Towards building a prototype spin-logic device

Since the late 1980s, several key discoveries, such as Giant and Tunneling Magne- toresistance, and advances in magnetic materials have paved the way for exponentially higher bit-densities in magnetic storage. In particular, the discovery of Spin-Transfer Torque (STT) has allowed information to be written to individual magnets using spin-currents. This has replaced the more traditional Oersted-field control used in field-MRAMs and allowed further scaling of magnetic-memories. A less obvious con- sequence of STT is that it has made possible a logic-technology based on magnets controlled by spin-polarized currents. Charge-coupled Spin Logic (CSL) is one such device proposal that couples a giant spin Hall effect(GSHE) write-unit with a Mag- netic Tunnel Junction read-unit. Several theoretical reports have demonstrated that a CSL-style device can function as a fundamental building block for neuromorphic computing by harnessing the intrinsic properties of magnets. This thesis describes the working of a CSL device. Experimental progress towards building the individual components of CSL and also our efforts to integrate these components into a CSL prototype will be presented. In addition to the integration effort, this work also explores spin-injection from a GSHE metal to a nanoscale magnet through an intermediate non-magnetic metal. Our results indicate that with the right choice of intermediate layers, the spin-angular mo- mentum absorbed by the magnet can be increased without engineering the intrinsic spin Hall angle of the GSHE metal. Finally, this work also proposes a Schottky-barrier model to describe the current flow through low-dimensional semiconductors and uses it to extract the band gap of black-phosphorus thin-films in an attempt to characterize novel 2D-materials

Limitations of the High-Low C-V Technique for MOS Interfaces With Large Time Constant Dispersion

We discuss the limitations of the high-low CV technique in evaluating the interface trap density (D-TT) in MOS samples with a large time constant dispersion, as occurs in silicon carbide (SiC). We show that the high-low technique can seriously underestimate D-IT for samples with large time constant dispersion, even if elevated temperatures are used to extend the range of validity

High Performance Multilayer MoS₂ Transistors with Scandium Contacts

While there has been growing interest in two-dimensional (2-D) crystals other than graphene, evaluating their potential usefulness for electronic applications is still in its infancy due to the lack of a complete picture of their performance potential. The focus of this article is on contacts. We demonstrate that through a proper understanding and design of source/drain contacts and the right choice of number of MoS₂ layers the excellent intrinsic properties of this 2-D material can be harvested. Using scandium contacts on 10-nm-thick exfoliated MoS₂ flakes that are covered by a 15 nm Al₂O₃ film, high effective mobilities of 700 cm²/(V s) are achieved at room temperature. This breakthrough is largely attributed to the fact that we succeeded in eliminating contact resistance effects that limited the device performance in the past unrecognized. In fact, the apparent linear dependence of current on drain voltage had mislead researchers to believe that a truly Ohmic contact had already been achieved, a misconception that we also elucidate in the present article

Improvement of Spin Transfer Torque in Asymmetric Graphene Devices

A graphene lateral spin valve structure with asymmetric contacts is presented for the first time, with enhancement of spin angular momentum absorption in its receiving magnet. The asymmetric device with tunneling barrier only at the injector magnet shows a comparable spin valve signal but lower electrical noises compared to the device with two tunneling barriers. We also report experimental measurements of spin transfer torque. Assisted by an external magnetic field of 2.5 mT, spin diffusion current-induced magnetization reversal occurs at a nonlocal charge current density of 33 mA/mu m(2), smaller than that needed in devices with two tunneling barriers

Spin Transfer Torque in a Graphene Lateral Spin Valve Assisted by an External Magnetic Field

Spin-based devices are widely discussed for post-complementary metal–oxide–semiconductor (CMOS) applications. A number of spin device ideas propose using spin current to carry information coherently through a spin channel and transfering it to an output magnet by spin transfer torque. Graphene is an ideal channel material in this context due to its long spin diffusion length, gate-tunable carrier density, and high carrier mobility. However, spin transfer torque has not been demonstrated in graphene or any other semiconductor material as of yet. Here, we report the first experimental measurement of spin transfer torque in graphene lateral nonlocal spin valve devices. Assisted by an external magnetic field, the magnetization reversal of the ferromagnetic receiving magnet is induced by pure spin diffusion currents from the input magnet. The magnetization switching is reversible between parallel and antiparallel configurations, depending on the polarity of the applied charged current. The presented results are an important step toward developing graphene-based spin logic and understanding spin-transfer torque in systems with tunneling barriers

High Performance Multilayer MoS2 Transistors with Scandium Contacts

While there has been growing interest in two-dimensional (2-D) crystals other than graphene, evaluating their potential usefulness for electronic applications is still in its infancy due to the lack of a complete picture of their performance potential. The focus of this article is on contacts. We demonstrate that through a proper understanding and design of source/drain contacts and the right choice of number of MoS2 layers the excellent intrinsic properties of this 2-D material can be harvested. Using scandium contacts on 10-nm-thick exfoliated MoS2 flakes that are covered by a 15 nm Al2O3 film, high effective mobilities of 700 cm(2)/(V s) room temperature. This breakthrough is largely attributed to the fact that we succeeded in eliminating contact resistance effects that limited the device performance in the past unrecognized. In fact, the apparent linear dependence of current on drain voltage had mislead researchers to believe that a truly Ohmic contact had already been achieved, a misconception that we also elucidate in the present article