Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
Scanning Probe Microscopy HT10 1
Other SPM Techniques
• Scanning Near-Field Optical Microscopy (SNOM)
• Scanning Capacitance Microscopy (SCM)
• Scanning Spreading Resistance Microscopy (SSRM)
• Multiprobe techniques
• Electrostatic Force Microscopy, Kelvin probe force microscopy
Scanning Probe Microscopy HT10 2
Scanning Probe Microscopy HT10 3
Scanning Near-Field Optical Microscopy (SNOM)
• Optical microscope resolution limit: 0.61λ (Abbè limit)
• Optical properties of nanostructures very important…..
• Light-emitting nanoparticles, molecules, devices..
• Inelastic scattering: Raman, fluorescence
• SNOM uses the optical near-field, non-radiative local electric field, evanescent at surfaces
• Near-field interactions lead to far-field changes
• Detector close to the surface, resolution in the 10-50 nm range
• Problem: tip preparation (standard SNOM)
Scanning Probe Microscopy HT10 4
SNOM basics
Dipole-dipole interaction (near field) lead to far-field changes
Overview of SNOM modes: a) Aperture SNOM b) scattering (aperture-less) SNOM c)-d): Related STM techniques
Scanning Probe Microscopy HT10 5
Aperture SNOM
• Optical fiber tips - tapered - metal coated - end aperture
• Many configurations for aperture SNOM
• Distance control crucial - tip shear forces
(NT-MDT design)
Scanning Probe Microscopy HT10 6
Aperture SNOM example Single dye molecules - fluorescence - 15 nm res.
Hosaka and Saiki, J. Microsc.202, 362 (2001)
Aperture: 20 nm
Illumination – collection mode
Resolution 15 nm better than fundamental limit in standard SNOM on metal surfaces (30-50 nm)
Scanning Probe Microscopy HT10 7
Scattering SNOM • Relies on field enhancement due to metallic tip - surface plasmons
• Raman scattering - SERS effect enhances signal up to 1011 -1014
• Raman spectroscopy -very useful for nanostructures, molecules, CNT
• Measures vibrational modes - ”fingerprint” for different bonds and molecules
• example: radial breathing mode in CNT - Raman shift direct measure of diameter (figure: diameters 1.51 – 1.22 nm)
CNT far-field Raman spectrum (a) near-field, (b) far-field, (c) difference
Scanning Probe Microscopy HT10 8
TERS - Tip-enhanced Raman spectroscopy
Images of a CNT bundle by ”standard” confocal Raman (left) and TERS Raman (right)
Branching CNT viewed by TERS Raman (left) and SFM (right)
Scanning Probe Microscopy HT10 9
Instrumentation for TERS
Example NT-MDT Ntegra Spectra
SFM
Micro-Raman spectrometer including:
• confocal microscope
• focused laser
• monochromator
• detectors
Scanning Probe Microscopy HT10 10
Scanning Capacitance Microscopy
• SCM useful for measuring dopant profiles on the nanoscale
• Model: MOS structure
!
1Ctot
=1
Cair+1Cox
+1CD
Scanning Probe Microscopy HT10 11
SCM
!
1Ctot
=1
Cair+1Cox
+1CD
(Note: sample bias!)
Scanning Probe Microscopy HT10 12
SCM instrumentation • Contact-mode SFM
• AC bias, measure dC/dV
• Special capacitance sensor
Scanning Probe Microscopy HT10 13
SCM examples
SCM of 0.6 µm n-channel MOSFET
From Edwards, APL 72, 698
SFM and SCM images of an SRAM device
Scanning Probe Microscopy HT10 14
Scanning Spreading Resistance Microscopy (SSRM)
• Measure spreading resistance by pressing the SFM tip into the sample - large force due to contamination
• Maxwell formula:
• Nanoscale contact: ballistic transport => Sharvin formula:
!
R = " 4a
!
RSharvin =h2e2
"F2
# 2a2
• Equal or better resolution than SCM
• more simple determination of doping level, but cannot determine carrier type
• surface damage due to indentation
Multiprobe techniques
Scanning Probe Microscopy HT10 15
Local conductivity measurements on nanostructures, nanoelectronic devices, organic layers, etc. Example: Single-electron transistor (SET)
Double-probe STM
Scanning Probe Microscopy HT10 16
Sharper tips
Scanning Probe Microscopy HT10 17
Figure 5 . Schematic diagram of minimum interprobe distance, Lmin , between two probes; a) two conventional W probes, b) two WOx nanorod probes grown at apexes of two W probes. SEM and TEM images of the WOx nanorods are shown in c) and d), respectively. An atomic-resolution STM image of a Si(111)7x7 surface obtained by using this WOx nanorod probe is shown in e).
Resistance measurements
Scanning Probe Microscopy HT10 18
Quadruple- probe
Scanning Probe Microscopy HT10 19
HOPG
V-I measurements on Si(111)4x1-In, a 1-dim. surface structure
Scanning Probe Microscopy HT10 20
Electrostatic Force Microscopy
• Special case of DFM
• Synonymous: Kelvin probe force microscopy
Scanning Probe Microscopy HT10 21
EFM, Kelvin probe measurements
!
Fel ="C"z
Vbias # VCPD +VAC cos$mt( )2
• Add AC signal to DC bias
!
Fel ="C"z
Vbias # VCPD( )2 + 2VAC cos$mt Vbias # VCPD( ) +VAC2 cos2$mt[ ]
• Force contains DC, ωm and 2ωm components
• ωm component zero when Vbias = VCPD
• ωm component zero detected by lock-in technique