is a schematic structural diagram of a green LED according to an embodiment of the present invention.

[0020] Figure 2 FIG. 1 is a schematic diagram of the energy band between the last QB and EBL in a green LED structure according to an embodiment of the present invention.

[0021] Figure 3 Schematic diagram of carrier movement among the n-GaN layer, the multi-quantum well MQWs layer, the SL EBL layer and the p-GaN layer in the green LED structure according to an embodiment of the present invention.

[0022] Figure 4 Schematic diagram of the structure of the MQW layer in a proportional green LED structure of the present invention.

[0023] Figure 5 Schematic diagram of the structure of the MQW layer in a proportional green LED structure of the present invention.

[0024] Figure 6 Schematic diagram of the structure of the MQW layer in a proportional green LED structure of the present invention.

[0025] Figure 7 FIG. 1 is a schematic diagram of the structure of a MQW combined with an SL EBL layer in a green LED structure according to an embodiment of the present invention.

[0026] Figure 8 This is a diagram of the light output power of LED D and LED A, LED B, and LED C of the present invention at different currents.

[0027] Fig. 9 The internal quantum efficiency diagrams of LED D, LED A, LED B, and LED C of the present invention at different currents.

[0028] Fig.10 This is a transient response curve diagram of LED D, LED A, LED B, and LED C of the present invention under a 60mA square wave current pulse. DETAILED DESCRIPTION

[0029] Next, the technical scheme in the embodiment of the present invention will be clearly and completely described in conjunction with the drawings of the present invention. The described embodiment is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiments of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention.

[0030] Spatially relative terms such as "below," "beneath," "below," "above," "upper," etc. are used in this specification to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures.

[0031] In addition, the use of terms such as "first", "second", etc. to describe various elements, layers, regions, sections, etc. is not intended to be limiting. The use of "having", "containing", "including", "comprising", etc. is open-ended, indicating the presence of the stated elements or features, but does not exclude additional elements or features. Unless the context clearly states otherwise.

[0032] The embodiment of the present invention provides a MINI green LED with high modulation bandwidth and high light output efficiency, and its structure is shown in FIG. Figure 1 The green light LED comprises a substrate 1, an n-type layer 2, a multi-quantum well layer, a superlattice electron blocking layer, a p-type layer 14 and a p-type electrode 15 which are sequentially stacked on the substrate 1.

[0033] The substrate 1 is a sapphire substrate or a silicon carbide substrate. The n-type layer 2 is a Si-doped n-type GaN layer. In a preferred embodiment, the doping concentration is 5×10 18 cm -3 , with a thickness of 4μm;. The multi-quantum well layer includes a quantum well structure of n periods, 2≤n≤5, and a single period quantum well structure is formed by stacking an InGaN layer, a first GaN layer, an AlGaN insertion layer and a second GaN layer in sequence; along the direction from the substrate to the p-type layer, the Al component of the AlGaN insertion layer increases layer by layer, and in the first period quantum well structure (i.e., the period adjacent to the n-type GaN), the Al component of the AlGaN insertion layer is 0.04, and in the last period quantum well structure (i.e., the period adjacent to the superlattice electron blocking layer), the Al component of the AlGaN insertion layer is 0.2, and the difference in the Al component of the AlGaN insertion layer in the quantum well structure of adjacent periods is constant; in a single quantum well period, the thickness of the InGaN layer is 3nm, the thickness of the first GaN layer is 3nm, the thickness of the AlGaN insertion layer is 1-2nm, and the thickness of the second GaN layer is 3nm. In a preferred embodiment, the multi-quantum well layer includes a 5-period quantum well structure, the Al components of the AlGaN insertion layers 6, 7, 8, 9, 10 are 0.04, 0.08, 0.12, 0.16, 0.2 respectively, and the thickness of the AlGaN insertion layer is 2 nm.

[0034] The superlattice electron blocking layer is composed of m periods of AlGaN / GaN stacking layers and an AlGaN thick layer disposed on the stacking layers, 2≤m≤5. In the AlGaN / GaN stacking layers, the thickness of the single AlGaN layer is 1-2nm, the thickness of the GaN layer is 1nm, and the doping type of the GaN layer is p-type with a doping concentration of 3×10 17 cm -3 The thickness of the AlGaN thick layer is 3 to 6 nm, its doping type is p-type, and the doping concentration is 3×10 17 cm -3 In a preferred embodiment, m is selected to be 5, and in the AlGaN / GaN stack, the Al composition of the AlGaN layer is constant and is selected to be 0.2, and the Al composition of the AlGaN thick layer is 0.2.

[0035] In the multi-quantum well structure, the GaN layer and the InGaN layer jointly provide space for the recombination of carriers. The change of their energy bands greatly affects the efficiency of carrier recombination. Since the electron mobility is much greater than the hole mobility, most electrons escape and leak without recombination in the potential well. The present invention adopts an AlGaN insertion layer to provide a gradually increasing height of the electron barrier, limit the electrons with high mobility to stay in the potential well as much as possible, so that they have more opportunities to recombine with holes, greatly improve the efficiency of carrier recombination, and further shorten the carrier life. The optoelectronic performance and communication performance of the green light LED device are further improved. The AlGaN insertion layer adopts a gradual change of Al component to make the carrier concentration in the multi-potential well more uniform, improve current congestion, especially for small-sized MINI-type LEDs, the impact of the current crowding effect is more significant. In addition, the AlGaN / GaN superlattice electron blocking layer is used to provide a lower height of the hole barrier, which helps to inject low-mobility holes into the potential well faster and improve the carrier recombination efficiency.

[0036] The P-type layer 14 is a p-type GaN layer doped with Mg, and its doping concentration is 3×10 17 cm -3 The thickness is 100 nm. An n-type electrode 16 is disposed on the n-type GaN layer. Both the p-type electrode 15 and the n-type electrode 16 are transparent electrodes, and ITO, conductive polymer PEDOT, metal grid, carbon nanorod, nano silver wire or graphene are selected.

[0037] Figure 2 The energy band diagram between the last QB and EBL of a green LED in a preferred embodiment of the present invention is shown in FIG. 1. Considering the sidewall defects of the MINI LED, the electron trap energy level is 0.24 eV (E c -0.24eV), the trap density and capture interface are 1×10 13 cm -3 and 3.4×10 -17 cm 2 The hole trap energy level is 0.46 eV (Ev+0.46 eV), and the trap density and capture interface are 1.6×10 13 cm -3 and 2.1×10 -15 cm 2 Then we get the energy band between QB and EBL as Figure 2 As shown, the electron barrier height between the last quantum barrier and the EBL of the structure of the present invention is the largest, reaching 462.4 meV, which can effectively prevent electron leakage. In addition, the hole barrier of the LED is reduced, proving that adding the AlGaN / GaN superlattice electron blocking layer structure can obtain higher hole injection.

[0038] Figure 3 The schematic diagram of carrier movement between the n-GaN layer, AlGaN / GaN multi-quantum well layer, SL EBL layer and p-GaN layer of a green light LED in a preferred embodiment of the present invention is shown in the figure. As shown in the figure, the period number of the multi-quantum well and the superlattice are both 5. When the forward working current is injected into the LED device, the electrons start to migrate from the n-GaN side, and some of the electrons are captured by the first potential well, while the other part of the electrons escape the first potential well and move to the second, third and subsequent potential wells. At this time, the gradually rising potential barrier slows down the speed of electron migration, so that the electrons are more evenly captured by multiple potential wells. The holes that start to migrate from p-GaN on the other side encounter a lowered potential barrier, which accelerates their own injection speed, so that the electrons captured by the potential wells combine with the holes injected into the potential well on the p side.

[0039] With regard to the light output power, internal quantum efficiency and modulation bandwidth of the green LED of the preferred embodiment of the present invention, the green LED of the present invention has obvious advantages over the other three conventional LED structures in terms of light output power, internal quantum efficiency and modulation bandwidth. It makes the distribution of carriers more uniform, increases the radiation recombination rate of carriers in the device, reduces the lifetime of carrier radiation recombination, and improves the light output efficiency and modulation bandwidth of the chip. In addition, electron leakage is reduced and hole injection is increased, and the current crowding effect is effectively alleviated, reducing the Droop effect under high current. The structure diagram of the active area of ​​the other three conventional LED structures LED A, LED B, LED C and the green LED of the present invention embodiment is shown in FIG. Figure 4-7 As shown, the structures of other layers are the same:

[0040] like Figure 4 As shown: LED A: Its MQW layer is an InGaN / GaN layer with 5 periods, the thickness of the InGaN layer is 3nm, and the thickness of the GaN layer is 3nm.

[0041] like Figure 5 As shown: LED B: Its MQW layer is based on the MQW layer of LED A, and an AlGaN barrier layer with a thickness of 1 to 2 nm and a fixed Al composition is inserted into the GaN layer, and the Al composition is 0.04 to 0.2.

[0042] like Figure 6 As shown: LED C: Its MQW layer is based on the MQW layer of LED A, and an AlGaN barrier layer with a thickness of 1 to 2 nm and increasing Al composition layer by layer is inserted into the GaN layer, and its Al composition is 0.04, 0.08, 0.12, 0.16, and 0.2 respectively.

[0043] like Figure 7LED D is the structure used in this embodiment. On the basis of LED C, a superlattice electron blocking layer consisting of an AlGaN / GaN layer with a fixed Al component and a p-AlGaN thick layer is added, wherein the thickness of the p-AlGaN thick layer is 3 to 6 nm, the thickness of the AlGaN layer is 1 to 2 nm, the thickness of the GaN layer is 1 nm, and the p-type doping concentration is 3×10 17 cm -3 . .

[0044] The device size of the four LED structures is 30um×30um.

[0045] Figure 8-Figure 10 Schematic diagram of the internal quantum efficiency, light output power and transient response of the above four MINI green LED-D and LED-A, LED-B, and LED-C.

[0046] The schematic diagram of the light output power of the MINI green LED-D and LED-A, LED-B, and LED-C under different currents in the embodiment of the present invention is shown in FIG. Figure 8 As shown, the light output power of LED A at an operating current of 200mA is only 47mW; LED B is based on LED A and an AlGaN barrier layer with a constant Al component of 1 to 2nm thickness is inserted. The lattice matching improves the crystal quality, thereby reducing the electrostatic field and increasing the wave function overlap of electrons and holes, so that the light output power at the same current is increased to 70mW; LED C is based on LED A and an AlGaN barrier layer with a gradually increasing Al component of 1 to 2nm thickness is inserted. The distribution of electrons and holes in the gradient barrier layer is more uniform, which reduces non-radiative recombination, so that the light output power at the same current is further increased to 179mW; LED D is based on LED C and a superlattice electron blocking layer composed of an AlGaN / GaN layer with a fixed Al component and a p-AlGaN layer is added, so that a lower hole barrier height is provided, which helps to inject low-mobility holes into the potential well faster, improve the carrier recombination efficiency, and finally increase the light output power to 224mW.

[0047] Consideration of the internal quantum efficiency of the MINI green LED of the preferred embodiment: Fig. 9The schematic diagram of the internal quantum efficiency of the MINI green LED structure of the present invention and the other three structures at different currents. It can be seen from the figure that the internal quantum efficiency of LED A and LEDB are 0.6 and 0.7 respectively, while the internal quantum efficiency of LED C and LED D is as high as 0.9. The InGaN / GaN multi-quantum well layer inserted with the Al component gradient AlGaN layer gradually raises the potential barrier of the active region, making the distribution of carriers more uniform and improving the internal quantum efficiency of the device. In addition, it can be seen that the Droop effect of LED A, LED B, LED C, and LED D under a high current of 200mA decreased by 86%, 79%, 58%, and 39% respectively, among which the Droop effect of LED D under a high current has the least impact, which is mainly due to the reduction of electron leakage and the increase of hole injection, and effectively alleviates the current crowding effect, reducing the Droop effect under a high current.

[0048] Regarding the communication performance of the MINI green LED of the embodiment, Fig.10 The transient response curves of the green LED structure of a preferred embodiment of the present invention and the above three comparative structures under a 60mA square wave current pulse are shown in the figure. From the figure, it can be seen that the rise time 1 of LED D is 6.38ns, the rise time 2 is 6.40ns, and the average rise time τ is 6.39ns. According to the bandwidth formula of the electro-optical-electrical modulation The modulation bandwidth of the structure is 93.8 MHz. The modulation bandwidths of the other three structures, LED A, LED B, and LED C, are 76.9 MHz, 75.4 MHz, and 91.5 MHz, respectively. It can be seen that the modulation bandwidth of the structure used in this embodiment is higher than that of the other three structures. The main reason is that there are more composite carriers inside the active region, and the injected carrier concentration is greater, so the recombination probability is greater, the carrier lifetime becomes shorter, and the modulation bandwidth is correspondingly higher, which is more suitable for communication system applications.

[0049] In optical communication applications, half-power bandwidth (or 3dB bandwidth) is of great significance for the frequency selectivity, signal integrity and performance evaluation of the system. According to the theoretical bandwidth formula of electrical-optical-electrical modulation It can be seen that π is the circumference, q is the basic electron charge, d is the thickness of the active layer, η is the carrier concentration of IQE under steady-state conditions, A is the Schottky recombination coefficient, B is the radiation recombination coefficient, and C is the Auger recombination coefficient. It can be seen that with the increase of the number of recombination carriers inside the active region, the probability of radiation recombination of carriers increases, the lifetime of radiation recombination becomes shorter, and the 3dB modulation bandwidth of the LED is correspondingly higher, which is consistent with the above-mentioned increase in the modulation bandwidth from 76.9MHz to 93.8MHz at a turn-on voltage of 3.86V and 60mA.

[0050] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A green light LED, characterized in that: It includes an n-type layer, a multi-quantum well layer, a superlattice electron blocking layer, a p-type layer and a p-type electrode which are sequentially stacked on a substrate; The multi-quantum well layer includes a quantum well structure of n periods, and a single period of the quantum well structure is formed by sequentially stacking an InGaN layer, a first GaN layer, an AlGaN insertion layer, and a second GaN layer. In the multi-quantum well layer, the Al component of the AlGaN insertion layer increases layer by layer along the direction from the substrate to the p-type layer, and 2≤n≤5; The superlattice electron blocking layer is composed of m periods of AlGaN / GaN stacking layers and an AlGaN thick layer arranged on the stacking layers, 2≤m≤5.

2. The green LED according to claim 1, characterized in that: In the multi-quantum well layer, the Al component of the AlGaN insertion layer in the quantum well structure of the first period is 0.04, the Al component of the AlGaN insertion layer in the quantum well structure of the last period is 0.2, and the difference of the Al component of the AlGaN insertion layer in the quantum well structure of adjacent periods is constant.

3. The green LED according to claim 1 or 2, characterized in that: In a single-period quantum well structure, the thickness of the InGaN layer is 3 nm, the thickness of the first GaN layer is 3 nm, the thickness of the AlGaN insertion layer is 1 to 2 nm, and the thickness of the second GaN layer is 3 nm.

4. The green LED according to claim 3, characterized in that: In the AlGaN / GaN stack, the thickness of the AlGaN layer is 1-2 nm, and the thickness of the GaN layer is 1 nm.

5. The green LED according to claim 4, characterized in that: In the AlGaN / GaN stack, the Al composition of the AlGaN layer is 0.

2.

6. The green LED according to claim 1, 2, 4 or 5, characterized in that: The thickness of the AlGaN thick layer is 3-6 nm, and the Al component of the AlGaN thick layer is 0.

2.

7. The green LED according to claim 6, characterized in that: The p-type layer is a p-type GaN layer.

8. The green LED according to claim 6, characterized in that: The n-type layer is an n-type GaN layer, and an n-type electrode is also arranged on the n-type GaN layer.

9. The green LED according to claim 6, characterized in that: The substrate is a sapphire or silicon carbide substrate.

10. The green LED according to any one of claims 1, 2, 4, 5, 7 to 9, characterized in that: The green light LED is a MINI green light LED.

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