Visualization of single subsurface impurity atoms and hidden nanoclusters in metals - by Dr. O. Kurnosikov o.kurnosikov@tue.nl Can STM see below a surface?! Yes!!! Is it really possible? Sure, down to 100 nm ! 1/31/2020 1 Introduction Absolutely pure materials ? Pure materials do not exist !!! It is bad! 1/31/2020 Impurity atoms It is good! Embedded (nano)clusters 2 Introduction How the impurities can be introduced in our structure? During the fabrication - Intended doping or - Undesirable contamination During the use - Usually undesirable - Diffusion from environment Formation, spatial distribution, and evolution of impurities atoms and clusters is of the highest interest Evolution of the impurities due to diffusion: - Can be redistributed within the bulk (solving, leaving the material through the surface) - Can form clusters We are interested in characterization of impurities and nanoclusters in metals 1/31/2020 3 Introduction More specifically - Gas impurities coming from a plasm or ion bombardment - Metal impurities formed at vacuum deposition The near-surface impurities Examples 1/31/2020 4 Introduction For ITER Degradation of W or Mo walls by implantation and growth of H2 and He-filled nanocavities: the growth of nanicavities can be visualizeed For micro- nanolithography Ar, Ne, He impurities in conducting layers (Al, Cu, Au, Ag, …) during plasma processing or magnetron sputtering deposition and other impurities during ion implantation or FIB operation For microelectronics Characterization of electromigration and sub-surface nanovoids formation in contacts and nanowires For solar cells and nanophotonics Ge nanoclusters and nanovoids in fused silica 1/31/2020 5 Introduction Methods of study Destructive or non-destructive - Non-local: Conduction measurements, XPS, AES, NMR, optical spectroscopy, mass-spectrometry, thermal desorbtion, chemical methods, isotopes method, magnetic measurements…. - Local: SEM, TEM (mesoscopic scale) and would be STM (atomic scale)? but it can see only a surface… We propose to use the STM method for 3D not-destructive characterization of : - Single atomic impurities hidden 2-3 nm below a surface - Nanoclusters burred up to 100 nm below a surface It is impossible in principle!!! Can we see with STM 1/31/2020 through a surface of metal? This is nonsense! Why not to try, but it will be no chance! 6 Introduction: Principle operation of STM The electrons with very low energy E~0.01…1.0 eV No ballistic penetration into the sample, just a tunnelling on the surface atoms electron φ 4 I eV S ( EF eV ) T ( EF ) M d 2 0 EF Tip Sample φ=4.5 eV : M~exp(-k d) ___________ k=ħ-1√2m(EF+φ − E) Δd=1Å I/Io≈10 Introduction: Scanning Tunneling Microscopy/Spectroscopy In selected points Spectroscopy (STS) Cu(111) Topography 7.8Å atomic resolution reflects the Density of States For each (x,y) Conductance mapping Introduction: common use of STM Co islands on Cu(111) 4 I eV S ( EF eV ) T ( EF ) M d 2 0 STM tip 42 x 42 nm2 Cu(001) ad-atom island step embedded atom 10 x 10 nm2 1/31/2020 9 Introduction: inconvenient use of STM STM tip Is it really possible? deep impurity atom cluster filled cavity “I am invisible, understand, simply because people refuse to see me.” (Prologue.1) Ralph Ellison Invisible Man 1/31/2020 10 Cu(111) surface Some ideas http://slideplayer.com/slide/6402875/ 1/31/2020 http://www.almaden.ibm.com/vis/stm/images/stm6.jpg 11 Friedel oscillations for 2-D electron gas Cu(111), Au(111), Ag(111) no bulk states but 2-D surface states Co atoms embedded into surface N. Knorr, M. A. Schneider, L. Diekhöner, P. Wahl, K. Kern PRL 88 (2002) 096804 1/31/2020 10 x 10 nm2 S.D. Kevan, PRL 50,526 (1983). 12 Interference of electrons scattered on the subsurface atom Expectations [011] Cu(110) [100] 1/31/2020 No anisotropy Anisotropy 13 Anisotropy + Focusing Surface interference due to subsurface scattering from a point defect: prediction for Cu(001), Cu(110), and Cu(111) [001] Point defect [110] Fermi surface of Cu [111] Y.S. Avotina, Y.A. Kolesnichenko, S.B. Roobol, and J.M. van Ruitenbeek, Low Temp. Phys. 34, 207 (2008). 1/31/2020 14 Buried atoms in metals STM tip Impurity atom of Co A. Weismann, M. Wenderoth, S. Lounis, P. Zahn, N. Quaas, R.G. Ulbrich, P.H. Dederichs, S. Blügel Science 323, 1190 (2009); Cu(111) Cu(001) Fermi surface Wave propagation Analogy: Ripples on water 1/31/2020 15 Buried atoms in metals Ripples on water Ripples on Cu(111) D~h Use for near-surface characterization of Cu T=20oC 1/31/2020 T>400oC T=350oC 16 Application: insight in near-surface physics Determining the distribution of Co atoms within subsurface atomic layers of Cu(001) Two terraces and an atomic step Ring-like interference pattern from subsurface buried atoms Distribution of Co atoms with depth Experimental Theoretical T. Siahaan et al. B 94, 195435 (2016) 1/31/2020 17 Considered system STM tip Embedding: 0.01-4.0 eV Matrix: Variation: Cu(001) Cu(111) Cu(110) 2-60 nm shape Ar Ne He or 2-15 nm Co Fe Important: Anisotropy of electron propagation O. Kurnosikov et al. T. Siahaan et al. 1/31/2020 PRB 77, 125429 (2008) PRL 102, 066101 (2009) PRL 106, 196803 (2011) PRB 84, 054109 (2011 ) PRB 90, 165419 (2014) Two cases 1. Nanocavities filled by Ar, Ne, He 2. Buried nanoclusters of Co and Fe 18 Quantum wells in thin metallic films Examples with steps at interface STM tip +5 V -5 V -5 V +5 V Pb 730 x 1100 nm I. B. Altfeder, K. A. Matveev, and D. M. Chen, PRL 78 (14) 2815 (1997) Si QW formation Tip Si Pb 1/31/2020 19 Nanocavity and buried nanocluster shape Anisotropic cases Isotropic case Wulff construction (110) metal-metal systems (111) 1/31/2020 (100) (111) ? 20 Intrinsic electron focusing Electron propagation in the bulk Focusing www.physik.tu-dresden.de/~fermisur/ k1 k2 2 T (rs ) 1 G(r s ri , E 1/31/2020 ri )P(rs ri , E ) Rn (rs ri ) w( E )dri dE Cu Ar 21 Noble gas impurities: Sample preparing Clean Cu crystal Ar+ bombardment @2-5 keV surface Cu(001) Annealing Implanted Ar 1/31/2020 800-1000K 22 Sample preparing Ar 1 2 3 4 XPS 1300 Ar 2p Intensity (Cyc) 1 – 300K 2 – 1000K 5 min 3 – 1050K 5 min 4 – 1075K 20 min Cu 1 1200 2 3 1100 4 250 245 240 Binding Energy (eV) 1/31/2020 23 STM results Cu(001) 1.0V 0.5V z 1 2 3 4 dI/dV Height, nm 330 x 330 nm2 0,3 4 3 0,05 2 0,00 0,2 1 10 2.5 A 0,1 0.2 A 0.5 A dI/dV (a.u.) Height (nm) 50 × 25 nm2 20 30 40 Distance (nm) 0,0 0 1/31/2020 100 200 Distance, nm 300 24 dI/dV map Ar in Cu(001) dI/dV map 600mV [110] 500mV 1/31/2020 [110] 60×60 nm2 25 Energy dependent dI/dV maps STS in the center of spot 1,6 [110] dI/dV (a.u.) 1,4 0.3V [110] 0.4V 0.5V 1,2 37.5 nm x 37.5 nm He 1,0 0,8 0,0 0,5 1,0 Bias voltage (V) ΔE≈0.2 eV 0.6V 0.7V 16.6 x 16.6 nm2 0.8V ΔE≈0.3 eV Ar View of the nanocavity from the (001) facet 1/31/2020 ΔE≈0.5 eV 26 dI/dV maps Overview of Ar nanocavities in Cu(110) 400mV 500mV 60×60 nm2 1/31/2020 27 Pattern and shape comparison a 4 b 3 2 1 (c) DE110 1 DEss 2 3 4 1/31/2020 28 The origins of the spots Diffraction from inclined interface Surface ≈0.25 nm (λF ≈ 0.49) 1/31/2020 29 Computer simulations: variation of the nanocavity shape Asymmetry of the nanocavity 1/31/2020 30 Ultimate depth using focusing effect 4.5 nm 12.5 nm Ar 22.4 nm Cu 32.5 nm 1/31/2020 20 x 20 nm2 31 Ultimate depth, continue 1.15 1.10 1.05 1.00 0.95 0.90 0.85 39.0 nm 0.80 Normalized differential conductance (arb.units) 0.0 0.5 1.0 1.5 1.00 52.9 nm 0.95 1.10 0.0 0.5 1.0 1.5 1.05 62.8 nm 1.00 0.95 0.0 0.5 1.0 1.5 1.00 80.0 nm 0.95 1/31/2020 0.0 0.5 Bias voltage (V) 1.0 1.5 20 x 20 nm2 32 Application: subsurface growth of nanocavities - Understanding of growth mechanism; - Determining of physical parameters 1/31/2020 33 Metallic subsurface nanoclusters Differences of the Ar-filled nanocavities and subsurface metallic nanoclusters 34 1/31/2020 Formation of the Fe or Co nanoislands in copper 0 nm 2-3 nm 50 x 50 nm 10-30 nm 30 x 30 nm 1/31/2020 35 Co nanoclusters in Cu(001) without focusing effect dI/dV maps Cu 3 nm Cu 6 nm dI/dV @ 1000mV Cu 10 nm dI/dV @ 400mV dI/dV @ 900mV 40 x 40 nm 1.4 1.2 dI/dV @ 600mV 1.1 1.2 0.8 -1.0 -0.5 0.0 0.5 1.0 Bias voltage (V) 1/31/2020 1.5 2.0 1.0 0.8 0.6 0.4 -1.0 -0.5 0.0 0.5 1.0 Bias voltage (V) 1.5 2.0 dI/dV/dI0/dV (a.u.) 1.0 dI/dV/dI0/dV (a.u.) 1.2 dI/dV/dI0/dV (a.u.) dI/dV/dI0/dV(a.u.) Cu 25 nm 1.0 0.9 -0.5 0.0 0.5 1.0 Bias voltage (V) 1.5 2.0 1.1 1.0 0.9 -1.0 -0.5 0.0 0.5 1.0 1.5 Bias voltage (V) 36 2.0 Fe in Cu(001) STS Maps: Indication of QW resonances, no focusing 6 nm of Cu above Fe nanoclusters dI/dV @ 300mV 4 nm of Cu 1/31/2020 dI/dV @ 400mV 6 nm of Cu 25 nm of Cu above Fe nanoclusters 30 x 30 nm dI/dV @ 70mV 9 nm of Cu 37 Application: self-burying during surface deposition T=350oC 1/31/2020 Experimental plots and fitting with different theoretical approaches 38 Conclusions - QW states below a surface can be used to identify subsurface atoms and nano-objects by STM - Anisotropy plays a remarcable role in forming spatial distributions of QW states - The ultimate depth of subsurface STM detection reaches a few tens of nanometers (80 nm is proved for the systems with focusing and 20 nm without focusing). 1/31/2020 39 Acknowledgments Muriel Sicot Timothy Siahaan Anrey Klavsyk Omer Adam Jochem Nietsch Can Avci Yuri Trushin Henk Swagten Bert Koopmans Wim de Jonge 1/31/2020 Questions? Comments? 40
