Formation and evolution of small B clusters in Si: Ion channeling study Lucia Romano, 1,2 Alberto Maria Piro, 1,2 Salvatore Mirabella, 1 and Maria Grazia Grimaldi 1,2 1 MATIS CNR-INFM, Via S. Sofia 64, I-95123 Catania, Italy 2 Dipartimento di Fisica e Astronomia, Università di Catania, Via S. Sofia 64, I-95123 Catania, Italy Received 5 December 2008; revised manuscript received 23 April 2009; published 17 February 2010 B off-lattice displacement in B-doped Si was observed under Si self-interstitials Is supersaturation induced by ion irradiation at room temperature. B lattice location has a characteristic channeling mark and was studied by nuclear reaction analyses and ion channeling technique, through the comparison of the performed angular scans along the 100 and 110 crystal axes and the simulated scans by FLUX code. Solid and liquid-phase epitaxies and molecular beam epitaxy were used to prepare B-doped Si samples in order to investigate samples with B concentration in the range between 10 19 and 10 21 at / cm 3 . B off-lattice displacement is limited by the fluence of excess Is per B atom. Small B-Is clusters BICs were formed as consequence of the interaction with Is produced during the ion irradiation. Clusters structures were investigated by simulating the channeling angular scans of cluster configurations predicted by theoretical calculations. In the early stage of Is injection, experimental observations are consistent with the presence of the predicted B 2 I clusters. These small BICs evolved into different structures under further ion irradiation. DOI: 10.1103/PhysRevB.81.075210 PACS numbers: 61.72.J-, 61.82.Fk, 61.85.+p, 85.40.Ry I. INTRODUCTION The excess of point defects, typically created by ion im- plantation, is the main cause of B clustering phenomena in crystalline Si. Dopant interaction with Si self-interstitials Is was pointed out as the origin of transient enhanced diffusion and deleterious dopant deactivation. In fact, it was exten- sively demonstrated that the presence of a supersaturation of Is induces dopant deactivation via the precipitation into elec- trically inactive Boron-Interstitial-Clusters BICs. 1–5 Clus- tering proceeds through the formation of immobile precur- sors by the usual mechanisms of Si self-interstitial diffusion and B kickout. The precursors of BICs form at concentra- tions far below the B solubility limit during implantation or in the very early stages of annealing, when Si interstitial supersaturation is very high. Then, they act as nucleation centers for the formation of B-rich clusters during annealing. B-rich clusters constitute the electrically inactive B compo- nent, so that the clustering process greatly affects both junc- tion depth and doping level in high-dose implants. Ab initio 6 and tight binding density functional methods 7,8 were used to calculate the stable configurations of the components of the large BICs family. Referring to the table by Liu et al. 6 BICs are cataloged by the number of B atoms and Si atoms in the cluster. The growth path was extensively inves- tigated in order to explain the deactivation mechanism that occurs for several B concentrations, implant and annealing conditions 3,9,10 by using Monte Carlo atomistic simulations. The direct observation of B clusters is an experimental chal- lenge that moves the TEM microscopists to improve the technique, 11,12 but the identification of small complexes is based only on indirect observations 6 and is still unclear. In particular, Pelaz’s model 9 is able to reproduce the diffusion profiles of B implanted Si and annealed at temperature about 800 ° C, while very few data are available about the clusters that can be formed at lower temperature 200 °C. Accord- ing to Liu et al., 6 we expect that large clusters will have higher energy configurations, moreover their formation can be kinetically precluded at low temperature. Channeling measurements have been widely used to de- termine the lattice location of foreign atoms in crystals. 13,14 We studied the early stage of B clustering in crystalline Si at room temperature, 15–17 through the detection of the B lattice location that is very sensitive to the formation of simple BIC structures, like the B-B pairs. In this paper, we review our recent results on BIC forma- tion induced by ion beam irradiation in B-doped Si samples in a wide range of B concentration 10 19 –10 21 at / cm 3 . At room temperature, B atoms undergo an off-lattice displace- ment under Is supersaturation induced by ion irradiation. The B displacement is limited by the fluence of excess Is per B atom and can be described using a simple model of B-Is interaction with the formation of small BICs. 15 In the early stage of Is injection, the channeling angular scans along the 100 and 110 crystal axes are consistent with the forma- tion of B 2 I clusters. The BICs evolution as function of in- jected Is will be discussed in comparison with the predicted growth paths. The details of the experimental procedure are reported in Sec. I. Experimental results about the B displacement in sev- eral samples with different B concentration are reported in Sec. II. First, we show unambiguously that B clustering is caused by the injection of Si point defects, since we im- planted Si ions in selected samples where the B-doped layer is spatially separated by the damaged region. Second, the B displacement as a function of the B concentration was stud- ied by using H irradiation. The lattice location of BICs was investigated by performing angular scans along 100 and 110 axes. Angular scans of BICs predicted in literature were simulated by the FLUX code 18 and are reported in Sec. III. The BICs formed as a consequence of ion irradiation were investigated with the help of channeling simulations. The mechanism of the BICs formation under ion irradiation is discussed in comparison with other experimental observa- tions and theoretical predictions. Section IV summarizes our conclusions. PHYSICAL REVIEW B 81, 075210 2010 1098-0121/2010/817/0752107 ©2010 The American Physical Society 075210-1