- Email: [email protected]
Ion-beam-induced surface modification and nanostructuring of AIIIBV semiconductors
Progress in Surface Science 74 (2003) 331–341 www.elsevier.com/locate/progsurf
Ion-beam-induced surface modification and nanostructuring of AIIIBV semiconductors M. Szymonski a
a,b,*
, F. Krok b, P. Struski a, J. Kolodziej a, B. Such a
Marian Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krak ow, Poland b Regional Laboratory for Physicochemical Analyses and Structural Research, Jagiellonian University, Ingardena 3, 30-060 Krak ow, Poland This paper is dedicated to the memory of late Professor Maria Steßslicka, a dear colleague and friend, a great scientist, and the charming Lady of Polish Physics
Abstract Ion-beam-induced modification of InSb (0 0 1) semiconductor surface has been studied by means of atomic force microscopy and Kelvin probe force microscopy. It was found that nonstoichiometric sputtering of the compound surface and beam enhanced surface diffusion led to unusual development of surface structures in the form of dots and wires with nanometer scale dimensions. The shape, the size and the surface density of nanostructures were investigated as a function of the beam flux and fluence, and as a function of the crystal orientation with respect to the ion-beam direction. The mechanisms involved in the surface modification were compared with results of Monte-Carlo computer simulations of anisotropic surface diffusion. 2003 Elsevier Ltd. All rights reserved. PACS: 61.16C; 61.72V Keywords: Ion-induced surface modification; Ion-enhanced surface diffusion; Surface nanostructuring; AIII BV semiconductor surfaces; Dynamic force microscopy; Kelvin probe force spectroscopy
1. Introduction Surfaces structured in nanometer-scale are manufactured for various important applications in contemporary electronics, materials science, molecular biology,
*
Corresponding author. Tel.: +48-12-6324888x5560/221033/142; fax: +48-12-6337086. E-mail address: [email protected] (M. Szymonski).
0079-6816/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsurf.2003.08.026
332
M. Szymonski et al. / Progress in Surface Science 74 (2003) 331–341
medicine, and other areas: e.g., low-dimensional mezoscopic systems (dots, wires) with unique ‘‘quantum’’ properties, atom aggregates for studying various phenomena, such as surface magnetism, catalysis, etc., and templates for growing or assembling nanostructures. The surfaces nanostructured by self-assembling of the deposited material, are usually presented in the literature as examples. However, another way of nanostructure manufacturing has been developed recently, i.e., using charge-particle beams for solid surface modification and nanostructuring [1]. Similarly, possibilities of surface nanostructuring by inelastic processes, induced by electrons or photons, have also been widely discussed [2]. Modification of materials by electronic excitation is becoming increasingly attractive, due to recent advances in laser and fiber optic technologies, as well as the wide availability of highly selective sources of radiation. Probing of solid surfaces with low-energy electrons and photons provide a new opportunity for the investigation of excited states at the mesoscopic level. For wide-band-gap materials, several electronic transitions from such localized excited states can lead to production of defects and emission of particles from a very thin layer in the surface region, which the new scanning-probe microscopies (AFM, DFM) can investigate. In the case of ion-bombarded solids, the main processes involved in the process of nanostructuring are: surface sputtering, mass-transport processes due to momentum transfer in the collision cascade, and diffusion (thermal, as well as beam-enhanced) on the irradiated surface. Typically, off-normal ion sputtering of amorphous solids causes development of ripples on the bombarded surface [3–5]. At grazing incidence, the ripples are oriented along the incidence beam direction, whereas at close to normal incidence angle the ripples are perpendicular to the beam. The first attempt to explain the mechanism of the ripple formation was based on Sigmund’s theory of sputtering [6]. Since the ion sputtering yield is determined by the surface density of the deposited energy, Bradley and Harper pointed out that the yield for trough-like regions will be higher than for the peak-like regions on a rough surface (see Fig. 1). Consequently, the surface front will be unstable and its roughness will be increased during continuous bombardment (troughs getting deeper and peaks getting higher and sharper on the relative scale of the moving surface front), unless some fast diffusion processes reverse the trend in the ion-beam morphology development. A stochastic nonlinear equation can be used to describe the evolution of the sputtering front represented by the local height function hðx; y; tÞ [7]: ohðx; y; tÞ ¼ vðhÞ þ mðhÞr2 hðx; yÞ Dr2 ðr2 hðx; yÞÞ þ g; ot
ð1Þ
where vðhÞ is the average erosion velocity of the surface, mðhÞ the negative surface tension, D the surface diffusion constant, and g the white-noise factor. It is believed that the competition between roughening by ion sputtering and smoothing by surface diffusion results in formation of the ripple morphology at the bombarded surface [1–6]. Two types of surface structures have been reported so far for AIII BV compound semiconductor surfaces subjected to ion bombardment. For normal incidence, either randomly distributed dots of nanometer dimensions, or filament-type structures were
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Fig. 1. Schematic illustration of ion beam enhanced roughening processes at bombarded surfaces.
observed [8–10], whereas, for oblique incidence, periodic height modulations (ripples) were observed [11,12]. In our recent studies [13,14], wire-like structures with the diameter of a few tens of nanometer were obtained at particular experimental conditions. In this paper, we report on some recent experimental findings obtained in our laboratory, which could exemplify the many aspects of beam-induced surface modification at the nanometer scale. In particular, heavy-ion bombardment of InSb compound semiconductor leading to nanowire and dot formation on the irradiated surface are presented.
2. Experimental A schematic geometry of the experiment with beam bombardment of InSb (0 0 1) surface is shown in Fig. 2. A detailed description of the UHV experimental system was given elsewhere [14]. Typically, an irradiation with 4 keV Arþ was performed at incidence angle of 50 off normal with the fluence between 1 · 1014 and 6 · 1016 ions/ cm2 and the ion flux in the range of 0.3–2.10 · 1015 ions/cm2 s. The preparatory work for obtaining clean and well-ordered InSb(0 0 1) c(8 · 2) surface involved several cycles of low energy (700 eV) off normal (±60) Arþ bombardment at a temperature of 700 K and annealing to 750 K for a few hours until a clear c(8 · 2) LEED pattern was obtained. For imaging of freshly prepared and beam-modified surfaces a dynamic (non-contact) atomic force microscopy (DFM) was performed using Park Scientific Instruments’ VP2 AFM/STM microscope equipped with NanoSurf easyPLL demodulator. All data were taken at room temperature a few hours after ionprocessing was performed. An atomic resolution image of the unmodified InSb surface is shown in Fig. 3. The freshly prepared surface consists of large, atomically flat terraces with edges oriented preferentially along h1 1 0i and h1 1 0i crystallographic directions. The atomic rows on the surface run along the h1 1 0i direction [15]. Interestingly, the
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Fig. 2. Schematic geometry of experiment for ion-beam modification of surfaces.
Fig. 3. 10 · 10 nm2 atomically resolved DFM image of freshly prepared, c(8 · 2) reconstructed surface of InSb(0 0 1).
surface crystallinity is preserved during the initial stages of ion-sputtering, until the fluence of 1016 ions/cm2 is achieved. Consequently, the features seen as a result of ion bombardment with the low fluence are dependent on the surface orientation with the respect to the ion beam. Some examples are discussed below.
3. Results and discussion In Fig. 4, a set of images of the ion-bombarded sample for four different ion fluences is shown. It is seen that for fluences below 1 · 1015 ions/cm2 , the surface is flat and only sparingly covered with white dots indicating points of higher elevation. Most likely, they represent In agglomerates assembled on the surface, due to nonstoichiometric surface erosion by ion bombardment. Above the dose of 1 · 1015 ions/ cm2 , the surface is randomly covered with elongated (wire-like) structures. The width
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Fig. 4. Set of AFM images of ion bombarded InSb (0 0 1) with various fluences. Ion-beam energy was 4 keV and the flux was equal to 2.15 · 1015 ions/cm2 s.
of the structures is of the order of a few tens of nanometers, and have several nanometers in height and their length is flux- and fluence-dependent. If the total fluence of the ion beam approaches 1016 ions/cm2 range, the lengths of the wires is of the order of a few microns. Furthermore, it was found that wire formation on the surface is strongly dependent on the ion flux. For fluxes lower than 1 · 1015 ions/cm2 s, dot-like structures are preferentially formed, with a diameter of the order of several tens of nanometers. For fluxes higher than 1015 ions/cm2 s, wires start to develop with increasing surface density, until the saturation density is obtained [16]. We studied the dependence of the nanowire formation process on the crystal orientation with respect to the ion beam. The results are shown in Fig. 5, where a set of AFM images obtained for different surface orientations is shown. Since, from our high resolution DFM measurements, we know precisely the orientation of the surface reconstruction rows (see Fig. 3), it can be concluded that the wires are formed along the direction perpendicular to the reconstruction rows. If the ion-beam direction is perpendicular to the rows, the parallel wires cover the surface with high density. If the beam projection is parallel to the rows, then only a few relatively short wires are seen with random orientation. The dominant features, in the latter case, are dots. At first glance, this result is surprising, since one would expect that the diffusion of adparticles at the reconstructed surface is easier along the rows than across them.
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Fig. 5. Set of AFM images of ion bombarded InSb (0 0 1) for various angles between ion-beam projection and sample crystallographic directions.
One should remember, however, that efficient, thermally assisted migration is drawing the adparticles away from the wire nucleation sites, thus degradating the wires rather than developing them. The role of anisotropic diffusion in the nucleation process of the wires is discussed in the next section. In order to learn more about the chemical composition of the surface structures formed on an ion-bombarded sample, we need a high-resolution chemical probe. A newly developed technique––Kelvin-probe force microscopy––offers means of finding the chemical sensitivity via surface contact potential difference measurements. We have recently introduced this technique in our laboratory [14], and in Fig. 6 a Kelvin-probe image of InSb surface subjected to ion bombardment is shown together with its topography image. The topography image shows a dot-like structure of several tens of nm developed on the surface after ion bombardment with the fluence of 2.5 · 1015 ions/cm2 . A high contrast Kelvin-probe image of the same structure indicates that the dot is made out of material which has clearly a different work function in comparison to the surrounding area of stoichiometric InSb. For comparison, the Kelvin probe force spectroscopy on a polycrystalline Au film grown on mica has been performed with the same tip. Knowing that for polycrystalline Au the work function is equal to 5.1 eV [17], the system has been calibrated for absolute work function measurements. It was determined that the work function of the ion bombarded (0 0 1) InSb was equal to 4.6 eV, and of the dot material 4.3 eV. This
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Fig. 6. DFM topography and KPFM image of dot-like structure. Cross-section represents contact potential difference along marked line.
could be compared with the elemental electron work function for In, equal to 4.12 eV and the InSb value of 4.2 eV [18]. In the last case, the structure and composition of the investigated InSb surface were not known precisely. Therefore, the value of work function for the ion bombarded (0 0 1) InSb, determined in our experiment, should not be expected to be the same as the one from reference [18]. Although the technique is unable to identify the elemental composition of the dot, there are several indications that it is made out of In or at least from a material highly enriched in In [19,20]. It is widely known that ion sputtering of AIII BV compounds is highly non-stoichiometric and the BV component is preferentially ejected [21,22]. Subsequently, surfaces enriched in AIII component are produced. Due to surface diffusion, the metal adatoms can agglomerate to form of droplets seen in both our topography and our Kelvin-probe images. Further studies of the wire-covered surfaces with Kelvin-probe force spectroscopy revealed that the work function of the wires is very close to that of the dot-like structures, indicating their similar chemical composition.
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4. Computer simulations with Monte-Carlo code In order to obtain more insight into the surface diffusion occurring on ionbombarded surface simple computer simulations of such processes were performed with the Monte-Carlo method. The following assumptions were made: 1. The InSb surface was represented by a 100 · 100 square-lattice model with cyclic boundary conditions. Surface reconstruction was not taken into account; 2. Beam-induced sputtering is assumed to be non-stoichiometric producing an In enriched surface. Adparticles remaining at the surface are migrating due to the beam-enhanced diffusion. Consequently, the direction of the primary ion momentum is assumed to be preferential for the surface hopping diffusion. The following steps were used in the simulation procedure: 1. Random number generation was used for surface adatom selection. 2. The hopping direction was selected with various preferences (from 1:3 to 20:1) for direction along the beam incidence. 3. The energy difference between the current and the neighbouring site for the selected adatom was calculated as proportional to the difference of coordination numbers for two nearest sites: DEk ¼ lk1 lk2 . 4. The hopping probability Pk was then calculated as follows: if DEk 6 0; or DEk P 0;
Pk ¼ 1; Pk ¼ ecDEk ; c ¼ 1=kT ;
the Boltzmann factor c was used as a simulation parameter. 5. The effect of the Ehrlich–Schwoebbel (E–S) barrier [23,24] on the hopping probability was taken into account. This barrier is known to occur where the diffusing particle on surface is jumping across the terrace steps, descending from the higher level or climbing up to the higher atomic plane. Such an energy difference was ta2 ken as equal to: DEE–S ¼ eaðh1 h2 Þ 1, where Dh ¼ h1 h2 is the terrace level difference and the a is taken as a simulation parameter. The corresponding hopping probability ‘‘correction’’ for the E–S barrier was calculated as P ¼ Pk ecDEE–S . 6. The decision about transferring the surface adatom to the site chosen in step 2 was taken, according to the calculated probability P . The points 1–6 were executed several times (40 jumps in the example shown below), then a new adatom was generated on a randomly selected free site of the surface matrix. In Fig. 7, the examples of such simulations are shown for three various values of the E–S barrier and a high preference for migration along the direction of the beam incidence (16:1). The results show that wires are simply formed due to anisotropic diffusion of adparticles on the surface with a preference rate for directions along the primary beam incidence starting from 2:1. Since it is known [25] that a fully developed collision cascade induced by 4 keV Ar ion bombardment
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Fig. 7. Results of surface diffusion simulations with Monte-Carlo code for different values of parameter a and total number of jumps equal to 100 000.
should be isotropic, we have to assume that the anisotropy of the diffusion is caused by the initial stages of the primary ion interaction with the adparticles. Primary knock-on particles created at this stage are known to conserve the direction of the ion momentum [25] and they should be considered as primary activation energy source for adparticle hoping leading to wire formation. The second aspect of the simulation result is that the E–S barrier seems to play an important role in wire formation. Lack of the barrier against diffusion across the step edges would result in the piling-up of the adparticle material in dot-like structures and/or fast dissipation of the wire like structures at the very early stage of their formation. In the simple model we used for the simulation, we were not able to monitor the effect of the surface reconstruction rows on the adparticle diffusion and the wire formation process. Our experimental results, however, imply that this should be still another important factor causing anisotropy in the surface diffusion of adparticles, at least in the early stage of ion bombardment.
5. Concluding remarks Beam-induced surface modification and nanostructuring of compound semiconductors involve the interplay of various surface processes dependent on particular experimental conditions, such as the sample temperature, its crystallinity, ion-beam current density and the angle of incidence of the primary ions. In fact, there are extremely interesting problems to be studied at atomic level by high-resolution scanning-probe microscopies. Some of the above discussed mechanisms occurring in compound solids are different from those in the elemental solids, where the ripple formation is the most visible mode of beam induced surface modification [1]. A primary reason for this difference is the non-stoichiometric sputtering of the compound surface, due to different thermodynamic properties of the components, such as their vapour pressure, surface tension, surface free energy, etc. The surface adparticles created by this that process can diffuse and nucleate with other adparticles to form small clusters subjected to further ion bombardment. It is unlikely, however, that wire-type structures could be developed primarily, due to non-uniform sputter erosion of the clusters related to sputter yield dependence on the angle of incidence.
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Our results strongly indicate that anisotropic diffusion of adparticles, activated by the primary ions and/or primary knock-on atoms carrying a fraction of the initial beam momentum, is responsible for the formation of wire-like structures on the modified surface. It has been found recently that surface modification of AIII –BV semiconductors can also take place due to electronic transitions. Weaver et al. [26] reported STM images of (1 1 0) GaAs surface subjected to electron bombardment with various fluences. It was demonstrated that electron bombardment causes surface erosion and one might expect this process to be non-stoichiometric. It would be interesting to explore whether surface structuring and self-organization could occur on compound semiconductor surfaces, due solely to inelastic processes.
Acknowledgements The experimental results presented in this paper were obtained as a part of research project supported by Polish Research Council (KBN) under the contract no. 2 P03B 065 23 and by the research grant of the Institute of Physics, Jagiellonian University.
References [1] F.B. de Mongeot, C. Boragno, U. Valbusa, in: M. Kortla et al. (Eds.), Atomistic Aspects of Epitaxial Growth, Kluwer Academic Publishers, Amsterdam, 2002, p. 221. [2] N. Itoh, A.M. Stoneham, Materials Modification by Electronic Excitations, Cambridge University Press, 2001. [3] S. Rusponi, G. Constantini, C. Boragno, U. Valbusa, Phys. Rev. Lett. 81 (1998) 2735. [4] S. Rusponi, G. Constantini, C. Boragno, U. Valbusa, Phys. Rev. Lett. 81 (1998) 4184. [5] S. Rusponi, G. Constantini, F.B. de Mongeot, C. Boragno, U. Valbusa, Appl. Phys. Lett. 75 (1999) 3318. [6] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol. A 6 (1988) 2390. [7] R. Cuerno, A.L. Barabasi, Phys. Rev. Lett. 74 (1995) 4746. [8] S. Facsko, T. Bobek, H. Kurz, S. Kyrsta, R. Cremer, Appl. Phys. Lett. 80 (2002) 130. [9] S. Facsko, T. Bobek, T. Dekorsy, H. Kurz, Phys. Stat. Sol. 224 (2001) 537. [10] M.P. Seah, S.J. Spencer, P.J. Cumpson, J.E. Johnstone, Surf. Interf. Anal. 29 (2000) 782. [11] F.A. Stevie, P.M. Kahora, D.S. Simons, P. Chi, J. Vac. Sci. Technol. A 6 (1998) 76. [12] C.M. Demanet, J.B. Malherbe, N.G. van der Berg, V. Sankar, Surf. Interf. Anal. 23 (1995) 433. [13] F. Krok, J.J. Kolodziej, B. Such, P. Piatkowski, M. Szymonski, R.Q. Odendaal, J.B. Malherbe, Opt. Appl. 32 (2002) 221. [14] F. Krok, J.J. Kolodziej, B. Such, P. Piatkowski, M. Szymonski, Appl. Surf. Sci. 210 (2003) 112. [15] J.J. Kolodziej, B. Such, M. Szymonski, F. Krok, Phys. Rev. Lett. 90 (2003) 226101. [16] F. Krok, J.J. Kolodziej, B. Such, P. Piatkowski, M. Szymonski, Nucl. Instr. Meth. B, in press. [17] D.E. Estman, Phys. Rev. B 2 (1970) 1. [18] B. Seroczynska, Acta Phys. Pol. A 52 (1977) 893. [19] M. Bouslama, C. Jardin, M. Ghamnia, Vacuum 46 (1995) 146. [20] M.O. Schweitzer, F.M. Leibsle, T.S. Jones, C.F. McConville, N.V. Richardson, Surf. Sci. 280 (1993) 63.
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[21] G.E. McGuire, M.G. Yang, Electrochemical Society Fall Meeting, Electrochem. Soc, Princeton, 1975, p. 247. [22] J.B. Malherbe, W.O. Bernard, I. Le, R. Strydom, C.W. Louw, Surf. Interf. Anal. 18 (1992) 491. [23] G. Ehrlich, F.G. Hudda, J. Chem. Phys. 44 (1966) 1039. [24] R.L. Schwoebel, J. Appl. Phys. 40 (1969) 614. [25] P. Sigmund, Rev. Roum. Phys. 17 (1972) 1079. [26] B.Y. Han, K. Nakayama, J.H. Weaver, Phys. Rev. B 60 (1999) 13846.
Ion-beam-induced surface modification and nanostructuring of AIIIBV semiconductors M. Szymonski a
a,b,*
, F. Krok b, P. Struski a, J. Kolodziej a, B. Such a
Marian Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krak ow, Poland b Regional Laboratory for Physicochemical Analyses and Structural Research, Jagiellonian University, Ingardena 3, 30-060 Krak ow, Poland This paper is dedicated to the memory of late Professor Maria Steßslicka, a dear colleague and friend, a great scientist, and the charming Lady of Polish Physics
Abstract Ion-beam-induced modification of InSb (0 0 1) semiconductor surface has been studied by means of atomic force microscopy and Kelvin probe force microscopy. It was found that nonstoichiometric sputtering of the compound surface and beam enhanced surface diffusion led to unusual development of surface structures in the form of dots and wires with nanometer scale dimensions. The shape, the size and the surface density of nanostructures were investigated as a function of the beam flux and fluence, and as a function of the crystal orientation with respect to the ion-beam direction. The mechanisms involved in the surface modification were compared with results of Monte-Carlo computer simulations of anisotropic surface diffusion. 2003 Elsevier Ltd. All rights reserved. PACS: 61.16C; 61.72V Keywords: Ion-induced surface modification; Ion-enhanced surface diffusion; Surface nanostructuring; AIII BV semiconductor surfaces; Dynamic force microscopy; Kelvin probe force spectroscopy
1. Introduction Surfaces structured in nanometer-scale are manufactured for various important applications in contemporary electronics, materials science, molecular biology,
*
Corresponding author. Tel.: +48-12-6324888x5560/221033/142; fax: +48-12-6337086. E-mail address: [email protected] (M. Szymonski).
0079-6816/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsurf.2003.08.026
332
M. Szymonski et al. / Progress in Surface Science 74 (2003) 331–341
medicine, and other areas: e.g., low-dimensional mezoscopic systems (dots, wires) with unique ‘‘quantum’’ properties, atom aggregates for studying various phenomena, such as surface magnetism, catalysis, etc., and templates for growing or assembling nanostructures. The surfaces nanostructured by self-assembling of the deposited material, are usually presented in the literature as examples. However, another way of nanostructure manufacturing has been developed recently, i.e., using charge-particle beams for solid surface modification and nanostructuring [1]. Similarly, possibilities of surface nanostructuring by inelastic processes, induced by electrons or photons, have also been widely discussed [2]. Modification of materials by electronic excitation is becoming increasingly attractive, due to recent advances in laser and fiber optic technologies, as well as the wide availability of highly selective sources of radiation. Probing of solid surfaces with low-energy electrons and photons provide a new opportunity for the investigation of excited states at the mesoscopic level. For wide-band-gap materials, several electronic transitions from such localized excited states can lead to production of defects and emission of particles from a very thin layer in the surface region, which the new scanning-probe microscopies (AFM, DFM) can investigate. In the case of ion-bombarded solids, the main processes involved in the process of nanostructuring are: surface sputtering, mass-transport processes due to momentum transfer in the collision cascade, and diffusion (thermal, as well as beam-enhanced) on the irradiated surface. Typically, off-normal ion sputtering of amorphous solids causes development of ripples on the bombarded surface [3–5]. At grazing incidence, the ripples are oriented along the incidence beam direction, whereas at close to normal incidence angle the ripples are perpendicular to the beam. The first attempt to explain the mechanism of the ripple formation was based on Sigmund’s theory of sputtering [6]. Since the ion sputtering yield is determined by the surface density of the deposited energy, Bradley and Harper pointed out that the yield for trough-like regions will be higher than for the peak-like regions on a rough surface (see Fig. 1). Consequently, the surface front will be unstable and its roughness will be increased during continuous bombardment (troughs getting deeper and peaks getting higher and sharper on the relative scale of the moving surface front), unless some fast diffusion processes reverse the trend in the ion-beam morphology development. A stochastic nonlinear equation can be used to describe the evolution of the sputtering front represented by the local height function hðx; y; tÞ [7]: ohðx; y; tÞ ¼ vðhÞ þ mðhÞr2 hðx; yÞ Dr2 ðr2 hðx; yÞÞ þ g; ot
ð1Þ
where vðhÞ is the average erosion velocity of the surface, mðhÞ the negative surface tension, D the surface diffusion constant, and g the white-noise factor. It is believed that the competition between roughening by ion sputtering and smoothing by surface diffusion results in formation of the ripple morphology at the bombarded surface [1–6]. Two types of surface structures have been reported so far for AIII BV compound semiconductor surfaces subjected to ion bombardment. For normal incidence, either randomly distributed dots of nanometer dimensions, or filament-type structures were
M. Szymonski et al. / Progress in Surface Science 74 (2003) 331–341
333
Fig. 1. Schematic illustration of ion beam enhanced roughening processes at bombarded surfaces.
observed [8–10], whereas, for oblique incidence, periodic height modulations (ripples) were observed [11,12]. In our recent studies [13,14], wire-like structures with the diameter of a few tens of nanometer were obtained at particular experimental conditions. In this paper, we report on some recent experimental findings obtained in our laboratory, which could exemplify the many aspects of beam-induced surface modification at the nanometer scale. In particular, heavy-ion bombardment of InSb compound semiconductor leading to nanowire and dot formation on the irradiated surface are presented.
2. Experimental A schematic geometry of the experiment with beam bombardment of InSb (0 0 1) surface is shown in Fig. 2. A detailed description of the UHV experimental system was given elsewhere [14]. Typically, an irradiation with 4 keV Arþ was performed at incidence angle of 50 off normal with the fluence between 1 · 1014 and 6 · 1016 ions/ cm2 and the ion flux in the range of 0.3–2.10 · 1015 ions/cm2 s. The preparatory work for obtaining clean and well-ordered InSb(0 0 1) c(8 · 2) surface involved several cycles of low energy (700 eV) off normal (±60) Arþ bombardment at a temperature of 700 K and annealing to 750 K for a few hours until a clear c(8 · 2) LEED pattern was obtained. For imaging of freshly prepared and beam-modified surfaces a dynamic (non-contact) atomic force microscopy (DFM) was performed using Park Scientific Instruments’ VP2 AFM/STM microscope equipped with NanoSurf easyPLL demodulator. All data were taken at room temperature a few hours after ionprocessing was performed. An atomic resolution image of the unmodified InSb surface is shown in Fig. 3. The freshly prepared surface consists of large, atomically flat terraces with edges oriented preferentially along h1 1 0i and h1 1 0i crystallographic directions. The atomic rows on the surface run along the h1 1 0i direction [15]. Interestingly, the
334
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Fig. 2. Schematic geometry of experiment for ion-beam modification of surfaces.
Fig. 3. 10 · 10 nm2 atomically resolved DFM image of freshly prepared, c(8 · 2) reconstructed surface of InSb(0 0 1).
surface crystallinity is preserved during the initial stages of ion-sputtering, until the fluence of 1016 ions/cm2 is achieved. Consequently, the features seen as a result of ion bombardment with the low fluence are dependent on the surface orientation with the respect to the ion beam. Some examples are discussed below.
3. Results and discussion In Fig. 4, a set of images of the ion-bombarded sample for four different ion fluences is shown. It is seen that for fluences below 1 · 1015 ions/cm2 , the surface is flat and only sparingly covered with white dots indicating points of higher elevation. Most likely, they represent In agglomerates assembled on the surface, due to nonstoichiometric surface erosion by ion bombardment. Above the dose of 1 · 1015 ions/ cm2 , the surface is randomly covered with elongated (wire-like) structures. The width
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Fig. 4. Set of AFM images of ion bombarded InSb (0 0 1) with various fluences. Ion-beam energy was 4 keV and the flux was equal to 2.15 · 1015 ions/cm2 s.
of the structures is of the order of a few tens of nanometers, and have several nanometers in height and their length is flux- and fluence-dependent. If the total fluence of the ion beam approaches 1016 ions/cm2 range, the lengths of the wires is of the order of a few microns. Furthermore, it was found that wire formation on the surface is strongly dependent on the ion flux. For fluxes lower than 1 · 1015 ions/cm2 s, dot-like structures are preferentially formed, with a diameter of the order of several tens of nanometers. For fluxes higher than 1015 ions/cm2 s, wires start to develop with increasing surface density, until the saturation density is obtained [16]. We studied the dependence of the nanowire formation process on the crystal orientation with respect to the ion beam. The results are shown in Fig. 5, where a set of AFM images obtained for different surface orientations is shown. Since, from our high resolution DFM measurements, we know precisely the orientation of the surface reconstruction rows (see Fig. 3), it can be concluded that the wires are formed along the direction perpendicular to the reconstruction rows. If the ion-beam direction is perpendicular to the rows, the parallel wires cover the surface with high density. If the beam projection is parallel to the rows, then only a few relatively short wires are seen with random orientation. The dominant features, in the latter case, are dots. At first glance, this result is surprising, since one would expect that the diffusion of adparticles at the reconstructed surface is easier along the rows than across them.
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Fig. 5. Set of AFM images of ion bombarded InSb (0 0 1) for various angles between ion-beam projection and sample crystallographic directions.
One should remember, however, that efficient, thermally assisted migration is drawing the adparticles away from the wire nucleation sites, thus degradating the wires rather than developing them. The role of anisotropic diffusion in the nucleation process of the wires is discussed in the next section. In order to learn more about the chemical composition of the surface structures formed on an ion-bombarded sample, we need a high-resolution chemical probe. A newly developed technique––Kelvin-probe force microscopy––offers means of finding the chemical sensitivity via surface contact potential difference measurements. We have recently introduced this technique in our laboratory [14], and in Fig. 6 a Kelvin-probe image of InSb surface subjected to ion bombardment is shown together with its topography image. The topography image shows a dot-like structure of several tens of nm developed on the surface after ion bombardment with the fluence of 2.5 · 1015 ions/cm2 . A high contrast Kelvin-probe image of the same structure indicates that the dot is made out of material which has clearly a different work function in comparison to the surrounding area of stoichiometric InSb. For comparison, the Kelvin probe force spectroscopy on a polycrystalline Au film grown on mica has been performed with the same tip. Knowing that for polycrystalline Au the work function is equal to 5.1 eV [17], the system has been calibrated for absolute work function measurements. It was determined that the work function of the ion bombarded (0 0 1) InSb was equal to 4.6 eV, and of the dot material 4.3 eV. This
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Fig. 6. DFM topography and KPFM image of dot-like structure. Cross-section represents contact potential difference along marked line.
could be compared with the elemental electron work function for In, equal to 4.12 eV and the InSb value of 4.2 eV [18]. In the last case, the structure and composition of the investigated InSb surface were not known precisely. Therefore, the value of work function for the ion bombarded (0 0 1) InSb, determined in our experiment, should not be expected to be the same as the one from reference [18]. Although the technique is unable to identify the elemental composition of the dot, there are several indications that it is made out of In or at least from a material highly enriched in In [19,20]. It is widely known that ion sputtering of AIII BV compounds is highly non-stoichiometric and the BV component is preferentially ejected [21,22]. Subsequently, surfaces enriched in AIII component are produced. Due to surface diffusion, the metal adatoms can agglomerate to form of droplets seen in both our topography and our Kelvin-probe images. Further studies of the wire-covered surfaces with Kelvin-probe force spectroscopy revealed that the work function of the wires is very close to that of the dot-like structures, indicating their similar chemical composition.
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4. Computer simulations with Monte-Carlo code In order to obtain more insight into the surface diffusion occurring on ionbombarded surface simple computer simulations of such processes were performed with the Monte-Carlo method. The following assumptions were made: 1. The InSb surface was represented by a 100 · 100 square-lattice model with cyclic boundary conditions. Surface reconstruction was not taken into account; 2. Beam-induced sputtering is assumed to be non-stoichiometric producing an In enriched surface. Adparticles remaining at the surface are migrating due to the beam-enhanced diffusion. Consequently, the direction of the primary ion momentum is assumed to be preferential for the surface hopping diffusion. The following steps were used in the simulation procedure: 1. Random number generation was used for surface adatom selection. 2. The hopping direction was selected with various preferences (from 1:3 to 20:1) for direction along the beam incidence. 3. The energy difference between the current and the neighbouring site for the selected adatom was calculated as proportional to the difference of coordination numbers for two nearest sites: DEk ¼ lk1 lk2 . 4. The hopping probability Pk was then calculated as follows: if DEk 6 0; or DEk P 0;
Pk ¼ 1; Pk ¼ ecDEk ; c ¼ 1=kT ;
the Boltzmann factor c was used as a simulation parameter. 5. The effect of the Ehrlich–Schwoebbel (E–S) barrier [23,24] on the hopping probability was taken into account. This barrier is known to occur where the diffusing particle on surface is jumping across the terrace steps, descending from the higher level or climbing up to the higher atomic plane. Such an energy difference was ta2 ken as equal to: DEE–S ¼ eaðh1 h2 Þ 1, where Dh ¼ h1 h2 is the terrace level difference and the a is taken as a simulation parameter. The corresponding hopping probability ‘‘correction’’ for the E–S barrier was calculated as P ¼ Pk ecDEE–S . 6. The decision about transferring the surface adatom to the site chosen in step 2 was taken, according to the calculated probability P . The points 1–6 were executed several times (40 jumps in the example shown below), then a new adatom was generated on a randomly selected free site of the surface matrix. In Fig. 7, the examples of such simulations are shown for three various values of the E–S barrier and a high preference for migration along the direction of the beam incidence (16:1). The results show that wires are simply formed due to anisotropic diffusion of adparticles on the surface with a preference rate for directions along the primary beam incidence starting from 2:1. Since it is known [25] that a fully developed collision cascade induced by 4 keV Ar ion bombardment
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Fig. 7. Results of surface diffusion simulations with Monte-Carlo code for different values of parameter a and total number of jumps equal to 100 000.
should be isotropic, we have to assume that the anisotropy of the diffusion is caused by the initial stages of the primary ion interaction with the adparticles. Primary knock-on particles created at this stage are known to conserve the direction of the ion momentum [25] and they should be considered as primary activation energy source for adparticle hoping leading to wire formation. The second aspect of the simulation result is that the E–S barrier seems to play an important role in wire formation. Lack of the barrier against diffusion across the step edges would result in the piling-up of the adparticle material in dot-like structures and/or fast dissipation of the wire like structures at the very early stage of their formation. In the simple model we used for the simulation, we were not able to monitor the effect of the surface reconstruction rows on the adparticle diffusion and the wire formation process. Our experimental results, however, imply that this should be still another important factor causing anisotropy in the surface diffusion of adparticles, at least in the early stage of ion bombardment.
5. Concluding remarks Beam-induced surface modification and nanostructuring of compound semiconductors involve the interplay of various surface processes dependent on particular experimental conditions, such as the sample temperature, its crystallinity, ion-beam current density and the angle of incidence of the primary ions. In fact, there are extremely interesting problems to be studied at atomic level by high-resolution scanning-probe microscopies. Some of the above discussed mechanisms occurring in compound solids are different from those in the elemental solids, where the ripple formation is the most visible mode of beam induced surface modification [1]. A primary reason for this difference is the non-stoichiometric sputtering of the compound surface, due to different thermodynamic properties of the components, such as their vapour pressure, surface tension, surface free energy, etc. The surface adparticles created by this that process can diffuse and nucleate with other adparticles to form small clusters subjected to further ion bombardment. It is unlikely, however, that wire-type structures could be developed primarily, due to non-uniform sputter erosion of the clusters related to sputter yield dependence on the angle of incidence.
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Our results strongly indicate that anisotropic diffusion of adparticles, activated by the primary ions and/or primary knock-on atoms carrying a fraction of the initial beam momentum, is responsible for the formation of wire-like structures on the modified surface. It has been found recently that surface modification of AIII –BV semiconductors can also take place due to electronic transitions. Weaver et al. [26] reported STM images of (1 1 0) GaAs surface subjected to electron bombardment with various fluences. It was demonstrated that electron bombardment causes surface erosion and one might expect this process to be non-stoichiometric. It would be interesting to explore whether surface structuring and self-organization could occur on compound semiconductor surfaces, due solely to inelastic processes.
Acknowledgements The experimental results presented in this paper were obtained as a part of research project supported by Polish Research Council (KBN) under the contract no. 2 P03B 065 23 and by the research grant of the Institute of Physics, Jagiellonian University.
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