MSE PhD Dissertation Defense: Don Koun Lee (Fri, November 9th, 10AM)

Department of Materials Science & Engineering

University PhD Dissertation Defense 

 

Nano-fabrication and Characterization of emerging memory technology specialized in spintronics

 

Don Koun Lee

Advisor: Prof. Shan X. Wang

 

Date: Friday, November 9th, 2012

Time: 10:00 AM (Refreshment served at 09:45 AM)

 

Location: Spilker Building (Center for Nanoscale Science and Engineering, Nano building)

Conference room # 232

 

Abstract

First part: Spin transfer torque (STT) devices with a nano aperture

Recent progresses in spin transfer torque (STT)-based random access memory make it a realistic contender in the race toward next generation solid state data storage devices. However, the relentless scaling-down of device dimensions stemming from the Moore's law mandates that the STT-based devices have a spin switching current density of well below 10⁶ A/cm² while maintaing thermal stability of data bits stored, which are usually two contradictory requirements that have spurred worldwide research for new approaches to boost STT with minimal spin currents. We demonstrate that a reduction of the switching current by a factor of >100 in a magnetic tunnel junction (MTJ) pillar of 200 × 400 nm² can be achieved with a nano-aperture of 30 × 30 nm². In the presence of the nano-aperture, there is a large component of current in the plane of the free layer that creates the adiabatic and non-adiabatic spin torques on the free layer in addition to the conventional spin torque of the tunnel current. A micromagnetic simulation including these competing spin torques confirms that the in-plane current induced spin torques generate spin waves that causes the dramatic reduction of the tunnel current required for switching. The nano-apertured MTJ pillars presented in this work provide a promising path to the large scale practical applications of STT devices since they retain their thermal stability over 10 years and simultaneously achieve a low switching tunnel current of the order of 10⁴ A/cm².

 

Second part: Low contact resistance in Ge/MgO/CoFeB spin diodes

While the rapid and continued progress in the scalability and the integration technology of conventional planar transistor facing the physical limitation, introduction of the new functionality into the transistors has emerged as one of the alternative approaches for the future integrated electronics technology. One of the current issues for improving the efficiency of the spin injection from the ferromagnetic materials (FM) into semiconductors (S) is the conductivity mismatch between both materials because the spin injection can be greatly affected by the ratio of both conductivities. Recent reports suggested that the improvement in the spin injection could be achieved by an ultra thin oxide layer between FM and S. In addition, the oxide barrier also reduces the metal-induced gap-state (MIGS) density at the interface between FM and S, which can release the Fermi level pinning and decrease the effective barrier height. We studied and compared the Schottky barrier height of FM/S and FM/oxide/S and demonstrated that the ultra thin oxide layer between FM and S can modulate the effective barrier height based on measurement of the current versus voltage characteristics and a thermionic emission model. Moreover, we will discuss how contact resistance of spin diodes can be optimized by fine control of MgO layer thickness. 

Don Koun Lee 
Ph.D. Candidate 
Materials Science and Engineering 
Stanford University