light-emitting diode

Abstract

light-emitting diode with - a first semiconductor body (10) comprising at least one active region (11) which is electrically contacted, wherein, during operation of the light-emitting diode, electromagnetic radiation (110) of a first wavelength range is generated in the active region (11), and - a second semiconductor body (20) attached to a top surface (10a) of the first semiconductor body (10), wherein the second semiconductor body (20) has an n-doped region (22), a p-doped region (23) and a re-emission region (21) with a multiple quantum well structure (213) arranged between the n-doped region (22) and the p-doped region (23), and wherein, during operation of the light-emitting diode, electromagnetic radiation (110) of the first wavelength range is absorbed in the re-emission region (21) and electromagnetic radiation of a second wavelength range (220) is re-emitted, and - a connecting material (30) arranged between the first (10) and second semiconductor body (20), wherein the connecting material (30) mechanically connects the first (10) and the second semiconductor body (20), wherein the re-emission region (21) comprises a multiple quantum well structure (213) with at least 20 quantum well layers and the multiple quantum well structure (213) is optically pumped during operation of the light-emitting diode.

Description

[0001] A light-emitting diode is specified.

[0002] Light-emitting diodes are known from documents EP 1 132 977 A2, US 2006 / 0 145 137 A1, WO 2006 / 062 588 A1, DE 10 2004 052 245 A1, WO 2007 / 146 860 A1, US 2005 / 0 023 545 A1, US 2008 / 0 035 944 A1, US 2004 / 0 041 164 A1 and US 2007 / 0 045 609 A1.

[0003] It has been found that the internal efficiency of light-emitting diodes (LEDs) based on materials such as InGaN decreases with increasing wavelength of the emitted electromagnetic radiation, from approximately 80% at 400 nm to approximately 30% at 540 nm. This means that the internal efficiency of LEDs suitable for producing green light is very low compared to LEDs emitting radiation in the UV or blue range.

[0004] One way to increase the internal efficiency of LEDs suitable for emitting green light could be to increase the number of electrically pumped quantum wells. However, this approach to solving the described problem has been found to have strict limitations due to the uneven charge carrier distribution during the electrical operation of the LED. According to current knowledge, a maximum of two quantum wells can be fully energized in green-light-emitting InGaN-based LEDs; adding further quantum wells does not appear to have a positive effect on the internal efficiency of the LED.

[0005] One task is to identify a light-emitting diode (LED) that can generate electromagnetic radiation with exceptional efficiency. Another task is to identify a light-emitting diode that can generate green light with exceptional efficiency.

[0006] The light-emitting diode (LED) comprises a first semiconductor body. This semiconductor body is, for example, epitaxially grown and can be based on the InGaN material system. The semiconductor body includes at least one active region, which is electrically contacted. During operation of the LED, electromagnetic radiation of a first wavelength range is generated in the active region of the first semiconductor body. This electromagnetic radiation is generated by electrically driving the active region. The electromagnetic radiation of the first wavelength range includes, for example, electromagnetic radiation from the ultraviolet (UV) range and / or blue light.

[0007] The light-emitting diode comprises a second semiconductor body attached to a top surface of the first semiconductor body. The second semiconductor body is also preferably epitaxially fabricated. It can be based on the InGaN or InGaAlP material system. The second semiconductor body includes a re-emission region with a multiple quantum well structure. The term "quantum well structure" here does not refer to the dimensionality of the quantization. It encompasses, among other things, quantum wells, quantum wires, and quantum dots, as well as any combination thereof.

[0008] During operation of the light-emitting diode, electromagnetic radiation of the first wavelength range is absorbed in the re-emission region, and electromagnetic radiation of a second wavelength range is re-emitted. The second wavelength range preferably comprises electromagnetic radiation with longer wavelengths than the first wavelength range. In particular, the second wavelength range includes electromagnetic radiation from the wavelength range of green and / or yellow and / or red light.

[0009] Particularly with regard to a second semiconductor body based on InGaAlP, the advantage is that, firstly, absorbing current-spreading layers and electrical contacts can be dispensed with. Secondly, the thermally activated loss current can be reduced by passivating the surface facing the first semiconductor body, thus reducing the temperature dependence of the efficiency.

[0010] The second semiconductor body is preferably arranged such that electromagnetic radiation of the first wavelength range can pass from the first semiconductor body into the second semiconductor body. For this purpose, the second semiconductor body is preferably arranged on a radiation emission surface of the first semiconductor body. A large proportion of the electromagnetic radiation generated in the first semiconductor body passes into the second semiconductor body. "A large proportion of the electromagnetic radiation" is understood to mean at least 50%, preferably at least 70%, and particularly preferably at least 85% of the electromagnetic radiation of the first wavelength range. The second semiconductor body is designed to be particularly large and preferably covers the entire radiation emission surface on the top side of the first semiconductor body.For example, the first and second semiconductor bodies may be flush with each other laterally, or the second semiconductor body may extend beyond the first semiconductor body laterally. The lateral direction is the direction that is perpendicular to an epitaxial growth direction of the first semiconductor body, or that runs parallel to a layer of the first or second semiconductor body.

[0011] A connecting material is arranged between the first and the second semiconductor bodies, the connecting material mechanically connecting the first and the second semiconductor bodies.

[0012] The connecting material can, for example, be a semiconductor material from which the first and second semiconductor bodies are formed. The first and second semiconductor bodies are then monolithically integrated together.

[0013] In this case, the first and second semiconductor bodies are, for example, manufactured in a single epitaxial growth process and thus formed as a single unit. Furthermore, it is possible that the first and second semiconductor bodies are joined together by a wafer bonding process. This wafer bonding process could be, for example, direct bonding or anodic bonding. The surfaces of the two semiconductor bodies facing each other are free of roughening and are smoothed, if necessary, before being joined.

[0014] Alternatively, the connecting material could be a transparent, electrically conductive material. For example, the connecting material could be a TCO (Transparent Conductive Oxide) material. In this case, the first and second semiconductor bodies could be connected to each other via anodic or direct bonding using the connecting material.

[0015] Alternatively, the connecting material can be electrically insulating. This connecting material could be, for example, a silicone, a high-refractive-index silicone with a refractive index greater than 1.5, an epoxy resin, silicon oxide, or silicon nitride. The first and second semiconductor bodies can then be joined together by gluing or bonding using this connecting material.

[0016] According to at least one embodiment of the light-emitting diode (LED), the LED comprises a first semiconductor body having at least one electrically contacted active region, wherein, during operation of the LED, electromagnetic radiation of a first wavelength range is generated in the active region, and a second semiconductor body attached to the top of the first semiconductor body, wherein the second semiconductor body has a re-emission region with a multiple quantum well structure, and wherein, during operation of the LED, electromagnetic radiation of the first wavelength range is absorbed and electromagnetic radiation of a second wavelength range is re-emitted in the re-emission region. The first and second semiconductor bodies are connected to each other by a bonding material arranged between them.

[0017] In the described light-emitting diode (LED), the re-emission region of the second semiconductor body is preferably not electrically contacted. This means that electromagnetic radiation in the re-emission region, i.e., the electromagnetic radiation of the second wavelength range, is not generated by electrically driving the multiple quantum well structure in the re-emission region, but rather by optically driving it. Thus, the LED is based, among other things, on the understanding that if the multiple quantum well structure is pumped optically rather than electrically, a uniform charge distribution within the structure is achieved.By directly arranging the first semiconductor body, which generates shorter-wavelength electromagnetic radiation during operation, with the second semiconductor body, which generates longer-wavelength electromagnetic radiation during operation, it is possible to utilize a maximum proportion of the electromagnetic radiation from the first wavelength range for the uniform generation of electron-hole pairs in the multiple quantum well structure of the re-emission region. Furthermore, such a light-emitting diode is characterized by particularly good spectral and thermal properties. This means, for example, that the active region of the first semiconductor body can be cooled very effectively, since the second semiconductor body acts as a kind of heat spreader for the first semiconductor body.

[0018] According to at least one embodiment of the light-emitting diode, the first semiconductor body has a plurality of first coupling structures on its upper surface facing the second semiconductor body. These first coupling structures can, for example, be roughened surfaces of the first semiconductor body. Furthermore, the first coupling structures can be pyramid-shaped or truncated pyramid-shaped protrusions on the upper surface of the first semiconductor body. The first coupling structures can be made of the same material as the semiconductor body and, for example, be structured from the material of the first semiconductor body. It is also possible that the first coupling structures are additional structures made of a material different from that of the first semiconductor body.The first output coupling structures preferably consist of a material whose optical refractive index differs from the refractive index of the first semiconductor body by no more than 30%.

[0019] According to at least one embodiment of the light-emitting diode, the interconnect material surrounds the first output structures at their exposed outer surfaces. That is, the interconnect material is inserted between the first and second semiconductor bodies and covers the first output structures. The interconnect material can then completely cover the first output structures at their exposed outer surfaces, so that the first output structures are embedded in the interconnect material. It is then possible that the first output structures do not touch the second semiconductor body at the top surface, but rather that interconnect material is arranged between the first output structures and the second semiconductor body.

[0020] Overall, the first coupling structures enable electromagnetic radiation of the first wavelength range to exit the first semiconductor body and enter the second semiconductor body with a greater probability than would be the case without them. For example, the first coupling structures reduce the probability of total internal reflection of electromagnetic radiation from the first wavelength range at the interface between the first and second semiconductor bodies.

[0021] According to at least one embodiment of the light-emitting diode, the second semiconductor body has a plurality of second output structures on its upper surface facing away from the first semiconductor body and / or on its lower surface facing the first semiconductor body. The second output structures can be identical or different from the first output structures of the first semiconductor body. That is, the second output structures can be structured from the same material as the second semiconductor body and thus consist of the same material. However, it is also possible for the second output structures to consist of a material different from the material of the second semiconductor body.

[0022] Preferably, the second semiconductor body has a plurality of second coupling structures on its upper surface facing away from the first semiconductor body and on its lower surface facing the first semiconductor body. The second coupling structures on the lower surface of the second semiconductor body advantageously reduce Fresnel losses at the interface between the second semiconductor body and the interconnect material.

[0023] In one embodiment, the second output coupling structures of the second semiconductor body consist of a material whose optical refractive index differs from the optical refractive index of the second semiconductor body by no more than 30%.

[0024] The second output coupling structures of the second semiconductor body increase the probability of light exiting the second semiconductor body.

[0025] The emitted light can be electromagnetic radiation from either the first or the second wavelength range. This means the LED can emit mixed light from both wavelength ranges. This mixed light could, for example, be white light.

[0026] However, it is also possible that the LED emits predominantly electromagnetic radiation from the second wavelength range. This means that the majority – for example, at least 90% – of the electromagnetic radiation from the first wavelength range that enters the second semiconductor body is absorbed there. In this way, it is possible for the LED to emit, for example, pure green, yellow, or red light.

[0027] According to at least one embodiment of the light-emitting diode, the material of the output coupling structures of the first and / or the second semiconductor body contains or consists of one of the following materials: titanium oxide, zinc selenide, aluminum nitride, silicon carbide, boron nitride, and / or tantalum oxide. These materials are characterized by having an optical refractive index that differs by no more than 30% from the refractive index of an InGaN-based semiconductor body.

[0028] According to at least one embodiment of the light-emitting diode (LED), a reflective layer is attached to the underside of the first semiconductor body facing away from the second. The reflective layer is, for example, a dielectric mirror, a Bragg mirror, a metallic mirror, or a combination of these. The reflective layer is designed to reflect electromagnetic radiation of the first wavelength range towards the second semiconductor body. This allows a particularly large proportion of the electromagnetic radiation of the first wavelength range to enter the second semiconductor body. Furthermore, the reflective layer can also reflect electromagnetic radiation of the second wavelength range, which is emitted from the second semiconductor body towards the first, back towards the second semiconductor body and thus out of the LED.

[0029] The multiple quantum structure of the re-emission region comprises at least 20 quantum well layers. These quantum well layers are arranged, for example, along a growth direction of the second semiconductor body and separated from each other by barrier layers. It has been shown that such a large number of quantum well layers can be uniformly populated with charge carriers by optical pumping, and that the efficiency of generating electromagnetic radiation in the second wavelength range increases significantly due to the high number of quantum well layers. Particularly in the full conversion of blue light or UV radiation to green light, the number of quantum well layers (also called quantum films) is important for the efficiency of light generation, since photons are only absorbed in the quantum well layers, and a high number of quantum well layers ensures a sufficient absorption cross-section.Furthermore, with a high number of quantum well layers, the lower charge carrier density in the individual wells results in a beneficial shift of the efficiency maximum to higher currents. Therefore, full conversion is possible at high current densities of > 100 A / cm². 2 be more efficient than a directly electrically pumped green LED.

[0030] The following section explains the light-emitting diode described here in more detail using exemplary embodiments and the corresponding figures. The Fig. 1A and Fig. Figure 1B shows the efficiency of electrically operated blue and green light-emitting diodes using graphical plots. With the Fig. 2A, Fig. 2B, Fig. 2C and Fig. In 2D, schematic cross-sectional views illustrate examples of the light-emitting diodes described here. Based on the graphic representations of the Fig. 3A, Fig. 3B, Fig. 4A, Fig. Section 4B explains in more detail the properties of the light-emitting diodes described here.

[0031] Identical, similar, or similarly effective elements in the figures are marked with the same reference symbols. The figures and the relative sizes of the elements depicted within them are not to be considered to scale. Rather, individual elements may be exaggerated for clarity and / or better understanding.

[0032] The Fig. Figure 1A shows, using a graph, the external efficiency (EQE) including optical losses and the internal efficiency (IQE) excluding optical losses for a light-emitting diode (LED) emitting electromagnetic radiation at a peak wavelength of 435 nm, i.e., blue light. The LED is electrically driven. As can be seen from the Fig. As can be seen in 1A, the internal efficiency is up to over 80%.

[0033] The Fig. Figure 1B shows, using a graph, the external efficiency (EQE) and the internal efficiency (IQE) for an electrically driven light-emitting diode emitting green light at a peak wavelength of 540 nm. As shown in the Fig. As can be seen in 1B, the maximum internal efficiency is less than 50%.

[0034] Overall, electrically pumped green LEDs are less efficient than electrically pumped blue LEDs or UV-emitting LEDs.

[0035] The Fig. Figure 2A shows a first embodiment of a light-emitting diode described here, illustrated by a schematic sectional view. The light-emitting diode of the Fig. Assembly 2A comprises a first semiconductor body 10 and a second semiconductor body 20. The first semiconductor body 10 and the second semiconductor body 20 are stacked one above the other. The second semiconductor body 20 follows the first semiconductor body 10 at its upper surface 10a. The radiation emission surface of the first semiconductor body 10 is also located at its upper surface 10a, through which all or a large part of the electromagnetic radiation 110 emitted from the first semiconductor body 10 exits.

[0036] The first semiconductor body 10 comprises a p-doped region 12 and an n-doped region 13. The active region 11 is arranged between the p-doped region 12 and the n-doped region 13. The active region 11 is electrically driven; the electrical connections are located in the Fig. 2A not shown (see the Fig. 2D). For example, the active region 11 comprises a pn junction, a single quantum well structure, or a multiple quantum well structure. On its upper surface 10a, the first semiconductor body 10 has first output coupling structures 14, which in this case are formed from the material of the first semiconductor body 10. For example, the first output coupling structures are a roughening produced by KOH etching. However, the first output coupling structures 14 can also be formed from other materials, as described above.

[0037] The second semiconductor body 20 comprises an n-doped region 22, a p-doped region 23, and a re-emission region 21 located between the two regions. The re-emission region 21 comprises a multiple quantum well structure. The re-emission region 21 is not electrically connected and is not electrically driven.

[0038] On its upper surface 20a, the second semiconductor body 20 comprises second output coupling structures 24, which are also structured into the semiconductor body 20 by means of KOH etching. The second output coupling structures 24 can also be formed from other materials, as described above. Second output coupling structures 24 can also be arranged on the lower surface 20b of the second semiconductor body 20 (not shown in the figure).

[0039] A connecting material 30, which in this case contains or consists of silicone, is arranged between the first semiconductor body 10 and the second semiconductor body 20. The connecting material 30 completely surrounds the first output coupling structures 14 of the first semiconductor body 10 at their exposed outer surfaces. The connecting material 30 is electrically insulating and forms a mechanical connection between the two semiconductor bodies.

[0040] The first semiconductor body 10 and the second semiconductor body 20 are presented here as separate epitaxially fabricated components and subsequently joined together by means of the connecting material 30. The second semiconductor body 20 and the first semiconductor body 10 are flush with each other at their side surfaces 20c and 10c, so that the semiconductor bodies 10 and 20 do not laterally overlap each other.

[0041] On the underside 10b of the first semiconductor body 10, facing away from the second semiconductor body 20, a mirror layer 40 is arranged, which in this case is designed as a metallic mirror, made, for example, of aluminum or silver. The mirror layer 40 is suitable for reflecting both electromagnetic radiation 110 from the first wavelength range and electromagnetic radiation 210 from the second wavelength range.

[0042] In the schematic sectional view of the Fig. Section 2B describes in more detail the multiple quantum well structure 213 of the re-emission region 21. The multiple quantum well structure 213 comprises a plurality of quantum well layers 211, which are separated from each other by barrier layers 212. Electromagnetic radiation of the first wavelength range 110 leads to a distribution of charge carriers 214 in the quantum well structures, which is uniform due to optical pumping.

[0043] In connection with the Fig. Figure 2C provides a schematic cross-sectional view illustrating a further embodiment of a light-emitting diode described herein. In this embodiment, the first semiconductor body 10 and the second semiconductor body 20 are monolithically integrated. This means, for example, that they are epitaxially deposited on top of each other in a single epitaxial device. Furthermore, it is possible that the first semiconductor body 10 and the second semiconductor body 20 are connected to each other by means of a wafer bonding process. In this embodiment, the connecting material 30 is formed by the semiconductor material 13, 22 of the first semiconductor body 10 and the second semiconductor body 20. Advantageously, the optical coupling between the active region 11 and the re-emission region 21 is better in this embodiment than in the embodiment that, for example, is combined with the Fig. 2A is described. A disadvantage is the more complicated manufacturing process in connection with the Fig. 2C shown embodiment.

[0044] A schematic 2D cross-sectional view illustrates one method for electrically contacting the active region 11 of the first semiconductor body 10. Channels 53 are introduced into the semiconductor body 10 from the underside 10b through the reflective layer. These channels are filled with an electrically conductive material that forms electrical contact points 51, 52 on the side of the reflective layer 40 facing away from the semiconductor body 10. Besides the embodiment shown, other connection options for electrically contacting the active layer 11 of the first semiconductor body 10 are also conceivable.

[0045] The graphic representation of the Fig. Figure 3A shows the absorption in the multiple quantum well structure 213 of the re-emission region 21 for the embodiment of the Fig. 2C (curve a) and the embodiment of the Fig. 2A (curve b) as a function of the wavelength λ of the electromagnetic radiation generated in the active layer 11. It can be seen that the absorption is optimal for electromagnetic radiation in the wavelength range of 400 nm, i.e., in the UV range. Preferably, electromagnetic radiation from the UV range is therefore generated in the active layer 11.

[0046] The Fig. Figure 3B shows a graphical representation of the efficiency plotted against the number of quantum well layers in the multiple quantum well structure 213. Curves a and b show the efficiency for the exemplary embodiments of the Fig. 2C and 2A, respectively. Curves c and d show the proportion of unconverted pump radiation still exiting the system for the embodiments of the Fig. 2C or 2A. However, there are also optical losses due to absorption, which in the variant according to Fig. 2C are higher than in the variant according to Fig. 2A. It can be seen that the efficiency increases with the number of quantum well layers 211 in the multiple quantum well structure 213. It should be noted that the monolithic structure, as it is associated with the Fig. 2C, as described in more detail, exhibits higher efficiency than the structure of Fig. 2A, in which silicon with a refractive index of approximately 1.4 is used as a connecting material 30 to connect the first semiconductor body 10 and the second semiconductor body 20.

[0047] The Fig. Figure 4A shows, using a graph, the efficiency versus the current at which the active area is operated. This is the internal efficiency, excluding optical losses. Since optical losses are not taken into account, the graph refers to the Fig. 4A both on the exemplary embodiment of the Fig. 2A as well as the exemplary embodiment of the Fig. 2C. Curve a shows the efficiency for five optically pumped quantum well layers, curve b for ten, curve c for 20, and curve f for 40 quantum well layers 211 in the multiple quantum well structure 213. Curve e shows the efficiency of the electrically pumped active region 11, which generates UV radiation. As can be seen from the Fig. As can be seen from the graph at 4A, the internal efficiency increases for higher currents. For currents above 200 mA, all curves for optically pumped multiple quantum well structures lie above the efficiency for an electrically pumped quantum well structure, as plotted in curve d.

[0048] The Fig. Figure 4B shows a graph of efficiency versus applied current, taking optical losses into account. The dashed lines refer to monolithically integrated embodiments, such as those used in conjunction with the Fig. 2C. The solid lines refer to embodiments in which the first semiconductor body 10 and the second semiconductor body 20 are manufactured separately, as described in connection with the Fig. 2A. A general trend can be seen in the improved efficiency of monolithically integrated LEDs due to lower optical losses. However, their manufacturing process is more complex.

[0049] Curve a shows the efficiency of an electrically pumped active region with a single quantum well layer generating green light for comparison. Curve b shows the situation for five quantum well layers, curve c for ten quantum well layers, curve d for 20 quantum well layers, and curve e for 40 quantum well layers, each with silicon as the connecting material 30 between the first semiconductor body 10 and the second semiconductor body 20.

[0050] Curve f shows the situation for five quantum well layers, curve g for ten quantum well layers, curve h for 20 quantum well layers, and curve i for 40 quantum well layers in the case where the first semiconductor body 10 and the second semiconductor body are monolithically integrated. Overall, the light-emitting diode exhibits a higher efficiency in the re-emission region 21 than the electrically pumped quantum well layer starting from approximately 20 optically pumped quantum well layers 211.

Claims

[1] Light-emitting diode with - a first semiconductor body (10) comprising at least one active region (11) which is electrically contacted, wherein, during operation of the light-emitting diode, electromagnetic radiation (110) of a first wavelength range is generated in the active region (11), and - a second semiconductor body (20) attached to a top surface (10a) of the first semiconductor body (10), wherein the second semiconductor body (20) has an n-doped region (22), a p-doped region (23) and a re-emission region (21) with a multiple quantum well structure (213) arranged between the n-doped region (22) and the p-doped region (23), and wherein, during operation of the light-emitting diode, electromagnetic radiation (110) of the first wavelength range is absorbed in the re-emission region (21) and electromagnetic radiation of a second wavelength range (220) is re-emitted, and - a connecting material (30) arranged between the first (10) and second semiconductor body (20), wherein the connecting material (30) mechanically connects the first (10) and the second semiconductor body (20), wherein the re-emission region (21) comprises a multiple quantum well structure (213) with at least 20 quantum well layers and the multiple quantum well structure (213) is optically pumped during operation of the light-emitting diode. [2] Light-emitting diode according to the previous claim, wherein the connecting material (30) is electrically insulating. [3] Light-emitting diode according to the previous claim, wherein the connecting material (30) is silicone or contains silicone. [4] Light-emitting diode according to one of the preceding claims, wherein the first semiconductor body (10) has a plurality of first output coupling structures (14) on its upper side (10a) facing the second semiconductor body (20). [5] Light-emitting diode according to the previous claim, wherein the connecting material (30) surrounds the first coupling structures (14) on their exposed outer surfaces. [6] Light-emitting diode according to one of the two preceding claims, wherein the first output coupling structures (14) consist of a material whose refractive index differs from the refractive index of the first semiconductor body (10) by no more than 30%. [7] Light-emitting diode according to the previous claim, wherein the second semiconductor body (20) has a plurality of second output coupling structures (24) on its upper side (20a) facing away from the first semiconductor body (10) and / or on its lower side (20b) facing the first semiconductor body (10). [8] Light-emitting diode according to the previous claim, wherein the second output coupling structures (24) consist of a material whose refractive index differs from the refractive index of the second semiconductor body (20) by no more than 30%. [9] Light-emitting diode according to one of the two preceding claims, wherein the material of the first output coupling structures (14) and the second output coupling structures (24) contains or consists of one of the following materials: TiO2, ZnS, AlN, SiC, BN, Ta2O5. [10] Light-emitting diode according to one of the preceding claims, in which a mirror layer (40) is attached to the underside (10b) of the first semiconductor body (10) facing away from the second semiconductor body (20). [11] Light-emitting diode according to one of the preceding claims, wherein the first wavelength range comprises electromagnetic radiation from the wavelength range of UV radiation and / or blue light. [12] Light-emitting diode according to the preceding claim, wherein the second wavelength range comprises electromagnetic radiation from the wavelength range of green light.

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