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