Two dimensional materials-based vertical heterojunction devices for electronics, optoelectronics and neuromorphic applications

Belete, Melkamu Adgo; Lemme, Max C. (Thesis advisor); Waser, Rainer (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Advancement of digital microelectronics relied for decades on the classical scaling philosophy guided by the famous evolutionary trend known as "Moore’s law". However, a slowdown of this relentless device scaling is becoming inevitable due to fundamental physical limits. This led to the rise of a new strategy called "more than Moore (MtM)" that targets on integrated circuit functionality diversification by promoting novel non-digital (analog) applications such as radio-frequency (RF) electronics, power management systems, optoelectronics, sensors, micro/nano electromechanical systems, next generation computing systems, etc. This strategy requires new device concepts and novel materials outperforming conventional ones. Novel two-dimensional (2D) materials such as graphene and molybdenum disulfide (MoS2) are suit- able for MtM applications due to their unique structural, electrical and optical properties. In line with the MtM goals, this thesis investigates vertical hybrid devices based on graphene, MoS2 and their heterostruc- tures integrated into conventional 3D silicon (Si) for applications to- ward RF electronics, optoelectronics and neuromorphic computing. The fabrication schemes used here are scalable and semiconductor process technology compatible. The primary milestone in this thesis has been the investigation of the potential of MoS2 as the emitter diode of graphene-base hot electron transistors (GBTs). GBTs are promising devices for high-speed analog electronics and they have a vertical architecture comprising a Si-emitter, a graphene-base and a metal-collector, each isolated by a thin barrier. Maximizing the performance of GBTs requires efficient hot-electron injection across the emitter-base-barrier. Theory suggests that this can be realized by using semiconducting barrier materials that form low energy barriers to promote thermionic emission. MoS2 is a good candidate in this regard owing to its semiconducting behavior with a bandgap and electron affinity values enabling band alignments providing a small barrier with respect to the Si-emitter. Hence, "Si/MoS2 using capacitance-voltage (C-V) and conductance-voltage (G-V) techniques. The static dielectric constant of MoS2 is obtained from the measured C- V data. Measurements under electric-field stress, verified by analytical simulations, have indicated the presence of interface states and mo- bile negative ions in MoS2. This observation was further supported by time-of-flight secondary ion mass spectroscopy analysis that showed hydroxyl ions (OH−) possibly originating from catalytic water splitting by MoS2. Furthermore, transmission electron microscopy studies reveal the structural properties of the film including its polycystallinity with vertically aligned layers. Next, charge carrier transport proper- ties were investigated across "n+-Si/MoS2/Graphene" vertical heterojunction diodes analogous to the emitter diodes of GBTs. Analyses of the measured temperature dependent I-V data in corroboration with analytical models confirmed that the electron transport across the n+- Si/MoS2 heterojunction barrier is dominated by thermionic emission. This fulfils the prerequisites for using MoS2 as the emission barrier of GBTs.The thesis also includes experiments on the "Si/MoS2/Metal" vertical heterojunctions for memristive switching. Static (DC) current-voltage (I-V) and resistive switching (RS) characterizations including endurance and state-retention tests demonstrate the memristive functionality of the devices. The switching tests exhibit a bipolar and nonvolatile RS behavior with encouraging endurance and state retention for at least 140 DC switching cycles and 2500 seconds, respectively. Controlled C-V, G-V and switching measurements in ambient and vacuum conditions, elucidated by analytical simulations, suggest that the observed RS behavior is due to electric field-driven movements of the mobile OH- ions along the vertical MoS2 layers and their influence on the potential barrier at the Si/MoS2 interface. In addition, electro-optical characterizations, in particular I-V measurements with and without white light illumination and spectral responsivity (SR) measurements, were carried out on vertical "n+-Si/MoS2/Graphene" heterojunction diodes, which exhibit broadband optical sensitivity. The SR data feature multiple peaks in the ultraviolet and visible regions indicating that the measured photocurrent is mainly due to excitations in the MoS2. In addition, an infrared response is observed for energies below the Si and MoS2 bandgaps. This may be attributed to absorption in the graphene and/or inter-layer transitions in a staggered band alignment or absorptions via midgap states in the MoS2 bandgap. In conclusion, the work and findings in this thesis can serve as a guideline for integrating 2D materials and their heterostructures into the existing Si platform to create hybrid heterojunction devices for potential electronic, optoelectronic and neuromorphic applications.

Institutions

• Chair of Electronic Devices [618710]