Deep levels of antisite defects clusters in ZnGeP 2
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Deep levels of antisite defects clusters in ZnGeP2 Valeriy G.Voevodin, Sergey N.Grinyaev Siberian Physico-Technical Institute 1 Revolution sq., 634050, Tomsk, Russia ABSTRACT On the basis of the methods of pseudopotential and large unit cell the research of deep levels of point and cluster defects in ZnGeP2 (ZGP) was carried out. It was shown that owing to lowering of the crystal lattice symmetry localised states of point defects in the ternary compound essentially differ from such states in its binary analogue GaP. Small quasicubic antisite defects clusters reduce the value of band gap, but render weak influence on the probability of optical transitions. But both single and the cluster antisite defects cause large modifications in deep levels of another point defects closed to them. On an example of phosphorus vacancy it was shown that intensive optical transitions with small energies can appear and cause an absorption in near IR – range because of antisite defects effect. INTRODUCTION Chalcopyrite semiconductor ZnGeP2 is the NLO material of choice for high-power tunability in spectral range of 2-5 µm. But optical absorption around 1-2 µm affects the usefulness of this perspective material. Admittedly the native acceptors such as VZn and ZnGe are responsible for this absorption. At the same time the calculations of formation energies of native defects [1] show that the least energy of formation has antisite pair ZnGe+GeZn. Domination of the given imperfection and the possibility of sphalerite phase existence in the crystal may follow from this. Transition disorder-order observable at temperature lowering by some authors, investigating the phase diagram [2], also testify to favour of this. Based on recent X-ray diffraction analysis of high-quality large ZGP [3], clusters of the disordered cubic ZGP structure assigned as β-ZGP were identified within the ordered tetragonal matrix. It was suggested that point defects, which were considered to be responsible for the large absorption coefficient in the near IR, also relate to the presence of the coherent β-ZGP clusters. Comparison of formation energies allows to suppose that the disordered cubic ZGP structure is connected with disorder in cation sublattice. However, because of small bond energy single antisite pair ZnGe-GeZn can not cause residual absorption. It is possible to assume, that more extended imperfection, such as antisite defects quasicubic clusters DQFUHDWHPRUHGHHSOHYHOVLQDIRUELGGHQUHJLRQLQDFRPSDULVRQZLWKVLQJOH defects. Moreover, such imperfections can vastly influence on the states, generated by other imperfections closed to them. Up to now theoretical studies of electronic structure of the crystal have been limited by perfect lattice only. The defect energies in ZGP were researched using atomistic simulation techniques based on the shell model [1]. In the given work we present the results of the study of electron states of ZGP containing the single antisite defects (ZnGe, GeZn, PZn, ZnP, GeP, PGe) and clusters of defects ZnGe, GeZn. Influence
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