318 research outputs found
Linear Stochastic Fluid Networks: Rare-Event Simulation and Markov Modulation
We consider a linear stochastic fluid network under Markov modulation, with a
focus on the probability that the joint storage level attains a value in a rare
set at a given point in time. The main objective is to develop efficient
importance sampling algorithms with provable performance guarantees. For linear
stochastic fluid networks without modulation, we prove that the number of runs
needed (so as to obtain an estimate with a given precision) increases
polynomially (whereas the probability under consideration decays essentially
exponentially); for networks operating in the slow modulation regime, our
algorithm is asymptotically efficient. Our techniques are in the tradition of
the rare-event simulation procedures that were developed for the sample-mean of
i.i.d. one-dimensional light-tailed random variables, and intensively use the
idea of exponential twisting. In passing, we also point out how to set up a
recursion to evaluate the (transient and stationary) moments of the joint
storage level in Markov-modulated linear stochastic fluid networks
Low temperature, postgrowth self-doping of CdTe single crystals due to controlled deviation from stoichiometry
Careful analysis of the Cd-Te pressure-temperature-composition phase diagram, shows a deviation of CdTe stoichiometry only in the Te-depletion direction between 450 and 550 degrees C. Combined control over the semiconductor composition, via intrinsic defects, and over the atmosphere and cooling rate can, therefore, yield a process for intrinsic doping of CdTe at these relatively low temperatures. We present results that support this. Quenching of CdTe, following its annealing in Te atmosphere at 400-550 degrees C, leads to p-type conductivity with a hole concentration of similar to 2 x 10(16) cm(-3). Slow cooling of the samples, after 550 degrees C annealing in Te or in vacuum, increases the hole concentration by one order of magnitude, as compared to quenching at the same temperature. We explain this increase by the defect reaction between Te vacancies and Te interstitials. Annealing in Cd at 400-550 degrees C leads to n-type conductivity with an electron concentration of similar to 2 x 10(16) cm(-3). Annealing at 450-550 degrees C in the equilibrium atmosphere, provided by adding CdTe powder, gives n-type material
Experimental Evidence for Defect Tolerance in Pb-Halide Perovskites
The term defect tolerance (DT) is used often to rationalize the exceptional
optoelectronic properties of Halide Perovskites, HaPs, and their devices. Even
though DT lacked direct experimental evidence, it became fact in the field. DT
in semiconductors implies tolerance to structural defects without the
electrical and optical effects (e.g., traps), associated with such defects. We
present first direct experimental evidence for DT in Pb HaPs by comparing the
structural quality of 2D, 2D_3D, and 3D Pb HaP crystals with their
optoelectronic characteristics using high sensitivity methods. Importantly, we
get information from the material bulk, because we sample at least a few 100
nm, up to several micrometer, from the sample surface, which allows assessing
intrinsic bulk (and not only surface) properties of HaPs. The results point to
DT in 3D, to a lesser extent in 2D_3D, but not in 2D Pb HaPs. We ascribe such
dimension dependent DT to the higher number of (near)neighboring species,
available to compensate for structural defect effects in the 3D than in the 2D
HaP crystals. Overall, our data provide an experimental basis to rationalize DT
in Pb HaPs. These experiments and findings can guide the search for, and design
of other materials with DT
- …