Graphene Based Heterojunctions for Nano-Electronic and Sensing Applications

Graphene, an atomically thin and semi-metallic two dimensional material, has been extensively researched over the past decade due to its superior intrinsic carrier velocity, electrical and chemically tunable work function, ability to form layered heterostructure with other materials, and relevant potential applications in electronics, sensing, optoelectronics, energy storage, etc. However, the confinement of charge carriers within one atomic layer results in an electrical transport that is extremely sensitive to the surrounding environment, which is beneficial for sensing applications, but at times unfavorable for electronic applications due to scattering from extrinsic impurities. In addition, due to its rather delicate structure, engineering a high quality gate dielectric without altering its characteristic electronic structure while enabling optimal surface passivation and gate control is one of the major challenges for graphene device development. Hexagonal Boron Nitride (hBN) has emerged as a possible option to meet the challenges, and has been exploited to alter graphene electronic structure by intentional crystallographic misalignment between the layers at the time of transfer or synthesis. The variation in electronic structure by hBN is possible due to its unique properties such as inert surface, similar hexagonal and nearly lattice matched structure with graphene and high surface optical phonon modes. Low temperature Pulsed laser deposition (PLD) grown amorphous BN on SiO2/Si, phase transformed to hBN by forming gas annealing, has been employed for graphene device application. Graphene field effect transistor (FET) fabricated from layered heterostructure of graphene/hBN on SiO2/Si exhibited electrical performance enhancement over graphene on SiO2/Si substrate in terms of mobility, carrier inhomogeneity and extrinsic doping. In a parallel effort, taking advantage of graphene’s tunable work function, a novel genre of sensor based on noble metal nanoparticle functionalized graphene/Si heterojunction Schottky diode has been developed for sensing non-polar H2, and enhancing response for polar NH3 molecular species. Reverse bias operation of the diode sensor exhibited orders of magnitude higher response compared to graphene FET based sensors due to exponential change in reverse current originated from interface barrier height change. The reverse bias operation also allows low power operation and modulation of the Fermi level of graphene, which can lead to the tuning of sensitivity and expansion of the dynamic range. Impedance Spectroscopic analysis of the diode sensor has been carried out to understand the underlying current transport mechanism. Fitting the impedance spectra for different gaseous exposure conditions with an equivalent circuit model, the changes in junction resistance and capacitance have been extracted. Along with these two parameters, experimentally obtained 3-dB cut off frequency for each gas exposure has been utilized for multimodal sensing by the diode sensor. Finally, temperature dependent magneto-transport study of PdHx passivated graphene has been carried out to elucidate the effect of metal nanoparticle assisted doping and molecular adsorption on graphene electrical transport properties. It has been observed from the systematic study that, the dominant scattering mechanism in bilayer graphene switched from coulomb scattering to thermal excited surface optical phonon scattering after PdHx passivation, and Hall mobility exhibited significant enhancement at the measurement temperature range of 298 to 10 K. Due to recent interests in exploiting metallic nanoparticles as dopant for 2D crystals, as well as enhancing sensitivity of chemical sensors and photodetectors, the findings are significant and would pave the way for future research efforts in this area