Pyramidal pits created by single highly charged ions inBaF2single crystals

Abstract

In various insulators, the impact of individual slow highly charged ions ͑eV-keV͒ creates surface nanostructures, whose size depends on the deposited potential energy. Here we report on the damage created on a cleaved BaF 2 ͑111͒ surface by irradiation with 4.5ϫ q keV highly charged xenon ions from a roomtemperature electron-beam ion trap. Up to charge states q = 36, no surface topographic changes on the BaF 2 surface are observed by scanning force microscopy. The hidden stored damage, however, can be made visible using the technique of selective chemical etching. Each individual ion impact develops into a pyramidal etch pits, as can be concluded from a comparison of the areal density of observed etch pits with the applied ion fluence ͑typically 10 8 ions/ cm 2 ͒. The dimensional analysis of the measured pits reveals the significance of the deposited potential energy in the creation of lattice distortions/defects in BaF 2 .Swift heavy ions ͑MeV-GeV kinetic energy͒ have become an important tool for structural modifications of various materials at the microscale and nanoscale for a wide range of applications in the last two decades. 1-4 One major limitation of using these high-energy ions is the damage creation in deep layers which in some applications should be avoided. The desire to confine the damage to the first few layers, which is essential for applications such as ion projection lithography, has stimulated the interest for the use of slow ͑eV-keV͒ highly charged ions ͑HCIs͒. 5 This type of ions is now readily available after recent developments in ion source technology leading to powerful ion sources such as the electron-beam ion trap ͑EBIT͒. 6,7 While electronic energy loss of swift heavy ions is the major cause of material modifications, 8,9 potential-energy deposition is dominating surface modifications by HCI. 10 During interaction with the solid surface HCI deposit their potential energy ͑the total ionization energy required for producing the high charge state from its neutral ground state͒ within a few femtosecond in a nanometer-sized volume close to the surface. 10-12 Initially the potential energy is deposited in the electronic subsystem of the target leading to strong electronic excitations. Strong electron-phonon coupling can then induce local surface modifications in various solids. Recently, HCI-induced surface modifications such as hillocks, 13-15 craters, 16 pits, 17 and calderalike structures 18 with nanometer dimensions have been demonstrated. 10 The study of nanostructure formation on surfaces induced by HCI is a relatively new field and still requires a detailed comparison between materials with common and different properties, in order to develop a more general understanding of the underlying mechanisms. For this aim and motivated by our recent findings regarding surface nanostructuring of CaF 2 by means of HCI ͑Refs. 13 and 14͒, we selected BaF 2 as one of the ionic alkaline-earth fluorides ͑along with CaF 2 and SrF 2 ͒ which have a wide range of potential applications in microelectronic ...

Highly charged ion induced nanostructures at surfaces by strong electronic excitations

“…8(b) and (e)) [77][78][79]. In these cases each individual ion impact develops into a pyramidal etch pit, as can be concluded from a comparison of the areal density of observed etch pits with the applied ion fluence.…”

“…In these cases each individual ion impact develops into a pyramidal etch pit, as can be concluded from a comparison of the areal density of observed etch pits with the applied ion fluence. A size analysis reveals the significance of the deposited potential energy in the creation of these pits [77,79]. Although the defects are not mobile enough to directly lead to desorption, the ion's impact region is structurally weakened by the potential energy deposition and therefore susceptible to chemical etching.…”

Phase Diagram for NanostructuringCaF2Surfaces by Slow Highly Charged Ions

“…3) to remain qualitatively valid. For BaF 2 (111) and KBr (001) for example, we have previously observed only the A and B phases [22,31]. The phase diagram predicts that by further increasing the potential energy of the HCI we should be able to reach region C, i.e.…”

“…The defect mediated desorption mechanism is less probable in CaF 2 , since color center recombination below the surface is much more likely [27] agglomerates [28,29]. The material in the vicinity of the impact region is not ablated but structurally weakened and forms the nucleus of an etchable defect subsequently removed by a suitable etchant [22]. The synergistic effect induced by the accompanying kinetic energy originates from kinetically induced defects created in the collision cascade which enhance the trapping of the color centers created by potential energy [30] and therefore increases defect agglomeration.…”

Charge Exchange and Energy Loss of Slow Highly Charged Ions in 1 nm Thick Carbon Nanomembranes

“…Therefore, the interaction of HCI with surfaces may not be described in terms of an equilibrium charge state dependent stopping force. Furthermore, due to the localization of the energy deposition slow HCI can be used as an efficient tool for surface nano-structuring [13][14][15][16][17][18][19][20][21][22][23][24] and tuning of the electrical properties of materials [25], as well as a probe for surface energy deposition processes [26,27]. Recently, it has been shown that slow HCI can create pores in 1 nm thick carbon nanomembranes (CNM) [28,29] mainly by deposition of their potential energy [30].…”