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mechanisms of formation and feature of a spatial distribution of dislocations

It is necessary to ooze some parents of formation of dislocations in the monocrystals depending on requirements of cultivation. The basic mechanisms at cultivation of monocrystals from a melt, a spotting dislocation density and allocation of dislocations on crystals it: "germination" from zatravochnogo a crystal; origin and reproduction of dislocations in the course of strain under the influence of thermal voltages; formation of dislocations under the influence of the voltages caused by the nonuniform allocation of impurities [32-40].

In slabolegirovannyh optical monocrystals of germanium (level of concentration of an impurity of all on 2... 3 orders exceed natural concentration of charge carriers) principal causes of formation of dislocations are the thermal voltages caused by the nonuniform allocation of temperatures in a crystal, and also possible penetration
Dislocations from a seeding agent. Possibility of prevention of "germination" of a dislocation from a seeding agent for reception bezdislokatsionnyh crystals is featured in operation [41]. It is shown, that reduction of diameter zatravochnogo a crystal before razrashchivaniem an ingot promotes an exit of dislocations on a surface of a growing crystal. Dislocations most effectively leave at crystal cultivation in a direction which forms a major corner with slip planes where dislocations are preferentially located. For crystals with granetsentrirovannoj a cubic lattice such optimum directions of growth are directions and .

Lack of "germination" of dislocations from zatravochnogo a crystal does not guarantee completely an irreproachable crystal as at certain level of thermal voltages of a dislocation can be generated and in bezdislokatsionnoj to a matrix. The primary goal at reception bezdislokatsionnyh monocrystals - decrease in thermal voltages to safe level. The thermal voltages promoting formation of dislocations, are spotted by temperature allocation in a growing ingot, therefore making of optimum thermal requirements of cultivation is closely related to examination of temperature fields in a crystal and a melt.

The considerable quantity of the operations linking a dislocation density in monocrystals, grown up of a melt, with temperature requirements, in the core, with lapse rates of temperatures in the field of crystallisation front is known.

In operation [42] data on interrelation of curvature of front of crystallisation and a dislocation density in germanium monocrystals are cited. It is shown, that change of curvature of front from 2 m "1 to 20 m ' 1 changes a dislocation density to 3 orders of magnitude: from IO2до IO5см" 2. In this connection such performances of process as velocities of gyration of a crystal and tiglja, length of a crystal and others influence a dislocation density inappreciablly.

In operation [43] cultivation key parametres bezdislokatsionnyh germanium monocrystals are spotted. It is a lapse rate of temperatures at the crystallisation front, not exceeding 300 Km ' 1 and supercooling
Melt at front within 2,5 ±1,0 K.Pri these requirements monocrystals with a dislocation density stablly gained than 10 sm ' 2 no more, and the basic part of monocrystals had 1... 3 dislocations on all section of a monocrystal. A method deficiency was restriction of diameter of monocrystals to 22... 24 mm. Among the basic technological requirements of process of growth level of supercooling of a melt was set.

It is known, that magnification of velocity of cultivation of monocrystals with other things being equal probably only at magnification of supercooling of a melt [44-47]. Supercooling change thus can lead to change of a dislocation density in monocrystals. Grown up bezdislokatsionnye germanium monocrystals were pyramids of growth of a singular facet {111}. Large-sized monocrystals of germanium gain usually in the form of a disk [31,48,49-51], and they are not pyramids of growth of a singular facet {111}, and represent their difficult combination. At orientation of a seeding agent it is a pyramid of growth of a facet {111} and in pairs alternating six pyramids of growth of facets {11Т} and {1 її}. Cultivation of large-sized monocrystals is process of formation of such structure. At inevitable oscillations of temperature and supercooling [52] alternating pyramids frequently grow together incoherently.

Thus, it is possible to speak about one parent of formation of dislocations at cultivation of crystals from a melt - incoherent accretion of pyramids of growth. In such cases dislocations pile-up and malouglovye boundaries are formed. Finally on boundaries of pyramids of growth of facets {11 Ї} and |1 її} can arise dvojnikovanie and there will be polycrystallization fields. The dislocation density on boundaries of pyramids of growth increases from crystal centre to periphery.

Formation of two-dimensional and three-dimensional flaws

Depending on temperature requirements in monocrystals there is the quantitative and qualitative transformation of the one-dimensional flaws
Structures - dislocations. In process of magnification of a lapse rate of temperatures and supercooling proportioned on volume of a crystal of a dislocation will be conversed to slip lines and malouglovye boundaries (MUG). Slide in structure of diamond type occur along planes to the peak reticular density {111}. Slip lines transit in directions with the peak density of knots of a lattice (110). At growth of large-sized monocrystals there is a quantum leap - dislocations concentrate on boundaries of joints of pyramids of growth of facets {111} in directions (112). Are formed malouglovye boundaries that is the first stage of transition of monocrystal structure in polycrystalline. The blocks of a monocrystal parted MUG on (112), razorientirovany from each other on small corners, not exceeding usually 5 °. At non-equilibrium increase in system (supercooling magnification) at cultivation of monocrystals of major diameters (from 50 to 600 mm) or in case of infringements of a stable mode, appear any way oriented MUG and dislocations pile-up. Finally the monocrystal will be conversed to a modular system becoming in essence, a polycrystal with irregular orientation of individuals. Razorientirovannye blocks are considered as three-dimensional flaws, but occurrence of two-dimensional flaws - two-dimensional boundaries between blocks [53-54] in effect result.

The similar pattern is characteristic for strongly alloyed crystals. In the alloyed monocrystals there can be three-dimensional flaws of type of inserts of the second phase [53,55]. The morphology of monocrystals thus is the person, crystals have a sinuosity on lateral surfaces and crystallisation front. For example, in germanium strongly alloyed by arsenic (level of concentration of an impurity - IO19см ' 3) inserts of the second phase in the central part border a pyramid of growth of a facet {111}. On insert periphery settle down along the directions (112) corresponding
To constants of pyramids {11T} and {1T1}, and also on the steps of front going in directions (11 θ) • Depending on orientation of a monocrystal of a pattern can change, but the scored basic features always are maintained. In case of a direction of growth (110) ledges of pyramids of growth of facets {111} settle down under a corner 54044 to a direction (ON). They transits lengthways (110). The arrangement of inserts of the second phase and morphology of fronts are rigidly interconnected; on them it is easily possible to judge a direction of orientation of a seeding agent.

Special view of two-dimensional flaws are dvojnikovye the boundaries related to supercooling of a melt - concentration or thermal. Dvojnikovanie in monocrystals occurs to a veering of growth with (Ill) on (115), there is a whole series of doubles, and the crystal finally becomes sferolitom. Thus, among all known parents of occurrence of all viewed types of flaws, the main things are supercooling of a melt and a lapse rate of temperatures to which the initial stage of formation of dislocations is closely related. A certain stage starts to play a role and incoherent accretion of pyramids of growth in crystal volume. Toting all these parents, obobshchenno they can be reduced to the classical parent, known plasticity from the theory - to formation in a crystal of mechanical voltages and their relaxation at excess of their critical level.

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A source: Ivanova Alexandra Ivanovna. Micromorphology of a surface and dislocation structure of large-sized optical crystals of germanium and paratellurita. The dissertation on competition of a scientific degree of the candidate of physical and mathematical sciences. Tver - 2015. 2015

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