David R. Myers April 11, 2007 EE 235 Ultimate Strength in Germanium Nanowires.
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Transcript of David R. Myers April 11, 2007 EE 235 Ultimate Strength in Germanium Nanowires.
David R. Myers
April 11, 2007
EE 235
Ultimate Strength in Germanium Nanowires
Motivation• In General
– Properties of thin films, nanowires are often different than bulk materials
– Need to characterize these properties to be able to design micro and nano systems
– Mechanical Strength for electromechanical device construction
• Nano-Specific– Can we get stronger materials?
Stress Intensity• Cracks cause local stress increases depending on
loading condition
Type1 Type2 Type3
1/ 2 3/ 20.43 1.9IK Pd Md 1/ 2 3/ 20.56 1.5IIK Pd Md
Type I Type II Type III
Thougless, et. al,1 found that for brittle plates, the stress intensity factors are:
Material Testing So Far…
• Before Study– Near Theoretical Strengths for composite
whiskers have been reported– Cracks & Imperfections typically cause deviation
from ultimate strength– Only about 50% theoretical strength for silicon
has been reported
• Germanium– High Mobility– Inferior Oxide– Wafers are brittle
Supercritical Fluid-Liquid-Solid Apparatus
• Monodisperse Gold nanocrystals as catalysts
• Hexane with tetraethylgermane and diphenylgermane (silicon precursor) at 365C, 5.5x104 mbar
• Very high throughput & 90% in <112> direction
Batch Reactor
Porous Alumina with Gold Nanocrystals
Supercritical Fluid-Liquid-Solid Mechanism
• Coexistance of Au:Ge Liquid and Germanium Solid at 360C, 28% Ge conc.
• Germanium most likely to dissolve into gold particle, until super-saturation, in which it will be expelled as nanowire
Device Testing Method
Sonicate IPA
Drop onto Trench
Pin with Platinum
Testing Method
Results & Comparison
Material Ultimate Strength
Bulk Silicon ~300 MPa - 1 GPa
Mild Steel ~450 MPa
Hard Steel ~1.3 GPa
Aluminum ~260 MPa
Ge Nanowire
~15 GPa
• Note Lack of Ductile Behavior Failure– Shows AFM application of force does not give rise to
defects– The ultimate theoretical strength of Ge is shown
Results for Various Radii
• Interesting to note max theoretical strength is 17GPa, but data gives 15 +/- 4 GPa
• Also Displacement at failure is a function of radius
Questions?
• Thanks!
Ductile Failure
Thin Film Application
• These Stress Intensity Factors (previous equations) have been derived for films on brittle materials2
0
1 0.279 ( 1)0.434IK
h I
0
1 0.217 ( 1)0.558IIK
h I
f
s
E
E
h
z
z
h I = Moment of Inertia
Note: Normally the stress intensity factor are determined by loading conditions. In this special case, the stress intensity factor is determined solely by the geometry.
Type II Stress Concentration• Type II Stress concentration determines direction of
crack propagation
KII < 0 KII = 0 KII > 0
Into substrate
Crack growth aimed away from surface
Crack growth aimed towards the surface
Crack growth parallel to surface
• Results from Drory2