About thermal stability of electrophysical properties of Si and Si

Full text: Decay of solid solutions of Ni and Cu in silicon is known to occur very intensely. This is due to small substitutional solubility of these impurities and their fast diffusion in silicon. The first circumstance causes high over saturation of the solution with substitutional component and the over saturation is driven force for decay process. The second circumstance facilitates movement of the released impurities toward sinks. However, decay process of Ni and Cu in silicon is not studied in detail as compared to other impurities, in particular in respect of its concentration dependence. The our studies shows that with the increase of Ni and Cu concentrations the decay temperature at isochronal annealing of p-Si and p-Si, as well as electrically active concentrations of Ni and Cu increase in both slow (I) and fast cooled (II) samples. However, trends of curves for samples of I and II types are quite different; the concentrations of Ni and Cu in Il-type samples increase with increase of temperature of thermal treatment and reach values of those for samples of I-type. From our point of view, the increase in electrical active concentration of Ni and Cu in silicon can be explained as follows. It is known that during thermal diffusion Ni and Cu impurities are agglomerated around energetically favorable lattice positions, where they are electrically neutral. Such positions are to be interstitial oxygen atoms or vacancies. Formation of impurity agglomerations, in its turn, leads to deformation of lattice and new structural defects. Such system approaches to equilibrium state at thermal processing, as a result the tension fields decrease and large amount of vacancies are created. At this a part of vacancies will be captured by Ni and Cu, enhancing their electrically active concentrations. The increase in decay temperature of Ni and Cu in p-Si and p-Si with increase of their concentrations can by explained by their inhomogeneous distribution in silicon bulk. The compensated material, obtained at high temperatures, contains a set of various regions with conductivity fluctuations, which differ from each other by sizes and degree of compensation, as well as by presence of precipitations (second phases) of different sizes. With increase of temperature the regions with smaller, fluctuations will decay first. Decay yields (vacancies and interstitial impurity atoms) can meet each other, creating additional electrically active complexes (1 channel). At the same time the regions with large fluctuations begin to decay (concentration fluctuations can be modeled as homogeneously charged spheres). However, inside these fluctuation regions the reverse reaction with respect to decay has also high probability (2 channel). With further increase of temperature the large fluctuations start to decay sectorially, decreasing concentration of impurity complexes; the reverse reaction occurs too (3 channel). From our point of view, as a result of realization of all these channels the decay process proceeds with delay and, consequently, the decay temperature increases in overcompensated silicon. This work was supported by the Center for Science and Technology at Ministerial Cabinet of the Republic of Uzbekistan (Grant No F-2.1.23) and the Supporting Fund for Fundamental Researches oi Uzbekistan Academy of Sciences (Grant No 9-04)