STM-TunnelingSTM-Tunneling

STM-TunnelingSTM-Tunneling

Why STM ? l The electronic microscopes gives ‘volume images’ (penetration depth) l In STM-no use of external particles l Principle-Electrons tunneling between an atomically sharp tip and a surface (animation-program files/netscape/communicator/program/stmanimation) STM-IntroductionSTM-Introduction

The STM combines three main concepts: l Scanning l Tunneling l Tip-point probing l Uniqueness: STM-IntroductionSTM-Introduction

l 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. l The breakthrough-atomic imaging in real space l The development of STM paved the way for a new family of techniques called : “scanning probe microscopy”. l 1986-Nobel prize to G. Binnig and H. Rohrer. STM-HistorySTM-History

Comparison of Characterization Techniques

Constant height vs constant current imaging STM Instrumentation

Constant Current Imaging (CCI)

Si (111) Surface STM: Si(7x7)

STM Images1. GaSb/InAs Only every-other lattice plane is exposed on the (110) surface, where only the Sb (reddish) and As (blueish) atoms can This 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 monolayers) 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 positioned 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 (reddish) and As (blueish) atoms can be seen. The atoms are 4.3 ֵ apart along the rows, with a corrugation of <0.5 ֵ. From work of W. Barvosa-Carter, B. R. Bennett, and L. J. Whitman.

The first STM image

NanolithographyNanolithography

Nanolithography: STM Here, the artist, shortly after discovering how to move atoms with the STM, found a way to give something back to the corporation which gave him a job when he needed one and provided him with the tools he needed in order to be successful. (Xe on Nickel, Nature 344, 524 (1990). Here they have positioned 48 iron atoms into a circular ring in order to "corral" some surface state electrons and force them 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)]

Pohl, D. W. IBM J. Res. Dev. 30, 417 (1986), Kuk and silverman, Rev. Sci. Instrum. 60, Mechanical Construction 2. Electronics 3. Data Acquisition 1.1 Vibration Isolation l To obtain vertical resolution of 0.01 , a stability of tip-sample ~  is required l Typical floor vibration ~  m, Hz, l  Vibration Isolation STM Instrumentation

l Damping: low frequency (<20 Hz, building)  tension wires or springs (resonance frequency ~1- 5 Hz), air table(res freq ~ 1 Hz). Medium frequency ( Hz,motors, acoustic noise)  mounting on plates. l External vibration isolation system+rigid STM design can reduce external vibrations by (“Interference filter”). STM Instrumentation (cont.)

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

STM Scanners h V l Piezoelectric bars l

STM-PrincipleSTM-Principle

Tube Scanners

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

“Inchworm”

STM Electronics (general)

STM Electronics (cont.) Carefull design because of low currents, stable feedback circuits. Sample and hold amplifier  local I-V characteristics Computer: 4-5 DAC’s, 2 ADC’s, real-time plane fit etc.

1.5 Tip preparation and characterization l Atomic resolution: single atom termination, macroscopic shape of little importance; 1  difference  1 order of magnitude in tunneling current. l Large scale structure: macroscopic tip shape of importance STM Tips

l Chemical composition of tip: oxide layer (jump to contact), atomic resolution - type of atom, higher resolution has been obtained by tunneling into or from d-orbitals (W). Tip material does not ‘dictate’ atom at the tip ! l Tip material: hard, UHV-W, Mo, Ir Air-Pt, Au (soft)  Pt-Ir. l Tip preparation: Electrolytical etching (drop-off technique), STM Tips (cont.)

l Disadvantage of electrolytical etching (drop-off technique), formation of oxide  ion milling: tip radii of 4 nm, cone angle ~ l In air : cutting of Pt-Ir wire (only for smooth surfaces) STM Tips (cont.)

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