Calculation Of Positron Characteristics In Silicon Carbide
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CALCULATION OF POSITRON CHARACTERISTICS IN SILICON CARBIDE Bernardo Barbiellini, Jan Kuriplach, Wolfgang Anwand , Gerhard Brauer Physics Department, Northeastern University, Boston, Massachusetts 02115 Charles University, Dept. of Low Temperature Physics, Prague, Czech Republic Research Center Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden, Germany
ABSTRACT Positron aÆnity calculations performed by a rst-principles approach based on density functional theory reveal, contrary to many other semiconductors, that free positrons and positronium atoms can escape from SiC. It is found that the treatment of the electronpositron interaction plays a crucial role when calculating the annihilation characteristics. These characteristics originating from both valence and core electrons, combined with the corresponding measurements, yield a very useful tool for surface studies and point defect identi cation in the bulk. Calculations will be compared with available experimental data. INTRODUCTION
Wide-band gap materials are excellent candidates for high power operations in aerospace, automotive, and petroleum industries. SiC, in particular, exists in a variety of di erent but closely related crystal structures, the so-called polytypes. These crystalline structures are never perfect on the atomic scale. The presence of point defects can dramatically change physical properties of these materials, even if their concentration is very low. Experimental methods based on positron annihilation [1, 2] can identify point defects in semiconductors in concentrations as low as 0.1 ppm. Because the energy-loss cross sections for positrons are high, they slow down to thermal energies before annihilating, even though they are formed with high energies in beta decay. The positron lifetimes in SiC materials can vary from 130 ps to 300 ps [3, 4, 5]. If the positron is localized in a vacancy-type defect, its lifetime increases from that of an extended (delocalized) positron state. Also the total momentum of the annihilation photon gives useful (chemical) information about the environment where the annihilation occurs [5, 6]. Thus these changes in the positron annihilation characteristics make the positron annihilation spectroscopies powerful to study defects in SiC. Moreover, positrons in SiC can also return to the surface by scattering or di usion and can be emitted into the vacuum either as bare particles or with an electron as an atom of Ps [7, 8]. Interestingly, Positronium velocity spectroscopy may be able to infer something about the electronic density of states [9]. Therefore one could use this tool for surface characterization. The goal of our work is to understand the positron behavior in SiC and to be able to extract useful information from positron studies in the SiC bulk and surfaces. We have used methods within Density Functional Theory (DFT) and the results were compared to the experiment. METHOD
The DFT can be generalized to positron-electron systems by including the positron density as we
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