Hard X-ray Photoelectron Spectroscopy Research at NIST beamline X24A Joseph C. Woicik NIST
Outline Introduction to hard x-ray photoelectron spectroscopy (HAXPES) Semiconductor gate stacks on silicon Epitaxial, room temperature feroelectric SrTiO 3 on silicon Site Specific XPS and the nature of the solid state chemical bond: Cu, GaAs, TiO 2 N doping of TiO 2
Hard X-ray Photoelectron Spectroscopy (HAXPES) Variable Kinetic Energy HAXPES Tune the HAXPES depth sensitivity (photoelectron kinetic energy) by tuning the X- ray excitation energy using a synchrotron X-ray beamline The U7A and X24A endstations form a unique measurement suite for surface to near bulk HAXPES by spanning X-ray excitation energy from 0.2 to 5 kev Example: Si electron binding energy 2p ~99 ev and 1s ~1840 ev, surface to bulk sensitivity (Lab source has fixed energy Al K Synchrotron Tool Photoelectron Kinetic Energy = h Binding Energy
Advantages of HAXPES
Advantages of HAXPES Study samples from air
Advantages of HAXPES Study samples from air; i.e., real samples!
Advantages of HAXPES Study samples from air; i.e., real samples! Tune λ for experimental system
Advantages of HAXPES Study samples from air; i.e., real samples! Tune λ for experimental system Variable kinetic energy XPS (VKE-XPS) for depth profiling
N o rm a liz e d In te n s ity (a rb. u n its ) P.S. Lysaght, J. Barnett, G.I. Bersuker, J.C. Woicik, D.A. Fischer, B. Foran, H.-H. Tseng, and R. Jammy Depth Sensitive VKE-XPS Si 1s h = 2100 ev h = 2500 ev h = 2600 ev HfO 2 SiO 2 h = 3000 ev 5 n m Si more oxidized less oxidized h = 3500 ev Si 2p "lab-like" spectrum h = 2100 ev -8-6 -4-2 0 2 4 Kinetic Energy (E-E c )
Advantages of HAXPES Study samples from air; i.e., real samples! Tune λ for experimental system Variable kinetic energy XPS (VKE-XPS) for depth profiling Bulk and surface sensitive core lines accessible for same element
Surface and Bulk Sensitive Core Lines Si 2p B.E. = 99 ev h = 2180 ev Si 1s B.E. = 1840 ev h = 2180 ev 2068 2072 2076 2080 Kinetic Energy (ev) 324 328 332 336 338 Kinetic Energy (ev)
Advantages of HAXPES Study samples from air; i.e., real samples! Tune λ for experimental system Variable kinetic energy XPS (VKE-XPS) for depth profiling Bulk and surface sensitive core lines accessible for same element Eliminate Auger interference
In te n s ity ( c o u n ts /s e c o n d ) Auger Interference 10000 Si KLL Auger 8000 6000 4000 O 1s 2000 0 1500 1550 1600 1650 1700 Kinetic Energy (ev)
Advantages of HAXPES Study samples from air; i.e., real samples! Tune λ for experimental system Variable kinetic energy XPS (VKE-XPS) for depth profiling Bulk and surface sensitive core lines accessible for same element Eliminate Auger interference Photoemission and other techniques (SSXPS)
What is a gate stack? Metal SiO 2 Si
What is a gate stack? Metal SiO 2 Metal SiO 2 ~ 2 nm Si Si
What is a gate stack? Metal C = ε r ε o A/d ~ 2 nm HfO 2 K ~ 20 Metal ~ 1 nm SiO 2 K ~ 4 SiO 2 ~ 2 nm Si Si
Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) SEMATECH: HfO 2 /SiO 2 /Si Gate Stacks hv = 2200 ev NH 3 postdeposition anneal Hf 4f HfO 2 Hf-N HfO 2 SiO 2 A 5 n m Si NH 3 predeposition anneal HfO 2 N 1s hv = 2200 ev Hf-N B NH 3 postdeposition anneal A NH 3 pre- & postdeposition anneal HfO 2 Hf-N NH 3 predeposition anneal Si-N B 2181 2183 2185 Kinetic Energy (ev) C 2187 NH 3 pre- & post-deposition anneal 1799 1781 1783 1805 1807 1809 Kinetic Energy (ev) P.S. Lysaght, J. Barnett, G.I. Bersuker, J.C. Woicik, D.A. Fischer, B. Foran, H.-H. Tseng, and R. Jammy C
In te n s ity (a rb. u n its ) Motorola: 5 ML (20 Å) SrTiO 3 films on Si(001) Si 2p h = 2210 ev SiO 2 /Si(001) SrTiO 3 /Si(001) 2104 2106 2108 2110 2112 Kinetic Energy (ev)
HfO 2 /SiGe/Si 2 nm HfO 2 950 C anneal as deposited (300 C) 30 nm SiGe Si
In te n s ity (a rb. u n its ) In te n s ity (a rb. u n its ) SiGe h = 2140 ev Si 2p SiGe h = 2140 ev Si 1s Ge 3d Ge 2p -8-6 -4-2 0 2 4 Kinetic Energy (E - E c ) -12-8 -4 0 4 Kinetic Energy (E - E c )
In te n s ity (a rb. u n its ) Hf 4f as deposited h = 2140 ev 950 C anneal 2116 2200 2204 Kinetic Energy (ev)
In te n s ity (a rb. u n its ) Si 2p h = 2140 ev 950 C anneal as deposited SiGe 2030 2034 2038 2042 Kinetic Energy (ev)
In te n s ity (a rb. u n its ) Si 1s h = 2140 ev 950 C anneal as deposited SiGe 288 292 296 300 304 Kinetic Energy (ev)
In te n s ity ( a rb. u n its ) Ge 3d h = 2140 ev 950 C anneal as deposited SiGe 2100 2104 2108 2112 Kinetic Energy (ev)
In te n s ity (a rb. u n its ) Ge 2p h = 2140 ev 950 C anneal as deposited SiGe 914 918 922 926 Kinetic Energy (ev)
Advantages of HAXPES Study samples from air; i.e., real systems! Tune λ for experimental system Variable kinetic energy XPS (VKE-XPS) for depth profiling Bulk and surface sensitive core lines accessible for same element Eliminate Auger interference Photoemission and other techniques (SSXPS)
Electronic Structure of GaAs J. Chelikowsky, D.J. Chadi, and M.L. Cohen
X-ray Standing Waves Bragg's Law: H H = k h - k o k o k h Electric Field: ^. ^. E(r,t) = [ E o e ik o r + E h e ik h r ] e -i t Electric Field Intensity:. I(r) = E E* = E o 2 [ 1 + R + 2 R cos( + H. r ) ] where E h / E o = R e i and 0 < <
Bragg Diffraction: = 2d sin( ) d R Reflectivity E B XSW Yield Y E B
In te n s i ty (a rb. u n i ts ) 2 1 0 Ga As d 111
In te n s ity (c n ts./s e c o n d ) 200 As 3d GaAs 160 120 Ga 3d 80 40 Valence Band 0-40 -30-20 -10 0 Kinetic Energy (ev) (E - E ) VBM
In te n s ity (a rb itra ry u n its ) V a le n c e B a n d T h e o ry E x p e rim e n t G a A s -1 6-1 2-8 -4 0 K in e tic E n e rg y (E - E ) V B M
Electronic Structure of Cu J. Friis, B. Jiang, K. Marthinsen, and R. Holmestad
In te n s ity (a rb. u n its ) Cu(11-1) Cu (valence) Cu 3p (core) 2970 2972 2974 2976 2978 Photon Energy (ev)
In te n s ity (a rb. u n its ) Cu(11-1) valence spectra 2954 2958 2962 2966 2970 Kinetic Energy (ev)
N o rm a liz e d In te n s ity (a rb. u n its ) Cu(11-1) valence spectra What happened to the free-electron theory of metals? -8-6 -4-2 0 2 Kinetic Energy (ev) (E - E f )
E.L. Shirley Photoemission process Feynman diagram: hole What is left behind. photoelectron What comes out of the crystal. photon What goes in the crystal. Electron/hole propagators are shown by solid lines. This process is forbidden for free electrons because of energy/momentum conservation.
E.L. Shirley Photoemission process Feynman diagram: hole What is left behind. photon What goes in the crystal. photoelectron What comes out of the crystal. Phonon lattice recoil Electron/hole propagators are shown by solid lines. This process is forbidden for free electrons because of energy/momentum conservation.
Recoil Effects of Photoelectrons in a Solid Y. Takata, Y. Kayanuma, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, Y. Harada, K. Horiba, S. Shin, S. Tanaka, E. Ikenaga, K. Kobayashi, Y. Senba, H. Ohashi, and T. Ishikawa
In te n s ity (a rb. u n its ) Electronic Structure of Rutile TiO 2 DOS theory expt. c-theory -8-6 -4-2 0 2 Energy (ev) (E - E ) VBM
O T i b c a
In te n s ity (a rb. u n its ) Ti 3p TiO (110) 2 O 2s VB -50-40 -30-20 -10 0 Energy (ev) (E - E ) VBM
In te n s ity (a rb. u n its ) (a) DOS theory expt. (b) pdos Ti expt. O x 4 expt. -8-6 -4-2 0 2 Energy (ev) (E - E ) VBM
Chemical Hybridization
In te n s ity (a rb. u n its ) (a) DOS theory expt. (b) pdos Ti σ π expt. O O-π x 4 expt. -8-6 -4-2 0 2 Energy (ev) (E - E ) VBM
In te n s ity (a rb. u n its ) (a) pdos Ti theory expt. (b) pdos O theory expt. -8-6 -4-2 0 2 Energy (ev) (E - E ) VBM
In te n s ity (a rb. u n its ) lpdos O 2p x 1.0 O 2s x 110 Ti 3d x 1.8 Ti 4p x 11 Ti 4s x 15-8 -6-4 -2 0 2 Energy (ev) (E - E ) VBM
Theoretical Atomic Cross Sections σti 4s 30! σ Ti 3d σo 2s 10! σ O 2p XPS favors states with smaller values of angular momentum! I(E,ħω) α i,l ρ i,l (E) σ i,l (E,ħω)
In te n s ity (a rb. u n its ) (a) pdos Ti c-theory expt. (b) pdos O c-theory expt. -8-6 -4-2 0 2 Energy (ev) (E - E ) VBM
In te n s ity (a rb. u n its ) Electronic Structure of Rutile TiO 2 DOS theory expt. c-theory -8-6 -4-2 0 2 Energy (ev) (E - E ) VBM
In te n s ity (a rb. u n its ) Ti 3p TiO (110) 2 O 2s VB -50-40 -30-20 -10 0 Energy (ev) (E - E ) VBM
N doped TiO 2 (anatase) Ti 2p
lpdos
Electronic Structure of TiO 2 (anatase)
lpdos
Electronic Structure of N doped TiO 2 (anatase)
Many Thanks Daniel Fischer (NIST) and Barry Karlin (NIST) Pat Lysaght (SEMATECH) Hao Li (Motorola Labs) Erik Nelson (SSRL), David Heskett (URI), and Lonny Berman (NSLS) Leeor Kronik (Weizmann) and Jim Chelikowsky (Austin) Abdul Rumaiz (NSLS) and Eric Cockayne (NIST)
In te n s ity (c n ts./s e c ) 3 0 G a 2 0 A s 1 0 to ta l 0-1 6-1 2-8 -4 0 K in e tic E n e rg y (E - E V B M ) e V
d d ik. < f e ( o r. p ) i > 2 H o = i p i 2 /2m + V( r ) H ' = A o. p E kin = h - E b (h >> E b ) f > = e ik f. r (k f >> k o ) [p,h] = -i h V( r ) d d < f. V( r ) i > 2
V a le n c e P h o to e m is s io n f > i > V ( r ) d d ik. o r ^. < f e ( p ) i > 2
Chemical Hybridization - + + + = - + 2p z λ2s λ2s + 2p z 2p λ2s - 2p z 2s λ2s + 2p z - + - + s-p hybrid H i,j = ψ i H ψ j dv Stronger Chemical Bond!