Hawkes P.W., Spence J.C.H. (Eds.) Science of Microscopy. V.1 and 2

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970 P. Sutter

technique, a historic overview may also be of limited value. Hence, I

have tried to identify representative examples in the current literature,

which are discussed brieﬂ y to illustrate the different uses of STM. The

connection to historic developments can in most cases be made easily

via literature references. For a more in-depth discussion of basic con-

cepts and application examples, several dedicated monographs on

STM and other scanning probe techniques (Chen, 1993; Wiesendanger,

1994; Güntherodt and Wiesendanger, 1992; Bonnell, 2001) represent an

invaluable resource.

This chapter is organized as follows. In Section 2 basic principles of

STM imaging are introduced, and the imaging methodology as well

as practical and instrumentation requirements are discussed. The

approach taken in surface imaging by STM is illustrated by the example

of silicon surfaces. Section 3 highlights an application of STM that has

gained ever-increasing importance: atomic scale spectroscopy. It pro-

vides a survey of the spectroscopy capabilities of STM, and introduces

a variety of techniques for local spectroscopy and spectroscopic

imaging. In addition, pathways toward obtaining chemical and element

speciﬁ city at the atomic scale—traditionally a weakness of STM—are

discussed. The extension of the operating conditions of STM to high

and low temperatures has opened up new avenues of investigation.

Variable temperature STM of dynamic surface processes as well as

atom and molecule manipulation at cryogenic temperatures are the

topics of Section 4. Section 5 discusses STM imaging and spectros-

copy on subsurface structures, using ballistic electrons to probe

buried interfaces or cross-sectional STM on cleavage faces of III–V

semiconductors to image embedded nanostructures. The chapter con-

cludes with a brief discussion of STM image simulation techniques in

Section 6.

2 Basic Principles of STM Imaging

2.1 Elastic Vacuum Tunneling and

Scanning Tunneling Microscopy

STM is based on the vacuum tunneling of electrons between two

solids, one of which is a sharp tip and the other a sample. To remove

an electron from a solid and bring it into vacuum with zero kinetic

energy requires energy equal to the work function φ = E

Vac

− E

, i.e., the

difference between the vacuum level, E

vac

, and the Fermi energy of the

material, E

. To move an electron from one solid to another, it has to

cross the same vacuum barrier. If the two solids are separated by a

microscopic distance of the order of the decay length of electronic

states into the vacuum (typically about 1–5 nm), electrons can cross the

barrier by quantum mechanical tunneling (Figure 15–1).

If, say, two metals are brought to within tunneling distance, a rapid

charge transfer takes place between them until an equilibrium state is

established in which their Fermi levels are aligned. Once in equilib-

rium, no net charge is transferred on average. Consider now a slightly

modiﬁ ed situation, in which metal A is biased relative to B by applying

Chapter 15 Scanning Tunneling Microscopy in Surface Science 971

a voltage, V

. The bias offsets the Fermi energies on either side of the

vacuum barrier by eV

. Under steady-state conditions, this potential

difference establishes a net tunneling current, I, between A and B.

In a simple planar tunneling model using the Wentzel–Kramers–

Brillouin (WKB) approximation, the tunneling current is given by an

integral over the energy range between the Fermi energies on either

side of the vacuum gap,

IEEeVTEeVdE

(

)

−

(

)

(

)

∫

ρρ

AAA

, (1)

where ρ

and ρ

denote the local density of states at energy E at the

surface of A and B, respectively, and T(E, eV

) is the tunneling transmis-

sion probability for electrons with energy E and applied bias V

TEeV

zm eV

E,exp

AB A

(

)

=−













+−

22

φφ

(2)

Here, φ

A,B

are the work functions of the two solids, z is their separation,

and m is the mass of the electron. Evaluation of T for positive (V

> 0)

and negative bias (V

< 0) shows that the transmission probability is

highest for electrons at the Fermi energy of the material that is nega-

tively biased, and falls off exponentially for lower energies down to a

lower cutoff at the Fermi energy of the positively biased electrode. This

general observation has important consequences in tunneling spec-

troscopy and spectroscopic imaging, as discussed in Section 3.

The tunneling current depends strongly on the separation, z, between

the two solids. To illustrate this fact, we simplify Eq. (1) by assuming

constant densities of states, independent of energy. In the limit of low

bias voltage, V

/φ

A,B

<< 1, the tunneling current is then given by

IV m z=−+

(

)

(

)

ρρ φ φ

ABA A B

exp /2



(3)

i.e., it depends exponentially on the separation. It is from this strong z

dependence of the tunneling current that STM derives its exquisite

height resolution, typically of the order of 0.1 pm, or below 1/100 mono-

layer for most solids.

Figure 15–1. Band diagrams (energy vs. distance along the tunneling direc-

tion) for vacuum tunneling between two metal electrodes, A and B.

972 P. Sutter

In actual STM imaging, an atomically sharp tip is used to obtain

laterally conﬁ ned tunneling at a well-deﬁ ned position of a sample. A

piezoelectric positioning element controls the in-plane (x, y) position

and height (z) of the tip with picometer resolution. While a constant

bias voltage is applied between sample and tip, the tip is scanned in a

line-by-line fashion to continuously change the tunneling position and

build up an image of a chosen area of interest on the sample. The strong

dependence of the tunneling current on the tip–sample separation can

be used for two basic modes of STM imaging: constant-current and

constant-height imaging (Figure 15–2).

In constant-current imaging, the tunneling current is measured at

each pixel of the scan and is compared with a chosen current set point.

Deviations between the measured tunneling current and the set point

are corrected by applying an appropriate voltage to the z-piezo, thus

adjusting the tip–sample separation. This feedback mechanism main-

tains a constant tunneling current during the scan, while the trajectory

z(x, y) followed by the tip is used to generate a map of the sample

surface or, more accurately, a map of a particular charge density contour

above the surface.

In constant-height mode, the tip height is not modiﬁ ed during the

scan. The tunneling current is again measured at each image pixel, but

is now used directly as a representation of the sample surface via a

current map, I(x, y). The current maps represent a cut through charge

density contours in the plane in which the tip is scanned above the

sample. Obviously, since the tip–sample separation is uncontrolled

during the scan, constant-height imaging is limited to samples with

low corrugation and/or small scan sizes. A more practical implementa-

tion of constant-height STM uses a slow feedback system to adjust the

tip–sample separation on time scales that are long compared to the

residence time at each pixel, thus compensating for sample surface

topography or sample tilt. As the current comparison and tip z cor-

rection are eliminated and only the tunneling current is measured,

data acquisition can be faster than in constant-current imaging. The

constant-height mode is thus preferred for fast STM image acquisition.

Figure 15–2. Schematic tip

trajectory for STM imaging in

constant current and constant

height mode.

Chapter 15 Scanning Tunneling Microscopy in Surface Science 973

2.2 Inelastic Vacuum Tunneling

The above considerations assumed elastic tunneling between two

materials, e.g., a sharp tip and the sample. In this case, the energy of a

tunneling electron is conserved in the transit through the vacuum

barrier. Only inside the opposite electrode the electron thermalizes

to the Fermi energy by phonon emission. Tunneling electrons can,

however, undergo inelastic scattering during the tunneling process.

Vibrational modes of molecules adsorbed on the sample surface, for

example, can be excited by inelastic scattering of a small fraction of the

tunneling electrons (Figure 15–3).

The onset of inelastic tunneling at characteristic energies can then

be detected and used as a ﬁ ngerprint to identify individual molecular

bonds. Such STM-based single molecule vibrational spectroscopy will

be discussed in Section 3. In addition, controlled amounts of energy

can be deposited locally into individual adsorbates, important for

the manipulation of adsorbed atoms or molecules, as discussed in

Section 4.

2.3 Practical Requirements: Tips, Samples, and

Operating Environment

Obtaining a highly localized tunneling contact requires very sharp

probe tips, ideally terminated by a single atom or a small cluster.

D-band metals (e.g., W) or alloys (PtIr) are generally thought to be

superior for obtaining high spatial resolution due to the strongly direc-

tional nature of the d-wave function. However, atomic resolution has

also been obtained with Au tips, i.e., tip materials with primarily s-

electrons at the Fermi level.

Electrochemically etched tungsten wires are often used for high-

resolution STM in vacuum, while less reactive PtIr or Au tips are

preferred for imaging under ambient conditions. Sharp W tips are

produced by electrochemical etching in ∼2 M KOH or NaOH solution,

using a stainless steel or Pt foil as a counter electrode. dc or ac etching

at typical bias voltages of 5–10 V can be used. Preferential etching at

the meniscus of the solution leads to a tapering of the immersed wire,

causing one end to eventually break off. To prevent further etching that

would blunt the tip apex, an electronic circuit detects the break-off

Figure 15–3. Band diagrams for elastic

and inelastic electron tunneling.

974 P. Sutter

current and switches the bias to zero within a few microseconds.

Etched tips are rinsed in deionized water to remove traces of the etch

solution, and loaded into ultrahigh vacuum (UHV). Contaminants and

surface oxide are desorbed from the tip by a heat treatment in UHV

prior to mounting the tip into the STM. Electron beam bombardment

of the tip apex provides an efﬁ cient local cleaning of the thin part of

the tip, and the applied electric ﬁ eld in this procedure may lead to an

additional tip sharpening (Kuk and Silverman, 1986). Alternative tip

cleaning and modiﬁ cation techniques while in tunneling contact

involve voltage pulses of the order of 4–10 V while scanning the sample,

or tip forming in the ﬁ eld emission regime (i.e., at tip–sample large

separation) with applied bias voltages between 10 and 100 V and cur-

rents of several nA. Ideally, the atomic conﬁ guration of the tip would

be characterized by ﬁ eld ion microscopy (FIM) prior to its use in the

STM. Attempts have been made to combine FIM with STM. However,

in practice such efforts are of limited beneﬁ t since tip changes occur

frequently during scanning, rendering a complete characterization of

the tip impractical.

To allow the controlled biasing of tip and sample, and to prevent

charging, conventional direct current STM is limited to metal or semi-

conductor samples. Alternating current STM (Kochanski, 1989) has

been demonstrated on insulators and organic layers, but is not widely

used. Clean and ordered crystalline surfaces with low surface rough-

ness are preferred for most studies, in particular for atomic-resolution

imaging. Reactive semiconductor or metal surfaces are best prepared

and imaged in UHV, but metal surfaces are also prepared and imaged

routinely in electrolytes (Gewirth and Niece, 1997). In vacuum, sample

preparation often involves a thermal cleaning step to remove contami-

nants. Flash cleaning to 1200°C for a brief period of time (few seconds)

while keeping the vacuum in the low 10

−9

torr range removes the native

oxide and provides atomically clean Si samples with a low density of

SiC contaminants. Metal surfaces (e.g., Cu, Al, Pt, Au), and some semi-

conductors (e.g., Ge) are prepared in a two-step procedure involving

repeated cycles of Ar

ion sputtering and annealing. In refractory

metals such as Ru, C contamination is removed by many (several

hundred) cycles of oxygen adsorption near room temperature and

ﬂ ashing to 1500°C. In situ cleavage can be a very effective method for

preparing sample surfaces that cannot be sputtered or heated to high

temperatures. Samples best prepared by cleavage include III–V com-

pound semiconductors (GaAs, InAs), as well as high-T

superconduc-

tors (Yba

7−x

, BiSr

8+x

). In situ cleavage is also the preparation

method of choice for cross-sectional STM (see Section 5), commonly

performed on (110) cleavage planes of III–V semiconductors. Bulk insu-

lating metal oxides, most prominent among them TiO

, can be imaged

successfully by STM following Ar

ion sputtering and annealing, which

generates oxygen vacancies and renders the surface region sufﬁ ciently

conductive for stable STM imaging. Au and highly oriented pyrolitic

graphite (HOPG) are among the few materials on which atomic resolu-

tion is obtained under ambient conditions. Well-ordered, (111) oriented

surfaces of Au single crystals and thin ﬁ lms on mica can be prepared

Chapter 15 Scanning Tunneling Microscopy in Surface Science 975

by ﬂ ame annealing in air (Robinson et al., 1992). Preparation of layered

materials, notably HOPG, is particularly simple. It involves the removal

of bundles of graphene layers by adhesive tape from the crystal. Since

the exposed graphene sheet has no dangling bonds and is very inert,

such samples can be imaged with atomic resolution in air for extended

time periods.

For many materials, in particular metals and alloys, electrochemical

STM is a powerful alternative to imaging in UHV. In this environment,

almost the entire portion of the tip that is immersed in the solution

must be coated by an insulating layer (e.g., wax or glass) to minimize

the faradaic (i.e., ionic) current, which is typically much larger than the

tunneling current and would otherwise dominate the measured signal.

Only a small fraction near the apex of the tip remains uncoated to

allow for tunneling between the tip and sample.

2.4 STM Instrumentation

In tunneling contact the probe tip is only a distance of the order of

1 nm away from the sample. As the tunneling current is exponentially

sensitive on the tip–sample separation, any mechanical vibrations of

the tip relative to the sample have to be minimized. To this end, a two-

fold strategy is commonly pursued. The microscope head itself is built

as compact and stiff as possible by closely integrating sample and tip

on a rigid platform. This results in a high resonance frequency for

oscillations of the tip relative to the sample. In addition, the entire

microscope is isolated mechanically from outside vibration sources,

e.g., by suspending it on soft extension springs or long bungy cords.

The overall goal is to achieve a maximal mismatch between the mech-

anical modes of a soft suspension system and the high resonance fre-

quency of the stiff microscope head. In addition, efﬁ cient vibration

damping is required, which can be achieved by magnetically induced

eddy currents. The springs used in many suspension systems have

their own mechanical modes that can be damped by polymer strips

woven into their coils. Direct coupling of acoustic noise into the micro-

scope, ﬁ nally, is a problem that is best solved by operating the STM in

vacuum (Figure 15–4).

A few additional components are part of any practical tunneling

microscope. A coarse approach mechanism has to be in place, which

moves the tip from an initial distance of several millimeters into

tunneling range without making physical contact with the sample.

Although this has been achieved by mechanical approach mechanisms

(Smith and Binnig, 1986), piezoelectric driven stick-slip motors are now

preferred since they allow the entire coarse approach to be performed

automatically under computer control. For the positioning and scan-

ning of the tip during STM imaging, a segmented piezoelectric tube

scanner is used (Binnig and Smith, 1986). By applying high voltages to

the individual segments, the scanner is independently actuated in the

(x, y) plane parallel to the sample surface and along the perpendicular

z axis with little crosstalk. The tunneling current in the pA to nA

range is converted into a voltage by a sensitive, low-noise ampliﬁ er

976 P. Sutter

with adjustable transconductance of 10

–10

V/A, whic h provides the

input signal for the tunneling current feedback electronics. Modern

STM controllers typically use digital feedback and scan modules imple-

mented by speciﬁ c software programs running on fast digital signal

processor (DSP) chips. The measured analog tunneling current signal

is digitized using an analog-to-digital (A/D) converter, and is com-

pared with the current set point by the feedback program on the

DSP. Necessary corrections in the tip height z are computed, and are

converted into analog voltages by digital-to-analog (D/A) converters.

These voltages may be ampliﬁ ed further by a high-voltage ampliﬁ er

stage to levels suitable to drive the tube scanner. The scan signal, i.e.,

independent voltage ramps driving the piezo scanner along the in-

plane x and y directions, is also generated digitally on a DSP, followed

by conversion and ampliﬁ cation steps analogous to those of the z

signal. A program running on a host computer and communicating

with the DSP, ﬁ nally, acquires, displays, and stores the measured tip

trajectory z(x, y) (in constant-current imaging) or position-dependent

tunneling current I(x, y) (in constant-height mode).

2.5 STM Imaging: Application Examples

High-resolution imaging of surface structure is probably the most

widespread application of STM. As a direct imaging technique, STM

has added signiﬁ cantly to the diffraction techniques used traditionally

to determine the structure of clean and adsorbate covered surfaces.

Examples of typical applications include the determination of the

structure of reconstructed surfaces of clean semiconductors, for

instance vicinal Si surfaces (Erwin et al., 1996) and oxides, such as TiO

(Diebold, 2003), and of adsorbate-induced reconstructions on metals

Figure 15–4. Schematic illustration of the various components of an STM

system, including piezoelectric tube scanner for tip positioning, and con-

trol electronics (current ampliﬁ er, feedback, scan generator, high-voltage

ampliﬁ er).

Chapter 15 Scanning Tunneling Microscopy in Surface Science 977

(e.g., O/Ru(0001); Meinel et al., 1997); the mapping of the evolution

of surface morphology during epitaxial growth (e.g., metal epitaxy;

Chambliss et al, 1995), etching (Boland and Weaver, 1998), and ener-

getic particle bombardment (e.g., electrons on Si; Nakayama and

Weaver, 1999); and the imaging and structural identiﬁ cation of non-

periodic structures such as surface defects, steps, as well as surface-

supported solid (e.g., silicide nanowires on Si; Chen et al., 2000)

and molecular nanostructures [e.g., molecular rotors on Cu(100);

Gimzewski et al., 1998]. Although electrically conducting samples are

a prerequisite for stable tunneling, high-resolution imaging is feasible

on ultrathin insulating ﬁ lms (for a recent review, see Schintke and

Schneider, 2004) and self-assembled organic monolayers [e.g., alkane-

thiol monolayers on Au(111); Cygan et al., 1998) supported by metal

substrates.

In view of the large number of materials systems to which STM has

been applied, a comprehensive survey would be beyond the scope of

this chapter. Instead, we discuss in some detail recent contributions of

STM imaging to one speciﬁ c material: silicon.

2.5.1 STM Imaging—Silicon Surfaces

Due to the important role of Si in electronics, extensive STM studies

on the structure of clean Si surfaces with different orientation have

contributed to making Si one of the best-characterized materials system.

Important early milestones in the characterization of clean Si surfaces

include the determination of surface bonding and reconstructions on

Si(111) (Binnig et al., 1983) and on Si(001) (Tromp et al., 1985) used as

a substrate in microelectronics. STM imaging served to establish the

thermodynamics of terraces and steps (Swartzentruber et al., 1990;

Men et al., 1988) on these surfaces, to study electromigration and step-

bunching (Yang et al., 1996), and to survey the stable facets involved

in the equilibrium shape of Si crystals (Gai et al., 1998).

Figure 15–5 shows an example of constant current STM images on

Si(111), a complex reconstructed surface whose geometric (Binnig and

Figure 15–5. (a) Ball-and-stick model of the Si(111)–(7 × 7) surface. Top: top view; bottom: side view.

a, adatom; r, rest atom; c, corner hole. (b) Filled-state constant-current STM on Si(111)–(7 × 7); V = −1.0 V;

I = 0.4 nA. (c) Empty-state constant-current STM; V = +1.2 V; I = 0.4 nA.

978 P. Sutter

Rohrer, 1983) and electronic (Hamers et al., 1986) structure was solved

in pioneering STM experiments, and which continues to play an impor-

tant role in STM technique development. Figure 15–5a shows a struc-

ture model of the (7 × 7) reconstruction (Takayanagi et al., 1985). Figure

15–5b, obtained at negative sample bias (V = −1.0 V), i.e., with electrons

tunneling from occupied sample states to unoccupied tip states, shows

the corrugation associated with ﬁ lled states of the sample. Conversely,

Figure 15–5c, obtained at positive sample bias (V = +1.2 V), maps the

empty states of the sample. Both images show atomic resolution, clearly

resolving the twelve adatoms (“a”) per unit cell. In addition character-

istic deep “corner holes” (“c”) bounding the diagonals of the 4.6 nm ×

2.9 nm rhombohedral unit cell are imaged.

While all adatoms are mapped uniformly in the empty state image,

the ﬁ lled state scan shows one-half of the unit cell somewhat higher

than the other, an effect on the charge density due to the different

stacking sequence of atomic layers in the two halves of the unit cell. A

comparison of the images obtained at opposite bias polarity suggests

that in addition to surface topography, the electronic structure of the

sample surface adds substantially to the contrast observed in STM

imaging. This is implicit also in Eq. (1) via the dependence of the tun-

neling current on the local densities of states of both tip and sample at

the tunneling contact. Note also that the rest atoms (“r”), a second

near-surface species with dangling bonds protruding into the vacuum,

are imaged neither at positive nor at negative sample bias. A detailed

discussion of bias-dependent imaging and other tunneling spectro-

scopy methods used to assess the local electronic structure of a sample

surface is given in Section 3.

Apart from the structure of clean Si surfaces, Si-based surface chem-

istry has been studied widely by STM. Initial studies provided a knowl-

edge base for technological processes, such as reactive ion etching,

doping, and chemical vapor deposition (CVD). STM was used to probe

interactions of Si with halogens (Boland, 1993) and with small mole-

cules involved in surface passivation (H

, H; Laracuente and Whitman,

2001), doping (PH

; Wang et al., 1994a), and CVD growth (Si

; Wang

et al., 1994b). More recent research directions include the integration

of molecular electronic elements with Si. Under this perspective, the

covalent bonding of a wide variety of organic molecules on Si has been

studied (for a review, see Wolkow, 1999), including small molecules

such as ethylene (Mayne et al., 1993) and acetylene (Li et al., 1997),

simple alkenes (e.g., propylene; Lopinski et al., 1998) and polyenes (e.g.,

1,3-cyclohexadiene; Hovis and Hamers, 1997), pentacene (Kasaya et al,

1998), and benzene (Borovski et al., 1998). On clean Si(001), adsorption

is from the gas phase in UHV. H-passivated Si can also be modiﬁ ed by

ex situ wet-chemical techniques, and reintroduced into UHV for STM

imaging.

Traditionally, an important aspect of the surface science of semicon-

ductors has been epitaxial growth. STM observations at initial growth

stages, i.e., at coverages of fractions of a monolayer (ML) up to several

ML, can provide direct insight into fundamental processes such as

adatom diffusion, incorporation into steps, and nucleation of mono-

Chapter 15 Scanning Tunneling Microscopy in Surface Science 979

layer islands (for a review, see Zhang and Lagally, 1998). Observations

of growth and equilibrium structures allow identifying kinetic and

thermodynamic factors affecting the growth process, as well as the role

of defects, of elastic strain, etc.

STM studies of Si epitaxy are commonly performed in either of two

modes. Conceptually preferred is the dynamic observation, via time-

lapse STM “movies,” of the growth surface with the sample held in the

microscope at high temperature and in the presence of a deposition

ﬂ ux. This difﬁ cult technique will be discussed in more detail in Section

4. As an alternative, STM imaging can be performed at room tempera-

ture on samples that have been quenched at key stages in the growth

process. If measures are taken to verify that the quenched-in morphol-

ogy is indeed representative of that at high temperature during growth,

this “quench-and-look” technique offers the advantage of higher spatial

resolution and larger scan sizes over high-temperature STM.

Early STM experiments focused on the initial stages of Si homoepi-

taxy and Ge/Si heteroepitaxy, mostly on the technologically important

(111) or (001) surfaces. Key results include the identiﬁ cation of regimes

of step-ﬂ ow growth and nucleation, determination of the activation

energy and atomistic pathway of surface diffusion (Mo et al., 1991),

explanation of growth and equilibrium shapes of two-dimensional

(2D) islands (Mo et al., 1989), and exploration of the modiﬁ cation of the

growth process by surfactants (Horn-von Hoegen, 1994). Figure 15–6

illustrates the identiﬁ cation of the initial stage of island formation in

Figure 15–6. STM images of 0.01 monolayers Ge on Si(001). The long diagonal

bands are substrate dimer rows. Filled-state images (A–C; V = −2 V; I = 0.2 nA)

and corresponding empty-state images (D–F; V = +2 V; I = 0.2 nA), showing

rows of symmetric (bean-shaped) and asymmetric (buckled) substrate dimers,

and Ge adatom-induced chain-like structures (marked by arrows). The chain-

like paired adatom structures are seen as metastable precursors to the forma-

tion of monolayer islands in Ge/Si homoepitaxy (Reprinted with permission