Direct Gap related Optical Properties of Ge/SiGe Multiple Quantum Wells – GOsPEL

To achieve GOSPEL project objective, material, optic and optoelectronic properties of Ge/SiGe quantum well structures are studied, both theoretically and experimentally, in order to improve the understanding of the physical properties in these structures.

As first experimental results we reported optoelectronic properties of Ge/SiGe multiple quantum well devices. An electro-absorption modulator, exhibiting 9 dB extinction ratio with 23 GHz bandwidth ans power consumption of 100 fJ/bit was demonstrated. A 10 Gbit/s photodetector, has been obtained. Finally as a light emitting diode, direct gap electroluminescence (EL) from the Ge/SiGe MQW waveguides has been experimentally demonstrated at room temperature.

Next steps rely first on a comparison between theoretical models and experimental results, in order to improve the performance of optoelectronic components by a detailed understanding of involved physical phenomena. The second objective is to integrate the optoelectronic devices with waveguides, in order to demonstrate high performance and low consumption optical links, based on Ge/SiGe quantum well structures for future communications systems.

P. Chaisakul, D. Marris-Morini, et al, IEEE Phot.Tech.Letters, 23 (20), 1430-1432 (2011).
P. Chaisakul, D. Marris-Morini, et al, Applied Physics Letters, 99, 141106, (2011).
P. Chaisakul, D. Marris-Morini, et al, Optics Express, 20 (3), 3219-3225, (2012)

Silicon photonics has generated an increasing interest in the recent years. The integration of optics and electronics on a same chip would allow the enhancement of integrated circuit performances, and optical telecommunications can benefit from the development of low cost and high performance solutions for high-speed optical links. Silicon based-optoelectronic devices are the key building blocks for the development of silicon photonics. Despite the demonstration of high performance silicon modulator, germanium photodetectors, and the achievement of optical sources using III-V material bonded on silicon, the integration of these different elements on an electronic chip is highly challenging due to the different materials and technologies for each building block. In addition, wideband silicon modulators require active regions longer than 1 mm which makes their integration on electronic chips difficult. Finally an effective silicon based light source is still the Holy Grail for silicon photonics researchers.
The real development of silicon photonics needs to solve these challenging points, and this will be possible only by using innovative breaking concepts. In this context, GOSPEL project propose to study direct gap-related optical properties of Ge/SiGe multiple quantum wells (MQW). Indeed, in 2005, photocurrent spectroscopy has been used to demonstrate that Quantum Confined Stark Effect (QCSE) can be obtained in Ge/SiGe quantum wells (QW), which is an important breakthrough as it was the first demonstration of using direct gap related effect in indirect bandgap materials. This demonstration has paved the way for a lot exciting works related to a good understanding of the mechanisms in these Ge/SiGe QW structures and for the achievement of innovative optoelectronic devices based on these mechanisms.
In this context the goal of GOSPEL project is to study physical, optical and optoelectronic properties of Ge/SiGe multiple quantum wells to go towards photonic devices. The structures are grown by low energy plasma enhanced chemical vapour deposition (LEPECVD), in L-Ness lab (Como - Politecnico di Milano, Italy) thanks to collaboration with Giovanni Isella’s group. Due to the large lattice mismatch between Si and Ge, a graded SiGe buffer is used, with Ge concentration of SiGe layer grown from zero to the final concentration with a continuous change to obtain a relaxed Ge-rich SiGe layer, where high quality Ge/SiGe quantum wells can be grown. Indeed the preliminary results allowed us to demonstrate QCSE at room temperature for light incident perpendicular and for the first time parallel to the QW planes, which is directly related to integrated photonic applications.

In GOSPEL project, material properties (energy and intensity of excitonic peaks, carrier dynamics and transport) and optoelectronic properties (influence of external electric field, luminescence) of Ge/SiGe MQW structures will be compared to theoretical results, in order to improve the understanding of physical properties in these structures. Devices based on these new effects will be designed and fabricated in order to experimentally observe light modulation, detection, and emitting properties of the Ge/SiGe MQW structures. GOSPEL project will then give answers on the possibilities and opportunities to develop a new and innovative Ge/SiGe photonics platform.