Investigation On The Elastic Constants And Dislocation Properties Of Ge And GaAs Using The First-principles Calculations

It is of great,scientific significance and practical value to study the elastic and plastic properties of semiconductor materials Ge and GaAs.In this paper,based on the linear and nonlinear elastic theory,the second-order elastic constants(SOECs)and third-order elastic constants(TOECs)of Ge and GaAs have been calculated by the first-principles calculations of density functional theory(DFT)method within the generalized-gradient-approximation(GGA)and the local-density-approximation(LDA)method,respectively.The generalized-stacking-fault energy(GSFE)sliding along(112){111} direction for Ge and GaAs and sliding along(110){111} direction for GaAs have been calculated with non-relaxed and atomic-relaxed calculations,and obtain the fitting curves of GSFE for Ge and GaAs.Based on the dislocation lattice theory,in combination with the SOECs and GSFE have been calculated,the mechanical properties of 90° partial dislocations in Ge and GaAs and shuffle 60° dislocations in GaAs have been studied.The following results show:The main results of the study on the elastic properties of Ge and GaAs are following:The lattice constants and the SOECs of Ge and GaAs are in good agreement with the experimental results and the previous results.The linear approach is not sufficient and the nonlinear elasticity theory must be considered for strains larger than approximately 1.5%.The TOECs have shown a very good agreement with the experimental and theoretical results except C456,and when considering the TOECs,the relationship of energy-strain is not symmetrical,and it is found that the energy of the positive strain is always less than that of the negative strain,which is why most of the TOECs are negative.The main results of the study on the dislocation properties of Ge and GaAs are following:The unstable stacking fault energy for glide dislocations in Ge and GaAs have been calculated by the GGA method and the LDA method,respectively.In comparison with the GGA method,the unstable stacking fault energy calculated from LDA method is higher,and the unstable stacking fault energy calculated from atomic-relaxed is lower than that calculated by non-relaxed,and the relaxed unstable stacking fault energy is about two-thirds of the non-relaxed.Considering the 90° partial dislocations sliding through the rotation of the bond in Ge and GaAs,a large degree of relaxation occurs in the sliding process.Therefore the GSFE obtained from atomic-relaxed is more reasonable.The unstable stacking fault energy of shuffle 60° dislocation in GaAs is calculated,and the unstable stacking fault energy calculate from LDA method is slightly larger than that calculated by using the GGA method;The unstable stacking fault energy calculated from atomic-relaxed is slightly smaller than that calculated by non-relaxed.For the dislocation width of 90° partials in Ge and GaAs,the dislocation width calculated from non-relaxed is narrower than calculated by atomic-relaxed,the dislocation width calculated from atomic-relaxed is about 1.4 times that calculated by non-relaxed.The Peierls stress calculated by atomic-relaxed is smaller than calculated from non-relaxed.For the dislocation width of 60° dislocation in GaAs,the dislocation width and Peierls stress calculated from atomic-relaxed method are close to that calculated by non-relaxed method.Thus,the unstable stacking fault energy is more lower,the dislocation width is more wider,the Peierls stress is more smaller,so the dislocation is more easier to slip;Glide dislocations in crystals with diamond and zinc-blende structure,it is necessary to consider the effect of atomic-relaxed on the dislocation core structure and Peierls stress;For the shuffle dislocation,the influence of atomic-relaxed on the results is very small and can be neglected.