Efficient Combination of Surface Texturing and Functional Coating for Very Low Secondary Electron Yield Surfaces and Rough Nonevaporable Getter Films

Abstract The formation of a fissured copper surface by picosecond pulsed laser irradiation is combined with functional coatings consisting of Ti and amorphous carbon layers or a Ti–Zr–V compound film to fabricate surfaces with the maximum of the secondary electron yield being as low as 0.4. By structural and spectroscopic analysis of the formed surfaces it is demonstrated that both coatings enclose the nanostructures generated by redeposition of metal structures from the laser‐induced plasma plume, keeping the initial topography intact. This allows an efficient elimination of secondary electron emission by combining the benefits from structural surface modification and adaption of electronic surface properties to efficiently dissipate the energy of impinging electrons. Thermal activation tests of the Ti–Zr–V nonevaporable getter films revealed that for films on nanostructured substrates, which have a much higher effective surface, a slight diminution of surface activation occurs at 160 and 200 °C, while this effect is completely compensated when heating up to 250 °C indicating promising pumping capabilities. Both examples highlight the benefits from combining 3D substrate patterning with classical 2D deposition technologies

Lattice location of impurities in silicon Carbide

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

Lattice sites of implanted Na in GaN and AlN in comparison to other light alkalis and alkaline earths

The lattice location of ion implanted radioactive 24Na (t1/2=14.96 h) in GaN and AlN was determined using the emission channeling technique at the ISOLDE/CERN facility. In the room temperature as-implanted state in both GaN and AlN, the majority of the sodium atoms are found on interstitial sites near the octahedral position, with a minority on cation Ga or Al substitutional sites. Following annealing at 800-900°C the interstitial fraction is reduced while the substitutional incorporation increases. Our results thus further establish the amphoteric character of Na in GaN and AlN, in analogy to the other light alkali Li, and alkaline earths Be and Mg. The site changes upon annealing are attributed to the onset of migration of interstitial Na, for which an activation energy of 2.2-3.4 eV is estimated in GaN and 2.0-3.1 eV in AlN, and its subsequent capture by cation vacancies resulting from the implan-tation. Comparison of the lattice site change behavior of Li, Be, Na and Mg shows that the onset of interstitial mi-gration correlates with the ionic radii of these elements

Lattice location of implanted transition metals in 3C–SiC

We have investigated the lattice location of implanted transition metal (TM) 56Mn, 59Fe and 65Ni ions in undoped single-crystalline cubic 3C–SiC by means of the emission channeling technique using radioactive isotopes produced at the CERN-ISOLDE facility. We find that in the room temperature as-implanted state, most Mn, Fe and Ni atoms occupy carbon-coordinated tetrahedral interstitial sites (TC). Smaller TM fractions were also found on Si substitutional (SSi) sites. The TM 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 occupying both SSi sites and random sites. An explanation is given according to what is known about the annealing mechanisms of silicon vacancies in silicon carbide. The origin of the observed lattice sites and their changes with thermal annealing are discussed and compared to the case of Si, highlighting the feature that the interstitial migration of TMs in SiC is much slower than in Si

Drawing the geometry of 3d transition metal-boron pairs in silicon from electron emission channeling experiments

Although the formation of transition metal-boron pairs is currently well established in silicon processing, the geometry of these complexes is still not completely understood. We investigated the lattice location of the transition metals manganese, iron, cobalt and nickel in n- and p+-type silicon by means of electron emission channeling. For manganese, iron and cobalt, we observed an increase of sites near the ideal tetrahedral interstitial position by changing the doping from n- to p+-type Si. Such increase was not observed for Ni. We ascribe this increase to the formation of pairs with boron, driven by Coulomb interactions, since the majority of iron, manganese and cobalt is positively charged in p+-type silicon while Ni is neutral. We propose that breathing mode relaxation around the boron ion within the pair causes the observed displacement from the ideal tetrahedral interstitial site. We discuss the application of the emission channeling technique in this system and, in particular, how it provides insight on the geometry of such pairs

Lattice location of Mg in GaN: a fresh look at doping limitations

Radioactive 27Mg (t1/2=9.5 min) was implanted into GaN of different doping types at CERN’s ISOLDE facility and its lattice site determined via beta− emission channeling. Following implantations between room temperature and 800°C, the majority of 27Mg occupies the substitutional Ga sites, however, below 350°C significant fractions were also found on interstitial positions ~0.6 Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p-type with 2E19 cm−3 stable Mg during epilayer growth, and lowest in Si-doped n-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350°C converts interstitial 27Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV

Emission Channeling with Short-Lived Isotopes (EC-SLI) at CERN’s ISOLDE facility

We give an overview on the historical development and current program for lattice location studies at CERN’s ISOLDE facility, where the EC-SLI (Emission Channeling with Short-Lived Isotopes) collaboration maintains several setups for this type of experiments. We illustrate that the three most decisive factors for the success of the technique are access to facilities producing radioactive isotopes, position-sensitive detectors for the emitted decay particles, and reliable simulation codes which allow for quantitative analysis

Identification of the interstitial Mn site in ferromagnetic (Ga,Mn)As

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (TAs) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (TGa) is negligible. TAs is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding TAs versus TGa occupancy of interstitial Mn in (Ga,Mn)As