Scanning Tunneling Microscopy

9

SCANNING TUNNELING MICROSCOPY M.S.Khaikin Institute of Physical Problems, USSR Academy of Sciences, Moscow A.M.Troyanovskii, V.S.Edelman Institute of Problems of Microelectronics Technology and Superpure Materials, USSR Academy of Sciences, 142432 Chernogolovka, USSR The principles of performance and the known designs of scanning tunneling microscopes (STM) are described. The paper reviews recent works on the STM studies of surface physical properties. Since the advent of the first scanning tunneling microscope (STM) designed by G.Binnig and H.Rohrer [1J tunneling microscopy has been finding ever increasing application in surface physics. This is due to its unique resolution which can approach hundreds of angstroms along the normal to the surface and several angstroms throughout the surface. Tunneling microscopes with a scanned area of tens of microns in size [2,3J have also been devised as well as those combined with a scanning electron ~,5J and scanning Auger microscope [4]. 1. PRINCIPLES OF

S~J PERFOID~~NCE

Let us assume that the sample is a continuous conducting medium, and a metal tip terminating in an atom is taken to the o sample surface for an interatomic distance of about 3 A. A potential difference ~ O. 01-1V applied between the sample and the tip will give rise to a current caused by the tunneling effect. The probability of tunneling is of the order of W~e

- ~ a

,1zmV

(1)

where V is the work fWlction amounting to a few electronvolts, and m ~10-27 g is the conduction electron mass. To estimate the tunneling current assume that the tip carries all the current due to the exponential dependence of W(a), i.e. the tunneling contact

10

area is ~10-16cm2. Taking the conduction electron density as 22 ,v10 cm- 3 and the electron velocity as ~108 cm/s, we obtain the tunneling current value J~1-10 nA which is well measurable. More detailed consideration shows that at low voltages the tunneling current is defined by the electron density of states f(E F , x,y,z), where EF is the Fermi energy for electrons in the sample, x,y are the plane coordinates, z is a coordinate along the normal to the surface [6J. V{hen a conventional method is used to study the tunneling effect in sample-dielectric-metallic electrode structures, the value of z is fixed (this is the dielectric film thickness), and the signal is averaged in the plane over sizes that are by far above the atomic one. As a contrast, for the tunneling microscope averaging occurs only within a single atom. Hence, when the tip scans throughout the surface with a constant surface-sample separation, the tunneling current appears to depend on the x,y coordinates and represents the surface electron structure. A different mode of operation is technically more convenient, namely, maintenance of constant current by a feedback circuit, i.e. constant electron density surface tracing and registration of the z(x,y) dependence. It is evident that the latter represents both the atomic structure and the macroscopic surface relief. 2. STM DESIGN Fig.1 shows the block diagram of the STM operating on the above principle. The tip is fixed on a three-coordinate piezo-

Fig. 1. STM block diagram [63].

11

electric scanner, the s&.Jple is positioned on a platform equipped with a step walker which allows brir~ing the sample to the tip with an accuracy of 1-0.1~m and choosing a region to be studied. An electronic unit controls the x,y piezoelectric scanners. Feedback voltage Vf. b. is applied to the z piezoelectric scanner to stabilize the current flowing through the tip at a given level. 'rhe dependence Vf.b.(x,y)GOz(x,y) can be registered by means of a computer which allows perfo~ling a flexible experlinental procedure, data processing (smoothing, Fourier transformation, etc.), storage and imaging in any convenient form. The STM is equipped with conventional electronics, and the specificity of the instrument is dictated by the design of the piezo-drives for the tip and the s~nple determining the parameters of the instrument. These devices must satisfy stringent and partly conflicting requirement: first, they should provide sufficient displacement at high rigidity for vibration protection, i.e. they should have rather high mechanical resonant frequencies which is also desirable to ensure fast operation. Second, preset displacements should be reproducible and linearly dependent on control voltage. Third, since even in a thermocompensated design local heat sources generate temperature gradients leading to thermal drift, it is essential that the power of control signals is decreased to the utmost. These requirements are most fully satisfied by using piezoelectric ceramic PZT with a righ coefficient of electromechanical energy conversion which may be as high as 40%. The grave disadvantage of piezocer&nics, namely, a pronounced hysteresis and its related nonlinearity (lng. 2), is unfortunately inherent in all

Fig. 2. Displacement - voltage dependences for PZT piezoceramics

[64J .

12

mechanical systems, for instance, electromagnetic or magnetostrictive ones. In principle, at the same mechanical rigidity, magnetic systems allow larger displacements than do piezoelectric systems. However, their performance involves fairly great power dissiFation resulting in heat drift problems. Arnong more than a dozen of described STM designs, but one incorporates a magnetic sweep system[71 which provides scanning within 10~ m with reproducibility of only 10- 5 em. A variety of designs of piezoelectric scanners have been described. Most of them are based on the transverse piezoelectric effect, i.e. elongation in the direction perpendicular to the electric field. Though in this case the piezoelectric coefficient is by a factor of 2-3 smaller than that for the longitudinal effect, sufficient displacement can be achieved at moderate control voltages using comparatively simple elements. There are three popular designs: a combination of three orthogonal piezoelectric bars used in the first works on STM DJ; a tubular element changing its length (z is the coordinate) and bending in two mutually perpendicular directiona (x,y are the coordinates) (5,8,9J; and, finally, a drive based on bDnorph piezoelements [2,3J. In this series the characteristic values of the o

trasformation constant increase from tens of A/V of control voltage (elongation of bars and tubes along the axis) to hundreds (bending o of tubes) and thousands of A/V (bimorphs). Hesonant frequencies decrease from tens of kilohertz to a few kilohertz, respectively. Since suppression of vibrations (with frequencies usually in the range of hertz to a few hundreds of hertz) is proportional to the square of the resonant frequency of the mechanical oscillator, it is clear that, with other conditions being equal, an increase of the scanning area leads to increasing noise. This make3 the construction of a universal STM problematic, and its design should be chosen in accordance with the requirements of the experiment. Operation of a real STM is largely dependent on the solution of a difficult problem, namely, first bringing the tip to the sample for a distance of N ~m (which is a characteristic displacement of the tip by the piezoelectric scanner). The tip must not touch the sample when brought to it. For this reason, purely mechanical systems (such as a micrometer screw [10J, a mechanical displacement transformer [9,11) are not the best designs. The "louse" on piezoceramic elements devised by Binnig and Rohrer [1J

1". )

and other similar designs (for instance, [12]) call for precise manufacturinr; to ensure a reliable electrostatic Clrollping. Electromagnetic (15) and piezocerronic pulse devices [5,14J have also been used. Overall, the problem of bringing the tip to the sample and, especially, choosing a region to be studied cannot be regarded as fully solved. An essential component of the STr.l is the tip. Large-scale o inhomogeneities ( . . . 10-100 A) can be well studied with the aid of tips produced by conventional electrochemical etchine(ECE). Atomic resolution images were obtained by EeE 0,11) tips,with ion etch tips [15, and even with mechanically made tips [1J. SO far, no warranted way of producing a tip has been found. ',mat is more, resolution can vary during operation which may be due to occasional capture (or failure) of microparticles or atoms from the sample surface onto the tip. Detailed information on various S'EM designs is available in IBM Journ. Res. & Develop., iss. 4,5 and Surface Science, 1/2, v.181 containing proceedings on international conferences on STM.

17J

3. RESULTS OBTAINED BY STM The most fascinating results have been obtained from studies of solids with atomic resolution. j,mong the materials for which atomic resolution has been achieved we should mention highly oriented pyrolitic graphite (HOPGj [5,18,20J, (111), (001) and some vicinal planes of silicon C1,11,21-2~, epitaxial germanium on silicon [25], GaAs [26,2V, gold [1,28], lamellar GaSe c omnound s [29J, 2H-'EaSe 2 and 2H- 'EaS 2 pO,3D ' 2H-NbSe 2 [32]. OXidation of nickel [33J, hydrogen sorbtion on nickel ~4Jand diffusion of oxygen atoms on Ni surface [35J were also observed. We should point out surface outlet of charge-density waves in 1T-TaSe 2 and 2H-TaSe 2 [Jo,3D . Ueasurement of the J-V characteristics routinely used in tunneling experiments allowed spectroscopy of surface electronic states in nickel L1], gold, palladium C36], silicon [1, GaAS{27), the phonon spectrum in graphite C37] and measuring local values of the energy gap in superconducting NbJSn [J8J and high-T c superconductors with a perovskite structure [39]. Further progress of the investieation of surface electronic states is connected with the possibility to vary the gap between the sample and the tip. '.rhe ob-

2J,

14

a

.........

l,um

b

Fig. 3. SEM photograph of Nb film structure on Si and,the STM tip brought to the microbridge (a) and STM image of the mlcrobrldge (bl

[46J.

served oscillations of tunneling gap resistance present information on surface state spectra [21,40, 41J • A number of papers on STM are concerned with studies of surface structure with nanometer resolution. In most cases their aim was to demonstrate the new possibilities given by STM. Among them are, for instance, investigation of the dependence of surface structure of silver films on their fabrication technology (42J, studies of silicon crystal surface topography [4JJ and its ion beam modification [44J. Muralt et ale developed a technique of measuring a local voltage drop alongside surface topography to study the current flow in island films [J] and GaAs structure with a p-n junction [45]. STM was applied [46J to observe submicron structures fabricated by photolithography (Fig.J). It was suggested to use STM both for control and microlithography [47,48]. Mention should be made of the works in which surface relief was related to electrophysical properties of samples. Such papers are few yet. Benistant et ale (49] has established a correlation between surface topography of silver single crystals of different orientation and a specular reflection degree of conduction electrons. Golyamina and Troyanovskii [50J have found that the structure of niobium films is bound up with their superconducting properties. It was also shown [51J that low temperature mobility of electrons o in the MOSFET inversion layer is restricted by large-scale ( JOO A) roughness of silicon surface. In addition to the foregoing STM applications, attempts have been made to use these techniques for investigation of surface impurity and defect states, molecules or biological structures

deposited amorphous periments mentioned 4.

sr~M

on solid substrate, Langmuir-Blodgett films, metallic structures, etc. More detailed information on the exalong with some references can be drawn from the journals in section 2 of this paper.

STUDIES OF GRAPHITE AND GALLIUM ARSENIDE

The potentialities of STM and related problems can be illuminated in more detail by studying two materials, namely, HOPG and GWlS. The list should also include silicon, but now it already deserves a separate review that will be beyond the scope of the present paper. Graphite. HOPG has been an object of n~~erous STM studies. It is most suitable for fabrication of atomically flat surfaces ~10J in size by splitting single crystals along the basal planeB~ Such flat areas permitted observation of atomic resolution images both in high vacuum (5,18,29,52] and in air [19,53-57] and even in distilled water [57J. Graphite inertia to atmosphere makes it a convenient test-object. STM images of graphite (0001) surface (Fig.4) reveal a number of unusual properties. On the one hand, the translation s~netry of the image structure consisting of centred hexagons does not conform to that of the honeycomb structure of a graphite monolayer

A

Fig. 4. STM images of (0001) graphite surface areas. The amplitude of corrugations is about 3-4 A.

Hi

(Fig. 5). The period and synunetry of the image indicate that only one from 01 and j3 physically none quivalent atoms is re.vealed in the image (see Fig.5). In principle, the same pattern would be observed if a "large" adsorbed atom were located in the centre of

(a)

--->R

" /' \, ---- - , ;'

(b)

C)::.,\,< \

,r-----·--------·-----...,,

~,

'13' a' ,~,.....--. ~

::i----4

.

J

"'-'--0

Fig. 5.

• •

(a) top and (b) side views on atoms in graphite lattice.

each cell. However, Auger analysis did not reveal foreign atoms on the surface of the sample prepared under ambient pressure and placed into a high vacuum chamber [18,58J. The difference between ot and f!> surface layer atoms is that in the second layer under the 0'- atom there is a carbon atom, whilst under the ~ atom there is none. As a result, the electron density, being almost synunetrically distributed in the vicinity of ~atoms, can be expected to concentrate mainly between t~e layers for dl atoms. This is confirmed by computer calculations WO,59,6~. 'rotal charge density is nearly the same for 01 and f points

p~(x, yl

at z = +3,2 a, u .

p(x,y,E F) at z '" 3,2 a.u .

x ra.u.)

Fig. 6. Maps of tqtal electron density pv(x,y) and graphite surface L20].

p

(EF,x,y) for

17

(Fig.6,a), yet, their density of states at the Fel~i level (EF,x,y) is greatly discriminated (Fig.6,b). Outside the sample the surfaces of the total constant electron density j'(x,y,z)=const. are almost flat: characteristic variations of z were calculated to o equalN 0.2 A which by an order of magnitude less than the observed corrugation. At the srone time for surfaces 0 (EF,x,y,z)=const. o - ,1 J variations of z amount to 1 A [20). Therefore, the experiment confirms that tunneling current is determined by electron density of states at the Fermi level. Another peculiarity is a great value of corrugation measured o as the displacement of the tip. It usually amounts to 2-4 A, but, depending on the experimental conditions, it can be as high as 8 A (Fig.7) [55] and even 30 A [56J. I t is evident that the values exceeding the interlayer distance in graphite cannot correspond to real modulation of the z-coordinate of surface j'(EF,x,y,z)=const. It was assumed [55J that Van der Waals forces between the tip and the sample are responsible for amplification of the apparent corrugation. The field of forces is determined by the total electron density j' (x,y,z) which is weakly dependent on x,y. Hence, when the tip is moved over the surface ~ (EF,x,y,z)=const., at constant

!

2mV

1.7 nA 2.7 nA

43 nA

20mV 1 nA 2.7 nA 7 nA

600mV

10

18 nA 29 nA

35 nA 10

20

(l\)

Fig. 7. Vertical displacement of the tip versus horizontal displacement along graphite surface at different currents and voltages in the tunneling gap [55].

Iii

tunneling current, it falls within areas with different values of f (z) due to variation of z. The force acting on the sample is changed affecting the macroscopic deformation of the sample, i.e. the surface "goes away" from the tip or "approaches" it. As a result, to obtain the wanted gap, the tip should be moved for a greater distance. Evidently, the effect is stronger, the closer, on the average, the tip to the sample. The tunneling gap resistance can be taken as a measure of proxDuity. As seen from Fig.7, the oscillation amplitUde increases with increasing tunneling current and decreasing voltage. The calculations made in [55) showed that, with reasonable assumptions, all the foregoing can account for the o increase of the oscillation amplitude up to ~ 10 A. The obtained force values 10- 9 N are close to the values 10- 8_10- 9 Nestablished by the direct experiment [61]. I t should be noted that the ascertainment of the presence of measurable mechanical forces motivated the development of a new instrument, namely, the atomic force scanning microscope [62J. Though its resolution is worse than that of STM, it is capable of studying not only conducting but any samples. The attempt to extend the above arguments to greater corrugations presents severe problems, since to provide wanted deformation it should be assumed that the tip radius amounts to hundreds of angstroms. Yet, an atomic resolution image requires a radius of no more than several angstroms. It has been suggested [56] that during operation a contamination layer (for instance, condensed moisture) is formed between the tip and the sample. (It can be easily estimated that at room temperature and normal moisture water is in equilo ibrium with steam in the gap 10-20 A thick) It is through this layer that the mechanical force is transferred. This mechanism is supported by the fact that evacuation to high vacuum and outgassing heating of the tip and the sample eliminate amplification of corrugation [56J. However, no complete clarity has yet been achieved, since the observed peCUliarities may be different in nature. On atomic scale there is a problem of the tip structure. Generally, one has to start from the only consideration that with atomic resolution of the image the tip may terminate in a single atom. Yet, in most cases, instead of the image shown in Fig.4,a, we observe images similar to those in Fig.4,b with the symnletry of spots being lower than the symmetry of their location. According to [20j, this may be

19

caused by surface defects shifting the upper atomic layer with respect to the lower one. But it will be more natural to suggest that the tip is not symmetrical and, what is more, it may be changed during the experiment when occasionally touching the sample. 1'his asswnption was confirmed in (37J. The work is concerned with the nonlinearity of volt-ampere characteristics of the tunneling current between a tungsten tip and HOPG at heliwll temperatures. The plot of d 2I/dU2 exhibited peaks corresponding to generation of phonons with knovm energies. Once the tip touched the surface, the tungsten phonon pecl(s were found to disappear, i. e. the tunneling gap was formed by a graphitegraphite pair by the transfer of a graphite microparticle onto the tip. In connection with the foregoing, it might be well to point out that the problem of a precise description of the tip on atomic scale or at least its guaranteed invariability is most vital for further development of scanning tunneling microscopy. Galliwn arsenide. The investigation of GaAs is of particular interest in view of the wide technological application of the semiconductor and its multilayer structures. Since a typical layer width in such structures is from tens to hundreds of angstroms, STM is an appropriate instrument for the studies. Yet, measurement of mUltilayer structure topography does not permit of revealing areas of different composition and the type of conductivity (Fig.8), for the topography does not display any distinguishing features.

100 I HA

i

3

i

-2 FiS· S. Topooram of cleaved GaAs multilayer structure. Fig. 9. J-V characteristics of the vacuum tunneling gaD for n-GaAs (1), n-AlGaAs (2), p-GaAs (3). .

~o

The use of 31'J',1 in the po t errt Lome t e i- mode [3] makes it possible to localize the p-n junction as a region where the current flowing alone the surface causes the voltage to drop [45J. More infonnation is provided by a method based on the difference between J-V characteristics for various regions (Fig.9). The characteristics were measured using the technique sugGested in [22J: at a fixed voltage between the tip and the sample the feedback system stabilized the current and thus the tunneling gap. For a short time the feeback waS off and, while the gap could not change quickly, the current J was measured at some voltage V. Then the initial state was restored and upon the next break of the feedback the current waS measured at a different voltage, etc. Without accounting for the mechanism of the formation of J-V characteristics, it should be emphasized that they are rather different for various regions. This enables the boundaries between them to be revealed. With this in view, the relief should be scaxmed along the coordinate when the voltage conforms to a tunneling current which is almost the same for all the regions (for instance, 1 V). At the instWlts of feedback break the voltage should be such that the current values are strongly distinguished. For example, to localize the bOillldaries of the p-region the voltage should be ~ 1 V (Fig.10). Application of this technique in STM imaging with atomic resolution allows revealing spatial distribution of electron den-

Fiq. 10. Z as a function of x for cleaved GaAs-based structure at the sample-tip voltage of 1.1 V (al and the tunnel current at 2.0 V (bl. Sharp increase of current corresponds to crossinq of the p-n junction.

Fig. 11. Constant current 3TH images acquired at sample voltages of (a) +1.9 and (b) -1.9 v. The surface height is given by a grey scale, ranging from 0 (black) to (a) 0.83 and (b) 0.65 A (white). (c) Top Vlew of the surface atoms. As atoms are represented by ODen clrcles and Ga atoms by closed circles. The rectanael indicates a unlt cell, whose position is the same in all three'figures [27J.

sity for different energy band states. It was used to study silicon [1,22J and Gru\s [27). For GaAs it was found that the maximum electron density of the valence band is shifted with reference o to that of the conduction band by 2.10 A (Fig.11). Comparison of this result with the calculated electron structure eave rise to a realistic model of (110) GaAs buckling surface. In surnrnary, STM, even as it is uae d now, has furnished fascinating results and, undoubtedly, holds the greatest promise for surface physics.

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