Surface Coatings Technology 203 (2009)2458-2462Contents lists available at ScienceDirectSurface Coatings TechnologyELSEVIERjournal homepage:www.elsevier.com/locate/surfcoatNanometer-scale surface modification of KBr (001)single crystal byAr ion bombardmentO.P.Sinha S.R.Saeed 2,F.Krok,M.Szymonski*Research Center for Nanometer-Scale and Advanced Materials (NANOSAM).Faculty of Physics,Astronomy and Applied Computer Science.ABSTRACTAvailable online 5 March 2009Morphological development on alkali halide crystal surfaces,under ion irradiation,has been investigated bymeans of Atomic Force Microscopy in UHV conditions.The KBr (001)surface has been exposed to Ar+beamwith varying incident angles(from5'to75off-normal)and energies(from 10to 5.0 keV).Different kinds ofSurface modificationnanostructures,depending on ion incident angle,have been observed on the irradiated surfaces such asSurface patterningnanocavities,or nanorings and nanodots,preferentially for close to normal incidence,and ripples for thelon irradiationoblique incidence.The formation ofripple structures on KBr surfaces as a function of the incidence angle andRipplesElectronic desorptionenergy has also been observed.The orientation of the ripples has been found to be incdent angle dependentand it is perpendicular,or parallel to the ion beam projection on the irradiated surface for angles of 30 andAtomic force microscopy75,respectively.Moreover,it has been observed that the wavelength of the ripples also depends on energy ofthe projectile ions.The AFM images acquired with atomic resolution reveal well ordered,crystalline structureof the ripples even after bombardment with high fluence.The results have been discussed in terms ofelectronic and ballistic processes of sputtering and existing theories for surface modifications.2009 Elsevier B.V.All rights reserved.1.Introductionto insulators [4-9].One such appealing morphology is the ripplestructure formation.The underlying mechanism for this evolution ofThe fabrication of nanoscale surface structures such as ripples,regular nanometer structure is a self-organization process caused bynanorods,quantum dots and wires have attracted considerablethe interplay and competition between surface roughening byattention due to their technological applications in optical andcurvature dependent sputtering and smoothening due to surfaceelectronic devices [1].Yet,the nanostructures obtained in various-diffusion process commonly attributed to the Bradley-Harper (B-H)assembled ways have a size distribution wider than required bytheory [10].Several researchers extended B-H theory by the inclusionapplications,and display random alignment.Lithographic methodsof viscous flow relaxation [6,11,12],ion enhanced or inhibited diffusionare often considered as prime candidates to overcome these short-[13].preferential sputtering without actual mass transport [14].comings,but due to their limited resolution they are open for furtherinterlayer diffusion (Ehrlich-Schwoebel barrier influence),crystal-challenges.Consequently,there is continued high demand forlographic dependence of the surface and bulk diffusion [15,16],thealternative methods that would allow low cost and efficient massrandom or noisy arrival of ions and the statistical variation of thefabrication of nanoscale surface structures.In the light of thesesputtering rate [17.and the effect of recoiling-adatom diffusiontechnological and scientific driving forces,the recent demonstrationinduced by ion irradiation [18].As a result,the exact mechanism forof nanostructure development by energetic ion beam has captured theripple formation remains controversial even after two decades frominterest of the scientific community as alternative and promising toolits first observation.Recently,Makeev,Cuemo and Barabasi (MCBfor the cost-efficient fabrication of large-area nanostructured surfacestheory )19 derived a stochastic nonlinear continuum equation tovia self-organization rather than lithography techniques [2,3].Variousdescribe the morphological evolution of amorphous surface eroded bymorphological evolutions of surfaces created by ion bombardment areion irradiation,adding nonlinear and fourth-order terms in surfaceof both fundamental and technological interest,ranging from metalsrelaxation to address the inadequacies of the B-H theory.The MCBtheory introduced an ion-induced smoothing mechanism via prefer-ential sputtering without mass movement on the surface.Being ioninduced,the smoothing remains effective when the thermal diffusionE-mail address:ufszymoncyf-kredu.pl (M.Szymonski).is negligible,and therefore makes the wavelength temperatureOn leave from Amity Institute of Nanotechnology.Amity University.UP.NOIDAIndia.independent at low temperatures.In addition,this theory has manycompelling trends including a linear increase of the ripple wavelengthSulaimani,Kurdistan region,Iraq.with ion energy,and saturation of the ripple amplitude at a longerdoi10.1016/j.surfcoat.2009.02.111O.P.Sinha et aL Surface Coatings Technology 203 (2009)2458-24622459nected with the redistribution chamber allowing for transferring ofthe samples between the chambers in UHV conditions.The SPMchamber is equipped with Omicron VT-AFM microscope.In thepreparation chamber the sample is mounted on the samplemanipulator,characterized by linear and rotated motion for experi-mental parameter adjustments.Prior to ion irradiation the sample isheated for 1 h at 425 K to remove the surface water overlayers.Thetemperature is measured with the thermocouple fixed to the sampleholder.The nanostructuring of the sample surface is performed withrastered Ar*focused beam (with the spot diameter about 1 mm).Thescanning area ofion beamirradiation exceeds the sample surface areain order to prevent the charging of the KBr surface during ion beambombardment.The Ar beam current density measured on amolybdenum plate is in the range of 10 uA/cm2 to 5.0 uA/cm2,depending on the ion energy.The secondary electron emission of theMo was suppressed by applying the positive bias to the plate.Theexperiments are performed with different parameters such as theincidence angle of the ion beam chosen in the range of 5-75 withUnbombardedrespect to the surface normal,and beam energy in the range of 1.0-5.0 keV with the fixed ion fluence of about 5x 1017 ions/cm2 at roomtemperature(RT).After the ion irradiation the sample is transferred tothe microscope chambers for imaging with UHV contact mode atomic5force microscope (C-AFM).The average patter wavelengths arededuced from AFM images,as well as from their two dimensional(2D)Fourier transformations (FFTs).The imaging is carried atdifferent places on the sample surfaces in order to get better statistics.The RMS surface roughness is measured directly from AFM topo-1.02.0graphy images.X (um)Fig.1.2 umx 2 um AFM image of typical KBr (001)substrate after cleavage in air3.Results and discussionfollowed by annealing in UHV at 425 K for 90 min.A typical image of KBr(001)surface after cleavage in air followedby annealing in UHV at 425 K for 1 h is shown in Fig.1.In this figure,time scale of irradiation.The well defined self-organized ripplethere is a clear observation of extended atomic terraces with typicalstructures appeared on CaF2 [8].It is suggested that the developedsizes exceeding 1.0 um separated by monoatomic step.The modifica-nanostructures arise from self-organization of the surface intotion of KBr(001)surfaces by 4 keV Ar ions for a fluence of 5x1017periodic stress domains after fluorine preferential erosion.Also,theions/cm as a function of incident angle of 5,15,20,45,60 and 75nanometric ripples were created on irradiated LiF surface by 800 eVArrespectively is presented in a sequence of images presented in Fig.2ion bombardment at an angle 35 and temperature about 80C[9].(a-f).The formation,shape and ordering of patterns depend onIn this context,alkali halides definitely appear as promisingprocess parameters like the ion beam incidence angle.By varying thematerials as they are transparent in nature acquiring a distinctiveion incidence angle,a change in the evolution of patterns on thecolor after an appropriate chemical or physical treatment.Althoughirradiated surfaces has been observed.Nanocavities and nanodotsdifferent kind of radiation can be exploited to induce the electronic(marked with circle)are the prominent features for 5 and 15 ofdefects,the use of low energy ions is particularly appealing because itincident angle irradiation.Moreover,it is interesting to see that theallows getting thin layers with high concentration of active centers atnanodots are accumulated to get the shape of initial nanorods forthe surface of the crystalline materials.Among the alkali halides,15 of the irradiation.Also,these nanorods seem to be connected topotassium bromide (KBr)is of special interest due to its numerouseach other.The developed nanostructures have dimension of 50-applications in research and technology.90 nm in width and 3-8 nm in height.In this paper,we report on the nanostructure formation on KBrFor the angle of incidence of the impinging ions increased to 30.(001)surface by Ar+beam irradiation as a function of varyingthere is clear observation of ripple structures which are perpendicularincident angle and energy.It is observed that the orientation of theto the projection of the ion beam on irradiated surface.The rippledeveloped anisotropic nanostructures is incident angle dependentwavelength is about 100 nm and the ripple height in the range ofwhile the wavelength and RMS surface roughness depend on the10 nm.Further increase in the angle of incident ions leads toprojectile energy.disappearance of the surface structures anisotropy(see Fig.2d withthe 6=45 )For the ion incidence angle approaching 75 off-normal,2.Experimentalthe clearanisotropy patter(ripple pattem)appears,which is parallelto ion beam projection on the surface.The observed change of rippleKBr crystals purchased from Kelpin Crystal(Neuhausen,Germany)orientation is consistent with the B-H theory.has been cleaved in air parallel to the (001)cleavage plane.TheFig.3 shows the (RMS)surface roughness (calculated from AFMsamples of the dimensions of 5x5 mm2 were mounted on aimages)versus the incidence angle.It is clear from the graph thatmolybdenum plate with the help of titanium clamps and fixed oninitially for incident angles up to 45,the RMS surface roughnessthe sample holder which can be resistively heated up to 1000 K.Thehardly depends on the angle,whereas for higher incidence angles itexperiments have been performed in an ultrahigh vacuum system,ncreases gradually by a factor of -2.with the base pressure in the range of low 10-10 mbar.The systemIn Fig.(4a-e)images of the KBr surface modified by Ar+beam forconsists of three main chambers for sample introduction (load-lock),varying energy of 1 keV,2 keV,3 keV,4 keV and 5 keV,respectively aresample preparation,and SPM imaging.The chambers are intercon-shown.The angle of incidence is kept at 75'off-normal and the fluence2460300661283000.40.81.00.40.81.00.40.81.0X(um)X(um)450600(wu)2054N10000.40.81.00.40.81.000.40.81.0X (um)X(um)En=4.0 keV and=5.0x10 ions/cm at room temperature.Arrows indicate the direction of the ion projectile and the insets are the 2D self-correlation maps.is 5x1017 ions/cm2.It is clear from the micrographs that the ripplecomponent known as effective surface diffusion (ESD)becomes theformation is occurring already at 10 keV energy.The ripples becomedominating smoothing mechanism.The variation of A with E revealsmore prominent and clear with increasing projectile energy.As thethat mostly the contribution of ion-induced component to ESD isangle of incidence is 75 off-normal,the observed ripples are found toconsistent with MCB theory.be parallel to the projection of the ion beam,demonstrating that theBasic assumptions of B-H and MCB approaches imply,that theripple orientation is energy independent.local erosion yield is continuously and smoothly varying with the localThe wavelength and RMS surface roughness dependences on Arsurface morphology and,as shown by Sigmund [22],the erosion isprojectile energy derived from the AFM images from Fig.4 are shownfaster in depressions than at elevations due to a spatial extension ofin Fig.5.It is clear from wavelength versus ion energy plot that thewavelength of the ripples is increasing with the increase of the ionenergy up to 5.0 keV (neglecting the wavelength for 1.0 keV as thestructure at this energy is not regular).The RMS surface roughnessincreases with increasing ion energy for the first four energies studiedand it approaches a maximum value at 4.0 keV,i.e.the ripple height isgreatest at 4.0 keV then tends to decrease at 5.0 keV following thetrend of the decreasing surface roughness.A standard approach to interpretation of various features of ion-induced ripples,their wavelength and the RMS surface roughness isbased upon B-H theory which states that there is interplay andcompetition between the dynamics of surface roughening as a resultof defect production by incident ion beam,associated with materialremoval by sputtering,and surface smoothening such as surfacediffusion and viscous flow.According to the Bradley and Harperoriginal work [10].the ripple wavelength,A-E-1/2,i.e.it is adecreasing function of ion energy,and the thermally activated surface20diffusion is the dominant process for the surface smoothening.Ion incidence angle (deg)However,the increase of wavelength with ion energy has beenexperimentally found by several groups [20,21].Accordingly,MakeevFig.3.Variation of the RMS surface roughness as a function of incidence angle obtainedCuerno and Barabasi (MCB)show [19]that A-E,i.e.,the wavelengthlinearly increases with ion energy.Furthermore,the ion-inducedions/cm at room temperature.