Thesis Defense - Li Zhang

 
Quasi-One Dimensional Nano-Materials for Nanoelectronic Devices
 
Li Zhang
Department of Chemistry, Stanford University
Advisor: Prof. Hongjie Dai
 
Date: Thursday August 27th, 2009
 
Time: 10:00 AM (Refreshments at 9:45 AM)
 
Location: BrauLec (in Mudd chemistry building)
 
 
Abstract
 

As the scaling of silicon based electronic devices is approaching limitation set by the physical and materials properties, several high mobility materials have gained much interest as possible substitutions of silicon for future electronics. This thesis focuses on single walled carbon nanotubes (SWNTs), graphene nanoribons (GNRs) and germanium nanowires (GeNWs) due to their unique properties.

Germanium nanowires (GeNWs) are one potential material to address the future device scaling limitation owing to their high hole and electron mobilities. However, the device performance is limited due to the insufficient electrostatic control over charge carriers in the channel in the typical back-gate or top-gate geometry. And the mobility analysis based on capacitance modeling alone without direct measurement could give errors due to a lot of uncertainties. In the first part of this dissertation, I will demonstrate a novel surround gate structure of GeNW FETs using a novel self-aligned fabrication approach. Individual SG GeNW FETs show improved switching over GeNW FETs with planar gate stacks owing to improved electrostatics. FET devices comprised of multiple quasi-aligned SG GeNWs in parallel afford on-currents exceeding 0.1 mA at low source-drain bias voltages. Direct experimental evidence show that SG nanowire transistors exhibit higher capacitance and better electrostatic gate control than top-gated devices, and are the most promising structure for future high performance nanoelectronics.
 

Single walled carbon nanotubes are molecular quantum wires (diameter ~ 1nm) which are highly chemically stable and exhibit outstanding electrical conductivity. However, typical synthesis of SWNTs yields a mixture of both metallic and semiconducting varieties with a range of diameters. Several methods have been reported to separate SWNTs and anion exchange (IEC) chromatography has shown the most promise for electronic type separation . In the second part of the dissertation, I will discuss the characterization of IEC separation efficiency by combining spectroscopy and electrical measurents. In the early experiement, the SWNTs were separated according to diameter and electronic types and the separation efficiency decreased with increasing tube diameter. The separation efficiency was much improved by using the new DNA sequence to suspend the SWNTs and single-chirality-enrichment were achieved.

Graphene is single layer graphite, which is predicted to exhibit bandgaps useful for room temperature transistor operations with excellent switching speed and high mobilities when made into narrow ribbons (sub-10 nm). And the all-semiconducting nature of sub-10 nm GNRs could bypass the problem of extreme chirality dependence of metal or semiconductor nature of carbon nanotubes (CNTs) for future electronics. Currently, making GNRs remains challenging by lithographic, chemical or sonochemical methods. It is difficult to obtain GNRs with controllable width at high yields. In the third part of the thesis, I will show an interesting approach to making GNRs by using plasma etching to unzip multiwalled carbon nanotubes partially embedded in a polymer film. The GNRs exhibit a narrow width distribution between 10-20 nm. Electrical transport measurements confirmed the bandgap opening in narrow GNRs.

 
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Li
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Li Zhang
Dai's Group
Department of Chemistry
Stanford University