Strained Gaas - Nanotechnology - Lecture Slides, Slides of Nanotechnology

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J. H. Woo,

Department of Electrical & Computer Engineering

Texas A&M University

GEOMETRIC RELIEF OF STRAINED GaAs

ON NANO-SCALE GROWTH AREA

Table of Contents

 INTRODUCTION

 BASIC PHYSICS ON EPITAXY

 SAMPLE DESCRIPTION

 RESULTS

 DISCUSSIONS

 FUTURE DEVELOPMENT

 CONCLUSIONS

Introduction

 Problems with III-V electron devices

 Substrate cost is much higher than Si

 Growth of III-V on Si is difficult and usually

defective

 For example, GaAs has 4% lattice mismatch to Si

Introduction

 Problems with GaAs epitaxy on Si

 No defect-free GaAs growth has been

experimentally demonstrated.

 4% misfit indicates that one dislocation will be

occupied in every 25 atomic planes 1

 Ge has almost the same lattice parameter as GaAs

and its critical thickness ( h c ) is ~2 nm on Si

2

 Ge is the optimum case as it is a unary material

 For binary materials, single crystal epitaxy is more

defective and therefore, the critical thickness is

higher.

Thin Film Epitaxy

and Applications

 Epitaxy

 The growth of a crystal of one material on the

crystal face of another material in such a way that

both materials have the same or similar structural

orientation.

 Applications of GaAs Epitaxy

 Solar cells

 Semiconductor Lasers

 High mobility devices

Lattice Mismatch

 Pseudomorphic growth: one-to-one matching

 Films strained due to misfit

 Misfit dislocation occurs with large strain

 ε// =(as -a f )/a f

 ε⊥ =(a f⊥ -a f )/a f where a f⊥ = a s

3

/a f

2

 Misfit %,

 Lattice mismatched when f is small

f s

f s

a a

a a f

− = 2 ×

Defects

 Formed during the relaxation of excessive

strain.

 Among many defect types, we are interested

in dislocations

Critical Thickness

 The maximum thickness before relaxation of

strain occurs leading to dislocations

 



 

 + 

  



= ln 1 8 ( 1 ) b

h

v f

b hc c π

Fabrication Method

 Number of possible fabrication method can be used

 The easiest method is to start from (110) Si substrate

 (110) Si is patterned into long, narrow patterns using

electron lithography

 The direction of this pattern was oriented so that (001)

surface is exposed on the side

 The patterned substrate is RIE etched to isolate the

epitaxy site

 GaAs is selectively grown on (110) surface only using

an MBE system

 The thickness is controlled carefully so that each batch of

sample has GaAs thickness of 20 Å to 100 Å

Fabrication Method

Si

Figure 2. Fabrication steps. (a) (110) Si, (b) Si patterned and RIE etched, (c) GaAs is selectively MBE grown on (110) surface

(a)

(b)

(c)

GaAs

Strain Simuation

 A study shows an equation which calculates

the stress on the SiGe film on Si

2

2

2 2

2 2

2 2

μ ( v )

μ ( v ) K

f(b) e e

f(a) e e

σ σ f (a) f (b) f(a)f(b)

f s

s f

πh

K(B b) πh

Kb

πh

K(A a) πh

Ka

x

---

− − −

− − −

Strain Simuation

σ_bar : effective stress σx : normal stress μf : Young’s Modulus for film μs : Young’s Modulus for substrate vf : Poisson’s ratio for film vs : Poisson’s ratio for substrate A : x dimension of epitaxy layer B : y dimension of epitaxy layer a : x position b : y position f(a) : stress as a function of position in x f(b) : stress as a function of position in y h : thickness of epitaxy layer

2

2

2 2

2 2

2 2

μ ( v )

μ ( v ) K

f(b) e e

f(a) e e

σ σ f (a) f (b) f(a)f(b)

f s

s f

πh

K(B b) πh

Kb

πh

K(A a) πh

Ka

x

---

− − −

− − −

Results

 Epi-layers with larger thicknesses showed

numerous dislocations at 60° to the surface

Figure 5. defective GaAs epitaxial layers at a larger thickness

Discussions

 The simulation result shows the relieving effect of the edges

 The average stress along the x direction was approximately 95%

of the original stress, yielding 5% of stress relief due to the finite

epitaxial site

Figure 6. effective stress plot