On-Chip Lasers Compute with Light

Semiconductor researchers have found a way to grow tiny lasers on silicon chips, opening the door to a future where all communication is done with photons instead of electrons, making mobile devices smaller, lighter, and ultra-low power.Silicon chip makers will soon be able to grow semiconductor lasers right on the same chip with processors, memory, and logic, enabling ultra-small, ultra-cheap, and ultra-low-power devices that compute with light instead of electrons, according to researchers at the University of California at Berkeley. Nearly all electronics chips today are composed of complementary metal oxide semiconductors (CMOS), which most people just call "silicon." However, the real magic of contemporary optical and wireless communications lies in the use of expensive alternatives to silicon CMOS, called gallium arsenide (GaAs—pronounced "gas") and other similar compounds of indium (InGaAs) and aluminum (AlGaAs).Together, these are called III-V (pronounced "three-five") compounds, because of their positions on the Periodic Table of Elements.

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  Nanoneedle-based lasers grown on silicon wafers form heterostructures—regions of dissimilar semiconductors—composed of III-V cores clad with insulators like aluminum-gallium-arsenide (AlGaAs).
  
  Today, optical signals are created by III-V chips made by semiconductor specialty manufacturers. Silicon CMOS kingpins like Intel Corp. (Santa Clara, Calif.) have announced prototype chips that can compute with light, but their on-chip lasers were merely flakes of III-V bonded to the chip—an expensive and tedious process that will likely never achieve commercialization.
  The University of California at Berkeley team, led by principal investigator and professor Connie Chang-Hasnain, on the other hand, has discovered a way of growing nanoscale lasers right on silicon chips, eliminating the need for expensive III-V chips and allowing on-chip computations to be accomplished with light instead of electricity.
  In the past, it has proven impossible for Intel and other silicon chip makers to grow III-V compounds on silicon substrates, because the underlying crystalline lattices of the two materials were too dissimilar, resulting in cracks if grown larger than a few nanometers in size. The UCB researchers, whose group included doctoral candidate Roger Chen, solved that problem by growing the lasers upward, thereby confining the interface between the two materials to a tiny nanoscale region that can tolerate the lattice mismatch.
  The second problem associated with adding III-V compounds atop silicon—that GaAs requires temperatures as high as 1,300 degrees Fahrenheit, which damages normal CMOS chips—was also solved by the UCB researchers, who fabricated their nanopillars at 750 degrees Fahrenheit, which silicon chips can tolerate without damage.
  The team used a variation of traditional metal-organic chemical vapor deposition (MOCVD), which enables the III-V materials to be fabricated atop silicon chips into nanopillars that use quantum-well confinement to produce lasing, promising a future in which optical signals are routinely used to pipe around high-speed data streams both between chips and even between cores on the same chip.
  Funding for the project was provided by the Defense Advanced Research Projects Agency (DARPA) and the Department of Defense (DoD).
   
  Quantum wells, formed by alternating layers of gallium arsenide (GaAs) and indium-gallium-arsenide (InGaAs), can produce lasers at various wavelengths and energy levels.
  

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