Spin Transport in Hybrid Organic-Semiconductor Nanostructures via Chirality-Induced Spin Selectivity

Semiconductor spintronics has been widely regarded as a viable pathway to overcome many limitations in the current microelectronics technology. Molecular spintronics, an emerging research area in spintronics, aims to incorporate the unique functionalities of organic components into the solid-state device architectures. The overarching theme of this dissertation research is to explore a new nonmagnetic pathway of spin injection and detection in semiconductor spintronics by employing chirality-induced spin selectivity (CISS) in chiral molecules.This dissertation begins with a study of spin transport in an inorganic semiconductor, the persistent photoconductor Si-doped Al0.3Ga0.7As. The persistent photoconductivity facilitates spin transport measurements on one and the same sample over a broad range of carrier densities across the insulator-metal transition (Chapter 3). Three-terminal (3T) and four-terminal (4T) Hanle measurements have been performed to ascertain the dependences of spin lifetimes on bias current and carrier density. The spin lifetimes in the AlGaAs channel are extracted from fittings to the Lorentzian function and one-dimensional spin drift-diffusion model. The comprehensive results from our experiments show broad similarities between 3T and 4T Hanle signals, providing evidence that 3T Hanle measurements in our systems do result from spin accumulation instead of spurious effects. The results provide a solid experimental basis for understanding the spin relaxation mechanisms in semiconductors. The second project (Chapter 4) focuses on the formation and micro/nano patterning of self-assembled monolayers (SAMs) of thiol molecules on the semiconductor GaAs. We demonstrated that by etching and passivating the GaAs surface with ammonium polysulfide, alkanethiol molecules can form high-quality SAMs by solution assembly. Moreover, using molecular patterning methods of dip-pen nanolithography and micro-contact printing, nanoscale and microscale thiol SAM patterns can be created directly on the prepared GaAs surface. The functionalities of the molecular SAMs were then demonstrated by using 4-aminothiophenol SAMs as templates for directed self-assembly of Au nanoparticles on GaAs. The experiments demonstrated the viability of the experimental techniques for fabricating molecular/semiconductor hybrid structures and bottom-up fabrication of nanoplasmonic structures. The unique combination of the physics knowledge and experimental skills from the first two projects laid the foundation for the implementation of my primary dissertation project: Utilization of CISS to generate spin polarization in semiconductors without using any magnetic materials. The two main methods of spin detection are the spin-valve effect and Hanle effect. For the spin-valve effect (Chapter 5), we fabricate vertical planar junctions of (Ga,Mn)As/α-helix L-polyalanine (AHPA-L) molecules/Au. The magnetoconductance (MC) of the junction is measured, which provided direct evidence for spin selective transport through chiral molecules assembled on a ferromagnetic semiconductor (Ga,Mn)As. Importantly, the junction structure in our experiments allowed for robust measurements of the bias dependences of MC, which show both a pronounced nonlinear-response component and a nontrivial linear-response component. Our results are in direct contradiction of the expectation of some theoretical models and provide an important constraint for a viable theory of CISS and its device manifestations. The Hanle effect, presented in Chapter 6, has been performed by the 3T configuration, in junctions of n-GaAs/AHPA-L molecules/Au, with the (Ga,Mn)As replaced by the non-magnetic Si-doped GaAs. The signals show two distinct components consistent with contributions from the Hanle effect and dynamic nuclear polarization respectively. The signals are observed to exhibit exceptional sensitivities to temperature, bias current, and the orientation of the applied magnetic field with respect to the junction plane. Details of the experimental results are not fully understood and warrant further experimental and theoretical investigation. The definitive demonstration of a CISS-induced Hanle effect in nonmagnetic semiconductors would be a significant step forward to a new paradigm in spintronics, namely, semiconductor spintronics free of any magnetic materials.