Mobility Enhancement of 2DHG in an In 0 2 Ga 0 in an In 0.24 Ga 0.76 - - PowerPoint PPT Presentation

Mobility Enhancement of 2DHG in an In0 2 Ga0

6As Quantum

in an In0.24Ga0.76As Quantum Well by <110> Uniaxial Strain

Ling Xia1, Vadim Tokranov2, Serge Oktyabrsky2 and Jesús del Alamo1

1MIT 2SUNY Alb 1MIT, 2SUNY Albany

05.25.2011

1

Motivation Motivation

• Improve p-channel InGaAs FETs for III-V CMOS

E h bi i l t i + i i l t i

• Enhance µ : biaxial strain + uniaxial strain

SixGe1-x pFET

Demonstrated: High-performance InGaAs nFET

1 μ ∂

Ge

1 μ π μ σ ∂ =− ∂

Ge

Wanted: Hi h f I G A FET

Jesus del Alamo, IEDM 2007, short course

πL_<110>(with σbi) > πL_<110> (without σbi)

2 Leonardo Gomez, EDL, 2010 2

High-performance InGaAs pFET

Experimental structure Experimental structure

• Biaxially

strained p-channel

• Typical output characteristics of

Biaxially strained p channel In0.24Ga0.76As QW: Typical output characteristics of fabricated QW-FET

• Channel strain :

1.7% biaxial compressive p

3

Experiment approach Experiment approach

• Apply uniaxial stress to GaAs chips

M f d H ll b

• Measure response of ungated Hall bars

– High IG prevents accurate C-V to extract CG and ps

• Mechanism to bend GaAs chips • Supporting mechanism • Stress and Hall bar orientations

4

p

• Can apply tensile or

compressive stress pp g and connections

Sheet hole density change Sheet hole density change

4

ps0=7.8e11 cm

• 2

4

R t 0 036% MP ps0=8.2e11 cm

• 2

Compressive Tensile Compressive Tensile

2

ps/ps0 (%)

σ⊥,[110]

Rate:

• 0.043%

per MPa

2

ps0 (%)

σ//, [110]

Rate: -0.036% per MPa

100 50 50 100

• 4
• 2

Δp σ//, [-110]

Channel along [-110]

Rate: 0.018% per MPa

100 50 50 100

• 4
• 2

Δps/p σ⊥,[-110]

Channel along [110]

Rate: 0.010% per MPa

• 100
• 50

50 100

Stress (MPa)

• 100
• 50

50 100

Stress (MPa) Solid lines: linear fittings to data Dashed lines: 1D SP simulation with piezoelectric effect

• Almost identical patterns in Δps for Hall bars along [110]

and [-110]

Δp determined by piezoelectric effect – Δps determined by piezoelectric effect – Similar to our previous p-channel GaAs study.

5

(L. Xia, to be published on TED)

Hole mobility change Hole mobility change

12

293

2/V

12

265

2/V

Compressive Tensile Compressive Tensile

4 8

(%)

σ⊥,[110]

Rate: 0.12% per MPa

µ0=293 cm

2/V.s

4 8

µ0=265 cm

2/V.s

σ⊥,[-110]

(%)

Rate: 0.046% per MPa

12

• 8
• 4

Δµ /µ0 σ//, [-110]

Channel along [-110]

Rate: -0.054% per MPa

12

• 8
• 4

Δµ /µ0 σ//, [110]

Channel along [110]

Rate: -0.071% per MPa

• 100
• 50

50 100

• 12

Stress (MPa)

• 100
• 50

50 100

• 12

Stress (MPa) Solid lines: linear fitting to data

µ // µ ⊥

• General trends of µh:

– Dominant factor: relative orientation of stress and transport direction Si il i Si d G

Tensile ↓ ↑ Compressive ↑ ↓

– Similar in Si and Ge

Sensitivities of µh to σ<110>

0.08% 0.12%

Sensitivities of µh to σ<110>

0.00% 0.04% (% per MPa)

• 0.08%
• 0.04%

(Δµ/µ)/σ

• 0.12%
• Preferred configuration: Compressive σ parallel to [-110] channel

σ//,[-110] σ//,[110] σ┴,[-110] σ┴,[110]

• Preferred configuration: Compressive σ parallel to [-110] channel
• Questions:

– Why π// different from π⊥ ? – Why |π//,[-110]| ≠ |π//,[110]|, and |π⊥,[-110]| ≠ |π⊥,[110]| ?

7

Anisotropy between π// and π⊥

• Dominated by in-plane VB dispersion anisotropy

– Simulation: 2D in-plane dispersion relation in QW by k p method

Anisotropy between π// and π⊥

Simulation: 2D in-plane dispersion relation in QW by k.p method No uniaxial stress With uniaxial [-110 ] compressive stress

σ = -112 MPa <110> directions symmetric

• Change of VB (m*) // or ⊥ to σ are different π// and π⊥ different
• Sign – opposite for Δm*// and Δm*⊥

8

g pp

// ⊥

• Magnitude – different (will show quantitatively later)
• Similar in Si or Ge

(S. Thompson, IEDM, 2004; O. Weber, IEDM, 2007)

Different π along the two <110> directions Different π along the two 110 directions

• Counterintuitive:

– Δm*// ( or Δm*⊥) should be the same for σ[-110] and σ[110]

• 1st effect : ps change due to piezoelectric effect (ps ↑ µh ↓)

– Partly explains π⊥,[-110] and π⊥,[110] difference – May have decreased π// [ 110] and increased π// [110] May have decreased π//,[-110] and increased π//,[110]

• 2nd effect: polarization-field-induced quantization change

Al0 42Ga0 58As 21 nm

Black: [110] -112 MPa Red: [-110] -112 MPa

In0.24Ga0.76As 9 nm Al0.42Ga0.58As 21 nm Al0.33Ga0.67As 80 nm

ϕ

2 (a.u.)

Red: [ 110] 112 MPa

S.I. GaAs Substrate GaAs buffer 70 nm

ϕ

9

S.I. GaAs Substrate

28 30 32 34 36 38 40 42

z along growth axis (nm)

Comparison between experiments d i l i and simulations

0.12%

|Δμ/μ0|

[-110] bar |Δμ/μ0|

Experiments

|Δµ/µ|/σ

• Extract average conductivity

m* by approximations:

a)

0.06% 0.08% 0.10%

|Δμ/μ0| |-Δm/m0|

[110] bar |-Δm/m0| |Δμ/μ0|

Simulations

|-Δm*/m*|/σ |Δµ/µ|/σ

2 2 *( )

2( ) = −

• i

v v

k m E E E y pp

(M. D. Michielis, TED, 2007)

(% per MPa

0.00% 0.02% 0.04%

*( ) ( )

( ) * ( ) ( )

∞ ∞

=

∑∫ ∑∫

vi

i i i E

m E f E g E dE m f E g E dE

|(Δµ/µ)/σ|

• Other sources of anisotropy:

Anisotropic scattering (e g polar optical phonon scattering) τ ≠ τ

σ//,[-110] σ//,[110] σ┴,[-110] σ┴,[110]

( ) ( )

∑∫

vi

i i E

f E g E dE

– Anisotropic scattering (e.g. polar optical phonon scattering) τ// ≠ τ⊥ when m*// ≠ m*⊥ (J. J. Harris, J. Phys. Chem. Solids, 1973) – Lateral composition modulation along [110] (K. Y. Cheng, Appl. Phys. Lett.,

1992) 1992)

– Strain relaxation along [110] (B. Bennett, J. Electron. Mater., 1991)

10

Comparison with other materials

Our experiments

Comparison with other materials

1 | | | | μ π ∂ =

Literat re Measured from

| | | | π μ σ = ∂

Literature 2DHG or inversion layers

ps = 6~8x1011 cm-2 For Ge, ps = 2x1012 cm-2

[1] 0.41 0.59 0.24 0.76

• Uniaxial strain is a viable path to enhance p-channel III-V FET performance

[1] [2]

This work

11

• Superposition of uniaxial strain on top of biaxial strain large improvement in µ

[1] L. Xia, APL, 2011. [2] L. Xia, to be published on TED