GaN-on-Si laser diode: open up a new era of Si-based optical interconnections open site

Date: Nov 1, 2016

As Si-based electronics technology approaches its scaling limits, it arises great interest in optical interconnections via Si photonics. However, Si with an indirect band-gap structure can hardly emit light. The lack of an efficient on-chip laser source remains as the major roadblock of Si photonics for decades, which recently has drawn renewed research interest. It is highly desirable to grow III–V semiconductor laser directly on Si for a monolithic integration with Si photonics to take the full advantage of low-cost large-scale fabrication platforms [1–3]. There had been only optically pumped lasing reported for GaN-based laser diodes (LDs) grown on Si [4–6]. Because of the large lattice mismatch between GaN and Si and the huge misfit in the coefficient of thermal expansion (CTE), direct growth of GaN on Si often encounters a very high density (109–1010 cm−2) of threading dislocations (TDs) and a strong tensile stress, even resulting in micro-crack network [7]. Moreover, the presence of AlGaN optical cladding layers grown on GaN contributes additional tensile stress. All these challenges have impeded the realization of electrically injected III-nitride laser directly grown on Si, because laser operation imposes a stringent requirement on defect density and stress management. In a paper recently published in Nature Photonics, Sun et al. [8] reported the first GaN-based blue–violet laser directly grown on Si, operating under a continuous-wave (CW) current injection at room temperature. This breakthrough will speed up the development of Si-based optoelectronics integration and open up a new era of optical interconnections via silicon photonics. To tackle the lattice mismatch and CTE misfit, a stress engineering buffer composed of a stack of Al-composition step down-graded AlN/AlxGa1−xN multi-layers was inserted between Si and GaN (Fig. 1a). They intentionally utilized the positive lattice mismatch to build up enough compressive strain in the GaN epitaxial film, in order to effectively compensate the tensile stress induced by the CTE mismatch during the cool-down after growth. Meanwhile, the compressive strain facilitates the TDs to incline and annihilate with each other, especially at the interfaces of adjacent buffer layers, giving a high quality GaN film on Si (Fig. 1a). The TD density (~5.8 × 108 cm−2) in the GaN film grown on Si is substantially lower than previous reports, as evidenced by the narrow linewidths (~260 arcsec) of double crystal X-ray rocking curves for GaN(0002) and GaN(101¯1¯1) diffractions, as shown in Fig. 1b.

Application: Others