No abstract

Germanium is listed as a critical raw material, and for environmental and economic sustainability reasons, strategies for lower consumption must be implemented. A promising approach is Ge lift‐off concepts, which enable to re‐use the substrate multiple times. Our concept is based on the Ge‐on‐Nothing approach that is the controlled restructuring at high temperature of a macroporous Ge surface, forming a Ge foil weakly attached to its parent wafer. Its suitability as III–V epitaxy seed and support substrate has previously been demonstrated with proof‐of‐concept solar cells. This work focuses on bringing this concept to the next level, by upscaling the detachable area to a full 200‐mm wafer scale, increasing foil thickness for sufficient light absorption in the Ge bottom cell, and improving the control on the strength that is bonding the suspended foil to its parent. By introducing a new high growth‐rate epitaxy process from GeCl4, and by engineering the GeON structure to introduce pillars with ad hoc density and shape, we fabricated P‐type foils with tunable boron doping up to 15 μm in thickness and 11 cm × 11 cm in area, for which the detachment strength could be adapted to the stresses induced by the solar cell process steps. The surface roughness and the electrical and crystal qualities of these foils were inspected by AFM, SIMS, SRP, ECCI, and TEM to check the GeCl4‐based epitaxy conditions and to check that the ad hoc pillars were not introducing any damage. Small‐area triple‐junction lattice‐matched GaInP/GaInAs/Ge solar cells were fabricated on 7‐μm‐thick Ge foils with various pillar densities and on a standard reference Ge wafer. The III–V layer nucleation was virtually the same on both substrates and the solar cells on the GeON foils performed in the same way as the cells on the Ge wafer, albeit a small loss in short‐circuit current and open‐circuit voltage that can be attributed to the thickness reduction and absence of rear‐side passivation. We conclude that it is possible to gain control on the GeON detachability and upscale the concept to areas relevant for the space PV industry, proving that porous germanium is a serious candidate for replacement of bulk Ge wafers in view of a more sustainable multijunction solar cell process.

No abstract

Silicon ring modulators with interdigitated diodes loss for 1 V pp , a 4.5 dB improvement over the lateral diode dSilicon ring optical modulators are attracting increasing interest co-integrated with CMOS electronics loss, high modulation efficiency, multi modulation has been reported in carrier [1, 2]. However, the obtained extinction ratios were limited to 5dB for i modulators are typically designed with the depletion area Modulators with junctions perpendicular to the waveguide ( increase the modulation efficiency and extinction ratio, however incompatible with CMOS [3, 4]. differing only in the diode design. We f achieving a 7.5 dB extinction ratio for 3 the lateral diode design. Open eye diagramsThe silicon ring modulators were fabricated crystalline silicon on 2 µm thick buried oxide etching of 70 nm Si. 248 nm lithography was subsequently used to define the Figure 1 Figure 1(a) shows the two diode designs that were investigated on ring modulators 450 nm waveguide width, 570 nm 0.8e18cm -3 for both p and n dopants. are parallel to the ring waveguide, and the second was the periodically interleaved perpendicularly to the ring. The widt n-doping, while the interaction length was of ~380 fF at 0 V for rings with the longitudinal resistance of ~20 Ohm for both. Transmission spectra for different bias conditions of the lateral and designs are shown in Figures 1(b) and 1(c) and interdigitated designs respectively. Consequently, the calculated 3 12.5 GHz for the lateral design, limited by the cavity photon lifetime, and 6 by the RC time constant when driven from a 50 Ohm transmission line Figure 2 shows the static extinction lateral and interdigitated modulator Silicon ring modulators with interdigitated diodes demonstrate 7.5 dB extinction ratios dB improvement over the lateral diode design, and can operate up to 10 Gb/s modulators are attracting increasing interest for low energy, small footprint electronics. Key requirements for such devices are high extinction ratio, low insertion loss, high modulation efficiency, multi-Gb/s speed and low energy per bit. Recently, 10 has been reported in carrier-depletion ring modulators using a 1 V peak-to-2]. However, the obtained extinction ratios were limited to 5dB for insertion losses lower than 3dB.designed with the depletion area parallel to the waveguide perpendicular to the waveguide (interdigitated diode design increase the modulation efficiency and extinction ratio, however so far they required highIn this work we compare the performance of two differing only in the diode design. We find that the interdigitated design results in higher modulation efficiency, dB extinction ratio for 3 dB insertion loss using 1 V pp , as opposed to only 3 ye diagrams were obtained at 10 Gb/s for both modulator modulators were fabricated on 200 mm Silicon-On-Insulator (SOI) wafers with 220 nm µm thick buried oxide. Rings of 40 µm radius were defined by 193 nm lithography was subsequently used to define the p-and n-doping patterns as shown in -of line W contacts and Cu met...