The relation between crystalline phase, electronic structure and dielectric properties in high-K gate stacks Safak Sayan*, R. Bartynski, E. Garfunkel, T. Emge, M. Croft, X. Zhao, D. Vanderbilt, N. Nguyen, J. Suehle, and J. Ehrstein Rutgers University, Depts. of Chemistry and Physics, EEEL, National Institute of Standards and Technology (NIST) Scaling the MOSFET Gate Stack Structure Evac Si3N4 Spacers Compound High-κ Dielectric Or Metal Barrier? Electrode φ φ e1 e2 SiO2 Dielectric? Ef Source Drain Silicon φ h2 φ h1 Future Gate Structure Metal Si-compatible high-k dielectric High-k Barrier layers and stacked structures SiO2 New electrode: Ge doped Si, Si metal, metallic oxide or nitride EOT 10⇒5Å? Benefit of integrating high-k material in CMOS devices ⎡ ⎛ qE ⎞ ⎤ ⎢ q⎜ − φ + ⎟ ⎥ ⎜ B 4πε ⎟ JαT 2 exp ⎢ ⎝ i ⎠ ⎥ ⎢ kT ⎥ ⎢ ⎥ ⎣⎢ ⎦⎥ ⎡ ⎛ qE ⎞ ⎤ ⎢ q⎜ − φ + ⎟ ⎥ ⎜ B πε ⎟ JαE exp ⎢ ⎝ i ⎠ ⎥ ⎢ kT ⎥ ⎢ ⎥ ⎣⎢ ⎦⎥ 3 3 2 ⎛ ⎞ q E − c()qφ 2 Jα exp⎜ B ⎟ φ ⎜ qE ⎟ B ⎝ ⎠ φ reduction in tunneling current B High-k SiO Less power dissipation 2 Trade-off: φB thickness G. D. Wilk, et.al, JAP 89, 5243 (2001) Benefits of using a high-k material G.D. Wilk How to locate the valence and conduction band positions? Si High-k Metal Band edges EF DOS •Theoretical density of states (probably the best method) ∞ VBM expt. N (E) = n (E ' )g(E − E ' )dE ' VBM theory-monoclinic v ∫ v 0 S VBM Response function Counts/DO Theoretical DOS -12 -10 -8 -6 -4 -2 0 Binding Energy (eV) Phase Diagram for HfO2 •5 identified phases of HfO2 Cubic Tetragonal Meta-stable Orthorhombic I Orthorhombic II May be stabilized by •film stress •grain-size effects •impurities Monoclinic - stable phase at ambient temperature and pressure Density of States for all phases of HfO2 VBM CBM Crystal Phase Band gap (eV) 20 Tetragonal E Orthorhombic II 2.94 10 g 0 Cubic 3.15 20 Orthorhombic I Monoclinic 3.45 10 0 Orthorhombic I 3.75 es 20 at Monoclinic Tetragonal 3.84 St 10 of 0 20 Cubic •Band gap values are Density 10 different for phases! 0 20 Orthorhombic II 10 0 -8 -6 -4 -2 0 2 4 6 8 10 12 •Effects on Energy (eV) band alignment? •Crystal structure can have considerable effect on measured band gap and hence barrier heights Approach: XRD, XAS, HRTEM, FTIR – for phase identification DFT calculations – to determine DOS for the identified phase Photoemission (PES) using soft x-rays – for mapping occupied densities of states Inverse Photoemission (IPES)– for unoccupied densities of states Sample preparation: CVD HfO2 using Hf-tetra-tert.-butoxide 400C in the thickness regime of 10 Å - 1µ ALD ZrO2 using ZrCl4/H2O chemistry at 300C in the thickness regime of 30-100 Å Photoemission Spectroscopy (PES): Brookhaven National Laboratories – beamline U8b (100-350 eV) Inverse Photoemission Spectroscopy IPES: Rutgers University (15-40 eV) Inverse Photoemission Spectroscopy (IPES) • IPES provides information on states above Fermi level • Mapping of unoccupied states is possible • Complementary technique to photoemission (PES) XRD Studies on HfO2/SiOxNy/Si gate stack Crystal structure – film thickness 1150 Å 450 Å 250 Å • Thicknesses are determined by Rutherford 125 Å Backscattering Spectroscopy (RBS) •The diffraction peak positions are consistent with monoclinic structure • As the films get thinner, average crystal size decreases, indicating more disorder Partial DOS for monoclinic HfO2 3-fold coordinated O (1) 4-fold coordinated O (2) •Valence band is predominantly composed of O-like bands •Valence band edge is determined by three-fold coordinated O •Conduction band is predominantly – Hf d-like bands Results of combined photoemission and inverse photoemission studies Photoemission Inverse Photoemission 1.0 Hf -d O -p 0.8 y t nsi 0.6 Inte ized 0.4 l a rm o N 0.2 0.0 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 VB CB Energy (eV) •Valence band is predominantly composed of O-like bands •Conduction band is predominantly – Hf d-like bands Locating Valence Band Maximum (VBM) and Conduction Band Minimum (CBM) 1.0 ∞ 0.8 N (E) = n (E' )g(E − E' )dE' v ∫ v 0.6 0 y 0.4 t 0.2 Response function 0.0 Theoretical DOS ed Intensi for monoclinic HfO iz 1.0 2 al 0.8 Norm 0.6 0.4 • Extract VBM and CBM from 0.2 band structure calculations 0.0 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 Energy (eV) • Agreement between theory and experiment is remarkable (width of bands, and main features) • Even though the DOS is not corrected for PES and IPES cross-sections • However note the disagreement between theory and experiment close the band edges Results from combined theory and experiment EV • Theoretical DOS is convoluted with the appropriate 2.18 response function 4.15 • VBM and CBM energy positions are extracted from band 1.97 structure calculations 0.26 • Electron affinity for HfO2 is ~2.2 eV 3.61 • Using the position of Fermi level within silicon gap 4.4 • Φn~2.0 eV and Φh ~3.6 eV – sufficiently large from leakage point of view HfO2 SiO2 Si Band-tail-States – “Effective” band gap VBM CBM Band-tail-states E =6.70 eV Order to disorder transition g ) .u Can result from: (a y t -10-8-6-4-2 0 2 4 6 810 •Crystal imperfections •Defects Intensi •Impurities •Stoichiometry E =5.84 eV g •Theory and experiment -10-8-6-4-2 0 2 4 6 810 comparison yielded a Energy (eV) gaussian distribution of these states centered close to band edges Band-tail-states Defect – edge transitions - the effective gap and barriers e- e- • XRD reveals that the films are predominantly monoclinic • Therefore finite crystallinity • On the other hand, theoretical calculations considers perfect crystal • Disorder in the film will result in appearance of band-tail states • Therefore intrinsically the perfect crystals have larger band gap • However due to the presence of band-tail-states, gap is reduced •Concept of “effective” gap as well as barrier heights can be more meaningful Energy band diagrams Intrinsic Including band-tail states 1.97 1.47 6.70 5.84 3.61 3.25 Si HfO2 • A perfect HfO2 crystal intrinsically has a larger band gap (than reported to date) • The presence of defect states close to band edges results in a smaller “effective” band gap and barrier heights Summary of band alignment for HfO2/SiO2/p-Si gate stack • Occupied and unoccupied densities of states of HfO2 is studied by PES and IPES • VBM and CBM are located by combining theoretical calculations with expt data • Comparison of theory and experiments suggested presence of band tail states in proximity of band edges • Due to the presence of band tail states, concept of “effective” band gap may be more meaningful • The extracted “effective” band gap, electron affinity and barrier heights are 5.84, 2.68, 1.47 and 3.25 eV Develop understanding of the electronic structure of different crystal phases of HfO2 and ZrO2 and the effect of annealing on physical and electronic properties Background: Compare HfO2 and ZrO2, and show results of relevance to phase and electronic structure issues in gate stack engineering One Problem: A broad range of dielectric constants, band gaps and barrier heights are reported in the literature. Question: Can the phase of ZrO2 (present in different films prepared by different deposition techniques and anneals) be responsible for the difference in measured values? Solution: Use array of experimental methods and DFT calculations to determine phase and electronic properties. DFT calculations: excellent for structure and energetics but underestimates the true band gaps however, consistent and comparable. Dielectric Constant for Different Phases of ZrO2 εo ~ 37 ~ 47 ~ 20 Static dielectric tensor = purely electronic screening (10-25%) + lattice contribution (IR-active phonons, 75-90%) Dielectric constant and Band Gap of all phases of HfO2 and ZrO2 ) HfO ε 2 ( 60 ZrO 2 ant t 40 Cons c i r t 20 ec el i D 0 4 ) 3 0.6 eV 2 and Gap (eV 1 B 0 Tetragonal Orthorhombic I Monoclinic Cubic Orthorhombic II • Tetragonal phase of both oxides have high dielectric constant and band gap • If tetragonal thin ZrO2 can be prepared, full advantage can be taken • HfO2 has a higher band gap than ZrO2 (~0.6 eV) for the same phase Summary of theoretical calculations •Crystal structure can have considerable effect on measured permittivity, band gap and barrier heights •Crystal structure of thin films needs to be determined and stated in studies •Tetragonal phase of both oxides have high dielectric constant and band gap •Permittivity •Band gap Thermal anneal effects •Barrier heights Stabilization of a certain phase due to: •film stress •grain-size effects •impurities • Can tetragonal ZrO2 be prepared (in gate stack)? ) HfO ε 2 ( t 60 ZrO 2 nstan 40 tric Co c 20 Diele 0 4 ) 3 V e 0.6 eV ( p a 2 G d n 1 Ba 0 Tetragonal Orthorhombic I Monoclinic Cubic Orthorhombic II • Tetragonal phase of both oxides have high dielectric constant and band gap Density of States for all phases of ZrO2 VBM CBM Crystal Phase Band gap (eV) 20 O -p Zr -d E 10 g Tetragonal Orthorhombic II 2.29 0 Cubic 2.63 20 10 Orthorhombic I Monoclinic 2.98 0 Orthorhombic I 3.06 ates 20 t 10 Monoclinic Tetragonal 3.31 of S y 0 Amorphous 2.71 20 Densit 10 Cubic 0 20 10 Orthorhombic II •Band gap values are 0 -8 -6 -4 -2 0 2 4 6 8 10 12 different for phases! Energy (eV) •Crystal structure can have considerable effect on measured band gap and hence barrier heights •Effects on band alignment ??? If tetragonal phase can be prepared, a high dielectric constant and band gap should be expected.