Germanium-on-Glass solar cells: fabrication and characterization

We report on Germanium on Glass solar cells realized by wafer bonding, layer splitting and epitaxial regrowth. We provide a detailed description of the layer transfer process and discuss the material characterization. The solar cells are fabricated and tested to extract the most significant figures of merit, evaluating their performance versus device area and operating temperature. The cells exhibit typical conversion efficiencies exceeding 2.4% under AM1.5 irradiation and a maximum efficiency of 3.7% under concentrated excitation. This Germanium on Glass approach is promising in terms of added flexibility in multi-junction engineering and allows a significant cost reduction thanks to the re-usability of the Ge substrates

Capacitive actuation and switching of add\u2013drop graphene-silicon micro-ring filters

We propose and experimentally demonstrate capacitive actuation of a graphene\u2013silicon micro-ring add/drop filter. The mechanism is based on a silicon\u2013SiO2\u2013graphene capacitor on top of the ring waveguide. We show the capacitive actuation of the add/drop functionality by a voltage-driven change of the graphene optical absorption. The proposed capacitive solution overcomes the need for continuous heating to keep tuned the filter\u2019s in/out resonance and therefore eliminates \u201cin operation\u201d energy consumption

Ultrafast, Zero-Bias, Graphene Photodetectors with Polymeric Gate Dielectric on Passive Photonic Waveguides.

We report compact, scalable, high-performance, waveguide integrated graphene-based photodetectors (GPDs) for telecom and datacom applications, not affected by dark current. To exploit the photothermoelectric (PTE) effect, our devices rely on a graphene/polymer/graphene stack with static top split gates. The polymeric dielectric, poly(vinyl alcohol) (PVA), allows us to preserve graphene quality and to generate a controllable p-n junction. Both graphene layers are fabricated using aligned single-crystal graphene arrays grown by chemical vapor deposition. The use of PVA yields a low charge inhomogeneity ∼8 × 1010 cm-2 at the charge neutrality point, and a large Seebeck coefficient ∼140 μV K-1, enhancing the PTE effect. Our devices are the fastest GPDs operating with zero dark current, showing a flat frequency response up to 67 GHz without roll-off. This performance is achieved on a passive, low-cost, photonic platform, and does not rely on nanoscale plasmonic structures. This, combined with scalability and ease of integration, makes our GPDs a promising building block for next-generation optical communication devices

Germanium on Silicon Near-Infrared Photodetectors

In recent years, the of Germanium on Silicon approach has been recognized as the best alternative to the well-established III-V technology for the fabrication of high performance near-infrared photodetectors. Recent results demonstrate that Ge heteroepitaxy on Si is by now mature to compete with standard III-V devices. Unfortunately, the integration of Ge-on-Si technology in standard CMOS process flows is still an open challenge due to the sophisticated growth techniques as well as the high thermal budget involved. This work proposes an alternative approach to the growth of Ge on Si for NIR optoelectronics applications. The first chapter introduces NIR detection for optical communication systems, with particular emphasis on Ge as a suitable material for the monolithic integration into NIR photodetectors on a Si platform. In the second chapter, the deposition process is described. Ge is deposited on Si by thermal evaporation, a very simple and low temperature (300◦ C ) technique suitable for both streamline process and back-end monolithic integration of Ge on Si CMOS electronics. Material characterization, both morphological and electrical, is also discussed. Raman and X-Ray analysis, as well as Transmission Electron Microscopy evidenced that Ge is epitaxially grown in a monocrystalline form with a high dislocation density. Hall measurements demonstrated high unintentional p -type doping (1017 ÷ 1018 cm−3 ) associated to the acceptor-like levels due to the large defect density. The transport and detection properties of evaporated Ge on Si heterojunctions are presented in the third chapter. Results demonstrated a trapassisted conduction mechanism explained by energy band pinning at the Ge/Si interface. The NIR detection properties were also investigated by illumination at normal incidence. The high doping together with the short diffusion length were found to drastically limit the responsivity of normal incidence devices. The last part of this work is dedicated to the design and fabrication of optimized NIR photodetector and their integration on SOI optical chips. Waveguide photodetectors (WPD) were fabricated to take advantage from the distributed absorption of light in guiding structures. WPD exhibit very promising performance with typical responsivities exceeding 0.2 A/W at 1 V reverse bias and 1.55 µm wavelength. These devices were monolithically integrated on SOI optical chips for the realization of channel monitors. The integrated devices exhibit very promising performance, with sensitivity of 10 nW and good linearity over about four orders of magnitude

Design optimization of single and double layer Graphene phase modulators in SOI

In this paper we report on an electro-refractive modulator based on single or double-layer graphene on top of silicon waveguides. The graphene layers are biased to the transparency condition in order to achieve phase modulation with negligible amplitude modulation. By means of a detailed study of both the electrical and optical properties of graphene and silicon, as well as through optimization of the geometrical parameters, we show that the proposed devices may theoretically outperform existing modulators both in terms of V(\u3c0)L and of insertion losses. The overall figures of merit of the proposed devices are as low as 8.5 and 2dB 19V for the single and double layer cases, respectively

Integrated silicon photonics ROADM for mobile network applications: System evaluation

The system performances of a micro-ring based silicon photonic integrated ROADM is evaluated experimentally. A theoretical modelling of the system allows to identify the fabrications errors' effect of single elements on the system performance

Design of erbium doped silicon nanocavities for single photon applications

Silicon-based quantum communication technologies are becoming a factual reality. However, the challenges related to an earth-space unifying technology are several, and nowadays an integrated source compatible with the CMOS technology is still missing. Here we present the design of a weak photon source consisting of a LED able to emit directly into the optical circuit and obtained through the doping of a portion of a silicon waveguide with ErOx complexes. To enhance the radiative emission, the source is placed inside a resonant cavity delimited by two waveguide Bragg mirrors. A study on the performance of the device is carried out as a function of different parameters, such as the geometry of the cavity and of the contacts used to electrically excite the defects, the doping level, and the characteristics of the mirrors. We design a prototype that guarantees a Purcell factor in the order of tens, emitting ideally 107-108 photons per second. The simulations provide a promising ground to further develop fully integrated single photon sources in silicon photonic circuits

Negligible power consumption add/drop switching properties of graphene on SOI microring resonators

We investigate the switching reconfiguration properties of graphene on SOI add/drop microring resonators. Graphene can effectively enable/disable the add/drop functionality with negligible power consumption with respect to the typical approach based on thermal tuning