Effect of the High-k Dielectric/Semiconductor Interface on Electronic Properties in Ultra-thin Channels

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1 Effect of the High-k Dielectric/Semiconductor Interface on Electronic Properties in Ultra-thin Channels Evan Wilson, Daniel Valencia, Mark J. W. Rodwell, Gerhard Klimeck and Michael Povolotskyi Electrical and Computer Engineering

2 Motivation As the size of devices decreases, the fraction of channel atoms in contact with gate oxide atoms increases Significant fraction of device atoms see oxide even for single gate, trend will be steeper for double gate, gate-all-around Evan Wilson ITRS projected gate length results for single-gate SOI

3 Motivation Traditional Network for Gate Computational Oxide Nanotechnology Gate (NCN) Dimensions, k, H-pass Model: Hard wall boundaries/wkb approximation in thin oxides +z Source Channel Drain E Semi. Channel Surface passivation with implicit oxide, dielectric constant, dimensions ψ Confinement Direction +z

4 Motivation Explicit Oxide Modeling More than just electrostatics Information about changes in the electronic structure m eff, DOS, band gap (confinement effects?) +z E In the future: traps, vacancies, impurities Confinement Direction +z

5 Can we gain something by doing explicit oxide modeling?

6 Semiconductor-oxide Interfaces L. Ye. et al Appl:interfaces 2013,5,8081 HfO 2 - InAs interface Oxides are often amorphous, multiple crystal phases with large Extended unit cells Huckel Theory tight-binding model selected as Interfaces compromise may between complex, computation with time strain and and generality large implemented in existing NEMO5 simulation tool suite rearrangements of atoms Need a model with flexible coupling calculation

7 Extended Hückel Hamiltonian Onsite elements Off diagonal elements Overlap matrix Diagonal elements are parameters; represent difference between ionization potential and electron affinity Coupling proportional to average of onsites and overlap of wavefunctions Overlaps computed with parameterized Slater-type orbitals for neighbors within specified interaction range Evan Wilson

8 EHT Parameterization a. b. TiO2 a.) and HfO2 b.) bulk band structures calculated with Extended Huckel Theory Oxide EHT parameters were fit to ab-initio density functional theory (DFT) band structures

9 Explicit oxide UTB channel simulation

10 TiO2 Heterostructure Bands Top: InAs/TiO2 super cell relaxed with DFT Bottom: Unrelaxed InAs semiconductor UTB with same crystal orientation TiO2-InAs interface UTB compared same UTB with Hydrogen passivation Heterostructure initial geometry produced by selecting low-strain commensurate lattice super cell Relaxed interface compared with H-passivated semiconductor UTB Slight reduction in band bap, large change in band curvature/dos in conduction band

11 HfO2 Heterostructure Bands InAs UTB E g =0.58 ev HfO2 Hetero. UTB E g =0.21 ev HfO2-InAs interface UTB compared same UTB with Hydrogen passivation Same procedure as in previous slide to produce HfO2 heterostructure Importance of explicit oxide can be seen in confinement effects in UTB s

12 Some visible effects like decreased confinement from explicit band structure, can we estimate effects on current?

13 TOB I-V Curves: TiO2 ~10% difference in 0.1V InAs UTB Channel TiO2/InAs Hetero UTB Limited/negligible effect of explicit oxide on current with this model

14 Conclusions Explicit Oxide Modeling: Limited effect observed for MOSFET current in this model Decreased confinement observed to reduce the band gap Effect on Tunnel FET devices? Sensitive to band gap Increased DOS in some cases Scattering rates may differ in heterostructure, bare UTB cases

15 Future Work EHT and Oxide Modeling Development Full self-consistent Schroedinger-Poisson device simulation with Recursive Green s Functions Scattering Parameterization New materials (oxides, II-VI s) Refine existing parameters

16 Questions?