Digital Theses Archive


Tesi etd-11162020-205553

Type of thesis
Integration on graphene technology on SI-based photonic platforms
Scientific disciplinary sector
Istituto di Tecnologie della Comunicazione, dell'Informazione e della Percezione - PH.D. PROGRAMME IN EMERGING DIGITAL TECHNOLOGIES (EDT)
Presidente Prof. SOREL, MARC
Membro Dott.ssa VITIELLO, MIRIAM
Membro D'AMORE, Vale
  • graphene
  • modulator
  • photo-thermoeelctric.
  • photodetector
  • silicon photonics
Exam session start date
Internet and data exchange are widespread aspects of nowadays life. It was only in the early 2000’s, no more than 20 years ago, that applications like video on demand, internet gaming, remote video-conferences and concepts like Internet of Things (IoT) were nothing more than concepts. Today, online applications are so pervasive that analysts forecast a total number of devices connected to the IP networks exceeding by three folds the world population by 2023. Connection speed will also increase to meet the requirements of these bandwidth-hungry applications, with 5G devices reaching an average connection speed of 575 Mbps by 2023.<br>High speed, large volume production and reduced power consumption are among the main factors that are driving the development of new Communication Technologies. Optical interconnects, adopted for long distance data transmission, strongly contributed to meet these speed requirements. However, technological solutions implemented for long haul communications are usually overengineered, too bulky and expensive to be suitable for the short reach communications, which are required by the growth of data centres. <br>Miniaturization of optical transceivers, critical components of an optical communication system, is a fundamental step for integration of optical interconnects within electronic components and for reduction of power consumption. In this scenario, integrated photonics plays a critical role as it can enable the integration of several functionalities on a single chip with reduced footprint at reduced cost. <br>InP photonic platform is likely the most complete technology providing integrated optical sources and on-chip amplification along with high-speed optical modulators and photodetectors. Nevertheless, the cost of an InP chip is not competitive with the cost of Si based photonics when dealing with large volume production, mainly because of the high cost and the small size of the InP substrates. Silicon photonics can take advantage of the maturity of existing CMOS processes to cost-effectively deliver the large number of devices required. However, several figures of merit of optical transceivers integrated on Si based photonic platform do not still fulfil the requirements in terms of speed and power consumption.<br>For example, electro-refractive modulators integrated on Silicon-on-Insulator (SOI) substrates struggle to reduce the V_π L parameter to less than 1 Vcm, resulting in a large driving voltage or in a reduced electro-optic bandwidth. When multi-wavelength data transmission is used, the reduction of the number of optical sources involved is of fundamental importance to reduce the power consumption and the system complexity. To this end, a large single channel bandwidth is highly desirable. Data rates exceeding 100 Gb/s have only been achieved by means of pre-emphasis techniques to compensate the limited electro-optic bandwidth of the transmitter, and by using high-order modulation formats to increase their spectral efficiency. Also, detection of an optical data stream by using an intensity modulation direct detection (IM-DD) scheme with Ge photodetectors integrated on SOI is limited to 90 Gb/s.<br>Graphene, a carbon allotrope and a 2D material, has recently emerged as a potential candidate to fill the gap between current technologies and the requirements for next generation photonic devices for datacom applications. Tunability of absorption coefficient and refractive index by means of electrostatic gating enables both electro-absorption and electro-refractive modulation. Theoretical achievable performance in terms of V_π L for electro-refractive modulation is about 0.16 Vcm, corresponding to almost a 10-fold improvement with respect to typical values for optical modulators integrated on SOI. The absence of a band-gap allows absorption in a broad optical spectrum, spanning from UV to the far infrared. High speed photodetectors have been demonstrated with optoelectronic bandwidths exceeding 100 GHz, thanks to the fast carrier dynamics upon optical excitation. Theoretical estimations for intrinsic response time of graphene-based photodetectors predict bandwidths larger than 260 GHz. Such a value can compete with unicarrier travelling wave photodetectors based on III-V semiconductors and outperform Ge based photodetectors.<br>The aim of this thesis is to demonstrate the potential impact of graphene technology through the realization of graphene-based photonic devices integrated on Si photonic platforms. The main part of the thesis is focused on high speed graphene photodetectors, however the chapter 6 is dedicated to the realization of a graphene electro-absorption modulator.<br>The first chapter describes the rationale of this work and introduces the basic concepts related to graphene used in the following chapters. A brief summary of state of the art of Ge and SiGe based active components integrated on a Silicon-on-Insulator (SOI) photonic platform is also given.<br>Chapter 2 analyses the photo-thermoelectric, the photo-bolometric, the photoconductive and the photovoltaic effects, i.e., high speed photoconversion mechanisms in graphene used in the field of optical communications and review the state of the art of graphene photodetectors with application in the optical communications field.<br>Chapter 3 describes the design of graphene photodetectors based on the photo-thermoelectric effect by using simplified formulas to outline the optimization strategy. The more accurate model described in APPENDIX B is used in chapter 4 and 5 to perform a quantitative analysis of the performance of the devices.<br>Chapter 4 describes the design, the fabrication and the experimental characterization of a PTE photodetector integrated on a SiN waveguide and using poly-vinyl alcohol as gate dielectric. A frequency response without evidence of roll-off up to 67GHz is here reported for the first time, suggesting an opto-electronic bandwidth larger than 67 GHz.<br>Chapter 5 describes the design, the fabrication and the experimental characterization of a PTE photodetector integrated on a SOI waveguide and using SiN as gate dielectric. Fabrication constraints posed by the use of PVA (detailed in chapter 4) are removed by the use of SiN as gate dielectric material. The detection of an optical data stream at 120 Gb/s has been demonstrated by using a 4-level Pulse Amplitude Modulation (PAM4) format and is, to date, the largest reported for a graphene PTE photodetector. This is the main experimental achievement of this thesis.<br>Chapter 6 reports on the design, the fabrication and the experimental characterization of a graphene electro-absorption modulator based on a double layer configuration. The transmission of a 50 Gb/s optical data stream is here reported for the first time by using an ON-OFF-Keying Non-return-to-Zero (OOK NRZ) modulation format for the first time.<br>Chapter 7 summarizes the results and concludes this work. <br><br>