Single Ion Impact Research Articles

Hierarchical Assembly of 2D Nanostructures of Relevance for Organic Solar Cell Design

AbstractThe first step in synthesizing a model film morphology via a surface-driven hierarchical assembly process is presented. The goal of the hierarchical assembly is the control of the morphology of complex molecular layers for the investigation of fundamental processes in organic solar cells. Using a focused ion beam (FIB) with Ga+ ions at 30 keV, the surface of highly oriented pyrolitic graphite (HOPG) is patterned with an array of local amorphous carbon ellipsoid spots (ACES), which provide preferential nucleation lines at their perimeter, and thus are instrumental in the control of fullerene island growth. On the undamaged surface regions outside the ACES pattern the fullerene island growth is unperturbed, and presents the well-known combination of round and fractal island shapes. The fullerene deposition at the periphery of the ACES pattern, which is characterized by single ion impact defects, results in stunted, smaller and irregular islands. Inside the ACES array, the C60 island growth is controlled by the shape of the ACES and is constrained to lobes which form around each ACES spot. The array and C60 lobe morphology and geometry are characterized and a subsequent understanding of the C60 diffusion fields and nucleation lines within the array is discussed.

Spatial extent of band bending in diamond due to ion impact as measured by secondary electron emission: Experiment and theory

Although hydrogentated diamond emits exceptionally high numbers of electrons upon single ion impact, the secondary electron yield decays at an extremely rapid rate as a function of ion fluence. We report measurements of this rapid decay at extremely low fluences where the ion tracks are widely separated and explain the results by a model based on the downwards bending of the conduction band edge, due to positive charge trapped within the ion track. The present work demonstrates the importance of charge trapping in explaining the electronic properties of diamond and other wide band gap materials.

Ar implantation of InSb and AlN at 15 K

Ar ion implantation-induced damage formation in InSb and AlN was studied by Rutherford backscattering spectrometry with both implantation and measurement being performed at 15K without change of the target temperature. InSb can be easily amorphised with ion fluences ⩾8×1012cm−2. AlN is not rendered amorphous for ion fluences up to 5×1015cm−2. The main reason for that difference is that in InSb each ion produces amorphous material, whereas in AlN only point defects or point defect complexes remain from a single ion impact. A comparison with the results for other III–V compounds is given.

Observation of surface modification and secondary particle emission in HCI–surface interaction

We have observed secondary particle emission in collision of HCI with Si and HOPG surfaces. For HOPG, we have measured the number of secondary electrons and the number of dot structures on the surface as the imprint of the incidence by STM. The single ion impact is surely observable with almost 100% efficiency by detecting an event of the secondary electron emission. For hydrogen terminated Si(111), H+, H2+, H3+, C+ and Si+ ions have been detected. The proton sputtering yield, which depends on the charge of HCI, has been examined.

Ion electron emission microscopy at SIRAD

In this contribution we describe how single event effect (SEE) studies will be performed with the ion electron emission microscope (IEEM) of the SIRAD irradiation facility located at the INFN Legnaro Laboratory. The IEEM will be used to locate with micrometric precision the impact points of impinging ions that give rise to SEE in an electronic device under test (DUT). In the IEEM technique the ion beam is not microfocused: the position of single ion impact is reconstructed by locating the secondary electron emission points on the DUT surface. We briefly review the original solutions under implementation at SIRAD, such as the opto-electronic approach used to reconstruct the secondary electron emission points by means of a fast, high-resolution imaging of point-like light sources, and a new IEEM test system under development based on a commercial memory array sensitive to single event upsets.

Ion beam induced charge and numerical modeling study of novel detector devices for single ion implantation

In the near future devices which are fabricated from shallow arrays of few and single atoms will exploit quantum mechanical rules to perform useful functions including quantum computation. Fabrication of these devices presents formidable technological challenges. We have developed a single ion implantation system that is capable of verifiable fabrication of single donor devices using 14keV 31P ions implanted into ultra-pure, high resistivity silicon substrates based on the technique of Ion Beam Induced Charge (IBIC). A detection system with integrated detector electrodes registers the charge transient from a single ion impact which is used to signal the implantation of an ion into the substrate. We describe here the use of IBIC with MeV ions to study the charge collection efficiency of the detector electrodes. By using three dimensional numerical technology computer-aided design (TCAD) models for the decrease in the IBIC signal as a function of distance from the detector electrode, we can obtain an accurate measurement of the resistivity of the silicon substrate, allowing confirmation of the values specified by the supplier, and providing us with confidence in the numerical models used by TCAD for simulation. This technique has advantages over resistivity measurements by four-point probes because it is spatially resolved, probes through the intact oxide, and can be done without making contact to the device in the area of the probe.

Raman scattering and photoluminescence studies on O + implanted porous silicon

The effects of 125 keV O + implantation on porous silicon (PS) in the fluence range from 10 14 to 10 16 cm −2 have been investigated using Raman scattering and photoluminescence techniques. PS has been prepared by anodically etching (100) cut n-type crystalline silicon (c-Si). Raman spectra of PS have been generated using a model of phonon confinement in Si nanocrystallites and compared with experiment. As the fluence increases, the Si nanocrystallite size estimated from the Raman spectra decreases. Unimplanted PS shows a visible photoluminescence (PL) peak at 2.02 eV, with 0.3 eV FWHM coming from electron confinement in Si nanocrystallites. Its intensity decreases due to the creation of non-radiative recombination centers and the peak position increases due to nanocrystallites size reduction with increase of fluence. On O + implantation, PS shows an additional visible PL at 2.35 eV with 0.3 eV FWHM due to amorphous silicon (a-Si) nanozones created by O + single ion impact. Evidence for this PL peak is provided from O + implanted c-Si. 2.35 eV PL intensity in O + implanted PS is 10 times higher than that in O + implanted c-Si.

Ion photon emission microscopy

A new ion-induced emission microscopy has been invented and demonstrated, which is called ion photon emission microscopy (IPEM). It employs a low current, broad ion beam impinging on a sample, previously coated or simply covered with a few microns of a fast, highly efficient phosphor layer. The light produced at the single ion impact point is collected with an optical microscope and projected at high magnification onto a single photon position sensitive detector (PSD). This allows maps of the ion strike effects to be produced, effectively removing the need for a microbeam. Irradiation in air and even the use of alpha particle sources with no accelerator are possible. Potential applications include ion beam induced charge collection studies of semiconducting and insulating materials, single event upset studies on microchips and even biological cells in radiobiological effectiveness experiments. We describe the IPEM setup, including a 60× OM-40 microscope with a 1.5 mm hole for the beam transmission and a Quantar PSD with 60 μm pixel. Bicron plastic scintillator blades of 10 μm were chosen as a phosphor for their nanosecond time resolution, homogeneity, utility and commercial availability. The results given in this paper are for a prototype IPEM system. They indicate a resolution of ∼12 μm, the presence of a spatial halo and a He-ion efficiency of ∼20%. This marks the first time that nuclear microscopy has been performed with a radioactive source.

Kinetics of the crystalline to crystalline phase transformation induced in pure zirconia by swift heavy ion irradiation

It is generally admitted that the irradiation of crystalline oxides with high energy heavy ions, where the slowing down is mainly due to electronic excitations or ionizations, often leads to the formation of highly defective or amorphous tracks. In the latter case, the amorphization process follows a single ion impact mechanism. However, recent results demonstrate that in certain circumstances crystalline to crystalline phase transitions can also occur. This is the case of pure zirconia which transforms from the monoclinic to the tetragonal phase when the deposited electronic energy loss exceeds an effective threshold near 13 keV nm −1. Moreover, recent experiments exploring a wide range of the deposited electronic energy loss demonstrate that the kinetics of phase transformation is always sigmoidal and is consistently characterized by a double ion impacts mechanism. Therefore, it seems that this process is a constant feature of this ion-beam induced crystalline to crystalline phase transition. The origin of this behaviour will be discussed and confronted with the existing models.

Swift heavy ion-induced modification of Al 2O 3 and MgO surfaces

Surface topography changes in single-crystal alumina and magnesium oxide samples irradiated with 245 MeV Kr and 128–710 MeV Bi ions have been studied by atomic force microscopy. The surface response consists of nanoscale hillock-like defects associated with single ion impact. These defects are observed on the surfaces of Al 2O 3 and MgO targets at ionizing energy loss values of about 25 and 15.8 keV/nm, respectively, which is less than the expected threshold values of amorphous latent track formation in these materials. Corresponding electron diffraction and high-resolution transmission electron microscopy studies show no evidence of an amorphous core of the ion track in sapphire irradiated with Bi ions at surface electronic stopping power of 41 keV/nm. Possible mechanisms of hillocks formation, alternative to crystalline–amorphous phase transition are discussed.

Ion-induced craters on the surface of benzoyl glycine single-crystals studied by scanning force microscopy

Crater-like defects on the surface of ion-irradiated organic single-crystals of benzoyl glycine (BG, hippuric acid) are imaged by scanning force microscopy (SFM). The crystals are irradiated by Xe, Pb, Bi and U with specific kinetic energy 11.4 MeV/u and fluences 5×10 9 and 1×10 10 ions/cm 2 . In this energy range, the energy loss along the projectile trajectory is almost exclusively electronic. The areal density of the craters matches with the ion fluence confirming that each crater is the result of a single ion impact. The mean crater diameters range from 25 to 50 nm and exhibit a linear dependence on the ion energy loss. Assuming that the shape of the craters is a section of a sphere, mean crater volumes are calculated using the mean diameters and depths extracted from SFM images. These volumes are of order 1 – 5×10 3 nm 3 and have a steep dependence on the energy loss. On average, 10 4 molecules have been ejected from each crater.

Desorption of molecules by cluster impact: A preliminary molecular dynamics study

A preliminary molecular dynamics computer simulation has been performed to look at the use of large cluster ions in the desorption of large intact molecules from surfaces. It is found that impacts of large clusters on graphite can produce pronounced acoustic waves and that it is possible that these waves can be used to desorb large molecules from the surface. By using the acoustic wave generated by the impact of a large cluster the molecule being desorbed is gently lifted from the surface in a co-ordinated fashion. This is in contrast to trying to utilise the collision cascade generated by a single ion impact. In the latter case, it is an infrequent event that produces a co-ordinated collision cascade which enables the intact ejection of a large molecule. In the former case, it is the most frequent occurrence, particularly if the energy of the bombarding cluster is low. In this case also very little fragmentation of the surface or molecules occurs.

Is ion sputtering always a "negative homoepitaxial deposition"?

We present a scanning tunneling microscopy study of the direct comparison between homoepitaxial deposition and surface ion sputtering on the Ag(001) system. At a temperature of 200 K, sputtering results in mound formation similar to the epitaxy case, while at higher temperatures an erosive regime sets in with the appearance of regular square pits. Contrary to the conventional wisdom, which considers ion sputtering as a deposition of vacancies, the analysis of single ion impact events reveals that the process produces both adatom and vacancy clusters. The key parameter determining the temperature dependence of surface morphology turns out to be the mobility of the adatom clusters which exceeds that of vacancy clusters.

Coulomb Explosion Sputtering, Crater and Blister Formation by HCI

A simple theoretical model based on gradual Auger neutralization of a highly charged ion as it approaches the surface, with consequent positive charge deposition in surface layers and their expansion due to Coulomb repulsion provides the means to make some estimates that could explain the creation of very shallow blisters and craters on surface, as well as sputtering of up to 103 atoms in a single ion impact. Calculation of the dependence of blister size on projectile charge, based on charge evolution, gives some results fitting the experimental data. The model deals not just with the conducting properties of the solid, but with its structure as well, for instance the layered structure of mica. While the general source of energy remains the same, the particular mechanism of its realization depends largely on the composition, structure and electronic properties of the solid. The composition of the ejecta is discussed within the framework of the shock wave approach.

STM Studies of HCI-induced Surface Damage on Highly Oriented Pyrolytic Graphite

Scanning tunneling microscopy (STM) with atomic scale resolution has been applied to study surface defects in highly oriented pyrolytic graphite (HOPG) which have been produced by impact of 150 eV singly and multiply charged Ar ions (charge state up to 9+). The most prominent surface defects are protrusions. Their area density is in good agreement with the applied ion dose, implying that about every single ion impact causes one protrusion. A (√3 × √3) R 30° surface reconstruction, as characteristic for interstitial defects in HOPG, is observed in the vicinity of most defects. As the most remarkable result we find that the measured size of the hillocks (mean diameter and height) increases with projectile charge state.

Nanoparticle ejection from Au induced by single Xe ion impacts

In situ transmission electron microscopy has been used to observe sputtered Au during Xe ion irradiation in transmission geometry. The sputtered Au was collected on an electron transparent carbon foil. Nanoparticles were observed on the collector foil after they were ejected by single ion impacts. The ejection is from the melt zone formed during the thermal spike phase of a displacement cascade produced near the surface by a single ion impact. Such single ion impacts are also capable of producing craters. Ejected nanoparticles can make a significant contribution to sputtering.

Scanning force microscopy of heavy-ion induced damage in lithium fluoride single-crystals

This report presents measurements on ion-irradiated single-crystals of lithium fluoride (LiF) by means of scanning force microscopy (SFM), complementing studies performed with optical spectroscopy, small-angle X-ray scattering, and surface profilometry. Freshly cleaved crystals were irradiated at room temperature with Au-, Bi- and U-ions (kinetic energies up to 11.4 MeV/u) at the linear accelerator UNILAC, and with 1.4 MeV/u Ni-, Sn-, Xe- and Au-ions at the high-charge injector HLI, both operated at GSI. Continuing previous SFM studies of small hillocks induced by single-ion impact, the hillock diameters vs. energy loss were reexamined with special regard to systematic errors arising from the finite sensor tip radius. Furthermore, on LiF (100) surfaces irradiated at grazing incidence, SFM images show linear latent tracks exhibiting a pearl chain of hillock-like protruding zones. They are reminiscent of the sequences of etch pits found by SFM after etching a LiF surface exposed by cleavage parallel to the ion trajectories.

STM observation of surface vacancies created by ion impact

The formation, structure and extinction of surface defects created at high temperature were observed with a variable temperature scanning tunneling microscope (VT-STM). A Si(111) 7×7 surface was irradiated with Xe+ ions and single ion impact traces of about 20 Å diameter were clearly observed with atomic resolution. The average size of the trace does not depend on the impact energy in the range from 1 to 5 keV. When the sample is annealed at 400°C, the vacancies created in subsurface by the impact start to diffuse toward the surface and appear on the surface, but the interstitial atoms generated together with the vacancies remain in the bulk. At 600°C, vacancy clusters are formed whose sizes correspond to the number of vacancies created near the surfaces. At 650°C, the interstitial atoms diffuse and recombine with surface vacancies and the size of the vacancy cluster decreases with annealing time.

Sputtering of Au Induced by Single Xe Ion Impacts

AbstractSputtering of Au thin films has been determined for Xe ions with energies between 50 and 600 keV. In-situ transmission electron microscopy was used to observe sputtered Au during deposition on a carbon foil near the specimen. Total reflection and transmission sputtering yields for a 62 nm thick Au thin film were determined by ex-situ measurement of the total amount of Au on the carbon foils. In situ observations show that individual Xe ions eject Au nanopanicles as large as 7 nm in diameter with an average diameter of approximately 3 nm. Particle emission correlates with crater formation due to single ion impacts. Nanoparticle emission contributes significantly to the total sputtering yield for Xe ions in this energy range in either reflection or transmission geometry.

Modification of alkanethiol self-assembled monolayers on Au by single-ion irradiation

We investigated the effects of single-ion impact on a self-assembled monolayer (SAM) made from octadecanethiol (CH 3(CH 2) 17SH) on Au (1 1 1), expecting that the ion impact processes could change the conductivities of SAMs, and could be used for device fabrication. The ion irradiation effect was characterized using Fourier transformation infrared spectroscopy (FTIR) and contact angle measurements. After 3.1 MeV Au ion irradiation, we found that the surface component changed from CH 3 to CH 2. The ion-affected regions at nano-scale were observed by Scanning tunnelling microscope (STM), the tunnelling conductance in these regions was larger than those in the non-affected regions.