Hard X-ray Photoelectron Spectroscopy Research at NIST beamline X24A. Joseph C. Woicik NIST

Transcription

1 Hard X-ray Photoelectron Spectroscopy Research at NIST beamline X24A Joseph C. Woicik NIST

2 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

3 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

4

5 Advantages of HAXPES

6 Advantages of HAXPES Study samples from air

7 Advantages of HAXPES Study samples from air; i.e., real samples!

8 Advantages of HAXPES Study samples from air; i.e., real samples! Tune λ for experimental system

9 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

10 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 Kinetic Energy (E-E c )

11 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

12 Surface and Bulk Sensitive Core Lines Si 2p B.E. = 99 ev h = 2180 ev Si 1s B.E. = 1840 ev h = 2180 ev Kinetic Energy (ev) Kinetic Energy (ev)

13 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

14 In te n s ity ( c o u n ts /s e c o n d ) Auger Interference Si KLL Auger O 1s Kinetic Energy (ev)

15 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)

16 What is a gate stack? Metal SiO 2 Si

17 What is a gate stack? Metal SiO 2 Metal SiO 2 ~ 2 nm Si Si

18 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

19 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 Kinetic Energy (ev) C 2187 NH 3 pre- & post-deposition anneal 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

20 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) Kinetic Energy (ev)

21 HfO 2 /SiGe/Si 2 nm HfO C anneal as deposited (300 C) 30 nm SiGe Si

22 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 Kinetic Energy (E - E c ) Kinetic Energy (E - E c )

23 In te n s ity (a rb. u n its ) Hf 4f as deposited h = 2140 ev 950 C anneal Kinetic Energy (ev)

24 In te n s ity (a rb. u n its ) Si 2p h = 2140 ev 950 C anneal as deposited SiGe Kinetic Energy (ev)

25 In te n s ity (a rb. u n its ) Si 1s h = 2140 ev 950 C anneal as deposited SiGe Kinetic Energy (ev)

26 In te n s ity ( a rb. u n its ) Ge 3d h = 2140 ev 950 C anneal as deposited SiGe Kinetic Energy (ev)

27 In te n s ity (a rb. u n its ) Ge 2p h = 2140 ev 950 C anneal as deposited SiGe Kinetic Energy (ev)

28 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)

29 Electronic Structure of GaAs J. Chelikowsky, D.J. Chadi, and M.L. Cohen

30 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 < <

31 Bragg Diffraction: = 2d sin( ) d R Reflectivity E B XSW Yield Y E B

32 In te n s i ty (a rb. u n i ts ) Ga As d 111

33 In te n s ity (c n ts./s e c o n d ) 200 As 3d GaAs Ga 3d Valence Band Kinetic Energy (ev) (E - E ) VBM

34 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 K in e tic E n e rg y (E - E ) V B M

35 Electronic Structure of Cu J. Friis, B. Jiang, K. Marthinsen, and R. Holmestad

36 In te n s ity (a rb. u n its ) Cu(11-1) Cu (valence) Cu 3p (core) Photon Energy (ev)

37 In te n s ity (a rb. u n its ) Cu(11-1) valence spectra Kinetic Energy (ev)

38 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? Kinetic Energy (ev) (E - E f )

39 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.

40 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.

41 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

42 In te n s ity (a rb. u n its ) Electronic Structure of Rutile TiO 2 DOS theory expt. c-theory Energy (ev) (E - E ) VBM

43 O T i b c a

44 In te n s ity (a rb. u n its ) Ti 3p TiO (110) 2 O 2s VB Energy (ev) (E - E ) VBM

45 In te n s ity (a rb. u n its ) (a) DOS theory expt. (b) pdos Ti expt. O x 4 expt Energy (ev) (E - E ) VBM

46 Chemical Hybridization

47 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 Energy (ev) (E - E ) VBM

48 In te n s ity (a rb. u n its ) (a) pdos Ti theory expt. (b) pdos O theory expt Energy (ev) (E - E ) VBM

49 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 Energy (ev) (E - E ) VBM

50 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)

51 In te n s ity (a rb. u n its ) (a) pdos Ti c-theory expt. (b) pdos O c-theory expt Energy (ev) (E - E ) VBM

52 In te n s ity (a rb. u n its ) Electronic Structure of Rutile TiO 2 DOS theory expt. c-theory Energy (ev) (E - E ) VBM

53 In te n s ity (a rb. u n its ) Ti 3p TiO (110) 2 O 2s VB Energy (ev) (E - E ) VBM

54 N doped TiO 2 (anatase) Ti 2p

55

56 lpdos

57 Electronic Structure of TiO 2 (anatase)

58 lpdos

59 Electronic Structure of N doped TiO 2 (anatase)

60 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)

61

62 In te n s ity (c n ts./s e c ) 3 0 G a 2 0 A s 1 0 to ta l K in e tic E n e rg y (E - E V B M ) e V

63 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

64 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

65 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!