Scanning Probe Microscopy

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1 Scanning Probe Microscopy Danny Porath 2003 (Eigler et.

2 Introduction to Scanning Probe Microscopy and applications. Julio Gómez Herrero. LNM. UAM SPM - the eyes to the Nano world.

3 With the help of. 1. Yosi Shacam TAU 2. Yossi Rosenwacks TAU 3. Julio Gomez-Herrero - UAM 4. Serge Lemay - Delft 5.

4 Outline SEM/TEM: 1. Examples, links and homework 2. STM principle, lab, Images 3. Tunneling 4. Instrumentation 5. Artifacts 6. Spectroscopy 7. Lithography

5 Books and Internet Sites Scanning Probe Microscopy and Spectroscopy, R. Wiesendanger (Cambridge U. Press) ttp://

6 Homework 5 1. Read the paper by: Crommie, Lutz & Eigler, Science 262, 218 (1993)] - Emphasize the lithography part. 2. Read the paper: Scanning Tunneling Microscope Instrumentation Kuk & Silverman 60, 165 (1989). 3. Find on the web, in a paper or in a book the 3 most impressive STM images: a. 1 - Technically b. 1 - Scientifically c. 1 - Aesthetically Explain your choice. If needed compare with additional images.

Scanning Tunneling Microscope (STM) Piezo Tip Sample Computer Electronics (Control) (Current+Feedback) I(V) ~ Ve -(ks) Matrix of heights (Image) Tunneling between a sharp tip and

37 Scanning Tunneling Microscope (STM) Piezo Tip Sample Computer Electronics (Control) (Current+Feedback) I(V) ~ Ve -(ks) Matrix of heights (Image) Tunneling between a sharp tip and conducting surface. Piezo enables xy and z movement. Working modes: constant current and constant height. The feedback voltage V z (x,y) is translated to heigh (topographic) information.

38 STM-Principle

39 STM Head רכיבי היחידה המרכזית ראש ה - STM חוד בידוד דגם בסיס גביש פייזו- אלקטרי בורג מיקרו- מטרי

40 Low Temperature STM

41 Low Temperature STM מחזיק הדגם רכיב קרוב גס קפיץ מוט חוד מחזיק הדגם גביש פייזואלקטרי ממברנה בורג מיקרומטרי

42 STM Laboratory STM אלקטרוניקה מערכת המחשב חיפוי עץ בסיס

43 STM Laboratory

44 Supercoiled DNA STM Images Gold monolayers Graphite atomic resolution

45 STM image of a Single-Wall Carbon Nanotube

46 Si (111) Surface (7x7 reconstruction)

STM Images Combination GaSb/InAs is color-enhanced 3-D rendered STM image shows the atomic-scale structure of the interfaces between GaSb and InAs in cross-section.

47 STM Images Combination GaSb/InAs is color-enhanced 3-D rendered STM image shows the atomic-scale structure of the interfaces between GaSb and InAs in cross-section. A superlattice of alternating GaSb (12 onolayers) and InAs (14 monolayers) was grown by molecular beam epitaxy. A piece of the wafer was cleaved in vacuum to expose the (110) surface, and then the tip was sitioned over the superlattice about 1 µm from the edge. Due to the structure of the crystal, only every-other lattice plane is exposed on the (110) surface, where only the Sb eddish) and As (blueish) atoms can be seen. The atoms are 4.3 Å apart along the rows, with a corrugation of <0.5 Å From work of W. Barvosa-Carter, B. R. Bennett, and L. J Only every-other lattice plane is exposed on the (110) surface, where only the Sb (reddish) and As (blueish) atoms can be seen

48 Inspection 20-25% of fabrication time! Tip techniques with CCD-camera for on-chip inspection Stylus (α-step) Height resolution 5 nm Lateral resolution 15 µm AFM Height resolution monolayer Lateral resolution nm Microscopy Optical microscopy (1 µm) Dark field, Interference contrast, luminescence Scanning Electron M. ( 1 nm)

49 STM-Introduction Why STM? The electronic microscopes gives volume images (penetration depth) In STM-no use of external particles Principle: Electrons tunnel between an atomically sharp tip and a surface

50 STM-Introduction The STM combines three main concepts: Scanning Tunneling Tip-point probing Uniqueness: Animation:

51 STM-History In March 1981, Gerd Binning, H. Rohrer, Ch. Gerber and E. Weibel observed electrons tunneling in vacuum between W tip and Pt; this in combination with scanning marked the birth of STM. The breakthrough: atomic-scale surface imaging in real space The development of STM paved the way for a new family of techniques called : scanning probe microscopy Nobel prize to G. Binnig and H. Rohrer.

52 The first STM image

53 Comparison of Characterization Techniques nalytical echnique Typical Application Signal Detected Elements Detection Limits Depth Resolution Lateral Probe Size XRF BS PS DAX EDS) Metal contamination Thin film composition Surface analysis Depth profiling elemental microanalysis X-rays S - U Atoms/cm 2 He atoms Li - U 1-10 at% (Z<20) Photoelectrons 10 mm 2-20 nm 2 mm (Z>20) Li - U at% 1-10 nm 10 µm 2 mm X-rays B - U at% 1 5 µm 1 µm uad IMS OF IMS Dopant profiling Surface microanalysis Surface microanalysis Secondary ions Secondary ions H - U H - U 10 8 Atoms/cm3 Atoms/cm 2 <1 <5 nm 1 µm (Imaging) 30 µm (D Profiling) monolayer 0.1 µm (Imaging)

54 Comparison of Characterization Techniques Analytical Technique Typical Application Signal Detected Elements Detection Limits Depth Resolution Lateral Probe Size AES HRAES Surface analysis and depth profiling Surface analysis, micro area depth Auger electrons profiling SEM Surface imaging Secondary & backscattered electrons Li - U at% <2 nm 100 nm Li - U at% 2-6 nm <15 nm 3 nm AFM Surface imaging Atomic forces 0.01 nm nm HRSEM STM High resolution surface imaging Secondary & backscattered electrons Surface imaging Tunneling currents 0.7 nm 0.01 nm 0.1 nm

55 STM-Tunneling

56 STM-Tunneling

57 STM-Tunneling Models Elastic vs. Inelastic (energy loss to phonons etc) tunneling processes One dimensional vs. three dimensional Barrier shape

58 Elastic Tunneling through One-Dimensional Rectangular Barrier א. בור פוט נצ י אל אי נ סופ י ח ד-מימדי V V V ( x) = 0 V o X < 0 X 0 X X = 0 X = L d 2ψ ( X ) 2 + k ψ ( X 2 dx ) = 0 k 1 = h [ 2m( E V )] 1 2 E ה אנרגיה הכל לי ת של האל קטרון, k -וקט ור הגל של ה אלקט רון m מ סת הא לקטרון

59 ב. מחסום פוטנציאל סופי חד-מימדי V(x) ( x) = 0 V o X < 0 X 0 V 0 e d 2ψ ( X ) 2 + k ψ ( X 2 dx ) = 0 מתכת x=0 אויר x k 1 E- האנרגיה הכללית של האלקטרון, = h [ 2m( E V )] 1 2 k -וקט ור הגל של ה אלקט רון m- מ סת הא לקטרון

60 מחסום פוטנציאל סופי חד-ממדי (המשך) x < 0; ψ ( x) ik x ik1x Ae 1 = + Be 1 ( k 1 = me h 1 2 [ 2 ] ) x 0; ψ ( x) = Ce i 2m( E V h 0 ) x + De i 2m( E V h 0 ) x = = Ce 2m( V h 0 E ) x + De + 2m( V h 0 E ) x V 0 e אויר מתכת

61 מחסום פוט נצ יאל צר x< 0; ψ( x) Ae ik1 x + Be ik1x = 0< x< s; ψ( x) = Ce 2m( V 0 E ) h x + + De 2m( V 0 E ) h x s< x; ψ( x) 1 = Fe ik x ψ ψ ת נאי שפ ה: (כדי ש משוואת שרדי נגר תתקי ים בנקודות אלה) ( x = 0, s) = Continous ( x = 0, s) = Continous s ik x 2 < x; ψ ( x) = Fe 1 ; F α e 2 2m( V0 E ) s h קבלנו גל (אלקטרון) בעל ערך מוחלט של אמ פ ליטודת ה ה סתברות הדועכת אקס פוננציאלי ת עם הגדל ת רוחב ה מ חסום a

62 V(x) מחסום פוט נצ יאל צר-מי נהור s V 0 e e מתכת אויר מתכת x=0 x=s 2 2m( V0 E ) ik x 2 s < x; ψ ( x) = Fe 1 ; F α e h = e 2κs x הסתברות למצוא את האלקטרון יורדת פי e כל פעם שמתרחקים 1 h מרחק של : = 2κ 2 2m( V 0 E )

63 מינהור ההסתברות למצוא אלקטרון בצד השני של מחסום ברוחב של Å 1 כאשר V o -E =1.0 ev היא 1/e עבור רוחב מחסום של ~Å 2 ההסתברות היא 1. המרחק הבין אטומי) (מרח ק ז ה הוא בקירוב V(x) V 0 Ψ (x) e מתכת אויר מתכת e x x=0 x=s x

64 TM-Tersoff and Hamann tunneling model Based on 1st order perturbation theory, taking into account density of states in tip and sample, assuming: Spherical symmetrical tip s-type tip wave function Small applied bias (unaffected wavefunctions)

65 TM-Tersoff and Hamann tunneling model I α U. n t (E F ) exp(2κr) n s (E F, r o ) U-Applied bias between tip and sample, n t (E F )-tip density of states at Fermi energy n s (E F,r o )-LDOS at the Fermi energy at r o κ = ( 2mφ ) / h φ - Effective local barrier height Since: n s α exp(-2κ(s+r)); (s wave function) I α exp(-2κs) Like the one dimensional infinite potential well!

66 The interpretation of STM images in light of Tersoff and Hamann tunneling model The STM image represents contour maps of constant surface LDOS at E F, evaluated at the center of the curvature of the tip. Higher wavefunctions (l,..) gave minor corrections, thus a good model for single atom tip.

67 Modifications of the simple model Effect of finite bias on tip (zero voltage wave functions): eu Iα nt ( ± eu ± E) ns ( E, ro ) de 0 U-Applied bias between tip and sample, n t (eu±e)-tip density of states at an energy E under bias U, n s (E,r o )-sample density of states at an energy E at the center of curvature of tip. (E f is taken =0)

68 Modifications of the simple model W and platinum-iridium are the most widely used for tip material, the density of states at E f is dominated by d states.

69 CCT Imaging with different tip atoms The 2x2 nm image of Au (111) changes during imaging probably due to change of effective orbital of the tip end

70 Conclusion the orbital at the end of the tip determines the spatial resolution in STM Comparison with theory Theoretical corrugation amplitude for s and d z 2 tip state on Al (111)

71 Constant current of Na tip over Na, S, and He adatoms Negative tip displacement for He: the closed valence shell of He produces a local decrease in density of states near EF; Reduced tunneling current => negative tip displacement in CCT. Bumps or holes in CCT may not correspond to presence or absence of atoms!

72 Common Tip Models ) Spherical potential well (continuum): Predicted resolution: 2 A o ( R+s) 1/2 Based on experimental results this model was not accurate enough ) Cluster of atoms or multiatom tip

73 Common Tips Models Single atom interacting tip ) Adsorbed atom on a metal electrode (Jellium model - free-electron metal substrate))

74 STM Working Modes Constant height vs constant current imaging

75 Constant Current Imaging (CCI)

76 STM Instrumentation Pohl, D. W. IBM J. Res. Dev. 30, 417 (1986), Kuk and silverman, Rev. Sci. Instrum. 60, 165 (1989) 1. Mechanical Construction 2. Electronics 3. Data Acquisition

77 Vibration Isolation To obtain vertical resolution of 0.01Å, a tip-sample stability of ~ Å is required Typical floor vibration ~ µm, Hz, Vibration Isolation

78 Damping low frequency (<20 Hz, building) tension wires or springs (resonance frequency ~1-5 Hz), air table (resonance frequency ~ 1 Hz). Medium frequency ( Hz,motors, acoustic noise) mounting on heavy plates. External vibration isolation system+rigid STM design can reduce external vibrations by a factor of ( Interference filter ).

79 STM design An example for an approach mechanism

80 STM Positioning Devices - Tripod Three dimensional movement of tip and sample Tip movement -piezoelectric drives. Sample-piezoelectric, magnetic (magnet inside a coil), mechanical.

81 STM Scanners - Piezoelectric bars h V l l = d31 l h V

82 Tube Scanners

83 Design Considerations: STM Scanners (cont.) Tube scanner: higher sensitivity due to thin walls, symmetrical vs. asymmetrical voltage. Single tube scanner: higher resonance frequency scan rate, stability voltage signals to produce a scan. Large piezoresponse. Low cross-talk between x,y and z piezodrives. Low nonlinearity, hysteresis, creep, and thermal drift. Sample-Tip Approach Mechanisms: Step size has to be smaller then the total range of z piezodrive; Step resolution of 50 Å,and dynamic range of cm is reuired! Inchworm (electrostrictive actuator), motion not limited in distance. (כינה) Inchworm = louse Two dimensional

84 Approach mechanism - Inchworm

85 STM Electronics (general)

86 STM Electronics (cont.) Careful design for low currents nad stable feedback circuits. Sample and hold amplifier local I-V characteristics Computer: 4-5 DAC s, 2 ADC s, real-time plane fit etc.

87 Tip preparation and characterization Atomic resolution: single atom termination, For atomic structures: macroscopic shape of little importance Large scale structure: macroscopic tip shape is important 1Å difference 1 order of magnitude in tunneling current.

88 Important factors: Chemical composition of tip Oxide layer (jump to contact) For atomic resolution - type of atom (tip material does not dictate atom at the tip!) Higher resolution has been obtained by tunneling into or from d-orbitals (W). Tip material: hard+low workfunction, UHV-W, Mo, Ir Air-Pt, Au (soft) Pt-Ir. Tip preparation: Electrolytic etching (drop-off technique) Cutting (with scissors) Tip preparation

89 Tip preparation - Electrolytic Etching (drop-off technique) The disadvantage of electrolytic etching: oxide formation

90 Tip Preparation Ion Milling & FIB Cutting Ion milling: tip radii of 4 nm, cone angle ~ 10 o. In air : cutting of Pt-Ir wire

91 STM Tips Preparation Tips are produced by electron beam deposition (EBD) inside SEM. Dissociation of chamber residual gases (H 2, O 2, CO, H 2 O)

92 Tip Shapes Etched Au Tip Ion milled W Tip Mechanically cut Pt Tip

93 Tip artifacts

94 Tip artifacts

95 Imaging Errors and Tip Convolution

96 Taking STM Images 1) Checking the tip Checking atomic resolution Chemical purity: dirty tip => lower sensitivity and resolution Checking vacuum gap: measuring I as a function of distance s 2 2m I = Aexp 2 2m( V0 E) h ln( I) = B s φ h d ln( I) 2 2m = ds h For a barrier height of 4 ev, and ds=2 Å, d(lni)=4.2, indicates a vacuum gap If the sample is covered by an insulator, the barrier height and sensitivity decreases (also due to compression of contaminants!) φ

97 Samples 1) Metals In air- atomic resolution is easily obtainable (only on thin films), but samples are usually contaminated. In UHV typical cleaning : ion sputtering, annealing at C (checked by AES or LEED) I-V curves can check cleanliness: at metals no energy gap around E f

98 Samples 2) Semimetals Graphite, and other layered transition metal dichalcogenides (WS 2, MoS 2 ) can be imaged with atomic resolution in air simply by cleaving or peeling (by adhesive tape). I-V curves can check cleanliness: at metals no energy gap around E f 3)Semiconductors usually capped with oxides -reduced resolution; atomic resolution is possible only with sub-monolayer oxygen coverage In UHV: three main preparation methods: Ion sputtering followed by annealing, cleavage, and thermal desorption (Si-flashing the sample for short periods at C). 4)Biological Specimens Usually droplet deposited on conducting substrates and dried. Freeze drying and then coating with metal film

99 Si (111) (7x7) The reconstruction: reduces the number of dangling bonds from 49 to 19 per 7x7 unit cell Reconstruction between atomic steps-depends on rate of cooling during Surface preparation

100 STM and Spectroscopy General Information of local electronic structure due to energydependent changes in density of states 1) STM does not reveal position of atoms 2) Spectroscopic information with atomic resolution Voltage-dependent Imaging Si (111) : 2V +2V

101 Scanning Tunneling Spectroscopy The tunneling from states near E F is dominant Therefore: Positive tip bias (in picture) shows tip empty states Positive sample bias shows sample empty states

102 STM and Spectroscopy More strongly bound electrons have shorter decay length Tunneling direction depend on bias between tip and sample, most of the tunneling is from electrons near E f spectroscopic information can be obtained by voltagedependent imaging Equlibrium Positive sample bias (Measuring the sample empty surface states) Negative sample bias ( Measuring the tip -sam. Surf states) Only electronic states between E f s of tip and sample contribute ϕ = A exp 2 2m( V0 E) z h

Si (111) (7x7) There are 12 adatoms per 7x7 unit cell Each adatom ties up 3 dangling bonds from underlying atomic layer Single dangling bond on each adatom The dangling bonds are partly filled and

103 Si (111) (7x7) There are 12 adatoms per 7x7 unit cell Each adatom ties up 3 dangling bonds from underlying atomic layer Single dangling bond on each adatom The dangling bonds are partly filled and therefore contribute to both filled and empty surface states The positions of the maxima do not depend on the applied bias Si (111) (7x7) is a special case where STM image provide position of surface

104 Si (111) (7x7) At positive sample bias all atoms are at equal height (empty surface states) Sample =1.5 V Sample =- 1.5 V At negative sample bias also rest atoms are observed Sample =-3 V At 3V rest atoms are visible

105 STM for Surface State Spectroscopy The disadvantage of succesive imaging (for surface state spec)-thermal drift, tip changes make direct comparison between successive images dificult The idea-the tunneling current increases if the applied voltage onsets a tunneling into unoccupied surface states di/dv(v) reflects surface states density ev Iα nt ( ± ev m E) ns( E, ro ) T ( E, ev ) de di dv 0 ev ev nt ( ± ev m E) ns( E, ro ) T( E, ev) de = [ nt ( ± ev m E) ns( E, ro ) T( E, ev) ]de V en( 0) n ( ev, r ) T( ev, ev) 0 t ev s o T = nt ( ± ev m E) ns( E, ro ) ( E, ev) de + V 0 Assuming dn t /dv 0 en (0) n t s ( ev, r o ) T( ev, ev)

106 STM for Surface State Spectroscopy For ev«φ, dt/dv = 0 di dv en ( 0) n ( ev, r ) T( ev, ev) t s Because T(eV) is a monotonic increasing function of V, structure in di/dv (V) is mainly due to changes in surface density of states of the sample. o In order to eliminate bacground (due to T(eV)), di/dv is normalized as: di/dv /I/V

107 STM Modulation Techniques Experimental Technique STS- scanning tunneling spectroscopy: imposing a small high frequency V and measuring with lock-in amp. Measurement gives : di/dv at the voltage V DC of the CCT Frequency range: too low-interfere with feedback electronics too high-displacement current High DC Voltage: high surface barrier forms a quantum well for surface electrons changes density of states. Low DC Voltage : surface density of states is obtained

108 Scanning Tunneling Spectroscopy of Si (111) (7x7) Top: I-V Measured at 3 locations of the Unit cell Middle: UPS measurement Bottom: di/dv/(i/v) averaged on several unit cells

109 STM Nanolithography

110 STM Nanolithography Xe on Nickel, The science of PR (Nature 344, 524 (1990).

STM Nanolithography 48 iron atoms in a circular ring the quantum corral" 48 iron atoms in a circular ring the quantum corral" The waves are surface state electrons forced into "quantum" states of the

111 STM Nanolithography 48 iron atoms in a circular ring the quantum corral" 48 iron atoms in a circular ring the quantum corral" The waves are surface state electrons forced into "quantum" states of the circular structure. The ripples in the ring of atoms are the density distribution of a particular set of quantum states of the corral. The artists were delighted to discover that they could predict what goes on in the corral by solving the classic eigenvalue problem in quantum mechanics -- a particle in a hard-wall box. [Crommie, Lutz & Eigler, Science 262, 218 (1993)]

The interpretation of STM images in light of Tersoff and Hamann tunneling model

The interpretation of STM images in light of Tersoff and Hamann tunneling model The STM image represents contour maps of constant surface LDOS at E F, evaluated at the center of the curvature of the tip.