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Study of the chemical and morphological evolution of InSb(001) surface under low energy ion bombardment
Vacuum 83 (2009) 745–751
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Study of the chemical and morphological evolution of InSb(001) surface under low energy ion bombardment Franciszek Krok Research Centre for Nanometer-Scale Science and Advanced Materials (NANOSAM), Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland
a b s t r a c t Keywords: Ion-induced surface modification AIIIBV semiconductor surfaces Nanostructures Atomic Force Microscopy Kelvin Probe Force Microscopy Contact potential
InSb(001) surfaces were subjected to 4 keV Arþ bombardment at oblique angles of incidence with ion fluences in the range of 9.0 1013–6.2 1017 ions/cm2. The evolution of bombardment induced surface structures and their chemical composition were studied with Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) in UHV, and ‘‘ex situ’’ with Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX). Various morphological features, such as small dots, wires and for very high ion fluence ripple-like structures were observed. It was found that both the initial surface crystallographic structure and the ion beam direction influence the developing anisotropic nanostructures on the irradiated surface. It was also found that the time evolution of the nanostructured surface in terms of surface roughness s, follows a power law s w tb. The surface nanostructures (dots and wires), at every stage of their development, are found to have different work functions in comparison to the surrounding InSb substrate. The results indicate that the nanostructures developed on the irradiated InSb surfaces consist of indium. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fabrication of well ordered nanostructures (nanodots and nanowires) has received wide attention due to possible applications in optical and electronic device manufacturing [1]. Ioninduced sputtering of semiconductor and metal surfaces can stimulate various processes resulting in regular pattern formation in which structures of nanometer or tens of nanometers sizes can be formed on the irradiated surfaces. Characteristic features of those nanostructures as well as their size depend on ion beam parameters, such as ion current density, fluence and angle of incidence. With regard to the limitations of the lithographic techniques, ion bombardment of solid surfaces is a competitive low cost method for producing large areas of nanopatterned surfaces. Facsko et al. [2] and Frost et al. [3] reported on the formation of ordered nanodots due to ion beam stimulated re-organization of GaSb and InP surface constituents. The repetition periods of nanodot patterns obtained for a normally incident of Arþ beam were easily tunable by varying the sputtering parameters, like sputter time, ion energy, and beam current density. Our recent studies [4,5] revealed that 4 keV Arþ bombardment at an oblique angle of incidence and
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with low ion current density led to creation of wire-like structures on an InSb(001) surface. The interaction of ions with solids is a complex phenomenon in which many mechanisms are involved. The main ones are surface sputtering, mass-transport processes due to momentum transfer in the collisions cascades, and diffusion (thermal or ion beam enhanced) on the irradiated surfaces. Bradley and Harper (BH) proposed in 1988 a stochastic continuum model for predicting the ripple wavelength and their orientation on amorphous metal and semiconductor surfaces [6]. The model describes the development of periodic structures on surfaces in terms of a roughening process driven by a curvature-dependent local sputter rate and a surface smoothing process due to surface self-diffusion. The surface height evolution h(x,y,t) is described by a linear equation
vhðx; y; tÞ ¼ vðqÞ þ nðqÞV2 hðx; yÞ DV2 V2 hðx; yÞ ; vt
(1)
where q is the ion incident angle, v(q) is the average erosion velocity of the surface, n(q) is the negative surface tension, and D is the surface diffusion constant. However, the model appeared to be oversimplified and failed to explain details of the ripple formation processes on metal surfaces, for cases in which the surface crystallinity is preserved during ion bombardment. For example, Constantini et al. [7,8] observed the development of periodic structures on metal irradiated surfaces whose direction does not follow the prediction of the BH theory. They explain the behaviour
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of the developing anisotropic structure in terms of Kuramoto– Sivashinsky equation [9] using crystallography-oriented diffusion on metals. Apart from morphological evolution, ion bombardment can also induce changes in the elemental composition of irradiated multicomponent surfaces. According to theoretical predictions [10] partial sputtering yields depend on atomic masses and surface binding energies of the constituents. Such theoretical predictions are consistent with an experimentally observed large nonstoichiometry only for InP surface [11], possibly because of the large mass difference of In and P atoms. However, for compounds with similar masses of constituents like GaAs or InSb, the theory does not give reliable estimation of partial yield ratios. For example, a gallium surface enrichment was observed experimentally for GaAs [12] in contrast to theoretical predictions. In the case of InSb(001), electron spectroscopy measurements (Auger Electron or Electron Energy Loss Spectroscopy) [13] proved that under low energy Arþ-bombardment at room temperature the surface becomes indium enriched. XPS and Low Energy Ion-Scattering Spectroscopy (LEISS) studies by Yu et al. [14] indicated that composition changes on irradiated AIIIBV semiconductor surfaces could be explained by taking ion beam-induced Gibbsian segregation into account. In the present paper we report on our recent experimental findings on InSb(001) surface nanostructuring by off-normal, low energy Arþ beam bombardment. We discuss the mechanisms leading to the development of anisotropic nanostructures on the irradiated surfaces. It will be shown that both, the direction of ion beam as well as the initial surface crystallographic structure have clear influence on the topography of nanostructured surfaces. Furthermore, we use Kelvin Probe Force Microscopy (KPFM) for assessment of the chemical composition of the nanostructures created on irradiated InSb(001). The KPFM is a newly developed technique, based on Atomic Force Microscopy, which allows to probe electronic and chemical surface properties at a nanometer scale. 2. Experiment A detailed description of our UHV experimental system was given elsewhere [4]. The substrate was an InSb epi-ready wafer purchased from TBL-Kelpin. Initially, the substrate surface was cleaned with a rastered 700 eV Arþ beam with an average current density of 0.7 mA/cm2, bombarding the surface at an angle of 60 off-normal for approx. 1 h. The wafer was kept at T ¼ 700 K during the ion beam cleaning. Following the ion cleaning, the wafer was annealed up to 750 K for a few hours. Previous work indicated [15] that such a cleaning procedure was sufficient for removing oxygen and carbon contaminants from the surface and for obtaining clear c(8 2) LEED pattern. Ion-induced surface modification was performed in consecutive cycles using a rastered focused Arþ beam of Eion ¼ 4 keV. The average current density of the nanostructuring beam was about 0.8 mA/cm2. The beam spot diameter was about 0.8 mm and the ion flux at the spot was about 400 mA/cm2. The angle of ion beam incidence, aion, was in the range of 45–80 with reference to the surface normal. Various polar angles 4, defined here as the angle between the projection of the beam on the surface and the C110D surface crystallographic direction, were chosen. Ion fluences in the range of 9.0 1013–4.5 1017 ions/cm2 were applied. The crystal was kept at room temperature during ion beam bombardment. After irradiation the samples were transferred in UHV into a microscope chamber for AFM imaging at room temperature. Atomic Force Microscopy images were obtained in contact mode (C-AFM) with a Park Scientific Instruments VP2 STM/AFM. Commercially available silicon piezoresistive cantilevers were used as probes. The applied force was typically about 100 nN.
In Kelvin Probe Force Microscopy (KPFM), the sample topography was acquired simultaneously from a map of the lateral distribution of surface contact potential. In general, the topography was measured using the non-contact AFM mode (NC-AFM) by driving the cantilever at its resonant frequency (f1 y 284 kHz) with pre-set detuning (FM mode). At the same time, the ac bias (at frequency u ¼ 0.4 kHz and amplitude of 0.5 V) with dc offset was applied to the sample to minimise an electrostatic interaction between the tip and the sample. The electrostatic interaction is induced by the work function (F) differences between the tip and the sample. The obtained map of Vdc represents the surface contact potential (CP) distribution. Other details of the technique and our FM–KPFM system were described elsewhere [16]. Ex situ Scanning Electron Microscopy was performed with a Jeol (JSM-5500LV) Microscope equipped with a silicon X-ray detector (IXRF Systems) for Energy Dispersive X-ray Analysis (EDX) at the Krakow Academy of Fine Arts.
3. Results and discussions 3.1. Atomic Force Microscopy measurements A clean, reconstructed InSb(001) surface composed of large atomically flat terraces is presented in Fig. 1. The surface is highly anisotropic with atomic rows aligned along the C110D crystallographic direction as shown by high resolution non-contact AFM measurements (cf. inset of Fig. 1). The irradiation with an Arþ beam at room temperature easily destroys the long-range atomic order on the sample surface even for a small ion fluence. For a fluence as low as 9.0 1013 ions/cm2, the LEED pattern characteristic for a c(8 2) reconstruction is transformed into that for a (1 1) structure. We have studied the dependence of the surface nanostructuring process on the crystal orientation with respect to the ion beam. In Fig. 2a–d, the InSb(001) surfaces bombarded with Eion ¼ 4 keV Arþ beam under aion ¼ 45 at low fluence (1 1016 ions/cm2) and polar angles of 4 ¼ 45 , 0 , 45 and 90 , respectively, are presented [17]. The main features developed on the irradiated surfaces are the dots and wires. From Fig. 2 it is clear that the wire formation is strongly dependent on the initial surface anisotropy. The wires are formed
Fig. 1. NC-AFM image of clean, reconstructed InSb(001) surface. The high resolution non-contact AFM image (12 12 nm2) with the surface atomic arrangement is presented in the inset of the figure.
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Fig. 2. C-AFM images of nanostructured InSb(001) surfaces performed with an Arþ beam (Eion ¼ 4 keV, aion ¼ 45 and fluence 1.2 1016 ions/cm2) at room temperature and polar angles 4 of (a) 45 , (b) 0 , (c) 45 and (d) 90 against the C001D direction. The black arrows indicate the projection of the ion beam on the irradiated surfaces (from Ref. [17]).
predominantly along the direction perpendicular to the reconstruction rows, i.e. along C110D. The wires cover the surface with the highest density if the ion beam direction is perpendicular to the rows (Fig. 3d), whereas, if the ion beam projection is parallel to the atomic rows only few relatively short wires are seen with random orientation (Fig. 3b). In the latter case, the dominant features are dots. For the system under study, the authors have already shown that surface diffusion of adatoms, generated by the ion impact, is the dominant process leading to development of anisotropic nanostructures (wires) on the irradiated surface [4]. Thus, the observed dependence of the direction of created wires on the surface anisotropy indicates that crystallographic-oriented diffusion is especially important for the initial stage of surface nanostructuring. Although long-range atomic ordering on the irradiated InSb(001) surface disappears already for very low ion fluence (as shown by the lack of the LEED pattern characteristic for the reconstructed surface), one cannot exclude that some short-range atomic ordering remains even for prolonged ion bombardment. In order to find out to which extent the surface crystallinity is preserved during ion irradiation we have studied the dependence of the wire formation on the ion beam fluence (the time of irradiation). The results are shown in Fig. 3, where a set of AFM images of InSb(001) surface irradiated with Eion ¼ 4 keV Arþ ions under aion ¼ 60 and at a polar angle 4 ¼ 25 with different fluences is shown. The surface irradiated with small ion beam fluence, of 3.5 1015 ions/cm2 (Fig. 3a), where the steps and terraces characteristic for non-irradiated surface can still be recognised, is covered with randomly spread clusters (dots) of approximately 50 nm in diameter and 5 nm in height. Rarely, elongated structures (wires) are seen which are aligned along the C110D direction, i.e. perpendicular to the initial reconstruction rows. For higher fluence, of about 4.2 1016 ions/cm2 (Fig. 3b), the wire-like structures are the dominant features on the surface. The wires are aligned along two different
directions. Most of wires are aligned along the projection of the ion beam on the surface. Those wires are typically over 1 mm long and are not uniformly wide at their base. They are broader at their front side (with respect to the ion beam) getting subsequently thinner. A few wires, however, are aligned along the C110D direction. Those wires are uniformly wide, about 70 nm in the base. Moreover, some of the observed wires are not straight with one part parallel to the ion beam projection, while the other part parallel to the C110D direction. Those wires most likely could result from an interconnection of two separated wires developing in two different directions. As the irradiation proceeds, the total number of wires increases with simultaneous increase of the number of wires aligned along the beam direction. For the fluence of 1.0 1017 ions/cm2 (Fig. 3c) the dense structure of wires aligned along the beam direction dominates the topography of the irradiated surface. For still higher fluence (3.7 1017 ions/cm2 – Fig. 3d) a clear ripple structure, parallel to the projection of the ion beam, is developed on the irradiated surface. The experimental findings indicate that the irradiated InSb(001) surface preserves its crystallinity during the initial stages of ion sputtering. The surface is assumed to become amorphous at a fluence of about 1.0 1017 ions/cm2 since for this fluence the developing anisotropic surface nanostructures are predominantly aligned along the direction of the ion beam. Also, Evdokimov et al. [18] concluded from low energy ion-scattering and ion-recoil spectroscopies that the InSb(001) surface did not appear amorphous even at very high (in the range of 1018 ions/cm2) ion fluence. Such fluences exceed well those required to produce amorphous surfaces of GaAs and Si. However, in their experiment the surface was bombarded with 4.8 keV Arþ ions at the grazing incidence (an angle of 10 to the surface). Thus the real amount of the ions penetrating the target was considerably smaller due to surface scattering. Once the InSb(001) surface becomes amorphous, the direction of the ion beam is the predominant factor influencing the anisotropy of the nanostructured topography. This is shown in Fig. 4a and b. Fig. 4a
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Fig. 3. C-AFM images of nanostructured InSb(001) surfaces performed with an Arþ beam (Eion ¼ 4 keV, aion ¼ 60 , 4 ¼ 25 ) at room temperature, with fluences of (a) 0.35, (b) 4.2, (c) 10.0 and (d) 62.5 1016 ions/cm2, respectively. The white arrow in (a) indicates the projection of the ion beam on the surface (the direction is common for all images).
depicts the InSb(001) surface bombarded to the stage when it is covered with closely packed wires parallel to the ion beam (fluence of 5.0 1016 ions/cm2). Consecutive irradiation of this surface after rotation by about 25 around the normal to the sample results in the topography shown in Fig. 4b for the total fluence of 1.1 1017 ions/ cm2. The surface is covered with closely packed wires whose direction is again parallel to the direction of the ion beam. For the present experimental conditions it has been found that the prolonged ion irradiation of the InSb(001) surface leads to the
development of a ripple-like structure similar to the one shown in Fig. 3d. The ripples are always parallel to the direction of the ion beam on the irradiated surfaces. In Fig. 5, the dependence of the ripple wavelength (l) on the ion incident angle and for the polar angles 0 and 90 is presented. For a given polar angle, a week dependence of the ripple wavelength on the incident angle is observed and the ripples have a maximum l when created with the aion ¼ 70 for 4 ¼ 0 . For the whole range of aion, the wavelength of ripples created with the ion beam directed parallel to the initial
Fig. 4. Two stages of development of the nanostructured InSb(001) surface irradiated with an Arþ beam. In (a) the surface is irradiated with ion beam fluence of 5.0 1016 ions/cm2 and in (b) the surface developed under successive irradiation (the total fluence of 1.1 1017 ions/cm2) but after rotation of about 25 around the normal to the sample surface. (CAFM images, 3 3 mm2). The relative change of white arrows (the beam direction) with respect to the image frames reflects the rotation of the sample between the two successive cycles of ion irradiation.
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a log–log plot of s versus the irradiation time, t, extracted from the surface evolution presented in Fig. 3, is shown. The rms roughness follows the power scaling relation s w tb with the growth exponent b ¼ 0.80 0.07. We have found similar scaling of the rms roughness with the irradiation time for InSb(001) surfaces at other incident and polar angles of the Arþ beam. According to the BH model, the amplitude (and correspondingly the rms roughness) of the periodic structures due to ion bombardment should depend exponentially on the irradiation time, s w exp(t). The scaling law s w tb observed here disagrees with the predictions of the BH model. As demonstrated theoretically, a power law scaling s w tb is a consequence of various nonlinear effects occurring on the irradiated surfaces [20]. In the present case, we associate the nonlinear behaviour in the evolution of InSb(001) surface morphology with the dominance of the ionenhanced surface diffusion of adatoms which is recognised as the dominant process influencing the surface nanostructuring [4]. Fig. 5. Dependence of the ripple wavelength developed on InSb(001) surfaces under Arþ bombardment on ion incident and polar angles (fluences in the range of few time 1017 ions/cm2).
atomic rows (4 ¼ 0 ) is higher (on average by about 40%) than l of ripples created for the ion beam direction perpendicular to the atomic rows (4 ¼ 90 ). The observed dependence of the ripple wavelength on the polar angle can be understood if one takes into account that before the ripple development, i.e. at the early stage of ion irradiation, the surface is covered predominantly with dots. It has been already shown, that the effect of ion irradiation results in the elongation of the initial dots transforming them into wires and finally, for the long irradiation time, closely packed wires (ripples) are developed [4]. From Fig. 2, it is obvious that the dimensions of the initial dots depend on the polar angle. The dots created with ion beam of direction parallel to the reconstruction rows (Fig. 2b) are larger (of 74.0 10.3 nm in diameter at their base) than the dots created with ion beam incidence perpendicular to the atomic rows (of 52.5 10.5 nm – Fig. 2d). Thus, the size of initial dots influences the transverse dimension of the wires and finally, the periodicity of the ripples. In order to describe the process of InSb(001) surface nanostructuring in a more quantitative manner and for comparison with the existing continuum models for surface erosion by ion sputtering, we have investigated the surface morphology evolution in terms of the surface root mean square (rms) roughness, s [19]. In Fig. 6,
Fig. 6. Log–log plot of the rms surface roughness, s, versus the irradiation time, t. The solid line represents the fit according to the scaling power law s w tb with the growth exponent b ¼ 0.80 0.07.
3.2. Scanning electron microscopy/energy dispersive X-ray analysis The chemical composition of the ion irradiated InSb(001) surfaces was investigated with Scanning Electron Microscopy (SEM) equipped with an EDX detector. In Fig. 7, a SEM image of InSb(001) irradiated with Arþ beam at aion ¼ 80 and fluence of 1016 ions/cm2 is shown. On the surface, the wires together with some large (of few microns) droplet-like structures are visible. The lateral resolution of the EDX technique is not sufficient for resolving the wire composition, as the X-rays are generated by the analyzing electron beam in a region of about 2 mm in depth. However, the composition of the droplets could easily be determined. The droplets, initially symmetric in shape as checked with SEM, originate from the cleaning procedure. The EDX spectra show that the droplets are composed of 99% indium. The excess In atoms, produced during the sputter cleaning performed at elevated temperature, are assumed to aggregate to droplets. Their elongated shape, seen in Fig. 7, is due to ion beam modification in the process of surface nanostructuring. The front side of the elongated droplets (the white part in Fig. 7) with respect to the beam direction is composed of In (99%) whereas the backside is composed of both, indium (52%) and antimony (48%). The same ratio of In/Sb has been found for the surrounding surface, thus the part assigned above as
Fig. 7. SEM image of the InSb(001) surface bombarded with an Arþ beam (Eion ¼ 4 keV, aion ¼ 80 , fluence of few time 1016 ions/cm2) at room temperature. Together with the wires big droplet-like structures are seen, which originate from the cleaning procedure. The white arrow indicates the projection of the ion beam on the surface. The zone marked with A depicts the surface area of the highest surface density of the wires.
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the backside of the droplets is actually the non-irradiated substrate surface area shadowed by the initial big droplet. From the investigation of the evolution of surface topography as a function of the irradiation time we concluded that the wires originate from the elongation (tailing) of initial dots. However, it was not clear wherever the dots are getting elongated upstream (towards the incoming ions) or vice verse, downstream the ion beam direction. From Fig. 7 (the zone marked with A) it is seen that the surface density of the wires created at the front side of the big droplets is higher than the average wire surface density on the whole irradiated surface. This gives evidence that during the ion irradiation adatoms diffuse in the direction of incident ion beam (ion-enhanced diffusion) and accumulate at the front side of the dots created at the initial stage of irradiation. As a result, the dots ‘‘move’’ towards the incoming ions to form the wires. As the irradiation proceeds, the wires are getting longer due to adatoms accumulation at their front side but, at the same time, the backside of the wires is getting thinner as it undergoes extensive ion sputtering. Such mechanism of dot elongation and wire creation, explains the observed shape of the wires being broader at their front side (with respect to the ion beam) and getting subsequently thinner (see Fig. 3b). 3.3. Kelvin Probe Force Microscopy measurements To get more information on the composition of the nanostructured surfaces Kelvin Probe Force Microscopy measurements were performed. In Fig. 8, the topographic and the contact potential (CP) images of an InSb(001) surface irradiated with a fluence of 2.1 1016 ions/cm2 are shown. In the topography image (Fig. 8a) there are two dots interconnected with the wire. There is also a small cluster close to the wire. In the CP image (Fig. 8b) the corresponding Kelvin probe signal contrast indicates that all the structures are made of a material with different work function with respect to the surrounding InSb substrate. For both structures (for the wire and the
Fig. 8. Topography (a) and contact potential (b) images of nanostructures developed on the irradiated InSb surface acquired with KPFM. In (c) the cross-section of the contact potential of the dots, taking along the line marked with A in (b), is shown.
dots) the work function is lower than that of the substrate areas. The measured decrease of the work function is size dependent as illustrated by the line profile across the surface contact potential image (Fig. 8c). The larger dot has lower potential by about 50 mV in comparison with the potential of the neighbouring smaller dot. The CP of the wire, on an average, is over two-fold lower than the CP of the dots (Figs. 8c and 9). However, the differences of the CP values cannot be directly associated with the differences in the relative contribution of In and Sb in the nanostructure composition. The electrostatic force is a long-range interaction and the obtained surface potential distribution can be affected by averaging effect due to a finite tip size. Results of several other experiments in which ion irradiated InSb surfaces without subsequent annealing induced modification were investigated with spectroscopic methods indicate that the surfaces are enriched with indium [13,14]. However, a lack of the lateral resolution in the spectroscopic measurements does not allow to determine in which form excess indium atoms are accumulated on the bombarded surface. We compare these results with our Kelvin Probe Force Microscopy measurements performed on the nanostructured InSb(001) surface. Although the KPFM is unable to identify directly the elemental composition of the nanostructures, it reveals that the wires and the dots are made of material with a work function being different from that of the irradiated InSb substrate (Fig. 8b). The lower work function of the nanostructures indicates an excess of indium in the developed surface nanostructures, since the work function of In (FIn ¼ 4.12 eV) is lower than that for the irradiated InSb(001) surface (Firrad.InSb ¼ 4.6 eV [5]). Indium enrichment of the wires and dots means that there is a metallic-type structure generated on the semiconductor substrate. In such a case, the condition of thermodynamic equilibrium implies that the Fermi levels of the two materials must coincide with each other. This condition is fulfilled by charge transfer and band bending near the interface in analogy with that on the interface between a bulk metal and a semiconductor (Schottky barrier model). The direction of electron flux in a metal– semiconductor contact depends upon the relative values of work functions of the two materials and the electrons will travel from the
Fig. 9. Line profiles of the wire topography (upper graph) and the CP signal (lower graph). The profiles are taken along the lines marked with B in Fig. 8a and b, respectively. In the upper graph the substrate region enriched with electrons is drawn schematically.
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material with the smaller work function to the material with the higher one. In the present case, Fnanostructure < Fsubstrate, thus, electrons are transferred from the nanostructures to the substrate. The metal–semiconductor interface is charged and there is a region in the semiconductor substrate of some thickness enriched with electrons [21]. In Fig. 9 the line profile (cross-section) of the wire (along the line B in Fig. 8) and the corresponding cross-section of the wire contact potential image is shown. The deep minimum in the CP signal corresponding to the potential of the wire is accompanied by two ‘‘shoulders’’ on both sides. The signal at the shoulders’ maxima is higher by about 50 mV in comparison to the potential of the InSb substrate. In the CP image (Fig. 8b) the ‘‘shoulders’’ are seen as two bright stripes aligned along the broad, dark stripe corresponding to the CP of the wire. The maximum of the ‘‘shoulders’’ correspond to the border between the wire and the substrate. It is likely that the increased CP signal in the ‘‘shoulders’’ reflects the charged metal–semiconductor interface. We have associated the width of the CP ‘‘shoulders’’ (from their onset to their maximum) with the thickness of the substrate region enriched with electrons. In Fig. 9, a cross-section of the substrate region enriched with electrons is drawn schematically. The termination of the enriched region on the surface implies that the substrate areas on both sides of the wire should be negatively charged. In fact, the increased magnitude of the CP signals on both sides of the wire support the concept that the areas around the wire are negatively charged since it was already shown by Sommerhalter et al. [22] that in the CP image the bright contrast corresponds to negatively charged areas. The results in Fig. 9 may, of course, be influenced by the lateral resolution of the KPFM instrument. 4. Conclusions Several aspects of ion bombardment induced surface modification caused by non-stoichiometric sputtering, beam enhanced surface diffusion and nanostructure development have been investigated for the InSb(001) surface. Firstly, we have observed that the ion irradiation led to creation of nanodots on the surface at the initial stage of irradiation, which subsequently, form nanowires as the time of irradiation increases. The wire development is explained as an effect of nanodot elongation upstream the ion beam direction due to the accumulation of the diffusing adatoms created by the ion impact. Secondly, we have found that two types of wires were formed with the shape and orientation dependent either on the primary beam orientation, or on the crystallographic orientation of the reconstruction rows at the (001) c(8 2) InSb surface. This finding indicates that, at the initial stage of ion irradiation, i.e. for fluences up to few times 1016 ions/cm2, anisotropic
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surface diffusion along the reconstruction rows plays an important role together with beam enhanced diffusion along the direction of the initial momentum transfer. Finally, we have found that ion sputtering of InSb at low fluences is highly non-stoichiometric. The contact potential measurements have shown that the surface nanostructures, at every stage of their development, had lower work functions in comparison to the surrounding amorphous InSb substrate. It is concluded, that the Sb component of the target is sputtered preferentially and the nanostructures developed on irradiated InSb surfaces consist of indium. Acknowledgements This work was supported by the European Commission under Contract No. MTKD-CT-2004-003132, 6th FP-Marie Curie action for Transfer of Knowledge: ‘‘Nano-engineering for Expertise and Development’’ – NEED. Careful reading of the manuscript by Professor M. Szymonski is kindly acknowledged. References [1] For review see: Jacak L, Hawrylak P, Wojs A. Quantum Dots. Berlin,: Springer; 1998. [2] Facsko S, Dekorsy T, Koerdt C, Trappe C, Kurz H, Vogt A, et al. Science 1999; 285:1551. [3] Frost F, Schindler A, Bigl F. Phys Rev Lett 2000;85:4116. [4] Krok F, Kolodziej JJ, Such B, Piatkowski P, Szymonski M. Nucl Instrum Methods B 2003;212:264. [5] Krok F, Kolodziej JJ, Such B, Piatkowski P, Szymonski M. Appl Surf Sci 2003; 210:112. [6] Bradley RM, Harper JM. J Vac Sci Technol A 1988;6:2390. [7] Constantini G, Rusponi S, Gianotti R, Boragno C, Valbusa U. Surf Sci 1998;416: 245. [8] Constantini G, Rusponi S, Buatier de Mongeot F, Boragno C, Valbusa U. J Phys Condens Matter 2001;13:5875. [9] Cuerno R, Barabasi A-L. Phys Rev Lett 1995;74:4746. [10] Sigmund P. Phys Rev 1969;184:383. [11] Massies J, Dazaly L. J Appl Phys 1984;55:3136. [12] Schweitzer MO, Leibsle FM, Jones TS, McConville CF, Richardson NV. Surf Sci 1993;280:63. [13] Bouslama M, Jardin C, Ghamnia M. Vacuum 1995;46:143. [14] Yu W, Sullivan JL, Saied SO. Surf Sci 1996;352–354:781. [15] Kolodziej JJ, Such B, Czuba P, Piatkowski P, Krok F, Szymonski M. Surf Sci 2002; 506:12. [16] Krok F, Kolodziej JJ, Such B, Struski P, Czuba P, Piatkowski P, et al. Surf Sci 2004; 566–568:63. [17] Szymonski M, Krok F, Struski P, Kolodziej JJ, Such B. Prog Surf Sci 2003;74:331. [18] Evdokimov IN, Valizadeh R, Armour DG, Richardson NV, McConville CF. Surf Sci 1994;318:281. [19] The rms surface roughness was calculated from the AFM images using the WSxM software (www.nanotec.es). [20] Makeev MA, Cuerno R, Barabasi AL. Nucl Instrum Methods B 2002;197:185. [21] Ioannides T, Verykios XE. J Catal 1996;161:560. [22] Sommerhalter Ch, Matthes ThW, Glatzel Th, Jaeger-Waldau A. Appl Phys Lett 1999;75:286.
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Study of the chemical and morphological evolution of InSb(001) surface under low energy ion bombardment Franciszek Krok Research Centre for Nanometer-Scale Science and Advanced Materials (NANOSAM), Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland
a b s t r a c t Keywords: Ion-induced surface modification AIIIBV semiconductor surfaces Nanostructures Atomic Force Microscopy Kelvin Probe Force Microscopy Contact potential
InSb(001) surfaces were subjected to 4 keV Arþ bombardment at oblique angles of incidence with ion fluences in the range of 9.0 1013–6.2 1017 ions/cm2. The evolution of bombardment induced surface structures and their chemical composition were studied with Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) in UHV, and ‘‘ex situ’’ with Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX). Various morphological features, such as small dots, wires and for very high ion fluence ripple-like structures were observed. It was found that both the initial surface crystallographic structure and the ion beam direction influence the developing anisotropic nanostructures on the irradiated surface. It was also found that the time evolution of the nanostructured surface in terms of surface roughness s, follows a power law s w tb. The surface nanostructures (dots and wires), at every stage of their development, are found to have different work functions in comparison to the surrounding InSb substrate. The results indicate that the nanostructures developed on the irradiated InSb surfaces consist of indium. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fabrication of well ordered nanostructures (nanodots and nanowires) has received wide attention due to possible applications in optical and electronic device manufacturing [1]. Ioninduced sputtering of semiconductor and metal surfaces can stimulate various processes resulting in regular pattern formation in which structures of nanometer or tens of nanometers sizes can be formed on the irradiated surfaces. Characteristic features of those nanostructures as well as their size depend on ion beam parameters, such as ion current density, fluence and angle of incidence. With regard to the limitations of the lithographic techniques, ion bombardment of solid surfaces is a competitive low cost method for producing large areas of nanopatterned surfaces. Facsko et al. [2] and Frost et al. [3] reported on the formation of ordered nanodots due to ion beam stimulated re-organization of GaSb and InP surface constituents. The repetition periods of nanodot patterns obtained for a normally incident of Arþ beam were easily tunable by varying the sputtering parameters, like sputter time, ion energy, and beam current density. Our recent studies [4,5] revealed that 4 keV Arþ bombardment at an oblique angle of incidence and
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with low ion current density led to creation of wire-like structures on an InSb(001) surface. The interaction of ions with solids is a complex phenomenon in which many mechanisms are involved. The main ones are surface sputtering, mass-transport processes due to momentum transfer in the collisions cascades, and diffusion (thermal or ion beam enhanced) on the irradiated surfaces. Bradley and Harper (BH) proposed in 1988 a stochastic continuum model for predicting the ripple wavelength and their orientation on amorphous metal and semiconductor surfaces [6]. The model describes the development of periodic structures on surfaces in terms of a roughening process driven by a curvature-dependent local sputter rate and a surface smoothing process due to surface self-diffusion. The surface height evolution h(x,y,t) is described by a linear equation
vhðx; y; tÞ ¼ vðqÞ þ nðqÞV2 hðx; yÞ DV2 V2 hðx; yÞ ; vt
(1)
where q is the ion incident angle, v(q) is the average erosion velocity of the surface, n(q) is the negative surface tension, and D is the surface diffusion constant. However, the model appeared to be oversimplified and failed to explain details of the ripple formation processes on metal surfaces, for cases in which the surface crystallinity is preserved during ion bombardment. For example, Constantini et al. [7,8] observed the development of periodic structures on metal irradiated surfaces whose direction does not follow the prediction of the BH theory. They explain the behaviour
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of the developing anisotropic structure in terms of Kuramoto– Sivashinsky equation [9] using crystallography-oriented diffusion on metals. Apart from morphological evolution, ion bombardment can also induce changes in the elemental composition of irradiated multicomponent surfaces. According to theoretical predictions [10] partial sputtering yields depend on atomic masses and surface binding energies of the constituents. Such theoretical predictions are consistent with an experimentally observed large nonstoichiometry only for InP surface [11], possibly because of the large mass difference of In and P atoms. However, for compounds with similar masses of constituents like GaAs or InSb, the theory does not give reliable estimation of partial yield ratios. For example, a gallium surface enrichment was observed experimentally for GaAs [12] in contrast to theoretical predictions. In the case of InSb(001), electron spectroscopy measurements (Auger Electron or Electron Energy Loss Spectroscopy) [13] proved that under low energy Arþ-bombardment at room temperature the surface becomes indium enriched. XPS and Low Energy Ion-Scattering Spectroscopy (LEISS) studies by Yu et al. [14] indicated that composition changes on irradiated AIIIBV semiconductor surfaces could be explained by taking ion beam-induced Gibbsian segregation into account. In the present paper we report on our recent experimental findings on InSb(001) surface nanostructuring by off-normal, low energy Arþ beam bombardment. We discuss the mechanisms leading to the development of anisotropic nanostructures on the irradiated surfaces. It will be shown that both, the direction of ion beam as well as the initial surface crystallographic structure have clear influence on the topography of nanostructured surfaces. Furthermore, we use Kelvin Probe Force Microscopy (KPFM) for assessment of the chemical composition of the nanostructures created on irradiated InSb(001). The KPFM is a newly developed technique, based on Atomic Force Microscopy, which allows to probe electronic and chemical surface properties at a nanometer scale. 2. Experiment A detailed description of our UHV experimental system was given elsewhere [4]. The substrate was an InSb epi-ready wafer purchased from TBL-Kelpin. Initially, the substrate surface was cleaned with a rastered 700 eV Arþ beam with an average current density of 0.7 mA/cm2, bombarding the surface at an angle of 60 off-normal for approx. 1 h. The wafer was kept at T ¼ 700 K during the ion beam cleaning. Following the ion cleaning, the wafer was annealed up to 750 K for a few hours. Previous work indicated [15] that such a cleaning procedure was sufficient for removing oxygen and carbon contaminants from the surface and for obtaining clear c(8 2) LEED pattern. Ion-induced surface modification was performed in consecutive cycles using a rastered focused Arþ beam of Eion ¼ 4 keV. The average current density of the nanostructuring beam was about 0.8 mA/cm2. The beam spot diameter was about 0.8 mm and the ion flux at the spot was about 400 mA/cm2. The angle of ion beam incidence, aion, was in the range of 45–80 with reference to the surface normal. Various polar angles 4, defined here as the angle between the projection of the beam on the surface and the C110D surface crystallographic direction, were chosen. Ion fluences in the range of 9.0 1013–4.5 1017 ions/cm2 were applied. The crystal was kept at room temperature during ion beam bombardment. After irradiation the samples were transferred in UHV into a microscope chamber for AFM imaging at room temperature. Atomic Force Microscopy images were obtained in contact mode (C-AFM) with a Park Scientific Instruments VP2 STM/AFM. Commercially available silicon piezoresistive cantilevers were used as probes. The applied force was typically about 100 nN.
In Kelvin Probe Force Microscopy (KPFM), the sample topography was acquired simultaneously from a map of the lateral distribution of surface contact potential. In general, the topography was measured using the non-contact AFM mode (NC-AFM) by driving the cantilever at its resonant frequency (f1 y 284 kHz) with pre-set detuning (FM mode). At the same time, the ac bias (at frequency u ¼ 0.4 kHz and amplitude of 0.5 V) with dc offset was applied to the sample to minimise an electrostatic interaction between the tip and the sample. The electrostatic interaction is induced by the work function (F) differences between the tip and the sample. The obtained map of Vdc represents the surface contact potential (CP) distribution. Other details of the technique and our FM–KPFM system were described elsewhere [16]. Ex situ Scanning Electron Microscopy was performed with a Jeol (JSM-5500LV) Microscope equipped with a silicon X-ray detector (IXRF Systems) for Energy Dispersive X-ray Analysis (EDX) at the Krakow Academy of Fine Arts.
3. Results and discussions 3.1. Atomic Force Microscopy measurements A clean, reconstructed InSb(001) surface composed of large atomically flat terraces is presented in Fig. 1. The surface is highly anisotropic with atomic rows aligned along the C110D crystallographic direction as shown by high resolution non-contact AFM measurements (cf. inset of Fig. 1). The irradiation with an Arþ beam at room temperature easily destroys the long-range atomic order on the sample surface even for a small ion fluence. For a fluence as low as 9.0 1013 ions/cm2, the LEED pattern characteristic for a c(8 2) reconstruction is transformed into that for a (1 1) structure. We have studied the dependence of the surface nanostructuring process on the crystal orientation with respect to the ion beam. In Fig. 2a–d, the InSb(001) surfaces bombarded with Eion ¼ 4 keV Arþ beam under aion ¼ 45 at low fluence (1 1016 ions/cm2) and polar angles of 4 ¼ 45 , 0 , 45 and 90 , respectively, are presented [17]. The main features developed on the irradiated surfaces are the dots and wires. From Fig. 2 it is clear that the wire formation is strongly dependent on the initial surface anisotropy. The wires are formed
Fig. 1. NC-AFM image of clean, reconstructed InSb(001) surface. The high resolution non-contact AFM image (12 12 nm2) with the surface atomic arrangement is presented in the inset of the figure.
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Fig. 2. C-AFM images of nanostructured InSb(001) surfaces performed with an Arþ beam (Eion ¼ 4 keV, aion ¼ 45 and fluence 1.2 1016 ions/cm2) at room temperature and polar angles 4 of (a) 45 , (b) 0 , (c) 45 and (d) 90 against the C001D direction. The black arrows indicate the projection of the ion beam on the irradiated surfaces (from Ref. [17]).
predominantly along the direction perpendicular to the reconstruction rows, i.e. along C110D. The wires cover the surface with the highest density if the ion beam direction is perpendicular to the rows (Fig. 3d), whereas, if the ion beam projection is parallel to the atomic rows only few relatively short wires are seen with random orientation (Fig. 3b). In the latter case, the dominant features are dots. For the system under study, the authors have already shown that surface diffusion of adatoms, generated by the ion impact, is the dominant process leading to development of anisotropic nanostructures (wires) on the irradiated surface [4]. Thus, the observed dependence of the direction of created wires on the surface anisotropy indicates that crystallographic-oriented diffusion is especially important for the initial stage of surface nanostructuring. Although long-range atomic ordering on the irradiated InSb(001) surface disappears already for very low ion fluence (as shown by the lack of the LEED pattern characteristic for the reconstructed surface), one cannot exclude that some short-range atomic ordering remains even for prolonged ion bombardment. In order to find out to which extent the surface crystallinity is preserved during ion irradiation we have studied the dependence of the wire formation on the ion beam fluence (the time of irradiation). The results are shown in Fig. 3, where a set of AFM images of InSb(001) surface irradiated with Eion ¼ 4 keV Arþ ions under aion ¼ 60 and at a polar angle 4 ¼ 25 with different fluences is shown. The surface irradiated with small ion beam fluence, of 3.5 1015 ions/cm2 (Fig. 3a), where the steps and terraces characteristic for non-irradiated surface can still be recognised, is covered with randomly spread clusters (dots) of approximately 50 nm in diameter and 5 nm in height. Rarely, elongated structures (wires) are seen which are aligned along the C110D direction, i.e. perpendicular to the initial reconstruction rows. For higher fluence, of about 4.2 1016 ions/cm2 (Fig. 3b), the wire-like structures are the dominant features on the surface. The wires are aligned along two different
directions. Most of wires are aligned along the projection of the ion beam on the surface. Those wires are typically over 1 mm long and are not uniformly wide at their base. They are broader at their front side (with respect to the ion beam) getting subsequently thinner. A few wires, however, are aligned along the C110D direction. Those wires are uniformly wide, about 70 nm in the base. Moreover, some of the observed wires are not straight with one part parallel to the ion beam projection, while the other part parallel to the C110D direction. Those wires most likely could result from an interconnection of two separated wires developing in two different directions. As the irradiation proceeds, the total number of wires increases with simultaneous increase of the number of wires aligned along the beam direction. For the fluence of 1.0 1017 ions/cm2 (Fig. 3c) the dense structure of wires aligned along the beam direction dominates the topography of the irradiated surface. For still higher fluence (3.7 1017 ions/cm2 – Fig. 3d) a clear ripple structure, parallel to the projection of the ion beam, is developed on the irradiated surface. The experimental findings indicate that the irradiated InSb(001) surface preserves its crystallinity during the initial stages of ion sputtering. The surface is assumed to become amorphous at a fluence of about 1.0 1017 ions/cm2 since for this fluence the developing anisotropic surface nanostructures are predominantly aligned along the direction of the ion beam. Also, Evdokimov et al. [18] concluded from low energy ion-scattering and ion-recoil spectroscopies that the InSb(001) surface did not appear amorphous even at very high (in the range of 1018 ions/cm2) ion fluence. Such fluences exceed well those required to produce amorphous surfaces of GaAs and Si. However, in their experiment the surface was bombarded with 4.8 keV Arþ ions at the grazing incidence (an angle of 10 to the surface). Thus the real amount of the ions penetrating the target was considerably smaller due to surface scattering. Once the InSb(001) surface becomes amorphous, the direction of the ion beam is the predominant factor influencing the anisotropy of the nanostructured topography. This is shown in Fig. 4a and b. Fig. 4a
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Fig. 3. C-AFM images of nanostructured InSb(001) surfaces performed with an Arþ beam (Eion ¼ 4 keV, aion ¼ 60 , 4 ¼ 25 ) at room temperature, with fluences of (a) 0.35, (b) 4.2, (c) 10.0 and (d) 62.5 1016 ions/cm2, respectively. The white arrow in (a) indicates the projection of the ion beam on the surface (the direction is common for all images).
depicts the InSb(001) surface bombarded to the stage when it is covered with closely packed wires parallel to the ion beam (fluence of 5.0 1016 ions/cm2). Consecutive irradiation of this surface after rotation by about 25 around the normal to the sample results in the topography shown in Fig. 4b for the total fluence of 1.1 1017 ions/ cm2. The surface is covered with closely packed wires whose direction is again parallel to the direction of the ion beam. For the present experimental conditions it has been found that the prolonged ion irradiation of the InSb(001) surface leads to the
development of a ripple-like structure similar to the one shown in Fig. 3d. The ripples are always parallel to the direction of the ion beam on the irradiated surfaces. In Fig. 5, the dependence of the ripple wavelength (l) on the ion incident angle and for the polar angles 0 and 90 is presented. For a given polar angle, a week dependence of the ripple wavelength on the incident angle is observed and the ripples have a maximum l when created with the aion ¼ 70 for 4 ¼ 0 . For the whole range of aion, the wavelength of ripples created with the ion beam directed parallel to the initial
Fig. 4. Two stages of development of the nanostructured InSb(001) surface irradiated with an Arþ beam. In (a) the surface is irradiated with ion beam fluence of 5.0 1016 ions/cm2 and in (b) the surface developed under successive irradiation (the total fluence of 1.1 1017 ions/cm2) but after rotation of about 25 around the normal to the sample surface. (CAFM images, 3 3 mm2). The relative change of white arrows (the beam direction) with respect to the image frames reflects the rotation of the sample between the two successive cycles of ion irradiation.
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a log–log plot of s versus the irradiation time, t, extracted from the surface evolution presented in Fig. 3, is shown. The rms roughness follows the power scaling relation s w tb with the growth exponent b ¼ 0.80 0.07. We have found similar scaling of the rms roughness with the irradiation time for InSb(001) surfaces at other incident and polar angles of the Arþ beam. According to the BH model, the amplitude (and correspondingly the rms roughness) of the periodic structures due to ion bombardment should depend exponentially on the irradiation time, s w exp(t). The scaling law s w tb observed here disagrees with the predictions of the BH model. As demonstrated theoretically, a power law scaling s w tb is a consequence of various nonlinear effects occurring on the irradiated surfaces [20]. In the present case, we associate the nonlinear behaviour in the evolution of InSb(001) surface morphology with the dominance of the ionenhanced surface diffusion of adatoms which is recognised as the dominant process influencing the surface nanostructuring [4]. Fig. 5. Dependence of the ripple wavelength developed on InSb(001) surfaces under Arþ bombardment on ion incident and polar angles (fluences in the range of few time 1017 ions/cm2).
atomic rows (4 ¼ 0 ) is higher (on average by about 40%) than l of ripples created for the ion beam direction perpendicular to the atomic rows (4 ¼ 90 ). The observed dependence of the ripple wavelength on the polar angle can be understood if one takes into account that before the ripple development, i.e. at the early stage of ion irradiation, the surface is covered predominantly with dots. It has been already shown, that the effect of ion irradiation results in the elongation of the initial dots transforming them into wires and finally, for the long irradiation time, closely packed wires (ripples) are developed [4]. From Fig. 2, it is obvious that the dimensions of the initial dots depend on the polar angle. The dots created with ion beam of direction parallel to the reconstruction rows (Fig. 2b) are larger (of 74.0 10.3 nm in diameter at their base) than the dots created with ion beam incidence perpendicular to the atomic rows (of 52.5 10.5 nm – Fig. 2d). Thus, the size of initial dots influences the transverse dimension of the wires and finally, the periodicity of the ripples. In order to describe the process of InSb(001) surface nanostructuring in a more quantitative manner and for comparison with the existing continuum models for surface erosion by ion sputtering, we have investigated the surface morphology evolution in terms of the surface root mean square (rms) roughness, s [19]. In Fig. 6,
Fig. 6. Log–log plot of the rms surface roughness, s, versus the irradiation time, t. The solid line represents the fit according to the scaling power law s w tb with the growth exponent b ¼ 0.80 0.07.
3.2. Scanning electron microscopy/energy dispersive X-ray analysis The chemical composition of the ion irradiated InSb(001) surfaces was investigated with Scanning Electron Microscopy (SEM) equipped with an EDX detector. In Fig. 7, a SEM image of InSb(001) irradiated with Arþ beam at aion ¼ 80 and fluence of 1016 ions/cm2 is shown. On the surface, the wires together with some large (of few microns) droplet-like structures are visible. The lateral resolution of the EDX technique is not sufficient for resolving the wire composition, as the X-rays are generated by the analyzing electron beam in a region of about 2 mm in depth. However, the composition of the droplets could easily be determined. The droplets, initially symmetric in shape as checked with SEM, originate from the cleaning procedure. The EDX spectra show that the droplets are composed of 99% indium. The excess In atoms, produced during the sputter cleaning performed at elevated temperature, are assumed to aggregate to droplets. Their elongated shape, seen in Fig. 7, is due to ion beam modification in the process of surface nanostructuring. The front side of the elongated droplets (the white part in Fig. 7) with respect to the beam direction is composed of In (99%) whereas the backside is composed of both, indium (52%) and antimony (48%). The same ratio of In/Sb has been found for the surrounding surface, thus the part assigned above as
Fig. 7. SEM image of the InSb(001) surface bombarded with an Arþ beam (Eion ¼ 4 keV, aion ¼ 80 , fluence of few time 1016 ions/cm2) at room temperature. Together with the wires big droplet-like structures are seen, which originate from the cleaning procedure. The white arrow indicates the projection of the ion beam on the surface. The zone marked with A depicts the surface area of the highest surface density of the wires.
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the backside of the droplets is actually the non-irradiated substrate surface area shadowed by the initial big droplet. From the investigation of the evolution of surface topography as a function of the irradiation time we concluded that the wires originate from the elongation (tailing) of initial dots. However, it was not clear wherever the dots are getting elongated upstream (towards the incoming ions) or vice verse, downstream the ion beam direction. From Fig. 7 (the zone marked with A) it is seen that the surface density of the wires created at the front side of the big droplets is higher than the average wire surface density on the whole irradiated surface. This gives evidence that during the ion irradiation adatoms diffuse in the direction of incident ion beam (ion-enhanced diffusion) and accumulate at the front side of the dots created at the initial stage of irradiation. As a result, the dots ‘‘move’’ towards the incoming ions to form the wires. As the irradiation proceeds, the wires are getting longer due to adatoms accumulation at their front side but, at the same time, the backside of the wires is getting thinner as it undergoes extensive ion sputtering. Such mechanism of dot elongation and wire creation, explains the observed shape of the wires being broader at their front side (with respect to the ion beam) and getting subsequently thinner (see Fig. 3b). 3.3. Kelvin Probe Force Microscopy measurements To get more information on the composition of the nanostructured surfaces Kelvin Probe Force Microscopy measurements were performed. In Fig. 8, the topographic and the contact potential (CP) images of an InSb(001) surface irradiated with a fluence of 2.1 1016 ions/cm2 are shown. In the topography image (Fig. 8a) there are two dots interconnected with the wire. There is also a small cluster close to the wire. In the CP image (Fig. 8b) the corresponding Kelvin probe signal contrast indicates that all the structures are made of a material with different work function with respect to the surrounding InSb substrate. For both structures (for the wire and the
Fig. 8. Topography (a) and contact potential (b) images of nanostructures developed on the irradiated InSb surface acquired with KPFM. In (c) the cross-section of the contact potential of the dots, taking along the line marked with A in (b), is shown.
dots) the work function is lower than that of the substrate areas. The measured decrease of the work function is size dependent as illustrated by the line profile across the surface contact potential image (Fig. 8c). The larger dot has lower potential by about 50 mV in comparison with the potential of the neighbouring smaller dot. The CP of the wire, on an average, is over two-fold lower than the CP of the dots (Figs. 8c and 9). However, the differences of the CP values cannot be directly associated with the differences in the relative contribution of In and Sb in the nanostructure composition. The electrostatic force is a long-range interaction and the obtained surface potential distribution can be affected by averaging effect due to a finite tip size. Results of several other experiments in which ion irradiated InSb surfaces without subsequent annealing induced modification were investigated with spectroscopic methods indicate that the surfaces are enriched with indium [13,14]. However, a lack of the lateral resolution in the spectroscopic measurements does not allow to determine in which form excess indium atoms are accumulated on the bombarded surface. We compare these results with our Kelvin Probe Force Microscopy measurements performed on the nanostructured InSb(001) surface. Although the KPFM is unable to identify directly the elemental composition of the nanostructures, it reveals that the wires and the dots are made of material with a work function being different from that of the irradiated InSb substrate (Fig. 8b). The lower work function of the nanostructures indicates an excess of indium in the developed surface nanostructures, since the work function of In (FIn ¼ 4.12 eV) is lower than that for the irradiated InSb(001) surface (Firrad.InSb ¼ 4.6 eV [5]). Indium enrichment of the wires and dots means that there is a metallic-type structure generated on the semiconductor substrate. In such a case, the condition of thermodynamic equilibrium implies that the Fermi levels of the two materials must coincide with each other. This condition is fulfilled by charge transfer and band bending near the interface in analogy with that on the interface between a bulk metal and a semiconductor (Schottky barrier model). The direction of electron flux in a metal– semiconductor contact depends upon the relative values of work functions of the two materials and the electrons will travel from the
Fig. 9. Line profiles of the wire topography (upper graph) and the CP signal (lower graph). The profiles are taken along the lines marked with B in Fig. 8a and b, respectively. In the upper graph the substrate region enriched with electrons is drawn schematically.
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material with the smaller work function to the material with the higher one. In the present case, Fnanostructure < Fsubstrate, thus, electrons are transferred from the nanostructures to the substrate. The metal–semiconductor interface is charged and there is a region in the semiconductor substrate of some thickness enriched with electrons [21]. In Fig. 9 the line profile (cross-section) of the wire (along the line B in Fig. 8) and the corresponding cross-section of the wire contact potential image is shown. The deep minimum in the CP signal corresponding to the potential of the wire is accompanied by two ‘‘shoulders’’ on both sides. The signal at the shoulders’ maxima is higher by about 50 mV in comparison to the potential of the InSb substrate. In the CP image (Fig. 8b) the ‘‘shoulders’’ are seen as two bright stripes aligned along the broad, dark stripe corresponding to the CP of the wire. The maximum of the ‘‘shoulders’’ correspond to the border between the wire and the substrate. It is likely that the increased CP signal in the ‘‘shoulders’’ reflects the charged metal–semiconductor interface. We have associated the width of the CP ‘‘shoulders’’ (from their onset to their maximum) with the thickness of the substrate region enriched with electrons. In Fig. 9, a cross-section of the substrate region enriched with electrons is drawn schematically. The termination of the enriched region on the surface implies that the substrate areas on both sides of the wire should be negatively charged. In fact, the increased magnitude of the CP signals on both sides of the wire support the concept that the areas around the wire are negatively charged since it was already shown by Sommerhalter et al. [22] that in the CP image the bright contrast corresponds to negatively charged areas. The results in Fig. 9 may, of course, be influenced by the lateral resolution of the KPFM instrument. 4. Conclusions Several aspects of ion bombardment induced surface modification caused by non-stoichiometric sputtering, beam enhanced surface diffusion and nanostructure development have been investigated for the InSb(001) surface. Firstly, we have observed that the ion irradiation led to creation of nanodots on the surface at the initial stage of irradiation, which subsequently, form nanowires as the time of irradiation increases. The wire development is explained as an effect of nanodot elongation upstream the ion beam direction due to the accumulation of the diffusing adatoms created by the ion impact. Secondly, we have found that two types of wires were formed with the shape and orientation dependent either on the primary beam orientation, or on the crystallographic orientation of the reconstruction rows at the (001) c(8 2) InSb surface. This finding indicates that, at the initial stage of ion irradiation, i.e. for fluences up to few times 1016 ions/cm2, anisotropic
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surface diffusion along the reconstruction rows plays an important role together with beam enhanced diffusion along the direction of the initial momentum transfer. Finally, we have found that ion sputtering of InSb at low fluences is highly non-stoichiometric. The contact potential measurements have shown that the surface nanostructures, at every stage of their development, had lower work functions in comparison to the surrounding amorphous InSb substrate. It is concluded, that the Sb component of the target is sputtered preferentially and the nanostructures developed on irradiated InSb surfaces consist of indium. Acknowledgements This work was supported by the European Commission under Contract No. MTKD-CT-2004-003132, 6th FP-Marie Curie action for Transfer of Knowledge: ‘‘Nano-engineering for Expertise and Development’’ – NEED. Careful reading of the manuscript by Professor M. Szymonski is kindly acknowledged. References [1] For review see: Jacak L, Hawrylak P, Wojs A. Quantum Dots. Berlin,: Springer; 1998. [2] Facsko S, Dekorsy T, Koerdt C, Trappe C, Kurz H, Vogt A, et al. Science 1999; 285:1551. [3] Frost F, Schindler A, Bigl F. Phys Rev Lett 2000;85:4116. [4] Krok F, Kolodziej JJ, Such B, Piatkowski P, Szymonski M. Nucl Instrum Methods B 2003;212:264. [5] Krok F, Kolodziej JJ, Such B, Piatkowski P, Szymonski M. Appl Surf Sci 2003; 210:112. [6] Bradley RM, Harper JM. J Vac Sci Technol A 1988;6:2390. [7] Constantini G, Rusponi S, Gianotti R, Boragno C, Valbusa U. Surf Sci 1998;416: 245. [8] Constantini G, Rusponi S, Buatier de Mongeot F, Boragno C, Valbusa U. J Phys Condens Matter 2001;13:5875. [9] Cuerno R, Barabasi A-L. Phys Rev Lett 1995;74:4746. [10] Sigmund P. Phys Rev 1969;184:383. [11] Massies J, Dazaly L. J Appl Phys 1984;55:3136. [12] Schweitzer MO, Leibsle FM, Jones TS, McConville CF, Richardson NV. Surf Sci 1993;280:63. [13] Bouslama M, Jardin C, Ghamnia M. Vacuum 1995;46:143. [14] Yu W, Sullivan JL, Saied SO. Surf Sci 1996;352–354:781. [15] Kolodziej JJ, Such B, Czuba P, Piatkowski P, Krok F, Szymonski M. Surf Sci 2002; 506:12. [16] Krok F, Kolodziej JJ, Such B, Struski P, Czuba P, Piatkowski P, et al. Surf Sci 2004; 566–568:63. [17] Szymonski M, Krok F, Struski P, Kolodziej JJ, Such B. Prog Surf Sci 2003;74:331. [18] Evdokimov IN, Valizadeh R, Armour DG, Richardson NV, McConville CF. Surf Sci 1994;318:281. [19] The rms surface roughness was calculated from the AFM images using the WSxM software (www.nanotec.es). [20] Makeev MA, Cuerno R, Barabasi AL. Nucl Instrum Methods B 2002;197:185. [21] Ioannides T, Verykios XE. J Catal 1996;161:560. [22] Sommerhalter Ch, Matthes ThW, Glatzel Th, Jaeger-Waldau A. Appl Phys Lett 1999;75:286.