Defects in SiC: from hetero-epitaxial growth to ion implantation

In the first part of the thesis, an extensive discussion on the research activity concerning the extended bidimensional defects within the 3C-SiC will be exposed. These defects are, in fact, one of the main causes that hinders the use of 3C-SiC heteroepitaxies grown on Si substrates. TEM investigations together with molecular dynamics simulations have been used to trace the structure of the main two-dimensional defects affecting the heteroepitaxial layers in 3C-SiC, with particular reference to Inverted Domain Boundaries (IDBs) and Stacking Faults (SFs). In this work Domain boundaries (DBs) generated during the growth of 3C-SiC on (001) Si and their interaction with stacking faults (SFs) will be studied. Direct Scanning Transmission Electron Microscopy (STEM) images will exhibit that DBs are inverted domain boundaries otherwise called antiphase boundary (IDBs or APB). The atomic arrangement of this IDB is different from the expected boundaries described in the literature, nevertheless, it has a highly coherent nature. The IDBs propagate in a complex way through the crystal. A close relationship will be identified between the IDBs lying on the (111) plane and the SFs, which appear to be coupled. Furthermore, it will be observed that the Partial dislocations surrounding the SFs possess an unconventional lying plane, i.e. [112], [123], [134]. Molecular dynamics simulations demonstrate the stability of the directions [110] as well as [112] direction, while the directions [123] and [134] of the partial dislocations (PDs) evolve towards [112] and [110]. The unusual directions will be identified by experimental investigation as they obey to rules of minimization of the strain energy, in partial dislocations. A structural characterization and distribution of SFs was performed by μ-Raman spectroscopy and room-temperature μ-photoluminescence. Two kinds of SFs, 4H-like and 6H-like, will be identified near the substrate interface. Each kind of SFs shows a characteristic photoluminescence emission of the 4H-SiC and 6H-SiC located at 393 and 425 nm, respectively. 4H-like and 6H-like SFs show different distribution along film thickness. The reported results were discussed in relation with the experimental data and theoretical models present in the literature. Investigations related to the temperature influence on the homo-epitaxial growth process of 3C-SiC will be presented. Different morphological analyses indicate that the growth temperature and the growth rate play a fundamental role in the stacking faults density. In details, X-ray diffraction and micro-Raman analysis show the strict relationship between growth temperature, crystal quality, and doping incorporation in the homo-epitaxial chemical vapor deposition CVD growth process of a 3C-SiC wafer. Furthermore, photoluminescence spectra show a considerable reduction of point defects during homo-epitaxy at high temperatures. Furthermore, properties of cubic silicon carbide (3C-SiC) grown epitaxially on a patterned silicon substrate composed by squared inverted silicon pyramids (ISP) will be shown. This compliant substrate prevents the stacking faults, usually found at the SiC/Si interface, to reach the surface. We investigated the effect of the size of the inverted pyramid on the epilayer quality. We noted that anti-phase boundaries (APBs) develop between adjacent faces of the pyramid and that the SiC/Si interfaces have the same polarity on both pyramid faces. Structure of the heterointerface will be investigated. Moreover, due to the merging of APB in the vertex of the pyramid, voids buried on the epilayer form. We will demonstrate that careful control of the growth parameters allows to modify the height of the void and the density of APBs, improving SiC epitaxy quality. It will be discussed the use of a buffer layer between the epitaxial layer and the substrate in order to reduce the defectiveness and improve the overall quality of the SiC epi-film. In particular, we find that the morphology and the quality of the epi-film depends on the carbonization temperature and the concentration of Ge in close proximity of the Si1-xGex/SiC interface. Ge segregation at the interface influences the film quality, and in particular a [Ge]>12% in close proximity to the interface leads to the formation of poly-crystals, while close to 10% induces a mirror like morphology. Moreover, by finely tuning the Ge concentration and carbonization temperature, crystal quality higher than that observed for SiC grown on bare silicon will be achieved. In chapter 3 we will study the crystal defectiveness subsequent to ion implantation and annealing by using various techniques including photoluminescence (PL), Raman spectroscopy and transmission electron microscopy (TEM). The aim of this work is to test the effectiveness of double step annealing to reduce the density of point defects generated during the annealing of a P implanted 4HSiC epitaxial layer. The outcome evidences that neither the first 1 hour isochronal annealing at 1650 - 1700 - 1750 °C, nor the second one at 1500 °C for times between 4 hour and 14 hour are able to recover a satisfactory crystallinity of the sample. Prismatic interstitial-type dislocation loops have been found as the dominant extended defect. Their average size are typically 1–20 nm in diameter and reside on the {0001} and {11-20} plane, consistent with Frank type dislocation behavior. Conventional annealing do not allow lattice relaxation towards low energy configurations. Anisotropic-next-nearest-neighbor Ising model demonstrated that evolution towards prismatic dislocation loops is highly energetic. Indeed, Raman E2(TO) mode displayed large in plane stress values up to 172 Mpa. This work will also describes the development of a new method for ion implantation induced crystal damage recovery using multiple XeCl (308 nm) laser pulses with a duration of 30 ns. Experimental activity was carried on single phosphorus (P) as well as double phosphorus and aluminum (Al) implanted 4H-SiC epitaxial layers. Samples will be characterized through μ-Raman spectroscopy, Photoluminescence (PL) and Transmission Electron Microscopy (TEM) and results were compared with those coming from P implanted thermally annealed samples at 1650–1700–1750 °C for 1 h as well as P and Al implanted samples annealed at 1650 °C for 30 min. The activity outcome shows that laser annealing allows to achieve full crystal recovery in the energy density range between 0.50 and 0.60 J/cm2. Moreover, laser treated crystal exhibits an almost stress-free lattice with respect to thermally annealed samples that are characterized by high point and extended defects concentration. Implanted area was almost preserved, except for some surface oxidation processes due to oxygen leakage inside the testing chamber. To work around surface erosion phenomena it will be proposed the practicability of coating systems designed to minimize the surface degradation of SiC following laser annealing process. This strategy based on a graphitic coating plays a key role along the annealing in a thermal nonequilibrium regime to obtain a high dopant activation. Thanks to its 3800 °C melting temperature and high absorption coefficient, graphite is, in fact, the best candidate to fully absorb laser radiation, allowing a heat diffusive regime along the underlying layers. Characterization will be performed by means of μ-Raman accurate analysis which allows to highlight the high crystalline quality and the absence of stress phenomena inside the crystal. A detailed portrait of the damage thresholds will be reported with particular emphasis on the thicknesses used to minimize sample erosion and, at the same time, ensure high activation of the implanted dopant. Circular Transfer Length Method (CTLM) measurements tests were carried out and will prove remarkable P activation on irradiated site. The 90 nm graphite coated sample will reveal at 0.5 J/cm2 activation values equal to 6.59·1019 ±1.95 e/cm3 which correspond to approximately 66% of P implanted activation. This is slightly higher than the value coming from the thermally annealed sample extracted from literature (around 62%). The results of this experimental activity give way to laser annealing process viability for damage recovery and dopant activation inside the implanted area. Starting from the state of the art about the formation of extended defects in ion implanted 4H-SiC homo-epitaxial wafers following high temperature post-implantation annealing, the improvement of the laser annealing technique is proposed as a turning point of post-implantation thermal processes. The study provided within this work has, therefore, as its main ambition to provide an accurate characterization of defects in SiC films and their behavior as a function of growth as well as in doping processes. The objective therefore concerns the removal of the relevant drawbacks in the development and the viability of reliable and efficient electronics devices.