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MIT’s Research Laboratory of Electronics (RLE) Develops LEDs on Si Substrates
LEDs are most often manufactured using sapphire substrates, requiring a liftoff process that eventually puts the LEDs on some type of Si driver. It’s tough to make them from silicon. That means LED sensors must be manufactured separately from their device’s silicon-based processing chip, often at a hefty price. But that could change, thanks to new research from MIT’s Research Laboratory of Electronics (RLE). Researchers have fabricated a silicon chip with fully integrated LEDs, bright enough to enable state-of-the-art sensor and communication technologies. The advance could lead to not only streamlined manufacturing, but also better performance for nanoscale electronics. Jin Xue, a PhD student in RLE, led the research, which is being presented at the IEDM conference this week. MIT co-authors included Professor Rajeev Ram, who leads the Physical Optics and Electronics Group in RLE, as well as Jaehwan Kim, Alexandra Mestre, Dodd Gray, Danielius Kramnik, and Amir Atabaki. Other co-authors included Kian Ming Tan, Daniel Chong, Sandipta Roy, H. Nong, Khee Yong Lim, and Elgin Quek, from the company GLOBALFOUNDRIES.
Silicon is widely used in computer chips and the capacity to switch between “off” and “on” underlies a computer’s ability to perform calculations. But despite silicon’s excellent electronic properties, it doesn’t quite shine when it comes to optical properties — silicon makes for a poor light source. So electrical engineers have turned away from the material when they need to connect LED technologies to a device’s computer chip. LED are made from III-V semiconductors, so called because they contain elements from the third and fifth columns of the periodic table. (Silicon is in the fourth column.) These semiconductors are more optically efficient than silicon — they produce more light from a given amount of energy. (You don’t see the light produced from the proximity sensor because it is infrared, not visible.) And while the proximity sensor is a fraction of the size of the phone’s silicon processor, it adds significantly to the phone’s overall cost. “There’s an entirely different fabrication process that’s needed, and it’s a separate factory that manufactures that one part,” says Ram. “So, the goal would be: Can you put all this together in one system?” Ram’s team did just that. Xue designed a silicon-based LED with specially engineered junctions — the contacts between different zones of the diode — to enhance brightness. This boosted efficiency: The LED operates at low voltage, but it still produces enough light to transmit a signal through 5 meters of fiber optic cable. Plus, GLOBALFOUNDRIES manufactured the LEDs right alongside other silicon microelectronic components, including transistors and photon detectors. While Xue’s LED didn’t quite outshine a traditional III-V semiconductor LED, it easily beat out prior attempts at silicon-based LEDs.
“Our optimization process of how to make a better silicon LED had quite an improvement over past reports,” says Xue. He adds that the silicon LED could also switch on and off faster than expected. The team used the LED to send signals at frequencies up to 250 megahertz, indicating that the technology could potentially be used not only for sensing applications, but also for efficient data transmission. Xue’s team plans to continue developing the technology. But, he says, “it’s already great progress.” t a microscopic scale, III-V semiconductors have nonideal surfaces, riddled with “dangling bonds” that allow energy to be lost as heat rather than as light, according to Ram. In contrast, silicon forms a cleaner crystal surface. “We can take advantage of those very clean surfaces,” says Ram. “It’s useful enough to be competitive for these microscale applications.” It allows silicon integrated circuits to communicate with one another directly with light instead of electric wires. This is somewhat surprising as silicon has an indirect bandgap and does not normally emit light.” There is an optical CPU architecture that the semiconductor industry has been dreaming of. The report of silicon-based micro-LEDs shows significant progress in these attempts.”
To date the light output of the Si based LED is relatively low, but pretty good for an initial result. French start-up, Aledia has independently started R&D to use 300mm Si substrates for their nano wire LEDs, with a target date for production of 2023.
LEDs are most often manufactured using sapphire substrates, requiring a liftoff process that eventually puts the LEDs on some type of Si driver. It’s tough to make them from silicon. That means LED sensors must be manufactured separately from their device’s silicon-based processing chip, often at a hefty price. But that could change, thanks to new research from MIT’s Research Laboratory of Electronics (RLE). Researchers have fabricated a silicon chip with fully integrated LEDs, bright enough to enable state-of-the-art sensor and communication technologies. The advance could lead to not only streamlined manufacturing, but also better performance for nanoscale electronics. Jin Xue, a PhD student in RLE, led the research, which is being presented at the IEDM conference this week. MIT co-authors included Professor Rajeev Ram, who leads the Physical Optics and Electronics Group in RLE, as well as Jaehwan Kim, Alexandra Mestre, Dodd Gray, Danielius Kramnik, and Amir Atabaki. Other co-authors included Kian Ming Tan, Daniel Chong, Sandipta Roy, H. Nong, Khee Yong Lim, and Elgin Quek, from the company GLOBALFOUNDRIES.
Silicon is widely used in computer chips and the capacity to switch between “off” and “on” underlies a computer’s ability to perform calculations. But despite silicon’s excellent electronic properties, it doesn’t quite shine when it comes to optical properties — silicon makes for a poor light source. So electrical engineers have turned away from the material when they need to connect LED technologies to a device’s computer chip. LED are made from III-V semiconductors, so called because they contain elements from the third and fifth columns of the periodic table. (Silicon is in the fourth column.) These semiconductors are more optically efficient than silicon — they produce more light from a given amount of energy. (You don’t see the light produced from the proximity sensor because it is infrared, not visible.) And while the proximity sensor is a fraction of the size of the phone’s silicon processor, it adds significantly to the phone’s overall cost. “There’s an entirely different fabrication process that’s needed, and it’s a separate factory that manufactures that one part,” says Ram. “So, the goal would be: Can you put all this together in one system?” Ram’s team did just that. Xue designed a silicon-based LED with specially engineered junctions — the contacts between different zones of the diode — to enhance brightness. This boosted efficiency: The LED operates at low voltage, but it still produces enough light to transmit a signal through 5 meters of fiber optic cable. Plus, GLOBALFOUNDRIES manufactured the LEDs right alongside other silicon microelectronic components, including transistors and photon detectors. While Xue’s LED didn’t quite outshine a traditional III-V semiconductor LED, it easily beat out prior attempts at silicon-based LEDs.
“Our optimization process of how to make a better silicon LED had quite an improvement over past reports,” says Xue. He adds that the silicon LED could also switch on and off faster than expected. The team used the LED to send signals at frequencies up to 250 megahertz, indicating that the technology could potentially be used not only for sensing applications, but also for efficient data transmission. Xue’s team plans to continue developing the technology. But, he says, “it’s already great progress.” t a microscopic scale, III-V semiconductors have nonideal surfaces, riddled with “dangling bonds” that allow energy to be lost as heat rather than as light, according to Ram. In contrast, silicon forms a cleaner crystal surface. “We can take advantage of those very clean surfaces,” says Ram. “It’s useful enough to be competitive for these microscale applications.” It allows silicon integrated circuits to communicate with one another directly with light instead of electric wires. This is somewhat surprising as silicon has an indirect bandgap and does not normally emit light.” There is an optical CPU architecture that the semiconductor industry has been dreaming of. The report of silicon-based micro-LEDs shows significant progress in these attempts.”
To date the light output of the Si based LED is relatively low, but pretty good for an initial result. French start-up, Aledia has independently started R&D to use 300mm Si substrates for their nano wire LEDs, with a target date for production of 2023.
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