Investigation On The Effects Of Intrinsic Defects And Impurities On The Mechanical Properties Of Silicon-based Micro-/Nano-Structures

MEMS/NEMS(Micro/Nano-electromechanical systems)have been widely applied in basic sciences and engineering fields.Silicon(Si)is one of the basic materials in MEMS/NEMS.The reliability of Si-based micro/nano structures directly restricts the mass production and marketization of MEMS/NEMS.It is also significant for equipment and personnel safety.With the expansion of the functions of Si-based MEMS/NEMS and the complexity of micro/nano structures,the reliability issues have attracted more and more attention.This dissertation takes the mechanical properties of Si,such as strength,plastic brittleness,fatigue,etc.,which are closely related to the failure modes,as the breakthrough point,and bulk Si,thin films,amorphous Si(a-Si)and nanowires as research subjects.The influences of common crystal defects and a typical electroactive impurity phosphorus(P)on the mechanical properties of Si micro/nano-structures are investigated to understand its reliability at atomic scale.Molecular dynamics(MD)is employed as main research tool,supplemented by first-principles calculations,analytical derivations and experiments.The research contents are listed as follows:1.Optimization of force field potential functionsThe force field formalism and corresponding parameter set largely determine the reliability of simulation results.The second-nearest-neighbor modified embedded atom method(2NN MEAM)is employed in this work.The 2NN MEAM parameter sets of pure Si,pure P and Si-P binary system are optimized by genetic algorithm(GA).The 2NN MEAM parameter set for pure Si is optimized mainly based on the properties of surface and stacking faults while that for pure P and Si-P binary system are optimized based on a wide range of basic physical properties,including bulk elastic modulus B and first order pressure derivative B’ for the common phases of P,and the cohesive energies,lattice constants,elastic constants,P-related point defects of typical Si-P compounds.Besides,the thermal stability of typical phases of P and Si-P under limited temperature and pressure are also tested to verify the usability of the newly developed potentials.2.Effects of dopant P on the tensile responses of crystalline Si(c-Si)and a-SiDopant P plays an important role in modifying the mechanical properties of c-Si and a-Si.MD is applied to investigate the underlying mechanism.It is found that the tensile fracture stress of bulk c-Si is obviously decreased by P impurities.In comparison,dopant P can effectively enhance the strength of notched c-Si by blocking the cracking of defects on surface and block the propagation of cracks.The latter mechanism plays a dominant role in strengthening the mechanical properties of Si-based micro/nano structures.Besides the fracture strength,this mechanism is also applicable for the enhancement of fatigue resistance.As for a-Si,the stiffness,yielding strength,and flow stress of bulk a-Si are increased by P doping.A transition from brittle to ductile of a-Si film by P doping is observed.Based on above conclusions,it is enlightening that processing techniques such as thermal diffusion and ion implantation can be used to increase the P doping concentration on the surface of Si micro/nano structures to improve the resistance to fracture,and that introducing P impurities can help to improve the ductility and yield strength of a-Si structures.3.Impacts of substitutional P on the tensile properties of Si nanowires with different growth orientationsThe effects of suface defects,crystal orientation,diameter,impurities etc.on the mechanical behaviors are systematicall studied.New findings are listed as follows:(1)The tensile strengths of undefected Si nanowires decrease with the increment of P concentration.Although P can lower the tensile strengths of intact nanowires,nanowires are practically accompanied with surface defects.P impurities located near the notch or in the propagation route can effectively prevent its cracking and expansion,thus enhancing its strength.(2)Undefected 2nm and 3nm [1 1 0]-Si nanowires exhibit plasticity under tension,but turn brittle when introduced surface defects.Due to the stress concentration,the crack is easier to nucleate and expand along cleavage pla ne.(3)The undefected 2nm and 3nm [1 1 0]-Si nanowires doped with P show a new failure mode.Although most of the samples are ductile,some are brittle.Interestingly,some of the 3nm samples with higher concentration fail by local amorphization.4.Influences of intrinsic defects and impurity P on the mobility of 60° dislocation in shuffle plane.The influences of crystal defects on the mobility of shuffle 60° dislocation are comprehensively discussed.It is found that Sp-<1 1 0> self-interstitial,substitutional P,Hexagonal interstitial P and PV pair can retard or even pin 60° dislocation from slipping.The underlying mechanism is the recombination of dangling bonds between defects and dislocation tip.Twins and stacking faults can prevent dislocations from crossing the twin or stacking fault interfaces even when the direction of the driven force exerted on dislocation is along the twisted shuffle plane.When subjected to compression or tension(perpendicular to the shuffle plane where the dislocation is located),the slipping velocity of the dislocation decreases.Tensile stress is more effective than compressive stress.The fatigue mechanism of Si at room or low temperature is modified.Evidences show that the activities of dislocation are prerequisites of fatigue failure.Crystal defects and impurities render the activities of dislocations complex and slow.Therefore,high-cycle is necessary to reach the ultimate fatigue failure.5.Experime ntal investigation on the effects of P on the mechanical properties of c-Si and a-Si structuresAn original multi-beam structure and the assorted primitive off-chip bending test system are designed.A great deal of tests show that P can effectively improve the bending strength and fatigue life of Si micro-beam,in accordance to the numerical simulation results above.Two a-Si film samples with different P concentrations are prepared using chemical vapor deposition.Indentation tests show that P can enhance its stiffness and strength.Based on above researches,the influences of intrinsic defects and dopant P on the failure behaviors of typical Si-based mechanical structures including bulk Si,Si films,amorphous Si and Si nanowires have been comprehensively investigated,which yield results with engineering and scientific significance.