Lattice location of impurities in silicon Carbide

Abstract

The presence and behaviour of transition metals (TMs) in SiC has been a concern since the start of producing device-grade wafers of this wide band gap semiconductor. They are unintentionally introduced during silicon carbide (SiC) production, crystal growth and device manufacturing, which makes them difficult contaminants to avoid. Once in SiC they easily form deep levels, either when in the isolated form or when forming complexes with other defects. On the other hand, using intentional TM doping, it is possible to change the electrical, optical and magnetic properties of SiC. TMs such as chromium, manganese or iron have been considered as possible candidates for magnetic dopants in SiC, if located on silicon lattice sites. All these issues can be explored by investigating the lattice site of implanted TMs. This thesis addresses the lattice location and thermal stability of the implanted TM radioactive probes 56Mn, 59Fe, 65Ni and 111Ag in both cubic 3C- and hexagonal 6H SiC polytypes by means of emission channeling experiments. For the Mn, Fe and Ni implanted probes in both polytypes, the occupation of displaced Si substitutional (near SSi) sites and tetrahedral interstitial carbon coordinated (ideal TC) sites was identified directly following room temperature (RT), with the majority of the TM probes found located in interstitial sites. The dependence of the identified lattice sites on annealing temperature was similar for Mn, Fe and Ni, hence the related complexes may be formed irrespective of the TM nature. The transition metal atoms partially disappear from ideal TC positions during annealing at temperatures between 500 °C and 700 °C which is accompanied by an increase in the TM fraction on SSi as well on random sites. The site changes were attributed to the onset of interstitial diffusion of the TMs, which allowed estimating values for the migration energies EM as EM(Mn)=1.9-2.7 eV, EM(Fe)=2.3-3.2 eV, and EM(Ni)=1.7-2.3 eV. The observed site change to random sites also happens in a temperature range where the literature suggests the transformation of the Si vacancy into a carbon vacancy antisite complex (VSiVC CSi), and consequently its unavailability as a major trap. Regarding the experiments with Ag, in 3C-SiC case, the 111Ag probes were found near Si substitutional sites and also a second fraction in the vicinity of substitutional C and bond-center (BC) sites, although emission channeling analysis was not able to clearly identify the exact location of this second fraction. As for the more complicated structure of 6H-SiC, where only ideal sites could be considered in experimental data analysis, it was found that the Ag probes are located both at SSi sites and BC sites. In 3C as well as in 6H-SiC the Ag probes occupying both types of sites showed high thermal stability, with the ones located at Si substitutional sites starting to disappear after annealing at 900 °C. From this an activation energy for the dissociation of substitutional Ag was estimated as 3.1 4.1 eV and identified as the onset of diffusion of substitutional Ag, for which two possible processes had been proposed in the literature: Franck-Turnbull diffusion and the so-called kick-out mechanism. The literature only presents a theoretical estimate of 3.35 eV for the activation energy of the kick-out process, which in fact fits quite well with the energy range estimated in this work. For semiconductors to be used effectively in applications need to be doped with shallow donors or acceptors. While for n type SiC with nitrogen or phosphorus two fairly shallow donors are available, for p SiC, the most commonly used acceptors aluminium or boron are relatively deep. Although theoretical calculations in the literature predict similar ionization energies as in the case of Al, the use of indium as acceptor in SiC has not been documented. Here, the lattice location of 124In in 3C-SiC and its thermal stability was studied as a function of the implantation temperature from RT to 800 °C. It was determined that an In fraction of 39% occupies near substitutional silicon sites after room temperature implantation, with the remaining In fraction sitting on “random” positions. For 600 °C implantation the In fraction located at near SSi sites shifted to ideal SSi sites, a similar result to the one obtained in this work for the other impurities, and related to the recovery of the host crystalline structure from implantation damage with the thermal treatments. Following implantation at 800 °C the In fraction sitting on ideal SSi sites reached its maximum value of 45%. Finally, lattice location results obtained in this thesis were compared to the ones for emission channeling studies in Si and diamond from the literature. The results for Fe in Si were also compared with Mössbauer spectroscopy studies, also available from the literature

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Last time updated on 18/06/2018

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