DESIGN AND MODELING OF 2D MATERIALS BASED OPTOELECTRONIC DEVICES

Abstract

Advances in power-efficient optical modulators and detectors are essential to make optical-electrical-optical (OEO) conversions highly efficient and ubiquitous for an efficient chip-integrated optical communication link. Silicon does, however, have several shortcomings as an active photonic material. Its basic form isn’t ideal for creating photodetectors and optical modulators. A solution to these problems is to develop hybrid systems using materials with properties offsetting the silicon limits while exploiting the complementary metal-oxide-semiconductor (CMOS) compatibility. To this end, two-dimensional (2D) materials add to the Si photonic platform fundamental properties like efficient light modulation and high-performance photodetection capabilities. Therefore, there is a requirement of novel device architectures to exploit the properties of 2D materials for optoelectronic applications. The unique band structure of graphene provides it with unusual electrical and optical properties, critical for the performance of electro-optic modulators. In addition to graphene, recently, transition metal dichalcogenides (TMDs) have also gained significant attention due to their interesting and rich optoelectronic properties, depending on composition and thickness. These materials exhibit strong light-matter coupling at excitonic resonances. The combination of photonic structures with graphene and TMDs could significantly extend their application in modulator design. A drawback of 2D materials for optic and photonic applications is the low modal overlap with any optical field originating from the atomic thickness of the monolayer material. Therefore, achieving a high modulation depth utilizing a monolayer or few layers of 2D material remains a challenge. The photo-field-effect transistor (photoFET) is a promising device architecture that can exploit the optical and electronic properties of 2D materials to simultaneously reduce the dark current and achieve high gain for photosensing applications. Despite the unique characteristics of these materials, efforts are still needed to overcome many intrinsic limitations. The limited optical absorption cross-section in these atomically thin materials leads to weak photogeneration, therefore, restricting the photosensitivity. Moreover, the zero bandgap of graphene leads to a considerable dark current. Further, the thickness-dependent spectral coverage in TMDs limits the absorption properties in the ultraviolet-visible part of the spectrum. Therefore, it is highly desirable to control the photocarrier behaviors of 2D materials to suit the needs of device functionalities. ii The expansions of well-established electronic/photonic devices to optoelectronic devices based on 2D material could resolve the issues involving low light absorption and short light-matter interaction length of 2D materials. This thesis work tailors the electronic and optical properties of 2D materials for the design of electro-optic modulator and photodetector. For an efficient modulator design, the effectiveness of a ring resonator structure has been utilized based on optimized graphene coverage using the field enhancement effect. The graphene integration in the ring provides tunability by means of altering the loss and coupling coefficient by changing the graphene coverage or its chemical potential. The effect of the build-up factor has been considered in evaluating the graphene induced losses and phase shift. For 1.55 μm radiation and a ring radius of 3.75 μm with 22% graphene coverage, a maximum modulation depth of 12.6 dB with an operational speed of 10 GHz is demonstrated based on electrostatic control of graphene chemical potential. The stable and strong tunable electro-optic property of different TMDs near the excitonic transitions has been utilized for the design of a visible-light modulator using the silicon nitride waveguide platform. A comparison of absorption changes due to electrostatically induced charges in MoS2, MoSe2, WS2, WSe2, and graphene has been presented for modulator design. The monolayer TMDs with a confinement factor of about 0.10% demonstrates 100x higher modulation strength as compared to the graphene-based modulator. In comparison with Mo based designs, the WS2 based modulator shows the highest modulation strength of around 68%, an improvement by a factor of 3.7. Further, the change in the spectral response of these materials with thickness and chemical composition has been exploited for the design of attenuator. The unique properties of TMDs and the substantial performance improvements in the low power electronics using tunnel field-effect transistor (TFET) predict an exciting future for TFET based devices in photosensing. The TMD TFET with high sensitivity in the sub-threshold regime has been exploited for the design of a highly sensitive photodetector by making use of photogating. The waveguide-integrated photodetector combines the advantage of MoS2 TFET with infrared detection capability of Germanium (Ge). The addition of the photovoltage to the gate voltage increases the band-to-band tunneling (BTBT) significantly, showing good photoresponse. At 1550 nm wavelength with an incident optical power intensity of 3 Wcm-2, a maximum responsivity of 2700 AW-1 and an on-off ratio of 107 are obtained. The device shows good spectral sensitivity with a low dark current. iii To further exploit the characteristics of TFET, a highly-sensitive and low power photosensor based on split gate TFET structure has been investigated for visible light detection. The optically tunable workfunction contrast between the two gates of the dual material gate (DMG) TFET achieves significant gains in the overall device characteristics. It demonstrates a high photo-to-dark current ratio of the order of 108 and a large variation in responsivity in the range between 103 AmW-1 to 10-5 AmW-1. The performance of the device has been investigated based on the position of the photogate on the tunneling or auxiliary side, the gate splitting ratio between the photosensing region and the metal, and the nTFET based device design.