high-temperature, high-power and high-frequency electronic
and opto-electronic devices, which can operate in harsh environments.
-#in fact light emission from SiC crystal rectifiers was observed
-#already in the very beginning of the 20th century
-#constituting the brirth of solid state optoelectronics.
-#and indeed, the first blue light emitting diodes in 1990 were based on SiC.
-#(nowadays superceded by direct band gap materials like GaN).
-
a SiC based inverter with an efficiency of almost 99% has been realized.
therefore, SiC constitutes a promising candidate to become the key technology
towards an extensive development and use of regenerative energies and emobility.
and precipitation occurs.
this is manifested by the disappearance of the dark contrasts in favor of
moire patterns, again due to the lattice mismatch of SiC and silicon.
-due to the slightly lower silicon density of SiC,
-precipitation is accompanied by the emission of only a few excess silicon atoms
-into the silicon host, since there is more space.
+the excess silicon atoms are released in the silicon host,
+since there is more space.
#it is worth to note that the hkl planes of substrate and SiC match.
slide 5
-however, controversial findings and conclusions exist in the literature.
+however, controversial findings exist in the literature.
instead of a carbon interstitial (Ci) based mechanism,
nejim et al propose a transformation based on substitutionally incorporated
carbon (Cs) and the generation of interstitial silicon,
slide 7
-in md, a system of n particles is described on the microscopic level
+in md, a system of n particles is described
by numerically integrating newtons equations of motion.
the particle interaction is given by an analytical interaction potential.
observables are obtained by taking time or ensemble averages.
roughly 6000 atoms were used to investigate defect structures
-and nearly a quater of a million atoms for the precipitation simulations.
+and nearly a quater of a million for the precipitation simulations.
the equations of motion are integrated by the velocity verlet algorithm
with a time step of 1 fs.
the interaction is decribed by a Tersoff-like short-range bond order potential,
developed by erhart and albe.
the short range character is achieved by a cutoff function,
which drops the interaction to zero inbetween the first and next neighbor atom.
-#the potential consists of a repulsive and an attractive part associated with
-#the bonding, which is limited by the bond order term, which takes
-#into consideration all atoms k influencing the bond of atoms i and j.
simulations are performed in the isothermal-isobaric ensemble
realized by the berendsen thermostat and barostat.
of the hk minimal principle, which maps the system of interacting electrons to
an auxillary system of non-interacting electrons in an effective potential.
however formally exact by introducing an energy functional,
-which accounts for the exchange and correlation energy.
-#the effective potential yields a ground-state density
-#for non-interacting electrons, which is equal to that for interacting electrons
-#in the external potential.
+which accounts for exchange and correlation.
the kohn sham equations need to be solved in a self consistency loop.
the vasp code is used for this purpose.
the diffusing atom is displaced stepwise from the starting
to the final position and relaxation is only allowed
perpendicular to the displacement direction.
-each step the configurational energy of the relaxed structure is recorded.
+each step the configurational energy is recorded.
slide 9
slide 10
as a next step, the Ci mobility is determined by the quantum mechanical method.
-two of the intuitively guessed migration pathways of a carbon 00-1 db,
-and the corresponding activation energies are shown.
+two of the intuitively guessed migration pathways of a carbon 00-1 db are shown.
in number one, the carbon atom resides in the 110 plane
crossing the bc configuration.
-due to symmetry it is sufficient to merely consider the migration into the bc
-configuration.
+due to symmetry it is sufficient to consider only the first half
+of the transition path.
an activation energy of 1.2 eV is obtained.
actually another barrier exists to reach a ground-state configuration.
slide 11
the situation changes completely for the classical description.
-path number one, from the db to the bc configuration
-shows the lowermost migration barrier of 2.2 eV.
+path number one, shows the lowermost migration barrier of 2.2 eV.
next to the fact, that this is a different pathway,
-the barrier is 2.4 times higher than ab inito result.
+the barrier is overestimated by a factor of 2.4.
moreover, the ea description predicts the bc configuration to be unstable
relaxing into the 110 db configuration.
as can be seen, the agglomeration of interstitial carbon is energetically
favorable.
the most favorable configuration shows a strong C-C bond.
-however, high migration barrier is necessary to obtain this configuration
+however, a high migration barrier is necessary to obtain this configuration
in contrast to the second most favorable configuration,
which additionally is represented 2 times more often in the systematically
investigated configuration space.
this is reinforced by the plot of the binding energy of Ci dbs
separated along the 110 direction.
-the interaction is found to be proportional to the reciprocal cube
-of the distance for extended separations and saturates for the smallest
-possible distance, i.e. the ground state.
a capture radius clearly exceeding 1 nm is observed.
-the interpolated graph suggests the disappearance of attractive forces
+however, the interpolated graph suggests the disappearance of attractive forces
between the two lowest separation distances of the defects.
this supports the assumption of C agglomeration and the absence of C clustering.
a situation absolutely conceivable in ibs,
structures are obtained, which exhibit low migration barriers
for the transition into the Cs configuration.
-high barriers are necessary for the reverse process.
+in contrast, high barriers are necessary for the reverse process.
based on this, a high probability of stable Cs configurations must be concluded.
slide 15
-additionally, it is instructive to look at combinations of Cs and Si_i,
+in addition, it is instructive to look at combinations of Cs and Si_i,
again, a situation which is very likely to arise in ibs.
Cs located right next to the 110 Si db within the 110 chain
constitutes the energetically most favirable configuration,
projects / lectures / latex.git / blobdiff