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Register a new userIdentifying vacancy complexes in compound semiconductors with positron annihilation spectroscopy: a case study of InN
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We present a comprehensive study of vacancy and vacancy-impurity complexes in InN combining positron annihilation spectroscopy and ab-initio calculations. Positron densities and annihilation characteristics of common vacancy-type defects are calculated using density functional theory and the feasibility of their experimental detection and distinction with positron annihilation methods is discussed. The computational results are compared to positron lifetime and conventional as well as coincidence Doppler broadening measurements of several representative InN samples. The particular dominant vacancy-type positron traps are identified and their characteristic positron lifetimes, Doppler ratio curves and lineshape parameters determined. We find that In vacancies and their complexes with N vacancies or impurities act as efficient positron traps, inducing distinct changes in the annihilation parameters compared to the InN lattice. Neutral or positively charged N vacancies and pure N vacancy complexes on the other hand do not trap positrons. The predominantly introduced positron trap in irradiated InN is identified as the isolated In vacancy, while in as-grown InN layers In vacancies do not occur isolated but complexed with one or more N vacancies. The number of N vacancies per In vacancy in these complexes is found to increase from the near surface region towards the layer-substrate interface.
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Small single crystals of Ga2(SexTe1-x)3 semiconductors, for x = 0.1, 0.2, 0.3, were obtained via modified Bridgman growth techniques. High-resolution powder x-ray diffractometry confirms a zincblende cubic structure, with additional satellite peaks observed near the (111) Bragg line. This suggests the presence of ordered vacancy planes along the [111] direction that have been previously observed in Ga2Te3. Defect studies via positron annihilation spectroscopy show an average positron lifetime of ~400 ps in bulk as-grown specimens. Such a large lifetime suggests that the positron annihilation sites in these materials are dominated by defects. Moreover, analyzing the electron momenta via coincidence Doppler broadening measurements suggests a strong presence of large open-volume defects, likely to be vacancy clusters or voids.
Understanding the nature and behavior of vacancy-like defects in epitaxial GeSn metastable alloys is crucial to elucidate the structural and optoelectronic properties of these emerging semiconductors. The formation of vacancies and their complexes is expected to be promoted by the relatively low substrate temperature required for the epitaxial growth of GeSn layers with Sn contents significantly above the equilibrium solubility of 1 at.%. These defects can impact both the microstructure and charge carrier lifetime. Herein, to identify the vacancy-related complexes and probe their evolution as a function of Sn content, depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to investigate GeSn epitaxial layers with Sn content in the 6.5-13.0 at.% range. The samples were grown by chemical vapor deposition method at temperatures between 300 and 330 {deg}C. Regardless of the Sn content, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, which confirmed the presence of open volume defects. The measured average positron lifetimes were the highest (380-395 ps) in the region near the surface and monotonically decrease across the analyzed thickness, but remain above 350 ps. All GeSn layers exhibit lifetimes that are 85 to 110 ps higher than the Ge reference layers. Surprisingly, these lifetimes were found to decrease as Sn content increases in GeSn layers. These measurements indicate that divacancies are the dominant defect in the as-grown GeSn layers. However, their corresponding lifetime was found to be shorter than in epitaxial Ge thus suggesting that the presence of Sn may alter the structure of divacancies. Additionally, GeSn layers were found to also contain a small fraction of vacancy clusters, which become less important as Sn content increases.
In this paper we examine the electronic and geometrical structure of impurity-vacancy complexes in Si and Ge. Already Watkins suggested that in Si the pairing of Sn with the vacancy produces a complex with the Sn-atom at the bond center and the vacancy split into two half vacancies on the neighboring sites. Within the framework of density-functional theory we use two complementary ab initio methods, the pseudopotential plane wave (PPW) method and the all-electron Kohn-Korringa-Rostoker (KKR) method, to investigate the structure of vacancy complexes with 11 different sp-impurities. For the case of Sn in Si, we confirm the split configuration and obtain good agreement with EPR data of Watkins. In general we find that all impurities of the 5sp and 6sp series in Si and Ge prefer the split-vacancy configuration, with an energy gain of 0.5 to 1 eV compared to the substitutional complex. On the other hand, impurities of the 3sp and 4sp series form a (slightly distorted) substitutional complex. Al impurities show an exception from this rule, forming a split complex in Si and a strongly distorted substitutional complex in Ge. We find a strong correlation of these data with the size of the isolated impurities, being defined via the lattice relaxations of the nearest neighbors.
Efficient ab initio computational methods for the calculation of thermoelectric transport properties of materials are of great avail for energy harvesting technologies. The BoltzTraP code has been largely used to efficiently calculate thermoelectric coefficients. However, its current version that is publicly available is based only on the constant relaxation time (RT) approximation, which usually does not hold for real materials. Here, we extended the implementation of the BoltzTraP code by incorporating realistic k-dependent RT models of the temperature dependence of the main scattering processes, namely, screened polar and nonpolar scattering by optical phonons, scattering by acoustic phonons, and scattering by ionized impurities with screening. Our RT models are based on a smooth Fourier interpolation of Kohn-Sham eigenvalues and its derivatives, taking into account non-parabolicity (beyond the parabolic or Kane models), degeneracy and multiplicity of the energy bands on the same footing, within very low computational cost. In order to test our methodology, we calculated the anisotropic thermoelectric transport properties of low temperature phase (Pnma) of intrinsic p-type and hole-doped tin selenide (SnSe). Our results are in quantitative agreement with experimental data, regarding the evolution of the anisotropic thermoelectric coefficients with both temperature and chemical potential. Hence, from this picture, we also obtained the evolution and understanding of the main scattering processes of the overall thermoelectric transport in p-type SnSe.
The attainability of modification of the apparent magnetic anisotropy in (III,Mn)V ferromagnetic semiconductors is probed by means of the finite-elements-based modelling. The most representative case of (Ga,Mn)As and its in-plane uniaxial anisotropy is investigated. The hysteresis loops of the continuous films of a ferromagnetic semiconductor as well as films structured with the elliptic antidots are modelled for various eccentricity, orientation, and separation of the anti dots. The effect of anti-dots on the magnetic anisotropy is confirmed but overall is found to be very weak. The subsequent modelling for (Ga,Mn)As film with the elliptic dots comprising of metallic NiFe shows much stronger effect, revealing switching of the magnetic moment in the ferromagnetic semiconductor governed by the switching behavior of the metallic inclusions.
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