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 In 0 2 Ga 0 in an In 0.24 Ga 0.76 As Quantum 6 As Quantum Well by <110> Uniaxial Strain Ling Xia 1 , Vadim Tokranov 2 , Serge Oktyabrsky 2 and Jesús del Alamo 1 1 MIT 2 SUNY Alb 1 MIT , 2 SUNY Albany 05.25.2011 1

Motivation Motivation • Improve p-channel InGaAs FETs for III-V CMOS • Enhance µ : biaxial strain + uniaxial strain E h bi i l t i + i i l t i Si x Ge 1-x pFET Demonstrated: High-performance InGaAs nFET ∂ ∂ μ μ 1 1 π =− μ ∂ σ 0 Ge Ge Jesus del Alamo, IEDM 2007 , short course Wanted: π L_<110> (with σ bi ) > π L_<110> (without σ bi ) Hi h High-performance InGaAs pFET f I G A FET Leonardo Gomez, EDL , 2010 2 2

Experimental structure Experimental structure • Biaxially • Typical output characteristics of Biaxially strained strained p channel p-channel Typical output characteristics of In 0.24 Ga 0.76 As QW: fabricated QW-FET - Channel strain : 1.7% biaxial compressive p 3

Experiment approach Experiment approach • Apply uniaxial stress to GaAs chips • Measure response of ungated Hall bars M f d H ll b – High I G prevents accurate C-V to extract C G and p s • Mechanism to bend GaAs chips • Supporting mechanism • Stress and Hall bar orientations p pp g • Can apply tensile or and connections compressive stress 4

Sheet hole density change Sheet hole density change Compressive Tensile Compressive Tensile 4 4 -2 p s0 =8.2e11 cm -2 p s0 =7.8e11 cm R t Rate: -0.036% per MPa 0 036% MP Rate: σ ⊥, [110] 2 σ //, [110] 2 p s / p s0 (%) -0.043% per MPa p s0 (%) 0 0 Δ p s / p Δ p σ //, [-110] σ ⊥, [-110] -2 Rate: 0.010% per MPa -2 Rate: 0.018% per MPa Channel along [110] Channel along [-110] -4 -4 -100 100 -50 50 0 0 50 50 100 100 -100 100 -50 50 0 0 50 50 100 100 Stress (MPa) Stress (MPa) Solid lines: linear fittings to data Dashed lines: 1D SP simulation with piezoelectric effect • Almost identical patterns in Δ p s for Hall bars along [110] and [-110] – Δ p s determined by piezoelectric effect Δ p determined by piezoelectric effect – Similar to our previous p-channel GaAs study. (L. Xia, to be published on TED ) 5

Hole mobility change Hole mobility change Compressive Tensile Compressive Tensile 12 12 2 /V 2 /V.s 2 /V 2 /V.s µ 0 =265 cm 265 µ 0 =293 cm 293 Rate: 0.12% per MPa σ ⊥, [-110] 8 8 Rate: 0.046% per MPa 4 4 σ ⊥, [110] (%) (%) 0 0 Δ µ / µ 0 Δ µ / µ 0 -4 -4 Rate: -0.054% per MPa σ //, [110] σ //, [-110] Rate: -0.071% per MPa -8 -8 Channel along [-110] Channel along [110] -12 12 -12 12 -100 -50 0 50 100 -100 -50 0 50 100 Stress (MPa) Stress (MPa) Solid lines: linear fitting to data µ // µ ⊥ Tensile ↓ ↑ • General trends of µ h : Compressive ↑ ↓ – Dominant factor: relative orientation of stress and transport direction – Similar in Si and Ge Si il i Si d G

Sensitivities of µ h to σ <110> Sensitivities of µ h to σ <110> 0.12% 0.08% (% per MPa) 0.04% 0.00% ( Δ µ / µ )/ σ -0.04% -0.08% -0.12% σ //,[-110] σ //,[110] σ ┴ ,[110] σ ┴ ,[-110] • • Preferred configuration: Compressive σ parallel to [-110] channel 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 ⊥ Anisotropy between π // and π ⊥ • Dominated by in-plane VB dispersion anisotropy – Simulation: 2D in-plane dispersion relation in QW by k p method 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 Δ m * // and Δ m * ⊥ g pp // ⊥ • Magnitude – different (will show quantitatively later) • Similar in Si or Ge (S. Thompson, IEDM , 2004; O. Weber, IEDM , 2007) 8

Different π along the two <110> directions Different π along the two 110 directions • Counterintuitive: – Δ m * // ( or Δ m * ⊥ ) should be the same for σ [-110] and σ [110] • 1 st effect : p s change due to piezoelectric effect ( p s ↑ � µ h ↓ ) – 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] • 2 nd effect: polarization-field-induced quantization change Black: [110] -112 MPa Al 0.42 Ga 0.58 As 21 nm Al 0 42 Ga 0 58 As 21 nm Red: [-110] -112 MPa Red: [ 110] 112 MPa In 0.24 Ga 0.76 As 9 nm 2 (a.u.) Al 0.33 Ga 0.67 As 80 nm ϕ ϕ GaAs buffer 70 nm S.I. GaAs Substrate S.I. GaAs Substrate 28 30 32 34 36 38 40 42 z along growth axis (nm) 9

Comparison between experiments and simulations d i l i • Extract average conductivity Experiments 0.12% [-110] bar m * by approximations: y pp | Δ µ / µ |/ σ | Δ µ / µ |/ σ | Δ μ / μ 0 | | Δ μ / μ 0 | a) | Δμ / μ 0| | Δμ / μ 0| (% per MPa 0.10% (M. D. Michielis, TED , 2007) |- Δ m */ m *|/ σ |- Δ m / m 0 | [110] bar |- Δ m/m0| Simulations 2 2 � k 0.08% * ( ) = m E v i − 2( E E ) 0.06% v |( Δ µ / µ )/ σ | ∞ 0.04% ∑∫ * ( ) ( ) m E f E g E dE ( ) i i 0.02% i E = m * vi ∞ ∑∫ ∑∫ 0.00% f E g E dE f E g E dE ( ) ( ) ( ) ( ) σ //,[-110] σ //,[110] σ ┴ ,[110] i σ ┴ ,[-110] i E vi • Other sources of anisotropy: – Anisotropic scattering (e.g. polar optical phonon scattering) τ // ≠ τ ⊥ 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 Comparison with other materials Our experiments ∂ μ 1 π π = = | | | | | | | | μ ∂ σ 0 Measured from Literat re Literature 2DHG or inversion layers p s = 6~8x10 11 cm -2 For Ge, p s = 2x10 12 cm -2 [1] [1] [2] 0.41 0.59 0.24 0.76 This work • Uniaxial strain is a viable path to enhance p-channel III-V FET performance • Superposition of uniaxial strain on top of biaxial strain � large improvement in µ [1] L. Xia, APL , 2011. 11 [2] L. Xia, to be published on TED