Electrical bistability in organic semiconductors and spin injection using organic magnetic semiconductor

by Bin Li

Institution: The Ohio State University
Department: Physics
Degree: PhD
Year: 2012
Keywords: Physics; organic semiconductor; organic magnet; organic electronics; organic spintronics
Record ID: 1951062
Full text PDF: http://rave.ohiolink.edu/etdc/view?acc_num=osu1334864514


Organic semiconductors have attracted much attention for their electronic and magnetic properties. They have served as a electronic functional part in light-emitting diodes and photovoltaic cells, as well as the bistable memory devices. In addition, the emergent organic-based magnetic semiconductor makes itself an alternative candidate for spintronic applications. This thesis is composed of two parts: organic bistable device and organic spin valve. In the first part, I will discuss a pentacene-based bistable memory device using Fe as the top electrode and compare it to the Al/pentacene/Al devices. The device displays stable switching from the low-current OFF state to the high-current ON state and long retention time. Our results suggest that Fe has the advantage over Al as the top electrode because it lowers the switching threshold voltage. The device is promising as a Write-Once Read-Many (WORM) memory. I will also discuss a study of the electrical bistability and bias-controlled spin valve effect in an organic device using rubrene (5,6,11,12 -tetraphenylnaphthacene) as an organic semiconductor channel. The half-metallic La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> (LSMO) and Fe are used as the two ferromagnetic electrodes. The device displays reproducible switching between a low-impedance (ON) state and a high-impedance (OFF) state by applying different polarities of high biases. In the ON state, the device shows a spin valve effect with magnetoresistance values up to 3.75 %. The observed spin valve effect disappears when the device recovers to the initial OFF state. In the second part, I will focus on the spin injection using organic-based magnetic semiconductor V[TCNE]<sub>x</sub> (x~2, TCNE: tetracyanoethylene). I will show the demonstration spin injection/detection in an all-organic-based magnetic tunnel junction using two organic-based V[TCNE]<sub>x</sub> magnets as the magnetic contacts and organic semiconductor rubrene as the spacer. For the V[TCNE]<sub>x</sub> film growth, we exploited two different growth techniques, chemical vapor deposition and molecular layer deposition, which result in different coercivities of V[TCNE]<sub>x</sub> films. The spin valve devices show negative magnetoresistance (MR), the sign of which does not change with temperature and bias. We propose that the unusual negative MR originates from the different spin polarizations of the molecular energy levels of V[TCNE]<sub>x</sub>. Our results show the significance of bias induced energy level shift in organic spintronic devices due to relatively narrow spin polarized bandwidths. I will also discuss the experimental results of room-temperature spin injection usignt V[TCNE]<sub>x</sub> as well as the effect of the organic spacer on the device performance.