Highly strained Ge micro-blocks bonded on Si platform for mid-infrared photonic applications

Abstract

Adding sufficient tensile strain to Ge can turn the material to a direct bandgap group IV semiconductor emitting in the mid-infrared wavelength range. However, highly strained-Ge cannot be directly grown on Si due to its large lattice mismatch. In this work, we have developed a process based on Ge micro-bridge strain redistribution intentionally landed to the Si substrate. Traction arms can be then partially etched to keep only localized strained-Ge micro-blocks. Large tunable uniaxial stresses up to 4.2% strain were demonstrated bonded on Si. Our approach allows to envision integrated strained-Ge on Si platform for mid-infrared integrated optics.Silicon photonics merges optical and electronic components to be integrated together onto a single microchip. Since the great interest in Si photonics has been demonstrated for telecommunication application more applications have emerged in the Mid-Infrared (MIR) wavelength range (2 to 5µm) like gas sensing [1]. Since the SiO 2 is transparent up to 3.5 µm wavelength [2], Silicon-On-Insulator (SOI) MIR spectrometers have been successfully demonstrated [3]. For longer wavelength, Ge on Si platform is instead used [4], [5]. However, the monolithically integration of active devices (sources and photodetector) are still missing to unlock all the possibilities for Si photonics in the MIR. The current solution consist of growing [6], [7] or bonding III-V materials [8], [9] on Si but these heterogeneous integration are not considered as Complementary Metal Oxide Semiconductor (CMOS) compatible by industrial foundries. For the group IV material sources, doped-Ge [10] and GeSn alloys [11] are envisioned to tackle the challenge to obtain a laser source CMOS compatible. Note that GeSn devices are also studied for photo-detection applications on Si substrate [12], [13]. Strained-Ge structures exhibits also a MIR direct bandgap [14], [15] but their integration, which require extra process step to localize and contact the strained-Ge, remains challenging. Such integration on a Si platform would open the way to MIR fully integrated active devices (source and detectors) which 2 are CMOS compatible. The availability of such approach would have many applications for sensing systems [2] or on-chip optical interconnects for high-performance computing [16].In this work, we propose to bond highly strained Ge directly on Si using a specific processing based on micro-bridge landing followed by the traction arm etching. Compared to free standing membranes, our landing approach will greatly facilitate the next process steps needed to go towards electrically pumped laser sources or integrated photodetectors based on strained-Ge for MIR applications. We measure a Raman spectral shift of around 8 cm -1 corresponding to 4.2% in a Ge micro-block directly bonded on Si which is higher than the first demonstration of 3 cm -1 using the biaxial approach [17]. Figure 1: Processing of a) Ge layer patterning; b) SiO 2 under-etching leading to concentrate the strain in the centre of landed micro-bridge; c) fina...