University Ph.D. Oral Examination : Paul W Leu

Semiconductor Nanowires: Modeling, Experiments, and Their Implications

 

Paul W. Leu

Department of Mechanical Engineering

 

Advisors: Prof. Paul C. McInyre and Prof. Kyeongjae (KJ) Cho

 

Friday, February 8th, 2008

3:00 PM (Refreshments served at 2:45 PM)

CIS-X 101

 

Semiconductor nanowires (NWs) have the potential for a variety of nanoelectromechanical and nanodevice applications. This presentation discusses the electromechanical properties and electrical transport of semiconductor NWs.

 

The first part of the talk covers the systematic study of the mechanical and electrical properties of small diameter (< 3 nm) silicon NWs under axial strain using ab initio density functional theory calculations. The values of Young's Modulus, Poisson ratio, band gap, effective mass, work function (WF) and deformation potentials were calculated for <110> and <111> oriented Si NWs. We performed a detailed study of the effects of axial strain on the band structure and electronic properties and attempt to predict the effect of strain on electronic transport properties. We found a dramatic decrease of the deformation potentials of Si NWs under strain, which may lead to a many fold increase of electron or hole mobilities. The decrease of deformation potentials occurs as NWs undergo a direct-to-indirect band gap transition and is concurrent to the increase of effective mass. We also found that quantum confinement in Si NWs acts as a built-in tensile strain, which splits conduction band valleys and decreases transport effective mass.

 

The second part of the talk covers the fabrication of vertically aligned germanium NW structures and their transport and doping characterization. Dense vertical Ge NWs were grown epitaxially on Ge <111> substrates from Au catalysts. The NWs could be doped by the deposition of a conformal shell of boron-doped Ge around the vertical NWs. The NWs were encapsulated in highly conformal silica using an alternating layer deposition process to isolate and support them. Chemical mechanical polishing was used to planarize the samples and expose the Ge NW tips. The post-CMP cleaning was found to be important in removing slurry particles and contamination from the samples while leaving the Ge NWs intact. We probed the topography and electrical transport properties of these encapsulated vertical Ge NW structures using a probe station and conductive AFM. The NWs were shown to exhibit resistor-like IV characteristics when grown on p-type substrates and p-n junction rectifying behavior when grown on n-type substrates. All processes took place at temperatures below 400^o C, a key requirement for monolithic 3-dimensional integration of semiconductor devices on Si integrated circuits.