NSF- NIRT: "Surface State Engineering" Charge Storage and Conduction in Organo-Silicon Heterostructures as a Basis for

Nanoscale Devices

John C. Bean (PI)1, Avik Ghosh1, Lloyd R. Harriott1, Lin Pu2, Keith Williams3

1Department of Electrical and Computer Engineering, 2Department of Chemistry, 3Department of PhysicsUniversity of Virginia, Charlottesville Virginia

Basic Idea:

Create highly-pure, highly-ordered bound organo-silicon hybrids

Molecules bound so intimately that electron waves pass easily between components

Yielding:

1) Precise charge transfer in to and out of Si surfaces

2) Conduction based on quantum mechanical resonance

In nanoscale MOSFETs this could:Supplant conventional trace donor/acceptor charge creation

Minimize ionized impurity scattering => Enhanced transport

Enable devices based on quantum interference phenomenon

Three pronged investigation strategy:

First principles modeling

Studies of trap-influenced transport in CNT & graphene surrogates

Development of full organo-Si MOSFET device structure

Multiscale Modeling of Trap Modified Transport:

Coupling weak and strong quantum correlation (wires vs. dots):

Density Functional Theory (Si) / Extended Hückel (molecules)

Within framework of Non-Equilibrium Green's Functions

Applied to Random Telegraph Signals (RTS) of molecules on FETs:

Could differentiate between molecules and alternate binding states:

Engineering Molecular Surface States on Silicon:

Task 1: Achieve dense high-quality organic attachment to Si

Achieved via UV-enhanced

UHV vacuum hydrosilylation

(e.g. alkyne => alkene + attachment):

Task 2: Identify organics capable of passivating Si surfaces

Hydrosilylated acetylene proved very effective as verified by pulsed

photoconductivity measurement of near surface Si minority carrier lifetimes

Task 3: Develop palette of organic surface state inducers

Surface dipoles: Surface charge transfer:

First achieved with variants of phenylacetylene:

(Engineering of Molecular Surface States on Si continued):

Fine-tuned with fluorine induced dipoles vs. quadrupoles:

Or via dilution of surface-state-inducing molecules with passivating molecules

Dipole induced surface band bending actually more intense in diluted layers:

Evidence that polar organics partially depolarize if in immediate proximity

Task 4: Fabrication of back-gated SOI MOSFET device structure

Goal: Assess channel modification via phenomena such as RTS signals

Pursuing both micron and deep sub-micron channel width device designs

Former: Easier to fabricate / Latter: More subtle surface state information

Component processes for both designs essentially complete:

Design Pads Channel Side-gates

Outreach:

Continuing Development of “Hands-on Intro to Nano” Class

Begun under 2005 NSF NUE grant

Complete class website (with visitors from over 1000 schools):

www.virlab.virginia.edu/Nanoscience_class/Nanoscience_class.htm

Cited in 2009 Award of IEEE Undergraduate Teaching Medal to PI

Plus ongoing summer Nano classes for K-12 science teachers

Recent Project Publications:Reversal of current blockade in nanotube-based field effect transistors through multiple trap correlations, J. Chan et al., Phys. Rev. B 80, 033402 (2009)

Controlling transistor threshold voltages using molecular dipoles, S. Vasudevan et al., J. Appl. Phys. 105, 093703 (2009)

Modeling electrostatic and quantum detection of molecules, S. Vasudevan et al., IEEE-Sensors Journal 8, 6 (2008)

Plus In-Press / In-Preparation:Quantum Transport through Metal-Molecule-Semiconductor heterostructures: the role of interfacial charges and barriers, A Bahuguna et al.

Coupling nano and microscale transport for electronic read-out of quantum dot dynamics, S. Vasudevan et al.

Single Molecule Characterization using Random Telegraph Signals, S. Vasudevan et al.

Microscopic model for trap-generated noise in nano-devices, S. Vasudevan et al.

Effect of Traps on CNT and Graphene Transport:

Basic studies + “concurrent engineering” of experimental techniques and modeling tools for later use with full SOI MOSFET device

Electron beam induced traps in CNT and graphene FETs:

RTS successfully modeled, including multi-trap interactions

Inter vs. intra layer transport in Graphene:

Despite common 2D idealizations, real data likely to include both!

Raman highly sensitive to layer edges => Decorate with metals

Colloidal Ag decoration Raman of Raman of Ag edgeof graphene edges normal HOPG decorated HOPG

Signature should facilitate deconvolution of transport modes