1. Dopant profiling and surface analysis of silicon nanowires using capacitance-voltage measurements Erik C. Garnett, Yu-Chih Tseng, Devesh R. Khanal, Junqiao Wu, Jeffrey Bokor, and Peidong Yang, Nature Nanotechnology, March 2009
Changhwan Shin
Department of Electrical Engineering and Computer Sciences
University of California, Berkeley, CA 94720
April 27, 2009

2. Outline Silicon nanowire structure and fabrication
Dopant profiling and surface analysis
C-V frequency-dependent measurement
Principle behind dopant profiling
Dopant profiling using high-frequency C-V measurement
FEM 3-D C-V simulation
Summary

3. Outline Silicon nanowire structure and fabrication
Dopant profiling and surface analysis
C-V frequency-dependent measurement
Principle behind dopant profiling
Dopant profiling using high-frequency C-V measurement
FEM 3-D C-V simulation
Summary 3

4. Completed silicon nanowire [SNW]-FET
Device structure
Gate dielectric
Al2O3 (15nm), ALD
Metal gate: Chromium
Diameter ~75nm S
Lg~2.3µm D
SOI
Device Fabrication
Growing SNW bridges epitaxially across patterend SOI trench,
Using gold nanoparticles and vapour-liquid-solid (VLS) mechanism
Hexagonal faceting of SNW ; <111> oriented wires
But the surfaces appear to be {211}, instead of {110} in theory
{211} commonly observed in micrometer-scale <111> oriented whiskers grown using the VLS mechnism.

5. Outline Silicon nanowire structure and fabrication
Dopant profiling and surface analysis
C-V frequency-dependent measurement
Principle behind dopant profiling
Dopant profiling using high-frequency C-V measurement
FEM 3-D C-V simulation
Summary 5

6. C-V Freq.-dependent Measurement
C-V measurement at 77K
Freq. dispersion in the depletion region
Substantial shift in C-V
Increased Low-Freq. cap.
Interface states (Not responding quickly to High-Freq. a.c.
Interface state density, Dit, vs. E-Ev
Using high-low method to extract Dit
By comparing high/low-freq. cap. , Dit as a function of E is extracted.
Due to underestimation of high-low method, lower bound of Dit is obtained.
Decoupling interface state effects from strain/chemical gating/surface roughness
1e13 cm-2ev-1
Mid-gap
4e11 cm-2ev-1

7. Outline Silicon nanowire structure and fabrication
Dopant profiling and surface analysis
C-V frequency-dependent measurement
Principle behind dopant profiling
Dopant profiling using high-frequency C-V measurement
FEM 3-D C-V simulation
Summary 7

8. Dopant profile using the high-Freq. C-V
“Principle” behind profiling
As SNW is depleted, the effective insulator thickness increases and the capacitance drops.
Voltage dependence of the cap. drop is related to the majority carrier density: Rapid (slow) drop – low (high) dopant concentration. 8

9. Outline Silicon nanowire structure and fabrication
Dopant profiling and surface analysis
C-V frequency-dependent measurement
Principle behind dopant profiling
Dopant profiling using high-frequency C-V measurement
FEM 3-D C-V simulation
Summary 9

10. Dopant profile using the high-Freq. C-V [2]
Radial dopant profile: Na diff/flat
The expected dopant diffusion profile simulated using the experimental doping conditions as inputs (Na-diff.).
Finite-element modeling (FEM) electrostatic simulations to calculate the corresponding majority carrier distribution (p(r) – diffusion).
Carrier concentration extracted from the theoretical C-V curve with the simulated boron profile as an input (p(r) – simulated)
divergence
divergence
Majority carrier profiling agreed well.
Experimental data matches well to simulated data, but start to diverge towards the core.
Divergence is expected according to the C-V dopant profile resolution limitation of twice the Debye screening length (Ld). Here, Ld is 2 and 13nm at the surface and core, respectively. 10

11. Outline Silicon nanowire structure and fabrication
Dopant profiling and surface analysis
C-V frequency-dependent measurement
Principle behind dopant profiling
Dopant profiling using high-frequency C-V measurement
FEM 3-D C-V simulation
Summary 11

12. Capacitance-Voltage simulation
FEM 3-D simulations
Ideal C-V curve generated with diffusion/flat profiles
In order to determine the flat band voltage (Vfb) and to validate the dopant profiling and Dit extraction.
Extra capacitance came from direct coupling between the surround gate and the nanowire leads.
Interface states causes minor deviation in slope.
20kHz
Extra capacitance 12

13. Summary
C-V measurement to determine the Dit profile as a function of position in the band-gap.
Comparable Dit of SNW to bulk MOSFET is critical for achieving high-performance electronic devices.
Radial boron doping profile was measured via C-V curve and matched the expected profile from dopant diffusion simulation. 13

14. Q & A
Thank you for your attention!!!
Questions?