Scanning probe microscopy of ion-irradiated materials

Nuclear Instruments and Methods in Physics Research B 151 (1999) 42±55

www.elsevier.nl/locate/nimb

Scanning probe microscopy of ion-irradiated materials R. Neumann

1

Gesellschaft f ur Schwerionenforschung (GSI), Planckstraûe 1, D-64291 Darmstadt, Germany

Abstract The modi®cations of solids induced by irradiation with energetic ions have been the subject of numerous studies with a large variety of methods, including in particular also microscopy. During the past decade, the techniques of scanning probe microscopy (SPM) opened up a novel access to the characterization of surfaces and interfaces before and after ion-beam exposure. Besides a very high magni®cation, under favourable conditions reaching even atomic resolution, also changes of properties such as surface topography, friction, and hardness became detectable on a nanometer scale. This report, not aiming to cite the complete literature, intends to give a representative overview of the investigations performed with scanning tunneling and scanning force microscopy on a broad spectrum of materials, including semiconductors, semimetals, ceramics, and polymers. The scienti®c goals, the solutions o€ered by SPM as well as the advantages and limits in comparison to other techniques will be addressed. A major aspect is to elucidate that SPM has the task to provide information complementary to the results of the more classical analytical tools. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 07.79.-v; 61.80.Jh

1. Introduction Energetic ions are suitable means for the modi®cation of the surface or bulk structure of solids. Depending on its mass, nuclear charge, and kinetic energy, an ion can create changes only within a thin surface layer, or is able to penetrate far into the bulk and to produce a long and narrow disordered zone along its trajectory. To choose an extreme case, a uranium ion, accelerated to a total kinetic energy of about 2.7 GeV by the Universal Linear

1 Tel.: +49-6159-71-2172; fax: +49-6159-71-2179; e-mail: [email protected]

Accelerator UNILAC of GSI, propagates along the ®rst tens of microns of its path with a velocity of approximately 15% of the velocity of light. The ion loses its energy almost exclusively by interaction with the electrons of the target, successively depositing roughly 30 keV/nm in time intervals of 2 ´ 10ÿ17 s. The ion passage induces very rapidly developing processes that cannot be observed during or immediately after their occurrence. But the properties of the remaining ®nal stage of damage, such as shape, size, and structure, store information about these processes. Depending on the sensitivity of the solid, the degree of disorder can range from point defects to a continuous amorphized zone along the ion path, commonly

0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 1 3 6 - 6

R. Neumann / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 42±55

called latent track. This historic name illustrates the diculty of visualizing ion tracks by means of classical tools. For numerous materials, the latent tracks have been analyzed with a broad variety of techniques: Small-angle neutron and X-ray scattering (SANS and SAXS) and optical spectroscopy extract information on the track size from a large number of tracks, the interpretation of the signal being based on speci®c model assumptions. The outer shape, i.e., the interface between the damage zones of single ion tracks and the intact surroundings can be made visible by precautionary etching of the central part. However, the shape at the surface area where the track starts and the inner track structure are destroyed by this procedure. Transmission electron microscopy (TEM) can image individual tracks with very high resolution, since it is capable of visualizing the atomic columns of a crystalline lattice and nanometersized disordered zones embedded in this lattice. Recently, also surface features of single ion tracks have been visualized by operating a transmission electron microscope in an out-of-focus mode [1,2]. During the past 15 years, the techniques of scanning tunneling and scanning force microscopy (STM and SFM) opened up a novel access to the characterization of surfaces and interfaces before and after ion beam exposure. Besides a very high magni®cation, under favourable conditions reaching even atomic resolution, also changes of properties such as surface topography, friction, and hardness became detectable on a nanometer scale. Together with scanning near-®eld optical microscopy (SNOM), STM and SFM represent the broad ®eld named SPM, including many speci®c techniques derived from the general principle, as for example magnetic force microscopy and scanning ion conductance microscopy. In the recent past, the number of publications dealing with SPM of ion-irradiated materials has grown enormously. Not aiming to cite the complete literature dealing with scanning probe microscopy of ion-irradiated substances, this report intends to give a representative overview of the investigations performed with scanning tunneling and scanning force microscopy on a broad spectrum of materials, including semiconductors, semimetals, ceramics, and polymers.

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The projectile ions used in the works being addressed in the present overview, cover the range of absolute kinetic energies from a few eV up to several GeV. In this broad regime, the primary energy loss of ions in solids is characterized by two fundamental mechanisms: nuclear energy deposition by elastic collisions between the ions and the cores of the target atoms occurs in the low-energy part. Causing damage by atomic displacement cascades, this e€ect becomes increasingly negligible above a few MeV. Already at signi®cantly lower energies, the ions start to transfer energy also to the target electrons by inelastic collisions, thereby exciting and ionizing atoms. This process, named electronic energy loss, is dominating from several 10 MeV upward, and induces the formation of latent tracks. In the following description of the results of numerous di€erent experiments, the absolute or speci®c kinetic energies of the ions are given without commenting which loss mechanism is accordingly e€ective. In many cases, however, conclusions or assumptions made by the authors about the process causing the damage under study will be mentioned explicitly. Most of the SPM measurements were performed in ambient air rather than under ultrahigh-vacuum conditions. One should therefore be aware that ion-induced damage may involve also a change of the chemical reactivity of the surface, causing further modi®cations of the damage when exposing the sample to air. 2. Conductive materials 2.1. Semiconductors A study of ion-irradiated silicon surfaces with STM has been performed already in 1985 [3,4]. Since high ¯uences of argon ions were applied, the damage zones overlapped. The authors observed hillocks of mean diameter 5 nm and mean height 1 nm. This early work has been followed a few years later by an extensive STM investigation of ionbombarded SiO2 /Si interfaces [5] and PbS(0 0 1), PbS(1 0 0), and Si(1 0 0) surfaces [6,7], focusing on the e€ect of single-ion impacts. While the silicon

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R. Neumann / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 42±55

samples were implanted through a 20 nm thick SiO2 layer that had to be removed by HF etching prior to STM imaging, the surfaces of single crystals were irradiated with ions brie¯y after cleavage. The authors registered atomically resolved images showing shallow ion-impact craters surrounded by lattice disorder such as the displacement of lead and sulphure atoms from their regular surface sites. Thorough scanning tunneling microscopy and spectroscopy in situ studies with atomic resolution of Si(1 1 1)±(7 ´ 7) and Si(1 0 0)± (2 ´ 1) surfaces irradiated by 3 keV Ar‡ ions (doses 6 1012 ions/cm2 ) under UHV conditions provided detailed information on surface damage as well as on annealing processes at 750°C [8]. The e€ects of the bombardment of GaAs(1 0 0) surfaces with 500 eV argon ions at ¯uences of about 1013 ions/cm2 were inspected with STM under UHV conditions [9]. Defects with a width  1 nm in the [1 1 0] direction supported the interpretation that single argon ion projectiles simultaneously removed the three arsenic dimers of a (2 ´ 4) unit cell. The irradiation of Si samples with 209 MeV Kr ions normal to the (0 1 0) plane allowed the SFM inspection of e€ects on the polished (1 0 0) surface plane caused by projectiles propagating parallel to the (1 0 0) lattice planes [10]. The authors report the creation of craters and hillocks in correspondence with the ion penetration depth. After bombarding samples of the layered crystal MoS2 with 13.4 MeV/n Au ions, STM imaging revealed craters with a width of about 2 nm and a depth of approximately 1 nm [11]. The authors propose that the material could be used for the detection of fragments from high-energy nucleus± nucleus collisions with a spatial resolution in the 3±5 nm regime. 2.2. Highly oriented pyrolytic graphite A separate chapter of this report is dedicated to STM of ion-irradiated highly oriented pyrolytic graphite (HOPG), since the number of articles dealing with this subject is quite large. One reason for this interest is based on HOPG properties that make it very suitable for STM imaging: HOPG is a layered material, consisting of parallel lattice planes with hexagonal ordering of the carbon at-

oms and exhibiting a high electric and thermal conductivity within these planes. Since only van der Waals forces are acting between neighbouring planes (connected with a much lower conductivity normal to the planes), the crystal can be cleaved easily parallel to the planes by pulling o€ a thin surface sheet by means of adhesive tape. A fresh clean surface provided in this way is atomically ¯at over hundreds of nanometers. It can be imaged by STM in air with atomic resolution, any disorder of the crystal lattice becoming thus very well detectable on a subnanometer scale. Another reason for numerous studies of the behaviour of graphite under irradiation originates from its use as a wall material in nuclear power plants. Radiation damage was analyzed by scanning electron microscopy (SEM), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), Rutherford backscattering (RBS), and electron spin resonance (ESR). Except SEM, these methods do not provide microscopic resolution. First STM measurements were dedicated to HOPG surfaces irradiated by 50 keV argon ions and 20 keV carbon ions at ¯uences up to 1 ´ 1013 ions/cm2 . They revealed hillocks with diameters of a few nanometers and heights of a fraction of a nanometer, embedded in the intact crystal lattice [12±14]. With increasing ¯uence, the surface became more and more corrugated and the STM was no longer able to image with atomic resolution [13]. In the neighbourhood of some hillocks, equidistant parallel lines were found, protruding from the average surface level and enclosing speci®c angles with the lattice vectors [14]. Similar superstructures were reported later also by other groups [15±17]. An STM image of a single heavyion track in HOPG is displayed in Fig. 1. The periodic lines show an orientation of about 30° with respect to the HOPG lattice. Small hillocks have been detected on HOPG surfaces also after irradiation with 15 MeV/n Au24‡ ions [18], and with 530 keV Au‡ and 4.5 MeV Au2‡ ions [17]. A STM analysis of damage produced by argon ions with energies in the range from 0.4 to 3 keV and ¯uences below 2 ´ 1012 ions/cm2 has been reported in [19]. The authors found that each ion caused a small protrusion whose volume increased with decreasing ion energy and incident angle to the

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Fig. 1. STM image (9 ´ 9 nm2 ) of a single heavy-ion track in HOPG [15,57].

surface. After irradiation with 215 MeV Ne ions, clusters (diameters 100±300 nm) of small hillocks (height 3±7 nm), elliptically shaped features (diameter 50±100 nm), each enclosing a crater, and well-ordered superstructures were found on the HOPG surface with STM [20]. An explanation on the basis of knock-on carbon atoms is presented. Further STM studies focused on surface and subsurface defects caused by irradiation at liquid± nitrogen temperature under normal and 75° tilted incidence with 50 keV Ne, Ar, Kr, Au, and Pb ions and with 50±90 and 150 keV Xe ions [21,22]. In the case of normal incidence, the authors found hillocks with various degrees of disorder. They assume that the defects originate from near-surface single carbon atom interstitials or interstitial clusters, in accordance with previous STM results obtained after irradiation of (0 0 0 1) HOPG with 50 eV Ar ions [23]. Under 75° tilted incidence of the projectiles, a sequence of hillocks occurred along the projection line of the ion trajectory, supposedly originating from recoil atoms deposit-

ed between the ®rst two surface lattice planes. Friction force microscopy was employed to image damage zones after bombardment with 3.1 MeV Au, Ag, Cu, and Si ions [24]. The ion tracks being areas of disorder exhibited a higher friction than the intact HOPG lattice. From a comparison of the calculated production rate of recoil atoms by nuclear collisions and the measured density of ion tracks, the authors conclude that the ion-induced damage is related to the occurrence of nuclear energy loss. Craters with rims caused by single-ion impact were detected with STM after irradiation of HOPG by 30±60 keV argon ions [25]. HOPG samples were irradiated also with 215 MeV Ne and 209 MeV Kr ions normal and parallel to the hexagonal lattice planes. Di€erent kinds of damage structures, such as craters and long, straight elevations (length exceeding 3 lm, width several tens of nm, height up to several nm) found by STM were attributed to recoil carbon atoms [26,27]. Another group investigated surface damage induced by 3.1 MeV Au ion irradiation,

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applying both STM and SFM in the noncontact mode under UHV conditions [28]. From SFM imaging the authors concluded that the disordered surface areas were topographically ¯at, but showed an increased friction. On the other hand, the damage zones exhibited a larger tunneling current than the intact lattice when imaged with constant-current STM, and the simultaneously acquired current-voltage characteristic of these regions di€ered from that of the intact HOPG surface. From these ®ndings the authors conclude that the disordered zones, rather than representing protrusions, possess a higher electronic density of states at the Fermi level than the intact surface, corresponding to a semimetal-to-metal transition. 2.3. Conductive materials other than semiconductors and HOPG This section addresses SPM work on some metals and on the classical superconductor NbSe2 . It should be mentioned that the number of studies is very limited, the reason being probably that ioninduced damage in metals is mainly restricted to the nuclear energy loss regime. Sputter damage on Au(1 1 1) and Pt(1 1 1) surfaces caused by 2.5 keV Ne‡ and 600 eV Ar‡ ions, respectively, was investigated with STM, revealing rich triangularand hexagonal-shaped morphological surface structures [29±31]. This work contains many interesting results about the evolution of the surface morphology as a function of ion irradiation and target temperature, taking into account the crystal structure and binding conditions of gold and platinum. The initial stages of sputtering on Au(1 1 1) with 600 and 3 keV Ne‡ ions were observed by STM with resolution of the doublestripe pattern of the (22 ´ 1) Au(1 1 1) reconstruction [32]. Inspite of the extraordinary amount of research focusing on high-Tc superconductors, NbSe2 with a low Tc is still attracting interest as a classical anisotropic model substance for type-II superconductors. NbSe2 crystals can be cleaved easily parallel to their layers, thus providing uncontaminated and atomically ¯at surfaces, suitable for SPM investigations. Latent tracks represent parallel columnar defects that can serve as pinning

centers for magnetic ¯ux lines, thereby limiting energy dissipation and allowing a higher critical current density. Therefore, NbSe2 samples irradiated by energetic heavy ions have been inspected by SFM in air to visualize the tracks [33] and by STM at 3 K to monitor the pinning behaviour [34]. Hillocks with diameters and heights of about 30 and 2 nm, respectively, were found both with STM and SFM on the surface of amorphous Fe85 B15 ribbons after irradiation by 850 MeV Pb and 350 MeV U ions [35]. Due to the much lower electronic energy loss, 750 MeV Kr ions did not cause a detectable surface change. Hollows detected in the neighbourhood of the hillocks were interpreted as indicating the beginning of sample growth. Thin (60 nm) ®lms of Fe (the upper 20 nm in fact consisting of FeO) deposited on Si substrates were irradiated by 243 MeV Au17‡ ions at ¯uences of 1.4 ´ 1013 and 4.6 ´ 1014 ions/cm2 [36]. The iron content as monitored with elastic recoil detection analysis (ERDA) turned out to remain constant. On the other hand, inspection of the surface with SFM before and after irradiation revealed a signi®cant growth of both grain size and roughness. The authors remark that the rearrangement of the atoms must be induced by electronic energy loss, since nuclear energy loss is two orders of magnitude smaller. 3. Insulators 3.1. Mica Similar to HOPG, also the SFM work on the mineral muscovite mica requires a separate section. Muscovite mica is a layered crystalline silicate cleavable easily between the two adjacent SiO4 -hexagonal lattice planes. This substance is very sensitive to irradiation with energetic heavy ions, and has therefore been used widely as a detector material. An overview of the historic development of studies of latent tracks in mica until about 1993 has been given in [37]. Already in the late ®fties and early sixties, tracks of ®ssion fragments in mica have been characterized with TEM [38±40], (see also [41]). But, since ion damage trails in mica (like in many other solids) tend to fade

R. Neumann / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 42±55

under the in¯uence of the TEM electron beam, the method was not applied to mica over about two decades. Only recently, latent ion tracks in mica have been reinspected successfully with TEM [42]. Measurements by means of X-ray [43±45] and neutron scattering [46,47], being sensitive to density changes, resulted in an average track diameter as a function of energy loss. Quite numerous SFM studies have been devoted to ion tracks in mica in order to re-examine the previous ®ndings, but also to exploit the new possibility of imaging individual ion tracks with very high resolution. It should be mentioned that SFM can provide micrographs with full resolution of the hexagonal lattice and was therefore employed also to study its undisturbed structure [48± 50]. Thus, the appearance of the SiO4 plane under SFM inspection is well established. The application of SFM to latent ion tracks in mica was pioneered in [51] (see also [16]), containing the ®rst direct track observation, high-resolution images of surface areas caused by individual ion impacts, a quantitative evaluation of the mean track diameter as a function of energy loss, the ®nding that the track cross sections represent areas of higher friction and of lower hardness, and the statement that the track diameters measured by SFM depend slightly on the loading force between the tip and the sample surface. The dynamic friction coecient inside and outside the latent track zones was investigated quantitatively in air by means of simultaneously recording topographical and lateral-force data. When mearuring with a Si3 N4 force sensor, the coecient was about a factor of 2 larger inside the track than on the intact SiO4 lattice plane [52]. The average diameter of the latent ion tracks as a function of electronic energy loss was extracted from measurements based on lateral-force microscopy, providing a data set [53] that complements the previous SFM results [16,51] and the X-ray and neutron scattering data [43±47]. In order to illustrate the amount of work that has been devoted to the measurement of ion track diameters vs. energy loss in mica, the data sets collected by several groups are summarized in Fig. 2. By modulating the distance and thereby also the force between probe tip and sample surface

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Fig. 2. Diameters of latent ion tracks in mica vs. energy loss: (d) SFM [53,59]; (s) SFM [51]; (+) X-ray and neutron scattering [45,46]; (r) TEM [42].

with the reference frequency of a lock-in ampli®er and by recording a lock-in ®ltered micrograph, the tracks appeared as areas softer than the undamaged lattice [54,55]. Such a force modulation image of Au-ion irradiated mica is shown in Fig. 3. The averaged track diameters obtained by force-modulation and by lateral-force imaging are in good agreement. When scanning a freshly cleaved surface with a bar-shaped force sensor perpendicularly to its long axis, the track cross sections did not appear in topographic images but only in the simultaneously recorded lateral-force micrographs. This means that the cross sections on cleaved surfaces are absolutely ¯at and manifest their existence only by a change in friction. However, small hillocks caused by the impacts of 11.4 MeV/n uranium ions were found on the mica surface that was directly exposed to ion irradiation [56,57]. The hillocks were roughly 2 nm high and had an average diameter of about 20 nm. Further studies of ion tracks in mica focused on the in¯uence of the loading force (that is the normal force acting between probe tip and sample) on the SFM imaging process, con®rming a linear relation between apparent track diameter and loading force [58,59]. The implications of the probe-sample contact area for the SFM imaging process were discussed in more detail within the framework of a generalized Hertzian theory [60,61]. Etch pits of tracks originating from 200 keV Ar ions and from recoiling daughter atoms emitted in alpha decay

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Fig. 3. Force-modulation image of Au-ion irradiated mica (170 ´ 170 nm2 ), acquired by lock-in ®ltered detection of the cantilever de¯ection [55]. The cross sections of ion tracks are areas of reduced surface elasticity as is indicated by the lower brightness. Their slightly elliptical shape originates from creep of the scanner piezo counter to the direction of slow scanning (from top to bottom). The error bars of the average track diameters cover this e€ect.

were studied in detail with SFM [62]. Patterns of 2 nm steps and bifurcations of these steps into pairs of 1 nm steps, re¯ecting the crystal structure, were visualized with very high resolution. Another group of SFM studies dealt with the e€ects of slow, but highly charged heavy ions such as Kr35‡ , Xe44‡ , Xe50‡ , U70‡ , and Th74‡ , impinging on a mica surface [63±65]. The kinetic energy and the potential energy of the ions were of the same order of magnitude, respectively amounting to several hundred keV. The ions were extracted from an electron beam ion trap (EBIT). When approaching the target, these projectiles draw electrons out of the surface and form socalled hollow atoms. During the subsequent penetration into the ®rst few monolayers of the solid, the ions dissipate their potential energy by emitting X-rays and Auger electrons. The authors suppose that the dramatic processes change the charge balance between neighbouring mica layers.

Inspection with SFM of mica surfaces irradiated with ¯uences of about 109 ions/cm2 showed hillocks with a height of a fraction of a nanometer and diameters of tens of nanometers. The authors characterize these defects as blister-like and report a roughly linear dependence of the blister volume on the potential energy of the incident ions [64]. Damage tracks on mica due to single 78.2 MeV iodine ions and 23 MeV C60 ions at di€erent angles of incidence have been studied extensively using tapping-mode SFM [66±68]. Both ion species caused large hillocks on the target surface, the hillocks being accompanied by raised tails in the case of grazing incidence. Also underlying mica surfaces, exposed by cleavage after irradiation, showed hillock-tail features, though less pronounced than on the surface. In some cases, even craters were found on the top of the surface hillocks. The ®nding that MeV C60 ions deposit their

R. Neumann / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 42±55

energy with a higher density than do MeV atomic ions is of particular importance. 3.2. Inorganic insulators other than mica This section is devoted to SPM of further dielectric materials, encompassing quartz, lithium niobate, sapphire, gypsum, and lithium ¯uoride. Subsequent to irradiation of SiO2 -covered Si chips with multiply-charged macromolecular ions (bovine insulin and others, with kinetic energies of about 100±800 keV), hillocks due to single-ion impacts were found by SFM [69]. Impact craters have been imaged by SFM on a thermally grown amorphous SiO2 ®lm after implantation of 40 keV As‡ ions at a ¯uence of about 1011 ions/cm2 [70]. In order to test the hypothesis that collision cascades of the projectile ions have locally increased the density of the a-SiO2 ®lm, a similar study has been performed with crystalline quartz, where a reduction in density due to ion impacts was expected [71]. In fact, asperities rather than craters have been found by SFM on ion bombarded surfaces. The SFM inspection of LiNbO3 single-crystals irradiated with 1.015 GeV tin ions revealed hillocks with a mean height of about 1.5 nm and a mean diameter of about 100 nm [72]. The topography of optically-polished single-crystal sapphire was examined by photon tunneling microscopy before and after implantation of 150 keV silicon, titanium, and chromium ions at ¯uences between 0.3 and 3 ´ 1017 ions/cm2 and at both room temperature and 800°C [73]. A part of the sample surface was masked during implantation, and the mask edge was imaged with the near-®eld optical microscope in order to determine the amount of material removed by sputtering and the change in surface roughness. The surface roughness remained below 3 nm in all cases. It should be mentioned here, that latent tracks in sapphire created by 20 MeV C60 ions were studied recently by RBS and TEM [74]. Comparative studies of ion tracks on crystal surfaces of gypsum (CaSO4 á 2H2 O) were performed using tapping-mode SFM and shadowreplica electron microscopy [75]. The damage trails with sizes of about 10±100 nm were caused by 78.2

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MeV iodine ions impacting at grazing incidence. While both techniques found equivalent dimensions for rim-to-rim crater widths and overall lengths, the defect heights were estimated somewhat larger by tapping-mode SFM than by the shadow-replica technique. For several decades, radiation damage in alkali halide crystals, caused by electrons, neutrons, Xand gamma rays, and also by energetic heavy ions has been investigated by many groups. A large fraction of the work concentrated on lithium ¯uoride which, among all ionic crystals, has the most pronounced ionic binding character and the highest binding energy. LiF single-crystal samples irradiated normal to the (1 0 0) lattice plane by di€erent heavy-ion species with kinetic energies of several tens of MeV to several GeV have been inspected by SFM under ambient conditions [76,77]. SFM intends to complement studies of ion-irradiated LiF performed with optical spectroscopy, SAXS, chemical etching, and surface pro®lometry (see Refs. 12±21 in Ref. [77]). On the original surface exposed to the ion beam, SFM revealed circular damage areas, consisting of small hillocks whose mean diameter depends linearly on the ionenergy loss and increases from about 20 nm for nickel ions to almost 70 nm for uranium projectiles. Two SFM images of a surface section of LiF irradiated with 5 ´ 1010 Au ions/cm2 are given in Fig. 4a and b. Fig. 4a is a topographic micrograph, whereas Fig. 4b represents a map of the lateral force. The diameter of the hillocks as a function of electronic energy loss is plotted in Fig. 5. The hillocks have average heights 0.5±2 nm. Interestingly, the mean diameter (determined by means of optical absorption spectroscopy [78]) of the cylindrical track zone that is dominated by single Fcenters, has the same order of magnitude as the hillock diameters. After cleaving a LiF crystal parallel to the ion trajectories and brief etching of the exposed surface, SFM found ion tracks consisting of a sequence of etch pits, as is illustrated in Fig. 6. 3.3. Organic materials For many years, organic materials have been employed as sensitive ion track detectors, or were

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Fig. 4. SFM images (1.5 ´ 1.5 lm2 ) of a surface section of a LiF single-crystal, irradiated with 5 ´ 1010 Au ions/cm2 . Fig. 4a shows the topography whereas Fig. 4b represents a lateral force micrograph [76].

irradiated by energetic ions, particularly, in order to produce ion track membranes. It is well known from numerous investigations that a heavy ion of

high kinetic energy, when passing through an organic solid, creates a damage trail along its trajectory characterized by severe structural changes.

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Fig. 5. Mean diameters vs. electronic energy loss of ion-induced hillocks on the surface of a LiF single-crystal [77].

These changes include, for example, surface sputtering, scission of polymer chains, production of new bonds and of chemical radicals, and are accompanied by the modi®cation of properties like

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crystallinity, density, etchability, optical transparency, and index of refraction. After irradiation of the polycarbonate LEXAN by highly charged ions whose kinetic energy was of the the same order of magnitude as their potential energy, that means, several hundred keV, SFM revealed blisters on the surface, having heights of about 6±7 nm and base diameters of 40±60 nm [63,65]. SFM imaging of etched ion tracks in polycarbonate and polyimid foils was reported in [79]. Several publications have been devoted to defects (with sizes up to about 100 nm) induced in single-crystals of the aminoacid L-valine by various ion species, including also biological macromolecules (bovine insulin and albumin) [69,80±84]. Craters were produced by irradiation with MeV ions of oxygen, sulphur, bromium, and iodine. In the case of grazing incidence of the projectiles, diameter and depth of the craters were larger than at normal incidence and a raised tail extending along the direction of incidence appeared behind

Fig. 6. Topographic SFM image (3 ´ 3 lm2 ) of etched ion tracks in LiF. Prior to etching, the crystal was cleaved parallel to the ion trajectories [76].

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each crater [80,83]. For 23 MeV C60 ions normally incident on L-valine, the crater volume was more than two orders of magnitude larger than for fast atomic ions. The crater width and length as a function of incident angle and the crater volume and width vs. ion energy loss were studied further in [81,82,84]. The authors considered a pressurepulse model to provide a qualitative description of the observations, but addressed also models assuming a shock wave or a thermal spike ([81,82,85] (see also Refs. therein)). The surface morphology of polyimide samples (with a degree of crystallinity below 10%) irradiated by 90 keV N‡ ions passes through drastic changes depending on the ion ¯uence, as was vividly illustrated by SFM micrographs [86]. At ¯uences from 1 ´ 1015 up to 1 ´ 1017 ions/cm2 an increased roughness develops, with lumps in some cases reaching diameters of several hundred nm, caused by carbonization that results in compactation and formation of carbonaceous clusters. SFM inspection of surface morphology changes of polyethylene caused by 63 keV Ar‡ and 155 keV Xe‡ ions at ¯uences of 1 ´ 1013 to 3 ´ 1015 ions/cm2 , visualized an increased surface roughness at higher ¯uences [87]. Samples of the polycarbonate CR39, a widely used track detector material, were irradiated by 126 MeV oxygen and 209 MeV krypton ions parallel and normal to the surface [88]. The authors found surface trails in the case of parallel irradiation, whose length agrees with the range calculated by means of the TRIM code, and they report the occurrence of craters and hillocks at normal incidence. Further SFM studies focused on ion-induced modi®cations of poly(methyl metacrylate) [89] and of polyethylene, polypropylene, polysterene, and polyimide [90]. The ¯uence-dependent development of polysterene surface structures by irradiation with low-energy Xe‡ ions was also documented with SFM [91]. Craters were observed with SFM on the surfaces of polystyrene, poly(methyl methacrylate), and polyethylene samples after bombardment with 20 MeV Au ions [92]. The authors chose di€erent molecular weights in order to study the in¯uence of the weight on the track size. The shapes of the damage structures are attributed to material ejection and plastic deformation.

4. Summary and conclusions The amount of works applying SPM to ion-irradiated materials is increasing rapidly. A principle reason is that this technique allows the analysis of speci®c surface characteristics not accessible by other microscopy methods. The surfaces under study may have been directly irradiated by ions or were exposed by cleavage of the sample after ion irradiation. Clearly, SPM comes up to limiting factors exceedable by other techniques. So, the main task is to provide complementary information rather than to displace well-established, successful analytical tools. In the following, the merits and limitations of SPM are summarized by recalling a few examples from the previous chapters. One major aspect is the extremely high resolution attainable by SPM. Except scanning near-®eld optical microscopy, light microscopy is not able to resolve structures on a nanometer scale. Even SEM can not resolve surface structures of this size, and is limited to electrically conductive materials, or requires the deposition of a thin conductive layer on the surface under study. To take mica as an example, neither optical microscopy nor SEM are capable of imaging the cross sections of latent tracks. The hillocks on a directly irradiated surface are so ¯at that a conductive layer would completely smear out these tiny protrusions. TEM can also visualize latent tracks in mica, and track diameters in mica have been measured by TEM and SFM with about the same precision. SFM has monitored the track cross sections only on a surface or on an interface exposed by cleavage, whereas the TEM images resulted from transmission of the electron beam through the bulk of a sample with a thickness of the order of a hundred nm. Due to the energy deposited by the electron beam, some change of the track size can not been excluded. On the other hand, track diameters derived from SFM measurements show some dependence on the loading force exerted on the surface by the force cantilever: the apparent track diameter becomes larger with increasing loading force. Besides the distinction between amorphized track areas and the surrounding intact lattice, SFM is sensitive also to di€erences in friction and in elasticity and hardness. Thus, the dynamic friction coecient was found to be higher

R. Neumann / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 42±55

within the track cross sections than on the undisturbed lattice, whereas the elasticity and hardness turned out to be lower inside than outside the tracks. It should be mentioned however, that TEM visualized subtle stress zones symmetrically distributed around the elliptically shaped cross sections of latent ion tracks in germanium sul®de [93]. On freshly cleaved surfaces of ion-irradiated HOPG, STM repeatedly revealed parallel and equidistant straight lines, leading o€ with decreasing amplitude from ion impact zones and enclosing speci®c angles with the lattice vectors. These oscillations, tentatively interpreted as periodic density changes of the two-dimensional electron gas in the hexagonal graphite lattice plane, originating from Bloch±wave scattering at an ion track, were so delicate that they tended to fade already under the in¯uence of the tunneling tip. To my understanding, there would be no chance to discover these sensitive features with any other microscopy method. Notwithstanding the enormous progress that has been made up to now, the application of SPM in the ®eld of ion-induced solid modi®cations is certainly still in the beginning, and many and diverse developments can be conceived. SPM under ultrahigh-vacuum conditions will make it possible to investigate samples prior and subsequent to ion irradiation under very clean conditions, that means, unperturbed by surface contaminations such as thin layers of water or hydrocarbons. Samples like thin ordered layers of organic or inorganic substances could be prepared, irradiated, and imaged in the same UHV chamber. By means of in situ UHV microscopy one can also think of monitoring the time development of ion tracks brie¯y after irradiation. Soft materials, such as, for example, polymer foils can be studied by SFM under liquid, preferably water. In this way, the force between cantilever and sample is reduced signi®cantly below 1 nN, and tip-induced deformations of the sample surface are avoided. Very faint topographic features, undetectable in air can thus be visualized. To conclude the outlook, one can also think of following the process of track etching by successive images acquired by submersing sample surface and cantilever in a suitable etchant.

53

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