Nanocharacterization (III)

Seeing at the Nano-scale Another way to see things at the nano-scale is to use probes which, themselves, are nano in size. These probe-based tools can be used to let us see size, shape, structure, composition, physical properties, and chemical properties.

One probe-based technique is the Atomic Force Microscope (AFM) One probe-based technique is the Atomic Force Microscope (AFM). This techniques uses the force between a nano-scale probe tip and the atoms of the specimen surface to create an image which can give size and shape.

Deflection of the cantilever due to varying forces between the nano-scale tip and the atoms of the surface is picked up by changes in the laser beam reflection and converted by a computer into a picture.

Schematic of an AFM Veeco Dimension 3100 http://www.veeco.com http://nue.clt.binghamton.edu/spm.html

Size and Shape Observations using an AFM DNA Nanolithograpy pattern

Another probe-based technique is the Scanning Tunneling Microscope (STM). This techniques uses the tunneling current between the tip and the atoms of a surface to create an image and even composition information. Tunneling current is a quantum mechanical phenomenon which depends extremely strongly on the distance between the tip and the atoms on the surface.

Picture of the Atoms on a Silicon Surface Imaged using STM surface. Note that you can see that Nature has made Some mistakes and atoms are missing. Scanning tunneling microscopes allow surfaces to be imaged at the atomic-scale Courtesy: Greg McCarty Schematic of the scanning tunneling microscope

Another probe-based technique is Nano-indentation Another probe-based technique is Nano-indentation. This technique presses a tip into a specimen surface at a specified rate and with a specified force. This technique can sample a nano-scale size region.

Nanoindentation Operation Scan surface of a specimen and find target spot. When the cantilever is located on the target spot, select indentation parameters (rate and force) and execute the indentation. Diamond cantilever tip is lowered and forced into the target surface causing the cantilever to deflect. By then knowing how far the tip is able to press into the surface for the specified rate and force, the material hardness can be determined. Imaging can then be done to determine location of test. typical indentation tip Indentations of two different diamond-like carbon films using three different forces (23,34, and 45uN) http://www.veeco.com

An Idea of How Small things can be and yet still be “seen” Some examples of how small the specimen (sample) can be and yet still be seen for some often-used techniques-- Transmission Electron Microscopy (TEM) Scanning Electron Microscopy (SEM) Field Emission Scanning Electron Microscopy (FE-SEM) Probe Techniques Atomic Force Microscopy (AFM) Scanning Tunneling Microscopy (STM)) Note: 10Angstoms equals 1 nanometer. http://www.eaglabs.com/en-US/services/bubblechart-102005-h.pdf

The Example of SiNW Characterization Microscope characterization Size characterization Field effect scanning electron microscopy (FESEM) for width Atomic force microscopy (AFM) for height Composition characterization Auger electron spectrum (ASE) Crystallinity characterization Raman spectroscopy Conductivity characterization I-V measurement

Microscope and Schematic Pictures SiNW/SiNR Gold caps Remaining Au slug 5% SiH4 in H2 Remaining Gold Gold cap (a) For long Au catalyst slug, SiNWs grow at each end of the Au slug Si absorption and diffusion coexist and compete each other. The top ends of Au slug get supersaturated before Si can diffuse to the center. Silicon locally saturates the tips of the gold slugs. Two SiNWs or SiNRs, each grow from Au slug ends, leaving a central slug

Microscope and Schematic Pictures SiNW/SiNR Gold caps Silicon substrate 5% SiH4 in H2 Gold cap (b) For short Au catalyst slug, SiNW/Rs grow from the center of Au slug Si absorption and diffusion coexist and compete each other. The whole Au slug gets supersaturated SiNW grows from the center and Au slug is split into two caps One SiNW or SiNR grow with two Au caps, leaving no Au central slug

Size Characterization Combination of FESEM and AFM FESEM offers width measurement AFM offers accurate height measurement Tip angle (20o) and radius size (15nm) Not good for width measurement

FESEM and AFM Results for Size b c 20nm nanowire group Width 35nm 200nm nanoribbon group Width 250nm e d Height 25nm Height 25nm

FESEM Results a Partially remove the capping layer and expose the SiNW/Rs SEM image of SiNW/R group series - widths from 200nm to 20nm Long Au slug case, with central Au slugs Bright wires: central Au slug left after LPCVD growth Dark wires: SiNWs or SiNRs

3-D AFM Result Cross sectional dimensions from FESEM and AFM SiNW/Rs inherit the exact shape of the nanochannels Growth front/catalyst caps are liquid Fill the full channel and minimize the contact resistance

AES Characterization for Composition Powerful surface characterization method Chemical and compositional properties of materials Samples a depth of typically 0.5 to 5 nm Auger scanning windows are smaller than 5 nm by 5 nm No influence from the Si substrate Insulation layer below SiNW/Rs

AES Characterization for 200nm SiNR Au: three characteristic peaks 1771ev, 2022ev, 2107ev Silicon: two characteristic peaks 96ev, and 1621ev multiple peaks C and O peaks Contamination Removed by ion-sputtering Silicon: two characteristic peaks 96ev, and 1621ev multiple peaks C and O peaks Contamination Removed by ion-sputtering No Au characteristic peaks

Auger Spectrum for 20nm SiNW 20nm Au wires 20nm Si wires The bright wires are composed of Au and silicon The dark wires are composed of silicon, confirming SiNW growth. No Au in dark wires at least to the detection limit of ASE

Raman Characterization for Crystallinity Optically isolate the underlying Si substrate Coating a reflective and opaque nickel layer (200nm) Inserted between the substrate and insulation layer

Y. Shan, A. K. Kalkan, C. Y. Peng, and S. J Y. Shan, A. K. Kalkan, C.Y. Peng, and S. J. Fonash, “From Si Source Gas Directy to Positioned, Electrically Contacted Si Nanowires: The Self-Assembling Grow-in-Place Approach”, Nano Letters, Vol. 4, p. 2085-2089, 2004

I-V Characterization for Conductivity Single nanowire/ribbon resistor structures

I-V Characterization I-V characteristics of a single 200 nm wide, 20 nm high channel without the SiNR (empty, 20 m contact spacing) and with a grown SiNR using contacts with 20 m () and 40 m () spacings. Linear I-V behavior, conductivity of 2 × 10-4 S/cm Resistance scales with contact spacings Contact resistance is not an issue Capable of achieving low Au doping Empty channel no dependence of I on V, but only noise

Key Ideas Nano-scale probes or beams of electrons, ions, or photons (light) can be used as our means to see at the nano-scale. Signals generated by the interaction of these probes or beams with a material can be computer processed into pictures we can see. These pictures can give us size, shape, structure, chemical and physical property information, and composition.