Possibility of subsurface investigations by scanning tunnelling microscope

Materials Science and Engineering B51 (1998) 192 – 196

Possibility of subsurface investigations by scanning tunnelling microscope V.Z. Chorniy *, C.J. Adkins Ca6endish Laboratory, Madingley Road, Cambridge CB3 0HE, UK

Abstract Scanning tunnelling microscope (STM) is a highly local probe of topology. It is also sensitive, however, to the three-dimensional electronic structure of the materials. We discuss how these features may be combined to allow the STM to be used for subsurface investigations. In particular, we discuss in detail the STM observations of quantum size effects (QSE). Supplementary techniques concerned with the Schottky barrier, connectivity and crystal orientation mapping are also discussed briefly. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Scanning tunnelling microscope; Scanning tunnelling spectroscopy; Quantum size effects; Tunnelling

1. Introduction Properties of thin metallic films are of great interest for both fundamental and applied science. Over the years an extensive range of surface-sensitive techniques has been established, which allow studies of surfaces with atomic resolution. In particular, the scanning tunnelling microscope (STM) is one of the most sensitive local probes of topology. Unfortunately, possibilities to study the interface between a film and a substrate by STM are far more limited. The only other scanning probe technique yielding microscopic information about sub-surface structures is ballistic electron emission microscopy (BEEM) which only probes certain kinds of buried interface [1] and requires modification of an STM. Therefore it would be very beneficial to extend the sensitivity of an STM to probe subsurface features. In this report we primarily concentrate on QSE but we also describe briefly some other techniques which can be used for such a purpose.

2. Quantum size effect

2.1. Theory In a thin metal film allowed electron energies will be * Corresponding author. 0921-5107/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 1 0 7 ( 9 7 ) 0 0 2 5 8 - 4

split into subbands corresponding to the formation of bulk standing waves (BSW) [2]. If the film has parallel surfaces and the thickness w, the perpendicular wavevector will have descreet allowed values kÞ =np/w and the spacing of the subbands will be given by a bulk dispersion relation. Each subband corresponds to a step in the density of states which results in periodic structure in the tunnelling conductance measured by STM (so their location is indicated by local maxima in the gradient of differential conductance curves s–V) [3–5]. The existence and spacing of these oscillations in principle carry information about film thickness, defects in the film and about the flatness and electronic properties of the interfaces. Similar oscillations can also be produced by single electron effects. These, however, can be distinguished by their characteristic energy spectrum or by moving tip around. For example, these oscillation could exist at a step on a sample’s surface, but BSW could not as will be explained below. This was verified experimentally by artificially creating those oscillations (i.e. an isolated particle was picked up from the surface by the tip to create double capacitance junction and then I–V measurements taken at different locations). There are four possible types of interface between a metal film and a substrate based on the type electron confinement in the well. The first type can occur at interfaces with vacuum, insulator or low-doped semiconductor [6] and corresponds to there being no states beyond the interface into which electrons can tunnel.

V.Z. Chorniy, C.J. Adkins / Materials Science and Engineering B51 (1998) 192–196

The second type is metal-degenerate semiconductor where the Schottky barrier and electronic mismatch provide confinement and the last two types are metal– metal: the third when the metal substrate has a gap in band structure in perpendicular direction in the energy range of interest and the fourth when it does not. In the former case electrons with small parallel momentum have no states to couple into. In the latter case a wave function from the film can couple with the substrate, but its ability to do so depends on the difference in the electronic structures. Here, a film’s wave function is only partially reflected from the interface. Noble metals on ferromagnets is a widely studied example of this group. As a general rule one will observe strong BSW for the first three types of interface and weak BSW for the last [7]. We have studied the first, second and forth kinds. Previously there were limited number of BSW observation by STM in special systems: NiSi2 and CoSi2 films on Si(111) [8], where QSE were used to image defect formation on interfaces, and elegant experiments with a ‘self-assembling’ structure consisting of bubbles implanted in a single-crystal Al film in UHV [9]. If BSW to be used for a general subsurface investigations by STM we have to consider limitations imposed by ordinary films. The films must be clean and defect free to ensure a mean free path greater then the thickness of the film and the upper and lower surfaces must be (atomically) parallel over a relatively large distance. The latter requirement proves to be most stringent, but an STM is a local probe, and such regions can be found. We analysed the problem to determine the degree of flatness (parallelism) required using a simple approximate argument based on an analogy with the treatment of evanescent modes in waveguides below the cut-off frequency. For ordinary metals, the minimum condition for formation of subbands comes out to be D/a\ 2(w/a)1/2

(1)

where D is the distance over which the surfaces of the film have to be atomically parallel, w is the film thickness and a the lattice spacing. For a 10 nm Au film, w/a: 25, and one obtains D \4 nm. If one would like to detect a step on a interface, atomically flat terraces of at least 10 nm extent are needed. Detailed calculations of electronic states near a step have been made by Ho¨rmandinger and Pendry [3] and they obtained similar results.

2.2. Experiment We used our prototype of the Oxford Instruments Mini CryoSTM in a continuous flow cryostat. The experiments were carried out at liquid helium temperature. The STM is not in a UHV environment and

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samples are exposed to atmosphere between preparation and measurement, so we are essentially restricted to using gold to minimise surface contamination. (Cut gold tips were used for the same reason.) The major problem with sample preparation is that the requirements of a large mean free path and the atomic parallelism of upper and lower surfaces over relatively large distances tend to be mutually exclusive. Films deposited on low-temperature substrates are relatively uniform but disordered, whereas films deposited at room temperature or above maybe relatively defect free but first form as island (discontinuous!) films and remain ‘rough’ to large thicknesses. Large atomically flat terraces require even higher temperatures (]300°C), which in turn require even higher film thickness to be continuous. The use of semiconductors or metals as interfaces brings the additional problem of interdiffussion affecting sharpness and reflectivity of an interface. In practice, as a compromise, we deposited at room temperature or  100°C above and sought areas where the thickness was constant over a sufficient area. A further difficulty with gold is that, in common with most fcc metals, the natural growth direction is (111). In this direction, the noble metals have an energy gap at the Fermi level, which for Au extends from 1 eV below EF to 3.6 eV above it. Probing states above 3.5 eV is very difficult, while bias dependency of a tunnelling barrier makes structures below − 1 eV to be seen with reduced sensitivity and can introduce features associated with the electronic structure of the tip. Also additional spectroscopic structure from surface states arises from formation of surface standing waves (SSW) near scattering boundaries or objects [10]. SSW’s will appear in spectra as irregular fluctuations in the energy range in which surface states exist, namely above about EF − 0.4 eV. Their presence was found to be a necessary condition for visibility of BSW states on Au(111) presumably because this indicates a locally good surface. BSW states, on the other hand, should be found at energies below EF − 1 eV (i.e. below the energy gap). Fig. 1 shows an example of structure observed in a 15 nm Au(111) film on glass, i.e. the first type of quantum well interface (films were grown in high vacuum by thermal evaporation onto a substrate at room temperature). The arrows show what we believe to be BSW band edges, indicated, as explained earlier, by local maxima in the gradient of the s–V curves. The first two subbands only are accessed in (a) with only the first remaining visible when the point is moved to a different position (b). This film was continuous but of uneven thickness from the joining of islands during growth. We selected terraces (atomically flat regions) close to the edges of ‘islands’ to find regions of small thickness. The

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Fig. 1. BSW band edges in a 15 nm Au(111) film on glass.

known dispersion relation [4] then allows one to estimate the local thickness. The largest thickness found in this way was 5.9 nm. Fig. 2 shows results for the quantum well interface of the second type obtained on an MBE film of (001) orientation (Au on GaAs(001) grown at 120°C). Here, there is no energy gap, but, as the film was just discontinuous, the resistance of the Schottky diode between an island and the substrate could make a large contribution to the total resistance measured by the STM. The diode accounts for the large non-linearity of the characteristics reflected in the V-shaped background of the s –V curve. In order to calculate the true energies of structures, the measured voltages have to be corrected by subtracting the potential drop across the diode. This correction was made as follows. By reducing the tunnelling gap, the resistance of the tunnelling junction could be made small compared with that of the diode. The measured I – V characteristic is then a good ap-

proximation to that of the diode plus a constant resistance which can be separated by fitting. For measurement of the density of states, the tip is withdrawn so that the tunnel resistance becomes important (the fact that by changing the tip-sample distance one can make tunnelling or Schottky barrier resistance to dominate is also of great importance for the techniques described below). Since the diode characteristic is now known, the voltage drop across it for each value of current can be subtracted from the measured total to obtain the true energies of the observed structures (which should be roughly equally spaced in this region). The marked regions of maximum slope in the s–V curves are then consistent with their derivation from the expected BSW structure. In these experiments, we chose the flat, large terraces on the tops of the islands so that the calculated thicknesses came out greater than the mean film thickness. The largest thickness calculated for the film of Fig. 2 which had a mean thickness of 8 nm, was 15 nm. We did not succeed in observing BSW for the forth type of interface (continuous film consisting of 6 nm Au on 2 nm Fe seed layer grown at 150°C on GaAs(001)). Although the limitations of BSW formation preclude interface imaging, the individual point measurements still in principle carry information about interface flatness, reflectivity, and electron lifetime at various energies. Even more important is that this information could be combined with information derived from a number of other methods, which we shall now describe briefly.

3. Connectivity mapping Conductivity of granular metals has been studied for several decades and is still the subject of current research [11]. These systems undergo metal-insulator transitions as the proportion of metal is varied. Close to the percolation threshold, the degree and nature of connectivity between neighbouring metal islands are important. We succeeded in imaging the connectivity in the discontinuous films of Fig. 2 by exploiting the non-linearity of the I–V characteristics of the Schottky diodes beneath the islands [12]. The topographic image of the film and corresponding connectivity map is shown in Fig. 3(a) and (b). Dark regions are more isolated than lighter ones.

4. Measurements on mesoscopic Schottky diodes

Fig. 2. BSW band edges in an 8 nm Au(001) film on GaAs(001).

We can examine the characteristics of the mesoscopic diodes formed between isolated gold islands and a GaAs substrate. We verified the accuracy of the standard diode equation I= I0[exp(aV)−1] for forward

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Fig. 3. All current-imaging tunnelling spectroscopy (CITS) images are for the same area 500 ×500 nm. (a) Topograph at − 700 mV/1 nA. The vertical range is 9.4 nm. (b) Current map at − 350 mV for − 700 mV/1 nA setting. (c) Current map at −700 mV for − 1500 mV/1 nA setting.

bias and used it in separating the diode and tunnelling resistances as already described. We were able to show qualitatively, by driving the tip into the film, how diode characteristics are sensitive to pressure, highlighting a problem of conventional point-contact probing of mesoscopic structures. At low temperatures, the diodes conduct in the field-emission regime and a depends only on effective mass, relative permittivity and doping concentration in the combination ND/orm* [13]. Measuring the diode characteristics in the STM therefore allows

these quantities to be probed on a nanometre scale. In principle, it should be also possible to measure the capacitance of these small diode structures.

5. Crystal orientation mapping It is believed that tunnelling in STM geometry weights the tunnelling probability towards the surface normal [14]. Thus, when tunnelling at low energies into

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a surface for which there is an energy gap for perpendicular momentum, tunnelling will tend to be suppressed, while at energies close to the band edge, tunnelling currents will rise more rapidly. (This, naturally, also contributes to our ability to detect BSW.) We have explored this idea in measurements on various Au films. Fig. 3(c) shows an orientation map, where (111) surfaces are darker (suppressed current) and not (111), a most of the surface, are lighter. This is a discontinuous film so tip-sample distance was set to make tunnelling resistance dominate. The degree of alignment is in agreement with X-ray data and is commensurate with BSW results. As a final confirmation that the effect is not connected with Schottky diodes we reduced the voltage and observed an additional pattern identical to Fig. 3(b) appear and become superimposed on the pattern of Fig. 3(c). In principle, this method, depending on a particular band structure, could be used to map other orientations or different metals [15]. In experiments of this sort we find that there appears to be a correlation between Au (111) orientation and connectedness — (111) areas tend to be less coupled to the rest of the film. This suggests either different Schottky barrier height or the angle of contact between Au and GaAs(001) for Au grains of different orientation (no correlation observed for (110) though), or, possibly, (111) as a preferred vertical growth direction (i.e. (111) islands tend to grow up instead of sideways).

6. Conclusion Although each of the presented methods has severe limitations, in combination they can provide useful information about properties of the film and interface, some of which could not be obtained in any other way.

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Acknowledgements The authors are grateful to Simon Gray for sample preparation, and acknowledge the technical support of Oxford Instruments and financial support of VZC through the Lundgren Fund.

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