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Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach

Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach. PhD Final Examination Anisur Rahman PhD Co-Advisors: Professor Mark Lundstrom and Professor Gerhard Klimeck School of Electrical and Computer Engineering Purdue University, West Lafayette, IN 47907.

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Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach

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  1. Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach PhD Final Examination Anisur Rahman PhD Co-Advisors: Professor Mark Lundstrom and Professor Gerhard Klimeck School of Electrical and Computer Engineering Purdue University, West Lafayette, IN 47907 PhD Defense, Anisur Rahman

  2. Acknowledgements • Doctoral Advisory Committee: • Professor Supriyo Datta (ECE) • Professor Ron Reifenberger (Physics) • Dr. Avik Ghosh (ECE) • Present Colleagues: • Dr. Diego Kienle • Dr. Jing Wang • Sayed Hasan • Neophytos Neophytou • Siyu Koswatta • Former Colleagues: • Dr. Zhibin Ren (IBM) • Dr. Ramesh Venugopal (TI) • Dr. Jung-Hoon Rhew (Intel) • Asst. Prof. Jing Guo (Univ. of Florida) Funding: SRC and MARCO/FCRP-MSD PhD Defense, Anisur Rahman

  3. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 PhD Defense, Anisur Rahman

  4. Background: CMOS Scaling • Historical Development: • Four decades of steady scaling • Present day MOSFETs are nanoscale (LG~30nm) • Scaling is motivated by performance and • integration density issues • Moore’s Law describes this steady growth • ITRS guides the future scaling trends Moore’s Law • Scaling Challenges: • Implementation of Moore’s law becoming challenging • Off-state leakage limits scaling of planar CMOS • Additional gates needed to curb SCE (dual/tri-gate) • Gate oxide scaling reached the limit (direct tunneling) • High-κ dielectric + metal gate in near future • Interface properties of Si-SiO2 becoming less critical 65nm Node Deviecs LG=35nm PhD Defense, Anisur Rahman

  5. Background: New Materials Electron Transport Property • Motivation for Novel Materirals: • Ge and III-V display very high mobility, • saturation velocity, and scattering • mean-free path • New process technology allows high-quality • channel-insulator interface • Ultra-high-speed,very-low-power logic • application • Experimental research is underway Ashley et al., ICSICT 2004 Current Calculation Qtop vinj • Key Device Physics Issues: • Treatment of quantum mechanical effects • Trade-off between high velocity and low DOS • Atomic level fluctuation become significant • Full-band treatment become necessary Source Drain Current=QtopX vinj Qtop=CG(VG-VT) where, CG<COX PhD Defense, Anisur Rahman

  6. Objective: • For New Channel Material Nanoscale CMOS Devices: • Develop simulation tools and theoretical approaches • Perform design studies, assess performance limits, and explore • scaling characteristics • Investigate relevance of carrier mobility • Identify key bandstructure related issues • Provide an improved understanding of their operation. • Theoretical Approaches: • Effective-mass-equation based quantum transport using NEGF formalism • for novel-channel material n-MOSFETs • Atomistic tight-binding approach for incorporating bandstructure effects in • deeply scaled ballistic n- and p-MOSFETs PhD Defense, Anisur Rahman

  7. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary and Conclusion • Future Work Chapter 2 Chapter 3 PhD Defense, Anisur Rahman

  8. Theory: Effective Mass Approach Ellipse Crystal in Effective Mass Equation (EME) Channel Bottom Gate Drain Top Gate Source • Key Features of EMA: • Most widely used approach for n-MOSFETs • Very successful to treat quantum transport • in (100) Si n-MOSFETs • NEGF and mode-space decomposition of the • transport problem is highly efficient Device Outstanding Issue: When DCS and ECS are not aligned, EMT is a full 3X3 matrix and solution of EME is very difficult PhD Defense, Anisur Rahman

  9. Theory: Generalized Effective Mass Unitary transformations conserves density of states and group velocity PhD Defense, Anisur Rahman

  10. Results: Ge n-MOSFETs LT=LG+2Lul Ch S D VD = 0.4 V Lul Lul LG NSD NSD IDS [μA/μm] Doping Density Profile Ballistic NEGF ID-VG Ballistic NEGF ID-VD VG = 0.4 V (4) (2) IDS [μA/μm] (1) Valley Deg. Si (001)/[100] Ge (111)/[110] Ge (001)/[100] • Simulation Setup: • End of ITRS 2001 UTB DG Ge n-MOSFETs • NEGF ballistic and scattering simulation • Design study performed • nanoMOS 2.5 modified to treat Ge n-MOSFETs • Ge devices on (100) and (111) wafers • Process variation and mobility effects examined EOT=0.6nm PhD Defense, Anisur Rahman Rahman et al., IEDM 2003

  11. Results: Ge n-MOSFETs Ch S D NSD NSD Energy μCh μSD μSD tbody Sensitivity to process variation NEGF Scattering Ballistic limit W/C device Y ch=1000cm2/V-sec Thick body ITRS 2016 IOFF [μA/μm] Nominal IDS [μA/μm] tbody = 2.5 nm ch=300cm2/V-sec Thin body VG = VD = 0.4V VD = 0.4 V; VG = 0 V Gate Length, LG [nm] S/D mobility, SD [cm2/V-sec] PhD Defense, Anisur Rahman

  12. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary and Conclusion • Future Work Chapter 4 PhD Defense, Anisur Rahman

  13. Top-of-the-Barrier Ballistic Model CQ Circuit Model for 2-D Electrostatics VG COX CS CD VS VD Gate Electrostatics • Key Features: • Semiclassical ballistic transport • Poisson equation solved only at top-of-barrier • Quantum capacitance, CQ, treated • Treats arbitrary bandstructure • Treats floating source potential • Two carrier fluxes, FS and FD present • Sum of carrier density in FS and FD is total charge • Difference of current from FS and FD is net current FS FD Source Drain UTB DG Model Device Top Gate Ch S D Rahman et al., TED, 2003 Bottom Gate PhD Defense, Anisur Rahman

  14. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 5 PhD Defense, Anisur Rahman

  15. Theory: Tight-binding (001) (111) (010) (110) a (100) Channel Drain Source Device — Ultra-thin Body Symmetric Dual-gate MOSFET Two FCC Lattices — Red andBlue • Challenges: • Appropriate TB model • Finite dimensional systems • Remove surface states • Treat electrostatics • Sparse matrix techniques • Computing I-V • Key Features: • Localized atomic orbital-like basis set • Suitable for modeling nanostructures • Correct full bandstructure • Strain, hetero-channel, novel materials PhD Defense, Anisur Rahman

  16. Approach: A Good TB Model Silicon CB ΔEc = +100meV • “Well-behaved” • TB Parameter Set • Manageable size — two • centerintegrals • Correctbandgap and • effective masses • Scalablefor strained • materials Vogl NEMO NN-sp3s* 5 orbitals NN-sp3d5s* 10 orbitals Boykin et al., PRB 69(11), 2004 PhD Defense, Anisur Rahman

  17. Approach: Band Bending TB Domain Calculate Bandstructure Transport Potential Charge EC VG Bulk/HOI MOSFETs EV nZ atoms Poisson Domain Poisson Charge Potential UTB MOSFETs TB Domain Self-consistence nZ atoms Band bending along thickness is less important in UTB. EC VG VG EV Band bending along thickness is important in bulk/HOI devices. PhD Defense, Anisur Rahman

  18. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Validity of single band parabolic E-k in Ge n-MOSFETs • Compare Si, Ge, GaAs, InAs as channel materials • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 6 PhD Defense, Anisur Rahman

  19. Study: Validity of Parabolic E-k in Ge • Simulation Setup: • Validity of parabolic E-k in UTB • unstrained Ge n-MOSFET examined • 20 band sp3d5s*-SO TB used • Three thicknesses: 16nm, 4nm, 2nm • Band bending not treated • I-V calculated from top-of-barrier model NN-sp3d5s*-SO ΔEc=300meV ΔEc=170meV • Bulk Ge Bandstructure: • The CB Δ, Λ,Г within 250meV. • Parabolic E-k valid at low energy • Quantum confinement can • alter the order of the bands. Red-Eff. mass Black-TB PhD Defense, Anisur Rahman Rahman et al., IEDM 2004

  20. Results: Thick Body Limit (16nm) L Г X 1 3 3 1 2D DOS 2 2 2D E-k • ~ 16nm (113 atomic layers) Ge body • Size of H ~ 2200 X 2200 • Electrostatic potential not treated • (100) wafer  quantization along [100] • Conduction band  subbands • Non-parabolicity important at high energy • I-V shown for (001)/[100] device [110] [100] Ballistic I-V VDS = 0.4V VT not adjusted PhD Defense, Anisur Rahman

  21. Results: Going Thinner (4nm) L Г X 1 3 1 3 2 2 2D E-k • ~ 4nm (30 atomic layers) Ge body • Size of H ~ 600 X 600 • Conduction band  Subbands • L and Г- Valleys came closer • Non-parabolicity affects ground state • Ge (001)/[100] device • VT shifted by 55 meV (not adjusted) [110] [100] Ballistic I-V 2D DOS 25% lower ION for eff. mass VDS = 0.4V VT not adjusted ΔVT=55 meV ΔVT PhD Defense, Anisur Rahman

  22. Results: Extreme Scaling (2nm) L Г X 1 3 1 3 2 2 2D E-k • ~ 2nm (12 atomic layers) Ge body • Size of H ~ 240 X 240 • Conduction band  Subbands • At Г, X Valleys form ground state • Strong non-parabolicity affects L-valleys • VT shifted by 570 meV (adjusted) [110] [100] Ballistic I-V 2D DOS 15% higher ION for eff. mass VT adjusted ΔVT=570 meV ΔVT PhD Defense, Anisur Rahman

  23. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Validity of single band parabolic E-k in Ge n-MOSFETs • Compare Si, Ge, GaAs, InAs as channel materials • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 6 PhD Defense, Anisur Rahman

  24. Study: New Channel Materials Group IV • Objective: • To explore and compare scaling characteristics of CMOS with Si, Ge, GaAs, InAs as channel materials. • Simulation Setup: • 2016 device specification from ITRS 2004 (22nm HP) • UTB DG with, body: 19 AL (~2.5nm), EOT=0.5nm • Unstrained material • Semiclassical, top-of-barrier, ballistic model Eeff = 1.3eV Si Energy, [eV] Eeff=1.04eV Ge Group III-V • Bandstructure: • CB and VB split into subbands • Quantum confinement increase effective band gap • Very high vinj expected for III-V • Lowest CB are X2(Si), L4(Ge), Г1(GaAs, InAs) • Si and Ge display higher CB DOS compared to III-V Eeff=1.72eV GaAs Energy, [eV] InAs Eeff=0.78eV Rahman et al., to appear in IEDM 2005. PhD Defense, Anisur Rahman

  25. Results: New Channel Materials ID-VD Ge Ge Si GaAs InAs vinj-VG • Injection velocity, vinj vs. VG: • Very high vinj for III-V materials • Electron vinj in InAs is highest, as expected • Beyond 0.4V, GaAs vinj drops (Г→L transfer) InAs GaAs Si Ge • Carrier density, Qtop vs. VG: • Ge: High CB DOS is key to its success • InAs: Electron Qtop and CG is strongly degraded • Beyond 0.4V, CG in GaAs improves (Г→L transfer) Qtop-VG Ge Si GaAs Low CB DOS strongly degrades deeply scaled III-V device performances InAs • Device: Deeply scaled body (19AL) • and oxide (0.5nm) • Ballistic ID-VD: • Ge performs best (n or p-FET) • GaAs or InAs cannot compete with Si or Ge • InAs n-FETs performs worst PhD Defense, Anisur Rahman

  26. Results: New Channel Materials Thin Oxide Ge tox=1.0nm InAs InAs & GaAs Ge Si GaAs Si tox=0.5nm InAs tox=0.5nm InAs GaAs GaAs Ge Ge Si Si Si tox=1.0nm • Thin EOT + Thick Body (Small CQ/COX): • Ge n-MOSFETs perform best • Qtops in III-V suffer, CG strongly degraded tox=0.5nm Ge Si Ge GaAs GaAs InAs InAs ID-VD Thick Oxide • Effects of ratioCQ/COXexplored: • 100 AL (~15nm) body thickness • Only n-MOSFETs treated • Ballistic transport using top-of-barrier model • Less QC effect, subbands closely separated • Thick body increase 2D DOS, consequently, CQ • EOT =1.0nm, and 0.5nm (thin) considered. vinj-VG • Thick EOT + Thick Body (Large CQ/COX): • III-V n-MOSFETs perform best • Silicon performs worst (slow X4 valleys) tox=1.0nm Qtop-VG PhD Defense, Anisur Rahman

  27. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Mobility behavior in strained bulk p- and n- MOSFETs • Hole density profile in HOI structure • Summary • Future Work Chapter 7 PhD Defense, Anisur Rahman

  28. Study: Strained Bulk p-MOSFETs Source Drain SiGe SiGe Si Substrate Induced Biaxial Tension Process Induced Uniaxial Compression Unstrained Intel 90 nm (Uniaxial) Rim et al. (Biaxial) 1995 Drain Source Source Drain Mobility [cm2/V-sec] Si Si Si Universal Hole Mobility (Unstrained) Si Si SiGe Si Rim et al. (Biaxial) 2002 Eeff [MV/cm] • Overview: • Devices on (100) wafers • Strain can be substrate-induced (biaxial) or • process-induced (uniaxial) • Strain deforms crystal by changing bond lengths • and bond orientations • Biaxial tensile strain: Hole mobility improves • at low gate bias but disappears at high bias • Uniaxial compressive strain: Hole mobility • improvement at low VG retained at high VG Experimental Hole Mobility Thompson et al., TED 04 PhD Defense, Anisur Rahman

  29. Consequence of Strain: sp3d5s*-SO Strained Bulk Si VB Strained Top VB HH Second VB LH Unstrained Bulk Si VB Unstrained PhD Defense, Anisur Rahman

  30. Results: Strained Bulk p-MOSFETs Intel 90 nm (Uniaxial) Rim et al. (Biaxial) 1995 2D E-k, Low VG and High VG Mobility [cm2/V-sec] Universal Hole Mobility (Unstrained) Rim et al. (Biaxial) 2002 Eeff [MV/cm] Experimental Hole Mobility Ballistic ION Ratio vs. Eeff Eeff=q(Ndep+p/3)/εSi • Bulk p-FETs: Self-consistent sp3d5s*-SO TB approach • Low VG: Top VB LH for both uniaxial and biaxial • High VG: Top VB HH for biaxial, LH for uniaxial • QC nullify strain splitting of LH,HH in biaxial case • Ballistic simulation explains mobility behavior in • strained p-MOSFETs PhD Defense, Anisur Rahman

  31. Study: Strained Bulk n-MOSFETs Process Induced Uniaxial Tension Source Drain SiGe SiGe Si • Mobility Behavior in n-MOSFETs: • Experimentally, both substrate-induced biaxial- • tension and process-induced uniaxial-tension • improves electron mobility • Electronic mobility enhancement is observed over the • entire range of gate bias • Such mobility enhancement is often explained in terms • of degeneracy removal of X2 and X4 valleys, which is • recently questioned (Fischetti et al., JAP, 2002.) • Simulation Setup and Observation: • Self-consistent sp3d5s*-SO TB model and the top-of- • barrier ballistic model does not show any enhancement • of strained device performance • Bandstructure alone cannot explain the electronic • mobility enhancement in strained planar n-MOSFETs Ballistic n-MOSFETs Eeff=q(Ndep+p/2)/εSi PhD Defense, Anisur Rahman

  32. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Mobility behavior in strained bulk p- and n- MOSFETs • Hole density profile in HOI structure • Summary • Future Work Chapter 7 PhD Defense, Anisur Rahman

  33. Study: Hole Profile in HOI Poly-Si ε-Si ε-SiGe Buried Oxide Schematic Representation • Heterostructure on Insulator (HOI) • Utilizes the high mobility central SiGe • channel to improve hole mobility • Band discontinuity moves holes to center • Band bending due to VG rearranges hole • profile TOX E 30 AL ~ 4 nm Biaxial tensile ε-Si (y=0.24) Biaxial compressive 21 AL ~ 3 nm ε-Si0.5Ge0.5 30 AL ~ 4 nm ε-Si (y=0.24) Biaxial tensile EV EC BOX • Simulation Setup: • The 20 band sp3d5s*-SO TB model with self- • consistent electrostatics. • Compressively strained Si0.5Ge0.5 sandwiched • between two Si layers under tensile strain. • Top and bottom oxides are 2 nm and 10 nm • thick, respectively. PhD Defense, Anisur Rahman Hoyt Group (MIT)

  34. Results: HOI Q-VG VB Profile Hole Profile EF = 0.45 eV Si/SiGe/Si BOX EF = 0.45 eV • Observations: • Tri-layer (Si-SiGe-Si) structure • tOX= 2 nm(top), tOX=10 nm(bot) • Low VG: band discontinuity • moves holes to central channel. • High VG: hole profile moves • near the surface. BOX Si/SiGe/Si Si/SiGe/Si EF = 0.45 eV BOX PhD Defense, Anisur Rahman

  35. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 8 PhD Defense, Anisur Rahman

  36. Summary: • Effective mass approach extended to treat n-MOSFETs on arbitrary wafer • orientations. • An NEGF study of LG=10nm end of ITRS Ge n-MOSFET reveals that: • Ge (001)/[100] device performs best and can meet target ION • Gate under-lap improves short-channel-effects • High mobility in the S/D region is crucial to limit RS degradation • A strict process tolerance in body thickness necessary to limit VT fluctuation across the chip. • A physics based top-of-the-barrier semiclassical ballistic transport model • developed and its application demonstrated. Continued to next slide PhD Defense, Anisur Rahman

  37. Summary (cont.): • A semi-empirical 20 band sp3d5s*-SO TB model used to assess strained and • unstrained UTB novel channel material CMOS devices, and was reveled that: • Ge n-MOSFETs: Below 4nm thickness, use of single band parabolic E-k is limited by non-parabolicity. Below 2nm thickness, its use is further limited by band reordering and multi-valley conduction. • A trade-off exists between high vinj and low DOS. III-V devices outperform Si or Ge n-MOSFETs only for thick body and thick EOT. For deeply scaled devices Ge displays the best ballistic performances. • Self-consistent gate electrostatics was treated in sp3d5s*-SO TB model • and was revealed that: • Experimental hole mobility behavior in strained planar p-MOSFETs can beexplained by bandstructure modulation. Similar behavior for electron mobility in n-MOSFETs cannot be explained by bandstructure alone. • HOI simulation shows that at high gate field, the hole-profile moves near the surface and not at the central high-mobility SiGe layer. PhD Defense, Anisur Rahman

  38. Future Work • Separate the effects of bandstructure and scattering in published experimental mobility data • Discretize the TB Hamiltonian for arbitrarily oriented wafers—Only (100) wafers treated here • Mode-space representation of TB Hamiltonian for UTB MOSFETs—A full 2D representation is not feasible, computationally • Employ zone-unfolding technique to treat SRS and random alloy effects • Employ self-consistent TB approaches for III-V HEMT and QWFET devices— Ballistic simulation is more relevant here due to their very high mobility PhD Defense, Anisur Rahman

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