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Photonic Integration on Silicon Germanium

Final Report Summary - PIOS (Photonic integration on silicon germanium)

Development of optoelectronic devices and material technologies based on enabling the integration of optical and electrical circuitry is crucial due to communication bottlenecks that arise from physical restrictions of electrical interconnects. A wide number of materials and processes are available for optical device and interconnect fabrication; however, monolithic integration silicon (Si) complementary metal-oxide-semiconductor (CMOS) microelectronics is the basic concern. Optoelectronic device technology, based on group III-V compounds and especially indium-gallium-arsenic (InGaAs), offers increased functionality and high performance. As a result, detectors based on these materials are the state-of-the-art optoelectronic devices for near infrared and especially optical fiber telecommunication bands (1.31 - 1.55 µm).

Despite its advantages, high-integration costs hinder the widespread use of III-V optoelectronics technology. Furthermore, construction of optical interconnection architectures requires materials with monolithic integration ability with standard Si CMOS technology and absorption capability that overcomes the spectral limitation of Si.

In this project, we develop technology for Si-germanium (SiGe)-based, integrated optoelectronic devices on multiple hydrogen anneal for heteroepitaxy (MHAH)-grown SiGe multi-quantum-well (MQW) structures. We demonstrate high-responsivity photodetectors and high-performance electro-absorption modulators exploiting quantum confined stark effect (QCSE). This project facilitated the transfer of the accumulated knowledge in this area from United State (US) institutions to the European Research Area (ERA), train researchers in the field and pave the way for the commercial exploitation of the concepts.

The project is executed in two phases; design work followed by experimental implementation of the design. One-dimensional (1D) quantum wells are designed to exploit QCSE and to construct electro-absorption modulators operational at the telecommunication C-band in a CMOS compatible fashion. A recently developed technique, MHAH, is utilised, where, buffer layers are used to provide high crystal quality. We developed a growth recipe to obtain very high quality Si and Ge layers with control over the layer thickness and composition. We verify the good crystal quality with transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray crystallography (XRD). The material composition of these structures is investigated by X-ray photoelectron spectroscopy (XPS) and SIMS studies.

Electro-absorption modulators are designed with multi-quantum-well (MQW) regions embedded in p-i-n diode architecture to provide for proper biasing of the MQW regions. Microfabrication recipes are developed and devices are manufactures in UNAM cleanroom facilities (UCF) at Bilkent University. An advanced infrared optoelectronic measurement set-up is designed and implemented for device characterisation. We have successfully demonstrated SiGe electro-absorption modulators based on QCSE working in the near infrared wavelengths. The modulators exhibit large figure of merit in the C-band with high contrast ratio at 'optically off' state and low background absorption at 'optically on' state. Based on our knowledge, demonstrated electro-absorption modulator is a novel, high performance device in terms of contrast ratio and insertion loss at C-band operation range. The SiGe growth recipe is designed for integration with a standard CMOS process. The anneal times and temperatures of the CMOS process are optimised to account for the high-temperature steps during the film growth. Furthermore, a selective area epitaxial growth (SEG) recipe is developed and demonstrated. SiGe layers compatible with CMOS are grown and characterised.

We are also glad to share outcomes that are not among the primary goals of this project but we believe are highly relevant with the scope of the work. We have observed record high temperature coefficient of resistance from MQW structures developed in this project. We recorded - 5.8 % / K temperature coefficient while the commercial materials today exhibit less than 3 % temperature coefficient of resistivity (TCR).

Traditionally, semiconductor nanocrystals are employed as engineered light absorbers for the wide spectrum of solar radiation. However, most of such efforts have produced limited results because the nanocrystals were obtained in an insulating matrix. Although nanocrystals absorb efficiently, the photo-generated carriers could not be collected efficiently. We have used the epitaxy technique developed in this project to grow germanium nanocrystals embedded in a Si matrix allowing charge carriers generated in the Ge nanocrystals to be transferred through the semiconducting host.

We also demonstrated a photodetector concept based on Si nanocrystals synthesised using laser ablation technique. This innovative approach can produce 20 - 150-nm-sized Si nanocrystals using laser ablation, which is potentially a low-cost and high-throughput method for synthesis of nanoparticles. It is believed that Si nanocrystals made in this manner could be useful for creating a variety of other optoelectronic devices, such as flexible and disposable sensors or cost-effective solar cells.

None of the above would be possible without the valuable contributions from the European Union (EU)'s Seventh Framework Programme (FP7) Marie Curie International Reintegration Grant (IRG). The results obtained for all activities are published in over 40 scientific journal and conference papers. The resources provided enabled Dr Okyay to travel to the US to maintain collaborations and establish new ones. The resources also facilitated the training of over 20 graduate students in integrated optoelectronics area. First class post-doctoral researchers are attracted from the US to ERA.

In addition to the scientific benefits, the results of this project can impact the European society and its way of life with the possibility of ultra-low cost and very high-speed communication solutions. Furthermore, handheld spectroscopy systems for mobile point-of-care applications could provide a paradigm shift in healthcare. The low-cost technologies developed here can transform machine vision and automotive industries. The potential exploitation of the technologies developed here will impact telecommunications, health and automotive industries securing European industrial leadership and competitiveness. Socio-economic impact of innovative technologies of this kind is exclusively beneficial and hard to overestimate.