Melt nonstoichiometry and defect structure of ZnGeP 2 crystals
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STRUCTURE OF CRYSTALS
Melt Nonstoichiometry and Defect Structure of ZnGeP2 Crystals G. A. Verozubovaa, A. Yu. Trofimova, E. M. Trukhanovb, A. V. Kolesnikovb, A. O. Okunevc, Yu. F. Ivanovd, P. R. J. Galtiere, and S. A. Said Hassanie a
Institute of Monitoring of Climatic and Ecological Systems, Siberian Branch, Russian Academy of Sciences, 10/3 ave Akademicheskii, Tomsk, 634021 Russia email: [email protected] b Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, pr. Akademika Lavrent’eva 13, Novosibirsk, 630090 Russia c Novgorod State University, ul. SanktPeterburgskaya 41, Veliky Novgorod, 173003 Russia d Institute of HighCurrent Electronics, Siberian Branch, Russian Academy of Sciences, Tomsk, 634021 Russia e Groupe d’Etude de la Matière Condensée (GEMaC) UMR 8635 CNRSUniversité de VersaillesSaint Quentin, 45, avenue des EtatsUnis78035 Versailles Cedex, France Received May 14, 2008
Abstract—The defect structure of ZnGeP2 crystals grown from a melt by the vertical Bridgman method has been investigated. A deviation of the melt composition from stoichiometric leads to the formation of stria tions and the inclusions of other phases which are observed as structures (chains) oriented parallel to the growth axis. According to the microanalysis data, the inclusion composition corresponds to a mixture of ZnGeP2, Zn3P2, and Ge. Nanoinclusions of germanium phosphide are detected by transmission electron microscopy. Xray topography reveals defects of four types. The main defects in the central part of an ingot are related to the composition fluctuations, and the newly formed dislocations are basically single ones. Most dislocations are formed at the crystal periphery. DOI: 10.1134/S1063774510010116
INTRODUCTION
time), show that three types of point defects affect the light absorption: GeZn substitutional defects are responsible for the transitions near the fundamental absorption edge; zinc vacancies exhibit strong mag netic circular dichroism near 0.7 and 1.24 μm; and phosphorus vacancies are detected in the entire spec tral range of 0.65–2.5 μm. Based on a comparison of the absorption spectra of ZnGeP2 samples recorded before and after 2 MeV electron irradiation and a mea surement of their conductivity, Brudnyi et al. [5, 6] suggested that the absorption is caused by clusters of zinc vacancies.
ZnGeP2 is a semiconductor with chalcopyrite V
structure belonging to the АIIВIV C 2 group. This mate rial is mostly used in nonlinear optics as an optical parametric oscillator (OPO) pumped by laser radia tion near 2 μm. One of the main factors that limits the wide application of ZnGeP2 in OPOs is optical absorption in the range of 0.65–2.5 μm, which is caused by intrinsic point defects. Figure 1 shows the spectrum of the optical absorption coefficient of as grown ZnGeP2, i.e., without aftergrowth treatments such as thermal annealing or highenergy electron irradiation. These spectra are typical of all ZnGeP2 crystals grown from melt. It can be seen in the figure that there i
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