Spatial distribution of Ar on the Ar-ion-induced rippled surface of Si Debi Prasad Datta and Tapas Kumar Chini* Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India Received 23 November 2004; revised manuscript received 25 February 2005; published 9 June 2005 We have measured spatial distribution of Ar atoms on the rippled surface generated on Si undergoing 60 keV Ar bombardment at a 60° angle of ion incidence. Elemental mapping and line scans using energy dispersive x-ray spectrometry attached in a scanning electron microscope confirmed that subsequent to the interpeak shadowing of incident ion flux, most of the argon atoms are incorporated around the middle part of the front slope of ripple facing the ion beam as compared to the rear slope. The spatial extension of the argon rich phase amounts about half of the ripple wavelength. The experimentally observed compositional heterogeneity be- tween the two faces of the ripples agrees reasonably good to the well-known Monte Carlo ion simulator TRIM based theoretical calculations. DOI: 10.1103/PhysRevB.71.235308 PACS numbers: 68.37.Hk, 68.35.Ct, 79.20.Rf, 81.16.Rf I. INTRODUCTION Development of periodic ripple morphology on solid sur- faces undergoing erosion by obliquely incident ion bombard- ment has become a subject of intense research 1,2 in recent years because the controllable micrometers to nanometers scale wavelength and amplitude of such self-organized pat- terns makes them good candidates for possible application such as x-ray/optical grating or templates for growing low dimensional structure for nanotechnology. 3 The linear insta- bility theory of ion bombarded surface, developed by Brad- ley and Harper BH, 4 predicts formation of sinusoidal ripples where wavelength remains constant but amplitude grows exponentially with bombardment time. Our recent atomic force microscopy AFM study 5 of the ripple mor- phology for 60 keV Ar → Si at 60° angle of ion incidence shows that as the bombardment time increases, a critical value of the ratio of amplitude to wavelength is reached as predicted by Carter’s geometrical argument 6 for which inter- peak shadowing of incident ion flux distorts the sinusoidal ripple habit to faceted one. Neither BH theory 4 nor the more generalized theory developed by Cuerno and Barabasi CB Ref. 7 and later by Makeev, Cuerno, and Barabasi MCB Ref. 8 could explain such dynamical behavior of ripple pattern where shadowing phenomena is observed. The ap- pearance of a ripple or a slope results immediately in local angles of ion incidence deviating from the overall one. Hence the local density of bombarding ions starts to deviate from the average one even much earlier than when the shad- owing occurs. When the shadowing condition is reached the front slope of the ripples facing the ion beam will have more ions with an angle of incidence close to the local surface normal than the opposite face where the local angle of ion incidence is close to grazing causing most of the ions to be reflected instead of being implanted. Consequently, the pen- etration depth of ions that enter the front slopes is larger than of those that enter the back slopes. So a larger depth of the front slope will be affected both structurally and composi- tionally as compared with the rear slopes. Indeed, a structural variation with the formation of a thicker surface amorphous layer on the slope facing the ion beam has been observed in our recent cross-sectional transmission electron microscopy XTEM study 9 of 50–120 keV Ar-ion incident at 60° induced Si surface ripples as depicted schematically in Fig. 1. However, the compositional variation over such medium kilo-electron-volt Ar-ion-induced Si surface ripples has not been addressed so far either theoretically or experimentally though such studies have been done at low 2–10 keV en- ergy for oxygen-ion-induced ripple formation on silicon by Homma et al. 10 For ripple generation using low kilo-electron-volt ion beams with low angle of ion incidence, interpeak shadowing of the incident ion flux is not observed in general because the amplitude is much smaller than the wavelength of the ripples. As a result, influence of surface composition change caused by local incorporation of implanting ions on ripple evolution has been ignored in many cases even though pos- sibilities of the existence of spatial inhomogeneities in ion- bombardment induced ripples was predicted 11 earlier. How- ever, recent simulation 12 shows that for technological application, such as to fabricate nanowires, one should use high kilo-electron-volt obliquely incident ion beam that gives rise to V-shaped ripple pattern. For such a high energy in- duced corrugated pattern with high amplitude, the simulation FIG. 1. Schematic diagram showing the structure and morphol- ogy of a ripple feature obtained from TEM measurements see Ref. 9 for details. PHYSICAL REVIEW B 71, 235308 2005 1098-0121/2005/7123/2353085/$23.00 ©2005 The American Physical Society 235308-1