Mar 15, 2017: Researchers of the University of California at Berkeley have demonstrated the design of III−V nanopillar LEDs with a Si-CMOS compatible optical lithography process. Nano LEDs are a key element for the effective integration of photonics into CMOS circuits for fast on-chip optical interconnects. In this experiment, the researchers could also control the growth location of these nano LEDs.
The article on this experiment, titled “Ultracompact Position-Controlled InP Nanopillar LEDs on Silicon with Bright Electroluminescence at Telecommunication Wavelengths”, reported the site-controlled growth of high-yield (90%) uniform arrays of InP nanopillars on silicon. This was grown under CMOS compatible conditions, which is low temperature without catalysts.
The researchers took a clean silicon wafer (111), and deposited 140nm of silicon dioxide (at 350°C) onto which they defined the nanoscale apertures of about 320nm in diameter. These were used to position the nanopillar nucleation sites, with a pitch varying from 1 to 40μm. The silicon surface was chemically roughened, and then the InP nanostructures were grew in a MOCVD chamber at temperatures between 450 and 460°C.
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The researchers found that the taper angle of the nanopillar was affected by growth temperature that yielded nanoneedles at 450°C but vertical pillar-shaped structures at 460°C.
The researchers incorporated five InGaAs quantum wells in the active region of a pn diode, on the basis of these nanopillars and through a concentrical core−shell growth forming electrically driven n-InP/InGaAs MQW/p-InP/p-InGaAs nano-LEDs.
The nanopillars expanded beyond the oxide opening and reached a final diameter of about 1μm. The n-doped core of the nanopillar was in direct contact with the n-Si substrate, and the p-doped shell grew over the oxide mask. This eliminated the shunt path from the p-doped shell and n-Si substrate. The 20/200nm of Ti/Au evaporated through angled electron beam evaporation onto a highly p-doped InGaAs contact layer and completed the devices to form electrical contacts, with a small region of the nanopillars and without metal as a window for the LED optical output.
The nanopillar LEDs emitted 1510nm with a high internal quantum efficiency of circa 30%. The nanopillar LEDs output 4μW of power was the highest reported light output from a nanopillar/nanostructure-based LED.