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

Scaling of silicon technology is near the end of its lifetime The newest Intel’s processor is fabricated

with 32nm nodes 22 nm in 2011, 16 nm in 2013 and 11 nm in

2016 Then what?

Faster performing device is needed III-V devices have been proposed

Higher electron mobility of GaAs can improve the speed of the transistors built on it

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 planes1

Ge has almost the same lattice parameter as GaAs and its critical thickness (hc) is ~2 nm on Si2

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.

Introduction

Proposed work Epitaxial layer in a metastable state

caused by strain may be able to extend the critical thickness

Study by Majhi, et. al. shows that Ge layer at metastable state showed higher critical thickness (Fig. 1)

GaAs growth on a limitedarea to relief the strainat the edge may helpto increase hc.

Figure 1. Dependence of critical thickness on the stability state2

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-af)/af

ε⊥ =(af⊥ -af)/af where af⊥ = as3/af

2 Misfit %,

Lattice mismatched when f is small

sf

sf

aa

aaf

2

Strain/Stress in Thin Films Mismatch means stress. af>as => film in compression, subs in

tension as>af => film intension, subs in

compression

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

1ln

)1(8 bh

fv

bh cc

SAMPLE DESCRIPTION

Various thicknesses of GaAs was selectively grown on (110) surface of Si 20Å, 40Å, 80Å and 100Å

The geometry of GaAs epitaxial site is limited to a long, narrow channel 20 nm in width and semi-infinitely long

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 on GaAs and Si

Lattice parameters GaAs – 5.65 Å Si – 5.43 Å

Si will be under tensile strain and GaAs under compressive strain due to their lattice parameters

Si

GaAs

Figure 3. Strain direction

Strain Simuation

A study shows an equation which calculates the stress on the SiGe film on Si3

)4(1

1

)3(1

)2(1

)1(

2

2

22

22

22

)v(μ

)v(μK

eef(b)

eef(a)

f(a)f(b)(b)f(a)fσσ

sf

fs

πh

b)K(B

πh

Kb

πh

a)K(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 layera : x position b : y positionf(a) : stress as a function of position in x f(b) : stress as a function of position in yh : thickness of epitaxy layer

)4(1

1

)3(1

)2(1

)1(

2

2

22

22

22

)v(μ

)v(μK

eef(b)

eef(a)

f(a)f(b)(b)f(a)fσσ

sf

fs

πh

b)K(B

πh

Kb

πh

a)K(A

πh

Ka

x

_

Results

A defect-free single crystal layer of GaAs at 20 Å of thickness has been demonstrated

Figure 4. defect-free single crystal 20-Å thick GaAs is grown on Si

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

Discussions

We can deduce that the 5% reduction in the stress was able to effectively reduce the strain in the film so that higher critical thickness can be achieved.

This led to the phenomenal result of the demonstration of growing defect-free single crystalline GaAs on Si

This could open up the research opportunity for higher performance electron devices

Future Research

Assuming that GaAs can now be successfully grown on Si, we can now design a GaAs device on Si and analyze the performance of such device

Synopsys Sentaurus TCAD can simulation 1D/2D/3D devices. The future research will involve the simulation using this software

Conclusion

Defect-free single crystalline GaAs was successfully grown on Si at a very small thickness but this could still lead to an opportunity for future electron devices as well as other applications

The simulation shows that the edge effect could reduce the stress on the film by 5% and this effectively led to an increase in the critical thickness of GaAs epitaxy on Si

References

1. Fischer, R., Morkoc, H., Neumann, D. A., Zabel, H., Choi, C., Otsuka, N., Longerbone, M., and Erickson, L. P., Journal of Applied Physics 60 (5) 1986.

2. P. Majhi, P. Kalra, R. Harris, K. J. Choi, D. Heh, J. Oh, D. Kelly, R. Choi,B. J. Cho, S. Banerjee, W. Tsai, H. Tseng, and R. Jammy, IEEE Electron Device Letters, 29 (1) 2008.

3. Fischer, A., Richter, H., Applied Physics Letters, 61 (22) 1992.