Budzulyak S. The mechanisms of tensoresistive effects in highly strained n-type silicon and germanium crystals

The results of complex investigation of main dependencies and peculiarities of tensoeffects in highly uniaxially strained n-type silicon and germanium crystals are presented. The methods of identification of tensoresistivity physical mechanism and determination of main electrophysical parameters of silicon and germanium are described. For heavily doped silicon at the condition of realization of the metal-insulator transition the dependence of electron effective mass vs. uniaxial pressure was obtained. As a result of careful fitting procedure, it was determined that the quadratic dependence of electron effective mass on the uniaxial pressure X || [111] describes the experimental data of measured pressure-tuned conductivity change in the range of strain-induced MI transition. Single linear dependence of increase of the electron effective mass on pressure can not describe the measured conductivity decrease at all. It was represented the conductivity stress dependence in form predicted by the scaling theory of localization taking into account the cubical form of relation between critical concentration Nc and electron effective mass, as follows from the Mott criteria. The best fit for n-Si(P) is achieved for critical exponent v = 0.5 (1.4 NP/Nc = 2.55). The value v = 0.5 for n-Si(P) is very close to that determined for conductivity change in slightly insulating n-Si(P) samples under the stress-induced insulator-to-metal transition at millikelvin temperatures and differs essentially from v = 1 predicted by the scaling theory of localization. Analysis of TR effect data in neutron transmutation doped (NTD) and gamma-irradiated silicon crystals allowed us to determine regularities of influence on the transport phenomena, additional strain-induced ionization of both radiation-induced energy levels (P, A-centres) and levels of high temperature thermodonors which are generated in NTD silicon with high oxygen concentration during the annealing. Enhanced contribution of additional strain-induced ionization of thermodonor levels to tensoeffects with decreasing phosphorus concentration is explained by approximately constant concentration of thermodonors in crystals with equal content of oxygen and identical conditions of annealing. It was obtained by analysing both temperature and pressure dependencies of resistivity, Hall effect and the ratio of electron concentration in different valleys of the C-band shifted by deformation that the tensoeffects mechanism of intervalley redistribution is inherent only for pure n-Si crystals doped by hydrogenlike donors in a narrow temperature range (78 - 100 K). The analysis of the obtained data testifies that the slope change of the linear part of dependencies lg(n2 / n1) = f(X) does not mean the change of magnitude of the deformation potential constant u in n-Si crystals doped by various methods.The peculiarities of impact ionization of shallow donors on the insulating side of MI transition for heavily doped silicon and germanium crystals were explained. Transformation of linear current-voltage characteristics into S-shape ones under influence of uniaxial pressure linear (for germanium) and quadratic (for silicon) dependencies Ebr = f(X) was explained in terms of different processes of energy gain by ionizing electron.