Congli Sun

Perpendicular magnetic tunnel junction with W seed and capping layers

C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program; MARCO; DARPA; National Science Foundation [ECCS-1310338]; Inston, Inc., through a Phase II Small Business Innovation Research award from the National Science Foundation

Metastable Intermediates in Amorphous Titanium Oxide: A Hidden Role Leading to Ultra-Stable Photoanode Protection

Metastable intermediates represent a non-equilibrium state of matter that may impose profound impacts to materials properties beyond our understandings of monolithic and equilibrium systems. Here, we report a discovery of hidden metastable intermediates in amorphous TiO2 thin films and their critical role in electrochemical damage. These intermediates have a non-bulk crystal-like structure and exhibit significantly higher electrical conductivity than both the amorphous and the crystalline phases. When these TiO2 films are applied to protect Si photoelectrochemical (PEC) photoanodes, the intermediates can induce localized high electrical currents that largely accelerate the etching of the TiO2 film and the Si electrode underneath. The intermediates can be effectively suppressed by raising their nucleation barrier via reducing the film thickness from 24 to 2.5 nm. The homogeneous amorphous TiO2-film-coated Si photoanodes achieved more than 500 h of PEC water oxidation at a steady photocurrent density of over 30 mA·cm-2.

Dielectric breakdown along c-axis boundaries in magnetoelectric O2O3 for spintronic devices

Voltage controlled boundary magnetism is crucial for spintronic devices with reduced power consumption. Magnetoelectric and antiferromagnetic Cr2O3 is an ideal material due to its electric field switching of nonvolatile boundary magnetism [1]. The boundary magnetization can switch an adjacent soft ferromagnetic layer via voltage-controlled exchange bias. Bulk Cr2O3 has a high dielectric breakdown field of 1000 KVmm and a large bandgap of 3.4 eV. However, even with a small electrode size of 0.04 mm and thick Cr2O3 film of 0.5 μm, the highest reported dielectric breakdown field of Cr2O3 films is only 200 KVmm [2]. The breakdown field drops rapidly to 8 KVmm if electrode size increases to 35 mm and film thickness decreases to 250 nm [2], making electric field induced switching of Cr2O3 based heterostructures very difficult. Here, we combine aberration corrected STEM characterization and spin polarized density functional theory (DFT) calculations to elucidate the structure, electronic properties, and magnetic properties of a new type of interface-stabilized planar crystallographic defect in Cr2O3 thin films that explains the structural origin of dielectric breakdown.