Materials Science Forum Vols. 645-648

Abstract: Calculations of lattice constant of 4H-SiC and diamond have been carried out. Lattice constant of 4H-SiC trends to decrease when nitrogen concentration increases. On the other hand, lattice constant of 4H-SiC trends to increase when aluminum concentration increases. Lattice constant of boron and phosphorus doped diamond trends to increase when impurity concentration increases. The effect of phosphorus on diamond lattice constant is about six times larger than that of boron.

Abstract: The electron backscatter diffraction (EBSD) detector placed inside a commercial scanning electron microscope (SEM) has been used to study of different SiC polytypes. Different growth conditions in chemical vapor deposition (CVD) method were applied to obtain the 3C- and 4H-SiC polytypes epitaxial layers. Growth processes were conducted on the Si-face on-axis 4H-SiC substrates. The growth temperature was in the range of 1300-1620°C and the reactor pressure was 75mbar. The initial C/Si ratio was varied from 0.075 reaching final value of 1.8. It was observed that intentional ramping of the C/Si ratio at the first stage of the growth clearly influences the 4H/3C factor. The growth temperature and ramping of the C/Si ratio were the main parameters to achieve a homogeneous 3C and 4H-SiC epitaxial layers.

Abstract: Raman microscopy has been used to study transport properties in hetero-epitaxial 3C-SiC/Si thin films. By an accurate analysis of the Longitudinal Optic phonon-plasmon coupled (LOPC) modes in n-type doped 3C-SiC films, free carrier density and mobility has been determined. A study of doped 3C-SiC reveals a strong relationship between the calculated free carrier density and both the C/Si ratio used during the epitaxial process and Silicon substrates orientation on which 3C-SiC thin films were grown (maintaining the N2 gas flow rate). The free carrier density obtained is in the range between 5x1016 cm-3 and 4x1018 cm-3. Epitaxial films grown on (111) Si substrates show a higher free carrier density and a lower dependence on C/Si ratios as compared to films grown on (100) Si substrates.

Abstract: Photoluminescence (PL) spectra of 3C-SiC(n,p)/6H-SiC(n) and 3C-SiC(p)/15R-SiC(n) heterostructures have been studied. A broad structureless band, absent in PL spectra of n–n heterostructures, was observed in the spectrum of p–n heterostructures in the energy range 2.2–2.4 eV. It is suggested that this band is due to the presence of a 2D electron gas in a quantum well near the heterointerface. High (~ 4000 cm2/s V) value of electron mobility and the absence of any significant fall in the mobility temperature dependence at liquid-nitrogen temperatures confirm the existence of a 2DEG in the QW at the heterointerface.

Abstract: Amorphous wide bandgap semiconductor thin films of the pseudobinary compound (SiC)1-x(AlN)x were grown by radio frequency dual magnetron sputtering on CaF2, MgO and glass substrates. We performed isochronical annealing steps up to 500°C. The optical bandgap is determined for each composition from spectroscopic transmission measurement in two different ways: according to Tauc and using the (αhν)2 plot. The dependence of the optical bandgap on the composition x can be described by Vegard’s empirical law for alloys.

Abstract: Organosilicate glasses (OSG) are of great technological interest as low-k dielectrics. We use ab initio simulations to analyze the influence of the C/Si ratio in OSG on the density and elastic properties of pore-free material to gain a detailed understanding of the origins of these properties in this class of materials. We find that the addition of carbon to amorphous silica leads to a stiffening of the material by removing the bending degrees of freedom of Si-O-Si bonds. This transition in mechanisms of stiffness, leads to a distinctly nonlinear behavior of the bulk and elastic modulus over the C/Si ratio. Experimental results do not contradict our findings, however dominant pore effects impede conclusive comparison.

Abstract: The recently developed technique of UVPL imaging has been used to track the path of basal plane dislocations (BPDs) in SiC epitaxial layers. The glide of BPDs during epitaxial growth has been observed and the role of this glide in forming half-loop arrays has been examined. The ability to track the path of BPDs through the epitaxy has made it possible to develop a BPD reduction process for epitaxy grown on 8° offcut wafers, which uses an in situ growth interrupt and has achieved a BPD reduction of > 98%. The images also provide insight into the strong BPD reduction that typically occurs in epitaxy grown on 4° offcut wafers.

Abstract: Since it was determined that the formation and expansion of intrinsic stacking faults (SFs) induced a drift in the forward voltage (Vf) in 4H-SiC bipolar devices, significant effort has been made to understand the driving force causing SF motion as well as the various associated luminescence processes. The observation that annealing of faulted SiC devices and epilayers induced SF contraction and a recovery of the Vf drift enabled the studying of the impact of various parameters such as temperature, injection level and operation time upon SF motion, the Vf drift and luminescence within the same device. However, these observations in many cases contradicted the previously reported driving force models. Here we report on a basic driving force model explaining SF expansion in hexagonal SiC as well as discuss the observation of green luminescence from C-core partial dislocations bounding the SFs that may indicate an enhanced mobility of point defects within forward biased SiC pin diodes.

Abstract: Shockley-type Stacking faults (SSF) in hexagonal Silicon Carbide polytypes have received considerable attention in recent years since it has been found that these defects are responsible for the degradation of forward I-V characteristics in p-i-n diodes. In order to extend the knowledge on these kind of defects and theoretically support experimental findings (specifically, photoluminescence spectral analysis), we have determined the Kohn-Sham electronic band structures, along the closed path Γ-M-K-Γ, using density functional theory. We have also determined the energies of the SSFs with respect to the perfect crystal finding that the (35) and (44) SSFs have unexpectedly low formation energies, for this reason we could expect these two defects to be easily generated/expanded either during the growth or post-growth process steps.

Abstract: In-grown stacking faults (IGSFs) in thick 4H-SiC epilayers grown at high growth rates have been characterized by micro-photoluminescence (micro-PL) spectroscopy and its intensity mapping. Strong PL emissions from the IGSFs are observed even at room temperature. Three kinds of IGSFs have been identified in the samples based on the micro-PL spectra. Each IGSF shows the specific PL emission peak located at 460 nm, 480 nm, and 500 nm, respectively. The shapes, distributions, and densities of IGSFs in the epilayers are revealed by the micro-PL intensity mapping. The stacking sequences of three IGSFs have been determined as (4,4), (3,5), and (6,2) in the Zhadonov’s notation, respectively, by high-resolution transmission electron microscopy observations. Three identified IGSFs are then classified as quadruple Shockley SFs, triple Shockley SFs, and double Shockley SFs, respectively, based on the shear formation model.