Beams of Atomic Clusters: Effects on Impact with Solids
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Rev.Adv.Mater.Sci.Beams of atomic clusters: 11 (2006) effects 19-45 on impact with solids 19 BEAMS OF ATOMIC CLUSTERS: EFFECTS ON IMPACT WITH SOLIDS V.N. Popok and E.E.B. Campbell Department of Physics, Göteborg University, 41296 Göteborg, Sweden Received: May 12, 2005 Abstract. A state-of-the-art review in the field of cluster beams and energetic cluster-surface interaction is presented. Ionised cluster beams are considered to be a controllable and versatile tool for modification and processing of surfaces and near-surface layers on an atomic scale as an alternative to ion implantation, ion assisted deposition and some other methods. A brief introduction to the history of cluster beams is given as well as a discussion of the capabilities of cluster beams for a variety of applications in material science, electronics and optics. Techniques for producing atomic and molecular clusters are described. Energetic cluster deposition and cluster implantation as well as related physical effects are the main emphasis of the review. In the final part of the paper the attention is focused on the study of surface erosion (crater formation) under energetic cluster ion impact. 1. INTRODUCTION tronic, optical, magnetic and other properties that are quite different from those of molecules or con- Atomic (or molecular) clusters are aggregates of a densed matter and that can be of great interest for few up to tens of thousands of atoms (or molecules) practical applications in nanotechnology [4-9]. With that show properties intermediate between those clusters consisting of up to thousands of atoms it is of individual atoms (or molecules), with discrete possible to transport and locally deposit a large energy states, and bulk matter characterised by amount of material, providing an advanced method continua or bands of states. More detailed informa- for growth of thin films which can be either porous or tion about the classification of clusters, their bond- very compact and smooth depending on the energy ing types and structure goes beyond the scope of regime used for cluster impact [10,11]. Low-energy this review and can be found elsewhere [1,2]. cluster implantation is found to be an efficient tool Cluster beams can be tools for manipulating for doping of shallow layers with suppression of dopant agglomerates of atoms providing the synthesis of diffusion during post-implantation annealing [12]. structures on the nm-scale. In particular, one can Cluster beams can also be used as very efficient control the cluster (nanoparticle) size, its impact tools for processing of surfaces (dry etching and energy and, to a certain extent, the spatial distribu- cleaning) or improving their surface topology tion of the deposited nanoparticles on a surface, for (smoothing) [13]. Current achievements in the ap- example, by preliminary processing or functio- plication of cluster beams are summarised in the nalisation [3]. Finite size effects can lead to elec- next section. Corresponding author: V.N. Popok, e-mail: [email protected] © 2006 Advanced Study Center Co. Ltd. 20 V.N. Popok and E.E.B. Campbell 2. BRIEF HISTORY AND PRESENT-DAY APPLICATIONS OF CLUSTER BEAMS The first mention of cluster beam production and investigation occurred at the end of the 50s and beginning of the 60s [14,15]. The possibility to sepa- rate small clusters of hydrogen, nitrogen and argon from non-condensed residual gas and transfer them into a high vacuum was shown. At the same time it was demonstrated with CO and H that cluster 2 2 beams can be ionised by electron bombardment enabling mass spectra to be obtained [15,16]. These experiments were followed by investigations of the Fig. 1. Photoluminescence spectra of thin films pro- cluster size distribution in a beam of (CO ) +, de- duced by deposition of size-selected silicon nano- 2 n pending on conditions of the cluster source [17] and crystals. The 488-nm line of an argon-ion laser was by development of methods to evaluate cluster used for excitation. Mean diameters of the sizes, for example, by scattering of a potassium nanocrystals are 2.47 (I), 3.65 (II), 3.92 (III), 4.95 atomic beam passing through a nitrogen cluster (VI) and 6.37 (VII) nm. Reprinted with permission beam [18]. Slightly later, first studies on the reflec- from [27]. Copyright 1997, American Physical So- tion of molecular nitrogen clusters from polished ciety. stainless steel, showing a strong maximum at 90° reflection angle independent of the angle of inci- dence, were carried out [19]. At the beginning of the 70s, experiments on the stopping of swift pro- ing compact and almost atomically smooth films. ton clusters in carbon and gold foils were performed This approach became of increasing interest in the [20]. The use of accelerated ionised clusters for prac- late 90s with intensive development of sub-micron tical applications has been pioneered by the Kyoto and nano-electronics. For example, growth of good- group. In the 70s they reported the development of quality Ge layers with a thickness of 100-400 nm a cluster source for ionised-cluster beam deposi- on a Si substrate at a temperature of 500 °C, which tion [21]. However, first published results on the is lower than the critical temperature of epitaxial deposition of Si, Au, and Cu suffer from a lack of growth by molecular beams or CVD, was reported confirmation concerning the cluster-to-monomer by using a germanium cluster beam at supersonic ratio in the beams. Further development of the tech- velocities [26]. Silicon films with tuneable photolu- nique in the 80s provided more controllable param- minescence properties were synthesised by low- eters of the beams and showed the applicability of energy deposition of size-selected Si clusters (Fig. ionised clusters for synthesis of thin metal films and 1); the obtained characteristics were well described heterostructures [22,23]. However, no clear picture in terms of zero-dimensional quantum dots [27]. By of the physical background emerged until the be- deposition of neutral carbon clusters it was possible ginning of the 90s when systematic experimental to grow nanostructured graphite-like films and pat- work corroborated by molecular dynamic simula- terns of three-dimensional objects using a mask (Fig. tions on thin film growth by energetic cluster depo- 2) [28,29]. This method of formation of patterned sition was published by the Haberland group, see carbon dots on the silicon surface followed by the for example [10,24,25]. thermal annealing was later employed to obtain ar- One of the important parameters in the practical rays of SiC dots for applications in electronics [30]. application of cluster beams is impact energy. By Recently, deposition of pure carbon clusters from a varying the cluster energy from low values (below or supersonic beam provided experimental evidence for on the level of eV/atom), corresponding to soft-land- the possibility of producing a carbyne-rich pure car- ing of a cluster on the surface, to high values (keV- bon solid [31]: before this publication, the existence MeV/atom) suitable for implantation one can develop of carbyne was strongly debated [32]. The cluster a variety of methods for synthesis and modification beam deposition technique was also found to be an of materials. efficient and powerful tool for fabrication of organic A large amount of mass and energy can be de- (polymer) thin films and various inorganic-organic posited locally when using charged clusters, form- Beams of atomic clusters: effects on impact with solids 21 Fig. 2. SEM images of (a) vertical and (b) 60o tilted tip deposited through superposition of two grids (see insertion in panel (a)) using a carbon cluster beam. Reprinted with permission from [29]. Copyright 2000, American Institute of Physics. complex nanometer-scale functional structures, see Energetic cluster deposition, so-called pinning, for example review [33]. Thus, the high deposition when the cluster is slightly embedded into a sub- rates as well as the control of the energy and clus- strate but preserves its composition gives an op- ter mass distribution makes the cluster beam tech- portunity to prevent cluster diffusion on the surface nique competitive with other synthetic routes for [41]. The pinned, size-selected clusters have po- materials where a well-defined morphology and tential applications in many fields, such as fabrica- structure is required. tion of semiconductor nanostructures and the A promising application of the low-energy clus- immobilisation and orientation of biological molecules ter beam deposition technique is the preparation of [42]. By initial processing of a surface, for instance optical, magnetic, and magneto-optic nanostructures by focused ion irradiation, it was found to be pos- formed by clusters of various metals on the surface sible to produce regular arrays of surface defects, of a dielectric matrix [4,5]. In the case of noble metal so-called surface functionalisation, by which depos- or copper nanoparticles, the films exhibit specific ited metal clusters can be organised on the sur- absorption bands in the near-UV, visible or near-IR face, giving control of the nanoparticle size, the ge- regions depending on the cluster species [6,7] due ometry, and the periodicity of the array (Fig. 3) to the collective excitation of conduction electrons [3,43]. (surface plasmon resonance). The position, shape Cluster ion beam processing in conjunction with and intensity of such absorption bands strongly one or several other methods may provide the op- depend on the size distribution, shape and volumetric portunity to realise otherwise difficult to obtain com- fraction of the metallic inclusions. In this context, positions or material phases. Cluster ion beam-as- the cluster beam technique offers some unique pos- sisted deposition has been used, for example, for sibilities for controlling these parameters and the growth of high-quality, thin and smooth films of synthesising composites with variable optical prop- ITO [44], Ta O , Nb O [45] and TiO [46].