High speed and multi-contact light emitting diode for data communication

Abstract

The LED can have a structure optimized for an operating speed of the LED. The LED can be a micro-LED. The LED can have a p-doped region with one or more quantum wells instead of an intrinsic region. The LED can have an etched via therethrough.

Description

Background Technology

[0001] This invention generally relates to LEDs (light-emitting diodes), and more specifically, to LEDs in optical communication systems.

[0002] Lasers are increasingly dominating optical communications due to their narrow linewidth, single-space-mode output, and high-speed characteristics. The narrow linewidth of lasers means that high-speed signals can travel long distances through dispersive media without pulse broadening. Long-distance fiber optic links are often dispersion-limited, and therefore narrow-linewidth lasers are essential for long-distance fiber optic links. The single-space-mode nature of lasers also makes them relatively easy to couple into single-mode fibers.

[0003] Stimulated emission of lasers also allows for high modulation speeds. Directly modulated optical links can easily operate at 25 Gb / s and potentially carry 50 Gb / s of information using PAM4 modulation.

[0004] However, the use of lasers can pose challenges for very short-range optical communications (such as chip-to-chip communications). Summary of the Invention

[0005] Some embodiments provide an LED configured for high-speed operation. In some embodiments, the LED serves as part of a data communication system. In some embodiments, the data communication system is an on-chip, inter-chip, or multi-chip intra-module communication system. In some embodiments, the LED is a microLED.

[0006] Some embodiments provide an optical communication system for transmitting information provided by a processor to another region of the processor or another module in a multi-chip module, the optical communication system comprising: an LED associated with the processor; an LED driver for modulating the output optical power of the LED such that the LED generates light based on data provided from the processor to the LED driver; a detector for performing photoelectric conversion using the light, the detector having, for example, an electrical output modulated by the optical power incident on the detector; and an optical waveguide for optically coupling light from the LED to the detector; wherein the LED comprises: a p-type layer; an n-type layer; and a lightly doped composite layer, the composite layer comprising at least one quantum well between the p-type layer and the n-type layer. Some embodiments provide an optical communication system for transmitting information provided by a first integrated circuit (IC) (e.g., a processor) to another area of ​​the first IC, or to a second IC in a multi-chip module. The optical communication system includes: an LED associated with the first IC; an LED driver for activating the LED to generate light based on data provided from the first IC to the LED driver; a detector for performing photoelectric conversion using the light; and an optical waveguide optically coupling the LED and the detector; wherein the LED includes a plurality of etched vias. In some embodiments, the first and / or second IC is a processor.

[0007] These and other aspects of the invention will be more fully understood upon review of this disclosure. Attached Figure Description

[0008] Figure 1 This is a block diagram illustrating an example of the use of an LED according to aspects of the present invention.

[0009] Figure 2A This demonstrates a typical pin LED structure, and Figure 2B exhibit Figure 2A The diagram shows the device.

[0010] Figure 3A An optimized doping structure for an LED according to aspects of the present invention is shown, and Figure 3B exhibit Figure 3A The diagram shows the device.

[0011] Figure 4 A microLED with etched through-holes according to an aspect of the present invention is shown.

[0012] Figure 5 It includes tables indicating the trade-offs in the design of microLEDs with typical parameters. Detailed Implementation

[0013] Figure 1 Examples of using LEDs are shown, which may be microLEDs in various embodiments, as discussed in various ways herein. Figure 1 In this process, the silicon processor 111 performs various operations on or with data. For example, the silicon processor may perform calculations on the data, perform switching functions, or perform other functions. The silicon processor provides at least some data to the LED driver 113. The LED driver activates the microLED 115 to optically provide at least some data from the processor, wherein the LED driver thereby modulates the output optical power of the microLED to optically provide at least some data from the processor. Light generated by the microLED is provided to the optical coupler 117, which transmits the light to the optical propagation medium 119. In some embodiments, the optical propagation medium, which may be, for example, a waveguide, may be used to transfer light from one region of the silicon processor to another region of the silicon processor. In other embodiments, the optical propagation medium may be used to transfer light from the silicon processor to, for example, in a multi-chip module ( Figure 1 (Not shown in the text) (where the term "chip" is generally used interchangeably with "integrated circuit" or "IC" unless the context clearly indicates otherwise) another silicon processor, or memory, or another chip. In doing so, an optical propagation medium can transfer light to another optical coupler 121, which in turn passes the light to a detector 123 (e.g., a photodiode) for photoelectric conversion. An electrical signal containing at least some data can be amplified by an amplifier 125 and provided to the silicon processor (or another chip in a multi-chip module). In some embodiments, the micro-LEDs and detectors can be individually coupled to the waveguide, and / or in some embodiments, they can be coupled in parallel as an array. In addition to transferring light and data from one location to another, the optical waveguide can also split light into two or more outputs, thereby allowing data fan-out. The optical waveguide or medium can also perform some kind of switching to direct the output from one receiver to another. As will be appreciated by those skilled in the art, the optical link can be full-duplex, such that when one or more links exist from the first chip to the second chip, one or more links can also exist from the second chip to the first chip.

[0014] In some embodiments, microLEDs differ from semiconductor lasers (SLs) as follows: (1) microLEDs do not have an optical resonator structure; (2) the optical output from a microLED is almost entirely spontaneous emission, while the output from an SL is primarily stimulated emission; (3) the optical output from a microLED is temporally and spatially incoherent, while the optical output from an SL is significantly temporally and spatially coherent; (4) microLEDs are designed to drive minimum currents as low as zero, while SLs are designed to drive minimum threshold currents, typically at least 1 mA. In some embodiments, microLEDs differ from standard LEDs in that: (1) they have an emission region smaller than 100 μm x 100 μm (in some embodiments, smaller than 10 μm x 10 μm); (2) they typically have positive and negative contacts on the top and bottom surfaces, while standard LEDs typically have both positive and negative contacts on a single surface; (3) they are typically used in large arrays for display and interconnect applications.

[0015] MicroLEDs and detectors can be individually coupled to a waveguide or coupled to a waveguide in parallel as an array. In some embodiments, the microLED is a microLED with a structure optimized for speed (e.g., high modulation speed). In some embodiments, the microLED is used to couple optical data into the waveguide, providing highly parallel communication between chips in some embodiments, such as on an inserter or through 3D optical structures, such as optical structures containing optical waveguides and / or free-space optical propagation using optical elements (e.g., lenses and holograms). GaN-based microLEDs have been developed for display applications, and a packaging ecosystem has been developed for mounting such devices on polycrystalline silicon backplanes on silicon or glass. With relatively minor modifications, the elements of this packaging ecosystem can be used to interconnect ICs together for chip-to-chip communication.

[0016] Furthermore, for chip-to-chip communication, the short distances mean that material dispersion, which is associated with the wide emission spectral width of LEDs, is not necessarily a problem. Simple calculations indicate that for a GaN LED with a center wavelength in the 400nm to 450nm range and a spectral width of 20nm, if the LED is modulated at 4Gb / s and propagates through a SiO2 waveguide or fiber, the waveguide or fiber can be up to 5 meters long with a dispersion power loss of less than 2dB. Since chip-to-chip communication within a multi-chip module (MCM) or across a PC board is typically less than tens of centimeters, the wide spectrum of LEDs is not a problem. Moreover, we can even use highly multimode waveguides into which the output light of the LED can be relatively easily coupled. Due to the short distances, mode dispersion in multimode waveguides is also not a problem. At a signal rate of 4Gb / s, even in a waveguide with a 10% core-cladding refractive index step of 0.67 NA, waveguide lengths can be up to 85cm with low dispersion power loss; smaller core-cladding refractive index steps generally have longer reachability. Therefore, in many embodiments, broadband LEDs and multimode waveguides are sufficient for chip-to-chip communication.

[0017] Furthermore, in various embodiments, the microLEDs are fabricated in very small sizes, with an emission area diameter of less than 2 μm. This tiny device exhibits very high brightness and can typically be coupled to multimode waveguides with high coupling efficiency. Although the output is typically Lambertian, with the appropriate use of reflectors, microlenses in some embodiments, and embedding the microLED within the waveguide in others, coupling efficiency can reach 30% or higher. MicroLEDs typically possess high quantum efficiencies, similar to or even exceeding those of lasers. Since they do not suffer much waveguide loss over short distances, even at blue or green wavelengths, low emission power is not required, and in some embodiments, a small microLED operating at less than 10 μA is sufficient.

[0018] Generally, the achievable modulation speed of microLEDs is limited by carrier lifetime (and, if the microLED is too large, by capacitance), and typically cannot reach the modulation speeds of lasers. However, clock speeds in microprocessors and logic appear to be limited to a few Gb / s. Input / output data in ICs is often accelerated using serializers / deserializers (SERDES) to create a smaller number of higher-speed channels. For example, commercially available switching ICs can currently operate at clock speeds of several GHz, but communicate with 256 or 512 channels at 50 Gb / s or 100 Gb / s per channel. These SERDES consume significant electrical power, and could be eliminated if the switching IC instead used a larger number of low-speed channels. Optical interconnects allow for greater parallelism and higher overall throughput by enabling the use of a larger number of channels, even at slower channel speeds. However, it is preferable to operate the LED at the highest possible modulation speed.

[0019] Furthermore, a significant advantage of microLEDs compared to lasers is their lack of a substantial threshold current. While quantum efficiency is a function of driving current, there is no significant threshold level, and microLEDs can operate at currents far lower than those of lasers. Given their usefulness in displays, a large infrastructure exists for mounting, connecting, and testing microLEDs on a variety of substrates. Moreover, GaN microLEDs typically exhibit much better high-temperature performance and reliability than semiconductor lasers.

[0020] Typically, GaN microLEDs optimized for display applications comprise cylindrical or quasi-cylindrical structures with pin doping distributions. The LED is switched on by forward-biasing a diode and injecting electrons from the n-region and holes from the p-region into the intermediate intrinsic region containing the InGaN quantum well. The p-contact is on one side of the structure, and the n-contact is on the other. In many applications, this cylinder is mounted on a chip, with the "bottom" side electrically contacting the chip and the "top" side contacting a common lead (e.g., ground or power lead). The top-side contact can be a transparent conductor, such as indium tin oxide (ITO). This "vertical" structure with contacts at both the top and bottom of the LED is generally preferred in microLEDs, but "lateral" structures with n- and p-contacts positioned on the same surface also exist. In any case, these structures do not need to be optimized for speed, as displays typically operate at 60Hz or 120Hz frame rates, rather than Gb / s.

[0021] We can make modifications to optimize the structure for speed. Generally, microLEDs are limited by the LED's capacitance and carrier recombination time. The capacitance, along with the driver's output impedance, forms an RC circuit, causing a roll-off at high frequencies. Carrier lifetime causes the LED to require a certain amount of time to turn off, because even after the electrical pulse ends, we must wait for most of the injected minority carriers to recombine, significantly reducing luminescence. Due to the small size of microLEDs, their capacitance (typically only a few nanofarads) does not significantly limit the modulation speed of the device; instead, the modulation speed is usually limited by the carrier lifetime. The modulation speed can be improved by applying a reverse bias to the microLED and electrically shaping the applied pulse to pull out the carriers, but structural modifications to the device can also improve the modulation speed.

[0022] A typical LED structure consists of a p-type region, an "active" region where carriers recombine and emit light, and an n-type region. Many different LED structures exist with varying structures in the active region. In some embodiments, the active region contains one or more quantum wells (QWs).

[0023] Generally, the speed of a microLED increases with current level. Carriers can recombine in an LED in three ways. At low current levels, recombination is trap-mediated (called SRH recombination). At higher current densities, these traps become saturated, and the quantum efficiency of the LED improves because radiative recombination dominates. As carrier density increases, this radiative recombination rate accelerates, increasing radiative efficiency but decreasing carrier lifetime. Therefore, the more difficult a microLED is to drive (e.g., at higher current densities), the faster it operates. At higher current densities, nonlinear nonradiative mechanisms, such as Auger recombination, further reduce carrier lifetime, but these non-emission mechanisms also reduce radiative quantum efficiency. For fast microLEDs with small diameters to increase current density at a given current, traps are relatively insignificant due to their saturation, and the relative importance of nonlinear nonradiative recombination versus radiative recombination rates determines the quantum efficiency.

[0024] Some embodiments utilize p, p-, n structures, where the "intrinsic region" is doped to a reasonable level of 10^16 / cm^3 to 10^17 / cm^3 in some embodiments to be p-type. In some embodiments, this results in a much narrower depletion width in the p-region compared to a pin structure. Electrons with high mobility are injected into the p-depletion region, which already has a high hole density. Since carrier recombination time is a function of carrier density, the device speed increases as the depletion width decreases. Carrier recombination time is also a function of the product of electron density and hole density, and p-doping in the depletion region increases the hole density, thus increasing the recombination rate and reducing the recombination time. A narrower depletion region may also have the undesirable effect of increasing the capacitance of the microLED, but this is not significant for structures with very small diameters because the RC time constant will still be much smaller than the recombination time.

[0025] Figure 2A This demonstrates a typical pin LED structure, and Figure 3A The optimized doped structure is shown, in which Figure 2B and 3B They also showcased Figure 2A and 3A The associated diagram of the device. Figure 2A The device has a p-doped GaN layer 211 and an n-doped GaN layer 215 sandwiching an intrinsic region 213 having an InGaN quantum well in between. Figure 3A The device also includes a p-doped GaN layer 251 and an n-doped GaN layer 255 sandwiching the intermediate region. However, in Figure 3A The intermediate region of the device is p-type doped and also contains an InGaN quantum well. In some embodiments, the quantum well is positioned physically closer to the p-doped GaN layer than the n-doped GaN layer.

[0026] Figure 2B The band diagram shows the conduction band 231 above the valence band 233 across the n-region 221, the intrinsic / depletion region 223, and the p-region 225. The band gap between the conduction and valence bands is generally constant across these regions, with energy levels typically increasing in the intrinsic / depletion region between the n-region and p-region, such that the energy levels in the p-region are higher than those in the n-region. Electrons are injected from the n-region 235 into the intrinsic / depletion region, and holes are injected from the p-region 237 into the intrinsic / depletion region, where recombination 239 occurs.

[0027] Figure 3B The band diagram also shows the conduction band 261 above the valence band 263, spanning n-region 221, depletion / p-regions 265a and 265b, and p-region 225 respectively. Figure 2B Compared to the images, in Figure 3B As can be seen, the depletion / p-region replaces the intrinsic / depletion region, where depletion region 265 is adjacent to the n-region, and p-region 265b is adjacent to p-region 225. The band gap between the conduction band and the valence band is generally constant across the regions, where the energy levels generally increase in the depletion / p-region (mainly in the depletion region), and are higher in p-region 225 than in n-region 221.

[0028] and Figure 2A compared to, Figure 3B It also demonstrates electron injection 275 above the thinner depletion region into the p-region, where recombination typically occurs. In GaN material systems, increasing the background doping can reduce the radiative recombination time by at least one or several orders of magnitude.

[0029] Although Figure 3AWe've shown p, p-, and n structures, but we can also dope the quantum well into n-type instead of p-type. This also increases the speed of the microLED compared to the pin structure. The advantage of n-doping compared to p-doping is that n-doping does not increase defects and does not reduce radiative efficiency. The doping level can be further increased to reduce carrier recombination time, at the cost of higher capacitance.

[0030] Some embodiments include... Figure 3B Further modifications to the doped structure can further improve performance. For example, some embodiments use an AlGaN barrier in the n-region to further enhance carrier injection into the p-doped recombination region and prevent hole injection into the n-type region. Some embodiments optimize the number, width, and strain of InGaN quantum wells in the p-region to reduce recombination time. For example, a lower In concentration that pushes the wavelength to a shorter wavelength also increases the rate. Thus, microLEDs with wavelengths between 380 nm and 430 nm can be inherently faster than microLEDs with longer wavelengths. For a given current, fewer quantum wells also increase the carrier density in the quantum wells. Carrier recombination time decreases more rapidly with increasing carrier density. Therefore, in some embodiments, the microLED has only one or a few quantum wells. In some embodiments, the quantum well width also becomes smaller. A smaller quantum well width brings electrons and holes closer together, where the overlap integral increases and the radiative recombination time decreases. Some embodiments use a suitable GaN substrate for growth to reduce the built-in electric field in the quantum well, increase the overlap integral between electrons and holes, and thus further reduce the carrier recombination time. We can also reduce the built-in electric field by decreasing the molar fraction of indium, thereby regaining a faster response in the short wavelength range. A lower indium concentration also reduces the Auger recombination rate, thus increasing the quantum efficiency of the LED.

[0031] In some embodiments, the structure optimized for high-speed operation has a small size, with a diameter less than about two micrometers, to increase current density and carrier density. In some embodiments, the structure optimized for high-speed operation has very few quantum wells, perhaps only one, to maximize carrier density at a given current density. In some embodiments, the indium concentration of the quantum wells is low, so the microLED will emit at a shorter wavelength, such as blue or ultraviolet wavelengths, because the lower indium concentration will give a lower piezoelectric field that increases the overlap integral of the hole-electron wavefunction and thus increases the recombination rate. In some embodiments, the quantum wells are small, typically 2 nm or smaller, to increase the overlap between electrons and holes. In some embodiments, the quantum wells are doped to p-type or n-type to increase the background carrier density.

[0032] Figure 5 Table I describes the trade-offs in the design of microLEDs with typical parameters.

[0033] Generally, higher doping levels also reduce nonradiative recombination time. This further shortens carrier lifetime and increases modulation speed, but at the cost of reduced quantum efficiency. Similarly, in very short-distance applications where waveguide propagation loss is minimal, quantum efficiency is less important than modulation speed. Fundamentally, there is a trade-off between quantum efficiency and modulation speed: increasing the nonradiative recombination rate increases the overall LED recombination rate, which in turn reduces quantum efficiency. Therefore, in some embodiments, the LED speed is increased at the expense of lower quantum efficiency, while in others it is increased significantly.

[0034] Rapid recombination centers can be induced in LEDs through several processes. These include low-temperature crystal growth in the intrinsic region, proton implantation, intentionally induced defect density using dislocations in the crystal lattice, roughening the etched surface, or increasing the exposed surface area through other techniques.

[0035] Typically, smaller microLEDs tend to have lower quantum efficiency because carriers diffuse and recombine on the etched outer surface. This reduces carrier lifetime and therefore also increases the speed of the microLED. This effect can be increased by etching structural holes or vias in the structure that expose more area in the sidewalls of vias, thereby creating more recombination centers. Figure 4 MicroLEDs with etched through-holes are on display. Figure 4 Examples show microLEDs with a generally cylindrical shape extending between a circular substrate 419 and a circular top 417. The microLED may include a base layer 413 (which may be, for example, an n GaN layer) extending upward from the circular substrate and a top layer 411 (which may be, for example, a p GaN layer) extending downward from the circular top. An intermediate layer 415 is located between the base layer and the top layer, and, as we understand it, the intermediate layer may provide an intrinsic depletion region or a p-depletion region.

[0036] Figure 4 The microLED also includes etched vias (e.g., etched via 421) extending from the circular top to the circular bottom. Therefore, the etched vias provide a perforation through the microLED from the top surface to the bottom surface. Figure 4 In this process, etched vias have a circular cross-section, thereby forming cylindrical vias, which are typically arranged in a square or rhomboid pattern. Etching vias can induce nonradiative recombination on the exposed surface, reducing carrier lifetime and thus increasing speed. This provides a more controllable method than proton implantation or cryogenic growth. In this case, various structures can be used to increase the surface area when etching the apparatus to form microLEDs. In some embodiments, these include etching multiple pillars and / or etching vias, as shown in the figure. In some embodiments, other shapes, such as star-shaped or rough-edged, may also be used or alternatively.

[0037] Although the invention has been discussed with respect to various embodiments, it should be recognized that the invention includes the novel and non-obvious claims supported by this disclosure.

Claims

1. An optical communication system for communicating information provided by a processor to another region of the processor or another chip in a multi-chip module, comprising: an LED associated with the processor; an LED driver for modulating optical output power of the LED so that the LED will generate light based on data provided to the LED driver from the processor; a detector for performing photoelectric conversion using the light; and an optical waveguide optically coupling light from the LED to the detector; wherein the LED comprises: a p-type GaN layer; an n-type GaN layer; and an intermediate region between the p-type GaN layer and the n-type GaN layer, the intermediate region being P-doped, the intermediate region including quantum wells, the quantum wells of the intermediate region being located physically closer to the p-type GaN layer than to the n-type GaN layer.

3. The system of claim 1, wherein the LED is a micro-LED.

2. The system of claim 1, wherein the p-doping of the intermediate region is in the range of 10 16 / cm 3 to 10 17 / cm 3 .

4. The system of claim 1, wherein the quantum wells comprise InGaN material.

5. The system of claim 1, further comprising: another LED associated with the other region of the processor or another chip in the multi-chip module; another LED driver for modulating optical output power of the other LED so that the other LED will generate light based on data provided to the other LED driver from the other region of the processor or another chip in the multi-chip module; and another detector for performing photoelectric conversion using the light from the other LED; wherein the other LED comprises: a p-type GaN layer; an n-type GaN layer; and an intermediate region between the p-type GaN layer and the n-type GaN layer, the intermediate region including quantum wells, the quantum wells of the intermediate region being located physically closer to the p-type GaN layer than to the n-type GaN layer.

6. The system of claim 5, wherein the optical waveguide optically couples light from the other LED to the other detector.

7. An optical communication system for communicating information provided by a processor to another region of the processor or another chip in a multi-chip module, comprising: an LED associated with the processor; an LED driver for activating the LED to generate light based on data provided to the LED driver from the processor; a detector for performing photoelectric conversion using the light; and an optical waveguide optically coupling the LED and the detector; wherein the LED includes: a plurality of etched vias for providing an aperture extending through the LED from a top surface of the LED to a bottom surface of the LED; a p-type GaN layer; an n-type GaN layer; and an intermediate region between the p-type GaN layer and the n-type GaN layer, the intermediate region being P-doped, the intermediate region including quantum wells, the quantum wells of the intermediate region being located physically closer to the p-type GaN layer than to the n-type GaN layer. ​ ​ 8. The optical communication system of claim 7, wherein the LED is a micro-LED.

9. The optical communication system of claim 7, further comprising: another LED associated with the other region of the processor or another chip in the multi-chip package; another LED driver for activating the other LED to produce light based on data provided to the other LED driver from the other region of the processor or another chip in the multi-chip package; and another detector for performing photoelectric conversion using the light from the other LED; wherein the other LED includes a plurality of etched vias.

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