Optical interconnect has great prospect to boost computation speed and reduce energy consumption for integrated circuits. This requires a laser source on silicon that operates at a silicon-transparent wavelength. However, all semiconductor materials that can emit beyond 1.1μm are severely lattice-mismatched to silicon, prohibiting high quality epitaxial growth on silicon. Although various semiconductor nanowires have been synthesized on silicon, their small lateral dimension leads to very weak light confinement at long wavelength. Therefore, no long-wavelength nanowire laser on silicon was ever reported.
Here, we demonstrate the first as-grown nanolaser on silicon with silicon-transparent wavelength, by directly synthesizing III-V micro-pillar on silicon-on-insulator (SOI) substrate. The cornerstone of our nanolaser is InP micro-pillar growth by catalyst-free metal-organic chemical vapor deposition. Due to its unique core-shell growth mode, micro-pillar can scale up to micron size in lateral dimension while maintaining superior quality in the bulk. InGaAs single/multiple quantum wells (QWs) are incorporated into InP micro-pillar as the active gain medium. High-resolution transmission electron microscopy reveals sharp InP-InGaAs-InP heterostructure interface and no defects are observed in QWs.
By tuning the indium composition, the emission wavelength can vary from 1.1μm to 1.5μm. In addition, various QW thicknesses (from 1nm to 5nm) lead to different emission wavelengths, in agreement with theoretical prediction of QW quantization effect. The micron size footprint of micro-pillar provides strong light confinement in transverse direction. In longitudinal direction, the buried oxide in SOI increases the bottom reflection, building up a vertical high-Q Fabry-Perot cavity on the as-grown micro-pillar. Therefore, under optical pumping, micro-pillar exhibits prominent Fabry-Perot modes with equal wavelength spacing. The mode peak spacing agrees well with the cavity length of micro-pillar. When pumping level reaches threshold, optically pumped laser is obtained with silicon transparent wavelength from 1.1μm up to 1.3μm, depending on the indium composition of QWs.
In conclusion, we achieve the first monolithic nanolaser on silicon with silicon-transparent wavelength using bottom-up approach. This is a crucial step towards practical integration of nano-photonics with silicon-based electronics.